Platinum electrodes pretreatments

A platinum electrode pretreated in the way as is described in Section 1.2 may show some minimal desorption of carbonaceous residues which may come from C-atoms diffusing from the bulk of Pt or from the rest of the gas in the UHV. A blank desorption experiment carried out by transferring a Pt electrode which was held at 450 mV in H2S04 for 120 s is shown in Fig. 2.4(a). 

The actual values of jo depended slightly on the pretreatment of the solid electrode. Indium-plated platinum electrode. 

Electrodes of 2 x 2 cm2 geometric area were used in the experiments. In the case of platinum, a pretreatment by fast triangular potential scans (200-300 V/s between 0.05 V and 1.5 V RHE) followed by heating up to 900 K in a 3 x 10 6 mbar 02 atmosphere was carried out. Electrodes, pre-treated in this way, can be emersed with a thin liquid film, which can easily be evaporated in the vacuum chamber. The heat treatment drastically reduces contamination of the platinum electrode by carbon. The roughness factor is usually in the order of three. 

Tlie same pretreatment was carried out on the smooth platinum electrode for in situ infiiared spectroscopic studies. The spectra of the Pt-Mo electrode with COad are shown in Fig. 4—22. C02 emerged at potentials as low as 250 mV. The oxidation does not, however, finish until 650 mV, which is the same potential as with piire platinum. The peak seen in the... 

At 500°C the reaction rate over the platinum electrode-catalyst appeared to be independent of the e.m.f. of the cell. At 550°C two reaction rate branches were observed, depending on whether the catalyst had been pretreated in oxidising or reducing conditions (see Figure 9). The e.m.f. of the cell also exhibited two branches dependent upon pretreatment (see Figure 9), in a similar manner to other SEP work on oxide catalysts.86,87 It was suggested that the catalyst state (i.e., catalyst oxygen content or 5) was a function of the catalyst history. Different catalyst states corresponded with different catalyst reactivities and the e.m.f. of the cell reflected the catalyst state. 

Activation (of noble metal electrodes) — Noble metal electrodes never work well without appropriate pretreatment. Polycrystalline electrodes are polished with diamond or alumina particles of size from 10 pm to a fraction of 1 pm to obtain the mirror-like surface. The suspensions of polishing microparticles are available in aqueous and oil media. The medium employed determines the final hydrophobicity of the electrode. The mechanical treatment is often followed by electrochemical cleaning. There is no common electrochemical procedure and hundreds of papers on the electrochemical activation of -> gold and platinum (- electrode materials) aimed at a particular problem have been published in the literature. Most often, -> cyclic and - square-wave voltammetry and a sequence of potential - pulses are used. For platinum electrodes, it is important that during this prepolarization step the electrode is covered consecutively by a layer of platinum oxide and a layer of adsorbed hydrogen. In the work with single-crystal (- monocrystal) electrodes the preliminary polishing of the surface can not be done. 

The Eh-pH and Eh-Es2- relations in the H2S-H2O system. The Eh values in the H2S-H20 system were found to be slightly dependent upon stirring ( 10 mV]. At 298 K the potentials of the platinum electrodes were established quite slowly [1 to 2 hr] at pH > 5. At pH < 5 it generally takes less than one hour. The potentials were independent of such electrode pretreatment as cathodic or anodic polarization. In Figure 4 we present the Eh-pH relations obtained in the H2S-H20-NaCl(0.7 j4] system. 

Cai WB, Ren B, Li XQ, She CX, Liu FM, Cai XW, Tian ZQ (1998) Investigation of surface-enhanced Raman scattering from platinum electrodes using a confocal Raman microscope dependence of surface roughening pretreatment. Surf Sci 406 9-22... 

The direct electron transfer between GOD and an electrochemically pretreated platinum electrode has been studied by Durliat and Comtat (1984). Spectroelectrochemical investigations showed that GOD was quantitatively reduced at the electrode. A GOD solution (100 pmol/1) was entrapped in a reaction chamber of 0.04 mm thickness in front of the electrode by a dialysis membrane. The electrode was pretreated by cyclic sweeping between -700 mV and +900 mV for 8 h. In anaerobic solution this sensor responded to glucose at a potential of +450 mV with a response time of 6 min and a linear concentration range between 0.01 and 7 mmol/1. 

The first enzyme electrode-based lactate analyzer was developed in 1976 by La Roche (Switzerland) (see Table 23). It uses cytochrome b2 in a tiny reaction chamber on top of a platinum electrode polarized at +0.25-0.40V. The solution for blood sample pretreatment recommended by the manufacturer has been improved by Soutter et al. (1978) by addition of cetyltrimethylammonium bromide. This compound hemo-lyzes the sample, stabilizes the lactate content, and leads to a good correlation with the spectrophotometric reference method using deproteinized blood ... 

Electrode pretreatment is critical for consistent results, so all electrodes were prepared in an identical manner before each experiment. The platinum surface was polished on a felt pad with 0.05 micron alumina and water. The surface was then rinsed with deionized water, wiped dry with a tissue, and the experiment begun as quickly as possible. 

A Ross reference cell is used for ORP measurements, with its potential calibrated after each reading using Light s solution ( ). Platinum electrodes were carefully pretreated (0 and kept in an oxygen-free solution for at least ten hours before each run, since initially adsorbed oxygen at the Pt electrode surface can give erroneous potential measurements for up to two hours, even in anaerobic systems (0. 

