Tuning the transition barrier of H2 dissociation in the hydrogenation of CO2 to formic acid on Ti-doped Sn2O4 clusters

Abstract

A density functional theory study has been performed to investigate cation-doped Sn₂O₄ clusters for selective catalytic reduction of CO₂. We study the influence of Si and Ti dopants on the height of the H₂ dissociation barrier for the doped systems, and then the subsequent mechanism for the conversion of CO₂ into formic acid (FA) via a hydride pinning pathway. The lowest barrier height for H₂ dissociation is observed across the ‘Ti–O’ bond of the Ti-doped Sn₂O₄ cluster, with a negatively charged hydride (Ti–H) formed during the heterolytic H₂ dissociation, bringing selectivity towards the desired FA product. The formation of a formate intermediate is identified as the rate-determining step (RDS) for the whole pathway, but the barrier height is substantially reduced for the Ti-doped system when compared to the same steps on the undoped Sn₂O₄ cluster. The free energy of formate formation in the RDS is calculated to be negative, which reveals that the hydride transfer would occur spontaneously. Overall, our results show that the small-sized Ti-doped Sn₂O₄ clusters exhibit better catalytic activity than undoped clusters in the important process of reducing CO₂ to FA when proceeding via the hydride pinning pathway.