Surface sensitivity catalysts

Two capabilities of ISS are important in applications to the analysis of ceramics. One of these is its surface sensitivity. Many catalyst systems use ceramics where the surface chemistry of the outer 50 A or less is extremely important to performance. Comparing the ratio of H and O to AI or Si is equally important for many systems involving bonding operations, such as ceramic detectors, thin films, and hydroxyapatite for medical purposes. 

Most of the published promotional kinetic studies have been performed on well defined (single crystal) surfaces. In many cases atmospheric or higher pressure reactors have been combined with a separate UHV analysis chamber for promoter dosing on the catalyst surface and for application of surface sensitive spectroscopic techniques (XPS, UPS, SIMS, STM etc.) for catalyst characterization. This attempts to bridge the pressure gap between UHV and real operating conditions. 

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. 

The dilemma is thus investigations of real catalysts under relevant conditions by in situ techniques give little information on the surface of the catalyst, while techniques that are surface sensitive can often only be applied on model surfaces under vacuum. Bridging the gap between UHV and high pressures and between the surfaces of single crystals and of real catalysts is therefore an important issue in catalysis. 

Surface Sensitivity. Using these Ideas, one can gather Information on a catalyst which can Indicate exactly which elements are on the surface and which are In second or deeper layers. To lllutstrate this Idea, consider MoS2 This material has a layered structure which has Mo sandwiched between sulfur layers. LEISS data reported elsewhere show that at low energies 600 eV) almost no Mo Is observed.(11) At higher energies,... 

Chapter 4 deals with several physical and chemical processes featuring various types of active particles to be detected by semiconductor sensors. The most important of them are recombination of atoms and radicals, pyrolysis of simple molecules on hot filaments, photolysis in gaseous phase and in absorbed layer as well as separate stages of several catalytic heterogeneous processes developing on oxides. In this case semiconductor adsorbents play a two-fold role they are acting botii as catalysts and as sensitive elements, i.e. sensors in respect to intermediate active particles appearing on the surface of catalyst in the course of development of catal rtic process. 

The development of experimental methods over the last 50 years has been at the forefront of new strategies that emerged, driven by the need to obtain molecular information relevant to the structure of catalyst surfaces and the dynamics of surface reactions. The ultimate aim was in sight with the atomic resolution that became available from STM, particularly when this was coupled with chemical information from surface-sensitive spectroscopies. 

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. 

Characterization techniques become surface sensitive if the particles or radiation to be detected come from the outer layers of the sample. Low energy electrons, ions and neutrals can only travel over distances between one and ten interatomic spacings in the solid state, implying that such particles coming off a catalyst reveal surface-specific information. The inherent disadvantage of the small mean free path is that measurements need to be carried out in vacuum, which conflicts with the wish to investigate catalysts under reaction conditions. 

Although valence band spectra probe those electrons that are involved in chemical bond formation, they are rarely used in studying catalysts. One reason is that all elements have valence electrons, which makes valence band spectra of multi-component systems difficult to sort out. A second reason is that the mean free path of photoelectrons from the valence band is at its maximum, implying that the chemical effects of for example chemisorption, which are limited to the outer surface layer, can hardly be distinguished from the dominating substrate signal. In this respect UPS, discussed later in this chapter, is much more surface sensitive and therefore better suited for adsorption studies. 

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... 

In conclusion, LEIS is extremely surface sensitive and suitable for determining outer layer compositions of catalysts and alloys, provided that proper calibration can be achieved. The shape of the background contains information about the vertical distribution of elements in the surface region. 

This chapter deals with the study of structural properties of catalysts and catalytic model surfaces by means of interference effects in scattered radiation. X-ray diffraction is one of the oldest and most frequently applied techniques in catalyst characterization. It is used to identify crystalline phases inside catalysts by means of lattice structural parameters, and to obtain an indication of particle size. Low energy electron diffraction is the surface sensitive analog of XRD, which, however, is only applicable to single crystal surfaces. LEED reveals the structure of surfaces and of ordered adsorbate layers. Both XRD and LEED depend on the constructive interference of radiation that is scattered by relatively large parts of the sample. As a consequence, these techniques require long-range order. 

The elemental composition, oxidation state, and coordination environment of species on surfaces can be determined by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) techniques. Both techniques have a penetration depth of 5-20 atomic layers. Especially XPS is commonly used in characterization of electrocatalysts. One common example is the identification and quantification of surface functional groups such as nitrogen species found on carbon-based catalysts.26-29 Secondary Ion Mass spectrometry (SIMS) and Ion Scattering Spectroscopy are alternatives which are more surface sensitive. They can provide information about the surface composition as well as the chemical bonding information from molecular clusters and have been used in characterization of cathode electrodes.30,31 They can also be used for depth profiling purposes. The quantification of the information, however, is rather difficult.32... 

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. 

Characterization of the Surfaces of Catalysts Measurements of the Density of Surface Faces for High Surface Area Supports. - It has always been a tenet of theories of catalysis that certain reactions will proceed at different rates on different surface planes of the same crystal. Experiments with metal single crystals have vindicated this view by showing that the rate of hydrogenolysis of ethane on a nickel surface will vary from one plane to another. In contrast the rate of methanation remains constant for the same planes.4 Because of this structure sensitivity of catalytic processes there is a requirement for methods of determining the number of each of the different planes which a catalyst and its support may expose at their surfaces. Electron microscopy studies of 5nm Pt particles supported upon graphite show them to be cubo-octahedra with surfaces bound by (111) and (100) planes.5 Similar studies of Pd and Pt prepared by evaporation reveal square pyramids of size 60-200 A bounded by incomplete (111) faces.6... 

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