Surface effects on the Formation of Hydrogen Peroxide from dissociated water vapour

The formation of hydrogen peroxide at 77°K from water vapour dissociated in a low-pressure electrodeless discharge was investigated as function of the surface-to-volume ratio. S/V, of both coated and uncoated tubular reactors interposed between the discharge exit and the coolant level. As S/V was increased, the peroxide yield increased to a maximum value and then decreased; this maximum shifted to much lower values of S/V for KCl-coated and H₃PO₄-coated surfaces. Compared with uncoated Pyrex surfaces, H₃PO₄-coatings gave higher H₂0₂ yields whereas KCl-coatings gave lower yields. Surfaces coated well with ice did not show much effect, but condensed water films decreased the H₂O₂ yield. Similar results were obtained when the surface area and the volume traversed by the dissociated vapour were varied at constant S/V. These surface effects are discussed to show that the formation of H₂O₂ occurs via the cold surface reactions H + O₂ → O_aH and H + 0₂H → H₂O₂ and not by the dimerization of OH radicals.