Effect of Ti3+ on TiO2-Supported Cu Catalysts Used for CO Oxidation

Jun 8, 2012 - ABSTRACT: In this paper, we have shown that Cu/TiO2 catalysts are highly active in CO oxidation. For instance, a 3.4% Cu/TiO2...
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Effect of Ti3+ on TiO2-Supported Cu Catalysts Used for CO Oxidation Ching S. Chen,†,* Tse C. Chen,‡ Chen C. Chen,† Yuan T. Lai,† Jiann H. You,§ Te M. Chou,†,§ Ching H. Chen,† and Jyh-Fu Lee †

Center for General Education and ‡Department of Chemical and Materials Engineering, Chang Gung University, 259 Wen-Hwa first Road, Kwei-Shan Tao-Yuan, Taiwan, 333, Republic of China § National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan, Republic of China S Supporting Information *

ABSTRACT: In this paper, we have shown that Cu/TiO2 catalysts are highly active in CO oxidation. For instance, a 3.4% Cu/TiO2 catalyst exhibits a higher turnover rate for the effective removal of CO in air than 3−5% Pt/TiO2 and 20% Cu/ZnO/Al2O3 catalysts. A small amount of Cu+ species is formed during the calcination treatment at 225 °C, which is the main active phase for the CO oxidation. However, it is proposed that some highly dispersed CuO can also form in the TiO2 lattice during the calcination treatment. Furthermore, a strong electron interaction between Cu2+ in highly dispersed CuO and Ti3+ on rutile TiO2 (Cu2++Ti3+→Cu++Ti4+) has been shown to occur. Overall, the reduction of Cu+ is a major factor that contributes to the reaction rate of the CO oxidation. photocatalytic activity of Cu/TiO2 for phenol oxidation.33 Well-dispersed copper particles are proposed to have partial positive charges and/or contain Cu+ species, because it is difficult to reduce isolated Cu2+ ions to Cu0.34 Interestingly, Boccuzzi et al. reported that the surface of Cu/TiO2 catalysts obtained by wet impregnation included both three-dimensional (3D) and two-dimensional (2D) copper particles.34,35 The surfaces of the 3D particles are highly reactive in CO oxidation, while the surfaces of the 2D particles are found to be the active sites in the CO−NO reaction. Recently, the effects of oxygenvacancy defects on TiO2-supported metal catalysts have also been reported in the literature, leading to high efficiencies in several water−gas shift and CO oxidation processes.36−38 In this work, TiO2 (Degussa Co. P25) was used as the support to prepare a series of Cu/TiO2 catalysts for the study of the CO oxidation reaction. The P25 TiO2 powder is composed of 25% rutile and 75% anatase. The anatase phase is usually attributed to a metastable state in the bulk, which tends to transform into the more stable rutile phase at temperatures above 500 °C.39 To date, the role of Ti3+ on rutile TiO2 in the formation of catalytic metal particles has attracted little attention. Herein, we demonstrate that the effects of Ti3+ on the TiO2 support can induce efficient catalytic activity for CO oxidation on Cu metal.

1. INTRODUCTION TiO2 is an important material that can be used in a variety of applications: photocatalytic reactions, gas-sensing systems, and heterogeneous catalytic reactions.1−9 The catalytic oxidation of carbon monoxide to carbon dioxide is an essential reaction for environmental protection due to its importance in emission control. Recently, this reaction has been studied intensively for its potential application in fuel-cell systems. Precious metals, such as Pt, Ru, and Au, have been used as oxidation catalysts because of their high activity and stability in exhaust-gas emissions processing. However, due to the high costs of these precious metals, some transition metals with high catalytic activity in CO oxidation have been evaluated as alternatives.10−14 For example, copper is a potential substitute for noble metals because of its high activity in CO oxidation and other pollutant-related reactions.15−21 Therefore, transition metals should be considered in CO oxidation applications related to exhaust-gas processing and CO removal. Some studies have shown that the reaction rate of CO oxidation depends strongly on the oxidation state and active sites of copper,22−25 but the active center of Cu for CO oxidation is controversial. In general, the coexistence of Cu+ and Cu0 is considered essential for catalytic activity. Huang et al. studied CO oxidation on CuO/TiO2, which was prepared by the deposition−precipitation method. They showed that an 8 wt % CuO/TiO2 system with near-monolayer dispersion had the highest activity among the CuO/TiO2 catalysts (Cu loading ranged from 2 to 12 wt %).26 Highly dispersed copper particles and Cu+ species on oxide supports have been suggested to be essential elements for several catalytic reactions, such as CO oxidation, CO−NO reduction, and steam reforming of methanol.27−32 In addition, highly dispersed copper in the form of Cu+ in a TiO2 matrix has been reported to facilitate the © 2012 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The Cu/TiO2 and Pt/TiO2 catalysts were prepared by impregnating TiO2 (Degussa P-25, 55 m2/g) with 20 mL of an aqueous solution of Cu(NO3)2·2.5H2O or H2PtCl6, respectively. The commercial CuO/ZnO/Al2O3 catalyst used in this Received: April 25, 2012 Revised: June 5, 2012 Published: June 8, 2012 9996

