Cobalt-aluminum mixed oxides prepared from layered double hydroxides for the total oxidation of benzene

Abstract

A series of Co-Al mixed oxides were prepared by co-precipitation method via Co-Al layered double hydroxides (LDHs) as precursors. The influence of chemical compositions of Co-Al LDHs on the structural and physicochemical properties of Co-Al mixed oxides as well as their catalytic performance for benzene total oxidation was investigated. The samples were characterized by using ICP, N2 physical adsorption, XRD, TG-DTA, SEM, TEM, Raman, H2-TPR, and XPS techniques. The characterization results showed that calcination of Co-Al LDHs gave rise to Co(Co, Al)2O4 spinel-like mixed oxide as the main phase. The crystallite size of Co(Co, Al)2O4 spinel (6∼19nm) decreased with decreasing the Co/Al molar ratio, suggesting the inhibition of crystal growth by the incorporation of Al3+ ions in the spinel phase. A drastic change in the state of Co-Al mixed oxide occurred at Co/Al=6, as indicated by H2-TPR and XPS. In benzene total oxidation, the activity of Co-Al mixed oxide increased with increasing the Co/Al molar ratio, with the highest activity at Co/Al=5; further increase in the Co/Al molar ratio to 6 led to significant decrease in the activity, properly caused by the change of surface state of mixed oxide. The 50h long-term stability test revealed that the optimized Co-Al mixed oxide was stable for the total oxidation of benzene.