Oxide Nanocrystal Model Catalysts - Accounts of Chemical

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Oxide Nanocrystal Model Catalysts Weixin Huang* Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Materials for Energy Conversion, Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China CONSPECTUS: Model catalysts with uniform and well-defined surface structures have been extensively employed to explore structure−property relationships of powder catalysts. Traditional oxide model catalysts are based on oxide single crystals and single crystal thin films, and the surface chemistry and catalysis are studied under ultrahigh-vacuum conditions. However, the acquired fundamental understandings often suffer from the “materials gap” and “pressure gap” when they are extended to the real world of powder catalysts working at atmospheric or higher pressures. Recent advances in colloidal synthesis have realized controlled synthesis of catalytic oxide nanocrystals with uniform and well-defined morphologies. These oxide nanocrystals consist of a novel type of oxide model catalyst whose surface chemistry and catalysis can be studied under the same conditions as working oxide catalysts. In this Account, the emerging concept of oxide nanocrystal model catalysts is demonstrated using our investigations of surface chemistry and catalysis of uniform and well-defined cuprous oxide nanocrystals and ceria nanocrystals. Cu2O cubes enclosed with the {100} crystal planes, Cu2O octahedra enclosed with the {111} crystal planes, and Cu2O rhombic dodecahedra enclosed with the {110} crystal planes exhibit distinct morphology-dependent surface reactivities and catalytic properties that can be well correlated with the surface compositions and structures of exposed crystal planes. Among these types of Cu2O nanocrystals, the octahedra are most reactive and catalytically active due to the presence of coordination-unsaturated (1-fold-coordinated) Cu on the exposed {111} crystal planes. The crystal-plane-controlled surface restructuring and catalytic activity of Cu2O nanocrystals were observed in CO oxidation with excess oxygen. In the propylene oxidation reaction with O2, 1-fold-coordinated Cu on Cu2O(111), 3-foldcoordinated O on Cu2O(110), and 2-fold-coordinated O on Cu2O(100) were identified as the active sites, respectively, to produce acrolein, propylene oxide, and CO2. Ceria rods enclosed with the {110} and {100} crystal planes, ceria cubes enclosed with the {100} crystal planes, and ceria octahedra enclosed with the {111} crystal planes exhibit distinct morphology-dependent oxygen vacancy concentrations and structures that can be well correlated with the surface compositions and structures of exposed crystal planes. Consequently, the metal−ceria interactions, structures, and catalytic performances of ceria-supported catalysts depend on the CeO2 morphology. Our results comprehensively reveal the morphology-dependent surface chemistry and catalysis of oxide nanocrystals that not only greatly deepen the fundamental understanding of oxide catalysis but also demonstrate a morphology-engineering strategy to optimize the catalytic performance of oxide catalysts. These results adequately exemplify the concept of oxide nanocrystal model catalysts for the fundamental investigations of oxide catalysis without the “materials gap” and “pressure gap”. With the structure− catalytic property relationships learned from oxide nanocrystal model catalyst studies and the advancement of controlledsynthesis methods, it is promising to realize the structural design and controlled synthesis of novel efficient oxide catalysts in the future.

1. INTRODUCTION

strategy has been developed in which uniform and well-defined surfaces are used as models of powder catalyst surfaces for the studies of surface structure−catalytic property relationships.2 Single-crystal-based materials have been extensively used as model catalysts,3−14 but there exist the “materials gap” and “pressure gap” between single-crystal-based model catalysts studied under ultrahigh vacuum (UHV) conditions and corresponding powder catalysts working at atmospheric or higher pressures. Consequently, fundamental understandings acquired with single-crystal-based model catalysts sometimes cannot be simply extended to working powder catalysts. Recent

Catalysts play a vital role in technologies for chemical industry and environmental remediation. The serious resource and energy shortage and environmental pollution require catalysts to be not only active but also selective. This poses a strict requirement on the uniformness of catalyst structure; meanwhile, fundamental understandings of structure−property relationships of catalysts are needed to rationally design the structure of selective catalysts. Catalytic properties of solid catalysts are cooperatively determined by the surface composition and surface electronic/geometric structure.1 It is difficult to unambiguously correlate the complex and nonuniform surface composition and structure of powder catalysts with the catalytic properties. In this respect, a model catalyst © 2016 American Chemical Society

Received: December 11, 2015 Published: March 3, 2016 520

DOI: 10.1021/acs.accounts.5b00537 Acc. Chem. Res. 2016, 49, 520−527

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Accounts of Chemical Research

