共查询到20条相似文献,搜索用时 734 毫秒
1.
Synthesis of liquid oxygenates from light alkanes (C 1--C 3) is achieved in a multifunctional three-phase catalytic membrane reactor (3PCMR) operating under mild conditions (T R, 80-120 °C; P R, 140 kPa). The features of superacid catalytic membranes mediated by the Me n+/H 2O 2 Fenton system in activating C 1-C 3 alkanes are presented. The effect of operating conditions ([H 2O 2], [Me n+]) on the catalyst activity is outlined. A general reaction pathway accounting for the activation of the CH bond of the alkane molecule on the superacid sites and the subsequent reaction of the activated alkane with primary reactive intermediates, generated from the Me n+/H 2O 2 system, is proposed. The suitability of the 3PCMR in enabling simultaneous reaction and product separation is discussed. 相似文献
2.
The present paper reviews our investigations concerning the mechanism of H 2 + O 2 reaction on the metal surfaces (Pt, Pd) at different structures: single crystals (Pt(1 1 1), Pt(1 0 0), Pd(1 1 0)); microcrystals (Pt tips); and nanoparticles (Pd–Ti 3+/TiO 2). Field electron microscopy (FEM), field ion microscopy (FIM), high-resolution electron energy loss spectroscopy (HREELS), XPS, UPS, work function (WF), TDS and temperature-programmed reaction (TPR) methods have been applied to study the kinetics of H 2 oxidation on a nanolevel. The adsorption of both O 2 and H 2 and several dissociative products (H ads, O ads, OH ads) was studied by HREELS. Using the DFT technique the equilibrium states and stretching vibrations of H, O, OH, H 2O, adsorbed on the Pt(1 1 1) surface, have been calculated depending on the surrounding of the metal atoms. Sharp tips of Pt, several hundreds angstroms in radius, were used to perform in situ investigations of the dynamic surface processes. The FEM and FIM studies on the Pt-tip surface demonstrate that the self-oscillations and waves propagations are connected with periodic changes in the surface structure of nanoplane (1 0 0)-(hex) ↔ (1 × 1), varying the catalytic property of metal. The role of defects (Ti 3+-□ O) in the adsorption centers formation, their stabilization by the palladium nanoparticles, and then the defects participation in H 2 + O 2 steady-state reaction over Pd–Ti 3+/TiO 2 surface have been studied by XPS, UPS and photodesorption techniques (PhDS). This reaction seems to involve the “protonate” hydrogen atoms (H +/TiO x) as a result of spillover effect: diffusion of H ads atoms from Pd particles on a TiO x surface. The comprehensive study of H 2, O 2 adsorption and H 2 + O 2 reaction in a row: single crystals → tips → nanoparticles has shown the same nature of active centers over these metal surfaces. 相似文献
3.
Hydrogen exchange between undeuterated and perdeuterated light alkanes (CD 4-C 3H 8, C 3D 8-C 3H 8) occurs on H-ZSM5 and on Ga- and Zn-exchanged H-ZSM5 at 773 K. Alkane conversion to aromatics occurs much more slowly because it is limited by rate of disposal of H-atoms formed in C-H scission steps and not by C-H bond activation. Kinetic coupling of these C-H activation steps with hydrogen transfer to acceptor sites (Ga
n+, Zn
m+) and ultimately to stoichiometric hydrogen acceptors (H +, CO 2,O 2, CO) often increases alkane activation rates and the selectivity to unsaturated products. Reactions of 13 CH 4/C 3H 8 mixtures at 773 K lead only to unlabelled alkane, alkene, and aromatic products, even though exchange between CD 4 and C 3H 8 occurs at these reaction conditions. This suggests that the non-oxidative conversion of CH 4 to higher hydrocarbons on solid acids is limited by elementary steps that occur after the initial activation of C-H bonds. 相似文献
4.
