Low-temperature de-NOx Performance Research Articles

Metals doping effect of perovskite–type La-Mn oxides for NH3-SCR reaction at low temperature

Doping of perovskites with the heteroatoms has emerged as an additional lever, a dimension beyond structural perfection and compositional distinction, for the alteration of many properties of perovskites. In this work, a series of different metals were doped via citric acid sol–gel method to investigate their influences on the structure and catalytic performance of La-Mn perovskite oxides for NH3-SCR reaction. It is found that Zr doped La-Mn oxides (LMZr) displays better low-temperature deNOx performance (T90 = 118 °C) than other metal doped catalysts. Such a result is mainly attributed to the increase of Lewis acid sites, surface Mn4+ and adsorbed oxygen species. Besides, La-Mn perovskite doped with Sr, Ce, Y and Al can also obviously improve low-temperature deNOx activity and N2 selectivity. These doped metals mainly change the electron transfer between different cations, thus changing the valence distribution of Mn on the surface, meanwhile, metal doping changes the crystalline size of the catalysts, thereby affecting the adsorption and activation of NH3 and O2 involved in the removal of NO. Although other metal doping alters the tolerance of the La-Mn catalyst to SO2 and/or H2O, SO2 is inevitably oxidized to S6+ and deposits on the catalyst surface. Zr or Ce doping significantly enhances the tolerance to SO2 and/or H2O, but they have a different mechanism. Zr reduces the deposition rate of sulfate while Ce reacts with SO2 to form Ce2(SOx)3 to protect Mn active species. This paper highlights the importance of Lewis acid, Mn4+ content and adsorbed oxygen in the NH3-SCR reaction.

Significant Effects of Adding Mode on Low-Temperature De-NOx Performance and SO2 Resistance of a MnCeTiOx Catalyst Prepared by the Co-Precipitation Method

Significant Effects of Adding Mode on Low-Temperature De-NOx Performance and SO2 Resistance of a MnCeTiOx Catalyst Prepared by the Co-Precipitation Method

Low-temperature deNOx performance and mechanism: a novel FeVO4/CeO2 catalyst for iron ore sintering flue gas

Low-temperature deNOx performance and mechanism: a novel FeVO4/CeO2 catalyst for iron ore sintering flue gas

Structural properties and low-temperature NH3-SCR activity of CeO2-MnOx mixed oxides catalyst in the microwave field

The NH3-SCR technology, based on CeO2-MnOx catalysts, is of great significance for NOx removal at low temperatures. Microwave irradiation, as an effective heating approach, can enhance the low-temperature NH3-SCR activity of catalysts. Herein, the structure-activity relationship of CeO2-MnOx oxides catalysts was explored under microwave irradiation. The MnOx catalyst and Ce-Mn(1:1) mixed oxide catalyst exhibited good low-temperature de-NOx performance in the microwave field. With the increase of microwave irradiation intensity and time, the structure and phase composition of MnOx material underwent significant changes. CeO2 played a key role in maintaining the structural stability of the Ce-Mn(1:1) catalyst. In addition, microwave irradiation affected the redox properties and acidity of the catalyst, regulating the catalytic performance. Suitable microwave operation conditions were conducive to increasing the number of acidic sites and enhancing the redox properties, which improved the low-temperature catalytic activity of the catalyst.

Collaborative optimization of de-NOx & NH4HSO4 decomposition over Cu-based LDO catalysts: Perspective from the acidic and redox properties

Low temperature de-NOx catalyst with excellent resistance to ammonium bisulfate (ABS) is of great practical significance. Study about the simultaneous optimization of de-NOx performance and ABS resistance by acidic and redox properties regulation is imperative. In this work, the influence of the acidic and redox properties to the de-NOx & ABS decomposition performance was investigated via three Cu-based LDO model catalysts with distinct surface chemical character. Test results indicated that the low temperature de-NOx performance of both the modified Cu2NiAl-LDO and Cu2CoAl-LDO catalysts was slightly improved, while their in situ ABS resistance behavior was remarkably different (Cu2CoAl-LDO > Cu3Al-LDO > Cu2NiAl-LDO). TPDC and TPSR test evidenced that the ABS thermal decomposition or its reactivity with NOx was facilitated over Cu2CoAl-LDO catalyst, but inhibited over Cu2NiAl-LDO catalyst. Characterizations revealed the Cu2CoAl-LDO catalyst with stronger redox property could contribute to ABS decomposition, while more weak acid sites on Cu2NiAl-LDO catalyst induced a negative effect for ABS decomposition. Our study might provide guidance on the rational design of de-NOx & anti-ABS catalysts for future studies.

