WO2016020656A2

WO2016020656A2 - Methanol production process

Info

Publication number: WO2016020656A2
Application number: PCT/GB2015/052226
Authority: WO
Inventors: Shik Chi Edman Tsang; Fenglin Liao
Original assignee: Isis Innovation Limited
Priority date: 2014-08-04
Filing date: 2015-07-31
Publication date: 2016-02-11
Also published as: WO2016020656A3; GB201413778D0

Abstract

The invention provides a process for producing methanol. The process comprises contacting a gas phase with a solid catalyst, wherein the gas phase comprises (a) H₂ and (b) C0₂, CO or a mixture of C0₂ and CO, and the catalyst comprises heterometallic particles comprising palladium and zinc. The invention also provides a catalyst which comprises heterometallic particles of palladium and zinc and a support material comprising zinc oxide. Also provided is a process for producing such a catalyst, the process comprising reducing a composition comprising palladium and zinc oxide. A catalyst obtainable by this process is also provided. The invention also relates to uses of the described catalysts for the hydrogenation of C0₂, CO, or a mixture of C0₂ and CO, to produce methanol, and, more particularly, for the hydrogenation of C0₂ to produce methanol. The invention also provides a composition which comprises palladium, zinc oxide and a compound suitable for promoting the reduction by hydrogen gas of zinc oxide to zinc metal.

Description

METHANOL PRODUCTION PROCESS

FIELD OF THE INVENTION

The invention relates to a process for producing methanol, a catalyst suitable for use in such a process, and a process for producing the catalyst.

BACKGROUND TO THE INVENTION

The catalytic production of methanol is one of the most important reactions in industry due to its huge annual production reaching 65 million tons. As a high energy density liquid, methanol can be used as a transportation fuel with good volatility (for combustion engines or fuel cells devices), and also act as a key platform chemical in the synthesis of numerous products. Given the fact that a long-term energy security is the key global challenge, the strategic position of methanol is expected to rise. Currently, methanol is synthesized on a large scale through the conversion of synthesis gas (CO and H₂, syn-gas) over CuZn/Al₂0₃. Presently, syn-gas is produced from themial processing of natural gas, which requires temperatures as high as 850 °C at 2MPa pressure; this is not only energy-inefficient but also highly dependent on non-renewable natural resources. The transformation of syn-gas into methanol currently requires an operation pressure of higher than 5MPa. The pressure gap between these two reactions requires expensive and cumbersome technology. Thus, the cost of pumping and the cumbersome equipment and procedures needed to couple syngas production with methanol synthesis significantly reduces profit margins. It is therefore an important challenge to provide new materials that can catalyse the synthesis of methanol from syn-gas, at pressures as low as 2MPa.

In addition, with the rapid development of the economy and the increasing population, the energy issue has become one of the most serious problems in the 21^st century. Currently, more than 80% of the world's energy consumption and production of chemicals originates from the use of fossil resources. With the ever-increasing demand for energy, such natural resources will run out in 70 years. Urgent action is therefore needed to explore new and green methanol synthesis routes to relieve the current dependence on nonrenewable energy resources. With the development of techniques for the large scale production of H₂ from renewable sources (solar energy, hydropower, biomass, excess chemical heat, etc.) the synthesis of methanol by hydrogenation of C0₂ becomes attractive from the point of a new sustainable economy. CO2 is an abundant carbon resource in nature and, in recent years, it has been receiving increasing attention due to its position as the primary greenhouse gas and the implication of its emissions on the problem of climate change. As shown in Figure 1, the catalytic hydrogenation of C0₂ to methanol and the consumption of methanol would compose a C0₂-neutral circle, which would not only reduce the dependence of methanol synthesis on non-renewable resources, but would also be of huge significance environmentally. Thus, if the C0₂ were obtained from biomass (i.e. glycerol) the recycled methanol fuel could be described as carbon neutral fuel. If the C0₂ were collected from atmosphere due to previous man-made activities (carbon capture from post combustion, etc.) a carbon-negative economy could be realized. Thus, there are strong environmental incentives to carry out catalytic hydrogenation of C0₂ to methanol.

At present, catalysts reported as being active for the hydrogenation of C0₂ to methanol include Cu/ZnO, Pd/Ga₂03 and Pd/ZnO. Of these, Cu/ZnO is used

commercially. However, the catalytic performance of Cu/ZnO for C0₂/E strongly depends on the reaction pressure; methanol selectivity decreases rapidly as the pressure is reduced. Even under a pressure of 50bar, the methanol selectivity predicted from thermodynamics is limited to from 30% to 50%, due to the existence of the reverse water gas shift reaction which produces CO as the main by-product if the reaction reaches equilibrium. This is evident from results of testing the Cu/ZnO catalyst, which are provided herein below. Thus, the commercial catalyst, Cu/ZnO, produces a poor methanol yield and has low selectivity for methanol under a pressure of 20 bar; high reaction pressures are required to raise the methanol selectivity thermodynamically, due to the low activity of Cu/ZnO. As stated above, however, high reaction pressure is a key factor that leads to increased costs and high energy consumption. In particular, the processing and transporting of C0₂ (carbon capture and storage) at high pressure becomes economically and technically very challenging. A new ultra low pressure methanol production process, able to produce methanol by hydrogenation of CO2, would therefore be extremely attractive to industry.

It should also be noted that C0₂/H₂ can now be produced from biomass or related chemicals by aqueous phase reforming (APR), which generates CO2/H2 at 2MPa (250°C) from alcoholic/acidic organic molecules (carbohydrates) in pressured liquid water. APR can reduce costs enormously as compared to corresponding traditional steam reforming reactions, as steam is no longer required. A new ultra low pressure process for producing methanol by C0₂ hydrogenation would therefore be extremely attractive to industry, especially if the operating temperature and pressure were comparable to those of the APR process. Also, current processes for producing bio-diesel, from methanol and plant or animal oils, generate glycerol as a by-product, and the generation of C0₂/H₂ by aqueous phase reforming (APR) of glycerol has also been recently demonstrated. Thus, a new ultra low pressure methanol production process, able to produce methanol by the hydrogenation of C0₂, would also be attractive to industry from the point of view of its utility (in combination with APR) to produce methanol for recycling into the biodiesel production process.

SUMMARY OF THE INVENTION

The invention provides a heterogeneous catalyst for the production of methanol by the hydrogenation of oxides of carbon, i.e. C0₂ and/or CO, which can occur via the following reactions:

C0₂ + 3H₂ CH3OH + H₂0

CO + 2H₂ -> CH3OH

The catalyst of the invention, which contains heterometallic particles comprising both palladium and zinc, has been found to provide an unexpectedly high activity and selectivity for the production of methanol by the hydrogenation of C0₂ and/or CO. It is for example surprisingly active at converting CO2 and H₂ into methanol at ultra-low reaction pressures, of around 2 MPa. The high activity of the catalyst at such low reaction pressures means that the pressure gap between the methanol synthesis reaction and the reaction to produce the synthesis gas (i.e. H₂ and C0₂ and/or CO) in the first place, either by traditional steam reforming or by aqueous phase reforming, is closed or at least reduced. This greatly enhances the economical viability of this particular route to methanol.

This represents a significant improvement over the present, industrial copper-based catalysts, which are unable to convert C0₂ and H₂ into methanol under such low pressure regimes, due to poor methanol activity and a relatively high activity for the competing reverse water-gas shift reaction (C0₂ + H2 -> CO + H₂0) at such pressures. It is thought that palladium provides a higher catalytic activity at moderate reaction conditions (low pressure, low temperature) because it has intrinsically stronger adsorptive properties than copper. However, both copper and palladium on their own are still not particularly selective for the production of methanol at such low pressures because both metals have a higher activity for the reverse water gas shift reaction, at pressures below 5 MPa, and therefore mainly produce CO and H2O. It is a surprising finding of the present invention, however, that modifying palladium metal with zinc metal, to give bimetallic PdZn, provides a catalyst that not only enhances methanol activity but also greatly suppresses the reverse water gas shift reaction at low pressures, leading to selectivities for methanol, at pressures of 2MPa, of greater than 70%.

Accordingly, the invention provides a process for producing methanol, which process comprises contacting a gas phase with a solid catalyst, wherein the gas phase comprises (a) H₂ and (b) C0₂, CO or a mixture of C0₂ and CO; and the catalyst comprises heterometallic particles, wherein each of the heterometalhc particles comprises palladium and zinc.

In another aspect, the invention provides a catalyst which comprises (a) a support material comprising zinc oxide, and (b) heterometallic particles, wherein each of the heterometallic particles comprises palladium and zinc.

In another aspect, the invention provides a process for producing a catalyst, which catalyst comprises (a) a support material comprising zinc oxide, and (b) heterometallic particles, wherein each of the heterometallic particles comprises palladium and zinc, which process comprises reducing a composition comprising: (a) palladium, and (b) a support material which comprises zinc oxide.

In another aspect, the invention provides a catalyst which is obtainable by the process of the invention for for producing a catalyst.

In another aspect, the invention provides the use of a catalyst which comprises heterometallic particles, wherein each of the heterometallic particles comprises palladium and zinc, for the hydrogenation of C0₂, CO, or a mixture of C0₂ and CO, to produce methanol.

In another aspect, the invention provides the use of a catalyst which comprises heterometallic particles, wherein each of the heterometallic particles comprises palladium and zinc, for the hydrogenation of C0₂ to produce methanol.

In another aspect, the invention provides a composition which comprises: (a) palladium, (b) zinc oxide, and (c) a compound suitable for promoting the reduction by hydrogen gas of zinc oxide to zinc metal.

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 is a schematic illustration of the neutral CO2 cycle comprising the hydrogenation of C0₂ and the consumption of methanol. Fig. 2 is a schematic illustration of the reduction of Pd-CdSe-ZnO by hydrogen gas to produce a catalyst of the invention.

Fig. 3 shows high resolution transmission electron microscopy (TEM) images of a ZnO support (0.0 wt% CdSe, sample 1, images c and d) and a CdSe-ZnO support (26.4 wt% CdSe, sample 4, images a and b).

