INFRARED SPECTROSCOPIC STUDY OF THE ISOCYANATE SURFACE COMPLEX OVER CU-ZSM-5 CATALYSTS

The adsorption of HNCO on H-ZSM-5 and on Cu-containing ZSM-5, SiO2, an d Al2O3 catalysts was investigated by means of Fourier transform infra red spectroscopy in the temperature range 200-673 K. HNCO readily adso rbs on ZSM-5 at 200-300 K to give an intense absorption band at 2260 c m(-1). This band is the asymmetric stretch of NCO bonded to Al3+. The formation of Si4+-NCO absorbing at 2300 cm(-1) requires a higher tempe rature, at least 373-473 K. The migration of NCO from Al3+ to Si4+ was observed. The location of absorption bands due to NCO bonded to Cu de pended on the state of the Cu; it was at 2230-2240 cm(-1) for Cu-0-NCO , 2200-2210 cm(-1) for Cu+-NCO and 2180-2185 cm(-1) for Cu2+-NCO. The isocyanate on Cu metal was found to be more stable than that on Pt met als. The spillover of NCO from Cu to the support also occurred, but it s extent was less than on the supported Pt metals. (C) 1995 Academic P ress, Inc.