大颗粒FCC汽油芳构化催化剂表面烧焦过程的数值模拟

综合考虑化学反应速率、内扩散和反应热的影响，建立单个催化剂颗粒表面烧焦过程的数学模型，模拟研究了粒径1000μm的球形催化剂的表面烧焦过程。研究表明，650℃再生，化学反应速率为控制步骤，催化剂的再生过程符合整体反应模型；800℃再生，内扩散影响严重，碳含量、氧分压沿径向变化剧烈。再生温度升高，催化剂颗粒的瞬时温升增大；催化剂初始碳含量越高、CO2/CO摩尔比越大，催化剂颗粒的瞬时温升越大，但再生过程所需时间增加；催化剂颗粒的径向温度分布均一。在排除外扩散影响的条件下，反应气速对再生过程影响很小。     

Kinetics of CO oxidation on high-concentration phases of atomic oxygen on Pt(111)

Temperature-programmed reaction spectroscopy (TPRS) and direct, isothermal reaction-rate measurements were employed to investigate the oxidation of CO on Pt(111) covered with high concentrations of atomic oxygen. The TPRS results show that oxygen atoms chemisorbed on Pt(111) at coverages just above 0.25 ML (monolayers) are reactive toward coadsorbed CO, producing CO(2) at about 295 K. The uptake of CO on Pt(111) is found to decrease with increasing oxygen coverage beyond 0.25 ML and becomes immeasurable at a surface temperature of 100 K when Pt(111) is partially covered with Pt oxide domains at oxygen coverages above 1.5 ML. The rate of CO oxidation measured as a function of CO beam exposure to the surface exhibits a nearly linear increase toward a maximum for initial oxygen coverages between 0.25 and 0.50 ML and constant surface temperatures between 300 and 500 K. At a fixed CO incident flux, the time required to reach the maximum reaction rate increases as the initial oxygen coverage is increased to 0.50 ML. A time lag prior to the reaction-rate maximum is also observed when Pt oxide domains are present on the surface, but the reaction rate increases more slowly with CO exposure and much longer time lags are observed, indicating that the oxide phase is less reactive toward CO than are chemisorbed oxygen atoms on Pt(111). On the partially oxidized surface, the CO exposure needed to reach the rate maximum increases significantly with increases in both the initial oxygen coverage and the surface temperature. A kinetic model is developed that reproduces the qualitative dependence of the CO oxidation rate on the atomic oxygen coverage and the surface temperature. The model assumes that CO chemisorption and reaction occur only on regions of the surface covered by chemisorbed oxygen atoms and describes the CO chemisorption probability as a decreasing function of the atomic oxygen coverage in the chemisorbed phase. The model also takes into account the migration of oxygen atoms from oxide domains to domains with chemisorbed oxygen atoms. According to the model, the reaction rate initially increases with the CO exposure because the rate of CO chemisorption is enhanced as the coverage of chemisorbed oxygen atoms decreases during reaction. Longer rate delays are predicted for the partially oxidized surface because oxygen migration from the oxide phase maintains high oxygen coverages in the coexisting chemisorbed oxygen phase that hinder CO chemisorption. It is shown that the time evolution of the CO oxidation rate is determined by the relative rates of CO chemisorption and oxygen migration, R(ad) and R(m), respectively, with an increase in the relative rate of oxygen migration acting to inhibit the reaction. We find that the time lag in the reaction rate increases nearly exponentially with the initial oxygen coverage [O](i) (tot) when [O](i) (tot) exceeds a critical value, which is defined as the coverage above which R(ad)R(m) is less than unity at fixed CO incident flux and surface temperature. These results demonstrate that the kinetics for CO oxidation on oxidized Pt(111) is governed by the sensitivity of CO binding and chemisorption on the atomic oxygen coverage and the distribution of surface oxygen phases.     

Effect of oxygen on the radical polymerization of acrylamide in ethanol and water

The polymerization of acrylamide has been studied in ethanol at 80°C with 2,2′-azobisisobutyronitrile (AIBN) as the initiator, and also in water at 60°C with 4,4′-azobis-4-cyanopentanoic acid (ACV) as the initiator. For the first system, it was found, approximately, that the well defined in duction period showed an inverse dependence on both the initial monomer and initiator concentrations, but was directly proportional to the square root of the initial oxygen concentration. For the second system, the rate of oxygen uptake was approximately directly proportional to the monomer and initiator concentration but independent of the oxygen concentration. The results obtained for these two systems are compared.     

