Activation energy for desorption

Thermal Desorption Temperature Maxima and Thermal Desorption Activation Energies for Saturated Hydrocarbons on Ru (0001)... 

Figure 4. Desorption activation energies for various probe Lewis bases on chemically modified Mo surfaces.

Figure 3.6 Catalytic ignition temperatures in a stagnation point flow over a platinum foil. (A) Measurements (symbols) and predictions (lines) for H2/O2/N2 mixtures with (Ph2+P02)/Ptotai =0.059). Dashed- and solid-line predictions for a<0.3 indicate bistability. (B) Measurements (symbols) and predictions (lines, for two values of the hydrogen desorption activation energy) for H2/air mixtures. Panel (A) is adapted from Behrendt et al. (1996) (with permission) and panel (B) is adapted from Vlachos and Bui (1996) (with permission).

This means that desorption activation energies can be much larger than those for adsorption and very dependent on 6 since the variation of Q with 6 now contributes directly. The rate of desorption may be written, following the kinetic treatment of the Langmuir model. 

Temperature progranuned desorption (TPD), also called thenual desorption spectroscopy (TDS), provides infonuation about the surface chemistry such as surface coverage and the activation energy for desorption [49]. TPD is discussed in detail in section B 1.25. In TPD, a clean surface is first exposed to a gaseous... 

On K modified Ni(100) and Ni(lll)62,63 and Pt(lll)64 the dissociative adsorption of hydrogen is almost completely inhibited for potassium coverages above 0.1. This would imply that H behaves as an electron donor. On the other hand the peaks of the hydrogen TPD spectra shift to higher temperatures with increasing alkali coverage, as shown in Fig. 2.22a for K/Ni(lll), which would imply an electron acceptor behaviour for the chemisorbed H. Furthermore, as deduced from analysis of the TPD spectra, both the pre-exponential factor and the activation energy for desorption... 

An increase in the enthalpy, H, of the adsorbate causes an equal decrease in its activation energy for desorption, Ed, i.e. AH = -AEd, thus ... 

Figure 5.26. Effect of catalyst potential on the oxygen desorption activation energy, Ed, calculated from the modified Redhead analysis for Pt, Ag and Au electrodes deposited on YSZ.44,46 Reprinted from ref. 44 with permission from the Institute for Ionics.

Table 6.2. Activation energies for NO dissociation and N2 desorption from two rhodium surfaces.

Suppose we successfully measured the sticking coefficient and the activation energy for adsorption of a certain molecule, as well as the rate of desorption. Is it then possible to estimate the equilibrium constant for adsorp-tion/desorption ... 

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. 

It is now assumed that the adsorption of NO on Rh(lOO) is associative and not activated. The sticking coefficients S(T) can be set to unity for all temperatures. Furthermore shall we assume that the activation energy for the desorption of NO from Rh(lOO) is 140 kj mol and that two Rh atoms constitute an adsorption site. 

Figure 5 presents data for the non-lnteractlng Rh/S102 catalyst at similar pressures and at 48, 158, and 333 °C. Even with the scatter In the 333°C data, there Is an obvious transition In the spectra as the temperature Is Increased. The predominant peak around 10 rad/sec diminishes and the one around. 5 rad/sec Increases to dominate the spectrum, a trend similar to that observed In the Rh/T102 spectra. Presumably, these trends are the result of differences In apparent activation energies for H2 adsorption and desorption on the various types of sites. 

Figure 2. Plot of the desorption rate, molecules/sec, (solid circles) and the Integrated number of molecules desorbed (solid line) for an adsorbate with a desorption activation energy of 20Kcal/mole and a preexponentlal of 10 sec-. The temperature jump shown In Figure 1 was used for this calculation.

A pre-exponential factor and activation energy for each rate constant must be established. All forward rate constants involving alkyne adsorption (ki, k2, and ks) are assumed to have equal pre-exponential factors specified by the collision limit (assuming a sticking coefficient of one). All adsorption steps are assumed to be non-activated. Both desorption constants (k.i and k ) are assumed to have preexponential factors equal to 10 3 sec, as expected from transition-state theory [28]. Both desorption activation energies (26.1 kcal/mol for methyl acetylene and 25.3 kcal/mol for trimethylbenzene) were derived from TPD results [1]. 

Another aspect of rate measurements that is useful in discriminating between the two types of adsorption involves studies of the rate of desorption. The activation energy for desorption from a physically adsorbed state is seldom more than a few kilocalories per mole, whereas that for desorption from a chemisorbed state is usually in excess of 20 kcal/mole. Consequently, the ease with which desorption occurs on warming from liquid nitrogen temperature to... 

Calculations carried out by Gottesfeld et al. [52], who borrowed from studies of thermal desorption of H2 from Cu [56, 57], indicate that H2 rather than H20 should be a product of formaldehyde oxidation at Cu at potentials up to ca. +0.4 V vs. RHE. This is provided conditions are such that the activation energy for hydrogen recombination and desorption does not exceed 10 kcal/mole. Obviously a relatively high activation energy (which appears never to be observed at Cu) favors eventual oxidation of adsorbed H atoms, before recombinative desorption can occur. Gottesfeld et al. s calculation is interesting, but perhaps not a definitive calculation since it is... 

For the present analysis, it is assumed that the activation energy for CO desorption decreases linearly with CO coverage. 

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