Low-temperature Selective Catalytic Reduction Research Articles

Wood-inspired high-performing hierarchical porous Ce0·7Zr0·3O2 catalyst for low-temperature selective catalytic reduction of NOx by NH3

Inspired by natural wood, novel Ce0·7Zr0·3O2 catalyst with hierarchical porous structure, W–CeZr, was fabricated using wood template method by utilizing catheter pores and pits in poplar. W–CeZr catalyst was then used for selective catalytic reduction of NO with NH3 for the first time. Compared with conventional Ce0·7Zr0·3O2 catalyst (C–CeZr), W–CeZr catalyst mimicked biological microstructure of wood and exhibited much greater low-temperature activity than C–CeZr, i.e., NO conversion for W–CeZr was 100% in the range of 250–350 °C, while that for C–CeZr was only 58.1% at 250 °C. Based on a series of characterization techniques, this effect can be attributed to an increase in surface area (m2·g−1), enrichment of Ce3+, and enrichment of surface-active oxygen species by 133.7%, 60%, and 59.5% compared to C–CeZr, respectively. In situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy results show that more activated reactants were present on the surface of W–CeZr because of its stronger surface acidity, as well as redox and oxygen storage abilities.

Facile modifications of HKUST-1 by V, Nb and Mn for low-temperature selective catalytic reduction of nitrogen oxides by NH3

HKUST-1 catalysts were impregnated with vanadium, niobium and manganese species and tested in NH3-SCR for NO removal. No reduction of NO by NH3 was registered for the unmodified HKUST-1 and disappearance of NO was attributed to adsorption. After a pre-treatment at 185 °C, the best catalytic performance was found for HKUST-1-Mn with 100% and 80% conversion of NH3 and NO, respectively at 185 °C. For the first time, the H2O tolerance was examined on the commercial HKUST-1 promoted with Mn. The addition of water vapor resulted in decrease of NO and NH3 conversions, which was immediately recovered when H2O feeding was stopped. The NO conversion dropped from 74% to 68% after 35 min of H2O addition.

Primary factors affecting denitrification efficiency of V-based catalysts in low-temperature selective catalytic reduction using NH3

The denitrification efficiency of vanadium-based catalyst for low-temperature selective catalytic reduction (SCR) using NH3 (<200 °C) highly depends on the structural species of vanadium oxide (VOx) and the physicochemical properties of the catalyst. In this study, we investigated the main factors affecting the denitrification efficiency of low-temperature NH3-SCR by adjusting synthetic factors such as calcination temperature and vanadium to oxalic acid (OA) ratio during the V/TiO2 synthesis. The main factors for enhancing the activity of low-temperature NH3-SCR with vanadium-based catalysts were identified to be (1) a high proportion of well dispersed monomeric VOx species (57.3%); (2) a large number of NH3-Lewis acid sites; and (3) a high oxygen mobility (surface adsorbed oxygen). In particular, the surface adsorbed oxygen species (Oα) of the catalyst was identified as a reactive factor in low-temperature NH3-SCR, which enhances the oxygen mobility of V/TiO2 in terms of turnover frequency (TOF). The NH3-Lewis acid sites significantly contribute to promoting the activity of low-temperature NH3-SCR at 180 °C owing to the strong adsorption strength of NH3 at a slow reaction rate.

Solid-phase impregnation promotes Ce doping in TiO2 for boosted denitration of CeO2/TiO2 catalysts

CeO2/TiO2 (denoted as CeTi) catalysts obtained by solid-phase impregnation behaved better in low-temperature selective catalytic reduction of NOx with NH3 (NH3-SCR) than that by conventional wet impregnation. To explore the main factors for activity distinction, the texture property, CeO2 dispersion and structure changes of TiO2 were comprehensively analyzed. It was found that surface changes of TiO2 had a significant impact on the improved activity. From results of inductively coupled plasma atomic emission spectrometer (ICP-AES), diffuse reflectance UV–vis spectroscopy (UV–vis-DRS) and Raman, it was inferred that Ce ions were partially incorporated into TiO2 lattice, accompanied with the formation of defects and vacancies during solid-phase impregnation. Accordingly, CeTi catalysts from solid-phase impregnation exhibited superiority in adsorption and activation of reactants. Further result from monitoring the preparation process indicated that the evolved NO played an important role in promoting Ce doping through depriving oxygen atoms on TiO2 surface. The interaction between Ce and Ti was enhanced. The catalyst performed better in NH3-SCR, especially at low temperature, which testified the solid-phase impregnation could be an effective method to modulate interface structure for designing efficient catalyst.

