In Situ Quantification of Reaction Adsorbates in Low-Temperature Methanol Synthesis on a High-Performance Cu/ZnO:Al Catalyst

The Enigma of Methanol Synthesis by Cu/ZnO/Al2O3-Based Catalysts

The activity and durability of the Cu/ZnO/Al2O3 (CZA) catalyst formulation for methanol synthesis from CO/CO2/H2 feeds far exceed the sum of its individual components. As such, this ternary catalytic system is a prime example of synergy in catalysis, one that has been employed for the large scale commercial production of methanol since its inception in the mid 1960s with precious little alteration to its original formulation. Methanol is a key building block of the chemical industry. It is also an attractive energy storage molecule, which can also be produced from CO2 and H2 alone, making efficient use of sequestered CO2. As such, this somewhat unusual catalyst formulation has an enormous role to play in the modern chemical industry and the world of global economics, to which the correspondingly voluminous and ongoing research, which began in the 1920s, attests. Yet, despite this commercial success, and while research aimed at understanding how this formulation functions has continued throughout the decades, a comprehensive and universally agreed upon understanding of how this material achieves what it does has yet to be realized. After nigh on a century of research into CZA catalysts, the purpose of this Review is to appraise what has been achieved to date, and to show how, and how far, the field has evolved. To do so, this Review evaluates the research regarding this catalyst formulation in a chronological order and critically assesses the validity and novelty of various hypotheses and claims that have been made over the years. Ultimately, the Review attempts to derive a holistic summary of what the current body of literature tells us about the fundamental sources of the synergies at work within the CZA catalyst and, from this, suggest ways in which the field may yet be further advanced.

Enhanced Methanol Synthesis from CO2 Hydrogenation Achieved by Tuning the Cu-ZnO Interaction in ZnO/Cu2O Nanocube Catalysts Supported on ZrO2 and SiO2

The nature of the Cu-Zn interaction and especially the role of Zn in Cu/ZnO catalysts used for methanol synthesis from CO2 hydrogenation are still debated. Migration of Zn onto the Cu surface during reaction results in a Cu-ZnO interface, which is crucial for the catalytic activity. However, whether a Cu-Zn alloy or a Cu-ZnO structure is formed and the transformation of this interface under working conditions demand further investigation. Here, ZnO/Cu2O core-shell cubic nanoparticles with various ZnO shell thicknesses, supported on SiO2 or ZrO2 were prepared to create an intimate contact between Cu and ZnO. The evolution of the catalyst's structure and composition during and after the CO2 hydrogenation reaction were investigated by means of operando spectroscopy, diffraction, and ex situ microscopy methods. The Zn loading has a direct effect on the oxidation state of Zn, which, in turn, affects the catalytic performance. High Zn loadings, resulting in a stable ZnO catalyst shell, lead to increased methanol production when compared to Zn-free particles. Low Zn loadings, in contrast, leading to the presence of metallic Zn species during reaction, showed no significant improvement over the bare Cu particles. Therefore, our work highlights that there is a minimum content of Zn (or optimum ZnO shell thickness) needed to activate the Cu catalyst. Furthermore, in order to minimize catalyst deactivation, the Zn species must be present as ZnOx and not metallic Zn or Cu-Zn alloy, which is undesirably formed during the reaction when the precatalyst ZnO overlayer is too thin.

Doubling the life of Cu/ZnO methanol synthesis catalysts via use of Si as a structural promoter to inhibit sintering

Cu/ZnO/Al2O3 catalysts used to synthesize methanol undergo extensive deactivation during use, mainly due to sintering. Here, we report on formulations wherein deactivation has been substantially reduced by the targeted use of a small quantity of a Si-based promoter, resulting in accrued activity benefits that can exceed a factor of 1.8 versus unpromoted catalysts. This enhanced stability also provides longer lifetimes, up to double that of prior generation catalysts. Detailed characterization of a library of aged catalysts has allowed the most important deactivation mechanisms to be established and the chemical state of the silicon promoter to be identified. We show that silicon is incorporated within the ZnO lattice, providing a pronounced improvement in the hydrothermal stability of this component. These findings have important implications for sustainable methanol production from H2 and CO2.

