Field of the invention The present invention relates generally to a process and an apparatus for pro¬ ducing concurrently hydrogen and metal oxide nanoparticles and to a process and an apparatus for producing ammonia.

Background of the invention It is known to produce hydrogen by reacting metals with steam. US 2,635,947 discloses a process in which metal particles are reacted in a fluidized bed with steam to generate hydrogen. However, the efficiency of such a method is lim¬ ited by the available specific surface area, since the oxidation of the metal to the metal oxide occurs principally on the surface of the metal particles.

It has also been proposed to generate hydrogen by bubbling steam through a bath of molten zinc (Berman A., Epstein M., The Kinetics of Hydrogen Produc¬ tion in the Oxidation of Liquid Zinc with Water Vapor. Int. J. Hydrogen Energy 2000; 25:957-67). However, the feeding of reactants and removal of products - especially the feeding of molten Zn and the removal of solid ZnO formed inside the Zn (liquid) bath - poses a technological difficulty for a continuous mode of operation. Furthermore, the reaction rate is inhibited by the formation of a ZnO (solid) layer around the steam bubbles. Further complications arise during start- ups and shutdowns because of the volume expansion and compression during zinc phase change. A process circumventing some of these problems has been proposed in US 3,928,550. Metallic zinc is combined with molten lead. The mixture is then sprayed under pressure into an oxidizer. There, the zinc is reacted with steam to yield hydrogen and zinc oxide. Again, this method may suffer from reduced efficiency due to the low specific surface area of the lead/zinc droplets and from the formation of a ZnO (splid) layer on the surface of the droplets.

In US 4,496,370 a process has been proposed in which zinc vapor is directly reacted with steam to yield zinc oxide and hydrogen. Such a reaction, however, is heterogeneous in nature and generally requires the presence of some sur¬ face, and may lead to undesired deposits of solid zinc and zinc oxide on the reactor walls, as the walls provide such a surface.

Conventional production of ammonia employs the Haber-Bosch process. An alternative is a two-step process, in which first a metal nitride is produced, which then is reacted with water to form ammonia. For example, it is known from WO 02/090255 to produce ammonia by reacting aluminum nitride with wa¬ ter. This process may suffer from low efficiency and yield due to passivation of the aluminum nitride particles.

Summary of the invention It is an object of the present invention to provide a process for producing hydro¬ gen which avoids the above-mentioned problems and enables high yields. This object is achieved by a process with the features of claim 1.

It is a further object of the present invention to provide a process for simultane¬ ous production of metal oxide nanoparticles along with hydrogen. This object is achieved by a process with the features of claim 10.

It is a further object of the present invention to provide a process for producing ammonia which avoids the above-mentioned problems and enables high yields. This object is achieved by a process with the features of claim 12. It is a further object of the present invention to provide apparatus for the produc¬ tion of hydrogen or ammonia and metal oxide nanoparticles which is suitable for carrying out the processes of the present invention. This object is achieved by an apparatus with the features of claims 15 or 24.

Both the processes for producing hydrogen and ammonia employ nanoparticles and/or nanodroplets comprising a metal or metal-containing compound, which are reacted with steam (i.e., water in the gas phase, water vapor). Specifically, for the production of ammonia, the metal-containing compound is a metal ni¬ tride. Nanoparticles and/or nanodroplets are advantageous because of their large specific surface area, which leads to an efficient reaction and high yields. The specific surface area of the nanoparticles and/or nanodroplets of the metal or metal-containing compound is significantly larger than for particles usually employed, e.g., in fluidized-bed reactors. Preferably, the nanoparticles and/or nanodroplets entirely consist of the metal or metal-containing compound, with¬ out admixtures or a carrier substance. However, the use of nanoparticles or nanodroplets comprising a carrier or admixtures is also within the scope of the present invention.

For the purposes of this document, "nanoparticles" are to be understood as solid particles having a BET surface area larger than 1 square meter per gram (m2/g). The term "nanoparticles" is thus defined by reference to a bulk property of a large collection (ensemble) of individual particles. Preferably, the BET sur- face area is larger than approximately 5 m2/g. More preferably, it is larger than 10 m2/g, and even more preferably larger than 20 m2/g.

For the purposes of this document, the term "BET surface area" is to be under¬ stood as the specific surface area as determined by gas adsorption and as de- fined in ISO 9277 ("Determination of the specific surface area of solids by gas adsorption using the BET method"). More specifically, the BET surface area is the specific surface area as determined by nitrogen adsorption at 77 K employ- ing the BET isotherm. For droplets (including nanodroplets), the BET surface area is to be understood as the BET surface area of the particles which are ob¬ tained after solidification of the droplets by rapid cooling without fusion of drop¬ lets. Thus, the term "nanodroplets" is to be understood as droplets of liquid whose solidification by rapid cooling without fusion yields nanoparticles in the sense of the above definition.

Assuming monodispersed spherical particles, the BET surface area, hereinafter referred to with the symbol SSA, is related to the average particle size d by the formula: d[μm] = 6/(p[g/cm3]xSSA[m2 /g]). Here, p is the density of the solid material. Hence, the definition of nanoparticles and nanodroplets by their BET surface area can directly be translated into a definition by average particle size, provided the density of the material is known.

For producing hydrogen, suitable metals or metal-containing compounds are metals or compounds which have a negative Gibbs free energy for the hydroly¬ sis reaction. In particular, the metal is preferably selected from the group con¬ sisting of Zn (zinc), Fe (iron), Mg (magnesium), Mn (manganese), Al (alumi¬ num), Ti (titanium), Si (silicon), and Ca (calcium). For the purposes of this document, the semi-metal silicon (Si) is to be understood as a metal. Preferred is Zn, Fe or Mg, most preferred is Zn. The metal-containing compound is pref¬ erably a pure or mixed low-valence metal oxide. The term "low-valence metal oxide" is to be understood as meaning a metal oxide in which at least one metal component is in a oxidation state lower than its maximum oxidation state. Suit- able pure metal oxides include FeO, SnO, TiO and TiOx where x represents a real number with 1 < x < 2, e.g. Ti2O3. Suitable mixed metal oxides include CoFexOy, MgFexOy, ZnFexOy, CdFexOy, MnFexOy, NiFexOy, InFexOy, SnFexOy, NiMn2FexOy, where x, y, and z represent real numbers and where at least one of the metal components is in an oxidation state below its maximum oxidation state. For producing ammonia, the metal nitride is preferably selected from the group consisting of aluminum nitride (AlN), silicon nitride (Si3N4), iron nitride (Fe2N), and titanium nitride (TiN). Preferred is aluminum nitride (AIN). The nanoparticles and/or nanodroplets may be provided in an immobilized form or in form of an aerosol. The aerosol form is preferred.

