Photolysis of Atomically Dispersed Rh/Al₂O₃ Catalysts: Controlling CO Coverage <i>in Situ</i> and Promoting Reaction Rates

Abstract

Atomically dispersed Rh active sites on oxide supports have gained significant traction in heterogeneous catalysis due to their unique reactivity. In many reactions of interest, carbon monoxide (CO) is involved as a reactant or intermediate, leading to the formation of highly stable gem-dicarbonyl species, Rh(CO)2, that kinetically limit reaction rates by requiring CO desorption to produce reactive monocarbonyl intermediates, Rh(CO). Here we report on the use of ultraviolet (UV) photon illumination to induce CO desorption from Rh(CO)2 species supported on Al2O3 and produce Rh(CO). In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to track the conversion of Rh(CO)2 to Rh(CO) under UV illumination and various environmental conditions. The kinetics of Rh(CO)2 photolysis and the resulting maximum yield of Rh(CO) produced via photolysis were influenced by temperature (143–423 K) and interactions between Rh(CO)x species and hydrated regions of the Al2O3 support. The formation kinetics and maximum yield of Rh(CO) were significantly promoted at elevated temperature and in water cofeed. DRIFTS studies coupled with in situ X-ray absorption spectroscopy measurements during photolysis suggested that UV photolysis of Rh(CO)2 to produce Rh(CO) is promoted by occurrence in hydroxylated regions of the support where coordination of Rh(CO) to support-derived OH and H2O stabilize the reactive intermediate. Kinetic and in situ DRIFTS studies of the water gas shift reaction (CO + H2O → CO2 + H2) on atomically dispersed Rh/Al2O3 under thermal conditions and during irradiation of varying wavelengths and photon flux support the hypothesis that UV photons can promote the rate of Rh(CO) formation from Rh(CO)2 to promote the water gas shift rate. These results suggest that UV illumination can be an effective strategy to promote chemistries on atomically dispersed Rh/Al2O3, where the rate is limited by desorption of CO from the coordinatively saturated Rh(CO)2 active site.

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