Ethylene oxidation on platinum

Table A.l. Electrochemical promotion studies of ethylene oxidation on platinum (ref. 43 in Chapter 11) ...

PtOx) in the Isothermal Rate Oscillations of Ethylene Oxidation on Platinum. J. Catal. 67, 348-361... 

Thus the binding of oxygen and hydrocarbons on the platinum surface is different from that for silver. Due to the great strength of oxygen-hydrocarbon bonds, the formation of ethylene oxide on platinum seems to be scarcely probable. It was found, moreover, that the propene oxidation reaction passed into the gas phase even at 70°. 

Valentinuzzi, M.E., Kohen, A.J., 2013. The mathemization of biology and medicine. IEEE Pulse, 50-56. Vayenas, C.G., Georgakis, C., Michaels, J., Tormo, J., 1981. The role of PtOx in the isothermal rate oscillations of ethylene oxidation on platinum. J. Catal. 67, 348-361. 

Compensation effects have been reported for the oxidation of ethylene on Pd-Ru and on Pd-Ag alloys (207, 254, 255) discussion of the activity patterns for these catalysts includes consideration of the influence of hydrogen dissolved in the metal on the occupancy of energy bands. Arrhenius parameters reported (208) for ethylene oxidation on Pd-Au alloys were an appreciable distance from the line calculated for oxidation reactions on palladium and platinum metals (Table III, H). Oxidation of carbon monoxide on Pd-Au alloys also exhibits a compensation effect (256). 

Systems (1) enter into class 3 (a PDE point is a PCB). Systems with linear reaction mechanisms belong to both class (2) and class (3) but these classes do not overlap since there are systems without intermediate interactions that do not satisfy the principle of complex balance (e.g. the Eley-Rideal mechanism for CO oxidation on platinum metal). On the other hand, there exist reaction mechanisms containing steps of "intermediate interactions but at the same time always having a PCB (e.g. the Twigg mechanism for ethylene hydrogenation on nickel). 

The glow electrolysis technique (electrolysis with an anode immersed in the solution and the cathode above the surface) at 600-800 V dc and 300-500 mA converts a solution of starch into ethylene, methane, hydrogen, and both carbon mono- and dioxides.323 Electrochemical methods for converting polysaccharides and other biomass-derived materials have been reviewed briefly by Baizer.324 These methods are mainly oxidations along a potential gradient, which decreases the activation energy of the reactants. Starch in 5 M NaOH solution is oxidized on platinum electrodes to carboxylic acids with an activation energy of about 10 kcal/mol. In acidic media oxidation takes place at C-l followed by decarboxylation and oxidation at the C-2 and C-6 atoms.325... 

A simple Langmuir-Hinshelwood model explains quantitatively the steady-state behavior (4) but it fails to explain the oscillatory phenomena that were observed. The origin of the limit cycles is not clear. Rate oscillations have not been reported previously for silver catalyzed oxidations. Oxidation of ethylene, propylene and ethylene oxide on the same silver surface and under the same temperature, space velocity and air-fuel ratio conditions did not give rise to oscillations. It thus appears that the oscillations are related specifically to the nature of chemisorbed propylene oxide. This is also supported by the lack of any correlation between the limits of oscillatory behavior and the surface oxygen activity as opposed to the isothermal oscillations of the platinum catalyzed ethylene oxidation where the SEP measurements showed that periodic phenomena occur only between specific values of the surface oxygen activity (6,9). 

Apart from poisoning by adsorbing impurities, the working electrode potential can also contribute to suppress electrocatalytic activity. Platinum metals, for instance, passivate or form surface oxygen and oxide layers above 1 V (Section IV,D), which inhibit Oj reduction (779,257,252) and oxidation of carbonaceous reactants (7, 78, 253, 254) however, decomposition of hydrogen peroxide on platinum is accelerated by oxygen layers (255). Some electrocatalysts may corrode or dissolve, especially in acidic electrolytes, while reactants may contribute to dissolution. Thus, ethylene oxidation on palladium to acetaldehyde proceeds via a Pd-ethylene complex, which releases colloidal palladium in solution (28, 29). Equivalent to this is the surface roughening and the loss of Pt in gas phase ammonia oxidation (256, 257). 

