Inverse Catalysts for CO Oxidation: Enhanced OxideâMetal

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Inverse Catalysts for CO Oxidation: Enhanced Oxide-Metal Interactions in MgO/Au(111), CeO2/Au(111) and TiO2/Au(111) Robert M. Palomino, Ramon A Gutierrez, Zongyuan Liu, Samuel Tenney, David Grinter, Ethan J. Crumlin, Iradwikanari Waluyo, Pedro J Ramirez, Jose A. Rodriguez, and Sanjaya D. Senanayake ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02744 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 27, 2017

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ACS Sustainable Chemistry & Engineering

Inverse Catalysts for CO Oxidation: Enhanced Oxide-Metal Interactions in MgO/Au(111), CeO2/Au(111) and TiO2/Au(111) Robert M. Palomino,1 Ramón A. Gutiérrez,2 Zongyuan Liu,1 Samuel Tenney,3 David C. Grinter, 1

Ethan Crumlin,4 Iradwikanari Waluyo,5 Pedro J. Ramírez,2,++ José A. Rodriguez,1* and Sanjaya D. Senanayake 1*

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Chemistry Department, Brookhaven National Laboratory, Upton, NY, 11973, United States 2 3

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Facultad de Ciencias, Universidad Central de Venezuela, Caracas 1020-A, Venezuela

Department of Chemistry, University of North Texas, Denton, TX, 76203, United States

Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, United States 5

National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, NY, 11973, United States

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Current address: Zoneca-CENEX, R&D Laboratories, Alta Vista, 64770 Monterrey, Mexico.

*Corresponding Authors: [email protected] , [email protected]

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ACS Paragon Plus Environment


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ABSTRACT: Au(111) does not bind CO and O2 well. The deposition of small nanoparticles of MgO, CeO2 and TiO2 on Au(111) produces excellent catalysts for CO oxidation at room temperature. In an inverse oxide/metal configuration there is an strong enhancement of the oxide-metal interactions and the inverse catalysts are more active than conventional Au/MgO(001), Au/CeO2(111) and Au/TiO2(110) catalysts. An identical trend was seen after comparing the CO oxidation activity of TiO2/Au and Au/TiO2 powder catalysts. In the model systems, the activity increased following the sequence: MgO/Au(111) < CeO2/Au(111) < TiO2/Au(111). Ambient pressure X-ray Photoelectron Spectroscopy (AP-XPS) was used to elucidate the role of the titania-gold interface in inverse TiO2/Au(111) model catalysts during CO oxidation. Stable surface intermediates such as CO(ads), CO32-(ads) and OH(ads) were identified under reaction conditions. CO32-(ads)

and OH(ads) behaved as spectators. The

concentration of CO(ad) initially increased and then decreased with increasing TiO2 coverage, demonstrating a clear role of the Ti-Au interface and the size of the TiO2 nanostructures in the catalytic process. Overall our results show an enhancement in the strength of the oxide-metal interactions when working with inverse oxide/metal configurations, a phenomenon that can be utilized for the design of efficient catalysts useful for green and sustainable chemistry. KEYWORDS: Low-temperature CO oxidation, Gold, Titania, Ceria, Magnesium oxide, Ambient pressure X-ray Photoelectron Spectroscopy, Inverse oxide/metal catalysts

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INTRODUCTION In our modern world, catalysis plays a significant role in the production of more than 80% of the chemicals and materials used in industrial operations. Heterogeneous catalysts are frequently used in proceses aimed towards green and sustainable chemistry. Most heterogeneous catalysts contain a combination of metals and oxides.1,2,3

