Synthesis and Treatment Parameters for Controlling Metal Particle

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Synthesis and Treatment Parameters for Controlling Metal Particle Size and Composition in Cu/ZnO MaterialsFirst Evidence of Cu3Zn Alloy Formation Salim Derrouiche,†,‡ Hélène Lauron-Pernot,† and Catherine Louis*,† †

Laboratoire de Réactivité de Surface, UMR CNRS 7197, Université Pierre et Marie Curie-UPMC, 4 place Jussieu, 75252 Paris cedex 05, France ABSTRACT: This paper deals with the study of the conditions of obtention of small metal copper particles on ZnO using easy synthesis methods and activation treatments, which were found “efficient” for the preparation of copper particles on silica, that is, deposition−precipitation with urea and incipient wetness impregnation. For the latter, four different copper precursors were used: copper(II) nitrate, acetylacetonate, bis-ethylenediamine, and tetraamine. Activation treatments were performed under different gas reducing conditions (H2 or CO in N2), temperatures between 280 and 350 °C, and isotherm between 1 min and 2 h, with or without a precalcination step. Transmission electron microscopy (TEM), UV−visible spectroscopy, and X-ray diffraction (XRD) techniques were used to characterize the samples at the different stages of the synthesis route. The TEM results showed that it is not as easy as on silica to make small copper particles on ZnO. However, some experimental conditions were found to lead to particles around 4 nm: impregnation with copper acetylacetonate or copper bis-ethylenediamine, followed by calcination before reduction in H2. The important result of this work is that depending on the final temperature of reduction under hydrogen, it was possible to form either metal Cu particles (280 °C) or Cu3Zn alloy particles (350 °C); this was directly proved by XRD and indirectly by UV−visible spectroscopy. However, the formation of Cu3Zn could be avoided if the reducing gas H2 was replaced by CO in N2. To our knowledge, this is the first time that the formation of Cu3Zn alloy particles is evidenced in Cu/ZnO samples. KEYWORDS: Cu/ZnO, nanoparticles, Cu, Cu−Zn, Cu1−xZnx, Cu3Zn, alloy, reduction conditions, impregnation, deposition−precipitation, copper nitrate, copper acetylacetonate, copper tetraamine, copper bis-ethylenediamine, catalyst, TEM, UV−visible, XRD Cu0,15−19 Cu0−Cu+,20 ZnOx species formed on top of the Cu surface21 and Cu−Zn surface and/or bulk alloys.22−25 In a very recent work, copper in the metallic state is proposed to be the active phase,26 but the role of ZnO through its interaction with copper particles is not fully elucidated. For those studies, simplified catalytic systems Cu/ZnO are prepared by coprecipitation,24,25,27−29 but copper can be embedded into ZnO,30 and the Cu particles are large (>10 nm). Deposition of Cu on a ZnO support seems therefore a more appropriate procedure. In spite of a thorough literature search, only a few papers report the use of such a procedure. Moreover, impregnation with copper nitrate31 leads to large Cu0 particles (20−50 nm). Other ones lead to smaller size, at least 10 nm size by precipitation32 and 7 nm by mechanochemical milling.33 The initial goal of this work was to investigate whether by using classical methods of preparation and playing with parameters of the preparation it was possible to get in f ine

1. INTRODUCTION Zinc oxide is a n-type semiconductor with a wide bandgap of 3.37 eV. When it is modified by copper, it constitutes very attractive materials owing to unique electrical, optical, and ferromagnetic properties.1,2 ZnO doped with Cu is potentially interesting for applications in spintronics3 and optical devices such as emitting diodes (LEDs) and laser diodes (LDs)4 and as semiconductors.5 In most cases, copper is present as Cu2+ ions into the ZnO lattice. Cu/ZnO is also the base for materials of high industrial interest in the field of catalysis. Cu/ZnO-based catalysts are very efficient for methanol synthesis,6,7 water gas shift,6,8 and steam reforming of methanol for hydrogen production for fuel cells.9−11 In industry, the catalysts for methanol synthesis and water gas shift are Cu/ZnO/Al2O3, they lead to optimum high activity and stability, and they are prepared by coprecipitation. Many academic works have been done to try to elucidate the nature of the active sites in catalysis and to study the mechanisms of reaction. Different types of active sites have been proposed for methanol synthesis: cationic copper species,12,13 copper species at the Schottky junction,14 © 2012 American Chemical Society

