Effect of rhodium modification on structures of sulfided Rh-MO-K/Al_2O_3 catalysts studied by XAFS

摘要

Local structures of the molybdenum sulfide species in the sulfided Rh-Mo-K/Al_2O_3 catalysts with various rhodium loadings have been investigated by XAFS. From the fitting results, the Mo-Mo coordination number for the sample free of rhodium (ca. 3.7) is similar to that for the crystalline MoS_2, indicating that the supported sulfided molybdenum species mainly present as large patches of MoS_2-like slabs. For the rhodium-modified samples, howerver, the amplitudes of the Mo-S and Mo-Mo coordination shells are sharply reduced, revealing a high dispersion of the MoS_2-like species in the catalysts. The scheme of the microstructures of the sulfided molybdenum species stabilized on the alumina support is discussed based upon the analysis of the EXAFS data.