Field

[0001] This disclosure relates to particulate catalysts useful for dry reforming of methane. The disclosure additionally relates to methods of preparing the particulate catalysts, and processes and uses for the particulate catalysts in dry reforming of methane.

Related applications

[0002] This application claims priority from Australian provisional patent application AU 2021902030, the entire contents of which are hereby incorporated by reference.

Background

[0003] Non-renewable fossil fuels are currently used as the major source of energy, making up about 85% of the fuel used worldwide. With the increasing demand for energy supply and rapid changes in the climate, there is an urgent need to develop alternative sources of energy. Hydrogen has long been considered an environmentally friendly energy carrier and has been widely used in various industrial processes, for example as a fuel in fuel-cells, as a feedstock in industrial oil refining, and as a raw material for ammonia production. Currently, hydrogen is mainly produced from hydrocarbon sources including coal, petroleum and natural gas such as methane. Since methane is the major component of petroleum reserves and landfill gas, there has been widespread interest in using methane as a source for producing hydrogen.

[0004] There are several processes for converting methane into synthesis gas (mixture of hydrogen and carbon monoxide, also called “syngas”) such as steam reforming, dry reforming, partial oxidation and autothermal reforming. Compared with other reforming processes, dry reforming of methane theoretically consumes no water but uses a large amount of carbon dioxide. Additionally, the required operating cost for dry reforming is 20% lower than other reforming processes. Therefore, dry reforming of methane is an attractive process in that it has the potential to not only relieve the pressure of greenhouse gas emission, but is also more economical.

[0005] In view of the interest in dry reforming of methane processes, there has been extensive research into the development of catalysts for dry reforming of methane. Catalysts are typically in the form of an active metal on a solid support. A problem commonly experienced with dry reforming catalysts is carbon deposition (coking) during the reaction, which may reduce the catalytic activity and stability of the catalyst and can also increase the risk of blocking of industrial equipment during operation of the dry reforming process. Further, while many dry reforming catalysts have been investigated on a small scale, they may not be suitable for large scale industrial application. For example, while noble metals have been identified as effective active metals, these metals are relatively expensive, and producing large quantities of catalyst containing these metals may not be economically feasible. Further, some catalysts may not have sufficient durability under the high temperatures and long reaction times used for industrial dry reforming of methane processes.

[0006] Accordingly, there is a need for alternative catalysts for dry reforming of methane, which may advantageously address one or more of the above identified problems.

[0007] Reference to any prior art in the specification is not an acknowledgment or suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant, and/or combined with other pieces of prior art by a skilled person in the art.

Summary

[000S] In one aspect, the present disclosure provides a particulate catalyst comprising: a porous metal oxide support comprising SiO ₂ ; and one or more supported active metals.

[0009] The particulate catalyst may be suitable for dry reforming of methane.

[0010] In some embodiments, the one or more supported active metals have an average domain size of less than about 5 nm, or less than about 3 nm.

[0011] In certain embodiments, the particulate catalyst has an active metal content of from about 0.5wt% to about 25wt%. [0012] In some embodiments, the one or more supported active metals comprise one or more transition metals.

[0013] In some embodiments, the one or more supported active metals comprise one or more group 8, group 9 and group 10 transition metals.

[0014] In some embodiments, the one or more supported active metals comprise one or more of nickel, cobalt and iron, especially one or both of nickel and cobalt.

[0015] In some embodiments, the one or more supported active metals comprise nickel.

[0016] In some embodiments, the one or more supported active metals consist of nickel.

[0017] In some embodiments, the particulate catalyst exhibits an X-ray diffraction (XRD) pattern that does not comprise peaks attributed to characteristic diffraction peaks of the one or more supported active metals. In certain embodiments, the particulate catalyst exhibits an XRD pattern that does not comprise peaks attributed to characteristic transition metal diffraction peaks, for example characteristic nickel diffraction peaks.

[0018] In some embodiments, the porous metal oxide support consists of a porous SiO ₂ support.

[0019] In some embodiments, the particulate catalyst has a specific surface area (SBET) of from about 600 m ² /g to about 1500 m ² /g.

[0020] In some embodiments, the particulate catalyst has an average pore size of from about 0.5 nm to about 3 nm.

[0021] In some embodiments, the particulate catalyst has an average particle size from about 100 nm to about 1000 nm.

[0022] In some embodiments, the particulate catalyst comprises a porous metal oxide support comprising SiO ₂ and one or more supported transition metals and further comprises any one or more of the following features: a) a specific surface area from about 600 to about 1500 m ² /g; b) an average particle size from about 100 nm to about 1000 nm; c) a weight percent transition metal(s) from about 0.5 wt.% to about 25 wt.%; d) the substantial absence of characteristic transition metal peaks in an XRD pattern of the particulate catalyst; e) at least a proportion of isolated transition metal atoms and/or isolated transition metal clusters; f) transition metal(s) having an average domain size of less than about 5 nm; and g) isolated transition metal atoms and/or isolated transition metal clusters having a diameter from about 0.5 nm to about 3 nm.

[0023] In certain embodiments, the one or more transition metals comprise or consist of nickel. In certain embodiments, the porous metal oxide support is a porous SiO ₂ support.

[0024] In another aspect, the present disclosure provides a method of preparing a particulate catalyst, the method comprising:

(i) combining a metal oxide precursor comprising one or more silica precursors, one or more surfactants and one or more bases in a solvent to provide a mixture; and

(ii) combining the mixture obtained in step (i) with one or more active metal precursors; thereby providing a particulate catalyst comprising a porous metal oxide support comprising SiO ₂ and one or more supported active metals.

[0025] In some embodiments, the mixture obtained in step (i) is in the form of a gel.

[0026] In certain embodiments, step (ii) is conducted immediately after the gel in step

(i) forms.

[0027] In some embodiments, the one or more active metal precursors comprise one or more transition metal precursors.

[0028] In some embodiments, the one or more active metal precursors comprise one or more group 8, group 9 and group 10 transition metal precursors. [0029] In some embodiments, the one or more transition metal precursors comprise one or more of a nickel precursor, a cobalt precursor and an iron precursor, especially one or both of a nickel precursor and a cobalt precursor.

[0030] In some embodiments, the one or more active metal precursors comprise a nickel precursor.

[0031] In some embodiments, the one or more active metal precursors consist of a nickel precursor.

[0032] In certain embodiments, the nickel precursor comprises one or more of NiCl2, NiCI ₂ .6H ₂ O, Ni(NOs) ₂ , Ni(NO ₃ ) ₂ .6H ₂ O, NiSO _4, NiSO ₄ .6H O, (NH ₄ ) ₂ Ni(SO ₄ ) ₂ .6H ₂ O, Ni(OCOCH ₃ ) ₂ .4H ₂ O, NiBra, NiCOa, NiFa, Nila, NiC ₂ O ₄ .2H ₂ O and Ni(CIO ₄ ) ₂ .6H ₂ O.

[0033] In some embodiments, the metal oxide support precursor consists of one or more silica precursors.

[0034] In some embodiments, the one or more silica precursors comprise one or more of tetraethyl orthosilicate (TEOS), tetramethoxy silane (TMOS), tetrakis(2- hydroxyethyl) orthosilicate (THEOS), trimethoxyvinylsilane (TMVS) and sodium silicate.

[0035] In some embodiments, the one or more surfactants comprise one or both cationic surfactant and non-ionic surfactant.

[0036] In some embodiments, the one or more surfactants comprise one or both of cetyltrimethylammonium bromide (CTAB) and cetyltrimetbylammonium chloride (CTAC).

[0037] In some embodiments, the one or more bases comprise one or both of sodium hydroxide and ammonium hydroxide.

[0038] In some embodiments, the method further comprises a step of quenching the particulate catalyst.

[0039] In some embodiments, the method further comprises a step of drying the particulate catalyst.

[0040] In some embodiments, the method further comprises a step of reducing the particulate catalyst. [0041] In some embodiments of the method, the particulate catalyst is the particulate catalyst as described herein.

[0042] In another aspect, the present disclosure provides the use of the particulate catalyst described herein or prepared by the method described herein for dry reforming of methane.

[0043] In another aspect, the present disclosure provides a process for dry reforming of methane, the process comprising: contacting methane and carbon dioxide in the presence of the particulate catalyst described herein or prepared by the method described herein; thereby producing hydrogen and carbon monoxide.

[0044] In some embodiments, the process is a continuous process.

[0045] In some embodiments of the process, the conversion of methane and/or carbon dioxide decreases by less than 10% relative to initial conversion after 1000 hours on stream at a temperature of about 500°C to about 800°C and a gas hourly space velocity (GHSV) of about 5.4x10 ⁴ mL g ^-1 h ^-1 .

[0046] In some embodiments of the process, the molar ratio of produced hydrogen to carbon monoxide is at least 0.7.

[0047] In some embodiments of the process, the molar ratio of produced hydrogen to carbon monoxide is at least 0.7, after 1000 hours on stream at a temperature of about 500°C to about 800°C and a gas hourly space velocity (GHSV) of about 5.4x10 ⁴ mL g ^-1 h ^-1 .

[0048] The present disclosure may offer one or more of the following advantages: in the case of particulate catalysts having one or more active metals comprising one or more period 4 transition metals (eg nickel, cobalt, iron), the silica support and active metal source of the particulate catalyst are relatively inexpensive; the particulate catalyst exhibits good catalytic activity, stability and durability under dry reforming of methane reaction conditions; the particulate catalyst exhibits resistance to coking; the method for preparing the particulate catalyst is relatively simple; the method for preparing the particulate catalyst is scalable; the particulate catalyst can be applied to large scale industrial processes.

[0049] Further aspects of the present disclosure and further embodiments of the aspects described in the preceding paragraphs will become apparent from the following description, given by way of example and with reference to the accompanying drawings.

Brief description of the drawings

[0050] Figure 1. Characterisation of 2.5%, 5% and 12.5% Ni/SiO ₂ catalysts. Figure 1 A: wide-angle XRD pattern of 2.5%, 5% and 12.5% Ni/SiO ₂ catalysts. Figures 1 B-D: N ₂ adsorption-desorption isotherms of (B) 2.5% Ni/SiO ₂ , (C) 5% Ni/SiO ₂ , and (D) 12.5% Ni/SiO ₂ catalysts.

[0051] Figure 2. Transmission electron microscopy (TEM) images of Ni/SiO ₂ samples. Figures 2A-2C: High angle annular dark filed scanning transmission microscopy (FIADDF-STEM) images of 2.5% Ni/SiO ₂ (Figure 2A), 5% Ni/SiO ₂ (Figure 2B) 12.5% Ni/SiO2 (Figure 2C) samples after 800°C H ₂ reduction. Figures 2D-2F: Energy-dispersive X-ray spectroscopy (EDS) mapping of prepared 12.5% Ni/SiO ₂ catalysts. The distribution of Ni is depicted in Figure 2E and the distribution of Si is depicted in Figure 2F.

[0052] Figure 3. Atomic Probe Tomography (APT) and APT simulations and analysis of 12.5% Ni/SiO ₂ catalysts. Figure 3A: APT for 12.5% Ni/SiO ₂ catalysts. Figure 3B: APT for 12.5% Ni/SiO ₂ catalysts with cluster identification. The enclosed areas represent the sites having more than 1 .937 Ni atoms per nm ² . Figure 3C: Ni radical distance function (RDF) based on the APT for 12.5% Ni/SiO ₂ . Figure 3D: Ni frequency distribution analysis (FDA) for 12.5% Ni/SiO ₂ .

