Dissociation on surfaces

DFT calculations [17,331,332] of 02/Pt(l 11) find the two different molecular precursors defined in the discussions above and observed by STM [153,326]. However, DFT calculations of 02 dissociation on surfaces are problematical and two different standard approximations for the exchange correlation functional (PW91 and RPBE) get somewhat different results. The DFT calculations are in only fair agreement with experiments. Most troubling is that all DFT barriers to dissociation from both molecular precursors are significant, 0.6-0.9 eV relative to the 02+ Pt(lll) asymptote. This is qualitatively inconsistent with experiment, which shows that thermal dissociation is energetically favored over desorption. 

The easiest test concerns Eq. (22a) describing the LJ-type dissociation. The equation establishes the linear correlation between the dissociation barrier A for a homonuclear admolecule A2 and the atomic heat of chemisorption Qh with the slope of k = 3/2. As seen from Table V, for H2, 02, and N2 dissociated on surfaces of metals as varied as Fe, Ni, Cu, W, and Pt, the experimental values of k lie within the range k = 1.4-1.7, that is, within 10-15% of the theoretical LJ value of k = 1.5. It should be stressed that, unlike similar linear relations between the activation barriers and the heats of reactions (Brpnsted, Polanyi, Frumkin-Temkin-Semye-nov, etc.), Eq. (22a) is not a postulate but a corollary of the general principle (BOC-MP) applied to the one-dimensional dissociation ABS As + Bs. 

Fig. 9. Avidity controlled interaction kinetics between the anti-biotin antibody 2F5 and the surface-confined biotin moieties. (A) Normalized SPR curves of the 2F5 association/dissociation process on surfaces with relatively high biotin densities. (B) Fluorescence kinetic curves of the 2F5 association/dissociation on surfaces with lower biotin densities.

So far, no exact 6D quantum dynamics calculation has been reported for diatomic dissociation on surface. But with modern computer power, such numerical endeavor will undoubtly be realized soon. We also note here recent mixed quantum/classical studies of Jackson who treated three COM coordinates classically and three internal molecular coordinates quantum mechanically for H2/Cu(100) (120). Such treatment seems quite promising for more complex systems. 

The calculated barrier for the reaction of CO and hydrogen to form the surface formyl intermediate over Ru is 143 kJ/mol (see Fig. 3.39). The subsequent CO bond activation of the the formyl intermediate on Ru is then only 30 kJ/mol. Hence, on the Ru terrace this scheme of first adding hydrogen before CO dissociation will be the preferred path to cleave the CO bond compared with the direct CO dissociation path. On the step, the barrier to formyl formation is similar to that on the terrace, hence higher than the barrier to direct CO dissociation. On surface steps the latter dissociation step path will be the preferred path for the formation of Ci adsorbed species. 

The most common case is the CO adsorption and dissociation on surfaces which depends on the type of metal and thus, on the electrons of the metal layer d. Thus, for example, during the adsorption and dissociation of CO with different metal levels d is connected directly to dissociation energy. The CO dissociation on copper is endothermic, on nickel is exothermic, and on cobalt is highly exothermic, as shown in Fig. 5.12. 

Surfaces in polar solvents and particularly in water tend to be charged, tlirough dissociation of surface groups or by adsorjDtion of ions, resulting in a charge density a. Near a flat surface, < ) only depends on the distance x from the surface. The solution of equation (C2.6.6) then is... 

The use of CO is complicated by the fact that two forms of adsorption—linear and bridged—have been shown by infrared (IR) spectroscopy to occur on most metal surfaces. For both forms, the molecule usually remains intact (i.e., no dissociation occurs). In the linear form the carbon end is attached to one metal atom, while in the bridged form it is attached to two metal atoms. Hence, if independent IR studies on an identical catalyst, identically reduced, show that all of the CO is either in the linear or the bricked form, then the measurement of CO isotherms can be used to determine metal dispersions. A metal for which CO cannot be used is nickel, due to the rapid formation of nickel carbonyl on clean nickel surfaces. Although CO has a relatively low boiling point, at vet) low metal concentrations (e.g., 0.1% Rh) the amount of CO adsorbed on the support can be as much as 25% of that on the metal a procedure has been developed to accurately correct for this. Also, CO dissociates on some metal surfaces (e.g., W and Mo), on which the method cannot be used. 

