Catalysis Communications

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1 Catalysis Communications 9 ( Contents lists available at ScienceDirect Catalysis Communications journal homepage: Effect of highly dispersed active sites of Cu/TiO 2 catalyst on CO oxidation Ching-Shiun Chen a, *, Jiann-Hwa You b, Jarrn-Horng Lin c, Yu-Yuan Chen b a Center for General Education, Chang Gung University, 259 Wen-Hwa 1st Road, Kwei-Shan Tao-Yuan 333, Taiwan, ROC b Department of Chemical and Materials Engineering, Chang Gung University, 259 Wen-Hwa 1st Road, Kwei-Shan Tao-Yuan 333, Taiwan, ROC c Department of Material Science, National University of Tainan, 33, Sec. 2, Shu-Lin Street, Tainan 7, Taiwan, ROC article info abstract Article history: Received 27 March 28 Received in revised form 28 May 28 Accepted 1 June 28 Available online 14 June 28 Keywords: Carbon monoxide oxidation Titanium dioxide Copper catalysts Temperature-programmed reduction The oxidation of CO over Cu/TiO 2 catalysts containing different Cu loadings was investigated. The activities of the Cu/TiO 2 catalysts were observed to be nearly constant for Cu contents between 3.7 wt% and 1.3 wt%. The characterization of the Cu/TiO 2 catalysts by H 2 temperature-programmed reduction (TPR suggested the presence of different CuO species with distinct catalytic activities, that is, isolated Cu atoms, highly dispersed CuO, and bulk CuO. The concentration of reducible Cu 2+ species at low temperatures (437 K was observed to be independent of Cu content and Cu atom aggregation due to a 773 K treatment. These reduced Cu species are potentially responsible for the activities of Cu/TiO 2 catalysts. Ó 28 Elsevier B.V. All rights reserved. 1. Introduction The catalytic oxidation of carbon monoxide to carbon dioxide is an essential reaction for environmental protection due to its importance in emission control. Precious metals, such as Pt, Ru, and Au, have been used as oxidation catalysts because of their high levels of activity and stability in controlling exhaust gas emissions. Unfortunately, these metals are prohibitively expensive, and therefore other transition metals with high catalytic CO oxidation activities have been investigated as alternatives. For example, copper is a potential substitute for noble metals due to its high activity towards CO oxidation and in other pollutant-destroying reactions [1 3]. The strong metal support interaction effect has been extensively studied in TiO 2 -supported group VIII metals; however, it is still controversial whether Cu/TiO 2 exhibits behavior similar to that of group VIII metals on TiO 2. Consequently, Cu/TiO 2 catalysts have attracted less attention. Hydrogen-treated TiO 2 facilitates the diffusion of oxygen from the bulk material, wherein subsequent oxygen surface depletion produces an n-type semiconducting species [4]. In general, Cu metal particles have a higher work function than TiO 2, which results in an electron transfer from the support to Cu. This charge transfer is expected to impart a negative charge on the small Cu particles, which may be strongly localized at the interface between the Cu and TiO 2 [4]. Boccuzzi et al. reported that the surface of Cu/TiO 2 obtained by wet impregnation included three-dimensional (3D and two-dimensional (2D Cu * Corresponding author. Tel.: x5685; fax: address: cschen@mail.cgu.edu.tw (C.-S. Chen. particles [5,6]. The surfaces of the 3D particles were highly reactive towards CO oxidation. The active sites in the CO NO reaction were observed to be on the surfaces of the 2D particles. Huang et al. studied CO oxidation on CuO/TiO 2 prepared by the deposition-precipitation method, and demonstrated that 8 wt% CuO/TiO 2 with a near-monolayer dispersion had the highest level of activity among the CuO/TiO 2 catalysts investigated with Cu loadings of 2 12 wt% [7]. Some reports from the literature have investigated CO adsorption to identify the active sites of Cu/TiO 2 catalysts [4 6,8,9]. Nevertheless, it should be considered that isolated Cu atoms and/or very small 2D Cu particles most likely do not retain copper s normal metallic properties. The highly dispersed and smaller CuO particles might reasonably be proposed