[0053] Figure 4. H ₂ -Temperature programmed reduction (TPR) profiles of prepared 2.5%, 5% and 12.5% Ni/SiO ₂ catalysts.

[0054] Figure 5. Dry reforming of methane (DRM) results of 2.5% Ni/SiO ₂ , 5%

Ni/SiO ₂ and 12.5% Ni/SiO ₂ catalysts. Figure 5A: CH ₄ conversion (circles), CO ₂ conversion (squares), and H ₂ /CO ratio (inset) of DRM for 2.5% Ni/SiO ₂ (dotted lines), 5% Ni/SiO ₂ (dashed lines) and 12.5% Ni/SiO ₂ (solid lines) catalysts at carrying temperatures from 500°C to 800°C. Figures 5B-5D: CH ₄ conversion (Figure 5B), CO ₂ conversion (Figure 5C), and FI2/CO ratio (Figure 5D) of DRM for 2.5% Ni/SiO ₂ (squares), 5% Ni/SiO ₂ (circles) and 12.5% Ni/SiO ₂ (stars) catalysts at 800°C over 20 hours reaction period. Reaction conditions: CH ₄ /CO ₂ =1 :1 , GHSV=5.4x10 ⁴ mL g ^-1 h ^-1 .

[0055] Figure 6. DRM results of 12.5% Ni/SiO ₂ catalysts. CH ₄ conversion (upper main trace), CO ₂ conversion (lower main trace) and FI2/CO ratio (inset) of DRM for the 12.5% Ni/SiO ₂ catalyst at 800°C over 1000 hours reaction period. Reaction conditions: CH ₄ /CO ₂ =1 :1 , GHSV=5.4x10 ⁴ ml_ g ^-- h ^-1 .

[0056] Figure 7. Analysis of used 2.5%, 5% and 12.5% Ni/SiO ₂ catalysts. Figure 7A: O ₂ - Temperature programmed oxidation (TPO) analysis. Figure 7B: Thermogravimetric analysis (TGA). Figure 7C: Wide-angle XRD analysis. Figures 7D-7F: FIRTEM images of 2.5% Ni/SiO ₂ (Figure 7D), 5% Ni/SiO ₂ (Figure 7E), and 12.5% Ni/SiO ₂ (Figure 7F) after 20 hours of reaction at 800°C with GSFIV=5.4x10 ⁴ mL g ^-1 h ^-1 .

[0057] Figure 8. Cluster analysis of APT of 12.5% Ni/SiO ₂ catalysts.

[0058] Figure 9. Schematic illustration of a hypothesised DRM reaction mechanism occurring over the Ni/SiO ₂ catalyst.

[0059] Figure 10. DRM results of a comparative 10% N1/SO ₂ catalyst prepared by incipient wetness impregnation method. Figures 10A-10C: CH ₄ conversion (Figure 10A), CO ₂ conversion (Figure 10B), and FI2/CO ratio (Figure 10C) of DRM at 800°C over 50 hours reaction period. Reaction conditions: CH ₄ /CO ₂ =1 :1 , GFISV=5.4x10 ⁴ mL g ^-1 h ^-1 .

[0060] Figure 11. DRM results of a 12.5% C0/SiO ₂ catalyst. Figures 11 A-11 C: CH ₄ conversion (Figure 11 A), CO ₂ conversion (Figure 11 B), and FI2/CO ratio (Figure 11 C) of DRM for 12.5% C0/SiO ₂ catalyst at 800°C over 100 hours reaction period. Reaction conditions: CH ₄ /CO ₂ =1 :1 , GHSV=5.4x10 ⁴ mL g ^-1 h ^-1 .

Detailed description of the embodiments

[0061] The following is a detailed description of the disclosure provided to aid those skilled in the art in practicing the present disclosure. Those of ordinary skill in the art may make modifications and variations in the embodiments described herein without departing from the spirit or scope of the present disclosure.

[0062] Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present disclosure, the preferred methods and materials are now described.

[0063] The following definitions are included to provide a clear and consistent understanding of the specification and claims. As used herein, the recited terms have the following meanings. All other terms and phrases used in this specification have their ordinary meanings as one of skill in the art would understand. Such ordinary meanings may be obtained by reference to technical dictionaries, such as Hawley's Condensed Chemical Dictionary 14th Edition, by R. J. Lewis, John Wiley & Sons, New York, N.Y., 2001.

[0064] As used in the specification and the appended claims, the singular forms ‘a’, ‘an’ and ‘the’ include plural referents unless otherwise specified. Thus, for example, reference to ‘a metal oxide’ may include one or more metal oxide(s) and reference to an ‘active metal’ may include at least one active metal, and the like.

[0065] As used herein, except where the context requires otherwise, the term ‘comprise’ and variations of the term, such as ‘comprising’, ‘comprises’ and ‘comprised’, are not intended to exclude further additives, components, integers or steps.

[0066] Unless specifically stated or obvious from context, as used herein, the term about’ is understood as within a range of normal tolerance in the art, for example within two standard deviations of the mean. ‘About’ can be understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated value.

Unless otherwise clear from context, all numerical values provided herein in the specification and the claim can be modified by the term ‘about’.

[0067] Ranges provided herein are understood to be shorthand for all of the values, including non-integer values, within the range. For example, a range of 1 to 50 is understood to include any number, combination of numbers, or sub-range from the group consisting 1 , 1.01 , 2, 2.2, 3, 3.45, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 , 32, 33, 34, 35, 36, 37, 38, 39, 40,

41 , 42, 43, 44, 45, 46, 47, 48, 49, or 50. [0068] It will be understood that the disclosure described and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the disclosure.

[0069] Context allowing, it is intended that any embodiment described herein may be combined with any other embodiment.

[0070] Any methods provided herein can be combined with one or more of any of the other methods provided herein.

Catalyst

[0071] The present disclosure provides a particulate catalyst comprising: a porous metal oxide support comprising SiO ₂ ; and one or more supported active metals.

[0072] The term “supported active metals” will be understood to mean that the one or more active metals are on the surface of the support, including in the pores of the support.

[0073] The one or more supported active metals may comprise one or more transition metals. The term “transition metal” will be understood to encompass an element in any one of groups 3 to 12 of the periodic table. In some embodiments, the supported active metal comprises one or more group 4, group 5, group 6, group 7, group 8, group 9, group 10, and group 11 transition metals, especially one or more group 5, group 6, group 7, group 8, group 9, group 10 and group 11 transition metals, more especially one or more group 8, group 9 and group 10 transition metals. Additionally, or alternatively, in some embodiments, the supported active metal comprises one or more period 4, period 5 and period 6 transition metals, especially one or more period 4 and period 5 transition metals, even more especially one or more period 4 transition metals. In this context, the term “period” will be understood to refer period (row) of the periodic table.

[0074] In some embodiments, the one or more supported active metals comprise one or more of nickel, cobalt and iron, especially one or both of nickel and cobalt. [0075] In preferred embodiments, the one or more supported active metals comprise nickel. In some embodiments, the one or more supported active metals consist of nickel. In this context, the term “consists of will be understood to imply that the particulate catalyst does not include any further supported active metals, that is, the particulate catalyst includes only includes nickel as the supported active metal.

[0076] The supported active metal may have an average domain size of less than about 5 nm. The term “domain size” will be understood to refer to the size of isolated atoms and/or isolated clusters of the supported active metal. Alternatively, or additionally, the supported active metal may be present in the form of single (isolated) atoms and/or single (isolated) clusters having an average size of less than about 5 nm. As shown in the Examples, the small size of atomically dispersed and isolated clusters of the supported active metal of the catalyst described herein may advantageously allow for high dispersion of the active metal on the support and/or reduced carbon (coke) depositing on the catalyst during dry reforming of methane. In some embodiments, the active metal has an average domain size of less than about 4 nm, about 3 nm, about 2 nm, or about 1 nm. In preferred embodiments, the active metal has an average domain size of less than about 2 nm, or about 1 nm. In some embodiments, the active metal has an average domain size of at least about 0.20 nm, about 0.25 nm, about 0.30 nm, about 0.35 nm, about 0.40 nm, about 0.45 nm, or about 0.50 nm. In some embodiments, the active metal has an average domain size of at least about the size of an isolated atom of any one of the one or more active metals. Any minimum and maximum value may be combined to form a range provided that the average size is within 0.20 nm (or the size of an isolated atom of any one of the one or more active metals) to 5 nm, for example from about 0.20 nm to about 2 nm or from about 0.20 nm to about 1 nm. The size may be determined by methods known in the art, for example measured from scanning transmission electron microscope (STEM) images or calculated using the Schemer equation from X-ray diffraction (XRD) patterns. It will be understood that the active metal sizes referred to in this paragraph relate to a catalyst which has not undergone reductive pre-treatment or been used in the dry reforming of methane.

[0077] Accordingly, in some embodiments, the particulate catalyst comprises: a porous metal oxide support comprising SiO ₂ ; and one or more supported active metals having an average domain size of less than about 5 nm.

[0078] In some embodiments, the particulate catalyst comprises: a porous metal oxide support comprising SiO ₂ ; and one or more supported active metals comprising one or more transition metals, especially one or more group 8, group 9 and group 10 transition metals, having an average domain size of less than about 5 nm.

[0079] In some embodiments, the particulate catalyst comprises: a porous metal oxide support comprising SiO ₂ ; and one or more supported active metals comprising one or more of nickel, cobalt and iron, especially one or both of nickel and cobalt, having an average domain size of less than about 5 nm.

[0080] Due to the small size of the one or more supported active metals, the active metal loading may have a negligible effect on the pore structure of the corresponding parent support (i.e. the support without active metal). Accordingly, in some embodiments, the particulate catalyst may have substantially the same average pore size as the porous support alone (ie the porous support prepared in the absence of supported active metal).

[0081] Due to the small size of the one or more supported active metals, an X-ray diffraction (XRD) spectrum of a particulate catalyst may not contain signals corresponding to characteristic diffraction peaks of the one or more active metals. Accordingly, in some embodiments, the particulate catalyst described herein exhibits an X-ray diffraction (XRD) pattern that does not comprise peaks attributed to characteristic diffraction peaks of the one or more active metals. It will be understood that these embodiments relate to a particulate catalyst which has not been used in the dry reforming of methane. These embodiments may apply to a particulate catalyst which has undergone reductive pre-treatment.

[0082] In some embodiments, the particulate catalyst has a supported active metal content of from about 0.5wt% to about 25wt%, especially from about 0.5wt% to about 15wt%, more especially from about 2.5wt% to about 12.5 wt%, based on the total weight of the catalyst. In certain embodiments, the particulate catalyst has an active metal loading of from about 0.5 wt% to about 25 wt%, especially from about 0.5wt% to about 15wt%, more especially from about 2.5wt% to about 12.5wt%. In certain embodiments, the particulate catalyst has an active metal loading of about 2.5 wt%, about 5.0wt% or about 12.5 wt%. In some embodiments, the particulate catalyst has an active metal loading of at least about 0.5 wt%, about 1 .0 wt%, about 1 .5 wt%, about 2.0 wt%, about 2.5 wt%, about 3.0 wt%, about 3.5 wt%, about 4.0 wt%, or about 4.5 wt%. In some embodiments, the particulate catalyst has an active metal loading of not more than about 25 wt%, about 24 wt%, about 23 wt%, about 22 wt%, about 21 wt%, about 20 wt%, about 19 wt%, about 18 wt%, about 17 wt%, about 16 wt%, about 15 wt%, about 14.5 wt%, about 14.0 wt%, about 13.5 wt%, about 13.0 wt%, or about 12.5 wt%. Any minimum and maximum value may be combined to form a range provided that the active metal loading is within 0.5wt% to 25wt%, for example from about 0.5wt% to about 15wt%, or from about 2.5wt% to about 12.5 wt%. The active metal content or active metal loading may be calculated or determined by methods known in the art, for example by inductively coupled plasma - optical emission spectrometry (ICP-OES).

