X‐ray Absorption Spectroscopy of an Fe‐Promoted Rh/TiO 2 Catalyst for Synthesis of Ethanol from Synthesis Gas

X‐ray absorption near‐edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS) have been used to elucidate the structural features of a 2 % Rh–2.5 % Fe/TiO 2 catalyst for syngas conversion to ethanol. The results from Rh K‐edge and Fe K‐edge spectroscopy on a fresh catalyst exposed to air indicated that both Rh and Fe were present as highly dispersed oxides on the titania, and that both adopted a +3 formal oxidation state. For the catalyst reduced in H 2 at 573 K, the Rh K‐edge EXAFS revealed RhRh interactions (coordination number N =6.1, RhRh distance r RhRh =2.67 Å) and a second feature arising from either RhTi or RhO coordination. The metal particles contained on an average about 20 atoms, which is consistent with a particle size of 1 nm. A shift in the Rh K‐edge XANES by −2 eV indicated that the reduced Rh particles were electron rich compared to bulk Rh metal. Although Rh was reduced completely to the metal by H 2 , the iron oxide promoter was reduced primarily from Fe III to Fe II oxide. The results from X‐ray absorption spectroscopy at both the Fe and Rh K‐edges were unchanged by exposure of the reduced catalyst to syngas (H 2 +CO) at 543 K. A structural model for this ethanol synthesis catalyst is proposed.