Gas-Surface Interaction

1. MATERIALS SCIENCE & ENGINEERING Part of A Learner’s Guide AN INTRODUCTORY E-BOOK Anandh Subramaniam & Kantesh Balani Materials Science and Engineering (MSE) Indian Institute of Technology, Kanpur- 208016 Email:anandh@iitk.ac.in, URL:home.iitk.ac.in/~anandh http://home.iitk.ac.in/~anandh/E-book.htm Adsorbates on Surfaces • Physisorption • Chemisorption Gas-Surface Interaction Please read before starting this chapter The reader may revise topics related to surface crystallography (& surfaces in general) Advanced Reading Reactions at Solid Surfaces Gerhard Ertl Wiley, Hoboken, New Jersey (2009)

2. Adsorption of Species on Surfaces • Atoms and molecules may adsorb on surfaces (metallic, semi-conductor, insulator). E.g. like Ar, CO, C2H4, Cl, etc. The adsorbed species is called an Adsorbate. • The species may have sub-monolayer, monolayer or multi-layer converge. • At very low coverages, the species may bunch together to form ‘islands’. • The species may get ordered at higher coverages (when the mean distance between the atoms/molecules ~5-10 Å). This may lead to the formation of adsorbate-surface superlattices (refer here: surface crystallograpy). • Attractive and repulsive forces operate between the adsorbed species atoms/molecules.

3. Gas-Surface Interaction: an Overview • When gas molecules impinge on a surface, they transfer momentum and give rise to ‘pressure’ on the surface. • Some of the molecules may stick on the surface and may be weakly bonded to the surface (by van der Waals forces) called Physisorption. (E.g. Ar on Pd surface) • If the interaction is stronger and it is referred to as Chemisorption. (E.g. CO on Ni(111)). • During chemisorption the entities of the molecule may break up  called Dissociative Chemisorption. (E.g. H2 on Pd surface).

4. Coverage • If adsorption is in equilibrium with desorption, at constant coverage () energy of the adsorption (Ead) is given by the Clausius-Clapeyron equation. • Thus, a plot of lnp with 1/T at constant coverage () will give us the isosteric heat of adsorption (at that coverage). • Fig.1 shows the Ead versus coverage for CO adsorbed on Pd(111). The adsorption energy remains constant up to  ~ 0.33 and then drops by 2 kcal/mol due to a change in the adsorption geometry, which arises due to repulsion between the adsorbed molecules. CO adsorbed on Pd(111)

5. Interaction between adsorbate molecules • The interactions between adsorbate molecules may be repulsive or attractive. • Repulsive interactions can arise in two ways: (i) Direct interaction or (ii) that mediated by the substrate. The direct interaction can arise from dipole–dipole interaction or from orbital overlap. The indirect nature of the interaction is mediated through the electronic structure of the substrate. The attractive interactions are usually mediated by the substrate. • Fig.1 shows the interaction potential between two oxygen atoms adsorbed on Ru (0001) surface, as a function of their separation (measured in terms of the lattice constant of the substrate surface).

6. Chemisorption • The Lennard-Jones diagram (Fig.1) helps us understand dissociative adsoption. If a • diatomic molecule A2 approaches a surface, it will first experience • (weak) bonding as A2,ad. Dissociation of the free molecule would • require the dissociation energy Ediss, and the two atoms would • then form strong bonds with the surface (Aad). The crossing point • of the two lines marks the activation energy for dissociative • adsorption and determines the kinetics of adsorption (see below), • while the adsorption energy Ead for A2 ! 2Aad is related to the • surface–adsorbatebondenergyES-A throughES-A ¼ 12 ðEad þ EdissÞ.

7. Chemisorption energy and surface diffusion • Table-1 lists the chemisorption energy of CO molecule on some metallic surfaces. The dissociation energy for the corresponding carbonyl compounds are also given in the table. • The dissociation energy for the carbonyl compounds larger by ~30–50%. This seems to be the case for both mono- or multinuclear compounds. The surface chemisorption energy is lower as the valency (no of unsaturated bonds) of the surface atoms is lower (as compared to a free atom). This also implies that the chemisorption energy should be higher on crystallographic planes with a lower density. • Surface steps and kinks offer preferred sites for adsorption with higher sorption energy. • The difference in the energy between different sites on the surface, determines the activation energy for surface diffusion. This activation energy is about 20% of the sorption energy and hence given sufficient kinetic activation the molecule will diffuse on the surface before desorption.