Nitrogen Oxides (NOx) are common toxic pollutants that are produced during high temperature oxidation, for example, in combustion engines. Catalytic NOx reduction by ammonia in metal-exchanged zeolites is a well-established method to convert these contaminants into environment-friendly species (N2 and H2O).1 Equimolar mixtures of NO/NO2 are desirable, because they can undergo the most effective fast NOx reduction reaction;2 yet, combustion processes typically produce more NO than NO2. Therefore, NO oxidation is a relevant step prior to NOx reduction and has been extensively studied. While NO oxidation may occur uncatalyzed as homogenous reaction, it has been shown that confinement and entropic effects in small pore zeolites increase the oxidation rate, with SSZ-13 demonstrating the best performance.3 When heterogeneously catalyzed over acidic zeolites, NO oxidation at low temperature (263-473 K) is unusual because its rate decreases with increasing temperature, resulting in a negative apparent activation barrier.3,4 At high temperature (&gt; 473 K), however, rate increases with increasing temperature.5 In this contribution, we use density functional theory (DFT) and statistical mechanics to shed some light on this puzzling observation and investigate the mechanistic differences at low and high temperature. Moreover, we study the zeolite acidity effects on the NO oxidation reaction free energy profiles within the low and high temperature regimes.

NO oxidation in Brønsted acidic chabazite within low and high temperature regimes: Theoretical study

2019 Fall National Meeting

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