Transient response techniques

Savatsky, B. J., and Bell, A. T. Studies of NO reduction by H2 using transient response techniques. This volume. 

Nitric Oxide Reduction by Hydrogen over Rhodium Using Transient Response Techniques... 

The present work was undertaken to investigate the applicability of transient response techniques [e.g., ref. 

Elements of response Wavefront analysis Conventional transient response techniques... 

Transient response techniques are used to investigate the activation of silver powder for ethylene epoxidation at vacuum and atmospheric pressures. Results indicate that the activation process is qualitatively the same in both pressure regimes. Numerical simulation of the process indicates that activation involves the concurrent incorporation of oxygen into surface and subsurface sites. The reaction selectivity parallels the incorporation of oxygen into the subsurface. 

Table 1 presents several examples of unsteady-state kinetics models. These models are presented in the form of rate dependencies for catalytic reaction stages and side processes. The parameters of the models, such as reaction rate constants and activation energies, are given in references (Table 1) and were determined mainly from experimental data using transient response techniques. For the reaction of CO oxidation over a supported platinum catalyst, the kinetic gas theory was applied for estimating the adsorption constants. 

Recently, Mathurand Wellek33 reported that there is a significant discrepancy in the values of the gas-phase Peclet number when the measurements are taken by the steady-state procedure which they (and also Brittan and Woodburn3) employed, as compared with the transient-response techniques employed by DeMaria and White13 and Sater and Levenspiel.43 Mathur and Wellek33 obtained... 

The same transient response techniques have been successfully applied in our laboratory to investigate also the kinetics of the surface reaction of NO with preadsorbed NH3. 

Thermal diffusivity measurements were performed on pellets by the use of a transient response technique. Pellets of cellulose or char were instrumented with thermocouples at known distances from the surface. Samples were then brought into contact with a hotplate whose surface temperature varied in an approximately sinusoidal manner. Samples of virgin cellulose could only be tested up to surface temperatures of about 500 K, to avoid pyrolysis. Char samples could be tested to temperatures up to those seen during pyrolysis (in excess of 750 K). Samples were covered by a bell jar, purged with nitrogen, in order to avoid reactions with oxygen. 

It is well established that ethylene does not adsorb on a pre-reduced silver surface.On a pre-oxidized silver surface, frontal chromatography has demonstrated that ethylene can be adsorbed in both a reversible and an irreversible form. However, there is also strong evidence, obtained using a transient response technique, that the reversibly adsorbed ethylene plays no part in the oxidation reaction at least in the temperature range 353—433 The same work showed that a stable intermediate is present, which leads to carbon dioxide. Such an intermediate was not seen for the epoxidation reaction. The intermediate had the stoicheiometry C H O of 1 2 (1—2). Although this species is formed by the interaction of ethylene with dissociatively adsorbed oxygen it is apparently decomposed to carbon dioxide and water by adsorbed molecular oxygen. 

A set of values of the reaction step rates (jf) at various steady-state concentrations of gases Cf) and surface species (0"), obtained at variation of reactants concentration and temperature, allows the kinetic expressions for the rates of steps r, = kfiiCfi) to be defined as well as estimation of the rate coefficients and activation energies. The principal difference of SSITKA from the conventional transient response technique implies that direct estimation of the concentration of active reaction intermediates is possible, which practically excludes a correlation between the values of kinetic parameters (coverages and rate coefficients). 

Winslow, P., Bell, A.T. (1984) Application of transient response techniques for quantitative determination of adsorbed carbon monoxide and carbon present on the surface of a ruthenium catalyst during Fischer-Tropsch synthesis. J. Catal, 86,158. 

Ghosh, A.K., and Agnew, J.B., Adsorption of acetylene, hydrogen chloride and vinyl chloride on activated carbons Kinetics and thermodynamics, using transient response technique, Chem. Eng. Commun., 40, 169-182 (1986). 

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