Process for preparing an oxidizing catalyst for reducing pollutants present in an exhaust gas essentially consisting of a catalytically effective amount of Mn 2 O 3, wherein the process comprises: a) preparing by mixing an aqueous solution comprising an amino acid and/or urea and/or ammonium nitrate, and a manganese salt; b) heat-treating the aqueous solution thus obtained in a pre-heated oven at a temperature comprised between 500° and 650°C for a time period comprised between 10 minutes and 1 hour so as to obtain a catalyst having: i) a surface area equal to or greater than 10 m 2 /g in a condition of post-synthesis before its first use, and ii) a surface area after ageing smaller than said surface area of the catalyst in the conditions of post-synthesis before its first use and equal to or greater than 2 m 2 /g, said ageing being carried out by: - placing an amount equal to 2 g of catalyst in said condition of post-synthesis before its first use, in a static air oven; - heating said amount of catalyst at a heating rate of 10°C/min up to an ageing temperature of 800°C; - maintaining said amount of catalyst at the ageing temperature for a time period of 12 hours; and - removing the sample from the oven at the end of the maintaining step. a) preparing by mixing an aqueous solution comprising an amino acid and/or urea and/or ammonium nitrate, and a manganese salt; b) heat-treating the aqueous solution thus obtained in a pre-heated oven at a temperature comprised between 500° and 650°C for a time period comprised between 10 minutes and 1 hour so as to obtain a catalyst having: i) a surface area equal to or greater than 10 m 2 /g in a condition of post-synthesis before its first use, and ii) a surface area after ageing smaller than said surface area of the catalyst in the conditions of post-synthesis before its first use and equal to or greater than 2 m 2 /g, said ageing being carried out by: - placing an amount equal to 2 g of catalyst in said condition of post-synthesis before its first use, in a static air oven; - heating said amount of catalyst at a heating rate of 10°C/min up to an ageing temperature of 800°C; - maintaining said amount of catalyst at the ageing temperature for a time period of 12 hours; and - removing the sample from the oven at the end of the maintaining step. i) a surface area equal to or greater than 10 m 2 /g in a condition of post-synthesis before its first use, and ii) a surface area after ageing smaller than said surface area of the catalyst in the conditions of post-synthesis before its first use and equal to or greater than 2 m 2 /g, said ageing being carried out by: - placing an amount equal to 2 g of catalyst in said condition of post-synthesis before its first use, in a static air oven; - heating said amount of catalyst at a heating rate of 10°C/min up to an ageing temperature of 800°C; - maintaining said amount of catalyst at the ageing temperature for a time period of 12 hours; and - removing the sample from the oven at the end of the maintaining step. - placing an amount equal to 2 g of catalyst in said condition of post-synthesis before its first use, in a static air oven; - heating said amount of catalyst at a heating rate of 10°C/min up to an ageing temperature of 800°C; - maintaining said amount of catalyst at the ageing temperature for a time period of 12 hours; and - removing the sample from the oven at the end of the maintaining step.