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

Using recent well-defined models of gamma-Al2O3 surfaces, the interactions of Ni-n(n = 1-7) clusters with different gamma-Al2O3 surfaces have been investigated in order to illustrate, by density functional theory periodic calculations, the effect of gamma-Al2O3 surface hydroxylation on the stability and nucleation of Ni in Ni/gamma-Al2O3 catalyst. Three types of gamma-Al2O3 surfaces, dehydrated gamma-Al2O3(100), dehydrated gamma-Al2O3(110) and hydrated gamma-Al2O3(110) were considered. Our results show that for the adsorption of Ni-n(n = 3-7) clusters, the gamma-Al2O3(110) surface is more favorable than the gamma-Al2O3(100) surface, however, for single Ni atoms and Ni-2 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 gamma-Al2O3 surfaces, is more favorable on the gamma-Al2O3(110) surface than on the gamma-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 gamma-Al2O3 surface. More importantly, the exothermicity of supported Ni-n(n = 2-7) clusters on different gamma-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.