Theoretical insights into H2O adsorption on CuAlO2(110) surfaces: from low to high coverage

Abstract

H₂O adsorption plays a pivotal role in the initial stages of methanol steam reforming (MSR), significantly influencing the subsequent reactions on the catalyst surface. In this work, periodic DFT + U + D3 calculations were employed to investigate the adsorption of H₂O on perfect and O-defective CuAlO₂(110) catalysts. The results reveal that the role of oxygen vacancies (O_v) in promoting H₂O dissociative adsorption is particularly pronounced at low coverage, diminishing as coverage increases. Within increasing the surface coverage (nH₂O, n = 1–18) on the perfect surface, the H₂O molecule undergoes three adsorption stages, dissociative, molecular, and physisorption, involving the formation of four types of hydrogen bonds: (i) between O_wH* and O_sH (n = 1–6), (ii) between molecularly adsorbed H₂O and surface O (n = 7–12), (iii) between physiosorbed H₂O and surface O (n = 13–18), and (iv) between physiosorbed H₂O and molecularly adsorbed H₂O (n = 13–18). Notably, oscillatory behavior of type-i hydrogen bonds induced by geometric effects of incoming H₂O molecules is observed in the 7–12 coverage range, along with the elimination of type-ii hydrogen bonds at the 12–13 coverage. The phase diagram of H₂O adsorption under typical MSR conditions indicates that the surface coverage falls within the 7–12H₂O range, providing crucial insights for understanding the surface nature to optimize MSR over CuAlO₂ catalysts. Our results highlight that the number and type of hydrogen bonds significantly impact H₂O adsorption energy, with low coverage H₂O activation being facilitated by O_v, thereby promoting the initial stage of MSR over CuAlO₂.