The Influence of ZnO−ZrO ₂ Interface in Hydrogenation of CO ₂ to CH ₃ OH

Docherty S.R., Copéret C.

Processes that rely on heterogeneous catalysts underpin the production of bulk chemicals and fuels. In spite of this, understanding of the interplay between the structure and reactivity of these complex materials remains elusive-rendering rational improvement of existing systems challenging. Herein, we describe efforts to understand complex materials capable of selective thermochemical conversion of CO2 to methanol using a surface organometallic chemistry (SOMC) approach. In particular, we focus on the remarkable, but often subtle, roles of metal-metal synergy and metal-support interfaces in determining the reactivity of many different systems for the conversion of CO2 to methanol. Specifically, we explore synthetic and analytical strategies for the systematic study of synergistic behaviors of multi-component catalytic systems in the context of CO2 hydrogenation, and we discuss how the insights obtained can inform the design of materials. We also address limitations of the approach employed and opportunities to expand upon the observations emerging from this work, before attempting to establish transposable and generalizable trends for Cu-based catalysts and beyond.

Meierhofer F., Fritsching U.

The gas synthesis of nanoparticles has gained major interest by different industries and research groups for the development of new materials and their subsequent implementation in numerous devices...

Lam E., Noh G., Larmier K., Safonova O.V., Copéret C.

• Cu-Zn/SiO 2 based heterogeneous catalyst for CO 2 reduction via surface organometallic chemistry. • Small and narrowly distributed CuZn x nanoparticles. • Reversible alloying-dealloying process. • Improved MeOH selectivity. • Formation of Zn II and CuZn x by in situ XAS. The hydrogenation of CO 2 to CH 3 OH is mostly performed by a catalyst consisting mainly of copper and zinc (Cu/ZnO/Al 2 O 3 ). Here, Cu-Zn based catalysts are generated using surface organometallic chemistry (SOMC) starting from a material consisting of isolated Zn II surface sites dispersed on SiO 2 – Zn II @SiO 2 . Grafting of [Cu(OtBu)] 4 on the surface silanols available on Zn II @SiO 2 followed by reduction at 500 °C under H 2 generates CuZn x alloy nanoparticles with remaining Zn II sites according to X-ray absorption spectroscopy (XAS). This Cu-Zn/SiO 2 material displays high catalytic activity and methanol selectivity, in particular at higher conversion compared to benchmark Cu/ZnO/Al 2 O 3 and most other catalysts. In situ XAS shows that CuZn x alloy is partially converted into Cu(0) and Zn(II) under reaction conditions, while ex situ solid state nuclear magnetic resonance and infrared spectroscopic studies only indicate the presence of methoxy species and no formate intermediates are detected, in contrast to most Cu-based catalysts. The absence of formate species is consistent with the higher methanol selectivity as recently found for the related Cu-Ga/SiO 2 .

Sha F., Han Z., Tang S., Wang J., Li C.

The increasing atmospheric CO2 level makes CO2 reduction an urgent challenge facing the world. Catalytic transformation of CO2 into chemicals and fuels utilizing renewable energy is one of the promising approaches toward alleviating CO2 emissions. In particular, the selective hydrogenation of CO2 to methanol utilizing renewable hydrogen potentially enables large scale transformation of CO2 . The Cu-based catalysts have been extensively investigated in CO2 hydrogenation. However, it is not only limited by long-term instability but also displays unsatisfactory catalytic performance. The supported metal-based catalysts (Pd, Pt, Au, and Ag) can achieve high methanol selectivity at low temperatures. The mixed oxide catalysts represented by Ma ZrOx (Ma =Zn, Ga, and Cd) solid solution catalysts present high methanol selectivity and catalytic activity as well as excellent stability. This Review focuses on the recent advances in developing Non-Cu-based heterogeneous catalysts and current understandings of catalyst design and catalytic performance. First, the thermodynamics of CO2 hydrogenation to methanol is discussed. Then, the progress in supported metal-based catalysts, bimetallic alloys or intermetallic compounds catalysts, and mixed oxide catalysts is discussed. Finally, a summary and a perspective are presented.

