Activity of iron pyrite towards low-temperature ammonia production

Abstract

In this work we report the characteristics of iron pyrite toward the production of ammonia at low temperatures under ultra-high vacuum conditions. We review (with additional unpublished details) our previous systematic study of nitrogen and hydrogen adsorption on single-crystal iron pyrite (FeS2) and summarise our earlier findings regarding the possibility of ammonia synthesis on this material. We also present new results concerning the adsorption of nitrogen and hydrogen on two related materials, namely molybdenum-treated iron pyrite surfaces and iron pyrite nanostructures deposited on a gold single-crystal. On the bare iron pyrite samples, ammonia is produced upon hydrogenation of preadsorbed N species at 230K, demonstrating that all hydrogenation steps are possible at low pressures and temperatures. Nitrogen adsorbs molecularly on FeS2{100} at low temperatures, desorbing at 130K, but does not adsorb dissociatively even at pressures up to 1bar. Adsorbed nitrogen species can, however, be obtained through exposure to excited nitrogen species. Hydrogen adsorbs on FeS2{100}, but only in the presence of an incandescent Ta filament. Recombinative desorption of H2 occurs at 225K and is accompanied by desorption of H2S at 260K. On the molybdenum-treated iron-pyrite, no appreciable Nads species were detected under the experimental conditions studied, and the same is true for iron pyrite nanostructures on Au{111}. We also provide further details of our efficient and reproducible method for preparing well-ordered stoichiometrically pure FeS2{100} suitable for surface science studies.