Unlocking the Facet-Dependent Ligand Exchange on Rutile TiO2 of a Rhenium Bipyridyl Catalyst for CO2 Reduction

Covalent attachment of molecular catalysts to electrode surfaces is an attractive approach to develop robust catalytic materials. Selectivity and tunability of the resulting catalytic surface can be achieved by ligand design, making surfaceattached CO2 catalysts of immense interest for zero carbon technologies. Unfortunately, the functionality of heterogenized catalysts strongly depends on the nature of the electrode surface and the specific binding mode of the catalyst on the electrode surface. Here, we perform experimental and theoretical vibrational sum-frequency generation spectroscopy (VSFG) to investigate the binding configuration of a popular molecular CO2 reduction catalyst, the Re(dcbpy)(CO)3Cl (dcbpy = 4,4 '-dicarboxy-2,2 ' bipyridine) complex (ReC0A), heterogenized on a 0.5% niobium (Nb)-doped rutile TiO2 (100) crystal. We find evidence of ligand exchange induced upon binding to the (100) TiO2 facet that was not observed on other TiO2 facets. The structural changes are induced by the sawtooth morphology of the TiO2 (100) facet, establishing interactions that lead to chloride (Cl-) ligand exchange with hydroxide (OH-) and formation of the Re(dcbpy)(CO)3OH (ReOH) adsorbate. DFT calculations show bidentate binding of ReOH through its carboxylate (COO-) groups in a flat-lying orientation stabilized by hydrogen-bonding of the OH- proton to the TiO2 surface. The OH-substituted site interacts strongly with the (100) TiO2 surface in a configuration unfavorable for the CO2 exchange that is necessary for catalytic functionality. These findings provide evidence of facet-dependent changes of the heterogenized molecular catalyst, underscoring the critical role of the surface facet while designing electrocatalytic materials.