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 cha racterised by several techniques (AA, ICP, XPS, BET, XRD and FTIR) whi ch evidenced that cesium is only on the surface of the catalyst and do es 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 th e catalytic behaviours in the oxidative dehydrogenation (ODH) of n-but ane. 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 conductiv ity and for C-4's selectivity. Cesium-promoted catalysts are much more conducting than unpromoted NiMoO4 due to the contribution of an addit ional surface ionic conductivity by mobile Cs+ ions to the overall con ductivity. Correlatively, there is a substantial decrease of the appar ent activation energy of conduction. Such surface ionic conductivity i s associated with labile O2- species which would be responsible for th e 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 an d selectivity to oxidative dehydrogenation products. From electrical c onductivity data, it has been proposed that butane ODH involves simult aneously basic O2- sites for the initial proton abstraction from the h ydrocarbon and anionic vacancies required (i) for the second H atom ab straction and (ii) for oxygen dissociative chemisorption necessary for the reoxidation of the solid, i.e. for the regeneration of surface an ions.