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UTA researchers demonstrate one-step solar process to convert CO2 and H2O directly into renewable liquid hydrocarbon fuels

Researchers at the University of Texas at Arlington have demonstrated a new solar process for the one-step, gas-phase conversion of CO2 and H2O to C5+ liquid hydrocarbons and O2 by operating the photocatalytic reaction at elevated temperatures and pressures.

The photothermocatalytic process for the synthesis of hydrocarbons—including liquid alkanes, aromatics, and oxygenates, with carbon numbers (Cn) up to C13—ran in a flow photoreactor operating at elevated temperatures (180–200 °C) and pressures (1–6 bar) using a 5% cobalt on TiO2 catalyst and under UV irradiation. A paper describing the process is published in Proceedings of the National Academy of Sciences (PNAS).

We are the first to use both light and heat to synthesize liquid hydrocarbons in a single stage reactor from carbon dioxide and water. Concentrated light drives the photochemical reaction, which generates high-energy intermediates and heat to drive thermochemical carbon-chain-forming reactions, thus producing hydrocarbons in a single-step process.

—Brian Dennis, UTA professor of mechanical and aerospace engineering and co-principal investigator of the project

The process uses cheap and earth-abundant catalytic materials. Further, the unusual operating conditions expand the range of materials that can be developed as photocatalysts. Although the efficiency of the current demonstration system is not commercially viable, they team noted, it is also far from optimized and it opens a promising new path by which such solar processes might be realized.

This simple and inexpensive new sustainable fuels technology could potentially help limit global warming by removing carbon dioxide from the atmosphere to make fuel. The process also reverts oxygen back into the system as a byproduct of the reaction, with a clear positive environmental impact, researchers said.

The eventual replacement of oil with fuels generated from sustainable and carbon-neutral sources is necessary if we are to avoid harmful climate change due to the buildup of greenhouse gases in the atmosphere. Advances in solar-based technologies are the most promising; however, these technologies generally produce either electricity or hydrogen, neither of which is an ideal replacement for liquid hydrocarbons. The least disruptive technology would replace oil-derived hydrocarbons with liquid hydrocarbon fuels derived from CO2, water, and a clean energy source, such as the sun, leading to a carbon-neutral fuel cycle.

Currently, there are a number of promising strategies to harness solar energy to generate high-energy molecules (fuels) from water and/or carbon dioxide, including (i) high-temperature thermochemical cycles, (ii) coupling photovoltaics to water electrolysis (PV-EC), (iii) developing single or tandem photoelectrochemical cells (PEC), or (iv) direct photochemical methods (PC) using semiconductor materials, often modified by added cocatalysts or nanostructuring techniques. Hydrogen, carbon monoxide, C1 hydrocarbons, and syngas are the most commonly produced fuels and are derived from water or water and CO2. Hydrogen produced via the water-splitting reaction (WSR) … is arguably the easiest to produce and stores the most energy on a mass basis (kJ/kg); however, it is not a particularly attractive replacement fuel for transportation, due to technological issues with low-volume energy density, safe storage, and transportation.

One commonly proposed solution to this dilemma is to use the H2 generated via the WSR, reaction 1, in combination with CO2 to synthesize liquid hydrocarbon fuels, using the reverse water–gas shift (RWGS), reaction 2, and Fischer–Tropsch synthesis (FTS), reaction 3. … We report here a photo-thermochemical process for driving the alkane reverse combustion (ARC) reaction (reaction 4) to produce C1 to C13 hydrocarbons in a single operation unit.

—Chanmanee et al.

H2O → H2+ ½O2 ΔG˚= 237.3kJ/mol WSR [1]
CO2 + H2 ⇌ CO + H2O ΔG˚= 25.2kJ/mol RWGS [2]
(2n +1)H2 + nCO → CnH(2n+2) + nH2O ΔG˚~ -99 n kJ/mol FTS [3]
(n+1)H2O + nCO2 → CnH2n+2 + (3/2n + ½)O2 ΔG˚~ 665 n kJ/mol ARC [4]
The solar photothermochemical alkane reverse combustion (SPARC) uses the sun process to provide both photons and heat. To examine the SPARC reaction, the team built a fixed-bed, tubular flow reactor in which the catalyst bed could be both heated and irradiated.

