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1.
The NO, NO/O 2, and NO/O 2/H 2O adsorption on MnO 2/NaY (5 and 15 wt.% MnO 2) composite catalyst and NaY has been studied by means of in situ FTIR and EPR spectroscopy at elevated temperatures and during heating under reaction-like conditions. NO adsorption and co-adsorption of NO and O 2 on NaY and MnO 2/NaY proceeds via oxidation of NO forming NO 2− and NO 3− species. Whereas the manganese dioxide preferably acts as oxidising agent, the zeolite stores the NO x species as nitrite and nitrate ions in the solid. In the presence of oxygen, the nitrate formation is enhanced due to additional oxidation of NO through gaseous oxygen leading to NO 2. Dimerisation of NO 2 to N 2O 4 and following disproportionation of the latter causes the formation of NO + and NO 3− species which are associated with nucleophilic zeolitic oxygen and especially alkali cations of the zeolite, respectively. The presence of oxygen facilitates reoxidation of Mn 2+ which keeps more Mn ions in the active state. Pre-adsorbed water and higher amounts of water vapour in the feed hinder the NO adsorption by blocking the adsorption sites and shift the nitrate formation to higher temperatures. The quantities and thermal stability of the nitrates formed during NO and NO/O 2 adsorption differs which points to a different mechanism of nitrate formation. In the absence of gaseous oxygen, nitrates are formed by participation of only lattice oxygen. In the presence of oxygen, nitrate formation by dimerisation and disproportionation reactions of NO 2 dominates. The manganese component of the composite catalyst supports the oxidation of NO to nitrite and subsequently to nitrate. During this process Mn 4+ is reduced to Mn 2+ as evidenced by in situ EPR measurements. 相似文献
2.
The effect of oxygen concentration on the pulse and steady-state selective catalytic reduction (SCR) of NO with C 3H 6 over CuO/γ-Al 2O 3 has been studied by infrared spectroscopy (IR) coupled with mass spectroscopy studies. IR studies revealed that the pulse SCR occurred via (i) the oxidation of Cu 0/Cu + to Cu 2+ by NO and O 2, (ii) the co-adsorption of NO/NO 2/O 2 to produce Cu 2+(NO 3−) 2, and (iii) the reaction of Cu 2+(NO 3−) 2 with C 3H 6 to produce N 2, CO 2, and H 2O. Increasing the O 2/NO ratio from 25.0 to 83.4 promotes the formation of NO 2 from gas phase oxidation of NO, resulting in a reactant mixture of NO/NO 2/O 2. This reactant mixture allows the formation of Cu 2+(NO 3−) 2 and its reaction with the C 3H 6 to occur at a higher rate with a higher selectivity toward N 2 than the low O 2/NO flow. Both the high and low O 2/NO steady-state SCR reactions follow the same pathway, proceeding via adsorbed C 3H 7---NO 2, C 3H 7---ONO, CH 3COO −, Cu 0---CN, and Cu +---NCO intermediates toward N 2, CO 2, and H 2O products. High O 2 concentration in the high O 2/NO SCR accelerates both the formation and destruction of adsorbates, resulting in their intensities similar to the low O 2/NO SCR at 523–698 K. High O 2 concentration in the reactant mixture resulted in a higher rate of destruction of the intermediates than low O 2 concentration at temperatures above 723 K. 相似文献
3.
The interactions between Pd/TiO 2 catalyst and the reactants and potential reaction intermediates present during aqueous nitrate reduction, including NO 3−, NO 2− and NO in the presence of H 2 and H 2O were studied by infrared spectroscopy. Adsorbed forms of NO, nitrite and nitrate could all be detected in the presence of water. In the presence of water/H 2, nitrate was the most stable surface species followed by nitrite and then highly reactive NO, suggesting that the reduction of nitrate to nitrite is the rate-limiting step. High concentrations of adsorbed nitrite appear to be linked to the detection of gaseous N 2O while the formation of ammonia is related to reactions on the Pd surface and the extent of formation is linked to high levels of adsorbed NO in addition to the surface hydrogen availability and the presence of water. 相似文献
4.