[0083] In preferred embodiments, the one or more supported active metals comprises or consists of nickel.

[0084] Accordingly, in some embodiments, the particulate catalyst comprises: a porous metal oxide support comprising SiO ₂ ; and one or more supported active metals comprising nickel having an average domain size of less than about 5 nm.

[0085] Further, in some embodiments, the particulate catalyst comprises: a porous metal oxide support comprising SiO ₂ ; and supported nickel having an average domain size of less than about 5 nm.

[0086] Due to the small size of the supported nickel, the nickel loading may have a negligible effect on the pore structure of the corresponding parent support (i.e. the support without nickel). Accordingly, in certain embodiments where the particulate catalyst comprises or consists of supported nickel as an active metal, the particulate catalyst may have substantially the same average pore size as the porous support alone (ie the porous support prepared in the absence of supported nickel). By way of example, a parent SiO ₂ support which does not comprise supported nickel may have an average pore size of about 1 .2 nm, and a particulate catalyst comprising or consisting of supported nickel as an active metal may have an average pore size of about 1 .2 nm.

[0087] Due to the small size of the supported nickel, an X-ray diffraction (XRD) spectrum of a particulate catalyst comprising or consisting of supported nickel as an active metal may not contain signals corresponding to characteristic diffraction peaks of nickel species, such as NiO and metallic Ni. Accordingly, in certain embodiments, the particulate catalyst described herein exhibits an X-ray diffraction (XRD) pattern that does not comprise peaks attributed to characteristic nickel diffraction peaks, including characteristic nickel diffraction peaks at 2Q between about 30° to about 80°. Examples of characteristic NiO diffraction peaks include peaks at 2Q of about 37°, about 43°, about 63°, about 75° and about 79°, which correspond to (111), (200), (220), (311), and (222) crystal planes of NiO respectively. Examples of characteristic metallic Ni diffraction peaks include peaks at 2Q of about 44°, about 52° and about 77°. In certain embodiments, the particulate catalyst exhibits an XRD spectrum which substantially corresponds to any one of the XRD spectra as shown in Figure 1 A. It will be understood that these embodiments relate to a catalyst which has not been used in the dry reforming of methane. These embodiments may apply to a catalyst which has undergone reductive pre-treatment.

[0088] A particulate catalyst comprising or consisting of supported nickel as an active metal may exhibit a hydrogen temperature-programmed reduction (H ₂ -TPR) spectrum having one or more peaks corresponding to moderate and/or strong metal-support interactions. Accordingly, in certain embodiments, the particulate catalyst described herein exhibits a H ₂ -TPR spectrum having a peak at from about 500°C to about 850°C, especially from about 580°C to about 800°C. In some embodiments, the particulate catalyst exhibits a H ₂ -TPR spectrum which substantially corresponds to any one of the H ₂ -TPR spectra as shown in Figure 4. It will be understood that these embodiments relate to a catalyst which has not undergone reductive pre-treatment or been used in the dry reforming of methane. Advantageously, as shown in the Examples, the presence of moderate to strong metal-support interactions may reduce carbon (coke) depositing on the catalyst, may prevent aggregation of Ni clusters, and/or may reduce metal sintering of the catalyst during dry reforming of methane.

[0089] In some embodiments, the particulate catalyst described herein does not comprise agglomerates of supported active metal, that is, supported active metal having an average domain size of greater than about 6 nm, for example greater than about 7 nm, greater than about 8 nm, greater than about 9 nm, or greater than about 10 nm. In certain embodiments, the particulate catalyst described herein does not comprise agglomerates of supported nickel, that is, supported nickel having an average domain size of greater than about 6 nm, for example greater than about 7 nm, greater than about 8 nm, greater than about 9 nm, or greater than about 10 nm. It will be understood that embodiments relate to a catalyst which has not undergone reductive pre-treatment or been used in the dry reforming of methane.

[0090] In some embodiments, the porous metal oxide support consists of a porous SiO ₂ support. In this context, “consists of will be understood to imply that the particulate catalyst does not include any further metal oxides in the support, that is, the particulate catalyst includes only SiO ₂ as the metal oxide in the support.

[0091] Accordingly, in some embodiments, the particulate catalyst comprises: a porous SiO ₂ support; and one or more supported active metals.

[0092] In some embodiments, the particulate catalyst comprises: a porous SiO ₂ support; and one or more supported active metals comprising one or more transition metals, especially one or more group 8, group 9 and group 10 transition metals.

[0093] In some embodiments, the particulate catalyst comprises: a porous SiO ₂ support; and one or more supported active metals comprising one or more of nickel, cobalt and iron, especially one or both of nickel and cobalt. [0094] In some embodiments, the particulate catalyst comprises: a porous SiO ₂ support; and one or more supported active metals comprising nickel.

[0095] In some embodiments, the particulate catalyst comprises: a porous SiO ₂ support; and supported nickel.

[0096] Further, in some embodiments, the particulate catalyst comprises: a porous SiO ₂ support; and one or more supported active metals having an average domain size of less than about 5 nm.

[0097] In some embodiments, the particulate catalyst comprises: a porous SiO ₂ support; and one or more supported active metals comprising one or more transition metals, especially one or more group 8, group 9 and group 10 transition metals, having an average domain size of less than about 5 nm.

[0098] In some embodiments, the particulate catalyst comprises: a porous SiO ₂ support; and one or more supported active metals comprising one or more of nickel, cobalt and iron, especially one or both of nickel and cobalt, having an average domain size of less than about 5 nm.

[0099] In some embodiments, the particulate catalyst comprises: a porous SiO ₂ support; and one or more supported active metals comprising nickel having an average domain size of less than about 5 nm. [0100] In some embodiments, the particulate catalyst comprises: a porous SiO ₂ support; and supported nickel having an average domain size of less than about 5 nm.

[0101] In some embodiments, the particulate catalyst has a specific surface area (SBET) of from about 600 m ² /g to about 1500 m ² /g, especially from about 900 m ² /g to about 1200 m ² /g, more especially from about 1000 m ² /g to about 1100 rm ² /g. In some embodiments, the particulate catalyst has a specific surface area of at least about 600 m ² /g, about 700 m ² /g, about 800 m ² /g, about 900 m ² /g, about 950 m ² /g, or about 1000 m ² /g. In some embodiments, the particulate catalyst has a specific surface area of not more than about 1500 m ² /g, about 1400 m ² /g, about 1300 m ² /g, about 1200 m ² /g, about 1150 m ² /g, or about 1100 m ² /g. Any minimum and maximum value may be combined to form a range provided that the specific surface area is within 600 to 1500 m ² /g, for example from about 900 m ² /g to about 1200 rm ² /g or from about 950 m ² /g to about 1150 m ² /g. The specific surface area may be determined by the methods known in the art, for example from nitrogen adsorption-desorption isotherms by the Brunauer-Emmett-Teller equation. Advantageously, having a relatively large surface area may allow for high dispersion of the one or more metal actives (eg nickel) on the catalyst.

[0102] In some embodiments, the particulate catalyst has a total pore volume of from about 0.60 cm ³ /g to about 1 .0 cm ³ /g, especially about 0.70 cm ³ /g to about 0.96 cm ³ /g, more especially from about 0.80 cm ³ /g to about 0.85 cm ³ /g. In some embodiments, the particulate catalyst has a total pore volume of at least about 0.60 cm ³ /g, about 0.62 cm ³ /g, about 0.64 cm ³ /g, about 0.66 cm ³ /g, about 0.68 cm ³ /g, about 0.70 cm ³ /g, about 0.72 cm ³ /g, about 0.74 cm ³ /g, about 0.76 cm ³ /g, about 0.78 cm ³ /g, or about 0.80 cm ³ /g. In some embodiments, the particulate catalyst has a total pore volume of not more than about 1.0 cm ³ /g, about 0.98 cm ³ /g, about 0.96 cm ³ /g, about 0.94 cm ³ /g, about 0.92 cm ³ /g, about 0.90 crm ³ /g, about 0.88 cm ³ /g, about 0.87 cm ³ /g, about 0.86 cm ³ /g, or about 0.85 cm ³ /g. Any minimum and maximum value may be combined to form a range provided that the total pore volume is within 0.60 cm ³ /g to 1 .0 cm ³ /g, for example from about 0.80 cm ³ /g to about 0.90 cm ³ /g. The total pore volume may be determined by methods known in the art, for example from nitrogen adsorption-desorption isotherms by the Brunauer-Emmett-Teller equation. [0103] In some embodiments, the particulate catalyst has an average pore size (diameter) of from about 0.5 nm to about 5 nm, especially about 0.5 nm to about 3 nm, more especially from about 1 nm to about 1 .5 nm, even more especially about 1 .2 nm. In some embodiments, the particulate catalyst has an average pore size of at least about 0.5 nm, about 0.6 nm, about 0.7 nm, about 0.8 nm, about 0.9 nm, about 1 .0 nm, about 1.1 nm, about 1 .2 nm, about 1 .3 nm, about 1 .4 nm, about 1 .5 nm, about 2.0 nm, about 2.5 nm, or about 3.0 nm. In some embodiments, the particulate catalyst has an average pore size of not more than about 5 nm, about 4.8 nm, about 4.6 nm, about 4.5 nm, about 4.4 nm, about 4.2 nm, about 4.0 nm, about 3.8 nm, about 3.6 nm, about 3.5 nm, about 3.4 nm, about 3.2 nm, about 3.0 nm, about 2.8 nm, about 2.6 nm, about 2.4 nm, about 2.2 nm, about 2.0 nm, about 1 .9 nm, about 1 .8 nm, about 1 .7 nm, about 1.6 nm, about 1.5 nm, about 1 .4 nm, about 1 .3 nm, or about 1 .2 nm. Any minimum and maximum value can be combined to form a range provided that the minimum value is smaller than the maximum value and the average pore size is within 0.5 to about 5 nm, for example from about 2.0 nm to about 3.0 nm or from about 1 .0 to about 1.4 nm. The pore size may be determined by methods known in the art, for example from nitrogen adsorption-desorption isotherms through the Barrett-Joyner-Halenda model.

[0104] In some embodiments, the one or more supported active metals are distributed on the support (ie distributed on the surface of the support, including in the pores of the support). In some embodiments, the one or more supported active metals are substantially uniformly distributed on the support. Advantageously, having the one or more active metals distributed on the support may reduce significant agglomeration of the one or more active metals on the catalyst during dry reforming of methane, which may improve the stability of the catalyst.

