Surface Reconstructions of Metal Oxides and the Consequences on

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Surface reconstructions of metal oxides and the consequences on catalytic chemistry Felipe Polo-Garzon, Zhenghong Bao, Xuanyu Zhang, Weixin Huang, and Zili Wu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b01097 • Publication Date (Web): 13 May 2019 Downloaded from http://pubs.acs.org on May 13, 2019

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ACS Catalysis

Surface Reconstructions of Metal Oxides and the Consequences on Catalytic Chemistry Felipe Polo-Garzon, Zhenghong Bao, Xuanyu Zhang,

,

Weixin Huang

*

and Zili Wu

*

Hefei National Laboratory for Physical Sciences at the Microscale, Key Laboratory of Surface and Interface Chemistry and Energy Catalysis of Anhui Higher Education Institutes, CAS Key Laboratory of Materials for Energy Conversion and Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China Chemical Science Division and Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831

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Abstract Catalysts are inherently dynamic in nature as they respond to the environments by changing the local and extended structures. Surface reconstruction is among such dynamic behaviors of catalysts and greatly impacts the physical, chemical and electronic properties of catalysts and consequently the catalytic performances. Thus understanding the nature of the catalytic sites of the reconstructed surfaces is essential for establishing the structure – catalysis relations and has attracted much interest in catalysis research. Knowledge of the reconstruction of metal oxides has been much limited compared to metal surfaces because the nature of oxide surfaces is generally more complex. Yet significant progresses have been made in recent years over oxide surfaces thanks to the advances in the ability to synthesize model oxide nanocrystals and to characterize the surface reconstruction behaviors under in situ and operando conditions with advanced spectroscopy and microscopy aided with computational modeling. In this Perspective, such advances in understanding the surface reconstruction behaviors of metal oxide nanoshapes and thin films with well-define surface structures under as-synthesized, various pretreatment, and reaction environments will be summarized, and the catalytic consequences of these reconstructions will be highlighted. The results from these studies clearly show that the combination of model oxides and in situ/operando investigations is imperative in shaping the fundamental understanding of the dynamic reconstructions of oxide surfaces. Yet it is still challenging to study the surface reconstructions of oxide catalysts with meaningful temporal and spatial resolution under operating conditions. Future opportunities are discussed on how to address the challenges and eventually help to design more efficient catalysts by taking advantages of the surface reconstructions. Key Words: metal oxide, nanoshapes, surface reconstruction, in situ, operando, structure, composition 2


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1. Introduction Surface reconstruction of solids can be defined as deviations of surface compositions and structures of solids from the bulk truncations resulting from different local coordination environments between surface atoms and bulk atoms and has been recognized to strongly affect surface chemistry and catalysis of solids.1 Reconstructed surfaces tend to adopt thermodynamically most stable surface compositions and structures in an equilibrium with the environments as long as the reconstruction kinetics is fast enough. The topmost atoms on a clean surface of most monoatomic solids would relax toward the bulk to result in shorter interlayer spacing to minimize the surface free energy. The rougher the surface, the larger the inward relaxation.2 In a reactive environment, chemisorption of molecules on solid surfaces occurs, likely driving massive relocations of the atoms in the solid substrate (adsorbate-induced surface reconstruction) to minimize the total free energy of the catalytic system. It was uncovered that not only the atoms in the solid substrate but also the atoms in the adsorbing molecule relocate to optimize the surface chemical bond before chemisorption.1 Thermodynamics drives the chemical rearrangements on both sides of the surface bond and the weakening of bonds among the substrate atoms and the rearrangement of the adsorbates are compensated by the exothermic heat of chemisorption. Adsorbate-induced surface reconstruction can be considered as a chemical reaction between molecules and solid surfaces. Thus the product, surface composition and structure of reconstructed surfaces, sensitively varies with the reactants, including surface composition and structure of unreconstructed surfaces and compositions and concentrations/pressures of molecules, and with the temperature.1, 3-6 Chemisorption of reactants on solid catalyst surfaces lays the foundation of heterogeneous catalysis, meanwhile, heterogeneous catalytic reactions occur under conditions up to several 3


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hundred atmospheres pressure and several hundred degrees Celsius temperature. Therefore, catalyst nanoparticles are anticipated to undergo surface reconstructions during heterogeneous catalytic reactions. It is thus critical to investigate reconstructing processes of catalyst surfaces under realistic reaction conditions to discern catalytic processes at the microscopic level. Furthermore, it would be advantageous to control the surface reconstruction behaviors to tune the catalytic performance of catalyst nanoparticles. However, it remains one of the daunting challenges in heterogeneous catalysis to identify surface reconstructions of working catalyst nanoparticles mainly due to the scarcity of surface-sensitive characterization techniques that can operate under conditions resembling real catalytic reactions. Particularly, the types and concentrations of adsorbates may vary among adsorption, surface reaction and desorption steps within a catalytic reaction cycle, likely resulting in differently reconstructed surfaces. Meanwhile, complex and non-uniform structures of catalyst nanoparticles add additional difficulty in correlating the original surface, reconstructed surface and the catalytic performance. While catalysts reconstruction during reaction processes has long been recognized and can be revealed by the evolution of catalytic performances, it is desirable to characterize the reconstructed state to reveal the structure of the working catalytic sites to enable new catalyst design. General pseudo in situ approaches include surface science studies of single crystals and thin films under UHV conditions 7-14 and surface characterizations of catalyst nanoparticles before and after catalytic reactions without exposures to ambient atmosphere.15-21 The former approach on monolithic surfaces can unambiguously identify compositions and structures of reconstructed surfaces, but suffers from the commonly named “materials gap” and “pressure gap” when extending the acquired information to working catalysts. The latter approach over particles has difficulty in identifying the distribution of the compositions and structures of reconstructed 4


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surfaces due to the heterogeneous structures of powder catalysts. These pseudo approaches are apparently not effective for understanding surface reconstructions induced by reversibly-adsorbed adsorbates. In situ characterizations of working catalysts with uniform structures are the ideal approach to study surface reconstructions and have recently become possible. On the one hand (tools side), various spectroscopic and microscopic techniques have been developed capable of probing the reconstruction of some catalysts under in situ and sometimes operando conditions with high spatial, energy, and even time resolutions.22-23 For examples, near-ambient pressure XPS is particularly suitable to study the oxidation state and coordination number of surface atoms under reaction conditions;6, 19, 24 Near-ambient pressure scanning tunneling microscopy, transmission or environmental scanning electron microscopy provides direct evidence of surface reconstructions;1, 25-26

