Carbon dioxide (CO2) is naturally present within the atmosphere, but has the potential to cause harm as it also accounts for over 80% of the harmful greenhouse gases that are emitted into the environment as a result of human activities, such as through the combustion of fossil fuels that are used for energy and transportation purposes.
The excessive emission of greenhouse gases, particularly CO2, is responsible for a number of deleterious effects to both human health and the environment; therefore, methods to reduce the presence of such gases are in high demand. The reduction of CO2 by use of photocatalysts has been a popular area of scientific investigation, however, the challenge to control the catalytic experiment to yield a specific product of interest has prevented this work from being applied to large-scale industrial applications.
The reduction of CO2 has the potential to yield products including formic acid (HCO2H), formaldehyde (HCHO), methanol (CH3OH), methane (CH4) and carbon monoxide (CO). To combat this issue, a team of International Researchers led by the Department of Energy’s Lawrence Berkeley National Laboratory and Nanyang Technological University (NTU) in Singapore have developed a unique photocatalytic material that is capable of converting CO2 to CO while simultaneously preventing the generation of unwanted byproducts in the process.
The photocatalyst composite material designed by the Researchers in this study is based on the metal-organic framework (MOF) that has been well documented in previous studies to effectively capture gas as its high surface area and adjustable pore size allows this category of materials to act as heterogenous catalysts. In the Berkeley Lab and NTU study, the photocatalyst they developed exhibits a spongy network structure, in which nickel (Ni2+) ions are present at the center, terephthalic acid (TPA) acts as its rigid organic linker and triethylene glycol (TEG) as its soft organic linker and dimethylformamide (DMF) as a solvent.
To initiate the desired reactions for the reduction of CO2, an unfocused infa-red laser was applied to the Ni(TPA/TEG) composite to allow for the metal ions within the metal to react with TEG and produce the distorted layered structure. The disordered structure of the composites allows for the enhanced accessibility of water molecules to penetrate the active sites of the material and thereby act as a catalyst for the reduction of CO2.
A number of confirmatory tests were performed in order to fully visualize the reduction of CO2 by this laser-chemical method, as well as provide a full three-dimensional image of the unique spongy architecture of the Ni(TPA/TEG) material. Structural analyzes of the Ni(TPA/TEG) composite were performed by three-dimensional electron tomographic results, which revealed various mesopores within the material, whereas scanning nanobeam diffraction data provided the Researchers with information on the specific particle properties.
X-ray diffraction (XRD) was performed to confirm the presence of TEG molecules on the Ni(TPA) framework when comparing the composite material with its control.
From the laser-chemical reaction conducted by this material in the study, the Researchers confirmed that CO was the only detectable gas produced from this reaction, thereby achieving 100% selectivity for CO. The proposed mechanism underlying the highly selective reaction shown in this experiment states that following the irradiation of the Ni(TPA/TEG) composite material by laser, the photosensitizer Ru(bpy)32+ is excited and then rapidly reduced and simultaneously transfers an electron to the Ni(TPA/TEG) material.
Upon this transfer of electrons, the spongy Ni(TPA/TEG) catalyzes the reduction of the fixed CO2 molecules to CO. The Researchers are hopeful that when used as a tandem catalyst, the Ni(TPA)TEG) composite material can rapidly convert CO2 into high-value liquid fuels following exposure to natural light.
Jeff Zehnder/ Shutterstock.com
- “A spongy nickel-organic CO2 reduction photocatalyst for nearly 100% selective CO production” K. Niu, Y. Xu, et al. Science Advances. (2017). DOI: 10.1126/sciadv.1700921.