As-synthesized Cu2O cubes (denoted as c-Cu2O) are capping ligand-free whereas as-synthesized Cu2O octahedra (denoted as o-Cu2O−PVP) and rhombic dodecahedra (denoted as dCu2O−OA) are capped with poly(vinylpyrrolidone) (PVP) and oleic acid (OA), respectively.35,36 A controlled oxidation process carried out under an atmosphere consisting of O2 and C3H6 with appropriate concentrations was developed to remove the capping ligands on o-Cu2O−PVP and d-Cu2O−OA without changing the morphologies, surface compositions, and structures.37,38 On one hand, capping ligands on Cu2O nanocrystals could be adequately oxidized by O2 at relatively low temperatures, avoiding the high temperature-induced morphology change; on the other hand, the coexistence of C3H6 and O2 in the atmosphere could cooperatively prevent Cu2O nanocrystals from both oxidation and reduction. Figure 1, panels A1 and A2, B1 and B2, and C1 and C2, show the

progress in colloidal synthesis has realized successful synthesis of catalytic nanocrystals with uniform composition and structure (size and morphology).15,16 The uniform composition and structure of these nanocrystals well fulfill the requirements of model catalysts; moreover, their surface chemistry and catalysis can be studied under the same conditions as working powder catalysts. Therefore, it is being recognized that uniform catalytic nanocrystals can be used as a novel type of model catalyst for the studies of structure−property relationships of powder catalysts without the “materials gap” and “pressure gap”.17−26 Oxides are widely used as catalysts and catalyst supports. Consisting of cations and anions, oxides exhibit more plentiful and complex surface compositions and structures than metals. Crystal planes exposed on an oxide particle determine not only the surface geometric structure but also the surface composition and strongly affect the catalytic properties. The crystal plane effect of oxide catalysts has been extensively reported using single-crystal-based model catalysts. Recently increasing results have been reported on morphology-dependent catalytic properties of uniform oxide nanocrystals.27−29 According to the Wulff’s rule,30 the morphology of a crystalline substance determines the crystal planes exposed on the surface. Thus, morphology-dependent catalytic properties of uniform oxide nanocrystals should microscopically demonstrate the effect of exposed crystal planes on the catalytic properties. Therefore, we consider oxide nanocrystals with uniform morphology that selectively expose one or two types of crystal planes as appropriate model catalysts for fundamental studies of the crystal plane effect. With such an idea, we have systematically investigated the surface chemistry and catalysis of uniform cuprous oxide (Cu2O) and ceria (CeO2) nanocrystals with various morphologies and successfully established the crystal plane−catalytic property relationships. Our results have nicely demonstrated the emerging concept of oxide nanocrystal model catalysts that is the topic of this Account. In the following sections, I will focus first on the morphology-dependent exposed crystal planes, surface compositions, and structures of uniform Cu2O and CeO2 nanocrystals, next on the crystal plane−surface composition/structure− surface chemistry/catalysis relationships of Cu2O nanocrystal model catalysts, and then on the CeO2 crystal plane−CeO2 oxygen vacancy concentration/structure−catalyst structure− catalytic property relationships of CeO2 nanocrystal-supported model catalysts. Finally, I will summarize the concept of oxide nanocrystal model catalysts and outlook for its effectiveness not only on the fundamental understandings of structure−catalytic property relationships of oxide catalysts but also on the innovation of selective oxide catalysts.

Figure 1. SEM and HRTEM images of capping-ligand-free Cu2O cubes (A1, A2), octahedra (B1, B2), and rhombic dodecahedra (C1, C2), and optimized structures of Cu2O(100) (A3), (111) (B3), and (110) (C3) surfaces. The insets show the electronic diffraction patterns; 0.43 and 0.30 nm correspond to the lattice fringe of {001} and {110} crystal planes, respectively, of cubic Cu2O. The red, brick red, and green balls represent oxygen, coordinated saturated copper, and coordinated unsaturated copper atoms, respectively.

