Platinum electrodes poisoning

In selecting reference electrodes for practical use, one should apply two criteria that of reducing the diffusion potentials and that of a lack of interference of RE components with the system being studied. Thus, mercury-containing REs (calomel or mercury-mercuric oxide) are inappropriate for measurements in conjunction with platinum electrodes, since the mercury ions readily poison platinum surfaces. Calomel REs are also inappropriate for systems sensitive to chloride ions. 

On the surface of metal electrodes, one also hnds almost always some kind or other of adsorbed oxygen or phase oxide layer produced by interaction with the surrounding air (air-oxidized electrodes). The adsorption of foreign matter on an electrode surface as a rule leads to a lower catalytic activity. In some cases this effect may be very pronounced. For instance, the adsorption of mercury ions, arsenic compounds, or carbon monoxide on platinum electrodes leads to a strong decrease (and sometimes total suppression) of their catalytic activity toward many reactions. These substances then are spoken of as catalyst poisons. The reasons for retardation of a reaction by such poisons most often reside in an adsorptive displacement of the reaction components from the electrode surface by adsorption of the foreign species. 

Palladium electrodes are also very active for formic acid oxidation, with higher current densities than platinum electrodes [Capon and Parsons, 1973c]. Oxidation occurs almost exclusively through the active intermediate path, without poison formation. The reaction is also very sensitive to the surface stmcture, and the activity of the... 

Smith PE, Ben-Dor KF, Abruna HD. 2000. Poison formation upon the dissociative adsorption of formic acid on bismuth-modified stepped platinum electrodes. Langmuir 16 787-794. 

More than a decade ago, Hamond and Winograd used XPS for the study of UPD Ag and Cu on polycrystalline platinum electrodes [11,12]. This study revealed a clear correlation between the amount of UPD metal on the electrode surface after emersion and in the electrolyte under controlled potential before emersion. Thereby, it was demonstrated that ex situ measurements on electrode surfaces provide relevant information about the electrochemical interface, (see Section 2.7). In view of the importance of UPD for electrocatalysis and metal deposition [132,133], knowledge of the oxidation state of the adatom in terms of chemical shifts, of the influence of the adatom on local work functions and knowledge of the distribution of electronic states in the valence band is highly desirable. The results of XPS and UPS studies on UPD metal layers will be discussed in the following chapter. Finally the poisoning effect of UPD on the H2 evolution reaction will be briefly mentioned. 

The work of Kunimatsu and Kita (1987) is very powerful evidence in favour of linearly adsorbed CO being the catalytic poison for methanol oxidation at a smooth platinum electrode in acid solution and has resulted in this hypothesis being generally accepted. However, there is some conflict between the IR results and those obtained by Vielstich and colleagues using chronocoulometry, ECTDMS and DEMS. 

Eh can conveniently be measured by inserting a platinum electrode into the soil or sediment and connecting this electrode to a reference electrode, such as the calomel electrode. The electro-motive potential (emf) generated can be measured on a suitable detector and the Eh calculated as the difference between this potential and the electrode potential of the reference electrode. However, the electrode must be kept clean during and after measurements to prevent poisoning of the electrode. This poisoning is due to the formation of a Pt-oxide coating. 

It was first shown by electrochemically modulated infrared reflectance spectroscopy (EMIRS) that the main poisoning species formed during the chemisorption and oxidation of methanol on a platinum electrode is carbon monoxide CO, either linearly bonded, or bridge bonded to the surface. The coverage degree of the electrode surface by linearly bonded CO can reach 90% on a pure platinum electrode, so that most of the active sites are blocked... 

This result is consistent with the observed effective poisoning of the CO oxidation reaction as reflected in the increased potential induced by bismuth in the cyclic voltammetry on the supported platinum electrodes (Figure 10a). The voltammetry of CO stripping on the supported catalysts indicates a similar behavior to that found on Pt(llO) in that bismuth results in a higher overpotential for CO oxidation. One must conclude that the morphology of the supported platinum catalyst results in facets more akin to the more open-packed Pt(l 10) surface than the Pt(lll) surface, a conclusion supported by comparison of the bismuth redox chemistry on the supported catalyst and the single-crystal surfaces [77]. 

