Surface investigations

Under catalytic reaction conditions, one should not necessarily expect species to proceed to the thermodynamic final state. An additional complication comes from the fact that the redox properties of catalytically active ceria and of ceria-zirconia mixed oxides appear to be quite different from the bulk thermodynamic values for ceria [37,38], For example, ceria films calcined above 1270 K no longer promote the WG8 [22] or steam-reforming reactions [20] and are much more difficult to reduce upon heating in vacuum [39]. These observations appear to be explained by calorimetric studies, which have shown that the heat of reoxidation for reduced Pd/ceria and Pd/ceria-zirconia catalysts is much lower than bulk thermodynamics would suggest [38]. Therefore, bulk thermodynamic information may not be entirely relevant for describing the nature of sulfur-containing species on catalytically active materials. 

There is surprisingly little information on the nature of species formed by contacting ceria with H28 or SO2 at the more moderate temperatures and pressures of the automotive exhaust environment. 8ome of the more comprehensive studies include those performed by Lavalley and coworkers [40-43], using primarily IR 

It is interesting to consider which factors are important in the formation of cerium sulfates. The presence or absence of Pt had little effect, showing that ceria is able to oxidize SO2 without an additional catalyst [42]. Furthermore, the oxidation of SO2 to S04 occurs on ceria to some extent without the addition of gas-phase O2, obviously with the simultaneous reduction of ceria [40]. Finally, bulk sulfates were only formed when the ceria samples were exposed to SO2 at temperatures above 250°C. 

All of this agrees with chemistry that can be inferred from data on Ce02 as an adsorbent for HjS. For adsorption at 700°C, Ce02 is completely converted to 02028 [31]. There are also indications that SO2 can react to form oxysulfides at room temperature on reduced ceria in ultra-high-vacuum experiments [46]. Interestingly, 62028 is oxidized back to e02 or 6303 upon exposure to 8O2 at 600° , producing 8O2 [31]. 

The above discussion examined results aimed at understanding the final products obtained after high-temperature adsorption or reduction. In order to 

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Brief description of new possibilities for surface investigations with higlily brilliant synchrotron x-ray sources. 

The infoiination obtained from an application of IR spectroscopy to a surface investigation includes the molecular structure, orientation, chemical reaction, conformation, crystallinity, and so on. 

X-ray reflectometry (XR) has already been described in Sect. 2.1 as a technique for polymer surface investigations. If a suitable contrast between components is present buried interfaces may also be investigated (Fig. 4d) [44,61,62]. The contrast is determined by the difference in electron density between materials. It is, in the case of interfaces between polymers, only achieved if one component contains heavy atoms (chlorine, bromine, metals, etc.). Alternatively the location of the interface may be determined by the deposition of heavy markers at the interface. 

X-ray scattering studies at a renewed pc-Ag/electrolyte interface366,823 provide evidence for assuming that fast relaxation and diffu-sional processes are probable at a renewed Sn + Pb alloy surface. Investigations by secondary-ion mass spectroscopy (SIMS) of the Pb concentration profile in a thin Sn + Pb alloy surface layer show that the concentration penetration depth in the solid phase is on the order of 0.2 pm, which leads to an estimate of a surface diffusion coefficient for Pb atoms in the Sn + Pb alloy surface layer on the order of 10"13 to lCT12 cm2 s i 820 ( p,emicai analysis by electron spectroscopy for chemical analysis (ESCA) and Auger ofjust-renewed Sn + Pb alloy surfaces in a vacuum confirms that enrichment with Pb of the surface layer is probable.810... 

Benninghoven, A., Surface Investigation of Solids by the Static Mmethod of Secondary Ion Mass Spectroscopy (SIMS), Surf Set., Vol. 35,1973, pp. 427-457. 

To verify that steady state catalytic activity had been achieved, the catalyst was allowed to operate uninterrupted for approximately 8 hours. The catalyst was then removed from the reactor and the surface investigated by XPS. The results are shown in Figure 2c. The two major changes in the XPS spectrun were a shift in the iron 2p line to 706.9 eV and a new carbon Is line centered at 283.3 eV. This combination of iron and carbon lines indicates the formation of an iron carbide phase within the XPS sampling volume.(J) In fact after extended operation, XRD of the iron sample indicated that the bulk had been converted to FecC2 commonly referred to as the Hagg carbide.(2) It appears that the bulk and surface are fully carbided under differential reaction conditions. 

Fuyukui, M., Watanabe, K. and Matsumoto, Y. (2006) Coherent surface phonon dynamics at K-coverd Pt(lll) surface investigated by time-resolved second harmonic generation. Phys. Rev. B, 74, 195412. 

The complexity and inhomogenicity of catalytic sites of metals and metal oxides make it difficult to interpret the mechanism of catalytic reactions on solid surfaces. Investigations that may lead to a better characterization of adsorbed species on catalytic sites could add much to our understanding of heterogeneous catalysis. 

Figure 3.28 Positive-going sweep of the voltammograms of Pt(lll) and Pt m(I11)x(1Q0) surfaces investigated with n = 2 — 9, 12, 14, 20, 26 and 40 in O.SM ll2S04. The sweep rate for all the experiments was 20mVs Displacements of the curves are arbitrary. The inset shows the evolution of the position of the step adsorption state for low values of n. After Parsons and...

The data in the Figs. 9.1,9.2 and 9.4 nicely illustrate the complementarity of XPS and SIMS and the possibilities that thin film oxide supports offer for surface investigations. Owing to the conducting properties of the support, charging is virtually absent and typical single crystal techniques such as monochromatic XPS and static SIMS can be applied to their full potential to answer questions on the preparation of supported catalysts. 

Figure A plot of the logarithm of the time corresponding to the maximum on the current transients plotted against the final potential for the three single crystal surfaces investigated.

For most of the surfaces from the [lio] zone, Fig. 2c, hydrogen adsorption is predominantly associated with the single peak occurring at almost the same potential for all the surfaces investigated. That peak, in the potential region... 

Joo, S.-W., Kim, W.-J., Yoon, W.S. and Choi, I.S. (2003) Adsorption of 4,4 -biphenyl diisocyanide on gold nanoparticle surfaces investigated by surface-enhanced Raman scattering. 

Pluchery, O., Chabal, Y. J. and Opila, R. L. Wet chemical cleaning of InP surfaces investigated by in situ and ex situ infrared spectroscopy. Journal of Applied Physics 94, 2707 (2003). 

Model Catalyst Surfaces Investigated by Scanning Tunneling Microscopy... 

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