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N2O/N2 gas stream according to the reaction 2Cu(s) + N2O → Cu2O(s) + N2. N2O chemisorption was performed with a 10% N2O/N2 mixture flowing at 30 mL/min at 353 K to form a monolayer of Cu2O on the catalyst surface. The oxidized Cu surface formed during the N2O chemisorption was reduced using a H2-TPR process. The TPR area of Cu2O was quantified by sampling 1 mL of 10% H2/N2 to calculate the amount of N2O consumed. The Cu0 surface area was calculated assuming that the N2O/Cu molar stoichiometry was 0.5. The average surface density for Cu metal was 1.46 × 1019 atoms/m2. The copper contents of the Cu/TiO2 samples were measured by inductively coupled plasma mass spectrometry (ICP/MS), which was performed at the High Valued Instrument Center in the National Sun Yat-sen University, Taiwan. Pt surface areas of 3% and 5% Pt/TiO2 catalysts used in this paper were measured by CO chemisorption at room temperature in a vacuum system. 2.8. Catalytic Activity Measurements. The CO oxidation activity measurements were performed in a fixed-bed reactor at atmospheric pressure. The gas reactants, containing 4.5% CO and 2.23% O2, were passed through the catalyst bed. All products were analyzed by gas chromatography on a 12 ft Porapak-Q column using a thermal conductivity detector (TCD). The turnover frequency (TOF) on Cu and Pt catalysts was calculated by the formula: TOF = [conversion × CO flow rate (mL/s) × 6.02 × 1023 (molecules/mol)]/ [24400 (mL/mol) × metal sites)]. 2.9. Electron Spin Resonance (EPR) Spectroscopy. X-band (9.0 GHz) continuous-wave EPR experiments were performed using a Bruker Elexsys E580 spectrometer equipped with a helium cryostat. The samples were measured at 77 K under photoirradiation with a 1000 W mercury lamp.

study was manufactured by Süd-Chemie Catalysts, Inc. (catalyst #G66B) with a Cu/Zn/Al molar ratio of 30:60:10. All Cu catalysts were calcined in air and were reduced in H2 at 573 K for 5 h. The Pt/ TiO2 catalysts were calcined in air and were reduced in H2 at 673 K for 5 h. 2.2. H2-Temperature-Programmed Reduction (H2-TPR). H2TPR of the catalysts was performed at atmospheric pressure using a conventional flow system. The catalysts were placed in a tube reactor and heated at a rate of 10 K/min in a 10% H2/N2 mixed gas stream flowing at 30 mL/min. The TCD current was 80 mA, and the detector temperature was 373 K. A cold trap containing a gel formed by adding liquid nitrogen to isopropanol in a Thermos flask was used to prevent water from entering the TCD. 2.3. X-ray Photoelectron Spectroscopy (XPS). The XPS spectroscopy was used to study the chemical composition and oxidation state of the catalyst surface. The XPS data were obtained using a Thermo VG-Scientific Sigma Probe spectrometer at the Precision Instrument Center of the College of Engineering in the National Central University, Taiwan. The spectrometer is equipped with an Al−K X-ray source (1486.6 eV, 1 eV = 1.602 × 10−19 J) operating at 108 W and a hemispherical analyzer operating at a pass energy of 50 eV. The instrument typically operates with an analysis chamber pressure of approximately 1 × 10−9 mbar. Binding energies for the catalyst samples were referenced to the C 1s line (284.6 eV) of the carbon overlayer. 2.4. In Situ X-ray Absorption Spectra (XAS) Measurements. The XAS spectra were recorded at the BL17C1 beamline at the National Synchrotron Radiation Research Center (NSRRC), Taiwan, where the electron storage ring is operated at 1.5 GeV. A double Si(111) crystal monochromator was employed for energy selection, which has a ΔE/E resolution of less than 1 × 10−4 at the Cu K-edge (8979 eV). All XAS powder studies for the nanoparticles were conducted in a homemade cell constructed of stainless steel. Two holes were made in the cell, one on the top and the other on one side. After placing the solid samples inside, the holes were closed with a Kapton film cap to avoid exposing the sample to the atmosphere. All spectra were recorded at room temperature in transmission mode, and higher harmonics were eliminated by detuning the double Si(111) crystal monochromator. Three gas-filled ionization chambers were used in series to measure the intensities of the incident beam (I0), the beam transmitted by the sample (It), and the beam subsequently transmitted by the reference foil (Ir). The third ion chamber was used in conjunction with the reference sample, a Cu foil, for Cu K-edge measurements. Soft XAS measurements were performed at the BL20A1 station at the NSRRC, and the measurements were performed in the total electron yield mode for the Cu and Ti Ledge spectra; each measurement was performed in an ultrahigh vacuum (UHV) chamber with a base pressure of 1 × 10−10 Torr. 2.5. In Situ X-ray Diffraction (XRD) Measurements. In situ XRD measurements for all Cu/TiO2 samples were performed at the high-energy beamline 01C2 at the National Synchrotron Radiation Research Center (NSRRC), Hisnchu. The beamline was operated at an energy of 25 keV. The XRD spectra were recorded using the wavelength λ = 0.5166 Å. The samples were loaded into capillary cells and heated to 573 K under air flow and H2 gas while recording the XRD patterns. 2.6. Thermal Gravimetric Analysis (TGA) and Raman Spectroscopy. The TGA experiments were performed on a Mettler Toledo TGA/DSC 1 thermogravimetric analyzer using a heating rate of 10 K/min and under an air flow of 50 mL/min. Raman spectra were obtained with a UniRaman instrument from Protrustech Co. equipped with a YAG laser (532 nm). The laser power at the sample was 15 mW. Raman scattering was measured for 100−2000 cm−1 with a resolution of 2 cm−1. Each spectrum was obtained with an exposure time of 3 and 30 scans. 2.7. Measurements of the Copper and Platinum Surface Areas. The specific Cu0 surface areas and dispersions of the Cu catalysts were determined by N2O chemisorption and H2-TPR. The Cu catalyst was reduced at 573 K in H2 gas for 5 h, and then all of the Cu0 present on the catalyst surface was carefully oxidized in a 10%

