Spectra thermal desorption

Figure 19. Thermal desorption spectra of water adsorbed on (I) Ag(l 10), (2) PK111), (3) Ru(001), and (4) Ni(UO). (Reproduced from P.A. Thiel and T.E Madey, Surf. Sci. Reports 7 258, Fig. 29,1987, Ci 1987 with permission of Elsevier Science.)...

Thermal desorption spectra, 171 Thermodynamic equilibrium, phase transitions at, 219 Thermodynamic phase formation, passivation potential and, 218 Time resolved measurements in the microwave frequency range, 447 photo electrodes and 493 Tin... 

Figure 2.2. Thermal desorption spectra of carbon monoxide, measured mass spectrometically at mass 28 (atomic units, a.u.), on a platinum (100) surface upon which potassium has been pre-adsorbed to a surface coverage of 0K.7 Reprinted with permission from Elsevier Science.

Figure 2.33. Thermal desorption spectra of oxygen from mixed oxygen-chlorine adlayers on Pt(100).9S The initial chlorine and oxygen concentrations as well as the dosing temperatures are indicated in the figure. Heating rate 20 K s 1.95 Reprinted with permission from Elsevier Science.

Figure 4.43. Thermal desorption spectra after gaseous oxygen adsorption on a Pt film deposited on YSZ at 673 K and an 02 pressure of 4x 10"6 Torr for 1800 s (7.2 kL) followed by electrochemical O2 supply (I=+15 pA) for various time periods.29-30 Reprinted from ref. 30 with permission from Academic Press.

Figure 4.45. Thermal desorption spectra (bottom) and corresponding catalyst potential variation (top) after electrochemical O2 supply to Ag/YSZ at 260-320°C at various initial potentials Uwr Each curve corresponds to different adsorption temperature and current, thus different values of Uwr, in order to achieve nearly constant initial oxygen coverage.31 Reprinted with permission from Academic Press.

Figure 5.3. Oxygen thermal desorption spectra after electrochemical O2 supply to Pt/YSZ at 673 K (I = +12 pA for 1800 s) followed by isothermal desorption at the same temperature at various times as indicated on each curve.4,7 Reprinted from ref. 7 with permission from Academic Press.

Figure 5.21. Experimental setup (inset) showing the location of the working (WE), counter (CE) and reference (RE) electrodes and of the heating element (HE) thermal desorption spectra after gaseous oxygen dosing at 673 K and an 02 pressure of 4x1 O 6 Torr on Pt deposited on YSZ for various exposure times. Oxygen exposure is expressed in kilo-langmuirs (1 kL=l0 3 Torrs). Desorption was performed with linear heating rate, ()=1 K/s.4 S Reprinted with permission from Academic Press.

Thermal desorption spectra of CO2 from a titania surface are shown in figure 2. It revealed two desorption peaks at temperature ca. 175 and 200 K. As reported, surface of titania have two structures which is similar to the results fomd by Tracy et al. [7]. Based on their study, it was confirmed that one peak at ca. 170 K was attributed to CO2 molecules bound to regular five-coordinate Ti site considered as the perfected titania structure. The second peak at ca. 200 K considered as the CO2 molecules bound to Ti referred to the... 

Fig. 2. Thermal desorption spectra for CO2 Adsorbed on titania samples...

The existence of various temperature intervals characterized by predominant manifestation of one of above interactions can be detected from thermal desorption spectra. For instance, the thermal desorption spectrum obtained in [71] for a cleaved ZnO (1010) monocrystal following its interaction with oxygen (Fig. 1.4) indicates the availability of such typical temperature intervals as interval of physical adsorption (a), chemisorption (b), interval of formation of surface defects (c) and, finally, the domain of formation of volume defects (d). 

Thermal desorption spectra of carbon monoxide on polycrystalline and on single crystal platinum are well known from experiments in the gas phase [48,49], The system is therefore appropriate to test the experimental setup. 

Fig. 2.3. Thermal desorption spectra after adsorption from the gas phase (a) adsorbed CO on Pt (b) H2 on Pt.

Fig. 2.4. (a) Thermal desorption blank experiment. The Pt electrode was held at 0.45 V vs. RHE in the base electrolyte (5 x 10 2 M H2S04) during 120 s and then transferred to the UHV. (b) Thermal desorption spectra of adsorbed CO on Pt after adsorption from an aqueous solution. Temperature scan 5 K/s. 

Figure 2.11 Thermal desorption spectra of silver from the close-packed surface of ruthenium for different initial Ag coverages. Desorption from the second layer of silver occurs at lower temperatures, indicating that Ag-Ag bonds are weaker than Ag-Ru bonds. Note the exponential increase of the low temperature sides of the peaks, indicating that the desorption follows zero-order kinetics (from Niemantsverdriet et al. [18]).

