Platinum surfaces experiment

The catalysts with the simplest compositions are pure metals, and the metals that have the simplest and most uniform surface stmctures are single crystals. Researchers have done many experiments with metal single crystals in ultrahigh vacuum chambers so that unimpeded beams of particles and radiation can be used to probe them. These surface science experiments have led to fundamental understanding of the stmctures of simple adsorbed species, such as CO, H, and small hydrocarbons, and the mechanisms of their reactions (42) they indicate that catalytic activity is often sensitive to small changes in surface stmcture. For example, paraffin hydrogenolysis reactions take place rapidly on steps and kinks of platinum surfaces but only very slowly on flat planes however, hydrogenation of olefins takes place at approximately the same rate on each kind of surface site. 

Pulsed current experiments of aqueous acetate solutions indicate that at least in aqueous solution a platinum oxide layer seems to be prerequisite for the da arboxy-lation to occur. Only at longer pulse durations (> 10 s) is ethane produced [73,74]. These are times known to be necessary for the formation of an oxide film. At a shorter pulse length (<10"" s) acetate is completely oxidized to carbon dioxide and water possibly at a bare platinum surface [75]. The potent dynamic response in the electrolysis of potassium acetate in aqueous solution also points to an oxide layer, whose... 

Experiments by Freund and Spiro/ with the ferricyanide-iodide system showed that the additivity principle held within experimental error for both the catalytic rate and potential when the platinum disk had been anodically preconditioned, but not when it had been preconditioned cathodically. In the latter case the catalytic rate was ca 25% less than the value predicted from adding the current-potential curves of reactions (15) and (16). This difference in behavior was traced to the fact that iodide ions chemisorb only on reduced platinum surfaces. Small amounts of adsorbed iodide were found to decrease the currents of cathodic Fe(CN)6 voltam-mograms over a wide potential range. The presence of the iodine couple (16) therefore affected the electrochemical behavior of the hexacyanofer-rate (II, III) couple (15). 

In order to assess the role of the platinum surface structure and of CO surface mobility on the oxidation kinetics of adsorbed CO, we carried out chronoamperometry experiments on a series of stepped platinum electrodes of [n(l 11) x (110)] orientation [Lebedeva et al., 2002c]. If the (110) steps act as active sites for CO oxidation because they adsorb OH at a lower potential than the (111) terrace sites, one would expect that for sufficiently wide terraces and sufficiently slow CO diffusion, the chronoamperometric transient would display a CottreU-hke tailing for longer times owing to slow diffusion of CO from the terrace to the active step site. The mathematical treatment supporting this conclusion was given in Koper et al. [2002]. 

The potential of the stripping peak, and hence the activity of the electrode for CO oxidation, also depends on the platinum surface structure in general and on the step density in particular. Based on the chronoamperometry experiments described in Section 6.2.1.1, one would expect the stripping peak to shift to lower potential with increasing step density. That this is indeed the case is shown in Fig. 6.6. Again, this... 

Figure 3.35 shows the potential dependence of the integrated band intensity of the linear CO observed in the experiment described above and the corresponding variation in the methanol oxidation current. The latter was monitored as a function of potential after the chemisorption of methanol under identical conditions to those employed in the IRRAS experiments. As can be seen from the figure the oxidation of the C=Oads layer starts at c. 0.5 V and the platinum surface is free from the CO by c. 0.65 V. The methanol oxidation current shows a corresponding variation with potential, increasingly sharply as soon as the CO is removed strong evidence in support of the hypothesis that the adsorbed CO layer established at 0.4 V acts as a catalytic poison for the electro-oxidation of methanol. 

The simulations are further conducted under the experimental conditions of Inada et al. (1985). In their experiments, 4.0 mm water droplets impact on a heated platinum surface at a temperature up to 420 °C. The subcooling degree... 

