Effects of Si/Al ratio on Cu/SSZ-13 NH3-SCR catalysts: Implications for the active Cu species and the roles of Brønsted acidity

In this review we focus on the catalytic removal of a series of N-containing exhaust gases with various valences, including nitriles (HCN, CH3CN, and C2H3CN), ammonia (NH3), nitrous oxide (N2O), and nitric oxides (NOx), which can cause some serious environmental problems, such as acid rain, haze weather, global warming, and even death. The zeolite catalysts with high internal surface areas, uniform pore systems, considerable ion-exchange capabilities, and satisfactory thermal stabilities are herein addressed for the corresponding depollution processes. The sources and toxicities of these pollutants are introduced. The important physicochemical properties of zeolite catalysts, including shape selectivity, surface area, acidity, and redox ability, are described in detail. The catalytic combustion of nitriles and ammonia, the direct catalytic decomposition of N2O, and the selective catalytic reduction and direct catalytic decomposition of NO are systematically discussed, involving the catalytic behaviors as ...

Selective Transformation of Various Nitrogen-Containing Exhaust Gases toward N2 over Zeolite Catalysts.

The development of Cu-chabazite (CHA) catalysts, i.e. Cu-SSZ-13 and Cu-SAPO-34, represents a significant technology breakthrough for the removal of NOx by selective catalytic reduction (SCR) with ammonia. Cu-CHA catalysts show an excellent hydrothermal stability towards high temperature aging and wide active temperature windows for the ammonia SCR reaction. This work summarizes the recent progress in the development of the Cu-CHA catalysts for the NH3-SCR reaction. The state of Cu in the reaction and the preparation methods on the catalytic performance are discussed. The advances in the understanding of the reaction mechanism are reviewed. The hydrothermal stability of the typical Cu-CHA catalysts are compared.

Recent advances in the selective catalytic reduction of NOx with NH3 on Cu-Chabazite catalysts

Nanometer-thick passive films on metals usually impart remarkable resistance to general corrosion but are susceptible to localized attack in certain aggressive media, leading to material failure with pronounced adverse economic and safety consequences. Over the past decades, several classic theories have been proposed and accepted, based on hypotheses and theoretical models, and oftentimes, not sufficiently nor directly corroborated by experimental evidence. Here we show experimental results on the structure of the passive film formed on a FeCr15Ni15 single crystal in chloride-free and chloride-containing media. We use aberration-corrected transmission electron microscopy to directly capture the chloride ion accumulation at the metal/film interface, lattice expansion on the metal side, undulations at the interface, and structural inhomogeneity on the film side, most of which had previously been rejected by existing models. This work unmasks, at the atomic scale, the mechanism of chloride-induced passivity breakdown that is known to occur in various metallic materials.

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Unmasking chloride attack on the passive film of metals

New insights into Cu/SSZ-13 SCR catalyst acidity. Part I: Nature of acidic sites probed by NH3 titration

In this study, we have developed a kinetic model for ammonia-SCR over a well-characterized Cu/SSZ-13 catalyst. It was found that a three-site model was needed in order to describe the ammonia temperature programmed desorption (TPD) with adsorption at 50 and 150 degrees C as well as ammonia oxidation, and NH3-SCR up to 600 degrees C. Based on literature studies, where detailed characterization of Cu/SSZ-13 have been conducted using several experimental techniques', we suggest the following physical interpretation of the S1 and S2 sites in the model. The S1 sites are associated with copper located in a six-membered ring, possibly slightly distorted due to interactions with-water and ammonia while, the S2 sites represent copper in the large cages or CuxOy species. In addition, ammonia is also stored on Bronsted acid sites, but in order not to complicate the model further, it was lumped together in the S1 and S2 sites. Finally, S3 sites have been added in order to describe the large amount of physisorbed ammonia at low temperature. This three-site model was capable of adequately describing the ammonia TPD experiments with the initial temperature of 50 and 150 degrees C. The heats of adsorption of ammonia on the Si and S2 sites were determined using micro-calorimeter experiments. Further, the main SCR reaction in the model occurs on S1 sites and the main ammonia oxidation reaction on S2 sites. However, due to the complex behavior associated with ammonia oxidation, where the conversion slightly decreased when the temperature was increased from 350 to 400 degrees C, an ammonia oxidation reaction occurring at low temperature with low rate needed to be introduced on S1. In a similar way, an added step was needed for ammonia-SCR on S2, which occurred at high temperature where the ammonia coverage on Si was low resulting in low conversion. To summarize, the-three-site model developed was capable of well describing the ammonia storage and release, ammonia oxidation as well as SCR and N2O formation across a broad temperature interval (100-600 degrees C).

