NOx Adsorption Capacity Research Articles

The functions of Pt located at different positions of HZSM-5 in H2-SCR

Series of Pt/HZSM-5 catalysts (x = 0.1–0.5) were prepared by impregnating H2PtCl6 or Pt(NO3)2 on HZSM-5 and characterized by FTIR and XPS. It was found that Pt is predominantly located on the zeolite outside surface for the catalysts prepared from H2PtCl6, whereas large portion of Pt existed in the zeolite channels for the catalysts prepared from Pt(NO3)2. In the H2-SCR, the Pt on the zeolite outside surface plays the role of activating H2, while that in the zeolite channels merely adsorbs NOx species for the reaction. For the catalysts, proper NOx adsorption capacity matching the property activating H2 is indispensable for increasing the H2 efficiency and the N2 selectivity. Some cations such as Al3+ capable of accepting more NOx species in the zeolite channels effectively improved the catalytic performance.

Enhancement of Low-Temperature Catalytic Activity over a Highly Dispersed Fe–Mn/Ti Catalyst for Selective Catalytic Reduction of NOx with NH3

A novel Fe2O3–MnO2/TiO2 catalyst was synthesized using a conventional impregnation method assisted with ethylene glycol and used for NH3–SCR. The catalyst exhibited superior low-temperature activity over a broad temperature window (100–325 °C), low apparent activation energy, and excellent sulfur-poisoning resistance. The characterization results revealed that the catalyst was greatly dispersed with smaller particles, and the partial doping of Fe into the TiO2 lattice thereby led to the formation of the Fe–O–Ti structure, which could strengthen the electronic inductive effect and increase the ratio of surface chemisorption oxygen, resulting in the enhancement of NO oxidation and favoring the low-temperature SCR activity via a “fast SCR” process. The in situ FTIR analysis showed that the NOx adsorption capacity was significantly improved due to the desired dispersion property, further helping both the SCR activity and reaction rate at low temperatures. The present work confirmed that more active sites can ...

Promoting Effects of Barium Substitution on the Catalytic Performances of FeCeO2−δ for Soot Oxidation

In this work, the promoting effects of the introduction of barium to FeCeO2−δ on soot oxidation were thoroughly studied. A series of FeCeO2−δ, BaFeCeO2−δ, and BaCeO2−δ was prepared by a self-propagating high-temperature synthesis method. The physiochemical properties of catalysts were investigated using Brunauer–Emmett–Teller, X-ray diffraction, transmission electron microscopy, NO temperature-programmed oxidation (NO-TPD), and NOx-TPD analysis. Ba(10%)FeCe displays the best catalytic performance and possesses the lowest soot ignition temperature of 282 °C in the presence of NO. However, FeCeO2−δ was the most active one, showing the lowest soot ignition temperature of 342 °C in the absence of NO. NO-TPD demonstrates that barium loading in FeCeO2−δ dramatically improves the NOx adsorption and desorption capacity. In addition, in situ diffuse reflectance infrared Fourier transform spectroscopy indicates that NO can be adsorbed on catalysts’ surfaces to form nitrite and nitrate species and that the correspon...

High-efficiency removal of NOx over natural mordenite using an enhanced plasma-catalytic process at ambient temperature

Plasma-assisted catalysis technology has been identified as one of the most efficient feasible routes for DeNOx. In this work, a combined NOx adsorption-CH4 plasma discharge process has been developed for DeNOx. The conversion of the adsorbed NOx on H-MOR and Co-MOR could reach 99.7% and 98.3% in the present process at ambient temperature, respectively. The results verified the feasibility of the present process using MORs as adsorbents and catalysts in theory. Then, the cost-effective natural mordenite (NMOR) was used as the adsorbent and catalyst instead of MOR. After acid treatment and ion-exchange, the modified Co-NMOR exhibited higher NOx adsorption capacity than MOR. The NOx conversion and N2 selectivity on Co-NMOR were 95.0% and 100.0% in the CH4-plasma discharge stage, respectively. Meanwhile, the cyclic operation of Co-NMOR for DeNOx has also been investigated.

