Platinum surface reaction

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 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. 

Surface Reactions on Clean Platinum and Rhodium at Low and High Pressures... 

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 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. 

Fig. 6 Amount of butane evolved (C4/Sn)during the reaction between SnBu4 and Pt/Si02 at 50 °C and then at 400 °C, under 50 mbar of H2. SnBu4 introduced (Snintr) per platinum surface atom (Pts) were 0.35 (0.25) 0.60 (0.57) 0.63 (0.69) and 0.85 (0.73). The numbers in brackets are the quantity of Sn fixed per Ptj, as measured by elemental analysis of the final solid...

Jarvi TD, Stuve EM. 1998. Fundamental aspects of vacuum and electrocatal)Tic reactions of methanol and formic acid on platinum surface. In Lipkowski J, Ross PN, eds. Electrocatalalysis. New York Wiley-VCH. p 75-154. 

Chemisorption of oxygen at Pt(lll) has been studied in detail by Ertl s group25 and the STM evidence is for complex structural features present in the temperature range 54M60K (Figure 4.14). The limitations of the Langmuir model, frequently invoked for reactions at platinum surfaces, is obvious from... 

Adsorbed carbon monoxide on platinum formed at 455 mV in H2S04 presents a thermal desorption spectrum as shown in Fig. 2.4b. As in the case of CO adsorption from the gas phase, the desorption curve for m/e = 28 exhibits two peaks, one near 450 K for the weakly adsorbed CO and the other at 530 K for the strongly adsorbed CO species. The H2 signal remains at the ground level. A slight increase in C02 concentration compared to the blank is observed, which could be due to a surface reaction with ions of the electrolyte. Small amounts of S02 (m/e = 64) are also observed. 

For forced-convection studies, the cathodic reaction of copper deposition has been largely supplanted by the cathodic reduction of ferricyanide at a nickel or platinum surface. An alkaline-supported equimolar mixture of ferri- and ferrocyanide is normally used. If the anolyte and the catholyte in the electrochemical cell are not separated by a diaphragm, oxidation of ferrocyanide at the anode compensates for cathodic depletion of ferricyanide.3... 

Subsequent to the discovery of skeletal rearrangement reactions on plati-num/charcoal catalysts, the reality of platinum-only catalysis for reactions of this sort was reinforced with the observation of the isomerization of C4 and C5 aliphatic hydrocarbons over thick continuous evaporated platinum films (68,108, 24). As we have seen from the discussion of film structure in previous sections, films of this sort offer negligible access of gas to the substrate beneath. Furthermore, these reactions were often carried out under conditions where no glass, other than that covered by platinum film, was heated to reaction temperature that is, there was essentially no surface other than platinum available at reaction temperature. Studies have also been carried out (109, 110) using platinum/silica catalysts in which the silica is catalytically inert, and the reaction is undoubted confined to the platinum surface. 

Since the early work of Langmuir (1), the chemisorption of carbon monoxide on platinum surfaces has been the subject of numerous investigations. Besides its scientific interest, an understanding of CO chemisorption on Pt is of considerable practical importance for example, the catalytic reaction of CO over noble metals (such as Pt) is an essential part of automobile emission control. 

EELS has been used to study the kinetics of relatively slow surface reactions, such as the hydrogen-deuterium exchange in benzene adsorbed on platinum [54], In... 

The kinetics of CO oxidation from HClOi, solutions on the (100), (111) and (311) single crystal planes of platinum has been investigated. Electrochemical oxidation of CO involves a surface reaction between adsorbed CO molecules and a surface oxide of Pt. To determine the rate of this reaction the electrode was first covered by a monolayer of CO and subsequently exposed to anodic potentials at which Pt oxide is formed. Under these conditions the rate of CO oxidation is controlled by the rate of nucleation and growth of the oxide islands in the CO monolayer. By combination of the single and double potential step techniques the rates of the nucleation and the island growth have been determined independently. The results show that the rate of the two processes significantly depend on the crystallography of the Pt surfaces. 

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