Although its use in operando conditions is rather limited, in situ Low energy ion scattering

(LEIS) is a powerful technique to probe compositions of surfaces and sub-surfaces with submonolayer precision27, better than common XPS (a few nm detection depth)28-30; Adsorption microcalorimetry combined with other techniques can give information about the nature, strength and number of sites on the reconstructed surface through the adsorption of probe molecules.31-32 On the other hand (materials side), progresses in controlled synthesis of inorganic materials have offered nanoshaped powder catalysts with uniform and well-defined facets 33-34, which can be used to greatly reduce the complexity of fundamental catalysis investigations35-39, including surface reconstructions. In addition, quantifications of energy barriers on these surfaces are possible with computational input from DFT. Metal oxides are frequently used as catalysts and catalysts supports. Consisting of metal cations and oxygen anions, metal oxide surfaces, particularly polar surfaces of metal oxides, are 5


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facile to undergo reconstructions 40,41,42. Surface reconstructions of oxide single crystals and thin films have been extensively explored under UHV conditions.43-44 Very recently, surface reconstructions of nanoshaped metal oxides under catalytic reaction conditions were also reported, which reveal actual active structures of catalytic oxides

45.

Herein we devote the present

Perspective to surface reconstructions of catalytic oxides with well-defined structures that are highly relevant to heterogeneous catalysis. The purpose of this work is not to recollect all literature regarding surface reconstructions of metal oxides, but rather to explore novel insights brought to heterogeneous catalysis by representative advances of the field using newly-emerged oxide nanoshapes (most cases) and traditional single crystals and thin films (a few cases). The Perspective is organized as the following: a review of surface reconstruction of as-synthesized oxide nanoshapes (cerium oxide and titanium oxide), and surface reconstruction of oxide nanoshapes under reactive environments (copper oxides, iron oxides, and perovskites), ended with the summary where future opportunities and challenges in understanding and capitalizing surface reconstruction phenomena for catalysis are outlined.

2. Surface reconstruction of as-synthesized oxide nanoshapes

The exposure of certain facets on oxide particles determine both surface geometric structure and element composition, and therefore strongly affect the catalytic performance.39 Although synthesized oxide nanoshapes often expose well-defined crystal planes, the composition of structure of exposed crystal planes cannot be assumed to adopt those of perfect crystal planes. For example, ceria cubes can have several types of defects like steps, voids, lattice distortion, bending and twinning46, which are generally caused by surface reconstructions during the catalyst 6


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synthesis, pretreatment and reaction process. Below we will show surface imperfections of assynthesized oxide nanoshapes using ceria and titania nanocrystals as examples. The term “assynthesized” refers to a state of the oxides after synthesis but may undergo an oxidation or reduction treatment.

2.1 CeO2 Ceria (CeO2) finds broad applications in catalysis, fuel cell and microelectronics. The morphologies of CeO2 nanocrystals are mostly reported in rods (typically {110} + {100} facets), cubes ({100} facet), and octahedra ({111} facet) at nanoscale,47-49 which often exhibit notable differences in many catalytic reactions. It is well known that the stability of these planes lies in the order of (100) < (110) < (111), and many investigations have been focusing on these three stable low index facets. Recently, it is noted that ceria rods could also expose {111} facets after annealing treatment at 973 K, which includes large amount of nanoscopic structural defects.50-54 CeO2 possess the fluorite crystal structure (Fm3m) with a cell parameter of 0.541 nm. In an ideal ceria, the Ce cations are aligned in a face-centered cubic (fcc) structure with O anions attached within the unit cell of a simple cubic configuration, giving the coordination of four-fold O anions and eight-fold Ce cations.55 As shown in Figure 1, an ideal ceria (111) surface displays three-fold coordinated O anions and seven-fold coordinated Ce cations, an ideal CeO2 (110) facet exposes three-fold coordinate oxygen anions and six-fold coordinate Ce cations, and an ideal CeO2 (100) surface exhibits two-fold coordinated oxygen anions and six-fold coordinated cerium cations.40, 56-57 The variation in number of surface coordinative unsaturation (cus) of Ce cations and O anions often leads to different interaction strength with surface adsorbates. The catalytic properties of ceria nanoshapes were often related to these surface atomic coordinations over ideal 7


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crystal planes. Yet it becomes increasingly clear that this kind of simple correlation cannot well explain experimental observations. Characterizations at both macroscopic (adsorption and reaction of probe molecules) and microscopic (electron microscopy) scales reveal that the surfaces of the ceria nanoshapes are not as perfect as they appeared.

Figure 1. Plain view of three typical ceria surfaces, (111), reconstructed (100) and (110) with the coordination unsaturation (cus) numbers marked for surface O and Ce. Reproduced with permission from 56. Copyright 2015 American Chemical Society.

Methanol is considered a “smart” reagent to probe the surface properties of oxides55 in the sense that its associated products can well probe the coordination situation of surface cations. In examining the nature of surface sites of oxidized ceria nanoshapes (rods, cubes and octahedra) by methanol adsorption coupled with IR spectroscopy,58 it was shown that on-top methoxy is shown on all three CeO2 shapes, while bridging and three-coordinated methoxy species are only revealed on ceria rods and cubes (Figure 2A). Considering the coordination status of surface cerium and oxygen atoms as well as their relative geometry on the ideal surfaces (Figure 1), it is expected that only on-top methoxy species could be generated upon dissociative adsorption of CH3OH on the (110) and (111) surfaces and only bridging methoxy on (100) surface. A well agreement is found between the IR observation and the prediction only for the (111) surface, indicating a perfect termination of the ceria octahedra. However, there are apparent discrepancies between the IR 8


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observations and the theoretic expectations on the categories of methoxy species on the (110) and (100) surfaces. The polar O-terminated ceria(100) surface is suggested to undergo a surface reconstruction to stabilize the surface polarity by taking away 50% of the O from the surface.40,41,42 However, even on such a reconstructed (100) facet, it might be difficult to generate an on-top methoxy on the cerium site owing to the steric hindrance of the surface O. It is possible that part of the (100) facet suffers further reconstruction due to the presence of defect sites.57,40 So some of the under-coordinated Ce cations are accessible to form on-top, the bridging and three-coordinated methoxies. Surface oxygen vacancies were also believed to be responsible for the generation of three-coordinated methoxy species observed on the ceria rods. Such surface reconstruction and oxygen vacancies were demonstrated in the recent electron microscopy studies shown later in this section.49, 59 Methanol adsorption results suggested imperfections of ceria rods and cubes with exposed high energy {110} and {100} planes. The dependence of defect sites on the surface and in the bulk on the shape of ceria was investigated by Raman spectroscopy coupled with O2 adsorption on both reduced and fully oxidized ceria nanocrystals. The amount of intrinsic defect (Frenkel-type) sites with a characteristic Raman band at 592 cm-1 on oxidized CeO2 nanoshapes follows the order of octahedra< cubes < rods, which is estimated from the relative intensity ratio of I592/I460 in the UV Raman spectra (Figure 2B). At reduced status, rods and cubes have similar amounts of formed defect sites (O-vacancies associated with Ce3+ defects) as probed by O2 chemisorption, but such defect sites are more clustered on rods than on cubes. Few defect sites could be created on the octahedra because of the least reducibility. The difference in the nature of defect sites induces different surface adsorbed O species on the nanoshapes: superoxide on 1-electron defect sites and

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Yang et al.

