A generalised model for the effect of a limiting non-key component in porous catalysts

A generalised model for pseudo first order catalytic reactions with limiting non-key reactant has been developed. The restriction of no external mass transfer resistance has been removed and a comparison is made with the earlier model which included this restriction. Results are presented for effectiveness factor, surface temperature and depth of penetration using this model.     

A high-speed method for obtaining kinetic data for exothermic or endothermic catalytic reactions under non-isothermal conditions illustrated for the ammonia synthesis

A detailed analysis of the method for fast acquisition of kinetic data by means of a polythermal-temperature-ramping reactor (PTR) as suggested by Wojciechowski in 1995 was performed. The ammonia synthesis reaction over a commercial Fe catalyst (BASF) was taken as an example. Kinetic data evaluation was based on a kinetic model as reported by Sehested et al. in 1999. As a result of data analysis, the experimental molar rate of change of ammonia could be described adequately over a wide range of N₂ and H₂ partial pressures as well as temperature. The pre-exponential factors, activation energies and adsorption enthalpies are in reasonable agreement with data for promoted iron catalysts reported in literature. Accordingly, the PTR method was evaluated as a efficient tool for kinetic data analysis. The number of experiments including different initial conditions can be significantly diminished since PTR experiments have not to follow the strict sequence of varying one experimental parameter after the other by keeping all others constant. The collected exit concentrations, exit temperature and contact time are correlated by spline-functions which allow the extraction of functional relationships required for derivation of rate expressions. If the steady-state of the catalyst is rapidly attained, temperature ramping allows fast continuous data acquisition. Thus, the PTR method has the potential of significantly reducing the time needed for determining reliable kinetics over a wide range of operating conditions.     

Microchemical engineering of catalytic reactions

The catalytic reduction of NO with H₂ or CO and the O₂+H₂ reaction have been investigated at low pressure (p < 10^-3 mbar) on microstructured bimetallic Pt(100)/Rh and Pt(100)/Ti surfaces prepared by lithographic techniques. Photoemission electron microscopy (PEEM) was the spatially resolving technique used. It is shown that diffusional coupling leads to dynamic effects which are size-dependent and thus can be controlled through the design of the surface microstructure. In connection with periodic parameter forcing these dynamic effects can potentially be exploited to improve the yield and selectivity of catalytic reactions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of adsorptive gaseous reactant on non-isothermal behavior for gas-solid reactions

The effect of adsorptive species on non-isothermal gas-solid reactions is studied on the basis of Langmuir-Hinshelwood kinetics. The concept of an effectiveness factor provides good information to ascertain the effect of adsorptive species and the transition of the rate controlling regime, in connection with the parameters, generally used in the analysis of non-isothermal behavior. For highly exothermic reactions, the effectiveness factor-Thiele modulus curves with multiple solutions are presented with respect to the modified adsorption equilibrium constant. The variations of the rate-controlling regime by the effect of adsorptive species are also discussed.     

金属氧化物纳米催化剂的晶面效应研究进展

纳米催化剂因尺寸效应具有优异的催化活性、选择性和稳定性,成为研究热点。在多相催化反应中,纳米催化剂不同的晶面因原子排布和密度不同,表现出不同的催化特性,即纳米催化剂的晶面效应。在纳米催化剂中,金属氧化物作为重要的催化剂和催化剂载体备受关注,通过对Ti O2、Co3O4和Ce O2金属氧化物特征晶面的表面结构进行分析,综述在特定催化反应中不同晶面催化性能和表面结构的关系以及不同晶面的表面结构在催化反应中的反应途径与机理。     

Periodic pressure forcing of catalytic reactions

Use of total pressure modulation to improve reactor performance has attracted attention for about three decades from a several different fields: chemical engineering, metallurgical engineering and biochemical engineering. Ultrasonic excitation of both mass transfer and reaction systems has been discussed in the literature extensively. Although ultrasonic pressure waves are low-amplitude, high-frequency, total-pressure oscillations, this review will consider such excitation just peripherally. Our attention will be focussed on forcing at higher amplitudes and much lower frequencies. These are directed at enhancing mass transfer in porous catalyst particles. High-amplitude forcing has been studied for other reactor operating variables such as feed composition and temperature. Modulation in which an abrupt pressure reduction is employed can stimulate reaction systems that are product-inhibited. There is a no literature in this area, but interest is growing because an extension of the principles involved leads to pressure-swing reactors.     

