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.     

The heterogeneous catalytic reduction of NO and N2O mixture by carbon monoxide

Kinetics of the simultaneous reduction N₂O and NO by CO on CuCo₂O₄ has been studied. The reactants are adsorbed onto the coordination-unsaturated cations of the catalyst. The studies showed that the reactions of N₂O and CO and of NO and CO occur between the adsorbed reactants on the catalyst surface; the catalyst surface is partially reduced during both these reactions. It was found that NO inhibits the reaction between N₂O and CO, because N₂O and NO compete for the active surface sites. The adsorption capacity of the catalyst is significantly higher for NO than for N₂O and hence NO displaces N₂Oads from the surface. The inhibition occurs on strongly localized sites and does not affect on the behaviour of the remaining free sites. At such blockage, the N₂O reduction rate decreases in direct proportion to the amount of adsorbed NO.     

REDUCTION OF NO WITH NH3 ON Fe-Cr OXIDES CATALYST

A catalyst consisting of a mixture of Fe₂O₃ and Cr₂O₃ (5/1 atomic ratio of Fe:Cr) supported on alumina pellets was examined for the complete reduction of NO with NH₃ to N₂ in an isothermal flow reactor. The reduction of NO was determined at various inlet concentrations of NO (approximating utility boiler emissions) and NH₃, and catalyst bed temperatures. In the absence of O₂, NO reduced completely to N₂ without N₂O formation. Correlation of the kinetic data considering both external and internal reactant mass transfer provided the following rate expression for the reduction of NO, r = 7.71 × 10⁴ exp. ( - 12,300/RT) P^0.9_NO P^0.1_NH3, moles / g.cat-sec atm.     

Combined plasma-catalytic processing of nitrous oxide

The gliding arc discharge, combined with a catalytic bed, has been applied for nitrous oxide processing in oxygen containing gases. It has been found that under conditions of the gliding arc, nitrous oxide in mixtures with oxygen or air not only decomposes to oxygen and nitrogen, but is also oxidised to nitric oxide. The overall conversion of nitrous oxide, as well as the degree of N₂O oxidation to NO were studied as a function of its initial concentration, flow rate, and discharge power. The overall N₂O conversion and degree of oxidation to NO decreased with increasing flow rate and initial N₂O concentration, and increased with increasing discharge power. The degree of N₂O oxidation to NO varied within 20–37%. The overall conversion and degree of N₂O oxidation increased when granular dielectric materials (TiO₂, SiO₂ (quartz glass), and γ-Al₂O₃) were introduced into the reaction zone. The energy efficiency and the overall conversion of N₂O were still further increased due to catalytic effects of a number of metal oxides (CuO, NiO, MnO₂, Fe₂O₃, Co₃O₄, ZrO₂) deposited on γ-Al₂O₃. The activity of the oxide catalysts within the active power range of 300–360 W decreased in the order: CuO>Fe₂O₃>NiO>MnO₂>Co₃O₄>ZrO₂. It has been concluded that the combined plasma-catalytic processing may be an efficient way for the reduction of N₂O emissions.     

钇掺杂三氧化二铁催化剂的脱硝性能研究

为提升三氧化二铁(Fe₂O₃)催化剂的脱硝性能,扩展催化剂的活性温度窗口,采用共沉淀法引入助剂钇(Y)元素对Fe₂O₃催化剂进行改性。通过X射线衍射(XRD)、氮气等温吸-脱附(N₂-BET)、X射线光电子能谱(XPS)、氨气程序升温脱附(NH₃-TPD)、氢气程序升温还原(H₂-TPR)等表征方法对样品进行了表征分析。XRD和N₂-BET结果表明,Y的掺杂使催化剂结构发生变化,比表面积增加、孔径减小。XPS和NH₃-TPD结果证明,Y掺杂Fe₂O₃具有更多的表面吸附氧(O_Ⅰ)、Fe²⁺以及更多的酸量。H₂-TPR结果表明,Y的掺杂使催化剂的氧化还原能力略有下降。测试了不同含量Y掺杂的Fe₂O₃催化剂在150～400℃的脱硝性能,其中Fe<...     

