X-ray Absorption Spectroscopy of an Fe-Promoted Rh/TiO2 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/TiO2 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 H2 at 573 K, the Rh K‐edge EXAFS revealed RhRh interactions (coordination number N=6.1, RhRh distance rRh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 H2, the iron oxide promoter was reduced primarily from FeIII to FeII 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 (H2+CO) at 543 K. A structural model for this ethanol synthesis catalyst is proposed. X‐ray vision: X‐ray absorption near‐edge structure and extended X‐ray absorption fine structure spectroscopies are used to elucidate the structural features of a Rh–Fe/TiO2 catalyst for syngas conversion to ethanol. Although both Rh and Fe are initially in the +3 oxidation state, heating the catalyst in H2 to 573 K completely reduces Rh to the metal, with a slight negative charge, and partially reduces the Fe promoter to FeII oxide.