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 design for hydrogenation of CO2 to HCOOH catalyst, but the nature of the active catalytic sites and the mechanism remain unknown. Two representative models have been investigated: (i) subnanometer cluster Pd5Zn/ZrO2 and (ii) PdZn(101) surface. DFT simulations with the following microkinetic calculations are used to identify the optimum structure and configurations for the reaction. Compared to the PdZn(101) surface, the Pd5Zn/ZrO2 system offers more stable adsorption and formation of intermediate species. Distinct results are shown in the preferable pathway on both systems. For PdZn(101), the formate route is more likely to proceed thermodynamically and kinetically. In contrast, supported cluster Pd5Zn/ZrO2 prefers hydrogenation reactions via the carboxyl pathway, where interface location 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, microkinetic simulation results show higher production of HCOOH in Pd5Zn/ZrO2 than PdZn surface at low and high temperatures.