H2 dissociative adsorption on

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Surfaces (PESs) to describe the interaction of H2 with a pseudomorphic monolayer of Cu or Pd on Ru(0001) by explicitly including the Ru sub- strate. We have ...
H2 dissociative adsorption on pseudomorphic monolayers grown on metal surfaces G. Laurent∗, 1 , C. D´ıaz∗ 2 , H.F. Busnengo† 3 , F. Mart´ın∗ 4 , ∗



Departamento de Qu´ımica C-9, Universidad Aut´onoma de Madrid, 28049 Madrid, Spain Instituto de F´ısica de Rosario and Universidad Nacional de Rosario, 2000 Rosario, Argentina

Synopsis We have evaluated dissociative adsorption probabilities as function of incidence energy for H2 impinging on pseudomorphic monolayers of Cu or Pd on Ru(0001) metal surfaces.

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E-mail: E-mail: 3 E-mail: 4 E-mail: 2

[email protected] [email protected] [email protected] [email protected]

between the molecule and the surface (direct process). Finally, a comparison to artificially compressed Pd(111) and expanded Cu(111) shows that, apart from strain, substrate electronic effects play a very important role. Since these effects vary from site to site, a fully 6D analysis of the H2 -surface interaction is necessary to understand the observed reactivity. Dissociative adsorption probability

A generally accepted picture in heterogeneous catalysis is that open surfaces, i.e., surfaces with relatively large lateral lattice constants, are more reactive than closed surfaces [1]. Hence, a natural way of tuning the catalytic activity of a surface would be to modify the lateral lattice constants by introducing strain. A simple way to design such surfaces is to deposit a metal onto single-crystal metal substrates with different lattice constants. Very often this leads to pseudomorphic monolayers that have the same lateral lattice constant as that of the underlying substrate. In this work, we have evaluated, by using the density functional theory (DFT), the 6D Potential Energy Surfaces (PESs) to describe the interaction of H2 with a pseudomorphic monolayer of Cu or Pd on Ru(0001) by explicitly including the Ru substrate. We have used these PESs to carry out 6D quasiclassical dynamics calculations and obtain the variation in dissociative adsorption probabilities with the incidence energy [2]. Ru(0001) has a lateral lattice constant of 2.73 ˚ A. The corresponding Cu(111) pseudomorphic monolayer is laterally expanded with respect to real Cu(111) (2.57 ˚ A), while the Pd(111) monolayer is compressed with respect to real Pd(111) (2.80 ˚ A). Both pseudomorphic bimetallic surfaces are currently produced in the laboratory (Cu/Ru [3] and Pd/Ru [4]) and are sufficiently stable to perform sticking measurements that can be directly compared to the present theoretical results. Although H2 dissociation is known to be non activated on Pd(111) and strongly activated on Cu(111), we have found that it is activated on both Pd/Ru(0001) and Cu/Ru(0001) (see Fig.1). Surprisingly, the minimum activation energy for H2 dissociation is very similar on both pseudomorphic monolayers, ∼ 100 meV. In both cases, H2 dissociation occurs after the first encounter

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H -Pd/Ru(0001) 2

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H -Pd(111) 2

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H -Cu/Ru(0001)

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H -Cu(111) 2

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Normal energy [eV]

Fig. 1. Calculated Dissociative adsorption probabilities vs normal incidence energy for H2 (v = 0, J = 0).

References [1] M. Gsell and P. Jakob and D. Menzel 1998 Science 280 717. [2] G. Laurent and H. F. Busnengo and P. Rivi`ere and F. Mart´ın 2008 Phys. Rev. B 77 193408. [3] R. Otero and F. Calleja and V. M. Garc´ıa-Su´arez et al. 2004 Surf. Sci. 550 65. [4] L. A. Kibler and A. M. El-Aziz and R. Hoyer and D. M. Kolb 2005 Angew. Chem. Int. Ed. 44 2080.