The effect of O 2 and N 2O on alkane reactivity and olefin selectivity in the oxidative dehydrogenation of ethane, propane, n-butane, and iso-butane over highly dispersed VO x species (0.79 V/nm 2) supported on MCM-41 has been systematically investigated. For all the reactions studied, olefin selectivity was significantly improved upon replacing O 2 with N 2O. This is due to suppressing CO x formation in the presence of N 2O. The most significant improving effect of N 2O was observed for iso-butane dehydrogenation: S(iso-butene) was ca. 67% at X(iso-butane) of 25%.Possible origins of the superior performance of N 2O were derived from transient experiments using 18O 2 traces. 18O 16O species were detected in 18O 2 and 18O 2–C 3H 8 transient experiments indicating reversible oxygen chemisorption. In the presence of alkanes, the isotopic heteroexchange of O 2 strongly increased. Based on the distribution of labeled oxygen in CO x and in O 2 as well as on the increased CO x formation in sequential O 2–C 3H 8 experiments, it is suggested that non-lattice oxygen species (possibly of a bi-atomic nature) originating from O 2 are non-selective ones and responsible for CO x formation. These species are not formed from N 2O. 相似文献
5.
The selective formation of CO and H 2 was observed by a molecular-beam catalytic reaction between n-C 4H 10 and O 2 on a Pt surface from around 1000 to 1500 K. The infrared emission of the product CO desorbed from the surface showed that the CO molecules are vibrationally substantially excited but rotationally very cool (rotational temperature; T
R = 360 K). The present molecular-beam study showed that CO and H 2 were formed directly from the hydrocarbon and O 2 without involving formation of CO 2 and H 2O as primary products. The implications of these results are discussed for the partial oxidation of methane (and other alkanes) to synthesis gas using practical supported metal catalysts. 相似文献
6.
The nature/concentration of halide promoters and influence of the Pd oxidation state on the promoted reaction system has been investigated on the direct H 2O 2 process over a 2.5 wt.% Pd/ZrO 2 catalyst in an aqueous acidic reaction medium. The oxidation state of Pd had a profound influence on the H 2O 2 synthesis process. Interestingly, the nature of the halide determined the magnitude/type of influence the Pd oxidation state exerted on the overall process. While the effect of the oxidation state on the H 2O 2 yields was large for the reaction systems containing F − or no halide, the effect was significantly smaller for the reaction systems containing Br − and Cl −. The nature of the halide also strongly influenced the H 2O 2 synthesis process. Br − strongly enhanced the H 2O 2 yields, while F − had a negative influence on the H 2O 2 yields. The ability of the halides to enhance the H 2O 2 process was found to strongly depend on its propensity to suppress the secondary H 2O 2 decomposition reaction. The influence of Br − and Cl −concentration studies revealed that the optimum halide concentration for the direct H 2O 2 synthesis process was dependent on the nature of the halide. While the maximum in H 2O 2 yields for the Br − containing reaction medium corresponded to a concentration of ∼0.9 mmol/dm 3 (KBr) the maximum for the Cl − containing solution was obtained at ∼1.5 mmol/dm 3 (KCl). Such knowledge is crucial from the viewpoint of optimization (catalyst/reaction system screening studies) of the direct H 2O 2 process. The qualitative trends (H 2O 2 selectivity/yield) observed in case of the incorporated halide catalysts were similar to those observed with halides in reaction medium over the Pd/ZrO 2 catalyst. 相似文献
7.
Palladium-based catalysts have been widely employed in the electro-Fenton process for in situ generation of H 2O 2. However, the process is still far from being practical on a large scale. In this work, a series of Cl xFePd/γ-Al 2O 3/Al catalysts were prepared by a three-step-impregnation method. They exhibited excellent activity in H 2O 2 in situ synthesis and high efficiency in phenol degradation. The characterization results showed that Cl could assist in increasing the content of Pd 0 and reducing the isoelectric point of catalysts, which led to the drastic promotion in the synthesis of H 2O 2. Theoretical calculations further demonstrated that Cl doping could facilitate the main reaction in H 2O 2 synthesis, as well as inhibit side reactions such as dissociation of the O O bond. Furthermore, kinetic models were proposed and fitted. A plausible reaction mechanism as well as degradation pathways were elaborated based on electron spin resonance and gas chromatography–mass spectrometry results. These findings illustrate the value of palladium-based Cl xFePd/γ-Al 2O 3/Al catalysts for their application in the electro-Fenton process. 相似文献
8.
