129 Xe NMR (nuclear magnetic resonance), a useful analytical tool for the investigation of zeolite pores, was evaluated as a novel technique for the analysis of active sites on Mo/Al2O3 hydrodesulfurization catalyst. 129 Xe NMR spectroscopy of Mo/Al2O3 catalyst detected a single peak attributed to xenon migrating in a few micropores on the surface of Al2O3. When a chemical shift δ of the peak was plotted against the amount of adsorbed xenon N in the NMR measurement, a nonlinear variation of δ appeared for sulfided Mo/Al2O3 catalyst. This result indicates that xenon strongly interacts electronically with molybdenum species on the surface. In addition, the term δ0 was calculated which mainly depends on collisions between xenon and the catalyst surface from the fitting of the plot to a theoretical equation. As a result, δ0 became larger with increased the molybdenum content. This result shows that migration of xenon was inhibited by molybdenum species on the surface. Increase in the sulfurization temperature also caused δ0 to increase and almost corresponded to the sulfurization degree of molybdenum measured by XPS (X-ray photoelectron spectroscopy). This indicates that δ0 is sensitive to formation of MoS2 crystallites on the surface. 129 Xe NMR can be a powerful tool for analysis of the formation of MoS2 crystallites on Mo/Al2O3 catalyst.

The present study evaluated 129 Xe NMR spectroscopy for the analysis of Co _ Mo/Al2O3 hydrodesulfurization catalyst. This study also reconsidered the conventional interpretation of 129 Xe NMR spectroscopy that had been used for zeolite micropore analysis. The chemical shift δ of an observed 129 Xe NMR peak varied nonlinearly against the amount of adsorbed xenon N for sulfided Mo/Al2O3 catalyst. In contrast, δ was almost constant against N for dried catalyst. This result suggests that the nonlinear variation of δ against N is mainly caused by electronic interactions between xenon and coordinatively unsaturated sites on the edge of MoS2 crystallites. In addition, the xenon diffusibility δ0 calculated from 129 Xe NMR spectra gradually increased with sulfidation temperature and approached the maximum value at more than 673 K, indicating that δ0 is closely related to the formation of MoS2 crystallites on the surface. δ0 became gradually larger with the cobalt loading and reached the maximum value at 5.7 mass％ for sulfided Co _ Mo/Al2O3 catalyst. On the other hand, δ0 increased from 0 mass％ to 2.4 mass％ and was almost constant at more than 2.4 mass％ for sulfided Co/Al2O3 catalyst. This observation is mainly caused by the differences in the magnetic susceptibility between Co _ Mo/Al2O3 catalyst and Co/Al2O3 catalyst after sulfidation. In other words, the formation of the antiferromagnetic Co _ Mo _ S phase causes increased magnetic susceptibility that greatly affects δ0. The slight decrease of δ0 at 7.3 mass％ for sulfided Co _ Mo/Al2O3 catalyst is closely related to the formation of Co9S8 prior to that of the Co _ Mo _ S phase. These findings strongly suggest that δ0 obtained from 129 Xe NMR spectroscopy is a sensitive indicator of the amount of the Co _ Mo _ S phase. Furthermore, the relative hydrodesulfurization activity of various Co _ Mo/Al2O3 catalysts was roughly correlated with δ0. This result also demonstrates that 129 Xe NMR spectroscopy is useful for analysis of the Co _ Mo _ S phase on Co _ Mo/Al2O3 hydrodesulfurization catalyst.

The effect of sulfi dation on 129 Xe NMR spectra of Co _ Mo/Al2O3 hydrodesulfurization catalysts was investigated. Before sulfidation, catalysts containing cobalt such as Co/Al2O3 and Co _ Mo/Al2O3 showed remarkable 129 Xe NMR peak broadening, whereas one sharp 129 Xe NMR peak was observed for Mo/Al2O3 and Al2O3, mainly caused by the paramagnetic effect of cobalt oxides such as CoAl2O4 and CoO. In contrast, after sulfi dation, the remarkable broadening of the 129 Xe NMR peak did not occur. We consider that the paramagnetic effect of cobalt was much reduced due to the transformation of paramagnetic oxides such as CoAl2O4 and CoO into antiferromagnetic sulfi des such as the Co _ Mo _ S phase and Co9S8. In addition, XPS of the sulfi ded catalysts showed that the Co 2p binding energy of the sharp peak for Co _ Mo/Al2O3 was 0.7 eV higher than that for Co/Al2O3. This result strongly suggests that cobalt was mainly present as the Co _ Mo _ S phase on the surface and is closely related to the observation that the 129 Xe NMR peak of sulfi ded Co _ Mo/Al2O3 was shifted further downfi eld in comparison with sulfi ded Mo/Al2O3. 129Xe NMR spectroscopy is sensitive to the formation of the Co _ Mo _ S phase on Co _ Mo/ Al2O3 hydrodesulfurization catalyst.

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