Molybdenum oxide as an efficient promoter to enhance the NH3-SCR performance of CeO2-SiO2 catalyst for NOx removal

Selective catalytic reduction (SCR) of NOx with NH3 has been widely used for the removal of NOx. Because of the inevitable disadvantages of conventional V2O5-WO3(MoO3)/TiO2 catalysts such as poor low-temperature SCR activity and toxicity of vanadium, as well as the high-cost and over-strong NH3 adsorption of zeolite catalysts, the development of efficient and non-toxic metal oxide SCR catalysts is highly demanded. Previously, we have developed an environmentally-benign CeO2-SiO2 mixed-oxide SCR catalyst (CeSi2), which exhibited superior SO2 resistance ability. However, the low-temperature SCR activity on CeSi2 was not that satisfactory. In this work, we proposed a new strategy of Mo doping to improve the low-temperature SCR activity on CeSi2, which was very crucial for its practical application. By a simple co-precipitation method, a homogenous Mo-Ce-Si mixed-oxide catalyst was prepared. The Mo doping could significantly enhance the NH3-SCR activity on CeSi2 below 250 °C, and the optimal catalyst was Mo0.1CeSi2, which could achieve 80% NOx conversion at 200 °C. Mo0.1CeSi2 also exhibited superior N2 selectivity and resistance to SO2/H2O poisoning. Via a series of characterizations, it was found that the redox properties and surface acidity on CeSi2 by Mo doping, which accounted for the enhanced low-temperature NH3-SCR activity on Mo0.1CeSi2. The reaction mechanism on Mo0.1CeSi2 was also fully revealed by in situ DRIFTS experiments. This work provided a new insight for the development of efficient low-temperature SCR catalysts with superior SO2 resistance ability.

Effects of Sm modification on biochar supported Mn oxide catalysts for low-temperature NH3-SCR of NO

In this study, Samarium (Sm) modification over Mn oxide biochar (BC)-based catalysts was synthesized and the activity of low-temperature selective catalytic reduction with NH3 (LT NH3-SCR) of NO was measured. Varies in surface area and pore volume, structure, morphology, and surface chemistry of Sm-modified Mn/BC catalyst were investigated to analyze the effect of Sm on the physiochemical properties on catalyst. The Sm(0.2)-Mn/BC catalyst showed the highest NH3-SCR activity with the NO conversion of 85% at 200 °C and best N2 selectivity in a broad temperature range of 75–250 °C. Doping with Sm species, Mn4+ + Mn3+/Mnn+ and Oα/Oβ increased greatly, and the surface adsorbed oxygen were enhanced in Sm(0.2)-Mn/BC catalyst, which could promote the oxidation of NO, the NO conversion and N2 selectivity. In addition, weak and medium acid increased markedly after the Sm species added on catalysts, causing more active sites for SCR reaction. The NH3-SCR reaction of Sm(0.2)-Mn/BC catalyst followed the L-H mechanism, and the generation of intermediate –NH2, NH4+ species and the NxOy species were observed which played vital part in the LT NH3-SCR reaction.

A novel CNTs functionalized CeO2/CNTs–GAC catalyst with high NO conversion and SO2 tolerance for low temperature selective catalytic reduction of NO by NH3

Low−temperature selective catalytic reduction of NOx by NH3 (NH3−SCR) for diminishing SO2 poisoning remains an issue in flue gas denitrification (DeNOx). Herein, A novel CNTs functionalized low temperature NH3−SCR catalyst CeO2/CNTs−GAC was prepared, which showed high NO conversion activity (100% at 150 °C) and SO2 resistance. The addition of CNTs restrained SO2 adsorption but improved the selective adsorption of NO, which restricted the deposition of (NH4)2SO4 and/or Ce2(SO4)3, and resulted in high SO2 resistance. The addition of CNTs facilitated the diffusion and transportation of NH3 and NO, and the electron transfer on CeO2/CNTs−GAC, leading to higher content of Ce3+ and adsorbed O species on the CeO2/CNTs−GAC surface and promoted formation of surface-adsorbed oxygen OA. Therefore, CeO2/CNTs−GAC provided abundant NO adsorption and activation sites, facilitating “fast SCR” reaction and enhancing the NH3−SCR reaction. The proposed CeO2/CNTs−GAC catalyst exhibited higher NH3−SCR activity, N2 selectivity, catalytic durability and SO2 resistance than CeO2/GAC.