Biosensor detecting techniques have attracted much attention in the content determination of H 2O 2, which has been used illegally as a food additive. An electrochemical biosensing membrane for the detection of H 2O 2 was developed with C6‐OH of chitosan immobilized cyclodextrin derivates (6‐CD–CTS), which possessed a high cyclodextrin loading capacity (2.12 × 10 ?4 mol/g), as the carrier. The biosensor was prepared through the inclusion of ferrocene as the electron mediator in a hydrophobic cavity of cyclodextrin and crosslinking catalase (CAT) to 2‐NH 2 of 6‐CD–CTS. The ferrocene‐included complex was evaluated by ultraviolet–visible spectrophotometry and thermogravimetric analysis. Its electrochemical behavior was also studied. The impact of the reaction conditions on the CAT immobilization capacity was evaluated. When previous membrane was used to detect the concentration of H 2O 2 ( CH2O2), we found that the catalysis of CAT and the signal amplification of ferrocene had a major impact on the cyclic voltammograms. The optimal working pH of the modified electrode was 7.0. The peak current ( I) had a linear relationship with the H 2O 2 concentration (CH 2O 2) in the range 1.0 × 10 ?4 to 1.0 × 10 ?3 mol/L. The linear regression equation was I = 0.00475 CH2O2 ? 0.03025. The detection limit was 10 ?6 mol/L. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41499. 相似文献
9.
The hydrogenation of CO 2 was investigated on Ga 2O 3-promoted Pd/SiO 2 catalyts and mechanical mixtures of Ga 2O 3/SiO 2 and Pd/SiO 2 catalysts (H 2/CO 2 = 3; P = 3.0 MPa; T = 523 K). By means of the latter it was possible to demonstrate that atomic hydrogen, H s, can be generated by Pd 0 far from Ga 2O 3, and move (spill-over) there to reach the other reactive species (formates) and complete the reaction cycle. The reaction results indicate that (as also evidenced by in situ FTIR) the Ga 2O 3-Pd/SiO 2 catalyst works as a true bi-functional system. The metal-promoter intimacy is not decisive in terms of the catalytic chemistry of the system, but the closeness between the Pd crystallites and the Ga 2O 3 surface patches boost the activity, owing to a minimized effort in the H s supply to the latter. 相似文献
10.
Both the conversion and H 2O 2 selectivity (or yield) in direct oxidation of H 2-to-H 2O 2 (using 1.7 mol% H 2 in O 2 as a feed) and also the H 2O 2 decomposition over zeolite (viz. H-ZSM-5, H-GaAlMFI and H-
) supported palladium catalysts (at 22 °C and atmospheric pressure) are strongly influenced by the zeolite support and its fluorination, the reaction medium (viz. pure water, 0.016 M or 1.0 M NaCl solution or 0.016 M H 2SO 4, HCl, HNO 3, H 3PO 4 and HClO 4), and also by the form of palladium (Pd 0 or PdO). The oxidized (PdO-containing) catalysts are active for the H 2-to-H 2O 2 conversion and show very poor activity for the H 2O 2 decomposition. However, the reduced (Pd 0-containing) catalysts show higher H 2 conversion activity but with no selectivity for H 2O 2, and also show much higher H 2O 2 decomposition activity. No direct correlation is observed between the H 2-to-H 2O 2 conversion activity (or H 2O 2 selectivity) and the Pd dispersion or surface acidity of the catalysts. Higher H 2O 2 yield and lower H 2O 2 decomposition activity are, however, obtained when the non-acidic reaction medium (water with or without NaCl) is replaced by the acidic one. 相似文献
11.
