Catalyst, catalyst layer, membrane-electrode assembly, electrochemical device, and method for producing catalyst

A catalyst comprising:

• a mesoporous material;
• and
• catalytic metal particles supported at least within the mesoporous material and containing platinum and a metal different from platinum,
• wherein the mesoporous material has mesopores with a mode radius of greater than or equal to 1 nm and less than or equal to 25 nm and a pore volume of greater than or equal to 1.0 cm3/g and less than or equal to 3.0 cm3/g before supporting of the catalytic metal particles, and has an average particle size of greater than or equal to 200 nm,
• a molar ratio of the metal different from platinum and contained in the catalytic metal particles relative to all metals contained in the catalytic metal particles is greater than or equal to 0.25, and among the catalytic metal particles, a volume ratio of catalytic metal particles having a particle size of greater than or equal to 20 nm is less than or equal to 10%.