Figure 13.6.1 Cyclic voltammogram for a smooth platinum electrode in 0.5 M H2SO4. Peaks formation of adsorbed hydrogen. Peaks H oxidation of adsorbed hydrogen. Peaks Oq formation of adsorbed oxygen or a platinum oxide layer. Peak Oc reduction of the oxide layer. Point 1 start of bulk hydrogen evolution. Point 2 start of bulk oxygen evolution. The shape, number, and size of the peaks for adsorbed hydrogen depend on the crystal faces of platinum exposed (62), pretreatment of electrode, solution impurities, and supporting electrolyte. See also Figure 13.4.4.

Castner, J. F., Wingard Jr., L. B., Alterations in Potentiometric Response of Glucose Oxidase Platinum Electrodes Resulting from Electrochemical or Thermal Pretreatments of a Metal Surface , Anal. Chem. 56 (1984) 2891-2896. 

The strong interactions with metal ions extend to the use of metal-modified electrodes in electrocatalysis. Catalysis has been demonstrated with four systems. Chromium treatment results in as much as a 200 mV positive shift in the reduction peak for lO in acetate buffer This has been compared to the necessity for prior oxidation of the platinum electrode surface Ruthenium pretreatment of (SN), electrodes results in a catalytic current for the I / couple in phosphate buffer, pH 7.6. These electrodes also photoelectrochemically reduce protons to hydrogen at —0.05 V versus SCE in dilute sulfuric acid solution Molybdate treated electrodes have been used to electrochemically reduce acetylene at potentials of 1.5 V versus SCE in borate and hydroxide solutions. Iron treated electrodes show some ability to facilitate this reaction, but the rate is slower than with the molybdate treated electrodes... 

In contrast to the d.m.e, very strong residual currents must be taken into account. These are due not only to the charging of the electrode and further to the electrochemical reactions of traces of impurities present in the solution, but, as follows from cyclic voltammograms with a platinum electrode, different regions can be observed such as those of oxide formation and reduction and the regions of H2 formation and its oxidation. For this reason a pretreatment [3] of a solid electrode is usually necessary this can be performed either mechanically or by the so-called "cycling. As regards the cathodic limit, one has to bear in mind that in aprotic solvents the proton availability is much lower than in water and consequently the cathodic limit is extended. 

A biamperometric method for the direct determination of pyrogallol compounds was based on the electrocatalytic oxidation of pyrogallol compounds at one pretreated platinum electrode and the reduction of platinum oxide at the other pretreated platinum electrode to form a biamperometric detection system with an applied potential difference of 10 mV. Three important compounds detected by the method were pyrogallol, gallic acid, and tannic acid. The linear relationships between currents and the concentrations of pyrogallol, gallic acid, and tannic acid were determined over the ranges 1.0 x 10 -1.0 X 10, 1.0 X 10 -1.0 X 10 and 1.0 x 10 -2.0 x 10 mol with detection limits... 

Pretreatment of platinum in hot chromic acid was shown [83] to be equivalent to anodic activation. The electrode has a large reactivity after reduction of the oxygen layer that was formed at open circuit. In contrast to the pretreatment with hot chromic acid, the i — U curve of the first sweep after treatment of the platinum electrode in hot nitric acid does not have the shape characteristic for a clean surface. An intermediate product in the reduction of nitric acid seems to be strongly adsorbed on platinum. It takes several hours of continuous cycling between 0.05 V and 1.4 V at 30mV/sec before the i- U curve regains the regular shape. 

Curves of the type a in Fig. 18 were also found [78] during the initial cycles on platinum electrodes that had been pretreated anodically and left at open circuit in electrolytic solutions for days. However, it took only 10 cycles to restore the regular shape of the i — U curves. Adsorption of impurities from the electrolyte is the cause of the loss of reactivity in the latter case. The comparison of Cp—U curves confirms this conclusion. The time in which the loss of reactivity occurs depends upon the amount... 

It was concluded first by Knorr and coworkers [1] that diffusion of molecular hydrogen is the rate-determining step of hydrogen evolution on smooth platinum and palladium electrodes pretreated anodically. The same conclusion holds for the anodic processes given by Eq. 1 and 2. The interpretation based on steady-state measurements [1] is valid at cathodic current densities — i < 1 mA/cm in strong acid electrolytes (0d.H+ 1)- The correctness of this interpretation has been substantiated by a number of independent techniques which allow the concentration Ch2 of H2 molecules adjacent to the surface to be determined ... 

Hoare found [86—88] that the potential of a bright platinum electrode which had been pretreated in concentrated nitric acid became steady at 1.23 V in purified 1 M H2SO4, saturated with O2, for more than 24 hrs. Traces of hydrogen peroxide were absent. He suggested that the pretreatment in nitric acid produced a film similar to the oxygen... 

The cleaning of the electrode surface presents a very difficult problem with the platinum electrodes. Some workers recommend cleaning with dilute sulphuric acid solution, others recommend treatment with chromic acid (which, of course, oxidizes the electrode surface). Still other workers recommend pretreatment at a constant potential for a certain length of time or at a series of potentials. The best way to be followed seems to be the one best suited to the problem at hand in that logical and reproducible results are obtained. A method of pretreatment, however, must be devised before one is able to produce data to any degree of reliability. 

BagotsMi, VasiPev, and co-workers [12, 22, 26] confirmed this conclusion by measurements on a smooth platinum electrode using pulse methods. Recently Biegler and Koch [50], using similar procedures, came to the conclusion that carbon monoxide was adsorbed on smooth platinum in methanol solutions. However, this conclusion is insufficiently convincing, since in [50] use was made of adsorption values obtained by other investigators [17, 33], and the electrodes were differently pretreated in [17, 33, 50]. 

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