In Situ XRD, XPS, TEM, and TPR Study of Highly Active in CO

Article pubs.acs.org/JPCC

In Situ XRD, XPS, TEM, and TPR Study of Highly Active in CO Oxidation CuO Nanopowders Dmitry A. Svintsitskiy,†,‡ Tatyana Yu. Kardash,†,‡ Olga A. Stonkus,† Elena M. Slavinskaya,† Andrey I. Stadnichenko,†,‡ Sergei V. Koscheev,† Alexei P. Chupakhin,‡ and Andrei I. Boronin*,†,‡ †

Boreskov Institute of Catalysis, Prospect Lavrentieva 5, 630090 Novosibirsk, Russia Novosibirsk State University, Pirogova Street 2, 630090 Novosibirsk, Russia

‡

S Supporting Information *

ABSTRACT: Copper(II) oxide nanopowders exhibit a high catalytic activity in CO oxidation at low temperatures. The combination of in situ XPS, XRD, and HRTEM methods was applied to investigate initial steps of CuO nanoparticles reduction, to identify oxygen and copper species and to revealed structural features in the dependence on reducing power of reaction medium. At the oxygen deficient surface of CuO nanopowders the metastable Cu4O3 oxide was formed under the mild reducing conditions −10−5 mbar CO or CO + O2 mixture with oxygen excess. Destruction of Cu4O3 structures in strong reducing medium (P(CO) ≥ 10−2 mbar) or under UHV conditions resulted in the formation of Cu2O which was epitaxially bounded with initial CuO particle. The reversible bulk reduction of CuO nanopowder to Cu2O at temperatures ∼150 °C can be explained by effortless propagation of Cu2O∥CuO epitaxial front inside the nanoparticle. The model of the surface restructuring along the {−111}CuO → {202}Cu4O3 → {111}Cu2O planes under the reduction of CuO nanopowders is proposed. The initial surface of CuO nanopowders is probably distorted and resembles Cu4O3-like structures that facilitates the CuOx ↔ Cu4O3 transition in mild reducing conditions. Such restructuring results in a unique electronic Cu4O3 structure with high oxygen deficiency and lowvalence Cu1+ sites stimulating the formation of highly reactive CO and O2 adsorbed species. It was shown that the most active oxygen species on the surface of CuOx is stabilized as O−, which was previously reported in papers by Roberts and Madix in their study of the copper−oxygen systems.

1. INTRODUCTION Copper-containing compounds are widely used in various applications such as material science, ecology, catalysis, and electronics. Numerous catalytic processes with copper being the active component including selective ammonia oxidation,1 phenol oxidation,2 the steam reforming of methanol,3 hydrocarbons oxidation,4 CO-PROX,5−7 and WGS reactions8−10 are well-known. The in situ11,12 and operando11,13 studies are prospective and essential for the catalytic reaction mechanisms understanding. Such experiments allow researchers to correlate a catalytic activity with molecular structure of active sites in approximate (in situ) or real catalytic conditions (operando). The active state on the surface is immediately formed in reaction conditions and can be changed after extraction from the reactor. For example, copper chromites and copper−zinc catalysts undergo the reversible structural and phase transitions.14,15 Using in situ approaches it was found that the active species of copper-ceria systems are different in WGS and CO-PROX reaction conditions: Cu0 closely interacting with oxygen vacancies of CeOx is an active site in WGS,10,16 and Cu1+−CO in the Cu−O−Ce interface in CO-PROX.17,18 Hence copper-containing systems should be studied by in situ methods due to the high sensitivity to redox conditions. © 2013 American Chemical Society

However, the major in situ/operando studies of coppercontaining catalysts have been provided with supported systems or mixed oxides (CuFe2O4,8 CuO/CeO2,17 and Cu/MoO219), whereas the literature devoted to the in situ study of individual copper oxides is poor. Rodriguez et al. have reported the dependence of the reduction pathway for CuO bulk powders on the flow rate of reducing agent (H2 or CO), the direct CuO to Cu0 for large supply and the CuO−Cu2O−Cu0 reduction in the case of limited supply.20−22 Additionally, Long et al. observed the Cu4O3 formation during reduction of CuO nanoparticles by electron beam.23 It is well-known that catalytic properties of copper oxides can be significantly modified by the variation of particle porosity,24 shape,25 defect structure,26 or particle size.27,28 For example, CuO nanoparticles are capable of CO oxidation at temperatures below 150 °C25,29,30 and even room temperature31 in contrast to bulk samples. Therefore nanosized copper(II) oxide is an attractive model object for physicochemical investigations. On the one hand, this system exhibits a high catalytic activity in oxidation of carbon Received: April 4, 2013 Revised: June 19, 2013 Published: June 20, 2013 14588

dx.doi.org/10.1021/jp403339r | J. Phys. Chem. C 2013, 117, 14588−14599

The Journal of Physical Chemistry C

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monoxide,25,29−31 methane,4 and methanol27 at temperatures comparable with real supported copper-containing catalysts. On the other hand, individual copper oxides can be unambiguously investigated with respect to the identification of active oxygen species and structural bulk transformations. The electronic structure of the CuO nanoparticle surface is significantly different from those of bulk samples because of a high oxygen deficiency.32 It gives rise to the stabilization of weakly charged oxygen species with a high reaction probability toward CO.33 In this paper redox properties of model CuO nanopowder were studied by combination of in situ physicochemical techniques such as XRD, XPS, and TEM. The interrelationship between surface and bulk reduction of nanosized copper(II) oxide was found at different CO partial pressure.

