The production of long-chain hydrocarbons from CO2 and water using solar light is a big challenge for the catalysis society. Cobalt nanostructures have been reported to catalyze the production of C3-C15 alkanes in a cobalt-water-CO2 three-phase system under white light, yet the proposed photocatalytic mechanism still lacks evidence. In this work, cobalt micro- and nanostructures with different sizes and morphologies are used as model catalysts to reproduce this reaction. Despite different activity, selectivity and stability, the catalysts are found to follow the same reaction mechanism: a CO2 assisted galvanic replacement reaction to produce H-2 followed by a thermal catalytic CO2 hydrogenation to produce alkanes; meanwhile, the Co catalysts are gradually but inevitably converted to CoCO3, leading to a slow deactivation. The low concentration of the in-situ formed H-2 favors the growth of C2+ alkanes. The incident light drives the reaction by heating the catalyst, rather than providing plasmonic hot electrons for CO2 reduction. (C) 2020 Elsevier Inc. All rights reserved.