Formation and partial self-healing of lattice defects during thermal treatments of GaZSM-5: An FTIR study using CO as a probe molecule
Room temperature adsorption of CO on H-GaZSM-5 occurs on both Brønsted and Lewis acid sites. On Brønsted acid sites, CO adsorption leads to an equilibrium between Z–OH···CO and Z–OH···OC species, where Z stands for the zeolite framework. Corresponding C–O stretching frequencies are 2168 and 2122 cm−1, respectively. Lewis acid sites, formed as a consequence of gallium loss during thermal treatment of the zeolite at 973 K in vacuum, appear to consist of Ga3+ and Ga+ ions; some zerovalent gallium can also be formed. Further heat treatment under oxygen seems to restore some of the gallium atoms to the zeolite framework, by a mechanism which is thought to involve the volatile gallium(I) oxide.