Beam experiments modeling

Figure 3.8. Kinetic data from molecular beam experiments with NO + CO mixtures on a Pd/MgO(100) model catalyst [70]. The upper panel displays raw steady-state C02 production rates from the conversion of Pco = PN0 = 3.75 x 10-8 mbar mixtures as a function of the sample temperature on three catalysts with different average particle size (2.8, 6.9, and 15.6 nm), while the bottom panel displays the effective steady-state NO consumption turnover rates estimated by accounting for the capture of molecules in the support. After this correction, which depends on particle size, the medium-sized particles appear to be the most active for the NO conversion. (Reproduced with permission from Elsevier, Copyright 2000).

The problem of quenching alkali resonance radiation in E-VR energy-transfer collisions with simple molecules is important as a model case for basic processes in photochemistry and serves its own right for a variety of practical applications, such as in laser physics. It has been studied for many years in the past, but only recent progress has led to information of the final internal energy of the molecule. In particular, crossed-beam experiments with laser-excited atoms allow a detailed measurement of energy-transfer spectra. There can be no doubt that the curve-crossing... 

All the experiments described in this chapter were performed on molecules in gas cells, and, unlike in beam experiments, the results obtained are averaged over all possible orientations of the molecules with respect to the relative velocity vector, and over the Maxwellian distribution of the relative velocities of the colliding particles. Nevertheless, the experimental data stimulated interest in the development of theoretical models of elastic... 

Figure 8 Trapping probability of 02/Pt(l 11) as a function of the kinetic energy for normal incidence. Results of molecular beam experiments for surface temperatures of 90 and 200 K (Luntz et al. [81]) and 77 K (Nolan et al. [87]) are compared to simulations in the hard-cube model (HCM).

This simple model would lead one to conclude that H2 dissociation on transition metals, where the unfilled d-states produce a low and early barrier (or even zero barrier), will show no vibrational enhancement, whereas dissociation on simple and noble metals, for which the barrier is high and late, will have vibrationally enhanced dissociation. This appears to be borne out in molecular beam experiments there is no observable increase in dissociation with internal state temperature for H2 on Ni(l 1 1), Ni(l 1 0), Pt(l 1 1) or Fe(l 1 0) [16-19], whereas dissociation on all surfaces of Cu shows an... 

This model is quite universal providing that the ion-induced reaction rates prevail on the thermal reaction rates. It has been thoroughly discussed for Si and SiCF etching by means of beam experiments [75-77], and has been checked in plasma environment [78]. It is also verified for other systems Si in Cl2, [79], SiGe alloys in SF5 [80], or InP in CH4—H2 [81]. 

Another effect of the nanometer size in reaction kinetics of catalytic reaction is the disappearance of the bi-stability in the CO oxidation as recently evidenced by molecular beam experiments on supported model catalysts [26]. 

In the reduction of NO by CO at low temperature, on Pd/MgO(100) model catalysts, the reaction rate is independent of CO pressure but increases with NO pressure [64,65], then the TON of the reaction is modified by the reverse-spillover effect and, in particular, depends on particle size. In that case, it is no longer possible to compare the TON value measured on different particle sizes, it is more appropriated to calculate the reaction probability [64], which takes into account for the real flux of molecules that reach the clusters which can be measured by molecular beam experiments. Reverse-spillover effects have also been recently observed for the CO oxidation on size-selected Pd clusters soft-landed on MgO epitaxial films [66]. 

These results suggest that the model is capable of reproducing the sustained chaos observed in the beam experiments. Figure 12.5.7 shows that there is good qualitative agreement between the experimental data (Figure 12.5.7a) and numerical simulations (Figure 12.5.7b). 

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