"Preference is given to catalysts which comprise, as catalytically active constituents, elements chosen from the group copper, silver, cobalt, nickel, ruthenium, rhodium, palladium, platinum, chromium and molybdenum, in particular chosen from the group copper, cobalt, nickel, in each case in metallic form (oxidation state 0) or in the form of compounds, such as oxides, which are reduced to give the" . . . . . .