Received: January 17, 2012 Revised: May 14, 2012 Published: May 18, 2012 2282

dx.doi.org/10.1021/cm3001915 | Chem. Mater. 2012, 24, 2282−2291

Chemistry of Materials

Article

350 °C for 2 h was compared to a one step-reduction under hydrogen at 350 °C. Preliminary experiments of temperature programmed reduction (TPR) conducted on the different samples indicate that a temperature of 280 °C was high enough to reduce copper to its zero valence state in agreement with published results.39 Alternatively, other conditions of thermal treatment were used, such as lower reduction temperatures for different durations or thermal reduction under 1% (v/v) CO/N2 mixture, but this will be presented in detail in section Results and Discussion. 2.2. Material Characterization. Chemical Analysis. Chemical analysis of Cu in the as-synthesized samples was performed by inductively coupled plasma atom emission spectroscopy at the CNRS Center of Chemical Analysis (Vernaison, France). The Cu loadings were expressed in weight percent, wt % Cu = [mCu/(mCu + mZnO)] × 100. The final copper content of all the Cu/ZnO samples were in the range 0.93−1.01 wt % for the nominal loading 1 wt %, and 8.6 wt % for that of the 10 wt % IWI-Nit sample. Transmission Electron Microscopy (TEM). TEM images of the samples were recorded using a Jeol 1100 (100 kV) microscope. The sample, sonicated in ethanol, was dispersed on a holey carbon grid prior to imaging. It is worth noting that the comparable size and mass−thickness contrast of Cu and ZnO nanoparticles may hamper their distinction in TEM images. However, the well faceted particles of ZnO support used in this study helped mitigate this problem at least when the Cu particles were small compared to the ZnO ones (30−200 nm). Statistical analysis of the particle size distributions (PSD) of the reduced samples was obtained by counting 100−400 particles, using the software ITEM. X-ray Diffraction Microscopy (XRD). The XRD data were recorded on a diffractometer (D8 Bruker Company) using the Cu Kα radiation (1.5418 Å; 40 kV and 30 mA) with a Ni filter. The crystallite sizes of the copper-containing compounds in Cu/ZnO samples were calculated using Scherrer’s equation,40 and correction for instrumental broadening was applied.41 Moreover, any variation observed in the diffraction peak positions was found to be much larger than the typical error associated with experimental uncertainties estimated from several measurements. In addition, the samples were spinned during XRD analyses, preventing preferential orientation effect on the XRD intensity and allowing quantitative exploitation of the data. Finally, all diffractograms were standardized relatively to the (202) peak at 2θ = 76.99° of pure ZnO since it does not interfere with any of the diffraction peaks of copper-containing compounds. UV−Visible Spectroscopy. UV−visible−NIR spectra were recorded on a Cary 5000 spectrometer in the range 1250−200 nm and at room temperature. BaSO4 was used as the standard. Prior to optical measurements, all the powder samples were finely crushed and lightly pressed into the sample holder of the diffuse reflectance cell (4 mm thick).

small metal copper particles. We used synthesis methods and activation treatments that were found “efficient” to prepare small metal copper particles on silica, that is, deposition− precipitation with urea (DPU)34 and incipient wetness impregnation (IWI).35 Several Cu/ZnO samples with a copper loading of 1 wt % were prepared according to these methods, and different copper precursors were used for IWI to study their influence on the final material properties: copper(II) nitrate, acetylacetonate, bis-ethylenediamine, and tetraamine. The as-synthesized Cu/ZnO samples were subjected to various post-synthesis treatments to optimize the metal particle size and their chemical nature; it was shown that copper and Cu− Zn alloy-type metals can be obtained depending on the reduction conditions, and we discovered that the Cu3Zn phase was formed under specific conditions; therefore, we also investigated under which conditions it could form and if its formation could be monitored.

2. EXPERIMENTAL SECTION 2.1. Preparation of Cu/ZnO Samples. The zinc oxide powder used was Kadox-911 from Horsehead Co.; its BET surface area was 8 m2/g after a treatment in oxygen at 400 °C for 2 h, which is also the calcination procedure used in this study (see below). Different Cu/ ZnO samples were synthesized according to the procedures described below. 2.1.1. Preparation of Cu/ZnO Samples by Incipient Wetness Impregnation. Four different impregnation solutions (0.53 M) containing the following copper(II)-precursors, copper nitrate (Sigma-Aldrich), copper acetylacetonate (Sigma-Aldrich), copper bisethylenediamine [Cu(en)2(H2O)2]2+, and copper tetraamine [Cu(NH3)4(H2O)2]2+, were prepared. Copper nitrate was dissolved in deionized water and copper acetylacetonate in dichloromethane (from Sigma-Aldrich). The copper bis-ethylenediamine solution was obtained by mixing a suitable amount of copper nitrate with a small excess of ethanediamine in deionized water (2.2 equiv.); the pH of the solution was 11.5. The copper tetraamine solution was obtained by mixing a suitable amount of copper nitrate with ammonia in deionized water (5 M); the pH of the solution was 11.5. To achieve a nominal Cu loading of 1 wt %, 3 mL of solution was added to zinc oxide powder (10 g) dropwise under stirring. The resulting solids were dried at room temperature (RT) for 12 h and then stored in a desiccator under vacuum at RT. The as-synthesized samples were denoted IWINit, IWI-AcAc, IWI-EN, and IWI-NH3, respectively. A 10 wt % IWINit sample was also synthesized following the same procedure. 2.1.2. Preparation of Cu/ZnO Samples by Deposition−Precipitation Method Using Urea as a Precipitant Reagent (DPU-Nit). A 1 wt % Cu/ZnO sample was prepared using the deposition− precipitation method with urea as a precipitant reagent according to a procedure developed by Geus.36 Briefly, 200 mL of an aqueous solution of copper nitrate (2.4 × 10−3 M) was introduced in a glass reactor containing 3 g of ZnO. The mixture was heated at 80 °C, and then 0.96 g of urea CO(NH2)2 (Sigma-Aldrich) was added to the mixture, and the temperature was kept at 80 °C for 20 h. The solid was recovered after centrifugation and then washed several times with distilled water and dried under vacuum at RT for 24 h prior to storage in a desiccator. The as-synthesized sample is denoted DPU-Nit. It is worth noting that, during the preparation, the color of the mixture gradually turned from pale blue to brownish, which is expected to be related to the formation of a copper oxide phase, as it will be discussed later. 2.1.3. Post-Synthesis Thermal Treatments. The as-synthesized samples were first dried at 120 °C for 12 h and then subjected to two standard post-thermal treatments under gases of high purity with a flow rate of 100 cm3/min and a heating rate of 5 °C/min up to a given temperature, according to the same experimental protocol as the one used in former study on Cu−Zn/SiO2 samples.37,38 A two-step treatment combining a calcination step under flowing oxygen at 400 °C for 2 h followed by a reduction step under flowing hydrogen up to

