Cu-exchanged zeolites have demonstrated widespread use as catalyst materials in the abatement of NOx, especially from mobile sources. Recent studies focusing on Cu-exchanged zeolites with the CHA structure have demonstrated them to be excellent catalysts in the ammonia-assisted selective catalytic reduction (NH3-SCR) of NOx. Thorough characterization of these materials using state-of-the-art techniques has led to a significant improvement in the understanding of active sites present, which contributes toward a fundamental understanding of the catalytic processes and the rational design of new materials; however, the availability of multiple techniques at our disposal has led to various observations and conclusions on the nature of the active sites. This article begins with a brief introduction to exhaust emission control in the mobile sector, followed by an overview of hydrocarbon-SCR and NH3-SCR; the former technology having found common use in light duty passenger vehicles, whereas the latter are applie...

Local Environment and Nature of Cu Active Sites in Zeolite-Based Catalysts for the Selective Catalytic Reduction of NOx

The effect of Si content in SAPO-34 on NO selective catalytic reduction over Cu/SAPO-34 catalyst was investigated. Three fresh catalysts with different Si contents, prepared by the same procedure, showed different NO conversions from 120 °C to 600 °C. These catalysts were characterized in detail by various techniques (ICP, XRD, SEM, NMR, NH 3 -TPD, H 2 -TPR and EPR). The NH 3 -TPD results indicate that the number of acid sites in Cu/SAPO-34 catalyst increases with increasing Si contents and the SCR activities of the samples correlate well with the number of acidic sites at low temperatures (200 °C). The EPR and H 2 -TPR results show that both the Si and the Al contents affect the number of isolated Cu 2+ ions in Cu/SAPO-34 catalysts. The catalysts aged at 750 °C for 12 h showed higher SCR activities than those of the untreated samples, consistent with the higher numbers of the isolated Cu 2+ ions.

The influence of silicon on the catalytic properties of Cu/SAPO-34 for NOx reduction by ammonia-SCR

A Mn–Fe/ZSM5 catalyst has been developed for removing NOx from diesel engine exhausts and its excellent low-temperature SCR activity and N 2 selectivity demonstrated in comparison with other representative SCR catalysts including CuZSM5 and a Cu-based commercial catalyst (COM). The well-dispersed MnO 2 and the high NH 3 adsorption capacity of the Mn–Fe/ZSM5 catalyst have been identified as the primary sources for its high deNOx activity for NH 3 /SCR. Hydrothermal stability and durability of the Mn–Fe/ZSM5 catalyst have been examined and compared to those of the CuZSM5 and COM catalysts. The hydrothermal stability of the catalyst improved upon the increase of Mn content and/or the addition of Er, the latter of which helps to stabilize the dispersion of MnOx on the catalyst surface during hydrothermal aging. The deNOx activity of the Mn–Fe/ZSM5 and its Er-promoted counterpart was less affected by HC poisoning, C 3 H 6 poisoning in particular, compared to the CuZSM5 and COM catalysts, mainly due to the excellent C 3 H 6 oxidation activity of MnO 2 . No poisoning of the Mn-based ZSM5 and CuZSM5 catalysts has been observed upon the addition of 2 wt.% of K + and Ca 2+ to their surface, primarily due to the high NH 3 adsorption capacity of the ZSM5 support, whereas the COM catalyst has been severely deactivated by the deposition of K + and Ca 2+ . The deNOx activity of the Mn-based ZSM5 catalyst, particularly the Er-promoted one, was less affected by SO 2 compared to the CuZSM5 and COM catalysts, although it was hardly regenerated at 500 °C. Formation of MnSO 4 on the catalytic surface appears to be the primary cause for the deactivation of the Mn-based ZSM5 catalysts in the presence of SO 2 in the feed gas stream.

Mn-Fe/ZSM5 as a low-temperature SCR catalyst to remove NOx from diesel engine exhaust

