Spatiotemporal Investigation of the Temperature and Structure of a Pt/CeO₂ Oxidation Catalyst for CO and Hydrocarbon Oxidation during Pulse Activation

Abstract

Reductive treatments with pulses of CO-rich atmosphere have been used to increase and maintain the low temperature activity of a Pt/CeO2-based oxidation catalyst. A combination of operando infrared thermography and spatiotemporal-resolved quick scanning extended X-ray absorption fine structure spectroscopy on a fixed bed microreactor unraveled that, apart from the pulse length, the reaction atmosphere, and the reactor temperature, also the emerging reaction heat during such activating pulses has a strong influence on the structure and catalytic performance of CO and propylene conversion in the axial direction of a fixed-bed and a monolithic reactor. The reductive pulse activation led to an increase of the integral catalyst activity as well as to the generation of zones of different particle sizes along the catalyst bed. In the case of an activation temperature between 250 and 350 °C and pulse lengths between 5 and 30 s, a hotspot of more than 80 K was observed at the beginning of the catalyst bed. Spatially resolved X-ray absorption spectroscopy indicates that larger and more reduced Pt particles are formed particularly at the beginning of the catalyst bed, whereas its subsequent part is less affected. Both the length of the reductive pulses and activation temperature have a distinct influence on the noble metal particle size. On the basis of these results, a Pt/CeO2 based honeycomb shaped substrate was activated in a similar manner. Spatially resolved gas phase profiling showed different reaction rates at the beginning of the reactor, which indicates that the concept can be transferred also to industrially relevant catalysts. In the future, such an activation procedure might open up the door to a new class of operation strategies, by which individual zones generated in the catalyst bed could be assigned for removal of specific pollutants in the exhaust stream.

Detailed microkinetics for the oxidation of exhaust gas emissions through automated mechanism generation

“…The addition of CO and H 2 to the CH 4 mixture decreases the onset temperature for CH 4 oxidation and the T 50 by 50 K, as seen in Figure 1c. Similarly, the T 50 for CO is decreased by 70 K when H 2 is added, also reported by Gänzler et al 75 Although the O 2 concentration is increased in the gas phase, the Pt crystals are locally exposed to a more reductive atmosphere (H 2 , CO), which can alter catalyst morphology and oxidation state 76,77 that affect the activity.…”

“…77 Moreover, the surface coverage and oxidation state of the Pt crystals can change across the packed-bed, as observed in operando experiments for the CO oxidation under lean combustion conditions. 76,77 Under stoichiometric conditions, these changes could be even more facile due to the complete depletion of the reductive gases and O 2 at full conversion. Although, the incorporation of surface oxidation can be done in a somewhat ad hoc manner by adjusting the active surface area, 9,73,92 a more rigorous and predictive description can only be achieved with ab-initio computations.…”

Detailed microkinetics for the oxidation of exhaust gas emissions through automated mechanism generation

“…noble metal's oxidation state. Therefore, this study underscores the importance of including morphological changes of the Pt crystals and the support, such as sintering, 73 cluster formation, 95 re-dispersion, and oxidation 76,77,94,97 and surface reconstruction 96 into the microkinetic model and the automated mechanism generation procedure. However, such an integration on a first-principles basis is beyond the scope of the present work.…”

Trendbericht Technische Chemie 2022

“…50,51) Zusätzlich ist der Katalysator nicht stationär und verändert seine Gestalt unter Reaktionsbedingungen. 52) Das Problem, diese Phänomene zu beschreiben, wird als Material Gap zusammengefasst. Um Experimente mit Ab-initio-Mikrokinetiken wiederzugeben, müssen alle möglichen aktiven Zentren berücksichtigt werden, was Arbeitsaufwand und Komplexität steigert.…”