CATALYST FOR DIRECT SYNTHESIS OF HYDROGEN PEROXIDE

TECHNICAL FIELD The present invention is directed to catalytic nanoparticles and catalysts comprising same which are suitable for direct synthesis of hydrogen peroxide, to a process for manufacturing catalysts comprising said catalytic nanoparticles, to the use of said catalysts for direct synthesis of hydrogen peroxide, and to a method of manufacturing hydrogen peroxide by direct synthesis using said catalysts.

BACKGROUND OF THE INVENTION

Hydrogen peroxide is a simple inorganic molecule and since its discovery has been an important commodity chemical with both industrial and domestic uses. In 2006 the annual production of H2O2 stood at around 4 million metric tonnes increasing by 4 % annually. Around 40 % of H2O2 is used in the pulp and paper industry as an alternative to chlorine-containing oxidants such as chlorine dioxide and sodium chlorate. Another major use is in water purification where H2O2 has been shown to destroy thiocyanate, nitrate, chlorine, hypochlorite and other potentially toxic chemicals which may be present in waste water.

Over 90 % of the world's H2O2 is manufactured by the indirect anthraquinone process, otherwise known as the auto-oxidation process. This process involves the hydrogenation of a substituted anthraquinone using a nickel or palladium catalyst to form a diol. The diol is then oxidised by an O2 rich aid feed to form the original anthraquinone and give H2O2 as a by-product. While this process is now large scale and energy efficient, there remain a number of problems associated with the anthraquinone process. The anthraquinone process is only viable on a large scale which means that concentrated H2O2 (70 wt%) solutions need to be stored and transported which can be hazardous, and the solutions may require the addition of acid or halide as stabilisers. While the process is operated at mild temperature and pressure, anthraquinone derivatives can be formed irreversibly which do not participate in the formation of H2O2. This means that the original anthraquinone molecule needs to be continually added to maintain the efficiency of the system. The use of a highly active hydrogenation catalyst can also result in the decomposition of the anthraquinone again reducing the efficiency of the system. The direct synthesis of H2O2 by the combination of molecular H2 and O2, removing the need for an anthraquinone intermediate would provide a much greener route to H2O2 as the reaction would be 100% atom efficient. While the combination of molecular H2 and O2 seems conceptually a simple reaction there are inherent problems associated with the process. One of these problems is that catalysts which are active for the synthesis of H2O2 also tend to be active for the hydrogenation of H2O2 and the direct combustion of H2 and O2, both reactions producing water. For example, monometallic Pd catalysts have been studied in the literature for the direct synthesis of H2O2. However, Pd catalysts which are active for H2O2 synthesis also tend to be active for the subsequent hydrogenation and decomposition of H2O2 resulting in lower overall yield and reduced H2O2 selectivity. The addition of halides and acids, either added to the reaction solution or incorporated into the catalyst, have been used to suppress the competing hydrogenation and decomposition reactions leading to improved yields of H2O2. It has been previously shown that Au-Pd bimetallic catalysts are more active and selective for the direct synthesis of H2O2 than monometallic palladium catalysts (see, for example, WO-A-2007007075 and WO-A-2012171892).

In order to scale up the direct process to an economically viable size, catalyst cost is an important factor that has to be taken into account and any reduction in the cost, while still maintaining the efficiency of the system, would be advantageous. There are various ways to achieve this including decreasing the amount of the active precious metals present in the catalyst. Another way would be to develop alternative Pd-based bimetallic catalysts containing an inexpensive second metal as a replacement for Au. However, to date, there are no such bimetallic catalysts reported that can compete with the best Au-Pd bimetallic catalysts for the direct synthesis of H2O2.

Another major issue is the stability of catalysts. Any commercially viable catalyst would need to be re-usable without major reduction in H2O2 productivity. Indeed, for any industrial catalytic process to be viable a stable catalyst is desirable to prevent the otherwise constant need to replace the catalyst and maintain a high yield of the desired product over time.

Thus, there is ongoing need to develop new catalyst for direct synthesis of H2O2 which are economically viable to produce and maintain. SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a catalytic nanoparticle formed during manufacture of a catalyst for direct synthesis of hydrogen peroxide, said catalytic nanoparticle comprising a first metal species and at least one second metal species, wherein the first metal species in monometallic form has an activity for hydrogenation, and the at least one second metal species in monometallic form does not decompose hydrogen peroxide, and wherein the first metal species and the second metal species are different, and if the catalytic nanoparticle comprises only one second metal species, the second metal species is other than tin.

According to a second aspect of the present invention, there is provided a catalyst for direct synthesis of hydrogen peroxide comprising catalytic nanoparticles according to the first aspect.

According to a third aspect of the present invention, there is provided a catalyst for direct synthesis of hydrogen peroxide, said catalyst comprising a first metal species and at least one second metal species, wherein the first metal species in monometallic form has an activity for hydrogenation, and the at least one second metal species in monometallic form does not decompose hydrogen peroxide, wherein if the catalyst comprises only one second metal species it is other than tin, wherein the combined amount of the first and second metal species is less than 15 wt. % of the catalyst, and wherein the catalyst comprises at least about 0.25 wt. % of the first metal species, based on the total weight of the catalyst.

According to a fourth aspect of the present invention, there is provided a method for making a catalyst comprising catalytic nanoparticles, the method comprising (i) preparing, providing or obtaining a catalyst precursor comprising catalyst support, a first metal species and at least one second metal species, wherein the first metal species in monometallic form has an activity for hydrogenation, and the at least one second metal species in monometallic form does not decompose hydrogen peroxide, (ii) treating the catalyst precursor under oxidizing conditions in a first step, (iii) treating the oxidised catalyst under reducing conditions in a second step, and (iv) treating the reduced catalyst under oxidising conditions at an elevated temperature in a third step (i.e., prior to use of the catalyst in the direct synthesis of hydrogen peroxide), wherein if the catalyst precursor comprises only one second metal species, the second metal species is other than tin.

According to a fifth aspect, there is provided a method of making hydrogen peroxide by direct synthesis, said method comprising converting hydrogen and oxygen to hydrogen peroxide in the presence of:

(i) catalytic nanoparticles comprising a mixed metal oxide and/or alloy of a first metal and at least one second metal, wherein the first metal in monometallic form has an activity for hydrogenation, and the at least one second metal in monometallic form does not decompose hydrogen peroxide; or

(ii) a catalyst comprising catalytic nanoparticles according to (i); or

(iii) a catalyst comprising a first metal species and at least one second metal species, wherein the first metal species in monometallic form has an activity for hydrogenation, and the at least one second metal species in monometallic form does not decompose hydrogen peroxide, wherein the combined amount of the first and second metal species is less than 15 wt. % of the catalyst, and wherein the catalyst comprises at least about 0.25 wt. % of the first metal, based on the total weight of the catalyst; or

(iv) a catalyst according to the second aspect or third aspect;

wherein, if the catalyst is (i), (ii) or (iii) and the catalyst comprises tin as the only second metal species, then:

(i) at least a portion of one of said hydrogen and oxygen is derived from an industrial process, and/or

(ii) at least a portion of said hydrogen and oxygen or any solvent, for example, methanol, used in the direct synthesis is generated or regenerated or recycled from a by-product or waste stream of an industrial process, and/or

(iii) at least a portion of said hydrogen is generated from the electrolysis of water, and/or

(iv) the method is conducted in the presence of contaminated water, and/or (v) the method further comprises using the hydrogen peroxide produced in an industrial process, for example, as a bleaching agent.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows the Temperature Programmed Reduction (TPR) profiles of monometallic Sn and Pd catalysts supported on ΤΊΟ2 and also a bare ΤΊΟ2 support after identical calcination treatment at 500°C for 3 hours. Figure 2 shows the TPR profile of various bimetallic catalysts of the invention supported on T1O2.

Figure 3 shows XRD patterns of monometallic 5% Pd, 5% Sn and bimetallic 2.5% Pd 2.5% Sn catalysts supported on silica after calcination at 500 °C for 3 hours.

Figure 4 shows a selection of XRD patterns for catalysts having variable Sn:Pd ratio.

Figure 5 shows a selection of XRD patterns obtained in order to demonstrate that the absence of PdO peaks in Pd/Sn/Si02 catalysts was not due to the Pd being below the detection limits of the technique.

Figure 6 shows the Pd(3d) XPS spectra of monometallic and bimetallic catalysts after calcination at 500 °C for 3 hours.

Figure 7 shows the Pd(3d) XPS spectra for a 1 % Pd / 4% Sn / Si0 ₂ catalyst after calcination at 500 °C for 3 hours and subsequent reduction at various temperatures for 2 h under 5% H2 / Ar. Figure 8 shows the Pd(3d) spectra of a reduced 1 % Pd / 4% Sn / S1O2 catalyst after subsequent heat treatment at 400 °C in air for 2 hours and 3 hours.

Figure 9 shows the Sn(3d ₅ / ₂ ) and Pd(3d) XPS spectra for a 3 % Pd / 2 % Sn / Ti0 ₂ catalyst following oxidation (O), oxidation and reduction (OR), oxidation-reduction- oxidation (ORO) and oxidation-reduction-oxidation-reduction-oxidation (ORORO).

Figure 10 shows a combined microscopy (by STEM) and EDS analysis of a 3 % Pd / 2 % Sn / T1O2 catalyst following oxidation-reduction-oxidation (ORO) treatment cycle. Figure 1 1 is a STEM image showing a Pd-Sn-Ox nanoparticle partially embedded in the amorphous Sn0 ₂ layer formed on a highly ordered Ti0 ₂ support during ORO treatment of a 3 % Pd / 2 % Sn / Ti0 ₂ catalyst. DETAILED DESCRIPTION OF THE INVENTION

Catalytic nanoparticles and catalyst comprising same

The catalytic nanoparticles are formed during manufacture of the catalyst in which they ultimately reside, the catalyst being suitable for direct synthesis of hydrogen peroxide. The catalytic nanoparticles are an active species for hydrogen peroxide production via direct synthesis. The catalytic nanoparticles and, thus, the catalyst comprising same, comprise at least two distinct metal species; a first metal species and at least one second metal species. The first and second metal species are different. A requisite characteristic of the first metal species is that, when in monometallic form, it has an activity for hydrogenation. In certain embodiments, the first metal species is selected from one or more of Pd, Pt, Ni, Ru, Rh, Os and Ir. In certain embodiments, the first metal species is selected from one or more of Pd, Pt and Ni. In certain embodiments, the first metal species is Pd or Pt or Ni. In certain embodiments, the first metal species is Pd. In certain embodiments, the first metal species forms, in addition to the catalytic nanoparticles, relatively smaller particles which are compositionally rich in the first metal species and substantially metallic, as may be determined by X-ray Photoelectron Spectroscopy (XPS). These relatively small particles may be up to about 5 nm in size, for example, up to about 3 nm in size, or up to about 2 nm in size, or up to about 1 nm in size, as may be determined, for example, using the STEM analysis methods described herein.