Fig. 4 shows the X-ray photoelectron spectra (XPS) of Zn 2pi/₂ for a series of Pd/CdSe/ZnO samples: a) samples 1-4 and b) their fitting curves.

Fig. 5 shows (a) EXAFS curves of Pd absorbing atoms in R space; and (b) the number of neighbour Pd atoms (descending with increasing CdSe concentration) and the number of neighbour Zn atoms (ascending with increasing CdSe concentration) for each absorbing Pd atom, derived from EXAFS fittings.

Fig. 6 shows graphs of methanol selectivity (a,b) and methanol time-space yield (c,d) for a series of PdZn catalysts and for the Cu-based industry catalyst under variable conditions. The dotted lines in graphs a and b indicate the methanol selectivity at equilibrium condition (upper line - 250 °C, lower line - 270 °C).

Fig. 7 shows graphs of (a) methanol selectivity, (b) methanol yield, and (c) CO yield, versus pressure, at 250 °C, for the PdZn catalyst of sample 4 (26.4 wt% CdSe) and for the Cu-based industry standard catalyst. The dotted line in each graph indicates equilibrium values.

Fig. 8 shows graphs of (a) methanol selectivity and (b) methanol space time yield of PdZn catalysts and the Cu-based industry standard catalyst under 2MPa pressure. The dashed line in Figure 8a indicates the change in methanol selectivity at equilibrium condition with temperature. The dotted lines in Fig. 8b show the equilibrium methanol yield with (lower dotted line) and without (upper dotted line) the reverse water gas shift equilibrium taken into account.

Fig. 9 is a graph showing calculated d band filling (ascending with increasing Zn concentration) and d band center (descending with increasing Zn concentration) of a series of PdZn alloys with variable composition.

Fig. 10 is a schematic illustration of the electronic orbital interactions between C0₂ and Pd molecules.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a process for producing methanol, which process comprises contacting a gas phase with a solid catalyst, wherein the gas phase comprises (a) H₂ and (b) C0₂, CO or a mixture of C0₂ and CO; and the catalyst comprises heterometallic particles, wherein each of the heterometallic particles comprises palladium and zinc.

The catalyst used in the process of the invention for producing methanol comprises heterometallic particles. The term "heterometallic particle" as used herein means a particle, for instance a nanoparticle or a cluster, which comprises more than one different metal, i.e. which comprises two or more different metals.

Typically, a heterometallic particle comprises only two different metals, in which case the heterometallic particle is a bimetallic particle (i.e. a heterobimetallic particle). However, in other cases, one or more further metals may be present too, i.e. the heterometallic particle may comprise three or more different metals.

A heterometallic particle may comprise, or consist of, an alloy of the two or more different metals, for instance an alloy in which atoms of the two or more different metals are essentially randomly distributed throughout the particle. Alternatively, a

heterometallic particle may have a "core-shell" structure. Thus, a heterometallic particle may have a central core, and a shell surrounding the core, wherein the core comprises a first metal (for instance palladium) and the shell comprises a second metal which is different from the first (for instance zinc). The word "surrounding" in this context means either completely surrounding or partially surrounding. The word is therefore intended to cover heterometallic particles in which the shell completely surrounds the core, such that the metal atoms on the surface of the particle are atoms of the second metal. However, it is also intended to cover heterometallic particles in which the shell is incomplete, such that some of the metal atoms on the surface of the particle are atoms of the second metal, but atoms of the first metal are also exposed. A heterometallic particle, as referred to herein, may be a heterometallic nanoparticle.

As used herein the term "nanoparticle" means a microscopic particle whose size is typically measured in nanometres (nm). A nanoparticle typically has a particle size of from 0.5 nm to 500 nm. For instance, a nanoparticle may have a particle size of from 0.5 nm to 200 nm. More often, a nanoparticle has a particle size of from 0.5 nm to 100 nm, or for instance from 1 nm to 50 nm.

A particle, for instance a nanoparticle, may be spherical or non-spherical. Non- spherical particles may for instance be plate-shaped, needle-shaped or tubular. The term "particle size" as used herein means the diameter of the particle if the particle is spherical or, if the particle is non-spherical, the volume-based particle size. The volume-based particle size is the diameter of the sphere that has the same volume as the non-spherical particle in question.

As used herein the term "cluster" refers to an ensemble of metal atoms where direct and substantial metal-metal bonding is present between the atoms. A cluster is typically intermediate in size between a small molecule (such as a water molecule or a glucose molecule) and a bulk solid, and may have a similar size to a nanoparticle. Thus, clusters often have particle sizes of from 0.5 nm to 500 nm. A cluster may for instance have a particle size of from 0.5 nm to 200 nm or, for instance, from 0.5 nm to 100 nm. Often, a cluster has a particle size of from 1 nm to 50 nm.

The term "heterometallic cluster" therefore refers to a cluster in which the metal atoms bonded to one another comprise atoms of a first metal and atoms of a second metal which is different from the first metal. Atoms of one or more further metals may optionally also be present. Thus, the heterometallic cluster may optionally comprise atoms of a third metal, or even atoms of a fourth metal, in addition to the atoms of the first and second metals. Often, however, a heterometallic cluster is a bimetallic cluster (i.e. a heterobimetallic cluster), whose metal atoms consist of atoms of a first metal (e.g.

palladium) and atoms of a second metal (e.g. zinc).

The heterometallic particles of the catalyst employed in the present invention may be formed by treating with hydrogen gas a composition comprising palladium particles supported on zinc oxide, in order to reduce Zn²⁺ ions in ZnO to zinc metal atoms, Zn(0). The Zn(0) atoms are thought to decorate the surfaces of the palladium particles and react with the Pd to form heterometallic particles comprising palladium and zinc. The extent to which the Pd nanoparticles react with Zn(0) may vary, depending, for example, on the quantity of Zn(0) produced by the reduction reaction and the quantity of Pd present in the first place. In some cases, for instance when the amount of Zn(0) produced is relatively low compared to the quantity of Pd present in the composition, the Zn(0) may simply decorate the surfaces of some or all of the Pd particles, to form Pd-Zn core-shell particles. Additionally or alternatively, the Zn(0) may react with the surfaces of some or all of the Pd particles to form small PdZn clusters on the surfaces of Pd particles. Unreacted Pd particles may remain. Accordingly, in addition to the heterometallic particles, the catalyst may further comprise palladium particles (typically palladium nanoparticles). These may for instance be unreacted Pd nanoparticles that were present in a precursor composition used to produce the catalyst. In other cases, e.g. when a relatively large amount of Zn(0) is produced compared to the Pd that is present, the Zn(0) may react with all of the Pd so that all of the Pd particles are converted into heterometallic nanoparticles or heterometallic clusters comprising both palladium and zinc.

The mean molar ratio of zinc metal to palladium metal in the heterometallic particles of the catalyst i s typically from 1 : 10 to 10 : 1. It may for instance be from 1 : 5 to 5:1. More typically, however, it is from 1 :3 to 3: 1.

Typically, the mean particle size of the heterometallic particles is less than 500 nm, and the heterometallic particles are therefore heterometallic nanoparticles. For instance the heterometallic nanoparticles may have a mean particle size of from 0.5 nm to 500 nm, or for instance from 1 nm to 500 nm. Typically, the particle size distribution of the heterometallic particles is such that 90 % of the particles have a particle size of less than 500 nm. More typically, said heterometallic particles have a mean particle size of less than or equal to 300 nm. For instance said heterometallic particles may have a mean particle size of from 0.5 nm to 300 nm, or for instance from 1 nm to 300 nm. Typically, the particle size distribution of the heterometallic particles is such that 90 % of the particles have a particle size of less than 300 nm.

Often, the heterometallic particles are, or comprise, heterometallic clusters.

Thus, the heterometallic nanoparticles may comprise heterometallic clusters.

Typically, each of the heterometallic clusters comprises metal atoms bonded to one another, which include palladium atoms and zinc atoms. Optionally, atoms of one or more further different metals may additionally be present in the cluster. Typically, however, the heterometallic clusters are bimetallic clusters, comprising atoms of palladium and zinc but no other metals. Often, such bimetallic clusters consist of palladium and zinc.

Often, the heterometallic particles have a mean particle size of less than or equal to 200 nm. For instance the heterometallic particles may have a mean particle size of from 0.5 nm to 200 nm, or for instance from 1 nm to 200 nm. Typically, the particle size distribution of the heterometallic particles is such that 90 % of the particles have a particle size of less than 200 nm. More typically, said heterometallic particles have a mean particle size of less than or equal to 100 nm. For instance said heterometallic particles may have a mean particle size of from 0.5 nm to 100 nm, or for instance from 1 nm to 100 mn.

Typically, the particle size distribution of the heterometallic particles is such that 90 % of the particles have a particle size of less than 100 nm. Said heterometallic particles may have a mean particle size of less than or equal to 50 nm. For instance said heterometallic particles may have a mean particle size of from 0.5 nm to 50 nm, or for instance from 1 nm to 50 nm. The particle size distribution of the heterometallic particles may be such that 90 % of the particles have a particle size of less than 50 nm.

In some embodiments, the heterometallic particles have a mean particle size of less than or equal to 10 nm. For instance the heterometallic particles may have a mean particle size of from 0.5 nm to 10 nm, or for instance from 1 nm to 10 nm. Typically, the particle size distribution of the heterometallic particles is such that 90 % of the particles have a particle size of less than 10 nm. Said heterometallic particles may have a mean particle size of less than or equal to 5 nm. For instance said heterometallic particles may have a mean particle size of from 0.5 nm to 5 nm, or for instance from 1 nm to 5 nm. The particle size distribution of the heterometallic particles may be such that 90 % of the particles have a particle size of less than 5 nm.

Typically, the heterometallic particles are supported on a support material.