Sm-MnOx catalysts for low-temperature selective catalytic reduction of NOx with NH3: Effect of precipitation agent

A series of Sm–Mn mixed oxide catalysts were prepared via precipitation using various precipitants, namely Na2CO3 (NH4)2CO3, and NH3·H2O, and evaluated for the selective catalytic reduction (SCR) of NOx with NH3 at low temperatures. Various characterisation techniques were used to determine the physicochemical properties of the catalysts, and it is found that their catalytic performance is greatly influenced by the nature of the precipitation agent used. It is found that Sm0.1Mn–Na2CO3 and Sm0.1Mn-(NH4)2CO3 exhibit superior catalytic performance in the SCR reaction to that of Sm0.1Mn–NH3·H2O due to an abundance of surface acid sites, high surface concentration of Mn4+, and high NO oxidation capacity. From in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) analysis, we conclude that the Sm–Mn catalysts follow both Eley-Rideal and Langmuir–Hinshelwood mechanisms, and that the Eley-Rideal mechanism is dominant at elevated temperatures.

A Probe into the Low-Temperature SCR Activity: NO Oxidative Activation to Nitrite-Intermediates

A Probe into the Low-Temperature SCR Activity: NO Oxidative Activation to Nitrite-Intermediates

Low temperature SCR of NOx over Mn/Fe mixed oxides catalyst: comparison of synthesis methods

AbstractBACKGROUNDWith the increasing environmental pollution issues arising from NOx, the low‐temperature selective catalytic reduction (LT‐SCR) of NOx by NH3 is becoming more important. Mn/Fe mixed oxides type catalysts have been proven to be particularly effective at low temperature reactions; however, the synthesis method can be a key factor determining the SCR activities.RESULTSA comprehensive study of LT‐SCR was carried out on a series of Mn/Fe mixed oxides catalysts prepared using diverse synthesis methods, namely, sonication assisted continuous coprecipitation (SACP), equilibrium adsorption (EA), precipitation method using either NaOH (P‐OH) or Na2CO3 (P‐CO) as precipitant, and one pot hydrothermal synthesis (CA). The morphology, composition, crystal property, oxidation status, and surface property were systematically characterised. LT‐SCR of the NOx performance of prepared Mn/Fe catalysts was conducted in temperatures from 75 to 225 °C. It was demonstrated that, at reaction temperatures under 125 °C, LT‐SCR performance showed the trend of P‐OH &gt; SACP &gt; EA ~ CA &gt; P‐CO; meanwhile, the trend was CA &gt; SACP &gt; P‐OH ~ EA &gt; P‐CO at temperatures over 125 °C. However, N2 selectivity showed the trend SACP &gt; CA &gt; P‐OH overall.CONCLUSIONThe characterisation results suggested that NOx conversion performance is controlled by several parameters, especially uniformity of materials, high surface area, Mn crystallinity, surface oxygen defects, Fe (III) on the surface; these are important parameters to achieve high denitrification performance as shown by P‐OH and SACP under low temperature. At higher temperatures, the stability of materials and high Brønsted acid sites on the surface are important, leading to better performance by CA and SACP.

