Ce-ZSM-5 catalysts for the selective catalytic reduction of NO x in stationary diesel exhaust gas

Abstract

This paper describes the preparation of Ce-ZSM-5 SCR catalysts for stationary diesel engine applications, such as marine diesels, and the performance of these catalysts with ammonia as a reducing agent in both simulated and real diesel exhaust gas. Catalysts prepared by ion exchange in aqueous solution, show an increasing NO x conversion with increasing cerium ion-exchange level at all temperatures tested (NO x conversion > 80% in the temperature range 350–500°C at a gas hourly space velocity (GHSV) of 40.000 h −1 with simulated diesel exhaust). Ce-ZSM-5 does not convert nitric oxide or ammonia into the greenhouse gas nitrous oxide. Moreover, Ce-ZSM-5 does not exhibit ammonia slip up to ammonia/nitric oxide feed ratios of 1.4, the excess ammonia being converted to nitrogen. Ce-ZSM-5 extrudates with 50 wt.% alumina binder do not show ammonia slip up to ammonia/nitric oxide feed ratios of at least 1.1. With diesel exhaust gases of a three-cylinder, 10 kW diesel engine, Ce-ZSM-5 can reach about 70% NO x conversion at 500°C at a GHSV of 50.000 h −1. Deactivation of the catalysts in real diesel exhaust occurs mainly in the first 60 h of operation resulting in stable catalysts which show about 40% NO x conversion at 450°C, at a GHSV of 32.000 h −1. The preparation of Ce-ZSM-5 via solid-state ion exchange results in a remarkably active catalyst.