EXAFS-XANES evidence of in operando Caesium reduction in Cs-Ru/C catalysts for ammonia synthesis

We present here a X-ray absorption spectroscopy (XAS) investigation on the local chemical order and electronic structure of Cs and Ba, promoters of the Ru/C catalysts for ammonia synthesis that attracted interest because of highly increased productivity. The role of the promoters is still largely unclear, although indirect evidence for Cs partial reduction has been obtained by this and other groups. Our XAS analysis with in situ H-2 reduction directly supports the partial Cs reduction in the promoted Ru/C catalysts, depending on the presence of Ru and on the graphitization degree of the support. Higher coordination of Ba was observed with respect to Cs in the reduced samples, without evidence of heavy atoms (Ru, Cs, and Ba) in the surroundings. Because of the strong electropositive nature of Cs, direct experimental evidence of its partial reduction is of outstanding significance also for other applications.