Fourier transform IR study of NOx adsorption on a CuZSM-5 DeNOx catalyst

The adsorption and coadsorption of selective catalytic reduction (SCR) reactants and reaction products on CuZSM-5-37 containing 11 wt.-% CuO have been studied by FTIR spectroscopy. The catalyst surface is characterized by both weak acidity and weak basicity as revealed by testing with probe molecules (CO₂, NH₃, H₂O). NO₂ adsorption results in formation of different kinds of nitrates. The same species are formed when NO is coadsorbed with oxygen at 180°C. NO adsorption at ambient temperature also leads to formation of nitrates as well as of Cu²⁺NO species. In the presence of oxygen the latter are converted according to the scheme: NO → N₂O₃ → N₂O₄ → NO₂ → NO₃. It is concluded that the surface nitrates are important intermediates in the SCR process. They are thermally stable and resistant towards interaction with CO₂, N₂, O₂, and are only slightly affected by H₂O and NO. However, they posses a high oxidation ability and are fully reduced by propane at 180°C. It is concluded that one of the most important roles of oxygen in SCR by hydrocarbons is to convert NO_x into highly active surface nitrates.     

A study of the mechanism of selective conversion of ammonia to nitrogen on Ni/γ-Al2O3 under strongly oxidising conditions

The activity and excellent selectivity (>90%) of γ-Al₂O₃-supported Ni for the selective catalytic oxidation (SCO) of NH₃ to N₂ with excess O₂ has been shown by microreactor studies. Further studies of the mechanism involved in this reaction have been carried out using TPD, TPO, TPReaction as well as DRIFTS. N₂H₄ and NO have been used to model the intermediates of the SCO mechanism (direct formation of N₂ via the recombination of two NH_x species) and of the in situ SCR mechanism (two-step formation of N₂ via the reduction of an in situ produced NO species by a NH_x species), respectively. Two IR absorption bands appear during the TPO of NH₃ in the temperature range of N₂ formation and have been assigned to stable bidentate nitrate surface species. This represents strong evidence that under the present conditions, formation of N₂ occurs via the in situ SCR mechanism. This also explains the sudden “NO jump” observed on various systems once the temperature is high enough to activate 50% of the NH₃ molecules fed to the catalyst. The fact that NO and NH₃ are able to react to give N₂ at low temperature (from 100°C) confirms that activation of NH₃ is the limiting step. In contrast, no evidence has been found to support the possibility of the SCO mechanism.     

The mechanism of the selective reduction of nitrogen oxides by hydrocarbons on zeolite catalysts

The mechanism of the selective catalytic reduction (SCR) of nitrogen oxides over 3d transition metal zeolites has been investigated in a variety of ways. The initial step is the abstraction of hydrogen from the hydrocarbon by adsorbed NO₂ species which is rate determining with methane but not with isobutane. The subsequent path appears to involve nitroso and/or nitro compounds. Comparative studies of the reactions of such compounds indicate that nitromethane is more likely to be an intermediate than nitrosomethane during the methane-SCR reaction over Co-MFI although the latter cannot be ruled out entirely. In both cases the predominant route to N₂ is an initial decomposition to carbon oxides and ammonia followed by the NH₃-SCR reaction. The isobutane-SCR reaction over Fe-MFI produces substantial amounts of hydrogen cyanide which disappears only at temperatures where all the hydrocarbon has been consumed. Hydrogen cyanide appears to arise from isobutyronitrile, the expected dehydration product if an initially formed nitroso compound undergoes tautomerism to an oxime. HCN is converted to N₂ largely by reaction with NO₂ which is fast well below 300°C in the absence of isobutane. The corresponding isobutane-SCR reaction over Cu-MFI gives rise to cyanogen (C₂N₂) rather than HCN. The general path is probably the same in the two systems with the difference arising from variation in the relative reactivity of HCN. The copper-containing catalyst is very effective at forming and dimerising adsorbed cyanide groups while the iron catalyst has higher activity for the oxidation of NO to the NO₂ needed to convert adsorbed cyanide to N₂. The difference between the apparent involvement of a nitro route in methane-SCR with Co-MFI, and a nitroso one with isobutane, is similarly explainable. The former reaction proceeds with simultaneous production of NO₂ which can participate in the intermediate chemistry that follows. However, the NO₂ concentration is low during the latter reaction over Cu-MFI and Fe-MFI as long as any hydrocarbon remains. This is due to the blocking of sites for NO oxidation by deposits and the recycling of NO₂ back to NO during hydrocarbon oxidation. Thus only NO is available and the nitroso route prevails. The extent to which this picture applies with other catalysts and other hydrocarbons remains to be established.     

