Oxygen Vacancy-Controlled Reactivity of Hydroxyls on an FeO(111

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Oxygen Vacancy-Controlled Reactivity of Hydroxyls on an FeO(111) Monolayer Film Lingshun Xu,†,‡,§ Wenhua Zhang,‡,|| Yulin Zhang,†,‡,§ Zongfang Wu,†,‡,§ Bohao Chen,†,‡,§ Zhiquan Jiang,† Yunsheng Ma,§ Jinlong Yang,*,†,§ and Weixin Huang*,†,‡,§ Hefei National Laboratory for Physical Sciences at the Microscale, ‡CAS Key Laboratory of Materials for Energy Conversion, Department of Chemical Physics, and Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China

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ABSTRACT: The reactivity of surface hydroxyls on FeO(111) monolayer films on Pt(111) with different oxygen vacancy concentrations has been investigated by means of X-ray photoelectron spectroscopy, thermal desorption spectroscopy, low energy electron diffraction, and density functional theory calculations. Surface hydroxyls on the FeO(111) monolayer films undergo two types of surface reactions: one type is surface reactions to form H2O and create oxygen vacancies; the other is surface reactions to form H2. Surface reactions to form H2O and create oxygen vacancies are preferred for surface hydroxyls on the stoichiometric FeO(111) monolayer film but get suppressed with the increasing of the oxygen vacancy concentration on the FeO(111) monolayer film. On the FeO0.67(111) monolayer film, surface hydroxyls prefer surface reactions to form H2. The accompanying DFT calculation results demonstrate that the thermodynamically favorable reaction between two OH(a) switches from the surface reaction to form H2O and oxygen vacancies on the stoichiometric FeO(111) monolayer film to the surface reaction to form H2 on the FeO0.75(111) monolayer film. These results reveal a novel concept of oxygen vacancy-controlled reactivity of surface hydroxyls in which the thermodynamically favorable reactions switch from reactions to form H2O and oxygen vacancies on the stoichiometric FeO(111) monolayer film to those to form H2 on the partially reduced FeO0.75(111) monolayer film. The interplay between oxygen vacancies and surface hydroxyls that both exert great influence on the physical chemistry and reactivity of oxide surface will greatly deepen the fundamental understanding of the relevant heterogeneous catalytic reaction systems involving transitional metal oxides.

1. INTRODUCTION Metal oxides play versatile and important roles in heterogeneous catalysis either as catalysts or as catalyst supports.1,2 Surface hydroxyls and point defects such as oxygen vacancies and F centers exert great influence on the physical chemistry and reactivity of oxide surfaces; however, the relevant fundamental studies remain challenging because of the complexity of these systems. Surface science studies of model systems for oxides, particularly well-defined oxide thin films, have been proven to be an effective approach.35 The effect of F centers on the physical chemistry and reactivity of MgO surface has been comprehensively investigated and understood employing MgO(001) thin films as model systems.610 The influence of oxygen vacancies on the physical chemistry and reactivity of reducible oxides such as TiO2 and CeO2 has also been extensively studied employing corresponding oxide single crystals and single crystal thin films.1117 F centers and oxygen vacancies on oxide surfaces are easily hydroxylated to form surface hydroxyls. Depending on the nature of metal oxides, surface hydroxyls can serve either as the Br€onsted acid sites or as the Br€onsted base sites, can result in the delocalization of electrons at metal oxide surfaces,18,19 can drive the surface reconstruction of metal oxide surfaces,2022 and can affect the dispersion and aggregation of the supported metal component.2327 Surface hydroxyls on metal oxides exclusively in the form of H bonded to surface lattice oxygen anion (herein defined as lattice surface hydroxyls) are also key r 2011 American Chemical Society

surface intermediates in several important heterogeneous catalytic reactions including H2 oxidation, methanol synthesis, dehydrogenation reaction, water gas shift reaction (WGS), and preferential oxidation of CO in H2 (PROX).2830 Therefore, it is of great importance to fundamentally understand the reactivity of lattice surface hydroxyls on metal oxides. The reactivity of hydroxyls on RuO2(110)/Ru(0001)3134 and ZnO(1010)3538 model surfaces have been quite well understood. During the hydrogen oxidation reaction catalyzed by RuO2(110)/Ru(0001), an interesting hydrogen transfer reaction pathway mediated by lattice surface hydroxyls was identified, in which the bridging O atoms harvest the hydrogen from the gas phase, and the on-top O atoms pick up chemisorbed hydrogen atoms from the bridging O atoms to produce water.34 Well-defined iron oxide thin films grown on Pt(111) including FeO(111) monolayer film, Fe3O4(111), R-Fe2O3(0001), and KxFe22O34(111) thin films compose a nice series of model surfaces for iron oxides.39 They have been successfully employed as model surfaces for the fundamental study of the industrial catalytic dehydrogenation process of ethylbenzene to styrene catalyzed by the iron oxide.3944 Recently, Pt-FeOx-based nanocatalysts have been Received: January 15, 2011 Revised: February 21, 2011 Published: March 14, 2011 6815

dx.doi.org/10.1021/jp200423j | J. Phys. Chem. C 2011, 115, 6815–6824

The Journal of Physical Chemistry C

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Figure 1. LEED patterns, Fe 2p3/2 and O 1s XPS spectra of the clean Pt(111) surface, the stoichiometric FeO(111) monolayer film, the FeO0.7(111) monolayer film, and the FeO0.67(111) monolayer film. The FeO0.7(111) and FeO0.67(111) monolayer films were prepared after the stoichiometric FeO(111) monolayer film experienced five cycles of 10 L D(g) TDS experiments at 130 K and three cycles of 10 L D(g) TDS experiments at RT without the reoxidation of the surface, respectively. The primary electron energy in the LEED measurements was 87 eV.

