Reaction-driven restructuring of Pt and Pd catalysts: In operando X-ray absorption spectroscopy study

The catalyzed hydrogenation of ethylene on supported metal catalysts has been intensively investigated, mainly because this reaction lies at the heart of many industrial processes. Most previous studies have been performed using surface science techniques in UHV. Therefor little is known about the nature of the active state of the catalyst at ambient pressure where the kinetics is very different. We employed operando X-ray absorption spectroscopy (XAS) to correlate the structural changes of SiO$_{\mathrm{2}}$-supported Pt and Pd catalysts with their activity for ethylene hydrogenation. The XAS experiments were performed at the beamlines X19A and X18B, NSLS, BNL. For both catalysts, strong and largely reversible transformations of the metal bonding were identified at about the maximum ethane conversion. The changes were different for Pt/SiO$_{\mathrm{2}}$ and Pd/SiO$_{\mathrm{2}}$ due to the ability of the latter to form bulk hydride, while the former can only adsorb hydrogen on the surface. As a result, Pt/SiO$_{\mathrm{2}}$ undergoes disordering of the surface, leading to a strong reduction of the Pt-Pt coordination number under H$_{\mathrm{2}}$-deficient conditions, while the main effect for Pd/SiO$_{\mathrm{2}}$ is the hydrogen uptake with concomitant increase in Pd-Pd bond length. The correlation between these different kinds of order transitions and differences in rates for these catalysts will be discussed.