They are usually prepared by

dispersing a small amount of metal on an oxide support.1,3 In these systems, the oxide phase can act as a simple template for the scattering of the metal phase or it can be a direct participant in the catalytic process.2 When the oxide is involved in the catalysis, there is a motivation to revamp the traditional configuration of industrial catalysts to exploit the intrinsic properties of metal oxides and obtain a superior performance.4,5,6,7 Due to their limited size and high density of defects, oxide nanoparticles can have special electronic and chemical properties.8 Catalysts generated by the deposition of oxide particles on a metal substrate have been studied for a long time in fundamental studies.9,10 Recent studies using high-resolution transmission electron microscopy (HRTEM) have observed overlayers of ZnO on top of the Cu particles present in copper/zinc oxide catalysts used for the industrial synthesis of CH3OH.11,12 In these industrial catalysts, the active phase probably has an inverse oxide/metal configuration.11,12 The same is valid in Cu/MoOx, Rh/TiO2, Pt/CeOx and Pt/TiO2 catalysts usually activated by pre-treatment (reduction) in hydrogen.9,11,12,13,14,15

Oxides that have a low surface free energy exhibit a

tendency to cover metals upon partial reduction. Thus, an oxide/metal configuration maybe more common in industrial heterogeneous catalysts than expected. In the area of green chemistry and the control of environmental pollution, inverse oxide/metal catalysts have shown an excellent performance for the oxidation of CO,7,10,16,17,18,19,20,21 the conversion of CO2 into alcohols,22,23,24,25 the production of hydrogen 3



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through the water-gas shift or the photocatalytic splitting of water,26,27 and the reduction of nitrogen oxides (DeNOx).5 They can help to minimize the impact of chemical processes on the environment and their efficiency and use of non-expensive oxide components are ideal for sustainable chemistry. Furthermore, inverse oxide/metal catalysts offer a convenient way to study the effects of the metal-oxide interface on the mechanism of many reactions important for the chemical industry.23-27 Are properties of the metal-oxide interface similar in inverse oxide/metal and conventional metal/oxide catalysts? This is an important issue for fundamental studies and practical applications. In this article we compare the performance for CO oxidation of catalysts in inverse and conventional configurations which combine Au with TiO2, CeO2 and MgO. Au(111) does not catalyze the oxidation of CO but becomes catalytically active upon the addition of oxide nanoparticles. Bulk TiO2, CeO2 and MgO also do not catalyze the oxidation of CO at low temperatures (< 400 K) but Au-TiO2, Au-CeO2 and Au-MgO interfaces do.28,29,30,31 The growth of titania nanoparticles on Au(111) has been studied extensively via scanning probe microscopies (STM, AFM).32,33,34,35,36,37,38 TiO2 grows on Au(111) forming twodimensional (2D) islands at low coverage with three-dimensional (3D) crystallites, in the rutile or anatase phases, appearing at high coverages.32-35 Inverse oxide/metal catalysts have been generated by depositing ceria particles on Au(111).26,39,40 Rough particles of ceria are common but the formation of well-ordered structures with a (111) fluorite termination has also been observed depending on the preparation mode.26,39,40 Compact islands of MgO with a (001) lattice configuration have been seen with STM on Au(111) upon the deposition of Mg under high oxygen pressures.41,42 Our studies show that the catalytic activity of these systems for CO oxidation increases following the sequence: MgO/Au(111) < CeO2/Au(111) < TiO2/Au(111). They all exhibit a reactivity larger than that of conventional metal/oxide catalysts. This is