Temvuttirojn C., Poo-arporn Y., Chanlek N., Cheng C.K., Chong C.C., Limtrakul J., Witoon T.

Methanol synthesis from CO2 hydrogenation at high temperatures was investigated over ZnOx/ZrO2 catalysts to illustrate the role of calcination temperatures (600–1000 °C) of the ZrO2 support. Charac...

Noh G., Docherty S.R., Lam E., Huang X., Mance D., Alfke J.L., Copéret C.

The selective hydrogenation of CO2 to CH3OH is a crucial part of efforts to mitigate climate change via the methanol economy. Understanding the nature and role of active sites is essential for desi...

Zhou C., Shi J., Zhou W., Cheng K., Zhang Q., Kang J., Wang Y.

Bifunctional catalysis coupling CO2 to methanol and methanol to hydrocarbons is a promising strategy for the direct hydrogenation of CO2 into high-value chemicals. However, bifunctional catalysts s...

Wang J., Tang C., Li G., Han Z., Li Z., Liu H., Cheng F., Li C.

Hydrogenation of CO2 to methanol utilizing the hydrogen from renewable energy sources offers a promising way to reduce CO2 emissions through the CO2 utilization as a carbon source. However, it is a...

Copéret C.

Heterogeneous catalysts are complex by nature, making particularly difficult to assess the structure of their active sites. Such complexity is inherited in part from their mode of preparation, which typically involves coprecipitation or impregnation of metal salts in aqueous solution, and the associated complex surface chemistries. In this context, surface organometallic chemistry (SOMC) has emerged as a powerful approach to generate well-defined surface species, where the metal sites are introduced by grafting tailored molecular precursors. When combined with thermolytic molecular precursors (TMPs) that can lose their organic moieties quite readily upon thermal treatment, SOMC provides access to supported isolated metal sites with defined oxidation state and nuclearity inherited from the precursor. The resulting surface species bear unusual coordination imposed by the surface that provides them high reactivity in comparison with their molecular precursor. In addition, these molecularly defined species bare strong resemblance with the active sites of supported metal oxides. However, they typically contain a higher proportion of active sites making structure-activity relationship possible. They thus constitute ideal models for this important class of industrial catalysts that are used in numerous applications such as olefin epoxidation (Shell process), olefin metathesis (triolefin process), ethylene polymerization (Phillips catalysts), or propane dehydrogenation (Catofin and related processes). This SOMC/TMP approach can thus provide detailed information about the structure of active sites in industrial catalysts, their mode of initiation and deactivation, as well as the role of the support and specific thermal treatment on the final activity of the catalysts. Nonetheless, these structurally characterized surface sites still exhibit heterogeneous environments borrowed from the support itself, that explain the intrinsic complexity of heterogeneous catalysis. Furthermore, SOMC/TMP can also be used to generate and investigate supported metal nanoparticles. Starting from the well-defined isolated sites, that also contain adjacent surface OH groups, one can graft a second metal and then generate after treatment under hydrogen small and narrowly dispersed alloys or nanoparticles with tailored interfaces that can show improved catalytic performances and are amiable to detailed structure-activity relationships. This approach is illustrated by two case studies: (1) formation of supported copper nanoparticles at tailored interfaces that contain isolated metal sites for the selective hydrogenation of carbon dioxide to methanol, allowing for a detailed understanding of the role of dopants and supports in heterogeneous catalysis, and (2) preparation of highly selective and productive propane dehydrogenation catalysts based on silica-supported Pt xGa y alloy. Overall, this Account shows how the combination of SOMC and TMP helps to generate catalysts, particularly suited for elucidating structural characterization of active sites at a molecular-level which in turn enables structure-activity relationship to be drawn. Such detailed information obtained on well-defined catalysts can then be used to understand complex effects observed in industrial catalysts (effects of supports, additives, dopants, etc.), and to extract information that can then be used to improve them in a more rational way.

