Desorption activity

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. 

In the case of nitrogen on iron, the experimental desorption activation energies are also shown in Fig. XVIII-13 the desorption rate was given by the empirical expression... 

J.L. Falconer, and R J. Madix, Flash desorption activation energies DCOOH decomposition and CO desorption fromNi(l 10), Surf. Sci. 48, 393-405 (1975). 

Figure 4.47. Effect of catalyst potential on the desorption activation energy of 02 from Pt/YSZ calculated from the modified Redhead analysis ( ) and from the initial slope of the TPD spectra (O).30 Reprinted with permission from Academic Press.

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.

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]. 

Campbell reported that propylene adsorbs weakly on gold surfaces and adsorbs moderately on T1O2 (11 0) with a desorption activation energy of 11.3kcal/mol and that propylene adsorbs most strongly at the perimeter of gold islands on Ti02 (11 0) [68]. 

In catalysis active sites are operative that allow for an alternative reaction path. For a satisfactory catalyst this alternative pathway leads to higher rates and higher selectivity. In heterogeneous catalysis reactant molecules adsorb at active sites on the catalyst surface at the surface sites reactions occur and products are desorbed subsequently. After desorption, active sites are again available for reactant molecules and the cycle is closed. In homogeneous catalysis the situation is essentially identical. Here complexation and decomplexation occur. A complication in heterogeneous catalysis is the need for mass transfer into and out of the catalyst particle, which is usually porous with the major part of the active sites at the interior surface. 

It follows from the formula that the experimentally evaluated value of / activation energy on the ZnO surface may be related to the activation energy of oxygen desorption from the zinc oxide surface. This value well agrees with the desorption activation energy measured with the aid of semiconductor detectors in work [109]. 

From this expression, one can show that, for reasonable parameter values, there can be no multiple steady states for very large Ts. This will also be the case, independent of Ts, if r is larger than both dA and dB. However, if Er is less than either desorption activation energy, then it is possible to reduce T below the critical value required for multiple steady states by decreasing the surface temperature. The actual values of the reactant partial pressure and surface temperature for which multiple steady states occur can vary by orders of magnitude due to the sensitivity of (46) to the parameters (and the fact that preexponential factors can vary by orders of magnitude between different systems). 

Figure 1 is an early representation of these three regimes with their distinctive physical and chemical phenomena (3). In this early picture, energy interconversion was considered as a form of isomerization—"energy isomerization"—leading to an expression of the excitation in a form more or less independent of the type of energy input. Vibrational excitation, especially of the lower frequency modes corresponding to intermolecular motion, was considered as the basis for desorption. Activation of surface phonons expresses these ideas in different currency. 

Here J, JQ and Ja are the statistical sums of activated complex and gas-phase molecules and of adsorbed atom (adatom), respectively, sA and eD the adsorption and desorption activation energies, a the area of adatom localization, h Planck s constant, and f. the parameters of the activated complex-adatom and adatom-adatom interactions (e < 0 for repulsion and e > 0 for attraction), A the contribution to the complete drop of adsorption heat AQ from the electron subsystem (for a two-dimensional free-electron gas model), x = exp (ej — e) — 1, jc, = = 0), / = 1/kT (k is the Boltzmann con-... 

The description of kinetic dependences is, however, facilitated considerably by the fact that the desorption activation energy is sufficiently high (> 50 kcalmol 1) for desorption to be neglected at T < 700 K. 

Fig. 8.6. Potential curves of Vvs. h in the transitive state model (a) and the collision model (b), where EA and Ed are the adsorption and desorption activation energies, respectively Q is the heat of adsorption, h the distance from the surface (horizontal axis of the figure).

Pai and Doren97 using DFT and cluster models found only the asymmetric transition state. They predicted the desorption activation energy to be 55 kcalmol-1 using LSD and... 

Reaction selectivity is observed on laser-induced desorption and dissociation of NO and CO chemisorbed on Ni, Pd, and Pt surfaces via the electronic transition using visible and ultraviolet nanosecond-pulsed lasers, as listed in Table 6. The open circle shows that desorption and dissociation have been observed, while the cross mark means that they have not been observed. These metals are isoelectronic and the band structure is very similar, but the activity on laser-induced desorption and dissociation is remarkably different. The origin of the different desorption activity between Pt and other transition metals of Ni and Pd may be closely related with the nature of the antibonding 2-ira state in adsorbed NO and CO [11]. 

