Energy desorption

Figure A3.10.8 Depiction of etching on a Si(lOO) surface, (a) A surface exposed to Br2 as well as electrons, ions and photons. Following etching, the surface either becomes highly anisotropic with deep etch pits (b), or more regular (c), depending on the relative desorption energies for different surface sites [28].

Figure 2.6. Effect of alkali coverage on (a) the alkali adatom dipole moment and alkali desorption energy (b) for Na, K and Cs adsorbed on Ru (0001) and corresponding effect of work function change AO on the alkali desorption energy (c).26 Reprinted with permission from Elsevier Science.

Hence, 0 needs to be known (or estimated, e.g. at half the initial coverage) at the point where Tm is reached. Again, a couple of iteration steps is usually sufficient to arrive at an estimate for the desorption energy. 

However, if the prefactor and the desorption energy depend on coverage, the derivative will take the form ... 

Such behavior is known as the compensation effecf . The important point is that if we ignore the additional term in Eq. (18), we essentially force the kinetic parameters to satisfy Eq. (19) resulting in a correlation between the prefactor and the desorption energy according to the compensation effect ... 

Figure 7.11. Effect of repulsive interactions betw/een N atoms and CO molecules. Left The TPD of CO from a clean Rh(lOO) surface is characterized by a desorption energy of 135 kj moT Right When CO desorbs out of a structure /here it is, on average, surrounded...

We now want to estimate the CO coverage when the catalyst is located in a plug-flow reactor with a partial pressure of Pqq = 0-01 bar at T= 1000 K. The desorption energy is estimated to be 147 kj mol and the pre-exponential factor is set to the usual 10 s , while the sticking coefScient is estimated to be 0.2 and independent of temperature. For simplicity we assume that each Ni atom can adsorb a CO molecule. 

Although the desorption energy at the metal-electrolyte interface should be different from that of the metal-gas phase, there is no reason to expect a different behavior for the change of d with 0. 

The Xj is a relative population of adsorption site of type i in the sample and cmax is the Cu+ ions concentration in the sample of the catalyst related to its volume V. F is the rate of flow of the carrier gas, e is a porosity of the layer of the catalyst bed. p is the rate of temperature change. The populations of the Cu+ site types and both desorption energies and desorption entropies for all Cu+ site types were optimized to obtain the best fit with the experimental data. All three experimental Cu-K-FER TPD curves were fitted at once together with all Cu-Na-FER previously measured TPD curves constraining the parameters AHads i and ASads,i to be the same for all samples. 

Due to the presence of low-temperature desorption peak a new desorption site was included to phenomenological model of TPD experiments previously used for the description of the Cu-Na-FER samples [5], The fit of experimental TPD curves was performed in order to obtain adsorption energies and populations for individual site types sites denoted A (A1 pair), B (sites in P channel (A1 at T1 or T2)), C (sites in the M channel and intersection (A1 at T3 or T4)) [3] and D (newly introduced site). The new four-site model was able to reproduce experimental TPD curves (Figure 1). The desorption energy of site D is cu. 82 kJ.mol"1. This value is rather close to desorption energy of 84 kJ.mol"1 found for the site B , however, the desorption entropy obtained for sites B and D are rather different -70 J.K. mol 1 and -130 J.K. mol"1 for sites B and D , respectively. We propose that the desorption site D can be attributed to so-called heterogeneous dual-cation site, where the CO molecule is bonded between monovalent copper ion and potassium cation. The sum of the calculated populations of sites B and D (Figure 2) fits well previously published population of B site for the Cu-Na-FER zeolite [3], Because the population of C type sites was... 

The difference in adsorption energy between a single Ag atom on Ru(001), 240 kJ/mol, and an Ag atom at the edge of an island, 290 kJ/mol, may be considered as the two-dimensional heat of Vaporization of Ag on Ru, and amounts to about 50 kJ/mol. Thus, the desorption energy depends not only on the strength of the bond between adsorbate and substrate, but also on interactions between the adsorbate atoms. Both contributions can be estimated from TDS. 

The difference in desorption energy between multilayer and monolayer Cu is essentially the same for Cu desorption from Ru in vacuum or by electrochemical stripping. 

In addition, a Temkin-type coverage dependence ofthe desorption energy had to be used to describe the data ... 

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