首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 31 毫秒
1.
A study of CO oxidation on LaCoO3 perovskite was performed in an ultrahigh vacuum system by means of adsorption and desorption. All gases were adsorbed at ambient temperature. Two adsorption states (α- and β-) of CO exist. The α-peak at 440 K is attributed to carbonyl species adsorbed on Co3+ ions while the β-peak at 663 K likely comes from bidentate carbonate formed by adsorption on lattice oxygens. CO2 shows a single desorption peak (β-state, 483 K) whose chemical state may be monodentate carbonate. A new CO2 desorption peak at 590 K can be created by oxidation of CO. O2 also shows two adsorption states. One desorbs at 600 K, which may reflect adsorption on Co3- ions. The other apparently incorporates with bulk LaCoO3 and desorbs above 1000 K. The two adsorption states of CO are oxidized via different mechanisms. The rate determining step in oxidation of a-CO is the surface reaction whereas for that of β-CO, it is desorption of product CO2.  相似文献   

2.
Fe–Ni–Cr alloy layers were prepared by electrodeposition from trivalent chromium plating bath in chloride-sulfate based solution. The influences of bath composition and plating parameters on the alloy electrodeposition process and the properties of deposited alloy were studied. The effects of plating parameters and bath composition such as current density, bath pH, bath temperature, the concentrations of FeSO4 · 7H2O and CrCl3 · 6H2O on the contents of Fe and Cr in Fe–Ni–Cr alloy layer were investigated. Electrodeposited Fe–Ni–Cr alloy layers on Fe substrate were characterized by X-ray diffraction (XRD), Electronic Differential System (EDS) and a CHI600B electrochemistry workstation. The composition of the Fe–Ni–Cr coatings depends on bath composition and plating conditions including pH, current density, and temperature. The internal structure of the alloy is nanocrystalline, the average grain size is 87 nm, and the corrosion resistance of the alloy layers is better than that of pure nickel layers.  相似文献   

3.
Burcham  Loyd J.  Deo  Goutam  Gao  Xingtao  Wachs  Israel E. 《Topics in Catalysis》2000,11(1-4):85-100
The application of in situ Raman, IR, and UV-Vis DRS spectroscopies during steady-state methanol oxidation has demonstrated that the molecular structures of surface vanadium oxide species supported on metal oxides are very sensitive to the coordination and H-bonding effects of adsorbed methoxy surface species. Specifically, a decrease in the intensity of spectral bands associated with the fully oxidized surface (V5+) vanadia active phase occurred in all three studied spectroscopies during methanol oxidation. The terminal V = O (∼1030 cm−1) and bridging V–O–V (∼900–940 cm−1) vibrational bands also shifted toward lower frequency, while the in situ UV-Vis DRS spectra exhibited shifts in the surface V5+ LMCT band (>25,000 cm−1) to higher edge energies. The magnitude of these distortions correlates with the concentration of adsorbed methoxy intermediates and is most severe at lower temperatures and higher methanol partial pressures, where the surface methoxy concentrations are greatest. Conversely, spectral changes caused by actual reductions in surface vanadia (V5+) species to reduced phases (V3+/V4+) would have been more severe at higher temperatures. Moreover, the catalyst (vanadia/silica) exhibiting the greatest shift in UV-Vis DRS edge energy did not exhibit any bands from reduced V3+/V4+ phases in the d–d transition region (10,000–30,000 cm−1), even though d–d transitions were detected in vanadia/alumina and vanadia/zirconia catalysts. Therefore, V5+ spectral signals are generally not representative of the percent vanadia reduction during the methanol oxidation redox cycle, although estimates made from the high temperature, low methoxy surface coverage IR spectra suggest that the catalyst surfaces remain mostly oxidized during steady-state methanol oxidation (15–25% vanadia reduction). Finally, adsorbed surface methoxy intermediate species were easily detected with in situ IR spectroscopy during methanol oxidation in the C–H stretching region (2800–3000 cm−1) for all studied catalysts, the vibrations occurring at different frequencies depending on the specific metal oxide upon which they chemisorb. However, methoxy bands were only found in a few cases using in situ Raman spectroscopy due to the sensitivity of the Raman scattering cross-sections to the specific substrate onto which the surface methoxy species are adsorbed. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

