| CPC B01J 23/10 (2013.01) [B01J 23/745 (2013.01); B01J 23/92 (2013.01); B01J 38/10 (2013.01); C01B 32/40 (2017.08); C01F 17/32 (2020.01)] | 12 Claims |

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1. A reducing agent for use in production of a product gas containing carbon monoxide, the reducing agent being brought into contact with a raw material gas containing carbon dioxide to reduce the carbon dioxide to produce the product gas,
the reducing agent comprising:
a composite metal oxide represented by Ce1-x(M)xOy, where M is a metal element with an ionic radius smaller than an ionic radius of Ce with an identical valence number and an identical coordination number, x represents a positive real number, and y represents a real number from 1 to 4, and
a support supporting the composite metal oxide, the support having oxygen ionic conductivity and having a function as an oxygen carrier, wherein the support comprises at least one selected from the group consisting of vanadium (V), manganese (Mn), molybdenum (Mo), yttrium (Y), chromium (Cr), lanthanum (La), cobalt (Co), nickel (Ni), copper (Cu), tungsten (W), and niobium (Nb).
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