Abstract
One of the outstanding challenges in diesel exhaust catalysis is to integrate oxidation chemistry, soot filtration and NOx reduction in a single aftertreatment unit, while avoiding the need for fuel injection to regenerate the filter. Here we show that destruction of trapped soot can be initiated catalytically at 200 °C when its oxidation is coupled with non-selective NOx reduction (using NH3 as reductant), which acts as an in-situ source of N2O. In laboratory tests over an extended temperature range (up to 800 °C), using supported silver as a catalyst for both non-selective NOx-reduction and soot oxidation, the conversion of immobilised soot to CO2 can be resolved into four consecutive steps as the temperature rises: catalysed oxidation by N2O; non-catalysed oxidation by NO2; catalysed oxidation by O2; non-catalysed oxidation by O2. Initial engine tests indicate that the critical first step (C + N2O) can be replicated in a diesel exhaust.
Highlights
Effective control of air pollution leads to substantial benefits whether measured in terms of human wellbeing or in value to the economy [1]
In common with other workers in the field [13,14,15,16], we have been investigating the potential for coupling catalytic soot oxidation with SCR. We were expecting this to require a combination of catalysts with high performance for each separate function, we have found that a silver catalyst can reduce NOx to N2 while at the same time oxidising soot, at the low temperatures typical of diesel exhaust, despite it being a poor SCR catalyst in the absence of soot
When a silver soot-combustion catalyst, Ag/CZA, was tested for its ability to catalyse the reduction of NOx by reaction with NH3 in the absence of soot, the NO concentration decreased by 50% between 150 °C and 310 °C, before beginning to increase
Summary
Effective control of air pollution leads to substantial benefits whether measured in terms of human wellbeing or in value to the economy [1]. We were expecting this to require a combination of catalysts with high performance for each separate function, we have found that a silver catalyst can reduce NOx to N2 while at the same time oxidising soot, at the low temperatures typical of diesel exhaust, despite it being a poor (ie non-selective) SCR catalyst in the absence of soot. This effect arises because the non-selective product of SCR is N2O Eq (4), which we have found to be effective at initiating the oxidation of trapped soot Eqs. As the exhaust-gas temperature rises, N2O formation diminishes as initially NOx reduction becomes more selective and the direct oxidation reactions of NO and NH3 predominate, but soot removal can continue through the well-established oxidation routes in which NO2 and O2 are the oxidants
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