representative SEM and HRTEM images with ED patterns of capping ligand-free c-Cu2O, Cu2O octahedra (denoted as oCu2O), and rhombic dodecahedra (denoted as d-Cu2O) nanocrystals, respectively. o-Cu2O, c-Cu2O, and d-Cu2O nanocrystals selectively expose Cu2O {111}, {100}, and {110} crystal planes, respectively. The optimized surface structures of Cu2O (111), (100), and (110) surfaces are shown in Figure 1, panels A3, B3, and C3, respectively, and their surface compositions and structural parameters are summarized in Table 1.35,36 The bulk Cu2O exhibits 2-fold coordinated Cu(I) (Cu CSA) and 4-fold coordinated oxygen (OCSA) with a Cu−O bond length of 1.85 Å. On Cu2O(111), the topmost layer consists of 3-fold coordinated O (O3c) and the second layer consists of CuCSA (75%) and 1-fold coordinated Cu (Cu1c) (25%) with a distance along the z direction of 0.23 Å. O3c is bonded to CuCSA with a bond length of 1.83 Å, and Cu1c is bonded to underneath OCSA

2. SURFACE COMPOSITIONS AND STRUCTURES OF Cu2O AND CeO2 NANOCRYSTALS Both Cu2O (space group of O4K-Pn3m) and CeO2 (space group of O5H-Fm3m) are cubic phase crystal structures and have important applications in heterogeneous catalysis. For cubic phase crystals following the Wulff’s construction,30 a cubic crystallite is enclosed with six {100} crystal planes, an octahedral crystallite is enclosed with eight {111} crystal planes, and a rhombic dodecahedral crystallite is enclosed with 12 {110} crystal planes. The recipes for the controlled synthesis of uniform Cu2O cubes, octahedra, and rhombic dodecahedra31,32 and uniform CeO2 cubes, octahedra, and rods33,34 were well established. 521


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vacancy concentrations and structures of various CeO2 nanocrystals (Table 2). The oxygen vacancy concentrations

Table 1. Exposed Crystal Planes on o-Cu2O, c-Cu2O, and dCu2O Nanocrystals and Their Surface Compositions and Structural Parameters Optimized by DFT Calculations nanocrystals o-Cu2O

exposed crystal planes Cu2O{111}

top-most layer 3-fold-coordinated O3c

second layer 2-fold-coordinated CuCSA (75%) 1-fold-coordinated Cu1c (25%) 2-fold-coordinated CuCSA 2-fold-coordinated CuCSA

c-Cu2O

Cu2O{100}

2-fold-coordinated O2c

d-Cu2O

Cu2O{110}

3-fold-coordinated O3c and 2-fold-coordinated CuCSA 4-fold-coordinated OCSA and 2-fold-coordinated CuCSA

bulk Cu2O

Table 2. Exposed Crystal Planes on o-CeO2, c-CeO2, and rCeO2 Nanocrystals and Their Oxygen Vacancy (Ov) Concentrations, Locations, and Structures

dCu−O (Å) 1.83

nanocrystals

1.91

exposed crystal planes

Ov concentrationsa

Ov locationsb

o-CeO2 c-CeO2

{111} {100}

0.23 0.60

subsurface surface

r-CeO2

{110}+{100}

0.76

surface

1.76 1.82

1.85

Ov structuresc not measured large oxygen vacancy clusters large oxygen vacancy clusters and small oxygen vacancies

a

Indicated by the intensity ratio between the defect-induced vibrational band and CeO2 F2g vibrational band in the Raman spectra of CeO2 nanocrystals probed with the excitation source at 325 nm (from ref 41). bIndicated by surface oxidation reactivity and surface hydroxyl reactivity of reduced CeO2 nanocrystals (from ref 43). c Indicated by PALS spectra of CeO2 nanocrystals (from ref 40).

with a bond length of 1.91 Å. On Cu2O(100), the topmost layer consists of 2-fold coordinated O (O2c) and the second layer consists of CuCSA. The bond length between O2c and CuCSA is 1.76 Å. On Cu2O(110), the topmost layer consists of 3-fold coordinated O3c and CuCSA and the second layer consists of CuCSA. The bond length between O3c and CuCSA is 1.82 Å. The Cu(I) sites on various Cu2O nanocrystals probed by CO adsorption follow the order of o-Cu2O > d-Cu2O ≫ c-Cu2O,39 consistent with the densities and coordination environments of Cu(I) on the optimized Cu2O(111), (110), and (100) surfaces. As-synthesized CeO2 rods, cubes, and octahedra denoted as r-CeO2, c-CeO2 and o-CeO2, respectively, are capping ligandfree and their representative TEM and HRTEM images are shown in Figure 2.40,41 c-CeO2 and o-CeO2 are enclosed with

