METHOD OF REGENERATING DETERIORATED CATALYST

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A method of regenerating a deteriorated denitrification catalyst by an easy and simple procedure. A catalyst which is for use in the reductional removal of NOx contained in a waste gas with ammonia as a reducing agent and has deteriorated in activity due to long-term use is washed with an aqueous acid solution containing vanadium and/or tungsten and having a pH of 4 or lower, preferably 2 or lower. This washing mainly dissolves away alkali metals, alkaline earth metals, arsenic, and sulfur, which are deteriorating ingredients, and the vanadium and tungsten which have deteriorated in activity. Simultaneously therewith, the vanadium and/or tungsten as an active ingredient is deposited. Examples of the alkali metals include potassium and sodium, while examples of the alkaline earth metals include calcium.