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1.
Metal–support interaction in the catalytic combustion of toluene was studied using metal oxides with different acid–base properties as supports for Pd. The catalytic performance was correlated with XPS data and the reaction order for oxygen. These studies revealed that the affinity for oxygen of Pd surface changed according to the acid–base character of metal oxide over MgO, Al 2O 3, SiO 2, SnO 2, Nb 2O 5, and WO 3. However, ZrO 2 exhibited exceptional character in that metal Pd was unusually stabilized, which was derived from the weak interaction between Pd and support surface. The reason for high toluene combustion activity of Pd/ZrO 2 was ascribed to the stabilization of metal Pd on ZrO 2. 相似文献
2.
The role of ceria, niobium and molybdenum oxides on the promotion of the NO reduction by CO was studied. A bifunctional mechanism was discussed as a function of both the nature of interaction between metal oxide and palladium and the redox properties of each metal oxide. The NO dissociation was better on the Pd/MoO3/Al2O3 catalyst than on the Pd/CeO2/Al2O3 and Pd/Nb2O5/Al2O3 catalysts. The explanation for the very high N2 production on Pd–Mo catalyst during the TPD analysis may be attributed to the NO+Meδ+ stoichiometric reaction. The promoting effect of a reducible oxide for the NO+CO reaction at low temperature can be ascribed mainly to its easiness for a redox interchange and its interaction with the noble metal particles. This would increase the surface redox ability and favor the dynamic equilibrium needed for high N2 selectivity. 相似文献
3.
Oxidation reactions in heterogeneous catalysis usually involve a Mars and van Krevelen mechanism which includes activation of the substrate on a metallic cation, insertion of oxygen from lattice oxygen ions, a redox mechanism on the catalyst surface, and the transfer of several electrons. It follows that such a reaction necessitates both acid-base and redox properties of a catalyst the acid site being of Lewis type (cations) and the basic sites being the surface O 2- or OH - species which could exhibit electrophilic or nucleophilic properties. The active site should be able to fulfil the following requirements: H abstraction from the substrate, oxygen insertion, and electron transfer. It has been shown to correspond to an ensemble of atoms of limited size in an inorganic molecular complex. It could correspond to local structural defects including steps, kinks, coordinatively unsaturated cations or to clusters of atoms on the surface. Some examples are described namely: - 1. (i) n-butane oxidation to maleic anhydride on (VO)2P2O7 catalyst where four dimers of vanadyl cations on the (100) face were suggested to form the active site;
- 2. (ii) isobutyric acid oxidative dehydrogenation to methacrylic acid on iron hydroxy phosphates where trimers of iron oxide octahedra were shown to constitute the most efficient and selective catalytic site while water was observed to be absolutely necessary to facilitate the reaction which corresponds to hydroxylated surface sites ensuring the redox mechanism;
- 3. (iii) propane oxidative dehydrogenation to propene on VMgO samples which was shown to depend both on VOx arrangements with respect to MgO and on the basicity of the material induced by MgO while vanadium cations induced acidic features.
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4.
The reactions of the chemical warfare agent sulfur mustard (HD) degradation over CaO, MgO, SiO 2, Al 2O 3, HZSM-5, A-Clays were studied. More than 10 kinds of products from the degradation of HD over these oxides were detected and identified by GC-FPD, GC–MS, NMR and UV–vis approaches. All the studied oxides can exhibit reactivity towards destroying HD molecules in air at room temperature. The acid and base sites over the oxides were not entirely poisoned by H 2O and CO 2 in air as evidenced by the acid–base property characterization results. The conserved acid and base sites over the oxides might be the reaction center for degradation of HD molecules. Both degradation activity and product distribution were strongly determined by the strength and density of the acid–base sites and the adsorbed water over the oxides. 相似文献
5.
The adsorption of NO on the oxygen site of several metal oxide surfaces is discussed. It is shown that the strength of the interaction and the variation of the bond lengths are not always correlated to the electron transfer from NO to the surface atoms. In cases of irreducible metal oxides, NO 22− may be strongly adsorbed. The formation of NO 2− on reducible metal oxide is difficult unless terminal oxygen is present on the surface. Then, the reduction of the surface by transferring the unpaired electron from the NO to the surface appears in DFT calculations (VASP code). 相似文献
6.
