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On the Nature of Interface Confinement Effect in Oxide/Metal Catalysts Yanxiao Ning, Mingming Wei, Liang Yu, Fan Yang, Rui Chang, Zhi Liu, Qiang Fu, and Xinhe Bao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09498 • Publication Date (Web): 13 Nov 2015 Downloaded from http://pubs.acs.org on November 14, 2015

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The Journal of Physical Chemistry

On the Nature of Interface Confinement Effect in Oxide/Metal Catalysts †

†

Yanxiao Ning,1, Mingming Wei,1, Liang Yu,1 Fan Yang,1 Rui Chang,2 Zhi Liu,2 Qiang Fu,1,* Xinhe Bao1,* 1

State Key Laboratory of Catalysis, iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P.R. China

2

Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA E-mail address: [email protected] (Qiang Fu), [email protected] (Xinhe Bao)

Abstract: Metastable oxide phases containing coordinatively unsaturated metal sites are highly active in many catalytic reactions. The stabilization of these nanostructures during reactions remains a major challenge. Here, we show that metastable two-dimensional (2D) FeO structures can be grown on Pt(111) and Au(111), but not on the graphene surface. The well-defined 2D structure is achieved by an interface confinement effect originated from the strong interfacial bonding between Fe atoms and substrate surface atoms. The stabilization effect has been described by the interface confinement energy (Econfinement), which is the energy difference lowered by interfacing the 2D structure with a substrate and decreases in the sequence of Pt(111) > Au(111) > graphene. This interface effect is widely present in many metal-oxide composite catalysts, and can be used to guide the rational design of catalytically active sites. † Both contribute equally to this work.

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1. INTRODUCTION Heterogeneous catalysts are often metal-oxide composites, which consist of metal nanostructures supported on oxides or decorated with oxide coatings. To understand the catalytic mechanisms and optimize the catalytic performance, extensive research efforts have been devoted to studying the metal-oxide composite catalysts. Despite the complexity of above systems, a few consensuses have been reached. For instance, the interaction between metal and oxide plays a critical role in the catalytic properties of supported metal catalysts.1-6 A classic example is that hydrogenation capabilities of noble metals supported on reducible oxides, such as iron oxides, are often determined by the reduction temperature of catalyst pretreatments.2 This phenomenon was termed as the strong metal-support interaction (SMSI) by Tauster et al. in the late 1970s, and the concept of SMSI has been successfully used in many metal/oxide catalysts.1 Related to the studies of SMSI, defects at oxide surfaces have been found to play an essential role in the catalytic chemistry of oxides, as well as their interaction with supported metal particles.7-9 Surface defects of metal oxides, mainly oxygen vacancies, form upon reduction treatments, in which the surface metal sites are coordinatively unsaturated (CUS). These CUS sites or the ensembles consisting of the CUS oxide sites and the neighboring metal atoms are usually catalytically active sites, whose understanding is of the general importance to the nature of catalysis.10-12 The role of oxide defects in heterogeneous catalysis has been extensively investigated in the past decades. For example, Freund and coworkers have done systematic studies in the structure-reactivity relationship of the oxide ultrathin films.13 They found that oxygen vacancies form on an O-Fe-O trilayer (FeO2) grown on Pt(111) with low formation energy, which are easily replenished via the dissociation of O2 molecules. The FeO2/Pt(111) surface can catalyze CO oxidation with high efficiency through the Mars-van Krevelen mechanism above 400 K.14,15 Our previous works have shown that an O-Fe bilayer (FeO) consisting of an oxygen-terminated layer and an interfacial Fe layer can be prepared on Pt(111). Below monolayer coverage, FeO islands on Pt(111) present Fe-terminated edges (FeO1-x/Pt(111)), which have only two neighboring oxygen atoms and are under coordinated.16,17 The island 2


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edges, like oxygen vacancy sites, are highly active to O2 activation and can produce atomic oxygen species, which further react with CO adsorbed on Pt surface sites nearby through the Langmuir-Hinshelwood mechanism. The FeO/Pt catalysts can selectively remove CO in excess of H2 at relatively low temperatures, e.g. 250 K.18,19 A key factor for the FeO/Pt(111) surface to catalyze the low temperature CO oxidation reaction is that the ferrous oxide (FeO) phase has not been transformed into the inactive ferric oxide (Fe2O3) phase in the presence of O2. Surface science measurements and density functional theory (DFT) calculations reveal that the stabilization of the monolayer FeO nanostructures relies on the strong bonding between Pt and Fe atoms at the FeO/Pt interface, in which extensive orbital hybridizations between Pt and Fe atoms occur.18 The strong interaction of FeO islands with Pt(111) induced by the interfacial Fe-Pt bonding was defined as the interface confinement effect, which has been applied to a number of oxide/metal inverse catalysts.16 In the present work, using both surface science experiments and DFT calculations we compared the different interface effects in the FeOx nanostructures on Pt(111), Au(111), and graphene surfaces. A general principle governing the interface confinement effect has been derived, which can facilitate our design of highly active FeO nanostructures stabilized on non-noble metals.

