Mechanistic causes of the hydrocarbon effect on the activity of Ag–Al2O3 catalyst for the selective reduction of NO

Ag–Al2O3 catalyst prepared by the sol–gel method was studied for the selective catalytic reduction (SCR) of NO by various hydrocarbons (HC-SCR). The rates of NO reduction and hydrocarbon conversion depend strongly on the type of hydrocarbon reductant; the rates were higher for the higher normal alkanes, higher for alkene than alkane, and higher for normal alkane than branched alkane. The formation and reaction of hydrocarbon-derived species adsorbed on the surface were investigated by in situ FTIR spectroscopy. The apparent rate of acetate formation in the hydrocarbon + O2 reaction and the rate of nitrate reaction in hydrocarbon + O2 also depend on the nature of the hydrocarbon with a similar activity pattern to that for the SCR reaction. For the various hydrocarbons used, the rate of nitrate reaction in hydrocarbon + O2 was close to the steady-state rate of NO reduction, indicating that the nitrate is a possible intermediate in this reaction. The reaction of nitrates with oxygenated hydrocarbon species, possibly the acetate, was proposed to be a crucial step in HC-SCR. A proposed mechanism explains the hydrocarbon effect on the de-NOx activity: the NO reduction activity depends on the reactivity of the hydrocarbon molecule, which affects the rate of hydrocarbon oxidation to oxygenates and hence the rate of nitrate reaction with oxygenated hydrocarbons.

[1]  K. Shimizu,et al.  Silver-alumina catalysts for selective reduction of NO by higher hydrocarbons: structure of active sites and reaction mechanism , 2001 .

[2]  K. Shimizu,et al.  Intermediates in the Selective Reduction of NO by Propene over Cu−Al2O3 Catalysts: Transient in−Situ FTIR Study , 2000 .

[3]  J. Ross,et al.  Mechanistic Aspects of the Selective Reduction of NO by Propene over Alumina and Silver–Alumina Catalysts , 1999 .

[4]  Tarik Chafik,et al.  Reactivity of surface isocyanate species with NO, O2 and NO+O2 in selective reduction of NOχ over Ag/Al2O3 and Al2O3 catalysts , 1998 .

[5]  Kazuyuki Ueda,et al.  Catalytic reduction system of NOx in exhaust gases from diesel engines with secondary fuel injection , 1998 .

[6]  P. Jacobs,et al.  Chromatographic Study of Adsorption of n-Alkanes on Zeolites at High Temperatures , 1998 .

[7]  R. Burch,et al.  The selective reduction of nitrogen oxides byhigher hydrocarbons on Pt catalysts underlean-burn conditions , 1997 .

[8]  G. Magnacca,et al.  A case study: surface chemistry and surface structure of catalytic aluminas, as studied by vibrational spectroscopy of adsorbed species , 1996 .

[9]  P. Millington,et al.  Selective reduction of nitrogen oxides by hydrocarbons under lean-burn conditions using supported platinum group metal catalysts , 1995 .

[10]  E. Lox,et al.  Catalytic reduction of nitrogen oxides in diesel exhaust gas , 1995 .

[11]  W. Hall,et al.  Reaction Studies of the Selective Reduction of NO by Various Hydrocarbons , 1994 .