Adsorbent lifetime

A major concern regarding the use of this technology under such rapidly fluctuating pressures is the stability of the adsorbent. Long-term testing of several types of synthetic zeolites and activated carbon in bench-scale, large-pilot-scale, and commercial units has demonstrated that adsorbent lifetimes of well over one year are to be expected in most applications. 

The trapping probability, a, is the probability of adsorption into a weakly held state on the surface and is measured under conditions where the adsorbate lifetime is small compared with the experimental time period. It is most readily measured by molecular beam techniques a typical experimental arrangement is depicted in Fig. 23. Depending on the sophistication of the equipment, the parameters which may be measured include the velocity distribution of incident and scattered particles and... 

The energetics and kinetics of film formation appear to be especially important when two or more solutes are present, since now the matter of monolayer penetration or complex formation enters the picture (see Section IV-7). Schul-man and co-workers [77, 78], in particular, noted that especially stable emulsions result when the adsorbed film of surfactant material forms strong penetration complexes with a species present in the oil phase. The stabilizing effect of such mixed films may lie in their slow desorption or elevated viscosity. The dynamic effects of surfactant transport have been investigated by Shah and coworkers [22] who show the correlation between micellar lifetime and droplet size. More stable micelles are unable to rapidly transport surfactant from the bulk to the surface, and hence they support emulsions containing larger droplets. 

It is the relationship between the bound potential energy surface of an adsorbate and the vibrational states of the molecule that detemiine whether an adsorbate remains on the surface, or whether it desorbs after a period of time. The lifetime of the adsorbed state, r, depends on the size of the well relative to the vibrational energy inlierent in the system, and can be written as... 

Such lifetimes vary from less than a picosecond to times greater than the age of the universe [29]. Thus, adsorbed states with short lifetimes can occur during a surface chemical reaction, or long-lived adsorbed states exist in which atoms or molecules remain attached to a surface indefinitely. 

Many of the fiindamental physical and chemical processes at surfaces and interfaces occur on extremely fast time scales. For example, atomic and molecular motions take place on time scales as short as 100 fs, while surface electronic states may have lifetimes as short as 10 fs. With the dramatic recent advances in laser tecluiology, however, such time scales have become increasingly accessible. Surface nonlinear optics provides an attractive approach to capture such events directly in the time domain. Some examples of application of the method include probing the dynamics of melting on the time scale of phonon vibrations [82], photoisomerization of molecules [88], molecular dynamics of adsorbates [89, 90], interfacial solvent dynamics [91], transient band-flattening in semiconductors [92] and laser-induced desorption [93]. A review article discussing such time-resolved studies in metals can be found in... 

Beokerle J D, Casassa M P, Cavanagh R R, Heilweil E J and Stephenson J C 1990 Ultrafast infrared response of adsorbates on metal surfaoes vibrational lifetime of CO/Pt(111) Phys. Rev. Lett. 64 2090-3... 

Another nonregenerative drying appHcation for molecular sieves is their use as an adsorbent for water and solvent in dual-pane insulated glass windows. The molecular sieve is loaded into the spacer frame used to separate the panes. Once the window has been sealed, low hydrocarbon and water dew points are maintained within the enclosed space for the lifetime of the unit. Consequently, no condensation or fogging occurs within this space to cloud the window. 

TBT exists in solution as a large univalent cation and forms a neutral complex with CH or OH . It is extremely surface active and so is readily adsorbed onto suspended particulate material. Such adsorption and deposition to the sediments limits its lifetime in the water column. Degradation, via photochemical reactions... 

The width and shape of the energy loss peaks in HREELS are usually completely determined by the relatively poor instrumental resolution. This means that no information can be obtained from HREELS about such interesting chemical physics questions as vibrational energy transfer, since the influence of the time scale and mechanism of vibrational excitations at surfaces on the lifetimes, and therefore the line widths and shapes, is swamped. (Adsorbates on surfaces have intrinsic vibra-... 

The spillover effect can be described as the mobility of sorbed species from one phase on which they easily adsorb (donor) to another phase where they do not directly adsorb (acceptor). In this way a seemingly inert material can acquire catalytic activity. In some cases, the acceptor can remain active even after separation from the donor. Also, quite often, as shown by Delmon and coworkers,65 67 simple mechanical mixing of the donor and acceptor phases is sufficient for spillover to occur and influence catalytic kinetics leading to a Remote Control mechanism, a term first introduced by Delmon.65 Spillover may lead, not only to an improvement of catalytic activity and selectivity but also to an increase in lifetime and regenerability of catalysts. 

Solid-surface room-temperature phosphorescence (RTF) is a relatively new technique which has been used for organic trace analysis in several fields. However, the fundamental interactions needed for RTF are only partly understood. To clarify some of the interactions required for strong RTF, organic compounds adsorbed on several surfaces are being studied. Fluorescence quantum yield values, phosphorescence quantum yield values, and phosphorescence lifetime values were obtained for model compounds adsorbed on sodiiun acetate-sodium chloride mixtures and on a-cyclodextrin-sodium chloride mixtures. With the data obtained, the triplet formation efficiency and some of the rate constants related to the luminescence processes were calculated. This information clarified several of the interactions responsible for RTF from organic compounds adsorbed on sodium acetate-sodium chloride and a-cyclodextrin-sodium chloride mixtures. Work with silica gel chromatoplates has involved studying the effects of moisture, gases, and various solvents on the fluorescence and phosphorescence intensities. The net result of the study has been to improve the experimental conditions for enhanced sensitivity and selectivity in solid-surface luminescence analysis. 

