Hydrogenation of CO2 to formic acid catalyzed by Co and Cu Single-atom catalysts supported on MOF-808: A DFT investigation

Abstract

In this work, DFT-based calculations and microkinetic modeling were employed to investigate CO2 hydrogenation to formic acid using H2 over Co and Cu single-atom catalysts supported on MOF-808. We investigated two pathways: one without the introduction of a second H2 molecule (pathway A) and another one with it (pathway B). Pathway B, which involves introducing the second H2 molecule alongside the formate intermediate from the first step, exhibits significantly lower energy barriers (three times lower) for the transformation into formic acid in the second step of CO2 hydrogenation. Moreover, pathway B shifts the reaction thermodynamics from endergonic to exergonic, highlighting its kinetic and thermodynamic advantages. Notably, we observed formate intermediates with quasi-bidentate geometry alongside the prevalent bidentate chelating geometry. Cu2+-MOF-808 exhibits superior catalytic activity compared to Co2+-MOF-808, attributed to Cu's stronger preference for stabilizing the transition state in its square planar geometry through the Jahn-Teller effect, which is less effective in Co. Furthermore, our microkinetic modeling consistently confirms that Cu2+-MOF-808 outperforms Co2+-MOF-808 at lower temperatures, with the rate of formic acid production depending on the concentration of H2. The desorption of formic acid is identified as the rate-determining step of the reaction, significantly impacting overall efficiency.