The steam may be provided to the reaction zone by generating steam remote from the reaction zone and injecting steam into the reaction zone or its vicinity, or by feeding liquid water into the reaction zone or its vicinity. The liquid water is then evaporated within the reaction zone or its vicinity to form steam because of the elevated temperatures generally present in the reaction zone and its vicinity. Of course, a combination of ex-situ and in-situ generation of steam is also pos¬ sible.

Preferably, the nanoparticles and/or nanodroplets and the steam are continu¬ ously fed into the reaction zone as streams, and any products of the reaction as well as any unreacted reactants are continuously removed from the reaction zone as a stream. In this way, a continuous process can be accomplished.

An advantage of the proposed processes for either hydrogen or ammonia pro¬ duction is that they can be carried out at moderate pressures. Advantageously, the process is carried out at a pressure below 10 bar, preferably below 2 bar, most preferred at ambient pressure or at less than 100 mbar above ambient pressure.

A very simple and highly efficient process results by generating the nanoparti- cles and/or nanodroplets in situ, i.e. in the reaction zone or in the vicinity of the reaction zone. Thus, the nanoparticles and/or nanodroplets may be generated in a formation zone upstream from or overlapping with said reaction zone by feeding a stream of a vapor of the metal or metal-containing compound into the formation zone, and cooling the vapor of the metal or metal-containing com- pound in the formation zone under conditions adapted for obtaining nanoparti¬ cles and/or nanodroplets. The nanoparticles/nanodroplets may be formed ho¬ mogeneously or heterogeneously on seeds provided to the reaction chamber from the outside. In particular, if the metal is Zn, seeds can be created be sus¬ pending a few tiny ZnO nanoparticles prior to the stream which contains the Zn vapor entering the reaction zone and contacting the steam. Alternatively, ZnO seed nanoparticles can be created in situ by sending a small oxygen flow to the Zn vapor before the latter enters the reaction zone and contacts the steam.

If nanoparticles are formed in situ, it may be advantageous to feed additional steam to the formation zone which is cooler than the vapor of the metal or metal-containing compound when being fed to the formation zone, in order to aid in rapid cooling ("quenching") of the vapor of the metal or metal-containing compound. It may be particularly advantageous to feed liquid water into the formation zone or into its vicinity. The water will then be evaporated in the for¬ mation zone or its vicinity to form steam. The heat of evaporation required for this will additionally aid in the cooling of the vapor of the metal or metal- containing compound.

Preferably, the stream of the vapor of the metal or metal-containing compound, before feeding the vapor into the formation zone, is at a temperature and pres¬ sure such that the temperature is above the saturation temperature of the va- por. In addition, it is preferred that, in the formation zone, the vapor is cooled to a temperature below the saturation temperature. Preferably, the cooling rate exceeds approximately 5000 K/s.

Alternatively, the nanoparticles and/or nanodroplets may be generated in a for- mation zone upstream from or overlapping with said reaction zone by feeding a stream of a precursor of the metal or metal-containing compound into the for¬ mation zone, feeding a stream of a reactant gas suitable for reacting with the precursor to yield the metal or metal-containing compound into the formation zone, and reacting the precursor and the reactant gas under conditions suitable to yield nanoparticles and/or nanodroplets comprising the metal or metal- containing compound. The precursor may itself be a metal or a metal-containing compound, preferably it is a metal. Advantageously, the precursor is provided as a vapor. The term "reactant gas" is to be understood as a gas or gas mixture suitable to react with the precursor. Specifically, the reactant gas may be a ni- triding gas. The term "nitriding gas" is to be understood as a gas or a gas mix¬ ture containing nitrogen and/or a nitrogen-containing compound suitable to re- act with the precursor to yield a metal nitride. Preferably, the nitriding gas com¬ prises nitrogen (N2).

Specifically, in the case of ammonia production, the precursor may be a metal, e.g., aluminum, preferably in the vapor state. Alternatively, the precursor may be a metal oxide, e.g. aluminum oxide (AI2O3), and the nitriding gas may addi¬ tionally comprise a reducing agent, e.g. carbon, methane, other hydrocarbons or biomass. The conditions in the formation zone are then chosen such that a reaction of the metal oxide with the nitriding gas mixture occurs, which yields the metal nitride. The metal oxide may be provided in the form of nanoparticles or nanodroplets, or nanoparticles and/or nanodroplets of the metal nitride may be formed in situ.

A side product of the proposed processes for producing hydrogen are metal oxide nanoparticles. Of course, these may as well be considered the main product rather than a side product. There is, e.g., a large market for such nanoparticles in the rubber industries, where they may be used, e.g., for improv¬ ing the properties of car tires. In this sense, the proposed processes for pro¬ ducing hydrogen may as well be considered as processes for producing metal oxide nanoparticles. In particular, the present invention provides a process for producing such nanoparticles (along with hydrogen) from a metal vapor and water/steam, the process comprising the steps of feeding a stream of a vapor of a metal into a formation zone; cooling said vapor of said metal in said formation zone to obtain metal nanoparticles and/or nanodroplets having a BET surface area of at least 1.0 m2/g, preferably by rapid quenching, i.e., by cooling from a temperature above the saturation temperature of the metal vapor to a temperature below this satu¬ ration temperature with a cooling rate exceeding approximately 5000 K/s; providing steam in a reaction zone downstream from the formation zone or overlapping with the formation zone; reacting the steam with the nanoparticles and/or nanodroplets in the re¬ action zone to obtain metal oxide nanoparticles and hydrogen.

Of course, any considerations made in connection with the process for produc¬ ing hydrogen equally well apply also to the process for producing metal oxide nanoparticles.

This process is particularly advantageous if the metal is Zn and the product of the process are ZnO nanoparticles.

The stream of the vapor of the metal, metal-containing substance or precursor may comprise a carrier gas. Likewise, any stream of steam or of a reactant gas may comprise a carrier gas. Advantageously, the carrier gas is substantially inert under the conditions of the process. The carrier gas may be a noble gas, specifically, argon.

Generally, the reaction of the steam with the metal or metal-containing com- pound is exothermic. Thermal energy released by this reaction may advanta¬ geously be employed for heating any of the streams fed to the formation or re¬ action zone (i.e., for heating any of the streams of steam, of the vapor of the metal, metal-containing compound or precursor, or of any other reactant gas, with or without carrier gases). Any thermal energy supplied to the process, e.g. for heating of the streams, may advantageously be provided by concentrated solar energy, nuclear process heat or by combustion of a portion of the hydro¬ gen generated.

