Methanol synthesis from CO2 and H2 using supported Pd alloy catalysts

A number of Pd based materials have been synthesised and evaluated as catalysts for the conversion of carbon dioxide and hydrogen to methanol, a useful platform chemical and hydrogen storage molecule. Monometallic Pd catalysts show poor methanol selectivity, but this is improved through the formation of Pd alloys, with both PdZn and PdGa alloys showing greatly enhanced methanol productivity compared with monometallic Pd/Al2O3 and Pd/TiO2 catalysts. Catalyst characterisation shows that the 1 : 1 β-PdZn alloy is present in all Zn containing post-reaction samples, including PdZn/Ga2O3, with the Pd2Ga alloy formed for the Pd/Ga2O3 sample. The heat of mixing was calculated for a variety of alloy compositions with high values determined for both PdZn and Pd2Ga alloys, at ca. -0.6 eV per atom and ca. -0.8 eV per atom, respectively. However, ZnO is more readily reduced than Ga2O3, providing a possible explanation for the preferential formation of the PdZn alloy, rather than PdGa, when in the presence of Ga2O3.

Methane Oxidation to Methanol

The direct transformation of methane to methanol remains a significant challenge for operation at a larger scale. Central to this challenge is the low reactivity of methane at conditions that can facilitate product recovery. This review discusses the issue through examination of several promising routes to methanol and an evaluation of performance targets that are required to develop the process at scale. We explore the methods currently used, the emergence of active heterogeneous catalysts and their design and reaction mechanisms and provide a critical perspective on future operation. Initial experiments are discussed where identification of gas phase radical chemistry limited further development by this approach. Subsequently, a new class of catalytic materials based on natural systems such as iron or copper containing zeolites were explored at milder conditions. The key issues of these technologies are low methane conversion and often significant overoxidation of products. Despite this, interest remains high in this reaction and the wider appeal of an effective route to key products from C-H activation, particularly with the need to transition to net carbon zero with new routes from renewable methane sources is exciting.

Advancing Critical Chemical Processes for a Sustainable Future: Challenges for Industry and the Max Planck–Cardiff Centre on the Fundamentals of Heterogeneous Catalysis (FUNCAT)

Catalysis is involved in around 85 % of manufacturing industry and contributes an estimated 25 % to the global domestic product, with the majority of the processes relying on heterogeneous catalysis. Despite the importance in different global segments, the fundamental understanding of heterogeneously catalysed processes lags substantially behind that achieved in other fields. The newly established Max Planck-Cardiff Centre on the Fundamentals of Heterogeneous Catalysis (FUNCAT) targets innovative concepts that could contribute to the scientific developments needed in the research field to achieve net zero greenhouse gas emissions in the chemical industries. This Viewpoint Article presents some of our research activities and visions on the current and future challenges of heterogeneous catalysis regarding green industry and the circular economy by focusing explicitly on critical processes. Namely, hydrogen production, ammonia synthesis, and carbon dioxide reduction, along with new aspects of acetylene chemistry.

Relationship between hydrothermal temperatures and structural properties of CeO2 and enhanced catalytic activity of propene/toluene/CO oxidation by Au/CeO2 catalysts

A simple hydrothermal synthesis of CeO₂ was implemented to obtain a series of CeO₂-supported gold (Au) catalysts, used for the total oxidation of propene/toluene/CO gas mixtures and the oxidation of CO. CeO₂ preparation started from a cerium hydrogen carbonate precursor using a range of different hydrothermal temperatures (HT) from 120 to 180°C. High-resolution transmission electron microscopy, X-ray diffraction, and H₂-temperature-programmed reduction data indicated that CeO₂ morphology varied with the HT, and was composed of the more active (200) surface. Following Au deposition onto the CeO₂ support, this active crystal plane resulted in the most widely dispersed Au nanoparticles on the CeO₂ support. The catalytic performance of the CeO₂-supported Au catalysts for both oxidation reactions improved as the reducibility increased to generate lattice oxygen vacancies and the number of adsorbed peroxide species on the CeO₂ support increased due to addition of Au. The Au catalyst on the CeO₂ support prepared at 120°C was the most active in both propene/toluene/CO oxidation and independent CO oxidation.

