Disclosed are: a catalyst which is capable of reducing and converting
nitrate ions into
ammonia without adding a
hydrogen gas thereto; a method for synthesizing
ammonia using the catalyst, wherein
nitrate ions are reduced without adding a
hydrogen gas thereto; and a method for decreasing
nitrogen oxide in water by reducing
nitrate ions contained in the water. Disclosed is a photocatalyst composition that is obtained by loading a photocatalyst, which is capable of photoreducing protons into a
hydrogen gas, with CuPd
alloy nanoparticles. The photocatalyst composition is used for the purpose of producing
ammonia by photoreducing
nitrogen that is coexistent with water and
nitrogen oxide. The CuPd
alloy nanoparticles are (1) CuPd
alloy nanoparticles having a B2 type
crystal structure and represented by CuxPd(1-x) (wherein 0.3<x<0.7), (2) CuPd alloy nanoparticles having a bcc type
crystal structure and represented by CuxPd(1-x) (wherein 0.3<x<0.7), (3) CuPd alloy nanoparticles having a fcc type
crystal structure and represented by CuxPd(1-x) (wherein 0.01≦x≦0.99), (4) CuPd alloy nanoparticles having an L12 type
crystal structure and represented by CuxPd(1-x) (wherein 0.7≦x≦0.98), or (5) CuPd alloy nanoparticles having a
crystal structure wherein at least two of a B2 type portion, a bcc type portion, an fcc type portion, an L12 type portion and a phase separation type portion are mixed. Disclosed are a method for synthesizing ammonia using the photocatalyst composition, and a method for decreasing
nitrogen oxide in water using the photocatalyst composition.