Platinum surface

Fig. VIII-2. Scanning tunneling microscopy images illustrating the capabilities of the technique (a) a 10-nm-square scan of a silicon(lll) crystal showing defects and terraces from Ref. 21 (b) the surface of an Ag-Au alloy electrode being electrochemically roughened at 0.2 V and 2 and 42 min after reaching 0.70 V (from Ref. 22) (c) an island of CO molecules on a platinum surface formed by sliding the molecules along the surface with the STM tip (from Ref. 41).

Ultraviolet photoelectron spectroscopy (UPS) results have provided detailed infomiation about CO adsorption on many surfaces. Figure A3.10.24 shows UPS results for CO adsorption on Pd(l 10) [58] that are representative of molecular CO adsorption on platinum surfaces. The difference result in (c) between the clean surface and the CO-covered surface shows a strong negative feature just below the Femii level ( p), and two positive features at 8 and 11 eV below E. The negative feature is due to suppression of emission from the metal d states as a result of an anti-resonance phenomenon. The positive features can be attributed to the 4a molecular orbital of CO and the overlap of tire 5a and 1 k molecular orbitals. The observation of features due to CO molecular orbitals clearly indicates that CO molecularly adsorbs. The overlap of the 5a and 1 ti levels is caused by a stabilization of the 5 a molecular orbital as a consequence of fomiing the surface-CO chemisorption bond. 

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. 

The kinetics of the oxidation of CO on a platinum surface indicate that CO and oxygen are adsorbed to about the same extent. The rate of oxidation depends on the oxygen partial pressure when CO is in excess, and on tire CO partial pressure when oxygen is in excess. 

The most commonly used catalysts are palladized charcoal or calcium carbonate and platinum oxide. For better isotopic purity, the use of platinum oxide may be preferred for certain olefins since the substrate undergoes fewer side reactions while being chemisorbed on the platinum surface as compared to palladium.Suitable solvents are cyclohexane, ethyl acetate, tetrahydrofuran, dioxane or acetic acid-OD with platinum oxide. 

The second aromatization reaction is the dehydrocyclization of paraffins to aromatics. For example, if n-hexane represents this reaction, the first step would be to dehydrogenate the hexane molecule over the platinum surface, giving 1-hexene (2- or 3-hexenes are also possible isomers, but cyclization to a cyclohexane ring may occur through a different mechanism). Cyclohexane then dehydrogenates to benzene. 

The formed methylcyclohexane carbocation eliminates a proton, yielding 3-methylcyclohexene. 3-Methylcyclohexene can either dehydrogenate over the platinum surface or form a new carbocation by losing H over the acid catalyst surface. This step is fast, because an allylic car-bonium ion is formed. Losing a proton on a Lewis base site produces methyl cyclohexadiene. This sequence of carbocation formation, followed by loss of a proton, continues till the final formation of toluene. 

Platinum is unaffected by most organic compounds, although some compounds may catalytically decompose or become oxidised on a platinum surface at elevated temperatures, resulting in an etched appearance of the metal. Carbon and sulphur do not attack platinum at any temperature up to its melting point. Molten platinum may dissolve carbon, but the solubility of the latter in solid solution is virtually zero. 

If no depolariser is added to an acidic chloride solution, corrosion of the anode occurs and the dissolved platinum is deposited on the cathode, leading to erroneous results and to destruction of the anode. A number of metals (for example, zinc and bismuth) should not be deposited on a platinum surface. 

These metals, particularly zinc, appear to react with the platinum in some way, for when they are dissolved off with nitric acid the platinum surface is dulled or blackened. Injury to the platinum can be prevented in these cases by first plating it with copper, and then depositing the metal on this. 

Constant current procedure. With the exception of lead, which from nitric acid solutions is deposited on the anode as Pb02, the ions listed in Table 12.1 are deposited as metal on the cathode. With the ions indicated by an asterisk in Table 12.1, it is advisable to use a platinum cathode which has been plated with copper before the initial weighing this is because, in these cases, the deposited metals cannot be readily distinguished on a platinum surface and it is difficult to be certain when deposition is complete. 

The platinum surface is intensively covered with adsorbed CO, especially at low temperatures and high concentration of CO where the Hinshelwood mechanism may dominate... 

The concentration dependence of CO oxidation over Pt at (02) (CO) l differs from the concentration dependence of CO oxidation over copper chromite at (02)°-2(C0). This can be explained by the fact that after the departure of a C02 molecule, the reoxidation of platinum surfaces is slow but the reoxidation of base metal oxide surfaces is fast. On the other hand,... 

