ENERGY SPECIAL ...........................New catalyst enables direct production of fuels from CO2 using solar energy

New catalyst enables direct production of fuels from CO2 using solar energy

The sun is a clean and inexhaustible source of energy with the potential to provide a sustainable answer to all future energy supply demands. There’s just one outstanding problem: the sun doesn’t always shine and its energy is hard to store. For the first time, researchers at the Paul Scherrer Institute (PSI) and the ETH Zurich have unveiled a chemical process that uses the sun’s thermal energy to convert carbon dioxide and water directly into high-energy fuels: a procedure developed on the basis of a new material combination of cerium oxide and rhodium. This discovery marks a significant step towards the chemical storage of solar energy. The researchers published their findings in the research journal Energy and Environmental Science. Several options to harnessing energy The sun’s energy is already being harnessed in various ways: whilst photovoltaic cells convert sunlight into electricity, solar thermal installations use the vast thermal energy of the sun for purposes such as heating fluids to a high temperature. Solar thermal power plants involve the large-scale implementation of this second method: using thousands of mirrors, the sunlight is focused on a boiler in which steam is produced either directly or via a heat exchanger at temperatures exceeding 500°C. Turbines then convert thermal energy into electricity. Researchers at PSI and the ETH Zurich have collaborated to develop a groundbreaking alternative to this approach. The new procedure uses the sun’s thermal energy to convert carbon dioxide and water directly into synthetic fuel. “This allows solar energy to be stored in the form of chemical bonds,” explains Ivo Alxneit, chemist at the PSI’s Solar Technology Laboratory. “It’s easier than storing electricity.” The new approach is based on a similar principle to that used by solar power plants.” Alxneit and his colleagues use heat in order to trigger certain chemical processes that only take place at very high temperatures above 1000°C. Advances in solar technology will soon enable such temperatures to be achieved using sunlight. Producing fuel with solar heat Alxneit’s research is based on the principle of the thermo-chemical cycle, a term comprising both the cyclical process of chemical conversion and the heat energy required for it. Ten years ago, researchers had already demonstrated the possibility of converting low-energy substances such as water and the waste product carbon dioxide into energy-rich materials such as hydrogen and carbon monoxide. This works in the presence of certain materials such as cerium oxide. When subjected to very high temperatures above 1500°C, cerium oxide loses some oxygen atoms. At lower temperatures, this reduced material is keen to re-acquire oxygen atoms. If water and carbon dioxide molecules are directed over such an activated surface, they release oxygen atoms. Water is converted into hydrogen and carbon dioxide turns into carbon monoxide (CO), whilst the cerium re-oxidizes itself in the process, establishing the preconditions for the cerium oxide cycle to begin all over again. The hydrogen and carbon monoxide created in this process can be used to produce fuel: specifically, gaseous or fluid hydrocarbons such as methane, petrol and diesel. Such fuels may be used directly but can also be stored in tanks or fed into the natural gas grid. One process instead of two Up to now, this type of fuel production required a second, separate process: the so-called Fischer-Tropsch Synthesis, developed in 1925. The European research consortium SOLAR-JET recently proposed a combination of a thermo-chemical cycle and the Fischer-Tropsch procedure. However, as Alxneit explains: “although this basically solves the storage problem, considerable technical effort is necessary to carry out a Fischer-Tropsch Synthesis.” In addition to a solar installation, a second industrial-scale technical plant is required. Direct production of solar fuel now possible By developing a material that allows the direct production of fuel within one process, the new approach developed by Ivo Alxneit and his colleagues dispenses with the Fischer-Tropsch procedure and hence also with the second step. This was accomplished by adding small amounts of rhodium to the cerium oxide. It has been known for some time that rhodium permits reactions with hydrogen, carbon monoxide and carbon dioxide. The resulting fuels are either used or stored and the cyclical process begins again once the cerium oxide is re-activated. “So far, our combined process only delivers small amounts of directly usable fuel,” concludes Alxneit. “But we have shown that our idea works and it’s taken us from the realms of science fiction to reality.”

CHEMWEEKLY 26JUL16