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.

Unveiling the Surface Structure of ZnO Nanorods and H2 Activation Mechanisms with 17O NMR Spectroscopy

ZnO plays a very important role in many catalytic processes involving H₂, yet the details on their interactions and H₂ activation mechanism are still missing, owing to the lack of a characterization method that provides resolution at the atomic scale and follows the fate of oxide surface species. Here, we apply ¹⁷O solid-state NMR spectroscopy in combination with DFT calculations to unravel the surface structure of ZnO nanorods and explore the H₂ activation process. We show that six different types of oxygen ions in the surface and subsurface of ZnO can be distinguished. H₂ undergoes heterolytic dissociation on three-coordinated surface zinc and oxygen ions, while the formed hydride species migrate to nearby oxygen species, generating a second hydroxyl site. When oxygen vacancies are present, homolytic dissociation of H₂ occurs and zinc hydride species form from the vacancies. Reaction mechanisms on oxide surfaces can be explored in a similar manner.

Elucidation of Cu-Zn Surface Alloying on Cu(997) by Machine-Learning Molecular Dynamics

The Cu-Zn surface alloy has been extensively involved in the investigation of the true active site of Cu/ZnO/Al2O3, the industrial catalyst for methanol synthesis which remains under controversy. The challenge lies in capturing the interplay between the surface and reaction under operating conditions, which can be overcome given that the explicit dynamics of the system is known. To provide a better understanding of the dynamic of Cu-Zn surface at the atomic level, the structure and the formation process of the Cu-Zn surface alloy on Cu(997) were investigated by machine-learning molecular dynamics (MD). Gaussian process regression aided with on-the-fly learning was employed to build the force field used in the MD. The simulation reveals atomistic details of the alloying process, that is, the incorporation of deposited Zn adatoms to the Cu substrate. The surface alloying is found to start at upper and lower terraces near the step edge, which emphasize the role of steps and kinks in the alloying. The incorporation of Zn at the middle terrace was found at the later stage of the simulation. The rationalization of alloying behavior was performed based on statistics and barriers of various elementary events that occur during the simulation. It was observed that the alloying scheme at the upper terrace is dominated by the confinement of Zn step adatoms by other adatoms, highlighting the importance of step fluctuations in the alloying process. On the other hand, the alloying scheme at the lower terrace is dominated by direct exchange between the Zn step adatom and the Cu atom underneath. The alloying at the middle terrace is dominated by the wave deposition mechanism and deep confinement of Zn adatoms. The short propagation of alloyed Zn in the middle terrace was observed to proceed by means of indirect exchange instead of local exchange as proposed in the previous scanning tunneling microscopy (STM) observation. The comparison of migration rate and activation energies to the result of STM observation is also made. We have found that at a certain distance from the surface, the STM tip significantly affects the elementary events such as vacancy formation and direct exchange.

Co-Production of Methanol and Methyl Formate via Catalytic Hydrogenation of CO2 over Promoted Cu/ZnO Catalyst Supported on Al2O3 and SBA-15

Cu/ZnO catalysts promoted with Mn, Nb and Zr, in a 1:1:1 ration, and supported on Al2O3 (CZMNZA) and SBA-15 (CZMNZS) were synthesized using an impregnation method. The catalytic performance of methanol synthesis from CO2 hydrogenation was investigated in a fixed-bed reactor at 250 °C, 22.5 bar, GHSV 10,800 mL/g·h and H2/CO2 ratio of 3. The CZMNZA catalyst resulted in higher CO2 conversion and MeOH selectivity of 7.22% and 32.10%, respectively, despite having a lower BET surface area and pore volume compared to CZMNZS. Methyl formate is the major product obtained over both types of catalysts. The CZMNZA is a promising catalyst for co-producing methanol and methyl formate via the CO2 hydrogenation reaction.

Hydrogenation of Formate Species Using Atomic Hydrogen on a Cu(111) Model Catalyst

The reaction mechanism of the CH₃OH synthesis by the hydrogenation of CO₂ on Cu catalysts is unclear because of the challenge in experimentally detecting reaction intermediates formed by the hydrogenation of adsorbed formate (HCOO_a). Thus, the objective of this study is to clarify the reaction mechanism of the CH₃OH synthesis by establishing the kinetic natures of intermediates formed by the hydrogenation of adsorbed HCOO_a on Cu(111). We exposed HCOO_a on Cu(111) to atomic hydrogen at low temperatures of 200-250 K and observed the species using infrared reflection absorption (IRA) spectroscopy and temperature-programmed desorption (TPD) studies. In the IRA spectra, a new peak was observed upon the exposure of HCOO_a on Cu(111) to atomic hydrogen at 200 K and was assigned to the adsorbed dioxymethylene (H₂COO_a) species. The intensity of the new peak gradually decreased with heating from 200 to 290 K, whereas the IR peaks representing HCOO_a species increased correspondingly. In addition, small amounts of formaldehyde (HCHO), which were formed by the exposure of HCOO_a species to atomic hydrogen, were detected in the TPD studies. Therefore, H₂COO_a is formed via hydrogenation by atomic hydrogen, which thermally decomposes at ∼250 K on Cu(111). We propose a potential diagram of the CH₃OH synthesis via H₂COO_a from CO₂ on Cu surfaces, with the aid of density functional theory calculations and literature data, in which the hydrogenation of bidentate HCOO_a to H₂COO_a is potentially the rate-determining step and accounts for the apparent activation energy of the methanol synthesis from CO₂ on Cu surfaces.

