Activation of Surface Oxygen Sites in a Cobalt ... - ACS Publications

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Activation of Surface Oxygen Sites in a CobaltBased Perovskite Model Catalyst for CO Oxidation Xiyang Wang, Keke Huang, Long Yuan, Shibo Xi, Wensheng Yan, Zhibin Geng, Yingge Cong, Yu Sun, Hao Tan, Xiaofeng Wu, Liping Li, and Shouhua Feng J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01623 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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The Journal of Physical Chemistry Letters

Activation of Surface Oxygen Sites in a Cobaltbased Perovskite Model Catalyst for CO Oxidation Xiyang Wang,† Keke Huang,† Long Yuan,† Shibo Xi,‡ Wensheng Yan,|| Zhibin Geng,† Yingge Cong,† Yu Sun,† Hao Tan,|| Xiaofeng Wu,† Liping Li† and Shouhua Feng*†

† State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, P.R. China ‡ Institute of Chemical and Engineering Sciences, A*STAR, 1 Pesek Road, Jurong Island, Singapore 627833, Singapore || National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, P. R. China

Corresponding Author Fax: (86)-431-85168624 Tel: (86)-431-85168661 Email: [email protected]

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ABSTRACT

Anionic redox chemistry is becoming increasingly important in explaining the intristic catalytic behavior in transition-metal oxides and improving the catalytic activity. However, it is a great challenge to activate lattice oxygen in noble-metal-free perovskites for obtaining active peroxide species. Here, we take La0.4Sr0.6CoO3-δ as a model catalyst and develop an anionic redox activity regulation method to activate lattice oxygen by tuning charge transfer between Co4+ and O2-. Advanced XAS and XPS demonstrate that our method can effectively decrease electron density of surface oxygen sites (O2-) to form more reactive oxygen species (O2-x), which reduces the activation energy barriers of molecular O2 and leads to a very high CO catalytic activity. The revealing of activation mechanism for surface oxygen sites in perovskites in this work opens up a new avenue to design the efficient solid catalysts. Furthermore, we also establish a correlation between anionic redox chemistry and CO catalytic activity.

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Perovskite oxides, as promising substitutes of noble metals for the oxidation of carbon monoxide and hydrocarbon-based fuels and electrochemical catalysis, have received significant attention owing to their abundant tunability of composition and structure, thermal stability and excellent catalytic activity.1,2 Activation of molecular oxygen is of vital importance in these catalytic reactions and usually undergoes a series of steps: O2→O2-→O22-→2O (ads).3,4 Consequently, the concentration and activity of surface oxygen species in perovskite oxides are two crucial factors that determine the catalytic performance. Even though increasing the amount of surface oxygen species such as doping other elements and modifying synthetic methods could partly improve the catalytic activity, their catalytic performances are still low comparing the expecting ones.5,6 On the other hand, enhancing the activity of surface oxygen species can efficiently reduce the activation energy barriers for the surface reaction.7-10 Therefore, it is of great significance to achieve highly active and stable oxygen species such as peroxide species on the surface of perovskite oxides. The activity and stability of oxygen species intimately depend on the electronic and geometric structure of the solid surface.4,11 Previous reports have shown that loading noble metals on the surface of perovskite oxides can easily obtain active O22- species due to strong metal-support interaction and more active sites, resulting in excellent catalytic activity.3,12,13 However, activating oxygen species of noble metal-free perovskites has always been a challenge and the formation mechanism of O22- species remains elusive, which severely restricts development of high-efficiency perovskite catalysts. Recent studies find that the covalency of metal–O bond in perovskite oxides can seriously affect the catalytic activity, and enhancing the interaction between B-site cation and O anion is very beneficial for activation of lattice oxygen.14,15 Moreover, the native surface of perovskite catalysts is usually dominated by segregated A-site


