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

Mechanistic Study of Carbon Dioxide Hydrogenation over Pd/ZnO-Based Catalysts: The Role of Palladium-Zinc Alloy in Selective Methanol Synthesis

Pd/ZnO catalysts show good activity and high selectivity to methanol during catalytic CO₂ hydrogenation. The Pd‐Zn alloy phase has usually been considered as the active phase, though mechanistic studies under operando conditions have not been conducted to verify this. Here, we report a mechanistic study under realistic conditions of methanol synthesis, using in situ and operando X‐ray absorption spectroscopy, X‐ray powder diffraction, and time‐resolved isotope labeling experiments coupled with FTIR spectroscopy and mass spectrometry. Pd‐Zn alloy‐based catalysts, prepared through reduction of a heterobimetallic Pd^IIZn^II acetate bridge complex, and which do not contain zinc oxide or any PdZn/ZnO interface, produce mostly CO. The Pd‐Zn phase is associated with the formation of CO, and does not provide the active sites required to produce methanol from the direct hydrogenation of carbon dioxide. The presence of a ZnO phase, in contact with a Pd‐Zn phase, is essential for efficient methanol production.

Highlights and challenges in the selective reduction of carbon dioxide to methanol

Carbon dioxide (CO₂) is the iconic greenhouse gas and the major factor driving present global climate change, incentivizing its capture and recycling into valuable products and fuels. The 6H⁺/6e^- reduction of CO₂ affords CH₃OH, a key compound that is a fuel and a platform molecule. In this Review, we compare different routes for CO₂ reduction to CH₃OH, namely, heterogeneous and homogeneous catalytic hydrogenation, as well as enzymatic catalysis, photocatalysis and electrocatalysis. We describe the leading catalysts and the conditions under which they operate, and then consider their advantages and drawbacks in terms of selectivity, productivity, stability, operating conditions, cost and technical readiness. At present, heterogeneous hydrogenation catalysis and electrocatalysis have the greatest promise for large-scale CO₂ reduction to CH₃OH. The availability and price of sustainable electricity appear to be essential prerequisites for efficient CH₃OH synthesis.

Influence of a Cu-zirconia interface structure on CO2 adsorption and activation

CO₂ adsorption and activation on a catalyst are key elementary steps for CO₂ conversion to various valuable products. In the present computational study, we screened different Cu-ZrO₂ interface structures and analyzed the influence of the interface structure on CO₂ binding strength using density functional theory calculations. Our results demonstrate that a Cu nanorod favors one position on both tetragonal and monoclinic ZrO₂ surfaces, where the bottom Cu atoms are placed close to the lattice oxygens. In agreement with previous calculations, we find that CO₂ prefers a bent bidentate configuration at the Cu-ZrO₂ interface and the molecule is clearly activated being negatively charged. Straining of the Cu nanorod influences CO₂ adsorption energy but does not change the preferred nanorod position on zirconia. Altogether, our results highlight that CO₂ adsorption and activation depend sensitively on the chemical composition and atomic structure of the interface used in the calculations. This structure sensitivity may potentially impact further catalytic steps and the overall computed reactivity profile.

Zeolite-Encapsulated Ultrasmall Cu/ZnOx Nanoparticles for the Hydrogenation of CO2 to Methanol