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probably a consequence of strong interactions at the oxide-metal interface (SOMIs). To gain insight into this phenomenon, ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) is used to study in detail the oxidation of CO on TiO2/Au(111) surfaces. EXPERIMENTAL SECTION Studies with single-crystal surfaces. The activity of the single-crystal catalysts systems was studied in a set-up that combines a ultra-high Vacuum (UHV) chamber for surface characterization (base pressure ~5×10-10 Torr) and a batch reactor for catalytic tests.23,27,31,43 The sample could be transferred between the reactor and the UHV chamber without exposure to air. The UHV chamber was equipped with instrumentation for XPS, ultraviolet photoelectron spectroscopy (UPS), low-energy electron diffraction (LEED), ion-scattering spectroscopy (ISS), and thermal-desorption mass spectroscopy (TDS).23,27,31,43 In the studies of CO oxidation, the sample was transferred to the reactor at ~ 300 K, then the reactant gases, 4 Torr of CO and 2 Torr of O2, were introduced.31 Product yields were analyzed by a mass spectrometer and/or a gas chromatograph.30,31 The amount of CO2 molecules produced in the catalytic tests was normalized by the active area exposed by the sample and the total reaction time.31 The kinetic experiments were done in the limit of low conversion (< 5%). The ambient pressure XPS experiments were performed at beamline 9.3.2 of the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. XPS analysis was performed using a VG Scienta R4000 HiPP analyzer and endstation with full surface preparation capabilities, details of which can be found elsewhere.44 The O 1s region was probed with a photon energy of 650 eV, and the C 1s, Au 4f, and Ti 2p regions with photon energy of 490 eV. The Au 4f7/2 photoemission line was used for binding energy calibration.

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The Au(111) single crystal was cleaned via Ar+-sputtering (2 x 10-5 Torr, 30 min) and annealing (823 K, 20 min) cycles until the C 1s spectrum was clean. The MgO/Au(111), CeO2/Au(111), and TiO2/Au(111) surfaces were prepared following procedures reported in the literature.37,39,42 The coverage of the oxide on the Au(111) surface was determined by means of ISS and/or XPS monitoring the attenuation of the corresponding signals for the gold substrate. Studies with high-surface area catalysts. Experiments for the oxidation of CO were also carried out using TiO2/Au and Au/TiO2 high-surface area powder catalysts. Using atomic layer deposition (ALD) small particles of TiO2 were deposited on unsupported Au nanocrystals with a size of about 20 nm.45 The details for the methodology followed in the ALD process are described elsewhere.7 After applying TiO2 ALD, 20-30% of the surface in the Au nanocrystals was covered by titania. TiO2 islands (1.5 to 2 nm in size) grew on corner and defect sites of the Au nanocrystals.7 The Au/TiO2 powder catalysts were prepared by gold deposition-precipitation (DP) on an anatase titania support.31,46 Loadings of 1 and 2 at. % of Au were dispersed on the titania. This led to catalysts with average Au particle sizes of 1.8 ± 0.6 nm and 3.1 ± 0.5 nm, respectively, as measured by TEM. The catalytic tests for CO oxidation were performed in a flow reactor (gas feed: 1%CO / 1%O2 / 98%N2, 20 mL/min, 100,000 h-1) under a steady-state mode between 225 and 500 K. RESULTS AND DISCUSSION Catalytic activity for CO oxidation of inverse oxide/gold catalysts. Au(111) is not active for the oxidation of CO. It binds CO weakly and does not dissociate the O2 molecule in an effective way.47 Figure 1 shows data for the catalytic activity of a gold surface pre-covered with different amounts of MgO, CeO2 and TiO2. All the oxides enhance the rate of CO oxidation. In a

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first stage, the catalytic activity increases with the addition of the oxide reaching a maximum when the gold surface is pre-covered 20-30% by MgO, CeO2 or TiO2. After that point, one sees a drop in the catalytic activity. The maximum catalytic activity is seen with small oxide nanoparticles and a large oxide-metal perimeter. Comparing the relative activities of the three oxides, one finds that nanoparticles of MgO substantially enhance the catalytic activity of Au(111), but the effects of CeO2 or TiO2 are much larger. In the case of TiO2/Au(111) the catalytic activity is higher than that of MgO/Au(111) by approximately a factor of two. Post-reaction characterization with XPS showed gold in a metallic state and no clear signs for the reduction of MgO, CeO2 or TiO2. In the case of the Ce4+ and Ti4+ cations, reduction to Ce3+ and Ti3+/Ti2+ was possible but it was not seen. In the C 1s region, Figure 2, peaks were found that denote the presence of adsorbed carbonates and atomic carbon.27 The reaction of CO with O2 can produce carbonates on oxide surfaces.17,48,49 In general, these carbonates can be seen as spectators or poisons of the reaction.48,49 The atomic C seen on the oxide/metal surfaces after CO oxidation has not been detected in experiments for conventional metal/oxide catalysts.31,48,49 It has also been detected in experiments for CO oxidation on Fe2O3/Au(111) surfaces.16,20 It is probably a consequence of the high reactivity of the oxide nanoparticles present in the catalysts. On the corner and edges of these oxide nanoparticles one could have full dissociation of CO or the Boudouard reaction: 2CO → C + CO2. The deposition of C may block some of the active sites at the oxide-metal interface, but all the systems in Figure 1 exhibit high catalytic activity. Figure 3 compares the CO oxidation activity of oxide/metal and conventional metal/oxide catalysts under the same reaction conditions.31,50 For the three oxide-metal interfaces investigated, the inverse oxide/metal configuration always displays the best performance. The 7



