Impacts of CO, H2 and C3H6 on the SCR reactions over a Cu-chabazite SCR catalyst

Yang Zheng, Michael P. Harold*, Dan Luss*
Department of Chemical &Biomolecular Engineering
University of Houston
Houston, TX 77204

Abstract

Upcoming EPA Tier 3 emission regulation sets a strict tailpipe NOx standard for light duty diesel and lean burn gasoline vehicles. A promising deNOx technology that helps automakers overcome the regulatory challenge is the combined LNT+SCR system, wherein the selective catalytic reduction (SCR) catalyst stores and utilizes reductants, generally NH3 , produced over the lean NOx trap (LNT) catalyst during rich purges for additional NOx reduction. The SCR component is periodically exposed to a rich feed typically containing a mixture of CO, H2 and hydrocarbons for LNT regeneration. This periodic exposure to reducing agents could somehow affect the SCR reactions, which are normally conducted under oxidizing conditions for stand-alone SCR system.
In order to understand the effect of rich feed on SCR catalysts, we study the impacts of CO, H2 and C3 H6 on the NH3 -SCR mechanisms over a commercial Cu-Chabazite (Cu- CHA; SSZ-13) SCR monolithic catalyst. The steady state reactions of NO, NO2 with the reducing agents (CO, H2 and C3 H6 ) are first examined. Although there is almost no reaction of NO alone with these reducing species, these reductants effectively promote the decomposition of NO2 to NO in the order of C3 H6 > CO > H2 , presumably through their oxidation which yields NO. Hence, such reducing agents are able to affect SCR reactions by varying the NO:NO2 ratio in favor of NO. We then examined the effects of CO, H2 and C3 H6 on the SCR reactions of NOx with NH3 under both steady state and cyclic conditions. As for steady-state standard and fast SCR reactions, CO and H2 only slightly affect the NOx conversion, while C3 H6 inhibits both SCR reactions due to an apparent poisoning effect of surface species formed between C3 H6 with NOx and O2 , as well as the efficient NO2 reduction to NO by C3 H6 at a rate of similar order of magnitude
as the fast SCR reaction. When it comes to slow SCR (NH3 + NO2 ), CO and C3 H6 enhance the NOx conversion at low temperatures (< 280 oC) due to the increased NO:NOx ratio that favors the fast SCR reaction. While increasing temperature above

300 oC to further raise NO:NOx ratio in favor of the standard SCR reaction, markedly decreases NOx conversion for C3 H6 case only. A similar but more complex trend is observed under cyclic conditions. Finally, the influence of these reductants on NH3 storage and oxidation is determined. Negligible change in NH3 storage capacity is observed in the presence of the reducing agents. NH3 oxidation is moderately inhibited in the presence of C3 H6 due to the aforementioned poisoning effect.

Fig.1 NO2 reactions with CO, H2 and C3 H6 . Feed condition is 500 ppm NO2 , 2.5% H2 O and 2% CO2 with either 1% CO, or 1% H2 or 500 ppm C3 H6 .

Fig.2 Steady-state slow SCR reaction in the presence of CO, H2 and C3 H6 . Feed condition for slow SCR is

500 ppm NO2 , 500 ppm NH3 , 2.5% H2 O and 2% CO2 .