Imaging Interface and Particle Size Effects by In Situ Correlative Microscopy of a Catalytic Reaction

Abstract

The catalytic behavior of Rh particles supported by three different materials (Rh, Au, and ZrO 2 ) in H 2 oxidation has been studied in situ by correlative photoemission electron microscopy (PEEM) and scanning photoemission electron microscopy (SPEM). Kinetic transitions between the inactive and active steady states were monitored, and self-sustaining oscillations on supported Rh particles were observed. Catalytic performance differed depending on the support and Rh particle size. Oscillations varied from particle sizeindependent (Rh/Rh) via size-dependent (Rh/ZrO 2 ) to fully inhibited (Rh/Au). For Rh/Au, the formation of a surface alloy induced such effects, whereas for Rh/ZrO 2 , the formation of substoichiometric Zr oxides on the Rh surface, enhanced oxygen bonding, Rh-oxidation, and hydrogen spillover onto the ZrO 2 support were held responsible. The experimental observations were complemented by micro-kinetic simulations, based on variations of hydrogen adsorption and oxygen binding. The results demonstrate how correlative in situ surface microscopy enables linking of the local structure, composition, and catalytic performance.

Catalysis in frequency space: Resolving hidden surface structure and activity oscillations

“…As an example, self-sustained oscillations of catalyst structure 5 were studied by Ertl et al already more than 30 years ago 6 . More recently, spatially resolving techniques carried out at reactant pressures in the 10 -6 mbar range have exhibited intriguing phenomena, including coexistence of oscillating and stationary surface structures on adjacent domains 7 , self-sustained oscillations that depend on particle size and support material 8 and multifrequential self-sustained oscillations on polycrystalline 9 and curved crystals 10 . Thus, the highly active minority structures are not static in time and several active structures might coexist with oscillating coverages on the catalyst surface making it extremely difficult to determine a correct structure-function relationship.…”