FIGURE 2-9 Repetitive cyclic voltanunograms illustrating the continuous growth of polyaniline on a platinum surface. 

M. Ayyoob, and M.S. Hegde, An XPS study of the adsorption of oxygen on silver and platinum surfaces covered with potassium or cesium, Surf. Sci. 133, 516-532 (1983). 

M. Peuckert, and H.P. Bonzel, Characterization of oxidized platinum surfaces by X-ray photoelectron spectroscopy, Surf. Sci. 145, 239-259 (1984). 

K. Franaszczuk, E. Herrero, P. Zelenay, A. Wieckowski, J. Wang, and R.I. Masel, A comparison of electrochemical and gas-phase decomposition of methanol on platinum surfaces, J. Phys. Chem. 96(21), 8509-8516 (1992). 

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... 

The adsorbed O atom and the adsorbed CO molecule then react on the surface to form CO2, which, being very stable and relatively unreactive, interacts only weakly with the platinum surface and desorbs almost instantaneously ... 

Oscillations may also occur on catalyst surfaces. As an example we briefly discuss the impressive work on oxidation of GO on platinum surfaces by Ertl and coworkers... 

Figure 7.7. Temperature-programmed desorption measurements corresponding to zero-, first-, and second-order kinetics of silver from ruthenium, CO from a stepped platinum surface, and N2from rhodium, respectively (data adapted from [J.W. Niemantsverdriet,...

Platinum serves as the catalyst for the oxidation of CO and hydrocarbons. It is relatively insensitive to contamination by lead or sulfur. At high temperatures it is not known to dissolve in the washcoat, but sintering into larger particles may lead to a substantial loss of platinum surface area with dramatic consequences for the overall oxidation activity. 

The synthesis of water from H2 and O2 on a platinum surface is assumed to proceed via the following elementary steps ... 

It is interesting to note that a dimer of Ce(IV) has also been invoked to account for observations on this exchange system at a platinum surface. The rate of exchange between the fully aquated ions of Ce(IV) and Ce(III) was concluded to be relatively slow. ... 

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). 

Iodide adsorbed on reduced platinum surfaces was found to affect several other systems. The most dramatic effect was shown when the couples U/r and O2/H2O were considered together. Addition of the current-potential curves of these two couples indicated that platinum should significantly catalyze the reaction... 

The additivity principle was well obeyed on adding the voltammograms of the two redox couples involved even though the initially reduced platinum surface had become covered by a small number of underpotential-deposited mercury monolayers. With an initially anodized platinum disk the catalytic rates were much smaller, although the decrease was less if the Hg(I) solution had been added to the reaction vessel before the Ce(lV) solution. The reason was partial reduction by Hg(l) of the ox-ide/hydroxide layer, so partly converting the surface to the reduced state on which catalysis was greater. 

The different species formed by steps (18) to (20) or (18 ) to (20 ) have been detected by in situ infrared reflectance spectroscopy, and such dissociative steps are now widely accepted even if the exact nature of the species formed during (20) or (20 ) is still a subject of discussion. Several groups proposed the species (COH)3js as the main, strongly adsorbed species on the platinum surface, even though no absorption infrared band can be definitely attributed to (COH),, . However, the formyl-like species ( CHO), , . has been formally identified, since it gives an IR absorption band ataroimd 1690cm . ... 

The crucial aspect is thus to determine the fate of the ( CHO), species. Possible mechanisms for its oxidative removal are schematically shown in Fig. 9. From this scheme, it appears that the desorption of the formyl species can follow different pathways through competitive reactions. This schematic illustrates the main problems and challenges in improving the kinetics of the electrooxidation of methanol. On a pure platinum surface, step (21) is spontaneously favored, since the formation of adsorbed CO is a fast process, even at low potentials. Thus, the coverage... 

Platinum is the only acceptable electrocatalyst for most of the primary intermediate steps in the electrooxidation of methanol. It allows the dissociation of the methanol molecule hy breaking the C-H bonds during the adsorption steps. However, as seen earlier, this dissociation leads spontaneously to the formation of CO, which is due to its strong adsorption on Pt this species is a catalyst poison for the subsequent steps in the overall reaction of electrooxidation of CHjOH. The adsorption properties of the platinum surface must be modified to improve the kinetics of the overall reaction and hence to remove the poisoning species. Two different consequences can be envisaged from this modification prevention of the formation of the strongly adsorbed species, or increasing the kinetics of its oxidation. Such a modification will have an effect on the kinetics of steps (23) and (24) instead of step (21) in the first case and of step (26) in the second case. 

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