Switchover of Reaction Paths in the Catalytic Decomposition of Formic Acid on TiO2(110) Surface

Abstract

The decomposition reaction of formic acid (DCOOD) was examined on a rutile TiO2(110) surface in catalytic and noncatalytic conditions. The kinetic behavior of the catalytic reaction was recorded by MS under DCOOD atmosphere of 10−6-10−3 Pa at 500-800 K, whereas TDS, LEED, AES, XPS, and UPS were used to chracterize adsorbed species derived from formic acid and their non-catalytic surface reactions under vacuum. Formic acid was dissociated to form formates and hydroxyl groups on TiO2(110) at 250 K. Bridge formates (0.5 ML) were arranged in a (2 × 1) order below 350 K. Formates (0.1 ML) were desorbed under vacuum at 350 K to relax the (2 × 1) overlayer. The evolution of D2 was observed at 400 K and assigned to recombination of the hydroxyl groups. Residual formates unimolecularly decompose at 570 K with an activation energy of 120 ± 10 kJ/mol and a pre-exponential factor of 2 × 109±1s−1, releasing a mixture of CO, CO2, D2, D2O, and DCOOD in TDS. On the other hand, it was found that TiO2(110) catalyzed two selective reactions: dehydration and dehydrogenation. TiO2(110) shows a preference for the dehydrogenation reaction into D2 + CO2 below 500 K. This is in contrast to the selective activity for the dehydration to D2O + CO reported on TiO2 powder catalysts. The rate of the catalytic dehydration was nearly independent of the pressure of formic acid below 700 K, but increased with surface temperature. An activation energy of 120 ± 10 kJ/mol was again observed in the catalytic dehydration reaction, suggesting that the unimolecular decomposition of formates at the surface is rate controlling. The rate of the catalytic dehydrogenation reaction depended on both the coverage of formate and the pressure of DCOOD, with a small activation energy of 15 ± 10 kJ/mol. The catalytic dehydrogenation reaction is thus suggested to proceed in a bimolecular process of a formate and a DCOOD molecule. A kinetic simulation supports the mechanisms, showing a switchover of the reaction paths by the second reactant molecule. These results are discussed, along with previous works on powder catalysts and single crystals.