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Chapter III. Chemical and Mechanistic Aspects of the Selective Catalytic Reduction of NOx by Ammonia over Oxide Catalyst

2012-9-27. 1. Introduction. 1.1 NOx formation: Combustion (thermal NOx)

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Chapter III. Chemical and Mechanistic Aspects of the Selective Catalytic Reduction of NOx by Ammonia over Oxide Catalyst

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    1. 2012-9-28 Chapter III. Chemical and Mechanistic Aspects of the Selective Catalytic Reduction of NOx by Ammonia over Oxide Catalysts

    2. 2012-9-28 1. Introduction 1.1 NOx formation: Combustion (thermal NOx) –high temperature N2 + O2 ? 2NO (1) NO + 1/2O2 ? NO2 (2) Oxidation of organic nitrogen in the fuel 1.2 NOx control Combustion control Post-combustion DeNOxing

    3. 2012-9-28 1.3 Combustion control

    4. 2012-9-28 1.4 Post-combustion DeNOxing Wet methods Sorption their use is not of commercial importance because of the high costs dry methods Catalytic heterogeneous reduction Catalytic homogeneous reduction

    5. 2012-9-28 1.5 Catalytic technologies Attractive: low cost and high efficiency 2NO ? N2 + O2: NOx decomposition very slow no sufficiently efficient catalysts 2NO ? N2: NOx catalytic reduction Reducing agent: CO Catalysts: Pt-Rh or Pd-based Al2O3 or ceramic monoliths Reducing agent: hydrocarbons such as methane, propane or propene no sufficiently active catalysts

    6. 2012-9-28 1.6 SCR (selective catalytic reduction) DeNOxing Reducing agent: NH3 first developed in Japan in the 1970s widely applied worldwide today reaction mechanism is still not unanimously recognized The corresponding aspects will be discussed as follows in details

    7. 2012-9-28 2. Metal oxide catalysts for SCR 2.1 Summary Including V2O5,V2O5/TiO2, V2O5-WO3/TiO2, V2O5-MoO3/TiO2 and so on; Supporters such as Al2O3, SiO2, ZrO2 are also used apart from TiO2 Catalytic active component can also be supported or mixed with iron, copper, chromium and manganese oxides Or introduced into zeolitic cavities, such as in the cases of X, Y and ZSM-5

    8. 2012-9-28 Structure of zeolites

    9. 2012-9-28 2.2 SCR catalysts characteristic active not only in oxidation catalysis but also in SCR reaction Catalyst components (V2O5, WO3 and MoO3) with a high selectivity in partial oxidation when supported on TiO2 give also rise to high selectivity in the SCR reaction

    10. 2012-9-28 3. The active phase and the role of the support and of promoters in the industrial catalysts 3.1 Function of active phase V2O5-WO3/TiO2, V2O5-MoO3/TiO2 and other industrial SCR catalysts. vanadium oxide generally very small <1% (w/w) vanadia is active not only for NOx reduction but also for the undesired oxidation of SO2 to SO3 WO3 increases catalyst activity and thermal stability

    11. 2012-9-28 3.2 Reasons of TiO2-anatase as the best support for SCR catalysts

    12. 2012-9-28 Reaction 2: Stability of vanadyl centers

    13. 2012-9-28 3.3 Function of promoters Example

    14. 2012-9-28 Results

    15. 2012-9-28 4. The SCR reaction stoichiometry and the by-reactions True reactants of SCR: NO2 before 1980s NO, due to the fact that the following reaction occurs negligibly for the diluted waste gases: NO + 1/2O2 ? NO2 (2) Therefore, generally: 4NH3+4NO+O2 ?4N2 + 6H2O (3) one atom of N2 from NO and the other from ammonia, according to isotope labelling

    16. 2012-9-28 Reaction (3) can occur over both V2O5-catalysts and Noble metal catalysts (1) Especially for V2O5-catalysts In the presence of O2 4NH3+ 2NO2 + O2 ? 3N2 + 6H2O (4) In the absence (or defect) of O2 4NH3 + 6NO ? 5N2 + 6H2O (5) four N2 molecules arise from a reaction between NO and NH3 (reaction (3)), one arises from two molecules of NO.