[0105] In some embodiments, the particulate catalyst has an average particle size (diameter) of from about 100 nm to about 1000 nm, for example from about 100 nm to about 900 nm, from about 100 nm to about 800 nm, from about 100 nm to about 700 nm, from about 100 nm to about 600 nm, or from about 100 nm to about 500 nm.

Catalyst preparation

[0106] The present disclosure provides a method of preparing a particulate catalyst. The method comprises the steps of: (i) combining a metal oxide support precursor comprising one or more silica precursors, one or more surfactants and one or more bases in a solvent to provide a mixture; and

(ii) combining the mixture obtained in step (i) with one or more active metal precursors; thereby providing a catalyst comprising a porous metal oxide support comprising SiO ₂ and one or more supported active metals.

[0107] In preferred embodiments, the mixture obtained in step (i) is in the form of a gel. In these embodiments, step (ii) may be conducted after the gel forms.

[0108] Accordingly, in some embodiments, the method comprises:

(i) combining a metal oxide support precursor comprising one or more silica precursors, one or more surfactants and one or more bases in a solvent to provide a gel; and

(ii) after the gel in step (i) is formed, combining the gel obtained in step (i) with one or more active metal precursors; thereby providing a catalyst comprising a porous metal oxide support comprising SiO ₂ and one or more supported active metals.

[0109] In some embodiments, step (ii) is conducted immediately after the gel forms.

In some embodiments, step (ii) is conducted within about 60 minutes, within about 45 minutes, within about 30 minutes, within about 15 minutes, within about 10 minutes, within about 5 minutes, within about 4 minutes, within about 3 minutes, within about 2 minutes, or within about 1 minute after the gel forms. In some embodiments, step (ii) is conducted before the metal oxide support is completely formed (established).

[0110] The one or more active metal precursors may comprise one or more transition metal precursors. In some embodiments, the active metal precursor comprises one or more group 4, group 5, group 6, group 7, group 8, group 9, group 10, and group 11 transition metal precursors, especially one or more group 5, group 6, group 7, group 8, group 9, group 10 and group 11 transition metal precursors, more especially one or more group 8, group 9 and group 10 transition metal precursors. Additionally, or alternatively, in some embodiments, the supported active metal comprises one or more period 4, period 5 and period 6 transition metal precursors, especially one or more period 4 and period 5 transition metals precursors, even more especially one or more period 4 transition metal precursors.

[0111] In some embodiments, the one or more active metal precursors comprise one or more of nickel, cobalt and iron precursors, especially one or both of nickel and cobalt precursors.

[0112] In preferred embodiments, the one or more active metal precursors comprise a nickel precursor. In some embodiments, the one or more active metal precursors consist of a nickel precursor. In this context, the term “consists of” will be understood to imply that the method (in particular step (ii)) does not include any further active metal precursors, that is, the method includes only includes the nickel precursor as the active metal precursor.

[0113] Accordingly, in some embodiments, the method comprises:

(i) combining a metal oxide support precursor comprising one or more silica precursors, one or more surfactants and one or more bases in a solvent to provide a mixture; and

(ii) combining the mixture obtained in step (i) with one or more active metal precursors comprising one or more transition metal precursors, especially one or more group 8, group 9 and group 10 transition metal precursors; thereby providing a catalyst comprising a porous metal oxide support comprising SiO ₂ and one or more supported transition metals, especially one or more group 8, group 9 and group 10 transition metals.

[0114] In some embodiments, the method comprises:

(i) combining a metal oxide support precursor comprising one or more silica precursors, one or more surfactants and one or more bases in a solvent to provide a mixture; and (ii) combining the mixture obtained in step (i) with one or more active metal precursors comprising one or more of nickel, cobalt and iron precursors, especially one or both of nickel and cobalt precursors; thereby providing a catalyst comprising a porous metal oxide support comprising SiO ₂ and one or more of supported nickel, cobalt and iron, especially one or both of supported nickel and cobalt.

[0115] In some embodiments, the method comprises:

(i) combining a metal oxide support precursor comprising one or more silica precursors, one or more surfactants and one or more bases in a solvent to provide a mixture; and

(ii) combining the mixture obtained in step (i) with one or more active metal precursors comprising a nickel precursor; thereby providing a catalyst comprising a porous metal oxide support comprising SiO ₂ and one or more supported active metals comprising supported nickel.

[0116] Further, in some embodiments, the method comprises:

(i) combining a metal oxide support precursor comprising one or more silica precursors, one or more surfactants and one or more bases in a solvent to provide a mixture; and

(ii) combining the mixture obtained in step (i) with a nickel precursor; thereby providing a catalyst comprising a porous metal oxide support comprising SiO ₂ and supported nickel.

[0117] The one or more supported active metals may have an average domain size of less than about 5 nm as described herein. Alternatively, or additionally, the one or more supported active metals may be present in the form of isolated atoms and/or isolated clusters having an average size of less than about 5 nm as described herein.

[0118] The one or more active metal precursors may be any suitable precursors capable of providing the one or more supported active metals known in the art. Examples of suitable nickel precursors include NiCI ₂ , NiCI ₂ .6H ₂ O, Ni(NO ₃ ) ₂ , Ni(NO ₃ ) ₂ .6H ₂ O, NiSO ₄ , NiSO ₄ .6H ₂ O, (NH ) ₂ Ni(SO ₄ ) ₂ .6H ₂ O, Ni(OCOCH ₃ ) ₂ .4H ₂ O, NiBr ₂ , NiCO ₃ , NiF2, NiI ₂ , NiC2O ₄ .2H ₂ O and Ni(CIO ₄ ) ₂ .6H ₂ O. In some embodiments, the nickel precursor comprises one or more of NiCI ₂ , NiCI ₂ .6H ₂ O, Ni(NO ₃ ) ₂ , Ni(NO ₃ ) ₂ .6H ₂ O, NiSO ₄ and NiSO ₄ .6H ₂ O. In preferred embodiments, the nickel precursor is selected from one or both of Ni(NO ₃ ) ₂ .6H ₂ O and NiSO ₄ .6H ₂ O. Examples of suitable cobalt precursors include C0CI ₂ , C0CI ₂ .6H ₂ O, Co(NO ₃ ) ₂ , Co(NO ₃ ) ₂ .6H ₂ O, C0SO ₄ , C0SO ₄ .7H ₂ O, C0CO3, CO(OCOCH ₃ ) ₂ .4H ₂ O, C0C2O ₄ .2H ₂ O and Co(C5H7O ₂ ) ₃ . Examples of suitable iron precursors include Fe(NO ₃ ) ₂ , Fe(NO ₃ ) ₂ .9H ₂ O, FeCI ₃ , FeCI ₃ , Fl2O, Fe2(SO ₄ ) ₃ , Fe2(SO ₄ ) ₃ . H ₂ O and Fe(CO ₃ ) ₃ .

[0119] Advantageously, the method disclosed herein may allow for one or more active metals (eg nickel) to be loaded on the porous silica support in the form of isolated atoms or isolated clusters. Without wishing to be bound by theory, the inventors hypothesise that the method described herein may involve the formation of a silica network by mixing the silica precursor, surfactant and base in the solvent. Subsequently adding one or more active metal precursors (eg a nickel precursor) to the silica network in situ may allow the metal(s) to be distributed on the silica support in the form of isolated atoms or isolated clusters. As shown in the Examples, atomically distributed and/or isolated clusters of the supported active metal of particulate catalysts prepared by the method described herein advantageously provide the catalysts with improved catalytic activity, stability and durability during dry reforming of methane. In contrast, a comparative catalyst which was prepared by a conventional incipient wetness impregnation method (loading a nickel precursor on a pre-formed solid S1O ₂ support) showed poorer catalytic activity and stability during dry reforming of methane compared to supported nickel particulate catalysts prepared by the method described herein.

[0120] In some embodiments, the metal oxide precursor consists of one or more silica precursors. In this context, “consists of” will be understood to imply that the method (in particular step (i)) does not include any further metal oxide support precursors, that is, the method includes only the one or more silica precursors as the metal oxide support precursor.

[0121] Accordingly, in some embodiments, the method comprises:

(i) combining one or more silica precursors, one or more surfactants and one or more bases in a solvent to provide a mixture; and (ii) combining the mixture obtained in step (i) with one or more active metal precursors; thereby providing a catalyst comprising a porous SiO ₂ support and one or more supported active metals.

[0122] In some embodiments, the method comprises:

(i) combining one or more silica precursors, one or more surfactants and one or more bases in a solvent to provide a mixture; and

(ii) combining the mixture obtained in step (i) with one or more active metal precursors comprising one or more transition metal precursors, especially one or more group 8, group 9 and group 10 transition metal precursors; thereby providing a catalyst comprising a porous SiO ₂ support and one or more supported transition metals, especially one or more group 8, group 9 and group 10 transition metals.

[0123] In some embodiments, the method comprises:

(i) combining one or more silica precursors, one or more surfactants and one or more bases in a solvent to provide a mixture; and

(ii) combining the mixture obtained in step (i) with one or more active metal precursors comprising one or more of nickel, cobalt and iron precursors, especially one or both of nickel and cobalt precursors; thereby providing a catalyst comprising a porous SiO ₂ support and one or more of supported nickel, cobalt and iron, especially one or both of supported nickel and cobalt.

[0124] In some embodiments, the method comprises:

(i) combining one or more silica precursors, one or more surfactants and one or more bases in a solvent to provide a mixture; and

(ii) combining the mixture obtained in step (i) with one or more active metal precursors comprising a nickel precursor; thereby providing a catalyst comprising a porous SiO ₂ support and one or more supported active metals comprising nickel.

[0125] In some embodiments, the method comprises:

(i) combining one or more silica precursors, one or more surfactants and one or more bases in a solvent to provide a mixture; and

(ii) combining the mixture obtained in step (i) with a nickel precursor; thereby providing a catalyst comprising a porous SiO ₂ support and supported nickel.

[0126] The silica precursor may be any suitable silica precursor capable of providing a SiO ₂ support. Examples of suitable silica precursors include inorganic silica sources, for example alkoxysilanes such as tetraethyl orthosilicate (TEOS), tetramethoxy silane (TMOS) and tetrakis(2-hydroxyethyl) orthosilicate (THEOS); alkoxyvinylsilanes such as trimethoxyvinylsilane (TMVS); and alkali metal silicates such as sodium silicate. In some embodiments, one or more silica precursors comprises one or more alkoxysilanes, for example one or more of tetraethyl orthosilicate (TEOS), tetramethoxy silane (TMOS) and tetrakis(2-hydroxyethyl) orthosilicate (THEOS). In preferred embodiments, the one or more silica precursors comprise TEOS.

[0127]The one or more surfactants may act as a templating agent. The one or more surfactants may comprise a cationic surfactant and a non-ionic surfactant. Examples of suitable cationic surfactants include tetraalkylammonium halides comprising at least one alkyl chain greater than 8 carbon atoms, for example cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC). Examples of suitable non-ionic surfactants include surfactants comprising a polyoxyethylene and/or polyoxypropylene chain, for example poloxamers such as Pluronic F123 and Pluronic F127 (poloxamer 407); polyoxyethylene stearyl ethers such as Brij-76; Triton X-100; any polysorbates such as polysorbate 20 (Tween 20), polysorbate 40 (Tween 40), polysorbate 60 (Tween 60) and polysorbate 80 (Tween 80). In some embodiments, one or more surfactant are selected from one or more cationic surfactants. In preferred embodiments, the one or more surfactants are selected from CTAB and CTAC, especially CTAB.