of r-CeO2 and c-CeO2 are much higher than that of o-CeO2, and oxygen vacancies are mainly located on the surfaces of cCeO2 and r-CeO2 but in the subsurface of o-CeO2. c-CeO2 mainly has large oxygen vacancy clusters while r-CeO2 has both large oxygen vacancy clusters and small oxygen vacancies. These results are consistent with previous DFT calculation results that the oxygen vacancy formation energies of CeO2(111), (100), and (110) surfaces followed the order of (111) > (100) > (110)44,45 and the surface oxygen vacancy is more stable than the subsurface oxygen vacancy on CeO2(110) and (100) but is less stable than the subsurface oxygen vacancy on CeO2(111).46,47

3. SURFACE CHEMISTRY AND CATALYSIS OF Cu2O NANOCRYSTAL MODEL CATALYSTS Cu(I) in Cu2O can either be reduced to Cu(0) or oxidized to Cu(II); thus Cu2O exhibits rich chemical reactivities. Chemical reactions of a solid particle initiate from the surface, and thus the chemical reactivities depend on the surface composition and structure. The reactivities of c-Cu2O, o-Cu2O−PVP and dCu2O−OA in typical redox reactions involving Cu2O were comparatively studied, and c-Cu2O was observed to be most stable.35,36,48−50 As shown in Figure 3A−C, c-Cu2O keeps the original cubic morphology while stepped layers of {100} facets develop at the expense of the original {111} and {110} crystal planes, respectively, exposed on o-Cu2O−PVP and d-Cu2O− OA during redox reactions in acetic acid and aqueous ammonia solutions.36,48 The morphology-dependent reactivity of Cu2O nanocrystals can be well correlated to the surface composition and structure of exposed crystal planes. The redox reactions need to break the surface Cu−O bond; therefore, the Cu2O(100) surface with the shortest surface Cu−O bond (Table 1) is reasonably more stable than the Cu2O(111) and (110) surfaces, resulting in the higher stability of c-Cu2O enclosed with {100} crystal planes than o-Cu2O−PVP enclosed with {111} crystal planes and d-Cu2O−OA enclosed with {110} crystal planes. During the redox reaction in aqueous ammonia solution, the formed [Cu(NH3)4]2+ further reacts with OH− to form the Cu(OH)2 precipitate whose nucleation sites were observed to

Figure 2. TEM (top) and HRTEM (bottom) images of CeO2 rods enclosed by {110} and {100} crystal planes, CeO2 cubes enclosed by {100} crystal planes, and CeO2 octahedra enclosed by {111} crystal planes.

the {100} and {111} crystal planes, respectively. Although under debate, r-CeO2 was considered to expose {110} and {100} crystal planes.29 The surface compositions and structures of CeO2(111), (100), and (110) surfaces have been much investigated both experimentally and theoretically employing single crystals and single crystal thin films, but their accurate atomic structures still remain unclear, particularly for the CeO2(100) and (110) surfaces.42 An important structural characteristic of CeO2 closely associated with its surface chemistry and catalysis in oxidation reactions is the nonstoichiometric composition. Our combined Raman spectroscopy and positron annihilation lifetime spectroscopy (PALS) results of CeO2 nanocrystals40,41 and surface oxidation reactivity and surface hydroxyl reactivity of reduced CeO2 nanocrystals43 demonstrate morphology-dependent oxygen 522


