担载型过渡金属催化剂上甲烷活化的TPSR研究

采用程序升温表面反应(TPSR)手段,系统地探讨了甲烷在过渡金属催化剂上的吸附活化特征,并对甲烷分解后产生的表面碳物种进行了考察。研究表明,CH_4确能在SiO_2担载的过渡金属(Co、Ni)催化剂表面发生解离吸附,生成复杂的表面碳物种,其中低温(＜400K)下的C_α是以原子态存在的碳化物型表面物种,它具有较高的配位不饱和度,因而具有较强的C—C成键能力。C_β(500~700K)是以M—CH_x(x≤3)形式存在的无定型表面碳物种,不同金属其特征亦不相同,C_γ(＞800K)是以石墨型存在的非活性表面碳物种。C_γ是催化剂失活的重要因素,高温有利于其生成。同时C_α的生成受制于催化剂表面碳物种的浓度分布,是一个动态平衡过程。

TPSR studies of the activation of methane on supported transition metal catalysts

The adsorptive activation of methane on transition metal catalysts was systematically investigated via TPSR, together with the surface carbonaceous species after the decomposition of methane. The results indicated that three kinds of carbonaceous species with different structures and properties were formed by the decomposition & adsorption of methane on the surface of SiO2 supported transition metal (Co, Ni)catalysts,among which Cα(<400K) was a carbide species in atomic state and its ability in C-C bonding was relatively strong owing to its highly unsaturated coordination ;Cβ( 500-700K) was an amorphous carbonaceous species in the form of M-CHx(x≤3), the characteristics of which .depends on the metal in it; and Cγ(>800K) was a graphite-type inactive carbonaceous species, which is an important factor for catalyst deactivation. High temperature is favourable to its formation. Furthermore, the formation of Cα,a dynamic equilibrium, is restricted by the d.stribution of carbonaceous species on catalyst surface.