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.

Insight into the low-temperature DeNOx performance of copper-based oxides: Perspective from the valence state distribution

Understanding the effect of CuOx valence state specifically is significant for constructing high-performance Cu based oxide catalyst for low-temperature selective catalytic reduction of NO with NH3 (NH3-SCR). In this work, CuOx with different valence state including Cu2O, Cu2O-CuO and CuO particles are synthetized as model catalysts, which present same variation laws in catalytic performance at 180 ℃, 210 ℃ and 240 ℃. NH3-SCR catalytic performance of Cu2O-CuO heterojunction is the best, CuO exhibits good catalytic activity, while Cu2O possesses excellent N2 selectivity. Experimental results combined with density functional theory (DFT) calculations reveal that Cu2+ favor to form monodentate nitrate and possess strong ability of activation and dehydrogenation of NH3. The energy barrier for the form of NH2* is 79.5 kJ/mol, which is much lower than that for Cu+. While Cu+ is beneficial to NH3 and NOx adsorption and the form of bidentate nitrate, which have a positive impact on improving the selectivity of N2. Moreover, the collaboration of Cu2+ and Cu+ facilitates the redox capability and the form of surface adsorbed oxygen species, thus the reaction progress is further promoted. The SCR reaction catalyzed by three samples are dominated mainly by Langmuir-Hinshelwood (L-H) mechanism at 180 ℃. While at 240 ℃, the reaction over Cu2O follows Eley-Rideal (E-R) mechanism, during which the rate-limiting step is the internal hydrogen transfer procedure (NH2NO*→NHNOH*). Over CuO, the reaction follows L-H mechanism, the process of atomic isomerism (NHNOH*+ *→N2*+H2O*) is the rate-determining step. The energy barriers are 136 and 146 kJ/mol, respectively. This work may supply important theoretical support for the preparation of high-performance Cu-based oxide catalysts.

Role of Ce in promoting low-temperature performance and hydrothermal stability of Ce/Cu-SSZ-13 in the selective catalytic reduction of NOx with NH3

With the increase of car ownership, new challenges are posed to the treatment of mobile source exhaust gas, especially denitration (de-NOx). It is urgent to develop highly efficient de-NOx catalyst with a wide working temperature window. In this work, a series of Ce modified Cu-SSZ-13 molecular sieves (Ce/Cu-SSZ-13) are prepared by ion exchange method with different exchange times (x = 1, 2, 4, 8 h), and the role of Ce in promoting low-temperature de-NOx performance and hydrothermal stability of Ce/Cu-SSZ-13 in the selective catalytic reduction (SCR) of NOx are investigated. Both the low-temperature de-NOx performance and the hydrothermal stability of Cu-SSZ-13 are enhanced due to the modification of Ce, especially the Ce/Cu-SSZ-13 with an exchange time of 2 h. Both Ce/Cu-SSZ-13 and Cu-SSZ-13 present more than 95 % NOx conversion in the temperature region of 210–600 °C, and Ce modification significantly improves the NOx conversion in the low-temperature range (120–210 °C). It is revealed that the acidity, the surface Cu2+/Cu+ ratio and the NH3 adsorption amount on Cu-SSZ-13 increase after Ce ion exchange, and the active component Cu2+ migrates to the more stable six-membered ring due to the modification of Ce, all of which have promote the improvement of NH3-SCR de-NOx performance. Furthermore, in the process of hydrothermal aging, the presence of Ce protects the framework of Cu-SSZ-13, acidic sites and active component Cu2+, which greatly improves the hydrothermal stability of Cu-SSZ-13 molecular sieve. In addition, in-situ DRIFTS shows that the developed Ce/Cu-SSZ-13 follows dual mechanisms of Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) to undergo NH3-SCR reaction. This work provides a new idea for improving the low-temperature de-NOx performance and hydrothermal stability of Cu-SSZ-13.