Selective hydrogenation of CO₂ to methanol is a "two birds, one stone" technology to mitigate the greenhouse effect and solve the energy demand-supply deficit. Cu-based catalysts can effectively catalyze this reaction but suffer from low catalytic stability caused by the sintering of Cu species. Here, we report a series of zeolite-fixed catalysts Cu/ZnO_x(Y)@Na-ZSM-5 (Y is the mass ratios of Cu/Zn in the catalysts) with core-shell structures to overcome this issue and strengthen the transformation. Fascinatingly, in this work, we first employed bimetallic metal-organic framework, CuZn-HKUST-1, nanoparticles (NPs) as a sacrificial agent to introduce ultrasmall Cu/ZnO_x NPs (∼2 nm) into the crystalline particles of the Na-ZSM-5 zeolite via a hydrothermal synthesis method. The catalytic results showed that the optimized zeolite-encapsulated Cu/ZnO_x(1.38)@Na-ZSM-5 catalyst exhibited the space time yield of methanol (STY_MeOH) of 44.88 g_MeOH·g_Cu^-1·h^-1, much more efficient than the supported Cu/ZnO_x/Na-ZSM-5 catalyst (13.32 g_MeOH·g_Cu^-1·h^-1) and industrial Cu/ZnO/Al₂O₃ catalyst (8.46 g_MeOH·g_Cu^-1·h^-1) under identical conditions. Multiple studies demonstrated that the confinement in the zeolite formwork affords an intimate surrounding for the active phase to create synergies and avoid the separation of Cu-ZnO_x interfaces, which results in an improved performance. More importantly, in the long-term test, the Cu/ZnO_x(1.38)@Na-ZSM-5 catalyst exhibited constant STY_MeOH with superior durability benefitted from its fixed structure. The current findings demonstrate the importance of confinement effects in designing highly efficient and stable methanol synthesis catalysts.

Operando high-pressure investigation of size-controlled CuZn catalysts for the methanol synthesis reaction

Although Cu/ZnO-based catalysts have been long used for the hydrogenation of CO₂ to methanol, open questions still remain regarding the role and the dynamic nature of the active sites formed at the metal-oxide interface. Here, we apply high-pressure operando spectroscopy methods to well-defined Cu and Cu_0.7Zn_0.3 nanoparticles supported on ZnO/Al₂O₃, γ-Al₂O₃ and SiO₂ to correlate their structure, composition and catalytic performance. We obtain similar activity and methanol selectivity for Cu/ZnO/Al₂O₃ and CuZn/SiO₂, but the methanol yield decreases with time on stream for the latter sample. Operando X-ray absorption spectroscopy data reveal the formation of reduced Zn species coexisting with ZnO on CuZn/SiO₂. Near-ambient pressure X-ray photoelectron spectroscopy shows Zn surface segregation and the formation of a ZnO-rich shell on CuZn/SiO₂. In this work we demonstrate the beneficial effect of Zn, even in diluted form, and highlight the influence of the oxide support and the Cu-Zn interface in the reactivity.

Methanol synthesis over Cu/CeO2–ZrO2 catalysts: the key role of multiple active components

Importance of well-dispersed copper species and well-developed ceria–zirconia surface during catalytic carbon dioxide hydrogenation to methanol.

Reverse water-gas shift reaction over Pt/MoOx/TiO2: reverse Mars–van Krevelen mechanism via redox of supported MoOx

Pt/MoO_x/TiO₂ shows excellent catalytic performance for the reverse water-gas shift reaction at 250 °C via reverse Mars–van Krevelen mechanism.

CO2 Hydrogenation to Methanol with Ga- and Zn-Doped Mesoporous Cu/SiO2 Catalysts Prepared by the Aerosol-Assisted Sol-Gel Process*

The preparation of copper-based heterogeneous catalysts dedicated to the hydrogenation of CO₂ to methanol typically relies on multi-step procedures carried out in batch. These steps are precisely tailored to introduce the active phase (Cu) and the promoters (e. g., zinc, gallium) onto a preformed support to maximize catalyst performance. However, each process step can be associated with the formation of waste and with the consumption of energy, thereby negatively impacting the environmental performance of the overall catalyst preparation procedure. Here, a direct and continuous production process is proposed for the synthesis of efficient catalysts for the CO₂ -to-methanol reaction. Gallium- and zinc-promoted mesoporous Cu-SiO₂ catalysts were prepared in one step by the aerosol-assisted sol-gel process. The catalysts consisted of spherical microparticles and featured high specific surface area and pore volume, with interconnected pores of about 6 nm. A strong promoting effect of Ga and Zn was highlighted, boosting the selectivity for methanol at the expense of CO. Upon calcination, it was shown that Cu species (initially trapped in the silica matrix) underwent a migration towards the catalyst surface and a progressive sintering. After optimization, the catalysts obtained via such direct, continuous, simple, and scalable route could compete with the best catalysts reported in the literature and obtained via multi-step approaches.