Rhodium Nanoparticles as a Carbon Dioxide Conversion Catalyst

By Benedette Cuffari, M.Sc.Sep 11 2017

Methanol is a commonly used solvent, feedstock for the production of ethylene and propylene and fuel additive that has shown to be an excellent and sustainable alternative fuel for chemical and energy industries.

Typically produced through a process referred to as the “syngas route,” where a fossil fuel will react with steam at an extremely high temperature in a reformer device in order to produce hydrogen, carbon monoxide (CO), methane (CH₄), and other useful products from a variety of hydrocarbon fuels¹.

While this industrial approach to produce methanol has been at the core of the methanol economy for quite some time, the production of methanol from carbon dioxide (CO₂) has found to be especially advantageous for a number of reasons.

Through a chemical reaction known as carbon hydrogenation, CO₂ can be directly converted to methanol without the requirement of a preliminary reduction of CO². This chemical reaction is advantageous for a number of reasons as it avoids the usage of carbon-containing fossil fuel sources, which in turn also avoids excessive emission of CO₂ that is often associated with global warming and the Greenhouse effect.

After being captured from a variety of natural or industrial sources, human activities or from the air, CO₂ hydrogenation requires the use of a highly selective catalyst to perform at its greatest potential. Several catalysts such as copper (Cu), zinc (Zn), chromium (Cr) and palladium (Pd) have been successful in minimizing byproduct formation, maximizing production of methanol and maintaining selectivity in the hydrogenation of CO₂².

While adequate in theory, these catalysts often require extremely high temperatures, which can therefore increase the required energy demand and shorten the lifetime of the catalyst. The photocatalytic activity of plasmonic metal nanoparticles has recently gained attention for their ability to drive chemical reactions as heat carriers through their ability to induce excitation of localized surface plasmon resonances (LSPRs), which describe the collective free electron oscillations⁴.

An ideal agent to cause such effects must not only exhibit good plasmonic behaviors, but must also be a highly catalytic agent in order to allow for an increase in the rate and selectivity of the reaction to occur.

Published on February 23^rd, 2017 in Nature Communications, Graduate Student Xiao Zhang under the guidance of Dr. Jie Liu at Duke Uniersity has been able to successfully utilize the photocatalytic properties of rhodium (Rh) nanoparticles in order to generate CH₄³. As one of the rarest elements on Earth, Rh plays an important role in the catalysis of a number of important industrial processes that are involved in the production of detergents, drugs, nitrogen fertilizer, as well as convertors in automobiles responsible for breaking down toxic pollutants³.

Experiments analyzed Rh nanoparticles derived from Rh nanocubes measuring at 334 nm in diameter in comparison to well-known plasmonic gold (Au) nanoparticles with an average diameter of 517 nm.

The photocatalytic reactions were performed in a fixed-bed reaction chamber where both types of nanoparticles were packed with a thickness of approximately 4 mm in order to ensure their complete absorption of light from the ultraviolet light source of a blue LED throughout the reaction⁴.

Upon illumination, Rh nanoparticles exhibit a seven-fold increase in the production of CH₄ as compared to when the reaction was conducted under traditional thermocatalytic mechanisms. Mass spectrometry was then conducted in order to measure a quantitative analysis of the gaseous products of the induced reaction.

With an almost immediate response to light, Rh nanoparticles also exhibited an impressive selectivity of approximately 95% in the production of CH₄⁴. Upon further investigation into the mechanism of action, it was found that CO₂ will initially undergo a dissociative adsorption onto the Rh surface, which will then create adsorbed carbon monoxide (CO) and oxygen (O), both of which can undergo further chemical reactions.

Of these reactions include the adsorption of CO to be hydrogenated to form an aldehyde (CHO), which can then be further dissociated and hydrogenated to selectively produce CH₄⁴.

The photocatalytic reaction rates exhibited in this study were greatly enhanced when conducted at the highest intensity of ultraviolet LED as compared to the thermocatalytic reaction rates conducted at the same temperature⁴. As a result of its photocatalytic behavior, the plasmonic Rh nanoparticles utilized in this experiment simultaneously showed a high selectivity in the production of CH₄ while also requiring much lower activation energies.

Image Credit:

Alexander A. Nedviga/ Shutterstock.com

References

1. Nice, Karim. "How Fuel Processors Work." HowStuffWorks. 4^th Oct. 2000. Web. http://auto.howstuffworks.com/fuel-efficiency/fuel-consumption/fuel-processor1.htm.
2. Jadhav, Suhas G., Prakash D. Vaidya, Bhalchandra M. Bhanage, and Jyeshtharaj B. Joshi. "Catalytic Carbon Dioxide Hydrogenation to Methanol: A Review of Recent Studies." Chemical Engineering Research and Design 92.11 (2014): 2557-567. Web.
3. "Light-driven Reaction Converts Carbon Dioxide into Fuel." ScienceDaily. ScienceDaily, 23^rd Feb. 2017. Web. https://www.sciencedaily.com/releases/2017/02/170223092159.htm.
4. Xiao Zhang, Xueqian Li, Du Zhang, Neil Qiang Su, Weitao Yang, Henry O. Everitt, Jie Liu. Product selectivity in plasmonic photocatalysis for carbon dioxide hydrogenation. Nature Communications, 2017

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