Zeolite Selective Catalytic Reduction Catalysts Research Articles

Modelling of the NOx storage behaviour during cold start of modern zeolite SCR catalysts

Further stringent emission regulations of modern diesel engines call for a more precise prediction of NOx emissions, thus enabling a better control of the exhaust-gas aftertreatment systems. A major part of the NOx emissions is emitted before the light-off temperature of the selective catalytic reduction (SCR) catalyst is reached. Therefore a precise emissions prediction is necessary during the cold start phase of a diesel passenger car. Recent measurements show that NOx emissions can be stored in the SCR catalysts during cold start. Furthermore a part of this stored NOx can be reduced during the driving cycle.This paper describes an empiric model predicting the NOx storage behaviour during vehicle cold start. In a previous work the main influence parameters on the NOx storage behaviour were investigated on a synthetic gas test bench. The knowledge gained from the previous research work defines the necessary input parameters for the NOx storage model. These investigations showed that the NOx storage effect strongly depends on the ammonia (NH3-) level stored in the catalyst, exhaust-gas mass flow, the water adsorbed (H2O) on the catalyst, and the temperature of the catalyst. The model was implemented for on-filter and flow-through SCR catalysts. There are two similar models, one for the close-coupled SCR system and the other one for the underfloor SCR system. Each NOx storage model is split into an adsorption part and a desorption part. For both parts the pre-conditioning from the previous driving cycle is taken into account, which means that the catalyst state at the end of the last driving cycle initializes the model data for the current cycle, in consideration of the downtime between the two cycles. The desorption part calculates the NOx conversion amount and defines the desorption mass flow of NOx resulting from the NOx storage effect. The developed NOx storage model has been validated with roller dynamometer measurements and with real world driving cycles.

Low-Temperature SCR Catalyst Development and Industrial Applications in China

In recent years, low-temperature SCR (Selective Catalytic Reduction) denitrification technology has been popularized in non-power industries and has played an important role in the control of industrial flue gas NOx emissions in China. Currently, the most commonly used catalysts in industry are V2O5-WO3(MoO3)/TiO2, MnO2-based catalysts, CeO2-based catalysts, MnO2-CeO2 catalysts and zeolite SCR catalysts. The flue gas emitted during industrial combustion usually contains SO2, moisture and alkali metals, which can affect the service life of SCR catalysts. This paper summarizes the mechanism of catalyst poisoning and aims to reduce the negative effect of NH4HSO4 on the activity of the SCR catalyst at low temperatures in industrial applications. It also presents the outstanding achievements of domestic companies in denitrification in the non-power industry in recent years. Much progress has been made in the research and application of low-temperature NH3-SCR, and with the renewed demand for deeper NOx treatments, new technologies with lower energy consumption and more functions need to be developed.

NH3 Storage on a Zeolite SCR Catalyst Measured Using a Microwave-based Method: Reduced Sensitivity for More Strongly Held NH3

One method for reduction of nitrogen oxides from the exhaust of modern diesel vehicles is the selective catalytic reduction (SCR) of NOx by NH3 injected onto a zeolite SCR catalyst. Knowledge of the amount of NH3 stored on these catalysts is useful for optimal conversion. Here, measurements of the NH3 storage on a Cu-chabazite zeolite SCR catalyst were done using the microwave-frequency resonator cavity method. Shown are results for NH3 loading on a previously empty catalyst and for the subsequent desorption of the more weakly held NH3. Temperature was held constant at 250°C. We demonstrate that a comparison of the frequency changes of the resonant modes can help determine variations in the spatial profile of the NH3 storage along the SCR axis. Analysis of desorption data shows that the sensitivity of the resonant frequency change per mass of NH3 stored on the catalyst is lower for that more strongly held.

Effects of Ce-Doping on the Structure and NH3-SCR Activity of Fe/Beta Catalyst

Abstract Fe/Beta zeolite selective catalytic reduction (SCR) catalysts supported on honeycomb ceramic were prepared by a standard water ion exchange method and evaluated in NOx conversion process. The zeolite material characters and effects of Ce-doping on the zeolite were investigated by Inductively Coupled Plasma Atomic Emission Spectrum (ICP-AES), low temperature N2 adsorption, X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM), NH3 Temperature-Programmed Desorption (NH3-TPD) and H2 Temperature-Programmed Reduction (H2-TPR). The results show that the Fe-Beta zeolite SCR catalysts with better zeolite structure and Ce-doping exhibit a more excellent catalytic ability on the NOx conversion temperature window, no matter for the fresh one or the one treated under hydrothermal or sulfur aged condition. It is deduced that the larger BET surface area, more uniform and smaller particles and more regular framework of zeolite as well as the unsaturated Ce4+ on zeolite have a great effect on the promotion of NH3-SCR performance.