Improved NH3-SCR deNOx activity and tolerance to H2O & SO2 at low temperature over the NbmCu0.1-mCe0.9Ox catalysts: Role of acidity by niobium doping

A series of NbmCu0.1-mCe0.9Ox catalysts were synthesized with the help of citric acid method, and used for the selective catalytic reduction of NOx with NH3 (NH3-SCR). The catalytic performance of the NbmCu0.1-mCe0.9Ox samples was greatly improved after the introducing of niobium. Among all catalysts, the Nb0.06Cu0.04Ce0.9Ox catalyst revealed the best low-temperature SCR activity, higher N2 selectivity and extraordinary SO2 and H2O durability with 100% NO conversion at 150–390 °C. The various characterization techniques were carried out to investigate the effect of niobium doping on the physical structures, surface properties and NH3-SCR activity of catalysts. Results reveal that the oxidation ability decrease but the acidity is enhanced with the Nb addition. The improved acidity by niobium doping is considered the primary cause for the superior deNOx performance of the Nb0.06Cu0.04Ce0.9Ox catalyst at low temperature, which can offset the negative effects of the reduced oxidation ability on activity. Furthermore, in situ DRIFTS results were applied for the investigation of the reaction process in detail.

New CeO2–TiO2, WO3–TiO2 and WO3–CeO2–TiO2 mesoporous aerogel catalysts for the low temperature selective catalytic reduction of NO by NH3

New mesoporous and well-structured aerogel catalysts (CeO2–TiO2, WO3–TiO2 and WO3–CeO2–TiO2) were elaborated via the sol–gel method, characterized by means of various techniques (XRD; N2-Physisorption at 77 K; NH3-TPD; H2-TPR; DRUV–Vis spectroscopy) and evaluated in the selective catalytic reduction (SCR) of NO by NH3. The results reveal that all the aerogel catalysts develop essentially the diffraction peaks of TiO2 anatase phase and are classified as mesoporous materials with a high surface area (70 < SBET < 106 m2 g−1), large porosity (0.27 < VPT < 0.46 cm3 g−1) and nanometer size of crystallites (8–15 nm). The addition of Ce and/or W influences differently the structure, texture, crystallites size, surface oxygen concentration, total acidity and redox ability of aerogel samples and clearly affects their NO-SCR activity which follows this order: TiO2 < WO3–TiO2 < CeO2–TiO2 < WO3–CeO2–TiO2. It was also found that cerium species are more active in the low temperature NO-SCR reaction than tungsten ones (NO conversions obtained at 300 °C using CeO2–TiO2 and WO3–TiO2 were 75 and 0%, respectively). On the other hand, it was suggested that the interactions between Ce and W species play a key role in improving the reactivity of WO3–CeO2–TiO2 catalyst in the SCR of NO by NH3. Interestingly, the NO conversion into N2 reaches 85% at 300 °C and exceeds 90% between 320 and 400 °C over this novel meso-structured aerogel catalyst.

Support promotion effect on the SO2 and K+ co-poisoning resistance of MnO2/TiO2 for NH3-SCR of NO

Mn-based catalysts are expected to be applied for removing NOx due to its excellent low-temperature activity. However, the practical use of these catalysts is extremely restricted with the co-poisoning of alkali metal and SO2 in the flue gas. Here the MnO2/TiO2 catalyst was employed to elucidate the co-poisoning mechanisms of K and SO2 for the low temperature selective catalytic reduction (SCR) of NO. The physicochemical properties of catalysts under different toxicity conditions were studied by experiments. The adsorption of NH3, SO2, NO, and K on active component (MnO2) and support (TiO2) was studied by density functional theory. This work unravels a promotion effect of support on the alkali and sulfur resistance. The SO2&K co-poisoning catalyst had higher SCR activity than the SO2-poisoned and K-poisoned catalyst alone. For a single toxic condition: (1) SO2 was preferentially bonded with the terminated O site of MnO2 inhibiting the dehydrogenation of NH3 and redox cycle. (2) The presence of Lewis base (K atom) on the catalyst decreased the binding energy of a Lewis base (NH3) and hindered the adsorption of NH3. For the synergistic effect of K and SO2, the majority of K adsorbed on the support (TiO2) lead to increase alkalinity, which could promote the adsorption of SO2 on the TiO2 and reduce the toxicity of the active component (MnO2).

