Supports surface composition

A few industrial catalysts have simple compositions, but the typical catalyst is a complex composite made up of several components, illustrated schematically in Figure 9 by a catalyst for ethylene oxidation. Often it consists largely of a porous support or carrier, with the catalyticaHy active components dispersed on the support surface. For example, petroleum refining catalysts used for reforming of naphtha have about 1 wt% Pt and Re on the surface of a transition alumina such as y-Al203 that has a surface area of several hundred square meters per gram. The expensive metal is dispersed as minute particles or clusters so that a large fraction of the atoms are exposed at the surface and accessible to reactants (see Catalysts, supported). 

A different type of low friction or low drag application is encountered with sliding doors or conveyor belts sliding on support surfaces. In applications like this the normal forces are generally quite small and the friction load problems are of the sticking variety. Some plastics exhibit excellent track surfaces for this type of application. TFEs have the lowest coefficient of any solid material and represent one of the most slippery surfaces known. The major problem with TFE is that its abrasion resistance is low so that most of the applications utilize filled compositions with ceramic filler materials to improve the abrasion resistance. 

The Effect of Support-Metal Precursor Interactions on the Surface Composition of Supported Bimetallic Clusters... 

The effect of precursor-support interactions on the surface composition of supported bimetallic clusters has been studied. In contrast to Pt-Ru bimetallic clusters, silica-supported Ru-Rh and Ru-Ir bimetallic clusters showed no surface enrichment in either metal. Metal particle nucleation in the case of the Pt-Ru bimetallic clusters is suggested to occtir by a mechanism in which the relatively mobile Pt phase is deposited atop a Ru core during reduction. On the other hand, Ru and Rh, which exhibit rather similar precursor support interactions, have similar surface mobilities and do not, therefore, nucleate preferentially in a cherry model configuration. The existence of true bimetallic clusters having mixed metal surface sites is verified using the formation of methane as a catalytic probe. An ensemble requirement of four adjacent Ru surface sites is suggested. 

It has generally been assumed that the most important consideration in the surface enrichment of one metal in preference to another in a supported bimetallic cluster is based on differences in the enthalpies of sublimation of the metals which comprise the cluster. In most cases, the surface composition is enriched in the metal having the lower enthalpy of sublimation (1 ). 

The role played by the support of influencing the surface composition of supported bimetallic clusters has only recently begun to receive some attention. Miura, a ( ) have shown that the nature of the support can play an important role in determining not only the surface composition of the supported bimetallic clusters but also the morphology of the particles. For silica-supported Pt-Ru... 

Surface Composition Measurements. The surface composition and metal dispersion for a series of silica (Cab-O-Sll) supported Ru-Rh bimetallic clusters are summarized In Table I. Surface enrichment In Rh, the element with the lower heat of sublimation, was not observed over the entire bimetallic composition range. In fact, to within the experimental limit of error of the measurements, surface compositions and catalyst compositions were nearly equal. A small local maximum In the dispersion was observed for the catalyst having a surface composition of 50% Rh. 

Table I. Catalyst Composition and Surface Composition for Silica-Supported Rh-Ru Bimetallic Catalysts...

The results of this study suggest that the dynamics of the nucleatlon process are of the utmost Importance In determining the structure and the surface composition of supported bimetallic clusters. Because the surface mobility of the metal phase during pretreatment is strongly influenced by the nature of the precursor-support Interaction, it is useful to consider this Interaction in some detail. 

While our discussion will mainly focus on sifica, other oxide materials can also be used, and they need to be characterized with the same rigorous approach. For example, in the case of meso- and microporous materials such as zeolites, SBA-15, or MCM materials, the pore size, pore distribution, surface composition, and the inner and outer surface areas need to be measured since they can affect the grafting step (and the chemistry thereafter) [5-7]. Some oxides such as alumina or silica-alumina contain Lewis acid centres/sites, which can also participate in the reactivity of the support and the grafted species. These sites need to be characterized and quantified this is typically carried out by using molecular probes (Lewis bases) such as pyridine [8,9],... 

As it can be observed in Table 13.1, Ir supported over pure oxides exhibited low acidity, but Ir supported on mixed Nb20s-Si02 displayed an important enhancement in the surface acidity with surface coverage by niobia increases. Binding energies (BE) of core-level electrons and metal surface composition were obtained from XP spectra. The BE values of Si 2p, Ti 2p3/2, Nb 3ds/2 were 103.4, 458.5 and 123 eV respectively, which are exactly the expected values considering the presence of oxides of Si (IV), Ti (IV) and Nb (V). With regard to Ir 4f7/2 core level, a... 

