Structure of Copper Oxide Species Supported on Monoclinic Zirconia

Article pubs.acs.org/JPCC

Structure of Copper Oxide Species Supported on Monoclinic Zirconia Vera P. Pakharukova,*,†,‡ Ella M. Moroz,† Dmitry A. Zyuzin,† Arcady V. Ishchenko,† Lidiya Yu. Dolgikh,§ and Peter E. Strizhak§ †

Boreskov Institute of Catalysis, SB RAS, Pr. Lavrentieva 5, 630090 Novosibirsk, Russia Novosibirsk State University, Pirogova Str. 2, 630090 Novosibirsk, Russia § Pisarzhevskii Institute of Physical Chemistry of the NAS, Pr. Nauki 31, 03039 Kiev, Ukraine ‡

S Supporting Information *

ABSTRACT: The structure of copper oxide species in Cu/ZrO2 catalysts with monoclinic zirconia as a support was studied. The catalysts with various copper loadings (3−40 wt % of Cu) prepared by impregnation technique were characterized using N2O titration, X-ray diffraction (XRD) analysis, radial distribution function (RDF) of electron density, and high-resolution transmission electron microscopy (HRTEM). It was established that the Cu/ZrO2 catalysts contained highly dispersed CuO particles along with the large ones. Geometrical similarity between the CuO and ZrO2 monoclinic structures favors stabilization of the highly dispersed CuO particles on the support with low specific surface area.

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INTRODUCTION It has been well documented that support materials in heterogeneous catalysts not only provide developed surface area for metal dispersion but also impact catalytic behavior. Questions regarding structural organization of catalysts, interaction between active component and support, and interface features have attracted considerable attention. Zirconium oxides have been suggested to be promising support materials because of unique combination of different properties, such as polymorphism, chemical resistance, toughness, and amphoteric nature.1,2 Zirconia-supported copper catalysts (Cu/ZrO2) have been employed in methanol synthesis,3,4 NO reduction by CO and hydrocarbons,5,6 CO oxidation,7,8 steam reforming of methanol,9,10 water−gas-shift (WGS) reaction,11,12 and so on. The zirconia polymorphism has been considered to be a key for controlling functional properties of the catalysts. Thus, it was reported that the Cu/ ZrO2 catalysts based on tetragonal (t) and monoclinic (m) zirconium oxides differed in activity for methanol synthesis.4 Effects of different zirconia phases on the catalytic performance were also observed in studies of ethanol dehydrogenation,13 WGS reaction,12 and CO oxidation.7 It is reasonable that chemical and structural features of different ZrO2 phases impact the dispersion of copper species. It was shown that the Cu/t-ZrO2 catalysts exhibited a higher copper dispersion than the Cu/m-ZrO2 systems.13−15 Temperature-programmed reduction (TPR), ultraviolet and visible diffuse reflectance spectroscopy (UV−vis−DRS), and X-ray absorption fine structure (XAFS) analysis revealed that the formation of copper oxide clusters preceded a growth of CuO bulk particles on the t-ZrO2 surface.9,16−18 Different acid−base properties of the zirconia polymorphs have been suggested to impact the copper dispersion.14,15 Rhodes et al. reported that a © XXXX American Chemical Society

larger net positive charge on the m-ZrO2 surface led to a weaker interaction between the support and Cu cations and caused a lower copper dispersion.14,19 A few data are currently available in the literature about the state and structure of copper species supported on the monoclinic zirconia. The Cu/m-ZrO2 catalysts were shown to exhibit preferred formation of bulk crystalline CuO particles;13−15,20 however, TPR results implied coexistence of highly dispersed copper oxide species and well-crystallized CuO particles on the m-ZrO2 surface.7,13,21−23 Araya et al.23 showed that an increase in Cu loading favored the formation of the large CuO particles, but concentration of the highly dispersed copper species remained constant. This result indicated a stabilization of the highly dispersed CuO species on some zirconia surface sites of limited concentration. We have undertaken a study to reveal the effects of the zirconia phases on the dispersion and structure of supported copper oxide species. Recently, we characterized the Cu/t-ZrO2 catalysts and demonstrated predominant formation of copper oxide chain clusters on the t-ZrO2 surface.24 This work is devoted to characterization of copper species supported on the monoclinic zirconia. Powder X-ray diffraction (XRD) analysis, N2O titration, radial distribution function (RDF) of electron density method, high-resolution transmission electron microscopy (HRTEM), and XANES (X-ray absorption near the edge structure) spectroscopy were used to gain insight into the structure of Cu/m-ZrO2 catalysts. Received: July 2, 2015 Revised: December 13, 2015

