Programmed Desorption

Figure 2. Variations of products as a function of the thermal desorption program following the reaction of ethane with 0 on MgO at 25°C. (BD indicates butadiene)(17).

Van Lieshont and co-workers [8] have adopted a high temperature programmable, temperature vapourisation injector for use as a multi-step thermal desorption programmed Py-GC to stndy polymer mixtures. They analysed a complex mixture of polymers and additives nsing this method. 

TPD Temperature-programmed desorption [171, 172] The surface is heated and chemisorbed species desorb at characteristic temperatures Characterization of surface sites and desorption kinetics... 

A powerful technique in studying both adsorption and desorption rates is that of programmed desorption. The general procedure (see Refs. 36, 84) is to expose a clean metal filament or a surface to a known, low pressure of gas that flows steadily over it. The pressure may be quite low, for example, 10 mm Hg or less, so that even nonactivated adsorption can take some minutes for... 

Temperature-programmed desorption (TPD) is amenable to simple kinetic analysis. The rate of desorption of a molecular species from a uniform surface is given by Eq. XVII-4, which may be put in the form... 

Mention was made in Section XVIII-2E of programmed desorption this technique gives specific information about both the adsorption and the desorption of specific molecular states, at least when applied to single-crystal surfaces. The kinetic theory involved is essentially that used in Section XVI-3A. It will be recalled that the adsorption rate was there taken to be simply the rate at which molecules from the gas phase would strike a site area times the fraction of unoccupied sites. If the adsorption is activated, the fraction of molecules hitting and sticking that can proceed to a chemisorbed state is given by exp(-E /RT). The adsorption rate constant of Eq. XVII-13 becomes... 

Studies to determine the nature of intermediate species have been made on a variety of transition metals, and especially on Pt, with emphasis on the Pt(lll) surface. Techniques such as TPD (temperature-programmed desorption), SIMS, NEXAFS (see Table VIII-1) and RAIRS (reflection absorption infrared spectroscopy) have been used, as well as all kinds of isotopic labeling (see Refs. 286 and 289). On Pt(III) the surface is covered with C2H3, ethylidyne, tightly bound to a three-fold hollow site, see Fig. XVIII-25, and Ref. 290. A current mechanism is that of the figure, in which ethylidyne acts as a kind of surface catalyst, allowing surface H atoms to add to a second, perhaps physically adsorbed layer of ethylene this is, in effect, a kind of Eley-Rideal mechanism. 

The temperature-programmed desorption of NO2 shows a maximum rate at 330 K using a heating rate of 7.53°C/sec. Take A to be lO sec , and assume that Eq. XVIII-1 applies. (Note Ref. 344.)... 

TPD Temperature programmed desorption After pre-adsorption of gases on a surface, the desorption and/or reaction products are measured while the temperature Increases linearly with time. Coverages, kinetic parameters, reaction mechanism... 

Thenual desorption spectroscopy (TDS) or temperature progranuned desorption (TPD), as it is also called, is a simple and very popular teclmique in surface science. A sample covered with one or more adsorbate(s) is heated at a constant rate and the desorbing gases are detected with a mass spectrometer. If a reaction takes place diirmg the temperature ramp, one speaks of temperature programmed reaction spectroscopy (TPRS). 

Fig. 15. Temperature-programmed desorption (TPD) spectra SWCNTs, (b) activated carbon and (c) open-tip SWCNTs [52].

B. Meng, W. H. Weinberg. Monte Carlo simulation of temperature programmed desorption spectra. J Chem Phys 700 5280-1589, 1994. 

In a recent paper [11] this approach has been generalized to deal with reactions at surfaces, notably dissociation of molecules. A lattice gas model is employed for homonuclear molecules with both atoms and molecules present on the surface, also accounting for lateral interactions between all species. In a series of model calculations equilibrium properties, such as heats of adsorption, are discussed, and the role of dissociation disequilibrium on the time evolution of an adsorbate during temperature-programmed desorption is examined. This approach is adaptable to more complicated systems, provided the individual species remain in local equilibrium, allowing of course for dissociation and reaction disequilibria. 

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. 

Application of a Temperature-Programmed Desorption Technique to Catalyst Studies... 

Erdal, B.R. Aguilar, R.D. Bayhurst, B.P. Daniels, W.R. Duffy, C.J. Lawrence, F.O. Maestas, S. Oliver, P.Q. Wolfsberg, K. "Sorption-Desorption Studies on Granite. I. Initial Studies of Strontium, Technetium, Cesium, Barium, Cerium, Europium, Uranium, Plutonium, and Americium", in "Proceedings of the Task 4 Waste Isolation Safety Assessment Program Second Contractor Information Meeting", Vol. II, Report PNL-SA-7352, Battelle Pacific Northwest Laboratory, 1978, pp. 7-67. 

Figure 2.27. Temperature programmed desorption (TPD) spectra of carbon monoxide (measured by Ap) as a function of temperature from nickel surfaces (a) Ni(l 11), (b) Ni(l 11) when the initially dosed surface has been subjected to an electron beam (150 pA for 10 minutes over an area of 1 mm2) and (c) a cleaved nickel surface.85 Reprinted with permission from Elsevier Science.

P. Berlowitz, C. Megiris, J.B. Butt, and H.H. Kung, Temperature-Programmed Desorption Study of Ethylene on a Clean, a H-Covered, and an O-Covered Pt( 111) Surface, Langmuir 1, 206-212 (1985). 

As already noted the strength of chemisorptive bonds can be varied in situ via electrochemical promotion. This is the essence of the NEMCA effect. Following initial studies of oxygen chemisorption on Ag at atmospheric pressure, using isothermal titration, which showed that negative potentials causes up to a six-fold decrease in the rate of 02 desorption,11 temperature programmed desorption (TPD) was first used to investigate NEMCA.29... 

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