A requisite characteristic of the second metal species is that, when in monometallic form, it does not decompose hydrogen peroxide. Additionally, it is desirable that the second metal species readily mixes with the first metal species to form a mixed oxide and/or alloy. Additionally, it is desirable that the second metal species is capable of forming an amorphous oxide layer on the surface of a catalyst support material during manufacture of the catalyst of the present invention and, more particularly, during the oxidation-reduction-oxidation treatment cycles by which the catalyst of the present invention may be manufactured. As described herein, the amorphous oxide layer may encapsulate the relatively smaller particles which are compositionally rich in the first metal species and substantially metallic. In certain embodiments, the at least one second metal species is selected from one or more of Ni, Zn, Sn, In, Ge, Ga, Cu, Fe, Co, Cr, Mo and W. In certain embodiments, the at least one second metal is selected from one or more of Ni, Zn and Sn, for example, Ni and Zn, Ni and Sn, or Sn and Zn. In certain embodiments, the at least one second metal is Ni (i.e., Ni is the sole second metal species). In certain embodiments, the at least one second metal is Zn. In certain embodiments, the at least one second metal is Sn. In certain embodiments, if the catalyst nanoparticle comprises only one second metal species, the second metal species is other than Sn.

For the avoidance of doubt, the reference to 'monometallic form' does not mean that the metal species is/are in monometallic form when present in the catalytic nanoparticles and catalysts. As explained herein, it is believed that at least a portion of both the first and second metals species are present in the catalytic nanoparticles in an oxidized form (i.e., have an assigned oxidation which is a positive integer, as determined by XPS).

In certain embodiments, the first metal species is Pd and the second metal species is one or more of Ni, Zn and Sn, or a combination of Sn and one or more of Ni, Zn, In, Ge, Ga, Cu, Fe, Co and Cr. In certain embodiments, the first metal species is Pd and the second metal species is Sn. In certain embodiments, the first metal species is Pd and the second metal species is Ni. In certain embodiments, the first metal species is Pd and the second metal species is Zn. In certain embodiments, the first and second metal species both have an assigned oxidation number (i.e., oxidation state) which is a positive number, for example, 1 +, 2+, 3+, 4+, etc, as may be determined by XPS (based on the Auger parameter). In certain embodiments, the actual oxidation state of the first and/or second metal species may vary from the assigned oxidation number, as may be determined by XPS. This may indicate a degree of electronic interaction between the metal species in the catalytic nanoparticle. In certain embodiments, it is found that the oxidation state of the first and metal species shifts (e.g., by up to about +/- 0.5 eV) following the oxidation-reduction- oxidation treatment cycle for manufacturing the catalyst nanoparticles and catalyst comprising same. In certain embodiments, the first and second metal species are present in the catalytic nanoparticle as a mixed metal oxide, alloy or both. The mixed metal oxide may be of the form M1 _x M2 _y O _z , wherein [(oxidation state of M x) + (oxidation state of M2.y) +(z.- 2) = 0], and wherein M1 = the first metal, and M2 = the second metal. Without wishing to be bound by theory, it is believed that the second metal species acts, in part, as a spacer at the surface of the catalyst nanoparticle, enabling better access to surface active sites for the hydrogen and oxygen species and subsequent formation of hydrogen peroxide. In certain embodiments, the catalytic nanoparticles are up to about 10 nm in size, for example, from about 2 nm to about 10 nm in size, as may be determined by any suitable scanning transmission electron microscopy (STEM) method, for example, STEM analysis performed on a JEOL 2200FS STEM equipped with a CEOS probe corrector and Thermo-Noran X-ray energy dispersive spectroscopy (XEDS) system. In certain embodiments, the catalytic nanoparticles are from about 2 nm to about 8 nm in size, or from about 2 nm to about 6 nm, or from about 2.5 nm to about 5 nm, or from about 2.5 nm to about 4 nm.

As described herein, the catalytic nanoparticles form during manufacture of the catalyst in which the catalytic nanoparticles ultimately reside following manufacture of the catalyst. In certain embodiments, the catalyst comprises a catalyst support (e.g., a crystalline titania or silica support material) with catalytic nanoparticles at or on the surface of the support. In certain embodiments, an amorphous oxide layer, of which at least a portion is an oxide of the at least one second metal species, forms on the catalyst support, and catalytic nanoparticles are present on or in the amorphous oxide layer. In certain embodiments, at least a portion of the catalytic nanoparticles are partially embedded or partially encapsulated in the amorphous layer. That is to say, the amorphous oxide layer may serve to anchor the catalyst particles to the support, leaving a surface of the catalyst nanoparticle exposed and available to catalyse the direct synthesis of hydrogen peroxide from hydrogen and oxygen. In certain embodiments, the amorphous oxide layer serves to wholly encapsulate the relatively smaller particles which are compositionally rich in the first metal species and substantially metallic, thereby suppressing the hydrogenation activity of theses metal rich species which would otherwise hydrogenate hydrogen peroxide. The thickness of the amorphous oxide may vary. For example, it may up to about 5 nm thick, or up to about 4 nm thick, or up to about 3 nm thick, or up to about 2 nm thick, or up to about 1 nm thick, as may be determined by STEM analysis. For example, in certain STEM images, the amorphous oxide layer may be seen as 'fuzzy' layer proximate a highly ordered support material (e.g., T1O2). It may form as a continuous layer upon the surface of the support. In certain embodiments, if may form as a discontinuous layer, by which is meant that 'islands' of the amorphous oxide layer form upon the surface of the support and that there are areas of the support surface which have no amorphous layer formed thereon.

The catalyst of the present invention may also be characterised compositionally. In certain aspects and embodiments, the catalyst for direct synthesis of hydrogen peroxide comprises a first metal species and at least one second metal species, wherein the first metal species in monometallic form has an activity for hydrogenation, and the at least one second metal species in monometallic form does not decompose hydrogen peroxide, wherein the combined amount of the first and second metal species is less than 15 wt. % of the catalyst, and wherein the catalyst comprises at least about 0.10 wt. % of the first metal species, for example, at least about 25 wt. % of the first metal species, based on the total weight of the catalyst. In certain embodiments in which the catalyst comprises only one second metal species, it is other than tin, i.e., the catalyst is free of tin. The catalyst comprises catalytic nanoparticles, as described herein.

In certain embodiments, the combined amount of the first and second metal species is less than about 14 wt % of the catalyst, for example, less than about 13 wt. % of the catalyst, or less than about 12 wt. % of the catalyst, or less than about 1 1 wt. % of the catalyst, or less than about 10 wt. % of the catalyst, or less than about 9.0 wt. % of the catalyst, or less than about 8.0 wt. % of the catalyst, or less than about 7.0 wt. % of the catalyst, or less than about 6.0 wt. % of the catalyst. In certain embodiments, the combined amount of the first and second metal species is at least about 0.5 wt. % of the catalyst, optionally subject to the proviso that the catalyst comprises at least about 0.25 wt. % of the first metal species.

In certain embodiments, the combined amount of the first and second metal species is at least about 0.75 wt. % of the catalyst, for example, at least about 1.0 wt. % of the catalyst, or at least about 1.5 wt. % of the catalyst, or at least about 2 wt. % of the catalyst, or at least about 2.5 wt. % of the catalyst, or at least about 3 wt. % of the catalyst, or at least about 4 wt. % of the catalyst. In certain embodiments, the combined amount of the first and second metal species constitutes from about 0.5 to less than 15 wt. % of the catalyst, for example, from about 1.0 to about 12 wt. % of the catalyst, or from about 2.0 to about 10 wt. % of the catalyst, or from about 3.0 to about 8 wt. % of the catalyst, or from about 4.0 to about 7 wt.% of the catalyst, or from about 4.0 to about 6.0 wt. % of the catalyst. In certain embodiments, weight ratio of the first metal species to the second metal species is from about 8: 1 to about 1 :8, for example, from about 6: 1 to about 1 :6, or from about 5: 1 to about 1 :5, or from about 4: 1 to about 1 :4, or from about 3: 1 to about 1 :3, or from about 2:1 to about 1 :2, or from about 1.5: 1 to about 1 : 1.5, or about 1 : 1.

In certain embodiments, the catalyst is a bimetallic catalyst comprising Pd (as first metal species) and Sn (as second metal species), with the proviso that the catalyst comprises at least about 0.25 wt. % Pd. In certain embodiments, the combined amount of Pd and Sn is less than about 14 wt % of the catalyst, for example, less than about 13 wt. % of the catalyst, or less than about 12 wt. % of the catalyst, or less than about 12 wt. % of the catalyst, or less than about 1 1 wt. % of the catalyst, or less than about 10 wt. % of the catalyst, or less than about 9.0 wt. % of the catalyst, or less than about 8.0 wt. % of the catalyst, or less than about 7.0 wt. % of the catalyst, or less than about 6.0 wt. % of the catalyst.

In certain embodiments, the combined amount of Pd and Sn is at least about 0.5 wt. % of the catalyst, subject to the proviso that the catalyst comprises at least about 0.25 wt. % of Pd. In certain embodiments, the combined amount of Pd and Sn is at least about 0.75 wt. % of the catalyst, for example, at least about 1.0 wt. % of the catalyst, or at least about 1.5 wt. % of the catalyst, or at least about 2 wt. % of the catalyst, or at least about 2.5 wt. % of the catalyst, or at least about 3 wt. % of the catalyst, or at least about 4 wt. % of the catalyst.