Low-temperature DeNOx characteristics and mechanism of the Fe-doped modified CeMn selective catalytic reduction catalyst

Low-temperature deNOx selective catalytic reduction (SCR) catalysts are of great significance to prolong catalyst life and reduce flue gas temperature. In this work, the low-temperature deNOx performance of different zFeCexMny catalysts was investigated on a simulated flue gas fixed-bed experimental bench. BET, XRD, XPS, NH3-TPD, and in situ FT-IR were used to characterize the physical-chemical properties of the samples. The optimum ratio and deNOx mechanism of the catalysts were explored. The results show 4FeMn7Ce3 doped with 4 wt% Fe based on Mn7Ce3 has the best deNOx performance. Its deNOx efficiency is over 90% at 120–220 °C and GHSV = 50,000 h−1, with the peak value of 98% at 150 °C. The Fe doping makes the oxide distributed uniformly on the catalyst's surface, promotes the valence cycle of the catalyst, yields more Mn4+ and Ce3+ species, and prompts the optimal deNOx temperature to the low temperature direction. Meanwhile, Lewis acid sites on the catalyst's surface are enhanced, which can promote the amide species formation that is beneficial for the deNOx reaction. Combined with in situ FTIR, the E-R mechanism and the L-H mechanism coexist in the deNOx process of zFeCexMny catalysts. These findings are helpful for the development of high-performance low-temperature deNOx catalysts.

Bimetallic modification of MnFeOx nanobelts with Nb and Nd for enhanced low-temperature de-NOx performance and SO2 tolerance

Nitrogen oxide (NOx) is one of the key factors contributing to air pollution, and selective catalytic reduction with ammonia (NH3-SCR) is widely used for denitrification (de-NOx). To effectively deal with the NOx pollution, it is urgent to develop efficient catalyst working at below 300 °C with a wide operating temperature window. Herein, a series of ferromanganese oxides (MnFeOx) nanobelts are bimetallically modified with niobium (Nb) and neodymium (Nd) for the first time via electrospinning method. The resultant MnFeNbNdOx catalysts present improved low-temperature de-NOx performance and sulfur dioxide (SO2) tolerance, particularly MnFeNb0.2Nd0.1Ox who achieves a NOx conversion up to over 90 % during the range of 120–330 °C and an enhanced tolerance of water (H2O) and SO2. Experimental and theoretical calculations confirm that strong adsorption of nitric oxide (NO) and ammonia (NH3) and weak adsorption of SO2 with the synergy of Nd, Nb and Mn enable MnFeNb0.2Nd0.1Ox excellent SCR activity and good SO2 tolerance. In-situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) further reveal that the de-NOx reaction over MnFeNb0.2Nd0.1Ox follows both Eley-Rideal (E-R) and Langmuir-Hinshel-wood (L-H) mechanisms. This study will inspire the further design and study of the synergy of transition metals and rare earth elements for de-NOx.

Boosting low temperature De-NOx performance and SO2 resistance over Ce-doped two dimensional Mn-Cr layered double oxide catalyst

In this paper, Mn-Cr mixed oxides with different structures are prepared by a co-precipitation method and used as active catalysts for low-temperature NH3 selective catalytic reduction (SCR) of NO. The Mn-Cr layered structure catalyst shows the highest activity and SO2 resistance in SCR. Furthermore, a Ce doping process on the catalyst further improves the NH3-SCR activity with SO2 resistance. The features of unique texture property, acidity, reducibility, and Mn-Cr redox cycle in the derived Mn-Cr layered structure catalyst should contribute to the improved catalytic activity for the SCR. The SO2 resistance of the catalyst is also improved by suppressing manganese sulfate formation. The improved catalytic activity and SO2 resistance due to the reducibility and sulfur resistance of Ce suggest that the Ce-doped Mn-Cr layered structure catalyst could be used as an effective catalyst for NH3-SCR.

Low temperature SCR de-NOx performance and mechanism of heteropolyacid VPO-Cr-PEG/TiO2 catalyst

In this paper, a heteropolyacid catalyst VPO-Cr-PEG/TiO2 was developed and the low temperature de-NOx performance of catalyst was studied. The experimental results showed that the de-NOx efficiency reached over 98% at the low temperature of 150–350 °C. The catalyst exhibited excellent sulfur and water resistance in the presence of 1000 ppm SO2 and 8 vol% H2O. In the presence of polyethylene glycol (PEG), the crystal surface energy of the catalyst decreased, and (VO)2P2O7 was the main active crystal phase of the catalyst. The selective catalytic reduction of NOx by NH3 over VP(1/5)O-Cr-PEG/TiO2 catalyst simultaneously followed the Langmuir-Hinshelwood and Eley-Rideal mechanisms.

