Ru/Rh catalyzed selective hydrogenation of CO2 to formic acid: a first principles microkinetics analysis

Abstract

Reverse water gas shift (RWGS, CO formation) and formic acid (FA) production are the two main reaction pathways during the initial hydrogenation stages of CO₂ methanation. Here, we predict that unsupported and oxygen deficient TiO₂ surface [TiO₂(v)] supported Ru_x/Rh_x nanoclusters can efficiently and selectively convert CO₂ into FA in comparison with CO. It has been revealed that unsupported Rh_x nanoclusters are better catalysts than Ru_x nanoclusters (x < 7); among them, Rh₄ exhibits the highest TOF value of 0.19 s⁻¹ at 450 K and 1 atm, typical experimental conditions for the CO₂ reduction reaction. In contrast, a supported Ru₄ catalyst exhibits a higher TOF value than Rh₄. Both CO₂ binding ability and H-migration rates are found to be accelerated by decreasing the size of the metal cluster on the oxide support. A dual atom Rh₂ site catalyst is predicted to be a better CO₂ hydrogenation catalyst than other larger sized supported clusters under ambient conditions. Microkinetics analyses suggest a very good product selectivity over the Rh₂@TiO₂(v) catalyst toward FA in comparison with CO. At 450 K, the predicted TOF and E^App are 2.11 × 10⁻⁶ s⁻¹ and 1.49 eV, respectively. The reaction passes through a sequential hydrogenation of into HCOO* followed by FA*, where desorption of FA* is found to be the rate determining step. However, above 500 K, the rate of FA production becomes faster over Ru₄@TiO₂(v).