Chemisorption trends

In this chapter, we shall use DFT to investigate the extent to which the oxide support alters the electronic structure of the deposited metal as a result of charge transfer at the metal-oxide interface. We will use CO chemisorption as a function of Pt film thickness to demonstrate how changes in the metal electronic structure can lead to chemisorption trends that deviate from expectations based on the current theory of molecular adsorption. 

Based on the reactivity of the species chemisorption trends and adopting the stoichiometry of HT OT Pt =1.5 0.75 1 established by Benson and Boudart (8) for chemisorption at both temperatures, dispersion values calculated are given in Table.3. Equations proposed for measuremnet of dipersion of different species are also listed in Table.3. It is seen that Pt(A) values calculated using three separate data and Sn(A) values obtained from two separate data match well with each other, indicating the validity of the method. The total Pt dipersion, which includes both alloyed and unalloyed forms, increases with increasing Sn/Pt ratio upto 8 beyond which there is a drastic decrease... 

The micropore volume varied from -0.15 to -0.35 cmVg. No clear trend was observed with respect to the spatial variation. Data for the BET surface area are shown in Fig. 14. The surface area varied from -300 to -900 mVg, again with no clear dependence upon spatial location withm the monolith. The surface area and pore volume varied by a factor -3 withm the monolith, which had a volume of -1900 cm. In contrast, the steam activated monolith exhibited similar imcropore structure variability, but in a sample with less than one fiftieth of the volume. Pore size, pore volume and surface area data are given in Table 2 for four large monoliths activated via Oj chemisorption. The data in Table 2 are mean values from samples cored from each end of the monolith. A comparison of the data m Table 1 and 2 indicates that at bum-offs -10% comparable pore volumes and surface areas are developed for both steam activation and Oj chemisorption activation, although the process time is substantially longer in the latter case. 

Equation (6.20) and the semiquantitative trends it conveys, can be rationalized not only on the basis of lateral coadsorbate interactions (section 4.5.9.2) and rigorous quantum mechanical calculations on clusters89 (which have shown that 80% of the repulsive O2 - O interaction is indeed an electrostatic (Stark) through-the-vacuum interaction) but also by considering the band structure of a transition metal (Fig. 6.14) and the changes induced by varying O (or EF) on the chemisorption of a molecule such as CO which exhibits both electron acceptor and electron donor characteristics. This example has been adapted from some rigorous recent quantum mechanical calculations of Koper and van Santen.98... 

At this point, it seems appropriate to consider the Co-, Ni-, and Cu-ethylene system as a whole, both to rationalize any spectral trends as a function of metal, and to evaluate the use of these complexes as local-ized-bonding models for chemisorption of C2H4. 

The copolymer-silica adhesives also follow a similar trend but fail much earlier than the terpolymer-based adhesives. This is because of two factors (1) the increase in the inherent strength of the adhesive due to more favorable terpolymer rubber-silica interaction and (2) chemisorption in much higher magnitude between the polar substrates and the nanocomposites. 

In the previous two sections we have described trends in the chemisorption energies of atoms and molecules on metallic surfaces. These express the final situation of the adsorption process. Here we consider what happens when a molecule approaches a surface. 

If we restrict ourselves to the late transition metals the trends will, as for the CO chemisorption energy, be dominated by the interaction of the antibonding orbital with the d band and the leading term is... 

Looking at the trends in dissociation probability across the transition metal series, dissociation is favored towards the left, and associative chemisorption towards the right. This is nicely illustrated for CO on the 4d transition metals in Fig. 6.36, which shows how, for Pd and Ag, molecular adsorption of CO is more stable than adsorption of the dissociation products. Rhodium is a borderline case and to the left of rhodium dissociation is favored. Note that the heat of adsorption of the C and O atoms changes much more steeply across the periodic table than that for the CO molecule. A similar situation occurs with NO, which, however, is more reactive than CO, and hence barriers for dissociation are considerably lower for NO. 

Copper clusters, as reported by the Rice group(lc), do not react with hydrogen. Hydrogen chemisorption on copper surfaces is also an activated process. Surface beam scattering experiments place this barrier between 4-7 kcal/mole(33). This large value is consistent with the activated nature oT hydrogen chemisorption on metal clusters, and the trend toward bulk behavior for relatively small clusters (>25 atoms in size). 

The similarity of the reactivity patterns for niobium and cobalt and the non-reacti vi ty of iron with nitrogen suggests that dissociative chemisorption is taking place. Dissociation of molecularly chemisorbed nitrogen is an activated process on all metals(35) and is most exothermic for the early metals in the periodic tab e(36). The limited observations on clusters seems to be consistent with these trends. 

In conclusion, we have seen that an applied held has the ability to strongly affect the chemisorption process. One trend, clearly observable in both Fig. 7.6(b) and Fig. 7.7(b), is that the sign of F determines whether or not Aq is enhanced by the presence of the held, i.e., Aq is increased (decreased) when F is positive (negative). More variable is the dependence of AE on F, due to the variability in the existence (or not) of Absolutions, which, when they do occur, represent the more stable interaction. Consequently, the presence of the held may either enhance or suppress the chemisorption process. 

Nalewajski, R. F. and A. Michalak. 1998. Charge sensitivity/bond-order analysis of reactivity trends in allyl-[Mo03] chemisorption systems A comparison between (010)- and (100)-surfaces. J. Phys. Chem. A 102 636-640. 

Using perturbation theory. Hammer and Nprskov developed a model for predicting molecular adsorption trends on the surfaces of transition metals (HN model). They used density functional theory (DFT) to show that molecular chemisorption energies could be predicted solely by considering interactions of a molecule s HOMO and LUMO with the center of the total d-band density of states (DOS) of the metal.In particular. 

The interaction of adsorbed reactants (phenol and methanol adsorbed separately and coadsorbed) and possible reaction products of phenol methylation with the Cul-xCoxFe204 system has been studied at temperatures between lOOoC and 350oC and probed by in situ FTIR spectroscopy. The spectra of adsorbed methanol, phenol and methylated products on catalyst surface, at lOOoC, did not possess much changes compared to the spectra of pure components that indicated the molecular adsorption of species on catalyst surface. The remarkable changes in the spectra occur, above 100°C due to the chemisorption of substrates, were observed and correlated with the observed reaction trend. 

It is also termed chemisorption (especially for gases), inner sphere adsorption and, in the case of ligands, ligand exchange. The binding constants, Kf and for the surface complexes show the same stability trend as do the constants for the equivalent complexation reactions in solution. 

In general, the trends noted above correspond to an increase in catalytic activity with increasing per cent d-character of metallic bonding or, roughly, with increasing work function. As noted earlier, this also implies an inverse dependence of catalytic activity on the heat of adsorption (i.e., on the strength of the chemisorptive bond). This is to be expected where dest>rp-tion is a rate-determining process. 

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