Method of Regenerating Thermally Deteriorated Catalyst

A catalyst that is used for a method of reduction removal of NOx in a exhaust gas by using ammonia as a reducing agent and has been deteriorated in activity by aggregation of vanadium oxide as an active component through long term use at a high temperature is washed with an acid aqueous solution having pH of 6 or less, and preferably 4 or less. The washing operation dissolves and removes away mainly the vanadium oxide as the aggregated active component, and then vanadium oxide as the active component is re-deposited thereon. The method of the invention enables regeneration of a thermally deteriorated catalyst, which has conventionally been impossible. The washing operation with the acid aqueous solution or an alkali aqueous solution does not influence the mechanical strength of the catalyst.