Influence of Hubbard U Parameter in Simulating Adsorption and Reactivity on CuO: Combined Theoretical and Experimental Study

Transition metal oxides are an important class of catalytic materials widely used in the chemical manufacturing and processing industry, owing to their low cost, high surface area, low toxicity, and easily tunable surface and structural properties. For these strongly correlated transition metal oxides, standard approximations in the density functional theory (DFT) exchange-correlation functional fail to describe the electron localization accurately due to the intrinsic errors arising from electron self-interactions. DFT+U method is a widely used extension of DFT, where the Hubbard U term is an onsite potential which puts a penalty on electron delocalization, successfully describing such systems at only slightly higher computational cost than standard DFT methods. The U-value is usually chosen based on its accuracy in reproducing bulk properties like lattice parameters and band structure. However, chemical reactions on transition metal oxide surfaces involve complex surface-adsorbate interactions, and using the bulk properties based U-values in a locally changing surface environment may not describe reaction energetics correctly. Hence, in the current DFT+U benchmarking work, using CuO as a model transition metal oxide, we perform DFT+U calculations to investigate the dissociative chemisorption of H-2 on it. It is observed that the U-value impacts computed adsorption enthalpies by over 100 kJ mol(-1). The DFT+U calculated adsorption enthalpy is compared with the experimental adsorption enthalpy, and equilibrium adsorption configurations are confirmed using infrared analysis. We reveal that the commonly used U-value of 7 eV (fitted against CuO bulk properties) overestimates the adsorption enthalpy by 20-40 kJ mol(-1). The U-value between 4.5 and 5.5 eV correctly predicts the adsorption of H-2 on CuO. The DFT+U benchmarking procedure elucidated in this article, encapsulates surface-adsorbate interactions, surface reactivity, and the dynamic surface reaction environment and, thus, provides an appropriate U-value to be used to model reactions on metal oxide surfaces.