CO2 and steam were flowed at 40 standard cubic centimeters per minute (sccm) over the 5% cobalt on a TiO2 catalyst bed, which was heated via an internal electric heater and irradiated with four surrounding 250-W Hg lamps.

Arc
Schematic diagram of photothermal flow reactor with cartoon picture of a single Co/TiO2 particle undergoing catalysis and TEM picture of cobalt on P25 TiO2 catalyst. Chanmanee et al. Click to enlarge.

The products were collected by passing the hot effluent gas through a condenser unit at 0 °C to capture condensable products; through a back-pressure regulator to drop the pressure to 1.0 bar; and then through a sampling loop of an automated online gas chromatograph.

A parametric study of temperature, pressure, and partial pressure ratio showed that temperatures in excess of 160 °C were needed to obtain the higher Cn products in quantity. The product distribution also shifted toward higher Cn products with increasing pressure. In the best run, more than 13% by mass of the products were C5+ hydrocarbons. Some of those, such as octane, are drop-in replacements for existing liquid hydrocarbon fuels.

At present, this gas-phase SPARC reaction is far from optimized and simply shows proof of principle. Higher productivities and better product distributions are likely to be realized as pressure, temperature, reactant ratio, space velocity, and catalyst are optimized.

… Although the current SPARC technology is currently im- practical on a commercial scale, it does offer a conceptually new and commercially promising solar fuels technology that would be simple and inexpensive relative to most PV-EC and PEC systems. The direct production of the value-added hydrocarbons liquid fuel minimizes the number of unit operations involved and the associated efficiency losses and capital expenses of each. In a field operation, it is easy to imagine the use of parabolic mirrors to focus and concentrate sunlight onto a catalyst bed, providing both the photons required for photoexcitation and the thermal energy needed to run the reaction. Assuming such a system may require active cooling, the excess thermal energy could be used for downstream product separations or other applications in which relatively low-grade heat can be applied. In this respect, an SPARC process can realize greater efficiencies than process requiring ambient or near-ambient temperatures in that the low-energy photons are used to help heat the SPARC reaction and to heat a working fluid to a more useful temperature (i.e., 200 °C).

—Chanmanee et al.

One of the next steps for the team is to develop a photo-catalyst better matched to the solar spectrum, said Frederick MacDonnell, UTA interim chair of chemistry and biochemistry and co-principal investigator of the project.

The research was supported by grants from the National Science Foundation and the Robert A. Welch Foundation.

Resources

  • Wilaiwan Chanmanee, Mohammad Fakrul Islam, Brian H. Dennis, and Frederick M. MacDonnell (2016) “Solar photothermochemical alkane reverse combustion” PNAS doi: 10.1073/pnas.1516945113

Comments

Roger K. Brown

Given the long dwell time of CO2 in the atmosphere we need to leave substantial amounts of fossil carbon in place (or practice effective long term CCS) if we want to mitigate global warming. Without an economic way of extracting CO2 from the atmosphere it is not clear whether or not this technology will do good or harm in this respect. I am not advocating against the development of this technology. I am simply questioning the easy optimism of many commenters that it represents a potential solution to the serious problems currently being faced by industrial civilization.

SJC

It is used in some fuels, it is also used in geothermal energy.

Engineer-Poet
In the best run, more than 13% by mass of the products were C5+ hydrocarbons.

Meaning that 84+% of the products by mass were H2, CO or C1-C4 hydrocarbons.

C3 and C4 hydrocarbons are useful motor fuel (LPG) but the rest needs help, at best.

Use CO2 twice, cut emissions in half.

95+% reductions are needed; use-twice is simply not going to cut it.

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