Researchers at the University of Toronto have developed a catalyst comprising of black silicon nanowire supported ruthenium ( Ru/SiNW) for the photochemical and thermochemical reduction of gaseous CO2 to methane (methanation) in the presence of hydrogen under solar-simulated light. An open access paper on their work is published in the new journal Advanced Science.
The Ru/SiNW catalysts activated the Sabatier reaction at a rate of 0.74 mmol g−1 h−1 under 14.5 suns intensity of solar-simulated irradiation in a hydrogen atmosphere at 15 psi and a H2:CO2 ratio of 4:1. The team suggested that much higher reaction rates could be achieved by optimizing the dispersion of the Ru over the SiNW support.
The results, suggested the researchers, represent a step towards engineering broadband solar fuels tandem photothermal reactors that enable a three-step process involving:
- CO2 capture;
- gaseous water splitting into H2; and
- reduction of gaseous CO2 by H2.
The concept of solar fuels is based on harnessing an abundant supply of energy from the sun and storing it in the form of chemical bonds. The most common solar fuel investigated in the literature is hydrogen gas generated from solar powered water splitting. Other solar fuels reactions involving the reduction of CO2 to generate carbon-based fuels such as carbon monoxide (CO), methane (CH4), and methanol (CH3OH) offer another source of energy with neutral CO2 emissions.
Here, we investigate the photoreduction of CO2 over a Ru catalyst supported by silicon nanowires (Ru/SiNW) in a hydrogen environment. We consider this solar assisted CO2 conversion as a complementary solar fuels reaction that can potentially use a renewable source of hydrogen to simultaneously reduce greenhouse gas emissions and provide methane to natural gas pipeline networks.—O’Brien et al.
The researchers sputtered ruthenium nanoparticles onto black silicon nanowire supports in a hydrogen environment. SiNWs are attractive as a support for solar-powered catalysis because, with a band-gap of 1.1 eV, they can potentially absorb 85% of the solar irradiance, the team noted. When vertically etched into a Si wafer these supports exhibit minimal reflection over a broad spectral range—such wafers are often referred to as “black silicon”.
In their work, the team found that these Ru/SiNW catalysts photoactivate the Sabatier reaction—the reaction of hydrogen with CO2 at elevated temperatures in the presence of a catalyst to produce methane—both thermochemically and photochemically.
From a thermochemical standpoint, the Ru/SiNW catalyst heats up when irradiated with solar-simulated light and methanation rates increase due to increased temperatures.
From a photochemical standpoint, at a set temperature, the rate of the Sabatier reaction increases proportionally to the number of incident photons with energy greater than the band-gap of Si.
…to put things into perspective, it should be noted that the rates reported herein (1 mmol/g·h) are still too low to reduce CO2 at globally significant rates. Further research is required to increase CO2 conversion rates over the Ru/SiNW catalyst reported herein and also to replace Ru with a less expensive catalyst such as Ni. Nevertheless, the discovery that CO2 can be reduced photochemically using a broad spectral range covering most of the solar spectrum is an important point to consider in designing solar fuels reactors.
For example, rather than heating the entire reactor, solar radiation can be focused onto the catalyst in order to reduce the heating load and also to use available land-space more efficiently. Furthermore, considering the Sabatier reaction rates…as an example, a given reaction rate can be attained at a lower temperature by photochemically driving the reaction. This ability to achieve higher reaction rates at lower temperatures may produce numerous advantages. For example, lower operating temperatures may reduce the deleterious effects of sintering, poisoning, mechanical degradation and eventual deactivation of the catalyst.—O’Brien et al.
The team also noted that the work shows that SiNWs can potentially be used as supports that provide heat from the sun to numerous other solar fuels catalysts loaded onto their surface to boost their reaction rates. As a proof of concept experiment, they showed that the reverse water gas shift can be activated over In2O3 nanoparticle photocatalysts loaded onto SiNW supports at ≈150 °C under irradiation from a Xe lamp). Also, SiNWs can be scaled to technologically significant proportions using well-known silicon wafer wet-chemistry processing.
Thus, on account of their large surface area, high absorption towards solar irradiation, and technologically mature background, black SiNW supports merit further investigation in the pursuit and development of practically useful gas-phase solar fuels catalysts.—O’Brien et al.
O’Brien P. G., Sandhel A., Wood T. E., Jelle A. A., Hoch L. B., Perovic D. D., Mims C. A., Ozin G. A. (2014) “Photomethanation of Gaseous CO2 over Ru/Silicon Nanowire Catalysts with Visible and Near-Infrared Photons” Adv. Sci., 1: 1400001. doi: 10.1002/advs.201400001