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DOI: 10.1021/acs.jpcc.5b06331 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. XRD patterns of the Cu/ZrO2 catalysts and m-ZrO2 support (A) and difference curves between the normalized XRD patterns of the catalysts and support (B). Diffraction lines of the m-ZrO2 and CuO phases are depicted.

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EXPERIMENTAL METHODS Synthesis of Catalysts. Commercial industrial monoclinic zirconia (State scientific enterprise “Zirconium”, Ukraine) was used as the support material. Its specific surface area was 15 m2 g−1. The catalysts were prepared by incipient wetness impregnation of m-ZrO2 with a solution of copper nitrate Cu(NO3)2. The impregnated materials were dried at 80 °C for 6 h and calcined in air at 300 °C for 6 h. The samples were designated as xCu/ZrO2, where x denotes the Cu loading (3, 5, 10, 20, and 40 wt %). The catalysts after 6 h of aging in a reaction of ethanol steam reforming (450 °C, C2H5OH/H2O/N2 1:19:18, preliminary reduction at 250 °C in the mixture H2/N2 1:1 for 2h.) were also considered. The details and results of the catalytic experiments are described elsewhere.25

By assuming a spherical shape of particles, the average size of copper particles (dCuN2O) was determined by the following expression N2O dCu (nm) =

CATALYST CHARACTERIZATION Chemical composition of the catalysts was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) on an Optima instrument. Specific surface area of metallic copper Cu0 in the catalysts was measured by N2O titration. The technique comprises two steps: oxidation of Cu0 to Cu2O using N2O and temperatureprogrammed reduction of Cu2O surface species (s-TPR).26,27 The Cu/ZrO2 samples (150 mg) were loaded in a flow microreactor. Before the analysis, prereduction was performed at 500 °C (a heating rate of 10°·min−1) in flowing H2/Ar mixture (10 vol % at 50 mL·min−1). Reduced samples were cooled to 60 °C and exposed to N2O for 45 min. s-TPR was carried out on the freshly oxidized Cu2O surface. The H2/Ar mixture (10 vol % at 50 mL·min−1) was passed through the samples. The temperature was increased at a rate of 10°·min−1 up to 500 °C, while a signal of a thermal conductivity detector was recorded. Copper dispersion (DCu), defined as a ratio of Cu exposed at the surface to total Cu, was calculated from the H2 uptake. The specific surface area of metallic copper (SCuN2O) was calculated using the following expression M H2 ·SF·NA 104 ·CM·WCu

(2)

where ρ is copper density (8.92 g·cm−3). XRD investigations were carried out at an ARL X’TRA diffractometer (Thermo, Switzerland) with a Si(Li) solid-state detector and Cu Kα radiation. The measurements were carried out in the 2θ range of 10−80° with a step of 0.1°. Phase analysis was performed using the ICDD PDF-2 database and the ICSD database.29 Average sizes of coherently scattering domain (CSD) dXRD were calculated by the line-broadening analysis according to the Scherrer equation,30 while for detected CuO phase a distinguishing between size and strain (Δd/d) contributions to the line broadening was performed by means of Williamson−Hall plots.31 The X’Pert HighScore Plus software was used for the analysis of peak profiles and for the Rietveld refinement with quantitative phase analysis.32 Visualization of crystal structures was performed using the program VESTA 3.33 Structural features of supported copper species were determined by analyzing the Fourier transformation of normalized X-ray diffraction scattering, which yields the radial distribution function of electron density 4πr2ρ(r)34−36 or the pair distribution function 4πr[ρ(r) − ρ0]37 (where ρ(r) is the electron density at distance r and ρ0 is the average electron density corresponding to a random atom distribution). The used calculation technique is described elsewhere.35,36,38 The procedure allows interatomic distances (r) and coordination numbers (CN) to be determined from position and area of peaks in the RDF curve. The analysis of RDF provides information about the local atomic structure and makes it possible to identify phases with a CSD size smaller than 3 nm. A comparison between the experimental and model RDFs is usually performed.36,38 The data required for modeling RDF, such as interatomic distances and coordination numbers, were calculated from structural data available in the ICSD database.29 The XRD patterns required for the RDF calculations were obtained using a high-resolution diffractometer at the Siberian Synchrotron and Terahertz Radiation Centre (SSTRC, Budker Institute of Nuclear Physics, Novosibirsk, Russia). The measurements were carried out with a step of 0.1° in the 2θ range of 3−145° at a wavelength of 0.703 Å or in terms of magnitude of the scattering vector, h = 4π sin(θ)/λ, in the range of 0.5−17 Å−1.