The kinetics of complex catalytic reactions

The mathematical and statistical tools for parameter estimation and model discrimination in complex kinetic models are briefly reviewed. Hougen-Watson type rate equations are extended to simultaneous reactions and to situations in which the active site concentration varies through the reactor as a consequence of changes in the gas phase composition. Examples are given of the kinetic modeling of processes based upno complex feedstocks. Lumping of components and reactions, as well as partitioning of the networks are illustrated. Finally, the benefit of expressing rates in terms of truly elementary mechanisms is discussed.     

Periodic temperature forcing of catalytic reactions

The exponential relationship of rate constant to temperature served as a magnet to attract the attention of researchers in the 1960-1970 to periodic temperature forcing. Early theoretical studies and simulation exercises suggested that significant improvements of rate and selectivity could be expected from this type of unsteady state operation. The first experiments in the late 1970s and early 1980 failed to confirm these expectations, so research on periodic temperature forcing languished. The current interest in microreactors with their greatly reduced thermal capacitance has revived activity in temperature modulation. Recent discoveries of the effect of temperature pulses in trickle-bed reactors have further spurred this activity. This review addresses the question of whether temperature modulation of chemical reactors can significantly affect reactor performance.     

Dynamics of catalytic reactions and reactors

Heterogeneous catalytic reactions and catalytic reactors are known to exhibit complex dynamic behavior and significant progress has been made, in the past two decades, in understanding this complexity using the tools of nonlinear dynamics and advanced experimental methods. This article presents an overview of complex dynamic behavior in heterogeneous catalysis and points out their intriguing nature along with practical implications. The issues discussed are the source of oscillatory behavior, classification of complex motions, transitions to chaotic solutions and spatiotemporal patterns in reactors of various geometries.     

煤中灰成分催化指数的相关性

总结国内外不同方法的煤中灰成分催化指数定义的基础上,对10种配合煤进行了试验,探讨了不同方法的催化指数之间的相关性。结果表明,MBI,MCI,MMCI,MCIy四者之间的相关性良好,MBI和MCI之间相关性系数为0.9905,MCIy和MBI之间相关性系数为0.9821,MCIy和MCI之间相关性系数为0.9986。在一定程度上,四者可以互换。     

Intrareactor heat transfer criterion—I. Zero order catalytic reactions

A lumped parameter model is used to describe temperature gradients in bench-scale, fixed-bed catalytic reactors cooled at the wall. A dimensionless criterion is derived for evaluating the significance of intrareactor heat transport limitations in cases of integral conversion and zero order reactions. The new criterion is of nearly the same form as an earlier “hot spot” criterion, but is less conservative. Application of the criterion to published data on methylcyclohexane dehydrogenation indicates significant influence of intrareactor heat transport on the kinetics.     

Dynamics of catalytic reactions on microdesigned surfaces

We report experimental and computational studies of reaction dynamics on Pt/Rh and Pt/TiO₂ microcomposite catalytic surfaces. Reaction fronts initiated at the material interface dominate both steady and dynamic behavior of the composite catalytic material. Our analysis links the transient phenomenon of front initiation to the bifurcations of reaction–diffusion systems with active boundaries.     

The effect of catalyst film thickness on the magnitude of the electrochemical promotion of catalytic reactions

The effect of catalyst film thickness on the magnitude of the effect of electrochemical promotion was investigated for the model catalytic reaction of C₂H₄ oxidation on porous Pt paste catalyst-electrodes deposited on YSZ. It was found that the catalytic rate enhancement ρ is up to 400 for thinner (0.2 μm) Pt films (40,000% rate enhancement) and gradually decreases to 50 for thicker (1 μm) films. The Faradaic efficiency Λ was found to increase moderately with increasing film thickness and to be described semiquantitatively by the ratio 2Fr _o/I ₀, where r _o is the unpromoted rate and I ₀ is the exchange current of the catalyst–electrolyte interface. The results are in good qualitative agreement with model predictions describing the diffusion and reaction of the backspillover O^2- species, which causes electrochemical promotion.     