    17. 2012-9-28 (2) Side reactions (N2O formation) Reaction of NO + ammonia 4NH3 + 4NO+ 3O2 ? 4N2O+ 6H2O (6) the converted NO/NH3 < 1 Ammonia oxidation: 2NH3 + 3/2O2 ? N2 + 3H2O (7) 2NH3 + 2O2 ? N2O + 3H2O (8) 2NH3 + 5/2O2 ? 2NO + 3H2O (9) oxygen instead of NO

    18. 2012-9-28 NO dismutation or disproportionation 2NO ? N2O + 1/2O2 (10) 3NO ? N2O + NO2 (11) Decomposition of N2O N2O ? N2 + 1/2O2 (12) Especially over transition metal oxide catalysts

    19. 2012-9-28 5. The catalysts behavior in the NH3+NO reaction Competitive reaction: NO reduction NH3 oxidation A good SCR catalyst has a higher selectivity of NO reduction than that of NH3 oxidation

    20. 2012-9-28 (A) V2O5-WO3-TiO2 catalyst at a space velocity of 7.4 L g-1 h-1. (B) Fe2O3-SiO2 catalyst at a space velocity of 160 L g-1 h-1. NO and NH3 = 500 Vppm; balance He with 2% O2

    21. 2012-9-28 (1) Effect of supports conversion of NO in copper samples (%wt CuO) on various oxide supports

    22. 2012-9-28 Experiences: Activity: silica-based catalysts < titania-based catalysts N2 selectivity: MoO3-SiO2, V2O5-SiO2 < MoO3-TiO2, V2O5-TiO2 However, the reverse is found for silica- and titania-supported tungsta

    23. 2012-9-28 (2) Catalytic active centers Catalytic species Mn based catalysts including MnOx-Al2O3, MnOx-WO3- Al2O3 and MnOx-TiO2 Cu based catalysts including CuO-carbon, and CuO-MgO-Al2O3 Fe2O3-based catalysts Cr2O3-based catalysts Fe-, Cr- and Mn-based oxides efficiently catalyze NH3 oxidation, therefore less selective for SCR reaction than V2O5-based catalysts

    24. 2012-9-28 Conclusions: V, Cu are found to be the best active centers ZSM-5 is the best support Cu-ZSM-5 and V-ZSM-5 are the most attractive catalysts for NOx removal in the future

    25. 2012-9-28 6. The observed reaction kinetics (1) Kinetic model: a value Reaction order on NO a = 1, mostly on Cr2O3/TiO2, Fe-Y zeolite, Cu-ZSM-5 and other Cu-exchanged zeolites 0.8 on Fe-ZSM5, 0.64 on ferric oxide and 0.4 on MnOx-WO3-Al2O3

    26. 2012-9-28 ß value ß was found to be 0, excess O2, no water vapor or with water contents > 5% and NH3/NO>1 on pure V2O5 or V2O5/TiO2, Fe-ZSM-5, Cu-ZSM-5 as catalysts ß =0.2 when NH3/NO <<1 However, ß <0 on ferric oxides and Fe-Y-zeolites, due to the competitive adsorption of NH3 with NO on the catalytic centers

    27. 2012-9-28 ? value Conflicting data were obtained to be 0-0.5 However, in practical conditions O2 is in large excess and oxygen is involved in the reoxidation, therefore O2 does interact with neither NO nor NH3 d value Water hinders the SCR reaction leading to d < 0 Most researchers thought that water improves N2O formation and has less action on the SCR reacction

    28. 2012-9-28 Conclusions of kinetics a = 1; ß, ?, d = 0 were commonly agreed Interpretations: Strong-adsorption of NH3, however weak-adsorption of NO from gas NO adsorption is rate-determining step a < 1 are because of the reaction of adsorbed NO with the adsorbed NH3 on the active sites