Perovskite Supported Catalysts for the Selective Oxidation of Glycerol to Tartronic Acid

Exceptional selectivity of LaMnO3 perovskite supported Au catalysts for the oxidation of glycerol to the dicarboxylate tartronic acid is reported. Through using monometallic Au, Pt or bimetallic Au:Pt nanoparticles the tartronic acid yield could be altered significantly, with a maximum yield of 44% in 6 h with Au/LaMnO3 and 80% within 24 h. These LaMnO3 supported catalysts were compared with conventionally TiO2 supported catalysts, which at comparable reaction conditions produced lactic acid, via a dehydration pathway, in high yield and a maximum tartronic acid yield of only 9% was observed. The LaMnO3 catalysts produced minimal lactic acid regardless of the supported metal, showing that the support structure influences the prevalence of dehydration and oxidation pathways. The choice of metal nanoparticle influenced product selectivity along the oxidation pathway for both LaMnO3 and TiO2 supported catalysts. Au catalysts exhibited a higher selectivity to tartronic acid, whereas AuPt catalysts produced glyceric acid and Pt catalysts produced predominantly C–C scission products.

Graphical Abstract

Heterogeneous Trimetallic Nanoparticles as Catalysts

The development and application of trimetallic nanoparticles continues to accelerate rapidly as a result of advances in materials design, synthetic control, and reaction characterization. Following the technological successes of multicomponent materials in automotive exhausts and photovoltaics, synergistic effects are now accessible through the careful preparation of multielement particles, presenting exciting opportunities in the field of catalysis. In this review, we explore the methods currently used in the design, synthesis, analysis, and application of trimetallic nanoparticles across both the experimental and computational realms and provide a critical perspective on the emergent field of trimetallic nanocatalysts. Trimetallic nanoparticles are typically supported on high-surface-area metal oxides for catalytic applications, synthesized via preparative conditions that are comparable to those applied for mono- and bimetallic nanoparticles. However, controlled elemental segregation and subsequent characterization remain challenging because of the heterogeneous nature of the systems. The multielement composition exhibits beneficial synergy for important oxidation, dehydrogenation, and hydrogenation reactions; in some cases, this is realized through higher selectivity, while activity improvements are also observed. However, challenges related to identifying and harnessing influential characteristics for maximum productivity remain. Computation provides support for the experimental endeavors, for example in electrocatalysis, and a clear need is identified for the marriage of simulation, with respect to both combinatorial element screening and optimal reaction design, to experiment in order to maximize productivity from this nascent field. Clear challenges remain with respect to identifying, making, and applying trimetallic catalysts efficiently, but the foundations are now visible, and the outlook is strong for this exciting chemical field.

Highly Active Co3O4-Based Catalysts for Total Oxidation of Light C1-C3 Alkanes Prepared by a Simple Soft Chemistry Method: Effect of the Heat-Treatment Temperature and Mixture of Alkanes

In the present work, a simple soft chemistry method was employed to prepare cobalt mixed oxide (Co₃O₄) materials, which have shown remarkably high activity in the heterogeneously catalyzed total oxidation of low reactive VOCs such as the light alkanes propane, ethane, and methane. The optimal heat-treatment temperature of the catalysts was shown to depend on the reactivity of the alkane studied. The catalytic activity of the Co₃O₄ catalysts was found to be as high as that of the most effective catalysts based on noble metals. The physicochemical properties, from either the bulk (using XRD, TPR, TPD-O₂, and TEM) or the surface (using XPS), of the catalysts were investigated to correlate the properties with the catalytic performance in the total oxidation of VOCs. The presence of S1 low-coordinated oxygen species at the near surface of the Co₃O₄-based catalysts appeared to be linked with the higher reducibility of the catalysts and, consequently, with the higher catalytic activity, not only per mass of catalyst but also per surface area (enhanced areal rate). The co-presence of propane and methane in the feed at low reaction temperatures did not negatively affect the propane reactivity. However, the co-presence of propane and methane in the feed at higher reaction temperatures negatively affected the methane reactivity.