C. Gutierrez, J.A. Caram, Electrooxidation of dissolved CO on a platinum electrode covered with a monolayer of the chemisorbed CO formerly considered to be poison. J. Electroanal. Chem. 1991, 308, 321-325. 

A major difficulty with contact potential work is the provision of a completely inert reference electrode. Poisoned nickel has been used, but aged gold or platinum is more widely applicable. Bewig and Zisman have described reference electrodes of gold and platinum coated with Teflon resin. These electrodes were more stable than the bare metals in wet and dry oxygen, nitrogen and in carbon dioxide, hydrogen and helium. 

Beck and Gerischer (34) used also the potentiometric method to study the kinetics of reduction of various simple-chain and cyclic olefins. Hydrogenation on a vibrating platinized platinum electrode was zero order in alkene and independent of pH in the region 2-8. In the presence of halide ions, specific catalyst poisoning caused decline of the reduction rate. 

The electrochemical oxidation of methanol has been extensively studied on pc platinum [33,34] and platinum single crystal surfaces [35,36] in acid media at room temperature. Methanol electrooxidation occurs either as a direct six-electron pathway to carbon dioxide or by several adsorption steps, some of them leading to poisoning species prior to the formation of carbon dioxide as the final product. The most convincing evidence of carbon monoxide as a catalytic poison arises from in situ IR fast Fourier spectroscopy. An understanding of methanol adsorption and oxidation processes on modified platinum electrodes can lead to a deeper insight into the relation between the surface structure and reactivity in electrocatalysis. It is well known that the main impediment in the operation of a methanol fuel cell is the fast depolarization of the anode in the presence of traces of adsorbed carbon monoxide. 

The oxidation reaction of glucose at 0.3 V versus RHE was carried out during long-term electrolysis. The activity of the platinum electrode decreased quickly because of poisoning adsorbed intermediates, a pulse potential set at 1.4 V was added to the time-program in order to reactivate in situ the platinum surface. 

This rather simple experiment demonstrates quite clearly the poisoning of the oxygen reduction process by copper. Incidentally, Cu(II) is a rather common impurity in distilled water and mineral acids, and this experiment demonstrates that underpotential deposition of a monolayer of copper from solutions containing as little as 1 ppm Cu can drastically affect the behavior of a platinum electrode. Adsorption of small amounts of other impurities (i.e., organic molecules) can also have an effect on solid-electrode behavior. Thus, electrochemical experiments often require making great efforts to establish and maintain solution purity. 

Low temperature carbon monoxide sensors based on the reversible carbon monoxide adsorptive poisoning of precious metal electrodes are also being developed by Los Alamos National Laboratory. The addition of metals such as ruthenium to the platinum electrode material greatly improves the hydrogen oxidation kinetics in the presence of CO. An amperometric sensor that senses the CO inhibition of the hydrogen oxidation can be fabricated from a platinum electrode, a proton conductor and a platinum ruthenium alloy electrode. While the... 

During the oxidation of formic acid and formaldehyde on platinum electrodes, an oscillatory behavior is frequently observed. " The surface poisoning species play a central role in the triggering of the oscillatory phenomena. Recent studies on formic acid and formaldehyde oxidation confirm this view. Inzelt and Kert sz reported that by the use of electrochemical quartz crystal microbalance technique (EQCM), the periodical accumulation and consumption of strongly bound species can be observed in the course of potential oscillation produced by the galvanostatic oxidation of formic acid. 

A detailed mechanism for the electro-oxidation of methanol on platinum electrodes in acidic media is shown in Scheme 2. The important features of the reaction mechanism include the formation of reactive intermediates, such as (CHO)a 5, which form on the electrode surface and are further oxidized to either (CO)ads, which leads to the poisoning species, or the adsorbed formyl species is oxidized to (COOH)ads or directly to COj. The mechanistic pathways described... 

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