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N2O SCu (m 2·g Cu−1) =

6 × 103 N2O SCu ·ρCu

(1)

,where MH2, SF, NA, CM, and WCu are moles of hydrogen consumed per unit mass of catalyst (μmol H2 g−1 cat), stoichiometric factor (2),28 Avogadro’s number (6.022 × 1023 mol−1), number of copper atoms per unit surface area (1.46 × 1019 atoms·m−2),27,28 and Cu content (wt %), respectively. B


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The Journal of Physical Chemistry C Table 1. Structural Characteristics of the Cu/ZrO2 Catalysts copper loading (wt %)

dCuOXRD (nm)a

3 5 10 20 40

23 33.0 ± 0.9 34.0 ± 0.5 38.0 ± 0.7

(Δd/d) XRD

dCuXRD (nm)b

dCuN2O (nm)

DCu (%)

SCuN2O (m2·g−1)

(8.3 ± 0.5) × 10−4 (3.6 ± 0.3) × 10−4 (1.9 ± 0.4) × 10−4

>100 >100 >100 >100

3 6 11 17 33

31.2 18.7 9.5 5.9 3.1

200 120 61 38 20

a

Average size of CuO crystallites (CSD size) in the fresh catalysts according to XRD data. bAverage size of Cu0 crystallites (CSD size) in the aged catalysts according to XRD data.

determination are applicable to crystallites with sizes in the range of 3−100 nm. The crystallites with sizes 100 nm). It should be noted that comparison of the dCuN2O and dCuXRD values is more correct because of similarity of treatment conditions (reducing atmosphere, temperatures of 500 and 450 °C, respectively). A disagreement between dCuN2O and dCuXRD values is considerable (Table 1). Much smaller dCuN2O values indicate that fine Cu0 particles remained on the support surface even after partial sintering under reaction conditions. Indeed, the quantitative XRD phase analysis did not detect a part of the loaded copper (Table 2). The HRTEM data confirmed that copper coexists in the highly dispersed and particulate states. The highly dispersed CuO particles were observed on the support surface. Despite the low surface area of the m-ZrO2 support (15 m2 g−1), the 5Cu/ZrO2 and 10Cu/ZrO2 catalysts contained the CuO particles of 2 to 3 nm in size (Figure 3). The highly dispersed Cu0 particles were also detected in the catalysts aged under the reducing reaction conditions. Figure 4 shows TEM and HRTEM images of the 5Cu/ZrO2 catalyst C


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interaction of the highly dispersed copper species with the mZrO2 surface was also suggested by the TPR findings.21,22,39 Indeed, the analysis of the HRTEM images revealed epitaxial bounding between the CuO nanoparticles and zirconia surface (Figure 5). The image was obtained after a short exposure to an

Table 2. Copper Loadings in the Catalysts Aged in the Reaction of Ethanol Steam Reforming [Cu]0 (wt %)a

[Cu] (wt %)b

[Cu]XRD (wt %)c

5 10 20

4.6 ± 0.1 12.2 ± 0.3 18.9 ± 0.3

3.3 ± 0.2 6.5 ± 0.3 18.0 ± 0.3

a

Nominal copper loading. bCopper loading according to the ICP-AES data. cCopper loading according to the Rietveld quantitative phase analysis.

