负载Ru催化剂金属表面活性结构的研究

运用原位FT-IR光谱和TPSR-MS等技术研究了负载Ru催化剂的金属表面状态. 结果表明催化剂中存在二类静态活性中心: (1)体现金属Ru本征特性的S_1中心, (2)金属与载体相互作用而产生的S_2中心. 在吸附CO及其加氢反应过程中, S_1中心上处于边、角、棱位置等配位不饱和的金属Ru原子或原子簇经CO剥蚀而产生的动态S_3活性中心. CO在S_1中心上以Ru~0—CO线式态吸附的, 其IR谱带位于1980～2060 cm~(-1)之间. Ru~0—CO在H_2流中进行程序升温加氢反应的TPSR-MS图上出现450 K左右的低温甲烷峰. 焙烧温度升高, 则在TPSR-MS谱图上出现两个甲烷峰, 600±50 K的高温甲烷峰归属为S_2中心上以Ru~(δ+)-CO线式态吸附CO加氢所致. IR谱图中的2075±50 cm~(-1)峰代表Ru~(δ+)-CO. IR谱中2135±5和2075±5 cm~(-1)这对峰的出现反映了S_3中心的形成.

INVESTIGATION ON SURFACE METALLIC ACTIVE STRUCTURE OF SUPPORTED RUTHENIUM CATALYST

The metallic surface structure of supported Ru catalysts, activated at different pretreatment conditions, was investigated by following the development of IR bands due to adsorbed CO, as well as by following the methane formation in temperature-programmed surface reaction(TPSR-MS) of adsorbed CO in a H_2 flow. It revealed that there existed three kinds of active sites, designated as S_1, S_2 and S_3 sites, respectively. The S_1 sites, characterized by low frequency(LF) bands of 1980～2060 cm~(-1) for linearly adsorbed CO, were attributed to metallic Ru particles with almost no interaction with the support. The TPSR-MS results showed that, when reaction with H_2, the adsorbed CO on these sites changed into CH_4 at a lower temperature of ca. 450 K, varying with activated conditions. On the other hand, the S_2 sites were visualized to be interfacial Ru particles in direct contact with supporting materials, and exhibited stronger interaction with support. As a result of this MSI effect, the S_2 sites would result in the formation of electron-deficient Ru~(δ+) sites, and maintain high dispersive states of metal catalysts, and also adsorb CO with configuation of Ru~(δ+)-CO at 2075±5 cm~(-1) mediun frequency (MF_2). Upon hydrogenation, the adsorbed CO of formed methane at a much higher temperature of ca. 650 K. Obviously, the S_1 and S_2 sites are static sites of the conventional nature.The interaction of CO with supported Ru catalysts led to the oxidative disruption of soluble surface Ru atoms or clusters with coordinatively unsaturation on the S_1 sites, as indicated by the slow transformation of the bands at 1980～2060 cm~(-1) due to Ru_x~0-CO to bands at 2135±5 cm (HF) and 2075±(MF_1) cm~(-1) attributed to surface-anchored carbonyl (support-O)_2Ru(CO)_3]_n (n=1,2…) species, referred to as S_3 sites. The oxidative corrosive carbonylation of Ru is both favored ther-modynamically and feasible kinetically under our experimental conditions. It was demonstrated that at higher temperature, the presence of CO, especially with high partial pressure of hydrogen, such as in the case of TPSR-MS experiments, causes the reductive agglomeration of the S_3 sites, and the soluble Ru particles or clusters on the S_1 sites will be restored. These observation led us to postulate that the S_3 sites were active sites formed dynamically during the course of the Fischer-Tropsch reaction.