Photoelectron diffraction determination of adsorbate structures

Abstract

Scanned-energy mode photoelectron diffraction (PhD) is a well-known method to determine quantitatively the local structure of adsorbates at surfaces. In this thesis, it has been employed to determine the adsorption site of a selection of molecules on surfaces. The adsorption on Cu(110), of methoxy (CH3O), an intermediate in the catalytic decomposition of methanol (CH3OH), has been studied. O 1s PhD spectra show the strongest modulation at 30° and 40° polar emission angles, both in the [1 1 0] azimuth, which is consistent with a bridge position adsorption site in the [1 1 0] azimuth. The subsequent analysis, as well as parallel DFT studies, confirms two bridge adsorption sites, with different bond lengths to the underneath copper atoms. A tilt of the molecules of 37° in the [1 1 0] azimuth is also observed, with the carbon atoms pointing in opposite directions for every adsorption site. This tilt creates a zig-zag model, which fits with an old STM [1] study. Formate (HCOO), a surface intermediate of the catalytic decomposition of formic acid (HCOOH), has been studied on two different faces of copper, Cu(110) and Cu(111). Although the adsorption sites obtained for both surfaces is similar, namely a short-bridge site slightly off atop, a significant difference of = 0.1 Å in the copper-oxygen bond lengths is found, being 1.99 Å for Cu(111) and 1.90 Å for Cu(110). In this thesis, it is demonstrated that it is possible, though very challenging, to perform PhD successfully under higher pressures. Methanol oxidation on Cu(110) has been studied under reaction conditions. At temperatures below = 450 K, the adsorption sites of methoxy and formate, the most important surface intermediates of this reaction, have been proved to be similar as in the previous studies performed in ultra high vacuum. A recent investigation of two different organic molecules, azobenzene (C12H10N2) and aniline (C6H7N), on rutile TiO2(110) and anatase TiO2(101) surfaces with scanning tunneling microscopy (STM) [2] indicates that both molecules lead the formation of the same superstructure, believed to be of a common species, phenyl imide (C6H5N). PhD has been exploited to determine the local adsorption site of adsorbed species formed by both molecules on rutile TiO2(110). N 1s photoelectron diffraction data are almost identical for both molecules, providing further support for a common surface species with the same, or a closely similar. Additional NEXAFS results support these results, implying that the local adsorption site of azobenzene and aniline is indeed the same. PhD results, which show the largest modulation amplitude at normal emission, suggests that the phenyl imide bonds via the N atoms atop a five-fold coordinated surface Ti atom, with the molecular plane tilted with respect to the surface normal, with a N-Ti bond length of 1.77 Å. [1] F. Leibsle, S. Francis, S.Haq, and M. Bowker, Aspects of formaldehyde synthesis on Cu(110) as studied by STM, Surf. Sci. 318, 46 (1994). [2] S.-C. Li, and U. Diebold, Reactivity of TiO2 rutile and anatase surfaces toward nitroaromatics, J. Am. Chem. Soc. 134, 64 (2010)