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cations and excessive surface segregation could hugely block electron transfer and anion oxygen activating pathways.16-18 These results show that the amount of surface oxygen species is significantly increased after modified, but to obtain more active peroxide species, electron density of surface oxygen sites needs to be further decreased.14 Hence, we propose engineering electronic structure combining these two aspects to activate lattice oxygen for improving the catalytic activity. Herein, we take highly active and metastable La0.4Sr0.6CoO3-δ (LSCO) as a model perovskite catalyst, and optimize the interaction between Co4+ ions and O2- ions using urea pyrolysis method that facilitates the enrichment and phase segregation of Sr. Further, we decrease coverage rate of surface segregation via chemical etching, which creates more active sites and retains original advantage structure. By using advanced surface-sensitive X-ray absorption and photoelectron spectroscopy, we find that this regulation method can efficiently improve charge transfer between Co4+ cations and O2- anions and the activation of surface oxygen sites facilitates the formation of active O22- species in the surface region. This work not only clearly illustrates the precise design route of activating lattice oxygen and relevant principle for the generation of surface O22- species, but also is a step towards substituting noble metals for CO removal in the future. Scheme 1a illustrates the designing route of activating lattice oxygen for perovskite LSCO. Firstly, we use urea pyrolysis method to produce surface Sr enriched perovskite (LSCO-Sr) that contains more surface oxygen species.19 Then, etching away superfluous surface segregation using acetic acid buffer solution, we can obtain target sample (LSCO-Sr-H+) having more surface peroxide species. Related details of these methods are shown in experimental section of supporting information. To verify the presence of nitrogen in perovskite oxides, we measure


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XPS data and EDS data of LSCO, LSCO-Sr and LSCO-Sr-H+. EDS data in Figure S1 shows that LSCO-Sr and LSCO-Sr-H+ have no N element in bulk structure and XPS results in Figure S2 also show that N element do not exist on the surface of LSCO, LSCO-Sr and LSCO-Sr-H+. These results demonstrate that reaction conditions of urea pyrolysis such as high temperature and flowing air cause complete decomposition of oxynitride species. Through two processes of urea pyrolysis and chemical etching, the electronic structure of LSCO is greatly optimized and energy level diagrams of Co–O bond covalency for three samples are shown in Scheme 1b. When more Sr enriches on the surface, the Co 3d band enters the ligand O 2p band and ligand holes can be formed by transferring electrons to the metal. After reducing surface segregation, charge transfer between Co4+ and O2- can be further enhanced and surface active sites also distinctly increase. Sufficient electron transfer between Co4+ ions and O2- ions can activate lattice oxygen in perovskite oxides, resulting in the formation of more surface peroxide species.

Scheme 1. a) Illustration of the regulation route for activation of surface oxygen sites. Blue spheres: surface secondary phases such as SrO/SrCO3/Sr(OH)2; green region: Sr enrichment regions; brownish black region: bulk structure. b) the schematic band structure of LSCO, LSCOSr and LSCO-Sr-H+ near the Fermi level.


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To demonstrate the influence of two step modifications on surface electronic structure, XAS is applied to characterize the three samples of LSCO, LSCO-Sr and LSCO-Sr-H+. The O-K edge XANES spectra normalized from 520 to 570 eV in Figure 1a show an increased white line intensity from LSCO, LSCO-Sr to LSCO-Sr-H+. Different from O K-edge, Co L-edge spectra in Figure 1b display a decreased intensity. The variations of white line intensities for O K-edge and Co L-edge reveal that the modified sample LSCO-Sr-H+ has lower electron density at the O site and a higher electron density at the Co site comparing to LSCO and LSCO-Sr, i.e. Sr enrichment and reduction of surface segregation can enhance the charge transfer between Co cations and O anions. When the content of Sr is high enough in perovskite LSCO, ionic state of the Co ions in perovskite is not really +4 (traditional format: SrCo4+O2-3), but rather that the charge state is compensated by O 2p holes (actually format: SrCo3+O-O2-2), namely electron at oxygen site is transferred into Co site.20 When the content of Sr is lower, charge transfer between Co4+ and O2is relatively difficult. So the charge transfer between Co4+ and O2- in perovskite oxides can be interpreted as the formation of Co3+ ions and O 2p hole due to a negative-charge-transfer energy and Sr enrichment has important effect on tuning surface charge transfer.21 Compared with LSCO and LSCO-Sr, sufficient charge transfer in LSCO-Sr-H+ can increase more O 2p holes doping and Co3+ ions (dummy Co4+ ions) on the surface and the formation of O 2p holes can transform lattice oxygen (O2-) in perovskite catalysts into active lattice oxygen (O-, O2-x), which decreases electron density at the O site and partly increases spare electrons at the Co site (Co4+→ Co3+, electron density: Co3+ (transfer) > Co4+ (origin)). Furthermore, the other reason for a higher electron density at the Co sites can be considered as the enhancement of covalence for Co-O bond.22 This apparent disappeared electron stems from exchange of La with Sr on the surface.


