Direct catalyst surface analysis

Controlled electrochemical experiments are designed to probe select aspects of the formic acid electrooxidation reaction as a function of material selection and/or experimental conditions. Unfortunately, the selected experimental technique employed imposes deviations from a complex three-dimensional catalyst layer used in an operational DFAFC and thus results in inconsistencies between techniques. Assuming the current-potential relationship is always directly correlated to Faraday s law for charge and CO2 production, the assessment techniques can be broken down into three general categories (1) indirect correlation, (2) desorbed product detection, and (3) direct catalyst surface analysis. 

As on previous occasions, the reader is reminded that no very extensive coverage of the literature is possible in a textbook such as this one and that the emphasis is primarily on principles and their illustration. Several monographs are available for more detailed information (see General References). Useful reviews are on future directions and anunonia synthesis [2], surface analysis [3], surface mechanisms [4], dynamics of surface reactions [5], single-crystal versus actual catalysts [6], oscillatory kinetics [7], fractals [8], surface electrochemistry [9], particle size effects [10], and supported metals [11, 12]. 

The withdrawn hquid-phase samples were analyzed with an HPLC (Biorad Aminex HPX-87C carbohydrate coluttm. 1.2 ttiM CaS04 in deionized water was used as a mobile phase, since calcium ions improve the resolution of lactobionic acid [17]). Dissolved metals were analysed by Direct Current Plasma (DCP). The catalysts were characterized by (nitrogen adsorption BET, XPS surface analysis, SEM-EDXA, hydrogen TPD and particle size analysis). 

Ion-beam thinning is usually used for dense bulk specimens where particular regions must be analyzed. It can be useful in AEM for thinning the same single crystals used in surface analysis to make direct comparisons with results from AES, XPS, etc. Ion-beam thinning can also be useful in analysis of interfaces and defects within bulk metallic catalysts such as Pt and Pd and their alloys. 

No precise information about the olefin polymerisation mechanism has been obtained from kinetic measurements in systems with heterogeneous catalysts analysis of kinetic data has not yet afforded consistent indications either concerning monomer adsorption on the catalyst surface or concerning the existence of two steps, i.e. monomer coordination and insertion of the coordinated monomer, in the polymerisation [scheme (2) in chapter 2], Note that, under suitable conditions, each step can be, in principle, the polymerisation rate determining step [241]. Furthermore, no % complexes have been directly identified in the polymerisation process. Indirect indications, however, may favour particular steps [242]. Actually, no general olefin polymerisation mechanism that may be operating in the presence of Ziegler-Natta catalysts exists, but rather the reaction pathway depends on the type of catalyst, the kind of monomer and the polymerisation conditions. 

The analysis of the effects of transport on catalysis has focused on a comparison of the availability of reacting species by diffusion to the rate of reaction on the catalytic sites. High-surface-area catalysts are usually porous. Comparison of transport to reaction rates has usually been based on Knudsen diffusion (by constricted collision with the pore walls) as the dominant mode of transport. DeBoer has noted that for small pores surface diffusion may dominate transport (192). Thiele modulus calculations may therefore not be valid if they are applied to systems where surface diffusion can be significant. This may mean that the direct participation of spillover species in catalysis becomes more important if the catalysts are more microporous. Generalized interpretations of catalyst effectiveness may need to be modified for systems where one of the reactants can spill over and diffuse across the catalyst surface. 

Although both the laboratory and industrial scale materials science of catalysts requires an integrated approach as already mentioned above, it is customary to classify the characterization methods by their objects and experimental tools used. I will use the object classification and direct the introductory comments to analysis, primarily elemental and molecular surface analysis, determination of geometric structure, approaches toward the determination of electronic structure, characterization by chemisorption and reaction studies, determination of pore structure, morphology, and texture, and, finally, the role of theory in interpreting the often complex characterization data as well as predicting reaction paths. 

High resolution transmission electron microscopy (TEM) (Jeol lOOCX) was employed to determine the size of the metal particles on the surface of the catalyst support, and the composition of individual metal particles was ascertained (for thin sections cut with an ultramicrotome) using a field-emission scaiming transmission electron microscope (STEM) (VG HB 501) (at 1.5 mm resolution) and an energy dispersive X-ray (EDX) analyser. The metal loading of catalysts was determined by ICP-AES (Spectro D), following dissolution in concentrated hydrochloric and sulphuric acids. Direct analysis of aqueous samples taken from the reaction medium, using the same analytical technique, allowed the corrosion of metallic components from the catalyst surface to be studied. 

The use of model systems amenable to detailed surface analysis provides a means for the direct examination of the association of lead wih the surface of noble metals [16], It immediately becomes apparent that in all the three supported noble metals the lead is directly associated with the noble metal sites and not with the support material, which in actual catalyst constitutes over 95% of the exposed BET area. This is shown o. Fig. 10 [16], for Pt supported on AI2O3, from the electron probe elemental maps. The Pt and Pb maps of samples exposed to simulated exhaust generated from combustion of iso-octane fuel containing 1.5 g Pb/gallon and 0.03 wt%S are exactly superimposed. The same obtains whether the support is 7-AI2O3, Ti02 or Zr02 on one hand or whether the metal is Pt, Rh or Pd. 

The effect of the reactant is basically a kinetic effect and is hence part of the kinetic analysis considered in Chapter 7. We confine our discussion in this section to the effect of solvent polarity, which has a direct influence on the performance of the catalyst. It is known (Thompson and Naipawer, 1973 Sehgal et al., 1975) that certain functional groups in a substrate can bind to the catalyst surface during a reaction in such a way as to enforce the addition of hydrogen from its own side of the molecule, which is opposite to that expected on the basis of steric hindrance, namely binding occurs from the opposite side. This effect, termed haptophilicity, can be exploited in selecting the right kind of solvent to enhance the selectivity to a desired product. 

A similar approach was used to validate the PGM kinetics data were collected over a PGM coated monolith and systematically compared with predictive model simulations. However, a different reactor model was used in this case. Indeed, simulation analysis of PGM coated monolith catalysts [20] pointed out that the extremely high reaction rates over this catalytic system result in full mass transfer control above 250 °C. For this reason, we assumed that in the case of washcoated monolith samples only the PGM washcoat surface was effectively active, thus we treated the PGM layer as a surface. The kinetics developed over the powdered PGM catalyst were thus referred to the catalyst surface and directly included in the PGM monolith model. 

---