demonstrated to be very active in PROX and WGS reactions.45,46 Sun et al. reported that the FeO(111) monolayer film on Pt(111) can catalyze CO oxidation at a temperature at which Pt is inactive.47,48 Xu et al. reported that lattice surface hydroxyls on FeO(111) monolayer islands can easily undergo the interfacial reaction with CO on Pt(111) to produce CO at room temperature.49 Fu et al. proposed that the interface between FeO(111) monolayer islands and the Pt(111) substrate is the active site for low temperature CO oxidation.50 All these surface science studies of FeO(111) monolayer structures on Pt(111) provide the compelling evidence that the ferrous iron oxide is the active Fe species in practical PtFeOx-based nanocatalysts for PROX and WGS reactions. The oxygen-terminated FeO(111) monolayer film consisting of a single iron layer and a single oxygen layer provides a very suitable model oxide surface for the investigation of properties of oxygen vacancies and surface hydroxyls. A unique advantage of the FeO(111) monolayer film is that once formed, oxygen vacancies on the FeO(111) monolayer film can keep stable even at elevated temperatures because of the lack of the fast diffusion process of lattice oxygen from the bulk to the surface that usually occurs in a thick oxide film. The FeO(111) monolayer film is very inert under ultrahigh vacuum (UHV) conditions, but Huang and Ranke43 first reported that the exposure of atomic hydrogen at room temperature (RT) could efficiently form lattice surface hydroxyls and simultaneously create oxygen vacancies, resulting in the partial reduction of the FeO(111) monolayer film. The structure of partially reduced FeO(111) monolayer film was further studied with scanning tunneling microscopy (STM) by Knudsen et al.51 In this paper, we reported a combined experimental and theoretical study of the reactivity of lattice surface hydroxyls on the FeO(111) monolayer films with different coverages of oxygen vacancies. We unambiguously demonstrate that the reaction pathways of lattice surface hydroxyls on the FeO(111) monolayer film are controlled by the oxygen vacancy concentration, switching from water formation on the stoichiometric surface to hydrogen formation on the FeO0.67 surface. Our results for the first time reveal the interplay between oxygen vacancies and surface hydroxyls that both exert great influence on the physical chemistry and reactivity of oxide surface. As far as we know, oxygen vacancy-controlled reactivity of hydroxyls on an oxide surface demonstrated in this paper is a novel concept. We believe that this concept will greatly deepen the fundamental

understanding of the relevant heterogeneous catalytic reaction systems involving transitional metal oxides.

2. EXPERIMENTAL SECTION All experiments were performed in a Leybold stainless-steel UHV chamber with a base pressure of 1.2 1010 mbar.49 The UHV chamber was equipped with facilities for X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and differential-pumped thermal desorption spectroscopy (TDS). The UHV chamber was also equipped with a QUAD-EV-S mini e-beam evaporator and a MGC75 thermal gas cracker both purchased from the Mantis Deposition Ltd. The Pt(111) single crystal purchased from MaTeck was mounted on the sample holder by two Ta wires spot-welded to the back side of the sample. The sample temperature could be controlled between 130 and 1473 K and was measured by a chromel alumel thermocouple spot-welded to the backside of the sample. Prior to the experiments, the Pt(111) sample was cleaned by repeated cycles of Arþ sputtering and annealing until LEED gave a sharp (1 1) diffraction pattern and no contaminants could be detected by XPS. The stoichiometric FeO(111) monolayer film was prepared on Pt(111) according to the well-established procedure.39 Highpurity iron (99.995%, Alfa Aesar China Co., Ltd.) was evaporated onto Pt(111) at RT followed by the oxidation in 1 106 mbar O2 at 850 K for 30 min. CO (>99.99%, Nanjing ShangYuan Industry Factory) and D2 (>99.8%, Nanjing ShangYuan Industry Factory) were used as received, and their purity was further checked by quadrupole mass spectrometer (QMS) prior to experiments. The exposure of D2 was accomplished by a switched-off MGC75 thermal gas cracker ended with an Ir capillary (diameter: 2 mm) that was positioned ∼8 cm in front of the sample. The thermal gas cracker in operation could generate gas phase atomic deuterium (D(g)) with a cracking efficiency of ∼60%. Other gases were dosed by backfilling. All exposures were reported in Langmuir (1 L = 1.0 106 torr 3 s) without corrections for the gauge sensitivity. During the TDS experiments, the sample was positioned ∼1 mm away from a collecting tube of the differential-pumped QMS and the heating rate was 3 K/s. XPS spectra were recorded using Mg KR radiation (hν = 1253.6 eV) with a pass energy of 50 eV. 6816

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Figure 2. (A) CO TDS spectra after the saturating exposure of CO (2 L) at 130 K and (B) D2 TDS spectra after the saturating exposure of D2 (100 L) at 130 K from the clean Pt(111) surface, the stoichiometric FeO(111) monolayer film, the FeO0.7(111) monolayer film, and the FeO0.67(111) monolayer film. The FeO0.7(111) and FeO0.67(111) monolayer films were prepared after the stoichiometric FeO(111) monolayer film experienced five cycles of 10 L D(g) TDS experiments at 130 K and three cycles of 10 L D(g) TDS experiments at RT without the reoxidation of the surface, respectively.

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Figure 4. (A) D2O and (B) D2 TDS spectra after various exposures of D(g) on the stoichiometric FeO(111) monolayer film at 130 K.

3.09 Å in order to match the FeO(111) layer. The FeO(111)/ Pt(111) film was simulated by a slab model containing three layers of Pt and one layer of Fe and O stacking as OFePt(3) in which iron occupied fcc threefold hollow sites on Pt (111) and oxygen also preferred fcc threefold hollow sites. In the calculations a (4 2) surface unit was employed, and Pt atoms were not allowed to relax.