    29. 2012-9-28 7. Adsorption and desorption studies Study methods: Spectroscopic technique Thermal desorption Tested Conditions: “Model” catalysts and “clean” conditions “clean” conditions: no water vapor and no SOx, and vacuum

    30. 2012-9-28 7.1. NH3 adsorption and desorption V2O5-based catalysts

    31. 2012-9-28 (1) Indicating three states adsorbed molecular ammonia: Lewis-type interaction ammonia adsorbed as ammonium ions: Br˘nsted acidic-OH surface hydroxy groups H-bonded NH3

    32. 2012-9-28 (2) Generation of the Br˘nsted acid sites

    33. 2012-9-28 (3) ammonia adsorption (a) Lewis-bonded NH3 at Ti sites; (b) H-bonded NH3 on oxide sites; (c) Lewis-bonded NH3 at vanadyl sites; (d) ammonium ions bonded at V Br˘nsted acid sites

    34. 2012-9-28 7.2. NO adsorption and desorption Weak or no adsorption of NO on V2O5, MoO3 and WO3-based catalysts Leading to the reaction of gas-phase NO with NH3 Stronger adsorption of NO on Fe2O3, CuO-, MnOx-based catalysts

    35. 2012-9-28 Other items Adsorption of impurities, such as hydrazine (NH2-NH2) and hydroxylamine (NH2OH) NH3 and NO co-adsorption

    36. 2012-9-28 8. Catalysts activity in other catalyzed reactions of NO (1) Cu species in Cu-ZSM-5, including bulk and supported Cu Decomposition of NO Reduction of NO with HCs Oxidation of NO to NO2 Reduction of NO with CO and H2

    37. 2012-9-28 (2) Mn-ZSM-5 & Fe-ZSM-5 Reduction of NO with HCs even in presence of O2 NO reduction by CO, CH4 and H2 V-based catalysts are inactive for the above reactions

    38. 2012-9-28 9. Proposed intermediate species, and reaction mechanisms for the SCR of NOx by NH3

    39. 2012-9-28 10. The active sites and mechanism of the NOx SCR by NH3 on V-based catalysts (1) Active sites Coordinated ammonia, amide, NH4+ or NH3+ ions---- existed NH3 types However, BrŘnsted acid mechanism was firstly favored due to the following reasons ammonium ions: only observed species from ammonia adsorption on V2O5 water was reported to accelerate the reaction

    40. 2012-9-28 very strong Br˘nsted acidic materials like sulfated titania and zeolite H-ZSM-5 show SCR activity additives increasing the Br˘nsted acidity, like WO3 and sulfates for V2O5-TiO2 and sulfates for TiO2, also increase catalytic activity IR studies showed that ammonium ion species are very evident on vanadia-based catalysts

    41. 2012-9-28 The reverse conclusions the presence of Lewis acidity, together with Br˘nsted acidity, on V-supported catalysts water inhibits the SCR reaction, likely due to a competition with ammonia on the Lewis sites Sulfation and addition of tungsten oxide increases also Lewis acidity of V2O5-TiO2 The IR data of Ramis et al. clearly show the reaction of Lewis bonded ammonia with NO

    42. 2012-9-28 (2) Reaction mechanism- Lewis mechanism (“amide-nitrosamide” mechanism , redox) NO does not necessarily adsorb before reaction amide species -NH2 from the adsorbed NH3 nitrosamide NH2NO formation from the reaction -NH2 with NO, due to the IR peak of -NH2 disappears after the NO addition and NH2NO can be detected by MS kinetic data can be successfully interpreted on the basis of this mechanism

    43. 2012-9-28 11. the active sites and mechanism on other transition metal-based catalysts Fe-, Cu-, Cr- and Mn-based catalysts adsorption of both NO and ammonia amide-nitrosamide was formed by the reaction of the adsrobed NO and the adsorbed ammonia

    44. 2012-9-28 12. Future research Catalysts characterization: monomeric species and polymeric species must be clearly tested Redox and acidic mechanism need be further affirmed A new experimental must be designed to investigate the real reaction under real conditions, such SOx and H2O so on The role of NO2 in SCR was not completely clear

    45. 2012-9-28

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