Supporting Information

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1 Supporting Information The Origin of Active Oxygen in a Ternary CuO x /Co 3 O 4 -CeO Catalyst for CO Oxidation Zhigang Liu, *, Zili Wu, *, Xihong Peng, ++ Andrew Binder, Songhai Chai, Sheng Dai *,, School of Chemistry and Chemical ngineering, Hunan University, Changsha 41008, China Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Department of Chemistry, University of Tennessee, Knoxville, Tennessee , United States ++ School of Letters and Sciences, Arizona State University, Mesa, Arizona 851, United States 1.Computational details The most stable surface CeO (111) was chose for this modeling. In a periodic slab model, the CeO (111) surface is represented using a supercell. The surface slab model was reduced from the ceria bulk with a lattice constant of a = Å, which was obtained using the computational criteria listed below and very close to the experimental value of Å. 1 A vacuum layer of about 14 Å was introduced in the z direction (perpendicular to the surface) to separate the slab from its images resulting from the periodic boundary conditions. We chose a slab thickness of 3 CeO trilayers (total 9 atomic layers) with 4 Ce atoms in each layer. The bottom trilayer (O-Ce-O stack) were fixed at their bulk positions to mimic the bulk, while the coordinates of the remaining ions were allowed to relax until the forces on each atom were less than 0.0 ev/å. One Co atom was introduced to the ceria surface to replace a Ce to generate the Co-doped ceria surface structure. Thus the supercell contains one Co and 11 Ce atoms, which corresponds to the Co doping rate of 8.33%. The Co-doped ceria surface is denoted as the Co 0.08 Ce 0.9 O system. 1

2 The first principles density-functional theory (DFT) calculations were performed using Vienna ab-initio Simulation Package (VASP) 3. The PB functional 4 and projector augmented wave (PAW) potentials 5 were used along with plane wave basis sets. The spin polarized calculation was performed. The reciprocal space was sampled using Monkhorst-Pack grid 6. The Gaussian smearing method was used to describe partial occupancies of orbitals, with width of the smearing set at 0. ev. The total energy in the self-consistent scheme was converged to within 0.01 mev. In the DFT calculations, the following electrons were treated as valence electrons: the 3d, 4s electrons in Co, 5s, 5p, 5d, 4f, 6s in Ce, s, p in O, and s, p in C. Since the standard DFT is known to not be able to correctly describe the highly localized d and f electrons in Co and Ce, the DFT+U method 7 was used to improve the prediction of electron correlation. In the DFT+U method, the on-site Coulomb integration among the localized d electrons in Co and f electron in Ce is treated explicitly with a Hubbard-Hamiltonian. And the Coulomb repulsion is characterized by a Hubbard parameter U. The U values selected for Co 3d and Ce 4f electrons are 3.3 ev 8 and 5.0 ev 9,10, respectively. Vacancy formation energy and adsorption energies of CO and O molecules on the ceria surface were calculated according to the following formulae. where The adsorption energy of CO molecule on the Co 0.08 Ce 0.9 O surface is calculated as ads slab + CO CO / slab = (1) slab and CO are the energies of the optimized Co 0.08 Ce 0.9 O system and the ground state CO molecule, respectively. / is the total energy of the geometrically optimized CO slab structure with a CO molecule adsorbed on the Co 0.08 Ce 0.9 O surface. The desorption energy of the product CO from the Co 0.08 Ce 0.9 O system is according to des = CO Ovac slab CO / Ovac slab ()

3 where CO / Ovac slab (or CO slab / ) is the energy of the adsorbed structure with a product CO on the ceria system, CO is the energy of the ground state CO molecule, and Ovac slab is the energy of the Co 0.08 Ce 0.9 O slab with one O vacancy formed on the surface. The formation energy of oxygen vacancy on the Co 0.08 Ce 0.9 O surface is calculated as Ovac 1 = slab O Ovac slab (3) in which O is the energy of gas phase O molecule. The adsorption energy of O molecule on the Co 0.08 Ce 0.9 O system with an O vacancy on the surface is calculated as, ads = Ovac slab + O O / Ovac slab (4) where O / Ovac slab is the total energy of the optimized structure with a O adsorbed at the site of O vacancy on the Co 0.08 Ce 0.9 O surface. The adsorption energy of CO molecule on the O incorporated Co 0.08 Ce 0.9 O system is calculated as, ads = O / Ovac slab + CO CO / O / Ovac slab (5) where CO / O / Ovac slab is the total energy of the optimized structure with a CO adsorbed on the O incorporated Co 0.08 Ce 0.9 O surface.. Simulation results:.1the perfect and Co doped ceria (111) surface Figure S1 (a) and (b) shows the perfect ceria (111) surface model containing 3 CeO trilayers. One surface Ce atom, indicated by * in Figure S1 (a), was replaced by a Co atom. The structure was relaxed within DFT to achieve the energy minimal. The optimized structure of the 3

4 Co doped ceria surface is presented in Figure S1 (c) and (d). Because the smaller size of Co atom compared to Ce, the Co atom sinks down into the material, which is clearly demonstrated in Figure S1 (c). In addition, the Co atom also shifts away from the center of three neighbored oxygen atoms, labeled as O1, O, and O3 in Figure S1 (c) and (d), toward O1. Therefore the bonding between Co and O/O3 is broken and O/O3 now is only two-fold bonded to the lattice Ce, making O/O3 relatively active. The other two surface oxygen atoms O1 and O4 are still three fold bonded to the system, while one bond of O1 is with Co instead of Ce. Figure S1 Optimized structures (a) side view and (b) top view of perfect ceria (111) surface, (c) side view and (d) top view of the Co doped ceria system. The diamond formed by black lines represents the supercell..co oxidation on the doped Co 0.08 Ce 0.9 O surface 4

