How does γ-irradiation influence the phase transition of α-NiMoO4?

An attempt has been made to stabilize the high temperature β-NiMoO4 phase which is catalytically more selective in oxidative dehydrogenation of alkanes at room temperature through irradiation by γ-rays. Different samples of stoichiomertic α-NiMoO4 were subjected to different doses of γ-irradiation in the range of 50–500 kGy. The effect of γ-irradiation on the α- to β-NiMoO4 transition temperature was studied using differential thermal analysis (DTA) technique with controlled rate of heating and cooling. The formation of β-NiMoO4 phase was investigated by means of XRD and FT-IR techniques. The catalytic performance of the resulting catalyst samples in the oxidative dehydrogenation of cyclohexane at 500 °C was investigated. The DTA investigation revealed that γ-irradiation effectively lowers the temperature of the α → β transition in NiMoO4 but did not affect the temperature of β-to-α return transition. A new peak characteristic of β-phase was detected at 2θ = 26.3° in the XRD patterns of γ-irradiated samples. FT-IR analysis also confirmed the formation of β-NiMoO4 where a new band characteristic of β-phase was observed at 879 cm−1 in the spectra of these samples. In view of the XRD results, the observed change in the α- to β-NiMoO4 transition temperature was attributed to the fact that radiation induced a strain in the α-NiMoO4 crystals. On the other hand, catalytic activity results indicated that γ-irradiated nickel molybdate catalysts showed a high selectivity towards cyclohexene at isoconversion (5%) in comparison with that of unirradiated parent α-NiMoO4.