Unveiling the CO Oxidation Mechanism over a Molecularly Defined Copper Single-Atom Catalyst Supported on a Metal-Organic Framework

Elucidating the reaction mechanism in heterogeneous catalysis is critically important for catalyst development, yet remains challenging because of the often unclear nature of the active sites. Using a molecularly defined copper single-atom catalyst supported on a UiO-66 metal-organic framework (Cu/UiO-66), allows a detailed mechanistic elucidation of the CO oxidation reaction. Based on a combination of in situ / operando spectroscopies, kinetic measurements including kinetic isotope effects, and density functional theory-based calculations, we identified the active site, reaction intermediates, and transition states of the dominant reaction cycle as well as the changes in oxidation/spin state during reaction. The reaction involves the continuous reactive dissociation of adsorbed O2, by reaction of O2,ad with COad, leading to the formation of an O atom connecting the Cu center with a neighboring Zr4+ ion as rate limiting step. This is removed in a second activated step.

Fundamental Aspects of Ceria Supported Au Catalysts Probed by In Situ/Operando Spectroscopy and TAP Reactor Studies

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 L_III 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.

Design and characterization of a temporal analysis of products reactor

We describe an improved temporal analysis of products (TAP) reactor design whose main new features in comparison to the recent TAP-2 design of Gleaves et al. [Appl. Catal. A 160, 55 (1997)] are the use of a turbomolecular pump, piezoelectrically driven pulse valves, and a newly designed, differentially pumped gate valve. The gate valve allows fast and simple changes between high pressure operation, in which in situ catalyst treatment can be performed, and the analytic mode with a direct line-of-sight connection to the analysis chamber and the mass spectrometer. The heating system and pulse valves are located outside the vacuum chamber, resulting in a system that is easy to operate and modify. The high stability and reproducibility of the pulse intensity allows for direct, quantitative evaluation of single-pulse and multipulse experiments. The performance of the system is demonstrated using the CO oxidation over a Au/TiO(2) catalyst as test reaction.

Scanning mass spectrometer for quantitative reaction studies on catalytically active microstructures

We describe an apparatus for spatially resolving scanning mass spectrometry which is able to measure the gas composition above catalytically active microstructures or arrays of these microstructures with a lateral resolution of better than 100 mum under reaction conditions and which allows us to quantitatively determine reaction rates on individual microstructures. Measurements of the three-dimensional gas composition at different vertical distances and separations between active structures allow the evaluation of gas phase mass transport effects. The system is based on a piezoelectrically driven positioning substage for controlled lateral and vertical positioning of the sample under a rigidly mounted capillary probe connecting to a mass spectrometer. Measurements can be performed at pressures in the range of <10(-2)-10 mbars and temperatures between room temperature and 450 degrees C. The performance of the setup is demonstrated using the CO oxidation reaction on Pt microstructures on Si with sizes between 100 and 300 mum and distances in the same order of magnitude, evaluating CO(2) formation and CO consumption above the microstructures. The rapidly decaying lateral resolution with increasing distance between sample and probe underlines the effects of (lateral) gas transport in the room between sample and probe. The reaction rates and apparent activation energy obtained from such measurements agree with previous data on extended surfaces, demonstrating the feasibility of determining absolute reaction rates on individual microstructures.