| RESEARCH |
At Northwestern University, chemistry professors Wolfgang M. H. Sachtler and Eric Weitz have teamed up to unravel reaction mechanisms that govern NOx reduction. The goal is to understand the chemistry in detail to maximize the effectiveness of NOx cleanup. Sachtler pointed out that, a few years ago, researchers found that NOx can be reduced efficiently by a Ba/Y-zeolite catalyst and acetaldehyde as the reducing agent at 200 șC, which is a typical diesel exhaust-gas temperature. So the Northwestern team probed the system by IR spectroscopy, isotopic labeling, and other methods. According to Sachtler, the investigation revealed a complex mechanism in which acetaldehyde is oxidized to acetic acid, which then reacts with NO2 to form the aci-anion of nitromethane (H2C=NOO). Nitromethane, in turn, is converted to isocyanic acid (HNCO), which can be hydrated to form ammonia. Ultimately, ammonia reacts with NOx to form ammonium nitrite (NH4NO2), which then decomposes to yield water and dinitrogen. OH, NO! Northwestern's Sachtler (left) and Weitz probe mechanistic details of NOx-cleanup chemistry. PHOTO BY MITCH JACOBY AFTER REVIEWING the main features of the 12-step mechanism, Sachtler remarked that "in 55 years of fundamental research in catalysis, I have never studied a reaction of such complexity in which every step has been reasonably understood and well-documented." The Northwestern study also addressed a puzzling feature regarding the ratio of reactants. Weitz noted that when ammonia is the reducing agent, NOx can be converted to N2 with nearly 100% efficiency on Ba/Y-zeolite if NO and NO2 are present in equimolar ratios. To understand the basis for that constraint, Weitz and Sachtler examined the role of NO in NOx reduction. It turns out that NO plays multiple mechanistic roles, Weitz said. For example, NO reduces HNO3 to HNO2 and reduces ammonium nitrate to ammonium nitrite. In addition, NO+ can react with water to form HNO2. Summarizing the chemical processes in a multistep reaction diagram, Weitz noted that all of the desired reactions lead to ammonium nitrite, which decomposes to form water and N2, the products of NOx abatement. He then explained that if all the reactions are combined to form a net reaction that yields water and N2, the stoichiometry indicates that NO and NO2 combine in a 1:1 ratio (J. Catalysis 2005, 231, 181). |
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