engleski

Anodic oxidation of formic acid

We continued the investigations of E.Müller1 and we are able to confirm that the experimental results obtained by this author are in agreement with ours, although E.Müller could not give a satisfactory explanation for anodic behavior of formic acid on Pt/Rh and Rh/Rh anodes. Using a lot better equipment, we obtained polarization curves with typical oscillations shown in Fig. la. The speed of polarization is essential for the formation of such kind of oscillations ; we used the speed at the rate of 0, 3 mVs l. The oscillations disappear at higher speeds of polarization. Fig. lb shows the case when the speed of polarization was 100 mVs-l. Obviously, a solid layer probably consisting a Rh-formate was formed at approximately 650 mV (NHP). This layer is not stable, and at the break down point, CO2 and H2 are formed in equimolar relation as stated by E.Müller who analized the gases formed in the anodic and cathodic electrolytic cell area. If the speed of polarization is greater than the break down speed, the periodic reaction should not be observed. Rhodium metal, Rh°, gives ions of different valencies. The most stable ion is Rh3+. But it was shown that the Rh2+ ion is thermodinamically stable2 and that a Rh(OOCH)2 can exist. Furthermore, it is possible to stabilize the formate with ligands (pyridine, or NH3). It is seen that the stabilizing action of pyridine inhibits the formation of the periodic Rh-formate break down by forming the stable Rh(OOCH)2 Py2-complex. The break down of Rh(OOCH)2 could be in the best way explained by using the Lewis notation. It is observed that after a longer time of polarization the rhodinized rhodium anode becomes brittle, and indicates the diffusion of hydrogen in metal. From 19655 to 1982 a lot of work has been done by investigating the anodic behaviour of HCOOH. But the methods and procedures vary in number of ways and therefore a comparison is extremely difficult. Our study gives an explanation for the mechanism of the HCOOH anodic behaviour that is possible without the assumption of problematic radicals.

anodic oxidation; formic acid; oscilations; rhodium

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