Catalyst, general surface measurements

The true intrinsic kinetic measurements require (1) negligible heat and mass transfer resistances by the fluids external to the catalyst (2) negligible intraparticle heat and mass transfer resistances and (3) that all catalyst surface be exposed to the reacting species. The choice of the reactor among the ones described in this section depends upon the nature of the reaction system and the type of the required kinetic data. Generally, the best way to determine the conditions where the reaction is controlled by the intrinsic kinetics is to obtain rate per unit catalyst surface area as a function of the stirrer speed. When the reaction is kinetically controlled, the rate will be independent of the stirrer speed. The intraparticle diffusional effects and flow uniformity (item 3, above) are determined by measuring the rates for various particle sizes and the catalyst volume, respectively. If the reaction rate per unit surface area is independent of stirrer speed, particle size, and catalyst volume, the measurements can be considered to be controlled by intrinsic kinetics. It is possible... 

Until Atkins and co-workers published that Cl atoms lowered the O2 desorption peak maximum temperature from 513 K (240 °C) to 481 K (208 °C) [3], there had been nothing published linking the effect of Cl atoms on any measurable physical property of the catalyst. Generally, the comments were nuanced along the lines that, because Cl atoms (which were considered to be on the surface of... 

In general, the solubility of an intermediate in the IL should be as low as possible (compared to the feed) to remove the intermediate from the IL phase that is in direct contact with the catalyst s surface to suppress further hydrogenation. The values, which were measured by the initial and final concentrations of COD, COE, or COA in the organic phase in contact with different ILs (Table 14.2), indicate that the feed COD (Kn.so C = bas a higher solubility in the IL [BMIM][0cS04] than the intermediate COE (fCN,50°c = 0.31). This was one of the reasons to choose this system for the initial investigations of the SCILL concept as this would increase the COE-selectivity if the catalyst is coated with an IL ( physical solvent effect ). 

The electrochemical active surface area (EASA) of fuel cell Pt-based catalysts could be measured by the electrochemical hydrogen adsorption/desorption method. For carbon supported Pt, Pt alloy, and other noble metals catalysts, the real surface area can be measured by the cyclic voltammetry method [55-59], which is based on the formation of a hydrogen monolayer electrochemically adsorbed on the catalyst s surface. Generally, the electrode for measurement is prepared by dropping catalyst ink on the surface of smooth platinum or glassy carbon substrate (e.g, a glassy carbon disk electrode or platinum disk electrode), followed by drying to form a catalyst film on the substrate. The catalyst ink is composed of catalyst powder, adhesive material (e.g., Nafion solution), and solvent. 

Physical adsorption is one of the important ways for the precise characterization of the surface structure of the catalyst. In the chemisorption, the interactive force between adsorption molecule and the solid surface is of the chemical affinity, which makes the chemical bond form between the adsorbed molecule and the solid surface. In general, they form covalent bond or coordinated bond containing enough parts of ion-bond on the metal surface, and obviously ionic bond on the surface of semiconductor oxide as well as some compounds, so chemisorption has significant selectivity. By the use of the selectivity of chemisorption, the surface area of metal components and the munber of active sites in the multi-component catalyst and supported catalyst can be measured. Thus a lot of useful information can be achieved. 

In ecent years the utility of extended X-ray absorption fine structure UXAFS) as a probe for the study of catalysts has been clearly demonstrated (1-17). Measurements of EXAFS are particularly valuable for very highly dispersed catalysts. Supported metal systems, in which small metal clusters or crystallites are commonly dispersed on a refractory oxide such as alumina or silica, are good examples of such catalysts. The ratio of surface atoms to total atoms in the metal clusters is generally high and may even approach unity in some cases. 

Atomic force microscopy (AFM) or, as it is also called, scanning force microscopy (SFM) is the most generally applicable member of the scanning probe family. It is based on the minute but detectable forces - order of magnitude nano-Newtons -between a sharp tip and atoms in the surface [39]. The tip is mounted on a flexible arm called a cantilever, and is positioned at a subnanometer distance from the surface. If the sample is scanned under the tip in the x-y plane, it feels the attractive or repulsive force from the surface atoms and hence is deflected in the z direction. Various methods exist to measure the deflection, as described by Sarid [40]. Before we describe equipment and applications to catalysts, we will briefly look at the theory behind AFM. 

It has been mentioned above that with nonuniform catalyst surfaces, as they mostly occur in practice, the above equations give merely an upper limit of the velocity although in such catalysts the true surface would be expected to be greater than the geometrical one. In nearly all the cases where a reaction has been measured on different catalysts, it has been found that fco is by no means a universal constant as would have to be expected from the simple theory. There is a regular connection between fco and the activation energy, of the general form ... 

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