负载Ru催化剂的CO吸附态的研究

Investigation on CO Adsorption over Supported Ruthenium Catalyst

本文运用原位FT-IR光谱系统研究了各种活化处理因素对负载Ru催化剂的CO吸附态的影响.发现2135±15(HF)和2075±5(MF)cm^(-1)IR谱峰产生于金属与载体接触的周边部位,反映了以多重态吸附的CO对称和反对称振动.金属载体间的相互作用的增强,HF和MF强度大幅度衰减.经纯O_2处理的催化剂,HF和MF峰位置没有发生变化,而峰强度随吸附时间的相对增长速率有很大的差异.对HF和MF作了归属,提出了6配位18电子紧壳层稳定结构的表面锚合的羟基氧桥或卤桥的多聚羰基化合物种CO吸附模式.

Infrared spectra of CO on supported Ru catalysts showed that there existed three kinds of IR spectral bands at 1980-2060, 2075±5 and 2135±5 cm-1, designated as LF. MF and HF bands, respectively. The first band became saturated almost instantly in excess CO and exhibited coverage-dependent frequencies at subsaturation which is consistent with its assignment to CO in on-top sites on the Ru crystallites. The latter two bands growed synchronously as a pair in CO at room and higher temperature with prolonged time of CO adsorption. Depending on the desorptive sequence of TPD-IR . spectra of adsorbed CO, the MF band was found to divid into two bands, referred to as MF1 and MF2. The latter is ascribed to adsorbed CO with the configuration Ru8+-CO and indicates that CO adsorbed on these sites where the interaction between metal and support was stronger. The pair of HF and MF1 are known to be associated with the Ru carbonyl structure of [(support -O)2Ru+2(CO)3]n) species. The intensity of these two bands was enhanced by calcining catalysts in inert atmosphere, instead of in air, before reduction, as well as by the presence of trace of H2O or/ and halide anions in the catalysts. The formation of HF and MF1 bands can be visualized as follows: the surface soluble Ru atoms or clusters with low coordination numbers (e. g. protruding atoms in the Ru crystallites) on Ru particles can be oxidized with surface hydroxyl groups, during the CO oxidative corrosive adsorption, to Ru+2 cation, and these Ru+2 cations combined with CO to form surface-grafted tricarbonyl species. These Ru+2 carbonyl species could be linked by bridging bonds of lone pair electrons of oxygen in the surface hydroxyl groups or halide anions to form a kind of polymeric carbonyl species with 6-coordinated, 18 electrons closed shell stable structure, which is known to have high barriers to ligand replacement via associative or dissociative mechanism.