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participation of the oxide in the catalysis increases when going from a bulk material to nanostructures and titania is the oxide most affected. We also studied the oxidation of CO on TiO2/Au and Au/TiO2 powders. Figure 4 compares their performances. The behavior seen here for the conventional Au/TiO2 catalysts compares well with that reported in previous studies.51,52 The Au/TiO2 catalyst with the best performance contained a small loading of 1 at. % of Au. An increase to 2 at. % of Au led to a decrease in catalytic activity. Results of TEM indicate that the drop in the catalytic activity correlates with an increase in the average Au particle size from 1.8 ± 0.6 nm to 3.1 ± 0.5 nm. A trend that also agrees with the results of previous studies.52 In any case, it is clear in Figure 4 that the inverse TiO2/Au catalyst is active at lower temperatures than the Au/TiO2 catalysts. This agrees with the results in Figure 3 for the model systems. In an oxide/metal arrangement, the metal-oxide interactions are stronger than in a metal/oxide arrangement due to the very low reactivity of bulk oxides where even the simple wetting of many metals is a problem.53,54 As mentioned above, the strong bonding associated with oxide/metal configurations can induce perturbations or modifications in the electronic properties of the oxide which eventually lead to novel chemical properties.55,56,57 In the next section, AP-XPS is used to study the chemistry for CO oxidation on TiO2/Au(111) surfaces. The oxide-metal interface and the mechanism for CO oxidation on inverse TiO2/Au(111) catalysts: AP-XPS studies. Figure 5 shows Ti 2p and O 1s spectra for TiO2/Au(111) surfaces where the fraction of the Au substrate covered by the oxide was 0.04, 0.25 and 0.4. In these systems the Au 4f peaks remained at the positions seen for metallic Au in clean Au(111). Figure 5A reveals that the as-deposited TiO2 is highly reduced (30-66 % of Ti3+ + Ti2+) as is evidenced by the appearance of significant amounts of Ti3+ and Ti2+, characterized by lower binding energy 8