to have a strong interaction with the TiO 2 support. Isolated Cu atoms might generate a partially positive charge, even if the Cu is reduced [4 6]. The well-dispersed copper particles have been suggested to have a partially positive charge and/or to be Cu + species, as it is difficult to reduce isolated Cu 2+ ions to Cu [5]. The effects of small copper particles on TiO 2 support for CO oxidation should not be neglected. Degussa P-25 TiO 2 (5 m 2 /g is typically the support used in the preparation of the catalyst. In this work, TiO 2 (Degussa Co., P-25 was used as the support to prepare a series of Cu/TiO 2 catalysts to study the CO oxidation reaction. We present the data from temperature-programmed reduction (TPR, X-ray diffraction (XRD, and Fourier transform infrared spectroscopy (FTIR studies of Cu/TiO 2 catalysts with different Cu loadings. These characterization techniques were used to discriminate the active phases of the Cu/TiO 2 catalysts. The relationship between the effects of small Cu particles on TiO 2 and the level of activity for CO oxidation is discussed /$ - see front matter Ó 28 Elsevier B.V. All rights reserved. doi:1.116/j.catcom

2 2382 C.-S. Chen et al. / Catalysis Communications 9 ( Experimental 2.1. Catalyst preparation The Cu/TiO 2 catalysts were prepared by impregnating TiO 2 (Degussa P-25 5 m 2 /g with 2 ml of an aqueous solution of Cu(NO H 2 O. The hydrogen (Sun-Fu Co., 99.99% pure and helium gases (Sun-Fu Co., % pure used in the reactions were passed through a deoxo purifier to remove residual impurities. Air (Sun-Fu Co., 99.99% pure was used as supplied. All Cu/TiO 2 catalysts were calcined in air and reduced in H 2 at 573 K for 5 h Catalytic activity measurements CO oxidation activity measurements were carried out in a fixedbed reactor at atmospheric pressure. The gas reactants consisted of 4.5% CO (5% CO/He at ml/min and 2.23% O 2 (air at 5.32 ml/ min. The reaction was performed under CO/O 2 (2:1, v/v at a total flow-rate of 5 ml/min. The amount of catalyst used in all reactions was 5 mg. All products were analyzed by gas chromatography in a 12 ft Porapak-Q column using a thermal conductivity detector (TCD Temperature-programmed reduction H 2 -TPR of catalysts was performed at atmospheric pressure in a conventional flow system. Catalysts were placed in a tube reactor and heated in a 1% H 2 /N 2 mixed gas stream flowing at 3 ml/ min at a heating rate of 1 K/min. The TCD current was 8 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 Measurement of the copper particle s surface area and size The specific Cu surface area and Cu catalyst dispersion were determined by N 2 O chemisorption and TPR. All Cu on catalysts were carefully oxidized in an N 2 O stream according to the reaction 2Cu ðsþ þ N 2 O! Cu 2 O ðsþ þ N 2 The monolayer of Cu 2 O on the catalyst surface after N 2 O chemisorption was reduced using a TPR process [1]. The TPR area of Cu 2 O was quantified by sampling 1 ml of 1% H 2 /N 2 to calculate the amount of N 2 O consumed. The Cu surface area was calculated, assuming a N 2 O/Cu molar stoichiometry of.5. The average surface density for Cu metal was copper atoms/m 2. The copper content of all catalysts was measured by inductively coupled plasma/mass spectrometry (ICP/MS. The Cu particle size was calculated from the Cu(111 peak of the XRD spectra for all copper samples according to the Scherrer equation, using the full width at half-maximum (FWHM values. The reduced Cu/TiO 2 catalysts showed an obvious diffraction peak at 2h = 43.4, which corresponds to the Cu(111 plane Measurements of FTIR spectra The catalysts with adsorbed CO were in situ DRIFT analyzed using a Nicolet 57 FTIR spectrometer fitted with a mercury cadmium telluride (MCT detector, and operating at a resolution of 1cm 1 for 256 scans. The DRIFT cell (Harrick Co. was equipped with ZnSe windows and a heating cartridge that permitted samples to be heated to 773 K. The CO adsorption was performed by passing a pure CO stream (flow-rate 3 ml/min over the Cu/TiO 2 catalysts at 298 K for 1 min, while the residual CO was purged by a stream of He (flow-rate 3 ml/min. 3. Results 3.1. Activity of Cu/TiO 2 and temperature-programmed reduction