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Figure 1. a) The O K-edge XANES, b) the Co L2, L3-edge XANES, c) the Co K-edge XANES and d) the R-space Fourier-transformed FT (k3χ(k)) of Co K-edge EXAFS of perovskite LSCO, LSCO-Sr and LSCO-Sr-H+. To clearly distinguish the variation of surface electronic structure, we do detailed analysis of O K-edge and Co L-edge spectra for the three samples. In O K-edge spectra of Figure 1a, the preedge peak A at 528.2 eV is assigned as charge-transfer band, which derives from introduction of extra holes on oxygen sites (3d6L electron configuration).21,23 Peak B at 530.4 eV is the overlapping bands between Co3d and O2p; peak C at 532.2 eV, only observed for sample LSCO-Sr-H+, can be designated as σ* resonance due to O1s→3σu (antibonding) transition of O22- species in the surface region; peak D at 533.4 eV can be assigned to a σ* resonance extinction due to O1s→3σu transition of O2- species; peak E at 536.4 eV is recognized as hybridization between Sr4d/La5d and O2p; peak F at 540.2 eV can be considered as O2-


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component of the Co4sp-O2p hybrid; and peak G at 542.6 eV originates from the hybridization between O2p and Co4sp with monoxidic species.24,25 Two-step modulation results in a shift of peak A to lower energy and peak B to higher energy. More importantly, the O2- species in LSCO and LSCO-Sr transform into O22- species in LSCO-Sr-H+ as evidenced by the shift of peak D toward lower energy to become Peak C.24-26 Peak E does not show any shift, suggesting that the hybridization between A-site cation and O anion remain unchanged. On the other hand, the gradually decreased intensity for peak F from LSCO to LSCO-Sr and finally merging into a single peak G for LSCO-Sr-H+ also demonstrate the enhancement of charge transfer between Co4+ and O2-. Further, infrared spectrum in Figure S3 shows that the three Co-O bond bending vibration peaks (Co4+-O bond stretching vibration around 662 cm-1 and two Co3+-O bond bending vibration around 418 cm-1 and 580 cm-1) shift to a higher frequency from LSCO to LSCO-Sr, to LSCO-Sr-H+. Raman peak (Co-O bending vibration in BO6 octahedron) in Figure S4 exhibits a red shift after modified.27-29 These results also indicate that Sr enrichment and reducing Sr segregation can increase the octahedral distortion and improve charge transfer between Co4+ and O2-. Comparing to the obvious variation of O K-edge, energy positions of peak I, II and III in the Co L-edge spectra for three samples have only subtle change. The intensity for the shoulder peak I relative to peak II slightly increases, suggesting that electrons in O anion are partly transferred into eg orbit of Co cation after two step modifications and the oxidation state of outermost layer Co cation remains the same due to Sr enrichment. Co 2p XPS were recorded in Figure S5 to further assess the oxidation state of surface Co ions. Similar characteristic parameters of three samples (including the peak position, half height width of Co2p3/2, the spin splitting energy between2p3/2 and 2p1/2, and the intensity ratio of satellite peak at 790.1 eV to main peak of Co2p3/2) has no observably change due to the complex spin state of Co cations in


