CO2 hydrogenation to HCOOH on PdZn surface and supported PdZn Cluster: A Comparative DFT study

Modifying heterogeneous catalysts for supported cluster-based types is important to design catalysts with better activity, stability, and selectivity. Alloying Pd with Zn and supported by ZrO₂ is a promising way to design catalysts for CO₂ hydrogenation to HCOOH, but the nature of the active catalytic sites and the mechanism remain unknown. Two representative models have been investigated: subnanometer cluster Pd₅Zn/ZrO₂ and PdZn(101) surface. DFT calculations combined with microkinetic simulations are used to identify the optimum structure and configurations for the reaction. Compared to the PdZn(101) surface, the Pd₅Zn/ZrO₂ offers much more stable adsorption and formation of intermediate species. Moreover, the formate route is more likely to proceed on PdZn(101) surface from the viewpoint of thermodynamic and kinetic. In contrast, the supported Pd₅Zn/ZrO₂ cluster prefers the carboxyl pathway, where the interface site between cluster-support is ascribed to a far more stable configuration. Electronic structure analysis reveals the nature of the transition state on intermediate formation, particularly the role of Pd and Zn edge atoms on the selectivity towards the carboxyl pathway on Pd₅Zn/ZrO₂. Finally, the comparison of microkinetic simulation results shows a preference for HCOOH formation on Pd₅Zn/ZrO₂ than PdZn(101) surface at medium to higher temperature.