Surface catalytic effect

For environmental reasons, burning should be smokeless. Long-chain and unsaturated hydrocarbons crack in the flame producing soot. Steam injection helps to produce clean burning by eliminating carbon through the water gas reaction. The quantity of steam required can be as high as 0.05—0.3 kg steam per kg of gas burned. A multijet flare can also be used in which the gas bums from a number of small nozzles parallel to radiant refractory rods which provide a hot surface catalytic effect to aid combustion. 

It was found that sorbed palladium might catalyse reaction of Mn(III) reduction by Cf not only after it s removing from coal, but AC with palladium, Pd/AC, has also his own catalytic effect. On the base of dependence between characteristics of AC, chemical state of palladium on AC surface and catalytic action of Pd/AC in indicator reaction it might establish, that catalytic action concerns only to non-reduced or partly reduced palladium ions connected with chloride ions on coal surface. The presence or absence of catalytic action of Pd/AC in above-mentioned reaction may be proposed for determination of chemical state of palladium on AC surface. Catalytic effect was also used for palladium micro-amounts determination by soi ption-catalytic method. 

The interaction between drug compounds and excipients, as these influence drug dissolution, can be successfully studied by means of reflectance spectroscopy. In one study concerning probucol and indomethacin, it was deduced that hydrogen bonding and van der Waals forces determined the physisorption between the active and the excipients in several model formulations [36]. Chemisorption forces were found to play only minor roles in these interactions. These studies indicated that surface catalytic effects could be important during the selection of formulation excipients. 

A surface-catalytic effect is observed, as mentioned above, when the surface of the solid substrate "matches well" with the crystal to be formed, i.e., when... 

Accumulation of the nucleophile OH - at the Al oxide/water interface is an important component of the observed surface catalytic effect. The OH - concentration at the plane of closest approach to the surface (at the diffuse layer, [OHj]), is higher than the concentration in bulk solution, because of favorable electrostatic interaction. 

A comprehensive study of PHP hydrolysis in the presence of various hydrous metal oxides has been completed (Torrents and Stone, 1991). Surface catalytic effects observed with PHP are in many ways distinct from those observed with MPT. Oxide surfaces can be neutral or negatively charged and still catalyze PHP hydrolysis. The Ti oxide suspensions, for example, can accelerate PHP hydrolysis at pH values at and above the pHzp. Iron oxides catalyze hydrolysis while Al oxides do not, despite very similar surface charge and surface proton level characteristics. 

Although the quantitative XP8 analyses may be burdened by a considerable systematic error ( ), the results summarized in Table II demonstrate convincingly that nearly equal surface concentrations of Na,K,Rb and Cs were achieved by the employed preparation method. The differences in surface catalytic effects summarized below are therefore caused by the different nature of the alkali ions and not by their different surface concentrations. 

To avoid the surface catalytic effects, a water-cooled probe must be used when sampling high temperature combustion products. This is of particular importance to oxyfuel combustion where measured species concentrations are much higher due to the elimination of nitrogen. Recommended probe materials and cooling requirements to avoid reaction of the various gases inside the probe are given elsewhere [48]. 

The major area of application for plastics in bioscience is in the two areas indicated. The plastics make interesting materials to be used for mechanical implants into all living systems, including animals and plants where they can serve as repair parts or as modifications of the system. Figure 18-4 shows a variety of plastics used in hospitals. The other applications are based on the membrane qualities of plastics which can control such things as the chemical constituents that pass from one part of a system to another, the electrical surface potential in a system, the surface catalytic effect on a system, and in some cases the reaction to specific influences such as toxins or strong radiation. A Dacron artery is shown in Fig. 18-5. 

Thus the authors are of the opinion that the initial exploratory work described in this paper underlines the significance and importance of defining the role of surface catalytic effects (if any) when interpreting chemical phenomena at sliding interfaces especially when the experiments are performed under U.H.V. conditions. Obviously to establish the existence or exact nature of any individual polymer/ iron interactions (where the metal has been stripped of its oxide) would require extensive further v/ork with the actual systems involved and it is hoped that this paper will stimulate research in this area. 

When the operating temperature exceeds ca 93°C, the catalytic effects of metals become an important factor in promoting oil oxidation. Inhibitors that reduce this catalytic effect usually react with the surfaces of the metals to form protective coatings (see Metal surface treatments). Typical metal deactivators are the zinc dithiophosphates which also decompose hydroperoxides at temperatures above 93°C. Other metal deactivators include triazole and thiodiazole derivatives. Some copper salts intentionally put into lubricants counteract or reduce the catalytic effect of metals. 

An effect which is frequently encountered in oxide catalysts is that of promoters on the activity. An example of this is the small addition of lidrium oxide, Li20 which promotes, or increases, the catalytic activity of dre alkaline earth oxide BaO. Although little is known about the exact role of lithium on the surface structure of BaO, it would seem plausible that this effect is due to the introduction of more oxygen vacancies on the surface. This effect is well known in the chemistry of solid oxides. For example, the addition of lithium oxide to nickel oxide, in which a solid solution is formed, causes an increase in the concentration of dre major point defect which is the Ni + ion. Since the valency of dre cation in dre alkaline earth oxides can only take the value two the incorporation of lithium oxide in solid solution can only lead to oxygen vacaircy formation. Schematic equations for the two processes are... 

