Rh/SiO~2和Rh/NaY催化剂上合成气反应的高压原位 红外光谱研究

采用高压原位FT-IR技术，对比研究了CO加H~2反应条件下Rh/SiO~2和Rh/NaY催化剂表面反应中间物种。在Rh/SiO~2表面上，无论在常压还是在1.0MPa合成气中，只观察到线式和桥式吸附CO。而在常压合成气中，Rh/NaY上不仅存在上述CO吸附物种，而且还有孪生型的Rh(Ⅰ)(CO)~2和少量Rh~6(CO)~1~6；当合成气压力升至1.0MPa后，Rh(Ⅰ)(CO)~2迅速转化成Rh~6(CO)~1~6和在2042cm^-^1产生吸收的单核羰基Rh物种，与此同时催化剂表面还生成了单齿和双齿乙酸根物种；这些在高压下生成的物种在合成气压力重新降回到常压时依然稳定存在。研究Rh/NaY上合成气反应表面物种与H~2的反应行为表明单齿乙酸根很可能是反应的活性中间物。这些结果说明Rh/NaY催化剂在高压合成气中的重构是诱发选择生成乙酸反应的基础。

In-situ FT-IR study of high pressure syngas conversion over Rh/SiO~2 and Rh/NaY catalysts

High pressure syngas V(CO):V(H~2)=1] conversion over unpromoted Rh catalyst supported on silica and zeolite NaY were studied at 250℃ with an in-situ IR cell that avoided contamination of iron carbonyls. Change of the syngas pressure produced no effect on the IR spectrum of Rh/SiO~2; bridged and linear CO on Rh clusters were the only detectable surface species under 0.1 to 1.0 MPa of flowing syngas. In addition to the bridged and linear CO species, two types of dicarbonyls Rh(Ⅰ)(CO)~2] and a small amount of Rh~6(CO)~1~6 were formed when Rh/NaY was exposed to 0.1 MPa syngas. Increasing of the syngas pressure to 1.0 MPa over Rh/NaY resulted in transformation of the dicarbonyls to Rh~6(CO)~1~6 and probably a mononuclear rhodium carbonyl featuring and absorption 2042 cm^-^1. The detectable reaction products adsorbed on Rh/NaY catalyst under 1.0 MPa were monodentate and bidentate acetates. These surface species were maintained even after releasing the syngas pressure back to 0.1 MPa. Thus, a remarkable difference exists in the effect of syngas pressure on the structure of Rh catalysts: reconstruction of Rh catalyst under high pressure of syngas occurs in zeolite NaY but not on silica. Reactivity of the adsorbed surface species toward hydrogen after the cagalyst reconstruction suggests that the monodentate acetate groups are responsible for the selective formation of acetic acid from syngas over the Rh/NaY catalyst.