Mechanistic and Kinetic Relevance of Hydrogen and Water in CO2 Hydrogenation on Cu-Based Catalysts: Article No. 115936

We ally steady-state kinetics, kinetic isotope effects, and density functional theory (DFT) calculations to illustrate that Cu-based catalysts remain saturated by H-adatoms (H*) and molecular formic acid (HCOOH**) during CO2 hydrogenation. High H* coverage under methanol synthesis conditions is evidenced by reverse water-gas shift (RWGS) rates that exhibit positive H2 reaction orders only at PH2 ~< 0.5 bar, above which methanol synthesis and RWGS rates exhibit first and zeroth order dependence on PH2, respectively. HCOOH** also accumulates on the surface with increasing PCO2 as informed by the Langmuir-type dependence on PCO2 (0.25-23 bar) for both methanol synthesis and RWGS. As both HCOOH** and H* have one H-atom per site occupied, the two species share the same PH2 dependence and give rise to CO2 reaction orders that are independent of PH2. Surface coverages determined based on kinetic analyses are further corroborated with DFT-derived adsorption energies that show favorable HCOOH** adsorbate-adsorbate interactions as well as repulsive interactions for bidentate formate (HCOO**) on H*-saturated surfaces. Methanol selectivity remains invariant with PCO2 and PCO despite CO inhibiting reaction rates, thereby demonstrating methanol synthesis and RWGS occur on the same active site. In contrast, water preferentially inhibits methanol synthesis rates, increases methanol synthesis H2 reaction order from 1.0 to 1.5, and alters the methanol synthesis H2/D2 kinetic isotope effect; the inhibitory effect of H2O thus cannot be attributed to competitive adsorption alone and instead reflects a change in the rate-determining step for methanol synthesis. The disparate kinetics of methanol synthesis and RWGS evince a branching pathway where methanol is formed from formates and CO is formed from carboxylates. The presented work thus identifies the relevant surface species, underscores the distinct catalytic role of water in branching methanol synthesis and RWGS pathways, and, in doing so, details a mechanistic picture that yields predictable rates and reaction orders for both methanol synthesis and RWGS on Cu-based CO2 hydrogenation catalysts.