61

also used CO chemisorption to probe the surface structure of CeO2

nanoshape. After the calibration of CO stretching frequencies using results obtained from different CeO2 single-crystal surfaces, they revealed that the rod-shaped ceria undergoes extensive restructuring and exposes {111} nanofacets, an observation also demonstrated in the atomic scale electron microscopy work49. As a result, the CeO2 rods exhibit a rather complex surface structure displaying a variety of defects (sawtooth-like facets, O vacancies, edges and corners), which can be responsible for the high activity. CO oxidation is a widely adopted model reaction to probe the redox property of ceriabased catalysts. The light-off activities of CO oxidation over three CeO2 nanoshapes follow the sequence of rods > cubes > octahedra. For example, the temperature for 20% CO conversion is 260 oC on rods, 320 oC on cubes, and 413 oC on octahedra. The CeO2 rods and cubes were found more reactive than octahedra, due partly to the formation of strongly bonded carbonate species on rods and cubes than on octahedra upon CO adsorption at room temperature. The lattice oxygen reactivity and mobility (accessed by CO-TPR and O isotopic exchange, respectively) are strongly surface-dependent, following the same tendency in CO oxidation reaction: rods > cubes > octahedra. Combined with reaction kinetic and spectroscopy studies, it was concluded that the driving forces for the surface-dependent behaviors of CO interaction with CeO2 are believed to be the differences in surface O vacancy formation energy, nature and amount of low coordinated sites and defect sites among the different CeO2 nanoshapes.62 Since CeO2 nanoshapes exhibit large differences in redox as well as acid-base properties,56 it is convenient to employ them to study these effects on the catalysis of supported metals without varying the support compositions.38,

51, 63-67

One of such examples is ceria

nanoshapes supported Ag catalysts where the defect sites in CeO2 support greatly impact the 11


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metal–ceria interaction, the structure and CO oxidation activity.68 ceria rods possess a higher oxygen vacancy concentration than that on CeO2 cubes, and have both small and large O vacancy clusters while ceria cubes are mainly dominated with large O vacancy clusters. Liu et al.

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also

reported small neutral Ce3+-O vacancy associates and larger size oxygen vacancy clusters on ceria rods. This leads to two types of supported silver species on CeO2 rods, positively charged Agn+ clusters at small O vacancy dominating in 1%-Ag/CeO2-rods and fine Ag nanoparticles at large O vacancy clusters dominating in 3%-Ag/CeO2-rods, but only Ag nanoparticles at large O vacancy clusters of cubes. Supported Ag nanoparticles are much more effective in activating the surface lattice oxygen, creating O vacancies, and promoting the surface reduction of ceria than supported Agn+ clusters. The average charge density of O vacancies and the ratio between large O vacancy clusters and small vacancies in ceria nanocrystals are reduced with supported Ag nanoparticles but enhanced with positively charged Agn+ clusters. Ag nanoparticles and ceria with a high O vacancy concentration and appropriate ratios between large O vacancy clusters and small vacancies compose the active structure in CO catalytic oxidation. Ceria cubes are better than rods to be a support to prepare Ag/CeO2 catalysts with a low Ag loading but a high catalytic activity.68 While some insights into the surface structure of the ceria nanoshapes have been gained from the macroscopic spectroscopy and reaction kinetic studies as described above, “seeing is believing” drives numerous efforts in atomic scale visualization of the surfaces of ceria via advanced electron microscopy. Lin et al. 49 determined the atomic surface structures of the (100), (110), and (111) surfaces observed on ceria cubes using aberration-corrected high-resolution electron microscopy (HREM) (Figure 3 Top). The predominantly exposed (100) surface was found to have a mixture of Ce, O, and reduced CeO terminations, underlining the complexity of this polar surface structure that was previously oversimplified. The (110) surface shows the 12


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“sawtooth-like” (111) nanofacets and flat CeO/Q terminations with oxygen vacancies. The (111) plane is ideally truncated with an O termination. These results shed light on that oxide nanocrystals are not always perfectly structured. In contrast, surface reconstruction, nanofaceting, and surface vacancies must be taken into consideration when correlating the structure-catalysis property for oxide nanomaterials. In situ phase contrast HREM with an aberration corrected imaging lens was applied to view the stability of CeO2 facets.70 Atomic hopping processes occur on the {100} type facets, while the {111} facets are found to be stable, consistent with the relative stability of these two surfaces. The instability of oxide surfaces and atomic reorganization often go side by side with electronic changes, e.g., Ce valence, associated O vacancies, and O transport pathways, which are key to the catalytic activities.71 Environmental transmission electron microscopy (ETEM) is an effective approach to directly view the surface reconstruction of CeO2 under chemical environment72-75. Crozier et al.76 applied in situ ETEM to study the dynamic changes occurred during redox reactions on CeO2 nanoparticles in a H2 atmosphere. It was found that a reversible phase transformation happened in pure ceria at 730 °C and O vacancies were introduced during the reduction process. The transformation takes place pretty fast with the cerium cation switching from 4+ to 3+ in the superstructure. The ceria (110) surface is originally constructed with a series of low-energy (111) nanofacets, which will be slowly transformed into a smooth (110) facet under strong reduction. The surface transformation enables the reduced surface to accommodate a high concentration of O vacancies without generating a strong perpendicular dipole moment. These atomic-level observations provide insights into the experimental results that ceria rods ((110) surface) can be

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relatively stable (due to the (111) nanofacets) yet still very reactive (due to the high density of O vacancies) in redox catalysis.