3. RESULTS AND DISCUSSION 3.1. Influence of the Preparation Method on the Nature of the Supported Phase. 3.1.1. Characterization of the As-Synthesized and Calcined 1 wt % Cu/ZnO Samples. The five Cu/ZnO samples synthesized using the procedures described in the Experimental Section were characterized after drying at RT and after calcination. Figure 1a,b shows the XRD patterns of the as-synthesized DPU-Nit and IWI-AcAc samples, respectively, and Figure 1c−g shows those of the five samples after calcination under flowing oxygen at 400 °C, along with the XRD pattern of the ZnO support alone (Figure 1h). Figure 1a,c shows that, apart from the ZnO peaks, both the as-synthesized and calcined DPU-Nit samples exhibit extra peaks that can be ascribed to the copper oxide CuO phase with the tenorite structure (JCPDS card No. 80-1916); the most intense peak of CuO detected at 2θ ≈ 38.78° results from the contribution of two peaks at 38.69° and 38.89° of CuO. Therefore, these results indicate that copper oxide is formed during the synthesis 2283



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The Cu(II) d−d transition band is observed at 765 nm (broad peak) for IWI-Nit (Figure 2A-e). It is shifted toward lower wavelengths, at 690 nm for IWI-NH3 (Figure 2A-d), at 660 nm for IWI-AcAc (Figure 2A-b), and at 545 nm for IWIEN (Figure 2A-c). The position of the d−d Cu(II) absorption band of IWI-Nit at 765 nm is consistent with former results obtained with Cu/SiO2 samples prepared by impregnation with copper nitrate35 and corresponds to Cu(II) coordinated by H2O ligands. The band of IWI-NH3 at 690 nm is close to that obtained on Cu/SiO2 prepared by adsorption of [Cu(NH3)4]2+ followed by drying at RT (705 nm).42 Note that this Cu/SiO2 sample before drying (so still wet) exhibited a d−d band at higher energy (630 nm), attributed to the adsorbed [Cu(NH3)4]2+ complex, and that the shift to 705 nm upon drying was assigned to the loss of two NH3 ligands. The band position of IWI-EN at 545 nm is roughly the same as that of Cu(II) in Cu/SiO2 prepared by adsorption of [Cu(en)2]2+ (570 nm).42 The higher energy of this transition compared to the other ones is due to the stronger ligand field induced by ethylenediamine, which is a strong chelating ligand. The band position of IWIAcAc (660 nm) is intermediate, which is consistent with the spectrochemical series of the ligands established according to their strength: EN > NH3 > AcAc > H2O.43 The UV−visible spectrum of DPU-Nit (Figure 2A-a) is different from the other ones and shows a kind of absorption edge with a threshold at ∼900 nm that may evoke the threshold of CuO35,44 and would be in agreement with the CuO diffraction peaks observed in Figure 1a. However, it is not strictly similar to the spectrum of CuO mechanically diluted in ZnO (Figure 2A-g). These differences are also attested by the different colors taken by the samples; as-synthesized IWI-Nit, IWI-AcAc, IWI-EN, and IWINH3 were cyan, blue violet, mauve, and pale blue, respectively, while the DPU-Nit sample was pale-gray-brown and the mechanical mixture was darker. After calcination, all the samples exhibit a UV−visible spectrum (Figure 2B) similar to that of as-synthesized DPUNit.35,44 Only that of IWI-Nit (Figure 2B-e) is slightly different and better looks like the typical absorption of CuO (Figure 2Bg) with a threshold at 900 nm that corresponds to the energy of the band gap (Eg = 1.4 eV) of CuO, which is a p-type semiconductor. This may be because of the presence of better crystallized or larger CuO particles. Indeed the XRD patterns of these calcined samples show that the CuO peak at 38.89° is more intense in IWI-Nit (Figure 1e) than in DPU-Nit or IWINH3 (Figure 1c,f). No CuO XRD peak is observed for IWIAcAc and IWI-EN. Therefore, the nontypical shape of the absorption band of CuO for all the calcined samples except IWI-Nit and the absence or the low intensity of the CuO XRD peak may be an indication of the presence of an amorphous CuO phase or of CuO particles of a few nanometers. The presence of the same absorption band, that is, the presence of a CuO-like compound, in as-synthesized DPU-Nit is more puzzling since amorphous hydroxynitrate was identified in Ni/SiO245 and Cu/SiO234 also prepared by DPU. Moreover, Cu/SiO2 prepared by DPU was blue after drying,34 whereas color change of the suspension was observed during the preparation of Cu/ZnO from pale blue to brownish, and the sample was pale-gray-brown after drying at RT; this is also an indication of the presence of CuO in this sample. It may be also underlined that the sample color does not change any longer upon calcination in contrast to the other samples, which were pale-blue to pale-violet after drying and turned pale-gray-brown

Figure 1. XRD patterns of 1 wt % Cu/ZnO samples as-synthesized: (a) DPU-Nit and (b) IWI-AcAc and after a calcination treatment at 400 °C for 2 h: (c) DPU-Nit, (d) IWI-AcAc, (e) IWI-Nit, (f) IWINH3, (g) IWI-EN; (h) XRD pattern of pure ZnO support. The line shows the most intense peaks of the JCPDS file of CuO (#80-1916).

of DPU-Nit (Figure 1a) and that no significant changes were evidenced after calcination (Figure 1c). It is worth noting that the intensity of the XRD peak of CuO is low because of the low copper loading (1 wt %) or because the Cu phase is not entirely crystallized. The XRD patterns of IWI-Nit and IWI-NH3 are similar and exhibit the CuO phase only after calcination (Figure 1e,f) while those of IWI-AcAc (Figure 1b,d, respectively) and IWI-EN (Figure 1g) do not show any CuO peaks. This may mean that either CuO does not form after calcination or the CuO particles are too small to be detected. Figure 2A displays the UV−visible spectra of all the assynthesized samples. The one of pure ZnO in Figure 2A-f shows an absorption edge around 400 nm ascribed to the bandgap of ZnO.

