Effect of Strain on the Reactivity of Metal Surfaces

The ability to grow and characterize one metal on top of another has developed rapidly over the last few years. In a number of cases it has become possible to epitaxially grow several layers of one metal on top of another. If the lattice constants of the two metals differ, strained overlayers are formed. It has been shown experimentally that such strained overlayers can have chemical properties that are significantly different from those of the pure overlayer metal [1‐ 3]. Most recently strain in the surface region has been introduced not just by growing one metal epitaxially on another, but by local deformation of a single metal phase [4]. Such strain has been shown to modify the chemisorption properties of the metal considerably. If strain generally induces changes in the ability of a surface to form bonds to adsorbed atoms or molecules, the possibility arises of using strain to manipulate the reactivity of a metal. In the present Letter we investigate the generality of the effect of strain on surface reactivity and its origin by performing a set of density functional (DFT) calculations. We study a metal [Ru(0001)] slab under compressive or tensile stress and show that both molecular (CO) and atomic (O) chemisorption energies as well as barriers for surface reactions (CO dissociation) vary substantially on strained lattices. We further proceed to show that this effect can be explained on the basis of shifts in the metal d bands induced by the stress. This allows us to develop a model for the effect which can be readily extended to several catalytically important systems. We used a three layer slab of Ru periodically repeated in a super cell geometry with five equivalent layers of vacuum between any two successive metal slabs. O adsorption and CO dissociation were treated within a s2 3 2d unit cell, whereas CO chemisorption was studied on a p 3 3 p 3 unit cell. These specific choices represent the most stable overlayer structures for the corresponding systems, as determined by experiments [5,6]. Adsorption is allowed on only one of the two surfaces exposed and the electrostatic potential is adjusted accordingly [7]. The top surface layer was relaxed for the atomic and molecular chemisorption problems, but kept fixed at its initial position for the calculation of the CO dissociation barrier. Ionic cores are described by ultrasoft pseudopotentials [8] and the Kohn-Sham one-electron valence states are expanded in a basis of plane waves with kinetic energies below 25 Ry. The surface Brillouin zone is sampled at 18 special k points. The exchange-correlation energy and potential are described by the generalized gradient approximation (PW91) [9,10]. The self-consistent PW91 density is determined by iterative diagonalization of the Kohn-Sham Hamiltonian, Fermi population of the KohnSham states (kBT › 0.1 eV), and Pulay mixing of the resulting electronic density [11]. All total energies have