Catalysis by Au nanoparticles has attracted considerable attention since it was discovered that Au particles, with a size less than a few nm, are active. Au nano­particles deposited on oxides can dissociate H2 heterolytically on sites involving one Au atom and a nearby surface oxygen atom. This is in contrast with other metals such as M = Ni, Pd, Pt, etc., where H2 is dissociated homolytically (in a symmetric fashion). The relatively poor activity of Au for hydrogenation reactions was attributed partly to the inability of Au to break the H-H bond and to the instability of Au hydrides (Au-H species), while various M-H species have been clearly evidenced by INS.

The INS spectra obtained after chemisorption of H2 on 3-4 nm Au particles supported on nanoparticulate ceria (5 nm) show apeak at400-600 cm-1 accompanied by a broad band from 750 to 1,200 cm-1. The first peak was assigned to bridging hydroxyl groups, and the second band to librational modes of water present on the catalyst surface and resulting from the reaction of hydrogen with oxygens of ceria [7]. The lack of observation of Au-H species by INS can be explained by the low per­centage of Au on the sample (Au loading: 0.48 wt%). On the other hand, the formation of Au-H could be observed by Fourier-transform infrared (FT-IR) spectroscopy, which seems to indicate a greater sensitivity of IR spectroscopy for this sample.