Figure 2. UV−visible spectra of samples 1 wt % Cu/ZnO assynthesized (A) and calcined (B): (a) DPU-Nit; (b) IWI-AcAc; (c) IWI-EN; (d) IWI-NH3; (e) IWI-Nit; (f) pure ZnO support; and (g) mechanical mixture of CuO (1 wt %) in ZnO.

The UV−visible spectra of all the as-synthesized IWI samples exhibit differences in the visible range 500−800 nm, that is, in the energy range of the Cu(II) d−d transitions, which reveals differences in the coordination sphere of the supported Cu(II) precursors (Table 1). Note that two d−d transitions are expected for Cu(II) complexes in octahedral symmetry. Moreover, because of the Jahn−Teller effect, the symmetry of the ligand field is lowered to square-planar or octahedral with tetragonal distortion (D4h), and three d−d transition bands are expected. In fact, because the bands are broad, only their envelope is observed. 2284



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Table 1. XRD, UV−Vis, and TEM Characteristics of the Five Cu/ZnO Samples (1 wt % Cu) calcineda

as-synthesized

reducedb

calcineda + reducedb

XRD DPU-Nit IWI-Nit IWI-NH3 IWI-AcAc IWI-EN DPU-Nit IWI-Nit IWI-NH3 IWI-AcAc IWI-EN a

no no no no

CuO diffraction diffraction diffraction diffraction

UV−visible CuO: edge at 900 nm Cu(II) d−d transition: 765 nm Cu(II) d−d transition: 690 nm Cu(II) d−d transition: 660 nm Cu(II) d-d transition: 545 nm

CuO CuO CuO no diffraction no diffraction

Cu3Zn Cu3Zn Cu3Zn Cu3Zn Cu3Zn

CuO: CuO: CuO: CuO: CuO:

25 nm (5−55 nm) 20.6 nm (7−36 nm)c 12.5 nm (3−25 nm) 9.1 nm (1−25 nm) 9.3 nm (1−25 nm)

(20 (20 (14 (10 (10

nm) nm) nm) nm) nm)

Cu3Zn (20 nm) Cu3Zn (20 nm) Cu3Zn (14 nm) no diffraction no diffraction TEM

edge at 900 nm Edge-like at 900 nm edge-like at 900 nm edge-like at 900 nm edge-like at 900 nm

25 nm (5−55 nm) 14 nm (3−30 nm)c 10 nm (2−29 nm) 3.8 nm (1−10 nm) 5.8 nm (1−17 nm)

Calcination O2, 400 °C, 2 h. bReduction H2, 350 °C, 2 h. cBimodal distribution.

after calcination. Hence, it seems that ZnO favors the formation of CuO during DPU. One can note that UV−visible acts as complementary of XRD since CuO is not detected by XRD in calcined IWI-AcAc (Figure 1d) and IWI-EN (Figure 1g); the higher sensitivity of UV−visible spectroscopy compared to XRD has already been observed by some of us in an earlier study on the Zn/SiO2 system.46 It can be concluded that the chelating ligands AcAc and EN prevent the formation of large CuO particles and lead to the formation of a more dispersed oxidized phase. In a former work on the preparation of Ni/SiO 2 samples prepared by impregnation, EN also provided much smaller Ni particles than water ligands of nickel nitrate.47 3.1.2. Influence of the Pretreatment Conditions on the Nature of the Supported Copper Phase. For the different Cu/ ZnO samples, the influence of the thermal treatments on the physicochemical properties of the copper-based particles was investigated. The as-synthesized Cu/ZnO materials were reduced under hydrogen at 350 °C, either directly or after previous calcination at 400 °C. As mentioned in the Experimental Section, these thermal treatments were the same as those used in previous studies of Cu−Zn/SiO2.37,38 Figure 3 shows the XRD patterns of the IWI-Nit and IWIAcAc samples after direct reduction (Figure 3b,d, respectively) and after calcination and reduction (Figure 3a,c, respectively). As a reference, the diffractogram of IWI-Nit calcined at 400 °C, reported in Figure 3e, shows the ZnO peaks and the extra peak of CuO. Apart from the ZnO peaks, the XRD patterns of reduced IWI-Nit (Figure 3a,b) show extra diffraction peaks at 2θ ≈ 42.67° (the most intense peak), 49.69°, and 88.33°. They correspond neither to metallic copper, nor to copper oxide, nor to metallic zinc, but they can be ascribed to the Cu3Zn phase (JCPDS file #65-6567: 42.673° (I = 100) and 49.686° (I = 43)). This indicates that a copper−zinc alloy is formed during the H2 thermal treatment at 350 °C in the IWI-Nit sample without any influence of the precalcination step. The DPU-Nit and IWI-NH3 samples behave the same as IWI-Nit. It is important to note that the formation of Cu3Zn in Cu/ZnO samples has never been reported before in the literature. The average size of the Cu3Zn nanoparticles in IWI-Nit was estimated from the Scherrer equation (see Experimental Section) to be around 20 nm regardless of the pretreatment (Figure 3a,b); DPU-Nit behave like IWI-Nit with the same XRD patterns and average particle size of 20 nm, independently of whether precalcination treatment was performed or not

