Selective catalytic reduction of NO with ammonia over V2O5 doped TiO2 pillared clay catalysts

A series of vanadia doped TiO₂-pillared clay (TiO₂-PILC) catalysts with various amount of vanadia were studied for selective catalytic reduction (SCR) of NO by ammonia in the presence of excess oxygen. It was found that the V₂O₅/TiO₂-PILC catalysts were highly active for the SCR reaction. The catalysts showed a broad temperature window, and the maximum NO conversion was higher than that on V₂O₅/TiO₂ catalyst and was the same as the commercial V₂O₅ + WO₃/TiO₂ catalyst. The V₂O₅/TiO₂-PILC catalysts also had higher N₂/N₂O product selectivities as compared to V₂O₅ doped TiO₂ catalysts. In addition, H₂O + SO₂ slightly increased the activities at high temperatures (>350°C) for the V₂O₅/TiO₂-PILC catalysts. Addition of WO₃ to V₂O₅ further increased the activities of the PILC catalysts. These results indicate that TiO₂-PILC is a good support for vanadia catalysts for the SCR reaction. In situ FT–IR experiment indicated that both Brønsted acid sites and Lewis acid sites exist on the catalyst surface, but with a large proportion being Brønsted acid sites at low temperatures (e.g., 100°C). The reaction path for NO reduction by NH₃ on the V₂O₅/TiO₂-PILC is similar to that on V₂O₅/TiO₂ catalyst, i.e., N₂ originates from the reaction between gaseous NO and NH₃ adspecies.     

The formation of N2O during the reduction of NO by NH3

The process of selective non-catalytic reduction of NO, SNCR, is important for limiting emissions of nitrogen oxides from coal-fired power plants. Such a process has been studied for many years, both in the laboratory and under practical conditions. This work was an attempt at elucidating some of the problems associated with the method when used under circulating fluidized bed (CFB) conditions and in particular, the formation of the N₂O by-product. The NO + NH₃ reaction has been studied in the laboratory, over quartz sand in a heated fixed bed flow reactor. In comparison with a combustion environment, the composition of the gas phase was drastically simplified and limited to NO and NH₃, in nitrogen as the carrier gas, with O2 added in some experiments. The product gases were analyzed for NO, N₂O and NH₃. The effects the following parameters were studied: temperature inside the reactor between 850 and 1250 K, height of the sand bed, NH₃/NO molar ratio over the range 0.54–2.0 and the addition of 1 or 2% of O₂ in volume. Baseline tests with an empty reactor were also made. With no sand in the reactor, the results were both qualitatively and quantitatively different. The sand helped to increase the efficiency of NO reduction, particularly at lower temperatures, but N₂O formation also appeared to be strongly enhanced, except at the highest temperatures. Higher molar NH₃/NO ratios favored NO reduction and N₂O production, both with and without sand. The reduction of NO did not appear to require the presence of O₂, but the introduction of 1% or 2% of O₂ gave some benefit. The results confirmed that under practical conditions more attention should be paid to the role of the bed solids in the SNCR process.     

NO presence effects on the reduction of N2O by CO over Al–Pd–Co oxide catalyst

Nitrous oxide (N₂O) is known as one of the greenhouse gases of which the emission levels are to be controlled and also an ozone-depleting material in the stratosphere. For more than a last couple of decades, various catalysts have been investigated for the reduction of N₂O emission. However, most of those catalysts require reaction temperatures as high as 450 °C even with the use of effective reducing agents. Furthermore, NO, which is usually present with N₂O, significantly interferes with the removal efficiency of N₂O. Al–Pd–Co oxide catalyst which is a type of mixed metal oxides (MMO) has shown 100% conversion of N₂O at temperatures as low as 200 °C using CO. This paper examines the effect of NO on the reduction of N₂O with CO over Al–Pd–Co oxide catalyst. The experiments were carried out in the temperature range of 100–500 °C with space velocity of 10,000–50,000 h^?1. Though the efficiency of N₂O reduction is decreased significantly when NO is present, the efficiency increases when sufficient CO is supplied. In this study, the reduction mechanisms of N₂O and NO by CO have been confirmed, and it was shown that MMO catalyst can simultaneously remove N₂O and NO using CO with high efficiency.     

