Reaction spectra, temperature programmed

Figure 1. Temperature programmed reaction spectra of C2H4 and H2 following saturation coverage of C2H4 on the clean and oxygen covered Pd(lOO) surface at 100 K. Atomic oxygen was generated on the surface by exposure to O2 at 300 K.

Temperature programmed reaction spectra depicting the reaction of H2O and OH groups with oxygen on Pd(lOO) are shown in fig. 3. Curve (a) was obtained for H2O adsorption on the clean surface and contains two peaks, the state at 167 K of multilayer H2O, and the <>2 state at 182 K due to H2O bound directly to the surface /7/. An additional state at 255 K, labelled y, is observed following coadsorption of 1 0 and 0 (fig. 3b). This state represents the reaction of OH groups III... 

Figure 4, Temperature programmed reaction spectra of CH3OH, H2CO and HpO following adsorption of multilayers of CH3OH on Pd(lOO) at 130 K precovered with 0.25 ML atomic oxygen. The oxygen was adsorbed at 300 K. All traces are on the same scale.

Figure 3. Temperature-programmed reaction spectra morutoring m/e = 73 (a cracking fragment of trimethylsilane) after adsorbing a saturation coverage of methyl groups on CusSi surfaces prepared by ion bombardment at 160 K (dotted curve) and 330 K (solid curve).

Figure 1. Temperature Programmed Reaction Spectra of Thiophene on Ni 111)...

Figure 6 Temperature-programmed reaction spectra of CeHg formation on Pd clusters of various size deposited on a MgO thin film. The bottom spectrum shows that for clean MgO(lOO) films no benzene is formed.

The ethylene temperature-programmed reaction spectrum of an oxygen-covered Cs/Ag/a-Al203 catalyst produced a peak for the co-evolution of EO and CO2 at 373 K (100 °C) with a selectivity to EO of 44%. The Cs had had no effect on the kinetics of desorption of oxygen from Ag(lll) nor on the amount of oxygen desorbing from that surface, nor on its selectivity in its oxidising ethylene to EO (Eig. 7.10). 

Figure I. Catalytic formation for different Pd cluster sizes obtained from temperature programmed reaction experiments. The bottom spectrum shows that for clean MgO(lOO) films no benzene is formed. Dots data full line data smoothing with adjacent averaging (25 points). Cluster coverage is 0,28 % of a monolayer for all cluster sizes, where one monolayer corresponds to 2.25x10 atoms/cm. ...

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. 

The effect of that Cs loading was studied by O2 TPD and by ethylene temperature-programmed reaction [3]. The O2 desorption spectrum obtained by Atkins and co-workers for the Cs/Ag/v-AhOa catalyst is shown in Fig. 7.9, lower curve. The upper curve with two peak maxima at 523 K (250 °C) and 573 K (300 °C) is that shown previously for O2 desorption from a fresh, unpromoted Ag/a-Al2O3 catalyst (Fig. 7.1). 

Figure 3. (a) Temperature-programmed reaction (TPR) spectra of QHg formed on Ag, Pd, and Rh atoms deposited on defect-rich MgO thin films grown on Mo(lOO) surfaces. As a comparison, the same experiment was performed on a clean defect-rich MgO film. Also shown is the calculated (C4H4)(C2H2)/Pdi/F5c intermediate of the cyclotrimer-ization reaction on Pd atoms adsorbed on an F center of the MgO (100) surface, (b) TPR spectra of CO2 from a defect-rich MgO thin film and deposited Pd atoms after exposure to O2 and CO (lower and upper spectrum), as well as from deposited Pd atoms after exposure to CO and O2. Calculated precursors of the two observed mechanisms (I) and (II) are also depicted. 

Pulse intensities in vacuum experiments range from 10 to 10 molecules per pulse with a pulse width of 250 ps and a pulse frequency of between 0.1 and 50 pulses per second. Such a spectrum of time resolution is unique among kinetic methods. Possible experiments include high-speed pulsing, both single-pulse and multipulse response, steady-state isotopic transient kinetic analysis (SSITKA), temperature-programmed desorption (TPD), and temperature-programmed reaction (TPR). 

There is of course attenuation of the signal, as shown in Fig. 5, taken from Joyner and Roberts (28) The gas phase spectrum will also be obtained, but this usually can be separated easily from the signal of the solid. This sample cell arrangement thus permits the study of the stationary-state surface during catalysis and also its evolution in response to pulses and step functions in the gas composition. The temperature of the sample should be controlled so that the surface can be studied during temperature-programmed desorption and reaction. 

