Zinc oxide deactivation

The space velocity was varied from 2539 to 9130 scf/hr ft3 catalyst. Carbon monoxide and ethane were at equilibrium conversion at all space velocities however, some carbon dioxide breakthrough was noticed at the higher space velocities. A bed of activated carbon and zinc oxide at 149 °C reduced the sulfur content of the feed gas from about 2 ppm to less than 0.1 ppm in order to avoid catalyst deactivation by sulfur poisoning. Subsequent tests have indicated that the catalyst is equally effective for feed gases containing up to 1 mole % benzene and 0.5 ppm sulfur (5). These are the maximum concentrations of impurities that can be present in methanation section feed gases. 

Later, better cements appeared based on c. 50% solutions of orthophosphoric acid. But even these were far from satisfactory. As always with dental cements, the problems revolved around the control of the setting reaction the reaction between zinc oxide and orthophosphoric acid was found to be far too fierce. By the time of Fleck s 1902 paper these problems had been solved. The importance of densifying and deactivating the zinc oxide powder to moderate the cement reaction had been recognized. Of equal importance was the realization that satisfactory cements could be produced only if aluminium was incorporated into the orthophosphoric acid solution. The basic science underlying this empirical finding was elucidated only in the 1970s. 

The zinc oxide used in ZOE cements differs entirely from that used in zinc phosphate cements. Whereas the latter has to be ignited to a very high temperature to deactivate it, the opposite is true of the zinc oxides used in the ZOE cement, which are of an activated variety. They are normally prepared by the thermal decomposition of zinc salts at 350 °C to 450 °C such oxides are yellow. Zinc oxides prepared by oxidizing zinc in oxygen may also be used these are white. 

Deactivation of Singlet Oxygen on a Surface of Zinc Oxide... 

The first step of surveying was represented by studying the nature of heterogeneous deactivation of singlet oxygen on a surface of oxidized and partially reduced zinc oxide [102] with the aid of semiconductor sensors. At room temperature and the pressure of O2 10 Torr, the value of y for de-exciting 02( A ) in ZnO is equal or approximately... 

Fig. 5.12. Coefficient of heterogeneous deactivation of singlet oxygen versus the temperature of zinc oxide surface at Pqj = 0.1 Torr [102]...

An important role in the mechanism is plaid by the applied particles of metal. One can conclude so from the experiments in studying heterogeneous deactivation of RGMAs on a pure and Au microcrystal-activated surfaces of glass and zinc oxide [162], The experiments were conducted by the Smith method, Au/ZnO sensors being used as RGMA detectors. The results of these investigations are tabulated in Tables 5.3. 

Zinc dust is frequently covered with a thin layer of zinc oxide which deactivates its surface and causes induction periods in reactions with compounds. This disadvantage can be removed by a proper activation of zinc dust immediately prior to use. Such an activation can be achieved by a 3-4-minute contact with very dilute (0.5-2%) hydrochloric acid followed by washing with water, ethanol, acetone and ether [/55]. Similar activation is carried out in situ by a small amount of anhydrous zinc chloride [156 or zinc bromide [157 in alcohol, ether or tetrahydrofuran. Another way of activating zinc dust is by its conversion to a zinc-copper couple by stirring it (180g) with a solution of 1 g of copper sulfate pentahydrate in 35 ml of water [/55]. 

Another difference between Co and Fe is their sensitivity towards impurities in the gas feed, such as H2S. In this respect, Fe-based catalysts have been shown to be more sulfur-resistance than their Co-based counterparts. This is also the reason why for Co F-T catalysts it is recommended to use a sulphur-free gas feed. For this purpose, a zinc oxide bed is included prior to the fixed bed reactor in the Shell plant in Malaysia to guarantee effective sulphur removal. Co and Fe F-T catalysts also differ in their stability. For instance, Co-based F-T systems are known to be more resistant towards oxidation and more stable against deactivation by water, an important by-product of the FTS reaction (reaction (1)). Nevertheless, the oxidation of cobalt with the product water has been postulated to be a major cause for deactivation of supported cobalt catalysts. Although, the oxidation of bulk metallic cobalt is (under realistic F-T conditions) not feasible, small cobalt nanoparticles could be prone to such reoxidation processes. 

The low-pressure methanol synthesis process utilizes ternary catalysts based on copper, zinc oxide, and another oxide, such as alumina or chromia, prepared by coprecipitation. Cu-Zn0-Al203 and Cu-Zn0-Cr203 are usually the most important industrial catalysts. A significant advance was made when a two-stage precipitation was suggested in which ZnAl2C>4, a crystalline zinc aluminate spinel, was prepared prior to the main precipitation of copper-zinc species.372 This alteration resulted in an increase in catalyst stability for long-term performance with respect to deactivation. Catalyst lifetimes industrially are typically about 2 years. 

