Characterization of CO–Rh species formed on Rh-Y zeolite by temperature-programmed desorption and infrared techniques

Abstract

The characterization of CO–Rh species which are formed on Rh-Y zeolite during exposure to CO has been carried out using the techniques of temperature-programmed description (t.p.d.) and infared spectroscopy (i.r.). The formation of three kinds of CO–Rh species was observed. The first is characterized by a t.p.d. peak at 473 K as well as i.r. bands at 2092 and 1834 cm^–1. The second species is characterized by a t.p.d. peak at 500 K and i.r. bands at 2098 and 1763 cm^–1. A similarity between the i.r. spectra of these species and those of the Rh₆(CO)₁₆/Na-Y system suggests that these two kinds of CO–Rh species are cluster-like species formed at the external surface or in the outermost supercages of the zeolite particles. The third species, characterized by a t.p.d. peak at 573 K and four i.r. bands at 2115, 2098, 2044 and 2019 cm^–1, is considered to be an Rh^I(CO)₂ species formed in the zeolite cavities. Conversion of the cluster-like species into the Rh^I(CO)₂ species is accelerated by higher temperatures, lower concentrations of CO and lower rhodium contents of the catalyst. In addition, the removal of water, physically adsorbed to fill the zeolite pores, accelerated the conversion. The rate of the exchange reaction between adsorbed and gaseous CO was much faster on the Rh^I(CO)₂ species than on the cluster-like species.