In certain embodiments, the combined amount of Pd and Sn comprises from about 0.5 to less than 15 wt. % of the catalyst, for example, from about 1.0 to about 12 wt. % of the catalyst, or from about 2.0 to about 10 wt. % of the catalyst, or from about 3.0 to about 8 wt. % of the catalyst, or from about 4.0 to about 7 wt.% of the catalyst, or from about 4.0 to about 6.0 wt. % of the catalyst. In certain embodiments, weight ratio of Pd to Sn is from about 8:1 to about 1 :8, for example, from about 6: 1 to about 1 :6, or from about 5: 1 to about 1 :5, or from about 4: 1 to about 1 :4, or from about 3: 1 to about 1 :3, or from about 2:1 to about 1 :2, or from about 1.5: 1 to about 1 : 1.5, or about 1 : 1. In certain embodiments, the catalyst comprises from about 0.5 wt.% to about 12 wt. % Sn and from about 0.25 wt. % to about 6 wt. % Pd, for example, from about 0.5 wt. % to about 8 wt. % Sn and from about 0.5 wt. % to about 4 wt. % Pd, or from about 1.0 wt. % to about 6 wt. % Sn and from about 0.5 wt. % to about 3. 5 wt. % Pd, based on the total weight of the catalyst. In certain embodiments, the catalyst comprises from about 2.0 wt. % to about 6.0 wt. % Pd and from about 0.5 wt. % to about 3 wt. % Sn, for example, from about 3.0 wt. % to about 5.0 wt. % Pd and from about 0.5 wt. % to about 2.0 wt. % Sn. In certain embodiments, the catalyst comprises about 4 % Pd and about 1 wt. % Sn, or about 3 wt. % Pd and about 2 wt. % Sn, or about 2 wt. % Pd and about 3 wt. % Sn, or about 1 wt. % Pd and about 2 wt. % Sn, based on the total weight of the catalyst.

In certain embodiments, the catalyst is a bimetallic catalyst comprising Pd (as first metal species) and Ni or Zn (as second metal species), optionally with the proviso that the catalyst comprises at least about 0.10 wt. % Pd, for example, at least about 0.20 wt. % Pd, or at least about 0.25 wt. % Pd. In the following section, the second metal is referred to as Ni. However, the amounts and relative amounts described apply equally to embodiments in which the second metal species is Zn. In certain embodiments, the combined amount of Pd and Ni is less than about 14 wt % of the catalyst, for example, less than about 13 wt. % of the catalyst, or less than about 12 wt. % of the catalyst, or less than about 12 wt. % of the catalyst, or less than about 1 1 wt. % of the catalyst, or less than about 10 wt. % of the catalyst, or less than about 9.0 wt. % of the catalyst, or less than about 8.0 wt. % of the catalyst, or less than about 7.0 wt. % of the catalyst, or less than about 6.0 wt. % of the catalyst.

In certain embodiments, the combined amount of Pd and Ni is at least about 0.5 wt. % of the catalyst, optionally subject to the proviso that the catalyst comprises at least about 0.10 wt. % Pd, for example, at least about 0.20 wt. % Pd, or at least about 0.25 wt. % Pd 0.25 wt. % of palladium. In certain embodiments, the combined amount of Pd and Ni is at least about 0.75 wt. % of the catalyst, for example, at least about 1.0 wt. % of the catalyst, or at least about 1.5 wt. % of the catalyst, or at least about 2 wt. % of the catalyst, or at least about 2.5 wt. % of the catalyst, or at least about 3 wt. % of the catalyst, or at least about 4 wt. % of the catalyst. In certain embodiments, the combined amount of Pd and Ni constitutes from about 0.5 to less than 15 wt. % of the catalyst, for example, from about 1.0 to about 12 wt. % of the catalyst, or from about 2.0 to about 10 wt. % of the catalyst, or from about 3.0 to about 8 wt. % of the catalyst, or from about 4.0 to about 7 wt.% of the catalyst, or from about 4.0 to about 6.0 wt. % of the catalyst. In certain embodiments, the weight ratio of Pd to Ni is from about 8:1 to about 1 :8, for example, from about 6: 1 to about 1 :6, or from about 5: 1 to about 1 :5, or from about 4: 1 to about 1 :4, or from about 3: 1 to about 1 :3, or from about 2: 1 to about 1 :2, or from about 1.5: 1 to about 1 : 1.5, or about 1 : 1. In certain embodiments, the catalyst comprises from about 0.1 wt.% to about 2 wt. % Pd and from about 0.5 wt. % to about 10 wt. % Ni, for example, from about 0.2 wt. % to about 1.5 wt. % Pd and from about 0.5 wt. % to about 7.5 wt. % Ni, or from about 0.25 wt. % to about 1 wt. % Pd and from about 1.0 wt. % to about 5.0 wt. % Ni, or from about 0.25 wt. % to about 1 wt. % Pd and from about 2.0 wt. % to about 5.0 wt. % Ni, or from about 0.25 wt. % to about 1 wt. % Pd and from about 3.0 wt. % to about 5.0 wt. % Ni, based on the total weight of the catalyst.

In certain embodiments, the catalyst support is an organic or inorganic support, for example, catalyst support selected from the group consisting of carbon supports, oxide supports and silicate supports, for example, from S1O2, ΤΊΟ2, AI2O3, CeC>2, Nb20s, W2O3, ZrC>2, Fe2C>3, silica-alumina, molecular sieves and zeolites, and mixtures thereof. Suitable carbon supports are graphite, carbon black, glassy carbon, activated carbon, highly orientated pyrolytic graphite, single-walled and multi-walled carbon nanotubes. In certain embodiments, the catalyst support comprises or is an oxide support, for example, an oxide support selected from S1O2, ΤΊΟ2, AI2O3, CeC>2, Nb20s, W2O3, ZrC>2, Fe2C>3 and mixtures thereof. In certain embodiments, the catalyst support is an acidic catalyst support. Acidic catalyst supports include, for example, niobic acid support, heteropolyacid-based support, acid-treated carbon support, sulfated zirconia/silica support, and a support comprising an oxide other than zirconium oxide (e.g., silica) and a precipitate layer of zirconium oxide. Heteropolyacid supports include supports of the formula Cs _x H3- _x PWi204o, where x is from about 2.0 to about 2.9, which may be prepared by the addition of a Cs source, such as CsNC , to aqueous H3PW12O40. In an advantageous embodiment, the catalyst support comprises or is S1O2. In another advantageous embodiment, the catalyst support comprises or is ΤΊΟ2. In certain embodiments, the catalyst support comprises a mixture of S1O2 and ΤΊΟ2. In certain embodiments, the catalyst does not comprise carbon supports. The catalyst support may comprise at least about 60 wt. % of the catalyst, based on the total weight of the catalyst, for example, at least about 70 wt. % of the catalyst, or at least about 80 wt. % of the catalyst, or at least about 85 wt. % of the catalyst, or at least about 90 wt. of the catalyst, or at least about 91 wt. % of the catalyst, or at least about 92 wt. % of the catalyst, or at least about 93 wt. % of the catalyst, or at least about 94 wt. % of the catalyst, or equal to or greater than about 95 wt.% of the catalyst. In certain embodiments, the catalyst support comprises from about 60 wt. % to about 99 wt. % of the catalyst, for example, from about 70 wt. % to about 99 wt. % of the catalyst, or from about 80 wt. % to about 97 wt. % of the catalyst, or from about 85 wt. % to about 95 wt. % of the catalyst, or from about 90 wt. % to about 95 wt. % of the catalyst.

In an advantageous embodiment, the catalyst is a bimetallic catalyst comprising Pd and Sn and the catalyst support comprises or is S1O2. In certain embodiments thereof, the catalyst comprises from about 0.25 wt. % to about 3.0 wt. % Pd and from about 2.0 wt. % to about 6.0 wt. % Sn, for example, from about 0.5 to about 2.0 wt. % Pd and from about 3.0 wt. % to about 5.0 wt. Sn, or from about 0.75 wt. % to about 1.25 wt. Pd and from about 3.5 wt. % to about 4.5 wt.% Sn, or about 1 wt.% Pd and about 4 wt. % Sn.

In another advantageous embodiment, the catalyst is a bimetallic catalyst comprising Pd and Sn and the catalyst support comprises or is ΤΊΟ2. In certain embodiments therefore, the catalyst comprises from about 1.5 wt. % to about 5 wt. % Pd and from about 0.25 wt. % to about 4.0 wt. % Sn, for example, from about 2.0 wt. % to about 4.0 wt. % Pd and from about 1.0 wt. % to about 3.0 wt. % Sn, or from about 2.5 wt.% to about 3.5 wt. % Pd and from about 1. 5 wt. % to about 2.5 wt. % Sn, or about 3 wt. % Pd and about 2 wt. % Sn.

