Reaction between NiO and Al2O3 in NiO/γ-Al2O3 catalysts probed by positronium atom

NiO/ γ -Al 2 O 3 catalysts with NiO content of 9 wt% and 24 wt% were prepared by solid state reaction method. They are annealed in air at temperatures from 100 °C to 1000 °C. Positron lifetime spectra were measured to study the microstructure variation during annealing process. Four positron lifetime components were resolved with two long lifetime τ 3 and τ 4 , which can be attributed to the ortho-positronium lifetime in microvoids and large pores, respectively. It was found that the longest lifetime τ 4 is rather sensitive to the chemical environment of the large pores. The NiO active centers in the catalysts cause decrease of both τ 4 and its intensity I 4 , which is due to the spin-conversion of positronium induced by NiO. However, after heating the catalysts above 600 °C, abnormal increase of the lifetime τ 4 is observed. This is due to the formation of NiAl 2 O 4 spinel from the reaction of NiO and γ -Al 2 O 3 . The generated NiAl 2 O 4 weakens the spin-conversion effect of positronium, thus leads to the increase of o-Ps lifetime τ 4 . Formation of NiAl 2 O 4 is further confirmed by both X-ray diffraction and X-ray photoelectron spectroscopy measurements.