Poisons, detection on metal surface

Many techniques have been used to detect the presence of poisons on metal surfaces, particularly by chemical analysis, but the effectiveness decreases with the poison content and special techniques become necessary for traces. For instance, special analytical (gas chromatography) capabilities to measure S concentrations below 5 ppb and CH4 below 1 ppm must be on-line (253). The sensitivity of spectroscopic methods becomes in those cases an important advantage. IR has often been employed to detect surface and/or bulk species by means of their character-... 

Concerning gaseous catalyst poisons, a distinction should be made between permanent poisons causing an irreversible loss of catalytic activity and temporary poisons which lower the activity while present in the synthesis gas. For a review of poisoning of synthesis catalysts, see [1,2]. Permanent poisons accumulate on the surface and may be detected by chemical analysis of the poisoned catalysts, whereas temporary poisons cause a partial coverage of the catalyst surface. Since oxygen is the most common temporary poison, it is difficult to detect the amount on the spent catalyst by analysis since the promoter phases are difficulty reducible metal oxides like alumina, magnesia, silica, and potassium oxide. 

By using different metal oxides on the surface, solid-state detectors can be made sensitive to many flammable and toxic gases. Also, they are not poisoned by silicones, lead, or halogens. As their response does not require oxygen to operate, solid-state detectors can be used to detect hazardous vapors in nitrogen or helium atmospheres.5... 

Thiophene metal poisoning as well as hydrogenation of ethylbenzene on metal catalysts require, as a first step, the chemisorption of both organic molecules on the metal active sites. Afterwards, catalyst deactivation can simply take place by the blocking of these sites or by further hydrogenolysis of thiophene and subsequent formation of an inactive surface metal sulfide. We believe that, in our conditions, this last mechanism is probably operating. This hypothesis is supported by the fact that butane was detected in our experiments and, furthermore, XPS analysis showed the formation of metal sulfides (S ) on the deactivated catalysts. 

Crosslinking of protein monolayers by mercuric ion (MacRitchie, 1970) and silicic acid (Minones et al., 1973) has been reported. These studies are relevant to poisoning by heavy-metal ions and to silicosis, effects that seem likely to result from attack on the cell membrane proteins. Crosslinking by mercuric ion was detected by a spectacular increase in surface viscosity and a decrease in compressibility when a number of proteins (BSA, insulin, ovalbumin, and hemoglobin) were spread on 0.001 M mercuric chloride solution. Poly-DL-alanine was unaffected whereas poly-L-lysine and poly-L-glutamic acid were affected in a similar manner to the proteins, indicating that mercuric ion interacts with the ionizable carboxyl and amino groups on the protein side—chains. Silicic acid similarly caused protein monolayers... 

The ability of the poison to block the active catalytic sites can be studied by determining a decrease in the catalytically active component on the surface. Selective chemisorption changes may be employed53 to detect changes in the accessible metal. Alternately, the covering of active species may be inferred from its decrease in detection from surface-sensitive techniques such as ESCA O. The approach is that the ESCA or Auger spectra for the active component decreases as it becomes covered with the poison. The escape depth for the emission from the active elements is limited and... 

When the purpose of the working electrode is to act as an inert electron sink, as in the detection of catecholamines, carbon is the preferred electrode material. On occasions when the electrode plays a direct role in the reaction, the precious metals are chosen. For example, silver can be oxidized to silver cyanide in the presence of cyanide ions. A major consideration when choosing an electrode material is its ability to maintain an active surface. Electrodes will develop a layer of surface oxide at positive applied potentials. The oxide layer will inhibit the oxidation of the analyte, and the response will decrease with repeated injections. The active surface can be renewed by polishing the electrode. Since glassy carbon electrodes are more resistant to poisoning by oxide formation, they are the electrode of choice for direct current amperometry. 

Polypyrrole was often used as support for platinum particles. Similarly to the case of polyaniline, the activity of such electrodes for the oxidation of methanol depends both on the amount of platinum and on the thickness of the polymer film [43]. In the same study, by using in-situ infrared spectroscopy, it was confirmed that linearly adsorbed CO species are the only detectable species present at the electrode surface. The authors attributed the enhancement of the overall activity observed to the high and uniform dispersion of the metallic particles with, possibly, an effect of the conducting polymer matrix itself. The same conclusions were drawn from another study [44] where the size of the particles obtained by electrodeposition was estimated at 10 nm. In this study, the Pt particles were entrapped into the polymer layer and showed a better activity than particles only deposited on the polymer surface. The authors interpreted their results as a decrease of the poisoning phenomenon in the 3D film in comparison to the only 2D deposit. 

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