An inventive apparatus comprises a reaction chamber with at least one first inlet for nanoparticles and/or nanodroplets comprising a metal or of a metal- containing compound or for a metal vapor, at least one second inlet for steam or liquid water, and at least one outlet for products of the reaction. It further com- prises a mixing zone adapted for mixing the streams entering through the first and second inlets, and a reaction zone downstream from and/or overlapping with the mixing zone adapted for reacting nanoparticles and/or nanodroplets with steam.

If nanoparticles are generated in situ in the reaction chamber, the reaction chamber further comprises a particle/droplet formation zone downstream from and/or overlapping with the mixing zone and upstream from and/or overlapping with the reaction zone. The particle/droplet formation zone is adapted for form- ing nanoparticles and/or nanodroplets from the metal vapor by rapid cooling.

Advantageously, the apparatus further comprises a metal evaporator for evapo¬ rating a metal vapor from a solid or liquid metal. The evaporator is connected to the first inlet of the reaction chamber for feeding the vapor to the first inlet and from there into the reaction chamber. A variety of embodiments for the evapora¬ tor may be envisaged. In the simplest case, the evaporator comprises a cham¬ ber in which a bath of liquid (molten) metal is provided, and means for passing a stream of a carrier gas such as argon over the surface of the liquid metal and towards the inlet of the reaction chamber. In a more sophisticated embodiment, the liquid metal might be sprayed through a carrier gas atmosphere, and the carrier gas together with the resulting metal vapor would then be fed to the re¬ action chamber. The metal vapor might also be generated by sublimation from solid metal at a sufficient temperature, where the solid metal is preferably finely dispersed to achieve a high specific surface.

Preferably, the apparatus further comprises a steam source, e.g., in the form of a steam generator, connected directly or indirectly to the second inlet of the re¬ action chamber.

In an advantageous embodiment, the steam is brought into thermal contact with the reaction chamber, in particular with the reaction zone, before being fed to the reaction chamber. This enables heat exchange between the reaction cham- ber (in which a generally exothermic reaction takes place) and the steam, which leads to heat removal from the chamber at the same time as heating of the steam to a temperature suitable for the subsequent reaction. To this end, the apparatus preferably comprises a steam conduit (steam feeding channel) in thermal contact with the reaction zone, whose inlet is connected to the steam source and whose outlet is connected to the second inlet of the reaction cham¬ ber.

The steam is advantageously fed into the reaction chamber in a manner that leads to rapid mixing with the nanoparticles or the metal vapor and to rapid cooling of same ("quenching") in order to obtain the nanoparticles and/or nano- droplets. This minimizes agglomeration of the nanoparticles to larger particles and deposition of the metal vapor or of nanoparticles on the walls of the reac¬ tion chamber. To this end, the apparatus preferably comprises a plurality of steam nozzles for injecting the steam in a manner that creates turbulent flow in the reaction chamber, in particular in the mixing and/or particle/droplet formation zones. Preferably the nozzles have a direction of ejection of the steam which is different from the direction of flow of the metal vapor or nanoparti- cles/nanodroplets, and which is preferably approximately at a right angle with the latter direction.

Preferably, the reaction chamber is elongated and has a plurality of inlets for said steam or liquid water and/or a plurality of inlets for said vapor of said metal or metal-containing compound distributed along a direction of elongation of the reaction chamber. In particular, preferably the steam is fed to the reaction chamber in such a distributed manner. Some or all of the inlets may again be designed as nozzles for creating turbulent flow.

Preferably, at least a portion of the reaction chamber has porous walls and means for feeding at least a portion of the steam or liquid water and/or at least a portion of the vapor of the metal, metal-containing compound or precursor to said reaction chamber through the porous walls. By this, deposits of the metal, the metal-containing compound, or of any reaction products on the walls of the reaction chamber can be reduced or completely avoided. Preferably, the steam is fed through the porous walls.

Alternatively, an inventive apparatus may comprise a reaction chamber with at least one first inlet for a precursor, at least one second inlet for a reactant gas (which can be a nitriding gas), at least one third inlet for steam or liquid water, and at least one outlet for products of the reaction. The reaction chamber fur¬ ther comprises a mixing zone adapted for mixing streams entering through at least said first inlet and said second inlet, a particle/droplet formation zone downstream from and/or overlapping with said mixing zone, adapted for forming nanoparticles and/or nanodroplets from said precursor and said reactant gas or said nitriding gas, and a reaction zone downstream from and/or overlapping with said mixing zone adapted for reacting nanoparticles and/or nanodroplets with steam.

The apparatus may further comprise a source of the precursor (such as an evaporator in the case of a gaseous precursor) connected to the first inlet, a source of the reactant gas (such as a pressure tank or compressor) connected to the second inlet, and/or a steam source (such as a steam generator) con¬ nected to the third inlet. Any other considerations made for the apparatus fur¬ ther above equally well apply, mutatis mutandis, to the present apparatus, in particular the considerations in relation to bringing the steam into thermal con¬ tact with the reaction chamber before feeding, the provision of steam nozzles for creating turbulent flow, the provision of inlets distributed along the length of the reactor, in particular, for the steam, or the provision of porous walls, in par¬ ticular for the steam.

Brief description of the drawings The invention will be described in more detail in connection with exemplary em¬ bodiments illustrated in the drawings, in which Fig. 1 shows a schematic representation of a solar two-step cycle for producing hydrogen; Fig. 2 shows a highly schematic representation of a reactor for producing hydrogen according to the present invention; Fig. 3 shows a highly schematic representation of a reactor for producing ammonia according to the present invention; Fig. 4 shows a schematic representation of a laboratory-scale apparatus for producing hydrogen or ammonia according to the present in¬ vention; Fig. 5 shows a schematic detail view of part of the apparatus of Fig. 4; Fig. 6 shows a schematic detail view of part of the apparatus of Fig. 4 according to a first embodiment; Fig. 7 shows a schematic detail view of the apparatus of Fig. 4 according to a second embodiment; Fig. 8 shows a highly schematic view of a laboratory-scale apparatus for producing nanoparticles; and Fig. 9 shows a highly schematic view of a laboratory-scale apparatus for producing hydrogen according to another embodiment of the pre¬ sent invention.

Detailed description of the invention Anthropogenic emissions of greenhouse gases and other pollutants can be sig¬ nificantly reduced or even completely eliminated by substituting fossil fuels by cleaner fuels, e.g. hydrogen. A review of the process technology for thermo- chemically producing hydrogen from water using solar energy is found in (Stein- feld A., Palumbo R., Solar Thermochemical Process Technology. In: Meyers RA, editor. Encyclopedia of Physical Science and Technology, 3rd edition, Vol¬ ume 15. San Diego, USA: Academic Press, 2002. p. 237-256).