燃煤飞灰中的未燃碳(unburned carbon,UBC)和金属氧化物因对汞有一定的吸附和氧化作用而被认为是燃煤电厂廉价的潜在脱汞吸附剂,但效率有待提高。文章选取两种UBC含量不同的飞灰,采用质量分数1%的NH 4Br溶液对其浸渍改性,利用固定床汞吸附实验台探究了烟气组分(O 2、SO 2和NO)以及飞灰中无机金属氧化物对汞吸附和氧化的影响,以期获得高效脱汞吸附剂吸附机理。结果表明,O 2对于溴素改性飞灰氧化汞有较小的促进效果,SO 2具有一定的抑制作用,NO促进Hg 0的氧化效果明显;溴素改性飞灰中的Fe 2O 3和TiO 2对Hg 0的氧化起着主要作用,原因在于溴素改性增加了金属氧化物Fe 2O 3和TiO 2中的晶格氧含量,促进了Hg 0的催化氧化,主要遵循Mars-Maessen机理。另外,UBC含量对飞灰脱除Hg 0的影响很大,溴素嵌入飞灰表面的UBC时,会使其邻域的活性加强,从而增强碳质表面对Hg 0的吸附能力,促进了后续反应的进行。 相似文献
7.
TM/TiO 2/SiO 2 photocatalysts were prepared by the photodeposition method using transition metal salts (TM=Fe 3+, Co 2+, Ni 2+ and Cu 2+) as precursors and the surface bond-conjugated TiO 2/SiO 2 as supporter in N 2 atmosphere, and were characterized by XRD, XPS, UV-Vis diffuse reflection and zeta-potential. Their photocatalytic activities were evaluated using reactive brilliant red K-2G (K-2G) and cationic blue X-GRL (CBX) showing different adsorption behavior on the oxides. Fe, Cu supported TiO 2/SiO 2 can efficiently extend the light absorption to the visible region. XPS analysis verified that the introduction of transition metal lead to the changes of the electronic environmental of Ti cations and the zeta-potential of oxides. As a result, K-2G has higher adsorption on the modified TiO 2/SiO 2 than that on the baked one, while the adsorption of CBX has a little change on the both oxides. At the same time, for the photodegradation of K-2G, Fe 3+, Co 2+, Ni 2+-modified catalysts show that their photoactivities are 3.3–2.2 times higher than the bare one. On the contrast, all transition-metal-supported catalysts have no significant activity improvement except that Fe/TiO 2/SiO 2 shows 1.68 times higher activity for the photodegradation of CBX. The results indicate that the photoactivity could be increased in photodegradation of dyes by changing the performances of adsorption to dyes and absorption to light of photocatalyst. 相似文献
8.
通过等体积浸渍法将金属组分引入到埃洛石纳米管(HNTs)上,研究金属组分对HNTs吸附脱硫性能的影响。采用X射线衍射(XRD)、吡啶吸附红外光谱(Py-IR)和N 2吸附-脱附测试对改性材料进行表征和分析。研究发现,3种金属组分在HNTs上主要以氧化物的形式存在,而且Co 3O 4、NiO和CuO的引入均能提高HNTs的吸附脱硫性能,其中负载量为10% NiO的HNTs脱硫率提高最多,由原管的30.10%提高到47.51%。金属氧化物的引入使HNTs表面的Lewis酸量增加,进而提高了HNTs的吸附脱硫性能。因此表明HNTs表面的Lewis酸量是影响其吸附脱硫性能的重要因素。 相似文献
9.
Novel MgO-doped CaO sorbent pellets were prepared by gel-casting and wet impregnation. The effect of Na + and MgO on the structure and CO 2 adsorption performance of CaO sorbent pellets was elucidated. MgO-doped CaO sorbent pellets with the diameter range of 0.5-1.5 mm exhibited an excellent capacity for CO 2 adsorption and adsorption rate due to the homogeneous dispersion of MgO in the sorbent pellets and its effects on the physical structure of sorbents. The results show that MgO can effectively inhibit the sintering of CaO and retain the adsorption capacity of sorbents during multiple adsorption-desorption cycles. The presence of mesopores and macropores resulted in appreciable change of volume from CaO (16.7 cm 3∙mol −1) to CaCO 3 (36.9 cm 3∙mol −1) over repeated operation cycles. Ca2Mg1 sorbent pellets exhibited favorable CO 2 capture capacity (9.49 mmol∙g −1), average adsorption rate (0.32 mmol∙g −1∙min −1) and conversion rate of CaO (74.83%) after 30 cycles. 相似文献
10.