Figure 5. HRTEM image of the CuO nanoparticle epitaxially bound with the zirconia surface in the 5Cu/ZrO2 catalyst.

electron beam. A prolongation of the exposure induced structural rearrangements in the CuO nanoparticles and comprehensive analysis of orientation relationships became complicated. A measured interplanar spacing of 2.75 Å was ascribed to a family of crystallographic planes (110) in the CuO structure. As seen in Figure 5, the (110) planes of CuO phase are parallel to (200) planes of the m-ZrO2 phase; epitaxial (110) CuO ∥ (200) ZrO2 contact is observed. The data on planes, which are exposed in m-ZrO2 materials, are controversial in the literature. The (111), (001), and (011) planes have been reported to be exposed in m-ZrO 2 samples.40,41 Warble reported that monoclinic zirconia has three main general surface types: (110), (100), and (111).42 The difference between the experimentally observed surfaces might be due to variation in synthesis techniques and treatments, which affect particle size and morphology. Thus, crystals of natural monoclinic zirconia (mineral baddeleyite) are commonly tabular on {100} and somewhat elongated along the [010] direction.43 Similarly, the monoclinic zirconia under study is natural material and the crystallites expose (200) faces. Additional information on the local structure of the copper species was obtained using RDF of electron density. It is difficult to analyze the overall RDF of electron density for the catalyst because it includes information on both the support and the supported component. Differential RDFs (d-RDFs) between the RDFs of the catalysts and support were used for analysis. As was previously shown,24,36,44 this approach allows the local structure of supported nanoparticles to be determined. To identify the supported copper species in the Cu/ZrO2 catalysts, we considered the local atomic arrangement in possible copper phases. Figure 6 shows the model RDFs constructed on the basis of known structural data of the CuO, Cu2O, and Cu0 phases. Significant differences between them allow one to distinguish the state and structure of copper species unambiguously. The d-RDFs of the catalysts are close to the model one for the CuO phase. Figure 7 presents the d-RDF for the 5Cu/ZrO2 catalyst in comparison with the model one for the CuO phase. Positions of maxima correspond directly to interatomic distances; areas are related to coordination numbers. The analysis of the d-RDF shows that the short-range atomic order in the copper species is close to that of the CuO phase. The

Figure 3. HRTEM images of the 5Cu/ZrO2 catalyst (A,B) and 10Cu/ ZrO2 catalyst (C,D). Some of the highly dispersed CuO particles are marked by arrows. Electron diffraction data, which are presented in the insets, show interplanar spacings of the m-ZrO2 phase.

after the catalytic reaction. The highly dispersed and large Cu0 particles are visualized.

Figure 4. TEM (right) and HRTEM (left) images of the 5Cu/ZrO2 catalyst aged in the reaction of ethanol steam reforming.

Thus, the data revealed heterogeneous size distribution of the copper oxide species on the m-ZrO2 surface: The fresh catalysts contained the highly dispersed CuO particles as well as the large ones. Moreover, the highly dispersed Cu0 particles remained on the support surface along with the sintered, large Cu0 particles after the reduction and catalytic testing. The results agree with the TPR data previously reported.7,13,21−23 It seems that large, XRD detectable CuO particles were reduced and sintered during the reduction and catalytic testing, while the highly dispersed CuO particles were resistant to the sintering. An interaction with the support probably prevents a segregation of the highly dispersed CuO particles. The D


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triangular and square oxygen layers and have 7-fold coordination. The CuO structure along the [110] direction comprises square oxygen networks (Figure 9). The Cu atoms have an octahedral distorted oxygen coordination: The four nearest O atoms are placed at distance of 1.96 Å and two O atoms are placed at distance of 2.78 Å.

Figure 6. Model RDFs for copper compounds.

Figure 9. Structure of the CuO phase along the [110] crystallographic direction. Distorted octahedral coordination of Cu atoms and structure of the oxygen sublattice are depicted.

The square oxygen networks in the CuO and m-ZrO2 oxides have similar atomic arrangement (Figure 10). There is a

Figure 7. Experimental d-RDF describing the copper local arrangement in the 5Cu/ZrO2 catalyst and model RDF for the CuO phase.