Fig. 1 shows a schematic representation of an example for a two-step thermo- chemical cycle for splitting water (H2O) into hydrogen (H2) and oxygen (O2). For an overview over such two-step water-splitting cycles, based on metal oxide redox reactions, see e.g. (Steinfeld A., Kuhn P., Reller A., Palumbo R., Murray J., Tamaura Y., Solar-processed Metals as Clean Energy Carriers and Water- Splitters. Int. J. Hydrogen Energy 1998; 23:767-74, and literature cited therein).

A first, endothermic step 102 comprises thermal reduction of metal oxide to the metal or the lower-valence metal oxide by providing solar energy 101. The sec¬ ond, exothermic step 103 comprises the hydrolysis of the metal to form hydro¬ gen and the original metal oxide; the latter is recycled to the first step. This solar thermochemical cycle is represented by:

1 st step (solar): MxOy -» xM + 0.5yO2 2nd step (non-solar): xM + yH2O -> MxOy + yH2

where M and MxOy denote the metal and the corresponding metal oxide. The net reaction is H2O -> H2 + 0.5 O2. Hydrogen and oxygen are formed in different steps, thereby eliminating the need for high-temperature gas separation.

In Fig. 1 , the metal oxide is zinc oxide (ZnO), which is reduced to elemental zinc (Zn) in the first solar step 102. The second, non-solar, exothermic step 103 is the hydrolysis of zinc at moderate temperatures to form hydrogen and zinc ox- ide. For the present example, the solar thermochemical cycle based on ZnO/Zn redox reactions is represented by:

1 st step (solar): ZnO -> Zn + 0.5O2 2nd step (non-solar): Zn + H2O ■* ZnO + H2.

The chemical aspects of the 1st step and the present status of the pertinent chemical reactor technology are summarized in (Steinfeld A., Solar Hydrogen Production via a 2-step Water-Splitting Thermochemical Cycle based on Zn/ZnO Redox Reactions, Int. J. Hydrogen Energy 2002; 27:611-19).

Other metals may also be employed in this redox (reduction-oxidation) cycle, such as Fe, Mg, Al, Ti, Si, and Ca. The temperature requirement for the produc- tion of metals from the metal oxides may be reduced by conducting the chemi¬ cal reduction electrolytically, or by making use of a reducing agent such as car¬ bon or methane (see e.g. Steinfeld A., Kuhn P., Reller A., Palumbo R., Murray J., Tamaura Y., Solar-processed Metals as Clean Energy Carriers and Water- Splitters. Int. J. Hydrogen Energy 1998; 23:767-74). Mixed metal oxides may be also employed in this redox cycle. The process may also be applied by reducing a metal oxide to a lower-valence metal oxide, instead of performing the com¬ plete reduction to the metal. For example, in the first step, Fe2O3 or Fe3O4 may be reduced to wustite (FeO), which has the capability of splitting water in the second step.

Ammonia can also be produced via a 2-step process. The first, high- temperature, endothermic step is the production of metallic nitrides by, e.g. car- bothermic reduction of their metal oxides in a N2 atmosphere. The second step is the hydrolysis of the metal nitride for the formation of ammonia and the origi¬ nal metal oxide; the latter is recycled to the first step. The reactions can be rep¬ resented by the following general equations:

1 st step (reduction): MxOy + C(gr) + N2 → Mx,Ny, + CO 2nd step (hydrolysis): Mx.Ny, + H2O → MxOy + NH '33

where M denotes the metal, and MX'Ny' and MxOy the corresponding metal ni¬ tride and oxide, respectively. The stoichiometry of theses reactions will depend on the metal. For example, for aluminum:

1st step (reduction): AI2O3 +3C(gr) + N2 → 2AIN + 3CO 2nd step (hydrolysis): 2AIN + 3H2O → AI2O3 +2NH3 The net reaction is: 3C(gr) + N2 + 3H2O → 3CO + 2NH3

Other metals may also be employed in this 2-step process, e.g. Si, Fe, Ti, etc. Other reducing agents than C(gr) (graphite) may also be employed for the re- duction step, e.g. methane, biomass, and other hydrocarbons.

Alternatively, ammonia can be produced via a 3-step process. The first step is the endothermic reduction of the metal oxide to the metal. The second step is the nitridation of the metal with a nitriding gas producing metal nitride. The third step is the hydrolysis of the metal nitride for the formation of ammonia and the original metal oxide.

In the present invention, it is suggested to employ nanoparticles and/or nano- droplets in a process for producing hydrogen or in a process for producing am¬ monia. Metal nanoparticles can be obtained advantageously by evaporation- condensation processes. In these processes, the metal is first evaporated from a melt. Metal nanoparticle formation then takes place by vapor condensation due to oversaturation during rapid cooling (quenching). The quenching can be achieved, e.g., by mixing of the metal vapor with a gas of lower temperature or by expansion (cf. Kodas T.T., Hampden-Smith M., Aerosol Processing of Mate¬ rials, Wiley- VCH, New York, 1999; Wegner K., Walker B., Tsantilis S., Pratsinis S. E., Design of Metal Nanoparticle Synthesis by Vapor Flow Condensation. Chem. Eng. ScL 2002; 57:1753-62). An inert gas is employed frequently to carry the metal vapor from an evaporation zone to a particle formation zone and the product particles to a collector. This process also has been employed suc¬ cessfully for the production of zinc nanoparticles (Daub O., Langel W., Reiner C, Kienle L., QMS-controlled Production of Nanocrystalline Metals by Inert Gas Condensation in a Flow System. Ber. Bunsenges. Phys. Chem. 1997; 101 :1753-56).

The evaporation-condensation process can be coupled with a chemical reaction by introduction of a reactant gas to produce metal oxides, metal nitrides, metal carbides, or other ceramic materials (see, e.g., Pratsinis S. E., Wang G., Panda S., Guiton T., Weimer A., Aerosol Synthesis of AIN by Nitridation of Aluminum Vapor and Clusters. J. Mater. Res. 1995; 10: 512-20). Generally speaking, nanoparticles of a metal or metal-containing compound can be produced in aerosol flow reactors by evaporation-condensation processes involving chemi¬ cal reactions. In these processes, a precursor is evaporated, and the vapor of the precursor is mixed with a reactant gas. Subsequently, particle formation and reaction from the precursor to the metal or metal-containing compound take place by vapor condensation during rapid cooling (quenching). In US Patent No. 5,525,320, such a process (with aluminum as precursor and nitrogen or ammo¬ nia as reactant gas) has been employed successfully for the production of alu¬ minum nitride nanoparticles.

An overview of the general process technology is given in (Kodas T.T., Hamp- den-Smith M., Aerosol Processing of Materials, Wiley-VCH, New York, 1999), and reference is made to this document for details of such processes.