Pd–Ce interactions and adsorption properties of palladium: CO and NO TPD studies over Pd–Ce/Al2O3 catalysts

We have examined the adsorption of CO and NO on powder Pd/Al₂O₃, Pd–Ce/Al₂O₃ and CeO₂/Al₂O₃ catalysts, using temperature-programmed desorption (TPD). For CO adsorption on oxidized and pre-reduced Pd–Ce/Al₂O₃ TPD profiles are identical to those observed for Pd/Al₂O₃, suggesting that interactions between ceria and Pd have a negligible effect on the adsorption properties of CO. It does, however, affect the oxidation state of the palladium particles. For NO, there are differences between Pd/Al₂O₃ and Pd–Ce/Al₂O₃. On oxidized catalysts, Pd/Al₂O₃ is more efficient for NO dissociation. However, pre-reduction increases the amount of NO that can adsorb on Pd–Ce/Al₂O₃ and react to N₂O and N₂. In comparison with Pd/Al₂O₃, reduced Pd–Ce/Al₂O₃catalysts dissociate NO at relatively high temperatures but they are more reactive and favor N₂ over N₂O.     

Transient analysis of the release and reduction of NOx using a Pt/Ba/Al2O3 catalyst

The release and reduction of NO_x in a NO_x storage-reduction (NSR) catalyst were studied with a transient reaction analysis in the millisecond range, which was made possible by the combination of pulsed injection of gases and time resolved time-of-flight mass spectrometry. After an O₂ pulse and a subsequent NO pulse were injected into a pellet of the Pt/Ba/Al₂O₃ catalyst, the time profiles of several gas products, NO, N₂, NH₃ and H₂O, were obtained as a result of the release and reduction of NO_x caused by H₂ injection. Comparing the time profiles in another analysis, which were obtained using a model catalyst consisting of a flat 5 nmPt/Ba(NO₃)₂/cordierite plate, the release and reduction of NO_x on Pt/Ba/Al₂O₃ catalyst that stored NO_x took the following two steps; in the first step NO molecules were released from Ba and in the second step the released NO was reduced into N₂ by H₂ pulse injection. When this H₂ pulse was injected in a large amount, NO was reduced to NH₃ instead of N₂.

A only small amount of H₂O was detected because of the strong affinity for alumina support. We can analyze the NO_x regeneration process to separate two steps of the NO_x release and reduction by a detailed analysis of the time profiles using a two-step reaction model. From the result of the analysis, it is found that the rate constant for NO_x release increased as temperature increase.     

Interactions NO—CO and O2—NO—CO on CuCo2O4/γ-Al2O3 and on γ-Al2O3- and CuCo2O4/γ-Al2O3-supported Pt, Rh and Pt—Rh catalysts, a transient response study

The interactions NO—CO and O₂—NO—CO have been studied onCuCo₂O₄γ-Al₂O₃ and on γ-Al₂O₃- and CuCo₂O₄γ-Al₂O₃-supported Pt, Rh and Pt—Rh catalysts. The deposition of noble metals (Pt, Rh and Pt—Rh) on CuCo₂O₄γ-Al₂O₃ instead of γ-Al₂O₃ is beneficial in: lowering the temperature at which maximum N₂O is formed and decreasing the maximum N₂O concentration attained; lowering the onset temperature of NO to N₂ reduction, and increasing the N₂ selectivity; preserving the activity towards NO to N₂ reduction on a higher level following the concentration step NO + COO₂+ NO + CO and changing the conditions from stoichiometric to oxidizing (50% excess of oxidants). The reason for this behaviour of the CuCo₂O₄γ-Al₂O₃-based noble metal catalysts is the formation (reversible) of a reduced surface layer on the CuCo₂O₄ supported spinel under the conditions of a stoichiometric NO + CO mixture.     

铁氧化物在流化床温度和CO气氛下的形态迁移及其对NO催化还原

利用小型固定床实验台实验研究了铁氧化物在典型流化床温度和CO还原性气氛下的形态迁移及其生成物对NO的催化还原作用,采用分级还原结合X射线衍射（XRD）表征分析,确定铁氧化物与CO和NO反应后生成物的价态及各种铁氧化物对NO的还原机制。结果表明,Fe₂O₃在实验条件下可依次被CO还原为Fe₃O₄、FeO和单质铁,反应过程中随着还原度的增加,还原速率逐级下降,从Fe₂O₃还原到Fe₃O₄的速率最高,FeO还原到Fe速率最低,在实验温度范围内,床温升高有利于提高Fe₂O₃到Fe₃O₄的还原速率和还原度。不同形态的铁氧化物对NO的催化还原特性不同,Fe₂O₃及其部分还原后生成的Fe₃O₄都不能直接与NO反应,Fe₂O₃对CO催化还原NO的效果很弱,而Fe₃O₄对CO还原NO的反应却有很强的催化作用,而进一步还原生成FeO与单质铁还可直接与NO反应。     