Shape-Dependent CO2 Hydrogenation to Methanol over Cu2O Nanocubes Supported on ZnO

The hydrogenation of CO₂ to methanol over Cu/ZnO-based catalysts is highly sensitive to the surface composition and catalyst structure. Thus, its optimization requires a deep understanding of the influence of the pre-catalyst structure on its evolution under realistic reaction conditions, including the formation and stabilization of the most active sites. Here, the role of the pre-catalyst shape (cubic vs spherical) in the activity and selectivity of ZnO-supported Cu nanoparticles was investigated during methanol synthesis. A combination of ex situ, in situ, and operando microscopy, spectroscopy, and diffraction methods revealed drastic changes in the morphology and composition of the shaped pre-catalysts under reaction conditions. In particular, the rounding of the cubes and partial loss of the (100) facets were observed, although such motifs remained in smaller domains. Nonetheless, the initial pre-catalyst structure was found to strongly affect its subsequent transformation in the course of the CO₂ hydrogenation reaction and activity/selectivity trends. In particular, the cubic Cu particles displayed an increased activity for methanol production, although at the cost of a slightly reduced selectivity when compared to similarly sized spherical particles. These findings were rationalized with the help of density functional theory calculations.

The state of zinc in methanol synthesis over a Zn/ZnO/Cu(211) model catalyst

The active chemical state of zinc (Zn) in a zinc-copper (Zn-Cu) catalyst during carbon dioxide/carbon monoxide (CO₂/CO) hydrogenation has been debated to be Zn oxide (ZnO) nanoparticles, metallic Zn, or a Zn-Cu surface alloy. We used x-ray photoelectron spectroscopy at 180 to 500 millibar to probe the nature of Zn and reaction intermediates during CO₂/CO hydrogenation over Zn/ZnO/Cu(211), where the temperature is sufficiently high for the reaction to rapidly turn over, thus creating an almost adsorbate-free surface. Tuning of the grazing incidence angle makes it possible to achieve either surface or bulk sensitivity. Hydrogenation of CO₂ gives preference to ZnO in the form of clusters or nanoparticles, whereas in pure CO a surface Zn-Cu alloy becomes more prominent. The results reveal a specific role of CO in the formation of the Zn-Cu surface alloy as an active phase that facilitates efficient CO₂ methanol synthesis.

Chemical Batteries with CO2

Efforts to obtain raw materials from CO2 by catalytic reduction as a means of combating greenhouse gas emissions are pushing the boundaries of the chemical industry. The dimensions of modern energy regimes, on the one hand, and the necessary transport and trade of globally produced renewable energy, on the other, will require the use of chemical batteries in conjunction with the local production of renewable electricity. The synthesis of methanol is an important option for chemical batteries and will, for that reason, be described here in detail. It is also shown that the necessary, robust, and fundamental understanding of processes and the material science of catalysts for the hydrogenation of CO2 does not yet exist.

Direct Synthesis of Dimethyl Ether from CO2: Recent Advances in Bifunctional/Hybrid Catalytic Systems

Dimethyl ether (DME) is a versatile raw material and an interesting alternative fuel that can be produced by the catalytic direct hydrogenation of CO2. Recently, this process has attracted the attention of the industry due to the environmental benefits of CO2 elimination from the atmosphere and its lower operating costs with respect to the classical, two-step synthesis of DME from syngas (CO + H2). However, due to kinetics and thermodynamic limits, the direct use of CO2 as raw material for DME production requires the development of more effective catalysts. In this context, the objective of this review is to present the latest progress achieved in the synthesis of bifunctional/hybrid catalytic systems for the CO2-to-DME process. For catalyst design, this process is challenging because it should combine metal and acid functionalities in the same catalyst, in a correct ratio and with controlled interaction. The metal catalyst is needed for the activation and transformation of the stable CO2 molecules into methanol, whereas the acid catalyst is needed to dehydrate the methanol into DME. Recent developments in the catalyst design have been discussed and analyzed in this review, presenting the different strategies employed for the preparation of novel bifunctional catalysts (physical/mechanical mixing) and hybrid catalysts (co-precipitation, impregnation, etc.) with improved efficiency toward DME formation. Finally, an outline of future prospects for the research and development of efficient bi-functional/hybrid catalytic systems will be presented.