Advantageously, the process for producing the nanoparticles/nanodroplets and the process for producing hydrogen employing these nanoparti¬ cles/nanodroplets are combined into a single process. A combined process for producing hydrogen then may comprise the following stages: Continuous pro¬ duction of steam and metal vapor in separate generators; continuous feeding of separate streams of steam and metal vapor into a mixing zone; mixing of steam and metal vapor inside the mixing zone to create a single stream of metal and steam; rapid cooling (quenching) of metal vapor by steam and simultaneous formation of nanoparticles and/or nanodroplets of metal; progression of the flow of steam laden with nanoparticles and/or nanodroplets of metal to a chemical reaction zone; chemical reaction of steam and metal, generating hydrogen and metal oxide particles along the chemical reaction zone; and continuous exit of products.

Not all of the above-mentioned stages need to be carried out as described; stages can overlap and/or can be skipped. Top candidate metals for the hydro- gen-production process are zinc, iron, and magnesium, but other metals can also be employed. The process may also be applied using a metal-containing compound instead of the metal, and a hydrogen-containing substance instead of water. In addition, the particles and/or droplets can form an aerosol or can be immobilized at a surface. In any of these possibilities, a key aspect is the com¬ bined formation of nanoparticles and/or nanodroplets with a BET-surface area above 1 m2/g, and their in-situ reaction to generate hydrogen. Alternatively, the nanoparticles and /or nanodroplets may be produced in separate upstream processes and fed as solids or liquids into the mixing zone.

Advantages of the proposed process for the production of hydrogen are at least three-fold: - Simple, controllable, and continuous feeding of reactants and removal of products during continuous mode of operation. Enhanced interface reaction kinetics and mass transfer by the high spe¬ cific surface area of nanoparticles and/or nanodroplets comprising a metal or metal-containing compound. - Elimination of the complication associated with the phase change of met¬ als in a bath.

Likewise, a combined process for the production of nanoparticles/nanodroplets of metal nitride and ammonia may comprise the following stages: Continuous production of steam, metal vapor, and a nitriding gas; Continuous feeding of separate streams of metal vapor and a nitriding gas into a mixing zone; Mixing of the metal vapor and the nitriding gas inside the mixing zone to create a single gas stream; Quenching of the vapor and simultaneous formation of nanoparti¬ cles and/or nanodroplets of metal nitride; Progression of the flow of nanoparti- cles and/or nanodroplets of metal nitride to a chemical reaction zone; Continu¬ ous feeding of steam into the chemical reaction zone; Chemical reaction of steam and metal nitride, generating ammonia and metal oxide particles along the chemical reaction zone; Continuous exit of products.

Again, not all of the above-mentioned stages must be carried out as described; stages can overlap and/or can be skipped. Specifically, metal nanoparticles may be produced in a separate upstream process, then contacted with a nitriding gas to form nanoparticles comprising the metal nitride, and these nanoparticles may then be fed into the mixing zone.

Alternatively, the metal vapor, the nitriding gas and the steam may first be mixed to form nanoparticles comprising metal nitride and steam, which then react in the chemical reaction zone downstream of the mixing zone to form metal oxide particles and ammonia. Top candidate metals for the ammonia- production process are aluminum, titanium, and silicon, but other metals can also be employed.

Advantages of the proposed process for the production of ammonia are: Simple, controllable, and continuous feeding of reactants and removal of products during continuous mode of operation. Enhanced interface reaction kinetics and mass transfer by the high spe¬ cific surface area of metal nitride nanoparticles and/or nanodroplets. Elimination of the complication associated with high pressure in conven¬ tional ammonia production, minimizing costs and safety concerns in the building and during operation of the plant. Elimination of the complication associated with the use of catalysts in the conventional ammonia production. Elimination of the need for syngas or hydrogen as the raw materials in the conventional ammonia production. - Utilization of widely available raw materials (C, N2, and water) via the 2- step process.

Fig. 2 shows a highly schematic view of an apparatus for producing hydrogen according to the present invention. The apparatus comprises an elongated re- action chamber 201 with one inlet for metal vapor and three inlets for steam. The metal vapor is provided in a metal vapor reservoir 203 and fed into the re¬ action chamber through a valve 207. The steam is provided in a steam reservoir 202 and fed into the reaction chamber through valves 204, 205 and 206. The valves are distributed along the direction of elongation of the reaction chamber. The reaction chamber may be thought to be divided into three (imaginary) zones: a mixing zone 211 , a particle/droplet formation zone 212, and a reaction zone 213. The borders of the zones are indicated by dashed vertical lines in Fig. 2. It should be emphasized that these borders may be only imaginary, i.e., there need not be any physical borders, and the borders may be gradual, i.e. they need not be well-defined.

For producing hydrogen, metal vapor and steam are fed into the mixing zone 211. There, intimate mixing between the metal vapor and the steam occurs. The temperature, mixing ratio and total pressure in the mixing zone are chosen such that the temperature is above the saturation temperature of the metal vapor at the partial pressure of the metal vapor in the mixing zone. The mixture of metal vapor and steam is then transferred to the particle/droplet formation zone 212. In this zone, the mixture is subjected to rapid cooling (quenching) to a tempera¬ ture below the saturation temperature of the metal vapor. By this, nanoparticles and/or nanodroplets of the metal are formed. The exact conditions under which such nanoparticle and/or nanodroplets formation occurs may depend on the type of metal, on the reactor design, size etc. (cf. Kodas T.T., Hampden-Smith M., Aerosol Processing of Materials, Wiley-VCH, New York, 1999; Wegner K., Walker B., Tsantilis S., Pratsinis S.E., Design of Metal Nanoparticle Synthesis by Vapor Flow Condensation. Chem. Eng. Sci. 2002; 57:1753-62). Examples for suitable conditions for a specific reactor design will be given below. Gener- ally, a cooling rate exceeding 5000 Kelvin per second (K/s or, equivalently, °C/s) is advantageously chosen. A higher cooling rate may be advantageous, as it may lead to smaller particle sizes. Therefore, more rapid cooling, e.g. at a rate exceeding 10000 K/s, is preferred, more preferred at a rate above 20000 K/s, even more preferred above 50000 K/s.

Additional steam may be fed to the particle/droplet formation zone 212 through valve 205. The steam may be at a lower temperature than at valve 204 in order to aid in rapid cooling of the metal vapor/steam mixture entering from the mixing zone.