4. RESULTS AND DISCUSSION

Figure 3. (A) O 1s XPS spectra following the increasing exposures of D(g) on the stoichiometric FeO(111) monolayer film at 130 K and (B) the corresponding normalized integrated peak areas of the individual O 1s components and total O 1s peak.

3. THEORETICAL METHODS All calculations were carried out using a program package Vienna ab initio simulation package (VASP),5255 which is a first-principles plan-wave code and treats the exchange and correlation in the DFT scheme. The spin-polarized PBE exchange-correlation function56 was used, and the electronion interaction was described by the projector augmented wave method.57 The one-electron wave function was expanded in a plane-wave basis with an energy cutoff of 450 eV. The atomic structures were optimized until residual atomic forces were less than 0.02 eV/Å. Gamma-centered MonkhorstPack k-point sampling with 0.04 and 0.02 Å1 spacings was utilized for the relaxation and energy calculations, respectively. The lattice mismatch between Pt(111) and FeO(111) results in a structure with a periodicity of ∼26 Å,58 which is hard work for DFT calculations. To reduce the complexity, we adopted the approximation employed by Galloway et al.58 and Shaikhutdinov et al.59 in which the lattice constant of Pt(111) substrate was extended from 2.77 to

4.1. Stoichiometric FeO(111) Monolayer Film. Figure 1 shows the LEED pattern, Fe 2p3/2 and O 1s XPS spectra of the freshly prepared stoichiometric FeO(111) monolayer film on Pt(111), whose results agree well with those reported in the literature.39 The stoichiometric FeO(111) monolayer film is inert to the chemisorption of CO and D2, as demonstrated by the CO and D2 TDS spectra shown in Figure 2. In our experiments, we employed reactive D(g) as the reactant. Figure 3A shows the O 1s XPS spectra following increasing exposures of D(g) on the stoichiometric FeO(111) monolayer film at 130 K. The stoichiometric FeO(111) monolayer film exhibits a single O 1s XPS peak with the binding energy at 529.6 eV. After the exposure of 1 L D(g), an O 1s shoulder peak with the binding energy at 531.4 eV appears in the O 1s XPS spectrum. Increasing the D(g) exposure to 2.5 L results in the growth of the O 1s component at 531.4 eV and the appearance of another O 1s component with the binding energy at 532.3 eV. With the further increasing of D(g), the O 1s component at 531.4 eV weakens but that at 532.3 eV keeps growing. The O 1s components at 531.4 and 532.3 eV can be reasonably assigned to OD(a) and D2O(a), respectively. Figure 3B presents the integrated peak areas of the individual O 1s components and total O 1s peak following various D(g) exposures at 130 K normalized to that of the clean stoichiometric FeO(111) monolayer film. With the increasing of the D(g) exposure, the total O coverage remains as 1 ML, the O coverage corresponding to FeO keeps decreasing, the OD(a) coverage initially increases and then slightly decreases, and the D2O(a) coverage keeps growing. These XPS results clearly demonstrate that the lattice oxygen in the stoichiometric FeO(111) monolayer film sequentially hydrogenates with D(g) at 130 K, forming OD(a) and D2O(a) according to D(g) þ Olattice f OD(a) and D(g) þ OD(a) f D2O(a). After the exposure of 10 L D(g) at 130 K, ∼0.26 ML lattice oxygen in the stoichiometric FeO(111) monolayer film hydrogenates into OD(a) and D2O(a). Figure 4 presents the D2O and D2 TDS spectra following various exposures of D(g) on the stoichiometric FeO(111) 6817



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Figure 5. (A) O 1s XPS spectra after the exposure of 5 L D(g) on the stoichiometric FeO(111) monolayer film at 130 K followed by annealing at different temperatures and (B) the corresponding normalized integrated peak areas of the individual O 1s components and total O 1s peak.

Figure 6. (A) O 1s XPS spectra after each exposure during the consecutive TDS experiments of 10 L D(g) exposure at 130 K without the reoxidation of the surface and (B) the corresponding normalized integrated peak areas of the individual O 1s components and total O 1s peak.

monolayer film at 130 K. The surface was reoxidized after each TDS experiment to restore the stoichiometric FeO(111) monolayer film. After the exposure of 1 L D(g), the D2O and D2 TDS spectra display a broad D2O and D2 desorption peak, respectively. Comparing the corresponding XPS result, the desorption of D2O and D2 could be identified as the surface reaction products of OD(a) on the stoichiometric FeO(111) monolayer film at elevated temperatures. When the D(g) exposure reaches 2.5 L, the D2O desorption spectrum displays a narrow peak at 184 K followed by a broad feature, and the D2 desorption spectrum displays a tiny peak at 160 K, a broad peak between 200 and 350 K, and a peak at 470 K. With the further increasing of the D(g) exposure, the D2O desorption peak at 184 K grows and shifts to higher desorption temperatures; the D2 desorption peak at 160 K grows and shifts to higher desorption temperatures, and the D2 desorption peak at 470 K also grows but does not shift. XPS was employed to study the annealing process of the stoichiometric FeO(111) monolayer film exposed to D(g) at 130 K. Figure 5A shows the O 1s XPS spectra after the surface was exposed to 5 L D(g) at 130 K followed by annealing at different temperatures, and Figure 5B shows the corresponding normalized integrated peak areas of the individual O 1s components and total O 1s peak. After the exposure of 5 L D(g) at 130 K, ∼0.17 ML lattice oxygen in the stoichiometric FeO(111) monolayer film hydrogenates, in which ∼0.08 ML lattice oxygen hydrogenates to OD(a) and ∼0.09 ML lattice oxygen hydrogenates to D2O(a). After annealing at 200 K, the O 1s feature corresponding to D2O(a) disappears on the surface, and meanwhile the peak intensities of OD(a) and FeO components do not change, indicating that D2O(a) entirely desorbs from the surface. Therefore, the D2O desorption peak at ∼190 K arises from the desorption of D2O(a). It is noteworthy that the desorption temperature of D2O(a) formed by the hydrogenation of lattice oxygen in the stoichiometric FeO(111) monolayer film in our case is similar to that of H2O(a) adsorbed on the stoichiometric FeO(111) monolayer film,39 suggesting that D2O(a) might migrate and adsorb on the oxygen sites after its formation. These XPS results also demonstrate that the D2 desorption peak at ∼170 K has nothing to do with any O-contained species on the surface. This D2 desorption peak is thus assigned to the recombinative desorption of D(a) chemisorbed on the Fe(II) sites exposed on the surface after the formation and migration of D2O(a). This assignment is consistent with the experimental observation that the D2O desorption peak at ∼190 K is

