Interplay of Pd ensemble sites induced by GaOx modification in boosting CO2 hydrogenation to formic acid

The interfacial modification of Pd nanoparticles supported on g-C3N4 (CN) was performed using highly dispersed amorphous MOx phase, where M represents Ga, Al, or B. The resulting Pd@MOx/CN exhibited enhanced activity in the hydrogenation of CO2 to yield formic acid. In particular, Pd@GaOx/CN displayed a maximum turnover number of 4540 based on the quantity of surface-exposed Pd atoms; this turnover number is more than six times higher than that of the unmodified catalyst. DFT calculations show that the presence of GaOx clusters on the Pd (111) surface produces the unique Pd ensemble sites, where electron-deficient Pd delta+ and electron-rich Pd delta- are adjacent. On the basis of kinetic and theoretical investigations, we propose a reasonable dual activation mechanism: the electron-deficient Pd delta+ species facilitates the adsorption of HCO3- ions, whereas the electron-rich Pd delta- species accelerates not only H-2 dissociation but also the attack of dissociated H atoms on C atoms in HCO3- ions.