The nanoparticles and/or nanodroplets generated in the particle/droplet forma- tion zone 212 are then transferred to the reaction zone 213. Here, conditions are chosen such that a reaction of the metal nanoparticles/nanodroplets with the steam can occur. Generally, pressure and temperature conditions are such that the temperature is below the saturation temperature of the metal vapor, such that no evaporation from the nanoparticles and/or nanodroplets can occur. At the same time, the temperature should be high enough that the reaction pro¬ ceeds rapidly enough. Additional steam can be fed to the reaction zone 213 through valve 206 for providing enough steam for a stoichiometric reaction and/or for regulating the temperature and pressure conditions in the reaction zone.

Finally, the reaction products (metal oxide-comprising particles and gaseous, elemental hydrogen) as well as any unreacted reactants (steam and/or metal nanoparticles/nanodroplets) are removed to form a product stream 208.

Specifically, if the metal is zinc (Zn) and the process is carried out at or near atmospheric pressure, the preferred conditions are as follows: The temperature in the mixing zone 211 should preferably exceed the saturation temperature of the zinc vapor. The saturation temperature depends on the partial pressure of Zn, which in turn depends on how much steam and Zn vapor is fed into the mix- ing zone. If the partial pressure approaches atmospheric pressure, the satura¬ tion temperature approaches 1180 K, and the temperature in the mixing zone should preferably exceed 1180 K. The temperature in the particle/droplet forma¬ tion zone and in the reaction zone should preferably be in the range between 800 K and 1180 K, when working at atmospheric pressure. In general, for any operating pressure, the temperature in the particle/droplet formation zone and in the reaction zone should be below the saturation temperature of zinc vapor. The cooling rate in the particle/droplet formation zone should preferably exceed 80000 K/s.

Various additions and modifications to the described process and apparatus are possible. Instead of single valves 204, 205, 206, at least a portion of the reac- tion chamber may be equipped with a porous wall, through which the steam is fed into the chamber. A porous wall with steam entering continuously has the advantage that reactions involving the chamber walls will be minimized, and solid material is prevented from precipitating on the walls.

Instead of feeding only steam to the reaction chamber in various places along the reaction chamber, also metal vapor can be fed to the reaction chamber in more than one place for optimizing the nanoparticle/nanodroplet formation.

Instead of or in addition to feeding steam to the reaction chamber, liquid water may be fed to the chamber. Due to the elevated temperatures in the reaction chamber, the water will readily evaporate in the chamber to form steam. This may be particularly advantageous if a separate stream of liquid water is fed to the particle/droplet formation zone 212 or its vicinity, e.g., through valve 205. While evaporating, the water will remove heat from the formation zone 212. This will aid in the rapid cooling (quenching) of the metal vapor and thus improve the formation of nanoparticles or nanodroplets.

In addition to steam, some other gas - which is preferably substantially inert un¬ der the conditions present within the reaction chamber - can be fed to the reac- tion chamber, either through additional inlets or together with the steam and/or the metal vapor. Feeding such an additional gas into the reaction chamber can improve the mixing and/or cooling process. Feeding of the metal vapor to the reaction chamber can be facilitated by employing the additional gas as a carrier gas for the metal vapor, i.e., by feeding such an additional gas to the reaction chamber together with the metal vapor. The additional gas can be, e.g., a noble gas, specifically argon. It may be advantageous to feed a metal as precursor, preferably in vapor form, and a reactant gas to the reaction chamber. The reactant gas reacts with the precursor to form a metal-containing compound. Alternatively, it may be advan¬ tageous to feed a metal-containing compound as precursor, preferably in vapor form, and a reactant gas to the reaction chamber. The reactant gas reacts with the precursor to form a metal or a different metal-containing compound. The reaction conditions are chosen such that the metal or metal-containing com¬ pound is obtained in the form of nanoparticles and/or nanodroplets. The precur¬ sor may be fed to the reaction chamber by means of a carrier gas.

An apparatus for carrying out a process in which a precursor is fed to a reaction chamber is shown in Fig. 3 in a highly schematic manner. Many components of this reactor are similar to the corresponding components of the reactor of Fig. 2 and serve similar purposes. Again, the apparatus comprises an elongated reac- tion chamber 301. The precursor is provided in a precursor reservoir 303 and fed into the reaction chamber through a valve 307. Steam is provided in a steam reservoir 302 and fed into the reaction chamber through valves 305 and 306. These two valves are at different positions along the direction of elongation of the reaction chamber. A reactant gas is provided in a reactant gas reservoir 310 and fed into the reaction chamber through a valve 304. The reaction cham¬ ber may again be thought to be divided into three (imaginary) zones: a mixing zone 311 , a particle/droplet formation zone 312, and a reaction zone 313.

The precursor and the reactant gas are fed into the mixing zone 311. There, intimate mixing between the precursor and the reactant gas occurs. The mixture of the precursor and the reactant gas is then transferred to the particle/droplet formation zone 312. In this zone, the mixture is subjected to rapid cooling (quenching) leading first to the formation of nanoparticles and/or nanodroplets of the precursor. Steam or liquid water may enter through valve 305 and may aid in the quenching process. The reactant gas then reacts with the precursor nanoparticles and/or nanodroplets to form nanoparticles and/or nanodroplets comprising the desired metal or metal compound. The exact conditions under which such nanoparticle and/or nanodroplet formation occurs will depend on the precursor, the reactant gas, on the reactor design, size etc.

The nanoparticles and/or nanodroplets generated in the particle/droplet forma- tion zone 312 are then transferred to the reaction zone 313. Here, conditions are chosen such that a reaction of the nanoparticles/nanodroplets with the steam can occur. Additional steam can be fed to the reaction zone 313 through valve 306 for providing enough steam for a stoichiometric reaction and/or for regulating the temperature and pressure conditions in the reaction zone. Finally, the reaction products as well as any unreacted reactants are removed to form a product stream 308. Again, at least a portion of the reaction chamber may be equipped with a porous wall, through which the steam is fed into the chamber.

Utilization of a precursor is especially advantageous in a process for producing ammonia according to the present invention. An example is the generation of aluminum nitride nanoparticles by feeding aluminum (as a precursor of alumi¬ num nitride) and a nitriding gas (as a reactant gas) to the reaction chamber, under conditions as e.g. described in US 5,525,320 or as in the references cited therein. Advantageously, the precursor is provided in the form of a vapor, and the nitriding gas is nitrogen.

