Polycrystalline platinum surfaces

There is a wealth of information available on CO chemisorption over single-crystal and polycrystalline platinum surfaces under ultrahigh-vacuum conditions research efforts in this area have gained a significant momentum with the advent of various surface analysis techniques (e.g., 2-8). In contrast, CO chemisorption on supported platinum catalysts (e.g., 9, 10, 11) is less well understood, due primarily to the inapplicability of most surface-sensitive techniques and to the difficulties involved in characterizing supported metal surfaces. In particular, the effects of transport resistances on the rates of adsorption and desorption over supported catalysts have rarely been studied. 

In a typical study of a polycrystalline platinum surface in contact with a solution of 0.5 M H2SO4, an electrode potential dependent intensity as shown in Fig. 5.132 was recorded [770]. 

Extensive characterization of these electrocatalysts for the fuel cell reactions has shown that the electrocatalytic activity of these nanoparticles is by a factor 10 lower than the activity of a polycrystalline platinum surface for the respective reactions, HOR or ORR [16], This difference is not fully understood yet. 

An interesting proprietary approach has been followed recently by the 3M Company, creating a continuous electrode area covered by nanoscale Pt-whiskers. Reactivity of these electrocatalytic layers is in the range of polycrystalline platinum surfaces [17]. 

Kraenert, R. (2005) Ammonia Oxidation over polycrystalline platinum surface morphology... 

The triioidode (l3 )/iodide (I) redox couple in l-butyl-3-methylimidazolium tetrafluorobo-rate ([BMImKBFJ) RTIL, the redox shuttle typically employed in dye-sensitized solar cells (DSSCs)," has also been investigated at polycrystalline platinum surfaces. RTILs have generally... 

Chronoamperometric transients for CO stripping on polycrystalline platinum were measured by McCallum and Fletcher [1977], Love and Lipkowski [1988] were the hrst to present chronoamperometric data for CO stripping on single-crystalline platinum. However, they interpreted their data on the basis of a different model than the one discussed above. Love and Lipkowski considered that the oxidation of the CO adlayer starts at holes or defects in the CO adlayer, where OH adsorbs. These holes act as nucleation centers for the oxidation reaction, and the holes grow as the CO at the perimeter of these holes is oxidized away by OHads- This nucleation and growth (N G) mechanism is fundamentally different from the mean held model presented above, because it does not presume any kind of mixing of CO and OH [Koper et ah, 1998]. Basically, it assumes complete surface immobility of the chemisorbed CO. 

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. 

It is interesting to note that at this high temperature the frequency of the Pt-CO band did not shift appreciably as the CO coverage varied, in contrast to observations made on a (111)-oriented platinum ribbon (6) or on a polycrystalline Pt surface (34) at or below room temperature. 

Adsorption of acetic acid on Pt(lll) surface was studied the surface concentration data were correlated with voltammetric profiles of the Pt(lll) electrode in perchloric acid electrolyte containing 0.5 mM of CHoCOOH. It is concluded that acetic acid adsorption is associative and occurs without a significant charge transfer across the interface. Instead, the recorded currents are due to adsorption/desorption processes of hydrogen, processes which are much better resolved on Pt(lll) than on polycrystalline platinum. A classification of adsorption processes on catalytic electrodes and atmospheric methods of preparation of single crystal electrodes are discussed. 

The polycrystalline platinum electrode was mounted in Kel>F resin and polished with a scries of alumina powders down to 0.05, resulting mirror finish. The apparent surface area was 1.85 cm. The electrode was washed with fuming sulfuric add and rinsed with ultra pure water prior to each measurement. 

At the second sweep, the oxidation current increased again above 1.05 V. This increase is probably due to the active siurface Pt-OH. At the reverse sweep, the current increase was seen at the potential range where the Pt-OH is reduced as usually seen on polycrystalline platinum. It is because Pt(lll) was rearranged by the Pt-OH formation and reduction and has become a more polycrystalline-like surface. 

The structure of a polycrystalline electrode depends on its preparation. Usually toe rough electrodes are prepared by electrochemical deposition of a given metal onto a suitable substrate. Microcrystals present in polycrystaUine samples are randomly oriented on the surface. Most likely, not only basal but also higher MiUer-index planes should be considered in anticipating toe final structure of the electrode surface. It was shown that the stmcture of the platinized platinum surface depends strongly on toe platinization conditions, e.g., on toe concentration of the platinization... 

We then designed model studies by adsorbing cinchonidine from CCU solution onto a polycrystalline platinum disk, and then rinsing the platinum surface with a solvent. The fate of the adsorbed cinchonidine was monitored by reflection-absorption infrared spectroscopy (RAIRS) that probes the adsorbed cinchonidine on the surface. By trying 54 different solvents, we are able to identify two broad trends (Figure 17) [66]. For the first trend, the cinchonidine initially adsorbed at the CCR-Pt interface is not easily removed by the second solvent such as cyclohexane, n-pentane, n-hexane, carbon tetrachloride, carbon disulfide, toluene, benzene, ethyl ether, chlorobenzene, and formamide. For the second trend, the initially established adsorption-desorption equilibrium at the CCR-Pt interface is obviously perturbed by flushing the system with another solvent such as dichloromethane, ethyl acetate, methanol, ethanol, and acetic acid. These trends can already explain the above-mentioned observations made by catalysis researchers, in the sense that the perturbation of initially established adsorption-desorption equilibrium is related to the nature of the solvent. 

It was found that changes in the voltam-metric response of polycrystalline platinum electrodes in the direction expected for preferred oriented surface electrodes can be achieved using a fast repetitive potential perturbation. 

Fig. 6.94. Comparison of adsorption properties of different electrode surfaces. Bisulfate adsorption as a function of electrode potential on different platinum planes (110), (111), (100) and on polycrystalline platinum. Data obtained by the radiotracer technique. (Re-printed from Y.-E. Sung, A. Thomas, M. Gamboa-Aldeco, K. Franaszczuk and A. Wieckowski, J. Electroanal. Chem. 378 131, copyright 1994, Figs. 14 and 15, with permission from Elsevier Science.)...

What does Eq. (6.246) mean This equation represents the adsorption process of ions on metallic surfaces. It includes several conditions that are characteristic of the adsorption process of ionic species, namely, surface heterogeneity, solvent displacement, charge transfer, lateral interactions, and ion size. However, is this equation capable of describing the adsorption process of ions In other words, what is the success of the isotherm described in Eq. (6.246) Figure 6.104 shows a comparison of data obtained experimentally for the adsorption of two ions—chloride and bisulfate—on polycrystalline platinum, with that obtained applying Eq. (6.246). The plots indicate that the theory is able to reproduce the experimental results quite satisfactorily. The isotherm may be considered a success in the theory of ionic adsorption. 

The catalytic oxidation of CO on iridium has not been as extensively studied as with palladium and platinum. However, as for these metals, both steady-state (40, 54, 124, 199-202) and nonsteady-state investigation (124, 200-203) have been carried out on both polycrystalline and single crystal surfaces. As the results are for the most part very similar to those obtained on palladium and platinum surfaces, only those results that shed additional light on the kinetics and mechanism basic to the reaction will be emphasized here. 

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