CO 2的捕获和分离具有重要的工业和环境意义。采用溶剂热法,以羧基和路易斯碱位点功能修饰的配体和锌离子构筑了阴离子型金属有机框架材料{[Zn 2(N)·(DMF) 3·(CH 3) 2NH 2]·(DMF) 2} n (NEM-7-Zn)。为了提高骨架的稳定性,通过金属离子置换工艺,将NEM-7-Zn转化为高稳定性的铜基框架材料NEM-7-Cu。采用EA、PXRD、TGA及比表面积分析等技术对多孔材料进行综合表征,并测定了NEM-7-Cu对二氧化碳、乙炔和甲烷单组分气体的吸附等温线。实验结果表明,NEM-7-Cu不仅具有较高的CO 2 吸附性能(74 cm 3·g -1),更表现出优异的CO 2/CH 4(11.5)和C 2H 2/CH 4(7.1)吸附选择性。通过巨正则Monte Carlo方法(GCMC)计算得到CO 2在NEM-7-Cu中的主要吸附位点为功能基团羧基与路易斯碱位点附近以及Cu的金属团簇附近。 相似文献
11.
Catalytic activities of various metal oxides for decomposition of nitrous oxide were compared in the presence and absence of methane and oxygen, and the general rule in the effects of the coexisting gases was discussed. The reaction rates of nitrous oxide were well correlated to the heat of formation of metal oxide, i.e., a V-shaped relationship with a minimum at −Δ Hf0 around 450 kJ (O mol) −1 was observed in N 2O decomposition in an inert gas. In the case of metal oxides having the heat of formation lower than 450 kJ (O mol) −1, CuO, Co 3O 4, NiO, Fe 2O 3, SnO 2, In 2O 3, Cr 2O 3, the activities were strongly affected by the presence of methane and oxygen. On the other hand, the activities of TiO 2, Al 2O 3, La 2O 3, MgO and CaO were almost independent. The reaction rate of nitrous oxide was significantly enhanced by methane. The promotion effect of methane was attributed to the reduction of nitrous oxide with methane: 4N 2O+CH 4→2N 2+CO 2+2H 2O. The activity was suppressed in the presence of oxygen on the metal oxides having lower heat of formation. On the basis of Langmuir–Hinshelwood mechanism, the effect of oxygen on nitrous oxide decomposition was rationalized with the strength of metal–oxygen bond. 相似文献
12.
The molecular structures and reactivity of the group V metal oxides (V 2O 5, Nb 2O 5 and Ta 2O 5) were compared. Their solid state structural chemistry, physical and electronic properties, number of active surface sites and their chemical reactivity properties were examined. For the bulk oxides, the solid state structural chemistry and the physical and electronic properties are well established. The number of active surface sites and the distribution of surface redox/acid sites were determined with methanol chemisorption and methanol oxidation, respectively. These studies revealed that the active surface sites present in pure V 2O 5 are primarily redox sites and the active surface sites in pure Nb 2O 5 are essentially acidic in nature. Furthermore, the surface redox sites present in pure V 2O 5 are orders of magnitude more active than the surface acid sites in pure Nb 2O 5. Consequently, the catalytic properties of bulk V 2O 5–Nb 2O 5 mixed oxides are dominated by the vanadia component. For the supported metal oxides, where the group V metal oxides are present as two-dimensional metal oxide overlayers, the structural and electronic properties are not well established in the literature. From a combination of molecular spectroscopic characterization methods (e.g., XANES, Raman, IR and UV–Vis DRS), it was possible to obtain this fundamental information. Methanol chemisorption studies demonstrated that a similar number of active surface sites are present in the supported vanadia and niobia catalyst systems. Similar to their bulk oxides, the surface vanadia species possess redox characteristics and the surface niobia species primarily possess acidic characteristics (Lewis acidity). The surface niobia species was a very sluggish redox site during oxidation reactions (e.g., methanol oxidation to formaldehyde and SO 2 oxidation to SO 3), but significantly promoted the surface vanadia redox sites for oxidation reactions that required dual surface redox and acid sites (e.g., butane oxidation to maleic anhydride and selective catalytic reduction of NO x by NH 3 to produce N 2). These new fundamental insights are allowing for the molecular engineering of group V metal oxide catalysts (especially vanadia and niobia). In contrast, the molecular structure and reactivity properties of Ta 2O 5 catalysts are not yet established and will require significant research efforts. 相似文献
13.
