Deactivation, adsorbates

Protect catalysts and adsorbents. Deactivated catalysts and adsorbents are solid wastes from the process. In some cases, relatively small amounts of contaminants can cause a load of catalyst or adsorbent to become useless. The catalyst... 

A few chromatographic systems exhibit //-type isotherms (Fig. 3-5) for certain samples. An //-type isotherm results whenever some (but not all) adsorption sites are so strong with respect to a particular compound as to adsorb that compound irreversibly, or effectively so. Commonly, these very strong sites form only a small part of the total adsorbent surface, and sometimes they are not affected by normal adsorbent deactivation. Because of the very large energies of adsorption on these sites, as well as their specificity for certain compounds, we are justified in referring to these strong interactions as chemisorption. In many cases the nature of the reaction between sample and adsorbent is fairly obvious. 

Sample reaction or chemisorption on the adsorbent bed (3) is generally associated with the strong adsorption sites or small pores which are responsible for isotherm nonlinearity (1) and excessively large sample retention volumes (2). The use of adsorbent deactivation or homogeneous surface adsorbents hence serves as a simultaneous answer to all three problems (1-3). Adsorbent standardization (4) is still a significant problem in both GSC and LSC, but much progress has been made recently (see Section 6-3). Once a satisfactory GSC column has been prepared, moreover, its lifetime will generally exceed that of a GLC column many times over. 

Langmuir isotherm, 55-58 linear capacity of, 80 Linear capacity Od.i, 78-79 vs. adsorbent deactivation, 87-89 vs. adsorbent type, 89-90 vs, band width, 90 vs. K 83-87 vs. sample structure, 86 vs. sample type, 90-91 vs. solvent, 87 vs. temperature, 86-87 Linear Elution Adsorption Chromatography (LEAC), 353-354 Linear isotherm, 77-79 See also Linear capacity advantages of, 77... 

A technique akin to adsorbent deactivation is thb modification of the total surface by covering the adsorbent with, sev al. monolayers of some compound or by reacting all the surface sites. In eitfier case a new surface results, of generally higher linear capacity and totally different adsorption characteristics. This general technique has so far beeit jeserved for adsorbents used in gas-solid chromatography (see Section 9-2B). 

Adsorbents. Deactivated adsorbents are the most suitable for fractionating ionic and neutral polar lipids. Examples are silica gel G containing 10% ammonium sulphate [67, 121] or 10% sodium acetate [67]. Especially good separations are achieved on air dried silica gel H... 

The presence of other functional groups ia an acetylenic molecule frequendy does not affect partial hydrogenation because many groups such as olefins are less strongly adsorbed on the catalytic site. Supported palladium catalysts deactivated with lead (such as the Liadlar catalyst), sulfur, or quinoline have been used for hydrogenation of acetylenic compound to (predominantiy) cis-olefins. 

Metals and alloys, the principal industrial metalhc catalysts, are found in periodic group TII, which are transition elements with almost-completed 3d, 4d, and 5d electronic orbits. According to theory, electrons from adsorbed molecules can fill the vacancies in the incomplete shells and thus make a chemical bond. What happens subsequently depends on the operating conditions. Platinum, palladium, and nickel form both hydrides and oxides they are effective in hydrogenation (vegetable oils) and oxidation (ammonia or sulfur dioxide). Alloys do not always have catalytic properties intermediate between those of the component metals, since the surface condition may be different from the bulk and catalysis is a function of the surface condition. Addition of some rhenium to Pt/AlgO permits the use of lower temperatures and slows the deactivation rate. The mechanism of catalysis by alloys is still controversial in many instances. 

Eor the selective pre-concentration of deactivated phenols a new silica-based material with the grafted 2,3,5-triphenyltetrazole was proposed. This method is based on the formation of molecular chai ge-transfer comlexes of 2,3,5-triphenyltetrazole (7t-acceptor) with picric acid (7t-donor) in the phase of the sorbent. Proposed SPE is suitable for HPEC analysis of nitrophenols after their desorption by acetonitrile. Test-system for visual monitoring of polynitrophenols under their maximum concentration limits was developed using the proposed adsorbent. 

Here Ny is the active center deactivated by Y. H20 is likely to be a Y-type inhibitor. To explain the steady-state period of polymerization it may be assumed that some quantities of Y are adsorbed on the support surface. 

