The role of ZnO in Cu/ZnO methanol synthesis catalysts

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.

Multiobjective Bayesian Optimization Framework for the Synthesis of Methanol from Syngas Using Interpretable Gaussian Process Models

Methanol production has gained considerable interest on the laboratory and industrial scale as it is a renewable fuel and an excellent hydrogen energy storehouse. The formation of synthesis gas (CO/H₂) and the conversion of synthesis gas to methanol are the two basic catalytic processes used in methanol production. Machine learning (ML) approaches have recently emerged as powerful tools in reaction informatics. Inspired by these, we employ Gaussian process regression (GPR) to the model conversion of carbon monoxide (CO) and selectivity of the methanol product using data sets obtained from experimental investigations to capture uncertainty in prediction values. The results indicate that the proposed GPR model can accurately predict CO conversion and methanol selectivity as compared to other ML models. Further, the factors that influence the predictions are identified from the best GPR model employing "Shapley Additive exPlanations" (SHAP). After interpretation, the essential input features are found to be the inlet mole fraction of CO (Y(CO, in)) and the net inlet flow rate (Fin(nL/min)) for our best prediction GPR models, irrespective of our data sets. These interpretable models are employed for Bayesian optimization in a weighted multiobjective framework to obtain the optimal operating points, namely, maximization of both selectivity and conversion.

Electronic Metal-Support Interactions Between CuxO and ZnO for CuxO/ZnO Catalysts With Enhanced CO Oxidation Activity

Metal-support interaction has been one of the main topics of research on supported catalysts all the time. However, many other factors including the particle size, shape and chemical composition can have significant influences on the catalytic performance when considering the role of metal-support interaction. Herein, we have designed a series of CuxO/ZnO catalysts as examples to quantitatively investigate how the metal-support interaction influences the catalytic performance. The electronic metal-support interactions between CuxO and ZnO were regulated successfully without altering the structure of CuxO/ZnO catalyst. Due to the lower work function of ZnO, electrons would transfer from ZnO to CuO, which is favorable for the formation of higher active Cu species. Combined experimental and theoretical calculations revealed that electron-rich interface result from interaction was favorable for the adsorption of oxygen and CO oxidation reaction. Such strategy represents a new direction to boost the catalytic activity of supported catalysts in various applications.

Methanol Synthesis by Hydrogenation of Hybrid CO2 -CO Feeds

The impact of carbon monoxide on CO₂ -to-methanol catalysts has been scarcely investigated, although CO will comprise up to half of the carbon feedstock, depending on the origin of CO₂ and process configuration. In this study, copper-based systems and ZnO-ZrO₂ are assessed in cycling experiments with hybrid CO₂ -CO feeds and their CO sensitivity is compared to In₂ O₃ -based materials. All catalysts are found to be promoted upon CO addition. Copper-based systems are intrinsically more active in CO hydrogenation and profit from exploiting this carbon source for methanol production, whereas CO induces surplus formation of oxygen vacancies (i. e., the catalytic sites) on ZnO-ZrO₂ , as in In₂ O₃ -based systems. Mild-to-moderate deactivation occurs upon re-exposure to CO₂ -rich streams, owing to water-induced sintering for all catalysts except ZnO-ZrO₂ , which responds reversibly to feed variations, likely owing to its more hydrophobic nature and the atomic mixing of its metal components. Catalytic systems are categorized for operation in hybrid CO₂ -CO feeds, emphasizing the significance of catalyst and process design to foster advances in CO₂ utilization technologies.

Investigation on the promotional role of Ga2O3 on the CuO-ZnO/HZSM-5 catalyst for CO2 hydrogenation

Dimethyl ether (DME) can be directly synthesized from carbon dioxide and hydrogen by mixing methanol synthesis catalysts and methanol dehydration catalysts. The activity and selectivity of the catalyst can be greatly affected by the promoter; herein, we presented a series of CuO-ZnO-Ga2O3/HZSM-5 hybrid catalysts, which were prepared by the coprecipitation method. The effect of the Ga2O3 content on the structure and performance of the Ga-promoted Cu-ZnO/HZSM-5 based catalysts was thoroughly investigated. The results showed that the addition of Ga2O3 significantly increased specific surface areas and Cu areas, decreased the size of Cu particles, maintained the proportion of Cu+/Cu0 on the surface of the catalyst, and strengthened the metal-support interaction, resulting in high catalytic performance.