accompanied by the D2 desorption peak at ∼170 K and is also supported by the TDS results of D2 chemisorption on the FeO(111) monolayer film with oxygen vacancies that will be discussed in the following part. The surface is the FeO0.91(111) monolayer film with 0.08 ML lattice oxygen in the form of OD(a) after the annealing at 200 K. After this surface was annealed at 350 K, the coverage of OD(a) decreases from 0.08 to 0.03 ML, the coverage of lattice O in FeO increases from 0.83 to 0.85 ML, and the total oxygen coverage on the surface decreases from 0.91 to 0.88 ML. These results demonstrate that the surface reaction of OD(a) between 200 and 350 K forms both D2 and D2O, giving rise to the broad desorption traces of both D2O and D2 in the TDS spectra. The further annealing of the surface at 600 K leads to a complete disappearance of OD(a); however, the total oxygen coverage on the surface remains as 0.88 ML and the coverage of lattice O in FeO increases from 0.85 to 0.88 ML. These results show that OD(a) reacts to exclusively form D2 above 350 K, agreeing with the TDS result that only the D2 desorption peak at 470 K was observed. Therefore, lattice oxygen in the stoichiometric FeO(111) monolayer film sequentially reacts with D(g) to form OD(a) and D2O(a) at 130 K. Upon heating, D2O(a) desorbs from the surface prior to 200 K, creating oxygen vacancies in FeO(111) monolayer film; OD(a) reacts to produce both D2 and D2O between 200 and 350 K, respectively, following 2OD(a) f 2O (lattice) þ D2 (g) and 2OD(a) f O (lattice) þ Ovacancy þ D2O (g); however, OD(a) that is stable on the surface above 350 K reacts to exclusively form D2. 4.2. FeO(111) Monolayer Films with Oxygen Vacancies. The above results clearly demonstrate that oxygen vacancies could be prepared in a controlled way on the stoichiometric FeO(111) monolayer film by the reaction with D(g). Moreover, the created oxygen vacancies can keep stable even at elevated temperatures because of the lack of the diffusion process of lattice oxygen from the bulk to the surface that usually occurs in a thick oxide film. This offers an appropriate model system to reliably study the reactivity of hydroxyls on oxide surfaces with oxygen vacancies. The consecutive TDS experiments of 10 L D(g) exposure at 130 K without the reoxidation of the surface were performed. Figure 6A shows the O 1s XPS spectra after the exposure of 10 L D(g) at 130 K in each TDS cycle, and Figure 6B shows the corresponding normalized integrated peak areas of the individual O 1s components and total O 1s peak. It can be seen clearly that the oxygen vacancy concentration on the 6818



Figure 7. (A) D2O and (B) D2 TDS spectra after each exposure during the consecutive TDS experiments of 10 L D(g) exposure at 130 K without the reoxidation of the surface.

FeO(111) monolayer film gets accumulated after each cycle of TDS experiment due to the formation and desorption of D2O(a). Calculated from the normalized peak area of total O 1s peak, the oxygen vacancy coverage after the first, second, third, fourth, and fifth cycles of TDS experiment is 0.09, 0.17, 0.22, 0.26, and 0.30 ML, respectively. The interaction of D(g) with the FeO(111) monolayer film at 130 K sensitively depends on the oxygen vacancy concentration. With the increasing of the oxygen vacancy concentration, the OD(a) formation is facilitated at the expense of the D2O(a) formation. On the FeO0.74 monolayer film, only OD(a) forms after the exposure of 10 L D(g) at 130 K. These results clearly demonstrate that the surface reaction of D(g) þ OD(a) f D2O(a) at 130 K is suppressed by the existence of oxygen vacancies on the FeO(111) monolayer film. Figures 7A and 7B respectively show the D2O and D2 thermal desorption traces in each TDS cycle. With the increasing of the cycle number, the desorption peak of D2O(a) at ∼190 K attenuates, and a novel peak at 213 K gradually develops that could be reasonably assigned to the surface reaction between D(a) on the exposed Fe(II) sites and OD(a) (D(a) þ OD(a) f Ovacancy þ D2O(g)); the D2 desorption trace grows and displays multipeaks. The D2 desorption peaks at ∼180 and ∼480 K arise from the recombinative desorption of D(a) on the exposed Fe(II) sites (2D(a) f D2(g)) and the surface reaction of OD(a) (2OD(a) f 2O þ D2(g)), respectively. Other D2 desorption peaks between 200 and 400 K might be contributed from several different surface reactions involving D(a) on the exposed Fe(II) sites and OD(a). After five cycles of TDS experiment following the exposure of 10 L D(g) at 130 K on the stoichiometric FeO(111) monolayer film without reoxidation, XPS results (Figure 1) indicate that the surface composition is FeO0.7 and the Fe 2p3/2 binding energy shifts to the lower binding energy. The corresponding LEED pattern (Figure 1) shows that the diffraction pattern arising from FeO(111) becomes diffuse and weak and a weak but visible p(2 2) pattern appears. The similar LEED pattern has been previously reported when the stoichiometric FeO(111) monolayer film was reduced by H(g) at room temperature43,51 and was proposed to correspond to a Fe3O2 structure by a combined STM, XPS, and DFT investigation.51 The chemisorption of CO and D2 was employed to probe the reactivity of the FeO0.7 monolayer film, and the results are presented in Figure 2. The FeO0.7 monolayer film exhibits a much enhanced reactivity as compared to the stoichiometric FeO(111) monolayer film, which can be rationally attributed to the existence of exposed Fe(II) sites. After an exposure of 2 L CO on FeO0.7 at 130 K, three CO desorption

