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 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 angstrom) 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.