Study of the thermal oxidation of polyolefins—III: Pressure and temperature dependence of the rate of oxygen absorption

The kinetics of oxygen absorption by high pressure polyethylene (PE) have been studied between 8 and 650 torr oxygen pressure and in the temperature range 140 to 180°. The mechanism of thermal oxidation has been described and the equation for the rate of oxygen absorption has been derived. The rate equation has been studied in the initial stage and at points of the maximum rate of oxygen absorption. Some rate constant combinations (characteristic of the process) and their temperature dependence have been determined from experimental data. The pressure dependence of the initial and maximum rates of oxygen absorption, in the pressure range investigated, is well described by the proposed mechanism.     

Gel permeation chromatography studies of chain scission and cross-linking during photo-oxidation of atactic polystyrene below and at tg

Photo-oxidative degradation of polystyrene in the form of film 20 μm thick was carried out in air using u.v. light of 254 nm at room temperature and at temperatures up to T_g. GPC was used to study changes of molecular weight distribution during the process. The GPC results were analysed using equations for an initially most probable distribution and non-uniform energy dissipation; the quantum yield values of chain scission and cross-linking of polystyrene during degradation were calculated. Initially, degradation progressed at high rate, connected with consumption of oxygen dissolved in the film. The slower subsequent degradation was connected with consumption of oxygen supplied during the reaction. An appreciable increase in the quantum yields for chain scission and cross-linking was observed just below and at T_g for the initial stage of photo-oxidative degradation. This increase of the quantum yield of photodegradation was caused by increased mobility of oxygen molecules in the film, connected with movement of polymer chain elements.     

The kinetics of living free-radical polymerization of styrene in the presence of alkoxyamine: Interpretation within the framework of secondary inhibition hypothesis

The kinetics of the free-radical polymerization of styrene at 120°C in the presence of alkoxyamine as a living polymerization agent has been studied. The alkoxyamine forms in situ with a 100-fold variation in the initial concentration of 2,2,6,6-tetramethyl-1-piperidinyloxy. The rate of the process at the stationary stage decreases with an increase in the initial concentration of 2,2,6,6-tetramethyl-1-piperidinyloxy. The computer simulation of the kinetic features of the process under study demonstrates that this tendency is related to the possible secondary inhibition of the process by alkoxyamine and/or hydroxylamine arising in the system in the course of polymerization.     

Geometric effects control isothermal oxidation of graphite flakes

The oxidation, in a neat oxygen atmosphere, of high-purity and highly crystalline natural graphite and synthetic Kish graphite was investigated. The physico-geometric model function of the kinetic rate equation was experimentally determined by isothermal thermogravimetric analysis at 650?°C. Analytic solutions for basic flake shapes indicate that this function strictly decreases with conversion. However, for both samples the experimental data trend was a rapid initial increase followed by the expected decrease to zero. High resolution field emission scanning electron microscopy, of partially oxidized flakes, provided plausible explanations for this discrepancy. Rapid development of macroscopic surface roughness during the initial stages of oxidation was evident and could be attributed to the presence of catalytic impurities. Large fissures along the planes of the natural graphite and the initiation, growth and coalescence of internal cavities in the Kish graphite were observed. Flake models incorporating the latter two features are difficult to analyse analytically. However, a facile probabilistic approach showed that reasonably good agreement with experimental data was possible.     

The Role of a Metallocene Additive in Radically Initiated Polymerization when Solving Direct and Inverse Kinetic Problems