In another advantageous embodiment, the catalyst is a bimetallic catalyst comprising Pd and Ni and the catalyst support comprises or is ΤΊΟ2. In certain embodiments therefore, the catalyst comprises from about 0.10 wt. % to about 2.5 wt. % Pd and from about 0.50 wt. % to about 6.0 wt. % Ni, for example, from about 0.25 wt. % to about 2.0 wt. % Pd and from about 1.0 wt. % to about 5.0 wt. % Ni, or from about 0.25 wt.% to about 1.5 wt. % Pd and from about 2.0 wt. % to about 5.0 wt. % Ni, or from about 0.25 wt. % to about 1.0 wt. % Pd and from about 3.0 wt. % to about 5.0 wt. % Ni, or about 0.5 wt. % Pd and about 4.5 wt. % Ni In another advantageous embodiment, the catalyst is a bimetallic catalyst comprising Pd and Zn and the catalyst support comprises or is ΤΊΟ2. In certain embodiments therefore, the catalyst comprises from about 0.10 wt. % to about 2.5 wt. % Pd and from about 0.50 wt. % to about 6.0 wt. % Ni, for example, from about 0.25 wt. % to about 2.0 wt. % Pd and from about 1.0 wt. % to about 5.0 wt. % Ni, or from about 0.25 wt.% to about 1.5 wt. % Pd and from about 2.0 wt. % to about 5.0 wt. % Ni, or from about 0.25 wt. % to about 1.0 wt. % Pd and from about 3.0 wt. % to about 5.0 wt. % Ni, or about 0.5 wt. % Pd and about 4.5 wt. % Ni. The catalyst of the invention may be characterised in terms of one or more properties, such as a Temperature Programmed Reduction (TPR) profile or X-ray powder diffraction pattern (XRPD). References to known X-ray powder diffraction patterns means, unless otherwise stated, the X-ray powder diffraction as according to the International Centre for Diffraction Data (ICDD) database (www.icdd.com). The ICDD meets the requirements of standard ISO 9001 :2008 pertaining to centralized assembling, recording, designing, editing and publishing of diffraction data for use by scientists worldwide and providing technical forums for promoting diffraction and related materials analysis techniques in science and technology. In certain embodiments in which the support comprises or is T1O2, the catalyst has a TPR profile, as determined in accordance with the method described herein, characterised by the presence of a relatively broad, positive peak (mV) between about 140°C and 200°C, for example, between about 150 °C and about 180 °C. In certain embodiments, said catalyst comprises from about 0.5 wt. % to about 5 wt. % Pd, and from about 0.5 wt. % to about 5 wt. % Sn. A TPR profile of an exemplary catalyst of the invention is depicted Figure 2.

In certain embodiments in which the support comprises or is S1O2, the catalyst has a powder X-ray diffraction pattern, as determined in accordance with the method described herein, characterised by the absence of any discernable peaks (2Θ) attributable to Pd-containing or Sn-containing species above the S1O2 support diffraction pattern, for example, the absence of any discernable peak at 34°±0.5 2Θ attributable to Pd-containing or Sn-containing species, e.g. PdO or SnO as according to the ICCD database. Preparation of catalytic nanoparticles and catalyst comprising same The catalytic nanoparticles, and the catalyst comprising same (that is, following manufacture of the catalyst), are in certain embodiments manufactured by a process comprising (i) preparing, providing or obtaining a catalyst precursor comprising catalyst support, a first metal species and at least one second metal species, wherein the first metal species in monometallic form has an activity for hydrogenation, and the at least one second metal species in monometallic form does not decompose hydrogen peroxide, (ii) treating the catalyst precursor under oxidizing conditions in a first step, (iii) treating the oxidised catalyst under reducing conditions in a second step, and (iv) treating the reduced catalyst under oxidising conditions at an elevated temperature (e.g., at a temperature higher than that which would be employed in the manufacture of hydrogen peroxide by direct synthesis, for example, an elevated temperature which is at least about 150 °C) in a third step. In certain embodiments, when the catalyst precursor comprises only one second metal species, the second metal species is other than tin. Without wishing to be bound by theory, it is believed this oxidation-reduction- oxidation treatment cycle (sometimes referred to herein as 'O-R-0 or 'ORO') serves to "lock-in" the favourable catalytic activity of the catalyst of the present invention, promoting activity for production of hydrogen peroxide and suppressing or even eliminating subsequent hydrogenation of hydrogen peroxide.

The catalyst precursor of step (i) may be prepared by any suitable preparative method, preferably starting from suitable metal precursors. For example, the first and second metal species, as catalytically active components, may be deposited onto a catalyst support in the form of metal oxides or metal ions, e.g., metal salt (i.e., nitrate, chloride, and the like), by any known method to form a catalyst precursor. The first and second metal species may be deposited simultaneously or sequentially, advantageously simultaneously.

After deposition of the metal precursors onto the catalyst support, a catalyst precursor may be recovered by any suitable separation method, such as evaporation, filtration, decantation and/or centrifugation. The recovered catalyst precursor may be washed and dried, for example, at a temperature of between about 50 °C and 150°C, typically greater than about 100°C, for example, greater than about 105°C, and typically, less than about 130 °C, for example, less than about 120 °C, e.g., a temperature of from about 105 °C to about 1 15 °C. Drying may be conducted over a suitable period of time, for example, up to about 24 hours, for example, from about 8 to 20 hours, or from about 12 to about 20 hours, or from about 12 to about 20 hours, or from about 14 to about 20 hours, or from about 12 to 18 hours, or from about 15 to about 17 hours.

In certain embodiments, the step of preparing, providing or obtaining a catalyst precursor comprising first and second metal species comprises depositing the first and second metal species onto the catalyst support in form of first metal/second metal oxides or first and second metal ions, for example, using a first metal salt and second metal salt, e.g., a Sn(IV) salt or a Sn(ll) salt, preferably a Sn(IV) salt). Exemplary salts include the nitrate or chloride or sulphate or carbonate. Preferably the salt is a chloride or nitrate, e.g., SnCI ₄ .xH ₂ 0, NiCI ₂ .xH ₂ 0, ZnCI ₂ .xH ₂ 0 and Pd (N0 ₃ ) ₂ .xH ₂ 0, wherein x is from 1 to 6, for example, SnCI ₄ .5H ₂ 0, NiCI ₂ .6H ₂ 0 and Pd(N0 ₃ ) ₂ .2H ₂ 0. The first and second metal may be deposited simultaneously or sequentially, advantageously simultaneously on the catalyst support. For example, an aqueous solution of the first metal salt and the second metal may be prepared, followed by addition of the catalyst support.

The relative amounts and weight ratios of first metal species, second metal species and catalyst support may be selected accordingly in order to obtain a catalyst having the desired catalyst support and relative amounts of first and second metal.

After deposition of the metal precursors onto the catalyst support, the catalyst precursor composition is recovered by any suitable separation method, such as evaporation, filtration, decantation and/or centrifugation. The recovered catalyst precursor may be washed and dried, for example, at a temperature of between about 50 °C and 150°C, typically greater than about 100°C, for example, greater than about 105°C, and typically, less than about 130 °C, for example, less than about 120 °C, e.g., a temperature of from about 105 °C to about 1 15 °C. Drying may be conducted over a suitable period of time, for example, up to about 24 hours, for example, from about 8 to 20 hours, or from about 12 to about 20 hours, or from about 12 to about 20 hours, or from about 14 to about 20 hours, or from about 12 to 18 hours, or from about 15 to about 17 hours.

The catalyst precursor is then transformed into the corresponding catalyst, generating the hereinbefore described catalytic nanoparticles, via the treatment cycle according to (ii), (iii) and (iv) above. Advantageously, the treatment cycle is conducted at elevated temperatures, for example, each of (ii), (iii) and (iv) are conducted at a temperature of at least about 175 °C, or at least about 200 °C.

The O-R-0 treatment cycle may be conducted under any suitable atmosphere such as, for example, oxygen containing atmosphere, inert atmosphere or reducing atmosphere. In certain embodiments, the treatment may be conducted under air, oxygen, nitrogen, argon, hydrogen or mixtures thereof.

In certain embodiments, oxidising step (i) is conducted at a temperature of from about 250 °C to about 800 °C, for example, from about 300 °C to about 700°C, or from about 350 °C to about 600°C, or from about 400 °C to about 550 °C, or from about 450 °C to about 550 °C, or from about 475 °C to about 525°C. The salt precursor will decompose at such temperatures. Heat treatment may be conducted under any suitable type of atmosphere such as, for example, oxygen containing atmosphere or inert atmosphere. In certain embodiments, the heat treatment is conducted under air, oxygen, nitrogen, argon, or mixtures thereof. Without wishing to be bound by theory, it is believed that the heat treatment leads to the formation of an second metal-first metal(ll) alloy in a mixed oxide type material and/or first metal(ll) oxide which is contact with the second metal oxide. As described above, and as exemplified below, the oxidation state of the first and second metal species be determined by chemical analysis such as XPS. Step (ii) may be considered an oxidation step as both the first and second metal species become associated with oxygen. Treatment step (ii) may be conducted for a period of time ranging from about 30 mins to about 10 hours, for example, from about 1 hour to about 8 hours, or from about 2 hours to about 6 hours, or from about 2 hours to about 5 hours, or from about 2 hours to about 4 hours, or from about 2.5 hours to about 3.5 hours, or about 3 hours.

In certain embodiments, step (ii) comprises or consists of oxidising the catalyst precursor at a temperature of from about 450 °C to about 550 °C for a period of from about 2 to about 4 hours.

Following step (ii) the oxidized catalyst is treated under reducing conditions, forming a reduced catalyst. Following reductive treatment, chemical analysis such as XPS, may indicate the presence of a metallic first metal species, which STEM analysis indicates is in the form of relatively smaller nanoparticles which are compositionally rich in the first metal species and substantially metallic. It has surprisingly been found that treating the oxidized catalyst under reducing conditions enhances the stability and, thus, re-usability of the catalyst. The reducing treatment, in some embodiments, have the effect of reducing the H2O2 productivity of the catalyst during direct synthesis. However, any modest decrease in H2O2 productivity is advantageously off-set by the increased stability and re-usability of the catalyst. Reducing conditions include heat treatment under reducing conditions, chemical reduction and electrodeposition. In certain embodiments, the oxidized catalyst from step (ii) is heat treated under reducing conditions, for example, a mixture of hydrogen and an inert gas such as argon or nitrogen, and at an elevated temperature of at least about 50 °C and for a period of time of from about 30 minutes to about 10 hours, for example, from about 30 minutes to about 8 hours, or from about 1 hour to about 6 hours, or from about 1 hour to about 4 hours, or from about 1 hour to about 3 hours, or from about 1.5 hours to about 2.5 hours, or about 2 hours. In certain embodiments, the temperature is from about 50 °C to about 350 °C, for example, from about 75°C to about 300°C, or from about 75 °C to about 275 °C, or from about 75 °C to about 250°C, or from about 100°C to about 300°C, or from about 150 °C to about 250 °C, or from about 175 °C to about 225 °C.

The mixture of hydrogen and argon may comprise up to about 20 vol. % hydrogen, for example, up to about 15 vol. % hydrogen, or up to about 10 vol. % hydrogen, or up to about 8 vol. % hydrogen, or up to about 6 vol. % hydrogen, or up to about 5 vol. % hydrogen. The mixture may comprise less than 5 vol. % hydrogen.

