Selective Catalytic Reduction of NOx by NH3

The ethanol dispersion method was employed to synthesize a series of MnO_x/SAPO-34 catalysts using SAPO-34 with the hierarchical pore structure as the zeolite carrier, which were prepared by facile acid treatment with citric acid. Physicochemical properties of catalysts were characterized by XRD, XPS, BET, TEM, NH₃-TPD, SEM, FT-IR, Py-IR, H₂-TRP and TG/DTG. NH₃-SCR performances of the hierarchical MnO_x/SAPO-34 catalysts were evaluated at low temperatures. Results show that citric acid etching solution at a concentration of 0.1 mol/L yielded a hierarchical MnO_x/SAPO-34-0.1 catalyst with ca.15 wt.% Mn loading, exhibiting optimal catalytic activity and SO₂ tolerance at low temperatures. Almost 100% NO conversion and over 90% N₂ selectivity at 120 °C under a gas hourly space velocity (GHSV) of 40,000 h⁻¹ could be obtained over this sample. Furthermore, the NO conversion was still higher than 65% when 100 ppm SO₂ was introduced to the reaction gas for 4 h. These could be primarily attributed to the large specific surface area, high surface acidity concentration and abundant chemisorbed oxygen species provided by the hierarchical pore structure, which could also increase the mass transfer of the reaction gas. This finding suggests that the NH₃-SCR activity and SO₂ poisoning tolerance of hierarchical MnO_x/SAPO-34 catalysts at low temperatures can be improved by controlling the morphology of the catalysts, which might supply a rational strategy for the design and synthesis of Mn-based SCR catalysts. Full article

Iron-based oxide catalysts for the NH₃–SCR (selective catalytic reduction of NO_x by NH₃) reaction have gained attention due to their high catalytic activity and structural adjustability. In this work, iron–niobium, iron–titanate and iron–molybdenum composite oxides were synthesized by a co-precipitation method with or without the assistance of hexadecyl trimethyl ammonium bromide (CTAB). The catalysts synthesized with the assistance of CTAB (FeM_0.3O_x-C, M = Nb, Ti, Mo) showed superior SCR performance in an operating temperature range from 150 °C to 400 °C compared to those without CTAB addition (FeM_0.3O_x, M = Nb, Ti, Mo). To reveal such enhancement, the catalysts were characterized by N₂-physisorption, XRD (Powder X-ray diffraction), NH₃-TPD (temperature-programmed desorption of ammonia), DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy), XPS (X-ray Photoelectron Spectroscopy), and H₂-TPR (H₂-Total Physical Response). It was found that the crystalline phase of Fe₂O₃ formed was influenced by the presence of CTAB in the preparation process, which favored the formation of crystalline γ-Fe₂O₃. Owing to the changed structure, the redox-acid properties of FeM_0.3O_x-C catalysts were modified, with higher exposure of acid sites and improved ability of NO oxidation to NO₂ at low-temperature, both of which also contributed to the improvement of NO_x conversion. In addition, the weakened redox ability of Fe prevented the over-oxidation of NH₃, thus accounting for the greatly improved high-temperature activity as well as N₂ selectivity. Full article

Copper and iron promoted ZrO₂ catalysts were prepared by one-pot synthesis using urea. The studied catalysts were characterized by XRD, N₂ physisorption, XPS, NH₃-TPD, and tested in the selective catalytic reduction of NO with NH₃ (NH₃-SCR) in the absence and presence of water vapor under the experimental conditions representative of exhaust gases from stationary sources. The influence of SO₂ on catalytic performance was also investigated. Among the studied catalysts, the Fe-Zr sample showed the most promising results in NH₃-SCR, being active and highly selective to N₂. The addition of SO₂ markedly improved NO and NH₃ conversions during NH₃-SCR in the presence of H₂O. The improvement in acidic surface properties is believed to be the cause. Full article