Matsuoka K, Iriyama Y, Abe T, Matsuoka M, Ogumi Z (2005) Electro-oxidation of methanol and ethylene glycol on platinum in alkaline solution poisoning effects and product analysis. Electrochim Acta 51(6) 1085-1090... 

Matsuoka, K., Iriyama, Y., Abe, T., Matsuoka, M. Ogumi, Z. (2005). Electro-oxidation of Methanol and Ethylene Glycol on Platinum in Alkaltne Solution Poisoning Effects and Product Analysis. Electrochimica Acta, Vol.51, N0.6, (November 2005), pp. 1085-1090, ISSN 00134686... 

Purely parallel reactions are e.g. competitive reactions which are frequently carried out purposefully, with the aim of estimating relative reactivities of reactants these will be discussed elsewhere (Section IV.E). Several kinetic studies have been made of noncompetitive parallel reactions. The examples may be parallel formation of benzene and methylcyclo-pentane by simultaneous dehydrogenation and isomerization of cyclohexane on rhenium-paladium or on platinum catalysts on suitable supports (88, 89), parallel formation of mesityl oxide, acetone, and phorone from diacetone alcohol on an acidic ion exchanger (41), disproportionation of amines on alumina, accompanied by olefin-forming elimination (20), dehydrogenation of butane coupled with hydrogenation of ethylene or propylene on a chromia-alumina catalyst (24), or parallel formation of ethyl-, methylethyl-, and vinylethylbenzene from diethylbenzene on faujasite (89a). 

Figure 12.8. Transient effect of an applied potential, UAP, between the two terminal gold electrodes (30 V) on the catalytic rate of ethylene oxidation (expressed in mol O/s) for dotted (filled circles) and multi-striped (open circles) platinum configuration.10 Reprinted with permission from Elsevier Science.

The shift in the C=C frequency, vi, for adsorbed ethylene relative to that in the gas phase is 23 cm-1. This is much greater than the 2 cm-1 shift that is observed on liquefaction (42) but is less than that found for complexes of silver salts (44) (about 40 cm-1) or platinum complexes (48) (105 cm-1). Often there is a correlation of the enthalpy of formation of complexes of ethylene to this frequency shift (44, 45). If we use the curve showing this correlation for heat of adsorption of ethylene on various molecular sieves (45), we find that a shift of 23 cm-1 should correspond to a heat of adsorption of 13.8 kcal. This value is in excellent agreement with the value of 14 kcal obtained for isosteric heats at low coverage. Thus, this comparison reinforces the conclusion that ethylene adsorbed on zinc oxide is best characterized as an olefin w-bonded to the surface, i.e., a surface w-complex. 

Fig. 7.109. Experimental current density-potential relationship for the oxidation of ethylene on platinum and an 80% Pt-20% Ru alloy.

The use of equation (3.2) to study the behaviour of catalysts is known as solid electrolyte potentiometry (SEP). Wagner38 was the first to put forward the idea of using SEP to study catalysts under working conditions. Vayenas and Saltsburg were the first to apply the technique to the fundamental study of a catalytic reaction for the case of the oxidation of sulfur dioxide.39 Since then the technique has been widely used, with particular success in the study of periodic and oscillatory phenomena for such reactions as the oxidation of carbon monoxide on platinum, hydrogen on nickel, ethylene on platinum and propylene oxide on silver. 

There have been used essentially only three catalysts foi the hydrogenation of ethylene oxides nickel, palladium on charcoal, and platinum black. Solvent normally employed include ethanol wait nickel, and ethanol, ethyl acetate, or acetic acid with the other. Reduction over platinum or palladium catalysts is usually conducted at room temperature and low pressure, whereas nickel catalysth Imvi-been employed in autoclaves at temperatures ranging from 3fT to nearly 200° and high pressures. For excellent general discussions ol catalytic redaction any of several outstanding sources14" 11-ltni m.i> be consulted. 

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