Active sites speciation of supported CoMoS phase probed by NO molecule: A combined IR and DFT study

By using infrared spectroscopy and second derivative analysis, we identify key vibrational components of NO adsorbed the sulfided phases present in hydrodesulfurization (HDS) supported catalysts: MoS2, cobalt promoted MoS2 (CoMoS) and Co9S8. With the help of density functional theory (DFT) calculations, five components (A, A', B, B', C) identified by the second derivative analysis are assigned to mononitrosyls and dinitrosyls species on the various Mo and Co sites of the MoS2, CoMoS and on Co9S8 phases. In particular, the component (A') at similar to 1846 cm(-1) is due to mononitrosyl species on Co sites of the partially promoted M-edge or to dinitrosyl species on Co sites of the fully promoted S-edge of the CoMoS phase. This component (A') appears as a descriptor of the promoted sites and is present on the delta theta- and gamma-Al2O3 supports. By contrast, on silica supported catalyst, (A') decreases significantly in favor of component (A) at 1859-1863 cm(-1) assigned mainly to nitrosyls on Co9S8 sites. Simultaneously, on silica, the component (B) at 1788-1790 cm(-1) is assigned to dinitrosyls on the Co promoted S-edge and/or on the Co9S8 phase. (B) can be distinguished from the component (B') at 1799 cm(-1) assigned to nitrosyls on the Co9S8 phase and/or to dinitrosyls on the non promoted Mo edge. A complementary chemometric analysis highlights that different evolution of the sites population as a function of NO contact time occurs as a function of the various sulfided phases and supports. In particular, chemometry suggests the presence of the two edges on the non-promoted MoS2 phase. We finally discuss the implication of these findings for HDS catalysts. (C) 2018 Elsevier Inc. All rights reserved.