2.5. X-ray Diffraction. Diffraction data were measured using Siemens (Germany) D500 diffractometer, (Cu Kα radiation, Bregg-Brentano focusing scheme), equipped with graphite monochromator on the diffracted beam and reaction camera with beryllium glasses. In situ experiments were carried out in the temperature range from room to 350 °C with atmospheric pressure of reaction mixtures −1% CO in He or 100% CO. The flow rate was maintained equal to 70 mL/min. In a typical experiment the CO supply was switched on, and thereafter the sample was heated to a given temperature in helium. The most informative part of the X-ray pattern in the range of 2θ from 36 to 45° was repeatedly scanned. The temperature of each experiment was chosen so that the time of the observed structural changes was significantly shorter than one scan period. Reoxidation of the sample was carried out in air after the reactor was uncapped. Processing of XRD data was performed using the MAUD program.36 2.6. X-ray Photoelectron Spectroscopy. Analysis of surface electronic structure by XPS was performed using VG ESCALAB HP photoelectron spectrometer equipped with the analyzer and the preparation chambers. To record the spectra Al Kα (hυ = 1486.6 eV) X-ray source with power less than or equal to 100 W was used. No sample reduction was observed during spectra acquisition. To remove surface admixtures CuO samples were heated in 100 Pa of O2 at 350 °C during 20 min. A chromel-alumel thermocouple was used to measure temperature. Only copper, oxygen and carbon were observed by XPS before and after the admixtures removal. The contribution of the residual carbonate oxygen in the O1s spectrum after surface decontamination was less than 2%. It was taken into account for quantitative calculations. The total gas pressure during in situ experiments was equal to 10−5 mbar. It provided the study of the initial stage of the interaction between CuO and CO. The reaction medium included pure CO or a CO + O2 mixture with ratios of 2:1 or 1:2. The temperature was varied from room to 250 °C. During the in situ experiments the intensity of photoelectron lines was stable providing measurements with high accuracy. To study deep CuO reduction the step-by-step procedure using CO titration was applied. During step-by-step titration the CuO sample was exposed by 1000 Pa of CO in the preparation chamber of photoelectron spectrometer during 10 min followed by the transfer of sample to the analyzer chamber. After spectra recording, the sample was returned to the preparation chamber for subsequent exposure. The spectrometer calibration was performed relative to bulk gold (Au4f7/2) and copper (Cu2p3/2) photoelectron lines with BE values of 84.0 and 932.7 eV, respectively. The O1s, Cu2p, C1s, and Cu LMM spectral regions were recorded to study a charge state of the corresponding elements. The chemical composition of surface was quantitatively determined from integral peak areas using standard atomic sensitivity factors (ASF).37 To calculate the O/Cu stoichiometry the ratio of O1sand Cu2p spectral areas was used. Processing of the obtained data and spectral analyses (curve fitting, area calculation, and difference spectra) were performed using the XPS-Calc program, which has been tested on a number of systems.38−40 The curve fitting procedure was performed using an approximation based on a combination of the Gaussian and Lorentzian functions with subtraction of a Shirley-type background. Before the curve fitting, all experimental spectra were smoothed using a Fourier filter. No considerable difference between the smoothed and experimental curves was observed; the mean-square deviation was less than 1%. The

2. EXPERIMENTAL PROCEDURES 2.1. Sample Preparation. The typical CuO nanopowder was prepared via precipitation from copper(II) sulfate solution using an excess of 40 wt % NaOH solution. The black precipitate was repeatedly washed with distilled water to neutral pH, filtered, and air-dried at room temperature for two days. The commercial cupric oxide (Urals Plant of Chemicals Reagents, V. Pyshma, Russia) designated as CuO-bulk was used as a reference bulk sample. The CuO nanopowder was airannealed at 650 °C for 2 h to prepare another reference specimen referred as CuO T = 650 °C. Polycrystalline massive copper foil (purity 99,99%) with a thickness of 20 μm as well as Cu2O prepared at the surface of such a foil were used as reference compounds in X-ray photoelectron studies. To prepare the Cu2O layer the foil was oxidized in 500 Pa of O2 at 350 °C followed by annealing at 700 °C in UHV conditions (P ≈ 10−8 mbar). 2.2. Specific Surface Area. The surface area values were calculated by BET theory based on the low-temperature N2 adsorption using a Sorbi N.4.1 surface analyzer; the relative error was ≤6%.34 2.3. Temperature-Programmed Reaction. The activity of CuO samples was measured by means of a temperatureprogrammed reaction of CO + O2 (TPR-CO + O2) using an automatic setup equipped with a flow reactor and a mass spectrometer for analysis of gaseous mixtures. An initial sample for TPR-CO+O2 analysis with a 0.5−0.25 mm size fraction was loaded into a stainless steel reactor. The reaction mixture that contained 0.2% CO, 1.0% O2, and 0.5% Ne (balance He) was introduced into the reactor at a rate of 1000 mL/min. The gas hour space velocity was equal to 240 000 h−1 for all tested catalysts. Each sample was twice heated from −10 to +450 °C at a heating rate of 10 °C/min and was intermediately cooled to room temperature in the reaction mixture. In a typical temperature-programmed reduction (TPR) experiment the reaction mixture contained 1% CO and 0.5% Ne balanced in He. The flow rate was maintained equal to 100 mL/min, whereas the heating rate was varied from 3.5 to 10 °C/min. 2.4. Transmission Electron Microscopy. The CuO samples were studied by high-resolution transmission electron microscopy using a JEM-2010 (JEOL, Japan) microscope with a lattice resolution of 0.14 nm and an accelerating voltage of 200 kV. The crystal lattice images were analyzed by the Fourier method. Carbon films of thickness ca. 10 nm fixed on a standard copper grid were used as substrates. The size distributions of the particles were constructed using the ImageJ freeware software program based on MS-Windows.35 14589