NO–NH3 coadsorption on vanadia/titania catalysts: determination of the reduction degree of vanadium

Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) has been used to study NH₃ and NO adsorption over a 15% w/w vanadia/titania catalyst. NH₃ is adsorbed as coordinate NH₃ and NH₄⁺ species over the oxidised catalyst, leading to the reduction of the vanadia surface. At 300°C, adsorbed nitrosyls species are detected, suggesting that the oxidation of gaseous or adsorbed ammonia species takes place over the V=O sites. Coadsorption experiments show that NO is able to reoxidise about the 57% of the reduced V=O groups, resulting in N₂, according to a NO+V_□→1/2N₂+V=O reaction. On the other hand, NO is only adsorbed over vanadia reduced surfaces. The measure of the area of the 2ν(V=O) bands results in an estimate of the oxidation state of vanadium. From this estimate it can be concluded that nitrosyls species are adsorbed on the catalyst surface for vanadium atoms having an oxidation state ranging from +4 to +3.1.     

Influence of NO2 on the selective catalytic reduction of NO with ammonia over Fe-ZSM5

The influence of NO₂ on the selective catalytic reduction (SCR) of NO with ammonia was studied over Fe-ZSM5 coated on cordierite monolith. NO₂ in the feed drastically enhanced the NO_x removal efficiency (DeNOx) up to 600 °C, whereas the promoting effect was most pronounced at the low temperature end. The maximum activity was found for NO₂/NO_x = 50%, which is explained by the stoichiometry of the actual SCR reaction over Fe-ZSM5, requiring a NH₃:NO:NO₂ ratio of 2:1:1. In this context, it is a special feature of Fe-ZSM5 to keep this activity level almost up to NO₂/NO_x = 100%. The addition of NO₂ to the feed gas was always accompanied by the production of N₂O at lower and intermediate temperatures. The absence of N₂O at the high temperature end is explained by the N₂O decomposition and N₂O-SCR reaction. Water and oxygen influence the SCR reaction indirectly. Oxygen enhances the oxidation of NO to NO₂ and water suppresses the oxidation of NO to NO₂, which is an essential preceding step of the actual SCR reaction for NO₂/NO_x < 50%. DRIFT spectra of the catalyst under different pre-treatment and operating conditions suggest a common intermediate, from which the main product N₂ is formed with NO and the side-product N₂O by reaction with gas phase NO₂.     

Active sites of Cu-ZSM-5 for the decomposition of acrylonitrile

The catalytic decomposition of acrylonitrile (AN) over Cu-ZSM-5 prepared with various Cu loadings was investigated. AN conversion, during which the nitrogen atoms in AN were mainly converted to N₂, increased as Cu loading increased. N₂ selectivities as high as 90–95% were attained. X-ray diffraction measurements (XRD) and temperature-programmed reduction by H₂ (H₂-TPR) showed the existence of bulk CuO in Cu-ZSM-5 with a Cu loading of 6.4 wt% and the existence of highly dispersed CuO in Cu-ZSM-5 with a Cu loading of 3.3 wt%. Electron spin resonance measurements revealed that Cu-ZSM-5 contains three forms of isolated Cu²⁺ ions (square-planar, square-pyramidal, and distorted square-pyramidal). The H₂-TPR results suggested that in Cu-ZSM-5 with a Cu loading of 2.9 wt% and below, Cu⁺ existed even after oxidizing pretreatment. The activity of AN decomposition over Cu/SiO₂ suggested that CuO could form N₂, but, independent of the CuO dispersion, nitrogen oxides (NO_x) were formed above 350 °C. Cu⁺ and the square-pyramidal and distorted square-pyramidal forms of Cu²⁺ showed low activity for AN decomposition. Temperature-programmed desorption of NH₃ suggested that N₂ formation from NH₃ proceeded on Cu²⁺, resulting in the formation of Cu⁺. The Cu⁺ ions were oxidized to Cu²⁺ at around 300 °C. Thus, high N₂ selectivity over Cu-ZSM-5 with a wide range of temperature was probably attained by the reaction over the square-planar Cu²⁺, which can be reversibly reduced and oxidized.     

