Investigation of Copper Carbonyl Species Formed upon CO Adsorption on Copper-Exchanged Zeolites by Diffuse Reflectance FTIR

Diffuse reflectance FTIR spectroscopy was applied to the study of the fundamental C−O bond stretching vibrations and their first overtones and combination modes of copper carbonyl species formed upon CO adsorption on aqueous ion-exchanged CuZSM-5, CuM, CuY, and CuL zeolites autoreduced at 673 K in vacuo and on Cu(I)Y (sse) zeolite prepared by solid-state ion exchange of NH4Y with CuCl. At low CO pressures, different kinds of Cu(I)(CO) monocarbonyl species are formed depending upon the types of the zeolites. With increasing the CO pressure, Cu(I)(CO)2 dicarbonyl species are formed on CuZSM-5, CuM, and CuY. After an evacuation of the preadsorbed CO at room temperature, the dicarbonyl species are transformed into monocarbonyl species. Such a transformation is reversible upon a change of the CO pressure. On the CuL zeolite, however, only one type of monocarbonyl is observed, no dicarbonyl species being detected even at high CO pressures. The frequency of the stretching vibration of the metal cation−carbon bond, i.e., Cu(I)−C bond, in the monocarbonyl species, which is difficult to detect directly by infrared measurements, can be calculated using the corresponding frequencies of the fundamental C−O bond stretching vibration and combination mode. It has been found that this frequency is much more sensitive to the electron donor−acceptor ability of the Cu(I) cation in the monocarbonyl species than that of the fundamental C−O stretching vibration alone. On all of the zeolites except CuL, the monocarbonyl species characterized by the frequencies of fundamental C−O and Cu(I)−C bond stretching vibrations of 2157 and ∼440 cm-1 is observed. Another type of monocarbonyl species with its frequencies of fundamental C−O and Cu(I)−C bond stretching vibrations of about 2143 and 500 cm-1 is only formed on the high silica containing zeolites, CuZSM-5 and CuM. The Cu(I) cation in this type of monocarbonyl species possesses a very high electron donor−acceptor ability. On CuY, CuL, and CuZSM-5 (with a low copper loading) one more type of Cu(I) cations with a low electron donor−acceptor ability is detected by the CO probe. Those cations are able to form monocarbonyl species characterized by the lowest frequency of Cu(I)−C bond stretching vibration ranging within 390∼420 cm-1.