Catalytic oxidation of soot on mesoporous ceria-based mixed oxides with cetyltrimethyl ammonium bromide (CTAB)-assisted synthesis

Mesoporous ceria and transition metal-doped ceria (M0.1Ce0.9O2 (M=Mn, Fe, Co, Cu)) catalysts were synthesized via CTAB-assisted method. The physicochemical properties of the prepared catalysts were characterized by XRD, DLS analysis, SEM, BET, Raman, H2-TPR and in situ DRIFT techniques. The catalytic activity tests for soot oxidation were performed under tight contact of soot/catalyst mixtures in the presence of O2 and NO+O2, respectively. The obtained results show that mesoporous ceria-based solid solutions can be formed with large surface areas and small crystallite size. Transition metals doping enhances the oxygen vacancies and improves redox properties of the solids, resulting in the increased NO oxidation capacity and NOx adsorption capacity. The soot oxidation activity in the presence of O2 is enhanced by doping transition metal, which may be related to their high surface area, increased oxygen vacancies and improved redox properties. The soot combustion is accelerated by the NO2-assisted mechanism under NO+O2 atmosphere, facilitating an intimate contact between the soot and the catalyst.

Design of highly efficient NOx storage-reduction catalysts from layered double hydroxides for NOx emission control from naphtha cracker flue gases

Although there are many reports on the removal of NOx from vehicle and power plant emissions, little effort has been devoted to the removal of NOx from cracker flue gases. Here we report a systematic investigation on the design of highly efficient NOx storage-reduction (NSR) catalysts using highly dispersed aqueous miscible organic-layered double hydroxides (AMO-LDHs) derived mixed oxides (LDOs) for NOx emission control from naphtha cracker flue gases. For a series of binary M2+Al3+ LDOs, the influence of six divalent cations (Ca, Zn, Cu, Ni, Co, and Mg) on the NOx adsorption capacity, NO oxidation property, and the thermal stability of adsorbed NOx was systematically investigated. The optimal NOx storage temperature range for each binary LDO was also determined. Based on the fundamental findings of binary LDOs, a ternary LDH CoMgAl-CO3 derived LDO CoMgAlOx with a significantly improved NOx storage capacity was designed. By tuning the Co/Mg ratio, the NOx storage capacity was greatly improved from 0.14mmol/g for Mg3Al1Ox to 0.50mmol/g for Co1Mg2Al1Ox at 300°C. By further doping Pt and K2CO3, a new NSR catalyst composed of 1wt% Pt/15wt% K2CO3/Co1Mg2Al1Ox was obtained, which exhibited a very high NOx storage capacity of 1.2mmol/g. Finally, the NSR cycling performance, and the CO2, SO2 and H2O resistance of this catalyst were also investigated. Thanks for its superior NOx storage capacity, NSR cycling performance, and CO2, SO2 and H2O resistance, this new NSR catalyst showed great potential for the NOx control from cracker flue gases.

Performance of Fe-Ba/ZSM-5 catalysts in NO + O2 adsorption and NO + CO reduction

Fe-Ba/ZSM-5 catalysts were prepared for NO + O2 adsorption and NO reduction by CO at 250–400 °C. NO adsorption and reduction were investigated for different Fe contents, while the Ba content was fixed at 5 wt.%. The catalysts were characterized by BET, SEM, XRD, XPS, H2-TPD, and in situ DRIFTS. The NO + CO activity tests of Fe-Ba/ZSM-5 catalysts showed that the NO reduction efficiencies were strongly affected by temperature. The NOx adsorption capacity of Fe-Ba/ZSM-5 catalysts increased with increasing Fe loading. The results of catalyst characterization showed that the dispersion of Fe species is one of the most important factors affecting NO reduction and adsorption performance. High Fe loading increased the amount of adsorbed NOx, while the NO + CO reaction was favored by good dispersion of Fe species. However, the NOx adsorption capacity decreased for Fe loadings above 8.3% due to the aggregation of metal oxides.

Influence of different supports on the physicochemical properties and denitration performance of the supported Mn-based catalysts for NH3-SCR at low temperature

The commonly used supports of SiO2, γ-Al2O3, TiO2, and CeO2 were synthesized, and used for preparing MnOx/SiO2, MnOx/γ-Al2O3, MnOx/TiO2, and MnOx/CeO2 catalysts with the purpose of investigating the influence of crystal structure and coordination status on the physicochemical properties and denitration performance of these supported Mn-based catalysts for low-temperature NH3-SCR. The obtained samples were characterized by XRD, Raman, BET, H2-TPR, NH3-TPD, in situ DRIFTS, NO+O2-TPD, XPS, and NH3-SCR model reaction. XRD results indicate that MnOx species can be highly dispersed on the surface of γ-Al2O3, TiO2, and CeO2, which is because that there are some octahedral and tetrahedral vacancy sites, octahedral vacancy site, and cubic vacancy site exist on the surface of defective spinel structure γ-Al2O3, anatase TiO2, and cubic fluorite-type structure CeO2, respectively. However, there is no any vacancy site on the surface of SiO2 due to its unique SiO4 tetrahedral structure, which results in the appearance of crystalline β-MnO2 on the surface of MnOx/SiO2 catalyst. Furthermore, H2-TPR results exhibit obvious different reduction behavior among these supported Mn-based catalysts, which is explained by the coordination status of Mn species. Finally, NH3-SCR model reaction results show that MnOx/γ-Al2O3 catalyst presents the best catalytic performance among these supported Mn-based catalysts due to its high dispersion, suitable reduction behavior, largest amount of acid sites, optimal NOx adsorption capacity, and abundant Mn4+ content.

