TiO2(110) for CO Oxidation: Alkali Enhanced

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High Activity of Au/K/TiO(110) for CO Oxidation: Alkali Enhanced Dispersion of Au and Bonding of CO Jose A. Rodriguez, David C Grinter, Pedro J Ramirez, Dario Stacchiola, and Sanjaya D. Senanayake J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11771 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 15, 2018

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The Journal of Physical Chemistry

High Activity of Au/K/TiO2(110) for CO oxidation: Alkali Enhanced Dispersion of Au and Bonding of CO José A. Rodriguez*1, David C. Grinter1,b, Pedro J. Ramírez2, Dario J. Stacchiola1,c and Sanjaya Senanayake1 1

Chemistry Department, Brookhaven National Laboratory, Upton, NY, 11973, United States 2 Facultad de Ciencias, Universidad Central de Venezuela, Caracas 1020 A, Venezuela *

Corresponding author, email [email protected]

b

Present address: Diamond Light Source, Didcot, Oxfordshire OX11 0DE, United Kingdom

c

Present address: Center for Functional Nanomaterials, Brookhaven National Laboratory,

Upton, NY 11973, United States.

Abstract Images of scanning tunneling microscopy show a high mobility for K on an oxidized TiO2(110) surface. At low coverages, the alkali occupies mainly terrace sites of the o-TiO2(110) system. The results of X-ray photoelectron spectroscopy indicate that K is fully ionized. The electron transferred from K to the titania affects the reactivity of this oxide favoring the dispersion of Au particles on the terraces of a oTiO2(110) surface. When small coverages of K and Au are present on the o-TiO2(110) system, only a few K-Au pairs are formed and the alkali affects Au chemisorption mainly through the oxide interactions. The addition of K to Au/o-TiO2(110) enhances the reactivity of the system opening new reaction paths for the adsorption and oxidation of carbon monoxide. CO can undergo disproportionation (2CO → Cads + CO2,ads) on K/o-TiO2(110) and Au/K/o-TiO2(110) surfaces. A Au-KOx interface binds CO much better than plain Au-TiO2 increasing the surface coverage of CO and facilitating its oxidation. Kinetic tests show that K promotes CO oxidation on Au/TiO2. Turnover frequencies of 2.1 molecules per Au site-1 s-1 and 10.8 molecules per Au site-1 s-1 were calculated for the oxidation of CO on Au/o-TiO2(110) and Au/K/o-TiO2(110) catalysts, respectively.

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1. INTRODUCTION For years, it has been known that alkali metals can act as promoters of catalytic or chemical reactions present in a large number of industrial processes (CO oxidation, Fischer-Tropsch synthesis, alcohol production from CO/CO2 hydrogenation, the watergas shift reaction, ammonia synthesis, olefin epoxidation, NO reduction, etc.).1,2

The

alkali metals can enhance the activity or selectivity of catalysts based on oxides, carbides or sulfides.1,2 The precise mechanisms behind the alkali promotional effects are not yet fully understood. In principle, an alkali can participate directly in a catalytic process, binding the reactants or products, or it can modify the chemical properties of the catalyst components through ensemble or electronic effects.1,2,3 Since the early work of Langmuir in 1923, the investigation of the co-adsorption of alkali metals and simple molecules on well-defined surfaces of transition metals has been a favored topic of research in the area of catalysis.1,2,3 To explain the effects of alkalis on the surface chemistry of metals, the following mechanisms have been proposed in the literature: (1) site-blocking effects, (2) through-the-metal electronic interactions, (3) through-the-space electronic interactions (e.g., electrostatic), (4) direct chemical interactions, and (5) alkali-induced surface reconstructions.1,2,3,4,5 In comparison, much less is known about the interaction of alkali metals with well-defined surfaces of oxides.6,7.8,9,10 In principle, the presence of O centers can lead to adsorption phenomena not seen on plain metals. The interaction of K with TiO2(110) and TiO2(100) substrates has been the subject of several works.6,11,12,13 Potassium deposition on TiO2(110) results in a reduction of the substrate, the alkali transfers almost a full electron into the titanium empty orbitals,6,8-13 and the formation of loosely bound potassium species which can move easily on the oxide surface to promote catalytic activity.12 The results of density 2 ACS Paragon Plus Environment

