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

Supported Pd catalysts are most promising in heterogeneousCO(2) hydrogenation into formic acid (FA), while their applicationsare immensely restricted due to relatively poorer activity and lowerutilization of Pd atoms compared to homogeneous catalysts. Herein,anatase TiO2{101}-supported Pd and PdAg catalysts wereused for the hydrogenation of CO2 into FA. The electronicproperties of supported Pd species were finely tuned by altering thePd contents and Pd:Ag ratios. As the Pd loading decreased, the weakenedmetallicity limited the hydrogenation property, resulting in the steepdecline of the reaction rate. The introduction of Ag not only improvesthe metallicity of supported Pd species but also promotes the utilizationof Pd atoms, jointly contributing to the reaction activity. However,more Ag loadings adversely suppress the H-spillover effect that blocksthe hydrogenation process. Consequently, an optimal Pd:Ag mole ratioof 5 over the Pd0.2Ag0.04/TiO2 catalystwith a Pd-Ag coordination number of 2 exhibits a very highFA yield, affording a value of 1429 h(-1). In situDRIFTS spectra coupled with kinetics results confirm the reactionproceeding through the bicarbonate intermediate, in which hydrogenationtoward formate is the rate-determining step. These results not onlydeepen the fundamental understanding of supported Pd catalysts inCO(2) hydrogenation but also broaden the concept of morphologyengineering strategy for developing efficient and low-cost heterogeneouscatalysts for FA production.

Automated summary

In this study, anatase TiO2{101}-supported Pd and PdAg catalysts were used for CO2 hydrogenation into formic acid. By finely tuning the electronic properties of the supported Pd species, it was found that the Pd0.2Ag0.04/TiO2 catalyst with a Pd:Ag mole ratio of 5 exhibited a very high formic acid yield.