Methane Oxidation to Methanol in Water

Methane represents one of the most abundant carbon sources for fuel or chemical production. However, remote geographical locations and high transportation costs result in a substantial proportion being flared at the source. The selective oxidation of methane to methanol remains a grand challenge for catalytic chemistry due to the large energy barrier for the initial C-H activation and prevention of overoxidation to CO₂. Indirect methods such as steam reforming produce CO and H₂ chemical building blocks, but they consume large amounts of energy over multistage processes. This makes the development of the low-temperature selective oxidation of methane to methanol highly desirable and explains why it has remained an active area of research over the last 50 years.The thermodynamically favorable oxidation of methane to methanol would ideally use only molecular oxygen. Nature effects this transformation with the enzyme methane monooxygenase (MMO) in aqueous solution at ambient temperature with the addition of 2 equiv of a reducing cofactor. MMO active sites are Fe and Cu oxoclusters, and the incorporation of these metals into zeolitic frameworks can result in biomimetic activity. Most approaches to methane oxidation using metal-doped zeolites use high temperature with oxygen or N₂O; however, demonstrations of catalytic cycles without catalyst regeneration cycles are limited. Over the last 10 years, we have developed Fe-Cu-ZSM-5 materials for the selective oxidation of methane to methanol under aqueous conditions at 50 °C using H₂O₂ as an oxidant (effectively O₂ + 2 reducing equiv), which compete with MMO in terms of activity. To date, these materials are among the most active and selective catalysts for methane oxidation under this mild condition, but industrially, H₂O₂ is an expensive oxidant to use in the production of methanol.This observation of activity under mild conditions led to new approaches to utilize O₂ as the oxidant. Supported precious metal nanoparticles have been shown to be active for a range of C-H activation reactions using O₂ and H₂O₂, but the rapid decomposition of H₂O₂ over metal surfaces limits efficiency. We identified that this decomposition could be minimized by removing the support material and carrying out the reaction with colloidal AuPd nanoparticles. The efficiency of methanol production with H₂O₂ consumption was increased by 4 orders of magnitude, and crucially it was demonstrated for the first time that molecular O₂ could be incorporated into the methanol produced with 91% selectivity. The understanding gained from these two approaches provides valuable insight into possible new routes to selective methane oxidation which will be presented here in the context of our own research in this area.

A combined periodic DFT and QM/MM approach to understand the radical mechanism of the catalytic production of methanol from glycerol

The production of methanol from glycerol over a basic oxide, such as MgO, using high reaction temperatures (320 °C) is a promising new approach to improving atom efficiency in the production of biofuels. The mechanism of this reaction involves the homolytic cleavage of the C₃ feedstock, or its dehydration product hydroxyacetone, to produce a hydroxymethyl radical species which can then abstract an H atom from other species. Obtaining a detailed reaction mechanism for this type of chemistry is difficult due to the large number of products present when the system is operated at high conversions. In this contribution we show how DFT based modelling studies can provide new insights into likely reaction pathways, in particular the source of H atoms for the final step of converting hydroxymethyl radicals to methanol. We show that water is unlikely to be important in this stage of the process, C-H bonds of C₂ and C₃ species can give an energetically favourable pathway and that the disproportionation of hydroxymethyl radicals to methanol and formaldehyde produces a very favourable route. Experimental analysis of reaction products confirms the presence of formaldehyde. The calculations presented in this work also provide new insight into the role of the catalyst surface in the reaction showing that the base sites of the MgO(100) are able to deprotonate hydroxymethyl radicals but not methanol itself. In carrying out the calculations we also show how periodic DFT and QM/MM approaches can be used together to obtain a rounded picture of molecular adsorption to surfaces and homolytic bond cleavage which are both central to the reactions studied.

Gas Phase Glycerol Valorization over Ceria Nanostructures with Well-Defined Morphologies

Glycerol solutions were vaporized and reacted over ceria catalysts with different morphologies to investigate the relationship of product distribution to the surface facets exposed, particularly, the yield of bio-renewable methanol. Ceria was prepared with cubic, rodlike, and polyhedral morphologies via hydrothermal synthesis by altering the concentration of the precipitating agent or synthesis temperature. Glycerol conversion was found to be low over the ceria with a cubic morphology, and this was ascribed to both a low surface area and relatively high acidity. Density functional theory calculations also showed that the (100) surface is likely to be hydroxylated under reaction conditions which could limit the availability of basic sites. Methanol space-time-yields over the polyhedral ceria samples were more than four times that for the cubic material at 400 °C, where 201 g of methanol was produced per hour per kilogram of the catalyst. Under comparable glycerol conversions, we show that the rodlike and polyhedral catalysts produce a major intermediate to methanol, hydroxyacetone (HA), with a selectivity of ca. 45%, but that over the cubic sample, this was found to be 15%. This equates to a 13-fold increase in the space-time-yield of HA over the polyhedral samples compared to the cubes at 320 °C. The implications of this difference are discussed with respect to the reaction mechanism, suggesting that a different mechanism dominates over the cubic catalysts to that for rodlike and polyhedral catalysts. The strong association between exposed surface facets of ceria to high methanol yields is an important consideration for future catalyst design in this area.