Temperature-Programmed Surface Reaction (TPSR) Experiments at 800 Torr. Pretreated and preoxidized silver exhibited no reactivity toward an ethylene/argon mixture at reaction temperatures (443 - 543 K) and atmospheric pressures (750-800 torr). The desorption spectrum of a pretreated sample showed no evidence of oxygen desorption when the sample was heated in vacuo to 673 K. These... 

Figure 6.18 illustrates the technique with a study on a proprietary cobalt on alumina Tropsch catalyst for Fischer-Tropsch synthesis (the reaction of synthesis gas, CO + Fl2, to hydrocarbon fuels) [55]. Trace amounts of platinum help to obtain an appreciable degree of reduction for the cobalt (similarly as in the temperature-programmed reduction of bimetallic Fe-Rh catalysts in Fig. 2.4). The left part of Figure 6.18 shows Co K-edge XANES of metal and oxide reference compounds, and illustrates the strong intensity of the white line region for ionic cobalt compounds. The XANES spectrum of the calcined CoPt/A Ch catalyst re-...

Isopropyl derivatives were introduced by Pettitt and Stouffer [287] and later studied by other workers [288]. They are prepared by reaction with 2-bromopropane in the presence of sodium hydride in dimethyl sulphoxide. The reaction scheme and the preparation procedure were given in Chapter 4 (see p. 64). Except for Arg, all amino acids under study provided the expected derivatives. The hydroxyl group of Hypro was, however, not protected. The derivatives were found to be stable for a reasonable period of time and were analysed on 3% of OV-17. The extension of this promising one-step method to all protein amino acids did not fulfill expectations, however [288]. Some amino acids (Gly, Gin, Asp and Asn) did not provide detectable derivatives and the others led to multiple peaks. Moreover, significant amounts of by-products were produced, which may interfere. Arg provided a single peak, the mass spectrum of which was identical with that of Orn both derivatives resulted from lactam formation. Isoprop derivatives of 23 common amino acids were separated on 5% of-Carbowax 20M on silanized Chromosorb G with temperature programming (50-240°C). 

Figure 12.18 shows a typical chromatogram obtained from an on-line product gas analysis for the oxidation of CH4. The chromatograms obtained from the TCD were typically characterized by steady baselines with well-resolved peaks. The product spectrum indicates that both partial and total combustion reactions of CH4 are present. Both CO2 and H2O are baseline-resolved using a Hayesep R column while the O2, N2, CH4, and CO are baseline-resolved on a molecular sieve column. The overall analysis time requires about 9 min. Some gaps exist in the retention times for the various components, but no attempt was made to reduce these by additional optimization of the temperature program. The overall analysis time could have been reduced if the dual-oven method had been used as described elsewhere (Delaney and Mills, 1999 Nicole et al., 2000). 

Figure 6.28. Temperature-programmed mass spectrometry of the decomposition of Pt +(NHs)4 ion exchanged in zeolite nZEM-Sl " . The corresponding reaction steps are indicated. Three peaks can be distinguished in the TPD spectrum. Oxygen is consumed in only two of the N2 formation peaks. The mechanism that explains the occurrence of these three peaks is consistent with proposals made earlier on the homogeneous oxidation of Ru-amine complexes in basic solutionl and the reaction of NO with Ru or Os complexes ].

By field ion mass spectroscopy of NH3 on Fe at room temperature, 4 -10"" torr, N Hy species and FeN Hy species are detected [510]. Secondary ion mass-spectroscopy of NH3/Fe(110) at 130 K shows FCnNH " with n, m = 1, 2, with n = 0,1,2, 3,4, H2 and Fe" [545]. The spectrum is interpreted as fragments of molecularily adsorbed NH3 [545]. When the temperature is increased, FeNH and FeNHl decrease smoothly, while Fe increases smoothly, illustrating the decrease in surface coverage and the reaction of NH3(g) with the surface at higher temperatures [545]. The intensity of NH3 with NH2 decreases steeply above 300 K consistent with the temperature programmed desorption studies [545]. Both ions are probably formed from adsorbed species [545]. The intensity of NH" decreases more slowly than NH2 and NH3 the reason is probably that NH" is formed from both NH3(g) and NH [545]. 

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