[0128] The one or more bases may be used to increase the pH of the mixture, for example to increase the pH of the mixture to a basic pH. Additionally, or alternatively, the one or more bases may act as a catalyst. Examples of suitable bases include hydroxide bases such as sodium hydroxide (NaOH) and ammonium hydroxide (NH ₄ OH) and amine bases such as triethanolamine (TEA) and diethanolamine (DEA). In some embodiments, the one or more bases comprise one or more hydroxide bases. In preferred embodiments, the one or more bases are selected from sodium hydroxide and ammonium hydroxide. In embodiments where the one or more bases are selected from one or more hydroxide bases, for example sodium hydroxide and ammonium hydroxide, the one or more hydroxide bases may be provided in the form of an aqueous hydroxide solution, for example an aqueous sodium hydroxide solution and an aqueous ammonium hydroxide solution. The aqueous hydroxide solutions may be provided in any concentration suitable for use in the mixture. In embodiments where the base comprises aqueous sodium hydroxide solution, the aqueous sodium hydroxide solution may have a concentration of about 0.1 M, about 0.2 M, about 0.3 M, about 0.4 M, about 0.5 M, about 0.6 M, about 0.7 M, about 0.8 M, about 0.9 M, about 1 .0 M, about 1 .1 M, about 1.2 M, about 1 .3 M, about 1 .4 M, about 1 .5 M, about 1 .6 M, about 1 .7 M, about 1 .8 M, about 1 .9 M, or about 2.0 M. Any two values may be combined to form a concentration range, for example a concentration of from about 0.1 M to about 2.0 M or from about 0.3 to about 1 .0 M. In preferred embodiments, the aqueous sodium hydroxide solution has a concentration of from about 0.5 M to about 1 .5 M. In embodiments where the base comprises aqueous ammonium hydroxide solution, the aqueous ammonium hydroxide solution may have a concentration of about 5 M, about 6 M, about 7 M, about 8 M, about 9 M, about 10 M, about 11 M, about 12 M, about 13 M, about 14 M, about 15 M, about 16 M, about 17 M, about 18 M, about 19 M, or about 20 M. Any two values may be combined to form a concentration range, for example a concentration of from about 10 M to about 20 M or from about 5 to about 15 M. In preferred embodiments, the aqueous ammonium hydroxide solution has a concentration of about 15 M to 20 M. In some embodiments, the aqueous ammonium hydroxide solution has a concentration of from about 25 vol% to about 33 vol%.

[0129] In some embodiments, the volume ratio of the silica precursor (eg TEOS) to surfactant (eg CTAC or CTAB) in step (i) is from about 0.5:1 .5 to about 1 .5:0.5, especially from about 0.8:1 .2 to about 1 .2:0.8, more especially about 1 :1 . The volume ratio of silica precursor and base may be suitably adjusted depending on the silica precursor and surfactant used. In some embodiments, the volume ratio of the silica precursor, surfactant and base (assuming the base is 14.7 M aqueous ammonium hydroxide solution) in step (i) is about 0.5 to about 1 .5, especially about 0.8 to about 1 .2, more especially about 1 for each component. For example, the volume ratio of the silica precursor, surfactant and base may be from about 1 :1 :0.5 to about 1 :1 :1 .5, especially from about 1 :1 :0.8 to about 1 :1 :1 .2, more especially about 1 :1 :1. The volume ratio of base may be suitably adjusted depending on the base (and in the case of an aqueous base solution, the concentration of base) used.

[0130] In some embodiments, the molar ratio of the silica precursor (eg TEOS) to surfactant (eg CTAC or CTAB) in step (i) is from about 0.5:0.30 to about 1 .5:0.10, especially from about 0.8:0.24 to about 1 .2:0.16, more especially about 1 :0.20. The molar ratio of silica precursor and surfactant may be suitably adjusted depending on the silica precursor and surfactant used. In some embodiments, the molar ratio of the silica precursor, surfactant and base (assuming the base is 14.7 M aqueous ammonium hydroxide solution) in step (i) is about 1 :0.1 :3 to about 1 :0.3:9, especially about 1 :0.15:3.5 to about 1 :0.2:5.2, more especially about 1 :0.2:4.5. For example, the molar ratio of the silica precursor, surfactant and base may be from about 1 :0.2:3.5 to 1 :0.2:5.2, especially about 1 :0.2:4.5. The molar ratio of base may be suitably adjusted depending on the base (and in the case of an aqueous base solution, the concentration of base) used.

[0131] In some embodiments, the solvent is capable of providing the mixture of the silica precursor, surfactant and base in the form of a gel. In some embodiments, the solvent is selected from one or more of water and ethanol. In preferred embodiments, the solvent is water.

[0132] Step (i) and step (ii) may independently be conducted at a temperature from room temperature up to a temperature below the boiling point of the solvent. In some embodiments, step (i) and step (ii) are independently conducted at a temperature from room temperature to about 80 °C, from room temperature to about 70 °C, from room temperature to about 60 °C, from room temperature to about 50 °C, from room temperature to about 40 °C, or from room temperature to about 30 °C. In some embodiments, step (i) is conducted at room temperature. In some embodiments, step (ii) is conducted at room temperature.

[0133] In some embodiments, the combining in step (i) comprises mixing (eg by stirring) the metal oxide support precursor, the one or more surfactants and the one or more bases in the solvent. In some embodiments, the combining in step (ii) comprises mixing (eg by stirring and/or sonicating) the mixture obtained in (i) with the one or more active metal precursors (eg a nickel precursor). Advantageously, sonicating the mixture obtained in (i) with the one or more active metal precursors may allow for improved distribution of the active metal on the support.

[0134] The method may further comprise, after step (ii), a step of isolating (separating) the particulate catalyst from the liquid phase. The particulate catalyst may be separated from the liquid phase by methods known in the art, for example by filtering or decanting.

[0135] The method may further comprise a step of washing the particulate catalyst to remove unreacted or excess reagents (eg unreacted or excess silica precursor, surfactant, base and/or active metal precursor). In some embodiments, the washing step comprises washing with one or more of water and ethanol.

[0136] The method may further comprise a step of drying the particulate catalyst. The drying step may be performed by methods known in the art, including methods described herein. The drying step may comprise storing the particulate catalyst at a temperature and/or for a duration sufficient to dry the catalyst (ie to remove residual solvent from the catalyst). In some embodiments, the temperature is about 60°C, about 70°C, about 80°C, about 90°C, about 100°C, about 110°C, or about 120°C. Any two values can be combined to form a temperature range, for example a temperature of from about 60°C to about 120°C. In preferred embodiments, the temperature is about 80°C. In some embodiments, the duration is about 8 hours, about 9 hours, about 10 hours, about 11 hours, about 12 hours, about 13 hours, about 14 hours, about 15 hours, about 16 hours, about 17 hours, about 18 hours, about 19 hours, or about 20 hours.

Any two values may be combined to form a time range, for example a duration of from about 8 to about 20 hours. In preferred embodiments, the duration is from about 12 hours to about 15 hours.

[0137] The method may further comprise a step of calcining the particulate catalyst. The calcining step may be performed by methods known in the art, including methods described herein. The calcining step be conducted at a temperature, at a heating rate, and/or for a duration sufficient to remove impurities or volatile substances. In some embodiments, the temperature is about 450°C, about 500°C, about 550°C, about 600°C, or about 650°C. Any two values may be combined to form a temperature range, for example temperature of from about 450°C to about 650°C. In preferred embodiments, the temperature is from about 500°C to about 600°C, especially about 550°C. The heating rate may be from about 1°C/min to about 4°C/min, for example a heating rate of about 1 °C/min, about 2°C/min, about 3°C/min or about 4°C/rmin. In preferred embodiments, the heating rate is from about 1°C/min to about 2°C/min, especially about 1 °C/min. In some embodiments, the duration is about 2 hours, 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours or about 8 hours. Any two values may be combined to form a time range, for example a duration of from about 2 hours to about 8 hours. In preferred embodiments, the duration is from about 4 hours to about 6 hours, especially for about 6 hours.

[0138] Prior to using the particulate catalyst in dry reforming of methane, the particulate catalyst may be subjected to reductive pre-treatment. Accordingly, the method may further comprise a step of reducing the particulate catalyst. The reducing step may be performed by methods known in the art, including methods described herein. The particulate catalyst may be reduced in situ, for example, the reducing step may be performed in a reforming reactor (prior to using the particulate catalyst for dry reforming of methane in that reactor). The reducing step may be conducted at a temperature, at a heating rate, for a duration, and/or under flow conditions sufficient to reduce the particulate catalyst. In some embodiments, the temperature is about 500°C, about 550°C, about 600°C, about 650°C, about 700°C, about 750°C, about 800°C, about 850°C or about 900°C. Any two values may be combined to form a temperature range, for example a temperature from about 500°C to about 900°C. In preferred embodiments, the temperature is from about 700°C to about 800°C, especially about 800°C. The heating rate may be about 10°C/min. In some embodiments, the duration is about 60 minutes, about 90 minutes, about 120 minutes, about 150 minutes, or about 180 minutes. Any two values may be combined to form a time range, for example a duration from about 60 minutes to about 180 minutes. In preferred embodiments, the duration is from about 60 minutes to about 120 minutes, especially about 120 minutes. The reducing agent may be hydrogen (H ₂ ) or a mixture of hydrogen and an inert gas, for example 5-10% H ₂ /N ₂ or 5-10% H ₂ /He. The reducing agent may be flowed at a rate of from about 20 mL/min to about 60 mL/min, for example about 20 mL/min, about 30 mL/min, about 40 mL/min, about 50 mL/min or about 60 mL/min. In preferred embodiments, the reducing step comprises flowing a 5% H ₂ /N ₂ mixture at a rate of about 50 mL/min. The reactor may optionally be purged after the reducing step to remove residual hydrogen, for example by purging with an inert gas (eg N ₂ or He) for a sufficient time to remove the residual hydrogen (eg about 30 minutes).

[0139] The present disclosure also provides the particulate catalyst prepared by the method described herein. The particulate catalyst prepared by the method disclosed herein may be the particulate catalyst as defined in any one or more of the herein disclosed embodiments.

Applications

[0140] The particulate catalyst described herein or prepared by the method described herein may be useful as a catalyst in dry reforming of methane.

[0141] Accordingly, the present disclosure provides the use of the particulate catalyst described herein or prepared by the method described herein for dry reforming of methane.

[0142] The present disclosure also provides a process for dry reforming of methane, the process comprising: contacting methane and carbon dioxide in the presence of the particulate catalyst described herein or prepared by the method described herein; thereby producing synthetic gas (syngas) comprising hydrogen and carbon monoxide.