组成改变对C9石油树脂的影响

C9馏分由于成分组成相当复杂,其各成分对合成石油树脂影响的了解还十分有限.本实验通过改变轻C9中双环戊二烯的含量,进行石油树脂催化聚合实验,观察轻C9组成变化对石油树脂的色相、软化点等性能的影响,考察重要组成的改变对石油树脂宏观性质的影响.     

乙炔氢氯化均相催化反应动力学的初步研究

初步研究了以PtCl₄为催化剂,在十二烷/JM-T盐酸盐液相体系中,乙炔氢氯化制氯乙烯的均相反应动力学模型。根据测量组分在十二烷/JM-T体系中物料的气液平衡和反应动力学数据,并对实验数据进行回归方差分析,比较两个模型（Ⅰ） 和（Ⅱ） ,认为模型Ⅰ与实验数据拟合得更好,其中k₁^′=5.2,k₂^′=3.7和k_－1=1287.8,回归分析的显著性检验F=3.801。根据模型Ⅰ初步推测该反应的反应过程是：C₂H₂先和催化剂Pt（Ⅳ）发生化学吸附,再和体系中的HCl反应;其中C₂H₂和催化剂络合为反应的控制步骤。     

Zeolite Y synthesized with FCC spent catalyst fines: particle size effect on catalytic reactions

In this study, Y zeolite with different particle sizes was synthesized with fines of Fluid Catalytic Cracking (FCC) spent catalyst. The effect of particle size on physicochemical properties of zeolite was systematically investigated. The results showed that zeolites synthesized via in situ crystallization technique exhibited large surface area, high relative crystallinity and high thermal stability. With a decrease of particle size of zeolite, both total acid density and B acid sites increased while acid L sites decreased. The cracking activity for heavy oil and coke resistance of ultra-fine zeolite catalysts were enhanced. Of note is that the desulfurization capability of superfine zeolite catalyst was found to be much higher than that of industrial catalyst.     

Periodic operation of transport reactors for catalytic reactions

Based on plug flow of gas and catalyst particles and concentration dependent deactivation kinetics, the performance of transport reactors under a periodic (rectangular pulse) inlet concentration is analysed for improvement in conversion and extent of catalyst decay. The effects of reaction and deactivation orders, reaction and deactivation constant groups, and γ (cycle split) on the performance of the reactors are evaluated theoretically. For reaction orders greater than one, periodic operation improves conversion. Resonance behaviour is observed for certain combinations of parameters. For identical operating conditions vertical upflow, downflow and horizontal flow reactors are compared. Conversion in upflow reactors is higher than that in either horizontal flow or downflow reactors. However, catalyst decay is the least in downflow reactors.     

New catalytic reactions of oxa- and azabicyclic alkenes

Bicyclic alkenes, including oxa- and azabenzonorbornadienes and their derivatives, can be readily activated by transition metal complexes face-selectively due to their unsymmetrical bicyclic structure and the intrinsic angle strain on the carbon-carbon double bond. We have developed several stereo-, regio-, and chemoselective reactions catalyzed by nickel and palladium complexes using these bicyclic alkenes as substrates, providing a unique means of constructing a variety of synthetically useful carbocycles and heterocycles with high efficiency not generally accessible by traditional methods. This Account outlines these new metal-catalyzed reactions that include couplings, cycloadditions, and cyclization reactions.     

Cu系催化剂在催化反应中的研究进展

Cu是常用的金属催化剂,具有加氢、脱氢和氧化等催化性能,铜基催化剂在化学工业中应用广泛。主要介绍了铜基催化剂在甲醇水蒸汽重整制氢、CO催化氧化消除、合成甲醇、草酸二甲酯加氢合成乙二醇、乙醇脱氢制乙酸乙酯等领域的研究进展。     

离子液体在两相催化反应中的应用

离子液体应用在两相催化反应中可以显著提高催化剂的活性、选择性,催化剂循环使用相对容易,而由离子液体和催化剂组成的具有高温均相低温两相功能的体系可使反应在均相进行,反应后产品易分离,此类离子液体体系具有广阔前景.本文介绍了常温和高温条件下离子液体在两相催化反应中的应用.