Temperature-programmed desorption spectra

B. Meng, W. H. Weinberg. Monte Carlo simulation of temperature programmed desorption spectra. J Chem Phys 700 5280-1589, 1994. 

Fig. 11. The temperature programmed desorption spectra for CO from Fe(lOO) following adsorption at 180 K. Curves (a)-(e) are arranged in order of increasing initial coverage by CO 45).

At intermediate coverages the dissociation starts between 200 and 450 K, then stops when the overall coverage becomes too high, and then restarts when new vacancies are formed due to NO desorption. The temperature-programmed desorption spectra also show a first-order-like N2 desorption peak... 

CO/Rh(100). - This system forms an example where we have determined lateral interactions by fitting temperature-programmed desorption spectra that were simulated using kinetic Monte Carlo to experimental spectra. For coverages below 5ML CO adsorbs at top sites, which form a square grid. CO desorption has a rate constant... 

Figure 13 Experimental (left) and simulated (right) temperature-programmed desorption spectra for COjRh( 100). The values on the right of each set of curves indicate initial coverages. The thin curves on the right are simulated spectra with the lateral interactions switched off. The heating rate is 5 Kjs ...

If we allow the lateral interactions to vary over a too large range during the optimization, then we occasionally get adlayer structures in the kMC simulations that differ from those found experimentally. This does not mean necessarily that very different temperature-programmed desorption spectra are... 

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 ...

Fig. 13. Temperature-programmed desorption spectra for CO on two Pt-Sn alloys. Only gas desorbed below 500 C is recorded, Adsorption temperature, 28 C cooling temperature, — 78" C heating rate, 145 C/min. From Verbeek and Sachtler (14).

B. Meng and W. H. Weinberg, Monte Carlo Simulations of Temperature-Programmed Desorption Spectra, J. Chem. Phys., 100 (1994) 5280. 

Fig. 1.12 Temperature-programmed desorption spectra of ethylene (0.5 L) on hydrogen-covered Pd(lll) as a function of hydrogen coverage. (Reprinted from [101] with permission from Elsevier.)...

To use Eq. (15), we need AE. This parameter can be estimated from the O2 temperature-programmed desorption spectra. Scrutinizing the spectra collected in [48], we obtain AE — 10 kcal/mol [the negative sign of AE means that oxygen adsorption is more favorable on the (100) face]. 

Fig. 1 Temperature-programmed desorption spectra of unpromoted and promoted nickel-alumina catalysts.

Figure 2 Temperature programmed desorption spectra of (a) H2 and (b) H20 from catalyst Pt-B. Data are shown for reduction in deuterium at 30(fC and 400PC...

Figure 27.1 Post-irradiation temperature-programmed desorption spectra of phenol adsorbed on Ag(lll) for distinct coverage and exposure time to 266 nm laser light. Reproduced from Lee et at, J. Chem. Phys., 2001, 115 10518, with permission of the American Institute of Physics...

The same reaction regenerates the Pt +(NH3)4 complex after reactions (6.32) and (6.35). In the presence of oxygen, hydrogen will be oxidized to form H2O, which drives this reaction thermodynamically. In the overall reaction scheme, N2 and N2O formation compete in the absence of water through reactions (6.32) and (6.36). In the presence of water, N2 formation is promoted because of reactions (6.33) and (6.35). There is ample evidence that the reduction of the zeolitic protons by reduced metal atoms such as Pt is actually an easy reaction stepl . In the temperature-programmed desorption spectra (Fig. 6.28), N2O formation occurs predominantly in the peak in which excess oxygen is consumed. The other two peaks follow N2 formation. In the absence of water, the low-temperature peak that corresponds to the reaction sequence initiated by reaction step (6.33) is suppressed. 

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)... 

Figure 3.2. Temperature programmed desorption spectra of hydrogen illustrate the decomposition of alkenes adsorbed on platinum (111) at 120 K (adapted from Creighton and White. 1983).

The heat of adsorption is increased by 6-10 kJ/mole for K promoted sites on Fe(l 00) [383,411] and 8 kJ/mole for K-promoted sites on Fe(l 11) [411]. Even at low coverages by K, the temperature programmed desorption spectra of hydrogen desorbing from H/K/Fe(l 00) or from H/K/Fe(l 11) are not split into peaks assignable to promoted or unpromoted sites respectively [411]. 

The number of sites found by CO chemisorption is used for the number of active sites for the catalyst [396]. Quantum mechanical calculations show that the effect of K is to stabilize N2. Experimentally the differences in sticking coefficient among the basal planes of Fe is small in the presence of K. The activity of all sites are assumed to be that of K/Fe(l 11) [396, 625, 645]. This may be further justified by the close similarity of the experimental temperature programmed desorption spectra of N2 and N for the industrial catalyst and for K/Fe(l 11) [631]. 

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