Impact of different forms of feed sulfur on small-pore Cu-zeolite SCR catalyst

• Extent of sulfur poisoning over SCR catalyst depends on temperature, time, presence of water, and SO3/SOx ratio.
• The mechanism of sulfur storage changes from physical adsorption to chemical adsorption with increase in catalyst temperature.
• Standard SCR reaction is drastically affected by the presence of sulfur, but fast SCR reaction has minimal impact.

Cu-exchanged small-pore zeolite catalysts, belonging to the structural family of chabazite (CHA), have achieved substantial commercial importance in the recent years as the materials of choice for selective catalytic reduction of oxides of nitrogen with ammonia in diesel exhaust. Their successful application was enabled by the understanding and management of the interactions of these catalysts with sulfur species present in the exhaust, mainly SO2 and SO3. While the latter was shown to be of particular importance, there remain very few literature studies on the impact of SO3 on the sulfation–desulfation behaviors of the Cu-zeolite catalysts of this class. This is due to the substantial experimental difficulties associated with using SO3 as a gas feed constituent and especially its accurate quantification. In this work, we have developed two specialized bench flow reactor systems, dedicated to generating controlled amounts of SO3, characterizing its impact on several facets of catalyst behavior, and quantifying the amount of stored sulfur species. Using the tools and methodology thus developed, we have investigated the interactions of a state-of-the-art Cu-zeolite catalyst with SO2 alone and with a mixture of SO2 and SO3, in the presence of water vapor. We have compared their impact at the lower- and upper-end of the practically relevant temperature range, namely at 200 and 400 °C. At the low temperature, the effect of sulfur poisoning was found to be modest and species-independent. However, at the higher temperature, presence of SO3 resulted in a substantially more significant impact on the catalyst performance, which was also more difficult to reverse. These findings indicate that sulfur poisoning of the Cu-zeolite catalyst can occur via different mechanisms, from indiscriminate adsorption of SOx species on Cu sites at lower temperatures, to a reversible chemical reaction of SO3 or H2SO4 produced in the wet feed, with the catalyst material at elevated temperatures.

Effect of 33 ppm SOx (SO2/SOx = 0.7) exposure at 400 °C for 24 h and subsequent regeneration steps on NOx conversion of Cu-CHA during standard SCR conditions.Figure optionsDownload high-quality image (148 K)Download as PowerPoint slide