3. RESULTS 3.1. H2-TPR. Figure 1 shows the H2-TPR profiles of the oxidized Cu/TiO2 catalysts with different Cu loadings that have

Figure 1. H2-TPR profiles of (a) 3.4% CuO/TiO2, (b) 5.2% CuO/ TiO2, (c) 7.3% CuO/TiO2, (d) 9.6% CuO/TiO2, and (e) reduced 7.3% Cu/TiO2 oxidized by a 30 mL/min N2O stream at 353 K for 180 s.

been calcined in the air at 573 K for 5 h. Broad TPR profiles of the oxidized 3.4−9.6 wt % Cu/TiO2 catalysts were observed at approximately 423−548 K. A low-temperature peak at 438 K was observed for all samples, and a broad reduction band centered at 478−491 K appeared with increasing Cu loading. The broad TPR profiles of oxidized Cu/TiO2 catalysts could be fitted to four principal peaks at approximately 438 K (α peak), 9997

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455 K (β peak), 484 K (γ peak), and 505 K (δ peak). The integrated area of four fitting peaks are listed in Table S1, wherein it can be observed that the reduction area of the β, γ, and δ peaks increased with increased Cu loading; however, the area of the α peak was stable for 3.4 to 9.6 wt % Cu/TiO2 catalysts. On the basis of our experience, CuO species are difficult to reduce below 473 K. Generally, the reduction of large CuO particles occurs between 473 and 523 K, while highly dispersed CuO particles reduce more readily.40 The reduction temperature of small particles of CuO is lower than that of bulk CuO.40 In the literature, the reduction temperature of a welldispersed (3−5 nm) CuO/SiO2 catalyst prepared by ionexchange method is below 523 K.41 Furthermore, in our previous studies, H2-TPR of 3 nm Cu nanoparticles revealed that the reduction peak of the CuO particles was ca. 513 K.42,43 The TPR profile of the reduced Cu/TiO2 catalyst oxidized by an N2O stream at 323 K (N2O + 2Cu → Cu2O + N2) exhibited a reduction peak for the Cu+ species at 423 K, as shown in Figure 1. The H2-TPR profiles of the CuO/TiO2 catalysts after calcination at 483 and 498 K are compared in Figure 2. Only

Figure 3. Raman spectra of 3.4−9.6% Cu on TiO2 for (a) the asimpregnated Cu2+/TiO2 and (b) Cu2+/TiO2 undergoing calcination at 573 K for 5 h.

dependence of Cu2+ ion-based CuO formation on TiO2 using a TGA technique in an air stream. Figure 4A,B display the TGA

Figure 2. H2-TPR profiles of 7.3% CuO/TiO2 at different calcination temperatures: (a) 483 K and (b) 498 K.

typical reduction of the CuO particles occurred when calcination was performed at 483 K; however, elevating the calcination temperature to 498 K caused low-temperature reduction to occur at 438 K. 3.2. Raman Spectroscopy. Raman spectroscopy was used to confirm the presence of the anatase and rutile phases of TiO2 in the as-impregnated Cu2+/TiO2 and calcined CuO/TiO2 samples by probing the excitation line at 532 nm, as shown in Figure 3. The anatase structure was characterized by bands at 397, 513, and 636 cm−1, corresponding to the B1g, A1g-B1g, and Eg modes, respectively. The peak positioned at 443 cm−1 was assigned to the Eg mode of the rutile phase for all asimpregnated Cu/TiO2 samples.44−49 However, after calcination at 573 K for 5 h, the spectra only retained the characteristic peaks of the anatase phase, and the peak corresponding to the rutile structure at 443 cm−1 vanished because CuO particles were generated. The disappearance of the rutile structure was investigated as a function of the calcination temperature using a thermal gravimetric analyzer technique (TGA) and Raman spectroscopy with the as-impregnated 7.3% Cu2+/TiO2 as the sample. A temperature-programmed oxidation of the as-impregnated 7.3% Cu2+/TiO2 sample was used to evaluate the temperature

Figure 4. (A) TGA curve of the as-impregnated 7.3% Cu2+/TiO2 with a heating rate of 10 K/min and under an air flow of 50 mL/min. (B) DTA curve of the (A) and (C) Raman spectra for 7.3% Cu2+/TiO2 calcined at (a) 483 K for 5 h and (b) 498 K for 5 h.

and DTA curves of the as-impregnated Cu2+/TiO2 sample. The weight loss for the sample was ascribed to the transformation of Cu2+ ions to CuO particles in the presence of air. Interestingly, the asymmetrical DTA curves fit well with the two peaks at 483 and 498 K. Raman spectroscopy was used to further investigate the structural changes of the TiO2 phase for the as-impregnated Cu2+/TiO2 samples undergoing calcinations at 483 and 498 K, as shown in Figure 4C. The peak of the rutile structure at 443 cm−1 remained when using a calcination temperature of 483 K but disappeared when the calcination temperature was increased to 498 K. 9998

