Local reactivity of bimetallic overlayer and cluster systems Ata Roudgar and Axel Groß Chemistry Department, Simon Fraser University, Burnaby, Canada Physik-Department T30, Technische Universit¨ at M¨ unchen, 85747 Garching, Germany I. Introduction Bimetallic surfaces are well-suited for the tailoring of the reactivity since they offer the possibility to prepare specific surface compositions and structures. In bimetallic systems, strain as well as electronic interaction effects modify the catalytic activity. We have tried to disentangle both effects by performing density functional theory calculations using the VASP code (G. Kresse and J. Furthm¨ uller, Phys. Rev. B 54, 11169 (1996)). H and CO adsorption energies have been determined as a local probe of the reactivity and were analyzed in terms of the d band model δE chem = V 2 |ε d -ε a | 2 δε d (B. Hammer, O. H. Nielsen, J. K. Nørskov, Catal. Lett. 46, 31 (1997)). II. Pd/Au(111) and Pd/Cu(111) overlayer: substrate and strain effects a Cu = 3.64 ˚ A < The calculated Pd lattice constant a Pd = 3.98 ˚ A < a Au = 4.18 ˚ A H and CO adsorption energy of Pd/Au(111) Number of Pd overlayers on Au 0 1 2 3 Pd@Au Pd Adsorption energy E ads (eV) -2.4 -2.1 -0.6 -0.3 0.0 0.3 0.6 CO fcc hollow H fcc hollow H hcp hollow H bridge H on-top (a) (111) d-band center on Pd/Au(111) 0 1 2 3 Pd@Au Pd Number of Pd over layers -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 d-band position (eV) Surface (111) Subsurface (111) Pd-Au interaction energy is 0.1 eV weaker than Pd-Pd therefore, both overlayer expansion and weak Pd-Au coupling lead to an effectively lower coordination of the Pd d band ⇒ higher d band center and larger adsorption energies H adsorption energies on Pd/Cu(111) Cu Pd/Cu Pd@Cu Pd Surface structure -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Adsorption energy E ads (eV) fcc site hcp site Bridge site on-top site d-band center of Pd/Au(111) Cu Pd/Cu Pd@Cu Pd -2.2 -2.1 -2.0 -1.9 -1.8 -1.7 -1.6 -1.5 -1.4 d-band center (eV) Pd-Cu interaction energy is 0.245 eV stronger than Pd-Pd therefore, both overlayer contraction and stronger Pd-Cu coupling lead to an effectively higher coordination of the Pd d band ⇒ lower d band center and lower adsorption energies III. Pd n cluster deposited on Au(111) There are two opposit effects: The lower coordination of cluster atoms and reduction of Pd-Pd distances Pd-Pd distances 2.76 2.76 2.65 2.76 2.76 2.77 (c) (d) (b) (a) NN distances: d Au =2.95 ˚ A, d Pd =2.80 ˚ A For Pd 3 two effects almost canceled each other while for Pd 7 one of them overcompensate the other H and CO adsorption on Pd 10 clusters (c) (b) (a) (d) ads E =-0.449 (-0.646) eV ads E =-0.468 (-0.559) eV E =-0.349 (-0.189) eV ads ads E =-0.358 (-0.669) eV H adsorption (a) (c) (b) (d) E =-1.744 (-2.469) eV ads E =-2.172 (-2.429) eV ads E =-2.147 (-1.741) eV ads E =-2.003 (-2.035) eV ads CO adsorption H and CO adsorption energies on Pd 10 /Au(111) (free Pd 10 ) clusters Adsorption energies on supported 3D clusters significantly reduced with respect to the overlayer system H and CO adsorption energies Pd Pd Overlayer Adsorption energy (eV) -2.4 -2.3 -2.2 -2.1 -0.7 -0.6 -0.5 H fcc H hcp CO fcc CO hcp 3 7 LDOS of Pd 3 cluster 0 2 4 6 8 Pd with substrate Au Pd without substrate Pd With substrate Pd 0 2 4 6 0 2 4 6 LDOS projected on atomic