A- Single Step Experiments. Potential step experiments were performed in order to determine the reaction mechanism and the reaction rate. As described above, the platinum surface was initially covered by a monolayer of CO at a controlled potential, Ef = 0.40 V (referred to as the initial potential) and then CO was removed from the bulk of the solution. Next, the electrode potential was suddenly changed to a more positive value, Ef, (referred to as the final potential) where the adsorbed CO was oxidized and the rate of oxidation was followed by recording the resulting current transients. 

Potential step experiments were conducted to study the electrocatalytic activities of a dean platinum surface. The appUed potential step sequence included 250-600 mV for the measurement after three pretreatment steps, i. e. 1550 mV for 5 s, 1050 mV for 20 s and 30 mV for 2 s. The last step for the pretreatment, 30 mV for 2 s, was added to the steps described earlier for other measmements in order to reduce Pt-OH,... 

For the Pt/cinchona catalysts only preliminary adsorption studies have been reported [30]. From the fact that in situ modification is possible and that under preparative conditions a constant optical yield is observed we conclude that in this case there is a dynamic equilibrium between cinchona molecules in solution and adsorbed modifier. This is supported by an interesting experiment by Margitfalvi [63] When cinchonine is added to the reaction solution of ethyl pyruvate and a catalyst pre-modified with cinchonidine, the enantiomeric excess changes within a few minutes from (R)- to (S)-methyl lactate, suggesting that the cinchonidine has been replaced on the platinum surface by the excess cinchonine. 

A comment needs to be made about the state of the platinum surface during these experiments. The solutions that were used contained 0.05 mol dm 3 HC104 to curtail iron(III) hydrolysis and most of the work was carried out at 5°C to decrease the contribution of the homogeneous reaction. Under these conditions the surface of the platinum at the catalytic potential was always in the reduced state, irrespective of its preconditioning treatment. As mentioned in Sect. 4.4, such a surface specifically adsorbs iodide ions [239, 264, 265]. In order to allow for their effect, the current-potential curves for the Fe(III)/Fe(II) couple had been determined in the presence of a small concentration (5 x 10 6 mol dm 3) of potassium iodide. It is interesting that this treatment was sufficient to maintain the validity of the additivity hypothesis, in contrast to the results obtained for the Fe(CN)g" +1 system on cathodically pretreated platinum surfaces (Fig. 25). 

Our studies of olefin and acetylene chemisorption states on platinum surfaces is presently incomplete. Ethylene and acetylene chemisorption on platinum (111) are complicated by the apparent presence of more than one chemisorption state (indicated by thermal desorption studies). When C2HH and C2D1, are chemisorbed on Pt(lll), the small fraction of ethylene thermally desorbed as ethylene comprises nearly a statistical mixture of all possible molecules. Thus we see here reversible C-H (and C-D) bond breaking on this flat platinum surface. In an analogous experiment with C2H2 and C2D2> only a small extent of H-D exchange was observed for the small fraction of acetylene molecules that reversibly desorb from this surface (11). 

The electrocatalytic oxidation of ethanol has been investigated for many years on different platinum-based electrodes, including Pt/X alloys (with X = Ru, Sn, Mo, etc ), and dispersed nanocatalysts. Pme platinum smooth electrodes are rapidly poisoned by some strongly adsorbed intermediates, such as carbon monoxide, resulting from the dissociative chemisorption of the molecule, as shown by the first experiments in infrared reflectance spectroscopy (EMIRS). Both kinds of adsorbed CO, either linearly-bonded or bridge-bonded to the platinum surface, are observed. Besides, oth-... 

A truly satisfactory explanation of these observations, which can be confirmed by independent experiments, has yet to be proposed. Tentatively it is argued that strongly adsorbed intermediates are formed during the oxidation of organic molecules. These intermediates block the surface and decrease its catalytic activity. Formation of a partial layer of lead atoms hinders the adsorption of such intermediates and thus allows the oxidation reaction to occur more readily on the remaining free platinum surface (or perhaps on parts of the surface covered by a monolayer of lead, which may retain much of the catalytic properties of the platinum substrate). This interpretation is attractive in that it is consistent with our general understanding of the electrocatalytic activity of metals (cf. Fig. 7F) but will have to be confirmed by other experiments before it can be accepted. 

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