The alkali promotion of CO dissociation is substrate-specific, in the sense that it has been observed only for a restricted number of substrates where CO does not dissociate on the clean surface, specifically on Na, K, Cs/Ni( 100),38,47,48 Na/Rh49 and K, Na/Al(100).43 This implies that the reactivity of the clean metal surface for CO dissociation plays a dominant role. The alkali induced increase in the heat of CO adsorption (not higher than 60 kJ/mol)50 and the decrease in the activation energy for dissociation of the molecular state (on the order of 30 kJ/mol)51 are usually not sufficient to induce dissociative adsorption of CO on surfaces which strongly favor molecular adsorption (e. g. Pd or Pt). 

Alkali promoted NO dissociation is clearly illustrated in the case of NO adsorption on K/Pt(lll), as NO is not adsorbed dissociatively on the alkali-clean surface. The dissociative adsorption of NO on K/Pt(l 11) takes place at temperatures higher than 300 K and the number of dissociated NO molecules... 

Figure 4.27 presents steady-state potentiostatic r vs 0Na results during NO reduction by H2 on Pt/p"-Al203f2 PInb values well in excess of 4000 are obtained for 0Na values below 0.002. This is due to the tremendous propensity of Na to induce NO dissociation on transition metal surfaces. Since Plj is often found to be strongly dependent on 0, (Figs. 4.26 and 4.27), it is also useful to define a differential promotion index pij from ... 

Figure 1.5 Plot of computed reaction barriers for dissociation at Eaa. for N2 dissociation as a function of nitrogen atom adsorption energy on surface terrace and stepped surface [2]. The upper curve is for surface terrace of (111) type of fee crystals, and the lower curve presents data on the stepped surfaces.

Figure 5.10. Defects consisting of oxygen vacancies constitute adsorption sites on a Ti02 (110) surface. Note how CO binds with its lone-pair electrons on a Ti ion (a Lewis acid site). O2 dissociating on a defect furnishes an O atom that locally repairs the defect. CO2 may adsorb by coordinating to an O atom, thus forming a carbonate group. [Figure adapted from W. Gopel, C. Rocher and R. Feierabend, Phys. Rev. B 28 (1983) 3427.]...

As explained in the previous chapters, catalysis is a cycle, which starts with the adsorption of reactants on the surface of the catalyst. Often at least one of the reactants is dissociated, and it is often in the dissociation of a strong bond that the essence of catalytic action lies. Hence we shall focus on the physics and chemistry involved when gases adsorb and dissociate on a surface, in particular on metal surfaces. 

Given a certain metal, what can we do to alter its reactivity First there is the structure of the surface. More open surfaces expose atoms of lower coordination. This narrows the d band, and shifts its position (up if it is more than half filled, down if the d band is less than half filled). To illustrate the point Tab. 6.2 shows experimentally determined activation energies of NO dissociation on the (111) and (100) surfaces of rhodium. 

Figure 7.12 neatly illustrates how dissociation reactions on surface are greatly affected by interactions with neighboring adsorbate species. This is because dissociations often require ensembles of sites in a specific arrangement. From our discus-... 

Next we will adopt a kinetic scheme and see if it describes the data of Fig. 7.16. Several treatments of HDS kinetics are available in the literature. Here we use a simplified scheme in which thiophene (T) exclusively adsorbs on sulfur vacancies, denoted by A, and H2 adsorbs dissociatively on all the sites (indicated by ) to form butadiene (B) and H2S in a rate-determining surface reaction (we ignore the kineti-cally insignificant hydrogenation steps of butadiene) ... 

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