Alumina-supported gallium oxide catalysts for NO selective reduction: Influence of the local structure of surface gallium oxide species on the catalytic activity

The surface structure of alumina-supported gallium oxide catalysts for selective catalytic reduction of NO by CH4 was studied. The XPS results showed that Ga atoms were dispersed well over the Al2O3 surface for the samples with Ga2O3 content below monolayer coverage (27 wt %) and that Ga2O3 particles appeared at higher content. The XANES and EXAFS results revealed that, below monolayer coverage, Ga atoms were located at the surface cation sites (tetrahedral and octahedral) of Al2O3, and the fraction of tetrahedral Ga species decreased as Ga content increased. The activity per surface area of the catalysts increased with Ga2O3 content up to around the monolayer coverage and declined at higher content. Below monolayer coverage, the activity per exposed Ga atom decreased as Ga content increased, which correlated well with the change in the fraction of tetrahedral Ga species. It was concluded that the present reaction is a structure-sensitive reaction depending on the local structure of Ga atom; GaO4 tetrahedra highly dispersed in the surface spinel are responsible for the activity.