Characterization and reactivity of pure TiO2–SO4 SCR catalyst: influence of SO4 content

The kinetics of the reaction of NO, N₂O and CO₂ with activated carbon without catalyst and impregnated with a precursor salt of vanadium (ammonium monovanadate) was investigated. The conversion of NO, N₂O and CO₂ was studied (450–900°C) using a TGA apparatus and a fixed bed reactor. The reactor effluents were analysed using a GC/MS on line. The addition of vanadium increased carbon reactivity and adsorption at lower temperatures. For NO and N₂O conversion the main products obtained were N₂, N₂O, CO and CO₂ but for CO₂ conversion only CO was detected. In situ XRD was a useful tool for interpreting catalyst behaviour and identifying phases present during reaction conditions. The catalytic effect of vanadium can be explained by the occurrence of redox processes in which the catalyst is reduced to lower oxidation states such as V₂O₅/V₆O₁₃.     

Vanadium loaded carbon-based catalysts for the reduction of nitric oxide

Carbon-based SCR catalysts for the reduction of NO with NH₃ at low temperatures have been prepared using activated carbons obtained from a local Spanish coal, doped with several vanadium compounds. Among them, the ashes of a petroleum coke (PCA) were also employed. Both the catalysts and the carbon supports have been characterized by means of N₂ and CO₂ physisorption, NH₃ and O₂ chemisorption and temperature programmed desorption (TPD). The activity of the catalysts has been tested in a laboratory-scale unit, measuring significant conversions of NO (above 50%) with almost 100% selectivity toward N₂ at 150 °C. The feasibility of using the petroleum coke ashes as the active phase was confirmed comparing the activity of the catalysts doped with these residues, with the one measured for the catalysts prepared using model vanadium compounds. The physical–chemical features of the carbon support resulted of key importance for achieving a considerable catalytic activity. The values of apparent energy of activation calculated for the catalysts presented in this paper were very similar to other carbon-based catalysts and smaller than the ones corresponding to TiO₂-supported systems. The gas residence time on the catalytic bed influences the catalytic activity to a great extent, thus being a determinant parameter for designing the SCR de-NO_x unit. To avoid ammonia slip, inlet concentrations of NH₃ has to be little under the stoichometric NH₃/NO ratio (0.7). The catalysts stability was tested in terms of carbon support gasification followed by termogravimetric analysis and gas chromatography. The activity of the catalysts was maintained at least over 24 h of reaction.     

Study of M-ZSM-5 nanocatalysts (M:Cu,Mn, Fe,Co...) for selective catalytic reduction of NO with NH3: Process optimization by Taguchi method

A series of different transition metals (V, Co, Cr, Mn, Fe, Ni, Cu and Zn) promoted H-ZSM-5 catalysts were prepared by impregnation method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The catalytic activity of these catalysts was evaluated for the selective catalytic reduction (SCR) of NO with NH₃ as reductant in the presence of oxygen. The results revealed that the catalytic activity of Cu-ZSM-5 nanocatalyst for NO conversion to N₂ was 80% at 300 ℃, which was the best among various promoted metals. Design of experiments (DOEs) with Taguchi method was employed to optimize NH₃-SCR process parameters such as NH₃/NO ratio, O₂ concentration, and gas hourly space velocity (GHSV) over Cu-ZSM-5 nanocatalyst at 250 and 300 ℃. Results showed that the most important parameter in NH₃-SCR of NO is O₂ concentration; followed by NH₃/NO ratio and GHSV has little importance. The NO conversion to N₂ of 63.1% and 94.86%was observed at 250 ℃ and 300 ℃, respectively under the obtained optimum conditions.     