Performance of V2O5/NPTiO2–Al2O3-nanoparticle- and V2O5/NTiO2–Al2O3-nanotube model catalysts in the SCR–NO with NH3

In this study, the NO reduction by NH₃ over V₂O₅/NPTiO₂–Al₂O₃ (nanoparticles) and V₂O₅/NTiO₂–Al₂O₃ (nanotubes) catalysts synthesized by the sol–gel method with 10 and 5 wt.% of Al₂O₃ and V₂O₅, respectively, is reported. The V₂O₅/NPTiO₂–Al₂O₃ and V₂O₅/NTiO₂–Al₂O₃ catalysts showed remarkable conversion, high acidity, structure stability, N₂ selectivity and a high resistance to deactivation in the presence of 10 vol.% of H₂O within the 200 to 500 °C temperature interval. The nanostructured catalysts developed in this work are an excellent alternative to improve the SCR–NH₃ process, both expanding its operation window and preventing deactivation by H₂O at high temperatures.     

Zr/HZSM-5 catalyst for NO reduction by C2H2 in lean-burn conditions

Zr-based zeolite catalysts were investigated for the first time in selective catalytic reduction of NO by hydrocarbon (HC-SCR). Highly dispersed zirconium species, especially the amorphous ultrafine zirconium oxide in the catalyst, considerably enhanced the activity for selective catalytic reduction of NO by acetylene (C₂H₂-SCR), both by accelerating the NO oxidation to NO₂ and enlarging the NO₂ adsorption capacity of the catalyst under the reaction conditions. Thus a durable and active Zr/HZSM-5 catalyst giving 89% of NO conversion to N₂ at 350 °C in 1600 ppm NO, 800 ppm C₂H₂, and 9.95% O₂ in helium was obtained. For the C₂H₂-SCR of NO, it was suggested that acidic sites with strong acidity on the Zr-based HZSM-5 catalysts are indispensable to initiate the aimed reaction via the route of NO oxidation to NO₂, which explains the higher activity for the reaction obtained over the Zr/HZSM-5 catalyst sample with lower SiO₂/Al₂O₃ ratio. The zirconium species could only functioned in the presence of protons in the C₂H₂-SCR of NO, so a synergistic effect between the zirconium species and protons of the Zr/HZSM-5 catalyst was proposed.     

Effect of NO on the catalytic removal of N2O over FeZSM-5. Friend or foe

Selective catalytic reduction (SCR) of N₂O with C₃H₈ over FeZSM-5 under excess oxygen is strongly inhibited by NO. The assistance of the hydrocarbon in N₂O reduction vanishes at high partial NO pressures, approaching the activity of the N₂O + NO system at molar NO/N₂O ratios of 1.5–2, with N₂O/C₃H₈=1. This effect differs from the NO promotion in direct N₂O decomposition over Fe-zeolite catalysts. The negative effect of NO on the N₂O reduction has a significant impact in the design and operation of catalytic reactors in tail-gases of nitric acid plants and other sources where NO comes along with N₂O.     

Promoting catalytic performances of Ni-Mn spinel for NH3-SCR by treatment with SO2 and H2O

Ni(0.4)-MnO_x catalyst was prepared by citrate combustion, which showed high catalytic performance for NH₃-SCR reaction. After the resistance tests of SO₂ and H₂O, Ni(0.4)-MnO_x-SH showed better NH₃-SCR activity than that of Ni(0.4)-MnO_x, when the temperature was > 240 °C. The characterizations suggest that Ni(0.4)-MnO_x-SH has more acid sites for ammonia adsorption and far weaker oxidation capacity for NH₃, which resulted in the high catalytic activity at middle-temperature.     