Imparato C., Fantauzzi M., Passiu C., Rea I., Ricca C., Aschauer U., Sannino F., D’Errico G., De Stefano L., Rossi A., Aronne A.

The functional properties of metal oxide semiconductors depend on intrinsic and extrinsic defects. The population of intrinsic defects is strongly affected by the synthesis method and subsequent treatments of the material, while extrinsic defects can originate from suitable doping. Stoichiometric ZrO2 is a nonreducible oxide with a large band gap. Therefore, controlling and modulating its defect profile to induce energy states in the band gap is the sole possibility to make it a photocatalyst responsive to visible light. We report a method, based on low temperature sol–gel synthesis coupled with treatments performed in mild conditions, to obtain undoped visible light-responsive ZrO2–x. The electronic structure of these materials is interpreted in relation to their oxygen vacancy defect population. On the basis of a wide set of experimental measurements (X-ray photoelectron, steady-state and time-resolved photoluminescence, electron paramagnetic resonance, and UV–visible diffuse reflectance spectroscopy) a...

Wang Y., Kattel S., Gao W., Li K., Liu P., Chen J.G., Wang H.

The synergistic interaction among different components in complex catalysts is one of the crucial factors in determining catalytic performance. Here we report the interactions among the three components in controlling the catalytic performance of Cu–ZnO–ZrO2 (CZZ) catalyst for CO2 hydrogenation to methanol. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements under the activity test pressure (3 MPa) reveal that the CO2 hydrogenation to methanol on the CZZ catalysts follows the formate pathway. Density functional theory (DFT) calculations agree with the in situ DRIFTS measurements, showing that the ZnO–ZrO2 interfaces are the active sites for CO2 adsorption and conversion, while the presence of metallic Cu is also necessary to facilitate H2 dissociation and to provide hydrogen resource. The combined experiment and DFT results reveal that tuning the interaction between ZnO and ZrO2 can be considered as another important factor for designing high performance catalysts for methanol generation from CO2. Despite great efforts, the reaction mechanism of CO2 hydrogenation to methanol and the nature of the active sites on Cu–ZnO–ZrO2 (CZZ) catalysts are still under debate. Herein, the authors report the interactions among the three components in controlling the catalytic performance of CZZ catalyst for CO2 hydrogenation to methanol.

Olah G.A., Goeppert A., Prakash G.K.

Lam E., Larmier K., Wolf P., Tada S., Safonova O.V., Copéret C.

Copper nanoparticles supported on zirconia (Cu/ZrO2) or related supported oxides (Cu/ZrO2/SiO2) show promising activity and selectivity for the hydrogenation of CO2 to CH3OH. However, the role of the support remains controversial because most spectroscopic techniques provide information dominated by the bulk, making interpretation and formulation of structure-activity relationships challenging. In order to understand the role of the support and in particular of the Zr surface species at a molecular level, a surface organometallic chemistry approach has been used to tailor a silica support containing isolated Zr(IV) surface sites, on which copper nanoparticles (∼3 nm) are generated. These supported Cu nanoparticles exhibit increased CH3OH activity and selectivity compared to those supported on SiO2, reaching catalytic performances comparable to those of the corresponding Cu/ZrO2. Ex situ and in situ X-ray absorption spectroscopy reveals that the Zr sites on silica remain isolated and in their +4 oxidation state, while ex situ solid-state nuclear magnetic resonance spectroscopy and catalytic performances show that similar mechanisms are involved with the single-site support and ZrO2. These observations imply that Zr(IV) surface sites at the periphery of Cu particles are responsible for promoting CH3OH formation on Cu-Zr-based catalysts and provide a guideline to develop selective CH3OH synthesis catalysts.

Wang J., Li G., Li Z., Tang C., Feng Z., An H., Liu H., Liu T., Li C.

Reduction of CO 2 to methanol using renewable hydrogen is a promising but challenging strategy for carbon capture and utilization.