4.
SiC-coated film onto carbon fibers as a barrier of oxidation resistance and reaction between carbon fibers and metals was investigated. The chemical vapor deposition of silicon carbide onto carbon fibers was performed at various temperatures ranging from 700 to 1000°C using triisopropylsilane vapor carried by hydrogen gas. The strength of the SiC-coated carbon fibers was decreased due to deterioration of fibers and chemical attack of hydrogen on the surface of carbon fibers during the coating process. The oxidation and the thermal resistance of the SiC-coated carbon fibers compared to the uncoated carbon fibers were improved at temperature range of 600–800°C and 1000–1200°C, respectively. Morphological change by air oxidation at temperature range of 500–800‡C was also investigated for the SiC-coated and the uncoated carbon fibers, respectively. The SiC-coated film between carbon fiber and aluminum was sufficient as a barrier of reaction on carbon fiber reinforced aluminum at temperature of above 1000°C.  相似文献   

5.
Absorption spectral analysis of fibers and films based on benzimidazole moieties of homo- and copolymeric systems showed that the vibronic state of the macromolecular chains of these systems were similar. The greatest difference of the two homopolymeric systems was observed in the region of N–H bending vibrations (1520 cm − 1). This enabled a procedure for spectral analysis of copolymers based on them to be developed. The ability to characterize the structural features in the copolymer molecular chains was investigated. It was shown that absorption bands of stretching vibrations in the region 3200–3500 cm − 1 deviated from additivity. This was consistent with a change in the nature of the intermolecular interaction.  相似文献   

6.
Mesoporous silica templated by Pluronic F68 was synthesized and characterized by TEM, N2 adsorption–desorption isotherms and FT–IR spectra. The sample had a high specific surface area (761 m2 g−1) and the mean pore diameter was 4.7 nm, indicating that it can be used as porcine pancreatic lipase (PPL) support. The physical adsorption of PPL on this mesoporous material in phosphate buffer solution with different pH values has been studied. The maximum adsorbed amount was observed at pH 7.0 and amounted to 826 mg g−1 and the maximum activity value of immobilized PPL was 227 μmol g−1 min−1. The optimal pH and temperature of the hydrolysis of triacetin for the immobilized PPL were at 8.0 and 45 °C, while they were at pH 7.0 and 35 °C for free PPL. The immobilized PPL showed excellent adaptability in higher pH and excellent heat resistance compared to free PPL. The retained activity of immobilized PPL was found to be ca. 50% of its original activity after the 5th reuse.  相似文献   

7.
F. Salvador  M.D. Merchán 《Carbon》1996,34(12):1543-1551
The thermal desorption in water of phenol, 2-dichlorophenol, 2,6-dichlorophenol, 4-nitrophenol and 2,6-dichloro-4-nitrophenol adsorbed onto an activated carbon is studied by temperature-programmed desorption (TPD) in liquid phase and by temperature-programmed adsorption-desorption (TPAD).The TPAD thermograms indicate that the desorption in water of the five compounds is complete before a temperature of 300 °C is reached. The five phenolic compounds are stable below 260 °C.Application of the interruption test shows that diffusion controls the desorption of these compounds in water. High-temperature adsorption isotherms and TPAD thermograms indicate that readsorption occurs during thermal desorption. Study of TPAD permits the calculation of the activation energies of adsorption and desorption. The values of these energies afford values for the adsorption bonding energy that lie within the range of the hydrogen bond.  相似文献   

8.
We have developed a methane sulfonic acid (MSA) based ternary electrolytic bath for co-deposition of the eutectic Sn–Ag–Cu films. The bath contains thiourea (TU), which functioned as an effective chelating agent in controlling the bath stability as well as the elemental and microstructural properties of the deposited film. A study of the bath behaviour at TU concentrations in the range 0.06–0.2 M is undertaken with the help of elemental and microstructure analysis, cyclic voltammetry (CV) and impedance analysis. The deposited films have close to eutectic composition with slightly higher Cu content for all the TU concentrations. On the other hand, the microstructure is found to be increasingly refined with increasing TU content. The CV and impedance analysis confirm chelation of Ag and Cu with TU and absence of such chelation with Sn ions. It also indicates close deposition potentials for each metal ion. Impedance analysis specifically reveals presence of an adsorbed insulating film on cathode surface, contributed by MSA or water. It also reveals competitive deposition between the insulating film and metal ions.  相似文献   

