Powder for catalysts and catalyst for exhaust gas purification

1. A catalyst powder comprising: a core portion that contains ceria and zirconia; and a surface layer portion that is located on the core portion and that contains ceria and zirconia,

wherein a ratio (M 2 /M 1 ) is 0.30 or more and 0.95 or less, the ratio (M 2 /M 1 ) being a ratio of a mole fraction M 2 (mol %) of cerium in the surface layer portion measured using X-ray photoelectron spectroscopy to a mole fraction M 1 (mol %) of cerium in the entire powder, and

wherein the core portion contains a rare earth element oxide excluding cerium and/or an alkaline earth metal oxide and the surface layer portion contains a rare earth element oxide excluding cerium and/or an alkaline earth metal oxide different from that or those of the core portion.

2. The catalyst powder according to claim 1 , comprising: a core particle that contains ceria and zirconia; and a supported metal oxide that is supported on a surface of the core particle and that contains ceria and zirconia,

wherein the surface layer portion constitutes at least a portion of the supported metal oxide.

3. The catalyst powder according to claim 1 ,

wherein M 4/2 (=M 4 /M 2 ) that is a ratio of M 4 and M 2 is greater than 1,

wherein M 4 (mol %) represents a mole fraction of zirconium in the surface layer portion measured using X-ray photoelectron spectroscopy, and M 2 (mol %) represents a mole fraction of cerium in the surface layer portion measured using X-ray photoelectron spectroscopy.

4. An exhaust gas purification catalyst comprising:

the catalyst powder according to claim 1 ; and

a noble metal that is supported on the catalyst powder.

5. The catalyst powder according to claim 1 , wherein the mole fraction M 2 (mol %) of cerium in the surface layer portion is 1.0 mol % or more and 30.0 mol % or less.

6. The catalyst powder according to claim 1 , wherein a ratio (M 4/2 /M 3/1 ) between M 3/1 and M 4/2 is 1.1 or more and 5.0 or less, wherein M 3/1 (=M 3 /M 1 ) represents a ratio between a mole fraction M 3 (mol %) of zirconium in the entirety of the powder and a mole fraction M 1 (mol %) of cerium in the entirety of the powder, and M 4/2 (=M 4 /M 2 ) represents a ratio between a mole fraction M 4 (mol %) of zirconium in the surface layer portions measured using X-ray photoelectron spectroscopy and a mole fraction M 2 (mol %) of cerium in the surface layer portions measured using X-ray photoelectron spectroscopy.

7. The catalyst powder according to claim 1 , wherein the catalyst powder contains 5 parts by mass or more and 20 parts by mass or less of a rare earth element oxide excluding ceria and/or an alkaline earth metal oxide, relative to 100 parts by mass of the entirety of the powder.

8. The catalyst powder according to claim 1 , wherein a ratio (M 5 /M 2 ) is 0.3 or more and 3.0 or less, the ratio (M 5 /M 2 ) being a ratio of a mole fraction M 5 of a rare earth element excluding cerium and/or an alkaline earth metal element in the surface layer portion measured using X-ray photoelectron spectroscopy to the mole fraction M 2 of cerium in the surface layer portions.