Monolithic Mn-Fe-Ce-Al-O catalyst with honeycomb cordierite ceramic as a carrier was reported for the first time for low temperature deNO_x application. In the reaction of selective catalytic reduction (SCR) of NO with NH₃, a NO conversion of above 80% at 100 °C was obtained. Notably, the catalyst also showed excellent resistance against SO₂ and H₂O. About 60% NO conversion was maintained after successive operation in the mixed stream of SO₂ and H₂O for 168 h. The Brunner−Emmet−Teller (BET) measurement, SEM, EDS, thermogravimetric analysis (TG), FT-IR, and XPS results of the used catalysts indicated that certain amounts of ammonium sulfate was formed on the surface of the catalyst. XPS results revealed that partial of Fe²⁺ was oxidized to Fe³⁺ during the reaction process, and Fe²⁺ species have strong redox ability, which can explain the decrease in activity after reaction. In addition, SO₂ and H₂O induced a transformation of Ce from Ce⁴⁺ to Ce³⁺ on the surface of the catalyst, which increased the amount of chemisorbed oxygen. Owing to these factors, the addition of Ce and Fe species contributes to excellent resistance of the catalyst to SO₂ and H₂O. Full article

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 NO_x 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 NO_x under lean-burn operation conditions are selective catalytic reduction (SCR) and the lean NO_x 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 NO_x and soot abatement strategies. The main focus of this review is on combined technologies for NO_x removal (i.e., LNT–SCR) and for the simultaneous removal of NO_x and soot, like SCR-on-Filter (SCRoF), in series LNT/DPF and SCR/DPF, and LNT/DPF and SCR/DPF hybrid systems. Full article

This paper reviews the recent advances in the management of nitrogen oxide (NOx) emissions from the internal combustion engine of light-duty and heavy-duty vehicles, addressing both technical and legal aspects. Particular focus is devoted to the often-virtuous interaction between new legislation imposing more restrictions on the permitted pollutant emission levels and new technologies developed in order to meet these restrictions. The review begins first with the American and then European directives promulgated in the 1970s, aimed at limiting emissions of pollutants from road transport vehicles. Particular attention is paid to the introduction of the Euro standards in the European Union for light- and heavy-duty vehicles, used as a legal and time frame reference for the evolution of emission aftertreatment systems (ATSs). The paper also describes governmental approaches implemented for the control of pollutant emissions in circulating vehicles, such as market surveillance and in-service conformity. In parallel, it is explained how the gradual introduction of small-scale devices aimed at the NOx control, such as lean NOx traps (LNTs) systems, and, most of all, the selective catalytic reduction (SCR) of NOx, permitted the application to road-transport vehicles of this ATS, originally designed in larger sizes for industrial usage. The paper reviews chemical processes occurring in SCR systems and their advantages and drawbacks with respect to the pollutant emission limits imposed by the legislation. Their potential side effects are also addressed, such as the emission of extra, not-yet regulated pollutants such as, for example, NH₃ and N₂O. The NOx, N₂O, and NH₃ emission level evolution with the various Euro standards for both light- and heavy-duty vehicles are reported in the light of experimental data obtained at the European Commission’s Joint Research Centre. It is observed that the new technologies, boosted by increasingly stricter legal limits, have led in the last two decades to a clear decrease of over one order of magnitude of NOx emissions in Diesel light-duty vehicles, bringing them to the same level as Euro 6 gasoline vehicles (10 mg/km to 20 mg/km in average). On the other hand, an obvious increase in the emissions of both NH₃ and N₂O is observed in both Diesel and gasoline light-duty vehicles, whereby NH₃ emissions in spark-ignition vehicles are mainly linked to two-reaction mechanisms occurring in three-way catalysts after the catalyst light-off and during engine rich-operation. NH₃ emissions measured in recent Euro 6 light-duty vehicles amount to a few mg/km for both gasoline and Diesel engines, whereby N₂O emissions exceeding a dozen mg/km have been observed in Diesel vehicles only. The present paper can be regarded as part of a general assessment in view of the next EU emission standards, and a discussion on the role the SCR technology may serve as a NOx emission control strategy from lean-burn vehicles. Full article