Electrochemical processes are always heterogeneous and confined to the electrochemical interface between a solid electrode and a liquid electrolyte (in this chapter always aqueous). The knowledge of the actual composition of the electrode surface, of its electronic and geometric structure, is of particular importance when interpreting electrochemical experiments. This information cannot be obtained by classical electrochemical techniques. Monitoring the surface composition before, during and after electrochemical reactions will support the mechanism derived for the process. This is of course true for any surface sensitive spectroscopy. Each technique, however, has its own spectrum of information and only a combination of different surface spectroscopies and electrochemical experiments will come up with an almost complete picture of the electrochemical interface. XPS is just one of these techniques. 

It is a matter of speculation as to whether or not the activity would pass through a significant maximum at a surface composition between 0 and 30% Rh. It is interesting to note in this connection that the magnetic susceptibility (156, 157) and the electronic specific heat coefficient (156) increase from low values at 60% Ag-Pd through pure palladium and reach a maximum at - 5% Rh-Pd, thereafter decreasing smoothly to pure rhodium. Activity maxima have also been reported for reduced mixed oxides and supported alloys of group VIII metal pairs. For example, in the... 

A question which has occupied many catalytic scientists is whether the active site in methanol synthesis consists exclusively of reduced copper atoms or contains copper ions [57,58]. The results of Szanyi and Goodman suggest that ions may be involved, as the preoxidized surface is more active than the initially reduced one. However, the activity of these single crystal surfaces expressed in turn over frequencies (i.e. the activity per Cu atom at the surface) is a few orders of magnitude lower than those of the commercial Cu/ZnO/ALO catalyst, indicating that support-induced effects play a role. Stabilization of ionic copper sites is a likely possibility. Returning to Auger spectroscopy, Fig. 3.26 illustrates how many surface scientists use the technique in a qualitative way to monitor the surface composition. 

In conclusion, AES is mainly used to study the elemental surface composition of conducting samples. It can be used to make chemical maps of heterogeneous surfaces and to study the vertical distribution of elements as a function of depth. The technique is of great importance in surface science and materials science, but less in the characterization of supported catalysts. 

The types of macrocycles most studied in which the active metal center is believed to be retained include Co, Fe, Ru porphyrins and related macrocycles. In these studies the optimal pyrolysis temperature is often reported to be between 400-800 °C. Above these temperatures, the active site begins to be destroyed, and activity decreases.49 An array of characterization techniques have been used to support these claims. XPS analysis has demonstrated that at the highest activity of samples, the surface composition of metal and nitrogen is also at its highest.78,96 Above the optimal treatment temperature, nitrogen and metal begin to disappear from the surface. Furthermore, Mossbauer spectroscopy and XAS have been used to... 

Two-phase particles ranging from 10 to 20 microns in size, supported on a graphite substrate, were observed in-situ in the UHV chamber of a scanning Auger microprobe. Both surface composition analysis and imaging of the particles could be undertaken. The preparation of the samples has been described in detail elsewhere. ... 

X-Ray Photoelectron Spectrometry. X-ray photoelectron spectrometry (XPS) was applied to analyses of the surface composition of polymer-stabilized metal nanoparticles, which was mentioned in the previous section. This is true in the case of bimetallic nanoparticles as well. In addition, the XPS data can support the structural analyses proposed by EXAFS, which often have considerably wide errors. Quantitative XPS data analyses can be carried out by using an intensity factor of each element. Since the photoelectron emitted by x-ray irradiation is measured in XPS, elements located near the surface can preferentially be detected. The quantitative analysis data of PVP-stabilized bimetallic nanoparticles at a 1/1 (mol/mol) ratio are collected in Table 9.1.1. For example, the composition of Pd and Pt near the surface of PVP-stabilized Pd/Pt bimetallic nanoparticles is calculated to be Pd/Pt = 2.06/1 (mol/ mol) by XPS as shown in Table 9.1.1, while the metal composition charged for the preparation is 1/1. Thus, Pd is preferentially detected, suggesting the Pd-shell structure. This result supports the Pt-core/Pd-shell structure. The similar consideration results in the Au-core/Pd-shell and Au-core/Pt-shell structure for PVP-stabilized Au/Pd and Au/Pt bimetallic nanoparticles, respectively (53). 

In dispersed-metal catalysts, the metal is dispersed into small particles, on the order of 5 to 500 A in diameter, which are generally located in the micropores (20-1000 A) of a high surface area support. This provides a large metal surface area per gram for high, easily measurable reaction rates, but hides much of the structural surface chemistry of the catalytic reaction. The surface structure of the small particles is unknown only their mean diameter can be measured and the pore structure could hide reactive intermediates from characterization. Some of the same difficulties also hold for thin films. However, we can accurately characterize and vary the surface structure of our single-crystal catalysts, and in our reactor the surface composition can also be readily measured both are prerequisites for the mechanistic study of the catalysis on the atomic scale. 

Anderson et al. (33) found that the surface composition can be a function of particle size in supported Pt-Cu and Rh-Ag alloys. Bartholomew and Boudart (17) did not find enrichment in highly dispersed Pt-Fe catalysts. 

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