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3.3. XRD Spectroscopy of Cu/TiO2 Catalysts. The XRD spectra of all oxidized CuO/TiO2 catalysts undergoing calcination at 573 K and the pure TiO2 support are displayed in Figure S1. Several diffraction patterns associated with CuO are present at 35.6° for the (111) plane, 58.6° for the (202) plane, 65.73° for the (022) plane, 66.48° for the (310) plane, and 68.04° for the (220) plane. The weak peak at 61.6° suggests that a crystalline Cu2O phase was formed during the calcination pretreatment in all CuO/TiO2 samples. Figure S2 compares the XRD spectra of the 7.3% CuO/TiO2 catalyst calcined at 483, 498, and 523 K. The CuO structure was first generated at 483 K; however, the formation of the diffraction peak at 61.6° was observed above 498 K. 3.4. XPS Spectroscopy. XPS spectroscopy was used to obtain information related to the oxidation state and the chemical environmental of the Cu surface. Figure 5A compares

loading were pretreated using the same methods that were used for the Cu/TiO2 samples, and a comparison of both support types was performed. The comparison showed that the typical CuO species displayed similar traits as the as-impregnated Cu2+/SiO2 sample after calcination at 573 K. Moreover, these findings are evidenced by the Cu 2P3/2 signal position at 934.0 eV and apparent satellite signals (Figure 5B). It is suggested that the TiO2 support may induce the oxidation state of the Cu oxides to be less than that of Cu2+ after calcination. Furthermore, the intensity ratios of Cu/Ti obtained from the XPS spectra for the calcined CuO/TiO2 catalysts could provide important information regarding the dispersion and crystalline size of the Cu oxides. Therefore, two apparently different linear dependences were plotted in the range 1.3−11.9% Cu loading, as shown in Figure 6. The results

Figure 6. XPS intensity ratios (ICu/ITi) as a function of the total CuO loading in various CuO/TiO2 catalysts.

Figure 5. Cu 2p3/2 XPS spectra of (A) 7.3% Cu on TiO2 and (B) 7% Cu on SiO2 using different pretreatments: (a) as-impregnated samples, (b) samples undergoing calcination at 573 K for 5 h, and (c) samples undergoing reduction at 573 K for 5 h after (b).

account for the differences between the dispersed and crystalline states of CuO on TiO2.54 However, it has been reported that the slope of the intensity ratio versus the CuO loading for highly dispersed CuO should be larger than the slope for bulk-type CuO. 3.5. X-ray Absorption near-Edge Structure (XANES). In XAS, the energy region near the absorption edge (0−50 eV) is referred to as XANES and generally provides a fingerprint for the oxidation state and site symmetry of the element from which the absorption spectrum was measured. Figure 7A shows the Cu K-edge XANES spectra of the calcined CuO/TiO2 samples with different Cu loadings. The spectra of the 10% CuO/SiO2 and CuO powder were used as the standards for comparison with these CuO/TiO2 samples. First-derivative curves are more sensitive than the integral curves for identifying the charge of the Cu particles. Figure 7B shows the firstderivative function of the rapidly rising edge step of the absorbance versus the energy in Figure 7A. In general, the maxima of the first-derivative curves yielded reference values of 8979 eV for metallic Cu0, 8982 eV for Cu2O, and 8984 eV for CuO.55 The first-derivative peaks of all calcined CuO/TiO2 samples were slightly shifted to lower energies by ca. 0.6 eV. These results indicate that Cu+/Cu2+ may be present in the CuO/TiO2 samples, even when calcined at 573 K for 5 h.

the XPS spectra of the Cu 2P3/2 signals of the 7.3% Cu/TiO2 sample after different pretreatments. The as-impregnated Cu2+/ TiO2 sample showed the principal XPS peak that is typical of the Cu2+ species at 933.2 eV, while also exhibiting satellite signals at 940.8 and 943.5 eV. These satellite peaks were attributed to shakeup transitions by a ligand-to-metal 3d charge transfer. However, the Cu 2P3/2 signals shifted to a lower binding energy of 932.8 eV with calcination pretreatment, and the peak of the reduced sample was present at 932.7 eV. The binding energy for Cu2+ in CuO has been reported to be 933.8 eV.50−52 Additionally, the peak at approximately 933.0 eV has been assigned to metallic Cu0 and/or Cu+ because the binding energies of Cu0 and Cu+ are not distinguishable based on the Cu 2P3/2 peak.50−52 The main Cu 2P3/2 peak of CuO was usually higher than that of Cu2O and Cu0 by approximately 1.3 eV.53 Furthermore, the major difference between CuO and Cu2O was the presence of satellite signals on the high-bindingenergy side of the Cu core line for the Cu2+ species. The characteristic shakeup peak was absent from the Cu+ and Cu0 species because these Cu species have completely filled 3d orbitals, and thus, the charge transfer process cannot occur. To understand these findings, SiO2 samples with a 7% Cu2+ ion 9999

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4sp1 transition at ca. 933.7 eV, belonging to the characteristic peak of the Cu2O species, was observed for all calcined CuO/ TiO2 samples. The concentration of the Cu2O species in CuO/ TiO2 remained constant, but the relative intensity of the peak at 933.1 eV decreased as the Cu content increased. The K-edges of the TiO2 spectra for P25 TiO2 and the calcined CuO/TiO2 catalysts are compared in Figure S3, and no significant change in intensity or energy shift could be observed. 3.6. Electron Paramagnetic Resonance (EPR) Spectroscopy. Figure 9 compares the EPR spectra of TiO2, the as-

Figure 7. (A) Cu K-edge XANES spectra of the CuO/TiO2 catalysts calcined at 573 K for 5 h and of the CuO standard. (B) First-derivative spectra of (A).