Catalytic decomposition of N2O over CeO2 promoted Co3O4 spinel catalyst

A series of CeO₂ promoted cobalt spinel catalysts were prepared by the co-precipitation method and tested for the decomposition of nitrous oxide (N₂O). Addition of CeO₂ to Co₃O₄ led to an improvement in the catalytic activity for N₂O decomposition. The catalyst was most active when the molar ratio of Ce/Co was around 0.05. Complete N₂O conversion could be attained over the CoCe0.05 catalyst below 400 °C even in the presence of O₂, H₂O or NO. Methods of XRD, FE-SEM, BET, XPS, H₂-TPR and O₂-TPD were used to characterize these catalysts. The analytical results indicated that the addition of CeO₂ could increase the surface area of Co₃O₄, and then improve the reduction of Co³⁺ to Co²⁺ by facilitating the desorption of adsorbed oxygen species, which is the rate-determining step of the N₂O decomposition over cobalt spinel catalyst. We conclude that these effects, caused by the addition of CeO₂, are responsible for the enhancement of catalytic activity of Co₃O₄.     

Study of the reaction of NOx and soot on Fe2O3 catalyst in excess of O2

This study addresses the catalytic reaction of NO_x and soot into N₂ and CO₂ under O₂-rich conditions. To elucidate the mechanism of the soot/NO_x/O₂ reaction and particularly the role of the catalyst -Fe₂O₃ is used as model sample. Furthermore, a series of examinations is also made with pure soot for reference purposes. Temperature programmed oxidation and transient experiments in which the soot/O₂ and soot/NO reaction are temporally separated show that the NO reduction occurs on the soot surface without direct participation of the Fe₂O₃ catalyst. The first reaction step is the formation of CC(O) groups that is mainly associated with the attack of oxygen on the soot surface. The decomposition of these complexes leads to active carbon sites on which NO is adsorbed. Furthermore, the oxidation of soot by oxygen provides a specific configuration of active carbon sites with suitable atomic orbital orientation that enables the chemisorption and dissociation of NO as well as the recombination of two adjacent N atoms to evolve N₂. Moreover, carbothermal reaction, high resolution transmission electron microscopy and isotopic studies result in a mechanistic model that describes the role of the Fe₂O₃ catalyst. This model includes the dissociative adsorption of O₂ on the iron oxide, surface migration of the oxygen to the contact points of soot and catalyst and then final transfer of O to the soot. Moreover, our experimental data suggest that the contact between both solids is maintained up to high conversion levels thus resulting in continuous oxygen transfer from catalyst to soot. As no coordinative interaction of soot and Fe₂O₃ catalyst is evidenced by diffuse reflectance infrared Fourier transform spectroscopy a van der Waals type interaction is supposed.     

High thermal conductive β-SiC for selective oxidation of H2S: A new support for exothermal reactions

This study aims at synthesizing a new by substituting 1 atom% Pd²⁺ in ionic state in TiO₂ in the form of Ti_0.99Pd_0.01O_1.99 with oxide-ion vacancy. The catalyst was synthesized by solution combustion method and was characterized by XRD and XPS. The catalytic activity was investigated by performing CO oxidation, hydrocarbon oxidation and NO reduction. A reaction mechanism for CO oxidation by O₂ and NO reduction by CO was proposed. The model based on CO adsorption on Pd²⁺ and dissociative chemisorption of O₂ in the oxide-ion vacancy for CO oxidation reaction fitted the experimental for CO oxidation. For NO reduction in presence of CO, the model based on competitive adsorption of NO and CO on Pd²⁺, NO chemisorption and dissociation on oxide-ion vacancy fitted the experimental data. The rate parameters obtained from the model indicated that the reactions were much faster over this catalyst compared to other catalysts reported in the literature. The selectivity of N₂, defined as the ratio of the formation of N₂ and formation of N₂ and N₂O, was very high compared to other catalysts and 100% selectivity was reached at temperature of 350 °C and above. As the N₂O + CO reaction is an intermediate reaction for NO + CO reaction, it was also studied as an isolated reaction and the rate of the isolated reaction was less than that of intermediate reaction.     

