Nanoscale Solvation Leads to Spontaneous Formation of a Bicarbonate Monolayer on Rutile (110) under Ambient Conditions: Implications for CO2 Photoreduction

The chemical state of a catalyst in operando is particularly important for catalysts that target minority species, such as atmospheric CO 2 which has a concentration of only 400 ppm. A reaction can be promoted by the selective binding of reactants or hindered by molecules that block active sites. We show that adsorbed CO 2, a very weakly bonded species on TiO 2, is unlikely to play the key role in CO 2 photoreduction under ambient conditions, at least on rutile (110), as the vast majority of unsaturated Ti sites are terminated by a different, much more strongly bound carbonaceous species: adsorbed bicarbonate (HCO 3 ). Using a combination of scanning tunneling microscopy (STM) and surface spectroscopies, we show that atmospheric CO 2 readily and stably displaces adsorbed H 2 O on rutile (110), creating a self-assembled monolayer of HCO 3 and H that is stable at room temperature even in vacuum. This reaction occurs on near-ideal, stoichiometric rutile (110) and does not require surface defects, such as O vacancies, Ti interstitials, or steps. This reaction is promoted both by the strong bidentate bonding of HCO 3 as well as the nanoscale H 2 O film that spontaneously forms on TiO 2 under ambient conditions. Density functional theory calculations show that the nanoscale water layer adsorbed to rutile (110) solvates the products and changes the reaction energetics significantly. The chemical state of the catalyst in operando will also be affected by the half-monolayer of adsorbed H produced by the reactive dissociation of H 2 O.