9.
The thermal chemistry of 1‐ and 2‐propyl moieties on Pt(111) was studied by using temperature‐programmed desorption (TPD) and reflection–absorption infrared spectroscopy (RAIRS). The propyl intermediates were prepared via thermal activation of the C–I bond of 1‐ and 2‐iodopropane adsorbed precursors, respectively. It was determined that the subsequent thermal activation of those propyl groups results in a competition between reductive elimination to propane, β‐hydride elimination to propene, and complete decomposition to propylidyne (and eventually to hydrogen and surface carbon). It was found that while the 2‐propyl intermediate favors propene production, 1‐propyl also yields significant amounts of propane. The formation of propene via β‐hydride elimination was identified by isotopic labeling TPD experiments, and directly about 200 K by RAIRS. Coadsorption experiments with hydrogen and deuterium were used to characterize hydrogenation and H–D reactions. All possible propene and propane isotopomers are formed from both 1‐ or 2‐iodopropane on the D/Pt(111) surface, indicating that exchange is likely to occur via a cyclic propyl–propene–propyl mechanism involving the formation of both 1‐ and 2‐propyl intermediates. Relative rates for 1‐ versus 2‐propyl conversion were estimated. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

10.
The paper describes the modification of PAN fibres with metallic nickel in order to give the fibre electroconductive properties. This is achieved by two consecutively executed processes. The first process is chemical metallization, involving the immersion of the PAN fibres in a bath containing 0.5 mol l−1 NiCl2 and 1.5 mol l−1 Rongalite at pH = 5.5 and T=288–293 K. Ni(II) is absorbed in the structure of the fibre and adsorbed at the surface through a complex reaction with the cyanide and carboxylic acid groups present in the PAN chemical structure and the Rongalite reduces the adsorbed Ni(II) to metallic Ni. The second process is galvanisation of the Ni-containing PAN surface (the Ni particles present at the surface form a “seed” layer) in an electrolyte bath using an electrolyte with 150 g l−1 of NiSO4 at pH 5–6 and room temperature. The resulting fibre showed a specific electrical resistance of the order of 10−6 Ω m, which is a promising result.  相似文献   

11.
Highly porous Poly (ε-caprolactone; PCL) microfibers were successfully fabricated by collecting the fibers into a water bath during electrospinning. The morphology of the fibers collected with and without the water bath was investigated. We observed that altering the pH of the water bath affected both the fiber diameter and the size of pores on the fibers. Acidic or basic condition was found to be more favorable than neutral conditions for the formation of well-porous fibers. The morphology and pore size of the microfibers were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The average diameter of the fibers and the pore size on the surface of the microfibers were found to be 12–14.5 and 0.3–0.7 μm, respectively. The crystallinity and thermal properties of the PCL mats were investigated by DSC. This highly porous nature of the microfibers makes PCL less crystalline and increases the surface to volume ratio of the mat. Therefore, the PCL mat obtained by water bath electrospinning may be more effective for tissue scaffolds and drug delivery than the mat obtained without water bath.  相似文献   

12.
Downstream processes following aqueous enzymatic extraction (AEE) of rapeseed oil and protein hydrolysates were developed to enhance the oil and protein yields as well as to purify the protein hydrolysates. The wet precipitate (meal residue) from the AEE was washed with twofold water at 60 °C, pH 11 for 1 h. Emulsions from the AEE and the washing step were pooled and submitted to a stepwise demulsification procedure consisting of storage-centrifugation and freezing–thawing followed by centrifugation. Aqueous phases were pooled and adsorbed onto macroporous adsorption resins (MAR) to remove salts and sugars. Following extensive rinsing with deionized water (pH 4), desorption was achieved by washing with 85% ethanol (v/v) to obtain crude rapeseed peptides (CRPs). In a separate experiment, stepwise desorption was carried out with 25, 55, and 85% ethanol to separate the bitter peptides from the other peptides. Using a combination of the AEE process, washing and demulsification steps, the yields of the total free oil and protein hydrolysates were 88–90% and 94–97%, respectively. The protein recovery was 66.7% and the protein content was enriched from 47.04 to 73.51% in the CRPs. No glucosinolates and phytic acid were detected in the CRPs. From the stepwise desorption, a non-bitter fraction RP25 (containing 64–66% of total desorbed protein) had a bland color and significantly higher protein content (81.04%) and hence was the more desirable product.  相似文献   

