Desorption, flash

Homes 30 Finland Tenax tubes Thermal desorption (flash desorption) GC-FID GC-MS [31]... 

TDS, FDS Thermal desorption spectroscopy. Flash desorption spectroscopy [173] Similar to TPD Similar to TPD... 

A more elaborate theoretical approach develops the concept of surface molecular orbitals and proceeds to evaluate various overlap integrals [119]. Calculations for hydrogen on Pt( 111) planes were consistent with flash desorption and LEED data. In general, the greatly increased availability of LEED structures for chemisorbed films has allowed correspondingly detailed theoretical interpretations, as, for example, of the commonly observed (C2 x 2) structure [120] (note also Ref. 121). 

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. 

In general, the flow rate F(t) consists of the following additive components the controlled flow rate Fd of the entering gas, the flow rate Fi which is due to parasitic leaks and/or diffusion, and the flow rate Fw resulting from possible adsorption-desorption processes on the system walls (in Section I, references are given to papers dealing with the elimination or control of the wall effects in the flash filament technique). In each of these flow rate components a particular ratio of the investigated adsorbate and of the inert gas exists and all these components contribute to the over-all mean values Fh(t) and F (t). 

The flash desorption technique is applied usually in ultrahigh vacuum conditions. Then all the mentioned contributions to S and F should be accounted for in the evaluation of the experimental desorption curves. The effect of Sw on the results of desorption measurements is discussed in... 

A different approach consists of stepwise changing the adsorbent temperature and keeping it constant at each of the prefixed values Tx, Ts,. . ., Tn for a certain time interval (e.g. 10 sec), thereby yielding the so-called step desorption spectra s(81-85). The advantage of this method lies in a long interval (in terms of the flash desorption technique) for which the individual temperatures Ti are kept constant so that possible surface rearrangements can take place (81-83). Furthermore, an exact evaluation of the rate constant kd is amenable as well as a better resolution of superimposed peaks on a desorption curve (see Section VI). What is questionable is how closely an instantaneous change in the adsorbent temperature can be attained. This method has been rarely used as yet. 

Modern Methods in Surface Kinetics Flash, Desorption, Field Emission Microscopy, and Ultrahigh Vacuum Techniques Gert Ehrlich... 

J.L. Falconer, and R J. Madix, Flash desorption activation energies DCOOH decomposition and CO desorption fromNi(l 10), Surf. Sci. 48, 393-405 (1975). 

Catalyst characterization - Characterization of mixed metal oxides was performed by atomic emission spectroscopy with inductively coupled plasma atomisation (ICP-AES) on a CE Instraments Sorptomatic 1990. NH3-TPD was nsed for the characterization of acid site distribntion. SZ (0.3 g) was heated up to 600°C using He (30 ml min ) to remove adsorbed components. Then, the sample was cooled at room temperatnre and satnrated for 2 h with 100 ml min of 8200 ppm NH3 in He as carrier gas. Snbseqnently, the system was flashed with He at a flowrate of 30 ml min for 2 h. The temperatnre was ramped np to 600°C at a rate of 10°C min. A TCD was used to measure the NH3 desorption profile. Textural properties were established from the N2 adsorption isotherm. Snrface area was calcnlated nsing the BET equation and the pore size was calcnlated nsing the BJH method. The resnlts given in Table 33.4 are in good agreement with varions literature data. 

LD), in which flash vaporisation of the sample is induced, may be applied. Other techniques which permit detection of less-volatile chemical species are FD (with simultaneous desorption/ionisation of molecules), FAB (with the sample dissolved (dispersed) in a suitable liquid) and SIMS (based on bombardment of a solid surface with high-energy ions). LD-FUCR-MS is superior to FAB-MS for polymer/additive identification because it gives molecular ion fragmentation [83],... 

Habenschaden, E. and Kiippers, J. (1984) Evaluation of flash desorption spectra , Surf. Sci., 138, L147. 

Eine wichtige Anwendung der thermischen Desorption im Gebiet sehr tiefer Drucke findet sich als flash-filament-Verfahren im Ab-schnitt IV C, S. 688. 

The purpose of this article is to review the results of transient low pressure studies of carbon monoxide oxidation over transition metal substrates. Particular emphasis is given to the use of in-situ electron spectroscopy, flash desorption, modulated beam and titration techniques. The strengths and weaknesses of these will be assessed with regard to kinetic insight and quantification. An attempt will be made to identify questions that are ripe for investigation. Although not limited to it, the presentation emphasizes our own work. A very recent review of the carbon monoxide oxidation reaction C l) will be useful to readers who are interested in a more comprehensive view. 

Throughout this section on pressure transients we have emphasized electron spectroscopy as a procedure for directly detecting surface species, and, with difficult calibration, their concentration. It is important to keep in mind that the detection limit for these is about 0.01 of a monolayer. Using flash desorption as a complementary technique this limit can be extended to 0.001 monolayer in certain cases. The fact remains that extremely labile chemisorbed species may be present in kinetically important but undetectable concentrations. Since residence times as short as 2 x 10 - seconds can be determined, molecular beam techniques, as described below, afford an alternative but indirect method of measuring the properties of these very reactive species. 

To increase the speed of the TIRF-based kinetic techniques, the perturbation can be optical rather than chemical. If the evanescent wave intensity is briefly flashed brightly, then some of the fluorophores associated with the surface will be photobleached. Subsequent exchange with unbleached dissolved fluorophores in equilibrium with the surface will lead to a recovery of fluorescence, excited by a continuous but much attenuated evanescent wave. The time course of this recovery is a measure of the desorption kinetic rate k2. This technique1-115) is called TIR/FRAP (or TIR/FPR) in reference to fluorescence recovery after photobleaching (or fluorescence photobleaching recovery). 

Examination of the effect of temperature (220°C-270°C) and the total heating time (20-45 minutes) on the thermal desorption process led us to adopt 2TO°C for 45 minutes as the optimum conditions for further studies. Assessment of the commercially available desorption systems in which there is flash heating of the sample, have shown that significant losses of some important compounds (p-ciesol, indole) occur on the surfaces of the metal connectors and sample transfer lines. It was to overcome these problems that the all silica system (Figure 2) was developed. 

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