coordination peaks corresponding to the CuO structure are well resolved. This result is in agreement with XANES data (Supporting Information). However, some significant discrepancies between the differential and model RDFs are observed. The main feature of the catalyst d-RDF is a presence of additional contributions around the distances 2.7, 4.0, 5.3, 6.0 Å, which are not typical of the CuO, Cu2O, Cu0 structures (Figures 6, 7). Appearance of the additional interatomic distances suggests possible deformation of the CuO structure. The 5Cu/ZrO2 catalyst contains the highly dispersed CuO particles. The supported CuO particles of 2 to 3 nm seem to have changes in the local atomic structure due to the interaction with the support surface. Both ZrO2 and CuO phases have a monoclinic crystal structure. Comprehensive consideration of these structures reveals a geometric similarity of some structural fragments. A crystal lattice of the m-ZrO2 phase along the [100] crystallographic direction is shown in Figure 8. Square and square-triangular oxygen networks alternate in the [100] direction. The Zr atoms are sandwiched between the

Figure 10. Structures of the square oxygen networks in the monoclinic CuO and ZrO2 oxides. In the case of zirconia, the square oxygen networks are located along the (200) planes.

proximity of angles and distances between the O atoms. The nearest interatomic distances are equal to 2.60, 2.73, 2.65 and 2.63, 2.9 Å in the m-ZrO2 and CuO structures, respectively. The differences do not exceed 10%. The good geometrical similarity between the CuO and ZrO2 crystal lattices favors the formation of the epitaxial contacts (110) CuO ∥ (200) ZrO2. A match between the lattices of nucleating crystal and support surface is critical for epitaxial growth.45 As was mentioned, the HRTEM data confirmed this epitaxial bounding (Figure 5). Strains in the CuO structure are expected because of existing lattice mismatches between the growing particle and support. These two phases share similar interfacial structure; the structure of CuO epilayers should be deformed to fit the ZrO2 surface structure. A cutoff of (110) CuO face is presented in Figure 11A. The arrangement of Cu atoms in the oxygen network is depicted. To conceive of an adjustment of the CuO lattice in the interface, we calculated the interatomic distances appearing at incorporation of Cu atoms into the oxygen networks of the ZrO2 structure (Figure 11 (B)). The estimated interatomic distances are equal to 1.8 to 1.95, 4.1 to 4.2 Å (Cu−O), 2.65, and 5.2 to 5.3 Å (Cu−Cu). These distances coincide with the additional interatomic distances observed in the experimental d-RDF (Figure 7). Thus, observed changes in the Cu atomic arrangement in the CuO particles are attributable to the formation of the epitaxial

Figure 8. Structure of the m-ZrO2 phase along the [100] crystallographic direction. The structure of the oxygen sublattice is presented on the right side. E


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Figure 11. Schematic top view of the Cu atomic arrangement in cutoffs of the (110) CuO (A) and (200) ZrO2 (B) faces. Cu−Cu and Cu−O interatomic distances are depicted.

bounding. The structure of the highly dispersed CuO particles is evidently strained from that in the bulk CuO. As shown in Table 1, noticeable diminishing microstrains within CuO crystallites is observed at increase in the copper loading from 10 to 40 wt %. This indicates a decrease in fraction of CuO particles being in interaction with the support surface with increase in the copper loading. The kind of ZrO2 phase is decisive for controlling the dispersion of the copper species. A smaller copper dispersion was observed in the Cu/m-ZrO2 systems.13−15,20 Our data confirmed preferable formation of the bulk CuO particles in the Cu/m-ZrO2 catalysts; however, the catalysts contained not only the large, XRD detectable CuO particles, but also the highly dispersed CuO particles revealed by HRTEM. The observed epitaxial CuO∥ZrO2 contacts can explain the presence of the highly dispersed CuO particles on the m-ZrO2 support possessing the low specific surface area. HRTEM study detected the epitaxial (110) CuO ∥ (200) ZrO2 contacts only, but other contacts also could take place. Nevertheless, we suggest that resemblance between CuO and ZrO2 structure fragments affects the formation and stabilization of CuO nanoparticles on the support surface. Thus, the results implied that geometrical similarity between the CuO and ZrO2 monoclinic structures favors interaction between the oxide particles. Undoubtedly, electronic structure, point defects, and acid−base properties of zirconia surface determine the interaction and interface organization; however, elucidating the interaction nature is a challenging task. Definitive conclusion about the interaction nature has not been yet made. Oxygen vacancies and coordinatively unsaturated Lewis acidic Zr4+ cations are abundant on the mZrO2 surface.14 It has been suggested that highly dispersed CuO species are stabilized over oxygen vacancies of m-ZrO2 surface.21,22 It was also reported that the interaction between copper species and zirconia complicated the reduction of highly dispersed CuO species21,39 and caused a high stability of Cu+ species on the m-ZrO2 surface under reduction conditions.13,15