Controlled reduction of aromaticity of alkylated polyaromatic compounds by selective oxidation using H2WO4, H3PO4 and H2O2: a route for upgrading heavy oil fractions

Selectivity in the oxidation of alkylated polynuclear aromatic hydrocarbon can be specifically controlled by the choice solvent.

The formation of methanol from glycerol bio-waste over doped ceria-based catalysts

A series of ceria-based solid solution metal oxides were prepared by co-precipitation and evaluated as catalysts for glycerol cleavage, principally to methanol. The catalyst activity and selectivity to methanol were investigated with respect to the reducibility of the catalysts. Oxides comprising Ce-Pr and Ce-Zr were prepared, calcined and compared to CeO₂, Pr₆O₁₁ and ZrO₂. The oxygen storage capacity of the catalysts was examined with analysis of Raman spectroscopic measurements and a temperature programmed reduction, oxidation and reduction cycle. The incorporation of Pr resulted in significant defects, as evidenced by Raman spectroscopy. The materials were evaluated as catalysts for the glycerol to methanol reaction, and it was found that an increased defect density or reducibility was beneficial. The space-time yield of methanol normalized to surface area over CeO₂ was found to be 0.052 mmol_MeOHm^-2h^-1, and over CeZrO₂ and CePrO₂, this was to 0.029 and 0.076 mmol_MeOHm^-2h^-1, respectively. The inclusion of Pr reduced the surface area; however, the carbon mole selectivity to methanol and ethylene glycol remained relatively high, suggesting a shift in the reaction pathway compared to that over ceria. This article is part of a discussion meeting issue 'Science to enable the circular economy'.

Enhancing the understanding of the glycerol to lactic acid reaction mechanism over AuPt/TiO2 under alkaline conditions

The oxidation of glycerol under alkaline conditions in the presence of a heterogeneous catalyst can be tailored to the formation of lactic acid, an important commodity chemical. Despite recent advances in this area, the mechanism for its formation is still a subject of contention. In this study, we use a model 1 wt. % AuPt/TiO₂ catalyst to probe this mechanism by conducting a series of isotopic labeling experiments with 1,3-¹³C glycerol. Optimization of the reaction conditions was first conducted to ensure high selectivity to lactic acid in the isotopic labeling experiments. Selectivity to lactic acid increased with temperature and concentration of NaOH, but increasing the O₂ pressure appeared to influence only the rate of reaction. Using 1,3-¹³C glycerol, we demonstrate that conversion of pyruvaldehyde to lactic acid proceeds via a base-promoted 1,2-hydride shift. There was no evidence to suggest that this occurs via a 2,1-methide shift under the conditions used in this study.

Enhancement in the rate of nitrate degradation on Au- and Ag-decorated TiO2 photocatalysts

The solar-driven reduction of nitrate to nitrogen has been studied in the presence of a formate hole scavenger, over a series of Au- and Ag-decorated TiO₂ catalysts.

Adipic acid formation from cyclohexanediol using platinum and vanadium catalysts: elucidating the role of homogeneous vanadium species

The applicability of platinum and vanadium catalysts for the production of adipic acid from cyclohexanediol is severely limited by the instability of the latter species in liquid phase reactions.

Efficient Elimination of Chlorinated Organics on a Phosphoric Acid Modified CeO2 Catalyst: A Hydrolytic Destruction Route

