Temperature programmed desorption single-crystal surfaces

Describe the experimental set-up for temperature-programmed desorption from a single crystal surface. 

Such a possibility has been recognized by early workers,9 but in spite of this intriguing possibility, only recently has such a metal surface been created. Chiral kink sites were created on Ag single crystal surfaces to produce the enantiomeric surfaces Ag(643)s and Ag(643)R however, no differences between (R)- and (S)-2-butanol were observed for either the temperature-programmed desorption from the clean surfaces or the dehydrogenation (to 2-butanone) from preoxidized surfaces.10 Unfortunately, Ag exhibits few catalytic properties, so only a limited array of test reactions is available to probe enantioselectivity over this metal. It would be good if this technique were applied to a more catalytically active metal such as Pt. 

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. 

Chapter 11. Temperature programmed desorption (TPD) is popular for determining concentrations of species adsorbed on the surfaces of single crystals and of real catalysts. In principle one can also determine the energy of the adsorption bond, however, only after applying rather involved computations [15]. We refer to Ref. [16] for a critical evaluation of TDS analysis. 

A variety of model catalysts have been employed we start with the simplest. Single-crystal surfaces of noble metals (platinum, rhodium, palladium, etc.) or oxides are structurally the best defined and the most homogeneous substrates, and the structural definition is beneficial both to experimentalists and theorists. Low-energy electron diffraction (LEED) facilitated the discovery of the relaxation and reconstruction of clean surfaces and the formation of ordered overlayers of adsorbed molecules (3,28-32). The combined application of LEED, Auger electron spectroscopy (AES), temperature-programmed desorption (TPD), field emission microscopy (FEM), X-ray and UV-photoelectron spectroscopy (XPS, UPS), IR reflection... 

Figure 1 Schematic of the experimental UHV/electrochemical transfer system used for studies on modified platinum single-crystal surfaces. (From Ref. 26.) The UHV system has facilities for X-ray photoelectron spectroscopy (XPS), low-energy ion scattering spectroscopy (LEISS), low-energy electron diffraction (FEED), and temperature-programmed desorption (TPS). The electrochemical chamber allows the electrochemical cell, 0 with integral counter, reference, and secondary working electrode, to be brought to the surface allowing contact of the electrolyte with the transferred surface.

The dehydrogenation reaction produces acetone and hydrogen, and is dominant over basic oxides ( ) The dehydration reaction produces propene and water, and is dominant over acidic oxides. It would be interesting to see if the competition between these two pathways depend on the exposed crystal planes of ZnO. We report here the results of such an investigation. 2-Propanol was decomposed on ZnO single crystal surfaces by the temperature programmed decomposition technique. To assist the interpretation of data, the temperature programmed desorption of propene and acetone were also studied. 

The temperature-programmed desorption of H2 turned out to be a new tool for the determination of Cu metal surface areas [6]. For industrial purposes, the interaction of hydrogen with copper surfaces is a significant elementary step in methanol synthesis. Moreover, adsorp-tion/desorption kinetics on copper single crystal surfaces is an interesting topic in surface physics... 

In previous papers by the authors [9,10], the temperature-programmed desorption of N2 from an iron-based catalyst has been studied experimentally. The microkinetic analysis of these results is based on the kinetic simulation of ammonia synthesis by Stoltze and Nprskov [22-24] using the approach by Dumesic and Trevino [2]. On Fe single crystal surfaces it was possible to detect a di-molecular precursor labelled a-N2 — forN2 dissociation... 

Table 4 gives an overview over the experimental and theoretical results for the adsorption energy of CO on the (100) plane of MgO. The most accurate experimental value has been determined by Wichtendahl et al. [73] by means of temperature programmed desorption (TPD) of CO from carefully prepared MgO(lOO) single crystal surfaces. The value of 0.14 eV indicates that this adsorption is a rather weak physisorption. It is in reasonable agreement with data from infrared (IR) spectroscopy experiments on MgO microcrystals and powder samples [74-77]. The data of He et al., obtained for the adsorption of CO on ultrathin MgO films on metallic support [74], seem to indicate a much stronger C0/Mg0(100) bond with an adsorption energy of 0.43 eV,but this is... 

Temperature programmed desorption (TPD) Single crystal MgO(lOO) surface 0.14 73... 

Temperature-programmed desorption of ammonia from iron single-crystal surfaces after high-pressure ammonia synthesis proves to be a sensitive probe of the new surface binding sites formed upon restructuring. Ammonia TPD spectra for the four clean surfaces are shown in Fig. 4.19. Each surface shows distinct desorption sites. The Fe(llO) surface displays one desorption peak with a peak maximum at 658 K. Two desorption peaks are seen for the Fe(lOO) surface p2 and P ) at 556 K and 661 K. The Fe(lll) surface exhibits three desorption peaks Pi, P2, and p ) with peak maxima at 495 K, 568 K, and 676 K, and the Fe(211) plane has two desorption peaks P2 and P ) at 570 K and 676 K. Temperature-programmed desorption spectra for the AljcO /Fe(110), A1 03,/Fe(100), and A1 0 /Fe(lll) surfaces restructured in 20torr of water vapor are shown in Fig. 4.20. A new desorption peak, P2 develops on the restructured Al fOy/Fe(110)... 

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