Álvarez A., Bansode A., Urakawa A., Bavykina A.V., Wezendonk T.A., Makkee M., Gascon J., Kapteijn F.

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.

Chang H., Gao F., Ma S., Zhu Y., Liu Z., Liu J., He H., Zhang K., Liu Y., Cao Y.

Zou T., Tazedaki E., Engel K.M., Chiang Y., Agrachev M., Raue K., Krumeich F., Eliasson H., Erni R., Stark W.J., Grass R.N., Araújo T.P., Pérez‐Ramírez J.

AbstractIndium‐zirconium (InZrOx) and zinc‐zirconium oxides (ZnZrOx) have emerged as highly selective and stable catalysts for CO2 hydrogenation to methanol, a versatile energy carrier. However, the disparity in synthesis methods, catalyst formulations, and structures previously studied precludes quantitative comparisons between the two families. Herein, a rigorous framework is pioneered to benchmark InZrOx and ZnZrOx materials prepared by a standardized flame spray pyrolysis synthesis platform, enabling consistently high surface areas and tunable metal speciation ranging from isolated atoms (<5 mol%) to predominantly nanoparticles (>10 mol%). Isolated indium and zinc species are commonly identified to be optimal for activity and methanol selectivity in their respective families, maximizing CO2 and H2 activation abilities. InZrOx outperforms ZnZrOx across speciations and is less structure sensitive, as deviations from atomic dispersion is less detrimental on performance for the former. Focusing on representative catalysts featuring saturation of isolated species, the higher activity of 5 mol% InZrOx over its ZnZrOx counterpart is linked to differences in surface oxygen vacancy chemistry, a lower degree of product inhibition, and more facile hydrogenation of the formate intermediate to methoxy. The identification of reactivity descriptors governing both families facilitates the development of unified guidelines in designing reducible oxide catalysts.

Schulte M.L., Sender V.C., Baumgarten L., Beck A., Nilayam A.R., Saraçi E., Grunwaldt J.

AbstractCO2 hydrogenation to methanol (MeOH) is a key transformation in the Power‐to‐liquid concept, which aims to store energy in chemical energy carriers and chemicals. Cu/ZnO/ZrO2 (CZZ) shows great promise due to its enhanced stability in the presence of water, a critical by‐product when utilizing CO2‐based feedstocks. The structure‐sensitivity of this reaction, especially for particle sizes below 10 nm and in three‐component systems, remains highly debated. Herein, we systematically prepared a series of CZZ catalysts by flame spray pyrolysis (FSP) to vary the crystallite size and to study its effect on methanol synthesis in this three‐component system. FSP enabled us to maintain a fixed Cu/Zn/Zr ratio close to the commercial composition (61/29/10 atomic ratio), while varying the precursor feed rate. This resulted in variation in the crystallinity. The characterization by X‐ray diffraction and electron microscopy revealed an increase in crystallite size with rising feed rate for Cu and t‐ZrO2, whereas ZnO remained mostly unaffected. The testing of the materials in methanol synthesis uncovered an increase in performance, higher space time yield and MeOH selectivity, with decreasing crystallite size for two (Cu, t‐ZrO2) of its three components. The increased selectivity with smaller sizes might be attributed to an increase in interfacial sites.

Zhang Q., Li D., Jiang Z., Gu H., Zhu M., Jin S., Zhu M.

Acknowledged as an ideal method for in situ hydrogen generation, methanol steam reforming (MSR) requires high-performance catalysts to enhance production efficiency. Herein, we prepared a series of Zr-modified Cu-based catalysts by a coprecipitation method and conducted a systematic analysis of the impacts of structural variations on MSR performance. Extensive characterization reveals a strong dependence of the catalyst's surface structure on Zr content. Introducing a moderate amount of Zr to the Cu/ZnO catalysts forms ZnZrOx solid solution and increases Cu dispersion, forming more Cu-ZnZrOx and Cu-ZnO interfacial sites with higher H2 production rate. Further increases in Zr content enlarge Cu nanoparticles and multiply Cu-ZrO2 interfacial sites. The optimal catalyst with a Zn/Zr molar ratio of 5, with the richest Cu-ZnO/Cu-ZnZrOx interfacial sites, achieves the highest H2 production rate of 117.4 mmolH2gcat-1h-1 at 200 °C, which is 1.3 times and 6.8 times higher than those of Cu/ZnO and Cu/ZrO2, respectively.

