Low-temperature NOx Removal Research Articles

Tunable Highly Dispersed Nia-xMnxAlOy Catalysts Derived from Layered Double Oxide for Low Temperature NOx Removal

Tunable Highly Dispersed Nia-xMnxAlOy Catalysts Derived from Layered Double Oxide for Low Temperature NOx Removal

Engineering the surface properties of CeVO4 catalysts to improve low temperature De-NOx performance through regulating pH value in synthesis process

The presence of quick electron transfer in metal oxides can well promote selective catalytic reduction (SCR) of NOx with NH3. Here, a series of CeVO4-X (X = 4, 6, 8, 10 and 12) catalysts with engineering the surface properties were successfully synthesized via controlling pH values in the hydrothermal process. The catalysts were evaluated in the NH3-SCR reaction in the temperature range of 90–420 °C. Among them, the CeVO4-8 catalyst with nanorods structure displayed the highest catalytic activity, achieving over 90 % NO conversion between 180 and 390 °C. It also showed high N2 selectivity and good tolerance to H2O and SO2. The effect of different pH values on the catalytic activity and microstructure of CeVO4 catalysts was comprehensively characterized using various techniques such as XRD, Raman, SEM, HR-TEM, N2 adsorption-desorption, H2-TPR, O2/NH3/NO-TPD, XPS, and in-situ DRIFTS. The characterization results revealed that the variation in catalytic activity among these catalysts is attributed to the difference in their surface properties, which were tuned by changing the pH values during synthesis process, despite having the same crystal phase of CeVO4. The superior catalytic performance of the CeVO4-8 catalyst, particularly at low temperature, can be attributed to its higher specific surface area, enhanced acidity, and the electron transfer between cerium and vanadium species (Ce4+ + V4+ ↔ Ce3+ + V5+), consequently resulting in increased redox capacity and facilitating the adsorption and activation of NH3 and NO molecules. This study broadens the applications of CeVO4 in the field of low temperature NOx removal.

Bulk tungsten-substituted vanadium oxide for low-temperature NOx removal in the presence of water

NH3-SCR (selective catalytic reduction) is important process for removal of NOx. However, water vapor included in exhaust gases critically inhibits the reaction in a low temperature range. Here, we report bulk W-substituted vanadium oxide catalysts for NH3-SCR at a low temperature (100–150 °C) and in the presence of water (~20 vol%). The 3.5 mol% W-substituted vanadium oxide shows >99% (dry) and ~93% (wet, 5–20 vol% water) NO conversion at 150 °C (250 ppm NO, 250 ppm NH3, 4% O2, SV = 40000 mL h−1 gcat−1). Lewis acid sites of W-substituted vanadium oxide are converted to Brønsted acid sites under a wet condition while the distribution of Brønsted and Lewis acid sites does not change without tungsten. NH4+ species adsorbed on Brønsted acid sites react with NO accompanied by the reduction of V5+ sites at 150 °C. The high redox ability and reactivity of Brønsted acid sites are observed for bulk W-substituted vanadium oxide at a low temperature in the presence of water, and thus the catalytic cycle is less affected by water vapor.

Mechanistic insight in NO trapping on Pd/Chabazite systems for the low-temperature NOx removal from Diesel exhausts

We have applied transient response methods to study Pd-CHA systems as Passive NOx Adsorbers (PNA) for the low-temperature NOx removal from Diesel exhausts. NO adsorption/TPD experiments were carried out at different temperatures in gaseous mixtures containing the main exhausts components (O2, CO, H2O) with the aim to clarify the mechanism determining the capability of Pd-CHA to trap and desorb NO. PdI was identified as the main NO storage site, both in mixtures containing a strong reductant like CO and in the presence of O2 only. When the oxidizing capability of the environment was enhanced, the Pd utilization, i.e. NO stored/Pdtot, dropped significantly: this loss is attributed to the competition between the NO storage on the reduced Pd sites and their re-oxidation. Pd oxidation played an important role also in the desorption phase: TPD experiments performed in gaseous mixtures with greater oxidizing strength enabled NO desorption at lower temperatures. The present data emphasize therefore the impact of the Pd redox state on the NO storage/release performances of Pd-CHA based PNAs.

Co-blending modification of activated coke using pyrolusite and titanium ore for low-temperature NOx removal

Activated coke (AC) has great potential in the field of low-temperature NO removal (DeNOx), especially the branch prepared by blending modification. In this study, the AC-based pyrolusite and/or titanium ore blended catalysts were prepared and applied for DeNOx. The results show blending pyrolusite and titanium ore promoted the catalytic performance of AC (Px@AC, Tix@AC) clearly, and the co-blending of two of them showed a synergistic effect. The (P/Ti-1/2)15@AC performed the highest NO conversion of 66.4%, improved 16.9% and 16.0% respectively compared with P15@AC and Ti15@AC. For the (P/Ti-1/2)15@AC DeNOx, its relative better porous structure (SBET = 364 m2/g, Vmic = 0.156 cm3/g) makes better mass transfer and more active sites exposure, stronger surface acidity (C–O, 19.43%; C=O, 4.16%) is more favorable to the NH3 adsorption, and Ti, Mn and Fe formed bridge structure fasted the lactic oxygen recovery and electron transfer. The DeNOx of (P/Ti-1/2)15@AC followed both the E–R and L–H mechanism, both the gaseous and adsorbed NO reacted with the activated NH3 due to the active sites provided by both the carbon and titanium.

