Low pressure CO 2 hydrogenation to methanol over gold nanoparticles activated on a CeO x /TiO 2 interface

Transcription

1 Low pressure CO 2 hydrogenation to methanol over gold nanoparticles activated on a CeO x /TiO 2 interface 1 Xiaofang Yang, 1 Shyam Kattel, 1 Sanjaya D. Senanayake, 2 J. Anibal Boscoboinik, 3 Xiaowa Nie, 4 Jesús Graciani, 1 José A. Rodriguez, 1 Ping Liu, 1 Darío J. Stacchiola * and 1,4 Jingguang G. Chen* 1 Chemistry Department, Brookhaven National Laboratory, Upton, NY Center for Functional Nanomaterials (CFN), Brookhaven National Laboratory, Upton, NY Dalian University of Technology, Dalian, China 4 Department of Physical Chemistry, University of Seville, E-41012, Seville, Spain 5 Department of Chemical Engineering, Columbia University, NY Experimental Details Preparation of Cu/TiO 2 and Cu/CeO x /TiO 2 surfaces A TiO 2 (110) crystal was cleaned by cycles of 1.0 kv, Torr Ar + sputtering, and annealing at 973 K. The surface was examined by XPS to ensure no carbon and sulfur impurities. Low coverage of CeO x (0.1 ML) was deposited at the TiO 2 surface at 623 K under Torr O 2. After deposition, the surface was continuously annealed at 673 K in Torr O 2 for 15 min and then cooled down to 300 K. Small coverages of Cu or Au (0.1 ML) were deposited on the clean TiO 2 surface or the CeO x pre-covered TiO 2 surface at 300 K in vacuum. In a separate UHV chamber, the surface morphology of CeO x /TiO(111) and Au/CeOx/TiO(111) was measured by scanning tunneling microscopy. More details of the preparation and characterization of the surfaces can be found in the previous report. 1 CO 2 hydrogenation and in-situ AP-XPS measurements The AP-XPS at Brookhaven National Laboratory is set up to obtain core-level photoelectron spectroscopy at gas pressures up to ~ 1 Torr and sample temperature up to 773 K heated with a button heater. Three differential pumping stages allow for gas pressures in the Torr range at the main chamber while maintaining ultra-high vacuum in the XPS analyzer. A separate small chamber equipped with a differentially pumped mass spectrometer is connected to the main chamber through a capillary tube, which allows to achieve a large pressure gradient between the main chamber (up to 1.0 Torr) and the mass spectrometer (10-6~7 Torr). The tube entrance inside the main chamber can be moved very close to the sample surface (<0.1 mm), so that reaction products mainly from the sample surface are monitored by the mass spectrometer. S1