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from bulk copper oxides, but a high-temperature treatment removed dissimilarity. Based on XRD data (see the Supporting Information, Figure S1) all studied samples corresponded to copper(II) oxide with tenorite structure (CC = 69757, ICSD database42). The morphological and redox properties of CuO samples are listed in Table 1. 3.2. Catalytic and Oxidative Properties of Copper(II) Oxide. The TPR-CO and TPR-CO + O2 curves of as-prepared CuO nanopowder, CuO T = 650 °C, and CuO-bulk samples are presented in Figure 2. This data characterize the catalytic properties and oxidizing ability to CO of copper(II) oxide with different dispersions. CuO T = 650 °C and CuO-bulk samples were reduced by CO at higher temperature, than nanodispersed powder. The mole ratio of total released CO2 to copper indicated the reduction of CuO to metal for all samples. During TPR-CO + O2 experiments the CuO nanopowders exhibited a lowtemperature activity (LTA) in catalytic CO oxidation in contrast to bulk samples (see Figure 2b). The temperature of 50% CO conversion (T50) for nanopowder was ∼108 °C, whereas bulk copper(II) oxides reacted with CO only above 150 °C (Table 1). Thus, catalytic properties of nanosized CuO were significantly different from classic bulk copper(II) oxide. Moreover, in Figure 2a, it can be observed the lowtemperature shoulder in the range from 80 to 130 °C as a fundamental feature of CuO nanopowders reduction. Such shoulder was repeatedly observed for all CuO nanopowders synthesized in different preparation conditions.32 The similar shoulder was found by other research groups in the same temperature range for both individual copper(II) oxide25,31 and supported systems.43−46 The decrease of heating rate from 10 °C/min to 3.5 °C/min resulted in a total shift of TPR-CO curve to lower temperatures and the splitting of main reduction peak into two peaks: at 118 and 130 °C (see Figure 3a). The difference between maxima of CO consumption peaks for TPRCO curves obtained with heating rates 10 °C/min (∼0.6 vol. %) and 3.5 °C/min (∼0.4 vol. %) can be observed (see Figures 2a and 3a) because the TPR curves were recorded in dependence on time. The decrease of heating rate resulted in the fall of reduction process speed and the decrease of CO consumption per min, respectively. The areas of TPR-CO curves in ≪CO consumption − recording time≫ axis were the same for different heating rate experiments. To interpret TPR-CO data, the reduction process of CuO nanopowder was simulated in photoelectron spectrometer. The step-by-step titration in 1000 Pa of CO, what corresponded to partial CO pressure during TPR-CO study (1% CO in He), was carried out. In Figure 3b,c the sets of Cu LMM and Cu2p spectra recorded during titration in the temperature range from 50 to 140 °C are presented. The spectra were recorded thereafter CuO nanopowder has been kept in CO at given temperature for 10 min. Such long CO exposures should be

maximum depth of XPS analysis is limited by 3-fold inelastic mean free path (IMFP) − 3λ. The IMFP values for O1s and Cu2p photoelectrons emitted from CuO were calculated as 8.8 and 6.7 nm, respectively.41 Therefore the surface of copper oxide can be considered as a layer with thickness less than 9 nm.

3. RESULTS 3.1. Characterization of CuO Samples. The specific surface area of the synthesized CuO nanopowder was equal to ∼89 m2/g, whereas the air-annealed sample CuO T = 650 °C was characterized by the surface area ∼4 m2/g. The latter was close to those of commercial bulk CuO (0.5 m2/g). In Figure 1,

Figure 1. TEM images of CuO nanopowder (a) in the as-prepared state and (b) after air-annealing at 650 °C. Panel c presents a typical electron diffraction pattern of CuO nanopowder. TEM image of CuObulk is presented in panel d.

TEM data for initial, air-annealed nanopowder and commercial bulk sample are presented. The as-prepared CuO nanopowder consisted of sheet-like particles with length from 45 to 165 nm, whereas particle width was varied in the range of 5−50 nm. The diagram of particle length distribution is shown in Figure 1a. The typical electron diffraction pattern of CuO nanoparticles agglomerate (see Figure 1c) was a ring-like type due to a merge of one-type reflections from chaotically orientated crystallites. The measured interplanar distances d110 = 2.75 Å, d002 = d−111 = 2.53 Å, and d111 = 2.32 Å were attributed to CuO phase. The air-annealing of nanopowder at 650 °C resulted in shapeless crystallites with size ≥250 nm (see Figure 1b). Commercial CuO-bulk sample also consisted of shapeless particles with size ≥450 nm (see Figure 1d). Hence the morphology of CuO nanopowders was significantly different Table 1. Quantitative Characteristics of Studied CuO Samples sample

CuO nanopowder

CuO T = 650 °C

CuO-bulk

S, m /g CSR, nma morphology (TEM) main TPR-CO peak, °C T50 (CO + O2), °C

89 15 nanosheets with lengths of 50−150 nm and widths of 5−50 nm 164 108

4 78 large particles ∼250 nm 318 234

0.5 ≈100 large particles ≥500 nm 336 273

2

a

CSR, coherent-scattering region. 14590



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Figure 2. (a) TPR-CO and (b) reheating TPR-CO + O2 curves of (1) as-prepared CuO nanopowder, (2) CuO nanopowder air-annealed at 650 °C, and (3) commercial bulk copper(II) oxide. The heating rate for all experiments was equal to 10 °C/min.