The development of efficient technologies to prevent the emission of hazardous chlorinated organics from industrial sources without forming harmful byproducts, such as dioxins, is a major challenge in environmental chemistry. Herein, we report a new hydrolytic destruction route for efficient chlorinated organics elimination and demonstrate that phosphoric acid-modified CeO₂ (HP-CeO₂) can decompose chlorobenzene (CB) without forming polychlorinated congeners under the industry-relevant reaction conditions. The active site and reaction pathway were investigated, and it was found that surface phosphate groups initially react with CB and water to form phenol and HCl, followed by deep oxidation. The high on-stream stability of the catalyst was due to the efficient generation of HCl, which removes Cl from the catalyst surface and ensures O₂ activation and therefore deep oxidation of the hydrocarbons. Subsequent density functional theory calculations revealed a distinctly decreased formation energy of an oxygen vacancy at nearest (V_O-1) and next-nearest (V_O-2) surface sites to the bonded phosphate groups, which likely contributes to the high rate of oxidation observed over the catalyst. Significantly, no dioxins, which are frequently formed in the conventional oxidation route, were observed. This work not only reports an efficient route and corresponding phosphate active site for chlorinated organics elimination but also illustrates that the rational design of the reaction route can solve some of the most important challenges in environmental catalysis.

Metal Triflate-Promoted Allylic Substitution Reactions of Cinnamyl Alcohol in the Presence of Orthoesters and Acetals

The product distribution of ethers formed from the reaction of cinnamyl alcohol with orthoesters in the presence of indium (III) triflate (InOTf)3 is dependent on both the reaction temperature and catalyst loading. Carrying out the reaction at room temperature under low loadings of the catalyst leads to a facile reaction generating the unexpected secondary allyl ether as the major product. In contrast, carrying out the reaction under higher catalyst loadings at elevated temperatures provides the expected primary linear ether in high yield and with excellent selectivity. The etherification reaction is also effective in the presence of acetals and ketals in place of orthoesters and allows for the development of the procedure to encompass a telescoped etherification protocol in which the acetal is generated in situ.

New insights for the valorisation of glycerol over MgO catalysts in the gas-phase

Aqueous glycerol solutions of up to 50 wt% were reacted over magnesium oxide catalysts at temperatures greater than 300 °C, the reactivity of which was compared to catalyst-free reactions.

Platinum Nanoparticle Inclusion into a Carbonized Polymer of Intrinsic Microporosity: Electrochemical Characteristics of a Catalyst for Electroless Hydrogen Peroxide Production

The one-step vacuum carbonization synthesis of a platinum nano-catalyst embedded in a microporous heterocarbon (Pt@cPIM) is demonstrated. A nitrogen-rich polymer of an intrinsic microporosity (PIM) precursor is impregnated with PtCl₆2- to give (after vacuum carbonization at 700 °C) a nitrogen-containing heterocarbon with embedded Pt nanoparticles of typically 1⁻4 nm diameter (with some particles up to 20 nm diameter). The Brunauer-Emmett-Teller (BET) surface area of this hybrid material is 518 m² g-1 (with a cumulative pore volume of 1.1 cm³ g-1) consistent with the surface area of the corresponding platinum-free heterocarbon. In electrochemical experiments, the heterocarbon-embedded nano-platinum is observed as reactive towards hydrogen oxidation, but essentially non-reactive towards bigger molecules during methanol oxidation or during oxygen reduction. Therefore, oxygen reduction under electrochemical conditions is suggested to occur mainly via a 2-electron pathway on the outer carbon shell to give H₂O₂. Kinetic selectivity is confirmed in exploratory catalysis experiments in the presence of H₂ gas (which is oxidized on Pt) and O₂ gas (which is reduced on the heterocarbon surface) to result in the direct formation of H₂O₂.

Mechanistic Insights into Selective Oxidation of Polyaromatic Compounds using RICO Chemistry

Ruthenium-ion-catalyzed oxidation (RICO) of polyaromatic hydrocarbons (PAHs) has been studied in detail using experimental and computational approaches to explore the reaction mechanism. DFT calculations show that regioselectivity in these reactions can be understood in terms of the preservation of aromaticity in the initial formation of a [3+2] metallocycle intermediate at the most-isolated double bond. We identify two competing pathways: C-C bond cleavage leading to a dialdehyde and C-H activation followed by H migration to the RuO_x complex to give diketones. Experimentally, the oxidation of pyrene and phenanthrene has been carried out in monophasic and biphasic solvent systems. Our results show that diketones are the major product for both phenanthrene and pyrene substrates. These diketone products are shown to be stable under our reaction conditions so that higher oxidation products (acids and their derivatives) are assigned to the competing pathway through the dialdehyde. Experiments using ¹⁸ O-labelled water do show incorporation of oxygen from the solvents into products, but this may take place during the formation of the reactive RuO₄ species rather than directly during PAH oxidation. When the oxidation of pyrene is carried out using D₂ O, a kinetic isotope effect (KIE) is observed implying that water is involved in the rate-determining step leading to the diketone products.