Preparation and properties of novel ceramic composites based on microencapsulated phase change materials (MEPCMs) with high thermal stability

Ceramic composites are widely used in medium/high temperature thermal energy storage (TES) and catalysis. Due to the high latent heat of phase change materials (PCMs), it is an effective method to improve the TES capacity by combining PCMs with ceramic materials. However, PCMs are easy to leak after being heated, so they need to be microencapsulated. Furthermore, for porous ceramic catalytic composites, the leakage of PCMs will block the pores, which seriously hinders their application. In this paper, a novel microencapsulated phase change material (MEPCM) with thermal expansion void was prepared using “double-layer coating, sacrificing inner layer” method. Based on that, two kinds of ceramic composites have been prepared. One is a TES material which composed of alumina, glass frit (GF) and MEPCMs. Thermal analysis results showed that the composite can still maintain stable heat storage performance after 200 melting-solidification cycles with little latent heat loss. Another is a multifunctional porous composite phase change material (CPCM) by loading Ce and Mn as catalyst via solution combustion synthesis (SCS) method, which can be used in low temperature SCR catalysis and other catalytic fields (100–300 °C). Based on MEPCMs with thermal expansion void, the two ceramic composites show great potential in energy storage and catalysis.

Insight into the Promoting Role of Er Modification on SO2 Resistance for NH3-SCR at Low Temperature over FeMn/TiO2 Catalysts

Er-modified FeMn/TiO2 catalysts were prepared through the wet impregnation method, and their NH3-SCR activities were tested. The results showed that Er modification could obviously promote SO2 resistance of FeMn/TiO2 catalysts at a low temperature. The promoting effect and mechanism were explored in detail using various techniques, such as BET, XRD, H2-TPR, XPS, TG, and in-situ DRIFTS. The characterization results indicated that Er modification on FeMn/TiO2 catalysts could increase the Mn4+ concentration and surface chemisorbed labile oxygen ratio, which was favorable for NO oxidation to NO2, further accelerating low-temperature SCR activity through the “fast SCR” reaction. As fast SCR reaction could accelerate the consumption of adsorbed NH3 species, it would benefit to restrain the competitive adsorption of SO2 and limit the reaction between adsorbed SO2 and NH3 species. XPS results indicated that ammonium sulfates and Mn sulfates formed were found on Er-modified FeMn/TiO2 catalyst surface seemed much less than those on FeMn/TiO2 catalyst surface, suggested that Er modification was helpful for reducing the generation or deposition of sulfate salts on the catalyst surface. According to in-situ DRIFTS the results of, the presence of SO2 in feeding gas imposed a stronger impact on the NO adsorption than NH3 adsorption on Lewis acid sites of Er-modified FeMn/TiO2 catalysts, gradually making NH3-SCR reaction to proceed in E–R mechanism rather than L–H mechanism.

Low-Temperature Selective Catalytic Reduction of NO x with NH3 over Mn-Ce Composites Synthesized by Polymer-Assisted Deposition.

The MnxCey binary catalysts with a three-dimensional network structure were successfully prepared via a polymer-assisted deposition method using ethylenediaminetetraacetic acid and polyethyleneimine as complexing agents. The developed pore structure could facilitate the gas diffusion and accelerate the catalytic reaction for NH3 selective catalytic reduction (SCR). Moreover, the addition of Ce is beneficial for the exposure of active sites on the catalyst surface and increases the adsorption of the NH3 and NO species. Therefore, the Mn1Ce1 catalyst exhibits the best catalytic activity for NOx removal with a conversion rate of 97% at 180 °C, superior water resistance, and favorable stability. The SCR reaction over the Mn1Ce1 catalyst takes place through the E–R pathway, which is confirmed by the in situ diffuse reflectance Fourier transform analysis. This work explores a new strategy to fabricate multimetal catalysts and optimize the structure of catalysts.

Assessment of copper-iron catalyst supported on activated carbon for low-temperature nitric oxide reduction by hydrogen

It is pertinent to assess the performance of a sustainable system that can treat nitrogen oxides (NO x) emissions from combusting biomass waste. Low-temperature selective catalytic reduction is attractive due to the longer catalyst lifetime and the possibility to use carbon-based catalysts. Hence, this study explores this system with the utilization of: (i) a cost-effective catalyst support, i.e. activated carbon derived from abundant biomass waste; (ii) a renewable reductant, i.e. hydrogen; and (iii) Earth-abundant metal catalysts, i.e. copper and iron. The catalyst was prepared by impregnating metal oxides (Cu and Fe) over palm kernel shell activated carbon (PKS). The catalyst was characterised by hydrogen-temperature programmed reduction (H 2-TPR) and nitric oxide-temperature programmed desorption (NO-TPD). H 2-TPR revealed an increase in the reducibility, attributed to the synergistic effects between Cu and Fe. However, these catalyst sites favour nitrous oxide (N 2O) formation as shown via NO-TPD. Meanwhile, the catalyst activity has also been investigated in a fixed-bed reactor. It showed that the 100% conversion can be achieved at 200°C, but the selectivity towards nitrogen formation is as low as 40%. Therefore, investigating the optimum design of PKSCuFe catalyst is justifiable to improve the performance of low-temperature selective catalytic reduction.

