Urea-SCR Technology for deNOx After Treatment of Diesel Exhausts

This chapter will provide an overview of the major legislative and technology developments related to SCR for mobile applications. Regulatory initiatives are moving forward in all markets in the light-duty (LD), heavy-duty (HD), and non-road (NR) (agriculture and construction) sectors. NOx regulations are quite tight, forcing deNOx aftertreatment in all US LD diesel applications, and most HD and NR applications in the US, Europe, and Japan. CO₂ regulations are also tightening worldwide, driving deNOx technologies on diesel engines. Knowledge of engine technologies will help define required deNOx aftertreatment needs. In the HD sector, because of the fuel consumption versus NOx trade-off, deNOx means deCO₂. As such, 98 % selective catalytic reduction (SCR) deNOx to result in elimination of EGR (exhaust gas recirculation) and minimum operating cost and CO₂ emissions is desired. The situation is similar on the LD side. Once SCR is added to the vehicle to meet the NOx regulations, it will likely be used to maximum efficiency in the context of consumer acceptance of ammonia consumption and replacement. On-board ammonia delivery systems for the SCR catalyst principally comprises liquid urea and gaseous ammonia types. Liquid ammonia systems are in their third generation of cost reduction and simplification. The gaseous ammonia system is attractive in LD applications due to the heavy weighting of cold start emissions in the regulations, and the ability to deliver ammonia at low exhaust temperatures. The field of catalysts, including oxidation catalysts, SCR catalysts, and ammonia slip catalysts is very dynamic. Oxidation catalysts are used to generate NO₂ for better low-temperature performance of certain SCR catalysts (like Fe-zeolites). However, they can have undesirable N₂O emissions, and NO₂ production decreases with palladium content, when replacing platinum. Zeolite SCR catalysts are evolving beyond the copper and iron varieties into other formulations with better low-temperature and/or high-temperature performance. Vanadia catalysts have made recent impressive gains in high-temperature durability. High-efficient SCR systems generally are run with excess ammonia. Ammonia slip catalysts (ASC) are becoming more selective towards nitrogen, minimizing N₂O, and NOx by-product formation. Finally, system design considerations are discussed, as well as emerging technologies. The author closes with comments on the future outlook and opportunities.

The term Off-Highway includes a great variety of diesel engine applications like propulsion of ships, mining trucks, harvesters, trains, power generation and pump drives, e.g. for hydraulic fracturing.

This overview summarizes the development and application of vanadia-based urea/NH₃-SCR for mobile applications. The focus is on heavy-duty diesel engines where this technology has been put to the market on a broad scale. A short history of vanadia SCR technology for diesel engines and how the technology emerged as the choice for mobile diesel engines is introduced together with related emission legislation. The layout of a typical mobile vanadia SCR system is described including different sensors and control strategies. Design considerations important for mobile vanadia SCR systems are discussed as well as washcoated and fully extruded catalysts. Some attention is put on enhancing NOx conversion by using vanadia-based SCR catalysts together with an oxidation catalyst. Durability and different deactivation mechanisms for vanadia-based SCR catalysts, relevant for mobile applications are discussed.

Since the introduction of the first emissions control regulations in the 1970s and 1980s [1], catalysis has been implemented extensively to maintain compliance and dramatically reduce the harmful pollutants emitted from combustion engines. For stoichiometric exhaust, primarily from gasoline-powered vehicles, precious metals, or platinum-group metals (PGM), such as Pt, Pd, and Rh, have been the hallmark of three-way catalysis, e.g., [2‐4], as they are highly active in oxidation of carbon monoxide (CO) and hydrocarbons (HCs) as well as the reduction of nitrogen oxides (NO_x). The chemistry behind these reactions is equilibrium driven, as the more benign products of CO₂, H₂O, and N₂ are thermodynamically favored. However, these catalysts only function properly if the exhaust is at or near stoichiometric conditions. As a result, gasoline vehicle manufacturers began designing their engine control systems to operate with stoichiometric air/fuel ratios to optimize catalyst performance and minimize emissions. The need for more fuel-efficient vehicles, both with respect to increasing fuel costs and future CO₂ emissions regulations, is driving vehicle manufacturers to investigate more efficient combustion strategies, such as lean-burn gasoline, or increase production of more fuel-efficient diesel vehicles.

