Structure and surface characteristics of Cu-based composite metal oxides derived from layered double hydroxides

Materials Chemistry and Physics 87 (2004) 402–410

Structure and surface characteristics of Cu-based composite metal oxides derived from layered double hydroxides Lihong Zhang, Feng Li∗ , David G. Evans, Xue Duan∗ Education Ministry Key Laboratory of Science and Technology of Controllable Chemical Reactions, Beijing University of Chemical Technology, P.O. Box 98, Beijing 100029, PR China Received 9 February 2004; received in revised form 2 June 2004; accepted 9 June 2004

Abstract Layered double hydroxides (LDHs) with Cu2+ /Zn2+ /Al3+ atomic ratios from 1:1:1 to 3:1:1 have been synthesized by coprecipitation method. The physicochemical properties of both the as-synthesized LDHs and their calcined products obtained at 773 K for 3 h have been characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR), chemical analysis, transmission electron microscopy (TEM), scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS), N2 adsorption–desorption experiments and thermogravimetric analysis (TGA). The chemical states of metal species on the surface of the calcined LDHs were also characterized by temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). The results indicate that Cu2+ /Zn2+ /Al3+ atomic ratio in the synthesis mixtures influences crystallinity, purity, and thermal stability of the LDHs, and hence, the composition of the resulting calcined LDHs. At room temperature and atmospheric pressure, oxidation of aqueous phenol solutions by hydrogen peroxide in the presence of the calcined LDHs was carried out. The results show that the calcined LDHs with the Cu2+ /Zn2+ /Al3+ atomic ratio of 1:1:1 has the highest catalytic activity for conversion of phenol, which may be related to the formation of a great amount of composite metal oxide containing Cu2+ ions and to good dispersion property of active Cu2+ centers present on the surface. © 2004 Elsevier B.V. All rights reserved. Keywords: Layered double hydroxide; Composite metal oxide; Surface characteristics; Dispersion

1. Introduction In recent years, there has been increasing interest in developing feasible methods for disposal of toxic wastewater that cannot be treated biologically. Catalytic wet oxidation (CWO) has been shown to be an effective technique for the treatment of organic compounds such as phenol and its derivatives [1,2], which represent an important class of environmental water pollutant found in the effluent from many industries such as petrochemicals, plastics, pharmaceuticals and fine chemicals. The essential idea of the CWO process is to enhance contact between the oxidant and the organic materials to be oxidized. Catalytic wet peroxide oxidation (CWPO) is one type of CWO and uses a liquid oxidizing agent (hydrogen peroxide), which eliminates the transfer problems. As a result, comparatively high oxidation efficiencies are obtained in CWPO and the process can be ∗ Corresponding authors. Tel.: +86-10-6441-2109; fax: +86-10-6442-5385. E-mail addresses: lifeng [email protected] (F. Li), [email protected] (X. Duan).

0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.06.010

performed under relatively mild conditions (such as atmospheric pressure and ambient temperature). Transition metal oxides, especially those containing copper, have shown out good catalytic activity in CWO [3,4]. Commercial processes rely on either supported precious metal and/or base metal oxide catalysts or on homogeneous catalysts containing iron or copper, and the most active catalysts seem to be supported precious metals and homogeneous Cu2+ systems [5]. It would be of interest to investigate in more detail the origin of the synergistic effects displayed by homogeneous Fe–Cu–Mn and heterogeneous Cu–Zn catalytic systems. More recently, other materials have been proposed as catalysts for the oxidation of substituted phenols (such as Cu/Fe supported on alumina or silica, iron/ZSM-5 zeolite, supported metalloporphyrins or metal phthalocyanins), but according to Barrault et al. [6] few of them can be used in practice due to a lack of stability and/or a low activity. Hydrotalcite, [Mg6 Al2 (OH)16 ](CO3 )·4H2 O, is an anionic clay mineral which is the parent of large class of isomorphic compounds known collectively as layered double hydroxides (LDHs), which have recently received significant attention [7,8]. LDHs are lamellar compounds with a struc-