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during the initial redox reaction involving Cu1c, the local OH− concentration at the liquid−o-Cu2O interface does not increase and the formed [Cu(NH3)4]2+ can migrate away from the surface into the aqueous solution, resulting in the nucleation of fewer Cu(OH)2 precipitates on o-Cu2O−PVP. Similar results were also observed in the redox reactions of cCu2O and o-Cu2O−PVP with Ce(IV) that form the CeO2 precipitate as one of the final products.49 As shown in Figure 3D,E, homogeneous nucleation and growth of CeO2 occur on c-Cu2O, developing smooth and dense CeO2 shells and eventually forming cubic Cu2O(core)/CeO2(shell) nanostructures; however, heterogeneous nucleation and growth of CeO2 occur on o-Cu2O−PVP, developing rough and porous CeO2 shells and eventually forming hollow octahedral CeO2 cages. In the reduction reactions of Cu2O by H2 and CO that need not only to break the surface Cu−O bond but also to adsorb and activate H2 and CO, the reduction temperature of oCu2O−PVP is lower than that of c-Cu2O by more than 120 °C, demonstrating much easier reduction of the Cu2O(111) surface than the Cu2O(100) surface (Figure 3F).35 On one hand, the Cu2O(100) surface is more stable than the Cu2O(111) surface; on the other hand, the Cu2O(111) surface with Cu1c is more capable of adsorbing and activating CO and H2 than the Cu2O(100) surface. c-Cu2O, o-Cu2O, and d-Cu2O nanocrystals exhibit morphology-dependent catalytic performances in CO oxidation with stoichiometric O239 and propylene oxidation with O2.37 oCu2O is most catalytically active in both reactions (Figure 4A), which could be attributed to the Cu1c sites of Cu2O(111) exposed on o-Cu2O. Moreover, morphology-dependent catalytic selectivities of Cu2O nanocrystals were observed in propylene oxidation (Figure 4B−D).37 o-Cu2O and c-Cu2O are selective in producing acrolein and CO2, respectively, while d-Cu2O shows comparative selectivities in producing acrolein, CO2, and propylene oxide (PO). Among all Cu2O nanocrystals, d-Cu2O exhibits the highest selectivity toward PO. The combined in situ DRIFTS and DFT calculation results identified the formation of Cu1c−C3H6(a) at Cu1c of Cu2O(111) (Eads = −1.53 eV) on o-Cu2O, bridge-adsorbed (Cu2c,O3c)−C3H6(a) at neighboring CuCSA and O3c of Cu2O(110) (Eads = −0.32 eV) and Cu2c−C3H6(a) at CuCSA of Cu2O(110) (Eads = −0.25 eV) on d-Cu2O, and bridgeadsorbed (O2c,O2c)−C3H6(a) at two neighboring O2c of Cu2O(100) (Eads = −2.85 eV) on c-Cu2O. Cu1c−C3H6(a) on o-Cu2O exhibits a slightly weakened CC bond and undergoes the C−H bond breaking reaction to produce acrolein with observed allyl, allene, and acrolein surface intermediates, as does Cu2c−C3H6(a) on d-Cu2O. Both (Cu2c,O3c)−C3H6(a) on d-Cu2O and (O2c,O2c)−C3H6(a) on c-Cu2O exhibit a significantly weakened CC bond and undergo the CC breaking reaction, but (Cu2c,O3c)−C3H6(a) and (O2c,O2c)−C3H6(a) undergo the epoxidation reaction and combustion reaction, respectively. O2c on Cu2O(100) is more electrophilic than O3c on Cu2O(110) so as to completely break the CC bond of (O2c,O2c)−C3H6(a), leading to the combustion reaction. The activation energies for the epoxidation and breaking reactions of the CC bond were calculated as 1.28 and 2.08 eV, respectively, for (Cu2c,O3c)− C3H6(a) on Cu2O(110) but as 2.09 and 1.02 eV, respectively, for (O2c,O2c)−C3H6(a) on Cu2O(100). Therefore, Cu1c− C3H6(a) on Cu2O(111), (Cu2c,O3c)−C3H6(a) on Cu2O(110), and (O2c,O2c)−C3H6(a) on Cu2O(100) are the active species to produce acrolein, PO, and CO2, respectively, in propylene

Figure 3. SEM images of c-Cu2O, o-Cu2O−PVP and d-Cu2O−OA nanocrystals etched in acetic acid solution (pH = 3.5) for 150 min (A1−A3) or in aqueous ammonia solution (pH = 10.5) for 10 min (B1−B3) and 150 min (C1−C3). TEM images of Cu2O−CeO2 nanocomposites prepared from c-Cu2O and o-Cu2O−PVP nanocrystals reacted with 0.025 (D1 and D2) and 0.1 mmol (E1 and E2) of (NH4)2Ce(NO3)6 in NaCl aqueous solution−ethanol mixed solution, and H2-TPR (F1) and CO-TPR (F2) spectra of c-Cu2O and o-Cu2O− PVP nanocrystals.

