Platinum surface impurities

The platinum electrode is also very convenient for investigating various adsorption phenomena in electrochemical systems. The surface of platinum is very stable and reproducible. As will be shown in what follows, the true working area can be determined with high accuracy for platinum surfaces with appreciable roughness and even for electrodes with highly dispersed platinum deposits. It is comparatively easy to clean the surface of adsorbed impurities and to control the state of the surface. 

There can be no doubt that even with a noble metal such as platinum, the surface can be heavily contaminated with carbon when the latter is used as a supporting material (51). This may be ameliorated by cautious treatment with oxygen which oxidizes this carbon impurity to carbon dioxide. Nevertheless, it is extremely doubtful if any platinum surface in platinum/carbon can be prepared without an appreciable, and perhaps substantial, amount of impurity. 

Platinum surfaces Mith (111) and (100) orientations treated in this May have been checked by using LEED characterization on as received" samples, both shoued the characteristic LEED pattern Hith their respective (lxl) surface symmetry. The non observation of the (5x20) symmetry for the (100) orientation Mas due to the presence of residual adsorbed impurities at the surface of as received samples. Simply this confirms the crystalline surface quality of the platinum samples prepared according to this technique (10). 

Nbdifying the platinum surface with Nafion enhanced the sustained ciurent. This effects appeared to stem from the protection effect from impurities. Supplying methanol fit>m the gas phase did not show much differoice in the methanol oxidation characteristics from supplying it 6rom the electrolyte phase. 

Modifying the platinum surface with Nafion enhanced the sustained current. These effects appeared to stem from the protection effect from impurities. 

All these photocorrosion processes are, of course, undesirable and it is obvious that their relative importance depends strongly on the presence of surface states which may facilitate recombination or redox reactions with adsorbed substrates. It is well known from ESR [69, 70, 94] and emission spectra [94] that most of these metal sulfide powders contain surface states. They are introduced during preparation of the powder as a result of lattice defects [72, 96], trapped holes [94], surface impurities [97] and metallization [38], and during the actual catalytic reaction as a consequence of irradiation and substrate adsorption. The stabilizing effect of plati-nization is exemplified by Figure 6 for the ZnS-catalyzed reduction of water in the presence of sodium formate [98]. Note that platinum does not accelerate the reaction but doubles the time of constant catalytic activity from 1 to 2 days. Similarly, the apparent product quantum yield of the 2,5-DHF dehydrodimerization is not increased but slightly decreases when platinizes ZnS is the photocatalyst [97]. 

Penetration of the acid through the recast film all the way to the platinum surface was supported by voltammetric evidence and was apparently assisted by the potential multicycling routine employed to maintain an impurity-free platinum surface [1]. Such a study of a filmed platinum electrode immersed in aqueous acid solution is, therefore, less than perfect for providing good data on the rate of ORR at the interface between platinum and hydrated ionomer in a fuel cell cathode, where the only interfacial liquid is distilled water. 

The region of the cyclic voltammogram, corresponding to anodic removal of Hathermal desorption spectra of platinum catalysts. However, unlikely the thermal desorption spectra, the cyclic-voltammetric profiles for H chemisorbed on Pt are usually free of kinetic effects. In addition, the electrochemical techniques offer the possibility of cleaning eventual impurities from the platinum surface through a combined anodic oxidation-cathodic reduction pretreatment. Comparative gas-phase and electrochemical measurements, performed for dispersed platinum catalysts, have previously demonstrated similar hydrogen and carbon monoxide chemisorption stoichiometries at both the liquid and gas-phase interfaces (14). 

The O2/H2O system is very slow so that the exchange current a I equilibrium is extremely low (10 /10 A cm 2) as a consequence, any other reaction at the electrode will hamper its study and that could be the reaction of impurities or other redox reactions involving the electrode itself. The so-called noble metals are not really inert and do interact with oxygen a platinum surface in contact with an O saturated solution adsorbs oxygeti as an electronically conducting monolayer but can be further oxidized to PIO, PtO . A detailed analysis of these phenomena, which falls outside the scope of the present review, can be found elsewhere [311. A platinum electrode, when a complete electronically conducting monolayer of I l—O is formed at the surface of the metal, behaves as an ideally inert electrode in such conditions, rest potentials dependent on pO2 and pH can be measured during a few hours, close to... 

Once again, Pt will be used as a sample system to illustrate the effect of one adspecies on the desorption of another. Single crystals of Pt often contain large surface impurities of S, P, Ca, C, Cl and O and these adatoms are often very difficult to remove. The influence of S at the surface on the desorption of CO from Pt is shown in Fig. 40 from the work of Bonzel and Ku [325]. Clearly, S substantially affects the desorption, decreasing both the desorption energy of the adsorbate and the amount adsorbed. Oxygen adsorbs on platinum to form an oxide and McCabe and... 

Silver and gold, which have relatively high vapor pressures, can be easily applied by vacuum evaporation, while metals of lower vapor pressure, such as platinum, palladium, and stainless steel, can be deposited by radio frequency sputtering in this case, however, the film thicknesses are limited to about 100 nm. Before electrode deposition, the samples must be cleaned and heat treated. Failure to ranove surface impurities will result in loss of adhesion when samples are subsequently heated during the measurement. 

Carbon supports typically undergo chemical or physical activation prior to platinum impregnation. The alteration of surface groups and functionalities on the carbon support can strongly influence the carbon-metal interaction that can directly affect the metal particle size, metal particle distribution, surface morphology of the carbon, and surface impurities that may be present. These parameters have been known to influence the catalytic metal stability and activity of the resulting catalyst. Common surface modifications strategies include chemical oxidation of the carhon or thermal activation to modify the surface structures. 

It should also be noted that the platinum surface is very sensitive to the presence of species in solution and to electrode pre-treatments (anodization, pre-reduction). Damjanovic etal reported a very strong dependence of the reaction pathway on the purity of the solution. They concluded that the oxygen reduction reaction occurred without hydrogen peroxide intermediate formation on a pre-reduced platinum electrode, and therefore that the production of hydrogen peroxide was effective only on sites affected by the presence of adsorbed impurities. 

Second, materials that may affect the ORR on platinum, organic impurities that come from the environment, MEA components, or fuels, are discussed. Alcohols or aldehydes are dissolved in H SO solutions, and the platinum RDE is investigated electrochemically under the ORR. The blockage of the platinum catalyst surface by these impurity molecules turns out to be a major cause of the degradation. 

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