Surface studies of ceria and mesoporous ceria powders by solid-state 1H MAS NMR

Herein is described the first detailed study of ceria surfaces using H-1 MAS NMR techniques to probe the nature of hydroxyl species formed after exposure to water. The H-1 MAS NMR signal deriving from adsorbed hydroxyl species was used to probe the surface of ceria and crystalline mesoporous powders as a function of calcination temperature. It was found that the signals from all samples were well-resolved and of high intensity and showed easily observed chemical shifts depending on their environment. High surface areas were evidenced by large hydroxyl-derived H-1 peak integrals for the mesoporous samples compared to that of ceria prepared by traditional precipitation techniques. It is suggested that surface-adsorbed hydroxyl species generally occupy two distinct structural environments (on-top and bridge) on ceria surfaces. Hydroxyl groups assigned to on-top positions were found to be more mobile and desorbed at lower temperatures than those held in bridging positions. In addition, the mesoporous surface appears to comprise of two distinct regions assigned to external surfaces and internal pore and cavity surfaces. We also highlight a more general point that adsorbate probe MAS NMR techniques can provide important analysis of surface chemistry in favorable circumstances. Ceria is an ideal example in which very well-resolved intense H-1 signals from adsorbed hydroxyl species are observed, and consequently MAS NMR methods provide unique details of the surface chemistry. The relevance of these data is discussed in the context of current knowledge of the properties of ceria.