X-RAY PHOTOELECTRON-SPECTRA OF HETEROGENEOUS CATALYSTS .3. CATALYSTS DERIVED FROM DIMERIC MOLYBDENUM(II) CARBOXYLATES ON SILICA AND THEIR REACTIONS WITH CARBON-MONOXIDE, NITRIC-OXIDE, AND HYDROGEN

The X-ray photoelectron spectra (XPS) have been recorded for a series of molybdenasilica catalysts (containing 4 or 10% Mo). These catalysts were prepared from the molybdenum (II) carboxylates Mo2(O2CR)4, where R = H, CH3, CF3, or C6H5, which contain a quadruple MoMo bond. The Mo 3d 1 2, 1 2 and O 1s binding energies were measured for the calcined catalysts before and after their reaction with NO and CO. XPS evidence supports the presence of a mixture of Mo (VI), Mo (V), and Mo (IV) sites on the surface of the calcined catalysts, while treatment with NO and CO leads to the formation of Mo (VI) and Mo (V), respectively. The redox characteristics of the catalyst (i.e., Mo (VI) Mo (V)) induced by NO and CO treatments are reversible as shown by cycling of the NO and CO in various sequences. ESR spectroscopy indicates that the surface changes monitored by XPS are also typical of the bulk, namely, Mo (V) with g = 1.93 is formed upon CO treatment but disappears in the presence of NO. The present XPS results are compared to relevant literature data for supported molybdena catalysts prepared from more conventional starting materials. It is suggested that Mo-SiO2 prepared from Mo2(O2CR)4 may have different metal-support interactions from other molybdena systems. © 1979.