Rh-Decorated Cu Alloy Catalyst for Improved C₂Oxygenate Formation from Syngas

Abstract

C 2 oxygenate (acetaldehyde, ethanol, etc.) formation from syngas (CO + H 2 ) is an important industrial process for the production of clean liquid energy fuels and valuable chemical feedstocks that are catalyzed industrially by Rh modified with Mn and Fe, etc. In an effort to identify catalysts based on less expensive metals and higher C 2 oxygenate selectivity, density functional theory (DFT) calculations were performed to tune the relative activity of the selectivity-determining steps, i.e., CO insertion in CH x ( x = 1, 2, 3) versus CH x hydrogenation by changing composition and structure of material. We find that the Rh-decorated Cu alloy catalyst has significantly lower CO insertion barriers compared to pristine Rh(111) and vicinal Rh(553) surfaces, whereas the variation of CH x hydrogenation barriers on the three surfaces is modest. A semiquantitative kinetic analysis based on DFT calculations shows that the C 2 oxygenate selectivity on RhCu(111) is substantially improved, with the production rate of C 2 oxygenates slightly higher than CH 4 under experimental conditions, compared with Rh(111) and Rh(553) that are highly selective to CH 4 . Our calculations suggest that the improved C 2 oxygenate selectivity on the RhCu alloy is primarily due to the fact that CO insertion is rather sensitive, whereas hydrogenation is insensitive to the ensemble effect. Furthermore, the Rh-decorated Cu alloy has stronger resistance toward coking and lower constituent cost compared to pure Rh catalysts and is thus a promising candidate for an improved C 2 oxygenate synthesis catalyst.