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functional calculations predict a large adsorption energy (~ 3.2 eV) with a small barrier (~ 0.25 eV) for diffusion on the oxide surface.12 In scanning tunneling microscopy (STM) images, the adsorbed alkali atoms lose their mobility when in contact with surface OH groups.12 Furthermore, K adatoms facilitate the dissociation of water on the titania surface. The K-(OH) species generated are good sites for the binding of gold clusters on the TiO2(110) surface producing Au/K/TiO2(110) systems with a high activity for the water-gas shift.12 In this article we investigate the oxidation of CO on Au/K/TiO2(110) surfaces. It is known that the Au/TiO2(110) system is a good catalyst for the low-temperature oxidation of CO.14 The K atoms can affect the dispersion of Au on a TiO2(110) surface.10,15 Our results show that Au/K/TiO2(110) surfaces bind well CO and O2 exhibiting and activity for CO2 oxidation that is 3-5 times higher than that of Au/TiO2(110).

2. EXPERIMENTAL The STM images for the deposition of K on oxidized TiO2(110) were collected in an ultrahigh vacuum (UHV) system with a base pressure of ~5×10-10 Torr which housed an Omicron variable temperature scanning tunnelling microscope (VT-STM). The single crystal TiO2(110) substrate was cleaned by repeated cycles of Ar sputtering and annealing in UHV to ~1000 K. Oxidised (o-TiO2(110)) surfaces containing a minimal concentration of surface oxygen vacancies were formed by exposing the as-prepared crystal to ~10 L O2 at 300 K. Potassium was deposited at 300 K from a getter source (SAES) and gold was evaporated in-situ (tip retracted ~100 nm) in the STM stage from an e-beam evaporator (Focus EFM-3) at room temperature.12 The K evaporation rate was calibrated in STM by comparison with deposition on to Cu2O/Cu(111).16 The coverage of potassium is given in ML, where one monolayer is defined as one adsorbate 3 ACS Paragon Plus Environment


per primitive unit cell of the TiO2(110) surface, a density of 5.2×1014 cm-2. The temperature of the sample was monitored with an infrared pyrometer during annealing and via a K-type thermocouple attached near to the sample plate during cooling. The catalytic tests for CO oxidation were carried out in a system which combines a batch reactor and a UHV chamber.26 This UHV chamber (base pressure ~ 1 x 10-10 Torr) was equipped with instrumentation for X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction, ion-scattering spectroscopy (ISS), and thermaldesorption mass spectroscopy. In this chamber, XPS spectra were acquired using Mg Kα radiation. The XPS binding energy scale was calibrated by setting the Ti 2p3/2 peak of Ti4+ in TiO2 at 459 eV.12 The Au/K/TiO(110) sample could be transferred between the UHV chamber and reactor without exposure to air. Typically, it was transferred to the batch reactor at ~ 300 K and then the reactant gases were introduced (4 Torr of CO and 2 Torr of O2). The amount of CO2 molecules produced was normalized by the active area exposed by the sample. Measurements of CO2 production were performed using a mass spectrometer. The kinetic tests were performed with a very low conversion of CO (< 5%) in the batch reactor. In our reactor, a steady-state regime for the production of CO2 was reached before ~ 2 min of reaction time.

3. RESULTS AND DISCUSSION 3.1 Growth of K on o-TiO2(110). Previous studies have examined the interaction of K with TiO2(110) surfaces which had a significant number of O vacancies or OH groups on the surface.6,9,12 This type of surface features serve as nucleation sites for potassium.6,9,12 Under CO oxidation conditions, not many O vacancies or OH groups are expected to be present on the TiO2(110) surface. Thus, we investigated the adsorption of K on heavily oxidized TiO2(110). The interaction of oxygen with the


