Desorption second order

Thus, for the second-order desorption kinetics and the hyperbolic heating schedule, the peaks are symmetric about Tm in the scale (1/T). The first-order peaks are asymmetric in this scale, exhibiting a steeper descent than ascent. These considerations suggest that the hyperbolic heating schedule is especially favorable for an analysis of the peak shapes and for the detection... 

Even if the peak behavior fits well for a given apparent desorption order, the real kinetic situation may be a different one. As a rate controlling step in a second-order desorption, random recombination of two particles is assumed most frequently. However, should the desorption proceed via a nonrandom recombination of neighboring particle pairs into an ordered structure, the resulting apparent first-order desorption kinetics is claimed to be possible (36). The term pseudo-first-order kinetics is used in this instance. Vice versa, second-order kinetics of desorption can appear for a nondissociative adsorption, if the existence of a dimer complex is necessary before the actual desorption step can take place (99). A possibility of switching between the apparent second-order and first-order kinetics by changing the surface coverage has also been claimed (60, 99, 100). 

In estimating the value of Ed by means of the transcendental equations (28), the circumstance utilized is that the variation of em for a given change in Tm is much less than the variation of exp(em) (31). Until now, only particular solutions have been available for the hyperbolic and linear heating schedules and for the first-order and second-order desorptions. They can be found for example in the fundamental papers by Redhead (31) and Carter (32) or in the review by Contour and Proud homme (106), and therefore will not be repeated here. Recently, a universal procedure for the... 

The paper by Dawson and Peng (98) can be quoted as an example of applying Eq. (58) to a kinetic analysis of both the first-order and second-order desorptions with an activation energy varying linearly with the surface coverage. 

Figure 7.7 shows three different sets of TPD measurements, corresponding to zero-, first- and second-order desorption processes. 

The right-hand part of Fig. 7.7 corresponds to the second-order desorption of nitrogen atoms from a rhodium surface. As the desorption reaction corresponds to N -I- N —> N2 -I- 2 the rate is indeed expected to vary with A characteristic feature of second-order desorption kinetics is that the peaks shift to lower temperature with increasing coverage, because of the strong dependence of the rate on coverage. 

A similar though more complicated expression exists for second-order desorption kinetics ... 

TPD of the nitrogen-saturated Fe(lOO) surface shows a symmetric feature with a peak maximum at 740 K if we use a heating ramp of 2 K s . Estimate the activation energy for desorption assuming second-order desorption. 

The results of a similar experiment with adsorbed hydrogen is shown in Fig. 2.3b. Only one desorption peak was observed in the temperature range studied [50], The desorption peak temperature lies at 420 K for the experiment with 0.8 L and is shifted to lower temperatures as the H2 concentration increases indicating second order desorption kinetics. Surface states with desorption temperatures at 165 K, 220 K, 280 K and 350 K were reported for the adsorption of H2 and D2 at 120 K [51]. Thermal desorption experiments after H2 adsorption at 350 K show only one desorption state at ca. 450 K [52],... 

Plot the surface coverage 6 versus temperature assuming second-order kinetics (n = 2) for the same initial coverages and parameters listed in part 4, above. Are the calculated TPD curves symmetric with respect to temperature Does Tp depend on initial surface coverage You will often see the comment that for second-order desorption kinetics, the surface coverage at Tp is 9P = 9 /2. Do your calculations bear this out ... 

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. 

Fewer nonsteady-state measurements have been carried out on iridium than on platinum and palladium. Figure 50 shows the results of a 02—CO coadsorption experiment on Ir(lll) (203). Initially 02 was adsorbed, followed by CO adsorption, after which the crystal was heated with a linear temperature rise. It is seen that the peak temperature for COz desorption is shifted to lower values with increasing CO coverage. This may be due to a second-order desorption effect (203) or a reduced activation energy for the reaction owing to interactions in the adlayer, as was found on Pd(lll) (176). 

The values represent the average and standard deviations for the areas under the specified m/e TPD feature normalized by the area under the ml e = 34 peak obtained in six independent measurements. b The peak areas were determined by fitting the TPD curves using a combination of a Gaussian and a Lorentzian. Fits to the formulas for first and second order desorption kinetics were also attempted but did not provide significantly better statistical results. 