Figure 3. XRD patterns of 1 wt % Cu/ZnO samples reduced under H2 at 350 °C for 2 h: IWI-Nit (a) with precalcination and (b) without a precalcination; IWI-AcAc (c) with precalcination and (d) without precalcination treatment; (e) XRD pattern of IWI-Nit calcined at 400 °C. The line shows the (111) peak of CuO from the JCPDS file of CuO (#80-1916); ▲ and ●: the most intense peaks from the JCPDS files of Cu0 (#04-0836) and Cu3Zn (#65-6567), respectively.

before reduction (Table 1). The particles in IWI-NH3 are smaller (14 nm). These results are confirmed by the particle measurements made by TEM (Figure 4 and Table 1). Note, however, that the IWI-Nit sample exhibits a bimodal distribution centered on 17 and 30 nm (Figure 4b) and that the precalcination step prior to reduction leads to small changes in the TEM particle size while XRD indicates almost no changes. In the case of the IWI-AcAc sample, the direct reduction at 350 °C leads to an XRD pattern (Figure 3d) similar to that obtained for IWI-Nit (Figures 3a,b), that is, to the formation of Cu3Zn, albeit the intensity of the main peak at ≈42.67° is significantly lower. In contrast, IWI-AcAc precalcined then reduced does not exhibit other XRD peaks than those of ZnO (Figure 3c). This may indicate that either Cu3Zn does not form or the particles or the crystallite domains are too small to be detected. IWI-EN behaves like IWI-AcAc, and the TEM analysis shows that after direct reduction, the metal particles are around 9 nm in both samples (Figure 5A-a,B-a, respectively, and Table 1) while after precalcination and reduction smaller metal particles are obtained, around 5 nm with narrower size distribution (Figure 5A-b,B-b, respectively, and Table 1). One can note that the metal particles are much larger on ZnO than on other oxide supports, regardless of the 2285



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Figure 4. TEM images of 1 wt % Cu/ZnO samples reduced under hydrogen at 350 °C for 2 h, along with their respective statistical size distribution: (a) DPU-Nit, (b) IWI-Nit, and (c) IWI-NH3.

particle size. A more detailed study of the conditions of formation of Cu3Zn is reported in the following section. 3.2. Experimental Evidence of the Formation of Cu3Zn Alloy Particles. 3.2.1. Influence of the Reduction Temperature on the Formation of Cu3Zn Alloy Particles. Here, we have investigated the influence of the H2-reduction conditions on the nature of the Cu-based compound present in Cu/ZnO samples. For this study, we chose a sample IWI-Nit with 10 wt % Cu to get more visible XRD lines of the metallic phase, but we checked that they were the same as for IWI-Nit with 1 wt % Cu. The sample was subjected to a calcination treatment under oxygen at 400 °C for 2 h followed by a reduction treatment under hydrogen at temperatures between 280 and 350 °C and isotherm between 1 min and 2 h. Each H2-reduction treatment was performed with the same batch of calcined sample. Figure 6 shows the corresponding XRD patterns in the region 2θ = 40− 55°, that is, in the range of the most intense diffraction peaks of Cu0 and Cu3Zn. The XRD pattern of the sample reduced at 350 °C for 2 h (Figure 6g) exhibits diffraction peaks at 2θ ≈ 42.70° and 49.72°, indicating the formation of the Cu3Zn alloy as previously discussed. In contrast, the XRD of the sample reduced at 280 °C for 1 min (Figure 6a) exhibits diffraction peaks at 2θ ≈ 43.30° and 50.43°, corresponding to those of the Cu0 phase. This indicates that a structural modification of the supported metal occurs between 280 and 350 °C. It is worth noting that a reduction at 280 °C for 2 h gives the same diffraction pattern (result not shown) as that obtained at 280 °C for 1 min (Figure 6a). This indicates that a temperature

preparation methods. For instance, by mere impregnation of silica with copper nitrate, copper particles of 3 nm could be obtained in spite of much higher reduction temperature (700 °C) whether the sample was precalcined or not.35 Other metal supported samples prepared by DPU, that is, Ni/SiO2,45,48 Cu/ SiO2,34 Au/TiO2, and Au/Al2O3,49−51 also led to the formation of small metallic particles (≤5 nm). Here, when DPU is applied to the synthesis of Cu/ZnO samples, large metal particles are obtained (Table 1). Let us also remember that we have shown above that ZnO favors the formation of CuO during DPU preparation (Table 1). The main difference between ZnO and the supports cited above is that ZnO is a basic support (PZC ∼ 9). So, the pH of the solution in the hydration layer of the support is certainly much higher, which favors the uncontrolled precipitation of copper hydroxide (precipitation uncontrolled because it is not controlled by the slow increase of pH resulting from the decomposition of urea; more information can be found in refs 45 and 48). The consequence is the formation of large particles of CuO and, after reduction, that of metal particles larger than in the other DPU samples cited above. Note that for some of the samples, IWI-AcAc and IWI-EN, a precalcination step prior to reduction leads to smaller Cu3Zn alloy nanoparticles than after direct reduction, whereas for the other ones, DPU-Nit, IWI-Nit, and IWI-NH3, no changes in the Cu3Zn nanoparticles size are observed. These experiments therefore show that appropriate choice of the method of preparation, of copper precursor, and of pretreatment conditions leads to a certain degree of control of the metal 2286



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Figure 5. TEM images of 1 wt % Cu/ZnO samples reduced under H2 at 350 °C for 2 h, along with their respective statistical size distribution: (A) IWI-AcAc: (a) without and (b) with a precalcination treatment under air at 400 °C for 2 h; (B) IWI-EN: (a) without and (b) with a precalcination treatment under air at 400 °C for 2 h.

