Mar
22
New Chromium Catalyst Can Form Ammonia From N2
March 22, 2016 | 1 Comment
Pacific Northwest National Laboratory (PNNL) scientists can now show how chromium forms ammonia from dinitrogen (N2) to make fuels and fertilizer. This work is a critical step in controlling a reaction that could store electrons from intermittent wind and solar stations in use-any-time fuels.
Scientists designed, created, and tested a chromium (Cr) complex, finding that a novel phosphorus-containing ring structure helps chromium turn dinitrogen and acid into ammonia. This work is part of efforts to develop molecular complexes to control electrons and protons for use in turning renewable energy into storable fuels. Artwork Image Credit: Jonathan Darmon, PNNL. Click image for the largest view.
Chromium always had a large potential compared to its heavier metal counterparts, and failed for more than 30 years to turn nitrogen gas into ammonia, a reaction that involves breaking one tough bond and making six new ones. The PNNL scientists at the Center for Molecular Electrocatalysis thought chromium was up to the job; it just needed a little support.
At the center, one of Department of Energy’s Energy Frontier Research Centers (EFRC), the scientists created a 12-atom ring structure called a ligand that partially surrounds the metal and offers a stable environment for the metal to drive the reaction. By creating this ligand structure, the team demonstrated the importance of the environment supporting chromium. Often a key to controlling metal reactivity, the structure encircling the chromium causes the normally unreactive dinitrogen to become more reactive when it binds to the metal.
Study leader Dr. Michael Mock said, “This research required the synergy of experimental and computational efforts in an EFRC. Studying this challenging reaction has benefited from the multiple years of funding that an EFRC enables.” The EFRCs are funded by the Office of Basic Energy Sciences at Department of Energy’s Office of Science.
Producing ammonia for fertilizer consumes vast quantities of natural gas to acquire the hydrogen, an issue that this work may one day help solve. However, this study is focused on another important challenge: storing intermittent wind and solar energy. Solar panels and wind turbines produce electrons that flow along power lines to energize appliances in our homes. But, solar power levels drop when clouds form over head. What if those electrons could be stored inside a chemical bond, as an energy-dense storage option? This study, which is complemented by two previous reports focused on understanding dinitrogen reactivity with chromium, may someday lead to the development of a system with this common metal as a hard-working catalyst.
Dr. Roger Rousseau, who led the computational studies said, “This research shows how important it is to move six electrons and six protons in the right order. It is rather like herding cats – and very difficult cats at that.”
There is a long tradition of turning dinitrogen (N2) into ammonia (NH3) using complex molecular catalysts, materials that reduce the roadblocks to make the reaction occur and aren’t consumed in the process. Of the metals studied in the column known as group 6 transition metals, chromium supported by phosphorus ligands didn’t work. In fact, papers from 1970 to the present day reported failures using chromium even in an environment that was thought to goad it into working.
But Dr. Mock and his team focused on the stabilizing effect from the phosphorus atoms of a 12-membered ligand that partially surrounded the chromium metal. Every fourth atom in the ring is a phosphorus atom that forms a bond with the chromium atom. The chemical bonds formed with three phosphorus atoms of the large ring together with two additional phosphorus donor atoms of a second ligand make the chromium atom very electron rich, which then can bind the dinitrogen. Once bound, the dinitrogen triple bond is weakened by coordination to the metal.
The team showed that the correct surroundings enhance chromium’s ability to bind and activate dinitrogen. In fact, the dinitrogen molecule in this case is more activated than in similar complexes with the heavier metals, molybdenum and tungsten, which have similar properties to chromium.
However, breaking the dinitrogen triple bond is still a delicate task.
The team found that managing the number of phosphorus atoms and the electron donating ability of these atoms was crucial. The team ran the reactions with acid at 50°C so that certain intermediate products containing nitrogen-hydrogen bonds didn’t fall apart. In these reactions, hydrogen ions from the acid surrounding the complex formed only a small amount of ammonia. They showed that adding acid caused the protons to favor binding with the metal, an unwanted connection. Additional optimization of the chromium complex and the conditions is required to control the formation of the desired nitrogen-hydrogen bonds.
The reaction still has secrets to reveal. The team is digging into two of them. First, how do the 12-membered rings that support the chromium form? In the experiments, the rings self-assemble around the chromium. What factors dictate that formation? Also, how can the protons be controlled to prevent them from binding to the electron-rich chromium and form additional bonds with nitrogen? Answering these questions could lead to learning how to control the reaction’s environs and lead to a catalyst that is fast, efficient, and long lasting, to convert the atmosphere’s dinitrogen to ammonia for the soil and fuels.
There is quite a way to go for a commercial and scaleable catalyst. On the other hand today there is a seemingly huge supply of natural gas. That supply is aggressively being burned away substituting for coal in power generation, consumed in mountains of ammonia fertilizer, lakes of anhydrous ammonia fertilizer and now exported chilled deep cold to liquid form. That huge supply isn’t going to last and the price will rocket up. All driven by short sighted government policy.
In an economy where government policy drives costs and choices in resources with little regard for the consequences to people and business, this technology can not come fast enough. Natural gas is after all, primarily used for home heating and industrial production.
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Congratulations to Dr. Michael Mock and his team for their brilliant work. This technology will have many other useful applications, and opens the way for catalyst research using other transition metals.