The electronic structural information of CuO on TiO2 was obtained by soft-XAS measurements, which can provide an increased probing depth relative to XPS. Figure 8 shows the Cu Figure 9. EPR spectra of TiO2 and 3.4% CuO/TiO2.

impregnated Cu2+/TiO2, and the calcined CuO/TiO2. The observed g-value at 1.975 was assigned to the paramagnetic Ti3+ centers in the rutile phase; however, the g-value of the Ti3+ centers in the anatase phase positioned at 1.991 was not observed in this experiment.4 The EPR signal at g = 2.02 was ascribed to atmospheric oxygen that adsorbed on Ti3+ sites to form O2−.57 The signal centered at g = 2.12, stemming from Cu2+ ions, was attributed to the dipolar interaction in Cu2+ aggregates.58 However, the characteristic peak of the Ti3+ centers in the rutile phase remained in the spectrum. With the calcined treatment at 573 K, the signal associated with the Ti3+ species completely disappeared. Overall, these results indicate that a strong interaction between Cu2+ and Ti3+ may occur during CuO particle aggregation. 3.7. Cu and Ti K-edge Extended X-ray Absorption Fine Structures (EXAFS). The FT k3-weighted EXAFS results at the Cu and Ti K-edge with phase correlation for the calcined CuO/TiO2 samples with different Cu loadings are shown in Figure 10. The k3x(k) spectra were obtained by comparing the FEFF theoretical fit with the back-transformed experimental EXAFS data, as shown in Figures S4 and S5. Here, the two-shell theoretical fit (scatter line) matched closely with the backtransformed experimental data (solid line). The structural parameters of Cu and Ti for the calcined CuO/TiO2 samples that were extracted from the best-fit EXAFS data are listed in Table 1. In Table 1, the coordination number of the NCu−Cu and Cu−Cu distances did not change with increased Cu loading, and the fitting range of the Cu metal for all CuO/TiO2 samples was 1−3.2 Å. The coordination number of the NCu−O decreased slightly with Cu content, but the Cu−O bond distance remained constant at 1.95 Å for all CuO/TiO2 samples. The results for CuO/TiO2 were closely

Figure 8. Cu K-edge XANES spectra of the Cu2O, CuO, and CuO/ TiO2 catalysts calcined at 573 K for 5 h.

L3-edge XANES spectra of the CuO/TiO2 samples, Cu2O and CuO. It is observed that two peak maxima occur at ca. 930.4 and 933.7 eV. However, the band at 933.7 eV became weaker in relative intensity with increasing Cu loading. The ground state of CuO mainly consists of a p6d9 state. Therefore, the band centered at 931.0 eV can be assigned to the 2P3/2→3d9 transition.56 For the Cu2O standard, the p6d9sp1 configuration contributed strongly to the L3 spectra of the Cu+ species.56 However, compared to the CuO standard, the peak at 933.1 eV for the Cu L3-edge of Cu2O was suggested to be the transition energy for an electron to the 4sp state. In Figure 8, the 2P3/2→ 10000

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TiO6 octahedron. Therefore, the calcined CuO particles here had the ability to lengthen the bond distance of Ti−O in TiO2. 3.8. Activity Tests for CO Oxidation. Figure 11 compares the temperature-dependent conversion for CO oxidation on the

Figure 10. Fourier transforms of k3-weighted EXAFS functions for the CuO/TiO2 catalysts for the (A) Cu K-edge and (B) Ti K-edge.

Table 1. Structural Parameters of Calcined CuO/TiO2 and P25 TiO2 Samples samples

shell

Na

R (Å)b

rc( × 10−2)

3.4% CuO/TiO2

Cu−O Cu−Cu Ti−Ti Ti−O (1) Ti−O (2) Cu−O Cu−Cu Ti−Ti Ti−O (1) Ti−O (2) Cu−O Cu−Cu Ti−Ti Ti−O (1) Ti−O (2) Cu−O Cu−Cu Ti−Ti Ti−O (1) Ti−O (2) Ti−Ti Ti−O (1) Ti−O (2)

4.3 2.1 3.5 3.9 1.5 3.9 2.1 3.5 3.9 1.5 3.6 2.1 3.7 3.9 1.7 3.5 2.2 3.7 3.8 1.8 3.7 3.6 1.8

1.95 2.69 3.02 2.00 2.11 1.95 2.67 3.03 2.00 2.11 1.95 2.67 3.03 1.99 2.10 1.95 2.64 3.04 1.98 2.09 3.03 1.94 2.05

3.9 2.4 5.1 4.3 5.1 1.8 1.4 4.0 2.2 4.3 3.2 2.2 3.3 1.9 2.7 1.1 2.0 1.5 1.9 1.5 4.4 5.3 4.1

5.2% CuO/TiO2

7.3% CuO/TiO2

9.6% CuO/TiO2

P25 TiO2

a

Figure 11. Temperature dependence of the conversion of CO oxidation for the oxidized, partially reduced, and fully reduced Cu/ TiO2 catalysts. All reactions were performed in gas reactants, containing 5% CO/He at 44.7 mL/min and air at 5.3 mL/min, at a total flow rate of 50 mL/min. The amount of catalyst used in all measurements was 50 mg.

oxidized, partially reduced, and fully reduced Cu/TiO2 catalysts. The partially reduced Cu/TiO2 catalyst was created by performing the H2-TPR treatment from 298 to 443 K. For the fully reduced and partially reduced Cu/TiO2 catalysts, a similar conversion for CO oxidation was observed. However, the oxidized Cu/TiO2 catalysts achieved a smaller CO conversion at low temperatures. Additionally, the catalytic activity could be significantly enhanced as the reaction temperatures increased to 423 K, yielding the undifferentiated CO conversion in comparison with the partially and fully reduced Cu/TiO2 catalysts. After comparing the results of H2TPR (Figure 1), it was suggested that the conversion of CO oxidation on Cu/TiO2 catalysts might strongly associate with the reduction of α-state. The XRD spectra of all reduced Cu/TiO2 samples are shown in Figure S6. The reduced Cu/TiO2 catalysts showed obvious diffraction patterns at 2θ = 43.4°, 50.2°, and 74.1° for the (111), (200), and (220) planes, respectively. The Cu loadings, Cu surface areas, and particle sizes examined in this work are given in Table 2.