Cu/zeolite catalysts have long been recognized to be highly active in the Selective Catalytic Reduction (SCR) of NOx with NH₃ [1–16]. Compared to titania supported vanadia SCR catalysts, which have been successfully commercialized for stationary NOx emission control since the 1970s and installed on certain Heavy Duty Diesel (HDD) vehicles to meet the NOx emission regulations since the early 2000s, Cu/zeolite SCR catalysts exhibit higher NOx conversion efficiency, particularly at low temperatures [11, 17]. In addition, Cu/zeolite SCR catalysts are more tolerant to high temperature excursions. For automotive applications, this is a critical requirement for the SCR component when it is combined with a Diesel Particulate Filter (DPF) in the emission control system. In order to effectively regenerate the DPF component, the entire system is exposed to temperatures above 600 °C periodically. Cu/zeolite SCR catalysts are significantly more stable than vanadium-based SCR catalysts at temperatures above 650 °C.

This review provides an introduction to the recent progress in the development of transition metal-exchanged zeolites and mixed oxide-based catalysts for the low-temperature selective catalytic reduction (SCR) of NO_x with ammonia. The nature of the different active species over different catalysts and the corresponding relationship between their performance and catalytic properties as well as the mechanism and kinetics of SCR reaction on different catalysts are discussed. In addition, recent developments of H₂-SCR are also reviewed.

In this chapter, the development of knowledge on the active sites of V2O5-WO3/TiO2, Fe- and Cu-modified zeolite catalysts, which are most frequently used for NH3-SCR, is reviewed and a short account on opinions regarding Mn- and Ce-based catalysts is given. In vanadia-based systems, binary sites combining two V ions via an oxygen bridge are thought to provide the highest activities although the reaction can proceed also on isolated V oxo sites. The W promoter has been found to separate V oxo surface species, which tend to form islands providing low activity, into smaller, more active entities. For Fe zeolites it has been shown that standard SCR, fast SCR and NO oxidation proceed on different sites, which discourages mechanistic concepts according to which NO2 formation by NO oxidation is the rate-determining step of standard SCR. Opinions on active sites for standard SCR range from “exclusively isolated sites” to “all accessible Fe sites.” Opposed to this, fast SCR is catalyzed by an isolated minority Fe site, most likely a type which is stabilized in the 2+ oxidation state by two close framework Al ions and can be oxidized to the 3+ state only by NO2. Standard SCR over Cu zeolites can proceed on isolated and clustered sites as well, but the high activities on the recently developed Cu chabazites originate from isolated Cu cations.

This chapter delineates the mechanistic aspects of the NO–NH₃–O₂ reacting system, also known as the standard SCR reaction. The standard SCR technology was first developed in the 1970s, and thus has a long history in the research area of catalyst development as well as the associated reaction mechanisms.

We review the NH₃–SCR catalytic chemistry, focusing specifically on the steps involving NO₂. For this purpose we use results from the mechanistic investigation of several SCR catalysts performed in our labs during the last decade, and complement them with other data from the literature. According to the resulting picture, the reactivity of NO/NO₂ + NH₃ over Fe- and Cu-promoted zeolites at low temperatures is consistent with previous mechanistic proposals for the Fast SCR chemistry over vanadium-based catalysts as well as over BaNa-Y zeolites. In such a general chemistry, the following roles are attributed to the three main SCR reactants: (1) NO₂ forms surface nitrates and nitrites via a disproportionation route; (2) NO reduces the nitrates to nitrites; (3) NH₃ decomposes/reduces the nitrites to N₂. The related individual reaction steps have been identified by transient reaction analysis and confirmed by in situ FT-IR techniques. Formation of unselective reaction products (NH₄NO₃, N₂O) is also explained by the nitrates reactivity. From a kinetic point of view, Step 2 is the most critical one at low temperature, being blocked by NH₃ below 140–160 °C. A red-ox interpretation of Step 2 is also discussed, wherein the very high Fast SCR reaction rates are attributed by the extreme activity of the nitrates in catalyst reoxidation. Altogether, the proposed mechanistic picture is able to explain the observed stoichiometry, selectivity, and kinetics of the NO/NO₂ + NH₃ reacting system over both V-based and metal-promoted zeolites catalysts.

