Poisoning effect of water

It seems very plausible that, on heating, water is evolved from neighboring surface hydroxyl groups leaving an 0 - ion and a hole in the coordination sphere of the Al + ions as shown in Fig. 5.1 his hole would be a powerful acceptor for nucleophilic reagents, especially in the case of tetrahedral coordination (Fig. 5b). The strong poisoning effect of water would be easily explained. 

The highly catalytic oxides are unstable chemically and decompose readily into free oxygen and a lower oxide of the metal. They can be readily reduced by CO. The reaction must, therefore, involve two steps the first, a process of bond formation between CO and the catalyst, followed by a splitting off of C02. In the second step, activated adsorption of oxygen on the unsaturated manganese atoms would regenerate the catalyst. Any proposed mechanism for the reaction must also explain the poisoning effects of water vapor and of adsorbed alkali. 

International Organization for Standardization (ISO) (2000c) Water Quality - Determination of the Non-poisonous Effect of Water Constituents and Waste Water to Duckweed (Lemna minor, Lemna gibba), ISO 20079. ISO, Paris. 

Single-Bed Isothermal Catalysts. Detailed analyses of exit gases from single-bed isothermal catalysts were determined with 2 g of red bauxite at 475°C. The inlet gas contained 3.4% sulfur dioxide, 5.9% carbon monoxide, and 90.7% helium. Figure 2 (Section A) shows that the sulfur dioxide analysis decreased from 3.4 to 0.8. In other words, about 76% of the sulfur dioxide was removed in the dry state at a carbon monoxide ratio, r, of 0.87. However, when 3% water vapor was added (Section B), the sulfur dioxide in the exhaust gas increased to 1.9%, illustrating the poisoning effect of water. When water vapor flow was stopped, the sulfur dioxide exhaust analyses decreased slowly (Section... 

C) but did not return to the initial value (Section A) within 40 min. The addition of 15% water vapor (Section D) further decreased the sulfur dioxide removal efficiency. Curve b in Figure 2 depicts the analyses of carbon dioxide when the bauxite catalyst was subjected to the water treatment. The mirror image resemblance of curves b and a in Figure 2 suggests that the reaction stoichiometry is closely represented by Equation 1 and that the poisoning effect of water is essentially caused by its competition for chemisorption on the alumina Lewis acid sites with the sulfur precursor of the intermediate (9) reductant carbonyl sulfide. 

There is a remarkable parallelism between the exchange and the hydrogenation reactions over alumina, although these reactions occur at temperatures several hundreds of degrees apart. For both reactions, the initial activity increases as the drying temperature is increased addition of 0.15 wt. % water to the dehydrated oxide results in almost complete loss of catalytic activity the full poisoning effect of water is exhibited only when the water is added at temperatures of about 300° or higher and the active sites... 

In the studies regarding the influence of various oxidants, chiral auxiliaries, solvents, and temperatures on enantioselectivity in the AE reaction, the use of tert-butyl hydroperoxide (TBHP), DET, and toluene at -20 °C gave the optimum results. However, in the bifunctional single-pot CSC followed by AE reaction (Table 5.14, Method B), the ee s and conversions are lower than in the independent AE reactions. This is due to the poisoning effect of water adsorbed on to the catalyst, which is formed in sim in the CSC reaction. When the catalyst was dried under a nitrogen flow for 1 h at 250 °C immediately after the CSC, similar enantioseleetivity in the AE reaction was obtained, supporting the above theory. 

The reduction of fused iron catalyst commences from the external surface of particles, and then expands inward. The reduction rate can be increased obviously by increasing the space velocity of reducing gas. The higher gas space velocity, the more favorable the reduction is, i.e., the lower the concentration of water vapor in gas, the faster the diffusion rate, the easier for the water molecules in the pore of catalyst to escape. As a result, the poisoning effect of water vapor is decreased to minimum. In addition, it is also conducive for the reduction reaction to move to the right and to raise the rate of reduction. However, when the space velocity continues to increase, the extent of increasing reduction rate will be minor. When it reaches the critical value, the space velocity of reductant gas on reduction rate has almost no impact. At the same time, in industrial production, increasing the space velocity is limited by the furnace heat supply and the temperature. 

Water vapor The poisoning effect of water vapor on the ammonia synthesis catalyst is similar to that of O2. The effect of water vapor on the activity of the catalyst relates to the concentration of water vapor, the temperature and pressure as follows. 

It has been published that hydrophobic-activated carbons can be suitable supports for noble metal species active for total oxidation. The catalytic behaviour of platinum and palladium supported on carbon-based monoUths was studied in the low temperature catalytic combustion of benzene, toluene and m-xylene, and compared with the corresponding behaviour of Pt-supported on y-Al203 coated monoliths. Carbon-based monoliths showed much better catalytic performance, which was ascribed to the fact that the carbon surface is more hydrophobic than the y-Al203, and the poisoning effect of water molecules produced during the combustion was... 

