Inhibition at Perimeter Sites of Au/TiO2 Oxidation Catalyst by Reactant Oxygen Isabel Xiaoye Green, Wenjie Tang, Monica McEntee, Mattew Neurock, and John T. Yates, Jr. Supporting Information Table of Contents: I. Instrumental. II. Synthesis Method. III. Model System and Calculation Details. IV. Additional Calculation Results. V. References for the Supporting Material. VI. Supporting Figures. S1
I. Instrumental. The in situ infrared (IR) measurements in this study were carried out in a stainless steel high vacuum transmission FTIR cell with a base pressure of 1 10-8 Torr. A top view schematic of the cell is shown in Figure S1a. A MKS Baratron capacitance manometer is used for high pressure measurements (0.001-100 Torr) during the catalytic reaction. The IR beam from a FTIR spectrometer (Bruker TENSOR 27) is directed to pass the center of the cell through two differentially pumped KBr windows. A residual gas analyzer (RGA) is connected to the cell for gas analysis and can be isolated from the cell by closing a gate valve. The samples were mounted on a l-n 2 cooled reentrant Dewar with Z-direction motion and were placed in the center of the cell at the IR beam focus. A schematic drawing of the sample holder is shown in Figure S1b. A piece of 0.003 in thick tungsten grid is employed to support the powdered samples and to provide uniform heating and cooling. 1 The samples, a Au/TiO 2 catalyst and a reference pure TiO 2 sample, were pressed into two 7 mm diameter circular spots with 33,500 psi pressure, one directly above the other. Each sample spot contains about 0.005 g of material. By moving the manipulator up and down in the Z-direction, both samples can be examined after the same experimental treatment. The tungsten grid is clamped between a pair of nickel bars and connected to the feedthrough via copper rods. A type K thermocouple is welded on the top of the tungsten grid, directly above the samples. It provides temperature readings to 0.1 K resolution. Temperature control of the samples is performed via a combination of resistive heating and l-n 2 cooling in the range 85-1000 K. A l-n 2 cooled MCT detector was employed for IR measurements. The FTIR spectrometer and the MCT detector together with the entire IR beam pathway were purged constantly by H 2 O- and CO 2 - free air. For each IR spectrum, a reference spectrum taken through the gas phase and the empty tungsten grid under the same experimental condition was subtracted to eliminate the influence of the gas phase and the tungsten grid. Each spectrum was taken in 52 s by averaging 128 interferograms at 2 cm -1 resolution. S2
II. Synthesis Method. The Au/TiO 2 sample was home-made following the deposition-precipitation protocol reported by Zanella et al., using hydrogen tetrachloroaurate (III) trihydrate (HAuCl 4 3H 2 O), TiO 2 powder (P 25, 49 m 2 /g), and urea. 2 The TiO 2 powdered sample was provided by Evonic Industries; urea (99.5%) and HAuCl 4 3H 2 O were purchased from Acros. Prior to deposition, the TiO 2 powder was activated at 373 K under 100 ml/min air flow for 24 h. The Au precursor solution consists 4.2 10-3 M HAuCl 4 3H 2 O and 4.2 M urea and is kept in dark throughout the synthesis procedure due to the known fact that it decomposes under illumination. The initial ph value of the precursor solution is ~2. Urea is used here to cause a slow release of ammonia to control the acidity of the solution. One gram of activated TiO 2 powder was added to 100 ml of the Au precursor solution for deposition. The mixture was kept at 353 K under vigorous stirring for 8 hours. The ph value of the mixture increased to ~7 at the end of the deposition. The solid (orange colored) was then separated from the liquid phase by a centrifuge at 3800 rpm for 20 min. The precipitate was then washed in 100 ml double-deionized water at 323 K using vigorous stirring for 10 min and centrifuged again. This washing cycle was repeated 5 times, in an attempt to remove the residual Cl - anions which are proposed to compete with O 2 for adsorption sites. 3,4 The cleaned Au/TiO 2 sample was dried at 373 K for 24 h under 100 ml/min air flow before installation into the vacuum cell. No chlorine was detected on this sample by Auger spectroscopy measured in a separate UHV system. The maximum Au loading on the catalyst was 8 wt%. Figure S2a shows a TEM image of the Au/TiO 2 catalyst after experiments. Figure S2b shows a Au particle diameter histogram where a distribution between 1-12 nm where a most probable diameter of 3 nm is found. This measurement is consistent with the Au particle size analysis reported by others from similarly-synthesized samples. 2 S3