This article considers Advanced Oxidation Processes involving O 3, O 3/UV, O 3/H 2O 2/UV, and H 2O 2/UV to destroy cyanide in jewelry manufacturing wastewaters. All experiments were performed in a semibatch reactor. The results showed that total cyanide can be reduced with different reaction rates, and the decrease of total cyanide can be described by pseudo–first-order kinetics. The reaction was performed under different pH values and H 2O 2 dosages to find the optimal conditions for the oxidation processes. The ozonation process destroyed total cyanide faster at a pH = 12, whereas ozonation combined with H 2O 2 and/or UV destroyed cyanide faster at a pH =10. The total cyanide destruction rate in the UV/H 2O 2 (700 mg/L) treatment was the highest among all studied processes, with removal efficiencies of 99% for CN ?, 99% for COD and 99% for TOC. 相似文献
12.
Aqueous-phase reforming of 10 wt% ethylene glycol solutions was studied at temperatures of 483 and 498 K over Pt-black and Pt supported on TiO 2, Al 2O 3, carbon, SiO 2, SiO 2-Al 2O 3, ZrO 2, CeO 2, and ZnO. High activity for the production of H 2 by aqueous-phase reforming was observed over Pt-black and over Pt supported on TiO 2, carbon, and Al 2O 3 (i.e., turnover frequencies near 8-15 min -1 at 498 K); moderate catalytic activity for the production of hydrogen is demonstrated by Pt supported on SiO 2-Al 2O 3 and ZrO 2 (turnover frequencies near 5 min -1); and lower catalytic activity is exhibited by Pt supported on CeO 2, ZnO, and SiO 2 (H 2 turnover frequencies lower than about 2 min -1). Pt supported on Al 2O 3, and to a lesser extent ZrO 2, exhibits high selectivity for production of H 2 and CO 2 from aqueous-phase reforming of ethylene glycol. In contrast, Pt supported on carbon, TiO 2, SiO 2-Al 2O 3 and Pt-black produce measurable amounts of gaseous alkanes and liquid-phase compounds that would lead to alkanes at higher conversions (e.g., ethanol, acetic acid, acetaldehyde). The total rate of formation of these byproducts is about 1-3 min -1 at 498 K. An important bifunctional route for the formation of liquid-phase alkane-precursor compounds over less selective catalysts involves dehydration reactions on the catalyst support (or in the aqueous reforming solution) followed by hydrogenation reactions on Pt. 相似文献
13.
Oxygen isotope exchange experiments, H 218O/H 216O (”wet” anneals) and 18O 2/ 16O 2 (”dry” anneals), were performed on single crystal samples of yttria‐stabilized zirconia (YSZ) at a temperature of T = 1073 K with subsequent determination of the oxygen isotope profiles in the solid by time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS). Such experiments yielded oxygen tracer diffusion coefficients ( D*) and oxygen tracer surface exchange coefficients ( k*), from both the polished (smooth) and unpolished (rough) sides of single crystal samples, as a function of water partial pressure pH 2O and oxygen partial pressure pO 2. Isothermal values of D* were found to depend on neither pO 2 nor pH 2O (nor surface roughness). Isothermal values of k*, in contrast, displayed a strong dependence on pO 2 or pH 2O; k* wet was, in addition, 2–3 orders of magnitude higher than k* dry. Surprisingly, surface roughness had little effect on k* wet, whereas rough surfaces exhibited much higher k* dry values than smooth surfaces. Data for k* wet obtained as a function of temperature at pH 2O = 18 mbar show a change in activation enthalpy at T ≈ 973 K. The behavior of k* is discussed in terms of surface composition, surface area and surface reaction mechanisms. 相似文献
14.
