HCHO , oxidation

Chain-bianching lequkements aie met by HOOH formed in reaction 25 and by HCHO oxidation (eqs. 30 and 31). 

Table 3.2 Catalytic activity supported Au and Pt catalysts for HCHO oxidation.

HCHO] (MX 10 ) Equiv. of Mn oxidized Equio. of HCHO oxidized Fi F, kuCHO ku i ... 

K. and Enyo, M. (1989) Surface species produced on Pt electrodes during HCHO oxidation in sulfuric add solution as studied by infrared reflection-absorption spectroscopy (IRRAS) and differential electrochemical mass spectroscopy (OEMS)./. Electroanal. Chem., 258, 219-225. 

Figure 13.6 Potential-step electro-oxidation of formaldehyde on a Pt/Vulcan thin-film electrode (7 p,gpt cm, geometric area 0.28 cm ) in 0.5 M H2SO4 solution containing 0.1 M HCHO upon stepping the potential from 0.16 to 0.6 V (electrolyte flow rate 5 pL at room temperature). (a) Solid line, faradaic current transients dashed line, partial current for HCHO oxidation to CO2 dotted line, difference between the net faradaic current and that for CO2 formation, (b) Solid line, m/z = 44 ion current transients gray line potential-step oxidation of pre-adsorbed CO derived upon HCHO adsorption at 0.16 V, in HCHO-free sulfuric acid solution, (c) Current efficiency transients for CO2 formation (dashed line) and formic acid formation (dotted line).

In separate experiments on HCHO oxidation over the same catalyst, CO was formed (but very little CO,). 

HOO + HCHO. One of the long-standing questions concerning the photo and thermal oxidation of HCHO in NO-free systems is the mechanism for the copious production of HC(0)0H [65,66], The results described above rule out the possibility of its formation as a primary product in the HO + HCHO reaction. Furthermore, the 02-reaction of the ensuing radical product HCO has been shown to yield CO and HOO rather than the adduct radical HC(0)00 which could, otherwise, be a likely candidate for a radical precursor of HC(0)0H. In contrast to the RC(0)00 radicals (R = alkyl group), the HC(0)00 radical is presumably thermochemically unstable in atmospheric pressures of air. Recently, the reaction of HOO with HCHO has been demonstrated to be sufficiently fast to contribute to various aspects of HCHO oxidation including the formation of HC(0)0H [67-70],... 

RHE) and coinciding with the peak (see Fig. 8) associated with the Ru(IV)/ Ru(VI) couple beginning at about 1.25 V (RHE) and the second coinciding with the peak usually associated with the Ru(VI)/Ru(VII) couple [209]. Formate was observed to be the product of HCHO oxidation in the first wave, while 4e oxidation of HCHO to carbonate was observed in the second wave. Despite its interesting catalytic features for the electro-oxidation of formaldehyde, Ru02 is not as active as the noble metal catalysts Pt and Pd for this reaction, however. 

Using the same approach and interpretation, values of — jq-ii.io o.44 jjj3 molecule s and Eub = 161.2 6.4 kJ mol were obtained [45] from studies of isobutene oxidation, as predicted by the similar thermochemistry and inert nature of methylallyl radicals due to electron delocalization. The agreement is good, and moreover the Arrhenius parameters are entirety consistent with Aif= 10 " cm molecule s and Elf = 163 kJ mot , which were obtained from studies of HCHO oxidation under conditions where the chain length was reduced virtually to zero. In the initial stages of reaction, the mechanism in KCl-coated vessels, where HO2 and H2O2 are efficiently destroyed at the vessel surface, is very simple. 

There was no HCHO oxidation on the Ru/Au(lll) electrode up to a potential of 0.25 V Starting from a potential of 0.25 V, the reaction occurred with increasing current up to a potential of 0.55 V, showing a peak with a maximum current density of 0.07 mA/cm2 at a potential of 0.43 V At higher potentials, as well as in the reverse sweep, the HCHO oxidation occurred following the same path as on the pure Au(l 11) surface, although with smaller currents, most likely due to the inhibiting effect of the present oxidized Ru species. 

The introduction of Zn(CH00)2 narrows the difference between the calculated value for 4> and the theoretical value of 1 for which the soluble Zn compounds have been totally accounted. However the introduction of CHOO" results in more anions than can be accounted for by the Zn concentration. A possible explanation is that HCOOH is a weak acid and that some of the acid remained undissociated. The presence of CHOO" in the dew is consistent with the findings of Knotkova, who observed that HCHO accelerated the corrosion of a number of metals, including galvanized steel, and that the corrosion products contained CHOO" (2 ). In addition to HCHO oxidation in solution, the possibility exists that CH3CHO coud be oxidized to acetic acid. However, no measurements were made during the experiments to evaluate this reaction. 

We see that CH4 oxidation results in a net gain of radicals when HCHO is photo-lyzed and turns equivalent OH into HO2 when HCHO oxidizes by OH. The gross budgets are ... 

Fig. 3.16 Illustration of the underlying mechanism of HCHO oxidation on a Pt Surface in aqueous electrolytes. Reprinted from [Kim and Kim (2007b)].

Fig. 3.17 shows a cyclic linear sweep voltammogram of the IPMC which confirms the HCHO oxidation. At the lower outer potential ranging below about 0.38 V, the dissociation of HCHO (i.e. dehydrogenation and CO adsorption) includes small anodic current. At about 0.4 V, the onset of... 

Poisoning effects during the electrooxidation of formaldehyde were mainly observed in acid solutions. Ad-atoms deposited at xmderpotentials enhance the electrocatalytic activity of the Pt electrode toward HCHO oxidation [81-83]. According to Motoo and Shibata [82], ad-atoms that do not adsorb oxygen (e.g. Pb, Tl, Bi) have only a small influence resulting... 

Machida K, Enyo M. In situ X-ray diffraction study of hydrogen entry into Pd and Pd-Au alloy electrodes during anodic HCHO oxidation. J Electrochem Soc... 

---