Maximov Anton L., Beletskaya Irina P.

Development of the "methanol" economy may be a way to establish the new chemistry under decarbonization conditions. Methanol here is used as a raw material for production of a wide range of chemicals, conventionally obtained from oil. The key process for the "methanol" economy is the reduction of CO2, which, along with renewable energy, is the main carbon-containing resource in the low-carbon industry. This review summarizes recent data on the main approaches to methanol production from CO2: catalytic hydrogenation of CO2 with hydrogen on heterogeneous or homogeneous catalysts; electrochemical reduction of CO2 to methanol; and CO2 conversion using photocatalysis. The main advantages and disadvantages of each method, the mechanisms of CO2 conversion taking into account the features of each type of catalysis, and the main approaches to the efficient catalysts are discussed.The bibliography includes 542 references.

Dostagir N.H., Tomuschat C.R., Oshiro K., Gao M., Hasegawa J., Fukuoka A., Shrotri A.

Zou T., Pinheiro Araújo T., Agrachev M., Jin X., Krumeich F., Jeschke G., Mitchell S., Pérez-Ramírez J.

While impregnation approaches are attractive, scalable routes for preparing supported oxide catalysts, achieving competitive methanol productivities over impregnated ZnO/ZrO2 remains challenging as they underperform state-of-the art systems with isolated zinc sites. Herein, by targeting the optimal active site structure, ZnO/ZrO2 systems prepared by impregnation achieve stable methanol space-time yields of 0.73 gMeOH h−1 gcat−1 during CO2 and hybrid CO-CO2 hydrogenation at suitable conditions. Notably, controlling the catalyst formulation using 5 mol% ZnO and a high surface area m-ZrO2 support fosters high zinc dispersion. In situ electron paramagnetic resonance and operando X-ray spectroscopy studies affirm the retention of isolated zinc species and facile generation of associated oxygen vacancies during the reaction. Analysis of pore structure and composition within the shaped bodies evidences abundant mesopores and a uniform zinc distribution, ensuring similar performance when translating from powder to technical forms. This work bridges fundamental understanding with practical demonstration of ZnO/ZrO2 systems.

Parra O., Portillo A., Ereña J., Aguayo A.T., Bilbao J., Ateka A.

The direct production of C5+ hydrocarbons from CO2/CO mixtures with methanol as intermediate is an attractive alternative for the production of gasoline from CO2 and syngas derived from biomass. With this purpose, the performance of CuO-ZnO-ZrO2 (CZZ), In2O3-ZrO2 (IZ) and ZnO-ZrO2 (ZZ) metallic oxides was compared by using them in tandem with a HZSM-5 zeolite. The catalysts were analyzed by means of N2 adsorption-desorption, XRD, XRF, H2-TPR and NH3-TPD. Two series of runs were performed in a packed bed reactor: (i) the methanol synthesis with the metallic oxides as catalysts, at 250–430 °C; 50 bar; CO2/COx, 0–1; H2/COx, 3; space time 6 gcat h molC−1; and (ii) the synthesis of hydrocarbons with the tandem catalysts with a metallic oxide/zeolite mass ratio of 1/1, at 340, 380 and 420 °C; 30 and 50 bar; CO2/COx, 0.5 and 1; H2/COx, 3; space time 12 gcat h molC−1. The results were quantified in terms of yield and selectivity of the product fractions and CO2 and COx (CO2 + CO) conversion. The higher methanol yield attained with the CZZ catalyst for the CO + H2 feed and its mixing with CO2 was faded by the problem of its sintering above 350 °C (minimum temperature required for the extent of methanol conversion to hydrocarbons). The IZ and ZZ catalysts were active, selective to methanol and stable both in the methanol synthesis and when used in IZ/HZSM-5 and ZZ/HZSM-5 tandem catalysts. Excellent results were obtained with the latter, which resulted in a 20.7% yield of C5+ hydrocarbon fraction at 420 °C and 50 bar, with CO2 and COx conversion of 39.7% and 28.4%, respectively. This fraction corresponded to isoparaffinic gasoline, with isoparaffin yield (mainly C5 and C6) surpassing 20% and low concentration of aromatics (0.1%) that led to a Research Octane Number of 91.8. This composition results attractive for its integration into the refineries gasoline pool. Furthermore, the changes of the CO2/COx ratio in the feed barely affected the yield and composition of the gasoline obtained with the ZZ/HZSM-5 catalyst, stating its great versatility.