In the actual process (Figure 10-5), the natural gas feedstock must first be desulfurized in order to prevent catalyst poisoning or deactivation. The desulfurization step depends upon the nature of the sulfur-containing contaminants and can vary from the more simple ambient temperature adsorption of the sulfur-containing materials on activated charcoal to a more complex high-temperature reaction with zinc oxide to catalytic hydrogenation followed by zinc oxide treatment. 

Fatty alcohols are obtained by direct hydrogenation of fatty acids or by hydrogenation of fatty acid esters. Typically, this is performed over copper catalysts at elevated temperature (170°C-270°C) and pressure (40-300 bar hydrogen) [26], By this route, completely saturated fatty alcohols are produced. In the past, unsaturated fatty alcohols were produced via hydrolysis of whale oil (a natural wax occurring in whale blubber) or by reduction of waxes with sodium (Bouveault-Blanc reduction). Today, they can be obtained by selective hydrogenation at even higher temperatures (250°C-280°C), but lower pressure up to 25 bar over metal oxides (zinc oxide, chromium oxide, iron oxide, or cadmium oxide) or partially deactivated copper chromite catalysts [26],... 

Bienholz A, Blume R, Knop-Gericke A, Giergsdies F, Behrens M, Claus P. Prevention of catalyst deactivation in the hydrogenolysis of glycerol by Ga203-modified copper/zinc oxide catalysts. J Phys Chem C. 2011 115 999-1005. 

The book also explores the application of various acidic catalysts, such as silica-alumina, zeolites (HY, HZSM-5, mordenite) or alkaline compounds such as zinc oxide. However, the main problem with catalytic cracking is that in the course of the cracking process all catalysts deactivate very quickly. Expensive zeolite catalysts increase the cost of waste plastics cracking process to the point where it becomes economically unacceptable since the catalyst becomes contained in coke residue and therefore cannot be recovered and regenerated. 

Some experiments on methanol decomposition over zinc oxide by Teller et al. (120) seem to demonstrate a remarkable transient activation, while similar ones over chromia show a similar marked deactivation. Several samples of each catalyst were studied in a flow system between about 300° and 430°, with irradiation during the reaction at a maximum dose rate of 8 x 10 ev gm i sec-i. The flow rate was such that a 10% conversion of the methanol would correspond to about 3 X 10 molecules decomposed per second per gram of catalyst. 

Organo-zinc compounds are commonly present in nearly all commercially lubricating oils which are intended for vehicle use. Unfortunately, these compounds are involved in the combustion process in the engine and are converted mainly into zinc oxide before entering the catalytic converters. This form of zinc usually deactivates the vehicle exhaust catalysts [1]. 

Extrapolated thermodynamic measurements for the sulfur-poisoned Hu/Al2°3 catalyst show that, given a sufficiently long exposure time, feedstock sulfur levels of 1 ppb will be enough to reach this threshold of sulfur coverage. With sulfur contamination above this level (conventional zinc oxide guard beds typically reduce sulfur levels to 0,1 ppm)t steam/carbon ratios of greater than or equal to 3.D and/or more H2 arc needed to prevent CCD in the critical inlet portions of the catalyst bed. Virtually complete sulfur removal is required to avoid c t Ly t coking and deactivation ... 

Because of the use of a hydrophilic solvent in the fabrication of the PLGA microparticles, pure insulin would exist in an amorphous or liquid state. This is not considered desirable since amorphous insulin has a much shorter duration of action than does crystalline insulin. In order to prevent this "deactivation" of insulin, zinc has been studied as a way to stabilize the insulin molecule in the presence of hydrophilic solvent. - Accordingly, an insulin preparation using zinc oxide is utilized in the preparation of the PLGA microcapsules. ... 

The zinc phosphate cement dates back to at least 1879, when Rollins reported a formulation based on syrupy orthophosphoric acid [13]. The first really satisfactory cement of this type was reported by Fleck in 1902 [14], His report described a paper based on zinc oxide that had been deactivated by heating, together with a solution of phosphoric acid modified by the inclusion of aluminium and zinc. These two approaches to moderating the reaction led to a setting process that took place at a sufficiently slow speed that a smooth paste could be prepared from the components, and there was time to apply it before hardening took place. 

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