[0143] The contacting step may be conducted at any suitable temperature, pressure and/or gas hourly space velocity (GHSV). The temperature may be from about 500°C to about 1000°C, especially from about 500°C to about 800°C. In some embodiments, the temperature is about 500°C, about 550°C, about 600°C, about 650°C, about 700°C, about 750°C, about 800°C, about 850°C, about 900°C, about 950°C, or about 1000°C. The pressure may be from about 0.1 MPa (about 1 bar) to about 3 MPa (about 30 bar), especially from about 0.1 MPa to about 0.5 MPa. In some embodiments, the pressure is about 0.1 MPa, about 0.2 MPa, about 0.3 MPa, about 0.4 MPa, about 0.5 MPa, about 1 .0 MPa, about 1 .5 MPa, about 2.0 MPa, about 2.5 MPa or about 3.0 MPa. The gas hourly space velocity (GHSV) may be from about 30000 ml_ g ^-1 h ^-1 to about 98000 ml_ g ^-1 h ^-1 , especially about 30000 mL g ^-1 h ^-1 to about 72000 rmL g ^-1 h ^-1 , more especially from about 36000 mL g ^-1 h ^-1 to about 60000 mL g ^-1 h ^-1 . In some embodiments, the GHSV is about 30000 mL g ^-1 h ^-1 , about 36000 mL g ^-1 h ^-1 , about 42000 mL g ^-1 h ^-1 , about 48000 mL g ^-1 h ^-1 , about 54000 mL g ^-1 h ^-1 , about 60000 mL g ^-1 h ^-1 , about 66000 mL g ^-1 h ^-1 , about 72000 mL g ^-1 h ^-1 , about 78000 mL g ^-1 h ^-1 , about 86000 mL g ^-1 h ^-1 , about 92000 mL g ^-1 h ^-1 , or about 98000 mL g ^-1 h ^-1 . In a preferred embodiment, the contacting step is conducted at a temperature from about 500°C to about 800°C at a pressure of about 0.2 MPa and GHSV of about 54000 mL g ^-1 h ^-1 .

[0144] The methane and carbon dioxide may be provided in any suitable gas flow mixture comprising methane and carbon dioxide, for example CH ₄ : CO ₂ : N ₂ or CH ₄ :

CO ₂ : He. In preferred embodiments, the ratio of methane to carbon dioxide in the gas flow mixture is about 1 :1 .

[0145] In some embodiments, the contacting step provides a CH ₄ conversion of from about 20% to about 90%, from about 40% to about 90%, from about 60% to about 90%, or from about 80 to about 90%. In some embodiments, the conversion of CH ₄ is at least about 20%, about 30%, about 40%, about 50%, about 60%, about 70%, about 80% or about 90%. These CH ₄ conversions may be under any one or more of the following conditions: a temperature of about 500°C to about 800°C, a pressure of about 0.2 MPa and GHSV of about 54000 mL g ^-1 h ^-1 .

[0146] In some embodiments, the contacting step provides a CO ₂ conversion of from about 50% to about 90%, from about 60% to about 90%, from about 70% to about 90%, or from about 80% to about 90%. In some embodiments, the conversion of CO ₂ is at least about 50%, about 60%, about 70%, about 80% or about 90%. These CO ₂ conversions may be under any one or more of the following conditions: a temperature of about 500°C to about 800°C, a pressure of about 0.2 MPa and GHSV of about 54000 mL g ^-1 h ^-1 .

[0147] In some embodiments, the contacting step produces syngas with a H ₂ /CO molar ratio of about 0.5 to about 0.8, from about 0.6 to about 0.8, or from about 0.7 to about 0.8. In some embodiments, the molar ratio of produced H ₂ /CO is at least about 0.5, about 0.6, about 0.7, about 0.8 or about 0.9. These molar ratios may be under any one or more of the following conditions: a temperature of about 500°C to about 800°C, a pressure of about 0.2 MPa and GHSV of about 54000 mL g ^-1 h ^-1 . [0148] Prior to using the particulate catalyst in dry reforming of methane, the particulate catalyst may be subjected to reductive pre-treatment. Accordingly, in some embodiments, the process further comprises, prior to the contacting step, a step of reducing the particulate catalyst. The reducing step may be performed as defined in any one or more of the herein disclosed embodiments. The particulate catalyst may be reduced in situ as defined in any one or more of the herein disclosed embodiments.

[0149] The process may further comprise the step of separating the hydrogen and the carbon monoxide.

[0150] The process may be conducted as a continuous process or as a batch process. In the case of a continuous process, the process may be conducted in a fixed bed or in a fluidised bed or in other reactor configurations well known in the art.

[0151] Advantageously, as shown in the Examples, the particulate catalyst described herein exhibited favourable catalytic activity, stability and durability (eg coke resistance) when used in continuous dry reforming methane reactions over an extended reaction duration (stream time) of up to 1000 hours.

[0152] Accordingly, in some embodiments, the particulate catalyst is substantially coke free after a duration of about 20 hours, about 30 hours, about 40 hours, about 50 hours, about 60 hours, about 80 hours, about 100 hours, about 200 hours, about 300 hours, about 400 hours, about 500 hours, about 600 hours, about 700 hours, about 800 hours, about 900 hours, about 1000 hours, about 1200 hours, about 1400 hours, about 1600 hours, about 1800 hours, or about 2000 hours on stream. The particulate catalyst may be substantially coke free for these stream times under any one or more of the following conditions: a temperature of about 500°C to about 800°C, a pressure of about 0.2 MPa and GHSV of about 54000 mL g ^-1 h ^-1 .

[0153] In some embodiments, the particulate catalyst is substantially stable after a duration of about 20 hours, about 30 hours, about 40 hours, about 50 hours, about 60 hours, about 80 hours, about 100 hours, about 200 hours, about 300 hours, about 400 hours, about 500 hours, about 600 hours, about 700 hours, about 800 hours, about 900 hours, about 1000 hours, about 1200 hours, about 1400 hours, about 1600 hours, about 1800 hours, or about 2000 hours on stream. The particulate catalyst may be substantially stable for these stream times under any one or more of the following conditions: a temperature of about 500°C to about 800°C, a pressure of about 0.2 MPa and GHSV of about 54000 mL g ^-1 h ^-1 .

[0154] In some embodiments of the process, the conversion of one or both of methane and carbon dioxide decreases by less than about 25%, about 20%, about 15%, about 12%, about 10%, about 9%, about 8%, about 7%, about 6%, or about 5% relative to initial conversion. These conversions may be after a duration of about 20 hours, about 30 hours, about 40 hours, about 50 hours, about 60 hours, about 80 hours, about 100 hours, about 200 hours, about 300 hours, about 400 hours, about 500 hours, about 600 hours, about 700 hours, about 800 hours, about 900 hours, about 1000 hours, about 1200 hours, about 1400 hours, about 1600 hours, about 1800 hours, or about 2000 hours on stream. These conversions may be under any one or more of the following conditions: a temperature of about 500°C to about 800°C, a pressure of about 0.2 MPa and a gas hourly space velocity (GHSV) of about 54000mL g ^-1 h ^-1 .

[0155] In some embodiments of the process, the molar ratio of produced hydrogen to carbon monoxide (H ₂ /CO) is at least about 0.5, about 0.6, about 0.7, about 0.8 or about 0.9. These molar ratios may be after a duration of about 20 hours, about 30 hours, about 40 hours, about 50 hours, about 60 hours, about 80 hours, about 100 hours, about 200 hours, about 300 hours, about 400 hours, about 500 hours, about 600 hours, about 700 hours, about 800 hours, about 900 hours, about 1000 hours, about 1200 hours, about 1400 hours, about 1600 hours, about 1800 hours, or about 2000 hours on stream. These molar ratios may be under any one or more of the following conditions: a temperature of about 500°C to about 800°C, a pressure of about 0.2 MPa and a gas hourly space velocity (GHSV) of about 54000mL g ^-1 h ^-1 .

[0156] Reference will now be made in detail to exemplary embodiments of the disclosure. While the disclosure will be described in conjunction with the exemplary embodiments, it will be understood that it is not intended to limit the disclosure to those embodiments. To the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the disclosure as defined by the appended claims.

Examples [0157] The present disclosure will be further described by way of non-limiting examples. It will be understood to persons skilled in the art of the present disclosure that many modifications may be made without departing from the spirit and scope of the present disclosure.

Example 1. Preparation of Ni/SiO ₂ catalysts

[0158] All chemicals used for the preparation of Ni/SiO ₂ were purchased from Sigma- Aldrich, such as tetraethylorthosilicate (TEOS, >98 %), ammonium hydroxide solution (33% NH ₃ in H ₂ O), sodium hydroxide (NaOH), hexadecyltrimethylammoniumchloride solution (CTAC, 25wt% in H ₂ O) or cetyltrimethylammonium bromide (CTAB), and nickel (II) nitrate hexahydrate or nickel(ll) sulfate hexahydrate.

[0159] Ni/SiO ₂ porous catalysts were synthesized as follows: in 100 ml_ deionized (Dl) water, 3 ml. of CTAC, ammonium hydroxide solution (or NaOH), and TEOS were vigorously stirred at room temperature (RT) to form a white gel. To prepare catalysts with different nNi/nSiO ₂ ratios, calculated amounts of nickel (II) nitrate were added just after the formation of the white gel under room temperature with vigorous stirring. In the same closed system, the mixture solution was ultrasonicated or strongly stirred for 20 min. After further vigorous stirring the gel for 1 hour at RT, the solid product was collected via vacuum filtration, followed by repeated washing by Dl water. The obtained solid product was then dried in an oven at 80°C overnight. Finally, at a heating rate of 1°C/min, the obtained Ni/SiO ₂ catalyst was calcined at 550°C for 6 hours in the presence of air. The catalysts were designated 2.5%, 5% and 12.5% N1/SO ₂ based on the loading of Ni content.

Example 2. Characterisation of Ni/SiO ₂ catalysts Methods

[0160] Powder X-ray diffraction (XRD) patterns were recorded by a Siemens D6000 with CuKα radiation at 45kV and 40mA in the range of 10-80°. High angle annular dark filed scanning transmission microscopy (HAADF-STEM) images were acquired from FEI Themis Z equipped with double spherical aberration corrector operating at 300kV. The morphology of the samples was characterized by Scanning Electron Microscope (SEM; FESEM, Zeiss Ultra+). The atom probe measurement was conducted on a CAMECA LEAP 4000X Si assisted with a 1 MHz 355 nm laser pulse, and specimen preparation was based on a combination of electrophoresis and FIB annular milling method. Inductively coupled plasma - optical emission spectrometry (ICP-OES) was used to measure the metal loading content. N ₂ adsorption-desorption isotherms were analysed by Autosor IQ-C system. Samples were degassed at 300°C for 4 hours under N ₂ backfill before measurements. Barrett-Joyner-Halenda (BJH) analysis was used to characterize the mesopores of the structure. H ₂ -temperature programmed reduction (H ₂ -TPR) was performed on a thermogravimetric analyser STD Q600. The samples were first heated at a heating ramp rate of 10°C /min to 300°C and held for 30 min in 50mL/min nitrogen (N ₂ ) to remove moisture, then with 50 mL/min total flow rate of gas mixture (10% hydrogen and 90% N ₂ ) the samples were heated to 800°C at a heating rate of 10°C/min. 02-temperature programmed oxidation (O ₂ -TPO) was performed on a thermogravimetric analyser STD Q600 under air to characterize the deposited carbon. The samples were heated at a heating ramp rate of 10°C/min to 800°C with 50 mL/min air flow rate.

Results and discussion

[0161] Ni/SiO ₂ catalysts with large Ni particles can contribute to large carbon deposits on the surface of large nickel nanoparticles (NiNP), resulting in the blockage of the active sites for DRM. The Ni/SiO ₂ catalysts of the present disclosure prepared by the method described in Example 1 form isolated atoms or isolated single clusters of nickel atoms on the surface and pores of the SiO ₂ support, which advantageously may be beneficial for preventing carbon deposit and metal sintering due to the strong metal- support interaction.