Promoting Ag/Al2O3 Performance in Low-Temperature H2-C3H6-SCR by Thermal Pretreatment of γ-Alumina in Water

The present work highlights for the first time that a thermal treatment of γ-Al2O3 in water at 80 °C for 24 h prior to Ag deposition (Ag/Al2O3-OH) leads to a drastic enhancement of H2-C3H6-SCR performance at low temperature. This enhancement is attributed to the higher NO x coverage on Ag/Al2O3-OH compared with that of Ag/Al2O3. The higher NO x adsorption capacity of the sample prepared from the γ-Al2O3 thermally-treated in water is proposed to result from the preferential interaction of Ag with newly-created Al2O3 anchoring sites (Al(OH)3 bayerite domains), which were not available on the untreated pristine support. Graphical Abstract

Co-doping a metal (Cr, Mn, Fe, Co, Ni, Cu, and Zn) on Mn/ZSM-5 catalyst and its effect on the catalytic reduction of nitrogen oxides with ammonia

Selective catalytic reduction (SCR) of NOx by NH3 over a series of Mn–M/Z catalysts (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, and Z = the ZSM-5 Zeolite) synthesized by wet impregnation method was investigated. Mn–Fe/Z, Mn–Co/Z, and Mn–Cu/Z catalysts exhibited approximately 100 % NOx conversion over a wide temperature range (200–360 °C) in a defined atmospheric condition, which was noticeably greater than that of Mn–Cr/Z (340–360 °C). Furthermore, the effect of addition of second metal oxide species to the initial Mn/Z catalyst on the structure of catalysts was studied by several characterization techniques. BET measurements revealed high surface area and pore volume of the Mn–Cu/Z catalyst. In addition, the XRD and UV–Vis DR results showed that addition of co-doped metal oxide species improved the dispersion of metal ions and inhibited crystallization of metal oxides. UV–Vis studies also were in good accordance with DTA/TG results confirming the formation of cobalt oxide and copper oxide clusters in Mn–Co/Z and Mn–Cu/Z catalysts, respectively. The FTIR spectra of pyridine adsorption, in addition, suggested the Mn–Cu/Z catalyst contained the most Lewis acid sites leading to more NOx adsorption capacity.

NO adsorption during lean-rich cycles on Pt–Ba/γ-Al2O3 traps in a gas stream simulating the exhaust of gasoline engines

The NO adsorption–desorption from traps composed of 8 wt% of Ba and 2 wt% of Pt deposited on γ-Al2O3 was studied. It was found that there was an important effect of the barium salt used (nitrate or acetate) on the trap performance. The exposition of the traps to NO presence at room temperature led to the formation of barium nitrites and nitrates, species that decomposed between 80 and 230 °C, and 400–580 °C, respectively. Furthermore, the NO adsorption capacity showed a complex dependence upon the O2 and SO2 content in the reaction feed stream, whereas the H2O presence had no observable effects. The NOx adsorption capacity on the tested traps increased with the content of oxygen in the feed stream, and decreased irreversibly with the SO2 presence.

NOx Removal over Modified Carbon Molecular Sieve Catalysts Using a Combined Adsorption-Discharge Plasma Catalytic Process

Carbon molecular sieves (CMS), 13X zeolite, and γ-Al2O3 were selected as catalyst support to investigate the NOx adsorption capacity, and a series of Cu modified CMS-based catalysts were used to investigate the NOx adsorption and discharge plasma catalytic removal capacity. Results showed that CMS has a larger NOx adsorption amount and lower desorption temperature in NOx temperature programmed desorption (TPD) process. The addition of Cu benefits the NOx adsorption and nonthermal plasmas (NTP) removal capacity, and the NOx removal capacity and the ratio of NTP/(NTP + TPD) achieved 96.2% and 68.39% over 15%Cu-CMS in 30 min. Water vapor has an obvious effect on the NOx adsorption and discharge plasma catalytic process. In cyclic operation, 15%Cu-CMS has a better NOx adsorption-discharge property. The Brunauer–Emmett–Teller (BET) method showed the average pore width, surface area, and pore volume of the sample after cyclic operation has no significant change. X-ray photoelectron spectroscopy (XPS) showed a n...

