DFT insights into doping and oxygen vacancy effects on CO and CO₂ adsorptions over CuAl2O4 spinel surfaces

Abstract

Introducing transition metals into CuAl₂O₄ spinel enhances catalyst stability and Cu sintering resistance in methanol steam reforming. Yet, the influence of doping on vacancy formation and the adsorption behaviors of CO₂ (the primary product) and CO (the notorious byproduct) remains unclear. Herein, we employed DFT + U to investigate CO and CO₂ adsorption on perfect, M-doped (Fe, Co, and Ni), and M-doped oxygen-deficient CuAl₂O₄ spinel (1 0 0) and (1 1 0) surfaces. We find that stronger CO adsorption on (1 0 0) than (1 1 0) surfaces across all M-doped surfaces, while CO₂ adsorbs more stronger on (1 1 0) surfaces. The weakened CO adsorptions are observed on Fe and Ni-doped surfaces, demonstrating that doping plays a significant role in improving the resistance to CO poisoning. Co-doping promotes CO adsorption via a CO₃-like structure on CuAl₂O₄(1 1 0) surface and boosts the CO oxidation. Furthermore, infrared spectroscopy simulation indicates that the vibrational frequencies for CO linear adsorption, formation of bent CO₂- and CO₃-like structures are within the ranges of 2042–2078, 1463–1566, and 1497–1816 cm⁻¹, respectively. In addition, Ov on Ni-doped surfaces can significantly strengthen the CO₂ adsorption by 0.6–1.3 eV, highlighting the doping and oxygen-defect engineering in enhancing the CO₂ capture. This research uncovers the critical impact of metal doping and oxygen vacancies on CO and CO₂ adsorptions over CuAl₂O₄ spinel catalyst, providing insights for developing catalysts with improved resistance to CO poisoning and enhanced CO oxidation which is vital for methanol steam reforming.