In certain embodiments, step (iii) comprises or consists of heat treating the heat-treated catalyst from step (ii) under a mixture of hydrogen and argon at a temperature from about 150 °C to about 250 °C for a period of time of from about 1 to about 3 hours. In such an embodiment, the mixture of hydrogen and argon may comprise up to about 10 vol. % hydrogen, for example, up to about 8 vol. % hydrogen, or up to about 6 vol. % hydrogen, or up to about vol. 5 % hydrogen. Chemical reducing agents are known in the art and may be selected, for example, from NaH, LiH, LiAlhU, NaBhU, KBH4, Cahb, SnC , diisobutylaluminium hydride, sodium citrate, disodium citrate, trisodium citrate, sodium formate, formic acid, hydrazine, methanol, and combinations thereof. Other reducing treatments include, for example, radiolysis in the presence of isopropanol. Following step (iii), the reduced catalyst is treated under oxidizing conditions. It has surprisingly been found that treating the reduced (and initially heat-treated) catalyst under oxidizing conditions may significantly suppress, or even completely inhibit, hydrogen peroxide hydrogenation and decomposition activity, without adversely affecting the stability of the catalyst. Without wishing to be bound by theory, it is believed that this further treatment results in the formation/build-up of a substantially amorphous layer of an oxide of the second metal species, which resides on the catalyst support and which encapsulates, in some embodiments, wholly, the relatively smaller nanoparticles described above which are compositionally rich in the first metal species and substantially metallic. The oxidizing conditions of step (iv) may comprise or consist of heating the reduced catalyst in an oxidizing atmosphere at an elevated temperature. For example, the reduced catalyst may be heated in oxygen, air or an atmosphere comprising higher levels of oxygen relative to air. The temperature may be from about 250 °C to about 800 °C, for example, from about 300 °C to about 700°C, or from about 350 °C to about 600°C, or from about 350 °C to about 550 °C, or from about 350 °C to about 500 °C, or from about 350 °C to about 450°C, or from about 375 °C to about 425 °C, or about 400 °C. The heat treatment (or oxidation step, or calcining step) may be conducted for a period of time ranging from about 30 mins to about 10 hours, for example, from about 1 hour to about 8 hours, or from about 2 hours to about 6 hours, or from about 2 hours to about 5 hours, or from about 3 hours to about 5 hours, or from about 3.5 hours to about 4.5 hours, or about 4 hours.

In certain embodiments, step (iv) comprises or consists of heating the reduced catalyst in air at temperature of from about 350 °C to about 450 °C for a period of time of from about 2 hours to about 4 hours or from about 3 hours to about 5 hours. In certain embodiments in which the support is S1O2, the period of time is from about 2 hours to about 4 hours, for example, 2.5 hours to about 3.5 hour, or no more than about 3 hours.. In certain embodiments in which the support is T1O2, the period of time is from about 3 hour to about 5 hours, for example, from about 3.5 hours to about 4.5 hours, or no more than about 4 hours.

Alternatively, the oxidizing conditions may comprise chemical oxidation, for example, oxidation of the reduced catalyst in the presence of chemical oxidizing agents such as, for example, perchloric acid, H2O2 and/or N2O. Following step (iv), the catalyst may be subjected to further reduction and oxidation treatment steps. For example, an O-R-0 may be followed by a subsequent reduction and oxidation step, i.e., and O-R-O-R-0 treatment cycle, and so on. The conditions of any subsequent reduction and oxidation treatment steps may be the same as those described in connection with steps (ii), (iii) and (iv) above.

In certain embodiments, the catalyst of the invention is obtainable or prepared by a process comprising: (i) depositing (optionally simultaneously) a first metal species and second metal species onto a catalyst support by co-impregnation, forming a catalyst precursor comprising catalyst support, first and second metal, (ii) oxidizing the catalyst precursor at a temperature of from about 450 °C to about 550 °C for a period of from about 2 hours to about 4 hours, forming an oxidized catalyst catalyst, (iii) treating the oxidized catalyst of step (ii) under a mixture of hydrogen and argon at a temperature of from about 150 °C to about 250 °C for a period of time of from about 1 hour to about 3 hours, forming a reduced catalyst, and (iv) treating the reduced catalyst of step (iii) under oxidising conditions by heating the reduced catalyst in air at a temperature of from about 350 °C to about 450 °C, or from 450 °C to 550 °C, for a period of time of from about 2 hours to about 4 hours, or from about 3 hours to about 5 hours. In such embodiments, the catalyst support may be S1O2 or ΤΊΟ2.

As discussed above, the catalyst of the invention may be re-usable without major reduction in H2O2 productivity. This indicates the catalyst is stable. Thus, in certain embodiments, the stability of a catalyst of the invention may be determined by comparing its H2O2 productivity on first use in a direct synthesis reaction and its H2O2 productivity on second use in a direct synthesis reaction (the conditions of the first and second direct synthesis reaction are identical). H2O2 productivity is determined in accordance with the method and calculations described in the Examples below. Thus, in certain embodiments, the catalyst has a stability of at least about 60 %, which is calculated as [(H2O2 productivity on second use)/(H2C>2 productivity on first use) x 100]. In certain embodiments, the catalyst has a stability of at least about 70 %, for example, at least about 80 %, or at least about 85 %, or at least about 90 %, or at least about 92 %, or at least about 94 %, or at least about 96 %, or at least about 98 %, or at least about 99 %. In certain embodiments, the catalyst may have a stability of 100 %, indicating no loss in H2O2 productivity between first and second use of the catalyst in a direct synthesis reaction. In such embodiments, the catalyst support may be S1O2 or Ti0 ₂ . Direct synthesis of hydrogen peroxide, H2O2

The process of manufacturing hydrogen peroxide by direct synthesis comprises converting hydrogen and oxygen to hydrogen peroxide in the presence of a catalyst of the invention, i.e., a catalyst according to the first and second aspects of the invention. The catalyst is heterogeneous. It has been surprisingly found that reacting hydrogen and oxygen in the presence of a catalyst of the invention enables the preparation of hydrogen peroxide with an increased productivity and/or suppressed hydrogenation activity and/or increased selectivity to hydrogen peroxide. Indeed, in certain embodiments, it is possible to suppress or even completely inhibit hydrogenation and decomposition of hydrogen peroxide.

Accordingly, there is provided a method of making hydrogen peroxide by direct synthesis, said method comprising converting hydrogen and oxygen to hydrogen peroxide in the presence of:

(i) catalytic nanoparticles comprising a mixed metal oxide and/or alloy of a first metal and at least one second metal, wherein the first metal in monometallic form has an activity for hydrogenation, and the at least one second metal in monometallic form does not decompose hydrogen peroxide; or

(ii) a catalyst comprising catalytic nanoparticles according to (i); or

(iii) a catalyst comprising a first metal species and at least one second metal species, wherein the first metal species in monometallic form has an activity for hydrogenation, and the at least one second metal species in monometallic form does not decompose hydrogen peroxide, wherein the combined amount of the first and second metal species is less than 15 wt. % of the catalyst, and wherein the catalyst comprises at least about 0.25 wt. % of the first metal, based on the total weight of the catalyst; or

(iv) a catalyst according to the second or third aspects of the present invention; wherein, if the catalyst is (i), (ii) or (iii) and the catalyst comprises tin as the only second metal species, then:

(i) at least a portion of one of said hydrogen and oxygen is derived from an industrial process, and/or

(ii) at least a portion of said hydrogen and oxygen or any solvent, for example, methanol, used in the direct synthesis is generated or regenerated or recycled from a by-product waste stream of an industrial process, and/or

at least a portion of said hydrogen is generated from the electrolysis of water, and/or

the method is conducted in the presence of contaminated water, and/or

the method further comprises using the hydrogen peroxide

produced in an industrial process, for example, as a bleaching agent.

The direct synthesis process may be conducted in any type of suitable reactor known in the art, for example, a stirred reactor, such as an autoclave equipped with stirring means, a loop reactor or a tube reactor. The process may be conducted batchwise, continuously or semi-continuously. The catalyst may be in the reactor as a fixed bed or fluidized bed.

In certain embodiments, the direct synthesis process is conducted at a temperature of from about -20 °C to about 100 °C, for example, from about -10 °C, to about 80 °C, or from about -5 °C to about 50 °C, or from about -2 °C to about 25 °C, or from about -1 °C to about 10 °C, or from about 0 °C to about 10°C, or from about 1 °C to about 50 °C, or from about 1 °C to about 25 °C, or from about 1 °C to about 10 °C, or from about 1 °C to about 5 °C, or from about 1 °C to about 3 °C, or at a temperature of about 0°C, or about 1 °C, or about 2 °C, or about 3 °C, or about 4 °C. In certain embodiments, for example, embodiments in which the first metal species is Pd and the second metal species is Ni, the direct synthesis process is conducted at a temperature of from about 10 °C to about 30 °C, for example, from about 15 °C to about 25 °C, or a temperature of about 20 °C.

In certain embodiments, the process is conducted in a liquid medium, for example, an aqueous liquid medium. The aqueous medium may comprise a water-miscible solvent including, for example, methanol, ethanol, isopropyl alcohol, acetone and glycols such as ethylene glycol, propylene glycol, diethylene glycol and dipropylene glycol, and mixtures thereof. In certain embodiments, the liquid medium is aqueous methanol. In certain embodiments, the weight ratio of water-miscible solvent to water is from about 10: 1 to about 1 : 10, for example, from about 8: 1 to about 1 :8, or from about 5: 1 to about 1 :5, or from about 4: 1 to about 1 :4, or from about 4: 1 to about 1 :2, or from about 4: 1 to about 1 : 1 , or from about 3: 1 to about 1 : 1 , or from about 2: 1 to about 1 : 1.

The total pressure in the reactor (measured at 20 °C) may vary according to the reaction conditions, amounts of starting materials and the type of reactor. In certain embodiments, the total pressure in the reactor is from about 0.1 to about 15 MPa, for example, from about 0.5 to about 10 MPa, or from about 1 to about 8 MPa, or from about 2 to about 6 MPa, or from about 3 to about 5 MPa, or from about 3.5 to about 4.5 MPa, or about 4 MPa.

