AN EXAFS STUDY ON OXIDIC AND SULFIDED K-MOO3 GAMMA-AL2O3 CATALYSTS/

By analyzing the extended X-ray absorption fine structure (EXAFS) of t he Mo K-absorption edge, structural information for both oxidic and su lfided K-MoO3/gamma-Al2O3 catalysts with different potassium content w as obtained. The oxidic samples show two backscatterer peaks in the ra dial distribution function (RDF), which correspond to the Mo-O coordin ations in the nearest Mo-O shell. The nearest oxygen atoms are present with large configurational disorder. The RDF for the K/Mo = 0 sample is significantly different from that for crystalline MoO3 and ammonium heptamolybdate. The RDFs for potassium promoted samples are, in some extent, similar to that for ammonium heptamolybdate. The sample with K /Mo = 0.8 and that with K/Mo = 1.5 do not show obvious difference in t heir local Mo-O structures. The EXAFS results support the earlier conc lusions from Raman spectroscopy studies on identical samples [7]. When the samples are sulfided, a rearrangement of the local neighbors arou nd Mo atoms takes place, to form small MoS2-like crystallites. The MoS and Mo-Mo coordination distances on these catalysts are the same as t hose in crystalline MoS2, but the coordination numbers are significant ly lower than in MoS2. The EXAFS results indicate that Mo species on t he K/Mo = 0 catalyst mainly consist of MoS-Mo units (the basic buildin g units of MoS2), which are highly dispersed and show a higher level o f disorder than in MoS2. With the modification by the potassium promot er, Mo species are significantly aggregated and their local neighbors are more similar to those in MoS2, but the Mo species still exist in a state of high dispersion.