The location of Co and Ni promoter atoms in industrial-style hydrotreating catalysts is examined by combining aberration-corrected scanning transmission electron microscopy and electron energy loss spectrum imaging. The observations unambiguously demonstrate that both Co and Ni promoter atoms occupy sites at all low-indexed edge terminations of hexagonally shaped multi-layer MoS2 nanocrystals. In contrast, similar observations for single-layer MoS2 nanocrystals show that Co-promoter atoms preferentially attach at the (-1 0 0) S-edge termination and are absent at the (1 0 0) Mo-edge termination. The apparent discrepancy between single- and multi-layer MoS2 nanocrystals can be explained by the 2H-MoS2 crystal structure, for which successive MoS2 layers alternatingly expose Mo- and S-edge terminations in any of the low-indexed directions. Thus, the multi-layer Co-Mo-S and Ni-Mo-S nanocrystals, formed in the present type of industrial-style hydrotreating catalyst, are consistently described as a superposition of single-layer Co-Mo-S and Ni-Mo-S structures, and in turn, provide promoted edge sites with different steric accessibility for the organic compounds in mineral oil distillates. (C) 2015 Elsevier B.V. All rights reserved.

We use scanning tunneling microscopy to investigate the morphology and atomic-scale edge structure of MoS2 nanoclusters synthesized on a gold single crystal as a model system for hydrodesulfurization catalysts using three different sulfur sources for sulfiding. Crystalline and triangularly shaped MoS2 nanoclusters are predominantly produced from sulfiding with H2S, dimethyl disulfide (DMDS) and dimethyl sulfide (DMS), but the detailed dispersion, stacking and distribution of active edge sites is sensitive to the selection of the sulfiding agent. The main effect of varying the sulfiding agent seems to be related to the variation in the reactivity of the sulfur, i.e. changes in the sulfur chemical potential. H2S and DMDS both yield fully sulfided edge structures, but lower sulfur content is obtained on the edges of MoS2 sulfided with DMS reflected by Mo-edges terminated by sulfur monomers. The present model studies demonstrate that MoS2 morphology and hereby also the dispersion, type and amounts of active edge sites can be manipulated by control of sulfiding conditions and choice of sulfiding agent. The findings are in line with previous reports on activity variations on technical catalysts which have undergone different sulfidation procedures.

Trends in hydrodesulfurization (HDS) activity are investigated on the basis of surface properties calculated by density functional theory for a series of HDS catalysts. It is shown that approximately linear correlations exist between HS group binding energies and activation barriers of key elementary reactions in HDS of thiophene. These linear correlations are used to develop a simple kinetic model, which qualitatively describes experimental trends in activity. The kinetic model identifies the HS-binding energy as a descriptor of HDS activity. This insight contributes to understanding the effect of promotion and structure–activity relationships.

Two-dimensional sheets of transition metal (Mo and W) sulfides are attracting strong attention due to the unique electronic and optical properties associated with the material in its single-layer form. The single-layer MoS2 and WS2 are already in widespread commercial use in catalytic applications as both hydrotreating and hydrocracking catalysts. Consequently, characterization of the morphology and atomic structure of such particles is of utmost importance for the understanding of the catalytic active phase. However, in comparison with the related MoS2 system only little is known about the fundamental properties of single-layer WS2 (tungstenite). Here, we use an interplay of atom-resolved Scanning Tunneling Microscopy (STM) studies of Au(111)-supported WS2 nanoparticles and calculated edge structures using Density Functional Theory (DFT) to reveal the equilibrium morphology and prevalent edge structures of single-layer WS2. The STM results reveal that the single layer S-W-S sheets adopt a triangular equilibrium shape under the sulfiding conditions of the synthesis, with fully sulfided edges. The predominant edge structures are determined to be the (101[combining macron]0) W-edge, but for the smallest nanoclusters also the (1[combining macron]010) S-edges become important. DFT calculations are used to construct phase diagrams of the WS2 edges, and describe their sulfur and hydrogen coordination under different conditions, and in this way shed light on the catalytic role of WS2 edges.