The reaction time may vary according to the reaction conditions and amounts of starting materials and may be adjusted accordingly. In certain embodiments, the reaction time is from about 30 seconds to about 10 hours, for example, from about 1 minute to about 5 hours, or from about 5 minutes to 300 minutes, or from about 10 minutes to about 240 minutes, or form about 10 minutes to about 180 minutes, or from about 15 minutes to about 120 minutes, or from about 15 minutes to about 90 minutes, or from about 15 minutes to about 60 minutes, or from about 15 minutes to about 45 minutes, or from about 20 minutes to about 60 minutes, or from about 25 minutes to about 50 minutes, or from about 25 minutes to about 40 minutes.

In certain embodiments, the reaction medium additionally comprises other components which further enhance the direct synthesis process. For example, The addition of halides and acids to the reaction may used to suppress the competing hydrogenation and decomposition reactions leading to improved yields of H2O2. For example, bromide ions, e.g., in the form of hydrogen bromide, may be added to reduce the tendency for H2O formation at higher temperatures. Additionally or alternatively, inorganic acid may be added to stabilize the hydrogen peroxide formed. Exemplary inorganic acids are sulphuric acid, nitric acid, hydrochloric acid and ortho-phosphoric acid. However, owing to advantageous properties, as described herein, of the catalysts of the invention, in certain advantageous embodiments, the process of manufacturing hydrogen peroxide by direct synthesis is conducted in the absence of halide and/or acid stabilizers.

In certain embodiments, at least a portion of one of said hydrogen and oxygen or any solvent (e.g., methanol and/or water) is derived from an industrial process. For example, in certain embodiments, at least a portion of said hydrogen and oxygen or any solvent, for example, methanol, used in the direct synthesis is generated or regenerated or recycled from a by-product or waste stream of an industrial process.

In certain embodiments, method further comprises using the hydrogen peroxide produced in an industrial process, for example, as a bleaching agent, disinfectant, or reagent in a chemical process. In certain embodiments, the industrial process may be an integrated process in which a reagent of the direct synthesis process, for example, hydrogen and/or methanol, is generated from a by-product or waste stream and used as a feed to the direct synthesis method along with oxygen, and the catalyst of the invention is used to generate hydrogen peroxide which, in turn, is used in the industrial process from which the by-product or waste stream is produced.

In certain embodiments, at least a portion of the hydrogen (and optionally oxygen) used in the direct synthesis is generated from the electrolysis of water.

In certain embodiments, the method is conducted in the presence of contaminated water, i.e., using contaminated water as solvent, optionally in conjunction with other solvents, such as clean water and/or methanol. The contaminated water may be contaminated with bacteria, which would be eradicated under the reaction conditions of the direct synthesis method, thereby producing a de-contaminated water in addition to hydrogen peroxide.

In other embodiments, the catalyst of the invention may also be used for the synthesis of hydrogen peroxide by indirect methods, such as by the anthraquinone process.

Embodiments of the present invention will now be described by way of illustration only, with reference to the following examples.

EXAMPLES

Catalyst preparation

In the following examples, unless stated otherwise, tin and palladium monometallic and bimetallic catalysts were prepared by impregnating or co-impregnating aqueous solutions of metal salts onto the appropriate catalyst support which included ΤΊΟ2 and S1O2. The catalysts prepared had a nominal total metal loading of 5 wt% unless otherwise stated. A typical preparation for 1 g of 2.5 % Pd / 2.5% Sn / ΤΊΟ2 was carried out as follows: 0.063 g Pd(N03)2.2H20 was first dissolved in 2 ml of de-ionised water and heated to 80 °C with stirring. 0.074 g of SnCU.5H20 was dissolved in a minimal amount of water and added to the aqueous palladium solution and left for 15 min. 0.95 g of the support was then added to the solution and the water allowed to evaporate until the mixture had formed a paste like consistency. Samples were then dried at 1 10 °C for 16 h and calcined in static air at various temperatures for 3 h with a ramp rate of 20 °C min ¹ . Unless stated otherwise, gold and palladium bimetallic catalysts were prepared by co-impregnating the appropriate catalyst support with solutions of PdCl2 and HAuCU. The catalysts contained a nominal metal content of 5 wt% unless otherwise stated. A typical preparation for 1 g of 2.5% Pd / 2.5% Au / ΤΊΟ2 was carried out according to the following procedure: 0.042 g of PdCl2 was added to 2.04 ml of HAuCU (12.25 g Au / 1000 ml) and heated to 80 °C with stirring and left until the PdCl2 had completely dissolved. 0.95 g of the desired support was then added to the solution and the water allowed to evaporate until the mixture formed a paste like consistency. The samples were dried at 1 10 °C for 16 h and then calcined in static air at various temperatures, typically 400 °C for 3 h with a ramp rate of 20 °C min ¹ .

Catalyst testing

- H2O2 synthesis in a batch system

Unless otherwise stated, the performance of each catalyst for the direct synthesis of H2O2 from H ₂ and O2 was determined using a Parr Instruments stainless-steel autoclave (equipped with an overhead stirrer and temperature/pressure sensors) with a nominal volume of 100 ml and a maximum working pressure of 14 MPa. During a standard synthesis test the autoclave was charged with 5.6 g of MeOH, 2.9 g of HPLC grade H ₂ 0 and 10 mg of catalyst. The autoclave was pressurised with 2.9 MPa 5% H2/CO2 and 1.1 MPa 25% O2/CO2 to give a total reaction pressure of 4 MPa. The autoclave was cooled to 2 °C and then stirred at 1200 rpm for 30 min. After the reaction was complete the solvents were filtered from the catalyst and 0.25 g aliquots of the solvent were titrated against a diluted Ce(S0 ₄ )2 solution acidified with 2% H ₂ S0 ₄ using ferroin as an indicator. The exact concentration of the Ce(S0 ₄ ) ₂ solution was determined by titration of a known amount of (NH ₄ ) ₂ Fe(S0 ₄ ) ₂ .6H ₂ 0 again using ferroin as an indicator.

To compare the performance of the catalysts over the 30 min reaction the average rate of H ₂ 0 ₂ production was calculated and normalised to catalyst mass to give a productivity value which is presented as mol _H2 o2 h ^"1 kg _ca t ^"1 . The wt % of H ₂ 0 ₂ was also determined for each reaction using the following calculations:

Volume Ce(S0 ₄ ) ₂ to titrate whole reaction solution = Titre x 8.5 (2.1 )

Sample mass

Moles Ce(S0 ₄ ) ₂ = Vol. Ce(SQ ₄ ) ₂ to titrate reaction solution x rCe(SQ ₄ ) ₂ l (2.2)

1000

Moles H ₂ 0 ₂ = Moles Ce(SQ ₄ ) ₂ (2.3)

2 Productivity = Mol H2O2 (2.4)

Catalyst mass (kg) x Reaction Time (h)

wt% H2O2 = Moles H2O2 x Mr H2O2 (2.5)

8.5 - H2O2 hydrogenation in a batch system

The hydrogenation activity of a catalyst was tested in a similar way to the direct synthesis activity. Unless otherwise stated, the autoclave was charged with 5.6 g of MeOH, 0.67 g 50 wt% H ₂ 0 ₂ and 2.23 g of HPLC grade H ₂ 0 and thoroughly mixed, after which 10 mg of catalyst was added. This solvent composition is equivalent of a 4 wt% H2O2 solution of the same volume used previously in the H2O2 synthesis experiments. 2 drops of the solvent solution each weighing around 0.04 g were removed and titrated with the acidified Ce(S0 ₄ )2 solution using ferroin as an indicator to determine accurately the initial H2O2 concentration. The autoclave was pressurised with 2.9 MPa 5% H2/CO2 and cooled to 2 °C and the reaction was carried out for 30 min at 1200 rpm stirring speed. After the reaction was complete the solvents were filtered from the catalyst and two -0.04 g aliquots of the solvent were titrated against an acidified dilute Ce(S0 ₄ )2 solution using ferroin as an indicator. The hydrogenation activity was calculated as ΓΠΟΙΗ202 IT ¹ kg _ca t ^{" 1} along with the percentage of the initial H2O2 which was present at the end of the reaction.

- Catalyst re-use in a batch system

Unless otherwise stated, catalyst reusability was tested by running a synthesis reaction as outlined above but increasing the amount of catalyst to 70 mg. After the reaction was complete the solvent was filtered from the catalyst, and the catalyst was allowed to dry overnight on the filter paper. Before being retested the catalyst was dried at 1 10 °C in an oven for 1 hour to ensure the sample was completely dry. Following this procedure a synthesis reaction was run as described above.

Catalyst characterisation - Temperature Programmed Reduction

Temperature programmed reduction (TPR) is a thermal analysis technique that allows H2 consumption to be measured while the sample undergoes a heating profile therefore indicating the temperatures at which the sample consumes H2 and has been used extensively to study catalytic systems. During a temperature programmed reduction the sample is heated under a flow of H2 containing gas and H2 consumption by the sample can be monitored using a TCD to analyse the gas that has passed through the sample cell. A plot of TCD signal against temperature can indicate at which temperature the sample undergoes reduction. A positive peak in the TCD signal indicates H ² consumption. TPR can give an indication as to what temperature a sample needs to be reduced at to generate the desired species on a catalyst and can also give an indication of the initial species present on the catalyst.

TPR profiles were recorded using a Thermo 1 100 series TPDRO. 0.1 g of sample was packed into the sample tube between quartz wool. Argon (15 ml min-1) was then passed through the system while it was heated from room temperature to 1 10 °C at 5 °C min ^"1 , where it was held for 60 min. The sample was allowed to cool to room temperature and following this the gas was switched to 10% H2 / Ar (15 ml min ^-1 ) and the sample was heated at 5 °C min ^-1 up to 800 °C. The profile was recorded using a TCD with positive polarity. - X-ray diffraction

Investigation of the bulk structure of the catalytic materials was carried out using a (Θ-Θ) PANalytical X'pert Pro powder diffractometer using a Cu Ka radiation source operating at 40 KeV and 40 mA. Standard analysis was performed using a 40 min scan between 2Θ values of 10-80° with the samples supported on an amorphous silicon wafer. Diffraction patterns of phases were identified using the ICDD data base.

