Platinum frequency response

Hudgins 17) has used a sinusoidally varying input in concentration to analyze the frequency response of a reactor for the catalytic dehydrogenation of ethanol using a conventional tubular reactor. Leder and Butt 18) have successfully used frequency response to analyze the dynamic behavior of a fixed-bed catalytic reactor used for the hydrogen-oxygen reaction over a supported platinum catalyst. Hydrogen inlet concentration was varied sinusoidally over frequencies from 2 cycles/ hour to 120 cycles/hour. 

The resistance of membranes can be measured by AC impedance methods [85,86], using the four-point-probe technique. The test membrane is placed in a cell consisting of two Pt-foil electrodes, spaced 3 cm apart, to feed the current to a sample of 3 x 1 cm and two platinum needles placed 1 cm apart, to measure the potential drop (see Fig. 4.3.26). The cell is placed in a vessel maintained at constant temperature by circulating water. The impedance measurements are then carried out at 1-10 kHz using a frequency-response analyzer (e.g., Solatron Model 1255HF frequency analyzer). After ensuring that there are no parasitic processes (from the phase angle measurements, which should be zero), one can measure the resistance directly. The membrane resistance can also be obtained directly from the real part of the impedance (see typical data in Fig. 4.3.27). 

Scheme 3 Terminal i (CO) frequencies (squares) versus redox potential for [Pt24(CO)3o]" the circles show the corresponding response of i>(CO) on poly crystalline platinum (from Ref 37 with permission).

The results of the DFT calculations for various stable C2H.V species and transitions states on Pt(lll) and Pt(211) are summarized in Table V, which also shows entropy changes for the various steps, as estimated from DFT calculations of the vibrational frequencies of the various adsorbed C2H species and transition states on 10-atom platinum clusters (55). Table V also includes estimates of the standard Gibbs free energy changes for the formation of stable C2H surface species and activated complexes responsible for C-C bond cleavage at 623 K. These estimates were made by combining... 

The specific conductance of a water sample provides a simple method to determine the total dissolved ionic solids present in the sample. It is also an inexpensive technique, which lends itself to continuous monitoring of a river or waste stream for the total ion content (Fig. 4.1), and can be easily used to check the accuracy of analyses conducted for specific ions. Specific conductance is measured via a pair of carefully spaced platinum electrodes, which are placed either directly in the stream to be measured or in a sample withdrawn from it [22]. The water temperature should be 25 °C, or the result corrected to this temperature. Voltages in the 12 to 14 range, and frequencies of 60 to 1000 Hz AC are used, plus a Wheatstone bridge circuit to obtain a conductivity reading in xmho/cm or xS/cm (microsiemen/cm). The response obtained is linear with the total ion content over a wide range of concentrations (Fig. 4.1). Examples of the conductance ranges and seasonal variation of some typical Canadian rivers are... 

Fare Thetal 1988 Quasi-static and high frequency C(V)-response of thin platinum... 

Te and Cu monolayers on gold, as well as Ag and Bi monolayers on platinum were obtained by cathodic underpotential deposition and investigated in situ by the potentiodynamic electrochemical impedance spectroseopy (PDEIS). PDEIS gives the graphical representation of the real and imaginary interfacial impedance dependencies on ac frequency and electrode potential in real-time in the potential scan. The built-in analyzer of the virtual spectrometer decomposes the total electrochemical response into the responses of the constituents of the equivalent electric circuits (EEC). Dependencies of EEC parameters on potential, especially the variation of capacitance and pseudocapacitance of the double layer, appeared to be very sensitive indicators of the interfacial dynamics. 

It is usually believed that high frequency capacitance obtained from impedance spectroscopy can represent ionic double layer capacity. However in general this is not the case for platinum electrode, this is the reason why one arrow in Fig. 1 (in the left) is crossed. In contrast to Cf measured under equilibrium conditions (by means of isoelectric potential shifts), non-equilibrium impedance response can contain a contribution from Ah (and/or Ao, surface concentration of oxygen-containing species). These contributions are determined by Ah and Ao potential derivatives and their free electrode charge derivatives, and in general can be either positive, or negative. 

Impedance is typically measured in a two-electrode configuration where the electrolyte is compressed between two blocking (steel, platinum) or nonblocking Li-electrodes (Qian et al. [2(X)2]). Analysis of electrolyte impedance in the presence of electrode impedance is complicated and usually assumes that the electrolyte is responsible for the highest frequency region of the spectrum, about IkHz. To improve confidence in the conductivity estimation, measurements with several layer thicknesses should be performed. To remove the effect of the electrode impedance in a test setup, four-electrode measurements have also been proposed (Bruce et al. [1988]). Typically, two pseudoreference electrodes made of Li-foil strips are pressed through a cavity in the middle of circular main electrodes to the surface of the polymer electrolyte under test. 

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