[0162] X-ray powder diffraction (XRD) was used to evaluate the crystallinity and the metal particle size of nickel in fresh Ni/SiO ₂ catalysts (ie catalysts which have not undergone reductive pre-treatment or been used in dry reforming of methane). The wide-angle XRD pattern of 2.5%, 5% and 12.5% Ni/SiO ₂ catalysts is shown in Figure 1A. As shown in Figure 1 A, each of the 2.5%, 5% and 12.5% Ni/SiO ₂ samples showed only one broad diffraction peak at about 23°, which is contributed by the amorphous mesoporous SiO ₂ support. Importantly, none of the samples showed any diffraction peaks corresponding to nickel (nickel diffraction peaks typically appear at 2Q between about 30-80°). The phenomenon of no obvious identified nickel diffraction peaks for all samples indicates that the Ni particles in the samples were either highly dispersed with very small particle sizes in the catalysts or maintained an amorphous form on the support.

[0163] The textural properties of the Ni/SiO ₂ catalysts were assessed by adsorption/desorption isotherm of N ₂ . The N ₂ adsorption-desorption isotherms of the 2.5%, 5% and 12.5% Ni/SiO ₂ samples are illustrated in Figures 1B, 1C and 1 D respectively. As shown in Figures 1 B-D, the loading of Ni content to the support contributed a negligible effect to the pore structure.

[0164] The corresponding pore size distribution of the Ni/SiO ₂ catalysts was calculated based on density functional theory (DFT). All Ni/SiO ₂ catalysts were shown to have narrow and relatively uniform micropores with diameter around 1 2nm.

[0165] The physicochemical properties of the Ni/SiO ₂ catalysts are summarized in Table 1. After loading nickel particles into the SiO ₂ support, the BET specific surface area decreased. This indicates that the nickel was successfully loaded into the pores of the SiO ₂ supports. As shown in Table 1 , the Ni metal loading calculated based on ICP is similar as the theoretical Ni metal loading based on calculation.

Table 1. Physiochemical properties of Ni/SiO ₂ catalysts aDetermined by nitrogen adsorption-desorption method. and pore volume were calculated via the Brunauer- Emmett-Teller equation, and the pore size was evaluated through the Barrett-Joyner-Halenda model. bDetermined by ICP-OES.

[0166] The morphology of the as-synthesized Ni/SiO ₂ catalysts was investigated by high angle annular dark filed scanning transmission microscopy (HAADF-STEM). In line with the wide-angle XRD and XAS results, no large Ni nanoparticles were identified by low magnification TEM images. Further, both 2.5% and 5% Ni/SiO ₂ catalysts were found to have a similar structure.

[0167] High-resolution TEM (HRTEM) spectroscopy images of the 2.5%, 5% and 12.5% Ni/SiO ₂ catalysts are shown in Figures 2A, 2B and 2C, respectively. In agreement with the XRD results, no lattice was identified for all samples under high- resolution TEM spectroscopy, confirming the overall amorphous structure of the catalysts. Additionally, isolated Ni atoms were present in the 2.5% Ni/SiO ₂ sample (exemplary isolated atoms are circled in Figure 2A), while small single (isolated) Ni clusters were identified in the 5% and 12.5% Ni/SiO ₂ samples (exemplary isolated clusters are depicted by arrows in Figures 2B and 2C).

[0168] The EDS mapping images of prepared 12.5% Ni/SiO ₂ catalysts is shown in Figures 2D, 2E and 2F, where the distribution of Ni is depicted in Figure 2E and the distribution of Si is depicted in Figure 2F. The EDS mapping images show that the small Ni clusters of the 12.5% Ni/SiO ₂ catalysts were uniformly distributed on the SiO ₂ support. Consistent with the EDS mapping result, an EDS line scan profile further confirmed the co-existence of the Ni and Si atoms on any spot of the sample. The signal of nickel atoms always appeared with the appearance of the signal of silicon atoms, indicating that there were no large Ni nanoparticles (NiNP) on the catalysts. At the same time, the EDS line scan profile showed that the ratio of Ni to Si at different sites was varied, indicating that although nickel cluster was uniformly distributed on the SiO ₂ support, the distribution was not in a random manner.

[0169] Atom probe tomography (APT) was used to further verify the Ni form in the 12.5% Ni/SiO ₂ catalysts. The APT results are shown in Figure 3A, with the position of Ni and Si atoms depicted. The 3D tomography clearly shows a non-uniform distribution of nickel in the Si matrix, which is in line with the variable Ni/Si ratio revealed by EDS. However, there is no large Ni segregation or agglomeration observed in the APT, which is consistent with the XRD measurements.

[0170] In order to quantify the Ni active sites in the 12.5% Ni/SiO ₂ catalysts, cluster identification was performed to identify areas with over 1 .937 Ni atoms per nm ² . The Ni rich areas exhibited by the iso-surface (over 1 .937 Ni atoms per nm ² ) is represented in the form of enclosed areas as shown in Figure 3B, representing the small Ni clusters in the SiO ₂ matrix. The APT analysis indicates that the nickel atoms could go into the catalysts by taking advantage of the mesopores of the support structure.

[0171] The Ni radical distance function (RDF) based on the APT for 12.5% Ni/SiO ₂ catalysts was determined and results are depicted in Figure 3C. The Ni RDF indicates that the Ni species have a clear Ni— Ni affinity, and the negative relationship between the distance and Ni atom concentration indicates that the Ni atoms in the 12.5% Ni/SiO ₂ catalysts were clustered instead of distributed atomically.

[0172] Ni frequency distribution analysis for 12.5% Ni/SiO ₂ catalysts was conducted and results are shown in Figure 3D. Compared with binomial model, the statistic frequency distribution analysis (FDA) showed a different count-concentration trend, further supporting that the distribution of Ni atoms was not in a random manner but mostly distributed in the pores of SiO ₂ supports.

[0173] Cluster analysis of the 12.5% Ni/SiO ₂ catalysts based on the APT results showed that most of the Ni atoms formed small clusters containing just a few nickel atoms inside. This is consistent with the results of FIRTEM. In HRTEM image, Ni clusters with an average diameter of around 1 nm were identified. The small Ni cluster condensed in the pores of SiO ₂ support indicate the existence of the metal-support interaction between the isolated clusters of Ni and SiO ₂ support with elevated temperature. This may advantageously inhibit the aggregation of the Ni cluster and contribute to enhanced catalyst stability.

[0174] H ₂ -Temperature programmed reduction (TPR) profiles were obtained to compare the strength of Ni metal-SiO ₂ support interactions of different samples and the reducibility of the Ni isolated atoms and isolated clusters on the calcined catalysts. The TPR profiles of 2.5%, 5% and 12.5% Ni/SiO ₂ catalysts are shown in Figure 4. The 2.5% Ni/SiO ₂ catalyst showed two peaks in the H ₂ -TPR profile. One weak peak appears at about 350°C and may be caused by the reduction of residual bulk NiO which had weak interaction with the silica shell. A second broad peak sits at 750-800°C, which is contributed by ionic nickel isolated atoms embedded in the SiO ₂ matrix and is ascribed to the reduction of NiO from the silica shell solid solution matrix. Increasing the nickel loading content to 5% caused the main broad peak to shift to a lower temperature of around 600-700°C. This may be because compared to the atomically dispersed Ni atoms on the support of the 2.5% Ni/SiO ₂ catalyst, the Ni cluster of the 5% Ni/SiO ₂ catalyst had a relatively weaker interaction with the SiO ₂ support, making it comparatively easier to be reduced. However, the Ni species was still more difficult to be reduced compared to the bulk NiO, indicating that a strong metal-support interaction still exist for NiSiO ₂ catalysts comprising small nickel clusters. Further increasing the Ni loading to 12.5%, the main peak shifted to about 580°C and the intensity of the peak was sharply increased. The increased peak intensity in the H ₂ -TPR indicates that with the increase of the Ni content, the exposure of Ni was also largely increased on the surface of support. Further, as shown in Table 1 , the 12.5% Ni/SiO ₂ has a comparatively larger specific surface area. Thus, the peak shift to lower temperature may be due to the increased Ni exposure and specific surface area. Consistent with the HRTEM and APT results, the absence of the 350°C peak in the TPR profiles of the 5% and 12.5% Ni/SiO ₂ catalysts indicates that no large Ni aggregation is present in the catalysts. Advantageously, the observed strong interaction between the Ni and the SiO ₂ support may be beneficial for durability of the catalysts, by enhancing resistance for carbon deposition and inhibiting the sintering and aggregation of the Ni cluster.

Example 3. Evaluation of Ni/SiO ₂ catalysts

Methods

[0175] Catalytic activity of 2.5%, 5% and 12.5% Ni/SiO ₂ catalysts were studied in a fixed-bed quartz tubular reactor (7 mm ID). 50 mg catalyst was held by two packs of quartz wool to prevent moving in the reactor tube. Reactant gases and carrier gas were controlled by mass flow controllers and fed under the condition of CH ₄ : CO ₂ : N ₂ = 1 :1 :1 (molar ratio), 0.2MPa, and feed GHSV=5.4x10 ⁴ mL g ^-1 h ^-1 . Prior to the reaction, the catalyst was reduced in situ with flowing 5% H ₂ /N ₂ mixture at a flow rate of 50mL/min. The temperature was ramped at 10°C/min to 800°C and held for 2 hours. After reduction, the system was held at 800°C under N ₂ (60 mL/min) for 0.5 hours to remove residual hydrogen. After introducing reactant gases, the system was retained at 800°C for 0.5 hours and the temperature subsequently decreased, in 50°C increments (holding time 0.5 hours until conversion had stabilized), to 500°C. Reaction products were analysed using an on-line micro gas chromatograph (Varian 4900) equipped with two TCD detectors (column: MS5A, PPQ). CH ₄ , CO ₂ conversions, selectivity of H ₂ and CO, H ₂ /CO ratio are defined as: where is conversion, S is selectivity, and F is mass flow.

[0176] Activity was also measured as a function of time on stream. Experiments were also carried out at a given temperature with same reduction procedure and feed GHSV.

Results and discussion

Dry reforming of methane over N1/SO ₂ catalysts

[0177] The catalytic DRM performance of 2.5%, 5% and 12.5% Ni/SiO ₂ catalysts was evaluated at varying temperature conditions from 500-800°C with gas hourly space velocity (GHSV) of 5.4x10 ⁴ mL g ^-1 h ^-1 . The results are illustrated in Figure 5A, which depicts CH ₄ conversion (circles), CO ₂ conversion (squares), and H ₂ /CO ratio (inset) for 2.5% Ni/SiO ₂ (dotted lines), 5% Ni/SiO ₂ (dashed lines) and 12.5% Ni/SiO ₂ (solid lines) catalysts. The results show that the CH ₄ conversions for each catalyst increased with the elevating temperature, which is due to the endothermic nature of DRM.

