CO interaction with zeolites studied by TPD and FTIR: Transition-metal ion-exchanged FAU-type zeolites

Abstract

The interaction of carbon monoxide with the cationic Lewis acid sites of transition-metal (Fe2+, Co2+, Ni2+) and Cu2+-exchanged Y and X zeolites was investigated by Fourier transform infrared spectroscopy (FTIR) and temperature-programmed desorption (TPD). The activation of the sample, the adsorption (at room temperature) of CO and the detection of desorbed molecular species were done in situ, either with FTIR or with a mass spectrometer, in gas-flow systems. During the adsorption of CO from the gas stream, a low surface coverage (between two and seven molecules per unit cell) was achieved. In the case of CuY and CuX zeolites, the TPD/MAS spectra exhibit only CO signals. During adsorption of CO, IR absorption bands in the 2200-2000 cm-1 spectral region, characteristic for symmetric stretching of CO adsorbed on zeolites, were collected in the FTIR spectra, indicating molecular adsorption of CO on Cu2+-exchanged samples. In the case of all other samples investigated, disproportionation of CO was noticed during its adsorption and temperature-programmed desorption. IR absorption bands near 2350 cm-1, characteristic for ν3 asymmetric stretching of weakly bound CO2 on zeolites, and IR absorption bands in the 2200-2000 cm-1 spectral region were collected during adsorption of CO on Fe2+-, Co2+- and Ni2+-exchanged samples. In the TPD/MAS spectra both CO and CO2 signals were monitored. Significant amounts of carbon deposits (between two and nine carbon atoms deposited per unit cell of the zeolite) were found after adsorption and TPD procedures.