The preparation of large surface area lanthanum based perovskite supports for AuPt nanoparticles: tuning the glycerol oxidation reaction pathway by switching the perovskite B site

Gold and gold alloys, in the form of supported nanoparticles, have been shown over the last three decades to be highly effective oxidation catalysts. Mixed metal oxide perovskites, with their high structural tolerance, are ideal for investigating how changes in the chemical composition of supports affect the catalysts' properties, while retaining similar surface areas, morphologies and metal co-ordinations. However, a significant disadvantage of using perovskites as supports is their high crystallinity and small surface area. We report the use of a supercritical carbon dioxide anti-solvent precipitation methodology to prepare large surface area lanthanum based perovskites, making the deposition of 1 wt% AuPt nanoparticles feasible. These catalysts were used for the selective oxidation of glycerol. By changing the elemental composition of the perovskite B site, we dramatically altered the reaction pathway between a sequential oxidation route to glyceric or tartronic acid and a dehydration reaction pathway to lactic acid. Selectivity profiles were correlated to reported oxygen adsorption capacities of the perovskite supports and also to changes in the AuPt nanoparticle morphologies. Extended time on line analysis using the best oxidation catalyst (AuPt/LaMnO₃) produced an exceptionally high tartronic acid yield. LaMnO₃ produced from alternative preparation methods was found to have lower activities, but gave comparable selectivity profiles to that produced using the supercritical carbon dioxide anti-solvent precipitation methodology.

The selective hydrogenation of furfural over supported palladium nanoparticle catalysts prepared by sol-immobilisation: effect of catalyst support and reaction conditions

Pd-TiO₂ nanoparticles prepared by sol-immobilisation are very active for selective hydrogenation of furfural under mild conditions, and addition of Pt enhances performance to achieve a 95% yield of tetrahydrofurfuryl alcohol.

Investigating the influence of acid sites in continuous methane oxidation with N2O over Fe/MFI zeolites

Methane oxidation using N₂O was carried out with Fe–MFI zeolite catalysts at 300 °C.

Oxidation of Polynuclear Aromatic Hydrocarbons using Ruthenium-Ion-Catalyzed Oxidation: The Role of Aromatic Ring Number in Reaction Kinetics and Product Distribution

Oxidation of aromatic hydrocarbons with differing numbers of fused aromatic rings (2-5), have been studied in two solvent environments (monophasic and biphasic) using ruthenium-ion-catalyzed oxidation (RICO). RICO reduces the aromaticity of the polyaromatic core of the molecule in a controlled manner by selective oxidative ring opening. Moreover, the nature of the solvent system determines the product type and distribution, for molecules with more than two aromatic rings. Competitive oxidation between substrates with different numbers of aromatic rings has been studied in detail. It was found that the rate of polyaromatic hydrocarbon oxidation increases with the number of fused aromatic rings. A similar trend was also identified for alkylated aromatic hydrocarbons. The proof-of-concept investigation provides new insight into selective oxidation chemistry for upgrading of polyaromatic molecules.

Supercritical Antisolvent Precipitation of Amorphous Copper-Zinc Georgeite and Acetate Precursors for the Preparation of Ambient-Pressure Water-Gas-Shift Copper/Zinc Oxide Catalysts

A series of copper–zinc acetate and zincian georgeite precursors have been produced by supercritical CO₂ antisolvent (SAS) precipitation as precursors to Cu/ZnO catalysts for the water gas shift (WGS) reaction. The amorphous materials were prepared by varying the water/ethanol volumetric ratio in the initial metal acetate solutions. Water addition promoted georgeite formation at the expense of mixed metal acetates, which are formed in the absence of the water co‐solvent. Optimum SAS precipitation occurs without water to give high surface areas, whereas high water content gives inferior surface areas and copper–zinc segregation. Calcination of the acetates is exothermic, producing a mixture of metal oxides with high crystallinity. However, thermal decomposition of zincian georgeite resulted in highly dispersed CuO and ZnO crystallites with poor structural order. The georgeite‐derived catalysts give superior WGS performance to the acetate‐derived catalysts, which is attributed to enhanced copper–zinc interactions that originate from the precursor.