2. EXPERIMENTAL METHODS Parts of the experiments were performed in an Omicron multi-probe ultrahigh vacuum (UHV) system with a base pressure of 2×10-10 mbar. The system has three main chambers containing a hemispherical energy analyzer (Omicron EA 125 5-channeltron) for X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS), a variable-temperature scanning tunneling microscopy (STM), and molecular beam epitaxy facility. Au(111) and Pt(111) single crystals were cleaned by Ar+ sputtering (1.5 keV) at room temperature followed by oxidation in 1.3×10-6 mbar O2 at 700 K, and a final UHV annealing at high temperature (850 K for Au(111) and 1080 K for Pt(111)). The surface quality and cleanness were checked by XPS and STM. The sample temperatures were measured 3



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by a type K thermocouple mounted on the backside of samples. Graphene overlayers on 6H-SiC(0001) was prepared by annealing n-type SiC(0001) wafer to 1500 K under UHV conditions. Fe was evaporated via resistive heating a tungsten filament wrapped with an Fe wire (0.1 mm diameter, purity of 99.998%). The deposition rate is about 0.025 monolayer (ML)/min. Here, 1 ML was determined by the amount of FeO fully covering the Pt(111) surface. The gas of O2 (purity of 99.999%) was dosed onto the substrate surfaces by backfilling the vacuum system using a leak valve. XPS data were collected using a Mg Kα X-ray source (1253.6 eV) and the binding energy values were calibrated with Pt 4f7/2 peak at 71.0 eV from the clean Pt(111) surface. The high resolution STM images were acquired in a Createc low-temperature STM system, which has been installed with a cryostat for LT-STM and an energy analyzer for XPS/UPS measurements. Fe evaporation was carried out using an effusion cell containing Fe slugs with the diameters of 3.175 mm and length of 3.175 mm (99.995%). STM measurements were performed at liquid nitrogen temperature in constant current mode using an electrochemically etched W tip. Near ambient pressure XPS (NAP-XPS) measurements were performed at beamline 9.3.2 at the Advanced Light Source, Berkeley. The FeO nanostructures were prepared by the similar recipe as we did in the other two systems. FeO coverage was calibrated by titration of CO on the surfaces at room temperature. XPS Fe 2p spectra were recorded in situ using a photon energies of 830 eV and Scienta 4000R-Hipp analyzer. We used the Vienna Ab-initio Simulation Package (VASP)20 for the DFT calculations. The projector augmented-wave pseudo potentials and a cutoff energy of 400 eV for the plane-wave basis set were adopted. The generalized gradient approximation (GGA) method with Perdew-Burke-Ernzerhof (PBE) functionals for the exchange-correlation term was used.21 The Monkhorst-Pack scheme was used for sampling the Brillouin zone. A triangular model of Fe10O6 nanoisland supported on a periodic Pt(111) or Au(111) slab surface is used. The vacuum thickness between the slabs was set as 11 Å. Spin polarization was considered throughout the calculations. 4


