Surface-sensitive techniques

The ideal technique for surface chemical analysis should provide a qualitative and quantitative elemental analysis of the first monolayer of the surface and identify the type of bonding at the surface. In addition, the measurement process should not alter the surface it should probe only a small area of the surface to resolve regions of inhomogeneity it should have nearuniform high sensitivity for all elements it should be suitable for any sample of interest and it should provide, or at least permit, a profile into the near-surface ( 1 xm) region of the sample. No one technique will satisfy all of these criteria. In some cases, it is not necessary that all be met, and one analytical technique can be chosen to answer all the questions raised. In other cases, the surface will have to be examined by more than one method here the complementary results obtained may lead to a more complete understanding of a problem than is possible with only one measurement. Section 4 will show examples of both types of investigation. 

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Xps is a surface sensitive technique as opposed to a bulk technique because electrons caimot travel very far in soHds without undergoing energy loss. Thus, even though the incident x-rays penetrate the sample up to relatively large depths, the depth from which the electron information is obtained is limited by the "escape depth" of the photoemitted electrons. This surface sensitivity of xps is quantitatively defined by the inelastic mean free path parameter which is given the symbol X. This parameter is defined to be the distance an electron travels before engaging in an interaction in which it experiences an energy loss. 

Then let us examine the rate relaxation time constant x, defined as the time required for the rate increase Ar to reach 63% of its steady state value. It is comparable, and this is a general observation, with the parameter 2FNq/I, (Fig. 4.13). This is the time required to form a monolayer of oxygen on a surface with Nq sites when oxygen is supplied in the form of 02 This observation provided the first evidence that NEMCA is due to an electrochemically controlled migration of ionic species from the solid electrolyte onto the catalyst surface,1,4,49 as proven in detail in Chapter 5 (section 5.2), where the same transient is viewed through the use of surface sensitive techniques. 

Because XPS is a surface sensitive technique, it recognizes how well particles are dispersed over a support. Figure 4.9 schematically shows two catalysts with the same quantity of supported particles but with different dispersions. When the particles are small, almost all atoms are at the surface, and the support is largely covered. In this case, XPS measures a high intensity Ip from the particles, but a relatively low intensity Is for the support. Consequently, the ratio Ip/Is is high. For poorly dispersed particles, Ip/Is is low. Thus, the XPS intensity ratio Ip/Is reflects the dispersion of a catalyst on the support. Several models have been reported that derive particle dispersions from XPS intensity ratios, frequently with success. Hence, XPS offers an alternative determination of dispersion for catalysts that are not accessible to investigation by the usual techniques used for particle size determination, such as electron microscopy and hydrogen chemisorption. 

Why are electron and ion spectroscopies generally surface-sensitive techniques when applied in the low-energy regime ... 

Secondary Ion Mass Spectroscopic Studies of Adsorption and Reaction at Metal Surfaces Correlations with Other Surface-Sensitive Techniques... 

The use and limitations of SIMS ion intensity distributions to provide quantitative and chemical state information for adsorption and reaction of small molecules at metal surfaces is discussed. We concentrate on well-defined surfaces where there is sufficient information on the adsorption system from other surface sensitive techniques to test the information content of SIMS. 

Experimentally, different structure- and surface-sensitive techniques such as in situ scanning tunnelling microscopy (STM), in situ X-ray diffraction (XRD), transition electron microscopy (TEM), and in situ infrared (IR) spectroscopy have been... 

In this chapter, we have summarized (recent) progress in the mechanistic understanding of the oxidation of carbon monoxide, formic acid, methanol, and ethanol on transition metal (primarily Pt) electrodes. We have emphasized the surface science approach employing well-defined electrode surfaces, i.e., single crystals, in combination with surface-sensitive techniques (FTIR and online OEMS), kinetic modeling and first-principles DFT calculations. 

Overall, this chapter aimed to emphasize and demonstrate the great potential of utilizing a multidisciplinary approach to bimetallic systems that combines computational methods with a number of highly sophisticated in situ and ex situ surface-sensitive techniques at electrified solid-liquid interfaces. Advances in the understanding of fundamental properties that govern catalytic processes at well-defined multimetallic... 

Optical second harmonic generation (SHG), which is the conversion of two photons of frequency u to a single photon of frequency 2co, is known to be an inherently surface-sensitive technique, because it requires a noncentrosymmetrical medium. At the interface between two centrosymmetrical media, such as the interface between two liquids, only the molecules which participate in the asymmetry of the interface will contribute to the SHG [18]. SHG has been used as an in-situ probe of chemisorption, molecular orientation, and... 

Table 7.83 lists the main characteristics of TLC-FAB-MS/LSIMS. A key difference between EI/CI and FAB/LSIMS/LD is the fact that sampling in FAB and LSIMS is from a specified location that corresponds to the impact footprint of the primary particle beam. The natural compatibility of FAB, LSIMS and LD with the direct mass-spectrometric analysis of TLC plates is readily apparent. Most mass-spectrometric measurements are destructive in nature, but FAB and LSIMS are surface-sensitive techniques in which the material actually consumed in the analysis is sputtered only from the top few microns of the sample spot. The underlying bulk is not affected, and can be used for further probing. The major limitation of TLC-FAB depends on the capability of the compounds to produce a good spectrum. 

XPS or ESCA (electron spectroscopy for chemical analysis) is a surface sensitive technique that only probes the outer atomic layers of a sample. It is very useful tool to study polymer surfaces [91]. An XPS spectrum is created by focusing a monochromatic beam of soft (low-energy) X-rays onto a surface. The X-rays cause electrons (photoelectrons) with characteristic energies to be ejected from an electronic core level. XPS, which may have a lateral resolution of ca. 1-10 pm, probes about the top 50 A of a surface. 

