Methanol synthesis from CO₂ hydrogenation over a Pd ₄/In₂O₃ model catalyst: A combined DFT and kinetic study

Methanol synthesis from CO₂ hydrogenation on a model Pd/In ₂O₃ catalyst, i.e. Pd₄/In₂O ₃, has been investigated using density functional theory (DFT) and microkinetic modeling. Three possible routes in the reaction network of CO ₂ + H₂ → CH₃OH + H₂O have been examined. Our DFT results show that the HCOO route competes with the RWGS route whereas a high activation barrier blocked the HCOOH route kinetically. The DFT results also suggest that H₂COO^* + H ^* ↔ H₂CO^* + OH ^* and cis-COOH^* + H^* ↔ CO^* + H₂O^* are the rate-limiting steps in the HCOO route and the RWGS route, respectively. Microkinetic modeling results demonstrate that the HCOO route is the dominant pathway for forming methanol from CO₂ hydrogenation. Furthermore, the activation of the H adatom on the Pd cluster and the presence of H₂O on the In ₂O₃ substrate play important roles in promoting methanol production. The hydroxyl adsorbed at the interface of Pd₄/In ₂O₃ induces structural transformation of the supported Pd₄ cluster from a butterfly shape into a tetrahedron one. This structural change not only indicates the dynamical nature of the supported nanocatalysts during the reaction but also causes the final hydrogenation step to change from CH₃O to H₂COH.