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Cu- and Fe-zeolite SCR catalysts emerged in recent years as the primary candidates for meeting the increasingly stringent lean exhaust emission regulations, due to their outstanding activity and durability characteristics. It is commonly known that Cu-zeolite catalysts possess superior activity to Fe-zeolites, in particular at low temperatures and sub-optimal NO₂/NOx ratios. In this work, we elucidate some underlying mechanistic differences between these two classes of catalysts, first based on their NO oxidation abilities, and then based on the relative properties of the two types of exchanged metal sites. Finally, by using the ammonia coverage-dependent NOx performance, we illustrate that state-of-the-art Fe-zeolites can perform better under certain transient conditions than in steady-state.

Nitrous oxide (N2O), with a global warming potential (GWP) of 297 and an average atmospheric residence time of over 100 years, is an important greenhouse gas (GHG). In recognition of this, N2O emissions from on-highway medium- and heavy-duty diesel engines were recently regulated by the US Environmental Protection Agency (EPA) and National Highway Traffic Safety Administration’s (NHTSA) GHG Emission Standards. Unlike NO and NO2, collectively referred to as NOx, N2O is not a major byproduct of diesel combustion. However, N2O can be formed as a result of unselective catalytic reactions in diesel aftertreatment systems, and the mitigation of this unintended N2O formation is a topic of active research. In this study, a nonroad Tier 4 Final/Stage IV engine was equipped with a vanadium-based selective catalytic reduction (SCR) aftertreatment system. Experiments were conducted over nonroad steady and both cold and hot transient cycles (NRSC and NRTC, respectively).

Ceria has become an increasingly important component in automotive exhaust catalysts over the past decade. Recently, with the proposal that measurements of oxygen storage be used for the on-board evaluation of catalyst performance for both low emission vehicles (LEV) and non-LEV vehicles, understanding the role of ceria and its deterioration with catalyst aging has become even more important. It is well established that ceria in an alumina support promotes oxygen storage/release by automotive catalysts under cycled air/fuel conditions, which in turn promotes the catalyst's conversion performance under those conditions. Another benefit of ceria is its enhancement of the catalytic activity for other reactions, such as the water-gas shift reaction under rich conditions. In addition, ceria may help catalyst durability by promoting precious metal dispersion and playing some role as a stabilizer of the support.

Diesel oxidation catalyst (DOC) is one of the critical catalyst components in modern diesel aftertreatment systems. It mainly converts unburned hydrocarbon (HC) and CO to CO2 and H2O before they are released to the environment. In addition, it also oxidizes a portion of NO to NO2, which improves the NOx conversion efficiency via fast SCR over the downstream selective catalytic reduction (SCR) catalyst. HC light-off tests, with or without the presence of NOx, has been typically used for DOC evaluation in laboratory. In this work, we aim to understand the influences of DOC light-off experimental conditions, such as initial temperature, initial holding time, HC species, with or without the presence of NOx, on the DOC HC light-off behavior. The results indicate that light-off test with lower initial temperature and longer initial holding time (at its initial temperature) leads to higher DOC light-off temperature.

Cu/CHA catalysts have been widely used in the industry, due to their desirable performance characteristics including the unmatched hydrothermal stability. While broadly recognized for their outstanding activity at or above 200°C, these catalysts may not show desired levels of NOx conversion at lower temperatures. To achieve high NOx conversions it is desirable to have NO2/NOx close to 0.5 for fast SCR. However even under such optimal gas feed conditions, sustained use of Cu/CHA below 200°C leads to ammonium nitrate formation and accumulation, resulting in the inhibition of NOx conversion. In this contribution, the formation and decomposition of NH4NO3 on a commercial Cu/CHA catalyst have been investigated systematically. First, the impact of NH4NO3 self-inhibition on SCR activity as a function of temperature and NO2/NOx ratios was investigated through reactor testing.

A study was performed to assess the effects of the catalyst volume and the ceria content in the washcoat on the aged emission performance of underfloor catalytic converters containing platinum and rhodium. Catalyst volumes of 1.4 L and 2.8 L were evaluated, while the ceria level was varied from 0 to 60% of the weight of the washcoat. The concentration of noble metals (g/L) was the same for both catalyst volumes, so the larger volume also contained more noble metal. Catalyst performance was evaluated on an air/fuel ratio sweep test, at steady-state conditions on an engine, and on the FTP test. In light of the new catalyst monitoring requirements for OBD II, each catalyst was also evaluated at steady-state conditions using a dual oxygen sensor technique in order to produce an O2 sensor index. The evaluations were performed at several intermediate stages as the catalysts were aged on engines using high temperature durability schedules intended to simulate high mileage conditions.

Sustained NOx reduction at low temperatures, especially in the 150-200 °C range, shares some similarities with the more commonly discussed cold-start challenge, however, poses a number of additional and distinct technical problems. In this project, we set a bold target of achieving and maintaining 90% NOx conversion at the SCR catalyst inlet temperature of 150 °C. This project is intended to push the boundaries of the existing technologies, while staying within the realm of realistic future practical implementation. In order to meet the resulting challenges at the levels of catalyst fundamentals, system components, and system integration, Cummins has partnered with the DOE, Johnson Matthey, and Pacific Northwest National Lab and initiated the Sustained Low-Temperature NOx Reduction program at the beginning of 2015 and completed in 2017.

