ARTICLE pubs.acs.org/JPCC
A Combined DFT and Statistical Mechanics Study for the CO Oxidation on the Au101 Cluster Nima Nikbin,† Giannis Mpourmpakis,*,†,‡ and Dionisios G. Vlachos† †
Department of Chemical Engineering, Center for Catalytic Science and Technology, University of Delaware, Newark, Delaware 19716, United States ‡ Institute of Electronic Structure and Laser, FORTH, Heraklion 71110, Crete, Greece
bS Supporting Information ABSTRACT: The CO oxidation reaction pathways on the negatively charged Au101 cluster are investigated using density functional theory calculations. Pre-exponential factors and reaction rate constants of each elementary step are calculated using statistical mechanics. Our results demonstrate that the reaction of CO and O2 on Au101 preferably proceeds through the formation of a 4-center intermediate rather than through O2 dissociation. The oxygen atom produced then reacts with adsorbed CO to form CO2. The ratedetermining step of the CO oxidation appears to be the CO2 release from the 4-center intermediate. Interestingly, it is shown that gaseous CO2, which binds near an adsorbed atomic oxygen, forms easily a bidentate CO3 species that can poison the catalyst. The reverse reaction (carbonate decomposition) occurs with a rate constant comparable to the rate-determining step. A rate constant comparison for various reactant arrangements suggests a site-dependent reactivity, even for this subnanometer Au101 catalyst.
I. INTRODUCTION In 1987, Haruta observed that nanometer gold (Au) exhibits high catalytic activity for the CO oxidation when supported on metal oxide surfaces.1 Since then, the activity of Au nanoparticles has received considerable attention, especially toward the lowtemperature CO oxidation reaction,2 due to CO being involved in several energy-related and environmental processes.3 Although several mechanisms have been proposed to explain Au’s activity, the reaction mechanism is still not well-understood. It is well accepted that the low-coordinated sites of the nanoparticles play a key role.4,5 The support can also strongly affect the catalytic activity, possibly due to its effect on the particles’ shape, and thus determine the number of the catalytically active (i.e., low-coordinated) sites.6 Furthermore, it has been suggested that the support can enhance the activity by inducing strain to the nanoparticles5 or by supplying oxygen for the reaction.7 However, the exchange of oxygen atoms between the particle and the support was found to be significantly less pronounced than the particle size effect.8 Reducible supports (e.g., TiO2, Fe2O3) are more reactive than irreducible supports (e.g., Al2O3, MgAl2O4, SiO2), and the CO oxidation turnover frequency (TOF) for particles of 2 nm exceeds that of 20 nm by 2 orders of magnitude.8 A large number of experimental and theoretical studies, summarized by Chen and Goodman,9 reported that significant charge transfer between the support and the Au catalyst is responsible for the observed catalytic activity. It was suggested that support defects provide anchoring and stabilization sites for nanoparticles;8 the increasing density of oxygen vacancies (F-centers) on the MgO support creates electron-rich nanoparticles, consistent with the increasing CO oxidation activity.10 Charge transfer r 2011 American Chemical Society
results in activation of adsorbed O2, enabling the CO oxidation at low temperatures.1113 Consistent with this hypothesis, gas-phase experiments showed that only anionic Au clusters exhibit significant O2 adsorption14 and CO oxidation activity.15 Regarding the CO oxidation mechanism, Liu et al. showed that, on Au(211) and Au(221), the reaction proceeds through the formation of a metastable, 4-center intermediate and proposed the same mechanism on nonreducible supports.7 Arenz et al. suggested that, on Au8/MgO, there are three possible CO oxidation mechanisms depending on the locations of the reactants: a Langmuir Hinshelwood mechanism with both reactants located at the nanoparticle surface (barrier: 0.1 eV), a mechanism with O2 at the supportnanoparticle interface and CO on the cluster (barrier: 0.5 eV), and a barrierless EleyRideal mechanism with O2 on the cluster and CO in the gas phase.11 The importance of each of these reaction sites for the CO oxidation is yet to be understood.16,17 Despite several theoretical papers,4,7,8,11,13,1824 the deactivation mechanism has not been investigated in detail. Furthermore, the site-dependent reactivity of small Au clusters has not been explored. In this study, we unravel the reaction mechanism on Au101 using density functional theory (DFT) and calculate reaction rate constants using statistical mechanics. This is a catalytically active15 cluster in the gas phase that also forms at an O-vacancy of a MgO support.11 The overall purpose of this work is to map the CO oxidation mechanism on the different sites of a small Au cluster, incorporating the carbonate formation pathway. Received: March 28, 2011 Revised: August 23, 2011 Published: September 08, 2011 20192
dx.doi.org/10.1021/jp206820t | J. Phys. Chem. C 2011, 115, 20192–20200
The Journal of Physical Chemistry C
ARTICLE
Figure 1. Planar, hexagonal ground state of Au101. The cluster has four distinct adsorption sites (according to the symmetry of the cluster), which are presented with Greek letters αδ. Both top and bridge adsorption configurations were taken into account.
II. COMPUTATIONAL DETAILS To reduce the computational cost while simulating the electronic effect of the support (charge transfer to the cluster),1113,15,25 we assume that the Au101 catalyst cluster is negatively charged and that the reactions occur on its surface. We used Becke’s three-parameter hybrid (B3LYP) functional with the Los Alamos National Laboratory second Double-Zeta (LanL2DZ) basis set as implemented in Gaussian 09.26 All calculations performed are spin-polarized. Energy and geometry optimizations of the adsorbed species were performed by taking into account various adsorption sites on the catalyst and different binding configurations of the adsorbates. Starting from a stable initial configuration of coadsorbed CO and O2 on the Au101 cluster, the potential energy curve leading to CO2 (e.g., the elongation of the oxygen bond) was calculated using the constrained minimization technique27 while the spin was conserved. In this way, we located the maximum of the potential energy curve along the reaction coordinate that corresponds to a candidate transition-state (TS) structure. The actual structure of the TS is located via full relaxation (optimization) to a saddle point (TS calculations). Minima were located with full relaxation of all bond lengths, angles, and dihedral angles. Vibrational frequencies were calculated for all the optimized systems. III. RESULTS AND DISCUSSION The Au101 cluster has a planar ground-state structure, in agreement with experimental and theoretical studies.19,20,28 We found the planar triangular and hexagonal structures to be almost isoenergetic, and they exhibit a doublet spin multiplicity in their ground state. For our analysis, we chose the hexagonal structure that corresponds to the experimentally determined ground state.19 The ground-state structure and the labeling of the adsorption sites are shown in Figure 1. Gaseous O2 and O exhibit a triplet multiplicity, whereas gaseous CO, a singlet. All Au10X systems (X = CO, O2, O, CO + O2, CO + O) were found to be doublets. The spin multiplicity was conserved along the reaction pathways. Energetics of Elementary Reactions. Following the adsorption of both reactants on the catalyst, the CO oxidation (reaction R1) proceeds by one of the following competing reactions (* denotes an adsorbed species) Formation of CO2 from adsorbed molecular O2 and CO ðR1Þ
O2 þ CO T COOO T þ O þ CO2
ðR1aÞ
OO þ CO T þ O þ CO2
ðR1bÞ
O2 þ CO T þ O þ CO2
ðR1cÞ
On the basis of different reaction coordinates, three possible pathways have been identified, as shown in Figure 2. In reaction R1a (Figure 2a), adjacent CO and O2 can form a 4-center intermediate (*CO*OO) with a barrier of