Figure 3. (Top row) A HREM image on a typical CeO2 cube at the [110] zone axis, showing (100), (110) and (111) facets; (Bottom) the mobility of cations at {100} surfaces under high vacuum (5 × 10Q2 mbar), 5 × 10Q/ mbar O2 and 5 × 10Q/ mbar CO2, respectively. Reproduced with permission from 49 and 70. Copyright 2014 and 2017 American Chemical Society.

Bugnet et al.

70

directly visualized the atomic mobility of Ce atoms on {100} facets of

ceria cubes at room temperature in high vacuum (HV), O2, and CO2 atmospheres in an ETEM (Figure 3. Bottom). The mobility of cerium atoms is considerable in HV, and weakens dramatically in an oxygen environment, then becomes undetectable in a carbon dioxide environment. This implies not only that the effects of electron-induced irradiation commonly

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observed in HV can be controlled and tuned in reactive environmental conditions, but also that ETEM can be an effective atomic-level approach to study surface-adsorbate interaction. Scanning tunneling microscopy (STM) was also applied to observe the surface reconstruction of ceria thin films. Pan et al.

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found that CeO2 (111) thin films could be

transformed into well-defined nanoshapes by annealing them in a lean O2 environment. The CeO2 nanoparticles undertake a distinct shape evolution that finally results in crystallites exposing large proportion of (100) planes with increasing temperature from 800 to 1100 K. The surface structure of the ceria (100) surface was further studied by STM coupled with density functional theory. Two surface reconstructions were figured out that both compensate for the electrostatic dipole of the bulk-cut (100) facet. Whereas the main (2×2) phase is consisted of pyramidal CeO4 units, similar to the Wolf reconstruction, an oxygen termination has been identified for a c(2×2) minority phase. Both structures are further stabilized by O removal from the stoichiometric surfaces. These STM observations of the surface structures of ceria (100) surface is complementary to those from the high resolution electron microscopy of ceria cubes,49 both indicating a complex reconstruction behavior of the polar surfaces of oxides. 2.2 TiO2 Titanium dioxide (TiO2) has become one of the most studied metal oxides in many function areas such as solar cell,78-81 photovoltaic cell,82-83 lithium ion batteries,84-87 photocatalysis88-90 and heterogeneous catalysis91-94. The superiorities of TiO2 include the high stability, low toxicity and natural earth abundance (low cost).95 The three major phase structures of TiO2 are rutile. anatase and brookite. Rutile is most stable, and anatase and brookite are readily transformed to rutile when heated.96 The pure brookite is rather difficult to prepare and often found as a mixture with the other phases.97 The basic building block in the bulk structures of both rutile and anatase composes of a 15


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Ti atom surrounded by six O atoms in an octahedral configuration. For rutile, previous DFT calculation results revealed that the stability of the low-index facets follows the sequence of (110) > (100) > (011) > (001).96,98,99 For anatase, the relative energies follow the sequence of (101) < (100) < (001).100-101 Various types of reconstructions of TiO2 surfaces have been observed, but the structures of reconstructed surfaces are still argued. Figure 4A1 shows the structure of ideal bulk-terminated rutile TiO2(110)-(1×1) surface. This surface contains two different O atoms, the three-fold coordination (O3c) in bulk and the two-fold coordinated bridge oxygen anions (Bridge O2c), and five-fold coordinated Ti cations (Ti5c) are perpendicular to the surface.96 Although the rutile (110) plane is most stable among all rutile surfaces, it is reduced upon annealing and reconstructs into in a (1×2) structure that has been explained with various types of models, including added Ti2O3 rows (Figure 4A2),102-104. “missing row”105, “missing unit”106 and added Ti3O6 rows107. The rutile TiO2(011)-(1×1) surface exposes top two-fold coordinated oxygen atoms at the apices and three-fold coordinated at the valleys (Figure 4B1). It tends to form a more stable reconstruction with a (2×1) structure after preparation in UHV condition, as shown in Figure 4B2.98,108 The TiO2 (001) surface possesses higher surface energies. The structure of TiO2 (001)-(1×1) surface is shown in Figure 4C1, on which all the surface titanium cations are four-fold coordinated and all the surface oxygen anions are two-fold coordinated.109 Upon annealing, various types of surface reconstructions were observed, and two typical structures of reconstructed surface were identified as {011} (Figure 4C2) and {114} (Figure 4C3) micro faceted phases.110

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a (1×n) reconstructed surface.114 The anatase (001) surface shows five-fold coordinated titanium cations, as well as two-fold and three-fold coordinated O anions, see Figure 4F1. However, the anatase TiO2(001)-(1×1) surface is not stable and reconstructs to a (1×4) structure (Figure 4F2) when heated to high temperatures.115-116 Photocatalysis and thermal catalysis of methanol and formaldehyde on TiO2 sensitively depend on the surface structures.117-125 Both methoxy and methanol species are photo-reactive on the rutile TiO2(110)-(1×1) surface, and methyl formate was observed to form as the product of the photocatalytic cross-coupling between chemisorbed formaldehyde and chemisorbed methoxy species.117 The HCHO adsorbed at Ti5c sites of rutile TiO2(110)-(1×1) can be photocatalytically oxidized into surface HCOO species via a transient HCO intermediate.118 On rutile TiO2(011)(1×2) reconstructed surface, the HCHO adsorbed at Ti5c sites is photoactive, whereas that at O2c sites is not.121 On anatase TiO2(001)-(1×4) reconstructed surface, methanol facilely dissociates at Ti4c sites to form strongly adsorbed methoxy species that undergo the dehydration coupling reaction to produce dimethyl ether,124 while photocatalytic oxidation reaction was observed specifically to occur for methoxy species at the Ti4c sites of the (1×4) added row reconstructed surface but not for methanol at the (1×1) basal surface.125 Thermal catalysis of methanol on two types of anatase TiO2 nanoshapes, nanoplates with higher content of {001} planes and truncated-bipyramidal nanocrystals with dominance of {101} facets, was investigated.126 It was shown that adsorbed methoxy species is dominated for all surface reactions, while the methanol dimer is a spectator species. Dimethyl ether was observed to form on both nanoshapes. The {001} facets on TiO2 nanoplate was proposed to undergo a similar surface reconstruction to the TiO2(001)-(1×4) single crystal surface124 and is responsible for the dimethyl ether formation. On the (101) surface exposed on truncated-bipyramidal nanocrystals, 18


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of surface F1+ color centers is lower on TiO2-{100} and TiO2-{101} than on TiO2-{001} facets. Under the same hydrogen treatment conditions, F1+ color centers and Ti3+ species are the superior defects created on TiO2-{001} NCs and are mainly located in the subsurface/bulk region; F1+ color centers, Ti3+ species, O2- species and Ti4+-O radical species are created on TiO2-{100} and spread from the surface to the subsurface/bulk region; and F1+ color center, Ti3+ species and O2- species are created on TiO2-{101} and distributed from the surface to the subsurface/bulk region. The created defects simultaneously enhance the light absorption/charge creation and the charge recombination probabilities, respectively beneficial and detrimental for the photocatalytic activity of TiO2 NCs. Thus H-TiO2-{100} and H-TiO2-{101} NCs without specific locations of defects exhibit similar photocatalytic activity to corresponding TiO2-{100} and TiO2-{101} NCs, respectively, but H-TiO2-{001} NCs with most defects in the subsurface/bulk exhibit an additional electric field between the stoichiometric surface and the reduced subsurface that enhances the charge separation efficiency and photocatalytic performance.