Cu-SSZ-13 catalysts are widely used for diesel aftertreatment applications for NOx (NO and NO2) abatement via selective catalytic reaction (SCR) due to their high conversion efficiency and excellent hydrothermal stability. Diesel engine exhaust contains small amounts of SOx due to the combustion of sulfur compounds in diesel fuel. The engine out SOx level mainly depends on the sulfur content in the diesel fuel. The presence of SOx from engine exhaust can deteriorate the SCR performance of Cu-SSZ-13 catalysts in real-world applications. This work is focused on the sulfur-induced deactivation process of a Cu-SSZ-13 catalyst under a range of simulated diesel engine operating conditions. Two catalyst deactivation modes, namely chemical poisoning and physical poisoning, are identified, primarily depending on the operating temperature. Chemical poisoning mainly results from the interaction between SOx and Cu species within the zeolite framework.

The low-temperature performance characteristics of a commercial lean NOX trap catalyst were evaluated using infra-red thermography (IRT) before and after a high-temperature aging step. Reaction tests included propylene oxidation, oxygen storage capacity measurements, and simulated cycling conditions for NOX reduction, using H₂ as the reductant during the regeneration step of the cycle. Testing with and without NO in the lean phase showed thermal differences between the reductant used in reducing the stored oxygen and that for nitrate decomposition and reduction. IRT clearly demonstrated where NOX trapping and regeneration were occurring spatially as a function of regeneration conditions, with variables including hydrogen content of the regeneration phase and lean- and rich-phase cycle times.

The high global warming potential of nitrous oxide (N2O) led to its inclusion in the list of regulated greenhouse gas (GHG) pollutants [1, 2]. The mitigation of N2O on aftertreatment catalysts was shown to be ineffective as its formation and decomposition temperatures do not overlap. Therefore, the root causes for N2O formation were investigated to enable the catalyst architectures and controls development for minimizing its formation. In a typical heavy-duty diesel exhaust aftertreatment system based on selective catalytic reduction of NOx by ammonia derived from urea (SCR), the main contributors to tailpipe N2O are expected to be the undesired reaction between NOx and NH3 over SCR catalyst and NH3 slip in to ammonia slip catalyst (ASC), part of which gets oxidized to N2O.

Lean NOx Traps (LNTs) are one type of lean NOx reduction technology typically used in smaller diesel passenger cars where urea-based Selective Catalytic Reduction (SCR) systems may be difficult to package . However, the performance of lean NOx traps (LNT) at temperatures above 400 C needs to be improved. The use of Rapidly Pulsed Reductants (RPR) is a process in which hydrocarbons are injected in rapid pulses ahead of a LNT in order to expand its operating window to higher temperatures and space velocities. This approach has also been called Di-Air (diesel NOx aftertreatment by adsorbed intermediate reductants) by Toyota. There is a vast parameter space which could be explored to maximize RPR performance and reduce the fuel penalty associated with injecting hydrocarbons. In this study, the mixing uniformity of the injected pulses, the type of reductant, and the concentration of pulsed reductant in the main flow were investigated.

An operational challenge associated with SCR catalysts is the NH3 slip control, particularly for commercial small pore Cu-zeolite formulations as a consequence of their significant ammonia storage capacity. The desorption of NH3 during increasing temperature transients is one example of this challenge. Ammonia slipping from SCR catalyst typically passes through a platinum based ammonia oxidation catalyst (AMOx), leading to the formation of the undesired byproducts NOx and N2O. We have discovered a distinctive characteristic, an overlapping NH3 desorption and oxidation, in a state-of-the-art Cu-zeolite SCR catalyst that can minimize NH3 slip during temperature transients encountered in real-world operation of a vehicle.

Mitigation of ammonia slip from SCR system is critical to meeting the evolving NH₃ emission standards, while achieving maximum NOx conversion efficiency. Ammonia slip catalysts (ASC) are expected to balance high activity, required to oxidize ammonia across a broad range of operating conditions, with high selectivity of converting NH₃ to N₂, thus avoiding such undesirable byproducts as NOx or N₂O. In this work, new insights into the behavior of an advanced ammonia slip catalyst have been developed by using accelerated progressive catalyst aging as a tool for catalyst property interrogation. The overall behavior was deconstructed to several underlying functions, and referenced to an active but non-selective NH₃ oxidation function of a diesel oxidation catalyst (DOC) and to the highly selective but minimally active NH₃ oxidation function of an SCR catalyst.