Polymerization of methyl methacrylate, initiated by benzoyl peroxide in the presence of titanocene dichloride, is considered from the point of view of formal kinetics. Based on the kinetic scheme of the process (which includes the reactions of classical radical polymerization, the reaction of benzoyl peroxide with titanocene dichloride, the reactions of the controlled radical polymerization of organometallic mediated radical polymerization (OMRP) and atom transfer radical polymerization (ATRP), the reaction of the formation of a coordinating active site and the coordinating chain propagation on a mathematical model of the kinetics of the process is created. This model also makes it possible to calculate the molecular-mass characteristics of poly(methyl methacrylate). As a result of the solution of the inverse kinetic problem at a temperature of 343 K, the values of the reaction rate constants of the kinetic scheme are found under which the discrepancy between the calculated models and experimental data is minimal. Using the developed model of the kinetics of the process, a numerical experiment is performed (i.e., a direct kinetic problem is solved). This problem revealed the following regularities of the process. (1) An increase in the initial concentration of titanocene dichloride at a constant initial concentration of benzoyl peroxide leads to an increase in the rate of consumption of benzoyl peroxide but not to an increase in the initial rate of the process compared to classical radical polymerization. (2) With an increase in the initial concentration of titanocene dichloride, the lifetime of the macroradicals at the initial stage of the process is reduced, and hence the molecular weight of the resulting polymethyl methacrylate is less than that of the polymethyl methacrylate obtained in the absence of titanocene dichloride, and it will increase during the process of approaching the final values. (3) During the polymerization of methyl methacrylate, initiated by benzoyl peroxide in the presence of titanocene dichloride, a smoothing gel effect (as in the case of the polymerization of methyl methacrylate initiated by benzoyl peroxide in the presence of ferrocene) does not occur since titanocene dichloride forms stable complexes with methyl methacrylate and, consequently, it participates in reactions consuming macroradicals to a lesser degree than ferrocene.     

Lead underpotential deposition on Au(110)

The electrochemical underpotential deposition (UPD) of lead on Au(110) was investigated by XPS using a custom‐built ultrahigh vacuum apparatus containing a chamber for electrochemical studies. A two‐step deposition process for lead UPD was confirmed. A large increase in the surface concentration of oxygen was found in solutions containing lead. The presence of lead was detected on the gold surface at all potentials within the range investigated (?500 mV to 1500 mV vs. Ag/AgCl). Degradation of chlorine by x‐rays was observed. The change in surface components with potential was investigated and linked to models of UPD and oxidation. The initial random deposition of lead from solution led to surface disordering. Copyright © 2003 John Wiley & Sons, Ltd.     

Aqueous Polymerization of Methyl Methacrylate Initiated by Potassium Peroxydisulfate-Ascorbic Acid Redox System

The aqueous polymerization of methyl methacrylate initiated by the redox system K₂S₂O₈-ascorbic acid has been studied at 35°C under the influence of oxygen. The rate of polymerization increases with increasing ascorbic acid concentration at low activator concentration, remains constant within the range 4.375 × 10^?3 to 11.25 × 10^?3 mole/liter, and at higher ascorbic acid concentration again decreases. The rate varies linearly with monomer concentration. The initial rate and the limiting conversion increase with increasing polymerization temperature. Organic solvents (water-miscible only) and small amounts of neutral salts like KC1 and Na₂SO₄ depress the initial rate and the maximum conversion. The addition of small amounts of salts like Cu²⁺ and Mn²⁺ increases the initial rate, but no appreciable increase in the limiting conversion is observed.     

重铬酸钾电催化合成反应过程阳极稳定性及析气效应研究

针对重铬酸钾传统生产技术存在的高污染、高消耗等问题,研究了电催化合成重铬酸钾绿色新技术。实验研究了自制钌/铱/钛氧化物复合阳极的电催化性能,表征了电极表面形貌。结果表明,该阳极在铬酸钾溶液中具有较低的析氧电位和稳定的高催化活性。实验测得不同反应条件下的阴极析氢速率,建立了析氢速率和析氢量随阳极液铬酸钾初始浓度、反应温度及反应时间变化的数学模型,指出可用宏观测得的阴极析氢量定量表征重铬酸钾电催化反应进程,并讨论了析氢速率和析氢量的影响因素。     

Chemiluminescence study of polymer oxidative stability

The application of the chemiluminescence technique to the study of polymer oxidative stability is discussed. For the initial stages of oxidation a sigmoidal change in the chemiluminescence intensity with time is justified when the experiment is performed isothermally at high oxygen pressure, whereas at low oxygen pressure and constant heating rate an exponential increase in the chemiluminescence intensity is expected. For advanced stages of oxidation depending on the ratio between the initial and equilibrium levels of hydroperoxides, the intensity of emitted light may either grow or decay until the equilibrium concentration of hydroperoxides is established. Two major parameters of an autocatalytic oxidation (induction time and oxidation rate) can be obtained by carrying out the experiment in an oxygen atmosphere at constant temperature. Alternatively, the extent of oxidation in a certain temperature region can be evaluated when a constant-heating-rate experiment is conducted under nitrogen.     