An extension of standard XRD is in-situ XRD which allows the study of the bulk phase to be monitored continuously as the sample is heated, pressurised or exposed to reaction conditions or a pressure of gas. Investigation of the bulk properties of the catalysts was carried out while increasing the temperature of the sample to mimic calcination conditions. An Xpert Pro XRD fitted with an Anton-Parr XRK900 in-situ cell (internal volume of 0.5 L) was used with XRD.

- X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) is a characterisation technique which can give information such as composition and oxidation state of species on the surface of the catalyst to a depth of around 10 nm. XPS was performed using a VG EscaLab 220i spectrometer, using a standard Al-Kcr X-ray source (300 W) and an analyser pass energy of 20 eV. Samples were mounted using double-sided adhesive tape, and binding energies were referenced to the C 1 s binding energy of adventitious carbon contamination, which was taken to be 284.7 eV.

- Microscopy

Scanning transmission electron microscopy (STEM) was performed on various samples of the Sn-Pd catalysts. STEM was performed using a JEOL 2200FS STEM equipped with a CEOS probe corrector and Thermo-Noran X-ray energy dispersive spectroscopy (XEDS) system. Specimens for scanning transmission electron microscopy (STEM) analysis were prepared by dry dispersing the catalyst powder onto a holey carbon film supported on a copper mesh TEM grid. Example 1

A series of monometallic and bimetallic catalysts were prepared by impregnation using both S1O2 and ΤΊΟ2 as support in accordance with the preparative methods described above. The composition of each catalyst is shown in Table 1 along with H2O2 productivity

Table 1.

Comparing the sum of the H2O2 productivity of the supported monometallic 2.5 wt% Pd, 2.5 wt% Sn with the bimetallic 2.5 wt% Pd / 2.5 wt% Sn catalysts, a clear synergistic effect between Pd and Sn when supported on both T1O2 and S1O2 is observed.

Example 2

A series of catalyst were prepared on S1O2 and T1O2 containing a total nominal content of 5 wt. % with varying Sn and Pd contents. A comparative catalyst comprising 5 wt. % Pd only was also prepared. A H2O2 hydrogenation test was carried out on each catalyst. The composition of the catalysts and H2O2 hydrogenation test results are summarized in Table 2.

The results show that irrespective of the Pd : Sn ratio, all Pd-Sn bimetallic T1O2 or S1O2 supported catalysts were less active for H2O2 hydrogenation/decomposition than the corresponding Pd monometallic catalysts. Table 2.

Example 3a The 1 ^st and 2 ^nd use productivity of the 1 % Pd / 4 % Sn / Si0 ₂ catalyst from Example 2 was determined. The catalyst was subjected to a reductive treatment at 200 °C for 2 hours under 5 % hb/Ar and its 1 ^st and 2 ^nd use productivity was determined along with hydrogenation activity. The catalyst was subsequently subjected to a heat treatment by calcination in air at 400 °C for 3 hours. Results are summarized in Table 3.

It is seen that the reductive treatment improves 2 ^nd use H2O2 productivity, but leads to an increase in H2O2 hydrogenation activity. The further heat treatment provides a stable catalyst and completely inhibits hydrogenation activity. Table 3.

Heat treatment 1 ^st use H2O2 2 ^nd use H2O2 H2O2

productivity productivity hydrogenated

(mol _H 202 h ^"1 kgcat ^"1 ) (mol _H 202 h ^"1 kgcat ^"1 ) (mol _H 202 h ^"1 kgcat ^"1 )

500°C/3h/air 66 22 66 (2 %)

500°C/3h/air

+ reduced 200 °C / 76 76 340 (10 %) 2 h

500°C/3h/air

+ reduced 200 °C /

50 50 0

2 h

+ 400°C/3h/air Example 3b

The 1 ^st and 2 ^nd use productivity of the 3 % Pd / 2 % Sn / Ti0 ₂ catalyst from Example 2 was determined. The catalyst was subjected to a reductive treatment at 200 °C for 2 hours under 5 % hb/Ar and its 1 ^st and 2 ^nd use productivity was determined along with hydrogenation activity. The catalyst was subsequently subjected to a heat treatment by calcination in air at 400 °C for 3 or 4 hours. Results are summarized in Table 4.

It is seen that the reductive treatment improves 2 ^nd use H2O2 productivity, but leads to an increase in H2O2 hydrogenation activity. The further heat treatment provides a stable catalyst and significantly suppresses (3 hour treatment) or completely inhibits (4 hour treatment) hydrogenation activity.

Table 4.

Example 4

Temperature programmed reduction (TPR) of a series of catalysts was carried out following calcination at 500 °C for 3 hours. Figure 1 shows the TPR profiles of monometallic Sn and Pd catalysts supported on T1O2 and also the bare T1O2 support after identical calcination treatment at 500°C for 3 hours. In the TPR profile of monometallic 5 % Pd / T1O2 the large negative response can be attributed to the evolution of hydrogen from the sample resulting from the decomposition of palladium-p-hydride.

Figure 2 shows the TPR profile of various bimetallic catalysts supported on ΤΊΟ2. All catalysts show a response at 90 °C; however the intensity of this signal is decreased considerably relative to the monometallic Pd catalyst. Without wishing to be bound by theory, this observation may either suggest that the presence of Sn inhibits the formation of palladium-p-hydride or that Pd is present in a different form, such as a Pd-Sn-O _x mixed oxide and/or alloy. Further, the TPR profiles of the bimetallic catalysts show a new unique feature at ca. 150 °C. This feature is not present in the TPR profiles of the monometallic catalysts and coincides with the decrease in the intensity of the palladium-p-hydride signal. Again, without wishing to be bound by theory, these observations may suggest that the unique TPR feature at 150 °C present in the Ti02-based Pd-Sn bimetallic catalysts could be related to a mixed Pd-Sn species, for example, a Pd-Sn-Ox mixed oxide and/or alloy phase which is believed to be active for the direct synthesis of H2O2. Example 5

X-ray diffraction patterns were obtained for selected Sn-Pd bimetallic catalysts supported on S1O2 and T1O2. Figure 3 shows XRD of the monometallic 5% Pd, 5% Sn and bimetallic 2.5% Pd 2.5% Sn supported on silica after calcination at 500 °C for 3 hours. The 5% Pd and 5% Sn materials both show a broad reflection at 22° from the poorly crystalline silica support. The monometallic Pd and Sn catalysts both show reflections characteristic of the PdO and SnO2 phases respectively. The bimetallic catalyst shows weak features of both PdO and SnO2. Figure 4 illustrates how the XRD pattern varies when the Sn:Pd ratio is varied. PdO features are indicated by the dashed line and as the Pd content is decreased from 5% the features become less intense. The same can be seen with the SnO2 reflections indicated by the dotted lines. The reflection at 34° is a combination of both SnO2 and PdO reflections. To check that the absence of PdO peaks in the XRD pattern was not due to the Pd being below the detection limits of the technique, standard samples were prepared with 1 % Pd and calcined at 500 °C for 3 hours, identical to that employed for the catalyst containing 1 % Pd / 4% Sn / S1O2. The XRD patterns are shown in Figure 5. The catalysts containing 1 % Pd with no Sn clearly show PdO reflections at 34. After reduction at 100 °C, the catalyst clearly showed reflections of Pd metal with similar particle sizes. These features are not present in the catalyst containing 1 % Pd / 4% Sn either after calcination or calcination and subsequent reduction at 100- 300 °C. Without wishing to be bound by theory, this may imply that Sn enhances the dispersion of Pd in the bimetallic catalysts or the Pd is incorporated into the Sn as an alloy, especially in the lower Pd content catalysts. After subsequent calcination after reduction of the catalyst no discernable features could be detected above the S1O2 diffraction patterns indicating that the metals are in a highly dispersed state.

XRD patterns (not shown) were recorded for the equivalent T1O2 samples. No reflections corresponding to Sn or Pd could be seen over the intense peaks of the T1O2 used as the support material for the catalyst. Example 6

XPS spectra were recorded for the various catalysts supported on S1O2 after various heat treatments and also for monometallic catalysts. Figure 6 shows the Pd(3d) XPS spectra of monometallic and bimetallic catalysts after calcination at 500 °C for 3 hours. The monometallic Pd sample shows peaks indicating the Pd is present in the 2+ oxidation state which agrees with the observation of PdO in the XRD analysis. The Pd(3d) XPS spectra of the bimetallic catalysts shows a shift to higher binding energy relative to the monometallic catalysts which indicates that the Pd is in a more oxidised state, possibly indicating electron transfer from Pd to Sn.

Figure 7 shows the Pd(3d) XPS spectra for the of 1 % Pd / 4% Sn / Si0 ₂ catalyst after calcination at 500 °C for 3 hours and subsequent reduction at various temperatures for 2 h under 5% H2 / Ar. A clear shift from Pd(ll) to metallic Pd can be seen after reduction at 100 °C and remains unchanged during reduction upto 200 °C.

Figure 8 shows the Pd(3d) spectra of the reduced 1 % Pd / 4% Sn / S1O2 after subsequent heat treatment at 400 °C in air for 2 hours and 3 hours. The spectra show that on heating under air at high temperatures for various times the Pd returns to the Pd(ll) oxidation state and the Pd(0) species begins to disappear; after 3 h the catalyst predominantly contains Pd(ll). Without wishing to be bound by theory, the observation that after a reduction and oxidation following an initial oxidation switches off the hydrogenation activity indicates a change in nanoparticle morphology or composition as the catalyst is substantially returned to its starting state in terms of oxidation state.