Figure 3. (a) TPR-CO curve for CuO nanopowder with a heating rate of 3.5 °C/min during exposure to 1% CO in He. Spectral (b) Cu LMM and (c) Cu2p regions recorded during titration of CuO nanopowder by 1000 Pa of CO.

Figure 4. In situ XRD reduction of CuO nanopowder in 100% CO at 40 °C; (a) a series of isothermal X-ray patterns and (b) the dependence of CuO and Cu0 content on reduction time.

spectral region and also the broadening of main Cu LMM peak at 917.9 eV to lower kinetic energies. It indicated the formation of monovalent copper at the surface of CuO nanopowder in the temperature range of 50−100 °C, what corresponds to lowtemperature shoulder of TPR-CO curve (see Figure 3a). Further increase of temperature from 120 to 140 °C resulted in the appearance of Cu LMM peaks at 918.5 and 921.3 eV attributed to metallic copper, whereas no shake-up satellite of Cu(II) species was detected in Cu2p spectra (see Figure 3b,c). In the range from 100 to 140 °C the surface of nanopowder was intensively reduced by CO to metallic state that is in good agreement with the TPR-CO data (see Figure 3a). Thereby the

correlated with the TPR-CO study at a minimal heating rate −3.5 °C/min. The Cu LMM and Cu2p spectra for pure bulk CuO, Cu2O, and copper foil were recorded and used as references (see Figure 3b,c). The simultaneous analysis of Cu LMM and Cu2p spectra allows distinguishing copper species at the surface reliably.47,48 No detected changes of Cu LMM line were found during CuO reduction by CO up to 50 °C, whereas some features in Cu2p spectra can be observed: the drop of shake-up satellite intensity and the shift of Cu2p5/2 peak to lower binding energy side (see Figure 3b,c). The increase of reduction temperature to 120 °C resulted in gradual intensification of observed changes in Cu2p 14591



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Figure 5. In situ XRD reduction of CuO nanopowder in 1% CO/He at 180 °C. (a) A series of isothermal X-ray patterns and (b) The dependence of phase content on reduction time.

Figure 6. In situ XRD reoxidation of CuO nanopowder preliminarily reduced in 1% CO/He at 180 °C to 10% of metallic copper. Reoxidation was carried out in air without preliminary cooling of the reactor; (a) X-ray patterns recorded during reoxidation in the temperature range from 180 to 350 °C; (b) dependence of phase content on reoxidation time.

TPR-CO peaks at 118 and 130 °C can be interpreted as the reduction of CuO nanoparticles to Cu2O state followed by reduction of Cu2O to metallic copper, respectively. Comparison of TPR-CO and TPR-CO + O2 curves obtained at the same heating rate (10 °C/min, see Figure 2a,b) showed that thte temperature range of the shoulder in TPR-CO curve (60−135 °C, marked with green color) correlates with growth of catalytic CO oxidation. The CO conversion at 135 °C was ∼95%. Thus, the nature of the low-temperature catalytic activity is related to the shoulder observed in the TPR-CO curve of CuO nanopowder. Based on Cu LMM and Cu2p spectral analysis, the appearance of shoulder was accompanied by surface Cu2+ to Cu1+ reduction. Meanwhile, the area of the shoulder was approximately ∼10 ± 3% with respect to the entire area of the TPR-CO curve. The removal of more than ∼13% of all oxygen resulted in bulk structural transitions. To study these bulk changes in the dependence on redox conditions, in situ X-ray diffraction was used. 3.3. In Situ X-ray Diffraction. For analysis of bulk redox processes during in situ X-ray diffraction investigation the fixed section of X-ray pattern with 2θ from 36 to 45° was used. This section includes the most intensive reflexes of CuO (111 and 200), Cu2O (200), and metallic copper (111) crystal structures (see Figure 4a, PDF-2 #05-0661, 05-0667, 04-0836 ICDD).49 The choice of this section was caused by a search of compromise between the highest level of information and minimal recording time simultaneously. During reduction of CuO nanopowders in 100% CO at 40 °C, an induction period followed by the appearance of (111) Cu0 reflex was revealed. Further interaction with CO resulted in the monotonic

decrease of CuO (111) and (200) reflexes accompanied by direct CuO to Cu0 transition without any intermediate phases. In Figure 4b the dependence of both CuO and Cu0 phase contents on reduction time is presented. The temperature of CuO nanopowders reduction in diffractometer camera (40 °C) was significantly lower than those during the TPR-CO experiment (150−200 °C). It can be explained by the difference of partial CO pressure: ∼105 Pa in case of in situ XRD study and ∼103 Pa during TPR-CO. When 1%CO/He mixture was used in situ XRD, reduction of CuO nanopowder was observed above 150 °C. Moreover, the reduction processed through intermediate Cu 2O phase formation, which is in good agreement with TPR-CO data (see Figure 3a). In the course of reduction in 1%CO/He the growth of Cu2O (200) peak intensity was observed. First, it accompanied the disappearance of CuO reflexes followed by the growth of metallic copper peaks (see Figure 5a). Previously, the change of CuO nanopowders reduction pathway in the dependence on partial CO pressure was also observed for bulk copper(II) oxide powders.22 The dependence of phase content (mass percents) on time during the reduction of CuO nanopowder at 180 °C in 1%CO/ He is presented in Figure 5b. When the Cu2O content reached ∼50 wt %, the slope of the curve changed indicating the alternation of the CuO to Cu2O reduction rate. Initially, the rate of Cu2O formation was higher than after half of transition. No similar features were found during reduction of bulk CuO powders in work by Wang et al.22 Moreover, in this work the appearance of metallic copper at the Cu2O content ≤20 wt % 14592



