The invention provides a method of reducing the amount of nitrogen oxide components in a process gas stream comprising: a) contacting a deNOX catalyst with the process gas in the presence of ammonia which results in the conversion of nitrogen oxide components as well as a decline in the NOX conversion over the deNOX catalyst; and b) regenerating the deNOX catalyst to improve the NOX conversion by contacting the deNOX catalyst at a temperature in the range of from 250 to 390° C. with a flow of ammonia that is reduced relative to the flow of ammonia in step a) and process gas, air or a mixture thereof.