Preparation and characterization of Ce-Zr and Ce-Mn based oxides for n-hexane combustion: Application to catalytic membrane reactors

Ce-Mn and Ce-Zr catalytic samples with different Ce/metal molar ratio have been prepared by coprecipitation followed by calcination at moderate temperatures (350–450 °C) and characterized by XRD, XPS, BET, TPR and SEM–EDX techniques. The preparation procedure of analogous catalytic membranes over γ-Al2O3 ceramic supports has been also studied in order to control loading, distribution and composition of the catalytic material inside the membrane thickness. To evaluate their catalytic performance, the combustion of n-hexane from air-diluted streams has been carried out in both a conventional fixed bed reactor and a flow-through catalytic membrane reactor operating in Knudsen diffusion regime. Surface area alone cannot account for the reaction performance achieved; however, the redox properties and oxygen mobility of the mixed oxides have been revealed as key parameters controlling the catalytic activity. Although Ce-Mn powdered catalysts appear more active in n-hexane combustion than their Ce-Zr counterparts; a strong deactivation phenomenon, more severe for Mn rich samples, is observed with time on stream in contrast with Ce-Mn based membranes showing catalytic stability under equivalent reaction conditions.