Hydride Pinning Pathway in the Hydrogenation of CO₂ to Formic Acid on Dimeric Tin Dioxide

Abstract

Capture of CO2 and its conversion into organic feedstocks are increasingly needed as society moves towards a renewable energy economy. Here, a hydride‐assisted selective reduction pathway is proposed for the conversion of CO2 to formic acid (FA) over SnO2 monomers and dimers. Our density functional theory calculations infer a strong chemisorption of CO2 on SnO2 clusters forming a carbonate structure, whereas heterolytic cleavage of H2 provides a new pathway for the selective reduction of CO2 to formic acid at low overpotential. Among the two investigated pathways for reduction of CO2 to HCOOH, the hydride pinning pathway is found promising with a unique selectivity for HCOOH. The negatively‐charged hydride forms on the cluster during the dissociation of H2 and facilitates the formation of a formate intermediate, which determines the selectivity for FA over the alternative CO and H2 evolution reaction. It is confirmed that SnO2 clusters exhibit a different catalytic behaviour from their surface equivalents, thus offering promise for future work investigating the reduction of CO2 to FA via a hydride pinning pathway at low overpotential and CO2 capturing.

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

“…This spontaneity was not observed in the case of undoped Sn 2 O 4 , where the required overpotential of HCOO* formation is 0.25 eV. 26 Comparing the present results with previous studies on the hydrogenation of CO 19 and illustrating that the reaction would proceed favourably.…”

“…Previously, undoped Sn2O4 clusters have been observed to dissociate H2 at Sn-O bridging sites, with the hydride pinning pathway leading to selective reduction of CO2 to HCOOH via a formate intermediate. 26 Considering this previous work, we calculated the transition states and barrier heights of H 2 dissociation on all possible bridging sites in the doped cluster; the subsequent optimized minima and transition state geometries are shown in Fig. 2, and the reaction energy profiles in Fig.…”

“…The transition barrier of the hydride transfer step in the doped system is lower than for the undoped cluster [1.15 eV (26.51 kcal/mol)], 26 which is attributed to the two factors: first, the weakness of the Ti-H bond relative to the Sn-H bond, which results in hydride transfer to the carbon being less energetically demanding; second, the Ti-centre in the doped SnTiO4 cluster retains its tetrahedral shape throughout i.e energy is not required for structure changes of SnTiO4, unlike for bare Sn2O4 where the Sn-centre changes its shape from tetrahedral to planar. 26 In the transition state TS1, the Ti-H bond elongates from 1.60 Å For the overall reaction, the hydride pinning (TS1) is the rate-determining step (RDS) and hence the free energy difference between IM2 / and IM1 equates to the necessary overpotential for realising the hydrogenation process. This overpotential factor can be calculated from the computational hydrogen electrode (CHE) model given by Nørskov et al, 37 which is discussed in the computational methodology.…”

“…17,,25 Similarly, another study on SnO2 reported a hydride pinning pathway whereby heterolytic H2 dissociation leads to the formation of an Sn-hydride bond, and CO2 activation occurs during the (rate determining) hydride transfer step; The observed outcome was total selectivity of HCOOH formation at an overpotential of 0.25 eV. 26 In the present work, we consider whether dopants in a model dimeric Sn2O4 cluster modify the key steps in this favourable hydrogenation pathway of CO2 to HCOOH. Structures for the Sn2O4 clusters have been taken from previous studies 26 and, for the newly considered doped system, we choose two dopants: Si, which is isovalent with Sn, with a stable +4 oxidation state as well as vacant 3d orbitals; and Ti, which has an atomic radius similar to Sn, vacant 3d electrons, and stability in the +4 oxidation state (though somewhat reducible).…”

“…26 In the present work, we consider whether dopants in a model dimeric Sn2O4 cluster modify the key steps in this favourable hydrogenation pathway of CO2 to HCOOH. Structures for the Sn2O4 clusters have been taken from previous studies 26 and, for the newly considered doped system, we choose two dopants: Si, which is isovalent with Sn, with a stable +4 oxidation state as well as vacant 3d orbitals; and Ti, which has an atomic radius similar to Sn, vacant 3d electrons, and stability in the +4 oxidation state (though somewhat reducible). As both dopants are considered formally in +4 oxidation states, their introduction to the cluster does not require compensating oxygen vacancies.…”