Abstract
Diesel engines operate under net oxidizing environment favoring lower fuel consumption and CO2 emissions than stoichiometric gasoline engines. However, NOx reduction and soot removal is still a technological challenge under such oxygen-rich conditions. Currently, NOx storage and reduction (NSR), also known as lean NOx trap (LNT), selective catalytic reduction (SCR), and hybrid NSR–SCR technologies are considered the most efficient control after treatment systems to remove NOx emission in diesel engines. However, NSR formulation requires high platinum group metals (PGMs) loads to achieve high NOx removal efficiency. This requisite increases the cost and reduces the hydrothermal stability of the catalyst. Recently, perovskites-type oxides (ABO3) have gained special attention as an efficient, economical, and thermally more stable alternative to PGM-based formulations in heterogeneous catalysis. Herein, this paper overviews the potential of perovskite-based formulations to reduce NOx from diesel engine exhaust gases throughout single-NSR and combined NSR–SCR technologies. In detail, the effect of the synthesis method and chemical composition over NO-to-NO2 conversion, NOx storage capacity, and NOx reduction efficiency is addressed. Furthermore, the NOx removal efficiency of optimal developed formulations is compared with respect to the current NSR model catalyst (1–1.5 wt % Pt–10–15 wt % BaO/Al2O3) in the absence and presence of SO2 and H2O in the feed stream, as occurs in the real automotive application. Main conclusions are finally summarized and future challenges highlighted.
Highlights
The concern about pollutants released by internal combustion engines has increased significantly since the late 1900s
Unburned hydrocarbons (HCs), carbon monoxide (CO), particulate matter (PM), sulfur oxides (SOx ), nitrogen oxides (NOx ), and soot are the main pollutants in the exhaust [1]
When selective catalytic reduction (SCR) system is placed downstream the NOx (Figure 12a) and NH3 (Figure 12b) outlet concentrations decrease drastically in comparison to single-NOx storage and reduction (NSR) system. These results suggest that most NH3 formed during regeneration of the NSR catalyst is adsorbed over SAPO-34 zeolite and in the subsequent lean period, reacts with the NOx slipping NSR system following SCR reactions [113]
Summary
The concern about pollutants released by internal combustion engines has increased significantly since the late 1900s. The ammonia generated from urea decomposition, which is stored on-board in a specific reservoir is the usual reductant in this technology (NH3 -SCR) [12,13] In this case, catalysts based on Cu or Fe exchanged on different zeolites are widely adopted as NH3 -SCR formulation [14,15,16]. NH3 -SCR involves a continuous admission of NH3 to reduce NOx , which is formed from the thermal decomposition of urea stored in on-board tank This fact limits the implementation of SCR technology in small vehicle due to the cost and space requirement. The amount of noble metal (Pt) should be reduced or replaced by less expensive and thermally more stable materials Taking into account these drawbacks, during the last decade, a great interest in developing perovskite-based formulation for NOx removal in diesel engines has grown.
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