In this paper we report the fabrication of model catalysts prepared to understand the structure of the BaO surface. This utilises the 'inverse' catalyst method, that is, the oxide layer is fabricated onto the top of a metal single crystal surface. We show that we can atomically resolve the surface structure of BaO(111) and that it presents a (2·2) reconstruction at its surface. Under other dosing conditions we can produce a layer which is metastable at 573K, which we believe to be the peroxide, BaO 2 . We have shown that the BaO layer can store NO x from a mix of NO and oxygen, even under the extremely low exposure conditions of UHV, proving that the NOx storage process is a facile one. The results indicate that it is not necessary to have NO2 in the gas phase in order to store NO x .KEY WORDS: NSR; NOx storage and reduction; SCR; STM; model catalysts; BaO model catalysts.NSR (NOx storage and reduction) catalysis is an important strategy to aid in the removal of pollutants from lean-burn type engines [1,2]. There is a considerable amount of work relating to this process in reactors and from infra-red measurements [3,4], but little which is devoted to the resolution of these processes at the ultra-nanoscale. To that end we here report on the first studies related to NSR using STM.Our STM has been described in detail elsewhere [5], but for brevity, suffice it to say that it has the capability of achieving atomic resolution, but also a near-unique capability of imaging at high temperature (up to 1000K), with relatively little thermal drift. It also has facilities for surface analysis (Auger electron spectroscopy) and surface cleaning, sputtering and gas treatment. All of the work below was carried out under ultrahigh vacuum conditions to ensure the purity of the surfaces studied and of the gases dosed.The strategy here is to attempt to make inverse model catalysts by depositing BaO onto the surface of Pt(111). The reason for using this strategy is that STM relies on having conductivity in the imaged material in order to obtain a tunnelling current. It has been shown that thin layers of wide band gap materials (even alumina [6,7]) are suitable for imaging, provided they are fabricated onto conducting support materials, usually single crystal metals [8]. Figure 1a and 2 shows two images obtained after the adsorption of Ba, followed by oxidation under different conditions. In Figure 1a we believe we have formed the BaO(111) surface [9], whereas in figure 1b we propose that BaO 2 is formed [10]. The reason for these assignments is that the structure in 1a) is thermally stable, and the atomic spacing corresponds with that expected for the BaO(111) surface with a (2 · 2) reconstruction. Note that the (111) BaO-(1 · 1) surface is polar and unstable and is therefore not expected to form, whereas theory predicts a (2 · 2) reconstruction for this surface, producing a near-neutral layer with low surface energy, figure 1b [9]. Other structures are identified under varying oxygen treatment conditions, which may be further recon...

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