Catalytic thermal reduction of CO2 has the potential to minimize the energy requirement in carbon capture and utilization. However, this process necessitates high energy with a reactant gas, heating, and pressurization. High-performance catalysts are thus needed to transform CO2 into value-added carbon products at low energy consumption. We here demonstrate the thermal reduction of CO2 to carbon products at 700 K and ambient pressure using nanoscale perovskite-type titanium nanocatalysts. The reactivity of CO2 was exponentially increased by decreasing the particle size until 10 nm. The reaction yields calculated from the weight changes of nanocatalysts under CO2 flow were very high, i.e., between 1600 and 3300 mu mol g(-1) h(-1) for nanocatalysts smaller than 20 nm, which were similar to those of CO2 reductions with a reactant gas into lower hydrocarbons and alcohols. A CO2 reductant was evaluated by transmission electron microscopy, chemical mapping using energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and Raman scattering spectroscopy. Graphitic carbons were coated on 9-12 nm nanocatalysts after CO2 reduction. Graphitic carbons were mainly observed on the 12 nm nanocatalyst. A nanodiamond-like structure was partly observed on the 9 nm nanocatalyst, as well as a graphitic structure. Nanodiamond was hardly produced in the mild condition at 700 K and ambient pressure, which was normally produced above 2 GPa and 1600 K. A nanodiamond was produced by quasi-high-pressure effect in graphene and amorphous carbons, which were previously produced on nanocatalysts. The low-temperature CO2 thermal reduction using perovskite-type titanium nanocatalysts at 700 K is thus a promising approach to CO2 reduction and novel way to produce nanodiamond-like structure.
