Theoretical and experimental study of H_2 adsorption on Pd atoms deposited on Cu(111)

MAXIMILIANO RAMOS ACEVEDO; MARINA MINITTI; DANIEL FARÍAS; RODOLFO MIRANDA; CRISTINA DÍAZ; ALEJANDRA ELISA MARTÍNEZ; HERIBERTO FABIO BUSNENGO

Edinburgo

Congreso; 29th European Conference on Surface Science (ECOSS-29); 2012

By evaporating small amounts of Pd on Cu(111), Pd atoms act as isolated active sites and make of Pd/Cu(111), an exciting system for single molecule chemistry and for real-world catalysis. In particular, STM experiments have shown that even ∼0.01 ML of Pd can tremendously increase the reactive sticking probability, S0 , of H2 on the otherwise inert Cu(111) surface. In this work we present a theoretical and experimental study of adsorption of H2 on Pd/Cu(111). By means of supersonic H2 molecular beam experiments and Helium Atom Scattering techniques, we have determined S0 as a function of the surface temperature, Ts , used during Pd evaporation and as a function of the molecular impact energy, Ei, for 0.01 ML and 0.1 ML of Pd. We also report DFT activation energy barriers for H2 dissociation: i) on Pd ad-atoms on Cu(111) terraces, ii) on substitutional topmost layer Pd atoms, and iii) on substitutional subsurface Pd atoms corresponding to the preferred positions of Pd deposited at low, intermediate (∼350 K) and higher Ts values respectively. In contrast with the usual belief, DFT calculations show that on Pd ad-atoms (characterized by the lowest coordination) the activation energy barrier for H2 dissociation is markedly larger than on substitutional topmost layer Pd atoms. This explains the intriguing non-monotonic dependence of the experimental S0 (Ts ) which reaches its maximum value for Ts ∼350K. Moreover, for the latter highest reactivity situation (i.e. for substitutional topmost layer Pd atoms) DFT-molecular dynamics results are also in good quantitative agreement with the experimental S(Ei) data.