withdrawn and subjected to activity testing under the above described conditions as follows with the progress of the experimental reaction a tellurium component and a phosphorus component were added to the reaction system in the order to te component p component te component according to the schedule indicated in table 5 below the changes in reaction results are shown in table 5 below also the p component used in this example was a phosphorus enriched catalyst prepared by impregnating phosphoric acid into the catalyst used here drying and calcining the te component used in this example was a tellurium enriched catalyst prepared by impregnating the catalyst used here with aqueous hydrogen peroxide having dissolved therein metallic tellurium with the aid of ammonium paramolybdate drying and calcining table 5__________________________________________________________________________ time elapsed from amount of te addition of yield or p added te or p total c sub 3 h sub 3 n catalyst atomic ratio reaction time c sub 3 h sub 3 n hcn hcnunder test per sb 100 hr hr __________________________________________________________________________fresh 76 6 5 2 81 8deteriorated 1 8 75 5 5 3 81 0 i after first 0 6 5 2 7 0 76 5 5 1 81 6addition of te 120 127 76.