Tailoring Electronic Properties and Atom Utilizations of the Pd Species Supported on Anatase TiO₂{101} for Efficient CO₂ Hydrogenation to Formic Acid

Abstract

Supported Pd catalysts are most promising in heterogeneous CO2 hydrogenation into formic acid (FA), while their applications are immensely restricted due to relatively poorer activity and lower utilization of Pd atoms compared to homogeneous catalysts. Herein, anatase TiO2{101}-supported Pd and PdAg catalysts were used for the hydrogenation of CO2 into FA. The electronic properties of supported Pd species were finely tuned by altering the Pd contents and Pd:Ag ratios. As the Pd loading decreased, the weakened metallicity limited the hydrogenation property, resulting in the steep decline of the reaction rate. The introduction of Ag not only improves the metallicity of supported Pd species but also promotes the utilization of Pd atoms, jointly contributing to the reaction activity. However, more Ag loadings adversely suppress the H-spillover effect that blocks the hydrogenation process. Consequently, an optimal Pd:Ag mole ratio of 5 over the Pd0.2Ag0.04/TiO2 catalyst with a Pd–Ag coordination number of 2 exhibits a very high FA yield, affording a value of 1429 h–1. In situ DRIFTS spectra coupled with kinetics results confirm the reaction proceeding through the bicarbonate intermediate, in which hydrogenation toward formate is the rate-determining step. These results not only deepen the fundamental understanding of supported Pd catalysts in CO2 hydrogenation but also broaden the concept of morphology engineering strategy for developing efficient and low-cost heterogeneous catalysts for FA production.