Hydrogen in methanol catalysts by neutron imaging

Although of pivotal importance in heterogeneous hydrogenation reactions, the amount of hydrogen on catalysts during reactions is seldom known. We demonstrate the use of neutron imaging to follow and quantify hydrogen containing species in Cu/ZnO catalysts operando during methanol synthesis. The steady-state measurements reveal that the amount of hydrogen containing intermediates is related to the reaction yields of CO and methanol, as expected from simple considerations of the likely reaction mechanism. The time-resolved measurements indicate that these intermediates, despite indispensable within the course of the reaction, slow down the overall reaction steps. Hydrogen-deuterium exchange experiments indicate that hydrogen reduction of Cu/ZnO nano-composites modifies the catalyst in such a way that at operating temperatures, hydrogen is dynamically absorbed in the ZnO-nanoparticles. This explains the extraordinary good catalysis of copper if supported on ZnO by its ability to act as a hydrogen reservoir supplying hydrogen to the surface covered by CO2, intermediates, and products during catalysis.

Methanol-Assisted Autocatalysis in Catalytic Methanol Synthesis

Catalytic methanol synthesis is one of the major processes in the chemical industry and may grow in importance, as methanol produced from CO₂ and sustainably derived H₂ are envisioned to play an important role as energy carriers in a future low-CO₂ -emission society. However, despite the widespread use, the reaction mechanism and the nature of the active sites are not fully understood. Here we report that methanol synthesis at commercially applied conditions using the industrial Cu/ZnO/Al₂ O₃ catalyst is dominated by a methanol-assisted autocatalytic reaction mechanism. We propose that the presence of methanol enables the hydrogenation of surface formate via methyl formate. Autocatalytic acceleration of the reaction is also observed for Cu supported on SiO₂ although with low absolute activity, but not for Cu/Al₂ O₃ catalysts. The results illustrate an important example of autocatalysis in heterogeneous catalysis and pave the way for further understanding, improvements, and process optimization of industrial methanol synthesis.

Intercalation of laminar Cu-Al LDHs with molecular TCPP(M) (M = Zn, Co, Ni, and Fe) towards high-performance CO2 hydrogenation catalysts

A confined space is broadly applied to enhance the dispersion and limit the aggregation of catalytically active sites, especially at high temperatures. In this work, we provided an efficient approach to immobilize transition metal ions (e.g., Zn2+, Co2+, Ni2+, and Fe2+) into the confined space of laminar Cu-Al layered double hydroxides (LDHs) using a range of molecular metalloporphyrins (viz., TCPP(M)) as shuttles. The deprotonated TCPP(M) not only provides nitrogen-based coordination sites to anchor a series of transition metal ions, but also intercalates and diffuses facilely into the interlayer gallery of LDHs by ion exchange. The obtained TCPP(M)@Cu-Al LDHs were then used as solid precursors for the fabrication of a series of heterogeneous catalysts for CO2 hydrogenation via high-temperature calcination. Two restriction forces contributed to the enhanced dispersion of the active species over the catalyst surface structures. Remarkably, the transition metals positioned within the confined space of LDHs significantly affected the catalytic performance of CO2 hydrogenation. Mainly CO, methanol, and methane were found as the C1 products, and their selectivities are highly dependent on the reaction intermediates, as suggested by the in situ DRIFTS study. Moreover, the designed catalysts fabricated via molecular TCPP(M) intercalation exhibited much better performance than the conventional catalysts derived from surface-supported CA-LDHs, due to their better metal dispersion and smaller particle size.

In[Ba3Cl3F6]: a novel infrared-transparent molecular sieve constructed by halides

A new metal halide molecular sieve In[Ba₃Cl₃F₆] has been synthesized by the hydrothermal method. It possesses a wide transparent window from 0.366 to 22 μm and can exhibit the good adsorption–desorption property.