Two nanocomposite photocatalysts were synthesized by forming highly dispersed Cu sites on surfaces. The synthesized materials were characterized with microscopy, UV-visible, and infrared spectroscopy, and tested in photocatalytic reduction. Computational modeling was conducted to elucidate the role of single Cu sites in promoting reduction on . The nanocomposites demonstrated significantly higher activity than bare in photocatalysis, and the activity was found to be related to the chemical state of surface species. According to our modeling studies, the highly dispersed Cu sites likely contributed to the improved photocatalysis by stabilizing surface adsorption of on . Our experimental and modeling studies further confirmed the direct involvement of surface Cu sites in CO generation via reduction.