engleski

Anodic oxidation of formic acid

In this paper an anodic oxidation of formic acid on rhodinized platinum electrodes, with and without pyridine addition is investigated. During anodic oxidation of HCOOH, in the potential range from 600 to 1000 mv(SHE), periodic oscillations occurred. As the speed of polarization is an essential for the oscillations occurrence ; the speed at the rate of 0, 3 mVs l was used. The oscillations disappear at higher speeds of polarization. We can therefore propose the following formulation for the anodic behaviour of formic acid on Pt (Rh) anodes: Rh° → Rh2+ + 2e- (1) (HCOOH)2 → 2 HCOOH (2) Formic acid exists as dimer, and during the contact with Rh, a monomerization occurs. 2 HCOOH → 2 HCOO- + 2H+ (3) 2 H+ goes to cathode and gives H2. Rh2+ + 2HCOO- → Rh(OOCH)2 (4) where Rh(OOCH)2 represents anodic layer: Further: Rh(OOCH)2 → Rh° + H2 + 2 CO2 (5) According to literature data it is possible to synthesise a stable complex of Rh(OOCH)2L2, where as a ligand is either pyridine or NH3. By pyridine addition into formic acid a decomposition of Rh(OOCH)2 into CO and H2, is prevented, although it caused a periodic oscillations. Our study gives an explanation for the mechanism of the HCOOH anodic behaviour that is possible without the assumption of problematic radical use, i.e. without very unstable Rh+. The interpretation of the mechanism of anodic process through Rh oxidation is also unlikely to occur, as the oxide formation occurs only at the very positive potential values.

anodic oxidation; formic acid; ocilations

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