Neodymium doped alkaline-earth oxide catalysts for propane oxidative dehydrogenation. Part I. Catalyst characterisation Original Research Article

Two series of alkaline earth oxides (AEO) of Mg, Ca, and Sr doped with 5 mol% Nd2O3 have been synthesised by two procedures: (i) evaporation of nitrate solutions with subsequent nitrate decomposition (designated Ev) and (ii) sol–gel method (designated SG). The catalysts samples characterisation was performed by ICP chemical analysis, BET specific surface area measurement, FTIR spectroscopy, CO2 TPD, powder XRD, and XPS. The XRD data have shown presence of solid solutions of both Nd in AEO and of alkaline earth elements in Nd2O3. The structural defects that appeared under Nd incorporation into AEO lattice were studied by detailed XRD peak profiles analysis. They were agglomerated or dispersed, their relative amounts depending on the preparation procedure and on the alkaline-earth ion size match with Nd3+. The dispersed local defects were favored by the sol–gel method and by close cation radii dimensions, whereas Nd3+ agglomerates formed mainly on Mg-containing samples and were favored by evaporation. The catalyst basicity has been observed to follow the expected order Mg < Ca < Sr and to increase under Nd3+ incorporation into AEO lattice, being stronger for SG samples than for Ev ones. The created cationic vacancies under neodymium incorporation increased the basic character of nearby oxygen anions by rendering them coordinatevely unsaturated. XPS results have shown presence of carbonate species and small enrichment in Nd on the surface.