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The importance of the low-temperature selective catalytic reduction (LT-SCR) of NOx by NH3 is increasing due to the recent severe pollution regulations being imposed around the world. Supported and mixed transition metal oxides have been widely investigated for LT-SCR technology. However, these catalytic materials have some drawbacks, especially in terms of catalyst poisoning by H2O or/and SO2. Hence, the development of catalysts for the LT-SCR process is still under active investigation throughout seeking better performance. Extensive research efforts have been made to develop new advanced materials for this technology. This article critically reviews the recent research progress on supported transition and mixed transition metal oxide catalysts for the LT-SCR reaction. The review covered the description of the influence of operating conditions and promoters on the LT-SCR performance. The reaction mechanism, reaction intermediates, and active sites are also discussed in detail using isotopic labelling and in situ FT-IR studies.

Lanthanum (La) ions are generally recognized to cause a decline of the catalytic performance for Cu-SSZ-13 zeolite in the selective catalytic reduction of NO x with NH 3 (NH 3 -SCR). Herein, we demonstrate that the NH 3 -SCR performance and hydrothermal stability of Cu-La-SSZ-13 zeolites can be enhanced with the incorporation of a small amount of La ions. The incorporation of La ions into SSZ-13 favors more Z 2 Cu 2+ ions at six-membered rings (6MRs), which results in higher hydrothermal stability of Cu-La-SSZ-13 than that of Cu-SSZ-13. The NO conversion of Cu-La-SSZ-13 achieves 5%–10% higher than that of Cu-SSZ-13 at the temperature range of 400–550 °C after hydrothermal ageing. While introducing excess amount of La ions in SSZ-13 may cause the formation of inactive CuO x , leading to the decrease of catalytic activity and hydrothermal stability. Notably, the low-temperature activity of Cu-SSZ-13 with a low Cu content (≤ 2 wt.%) can be boosted by the introduction of La ions, which is largely due to the improved redox ability of Cu active sites modified by La ions. Density functional theory (DFT) calculations indicate that La ions prefer to locate at eight-membered rings (8MRs) and thus promoting the formation of more Z 2 Cu 2+ ions. Meanwhile, the existence of La ions in SSZ-13 inhibits the dealumination process and the transformation from Z 2 Cu 2+ to CuO x , resulting in its enhanced hydrothermal stability. The present work sheds a new insight into the regulation of secondary metal cations for promoting high NH 3 -SCR performance over Cu-SSZ-13 zeolite catalysts.

Mn-based catalysts have attracted significant attention in the field of catalytic research, particularly in NOx catalytic reductions and CO catalytic oxidation, owing to their good catalytic activity at low temperatures. In this review, we summarize the recent progress of Mn-based catalysts for the removal of NOx and CO. The effects of crystallinity, valence states, morphology, and active component dispersion on the catalytic performance of Mn-based catalysts are thoroughly reviewed. This review delves into the reaction mechanisms of Mn-based catalysts for NOx reduction, CO oxidation, and the simultaneous removal of NOx and CO. Finally, according to the catalytic performance of Mn-based catalysts and the challenges faced, a possible perspective and direction for Mn-based catalysts for abating NOx and CO is proposed. And we expect that this review can serve as a reference for the catalytic treatment of NOx and CO in future studies and applications.

Selective catalytic reduction of nitrogen oxides (NOx) with ammonia (NH3-SCR) has been implemented in response to the regulation of NOx emissions from stationary and mobile sources above 300 °C. However, the development of NH3-SCR catalysts active at low temperatures below 200 °C is still needed to improve the energy efficiency and to cope with various fuels. In this review article, recent reports on low-temperature NH3-SCR catalysts are systematically summarized. The redox property as well as the surface acidity are two main factors that affect the catalytic activity. The strong redox property is beneficial for the low-temperature NH3-SCR activity but is responsible for N2O formation. The multiple electron transfer system is more plausible for controlling redox properties. H2O and SOx, which are often found with NOx in flue gas, have a detrimental effect on NH3-SCR activity, especially at low temperatures. The competitive adsorption of H2O can be minimized by enhancing the hydrophobic property of the catalyst. Various strategies to improve the resistance to SOx poisoning are also discussed.

Cu/SSZ-13 Selective Catalytic Reduction (SCR) catalysts have been extensively studied for the past five-plus years. New and exciting fundamental and applied science has appeared in the literature quite frequently over this time. In this short review, a few topics specifically focused on a molecular-level understanding of this catalyst are summarized: (1) The nature of the active sites and, in particular, their transformations under varying reaction conditions that include dehydration, the presence of the various SCR reactants and hydrothermal aging; (2) Discussions of standard and fast SCR reaction mechanisms. Considerable progress has been made, especially in the last couple of years, on standard SCR mechanisms. In contrast, mechanisms for fast SCR are much less understood. Possible reaction paths are hypothesized for this latter case to stimulate further investigations; (3) Discussions of rational catalyst design based on new knowledge obtained regarding catalyst stability, overall catalytic performance and mechanistic catalytic chemistry.

