Emissions from vehicles contain a variety of pollutants that must be either oxidized or reduced efficiently in the catalytic converter. Improvements to the catalyst require knowledge of the microkinetics, but the complexity of the exhaust gas mixture makes it challenging to identify the reaction network. This complexity was tackled by using the “Reaction Mechanism Generator” (RMG) to automatically generate microkinetic models for the oxidation of combustion byproducts from stoichiometric gasoline direct injection engines on Pt(111). The possibilities and the limitations encountered during the generation procedure are discussed in detail. A combination of first-principles-based mechanism construction and top-down parameter refinement allows a description of experimental results obtained by kinetic testing of a Pt/Al2O3 monolith under stoichiometric conditions. The study can serve as a blueprint for the usage of RMG for other challenging heterogeneously catalyzed reactions.

Pt/CeO2-based catalysts are attractive for emission control applications, because of their outstanding CO oxidation performance at low temperature compared to the Al2O3-based counterpart. A detailed study of the Pt electronic structure by means of operando X-ray methods together with a spatially resolved assessment of the catalytic activity along the catalytic reactor allowed to spectroscopically identify the catalytically active Pt species at low temperature in Pt/CeO2 as reduced and CO-covered Pt nanoparticles with active metal-support interface sites. The insight was gained by spatially resolved operando X-ray absorption near edge structure (XANES) experiments in high energy resolution fluorescence detection (HERFD) mode combined with CO concentration profiles along the catalyst bed using a capillary technique. The selection of methods allowed to elucidate the interplay between catalytic activity, oxidation state and CO coverage on Pt nanoparticles in Pt/CeO2 and to unravel the special nature of Pt/CeO2 by comparison to Pt/Al2O3. At low temperature, the Pt surface is covered by CO in both catalysts according to HERFD-XANES. In contrast to Pt/Al2O3, Pt/CeO2 converts CO in the presence of CO covered Pt nanoparticles. CO concentration profiles reveal the contribution of the Pt-CeO2 interface sites to be crucial and they allow a higher CO oxidation rate of Pt/CeO2 compared to Pt/Al2O3 along the catalytic reactor at low temperature. This demonstrates the importance of correlative and advanced operando spectroscopic techniques in a spatially resolved manner. Furthermore, it consolidates the outstanding role of the noble metal-support interface in Pt/CeO2-based catalyst systems for low temperature CO oxidation.

Pd/Al2O3 and Pd/CeO2 catalysts were investigated for methane oxidation at conditions typical for the exhaust of lean burn gas engines. The results show that catalyst prereduction significantly increases the catalytic activity during the light-off irrespective of the gas composition. Operando X-ray absorption spectroscopy revealed a fully reduced catalyst state after the reductive pretreatment, which is reoxidized with increasing temperature in a lean reaction mixture, resulting in bulk PdO formation at 350 °C. The correlation of catalytic activity with oxidation state during light-off tests led to the conclusion that PdO is a mandatory species for methane oxidation. We attribute the increased conversion after prereduction to the slight sintering of Pd particles and higher reactivity of the formed PdO surface species. Additionally, the H2O inhibition effect was found to be retarded under dry conditions due to the relatively slow palladium reoxidation. The results presented are in particular relevant for the activity of methane oxidation catalysts at low temperature and under dynamic conditions.

No abstract

This study investigates possible pathways to reduce water inhibition over Pd−Pt methane oxidation catalysts supported on alumina and a ceria‐zirconia mixed oxide under conditions typical for lean burn gas engines. Spatially resolved concentration and temperature profiles reveal that addition of hydrogen to the lean reaction mixture leads to significant heat production and an increase in methane conversion. Moreover, reductive pulses during operation are not only able to regenerate the catalyst deactivated by water by removal of OH‐groups from the catalyst surface, but even promote its activity after repeated application of pulsing for several hours. X‐ray absorption spectroscopy reveals the formation of a partially reduced PdO−Pd mixture during the pulsing, which explains the increase in activity. This state of high activity is stable for several hours under the tested lean conditions. The results presented in this study suggest an efficient in situ activation strategy to overcome water inhibition of methane total oxidation over Pd−Pt catalysts by careful process control.