[0178] At a reaction temperature of 800°C, the 2.5% Ni/SiO ₂ catalysts exhibited a CH ₄ conversion of 40.9%. Increasing the Ni content loading to 5%, the conversion of CH ₄ only increased to 45.2%. Therefore, despite the halved Ni content on the 2.5% Ni/SiO ₂ catalysts compared with 5% Ni/SiO ₂ catalysts, the comparable initial activity of the samples indicates that the isolated Ni atoms on the SiO ₂ support of the 2.5%

Ni/SiO ₂ catalysts have comparably higher initial activity. The initiation of dry reforming for 2.5% Ni/SiO ₂ catalysts occurred from about 650°C, while the initiation for 5% and 12.5% Ni/SiO ₂ catalysts was shown to occur from lower temperature of about 500- 550°C. This is in line with the H ₂ -TPR result, which indicated that the 2.5% Ni/SiO ₂ catalysts had a comparably stronger interaction with the support, indicating it may be more difficult to reduce the NiO and expose the active sites, and lead to the activation of the dry reforming of methane.

[0179] As shown in Figure 5A, the 12.5% Ni/SiO ₂ catalyst was shown to have the best DRM performance. At 800°C, the 12.5% Ni/SiO ₂ catalyst had more than doubled conversion of reactant compared to the 5% and 2.5% Ni/SiO ₂ catalysts. This may be attributed to the fact gleaned by above mentioned characterization that the larger Ni cluster in 12.5% Ni/SiO ₂ may be easier to be reduced, and that more active Ni sites are exposed for DRM under the same reaction conditions, leading to a higher methane conversion. The positive relationship between the increased catalytic activity and the exposed surface Ni sites demonstrates that the Ni sites on the catalysts were involved in the catalytic cycle. The higher CO ₂ conversion than that of CH ₄ indicated the presence of the side reverse water gas shift reaction (RWGS), which also affected the syngas ratio.

[0180] The stability of the catalysts was evaluated at 800°C with identical reaction conditions over a 20 hour reaction period. Figures 5B, 5C and 5D illustrate the CH ₄ conversion, CO ₂ conversion, and FI2/CO ratio, respectively, of DRM for 2.5% Ni/SiO ₂ (squares), 5% Ni/SiO ₂ (circles) and 12.5% Ni/SiO ₂ (stars) catalysts at 800°C over the 20 hour reaction period. As shown in Figures 5B-5D, the CH ₄ and CO ₂ conversion of the 2.5% Ni/SiO ₂ catalysts decreased by approximately 15% over the reaction period. However, the catalytic activity of the 2.5% Ni/SiO ₂ catalysts became stable after about the first 10 hours. In contrast, the 5% and 12.5% Ni/SiO ₂ catalysts maintained a relatively stable activity throughout the 20 hour DRM reactions. Referring the HRTEM characterizations (Figure 2), compared with 5% Ni/SiO ₂ sample, the cluster size was not observably increased, but there was increased the coverage on the SiO ₂ support of isolated nickel clusters in the 12.5% Ni/SiO ₂ . To confirm this, the activity of the 2.5%,

5% and 12.5% Ni/SiO ₂ catalysts were compared based on the conversion of CH ₄ per amount of Ni loading. In line with the morphology characterizations, the 5% and 12.5% Ni/SiO ₂ catalysts showed a similar catalytic efficiency at varied temperatures, indicating that for both the 5% and 12.5% Ni/SiO ₂ catalysts, the same active sites were functionalized during the dry reforming of methane. At the 20 ^th hour of reaction, the conversion efficiency of 2.5% Ni/SiO ₂ catalysts was found to be similar as those for the 5% and 12.5% Ni/SiO ₂ catalysts. The comparable efficiency indicated that the unstable isolated Ni atoms of the 2.5% Ni/SiO ₂ catalysts were sintered during the DRM, and which formed the same active sites as 5% and 12% Ni/SiO ₂ catalysts after 10 hours of reaction. This may be contributed by the collapse the nanoporous SiO ₂ support at high temperature. Due to the presence of RWGS, the surface Si-O-Si bond might be partially hydrolysed by inevitably present steam during the high temperature reaction.

[0181] The stability of the 12.5% Ni/SiO ₂ catalyst was evaluated over a 1000 hour reaction period at 800 °C with gas hourly space velocity (GHSV) of 5.4x10 ⁴ mL g ^-1 h ^-1 . The results are shown in Figure 6, which illustrates the CPU conversion (black), CO ₂ conversion (grey) and H ₂ /CO ratio (inset) of DRM for the 12.5% Ni/SiO ₂ catalyst over the 1000 hour reaction period. The results show that the 12.5% Ni/SiO ₂ catalyst remained relatively stable over the 1000 hour DRM reaction period.

Investigating catalyst stability

[0182] At DRM reaction temperatures, both methane decomposition and Boudouard reactions could contribute to coke formation, which eventually cover all the Ni active sites leading to deactivation of the catalysts. To investigate the superior stability of the 2.5%, 5% and 12.5% Ni/SiO ₂ catalysts, the catalysts which has been evaluated in the 20 hour DRM reaction at 800°C were studied by temperature programmed oxidation (TPO) and thermogravimetric analysis (TGA).

[0183] The O ₂ -TPO analysis and TGA analysis of the used 2.5%, 5% and 12.5% Ni/SiO ₂ catalysts are shown in Figures 7 A and 7B respectively. As expected, due to the small size of the Ni and comparatively large surface area of the support, the O ₂ -TPO showed a zero-degree of coking for all catalysts. The TGA result further confirmed that there was no carbon deposit on the catalysts. The excellent coke resistance largely explains the superior stability of the Ni/SiO ₂ catalysts. In addition, the negligible carbon deposition on the 2.5%Ni/SiO ₂ catalysts indicated that the decrease of the catalytic activity at the first 10 hour of DRM may result from sintering of isolated Ni active atoms. Advantageously, the large surface area of the catalysts may allow for good metal dispersion, and the small sizes of Ni active sites may also prevent carbon depositing on the surface of active sites. [0184] The wide-angle XRD pattern of the used 2.5%, 5% and 12.5% Ni/SiO ₂ catalysts is shown in Figure 7C. The results show that after 20 hours of reaction at 800°C, reduced Ni peaks, namely metallic Ni (1 1 1) and Ni (2 0 0), can be observed in the XRD patterns in each of the used catalysts between 40-55°, the broad peak at about 23° being attributed to the amorphous silica support. Estimated by Schemer’s equation, the mean sizes for the supported nickel on the used 2.5%, 5% and 12.5% Ni/SiO ₂ catalysts were 5.9nm, 5.7nm, 5.7nm, respectively. Consistent with the CH ₄ efficiency evaluation, the 2.5% Ni/SiO ₂ showed the similar Ni active sites compared with the 5% and 12.5% Ni/SiO ₂ catalysts having higher Ni content loading.

[0185] HRTEM spectroscopy images of the used 2.5%, 5% and 12.5% Ni/SiO ₂ catalysts are shown in Figures 7D, 7E and 7F, respectively. In line with the XRD results, HRTEM further confirmed that all used catalysts maintained Ni clusters with similar sizes. As shown in the DRM results in Figures 5B-5D, in the first hour of reaction both 5% and 12.5% Ni/SiO ₂ catalysts showed a small catalytic deactivation. It is hypothesised that the Ni active sites were aggregated to around 5-6 nm in the first hour of reaction, and then maintained a stable structure which contributed to no further deactivation. In the case of the 2.5% Ni/SiO ₂ catalyst, due to the strong interaction between the isolated Ni atoms with the support, the 2.5% Ni/SiO ₂ may have taken up to about 10 hours to achieve an ultra-stable structure.

[0186] Figure 8 illustrates the cluster analysis of APT of fresh prepared 12.5% Ni/SiO ₂ catalysts (ie catalysts which have not undergone reductive pre-treatment or been used in dry reforming of methane), which shows the observed Ni, Si and O atoms concurrently. The cluster analysis results also indicated the presence of a Ni-O-Si bond. This could suggest that the Ni-O-Si bond was not formed during the DRM but is present after synthesis of the catalyst. The observation of Ni-O-Si bond in the 12.5% Ni/SiO ₂ catalyst suggests the site confinement of nickel isolated clusters maybe the key for the proceeding of the DRM. This is in accordance with the stability test results, due to the existence of the super stable Ni cluster, the 5% and 12.5% Ni/SiO ₂ catalysts showed excellent stability through the DRM process.

[0187] In the case of the 2.5% Ni/SiO ₂ catalyst, the phenomenon that the reactivity decreased in the first 10 hours and subsequently became stable indicates that Ni-O-Si bonds may be formed with the formation of small Ni clusters. Over time, at the DRM reaction temperature, the atomically dispersed Ni on the silica solid solution matrix may transform into ultra-stable small Ni clusters, forming new nickel silica species and maintaining strong interaction with support, which is supported by the efficiency evaluation. Then the ultra-stable small Ni clusters instead of single Ni atoms on the SiO ₂ support may become active sites for DRM, resulting in no further deactivation.

[0188] Without wishing to be bound by theory, the inventors hypothesise that process of DRM can be considered as a sequential redox reaction as shown in the schematic in Figure 9. Initially, the partial reduction of catalyst occurs as a result of the oxidation by CH ₄ , forming absorbed CHx ^* and Hx ^* species on the surface of catalyst along with the production of oxygen vacancies (Ov). The mobile oxygen replenishment is then conducted by the dissociation of CO ₂ , which produces absorbed COx ^* and Ox ^* species over the surface of support or at the metal-support interface. The plausible dehydrogenation of CH ₃ to CO is then conducted via CH ₃ oxidation, which generates CH ₃ O ^* and leaves no carbon deposit. The absorbed carbonate species then generates syngas with high selectivity.

Example 4. Comparative example - Ni/SiO ₂ catalyst prepared by incipient wetness impregnation method

[0189] A comparative catalyst was prepared by the incipient wetness impregnation method by loading a nickel precursor onto a solid SiO ₂ support at 10% loading of nickel on SiO ₂ . The catalytic DRM performance and stability of the comparative catalyst was evaluated over 50 hours at 800 °C with gas hourly space velocity (GHSV) of 5.4x10 ⁴ mL g ^-1 h ^-1 . Figures 10A, 10B and 10C illustrate the CH ₄ conversion, CO ₂ conversion, and H ₂ /CO ratio, respectively, of DRM for the comparative catalyst over the 50 hour reaction period. As shown in Figures 10A-10C, the CH ₄ and CO ₂ conversion of the comparative catalyst steadily decreased over the reaction period and the catalytic activity did not appear to stabilise over the reaction period. The CH ₄ conversion of the catalyst decreased by about 33.1% over the reaction period.

Example 5. Preparation and evaluation of C0/SiO ₂ catalyst

[0190] A C0/SiO ₂ catalyst was prepared using the procedure described in Example 1 but using cobalt as the active metal instead of nickel. The comparative catalyst was prepared at 12.5% loading of cobalt on SiO ₂ , where Co(NO ₃ ) ₂ -6H ₂ O was used as the cobalt precursor. The catalytic DRM performance and stability of the comparative catalyst was evaluated over 100 hours at 800 °C with gas hourly space velocity (GHSV) of 5.4x10 ⁴ mL g ^-1 h ^-1 . Figures 11 A, 11 B and 11C illustrates the CH ₄ conversion, CO ₂ conversion and H ₂ /CO ratio, respectively, of DRM for the 12.5% C0/SiO ₂ catalyst over the 100 hour reaction period. As shown in Figures 11 A and 11 B, the CH ₄ conversion decreased by about 8.7% and the CO ₂ conversion decreased by about 8.1% over the reaction over the reaction period.