Origin of the Activity Trend in the Oxidative Dehydrogenation of Ethanol over VOx /CeO2.

Supported vanadia (VOx ) is a versatile catalyst for various redox processes where ceria-supported VOx have shown to be particularly active in the oxidative dehydrogenation (ODH) of alcohols. In this work, we clarify the origin of the volcano-shaped ethanol ODH activity trend for VOx /CeOx catalysts using operando quick V K- and Ce L3 - edge XAS experiments performed under transient conditions. We quantitatively demonstrate that both vanadium and cerium are synergistically involved in alcohol ODH. The concentration of reversible Ce4+ /Ce3+ species was identified as the main descriptor of the alcohol ODH activity. The activity drop in the volcano plot, observed at above ca. 3 V nm-2 surface loading (ca. 30 % of VOx monolayer coverage), is related to the formation of spectator V4+ and Ce3+ species, which were identified here for the first time. These results might prove to be helpful for the rational optimization of VOx /CeO2 catalysts and the refinement of the theoretical models.

Drastic Events and Gradual Change Define the Structure of an Active Copper-Zinc-Alumina Catalyst for Methanol Synthesis.

The copper-zinc-alumina (CZA) catalyst is one of the most important catalysts. Nevertheless, understanding of the complex CZA structure is still limited and hampers further optimization. Critical to the production of a highly active and stable catalyst are optimal start-up procedures in hydrogen. Here, by employing operando X-ray absorption spectroscopy and X-ray diffraction, we follow how the industrial CZA precursor evolves into the working catalyst. Two major events in the activation drastically alter the copper- and zinc-containing components in the CZA catalyst and define the final working catalyst structure: the reduction of the starting copper(II) oxide, and the ripening and re-oxidation of zinc oxide upon the switch to catalytic conditions. These drastic events are also accompanied by other gradual, structural changes. Understanding what happens during these events is key to develop tailored start-up protocols that are aimed at maximal longevity and activity of the catalysts.

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 PdII ZnII 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.

In situ formation of surface and bulk oxides in small palladium nanoparticles.

Evolution of surface and bulk palladium oxides in supported palladium nanoparticles was followed in situ using X-ray absorption spectroscopy. The surface oxide was found to be easily reducible in hydrogen at room temperature, while removal of bulk oxide required heating in hydrogen above 100 °C. We also found that the co-presence of hydrogen and oxygen favours a stronger oxidation of palladium particles compared to pure oxygen.

The dynamics of overlayer formation on catalyst nanoparticles and strong metal-support interaction.

Heterogeneous catalysts play a pivotal role in the chemical industry. The strong metal-support interaction (SMSI), which affects the catalytic activity, is a phenomenon researched for decades. However, detailed mechanistic understanding on real catalytic systems is lacking. Here, this surface phenomenon was studied on an actual platinum-titania catalyst by state-of-the-art in situ electron microscopy, in situ X-ray photoemission spectroscopy and in situ X-ray diffraction, aided by density functional theory calculations, providing a novel real time view on how the phenomenon occurs. The migration of reduced titanium oxide, limited in thickness, and the formation of an alloy are competing mechanisms during high temperature reduction. Subsequent exposure to oxygen segregates the titanium from the alloy, and a thicker titania overlayer forms. This role of oxygen in the formation process and stabilization of the overlayer was not recognized before. It provides new application potential in catalysis and materials science.

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.