Strict adherence to pollution limits poses a risk to light-duty diesel engines due to challenges in post-treatment procedures, product limitations, and emission criteria. This paper aims to determine the underlying principles for the technological development of an after-treatment technique that incorporates a diesel oxidation catalyst (DOC), catalytic Diesel Particle Filter (DPF), urea injector, and catalytic urea selective reduction (SCR). Implementing this selective catalytic reduction (SCR) technique would greatly enhance the catalyst’s ability to convert NOx by regulating the evaporation of urea and avoiding a decrease in exhaust temperature and mixing efficiencies. Moreover, the uniformity of the NH3 concentration distribution over the catalyst surface is advantageous. This study explored the concept of an electrically evaporated urea-dosing device. The investigation revealed that heated urea had a beneficial effect on improving the elimination of NOx from both continuous and intermittent motor operations before its application to the gas exhaust. The cylindrical urea evaporative heating chamber was equipped with a venturi jet that directed urea vapour down the exhaust drain. The urea solution dosing technique, administered by spraying, was a customised method more advantageous than the conventional liquid dosage system.

γ-Fe 2 O 3 catalyst showed higher catalytic activity than α-Fe 2 O 3 in 150–300 °C. Both NH 3 and NO x species adsorbed and reacted easily on γ-Fe 2 O 3 . However, NH 3 adsorbed on α-Fe 2 O 3 and then reacted with gaseous NO x . Gaseous NO x was more easily adsorbed on α-Fe 2 O 3 than gaseous NH 3 , consequently, stable nitrates formed and blocked the active sites, which affected the SCR reaction badly. Graphical Abstract

Diesel-fueled vehicles have classically had high particulate and NOx emissions. The introduction of Diesel Particulate Filters (DPFs) and Selective Catalytic Reduction for NOx (SCR) systems have decreased the Particle Number (PN) and NOx emissions, respectively, to very low levels. However, there are concerns regarding the emissions released during the periodic DPF regenerations, which are necessary to clean the filters. The absolute emission levels and the frequency of the regenerations determine the contribution of regenerations, but where they happen (city or highway) is also important due to different contributions to human exposure. In this study, we measured regulated and non-regulated emissions of a Euro 6d-temp vehicle both in the laboratory and on the road. PN and NOx emissions were similar in the laboratory and on-the road, ranging around 1010 p/km and 50 mg/km, respectively. Six regeneration events took place during the 1300 km driven, with an average distance between regeneration events of only 200 km. During regeneration events, the laboratory limits for PN and NOx, although not applicable, were exceeded in one of the two measured events. However, the on-road emissions were below the applicable not-to-exceed limits when regenerations occurred. The weighted PN and NOx emissions over the regeneration distance were approximately two times below the applicable limits. The N2O emissions were <14 mg/km and NH3 at instrument background level (<1 ppm), reaching 8 ppm only during regeneration. The results of this study indicate that due to the short interval between regenerations, studies of diesel vehicles should report the emissions during regeneration events.

In this work, two palladium-based catalysts with either ZSM-5 or Zeolite Y as support material are tested for their performance in selective catalytic reduction of NOx with hydrogen (H 2 -SCR). The ligh-toff measurements in synthetic exhaust gas mixtures typical for hydrogen combustion engines are supplemented by detailed catalyst characterization comprising N 2 physisorption, X-ray powder diffraction (XRD), hydrogen temperature programmed reduction (H 2 -TPR) and ammonia temperature programmed desorption (NH 3 -TPD). Introducing 10% or 20% TiO 2 into the catalyst formulations reduced the surface area and the number of acidic sites for both catalysts, however, more severely for the Zeolite Y-supported catalysts. The higher reducibility of the Pd particles that was uncovered by H 2 -TPR resulted in an improved catalytic performance during the light-off measurements and substantially boosted NO conversion. Upon exposition to humid exhaust gas, the ZSM-5-supported catalysts showed a significant drop in performance, whereas the Zeolite Y-supported catalyst kept the high levels of conversion while shifting the selectivity from N 2 O more toward NH 3 and N 2 . The 1%Pd/20%TiO 2 /HY catalyst subject to this work outperforms one of the most active and selective benchmark catalyst formulations, 1%Pd/5%V 2 O 5 /20%TiO 2 -Al 2 O 3 , making Zeolite Y a promising support material for H 2 -SCR catalyst formulations that allow efficient and selective NOx-removal from exhaust gases originating from hydrogen-fueled engines.

The application of secondary NOx control methods in medium to low-capacity furnaces is a relatively new topic on the energy market and thus requires further research. In this paper, the results of full-scale research of SNCR and hybrid SNCR + SCR methods applied into a 29 MWth solid fuel fired stoker boiler is presented. The tests were performed for a full range of boiler loads, from 33% (12 MWth) to 103% (30 MWth) of nominal load. A novel SNCR + SCR hybrid process was demonstrated based on an enhanced in-furnace SNCR installation coupled with TiO2-WO3-V2O5 catalyst, which provides extra NOx reduction and works as an excess NH3 “catcher” as well. The performance of a brand-new catalyst was evaluated in comparison to a recovered one. The emission of NOx was reduced below 180 mg NOx/Nm3 at 6% O2, with ammonia slip in flue gas below 10 mg/Nm3. Special attention was paid to the analysis of ammonia slip in combustion products: flue gas and fly ash. An innovative and cost-effective method of ammonia removal from fly ash was presented and tested. The main idea of this method is fly ash recirculation onto the grate. As a result, ammonia content in fly ash was reduced to a level below 6.1 mg/kg.