5 The adsorption of a CO molecule on the doped Co 0.08 Ce 0.9 O surface was explored. The sites of three surface oxygen atoms, namely O1, O3 and O4, were studied (note that O site is equivalent to O3 site). In all three cases, a product of CO was formed. The optimized structures are presented in Figure S. The CO adsorption energy calculated from q. (1) are.87 ev, 3.06 ev, and. ev for the sites of O1, O3, and O4, respectively, indicating that O3 is the most active and O4 is the least active surface oxygen, respectively. The findings presented in Figure S1 and S agree qualitatively with the Cu doped ceria system by Yang et al. 10 Figure S Optimized structures of CO adsorbed (CO formation) on the Co doped ceria system. CO adsorps the active surface oxygen on the site (a) O1 (b) O3 (c) O4 to form product CO. Note the 4 th active surface oxygen on the site is equivalent to O3. The bond length and distance are in unit of Å. 5

6 .3CO product desorption and O vacancy formation on the Co 0.08 Ce 0.9 O surface Removing the product CO from the optimized structures in Figure S results in an O vacancy formed on the surface of the Co 0.08 Ce 0.9 O system. These reduced surfaces were further relaxed within the scheme of DFT. According to q. (3), the formation energy of O vacancy was calculated to be ev, ev, and ev for the O vacancy on sites of O1, O3, and O4, respectively. The relatively small formation energy of O vacancy at sites O1 and O3 indicates that the CO product in experiments mainly from these two sites. The desorption energy of CO from the reduced surface, using q. (), is ev and ev from the O1 and O3 sites, respectively, further confirming that the CO oxidation was primarily taken place on O1 and O3 sites..4o adsorption on the reduced Co 0.08 Ce 0.9 O surface We further studied the gas phase O adsorption on the reduced Co 0.08 Ce 0.9 O surface. As an example, the O molecule adsorbed at the O3 vacancy is presented in Figure S3. One oxygen atom (labeled as O3* in Figure S3) in O occupies the O3 vacancy site, and the other O (labeled as O5) is tilted from the surface with a tilting angle of The bond length of the adsorbed O molecule is 1.33 Å. The O-O bond length in the ground state of O molecule is 1.3 Å from the DFT calculation. Thus the O-O bond in the adsorbed O is weakened compared to the gas phase O. The adsorption energy is calculated to be 1.89 ev, according to q. (4), indicating a strong adsorption of O. 6

7 Figure S3 Optimized structure of O adsorbed at the O3 vacancy site on the reduced Co 0.08 Ce 0.9 O surface. The bond length is in unit of Å..5CO oxidation on the O incorporated Co 0.08 Ce 0.9 O surface CO adsorption on the O incorporated Co 0.08 Ce 0.9 O surface was further explored. At the site of incorporated O, CO reacts with the surface in two scenarios: (a) forming a carbonate and (b) formation of CO. The optimized structures are presented in Figure S4. The adsorption energies calculated from q. (5) are 3.4 ev and.95 ev for carbonate and CO, respectively. CO adsorption on the top of lattice oxygen O1, O, and O4 (see label in Figure S3) were also investigated. CO formation was found at those sites. However, the CO adsorption energies are generally smaller than the corresponding cases in Figure S. Giving an example of CO adsorbed on top of site O4, the adsorption energy is.1 ev in Figure S (without O incorporated) while the adsorption energy is only 1.3 ev for the case of O incorporated Co 0.08 Ce 0.9 O surface. 7

8 Figure S4 Optimized structures of CO oxidation on the O incorporated Co 0.08 Ce 0.9 O surface. (a) formation of carbonate (b) formation of CO The bond length and distance are in unit of Å. Reference: (1) Duclos, S.J. ; Vohra, Y.K. ; Ruoff, A.L. ; Jayaraman, A. ; spinosa, G.P. High-pressure x-ray diffraction study of CeO to 70 GPa and pressure-induced phase transformation from the fluorite structure. Physical Review B 1988, 38, () Kohn, W. ; Sham, L.J. Self-consistent equations including exchange and correlation effects. Physical Review 1965, 140, A1133. (3) Kresse, G. ; Furthmuller, J. fficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Computational Materials Science 1996, 6, (4) Perdew, J.P. ; Burke, K. ; rnzerhof, M. Generalized gradient approximation made simple. Physical Review Letters 1996, 77, (5) Kresse, G. ; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Physical Review B 1999, 59, (6) Monkhorst, H.J. ; Pack, J.D. Special points for Brillouin-zone integrations. Physical Review B 1976, 13, (7) Dudarev, S.L. ; Botton, G.A. ; Savrasov, S.Y. ; Humphreys, C.J. ; Sutton, A.P. lectron-energy-loss spectra and the structural stability of nickel oxide: An LSDA+U study. Physical Review B 1998, 57, (8) Wang, L. ; Maxisch, T. ; Ceder, G. Oxidation energies of transition metal oxides within the GGA+ U 8

9 framework. Physical Review B 006, 73, (9) Castleton, C.W.M. ; Kullgren, J. ; Hermansson, K. Tuning LDA+ U for electron localization and structure at oxygen vacancies in ceria. Journal of Chemical Physics 007, 17, (10) Yang, Z. ; He, B. ; Lu, Z. ; Hermansson, K. Physisorbed, chemisorbed, and oxidized CO on highly active Cu CeO (111). The Journal of Physical Chemistry C 010, 114,