Characterization of Co-Mo/Al<sub>2</sub>O<sub>3</sub> Hydrodesulfurization Catalyst by <sup>129</sup>Xe NMR Spectroscopy

Abstract

The present study evaluated 129Xe NMR spectroscopy for the analysis of Co-Mo/Al2O3 hydrodesulfurization catalyst. This study also reconsidered the conventional interpretation of 129Xe NMR spectroscopy that had been used for zeolite micropore analysis. The chemical shift δ of an observed 129Xe 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 129Xe 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 129Xe 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 129Xe NMR spectroscopy is useful for analysis of the Co-Mo-S phase on Co-Mo/Al2O3 hydrodesulfurization catalyst.