Atomic Layer Deposition of ZnO on Mesoporous Silica: Insights into Growth Behavior of ZnO via In-Situ Thermogravimetric Analysis

ZnO is a remarkable material with many applications in electronics and catalysis. Atomic layer deposition (ALD) of ZnO on flat substrates is an industrially applied and well-known process. Various studies describe the growth of ZnO layers on flat substrates. However, the growth characteristics and reaction mechanisms of atomic layer deposition of ZnO on mesoporous powders have not been well studied. This study investigates the ZnO ALD process based on diethylzinc (DEZn) and water with silica powder as substrate. In-situ thermogravimetric analysis gives direct access to the growth rates and reaction mechanisms of this process. Ex-situ analytics, e.g., N₂ sorption analysis, XRD, XRF, HRTEM, and STEM-EDX mapping, confirm deposition of homogenous and thin films of ZnO on SiO₂. In summary, this study offers new insights into the fundamentals of an ALD process on high surface area powders.

CO2 Hydrogenation to Methanol over La2O3-Promoted CuO/ZnO/Al2O3 Catalysts: A Kinetic and Mechanistic Study

The hydrogenation of CO2 to methanol has been investigated over CuO/ZnO/Al2O3 (CZA) catalysts, where a part of the Al2O3 (0, 25, 50, 75, or 100%) was substituted by La2O3. Results of catalytic performance tests obtained at atmospheric pressure showed that the addition of La2O3 generally resulted in a decrease of CO2 conversion and in an increase of methanol selectivity. Optimal results were obtained for the CZA-La50 catalyst, which exhibited a 30% higher yield of methanol, compared to the un-promoted sample. This was attributed to the relatively high specific surface area and porosity of this material, the creation of basic sites of moderate strength, which enhance adsorption of CO2 and intermediates that favor hydrogenation steps, and the ability of the catalyst to maintain a large part of the copper in its metallic form under reaction conditions. The reaction mechanism was studied with the use of in situ infrared spectroscopy (DRIFTS). It was found that the reaction proceeded with the intermediate formation of surface formate and methoxy species and that both methanol and CO were mainly produced via a common formate intermediate species. The kinetic behavior of the best performing CZA-La50 catalyst was investigated in the temperature range 190–230 °C as a function of the partial pressures of H2 (0.3–0.9 atm) and CO2 (0.05–0.20 atm), and a kinetic model was developed, which described the measured reaction rates satisfactorily.

Microemulsion solventing-out co-precipitation strategy for fabricating highly active Cu–ZnO/Al2O3 dual site catalysts for reverse water gas shift

A new strategy for the preparation of dual site catalysts is introduced, which combines microemulsion technology and anti-solvent extraction technology.

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.

Methanol Synthesis from CO2 Hydrogenation

In the future we will be phasing out the use of fossil fuels in favour of more sustainable forms of energy, especially solar derived forms such as hydroelectric, wind and photovoltaic. However, due to the variable nature of the latter sources which depend on time of day, and season of the year, we also need to have a way of storing such energy at peak production times for use in times of low production. One way to do this is to convert such energy into chemical energy, and the principal way considered at present is the production of hydrogen. Although this may be achieved directly in the future via photocatalytic water splitting, at present it is electrolytic production which dominates thinking. In turn, it may well be important to store this hydrogen in an energy dense liquid form such as methanol or ammonia. In this brief review it is emphasised that CO2 is the microscopic carbon source for current industrial methanol synthesis, operating through the surface formate intermediate, although when using CO in the feed, it is CO which is hydrogenated at the global scale. However, methanol can be produced from pure CO2 and hydrogen using conventional and novel types of catalysts. Examples of such processes, and of a demonstrator plant in construction, are given, which utilize CO2 (which would otherwise enter the atmosphere directly) and hydrogen which can be produced in a sustainable manner. This is a fast-evolving area of science and new ideas and processes will be developed in the near future.

Challenges in the Greener Production of Formates/Formic Acid, Methanol, and DME by Heterogeneously Catalyzed CO2 Hydrogenation Processes

The recent advances in the development of heterogeneous catalysts and processes for the direct hydrogenation of CO₂ to formate/formic acid, methanol, and dimethyl ether are thoroughly reviewed, with special emphasis on thermodynamics and catalyst design considerations. After introducing the main motivation for the development of such processes, we first summarize the most important aspects of CO₂ capture and green routes to produce H₂. Once the scene in terms of feedstocks is introduced, we carefully summarize the state of the art in the development of heterogeneous catalysts for these important hydrogenation reactions. Finally, in an attempt to give an order of magnitude regarding CO₂ valorization, we critically assess economical aspects of the production of methanol and DME and outline future research and development directions.