Molecular DFT modeling combined with computational spectroscopy (EPR and IR) were applied for analysis of the NO bond breaking and NN and OO bond making in the context of deNO x and deN 2O reactions. Interaction of NO, N 2O and NO 2 with cationic (transition metals) and anionic (surface O 2− ions) centers was explored at the molecular level. The elementary events such as reactant coordination, charge and spin redistributions, which are principal molecular constraints for efficient decomposition of the nitrogen oxides (N 2O and NO) were discussed. Particular attention was paid to dynamics of the NO bond cleavage in N 2O molecule through electron and oxygen atom transfer routes, evaluation of preferable coordination modes of NO, discrimination between inner- and outer-sphere mechanism of NN bond formation, and the influence of spin and electronic redistribution on the reaction course (spin catalysis). Owing to their simplicity and well known surface chemistry, model systems selected for studies of such processes include MoO x/SiO 2, MgO and ZSM-5 zeolite exchanged with various transition metal ions (TMI) of different electron configuration and spin multiplicity: Mo 5+ (d 1, 2D) Fe 3+, Mn 2+, Cr + (d 5, 6S), Fe 2+ (d 6, 5D), Co 2+ (d 7, 4F), Ni 2+ (d 8, 3F), Cu 2+ (d 9, 2D) and Cu +, Zn 2+ (d 10, 1S). 相似文献
5.
The hydroxyapatite (HAP) is prepared by precipitation method and examined for the photocatalytic degradation of calmagite, a toxic and non-biodegradable azo-dye compound. The physicochemical properties of hydroxyapatite material were characterized using BET surface area, XRD, FT-IR, and SEM analysis. The FT-IR analysis of the hydroxyapatite revealed that the peak intensity due to absorbance of surface PO 43− group centered at wave number 1030 cm −1 is drastically decreased upon exposure to UV for 1 h. The study includes dark adsorption experiments at different pH conditions, influence of the amount of catalyst, and effect of pH on photocatalytic degradation of dye, chemical oxygen demand (COD) removal, biological oxygen demand (BOD 5) increase and SO 42− and NO 3− ions evolution during the degradation. At optimum photocatalytic experimental conditions the same is compared with commercial degussa P-25 TiO 2. The photocatalytic treatment significantly reduced the COD (92% removal) and increased the BOD 5/COD ratio to 0.78. Considerable evolution of SO 42− (8.5 mg L −1) and NO 3− (12.2 mg L −1) ions are achieved during the degradation process, thus reflecting the usefulness of the hydroxyapatite photocatalytic treatment in calmagite removal in wastewater. 相似文献
6.
The reaction mechanism of the reduction of NO by propene over Pd-based catalysts was studied by FTIR spectroscopy. It was observed that the reaction between NO and propene most probably goes via isocyanate (2256–2230 cm −1), nitrate (1310–1250 cm −1) and acetate (1560 and 1460 cm −1) intermediates formation. Other possible intermediates such as partially oxidized hydrocarbons, NO 2, and formates were also detected. The reaction between nitrates and acetates or carbonates reduced nitrates to N 2 and oxidized carbon compounds to CO 2. In situ DRIFT provides quick and rather easily elucidated data from adsorbed compounds and reaction intermediates on the catalyst surface. The activity experiments were carried out to find out the possible reaction mechanism and furthermore the kinetic equation for NO reduction by propene. 相似文献
7.
The electrodeposition of metallic rhodium on pyrolytic graphite from 10 mM Na 3RhCl 6 + 0.5 M NaCl aqueous solution was studied by potentiostatic method with the use of a double-pulse technique involving nucleation and growth pulses. Physico-chemical properties of Rh deposits were investigated by electrochemical methods and scanning electron microscopy. The activity of Rh-modified graphite electrodes towards nitrate reduction in neutral medium was demonstrated, the activation energy of nitrate reduction and NO 3− Langmuir adsorption constant on Rh deposits were determined. The use of double-pulse technique resulted in enhanced surface coverage in comparison with usual potentiostatic deposition and in decreasing the mean particle size down to 30 nm, while the specific catalyst surface area attains 32 m2 g−1. The increase in the nucleation pulse duration from 20 to 100 ms enhances the mass catalytic activity towards NO3− reduction, which reaches 175 A g−1 for the best samples. Irrespectively of electrodeposition parameters, only NH3 and NO2− were detected as nitrate reduction products. The rate of NO3− destruction was equal to which is much higher than that of most of Pd/Cu-based nitrate hydrogenation systems and Ag/TiO2 photocatalysts. 相似文献
8.
