Desorption experiments

Current use of statistical thermodynamics implies that the adsorption system can be effectively separated into the gas phase and the adsorbed phase, which means that the partition function of motions normal to the surface can be represented with sufficient accuracy by that of oscillators confined to the surface. This becomes less valid, the shorter is the mean adsorption time of adatoms, i.e. the higher is the desorption temperature. Thus, near the end of the desorption experiment, especially with high heating rates, another treatment of equilibria should be used, dealing with the whole system as a single phase, the adsorbent being a boundary. This is the approach of the gas-surface virial expansion of adsorption isotherms (51, 53) or of some more general treatment of this kind. 

This model system corresponds to the conditions under which flash desorption experiments are performed. The temperature programed desorption of Amenomyia and Cvetanovi6 is based on different model requirements as will be dealt with in Section IV.B. Therefore, the following treatment in the present section is pertinent only to the flash desorption conditions. 

Modification techniques for activated carhon were used to increase the removal capacity by surface adsorption and to improve the selectivity to volatile organic compounds (VOCs). Modified activated carbons (MACs) were prepared by modifying the purified activated carbon with various acids or bases. The effects of adsorption capacity and modified contents on the textural properties of the MACs were investigated. Furthermore, VOC adsorption and desorption experiments were carried out to determine the relationship between the adsorption capacity and the chemical properties of the adsorbents. High adsorption capacity for the selected VOCs was obtained over lwt%-H3P04/AC (lwt%-PA/AC). As a result, MAC was found to be very effective for VOC removal by adsorption with the potential for repeated use through desorption by simple heat treatment. 

NO is now chemisorbed on the Rh particles at a temperature where it does not adsorb on the AI2O3. The saturation coverage of NO on Rh(lOO) corresponds to one NO molecule per two rhodium surface atoms, with NO sitting in a c(2x2) surface structure. After having saturated the catalyst with NO, a temperature-programmed desorption experiment (TPD) is performed with a heating rate of 2 K min". NO is seen to desorb with a maximal rate at 460 K. The total NO gas that desorbs amounts to 18.5 mL per gram catalyst (P = 1 bar and T = 300 K). It can be assumed that NO does not dissociate on the Rh(lOO) surface. 

The laser desorption experiments which we describe here utilize pulsed laser radiation, which is partially absorbed by the metal substrate, to generate a temperature jump in the surface region of the sample. The neutral species desorbed are ionized and detected by Fourier transform mass spectrometry (FTMS). This technique has... 

SEM and" SEM-EDX analyses have been used in order to observe how and where the new material forms on the alumina support. XRD and MASNMR studies have been performed for its identification. Porous characteristics of the composite material have been explored using N2 adsorption-desorption experiments (Micromeritics ASAP 2000M)... 

All of the Au/metal oxide catalysts deactivate quickly, under the conditions shown in Figure 4. In addition, the deactivation of the Au/metal oxide catalysts appears to be enhanced in the presence of COj. In support of the theory that increased basicity of the metal oxides leads to lower stability, we carried out COj temperature programmed desorption experiments on the various catalysts. The COj TPD data also confirmed that an increase in the basicity of the metal oxides leads to an increase in the amount of COj adsorption on the catalysts. 

The results of a similar experiment with adsorbed hydrogen is shown in Fig. 2.3b. Only one desorption peak was observed in the temperature range studied [50], The desorption peak temperature lies at 420 K for the experiment with 0.8 L and is shifted to lower temperatures as the H2 concentration increases indicating second order desorption kinetics. Surface states with desorption temperatures at 165 K, 220 K, 280 K and 350 K were reported for the adsorption of H2 and D2 at 120 K [51]. Thermal desorption experiments after H2 adsorption at 350 K show only one desorption state at ca. 450 K [52],... 

A platinum electrode pretreated in the way as is described in Section 1.2 may show some minimal desorption of carbonaceous residues which may come from C-atoms diffusing from the bulk of Pt or from the rest of the gas in the UHV. A blank desorption experiment carried out by transferring a Pt electrode which was held at 450 mV in H2S04 for 120 s is shown in Fig. 2.4(a). 