Ta and Mo oxides supported on CeO2-TiO2 for the selective catalytic reduction of NOx with NH3 at low temperature

The SCR of NOx with NH3 is the most efficient technology to reduce the emission of nitrogen oxides from mobile and stationary sources. Ta and Mo catalysts supported on ceria-titania (molar ratio 40:100) were prepared via sol-gel (SG) and wet impregnation (WI) methods and tested for low-temperature NH3-SCR. The preparation methods led to bulk or core-shell catalysts and the effect of these features on performance was investigated in detail. The catalysts were systematically characterized using methods such as ICP-OES, N2 physisorption (BET), XRD, H2-TPR, NH3-TPD, XPS, Raman, in situ EPR, and in situ DRIFT spectroscopy. Among all the catalysts, Mo5/Ce40/Ti100 (SG) achieved the highest NOx conversion (80-95%) and N2 selectivity (>97%) over a wide temperature range of 175-400 °C. This material outperforms other reported Ce-Ti-based catalysts especially at low temperature. Highly dispersed active species, large BET surface area, high fractions of Ce3+ and enough chemisorbed oxygen species, Lewis acid sites, and good reducibility of the catalyst contribute to the excellent low-temperature DeNOx performance of this catalyst. The effect of H2O and SO2 in feed on the activity was investigated.

Insights into Reaction Conditions for Selective Catalytic Reduction of NOx with a MnAl Oxide Catalyst at Low Temperatures

ABSTRACT MnOx catalysts have been proven to have superior activity at low temperatures (LT) for selective catalytic reduction (SCR) of NOx. In this study, spherical mesoporous MnAlx catalysts with different molar ratios were prepared using a simple precipitation method. In addition, their NOx catalytic performance, along with relevant main factors (reaction temperature, molar ratio, particle size, space velocity, and O2 content), was investigated in a simulated fixed bed reactor. The relationship between the main factors and DeNOx efficiency was evaluated by analyzing two parameters: the NOx conversion rate and N2 selectivity. The results showed that the MnAl0.1 catalyst had the optimal low temperature DeNOx performance and had a relatively stable NOx conversion rate of more than 92% in a space velocity range of 14331 h–1 to 23885 h–1, where smaller catalyst particle size led to higher NOx conversion rate and N2 selectivity as high as 100%. In addition, the appropriate oxygen concentration was found to increase NOx conversion and N2 selectivity. This work would be beneficial to further optimization of LT SCR catalyst systems for practical applications.

NiMn mixed oxides with enhanced low-temperature deNOx performance: Insight into the coordinated decoration of MnOx by NiO phase via glycine combustion method

Herein, a facile glycine combustion method was utilized to prepare a series of Ni and Mn based single/double metal oxides, which were evaluated as catalysts for low temperature selective catalytic reduction of NO with NH3. The NiMn-T samples presented superior catalytic performance especially for NiMn-400, with ∼100 % NOx conversion, >85 % N2 selectivity within 90-300 °C, and better SO2 resistance. The superior catalytic activity might be related to the coordination of Ni and Mn, which afforded higher Mn4+/Mnn+ ratio, larger SBET, more suitable acid site amounts and redox capacity. The improved SO2 resistance of NiMn-400 catalyst can be ascribed to the less ammonium (bi)sulfate deposition and metal sulfation. In-situ DRIFTS revealed that the Ni doping could deliver more reactive species (NH2, monodentate nitrite, bidentate nitrate), and the surface acidity is less affected by SO2, which can account for the enhanced low temperature activity and SO2 resistance of the NiMn-400 catalyst.

Influencing factors on low-temperature deNOx performance of foamed metal based catalytic filter

A series of supported vanadium-titanium denitrification catalysts were prepared by impregnation method with foam metal as carrier for NH3-SCR of NOx at low temperature. Effects of pH, slurry concentration, catalyst loading rate, face velocity and O2 concentration on catalytic performance were studied. The results showed that when immersion concentration was 30g/100mL and filtration speed was 0.7m/min, the pressure drop was 300Pa and NOx removal efficiency was 98% at 200°C. The foam catalytic filter was put into the bag dust collector, which had the function of simultaneous removing particulate and NOx. When filtration velocity was 0.7m/min, NOx removal efficiency reached 95.5% at 200°C and it had good water resistance. The results provided the basis for further pilot experiment.