depend on the Cu2O morphology.48 The Cu(OH)2 precipitates selectively nucleate on c-Cu2O (Figure 3B1) and then form a dense Cu(OH)2 shell (Figure 3C1) that prevents the redox reaction of c-Cu2O. The Cu(OH)2 precipitates initially could not be observed on o-Cu2O−PVP (Figure 3B2) and then sparsely appear with the reaction proceeding (Figure 3C2). These observations could also be explained by the Cu2O surface compositional stoichiometry-dependent local concentrations of [Cu(NH3)4]2+ and OH− at the liquid−Cu2O interface. As shown in Figure 1A3, O2c in the topmost layer of Cu2O(100) exposed on c-Cu2O gets released and undergoes the hydrolysis reaction to produce OH− as soon as CuCSA in the second layer undergoes the redox reaction to produce [Cu(NH3)4]2+, resulting in a high local OH− concentration at the liquid−c-Cu2O interface that facilitates the precipitation reaction of [Cu(NH3)4]2+ to form the Cu(OH)2 nuclei on cCu2O. As shown in Figure 1B3, Cu1c in the second layer of Cu2O(111) exposed on o-Cu2O−PVP is more reactive than CuCSA due to its weak Cu−O bond and preferentially undergoes the redox reaction, but its reaction only reduces the coordination number of OCSA in the third layer to which it is bonded and does not release any free O anions. Therefore, 523


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Figure 5. High-resolution TEM images of CuO thin films grown on cCu2O (A1) and o-Cu2O−PVP (A2) during CO oxidation with excess O2, and catalytic performances (B1) and Arrhenius plots (B2) of CuO thin films grown on c-Cu2O (CuO/c-Cu2O) and o-Cu2O (CuO/oCu2O) during CO oxidation with excess O2, and (C) optimized structures of CuO thin films grown on Cu2O(100) (CuO/Cu2O(100)) and Cu2O(111) (CuO/Cu2O(111)).

Figure 4. (A) CO conversion in CO oxidation with stoichiometric O2 and C3H6 conversion rate in propylene oxidation with O2 of c-Cu2O, o-Cu2O, and d-Cu2O nanocrystals, catalytic selectivity of (B) c-Cu2O, (C) o-Cu2O, and (D) d-Cu2O nanocrystals in propylene oxidation with O2, and (E) schematic illustration of crystal plane-controlled selectivity of Cu2O catalysts in propylene oxidation with O2.

ing strong connections between the surface structures of the original catalyst nanoparticle and the restructured working catalyst nanoparticle. The crystal-plane-controlled surface restructuring process not only deepens fundamental understandings of the surface restructuring process but also provides a strategy to control the surface structure and catalytic performance of restructured catalyst nanoparticles.

oxidation with O2 catalyzed by Cu2O (Figure 4E). These results not only evidence that heterogeneous catalytic reactions as complex as propylene oxidation can be fundamentally understood at the molecular level employing oxide nanocrystal model catalysts but also reveal a morphology/crystal plane-engineering strategy of oxide catalysts for the innovation of selective catalysts. The surfaces of c-Cu2O and o-Cu2O−PVP used as the catalysts for CO oxidation with excess O2 were observed to be oxidized into the CuO thin film (Figure 5A), and the CuO thin film formed on o-Cu2O−PVP exhibits a much higher catalytic activity and a lower apparent activation energy than that formed on c-Cu2O (Figure 5B).51 DFT calculation results demonstrate that the CuO thin film on Cu2O(111) is terminated with 3-fold coordinated Cu3c and 3-fold coordinated O3c, while that on Cu2O(100) is terminated with 2-fold coordinated O2c (Figure 5C). The coordination unsaturated Cu3c of the CuO thin film on Cu2O(111) is more active in catalyzing CO oxidation than coordination unsaturated O2c of the CuO thin film on Cu2O(100). These findings reveal a crystal-plane-controlled surface restructuring process of catalyst nanoparticles, convey-

4. SURFACE CHEMISTRY AND CATALYSIS OF CeO2 NANOCRYSTAL-SUPPORTED MODEL CATALYSTS CeO2-supported catalysts have important catalytic applications and the metal−CeO2 interaction is an important factor affecting the structure and catalytic performance. Oxygen vacancies on CeO2 are believed to play a key role in the metal−CeO2 interaction. o-CeO2, c-CeO2, and r-CeO2 nanocrystals with different oxygen vacancy concentrations and structures have been demonstrated to exhibit morphology-dependent surface chemistry and catalysis. 52,53 We employed these CeO 2 nanocrystals as model CeO2 supports for the studies of oxygen vacancy effect on the metal−CeO2 interactions. CeO2 morphology-dependent metal−CeO2 interactions were observed in CeO2 nanocrystal-supported Ag40 and Pt41 model catalysts prepared by conventional wetness incipient impregnation method followed by calcinations at 773 K and by H2 reduction at 200 °C, respectively. Positively charged Agn+ 524