An extensive investigation of the chemistry, the mechanism, and the kinetics of the NO-NO₂/NH₃ SCR reactions for mobile applications was performed over a commercial vanadium-based catalyst. On the basis of the collected results, a Mars-Van Krevelen dynamic kinetic model was derived, which unifies the rates of the Standard and the Fast SCR into a single Redox-type approach.

Rising transportation fuel costs have increased the use of diesel-powered vehicles, which are more fuel efficient than their gasoline counterparts. But the lean diesel exhaust contains NOx (NO + NO₂) which is notoriously difficult to reduce in the presence of excess O₂. Selective catalytic reduction (SCR) of NOx with NH₃ generated

In this chapter, kinetic models for ammonia SCR over Cu-zeolites are described. Both global and detailed models are presented and both commercial materials as well as model samples are used. In the SCR system, several reactions are important in order to describe the mechanism and these reactions are discussed below.

Selective catalytic reduction (SCR) is emerging as the most promising technology for the abatement of NO _x emissions from diesel vehicles. Given the high NO _x conversion efficiency requirements and the need for shorter development cycles and cost reduction, the use of numerical simulation is nowadays well established. In this work, the fundamentals of flow-through and wall-flow SCR converters are presented. The model parameter calibration, the development of SCR kinetic models from small-scale reactors, and their applicability to real-world full-scale applications are discussed. Since modern exhaust systems usually consist of more than one aftertreatment device, the last part is dedicated to the modeling of combined SCR-DPF systems, either in-series or as a multilayered catalyst.

Diesel engine urea-SCR system control has been a great challenge ever since the start of SCR mobile vehicle applications. Open-loop urea dosing is difficult to meet the tailpipe NO_x and ammonia emission requirements primarily due to the complex SCR dynamics in engine transient operations. Closed-loop SCR urea dosing control is thus more preferable in order to achieve high SCR operational NO_x conversion efficiency and low tailpipe ammonia slip. However, the highly nonlinear dynamics, limited measurements, and transient operating conditions make the closed-loop control of on-vehicle SCR systems challenging. Many studies on SCR system control-oriented modeling, measurement systems, and controller designs have been proposed in recent years. While several major SCR control related issues have been addressed to some extents, many vehicle SCR real-time control challenges remain open. The objective of this chapter is to summarize the vehicle SCR system control challenges and review the possible solutions in the aspects of control-oriented modeling, measurements, and control for mobile vehicle applications. The main intent is to provide an introduction to the current vehicle SCR control system developments.

As emissions regulations become more stringent, new aftertreatment technologies are necessary. To assist in the reduction of NO_x emissions, liquid-based SCR technologies have been adopted globally. The most common technique used to introduce ammonia (NH₃) into the exhaust stream is via a dosing system metering diesel exhaust fluid (DEF). SCR system efficiency relies heavily on how uniformly the NH₃ is distributed across the catalyst face.

In mobile SCR applications, the most widespread reducing agent is ammonia. However, due to its toxicity it is not stored directly as pressurized or liquefied gas. Instead, an aqueous solution of 32.5 wt % urea is commonly used as ammonia precursor. The urea solution can be dosed into the main exhaust pipe, where it decomposes in the hot exhaust gas and on the SCR catalyst to yield ammonia and carbon dioxide. Urea as ammonia precursor and its decomposition will be discussed in the first part of this chapter, with a focus on catalytic decomposition and byproduct formation. In the second part, alternative ammonia precursor compounds, including solid ammonia precursors will be presented. These exhibit a higher ammonia storage density, a lower melting point and/or a higher stability when stored at elevated temperatures compared to urea solution.

This chapter describes the numerical models adopted for the simulation of the internal flows inside exhaust systems of internal combustion engines, with particular focus on the part of the system upstream of the catalytic converter. Particular attention is paid to the modeling of the dosing system, which requires a correct numerical description of the spray evolution inside the gas stream, and its subsequent interaction with pipe walls (or eventual mixing device) and the dynamics of the liquid film. The simulation of all these processes is mandatory when an optimization of the exhaust systems is addressed in order to improve the abatement efficiency of the SCR system.

