Catalyst poisons, detection

Catalyst Poisons. It is well known that sulfur, chlorine, etc. are strong poisons for nickel catalyst. Chlorine was not detectable in the synthesis gas downstream of the Rectisol in the SASOL plant. The total sulfur content of this gas—in the form of H2S, COS, and organic sulfur components—averaged 0.08 mg/m3 with maximum values of 0.2 mg total sulfur/m3. 

These instruments employ a continuous flow of persulfate solution to promote oxidation prior to ultraviolet irradiation, and have a low system blank and low detection limit. Since all reactions take place in the liquid phase, problems suffered by combustion techniques, such as catalyst poisoning, reactor corrosion, and high-temperature element burnouts, are obviated. However, the ultraviolet-promoted chemical oxidation technique is not designed to handle particulate-containing samples, and tends to give incomplete oxidation for certain types of compounds such as cyanuric acid. 

We shall omit the description of the numerous cases in which a decrease or a complete annihilation of catalytic action by a second substance was observed, an effect which has been called catalyst poisoning since the investigations of Knietsch and Bredig (13). Effects of this type have been encountered again and again since the first report of Dobereiner they were often due to the unintentional presence of harmful substances in the catalysts or to contaminations of the reacting components. There are, of course, also numerous cases in which added substances exert no detectable influence on a catalyst. II. 

One reason that is often cited for studying supported homogeneous catalysts is the relative ease of separation of catalyst from reactants. However for this factor to have practical significance it is essential that metal elution is minimal and very few studies on elution have been carried out in the past. ° Elemental analyses on the catalyst before and after reaction are insufficiently accurate to detect small losses. Moreover, arguments for low Rh loss on the basis of no change in activity on re-use of catalyst are not valid as the reaction is normally under mass transfer control. As many catalyst-support systems contain residual halide which is a known catalyst poison, if simultaneous leaching of halide and metal occurs then little change in activity may result. 

To address these limitations, additional -tests have been designed. In one experiment called the "three-phase test," a substrate is attached to an insoluble support, such as a polymer. If the catalyst is also a solid, then the supported substrate reacts much more slowly than the analogous soluble substrate, but if the catalyst is dissolved, then the two substrates will react with more similar rates. The polymer-bonded substrate must be swollen in a solvent which is compatible with the homogeneous catalyst. Applications of this test for homogeneity are Illustrated in Equation 10.54. The virtue of this test is that it is based on the catalytic process itself, rather than on detection of hypothetical catalysts. A variation of this theme involves the use of polymeric catalyst poisons, such as polythiols. These were found to have no effect on heterogeneous catalysts but to retard homogeneous catalysts.- ... 

Le Canut, J. M. Abouatallah. 2006. Detection of membrane drying, fuel cell flooding, and anode catalyst poisoning on PEMFC stacks by electrochemical impedance spectroscopy. /. Electrochemical Society 153 A857-A864. 

Nitrogen species in crude oils can cause catalyst poisoning. ASTM D 3228 or ASTM D 4629 normally determines nitrogen content. Either a syringe inlet or boat inlet analyzes distillate cuts by Oxidative Combustion and Chemiluminescence detection. Whole crude, atmospheric and vacuum... 

In the case of gaseous catalyst poisons, a distinction can be made between permanent poisons causing an irreversible loss of catalytic activity and temporary poisons which lower the activity only while present in the synthesis gas. This distinction is fully discussed in the book by Nielsen. Permanent poisons such as sulfur accumulate upon the catalyst surface and may be detected by chemical and spectroscopic analysis, while temporary poisons do not interact nearly as strongly with the catalyst. It is very difficult to detect temporary poisons by means of post-analytical methods. The principal temporary poisons are oxygen, carbon oxides, and water. Since the catalyst also contains percent amounts of oxygen... 

Finally, Fig. 3b shows the impact of Cl poisoning on the functioning of an air/H2 fuel cell. The cathode of the Membrane-Electrode Assembly (MEA) was processed either with sample C-200-1, or with catalyst C-450-5. The catalyst poisoning dramatically decreases the current produced at a fixed potential. However, further calculation shows that the decrease of accessible Pt surface due to Cl coverage is not sufficient to explain the poor performance of catalyst C-200-1. Much probably, the presence of chlorine also hampers the contact between the Pt particles and the ionomer i.e., Nafion ), and decreases thus further the amount of Pt atoms that are tmly available for the oxygen reduction in the monocell. This was checked by in situ cyclic voltammetry the detected Pt surface per mass unit of metal was lower in the processed MEA than in the initial catalyst powder. 

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