Formaldehyde oscillating oxidation

Ethane. Ethane VPO occurs at lower temperatures than methane oxidation but requires higher temperatures than the higher hydrocarbons (121). This is a transition case with mixed characteristics. Low temperature VPO, cool flames, oscillations, and a NTC region do occur. At low temperatures and pressures, the main products are formaldehyde, acetaldehyde (HCHOiCH CHO ca 5) (121—123), and carbon monoxide. These products arise mainly through ethylperoxy and ethoxy radicals (see eqs. 2 and 12—16 and Fig. 1). 

Mishina F, Karantonis A, Yu Q-K, Nakabayashi S. 2002. Optical second harmoitic generation during the electrocatalytic oxidation of formaldehyde on Pt(lll) Potentiostatic regime versus galvanostatic potential oscillations. J Phys Chem B 106 10199-10204. 

As already mentioned, also for the other oxygenated Cl compounds, i.e. formaldehyde [118, 138-147] and methanol [148-154], as well as for larger organic molecules, dynamic instabilities are reported. Many of them are compiled in Ref. [154], for formaldehyde oxidation on Rh and Pt [147] and methanol oxidation on Pt [155] the oscillations could be clearly identified as HN-NDR type oscillations. However, in view of the number of reaction steps involved in these oxidation reactions and of the possible complexity of the interaction of the supporting electrolyte with the dynamics even in the much simpler formic acid oxidation, it is not astonishing that any quantitative considerations should still be missing. There are some attempts to qualitatively explain the observed phenomena with reaction mechanisms that go beyond the simple dual-path model described above. However, at the time being, they are quite speculative. Therefore I shall not discuss them in more detail in this article. A summary of these works can be found in [156],... 

During the oxidation of formic acid and formaldehyde on platinum electrodes, an oscillatory behavior is frequently observed. " The surface poisoning species play a central role in the triggering of the oscillatory phenomena. Recent studies on formic acid and formaldehyde oxidation confirm this view. Inzelt and Kert sz reported that by the use of electrochemical quartz crystal microbalance technique (EQCM), the periodical accumulation and consumption of strongly bound species can be observed in the course of potential oscillation produced by the galvanostatic oxidation of formic acid. 

The phenomenon of oscillating reactions is widely recognized in chemistry. In those cases in which a surface-boxmd species or the surface itself is involved in the complex sequence of steps, the EQCM offers the prospect of additional information. An example of this is the role of an oxide layer in the oxidation of formaldehyde at Pt and Rh [174], Similarly, in the oxidation of 2-propanol at Pt electrodes, it was found that the oscillations (in potential and mass, at constant ourent) increased in amphtude until the positive extreme of the potential excursion reached a value consistent with PtOH and/or PtO formation [175]. Oscillations in mass at open-circuit potential were also observed during the dissolution of Cu in sulfate media when the solution concentration of Cu " " was sufficiently high (c > 0.045 mol dm ) [176], although in this case the potential excursions were such that oxide formation/dissolution was ruled out. 

In the present oscillation phenomena the CO2 volume fraction oscillates with substantial amplitudes, whereas the formaldehyde shows smaller or no oscillations. Therefore, the overall oscillations are mainly due to a variation of the total oxidation activity (cf. Equation 3 and 4) whereas the changes of the partial oxidation activity are of minor importance which is due to the choice of a lean reaction mixture. 

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