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features.34,37 The ratio of TiO2 species changes with coverage: A 0.04 coverage has an approximate Ti2+:Ti3+:Ti4+ ratio of 1:1:1, a 0.25 coverage has 1:3:7, and a 0.4 coverage has 1:5:14. Thus, a reduction in the size of the TiO2 particles increases their reducibility. The O 1s spectra for each TiO2 coverage exhibits a shoulder at ~532 eV that is indicative of hydroxyls on the TiO2 surface58 or oxygen spillover on the Au(111) surface near the TiO2 islands38 whose intensity increases proportionally with TiO2 coverage. Figure 6 displays Ti 2p spectra for each TiO2 coverage on Au(111) under different reaction conditions: 100 mTorr of CO at 300 K (panel A), and 100 mTorr of CO + 100 mTorr of O2 at 300 K (panel B) or 400 K (panel C). Upon exposure to 100 mTorr of CO, Ti is relatively unperturbed in comparison to the freshly deposited films, except for the 0.04 TiO2 coverage that underwent a mild oxidation. Upon the addition of 100 mTorr O2 at 300 K, several features change/appear. The Ti 2p spectra shown in Figure 6B indicate that the addition of O2 oxidizes the TiO2 as the shoulder at 457 eV is reduced to ~¼ of its previous intensity. The ability to dissociate O-O bonds and heal O vacancies is thus clear, mirroring the behavior on the surface of bulk TiO2-x(110).59 Annealing to 400 K (Figure 6C) results in a near complete oxidation to Ti4+ for all TiO2 coverages studied, no Ti2+ remains and only a small fraction of Ti3+ exists. Figure 7 shows the evolution of the oxidation state of Ti in the clean or as-prepared TiOx/Au(111) systems and after exposure to CO or a CO+O2 mixture. When comparing the Ti 2p spectra for each condition, it becomes clear that the oxidation state of the Ti atoms in the titania deposits was changing under the different reaction conditions. The fact that Ti is almost completely oxidized during CO oxidation conditions indicates the relative ease of oxidation by O2 compared to the reducing power of CO. Additionally, the interaction of CO with all TiO2 films did not result in a reduction of TiO2, but instead an oxidation occurred. CO oxidation on 9



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conventional metal/oxide catalysts is thought to proceed via one of two main reaction mechanisms: Associative and Mars-van Krevelen.60,61 In the associative mechanism, molecularly bound oxygen adsorbed on the support can oxidize CO bound at or near the supported metal to yield CO2. Conversely, in the Mars-van Krevelen mechanism, the weakly bound oxygen from the support oxidizes CO and O2 heals the created vacancies created in the support.

This

mechanism is most easily observed by the reduction of the oxide film in the presence of CO following CO2 formation that is later deactivated by the lack of weakly bound oxygen species until the surface is re-oxidized by the addition of oxygen. The trends seen in Figure 7 for the inverse TiO2/Au(111) catalysts rule out the Mars-van Krevelen type mechanism and suggests that an associative mechanism is more likely. Previous studies have shown that CO oxidation on Au/TiO2(110) and TiO2/Au(111) goes much faster at 400 K than at 300 K.35 At 400 K, one is dealing with almost fully oxidized titania particles (Figure 6C). At the TiO2-Au interface, there is fast dissociation of O2 and reaction with CO. The C 1s spectra under reaction conditions (Figure 8) reveal a number of surface bound species. Under 100 mTorr of CO at 300 K (Figure 8A), gas phase CO can be found at 291 eV and a surface bound CO peak is seen at 285 eV. Graphitic carbon (284 eV) is observed in all spectra, which is consistent with the XPS results in Figure 2. The Au(111) surfaces with 0.04 and 0.25 coverages of TiO2 display features near to 286 and 287.5 eV that can be attributed to adsorbed CO and the formation of carbonates.23,27,43 Initially these TiO2/Au(111) surfaces had a large concentration of Ti3+ and Ti2+ sites (see Figure 5A). The highly defective TiO2 in these systems bound the CO molecule and in some cases was able to dissociate it. The Au(111) surface pre-covered by 0.25 of TiO2 exhibited a very rich surface chemistry. CO was bound to the TiO2Au interface and reacted with atoms of the oxide to give a small amount of carbonate species. 10