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TiO2(110) surface has been extensively studied and is rather complex,12,17,18,19,20,21 as shown in a recent paper.21 The reviews by Pang et al.18,19 and references within discuss various methods for forming oxidised TiO2 surfaces. In our experiments, we wished to minimize the significant influence of hydroxyl species and oxygen vacancies on the adsorption of potassium on TiO2(110) which was demonstrated in our earlier work.12 As a result we have chosen a recipe (10 L O2 at 300 K) that fills the majority of any surface oxygen vacancies present as well as reacting away most of the surface hydroxyl groups, without inducing additional diffusion of subsurface Ti species, or other unwanted effects (e.g. oxygen adatoms) that may be observed under higher temperatures or oxygen exposures/chemical potentials.18,19,21 Figure 1 compares an as-prepared surface (a) which was hydroxylated by water from the background vacuum with an oxidised surface (b) prepared by exposure to O2 at 300 K, and results in a nearly hydroxyl-free surface. On this kind of surface, the dosed K became fully oxidized and in XPS showed the K 2p positions expected for K+1 (see below). The adsorbed potassium cations had a high mobility on the o-TiO2(110) surface and it was not easy to image them with STM as seen in a previous study.12 Figure 2 displays a STM image collected after depositing a very small amount of potassium onto o-TiO2(110) at 300 K. The calibrated dose of potassium in Figure 2 was 0.006 ML, however in the measure by STM only about 0.003 ML was observed. This is likely due to high mobility of the potassium species on the oxidised surface, especially under the measurement conditions where the STM tip can easily move loosely-bound adsorbates and does not image them as a result.12 In Figure 2, most of the atoms or clusters of potassium appear on terraces of the surface. An increase in the coverage of potassium led to agglomeration of the alkali into clusters as seen in previous studies for titania surfaces with O vacancies and OH groups.6,9,12



3.2 Co-deposition of Au and K on o-TiO2(110). Figure 3 compares the dispersion of Au on o-TiO2(110) and K/o-TiO2(110) surfaces. From the STM data, the number densities are as follows: 0.019 ± 0.002 clusters/nm2 for Au/o-TiO2(110) and 0.015 ± 0.002 clusters/nm2 for Au/K/o-TiO2(110). Without the alkali on the oxide surface, the Au particles prefer binding to step and defect sites, with a few particles present on the flat terraces of the surface. A similar result has been found in previous studies for regular or hydroxylated TiO2(110).14 Due to the difference in height between the potassium (0.1-0.2 nm) and the gold clusters (0.5-1 nm), in a first approximation, one can identify the Au clusters but it is difficult to assign the position of the potassium species after Au deposition from these STM images. The smaller protrusions visible in Figure 3b are likely to be a mixture of potassium and other small defects such as hydroxyl groups. In the case of Au deposition on K/o-TiO2(110), one sees a larger dispersion of Au particles on the terraces of the surface (Figure 4). This larger dispersion could lead to an enhancement of catalytic activity. The data in Figure 4 is derived from an analysis of 12 STM images (see supplemental Figures S1 and S2) containing 1355 gold clusters across a total area of 8.3x105 nm2. The error bars in Figure 4 are the variance between the edge/terrace ratios between different STM images. The larger dispersion of Au on o-TiO2(110) could be a consequence of direct bonding of Au with the alkali cations or a product of the K→TiO2 charge transfer which perturbs electronically the titania to facilitate bonding of Au particles.12 Figure 5 compares STM images recorded before and after depositing Au on a K/o-TiO2(110) surface, with the locations of the potassium species highlighted in both images. Most of the Au particles bind to the terraces of the titania but there is a very low coincidence rate between the Au cluster positions and the K, ~5% across the many images we


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collected. Thus the modification of the dispersion for the admetal could be a consequence of an indirect or long-range interaction though the oxide.12 A similar phenomenon has been seen for the deposition of alkalis on metals where long range interactions have been found for coadsorption with CO and NO.1-3 Part of the Au adatoms could be nucleating on K cations which had a high mobility and were not detected by STM. Furthermore, as a consequence of compound formation with an adsorbate, species such as AuKCOx and AuKOx are formed on the titania surface upon exposure to CO and O2 (see below). The K cations probably serve as anchor sites for binding Au and adsorbates.

3.3 Adsorption of CO on Au/ o-TiO2(110) and Au/K/o-TiO2(110) surfaces. CO binds weakly to Au(111) or o-TiO2(110) desorbing at temperatures below 200 K.22,23 The binding energy of CO on a non-defective TiO2(110) surface is 9.9 kcal/mole in the limit of zero coverage. Desorption of the molecule is observed at temperatures between 130 and 170 K.23 When anion vacancy sites are produced under controlled annealing conditions, a significant increase in the desorption temperature of a portion of the chemisorbed CO is observed with the molecule still present on the oxide surface until temperatures close to 350 K.23 CO probably adsorbs more strongly on lattice Ti sites in the vicinity of anion vacancy sites.23 The top of Figure 6 shows a XPS spectrum covering the K 2p and C 1s regions of a system generated by dosing 50 L of CO to a o-TiO2(110) surface pre-covered by 0.1 ML of potassium. On the plain K/o-TiO2(110), the K 2p3/2 peak appears near 294.5 eV, the position expected for K+ ions dispersed on titania,24,25 HO-TiO2(110)12 or other oxide surfaces.26,27 Upon the dosing of CO, clear features are detected for adsorbed



