Flash desorption measurements

Quantitative Flash Desorption Measurements from Tungsten... 

The quantitative flash desorption measurements therefore establish that at the temperature of evolution f nitrogen is present on the surfaces as atoms. Desorption then involves the collision of two energetic adatoms with one another and their evaporation as a molecule. 

An even closer look at the atomic details of the interaction of xenon with tungsten cannot be achieved the binding is so weak that observation in the field ion microscope does not appear feasible. However, flash desorption measurements extended to crystals with a single surface... 

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... 

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. 

Experimentally, information about the adsorption and desorption rates is obtained with the help of programmed desorption. One procedure is flash desorption A surface is instantaneously heated up (normally in vacuum) and we measure the temporal desorption of material, for instance with a mass spectrometer. Heating is usually done with a laser pulse (PLID, pulsed laser induced thermal desorption). 

Yates et al. (66) measured change in ion currents from the surface relative to the concentration of CO adsorbed in the a and j8 phases, as determined by flash desorption. They found that, like the a phase, there was a delay in build-up of the ion current until after adsorption into the states has occurred extensively (Fig. 13). This supported the view of Menzel and Gomer (51-53) that the a phase contributes largely to the... 

It is agreed by Degras and Lecante (81) and Lichtman and co-workers (68) that the predominant ionic species electronically desorbed from CO adsorbed on molybdenum at room temperature is O. The latter workers (68) showed that the a-CO, as observed by them using flash filament measurements, had the largest 0+ desorption cross-section toward 100-eV electrons, as had proved to be the case for tungsten. The j8 phase was shown however to contribute up to a few percent to the O signal as illustrated in Pig. 21. The /3 phase contribution to the... 

The flash desorption results agree well with the heats of adsorption measurements on evaporated nickel films (80,137) (Fig. 20). McBaker and Rideal (137) reported however that under certain conditions, CO undergoes disproportionation over evaporated nickel films at temperatures above 190°C to give carbon and carbon dioxide, the rate of the... 

Flash desorption, although still dependent upon macroscopic wire samples, has made it possible to quantitatively measure rate processes involving the transfer of molecules between the gas phase and the solid. In principle, even the dependence of surface kinetics on atomic structure could be established by studies on macroscopic samples. The specification of surface features below 100A is difficult on such samples. For this purpose measurements in the field emission and ion microscope are more convenient and powerful—they afford a view of the surface on a scale approaching atomic dimensions. However, such work can only be properly carried out against a background of detailed macroscopic information, and it is this sequence from macroscopic measurements to direct observation of atoms that will be followed here. 

This limitation can be overcome by separating the heating and measuring function as in the desorption spectrometer, shown in Fig. 27, which was suggested by Rodbell and employed in the 1st quantitative flash desorption studies (6). Here the sample is heated by direct current,... 

Flash desorption, as well as other kinetic measurements, are fruitful sources of information on the energetics of surface processes. Indeed, for some systems, especially those in which processes occur at high temperatures, the traditional techniques such as calorimetry and isotherm determinations are difficult to execute and interpret. In order to compare the results obtained by flash desorption with kinetic and equilibrium measurements by more standard techniques, a sketch of the interrelations between energy parameters (29) is in order. 

Another example of physical interest arises in flash desorption. There the population and desorption energy of molecules held in different states of binding can be determined in detail. To compare these measurements with the results of calorimetric and isotherm studies, the desorption energy for each state must first be properly weighted by its population. This is illustrated for the adsorption of CO on tungsten in Fig. 31. There a diminution of the differential heat of adsorption... 

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. 

The power of the flash desorption technique lies in its ability to give a rapid, direct count of the number of adsorbed species on a surface. However, information on the properties of the adsorbed layer is obtained only indirectly, by deduction from adsorption and desorption measurements. To supplement these indirect studies, there are needed techniques that yield information on the properties of the adsorbed material by direct observation of the gas layer. 

Low field or contact potential measurements on well-defined macroscopic surfaces have an advantage here. The total amount of adsorbed material can be measured separately by flash desorption. Moreover, the contact potential A corresponds to an area average, which is also approached in low field emission measurements. The change in the contact potential in adsorption can therefore be unequivocally related to the dipole moment per adatom through Eq. (32). The difficulty in this approach lies in the preparation of a truly uniform surface of macroscopic size, which has not as yet been accomplished. 

Flash desorption does not provide information on surface diffusion, or on the crystal dependence of the binding energy a decision on the mechanism of adsorption based solely on this one type of measurement is therefore not possible. 

The presence of multiple states in flash desorption, which in Section II, C, 1, b, was assigned to a structural effect, has been confirmed by the observation of significant variations in the binding energy of Xe over the surface. More than that, the diminution of the heat of adsorption, deduced from macroscopic measurements, likewise appears to be dependent upon the structure of the surface—in the field emission microscope, the Ilf planes were found to fill in last and with a lower binding energy than typical of rougher planes. 

The mechanism described above, with irreversible adsorption of reactants and irreversible desorption of the Mari explained very satisfactorily all the experimental data of Tamaru et and Boudart et for the decomposition rate of ammonia at high temperatures and low pressures on tungsten and molybdenum respectively with simultaneous measurement of the surface concentration of N by Auger electron spectroscopy. Disturbing the stationary state by flashing desorption of the metal catalysts, the rate of returning to steady state of the two postulated irreversible steps of adsorption and desorption could be evaluated separately. 

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. 

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