The bandwidth of the d band is considered as one of the candidates for the origin for the desorption activity [11]. The bandwidth of the Pt d band ( 8 eV) is wider than those of Ni and Pd ( 5 eV). If the bandwidth is wider than 8 eV, the 5a state of NO or the bonding 5ab state of CO is resonant with the d state of the substrate metal. The strong resonant 5a-d interaction causes attenuation of the 2-Tr-d interaction [84]. Therefore, it is considered that excitation to the 2-ira state brings longer lifetime of the... 

The Pt(l 11) surface alloyed by a small amount of Ge is a nice alloy substrate for studying the origin of the desorption activity in UV laser-induced desorption [87]. This surface alloy is prepared by repeated cycles of deposition of a few ML Ge and subsequent annealing to 1100°C until a constant Ge Auger electron signal was obtained. The total amount of Ge contained in several surface layers of this alloy is 0.1 ML and the Ge coverage in the top layer is 0.04 ML due to a 5 x 5 structure observed by STM [88]. Fukutani et al. call this surface the Pt(l 1 1)-Ge surface alloy. 

When ArF excimer laser irradiates the Pt( 111 )-Ge surface alloy saturated by NO or CO at 80 K, desorbed NO molecules are detected by the REMPI method, while no CO desorption is observed [87]. Only a little modification of the Pt(l 1 1) surface brings such a remarkable change of the desorption activity. TDS of NO from the alloy at various NO coverages is shown in Fig. 29. Every spectrum has a prominent peak at 220 K, and NO is saturated at 0.2 L exposure in contrast with Pt(l 1 1), on which NO is saturated at 2 L exposure and saturation coverage is 0.75 ML. 

The peak temperature in TDS of NO desorption from the alloy is 220 K in contrast with 310 K for the on-top species from Pt(l 11), i.e. lower by 90 K due to alloying. This lowering of the TDS peak is also observed for CO on the alloy at saturation, which reveals the peak at about 330 K, lower by 80 K than that on Pt(l 1 1) [87]. This weakening of the chemisorption s bond for the on-top species is consistent with the attenuation of the interaction with the dyy and dy / orbitals caused by the d band filling of the eg state. Flowever, laser-induced desorption of CO from the alloy is not observed. Therefore, the desorption activity in the DIET process is indifferent to the strength of the chemisorption bond. 

Finally, we would like to make a scenario of the desorption activity for NO and CO desorption from Pt(l 1 1) and Pt(l 1 1)-Ge surface alloy. This scenario will be extended to a general concept of desorption in the DIET process of simple molecules from metal surfaces. The lifetime and the critical residence time in the intermediate excited state followed by desorption are important keys for solving what is the origin of the desorption activity in the DIET process from metal surfaces. The excited molecules are not desorbed, if the residence time in the excited state is shorter than the critical residence... 

Adsorbed Species, Coverages, and Desorption Activation Energies"... 

The chemical composition can be measured by traditional wet and instrumental methods of analysis. Physical surface area is measured using the N2 adsorption method at liquid nitrogen temperature (BET method). Pore size is measured by Hg porosimetry for pores with diameters larger than about 3.0 nm (30 A) or for smaller pores by N2 adsorp-tion/desorption. Active catalytic surface area is measured by selective chemisorption techniques or by x-ray diffraction (XRD) line broadening. The morphology of the carrier is viewed by electron microscopy or its crystal structure by XRD. The active component can also be measured by XRD but there are certain limitations once its particle size is smaller than about 3.5 nm (35 A). For small crystallites transmission electron microscopy (TEM) is most often used. The location of active components or poisons within the catalyst is determined by electron microprobe. Surface contamination is observed directly by x-ray photoelectron spectroscopy (XPS). 

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

Figure lO A plot of desorption activation energy indicated an enhanced binding energy of -6.2 kJ/mol for the desorption peak centered at -50 °C, as shown in Fig. 7. 

Repulsive interactions between NO and CO molecules were assumed and the activation energy for desorption was given by Eact,No,co(0) = Eaci(O) — k (0NO + 0co). Although oscillations similar to those observed in experiments were predicted by this model, no justification was given for the unusual quadratic dependence on surface coverage for the desorption activation energy, and experiments in which CO and NO have been coadsorbed on Pt(lOO) have shown small or attractive interactions between the adsorbates (300). 

Simultaneously, ethylene molecules are chemisorbed to the surface. If through diffusion a defect (ejH jA) appears at an interstitial site immediately below a carbon atom of the ethylene the electron bound in the field of the proton will interact with the p electron of that carbon atom. There will be a finite probability of bond formation at this stage with resultant desorption of that end of the ethylene molecule. If during the period of this process a second defect appears under the other carbon atom, the hydrogenated product, ethane, will form and be desorbed if the desorption activation energy is available to it. The reaction is then... 

Fig. 3 Effect of catalyst potential on the oxygen desorption activation energy,...

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