Selective catalytic reduction of NO by ammonia using mesoporous Fe-containing HZSM-5 and HZSM-12 zeolite catalysts: An option for automotive applications

Mesoporous and conventional Fe-containing ZSM-5 and ZSM-12 catalysts (0.5–8 wt% Fe) were prepared using a simple impregnation method and tested in the selective catalytic reduction (SCR) of NO with NH₃. It was found that for both Fe/HZSM-5 and Fe/HZSM-12 catalysts with similar Fe contents, the activity of the mesoporous samples in NO SCR with NH₃ is significantly higher than for conventional samples. Such a difference in the activity is probably related with the better diffusion of reactants and products in the mesopores and better dispersion of the iron particles in the mesoporous zeolite as was confirmed by SEM analysis. Moreover, the maximum activity for the mesoporous zeolites is found at higher Fe concentrations than for the conventional zeolites. This also illustrates that the mesoporous zeolites allow a better dispersion of the metal component than the conventional zeolites. Finally, the influence of different pretreatment conditions on the catalytic activity was studied and interestingly, it was found that it is possible to increase the SCR performance significantly by preactivation of the catalysts in a 1% NH₃/N₂ mixture at 500 °C for 5 h. After preactivation, the activity of mesoporous 6 wt% Fe/HZSM-5 and 6 wt% Fe/HZSM-12 catalyst is comparable with that of traditional 3 wt% V₂O₅/TiO₂ catalyst used as a reference at temperatures below 400 °C and even more active at higher temperatures.     

Thermogravimetric study on pyrolysis of biomass with Cu/Al2O3 catalysts

The Cu/Al₂O₃ catalysts of three different compositions (10, 20 and 30 wt.% Cu loading), have been investigated with regard to their catalytic effects on pyrolysis of paper biomass species (up to 800 °C) by thermogravimetric analysis (TGA) experiments. The results show that catalysts made devolatilization at lower (below 200 °C) and middle temperature (200–400 °C) regions in the pyrolysis of the biomass species, and the temperature reduction effects follow the order: 30 > 20 > 10 wt.% copper loading. Although the catalysts with 10 and 20 wt.% copper have shown almost similar activity, whereas dehydration reaction was enhanced almost 40% in the presence of 30 wt.% copper-loaded catalyst. At the same time, the amount of residue at the end of the reaction also decreased with increase in the copper loading from 10 to 30 wt.%. At higher temperatures (above 400 °C), the catalyst with greater copper loaded worked more nicely possibly due to the enhancement of the depolymerization reaction over dehydration of cellulose in presence of more basic catalysts. The catalysts were characterized by using X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area analysis and scanning electron microscopy (SEM). XRD results show the formation of CuAl₂O₄ spinel and Cu₂O phase in the catalysts.     

Reduction of N2O by NH3 on polycrystalline copper and Cu(1 1 0): A combined XPS,FT-IRRAS and kinetics investigation

Kinetics of N₂O decomposition and catalytic reduction of N₂O by NH₃ in the presence or absence of oxygen have been studied on polycrystalline Cu planar chip (3 cm × 3 cm × 0.1 cm) or Cu(1 1 0) single crystal, using catalytic test equipment, XPS and FT-IRRAS techniques. It has been shown that N₂O decomposes on metallic Cu, but gives then Cu₂O, which is detrimental to N₂O decomposition. The presence of a reductant, such as NH₃, allowed N₂O to react leading to its catalytic reduction to N₂; 500 °C is the best temperature for catalytic reduction alone, i.e. with low additional self-decomposition of N₂O or NH₃. The presence of oxygen, in amount less than that of NH₃, leads to more efficient NH₃ oxidation, oxygen being observed to be more reactive than N₂O on NH₃. XPS results enabled to identify the active surface as metallic Cu and Cu₃N for NH₃ oxidation and NH₂, NH, N adsorbed species as intermediates of the reaction. At room temperature, in the presence of N₂O, O₂ and NH₃, FT-IRRAS allowed to show the formation of NH₂ and NH species (bands at 1550 and 1440 cm⁻¹, respectively) and of two N₂^δ− species (bands at 2170 and 2204 cm⁻¹), the latter one corresponding to adsorbed N₂^δ− species close to adsorbed electron accepting O or OH species. This study demonstrated that N₂O decomposed to N₂ and O species during SCR reaction; it enabled to identify several adsorbed surface species (N, NH, NH₂, N₂^δ−), both by XPS after catalytic reaction at 500 °C on the polycrystalline Cu chip and by IRRAS on Cu(1 1 0) single crystal in the presence of the reactants at room temperature. In addition, it was shown that N₂ is a powerful IR probe to characterise the surrounding environment of surface sites that cannot be identified by any other way.     

