Infrared Spectroscopy of Ammonia on Iron: Adsorption, Synthesis and the Influence of Oxygen P. Iyngaran, D.C. Madden, D.A. King and S.J. Jenkins ∗ Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK (Dated: October 5, 2017) We report on reflection absorption infrared spectroscopy (RAIRS) investigations into the influ- ence of oxygen on the surface chemistry of NH3 on Fe{111}. Pre-adsorption of oxygen is found to strengthen the interaction between ammonia and the surface, albeit at the lowest oxygen coverage examined the effect is only notable when the ammonia coverage becomes high. At higher oxygen coverage, the same effect is observed even with very low ammonia coverage. In cases where the oxy- gen overlayer is ordered, the effects are seen quite clearly, but for disordered overlayers the surface heterogeneity makes assignment of absorption features more difficult; nevertheless, comparison with the ordered examples allows us to identify the same underlying behaviour. When potassium is coad- sorbed with oxygen, the effect on ammonia adsorption is threefold, including potassium-dominated and oxygen-dominated features, alongside features that suggest either cancelling or absent influences from the atomic adsorbates. Synthesis of ammonia from nitrogen adatoms in 0.6 mbar H 2 shows clear evidence of very similar surface interactions. I. INTRODUCTION Ammonia synthesis via the Haber-Bosch process is one of the most significant chemical processes underpinning global economic and population stability. Essential for the production of artificial fertilisers and a vital nexus in the nitrogen value chain of the chemical industry, am- monia is responsible both for feeding the world and for providing many of its most sought after commercial ma- terial products. The synthesis reaction proceeds over an iron catalyst, which dissociatively adsorbs both nitrogen and hydrogen, allowing the resulting adatoms to react and hence form ammonia, which then desorbs 1 . Considerable effort has been devoted to understanding the details of ammonia synthesis over iron, initially via experiments conducted with high-area catalysts 2–7 and latterly with single-crystal samples 8–13 . The consensus that emerges from this body of work is that the ini- tial dissociative adsorption of nitrogen is generally the rate-limiting step, although self-poisoning can be domi- nant when the partial pressure of ammonia is high and the product blocks sites at which dissociation can occur. Potassium is commonly used as a promoter, acting both directly to facilitate nitrogen dissociative chemisorption and indirectly to enhance the rate of ammonia desorp- tion. This latter effect was seen very clearly in our previ- ous reflection absorption infrared spectroscopy (RAIRS) experiments, which demonstrate a measurable redshift in the ammonia umbrella mode frequency, indicative of a weaking in the ammonia–iron bond, due to coadsorp- tion with potassium 14 . Furthermore, we have also shown that potassium promotes the intermediate hydrogenation steps that sequentially convert N to NH to NH 2 to NH 3 on the surface, so the alkali metal is found to be beneficial at every stage in the ammonia synthesis mechanism 15 . In commercial reactors, however, the potassium pro- moter is not the only species added to the iron surface. In particular, the alkali metal is stabilised on the surface by the presence of oxygen, so that the working promoter is could arguably be considered more akin to a potassium oxide or sub-oxide than an array of separate adatoms. On the other hand, oxygen is generally considered to be a poison for the Haber-Bosch process 16 so its presence on the surface is potentially double-edged – stabilising the alkali metal promoter, but poisoning sites itself. Accordingly, we here investigate, via RAIRS, the role of oxygen in modifying the behaviour of ammonia on the Fe{111} surface, initially on its own and subsequently upon coadsorption with potassium. II. EXPERIMENTAL METHOD A. RAIRS Apparatus The experiments described below were carried out un- der ultra-high-vacuum (uhv) conditions, using apparatus described in detail elsewhere 17,18 . The main chamber is equipped for cleaning and characterisation of the sam- ple, boasting an ion gun, Auger spectrometer, mass spec- trometer, and low-energy electron diffraction facilities. A side chamber capable of operation up to atmospheric pressure is arranged so as to allow an infrared beam to strike the sample at grazing incidence for RAIRS exper- iments. The sample may be shuttled between the side and main chambers as necessary, by means of a mag- netic transfer rod. Spectra presented below are shown as a ratio against a reference spectrum obtained from the clean surface immediately before each set of experiments. The spectrometer is purged with dry nitrogen, to keep the beam path free of atmospheric molecules, but varia- tions in the quality of the purge subsequent to obtaining the reference spectrum give rise to spurious spikes in the range 1400-1800 cm −1 , corresponding predominantly to