Len T., Luque R.

Addressing the CO2 challenge is mandatory for the well-being of Earth's ecosystem and humanity. CO2 catalytic hydrogenation is a suitable solution.

Chen H., Gao P., Liu Z., Liang L., Han Q., Wang Z., Chen K., Zhao Z., Guo M., Liu X., Han X., Bao X., Hou G.

Surface metal hydrides (M-H) are ubiquitous in heterogeneous catalytic reactions, while the detailed characterizations are frequently hindered by their high reactivity/low concentration, and the complicated surface structures of the host solids, especially in terms of practical solid catalysts. Herein, combining instant quenching capture and advanced solid-state NMR methodology, we report the first direct and unambiguous NMR evidence on the highly reactive surface gallium hydrides (Ga-H) over a practical Ga2O3 catalyst during direct H2 activation. The spectroscopic effects of 69Ga and 71Ga isotopes on the 1H NMR signal are clearly differentiated and clarified, allowing a concrete discrimination of the Ga-H signal from the hydroxyl crowd. Accompanied with quantitative and two-dimensional NMR spectroscopical methods, as well as density functional theory calculations, information on the site specification, structural configuration, and formation mechanism of the Ga-H species has been revealed, along with the H2 dissociation mechanism. More importantly, the successful spectroscopic identification and isolation of the surface Ga-H allow us to clearly reveal the critical but ubiquitous intermediate role of this species in catalytic reactions, such as propane dehydrogenation and CO2 hydrogenation reactions. The analytic approach presented in this work can be extended to other M-H analysis, and the insights will benefit the design of more efficient Ga-based catalysts.

Halawy S.A., Osman A.I., Rooney D.W.

Herein we demonstrate the preparation and characterization of nanocrystalline ZnO, either pure or promoted with 1–10 wt.% K2O. All catalysts calcined at 400°C were in the nano-crystallite scale as confirmed by X-ray powder diffraction analysis in the 22.9–28.0 nm range. According to the CO2-temperature-programmed desorption study using thermogravimetric analysis and differential scanning calorimetry techniques, they have a broad spectrum of surface basic sites. Because of the significance of methyl ethyl ketone (MEK) as a next-generation biofuel candidate with high-octane, low boiling point, and relatively high vapor pressure. The prepared catalysts were examined during the direct production of MEK via 2-butanol (2B) dehydrogenation. Among catalysts tested, ZnO promoted with 1% K2O showed a superior catalytic activity towards the conversion of 2B to MEK, that is, 71.7% at a reaction temperature of 275°C. The selectivity for the production of MEK over all catalysts was ≥95% across all catalysts when using N2-gas as a carrier. The use of airflow in this reaction resulted in a clear loss of selectivity toward MEK production as well as the appearance of undesirable products such as acetone and methanol. All catalytic properties of catalysts, particularly those of moderate strength, were highly correlated with the distribution of surface basic sites. Finally, a reaction mechanism was proposed for the dehydrogenation of 2B, followed by the partial oxidation of MEK.