Coordination number. bBond distance. cResidual factor.

related to the typical CuO structure. However, the variation of NCu−O with Cu loading could be associated with the change in size of the CuO particle. Table 1 also shows the bond distances and coordination number of the three shells (Ti−O and Ti− Ti) in the range of 1.4−3.5 Å. The P25 TiO2 was used as the standard to which all CuO/TiO2 samples were compared. Overall, the bond length and coordination number of Ti−Ti in TiO2 were not influenced by the CuO formed on TiO2. However, the coordination environments of Ti−O had bond distances of 1.94 Å and 2.05 Å, yielding coordination numbers of ∼4.0 and ∼2.0, respectively. This result implied that a 6-foldcoordinated Ti4+ ion in a TiO2 specimen resulted in a distorted

Table 2. Comparison of the Cu/TiO2 Catalysts after Calcination in Air and Reduction in H2 at 573 K for 5 h

10001

Cu content (wt %)

particle size (nm)

Cu surface area (m2/g)

3.4 5.2 7.3 9.6

15.9 17.9 19.8 22.3

1.5 2.1 2.6 3.1

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Figure 12. Temperature dependence of TOF and Arrhenius plots of CO oxidation on fully reduced Cu/TiO2 catalysts. All reactions were performed in gas reactants, containing 5% CO/He at 89.4 mL/min and air at 10.6 mL/min, at a total flow rate of 100 mL/min. The amount of catalyst used in all measurements was 2 mg.

Figure 13. Temperature dependence of the activity of CO oxidation for the reduced 3.4% Cu/TiO2, 20% Cu/ZnO/Al2O3, and a series of TiO2supported Pt catalysts. All reactions were performed in gas reactants, containing 5% CO/He at 89.4 mL/min and air at 10.6 mL/min, at a total flow rate of 100 mL/min. The amount of catalyst used in all measurements was 2 mg.

than the Cu/ZnO/Al2O3 or Pt metal catalysts. The Cu/ZnO/ Al2O3 catalyst gave the highest Cu surface area (23 m2/g), but exhibited poor reaction activity for CO oxidation.

The kinetic parameters of CO oxidation on all Cu/TiO2 catalysts could be further obtained, based on the total Cu surface area. All CO oxidation reactions were carried out below 10% conversion, in order to obtain confidential data for kinetic analysis. Figure 12 presents the turnover frequency (TOF) of 3.4−9.6% Cu/TiO2 catalysts for CO oxidation as a function of temperature in the range of 373−473 K. It was observed that the turnover rates of CO oxidation decreased with Cu loading increasing. The Arrhenius plots for 3.4−9.6% Cu/TiO2 give the activation energies in the range of 24.9−27.6 kJ/mol at 398− 473 K. Figure 13 compares the TOF of CO oxidation as a function of temperature for the reduced 3.4% Cu/TiO2, 20% Cu/ZnO/ Al2O3, and a series of TiO2-supported Pt catalysts. Notably, the Cu/TiO2 catalysts had a significantly higher level of specific rate

4. DISCUSSION As shown in Figure 5, the XPS spectra of the Cu/TiO2 catalyst reveal that the binding energy of the Cu 2P3/2 signals of the asimpregnated Cu/TiO2 sample at 933.2 eV for the Cu2+ species shifted to an energy of 932.8 eV. Additionally, the disappearance of the satellite shakeup peak was observed after calcination at 573 K for 5 h, which was not observed for the Cu/SiO2 sample. Thus, it is suggested that the TiO2 support may reduce the positive charge of Cu2+ through a process occurring during the calcination treatment. The Cu K-edge XANES spectra also indicated that the charge of CuO on TiO2 10002

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Scheme 1. Proposed Mechanism of Cu2O Formation on TiO2 as the Active Phase for CO Oxidation

CuO species with CuO loading would be faster than that of the bulk CuO particles. In Table 1, the ratio of the Cu−O to Cu− Cu coordination numbers (CN) decreased as the Cu loading increased; however, the bond distances for all samples were almost identical. These results were quite similar to the bulk CuO structure. The distinct difference in the ratio of the Cu−O to Cu−Cu coordination numbers was associated with the change in size of the CuO particle. Smaller nanosized particles offer higher surface areas and usually have excess oxygen on their surface. The EXAFS analysis of the Ti−O coordination in Table 1 shows that the distance of the Ti−O bond can increase slightly within the range of 3.4−9.6% Cu loading and that the coordination numbers for both Ti−O bonds remained constant. Thus, it was reasonable to suggest that the Cu2+ ions could enter the TiO2 lattice and that the highly dispersed CuO was generated within the TiO2 lattice during the calcination treatment, leading to the longer Ti−O bond. However, the smaller lattice spacings may also restrict the Cu particle size, resulting in smaller particles during Cu atom aggregation. When analyzing the H2-TPR profiles in Figure 1, it was apparent that the peak for the 3.4% CuO/TiO2 sample mainly occurred at 438 K due to the reduction of the Cu+ species. As CuO loading increased, most of the Cu species in the oxide samples were present in the +2 oxidation state and a small amount was in the +1 oxidation state. Notably, the 3.4% CuO/ TiO2 sample mainly contained the Cu+ species and exhibited a longer bond distance for the Ti−O coordination than P25 TiO2. However, these results for low Cu loading indicate that the Cu+ species should have been generated from Cu2+ in the highly dispersed CuO interacting with Ti3+ in rutile TiO2. The presence of the rutile phase has been reported to imply the existence of defect sites.44 Therefore, CuO may strongly intact with the rutile structure. However, the formation of Cu2O in CuO/TiO2 samples has been ascribed to the presence of oxygen-defect vacancies (□s) in the TiO2 structure.36,38 The oxygen-defect vacancies are proposed to interact strongly with dispersed crystalline metals and form several new sites at the metal−support interface (such as M-□s-Ti3+).36,38 The effect of the oxygen-defect vacancies has been the subject of much discussion, and CuO has been proposed to interact with TiO2, resulting in charge transfer from the support to metallic particles. Our studies indicate that the CuO particles can form before Cu2O formation, especially with increasing temperature. Furthermore, the oxygen-defect vacancies (□s-Ti3+) may cause partial reduction of CuO according to 2CuO + □s → Cu2O + Os, which when combined with high-temperature calcination may enhance the formation of Cu2O from CuO. Figure 11 clearly shows that the fully and partially reduced Cu/TiO2 catalysts exhibit small differences in the conversion of CO oxidation. On the basis of the H2-TPR profiles, the efficiency of the Cu/TiO2 catalysts for CO oxidation strongly