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of CuO nanopowders in the 1%CO/5%O2/He mixture, which imitates catalytic conditions, no bulk structural transformations were observed up to 350 °C (see the Supporting Information, Figure S2). However, based on our previous results the exposure of the CO + O2 mixture modifies the surface of CuO nanopowders significantly.32 3.4. Transmission Electron Microscopy. Copper(II) oxide nanopowders were studied by TEM before and after catalytic tests. The treatment in reaction conditions resulted in the changes of nanoparticle shape from sheet-like to roundlike.32 Moreover, intensive exposure by electron beam induced a structural transition of initial CuO nanoparticles. On the one hand, it complicated analysis of the TEM data, but on the other hand, it allowed us to obtain worthwhile knowledge. The reduction of CuO nanoparticles by electron beam was in situ carried out in the chamber of an electron microscope. The TEM image of the initial CuO nanoparticle obtained after exposure by a low intensity electron beam during a short time is presented in Figure 8a. Increase of beam intensity resulted in the appearance of extended islands with changed crystal structure at the surface of initial nanoparticle (see Figure 8b). The analysis of high resolution images revealed the formation of Cu4O3 structures. In Figure 8c the fast Fourier transform (FFT) image of Cu4O3/CuO particle is presented. The low-intensive reflex with interplanar spacing ∼3.16 Å (marked by red ring) can be interpreted only as the (112) reflex of the Cu4O3 lattice because no similar values were found for CuO or Cu2O lattices. The observed (111) reflex for initial CuO nanoparticle (see Figure 8a) was broadened as the thickness of the Cu4O3 surface layer increased. At certain thickness the new reflex with interplanar spacing ≈ 2.51 Å attributed to d202 Cu4O3 was observed. As the interplanar spacing of (−111) CuO ≈ 2.53 Å is very similar to those of (202) Cu4O3 equal to 2.51 Å, these reflexes is not distinguished in FFT image. The area with Cu4O3−CuO interface from Figure 8b (marked by dash line) is presented in Figure 8d after Fourier filtration procedure. The observed thickness of Cu4O3 layer was equal to ∼1.3 nm and resulted in the appearance of moiré fringes due to the overlapping of CuO and Cu4O3 lattices. Prolongation of electron beam exposure destroyed the Cu4O3 layer followed by the formation of 2−3 nm Cu2O structures with typical interplanar spacing of 2.46 Å. It was attributed to the family of crystallographic planes {111} of Cu2O. It should be noted, that formed Cu2O structures were epitaxially bounded with initial CuO nanoparticle. The same structures were observed at the CuO nanopowder surface after exposure of reaction CO+O2 mixture in reactor (see Figure 9). Figure 9b,c shows typical FFT images for CuO and Cu2O segments of epitaxy. The interplanar spacing 2.53 Å of the CuO lattice was identified as the family of crystallographic planes {−111}. As seen in Figure 9a, the (111) plane of Cu2O is parallel to (−111) faces of CuO, which confirms the epitaxial bounding between these lattices. Such epitaxial (111)Cu2O∥(−111)CuO structures were stable under electron beam exposure. It should be noted that we checked the possible influence of atmospheric air exposure on the state of the samples reduced by reaction media. XPS of CuO nanopowders after CO + O2 treatment in the reactor showed that spectra obtained reveal great amounts of Cu1+ ions. So, air exposure at room temperature does not result in fast reoxidation of Cu2O epitaxial structures to the initial CuO species.

was observed, whereas in case of CuO nanopowders metal species was only appeared at the Cu2O content ∼90 wt %. It emphasizes a difference between properties of bulk and nanosized systems. The authors also discussed a possibility of metastable oxide formation at the beginning of CuO reduction by CO. The generation of metastable phases, for example Cu4O3, can become apparent for CuO nanoparticles because of their high specific surface area. The most intensive (202) reflex of Cu4O3 corresponding to 2θ = 35.8° is similar to the intense (−111) reflex of CuO with 2θ = 35.4° and (111) reflex of Cu2O with 2θ = 36.4°.49 It is very difficult to detect a small quantity of the Cu4O3 phase when broadened peaks from CuO and Cu2O are also present in the XRD pattern. Thus, the formation of Cu4O3 could not be ruled out during CuO nanopowders reduction. Another possibility is a formation of a metastable X-ray amorphous Cu4O3 phase as a thin surface layer. Figure 6a presents the X-ray patterns recorded during reoxidation of preliminary reduced CuO nanopowder to metallic copper. Before reoxidation the reduced sample contained three phases: w(Cu2O) ≈ 80%, w(CuO) ≈ 10%, and w(Cu) ≈ 10%. Such a sample could not be fully reoxidized after 4 h of air exposure and a temperature increase up to 350 °C. The dependence of phase content on reoxidation time is presented in Figure 6b. When reduction of CuO nanopowders was stopped at the stage of CuO to only Cu2O (up to 50 wt %) transition without the metallic copper appearance, the time of full reoxidation in air was significantly less than the registration time of one X-ray pattern scan (see Figure 7).

