Zirconia phase effect in Pd/ZrO2 catalyzed CO2 hydrogenation into formate

In heterogeneous catalysis, transformation of CO2 into formate largely relies on supported palladium catalyst, in which the support and in particular the support basicity plays a critical role. In this study, we prepared a series of Pd catalysts loaded on ZrO2 bearing three different crystal phases, namely the tetragonal phase (ZrO2-T), the monoclinic phase (ZrO2-M), and the hybrid tetragonal and monoclinic phase (ZrO2-M&T). With the same metal loading, ZrO2-T supported Pd catalyst exhibited considerably higher intrinsic activity in CO2 hydrogenation into formate compared with the other two catalysts. Pd/ZrO2-T with 2 wt% Pd was the most active one in the series, affording a TOF of 2817 h(-1) at 373 K. In situ DRIFTS spectroscopy, kinetic study and CO2-TPD analysis provided evidence that all Pd/ZrO2 catalysts follow a similar catalytic mechanism, and the origin of the superior activity for Pd/ZrO2-T is the higher density of weak basic sites on ZrO2-T, which facilitates the transformation of CO2 into the desirable surface adsorbed bicarbonate as the precursor for formate. This work highlights the importance of crystal-phase engineering of the support in tuning the catalytic activity of Pd based catalysts for CO2 hydrogenation.