The same trends regarding the effect of sulfur have been reported for NO adsorption on Pt(lOO)90 and Rh(100).6 In the case of Pt(100) dissociative adsorption is completely inhibited upon formation of a p(2x2) overlayer at a sulfur coverage equal to 0.25, while the binding strength of molecularly adsorbed NO is lowered by more than 50 kJ/mol, as calculated by analysis of NO TPD data. Due to this complete inhibition of dissociative adsorption, the CO+NO reaction is completely deactivated, although it proceeds easily on sulfur free Pt(100). In the case of Rh(100) a sulfur coverage of only 0.08 suffices to completely inhibit NO dissociation at 300 K. 

If the mixture to be separated contains fairly polar materials, the silica may need to be deactivated by a more polar solvent such as ethyl acetate, propanol or even methanol. As already discussed, polar solutes are avidly adsorbed by silica gel and thus the optimum concentration is likely to be low, e.g. l-4%v/v and consequently, a little difficult to control in a reproducible manner. Ethyl acetate is the most useful moderator as it is significantly less polar than propanol or methanol and thus, more controllable, but unfortunately adsorbs in the UV range and can only be used in the mobile phase at concentrations up to about 5%v/v. Above this concentration the mobile phase may be opaque to the detector and thus, the solutes will not be discernible against the background adsorption of the mobile phase. If a detector such as the refractive index detector is employed then there is no restriction on the concentration of the moderator. Propanol and methanol are transparent in the UV so their presence does not effect the performance of a UV detector. However, their polarity is much greater than that of ethyl acetate and thus, the adjustment of the optimum moderator concentration is more difficult and not easy to reproduce accurately. For more polar mixtures it is better to explore the possibility of a reverse phase (which will be discussed shortly) than attempt to utilize silica gel out of the range of solutes for which it is appropriate. 

Thus, the column should completely resolve about 14 equally spaced peaks. It is seen from figure 1 that a peak capacity of 14 is not realized although most of the components are separated. This means that the column may not have been packed particularly well and/or the flow rate used was significantly above the optimum velocity that would provide the maximum efficiency. The mobile phase that was used was tetrahydrofuran which was sufficiently polar to deactivate the silica gel with a layer (or perhaps bilayer) of adsorbed solvent molecules yet was sufficiently dispersive to provide adequate sample... 

Room temperature CO oxidation has been investigated on a series of Au/metal oxide catalysts at conditions typical of spacecraft atmospheres CO = 50 ppm, COj = 7,000 ppm, H2O = 40% (RH) at 25 C, balance = air, and gas hourly space velocities of 7,000- 60,000 hr . The addition of Au increases the room temperature CO oxidation activity of the metal oxides dramatically. All the Au/metal oxides deactivate during the CO oxidation reaction, especially in the presence of CO in the feed. The stability of the Au/metal oxide catalysts decreases in the following order TiOj > FejO, > NiO > CO3O4. The stability appears to decrease with an increase in the basicity of the metal oxides. In situ FTIR of CO adsorption on Au/Ti02 at 25 C indicates the formation of adsorbed CO, carboxylate, and carbonate species on the catalyst surface. 

The activity and stability of catalysts for methane-carbon dioxide reforming depend subtly upon the support and the active metal. Methane decomposes to carbon and hydrogen, forming carbon on the oxide support and the metal. Carbon on the metal is reactive and can be oxidized to CO by oxygen from dissociatively adsorbed COj. For noble metals this reaction is fast, leading to low coke accumulation on the metal particles The rate of carbon formation on the support is proportional to the concentration of Lewis acid sites. This carbon is non reactive and may cover the Pt particles causing catalyst deactivation. Hence, the combination of Pt with a support low in acid sites, such as ZrO, is well suited for long term stable operation. For non-noble metals such as Ni, the rate of CH4 dissociation exceeds the rate of oxidation drastically and carbon forms rapidly on the metal in the form of filaments. The rate of carbon filament formation is proportional to the particle size of Ni Below a critical Ni particle size (d<2 nm), formation of carbon slowed down dramatically Well dispersed Ni supported on ZrO is thus a viable alternative to the noble metal based materials. 

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