Our XPS results (Supporting Information) also indicated interaction of copper oxide particles with zirconia surface. The obtained structural data are useful for understanding existing structure−performance relationships. Thus, the present results allow us to explain our recent data on catalytic performance of the Cu/m-ZrO2 and Cu/t-ZrO2 systems in ethanol steam reforming. Series of catalysts with nearly identical surface area of the supports (16 to 17 m2 g−1) were tested. At equal copper loading, the copper particles had somewhat lower surface area and larger average size on the m-ZrO2 surface; however, the Cu/m-ZrO2 catalysts showed catalytic performance comparable to that of Cu/t-ZrO2 systems.25,46 The high activity seems to be related to stabilization of highly dispersed copper species on the m-ZrO2 surface. Recently, we tested the activity of CuO-CeO2 catalytic systems supported on different oxides (ZrO2, TiO2, MnO2, Al2O3) in preferential CO oxidation in hydrogen-rich gas mixtures (PROX) and showed that monoclinic zirconia was the most effective support.47,48 Á guila et al. also claimed high activity of the ternary CuO-CeO2/m-ZrO2 systems in the PROX process.49 The ability of m-ZrO2 oxide to disperse copper species has been suggested.47,49 The present study of the binary Cu/m-ZrO2 system supports this suggestion. It was also reported that metal−support interaction and high dispersion of copper species defined the high catalytic activity of the Cu/m-ZrO2 catalysts in methane oxidation,23 WGS reaction,21,22 and CO oxidation.7 Thus, the ability of monoclinic zirconia to stabilize the highly dispersed CuO particles determines its efficiency as the support or promoter in copper containing catalysts for different processes.

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CONCLUSIONS The structure of supported copper species in the Cu/mZrO2 catalysts was studied using a complex of physicochemical methods. The Cu/m-ZrO2 catalysts contained the highly dispersed CuO particles along with the large, well-crystallized CuO particles. Geometrical similarity between the CuO and ZrO2 monoclinic structures favors the formation of epitaxial F