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COx species and atomic C.12,28 The C and some of the COx species are probably generated by the disproportionation of carbon monoxide: 2COads  Cads + CO2,ads

(1)

A strong interaction between CO and K leads to a rupture of C-O bonds. A similar phenomenon has been observed for the coadsorption of K and CO on metal surfaces and it may reflect an intrinsic property of the alkali.1-3 Part of the generated COx could come from a direct reaction of CO with surface oxygen sites facilitated by the presence of K cations. Potassium carbonate-like species are very stable compounds.29 Heating to temperatures of 400-500 K did not affect the intensity on the COx and C species formed on CO/K/o-TiO2(110). The bottom of Figure 7 displays a C 1s XPS spectrum collected after dosing 50 L of CO at 200 K to a o-TiO2(110) surface with 0.2 ML of gold. There is no visible peak for adsorbed CO. A similar result was found for plain o-TiO2(110) and for titania surfaces with Au coverages between 0.3 and 1.5 ML. On these systems the CO bonding interactions were so weak that the molecule desorbed at temperatures below 200 K. A drastic change in the surface chemistry of CO was seen after co-adsorbing K and Au on the titania substrate. The top of Figure 7 shows C 1s XPS spectra collected following the dosing 50 L of CO to a Au/K/o-TiO2(110) surface at 200 K with subsequent heating to 300 K. For this system, one can see peaks for adsorbed atomic C at ~ 284.5 eV, CO at ~ 286 eV and a COx species at ~ 289 eV.12,28 The C and COx species are probably generated by the disproportionation of carbon monoxide, equation (1). These species were also observed after dosing CO to K/o-TiO2(110) surfaces, Figure 6. Again a strong interaction between CO and K leads to dissociation of C-O bonds but the presence of Au on the surface reduces the number of adsorbed molecules which decompose. In Figure 7, the C 1s peak for adsorbed CO is characteristic of


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Au/K/o-TiO2(110) surfaces. This peak probably comes from CO adsorbed at Au-KOx interfaces. This adsorbed CO is bound stronger than on o-TiO2(110)18 or Au/oTiO2(110) but eventually desorbs upon heating the sample to 300 K. Nevertheless, this state could be populated under a background pressure of CO, and it likely plays an important role on the oxidation of CO on Au/K/o-TiO2(110) at room temperature.

3.4 CO oxidation on on Au/ o-TiO2(110) and Au/K/o-TiO2(110) surfaces. In recent years the oxidation of CO on Au/TiO2 has received a lot of attention.17,30 The reaction probably takes place at the gold-titania interface and the exact configuration of the active sites and mechanism probably change with the reaction conditions.17,21 Some studies have proposed that the enhanced activity is from uncoordinated sites on the nanoconfined Au, while the support stabilizes the size of the particle and provides oxygen for the oxidation of CO on Au sites.31,32 The reaction could be catalyzed by a transient Au-CO species that migrates from a Au-cluster onto a surface oxygen adatom.21 It subsequently reacts with the TiO2 support via a Mars van Krevelen mechanism to form CO2 and finally the Au atom reintegrates back into the gold cluster to complete the catalytic cycle.21 From the results presented in the previous sections, K could affect the oxidation of CO by enhancing the dispersion of Au particles on TiO2(110) and by facilitating the binding of CO. In test experiments, we found that o-TiO2(110) and K/o-TiO2(110) were not able to catalyse the oxidation of CO at room temperature. Figure 8 compares the CO oxidation activity of Au/o-TiO2(110) and Au/K/o-TiO2(110) surfaces. In the top-panel one can observe steady-state production of CO2 with the rate of reaction increasing when 0.02 and 0.1 ML of potassium are present on the surface. After adding 0.02 ML of K, the rate of CO oxidation increased by a factor of ~ 2.6, being over 10 times larger in