Selective catalytic reduction of NOx with NH3 over Cu-ZSM-5—The effect of changing the gas composition

The selective catalytic reduction of nitrogen oxides (NO_x) with ammonia over ZSM-5 catalysts was studied with and without water vapor. The activity of H-, Na- and Cu-ZSM-5 was compared and the result showed that the activity was greatly enhanced by the introduction of copper ions. A comparison between Cu-ZSM-5 of different silica to alumina ratios was also performed. The highest NO conversion was observed over the sample with the lowest silica to alumina ratio and the highest copper content. Further studies were performed with the Cu-ZSM-5-27 (silica/alumina = 27) sample to investigate the effect of changes in the feed gas. Oxygen improves the activity at temperatures below 250 °C, but at higher temperatures O₂ decreases the activity. The presence of water enhances the NO reduction, especially at high temperature. It is important to use about equal amounts of nitrogen oxides and ammonia at 175 °C to avoid ammonia slip and a blocking effect, but also to have high enough concentration to reduce the NO_x. At high temperature higher NH₃ concentrations result in additional NO_x reduction since more NH₃ becomes available for the NO reduction. At these higher temperatures ammonia oxidation increases so that there is no ammonia slip. Exposing the catalyst to equimolecular amounts of NO and NO₂ increases the conversion of NO_x, but causes an increased formation of N₂O.     

Alumina- and titania-based monolithic catalysts for low temperature selective catalytic reduction of nitrogen oxides

The selective catalytic reduction of NO+NO₂ (NO_x) at low temperature (180–230°C) with ammonia has been investigated with copper-nickel and vanadium oxides supported on titania and alumina monoliths. The influence of the operating temperature, as well as NH₃/NO_x and NO/NO₂ inlet ratios has been studied. High NO_x conversions were obtained at operating conditions similar to those used in industrial scale units with all the catalysts. Reaction temperature, ammonia and nitrogen dioxide inlet concentration increased the N₂O formation with the copper-nickel catalysts, while no increase was observed with the vanadium catalysts. The vanadium-titania catalyst exhibited the highest DeNO_x activity, with no detectable ammonia slip and a low N₂O formation when NH₃/NO_x inlet ratio was kept below 0.8. TPR results of this catalyst with NO/NH₃/O₂, NO₂/NH₃/O₂ and NO/NO₂/NH₃/O₂ feed mixtures indicated that the presence of NO₂ as the only nitrogen oxide increases the quantity of adsorbed species, which seem to be responsible for N₂O formation. When NO was also present, N₂O formation was not observed.     

NO decomposition and reduction by CH4 over Sr/La2O3

Both NO decomposition and NO reduction by CH₄ over 4%Sr/La₂O₃ in the absence and presence of O₂ were examined between 773 and 973 K, and N₂O decomposition was also studied. The presence of CH₄ greatly increased the conversion of NO to N₂ and this activity was further enhanced by co-fed O₂. For example, at 773 K and 15 Torr NO the specific activities of NO decomposition, reduction by CH₄ in the absence of O₂, and reduction with 1% O₂ in the feed were 8.3·10⁻⁴, 4.6·10⁻³, and 1.3·10⁻² μmol N₂/s m², respectively. This oxygen-enhanced activity for NO reduction is attributed to the formation of methyl (and/or methylene) species on the oxide surface. NO decomposition on this catalyst occurred with an activation energy of 28 kcal/mol and the reaction order at 923 K with respect to NO was 1.1. The rate of N₂ formation by decomposition was inhibited by O₂ in the feed even though the reaction order in NO remained the same. The rate of NO reduction by CH₄ continuously increased with temperature to 973 K with no bend-over in either the absence or the presence of O₂ with equal activation energies of 26 kcal/mol. The addition of O₂ increased the reaction order in CH₄ at 923 K from 0.19 to 0.87, while it decreased the reaction order in NO from 0.73 to 0.55. The reaction order in O₂ was 0.26 up to 0.5% O₂ during which time the CH₄ concentration was not decreased significantly. N₂O decomposition occurs rapidly on this catalyst with a specific activity of 1.6·10⁻⁴ μmol N₂/s m² at 623 K and 1220 ppm N₂O and an activation energy of 24 kcal/mol. The addition of CH₄ inhibits this decomposition reaction. Finally, the use of either CO or H₂ as the reductant (no O₂) produced specific activities at 773 K that were almost 5 times greater than that with CH₄ and gave activation energies of 21–26 kcal/mol, thus demonstrating the potential of using CO/H₂ to reduce NO to N₂ over these REO catalysts.     

