The effect of γ-Al2O3 surface hydroxylation on the stability and nucleation of Ni in Ni/γ-Al2O3 catalyst: a theoretical study

Abstract

Using recent well-defined models of γ-Al₂O₃ surfaces, the interactions of Ni_n(n = 1–7) clusters with different γ-Al₂O₃ surfaces have been investigated in order to illustrate, by density functional theory periodic calculations, the effect of γ-Al₂O₃ surface hydroxylation on the stability and nucleation of Ni in Ni/γ-Al₂O₃ catalyst. Three types of γ-Al₂O₃ surfaces, dehydrated γ-Al₂O₃(100), dehydrated γ-Al₂O₃(110) and hydrated γ-Al₂O₃(110) were considered. Our results show that for the adsorption of Ni_n(n = 3–7) clusters, the γ-Al₂O₃(110) surface is more favorable than the γ-Al₂O₃(100) surface, however, for single Ni atoms and Ni₂ clusters, the reverse becomes true. Meanwhile, for the adsorption of Ni_n(n = 2–7) clusters, the hydrated (110) surface is not favorable compared to the dehydrated (110) surface, due to the presence of surface hydroxyls on the former. The reverse is true for single Ni atoms due to weaker surface deformation. Further, the support stabilizes Ni_n(n = 2–7) clusters well in the supported state, in which the presence of surface hydroxyls reduces the stability of the supported Ni_n clusters. On the other hand, the nucleation ability of Ni_n clusters on different γ-Al₂O₃ surfaces, is more favorable on the γ-Al₂O₃(110) surface than on the γ-Al₂O₃(100) surface, and the dehydrated (110) surface is more favorable than the hydrated (110) surface due to the presence of surface hydroxyls, namely, surface hydroxylation reduces the nucleation ability of Ni_n clusters on the γ-Al₂O₃ surface. More importantly, the exothermicity of supported Ni_n(n = 2–7) clusters on different γ-Al₂O₃ surfaces is lower than that of isolated Ni_n clusters, indicating that the support is not favorable for the nucleation of Ni_n(n = 2–7) clusters, as a result, the support can inhibit the aggregation of clusters, and favors the formation of small clusters.