The catalyst is treated with a gas containing a high but limited volume of O2. Ordinarily such a concentration of O2 would overheat the catalyst. However, the gas is put through at such a velocity that it carries off the generated heat without raising its own (gas) temperature excessively. The gas leaving the catalyst room is divided into 2 parts. Of these, the larger is cooled and recycled. Fresh gas is admixed periodically or continuously.