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results of CeO2 nanocrystal-supported Ag model catalysts. However, the Pt-mass specific reaction rates of Pt/r-CeO2 and Pt/c-CeO2 catalysts in PROX reaction follow the order of 0.5%-Pt/CeO2 only containing Pt2+ > 1%-Pt/CeO2 containing both Pt2+ and Pt nanoparticles > 0.2%-Pt/CeO2 only containing Pt2+, indicating that the oxidation of CO in PROX reaction should involve reaction pathways other than that in CO oxidation. These results provide evidence for the H2assisted CO oxidation mechanism proposed for Pt/CeO2 catalysts in which the spillover of H(a) formed by H2 dissociation on Pt surface to the CeO2 support forms oxygen vacancies and hydroxyl groups, respectively, to promote the O2 activation and to supply additional oxidizing species.54 Our results of FeO(111)/Pt(111) inverse model catalysts directly prove that hydroxyl groups on oxide can oxidize CO(a) on Pt at the Pt−FeO interface to produce CO2 and such an interfacial CO + OH oxidation pathway is favored by the presence of oxygen vacancies in oxide and enhanced by the spillover of H(a) from the Pt surface to the Pt−FeO interface.55−58 Since the Pt nanoparticle−CeO2 interface is more active than the Pt2+−CeO2 interface in catalyzing the oxidation of CO following the mechanism in CO oxidation, the higher Ptmass specific reaction rate in PROX reaction of 0.5%-Pt/ r(c)CeO2 than 1%-Pt/r(c)-CeO2 indicates that the Pt2+−CeO2 interface should be more active than the Pt nanoparticle−CeO2 interface in catalyzing the oxidation of CO following the H2assisted CO oxidation mechanism. Employing CeO2 nanocrystal-supported model catalysts, we demonstrate a CeO2 morphology/crystal plane-dependent interplay between the CeO2 oxygen vacancies and the metal− CeO2 interaction in metal/CeO2 catalysts. The oxygen vacancy concentration and structure of starting CeO2 support strongly affect the metal−CeO2 interaction and the metal structure in the metal/CeO2 catalyst, but meanwhile the resultant metal− CeO2 interaction and the metal structure change the oxygen vacancy concentration and structure of CeO2 support in the metal/CeO2 catalyst. Similar results were also reported for Au/ CeO2 catalysts.59 Therefore, engineering the morphology of CeO2 support offers a facile strategy to fabricate CeO2supported model catalysts for both fundamental understandings and structural optimizations without the change of catalyst compositions.

clusters could be stabilized by small oxygen vacancies on rCeO2 in 1%-Ag/r-CeO2, while only supported Ag nanoparticles form on c-CeO2 with large oxygen vacancy clusters (Figure 6A1). The impregnated Pt precursors interact more strongly

Figure 6. (A1) Ag 3d XPS spectra and (A2) Ag-mass specific reaction rates in CO oxidation of various Ag/r-CeO2 and Ag/c-CeO2 catalysts. (B1) Pt0/Pt2+ atomic ratios estimated from the Pt 4f XPS spectra and (B2) Pt-mass specific reaction rates in CO oxidation and PROX reaction of various Pt/r-CeO2, Pt/c-CeO2, and Pt/o-CeO2 catalysts.

with r-CeO2 and c-CeO2 with larger oxygen vacancy concentrations than with o-CeO2 and consequently are less reduced, leading to the largest fraction of Pt nanoparticles in the Pt/o-CeO2 catalyst (Figure 6B1). Meanwhile, the resultant metal species and CeO2 morphology cooperatively affect the oxygen vacancy concentration and structure of CeO2 in CeO2supported model catalysts. Supported Ag or Pt nanoparticles are more capable of creating oxygen vacancies in CeO2 than supported Agn+ clusters or Pt2+ cations. Catalytic performances of CeO2 nanocrystal-supported Ag and Pt model catalysts were evaluated in CO oxidation and PROX reaction. The silver mass-specific reaction rate in CO oxidation follows the order of 1%-Ag/c-CeO2 > 3%-Ag/c-CeO2 ≈ 3%-Ag/r-CeO2 ≫ 1%-Ag/r-CeO2 (Figure 6A2). Thus, the Ag nanoparticle−CeO2 interface is more active than the Agn+− CeO2 interface to catalyze CO oxidation, and c-CeO2 is a better support than r-CeO2 to prepare Ag/CeO2 catalysts with high silver mass-specific catalytic activities. With the same Pt loading, the catalytic performances follow the order of Pt/r-CeO2 > Pt/ c-CeO2 > Pt/o-CeO2 in both CO oxidation and PROX reaction (Figure 6B2), consistent with the orders of the reducibility and oxygen vacancy concentration of various CeO2 supports. With the same CeO2 morphology, the catalytic performances of Pt/ CeO2 catalysts in CO oxidation and PROX reaction at low temperatures vary with the Pt species in different ways. The Ptmass specific reaction rates of Pt/r-CeO2 and Pt/c-CeO2 catalysts in CO oxidation increase with the Pt loading and the fraction of Pt nanoparticles, demonstrating that the Pt nanoparticle−CeO2 interface is more active than the Pt2+− CeO2 interface in catalyzing CO oxidation. This agrees with the