Accordingly, the catalyst employed in the process of the invention for producing methanol typically further comprises a support material. Any suitable support material may be employed; the skilled person knows of a wide range of materials that are suitable for supporting metal particles in a heterogeneous catalyst. The support material may for instance comprise a metal oxide, carbon or a metal nitride. Typically, the support material comprises a metal oxide.

Usually, the support material comprises zinc oxide, as one convenient route to the catalyst employed in the present invention is to treat with hydrogen (or to otherwise reduce) a precursor composition comprising palladium supported on zinc oxide, in order to reduce some of the zinc ions of the oxide to produce zinc metal that then reacts with the palladium to form the heterometallic particles, which are then supported on the zinc oxide.

Typically, when the support material comprises zinc oxide, it further comprises a compound suitable for promoting the reduction of zinc oxide to zinc metal. Any compound suitable for promoting the reduction of metal ions may be employed, such compounds being well known to the skilled person. Usually, however, a quantum dot material is employed. For instance, the compound suitable for promoting the reduction of zinc oxide to zinc metal may be CdS, CdSe or CdTe. Alternatively, it may be a non-Cd quantum dot material, for instance ZnS, ZnSe, ZnTe, ZnP, InP, CuInS or TIP, or for instance a Ga-containing quantum dot material, for instance GaAs, GaP, GaN or GaSb. Typically, however it is CdSe. A mixture of any two or more of the aforementioned compounds may also be employed. The reduction itself may be achieved by treating a precursor composition, comprising palladium supported on zinc oxide, with a reducing agent. Typically, the reducing agent is hydrogen, for instance hydrogen gas. The hydrogen may be in the form of neat H₂ or "dilute" H₂ (for instance it may be a mixture of H₂ and an inert gas such as N₂ or a noble gas like argon). Alternatively, the reducing agent may be a chemical reductant, such as for instance sodium borohydride, aluminium hydride, or an active metal. The reduction reaction may be activated by any suitable activation means. Typically, it is activated thermally (i.e. by heating). Other suitable activation means include, but are not limited to, photo-irradiation (i.e. by irradiating with light, for instance visible or UV light), electrical activation (i.e. by applying electricity, which may be pulsed or static), or acoustic or ultrasonic activation (e.g. by sonication).

Often, when the support material comprises zinc oxide, the support material further comprises nanoparticles of the compound suitable for promoting the reduction of zinc oxide to zinc metal, which nanoparticles are in contact with the zinc oxide. The zinc oxide may, for instance, form a shell around the nanoparticles. Typically, therefore, the support material further comprises nanoparticles of a quantum dot material. Usually, the nanoparticles comprise the quantum dot material CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnP, InP, CuInS, TIP, GaAs, GaP, GaN or GaSb. Often, however, the nanoparticles comprise CdSe.

The nanoparticles of the compound suitable for promoting the reduction of zinc oxide to zinc metal, which is usually a quantum dot material, typically have a mean particle size of less than or equal to 100 nm. For instance said nanoparticles may have a mean particle size of from 0.5 nm to 100 nm, or for instance from 1 nm to 100 nm.

Typically, the particle size distribution of the nanoparticles is such that 90 % of the nanoparticles have a particle size of less than 100 nm. Said nanoparticles of the compound suitable for promoting the reduction of zinc oxide to zinc metal, which is usually a quantum dot material, may for instance have a mean particle size of less than or equal to 50 nm. For instance said nanoparticles may have a mean particle size of from 0.5 nm to 50 nm, or for instance from 1 nm to 50 nm. The particle size distribution of the nanoparticles may be such that 90 % of the particles have a particle size of less than 50 nm.

In some embodiments, the nanoparticles of the compound suitable for promoting the reduction of zinc oxide to zinc metal, which is usually a quantum dot material, have a mean particle size of less than or equal to 20 nm. For instance the nanoparticles may have a mean particle size of from 0.5 nm to 20 nm, or for instance from 1 nm to 20 nm. Typically, the particle size distribution of the nanoparticles is such that 90 % of the particles have a particle size of less than 20 nm. Said nanoparticles of the compound suitable for promoting the reduction of zinc oxide to zinc metal, which is usually a quantum dot material, may have a mean particle size of less than or equal to 10 nm. For instance said nanoparticles may have a mean particle size of from 0.5 nm to 10 nm, or for instance from 1 nm to 10 nm. The particle size distribution of the nanoparticles may be such that 90 % of the particles have a particle size of less than 10 nm. Often, said nanoparticles have a mean particle size of less than or equal to 5 nm. For instance said nanoparticles may have a mean particle size of from 0.5 nm to 5 nm, or for instance from 1 nm to 5 nm. The particle size distribution of the nanoparticles may be such that 90 % of the particles have a particle size of less than 5 nm.

The amount of compound suitable for promoting the reduction of zinc oxide to zinc metal, which is usually a quantum dot material, present in the catalyst is typically less than 50 weight %, based on the total weight of the catalyst, for instance less than 40 weight %. Usually, the catalyst comprises from 5 to 45 weight % of the compound (e.g. the quantum dot material) based on the total weight of the catalyst. Often, for instance, the catalyst comprises from 20 to 30 weight % of the compound suitable for promoting the reduction, based on the total weight of the catalyst, or for instance from 23 to 27 weight %, based on the total weight of the catalyst. The support material may for instance comprise from 20 to 30 weight % CdSe based on the total weight of the catalyst.

Usually, when the catalyst comprises a support material which comprises zinc oxide, the molar ratio of Zn(0) to Zn²⁺ in the catalyst is from 1 :10 to 10:1. The molar ratio of Zn(0) to Zn²⁺ in the catalyst may for instance be from 1 :2 to 5 : 1 , or for instance from 2:1 to 7:2. In one embodiment, the molar ratio of Zn(0) to Zn²⁺ in the catalyst is from 2.8:1.

The amount of palladium present in the catalyst is usually from 0.1 to 10 weight % based on the total weight of the catalyst. It may for instance be from 0.5 to 8 weight %, or for instance from 1 to 8 weight %, e.g. from 3 to 7 weight %, based on the total weight of the catalyst.

The catalyst employed in the process of the invention for producing methanol is typically a catalyst which is obtainable by a process of the invention for producing a catalyst, i.e. it is typically obtainable by reducing a composition comprising: (a) palladium, and (b) a support material which comprises zinc oxide. The catalyst may for instance be one which is obtained by a process of the invention for producing a catalyst, i.e. by reducing a composition comprising: (a) palladium, and (b) a support material which comprises zinc oxide. The process by which the catalyst is obtained or obtainable may be as further defined anywhere herein for the process of the invention for producing a catalyst.

Typically, in the process of the invention for producing methanol, the step of contacting the gas phase with the solid catalyst is performed at a pressure of from 0.1 MPa (i.e. 1 atmosphere) to 10 MPa. It may for instance be performed at a pressure of from 0.1 MPa to 5 MPa. Preferably, however, the step of contacting the gas phase with the solid catalyst is performed at a pressure of less than 5 MPa, which is made possible by employing the PdZn catalyst described herein. Thus, preferably the step of contacting the gas phase with the solid catalyst is performed at a pressure of less than 4 MPa or, for instance, a pressure of less than 3 MPa. The step of contacting the gas phase with the solid catalyst may for instance be preformed at a pressure of from 0.1 MPa to 4 MPa, or for instance from 0.5 MPa to 4 MPa, or for example from 1 MPa to 3 MPa. In some embodiments, it is performed at a pressure of from 1.5 MPa to 3 MPa.

The step of contacting the gas phase with the solid catalyst may for instance be performed at a pressure of less than or equal to 2.5 MPa, for instance a pressure of from 0.1 MPa to 2.5 MPa. The step may for instance be performed at a pressure of from 1.0 MPa to 2.5 MPa, for instance from 1.5 MPa to 2.5 MPa, e.g. a pressure of about 2 MPa.

The step of contacting the gas phase with the solid catalyst may for instance be performed at a pressure of less than or equal to 2.0 MPa, for instance at a pressure of from 0.1 MPa to 2.0 MPa. It may for instance be performed at a pressure of from 1 MPa to 2 MPa, or for instance from 1.5 MPa to 2 MPa. It may for example be performed at a pressure of about 2 MPa.

The step of contacting the gas phase with the solid catalyst is typically performed at a temperature of less than or equal to 500 °C, for instance at a temperature of from 100 °C to 500 °C, or for example from 150 °C to 500 °C. It may for instance be performed at a temperature of less than or equal to 400 °C, for instance at a temperature of from 100 °C to 400 °C, or for example from 150 °C to 400 °C.

The step of contacting the gas phase with the solid catalyst is in some embodiments performed at a temperature of less than or equal to 350 °C, for instance at a temperature of from 100 °C to 300 °C, or for example from 150 °C to 300 °C. It may for instance be performed at a temperature of less than or equal to 275 °C, or for instance at a temperature of less than or equal to 250 °C. For example, the step of contacting the gas phase with the solid catalyst is in some cases performed at a temperature of from 100 °C to 275 °C, or for example from 150 °C to 250 °C.

Usually, the step of contacting the gas phase with the solid catalyst is performed at a temperature of less than or equal to 600 °C and at a pressure of less than or equal to 5 MPa. Thus, for instance, the the step of contacting the gas phase with the solid catalyst may be performed at a pressure of from 0.1 MPa to 6 MPa and a temperature of from 100 °C to 600 °C. It may for instance be performed at a pressure of from 0.5 MPa to 6 MPa and a temperature of from 100 °C to 600 °C.

The step of contacting the gas phase with the solid catalyst may for instance performed at a temperature of less than or equal to 500 °C and at a pressure of less than or equal to 3 MPa. Thus, for instance, the the step of contacting the gas phase with the solid catalyst may be performed at a pressure of from 0.1 MPa to 3 MPa and a temperature of from 100 °C to 500 °C. It may for instance be performed at a pressure of from 0.5 MPa to 3 MPa and a temperature of from 100 °C to 500 °C.