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Figure 8. (A) O 1s XPS spectra after each exposure during the consecutive TDS experiments of 10 L D(g) exposure at RT without the reoxidation of the surface and (B) the corresponding normalized integrated peak areas of the individual O 1s components and total O 1s peak. The O 1s XPS spectrum of the final surface is also included.

peaks at 146, 272, and 433 K (weak) were observed in the CO TDS spectrum. Lemire60 et al. studied the chemisorption of CO on an Fe3O4(111) thick film on Pt(111) terminated with 1/4 ML octahedral Fe(II) situated above another 1/4 ML tetrahedral Fe(III) and also observed CO desorption peaks at 150 and 250 K. The FeO0.7 monolayer film is also active for the D2 chemisorption. Two D2 desorption peaks appear at 171 and 240 K after an exposure of 100 L D2 at 130 K. No formation of OD(a) and D2O(a) was observed by XPS upon the chemisorption at 130 K, indicating that D(a) chemisorbs on the exposed Fe(II) sites, whose recombinative desorption gives rise to the observed D2 desorption peaks. These results support our above arguments that the D2 desorption peak at ∼180 K observed after the exposure of D(g) on the stoichiometric FeO(111) monolayer film at 130 K arises from the recombinative desorption of D(a) on the exposed Fe(II) sites due to the formation and migration of D2O(a). In our previous work,43 we observed that the exposure of H(g) at RT leads to the partial reduction of the stoichiometric FeO(111) monolayer film and that the reactivity of OH(a) on the FeO(111) monolayer film depends on the oxygen vacancy concentration. The reaction pathway of OH(a) þ OH(a) switches from the H2O formation to the H2 formation with the increasing of the oxygen vacancy concentration on the FeO(111) monolayer.43 The current experimental results clearly prove that the partial reduction of the stoichiometric FeO(111) monolayer film caused by the exposure of H(g) at RT is due to the formation and desorption of H2O and further reveal that the reactivity of OH(a) on the FeO(111) monolayer film toward H(g) also depends on the oxygen vacancy concentration. The surface reaction of H(g) þ OH(a) f H2O(a) gets suppressed with the accumulation of the oxygen vacancy on the FeO(111) monolayer. We have also performed the consecutive TDS experiments of 10 L D(g) exposure at RT without the reoxidation of the surface. Figure 8A shows the O 1s XPS spectra after the exposure of 10 L D(g) at RT in each cycle, and Figure 8B shows the corresponding normalized integrated peak areas of the individual O 1s components and total O 1s peak. Only OD(a) forms and remains on the surface after the exposure of D(g) at RT because D2O desorbs from the surface as soon as it forms. Calculated from the normalized peak area of total O 1s peak, the oxygen vacancy coverage after the first, second, and third cycle of the TDS experiment is 0.20, 0.32, and 0.33 ML, respectively. Figures 9A and 9B respectively show the D2O 6819



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Figure 9. (A) D2O and (B) D2 TDS spectra after each exposure during the consecutive TDS experiments of 10 L D(g) exposure at RT without the reoxidation of the surface.

and D2 thermal desorption traces in each TDS cycle. Only a very weak D2O desorption peak was observed during the first cycle of the experiment, and no D2O desorption peak was observed during the following cycles of the experiment. The D2 desorption gives two peaks at 366 and 463 K. These results demonstrate that OD(a) on the FeO0.67 monolayer film selectively undergoes surface reactions to form D2. In other words, 0.33 ML is the saturating coverage of oxygen vacancy on the FeO monolayer film, and the FeO0.67 monolayer film is resistant to the further reduction, consistent with our previous results.43 We also characterized the structure of the FeO0.67 monolayer film with LEED, XPS, and chemisorption of CO and D2. The structure of the irreducible FeO0.67 monolayer film obtained after three cycles of TDS experiment following the exposure of 10 L D(g) at RT is slightly different from that of the FeO0.7 monolayer film obtained after five cycles of TDS experiment following the exposure of 10 L D(g) at 130 K. The FeO0.67 monolayer film exhibits the same LEED pattern (Figure 1) as the stoichiometric FeO(111) monolayer film, implying that it might consist of large FeO(111) domains and irregular-distributed oxygen vacancies. Both the CO desorption trace (Figure 2A) and the D2 desorption trace (Figure 2B) exhibit a broad desorption peak after the saturating exposure of CO (2 L) and D2 (100 L) at 130 K, respectively. These results also suggest that the distribution of Fe(II) sites on the irreducible FeO0.67 monolayer film is different from that on the FeO0.7 monolayer film. 4.3. DFT Calculations. DFT calculations were performed to study the chemisorption of H(g) on the FeO(111) monolayer film with different oxygen vacancy concentrations and the reactivity of formed OH(a). Figure 10 presents the top view and side view of the optimized structure model of the stoichiometric FeO(111) monolayer film on Pt(111). The PtFe interlayer distance is 1.96 Å, and the FeO interlayer distance is 0.75 Å which corresponds to the FeO distance of 1.94 Å. These calculated values of the FeO interlayer distance and the FeO distance (0.75 Å/1.94 Å) agree well with previous calculation values of Galloway et al. (0.65 Å/1.90 Å)58 and Shaikutdinov et al. (0.63 Å/1.86 Å).59 A (4 2) surface unit was used in the calculations. An FeO0.875(111) surface with θ(Ovacancy) = 1/8 ML, an FeO0.75(111) surface with θ(Ovacancy) = 1/4 ML, and an FeO0.625(111) surface with θ(Ovacancy) = 3/8 ML were generated by creating an oxygen vacancy on the O(v1) site, creating two oxygen vacancies on the O(v1) and O(a1) sites,