The Cu loading, BET surface area, Cu surface area, and particle size of all Cu/TiO 2 species used in this work are summarized in Table 1. All samples were fresh and pre-calcined and pre-reduced at 573 K for 5 h. It was found that the BET surface area decreased with Cu loading, while the Cu surface area and the Cu/TiO 2 particle size increased with Cu content. Fig. 1 depicts the dependence of the initial CO oxidation rate on the Cu loading of the Cu/TiO 2 catalysts at different temperatures. It is important to note that the catalytic activity was nearly constant between 3.7 and 1.3 wt% Cu. The Cu/ TiO 2 catalysts were further characterized by TPR experiments. Fig. 2A depicts the H 2 -TPR profiles of the oxidized Cu/TiO 2 catalysts with different Cu loadings after calcination in air at 573 K for 5 h. The broad TPR profiles of the wt% oxidized Cu/TiO 2 catalysts can be fit to four principal peaks at approximately 437 K (a peak, K (b peak, 483 K (c peak, and K (d peak. The integrated area of TPR of the four peaks as a function of Cu loading is depicted in Fig. 2B, wherein it can be observed that the reduction area of the b, c, and d peaks increased with increased Cu loading; however, the area of the a peak was stable for wt% Cu/TiO 2 catalysts. Fig. 3 compares CO oxidation at 373 K using 5.6 wt% Cu/TiO 2 with different pre-treatments. The oxidized Cu/TiO 2 catalyst achieved less CO conversion (curve a, while the catalytic activity was enhanced significantly by a factor of 2.2 as the Cu/TiO 2 catalyst was partially reduced by H 2 -TPR at temperatures from 298 to 448 K (curve b. Interestingly, the partially reduced Cu/TiO 2 Table 1 The comparison of the Cu/TiO 2 catalysts after calcination in air and reduction in H 2 at 573 K for 5 h Cu content (wt% BET surface area (m 2 /g Cu surface area (m 2 /g Formation Rate of CO 2 (μmol s -1 gcat (d (c Cu particle size (nm Cu Content/wt % Fig. 1. Dependence of CO conversion on Cu/TiO 2 catalysts as a function of Cu loading for 2:1 (v/v CO/O 2 and 5 ml/min over.2 g of catalyst at 413 K, 433 K, (c 453 K, and (d 473 K.

3 C.-S. Chen et al. / Catalysis Communications 9 ( A B δ peak H 2 Consumption (Arb. Unit (d (c Integrated Area (Arb. Unit γ peak β peak α peak Temperature (K Cu Content (wt % Fig. 2. (A TPR profiles of all Cu/TiO 2 catalysts after calcination in air at 573 K for 5 h: 3.7 wt%, 5.6 wt%, (c 8.3 wt%, and (d 1.3 wt%. (B Peak intensities for the TPR peaks in (A as a function of Cu concentration. The weight of catalyst used was.5 g. CO Conversion (% Time (min Fig. 3. Comparison of CO conversion on 5.6 wt% Cu/TiO 2 catalysts with different pre-treatments: Cu/TiO 2 catalysts after calcination in air at 573 K for 5 h; Cu/ TiO 2 catalyst after calcination in the air at 573 K for 5 h, then reduction using H 2 - TPR reduction by passing H 2 at 1 ml/min from 298 K to 448 K; (c Cu/TiO 2 catalyst after calcination and reduction at 573 K for 5 h. catalyst had a higher activity than the fully-reduced Cu/TiO 2 catalyst (curve c. In order to prove the relationship between the a state and the Cu activity, we used a TiO 2 support (Fisher Scientific Co., T315-5 with a specific surface area of 9.8 m 2 /g to prepare a 7.3 wt% Cu/TiO 2 catalyst containing 1.15 m 2 /g of Cu surface area. Fig. 4A compares the catalytic activities and TPR profiles of the 7.3 wt% Cu/TiO 2 F catalyst to the 3.7 wt% Cu/TiO 2 (Cu surface area 1.22 m 2 /g. The 7.3 wt% Cu/TiO 2 F catalyst, which showed no a state, had a relatively poor reaction rate for CO oxidation, despite that the catalysts had similar Cu surface areas. Fig. 4B compares the TPR profiles and catalytic activities of the 5.6 wt% Cu/TiO 2 catalysts treated at 573 K and 773 K. Fig. 4B depicts the TPR profiles of the 5.6 wt% CuO/TiO 2 catalysts calcinated in air for 5 h at 573 K and 773 K. From this figure, it can be seen that the c and d peak areas increased as the area of the b peak decreased. The reduction temperature of the a peak was independent of the pre-treatment temperature, while the high-temperature treatment slightly increased the intensity of the a peak. Pre-treatment by calcination and reduction at 773 K for 5 h resulted in a slightly higher activity for (c CO