Catalyst based on oxides of Fe, Co, Bi and Mo

A catalyst of the formula I

• (A.sub.a B.sub.b O.sub.x).sub.p (C.sub.c D.sub.d Fe.sub.e Co.sub.f E.sub.i F.sub.j O.sub.y).sub.q I,
• where
• A is bismuth, tellurium, antimony, tin and/or copper,
• B is molybdenum and/or tungsten,
• C is an alkali metal, thallium and/or samarium,
• D is an alkaline earth metal, nickel, copper, cobalt, manganese, zinc, tin, cerium, chromium, cadmium, molybdenum, bismuth and/or mercury,
• E is phosphorus, arsenic, boron and/or antimony,
• F is a rare-earth metal, vanadium and/or uranium,
• a is from 0.01 to 8,
• b is from 0.1 to 30,
• c is from 0 to 4,
• d is from 0 to 20,
• e is from 0 to 20,
• f is from 0 to 20,
• i is from 0 to 6,
• j is from 0 to 15,
• x and y are numbers determined by the valency and frequency of the elements other than oxygen in I, and p and q are numbers whose ratio p/q is in the range from 0.001 to 0.099,
• obtained by the following steps
• (a) preparation of a key phase having the composition A.sub.a B.sub.b O.sub.x,
• (b) preparation of a guest phase having the composition C.sub.c D.sub.d Fe.sub.e Co.sub.f E.sub.i F.sub.j O.sub.y or water-soluble salts of elements of the guest phase or a mixture of at least one water-soluble salt of the elements of the guest phase and at least one calcined phase of one of the elements of the guest phase which is not in the form of a water-soluble salt, and
• (c) mixing of the key phase with the guest phase, and
• (d) optionally drying the mixture obtained in (c), optionally drying the mixture obtained in (c), optionally followed by calcination and optionally followed by shaping of the catalyst composition
• the mean particle size of at least one of a guest phase or a key phase composition being in the range from 10 nm to less than 1.mu.m.