Structural Features of Fe-Promoted Rh/Titania Catalyst for Direct Synthesis of Ethanol From Syngas as Elucidated by XANES and EXAFS

In a recent contribution from our group [1], the influence of support and promoter metal on the activity and selectivity of Rh-based catalysts for the direct synthesis of ethanol from syngas was explored.  The reactions were performed in a fixed-bed reactor system typically operating at 543 K, 20 atm, with a WHSV of 133 mL (STP) syngas gcat^-1 min^-1 with a H₂/CO ratio of 1:1.  As a result of screening studies of differently-supported and differently-promoted catalysts, a 2%Rh-2.5%Fe/TiO₂ prepared by incipient wetness impregnation was found to be optimal in terms of activity (CO conversion), selectivity (to ethanol), and turnover frequency.  In our current work, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) have been used to elucidate the structural features of this 2%Rh-2.5%Fe/TiO₂ catalyst for the direct conversion of syngas to ethanol. 

The results from Rh K-edge and Fe K-edge X-ray absorption spectroscopy on a fresh catalyst exposed to air indicate that both Rh and Fe are present as highly dispersed oxides on the titania, both residing in a +3 formal oxidation state.  For the catalyst reduced in H₂ at 573 K, the Rh K-edge EXAFS revealed Rh-Rh interactions (N = 6.1, R_Rh-Rh = 2.67 Å) and a second feature from either Rh-Ti or Rh-O coordination.  Based on this coordination number, the metal particles contained on an average about 20 atoms, which is consistent with a particle size of about 1 nm.  A shift in the Rh K-edge XANES spectra (plotted as derivative of normalized μ(E) vs. edge shift E-E₀) by -2.1 eV indicated the reduced Rh particles were electron-rich compared to the bulk Rh metal.  Although Rh was reduced completely to the metal by H₂, the iron oxide promoter 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 (H₂+CO) at 543 K.  A structural model for this ethanol synthesis catalyst, including the promotional roles of Fe and titania, will be discussed.

1.  M.A. Haider, M.R. Gogate, and R.J. Davis, J. Catal. 261 (2009) 9.