3. Surface reconstruction of oxide nanoshapes under reactive environments Reactive environments can be decisive for reconstructions of metal oxide catalyst surfaces. Detailed work on nanocrystals and thin films of both binary and ternary oxides have successfully shown the reconstructions of the outermost layer by inclusion/removal of cations and/or oxygen ions, and these reconstructions have been shown to depend on the chemical potential of the reactive species. At the same time, surface reconstructions can hinder or boost the catalytic evolution of reactants, depending on the activation barriers over the newly formed active sites. Details on these

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findings, and how they have contributed to progressing the field, are presented in this section with a few representing examples including copper oxides, iron oxides and perovskites. 3.1 Copper oxides Copper oxides are present in catalysts used for important catalytic reactions, such as watergas shift (WGS), methanol synthesis, and propylene expoxidation.20,

132-134

The active copper

species during the catalytic reactions, i.e. Cu(0), Cu(I) and Cu(II), is always an issue under debating. Recent works combining Cu2O nanoshapes and in situ characterizations demonstrate that the reconstruction of copper species depends on both reactive environments and catalyst structures. Catalytic performances of uniform octahedra, cubic and rhombic dodecahedra Cu2O nanocrystals respectively enclosed with {111}, {100} and {110} facets were evaluated in CO oxidation with stoichiometric O2 (2% CO and 1% O2 balanced with N2), CO oxidation with excess O2 (1% CO in dry air), propylene oxidation with O2 (C3H6/O2/N2 = 2/1/22), and WGS reaction (CO/H2O=1/2). The surface of various Cu2O nanocrystals does not reconstruct in CO oxidation with stoichiometric O2 135 and propylene oxidation with O2 136. In CO oxidation with excess O2, the surfaces of c-Cu2O and o-Cu2O oxidize into CuO thin films. The CuO thin film formed on oCu2O had much higher catalytic activity and a lower apparent activation energy than that formed on c-Cu2O.137 Accompanying DFT calculations demonstrated that the CuO thin film on Cu2O(111) is terminated with three-fold coordinated Cu3c and three-fold coordinated O3c, whereas that on Cu2O(100) is terminated with two-fold coordinated O2c. The coordination unsaturated Cu3c of the CuO thin film on Cu2O(111) is more active than the coordination unsaturated O2c of the CuO thin film on Cu2O(100) for catalyzing CO oxidation. Very recently, uniform Cu2O nanocrystals with different sizes between 1029 and 34 nm were used to catalyze CO oxidation in excess O2 138. As the Cu2O size decreases, the surface reconstruction extent increases, and the dominant catalytic 21


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active sites switch from face sites (CuO on Cu2O(100)) to edge sites (CuO on Cu2O(110)), highlighting the significance of size effect of Cu2O on the surface reconstruction behavior and on the reaction mechanism. During WGS reaction, Cu2O nanocrystals were observed to undergo full reduction into Cu nanocrystals with Cu(0) and CuxO (x 10) coexisting on the surface while conserving their nanoshape.45 Such a morphology-preserved reconstruction allows the preparation of uniform Cu nanocrystals from corresponding Cu2O nanocrystals (Figure 6A).* For different facet terminations, the catalytic performance of Cu NCs in low-temperature WGS reaction was evaluated. Cu cubes enclosed with {100} facets are very active while Cu octahedra enclosed with {111} facets are inactive (Figure 6B). The Cu-CuxO interface on Cu surface was identified as the active site, and it exhibits the highest catalytic activity on Cu(100). However, the Cu-CuxO interface on Cu(111) becomes self-poisoned by the very stable surface formate species, which prevents the catalytic cycle from completion. The identification of the active Cu (100) facet led to the design of a superior ZnO/cubic-Cu catalyst in low-temperature WGS reaction.45

*

As observed in Figure 6, the synthesized particles can be up to roughly 1000 nm in size, which lays outside the commonly accepted definition of nanoparticle (~ up to 200 nm). We refer to these particles as nano for the sake of consistency with the references and we encourage the reader to treat the term ‘nanoparticle’ as a general definition. 22


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Figure 6. Representative SEM images of a Cu2O cubes, b Cu2O octahedral, and c Cu2O rhombic dodecahedra. d XRD patterns, e Cu LMM AES spectra measured without exposure to air, and f in situ DRIFTS spectra of CO adsorption at – 150 oC of Cu2O cubes (a1), octahedra (b1), rhombic dodecahedra (c1) and Cu cubes (a2), octahedra (b2) and rhombic dodecahedra (c2). Representative SEM, TEM and HRTEM images of (g1–g3) Cu cubes, (h1–h3) Cu octahedra and (i1–i3) Cu rhombic dodecahedra. The insets in the HRTEM images show the ED patterns of corresponding Cu nanocrystals. The lattice fringes of 1.80 and 2.08 Å, respectively, correspond to the spacing of Cu(200) and (111) crystal planes (JCPDS card NO. 89-2838). The scale bars of a–c and (g1–i1) correspond to 1000 nm, that of (g2–i2) correspond to 500 nm, and that of (g3–h3) correspond to 2 23


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nm. Reproduced with permission from 45. Copyright 2017 Creative Commons License (http://creativecommons.org/licenses/by/4.0/).

