Novel photochemical ammonia synthesis

22 April 2016 - The biohybrids are coming

AmmoniaSynthBrown2016.PNGIn nature, bacteria convert nitrogen to ammonia using a iron-molybdenum cofactor (FeMoCo) and a lot of ATP. As ammonia is valuable, chemists have been working on a synthetic ammonia synthesis process for ages that thus far has only yielded the brute-force high-temperature high pressure Haber process. A new effort by the King team at the National Renewable Energy Laboratory reported in Science (Brown et al. DOI) describes a photochemical version. In it FeMoCo stays but electron-donor ATP is replaced by cadmium sulfide nanocrystals. When exposed to light this material can deliver electrons just as well.

For this study the bacterium Azotobacter vinelandii produced the required quantities of FeMoco. CdS nanorods were produced in a complex solid-state high temperature synthesis with calcium oxide, octadecylphosphonic acid, trioctylphosphine oxide, hexamethyldisilathiane and tributylphosphine. A mixture of CdS nanocrystals and FeMoCo with add buffer HEPES was irradiated at 405 nm and ammonia generation measured: TOF = 75 per minute. This compares well with the biochemical ATP process with a TOF of 119 per minute. The reported quantum yield is 3.3%. The usual FeMoCo inhibitors (CO, acetylene) also inhibited the photochemical process. Downside is that HEPES is a sacrificial catalyst in this system. The authors note that other more qualified reducing agents exist but the CdS nanorods deliver electrons more rapidly. This prevents the competing process of hydrogen generation before nitrogen binds.