Information obtained by atom-resolved scanning tunneling microscopy (STM) on the morphology and edge structures of single-layer MOSsub(2) and promoted MoS2 nanoparticles in hydrodesulfurization catalysts is reviewed with emphasis on the role of sulfiding conditions, promoters and substrate interaction.

Sterical hindrance of large S-containing molecules is known to be a significant cause of low hydrodesulfurization activity. To shed light on the atomistic aspects of such steric effects, we use atom-resolved scanning tunneling microscopy (STM) to probe the detailed adsorption configurations of dibenzothiophene (DBT) and alkyl-substituted DBT molecules on both unpromoted and Co-promoted MoS2 nanoclusters. We show that the unsubstituted DBT molecule can adsorb directly through its sulfur atom at sulfur vacancies on the nanocluster. However, only vacancies at corner sites are found to be accessible for DBT adsorption. In a similar way, DBT is found to adsorb on the Co–Mo–S sites exposed at the corners of the Co-promoted nanoclusters. In contrast, the 4,6-DMDBT molecule adsorbs in a flat configuration on the brim sites of the MoS2 clusters without the requirement of a nearby vacancy. Our findings support a model where hydrogenation and sulfur extrusion may take place on different sites.

S2-basierte Nanokatalysatoren werden zur Hydrodesulfurierung in Ölraffinerien eingesetzt. In der Zuschrift auf S. 10 335 ff. präsentieren S. Helveg et al. einzelatomempfindliche Transmissionselektronenmikroskopieaufnahmen industriell hergestellter Nanokatalysatoren. Die beobachteten katalytisch wichtigen Kantenabschlüsse stimmen mit Voraussagen aus Modellstudien überein. Bild: P. Møller und S. Nygaard, Haldor Topsøe A/S.

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.

Atoms on the edge: The atomic edge structure of industrial-style MoS2 nanocatalysts was imaged using single-atom sensitive electron microscopy (see picture). The observed industrial-style edge terminations match predictions of model catalyst studies and thus address the so-called "materials gap" in catalysis.

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.

In hydrodesulfurization (HDS) of fossil fuels, the sulfur levels are reduced by sulfur extraction from hydrocarbons through a series of catalyzed reaction steps on low-coordinated sites on molybdenum disulfide (MoS2) nanoclusters. By means of scanning tunneling microscopy (STM), we show that the adsorption properties of MoS2 nanoclusters toward the HDS refractory dibenzothiophene (DBT) vary dramatically with small changes in the cluster size. STM images reveal that MoS2 nanoclusters with a size above a threshold value of 1.5 nm react with hydrogen to form so-called sulfur vacancies predominately located at edge sites, but these edge vacancies are not capable of binding DBT directly. In contrast, MoS2 nanoclusters below the threshold perform remarkably better. Here, sulfur vacancies form predominantly at the corner sites, and these vacancies show a high affinity for DBT. The results thus indicate that very small MoS2 nanoclusters may have unique catalytic properties for the production of clean fuels.

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. (C) 2010 Elsevier Inc. All rights reserved.