The above results of reconstruction of Cu2O nanocrystals reveal a connection between the crystal plane exposed and the reconstruction process of catalyst nanoparticles; thus, a strong connection exists between the structures of the original catalyst nanoparticle and the reconstructed working catalyst nanoparticle. Such a crystal plane-controlled reconstructing process not only enriches the fundamental understanding of catalyst reconstruction under different atmospheres but also provides a way to tune the structure and catalytic performance of reconstructed catalyst nanoparticles. Reconstruction was also observed on Cu single crystal surfaces under reactive conditions. For example, an in situ near ambient-pressure STM & XPS, and infrared reflection absorption spectroscopy (IRRAS) study of CO oxidation over a Cu(111) single crystal revealed the dynamic nature of the catalyst.9 Metallic copper was reported as the most active phase for CO oxidation via a Langmuir-Hinshelwood mechanism, although it is not stable under reaction conditions, as the surface reconstructs in a redox cycle between CuO, Cu2O and Cu. During the redox reconstruction process, Cu atoms are detached from terraces and step edges during oxidation and rejoin those edges during reduction. Inverse catalysts, where metal-oxide nanoparticles are deposited on metal substrates, are useful systems to study the reconstruction of active sites. “Massively reconstructed” CeO2X/Cu(111)

inverse catalyst possesses copper microterraces that showed enhanced performance for

the water-gas shift reaction (WGS) when compared to Cu(100), Cu(111), and “regular” as-grown CeO2-X/Cu(111) catalysts.10-11 The “massively reconstructed” CeO2-x/Cu(111) was acquired via 24


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oxidation of the as-grown catalysts in O2 followed by subsequent reduction in CO. Such treatments were observed to induce significant morphological changes of the catalyst that dramatically increase the catalytic activity. Surface reconstruction was recently found on an industrially used Cu/ZnO/Al2O3 catalyst for methanol synthesis through the hydrogenation of CO and CO2. This catalyst is a complex system where Cu and ZnO present a synergistic interaction. As shown recently, the reduced catalyst presents a core-shell-like structure due to the so-called strong metal support interaction (SMSI),15 in which the Cu particles are coated by “graphitic-like” layers of ZnO, a metastable structure (wurtzite ZnO is the thermodynamically stable structure). Such reconstructed unit as the active structure for methanol synthesis was also recently demonstrated over ZnCu(111) model surface during CO2 hydrogenation.139 A recent computational study of Cu/Co systems showed that for the clean surface, Cu completely segregates to form a core-shell-like structure.140 However, upon CO adsorption, some Co atoms swap with surface Cu atoms, evidencing an adsorbateinduced reconstruction. 3.2 Iron oxides Iron oxide catalysts are of paramount importance for industrial processes such as ammonia production, the water-gas shit reaction (WGS), Fischer-Tropsch synthesis, and non-oxidative dehydrogenation reaction of ethylbenzene to styrene.20, 141-142 Reconstruction of working catalysts among metallic iron, various iron oxides and various iron carbides have been often observed, leading to debates on the nature of active sites. For example, Cr and Cu-doped iron-oxide powder catalysts have shown reconstruction properties under high-temperature WGS conditions. Under reaction conditions, iron oxide transitions from Fe2O3 to Fe3O4 and Cu ex-solutes as Cu nanoparticles coated with an iron oxide layer. The inclusion of Cu in the Cu-promoted CrFeOX 25


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catalysts has been shown to enhance turnover frequencies.19-21 Despite the significant advances in understanding this system of industrial relevance, the consequences of the SMSI phenomenon on the activation energies of elementary reaction steps are yet to be unveiled. The Freund group has recently endeavored to elucidate the reconstruction process microscopically and identify the active site of a FeO(111) bilayer film deposited on a Pt(111) single crystal in CO oxidation. The FeO(111) bilayer film deposited on a Pt(111) single crystal is active in catalyzing CO oxidation. It was found via high-resolution STM, that upon O2 exposure at 177 oC, and at least at 20 mbar, the FeO bilayer reconstructs to the O-Fe-O trilayer (FeO2).12 DFT calculations demonstrated that such a reconstruction proceeds via low activation barriers and the O-Fe-O trilayer favors the oxidation of CO via the Mars-van Krevelen mechanism. Such a fundamental understanding was found effective to explain the SMSI effect for Pt deposited on Fe3O4(111) film.13

Figure 7. A) a) Typical morphology of oxidized FeO2-x films on Pt(111) at sub-monolayer coverages. The cross-view of a FeO2-x/Pt(111) film is shown below. b)–d) STM images of 0.5 ML FeO2-x/Pt(111) film exposed to 10-6 mbar CO at 177 oC for 2 min (b), additional 2 min (that is, 4 min in total) (c), and 8 min in total (d); Image size is 150 nm x 150 nm; tunneling bias and current are -3 V and 0.1 nA, respectively. e) The FeO2-x surface area normalized to the area in the as26


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prepared sample as a function of the accumulative exposure time. B) Scenario for CO reaction with FeO2-x/Pt(111) islands. CO adsorbed on Pt reacts with the O atom at the island edge to form CO2. The atoms surrounding the O vacancy relax and locally form a FeO bilayer structure. Further reaction occurs at the interface between the original oxidized FeO2 and the reduced FeO-like phases, providing the weakest bonded oxygen. Reproduced with permission from ref 14. Copyright 2018 John Wiley and Sons.

In a follow-up study, it was found that CO oxidation proceeds most easily at the interface between the reduced (FeO) and oxidized (FeO2) phases of the iron oxide film islands deposited on Pt(111) (Figure 7).14 The reduced phase (FeO) is initiated via the oxidation of adsorbed CO (on Pt) with an oxygen of FeO2 at the edge of the FeO2 islands. This generates an oxygen vacancy that induces the reconstruction of the edge of the FeO2 island into the FeO phase. At the generated FeO-FeO2 interface, CO oxidation proceeds with the lowest barrier. STM images, along with reaction experiments, showed that the density of the active sites remains constant under reaction condition while it reduces upon exposure to CO without co-feeding O2 due to the shortening of the FeO-FeO2 perimeter. The FeO-FeO2 active sites are sensitive to the reaction atmosphere and cannot be recovered upon exposure to oxygen-rich mixtures with CO. Surface oxygen vacancies are a common structural characteristic of reductive reconstruction of iron oxide catalysts, and hydroxyl groups are important intermediates in reactions catalyzed by iron oxide involving H2O, H2 and hydrocarbons, such as WGS reaction and nonoxidative dehydrogenation reaction of ethylbenzene (EB) to styrene. A phenomenon of surface oxygen vacancy-controlled reactivity of hydroxyl groups on iron oxides has been well-established, in which the presence of surface oxygen vacancies locally and thermodynamically suppresses the 27


