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J. Phys. Chem. C 2009, 113, 15266–15273
Intermediates and Spectators in O2 Dissociation at the RuO2(110) Surface Hangyao Wang,† William F. Schneider,*,†,‡ and David Schmidt† Department of Chemical and Biomolecular Engineering, and Department of Chemistry and Biochemistry, UniVersity of Notre Dame, Notre Dame, Indiana 46556 ReceiVed: April 9, 2009; ReVised Manuscript ReceiVed: June 26, 2009
Using plane wave DFT calculations, we identify various configurations of peroxo-like and superoxo-like molecular oxygen adsorbed at the RuO2(110) surface. All configurations are found to be metastable to dissociative oxygen adsorption in the presence of pairs of vacant surface sites. Transition state calculations show that the barrier to dissociation across adjacent vacant sites is small. Because of the configurational constraints of the RuO2(110) surface and the low mobility of O atoms in bridge or cus sites, however, we find that dissociative O2 adsorption will tend to generate single, “stranded” vacant sites that act as stable adsorption sites for molecular O2. Simulated temperature-programmed desorption spectra are in good agreement with experimental observation and identify signatures of these O*2 spectators as well as O*2 intermediates to dissociation. 1. Introduction O2 adsorption and dissociation are essential elements of heterogeneous catalytic reactivity, and thus the nature of surface-O2 interactions is one of the most fundamental issues in catalysis.1,2 In the Langmuir-Hinshelwood model of catalytic oxidations at metal surfaces, reactive O2 is conceived to adsorb dissociatively,3 possibly via a relatively weakly bound, mobile, and transient molecularly adsorbed state.4-6 Transition metal oxides similarly catalyze many types of oxidation reactions,7 and even on nominally “noble” metals, metal oxide surface films have been proposed to play a role in catalytic oxidation activity.8,9 Catalytic oxidations on metal oxide surfaces are typically described within a Mars-van Krevelen model (M-vK),10 in which reductants react with metal oxide lattice oxygen, and the resultant oxygen vacancies are filled by dissociative adsorption of O2. Molecularly adsorbed O2 is a more common and robust feature of metal oxide surface chemistry than of metals. These adsorbed species exist as superoxides (O-2 ) 2ions in the metal and peroxides (O22 ) that take the place of O oxide lattice.7,11 The relation between these molecular species, O2 dissociation, and catalytic activity is in general not well understood. Ruthenium dioxide (RuO2) is a popular and convenient model system for studying catalytic reactions at transition metal oxide surfaces, because the (110) oxide surface is both catalytically active and readily amenable both to detailed surface science characterization and to electronic structure modeling.12-19 CO catalytic oxidation on this surface was originally proposed to proceed by a M-vK mechanism,12 but more recent work suggests a Langmuir-Hinshelwood model in which both CO and O2 compete for surface sites.18,20,21 Two types of surface-adsorbed O are thought to contribute to overall activity and are proposed to be replenished by O2 adsorption and dissociation.12,17,19,21-25 Molecularly adsorbed O2 states evident as a low-temperature feature in temperature-programmed desorption (TPD) experiments26 and inferred from scanning tunneling microscopy (STM) * E-mail:
[email protected]. † Department of Chemical and Biomolecular Engineering. ‡ Department of Chemistry and Biochemistry.
Figure 1. Ball and stick representation of RuO2(110) surface with red balls representing oxygen and gray balls ruthenium. The directions a, b, and c are [001], [1j10] and [110], respectively. The definitions of Obr, O3f, and Rucus are indicated in the figure.
experiments27 have been proposed as precursors to dissociative adsorption. However, the nature of these molecularly adsorbed states, their relation to other possible surface-adsorbed molecular O2 states, and the roles of these states in O2 activation and catalytic reactivity are not well established. In this work we use plane-wave, supercell density functional theory (DFT) modeling to enumerate the structures, vibrational spectroscopic signatures, and stabilities of possible O2 adsorbate configurations on the RuO2(110) surface. The stoichiometric RuO2(110) surface exposes one-dimensional arrays of bridging (Obr) and three-fold coordinated (O3f) oxygen as well as fivefold coordinated Ru ions (Figure 1), typically referred to as coordinatively unsaturated sites, or Rucus.12 We consider both bridge and cus sublattices as potential sites for molecularly adsorbed O2 and identify a variety of adsorbed O*2 configurations with features characteristic of superoxides or peroxides. Both experimental and computational evidence indicates that in equilibrium with gas-phase O2, the stoichiometric RuO2(110) surface can accommodate additional atomic oxygen up to complete filling of cus sites.26,28,29 Consistent with these results, thermodynamic stability analysis shows that all identified molecular O2 configurations are energetically metastable to dissociation into atomic O at pairs of vacant sites. Further, transition state calculations show that the barrier to dissociation across adjacent vacant sites is small. However, because of the
10.1021/jp903304f CCC: $40.75 2009 American Chemical Society Published on Web 07/31/2009
Intermediates and Spectators in O2 Dissociation
Figure 2. Top-view of RuO2(110) surface. Large gray circles represent oxygen and small black circles represent Ru. Solid box represents 3 × 1 cell and dashed box 4 × 1 cell.
configurational constraints of the RuO2(110) surface and the low mobility of O atoms in bridge or cus sites,19 we find that dissociative O2 adsorption will tend to generate single, “stranded” vacant sites along the cus rows that act as stable adsorption sites for molecular O*2 . Simulated TPD spectra provide strong evidence that these stranded molecular O2 configurations account for the experimentally observed molecular O2 state.26 These O*2 bound at single cus sites are surface spectators, rather than intermediates, to O2 dissociation. O*2 bound across two cus sites or across cus and br sites are the transient intermediates responsible for dissociative adsorption and recombinative desorption of O2. The results illustrate the relatively complex interplay of O2 molecular adsorption and dissociation chemistry at this metal oxide surface.
J. Phys. Chem. C, Vol. 113, No. 34, 2009 15267 Pack mesh is used to sample the first Brillouin zone (12 symmetry-unique k-points). Sufficiently large grids are used to avoid so-called “wrap-around” errors in fast Fourier transforms (FFTs). Some calculations were performed using a 4 × 1 tetragonal supercell to explore wider adsorbate coverage ranges. These calculations are performed with a 4 × 8 Monkhorst-Pack mesh. These parameters were sufficient to converge calculated energies to