Enhancing the Catalytic Activity of Zn-Containing Magnetic Oxides in a Methanol Synthesis: Identifying the Key Factors

A new family of Ni-, Co-, and Cr-doped Zn-containing magnetic oxide nanoparticles (NPs) stabilized by polyphenylquinoxaline (PPQ) and hyperbranched pyridylphenylene polymer (PPP) has been developed. These NPs have been synthesized by thermal decomposition of Zn and doping metal acetylacetonates in the reaction solution of preformed magnetite NPs, resulting in single-crystal NPs with spinel structure. For the PPQ-capped NPs, it was demonstrated that all three types of metal species (Fe, Zn, and a doping metal) reside within the same NPs, the surface of which is enriched with Zn and a doping metal, while the deeper layers are enriched with Fe. The Cr-doped NPs at the high Cr loading are an exception due to favored deposition of Cr on magnetite located in the NP depth. The PPP-capped NPs exhibit similar morphology and crystallinity; however, the detailed study of the NP composition was barred due to the high PPP amount retained on the NP surface. The catalyst testing in syngas conversion to methanol demonstrated outstanding catalytic properties of doped Zn-containing magnetic oxides, whose activities are dependent on the doping metal content and on the stabilizing polymer. The PPP stabilization allows for better access to the catalytic species due to the open and rigid polymer architecture and most likely optimized distribution of doping species. Repeat experiments carried out after magnetic separation of catalysts from the reaction mixture showed excellent catalyst stability even after five consecutive catalytic runs.

The role of copper particle size in low pressure methanol synthesis via CO2 hydrogenation over Cu/ZnO catalysts

The size of copper nanoparticles exerts a significant influence on the selectivity of the hydrogenation of carbon dioxide to methanol.

Effect of the components' interface on the synthesis of methanol over Cu/ZnO from CO2/H2: a microkinetic analysis based on DFT + U calculations

During the methanol synthesis over Cu/ZnO catalysts, the phase interface was observed to supply spillover hydrogen to active copper sites.

The active site of methanol synthesis over Cu/ZnO/Al2O3 industrial catalysts

One of the main stumbling blocks in developing rational design strategies for heterogeneous catalysis is that the complexity of the catalysts impairs efforts to characterize their active sites. We show how to identify the crucial atomic structure motif for the industrial Cu/ZnO/Al(2)O(3) methanol synthesis catalyst by using a combination of experimental evidence from bulk, surface-sensitive, and imaging methods collected on real high-performance catalytic systems in combination with density functional theory calculations. The active site consists of Cu steps decorated with Zn atoms, all stabilized by a series of well-defined bulk defects and surface species that need to be present jointly for the system to work.

High-pressure CO adsorption on Cu-based catalysts: Zn-induced formation of strongly bound CO monitored by ATR-IR spectroscopy

CO adsorption at 1 MPa on Cu-Zn stearate colloids and supported Cu catalysts was studied in situ by attenuated total reflection infrared (ATR-IR) spectroscopy. Subsequent to thorough reduction by H(2), the IR band at 2110-2070 cm(-1) due to linearly adsorbed CO on clean metallic Cu was always observed initially on all Cu catalysts. During the exposure of Zn-containing samples to CO at high pressure, a new IR band at ca. 1975 cm(-1) appeared in addition and increased in intensity even at room temperature. The detailed analysis of the IR spectra showed that the new IR band at ca. 1975 cm(-1) was not related to coadsorbed carbonate/formate-like species, but to the content of Zn in the samples. This IR band was found to be more stable than that at 2110-2070 cm(-1) during purging with inert gas. It disappeared quickly in synthetic air, pointing to a strongly reduced state of the Zn-containing Cu catalysts achieved during high-pressure CO exposure. It is suggested that CO can reduce ZnO to Zn in the presence of Cu, resulting in the formation of a CuZn(x) surface alloy. As the CO species with the characteristic IR band at ca. 1975 cm(-1) binds more strongly to this CuZn(x) alloy than the linearly adsorbed CO to pure Cu, it is suggested to be adsorbed on a bridge site.