Selective methane production from visible-light-driven photocatalytic carbon dioxide reduction using the surface plasmon resonance effect of superfine silver nanoparticles anchored on lithium titanium dioxide nanocubes (Ag@Li_xTiO₂)

This study focused on the results of applying the strong surface plasmon resonance (SPR) effect of silver (Ag) particles anchored on cubic phase Li_xTiO₂ to the carbon dioxide (CO₂) photoreduction reaction. The study demonstrated the importance of three aspects: First, the cubic TiO₂, which activated the [101] facet, was successfully produced. Secondly, Li⁺ ions were introduced as Frenkel defects in some lattices to create oxygen defects. These vacancies increased the adsorption of carbon dioxide and sped up the rate-determining step in the CO₂ reduction reaction. In other words, they induced the easy conversion of CO₂ to CO, which is the first reduction product. Finally, the loading of Ag nanoparticles onto the Li_xTiO₂ cubic surface the improved photocatalytic activity through SPR effects, and in particular led to selective conversion of CO₂ to methane (CH₄). Quantitatively, the yield of CH₄ from CO₂ using the Ag@Li_0.075TiO₂ particles was 49 μmol/g after 10 h of reaction, which was 8.2 and 1.5 times higher than that of cubic TiO₂ (6 μmol/g) and Li_0.075TiO₂ (33 μmol/g) under UV-light. Additionally, its activity did not decrease under visible lights of 420 and 620 nm with the similar CH₄ yields of 42 and 34 μmol/g after 10 h, respectively. In particular, the production ratio of CH₄ and CO using cubic TiO₂ and Li_xTiO₂ were about 1:1, with no selectivity for either product. However, after metallic Ag nanoparticles were loaded, the product selectivity shifted towards CH₄, and the product ratio of CH₄ to CO was about 3:1. Furthermore, the Ag@Li_0.075TiO₂ particles exhibited a strong SPR effect (in particular, direct electron transfer), which contributed to maintaining the charge separation and the lifetime of the catalyst over a long period. Catalytic deactivation was not observed during five cycles of recycling tests.