Abstract: Carbonyl sulfide and/or hydrogen cyanide contained in a mixed gas are/is converted by contacting the mixed gas with an alkalized chromium oxide-aluminum oxide catalyst in the presence of steam, wherein the mixed gas and the steam at a volume ratio of 0.05≦steam/mixed gas≦0.3 are contacted with the alkalized chromium oxide-aluminum oxide catalyst at a gas hourly space velocity no less than 2000 h−1 at a temperature in the range of 150° C. through 250° C. In this case, the alkalized chromium oxide-aluminum oxide catalyst is set to have a grain size in the range of 1 mm through 4.5 mm. With this arrangement, since the surface area of a catalyst can be increased to a certain degree, the activity of the catalyst is increased to achieve the high processing speed, while since generation of a side reaction can be suppressed, lowering of the conversion rate of COS and/or HCN caused by the side reaction can be suppressed.

Abstract: The present invention is directed to a harmful gas removing agent used for removing harmful gas such as CO, NO.sub.x, and O.sub.3 from exhaust gases ventilated from automobile tunnels and underground parking, thereby preventing air pollution. The harmful gas removing agent, solving the problems of the conventional adsorbents and catalysts, is capable of efficiently removing NO.sub.x with no need of pre-treatment of adding O.sub.3 to the exhaust gas being treated, as well as being capable of satisfactorily removing CO and O.sub.3 from exhaust gases even at high humidity, and its removing activity lasts for a long time. Furthermore, the harmful gas removing agent can be regenerated by being exposed to hot air at relatively low temperature, thereby being repeatedly used, and is hard to be poisoned by SO.sub.x contained in the exhaust gas.