Effect of Co/Ni ratio and Ce amount in cobalt nickel mixed oxide catalyst for methane combustion

Abstract

In view of the low emissions of nitrogen oxides, carbon monoxide and toxic hydrocarbons, natural gas vehicles (NGVs) have been attracted considerable attention as one of the substitutes for gasoline and diesel vehicles. However, the emission of unburned methane from NGVs heavily contributes to greenhouse effect since the global warming potential of methane is 21 times higher than that of carbon dioxide. This study aimed at investigating the effect of Co/Ni ratio of cobalt nickel mixed oxides and cerium doping on CoNi (50:50) catalyst for methane combustion to effectively reduce unburned methane. A series of cobalt nickel mixed oxide catalysts were prepared by co-precipitation method with the Cobalt oxide, Nickel oxide, CoNi (75:25), CoNi (67:33), CoNi (50:50) and CoNi (33:67). The notation of CoNi (X:Y) was used to designate the catalyst with the ratio of Co:Ni = X:Y. In addition, a series of cerium doped CoNi (50:50) catalysts were prepared by wet impregnation method with the cerium loading of 5 wt%, 10 wt% and 20 wt%. The catalytic combustion of methane was performed among all prepared catalysts. Various characterizations were performed such as N2 adsorption-desorption with BET method, ICP-AES, XRD, EXAFS, XPS and H2 TPR. It was found that CoNi (50:50) and CoNi (67:33) catalyst exhibit the superior activity for methane combustion. Both catalysts contain NiCo2O4 spinel structure in largely distorted form. Such structure disorder contributes to improvement for the adsorption of surface oxygen species and reducibility of NiCo2O4. In addition, cerium doped catalyst demonstrates the enhancement of the activity. Especially, 10 wt% cerium doped catalyst indicates the highest activity among the catalysts. It implies that cerium doping as well as structure disorder plays an important role for the methane combustion. Both characterization and reaction results lead us to the conclusion that not only optimized cobalt nickel mixed oxide but also proper amount of cerium doping on CoNi (50:50) can improve the activity of methane combustion.