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the perovskite. However, shoulder peak in Co 2p 2p3/2 and 2p1/2 spectra for LSCO-Sr and LSCO-Sr-H+ shifts to lower energy compared with perovskite LSCO. Co L-edge XANES and Co K-edge XANES also show that their absorption peak and edge shift to lower energy shift (namely decrease of Co valence state). These experimental results are coincident with above explanation for the change of surface electronic structure. The Co K-edge absorption spectrum is measured to exam the fine structure. The XANES spectrum that involves the transitions from Co 1s to Co 3d and Co 1s to Co 4p is shown in Figure 1c. The peak intensities of LSCO-Sr and LSCO-Sr-H+ in 1 region (inset of Figure 1c) become weak compared to LSCO, which stems from charge transfer between Co and O as observed in O K-edge and Co L-edge spectra. From Co K-edge XANES spectrum in 2 region of Figure 1c, we can observe that absorption edge shifts to lower energy for two modified samples comparing to LSCO sample, indicating that urea pyrolysis modification reduces the oxidation state of Co cations in bulk structure. The nearly same absorption edge for LSCO-Sr and LSCO-Sr-H+ shows that Co ions in both sample have very closer oxidation state, indicating that chemical etching for Sr segregation is mainly limited on few surface atom layers and has less effect on bulk structure. The R-space Fourier-transformed FT (k3χ (k)) of Co K-edge EXAFS for three samples are shown in Figure 1d. Two dominate shells observed for three samples at ~1.83 Å and ~3.7 Å are attributed to Co-O bond and Co-La/Sr/Co bonds in corner-sharing octahedra, while the new appeared shell for two modified sample of LSCO-Sr and LSCO-Sr-H+ at ~2.7 Å is assigned to Co-Co bond in edge-sharing octahedra.29,30 The process of urea pyrolysis is vital to the formation of corner-sharing octahedra, while reducing Sr segregation only a little enhances the charge transfer and mainly increases the number of active sites. Moreover, the peak position of Co-O bond for three samples remains accordant, but the peak intensity gradually increases from LSCO


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to LSCO-Sr-H+, suggesting a decrease of disorder degree in the case of remaining the fixed CoO bond length. For Co-La/Sr and Co-Co bonds in the third shell, the peak intensity gradually decreases and the peak position visibly shifts to higher bond length from LSCO, LSCO-Sr to LSCO-Sr-H+, which could be related with the presence of edge-sharing octahedra and octahedral distortion.

Figure 2. The O 1s XPS spectra of (a) LSCO, (b) LSCO-Sr and (c) LSCO-Sr-H+; the Sr 3d XPS spectra of (a) LSCO, (b) LSCO-Sr and (c) LSCO-Sr-H+


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Figure 3. a) the ratios of “Surface” Sr /“lattice” Sr for LSCO, LSCO-Sr and LSCO-Sr-H+, b) the contents of active oxygen species in perovskite oxides. Error bar: multiple repeated measurement and fit results for XPS data. To verify our theoretical model for the variations of surface composition, XPS data of these samples are recorded. Multi-peak fitting results of Sr3d and O1s spectrum are shown in Figure 2 and detail information are displayed in surface composition analysis of supporting information. The Sr3d spectra in Figure 2 (a-c) can be well fitted by two double spin-orbital splitting of 3d 5/2 and 3d 3/2. The ones (blue solid lines) at lower binding energy (~131.6–132.7 eV) are assigned to the contribution of bulk Sr ions (denoted as "Lattice Sr"), while that (pink solid lines) in high


