Role of catalyst shape and reactor performance for relating the catalytic activity and the chemical structure of active sites for emission control catalysis

Packed powder beds and coatings are two relevant forms of catalysts, applied in industrial heterogeneous catalysis. Both types have their individual advantages and disadvantages in regard to the performance and characterisation causing some complexity in the resulting flow patterns, possible temperature inhomogenities in the reactor and the dynamic evolution of chemical state of the noble metal during the reaction along the catalytic bed. The well-known CO oxidation reaction over Pt/Al2O3 catalysts was used in this study to uncover the influence of the gas phase compositions, the influence of the temperature and the evolution of the electronic structure of Pt for powdered and coated catalysts at comparable length scales. Advanced operando investigations were used to demonstrate the influence of spatial gradients in the gas phase for washcoatings in contrast to packed powder beds. Additionally, transient gradients in the chemical state of Pt, which occurred more pronounced for packed powder beds than for coated monoliths were followed and traced back to heat and mass transfer effects. Finally, the catalytic activity can be linked to the temperature distributions for both types of samples. These findings will be valuable for planning and evaluating future combinations of spectroscopic and catalytic experiments on industrially relevant systems.