线式和桥式中心上一氧化碳和氢相互作用研究

A Study of Interaction of CO and Chemisorbed H on Linear Sites and Bridged Sites

本文利用TP(程序升温)-IR动态方法研究了3%Rh_2/Al_2O_3的CO吸附,CO和H_2共吸附的动态行为.结果表明3%Rh_2/Al_2O_3上有孪生,线式和桥式三种吸附方式.当CO单独暴露于催化剂时,三种吸附中心上CO歧化在260℃以上发生.CO和H_2在室温下共吸附,在2050,1995和1815cm^(-1)出现谱带.可分别归属为Rh^1中心上的羰基氢化为(?),线式中心上的羰基氢化物(?)和桥式中心上的羰基氢化物(?).动态过程中这些羰基氢化物谱带向低频位移的行为也可归结为相应的多氢羰基氢化物的形成.与CO单独吸附的行为相比较,作者解释为在CO+H_2反应条件下,Rh_2/Al_2O_3的三种吸附中心上C-O键的削弱和断裂是经由相应的羰基氢化物-多氢羰基氢化物途径,而不是经由C-O键的直接断裂途径(CO解离或Bouduard反应).

The dynamic behavior of CO adsorption and of CO and H2 coadsorption on 3% Rh2/Al2O3 have been investigated by TP(or TPD)-IR dynamic methods. The results of CO adsorption indicated that on 3 % Rh2/A12O3 gem-dicarbonyl, linear CO and bridged CO are formed and Rh1 sites, linear sites and bridged sites are stable. When the catalyst is exposed to pure CO at 20- 260℃ , all bands of the chemisorbed CO practically did not change in wavenumber and intensities indicating that C -O bond was not weakened. While above 260℃ the intensities of gem-dicarbonyl bands greatly decreased without any shift in wavenumber and the bands disappeared after a longer treatment in CO; and the intensities of linear CO and bridged CO bands decreased with a simultaneous shift to lower wavenumber. The decrease of the intensities of gem-dicarbonyl, linear CO and bridged CO bands above 260℃ in CO was attributed to the formation of surface carbon. In coadsorption of CO and H2 three new bands at 2050, 1995 and 1815 cm-1 appeared at 20℃. Which were attributed to on Rh'sites, on linear sites and on bridged sites respectively. The results of CO and H2 coadsorption indicated that the bands at 2050, 1995 and 1815 cm-1 gradually shifted to lower wavenumber and disappeared above 200℃. These facts were more remarkable during CO adsorption-TP in flowing H2. The facts that these bands gradually shifted to lower wavenumber show that the multi-hydrogen carbonyl hydrides were formed during CO and H2 coadsorption. In these processes C-O bonds are gradually weakened with a gradual reduction of stretching frequency. Above 200℃, the intensities of these new bands greatly decreased; meanwhile CH4 was detected in gas phase. This indicates that the C - O bonds in multi-hydrogen carbonyl hydride have been broken. On comparison of these results with that obtained in CO adsorption alone we believe that the scission of C - O bond is via multi-hydrogen carbonyl hydride.