Effects of Excess Cobalt Oxide Nanocrystallites on LaCoO_3 Catalyst on Lowering the Light off Temperature of CO and Hydrocarbons Oxidation

摘要

Catalysts with the formula of LaCo_((1+x))O_((3+δ)), where 0 ≤ x ≤ 1, were studied for oxidation of CO and C_2H_6 in a synthetic exhaust gas, comprising 6.0 % CO and 0.2 % C_2H_6 in Ar. Ethane was selected as a model for hydrocarbons in the exhaust gas. The performance of catalysts is correlated to their properties, particularly their redox in oxidizing and reducing atmospheres. XRD patterns show perovskite structure for all catalysts. The Co_3O_4 crystallite size was calculated using the Scherrer's equation. TPR results show the reduction of Co_3O_4 and cobalt oxide in perovskite structure in the range of 330 - 475 ℃ and 550 - 650 ℃, respectively. Disappearance of the cobalt oxide structure in XRD patterns of LaCo_(1≤x≤1.3)O_((3+δ)) catalysts are attributed to small size of the cobalt oxide crystallites. Redox properties of catalysts were also studied by electrical conductivity measurements. Similar Arrhenius-type electrical conductivity behaviors of catalysts with that of cobalt oxide indicates that the cobalt component is essential for charge carrier mobility in LaCo_((1+x))O_((3+δ)) catalysts. Catalyst with 0.3 mole excess cobalt, i.e. LaCo_(1.3)O_((3+δ)), which shows the lowest activation energy of electrical conductivity (E_c) and the lowest ratio of conductivities in reducing to oxidizing atmospheres, has the lowest light off temperatures for oxidation of both CO and ethane. High ability of catalysts in gas phase-lattice oxygen transfer is evidenced by the fast reduction and oxidation behaviors of catalysts in CO and air atmosphere, respectively.