Novel Concepts for Low Temperature Hydrocarbon SCR Catalyst Development

The removal of nitrogen oxides (NOx) from lean exhaust gas constitutes a major technological challenge. One solution suited for mobile applications is based on Selective Catalytic Reduction (SCR) employing hydrocarbon reductants. Currently, hydrocarbon SCR (HC-SCR) finds limited application in the automotive sector, mainly in the form of so-called passive deNOx. This refers to the limited NOx reduction "10%) that occurs over diesel exhaust oxidation catalysts, residual hydrocarbons in the exhaust gas functioning as the reductant. Active hydrocarbon SCR systems, in which additional diesel fuel is added to the exhaust gas to function as the NOx reductant, have to date been little used. This can be attributed to the limited NOx conversion levels obtained with current catalysts and the lack of adequate catalyst durability. However, this situation may change with the switch to low sulfur diesel fuel in the U.S. in 2007, which should result in improved catalyst durability. Indeed, hydrocarbon SCR is being promoted for retro-fit applications on heavy duty diesel (HDD) trucks in areas where low sulfur diesel is already available [1]. The ease of installation and operation compare very favorably with other NOx reduction technologies such as ammonia/urea SCR or NOx adsorber catalysts, albeit that HC-SCR cannot provide such high NOx conversion levels as these other methods. Additionally, HC-SCR may find application on new vehicles when more stringent Federal emission standards for HDD and light duty diesel (LDD) vehicles take effect in the 2007-2010 timeframe [2]. Although HC-SCR by itself is unlikely to provide sufficient NOx reduction to satisfy these emission standards, it may provide a useful tool for combination with other NOx reduction strategies [3]. Against this background we set out a new strategy in this proposal, aimed at optimizing HC-SCR performance. Given that one of the main weaknesses of HC-SCR technology is the very limited temperature range of operation of HC-SCR catalysts (as discussed below), we propose an approach based on dual SCR catalysts, corresponding to low temperature (L 1) and high temperature (H1) formulations, with exhaust gas switching to utilize the appropriate catalyst based on gas temperature. Further, we propose developing a LT catalyst tailored to this purpose, for use in LDD applications (although we note that the exhaust gas switching concept could equally well be applied to lean burn gasoline vehicles). Based on current leads, platinum supported on activated carbon appears to be the most active HC-SCR catalyst in the low temperature range (150-250 0q, while possessing the highest selectivity to N2 (as opposed to N20) in comparison with other supported Pt catalysts [4,5]. Alloying Pt with other metals, such as Rh or Ir, may additionally help to improve the selectivity to N2, as well as being beneficial for HC-SCR activity. However, a major concern associated with the use ofPt/carbon catalysts is the propensity of the carbon support to undergo N02-assisted combustion. For this reason we propose using multi-walled carbon nanotubes (MWNTs) as the support. MWNTs represent a relatively "new" form of carbon, given that their existence was unknown until 1991 [6]. Whilst a number of studies have shown them to be excellent catalyst supports, of greater significance in the present context is the fact that they are less readily oxidized than amorphous forms of carbon. This fact, together with the use of exhaust gas switching, renders the application of MWNTs to HC-SCR catalyst design viable. We therefore intend to develop Pt/ MWNT-based catalYsts as active, selective and oxidation-resistant LT HC-SCR catalYsts. A further olycctive is to stutfy structure-activitY relationships in Pt-based metal alloy catalYsts, so as to derive fundamental insights which mqy aid the future design of H C-SCR catalYsts. 2. Hydrocarbon SCR catalysts: state of the art Zeolite-based catalysts such as Cu-ZSM-5 initially showed the most promise for hydrocarbon SCR