The catalytic activity of polycrystalline Pd films deposited on 8 mol% Y 2O 3-stabilized–ZrO 2 (YSZ), an O 2–-conductor, can be altered reversibly by varying the potential of the Pd catalyst film via the effect of nonfaradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion. The complete oxidation of ethylene was investigated as a model reaction in the temperature range 290–360 °C and atmospheric total pressure. The rate of C 2H 4 oxidation can be reversibly enhanced by up to 45% by supplying O 2– to the catalyst via positive current application. The steady-state rate change is typically 10 3–10 4 times larger than the steady-state rate I/2 F of electrochemical supply or removal of promoting oxide ions. The observed behaviour is discussed on the basis of previous NEMCA studies and the mechanism of the reaction. 相似文献
15.
Poor H 2O 2-resistance by enzymes is a key bottleneck in the epoxidation process of oil by enzymatic methods. In this study, the stability of three lipases, from Aspergillus oryzae lipase (AOL), Aspergillus fumigatus lipase B (AflB), and marine Janibacter (MAJ1), in the presence of H 2O 2 was evaluated in different types of natural deep eutectic solvents (NADES). This stability was strengthened significantly in the NADES compared to the buffer. Specifically, AOL retained 84.7% of its initial activity in the presence of choline chloride/sorbitol (1:1 M ratio) and 3 mol L −1 H 2O 2 after 24 h incubation at 40°C. In addition, the two-phase epoxidation process was optimized with AOL in ChCl/sorbitol to reach up to 96.8% conversion under the optimized conditions (molar ratio of octanoic acid/H 2O 2/C=C-bonds = 0.3:1.5:1, enzyme loading of 15 U g −1 of soybean oil, ChCl/sorbitol content of 70.0% of the weight of hydrophilic phase, and reaction temperature of 50°C). Moreover, the lipase dispersed in NADES retained approximately 66% of its initial activity after being used for seven batch cycles. Overall, NADES-based enzymatic epoxidation is a feasible and promising strategy for the synthesis of epoxidized oils. 相似文献
16.
The influence of the O 2/H 2 mole ratio in the gaseous feed and also those of other reaction conditions [viz. concentration of H 3PO 4 (0–5 mol/dm 3), temperature (5–50 °C), gas (H 2 and O 2) space velocity (5.8–23.4 h ?1) and reaction time (0.1–8 h)] on the H 2O 2 formation in the H 2-to-H 2O 2 oxidation over the Br(1 wt%)–F(1 wt%)–Pd(5 wt%)/Al 2O 3 catalyst in an aqueous acidic (H 3PO 4) medium have been thoroughly investigated. The effects of the O 2/H 2 ratio, reaction temperature and acid concentration on the destruction of H 2O 2 by its decomposition and/or hydrogenation reactions over the catalyst in the acidic reaction medium have also been studied. The net H 2O 2 formation (H 2O 2 yield) over the catalyst passed through a maximum with increasing the acid concentration, the temperature or the O 2/H 2 feed ratio. However, it decreased markedly with increasing the gas space velocity or the reaction period. The H 2O 2 decomposition and hydrogenation activities of the catalyst increased appreciably with increasing the reaction temperature and decreased with increasing the acid concentration. The H 2O 2 destruction during the H 2-to-H 2O 2 oxidation increased with increasing the concentration of H 2 (relative to that of O 2) due to the increased H 2O 2 hydrogenation rate over the catalyst. The net H 2O 2 formation in the H 2-to-H 2O 2 oxidation decreased sharply with increasing the initial amount of H 2O 2 present in the reaction mixture. The presence of H 2O 2 and the higher H 2/O 2 ratios have detrimental effects on the net formation of H 2O 2. 相似文献
17.
The kinetic behavior in the direct synthesis of H 2O 2 with Pd–Me (Me = Ag, Pt) catalysts prepared by depositing the noble metals by electroless plating deposition (EPD) or deposition–precipitation
(DP) methods on α-Al 2O 3 asymmetric ceramic membrane with or without a further surface coating by a carbon thin layer is reported. The effect of the
second metal with respect to Pd-only catalysts considerably depends on the presence of the carbon layer on the membrane support.
