First-Principles Calculations of Condition-Dependent Cu/Fe Speciation in Sulfur-Poisoned Cu- and Fe-SSZ-13 Catalysts

Abstract

A combination of first-principles thermodynamics and density functional theory (DFT) was applied for the prediction of sulfur-poisoned monomeric Cu/Fe species formed in the SSZ-13 catalyst framework under selective catalytic reduction (SCR)-relevant conditions in the presence of sulfur dioxide, ammonia, oxygen, and water. Differences in fresh and sulfurpoisoned species were found for Cu-and Fe-SSZ-13 catalysts containing one Al (1Al sites) or two Al (2Al sites) in 6-membered rings (6MRs) or 8membered rings (8MRs). The impact of ammonia concentration during lowand high-temperature sulfur-poisoning on Cu-and Fe-speciation was also investigated. SCR-relevant concentrations of ammonia in the gas mixture led to the formation of ammonium sulfates over copper in 2Al and 1Al sites of Cu-SSZ-13, while bisulfate and sulfuric acid species were predicted at these copper sites either in the absence of ammonia or at negligible concentrations of ammonia during low-and high-temperature poisoning. The absence of ammonia in the gas mixture led to the formation of iron-bisulfates at 2Al sites of Fe-SSZ-13 during lowtemperature poisoning, while the formation of ammonium sulfates was favorable under SCR-relevant conditions. In contrast to the facile formation of ammonium sulfates at copper sites of Cu-SSZ-13, only ammonium-free iron-sulfates formed at 1Al sites in Fe-SSZ-13 under realistic operational conditions. The regeneration of 2Al sites of Cu-SSZ-13 was predicted to occur at higher temperatures compared to 2Al sites in Fe-SSZ-13, whereas the opposite was predicted for 1Al sites. The analysis of fresh and regenerated Cu/Fe species was carried out as well. These theoretical results on model catalysts provide a first step in the understanding of sulfur-poisoning in Fe-SSZ-13 catalysts, supporting further experimental investigations to improve NH 3 -SCR catalysts for meeting future emission standards.

Enhanced hydrothermal stability and SO2-tolerance of Cu–Fe modified AEI zeolite catalysts in NH3-SCR of NOx

“…Previous studies have suggested that ammonium sulfate species tend to form on 2Z-Cu 2+ at 6MR, 8 while the Cu 2+ ions preferentially located at 8MR in our Cu0.42/Fe0.16-AEI would be expected to lead to fewer deposited ammonium sulfate species. Furthermore, a recent study Mesilov et al 46 proposed that the formation of ammonium sulfates was less favorable for Fe-SSZ-13 compared to Cu-SSZ-13 under the same conditions according to first-principles calculations, which is also in conformity with the TGA results from our SSZ-39 based catalysts (Fig. 12).…”

“…While copper sulfate species tended to form on Z-CuOH + sites at 8MR, copper sulfate formation on these sites may have been inhibited to some extent in Cu0.42/Fe0.16-AEI due to the presence of Fe ions. As pointed out by Mesilov et al , 46 removal of metal sulfates in small pore Fe/zeolites requires higher temperatures than in Cu-containing counterparts. This indicates that Fe ions might be preferentially sulfated according to the theory of ionic polarization.…”

Computational Study of Noble Metal CHA Zeolites: NO Adsorption and Sulfur Resistance

“…A similar approach has been widely used previously, 64,87,93−95 including our own study on Cu-and Fe-SSZ-13. 96 3. RESULTS AND DISCUSSION 3.1.…”

“…We refer to Section S5 for more details on the methodology and approximations used for calculations of Gibbs free energies. A similar approach has been widely used previously, ,,− including our own study on Cu- and Fe-SSZ-13 …”

Impact of Biodiesel-Based Phosphorus and Sulfur on Copper Speciation of Cu-SSZ-13 Catalysts: XAFS Scanning during H2-TPR

“…Abundant studies on sulfur poisoning of Cu-SSZ-13 have been performed. − The sulfur poisoning led to decreased deNO x activity at temperatures below 350 °C . This deactivation was mainly associated with the poisoning of active Cu sites .…”