An apparatus which has been employed successfully in laboratory-scale ex¬ periments for producing hydrogen according to the principles of the present in¬ vention is shown in Fig. 4 in a highly schematic manner. The apparatus can be modified for producing ammonia. The apparatus comprises a tubular reactor 401 manufactured from Inconel® alloy (length 1000 mm, inner diameter 52.5 mm). The reactor may be heated by a furnace 402 (Carbolite® CFM 14/1). A stainless-steel cap 405 divides the reactor into an evaporation zone to the left of cap 405 and a reaction chamber 406 to the right of cap 405. The cap 405 has a conical inner surface with a central opening of approximately 2 mm, thereby forming a nozzle. A ceramic crucible 404 (Alsint® 99.7) with molten metal is placed in the evaporation zone of the reactor 401. An inlet tube 403 (inner di- ameter: 2.2 mm) is provided for feeding a carrier gas to the reactor, which is directed to flow along the surface of the metal in the crucible 404. Pressure in the evaporation zone is monitored by a pressure transducer 430.

Gases (steam and argon) are supplied to the reactor by the following means: An argon reservoir 420 is provided. Argon is fed to the inlet tube 403 to gener¬ ate a carrier gas stream, via a first mass flow controller 421 (Bronkhorst® EL- Flow 201 A). Steam is generated in a steam generator 424 with the aid of a wa¬ ter pump 423 by known means. Argon gas is fed to the steam generator from the reservoir 420 through a second mass flow controller 422. An argon/steam mixture (hereinafter referred to as the mixing gas stream) is fed to the reactor 401 through a superheater 425.

Reaction products are treated and analyzed as follows. Particles produced in the reactor 401 are collected on a glass fiber filter 407 (Whatman® GF/A, di¬ ameter 150 mm). Gases exiting filter 407 are cooled in a condenser 412 by cooling water (cooling water inlet 410, cooling water outlet 411), the condenser additionally comprises a water trap (not shown in Fig. 4). Gases leaving the condenser 412 can be removed by a pump 414 (Vacuubrand® RZ 16), con- nected to the condenser 412 via pressure control valve 413. By means of a gate valve 415, gases may alternatively enter a gas chromatograph (GC) 416 (MTI Analytical Instruments P200 Gas Analyzer).

Figs. 5 to 7 show some details of the reactor 401. As best seen in Fig. 5, a lance comprising four concentric tubes 510, 511 , 512, 513 is provided in the reaction chamber 406. Steam and argon enter the reaction chamber from the right end as a mixing gas stream 505, flowing in the space between tubes 511 and 512. The mixing gas stream 505 is discharged to a mixing zone 503 near the nozzle of the cap 405, at the left end of the reaction chamber 406. In the mixing zone, the mixing gas stream 505 hits the argon/metal vapor stream 502 exiting the nozzle of cap 405 to the right. Both streams are intimately mixed and then flow to the right in Fig. 4 and 5, into a particle/droplet formation and reac- tion zone 501 within the inner tube 510 (inner diameter: 16.5 mm). In this spe¬ cific design, the nanoparticle/nanodroplet formation zone and the reaction zone overlap. Means for controlling the temperature within the reaction chamber 406, specifically within tube 510, may be provided (not shown in the Figures). These means will ensure that the inner wall of tube 510 is sufficiently thermostated to assure maximum metal to metal oxide conversion and therefore maximum hy¬ drogen production. The inner wall of tube 510 is colder than the argon/metal vapor mixture in stream 502 at the position where the combined gas streams enter the mixing zone 503. Thereby, any heat of reaction may be removed. Re- action products and unreacted reactants leave the reaction chamber 406 to the right as stream 504.

Figs. 6 and 7 illustrate two different geometries for the mixing zone. In Fig. 6, the streams 502 and 505 hit each other in a "T-mode" at approximately right angles: Stream 505 is directed radially inward, while stream 502 enters the re¬ action chamber centrally in axial direction. The length of the mixing zone can be varied by inserting or removing distance rings 601 with a thickness of 6 mm each. In Fig. 7, the streams hit each other head-to-head in a ring-shaped off- axis region due to the presence of a bluff body 701. The configuration of Fig. 6 yielded better results than the configuration of Fig. 7, and therefore the configu¬ ration of Fig. 6 is preferred.

Experiments were carried out utilizing the apparatus of Fig. 4. Prior to every experiment, the reactor was evacuated to 25 mbar and subsequently purged with argon in order to remove as much oxygen from the system as possible. The gate valve 415 remained closed throughout the evacuation and purging process in order to protect the GC. This procedure was repeated several times until the amount of oxygen detected by the GC remained at a constant value of 0.3 - 0.4 %. It was not possible to ensure a completely oxygen-free system, as the reactor was not fully gas-tight. To hinder a further increase in O2 concentra¬ tion by diffusion of the ambient O2 into the system, the reactor was kept at 1100 mbar during warm up. As soon as steady state at the desired temperature was reached, the carrier gas was turned on followed by the water for steam generation and the mixing gas. The carrier gas was switched on first to prevent a steam backflow into the evaporation zone, which would have led to a fast oxidation of the metal melt surface. Samples were taken by the GC every 30 seconds for the duration of an experiment. The sampling phase was 10 seconds, the analyzing phase lasted 20 seconds. The concentration of H2 was displayed in % and logged in a file. The accuracy of the GC used to determine the H2 concentration of the product gas was approved by analyzing a gas mixture (Scott Specialty Gases) contain¬ ing 1% of each O2, H2, CO, CO2, and CH4 with the balance of N2 previous to the first run.

In one experiment, zinc (Zn) was placed into the crucible 404. Argon was fed to the inlet tube 403 at a carrier gas flow rate of 0.5 liters per minute (l/min) and passed over the crucible with the molten zinc. The temperature in the inside of the furnace 402 was set to 1023 K. Steam was generated at a rate of 10.2 grams per hour (g/h) and was mixed with argon at a rate of 1.5 l/min and fed to the reaction chamber as mixing gas stream 505. The pressure in the reactor 401 was only slightly above atmospheric pressure (1050 mbar absolute pres¬ sure). The particles collected on the filter were analyzed by X-ray diffraction (XRD). XRD patterns were analyzed using the software Topas® (Version 2.0) to obtain the exact particle composition (the Zn to ZnO ratio). The same software was also used to determine the crystal size of Zn and ZnO. A hydrogen yield (moles of hydrogen produced per moles of zinc evaporated) of up to 68% was achieved with these or similar parameters. With these or similar conditions, the size of the collected Zn and ZnO crystals, as determined by X-ray diffraction, varied between approximately 10 nanometers (nm) and 130 nm.

The apparatus shown in Fig. 4 can be modified for producing ammonia by pro¬ viding an additional inlet for a nitriding gas (e.g., nitrogen). This inlet may re¬ place the steam inlet such that stream 505 would be a stream of nitriding gas. A precursor (e.g., aluminium) would then be vaporized in the evaporation zone and mixed with the nitriding gas in the mixing zone 503. Steam is introduced to the formation/reaction zone 501 by means not shown in Fig. 5 (e.g., mixed with the nitriding gas or fed through the annular region between tubes 510 and 511 , with a plurality of radial openings in tube 510 to allow the steam to be trans¬ ferred to the reaction zone 501).