Angewandte Chemie International EditionVolume 49, Issue 15 p. 2708-2710 Communication Imaging MoS2 Nanocatalysts with Single-Atom Sensitivity† Christian Kisielowski Dr., Christian Kisielowski Dr. National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94708 (USA)Search for more papers by this authorQuentin M. Ramasse Dr., Quentin M. Ramasse Dr. National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94708 (USA)Search for more papers by this authorLars P. Hansen Dr., Lars P. Hansen Dr. Haldor Topsøe A/S, Nymøllevej 55, 2800 Kgs. Lyngby (Denmark)Search for more papers by this authorMichael Brorson Dr., Michael Brorson Dr. Haldor Topsøe A/S, Nymøllevej 55, 2800 Kgs. Lyngby (Denmark)Search for more papers by this authorAnna Carlsson Dr., Anna Carlsson Dr. FEI Company, Achtseweg Noord 5, 5651 GG Eindhoven (The Netherlands)Search for more papers by this authorAlfons M. Molenbroek Dr., Alfons M. Molenbroek Dr. Haldor Topsøe A/S, Nymøllevej 55, 2800 Kgs. Lyngby (Denmark)Search for more papers by this authorHenrik Topsøe Dr., Henrik Topsøe Dr. Haldor Topsøe A/S, Nymøllevej 55, 2800 Kgs. Lyngby (Denmark)Search for more papers by this authorStig Helveg Dr., Stig Helveg Dr. [email protected] Haldor Topsøe A/S, Nymøllevej 55, 2800 Kgs. Lyngby (Denmark)Search for more papers by this author Christian Kisielowski Dr., Christian Kisielowski Dr. National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94708 (USA)Search for more papers by this authorQuentin M. Ramasse Dr., Quentin M. Ramasse Dr. National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94708 (USA)Search for more papers by this authorLars P. Hansen Dr., Lars P. Hansen Dr. Haldor Topsøe A/S, Nymøllevej 55, 2800 Kgs. Lyngby (Denmark)Search for more papers by this authorMichael Brorson Dr., Michael Brorson Dr. Haldor Topsøe A/S, Nymøllevej 55, 2800 Kgs. Lyngby (Denmark)Search for more papers by this authorAnna Carlsson Dr., Anna Carlsson Dr. FEI Company, Achtseweg Noord 5, 5651 GG Eindhoven (The Netherlands)Search for more papers by this authorAlfons M. Molenbroek Dr., Alfons M. Molenbroek Dr. Haldor Topsøe A/S, Nymøllevej 55, 2800 Kgs. Lyngby (Denmark)Search for more papers by this authorHenrik Topsøe Dr., Henrik Topsøe Dr. Haldor Topsøe A/S, Nymøllevej 55, 2800 Kgs. Lyngby (Denmark)Search for more papers by this authorStig Helveg Dr., Stig Helveg Dr. [email protected] Haldor Topsøe A/S, Nymøllevej 55, 2800 Kgs. Lyngby (Denmark)Search for more papers by this author First published: 26 March 2010 https://doi.org/10.1002/anie.200906752Citations: 94 † Microscopy was performed at NCEM, which is supported by the Office of Science, Office of Basic Energy Sciences of the U.S. Department of Energy under contract no. DE-AC02-05CH11231. The research was carried out for the Helios Solar Energy Research Center, which is funded under contract no. DE-AC02-05CH11231. Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Graphical Abstract Close up: The atomic arrangement of industrial-style MoS2 nanocatalysts can be scrutinized by using single-atom-sensitive electron microscopy (see picture). This technique, which provides an excellent agreement between simulation and experiment, allows element detection in compound materials and permits the type and the concentration of the catalytically important edge sites to be estimated. References 1R. Tenne, Angew. Chem. 2003, 115, 5280– 5289; Angew. Chem. Int. Ed. 2003, 42, 5124– 5132; R. Tenne, Nat. Nanotechnol. 2006, 1, 103– 111. 2R. Prins, Adv. Catal. 2001, 46, 399– 464. 3B. Hinnemann, P. G. Moses, J. Bonde, K. P. Jørgensen, J. H. Nielsen, S. Horch, I. Chorkendorff, J. K. Nørskov, J. Am. Chem. Soc. 2005, 127, 5308– 5309. 4J. P. Wilcoxon, T. R. Thurston, J. E. Martin, Nanostruct. Mater. 1999, 12, 993– 997. 5 5aK. P. de Jong, L. C. A. van den Oetelaar, E. T. C. Vogt, S. Eijsbouts, A. J. Koster, H. Friedrich, P. E. de Jongh, J. Phys. Chem. 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Filename Description anie_200906752_sm_miscellaneous_information.pdf52.1 KB miscellaneous_information Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article. Volume49, Issue15April 1, 2010Pages 2708-2710 ReferencesRelatedInformation

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. (C) 2010 Elsevier Inc. All rights reserved.

Atome bei der Arbeit:Nanokatalysatoren auf MoS2-Basis werden für Hydrodesulfurierungen in Ölraffinerien eingesetzt. In ihrer Zuschrift auf S. 2768 ff. machen S. Helveg und Mitarbeiter die Anordnung der einzelnen Atome in solchen Nanokatalysatoren mithilfe von hochauflösender Transmissionselektronenmikroskopie sichtbar. (Bild: P. Møller und S. Nygaard, Haldor Topsøe A/S.)