Iron and copper-based zeolites are effective catalysts for the lean selective catalytic reduction (SCR) of NOx with NH3. Cu-zeolites are more active at lower temperatures (≤350 °C) while Fe-zeolites are more active at higher temperatures (≥400 °C). The effectiveness of a catalytic system comprising Fe- and Cu-based zeolites was examined for the standard (NO + O2 + NH3) and fast (NO + NO2 + NH3) SCR reactions. Experiments carried out with in-house and commercial Fe- and Cu-zeolite monoliths of varying lengths quantified their relative SCR activities. The commercial Cu-zeolite achieved complete NOx conversion for the standard SCR at 250 °C while the commercial Fe-zeolite achieved high NOx conversion at higher temperatures (≥400 °C) where it out-performed the Cu-zeolite. Subsequently, three configurations of combined Fe and Cu-zeolite catalysts were compared: • “Sequential brick” catalyst comprising Fe-zeolite and Cu-zeolite monolith. • “Mixed washcoat” catalyst comprising a washcoat layer having equal mass fractions of Fe- and Cu-zeolites. • “Dual layer” catalyst comprising monolith coated with individual layers of Fe- and Cu-zeolites of different thicknesses and mass fractions. The sequential brick design with Fe-zeolite brick followed by a Cu-zeolite brick gave a higher conversion than the Cu/Fe sequence of equal loadings with the Fe(33%)/Cu(67%) achieving the highest NOx conversion over a wide range of temperatures. The mixed washcoat catalyst achieved NOx conversion that was nearly an average of the individual Fe-only and Cu-only catalysts. The dual layer catalyst with a thin Fe-zeolite (33% of the total washcoat loading) layer on top of a thicker Cu-zeolite layer (67%) resulted in very high NOx removal efficiencies over a wide temperature range for both the standard and fast SCR reactions. The performance of this dual-layer system was comparable to the series arrangement of Fe and Cu-bricks. The Cu-zeolite on Fe-zeolite dual layer catalyst was not nearly as effective for the same loadings. The Fe/Cu dual layer catalyst also exhibited superior performance for the fast SCR reaction. The results are interpreted in terms of the activities of each catalyst for SCR and ammonia oxidation. An assessment of the extent of washcoat diffusion limitations shows that the dual layer configuration is superior to the sequential brick configuration. The existence of an optimal loading distribution of the Fe- and Cu-zeolite catalysts as well as other intangible benefits of the dual layer SCR catalyst are discussed.

Selective catalytic reduction of NOx on combined Fe- and Cu-zeolite monolithic catalysts: Sequential and dual layer configurations

Ceria is a common component of engine aftertreatment catalysts due to its oxygen storage ability, its redox properties, and its role in stabilizing Pt against sintering. The interactions between ceria and NH3 or NOx were investigated to better understand the role of ceria in oxidation reactions occurring over a diesel oxidation catalyst, in the reduction of NOx on lean NOx traps, and in the selective catalytic oxidation (SCO) of NH3. Ceria proved active in NO oxidation, selective catalytic reduction of NO by NH3 (NH3–SCR), and NH3–SCO reactions. Between 100 and 450 °C, both NH3 and NOx adsorbed on ceria simultaneously. In the absence of NOx, NH3 was oxidized over CeO2 forming N2 via a two-step selective catalytic reduction mechanism at low temperature and NOx at high temperature. In the presence of NOx, NH3 reacted with adsorbed NOx species, again forming N2 at lower temperatures (250–450 °C), while at higher temperature, a significant portion of the NH3 was oxidized, with product NO formed.

Characterization of Ceria’s Interaction with NOx and NH3

There have been several recent reports regarding the spatial resolution of gas species in monolith-supported automobile exhaust catalysts, with examples including characterization of DOCs (diesel oxidation catalysts) and LNTs (lean NOx traps). However, spatially resolving gas concentrations in NH3-SCR (selective catalytic reduction) applications is limited due to the difficulty in ppm-level NH3 detection in the presence of percent levels of water and N2 using mass spectrometry. In this study, a gas-phase FT-IR analyzer was used for the first time to spatially resolve gas concentrations in a monolith-supported SCR catalyst. The reactions analyzed include standard SCR, fast SCR and SCR with NO2. The results show that the three SCR reactions proceed at significantly different rates, especially at temperatures below 300 °C, and can be correlated to the amount of catalyst used. For example, the catalyst lengths needed to achieve 80% NOx conversion at 300 °C are 2.4, 1.2 and 0.5 cm for conditions that meet the standard SCR, NO2-SCR and fast SCR reaction stoichiometries, respectively. For the standard SCR reaction, kinetic analysis, and spatially resolved NO oxidation and SCR results consistently indicate that the rate-determining step is NO oxidation. NH3 has an inhibition effect, as it suppresses NO oxidation by competitive adsorption on the active sites. At 300 °C, the outlet NOx conversion is not limited by the reaction kinetics, but by insufficient NH3 supply, since part of the NH3 is oxidized by O2. Compared with 300 °C, higher NOx conversions are attained at 400 or 500 °C, which is due to significantly enhanced NO oxidation, and the resulting increase in NH3 reacting with NOx via SCR rather than O2 via NH3 oxidation. For NO2-SCR, a considerable amount of N2O was formed at 250 °C but decreased with increasing temperature. The decreased N2O is due to improved selectivity in the NO2-SCR to N2, as well as N2O decomposition at the back part of the catalyst at high temperature. Finally, different SCR reaction patterns were identified when testing with NO:NO2 = 3:1 and 1:3. For NO:NO2 = 3:1, the SCR reactions proceed in series, namely through the fast reaction first, followed by standard SCR. The fast SCR and NO2-SCR reactions proceed in parallel for NO:NO2 = 1:3. The results indicate that if NO2 is the limiting reactant, fast SCR dominates, but if excess NO2 is available, the NO2 SCR reaction can proceed in parallel.

Prasanna Wijayakoon

Papers

Spatially resolving SCR reactions over a Fe/zeolite catalyst