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ture derived from that of brucite, which consists of M2+ ions coordinated octahedrally by hydroxyl groups, with the octahedral units sharing edges to form infinite, charge neutral layers. In an LDH, isomorphous replacement of a fraction of the divalent cations with trivalent cations occurs and generates a positive charge on the layers that necessitates the presence of interlayer charge-balancing anions. Water molecules generally occupy the remaining free space of the interlayer domain. LDHs may be represented by the general formula [M2+ 1−x M3+ x (OH)2 ]x+ (An− )x/n ·mH2 O, where M2+ and M3+ are divalent and trivalent cations, respectively, x is equal to the ratio M2+ /(M2+ + M3+ ), and An− is the interlayer anion with charge n. A wide range of composition can be obtained for these materials by changing the nature of the metal cations and the ratio M2+ /M3+ as well as the type of the intercalated anion. Reported M2+ and M3+ species include Mg2+ , Fe2+ , Co2+ , Cu2+ , Ni2+ , or Zn2+ and Al3+ , Cr3+ , Ga3+ , Mn3+ or Fe3+ , respectively [9–14], and An− can be simple anions [14,15] such as CO3 2− , SO4 2− , NO3 − , F− , Cl− or PO4 3− , or larger species such as heteropolyanions [16,17], organic anions [18–21] or complex anions [22,23]. Hence, their chemical and physical properties can be varied widely. A particularly interesting aspect of LDHs chemistry is their use as catalysts, either as-synthesized or after a controlled thermal treatment, generally around 450–500 ◦ C [8,24–27]. At these temperatures LDHs lose their layer structure and form highly active composite metal oxides with high thermal stability, high surface area, and good metal dispersion which are all very important attributes for a potential catalyst. In this paper, we report the synthesis of LDHs containing Cu2+ , Zn2+ and Al3+ cations using a coprecipitation method. The physicochemical properties of both the LDHs and the calcined products at 773 K have been characterized, and the calcined LDHs were investigated as potential catalysts in the CWPO process for aqueous phenol solution.

2. Experimental 2.1. Sample preparation CuZnAl-LDH carbonates containing Cu2+ , Zn2+ and cations with different Cu2+ /Zn2+ /Al3+ atomic ratios were prepared by coprecipitation method. A solution of NaOH and Na2 CO3 ([CO3 2− ] = 2[M3+ ], [OH− ] = 2{2[M2+ ] + 3[M3+ ]}) was added dropwise with vigorous stirring to 100 ml of an aqueous solution of Cu(II), Zn(II), and Al(III) nitrates (total cation concentration of 1.2 M) with the Zn2+ /Al3+ atomic ratio of 1:1. The addition was monitored by a pH regulator via a pH electrode immersed in the reagent solution and ended at a desired value of pH in the reaction mixture. The resulting precipitate was aged for a fixed period of time at 60 ◦ C with stirring, then recovered by four dispersion and centrifugation cycles in deionized water, and the resulting gelatinous precipitate was Al3+