Investigation of CO oxidation and NO reduction on three-way monolith catalysts with transient response techniques

The kinetics of CO oxidation and NO reduction reactions over alumina and alumina-ceria supported Pt, Rh and bimetallic Pt/Rh catalysts coated on metallic monoliths were investigated using the step response technique at atmospheric pressure and at temperatures 30–350°C. The feed step change experiments from an inert flow to a flow of a reagent (O₂, CO, NO and H₂) showed that the ceria promoted catalysts had higher adsorption capacities, higher reaction rates and promoting effects by preventing the inhibitory effects of reactants, than the alumina supported noble metal catalysts. The effect of ceria was explained with adsorbate spillover from the noble metal sites to ceria. The step change experiments CO/O₂ and O₂/CO also revealed the enhancing effect of ceria. The step change experiments NO/H₂ and H₂/NO gave nitrogen as a main reduction product and N₂O as a by-product. Preadsorption of NO on the catalyst surface decreased the catalyst activity in the reduction of NO with H₂. The CO oxidation transients were modeled with a mechanism which consistent of CO and O₂ adsorption and a surface reaction step. The NO reduction experiments with H₂ revealed the role of N₂O as a surface intermediate in the formation of N₂. The formation of NN bonding was assumed to take place prior to, partly prior to or totally following to the NO bond breakage. High NO coverage favors N₂O formation. Pt was shown to be more efficient than Rh for NO reduction by H₂.     

Selective catalytic reduction of N2O in industrial emissions containing O2, H2O and SO2: behavior of Fe/ZSM-5 catalysts

The catalytic behavior in N₂O reduction by propane in the presence of O₂, H₂O and SO₂ of Fe/ZSM-5 catalysts prepared by ion exchange and chemical vapour deposition (CVD) is reported. The catalyst prepared by CVD shows a lower dependence of the rate of selective N₂O reduction on the decrease in C₃H₈ to N₂O ratio in the feed and a higher resistance to deactivation by SO₂ in accelerated durability tests with high SO₂ concentration (500 ppm). This catalyst shows stable catalytic behavior in the presence of SO₂ for more than 600 h of time-on-stream. Characterization of the catalysts by UV–VIS–NIR diffuse reflectance indicates that the poor performances of the sample prepared by ion exchange could be related to the presence of highly clustered Fe³⁺ species, in this catalyst. On the other hand, Fe₂O₃ particles are not present in the sample prepared by CVD while mainly isolated Fe³⁺ ions and iron-oxide nanoclusters are present.     

稻壳白炭黑负载Fe2O3的气相芬顿反应NO预氧化

以稻壳为硅源,采用直接煅烧法制备白炭黑,以其为载体,采用共浸渍法制备Fe₂O₃/SiO₂催化剂;并采用同样方法以商用二氧化硅为载体制备Fe₂O₃/C-SiO₂催化剂,将二者用于催化H₂O₂预氧化NO的实验。探究不同工况（负载量、催化温度、H₂O₂汽化温度、H₂O₂流量和水汽浓度）对NO预氧化的影响,并对催化剂进行表征,分析其物理化学性质对催化性能的影响。结果表明,在负载量为50%、催化温度为140℃、H₂O₂汽化温度为120℃、H₂O₂流量为2.5mL/h时,达到最佳工况,NO氧化度能达到73%;在相同实验条件下Fe₂O₃/SiO₂催化剂的预氧化效果要比Fe₂O₃/C-SiO₂催化剂高20%左右。TPR结果表明载体可以降低活性组分的还原温度,减少活性组分的团聚;催化剂的晶相结构稳定,机械强度及热稳定性良好;ESR和XPS结果显示Fe₂O₃/SiO₂催化剂的催化性能优于Fe₂O₃/C-SiO₂催化剂,能够更好地催化分解H₂O₂产生·OH。     

Effect of the pretreatment with oxygen and/or oxygen-containing compounds on the catalytic performance of Pd-Ag/Al2O3 for acetylene hydrogenation

Catalytic performance of Pd-Ag/-Al₂O₃ was studied for the selective hydrogenation of acetylene in the presence of excess ethylene. The catalyst activation was undertaken prior to the reaction test by the pretreatment with oxygen and/or oxygen-containing compounds, i.e. O₂, NO, N₂O, CO and CO₂. The enhancement of catalytic performances by the pretreatment was a consequence of an increase in accessible Pd sites responsible for acetylene hydrogenation to ethylene. Furthermore, the sites involving direct ethane formation from acetylene could be suppressed by NO_x treatment.     