In materials investigations surface-sensitive techniques are of special interest. The major contribution of infrared spectroscopy to this field is internal reflection spectroscopy (IRS), often called the "attenuated total reflection" (ATR) technique. To describe theory and principle, electromagnetic wave theory must be apphed [33]. 

The nature of the final state depends upon the energy, hv, of the exciting photons. In X-ray photoelectron spectroscopy (XPS) the exciting photons are provided by sources such as A1 Ka (1,486 eV) or Mg Ka (1,253 eV) and excitation of the core electrons of the molecules is observed. In UV photoelectron spectroscopy (UPS), Hel (21.2eV) or Hell (40.8 eV) radiation is used and excitation from the valence region of the neutral molecule is observed. XPS and UPS are surface-sensitive techniques, which are capable of providing extremely useful information on the chemical nature of a surface or interface and, in the case of the XPS, the conformational state of the molecules at the surface [64]. 

Stipp S.L., Hochella M.F.Jr., Parks G.A., Leckie J.O. Cd2+ uptake by calcite, solid-state diffusion, and the formation of solid-solution Interface processes observed with near-surface sensitive techniques (XPS, LEED, and AES). Geochim Cosmochim Acta 1992 56 1941-1954. 

Time resolved ellipsometry is a surface sensitive technique that can be used to study the kinetics and mechanism of electrochemical deposition of photopolymer films. 

There is a wealth of information available on CO chemisorption over single-crystal and polycrystalline platinum surfaces under ultrahigh-vacuum conditions research efforts in this area have gained a significant momentum with the advent of various surface analysis techniques (e.g., 2-8). In contrast, CO chemisorption on supported platinum catalysts (e.g., 9, 10, 11) is less well understood, due primarily to the inapplicability of most surface-sensitive techniques and to the difficulties involved in characterizing supported metal surfaces. In particular, the effects of transport resistances on the rates of adsorption and desorption over supported catalysts have rarely been studied. 

Recently there has been a growing emphasis on the use of transient methods to study the mechanism and kinetics of catalytic reactions (16, 17, 18). These transient studies gained new impetus with the introduction of computer-controlled catalytic converters for automobile emission control (19) in this large-scale catalytic process the composition of the feedstream is oscillated as a result of a feedback control scheme, and the frequency response characteristics of the catalyst appear to play an important role (20). Preliminary studies (e.g., 15) indicate that the transient response of these catalysts is dominated by the relaxation of surface events, and thus it is necessary to use fast-response, surface-sensitive techniques in order to understand the catalyst s behavior under transient conditions. 

The development of surface-sensitive techniques. The classical electrochemical methods involve the measurement of potential and current. While these are extremely useful in the study of reaction rates and mechanisms, they give no information on the structure of the interface. A variety of surface-sensitive techniques has now been adapted to the electrochemical situation and applied to the investigation of electrode surface structure. 

Because XPS is a surface sensitive technique, it recognizes how well particles are dispersed over a support. Figure 3.8 schematically shows two catalysts with the... 

The experimental approach discussed in this article is, in contrast, particularly amenable to investigating solvent contributions to the interfacial properties 131. Species, which electrolyte solutions are composed of, are dosed in controlled amounts from the gas phase, in ultrahigh vacuum, onto clean metal substrates. Sticking is ensured, where necessary, by cooling the sample to sufficiently low temperature. Again surface-sensitive techniques can be used, to characterize microscopically the interaction of solvent molecules and ionic species with the solid surface. Even without further consideration such information is certainly most valuable. The ultimate goal in these studies, however, is to actually mimic structural elements of the interfacial region and to be able to assess the extent to which this may be achieved. 

In this chapter, we introduce some of the most common spectroscopies and methods available for the characterization of heterogeneous catalysts [3-13], These techniques can be broadly grouped according to the nature of the probes employed for excitation, including photons, electrons, ions, and neutrons, or, alternatively, according to the type of information they provide. Here we have chosen to group the main catalyst characterization techniques by using a combination of both criteria into structural, thermal, optical, and surface-sensitive techniques. We also focus on the characterization of real catalysts, and toward the end make brief reference to studies with model systems. Only the basics of each technique and a few examples of applications to catalyst characterization are provided, but more specialized references are included for those interested in a more in-depth discussion. 

As mentioned above, most modem surface-sensitive techniques operate under vacuum, and are often used for studies in model systems. Nevertheless, there have been recent attempts to extend that work to more relevant catalytic problems. Great advances have already been made to bridge the so-called pressure and materials gaps, that is, to address the issues related to the differences in catalytic behavior between small simple samples (often single crystals) in vacuum, and supported catalysts under higher (atmospheric) pressures [155-157], Nevertheless, more work is still needed. 

There are numerous surface-sensitive techniques that can be applied to the study of catalyst surfaces in fact, a complete treatment of these is beyond the scope of this discussion. Therefore, the reader is directed toward some excellent resources for a more complete discussion.1 26-29 Here, we aim only to introduce some of the more popular techniques as well as to familiarize the reader with the alphabet soup of surface-science acronyms that will be used below. 

In situ studies of catalytic reactions have also been a prime focus of our group. The high-pressure spectroscopic technique used in our research is polarization modulation IR reflection absorption spectroscopy (PM-IRAS). Like SFG, PM-IRAS is a highly surface-sensitive technique that yields vibrational information about adsorbed surface species. Unlike SFG, however, PM-IRAS... 

One of the primary advantages of AES is that it is a surface-sensitive technique. Auger electrons with energies between 50-100 V have mean free... 

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