Continuing a comprehensive study of the reduction of COx over supported Ru catalysts, we explored the interaction of CO2 with Ru/g-Al2O3 by TAP reactor measurements, focusing on dynamic aspects in adsorption/desorption, reaction and oxygen exchange processes. Pulse shape analysis in H2/CO2 multipulse sequences provides information on the interaction of reactant/product species with the catalyst. The measurements provide information on the dynamic build-up of reaction intermediates and more stable adspecies during pulsing, and its relation to CH4 formation. Facile oxygen exchange between CO2 and catalyst, followed by isotope labelling experiments, is quantitatively reconciled in a simple model, relating the ratio between different CO2 isotopologues to the 18O : 16O ratio in the total exchangeable oxygen on the surface and in the CO2 pulse. The results provide detailed insight into various aspects of the interaction between CO2 and Ru/Al2O3 catalysts important for a mechanistic understanding of various catalytic reactions involving CO2.

As part of a comprehensive study on the reduction of COx on supported Ru catalysts we systematically investigated the dynamic interaction of CO2 with a Ru/γ-Al2O3 catalyst in a temporal analysis of products (TAP) reactor, focusing on the redox properties of the catalyst, specifically on the deposition of active oxygen (Oact) from CO2. This was investigated by exposing the pre-reduced catalyst to CO2 pulses and subsequent titration of the deposited Oact by CO pulses. CO2 is much less active for Oact deposition than O2. Reductive co-reactants reduce (CO) or increase (H2) the activity of CO2 for Oact deposition. 18O-marked CO2 pulses show facile oxygen exchange of CO2 with the catalyst, despite the inertness of the support. Based on excess effluent CO2 we conclude that surface carbonates can both build up on the surface upon interaction with CO or CO2 and also decompose upon O2 or CO2 pulsing, releasing additional CO2.

The influence of Si doping of high specific-surface-area Ru/TiO2 catalysts with similar structural properties on the CO2 reduction reaction was systematically investigated by kinetic measurements, combined with microscopic and spectroscopic methods for ex situ and in situ /operando catalyst characterization. While for undoped Ru/TiO2 a high-temperature treatment (350 degrees C) in reaction atmosphere results in a pronounced change of the selectivity from methanation to CO formation via the reverse water-gas shift (RWGS) reaction, CH4 formation is stabilized by Si doping of the catalyst support. For doping levels around 8 wt%, almost 100% CH4 selectivity is maintained. Comprehensive catalyst characterization is employed to identify trends in the physical and chemical properties with increasing Si doping and thus physical reasons responsible for the distinct differences in catalyst performance and stability. This work opens a route for improving the stability and selectivity of Ru/TiO2 catalysts in the CO2 hydrogenation reaction, a highly relevant application.

As part of an ongoing study on the performance and function of highly active Au/ZnO catalysts in the green synthesis of methanol, from H2 generated using renewable energies and CO2, we have systematically investigated the influence of water vapor on the reaction behavior. Water is a stoichiometric byproduct both in methanol formation and in the competing reverse water-gas shift (RWGS) reaction, and therefore its formation together with methanol formation is unavoidable. Changes in the reaction behavior, and possible physical or mechanistic reasons underlying these changes were addressed by high-pressure kinetic measurements, ex situ catalyst characterization and temporal analysis of products (TAP) reactor measurements. Already 2 % of water vapor led to a substantial but reversible lowering of both the methanol and the CO formation rate, and they decreased even more upon adding larger amounts. Possible reasons for this and consequences on the performance of Au/ZnO catalysts in this reaction are discussed.

Abstract The discovery of the activity of dispersed gold nanoparticles three decades ago paved the way for a new era in catalysis. The unusual behavior of these catalysts sparked many questions about their working mechanism. In particular, Au/CeO2 proved to be an efficient catalyst in several reactions such as CO oxidation, water gas shift, and CO2 reduction. Here, by employing findings from operando X‐ray absorption spectroscopy at the near and extended Au and Ce LIII energy edges, we focus on the fundamental aspects of highly active Au/CeO2 catalysts, mainly in the CO oxidation for understanding their complex structure‐reactivity relationship. These results were combined with findings from in situ diffuse reflectance FTIR and Raman spectroscopy, highlighting the changes of adlayer and ceria defects. For a comprehensive understanding, the spectroscopic findings will be supplemented by results of the dynamics of O2 activation obtained from Temporal Analysis of Products (TAP). Merging these results illuminates the complex relationship among the oxidation state, size of the Au nanoparticles, the redox properties of CeO2 support, and the dynamics of O2 activation.

While being highly active for the CO oxidation reaction already at low temperatures, Au/CeO2 catalysts suffer from continuous deactivation with time on stream, with the activity and deactivation depending on the initial catalyst activation procedure. In previous X-ray absorption measurements at the Au LIII edge, which focused on changes in the electronic and geometric changes of Au, we found a modest increase of the Au particle size during reaction, with the Au nanoparticles (NPs) present in a dominantly metallic state during reaction, regardless of the pretreatment. Here we aim at expanding on these insights by examining the changes in electronic and chemical composition of the CeO2 support induced by different pretreatment procedures and during subsequent CO oxidation at 80 °C, by following changes at the Ce LIII near edge region in time-resolved operando X-ray absorption measurements. The results indicate a strong dependence of the initial concentration of Ce3+ ions on the pretreatment, while during subsequent reaction this rapidly approaches a steady-state value which depends on the oxidative/reductive character of the reaction gas mixture, but is largely independent of the pretreatment. These results are discussed and related to earlier finding on the electronic properties of Au nanoparticles under identical reaction conditions.