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This rich chemistry disappeared when the titania coverage was increased to 0.4. As the size of the titania particles increases they adopt a more bulk-like structure with less defects or imperfections and their reactivity drops.34,37 One can see drastic changes upon the addition of oxygen (Figure 8B). The C1s spectrum for a coverage of 0.4 shows a peak assigned as adsorbed CO32- at 289 eV not seen when CO is the only reactant (Figure 8A). This assignment is based on a similar binding energy seen for carbonate species bound to CeO2 at 290.3 eV. 23,27,43 At the same time, there is adsorption of CO and deposition of C on the TiO2/Au(111) surface. Thus, the presence of O2 opens new reaction channels not detected when CO is the only reactant. Even more important: the surface chemistry changes depending of the coverage (and particle size) of the titania.34,37 The best TiO2/Au system for the chemisorption of CO is the one with a titania coverage of 0.25. An increase in the TiO2 coverage reduces the chemisorption of CO and favors carbonate formation, Figure 9. Indeed, after exposing bulk-like powders of TiO2 to atmospheric pressures of CO at moderate temperatures, one sees the formation of carbonates.7,29,30 At 400 K (Figure 8C) the relative amounts of carbonate are slightly larger. Again, one sees that the coverage of TiO2 (which affects the size of the oxide particles)34,37 has a strong impact on the type of species adsorbed on the surface. In Figures 8 and 9, the amount of CO(ad) increases significantly upon the introduction of TiO2, peaks at a titania coverage of 0.25, and then drops when more titania is added. Bonding of CO is occurring on an oxide-metal interface where the titania has special chemical properties due to its size. Green et. al. concluded that the enhanced CO oxidation activity seen from Au/TiO2 at 120 K was the result of mobile CO species bound to TiO2 that migrated to the interface between TiO2 and Au and reacted with the oxygen that was activated at the interface.29,30 While a similar correlation between CO oxidation activity to interface sites is observed on the inverse catalyst, 11



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the results reported here indicate that a special interface is probably responsible for binding CO, which differs from the conventional catalyst where CO is bound to bulk TiO2.29,30 One of the features observed in the C 1s XPS spectra of Figures 2 and 8 is a surface bound carbonate species, CO32-. These surface carbonates have previously been reported either as intermediates in CO oxidation or spectator species, which can readily cover the catalyst surface ultimately leading to deactivation.23,27,43 In Figure 1 the catalytic activity substantially decreases when going from a titania coverage of 0.25 to 0.4. However, in Figure 8 the amount of carbonate present at these two coverages is very similar. This suggests that the carbonate species is a spectator, but there is no conclusive evidence from this study to label it as a poison. Figure 10 shows the O 1s spectra for each film under: 100 mTorr of CO at 300 K (A), 100 mTorr of CO + 100 mTorr of O2 at 300 K (B) and 400 K (C). Under CO atmosphere (Figure 10A), no apparent shift in the binding energy of O from TiO2 is observed. Surface hydroxyls and adsorbed CO (CO(ad)) are almost indistinguishable from their binding energies,58,62 therefore the amount of CO(ad) cannot be estimated from the O 1s spectra. The peak at ~537.8 eV is attributed to gas phase CO. With the addition of O2 (Figure 10B), two important new features appear: Gas phase O2 peaks at 538.5 eV and 539.5 eV. The addition of O2 has a minor effect on the amount of OH groups present on the surface. At 400 K, the O 1s spectra (Figure 10C) point to a clear decrease in the amount of OH present on the surface with respect to the coverage seen at 300 K (Figure 10A,B). The adsorbed OH is probably a spectator since there is not drop in catalytic activity when raising the temperature (see Figure 4). The specific mechanism and active phase of CO oxidation over Au/TiO2 has remained under heavy debate even after more than two decades of research. A general consensus has been formed that the interface between Au and the metal oxide is responsible for the exceptional 12


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activity, since bulk Au and the metal oxides alone show little to no activity towards CO oxidation.7,18,29-31,49,51 While the importance of the interface is accepted, the role of the support and the specific nature of interfacial interactions are open issues. 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.28,60 This phenomenon is not occurring in the type of catalysts under study here since they contain bulk Au(111). Other authors argue that CO and O2 are adsorbed on the Au surface and the interface between Au and the metal oxide facilitates the reaction between CO and O2.63,64 Most recently, the interface was thought to be responsible for both the adsorption of reactants and the active sites for the oxidation of CO.29,30,65 Additionally, water and surface hydroxyls have been found crucial to low temperature CO oxidation (

Inverse Catalysts for CO Oxidation: Enhanced OxideâMetal

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