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

► We studied the reaction between NiO and Al2O3 in NiO/γ-Al2O3 catalysts using positronium atom as a new probe. ► After annealing the catalyst above 600 °C, reaction between NiO and Al2O3 occurs, which leads to the formation of NiAl2O4. ► The NiAl2O4 resultant weakens the spin-conversion effect of positronium, which results in the abnormal increase of ortho-positronium lifetime. ► Our results show that the positronium can be used as a sensitive probe for the detection of chemical reaction occurred in catalysts.