Unusual H₂‐forming chain reaction in the 3130‐Å photolysis of formaldehyde‐oxygen mixtures at 25°C

Abstract

Abstract A kinetic study has been made of the 3130‐Å photolysis of CH 2 O (8 torr) in O 2 ‐containing mixtures (0.02–8 torr) and in the presence of added CO 2 (0–300 torr) at 25°C. Quantum yields of formation of H 2 , CO, and CO 2 and the loss of O 2 were measured. Φ and Φ CO were much above unity. In an explanation of these unexpected results, a new H‐atom‐forming chain mechanism was postulated involving HO 2 and HO addition to CH 2 O: CH 2 O + h ν → H + HCO (1) H + CH 2 O → H 2 + HCO (3) H + O 2 + M → HO 2 + M (6) HCO + O 2 → HO 2 + CO (8) HO 2 + CH 2 O → (HO 2 CH 2 O) → HO + HCO 2 H (15) HO + CH 2 O → H 2 O + HCO† (16); HCO† → H + CO (19) HO + CH 2 O → H 2 O + HCO (17) and HO + CH 2 O → HCO 2 H + H (18). When the results are rationalized in terms of this mechanism, the data suggest k 16 ≫ k 17 and k 16 / k 18 ≅ 0.5. The data require that a reassessment of the relative rates of reactions (7) and (8) be made, since in the previous work HCO 2 H formation was used as a monitor of the rate of reaction (7) HCO + O 2 + M → HCOO 2 + M (7). The present data from experiments at P = 8 torr and P = 1–4 torr give k 7 [M]/( k 7 [M] + k 8 ) ≥ 0.049 ± 0.017. These data coupled with the k 8 estimates of Washida and coworkers give k 7 ≥ (4.4 ± 1.6) × 10 11 l 2 /mol 2 ·sec for M = CH 2 O. The reaction sequence proposed here is consistent with the observed deterimental effect of O 2 addition on the laser‐induced isotope enrichment in HDCO. In additional studies of CH 2 O‐O 2 ‐isobutene mixtures it was found that Φ was equal to ϕ 2 as estimated in O 2 ‐free CH 2 O‐isobutene mixtures. These results suggest that the increase in CO (ν = 1) product observed with O 2 addition in CH 2 O photolysis does not result from perturbations in the fragmentation pattern of the excited CH 2 O, but it is likely that it originates in the occurrence of the exothermic reaction HCO + O 2 → HO 2 + CO (ν = 1).