Figure 7. In situ XRD reduction and reoxidation of CuO nanopowder at 180 °C. (1) X-ray pattern of the initial sample. (2−4) X-ray patterns recorded during reduction in 1% CO/He. (5) The first X-ray pattern scan after reoxidation in air. The full reoxidation time was less than 5 min.

Therefore the reoxidation of preliminary reduced CuO nanopowders without the metallic copper appearance was easier and faster than in case of Cu0 appearance. It could be related to an inflection of Cu2O curve observed in Figure 5b. Such inflection is probably caused by the stabilization of metastable oxides, for example Cu4O3, at the surface. Moreover, low-temperature catalytic activity of CuO nanopowders can also be explained by the presence of active metastable phases at the surface of CuO nanoparticles. During the in situ XRD study 14593



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Figure 8. (a) HRTEM image of an initial CuO nanoparticle together with the corresponding fast Fourier transform (FFT) image; (b) HRTEM image of a CuO nanoparticle after prolonged exposure by the electron beam; (c) FFT image of the area in panel b marked by dashed lines; (d) The area in panel b marked by dashed lines after the Fourier filtration procedure. Schematic images of the reciprocal lattice of CuO (e) and Cu4O3 (f) are also presented.

Figure 9. (a) HRTEM image of a Cu2O particle epitaxially bound with CuO after reaction exposure by CO + O2 in the reactor. FFT images of corresponding (b) CuO and (c) Cu2O crystal lattices are also presented.

Figure 10. Crystal lattices of CuO, Cu4O3, and Cu2O along fixed crystallographic directions: [−111], [202], and [111], respectively (indicated by the upright arrow). On the right side of the figure, the cutoffs of the (−111), (202), and (111) faces are presented. To build this figure, the ICSD database was used: CuO (CC = 6975750), Cu4O3 (CC = 7767551), and Cu2O (CC = 5204352).

Although the epitaxial patches were found at the CuO nanoparticle surface after catalytic tests, they cannot be considered as active species in reaction conditions immediately. Such structures were likely formed as a result of the active sites destruction during instrumental characterization, for example, during evacuation or electron beam exposure. The crystal lattices of CuO, Cu4O3, and Cu2O along [−111], [202], and [111] directions, respectively, are presented in Figure 10. On the right side of the image the (−111) CuO, (202) Cu4O3, and (111) Cu2O planes are shown. The crystal structure of Cu4O3 along the [202] direction contained alternate layers of (−111) CuO-like (with all oxygen atoms) and (111) Cu2O-like planes. Figure 10 shows the (202) plane of Cu4O3 similar to the (−111) CuO face. The interplanar spacings of (−111) CuO, (202) Cu4O3, and (111) Cu2O faces are close and equal to 2.524, 2.508, and 2.464 Å, respectively. Moreover, the arrangement of copper atoms in these faces is similar (see Figure 10). According to crystallographic data and

in situ TEM study the reduction pathway during electron beam exposure was concluded to be as follows: {−111}CuO → {202}Cu4O3 → {111}Cu 2O

Destruction of the Cu4O3 layer resulted in the formation of (111)Cu2O∥(−111)CuO epitaxy. The good geometrical similarity between (−111) CuO, (202) Cu4O3, and (111) Cu2O planes can explain an effortlessness of transitions between them and, as a consequence, a high oxygen mobility of CuO nanopowders at low temperatures as provided by TPRCO study. The formation of epitaxial structures can greatly facilitate the CuO ↔ Cu2O interconversion which is in good agreement with in situ XRD data. Thus, HRTEM study of CuO nanopowder indicated significant surface modification during reaction exposure. To study an evolution of electronic states at the surface of CuO 14594



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945 eV (see Figure 11b) as the changes in Cu LMM spectra (not presented) indicated the appearance of Cu1+ ions at the surface of CuO nanopowder during in situ XPS study. For example, the maximum quantity of monovalent copper at the surface in pure CO at 250 °C was equal to ∼38% (see Table 2). However, no shifts of the lattice oxygen peak with binding energy 529.6 eV in O1s-spectra was observed during the appearance of surface Cu1+ ions (see Figure 11a). The lattice oxygen of the Cu2O phase is characterized by the binding energy of ∼530.2 eV.37,53 Thus, the absence of shifts in O1s spectra indicated that no Cu2O structures including Cu2O/ CuO epitaxies were formed at the surface of CuO nanopowder during exposure in pure CO or CO + O2 reaction mixtures. The first step of CuO nanopowder reduction in the reaction conditions occurred with preservation of initial surface structure or it is negligible changes. It should be noted that the lowtemperature shoulder in the TPR-CO curve (see Figures 2a and 3a) was also accompanied by Cu(II) to Cu(I) surface reduction without bulk transitions. The quantitative distribution of oxygen species is presented in Table 2. It contains data about CuO nanopowder in the initial state, during the in situ XPS study and after pumping out the reaction mixture. According to section 3.1, CuO nanopowder was annealed in 100 Pa of O2 at 650 °C for 1 h to prepare the bulk reference sample. The surface O/Cu ratio of the resulting bulk copper(II) oxide was assumed to be equal to one. Thereby the surface of CuO nanopowders was characterized by high oxygen deficiency (O/Cu ≈ 0.85) even taking into account the additional O1s shoulder in the region of 531.4 eV.33 The high oxygen deficiency should result in significant modification of electronic and geometric surface properties. It explains the stabilization of reactive weakly charged oxygen species (Eb(O1s) = 531.4 eV). Previously, similar O−-like oxygen at the surface of CuO with ∼5% of oxygen deficiency was also observed.54 The quantity of lattice oxygen with Eb(O1s) = 529.6 eV at the initial surface of CuO nanoparticles was equal to ∼0.76 that is close to stoichiometry of Cu4O3 (O/Cu = 0.75). This structure was detected by HRTEM during the reduction of CuO by electron beam. Moreover, the total oxygen quantity during prolonged reaction exposure was also similar to ∼75% (see Table 2). The pumping out the reaction mixture up to 10−8−10−9 mbar with the simultaneous cooling of the sample to room temperature resulted in significant changes in O1s- and Cu2pspectra. As seen in Figure 11a and Table 2, the reduction of Cu(II) to Cu(I) was accompanied by the shift of the O1s-peak to higher binding energies up to ∼530.0 eV after evacuation. It indicated the formation of Cu2O surface structures. Since the observed shift of the O1s-peak introduced a great error to

