Indium-doped Co3O4 nanorods for catalytic oxidation of CO and C3H6 towards diesel exhaust

Abstract

Platinum-group metals are widely used as diesel oxidation catalysts (DOCs) for exhaust control. It is a challenge to improve performance and reduce the cost of DOCs, while also to avoid interference of hydrocarbons and sintering of platinum metals at high temperatures. We present here an indium-doped Co3O4 uniform nanorod catalyst whose catalytic performance in CO, C3H6, and even NO oxidation is comparable to platinum-group DOCs in diesel exhaust. No obvious deactivation was observed in long-term stability tests under simulated diesel exhaust conditions. These indium-doped Co3O4 nanorods might open a promising pathway towards low-cost efficient diesel exhaust control systems. Characterization results indicated that lattice oxygen could be much more easily abstracted by hydrogen or carbon monoxide from indium-doped Co3O4 than from Co3O4 and the physical mixture Co3O4 and In2O3. The presence of indium with its large cation radius could influence the chemical status of surface/chemisorbed oxygen in Co3O4-In2O3 nanorods, thereby increasing the mobility of lattice oxygen involved in the catalytic oxidation reaction. The reaction mechanism of catalytic oxidation of CO and C3H6 were evaluated based on kinetic and FTIR studies. For CO oxidation, activated CO3* reduced by adsorbed CO* in an irreversible step to generate the final product of CO2 could be considered as the kinetically-relevant step. DRIFT spectroscopy confirmed that only stable carbonate species were observed over Co3O4-In2O3 nanorods that might be further reduced by CO to form CO2. For C3H6 oxidation, the incorporation of activated oxygen (O*) into anion vacancy of catalyst surface was the kinetically-relevant step, while the active sites on catalyst surface should be totally covered by the intermediates of C3H6 or its generated species, which actually acted as the most abundant surface intermediates (MASI). DRIFT spectroscopy confirmed that C3H6 and its related intermediates like formate, acetate, and acetone species would be formed over Co3O4-In2O3 nanorods.