Mechanistic investigation of carbon dioxide hydrogenation on copper-based catalysts

Abstract

Thermocatalytic CO2 hydrogenation is limited by equilibrium constraints that favor the reactants, high energy barriers to activate recalcitrant C=O double bonds, and branching reaction pathways that decrease selectivity toward the product of interest. The presence of an interconnected reaction network for CO2 hydrogenation further necessitates re-evaluation of the validity of existing functional forms of rates and reversibility originally derived for single-path sequences in describing rates and reversibility of interconnected networks. To this end, we first demonstrate the connectivity-dependent nature of rates and the connectivity-independent nature of reversibility through mathematical formalisms where kinetic resistances unambiguously parse both rates and degrees of rate control in terms of contributions from constitutive steps in the pathway of interest and contributions from branching steps. Further analysis on the degrees of rate control shows that stoichiometric regularity ensures the equality of the forward and reverse degrees of rate control at all extents of reaction and that symmetry in stoichiometry, rate constant, and concentration for reaction pathways about the branching intermediate results in degrees of rate control that exhibit a constant offset. Reversibility-based formalisms are then leveraged to inform a branching reaction network with distinct intermediates for the methanol synthesis and reverse water-gas shift (RWGS) pathways during CO2 hydrogenation on Cu-based catalysts. By deconvoluting the underlying kinetic and thermodynamic driving forces, we identify hydrogen and water as the salient species governing methanol selectivity and yield where hydrogen preferentially promotes methanol synthesis rates and water preferential inhibits methanol synthesis rates. These observed trends in rates, in combination with kinetic isotope effects and density functional theory calculations, illustrate that catalysis occurs on H*- and HCOOH**-saturated surfaces where methanol synthesis proceeds through a formate pathway with rate-determining steps that are dependent on water partial pressures while RWGS proceeds through a carboxylate pathway with one sole rate-determining step. We lastly detail experimental and analytical protocols useful in assessing the network connectivity and intrinsic kinetics of interconnected networks in the context of CO2 hydrogenation to complement existing efforts in catalyst development and advance the field of CO2 valorization.