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3. RESULTS AND DISCUSSION Growth of FeOx nanostructures on various substrates. 0.5 ML FeOx nanostructures were grown on various substrates by evaporation of Fe in O2 atmospheres at room temperature followed with annealing in UHV. STM images acquired from the surfaces were displayed in Figure 1a-c. The FeOx/Pt(111) surfaces prepared in 1.3 × 10-7 and 1.3 × 10-6 mbar O2 present the similar surface structure, which consist of 2D nanoislands having a surface lattice of 3.1 Å (inset of Figure 1a) and a Moiré pattern with the periodicity of 25 Å. These nanostructures can be ascribed to single-layer FeO structure terminated with surface oxygen layer.17,22 The monolayer FeOx nanoislands have been obtained on Au(111) when grown in 1.3 × 10-7 mbar O2 (Figure 1b), which present a surface lattice of 3.4 ± 0.1 Å (inset of Figure 1b). The similar FeOx nanostructures were identified by Khan et al. as to be single-layer FeO.23 The FeOx overlayer grown on Au(111) in 1.3 × 10-6 mbar O2 contains a higher density of nanoislands with various heights (Figure 1c). The line profile in Figure 1d shows that the lowest islands are 1.3 Å high, comparable to the single-layer FeO, and others have the heights of 3.8, 6.5, and 8.0 Å, respectively. It has been previously demonstrated that growth of Fe oxide on graphitic carbon surfaces under the similar conditions always produces 3D Fe2O3 nanoislands.17,24 The present STM image from Fe oxide deposited on the graphene surface also indicates the island structure (Figure S1). XPS Fe 2p spectra of the 0.5 ML FeOx overlayers supported on Pt(111), Au(111), and graphene surfaces were shown in Figure 1e and Figure S2. On the Pt(111) surface, the formed FeOx overlayers always have the Fe 2p3/2 binding energies (BEs) at 709.3 eV, characteristic for Fe2+ species.17 In 1.3 × 10-7 mbar O2 FeO can form on Au(111) as well, while a BE of 710.6 eV was observed for FeOx grown in 1.3 × 10-6 mbar on Au(111) which is close to that of Fe3+ species in Fe3O4.25 For FeOx nanostructures grown on the graphene/SiC(0001) surface Fe 2p3/2 peaks were always located at 711.0 eV, which is from Fe2O3 structure. Accordingly, the FeOx nanostructures grown on the various supports can be 5



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illustrated by the schemes in Figure 1e. The FeO single-layer formed on Pt(111) in both 1.3 × 10-7 and 1.3 × 10-6 mbar O2. The same FeO structure was grown on Au(111) in 1.3 × 10-7 mbar O2, while planar Fe3O4 nanocrystals were obtained in 1.3 × 10-6 mbar O2. 3D Fe2O3 structure was observed on the graphene surface under the same growth conditions. Overall, the Pt(111) surface exerts the strongest effect on FeOx growth, which is favoring the formation of the metastable FeO monolayer structures. The carbon surface plays the least role in the FeOx growth, and the most stable Fe2O3 islands form on the surface.

Figure 1 Growth of FeOx nanostructures on various substrates. a-c, STM images (86 nm × 86 nm) of 0.5 ML FeOx nanostructures grown on Pt(111) in 1.3 × 10-7 mbar O2 (a, Vs = 0.3 V, I = 0.1 nA), on Au(111) in 1.3 × 10-7 mbar O2 (b, Vs = 0.3 V, I = 0.1 nA), and on Au(111) in 1.3 × 10-6 mbar O2 (c, Vs = 0.2 V, I = 0.1 nA). Insets in (a) (Vs = 0.02 V, I = 3.0 nA) and (b) (Vs = -0.04 V, I = 1.5 nA) are the atomic resolution images of FeO islands (7.3 nm × 7.3 nm). d, Scanning profiles along the lines shown in (a-c), respectively. e, XPS Fe 2p3/2 peak positions of the FeOx nanostructures formed on various substrates at different oxygen atmosphere. Insets are the corresponding schemes. Red: Fe; orange: O; dark blue: Pt; yellow: Au; purple: Si: 6


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grey: C. Stability of FeO nanostructures supported on Au(111) and Pt(111). To understand the effect of the metal substrate on the interaction between FeO and metal, 0.5 ML FeO/Pt(111) and 0.5 ML FeO/Au(111) surfaces, which were prepared by evaporation of Fe in 1.3 × 10-7 mbar O2 at room temperature, were annealed in various O2 atmospheres and temperatures which were investigated by STM and XPS (Figure 2 and Figure S3). Both surfaces were exposed to 1.3 × 10-6 mbar O2 at elevated temperatures. At room temperature, Fe 2p3/2 binding energies are located at 709.3 eV, which combined with STM images confirm the presence of monolayer FeO nanoislands on two substrates. Upon oxidation of the FeO/Pt(111) surface at 473 K the Fe 2p peak starts to shift to a higher binding energy position, and the main peak gets stabilized at 709.8 eV when oxidizing at 573 K (Figure 2a). STM image of the oxidized surface demonstrates that the surface nanoislands only get big but still present monolayer morphology with an identical height of 1.9 Å (Figure 2b, Figure S4). These results indicate that the oxidation treatment has induced a structural transformation from FeO to FeO2.15,26 On the Au(111) surface, oxidation at 573 K has already induced a large shift of the Fe 2p3/2 binding energy to 710.9 eV, and STM image of the surface shows that FeOx nanoislands have the heights of 3.5, 6.0, and 8.0 Å (Figure 2c, Figure S4). Both XPS and STM data confirm that that FeO nanoislands on Au(111) have been oxidized to Fe3O4 nanoislands in 1.3 × 10-6 mbar O2 at 573 K.27 We further studied structural transformation of FeO on two substrates in higher O2 pressure at room temperature, using in-situ NAP-XPS at Advanced Light Source, Berkeley.28 As shown in Figure 2d and Figure S5, the as-prepared FeO/Pt(111) and FeO/Au(111) surfaces have the same Fe 2p3/2 peak position at 709.3 eV in 1.3 × 10-6 mbar O2. When the O2 partial pressure increases up to 1.3 × 10-4 mbar, the Fe 2p3/2 peak of the FeO/Au(111) surface has shifted by 1.5 eV, reaching to 710.8 eV, which is characteristic for Fe3O4.27 On the FeO/Pt(111) surface the chemical state of surface Fe species does not change till the partial pressure of O2 was raised to 1.3 mbar, which is four orders of magnitude higher than that on the Au(111) surface. Meanwhile, the BE 7