In some embodiments, the step of contacting the gas phase with the solid catalyst is performed at a temperature of less than or equal to 350 °C and at a pressure of less than or equal to 3 MPa. Thus, for instance, the the step of contacting the gas phase with the solid catalyst may be performed at a pressure of from 0.1 MPa to 3 MPa and a temperature of from 100 °C to 300 °C. It may for instance be performed at a pressure of from 0.5 MPa to 3 MPa and a temperature of from 100 °C to 300 °C.

The step of contacting the gas phase with the solid catalyst may for instance be performed at a temperature of less than or equal to 300 °C and at a pressure of less than or equal to 2.5 MPa. Thus, for instance, the step of contacting the gas phase with the solid catalyst may be performed at a pressure of from 0.1 MPa to 2.5 MPa and a temperature of from 100 °C to 300 °C. Typically, it is performed at a pressure of from 1.0 MPa to 2.5 MPa and a temperature of from 100 °C to 300 °C. It may for instance be performed at a pressure of from 1.5 MPa to 2.5 MPa and a temperature of from 150 °C to 280 °C.

In one embodiment, the step of contacting the gas phase with the solid catalyst is performed at a temperature of less than or equal to 280 °C and at a pressure of less than or equal to 2 MPa, for instance at a temperature of less than or equal to 260 °C and at a pressure of less than or equal to 2 MPa. It may for instance be performed at a pressure of from 0.1 MPa to 2.0 MPa and a temperature of from 150 °C to 280 °C, or for instance at a pressure of from 1.0 MPa to 2.0 MPa and a temperature of from 200 °C to 280 °C, or at a pressure of from 1.5 MPa to 2.0 MPa and a temperature of from 200 °C to 280 °C. The catalyst employed in the present invention is especially efficient at converting C0₂ and H₂ into methanol. Accordingly, often, in the process of the invention, the gas phase comprises H₂ and C0₂. Of course, the gas phase may further comprise CO in this embodiment.

The catalyst can also be used to convert very efficiently syn-gas, i.e. CO and H₂, into methanol. Accordingly, the gas phase may comprise H2 and CO.

Typically, the process of the invention for producing methanol further comprises recovering said methanol.

When the gas phase employed in the process of the invention comprises H₂ and CO2, the C0₂ may be obtained by combustion, for instance by combustion of a fuel, such as a fossil fuel. For instance, it may be obtained by combustion of natural gas, coal, petroleum, or a derivative of natural gas, coal or petroleum (i.e. a petroleum derivative). Alternatively, the C0₂ may be collected from the environment. Thus, the C0₂ may be atomspheric CO2. Other sources for the C0₂, the H₂ or both the C0₂ and H₂ include biomass, bio-gases and bio-fuels. Accordingly, the C0₂ and/or H₂ employed in the process of the invention may be obtained by processing a biomass, a bio-gas or a bio-fuel. The C0₂ and/or ¾ employed may for instance be obtained by steam reforming or by aqueous phase reforming a said biomass, bio-gas or bio-fuel. Any suitable biomass may be employed, for instance a biomass which comprises wood, lignin or a carbohydrate.

The H₂ used in the process of the invention may be obtained by electrolysis of water, or for instance by photolysis of water. Alternatively, it may be obtained by thermal (ground heating), nuclear or mechanical (wind) assisted hydrogen production, or for instance by hydrocarbon processing and/or decomposition.

The gases in the gas phase employed in the process of the invention may be produced by steam reforming, e.g. by steam reforming of hydrocarbon fuels such as natural gas, or for instance by steam reforming of coal, oil or biomass. Thus, steam reforming may be used to produce the mixture of H₂ and CO employed in the gas phase (or the mixture of ¾ and CO2, or the mixture of H₂, C0₂ and CO, as the case may be). The pressure of the steam reforming may be less than or equal to about 3 MPa, or for instance less than or equal to about 2.5 MPa. The steam reforming may for example be carried out at a pressure of from about 1.5 MPa to about 3 MPa, or for instance at a pressure of from about 1.5 MPa to about 2.5 MPa, for example at about 2 MPa. The temperature at which the steam reforming is performed depends on the pressure but may for instance be from about 500 °C to about 1000 °C, for instance about 850 °C. Alternatively, the gases in the gas phase employed in the process of the invention may be produced by aqueous phase reforming, e.g. by aqueous phase reforming of sugars, sugar alcohols such as for instance glycerol, or biomass. Often, for instance, the gas phase comprises H₂ and CO, and the H₂ and C0₂ in the gas phase are produced by said aqueous phase reforming. The pressure of the aqueous phase reforming may be less than or equal to about 3 MPa, or for instance less than or equal to about 2.5 MPa. The aqueous phase reforming may for example be earned out at a pressure of from about 1.5 MPa to about 3 MPa, or for instance at a pressure of from about 1.5 MPa to about 2.5 MPa, for example at about 2 MPa. The temperature at which the aqueous phase reforming is performed may for instance be from about 150 °C to about 300 °C, for instance about 250 °C. Typically, alcoholic and/or acidic organic molecules are converted into said gases in the gas phase by said aqueous phase reforming. For instance, one or more carbohydrates or sugar alcohols may be converted into said gases in the gas phase by said aqueous phase reforming.

Accordingly, in some embodiments, the process of the invention for producing methanol further comprises producing (a) the H₂ and (b) the C0₂, CO or mixture of C0₂ and CO, of said gas phase, by steam reforming or by aqueous phase reforming.

Often, the gas phase comprises CO2 and H₂ and the process further comprises producing the C0₂ and H₂ of the gas phase by the aqueous phase reforming of glycerol.

The glycerol may for instance be a by-product of a biodiesel production process. Thus, often, the gas phase comprises C0₂ and H₂ and the process further comprises: performing a biodiesel production process, comprising reacting methanol with a plant oil or an animal oil to produce (i) biodiesel and (ii) glycerol as a by-product; and producing the C0₂ and ¾ of the gas phase by aqueous phase reforming of said glycerol by-product.

The process may optionally further comprise recovering the methanol produced by the process of the invention for producing methanol, and recycling said methanol into said biodiesel production process.

The pressure of the aqueous phase reforming of glycerol may be less than or equal to about 3 MPa, or for instance less than or equal to about 2.5 MPa. The aqueous phase reforming of glycerol may for example be earned out at a pressure of from about 1.5 MPa to about 3 MPa, or for instance at a pressure of from about 1.5 MPa to about 2.5 MPa, for example at about 2 MPa. The temperature at which the aqueous phase reforming of glycerol is performed may for instance be from about 150 °C to about 300 °C, for instance about 250 °C. Thus, typically, the entire process is performed at a pressure that does not exceed 3.0 MPa. For instance, the entire process may be performed at a pressure that does not exceed 2.5 MPa, or for instance at a pressure that does not exceed about 2 MPa. Often, in this process, the step of contacting the gas phase with the solid catalyst is performed at a temperature of from 200 °C to 300 °C.

Catalysts which are typically employed in the process of the invention are novel composition per se. Accordingly, the invention also provides a composition, which composition comprises (a) a support material comprising zinc oxide, and (b) heterometallic particles, wherein each of the heterometallic particles comprises palladium and zinc. The composition is usually employed as a catalyst, and so the composition is generally referred to herein as a "catalyst". Accordingly, the invention provides a catalyst which comprises (a) a support material comprising zinc oxide, and (b) heterometallic particles, wherein each of the heterometallic particles comprises palladium and zinc.

The catalyst (or composition) of the invention may be as further defined anywhere herein. For instance, the catalyst of the invention may as further defined anywhere hereinbefore for the catalyst employed in the process of the invention for producing methanol. In particular, the heterometallic particles of the catalyst of the invention may be as further defined anywhere hereinbefore. Also, the support material comprising zinc oxide, in the catalyst of the invention, may be as further defined anywhere hereinbefore.

Typically, for instance the support material in the catalyst of the invention further comprises a quantum dot material. Usually, the quantum dot material is CdS, CdSe or CdTe. Alternatively, it may be a non-Cd quantum dot material, for instance ZnS, ZnSe, ZnTe, ZnP, InP, CuInS or TIP, or for instance a Ga-containing quantum dot material, for instance GaAs, GaP, GaN or GaSb. In some embodiments, the quantum dot material is CdSe. Usually, the quantum dot material comprises nanoparticles of the quantum dot material in contact with the zinc oxide. The, or at least some of the, zinc oxide may form a shell around the nanoparticles of the quantum dot material. The nanoparticles of the quantum dot material typically have a mean particle size of less than or equal to 100 nm. For instance the nanoparticles may have a mean particle size of from 0.5 nm to 100 nm, or for instance from 1 nm to 100 nm. Typically, the particle size distribution of the nanoparticles is such that 90 % of the nanoparticles have a particle size of less than 100 nm. Said nanoparticles of the quantum dot material may for instance have a mean particle size of less than or equal to 50 nm. For instance said nanoparticles may have a mean particle size of from 0.5 nm to 50 nm, or for instance from 1 nm to 50 nm. The particle size distribution of the nanoparticles may be such that 90 % of the particles have a particle size of less than 50 nm. In some embodiments, the nanoparticles of the quantum dot material have a mean particle size of less than or equal to 20 nm. For instance the nanoparticles may have a mean particle size of from 0.5 nm to 20 nm, or for instance from 1 nm to 20 nm. Typically, the particle size distribution of the nanoparticles is such that 90 % of the particles have a particle size of less than 20 nm. Said nanoparticles of the quantum dot material may have a mean particle size of less than or equal to 10 nm. For instance said nanoparticles may have a mean particle size of from 0.5 nm to 10 nm, or for instance from 1 nm to 10 nm. The particle size distribution of the nanoparticles may be such that 90 % of the particles have a particle size of less than 10 nm. Often, said nanoparticles have a mean particle size of less than or equal to 5 nm. For instance said nanoparticles may have a mean particle size of from 0.5 nm to 5 nm, or for instance from 1 nm to 5 nm. The particle size distribution of the nanoparticles may be such that 90 % of the particles have a particle size of less than 5 nm.