2 For a short time measurement, the reaction chamber is in analogy to a flow reactor, in which the reactants enter the chamber through two leak valves and the gas concentrations near the catalytic surface (reactant and products) are sampled and analyzed by the mass spectrometer. However, for a long time reaction, the chamber becomes a combination of a flow reactor and a batch reactor because parts of the products are accumulated inside the chamber and parts of the products are pumped to the mass spectrometer. Therefore, to obtain an accurate kinetic data, all the measurements in this study are only carried out for a short period of time (4 min) and the CO and methanol concentrations can be considered to be approximately at steady state. During the reaction, 100 mtorr CO 2 and 700 mtorr H 2 were leaked into the main chamber at 300K. Then, the sample was linearly heated up to 573K in the CO 2 /H 2 mixtures. The reaction products were recorded by mass spectrometer and the surface was examined by XPS. Intensities of ions (na) with m/e of 44, 31, 28, 18 and 15 were recorded as a function of time. The quantification of products is based on the calibration plot, which shows a linear relationship between actual pressure of gases in the reaction chamber and the corresponding mass spectrometric signals in the mass spectrometer. The calibration plot was obtained by adding different levels of a known pressure of gas in the reaction chamber and then measuring its ion intensity in mass spectrometry. During the reaction, the O1s and C1s in XPS were recorded at the photon energy of 700 ev and 538 ev, respectively. All the reported binding energy was calibrated with the Au4f7/2 peak of a gold foil at 84.0 ev. Ambient Pressure XPS (AP-XPS) Figure S1 shows the C1s regions in the AP-XPS for both Cu/TiO 2 and Cu/CeO x /TiO 2 during CO 2 hydrogenation. Peak at ev is assigned to the atomic carbon co-deposited with Au and Cu. After exposing 100 mtorr CO 2 and 700 mtorr H 2 at 300 K, three largely separated C1s peaks can be identified in the XPS spectra and their intensities vary for each surface. The peak at ~ ev is due to the gas phase CO 2. The peak at ev could be assigned as carbonate and carboxyl (CO δ- 2 ) which have been identified by infrared reflection-absorption spectroscopy (IRRAS) experiments. 2 The observation of carbonates/carboxyl indicates that both surfaces are active in activating CO 2 at room temperature. It is worthy to notice that the peak intensity at ev is much larger on the CeO x modified surfaces. Meanwhile, negligible amount of carbonate/carboxyl is formed on either TiO 2 or CeO x /TiO 2 surfaces under the same ambient pressures of CO 2. Therefore, effective activation of CO 2 requires both the metals and oxide supports and the active sites are most likely located at the interfacial regions between Cu and CeO x. The C1s peaks in spectra 2 in all figures are compared to show the surface species after the surfaces were heated to 573 K in CO 2 /H 2 gases and then cooled down to 323 K. Under these conditions, the peak at ev could be assigned as the adsorbed HCOO, which was formed through the hydrogenation of adsorbed carbonate and carboxylate species. 2 The C1s peak intensity from Cu (or Au)/ CeO x /TiO 2 is much larger than that from Cu (or Au)/TiO 2, indicating that more formate was present at the CeO x modified surfaces. Another interesting feature on this surface is the appearance of a small new peak at ev. This peak is probably S2

3 related to the adsorbed CO or O-CH 3 species as both CO and methanol were formed in the reaction. C1s spectra (A-3, B-3, C-3 and D-3) with the surfaces being heated at 573 K were compared for all surfaces. Under this condition, carbonate should decompose. As is shown in Figure 1A-3, the surface carbon species on the Cu/TiO 2 surface completely disappeared. The C1s peak on the Cu/TiO 2 surface nearly disappeared by annealing to 573 K. However, the formate/carboxyl on the Cu/CeO x /TiO 2 was still visible in Figure 1B-3. Similar results for Au were observed. A B C D S3

4 Figure S1. XPS of C1s region under ambient pressure conditions. A). Cu/TiO 2, B). Cu/CeO x /TiO 2, C). Au/TiO 2 and D). Au/CeO x /TiO mtorr CO 2, 700 mtorr H 2 at 300 K 2. Heating to 573 K then cooling down to 323K, 3. Maintaining the sample temperature at 573 K. Computational Methods Density functional theory (DFT) 4,5 calculations were performed using plane wave basis set Vienna Ab-initio Simulation Package (VASP) code. 6,7 A plane wave cut-off energy of 400 ev was used for total energy calculations. The Brillouin zone was sampled using a single gamma point. The interactions between electrons and nuclei were treated with all electron like projector augmented wave (PAW) potentials 8,9 with the generalized gradient approximation (GGA) using PW91 functionals. 10 Ionic positions were optimized until Hellman-Feynman force on each ion was smaller than 0.02 ev/å. The TiO 2 support was modeled as a periodically repeating five layer 5 2 unit cell of rutile TiO 2 (110) surface. A vacuum layer of ~14 Å thick was added in the slab cell along the direction perpendicular to the surface in order to minimize the artificial interactions between the surface and its periodic images. The electronic structure of Ce (4f electrons) and Ti (3d electrons) were treated in DFT+U formalism. For Ce and Ti, we have used the U eff = 4.5 ev following our previous work. 11 During calculations, atoms in the top two layers were allowed to relax while the atoms in the bottom three layers were fixed. The possible mechanism for CO 2 hydrogenation on Au supported on TiO 2 was studied by depositing a small Au 3 cluster on an oxygen vacancy of TiO 2. And to further study the CO 2 hydrogenation mechanism on Au supported on CeO x /TiO 2 mixed oxide, a Ce 2 O 3 dimer was first deposited on TiO 2 following the procedure in our previous work. 11 Then Au 3 cluster was deposited on CeO x /TiO 2 mixed oxide. The binding energy of Au 3 cluster on TiO 2 and Ce 2 O 3 modified TiO 2 support is calculated as BE =E(Au 3 +support)-e(au 3 )-E(support) where E(Au 3 +support), E(Au 3 ) and E(support) are the total energies of Au 3 adsorbed support, Au 3 cluster in gas phase and clean support, respectively. The transition state of a chemical reaction was located using the climbing image nudged elastic band (CI-NEB) method. 12 S4