plateau of 1 min at 280 °C is sufficient to reduce the copper oxide particles formed during the precalcination step but that 280 °C is not a temperature high enough to allow the formation of Cu3Zn alloy particles. This structural phase transformation of Cu0 into Cu3Zn can be followed through the comparison of the XRD patterns obtained after reduction under H2 at temperatures between 280 and 350 °C. A shift of the (111) diffraction peak of the Cu0 phase is observed from 2θ ≈ 43.30° to 43.10° (Figure 6a−d) when the reduction temperature is increased from 280 to 350 °C (with 1 min at the final temperature). In parallel, the intensity of this peak progressively decreases along with the appearance and progressive increase of a shoulder at 2θ ≈ 43.00° after reduction at 350 °C (Figure 6d). Afterward, the peak continues to broaden after 5 min at 350 °C (Figure 6e). Then, as the reduction time at 350 °C is still increased, the peak becomes thinner, significantly shifts from 2θ ≈ 43.10° (1 min) to 42.70° (2 h) (Figure 6f,g), and increases in intensity. Higher reduction temperatures up to 500 °C for 1 min do not lead to significant changes (XRD not shown), indicating that the

Figure 6. XRD patterns (in the range 2θ = 40−55°) of sample 10 wt % IWI-Nit calcined at 400 °C followed by H2 reduction at different temperatures and for different times: 1 min at (a) 280 °C, (b) 330 °C, (c) 340 °C, and (d) 350 °C and at TR = 350 °C for (e) 5 min, (f) 30 min, and (g) 2 h.▲ and ●: the most intense peaks from the JCPDS files of Cu0 (#04-0836) and Cu3Zn (#65-6567), respectively.

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Cu3Zn particles formed after H2 reduction at 350 °C are stable in composition and size at least up to 500 °C. While the diffraction peaks at 43.30° and 42.70°, observed in Figure 6a,g, can be ascribed to Cu0 and Cu3Zn, respectively, those observed in Figure 6c−f may correspond to Cu−Zn compounds of composition intermediate between Cu0 and Cu3Zn, that is, Cu1‑xZnx with 0 < x < 0.25. Cokoja et al. observed shifts of the XRD peaks of colloidal α-Cu1−xZnx synthesized following an organometallic route with different compositions: x = 0.09, 0.17, and 0.33.52 The largest shift of −0.456° with respect to the (111) Cu0 peak was measured by the authors for the Cu0.67Zn0.33 colloids. In spite of the higher Zn content, this shift is smaller than the one of −0.598° observed between Cu0 and Cu3Zn in our samples. We can therefore propose that the formation of Cu3Zn alloy passes through the formation of Cu0 phase, followed by a gradual enrichment by Zn coming from superficial ZnO reduction between 280 and 350 °C, and the intermediate formation of Cu−Zn compounds with variable compositions (Cu1−xZnx (x < 0.25)). It is worth noting that the FWHM of the (111) peak ascribed to Cu3Zn (Figure 6g) is barely different from that of the Cu0 phase (Figure 6a), indicating that the size of Cu3Zn particles is conditioned by that of the Cu0 particles; here, both average sizes are 20−22 nm. Complementary characterization of this phase transition was made by UV−visible spectroscopy on the 10 wt % IWI-Nit sample precalcined at 400 °C (Figure 7). After reduction at 280

7g) can be attributed to Cu0 and Cu3Zn, respectively, while the other intermediate bands (Figure 7b−f) can be related to the coexistence of both Cu0 and Cu1−xZnx with x < 0.25, as noted previously in the XRD pattern (Figure 6b−f) where a shoulder or a peak broadening at 43.30° is detected. Blue-shift was also observed by Cokoja et al. with colloidal α-Cu1−xZnx from 573 nm (pure copper nanoparticles) to 535 nm (Cu0.5Zn0.5 colloids).52 3.2.2. Discussion on the Formation of Cu3Zn. To our knowledge, the formation of Cu3Zn has never been observed before in Cu/ZnO-based samples. However, according to van Herwijnen and de Jong,56 who studied the stability of a commercial catalyst, 32 wt % CuO/62 wt % ZnO/2 wt % Fe2O3, under 5% H2/N2, zinc oxide reduction, proceeds at temperatures above 300 °C with the concomitant formation of α-brass, for example, Cu−Znx with x = 0.045 at 350 °C. According to the authors, at lower temperatures, zinc diffusion limitations would limit the zinc content into copper to x = 0.01 at 200 °C; note that this temperature may be also too low to reduce Zn(II). However, Fujitani and Nakamura24 reported the formation of Cu−Zn alloy in Cu/ZnO samples prepared by coprecipitation and reduced under 50 atm of 10% H2 in He at 250 °C through an increase of the lattice constant of Cu0 with the increase of ZnO content. For ZnO content higher than 40%, they found that the lattice constant of Cu reached a maximum of about 3.64 Å, which was estimated to correspond to a Zn content of 13−17% of Zn in the alloy. In our case, a higher zinc content is achieved, 25%, with the formation of the definite Cu3Zn alloy at 350 °C. Sanches et al.25 also reported the formation of Cu−Zn alloy but not that of Cu3Zn in Cu/ ZnO materials prepared by precipitation after calcination at 400 °C and reduction at 250 °C under H2. Cu and Zn are well-known to form bulk alloy, that is, brass, and the Cu−Zn bulk phase diagram indicates high miscibility of zinc and copper and the existence of several ordered phases.57 During alloy formation in Cu/ZnO58 and in Zn deposited on Cu(111),59 several processes would coexist, such as diffusion and dissolution. According to Spencer,58,60 the formation of brass is limited by the diffusion rate of Zn in Cu particles and becomes fast above 400 °C. Grunwaldt et al.27 proposed a model for the mechanism of formation on Cu−Zn alloy based on an EXAFS study of Cu/ZnO sample prepared by coprecipitation. Based on the presence of Zn0 species and oxygen vacancies at the boundary of the Cu0 particles, they propose the formation of surface Cu−Zn alloy on Cu particles after reduction (10% CO in H2) at 300 °C and that of bulk Cu−Zn alloy particles under stronger reducing condition at 600 °C, that is, at temperature significantly higher than in the present study. One can note that Burch et al.61 proposed that during thermal treatment of Cu-ZnO catalyst under hydrogen, hydrogen spills over ZnO, allowing ZnO to act as a hydrogen reservoir. This would allow the generation of reduced Zn0 species and oxygen vacancies at the boundary of the Cu0 particles. From these results, we anticipate that a slow kinetic process of transformation of Cu0 into Cu3Zn occurs and depends on the reduction temperature. Temperature increase leads to the reduction of zinc species and then to the diffusion of zinc into the copper particles. The kinetics of diffusion/dissolution of zinc into the copper particles is probably also dependent on the initial size of the Cu0 particles, and since there is a distribution of Cu0 particle size in the sample, this may explain the coexistence of Cu0 (probably the largest ones) and Cu1−xZnx

