Spectroscopy, microscopy and theoretical study of NO adsorption on MoS2 and Co–Mo–S hydrotreating catalysts

Abstract

Infrared (IR) spectroscopy using NO as a probe molecule has been one of the important methods for characterizing hydrotreating catalysts, since this technique provides information on the nature and quantity of active edge sites of these catalysts. However, due to the strong adsorption of NO, which may lead to significant edge reconstructions, it has not been clear, how the characteristics of the adsorption complexes may reflect the nature of the original edge sites. By combining IR spectroscopy measurements with scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations, we present new atomic-scale insight into the nature of NO adsorption on MoS 2 and Co-Mo-S nanoclusters. The DFT calculations and STM experiments show that NO does not adsorb at fully sulfided MoS 2 edges not containing hydrogen. However, typical sulfided catalysts will have hydrogen present at the edge in the form of S-H groups. For such samples, the results indicate a ''push-pull'' type mechanism involving simultaneous vacancy creation, NO adsorption and H 2 S release. This mechanism is observed to dominate in the IR experiments. In STM experiments, stable vacancies can be generated by dosing atomic hydrogen, and these vacancies are observed to adsorb NO dimers. The detailed nature of the adsorption is revealed by DFT. IR measurements recorded during temperature-programmed desorption (TPD) show the presence of several NO adsorption complexes and the assignment to specific species is achieved by comparison to calculated frequencies and adsorption energies obtained from DFT. The results show that mononitrosyl species dominate at the Mo-edges, whereas stable dinitrosyl species are found at both the unpromoted and the Co-promoted S-edges. Thus, based on the present results, it is possible to use NO as a probe molecule to obtain detailed atomic-scale information on hydrotreating catalysts and the origins of activity differences.

Atom-resolved scanning tunneling microscopy investigations of molecular adsorption on MoS2 and CoMoS hydrodesulfurization catalysts

“…Topsøe, B. Clausen, S. Helveg, and others at Haldor Topsøe A/S, we have, in the last decade, published a series of studies where scanning tunneling microscopy (STM) was applied as the principal technique to study the adsorption effects relevant to HDS reactions on MoS 2 nanoparticles and we have provided the hitherto most detailed atomic-level experimental view on molecular interactions on the active MoS 2 and CoMoS nanoparticles [26][27][28][29][30][31][32].…”

Functionalisation of Colloidal Transition Metal Sulphides Nanocrystals: A Fascinating and Challenging Playground for the Chemist

“…Among disulphides, MoS 2 is interesting both as a hydro-treating and as a desulphurisation catalyst. The complicated chemistry of its edge surfaces has stimulated several theoretical and experimental investigations [177][178][179][180][181][182]. More systematic studies involving a broader range of metal sulphides [183] are also reported.…”

An industrial perspective on the impact of Haldor Topsøe on computational chemistry

“…The catalytic active phase termed CoMoS (or NiMoS for Ni-promoted MoS 2 ) is MoS 2 nanoparticles with the edges decorated by Co [33]. The exact location and local environment of Co have been determined by a combination of EXAFS, IR, DFT, STM, and TEM studies [34][35][36][37][38]. The close agreement between experimental observations and thermodynamics based on DFT gives confidence of DFT's ability to predict the structure and energetics of these rather complicated structures [35,37,[39][40][41].…”