(304h) The Influence of Hubbard U Parameter in Simulating Adsorption and Reactivity on CuO Surface(s): A Combined Theoretical and Experimental Study
AIChE Annual Meeting
Tuesday, October 31, 2017 - 10:13am to 10:30am
Transition metal oxides are an important class of catalytic materials widely used in the chemicals manufacturing and processing industry owing to their low cost, high surface area, low toxicity and easily tuneable surface and structural properties. For these strongly correlated transition metal oxides, standard approximations in the density functional theory (DFT) exchange-correlation functionals 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 these 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 benchmarking work, using CuO as a model transition metal oxide, we perform DFT+U calculations to investigate the dissociative chemisorption of H2 on it. it is observed that the U-value changes the calculated adsorption energies by over 100 kJ mol-1. Chemisorption experiments are performed at 30-50 °C to ensure that adsorption products do not leave the system. The (dissociative) adsorption configurations along the reaction coordinate are determined by performing in-situ infrared analysis at adsorption temperatures. For the experimentally validated equilibrium configuration, adsorption energies are comupted using DFT+U for progressive U values and compared with experimental adsorption energies. We reveal that the commonly used U-value of 7 eV (fitted against CuO bulk properties) overestimates the adsorption energy by 20-40 kJ mol-1. The U-value between 4.5-5.5 eV correctly predicts the adsorption of H2 on CuO. The DFT+U benchmarking procedure elucidated in this paper, encapsulates surface-adsorbate interactions, surface reactivity and dynamic surface reaction environment and thus, provides an appropriate U-value to be used to model reactions on metal oxide surfaces.