Temperature programming theory

This equation, although originating from the plate theory, must again be considered as largely empirical when employed for TLC. This is because, in its derivation, the distribution coefficient of the solute between the two phases is considered constant throughout the development process. In practice, due to the nature of the development as already discussed for TLC, the distribution coefficient does not remain constant and, thus, the expression for column efficiency must be considered, at best, only approximate. The same errors would be involved if the equation was used to calculate the efficiency of a GC column when the solute was eluted by temperature programming or in LC where the solute was eluted by gradient elution. If the solute could be eluted by a pure solvent such as n-heptane on a plate that had been presaturated with the solvent vapor, then the distribution coefficient would remain sensibly constant over the development process. Under such circumstances the efficiency value would be more accurate and more likely to represent a true plate efficiency. 

All of the Au/metal oxide catalysts deactivate quickly, under the conditions shown in Figure 4. In addition, the deactivation of the Au/metal oxide catalysts appears to be enhanced in the presence of COj. In support of the theory that increased basicity of the metal oxides leads to lower stability, we carried out COj temperature programmed desorption experiments on the various catalysts. The COj TPD data also confirmed that an increase in the basicity of the metal oxides leads to an increase in the amount of COj adsorption on the catalysts. 

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. 

Flowers, M. C., Jonathan, N. . H., Yong, L. and Morris, A. Temperature programmed desorption of molecular hydrogen from a Si(100)-2 x 1 surface theory and experiment. Journal of Chemical Physics 99, 7038 (1993). 

The first chapter (Chapter 10) in the section on catalyst characterization summarizes the most common spectroscopic techniques used for the characterization of heterogeneous catalysts, such as XPS, Auger, EXAFS, etc. Temperature programmed techniques, which have found widespread application in heterogeneous catalysis both in catalyst characterization and the simulation of pretreatment procedures, are discussed in Chapter 11. A discussion of texture measurements, theory and application, concludes the section on the characterization of solid catalysts (Chapter 12). 

Elliott J.A.W. and Ward C.A., Temperature programmed desorption A statistical rate theory approach, J. Chem. Phys. 106 (1997) pp.5677-5684. 

The basic theory for nonisothermal prediction of degradation rate was established in the 1950s.320-321 Its application to the stability prediction of pharmaceuticals was reported by Rogers322 and extended by Eriksen and Stelmach.323 Initial temperature programs or algorithms used relationships that could easily be integrated when inserted into Eq. (2.84). 

The theory of desorption of atoms and molecules by temperature-programmed desorption has been reviewed in references [61, 74-76, 124-126]. Discuss the assumptions made in deriving the first- and second-order desorption rates and their correlation to the temperature of the maximum desorption rates. How does the magnitude of the preexponential factor reflect the assumption of (a) a mobile adsorbate layer or (b) an immobile adsorbate layer ... 

Composites containing different types of guests (metal or alloy particles, oxides, sulfides, complexes, polymers) in the cavities of zeolite hosts are prepared for various appHcations in materials research and catalysis. Except for quality assessment by detection of extra-zeolite material after synthesis or thermal treatments, photoemission plays a largely auxiliary role in this area, cooperating with bulk techniques such as X-ray absorption, UV-Vis, IR of probe molecules, and temperature-programmed reduction. The attention drawn to the significance of intra-zeolite potentials by XPS studies [12] has, however, contributed to the elaboration of a new theory of metal-support interactions [18,19]. 

A comparison between experimental adsorption energies for different adsorbates on different metal surfaces estimated from UHV temperature-programmed desorption studies and those calculated using density functional theory is shown in Fig. 1.1a. Although this is a very useful first step, the differences are certainly not within the 5 kJ/mol engineering accuracy that one would like. Figure 1. lb shows a comparison between HREELS and DFT calculated vibrational frequencies for maleic anhydride adsorbed on Pd(lll). 

Oxide Growth Kinetics and Mechanism. Formation of oxide films by potentiostatic polarization and their characterization by CV enables distinction of various oxide states as a function of the polarization conditions, here Ep, tp and T. This method allows precise determination of the thickness of oxide films with accuracy comparable to the most sensitive surface science techniques 4-7J1-20), CV may be considered the electrochemical analog of temperature programmed desorption, TPD, and one may refer to it as potential programmed desorption, PPD. Theoretical treatment of such determined oxide reduction charge densities by fitting of the data into oxide formation theories leads to derivation of important kinetic parameters of the process as a function of the polarization conditions. The kinetics of electro-oxidation of Rh at the ambient temperature were studied and some representative results are reported in ref 24. The present results are an extension of the previous experiments and they involve temperature dependence studies. 

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