Switchover of reaction mechanism for the catalytic decomposition of HCOOH on a TiO2(110) surface

Catalytic decomposition of HCOOH on a TiO2(110) surface switches over between unimolecular dehydration (HCOOH -> H2O + CO) and bimolecular dehydrogenation (HCOOH -> H-2 + CO2), depending on the reaction conditions. As the dehydration and dehydrogenation, reactions proceed on acidic and, basic oxide catalyst surfaces, respectively, the aspect observed on the same single crystal surface seems to be not compatible with the conventional acid-base catalysis concept. To clarify the origin of the switchover of the acid-base catalysis, the reaction mechanism of the HCOOH dehydrogenation was studied by density functional theory (DFT) calculations. It was concluded from the DFT calculations together with the rate equation and experimentally determined activation energy that the bimolecular dehydrogenation proceeds between a strongly adsorbed bridging formate anion and a weakly adsorbed HCOOH molecule by cooperative catalysis of three adjacent surface Ti4+ ions as Lewis acidic sites on the surface. This mechanism is entirely different from the previous dehydration mechanism that the dehydration occurs on an oxygen point defect (basic character) formed in situ by H2O desorption from two OH under the catalytic dehydration reaction conditions. Thus, the TiO2(110) surface provides two kinds of active sites for the HCOOH decomposition in a manner different from the traditional acid-base catalysis concept.