Obviously, desorption of COHad at a given constant potential requires the break of the triple bond of COHad to Pt and the ionization of hydrogen. As was shown above in thermal desorption experiments H2 and CO are formed. The desorption process of COHad could require a higher activation energy than the desorption of... 

In order to test the reversibility of metal-bacteria interactions, Fowle and Fein (2000) compared the extent of desorption estimated from surface complexation modeling with that obtained from sorption-desorption experiments. Using B. subtilis these workers found that both sorption and desorption of Cd occurred rapidly, and the desorption kinetics were independent of sorption contact time. Steady-state conditions were attained within 2 h for all sorption reactions, and within 1 h for all desorption reactions. The extent of sorption or desorption remained constant for at least 24 h and up to 80 h for Cd. The observed extent of desorption in the experimental systems was in accordance with the amount estimated from a surface complexation model based on independently conducted adsorption experiments. 

However, due to the difficulties in calculating ion yields in SIMS, quantitation of the data is not very reliable, and their work was not conclusive. We have determined here that the reaction of chemisorbed ethylene to form ethylidyne is first order in ethylene coverage. A noticeable isotope effect was observed, with activation energies of 15.0 and 16.7 Kcal/mole for C H and 02 respectively. These values are smaller than those calculated from TDS, but the differences can be reconciled by including the recombination of hydrogen atoms on the surface in the interpretation of the thermal desorption experiments. 

These experiments also show the value of NEXAFS as a technique for following the kinetics of surface processes. We have shown that experiments can be tailored so a specific reaction can be studied, even if gas evolution is not involved. This represents an advantage over thermal desorption experiments, where several steps may be required in order to desorb the products to be detected. Another advantage of NEXAFS is that rates are measured isothermally, so the kinetic parameters can be determined with accuracy. Finally, NEXAFS is not a destructive technique, so we need not to worry about modifying the surface compounds while probing the system, as would be the case with other techniques such as Auger electron spectroscopy. 

As stated above, when probes with specific adsorption characteristics are used, additional chemical information can be extracted from adsorption-desorption experiments. Temperature-programmed desorption (TPD) in particular is often employed to obtain information about specific sites in catalysts [55,56], The temperature at which desorption occurs indicates the strength of adsorption, whereas either the amount of gas consumed in the uptake or the amount of desorption upon heating attests to the concentration of the surface sites. The most common molecules used in TPD are NH3 and C02, which probe acidic and basic sites, respectively, but experiments with pyridine, Oz, H2, CO, H20, and other molecules are often performed as well [57-59], As an example, the ammonia... 

Figure 3.37. Pd-H phase diagram. T-Xprojection (onto a plane at constant pressure P = 100 Pa) obtained from the experimental P—x isotherms. Because of hysteresis the data obtained in absorption or desorption experiments are slightly different.

Desorption experiments indicated that the pigments are very strongly bonded to SPE sorbents X and XI, while they can be easily eluted from the octadecylsilica SPE cartridge. Because of their high sample capacity and high preconcentration factor, ODS SPE cartridges were employed in further experiments. RP-HPLC-DAD analyses were carried out in an ODS column (250 X 4mm i.d. particle size 5/an) at ambient temperature. Solvents A and B were... 

In line with the general objectives in the present chapter we propose to discuss only the leaching and sorption/desorption experiments conducted by Aboul-Kassim [1] and Eldin et al. [66, 67]. In addition, the approach taken by these authors to predict the behavior of toxic compounds in the leachates from various SWMs/COMs is also discussed. 

During desorption experiments with PtieAuie (Fig. 7.7), the Au-CO band disappears quickly, and coincides with an increase in the Pt-CO band intensity. Closer inspection of the PtigAuig desorption experiment suggests that the broad room temperature Pt-CO band may be better described as two bands at 2065 and 2050 cm. ... 

Davis, S.C. Natoli, V. Neumann, G.M. Derrick, P.J. A Model of Ion Evaporation Tested Through Field Desorption Experiments on Glucose Mixed With Alkali Metal Salts. Int. J. Mass Spectrom. Ion Proc. 1987, 78, 17-35. 

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