(755b) Nature of Active Sites for Passive NOx Adsorption on Pd-SSZ-13

Lean-burn designs of higher fuel-efficient diesel engines have resulted in exhaust emissions at considerably lower temperatures. However, leading emission control technologies (three-way catalysis, selective catalytic reduction, lean NO_x traps) available to abate NO_x,CO and hydrocarbon (HC) emission levels function at temperatures typically above 200 °C. The time required to reach this light-off temperature is referred to as the cold-start period when significant amounts of the mentioned pollutants get released into the atmosphere. In order to meet the increasingly stringent emission limits imposed by US and European legislation, passive NO_x adsorbers (PNAs) and hydrocarbon traps (HCTs) have attracted much attention in recent years. A required characteristic of these materials is to trap NO_x, CO and HC at low temperatures and release them at elevated temperatures for downstream conversion once the light-off temperature is reached. Metal-exchanged zeolites are of particular interest as PNA material, because they exhibit a variety of dynamically changing active sites that contribute to their overall performance.

Pd-exchanged SSZ-13 with CHA framework has shown great promise in trapping NO_x at temperatures below 150 °C and releasing them above 200 °C. With the help of density functional theory (DFT) simulations combined with experimental techniques including temperature programmed desorption (TPD) and temporal analysis of products (TAP) on 1% Pd-SSZ-13, we have investigated the nature of these active sites. Based on their transient responses to temperature and feed changes, we propose a viable mechanism of elementary reaction steps describing to the trapping and release of NO_x.

Our DFT results indicate that NO adsorption is the strongest on the partially reduced ZPd^I sites, with a strength of ca. -250 kJ/mol. While the presence of H₂O weakens the adsorption strength to -200 to -220 kJ/mol, NO adsorption remains relatively strong. The ZPd^I sites, however, are not as stable as the ZPd^IIOH or Z₂Pd^II sites, suggesting that ZPd^I is a minority site that may dynamically form in response to temperature or feed changes. We investigated the formation of ZPd^I sites and considered a two-step reduction mechanism converting two adjacent ZPd^IIOH sites to ZPd^I sites. In the first step the adjacent ZPd^IIOH sites dehydrate to form the bridged Z₂[Pd^II-O-Pd^II] species, which is subsequently reduced in the presence of NO to form two ZPd^I sites. With NO as the only reductant the formation of a ZPd^I site from ZPd^IIOH is endothermic by ca. 70 kJ/mol, but in the presence of stronger co-reductants, such as ethylene or CO, the energetics become more favorable: ethylene lowers the energy required to reduce ZPd^IIOH to ca. 10 kJ/mol and the even stronger reductant CO renders the formation of ZPd^I exothermic by ca. -35 kJ/mol.

Our TPD results for 1wt% Pd-SSZ-13 (Si/Al ~12-25) with NO_X uptake at 100 °C followed by desorption with a temperature ramp of 20 °C /min indicate good NO uptake with a NO:Pd ratio of ~1 in presence of water. Higher uptake is possible by adsorption on Brønsted acid sites, which become available when water is removed from the feed. The presence of other reductants, such as CO or ethylene, also leads to a significant increase in NO uptake. These observations are in good qualitative agreement with the conclusions drawn from our DFT calculations. TAP analysis using isotope labelling studies in the temperature range of 100-450 °C with ¹⁶O₂ pretreated catalyst,pulsed with ¹⁵N¹⁸O, indicates the formation of ¹⁵N¹⁶O¹⁸O.The incorporation of the labelled oxygen in NO₂ is consistent with the second step in the two-step mechanism proposed in DFT calculations.

Our detailed mechanistic insight into the elementary steps describing the dynamic changes of active sites during NO trapping and NO₂ production suggests that the relative stability of reduced, oxidized and hydrated/hydroxylated cationic sites is paramount. This knowledge provides the basis for future improvements of PNA materials and ultimately, cleaner automotive emissions.