Proposed carbon capture and utilization (CCU) system based on catalytic CO2 conversion to liquid hydrocarbon fuels with hydrogen supplied from solar water splitting. Source: Choi et al. Click to enlarge.

Making hydrocarbons out of CO₂ must overcome two challenges—chemical stability of the CO₂ molecule and a cheap and renewable hydrogen source. Catalytic hydrogenation of CO₂ has been studied intensively, but the products of direct hydrogenation are limited mostly to low molecular weight (C₁-C₄) hydrocarbons or oxygenates (CO, CH₃OH, HCOOH, CH₃OCH₃, etc.) instead of heavier liquid hydrocarbons more suitable for transportation fuels.

Here we propose a new path to CCU—direct CO₂ conversion to liquid fuels with renewable hydrogen produced by solar water splitting… Thus, CO₂ emitted from industrial sources like coal power plants, steel mills, or chemical plants is captured and reacts with H₂ generated from solar hydrogen plant to produce liquid fuels in a single step. Our direct CO₂-FT synthesis is different from the CO₂-to-diesel conversion process recently announced by Audi, which actually involves two steps—reverse water gas shift (RWGS) reaction to CO followed by CO Fisher-Tropsch (FT) synthesis. The solar water splitting to produce hydrogen is developing rapidly lately and highly efficient PV-electrolysis or photoelectrochemical systems have been developed. Hence, this report focuses on the catalytic CO₂-to-liquid fuel conversion (CO₂ FT synthesis) with renewable H₂.

—Choi et al.

The team developed a novel Cu-Fe catalyst derived from delafossite-CuFeO₂; the new catalyst produces heavy hydrocarbons from CO₂ hydrogenation in the same manner as from conventional CO-FT synthesis.

The catalyst was prepared by reduction of delafossite-CuFeO₂ and in-situ carburization to Hägg carbide (χ-Fe₅C₂), the active phase for heavy hydrocarbon formation. The reference catalysts derived from bare Fe₂O₃, CuO-Fe₂O₃ mixture, and spinel CuFe₂O₄ are much less active and produce mainly light hydrocarbons, highlighting the critical role of delafossite-CuFeO₂ as the catalyst precursor.

The main products produced by the new catalyst cover the gasoline (C₅-C₁₁) and diesel (C₁₂-C₂₁) ranges, while waxy hydrocarbons (C₂₅₊) are ∼15 wt%. This product distribution is very similar to the one typically observed in CO-FT synthesis over iron catalysts.

This represents the first demonstration that liquid fuels and olefins of high value and large market could be obtained directly from CO₂, the most troublesome greenhouse gas.

—Choi et al.

This study has been supported by both the Climate Change-Response Tech Development Project and Mid-Career Researcher Program by Ministry of Science, ICT and Future Planning (MSIP), South Korea.

Resources

• Yo Han Choi, Youn Jeong Jang, Hunmin Park, Won Young Kim, Young Hye Lee, Sun Hee Choi, and Jae Sung Lee (2016) “Carbon dioxide Fischer-Tropsch synthesis: A new path to carbon-neutral fuels”, Applied Catalysis B: Environmental doi: 10.1016/j.apcatb.2016.09.072