Oxide core-level shifts

First, the chemical shifts for Pt oxides and hydroxides are typically about 3 eV (for the stoichiometric compounds) and therefore significantly larger than the measured maximum shift (1.3eV for the Pt atom). Second, comparing the results to studies on commercial Pt catalysts, these systems exhibit an oxide core-level shift of about 2 eV, again larger than the observed values. 

Since most "surface-sensitive" techniques sample at least a few atomic planes into the sample, it is difficult to experimentally separate the electronic structure of the outermost plane of atoms from that of the planes below. Theoretical calculations are able to clearly separate surface from bulk electronic structure, of course it is common to calculate a separate electronic density-of-states for each plane in the crystal structure ("layer density-of-states"). Significant changes from the bulk electronic structure are sometimes found for the surface planes in calculations. However, it is difficult to confirm those results experimentally [1]. In some oxides, the bandgap at the surface has been observed to narrow compared to that of the bulk. The measured core-level binding energies of partially coordinated surface atoms are often shifted, by as much as an eV, from their bulk values [32] these are referred to as "surface core-level shifts". However, the experimental separation of surface from bulk electronic structure is at present far from satisfactory. 

The interaction of a Pd4 (C4v) cluster with the oxide surface was analyzed in more detail with the help of electron density difference plots and other theoretical tools, such as population analysis, core level shifts as well as induced and dynamic dipole moments [175]. Three interaction mechanisms were found to contribute to different extent metal polarization with the subsequent electrostatic attraction, Pauli repulsion, and covalent orbital interactions. Electrostatic interactions make up a sizeable fraction of the adhesion energy the polarization of the metal adsorbate by the surface electric field provides an important bonding mechanism. For the adsorption of Pd on-top or in the vicinity of the surface Mg " cations this electrostatic interaction accounts for almost the entire adsorption energy, albeit counteracted by Pauli repulsion. For adsorption on-top 0 , on the other hand, mixing of adsorbate and substrate orbitals becomes noticeable. This hybridization or covalent bonding at the interface with the oxide anions is complemented by electrostatic polarization. Further work is required to establish in a more quantitative way the relative importance of electrostatic and chemical bonding contributions. However, in line with our other studies of... 

Fig. 26. Si 2p core level shifts related to oxide formation (a) a clean silicon surface (b) adsorption of atomic oxygen (c) first indication of Si02 formation following exposure to excited oxygen (d) continued exposure of (c) to non-excited 02 (after Garner et al. [232]).

In order to investigate further on the structures and reactivity of surface oxide, DFT model calculations are carried out and their results are compared to the experimental results. Figure 9.13 shows the various surface oxide structures of test models and Table 9.1 displays their comparison to experimental results with the degrees of surface core-level shifts. From this comparison, the measured surface oxide in both AP-XPS and STM matches fairly well with a-Pt02-like nanoscale surface oxide. Finally, in order to find out the surface reactivity of surface oxide, CO titration... 

Core level shift Precise photoelectron binding energy peak positions of the peak centers denote the chemical oxidation and/or electronic states of orbitals from which the photoelectrons emanate. Minute variations in the binding energy arising from differing electronic environments of the ejected photoelectrons are manifested as shifts in the peak position (also referred to as chemical shifts). 

The relatively poor resolution of the XPS systems has lead to an extensive use of deconvolution techniques in order to prove the presence of shifted core levels of low intensity in the presence of unshifted levels (thin oxide layers on metal substrates). Deconvolution techniques should be used only in those cases where the presence of multi components is shown up by a shoulder in the intensity distribution. Interpretation of asymmetric peaks in terms of chemical shifts can be misleading in some cases because the asymmetry may change due to a change of the electron population at the Fermi level as was demonstrated for the metallic oxide Ir02 [23, 24],... 

X-ray photoelectron spectroscopy also provides information on the chemical composition of a surface. An incoming photon causes electrons to be emitted from atomic core levels, which are then analyzed as a function of kinetic energy. The shifts of these core-level energies provide information about the chemical environment surrounding the excited atom. This information also includes changes in the oxidation state of the sample. 

In the following section, we describe the case of adsorption of a Sn complex onto a palladium oxide suspension. In an alkaline medium (a basic PdO hydrosol), chlorides in the SnCL complex are substituted in the coordination sphere of tin(IV) by hydroxo anions, which are in excess, yielding the stannate Sn(OH)g complex. The Sn Mossbauer spectroscopy spectrum of a bimetallic sol (frozen in liquid nitrogen) is compared with a true stannic solution. At the same tin concentration, it shows the changes in the Sn environment due to adsorption onto the PdO surface (Fig. 13.27). The isomer shift S is found to be close to zero for the stannate solution and increases when contacted with the PdO suspension, indicating a modification of the coordination sphere of tin. The increase in 5 can be correlated to an increase in the core level electronic density of tin. The quadrupole splitting A, is related to a modification of the symmetry of the close environment of tin, due to adsorption of Sn(OH)g complexes onto the PdO colloidal nanoparticles. 

We shall not discuss here oxygen interaction with nickel since, although it is one of the most extensively studied systems (59), there is still considerable ambiguity and confusion regarding possible models. Two systems that, however, have lent themselves to investigation by XPS are the oxidation of lead (60, 60a) and aluminum surfaces (10). In both cases substantial shifts were observed in the metal core levels, which as emphasized previously is more the exception than the rule. 

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