Evolution curves for first and second order desorption are shown in Fig. 7. They differ qualitatively in shape and their dependence on the initial concentration of... 

Fig. 7. Depoidence of evolution curve on rate law for desorption. First order versus second order desorption = 80 kcal. mole- = 3.54 x 10 > sec- v, = 8 X IQ- molecules-. sec. cm .

The observed second-order desorption of pyridine is believed to be evidence that the majority of surface silanols are paired on the surface in either a vicinal or geminal configuration. Further evidence for the pairing of the majority of silanols on the silica surface was presented elsewhere... 

Evolution curves for first- and second-order desorption with the same activation energy and heating curve (l/T linear in time) are shown in Fig. 11. They differ qualitatively in shape and in their dependence upon the initial concentration of adsorbed gas, n(tj. The second order curve is shifted to lower times (that is, to lower temperatures) as the initial concentration is increased, whereas the first order desorption is, of course, independent of initial concentration. The second order curve also has a characteristic s shape not found from first-order evolution. 

Fig. 11. Dependence of evolution curve on rate law for desorption, (a) First- versus second-order desorption E D = 80 kcal/mole, v, = 3.54 X 101 sec-1, v, = 8 X 10" molecules-1 sec-1 cm. For variable heat, ED = E°B — ifn E°D = 80 kcal/mole, ij = 0.3 kcal/mole per 10,s molecules/cm. (b) First-order desorption with concentration dependent desorption energy. E B = 110 kcal/mole, ij = 0.3 kcal/mole per 10u molecules/ cm, v, = 3.54 x 10 sec-1. Heating schedule 1/T = a + bt a = 9.95 X 10- (°K)-1, -b = 1.192 X 10- (°K sec)-1.

Fig. 12. Effect of heating schedule on evolution curve. First- and second-order desorption (with rate parameters as in Fig. 11) are shown in (a) for heating curves indicated in (b). Second-order reaction maintains s shape.

Fig. 17. Analysis of pressure-temperature curves for ft nitrogen, indicating second-order desorption kinetics.

For a second-order desorption process, both the desorption-peak maximum temperature Tp and the half-width change with increasing initial coverage. An example of this type of behavior is the desorption of N2 from W(IOO) (Figure 4.17), clearly indicating the dissociative adsorption of nitrogen before desorption under the conditions of the experiment. 

The theory of desorption of atoms and molecules by temperature-programmed desorption has been reviewed in references [61, 74-76, 124-126]. Discuss the assumptions made in deriving the first- and second-order desorption rates and their correlation to the temperature of the maximum desorption rates. How does the magnitude of the preexponential factor reflect the assumption of (a) a mobile adsorbate layer or (b) an immobile adsorbate layer ... 

Shortly after the work of Koehler et al. on Si(lll) appeared, Sinniah et al. [33] reported the unexpected result that desorption from Si(lOO) followed first-order kinetics. These experiments used an isothermal technique like that of Koehler et al. However, the measurements of Sinniah et al. were sensitive to coverages as low as 0.006 ML, allowing them to use initial coverages as low as 0.06 ML. This result demonstrates that the mechanism of recombi-native desorption on Si(lOO) is qualitatively different from that on metal surfaces, where kinetics are second order [34]. It is easy to rationalize second-order desorption kinetics by a mechanism in which two independently diffusing H atoms must approach one another to recombine. A mechanism that yields first-order kinetics must imply either (a) some interaction between the H atoms (so that their positions are correlated) or (b) a multistep mechanism where the rate-limiting step involves motion of only one H atom. 

The relative acidity of the silanol sites on the two types of silica surfaces studied can be compared with respect to the rate constants of the desorption process, and the amount of pyridine retained on the surface at the end of the desorption study. For the samples prepared at 200°C, the second order desorption rate constant was 7.42 X 10 for Davisil and 15.9 x 10 for Zorbax. Pyridine was removed more quickly from silanol sites on the... 

Although the kinetic data derived in these experiments can be well represented by a singular second order desorption model, closer inspection of the data presents evidence for three different, and successive second order... 

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