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Project Progress

In addition to the FTIR spectroscopic and SEM imaging techniques we have extended the characterization techniques of surface species to multi-edge X-ray spectroscopy. This emerging spectroscopic technique uses high intensity, tunable X-rays from both the traditional soft (<1 keV) and hard (> a few keV) X-rays, as well as the intermediate ‘tender’ X-ray energy range to obtain complementary structural information about the structure of a given absorber (Fe or S). Furthermore, we exploited the benefits of using various detection techniques that allowed us to gain insights into the depth profile of the molecular beam exposed pyrite samples. After exposure to D/Ar beam, the pyrite samples were removed from the vacuum chambers and exposed to air. The unavoidable oxidation of the top surface turned out to be beneficial due to formation of a protective cap for preserving the catalytically active partially reduced Fe-S layer.

High energy (7 keV) Fe K-edge, intermediate energy (3 keV) S K-edge, and low energy 0.8 keV Fe L-edge X-ray absorption spectroscopic measurements together revealed an intriguing layered structure of the pyrite samples created in D/Ar beam surface scattering experiments. As expected upon exposure to air the sulfur depleted surface will adopt the same structure and composition as the surface of metallic iron powder. Beneath this protective, passivating layer is a novel Fe-S phase that shows spectral features consistent with formally Fe(I) oxidation state surrounded with sulfide (S^2-) ligands. This layer sits on the bulk pyrite with Fe(II) and persulfide (S₂^2-) ligands. We have collected complementary data from both the Fe and S points of view that unambiguously support the above assignments. The presence of the tentatively assigned Fe(I)2S phase next to a Fe(II)S₂ is relevant to the catalytically active site of the hydrogenase enzyme with Fe(I) and Fe(II) centers linked by sulfur atoms.