Modeling temperature programmed desorption

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. 

The SCR catalyst is considerably more complex than, for example, the metal catalysts we discussed earlier. Also, it is very difficult to perform surface science studies on these oxide surfaces. The nature of the active sites in the SCR catalyst has been probed by temperature-programmed desorption of NO and NH3 and by in situ infrared studies. This has led to a set of kinetic parameters (Tab. 10.7) that can describe NO conversion and NH3 slip (Fig. 10.16). The model gives a good fit to the experimental data over a wide range, is based on the physical reality of the SCR catalyst and its interactions with the reacting gases and is, therefore, preferable to a simple power rate law in which catalysis happens in a black box . Nevertheless, several questions remain unanswered, such as what are the elementary steps and what do the active site looks like on the atomic scale ... 

Figure 9.15. Comparison of the total ammonia adsorption of coated and extruded V2O5/WO3—Ti02 catalysts. Catalyst volume = 7 cm3. Model gas for loading 10% 02, 5% H20, NH3 = 1000ppm, and balance N2. GHSV = 52000h 1. Model gas for temperature-programmed desorption (TPD) experiment 10% 02, 5% H20, NO = 1000 ppm, NH3 = 1000 ppm, and balance N2. NH3 desorbed is calculated as the sum of thermally desorbed NH3, directly measured at the catalyst outlet, and chemically desorbed NH3, measured by the reduction of the NO concentration due to the SCR reaction.

Figure 16 Simulated and experimental temperature-programmed desorption spectra for OlPt(lll). The solid lines are experimental spectra. The crosses indicate simulated spectra for a model of the lateral interactions with nearest and next-nearest pair interactions, and also a linear 3-particle interaction. The O2 is formed from two atoms at next-nearest-neighbor positions. The kinetic parameters are — 206.4 kj/mol, v = 2.5 x 10 s a = 0.773, cpxN — 19.9 kjjmol, tp NN = 5.5 kjjmol, and (punear = 6.1 kJImol. In each plot the curves from top to bottom are for initial oxygen coverage of 0.194, 0.164, 0.093, and 0.073 ML, respectively. The heating rate is 8 Kjs ...

Koehler, B.G., Middlebrook, AM., and Tolbert, M.A. (1992) Characterisation of model polar stratospheric cloud forts using Fourier transform infrared spectroscopy and temperature programmed desorption, J. Geophys. Res. 97,8065-8074. 

In the last decade, a number of publications has been devoted to this subject. These studies are either based on spectroscopic techniques (IR, NMR) or on desorption techniques (temperature programmed desorption of pyridine and water). In all of these models the distinction between free and bridged silanols, trapped water and intraglobular hydroxyls is the key problem. 

Mechanistic studies start with determination of the kinetic rate law and the rate-limiting step information on heat and mass transfer is also needed. These studies may use such techniques as isotopic labeling, chemisorption measurements, surface spectroscopy, temperature-programmed desorption, and kinetic modeling experiments. 

In contrast to the acetaldehyde decarbonylation, reactions with ethanol over Rh (111) did not lead to formation of methane but rather to an oxametallocycle via methyl hydrogen abstraction. These data suggest that ethanol formed over supported rhodium catalysts may not be due to hydrogenation of acetaldehyde. This study shows how surface science studies of model catalysts and surfaces can be used to extract information about reaction mechanisms since the nature of surface intermediates can often be identified by methods such as temperature programmed desorption and high resolution electron energy loss spectroscopy. 

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... 

Until now, in order to select a hydrocarbon adsorber with higher hydrocarbon trapping and conversion efficiency, experimental tests using vehicle and engine dynamometer has been employed. A model. Temperature Programmed Adsorption (TPA), was proposed by Kim, et al. [4-S] to save cost and time. The model has an advantage to analysis adsorption, desorption and conversion of hydrocarbons simultaneously. 

A specific model explaining the role of different catalytic sites on B-ZSM 5 and B-ZSM-11 is shown in Figure 1. Quantitative Temperature Programmed Desorption (TPD) data have been used to identify 3 types (different steric... 

Since zeolites are typical acid-base catalysts, their acid-base properties are of great importance in investigating the catalytic decomposition of hydrocarbons. Three methods — titration, temperature-programmed desorption, and characterization by test reaction — are employed to measure acid-base properties. In this study, n-hexane was used as a model hydrocarbon and its decomposition over HY, HCeY, HSmY, and HCuY zeolites was investigated. Depending on the metal exchanged, n-hexane conversion and product distribution were observed to vary in the higher ccmversion r ion. The relation between product distribution and the acid-base properties of the zeolites are discussed. 

Agreement of a spatially averaged quantity, such as a reaction rate and a temperature-programmed desorption spectrum, is often insufficient to test whether the underlying mechanisms and input in a molecular model are correct. Ideally, validation of molecular simulation predictions demands spatiotemporal data over a multitude of length and time scales. Unfortunately, such data are rarely available (see the membrane application example earlier). However, advances in scanning probe techniques start rendering such comparisons feasible. 

To measure the solid acidity of supports, NHs-temperature-programmed-desorption (TPD) experiments of zeolite supports were carried out on a special NH3-TPD apparatus (Ohkura Riken, Model ATD700A) interfaced to a personal computer. TPD profiles were obtained under vacuum conditions with the temperature varying from 100 to 600°C at 10°C/min in the TPD process. Samples were degassed at 500°C for 1 h under vacuum condition before measurement. The surface area and micropore volume of the zeolite... 

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