Influence of support properties on H2 selective catalytic reduction activities and N2 selectivities of Pt catalysts

The selective catalytic reduction of NOx by H2 (H2-SCR) was studied over Pt/MgO, Pt/γ-Al2O3, Pt/ZrO2, and Pt/HZSM-5 catalysts. The H2-SCR activities and N2 selectivities of the catalysts were strongly influenced by the amounts of Pt metal in the catalysts and the NOx adsorption capacities of the supports. The acidic surface of HZSM-5 increased the amount of metallic Pt on the support, decreasing the NOx adsorption capacity, resulting in much higher H2-SCR activity and N2 selectivity. The inferior activities of Pt/MgO and Pt/γ-Al2O3 are ascribed to the low amounts of metallic Pt and large NOx adsorption capacities of the supports as a result of their basic surfaces. Based on these results and in situ Fourier transform infrared spectroscopic studies of the reaction, it is proposed that the reduction products of nitrite/nitrate species at the Pt/support interface are N2 or N2O, depending on the relative amounts of active hydrogen and nitrous species involved in the reduction.

Catalytic NOx Reduction in a Novel i-CFB Reactor: II. Modeling and Simulation of i-CFB Reactors

A mathematical model has been developed for the i-CFB deNOx reactor, which is designed for the hydrocarbon-based selective catalytic reduction (HC-SCR) process. The i-CFB model consists of three submodels: hydrodynamics, NOx adsorption, and reaction kinetics. The modeling results show good agreement with the experimental data. It is observed from the simulation that the performance of the i-CFB reactor is very sensitive to gas bypass from the draft tube to the annulus (Rd-a) but is less sensitive to the converse (Ra-d). The solids circulation rate (Gs) has little effect on the overall deNOx efficiency. The analysis of NOx adsorption and reduction at both the annulus and the draft tube zones revealed that solids residence time in the reduction zone is too short for the NOx reduction reaction for the current i-CFB reactor design. A large reduction zone could significantly enhance the overall deNOx efficiency. The optimum reduction zone area ratio (AR/Atotal) should be ∼0.65–0.7. Further increases in the AR/Atotal ratio will decrease NOx conversion. It is also observed that the performance of the i-CFB at higher AR/Atotal ratios is less sensitive to gas bypass from the reduction zone to the adsorption zone. The deNOx efficiency of i-CFB reactor becomes more sensitive to NOx adsorption capacity at higher AR/Atotal ratios.

In situ DRIFTS study of NOx adsorption behavior on Ba/CeO2 catalysts

A series of Ba/CeO2 catalysts with different Ba loading amounts were prepared by incipient wetness impregnation. Their NOx adsorption behaviors under NO and NO+O2 conditions were investigated by in situ DRIFTS. It was found that NOx was adsorbed and stored in the form of nitrites and nitrates on both Ba and Ce sites on the surface of the catalysts. The less thermally stable BaCO3 was suggested to be the main active phase for NOx trapping. Ceria served primarily as an oxygen supplier in the absence of O2, and the reaction from nitrites to nitrates on Ba sites was the key step in this case. In the presence of O2, however, gaseous O2 became the main oxygen source. The NOx adsorption capacity of the catalyst was dominated by the Ba content. Moreover, the stability of nitrites and nitrates formed on Ce sites was found to be lower than those formed on Ba sites which existed in the form of the ionic barium nitrate species.

A Ce–Sn–O catalyst for the selective catalytic reduction of NO with NH3

Abstract A series of Ce–Sn–Ox catalysts prepared by the facile coprecipitation method exhibited good catalytic activity in a broad temperature range from 100 °C to 400 °C for the selective catalytic reduction of NOx with NH3 at the space velocity of 20,000 h− 1. The Ce4Sn4Ox catalyst calcined at 400 °C showed high resistance to H2O, SO2, K2O and PbO under our test conditions. The better catalytic performance was associated with the synergistic effect between CeO2 and SnO2, which strengthened the NH3 and NOx adsorption capacity on the surface of the catalyst.