Effects of WO3 doping on stability and N2O escape of MnOx–CeO2 mixed oxides as a low-temperature SCR catalyst

MnO_x–WO_x–CeO₂ catalysts synthesized using a sol–gel method were investigated for the low-temperature NH₃-SCR reaction. Among them, W_0.1Mn_0.4Ce_0.5 mixed oxides exhibited above 80% NO_x conversion from 140 to 300 °C. In addition, this catalyst exhibited high stability and CO₂ tolerance in a 50 h activity test at 150 °C. Substantially reduced N₂O production and enhanced N₂ selectivity were achieved by WO₃ doping, which was due to the weakened reducibility and increased number of acid sites. The decreased SO₂ oxidation activity as well as the reduced formation of ammonium and manganese sulfates resulted in a high SO₂ resistance of this catalyst.     

HC-SCR reaction pathways on ion exchanged ZSM-5 catalysts

Metal cation (metal = Cu, In and La) ion exchanged ZSM-5 zeolites as catalysts for the NO selective reduction by propane and propene in excess oxygen. The surface reactions of HC-SCR over catalysts were investigated through in situ DRIFTS method. For C₃H₈-SCR, adsorbed nitrate species (–NO₃) were observed as main reaction intermediates and they could react with gaseous propane to produce N₂, H₂O and CO₂. While for C₃H₆-SCR, adsorbed amine species (–NH₂) were observed as main reaction intermediates and they could react with NO or NO₂ to produce the final products. The different reaction pathways for C₃H₈-SCR and C₃H₆-SCR over catalysts were proposed based on the DRIFTS results and the main factors controlling the activities of catalysts were discussed in details. The competing adsorption between NO–O₂ and HC–O₂ on the Brønsted acid sites of catalysts was responsible for the different reaction pathways in HC-SCR.     

Preparation of Mn–FeOx/CNTs catalysts by redox co-precipitation and application in low-temperature NO reduction with NH3

A novel redox co-precipitation method was firstly adopted to prepare the Mn–FeO_x/CNTs catalysts for use in low-temperature NO reduction with NH₃. The catalysts were possessed of amorphous structure and exhibited 80–100% NO conversion at 140–180 °C at a high space velocity of 32,000 h^− 1.     

Regeneration of sulfur-poisoned CeO2 catalyst for NH3-SCR of NOx

The effects of regeneration on the activities and structure of CeO₂ catalysts for NH₃-SCR of NO_x have been studied in this article. CeO₂ catalyst is deactivated by SO₂ for NH₃-SCR of NO_x in a 200 h long-term operation at 350 °C due to the formation of sulfates, and its NO_x conversion decreases from 100% to 83% gradually. However, sulfates can be removed from sulfur-poisoned CeO₂ catalysts under high temperature thermal treatment in air. After regeneration, NO_x conversion of sulfur-poisoned CeO₂ catalyst is recovered to about 100% at 350 °C. Moreover, the regeneration temperature is related to the nature of the sulfates formed on the sulfur-poisoned CeO₂ catalysts.     

Synergetic catalytic removal of Hg0 and NO over CeO2(ZrO2)/TiO2

A series of CeO₂(ZrO₂)/TiO₂ monolith catalysts were investigated for catalytic oxidation of Hg⁰ and NH₃-SCR of NO. Effect of flue gases components on catalytic oxidation of Hg⁰ was mainly studied. Results showed that the CeO₂(ZrO₂)/TiO₂ catalyst exhibited high efficiency for catalytic oxidation of Hg⁰ at 240–400 °C without adding other oxidant, and its catalytic performance for NH₃-SCR of NO was not affected. NH₃ had slight inhibitory effect while SO₂ and NO had no influence on catalytic oxidation of Hg⁰, but O₂ obviously improved catalytic oxidation of Hg⁰ for its oxidation susceptibility.     

