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| - According to the invention preferred promoters are the elements lithium, potassium, sodium, rubidium, cesium, thallium, molybdenum, zinc, hafnium, zirconium, titanium, chromium, manganese, nickel, copper, iron, boron, silicon, tin, niobium, cobalt and bismuth under which in addition to iron, in particular niobium, molybdenum, zinc and bismuth are preferred.
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