Photooxidation of Formaldehyde at Low Partial Pressures of Aldehyde T. C. Purcell and I. R. Cohen National Center for Air Pollution Control, U. S. Department of Health, Education, and Welfare, 4676 Columbia Parkway, Cincinnati, Ohio 45226
Experimenta The oxidant produced by the photooxidation of formaldehyde at low partial pressures of aldehyde has been identified as hydrogen peroxide. The apparent molecular imbalance between the peroxide formed and the formaldehyde consumed is attributable. at least in part, to surface effects related to the reaction container. H
A
lthough the photooxidation of aldehyde has been investigated by several research groups in recent years, the bulk of the work has been carried out at relatively high concentrations of aldehyde. Recent investigation has shown that the mechanism for the photooxidation of propionaldehyde is somewhat different at low partial pressures of aldehyde (Altshuller, Cohen, et al., 1966). This type of research is significant in relation to atmospheric chemistry and suggests the need for more detailed investigations at low reactant concentrations. Carruthers and Norrish (1936) were the first to report on the photooxidation of formaldehyde, including as products formic acid, water, carbon monoxide, carbon dioxide, hydrogen, and polymer. Later, Horner and Style, working in the millimeter range of partial pressure with formaldehyde, found hydrogen, carbon monoxide, carbon dioxide, and formic acid, but no trace of performic acid or hydrogen peroxide (Horner and Style, 1954). Recently, Norrish and Thomas (1966) found hydrogen peroxide as a product of the thermal oxidation of gaseous formaldehyde in the millimeter range of partial pressure. With a matured quartz vessel, 70% conversion of formaldehyde to hydrogen peroxide was observed. The absence of performic acid as an intermediate in the reaction was confirmed by mass spectrometric analysis.
Formaldehyde, at concentrations ranging from 1 to 30 p.p.m. in air, was irradiated in reaction containers fabricated from fluorinated ethylene-propylene copolymer (FEP Teflon, Du Pont). The irradiations were carried out at 23 O + 1 C. in the same thermostated chamber used previously (Altshuller and Cohen, 1964). Mixed banks of sunlight fluorescent (wavelength maximum at 3100 A,) and blacklight fluorescent lamps (3600 A.) were ordinarily used. Formaldehyde-air mixtures in FEP containers also were subjected to solar radiation during the summer and fall months. Concentrations of formaldehyde were determined by a modification of the chromotropic acid method (Altshuller and Cohen, 1964). Hydrogen peroxide concentrations were determined by the 8-quinolinol (Cohen and Purcell, 1967) and catalyzed-KI methods (Cohen, Purcell, et ai., 1967). Results and Discussion Previously, the photooxidation of propionaldehyde ai low partial pressures had been found to give rise to a slow oxidant, which was identified by gas chromatography and kinetic colorimetry as ethyl hydroperoxide (Altshuller, Cohen, et al., 1966). In a similar way, photooxidation of low partial pressures of acetaldehyde produced methyl hydroperoxide (Purcell and Cohen, 1967). The oxidant produced from the photooxidation of low partial pressures of formaldehyde probably would be hydrogen peroxide. Colorimetry with neutral potassium iodide reagent indicates that no peroxy acid is present in the reaction vessel. This may mean that performic acid has only transient existence (Dever and Calvert, 1962), if indeed it is involved at all. On the other hand, the only identifiable oxidant produced in the photooxidation system is hydrogen peroxide. To the authors’ knowledge, this is the first time hydrogen peroxide has been Volume 1, Number 10, October 1967 845
reported as the oxidant produced by the photooxidation of formaldehyde. The primary process for the photolysis of formaldehyde at 3100 A. is reported (Calvert and Steacie, 1951) as follows. hu
HCHO + HCHO* + H
+ CHO
In the presence of high partial pressures of oxygen, the hydroperoxyl radical should be formed.
+ CHO + H
0 2
+
0 2 +
HOy
HO?
+ CO
The subsequent formation of hydrogen peroxide can be explained as follows.
+ HCHO HOn + HCHO HOa + HCO HOC + HOe
HOa
I 9' !I
,.