Figure 9 shows the Sn(3d ₅ / ₂ ) and Pd(3d) XPS spectra for a 3 % Pd / 2 % Sn / TiO ₂ catalyst following oxidation at 500 °C for 3 hours in air (O), subsequent reduction at 200°C for 2 hours under 5% H2 / Ar (OR), a further oxidation treatment at 400 °C for 3 hours in air oxidation (ORO) and further sequential reduction (at 200°C for 2 hours under 5% H2 / Ar) and then oxidation (at 400 °C for 3 hours in air oxidation) i.e., an ORORO process. This analysis reveals that Sn is present in the +4 oxidation state through the treatment cycles and that the Pd was in the +2 oxidation state after the initial oxidation (O) and with some metallic character after the reduction step (OR). However, on reduction (OR), the binding energy of Sn shifted -0.5 eV and the Pd +0.5 eV than the expected values for Sn4+ and metallic Pd, indicating an enhanced interaction between the metal species. This shift remains on further oxidation (ORO) compared to the previous values for Pd 2+ and Sn 4+ observed for the oxidised only (O) samples indicating that this interaction was "locked-in".

Microscopy and XPS reveals that there are three kinds of structures present in the catalyst after the O-R-O treatment cycle: catalytic nanoparticles (-2-10 nm) that contain both Sn and Pd in varying ratios, smaller Pd rich species and a SnO ₂ amorphous layer which covers the TiO ₂ support in places. It is observed that the SnO ₂ amorphous layer becomes thicker (initially - 1 nm after first oxidation step then increasing to 2-3 nm after ORO). The amorphous layer appears to encapsulate the smaller Pd-rich species. Figure 10 shows the smaller Pd-rich particles (box 2) encapsulated by the amorphous Sn0 ₂ layer (box 1 ). Box 3 in Figure 10 is included as a background reference. Without wishing to be bound by theory, it is believed that the Pd-Sn-O _x nanoparticles (wherein 'x' may vary to maintain charge neutrality) are the main actives for hydrogen peroxide production, with Sn acting as a spacer splitting up the Pd surfaces. These species are believed to be mixed oxide particles as no significant contribution from metallic species is seen in the XPS. The smaller, Pd- rich, species are active species for hydrogenation and decomposition. However, after the ORO treatment cycle and due to strong metal-support interaction, these smaller species are encapsulated by the Sn0 ₂ amorphous layer, preventing them from hydrogenating hydrogen peroxide. The larger catalyst nanoparticles, on the other hand, are not so readily encapsulated by the amorphous oxide layer and, thus, remain available for reaction. Figure 1 1 is a STEM image showing a Pd-Sn- Ox nanoparticle (2) partially embedded in the amorphous Sn0 ₂ layer (4) which "sits on" the highly ordered Ti0 ₂ support (6).

Example 7 - comparative catalysts

A bimetallic catalyst comprising 0.5 % Pd and 15 % Sn (total nominal metal = 15.5 wt. %) was prepared based on the method described in Example 3 or US-A- 537840. The obtained powder was subjected to various heat treatments: (i) calcination at 500 °C in air for 3 hours followed by reductive treatment at 200°C for 2 hours in 5 % H ₂ /Argon; and (ii) calcination at 500 °C in air for 3 hours, followed by reductive treatment at 200°C for 2 hours in 5 % H ₂ /Argon, followed by heat treatment at 400°C for 4 hours in air. The heat treated catalysts were tested in a direct synthesis reaction in accordance with the method described above. 1 ^st use productivity, 2 ^nd use productivity and hydrogenation activity was determined for each catalyst. In fact, it was not possible to determine 2 ^nd use productivity data for the this catalyst as it was not recoverable following use in the first direct synthesis reaction. It is noted in this respect that the hydrogen peroxide reaction described in Example 3 of US-A-5378450 for this catalyst required the addition of an sulphuric acid and so it appears this catalyst requires an acid stabilizer. A 2.5% Pd / 2.5% Au / ΤΊΟ2 catalyst was prepared in accordance with the method described above. The catalyst precursor was subjected to the same heat treatments (i) and (ii) described above, save that in each case the first calcination was carried out at 400°C, and tested for 1 ^st use productivity, 2 ^nd use productivity and hydrogenation activity.

The results are summarized in Table 5 below, together with results for various bimetallic catalyst of the invention.

Table 5.

Example 8 - Pd-Ni catalyst preparation, analysis and testing

- catalyst preparation All catalysts were prepared by conventional impregnation. A palladium nitrate solution in water (12.5 mg/ml, confirmed by MP-AES) and a nickel chloride solution in water (12.5 mg/ml, confirmed by MP-AES) were combined with titania support in the required ratio and heated with stirring until a thick paste was achieved. This thick paste was dried (110°C, 16h) followed by heat treatment as described for each sample.

- catalyst metal ratio studies

Pd-Ni/TiC>2 catalysts were prepared with varied Pd:Ni ratios and a total metal loading of 5 wt%. The metal precursors used were Pd(NOs)2 and NiC . 6H2O, all catalysts were oxidised at 500°C for 3 hours.

Productivity was tested under the following conditions: 0.01 g catalyst, 5% H2/CO2 (2.9 MPa ) and 25% O2/CO2 (1.1 MPa), 1200 rpm, 8.5 g water solvent, 20°C, 30 mins.

Degradation was tested under the following conditions: 0.01 g catalyst, 5% H2/CO2 (2.9 MPa), 1200 rpm, 0.68 g 50 wt% H2O2, 7.82 g water solvent, 20°C, 30 mins.

Results are summarised in Table 6. Table 6.

N.B. for a 30 minute reaction, a productivity of 22 mol kg _ca t ^"1 hr ¹ (as shown by 0.5% Pd 4.5%Ni/TiC>2) equates to approximately 450 ppm H2O2. - catalyst re-use studies

As seen in Table 7 below, there is a significant fall in productivity upon re-use of the Pd-Ni catalysts which had been heat treated in air at 500°C.

Table 7.

Conditions: 30 min, 8.5 g water, 20 °C, 1200 rpm, 10 mg catalyst, 2.9 MPa 5 %

H2/CO2, 1.1 MPa 25 % O2/CO2 To further investigate whether these catalysts could be made stable upon re-use, 0.5%Pd 4.5%Ni/TiC>2 catalysts were prepared by impregnation, oxidised or reduced at various temperatures and then tested. Conditions and results are summarized in Table 8. Table 8.

Conditions: 30 min, 8.5 g water, 20°C, 1200 rpm, 10 mg catalyst, 2.9 MPa 5% H2/CO2,

High temperature (≥700°C) oxidative heat treatment produces catalysts which appear to be largely stable, however these have relatively poor productivity. No reductively treated catalysts appear to be stable. - Oxidation-Reduction-Oxidation treatments

An oxidation-reduction-oxidation treatment, which proved successful for making Pd-Sn catalysts stable, was also investigated. A 0.5%Pd 4.5%Ni/TiC>2 catalyst was heat treated in air at 500°C for 3 hr, followed by heating in H ₂ /Ar at 200°C for 2 hr, followed again by heating in air at 500°C for 3 hr. Results, including comparison with catalysts prepared by oxidation only, and oxidation-reduction, only, are summarized in Table 9.

Table 9.

Conditions: 30 min, 8.5 g water, 20 °C, 1200 rpm, 10 mg catalyst, 2.9 MPa 5 %

H ₂ /C0 ₂ , 1.1 MPa 25 % 0 ₂ /C0 ₂

MP-AES analysis of the post-reaction solution has shown that there is no Ni or Pd present after a 30 minute reaction using the O-R-0 treated catalyst. Further, activity is maintained upon re-use of the catalyst. Thus, the O-R-0 treated catalyst is stable.

Due to the low hydrogenation activity and stability of the catalyst, multiple reactions were performed in succession to investigate whether it was possible to produce higher concentrations of hydrogen peroxide. After completion of a 30 minute reaction the autoclave was vented and re-charged with gases while remaining sealed; the concentration of hydrogen peroxide was then tested after 3 or 5 subsequent reactions. Results are summarized in Table 10.

Table 10.

Conditions: 30 min (repeated as shown), 8.5 g water, 20 °C, 1200 rpm, 10 mg catalyst, 2.9 MPa 5 % H ₂ /C0 ₂ , 1.1 MPa 25 % O2/CO2 (recharged for each 30 min reaction)

These results indicate that this catalyst can additively produce ca. 200 ppm per 30 minute reaction with little breakdown of the hydrogen peroxide present in solution.

The catalyst was also tested under 'standard' hydrogen peroxide synthesis conditions, as summarised in Table 1 1.

Table 11.

Conditions: 30 min, 2.9 g water 5.6 g methanol, 2 °C, 1200 rpm, 10 mg catalyst, 2.9 MPa 5 % H2/CO2, 1.1 MPa 25 % O2/CO2

As can be seen, the catalyst is not only productive with only 0.5% Pd, but importantly, testing indicates that it has a very low to no activity for the hydrogenation/decomposition of hydrogen peroxide, enabling the production of high concentrations of hydrogen peroxide with repeated reactions.

Due to the increase in stability after treating 0.5% Pd 4.5% Ni/Ti02 with an oxidation- reduction-oxidation cycle, different metal ratio Ni-Pd/Ti02 catalysts were treated in this way and tested. Results are summarized in Table 12. Table 12.

1.1 M Pa 25 % 0 ₂ /C0 ₂ Example 9 - Pd-Zn catalyst preparation

All catalysts were prepared by conventional impregnation. A palladium nitrate solution in water (12.5 mg/ml, confirmed by MP-AES) and a zinc chloride solution in water (12.5 mg/ml, confirmed by MP-AES) were combined with titania support in the required ratio and heated with stirring until a thick paste was achieved. This thick paste was dried (1 10°C, 16h) followed by heat treatment as described for each sample.

Pd-Zn/Ti0 ₂ catalysts were prepared with varied Pd:Zn ratios (as summarised in Table 13) and a total metal loading of 5 wt%. The metal precursors used were Pd(NOs) ₂ and ZnCI ₂ . Catalysts were either oxidised at 500°C for 3 hours (i.e., Ό'), or oxidised at 500°C and reduced in H ₂ /Ar at 200°C for 2 hr (i.e., 'OR'), or oxidised at 500°C for 3 hours, reduced in H ₂ /Ar at 200°C for 2 hr followed again by oxidizing in air at 500°C for 3 hr (i.e., ORO'). Table 13.

Pd wt.% Zn wt.%

0 5

0.25 4.75

0.5 4.5

1 4

2.5 2.5

4 1

5 0