Transient methods

The most widely used transient method is the temperature-jump (T-jump) method. This is based on the van t Hoff equation, which describes the temperature dependence of the equilibrium constant. 

Measurements Using Liquid-Phase Reactions. Liquid-phase reactions, and the oxidation of sodium sulfite to sodium sulfate in particular, are sometimes used to determine kiAi. As for the transient method, the system is batch with respect to the liquid phase. Pure oxygen is sparged into the vessel. A pseudo-steady-state results. There is no gas outlet, and the inlet flow rate is adjusted so that the vessel pressure remains constant. Under these circumstances, the inlet flow rate equals the mass transfer rate. Equations (11.5) and (11.12) are combined to give a particularly simple result ... 

The hot-wire method is a transient method and can yield an absolute value. A long, thin wire, through which electric current is passed stepwise, is placed in a melt, when... 

For the individual types of transient measuring techniques, special names exist but their terminology lacks uniformity. The potentiostatic techniques where the time-dependent current variation is determined are often called chronoamperometric, and the galvanostatic techniques where the potential variation is determined are called chronopotentiometric. For the potentiodynamic method involving linear potential scans, the term voltammetry is used, but this term is often used for other transient methods as well. 

The second limitation is concerned with the high time constant Tceii determined by the high capacitance of the CLs and by substantial cell resistance. Tceu for a thick MEA of large geometric area may exceed several minutes, thus strongly limiting the application of transient methods and methods based on the concept of impedance. 

The transient method characterized by linearly changing potential with time is called potential-sweep (potential-scan) voltammetry (cf. Section 5.5.2). In this case the transport process is described by equations of linear diffusion with the potential function... 

The electrical activity of a defect is characterized in part by its electrical level position, which can be determined by capacitance transient methods. When the capacitance transient spectra are monitored before and after exposure to atomic hydrogen, it is found in many systems that these levels disappear. This phenomenon has been associated with the formation of electrically inactive hydrogen-impurity complexes as summarized by Pear-ton et al. (1987) and in Chapter 5 of this volume. 

Understanding Heterogeneous Catalysis Through the Transient Method... 

Gas-phase Kinetics. A better appreciation of the experiments to be discussed later will be obtained after a review of some experimental aspects of the transient method. Here we deal with experiments at atmospheric pressure. A flow sheet for kinetic measurements is given in Fig. 1, a descendant of that first given by Bennett et al. (15). Chemical analysis of the gases during transients is ideally done by a mass spectrometer, although Kobayashi and Kobayashi (4 ) used a number of gas chromatographs in order to get samples sufficiently frequently. 

Some of the first transient infrared data on heterogeneous catalysis were gathered by Heyne and Tomkins in 1966 (16). The method has been advocated and used by Tamaru, who has recently reviewed his work since 1967 in this field (17). Dent and Kokes used the transient method in IR studies in 1970 (18). Since then the method has been applied to kinetic studies in particular by Ueno et al. (19, 20) and most recently by Savatsky and Bell (21). 

This subject has been thoroughly reviewed by Engel and Ertl (48) and most recently by White (49). Here the intention is to describe some contributions of the transient method to our understanding of this classical problem. Only the temperature range below 450K is discussed here. 

Recently there has been a growing emphasis on the use of transient methods to study the mechanism and kinetics of catalytic reactions (16, 17, 18). These transient studies gained new impetus with the introduction of computer-controlled catalytic converters for automobile emission control (19) in this large-scale catalytic process the composition of the feedstream is oscillated as a result of a feedback control scheme, and the frequency response characteristics of the catalyst appear to play an important role (20). Preliminary studies (e.g., 15) indicate that the transient response of these catalysts is dominated by the relaxation of surface events, and thus it is necessary to use fast-response, surface-sensitive techniques in order to understand the catalyst s behavior under transient conditions. 

As mentioned in potentiostatic current transient method, when the fractal dimension is determined by using diffusion-limited electrochemical technique, the diffusion layer length acts as a yardstick length.122 In the case of cyclic voltammetry, it was... 

In general, experiments using transient methods utilize solutions to Eq. (92) (Sect. 4.2.2.3) to obtain so-called experimentally derived diffusion coefficients. The following sections will show briefly the common transient methods of experimentation used to obtain test data for calculations of the transport coefficient. 

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