High Coverage Carbon Monoxide Adsorption on a ZnAl₂O₄ Surface: A DFT Study

ZnAl₂O₄ spinel is widely used as a support in various reactions due to its thermal stability. Investigating the interaction between CO and ZnAl₂O₄ is essential for understanding the surface reaction mechanisms. In the present work, density functional theory (DFT) and ab initio atomistic thermodynamics were employed to investigate the adsorption of CO on perfect and O-defective ZnAl₂O₄ surfaces at varying CO coverages. The results reveal the formation of C* species and the subsequent aggregation can be hindered by the concentration of surface oxygen vacancy (Ovac) at low CO coverages. Notably, at high CO coverages, carbon deposition (CD) occurs on the ZnAl₂O₄ surfaces. Δμ_CO is regulated below its threshold based on surface Ovac concentrations to control CO coverage and thereby suppress CD. At 523 K, the CO partial pressures should be maintained below 100, 10^-3, 100, and 10^-6 atm for surface with Ovac concentrations of 0, 12.5, 25, and 37.5%, respectively, to prevent CD on the surface. DFT-simulated IR spectra show that varying Ovac concentrations significantly affects the CO adsorption peak types and shifts, causing red shifts in vibrational frequencies for linearly adsorbed CO. These findings provide guidance for synthesizing ZnAl₂O₄ with varying Ovac concentrations to enhance catalyst efficiency and durability while mitigating CD.