Solid-surface fluorescence and phosphorescence quantum yield values were obtained from +23° to -180°C for the anion of p-aminobenzoic acid adsorbed on sodium acetate (11). Fhosphorescence lifetime values were also obtained for the adsorbed anion from +23° to -196°C. Table 1 gives the fluorescence and phosphorescence quantum yield values acquired. The fluorescence quantum yield values remained practically constant as a function of temperature. However, the phosphorescence quantum yield values changed substantially with temperature. The phosphorescence lifetime experiments indicated two decaying components. Each component showed a gradual increase in phosphorescence lifetime with cooler temperatures, but then the increase appeared to level off at the coldest temperatures. 

The phosphorescence lifetimes for the p-aminobenzoic acid anion adsorbed on sodium acetate as a function of temperature were evaluated in a manner similar to the one discussed by Oelkrug and coworkers (,28-30) for polycyclic aromatic hydrocarbons adsorbed on y-alumina. In general, the solid-surface phosphorescence lifetime cutrves for the anion of p-aminobenzoic acid followed Equation 2. 

Sodium Acetate-Sodium Chloride Mixtures. Ramasamy and Hurtubise (12) obtained RTF and RTF quantum yields, triplet formation efficiency, and phosphorescence lifetime values for the anion of p-aminobenzoic acid adsorbed on sodium acetate and on several sodium acetate-sodium chloride mixtures. Rate constants were calculated for phosphorescence and for radiationless transition from the triplet state. The results showed that several factors were important for maximum RTF from the anion of p-aminobenzoic acid. One of the most important of these was how efficiently the matrix was packed with sodium acetate molecules. A similar conclusion was found for RTF however, the RTF quantum yield increased more dramatically than the RTF quantum yield. 

Burrell and Hurtubise (.32) investigated calibration curves extended well beyond the normal linear range for RTF and RTF of benzoCf)quino-line adsorbed on a silica gel chromatoplate under neutral and acidic conditions. As the benzoCf)quinoline concentration increased, the RTF curves leveled off, whereas the RTF curves passed through a maximum and then decreased. The extended calibration curves along with fluorescence and phosphorescence spectra and phosphorescence lifetimes for benzoCf)quinoline revealed differences in the RTF and RTF phenomena. For example, it was determined that RTF could arise from molecules adsorbed on the surface and in multilayers of molecules, whereas phosphorescence was only generated from molecules adsorbed on the surface of the chromatoplate and not in the multilayers. ... 

Filaments are usually refractory metals such as tungsten or iridium, which can sustain high temperatures for a long time (T > 3000 K). The lifetime of filaments for electron sources can be prolonged substantially if an adsorbate can be introduced that lowers the work function on the surface so that it may be operated at lower temperature. Thorium fulfills this function by being partly ionized, donating electrons to the filament, which results in a dipole layer that reduces the work function of the tungsten. In catalysis, alkali metals are used to modify the effect of the work function of metals, as we will see later. 

An excited particle that is to become involved in the electrochemical reaction must be sufficiently close to the electrode surface to diffuse to the surface within the lifetime of its excited state. It is better yet when it is present on the surface as an adsorbate. Sometimes, dyes are applied to the surface which are not themselves involved in any electrochemical reaction but which when excited react with the solution to produce a soluble secondary substance that will react (sensitization of the electrode surface). 

TR-SFG seems to be an ideal tool to study the surface dynamics of adsorbed CO at solid/liquid interfaces. Although there are several reports of TR-SFG studies on an electrode, they are only of investigations of vibrational relaxation lifetime by IR excitation [34, 65, 67]. 

The majority of heterogeneous chemical and physical-chemical processes lead to formation of the intermediate particles - free atoms and radicals as well as electron- and oscillation-excited molecules. These particles are formed on the surface of solids. Their lifetime in the adsorbed state Ta is determined by the properties of the environment, adsorbed layer, and temperature. In many cases Ta of different particles essentially affects the rate and selectivity of heterogeneous and heterogeneous-homogeneous physical and chemical processes. Therefore, it is highly informative to detect active particles deposited on surface, determine their properties and their concentration on the surface of different catalysts and adsorbents. 

Lifetimes of free atoms and radicals account for the degree of interaction of these particles with an ambient medium and with each other. Due to high reaction capability of active particles in gaseous and, especially, in liquid media, their lifetimes are rather small. In gaseous phase, at small pressures these lifetimes are determined by heterogeneous recombination of these particles on vessel walls and by interaction of these particles with an adsorbed layer. At high gas pressures, the lifetimes are determined by bulk recombination and chemical interaction with ambient molecules. 

Henry constant F can be evaluated by the method of Tsvitering and Krevelyn (see [105]) based on the concept of mean lifetime of a molecule absorbed on a surface proposed by De Boer [106]. The number of molecules adsorbed on a surface is... 

Adsorbed ethanol (CHsCHzOHad) ean be converted to CHsCHOad by hydrogen abstraction, as shown in path A. CH3CH20Had may react with h to produce Itrani, Itran2, and Itrans, wMch represent transient intermediates with lifetimes that could be too short and concentrations that could be too small to be detected by... 

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