Adsorption probability

NO, the monomer C is CO, and the products are A2 = N2 and CB = CO2. The adsorption probability of C species (Fc) is the parameter of the model. The slow rate-determining step in this sequence is the dissociation of NO which requires a neighboring site to proceed. Since product formation liberates more vacant sites than those necessary for the dissociation of NO, an autocatalytic production of vacant sites takes place. 

Adsorption probability at an active site of the surface Sc Schmidt number... 

The effect of oxidizing atmospheres on the reduction of NO over rhodium surfaces has been investigated by kinetic and IR characterization studies with NO + CO + 02 mixtures on Rh(lll) [63], Similar kinetics was observed in the absence of oxygen in the gas phase, and the same adsorbed species were detected on the surface as well. This result contrasts with that from the molecular beam work [44], where 02 inhibits the reaction, perhaps because of the different relative adsorption probabilities of the three gas-phase species in the two types of experiments. On the other hand, it was also determined that the consumption of 02 is rate limited by the NO + CO adsorption-desorption... 

Figure 3.11. Typical experimental behaviors for dissociative adsorption probabilities S with respect to incident energy Et in (a) and with respect to Ts in (b) for limiting dissociation behaviors. The solid lines are for direct (weakly activated) dissociative adsorption and the dashed lines are for a precursor-mediated dissociation.

Figure 3.20. Molecular adsorption probability S0 (= a in the notation of this chapter) for NO on Pt(l 11) vs. incident translational energy ,. Solid circles are for 0, = 0° and open squares are for , = 60°. From Ref. [193].

Figure 3.32. H2 Sticking (dissociative adsorption) probability S on Pd(100) as a function of incident normal kinetic energy Et = En. Circles are experiment [304], dashed and solid line are 6D first principles quantum dynamics with H2 in the ground state and a thermal distribution appropriate to the experiments, respectively [109]. The inset is also 6D first principles quantum dynamics but based on a better PES [309]. From Ref. [2].

We have studied above a model for the surface reaction A + 5B2 -> 0 on a disordered surface. For the case when the density of active sites S is smaller than the kinetically defined percolation threshold So, a system has no reactive state, the production rate is zero and all sites are covered by A or B particles. This is quite understandable because the active sites form finite clusters which can be completely covered by one-kind species. Due to the natural boundaries of the clusters of active sites and the irreversible character of the studied system (no desorption) the system cannot escape from this case. If one allows desorption of the A particles a reactive state arises, it exists also for the case S > Sq. Here an infinite cluster of active sites exists from which a reactive state of the system can be obtained. If S approaches So from above we observe a smooth change of the values of the phase-transition points which approach each other. At S = So the phase transition points coincide (y 1 = t/2) and no reactive state occurs. This condition defines kinetically the percolation threshold for the present reaction (which is found to be 0.63). The difference with the percolation threshold of Sc = 0.59275 is attributed to the reduced adsorption probability of the B2 particles on percolation clusters compared to the square lattice arising from the two site requirement for adsorption, to balance this effect more compact clusters are needed which means So exceeds Sc. The correlation functions reveal the strong correlations in the reactive state as well as segregation effects. 

In [4] we have introduced a CA model for the NH3 formation which accounts only for a few aspects of the reaction system. In our simulations the surface was represented as a two-dimensional square lattice with periodic boundary conditions. A gas phase containing N2 and H2 with the mole fraction of t/N and j/h = 1 — j/n, respectively, is above this surface. Because the adsorption of H2 is dissociative an H2 molecule requires two adjacent vacant sites. The adsorption rule for the N2 molecule is more difficult to be described because experiments show that the sticking coefficient of N2 is unusually small (10-7). The adsorption probability can be increased by high energy impact of N2 on the surface. This process is interpreted as tunnelling through the barrier to dissociation [32]. 

Another possibility to increase the adsorption probability is via electronic promoters (K2O). It is believed that this promoter, which is enriched on the... 

Let us discuss now a more realistic case in which not all surface sites are activated. This means a reduction of the adsorption probability of the N2 molecules. In the following we assume that 5=1/8. 

Curve 3 shows the diagram for EA = 0 and EAb — 1- The parameter Eab shifts the critical point y dramatically to larger values of Yco which means that the complete occupation of the lattice by B easily takes place. This is understandable from the fact that the A-adsorption probability is decreased by the repulsion with the B particles, which are the dominant species on the lattice. Therefore the tendency of the B particles to form large clusters is enhanced. This can be seen from Rco2 which is for Yco < yi nearly zero and rises sharply at Yco Vi-... 

Porstendorfer, J. Mercer, T.T. (1978) Adsorption probability of atoms and ions on particulate surfaces in submicrometer size range. Journal of Aerosol Science, 9, 469-74. 

In the case of dissociative adsorption on surfaces there is an additional channel into which energy can be transfered, namely the conversion of the kinetic and internal energy of the molecule into translational energy of the fragments on the surface with respect to each other. In fact, in the dissociation of light molecules such as 1I2 on metal surfaces the dissociative adsorption probability is... 

Figure 6 Dissociative adsorption probability of H2 on Cu(l 00) as a function of the incident kinetic energy determined by six-dimensional quantum wave-packet calculations for molecules initially in the vibrational ground state and first excited state, respectively [32], For the vibrational ground state, the calculations are compared to experimental results derived from an analysis of adsorption and desorption experiments [27].

Information from beam exposures of the sample can be obtained a number of ways. For surface adsorption probabilities greater than 0.03, the King and Wells method [16] can accurately be employed to determine initial adsorption probabilities of a gas species. In short, a King and Wells measurement monitors the partial pressure of the scattering chamber before, during, and after exposure of the sample to the beam flux. A typical reflectivity measurement can be seen in Fig. 2. 

Figure 23 Adapted from Guo et al. [83], Initial dissociative adsorption probability as a function of normal translational energy for O2 on Pt(l 0 0)-hex-R0.7° for various surface temperatures, Ts.

Molecular adsorption probabilities for 02 as a function of normal translational energies at Ts = 77 K were also measured by Nolan et al., and are shown in Fig. 24, along with dissociative adsorption probabilities obtained by Guo et al. [83] at Ts = 573 K. 

Apparent from Fig. 24 is the self-similarity of the curves for molecular adsorption and dissociative adsorption which Nolan et al. argues provides strong evidence that both processes occur via a similar initial step, specifically, a molecularly adsorbed precursor. Further, the adsorption probability at 71 = 77 K appears to scale with normal energy, mirroring the normal energy scaling of the dissociative probability curve. 

---