Photoconversion of CO2 into fuels completes the carbon neutral cycle in a sustainable society. To exclude the contribution of adventitious carbon, monitoring the time course of (CO2)-C-13 conversion into C-13-fuel is essential, but has been rarely reported. In the present work, a composite of Au nanoparticles with ZrO2 was found to be effective in converting (CO2)-C-13 into (CO)-C-13 at a rate of 0.17 mu mol h(-1) g(cat)(-1) in the presence of H-2 and UV-vis light. The detected (CO)-C-12 as a minor byproduct (11.9 %) was identified as due to adsorbed (CO2)-C-12 from the air. The C-12 ratio in the total amount of CO2 was evaluated based on a (CO2)-C-13 photoexchange reaction (8.7 %). The discrepancy between these values suggested a slower exchange reaction step between the chemisorption site for CO2 reduction and the physisorption site for CO2 compared to the reduction step to CO. Furthermore, based on in-profile kinetic studies using sharp-cut filters and control reactions in the dark, the contribution ratio for CO2 conversion was determined to be via charge separation at the band-gap of ZrO2 (lambda < 320 nm): 69 % and via ambient heat (1/2kT): 31 %. Localized surface plasmon resonance (LSPR) absorption of Au and infrared absorption in the range of lambda > 320 nm did not promote catalysis. The LSPR absorption was further investigated by Au L-3-edge extended X-ray absorption fine structure analysis. Ambient heat on the Au nanoparticles should have promoted H-2 activation enough, supplying protons to the CO2 reduction sites over ZrO2; however, a temperature increase of 26 K on the Au surface was marginal for further H-2 activation. CO2 photoconversion with added moisture was also attempted; the CO formation rate using ZrO2 under these conditions was 0.15 mu mol h(-1) g(cat)(-1). However, 47 % was characterized as (CO)-C-12 originating from chemisorbed (CO2)-C-12, and H-2 was also formed at a comparable rate of 0.14 mu mol h(-1) g(cat)(-1) from a competing reaction. The addition of Au to ZrO2 was found to suppress CO formation and promote H-2 formation, and Mg2+ addition to Au-ZrO2 effectively suppressed H-2 formation directing to the CO formation.