Enhancement effects of [rad]O2− and [rad]OH radicals on NOX removal in the presence of SO2 by using an O3/H2O2 AOP system with inadequate O3 (O3/NO molar ratio = 0.5)

The O3/H2O2 advanced oxidation process (AOP) was, for the first time, utilized for low-temperature NOX removal. The present method has a high NOX removal, which is 58% higher than the sum of that for single O3 and H2O2 method with O3/NO molar ratio of 0.5. Besides, much O3 consumption, together with operating costs will be saved. The improved activity in O3/H2O2 is due to the increased generation amount of OH and O2−/HO2 radicals than the consumption amount of O3 when the molar ratio of O3/NO < 1, and the O2−/HO2 radical can selectively oxidize NO into HNO3 with a fast rate constant in a single step, while OH can non-selectively oxidize NO and NO2 also at a fast rate constant. The SO2 in the flue gas even poses a positive effect on the NOX removal. Absorption by NaOH solution is generally followed by the O3/H2O2 method, resulting in the final 95% NOX removal with the O3/NO molar ratio of 0.5. Only NO3− ions can be detected as the liquid production through ion chromatography for the O3/H2O2 method. We expect that this study can shed some lights on the route design for simultaneous NOX and SO2 removal.

On the hydrothermal stability of Cu/SSZ-13 SCR catalysts

Cu/SSZ-13 SCR catalysts have been extensively studied in the past decade or so. Hydrothermal stability of these catalysts has been identified as the most important criterion for application. In this perspective, we describe recent atomic-level understanding of their hydrothermal stability. In particular, electron paramagnetic resonance (EPR) is shown to rather accurately quantify isolated Cu(II) ions and CuO clusters in fresh and aged catalysts to demonstrate the remarkable hydrothermal stability for Cu2+ ions located in 6-membered ring windows, and the conversion of [Cu(OH)]+ ions in Chabazite cages to CuO clusters. The hydrothermal stability difference of the two isolated Cu(II) ions is confirmed with DFT simulations and the conversion of [Cu(OH)]+ to CuO is proposed to involve formation, migration and condensation of Cu(OH)2 intermediates. The structural destructive role of CuO clusters is attributed to mesopore formation from their migration, which more severely damages the catalysts than dealumination. Finally, perspectives are given on new strategies for low-temperature NOx removal, rational design and refinement of Cu/SSZ-13, and development of new Cu/zeolite SCR catalysts with even better performance than the state-of-the-art Cu/SSZ-13.

Catalytic ozonation for low-temperature NOX (x = 1, 2) removal with [rad]OH radicals over Cu doped Ce0.90Co0.10O2 − δ catalysts and mechanism analysis

A series of Ce0.90Co0.10O2−δ and Ce0.90−xCuxCo0.10O2−δ (x=0.03, 0.07 and 0.10) catalysts synthesized by an alkaline hydrothermal method were utilized as ozonation catalysts for the NOX (x=1, 2) removal at low temperatures. A novel catalyst-duct separation apparatus for denitrification by catalytic ozonation was developed by our group. Ce0.83Cu0.07Co0.10O2−δ exhibits the highest catalytic activity (91.5% removal at 120°C), whereas Ce0.80Cu0.10Co0.10O2−δ presents the lowest (74.1% removal at 120°C). Only NO3− is detected in the tail solutions. The catalytic performance presents a positive relationship with the corresponding OH concentration. OH radical can be “transferred” from catalyst surface to the duct, prolonging activation time through its own reproduction reaction. The surface –OH activation, not the surface –OH density determines the OH concentration in the present method. Ce0.83Cu0.07Co0.10O2−δ contains a large number of high-activation bridging –OH, whereas the bridging –OH is absent in Ce0.80Cu0.10Co0.10O2−δ. Therefore, Ce0.83Cu0.07Co0.10O2−δ shows much higher activation for promoting the formation of OH than Ce0.80Cu0.10Co0.10O2−δ.

Low-Temperature NOX (x = 1, 2) Removal with •OH Radicals from Catalytic Ozonation over α-FeOOH

ABSTRACTFeOOH(H), FeOOH(P) and FeOOH(O) prepared by hydrothermal, precipitant-hydrolysis and oxidation-hydrolysis methods were tested as ozonation catalysts for the low-temperature NOX (x = 1, 2) removal. FeOOH(H) exhibits higher catalytic activities than FeOOH(P) and FeOOH(O), achieving 85.6% of NOX removal efficiency with a low ozone concentration. Compared to FeOOH(P) and FeOOH(O), FeOOH(H) shows much higher BET surface areas and higher density of surface -OH, both of which are critical for the •OH radical generation over the catalysts. These radicals can be successfully transferred into the duct under the coupling effect of hydroxyl radicals and O3, oxidizing and removing the NOX (x = 1, 2). Results of ion chromatography (IC) indicate that the oxidation products are all NO3− without any NO2− in the tail solutions.

