Abstract: A process for preparing formaldehyde by oxidative dehydrogenation of methanol comprises passing a gas mixture (i) comprisinga) from 0.1 to 50% by volume of methanol,b) from 0.1 to 30% by volume of oxygen,c) from 0 to 50% by volume of nitrogen oxide andd) from 0 to 60% by volume of water at from 150 to 800.degree. C. through a phosphorus-doped silver catalyst fixed bed (a) and applying from 0.01 to 100 ppm by weight of phosphorus, based on the phosphorus-doped silver catalyst fixed bed, in the form of a finely divided phosphorus compound having a melting point or decomposition temperature of more than 500.degree. C. (phosphorus compound P) to the phosphorus-doped silver catalyst fixed bed (a) per kg of methanol which is passed through in the form of the gas mixture (i) per cm.sup.2 of the cross-sectional area of the phosphorus-doped silver catalyst fixed bed (a).

Abstract: A phosphorus-doped silver catalyst fixed bed is obtainable byI. arranging silver crystals obtained by electrolytic deposition of silver from an aqueous silver salt solution to form a starting silver catalyst fixed bed,II. producing an activated silver catalyst fixed bed from the starting silver catalyst fixed bed by passing through the latter, at from 150 to 800.degree. C., a gas mixture comprising methanol and oxygen (gas mixture M), andIII. bringing the activated silver catalyst fixed bed into contact with from 1 to 20,000 ppm by weight of phosphorus, based on the silver, in the form of a finely divided phosphorus compound having a melting point or decomposition temperature of more than 500.degree. C. (phosphorus compound P).

Abstract: A process for preparing phosphorus-doped silver catalysts from silver contaminated with phosphorus compounds, which comprisesa) heating the silver which is contaminated with phosphorus compounds to temperatures at which the silver is liquid and subsequently cooling to temperatures below the melting point,b) preparing silver crystals from the silver obtained in stage a) by anodically oxidizing it to silver ions in an electrolysis cell with an aqueous electrolyte, and cathodically reducing the silver ions again to elemental silver, andc) contacting the silver crystals with a finely divided phosphorus compound with a melting point or decomposition temperature above 500.degree. C. (phosphorus compound P).

Abstract: Formaldehyde is prepared by oxidative dehydrogenation of methanol in the gas phase on a silver or silver-containing catalyst by means of an oxygen-containing gas which contains dinitrogen oxide.