nanopowder, in situ X-ray photoelectron spectroscopy (XPS) was applied. 3.5. In Situ X-ray Photoelectron Spectroscopy. Copper(II) oxide nanopowders were studied by in situ XPS at the temperature from room to 250 °C in various reaction media: pure CO, CO + O2 = 2:1, CO + O2 = 1:2. The total gas pressure was ∼10−5 mbar in all experiments. In situ O1s and Cu2p spectra of nanodispersed CuO after removal of surface admixtures (see Experimental Procedures) are presented in Figure 11. These spectra were acquired in the CO:O2 = 2:1

Figure 11. In situ (a) O1s and (b) Cu2p spectra of CuO nanopowder in reaction CO:O2 = 2:1 mixture (total pressure of gas phase was 10−5 mbar) at (●) 50 and (▲) 250 °C. Additionally, (▼) the corresponding spectra after outgassing of reaction medium are presented. (■) The difference in the O1s spectrum between the 50 and 250 °C states is given with 2-fold magnification.

mixture at temperature from 50 to 250 °C. Evolution of spectra in experiments with pure CO or CO:O2 = 1:2 mixture was identical to those presented in Figure 11 with the difference that the quantity of monovalent copper at surface diminished as the ratio of oxygen in reaction mixture increased (see the Supporting Information, Figure S3). The increase of temperature from 50 to 250 °C resulted in the changes of O1s spectra at 529.6 and 531.4 eV regions (see Figure 11a) corresponded to CuO lattice oxygen Olattice (Ebin(O1s) ≈ 529.5 eV) and weakly charged oxygen species Oadd (Eb(O1s) ≈ 531.3 eV). The latest oxygen form was found to be highly reactive toward CO oxidation at 80 °C.32,33 Its reaction probability was higher by 4 orders of magnitude than those of lattice CuO oxygen. The simultaneous drop of signal intensity in the O1s spectral regions of lattice and weakly charged oxygen species was observed during exposure by pure CO or CO + O2 mixtures (see Figure 11a). It confirmed a high reaction probability of CuO nanoparticles toward CO. The shift of the Cu2p3/2 peak from 933.5 to 932.6 eV and the intensity drop of the shake-up satellite in the region of 940−

Table 2. Relative Concentrations of Oxygen and Copper Species at the Surface of CuO Nanopowder during in Situ XPS Study CuO nanopowder sample total oxygen quantity, % Olattice, % Oadd., % Cu1+ quantity, % a

bulk CuO

initial state

10−5 mbar of CO, T = 250 °C

After pumping out 10−5 mbar of CO

10−5 mbar of CO + O2 (2:1), T = 250 °C

After pumping out 10−5 mbar of CO + O2 (2:1)

100

85

75

72

76

71

100 0 0

76 9 0a

71 4 38

50

73 3 23

48

Absence of the Cu1+ state at the surface of this sample was discussed in ref 33. 14595



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Figure 12. Scheme of reduction surface/bulk processes for CuO nanopowder in dependence on reaction conditions.

surface of the initial CuO nanoparticle along fixed crystallographic direction (111)Cu2O∥(−111)CuO. Although no bulk structural transitions of CuO nanopowders occurs up to 350 °C, significant modification of the surface in the CO + O2 reaction medium was found. On the basis of all obtained data, we suppose that catalytically active surface state of CuO nanoparticles is layer or islands of metastable Cu4O3 oxide formed during exposure of the reaction mixture. Such Cu4O3 sites include low valence Cu1+-like species, which facilitate CO adsorption, and oxygen vacancies, which stabilize reactive oxygen forms. The analysis of the Cu4O3 crystal structure (CC = 7767551) shows the presence of two types of oxygen ions in lattice in contrast to copper(II) and copper(I) oxides. Electronic properties of these oxygen ions should be different as well as CuO lattice oxygen and weakly charged oxygen species observed by XPS at the surface of CuO nanopowders. The confirmation of high catalytic activity of paramelaconite phase (Cu4O3) was reported in ref 57 in complete isopropanol oxidation that supports our hypothesis of active site with Cu4O3-like structure. The revealed low-temperature shoulder of TPR-CO curves for CuO nanopowders can be associated with formation of metastable surface Cu4O3 structures at the first step of reduction (see Figures 2a and 3a). Temperature ranges of the TPR-CO shoulder and CO conversion catalytic curve are similar (see Figure 2, marked by green regions). It confirms the relationship between low temperature catalytic activity of CuO nanoparticles and formation of metastable Cu4O3 oxide, which was characterized by active sites for catalysis. It should be noted that researchers have identified LTA of supported copper catalysts observed in TPR curves lowtemperature shoulder.43−46 This shoulder was attributed to copper species strongly interacting with the support, for example, ceria containing a lot of oxygen vacancies within the interface layers. Formation of metastable and nonstoichiometric phases that exhibit low reduction temperatures is discussed in the literature devoted to individual copper oxides.25,31 A high oxygen mobility of oxygen at the surface of such metastable phases can cause the high catalytic activity in CO oxidation.58 Wang et al. have discussed the possibility of oxygen vacancies to be accumulated at the surface of bulk copper oxide powders during interaction with CO.22 However, no Cu4O3 phase was found during bulk CuO investigation in contrast to nano-