5 Figure S2. Partial density of states (PDOS) of Au atoms in Au 3 /TiO 2 (110) and Au 3 /CeO x /TiO 2 (110). Figure S3. (a) Au 3 cluster supported on TiO 2 (110) (top: side view and bottom: top view) (b)-(j) most stable adsorption configurations (top: side view, bottom: top view) of *H, *CO 2, **CO 2, *HOCO, *CO, *HCO, *H 2 CO, *H 3 CO and *CH 3 OH on Au 3 TiO 2 (110), respectively. Light blue: Ti, yellow: Au, red: O and white: H atoms, respectively. S5

6 Figure S4. (a) Au 3 cluster supported on Ce 2 O 3 modified TiO 2 (110) (top: side view and bottom: top view) (b)-(j) most stable adsorption configurations (top: side view, bottom: top view) of *H, *CO 2, **CO 2, *HOCO, *CO, *HCO, *H 2 CO, *H 3 CO and *CH 3 OH on Au 3 /CeO x /TiO 2 (110) respectively. Light blue: Ti, yellow: Au, green: Ce, red: O and white: H atoms, respectively. References (1) Park, J. B.; Graciani, J.; Evans, J.; Stacchiola, D.; Senanayake, S. D.; Barrio, L.; Liu, P.; Sanz, J. F.; Hrbek, J.; Rodriguez, J. A. J. Am. Chem. Soc. 2009, 132, 356. (2) Mudiyanselage, K.; Senanayake, S. D.; Feria, L.; Kundu, S.; Baber, A. E.; Graciani, J.; Vidal, A. B.; Agnoli, S.; Evans, J.; Chang, R.; Axnanda, S.; Liu, Z.; Sanz, J. F.; Liu, P.; Rodriguez, J. A.; Stacchiola, D. J. Angew. Chem. Int. Ed. 2013, 52, (3) Mullins, D. R.; Overbury, S. H.; Huntley, D. R. Surf, Sci. 1998, 409, 307. (4) Hohenberg, P.; Kohn, W., Phys. Rev. B. 1964, 136, B864. (5) Kohn, W.; Sham, L.J. Phys. Rev. 1965,140, (6) Kresse, G.; Furthmuller,J. Comp. Mater. Sci.1996, 6, 15. (7) Kresse, G.; J. Hafner, J. Phys. Rev. B. 1993, 48, (8) Blochl, P. E. Phys. Rev. B 50, (9) Perdew, J. P.; Wang, Y. Phys. Rev. B. 1992, 45, (10) Kresse G.; Joubert D. Phys. Rev. B. 1999, 59, (11) Park, J. B.; Graciani, J.; Evans, J.; Stacchiola, D.; Ma, S.; Liu, P.; Nambu, A.; Sanz, J. F.; Hrbek, J.; Rodriguez, J. A. Proc. Natl. Acad. Sci. 2009, 106, (12) Henkelman, G.; Uberuaga, B. P.; Jonsson, H. J. Chem. Phys. 2000, 113, S6