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 Nin(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 Nin(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 Nin(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 Nin(n = 2–7) clusters well in the supported state, in which the presence of surface hydroxyls reduces the stability of the supported Nin clusters. On the other hand, the nucleation ability of Nin 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 Nin clusters on the γ-Al2O3 surface. More importantly, the exothermicity of supported Nin(n = 2–7) clusters on different γ-Al2O3 surfaces is lower than that of isolated Nin clusters, indicating that the support is not favorable for the nucleation of Nin(n = 2–7) clusters, as a result, the support can inhibit the aggregation of clusters, and favors the formation of small clusters.