(24d) Unusual Low-Temperature Ammonia Oxidation Behavior In Zeolite-Based SCR Catalysts

Selective catalytic reduction of NOx with NH3 (SCR) has emerged in recent years as a primary technology for meeting the increasingly stringent environmental regulations for diesel exhaust emissions. This was enabled by a new class of SCR catalysts, introduced in the US in the recent years, based on metal-exchanged zeolites. These catalysts combine a number of beneficial characteristics, including excellent activity across a broad range of conditions and outstanding hydrothermal stability.

Our detailed characterization of the individual redox and acid-base functions of these catalysts showed that they possess poor activity for NH3 oxidation by O2, with no appreciable conversion up to ~ 400C, as expected from catalysts designed to selectively react NH3 with NOx in the presence of vast excess of O2. On the other hand, we discovered that, under the SCR reaction conditions unexpectedly large NH3 oxidation by O2 (parasitic NH3 oxidation) was observed, even at temperatures as low as ~250C. Further study of this unexpected phenomenon by systematically varying inlet gas composition has confirmed that such parasitic NH3 oxidation has only occurred in the presence of NOx in the gas feed. The data collected to date indicate that NOx modifies oxidation state of the exchange metal sites in zeolite, leading to their increased activity in the ammonia oxidation process.