Trends in Chemisorption

In the following, we first consider trends in chemisorption energies. Similar trends in activation barriers will be dealt with in the next section. 

As we go from left to right across the transition metals in the periodic table, the metal atoms become smaller, much as in the lanthanide contraction (Section 2.6). Furthermore, the atoms of elements of the first transition series are smaller than those of corresponding members of the second and third. Consequently, interstitial carbides are particularly important for metals toward the lower left of the series, as with TiC, ZrC, TaC, and the extremely hard tungsten carbide WC, which is used industrially as an abrasive or cutting material of almost diamond like hardness. The parallel with trends in chemisorption (Section 6.1) will be apparent. 

To evaluate whether trends in chemisorption energies on Pt nanoparticles are consistent with the d-band model, d-band densities of states were projected out for different adsorption sites to determine the corresponding d-band centers relative to the Fermi level. No correlation was observed between the adsorption energies and site-specific d-band centers. Even though metal nanoparticles possess a continuous electronic band structure and, thus, metal-like electronic properties, their catalytic surface properties are not controlled by band structure effects but by the local electronic structure of the adsorption sites. An important conclusion from this study is that... 

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. 

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. 

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. 

As a first example of the use of the d band model, consider the trends in dissociative chemisorption energies for atomic oxygen on a series of 4d transition metals (Figure 4.6). Both experiment and DFT calculations show that the bonding becomes... 

A large number of studies have focused on the chemisorption of molecular oxygen on well-defined Pt and Pt alloys. Chemisorption is the first step in the ORR process, and the structure, bonding type and energy are key elements for understanding trends in ORR activity as function of electrode composition and structure. 

One of the primary aims of the research program described in this review has been to formulate adequate two-reactant reactivity concepts, and the underlying coordinate systems, which can be used to diagnose reactivity and selectivity trends in systems of very large donor/acceptor reactants, e.g., chemisorption systems. The CSA approach [52], which provides the basis for the present work, is both relevant and attractive from the chemist s point of view, since many branches of chemistry—the theory of chemical reactivity in particular—consider responses of chemical species to perturbations of the external potential and... 

This alternative is quite attractive from another standpoint. If the generation of an active site is the slow step in certain chemisorption processes, what is the nature of this site generation An answer must be found in the chemistry of the solid state so that an inquiry into the detailed kinetics of adsorption rejoins a current active trend in catalytic research. While interesting models involving the electronic behavior of adsorbents and catalysts have been proposed during the last ten years, none can be considered as definitely proved and progress has been slow because of the usual experimental difficulties in surface definition and reproducibility. 

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