In sliding electrical contact applications, palladium plating has been criticised on the basis of a tendency due to its catalytic activity to cause polymerisation of organic vapours from adjacent equipment with the formation of insulating films on the surface. This effect is important in certain circumstances, but is not serious in many practical applications... 

The catalytic effect of various surfaces was also investigated and showed that electron-deficient sites were responsible for promoting the condensation. Basic substances and small amounts of water were found to considerably reduce the rate of resinification. 

Although NEMCA is a catalytic effect taking place over the entire catalyst gas-exposed surface, it is important for its description to also discuss the electrocatalytic reactions taking place at the catalyst-solid electrolyte-gas three phase boundaries (tpb). This means that the catalyst-electrode must also be characterized from an electrochemical viewpoint. When using YSZ as the solid electrolyte the electrochemical reaction taking place at the tpb is ... 

Attempts have also been made to obtain the radicals (CF3)3C and CeFs as products of vacuum pyrolysis of (CF3)3CI and CeFsI (Butler and Snelson, 1980b). However, only perfluoroisobutene was observed in an IR spectrum of pyrolysis products of (CF3)3CI. Thermolysis of CeFsl led to formation of CF4, CF3 and CF2 as a result of decomposition of the aromatic ring. This behaviour was explained as due to catalytic effects which take place on the platinum reactor surface. 

High resolution electron microscopy has recently demonstrated the capability to directly resolve the atomic structure of surfaces on small particles and thin films. In this paper we briefly review experimental observations for gold (110) and (111) surfacest and analyse how these results when combined with theoretical and experimental morphological studies, influence the interpretation of geometrical catalytic effects and the transfer of bulk surface experimental data to heterogeneous catalysts. 

There are a number of concepts concerning the structure of small particles which have a bearing upon geometrical catalytic effects (e.g. 41-43). These follow both from the surface imaging results, and a detailed experimental (13-15) and theoretical (44-47) study of particle morphologies. 

We have discussed here, very briefly, some recent observations of small particle surfaces and how these relate to geometrical catalytic effects. These demonstrate the general conclusion that high resolution imaging can provide a direct, structural link between bulk LEED analysis and small particle surfaces. Apart from applications to conventional surface science, where the sensitivity of the technique to surface inhomogenieties has already yielded results, there should be many useful applications in catalysis. A useful approach would be to combine the experimental data with surface thermodynamic and morphological analyses as we have attempted herein. There seems no fundamental reason why results comparable to those described cannot be obtained from commercial catalyst systems. 

GL 3] [R 5] [P 4] A catalytic effect of the fluorinated metal surface of the micro channel was clearly determined [15],... 

In any case, it is interesting to note that catalytic efficacy has been observed with nano- or mesoporous gold sponges [99-101, 145] suggesting that neither a discrete particle nor an oxide support is actually a fundamental requirement for catalysis. An alternative mechanism invokes the nanoscale structural effect noted in Section 7.2.2, and proposes that the catalytic effect of nanoscale gold structures is simply due to the presence of a large proportion of lowly-coordinated surface atoms, which would have their own, local electronic configurations suitable for the reaction to be catalyzed [34, 49,146] A recent and readily available study by Hvolbaek et al. [4] summarizes the support for this alternate view. 

Sometimes anodic protection is used, in which case the metal s potential is made more positive. The rate of spontaneous dissolution will strongly decrease, rather than increase, when the metal s passivation potential is attained under these conditions. To make the potential more positive, one must only accelerate a coupled cathodic reaction, which can be done by adding to the solution oxidizing agents readily undergoing cathodic reduction (e.g., chromate ions). The rate of cathodic hydrogen evolution can also be accelerated when minute amounts of platinum metals, which have a stroug catalytic effect, are iucorporated iuto the metaf s surface fayer (Tomashov, 1955). 

In some cases, adsorbed foreign species may give rise to acceleration of a reaction. In the hterature, a few cases have been described where strong catalytic effects were observed on catalytically inactive electrodes when small amounts of platinum species that had by accident arrived in the electrolyte solution became adsorbed on their surface. 

At present, most workers hold a more realistic view of the promises and difficulties of work in electrocatalysis. Starting in the 1980s, new lines of research into the state of catalyst surfaces and into the adsorption of reactants and foreign species on these surfaces have been developed. Techniques have been developed that can be used for studies at the atomic and molecular level. These techniques include the tunneling microscope, versions of Fourier transform infrared spectroscopy and of photoelectron spectroscopy, differential electrochemical mass spectroscopy, and others. The broad application of these techniques has considerably improved our understanding of the mechanism of catalytic effects in electrochemical reactions. 

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