An ammonia reduction-denitration process for exhaust gases containing NO.sub.x consisting mainly of NO.sub.2, and an apparatus therefor are provided, which process comprises treating the exhaust gases with ammonia using a catalyst which is active to both a reaction of (NO.sub.2 +NH.sub.3 .fwdarw.N.sub.2 +N.sub.2 O+H.sub.2 O) and a reaction of (N.sub.2 O+NH.sub.3 .fwdarw.N.sub.2 +H.sub.2 O) at an area velocity of 3 m/h or less and in a concentration ratio of NH.sub.3 /NO.sub.2 of 1.3 or more. According to this process, it is possible to reduce NO.sub.2 contained in the exhaust gases with ammonia at one stage without by-producing N.sub.2 O.

A supported silver-containing catalyst for the decomposition of dinitrogen monoxide as pure gas or in admixture with other gases and having an aluminum oxide support which has a specific surface area of from 26 to 350 m.sup.2 /g, and a process for the selective catalytic decomposition of dinitrogen monoxide as pure gas or in admixture with other gases at elevated temperature, in which the said supported catalyst is used.

Impure gas streams containing contaminating amounts of NO.sub.x, e.g., automotive exhaust fumes and industrial waste gases, are purified by contacting same, in the presence of ammonia, at an elevated temperature, with a catalyst composition which comprises an inorganic oxide support substrate having a catalytically effective amount of a metal oxide active phase deposited thereon, such support substrate comprising at least one alumina, aluminate, titanium dioxide and/or zirconium dioxide and such catalytically active phase comprising at least one vanadium oxide and/or molybdenum oxide and/or tungsten oxide, the surface of the support substrate being chemically bonded to the metals V and/or Mo and/or W and the catalyst being devoid of V.sub.2 O.sub.3 and/or MoO.sub.3 and/or WO.sub.3 crystalline phases, and thereby selectively reducing such NO.sub.x values while minimizing the formation of N.sub.2 O.

Impure gas streams containing contaminating amounts of NO.sub.x, e.g., automotive exhaust fumes and industrial waste gases, are purified by contacting same, in the presence of ammonia, at an elevated temperature, with a catalyst composition which comprises an inorganic oxide support substrate having a catalytically effective amount of a metal oxide active phase deposited thereon, such support substrate comprising at least one alumina, aluminate, titanium dioxide and/or zirconium dioxide and such catalytically active phase comprising at least one vanadium oxide and/or molybdenum oxide and/or tungsten oxide, the surface of the support substrate being chemically bonded to the metals V and/or Mo and/or W and the catalyst being devoid of V.sub.2 O.sub.3 and/or MoO.sub.3 and/or WO.sub.3 crystalline phases, and thereby selectively reducing such NO.sub.x values while minimizing the formation of N.sub.2 O.

Disclosed are a catalyst for selective catalytic reduction of nitrogen oxides and a method for preparing the same. Useful for the removal of nitrogen oxides is a catalyst prepared using spent catalysts having been absorbed with vanadium, nickel and sulfur in the hydro-desulfurization line of an oil refinery in which a catalyst for the hydro-desulfurization contains molybdenum, iron, cobalt and silicon on the alumina support in accordance with the present invention. The present catalyst can remove nitrogen oxides at a level of 90% or higher, exhibiting a 10% or more increase in efficiency of the catalyst performance. Additionally, the catalyst can increase the efficiency of spent catalyst reclamation by 250%.