Size and oxidation state tracking of dynamic Rh catalysts on rutile TiO2 by ambient-pressure XPS

초록

Rhodium supported on titania (Rh/TiO2) is an active catalyst for the reverse water gas shift reaction, yet the nature of the active sites for this reaction and others remain under debate. Single atom Rh sites have frequently been proposed as key sites, making it essential to monitor size changes of Rh species under in situ conditions to establish the structure–function relationship. However, surface-sensitive in situ measurements of nanosized particles remain experimentally challenging and have focused on metal oxide single crystal model systems. Here, we apply ambient pressure X-ray photoelectron spectroscopy (APXPS) to Rh/TiO2 powdered catalysts under oxidizing and reducing environments. We find size-dependent binding energy shifts in both oxidized and reduced Rh species. By deconvoluting this size effect from oxidation state core level shifts, APXPS can provide qualitative evidence of Rh cluster size changes. Potential electronic effects responsible for these shifts are explored with density functional theory calculations. Ex situ transmission electron microscopy gives insight into particle size while Raman spectroscopy identifies Rh oxide phases. Correlative X-ray absorption spectroscopy and pair distribution function (PDF) measurements confirm the structural changes in the APXPS results. These findings offer direct insight into the dynamic behavior of Rh catalyst sintering and fragmentation based on core-level shifts. This journal is © The Royal Society of Chemistry, 2026.