Great quantities of hydrocarbon fuels will be needed for the foreseeable future, even if electricity-based energy carriers begin to partially replace liquid hydrocarbons in the transportation sector. Fossil fuels and biomass are the most common feedstocks for production of hydrocarbon fuels. However, if you use renewable energy, carbon dioxide and water can be recycled into sustainable hydrocarbon fuels in non-biological processes which remove oxygen from CO2 and H2O (the reverse of fuel combustion). Also, the capture of CO2 from the atmosphere could enable a closed-loop carbon-neutral fuel cycle.
Now (2020), researchers at the National Renewable Energy Laboratory (NREL) and the University of Southern California (USC) have invented a new process to synthesize metal carbide nanoparticles that can be used to transform waste carbon dioxide into hydrocarbon fuels or precursors to chemical products. Metal carbides, like the molybdenum carbide nanoparticles developed by NREL and USC, are compounds of metal and carbon that are known for their potential in a wide range of catalytic applications such as converting carbon dioxide to carbon monoxide or hydrocarbons for use as fuels or solvents. In search of a milder, more scalable process, the research team have developed a solution-phase route to prepare metal carbide nanoparticles using a continuous flow millifluidic reactor at temperatures nearly half of what was typically required. The extreme conditions of carburization also limit the scope of synthetic control over the physical properties of metal carbides. This novel synthesis route could enable scientists to tune certain physical properties of their nanoparticles like composition, size, and morphology, resulting in improved catalytic performance.
For years scientists have tried to find effective and sustainable ways to recycle hydrocarbons into useful fuels. In 2017, an efficient, stable and multifunctional Na–Fe3O4/HZSM-5 catalyst was developed, which could directly convert CO2 to gasoline-range (C5–C11) hydrocarbons with a selectivity up to 78% of all hydrocarbons, while only 4% methane at a CO2 conversion of 22% under industrial relevant conditions. This was achieved by a multifunctional catalyst providing three types of active sites (Fe3O4, Fe5C2 and acid sites), which in combination catalysed a tandem reaction. More significantly, the appropriate proximity of three types of active sites played a crucial role in the successive and synergetic catalytic conversion of CO2 to gasoline. The multifunctional catalyst, exhibiting a remarkable stability for 1,000 h on stream, had the potential to be a promising industrial catalyst for CO2 utilization to liquid fuels.
Also in 2017, a cheap new chemical catalyst converted CO2 into fuels, using electricity from a solar cell to split CO2 into energy-rich carbon monoxide (CO) and oxygen. Although the conversion was not efficient enough to compete with fossil fuels such as gasoline, it was an important step towards making essentially unlimited amounts of liquid fuels from sunlight, water, and CO2, the chief culprit in global warming.The transformation began when CO2 was broken down into oxygen and CO, the latter of which can be combined with hydrogen to make a variety of hydrocarbon fuels. Adding four hydrogen atoms, for example, created methanol.
In 2019, scientists at Stanford University designed a method to combine the two-step process of converting CO2 into useful fuels whereby CO2 is first reduced to carbon monoxide and then gets further processed in a single reaction, and set about creating a new catalyst that could simultaneously strip an oxygen molecule off CO2 and combine it with hydrogen. The team succeeded by combining ruthenium and iron oxide nanoparticles into a catalyst. The scientists believed that the ruthenium made the hydrogen chemically ready to bond with the carbon from CO2. The hydrogen then spilt onto the iron shell, which made the carbon dioxide more reactive.
There are several advantages to using this new catalyst for fuel production: preparing metal carbides for catalysis has typically involved extreme environments, with high concentrations of flammable gases and high temperatures (593°C).The extreme conditions of carburization also limit the scope of synthetic control over the physical properties of metal carbides. This novel synthesis route may enable researchers to tune certain physical properties of their nanoparticles like composition, size, and morphology, resulting in improved catalytic performance.
The scientists at NREL hope that by continuing to develop more mild and controlled methods of preparing nanostructured carbide catalysts they can help facilitate the commercialization and deployment of these innovative materials.