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shift is only 1.0 eV, approaching that of FeO2 structure.26 Based on the above results, we conclude that the metastable FeO nanoislands present higher stability on Pt(111) than that on Au(111), which indicates that the Pt(111) surface exerts much stronger interface effect on the FeO nanostructures than Au(111).

Figure 2. Stability of FeO nanostructures on Au(111) and Pt(111). a) Evolution of Fe 2p3/2 BEs with the oxidation temperature and the Fe 2p spectra were given in Figure S3. The oxidation experiments were performed by exposing the 0.5 ML FeOx overlayers on Pt(111) and Au(111) to 1.3 × 10-6 mbar O2 at various temperatures. b, c) STM images of oxidized FeO nanoislands on Pt(111) (b, 130 nm × 130 nm) and Au(111) (c, 86 nm × 86 nm) after treatment in 1.3 × 10-6 mbar O2 at 573 K. The line profiles are shown in Figure S4. d) Evolution of Fe 2p3/2 BEs with the oxygen partial pressures measured by NAP-XPS at room temperature. The Fe 2p spectra were displayed in Figure S5. Effect of metal support on the stability of FeO nanostructures. As discussed above, the strong interaction between surface oxides and the metal substrate, i.e. the interface effect between the oxide and the metal, originates from the interfacial metal-metal binding, such as Fe-Pt binding at FeO/Pt(111) surface and Fe-Au bonding 8


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at FeO/Au(111) surface. Using a Fe10O6 model we calculated the binding energy (Eb) of Fe10O6 on the Pt(111) and Au(111) surfaces (Figure S6). Eb on Pt(111) is 0.53 eV lower than that on Au(111) (Figure 3). Furthermore, oxidation of the metastable FeO structure into the more stable high-valence iron oxide was also calculated. The reaction formula were as follows: Fe10O6/Pt(111) + 6O2 → Fe10O18/Pt(111)

(1)

Fe10O6/Au(111) + 6O2 → Fe10O18/Au(111) (2)

Figure 3. DFT calculations of the FeO oxidation on Au(111) and Pt(111) surfaces. a) Fe10O6 (top) and Fe10O18 (bottom) structures on Pt(111) surface. b) Energy scheme for structural transformation of metastable Fe10O6 structure to the stable Fe10O18 structure on Au(111) and Pt(111) surfaces, Fe10O6 + 6O2 → Fe10O18. The metal surface exerts a confinement effect on the surface oxide lowering the energy of Fe10O18, which is termed as interface confinement effect (Econfinement). The calculation results show that reaction energy (∆G) for reaction (1) is -1.06 eV and -1.44 eV for the reaction (2), whose difference is 0.38 eV (Figure 3). The energy barrier for the oxidation of FeOx nanostructures can be qualitatively estimated using the Brønsted-Evans-Polanyi (BEP) relation,29,30 in which a reaction activation energy is often linearly dependent on the reaction energy. Accordingly, the energy 9