Figure 10. Top view and side view of the optimized structure model of the stoichiometric FeO(111) monolayer film on Pt(111) employed in the DFT calculations. The blue, purple, and red balls represent the Pt, Fe, and O atoms, respectively.

and creating three oxygen vacancies on the O(v1), O(a1), and O(a4) sites, respectively. We herein define the formation energy of oxygen vacancy on an oxide surface to be the energy difference (E(final surface) E(initial surface)) between the initial surface and the final surface with one additional oxygen vacancy. Therefore, more positive the formation energy of oxygen vacancy on an oxide surface is, the more difficult it is to create an oxygen vacancy on the oxide surface. From the calculated total energy of FeO(111)/Pt(111), FeO0.875(111)/Pt(111), FeO0.75(111)/ Pt(111), and FeO0.625(111)/Pt(111) surfaces, the formation energy of oxygen vacancy is 5.88, 7.42, and 8.79 eV on FeO(111)/ Pt(111), FeO0.875(111)/Pt(111), and FeO0.75(111)/Pt(111) surfaces, respectively. This indicates that creating an oxygen vacancy on FeO(111)/Pt(111) becomes more difficult with the increasing of oxygen vacancy concentration in FeO. The adsorption energy of H(g) on FeO(111), FeO0.875(111), and FeO0.75(111) surfaces was calculated, and the results are summarized in Table 1. On the stoichiometric FeO(111) monolayer film, only O sites are available for the H(g) adsorption. The adsorption energy of one H(g) atom on the O-site was calculated to be 3.11 eV, demonstrating that the formation of the surface hydroxyl is thermodynamically very favorable. This also indicates that the stoichiometric FeO(111) monolayer film on Pt(111) prefers the hydroxylation reaction. On the FeO0.875(111) monolayer film, the calculated adsorption energy of one H(g) atom on the O(a1) and O(a3) sites is 1.67 and 1.63 eV, respectively. The Fe(II)(v1) site on FeO0.875(111) is also accessible to H(g), and the calculated adsorption energy of one H(g) atom on the Fe(II) site is 0.81 eV, demonstrating that H(g) can chemisorb on the Fe(II) site, supporting our experimental results. However, H(a) on the Fe(II) site is less stable than that on the O site. We also calculated the adsorption energy of two H(g) atoms on the surface. The adsorption energy of two H(g) atoms on the neighboring O(v1) and O(a3) sites on the stoichiometric FeO(111) monolayer film is 6.10 eV, which is roughly twice that of one H(g) atom on the O(v1) site, but interestingly, the adsorption energy of two H(g) atoms on 6820



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Table 1. Adsorption Energy (Eads) of H(a) on the FeO(111) Monolayer Film with Different Oxygen Vacancy Concentrations surface FeO0.875(111)

FeO(111)

a

a

FeO0.75(111)

chemisorption sites

v1

v1a3

v1

a3

a1

v1a3

a1a3

a3a4

a1a2

a3a4

Eads (eV)

3.11

6.10

0.81

1.63

1.67

3.89

4.64

4.59

4.65

4.41

The v1 site on FeO0.875(111) corresponds to the oxygen vacancy site, i.e., the Fe(II) site.

Table 2. Thermodynamics (ΔH) of OH(a) þ OH(a) f O þ Ovacancy þ H2O(g) and OH(a) þ OH(a) f 2O þ H2(g) on the FeO(111) Monolayer Film with Different Oxygen Vacancy Concentrations surface reactions

FeO(111)

FeO0.875(111)

FeO0.75(111)

OH(a) þ OH(a) f O þ Ovacancy þ H2O(g)

0.01 eV

0.08 eV

1.24 eV

OH(a) þ OH(a) f 2O þ H2(g)

