Researchers produce green syngas using CO2, water and sunlight

Researchers from the University of Michigan and McGill University in Canada report photochemical syngas synthesis using a core/shell Au@Cr₂O₃ dual cocatalyst in coordination with multistacked InGaN/GaN nanowires (NWs) with the sole inputs of CO₂, water, and solar light. Their paper is published in Proceedings of the National Academy of Sciences (PNAS).

If we can generate syngas from carbon dioxide utilizing only solar energy, we can use this as a precursor for methanol and other chemicals and fuels. This will significantly reduce overall CO₂ emissions.

—Zetian Mi, professor of electrical and computer engineering at the University of Michigan, who led the study

To create a process that uses only solar energy, Mi’s group overcame the difficulty of splitting carbon dioxide molecules, which are among the most stable in the universe. For this, they peppered a forest of semiconductor nanowires with nanoparticles. Those nanoparticles, made of gold coated with chromium oxide, attracted the carbon dioxide molecules and bent them, weakening the bonds between the carbon and oxygen.

A diagram of the semiconductor nanowires made of indium, gallium and nitrogen—decorated with gold and chromium oxide nanoparticles. When the light hits the nanowire, it frees up electrons and the positively charged “holes” that electrons leave behind. On the nanowire itself, the holes oxidize water into protons (hydrogen) and oxygen. Meanwhile, some electrons are drawn into the metal nanoparticles, where they break apart carbon dioxide. The molecules recombine into the carbon monoxide, hydrogen and methane molecules that make up syngas. Image credit: Roksana Rashid, McGill University

First-principle density functional theory calculations revealed that Au and Cr₂O₃ are synergetic in deforming the linear CO₂ molecule to a bent state with an O-C-O angle of 116.5°, thus significantly reducing the energy barrier of CO₂RR compared with that over a single component of Au or Cr₂O₃. Hydrogen evolution reaction was promoted by the same cocatalyst simultaneously.

By combining the cooperative catalytic properties of Au@Cr₂O₃ with the distinguished optoelectronic virtues of the multistacked InGaN NW semiconductor, the developed photocatalyst demonstrated high syngas activity of 1.08 mol/g_cat/h with widely tunable H₂/CO ratios between 1.6 and 9.2 under concentrated solar light illumination.

Nearly stoichiometric oxygen was evolved from water splitting at a rate of 0.57 mol/g_cat/h, and isotopic testing confirmed that syngas originated from CO₂RR. The solar-to-syngas energy efficiency approached 0.89% during overall CO₂ reduction coupled with water splitting. The work paves a way for carbon-neutral synthesis of syngas with the sole inputs of CO₂, H₂O, and solar light.

—Rashid et al.

The gallium nitride nanowires used the light energy to free electrons and the positively charged spaces they leave behind (holes). The holes split water molecules, separating the protons (hydrogen) from the oxygen. Then, at the metal catalysts, the electrons split the carbon dioxide, producing carbon monoxide and sometimes drawing in the free hydrogen to make methane. Processes are under development to separate the oxygen from the other gases.

By changing the ratio of gold to chromium oxide in the nanoparticles, Mi’s team was able to control the relative amounts of hydrogen and carbon monoxide produced in the reaction. The ratio of hydrogen to carbon monoxide affects how easy it is to produce a type of fuel or chemical.

What is surprising is the synergy between gold and chromium oxide to make the CO₂ reduction to syngas efficient and tunable. That was not possible with a single metal catalyst. This opens up many exciting opportunities that were not previously considered.

—Zetian Mi

Mi’s tunable syngas setup uses standard industrial manufacturing processes, and is scalable. While Rashid used distilled water in this experiment, seawater and other electrolyte solutions are also expected to work, and Mi has used them in related water-splitting studies.

Mi’s next goal is to increase the efficiency of the device. When 10% of the light energy is converted to chemical energy, he hopes that the technology could see the technology be adopted for renewable energy, similar to solar cells.

The project was supported through the Emission Reduction Alberta ERA, based at McGill University in Canada, former home of Mi.

A licensing agreement for intellectual property developed in this study is in the process of being negotiated in order to bring the technology to market and make a positive environmental impact. The two companies are NS Nanotech Inc. and NX Fuels Inc, both co-founded by Mi. The University of Michigan and Mi have a financial interest in these companies.

Resources

• Roksana Tonny Rashid, Yiqing Chen, Xuedong Liu, Faqrul Alam Chowdhury, Mingxin Liu, Jun Song, Zetian Mi, and Baowen Zhou (2022) “Tunable green syngas generation from CO₂ and H₂O with sunlight as the only energy input” PNAS doi: 10.1073/pnas.2121174119