the system with 0.1 ML of K. The bottom panel of Figure 8 displays the rate of CO2 production on Au/K/o-TiO2(110) surfaces as a function of Au coverage with a K fixed coverage of 0.1 ML. A maximum in the production of CO2 is observed at gold coverages of 0.3-0.4 ML. Previous studies have shown that there is a marked size effect on the Au catalytic activity with metal clusters in the range of 2-3.5 nm exhibiting the maximum reactivity. 14,21-23 Independently of Au coverage, Au/K/o-TiO2(110) is a much better catalyst for the oxidation of CO than Au/o-TiO2(110). Turnover frequencies (TOFs) for CO oxidation can be estimated assuming a total dispersion on the Au on the oxide substrate.14 Following this approach, using the data in the bottom panel of Figure 8, we estimate maximum TOFs of 2.1 molecules per Au site-1 s-1 and 10.8 molecules per Au site-1 s-1 for Au/o-TiO2(110) and Au/K/o-TiO2(110), respectively. These should be taken as lower limits for the TOFs because we probably overestimated the number of exposed Au active sites.14 In any case, the TOF of Au/K/o-TiO2(110) is already significantly larger than the TOFs reported for other Au/TiO2 catalysts under similar conditions (< 4 molecules per Au site-1 s-1). 32,33 XPS was used to characterize the Au/o-TiO2(110) and Au/K/o-TiO2(110) catalysts after reaction. Figure 9 shows C 1s XPS spectra collected after exposure of the catalysts to CO and a mixture of CO/O2. In the C 1s region, there are peaks that correspond to adsorbed COx species and C. On the Au/o-TiO2(110) catalyst, only a small amount of COx is seen. The addition of K enhances the reactivity of the system changing the surface chemistry. The spectrum obtained after exposing the Au/K/oTiO2(110) catalyst to 4 Torr of CO is consistent with the occurrence of reaction (1) and the formation of some COx by reaction of CO with O sites on the oxide. With respect to the spectra in Figure 7, there is an increase in the coverages of COx and C close to a factor of 2 due to the higher pressure of CO. After the CO oxidation reaction, only COx


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species were present on the Au/K/o-TiO2(110) catalyst with a coverage close to 0.5 ML which is bigger than those seen after exposure of the surfaces to only CO. In the corresponding Au 4f spectrum, which was very similar to that reported for a clean Au/TiO2(110) surface,34 we saw small binding energy shifts (< 0.2 eV) implying that gold remained in a metallic state. The COx species was probably bounded to K and adsorption sites of the TiO2(110) support. On the basis of the results in Figures 8 and 9, we can conclude that K promotes CO oxidation on Au/TiO2. An indirect or through-oxide effect could occur because K affects the dispersion of gold on titania, Figures 4 and 5, but it is clear that the alkali is directly involved in the chemistry. A Au-KOx interface binds CO much better than plain Au-TiO2 facilitating CO oxidation. Recently, it has been shown that the addition of sodium or potassium to water-gas shift (WGS, H2O + CO → H2 + CO2) catalysts which contain gold or platinum produces a substantial increase in catalytic activity by inducing the formation of single-site (Pt or Au)-O(OH)x-(Na or K) species.35,36 A similar phenomenon could be taking place in the Au/K/o-TiO2(110) system.

4. CONCLUSIONS Results of STM show a high mobility for K on an o-TiO2(110) surface. At low coverage, K is fully oxidized and occupies mainly terrace sites of the o-TiO2(110) surface. The K deposited on the titania affects the reactivity of this oxide favoring the dispersion of Au particles on the terraces of a o-TiO2(110) surface. The addition of K to Au/o-TiO2(110) enhances the reactivity of the system opening new reaction paths for the adsorption and oxidation of CO. CO can undergo disproportionation (2CO → Cads + CO2,ads) on K/o-TiO2(110) and Au/K/ o-TiO2(110) surfaces. A Au-KOx interface binds CO much better than plain Au-TiO2 facilitating CO oxidation. Kinetic tests show that K



promotes CO oxidation on Au/TiO2. TOFs of 2.1 molecules per Au site-1 s-1 and 10.8 molecules per Au site-1 s-1 were calculated for the oxidation of CO on Au/o-TiO2(110) and Au/K/o-TiO2(110) catalysts, respectively.

ASSOCIATED CONTENT Supporting Information is available Set of STM images for Au/o-TiO2(110) and Au/K/o-TiO2(110)

AUTHOR INFORMATION Corresponding author E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The experimental studies carried out at Brookhaven National Laboratory were supported by the US Department of Energy, Chemical Sciences Division (DESC0012704). The work of PJR was in part financed by research grants of BID and Metallurgia.


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Hardman, P.J.; Casanova, R.; Prabhakaran, K.; Muryn, C.A.; Wincott, P.L.; Thornton, G. Electronic Structure Effects of Potassium Adsorption on TiO2(100), Surf. Sci. 1992, 269/270, 677-681.