For designing a continuous reactor for carrying out the process of the present invention commercially, the following design guidelines should be contemplated: • Clear separation of evaporation and reaction zone; • Continuous supply of metal vapor; • Continuous supply of water vapor; • Additional continuous supply of nitriding gas in the case of ammonia pro¬ duction; • Continuous removal of metal oxide particles and unreacted metal or metal-containing particles. • Continuous removal of hydrogen and unreacted water vapor in the case of hydrogen production; • Continuous removal of ammonia, unreacted water vapor and unreacted nitriding gas in the case of ammonia production.

A clear separation of the evaporation zone and the reaction chamber avoids steam backflow, which is the main reason for a fast oxidation of the metal melt surface. The separation should be done in a manner allowing for independent temperatures of the evaporation zone and of the reaction zone. Of course, con¬ densation of metal inside the interconnection of the two zones should be avoided. With a controllable and continuous supply of metal, it is possible to control the evaporation rate of metal such that it does not decrease over time. If oxidation of the metal melt surface which is responsible for reduction of the evaporation rate cannot be avoided, despite the separation of evaporation and reaction zone, the forming oxide layer on the melt surface should be removed. This could be done mechanically, for instance by using a stirrer. Continuous supply of steam can be achieved by a steam generator as is well known in the art.

In the embodiment of Fig. 4, nanoparticles are generated in situ, i.e. in the chamber 406 in which also the reaction of the nanoparticles and the steam takes place. However, nanoparticles of a metal or metal-containing compound may also be produced in a process separate from the reaction with the steam. An apparatus which has been employed successfully for producing nanoparti¬ cles is shown in Fig. 8 in a highly schematic manner. This apparatus is a slight modification of the apparatus of Fig. 4, and many parts of this apparatus are identical to the corresponding parts of the apparatus in Fig. 4 and serve similar purposes. These parts carry the same reference numbers as in Fig. 4 and will not be explained again for the sake of brevity. The main differences are the fol¬ lowing: Cooling water 810, 811 is provided to the reactor between tubes 510 and 511 and between tubes 512 and 513. The condenser is replaced by a heat exchanger 812. Pure argon gas is provided as the mixing gas stream 505 instead of an argon/steam mixture. Thus, a steam generator is unnecessary and is left away.

With this setup, Zn nanoparticles were produced. The flow rate of the argon car¬ rier gas fed to tube 403 varied between 0.5 and 3 l/min. The flow rate of the ar¬ gon mixing gas stream 505 varied between 10 and 30 l/min. The furnace tem¬ perature varied between 923 and 1023 K. Other parameters (number of cruci¬ bles 404, mixing mode, mixing zone length, diameter of the hole in cap 405) were also varied. BET surface areas between 1.9 m2/g and 11.2 m2/g, corre¬ sponding to BET-equivalent particle diameters between 425 nm and 72 nm, were obtained. In the investigated range, the conditions leading to the largest BET surface area of 11.2 m2/g (i.e., to the smallest particle size) were a furnace temperature of 1023 K, a carrier gas flow rate of 1.5 l/min, a mixing gas flow rate of 30 l/min, one single crucible, T-mode mixing with one distance ring 601 of 6 mm thickness to define the mixing zone, and 6 mm hole diameter in cap 405. The Zn nanoparticles thus obtained may be collected and used in a fluid- ized-bed reactor to produce hydrogen in a process according to the present in¬ vention or could be directly fed to the mixing zone 503 of a modified reactor similar to the reactor 401 without melt evaporation 404.

Fig. 9 shows another embodiment of a reactor successfully employed in labora¬ tory-scale investigations. Identical parts carry the same reference numbers as in Figs. 4 to 8. The reactor, broadly speaking, comprises three regions: an evapo¬ ration stage 214, a cooling stage 215, and a reaction stage 216. The reaction stage 216 itself may be considered as being split up into a mixing zone (imme- diately adjacent to the steam nozzles 910 to be described below), a nanoparti- cle formation zone (overlapping with the mixing zone and extending slightly downstream of same) and a reaction zone (the entire region extending down¬ stream from the steam nozzles).

The evaporation stage 214 (which may be considered a special embodiment of an evaporator) comprises a first furnace 402a in which a first reactor tube is disposed. A holder with an attached crucible 404 for receiving molten metal ex¬ tends into the reaction tube. The holder is attached to a balance 901 for meas¬ uring the evaporation rate, enclosed in a balance box 900. Argon gas serving as a carrier gas is fed to the balance box by means of a mass flow controller (MFC) 421. The carrier gas streams into the balance box and from there into the reactor tube, being heated by the furnace 402a and carrying with it metal vapor evaporating from the crucible 404. The vapor-laden carrier gas passes through the cooling stage 215 and is cooled to a temperature just above the saturation temperature of the metal vapor in the carrier gas. From there, it passes into the reaction stage 216.

The reaction stage comprises a second furnace 402b. Disposed in the second furnace is a second reactor tube connected to the first reactor tube. One or more steam conduits 911 extend through the second reactor tube in its longitu¬ dinal direction. An argon/steam mixture is fed from a steam generator 424, which in turn is fed by argon gas through an MFC 422 and by water from a wa- ter pump 430, to these conduits from the downstream end of the reactor to steam nozzles 910. There, the steam is injected into the stream of the carrier gas carrying the metal vapor, the injection direction being at approximately right angles to the longitudinal direction of the reactor. The steam creates a turbulent flow in the region around the steam nozzles, thoroughly mixing the steam and the vapor-laden carrier gas and, at the same time, quenching the metal vapor (i.e., rapidly cooing it to below the saturation temperature of the metal), such that metal nanoparticles are formed. The stream of carrier gas, metal nanoparti- cles and steam is then carried down an inner tube 912 disposed within the sec- ond reactor tube, where the hydrolysis reaction takes place. A mixture contain¬ ing metal oxide nanoparticles and hydrogen exits the inner tube through filter 407.

By sharing a wall with the inner tube 912, the steam conduits 911 are in thermal contact with the inner tube. Heat exchange takes place between the steam be¬ ing passed to the steam nozzles 910 and the gas stream in inner tube 912. Thereby, the steam is preheated, and heat is removed from the (exothermic) reaction taking place in the inner tube.

Instead of a plurality of steam conduits, an arrangement of concentric tubes as in Fig. 5 may be employed.

It should be emphasized that, while the invention has been described in con¬ junctions with specific embodiments, many alternatives, modifications and varia- tions are possible, and the invention is not limited to the embodiments de¬ scribed above.