Aftertreatment Technologies for Diesel Engines: An Overview of the Combined Systems

Abstract

The abatement of the pollutants deriving from diesel engines in the vehicle sector still represents an interesting scientific and technological challenge due to increasingly limiting regulations. Meeting the stringent limits of NOx and soot emissions requires a catalytic system with great complexity, size of units, and number of units, as well as increased fuel consumption. Thus, an after-treatment device for a diesel vehicle requires the use of an integrated catalyst technology for a reduction in the individual emissions of exhaust gas. The representative technologies devoted to the reduction of NOx under lean-burn operation conditions are selective catalytic reduction (SCR) and the lean NOx trap (LNT), while soot removal is mainly performed by filters (DPF). These devices are normally used in sequence, or a combination of them has been proposed to overcome the drawbacks of the individual devices. This review summarizes the current state of NOx and soot abatement strategies. The main focus of this review is on combined technologies for NOx removal (i.e., LNT–SCR) and for the simultaneous removal of NOx and soot, like SCR-on-Filter (SCRoF), in series LNT/DPF and SCR/DPF, and LNT/DPF and SCR/DPF hybrid systems.

Computational Study of Noble Metal CHA Zeolites: NO Adsorption and Sulfur Resistance

“…The presence of both Pd and Pt can lead to a higher amount of adsorbed NO 2 compared to Pd-only formulations, as was observed for non-zeolite LTNA previously . This NO 2 is crucial for regeneration of downstream diesel particulate filter via soot oxidation. − A combination of platinum group metals (PGMs) in zeolites can be promising for further improvements in the PNA technology. To begin exploring this area, a comparison of Pd, Pt, and Ru zeolites can be done using first-principles modeling approaches.…”

Impact of Biodiesel-Based Phosphorus and Sulfur on Copper Speciation of Cu-SSZ-13 Catalysts: XAFS Scanning during H2-TPR

“…Our present study is relevant for the configuration with DPF placed downstream of both DOC and SCR catalysts (DOC-SCR-DPF) as well as for the configuration with a SCR catalyst coated on a DPF (DOC-SCRF). 90,91 additionally decrease the number of contaminants reaching the SCR catalyst. This was found previously for the aging of Cu zeolite catalyst using Na-contaminated biodiesel.…”

The Effect of Potassium Inclusion in a Silver Catalyst for N2O-Mediated Oxidation of Soot in Oxidising Exhaust Gases

“…By contrast, a diesel engine requires a more complex aftertreatment system, mainly because the exhaust gas is cooler and it contains (i) soot particulate (which requires trapping before it can be oxidised) and (ii) a high concentration of O 2 (which makes the reduction of NOx particularly challenging) [6]. Most commercial systems currently consist of a series of consecutive units [7,8], e.g., a diesel oxidation catalyst (DOC) for the removal of carbon monoxide and hydrocarbons, a diesel particulate filter (DPF) for trapping soot, and an SCR catalyst for the selective reduction of NOx by reaction with ammonia which is formed in situ by dosing aqueous urea into the exhaust gas. The DOC has the additional function of intermittently generating a large exotherm to induce combustion of the trapped soot, which regenerates the DPF.…”

“…Although an SCR catalyst can be coated onto a DPF to form an integrated unit (referred to as SCRF [7], SCRoF [8] or SPDF [9]), the filter still requires active regeneration by the injection of fuel into an upstream DOC. Therefore, the challenge remains to design a catalysed filter in which the chemistry of NOx-reduction is coupled with that of continuous soot oxidation [10] (e.g., using perovskites [11] or 3DOM mixed-metal oxides [12]), so avoiding the requirement for intermittent regeneration of the filter.…”