A systematic investigation of the surface morphology and of the vibrational properties of CO and NO adsorbed on simple oxides microcrystals (like MgO, NiO, NiO-MgO, CoO-MgO, ZnO, ZnO-CoO, -Cr 2O 3, -Al 2O 3, MgAl 2O 4 and other spinels, TiO 2, ZrO 2 and other oxides of a similar structure) with regular crystalline habit and exposing thermodynamically stable and neutral faces, is presented with the aim to elucidate the spectroscopic manifestations of CO and NO adsorbed on well defined crystallographic positions. In particular the structure of CO and NO adsorbed on the cationic sites of extended faces of these model solids is presented and discussed with the aim of elucidating the nature of the Mex+··· CO/NO bond (Mex+ = non transition metal ion or transition metal ion). When non transition metal ions are involved, the molecule-cation interaction is predominantly electrostatic. This leads to an increase of the CO stretching frequency, which is roughly proportional to the polarizing field. On the contrary, when transition metal ions are involved, beside the predominant electrostatic interactions, a small contribution to the bond stability comes also from d-π overlap forces, which, although not very important from the energetic point of view, greatly influence the static and dynamic dipoles localized on the adsorbed molecules. Consequently, the strength of the dipole-dipole interactions occurring in the ordered adlayers of CO and NO adsorbed on transition and non transition metal oxide surfaces are resulted remarkably different. On these well defined surfaces, the effects influencing the half-width (FWHM) of the CO and NO stretching peaks have also been considered. It has been calculated that the FWHM is a very sensitive parameter of the surface perfection. In a few cases (ZnO, -Cr2O3, etc.) FWHM values comprised in the 1.5–3.7 range have been obtained, which are indicative of a single-crystal quality of the exposed faces. These spectroscopic results were compared with those obtained with quantum calculations. Finally the activity towards CO and NO of perfect, low index faces and of more defective situations (like those associated with edges, steps and corners) are compared, in order to have a better insight on the role of surface defectivity in catalytic reactions. 相似文献
14.
The surface properties of a series of V 2O 5 catalysts supported on different oxides (Al 2O 3, H–Na/Y zeolite, MgO, SiO 2, TiO 2 and ZrO 2) were investigated by transmission electron microscopy and FTIR spectroscopy augmented by CO and NH 3 adsorption. In the case of the V 2O 5/SiO 2 system TEM images evidenced the presence of V 2O 5 crystallites, whereas such segregated phase was not observed for the other samples. VO x species resulted widely spread on the surface of Al 2O 3, H–Na/Y zeolite, MgO and SiO 2, whereas on TiO 2 and ZrO 2 they are assembled in a layer covering almost completely the support. Furthermore, evidences for the presence in this layer of V–OH Brønsted acid sites close to the active centres were found. It is proposed that propene molecules primarily produced by oxydehydrogenation of propane can be adsorbed on this acid centres and then undergo an overoxidation by reaction with redox centres in the neighbourhood. This features could account for the low selectivity of V 2O 5/TiO 2 and V 2O 5/ZrO 2 catalysts. 相似文献
15.
Nb 2O 5 loaded on the supports and mixed with oxides was studied to investigate the activity and acidity for Friedel-Crafts benzylation of anisole. From the study on the loaded catalysts, a preliminary conclusion for the selection of metal oxide was obtained; namely, such an acidic oxide as silica was suitable for the support of Nb 2O 5. Then, MoO 3 and WO 3 were mixed with Nb 2O 5, and prominent high catalytic activity and acidities were observed. Both oxides of Nb 2O 5-MoO 3 and Nb 2O 5-WO 3 showed almost similar behavior with respect to characterization and catalytic activity. Surface area increased, X-ray diffraction (XRD) and Raman bands were lost, acid sites, both Brønsted and Lewis characters generated, and surface acid site density was as high as 2–4 nm −2. The acid sites were generated on the amorphous metal oxides consisting of Nb and Mo or W oxides, different in nature from those of Nb 2O 5 calcined and un-calcined, and active for Friedel-Crafts benzylation. 相似文献
16.