The samples were then mounted into the vacuum IR cell as depicted in Figure S1. Before initial use, the samples were heated to 473 K in vacuum for 30 min, followed by treating with 20 Torr of O 2 at 473 K for 90 min. After evacuation, the samples were kept at 473 K in vacuum for another 30 min before they were cooled to the desired temperature. The heating/cooling rate is set at 12 K/min for sample pre-treatment unless mentioned otherwise. This treatment removed most of the impurity hydrocarbon on the sample, judging from IR observation. For this study, samples were not heated to temperatures higher than 473 K to avoid Au agglomeration. Complete removal of hydroxyl species from the sample surfaces requires higher temperature treatment. Thus, the samples were not hydroxyl free. After the initial treatment, before each separate experiment, the samples were heated to 473 K in vacuum for 15 min to regenerate the surface, removing any impurity accumulated between experiments. III. Model System and Calculation Details. The adsorption and activation of O 2 as well as the adsorption and oxidation of CO were carried out on the model Au/TiO 2 system as briefly described in the main text and depicted in Figure S5. All of the calculations reported herein were carried out using density functional theory (DFT) calculations as implemented in the Vienna ab initio Simulation Package (VASP). 5 The core electrons were treated by pseudopotentials built with the projector augmented-wave (PAW) method 6,7. The valence electrons were described with Kohn-Sham single-electron wave functions and expanded in plane-wave basis with energy cutoff of 400 ev. The exchange-correlation energy was described by the PW91 gradient approximation (GGA) functional. 8 The on-site Coulomb interactions was corrected by the DFT+U method, 9 in which the value of U was chosen to be 4.0 ev in order to generate the experimentally-observed band-gap structure. 10 Spin-polarization was considered for all calculations and was used when necessary. A vacuum gap of 10 Å was used in S4
the Z-direction between slabs. The first Brillouin zone was sampled with (2 2 1) k-point mesh. 11 Geometries were considered as optimized when the force on each atom was less than 0.03 ev/å. The reaction pathway and activation barriers were found by the nudged elastic band (NEB) method with image climbing, 12,13 combined with the dimer method. 14 The NEB method was used to follow the minimum energy path between the reactant and product states. When the perpendicular forces on all of the images along the band were lower than 0.1 ev/å, the dimer method was subsequently used to refine the transition state to the point where the force acting on the transition state dimer was lower than 0.03 ev/ Å. The charge state of the Au atoms was analyzed by the Bader charge analysis method. 15,16 Before the catalyst was oxidized, the surface Au atoms were neutral with the exception at the perimeter where the Au atoms bound to the bridge oxygen of the TiO 2 and were slightly positively charged (Figure S7), similar to previous theoretical results for small Au clusters on TiO 2. 17 IV. Additional Calculation Results. A. Au oxidation. Both previous experimental results 18 and our calculations found that there was no adsorption of O 2 on the ideal rutile TiO 2 (110) surface, and as such, Ti 5c sites far away from the Au were not considered. A thorough search for the adsorption of O 2 at all of the characteristically different sites on the model Au/TiO 2 was carried out. The perimeter site of the Au/TiO 2 was found to be the most favorable site for O 2 adsorption (Figure S5), which is also the most active site for O 2 activation (Figure S6). Oxygen, which adsorbs at the perimeter Ti-Au site with a di-σ binding structure, dissociates with a barrier ~0.4-0.6 ev. The resulting O atom binds to the three-fold Au S5
hollow (Figure S6a) or the bridge of a Au edge site (Figure S6b) nearby. The charge state changes on the Au nanorod due to the oxidation are shown in Figure S7. It is clear that the oxidation increases positive charge on the neighboring Au atoms. B. O atoms on Au/TiO 2. The oxidation is initiated at the dual perimeter and can be expanded to the Au sites further away from the perimeter by O atom surface diffusion. The O atoms can move on the Au rod with diffusion barriers from 0.2 to 0.7 ev, oxidizing those neutral Au atoms above the perimeter. Favorable binding site for the O atom is either on a FCC three-fold hollow or the edge bridge site, as shown in Figure S8a-c. The binding on the FCC hollow is slightly preferred as the binding energy of the O atom is ~0.1 ev stronger (-0.42 ev) as compared to binding on bridge site on the edge (-0.32 ev). The O atom left on the Ti 5c site can also diffuse along the Ti-row on the oxide with a barrier of 0.75 ev (Figure S8d). Once the O atoms move out of the perimeter region, the interaction between the O atom and the oxide decreases. Consequently, two O atoms on the Ti 5c sites can recombine to produce O 2 and desorb, with a barrier of only 0.2 ev (Figure S8e). C. The effect of surface coverage In Figure 7, we only show the case at low CO and O coverage, which may differ from the real surface coverage for CO molecules and O atoms under working conditions. However, this should not affect our conclusions, as adsorbates on oxides are far apart and have very weak if any lateral interactions, thus resulting in very small changes on the activation barrier and reaction energy with coverage. 