Selective adsorption of oxygen

J. Dewar Found selective adsorption of oxygen from a mixture with nitrogen, during the uptake of air by charcoal 1904... 

Figure 12.8 PAES spectra for a Si(100) surface covered with 0.3 ML of Au (a) before and (b) after exposure to 400 Langmuir of 02- The oxygen exposure caused a decreas in the Si peak but not the Au peak. This indicates that the positrons are being pushed away from the Si due to selective adsorption of oxygen at exposed Si sites. A schematic drawing indicating this mechanism is shown in Figure 12.9 below.

Carbon molecular sieves (CMS) adsorbents are produced by pyrolysis of carbonaceous materials followed by carefully controlled deposition of carbon within the pores [43]. In contrast to activated carbons which have a broad distribution of micropore size (generally in the 10 - 100 A range) the pores of a carbon molecular sieve are very small (< 10 A) and the pore size distribution in narrow. As a result the adsorption behavior is similar to that of a zeolite. Carbon molecular sieves are widely used for production of nitrogen from air (by selective adsorption of oxygen). There is little difference between the equilibrium isotherms of O2 and N2 on CMS but as a result of its slightly smaller molecular size oxygen is adsorbed very much faster (diffiisivity ratio 10 - 100). The sorption kinetics show some interesting features. 

The rates of production are reported as turnover rates based on the number of V metal atoms as titrated by the adsorption of oxygen (77). These rates represent the number of product molecules produced per site per unit time and thus are measures of the actual product yield (conversion x selectivity) of the catalytic site. Conversions were less than 10% so that conclusions are derived for primary oxidation products. 

Dissociative adsorption occurring on four adjacent silver atoms (AgadJ) is responsible for nonselective oxidation to yield CO2. An optimum chlorine coverage of about 25% of surface silver atoms effectively blocks dissociative adsorption of oxygen by physical blocking ensuring increased selectivity. 

In addition, the large pore volume, pore size flexibility, and structural variety of MCM-41 can be extensively used for the selective adsorption of a diversity of gases and liquids [39,40], An extremely high sorption capacity for benzene has been demonstrated [40], Widespread work has been carried out on the sorption properties of some adsorbates, such as nitrogen, argon, oxygen, water, benzene, cyclopentane, toluene, and carbon tetrachloride, as well as certain lower hydrocarbons and alcohols on MCM-41 [122],... 

Early work of Barrer (3), McKee (4) and Domine and Hay (5) showed that calcium A, calcium X, mordenite and several types of natural zeolites could be used to enrich air by a selective adsorption of nitrogen. Several pressure-swing-adsorption processes utilizing zeolite adsorbents have been developed which yield a product containing up to 95% oxygen at rates of 20 tons per day (6,7). 

Oxidation of ethene on silver catalysts to yield ethene oxide is a good example of an industrial catalytic process with a high selectivity. In order to confirm a possible correlation between the catalysts affinity towards oxygen and their activity in ethene epoxidation, a heat-flow microcalorimeter equipped with a pulse flow reactor has been used to study the reaction of oxygen at 473 K with a series of silica-supported silver catalysts [71]. At 473 K, adsorption of oxygen at the surface of silver is a fast process incorporation of oxygen into deeper metal layers, though present, is a slow process. 

A reaction mechanism for the OXD of cyclohexane over Cu(II)-Y Zeolite, has been proposed in which the adsorption of oxygen was found to be the rate determining step [35]. A similar trend was observed for the OXD of cyclohexene over Co(II)-Y Zeolite [33], as a high adsorbed oxygen reactivity and benzene selectivity were found in both reactions. 

The effect of oxygen might be explained in two ways. First, one might suppose that the sites that were last (most difficult) to reduce, which also produce high-MW polymer, would be most reactive with oxygen. Therefore, the adsorption of oxygen might be considered as selectively... 

Sample Preparation. Samples were prepared by exposing the silica gel, previously dried at 150 °C for 24 h, to either cyclohexane or pentane solutions containing selected amounts of the fluorophore. The solvent was carefully removed under vacuum when required. Complete probe adsorption of liquid-solid samples was verified with absorption spectroscopy. Less than 0.07% of the silica surface was typically covered by the probe. Immediately before data collection, dry samples were evacuated under vacuum at 125-130 °C for 30 min. The total dehydration procedure was sufficient to remove the physisorbed water while leaving the surface silanol functionality intact (12). Selected amounts of oxygen were introduced into a... 

A in their smallest dimension, in that range, could be completely separated by selective adsorption. This high resolution is not achieved by homogeneous enlargement of the pore dimensions since the activating process involves chemical adsorption of oxygen molecules and desorption of a 3k entity like CO or CO2 molecules. In essence it is a kinetic separation (1) with a... 

---