Simple catalytic mechanism permitting a multiplicity of catalyst steady states

The conditions of the existence of several steady states for a three-step mechanism, viz. a catalytic trigger (consecutive), are discussed.

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Dynamic properties of a heterogeneous catalytic reaction with several steady states

For the simplest catalytic mechanism (three-stage) permitting several steady states of the surface, a qualitative analysis of the properties of its kinetic model trajectories has been performed. Types of the steady states have been determined. Absence of oscillations (damped and undamped) has been shown.

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Для простейшего каталитического механизма (трехстадийного), допускающего несколько стационарных состояний поверхности, проведен качественный анализ свойств траекторий его кинетической модели. Определены типы стационарных состояний. Показано отсутствие осцилляций (затухающих и незатухающих).

     

Criterion of multiplicity of steady states and the explicit expression for bifurcation set in the simplest model of a catalytic reaction with an arbitrary isotherm

The following criterion of multiplicity of steady states (m.s.s.) for the simplest model of an open heterogeneous catalytic system (monomolecular isomerization in CSTR) with an arbitrary isotherm was obtained in this paper:

On the multiplicity criterion for steady states in catalytic reactions

CNDO/2 studies of NH₃ adsorption in terms of cluster models of the-Al₂O₃ surface sites have revealed that coordinatively unsaturated aluminium ions occupying tetrahedral and octahedral sites are Lewis acids that are stronger in the former case. NH₃ molecule adsorption on these sites is accompanied by an increase in the H–N–H bond angle and a weakening of the N–H bond.

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/2 , -Al₂O₃ , , , , . NH₃ H–N–H N–H.

     

Analysis of fluctuations in chemical reactions with several steady states

The effect of fluctuations in nonequilibrium systems is treated in terms of the stochastic theory. A solution of the fundamental equation is analyzed using the Monte-Carlo method /1, 2/. The results are compared with the conclusions following from the equations corresponding to a phenomenological model.

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The concepts of surface states

Electronic surface states are introduced from the tight binding approach as well as in the nearly free electron approximation. The localization mechanism and the criteria for Tamm and Shockley states are demonstrated. These concepts yield a new way of estimating the image potential state energies.     

A parametric analysis of the basic models of the mechanisms of the simplest catalytic reactions

A series of the simplest thermokinetic models, in which the kinetic subsystem corresponds to traditional catalytic mechanisms, such as the Eley-Rideal mono- and bimolecular mechanisms and the Langmuir-Hinshelwood mechanism, and autocatalytic mechanisms characterized by multiplicity of stationary states and autooscillations in the kinetic region, were considered. The method of continuation along a parameter was used for studying these basic models of thermokinetics. The parametric analysis performed allowed us to identify parameter regions characterized by critical effects (multiplicity of stationary states and autooscillations).     

The surface catalytic mechanism: a comparative study with square-wave and staircase cyclic voltammetry

A pseudo-first-order catalytic mechanism in which both reactant and product of a redox reaction are strongly immobilized on an electrode surface is theoretically analysed under conditions of square-wave (SWV) and staircase cyclic voltammetry (SCV). A mathematical procedure is developed under diffusionless conditions. The relationships between the properties of the voltammetric response and both the kinetic parameters of the redox reaction and the parameters of the excitation signal are studied. The phenomenon of the quasi-reversible maximum is discussed. A comparative study between SWV and SCV is presented and the limitations and advantages of both techniques, from analytical and kinetic points of view, are discussed. The theoretical predictions are experimentally confirmed by the redox reaction of azobenzene in the presence of hydrogen peroxide as an oxidizing agent. Electronic Publication     

Positions of steady states in two-component chemical systems

The general kinetic equation for two-component chemical systems is analyzed. It is shown that the positions of steady states in concentration spaces can be detected by a qualitative analysis of the chemical mechanism.

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Dynamics of chemical reactions and nonphysical steady states

Data on the position of nonphysical (lying beyond the region of determination) steady states are shown to be of use for understanding the dynamic behavior of chemical reactions, in particular, the reasons for slow relaxations.

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Temporal asymmetry of fluctuations in nonequilibrium steady states

Temporal asymmetries of fluctuation paths in nonequilibrium microscopic shearing systems are observed for the first time. Inspired by theories that predict asymmetry of fluctuation paths in stochastic dynamics, we focus on deterministic reversible particle models, which represent a small part of a macroscopic system. We have monitored and measured the asymmetry of the fluctuation paths of various observables as they go away from and towards the mean. The understanding of such asymmetries may scatter light on how irreversibility emerges from the microscopic reversible dynamics and on the behavior of mesoscopic (nanoscale) systems.     

Multiplicity of steady states in oxidative heterogeneous catalysis

Typical mechanisms of heterogeneous catalytic reactions and their kinetíc models that are characterized by a plurality of steady states are considered. Models of adsorption that lead to a plurality of equilibrium adsorption states are discussed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, No. 4, pp. 285–300, July–August, 1992.     

Modelling the effect of the electrode potential on the metal-adsorbate surface states: relevant states in the charge transfer mechanism of SERS

Quantum mechanical calculations of the ground and excited electronic states of several [Ag(n)-pyridine](q) complexes yield a linear dependence of the energies of the surface states, especially the metal-to-molecule charge transfer states, on q(eff) = q/n. This is the first theoretical approach to modelling the effect of the electrode potential on SERS.     

The mechanism of cyclic nucleotide hydrolysis in the phosphodiesterase catalytic site

The cyclic nucleotide phosphodiesterase superfamily of enzymes (PDEs) catalyzes the stereospecific hydrolysis of the second messengers adenosine and guanosine 3',5'- cyclic monophosphate (cAMP, cGMP) to produce 5'-AMP and 5'-GMP, respectively. The PDEs are targets of high-throughput screening to determine selective inhibitors for a variety of therapeutic purposes. The catalytic pocket where the hydrolysis takes place is a highly conserved region and has several residues which are absolutely conserved across the PDE families. In this study, we consider a model cyclic substrate in which the adenine/guanine base has been replaced with a hydrogen atom, and we present results of a quantum computational investigation of the hydrolysis reaction as it occurs within the PDE catalytic site using the ONIOM hybrid (B3LYP/6-31g(d):PM3) method. We characterize the bound substrate, the bound hydrolyzed product, and the transition state which connects them for our model cyclic substrate placed in a truncated model of the PDE4D2 catalytic site. We address the role that the conserved histidine proximal to the bimetal system of the catalytic site, along with its conserved glutamine partner, plays in the generation of the hydroxide nucleophile. Our study provides computational evidence for several key features of the cAMP/cGMP hydrolysis mechanism as it occurs within the protein environment across the PDE superfamily.     

Synthesis of hierarchical Polystyrene/Polyaniline@Au nanostructures of different surface states and studies of their catalytic properties

We synthesized hierarchical Polystyrene/Polyaniline@Au(PS/PANI@Au) catalysts through a seeded swelling polymerization and in-situ reduction procedure. PS/PANI@Au catalysts possess a core of PS as seed and template, a PANI shell with fibers and uniform gold nanoparticles on the surface. The configuration changes of the PANI chains resulting from the doping/ dedoping procedure led to various loading amounts of Au nanoparticles. Reduction of 4-nitrophenol was chosen as the probe reaction to evaluate the catalytic activity of supported Au nanocatalysts. The catalytic results indicated that dedoping treatment of the PS/PANI supports provides stronger coordinative ability to metal nanoparticles as well as more –N= groups, which results in a better catalytic performance towards the reduction of 4-nitrophenol.