13.
The thermodynamics of a water-vapor-E-glass system were studied by adsorption-isotherm analysis. The seven isotherms completed were Brunauer type IV with permanent hysteresis. The first adsorbed water layer could not be removed by pumping, indicating permanent hydrolysis. Application of heat removed the remaining water to close the hysteresis loop. Calculation of isosteric heat of adsorption indicates that the adsorbed state is between the solid and the liquid states, equivalent to the loss of one vibrational and one translational degree of freedom. A new plot of energy vs thickness is proposed which describes the adsorption independent of surface area, as contrasted to the usual energy-vs-volume plot which was found to depend on surface area. A modified Clausius-Clapeyron equation is proposed for computing the isosteric heat of adsorption.  相似文献   

14.
We consider the effect of the relative velocity of a carbon particle on the specific features of the time dependences of the temperature and dimater of the particle at given temperatures of air and the wall of a reaction device and at a specified oxygen concentration in the medium. The stable and critical values of the temperature and combustion rate of the particle versus the initial particle diameter at its given velocity and the relative velocity of a particle of given size are studied. The effect of the diameter and velocity of the particle on the critical values of its initial temperature, which determine ignition inside hysteresis loops, is analyzed. The dependences of the critical particle diameter at which the thermal regime changes (ignition and extinction) on the relative particle velocity are derived. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 1, pp. 25–30, January–February, 1998.  相似文献   

15.
Ruthenium supported on magnesia was found to be a highly active and selective catalyst for the reduction of NO to N2 with H2. The adsorption of NO on Ru/MgO was studied at room temperature by applying frontal chromatography with a mixture of 2610 ppm NO in He. Subsequently, temperature‐programmed desorption (TPD) and temperature‐programmed surface reaction (TPSR) experiments in H2 were performed. The adsorption of NO was observed to occur partly dissociatively as indicated by the formation of molecular nitrogen. The TPD spectrum exhibited a minor NO peak at 340 K indicating additional molecular adsorption of NO during the exposure to NO at room temperature, and two N2 peaks at 480 K and 625 K, respectively. The latter data are in good agreement with previous results with Ru(0001) single‐crystal samples, where the interaction with NH3 was found to lead to two N2 thermal desorption states with a maximum coverage of atomic nitrogen of about 0.38. Heating up the catalyst after saturation with NO at room temperature in a H2 atmosphere revealed the self‐accelerated formation of NH3 after partial desorption of N2, whereby sites for reaction with H2 become available. As a consequence, the observed high selectivity towards N2 under steady‐state reduction conditions is ascribed to the presence of a saturated N+O coadsorbate layer resulting in an enhanced rate of N2 desorption from this layer and a very low steady‐state coverage of atomic hydrogen. The formation of H2O by reduction of adsorbed atomic oxygen is the slow step of the overall reaction which determines the minimum temperature required for full conversion of NO. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

16.
Induced electrodeposition of Ni–W alloys was carried out onto steel substrates from acidic citrate baths (pH 4.5) under different conditions of concentration of electrolyte, current density and temperature. Bright and highly adherent Ni–W deposits were successfully obtained with a relatively high cathodic current efficiency CCE (80–85%). The CCE increases greatly with increasing pH and Ni2+ ion content in the bath. The W% in the alloy deposits is in the range of 4–20 wt% depending on the operating condition. The W content in the deposit was found to increase with an increase in Ni2+ ion content, pH and temperature. The surface morphology was examined by scanning electron microscopy while the structure of the alloy was examined by X-ray diffraction analysis.  相似文献   

17.
A series of SiO2–SnO2 samples with various Sn/Si molar ratios (0.05–1.0) have been synthesized by the sol–gel technique from (Tetraethylorthosilicate) TEOS and Sn(CH3COO)4 precursors in water free conditions. The synthesis applied allowed obtaining the final product in monolithic (nonfractured upon drying) form with no use of drying control chemical additives. All samples are characterized by thermal analysis, XRD, and FTIR. The low temperature nitrogen adsorption measurements indicate the presence of both micro and mesopores. The samples containing less than 20 wt% of SnO2 show much higher surface area than SiO2 gel. The appearance of new bands at 1,048 and 882 cm−1 in the FTIR spectra could be related to stretching vibrations of the three dimensional Si–O–Sn network, which suggests that tin component has replaced silicon atoms in Si–O–Si structure.  相似文献   

18.