Figure 9.14 Thermal desorption spectra of CO from clean (left) and potassium-promoted Ni (110) (middle and right) mea-sured at a heating rate of 13 K/s. The spectra exhibit two desorption states for CO on promoted surfaces and indicate that CO binds more strongly to sites adjacent to potassium (from Whitman and Desorption Temperature (K) Ho [46]).

Fig. 5. H2 (2 amu) and D2 (4 amu) thermal desorption spectra from chemisorbed and on Pt(lll), respectively.

Temperature Dependence of Secondary Ions. We now consider the relationship between SIMS spectra and desorbable hydrogen and hydrocarbon species in more detail by comparing the temperature dependence of the various hydrocarbon containing ions and RU2C2 with the thermal desorption spectra of Figure 1. 

Valuable information can be obtained from thermal desorption spectra (TDS) spectra, despite the fact that electrochemists are somewhat cautious about the relevance of ultrahigh vacuum data to the solution situation, and the solid/liquid interface in particular. Their objections arise from the fact that properties of the double layer depend on the interaction of the electrode with ions in the solution. Experiments in which the electrode, after having been in contact with the solution, is evacuated and further investigated under high vacuum conditions, can hardly reflect the real situation at the metal/solution interface. However, the TDS spectra can provide valuable information about the energy of water adsorption on metals and its dependence on the surface structure. At low temperatures of 100 to 200 K, frozen molecules of water are fixed at the metal. This case is quite different from the adsorption at the electrode/solution interface, which usually involves a dynamic equilibrium with molecules in the bulk. 

Figure 4. Thermal desorption spectra of K on Ni(lOO) and on NiO/Ni(100). The NiO/Ni(100) layer was prepared by 300 L O2 exposure at 300 K followed by annealing to 800 K. The potassium-deposition was carried out at 90 K.

If r0 and m are known quantities, the activation energy for desorption may be simply determined from the temperature, Tp, at which the maximum rate of desorption occurs (117). For associatively adsorbed CO the reaction order for desorption may be safely assumed to be one and frequently vo = 1013 sec-1 is assumed to be a reasonable value. If the resulting data for d are compared with values for the isosteric heats of adsorption a (these should be equal since the kinetics of adsorption is nonactivated), very often deviations by several kcal/mol occur (91) that indicate the weakness of this assumption. More sophisticated techniques for analyzing thermal desorption spectra (118-121) allow the independent determination of both parameters, v and d. The results demonstrate that vQ may deviate considerably from 1013 sec-1. For example, for the system CO/Ru(001) Menzel et al. (122) came to the conclusion that v0 may reach values up to 1018 sec-1, whereas a rather small number of 1011 sec-1 was derived by Weinberg et at. (76) for CO desorption from an oxidized Ir(l 10) surface. An additional complication arises from the fact that analysis of thermal desorption spectra on the basis of (4) may yield misleading results if desorption takes place via transition to a precursor state (102). which may be the case for adsorbed CO. 

Fig. 22. Thermal desorption spectra for the evolution of C02 from Ir(110). The oxygen covered surface was annealed at various temperatures and was subsequently reacted off with CO (124).

The reconstruction of the Ir and Pt surfaces also complicates the adsorption behavior on these metals. Exposure of the reconstructed 1 x 2 Ir(110) surface to oxygen results in a 2 x 2 pattern (54. 124) which on the basis of thermal desorption spectra has been assigned to a coverage of 0 = 0.25 (124), whereas adsorption on the reconstructed 1 x 2 Pt surface leads to a 1 x 2 structure with streaks in the (100) direction (134). Adsorption of 02 on the metastable 1 x 1 Ir(l 10) surface, which is stabilized by the random... 

Fig. 28. Thermal desorption spectra for oxygen adsorption on Pd(lll). The adsorption temperature increases in sequence 1-6 (13).

Since oxygen is dissociatively adsorbed and desorbs as 02, one would expect the thermal desorption spectra to show a shift in the peak temperature to lower values with increasing initial coverage as is to be expected for second-order desorption (117). This is, however, not always the case as can be seen in the desorption spectra for 02 from Pt(10) (125) shown in Fig. 29. Initially the peak shifts to lower temperatures, but further increases in the coverage leave the peak temperature unchanged. 

Figure 23.4 Thermal desorption spectra for CO adsorption on Mo2C at 96 K with CO exposure (a) 2.15 L, (b) 0.25 L. Heating rate is 1.5 Ks. ...

Figure 23.6 Thermal desorption spectra acquired following the interaction of 20 L of NO with Mo2C at 108 K. Heating rate is 1.5 Ks-1.

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