The insight into the role of CeO2 in improving low-temperature catalytic performance and SO2 tolerance of MnCoCeOx microflowers for the NH3-SCR of NOx

A new MnCoCeOx microflower is rationally designed by a series of elaborate steps for the selective catalytic reduction (SCR) of NOx with NH3. The MnCoOx microflower is firstly synthesized by the self-assembly of Mn-Co mixed oxide nanosheets, and then CeO2 nanoparticles are in-situ grown on the surface of Mn-Co mixed oxide nanosheets by dipping MnCoOx microflower in Ce(NO3)3 solution followed by a heat-treatment. The resultant MnCoCeOx microflower presents significantly enhanced low-temperature catalytic performance and SO2 tolerance. It is revealed that the attached CeO2 plays several important roles in the improvement of low-temperature de-NOx performance, such as decreasing the apparent activation energy, increasing the ratios of Ce3+/Cen+, Mn4+/Mnn+ and Oα/(Oα + Oβ), enhancing the oxidation ability of MnCoOx at low temperatures. Moreover, by preventing MnCoOx from being vulcanized into metal sulfate species, CeO2 plays an important role in enhancing the resistance to SO2 poisoning. The in-situ DRIFTS results disclose that the NH3 coordinated on Lewis acid sites, NH4+ bound to Brønsted acid sites, the NO2 and bidentate nitrates linked on metal oxides are the major reactive species on MnCoCeOx catalyst, which occur SCR de-NOx reaction following both Eley-Rideal and Langmuir-Hinshelwood mechanisms.

Two-dimensional MnFeCo layered double oxide as catalyst for enhanced selective catalytic reduction of NOx with NH3 at low temperature (25–150 °C)

Low temperature catalysts for selective catalytic reduction of NOx with NH3 plays an important role in flue gas treatments. Here, a two-dimensional MnFeCo layered double oxide were synthetized, characterized and employed as NH3 selective catalytic reduction catalyst, showing excellent low temperature de-NOx performance. MnFeCo layered double oxide exhibited a specific surface area of 92 m2/g with average pore size and volume of 2 nm and 1.5 cm3/g. At 100 °C, the MnFeCo layered double oxide performed an efficient NO conversion of 100 %. Even at 25 and 50 °C, NO conversions reached values of 49 and 86 %, respectively. Furthermore, the NO conversion reached an efficiency of 89 % in the presence of H2O vapor (5 %) and SO2 (100 ppm). Based on that, it can be said that this NH3 selective catalytic reduction catalyst shows great potential in NOx denitration of stationary industrial installations at a low temperature range.

The deposition of VWOx on the CuCeOy microflower for the selective catalytic reduction of NOx with NH3 at low temperatures

NOx emissions are a major environmental problem, and the selective catalytic reduction (SCR) is the most effective method to convert NOx in flue gas into harmless N2 and H2O. In this work, a new carrier, CuCeOy microflower assembled from a large number of copper-cerium mixed oxide nanosheets, is firstly developed to load vanadium-tungsten mixed oxides (VWOx) for the SCR of NOx with NH3. The resultant optimal VWOx/CuCeOy catalyst exhibits significantly enhanced low-temperature de-NOx performance with the NOx conversion of 60% at 180 °C, over 90% from 240 °C to 390 °C under the gas hourly space velocity (GHSV) of 36,000 h−1. The reason can be mainly attributed the fact that the transfer of electrons among Ce, Cu and V ions is very easy to occur via the following equations Ce3++Cu2+ ↔ Ce4++Cu+, V5+ + Cu+ ↔ V4+ + Cu2+, V4+ + Ce4+ ↔ V5+ + Ce3+, which effectively decreases the apparent activation energy (Ea = 16.59 kJ/mol) of NH3-SCR de-NOx reaction. In addition, the enhanced reducibility and a large number of Brønsted acid sites also contribute the low-temperature de-NOx performance. Both Eley-Rideal and Langmuir-Hinshelwood mechanisms are included in the NH3-SCR de-NOx reaction over the VWOx/CuCeOy catalyst.

Comprehensive understanding the promoting effect of Dy-doping on MnFeOx nanowires for the low-temperature NH3-SCR of NOx: An experimental and theoretical study

Recently, Mn-based catalysts doped/modified by rare earth elements for the selective catalytic reduction (SCR) of NOx with NH3 are becoming a new research hotspot. In current work, Dy-doped MnFe oxides with the morphology of one-dimensional nanowire are successfully prepared by using electrospinning method for the first time, and the catalytic performances for the NH3-SCR of NOx are investigated in detail. It is found several positive factors are introduced due to the doping of Dy with an appropriate amount, such as the enhanced surface Mn4+ concentration and surface chemical adsorption of oxygen, the increased adsorption capacity for NH3 and NO. Theoretical calculation reveals that the Dy-doping causes the larger depletion of electron density of mainly O atoms on the surface of MnFeDy than that of MnFe case, which energetically favors the adsorption of NH3 and NO molecules. Our results fully confirm that Dy is an effective doped element of Mn-based catalysts for developing SCR catalysts with excellent low-temperature de-NOx performance.