Abstract
Changes of the surface structure of bismuth molybdate catalysts Bi 2MoO 6, Bi 2Mo 2O 9, and Bi 2(MoO 4) 3 after outgassing, after interaction with hydrogen and after interaction with propylene-oxygen mixture have been studied by XPS and UPS. Outgassing of MoO 3 and Bi 2MiO 6 results in surface reduction of Mo 6+ to Mo 4+ions, Mo 4+ constituting two kinds of donor center above the valence band. This process is enhanced on interaction with H 2, but it is only in later stages of the reduction that Bi 3+ ions become reduced. In the conditions of the catalytic oxidation of propylene the surface composition of the three bismuth molybdate phases reflects that of the bulk, whereas after vacuum or reducing treatment the three compounds show similar values of the Bi Mo ratio.
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