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Sub-stoichiometric Molybdenum Sulfide Phases with Catalytically Active Basal Planes Yang Bao, Ming Yang, Sherman Jun Rong Tan, Yanpeng Liu, Hai Xu, Wei Liu, Chang Tai Nai, Yuan Ping Feng, Jiong Lu, and Kian Ping Loh J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b09042 • Publication Date (Web): 03 Oct 2016 Downloaded from http://pubs.acs.org on October 5, 2016
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Journal of the American Chemical Society
Sub-stoichiometric Molybdenum Sulfide Phases with Catalytically Active Basal Planes Yang Bao1,2,3†, Ming Yang3,4†, Sherman Jun Rong Tan1,2†, Yan Peng Liu1, Hai Xu1, Wei Liu1,2,3, Chang Tai Nai1, Yuan Ping Feng3,5, Jiong Lu1,3, Kian Ping Loh1,3⋆
1.
Department of Chemistry, National University of Singapore, Singapore 117543, Singapore. 2.NUS
Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore 117456, Singapore. 3.Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore 117546, Singapore.4.Institute of Materials Research and Engineering, Agency for Science, Technology and Research, Singapore 117602, Singapore. 5.Department of Physics, National University of Singapore, Singapore 117551, Singapore.
⋆email:
[email protected] ABSTRACT: Molybdenum sulfide (MoS2) is widely recognized for its catalytic activities where the edges of the crystals turn over reactions. Generating sulfur defects on the basal plane of MoS2 can improve its catalytic activity, but generally there is a lack of model system for understanding metal-centered catalysis on the basal planes. Here, we synthesized a new phase of sub-stoichiometric molybdenum sulfide (s-MoSx) on sulfur-enriched copper substrate. The basal plane of s-MoSx contains chemically reactive Mo-rich sites that can undergo dynamic dissociative adsorption/desorption processes with molecular hydrogen, thus demonstrating its usefulness for hydrogen-transfer catalysis. In addition, scanning tunneling microscopy was used to monitor surface-directed Ullmann coupling of 2,8-dibromo-dibenzothiophene (DBDBT) molecules on s-MoSx nanosheets, where the four-fold symmetric surface sites on s-MoSx direct C-C coupling to form cyclic tetramers with high selectivity.
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pling reactions. STM studies show that the four-fold symmetric Mo-rich sites allow the adsorptive preassembly of DBDBT molecules and catalyze Ullmann type aryl-aryl coupling to form cyclic tetramers with high selectivity.
Introduction Molybdenum-sulfur compounds are widely recognized as an important class of non-noble metal catalysts. For example, alumina supported molybdenum sulfides are important industrial catalysts in hydrodesulfurization (HDS) processes1. Recently, molybdenum sulfide species in the molecular (CP2Mo2S4, Mo3S44+, Mo3S132- etc.)2-4, nanoparticle5,6 or amorphous (MoS2, MoS3)7,8 forms have emerged as efficient catalysts in hydrogen evolution reaction (HER). To design more effective Mo-S catalysts, fundamental insights on the nature of the active sites and the reaction mechanisms are needed. In view of the complexity of industrial catalysts, the study of catalyst operation is often undertaken with model systems, on which surface characterization techniques are applied to interrogate the adsorption of molecular species on catalytic sites. Among the characterization techniques, scanning tunneling microscopy (STM) is particularly informative, since it allows imaging of the active sites and active site-reactant interaction with atomic resolution.
Experimental section Sample preparation. Experiments were performed in an ultrahigh vacuum chamber (UHV, base pressure of 10-10 mbar) with a UHV STM unit (high temperature-STM 150 Aarhus, SPECS, GmbH). The Cu(111) substrate (Mateck, GmbH) was cleaned by repeated argon ion sputtering at p(Ar)= 1 × 10-5mbar, 1.5keV, followed by annealing at 600℃. Growth of MoS2 or s-MoSx islands were carried out by evaporating Mo atoms onto the substrates, followed by annealing in H2S at elevated temperatures (400-700℃). Interaction of s-MoSx with H2 and O2. Hydrogenation experiment was performed by dosing H2 at 1x10-6mbar while keeping the sample at temperatures ranging from room temperature to 80℃. Hydrogen desorption was performed by annealing the hydrogenated sample at 200℃ in vacuum. Oxygenation experiment was performed by dosing O2 at 1x10-6mbar while keeping the sample at room temperature.
Besenbacher and coworkers have pioneered the STM studies of substrate-supported MoS2, which was used as a model system to understand catalytic activity on coordinatively unsaturated Mo edge sites.5,9-13 Sub-stoichiometric MoSx (x