was lower than that of CuO powder. Furthermore, the Cu L3edge XANES spectra clearly provided evidence that the characteristic 2P3/2→4sp1 transition peak of Cu2O could be found in all CuO/TiO2 samples. However, the XRD spectra in Figures S1 and S2 have also revealed that the Cu2O structure may be generated in the CuO/TiO2 catalysts after the calcination pretreatment. Specifically, the calcined temperature-dependent XRD spectra suggest that the Cu2+ species were first aggregated into CuO particles at ca. 483 K, and then the conversion to a Cu2O structure occurred when the temperature was increased to 498 K. On the basis of these results, it is proposed that some Cu+ species could be present in the CuO/TiO2 samples, even after calcination at 573 K for 5 h. Furthermore, the TiO2 support undoubtedly depends closely on the formation of Cu+ species in the CuO/TiO2 catalyst. Similar results have been reported in the literature regarding the observation of a lower oxidation state for Cu at higher calcination temperatures (723 K), but no further discussion of the mechanism of Cu2O formation is available.59 Colόn et al. reported that Cu-doped TiO2 that was calcined at 773 K could form Cu2O, indicating that the Cu2+ species easily accepts photogenerated electrons from the TiO2 conduction band, leading to Cu+ formation.53 In the present study, the Raman spectra of the Cu2+ species on TiO2 (Figure 4) proved that the weak peak belonging to the rutile phase in TiO2 could completely disappear after calcination treatment at 498 K. The XRD spectra also clearly showed that the Cu2+ ions on TiO2 supports could aggregate to form a CuO structure at ca. 483 K and that Cu2O was generated at 498 K. Additionally, the EPR spectra also indicated that the Ti3+ centers in the rutile phase could disappear in conjunction with CuO formation. On the basis of these results, the calcination treatment probably could induce the strong electron interaction between the Cu2+ and Ti3+ ions, leading to the following charge transfer reaction: Cu2+ + Ti3+ → Cu+ + Ti4+. However, the TPR results in Figure 2 clearly show that calcination at 498 K could enhance the formation of a lowtemperature reduction peak at 438 K. Therefore, it is suggested that reduction of the Cu+ species may have occurred during the H2-TPR of the CuO/TiO2 sample after calcination at 573 K for 5 h, as evidenced by a low-temperature peak at 438 K. Furthermore, the TPR profiles of the oxidized Cu/TiO2 catalysts with a low-temperature peak at 438 K may be associated with the reduction of the Cu+ species in the oxidized Cu/TiO2 catalysts. These results undoubtedly suggest that the formation of the Cu2O structure is closely related to the disappearance of the Ti3+ ions in the rutile phase of TiO2. The metal-to-support intensity ratios obtained by XPS spectra (Figure 6) depicted two linear dependencies on CuO loading for the highly dispersed and crystalline CuO samples with an interception point at ca. 5.5%. It is reasonable to expect that the increase in the intensity ratio for the highly dispersed 10003