Nitrogen dioxide (NO2) is known to pose a risk to human health and contributes to the formation of ground level ozone. In recognition of its human health implications, the American Conference of Governmental Industrial Hygienists (ACGIH) set a Threshold Limit Value (TLV) of 0.2 ppmv NO2 in 2012. For mobile sources, NO2 is regulated as a component of NOx (NO + NO2). In addition, the European Commission has indicated it is considering separate Euro 6 light-duty diesel and Euro VI heavy-duty diesel NO2 emissions limits likely to mitigate the formation of ground level ozone in urban areas. In this study, we conduct component-level reactor-based experiments to understand the effects that various aftertreatment catalyst technologies including diesel oxidation catalyst (DOC), diesel particulate filter (DPF), selective catalytic reduction (SCR) catalyst and ammonia oxidation (AMOX) catalyst have on the formation and mitigation of NO2 emissions.

The high global warming potential of nitrous oxide (N₂O) led to its recent inclusion in the list of regulated pollutants under the emerging greenhouse gas regulations. While N₂O can be present in small quantities among the combustion products, it can also be generated as a minor byproduct in various types of aftertreatment systems. In this work, a systematic review of sources of N₂O is presented, along with the potential mechanisms of formation in a typical selective-catalytic-reduction-based diesel exhaust aftertreatment system. It is demonstrated that diesel oxidation catalysts (DOC), selective catalytic reduction (SCR) catalyst, and ammonia slip catalyst (ASC) can all potentially contribute to N₂O formation, depending on the catalyst material and exhaust gas conditions, as well as aftertreatment operation strategies. Furthermore, catalysts used in SCR aftertreatment system are also shown to decompose and/or reduce N₂O to N₂ under select conditions.

This paper presents a monitoring, feedback and control system for SCR urea dosing utilizing an embedded model and NH3 sensing after the SCR for loop closing control. A one-dimensional SCR model was developed and embedded in a Simulink/Matlab environment. This embedded model is utilized for on-line, real time control of 32.5% aqueous urea dosing in the exhaust stream. Engine testing and simulation are used with the embedded SCR model and NH3 sensor closed loop feedback to demonstrate the advantages of this control approach for meeting both NOx emission requirements and NH3 slip targets. The paper explores these advantages under heavy duty FTP cycle conditions. The potential benefits include SCR size optimization and fuel consumption rate reduction under certain operating conditions.

Several reactor systems have been used to study the catalytic chemistry of a particular proprietary zeolitic catalyst in conditions that mimic those found in light-duty diesel exhaust after a non-thermal plasma generator. Very similar catalytic results were obtained with NO + plasma or NO2 as the source of NOx using propene (C3H6) as the reductant. The formation of nitrogen, carbon dioxide, and other products were studied from 150°C to 250°C using a He balance gas and NOx in the form of NO2. The results demonstrate that nitrogen is formed by the selective catalytic reduction of NO2 by propene. The highest activity for N2 formation from NO2 was near 50% conversion at 200°C for a space velocity of 12,600 h-1. The NOx conversion by adsorption and by catalytic reduction was quantified. By performing studies with and without the presence of water, a clear separation in behavior between adsorption processes and catalytic reaction was observed.

Worldwide, regulations continue to drive reductions in brake-specific emissions of nitric oxide (NO) and nitrogen dioxide (NO2) from on-highway and nonroad diesel engines. NOx, formed as a byproduct of the combustion of fossil fuels (e.g., natural gas, gasoline, diesel, etc.), can be converted to dinitrogen (N2) through ammonia (NH3) selective catalytic reduction (SCR). In this study, we closely examine the low-temperature storage, isothermal desorption, reactive consumption, and thermal release of NH3 on commercial Cu-SSZ-13 and V2O5-WO3/TiO2 SCR catalysts. Catalyst core-reactor, N2 adsorption (BET) surface area, and in-situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) experiments are utilized to investigate the fundamental chemical processes relevant to low-temperature (T < 250°C) NH3 SCR.

Cu-SSZ-13 selective catalytic reduction (SCR) catalysts are broadly applied in diesel aftertreatment systems for the catalytic conversion of oxides of nitrogen (NO + NO2). Diesel exhaust contains a wide range of water vapor concentrations depending on the operating condition. In this study, we evaluate the impact of water vapor on the relevant SCR catalytic functions including NOx conversion, NO oxidation, NH3 oxidation, and N2O formation under both standard and fast SCR conditions. Reactor-based experiments are conducted in the presence and absence of water vapor. Results indicate that water vapor can have both a positive and negative impact on low temperature NOx conversion for standard SCR reaction. At low inlet NOx concentrations, the presence of water vapor has a negative effect on NOx conversion, whereas, at high inlet NO concentrations, water vapor improves NOx conversion.

The ammonia slip catalyst (ASC), typically composed of Pt oxidation catalyst overlaid with SCR catalyst, is employed for the mitigation of NH3 slip originating from SCR catalysts. Oxidation and SCR functionalities in an ASC can degrade through two key mechanisms i) irreversible degradation due to thermal aging and ii) reversible degradation caused by sulfur-oxides. The impact of thermal aging is well understood and it mainly degrades the SCR function of the ASC and increases the NH3 conversion to undesired products [1]. This paper describes the impact of sulfur-oxides on critical functions of ASC and on NH3 oxidation activity and selectivity towards N2, NOx and N2O. Furthermore impact of desulfation under selected conditions and its extent of ASC performance recovery is explained.