Generally, exhaust gases discharged from various combustion devices contain NO.sub.x in conjunction with sulfur oxides (hereinafter referred to as SO.sub.x) and carbon monoxide. The NO.sub.x is the principal cause for the so-called photochemical smog which has been posing a serious social problem in the recent years. For prevention of the air pollution, it is necessary either to decrease the NO.sub.x content in exhaust gases or to remove NO.sub.x from exhaust gases.
With a view to decreasing the NO.sub.x content in exhaust gases, measures have been taken to improve the burners of combustion devices and to adjustment of combustion conditions. Usually, these measures result in a degradation in the combustion efficiency and a decline in the combustion temperature. Thus, the measures cannot be called a general solution applicable to all the combustion devices. There has been proposed a method which, for the purpose of removing NO.sub.x from an exhaust gas, effects catalytic reduction of NO.sub.x in the exhaust gas with ammonia, in the presence of a catalyst formed by depositing a noble metal such as platinum on .gamma.-alumina (activated alumina) or in the presence of a catalyst of an oxide of metal such as copper or chromium supported on an activated alumina. When, in this method, the reduction is carried out by using the catalyst containing the deposited noble metal and when the exhaust gas happens to contain SO.sub.x, then the noble metal is poisoned by the SO.sub.x and consequently prevented from manifesting the catalytic activity effectively, with a disadvantageous result that the catalyst will fail to endure prolonged use. In addition, this catalyst has another economical disadvantage in that it is rather expensive. When, in this method, the reduction is performed by using the catalyst having the oxide of metal deposited on activated alumina, there is also involved a disadvantage that the catalytic activity of the catalyst, the optimum reaction temperature and other similar factors are notably affected by the composition of the exhaust gas; the composition in the sense of presence or absence of SO.sub.x, for example. Accordingly, where the NO.sub.x in the exhaust gas is to be removed by the catalytic reduction thereof with ammonia, the above-mentioned known catalysts are not expected to serve their purposes satisfactorily.