Water: A promoter of ammonia selective catalytic reduction over copper-exchanged LTA zeolites

Negative effects of water on the hydrothermal stability of ammonia (NH3) selective catalytic reduction (SCR) catalysts are well known. Thus, developing water-resistant, zeolite-based transition metal catalysts like copper (Cu) for mitigating nitrogen oxides from mobile sources is very challenging. Here we show that water markedly facilitates the migration of copper ions from sterically inaccessible sod cage to the readily accessible lta one through six-membered rings within a copper-exchanged high-silica Cu-LTA zeolite. We also show that this ubiquitous molecule promotes the low-temperature SCR activity of the Cu-LTA catalyst with a plausible reaction [H2O-Cu-NH3]+ intermediate. We anticipate that water might play a beneficial role in nitrogen oxides removal over metal-containing zeolites for cleaner air via tuning the active sites and reaction pathways.

MnOx location on MnOx-ZSM-5 to influence the catalytic activity for selective catalytic reduction of NOx by NH3

MnOx-ZSM-5 catalysts with different MnOx location were synthesized and their catalytic performances are investigated for selective catalytic reduction of NOx with NH3 (NH3-SCR). MZ-150 that MnOx clusters are dispersed on the outside of ZSM-5 exhibits higher catalytic activity at low temperature than MZ-25 of MnOx nanoparticles encapsulated in the channels of the support. The characteristics results show that the excellent redox ability and the dominant Mn3+ species over MZ-150 improve the oxidation of NO to NO2 and the formation of unstable NOx intermediates, enhancing the low-temperature SCR activity. For MZ-25, the confinement effect of ZSM-5 leads to a large NOx adsorption capacity, but the generated NOx species are stable, which could be the important reason for the poor activity at low temperature. This work reveals the relationship between the location of MnOx nanoparticles and the SCR properties, which can provide a new idea for designing the low-temperature zeolite catalysts.

A strategy for constructing highly efficient yolk-shell Ce@Mn@TiOx catalyst with dual active sites for low-temperature selective catalytic reduction of NO with NH3

In this work, Ce@MnOx catalysts with core-shell structure were obtained via simple hydrothermal treatment, and the redox capacity can be effectively improved through co-existence of Ce and Mn. Since the strong oxidation capacity of MnOx can lead to the peroxidation of ammonia, the redox capacity can be suppressed with the decrease of manganese content. However, in the case of low manganese content, it is discovered that the surface acidity is very weak and the ammonia adsorption behavior on the catalyst is limited, which is detrimental to the NH3-SCR process. To solve this problem, a series of yolk-shell structured Ce@Mn@TiOx were accordingly constructed, where the TiO2 shell can provide more surface acid sites to promote the adsorption and activation of ammonia. As expected, the Ce@Mn@TiOx exhibits excellent catalytic performance for NH3-SCR at low temperature and possesses a relatively wide working temperature window. More importantly, the TiO2 shell could protect the active sites from water vapor competitive adsorption as well as sulfur attack. Thus, the Ce@Mn@TiOx-10% catalyst exhibited satisfied water vapor resistance in the SCR process, and its sulfur tolerance was also improved to a certain extent. In situ DRIFTs analysis demonstrated that the Ce@Mn@TiOx-10% catalyst mainly follows the L-H reaction mechanism. Consequently, the separation of acid and redox sites has been successfully accomplished by the construction of Ce@Mn@TiOx with yolk-shell structure, where the outer shell (TiO2) mainly enhanced the adsorption capacity of ammonia and protected active centers from attacking by moisture and even sulfur species, while the MnOx interlayer enhanced the interfacial electronic interaction of Ce/Mn and ensured efficient redox cycle during the NH3-SCR process.