Infrared study of ozone adsorption on CeO2

Ozone (O-3) adsorption on CeO2 pretreated under different conditions and characterized by low-temperature CO adsorption was studied by Fourier transform infrared (FTIR) spectroscopy at 77-300 K. Preliminary exposure to CO2, pyridine, acetonitrile, or methanol at 293 K or to CO at 77 K, as well as adsorption of O-18 substituted O-3 were used to clarify the nature of adsorption sites and the structure of the surface species. In addition to physisorption and weak H-bonding with the most acidic OH groups of the ceria surface, O-3 forms coordinate complexes with two kinds of Lewis acid sites. These complexes have the bands of the nu(1) and nu(3) modes of O-3 at 1096 and 1018-1015 cm(-1), or 1104 and 1012-1008 cm(-1), respectively. The coordinated O-3 molecule is no longer symmetric but is bound to a surface Ce4+ ion via one of its terminal oxygen atoms. Basic surface sites of the samples pretreated at 773 K account for O-3 decomposition, which occurs almost explosively at 77 K but could be inhibited if O-3 adsorption is performed from the solution in liquid oxygen. Formation of ozonide O-3(-) (bands at 792 and 772 cm(-1)) and superoxide O-2(-) (band at 1128 cm(-1)) species was detected; these species are believed to be the intermediates of O-3 decomposition on basic sites. On ceria, O-3 does not react at 77 K with adsorbed CO, but ozonolysis of surface methoxy groups proceeds slowly, leading to a formate surface species.