Figure 7. UV−visible spectra of sample 10 wt % IWI-Nit calcined at 400 °C followed by H2 reduction at different temperatures and for different times: 1 min at (a) 280 °C, (b) 330 °C, (c) 340 °C, and (d) 350 °C and at 350 °C for (e) 5 min, (f) 30 min, (g) 2 h; (h) pure ZnO; (i) IWI-Nit calcined at 400 °C followed by CO-reduction at 350 °C for 2 h; and (j) IWI-Nit calcined at 400 °C followed by a successive reduction under H2 at 350 °C for 2 h and then CO/N2 at 350 °C for 2 h.

°C for 1 min, an absorption band is detected at 556 nm (Figure 7a), which can be attributed to the surface plasmon resonance (SPR) of metallic copper particles.53−55 Note that the XRD patterns were also recorded in air and showed peak characteristics of metal particles, which proves that the absorption band observed in Figure 7 cannot result from that of Cu(II) species (Figure 2a). When the reduction temperature is increased up to 350 °C and the duration of the temperature plateau is increased, the band initially at 556 nm (Figure 7a) shifts toward lower wavelengths: 552, 545, 532, 524, 511, and 503 nm (Figure 7b−g, respectively). By analogy with the XRD results, the bands at 556 nm (Figure 7a) and 503 nm (Figure 2288



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with H2, which favored the formation of Cu−Zn alloy in Cu/ ZnO. Upon subsequent H2-reduction at 350 °C for 2 h (Figure 8b), the two XRD lines shift to lower angle close to those of Cu3Zn (Figure 6g), albeit with Δθ of ≈0.15° compared to Cu3Zn, indicating the formation of Cu−Zn alloy with a stoichiometry close to Cu3Zn. Conversely, Figure 8c shows the XRD pattern of the sample treated under 1% (v/v) CO/N2 after the former reduction under H2 at 350 °C for 2 h. It exhibits peak positions intermediate between those of Cu0 and those of Cu3Zn, that is, of Cu1−xZnx with x < 0.25. UV−visible spectroscopy was also performed on the same sample. Figure 7i shows the UV−visible spectrum of 10 wt % IWI-Nit calcined at 400 °C then reduced under 1% (v/v) CO/N2 at 350 °C for 2 h. It exhibits an absorption band at 559 nm, very close to that at 556 nm of Cu0 obtained after reduction under H2 at 280 °C (Figure 7a). After further H2 reduction at 350 °C for 2 h, the band is at 503 nm (Figure 7g) and can be attributed to Cu3Zn. Another subsequent CO reduction at 350 °C for 2 h leads to a band at 538 nm (Figure 7j) close to that obtained after H2 reduction at temperatures between 330 °C (545 nm, Figure 7c) and 340 °C (532 nm, Figure 7d), which suggests the formation of Cu1−xZnx with x < 0.25. As in Section 3.1, the UV−visible results are consistent with the XRD data. This set of experiments show that, in severe temperature reduction conditions (350 °C), the formation of CuZn surface alloy can be avoided by the choice of an appropriate reductant (here CO in N2). Moreover, once the Cu3Zn phase is formed, it does not undergo complete reversible transformation back to the Cu0 phase in 1% CO/N2 gas mixture. It is worth noting that the UV−visible bands or XRD peaks linked to the Cu3Zn phase do not vanish or shift after several days of air exposure, suggesting that the particles are not as sensitive to air as Cu0 particles for which fast reoxidation process was evidenced.52,65 We also observed that a subsequent calcination at 400 °C reoxidizes the particles (Cu0 and Cu3Zn) into copper oxide but that a further reduction under H2 or under 1% CO/N2 at 350 °C for 2 h restores the Cu3Zn and Cu0 phase, respectively, with identical particle size, as evidenced both by TEM and XRD (results not shown). According to the XRD and UV−visible results (Figures 6 and 7), Cu/ZnO samples exhibit a Cu0 supported phase after reduction under H2 at temperature lower than ∼300 °C, a Cu− Zn alloy phase for reduction at temperature higher than ∼300 °C, and Cu3Zn at 350 °C. When the reduction gas is changed, from H2 to CO/N2 at 280 °C, this does not lead to further modification of the Cu0 phase (Figures 6a and 8a). Therefore, under H2, the temperature of 280 °C is certainly low enough for the reduction and diffusion of zinc into the copper particles to be neglected. As the reduction temperature increases, the diffusion of zinc into the copper becomes possible, and alloying starts to form. However, this does not occur with 1% CO/N2 but only with H2. This suggests that even if the temperature is high enough for zinc migration to occur significantly, this is the nature of the gas that governs the formation of the Cu−Zn alloys.

