What are the important factors determining the state of copper ion on various supports? Analysis using spectroscopic methods and adsorption calorimetry

Abstract

The important factors that determine the state of copper ion supported on the SiO₂·Al₂O₃, SiO₂ and ZSM-5 samples have been elucidated by using various spectroscopic techniques and adsorption calorimetry. When CO was adsorbed on the copper ion supported SiO₂·Al₂O₃ (Cu/SiO₂·Al₂O₃) sample or the copper ion exchanged ZSM-5 (CuZSM-5) sample which had been evacuated at 873 K in advance, a band was observed at around 2155 cm^-1 which can be assigned to the CO species adsorbed onto the monovalent copper ion in these samples. In the case of CO adsorption on the copper ion deposited SiO₂ (Cu/SiO₂) sample, the band due to the adsorbed CO species appeared at 2132 cm^-1. The differential heat of adsorption (H_d) of CO on Cu/SiO₂·Al₂O₃ gave a value of ca. 100 kJ mol^-1 at the initial adsorption stage and it gradually decreased with increasing amount adsorbed. The same relationship in the H_d–ν_CO (wavenumber of absorption band due to the C–O stretching vibration) plots was observed in the systems of Cu/SiO₂·Al₂O₃–CO and CuZSM-5–CO, which indicates that the same σ bonding interaction is operative in these systems. In the case of CO adsorption on the Cu/SiO₂ sample, the amount adsorbed is too small to get meaningful values of the adsorption heat for the discussion of the bonding nature between the copper ions and CO molecules, and we speculated that the same σ-bonding interaction is operative. The existence of the Brönsted acid sites on the original proton-type SiO₂·Al₂O₃ and ZSM-5 samples was confirmed by the IR spectra using CO as a probe molecule and by the measurement of solid NMR spectra. These data provide an explanation for the appearance of an IR band (2155 cm^-1) due to the CO species adsorbed on the Cu/SiO₂·Al₂O₃ and CuZSM-5 samples. The existence of Brönsted acid sites, due to the existence of Al in the lattice, can be regarded as an important factor in their role as catalysts in the various reactions. The state of copper ions that act as the active sites in the catalytic reactions is different, depending on the Si:Al ratio of the sample; the Cu²⁺ species supported on the SiO₂·Al₂O₃ sample having a lower Si:Al ratio resist reduction, because the exchanged divalent ions may occupy two exchangeable sites simultaneously. It seems that the higher Si:Al ratio is a necessary condition for keeping an amount of copper ion deposited on the support sufficient for redox reaction as well as for acting as a good NO-decomposition catalyst. From the spectroscopic observations such as IR, emission, X-ray absorption, and electron paramagnetic resonance spectra, it is also found that the copper ions on the SiO₂ sample reduced in the evacuation process are dispersed appropriately in Cu₂O-like sites.