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Abstract: In this study, we used scanning tunneling microscopy (STM) and density functional theory (DFT) to investigate the atomic-scale structure of the active Co- or Ni-promoted MoS2 nanoclusters in hydrotreating catalysts. Co-promoted MoS2 nanoclusters (Co–Mo–S) are found to adopt a hexagonal shape, with Co atoms preferentially located at () edges with a 50% sulfur coverage. The first atom-resolved STM images of the Ni-promoted MoS2 nanoclusters (Ni–Mo–S) reveal that the addition of Ni also leads to truncated morphologies, but the degree of truncation and the Ni sites are observed to depend on the nanocluster size. Larger clusters (type A) are structurally similar to Co–Mo–S exposing fully Ni-substituted () edges with a 50% S coverage. Smaller clusters (type B) show dodecagonal shapes terminated by three different edges, all of which contain Ni-promoter atoms fully or partially substituting the Mo atoms. The findings may shed more light on the different selectivities observed for the Co- and Ni-promoted hydrotreating catalysts. [Copyright &y& Elsevier]

Abstract: From an interplay of Scanning Tunneling Microscopy (STM), X-ray Photoelectron Spectroscopy (XPS) experiments, and density functional theory (DFT) calculations, we investigate the fundamental effect of the use of organosulfur compounds for the sulfidation of MoS2 nanoclusters in the hydrotreating catalyst. Our results reveal that incorporation of carbon in MoS2-based hydrotreating catalysts as carbide-type phase is not favorable when synthesized with or exposed to dimethyl disulfide (DMDS) or dimethyl sulfide (DMS). These results suggest that substitution of sulfur with carbon on the predominant type of edge structures in MoS2 nanoclusters is thermodynamically unfavorable, a result that is confirmed by DFT to be valid for all edge structures of MoS2 exposed under catalytically relevant conditions. However, the results show that the choice of the sulfiding agent can strongly influence the morphology and dispersion of the sulfided phase, and such effects are therefore expected to be of major importance for the activity of the freshly sulfided catalyst. [Copyright &y& Elsevier]

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 MoS2 and Co–Mo–S nanoclusters. The DFT calculations and STM experiments show that NO does not adsorb at fully sulfided MoS2 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 H2S 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. [Copyright &y& Elsevier]

Abstract: The promotion of the activity of MoS2-based hydrotreating catalysts by various first-row transition metals exhibits a typical variation referred to as a volcano plot. Co and Ni are seen to substantially promote the catalytic activity of MoS2, whereas the neighboring first-row metals promote reactivity to a much lesser extent, or not at all. In order to provide a better atomistic understanding of the catalytic synergies, we perform here a comparative scanning tunneling microscopy (STM) analysis of the atomic-scale structure and morphology of MoS2 nanoclusters doped with the first-row transition metals: Fe, Co, Ni, and Cu. We reveal that addition of all four dopant metals results in the formation of mixed-metal “Co–Mo–S”-type structures shaped as single-layer hexagonally truncated triangular MoS2-like nanoclusters. The modification of the preferred nanocluster equilibrium morphology is explained as a direct consequence of a favored substitution of dopant metal atoms into the S-edges of MoS2. The degree of truncation and the edge dispersion are, however, found to depend greatly on the type of dopant atom since the relative length of the dopant-stabilized edges decreases with the number of valence shell electrons of the dopant transition metal. A comparison of the observed atomic structure and morphology with the hydrotreating activity measured for industrial-style prepared Me–Mo–S catalysts (Me=Fe, Co, Ni, and Cu) supported on carbon reveals that two parameters are relevant to describe the promotional behavior: (i) a geometric parameter, which relates to the relative number of promoted and unpromoted sites in the Me–Mo–S nanoclusters, and (ii) a more conventional parameter relating to bonding and adsorption strength, i.e., describing the intrinsic activity of the particular Me-doped S-edge. [Copyright &y& Elsevier]

Abstract: Basic nitrogen-containing compounds such as pyridine are well known to be inhibitors of the hydrodesulfurization (HDS) reaction for the MoS2-based catalysts. From an interplay of scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations, atomic-scale insight into pyridine adsorption on MoS2 is obtained. In agreement with previous IR-spectroscopy and DFT studies, the STM results show that the pyridine molecule itself interacts weakly or not at all with the MoS2 nanoclusters. However, in the presence of hydrogen at the MoS2 edges, adsorbed species are revealed by STM also at the edges. The calculated DFT energies and simulated STM images allowed us to conclude that these species are pyridinium ions located at the catalytically active brim sites. Furthermore, the DFT results for the vibrational modes of the adsorbed pyridinium species agree well with those observed in earlier IR experiments on high surface alumina-supported MoS2 catalyst. The adsorption sites appear to be very similar to the brim sites involved in hydrogenation reactions in HDS. Thus, the combined STM and DFT results provide new atomic-scale insight into the inhibition effect of basic N-compounds in HDS and the first direct observation of the adsorption mode of basic N-compounds on the catalytically active MoS2 edges. Our results lend further support to previously reported correlations between inhibiting strength and proton affinity for the N-containing compounds. [Copyright &y& Elsevier]