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Heise, R.; Courths, R. A Photoemission Investigation of the Adsorption of Potassium on Perfect and Defective TiO2(110) Surfaces, Surf. Sci. 1995, 331/333, 1460-1466.

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Wilde, M.; Beauport, I.; Stuhl, F.; Al-Shamery, K.; Freund, H.-J. Adsorption of Potassium on Cr2O3(0001) at Ionic and Metallic Coverages and UV-laser Induced Desorption, Phys. Rev. B. 1999, 59, 13401-13412.


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Figures and captions

Figure 1. STM images of (a) as-prepared h-TiO2(110) and (b) oxidised o-TiO2(110). (20 × 10 nm2, V = +1.74 V, I = 0.1 nA)

Figure 2. STM image of 0.006 ML potassium deposited onto o-TiO2(110) at 300 K. The positions of the potassium atoms/clusters are highlighted with black circles. (100 × 100 nm2, V = +1.51 V, I = 0.08 nA)


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Page 17 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. STM images of 0.1 ML Au supported on (a) o-TiO2(110) and (b) 0.006 ML K/o-TiO2(110). (100 × 63 nm2, V = +1.29 V, I = 0.08 nA)

Figure 4. Plot showing the relative locations of the Au clusters on bare and potassium-modified oTiO2(110).



Figure 5. STM images before and after gold deposition: (a) 0.006 ML K/o-TiO2(110) and (b) 0.1 ML Au on K/o-TiO2(110). (40 × 40 nm2, V = +1.51 V, I = 0.08 nA) The positions of K atoms and Au clusters are highlighted with green and yellow circles, respectively.

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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K 2p & C 1s XPS K/o-TiO2(110)

K COx

C + 50L CO, 200K

as-prepared 300

298

296

294

292

290

288

286

284

282

Binding Energy (eV) Figure 6. C 1s XPS spectra (K 2p and C 1s regions) collected after depositing 0.1 ML of K on oTiO2(110) with subsequent dosing of 50 L of CO at 200 K.


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C 1s XPS 50 L of CO at 200 K

COx


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C Au/K/TiO (110) 2

300 K

CO

Au/K/TiO2(110)

+ CO, 200 K

Au/TiO (110)

+ CO, 200 K

2

292

290

288

286

284

282

Binding Energy (eV) Figure 7. C 1s XPS spectra collected after dosing 50 L of CO at 200 K to Au/o-TiO2(110) and Au/K/oTiO2(110) surfaces. 0.1 ML of K and/or 0.2 ML of Au were deposited to the titania substrate.


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16

K cov

4 Torr of CO, 2 Torr of O2

14

300 K, 0.2 ML of Au

0.10

12 10 8 6 4 0.02 2

0.00

0 0

1

2

3

4

5

6

5

4 Torr CO, 2 Torr O2 300 K

4 Au/K/TiO2(110)

15

-1

molec sec cm

-2

Time (minutes)

CO2 molecules produced / 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

CO2 molecules produced / 1017 molec cm-2


3

2

1

Au/TiO2(110)

0 0.0

0.4

0.8

1.2

1.6

2.0

2.4

Au coverage / ML Figure 8. Top panel: Accumulation of CO2 molecules as a function of time during the oxidation of CO on plain Au/o-TiO2(110) and Au/K/o-TiO2(110) surfaces with K coverages of 0.02 and 0.1 ML. The Au coverage in all of these systems was 0.2 ML. Bottom panel: Rates for the production of CO2 during the oxidation of CO on Au/K/o-TiO2(110) as a function of gold coverage. The reported values for the


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production of CO2 were obtained after exposing the catalysts to 4 Torr of CO and 2 Torr of O2 at 300 K. The number of CO2 molecules produced is normalized by the sample surface area.

C 1s XPS Post-reaction at 300 K


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4 Torr CO, 2 Torr O2

Au/K/TiO2(110)

4 Torr CO, 2 Torr O2

Au/TiO2(110) COx C

4 Torr CO

Au/K/TiO2(110)

4 Torr CO 294

292

Au/TiO2(110) 290

288

286

284

282

Binding Energy (eV) Figure 9 C 1s XPS spectra collected after exposing Au/o-TiO2(110) and Au/K/o-TiO2(110) surfaces to 4 Torr of CO or a reaction mixture of 4 Torr of CO and 2 Torr of O2 at 300 K for 5 minutes. The coverages of Au and K were 0.2 and 0.1 ML, respectively.



TOC Graphic


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TiO2(110) for CO Oxidation: Alkali Enhanced

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