Rh~Ⅰ中心上一氧化碳和氢相互作用研究

A STUDY OF INTERACTION OF CO AND CHEMISORBED H_2 ON Rh^I SITES

利用TP-IR动态方法研究了0.7％Rh_2/Al_2O_3上CO吸附,CO和H_2共吸附的动态行为。结果表明,0.7％Rh_2/Al_2O_3上CO只有孪生吸附态,谱线出现在2087和2014cm^(_1)且孪生吸附态对应的Rh^I中心是稳定的。CO单独暴露于催化剂时,CO歧化在260℃以上发生。在室温CO和H_2(1:1)共吸附时,2050cm^(_1)新谱带出现,归属为Rh^I中心上形成羰基氢化物。动态过程中2050cm^(_1)谱带逐渐向低波数位移的行为归结为多氢羰基氢化物的形成。与CO单独吸附的结果相比较,作者认为在CO+H_2反应条件下,C—O键的削弱和断裂是经由多氢羰基氢化物途径,而不是经由C—O键直接断裂(CO解离或Bouduarol反应)途径。

0.7% Rh2/Al2O3 was prepared by the impregnation of γ-Al2O3 with a hexane solution of Rh2(CO)4Cl2. The dynamic behavior of CO adsorption and coadsorption of CO and H2 have been investigated by using TPD-IR dynamic method. The results of CO adsorption indicate that only gem-dicarbonyl species is formed and RhI sites are stable. When 0.7% Rh2/Al2O3 was exposed to pure CO above 260 ℃ the intensities of gem-dicarbonyl bands greatly decreased, and the bands disappeared after a longer treatment in CO. In the coadsorption of CO and H2 the intensities of gem-dicarbonyl band declined incomparison with CO adsorption alone and a new band at 2050cm-1 appeared at 20℃, which is attributed to the carbonyl hydride on RhI sites.The results of the IR study of the coadsorption of CO and H2 in the dynamic process indicate that, there are two simultaneous processes, one of which is the gradual transformation -of the gem-dicarbonyl band into the new band;the other is the shift of new band to lower wavenumber. The fact that the new band at 2050cm-1 gradually shifted to lower wavenumber shows that rhodium carbonyl polyhydride is formed during coadsorption of CO and H2 followed by temperature programmed heating in CO and H2 or CO adsorption follwed by temperature programmed heating in flowing H2. In thiss proces C-O bond is gradually weakened with a reduction in C-O stretching frequency. At or above 200℃, the C-O bond in rhodium carbonyl polyhydride has broken down. When these results are compared with that obtained in the IR study of CO adsorption alone during the dynamic process of temperature programmed heating in CO, we believe that in CO+H2 reaction the scission of C-O bond is via the intermediate of a carbonyl polyhydride.