TAP study of NOx storage and reduction on Pt/Al2O3 and Pt/Ba/Al2O3

A systematic mechanistic study of NO storage and reduction over Pt/Al₂O₃ and Pt/BaO/Al₂O₃ is carried out using Temporal Analysis of Products (TAP). NO pulse and NO/H₂ pump-probe experiments at 350 °C on pre-reduced, pre-oxidized, and pre-nitrated catalysts reveal the complex interplay between storage and reduction chemistries and the importance of the Pt/Ba coupling. NO pulsing experiments on both catalysts show that NO decomposes to major product N₂ on clean Pt but the rate declines as oxygen accumulates on the Pt. The storage of NO over Pt/BaO/Al₂O₃ is an order of magnitude higher than on Pt/Al₂O₃ showing participation of Ba in the storage even in the absence of gas phase O₂. Either oxygen spillover or transient NO oxidation to NO₂ is postulated as the first steps for NO storage on Pt/BaO/Al₂O₃. The storage on Pt/Ba/Al₂O₃ commences as soon as Pt–O species are formed. Post-storage H₂ reduction provides evidence that a fraction of NO is not stored in close proximity to Pt and is more difficult to reduce. A closely coupled Pt/Ba interfacial process is corroborated by NO/H₂ pump-probe experiments. NO conversion to N₂ by decomposition is sustained on clean Pt using excess H₂ pump-probe feeds. With excess NO pump-probe feeds NO is converted to N₂ and N₂O via the sequence of barium nitrate and NO decomposition. Pump-probe experiments with pre-oxidized or pre-nitrated catalyst show that N₂ production occurs by the decomposition of NO supplied in a NO pulse or from the decomposition of NOx stored on the Ba. The transient evolution of the two pathways depends on the extent of pre-nitration and the NO/H₂ feed ratio.     

Rôles of catalytic oxidation in control of vehicle exhaust emissions

Catalytic oxidation was initially associated with the early development of catalysis and it subsequently became a part of many industrial processes, so it is not surprising it was used to remove hydrocarbons and CO when it became necessary to control these emissions from cars. Later NOx was reduced in a process involving reduction over a Pt/Rh catalyst followed by air injection in front of a Pt-based oxidation catalyst. If over-reduction of NO to NH₃ took place, or if H₂S was produced, it was important these undesirable species were converted to NOx and SOx in the catalytic oxidation stage. When exhaust gas composition could be kept stoichiometric hydrocarbons, CO and NOx were simultaneously converted over a single Pt/Rh three-way catalyst (TWC). With modern TWCs car tailpipe emissions can be exceptionally low. NO is not catalytically dissociated to O₂ and N₂ in the presence of O₂, it can only be reduced to N₂. Its control from lean-burn gasoline engines involves catalytic oxidation to NO₂ and thence nitrate that is stored and periodically reduced to N₂ by exhaust gas enrichment. This method is being modified for diesel engines. These engines produce soot, and filtration is being introduced to remove it. The exhaust temperature of heavy-duty diesels is sufficient (250–400 °C) for NO to be catalytically oxidised to NO₂ over an upstream platinum catalyst that smoothly oxidises soot in the filter. The exhaust gas temperature of passenger car diesels is too low for this to take place all of the time, so trapped soot is periodically burnt in O₂ above 550 °C. Catalytic oxidation of higher than normal amounts of hydrocarbon and CO over an upstream catalyst is used to give sufficient temperature for soot combustion with O₂ to take place.     

SELECTIVE CATALYTIC REDUCTION OF NITRIC OXIDE BY AMMONIA ON PILLARED CLAY CATALYSTS: POSSIBLE MECHANISM AND PROMOTERS

The delaminated Fe₂0₃-pillared clay shows high activities for selective catalytic reduction (SCR) of NO by NH₂. Temperature program desorption (TPD) studies show that large amounts of NO., are adsorbed on the pillared clay catalyst at the SCR reaction temperatures (i.e. near 400°C). This result indicates that a Langmuir-Hinshelwood type mechanism (for reaction between chemisorbed NO, and NH, on the surface to form N₂) is operative for the pillared clay catalyst, which is in contrast to the SCR reaction on the commercial vanadia-based catalysts. The SCR activities for the delaminated Fe₂0₃-pillared clay catalyst are higher than that of a commercial-type V₂O₅ + WO₃/TiO₂ catalyst under SO₂ + H₂0 free conditions, but became lower in the presence of SO₂ +l H₂0. However, when promoted by doping 1-3% Cr₂0₃, the pillared clay catalyst exhibits higher SCR activities than the commercial-type catalyst in the presence of S0₂ + H₂O at all practical SCR reaction temperatures     

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.