Durable Zeolite Based Catalyst Systems for Diesel Emission Control

Diesel vehicles are widely used for transportation of people and goods, which is responsible for a significant consumption of diesel, and the associated release of mainly CO2, but also pollutantssuch as CO, hydrocarbons, soot particles and NOx (x = 1,2). These pollutants threat the health of humans and negatively affect the environment, and therefore, abatement is enforced by legislation, which is practically handled by installation of exhaust after treatment systems in passenger cars aswell as in heavy-duty vehicles.

A crucial component of the after treatment systems is the catalystfor selective catalytic reduction of NOx with NH3 (NH3-SCR). Current state-of-the-art SCR catalysts are zeolite-based Cu-CHA materials, which is mainly due to their unmatched low temperature activity. Unfortunately, the presence of 0.5-2 ppmv of SO2 in diesel exhaust significantly inhibits the low-temperature activity Cu-CHA catalysts, which diminishes the NOx removal efficiency.

In order to comply with current and future NOx emission limits, development of more SO2 resistant Cu-CHA catalyst systems is necessary, which requires a better fundamental understanding of the deactivation of Cu-CHA catalysts by SO2. In this work, Cu-CHA catalysts have been produced and exposed to SO2 in various gas mixtures at different temperatures (200-550 °C), and exposure times, to investigate the effects of SO2 at the various conditions of an after treatment system.

The uptake of S, determined from elemental analysis and adsorption/desorption measurements, has been compared to the impact on the catalytic performance in the NH3-SCR reaction after the different SO2 exposures, and after regeneration at 550 °C in SO2-free gas. In parallel, characterization with scanning transmissionelectron microscopy – energy dispersive X-ray (STEM-EDX) spectroscopy and electron paramagnetic resonance (EPR) spectroscopy have been used to assess the location of S, and densityfunctional theory (DFT) calculations have been carried out to determine possible Cu,S species.

The deactivation is established to be the result of formation of Cu,S species, and not a consequence of ammonium sulfate precipitation, since the S/Cu ratio has not been observed to significantly exceed 1. Some Cu,S species decompose below 550 °C (reversible), while a more stable Cu sulfate species (irreversible) that decomposes around 650 °C, can form on a restricted fraction of the Cu.

Formation of the different Cu,S species is dependent on several conditions such as the oxidation state of Cu, the temperature, and the presence of H2O and SO3. DFT calculations suggested that SO2 adsorbs more stably on CuI, while SO3 preferably reacts with CuII, which was consistent with experimental data.

At 200 °C it was observed that the formation of Cu,S species is enhanced by co-feeding SO3, whereas at 550 °C there is no measurable impact. In the same experiments, the presence of H2O enhanced the formation of irreversible Cu sulfate at both 200 and 550 °C, but had no impact on the formation of reversible Cu,S species.

While there is no apparent impact of the chemical composition of the CHA framework (HnAlnSi1-nO2 vs HnSinAlP1-nO4), the CuII sites associated with one or two framework Al centers, Z-CuO Hand Z2-Cu, respectively, have different resistance towards SO2, as indicated by DFT calculations.The EPR characterization indirectly showed that mainly Z-CuOH reacts with SOx to formation of the reversible Cu,S species, whereas certain Z2-Cu sites were directly seen to participate in the formation of irreversible Cu sulfate.

Finally, other Z2-Cu sites were inert to SO2 exposure, whichexplains why a 100% deactivation has not been observed. In terms of the impact of reversible and irreversible Cu,S species on the NH3-SCR activity, the deactivation inferred by the reversible Cu,S species was always disproportionately larger than theS/Cu ratio, and caused a lowering of the apparent SCR activation energy with increasing S/Curatio.

In contrast, the remaining irreversible Cu,S species after regeneration exhibited a 1:1correlation between the deactivation and S/Cu ratio, as well as the apparent activation energieswere restored to the same level as the fresh catalyst. The deactivation occurs by exposure to 1.5ppmv SO2, and by increasing the SO2 concentration and simultaneously decreasing the exposuretime correspondingly, similar deactivation levels are reached.

Hence, it appears to depend on theproduct of the SO2 concentration and exposure time. Accelerated SO2 exposures showed that the deactivation occurs fast, reaching at least 80% before 5% of the lifetime SO2 exposure. However, the deactivation could at all times be lowered to about 20%, which is probably dependent on thespecific Cu-CHA catalyst.

Thus, the application of Cu-CHA catalysts in aftertreatment systems iscontingent on regeneration. A new method to quantify the active amount of Cu in Cu-CHA catalysts by measuring the NO consumption during a temperature-programmed reduction in NO+NH3 has been developed (NOTPR). The method is applicable on regenerated catalysts, and potentially also on SO2 exposed catalysts.