Regeneration of a vanadium pentoxide supported activated coke catalyst-sorbent used in simultaneous sulfur dioxide and nitric oxide removal from flue gas: Effect of ammonia

A vanadium pentoxide supported activated coke (V₂O₅/AC) catalyst-sorbent has been reported to be very active for simultaneous removal of SO₂ and NO under dry conditions at temperatures of 200 °C and below. Regeneration of the SO₂-captured catalyst-sorbent is a key step in operation of such a process, which influences the catalyst-sorbent's SO₂ and NO removal activities, lifetime, as well as recovery of sulfur. Due to limited information in this regard, this paper studies thermal regeneration of a V₂O₅/AC catalyst-sorbent with emphases on the effect of atmosphere. The optimum regeneration temperature is found to be 380 °C in an Ar stream and 300 °C in a 5% NH₃/Ar stream. Compared to the fresh V₂O₅/AC, the V₂O₅/ACs regenerated in Ar show lower SO₂ adsorption capacities and higher NO removal activities, while the regenerated V₂O₅/ACs by 5% NH₃/Ar show higher and stable SO₂ adsorption capacities and higher NO removal activities. Two types of reactions occur during the regeneration: reduction of the adsorbed sulfur species by carbon to SO₂ and CO₂, and oxidation of carbon by oxygen in the V₂O₅/AC to CO₂. The carbon consumption of the latter is much more than that of the former in an Ar atmosphere, but fully suppressed by the presence of 5% NH₃. Detailed analysis and characterization of the V₂O₅/AC subjected to the regenerations are presented.     

单空缺石墨烯负载的Pd单原子催化剂上NO还原的密度泛函理论研究

采用密度泛函理论（DFT）方法对单空缺石墨烯负载的Pd单原子（Pd/SVG）催化剂上H₂还原NO的反应进行了研究,探究了Pd/SVG上NO还原生成N₂和NH₃的路径。在Pd/SVG上NO容易加氢形成HNO,需要的活化能为67.0 kJ·mol^-1,显示了极高的催化活性。N₂生成的有利路径为NO活化生成HNO后,HNO继续加氢生成中间体NH₂O和NH₂OH,然后NH₂OH解离生成NH₂和OH,生成的NH₂中间体结合NO形成NH₂NO,然后NH₂NO异构化形成的NHNOH再经解离生成N₂与H₂O,这个过程中的决速步骤为NH₂NO分子内氢转移生成NHNOH,能垒为144.3 kJ·mol^-1。对于NH₃的生成,从NO的活化到中间体NH₂的形成与N₂的形成过程相同,最后NH₂加氢即可形成NH₃,这个过程中的决速步骤为NH₂O加氢生成NH₂OH,能垒为86.4 kJ·mol^-1。比较生成N₂和NH₃的决速步能垒可见,Pd/SVG催化剂上NO经H₂还原更容易形成NH₃。本研究为石墨烯负载型Pd基催化剂上H₂还原NO的实验及工业应用提供理论参考。     

Formation of nitrogen oxides from fuel-N through HCN and NH3: a model-compound study

The conversion of fuel-nitrogen to HCN and NH₃ and to nitrogen oxides was studied with nitrogen-containing model compounds, chosen to represent the main nitrogen and oxygen functionalities in fossil fuels. Two kinds of experiments were performed in an entrained-flow reactor at 800 °C. The conversion of model-compound-N to HCN and NH₃ was determined under inert conditions, and the formation of NO, N₂O and NO₂ was determined under oxidizing conditions. In inert atmosphere, oxygen-containing functional groups had an important effect on the ratio of HCN to NH₃. In particular, OH groups bound directly in the ring structure increased the conversion of nitrogen to NH₃. In oxidizing atmosphere, the conversions of model-compound-N to N₂O were high, but the substituent groups had no well-defined effect on the ratio of N₂O to NO. The formation of NO₂ was insignificant.     