A new class of Cu/ZnO catalysts derived from zincian georgeite precursors prepared by co-precipitation

Zincian georgeite, an amorphous copper–zinc hydroxycarbonate, has been prepared by co-precipitation using acetate salts and ammonium carbonate.

Zincian georgeite, an amorphous copper–zinc hydroxycarbonate, has been prepared by co-precipitation using acetate salts and ammonium carbonate. Incorporation of zinc into the georgeite phase and mild ageing conditions inhibits crystallisation into zincian malachite or aurichalcite. This zincian georgeite precursor was used to prepare a Cu/ZnO catalyst, which exhibits a superior performance to a zincian malachite derived catalyst for methanol synthesis and the low temperature water–gas shift (LTS) reaction. Furthermore, the enhanced LTS activity and stability in comparison to that of a commercial Cu/ZnO/Al₂O₃ catalyst, indicates that the addition of alumina as a stabiliser may not be required for the zincian georgeite derived Cu/ZnO catalyst. The enhanced performance is partly attributed to the exclusion of alkali metals from the synthesis procedure, which are known to act as catalyst poisons. The effect of residual sodium on the microstructural properties of the catalyst precursor was investigated further from preparations using sodium carbonate.

The effect of sodium species on methanol synthesis and water–gas shift Cu/ZnO catalysts: utilising high purity zincian georgeite

The effect of sodium species on the physical and catalytic properties of Cu/ZnO catalysts derived from zincian georgeite has been investigated. Catalysts prepared with <100 ppm to 2.1 wt% Na⁺, using a supercritical CO₂ antisolvent technique, were characterised and tested for the low temperature water–gas shift reaction and also CO₂ hydrogenation to methanol. It was found that zincian georgeite catalyst precursor stability was dependent on the Na⁺ concentration, with the 2.1 wt% Na⁺-containing sample uncontrollably ageing to malachite and sodium zinc carbonate. Samples with lower Na⁺ contents (<100–2500 ppm) remained as the amorphous zincian georgeite phase, which on calcination and reduction resulted in similar CuO/Cu particle sizes and Cu surface areas. The aged 2.1 wt% Na⁺ containing sample, after calcination and reduction, was found to comprise of larger CuO crystallites and a lower Cu surface area. However, calcination of the high Na⁺ sample immediately after precipitation (before ageing) resulted in a comparable CuO/Cu particle size to the lower (<100–2500 ppm) Na⁺ containing samples, but with a lower Cu surface area, which indicates that Na⁺ species block Cu sites. Activity of the catalysts for the water–gas shift reaction and methanol yields in the methanol synthesis reaction correlated with Na⁺ content, suggesting that Na⁺ directly poisons the catalyst. In situ XRD analysis showed that the ZnO crystallite size and consequently Cu crystallite size increased dramatically in the presence of water in a syn-gas reaction mixture, showing that stabilisation of nanocrystalline ZnO is required. Sodium species have a moderate effect on ZnO and Cu crystallite growth rate, with lower Na⁺ content resulting in slightly reduced rates of growth under reaction conditions.

Stable amorphous georgeite as a precursor to a high-activity catalyst

Copper and zinc form an important group of hydroxycarbonate minerals that include zincian malachite, aurichalcite, rosasite and the exceptionally rare and unstable--and hence little known and largely ignored--georgeite. The first three of these minerals are widely used as catalyst precursors for the industrially important methanol-synthesis and low-temperature water-gas shift (LTS) reactions, with the choice of precursor phase strongly influencing the activity of the final catalyst. The preferred phase is usually zincian malachite. This is prepared by a co-precipitation method that involves the transient formation of georgeite; with few exceptions it uses sodium carbonate as the carbonate source, but this also introduces sodium ions--a potential catalyst poison. Here we show that supercritical antisolvent (SAS) precipitation using carbon dioxide (refs 13, 14), a process that exploits the high diffusion rates and solvation power of supercritical carbon dioxide to rapidly expand and supersaturate solutions, can be used to prepare copper/zinc hydroxycarbonate precursors with low sodium content. These include stable georgeite, which we find to be a precursor to highly active methanol-synthesis and superior LTS catalysts. Our findings highlight the value of advanced synthesis methods in accessing unusual mineral phases, and show that there is room for exploring improvements to established industrial catalysts.