Catalyst Composition

A catalyst composition comprising an Rh-containing perovskite-type composite oxide represented by the following general formula (I) and/or a Pt-containing perovskite-type composite oxide represented by the following general formula (II), and a thermostable oxide optionally containing a noble metal,

• A1xA2wB11−(y+z)B2yRhzO3±δ (I)
• wherein A1 represents at least one element selected from alkaline earth metals;
• A2 represents at least one element selected from rare earth elements;
• B1 represents at least one element selected from Ti, Zr, Hf, and tetravalent rare earth elements;
• B2 represents at least one element selected from Al and transition elements (excluding tetravalent rare earth elements, Ti, Zr, Hf, and Rh);
• x and w represent atomic ratios satisfying the following conditions:
• 0.8≦(x+w)≦1.3 (0.6≦x≦1.3, and 0≦w≦0.4);
• y represents an atomic ratio satisfying the following condition:
• 0≦y<0.5;
• z represents an atomic ratio satisfying the following condition:
• 0<z≦0.5;
• δ represents an oxygen excess or an oxygen deficiency,
• A3rA4sB31−(t+u)B4tPtuO3±δ′ (II)
• wherein A3 represents at least one element selected from alkaline earth metals;
• A4 represents at least one element selected from rare earth elements;
• B3 represents at least one element selected from Ti, Zr, Hf, and tetravalent rare earth elements;
• B4 represents at least one element selected from Al and transition elements (excluding tetravalent rare earth elements, Ti, Zr, Hf, and Pt);
• r and s represent atomic ratios satisfying the following conditions:
• 0.8≦(r+s)≦1.3 (0.6≦r≦1.3, and 0≦s≦0.4);
• t represents an atomic ratio satisfying the following condition:
• 0≦t<0.5;
• u represents an atomic ratio satisfying the following condition:
• 0<u≦0.5;
• and δ′ represents an oxygen excess or an oxygen deficiency.