We have obtained mass spectra of negative ions produced by rays in artificial air at atmospheric pressure (N 2: 80%, O 2: 20%, H 2O: 20–1500 ppm, CO 2: 0.2–300 ppm, NO, NO 2 0.02 ppm). We observed two main categories: hydrates built on simple ions (O 2−, O 3−, OH −, CO 3−, CO 4−, HCO 3−, NO 2−, NO 3−), hydrates built on complex ions (NO x−, HNO γ, HCO 3−HNO x, x = 2,3; Y = 2, 3). For high values of hygrometry, CO 2 content and ageing time (5 msec) we observe the disappearance of O 2−, O 3−, OH − hydrates whereas the major part of the spectrum consists of complex ions. 相似文献
9.
Ag-based catalysts supported on various metal oxides, Al 2O 3, TiO 2, and TiO 2–Al 2O 3, were prepared by the sol–gel method. The effect of SO 2 on catalytic activity was investigated for NO reduction with propene under lean burn condition. The results showed the catalytic activities were greatly enhanced on Ag/TiO 2–Al 2O 3 in comparison to Ag/Al 2O 3 and Ag/TiO 2, especially in the low temperature region. Application of different characterization techniques revealed that the activity enhancement was correlated with the properties of the support material. Silver was highly dispersed over the amorphous system of TiO 2–Al 2O 3. NO 3− rather than NO 2− or NO x reacted with the carboxylate species to form CN or NCO. NO 2 was the predominant desorption species in the temperature programmed desorption (TPD) of NO on Ag/TiO 2–Al 2O 3. More amount of formate (HCOO −) and CN were generated on the Ag/TiO 2–Al 2O 3 catalyst than the Ag/Al 2O 3 catalyst, due to an increased number of Lewis acid sites. Sulfate species, resulted from SO 2 oxidation, played dual roles on catalytic activity. On aged samples, the slow decomposition of accumulated sulfate species on catalyst surface led to poor NO conversion due to the blockage of these species on active sites. On the other hand, catalytic activity was greatly enhanced in the low temperature region because of the enhanced intensity of Lewis acid site caused by the adsorbed sulfate species. The rate of sulfate accumulation on the Ag/TiO 2–Al 2O 3 system was relatively slow. As a consequence, the system showed superior capability for selective adsorption of NO and SO 2 toleration to the Ag/Al 2O 3 catalyst. 相似文献
10.
The pathway for selective reduction of NO x by methane over Co mordenite cataysts has been studied by comparing the rates of the individual reactions (NO oxidation, CH 4 oxidation, NO 2 reduction) with that of the combined reaction (NO + O 2 + CH 4). Co (+2) was exchanged into H-MOR and Na-MOR to give catalysts with different metal loading and number of support protons. Additionally, exchanged Co (+2) ions were precipitated with NaOH to produce dispersed cobalt oxide on Na-MOR. The NO oxidation rate is the same for ion exchanged Co (+2) ions in H-MOR and Na-MOR, but the rate of Co (+2) ions is much lower than that of cobalt oxide. NO oxidation equilibrium is obtained only for those catalysts with high metal loading, cobalt oxide or run at low GHSV. Under the conditions of selective catalytic reduction, methane oxidation by O 2 is low for all catalysts. The turnover frequency of Co on Na-MOR, however, is higher than that on H-MOR. The rate of NO 2 reduction to N 2 is directly proportional to the number of support acid sites and independent of the amount of Co. Comparison of the rates and selectivities for the individual reactions with the combined reaction of NO + O 2 + CH 4 indicates that there are two types of catalysts. For the first, the NO oxidation is in equilibrium and the rate determining step is reduction of NO 2. For these catalysts, the rate (and selectivity) for formation of N 2 is identical from NO + O 2 + CH 4 and NO 2 + CH 4. These catalysts have high metal loading and few acid sites. Nevertheless, the rate of N 2 formation increases with increasing number of protons. For the second type of catalyst, NO oxidation is not in equilibrium and is the rate limiting step. For these catalysts the rate of N 2 formation increases with increasing metal loading. Neither catalyst type, however, is optimized for the maximum formation of N 2. By using a mixture of catalysts, one with high NO oxidation activity and one with a large number of Brønsted acid sites, the rate of N 2 is greater than the weighted sum of the individual catalysts. The current results support the proposal that the pathway for selective catalytic reduction is bifunctional where metal sites affect NO oxidation, while support protons catalyze the formation of N 2. 相似文献
11.