The amount of quantum dot material present in the catalyst is typically less than 50 weight % based on the total weight of the catalyst, for instance less than 45 weight %, e.g. less than 40 weight %. Usually, the catalyst comprises from 5 to 45 weight % of the quantum dot material based on the total weight of the catalyst. Often, for instance, the catalyst comprises from 20 to 30 weight % of the material based on the total weight of the catalyst, or for instance from 23 to 27 weight %, based on the total weight of the catalyst. The support material may for instance comprise from 20 to 30 weight % CdSe based on the total weight of the catalyst.

Usually, the molar ratio of Zn(0) to Zn ⁺ in the catalyst of the invention is from 1 : 10 to 10:1. The molar ratio of Zn(0) to Zn²⁺ in the catalyst may for instance be from 1 :2 to 5:1, or for instance from 2:1 to 7:2. In one embodiment, the molar ratio of Zn(0) to Zn²⁺ in the catalyst of the invention is from 2.8:1.

The amount of palladium present in the catalyst of the invention is usually from 0.1 to 10 weight % based on the total weight of the catalyst. It may for instance be from 0.5 to 9 weight %, or for instance from 1 to 8 weight %, e.g. from 3 to 7 weight %, based on the total weight of the catalyst.

The catalyst of the invention as defined above is typically a catalyst which is obtainable by the process of the invention for producing a catalyst. Thus, the catalyst of the invention as defined above is typically a catalyst which is obtainable by a process which comprises reducing a composition comprising (a) palladium, and (b) a support material which comprises zinc oxide. The process by which the catalyst is obtainable may be as further defined herein for the process of the invention for producing a catalyst.

The invention also provides a process for producing a catalyst, which catalyst comprises (a) a support material comprising zinc oxide, and (b) heterometallic particles, wherein each of the heterometallic particles comprises palladium and zinc, which process comprises reducing a composition comprising: (a) palladium, and (b) a support material which comprises zinc oxide.

The term "reducing a composition", as used herein, means treating the composition with a reductant under conditions which cause the composition to be reduced. As the skilled person will appreciate, when a composition comprising (a) palladium and (b) zinc oxide is reduced with a reductant, such as for instance hydrogen gas, zinc oxide will be reduced to produce zinc metal (i.e. Zn²⁺ will be reduced to Zn°). Typically, therefore, the phrase "reducing a composition comprising (a) palladium, and (b) a support material which comprises zinc oxide" means treating the composition with a reductant under conditions which cause reduction of zinc oxide to zinc metal.

Typically, the reductant is hydrogen, for instance hydrogen gas. Accordingly, the process typically comprises reducing with hydrogen gas a composition comprising: (a) palladium, and (b) a support material which comprises zinc oxide. In other words, the process typically comprises treating with hydrogen gas a composition comprising: (a) palladium, and (b) a support material which comprises zinc oxide. The hydrogen may be neat H₂ or "dilute" H₂. For instance, the hydrogen may be present in a mixture of H₂ with one or more other gases, for instance in a mixture of H₂ and an inert gas such as, for instance, N₂ or a noble gas, such as argon.

Alternatively, the reductant may be a chemical reagent, such as for instance a hydride. The hydride may for instance by sodium borohydride, aluminium hydride, or a metal hydride.

The reduction reaction may be activated by any suitable activation means.

Typically, it is activated thermally (i.e. by heating). Other suitable activation means include, but are not limited to, photo-irradiation (i.e. by irradiating with light, for instance visible or UV light), electrical activation (i.e. by applying an electric current, which may be pulsed or static), or acoustic or ultrasonic activation (e.g. by sonication). Accordingly, in the process for producing a catalyst, reducing said composition may further comprise heating the composition, irradiating the composition with light, applying an electrical current to the composition, or sonicating the composition. Treating the composition with hydrogen gas usually comprises passing hydrogen gas over the catalyst.

Usually, the composition is treated with hydrogen gas at a temperature of from room temperature (e.g. from about 20 °C) to 500 °C, or for instance from about 20 °C to 400 °C.

Typically, the composition is treated with hydrogen gas (which may be neat or dilute hydrogen gas) at a pressure of from 0.1 MPa to 10 MPa, or for instance from 0.1 MPa to 5 MPa.

Usually, the composition is treated with hydrogen gas for at least 1 minute. Often, it is treated with hydrogen gas for at least 0.1 hours, or for instance for at least 1 hour. The composition may for instance be treated with hydrogen gas for from 0.1 to 1 hours.

Thus, typically, said composition is treated with said hydrogen gas at a temperature of from 20 °C to 500 °C, at a pressure of from 0.1 MPa to 10 MPa, for at least 1 minute.

Usually, for instance, the composition is reduced, at a temperature of about 250 °C, and a pressure of about 1 atmosphere, with a gas comprising 99.9 vol. % H₂ or

alternatively with dilute H₂, for at least 0.1 hours or more typically for at least 1 hour.

The composition may be treated with hydrogen gas alone, i.e. neat hydrogen gas, or a mixture of hydrogen gas with one or more other gases. Typically, the one or more other gases are one or more inert gases such as, for instance, one or more gases selected from nitrogen and the noble gases, for instance from nitrogen, argon and helium. The catalyst may be treated with hydrogen gas substantially in the absence of oxygen.

Usually, the support material in the composition to be reduced with hydrogen gas further comprises a compound suitable for promoting the reduction of zinc oxide to zinc metal. Any compound suitable for promoting the reduction of metal cations, and in particular zinc cations, may be employed, such compounds being well known to the skilled person. Usually, however, a quantum dot material is employed. Thus, the compound suitable for promoting the reduction of zinc oxide to zinc metal typically comprises a quantum dot material. For instance, the compound suitable for promoting the reduction of zinc oxide to zinc metal may comprise CdS, CdSe or CdTe. Alternatively, it may comprise a non-Cd quantum dot material, for instance ZnS, ZnSe, ZnTe, ZnP, InP, CuInS or TIP, or for instance a Ga-containing quantum dot material, for instance GaAs, GaP, GaN or GaSb. Typically, however it comprises CdSe.

Often, when the support material in the composition further comprises a compound suitable for promoting the reduction of zinc oxide to zinc metal, it comprises nanoparticles of said compound. Thus, often, the support material in the composition further comprises nanoparticles of a quantum dot material. Typically, the nanoparticles of the compound suitable for promoting the reduction of zinc oxide to zinc metal are in contact with the zinc oxide. The nanoparticles may for instance be coated with said zinc oxide. In other words, the zinc oxide may form a shell around the nanoparticles.

Usually, the nanoparticles comprise CdS, CdSe or CdTe. Alternatively, they may comprise a non-Cd quantum dot material, for instance ZnS, ZnSe, ZnTe, ZnP, InP, CuInS or TIP, or for instance a Ga-containing quantum dot material, for instance GaAs, GaP, GaN or GaSb. Often, however, the nanoparticles comprise CdSe.

The nanoparticles of the quantum dot material in the support material of the composition typically have a mean particle size as defined above, for the catalyst of the invention. Typically, for instance, they have a mean particle size of less than or equal to 100 nm, for instance less than or equal to 10 nm.

The amount of the compound suitable for promoting the reduction of zinc oxide to zinc metal (i.e. usually, the amount of the quantum dot material) in the composition to be reduced typically governs the extent of reduction that takes place. Indeed, the calculated Zn(0)/Zn(2+) ratio in the resulting catalyst increases with increasing concentration of quantum dot material in the composition that is reduced (see Table 1 in the Example below).

Thus, the amount of compound suitable for promoting the reduction of zinc oxide to zinc metal, which is usually a quantum dot material, present in the composition to be reduced is typically less than 50 weight % based on the total weight of the composition, for instance less than 40 weight %. Usually, the composition comprises from 5 to 45 weight % of the compound suitable for promoting the reduction of zinc oxide to zinc metal (e.g. the quantum dot material). Often, for instance, the composition comprises from 20 to 30 weight % of the compound, based on the total weight of the composition, or for instance from 23 to 27 weight %. The composition may for instance comprise from 20 to 30 weight % CdSe, or for instance from 23 to 27 weight % CdSe.

The amount of palladium present in the composition that is reduced with hydrogen, in the process of the invention for producing the catalyst, is usually from 0.1 to 10 weight % based on the total weight of the catalyst. It may for instance be from 0.5 to 9 weight %, or for instance from 1 to 8 weight %, e.g. from 3 to 7 weight %, based on the total weight of the catalyst. Usually, the palladium is present in the composition in the form of particles comprising said palladium, which particles are supported on the support material. Usually, the particles comprising said palladium are nanoparticles.

The process of the invention for producing a catalyst may further comprise producing the composition comprising (a) palladium, and (b) a support material which comprises zinc oxide. Typically, the composition is produced by impregnating a support material which comprises zinc oxide, with a composition comprising palladium. The support material which is impregnated may be as defined hereinbefore, i.e. it typically further comprises a compound, such as a quantum dot material, suitable for promoting the reduction of zinc oxide to zinc metal. Such a further compound and zinc oxide can be added to one another using known methods (for example by solution impregnation, or by sequential growth of the further compound and zinc oxide).

The term "impregnating", as used herein, typically refers to solution impregnation, and so the composition comprising palladium, with which the support material is impregnated, is typically a composition comprising said palladium and a solvent. The composition may be a solution of a palladium salt in said solvent, or a suspension, for instance a colloidal suspension, of particles (e.g. nanoparticles) comprising palladium in said solvent. An aqueous solution of palladium nitrate may for instance be employed. The support material is typically treated with said composition comprising said palladium and a solvent. A solvent removal step is typically perfomed thereafter. For instance, the solvent may be removed by decanting after a filtration or centrifugation step, and/or instance by evaporation (e.g. under reduced pressure or by heating). Calcination (heating the resulting product in air) may then be performed.

The catalyst, once produced, may be used as further described herein, for the hydrogenation of CO2, CO, or a mixture of CO2 and CO, to produce methanol.