Reaction of ethylene with oxygen adsorbed on a silver film. Dependence of the selectivity to ethylene oxide on the ethylene pressure

Kinetics of the reaction of ethylene with oxygen adsorbed on a silver film was studied by a static method at 333 K, initial surface coverage with oxygen close to 0.8 and initial ethylene pressures of 1.1 and 4.4 Pa. The ethylene oxide selectivity was found to increase with increasing ethylene pressure.     

Kinetics and mechanism of cyclohexane oxidation with molecular oxygen in the presence of propionic aldehyde

The kinetics and mechanism of the liquid-phase oxidation of cyclohexane with molecular oxygen in the presence of the additives of propionic aldehyde are studied at 303.0, 322.5, and 341.5 K by measuring the rates of oxygen and propionic aldehyde consumption and the yields of the main reaction products (cyclohexanol (COL), cyclohexanone (CON), cyclohexyl hydroperoxide, and propionic acid and peracid). A kinetic scheme is proposed and rate constants of elementary reactions are estimated based on the analysis of their rates and the yields of the main cyclohexane products. The key reactions of the main steps (including chain initiation, propagation, and termination) are determined. An increase in the rate of cyclohexane oxidation and the yield of the target products (cyclohexanol, cyclohexanone, and cyclohexyl hydroperoxide) in the presence of propionic aldehyde suggests that highly active acylperoxy radicals participate in chain propagation. The [CON]/[COL] ratio indicates that these products are mainly formed in chain propagation. The strong effect of the Baeyer-Villiger rearrangement on both the rate of oxygen consumption and the yield of the target products at the initial stages of the process and at high propionic aldehyde concentrations is explained.     

Coverage dependence of oxygen decomposition and surface diffusion on rhodium 111: a DFT study

A systematic study of oxygen adsorption, decomposition and diffusion on Rh111 and its dependence on coadsorbed oxygen molecules has been performed using density functional theory calculations. First, the bonding strength between metal surface and adsorbed oxygen molecules has been studied as a function of initial oxygen coverage. The bonding strength decreases with increasing oxygen coverage, which points towards a self-inhibition of the adsorption process. The potential energy hypersurface (PES) for the dissociation of oxygen molecules adsorbed on a threefold fcc position perpendicular to the surface was calculated using a combined linear/quadratic synchronous transit method with conjugate gradient refinements. The results indicate that a minor amount of oxygen on the surface enhances the decomposition of further oxygen molecules, while this process is inhibited at higher coverage. Moreover, PES calculations of a single site jump of atomic oxygen on rhodium 111 indicate that the activation energy increases as well with increasing oxygen coverage. All results are discussed with respect to a rhodium based catalytic NOx reduction/decomposition system proposed by Nakatsuji, which decomposes nitrogen oxides in oxygen excess.     

Surface‐Roughness Evolution During the Cementation of Silver Ions on Solid Copper in Aqueous Sulfuric Acid Solution

The evolution of the surface roughness during cementation of Ag⁺ conducted either in O₂‐free or O₂‐saturated aqueous H₂SO₄/CuSO₄ was investigated at two different initial concentrations of Ag⁺. The kinetics data of the process determined previously in the rotating cylinder were linked directly with scanning‐electron‐microscope (SEM) images and surface‐height‐distribution diagrams calculated for various cementation times. It was found that, at the beginning of the process, the surface roughness decreases due to formation of a flat Ag layer on the top of the surface, independent of the presence or absence of O₂ in the system. With increasing reaction time, an increase in the surface roughness was observed. The rate enhancement of the process is mainly responsible for the increase of the surface roughness in the O₂‐saturated solutions, especially at the higher initial Ag⁺ concentration (100 mg/dm³). The rate enhancement observed at a latter stage of the process, connected with the increase of the effective surface area of the cathodic sites, was separated from the rate enhancement induced by the competitive chemical process occurring in O₂‐free solution. The difference in the mechanisms of the processes conducted under aerobic and anaerobic conditions was reflected in the surface‐heigth distributions calculated from the SEM images.     

