Electrical conductivity, basicity and catalytic activity of Cs-promoted alpha-NiMoO4 catalysts for the oxidative dehydrogenation of n-butane

Unpromoted and cesium-promoted alpha-NiMoO4 have been prepared and characterised by several techniques (AA, ICP, XPS, BET, XRD and FTIR) which evidenced that cesium is only on the surface of the catalyst and does not affect the molybdate structure. Catalysts with Cs loadings of 0%, 1%, 3% and 6% (surface atomic ratio Cs/Mo) were investigated by CO2-TPD and electrical conductivity techniques in order to rationalise the catalytic behaviours in the oxidative dehydrogenation (ODH) of n-butane. CO2-TPD data showed an increase in the catalyst basicity with Cs content up to 3%, but a subsequent decrease was observed fbr the 6% Cs-NiMoO4 sample. A similar trend was found for the electrical conductivity and for C-4's selectivity. Cesium-promoted catalysts are much more conducting than unpromoted NiMoO4 due to the contribution of an additional surface ionic conductivity by mobile Cs+ ions to the overall conductivity. Correlatively, there is a substantial decrease of the apparent activation energy of conduction. Such surface ionic conductivity is associated with labile O2- species which would be responsible for the higher selectivity of Cs-promoted catalysts. At high Cs loadings (6%), the growth of cesium oxide particles leads to a loss of dispersion and consequently to a decrease of basicity, electrical conductivity and selectivity to oxidative dehydrogenation products. From electrical conductivity data, it has been proposed that butane ODH involves simultaneously basic O2- sites for the initial proton abstraction from the hydrocarbon and anionic vacancies required (i) for the second H atom abstraction and (ii) for oxygen dissociative chemisorption necessary for the reoxidation of the solid, i.e. for the regeneration of surface anions.