From previous work, the adsorption of anions is regarded as an essential factor for the different corrosion behaviour of metals in solutions containing different anions. Adsorption is measured by means of 36Cl −, 82Br −, 18 F −, 36ClO 4−, 35O 42−, H 35S − and 14CN − on Pt, Ni and Fe in the form of sheets and evaporated films. Besides the determination of the adsorption after dipping into the solution, a method has been developed for the measurement of adsorption in contact with the solution and for the determination of its kinetics. The method can also be applied to O 2-free metal surfaces produced under vacuum. In this case, however, very rapid adsorption is observed, whereas normally saturation is reached only after many hours. It is concluded that, in general, exchange between oxygen on the metal and the anion takes place rather than simple adsorption. The distribution of the anions adsorbed on the metal surface has been studied by autoradiography; adsorption takes place preferentially at the grain boundaries and increases when the crystal size decreases. These results confirm the interpretation of passivation as a competition between various processes: metal dissolution, coverage by a passivating oxide film, and displacement of oxygen by anions. 相似文献
12.
The effect of various metal additives on the catalytic performance of carbon during the selective reduction of NO with methanol has been studied in the absence and in the presence of gas-phase oxygen. The mechanism of this reaction in the use of carbon-supported catalysts was studied by means of in situ FTIR combined with the measurements of catalytic activity. In the absence of oxygen, NO is adsorbed in a very small amount on the surface of carbon. The reaction of NO with the products of the methanol decomposition, results in the formation of adsorbed isocyanate (NCO) species at 2229 cm −1. Formation of gaseous NH 3 as a product of the reaction between NO and methanol was observed. Oxygen-containing surface functional groups of carbon effectively promotes the reduction of NO with methanol. IR spectroscopic results of this study show that in the presence of O 2 nitric oxide is catalytically oxidized to nitrogen dioxide, which is chemisorbed on the surface of carbon. The surface nitrogen species were identified by FTIR spectroscopy. 相似文献
13.
Copper ion-exchanged zeolites ZSM5 with SiO 2:Al 2O 3 molar ratios 33 and 53 have been subjected to activity tests for direct decomposition of NO (2000 ppm, GHSV 560–5400 h −1). In situ infrared measurements were used to follow the reaction and surface and gas phase compositions. IR studies were also done in excess oxygen with rapid NO 2 formation in the gas phase. A high level of overexchange of copper in the zeolite in combination with a low concentration of acid sites, concurrent with a high SiO2:Al2O3 ratio, enhances the conversion of NO. A vibrational band at 1631 cm−1 is observed below the light-off temperature and interpreted as a bridged nitrato group bound to Cu2+–O–Cu2+ dimers. This band disappears above the light-off temperature but the intensity below this temperature correlates with the catalytic activity. We interpret that these bridge bound nitrato groups act as siteblockers on the active sites for NO conversion and that a tentative reaction intermediate, N2O3, also binds in a bridge configuration to the same Cu2+–O–Cu2+ dimers. A second nitrato group with unidentate coordination and vibrational bands at 1598/1575 cm−1 probes isolated copper ions. A third infrared band at 2130 cm−1 confirms previous observations of
-ions bound to the zeolite. We conclude that these species are coordinated to deprotonated and negatively charged sites on the zeolite and that these sites for
adsorption are blocked by Cu2+ ion-exchange. The 2130 cm−1 species appear to have no role in direct NO decomposition but the adsorption sites are crucial for the stability of the zeolite and intimately related to ion mobility in the lattice. Prolonged immersion of the zeolite in dilute solutions of copper ions improves the catalyst performance by copper hydroxylation leading to enhanced formation of the above dimers. A high SiO2:Al2O3 ratio leads to more stable catalysts, particularly in combination with a modest overexchange of copper ions. Excessive amounts of copper escalates the deactivation of the Cu-ZSM5 catalyst through the migration and sintering of cupric oxide crystallites. 相似文献
14.
