Insights on Carbon Neutrality by Photocatalytic Conversion of Small Molecules into Value-Added Chemicals or Fuels.

Photocatalytic conversion of small molecules (including H 2 O, CO 2 , N 2 , CH 4 , and benzene) into value-added chemicals or fuels (e.g., H 2 , NH 3 , C 2 + , etc.) is a promising strategy to cope with both the worldwide increasing energy demand and greenhouse gas emission in both energy sectors and chemical industry, thus paving an effective way to carbon neutrality. On the other hand, compared with conventionally thermo- or electrocatalytic processes, photoactivation can convert these very stable small molecules by the unexhausted solar energy, so leading to store solar energy in chemical bonds. Thus, it can effectively reduce the reliance on the nonrenewable fossil fuels and avoid the substantial emission of hazardous gases such as CO 2 , NO x , and so on while producing valued-added chemicals. For example, semiconductors can absorb solar light to split H 2 O into H 2 and O 2 or convert CO 2 to alcohols, which can then be used as zero or neutral carbon energy sources. Although many efforts have already been made on photocatalytic small molecule activation, the light-energy conversion efficiency is still rather moderate and the yield of aimed value-added chemicals cannot meet the requirement of large-scale application. The core for these artificial photocatalytic processes is to discover a novel photocatalyst with high efficiency, low cost, and excellent durability. Over the past two decades, the Tang group has discovered a few benchmark photocatalysts (such as dual-metal-loaded metal oxides, atomic photocatalysts, carbon-doped TiO 2 , and polymer heterojunctions, etc.) and investigated them for photocatalytic conversion of the above-mentioned five robust molecules into value-added chemicals or liquid fuels. Besides, advanced photocatalytic reaction systems including batch and continuous flow membrane reactors have been studied. More importantly, the underlying reaction mechanism of these processes has been thoroughly analyzed using the state-of-the-art static and time-resolved spectroscopies. In this Account, we present the group's recent research progress in search of efficient photocatalysts for these small molecules' photoactivation. First, the strategies used in the group with respect to three key factors in photocatalysis, including light harvesting, charge separation, and reactant adsorption/product desorption, are comprehensively analyzed with the aim to provide a clear strategy for efficient photocatalyst design toward small and robust molecule photoactivation under ambient conditions. The application of in situ and operando techniques on charge carrier dynamics and reaction pathway analysis used in the group are next discussed. Finally, we point out the key challenges and future research directions toward each specific small molecule's photoactivation process.