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.

Atomically Dispersed Zn-Stabilized Niδ+ Enabling Tunable Selectivity for CO2 Hydrogenation

For a heterogeneous catalytic process, the performance of catalysts could be improved by modifying the active metal with a second element. Determining the enhanced mechanism of the second element is essential to the rational design of catalysts. In this work, Zn was introduced as a second element into Ni/ZrO₂ for CO₂ hydrogenation. In contrast to Ni/ZrO₂ , the selectivity of NiZn/ZrO₂ is observed to shift from CH₄ to CO. A series of structural characterization results reveals that Zn is atomically dispersed in the NiO and ZrO₂ phases as NiZnO_x and ZnZrO_x , respectively during CO₂ hydrogenation, stabilizing a higher valence state of Ni (Ni^δ+ ) under a hydrogenation atmosphere over Ni-O-Zn site and thus promoting the generation of CO. These findings shed light on the O-mediated bimetallic effect of NiZn/ZrO₂ and bring new insight into the rational design of more efficient heterogeneous catalysts.

Catalytic Hydrogenation of CO2 to Methanol: A Review

High-efficiency utilization of CO2 facilitates the reduction of CO2 concentration in the global atmosphere and hence the alleviation of the greenhouse effect. The catalytic hydrogenation of CO2 to produce value-added chemicals exhibits attractive prospects by potentially building energy recycling loops. Particularly, methanol is one of the practically important objective products, and the catalytic hydrogenation of CO2 to synthesize methanol has been extensively studied. In this review, we focus on some basic concepts on CO2 activation, the recent research advances in the catalytic hydrogenation of CO2 to methanol, the development of high-performance catalysts, and microscopic insight into the reaction mechanisms. Finally, some thinking on the present research and possible future trend is presented.

Efficient CO2 catalytic hydrogenation over CuOx–ZnO/silicalite-1 with stable Cu+ species

Efficient and stable CuOx–ZnO/S-1 catalysts for CO2 hydrogenation were inexpensively prepared, in which the ZnO–Cu2O interface and silanol nests play key roles.

The unique interplay between copper and zinc during catalytic carbon dioxide hydrogenation to methanol

In spite of numerous works in the field of chemical valorization of carbon dioxide into methanol, the nature of high activity of Cu/ZnO catalysts, including the reaction mechanism and the structure of the catalyst active site, remains the subject of intensive debate. By using high-pressure operando techniques: steady-state isotope transient kinetic analysis coupled with infrared spectroscopy, together with time-resolved X-ray absorption spectroscopy and X-ray powder diffraction, and supported by electron microscopy and theoretical modeling, we present direct evidence that zinc formate is the principal observable reactive intermediate, which in the presence of hydrogen converts into methanol. Our results indicate that the copper–zinc alloy undergoes oxidation under reaction conditions into zinc formate, zinc oxide and metallic copper. The intimate contact between zinc and copper phases facilitates zinc formate formation and its hydrogenation by hydrogen to methanol.

In spite of numerous works, the nature of high activity of Cu/ZnO catalyst in methanol synthesis remains the subject of intensive debate. Here, the authors study the carbon dioxide hydrogenation mechanism using high-pressure operando techniques which allow them to unify different, seemingly contradicting, models.

State of the art and perspectives in heterogeneous catalysis of CO2 hydrogenation to methanol

The ever-increasing amount of anthropogenic carbon dioxide (CO₂) emissions has resulted in great environmental impacts, the heterogeneous catalysis of CO₂ hydrogenation to methanol is of great significance.

Influence of CO on the Activation, O-Vacancy Formation, and Performance of Au/ZnO Catalysts in CO2 Hydrogenation to Methanol

The impact of CO on the activation and reaction characteristics of Au/ZnO catalysts in methanol synthesis from a CO₂/H₂ mixture was studied by kinetic, near ambient pressure X-ray photoelectron spectroscopy and X-ray absorption spectroscopy at the O K-edge, together with in situ Foureir transform infrared measurements. Transient measurements under up to industrial reaction conditions (50 bar, 240 °C) show a pronounced transient increase of the activity for methanol formation from CO₂/H₂ after exposure to a CO/H₂ reaction gas mixture, while the steady-state activity is similar to that observed directly after oxidative pretreatment. For the reaction in CO/H₂, the much longer activation phase is accompanied by formation of CO₂ due to reaction of CO with the ZnO catalyst support. This leads to O-vacancy formation on the support at an extent significantly higher than in CO₂/H₂. The consequences of these findings on the mechanistic understanding of methanol formation from CO₂/H₂ on Au/ZnO and for ZnO-supported catalysts in general are discussed.

Steps Control the Dissociation of CO2 on Cu(100)

CO₂ reduction reactions, which provide one route to limit the emission of this greenhouse gas, are commonly performed over Cu-based catalysts. Here, we use ambient pressure X-ray photoelectron spectroscopy together with density functional theory to obtain an atomistic understanding of the dissociative adsorption of CO₂ on Cu(100). We find that the process is dominated by the presence of steps, which promote both a lowering of the dissociation barrier and an efficient separation between adsorbed O and CO, reducing the probability for recombination. The identification of steps as sites for efficient CO₂ dissociation provides an understanding that can be used in the design of future CO₂ reduction catalysts.