Mechanism of nitrosylmyoglobin autoxidation: temperature and oxygen pressure effects on the two consecutive reactions

As shown by singular value decomposition and global analysis of the absorption spectra, oxidation of nitrosylmyoglobin, MbFe(II)NO, by oxygen occurs in two consecutive (pseudo) first-order reactions in aqueous air- saturated solutions at physiological conditions (pH 7.0, I=0.16 m (NaCl)). Both reaction steps have a large temperature dependence with the following activation parameters: DeltaS++(1) = 121+/-7 and DeltaS++(1) = 23+/-29; and DeltaS++(2) = 88+/-14 kJ mol(-1) and DeltaS++(2)-63+/-51 J(-1) K(-1) mol(-1) at 25 degrees C for the first and second step, respectively. At physiological temperature, the initial reaction is faster, while at lower temperatures, the first reaction is slower and rate-determining. The rate of the first reaction is linearly dependent on oxygen pressure at lower pressures, while for oxygen pressures above atmospheric, the rate exhibits saturation behaviour. The second reaction is independent of oxygen pressure. The rate of the second reaction increases when oxymyoglobin is added. In contrast, the rate of the first reaction is independent of the presence of oxymyoglobin. The observed kinetics are in agreement with a reaction mechanism in which the nitric oxide that is initially bound to the Fe(II) centre of myoglobin is displaced by oxygen in a reversible ligand-exchange reaction prior to an irreversible electron transfer. The ligand-exchange process is dissociative in nature and depends bond breaking, and nitric oxide is suggested to be trapped in a protein cavity. The absorption spectrum of the intermediate, as resolved from the global analysis, is in agreement with a peroxynitrite complex, and the initial process must involve partial electron transfer.     

Kinetics and mechanism of water catalytic oxidation by a Ru3+(bpy)3 complex in the presence of colloidal cobalt hydroxide

Kinetics of the catalytic oxidation of water to molecular oxygen by a tris(bipyridyl) Ru(III) complex is studied in the presence of colloidal cobalt hydroxide stabilized by starch. Oxidant consumption follows the first-order rate law with respect to the oxidant concentration. The dependence of the apparent rate constant of this process on the catalyst concentration, initial oxidant concentration, and initial concentration of its reduced form was determined. The dependence of the oxygen yield on H⁺ at pH 7–11 and a catalyst concentration of 10-⁷-10-³ mol¹ is studied. An intermediate product of the reaction was found, which is probably a bridged peroxo complex of cobalt. The kinetic scheme and mechanism of the reaction is proposed, which agree with experimental observations.     

Kinetic models of sorption: a theoretical analysis

The kinetics of sorption from a solution onto an adsorbent has been explored theoretically. The general analytical solution was obtained for two cases. It has been shown that at high initial concentration of solute (sorbate) the general equation converts to a pseudo-first-order model and at lower initial concentration of solute it converts to a pseudo-second-order model. In other words, the sorption process obeys pseudo-first-order kinetics at high initial concentration of solute, while it obeys pseudo-second-order kinetics model at lower initial concentration of solute. The theoretical results (derived equations) show that the observed rate constants of pseudo-first-order and pseudo-second-order models are combinations of adsorption and desorption rate constants and also initial concentration of solute. The obtained theoretical equations are used to correlate experimental data for sorption kinetics of some solutes on various sorbents. The predictions of the theory are in excellent agreement with the experimental data.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 31: Formal kinetics of γ-lactone formation from additional primary products

The potential contribution of primary oxidation products to γ-lactone formation in polyethylene is discussed. The 1,4,6-hydroperoxy-keto-hydroperoxides and the 1,2,5-tris-hydroperoxides are investigated in this work. Their formation and decomposition is examined with respect to γ-lactone formation at increasing rates in the initial stages and possibly at constant rates in the advanced stages of polyethylene processing. The formal kinetics based on the mechanisms is used to check the effect of the temperature and of the oxygen concentration. It is found that the activation energy calculated for the two mechanisms envisaged can account for different experimental values valid in the initial or advanced stages of polyethylene processing. However, the calculated increase of the rate on passing from air to pure oxygen is always much larger than the experimental value. Hence, the mechanisms examined can contribute to part only of the γ-lactone found experimentally. They should necessarily be complemented by additional mechanisms that show smaller increase with the oxygen concentration than that found experimentally.