oxidation for the 5.6 wt% Cu/TiO 2 catalyst compared to that treated at 573 K FTIR of CO adsorbed on Cu/TiO 2 Based on the aforementioned experimental results, it is reasonable to deduce that the reduction of Cu 2+ in the a state might be closely associated with the activity of Cu/TiO 2 for CO oxidation. We used CO as a probe molecule to identify the active sites of Cu/TiO 2 catalysts since CO is a good probe molecule for vibrational spectroscopy, and can usually provide important information about the surface sites of adsorbed species. Fig. 5 depicts the IR spectra of CO adsorbed on the 5.6 wt% Cu/TiO 2 as a function of different pre-treatments. No detectable IR spectrum alluding to CO adsorption on the pure TiO 2 support was observed at 298 K. The spectrum of CO adsorbed on the oxidized Cu/TiO 2 catalyst had a broad IR band corresponding to a linear CO adsorption on the Cu surface (spectrum a in Fig. 5, which could be fit to three principal peaks positioned at 261 cm 1 (the L 1 state, 215 cm 1 (the L 2 state, and 2117 cm 1 (the L 3 state. When the Cu/TiO 2 catalyst was partially reduced by H 2 -TPR at temperatures ranging from 298 to 448 K, a weak CO shoulder peak positioned at 2124 cm 1 (the L 4 state was observed, as shown in Fig. 5b. The IR intensity significantly increased when the Cu/TiO 2 catalyst was reduced at 573 K for 5 h (spectrum c in Fig. 5. The IR bands for the reduced Cu/TiO 2 exhibited lower CO stretching frequencies at 25 cm 1 (L 1, 297 cm 1 (L 2, and 2112 (L 3. The L 4 -CO retained its stretching frequency at 2123 cm 1. The red shift of the L 1 -CO, L 2 -CO, and L 3 -CO peaks may be caused by the reduced metal surface that provided stronger back donation to the anti-bonding 2p orbitals of the CO, resulting in a strengthening of the metal carbon bond, while simultaneously weakening the C O bond. 4. Discussion The reduction of TPR peaks below 473 K has been interpreted by some authors to be the result of smaller CuO particles and/or highly dispersed CuO particles [4,11,12]. The higher reduction temperature (above 473 K has been proposed to be associated with larger particles of bulk CuO on the TiO 2 surface. It is commonly believed that finely dispersed Cu is the active phase for CO oxidation

4 2384 C.-S. Chen et al. / Catalysis Communications 9 ( TCD Signal (Arb. Unit CO Conversion (% α state Temperature/K Temperature (K Fig. 4. (A Comparison of light-off curves for CO oxidation on different TiO 2 -supported Cu catalysts after calcination in the air and reduction in H 2 at 573 K for 5 h, and the TPR profiles of both catalysts: 3.7 wt% Cu/TiO 2 (P-25 TiO 2 ; 7.3 wt% Cu/TiO 2 (Fisher Scientific Co.. (B Comparison of light-off curves for CO oxidation on 5.6 wt% Cu/TiO 2 catalyst after calcination and reduction at 573 K and 773 K and TPR profiles of 5.6 wt% Cu/TiO 2 catalyst after calcination in air at 573 K for 5 h, 773 K for 5 h. The weight of catalyst used was.5 g. Absorbance (c cm -1 (L cm -1 (L cm -1 (L 2 25 cm -1 (L Wavenumbers (cm -1 2 Fig. 5. IR spectra of CO adsorbed on the 5.6 wt% Cu/TiO 2 : the sample was calcinated in air at 573 K for 5 h; the sample was partially reduced by H 2 -TPR at temperatures from 298 K to 448 K; (c the sample was reduced in H 2 at 573 K for 5h. [13]. The a and b peaks could be associated with highly dispersed and/or smaller CuO particles, while the c and d states could be due to the reduction of larger particles of bulk CuO on the TiO 2 surface [13,14]. Interestingly, the concentration of Cu 2+ in the a state was independent of Cu loading. This partial reduction by H 2 -TPR produced the expected reduction of the a peak, while the larger particles of copper and bulk CuO still remained predominantly oxidized states. Similar TPR results were also observed by Coloma et al. [11]. Figs. 3 and 4 clearly show that the reduction of Cu 2+ in the a state significantly enhances the catalytic activity of the Cu/TiO 2 for CO.2 oxidation. Thus, it is reasonable to deduce that the activity of the Cu/TiO 2 is correlated with the magnitude of the reduced a peak at 433 K. The active sites of Cu/TiO 2 were probed by CO adsorption. The IR bands corresponding to CO adsorbed on TiO 2 -supported Cu have been