Experimental and Modeling Study of NH <sub>3</sub> -SCR on a Hydrocarbon-Poisoned Cu-CHA Catalyst

Abstract

<div class="section abstract"><div class="htmlview paragraph">A urea-selective catalytic reduction (SCR) system is used for the reduction of NOx emitted from diesel engines. Although this SCR catalyst can reduce NOx over a wide temperature range, improvements in NOx conversion at relatively low temperatures, such as under cold-start or low-load engine conditions, are necessary. A close-coupled SCR (cc-SCR), which was set just after the engine exhaust manifold, was developed to address this issue. The temperature of the SCR catalyst increases rapidly owing to the higher exhaust temperatures, and NOx conversion is then enhanced under cold-start conditions. However, since the diesel oxidation catalyst is not installed before the SCR catalyst, hydrocarbon (HC) emissions pass directly through the SCR catalyst and poison it, leading to lower NOx conversion. Therefore, the mechanism of NOx conversion reduction on HC-poisoned SCR catalysts are required to be studied. In this study, the effects of HC poisoning on the NOx conversion of Cu-CHA catalysts experimentally investigated using propene, n-decane, and 1-methylnaphtalene. In addition, a kinetic model of NH<sub>3</sub>-SCR over the HC-poisoned Cu-CHA catalyst was constructed. When 500 ppm propene was passed through the SCR catalyst, the coke was found to be formed on the catalyst, which led the decrease of the NOx conversion (maximum 75% reduction at 210 °C). Conversely, when n-decane or 1-methylnaphthalene was used, no coke was formed at temperatures below 500 °C, and the NOx conversion was unaffected. Even when coke was formed, it decomposed above 350 °C, and the NOx conversion was equivalent to that of a fresh catalyst. Based on the experimental results, a model for NH<sub>3</sub>-SCR over an HC-poisoned Cu-CHA catalyst was constructed. The reactor model was the one channel model and one-dimensional mass, momentum, energy and species balances were solved in the channel gas phase, assuming a quasi-steady state. The model reproduced the experimental results reasonably well, including the recovery of the catalyst from poisoning at relatively high temperatures.</div></div>