Comparison of CuxZnAlO mixed oxide catalysts derived from multicationic and hybrid LDH precursors for methane total oxidation

Abstract

CuxZnAlO mixed oxide catalysts for methane total oxidation have been obtained by thermal activation of layered double hydroxides (LDHs) precursors. LDHs have been prepared following two different routes: (i) conventional coprecipitation (pH=8 or 10) and (ii) hybridation achieved by intercalation of Cu-containing anionic complexes ([Cu-EDTA]2− and [Cu-citrate]−) into host ZnAl-LDH. The mixed oxide catalysts showed only ZnO phase at Cu<5 at.% and a mixture of ZnO and CuO phases at higher Cu content. CuO particles differently interacting with the oxide supports were revealed by TPR analysis. They were reduced at lower temperature but with a lower H2 consumption in the mixed oxides derived from the hybrids suggesting lower accessibility. The activity of the catalysts in the total oxidation of methane strongly depended on the method of preparation of the LDH precursors and on the Cu content. Cu10ZnAlO (10 at.% Cu) derived from a coprecipitated LDH showed the highest intrinsic activity and a good stability suggesting an optimum dispersion of the copper-containing active species. Among the catalysts derived from the hybrid precursors, the higher activity of the Cu(EDTA)-ZnAl ones may be due to their higher Cu content. The lower intrinsic activities and higher activation energies of this series of catalysts compared to the coprecipitated ones may be due to the lower accessibility to the CuO active sites.