Composition dependent performance of alumina-based oxide supported WO3 catalysts for the NH3-SCR reaction and the NSR + SCR coupled process

Among the specific technologies to reduce NOx from automobile lean exhaust gases, the NOx storage reduction (NSR) and the selective catalytic reduction (SCR) by urea/NH3 are the two main effective proposed processes. During the short rich excursions of the NSR process, emissions of undesired NH3 can occur and the combination of the NSR and a NH3-SCR catalyst has been proposed to improve the global treatment efficiency. With the aim to develop non zeolite SCR catalysts, materials based on modified alumina were prepared by sol–gel method. Necessary acidic and redox properties were expected by incorporation of metal ions such as cerium (for oxygen mobility), zirconium, titanium, and silica (acidic behavior). The influence of each element incorporated in the host alumina and their combination were evaluated in NH3-SCR before and after 9wt% WO3 impregnation. Among the 15 synthetized supports, three WO3 supported catalysts exhibited promising SCR activities were selected (WO3/Al0.2Ce0.4Ti0.4, WO3/Al0.2Ce0.16Zr0.32Ti0.32 and WO3/Al0.1Si0.1Ce0.16Zr0.32Ti0.32) and characterized (N2 adsorption, XRD analysis, NH3-SCO reactivity, NH3 storage capacity, pyridine adsorption monitored by IR spectroscopy, H2-TPR, OSC) and then associated downstream to a model Pt/Ba/Al2O3 NSR catalyst. The use of ammonia emitted from the NSR catalyst during the rich pulses was studied taking into accounts three possibilities: the NH3-SCR reaction (NH3+NOx→N2), the NH3-SCO reaction (NH3+O2→N2) and the unconverted NH3. Only the NH3-SCR reaction occurs at 200°C; whereas the three pathways take place at 300 and 400°C. In addition, a lack of strong acidic storage sites results in unconverted ammonia at high temperature.

Effect of Hydrocarbon Emissions From PCCI-Type Combustion on the Performance of Selective Catalytic Reduction Catalysts

Core samples cut from full size commercial Fe-and Cu- zeolite selective catalytic reduction catalysts were exposed to a slipstream of raw engine exhaust from a 1.9-liter 4-cylinder diesel engine operating in conventional and premixed charge compression ignition (PCCI) combustion modes. Subsequently, the NOx reduction performance of the exposed catalysts was evaluated on a laboratory bench-reactor fed with simulated exhaust. The Fe-zeolite NOx conversion efficiency was significantly degraded, especially at low temperatures (<250 °C), after the catalyst was exposed to the engine exhaust. The degradation of the Fe-zeolite performance was similar for both combustion modes. The Cu-zeolite was much more resistant to hydrocarbon (HC) fouling than the Fe-zeolite catalyst. In the case of the Cu-zeolite, PCCI exhaust had a more significant impact than the exhaust from conventional combustion on the NOx conversion efficiency. For all cases, the clean catalyst performance was recovered after heating to 600 °C. Gas chromatography mass spectrometry analysis of the HCs adsorbed to the catalyst surface provided insights into the observed NOx reduction performance trends.

Experimental Demonstration of a New Model-Based SCR Control Strategy for Cleaner Heavy-Duty Diesel Engines

Selective catalytic reduction (SCR) is a promising diesel after treatment technology that enables low nitrogen oxides (NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ) tailpipe emissions with relatively low fuel consumption. Future emission legislation is pushing the boundaries for SCR control systems to achieve high NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> conversion within a tailpipe ammonia (NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) slip constraint, and to provide robustness to meet in-use compliance requirements. This work presents a new adaptive control strategy that uses an ammonia feedback sensor and an online ammonia storage model. Experimental validation on a 12-liter heavy-duty diesel engine with a 34-liter Zeolite SCR catalyst shows good performance and robustness against urea under- and over-dosage for both the European steady-state and transient test cycles. The new strategy is compared with a NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> sensor-based control strategy with cross-sensitivity compensation. It proved to be superior in terms of transient adaptation and taking an NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> slip constraint into account.

