| Summary: | Commercial TiO<sub>2</sub> (anatase) was successfully modified with Ag nanoparticles at different nominal loadings (1%–4%) using a liquid impregnation method. The prepared materials retained the anatase structure and contained a mixture of Ag<sup>0</sup> and Ag<sup>I</sup> species. Samples exhibited extended light absorption to the visible region. The effect of Ag loading on TiO<sub>2</sub> is studied for the photocatalytic reduction of CO<sub>2</sub> to CH<sub>4</sub> in a gas–solid process under high-purity conditions. It is remarkable that the reference TiO<sub>2</sub> used in this work is entirely inactive in this reaction, but it allows for studying the effect of Ag on the photocatalytic process in more detail. Only in the case of 2% Ag/TiO<sub>2</sub> was the formation of CH<sub>4</sub> from CO<sub>2</sub> observed. Using different light sources, an influence of the localized surface plasmon resonance (LSPR) effect of Ag is verified. A sample in which all Ag has been reduced to the metallic state was less active than the respective sample containing both Ag<sup>0</sup> and Ag<sup>+</sup>, indicating that a mixed oxidation state is beneficial for photocatalytic performance. These results contribute to a better understanding of the effect of metal modification of TiO<sub>2</sub> in photocatalytic CO<sub>2</sub> reduction.
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