Using cobalt oxide as a catalyst 

Carbon dioxide is a greenhouse gas. To reduce its heating effect on the earth, it is captured and stored in deep underground facilities. This involves a large amount of energy for excavating and drilling rocks as well as for applying pressure to reduce the volume of the gas for storage. So researchers are exploring alternatives. 

Using catalysts, carbon dioxide can be converted to methanol, a potential biofuel. Cobalt oxide, in its nanostructured form, is a good choice as catalyst. Cobalt oxide nanostructures can be cubes, rods or sheets. Which morphology of nano-cobalt oxide is best for reducing carbon dioxide to methane?

Researchers from CSIR-NCL, Pune and IIT Palakkad collaborated to tackle the question.

To create cubic structures, they used cobalt nitrate hexahydrate and sodium hydroxide as reducing agent in a hydrothermal reactor. For rod- and sheet-like structures, they used different volumes of one molar sodium carbonate. By strategically manipulating parameters such as pH, temperature, pressure, and concentration, the researchers fabricated nano-cobalt oxide with cube, rod, and sheet morphologies, and confirmed the morphology using an electron microscope. 

To test the performance of the nanostructures, they passed carbon dioxide and hydrogen through a layer of each of the nanostructures at ambient pressure. For catalysis, hydrogen and carbon dioxide have to be adsorbed next to each other on the cobalt oxide surface and this may be influenced by temperature. So the researchers varied the temperature of the reaction and found that the maximum catalytic activity was at 400 degree Celsius.

The material with rod-shaped structure had the highest catalytic activity. At 400 degree Celsius, the rod-shaped cobalt oxide converted nearly one-fourth of carbon dioxide into methanol.

This was partly because the rod-like nanostructures had more surface area, say the researchers. There were other factors at play too. The mechanism of the reaction depends on the oxidation state of the cobalt metal: the higher the oxidation state, the greater the hydrogenation.  

The researchers used X-ray photoelectron spectroscopy and, from the photoelectrons emitted by the nanomaterials in response to X-rays, they found that the oxidation state of cobalt in cobalt oxide varied with the morphology of the nanostructures and that the rod-like nano-cobalt had higher oxidation states. 

Industries could use rod-shaped nano-cobalt oxide to convert carbon dioxide into methanol at ambient pressure, and reduce the emission of carbon dioxide into the atmosphere. In the process, they could generate methanol, and recover the costs. 

DOI: 10.1021/acs.jpcc.3c02857;
J. Phys. Chem. C 127: 13055−13064 (2023)

Reported by Ajesh K Zachariah
Mar Thoma College, Tiruvalla

*This report was written during the 4th online workshop on science writing organised by Current Science.

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Tagged as: C P Vinod, cobalt oxide nanorods, CSIR-NCL, Dinesh Jagadeesan, IIT Palakkad, Preeti Jain, Sharad Gupta

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