Investigation of PCDD/F Emissions from Mobile Source Diesel Engines: Impact of Copper Zeolite SCR Catalysts and Exhaust Aftertreatment Configurations

This study investigated the impact of copper zeolite selective catalytic reduction (SCR) catalysts and exhaust aftertreatment configurations on the emissions of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) from mobile source diesel engines. Emissions of PCDD/Fs, reported as the weighted sum of 17 congeners called the toxic equivalency quotient (TEQ), were measured using a modified EPA Method 0023A in the absence and presence of exhaust aftertreatment. Engine-out emissions were measured as a reference, while aftertreatment configurations included various combinations of diesel oxidation catalyst (DOC), diesel particulate filter (DPF), Cu-zeolite SCR, Fe-zeolite SCR, ammonia oxidation catalyst (AMOX), and aqueous urea dosing. In addition, different chlorine concentrations were evaluated. Results showed that all aftertreatment configurations reduced PCDD/F emissions in comparison to the engine-out reference, consistent with reduction mechanisms such as thermal decomposition or combined trapping and hydrogenolysis reported in the literature. Similarly low PCDD/F emissions from the DOC-DPF and the DOC-DPF-SCR configurations indicated that PCDD/F reduction primarily occurred in the DOC-DPF with no noticeable contribution from either the Cu- or Fe-zeolite SCR systems. Furthermore, experiments performed with high chlorine concentration provided no evidence that chlorine content has an impact on the catalytic synthesis of PCDD/Fs for the chlorine levels investigated in this study.

Deactivation of accelerated engine-aged and field-aged Fe–zeolite SCR catalysts

A single-cylinder diesel engine with an emissions control system – diesel oxidation catalyst (DOC), Fe–zeolite selective catalytic reduction (SCR) catalyst, and diesel particulate filter (DPF) – was used to perform accelerated thermal aging of the SCR catalyst. Cyclic aging is performed at SCR inlet temperatures of 650, 750 and 850 °C for up to 50 aging cycles. To assess the validity of the implemented accelerated thermal aging protocol, a field-aged SCR catalyst of similar formulation was also evaluated. The monoliths were cut into sections and evaluated for NO x performance in a bench-flow reactor. While the rear section of both the field-aged and the accelerated engine-aged SCR catalysts maintained high NO x conversion, 75–80% at 400 °C, the front section exhibited a drastic decrease to only 20–35% at 400 °C. This two-tiered deactivation was also observed for field-aged samples that were analyzed in this study. To understand the observed performance changes, thorough materials characterization was performed which revealed two primary degradation mechanisms. The first mechanism is a general Fe–zeolite deterioration which led to surface area losses, dealumination of the zeolite, and Fe 2O 3 crystal growth. This degradation accelerated above 750 °C, and the effects were generally more severe in the front of the catalyst. The second deactivation mechanism is linked to trace levels of Pt that are suspected to be volatizing from the DOC and depositing on the front section of the SCR catalyst. Chemical evidence of this can be seen in the high levels of NH 3 oxidation (80% conversion at 400 °C), which coincides with the decrease in performance.

Mechanistic study of the low temperature activity of transition metal exchanged zeolite SCR catalysts

Reactor activity measurements and FTIR spectroscopy were applied to study the transient and steady state behavior of Cu and Fe exchanged zeolite catalysts for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) with NH 3 at 200 °C. Different surface NOx adsorption complexes were observed on the two catalysts. IR bands assigned to surface nitrate/nitrite groups were apparent on the Cu catalyst under most testing conditions. On the Fe catalyst, IR bands assigned to surface nitro groups were also observed, in addition to nitrate/nitrite groups. Both the nitrate/nitrite and the nitro group showed high reactivity with NH 3, which explained similar initial transient SCR activity of the two catalysts. The presence of NH 3 was found to block the Fe sites from catalyzing the oxidation of NO and severely inhibited the overall SCR activity when there was no NO 2 in the gas mixture. Addition of NO 2 improved the low temperature SCR activity of both catalysts. The higher SCR activity on the Fe catalyst at the optimum NO:NO 2 = 1:1 was attributed to the high intrinsic activity of the nitro groups formed on the Fe catalyst.

A study of chemical aging effects on HDD Fe–zeolite SCR catalyst

The effect of phosphorus (P) on iron (Fe)–zeolite based selective catalytic reduction (SCR) monolith catalysts was studied in the absence and the presence of sulfur, and at two different exposure temperatures (220 and 450 °C). It was found that exposure of Fe–zeolite SCR catalyst to P resulted in a loss of NOx conversion and an increase of NH 3 slip relative to a degreened baseline sample. Some of the increased slip was attributed to a loss of NH 3 storage capacity due to P blockage of adsorption sites. Additional increased slip resulted from a rise in the slip to storage ratio, likely due to a shorter effective channel length for NH 3 adsorption since most of the P blocked sites at the sample inlet. DeNOx activity was then reduced due to the decreased amount of adsorbed NH 3. P exposure using zero sulfur fuel resulted in the same amount of deactivation as aging done using low sulfur (350 ppm S) fuel, suggesting that this amount of S did not affect this formulation. An initial loss of activity proved to be due to hydrocarbon exposure, and was reversible simply by ramping the sample up in temperature during the evaluation test. Increasing the exposure temperatures also increased the loss of DeNOx performance. It is proposed that P impacts these catalysts by physically blocking adsorption sites for NH 3, primarily at the monolith channel inlet.