5. SUMMARY AND OUTLOOK Uniform Cu2O and CeO2 nanocrystals exhibit distinct morphology-dependent surface chemistry and catalysis that can be well correlated to the surface compositions and structures of exposed crystal planes. Thus, uniform oxide nanocrystals selectively exposing one or two types of crystal planes are suitable oxide model catalysts for the fundamental studies of the crystal plane effect of oxide catalysts without the “materials gap” and “pressure gap”. These results adequately exemplify the concept of oxide nanocrystal model catalysts, adding a novel type of model catalyst for the fundamental studies of oxide catalysis. However, great challenges still remain for synthesis of uniform oxide nanocrystals not only with controllable morphology but also with controllable size, and the complexity of surface composition and structure of oxide nanocrystals must be comprehensively considered. In addition to terrace sites, edge sites and corner sites are exposed on threedimensional nanocrystals and the densities increase with the decrease of nanocrystal size. Although negligible for our Cu2O nanocrystals with large sizes, their contributions to surface 525


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Accounts of Chemical Research chemistry and catalysis need to be considered for fine-sized nanocrystals. Nanocrystals synthesized by the wet-chemistry methods generally exhibit surface compositions and structures different from bulk truncation and are always contaminated with surface hydroxyl groups and carbonates. Nanocrystals with finite sizes also facilely undergo surface restructuring during catalytic reactions. With this in mind, we propose an approach to employ model catalysts ranging from single crystals to uniform nanocrystals to comprehensively study various structural effects of catalytic nanoparticles on the catalytic property and the reaction mechanisms at different levels of complexity (Figure 7). Such an approach can lead to the fundamental understandings of working catalysts at the molecular level.

Biography Weixin Huang, born in 1974, received his B.S. from University of Science and Technology of China (USTC) in 1996 and his Ph.D. from Dalian Institute of Chemical Physics in 2001. He then worked at University of Texas at Austin and Fritz-Haber-Institut der MPG. He became a professor of USTC in 2004. His research activity focuses on surface chemistry and catalysis of model catalysts ranging from single crystals to nanocrystals. He is an editor of Applied Surface Science and editorial board members of Catalysis Letters and Topics in Catalysis.

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Figure 7. Model catalysts ranging from single crystals to uniform nanocrystals for fundamental studies of structure−property relationships and reaction mechanisms of working catalysts at the molecular level.

Oxide nanocrystals act more than as model catalysts. Morphology-dependent catalytic performances of uniform oxide nanocrystals provide a morphology-engineering strategy to optimize catalytic activity of oxide catalysts and innovate selective oxide catalysts. Selective oxide catalysts must exhibit uniform composition and structure so as to expose a single type of active site specifically catalyzing the target reaction, which can be fulfilled by uniform oxide nanocrystals. Therefore, catalysis by oxide nanocrystals with uniform and well-defined structures is becoming an emerging frontier of oxide catalysis, and the advances will realize the structural design and controlled synthesis of novel efficient oxide catalysts: the ultimate goal of catalysis research.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

This work was financially supported by National Basic Research Program of China (Grant 2013CB933104), National Natural Science Foundation of China (Grants 21525313, 21173204, and U1332113), MOE Fundamental Research Funds for the Central Universities (Grant WK2060030017), and Collaborative Innovation Center of Suzhou Nano Science and Technology. Notes

The authors declare no competing financial interest. 526


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Oxide Nanocrystal Model Catalysts - Accounts of Chemical

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