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low-barrier water formation pathway of hydroxyl reaction and consequently opens up other reaction pathways involving hydroxyl groups, such as the formation of hydrogen and the reaction of hydroxyl groups with surface CO at metal-oxide interfaces to form CO2.143-147

Such a

phenomenon nicely connects surface oxygen vacancies and reactivity of hydroxyl groups, which provides a profound insight on active structures of iron oxides in catalyzing reactions involving H2O, H2 and hydrocarbons. The active structure of iron oxides in reactions with H2 as a product of hydroxyl reaction cannot be stoichiometric, instead, there must be enough neighbouring surface oxygen vacancies. These results suggest that appropriate elementary surface reactions can be developed to probe whether oxide catalysts undergo surface reconstruction in catalytic reactions or not. 3.3 Perovskites Perovskites have the general formula ABO3, where A is an alkali, alkaline or lanthanide metal, and B is a transition metal. Thanks to the wide range tunability of composition and structure, perovskites have found extensive applications in thermal catalysis, electrocatalysis and photocatalysis.148-149 A stoichiometric ratio of the metal cations at the surface of perovskites seems to be rather the exception, not the norm. The diffusion and segregation of the cations constituting the perovskite catalyst under reactive environments is very much relevant for catalysis. Peng et al.150 showed that a treatment of La0.5Sr0.5CoO3 perovskite with HNO3 successfully removes inactive SrCoO3 and creates surface defects. These defects provoke the migration of Sr from the bulk to the surface during NO storage, enhancing Sr2+ sites active for NOx storage and subsequently catalytic activity for NO oxidation to NO2, related to the Co2+-Co3+ redox couple. Oxygen-treatments of SrTiO3 (STO) and BaZrO3 nanoparticles at high temperatures (> 500 °C) were showed to cause exposures of Sr or Ba at the surface, respectively.151 Strategies of 28


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oxygen-treatments at high temperature and HNO3 treatments were combined to successfully tune the surface termination of STO and BaZrO3 nanoparticles for acid-base catalysis in the conversion of 2-propanol.151 2-propanol dehydration and dehydrogenation are favored over acid (Ti or Zr termination) and basic sites (Sr or Ba termination) respectively to produce propene and acetone. The shape-effect of STO nanocrystals on acid-base catalysis was further studied.152 Three STO nanoshapes, including cubes, truncated cubes and dodecahedra with decreasing ratio of {100} facets to {011} facets, were synthesized and treated in HNO3 to expose Ti at the surface. LEIS measurements showed surface Sr segregation upon treatment at 550 °C in oxygen for all shaped STO. These nanoshapes were tested for the dehydrogenation of ethanol. A correlation between turnover rates and the ratio of the facets, (110) and (001), was not observed. Instead, the rates could be related to the strength and distribution of acid/base sites which are determined by the degree of surface reconstruction after high temperature O2 treatment or HNO3 treatment. The effect of the facet exposed in perovskites was undermined by the complex surface reconstruction behaviors which presumably vary with different facets. The surface reconstruction of STO was also explored for reactions occurring at higher temperatures and of redox-character: CH4 combustion153 and oxidative coupling of methane (OCM)154. The surface termination of multiple STO samples was fine-tuned by means of incipient wetness impregnation of Sr, thermal treatment and acid leaching. It was found that the composition of subsurface layers correlates with the composition of the top-surface, highlighting that although catalysis is a surface phenomenon, the bulk plays a role in stabilizing the surface structure. Two sets of STO catalysts, treated commercial samples and in-house synthesized samples via hydrothermal-synthesis, were studied. It was found that the rate for CH4 combustion is proportional with the concentration of Sr at the surface for both sets of STO samples. The set of 29


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samples synthesized in-house showed higher rates than the other set, as they developed higher density of the active Sr-terminated step-sites visualized by STEM imaging (Figure 8) and supported by DFT modelling. Although the set of samples derived from commercial STO showed increase in combustion and OCM rates as the concentration of surface Sr increased, for Srconcentration greater than 66% the rates plateau out. It was found that the density of basic sites on the surface increases with Sr precisely up to a concentration of 66%. Thus, the concentration of basic sites on the reconstructed surfaces, was put forth, along with the surface concentration of Sr, as a promising reactivity descriptor. These work highlights how one can make use of surface reconstruction of perovskites to create new active sites, the Sr-terminated step-sites, for C-H bond activation of methane. Similar effect was also observed in CO oxidation using a series of catalysts LaxSr1-xCoO3^

(x=0.7, 0.5, 0.3).155 It was observed that increased exposure of Sr at the surface enhanced

conversions for CO oxidation. A treatment in a KBH4 solution was found to increase the amount of Sr and oxygen vacancies in the near-surface region, which further boosted catalytic activity for CO oxidation.

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catalysts used to convert exhaust gases into less harmful species are subjected to long-term oxidation-reduction cycles that have been shown to reduce the catalyst efficiency over time. Such a performance deterioration can be largely alleviated by using the “intelligent catalysts” in which nanoparticles of the active precious metal segregate during the reducing step and these metals reincorporate into the bulk of the catalyst during the oxidation step. Such a dynamic behavior was shown to prevent the agglomeration of metal nanoparticles at the surface, and thus guarantee sustained performance of the catalyst.17,

156

Perovskites were found to be reliable three-way

intelligent catalysts, and LaFe0.95Pd0.05O3 has been used for gasoline automotive emission control since 2002.17 DFT simulations by Hamada et al.157 showed that for LaFe1-xMxO3-y (M=Pd, Rh, Pt) perovskites, Pd and Pt segregation at the surface is favored upon the creation of oxygen vacancies (Pd segregates easier than Pt), whereas Rh prefers to form the solid-solution. “Intelligent” LaPrNiO3 catalyst was found very stable during oxidative reforming of methane (ORM, feed: CH4+CO2+O2) due to the cycling diffusion of Ni in and out of the lattice during the reaction.18 The degree of non-stoichiometry of perovskite catalysts was used to control ex-solution of nanoparticles. A cation and oxygen-deficient perovskites (A8 _BO0 ^) have shown ex-solution properties of the B cation (or a dopant in the B position) that allow controlled creation of terraces and strongly-anchored nanoparticles with remarkably homogeneous dispersion.158 Gorte and co-workers 159 have recently shown that the surface area of perovskites can be increased by depositing perovskite layers via atomic layer deposition (ALD) on a high-surface area support (MgAl2O4), and that those perovskites still act as “intelligent” catalysts. Ni/MgAl2O4 and Ni-CaTiO3/MgAl2O4 catalysts were tested for DRM and steam methane reforming (SMR). Although the catalytic performance, after high temperature activation (1073 K) under reducing conditions, was similar for the catalysts with or without the CaTiO3 layer, the Ni-CaTiO3/MgAl2O4 32


