In thermal desorption spectroscopy

Fig. 3. Schematic illustration of a second order desorption process in thermal desorption spectroscopy, TDS. Upon heating the adsorbed atoms form molecules before they desorh from the surface and are detected in amass spectrometer (MS).

While A

metal-water interactions are better probed by thermal desorption spectroscopy (TDS) in which heat is used to detach molecules from a surface. TDS data are in parallel with A (and AX) data. This is illustrated in Fig. 19.35 The spectrum of Ag(110) shows only one peak at 150 K, corresponding to ice sublimation. This means that Ag-H20 interactions are weaker than H20-H20 interactions (although they are still able to change the structure of the... 

Desorption is important both because it represents the last step in a catalytic cycle and because it is also the basis of temperature-programmed desorption (TPD), a powerful tool used to investigate the adsorption, decomposition and reaction of species on surfaces. This method is also called thermal desorption spectroscopy (TDS), or sometimes temperature programmed reaction spectroscopy, TPRS (although strictly speaking the method has nothing to do with spectroscopy). 

We have undertaken a series of experiments Involving thin film models of such powdered transition metal catalysts (13,14). In this paper we present a brief review of the results we have obtained to date Involving platinum and rhodium deposited on thin films of tltanla, the latter prepared by oxidation of a tltanliua single crystal. These systems are prepared and characterized under well-controlled conditions. We have used thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES) and static secondary Ion mass spectrometry (SSIMS). Our results Illustrate the power of SSIMS In understanding the processes that take place during thermal treatment of these thin films. Thermal desorption spectroscopy Is used to characterize the adsorption and desorption of small molecules, In particular, carbon monoxide. AES confirms the SSIMS results and was used to verify the surface cleanliness of the films as they were prepared. 

The apparatuses used for the studies of both ammonia synthesis emd hydrodesulfurization were almost identical, consisting of a UHV chamber pumped by both ion and oil diffusion pumps to base pressures of 1 x10 " Torr. Each chamber was equipped with Low Energy Electron Diffraction optics used to determine the orientation of the surfaces and to ascertain that the surfaces were indeed well-ordered. The LEED optics doubled as retarding field analyzers used for Auger Electron Spectroscopy. In addition, each chamber was equipped with a UTI 100C quadrupole mass spectrometer used for analysis of background gases and for Thermal Desorption Spectroscopy studies. 

The variations in the kinetic parameters (E, m,n) with chlorine coverage shown in Fig. 5 are entirely consistent with our studies by thermal desorption spectroscopy, which show the effects of chlorine... 

Thermal desorption spectroscopy and temperature programmed reaction experiments have provided significant insight into the chemistry of a wide variety of reactions on well characterized surfaces. In such experiments, characterized, adsorbate covered, surfaces are heated at rates of 10-100 K/sec and molecular species which desorb are monitored by mass spectrometry. Typically, several masses are monitored in each experiment by computer multiplexing techniques. Often, in such experiments, the species desorbed are the result of a surface reaction during the temperature ramp. 

One of the standard surface science methods for assessing the concentration and stability of a chemisorbed species is thermal desorption spectroscopy (TDS). An early paper by Redhead ( 7) developed the conceptual framework for certain cases. Many papers since then have expanded the applicability of this method. Recent work of Madix Q8) > Weinberg (9) and Schmidt CIO) is particularly noteworthy. Most of this work focuses on the desorption of a single molecular species and not on reactions in desorbing systems. However, qualitative features of the temperature dependence of reactions can be assessed using this method. Figures 1 and 2 taken from the... 

Temperature programmed desorption (TPD) or thermal desorption spectroscopy (TDS), as it is also called, can be used on technical catalysts, but is particularly useful in surface science, where one studies the desorption of gases from single crystals and polycrystalline foils into vacuum [2]. Figure 2.9 shows a set of desorption spectra of CO from two rhodium surfaces [14]. Because TDS offers interesting opportunities to interpret desorption in terms of reaction kinetic theories, such as the transition state formalism, we will discuss TDS in somewhat more detail than would be justified from the point of view of practical catalyst characterization alone. 

CO oxidation, 38 236 differential heat of adsorption, 38 217 Biphasic systems, catalysis see Multiphase homogeneous catalysis BiPMo catalysts, 34 39 in formamide to nitrile reaction, 34 39 Bi-postdosing thermal desorption spectroscopy cyclohexene, 42 240... 

J.P. Maehlen, V.A. Yartys, R.V. Denys, M. Fichtner, C. Frommen, B.M. Bulychev, P. Pattison, H. Emerich, Y.E. EiUnchuk, D. Chernyshov, Thermal decomposition of AlH by in situ synchrotron X-ray diffraction and thermal desorption spectroscopy, J. Alloys Compd 446-447 (2007) 280-289. 

Adsorption at Low Pressure (P < 10" Torr). The adsorption of propene has been studied with thermal desorption spectroscopy (TDS) on all of the different forms of the (100) and (111) surfaces and under several different conditions of exposure. For exposures at low pressure (P< 10 Torr), no selective oxidation is observed. For small exposures (< 5 L) at low-temperature (100K-120K), four propene desorption states are observed from the Ci O(lll) surface comparecf to two desorption states from the Cu9O(100)-Cii surface. These TDS results are shown in Figure 3, and give a cfear indication of a structure-sensitive interaction of propene with Cu20. 

Temperature control in electrode kinetics, 1121 Terraces, electrodepositon, 1307, 1336 Thermal desorption spectroscopy (TDS), 787 Thermal reactions in semiconductors, definition, 1088... 

A similar analysis was performed for 0/Rh(l 11) where in the limit of zero coverage i>d = 2.5 x 10-3 cm2 sec-1 and d = 56 2 kcal/mol were derived (146). The latter value is similar to the desorption energies determined by thermal desorption spectroscopy for Pd(lll) [55 kcal/mol (130)] and for Ir(lll) [65 kcal/mol (133)]. A somewhat higher value (80 kcal/mol) was reported for Ru(0001) (148), which probably accounts for the smaller reactivity of this metal in the CO oxidation reaction. Isosteric heats of adsorption were only performed with Pd(100) [60 kcal/mol at medium coverages (756)] and with Pd(110) (2). In the latter case the adsorption energy was found to vary between 80 and 48 kcal/mol with increasing coverage which is similar to the TDS data derived for Ir(l 10) (124). 

The TPD experimental technique is alternatively, but less suitably, termed thermal desorption spectroscopy (TDS). It is a very useful complement to vibrational spectroscopy and can be applied to adsorption on single-crystal or finely divided metal surfaces. TPD involves the dynamic analysis, usually by mass spectrometry, of the gases desorbed from the surface as the temperature is raised at a uniform rate, starting from a known state of adsorption. In addition to... 

In this chapter, we discuss TPR and reduction theory in some detail, and show how TPR provides insight into the mechanism of reduction processes. Next, we present examples of TPO, TP sulfidation (TPS) and TPRS applied on supported catalysts. In the final section we describe how thermal desorption spectroscopy reveals adsorption energies of adsorbates from well-defined surfaces in vacuum. A short treatment of the transition state theory of reaction rates is included to provide the reader with a feeling for what a pre-exponential factor of desorption tells about a desorption mechanism. The chapter is completed with an example of TPRS applied in ultra-high vacuum (UHV), in order to illustrate how this method assists in unraveling complex reaction mechanisms. 

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