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barrier for reaction (1) is expected to be larger than the reaction (2), and it can be inferred that the low-valence iron oxide presents a higher stability on Pt(111) than Au(111). The decrease in total energy due to the oxide-metal interaction helps to stabilize the metastable oxide phase, which can be termed as the interface confinement energy (Econfinement). In many circumstances this energy can be used as the descriptor to indicate the stability of an oxide supported on a metal. It is well known that reducible metal oxides containing CUS metal sites are very reactive for molecule adsorption and catalytic reactions. However, such metastable and active structures are difficult to maintain in reactions, particularly in the oxidative reaction environments. Taking FeO nanoislands as an example, the high activity originates from the coordinatively unsaturated Fe sites at the island edges and the active ensemble is TM-Fe-O (TM = transition metals other than Fe).31 In this structure, the TM-Fe bonding is critical to the stability of the active ensemble. For the weak TM-Fe interaction atomic oxygen species may break the TM-Fe bonds and insert between the TM and Fe atoms forming the TM-O-Fe-O structures. Consequently, the coordinative unsaturation of the edge Fe atoms may not be present, which finally deteriorate the catalytic activity.26 Based on this consideration, we suggest that the interface confinement effect is determined by the competition between TM-Fe bonding and TM-O bonding. That is, the stronger TM-Fe bonding, the higher possibility to have the strong interface confinement effect. Vice versa, the strong TM-O bonding results in the weak interface confinement effect and it is difficult to keep the active FeO structure in reactions.

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Figure 4. Interface confinement effect in FeOx/TM inverse systems. Influence of the enthalpy of formation of Fe-TM binary compounds32 and the oxygen affinity of metals33 on the interface effect is illustrated. In the left-upper corner region the metals tend to form strong bonding with Fe and bond weakly with oxygen, in which the strong interface confinement between FeO and metal is expected. In contrast, the right lower corner contains those metals having weak interface confinement effect on FeO. The dashed line depicts the borderline between the strong and weak interface confinement effects. Therefore, strong TM-Fe bonding but weak TM-O binding are the prerequisite conditions to have a strong interface confinement effect. Noble metals such as Pt meet this criterion and it has been shown that the Pt surface exerts strong confinement on the FeO structure. Guided by this principle the metals other than Pt which can stabilize the FeO active structure may be predicted. As a rule of thumb, the enthalpy of formation of Fe-TM alloy can be used to characterize the bonding strength of the metal with Fe,32 and the oxygen affinity characterizes the bonding strength of oxygen with the metal33. Figure 4 compile the enthalpy of formation of Fe-TM binary compounds and the oxygen affinity of most transition metals. We suggest that metals in the left upper corner of the Figure 4 should have strong interface confinement 11



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effect on the FeO structure, which has been confirmed by many previous works.23,34-36 On the other hand, it is expected that all metals in the right lower corner cannot exert the strong interface confinement effect on the FeO formation and the FeO/TM structures are not stable. Using the same principle, the interface confinement effect on other TM oxide structures can be predicted, and the similar argument might be applicable to other oxides, such as cobalt oxide (CoOx) and cerium oxide (CeOx) (Figure S7).

4. CONCLUSIONS In summary, the comparative studies in the growth and stability of metastable FeO nanoislands on Pt(111), Au(111), and graphene surfaces allow us to conclude that the interfacial bonding between Fe atoms and substrate surface atoms is controlling the interface confinement effect, which stabilizes the metastable Fe oxide phase and CUS Fe active sites in a wide range of temperature and O2 pressure. The stabilization effect has been described by the interface confinement energy (Econfinement), which decreases in the sequence of Pt(111) > Au(111) > graphene. This interface confinement effect may be widely present in many metal-oxide composite catalysts, and understanding the nature of this interface effect would facilitate the rational design of active sites and the search for the replacement of noble metal catalysts.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Qiang Fu). Telephone: +86-411-84379253. * E-mail: [email protected] (Xinhe Bao). Telephone: +86-411-84379128. Author Contributions Q. F. and X. B. conceived and supervised the project. Y. N. and M. W. performed XPS and STM experiments. L. Y. carried out DFT the calculations. Q. F., F. Y., R. C., and Z. L. performed the AP-XPS experiments. Y. N., Q. F., and X. B. wrote the paper and all authors discussed and revised the final manuscript. Notes 12


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The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21222305 and 21103171), Ministry of Science and Technology of China (No. 2013CB834603, 2013CB933100, and 2011CBA00503), and the Key Research Programme of the Chinese Academy of Science (Grant No. KGZD-EW-T05). The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

ASSOCIATED CONTENT Supporting Information The Supporting information is available free of charge on the ACS Publications website at DOI: STM image of FeOx nanostructures grown on graphene/SiC(0001), XPS Fe 2p spectra of FeOx on various substrates and oxidation conditions, DFT calculation of bonding and oxidation of Fe10O6 on metals.

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Gustafson, J.; Lundgren, E., Growth of Ultra-thin Iron Oxide Films on Ag (100), J. Phys. Chem. C, 2015, 119, 2572–2582.

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