quantitative characterization of oxygen species, this data is not presented in Table 2 for corresponding samples. Formation of surface copper(I) oxide after pumping out the reaction mixture is in good agreement with the appearance of Cu2O∥CuO epitaxial structures as provided by HRTEM. Such structures were also found at the surface of CuO nanopowders after catalytic CO + O2 exposure followed by the evacuation in the chamber of the electron microscope.

4. DISCUSSION Morphological properties of highly dispersed CuO nanopowders differ from those of bulk copper(II) oxide. Electronic structure of the nanoparticles surface is significantly modified inducing changes of their redox properties.32 CuO nanopowders exhibited catalytic activity in CO oxidation at 60−70 °C. This fact is in good agreement with the literature data,25,31 and the temperature range of the catalytic CO oxidation by CuO nanopowders is similar to those of copper-ceria systems (see Figure 2b).6,55,56 Thus, copper(II) oxide nanopowder is an attractive model object for simulative investigations. In this paper in situ study of surface and bulk reduction of nanosized CuO was carried out in dependence on redox conditions. Obtained results are schematically summarized in Figure 12. The surface of CuO nanoparticles was characterized by high oxygen deficiency up to 15% that resulted in the stabilization of weakly charged oxygen species with a high reaction probability toward CO at low temperature (80 °C32,33). The combined XPS and TPR-CO investigation (see Figures 3 and 2a) revealed a high mobility of surface oxygen at 60−120 °C without bulk phase transformations. In the mild reducing conditions (P(CO) ≤ 10−5 mbar or exposure by low intensity electron beam) deficient CuOx surface of nanoparticles was covered by thin layer of Cu4O3 as evidenced by in situ HRTEM and XPS. At such surface there was significant Cu1+ quantity and the O/Cu stoichiometry was similar to 0.75 (see Table 2 and Figure 11). The surface Cu4O3 layer is metastable and could be destroyed easily by an increase in reducing power of reaction conditions: the increase of CO partial pressure in the reaction mixture, prolonged exposure by electron beam, or evacuation. Destruction of Cu4O3 structures resulted in the formation of Cu2O islands or thin Cu2O layer which was bounded to the 14596



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particles.23 Formation of Cu4O3 at the surface of CuO nanopowders can be caused by their unique electronic structure and, as a consequence, high oxygen deficiency. Initial surface of CuO nanopowders is probably distorted and resembling Cu4O3-like structure, that facilitates CuOx ↔ Cu4O3 transition in mild reducing conditions. XPS data show the presence of shoulder with Eb(O1s) = 531.3 eV in O1s spectra of oxidized samples indicating the stabilization of weakly charged oxygen species like O− or Oδ−. Such oxygen species in copper−oxygen systems were earlier studied in works by Roberts59−61 and Madix.62,63 They showed the high reactivity toward CO of weakly charged oxygen species at low temperatures. The stabilization of such species was not possible at the ordered oxide surfaces, so the presented results on the stabilization of O− species at the surface of distorted CuOx are in a full accordance with results of Roberts and Madix.59−63 Thus, the general concept of Roberts and Madix on the key role of weakly charged oxygen species in the oxidation reactions can be extended to CuO nanoparticles. Comparison of obtained data (see Figure 5) with the results of the Wang et al. study22 allows us to suppose different reduction by CO behavior for bulk and nanosized CuO. The reduction of CuO nanoparticles to Cu2O was characterized by two steps with different rates in contrast to the bulk sample. It can be explained in terms of surface Cu4O3 appearance. Formation of Cu4O3 surface structures from deficient distorted CuO lattice is likely accompanied by easier oxygen release than further Cu4O3 to Cu2O∥CuO destruction. The problem of bulk Cu4O3 stabilization can be caused by difficulties in oxygen vacancies ordering in nondeficient bulk of CuO particles. The formation of epitaxial Cu2O∥CuO structures observed by HRTEM at the surface of CuO nanopowder can explain the difference of temperature ranges for bulk and nanosized CuO samples. The reduction of CuO nanopowders in 1%CO/He mixture occurred at 150−180 °C whereas bulk samples reduced only above 250 °C as provided by the TPR-CO study (see Figure 2). We assume that formation of Cu2O∥CuO epitaxy facilitates bulk CuO ↔ Cu2O transition due to high mobility of epitaxial front throughout CuO nanoparticles. It is also shown to facilitate the reoxidation process as presented in Figure 7. Reduction to metallic copper resulted in difficult prolonged reoxidation with the necessity of temperature increase up to 350 °C. The appearance of metallic copper was observed when the quantity of residual CuO phase was

In Situ XRD, XPS, TEM, and TPR Study of Highly Active in CO

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