19 It is also shown in our previous study that the changes in the calculated barriers and reaction energies are less than 0.05 ev as we move from low coverage to high coverage TiO 2. 20 S6
V. References for the Supporting Material. (1) Basu, P.; Ballinger, T. H.; J. T. Yates, Jr. Rev. Sci. Instrum. 1988, 59, 1321-1327. (2) Zanella, R.; Giorgio, S.; Henry, C. R.; Louis, C. J. Phys. Chem. B 2002, 106, 7634-7642. (3) Laursen, S.; Linic, S. Phys. Chem. Chem. Phys. 2009, 11, 11006-11012. (4) Kung, M. C.; Davis, R. J.; Kung, H. H. J. Phys. Chem. C 2007, 111, 11767-11775. (5) Kresse, G. Phys. Rev. B 2000, 62, 8295-8305. (6) Blöchl, P. E. Phys. Rev. B 1994, 50, 17953-17979. (7) Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1758-1775. (8) Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 45, 13244-13249. (9) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Phys. Rev. B 1998, 57, 1505-1509. (10) Morgan, B. J.; Watson, G. W. Surf. Sci. 2007, 601, 5034-5041. (11) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188-5192. (12) Henkelman, G.; Jónsson, H. J. Chem. Phys. 2000, 113, 9978-9985. (13) Henkelman, G.; Uberuaga, B. P.; Jónsson, H. J. Chem. Phys. 2000, 113, 9901-9904. (14) Henkelman, G.; Jónsson, H. J. Chem. Phys. 1999, 111, 7010-7022. (15) Bader, R. F. W. Atoms in Molecules: A Quantum Theory; Oxford University Press, New York, 1990. (16) Tang, W.; Sanville, E.; Henkelman, G. J. Phys.: Condens. Matter 2009, 21, 084204-1-7. (17) Wang, J. G.; Hammer, B. Phys. Rev. Lett. 2006, 97, 136107-1-4. (18) Pan, J.-M.; Maschhoff, B. L.; Diebold, U.; Madey, T. E. J. Vac. Sci. Technol., A 1992, 10, 2470-2476. (19) Reuter, K.; Frenkel, D.; Scheffler, M. Phys. Rev. Lett. 2004, 93, 116105-1-4. (20) Green, I. X.; Tang, W.; Neurock, M.; Yates, J. T., Jr. Science 2011, 333, 736-739. S7
VI. Supporting Figures. Figure S1. Schematic drawings of the high vacuum system for transmission IR studies. a. Top view of the IR cell. b. A closeup view of the sample holder. S8
a Number of Au particles 80 70 60 50 40 30 20 10 0 b 1 2 3 4 5 6 7 8 9 10 11 12 Diameter of Au particles (nm) Figure S2. a. TEM image of the Au/TiO 2 catalyst. Image was taken after all experiments are done. b. Histogram of the Au particle size distribution. S9
Absorbance ~2206cm -1 CO/TiO 2 0.5 2179cm -1 CO/TiO 2 ~2126cm -1 CO/Au δ+ -1 ~2106cm CO/Au 0 P(CO)=0.07 Torr T IR =120 K iv. fully-oxidized Au δ+ /TiO 2 iii. oxidized TiO 2 ii. fully-reduced Au 0 /TiO 2 i. reduced TiO 2 2250 2200 2150 2100 2050 2000 Frequency (cm -1 ) Figure S3. Comparison of IR spectra of 120 K-CO saturation coverage on the Au/TiO 2 catalyst and TiO 2 blank sample after the same pre-treatment. i and ii, after 295 K-CO reduction followed by 473 K evacuation. iii and iv, after 473 K-O 2 oxidation followed by 473 K evacuation. S10
T = 120 K Absorbance 0.5 vi iv v iii 1 Torr O 2 1200 s 1 Torr O 2 1200 s 3rd cycle 2nd cycle O 2 evacuation and CO readsorption ii i 1 Torr O 2 1200 s 1st cycle 2400 2300 2200 2100 2000 Frequency (cm -1 ) Figure S4. IR spectra of repeated CO oxidation cycles on a fully-oxidized Au δ+ /TiO 2 catalyst at 120 K. i, initial saturation of the Au δ+ /TiO 2 catalyst by CO followed by evacuation. ii, after 1200 s of CO oxidation reaction on the catalyst shown in spectrum i, in 1 Torr of O 2 (first catalytic cycle). iii, IR spectrum taken after the catalyst shown in spectrum ii had gone through O 2 evacuation, CO readsorption, and CO evacuation. iv, after 1200 s of CO oxidation reaction on the catalyst shown in spectrum iii, in 1 Torr of O 2 (second catalytic cycle). v, IR spectrum taken after the catalyst shown in spectrum iv had gone through O 2 evacuation, CO readsorption, and CO evacuation. vi, after 1200 s of CO oxidation reaction on the catalyst shown in spectrum v, in 1 Torr of O 2 (third catalytic cycle). S11
Figure S5. O 2 adsorption energy at different sites. E ads refers to the adsorption energy. Some data from ref [ 20 ]. S12
Figure S6. Calculated barrier for Au oxidation at different sites. S13
Figure S7. Au charge state changes on (a) Au-Site I and (b) Au-Site II. S14
Figure S8. (a-c)-o atom diffusion on Au nanorod; (d)-o atom diffusion on Ti 5c and (e)- recombination to O 2. H refers to the reaction energy. The O atom diffusion on Ti 5c data comes from ref [ 20 ]. S15