FeO x/ZrO 2 samples, prepared by impregnation with Fe(NO 3) 3, were characterised by means of DRS, XRD, FTIR, redox cycles and volumetric CO adsorption. Volumetric CO adsorption, combined with FTIR, showed that 45% of iron in the sample containing 2.8 Fe atoms nm −2 was capable of forming iron carbonyls. DRS evidenced Fe 2O 3 on samples with Fe-content≥2.8 atoms nm −2. The selective catalytic reduction of NO with C 3H 6 in the presence of O 2 was studied with a reactant mixture containing NO=4000 ppm, C 3H 6=4000 ppm, O 2=2%. The dependence on iron-content suggests that only isolated iron, prevailing in dilute FeO x/ZrO 2, is active for NO reduction, whereas iron on the surface of small oxide particles, prevailing in concentrated FeO x/ZrO 2, is active for C 3H 6 combustion. 相似文献
15.
采用Co(NO_3)_2对活性炭进行改性,考察浸渍浓度和吸附温度等条件对活性炭吸附NO性能的影响,并对已吸附NO的0.3 mol·L~(-1)的Co(NO_3)_2改性活性炭进行再生。通过BET、SEM、吸附等温线和FT-IR表征样品的比表面积、颗粒形貌和表面官能团。结果表明,当浸渍溶液浓度为0.3 mol·L~(-1)时,吸附效果最佳,80 min时吸附效率达88.90%。活性炭的吸附效率随着温度升高而降低,用0.3 mol·L~(-1)Co(NO_3)_2改性的活性炭在200℃时的吸附效率大于90%,并可持续50 min。SEM和FT-IR表征结果表明,在Co(NO_3)_2改性的活性炭表面和孔隙生成了Co_3O_4,促进NO催化氧化为NO_2并进行吸附。加热再生后的0.3 mol·L~(-1)Co(NO_3)_2改性活性炭对NO的吸附效率在60 min内仍高于88.90%,再生效果较好,可持续再生利用。 相似文献
16.
The interaction of NO with Co 2+/Co 3+ redox sites in CoAPO-18 and CoAPO-5 catalysts was studied by means of FTIR and diffuse reflectance UV–Vis spectroscopy both at 298 and 85 K. Two families of Co 2+ sites were found in the CoAPO-18 structure. (A) Ions in framework [Co 2+(OH)P], associated with Brønsted acid sites which adsorb NO to produce dinitrosyls absorbing at 1903 and 1834 cm −1; these dinitrosyl complexes are reactive, in that Co 2+ is oxidized to Co 3+ and N 2O is formed. (B) S tructural defects Co 2+ (Lewis acid sites) which stabilize dinitrosyls absorbing at 1900 and 1813 cm −1. The NO adsorption both on reduced and, more significantly, on oxidised CoAPO-18 also leads to the formation of NO 2δ+ adsorbed species. It was found that the two kinds of dinitrosyl complexes have different reactivity in presence of oxygen. Both families of sites are also present in CoAPO-5 catalysts on which, however, the redox reaction upon NO adsorption does not occur significantly. 相似文献
17.
