Mechanistic Studies of the Reduction of NOx by Oxygenates

Eric  Weitz, Northwestern University

Younghoon Yeom, Meijun Li, Aditya Savara, Wolfgang Sachtler and Eric Weitz
Institute for Catalysis in Energy Processes and Department of Chemistry
Northwestern University
Evanston, IL, USA 60208

Using principally FTIR spectroscopy we have elucidated the multi-step mechanisms for the reduction of NOx in the presence of acetaldehyde over BaNa/Y zeolite [1] as well as for NOx reduction in the presence of ethanol over silver exchanged Y zeolite (Ag/Y) [2].  Surface acetate ions, formed from the oxidation of acetaldehyde, react with NO2 to yield nitromethane: a critical intermediate in subsequent deNOx chemistry.  Nitromethane, which is likely in equilibrium with its aci-anion, reacts with NO2.  Our data indicate that this reaction leads to a dinitromethane intermediate, which then dissociates to form HNCO. HNCO can then be hydrolyzed to produce ammonia.  The subsequent chemistry to form ammonium nitrite and then N2 by decomposition of ammonium nitrite is well known.  The major qualitative difference in these systems is the temperature dependence of the reaction of acetate with NO2.  Nitromethane is a critical intermediate in the pathway for NOx reduction with both ethanol and acetaldehyde.  When introduced as a reductant, yields for the reduction of NO2 to N2 approach 100% at temperatures as low as 140 C [3].  The implications of these observations for the rate limiting step in the mechanism for NOx reduction in these systems will be discussed.  If time permits, the role of NO in the reduction of NOx with added NH3 will also be discussed [4].

1.  Yeom Y., Wen B., Sachtler W.M.H., and Weitz E. J. Phys. Chem., B, 108 5386 (2004)
2.  Yeom. Y, M. Li, W. M.H. Sachtler, and E. Weitz, J. Catalysis 238 110-110 (2006)
3. Yeom. Y, M. Li, W. M.H. Sachtler, and E. Weitz,, Catalysis Letters 118, 173-179 (2007).
4.  Yeom Y., Henao J., Li M., Sachtler W.M.H., and Weitz E.  J. Cat. 231, 181-193 (2005)