AN INVESTIGATION OF THE PUTATIVE PHOTOSYNTHESIS OF AMMONIA ON IRON-DOPED TITANIA AND OTHER METAL-OXIDES

The reported heterogeneous photocatalytic synthesis of NH3 from N-2 and H2O in the presence of iron-doped TiO2 or other semiconducting metal oxides has been investigated. Skepticism about this thermodynamically and mechanistically daunting process is appropriate, and the results reported here support a skeptical position. To assess the reproducibility of the results obtained by previous workers, we performed a large number of experiments under various conditions similar to those employed with apparent success by others. We also carried out N-15 isotope labelling experiments in which the lower limit of detection was around 0.2-1 nmol of added (NH3)-N-15 in 10 mu mol of natural ammonia, i.e. two to three orders of magnitude below the levels reported in successful syntheses by others. All the catalysts tested promoted the photo-oxidation of ammonia under the conditions said to give ammonia photosynthesis. Photocatalytic nitrogen fixation was not found in any of the experiments, although in some cases results were obtained that could have been mistaken for nitrogen fixation. Our results suggest that the reported putative reaction does not occur. We recommend that any further research in this area should be based on standards of demonstration as rigorous as those applied in experiments on the biological fixation of nitrogen.