By taking cues from two biological processes, researchers have made a catalytic material that converts nitrogen to ammonia when irradiated by white light (J. Am. Chem. Soc. 2015, DOI: 10.1021/ja512491v). The new strategy may one day help scientists achieve energy savings in various catalytic processes by capitalizing on abundant sunlight to produce valuable chemicals.
Manufacturers worldwide produce some 200 million tons of ammonia annually, mainly for use as fertilizer and for making nitrogen-containing compounds. The standard industrial process, the Haber-Bosch method, involves reacting nitrogen, which is relatively inert, with hydrogen at 400 °C and at a pressure roughly 250 times atmospheric pressure in the presence of an iron-based catalyst. It is, of course, highly energy intensive.
Nature also converts nitrogen to ammonia, albeit far more slowly, through a process known as nitrogen fixation. The reaction, which runs under much milder conditions, occurs in microbes containing nitrogenase enzymes. These catalysts tend to include a reactive cluster of iron, molybdenum, and sulfur.
In an effort to understand nature’s energy-efficient ways, researchers previously made synthetic analogs of these clusters. They found that a few of them can catalyze ammonia production from nitrogen under strongly reducing conditions.
A team of Northwestern University chemists, including Abhishek Banerjee and Mercouri G. Kanatzidis, has now demonstrated a potentially more useful catalyst: one that can be switched on and driven by light to mediate ammonia production at room temperature and ambient pressure. Dubbed a chalcogel, the material, which mimics some aspects of nitrogen fixation and photosynthesis, is a light-absorbing, porous, amorphous solid composed of Mo2Fe6S8 clusters linked by Sn2S6 ligands.
The team bubbled nitrogen through aqueous solutions containing the chalcogel, a proton source (pyridinium hydrochloride), and an electron donor (sodium ascorbate). They detected ammonia shortly after aiming a white light source at the gel and report that during irradiation the chemical’s concentration increased continuously.
Control tests show that solutions lacking the catalyst and those kept in the dark do not produce ammonia. The team acknowledges that the chalcogel evaluated in this study produces ammonia too slowly for industrial use but notes that the material remained stable with no loss of activity during a 72-hour test.
“This is extremely elegant work,” says University of Liverpool chemistry professor Matthew J. Rosseinsky. He notes that the study paves the way for further research projects to determine the catalyst’s active site structure and the role of the Sn2S6 ligands. Rosseinsky also wonders whether there is a relationship between the electronic structure of the cluster and a wavelength dependence of the catalytic activity.