Abstract
A series of alumina-supported MoO/sub 3/, NiO, and NiO-MoO/sub 3/ catalysts with varying amount of MoO/sub 3/ and/or NiO has been examined by low energy ion scattering spectroscopy. The evolution of the surface structure of a MoO/sub 3//Al/sub 2/O/sub 3/ sample during the preparation and after the addition of the promoter (cobalt or nickel), as well as the effect of the calcination temperature, have been studied. The results, when compared with previous studies by X-ray photoelectron and laser Raman spectroscopy, confirm the description of the supported species as small oxomolybdenum entities, well dispersed and occupying only a small fraction of the support surface. A strong shielding effect of the Mo element by the promoter Co or Ni has been detected; this gives direct evidence of a chemical interaction between the promoter and the oxomolybdenum species. A description of this association as an isopolymolybdate salt of Co or Ni is proposed.
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