Sensor and method for determining the ammonia loading of a zeolite SCR catalyst

A sensor and a method for determining the NH3 loading of a base-metal zeolite catalyst are described. This catalyst material is used to reduce NOx emissions in diesel exhaust via reaction with injected NH3, a process known as selective catalytic reduction (SCR). Some of the injected NH3 is stored on the catalyst surface before it reduces NOx. We demonstrate that the catalyst material itself can be used as part of a sensor measuring the level of stored NH3. The AC conductivity of a thick film of the catalyst material was measured at 4 Hz and was found to increase when NH3 was introduced into the gas phase, eventually saturating presumably as the material reached its maximum NH3 storage capacity. The conductivity was monitored as the temperature was raised sufficiently to desorb (or oxidize) most of the stored NH3. The resulting change in the conductivity upon heating was used as a measure of the NH3 loading level on the zeolite catalyst material. We demonstrate this parameter is dependent on NH3 concentration, NH3 loading time and gas temperature.

Testing zeolite SCR catalysts under protocol conditions for NO x abatement from stationary emission sources

In developing higher efficiency catalysts for NO x abatement from stationary emission sources utilizing selective catalytic reduction (SCR), comparison test conditions, including gas composition and gas flow rates, are needed that approximate industrial operating conditions. Standard tests will be fuel-dependent, i.e. typical flue gas from a coal-fired electrical utility plant might consist of 150–1000 ppm NO, 5% O 2, 13% CO 2, 8% H 2O, and 200–2000 ppm SO 2, while that from a plant utilizing natural gas would contain appreciably more O 2, less NO, and very low SO 2. For low temperature catalysts developed for placement after the electrostatic precipitator (ESP) and desulfurization units, flue gas would essentially be dust- and sulfur-free. Protocols are given for comparison testing of NO x abatement catalysts using ammonia as the reductant. These test conditions are used in the comparison of Cu-, Fe-, and V- containing ZSM-5 and mordenite catalysts. It was found that initial NO conversion levels were lower than those achieved after a thermal pretreatment of the catalysts. Under the reaction conditions employed, moisture was an inhibitor for the Fe- and V-containing catalysts but was a promoter for the Cu-mordenite catalyst. The V-ZSM-5 catalyst exhibited a stable activity for NO reduction in the presence of H 2O and SO 2 but rapid deactivation was observed in the presence of 10 ppm SO 3.

Sorbent and ammonia injection at economizer temperatures upstream of a economizer temperatures upstream of a high—temperature baghouse

AbstractThe current technology of choice—world–wide—for post–combustion NOx control is selective catalytic reduction (SCR) with ammonia. The application of SCR to coal–fired units has proven to be somewhat more difficult than its application to natural gas and oil–fired units due to SO2 poisoning of the catalyst, catalytic oxidation of SO2 to SO3, and erosion and fouling of the catalyst by fly ash [1]. These problems could be potentially diminished if SO2 and particulate removal systems were placed upstream of the SCR reactor. However, this is not easily accomplished since SCR systems operate best in the temperature range of 600°F–800°F (315°C–425°C). Several SCR installations in Europe employ conventional flue gas desulfurization (FGD) and particulate control upstream of flue gas reheat and SCR. However, the flue gas reheat typically imposes a 1%–2% energy penalty [2]. Several recent technical developments now offer a technology whereby these limitations of SCR for coal applications can be alleviated.These technical developments are in the areas of high temperature filtration, improved SCR catalysts, and advances in sorbent injection. These advances have been incorporated into a patented process known as the SOxNOxRox Box™ (SNRB) process. Briefly, this process consists of a “hot” baghouse employing woven ceramic fabric bags, a zeolite SCR catalyst incorporated into the baghouse, and the injection of either calcium–or sodium–based sorbent upstream of the baghouse (see Figure 1).This paper deals with the status and development of the SNRB process, including some results from recently completed pilot–plant tests performed under a contract with the Ohio Coal Development Office (OCDO).