Abstract  

Temperature programmed desorption (TPD) of ethanol, as well as ethanol and methanol dehydration reactions were studied on γ-Al2O3 in order to identify the active catalytic sites for alcohol dehydration reactions. Two high temperature (>473 K) desorption features were observed following ethanol adsorption. Samples calcined at T ≤ 473 K displayed a desorption feature in the 523–533 K temperature range, while those calcined at T ≥ 673 K showed a single desorption feature at 498 K. These two high temperature desorption features correspond to the exclusive formation of ethylene on the Lewis (498 K) and Br?nsted acidic (~525 K) sites. The amount of ethylene formed under conditions where the competition between water and ethanol for adsorption sites is minimized is identical over the two surfaces. Furthermore, a nearly 1-to-1 correlation between the number of under-coordinated Al3+ ions on the (100) facets of γ-Al2O3 and the number of ethylene molecules formed in the ethanol TPD experiments on samples calcined at T ≥ 673 K was found. Titration of the penta-coordinate Al3+ sites on the (100) facets of γ-Al2O3 by BaO completely eliminated the methanol dehydration reaction activity. These results demonstrate that in alcohol dehydration reactions on γ-Al2O3, the (100) facets are the active catalytic surfaces. The observed activities can be linked to the same Al3+ ions on both hydrated and dehydrated surfaces: penta-coordinate Al3+ ions (Lewis acid sites), and their corresponding –OH groups (Br?nsted acid sites), depending on the calcination temperature.  相似文献   

19.

Abstract  

Synthesis of trioctylamine by the amination of octanol and ammonia under atmospheric pressure over an excellent Ni–Cu catalyst supported on diatomite is studied in this article. The key factor for the synthesis is the preparation of catalyst with a high activity and selectivity. The activity and selectivity can be adjusted by varying the Ni to Cu ratios. The optimum molar ratio of Ni to Cu was 1.25:1. For the catalyst with a Ni/Cu ratio of 1.25:1, the conversion of octanol and the selectivity of trioctylamine reached 100 and 97.3%, respectively, at 5 h. The reaction of dioctylamine with octanol was the rate-determining step for the formation of trioctylamine. The Physical properties of catalysts, such as particle size, Brunauer–Emmett–Teller (BET) surface area, valence state of catalyst elements, morphology and reduction properties of catalysts were investigated by using X-ray diffraction, nitrogen adsorption–desorption isotherms (BET), X-ray photoelectron spectroscopy, Transmission electron microscopy, and temperature programmed reduction, respectively. The reaction scheme of catalytic amination of octanol with ammonia was discussed.  相似文献   

20.
Nickel coating on the carbon–polythene composite plate was prepared by electrodeposition in a nickel sulfate solution in this work. The morphology and cross-sectional microstructure of the nickel coating were examined by scanning electron microscope (SEM) and optical microscope (OM), respectively. The influence of bath temperature on the nickel deposition rate was investigated experimentally. The adhesion between the coating and the substrate was evaluated by the pull-off test. The corrosion behavior of the coating in an aqueous solution of NaCl was studied by electrochemical methods. The results showed that the nickel electrodeposition rate could reach up to 0.68 μm min−1 on average under conditions of cathodic current density of 20 mA cm−2 and bath temperature of 60 °C. It was confirmed that increasing the bath temperature up to 50 °C had a positive effect on the nickel deposit rate, while an adverse effect was observed beyond 60 °C. The adhesion strength between the nickel coating and the substrate can be more than 2.3 MPa. The corrosion potential of the bright coating in the NaCl solution was more positive than that of the dull coating, and the anodic dissolution rate of the bright coating was also far lower at the same polarization potential compared with the dull coating.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号