Mercury oxidation by hydrochloric acid over TiO2 supported metal oxide catalysts in coal combustion flue gas

Mercury oxidation by hydrochloric acid over the metal oxides supported by anatase type TiO₂ catalysts, 1 wt.% MO_x/TiO₂ where M = V, Cr, Mn, Fe, Ni, Cu, and Mo, was investigated by the Hg⁰ oxidation and the NO reduction measurements both in the presence and absence of NH₃. The catalysts were characterized by BET surface area measurement and Raman spectroscopy. The metal oxides added to the catalyst were observed to disperse well on the TiO₂ surface. For all catalysts studied, the Hg⁰ oxidation by hydrochloric acid was confirmed to proceed. The activity of the catalysts was found to follow the trend MoO₃ ~ V₂O₅ > Cr₂O₃ > Mn₂O₃ > Fe₂O₃ > CuO > NiO. The Hg⁰ oxidation activity of all catalysts was depressed considerably by adding NH₃ to the reactant stream. This suggests that the metal oxide catalysts undergo the inhibition effect by NH₃. The activity trend of the Hg⁰ oxidation in the presence of NH₃ was different from that observed in its absence. A good correlation was found between the NO reduction and the Hg⁰ oxidation activities in the NH₃ present condition. The catalyst having high NO reduction activity such as V₂O₅/TiO₂ showed high Hg⁰ oxidation activity. The result obtained in this study suggests that the oxidation of Hg⁰ proceeds through the reaction mechanism, in which HCl competes for the active catalyst sites against NH₃. NH₃ adsorption may predominate over the adsorption of HCl in the presence of NH₃.     

Synthetic coal chars for the elucidation of NO heterogeneous reduction mechanisms

In the present work, the mechanisms involved in NO–char heterogeneous reduction have been studied using a synthetic coal char (SC char) as carbon source. Another synthetic char (SN char) without nitrogen in its composition has also been employed in these studies. Isothermal reduction tests at different temperatures have been carried out. Two temperature regimes were considered: low temperature (T < 250 °C) where NO chemisorption takes place and high temperature (T > 250 °C) where NO–C reaction occurs. Step response experiments combining consecutive reaction stages with NO and ¹⁵NO were performed in order to determine the role of nitrogen surface complexes, C(N), in the reduction process. The results revealed N₂ and CO₂ to be the main reduction products under the experimental conditions employed in this work. NO chemisorption at lower temperatures results in N₂ emission and surface complexes (mainly oxygenated) formation, while char gasification by NO involves a direct NO attack on the char surface to form surface complexes. As a consequence of desorption of these complexes new sites of reaction are created.     

Low-temperature selective catalytic reduction of NO over MnOx/CNTs catalysts: Effect of thermal treatment condition

Carbon nanotubes (CNTs) supported manganese oxide catalysts were prepared through different thermal treatment routes and used for low-temperature selective catalytic reduction of NO with NH₃. The MnO_x/CNTs catalyst prepared by calcined the precursor in air at 300 °C showed lower NO conversions than that treated at 250 °C, while it showed higher NO conversions than the one calcined in nitrogen. BET, TGA, XRD and H₂-TPR results indicated that CNTs may impose effects on the oxidation state and redox ability of the manganese oxide and hence on the catalytic activity during the calcination process at given temperatures.     

A novel one-pot synthesized CuCe-SAPO-34 catalyst with high NH3-SCR activity and H2O resistance

CuCe-SAPO-34 catalysts based on the one-pot hydrothermal synthesis method were prepared for the first time. The addition of Ce suppressed the formation of CuO and increased the amount of active Cu^2 +, resulting in better NH₃-SCR activity than Cu-SAPO-34. Ce greatly improved the H₂O resistance during the SCR process by stabilizing the zeolite structure and obstructing the transformation of active Cu^2 + into inactive forms.     

Size controlled ZSM-5 on the structure and performance of Fe catalyst in the selective catalytic reduction of NOx with NH3

Fe/ZSM-5 catalysts with various morphologies and sizes were prepared and the catalytic properties in NH₃-SCR were also investigated. The different ZSM-5 morphologies and sizes indeed influence the dispersion of Fe species. The Fe/ZSM-5 catalyst, which was cauliflower-like morphology of ZSM-5 support aggregated by small nano-crystal zeolite with crystallite size of about 50 nm, exhibited the best NH₃-SCR activity (T_≥ 90% = 280–650 °C). This specific morphology and size of ZSM-5 support were considered to benefit the distribution of isolated Fe^3 + species, which was proved to be the main active sites in SCR reaction.