Topics in Catalysis

, Volume 11, Issue 1–4, pp 263–270

On the mechanism of the selective catalytic reduction of NO to N2 by H2 over Ru/MgO and Ru/Al2O3 catalysts

  • A. Hornung
  • M. Muhler
  • G. Ertl


Steady-state and transient kinetic experiments were performed in a versatile microreactor flow set-up with magnesia- and alumina-supported ruthenium catalysts in order to elucidate the mechanism of the selective catalytic reduction (SCR) of nitric oxide with hydrogen. Both Ru/MgO and Ru/γ-Al2O3 were found to be highly active catalysts converting NO and H2 into N2 and H2O with selectivities close to 100% at full conversion, although Ru-based catalysts are known to be active in the synthesis of NH3 from N2 and H2. Frontal chromatography experiments with NO at room temperature revealed that NO and its dissociation products displace adsorbed atomic hydrogen (H−*) almost completely from hydrogen-precovered Ru surfaces. Obviously, NO and H2 compete for the same adsorption sites, H−* being the weaker bound adsorbate. Temperature-programmed surface reaction (TPSR) experiments in H2 subsequent to NO exposure demonstrated that higher heating rates and lower partial pressures of H2 shift the selectivity from NH3 to N2. Therefore, the coverage of H−* is concluded to govern the branching ratio between the rate of associative desorption of N2 (2N−*→N2 + 2*) and the rate of hydrogenation of N−* (N−* + 3H–* →NH3 + 4*). Finally, the steady-state coverages of N- and O-containing adsorbates were derived by interrupting the SCR reaction and hydrogenating the adsorbates off as NH3 and H2O. By solving the site balance, the Ru surfaces were found to be essentially N2 is attributed to the very low coverage of H−* due to site blocking by a N + O coadsorbate layer, favouring the recombination of N−* instead of its hydrogenation to NH3.

nitric oxide selective catalytic reduction supported ruthenium catalysts frontal chromatography temperature-programmed surface reaction reaction mechanism 


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  1. [1]
    H. Bosch and F. Janssen, Catal. Today 2 (1988) 369.CrossRefGoogle Scholar
  2. [2]
    A. Hornung, M. Muhler and G. Ertl, Catal. Lett. 53 (1998) 77.CrossRefGoogle Scholar
  3. [3]
    M. Shelef and H.S. Gandhi, Ind. Eng. Chem. Prod. Res. Dev. 11 (1972) 393.CrossRefGoogle Scholar
  4. [4]
    G.L. Bauerle, S.C. Wu and K. Nobe, Ind. Eng. Chem. Prod. Res. Dev. 14 (1975) 123.CrossRefGoogle Scholar
  5. [5]
    K.C. Taylor and R.L. Klimisch, J. Catal. 30 (1973) 478.CrossRefGoogle Scholar
  6. [6]
    K.C. Taylor, R.M. Sinkevitch and R.L. Klimisch, J. Catal. 35 (1974) 34.CrossRefGoogle Scholar
  7. [7]
    R.L. Klimisch and K.C. Taylor, Ind. Eng. Chem. Prod. Res. Dev. 14 (1975) 26.CrossRefGoogle Scholar
  8. [8]
    R.J. Voorhoeve and L.E. Trimble, J. Catal. 38 (1975) 80.CrossRefGoogle Scholar
  9. [9]
    K. Otto and M. Shelef, Z. Phys. Chem. NF 85 (1973) 308.Google Scholar
  10. [10]
    S.L. Matson and P. Harriot, Ind. Eng. Chem. Prod. Res. Dev. 17 (1978) 322.CrossRefGoogle Scholar
  11. [11]
    M. Uchida and A.T. Bell, J. Catal. 60 (1979) 204.CrossRefGoogle Scholar
  12. [12]
    F. Rosowski, O. Hinrichsen, M. Muhler and G. Ertl, Catal. Lett. 36 (1996) 229.CrossRefGoogle Scholar
  13. [13]
    O. Hinrichsen, F. Rosowski, A. Hornung, M. Muhler and G. Ertl, J. Catal. 165 (1997) 33.CrossRefGoogle Scholar
  14. [14]
    H. Dietrich, K. Jacobi and G. Ertl, J. Chem. Phys. 105 (1996) 8944.CrossRefGoogle Scholar
  15. [15]
    S. Schwegmann, A.P. Seitsonen, H. Dietrich, H. Bludau, H. Over, K. Jacobi and G. Ertl, Chem. Phys. Lett. 264 (1997) 680.CrossRefGoogle Scholar
  16. [16]
    C. Nagl, R. Schuster, S. Renisch and G. Ertl, Phys. Rev. Lett. 81 (1998) 3483.CrossRefGoogle Scholar
  17. [17]
    P.J. Shires, J.R. Cassata, B.G. Mandelik and C.P. van Dijk, US Patent 4,479,925 (1984).Google Scholar
  18. [18]
    M. Muhler, F. Rosowski, O. Hinrichsen, A. Hornung and G. Ertl, Stud. Surf. Sci. Catal. 101 (1996) 317.CrossRefGoogle Scholar
  19. [19]
    F. Rosowski, A. Hornung, O. Hinrichsen, D. Herein, M. Muhler and G. Ertl, Appl. Catal. A 151 (1997) 443.CrossRefGoogle Scholar
  20. [20]
    R.A. Dalla Betta, J. Catal. 34 (1974) 57.CrossRefGoogle Scholar
  21. [21]
    M. Muhler, F. Rosowski and G. Ertl, Catal. Lett. 24 (1994) 317.CrossRefGoogle Scholar
  22. [22]
    O. Hinrichsen, A. Hornung and M. Muhler, Chem. Eng. Technol. 22 (1999) 1039.CrossRefGoogle Scholar
  23. [23]
    T. Zambelli, J. Wintterlin, J. Trost and G. Ertl, Science 273 (1996) 1688.Google Scholar
  24. [24]
    R. Burch, P.J. Millington and A.P. Walker, Appl. Catal. B 4 (1994) 65.CrossRefGoogle Scholar

Copyright information

© Kluwer Academic Publishers 2000

Authors and Affiliations

  • A. Hornung
    • 1
  • M. Muhler
    • 1
  • G. Ertl
    • 2
  1. 1.Lehrstuhl für Technische ChemieRuhr-Universität BochumBochumGermany
  2. 2.Fritz-Haber-Institut der Max-Planck-GesellschaftGermany

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