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finally dried at 60 ◦ C overnight. The LDHs were calcined at 500 ◦ C for 3 h. Synthesized LDHs were denoted LDH-M/N (M/N means the Cu2+ /Zn2+ atomic ratio in the synthesis mixture), and corresponding calcined LDHs were denoted CLDH-M/N. The control sample denoted CS was obtained by calcining a mixture of Cu(II), Zn(II), and Al(III) nitrates (atomic ratio of Cu2+ /Zn2+ /Al3+ = 1:1:1) at 500 ◦ C for 3 h. 2.2. Characterization Powder X-ray diffraction (XRD) patterns of the samples were recorded using a Shimadzu XRD-6000 diffractometer under the following conditions: 40 kV, 30 mA, Cu K␣ radiation. The samples, as unoriented powders, were step-scanned in steps of 0.04◦ (2θ) in the range from 3◦ to 70◦ using a count time of 10 s step−1 . Thermogravimetric analysis (TGA) was carried out on a PCT-1A thermal analysis system produced locally. Samples of around 10.0 mg were heated at 10 ◦ C min−1 to a maximum temperature of 873 K. Elemental analysis was performed on a Shimadzu ICPS-75000 model inductively coupled plasma emission spectrometer (ICP-ES). Samples were dried at 100 ◦ C for 24 h prior to analysis, and solutions were prepared by dissolving the samples in dilute hydrochloric acid (1:1). Surface areas, pore volumes were determined from N2 adsorption–desorption measurements in a Sorptomatic1990 automatic gas adsorption instrument using the BET equation for surface area. Prior to the measurement, the samples were outgassed at 473 K under vacuum for 2 h. Fourier transform infrared spectra (FT-IR) were obtained in the range of 4000–400 cm−1 using the KBr pellet technique on a Bruker Vector-22 Fourier transform spectrometer. Transmission electron microscopy (TEM) was performed using a Hitachi H-800 electron microscope. The accelerating voltage applied was 100 kV. Scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS) analysis was performed using a JSM-630/F apparatus with the analytical software INCA. The accelerating voltage applied was 15 kV. The properties of the catalysts were investigated by means of temperature-programmed reduction experiments (TPR). The calcined LDHs (50 mg) were placed in a quartz reactor and reduced in a stream of H2 (4% H2 + 96% N2 ) with a heating rate of 10 ◦ C min−1 up to 600 ◦ C and held at this temperature for 20 min. Hydrogen consumption due to the reduction of Cu-containing phases was monitored continuously by a gas chromatograph. Relative hydrogen consumption was determined from the area of the TPR peak. The experimental curve fitted with a program that made use of a combination of Gaussian–Lorentzian lines using linear baseline. The XPS results were carried out with a V.G. Scientific ESCALAB Mark II system. A Mg K␣ (hν = 1253.6 eV) was used as X-ray source. The base pressure in the apparatus was about 2 × 10−6 Pa during analysis. Due to the

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C-18 column, with a mixture of 30% methanol and 70% of redistilled water as a mobile phase at a total flow rate of 0.8 ml min−1 . The absorbance at 280 nm was used to measure the distributions of products. The analysis of the total organic carbon (TOC) was conducted with a Shimadzu TOC-5000 analyzer. 3. Results and discussion 3.1. Structural properties of the LDHs and calcined LDHs

Fig. 1. XRD patterns of the CuZnAl-LDHs.

rather high conductivity of the samples, the spectra were recorded at room temperature without further sample treatment and in about 20 min to avoid X-ray induced reduction of the Cu(II) species. All binding energy (BE) values were charge-corrected to the C1s signal which set at 284.6 eV of the carbon overlayer and the standard deviation of the peak position was within ±0.1 eV. This reference gave BE values within an accuracy of ±0.2 eV. Samples were analyzed as powders dusted onto double-sided sticky tape. The hemispherical analyzer functioned with constant pass energy of 50 eV for high-resolution spectra. The experimental bands were fitted with a combination of Gaussian–Lorentzian lines using linear baseline. 2.3. Catalytic reactions The catalytic oxidation process was carried out in a 250 ml three-neck glass flask. Hydrogen peroxide solution (30% (w/v), 1 ml) was added to an aqueous phenol solution (100 mg l−1 , 100 ml) containing 0.2 g of catalyst. The mixture was kept at room temperature for 60 min. The phenol conversion and product distribution were determined by HPLC. Aliquots of 5 ␮l were injected into a reverse-phase