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(11) Tang, Q. L.; Liu, Z. P. Identification of the Active Cu Phase in the Water−Gas Shift Reaction over Cu/ZrO2 from First Principles. J. Phys. Chem. C 2010, 114, 8423−8430. (12) Aguila, G.; Guerrero, S.; Araya, P. Influence of the Crystalline Structure of ZrO2 on the Activity of Cu/ZrO2 Catalysts on the Water Gas Shift Reaction. Catal. Commun. 2008, 9, 2550−2554. (13) Sato, A. G.; Volanti, D. P.; Meira, D. M.; Damyanova, S.; Longo, E.; Bueno, J. M. C. Effect of the ZrO2 Phase on the Structure and Behavior of Supported Cu Catalysts for Ethanol Conversion. J. Catal. 2013, 307, 1−17. (14) Rhodes, M. D.; Bell, A. T. The Effects of Zirconia Morphology on Methanol Synthesis from CO and H2 over Cu/ZrO2 Catalysts: Part I. Steady-State Studies. J. Catal. 2005, 233, 198−209. (15) Ma, Z.-Y.; Yang, C.; Wei, W.; Li, W.-H.; Sun, Y.-H. Catalytic Performance of Copper Supported on Zirconia Polymorphs for CO Hydrogenation. J. Mol. Catal. A: Chem. 2005, 231, 75−81. (16) Szizybalski, A.; Girgsdies, F.; Rabis, A.; Wang, Y.; Niederberger, M.; Ressler, T. In Situ Investigations of Structure−Activity Relationships of a Cu/ZrO2 Catalyst for the Steam Reforming of Methanol. J. Catal. 2005, 233, 297−307. (17) Kundakovic, Lj.; Flytzani-Stephanopoulos, M. Reduction Characteristics of Copper Oxide in Cerium and Zirconium Oxide Systems. Appl. Catal., A 1998, 171, 13−29. (18) Liu, Z.; Amiridis, M. D.; Chen, Y. Characterization of CuO Supported on Tetragonal ZrO2 Catalysts for N2O Decomposition to N2. J. Phys. Chem. B 2005, 109, 1251−1255. (19) Ardizzone, S.; Bianchi, C. L. Electrochemical Features of Zirconia Polymorphs. The Interplay between Structure and Surface OH Species. J. Electroanal. Chem. 1999, 465, 136−141. (20) Wang, L.-C.; Liu, Q.; Chen, M.; Liu, Y.-M.; Cao, Y.; He, H.-Y.; Fan, K.-N. Structural Evolution and Catalytic Properties of Nanostructured Cu/ZrO2 Catalysts Prepared by Oxalate Gel-Coprecipitation Technique. J. Phys. Chem. C 2007, 111, 16549−16557. (21) Chen, C.; Ruan, C.; Zhan, Y.; Lin, X.; Zheng, Q.; Wei, K. The Significant Role of Oxygen Vacancy in Cu/ZrO2 Catalyst for Enhancing Water−Gas-Shift Performance. Int. J. Hydrogen Energy 2014, 39, 317−324. (22) Zhang, Y.; Chen, C.; Lin, X.; Li, D.; Chen, X.; Zhan, Y.; Zheng, Q. CuO/ZrO2 Catalysts for Water−Gas Shift Reaction: Nature of Catalytically Active Copper Species. Int. J. Hydrogen Energy 2014, 39, 3746−3754. (23) Aguila, G.; Gracia, F.; Cortes, J.; Araya, P. Effect of Copper Species and the Presence of Reaction Products on the Activity of Methane Oxidation on Supported CuO Catalysts. Appl. Catal., B 2008, 77, 325−338. (24) Pakharukova, V. P.; Moroz, E. M.; Kriventsov, V. V.; Larina, T. V.; Boronin, A. I.; Dolgikh, L. Yu.; Strizhak, P. E. Structure and State of Copper Oxide Species Supported on Yttria-Stabilized Zirconia. J. Phys. Chem. C 2009, 113, 21368−21375. (25) Dolgikh, L. Yu.; Pyatnitskii, Yu. I.; Reshetnikov, S. I.; Deinega, I. V.; Staraya, L. A.; Moroz, E. M.; Strizhak, P. E. Effect of Crystalline Modification of the Support on the Reduction and Catalytic Properties of Cu/ZrO2 Catalysts in the Steam Reforming of Bioethanol. Theor. Exp. Chem. 2011, 47, 324−330. (26) Gervasini, A.; Bennici, S. Dispersion and Surface States of Copper Catalysts by Temperature-Programmed-Reduction of Oxidized Surfaces (s-TPR). Appl. Catal., A 2005, 281, 199−205. (27) Guerreiro, E. D.; Gorriz, O. F.; Rivarola, J. B.; Arrua, L. A. Characterization of Cu/SiO2 Catalysts Prepared by Ion Exchange for Methanol Dehydrogenation. Appl. Catal., A 1997, 165, 259−271. (28) Evans, J. W.; Wainwright, M. S.; Bridgewater, A. J.; Young, D. J. On the Determination of Copper Surface Area by Reaction with Nitrous Oxide. Appl. Catal. 1983, 7, 75−83. (29) Inorganic Crystal Structure Database (ICSD-for-WWW); Fachinformationszentrum (FIZ): Karlsruhe, Germany, 2007. (30) Scherrer, P. Estimation of Size and Internal Structure of Colloidal Particles by Means of X-Rays. Göttinger Nachrichten 1918, 2, 98−100.

CuO∥ZrO2 contacts and stabilization of the highly dispersed CuO particles on the on the m-ZrO2 support with low specific surface area. In addition to detection of the highly dispersed CuO particles, identification of their local structure was performed. The atomic arrangement in the CuO particles was strained by the requirement of the epitaxial growth.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b06331. S1. XPS data. S2. The Williamson−Hall plots for determination of average CSD size and microstrain within CuO crystallites. S3. XANES data. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +7 (383) 326 95 97. Fax: +7 (383) 330 80 56. E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research activity is financially supported by the state project V.44.1.17. We also thank V. V. Kriventsov and A. I. Boronin for XANES and XPS studies, respectively.

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REFERENCES

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Structure of Copper Oxide Species Supported on Monoclinic Zirconia

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