Several factors in the preparation of these membranes as well as the reaction conditions (temperature, concentration of Br −, pH) determine the selectivity in H 2O 2 formation, influencing the rate of the consecutive reduction of H 2O 2 (which is faster with respect to H 2O 2 decomposition on the metal surface) and/or of direct H 2 + O 2 conversion to H 2O. Defective Pd sites are indicated to be responsible for the two unselective reactions leading to water formation (parallel
and consecutive to H 2O 2 formation), but the rate constants of the two reactions are differently influenced from the catalytic membrane characteristics.
Increasing the noble metal loading on the membrane not only increases the productivity to H 2O 2, but also the selectivity, due to the formation of larger, less defective, Pd particles. 相似文献
18.
Single- and double-stage catalytic preferential CO oxidation (CO-PrOx) over-Fe 2O 3-promoted CuO–CeO 2 in a H 2-rich stream has been investigated in this work. The catalyst was prepared by the urea-nitrate combustion method and was characterized by X-ray diffractometer (XRD), X-ray fluorescence (XRF), Brunauer–Emmet–Teller (BET), transmission electron microscope (TEM), and scanning electron microscope (SEM). The catalytic activity tests were carried out in the temperature range of 50–225 °C under atmospheric pressure. The results of the single-stage reaction indicated that complete CO oxidation was obtained when operating at a O 2/CO ratio of 1.5, W/ F ratio of 0.36 g s/cm 3, and at a reaction temperature of 175 °C. At these conditions, H 2 consumption in the oxidation was estimated at 58.4%. Applying the same conditions to the double-stage reaction, complete CO oxidation was found and H 2 consumption in the oxidation was reduced about 4.9%. When decreasing the double-stage reaction temperature to 150 °C, the results elucidated that CO could be converted to CO 2 completely while H 2 consumption in the oxidation was further reduced to 33.5%. A temperature blocking 2 2 factorial design has been used to describe the importance of the factors influencing the catalytic activity. The factorial design was according to the experimental results. When adding CO 2 and H 2O in feed, reduction of CO conversion for single- and double-stage reaction is obtained due to a blocking of CO 2 and H 2O at a catalytic active site. Comparing CO conversion obtained when operating with/without CO 2 and H 2O in feed for single- and double-stage reaction, less reduction is achieved when operating in double-stage reaction. 相似文献
19.
A mechanism for the electrodeposition of acrylic resin on aluminium is proposed, based on experimental studies of acid value, anodic gas evaluation and anodic film resistance. The mechanism can be expressed as Al f Al 3+ + 3e 2Al 3+ + 3H 2O f Al 2O 3 + 6H + 2Al 3+ + 6H 2O f 2Al(OH) 3 + 6H + H + + RCOC –
f RCOOH. This is different from the mechanism for zinc and steel, where it is metal ions from anodic dissolution which neutralize the macro-ions and cause a deposit on the anode surface. 相似文献
20.
The efficiency of ozonation and advanced oxidation processes such as ozone/UV, ozone/H 2O 2 and H 2O 2/UV was assessed for chlorinated hydrocarbons using a closed batch-type system. 1,1-Dichloropropene (DCPE), trichloroethylene (TCE), 1-chloropentane (CPA), and 1,2-dichloroethane (DCA) were used as model compounds. The direct reaction between substrates and ozone predominated at lower pH, which resulted in the efficient oxidation of the olefin, DCPE. At higher pH, ozonation resulted in more efficient oxidation of the chlorinated alkanes, with a corresponding decrease in the efficiency of DCPE oxidation. Consistent results were observed for ozone/H2O2 and ozone/UV treatment. Due to slow UV-induced decomposition of H2O2, the process using H2O2/UV (254 nm) resulted in very slow oxidation of all four compounds. The total ozone requirement to achieve a given degree of elimination (to 37% of the original concentration), δ0.37, was used to assess the combined effects of the direct and indirect reactions for different types of waters. 相似文献
|