INTERNATIONAL JOURNAL OF ENERGY RESEARCH Int. J. Energy Res., 23, 351 — 369 (1999) CATALYTIC CONTROL OF AUTOMOTIVE NO  : A REVIEW SOUVIK BHATTACHARYYA * AND RANDIP K. DAS Mechanical Engineering Department, University of Canterbury, Private Bag 4800, Christchurch, New Zealand R&D Centre, Mahindra & Mahindra Ltd, Nashik, India SUMMARY This article summarizes several technical studies reported in the literature on catalytic conversion technology to control pollution due to automotive exhausts with specific focus on NO  reduction. While simple theoretical reactions are stated, the review concisely presents the various techniques available with their specifications and performances. Noble-metal converters, in spite of their proven-technology advantage, are considered expensive while zeolite-based catalysts are preferred today as increasingly more research findings have made this technology more mature. Conclusion and recommendations on specific applications have been presented as well. Copyright  1999 John Wiley & Sons, Ltd. KEY WORDS: catalytic converter; NO  control; zeolite; emissions 1. INTRODUCTION The exhaust gases from automobile engines and industrial boilers contain mainly oxides of carbon (CO and CO  ), oxides of nitrogen (NO  ), hydrocarbons (HC), sulphur dioxide (SO  ), particulates and soot. One of the most urgent problems is removal of NO  , typically produced during high-temperature combustion and contribute to formation of smog and ground-level ozone by reacting with hydrocarbons in presence of sunlight. Increasingly stringent pollution control standards are being promulgated everyday necessitating development of more efficient emission control systems. This article highlights several technical studies reported in the literature on catalytic conversion technoogy to control pollution due to automotive exhausts with specific focus on NO  emission reduction. A recent review (Heimrich, 1996) has been reported specifically for diesel exhausts and only reviewed studies made on NO  control systems tested using diesel engine exhaust. Another recent (Koltsakis and Stamelos, 1997) article has reviewed catalytic automotive exhaust aftertreatment in a more general perspective with less emphasis on specific details of the technologies involved. The present review attempts at a more complete picture of the phenomenon with more elaborate insight including studies made with engine exhausts as well as in chemical engineering research specializing in catalysts. Different techniques for controlling exhaust emissions can be grouped into two broad categories: combus- tion modification, methods which are adopted before or during combustion and exhaust after-treatment, which are provided to the exhaust gases after the combustion is complete. 2. COMBUSTION MODIFICATIONS Different techniques such as, spark retard, rich or lean A/F mixture, water injection, exhaust gas recirculation (EGR), burn rate modification, etc. have been tested for keeping the pollutants under control. HC and CO emissions can be checked up to certain extent by using lean A/F mixture while by using rich mixtures NO  *Correspondence to: Souvik Bhattacharyya, Department of Mechanical Engineering, University of Canterbury Private Bag 4800, Christchurch, New Zealand. Email: souvik@mech.canterbury.ac.nz CCC 0363-907X/99/040351— 19$17.50 Received 16 October 1998 Copyright  1999 John Wiley & Sons, Ltd. Accepted 28 October 1998