Surface differential diffraction

Lee, D., Ndieyira, J.W. and Rayment, T. (2002) In situ time-resolved structural study of an electrode process by surface differential diffraction, in Spectroscopic tools for the analysis of electrochemical systems (ed. J. McBreen), Electrochemical Society Proceedings, vol. 99 (15),... 

The scattered X-ray from crystal planes or a thin metal film can interfere coherently with scattered X-rays from an adsorbate layer. This phenomenon is called surface differential diffraction SDD or Bragg peak interference. The theory has been described elsewhere in detail [40, 41]. Erom SDD measurements information on the relative amount of material in the adsorbate layer (coverage) and the distance between atoms in the top layer of the substrate and in the adsorbate layer as a function of electrode potential can be derived. 

X-ray diffraction patterns of powdered catalysts were recorded with a Rigaku RINT 1200 diffractometer using a radiation of Ni-filtered Cu-Ka. BET surface area and pore size distribution were calculated from the adsorption isotherm of N2 at 77 K. The BJH method was used for the latter. Aluminum content was determined by ICP spectrometer. FTIR spectra of adsorbed NH3 were recorded with a JASCO FT/IR-300 spectrometer. The self-supporting wafer was evacuated at prescribed temperatures, and 25 Torr of NH3 was loaded at 473 K. After NH3 was allowed to equilibrate with the wafer for 30 min, non-adsorbed NH3 was evacuated and a spectrum was collected at 473 K. The differential heat of adsorption of NH3 was measured with a Tokyo-riko HTC-450. The catalyst was pretreated in the presence of 100 Torr oxygen and evacuated at 873 K. The measurements were run at 473 K. 

Polycrystalline Ni-Sb alloys, unsupported Sb-loaded Ni powders, artificially contaminated as well as equilibrium FCC (from a heavy oil cracker) have been studied using X-ray diffraction (XRD), Auger spectroscopy and x-ray photoelectron spectroscopy (XPS) by workers at Phillips in what was one of the first examples of a multi-characterization technique approach to FCC study (39,40). XPS results have indicated that in an equilibrium FCC, 50 to 80% of the surface Sb and 30% to 50% of the surface Ni could be reduced to metal. This XPS data suggested that Ni and Sb were present on at least two different sites (that can be differentiated by their reducibility at 500°C/H2) and that reduction caused a twofold increase in the Ni/Sb ratio due to greater Ni dispersion. 

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