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catalyst showed much less carbon deposition after treatment in dry CH4. Pd-LaFeO3/MgAl2O4 was also synthesized and tested for CH4 and CO combustion.160 This catalyst exhibited high and low activity upon reduction and oxidation, respectively, typical of an “intelligent” catalyst that exsolves Pd upon reduction and reincorporates it into the lattice upon oxidation. PdLaFeO3/MgAl2O4 showed higher rates than bulk LaFe0.97Pd0.03O3 for methane oxidation, after activation in H2, possibly due to the short diffusion path in the thin perovskite film and thus more versatile intelligent behavior. Surface reconstruction of Sr-containing perovskites have been also reported when used as electrocatalysts. For instance, segregation of Sr occurred at the surface of a thin film solid oxide fuel cell electrode, SrTi1-xFexO0 ^.7 Furthermore, it was observed that SrIrO3 thin films reconstruct into IrOx/SrIrO3 due to Sr-leaching during electrochemical tests for the oxygen evolution reaction (OER), and this reconstructed catalyst outperformed conventional catalysts.161 Also for OER, SrTiO3 terminated with a TiO2 double-layer showed enhanced activity.162 Another example of a B-terminated active electrocatalyst was reported by Fabbri et al.24 A reconstructed (Co/Fe)O(OH) layer on the surface of Ba0.5Sr0.5Co0.8Fe0.2O0 ^ was identified, by means of operando X-ray absorption spectroscopy, as the active catalytic stage for OER. The assemble of this layer occurs upon leaching of Ba2+ and Sr2+ in the electrolyte and leads to outstanding electrocatalytic performance. When comparing La0.8Sr0.2CoO0 ^ (LSC) film and powder at elevated temperatures (520 °C), Crumlin et al.163 observed near-surface Sr enrichment and less secondary phases in the film. In a later study, it was shown that the introduction of less reducible cations (Hf, Ti) to the perovskite structure of LSC films, limited the segregation of Sr at the surface (promoted upon thermal treatment up to 550 °C in O2), and reduced the amount of oxygen vacancies, which in turn increased the surface oxygen exchange.8 33


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Recently, Kwon et al.

164

reported a good agreement of the calculated ‘co-segregation

energy’ (referred to the segregation of the dopant and an oxygen vacancy) for a set of layered perovskites, PrBaMn2ODK^ and PrBaMn1.7T0.3ODK^ (T= Co, Ni, Fe) with the ex-solution trend observed experimentally after reduction in humidified H2 at 800 °C (Co-doped>Ni-doped> PrBaMn2ODK^ >Fe-doped). The ex-solved Co and Ni systems showed outstanding performance in H2 fuel cells when compared with other materials reported in the literature.

4. Summary and outlook In this perspective, we briefly introduce the concept of surface reconstruction in heterogeneous catalysis and review representative examples of surface reconstruction of metal oxide catalysts, including surface construction of as-synthesized ceria and titania under different treatments, and surface reconstruction of copper oxides, iron oxides and perovskites under reactive environments. It can be seen that surface reconstruction of oxide catalysts is a common phenomenon under not only regular treatments (oxidation and reduction) but more significantly working conditions and can significantly impact the catalytic performance. Depending on the reactants, including surface composition and structure of unreconstructed surfaces and compositions and concentrations/pressures of molecules, and the temperature, oxide catalysts can undergo extensive reconstructions including phase segregation, formation of defects, vacancies, surface oxidation and reduction, and even bulk oxidation and reduction. It stresses that the structure of reconstructed oxide catalysts should be used to correlate with the catalytic performance, and the characterizations should be done in situ for working catalysts. Such a goal can now be partly achieved with the present status of in situ and operando characterization

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techniques, nanoshaped powder catalysts with uniform and well-defined structures, and theoretical calculations. Substantial challenges still remain in understanding the complex reconstruction of heterogeneous catalysts. Firstly, the present in situ and operando characterization techniques are mainly spectroscopies that are able to indicate if the reconstruction occurs or not but cannot give exact local atomic structure of the reconstructed surfaces. Thus in situ and operando microscopies capable of characterizing working catalysts need to be developed to provide atomic resolution of the working structure. Current in situ electron microscopy at most provides a side view of the working surface while scanning probe microscopy capable of surface spatial resolution has difficulty in studying powder catalysts under reaction conditions. Meanwhile, new in situ and operando characterization techniques also need to be developed for catalytic reactions with very harsh reaction conditions such as high temperatures and pressures where catalyst reconstruction is even more severe. Secondly, since surface reconstruction is closely related with the type and coverage of adsorbates, it is expected that surface reconstruction might vary among various steps within a catalytic reaction cycle, including adsorption, surface reaction and desorption, thus in situ and operando characterization techniques with fast temporal solution enough to follow the surface reconstruction process within a catalytic reaction cycle need to be developed. Thirdly, nanoshaped powder catalysts with uniform, well-defined and tunable structures need to be fabricated not only to facilitate the unambiguous establishment of structure-catalytic performance of reconstructed catalysts but also to understand the structural effect of starting catalysts on the reconstructed catalysts so as to control the surface reconstruction process. Finally, the capacity of theoretical calculations needs to be powerful enough to simulate the surface reconstruction process of working catalysts to provide microscopic pictures of both the surface and the subsurface and even the bulk. 35


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Therefore, future research work should be dedicated to understanding the dynamic nature of surface reconstruction process of working catalysts, the factors affecting the surface reconstruction process, and the structure-catalysis relationship of reconstructed catalysts. With these fundamental understandings, strategy can be developed to realize efficient catalysis over new catalysts with desirable and controllable reconstruction behaviors. Ultimately, novel “smart” catalysts may be developed that spontaneously adopt optimum structures for a given set of reaction conditions, even can detect changes in the reaction conditions and adapt to them.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (WH) and [email protected] (ZW). Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS WH acknowledges financial supports by the National Key R & D Program of Ministry of Science and Technology of China (2017YFB0602205), the Chinese Academy of Sciences, the National Natural Science Foundation of China (21525313, 91745202, 21703227), the Changjiang Scholars Program of Ministry of Education of China. FPG, ZB and ZW are supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy

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Sciences, Chemical Sciences, Geosciences, and Biosciences Division. XZ acknowledges the fellowship from China Scholarship Council.

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