NOx reduction from diesel emissions over a nontransition metal zeolite catalyst:: A mechanistic study using FTIR spectroscopy

The reactions of acetaldehyde. acetic acid, and nitromethane with NO2 on a BaNa-Y zeolite are monitored with FTIR spectroscopy, typically at 473 K. Isotopic labeling was used to help elucidate reaction pathways and intermediates. The mechanism that has been deduced for the reaction of NO2 with acetaldehyde starts with the formation of acetate ions and subsequently the aci-anion of nitromethane. NO can react with the aci-anion of nitromethane, which is stabilized in the ionic environment of the zeolite to form a proposed O2NCH2NO2- intermediate. At 473 K. the O2NCH2NO2- intermediate is expected to swiftly lose NO2 and H2O yielding the formonitrile oxide anion, CNO-, which can isomerize to NCO- and react with a proton to form HNCO. The formation of an O2NCH2NO2- intermediate is consistent with the observation of both isotopomers of HNCO when (NO2)-N-15 is a reactant. Also, as expected for an O2NCH2NO2- intermediate, all four isotopomers of N2O are observed. In the next step, NH3 and CO2 are formed from HNCO + H2O. Thus, in this system ammonia is formed by the sequence: CH3CHO --> CH3COO- --> CH2NO2- --> O2NCH2NO2- --> CNO- --> HNCO --> NH3. NO2 is the reaction partner in the first three steps; a proton is added in the fifth step and H2O is the co-reactant in step 6. Once NH3 is formed, well-known chemistry can lead to the formation of N-2. Remarkably, in this system N-2 formation as a result of reduction of NOx occurs in the absence of a transition metal. The formation of nitromethane is observed when acetaldehyde reacts with NO2. When added nitromethane reacts with (NO2)-N-15, formation of (NO2)-N-14 is observed, which is also consistent with formation of an O2NCH2NO2- intermediate. The reaction channel outlined above is also followed by acetic acid and results in a very high CO2/CO ratio (>20:1). The reaction of either acetic acid or acetaldehyde with NO2 gives a similar set of products, except that the CO to CO2 ratio is higher and methanol is formed when acetaldehyde is the reactant. These observations suggest that a parallel reaction channel exists for acetaldehyde which could involve free radicals. Such a path would likely be initiated by NO2 abstracting a H atom from acetaldehyde. One possible reaction path for the resulting acetyl radical is dissociation to CO and a methyl radical. Gas-phase nitromethane is also observed with acetaldehyde but not with acetic acid as the reactant. Nitromethane could come from neutralization of an ionic precursor or it could be a product of the additional reaction channel for acetaldehyde alluded to above.