Flash desorption method

The energy parameters for chemisorption derived from different experimental measurements are therefore comparable, provided that for nonequilibrium determinations both in calorimetric and in rate studies the molecular processes are properly identified. Here the detailed insights achievable with flash desorption methods are particularly important. 

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... 

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. 

Application of Spectrophotometry to the Study of Catalytic Systems H. P. LeftinandM. C, Hobson,Jr. Hydrogenation of Pyridines and Quinolines Morris Freifelder Modern Methods in Surface Kinetics Flash, Desorption, Field Emission Microscopy, and Ultrahigh Vacuum Techniques Gert Ehrlich... 

In a determination of s2 involving the adsorption of hydrogen at 77 K, a value of 0.07 was obtained. The more accurate method of flash desorption gave s2 = 0.1. These experiments establish that s2 is virtually temperature independent. For reasons of self-consistency, the value s2 = 0.05 was used in implementing eqns. (80) and (82). 

Another method, not technically related, that shows promise and that has been used more extensively than PD in analysis of oligosaccharides, although still not enough to be fully evaluated, is that of flash desorption (78JA1974 79ACR359 82SIJ110). In these papers, Daves and co-workers have clearly shown that this technique has some value. It appears unlikely, however, that it will develop into a standard analytical method. 

In 1961, two researchers published reports of systematic studies of CO on poly crystalline tungsten using flash desorption experiments. Ehrlich (27) used a technique involving the very rapid flash, and Redhead (22) the slower desorption method. The results obtained from these two studies, and from later reports by Ehrlich (16, 28), were in good agreement, bearing in mind the limitations of the technique and the undefined nature of the poly crystalline substrates. These results and more recent studies will be discussed in the next two sections. 

A proper judgment of the validity of these findings, as well as any extension of such work, must rest upon a detailed appreciation of the experiments involved. It is the aim of this article to review the experimental methods upon which these advances have been based—the flash filament technique, flash desorption, field emission and field ion microscopy, and the use of ultrahigh vacuum procedures. 

This was the method employed in the first quantitative study of flash desorption. 

Approach to equilibrium. In flash desorption the activation energy should be completely equivalent to that found by standard methods. The extent to which an equilibrium distribution of adatoms over the surface can be obtained may, however, remain in question. 

The relative merits of flash desorption and field emission, as well as the problems encountered in the study of adsorption phenomena by each can best be appreciated by sketching the information attained on one particular system. Here we will consider the data on the interaction of xenon with W, obtained by both methods (7, 45). 

Various techniques are used to obtain information on the active centers of catalysts, such as selective poisoning, measurement of the catalyst acidity and its strength, field electron and ion microscopy, infrared spectroscopy, fiash-filament desorption, differential isotopic method, etc. A temperature-programmed desorption method, which will be described and discussed in the present article, is in principle similar to the fiash-filament desorption method, reviewed recently by Ehrlich (1). It differs, however, from it in several respects. Modifications have been necessary in order to make the construction and operation of the apparatus easier and to adapt it to studies of materials other than metals, for example the conventional oxide catalysts. The conditions employed are much more similar to those ordinarily used in catalytic reactions than is the case with the fiash-filament method. An additional important feature of the modified technique is that it permits in some cases simultaneous study of a chemisorption process and the surface reaction which accompanies it. At the same time the modifications made have sacrificed some of the simplicity of the flash-filament method. For example, an obvious complication may arise from the porous structure of the conventional catalytic materials, in contrast to the relatively smooth surfaces of metal filaments. The potential presence of this and other complications requires extension of the relatively simple theoretical treatment of flash-filament desorption to more complicated cases. 

Temperature-programmed desorption (TPD) (Cvetanovic and Amenomiya 1972) yields not only the heat of adsorption but also information on groups of different active sites, which are useful in characterizing reactions. While the flash filament method in which an adsorbate is desorbed from a rapidly heated filament in an ultra-high vacuum environment has been used for a long time, the usefulness of the TPD method lies in the fact that the adsorbed gas is desorbed in a programmed... 

G. Ehrhch, Modern Methods in surface kinetics flash desorption, field emission microscopy, and ultrahigh vacuum techniques. Adv. Catal. 14, 255 27 (1963). doi 10.1016/S0360-0564(08)60341-7... 

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. 

The extraction of environmental or geological particulate matter with solvents, solvent mixtures or supercritical carbon dioxide is the preferred method, because it results in minimum alteration of its polar components, avoids hydrolysis of anhydrides, esters, etc. and is highly efficient for most organic compounds (even sugars). Other workers have reported the direct vaporization by thermal desorption or flash pyrolysis of organic compounds from particulate matter into GC, MS, or GC-MS instruments. Those methods work fine for neutral compounds (e.g. hydrocarbons) but should be used with caution when analyzing polar or labile compounds. 

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