The reduction of nitric oxide by propene in the presence of oxygen over platinum-group metals supported on TiO 2, ZnO, ZrO 2, and Al 2O 3 has been investigated by combined diffuse reflectance FT-IR spectroscopy and catalytic activity studies under flow reaction conditions at 523–673 K and atmospheric pressure. The catalytic activity for the selective reduction of nitric oxide and the intensity of the IR bands due to reaction species depended strongly on the nature of the support, type of supported metal, reaction time and temperature. The main surface species detectable by IR were adsorbed hydrocarbons (2900–3080 cm −1), isocyanate (2180, and 2232–2254 cm −1), cyanide (2125 cm −1), nitrosonium (1901 cm −1), CO 2 (2343–2357 cm −1), CO (2058 cm −1) and carbonate (1300–1650 cm −1) species. In the case of rhodium containing catalysts, when supported on Al 2O 3, they exhibited both the highest concentration of surface species and the highest activity for nitric oxide reduction and selectivity to nitrogen. The catalytic activity and the IR intensities of the nitrosonium and isocyanate bands increased with reaction temperature, reached their maximum between 570 and 620 K, and then decreased at higher temperatures. The IR band intensities due to nitrogen containing surface species were found to be strongly correlated to the activity for nitric oxide conversion and only slightly related to the selectivity to dinitrogen. 相似文献
18.
The activity of several catalysts are studied in the soot combustion reaction using air and NO/air as oxidising agents. Over Al 2O 3-supported catalysts NO (g) is a promoter for the combustion reaction with the extent of promotion depending on the Na loading. Over these catalysts SO 42− poisons this promotion by preventing NO oxidation through a site blocking mechanism. SiO 2 is unable to adsorb NO or catalyse its oxidation and over SiO 2-supported Na catalysts NO (g) inhibits the combustion reaction. This is ascribed to a competition between NO and O 2. Over Fe-ZSM-5 catalysts the presence of a NO x trapping component does not increase the combustion of soot in the presence of NO (g) and it is proposed that this previously reported effect is only seen under continuous NO x trap operation as NO 2 is periodically released during regeneration and thus available for soot combustion. Experiments during which the [NO] (g) is varied show that CO, rather than an adsorbed carbonyl-like intermediate, is formed upon reaction between NO 2 (the proposed oxygen carrier) and soot. 相似文献
19.
The catalytic decomposition of nitrous oxide to nitrogen and oxygen was studied overRh/ZnO, Rh/CeO 2, Rh/ZSM-5, CuZSM-5 and CoAlCO 3HT (hydrotalcite). The effects of metal loading and calcination conditions upon the catalytic performance were examined on Rh/ZnO. A 0.5 wt.% Rh/ZnO catalyst was found to be the most active catalyst, whose reaction rate was 4.0 × 10 4 μmol(N 2O) · g −1 · h −1 under the conditions of 950 ppm N 2O and 5% O 2 at 300°C. The oxidized Rh/ZnO showed a higher activity than that calcined in a reducing atmosphere. The TEM and EDX observations revealed the formation of particles of ca. 50Åin diameter. They consisted of rhodium and zinc oxides as major and minor components, respectively. The activities of all these catalysts decreased when NO 2 and H 2O were added to the feed. 相似文献
20.
N 2O 5 reacts with O 2− ion in LiCl---KCl eutectic at 450° to give NO 3−. By analogy to the salts of the other oxides of Group V, NO 3− can be considered as metanitrate and is expected to give—under appropriate conditions—the corresponding pyro-salt. Experiments are described in which the O 2− ion in LiCl---KCl melt is potentiometrically titrated with KNO 3. The titration curves show an inflexion at the composition corresponding to pyronitrate, N 2O 74−. The formation of pyronitrate in KNO3 melts is also established. Strong oxide-ion donors, eg Na2O2 or NaOH, or electrolytically generated O2− ion, react slowly with the melt to produce a compound of less basic character. The reaction is zero-order with respect to O2− and has an activation energy of ca 6·17 Kcal/mole. Pyronitrate in molten KNO3 possesses a basicity comparable to that of the carbonate ion in the same melt. It readily lends its oxide ion to strong acids eg, Cr2O72− and PO3−. X-ray diffraction patterns of NO3−-N2O74− mixtures show peaks that can be correlated to the new anion. 相似文献
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