The XRD patterns of the LDHs with different Cu2+ /Zn2+ atomic ratios in the range of 1:1–3:1 are shown in Fig. 1 and the relevant structural parameters given also in Table 1. It can be seen out that the Cu2+ /Zn2+ /Al3+ ratios in the LDHs are similar to those employed in the synthesis mixture, indicating essentially complete precipitation of the metal ions. The LDHs phase (JCPDS file no. 38-0487) were observed as the major phase in all samples [28]. In each case, the XRD patterns exhibit the characteristic reflections of LDH materials with a series of (0 0 l) peaks appearing as narrow symmetric lines at low angle, corresponding to the basal spacing and higher order reflections. Particularly, a single crystalline LDH phase is obtained in LDH-2/1 and LDH-1/1 samples. When the Cu2+ /Zn2+ atomic ratio increases to 3:1, other phases such as copper–zinc carbonate hydroxide (CZH; JCPDS file no. 18-1059) and malachite (MT; JCPDS file no. 41-1390) are formed, as indicated by their characteristic reflections at 2θ values [29] of 14.67◦ and 17.60◦ [30], respectively. This result can be explained by the Jahn–Teller distortion of the coordination environment around the Cu2+ cations leading to instability of the brucite-like layers when the Cu2+ /Zn2+ atomic ratio is high and consequent segregation of other copper-containing phases. Assuming a 3R stacking of the layers and from the positions of the (0 0 3), (0 0 6) and (1 1 0) reflections, the lattice parameters a and c of LDH phases may be calculated [31]. The unit cell parameter a is the average distance between two metal ions in the layers and c is three times the distance from the center of one layer to the next. The value of a (=2d1 1 0 ) is a function of the average radius of the metal

Table 1 Structure and composition parameters of CuZnAl-LDHs prepared using the coprecipitation method Sample

Cu/Zn/Al atomic ratio XRD phases obtaineda Lattice parameter ab (nm) Lattice parameter cc (nm) Ld (nm)

LDH-3/1

LDH-2/1

LDH-1/1

2.83:0.95:1.00 LDH + CZH + MT 0.308 2.292 17.5

1.92:0.96:1.00 LDH 0.306 2.279 17.1

1.03:1.03:1.00 LDH 0.302 2.278 29.5

LDH = layered double hydroxide; CZH = copper–zinc carbonate hydroxide ((Cu, Zn)2 CO3 (OH)2 ); MT = malachite (Cu2 (CO3 )(OH)2 ). a = 2d1 1 0 . c Average value calculated from (0 0 3) and (0 0 6) reflections. d Crystallite size in c direction calculated using the Scherrer equation and the FWHM of (0 0 3) and (0 0 6) reflections (see text). a

b

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cations, and increases with increasing Cu2+ /Zn2+ atomic ratios, reflecting the fact that the ionic radii (Shannon ionic radii [32]) for Cu2+ , Zn2+ and Al3+ are 0.073, 0.074 and 0.054 nm, respectively. The value of c (=(d0 0 3 + 2d0 0 6 + 3d0 0 9 )) is a function of a number of factors including the average charge of the metal cations, the nature of the interlayer anion, and the water content, and hence no pattern is apparent. The average crystallite size in the c direction (the stacking direction, perpendicular to the layers) may be estimated from the values of the full width at half-maximum (FWHM) of the (0 0 3) and (0 0 6) diffraction peaks by means of the Scherrer equation [L = 0.89λ/β(θ) cos θ], where L is the crystallite size, λ the wavelength of the radiation (0.15418 nm) of the radiation used, θ the Bragg diffraction angle, and β(θ) the FWHM [31]. Hence, the crystallite sizes in the c direction are the highest when the Cu2+ /Zn2+ atomic ratio is 1:1. This is because of the fact that the presence of a larger number of Al3+ ions in the layers presumably accelerates the rate of stacking of the layers. Although the average crystallite size in the a direction is sometimes estimated from the FWHM of the (1 1 0) peak [33], the low intensity of this peak coupled with the approximations inherent in the Scherrer equation introduce a large uncertainty into the calculated value. The FT-IR spectra of the LDHs (see Fig. 2) resemble those of other hydrotalcite-like phases [26,34]. Typical of these spectra are the strong broad absorbance band between 3600 and 3200 cm−1 associated with the stretching mode of hydrogen-bonded hydroxyl groups from both the hydroxide layers and interlayer water. A bending vibration band corresponding to a water deformation band, δ(H2 O), is seen at 1630 cm−1 . In most of the samples, there are three IR active absorption bands arising from the carbonate anion observed at 1350–1380 (ν3 ), 850–880 (ν2 ) and 670–690 cm−1 (ν4 ). However, in LDH-2/1 and LDH-3/1, two bands are observed in the region 1350–1400 cm−1 ,