A chemically and physically consistent mathematical model of a commercial dual-layer (SCR + PGM) monolith catalyst for control of the NH₃ slip (ASC, Ammonia Slip Catalyst) from NH₃/urea-SCR converters is herein presented. In a first stage NH₃/O₂/NO–NO₂–N₂O steady-state and transient kinetic runs were performed at high space velocities in a representative temperature range (150–550 °C) over each one of the two ASC component catalysts (Fe-zeolite SCR and Pt/Al₂O₃ PGM) in the form of powders to provide intrinsic kinetic information on the prevailing reaction pathways and to enable the development of original kinetic models for each catalytic phase. In a subsequent stage the two powdered catalysts were also tested jointly, both in a sequential dual-bed configuration and in the form of a mechanical mixture, thus acquiring information on the interactions between the SCR and the PGM catalytic chemistries. Two global kinetic models were fitted to the SCR and to the PGM catalyst data, respectively, and then suitably combined to generate predictive simulations of the runs over the double bed and over the mechanical mixture. Lab-scale NH₃/O₂/NO–NO₂ catalytic activity experiments were performed over washcoated core monolith single-layered SCR and PGM catalysts and over a full dual-layer ASC honeycomb catalyst in order to evaluate the importance of transport phenomena and to scale-up the kinetics. Finally, an original heterogeneous 1D + 1D dynamic mathematical model of a single monolith channel with dual-layer washcoat was developed for the simulation of ASC catalysts. The rate parameters derived from the kinetic analysis of the powdered SCR and PGM catalyst data were incorporated, together with the relevant geometrical and morphological catalyst properties, into such a model, which was eventually validated by simulating data from engine test bench runs, collected over a full scale dual-layer ASC monolith catalyst using real exhausts from a Diesel engine.

This chapter gives a critical overview of the recent advances in NOx abatement in excess of oxygen based on the combination of the NOx storage-reduction (NSR) and Selective Catalytic Reduction (SCR) processes. Ammonia may be produced during the regeneration step of NSR catalyst, by the direct reaction (NOx + H₂) or/and the isocyanate route. Recent literature highlights that the ammonia production rate is higher than the ammonia reaction rate with the remaining NOx in order to form N₂. In order to optimize the use of the in situ produced ammonia, a catalyst dedicated to the NOx–SCR by NH₃ can be added. Zeolites are the main studied materials for this application. Catalytic reduction of NOx by NH₃ relates a complex mechanism, in which the nuclearity of the active sites is still an open question. Over zeolites, the NO to NO₂ oxidation step is reported as the rate-determining step of the SCR reaction, even if the first step of the reaction is ammonia adsorption on zeolite Brønsted acid sites. Thus, the addition of a NH₃–SCR material to the NSR catalyst is a possible way to increase the global NOx abatement and maximize the N₂ selectivity, together with the prevention of the ammonia slip.

Most modern diesel engine aftertreatment systems comprise functions to oxidize hydrocarbons and CO, reduce particulate emissions (mass and number), and reduce NOx enabling compliance with ever tightening regulations. A typical system layout based on SCR as a means to reduce NOx is shown in Fig. 20.1a.

The choice to pursue Selective Catalytic Reduction (SCR) using aqueous urea as a NO_x reductant for lean NO_x control on diesel vehicles was not an easy one. It was difficult to imagine an infrastructure for delivery of aqueous urea to diesel vehicles. In the 1990s, SCR technology was best known for its use in stationary source control of NO_x. There were published studies on its potential effectiveness for diesel vehicle NO_x control at steady state, most notably by Degussa [13], Volkswagen [23], and Hug Engineering [25]. Earlier, interesting work was also being done at the Paul Scherrer Institute in Switzerland [28]. Ford applied urea SCR to a light-duty truck and tested it with success on transient cycles [43]. The SCR catalysts available at that time included vanadia/titania, and base metal/zeolite formulations using copper or iron, usually using ZSM-5. It was unknown if these catalyst types would ever be durable enough for a vehicle application, not only hydrothermally but with diesel fuel sulfur contents on the order of 500 ppm, or approximately 50 times the current diesel sulfur level in the U.S. and Europe today. There was also uncertainty about integration of particulate traps into SCR systems, low temperature/high space velocity applications, and aqueous urea handling issues, including freeze point and replenishment.

One major challenge for car manufacturers since several years is to be compliant with the stringent emission standards for internal combustion engines, especially in Europe and in the US. Besides the treatment of standard pollutants like CO, NOx, and HC, the emission of CO₂ has gained increasing environmental relevance due to its greenhouse potential. Further reduction of CO₂ emissions for lean-burn engines is, in general, correlated with an increase of NOx raw emissions and requires improved aftertreatment systems.