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 b een 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 diffus ion of O atoms generated by dissociative adsorption on the high pressure si de at temperatures > 500 K. Methanol pressures were kept sufficiently low ( less than or equal to 10(-5) Torr) to ensure collision free sampling of the products using electron-impact mass spectrometry. Starting with a new Ag-f oil, reaction rates were found to increase over time, with maximum rates ac hieved after activation times of several hours. Activation is accompanied b y significant changes in membrane morphology. For a methanol pressure of 1 x 10(-6) Torr, the maximum rate of formaldehyde formation is 7 x 10(14) mol /s-cm(2) at 900 K and only decreases by a factor of 2 as the foil temperatu re is reduced to 300 K. Reaction at room temperature is attributed to the h igh diffusion rate of O atoms dissolved in the bulk which maintains the sur face oxygen concentration sufficiently high to achieve reaction rates of 5 x 10(14) mol/s-cm2 for hours when the methanol pressure is held at 2 x 10-6 Torr. Attempts to titrate all the membrane O atoms through reaction with m ethanol were unsuccessful, and suggest that the loading must be greater tha n 10(19) O atoms/gram of Ag.