Accordingly, the invention provides the use of a catalyst as defined herein as a catalyst for the hydrogenation of C0₂, CO, or a mixture of C0₂ and CO, to produce methanol.

The catalyst is particularly advantageous and effective for producing methanol by the hydrogenation of C0₂. Accordingly, the invention provides the use of a catalyst as defined herein as a catalyst for the hydrogenation of CO2 to produce methanol.

However, the catalyst is also useful for producing methanol from traditional syngas. Accordingly, the invention also provides the use of a catalyst as defined herein as a catalyst for the hydrogenation of CO to produce methanol. Precursor compositions which are typically employed to produce the catalyst of the invention (by reduction with hydrogen) are also novel. Thus, the invention also provides a composition which comprises: (a) palladium, (b) zinc oxide, and (c) a compound suitable for promoting the reduction by hydrogen gas of zinc oxide to zinc metal. Usually, the compound suitable for promoting the reduction by hydrogen gas of zinc oxide to zinc metal is a quantum dot material, and the invention therefore provides a composition which comprises: (a) palladium, (b) zinc oxide, and (c) a quantum dot material. The quantum dot material is typically CdS, CdSe or CdTe. Alternatively, it may be a non-Cd quantum dot material, for instance ZnS, ZnSe, ZnTe, ZnP, InP, CuInS or TIP, or for instance a Ga- containing quantum dot material, for instance GaAs, GaP, GaN or GaSb. Usually, however it is CdSe.

Usually, the quantum dot material is in the form of nanoparticles, supported on the zinc oxide. Thus, the composition usually comprises: (a) palladium, (b) zinc oxide, and (c) nanoparticles of said quantum dot material. The nanoparticles of said quantum dot material are usually in contact with the zinc oxide. The nanoparticles of the quantum dot material typically have a mean particle size as defined above, for the catalyst of the invention. The amount of the quantum dot material is also typically as defined above, for the catalyst of the invention, in terms of weight % based on the total weight of the composition.

The palladium is also typically present in the form of particles, for instance nanoparticles. Thus, usually, the composition of the invention comprises: (a) nanoparticles of palladium, (b) zinc oxide, and (c) nanoparticles of the quantum dot material. The nanoparticles of the quantum dot material are often nanoparticles of CdSe. The amount of palladium present in the composition is usually from 0.1 to 10 weight % based on the total weight of the composition. It may for instance be from 0.5 to 9 weight %, or for instance from 2 to 8 weight %, e.g. from 3 to 7 weight %, based on the total weight of the composition.

The composition of the invention may be as further defined hereinbefore for the composition that is reduced with hydrogen in the process of the invention for producing a catalyst.

The present invention is further illustrated in the Example which follows: EXAMPLE

It is demonstrated in this Example that a series of supported PdZn bimetallic clusters with variable composition were synthesized successfully and that they work well as the active sites in C0₂ hydrogenation. With the increasing concentration of Zn(0) in the bimetallic cluster, a methanol selectivity over 75% has been achieved at 5MPa, with a much higher methanol space-time yield compared with a standard industrial Cu-based catalyst. Even more, the PdZn catalyst can operate under a pressure at 2 MPa, and the methanol selectivity can be maintained at greater than 70% with a decent space-time yield.

Materials and Methods

Catalyst production

The CdSe-ZnO support is synthesized by a sequential growth of CdSe and ZnO. CdSe particles are obtained through the reaction of Na₂SeS0₃ and Cd(N03)2.4H₂0. The

Na₂SeS03 aqueous solution was prepared by refluxing 0.6g Se powder in 100ml aqueous solution containing 4g Na₂S0₃ at 80 °C overnight. 0.64 g Cd(N0₃)₂.4H₂0 and 14.70 g sodium citrate dehydrate were dissolved into 100 mL water to form a solution which was mixed with freshly prepared 0.1 M Na₂SeS0₃ (100 mL) into a flask and heated in water bath at 60 °C for 15 minutes. The red precipitate was collected by centrifugation at 5000 rpm for 10 minutes and extensively washed, after which the supernatant was decanted and discarded. The addition of ZnO was carried out as follows: 1.487 g zinc nitrate [Zn(N0₃)₂. 6H₂0] and 6.000 g NaOH were dissolved in 10 mL deionized water (the molar ratio of Zn² to OH^" was 1 :30). Some CdSe particles produced previously were dispersed into 100 mL ethanol which was added to the solution containing Zn precursor. 5 mL ethylenediamine was also put into the mixture, which was then transferred to a 250mL covered plastic container. This was kept at room temperature (1 atmosphere) under constant stirring until the red mixture turned white. The white crystalline product was then collected by centrifugation and was washed with deionized water and pure ethanol. The final product was dried in an oven at 60 °C for 12 h. The loading of 5 wt% Pd onto above synthesized CdSe-ZnO support was achieved by the impregnation method; 0.3308g Palladium nitrate aqueous solution (15 wt% Pd) was diluted in 3ml distilled water, lg of the synthesized support was immersed into the aqueous solution containing Pd. The mixture was kept stirring at 50 °C to evaporate the solvent at 1 atmpsphere. Yellow products were harvested after the impregnation and all the samples were calcined at 450 °C, 1 atmosphere with an air flow rate of 30 ml/min for 2h. The catalyst is pre-reduced at 250 °C, 1 atmosphere with 99.9% H₂ for at least 1 hour. The flow rate was set to be 30ml/min.

CO2 hydrogenation experiments

O.lg sample mixed with O.lg A1₂0₃ was loaded to a tubular fixed bed reactor (12.7 mm outside diameter) under a flowing stream of reactants. The tests in the hydrogenation of CO₂ were earned out at a total pressure from 2.0 MPa to 4.5MPa with a temperature range from 190 °C to 270 °C. CO2/H₂ reaction mixtures with molar ratios of 1 :3 were fed at a rate of 30 stp mL/min (stp=standard temperature and pressure; P=l atmosphere, T=298 K) through the catalyst bed. Before each test, the catalyst was pre-reduced at 523 K for 2 h under a H₂ flow of 30 stp mL/min.

Results and Discussion

A series of core-shell CdSe-ZnO supports were first synthesized. The CdSe quantum dots were added to promote the reduction of ZnO in ¾ atmosphere through a type II hetero- junction (Figure 2). The high resolution transmission electron microscopy images of ZnO support and one of the CdSe-ZnO supports are displayed in Fig. 3 (the concentration of CdSe is verified by inductive coupled plasma - atomic emission spectrometry (ICP-AES). Without the addition of CdSe quantum dot, rod ZnO are clearly found and the lattice fringe of 0.26nm was assigned to the lattice distance of ZnO(002) planes. From the images of CdSe-ZnO support (Fig. 3a,b), the preformed 4±1 nm CdSe nanoparticles are embedded and dispersed in rod ZnO and an intimate interface between ZnO and CdSe is observed (Fig. 3b); the neighbor lattice fringes of 0.26nm and 0.32nm are accounting for the lattice distance of Zn0(002) and that of CdSe(lOl) planes. This suggests that our sequential synthesis method was successful in burying CdSe into ZnO rod crystals. The Pd particles were loaded on the support by an impregnation method.

As shown in Figure 2, the expected energy levels (valence and conduction orbitals) of CdSe are staggered with those of ZnO. When the material is thermally excited, the electrons are prone to be accumulated in the ZnO rich areas while the holes in CdSe rich areas. With the spatial separation of these excitons, the lifetime of excitons (excited electrons and holes) extends which then facilitates surface chemical reactions. With the presence of Pd particles in proximity, the activated atomic H can react with the holes existing as [0]/[Se] with lower formal charge to produce H₂0/H₂Se and then the excited electrons reduce Zn²⁺ to Zn(0). Therefore, the amount of produced Zn(0) increases with the concentration of the CdSe-ZnO interfaces. These produced Zn(0) atoms decorate on the Pd nanoparticles to form PdZn bimetallic clusters and the composition is governed by the ratio of CdSe: ZnO in the support accordingly.

The X-ray photoelectron spectra (XPS) of Zn 2p for a series of reduced Pd/ZnO/CdSe samples are shown in Fig. 4. The peak at around 1021 eV is assigned to Zn 2pm. Clearly, the peak shifts to a lower binding energy with the increasing concentration of CdSe. The binding energy values for Zn(0) 2p 1/2 and ZnO 2p m are 1020.1 eV and 1021.5 eV, respectively and the lowering binding energy indicates an increase in concentration of Zn(0) in the samples. From the fitting results, all the curves split into two peaks, one is located at 1020.1 eV corresponding to the signal of Zn(0) and the other one around 1021.3 eV is assigned to Zn(II) in ZnO. According to the integration of the peaks, the ratio of Zn(0)/Zn(2+) can be calculated. As shown in table 1, the value increases with the concentration of added CdSe and reaches the peak in sample 4 with the highest concentration of CdSe, which implies the strong promotion effect of CdSe on the reduction of ZnO to Zn(0).

Table 1. Calculated ratio of Zn(0): Zn²⁺ from the XPS results for a series of

Pd/CdSe-ZnO samples.

Given the fact that Pd(0) is the active site to supply atomic hydrogen for the reduction process, it is expected that Zn will decorate or react with Pd nanoparticles to form PdZn bimetallic clusters at a close proximity on the surface of the hetero-j unction support. X-ray absorption spectroscopy (XAS) of the reduced samples was therefore carried out at the Pd K-edge (24350 eV). The extended X-ray absorption fine structure (EXAFS) fitting results are summarized in Fig. 5 and Table 2. The total coordination number of 6-7 over all the samples agrees with those result of HRTEM indicating that the average metal particle size is very small (ca. l-2nm). Pd-Zn scattering path with a distance of 2.56A is clearly observed and found increasing in the samples at increasing CdSe content. This indicates the increasing formation of PdZn bimetallic cluster at the expense of Pd-Pd lattice while the overall particle size is kept constant. Also, Sample 4 (26.4 wt% CdSe) gives the highest number of first shell neighbour Zn to Pd than all the other samples indicating its highest concentration of Zn(0) in the PdZn bimetallic clusters due to its highest extent of ZnO-CdSe interfaces.

