Evidence for the dynamic change of surface structure of Cu-Al spinel: a DFT study

Abstract

Using density functional theory with dispersion correction and U-parameter (DFT + U-D3) method, the Cu release process from spinel structure is simulated by stepwise removal of CuO from spinel-structured Cu₁₆Al₃₂O₆₄, forming Cu_16-nAl₃₂O_64-n with CuO defects (V_CuO)_n (n = 1–8). The stepwise formation energies of the Cu_16-nAl₃₂O_64-n exhibit a complex variation trend with the increasing n, revealing an initial decline from n = 1 to n = 4 and increase with n = 5–7 followed by sharp decrease at n = 8. Interestingly, the stepwise formation energies even become negative (−0.26 eV for n = 4 and −1.63 eV for n = 8, respectively). Hence, the total formation energies of the Cu_16-nAl₃₂O_64-n system demonstrate an increasing trend with two energy reductions at n = 4 and n = 8, respectively. It was also found that the formation of CuO defect induced significant changes in the cationic coordination environment, leading to atomic rearrangement and lattice distortion. As the number of defects increases, the coordination number of some Al atoms decreases from hexa-coordination to lower-coordinations (primarily tetra-coordination, followed by penta-coordination and rarely tri-coordination).The findings of this study are consistent with the experimental data obtained during the sustained release catalysis, validating the dynamic changes in the surface structure during the catalytic process and laying a foundation for further research on the releasing mechanism of Cu-Al spinel.