Horowitz, A., Su, F., and Calvert, J.G. (1978)
Unusual H2-forming Chain Reaction in the 3130-A Photolysis of Formaldehyde-Oxygen Mixtures at 25 C
Int. J. Chem. Kinet. 1978, 10, 1099.
Abstract
A kinetic study has been made of the 3130-A photolysis of CH2O (8 torr) in O2-containing mixtures (0.02-8 torr) and in the presence of added CO2 (0-300 torr) at 25 C. Quantum yields of formation of H2, CO, and CO2 and the loss of O2 were measured. F(H2) and F(CO) were much above unity. In an explanation of these unexpected results, a new H-atom-forming chain mechanism was postulated involving HO2 and OH addition to CH2O (reactions (A), (58), (33), (168), (B), (C), (D), (101), and (E)):
(A) CH2O + photon -> H + HCO
(B) HO2 + CH2O -> (HO2CH2O) -> OH + HCO2H
(C) OH + CH2O -> HCO* + H2O
(D) HCO* -> H + CO
(E) OH + CH2O -> H + HCO2H
(F) HCO + O2 + M -> HCOO2 + M
When the results are rationalized in terms of this mechanism, the data suggest k(C)>>k(101) and k(C)/k(E)=0.5. The data require that a reassesment of the relative rates of reactions (F) and (168) be made, since in the previous work HCO2H formation was used as a monitor of the rate of reaction (F). The present data from experiments at P(CH2O)=8 torr and P(O2)=1-4 torr give k(F)[M]/{k(F)[M]+k(168)}>0.049±0.017. These data coupled with the k(168) of Washida and coworkers (Washida et al. (1974)) give k(F)>(4.4±1.6)E+11 l^2/mol^2 s for M=CH2O. The reaction sequence proposed here is consistent with the observed detrimental effect of O2 addition on the laser-induced isotope enrichment in HDCO. In additional studies of CH2O-O2-isobutene mixtures it was found that F(H2) was equal to F(2) as estimated in O2-free CH2O-isobutene mixtures. These results suggest that the increase in CO(v=1) product observed with O2 addition in CH2O photolysis does not result from perturbations in the fragmentation pattern of the excited CH2O, but it is likely that it originates in the occurrence of the exothermic reaction HCO + O2 -> HO2 + CO(v=1).

REACTION RATE COEFFICIENTS SUGGESTED IN THIS WORK:

GRI-Mech
Number
('-' sign
means
reverse)		 Reaction Rate Coefficient
A T^n exp(-E/RT)		 Temperature
Range
(K)
A
(mol,cm3,s)		 n
(T in K)		 E
(cal/mol)
B HO2 + CH2O -> OH + HCO2H 7.3E+7     298
C OH + CH2O -> HCO* + H2O 2.92E+12     298
D* HCO* -> H + CO 7.0E+8     298
E OH + CH2O -> H + HCO2H 5.64E+12     298
F** HCO + O2 + M -> HCOO2 + M 4.2E+17     298

* dimensions = s-1
** M = CH2O, dimensions = cm^6/mol^2 s