A special air burner at the top of the catalyst bed could increase the gas temperature and was used until 1922, despite the poisoning effect of water on the catalyst. 

For example, in Tajikistan in 1980, more than 10% of local fish species were threatened by extinction as a result of poisoned bodies of water [3]. In the Nizhegorodsk oblast, 21 of 57 fish species disappeared by 1980, mainly due to the effects of agricultural run-off. On average, about 30% of the cases of fish death in freshwater reservoirs in the central belt of Russia are due to pesticide contamination of those bodies of water [1]. 

It is usually difficult to discuss unambiguously on the role of the formation of sulphate, which may explain the deactivation. Their formation can equally occur on the support and on the noble metals. The poisoning effect of S02 has been reported by Qi el al. on Pd/Ti02/Al203 [112], However, in the presence of water, the stabilisation of hydroxyl groups could inhibit the adsorption of S02 [113], Burch also suggested a possible redispersion of palladium oxide promoted by the formation of hydroxyl species [114], Such tentative interpretations could correctly explain the tendencies that we observed irrespective to the nature of the supports, which indicate an improvement in the conversion of NO into N2 at high temperature. Nevertheless, the accentuation of those tendencies particularly on prereduced perovskite-based catalysts could be in connection with structural modifications associated with the reconstruction of the rhombohedral structure of... 

Some work on the electrooxidation of the methanol and hydrazine has been published (23,26). For methanol (24), the electrooxidation was studied in presence of different anions to evaluate the poisoning effect of several of them. Also, some work on anhydrous methanol was aimed at showing the role of water in the reaction (23). 

Equation (341) solves the task of quantitatively describing the effect of water vapor, and also oxygen gas (the last being rapidly converted to water vapor at the conditions of the reaction), on the activity of commercial ammonia synthesis catalysts. This result is of practical importance for ascertaining the necessary degree of purity of the inlet gas mixture with respect to poisons containing oxygen (122). 

The effect of water content on the acidity of kaolinite was examined by Solomon and co-workers (235, 236). Kaolinite dried at 110°C (0% water) had strongly acidic sites, comparable to 90% sulfuric acid (pKa < —8.2), as indicated by the Hammett indicator method. The strongest sites were readily poisoned by water. At 1% wt water content the strongest acid sites were equivalent in strength to 48% sulfuric acid (pKa = 3.0). With... 

DEV L30 - DIN 38 412 Part 30 (1989) German standard methods for the examination of water, wastewater and sludge - Determination of the non-poisonous effect of waste water to Daphnia by dilution limits, Beuth Verlag, Berlin, Germany. 

Parera and his co-workers (359-362) have studied the poisoning effect of amines, pyridine, phenol, and acetic acid. A reduced rate of ether formation from methanol at the standard temperature of 230°C was observed when the poisons were present in the feed. In most cases the original activity was recovered, although rather slowly. Most probably the poisons were either displaced by alcohol and/or water or removed from the surface by chemical transformations. 

The deleterious effect of water vapor was speculated to be due to its inhibition of carbon formation freeing the metal surface for interaction by H2S. Thus, sulfur poisoning of nickel at high temperature (above 673 K) may be more representative of a carbon-fouled surface, whereas at low temperatures it may be more characteristic of the clean metal surface. Again, this needs to be confirmed by direct measurements of carbon and sulfur adsorption. For Ni/Al203 and Ni/ZrOz the extent of sulfur deactivation was about fiftyfold at 673 K at 523 K the extent of deactivation was about 1000-fold. However, for Raney Ni the extent of sulfur deactivation was tenfold higher at 673 K than at 523 K this difference in behavior also needs confirmation and explanation. 

The autocatalytic nature of the reaction, described by Hinshelwood and Williamson [1], is in sharp contrast with the effect of water on the surface reaction at lower temperatures, which is poisoned by steam, and also with the inhibiting effect of water vapour on the second limit explosions. The autocatalysis has been studied in some detail by Chirkov [36], who used a reaction vessel of Durobax glass with diameter 5 cm and volume 200- 250 cm. For hydrogen oxygen ratios of about 2 1 at 550 torr initial pressure and 524 °C, he found the reaction rate w (torr sec ) to be given in terms of the initial pressure p and the amount of gases reacted x by... 

The importance of the details of pretreatment of chromia catalysts has often not been recognized nor, in some of the earlier work, the strong poisoning action of water vapor. Thus, the temperatures listed in Table I may be higher than necessary under optimum conditions. For example, one would conclude from Lazier and Vaughen (2) and Frey and Huppke (3), that the hydrogenation of olefins on chromia would require temperatures of about 300°. However, Weller and Voltz (8) discovered that the hydrogenation of ethylene could be effected at — 78° on well-activated chromia under anhydrous conditions. 

STABILITY POISONS When water vapor is present in the sulfur dioxide-air mixture supplied to a platinum-alumina catalyst, a decrease in oxidation activity occurs. This type of poisoning is due to the effect of water on the structure of the alumina carrier. Temperature has a pronounced ejffect on... 

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