The effect of water vapor on NOx storage and reduction in combination with plasma

In this study, the effect of water vapor on NOx storage and reduction in combination with non-thermal plasma was investigated. The results indicated that H2O could lower NOx adsorption capacity of H-ZSM-5 at the adsorption stage, due to the competitive adsorption between H2O and NOx on the surface of H-ZSM-5. When the content of H2O was above 2%, the NOx breakthrough adsorption capacity of H-ZSM-5 remained stable at 2.0mLg−1. At the discharge stage, the introduction of H2O decreased the total NOx conversion from 98.7% to 94.2%, while the conversion of adsorbed NOx to N2 from 94.0% to 83.9%. When increasing the CH4 concentration and the discharge power, the total NOx conversion and the conversion of adsorbed NOx to N2 moved up to 99.5% and 97.4%, respectively. In addition, although Ni-ZSM-5 exhibited better resistance to H2O than H-ZSM-5 for NOx adsorption, it showed inferior efficiency for NOx removal and nearly 15% NOx remained after the discharge stage.

Influence of the Alkali Promoter on the Activity and Stability of Transition Metal (Cu, Co, Fe) Based Structured Catalysts for the Simultaneous Removal of Soot and NOx

Structured catalysts prepared by means of coating cordierite monoliths with alumina-based suspensions containing transition metals such as Cu, Co and Fe and alkali/alkali-earth promoters such as K and Ba. Textural and structural features of these catalysts were analyzed by means of N2 adsorption and SEM. Their activity in the simultaneous removal of soot and NOx was assayed in a lab-scale installation, using a carbon black as diesel surrogate. Catalysts exhibited significant activity in deNOx and soot oxidation. K and Ba enhanced both NOx adsorption and soot–catalyst contact. However Ba contributed to a greater extent to the adsorption of N-species, which moreover presented higher thermal stability than on K-catalysts, and K showed higher mobility than Ba. Thus, Ba-containing catalysts showed increased activity towards NOx reduction but shifted to higher temperatures in comparison to K-catalysts, which on the other hand resulted more active towards soot oxidation than Ba-ones. Fe-based catalyst turned out to be less active both in soot oxidation and NOx reduction than Co and Cu-based ones. Intensive calcination of the catalysts at 800 °C for 5 h resulted in substantial loss of K and Ba. Loss of promoter depends, however, on the metal contained in the catalyst. In this sense Fe-containing catalysts showed higher stability. Calcination has a substantial effect on catalytic activity. Catalyst significantly lost their NOx adsorption capacity and showed similar activity than a catalyst prepared in absence of promoter, pointing to a substantial change in reaction mechanism and reaction predominantly occurring on metallic sites upon the loss of alkali/alkali-earth compound.

The influence of chromium substitution on an iron–titanium catalyst used in the selective catalytic reduction of NO

A series of Cr-substituted Fe–Ti compounds were prepared by the co-precipitation method and were systematically investigated as catalysts for the selective catalytic reduction (SCR) of NO by NH3. A variety of analytical techniques revealed that the Cr substitution amount affects the N2 selectivity, SCR activity, redox behavior of NH3/NOx, adsorption ability, and structure of catalysts in terms of surface properties, porosity, mobility of lattice oxygen, oxidative ability of Cr species, ratio of Brønsted acid sites and Lewis acid sites, NOx adsorption capacity, and structural disorder and distortion. In a series of FeaCr1−aTiOx (a=1, 0.75, 0.5, 0.2, 0) catalysts, Fe0.5Cr0.5TiOx showed the highest activity because of the optimized interactions of Fe, Cr, and Ti species in this catalyst.

Investigation of Cs–Cu/ZrO2 systems for simultaneous NOx reduction and carbonaceous particles oxidation

The effect of Cs and Cu contents in Cs–Cu/ZrO2 catalysts has been studied in the simultaneous abatement of NOx and carbon black (CB), used as a model of soot. Cu/ZrO2 and Cs–Cu/ZrO2 catalysts with a low Cs content are moderately active but show a beneficial role of NOx in CB elimination. Cs–Cu/ZrO2 catalysts with high amount of cesium are the more active systems towards both CB oxidation and NOx reduction. The increase in the amount of Cs leads to the formation of a higher proportion of CuO crystallites which are active for CB oxidation. Cesium in Cs–Cu/ZrO2 also acts as a promoter favouring the contact between reactants: on the first hand it increases the exchange surface between CB and the catalyst surface and on the other hand, it enhances the NOx adsorption capacity of Cs–Cu/ZrO2 and the further NOx reduction.