Mixed oxides of Co 3O 4–TiO 2 have shown the highest catalytic activity for the reduction of SO 2 by CO among catalysts that have been developed so far. Almost zero conversion was observed with cobalt alone, whereas a high conversion was obtained with TiO 2 especially at high temperatures. There existed a strong synergistic promotional effect in the conversion of SO 2 when cobalt was mixed with TiO 2. The synergistic effect observed with mixed oxides is caused by simultaneous contributions from two different reaction routes via COS intermediate mechanism and modified redox mechanism. The synergistic effect that is caused by the COS mechanism has a smaller amount of contribution in the conversion increase and remains almost constant with an increase in the reaction temperature. A larger portion of the synergistic effect is contributed from the modified redox mechanism especially at low temperatures, but the effect disappears at temperatures above 450°C. It is found that the introduction of cobalt into TiO 2 produces COS by the reaction between sulfided CoS 2 and CO even at low temperatures. The COS intermediate can react with SO 2 to produce an additional sulfur via the COS intermediate mechanism, and also behaves as a strong reductant to keep oxygen vacancies on the TiO 2 in a high concentration for the production of sulfur via modified redox mechanism. 相似文献
17.
Previous experiments showed that γ-Al 2O 3-modified Al powder could continuously react with water and generate hydrogen at room temperature under atmospheric pressure. In this work, a possible physicochemical mechanism is proposed. It reveals that a passive oxide film on Al particle surfaces is hydrated in water. OH − ions are the main mobile species in the hydrated oxide film. When the hydrated front meets the metal Al surface, OH − ions react with Al and release H 2. Because of the limited H-soluble capacity in small Al particles and the low permeability of the hydrated oxide film toward H + species, H 2 molecules accumulate and form small H 2 gas bubbles at the Al:Al 2O 3 interface. When the reaction equilibrium pressure in H 2 bubbles exceeds a critical gas pressure that the hydrated oxide film can sustain, the film on the Al particle surfaces breaks and the reaction of Al with water continues. As the surface oxide layer on modified Al particles has a lower tensile strength, the critical gas pressure in H 2 bubbles is lower so that under an ambient condition, the reaction of modified Al particles with water is continuous. The proposed mechanism was further confirmed by a new experiment showing that the as-received Al powder could continuously react with water at temperatures above 40°C and under low vacuum, because the vacuum makes the critical gas pressure in H 2 bubbles decrease as well. 相似文献
18.
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
19.
This work aims at exploring the thermal ageing mechanism of Pt on ceria-based mixed oxides and the corresponding effect on the oxygen storage capacity (OSC) performance of the support material. Pt was supported on low-surface-area CeO 2–ZrO 2–La 2O 3 mixed oxides (CK) by impregnation method and subsequently calcined in static air at 500, 700 and 900 °C, respectively. The evolutions of textural, microstructural and redox properties of catalysts after the thermal treatments were identified by means of X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and high-resolution transmission electron microscope (HRTEM). The results reveal that, besides the sintering of Pt, encapsulation of metal by the mixed oxides occurs at the calcination temperature of 700 °C and above. The burial of Pt crystallites by support particles is proposed as a potential mechanism for the encapsulation. Further, the HRTEM images show that the distortion of the mixed oxides lattice and other crystal defects are distributed at the metal/oxides interface, probably indicating the interdiffusion/interaction between the metal and mixed oxide. In this way, encapsulation of Pt is capable to promote the formation of Ce 3+ or oxygen vacancy on the surface and in the bulk of support. The OSC results show that the reducibility and oxygen release behavior of catalysts are related to both the metal dispersion and metal/oxides interface, and the latter seems to be more crucial for those supported on low-surface-area mixed oxides. Judging by the dynamic oxygen storage capacity (DOSC), oxygen storage capacity complete (OSCC) and oxygen releasing rate, the catalyst calcined at 700 °C shows the best OSC performance. This evident promotion of OSC performance is believed to benefit from the partial encapsulation of Pt species, which leads to the increment of Ce 3+ or oxygen vacancies both on the surface and in the bulk of oxides despite a loss of chemisorption sites on the surface of metal particles. 相似文献
20.
The adsorption of CO at 130 K has been studied on Co-H-FER, Co-H-MFI and Co-H-MOR, as well as on Co-silica–alumina and on Co-containing mesoporous materials. Over Co-H-MFI also the adsorption of NO and of ammonia and the coadsorption of ortho-toluonitrile and CO have been investigated. The data show that on all samples Lewis acidic isolated Co 2+ species are predominant. However, small amounts of oxidizing sites, possibly Co 3+ and/or cobalt oxide particles also exist. This is shown by the oxidation of part of CO to CO 2 and of NO to NO + at very low temperature as well as by the formation of Co 3+ mononitrosyls upon adsorption of NO. These sites, although difficult to be evidenced by direct spectroscopic measurements, are likley the active sites for CH 4-SCR, where NO is activated as an adsorbed N xO y species able to react selectively with methane. 相似文献
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