Table 2. Average coordination environment of a Pd absorbing atom from the EXAFS results for a series of Pd/CdSe-ZnO samples

Through the strong interaction between Zn and Pd atoms the electronic and geometric structure of Pd can be modified in subtle manner, which then influences the catalytic performance. The catalytic results of a series of reduced Pd/CdSe-ZnO samples in the hydrogenation of C0₂ are summarized in Fig. 6. The traditional Cu/ZnO based catalyst was conducted as a reference. Clearly, Pd-based catalysts show both much higher methanol selectivity and methanol space-time yield in the hydrogenation of C0₂ comparing with Cu-based one indicating the superior activity of Pd based catalyst.

According to Fig. 6a, b, methanol selectivity increases with the concentration of Zn in PdZn bimetallic clusters under a series of variable reaction conditions. When sample 4 with the highest concentration of Zn(0) (26.4 wt% CdSe) was employed, methanol selectivity reaches 75 % at 5MPa. Whereas, for the Cu based industry catalyst, methanol selectivity is limited to 30% by the readily production of CO. The corresponding methanol time-space yields are displayed in Fig.6c,d. The values also increase with the concentration of Zn in PdZn bimetallic cluster with a tendency of leveling off. With regards to the optimized catalyst (sample 4 (26.2 wt% CdSe), the activity of Pd with modification of Zn is estimated to be 20- 100 times per gram basis higher than that of Cu in the industry catalyst.

Compared with the results at 5MPa and 2MPa, it is noted that the methanol selectivity for Pd catalyst is maintained while for the industry catalyst, the values decreases strongly; the methanol selectivity is under 10% at 2MPa (in a temperature range of 250-270°C) which implies that the industry catalyst cannot work at a low pressure, as stated in the introduction. Meanwhile, for the optimized PdZn catalyst (sample 4 (26.2 wt% CdSe)), a 70% methanol selectivity was observed at 2MPa, 270°C with a decent space-time yield. This value is well beyond the methanol selectivity under equilibrium condition which implies that the reversed water gas shift reaction is strongly suppressed on the surface of sample 4. Therefore, PdZn with high concentration of Zn is a promising low-pressure catalyst for methanol synthesis from CO2 hydrogenation.

To investigate the detailed catalytic behavior of catalysts with variable pressures, a series of tests were conducted at 250°C. The catalytic performances of sample 4 (26.4 wt% CdSe) and Cu-based industrial catalyst under different pressures are shown in Fig. 7. For Cu-based industrial catalyst, both methanol selectivity and methanol yield decrease while CO yield rises at decreasing reaction pressure. All these results show the same trend with the values predicted by thermodynamic equilibrium (the dash lines in Fig. 7), which implies that the catalytic performance of Cu based industry catalyst is strongly influenced by the thermodynamic values. As to the optimized PdZn catalyst (sample 4 (26.4 wt% CdSe)), methanol selectivity is kept stable with the decreasing of reaction pressure while methanol yield and CO yield decreases slightly. As shown in Fig. 7c, the values of CO yield are far away from the equilibrium position indicating the extremely slow rate of the reverse water gas shift reaction in the PdZn system, thus the system is clearly under a kinetic control regime. The observed high methanol selectivity is due to the relatively fast production rate of methanol than that of CO production. Compared with the Cu based industry standard catalyst, the catalytic performance of the PdZn catalyst with a high concentration of Zn is much more independent of the applied pressure.

The high methanol selectivity and its stability with reducing reaction pressure of the PdZn catalyst (sample 4) provides an exciting possibility to explore a new catalyst working under low pressure with a decent time-space methanol yield. The catalytic performances of the PdZn catalysts (sample 1 and sample 4) and the Cu-based catalyst at 2MPa with variable temperatures are shown in Fig. 8. For all the samples, methanol selectivity decreases with rising temperature due to the effect of thermodynamics (C0₂+3H₂→

CH₃OH+H₂O, AH=-49.5KJ/mol; C0₂+H₂→CO+H₂0, AH=41.2KJ/mol). In a temperature range of 210-270 °C, the PdZn based catalysts show much higher methanol selectivity than Cu based catalyst especially for sample 4 with a high concentration of Zn(0) in PdZn bimetallic cluster. The methanol space-time yield of the Cu based industry catalyst decreases with rising temperature, which is influenced by the thermodynamic equilibrium (the reaction of C0₂ + 3H₂→CH₃OH + H₂0 is strongly exothermic). Despite the fact that sample 4 shows low activity below 230 °C, the activity rises sharply with increasing temperature through the activation of the catalyst, and the methanol space-time yield reaches 20 times that of the optimized value of the Cu-based catalyst with a much higher methanol selectivity at 270 °C. It is noted that the methanol yield of sample 4 is higher than the theoretical value predicted at thermodynamic equilibrium conditions taking the reverse water gas shift (RWGS) reaction into account, and is approaching the new equilibrium value with no account for RWGS reaction. This implies that PdZn has a superior ability not only to catalyze the formation of methanol selectively, but also to suppress greatly the RWGS reaction.

With the combined catalytic results, it can be concluded that the PdZn catalyst exhibits both higher methanol selectivity and activity than the traditional Cu-based catalyst in the hydrogenation of CO_¾ and can also operate under pressures as low as 2MPa with a decent methanol space-time yield. Additionally, the catalytic ability in producing methanol selectively is enhanced by increasing the concentration of Zn(0) in the PdZn bimetallic cluster, and this effect should be assigned to the modification of Zn on the electronic and geometric structure of Pd. The calculated d band structures of Pd in a series of PdZn alloys with variable composition are shown in Fig. 9. As the concentration of Zn atom increases, the d band filling shifts progressively to a higher value indicating the gaining of electrons in d band for the Pd atoms while the d band center shifts to a lower position. From the viewpoint of electronic structure, Zn has a configuration of 3d¹⁰ 4s² and the valence orbitals are completely fulfilled. Meanwhile, the electronic configuration of Pd is 4d¹⁰ 5s°, through a hybrid of s-d orbitals, some orbital vacancies are introduced in the d band of Pd. Therefore, when Zn atoms with fulfilled d orbitals are introduced into the lattice of Pd particles, their d electrons flow into the vacancies in d band of Pd, and raise the d band filling. On the other hand, the progressive increase of Zn concentration in the PdZn cluster can cause a lattice shrink due to the smaller size of Zn comparing with Pd, which makes the orbital overlapping of Pd with each other stronger. Consequently, the d band center shifts to a lower position.

It is thought that the modified d band structure of Pd could have altered the adsoiption mode of CO2 on its surface and consequently have influenced the catalytic result. The diagram illustrating the electronic interactions of CO2 and Pd is displayed in Figure 10. The lowest unoccupied molecular orbitals (LUMO) of CO2 contain two anti-bonding π orbitals. When the d band filling of Pd increases by the Zn incorporation, the π back donation to CO2 is enhanced which then increases the π adsorption mode of CO2 on its surface (Fig. 10). This will facilitate the formation of surface formate specie (partial hydrogenation of CO2), which is an intermediate to methanol. The inhibition of CO formation from CO2 through RWGS could be attributed to the weakening of the Pd-0 bonds due to the lowered d-band centre (weakening the sigma interaction between the C0₂ and Pd surface). Eventually, methanol selectivity increases with the concentration of Zn(0) PdZn bimetallic cluster. The preparations of high Zn atoms content on Pd nanoparticle surface while maintaining the small particle size by our new heteroj unction technique will render the formation of extensive PdZn on support which could not easily obtained by other alternative deposition techniques. In addition, the selective poisoning of high indexed Pd sites (stepped, edges, corners, adatoms) on the nanoparticle by the added Zn atoms may also cause the suppression of CO formation from RWGS.

Claims

1. A process for producing methanol, which process comprises contacting a gas phase with a solid catalyst, wherein:

the gas phase comprises (a) H₂ and (b) C0₂, CO or a mixture of C0₂ and CO; and the catalyst comprises heterometallic particles, wherein each of the heterometallic particles comprises palladium and zinc.

2. A process according to claim 1 wherein the heterometallic particles are

heterometallic nanoparticles.

3. A process according to claim 2 wherein the heterometallic nanoparticles comprise heterometallic clusters, wherein each of the heterometallic clusters comprises palladium atoms and zinc atoms.

4. A process according to claim 3 wherein the heterometallic clusters are bimetallic clusters, consisting of palladium atoms and zinc atoms.

5. A process according to any one of claims 1 to 4 wherein the heterometallic particles have a mean particle size of less than or equal to 100 nm.

6. A process according to any one of claims 1 to 4 wherein the heterometallic particles have a mean particle size of less than or equal to 10 nm.

7. A process according to any one of claims 1 to 4 wherein the heterometallic particles have a mean particle size of less than or equal to 5 nm.

8. A process according to any one of the preceding claims wherein the catalyst further comprises a support material.

9. A process according to claim 8 wherein the support material comprises zinc oxide.

10. A process according to claim 9 wherein the support material further comprises a quantum dot material.

11. A process according to claim 10 wherein the quantum dot material is CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnP, InP, CuInS, TIP, GaAs, GaP, GaN or GaSb.

12. A process according to claim 10 or claim 11 wherein the quantum dot material is CdSe.

13. A process according to any one of claims 10 to 12 wherein the quantum dot material comprises nanoparticles of the quantum dot material in contact with the zinc oxide.

14. A process according to claim 13 wherein the nanoparticles of the quantum dot material have a mean particle size of less than or equal to 10 nm.

15. A process according to any one of claims 10 to 14 wherein the catalyst comprises from 5 to 45 weight % of the quantum dot material, based on the total weight of the catalyst.

16. A process according to any one of claims 10 to 14 wherein the catalyst comprises from 20 to 30 weight % of the quantum dot material, based on the total weight of the catalyst.

17. A process according to any one of claims 9 to 16 wherein the molar ratio of Zn(0) to Zn²⁺ in the catalyst is from 1 : 10 to 10:1.

