Composition dependent performance of alumina-based oxide supported WO3 catalysts for the NH3-SCR reaction and the NSR + SCR coupled process

Abstract

Among the specific technologies to reduce NOx from automobile lean exhaust gases, the NOx storage reduction (NSR) and the selective catalytic reduction (SCR) by urea/NH3 are the two main effective proposed processes. During the short rich excursions of the NSR process, emissions of undesired NH3 can occur and the combination of the NSR and a NH3-SCR catalyst has been proposed to improve the global treatment efficiency. With the aim to develop non zeolite SCR catalysts, materials based on modified alumina were prepared by sol–gel method. Necessary acidic and redox properties were expected by incorporation of metal ions such as cerium (for oxygen mobility), zirconium, titanium, and silica (acidic behavior). The influence of each element incorporated in the host alumina and their combination were evaluated in NH3-SCR before and after 9wt% WO3 impregnation. Among the 15 synthetized supports, three WO3 supported catalysts exhibited promising SCR activities were selected (WO3/Al0.2Ce0.4Ti0.4, WO3/Al0.2Ce0.16Zr0.32Ti0.32 and WO3/Al0.1Si0.1Ce0.16Zr0.32Ti0.32) and characterized (N2 adsorption, XRD analysis, NH3-SCO reactivity, NH3 storage capacity, pyridine adsorption monitored by IR spectroscopy, H2-TPR, OSC) and then associated downstream to a model Pt/Ba/Al2O3 NSR catalyst. The use of ammonia emitted from the NSR catalyst during the rich pulses was studied taking into accounts three possibilities: the NH3-SCR reaction (NH3+NOx→N2), the NH3-SCO reaction (NH3+O2→N2) and the unconverted NH3. Only the NH3-SCR reaction occurs at 200°C; whereas the three pathways take place at 300 and 400°C. In addition, a lack of strong acidic storage sites results in unconverted ammonia at high temperature.