Desorption peak

Figure Bl.26.8. Adsorption/desorption peaks for nitrogen obtained with the continuous flow metiiod (Nelsen F M and Eggertsen F T 1958 Anal. Chem. 30 1387-90).

Ultrasound frequencies can be introduced into the walls of the vacuum system. If a source of ultrasound is placed on the wall of an ultrahigh vacuum system, a large hydrogen peak is observed. Related phenomena, presumably from frictional effects, are observed if the side of a vacuum system is tapped with a hammer a desorption peak can be seen. Mechanical scraping of one part on another also produces desorption. 

With correct experimental procedure TDS is straightforward to use and has been applied extensively in basic experiments concerned with the nature of reactions between pure gases and clean solid surfaces. Most of these applications have been catalysis-related (i. e. performed on surfaces acting as models for catalysts) and TDS has always been used with other techniques, e.g. UPS, ELS, AES, and LEED. To a certain extent it is quantifiable, in that the area under a desorption peak is proportional to the number of ions of that species desorbed in that temperature range, but measurement of the area is not always easy if several processes overlap. 

Fig. 1. Effect of pumping speed on a desorption peak at a fixed heating rate. Experimental parameters given in the text. Reproduced from Ehrlich (27), with permission.

Fig. 2. Effect of heating rate on a desorption peak at a fixed pumping speed. Hyperbolic heating schedule, l/To = 9.95 X 10-1 K-1 S/V = 4.8 sec-1 E — 80 kcal mole-1 x = 1. Reproduced from Hansen and Mimeault (30), with permission.

The order of the desorption process is estimated in the first place from the shape of the desorption peak, preferably in the l/T scale. The first-order peaks here are clearly asymmetric, the falling branch being steeper than the ascending one. The second-order peaks are near symmetric and are broader. The third-order peaks are even broader and are again asymmetric, but in this case the ascending branch is steeper than the falling one. 

For estimates of both Ed and fcd in the Arrhenius equation, in principle two different points on a desorption peak or two runs with different heating factors o2 are required. One obvious point is the maximum of the peak, and very often only this is used while the value of kd is supposed to be of the order of magnitude 1012 to 1013 sec-1. As seen from Eq. (28), the location of Tm depends but weakly on fcd as compared to its dependence on Ed, so that an uncertainty in the value of kd of one order of magnitude does not affect the estimated value of Ed appreciably. This has been clearly illustrated by analogue simulation of the thermodesorption processes (104). On the other hand, the said fact causes the estimates of kd to be very uncertain. A recently published computational analysis of the peak location behavior shows the accuracy of the obtained values of Ed (105). 

Estimates of Ed can be obtained also from the height of the desorption peak. From Eqs. (32) and (33) we get... 

Parsons-Zobel plot for NaF solutions was linear (Table 12). The value of Cf"0,determined by the extrapolation of the Cl, Q1 curve to Cjl = 0 and corrected by the value of /pz, has been obtained (Cf 0 - 0.32 F m 2). Adsorption studies of (C l at a polished pc-Pb show splitting of the adsorption-desorption peaks, which can be explained by the energetic inhomogeneity of the surface. The difference between Ea=Q values of various Pb faces has been estimated to be on the order of50-60 mV.604... 

Studies in surface-inactive electrolyte solutions with various organic compounds (cyclohexanol, 1-pentanol, 2-butanol, camphor, tetra-buthyl ammonium ion, TBN+) show that the adsorption-desorption peak shifts to more negative potentials in the order (0001) < (1010) < (1120) this was explained by the increasing negative value of Eff=0 in the same direction.259 629-635... 

The R of electropolished Zn single-crystal face electrodes has been obtained from the shape of the adsorption-desorption peak of cyclohex-anol at various Zn and Hg surfaces.154 The roughness factor of Zn electrodes has been found to increase in the order Zn(0001) < Zn(lOlO) < Zn(llZO) with values in the range 1.1 to 1.25. 

Adsorption of aliphatic alcohols and tetra-alkylammonium cations from Na2S04 + HjO solutions on Sb electrodes has been investi-gated.721 724 Splitting of the adsorption-desorption peak into two independent maxima has been found725,726 for cyclohexanol adsorption at an electrochemically polished pc-Sb electrode accordingly, the difference between the [Pg.120]

Renewed Sn + Cd alloy surfaces have been studied by Safonov and Choba821 by impedance. The has been found to shift toward more negative E with time, suggesting that the content of Cd at the Sn + Cd alloy surface increases with time. For the alloy with 10% Cd, the time dependence of C for adsorption of organic substances is significantly different from that for Sn + Pb alloys. At relatively short times, E"1 shifts in the negative direction, which shows the increase of the Cd content in the Sn + Cd alloy surface layer. At longer times, an additional adsorption-desorption peak (step) has been observed, which has been explained by the formation of rather wide two-dimensional areas of Cd microcrystals at the alloy surface.824... 

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... 

Substrate Anneal Temp (K) Desorption Peak (T) Total Peak Area... 

Temperature-programmed desorption of mesitylene shows a marked difference to the catalysts prepared on MgCl2 surfaces. The spectrum contains only one desorption peak at aroimd 250 K. Due to the similar desorption temperature to the peak observed for MgCl2-based films, this peak was assigned to desorption from low coordinated or defect sites [118]. 

Kim and Somorjai have associated the different tacticity of the polymer with the variation of adsorption sites for the two systems as titrated by mesitylene TPD experiments. As discussed above, the TiCl >,/Au system shows just one mesitylene desorption peak which was associated with desorption from low coordinated sites, while the TiCl c/MgClx exhibits two peaks assigned to regular and low coordinated sites, respectively [23]. Based on this coincidence, Kim and Somorjai claim that isotactic polymer is produced at the low-coordinated site while atactic polymer is produced at the regular surface site. One has to bear in mind, however, that a variety of assumptions enter this interpretation, which may or may not be vahd. Nonetheless it is an interesting and important observation which should be confirmed by further experiments, e.g., structural investigations of the activated catalyst. From these experiments it is clear that the degree of tacticity depends on catalyst preparation and most probably on the surface structure of the catalyst however, the atomistic correlation between structure and tacticity remains to be clarified. 

Figure 12.5 CO stripping voltammogram with a CO- tee 0.1 M H2SO4 electrolyte. Compare the data in Fig. 12.4 the CO oxidation region begins at V = 0.43 V. After CO stripping, hydrogen adsorption/desorption peaks and the beginning of the Pt oxidation range are shown.

Influence of Cflf, concentration. As seen in Figure 16.5, an increase of the CgHg concentration from 1 to 20 mM (the benzene saturation concentration is 22 mM) leads to (i) a shift of the Hupd adsorption and desorption peaks toward less-positive potentials (ii) a decrease in Hupd charge density and (iii) the eomplete... 

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