Transient techniques

Gerstein B C and Dybowski C 1985 Transient Techniques in NMR of Solids (New York Academic)... 

The transient techniques can be grouped according to various criteria (Fig. 12.6), as follows ... 

Step- and impulse-response methods. Chemisorption can conveniently be measured under flow conditions using transient techniques, in particular, step-response and impulse-response measurements. After pretreatment, pulses of probe gas are injected into a carrier gas stream passing through the reactor that contains the pre-treated. sample. The response is detected at the reactor exit. 

Macdonald, D. D., Transient Techniques in Electrochemistry, Plenum Press, New York, 1977. 

In order to get closer from the catalytic conditions (for example in DeNO reaction exhaust gases are to be treated) investigators developed reactor cells allowing the infrared study of catalysts underflow. The principle of transient technique is then to introduce... 

Wavefront Analysis—A Special Transient Technique for Kinetic Investigations in Distributed Catalytic Systems... 

Our main motivation to develop the specific transient technique of wavefront analysis, presented in detail in (21, 22, 5), was to make feasible the direct separation and direct measurements of individual relaxation steps. As we will show this objective is feasible, because the elements of this technique correspond to integral (therefore amplified) effects of the initial rate, the initial acceleration and the differential accumulative effect. Unfortunately the implication of the space coordinate makes the general mathematical analysis of the transient responses cumbersome, particularly if one has to take into account the axial dispersion effects. But we will show that the mathematical analysis of the fastest wavefront which only will be considered here, is straight forward, because it is limited to ordinary differential equations dispersion effects are important only for large residence times of wavefronts in the system, i.e. for slow waves. We naturally recognize that this technique requires an additional experimental and theoretical effort, but we believe that it is an effective technique for the study of catalysis under technical operating conditions, where the micro- as well as the macrorelaxations above mentioned are equally important. 

As for any transient technique, it is essential also for wavefront analysis... 

As the field of electrochemical kinetics may be relatively unfamiliar to some readers, it is important to realize that the rate of an electrochemical process is the current. In transient techniques such as cyclic and pulse voltammetry, the current typically consists of a nonfaradaic component derived from capacitive charging of the ionic medium near the electrode and a faradaic component that corresponds to electron transfer between the electrode and the reactant. In a steady-state technique such as rotating-disk voltammetry the current is purely faradaic. The faradaic current is often limited by the rate of diffusion of the reactant to the electrode, but it is also possible that electron transfer between the electrode and the molecules at the surface is the slow step. In this latter case one can define the rate constant as ... 

Shen et al. (142) used an isotopic transient technique and XPS to investigate the partial oxidation of CH4 to synthesis gas on a Ni/Al203 catalyst at 973 K. The results show that CH4 can decompose easily and quickly to give H2 and Ni C on the reduced catalyst, and that Ni vC can react rapidly with NiO, formed by the oxidation of nickel by 02 to give CO or C02, depending on the relative concentration of Ni,C around NiO on the catalyst surface. The conclusion drawn by the authors (142) was not only that H2 and CO are primary products in the partial oxidation of CH4, but also that most of the CO2 is also the primary product of the surface reaction between Ni,C and NiO. In contrast, the kinetics results of Verykios et al. (143) indicated that the reaction on the Ni/La203 catalyst mainly takes place via the sequence of total oxidation to CO2 and H20, followed by... 

Fractal Dimensions of the Profiles h(x) at Various Morphological Amplitudes rj in h(x) = 77/wsCv) Determined by the Current Transient Technique (2nd Column) and the Triangulation Method (3rd Column). Here, /ws(-v) Means the Weierstrass Function with a Self-Affine Fractal Dimension t/Fsa = 1.5 ... 

XsNv dp df Determined by the current transient technique dF.ss Determined by the triangulation method... 

In practical application, it was reported that the platinum particles dispersed in highly porous carbonized polyacrylonitrile (PAN) microcellular foam used as fuel-cell electrocatalyst160 have the partially active property. The fractal dimension of the platinum particles was determined to be smaller than 2.0 by using the potentiostatic current transient technique in oxygen-saturated solutions, and it was considered to be a reaction dimension, indicating that not all of the platinum particle surface sites are accessible to the incoming oxygen molecules. 

The integral relationships above are valid for any transient technique other than cyclic voltammetry, since at this stage of the derivation, the fact that the potential is a linear function of time has not yet been introduced. It is also valid in the case where charge transfer is not fast and together with diffusion, kinetically governs the electrochemical response. In the present case, the linear relationship between potential and time comes into play through Nernst s law, leading to... 

Britz, D., Digital Simulation in Electrochemistry, Springer-Verlag, Berlin, 1981 and MacDonald, D., Transient Techniques in Electrochemistry, Plenum Press, New York, 1977. Both of these books contain copious details concerning electrochemical simulations. Although these texts are extremely mathematical (as all simulation work has to be), the basic concepts are not too difficult to follow. The application notes to Condecon (see URL on page 301) are also a feast of detail. 

The simplest and most commonly used method for determination of a double layer s capacitance is the galvanostatic (constant-current) transient technique. 

Galvanostatic Transient Technique. In the galvanostatic method a constant-current pulse is applied to the cell at equilibrium state and the resulting variation of the potential with time is recorded. The total galvanostatic current ig is accounted for (1) by the double-layer charging, /ji, and (2) by the electrode reaction (charge transfer), i. ... 

The plot V = f(t), in the microseconds range, is a straight line with slope dVIdt. Thus, the double-layer capacitance Qj may be calculated by means of Eq. (4.17) using the slope (dV/dt) provided by the experimental data. One example of such calculations is presented in Problem 4.2. Galvanostatic transient technique is discussed in detail in Section 6.9. 

Study of the charge-transfer processes (step 3 above), free of the effects of mass transport, is possible by the use of transient techniques. In the transient techniques the interface at equilibrium is changed from an equilibrium state to a steady state characterized by a new potential difference A(/>. Analysis of the time dependence of this transition is the basis of transient electrochemical techniques. We will discuss galvanostatic and potentiostatic transient techniques for other techniques [e.g., alternating current (ac)], the reader is referred to Refs. 50 to 55. 

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