Dynamic Structure of a Working Methanol Steam Reforming Catalyst: In Situ Quick-EXAFS on Pd/ZnO Nanoparticles

Combining time-resolved X-ray absorption spectroscopy (XAS) with simultaneous mass spectrometric activity measurements, both performed during catalytic methanol steam reforming, we have studied the dynamic changes of Pd nanoparticles supported on ZnO. The formation of the catalytically active phase PdZn was unambiguously monitored in real time. We thus directly prove for the first time that the PdZn alloy formed in the course of the catalytic reaction is responsible for the excellent selectivity to H-2 and CO2. Alloying started at the nanoparticle surface and proceeded "inwards" toward the particle center. On the basis of extended X-ray absorption fine structure (EXAFS) analysis, the extent of PdZn alloy formation was estimated. Alloying was reversible. In contact with oxygen, PdZn segregated into palladium metal and ZnO. The XAS data were corroborated by Fourier transform infrared (FTIR) spectroscopy. It is demonstrated that the application of operando quick-EXAFS is a powerful tool for studying nanomaterials and their dynamic adaptation to reactive environments.