Hydrogen Promotion of Low-Temperature Passive Hydrocarbon-Selective Catalytic Reduction (SCR) over a Silver Catalyst K. Theinnoi, † S. Sitshebo, † V. Houel, ‡ R. R. Rajaram, ‡ and A. Tsolakis* ,† School of Mechanical Engineering, UniVersity of Birmingham, Birmingham B15 2TT, United Kingdom, and Johnson Matthey Technology Centre, Blount’s Court, Sonning Common, Reading RG4 9NH, United Kingdom ReceiVed June 12, 2008. ReVised Manuscript ReceiVed July 31, 2008 Relatively small amounts of H 2 (500-3000 ppm) have been added to a real diesel exhaust gas during passive hydrocarbon-selective catalytic reduction (SCR) over a silver catalyst. The added H 2 is very effective at promoting low-temperature activity and at sustaining it by removing carbon-rich poisoning species from the catalyst surface. NO 2 production by the catalyst appears to be a key step in the self-cleaning process, but it is highly dependent upon the gas-phase concentrations of H 2 , total NO x , and hydrocarbons and the exhaust-gas temperature. Careful control of H 2 addition is therefore required in response to changes in conditions, to maintain suppression of the carbon-rich species without releasing NO 2 from the tailpipe. The highest overall NO x reduction can be achieved by operating the engine with exhaust gas recirculation in combination with H 2 -promoted passive hydrocarbon-SCR as an aftertreatment. 1. Introduction Growing concerns about climate change and the decline in fossil fuel resources are contributing to the increasing use of diesel technology. With its superior fuel economy and its ability to run on a range of renewable fuels, the diesel engine is no longer limited to its traditional applications (heavy-duty trucks, buses, off-road vehicles, and machinery) but is steadily gaining a greater share of the massive passenger-car market, particularly in Europe. 1 However, oxygen-rich combustion of diesel fuel leads to specific emission-control challenges, which are not encountered with gasoline spark ignition. These challenges relate to the removal of particulate matter (PM) and the reduction of NO x (NO and NO 2 ) under strongly oxidizing conditions. 2 Furthermore, at the same time as the diesel engine population is growing, more stringent regulations are being introduced to control PM and NO x emissions, e.g., Euro 6 and U.S. Tier 2-Bin 5 for passenger cars and light-duty vehicles, and US-2010 for trucks. These regulations will require the introduction of new exhaust-gas aftertreatment systems, such as a combination of diesel particulate filter (DPF) and NO x -reduction catalyst. While catalyzed DPFs are already being used to remove PM, CO, and hydrocarbons, 3 the main contenders for future NO x control are urea-SCR 5,6 and NO x adsorption, 6,8 probably in conjunction with exhaust-gas recirculation (EGR). By compari- son, selective catalytic reduction of NO x using hydrocarbons (known as hydrocarbon-SCR) is considered to be much more difficult to operate successfully, yet it has the clear advantage of being simpler to implement, especially if it were to be used passively. Among the potential hydrocarbon-SCR catalysts, supported silver is one of the most promising. It forms N 2 selectively and is durable, and although inhibited by sulfur species, the effect is reversible. However, at typical diesel exhaust-gas temperatures (e300 °C), its activity can be low. The main causes of this low rate of reaction are (i) the silver can self-poison by forming stable surface nitrates and (ii) gas-phase hydrocarbons can transform into carbon-rich surface species that block the active sites. In common with other workers in the field, 2,9-19 we have found that active hydrocarbon-SCR (using injected fuel as the * To whom correspondence should be addressed. Telephone: +44121- 4144170. Fax: +44121-4143958. E-mail: a.tsolakis@bham.ac.uk. † University of Birmingham. ‡ Johnson Matthey Technology Centre. (1) Schindler, K. P. Advances in diesel engine technologies for European passenger vehicles. U.S. Department of Energy (DOE), 8th Diesel Emissions Reduction Conference (DEER), San Diego, CA, Aug 2002, http://www. osti.gov/fcvt/deer2002/schindler.pdf. (2) Satokawa, S. Enhancing the NO/C 3 H 8 /O 2 reaction by using H 2 over Ag/Al 2 O 3 catalysts under lean-exhaust conditions. Chem. Lett. 2000, 294. (3) Chatterjee, S.; Walker, A. P.; Blakeman, P. G. Emission control options to achieve Euro IV and Euro V on heavy duty diesel engines. SAE Paper 2008-28-0021, 2008. (4) Saito, S.; Shinozaki, R.; Suzuki, A.; Jyoutaki, H.; Takeda, Y. Development of urea-SCR system for commercial vehiclesBasic charac- teristics and improvement of NO x conversion at low load operation. SAE Paper 2003-01-3248, 2003. (5) Koebel, M.; Elsener, M.; Kleemann, M. Urea-SCR: A promising technique to reduce NO x emissions from automotive diesel engine. Catal. Today 2000, 59, 335. (6) Mahzoul, H.; Brilhac, J. F.; Gilot, P. Experimental and mechanistic study of NO x adsorption over NO x trap catalysts. App. Catal., B 1999, 20, 47. (7) Ohno, H.; Takanohashi, T.; Takaoka, N.; Kuroda, O.; Iizuka, H. NO x conversion properties of a mized oxide type lean NO x catalyst. SAE Paper 2000-01-1197, 2000. (8) Guyon, M.; Blanche, P.; Bert, C.; Philippe, L.; Messaoudi, I. NO x - trap system development and characterization for diesel engines emission. SAE Paper 2000-01-2910, 2000. (9) Richter, M.; Bentrup, U.; Eckelt, R.; Schneider, M.; Pohl, M.-M.; Fricke, R. The effect of hydrogen on the selective catalytic reduction of NO in excess oxygen over Ag/Al 2 O 3 . App. Catal., B 2004, 51, 261. (10) Sazama, P.; Capek, L.; Drobna, H.; Sobalik, Z.; Dedecek, J.; Arve, K.; Wichterlova, B. Enhancement of decane-SCR-NO x over Ag/alumina by hydrogen. Reaction kinetics and in situ FTIR and UV-vis study. J. Catal. 2005, 232, 302. Energy & Fuels 2008, 22, 4109–4114 4109 10.1021/ef8004515 CCC: $40.75  2008 American Chemical Society Published on Web 10/08/2008