March 28, 2021. By Kolemann Lutz
Researchers from Saudi Arabia demonstrated a cost efficient and environmentally friendly route for the hydrogenation and solar-driven methanation of carbon dioxide (CO2) and hydrogen (H2) into methane (CH4) with a photothermal catalyst at nearly 100% selectivity and without any external energy, or heat sources.
As a fuel for ovens, homes, water heaters, automobiles, turbines, and much more, CH4
is the intended propellant on Mars and also enables many chemical reactions, products, and a variety of industrial uses including propellant for other vehicles, to make heat and electricity, manufacture organic hydrocarbons such as methanol, formaldehyde, and formic acid.
The capture and conversion of CO2 into carbon-based products is of significant interest for researchers and organizations worldwide. As traditional photocatalysts are based on wide bandgap inorganic semiconductors, the efficiency of photocatalytic CO2 conversion using H2O or H2 remains very low.
Organic semiconductors or photocatalysts can be used to generate heat from the interaction of absorbed light with the catalyst, also known as the photothermal effect, without the need for any external energy or heat sources. Other industrial approaches to methanation or hydrogenating carbon dioxide typically occurs through the Sabatier process, which demands external energy sources to attain temperatures ranging from 300–400 °C and sometimes as high as 500 degrees Celsius. Thus, there is a need for alternative low-energy methanation production methods at industrial-scale.
In 2014, a research team demonstrated the application of the photothermal effect to efficiently convert CO2 to CH4 using Group VIII metals as the catalyst. Inspired to develop new efficient photothermal systems based on abundant, noncritical first-raw transition metals, researchers from KAUST University built a photocatalyst from nickel nanoparticles on a layer of barium titanate to absorb the light in a way that excites electrons into high energy states, or hot electrons. These hot electrons initiate the chemical reaction that converts carbon dioxide into CH4. Under optimum conditions, researchers demonstrated that the catalyst generates methane with production rates as high as 103.7 mmol g-1h-1 with nearly 100% selectivity and with impressive efficiency using only the photothermal effect UV-visible and visible solar irradiation.
Additionally, researchers conducted a control experiment under dark conditions with a
with 250 deg C inside the photoreactor, resulting in a 75% CO2 conversion after 20 min of the photothermal reaction.
A major advantage is that the photocatalyst absorbs all visible wavelengths of the light spectrum compared to most catalysts that are restricted to ultraviolet rays. On Earth, this is significant since ultraviolet (UV) light comprises only 4 to 5% of the energy available in sunlight. On Mars, UV flux is primarily in the near (300 um) and far (200um) wavelengths due to the absorption and scattering in the Martian atmosphere.
In hindsight, researchers established a new record for solar-drive CH4 production of 1.661 grams/hr of CH4, or 25 Earth days to produce one kg of CH4 using the small scale experiment demonstration, for photocatalyst materials in direct methanation of carbon dioxide.
For comparison, a 50kg small-scale Sabatier reactor can produce around one kg per day of O2:CH4 propellant with important liquid hydrogen while operating at ~17 kWh per day or one tonne of propellant per 17 MWh energy. Considering high-energy Sabatier reactors demand elevated temperatures ranging from 300–400 °C and the propellant plant has well over one Earth year to synthesize CH4 to fill a 240,000kg (in 1G environment) liquid CH4 carrying capacity in the propellant chamber for ascent vehicle, the photocatalyst approach provides a low mass, low power, and affordable solution to passively produce and supplement CH4 production nearby the methane feedstock storage containers.
This approach provides a sustainable way to convert harmful greenhouse gases into valuable fuel as CO2 could be repeatedly recycled from the atmosphere to fuel and back again. The researchers aim to widen the applications of their approach and toward scaling up the synthesis and reactor system on Earth. "One strategy for our future research is to move towards producing other valuable chemicals, such as methanol," says Jorge Gascon, who led the research team. The researchers also see potential for using light energy to power the production of chemicals that don't contain carbon, such as ammonia (NH3).”
Ideally, a consortium of research organizations are involved in testing and optimising a slew of CH4 production experiments and simulations on the surface with methane production prototypes and larger-scale plant designs.
“If the amount of sunlight, 590W/m2 that reaches Mars is insufficient, photocatalysts could certainly become a preferred method for hydrocarbon production in orbit, lunar surface, and places like the clouds of Venus”, mentioned by Kolemann Lutz, Cofounder at MarsU. The Venetian atmosphere receives 2,600 watts per square meter, or on average 2.6X greater solar intensity than on Earth’s surface.
Future research could also be geared toward the systems and architecture involved in integrating the photothermal systems at CH4 production plants and Sabatier reactors and even magnifying or concentrating the photothermal effect to make better use of available sunlight, similar to Focused ultrasound (FUS).
Industrial scale solar-driven methanation of carbon dioxide (CO2) and hydrogen (H2) holds the potential to passively generate abundant quantities of methane, compounds and other hydrocarbons in-situ at close to no additional energy and cost during operations.
Diego Mateo, et al. Efficient Visible-Light Driven Photothermal Conversion of CO2 to Methane by Nickel Nanoparticles Supported on Barium Titanate. Journal of Advanced Functional Materials. December 4, 2020.