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    Active and stable Fe-based catalyst, mechanism, and key role of alkali promoters in ammonia synthesis

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    Type
    Article
    Authors
    Almaksoud, Walid
    Rai, Rohit Kumar cc
    Morlanes, Natalia Sanchez cc
    Harb, Moussab cc
    Ahmad, Rafia
    Ould-Chikh, Samy cc
    Anjum, Dalaver H. cc
    Hedhili, Mohamed N. cc
    Al-Sabban, Bedour E. cc
    Albahily, Khalid
    Cavallo, Luigi cc
    Basset, Jean-Marie cc
    KAUST Department
    KAUST Catalysis Center (KCC)
    Physical Science and Engineering (PSE) Division
    Imaging and Characterization Core Lab
    Electron Microscopy
    Surface Science
    Chemical Science Program
    SABIC Corporate Research and Development Center at KAUST, Saudi Basic Industries Corporation, Thuwal 23955, Saudi Arabia.
    Date
    2020-11-13
    Online Publication Date
    2020-11-13
    Print Publication Date
    2020-11
    Embargo End Date
    2021-11-13
    Submitted Date
    2020-10-07
    Permanent link to this record
    http://hdl.handle.net/10754/665972
    
    Metadata
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    Abstract
    Worldwide NH3 production reached 0.18 Gton in 2019, and 1-2 % of the global CO2 emissions are due to large-scale NH3 synthesis (1 billion tons of CO2 / year). A catalyst for ammonia synthesis has been obtained by pyrolysis of iron phthalocyanine (FePc) precursor under N2, followed by impregnation with alkali metals (Na, Li, K, and Cs) and H2 treatment. Characterization (XPS, XRD, HR-TEM, ICP-OES, TGA, CHNS analysis, and BET) revealed nano-sized core-shell structures formed during H2 treatment, with Fe in the core and promoters (“Cs2O” and “K2O”) with carbon on the shell. The alkali metals partially inhibit the methanation process of carbon. These Fe NPs were found to be very active and stable catalysts, as compared to the commercial iron-based catalyst KM1 (Haldor-Topsoe). Activities of promoted catalysts follow the order: K>Cs>Na∼Li, with more than 6% of NH3 at 400 °C and 7 MPa, and contact time (WHSV) of 12000 ml.g-1.h-1 with K. The apparent activation energy was found to be 31 kJ.mol-1 and 34 kJ.mol-1 for 3-K-FePc700 and 10-Cs-FePc700 suggesting the facile activation of N2 on the catalysts surface. DFT-based predicted atomic and electronic structures reveal a similarity in the partial charge distribution on surface Fe species with K or Cs. Surprisingly the main effect of alkali is related to the geometrical repartition of alkali, leading to a larger number of exposed iron atoms, active sites, in the case of K than Cs. The alkali (present as metal oxide) leaves at medium coverage of the surface some exposed Fe(0) for N2 non-dissociative chemisorption (end-on type). The free energy profile demonstrates that the thermodynamic stability of the reaction intermediates for nitrogen reduction reaction (NRR) increases with pressure indicating better feasibility of the reaction at higher pressures.
    Citation
    Al Maksoud, W., Rai, R. K., Morlanés, N., Harb, M., Ahmad, R., Ould-Chikh, S., … Basset, J.-M. (2020). Active and stable Fe-based catalyst, mechanism, and key role of alkali promoters in ammonia synthesis. Journal of Catalysis. doi:10.1016/j.jcat.2020.10.031
    Sponsors
    This research was supported by SABIC Company. The authors acknowledge the resources and facilities provided by the King Abdullah University of Science and Technology. The authors acknowledge as well the KAUST Analytical Core Lab (ACL), particularly the scientists Mohammed Khalid and Dr. Omar El Tall. The authors acknowledge as well the KAUST Imaging Core Lab, particularly Dr. Alessandro Genovese. M.H., R.A., and L.C. acknowledge the KAUST Supercomputing Laboratory for providing computational resources using the supercomputer Shaheen II.
    Publisher
    Elsevier BV
    Journal
    Journal of Catalysis
    DOI
    10.1016/j.jcat.2020.10.031
    Additional Links
    https://linkinghub.elsevier.com/retrieve/pii/S0021951720304449
    ae974a485f413a2113503eed53cd6c53
    10.1016/j.jcat.2020.10.031
    Scopus Count
    Collections
    Articles; Imaging and Characterization Core Lab; Physical Science and Engineering (PSE) Division; Chemical Science Program; KAUST Catalysis Center (KCC)

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