Anomalous Reaction Kinetics Research Articles

Anomalous kinetics in diffusion limited reactions linked to non-Gaussian concentration probability distribution function

We investigate anomalous reaction kinetics related to segregation in the one-dimensional reaction-diffusion system A + B → C. It is well known that spatial fluctuations in the species concentrations cause a breakdown of the mean-field behavior at low concentration values. The scaling of the average concentration with time changes from the mean-field t(-1) to the anomalous t(-1/4) behavior. Using a stochastic modeling approach, the reaction-diffusion system can be fully characterized by the multi-point probability distribution function (PDF) of the species concentrations. Its evolution is governed by a Fokker-Planck equation with moving boundaries, which are determined by the positivity of the species concentrations. The concentration PDF is in general non-Gaussian. As long as the concentration fluctuations are small compared to the mean, the PDF can be approximated by a Gaussian distribution. This behavior breaks down in the fluctuation dominated regime, for which anomalous reaction kinetics are observed. We show that the transition from mean field to anomalous reaction kinetics is intimately linked to the evolution of the concentration PDF from a Gaussian to non-Gaussian shape. This establishes a direct relationship between anomalous reaction kinetics, incomplete mixing and the non-Gaussian nature of the concentration PDF.

Statistical Models for Surface Catalysis: Evidence for Anomalous Reaction Rates

both cases we use Monte Carlo simulations to exhibit anomalous reaction kinetics which is the expression of density fluctuations and self-organization of reactants in the system. We show the influence of this self-organization on the macroscopic behavior of the system. Catalytically activated processes have been known for a very long time now but still present a challenge for fundamental research. Recently, statistical physics concepts have been used to describe collective phenomenon in catalysis such as pattern formation in excitable media' or nonequilibrium phase transi- tions.l.2 For example, a great deal of work has been done on pattern formation associated with nonlinear effects [see, for example, refs

Non-Classfcal Kinetiics in Hydrogen-Deuterium Exchange Reaction on Metal Catalyst

AbstractMonte Carlo simulations of the A2 + B2→ P (with P = AA, AB, and BB) reaction on various monodisperse island systems provided various models for the reaction H2 + D2→ HD on alumina supported platinum catalysts. Anomalous reaction kinetics and reaction orders due to the effects of diffusion, reactant correlation, and ensemble character of the catalyst in low dimensional systems are observed not only at early time but also at steady state. Concentration effects and spillover studies indicate that surface diffusion and hydrogen spillover may be responsible for the anomalous behaviors.

Low dimensional reaction kinetics and self-organization

Diffusion-limited reaction kinetics becomes anomalous not only for fractals, with their anomalous diffusion, but also for low-dimensional (one and two) and disperse media, where the random walk is compact. We focus on annihilation, recombination and trapping reactions under non-equilibrium steady state (steady source) or batch (big bang) conditions. The typical reactions are: A + A → Products, A + B → Products and A + C → Products. We are interested in the global rate laws, and their relation to particle-particle distributions (e.g., pair-correlation and nearest-neighbor distribution functions) and in local rate laws (if definable). Anomalous reaction kinetics (more than classical kinetics) is particularly sensitive to initial conditions, source term structure, conservation laws (e.g., equal densities for A and B), excluded volume effects, and medium size, dimensionality and anisotropy. Analytical formalisms, scaling arguments, computer (and supercomputer) simulations and experiments (on chemical and physical reactions) all play an important role in the newly emerging picture.

Reaction Kinetics in Restricted Spaces

AbstractReactions in restricted spaces rarely get stirred vigorously by convection and are thus controlled by diffusion. Furthermore, the compactness of the Brownian motion leads to both anomalous diffusion and anomalous reaction kinetics. Elementary binary reactions of the type A + A → Products, A + B → Products, and A + C → C + Products are discussed theoretically for both batch and steady‐state conditions. The anomalous reaction orders and time exponents (for the rate coefficients) are discussed for various situations. Global and local rate laws are related to particle distribution functions. Only Poissonian distributions guarantee the classical rate laws. Reactant self‐organization leads to interesting new phenomena. These are demonstrated by theory, simulations, and experiments. The correlation length of reactant production affects the self‐ordering length scale. These effects are demonstrated experimentally, including the stability of reactant segregation observed in chemical reactions in one‐dimensional spaces, e.g., capillaries and microcapillaries. The gap between the reactant A (cation) and B (anion) actually increases in time and extends over millimeters. Excellent agreement is found among theory, simulation, and experiment for the various scaling exponents.

Kinetics and self-ordering of excitations in low dimensions

Abstract Diffusion-limited reaction kinetics becomes anomalous not only for fractals, with their anomalous diffusion, but also for low-dimensional (one and two) and disperse media, where the random walk is compact. We focus on annihilation, recombination and trapping reactions under non-equilibrium steady state (steady source) or batch (big bang) conditions. The typical reactions are: A + A → Products, A + B → Products and A + T → T + Products. We are interested in the global rate laws, and their relation to particle-particle distributions (e.g., pair-correlation and nearest-neighbor distribution functions) and in local rate laws (if definable). Anomalous reaction kinetics (more than classical kinetics) is particularly sensitive to initial conditions, source term structure, conservation laws (e.g., equal densities for A and B), excluded volume effects, and medium size, dimensionality and anisotropy. Analytical formalisms, scaling arguments, computer (and supercomputer) simulations and experiments (on chemical and physical reactions) all play an important role in the newly emerging picture.

Further proof for the catalytic role of the larger subunit in the spinach leaf ribulose-1,5-diphosphate carboxylase

The catalytically active oligomeric form of the larger subunit, A m, obtained from spinach leaf ribulose-1,5-diphosphate carboxylase by pretreatment with p-mercuribenzoate at pH 7.5 followed by incubation at pH 9.0, was free of the smaller subunit based on C-terminal amino acid analyses. Valine was the predominant C-terminus of the A m preparations, the release of tyrosine being negligibly small [cf. Sugiyama and Akazawa, Biochemistry 9 (1970) 4499]. The pH optimum of the ribulose-1,5-diphosphate carboxylase reaction by A m was about 8.5, in comparison to the native enzyme which showed an alkaline pH optimum only in the absence of Mg 2+. The substrate saturation curve of the catalytic subunit with respect to bicarbonate followed the Michaelis-Menten equation, as contrasted to the anomalous reaction kinetics of the native ribulose-1,5-diphosphate carboxylase molecule reported previously. These overall results indicate that the allosteric properties of spinach ribulose-1,5-diphosphate carboxylase are possibly conveyed by a unique structural conformation that requires the presence of the smaller subunit in association with the larger catalytic subunit component of the enzyme molecule.

A crude nuclease preparation suitable for use in DNA reassociation experiments

A crude preparation of nuclease S 1 from Aspergillus oryzae is shown to have a strong preference for heat-denatured DNA as substrate, to be active over a range of ionic strengths, to degrade a variety of single-stranded DNA's to better than 97 % acid-soluble materials, and to be capable of removing the single-stranded regions from reassociated DNA molecules at a range of temperatures without extensively degrading the reassociated duplex regions. Nuclease S 1 displays anomalous reaction kinetics at low substrate concentrations.