Study of the partial oxidation of methanol to formaldehyde on a polycrystalline Ag foil

A study of the partial oxidation reaction of methanol to formaldehyde has been carried out on the surface Of an oxygen permeable, Ag-membrane. Surface oxygen species are formed on the high vacuum side of the Ag-foil by diffusion of O atoms generated by dissociative adsorption on the high pressure side at temperatures > 500 K. Methanol pressures were kept sufficiently low (≤10^-5 Torr) to ensure collision free sampling of the products using electron-impact mass spectrometry. Starting with a new Ag-foil, reaction rates were found to increase over time, with maximum rates achieved after activation times of several hours. Activation is accompanied by significant changes in membrane morphology. For a methanol pressure of 1 × 10^-6 Torr, the maximum rate of formaldehyde formation is 7 × 10¹⁴ mol/s-cm² at 900 K and only decreases by a factor of 2 as the foil temperature is reduced to 300 K. Reaction at room temperature is attributed to the high diffusion rate of O atoms dissolved in the bulk which maintains the surface oxygen concentration sufficiently high to achieve reaction rates of 5 × 10¹⁴ mol/s-cm² for hours when the methanol pressure is held at 2 × 10^-6 Torr. Attempts to titrate all the membrane O atoms through reaction with methanol were unsuccessful, and suggest that the loading must be greater than 10¹⁹ O atoms/gram of Ag.