Effect of fluoride doping for catalytic ozonation of low-temperature denitrification over cerium–titanium catalysts

Nanostructure cerium–titanium catalysts including fluoride (F−) doped ones were prepared by impregnation (Ce/TiO2 and Ce/F–TiO2) and co-precipitation (CeTi and CeTiF) method, and tested as ozonation catalysts for low-temperature NOX removal ranging from 40 to 180 °C. Interestingly, the doping of F− promotes the NOX removal for CeTi whereas inhibits the removal for Ce/TiO2. The NOX removal presents linear relationship with the concentration of hydroxyl radicals (OH). The surface –OH linked to Ce atoms as well as the H2O adsorption over Ce atoms play the important role in the generation of ·OH radicals. The doping of F− into CeTi improves the activation of Ce–OH linkage bonds and the adsorption of H2O over Ce atoms, thus increasing the concentration of OH radicals and catalytic activities. Although the doping of F− into Ce/TiO2 enhances the density of surface –OH, the ones linked to Ce atoms remain constant and the H2O adsorption over Ce atoms are declined, finally declining the catalytic activities.

Structural characterizations of fluoride doped CeTi nanoparticles and its differently promotional mechanisms on ozonation for low-temperature removal of NOx (x = 1, 2)

Nanostructured cerium and titanium mixed oxides (CeTi) as well as fluoride (F−) doped one (CeTiF) prepared by coprecipitation method were tested as ozonation catalysts for the low-temperature NOx (x=1, 2) removal. The CeTiF exhibits higher catalytic activities than CeTi, which are found to present linear relationship with the OH radical concentration. The F− is for the first time to be doped into CeTi as the ozonation catalyst. The doping of F− does not increase the density of surface –OH or surface areas, whereas it lowers the binding energies of surface –OH, promoting its reaction activities and thus the production of OH radicals. More importantly, the F− mainly substitutes the O2− in Ce–O–Ti linkage bonds (Ce–F−–Ti formation) rather than Ti–O–Ti or Ce–O–Ce, producing oxygen vacancies in the neighboring Ce–O–Ti. These vacancies benefit the adsorption of sufficient H2O to yield active surface –OH in the reaction process, promoting the adsorption of O3 and thus the production of OH radicals. The isotopic method is utilized to elaborate the catalytic mechanisms. The results show that the adsorption of H2O on oxygen vacancies plays a more critical role in the OH production, thus NOx (x=1, 2) removal.

NOx conversion properties of a novel material: Iron nanoparticles stabilized in carbon

Nitrogen oxides (NOx) belong to the most common pollutants from combustion processes and are a major threat to human health. Carbon-based catalysts exhibit strong advantages for NOx removal like low-toxic application and easy handling. However, gasification of the carbon matrix at elevated temperatures is still one of the greatest concerns. Hence, we have directed our focus on especially low temperature NOx-removal using a novel material, iron nanoparticles stabilized in a carbon matrix (nano-Fe/C). The investigations included NO2 uptake properties and catalytic conversion of NO2 in recycle flow at 425K and 328K, scanning transmission electron microscopy and 77K-N2-adsorption.Nano-Fe/C exhibits superior NOx-removal properties compared with untreated or iron-infiltrated activated carbon or magnetite reference catalysts. No severe catalyst deactivation or catalyst aging at 425K is observed. Even at 328K nano-Fe/C still exhibits NO2-conversion, although without converting the product NO. NO2 adsorption at 297K is suggested to occur in three stages with different kinetics: (1) NO2 adsorption and reduction to NO, (2) physisorption on the oxidized catalyst surface and (3) saturation of the catalyst and diffusion into the substrate matrix. At 425K, NO2 is quickly reduced to NO and the resulting NO is further converted to N2O. After complete consumption of NO, the residual NO2 is also converted to N2O. A possible reaction mechanism is suggested based on the conversion kinetics.

Effect of Ozone Addition to Lean NOx Trap Method Using Plasma-Catalyst System

Although diesel vehicles have high energy efficiency, the removal of emitted NOx at low temperature persists as an important issue. This report presents an examination of low-temperature NOx removal using ozone injection and a plasma-lean NOx trap (LNT) catalyst system. Ozone is readily producible by a dielectric barrier discharge reactor using oxygen in air, and its injection to emitted NO enables oxidation to NO2 even at low temperatures. Results show that synergetic effects of ozone injection and plasma-LNT catalyst system enable low-temperature NOx removal.

Toward Improved Catalytic Low‐Temperature NOx Removal in Diesel‐Powered Vehicles

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