Subsurface Incorporation of Oxygen into Palladium(111): A Theoretical Study of Energetics and Kinetics Ernst D. German* and Moshe Sheintuch Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel Alexander M. Kuznetsov † A. N. Frumkin Institute of Physical Chemistry and Electrochemistry, 31 Leninsky Prospect, Moscow 119991, Russia ReceiVed: May 21, 2009; ReVised Manuscript ReceiVed: July 15, 2009 The kinetic and thermodynamic characteristics of oxygen penetration of palladium (111) surface are studied using the DFT cluster method. The activation energy and the energy difference between the adsorbed and subsurface states of oxygen atom as well as the corresponding frequency and geometrical characteristics are computed from the minimum energy paths describing the penetration process. A simple analytical model of this process is suggested and is applied in order to gain insight into its mechanism and estimate its rate. This approach is used to predict the trends in the activation barriers for the penetration of a single oxygen atom on several transition metals showing Pd to have the lowest barrier (37 kcal/mol as opposed to 44-46 kcal/ mol with Pt, Ru, Rh, and Ir). It is also applied to show that the dependence of the activation energy on the reaction heat is described by the linear BEP relation and that the activation barrier declines with the oxygen coverage of (111) palladium surface. 1. Introduction It has been known for some time that adsorbed oxygen can penetrate the metal surface under reaction conditions, in some cases causing significant changes in reaction dynamics and catalytic activity. Obviously, understanding of the mechanism of surface oxidation should be of importance for harnessing many technological processes, such as catalysis and metal passivation 1 or oxygen storage at a metal-oxide interface of catalyst nanoparticles, 2 and knowledge of the corresponding reaction rates and activation energies is required for that purpose. Therefore, theoretical estimation of these kinetic characteristics is of great interest. Moreover, the interaction of surface oxidation and reduction, coupled with surface reaction, is believed to be the main mechanism for kinetic oscillations under commercial conditions (atmospheric pressure, supported catalysts), observed in so many oxidation reactions (see references below). The period of these oscillations should be similar to the time scale of subsurface oxidation, motivating us to estimate the corre- sponding rate constants. Since our interests lie mainly in CO oxidation on Pd, we review first the works relevant to this metal but briefly review other group VIII metals as well. Evidence for oxygen migration in Pd(111) was provided by molecular beam adsorption, thermal desorption (TDS), low energy electron diffraction (LEED), and high-resolution electron energy loss spectroscopy (HREELS). 3 The barrier for oxygen atoms to penetrate the surface layer of clean Pd(111) was estimated to be about 36 kcal/mol. 3 Subsur- face oxygen formation in Pd(111) was studied using scanning tunneling microscopy (STM) 4 showing that it occurs simulta- neously with slow oxygen uptake into chemisorbed states above 0.25 ML. Subsurface oxygen was found to be extremely unreactive toward CO on the surface. These experiments performed at 523 K clearly demonstrated that the loss of chemisorbed oxygen from the solid bulk plays a significant role in the adsorption experiments. Investigation of the interaction of oxygen with Pd(110), using a wide range of experimental methods 5 at 100 < T < 900 K, were also explained by migration of surface oxygen into the subsurface region. Subsurface oxygen was observed in photoemission electron microscopy (PEEM) images of pattern formation during CO oxidation on Pt(110). 6-8 The effective activation energy of subsurface oxygen formation was estimated from the suggested kinetic model to be 6-8 kcal/mol. Direct observation of the conversion of adsorbed oxygen on Pt(100) into the subsurface state by PEEM, 9,10 which reflects changes in surface work function, was based on the negative effect of subsurface oxygen on the work function of the clean surface due to an inversion in the surface dipole moment. The associated activation energy in this case was found to be about 15 kcal/mol. Subsurface oxygen was detected 11 by TDS after O 2 adsorption on Pt(110) in the temperature range from 875 to 895 K with an estimated E a ) 24.5 kcal/mol. X-ray photoelectron diffraction method (XRPED) at T ) 470 K was employed to show 12 that on Rh(111) ∼5% of a monolayer of oxygen atoms occupy octahedral interstitial sites underneath fcc adsorption sites. Various experimental techniques 13-15 were used to demon- strate O penetration from the Rh(111) surface into the bulk, with an associated barrier of 4.3 kcal/mol. 13 Evidence of the formation of subsurface oxygen was obtained on O/Ru (0001) system using TDS, LEED, ultraviolet photoelectron spectros- copy (UPS), and Auger electron spectroscopy (AES) experi- ments. 16-18 Subsurface oxygen formation proceeds according to the Arrhenius law with the activation energy of 13.8 kcal/ mol but the authors note that this value represents an average activation barrier over a chain of elementary reaction steps and cannot be ascribed to a well-defined elementary step. Only few theoretical works have addressed formation of subsurface oxygen. Phonon activated diffusion of an oxygen † Deceased. J. Phys. Chem. C 2009, 113, 15326–15336 15326 10.1021/jp904758x CCC: $40.75  2009 American Chemical Society Published on Web 08/04/2009