18. A process according to any one of claims 9 to 16 wherein the molar ratio of Zn(0) to Zn²⁺ in the catalyst is from 1 :2 to 5:1.

19. A process according to any one of claims 9 to 16 wherein the molar ratio of Zn(0) to Zn²⁺ in the catalyst is from 2: 1 to 7:2.

20. A process according to any one of the preceding claims wherein the amount of palladium present in the catalyst is from 0.1 to 10 weight %, based on the total weight of the catalyst.

21. A process according to any one of the preceding claims wherein the catalyst is obtainable by a process as defined in any one of claims 57 to 70.

22. A process according to any one of the preceding claims which further comprises producing the catalyst by a process as defined in any one of claims 57 to 70.

23. A process according to any one of the preceding claims wherein the step of contacting the gas phase with the solid catalyst is performed at a pressure of from 0.1 MPa to 6 MPa.

24. A process according to any one of the preceding claims wherein the step of contacting the gas phase with the solid catalyst is performed at a pressure of less than 3 MPa.

25. A process according to any one of the preceding claims wherein the step of contacting the gas phase with the solid catalyst is performed at a pressure of less than or equal to 2 MPa.

26. A process according to any one of the preceding claims wherein the step of contacting the gas phase with the solid catalyst is performed at a temperature of from 150 °C to 500 °C.

27. A process according to any one of the preceding claims wherein the step of contacting the gas phase with the solid catalyst is performed at a temperature of less than or equal to 300 °C.

28. A process according to any one of the preceding claims wherein the step of contacting the gas phase with the solid catalyst is performed at a temperature of less than or equal to 260 °C.

29. A process according to any one of the preceding claims wherein the step of contacting the gas phase with the solid catalyst is performed at a temperature of less than or equal to 300 °C and at a pressure of less than or equal to 2.5 MPa.

30. A process according to any one of the preceding claims wherein the step of contacting the gas phase with the solid catalyst is performed at a temperature of less than or equal to 260 °C and at a pressure of less than or equal to 2 MPa.

31. A process according to any one of the preceding claims wherein the gas phase comprises H₂ and C0₂.

32. A process according to claim 31 wherein the gas phase further comprises CO.

33. A process according to any one of claims 1 to 30 wherein the gas phase comprises H₂ and CO.

34. A process according to any one of the preceding claims which further comprises recovering said methanol.

35. A process according to claim 31 or claim 32 wherein:

(a) the C0₂ is obtained by combustion, optionally by combustion of natural gas, coal, petroleum or a derivative of natural gas, coal or petroleum;

(b) the C0₂ is collected from the environment, optionally, wherein the CO2 is atomspheric C0₂;

(c) the CO2 and/or H2 are obtained by processing a biomass, a bio-gas or a bio-fuel, optionally by steam reforming or aqueous phase reforming a said biomass, bio-gas or bio- fuel, optionally wherein the biomass comprises wood, lignin or carbohydrate;

(d) the H2 is obtained by electrolysis of water; by photolysis of water; by thermal (ground heating), nuclear or mechanical (wind) assisted hydrogen production; or by hydrocarbon processing or decomposition.

36. A process according to any one of the preceding claims which further comprises producing (a) the ¾ and (b) the CO2, CO or mixture of CO2 and CO, of said gas phase by steam reforming or aqueous phase reforming.

37. A process according to claim 31 or claim 32 which further comprises producing the C0₂ and H₂ of the gas phase by the aqueous phase reforming of glycerol, optionally wherein said glycerol is a by-product of a biodiesel production process.

38. A process according to claim 31 or claim 32 which further comprises:

performing a biodiesel production process, comprising reacting methanol with a plant oil or an animal oil to produce (i) biodiesel and (ii) glycerol as a by-product; and producing the C0₂ and H₂ of the gas phase by aqueous phase reforming of said glycerol by-product.

39. A process according to claim 38 which further comprises recovering methanol produced by the process for producing methanol, and recycling said methanol into the biodiesel production process.

40. A process according to any one of claims 36 to 39 wherein the entire process is performed at a pressure that does not exceed 3.0 MPa.

41. A process according to any one of claims 36 to 39 wherein the entire process is performed at a pressure that does not exceed 2.5 MPa.

42. A process according to any one of claims 36 to 39 wherein the entire process is performed at a pressure that does not exceed 2.0 MPa.

43. A process according to any one of claims 40 to 42 wherein the step of contacting the gas phase with the solid catalyst is performed at a temperature of from 200 °C to 300 °C.

44. A catalyst which comprises (a) a support material comprising zinc oxide, and (b) heterometallic particles, wherein each of the heterometallic particles comprises palladium and zinc.

45. A catalyst according to claim 44 wherein the wherein the support material further comprises a quantum dot material.

46. A catalyst according to claim 44 or claim 45 wherein the quantum dot material is CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnP, InP, CulnS, TIP, GaAs, GaP, GaN or GaSb.

47. A catalyst according to any one of claims 44 to 46 wherein the quantum dot material is CdSe.

48. A catalyst according to any one of claims 44 to 47 wherein the quantum dot material comprises nanoparticles of the quantum dot material in contact with the zinc oxide.

49. A catalyst according to any one of claims 44 to 48 wherein the nanoparticles of the quantum dot material have a mean particle size of less than or equal to 10 nm.

50. A catalyst according to any one of claims 44 to 49 which comprises from 5 to 45 weight % of the quantum dot material, based on the total weight of the catalyst.

51. A catalyst according to any one of claims 44 to 50 which comprises from 20 to 30 weight % of the quantum dot material, based on the total weight of the catalyst.

52. A catalyst according to any one of claims 44 to 51 wherein the molar ratio of Zn(0) to Zn²⁺ in the catalyst is from 1 : 10 to 10:1.

53. A catalyst according to any one of claims 44 to 51 wherein the molar ratio of Zn(0) to Zn²⁺ in the catalyst is from 1 :2 to 5 : 1.

54. A catalyst according to any one of claims 44 to 51 wherein the molar ratio of Zn(0) to Zn²⁺ in the catalyst is from 2:1 to 7:2.

55. A catalyst according to any one of claims 44 to 54 wherein the amount of palladium present in the catalyst is from 0.1 to 10 weight %, based on the total weight of the catalyst.

56. A catalyst according to any one of claims 43 to 53 which is obtainable by a process as defined in any one of claims 57 to 70.

57. A process for producing a catalyst which comprises (a) a support material comprising zinc oxide, and (b) heterometallic particles, wherein each of the heterometallic particles comprises palladium and zinc, which process comprises reducing a composition comprising:

(a) palladium, and

(b) a support material which comprises zinc oxide.

58. A process according to claim 57 wherein the support material further comprises a compound suitable for promoting the reduction of zinc oxide to zinc metal.

59. A process according to claim 58 wherein the compound suitable for promoting the reduction of zinc oxide to zinc metal comprises a quantum dot material.

60. A process according to claim 59 wherein the compound suitable for promoting the reduction of zinc oxide to zinc metal comprises nanoparticles of a quantum dot material in contact with the zinc oxide.

61. A process according to claim 60 wherein the nanoparticles of the quantum dot material have a mean particle size of less than or equal to 10 rrm.

62. A process according to any one of claims 57 to 61 wherein the quantum dot material is CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnP, InP, CuInS, TIP, GaAs, GaP, GaN or GaSb.

63. A process according to any one of claims 57 to 61 wherein the quantum dot material is CdSe.

64. A process according to any one of claims 57 to 63 wherein the amount of quantum dot material present in the composition is from 5 to 45 weight % based on the total weight of the composition.

65. A process according to any one of claims 57 to 64 wherein the amount of quantum dot material present in the composition is from 20 to 30 weight % based on the total weight of the composition.

66. A process according to any one of claims 57 to 65 wherein the amount of palladium present in the composition is from 0.1 to 10 weight % based on the total weight of the composition.

67. A process according to any one of claims 57 to 66 wherein the palladium is present in the composition in the form of particles comprising said palladium, which particles are supported on the support material.

68. A process according to claim 67 wherein the particles comprising said palladium are nanoparticles.

69. A process according to any one of claims 57 to 68 wherein reducing the

composition comprises treating the composition with hydrogen gas.

70. A process according to claim 69 wherein said composition is treated with said hydrogen gas at a temperature of from 20 °C to 500 °C, at a pressure of from 0.1 MPa to 10 MPa, for at least 1 minute.

71. A process according to any one of claims 57 to 70 further comprising producing the composition comprising (a) palladium, and (b) a support material which comprises zinc oxide, by impregnating the support material with a composition comprising palladium.

72. Use of a catalyst as defined in any one of claims 1 to 56 as a catalyst for the hydrogenation of C0₂, CO, or a mixture of C0₂ and CO, to produce methanol.

73. Use of a catalyst as defined in any one of claims 1 to 56 as a catalyst for the hydrogenation of C0₂ to produce methanol.

74. A composition which comprises:

(a) palladium, (b) zinc oxide, and

(c) a compound suitable for promoting the reduction by hydrogen gas of zinc oxide to zinc metal.

75. A composition according to claim 74 wherein the compound suitable for promoting the reduction by hydrogen gas of zinc oxide to zinc metal is a quantum dot material.

76. A composition according to claim 75 wherein the compound suitable for promoting the reduction of zinc oxide to zinc metal comprises nanoparticles of the quantum dot material in contact with the zinc oxide.

77. A composition according to claim 75 or claim 76 wherein the quantum dot material is CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, ZnP, InP, CuInS, TIP, GaAs, GaP, GaN or GaSb.

78. A composition according to claim 75 or claim 76 wherein the quantum dot material is CdSe.

79. A composition according to any one of claims 74 to 78 wherein the palladium is present in the composition in the form of particles comprising said palladium.

80. A composition according to claim 79 wherein the particles comprising said palladium are nanoparticles.

81. A composition according to any one of claims 74 to 80 which is as further defined in any one of claims 61 and 64 to 66.