1.56 eV

0.12 eV

0.12 eV

the neighboring sites on the FeO0.875(111) monolayer film is much larger than the sum of the adsorption energy of an individual H(g) atom on each site; for example, the adsorption energy of two H(g) atoms on the neighboring O(a1) and O(a3) sites (4.64 eV) is much larger than the total adsorption energy (3.30 eV) of one H(g) atom on the O(a1) site and the other on the O(a3) site, and the adsorption energy of two H(g) atoms on the neighboring O(a1) and Fe(II)(v1) sites (3.89 eV) is much larger than the total adsorption energy (2.48 eV) of one H(g) atom on the O(a1) site and the other on the Fe(II)(v1) site. These calculation results suggest that H(a) has a larger tendency to island on the FeO0.875(111) monolayer film than on the stoichiometric FeO(111) monolayer film. On the FeO0.75(111) monolayer film, the adsorption energy of two H(g) atoms on the neighboring O(a3) and O(a4) sites was calculated to be 4.41 eV, also indicating a similar tendency of H(a) island formation. The influence of oxygen vacancies on the reactivity of OH(a) on the FeO(111) monolayer film was examined by calculating the thermodynamics of the OH(a) þ OH(a) f O (lattice) þ Ovacancy þ H2O(g) and OH(a) þ OH(a) f 2O (lattice) þ H2(g) reactions on the FeO(111) monolayer films with different oxygen vacancy concentrations. The results are summarized in Table 2. On the stoichiometric FeO(111) monolayer film, the final state energy of the H2O formation reaction is the same as the initial state energy whereas that of the H2 formation reaction is 1.56 eV above the initial state energy, suggesting that the H2O formation is thermodynamically highly favorable over the H2 formation when two OH(a) react on the stoichiometric FeO(111) monolayer film. The case changes on the FeO0.875(111) monolayer film. Both reactions are endothermic, but the ΔH values for the H2O formation reaction (0.08 eV) and the H2 formation reaction (0.12 eV) are comparable, demonstrating that the H2O formation reaction and the H2 formation reaction thermodynamically compete for the reaction of two OH(a) on the FeO0.875(111) monolayer film. With the oxygen vacancy concentration increasing to 0.25 ML (the FeO0.75(111) monolayer film), the reaction thermodynamics of two reaction routes for OH(a) completely inverts. The H2 formation reaction is slightly exothermic (ΔH = 0.12 eV), but the H2O formation reaction is strongly endothermic (ΔH = 1.24 eV). This suggests that the H2 formation reaction is thermodynamically highly favorable over the H2O formation reaction when two OH(a) react on the FeO0.75(111) monolayer film. These DFT calculation results demonstrate that the thermodynamically favorable reaction of two OH(a) on the FeO(111) monolayer film switches from the H2O formation

reaction to the H2 formation reaction with the increasing of the oxygen vacancy concentration and that the FeO0.75(111) monolayer film is relatively resistant to the further reduction, agreeing well with the experimental results. 4.4. Oxygen Vacancy-Controlled Reactivity of Surface Hydroxyls on the FeO(111) Monolayer Films. The current combined experimental and theoretical results, together with our previous experimental results,43 reveal a novel concept that the reactivity of lattice OH(a) on the FeO(111) monolayer films is controlled by the oxygen vacancy concentration (Figure 11). Lattice surface hydroxyls on the FeO(111) monolayer film undergo two types of surface reactions: one is to form H2O and create oxygen vacancies in the oxide surface; the other is to produce H2 and restore the oxide surface. Surface reactions to form H2O and create oxygen vacancies are preferred for surface hydroxyls on the stoichiometric FeO(111) monolayer film. OH(a) can react with H(g) to form H2O(a) at 130 K that desorbs from the surface upon heating, creating oxygen vacancies on the FeO(111) monolayer film; at elevated temperatures, OH(a) undergoes surface reactions to form both H2O and H2, in which the H2O formation creates more oxygen vacancies. However, these reaction pathways of OH(a) to form H2O and create oxygen vacancies are suppressed gradually with the increasing of the oxygen vacancy concentration on the FeO(111) monolayer film. On the FeO0.67(111) monolayer film, surface hydroxyls prefer surface reactions to form H2. OH(a) does not react with H(g) to form H2O(a) at 130 K; upon heating, OH(a) exclusively undergoes surface reactions to form H2. The DFT calculation results also demonstrate that the thermodynamically favorable reaction between two OH(a) switches from the surface reaction to form H2O and oxygen vacancies on the stoichiometric FeO(111) monolayer film to the surface reaction to form H2 on the FeO0.75(111) monolayer film with θ(Ovacancy) = 1/4 ML. Therefore, oxygen vacancies in the FeO(111) monolayer film control the reactivity of surface hydroxyls by switching the thermodynamically favorable reactions from reactions to form H2O and oxygen vacancies on the stoichiometric FeO(111) monolayer film to those to form H2 on the partially reduced FeO0.75(111) monolayer film. This can be reasonably understood in the view that the oxygen vacancy is one of the products in the H2O formation reactions from surface hydroxyls. Although the influence of oxygen vacancies and surface hydroxyls on the physical chemistry of oxide surfaces has been extensively studied, our results provide for the first time the solid evidence for the interplay between oxygen vacancies and lattice surface hydroxyls. Because of the structural similarity of 6821



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Figure 11. Schematic illustration of oxygen vacancy-controlled reactivity of lattice surface hydroxyls on FeO(111) surfaces.

transitional metal oxides, the concept of oxygen vacancy-controlled reactivity of lattice surface hydroxyls might be extended from the FeO(111) monolayer films to other transitional metal oxide surfaces and provide novel insights into the fundamental understanding of the physical chemistry of transitional metal oxide surfaces. 4.5. Implications for Practical Catalytic Reactions. Our findings have important implications for practical catalytic reactions over transitional metal oxides involving lattice surface hydroxyls. The very important information conveyed by our findings is that the initial reduction of a stoichiometric metal oxide surface is facile via the surface reactions of lattice surface hydroxyls, but there exists a relatively reduction-resistant metal oxide surface with an appropriate concentration of oxygen vacancies on which lattice surface hydroxyls selectively react to form H2. The reduction behavior of transitional metal oxides by H2 is an important factor affecting their catalytic performance, but its mechanism and kinetics have been poorly understood. Inferred from our results, the reduction of the stoichiometric metal surface might be facile until the formation of the reduction-resistant metal oxide surface whose further reduction is rate-determining. A likely mechanism after the formation of the reduction-resistant metal oxide surface is that lattice oxygen ions in the subsurface and bulk of metal oxide migrate to the surface and get reduced in the case that the migration activation energy is lower than the reduction activation energy of the reduction-resistant metal oxide surface. This will lead to the reduction of the subsurface and bulk of metal oxide prior to the full reduction of the surface of metal oxide. Our results also provide deep insights into the active surface structure of metal oxide used as the catalyst for nonoxidative dehydrogenation reactions in which the lattice surface hydroxyls

