The achievement marks a significant step forward in developing technology that could help generate fuel and other energy-rich products using a solar-powered catalyst while mitigating levels of a potent greenhouse gas.

We show a near 100 percent selectivity of CO production, with no detection of competing gas products like hydrogen or methane. That’s a big deal. In carbon dioxide reduction, you want to come away with one product, not a mix of different things.

—Haimei Zheng, staff scientist in Berkeley Lab’s Materials Sciences Division and co-corresponding author of the study

In chemistry, reduction refers to the gain of electrons in a reaction, while oxidation is when an atom loses electrons. Among the well-known examples of carbon dioxide reduction is in photosynthesis, when plants transfer electrons from water to carbon dioxide while creating carbohydrates and oxygen.

Carbon dioxide reduction needs catalysts to help break the molecule’s stable bonds. Interest in developing catalysts for solar-powered reduction of carbon dioxide to generate fuels has increased with the rapid consumption of fossil fuels over the past century, and with the desire for renewable sources of energy.

Researchers have been particularly keen on eliminating competing chemical reactions in the reduction of carbon dioxide.

Complete suppression of the competing hydrogen evolution during a photocatalytic CO₂-to-CO conversion had not been achieved before our work.

—Haimei Zheng

At Berkeley Lab, Zheng and her colleagues developed an innovative laser chemical method of creating a metal-organic composite material. They dissolved nickel precursors in a solution of triethylene glycol and exposed the solution to an unfocused infrared laser, which set off a chain reaction in the solution as the metal absorbed the light. The resulting reaction formed metal-organic composites that were then separated from the solution.

When we changed the wavelength of the laser, we would get different composites. That’s how we determined that the reactions were light-activated rather than heat-activated.

—co-lead author Kaiyang Niu

The researchers characterized the structure of the material at the Molecular Foundry, a DOE Office of Science User Facility at Berkeley Lab. The nickel-organic photocatalyst had notable similarities to metal-organic frameworks, or MOFs. While MOFs have a regular crystalline structure with rigid linkers between the organic and inorganic components, this new photocatalyst incorporates a mix of soft linkers of varying lengths connected with nickel, creating defects in the architecture.

The resulting defects are intentional, creating more pores and sites where catalytic reactions can occur, said Kaiyang Niu. The new material is more active and highly selective compared with MOFs made by traditional heating, he added.

Scientists at NTU tested the new material in a gas chamber filled with carbon dioxide, measuring the reaction products using gas chromatography and mass spectrometry techniques at regular time intervals. They determined that in an hour at room temperature, 1 gram of the nickel-organic catalyst was able to produce 16,000 micromoles, or 400 milliliters, of carbon monoxide. Moreover, they determined that the catalyst had a promising level of stability that allowed it to be used for an extended time.

The reduction of carbon dioxideCO₂ by catalysts is not new, but other materials typically generate multiple chemicals in the process. The near-total production of carbon monoxide with this material represented a new level of selectivity and control, the researchers emphasized.

The researchers have some thoughts about how this selectivity occurs. They suggest that the architecture of their photocatalyst makes it easier for carbon dioxide anions to bind to reaction sites, leaving little space for hydrogen radicals to land. This would limit the proton transfers necessary to form hydrogen gas, the researchers said.

Further, the researchers found when the spongy Ni-organic catalyst was enriched with Rh or Ag nanocrystals, the controlled photocatalytic CO₂ reduction reactions generate formic acid and acetic acid. More importantly, the researchers noted, the molecules of these products are characterized by two-carbon links, a step toward the generation of higher-energy liquid fuels with more carbon bonds.

Proposed mechanisms for the photocatalytic conversion of CO2 to CO and of CO to other liquid products. (A) Visible light reduction of the photosensitizer [Ru(bpy)3]2+, which transfers an electron to the Ni(TPA/TEG) catalyst to convert CO2 to CO (B) and to Ni(TPA/TEG)-(Ag/Rh) catalysts for the generation of HCOOH, CH3COOH, and CH3CH2OH from further reduction of CO (C). (D) Possible conversion pathways leading to the formation of HCOOH, CH3COOH, and CH3CH2OH via proton-coupled one-, four-, and eight-electron steps, respectively. Niu et al. Click to enlarge.

By assuming that the evolved CO may be further consumed for the production of acids, we also conduct control experiments using CO, instead of CO₂, as the gas feedstock for the photocatalytic reduction reaction. As a result, largely enhanced yields of acids are obtained. For instance, the amount of HCOOH evolved from the CO reduction is 24 times higher than that from the CO₂ reduction on the Ni(NPA/TEG) catalyst, and the amount of CH3COOH produced from the CO reduction reaction for the Ag/Rh-decorated Ni(NPA/TEG) catalyst exhibits a sixfold increase over the results from the CO₂ reduction reaction. Besides the increase in acid production, another important C₂ product, that is, ethanol [with a concentration of 270.6 μM for Ni(NPA/TEG)-Rh and 262.2 μM for Ni(NPA/TEG)-Ag], has also emerged from the 6-hour photocatalytic CO reduction reaction.

—Niu et al.

The world right now is in need of innovative ways to create alternatives to fossil fuels, and to stem the levels of excessive CO₂ in the atmosphere. Converting CO₂ to fuels using solar energy is a global research endeavor. The spongy nickel-organic photocatalyst we demonstrated here is a critical step toward practical production of high-value multi-carbon fuels using solar energy.

—Haimei Zheng

Other authors on this paper include co-corresponding author Rong Xu, NTU associate professor of chemical and biomedical engineering; You Xu, NTU research fellow in Xu’s lab; visiting scholar Haicheng Wang and scientist Joel Ager at Berkeley Lab’s Materials Sciences Division; and Karen Bustillo, a scientific engineer at the Molecular Foundry.

The DOE Office of Science supported this work. Additional characterization work was done at the Center for Functional Nanomaterials at Brookhaven National Laboratory, also a DOE Office of Science User Facility.

Resources

• Kaiyang Niu, You Xu, Haicheng Wang, Rong Ye, Huolin L. Xin, Feng Lin, Chixia Tian, Yanwei Lum, Karen C. Bustillo, Marca M. Doeff, Marc T. M. Koper, Joel Ager, Rong Xu and Haimei Zheng (2017) “A spongy nickel-organic CO₂ reduction photocatalyst for nearly 100% selective CO production” Science Advances Vol. 3, no. 7, e1700921 doi: 10.1126/sciadv.1700921