Catalyst surface, accessibility

Fig. 42 O2 reduction kinetics at a dispersed Pt Catalyst in a PEFC cathode operating at 80 °C 100% RH (a). Pt utilization , that is, Pt catalyst surface accessed by protons and electrons was...

The stereoselectivity of this reaction depends on how the alkene approaches the catalyst surface As the molecular model m Figure 6 3 shows one of the methyl groups on the bridge carbon lies directly over the double bond and blocks that face from easy access to the catalyst The bottom face of the double bond is more exposed and both hydrogens are transferred from the catalyst surface to that face... 

Another complication arises when not all of the internal surface of a porous catalyst is accessed. Then a factor called the effectiveness T is apphed, making the power law equation, for instance,... 

Steric hindrance around an allylic function will diminish its hydrogenolysis as access of the function to the catalyst surface is impeded. Reduction of 5-methylthebaine (32) proceeds smoothly over Pd-on-C in ethanol at 1 atm to afford 5-methyldihydrothebaine (33), whereas reduction of thebaine itself is less clean and gives dihydrothebainol, dihydrothebainone, and dihydro-thebaine (/b). 

Catalytic reactors can roughly be classified as random and structured reactors. In random reactors, catalyst particles are located in a chaotic way in the reaction zone, no matter how carefully they are packed. It is not surprising that this results in a nonuniform fiow over the cross-section of the reaction zone, leading to a nonuniform access of reactants to the outer catalyst surface and, as a consequence, undesired concentration and temperature profiles. Not surprisingly, this leads, in general, to lower yield and selectivity. In structured reactors, the catalyst is of a well-defined spatial structure, which can be designed in more detail. The hydrodynamics can be simplified to essentially laminar, well-behaved uniform fiow, enabling full access of reactants to the catalytic surface at a low pressure drop. 

Catalyst layer architecture As a consequence of the diminishing remrns from ever higher dispersion, the effort to increase the active catalyst surface area per unit mass of Pt has centered in recent years primarily on optimization of catalyst layer properties, aiming to maximize catalyst utilization in fuel cell electrodes based on Pt catalyst particle sizes of 2-5 nm. High catalyst utilization is conditioned on access to the largest possible percentage of the total catalyst surface area embedded in a catalyst... 

Although carbon has many important qualities for a support material, it also appears to play a role in the access of the substrate to the active sites. Activated carbon preferentially adsorbs organic material from aqueous solutions. Thus the local concentration of reactants and products can be quite different at the catalyst surface than in the bulk solution. 

It ought to be verified, however, in all cases, that the experimental Q-9 curve truly represents the distribution of surface sites with respect to a given adsorbate under specified conditions. The definition of differential heats of adsorption [Eq. (39) 3 includes, in particular, the condition that the surface area of the adsorbent A remain unchanged during the experiment. The whole expanse of the catalyst surface must therefore be accessible to the gas molecules during the adsorption of all successive doses. The adsorption of the gas should not be limited by diffusion, either within the adsorbent layer (external diffusion) or in the pores (internal diffusion). Diffusion, in either case, restricts the accessibility to the adsorbent surface. 

For a solid-catalyzed gas-phase reaction, the catalyst is commonly in the form of particles or pellets of various possible shapes and sizes, and formed in various ways. Such particles are usually porous, and the interior surface accessible to the reacting species is usually much greater than the gross exterior surface. 

Molecular-level studies of mechanisms of proton and water transport in PEMs require quantum mechanical calculations these mechanisms determine the conductance of water-filled nanosized pathways in PEMs. Also at molecular to nanoscopic scale, elementary steps of molecular adsorption, surface diffusion, charge transfer, recombination, and desorption proceed on the surfaces of nanoscale catalyst particles these fundamental processes control the electrocatalytic activity of the accessible catalyst surface. Studies of stable conformations of supported nanoparticles as well as of the processes on their surface require density functional theory (DFT) calculations, molecular... 

Fouling Salt formation can build-up on the catalyst surface effectively limiting accessibility. Ammonium bisulfate can form at low temperatures. This foulant can be removed by increasing temperature and is considered a temporary poison. 

As compared to conventional petrochemicals, the significant hindrance of carbohydrates induces many diffusional limitations and activity of solid catalysts is obviously strictly governed by the accessibility of the catalytic sites. In this context, the porosity of commonly used siliceous-based catalysts or metal oxides is not crucial since, because of the steric hindrance of carbohydrates, the catalytic reaction mainly takes place on the catalyst surface. In the case of organic polymers, utilization of flexible polymeric chains considerably improves the accessibility of the catalytic sites. 

Ammonia Chemisorption. Ammonia gas has been widely employed as a basic adsorbate to count the number and strength of acid sites on various solid surfaces (28,29). The nature and strength of these sites may relate the activity and selectivity character of the catalysts. Further, all acid sites on catalyst surface are easily accessible to the small molecules of NH (kinetic dla 0.26 nm) and... 

Although there still remains some "art" in the production of high activity catalysts, surface area, pore size and other factors relevant to the accessibility of the reactant gases to the catalytic sites are clearly of primary importance. Thus any deposition of product or by-product within the catalyst pore structure is undesirable. In recent years the relationship between impure Claus feed containing hydrocarbons and catalyst lifetime has been well demonstrated (32). Carbon of hydrocarbon polymer deposition on the catalyst usually results in blocking access of the reactant gases to the internal catalytic sites. Product sulfur depos-... 

Percentage exposed in metallic catalysts. The accessibility of the atoms of metal in metallic catalysts, supported or unsupported, depends upon the percentage of the total atoms of metal which are surface atoms. It is recommended that the term percentage exposed be employed for this quantity rather than the term dispersion which has been frequently employed. 

All the surface processes on automotive catalysts which have been tested for the effects of lead poisoning are affected by the access of lead to the catalyst surface. The effect will differ, though, for different surface processes. Oxidation of hydrocarbons has been found repeatedly to be more vulnerable than oxidation of carbon monoxide to lead poisoning (10, 19, 25). The initial oxidation activity of noble metal catalysts, never exposed to poisons, is higher for CO than for hydrocarbons (54). Therefore, it is best to use the effect of lead on hydrocarbon oxidation for assessing the susceptibility of a given oxidation catalyst to this type of poisoning. 

In any gas/solid catalytic system, the reactant must first be adsorbed on the catalyst surface. This is why surface characterization is so important. Studying the adsorption of various molecules under controlled conditions yields information regarding the catalyst surface area, pore volume, and pore size distribution [80]. The key factor here is accessibility. Sophisticated spectroscopic analysis of single-crystal models can tell us a lot about what goes on at the active site, but the molecules must get there first. 

Heterogeneous catalysis is a surface phenomenon, therefore the overall kinetic parameters are dependent on the real exposed catalyst surface area. In the supported systems only a part of the photocatalyst is accessible to light and to substrate. Besides, the immobilized catalyst suffers from the surface deactivation since the support could enhance the recombination of photogenerated electron-hole pairs and a limitation of oxygen diffusion in the deeper layers is observed. 

---