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.

Key words: RHODIUM, SILICON DIOXIDE, ZEOLITE, INFRARED SPECTROPHOTOMETRY, FOURIER TRANSFORM, SYNTHETIC GAS