suggested in [11,15]. The IR bands below 21 cm 1 (L 1 and L 2 correspond to CO adsorbed on low index planes, such as the (111 and (1 faces [1,11,15]. The band at cm 1 (L 3 corresponds to CO adsorbed on imperfect sites, such as step and edge sites. The CO bands at cm 1 (L 4 may be from CO adsorbed on isolated Cu atoms and/or small 2D Cu particles on the TiO 2 surface [5,6,11]. Boccuzzi et al. reported that small 2D Cu particles in Cu/TiO 2 are partially electropositive as a result of interactions with oxygen atoms on the surface of the support [4,5,16]. The phase of highly dispersed Cu may cause isolated Cu to partially dissolve at the TiO 2 surface during wet impregnation [4]. Therein, wet impregnation-prepared Cu/TiO 2 may produce a sample with isolated or 2D clusters of Cu [4]. By comparing Figs. 3 and 5, it can be observed that sites L 1,L 2, and L 3 are associated with a low reactivity for CO oxidation. The reduction of the a state evidently causes CO to be adsorbed on the L 4 site (2124 cm 1, Fig. 3b, subsequently increasing the activity of the Cu/TiO 2, even if the other sites (L 1,L 2, and L 3 remain in the oxidized state (see Fig. 5b. In other words, these phenomena imply that the a state of TPR depends on the L 4 site, the isolated Cu atoms, and/or small 2D Cu particles on the TiO 2 surface. The results illustrated by Fig. 4 reveal that the presence of Cu in the a state is potentially associated with a higher level of activity. To our knowledge, there are no reports in the literature that discuss the a-cu/tio 2 reduction peak at 433 K. Cu/CeO 2 catalysts have similar characteristics at reduction temperature of K, which is 4 5 K lower than the peak temperature for the reduction of larger bulk CuO particles. This reduction peak was proposed to arise from the reduction of Cu 2+ entering the CeO 2 lattice [6]. In the present study, the a-cu/tio 2 reduction peak could potentially derive from Cu 2+ ions located in the TiO 2 lattice. The smaller lattice spacing may restrict the Cu particle size, resulting in smaller particles during Cu atom aggregation. These small Cu particles in the a state dominated the catalytic activity for CO oxidation.

5 C.-S. Chen et al. / Catalysis Communications 9 ( Conclusions We have demonstrated that Cu/TiO 2 catalysts possess high levels of catalytic activity for CO oxidation. The activity of Cu/TiO 2 catalysts was not enhanced by increased Cu surface area or by Cu content, and was almost constant for wt% Cu. The results of H 2 -TPR differentiated the different CuO species, including isolated Cu atoms, highly dispersed CuO (the a and b states, and bulk CuO (the c and d states. Cu loading and Cu atom aggregation due to the 773 K treatment did not influence the concentration of Cu 2+ in the a state. Activity tests and CO-FTIR have demonstrated that the Cu 2+ reduction at 437 K (a state is likely to be the main factor in determining the activity of the Cu/TiO 2 catalysts. Acknowledgements Financial support from the National Science Council of the Republic of China (NSC M is gratefully acknowledged. References [1] H.G. El-Shobaky, Y.M. Fahmy, Appl. Catal. B 63 ( [2] P.W. Park, J.S. Ledford, Appl. Catal. B 15 ( [3] A. Martínez-Arias, M. Fernández-García, J. Soria, J.C. Conesa, J. Catal. 182 ( [4] F. Boccuzzi, A. Chiorino, G. Martra, M. Gargano, N. Ravasio, B. Carrozzini, J. Catal. 165 ( [5] F. Boccuzzi, S. Coluccia, G. Martra, N. Ravasio, J. Catal. 184 ( [6] F. Boccuzzi, G. Martra, C. Partipilo Papalia, N. Ravasio, J. Catal. 184 ( [7] J. Huang, S. Wang, Y. Zhao, X. Wang, S. Wang, S. Wu, S. Zhang, W. Huang, Catal. Commun. 7 ( [8] F. Boccuzzi, E. Guglielminotti, G. Martra, G. Gerrto, J. Catal. 146 ( [9] T. Venkov, K. Hadjiivanov, Catal. Commun. 4 ( [1] C.J.G. Van Der Grift, A.F.H. Wielers, B.P.J. Joghi, J.V. Beijnum, M.E. Boer, M.V. Helder, J.W. Geus, J. Catal. 131 ( [11] F. Coloma, F. Marquez, C.H. Rochester, J.A. Anderson, Phys. Chem. Chem. Phys. 2 ( [12] F.S. Dels, A. Vavere, J. Catal. 85 ( [13] M.F. Luo, J.M. Ma, J.Q. Lu, Y.P. Song, Y.J. Wang, J. Catal. 246 ( [14] X. Tang, B. Zhang, Y. Li, Y. Xu, Q. Xin, W. Shen, Appl. Catal. A 288 ( [15] P. Hollins, Surf. Sci. Rep. 16 ( [16] C.T. Campbell, Surf. Sci. Rep. 27 (