A multi-site kinetic model for NH3-SCR over Cu/SSZ-13

Comparison of Cu/BEA, Cu/SSZ-13 and Cu/SAPO-34 for ammonia-SCR reactions

The sensitivity of NH3-SCR activity to water vapour at 70 degrees C is tested for Cu/SAPO-34 (morph.) catalysts. The synthesised Cu/SAPO-34 (morph.) is remarkably susceptible to low-temperature activity loss. After only 3 hat 70 degrees C, the NOx conversion decreases from an initial 87% to 66% at 200 degrees C, after another 9 h to 6%. This deactivation is accompanied by a partial loss of microporous volume, but the SAPO-34 framework remains crystalline. We postulate that a transformation of the active copper sites into an inactive form could be the cause for the deactivation. Regeneration of the catalyst was attempted, but had no impact on activity.

Deactivation of Cu/SAPO-34 during low-temperature NH3-SCR

Deactivation of Cu-SSZ-13 by SO2 exposure under SCR conditions

In this study, we have developed a multi-site kinetic model that describes the sulfur poisoning and gradual sulfur removal over Cu/SSZ-13 used for NH3-SCR. Sulfur poisoning was conducted under SCR conditions and thereafter, repeated SCR experiments were conducted to examine the effect of such poisoning and the subsequent gradual removal of sulfur. In addition, the effect of sulfur poisoning was examined on NH3 TPD and ammonia oxidation experiments. The following sites were used in the kinetic model: copper in the six-membered rings as described by S1Cu, copper in the larger cages with S2 and S3 as a site where physisorbed ammonia can attach. Further, ammonia was also adsorbed on the Bronsted sites, represented by S1Bron in the model, but in order not to further complicate the model, small amounts of ammonia storage on Bronsted sites were also lumped into S2. In the model, SO2 was adsorbed on the sites containing copper, which are S1Cu and S2. It should be noted that S1Cu and S2 represents hydrated copper sites. Interestingly, we observed experimentally that ammonia storage was larger after sulfur poisoning compared to before, which is why we added ammonia storage and desorption to the S1Cu-SO2 and S2-SO2 sites. However, ammonia was already adsorbing on the copper site; thus, these steps did not result in increased storage. Consequently, reaction steps were added where additional ammonia was adsorbed to form S1Cu-SO2-(NH3)2 and S2-SO2-(NH3)2 species, which could be interpreted as precursors to ammonium sulfates. Another aspect that must be addressed in the model is the observation in the literature that SO2 is more easily desorbed in SO2+NH3+O2 TPD than SO2+O2 TPD. Reversible reaction steps were therefore added whereby the S1Cu-SO2-NH3 and S2-SO2-NH3 species were decomposed to form SO2. A final reaction step was incorporated into the model to describe the SCR reaction with ammonia attached to the sulfur sites. The developed model could well describe the sulfur poisoning and gradual regeneration during repeated SCR experiments. In addition, the model well described the NH3 TPD and NH3 oxidation before and after sulfur poisoning.

Kirsten Leistner

Papers

A multi-site kinetic model for NH3-SCR over Cu/SSZ-13

Comparison of Cu/BEA, Cu/SSZ-13 and Cu/SAPO-34 for ammonia-SCR reactions

Deactivation of Cu/SAPO-34 during low-temperature NH3-SCR

Deactivation of Cu-SSZ-13 by SO2 exposure under SCR conditions

A kinetic model for sulfur poisoning and regeneration of Cu/SSZ-13 used for NH3-SCR