PdO/Pd0/TiO2 Nanocatalysts Engineered by Flame Spray Pyrolysis: Study of the Synergy of PdO/Pd0 on H2 Production by HCOOH Dehydrogenation and the Deactivation Mechanism

Abstract

Palladium-based catalysts are among the most efficient for H2 production via HCOOH (FA) dehydrogenation at near-ambient pressure and temperature. Herein, we show that [PdO/Pd0/TiO2] nanocatalysts bearing a tetragonal PdO nanophase can be optimized for enhanced FA dehydrogenation via engineering of the [PdO:Pd0] ratio on the TiO2 support. We have developed a sequential-deposition flame spray pyrolysis (SD-FSP) technique for deposition of Pd on TiO2 at a high [PdO:Pd0] ratio up to 75%. In addition, we have synthesized low-[PdO:Pd0]-ratio catalysts using an oxygen-lean FSP protocol. The SD-FSP-made [PdO/Pd0/TiO2] nanocatalysts with a high [PdO:Pd0] ratio >70% can achieve a high H2 gas production rate of 534 mmol/g of Pd/min that supersedes by >300% the efficiency of [Pd0/TiO2] nanocatalysts with low PdO content. The thermodynamic basis of the role of [PdO:Pd0] was investigated by an Arrhenius study, which reveals that the activation energy barrier Ea is decreasing significantly, i.e., up to 50%, upon an increase of the [PdO:Pd0] ratio. A reduction of surficial PdO toward Pd0, by in situ generated H2, exerts a strong inhibitory effect on the catalyst. Overall, the present data indicate that both [i] maximization of the [PdO:Pd0] ratio and [ii] minimization of PdO reduction during H2 production are key prerequisites for enhanced FA dehydrogenation by TiO2-supported/Pd catalysts.