orbitals 0 2 4 6 8 -4 -3 -2 -1 0 1 0 2 4 6 8 Pd (d xy ) Pd (d yz ) Pd (d 3z^2-r^2 ) Pd (d xz ) Pd (d x^2-y^2 ) d band LDOS of the Pd 3 cluster: d states with z component broad- ened ⇒ metallic character IV. Pd/Au(111) overlayer in the presence of the water Water monomer and dimer on Pd/Au(111) E mono ads = -0.316 eV, E dimer ads = -0.427 eV/H 2 O On-top site is the most favorable site for water adsorption E H-bond = (E dimer ads - E mono ads ) × 2 ⇒ E H-bond = -0.222 eV < E Gas H-bond Water adsorption energy as a function of water coverage θ : 2/3 1 3/4 1/2 1/3 1/4 E ads : ⇓ +3.135 -0.465 -0.419 -0.295 -0.308 H 2 O adsorption energies in eV/H 2 O Possible structures for θ =2/3 structure E ads (eV/H 2 O) H-down bilayer -0.536 H-down with shifted bilayer -0.512 H-up bilayer -0.506 H-up with shifted bilayer -0.462 half-dissociation bilayer -0.321 H 2 O adsorption energies in eV/H 2 O The interaction between hexagonal water ring with metal is significantly small E ads [M-H 2 O] = -0.12 eV/H 2 O H-down bilayer for different substrates structure E ads (eV/H 2 O) L ( ˚ A) Pd/Au -0.536 1.95 Pd/Pd@Au -0.520 1.95 Pd -0.552 1.84 L=Hydrogen bond length ( ˚ A) Different water structures for θ =2/3 (a) (b) (d) (c) Water structure: a) H-down bilayer, b) H-up bilayer, c) half-dissociation bilayer, d) H-down with shifted bilayer Lattice expansion ⇒ H-bond length ↑ ⇒ H-bond energy ↓ ⇒ E ads ↓ H and CO adsorption on Pd/Au(111) (a) (b) (d) (c) H and CO adsorption: a) H adsorption on fcc site, b) H adsorption on hcp site (black circles are H adsorbate), c) CO adsorption on-top site, d) CO adsorption on fcc site For CO adsorption, H-down and H-up with shifted bilayer was used for fcc and hcp sites (the most favorite) H and CO adsorption energies H adsorption energies E ads [H 2 O] ↓ E ads [H 2 O] ↑ E ads [Clean] Fcc -0.661 -0.660 -0.690 Hcp -0.596 -0.595 -0.655 On-top 0.155 — 0.075 CO adsorption energies Fcc -1.831 -1.894 -2.023 Hcp -1.866 -1.923 -2.043 On-top -1.243 -1.317 -1.413 All adsorption energies are in eV CO adsorption energies for H-down and H-up bilayer are different ⇒ relatively small dipole-dipole interaction H and CO adsorption energies changed by 30-50 meV (5%) and 170-200 meV (10%) respectively in the presence of water. i.e. they are only slightly modified by the presence of water. V. Conclusions and publications • We have presented DFT calculations for the adsorption of atomic H and molecular CO on the Pd/Au overlayer and cluster in the presence and absence of the water. • For Pd/Au(111) overlayer both lattice expansion as well as Pd-Au interaction lead to a higher adsorption energy. • For Pd cluster deposited on Au(111) we found a lower adsorption energy compare to Pd/Au(111) overlayer. • For Pd/Au(111) overlayer in the presence of the water we found that the modification of H and CO adsorption energies due to the relatively weak interaction of water are reduced by less than 50 meV and 200 meV respectively. A. Roudgar and A. Gro, Local reactivity of metal overlayers: Density functional theory calculations of Pd on Au, Phys. Rev. B 67, 033409 (2003). A. Roudgar and A. Groß, Local reactivity of thin Pd overlayers on Au single crystals, J. Electroanal. Chem. 548, 121 (2003). A. Roudgar and A. Groß, Local reactivity of supported metal clusters: Pd on Au(111), Surf. Sci. 559, L180-L186 (2004).