(probably the smallest ones) in the sample after moderate reducing conditions (Figures 6 and 7). It is possible that the mechanism of Cu−Zn alloy formation is still more intricate and is also determined by the nature of the precursor. Indeed, Sanches et al.25 also reported the formation of Cu−Zn alloy in Cu/ZnO as a result of the presence of aurichalcite ((Zn,Cu)5(CO3)2(OH)6) formed during coprecipitation using Na2CO3 as precipitating agent. This is a point that was not checked by the former authors cited above. In the present system where copper is deposited on ZnO, the formation of mixed compound is less probable. 3.3. Implication of the Nature of the Reduction Route on the Changes in Surface Structures. Cu3Zn particles have been identified after H2-reduction pretreatment at 350 °C for 2 h whereas Cu1−xZnx of intermediate stoichiometry was obtained under softer reduction conditions (lower temperature or shorter reduction time at 350 °C). However, it must be kept in mind that the chemical state of catalyst during reaction can differ from its state after pretreatment, as pointed out for instance by Topsøe et al.62,63 Since carbon monoxide is also one of the reactants in methanol synthesis, its possible influence on the formation and stability of the Cu-based supported phase was investigated. Figure 8a shows the XRD pattern of the 10 wt % IWI-Nit sample (in the range 2θ = 40−55°) obtained after calcination at

Figure 8. XRD patterns (in the range 2θ = 40−55°) of 10 wt % IWINit calcined at 400 °C followed by reduction under different gas: (a) 1% CO/N2 at 350 °C for 2 h; (b) 1% CO/N2 at 350 °C for 2 h and then H2 at 350 °C for 2 h; (c) H2 at 350 °C for 2 h and then 1% CO/ N2 at 350 °C for 2 h. ▲ and ●: the most intense peaks from the JCPDS files of Cu0 (#04-0836) and Cu3Zn (#65-6567), respectively.

400 °C for 2 h and reduction under 1% (v/v) CO/N2 gas mixture at 350 °C for 2 h. It shows the (111) peak at 43.30° and the (200) peak at 50.43°, characteristic of the Cu0 phase. Similar XRD patterns with almost the same intensities are obtained after reduction at lower temperature, between 280 and 350 °C. This indicates that the Cu0 particles, which are formed at 280 °C under 1% (v/v) CO/N2, remain stable up to at least 350 °C and that Cu3Zn does not form under this gas. This may be due to the lower reduction potential of CO diluted in N2 compared to H2 as attested by an elementary thermodynamic calculation. Consistently, Liu et al. also observed the formation of Cu0 particles (XRD and XPS techniques) in Cu/ZnO catalysts with 5−44 wt % Cu calcined in air at 350 °C and then reduced in 15% CO/N2 at 300 °C for 1 h,8 and Wagner et al.64 reported a larger reduction potential of H2/CO as compared

4. CONCLUSION In this contribution, TEM, XRD, and UV−visible techniques were combined to investigate the influence of different conditions of preparation of Cu/ZnO materials. The assynthesized samples were subjected to various post-synthesis treatments, which were found to result in metal particles of 2289



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different nature and size. The Cu/ZnO samples reduced under H2 at 280 °C for 2 h lead to the formation of Cu0 particles. As reduction temperature increases up to 350 °C and duration at 350 °C increases, XRD and UV−visible techniques reveal a progressive transformation of the metallic phase into Cu−Zn alloy: Cu0 → Cu1−xZnx (x < 0.25) → Cu3Zn. To our knowledge, this is the first time that the Cu3Zn alloy is evidenced in a Cu/ZnO sample. It was also found that at 350 °C, the formation of Cu3Zn can be avoided by the choice of an appropriate reductant (here 1% (v/v) CO/N2). The size of the Cu3Zn particles in Cu/ZnO is determined by the size of Cu0 particles and can be tuned from 22 to 4 nm, depending on the synthesis route and the copper precursor: DPU-Nit > IWI-Nit > IWI-NH3 > IWI-EN > IWI-AcAc, and this has been related to the strength of the ligand field of the copper precursor. A precalcination step prior to reduction significantly affects the particle size distribution in some of the samples, IWI-EN and IWI-AcAc, leading to smaller particles of 6 and 4 nm, respectively; for the other samples, DPU-Nit, IWI-Nit, and IWI-NH3, no changes are evidenced. In following of this work, the catalytic properties of Cu3Zn/ZnO and Cu0/ZnO will be compared in a suitable reaction, such as methanol synthesis.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], tel.: +33 1 44 27 30 50, fax: +33 1 44 27 60 33. Present Address

‡ On leave at Veolia Environnement Recherche & Innovation, Département Génie des Procédés, Centre de Recherche de Maisons-Laffitte, Chemin de la Digue − B.P. 76, 78603 Maisons-Laffitte Cedex, France.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the ANR (Agence Nationale pour la Recherche), which sponsored this work (program ANR-08EFC). We would also like to thank colleagues of our laboratory, Dr. Laurent Delannoy, Dr. Camille Lafontaine, Dr. Guylène Costentin, and Sandra Casale for fruitful discussions.

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REFERENCES

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