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Nanowires Analyzed by X-ray Absorption Spectroscopy. Chem. Mater. 2011, 23, 3636−3644. (4) Dimitrijevic, N. M.; Vijayan, B. K.; Poluektov, O. G.; Rajh, T.; Gray, K. A.; He, H.; Zapol, P. Role of Water and Carbonates in Photocatalytic Transformation of CO2 to CH4 on Titania. J. Am. Chem. Soc. 2011, 133, 3964−3971. (5) Jiang, B.; Tian, C.; Pan, Q.; Jiang, Z.; Wang, J. Q.; Yan, W.; Fu, H. Enhanced Photocatalytic Activity and Electron Transfer Mechanisms of Graphene/TiO2 with Exposed {001} Facets. J. Phys. Chem. C 2011, 115, 23718−23725. (6) Braun, A.; Akurati, K. K.; Fortunato, G.; Reifler, F. A.; Ritter, A.; Harvey, A. S.; Vital, A.; Graule, T. Nitrogen Doping of TiO2 Photocatalyst Forms a Second eg State in the Oxygen 1s NEXAFS Pre-edge. J. Phys. Chem. C 2010, 114, 516−519. (7) Liou, Y. J.; Hsiao, P. T.; Chen, L. C.; Chu, Y. Y.; Teng, H. Structure and Electron-Conducting Ability of TiO2 Films from Electrophoretic Deposition and Paste-Coating for Dye-Sensitized Solar Cells. J. Phys. Chem. C 2011, 115, 25580−25589. (8) Hsiao, P. T.; Lu, M. D.; Tung, Y. L.; Teng, H. Influence of Hydrothermal Pressure during Crystallization on the Structure and Electron-Conveying Ability of TiO2 Colloids for Dye-Sensitized Solar Cells. J. Phys. Chem. C 2010, 114, 15625−15632. (9) Liu, X.; Wang, A.; Li, L.; Zhang, T.; Mou, C. Y.; Lee, J. F. Structural Changes of Au−Cu Bimetallic Catalysts in CO Oxidation: In situ XRD, EPR, XANES, and FT-IR Characterizations. J. Catal. 2011, 278, 288−296. (10) Marba’n, G.; Fuertes, A. B. Highly Active and Selective CuOx/ CeO2 Catalyst Prepared by a Single-step Citrate Method for Preferential Oxidation of Carbon Monoxide. Appl. Catal., B 2005, 57, 43−53. (11) El-Shobaky, G. A.; Ghozza, A. M. Effect of ZnO Doping on Surface and Catalytic Properties of NiO and Co3O4 Solids. Mater. Lett. 2004, 58, 699−705. (12) Zheng, X.; Wang, S.; Wang, S.; Zhang, S.; Huang, W.; Wu, S. Copper Oxide Catalysts Supported on Ceria for Low-Temperature CO Oxidation. Catal. Commun. 2004, 5, 729−732. (13) Federico, G.; Marta, M. N.; Antonella, G. Low Temperature Oxidation of Carbon Monoxide: the Influence of Water and Oxygen on the Reactivity of a Co3O4 Powder Surface. Appl. Catal., B 2004, 48, 267−274. (14) Tascon, J. M. D.; Tejuca, L. G.; Rochester, C. H. Surface Interactions of NO and CO with LaMO3 Oxides. J. Catal. 1985, 95, 558−566. (15) El-Shobaky, H. G.; Fahmy, Y. M. Nickel Cuprate Supported on Cordierite as an Active Catalyst for CO oxidation by O2. Appl. Catal., B 2006, 63, 168−177. (16) Park, P. W.; Ledford, J. S. The Influence of Surface Structure on the Catalytic Activity of Alumina Supported Copper Oxide Catalysts. Oxidation of Carbon Monoxide and Methane. Appl. Catal., B 1998, 15, 221−231. (17) Martínez-Arias, A.; Fernández-García, M.; Soria, J.; Conesa, J. C. Spectroscopic Study of a Cu/CeO2 Catalyst Subjected to Redox Treatments in Carbon Monoxide and Oxygen. J. Catal. 1999, 182, 367−377. (18) Skårman, B.; Grandjean, D.; Benfield, R. E.; Hinz, A.; Andersson, A.; Wallenberg, L. R. Carbon Monoxide Oxidation on Nanostructured CuOx/CeO2 Composite Particles Characterized by HREM, XPS, XAS, and High-Energy Diffraction. J. Catal. 2002, 211, 119−133. (19) Taylor, S. H.; Hutchings, G. J.; Mirzaei, A. A. The Preparation and Activity of Copper Zinc Oxide Catalysts for Ambient Temperature Carbon Monoxide Oxidation. Catal. Today 2003, 84, 113−119. (20) Luo, M. F.; Zhong, Y. J.; Yuan, X. X.; Zheng, X. M. TPR and TPD Studies of CuO/CeO2 Catalysts for Low Temperature CO Oxidation. Appl. Catal., A 1997, 162, 121−131. (21) Manzoli, M.; Monte, R. D.; Boccuzzi, F.; Coluccia, S.; Kašpar, J. CO Oxidation over CuOx-CeO2-ZrO2 Catalysts: Transient Behaviour and Role of Copper Clusters in Contact with Ceria. Appl. Catal., B 2005, 61, 192−205.

depends on the reduction of the Cu oxides at 438 K. Thus, it was concluded that the reduction of Cu+ at 438 K may be the important effect the CO oxidation reaction. Scheme 1 illustrates the steps of Cu2O formation as the active phase for CO oxidation.

5. CONCLUSIONS In this paper, we discussed the higher turnover rate for CO oxidation of a series of Cu/TiO2 catalysts and compare the prepared catalysts with 3−5% Pt/TiO2 and 20% Cu/ZnO/ Al2O3 catalysts. It has been shown that the highly efficient CO oxidation reaction on the Cu/TiO2 catalysts may strongly depend on the formation of Cu+ species during the aggregation of Cu2+ species into CuO particles. We have used XPS and XANES spectroscopy to prove the presence of the Cu+ species in the CuO/TiO2 catalysts, and the highly dispersed CuO particles may be nucleated in the TiO2 lattice during the CuO formation process. Furthermore, the calcination treatment likely induces a strong electron interaction between Cu2+ in the highly dispersed CuO particles and Ti3+ ions in rutile TiO2, leading to the charge transferred reaction: Cu2+ + Ti3+ → Cu+ + Ti4+. The temperature-dependent XRD spectra provided evidence that the Cu2+ species first transformed into CuO particles at ca. 483 K, after which the Cu2O structure was generated by increasing the temperature of the calcination process to 498 K. The small amount of Cu+ species present in the catalyst can be reduced above 438 K, which is a major factor that contributes to the reaction rate of CO oxidation.

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ASSOCIATED CONTENT

S Supporting Information *

Addional spectra as described in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +886-32118800x5685. Fax: +886-32118700. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Science Council of the Republic of China (NSC100-2113-M-182-001-MY3) and Chang-Gung Memorial Hospital (CMRPD190043) are gratefully acknowledged. We are also grateful for the in situ X-ray absorption spectroscopy and X-ray diffraction (XRD) measurements performed by the National Synchrotron Radiation Research Center (NSRRC).

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