Selective removal of oxides of nitrogen from gas mixtures containing oxygen

Oxides of nitrogen are selectively removed from gas mixtures containing oxygen by reduction with at least the stoichiometric amount of ammonia in the presence of a catalyst comprising one or more of the oxides of vanadium, molybdenum or tungsten. Preferred oxides are V2O5, MoO3 and WO3. The gas mixture i passed over the catalyst at space velocities of from 3000 to 80,000, preferably from 10,000 to 30,000, parts by volume of gas mixture per hour per part by volume of catalyst, the temperature of the gas mixture prior to reduction and upon admission to the catalyst being between 150 DEG and 400 DEG C., preferably between 200 DEG and 350 DEG C. The catalyst may be a supported catalyst which contains from 2 to 50% by weight of the oxide or oxides and in which the carrier is aluminium oxide, for example alpha-alumina, and/or silicic acid. The catalyst may be prepared by impregnating a moulded carrier one or more times with a solution of a salt containing one or more of the metals, for example, of vanadium oxysulphate, of V2O5 in oxalic acid, or of ammonium molybdate or tungstate, drying and heating in a current of air for several hours at a temperature of from 450 DEG to 600 DEG C. to form the corresponding metal oxide or oxides; the latter may be converted to a lower oxidation state by prolonged heating in a current of hydrogen at a temperature of at least 400 DEG C.