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binding energy region (~133.4–133.6 eV) are surface Sr species (market as "surface Sr").31,32 To intuitively analyze surface Sr contents, the ratios of Srsurface / Srlattice for these samples are calculated and shown in Figure 3a. The Srsurface / Srlattice ratio of LSCO-Sr increases from 3.76 (LSCO) to 4.68 after Sr enrichment. When LSCO-Sr continues to be treated with diluted acetic acid, the ratio decreases to 3.56 for LSCO-Sr-H+. We also observed surface segregation phase SrCO3 in XRD patterns of the LSCO-Sr and LSCO-Sr-H+ samples due to urea pyrolysis (Figure S9). The O 1s spectrum in Figure 2 (d-f) for each sample can be de-convoluted into four components. The first component located at ~528.6 eV is bulk O2- (Olattice), and the one at ~529.6 eV stems from perovskite lattice termination layer such as O22-/O- ions. The other two are surface oxygen species (Osurface) that involve surface secondary phases (such as SrO and Sr(OH)2 at ~531.3 eV) and surface adsorbed oxygen species (such as SrCO3 at ~533.2 eV).32,33 The content of O22-/O- ions has very important effect on the formation of surface peroxide species and is very beneficial to enhance electron transfer between perovskite surface and adsorbed oxygen molecule.34,35 It is well known that O22-/O- ions have higher chemical activity than O2- ions and surface absorbed oxygen species such as H2O, CO2, SrO, Sr(OH)2, SrCO3 etc. Increasing O22-/Oions in catalysts can create more active oxygen sites and improve the activation ability of adsorbed oxygen molecule. Therefore, XPS was generally used to quantify these oxygen species to confirm the activity and quantity of surface oxygen species. Calculation for O1s in Figure 3b shows that relative contents of O22-/O- ions are 16.5% for LSCO-Sr and 19.8% for LSCO-Sr-H+, almost 2 times higher than 8% for original LSCO. These suggest that urea pyrolysis leads to surface Sr enrichment and enhances charge transfer between Co4+ and O2-, which increases more oxygen species and produces more useless Sr segregation. The chemical etching in the second step reduces some surface segregations and maintains original dominant structure by regulating


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reaction time and pH value (the process is shown in experimental details of supporting information), which creates more active sites and further improves the interaction between Co4+ ions and O2- ions. Therefore, perovskite LSCO-Sr-H+ possesses more active oxygen species and less Sr segregation on the surface.

Figure 4. a) O2-TPD profiles, b) H2-TPR profiles, c) CO oxidation activity and d) 2D histogram of the 10%, 50%, 90% CO conversion vs. reaction temperatures for LSCO, LSCO-Sr, and LSCO-Sr-H+. The O2-TPD was carried out to detect oxygen species of three samples (Figure 4a). Usually, adsorbed oxygen (Oads) is released at temperature below 400ºC, while the peak observed at temperature >400ºC is related with desorption of lattice oxygen.2,10,36 LSCO-Sr has lower Oads desorption temperature (87ºC) compared to LSCO (380ºC) and LSCO-Sr-H+ (179 ºC), and


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LSCO-Sr and LSCO-Sr-H+ exhibit similar onset temperature, obviously lower than LSCO. Moreover, we also observe that LSCO-Sr-H+ has the highest adsorption capacity (absorption peak area) of active oxygen species at < 200 ºC. This indicates that surface oxygen mobility of perovskite LSCO is poorer than other two treated samples. We used H2-TPR to further analyze chemical adsorption capacity and redox behavior. According to previous studies,10,36 the reduction process of Co-based perovskites is divided into two steps: 1) Co4+/3+→Co2+ (~665K), 2) Co2+→Co0 (~773K). In Figure 4b, we can observe that the reducing temperature of first peak for LSCO-Sr-H+ is 311 ºC, lower than that for LSCO (364 ºC) and LSCO-Sr (335 ºC), i.e. Reactivity and adsorption capacity of surface oxygen species increase in the sequence of LSCO

Activation of Surface Oxygen Sites in a Cobalt ... - ACS Publications

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