are formed as the surface intermediates. A typical example is the nonoxidative dehydrogenation reaction of ethylbenzene to styrene catalyzed by K-promoted hematite.61 Our results infer that the stoichiometric metal oxide surface might not be the likely active surface structure because the H2O formation is highly thermodynamically favorable as the reaction product of lattice surface hydroxyls on such a surface. Instead, a stable partially reduced metal oxide surface on which lattice surface hydroxyls selectively react to form H2 is probably the active surface structure of the catalyst. Oxygen vacancies and surface hydroxyls are very common on metal oxide surfaces. We believe that the interplay between oxygen vacancies and lattice surface hydroxyls might be a general phenomenon occurring during the course of surface reactions on transitional metal oxide surfaces involving surface hydroxyls such as methanol synthesis, dehydrogenation reaction, WGS reaction and PROX reaction, and even water splitting.

5. CONCLUSION We have experimentally and theoretically investigated the reactivity of surface hydroxyls on the FeO(111) monolayer film on Pt(111) with different oxygen vacancy concentrations. On the stoichiometric FeO(111) monolayer film, surface hydroxyls prefer surface reactions to form H2O and create oxygen vacancies. However, these surface reactions are suppressed gradually with the increasing of the oxygen vacancy concentration on the FeO(111) monolayer film, and on the FeO0.67(111) monolayer film, surface hydroxyls prefer surface reactions to form H2. The DFT calculation results also demonstrate that the thermodynamically favorable reaction between two OH(a) switches from the surface 6822


The Journal of Physical Chemistry C reaction to form H2O and oxygen vacancies on the stoichiometric FeO(111) monolayer film to the surface reaction to form H2 on the FeO0.75(111) monolayer film. These results reveal a novel concept of oxygen vacancy-controlled reactivity of surface hydroxyls in which the thermodynamically favorable surface reactions switch from reactions to form H2O and oxygen vacancies on the stoichiometric metal oxide surface to those to form H2 on the partially reduced metal oxide surface with an appropriate amount of oxygen vacancies. The interplay between oxygen vacancies and surface hydroxyls that both exert great influence on the physical chemistry and reactivity of oxide surface will greatly deepen the fundamental understanding of the relevant heterogeneous catalytic reaction systems involving transitional metal oxides.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (J.Y.); [email protected] (W.H.).

’ ACKNOWLEDGMENT This work was financially supported by National Natural Science Foundation of China (grants 20773113, 20803072), the solar energy project of Chinese Academy of Sciences, National Basic Research Program of China (2010CB923302), MOE program for PCSIRT (IRT0756), the Fundamental Research Funds for the Central Universities, and the MPG-CAS partner group program. ’ REFERENCES (1) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, U.K., 1994. (2) Noguera, C. Physics and Chemistry of Oxide Surfaces; Cambridge University Press: Cambridge U.K., 1994. (3) Freund, H.-J.; Goodman, D. W. In Handbook of Heterogeneous Catalysis, 2nd ed.; Ertl, G., Kn€ozinger, H., Eds.; VCH: Weinheim, Germany, 2008; Vol. 3, p 1309. (4) Freund, H.-J.; Pacchioni, G. Chem. Soc. Rev. 2008, 37, 2224. (5) Freund, H.-J. Chem.—Eur. J. 2010, 16, 9384. (6) Sterrer, M.; Nowicki, M.; Heyde, M.; Nilius, N.; Risse, T.; Rust, H.-P.; Pacchioni, G.; Freund, H.-J. J. Phys. Chem. B 2006, 110, 46. (7) Sterrer, M.; Risse, T.; Martinez Pozzoni, U.; Giordano, L.; Heyde, M.; Rust, H.-P.; Pacchioni, G.; Freund, H.-J. Phys. Rev. Lett. 2007, 98, 096107. (8) Risse, T.; Shaikhutdinov, S.; Nilius, N.; Sterrer, M.; Freund, H.-J. Acc. Chem. Res. 2008, 41, 949. (9) K€onig, T.; Simon, G. H.; Rust, H.-P.; Heyde, M.; Freund, H.-J. J. Am. Chem. Soc. 2009, 131, 17544. (10) Lin, X.; Yang, B.; Benia, H.-M.; Myrach, P.; Yulikov, M.; Aumer, A.; Brown, M. A.; Sterrer, M.; Bondarchuk, O.; Kieseritzky, E.; Rocker, J.; Risse, T.; Gao, H.-J.; Nilius, N.; Freund, H.-J. J. Am. Chem. Soc. 2010, 132, 7745. (11) Wang, Q.; Biener, J.; Guo, X.-C.; Farfan-Arribas, E.; Madix, R. J. J. Phys. Chem. B 2003, 107, 11709. (12) Wendt, S.; Schaub, R.; Matthiesen, J.; Vestergaard, E. K.; Wahlstr€om, E.; Rasmussen, M. D.; Thostrup, P.; Molina, L. M.; Lægsgaard, E.; Stensgaard, I.; Hammer, B.; Besenbacher, F. Surf. Sci. 2005, 598, 226. (13) Esch, F.; Fabris, S.; Zhou, L.; Montini, T.; Africh, C.; Fornasiero, P.; Comelli, G.; Rosei, R. Science 2005, 309, 752. (14) Campbell, C. T.; Peden, C. H. F. Science 2005, 309, 713. (15) Polarz, S.; Strunk, J.; Ischenko, V.; Berg, M. W. E. v. d.; Hinrichsen, O.; Muhler, M.; Driess, M. Angew. Chem., Int. Ed. 2006, 45, 2965.

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Oxygen Vacancy-Controlled Reactivity of Hydroxyls on an FeO(111

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