Temperature-programmed experiment

The kinetics of a first-order reaction are very similar to those represented by the contracting volume equation [70], except in the final stages of reaction when a approaches 1.00. In measurements of reactivity, or in comparisons of properties of similar substances, the first-order expression can sometimes be used as a convenient empirical measurement of rate. The assumption of first-order behaviour is often made in the kinetic analyses of programmed temperature experiments (see Chapter 5). The software supplied with many commercial instruments often provides only order-based equations for kinetic analysis of data, whereas other equations more obviously applicable to solids, such as those given here, are not tested. 

Yield-temperature curves, typically obtained from programmed temperature experiments using the techniques of thermal analysis, are illustrated in Chapter 5, after consideration of the effects of temperature on the rates of solid state reactions in Chapter 4. 

The experimental data used in such an analysis may be (i) several sets of measurements of some property which has been demonstrated to be proportional to either the fractional extent of reaction, a, or the rate of reaction, AcdAt, against time t, at a series of different isothermal temperatures or (ii) similar measurements made during programmed temperature experiments, possibly at a series of different but constant heating rates. 

The techniques of thermal analysis stimulated an interest in the estimation of reaction kinetic parameters from programmed temperature experiments. Some of this background was covered by Brown and Galwey in Chapter 3 of Volume 1 [Vol.l, Ch.3]. A major advance was... 

Figure 7.15. Temperature-programmed SIMS experiment showing the surface reaction between adsorbed C and N atoms to give a surface cyanide species at 475 and 600 K decomposition of CN into C -t N, followed by instantaneous desorption of N2, occurs at...

NO is now chemisorbed on the Rh particles at a temperature where it does not adsorb on the AI2O3. The saturation coverage of NO on Rh(lOO) corresponds to one NO molecule per two rhodium surface atoms, with NO sitting in a c(2x2) surface structure. After having saturated the catalyst with NO, a temperature-programmed desorption experiment (TPD) is performed with a heating rate of 2 K min". NO is seen to desorb with a maximal rate at 460 K. The total NO gas that desorbs amounts to 18.5 mL per gram catalyst (P = 1 bar and T = 300 K). It can be assumed that NO does not dissociate on the Rh(lOO) surface. 

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. 

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. 

Figure 5. n-Butane conversion to isobutane as a function of temperature in a temperature programmed reaction experiment conducted over 0.4 g of INiSZ(s) catalyst under an n-butane/ hydrogen mixture (n-C4 molar fraction = 0.34) at a constant heating rate of 2C/min... 

Figure 12.4 Scheme of an ideal DSC curve for the study of an endothermic process by the dynamic method. Also shown are the programmed temperature (fp) line and the zero line. The zero line is obtained in a separate experiment, where the sample and the reference crucibles are empty and the heating program used in the main experiment is maintained. 

Let us first address the question of the accurate measurement of the temperature of the sample in DSC experiments [252-255], As illustrated in figure 12.4, the programmed temperature, Tp, usually varies linearly with the time t, and can... 

Figure 3 Sketch of an example of the evolution of a system during a temperature-programmed desorption experiment in the system s phase diagram. The fat line indicates the change of the temperature and coverage during the experiment, and the thin lines indicate the phase transitions (see text). The snapshots below the order-disorder transition line are taken during a simulation of the experiment. The coverages are 0.3, 0.5, and 0.7 ML. The snapshots above the order-disorder transition line show adlayers of 0.3 and 0.7 ML at high temperatures...

The production of butadiene from butene involves at least three surface intermediates adsorbed butene, 7t-allyl, and butadiene. One or more of these may be particularly vulnerable to attack by gas-phase oxygen on a-Fe203. From the temperature programmed desorption experiments, it was found that the products of isomerization, selective oxidation, and combustion... 

Tsuchiya et al.36 performed temperature-programmed desorption experiments of H2 chemisorbed on platinum. This gave rise to several desorption maxima. To explain different... 

According to eqn.(5.4), if the result of a programmed temperature scanning experiment in GC is a bunch of peaks eluted around a column temperature of 195 °C, then a chromatogram in which all the peaks appear with roughly optimal capacity factors may be expected to result from an isothermal experiment at 150 °C. 

The obvious alternative to the sequential optimization methods is the use of an interpretive optimization method. In such a method a limited number of experiments is performed and the results are used to estimate (predict) the retention behaviour of all individual solutes as a function of the parameters considered during the optimization (retention surfaces). Knowledge of the retention surfaces is then used to calculate the response surface, which in turn is searched for the global optimum (see the description of interpretive methods in section 5.5). For programmed temperature GC the framework of such an interpretive method has been described by Grant and Hollis [614] and by Bartu [615]. 

Temperature Programmed Reduction and Thermo-Magnetic Analysis. The results of temperature programmed reduction experiments performed on a catalyst prepared by incipient wetness impregnation and the physical mixture are presented in Fig. 1. 

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