Influence of CO2 and H2O on NOx storage and reduction on a Pt–Ba/γ-Al2O3 catalyst

In this paper, the effect of CO₂ and H₂O on NO_x storage and reduction over a Pt–Ba/γ-Al₂O₃ (1 wt.% Pt and 30 wt.% Ba) catalyst is shown. The experimental results reveal that in the presence of CO₂ and H₂O, NO_x is stored on BaCO₃ sites only. Moreover, H₂O inhibits the NO oxidation capability of the catalyst and no NO₂ formation is observed. Only 16% of the total barium is utilized in NO storage. The rich phase shows 95% selectivity towards N₂ as well as complete regeneration of stored NO. In the presence of CO₂, NO is oxidized into NO₂ and more NO_x is stored as in the presence of H₂O, resulting in 30% barium utilization. Bulk barium sites are inactive in NO_x trapping in the presence of CO₂·NH₃ formation is seen in the rich phase and the selectivity towards N₂ is 83%. Ba(NO₃)₂ is always completely regenerated during the subsequent rich phase. In the absence of CO₂ and H₂O, both surface and bulk barium sites are active in NO_x storage. As lean/rich cycling proceeds, the selectivity towards N₂ in the rich phase decreases from 82% to 47% and the N balance for successive lean/rich cycles shows incomplete regeneration of the catalyst. This incomplete regeneration along with a 40% decrease in the Pt dispersion and BET surface area, explains the observed decrease in NO_x storage.     

非热等离子体强化TiO2催化尿素分解副产物水解性能的研究

尿素-选择性催化还原技术低温下运行时尿素分解不彻底,易形成缩二脲、三聚氰酸和三聚氰胺等副产物。本研究将TiO₂催化剂与介质阻挡放电等离子体相结合,在程序升温条件下考察了载气中有无O₂时引入等离子体前后TiO₂催化尿素分解副产物水解的性能。结果表明：TiO₂表面缩二脲、三聚氰酸和三聚氰胺分别在43~261℃、217~300℃和199~300℃水解生成NH₃和CO₂,载气中有无O₂对催化水解过程几乎无影响。引入等离子体后缩二脲、三聚氰酸和三聚氰胺水解所需温度显著降低,载气中无O₂时引入等离子体NH₃产率变化不大,副产物仅有少量N₂O和NO,有O₂时NH₃产率显著降低,且生成较多N₂O、NO、NO₂及少量NH₄NO₂和NH₄NO₃。未来需从优化放电条件和催化剂组成等方面解决引入等离子体导致副产物形成等问题。     

Inhibition effect of H2O on V2O5/AC catalyst for catalytic reduction of NO with NH3 at low temperature

The inhibition effect of H₂O on V₂O₅/AC catalyst for NO reduction with NH₃ is studied at temperatures up to 250 °C through TPD, elemental analyses, temperature-programmed surface reaction (TPSR) and FT-IR analyses. The results show that H₂O does not reduce NO and NH₃ adsorption on V₂O₅/AC catalyst surface, but promotes NH₃ adsorption due to increases in Brønsted acid sites. Many kinds of NH₃ forms present on the catalyst surface, but only NH₄⁺ on Brønsted acid sites and a small portion of NH₃ on Lewis acid sites are reactive with NO at 250 °C or below, and most of the NH₃ on Lewis acid sites does not react with NO, regardless the presence of H₂O in the feed gas. H₂O inhibits the SCR reaction between the NH₃ on the Lewis acid sites and NO, and the inhibition effect increases with increasing H₂O content. The inhibition effect is reversible and H₂O does not poison the V₂O₅/AC catalyst.     

On the quantitative aspects of hydrolysis of isocyanic acid on TiO2

The selective catalytic reduction with aqueous solutions of urea is currently seen having the highest potential to reduce NO_x and particulate emissions for commercial diesel powered vehicles. Ammonia as the actual reduction medium is formed from urea in two consecutive reactions, i.e. via the thermolysis of urea to isocyanic acid and NH₃ and the catalyzed hydrolysis of HNCO over TiO₂ to NH₃ and CO₂. A kinetic model for the hydrolysis reaction was derived for a reaction scheme comprising a set of elementary steps. To minimize the number of unknown variables in the kinetic model for the overall rate, the equilibrium constants for both reactants (HNCO and H₂O) and products (NH₃ and CO₂) were determined from adsorption isotherms using Langmuir and multilayer adsorption models. A data set consisting of 49 data points for the rate determined at varying reactant concentrations was fitted with the kinetic model using a non-linear least mean squares regression analysis.