which can be attributed either to the disordered nature of the interlayer or to a lowering of the symmetry of the carbonate anions from D3h to C2v in the interlayer, which lifts the degeneracy of the ν3 mode. Observation of the ν1 mode around 1050 cm−1 also suggests a lowering of the symmetry of the carbonate ion. The bands observed in the low-frequency region of the spectrum correspond to lattice vibration modes and can be attributed to cation–oxygen (M–O) vibrations from 850 to 620 cm−1 and O–M–O vibrations around 440 cm−1 . The TGA/DTG traces (see Fig. 3) for the LDHs indicate two general regions of mass loss. The first one at low temperature, corresponding to the reversible removal of physisorbed and interlayer water (from both the internal gallery surfaces and the external (non-gallery) surfaces) without collapse of the structure, extends from approximately 100 to 200 ◦ C. The second at higher temperature in the range of 200–400 ◦ C arises from the dehydroxylation of the lattice as well as decomposition of the interlayer carbonate anions. Furthermore, the peak temperatures corresponding to the two steps of mass loss in DTG curves shift to lower temperature with decreasing Cu2+ /Zn2+ atomic ratio, suggesting that the LDH sample with low Cu2+ /Zn2+ atomic ratio can be turn into other phases more easily. Fig. 4 shows the XRD patterns of the calcined LDHs. Clearly, calcination has destroyed the layered structure since no characteristic reflections of LDHs are present in the XRD patterns, but some broad diffraction peaks that are obviously different from that of single metal oxide phases such as zinc oxide and copper oxide are detectable in the XRD patterns. It indicates that after calcination composite metal oxide phase containing Cu2+ , Zn2+ and Al3+ cations have developed, although other amorphous phases may also be present but not observed by XRD. When the Cu2+ /Zn2+ atomic ratio is 1:1, dominant composite metal oxide phase is present in addition to little zinc oxide and copper oxide phases.

Fig. 2. FT-IR spectra of CuZnAl-LDHs.

Fig. 3. TG and DTG plots for the CuZnAl-LDHs.

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Fig. 4. XRD patterns of the samples. Fig. 6. IR spectra of the samples.

Furthermore, the expected hexagonal platelet nature of the composite metal oxide phases can be found apparently from the TEM micrographs of the calcined samples illustrated in Fig. 5, and the higher are the Cu2+ /Zn2+ atomic ratios, the more are metal oxide phases. This result is in agreement with the above analysis from the XRD patterns of the calcined LDHs. The FT-IR spectra of the calcined LDHs are shown in Fig. 6. The spectra show a band at 3300–3700 cm−1 , which may be assigned to the stretching mode of the remaining hydrogen-bonded hydroxyl groups. Compared with the as-synthesized LDHs, the intensity of the band is reduced as a result of the removal of physisorbed and interlayer water and the dehydroxylation of the lattice. Significant changes are observed in the low frequency region associated with changes in lattice structure and the intensity of the cation–oxygen vibrational bands is significantly increased compared with the LDH precursors. As expected, IR absorption bands arising from the carbonate anion of the LDHs have decreased in intensity because of decomposition of the interlayer anion.

3.2. Study of surface chemistry of the calcined LDHs The XPS analysis of the Cu2p3/2 region for the calcined LDHs displayed in Fig. 7(a) shows the presence of four peaks in the range of 930–945 eV. However, various studies [35–38] indicated that the position of these peaks depends on the chemical composition of sample and particularly on the near environment of the copper cations. In our case it is possible to distinguish between copper species present in composite metal oxide phase (CuA 2+ ) and copper species in CuO phase (CuB 2+ ), and signals in the calcined LDHs are attributed as it is reported in Table 2. It can be noted that the less Cu content in the calcined LDHs favors thus CuA 2+ rather than CuB 2+ cations. It is well known that ZnO with Zn2+ partially replaced by Al3+ formed n-type semiconductor oxide and then became the acceptor of electrons that results in the decreasing of binding energy [39]. In contact with XPS spectra of Zn 2p regions of the calcined samples presented in Fig. 7(b), the binding energy of Zn 2p decreases as the Cu2+ /Zn2+ atomic ratio decreases,

Fig. 5. TEM micrographs of selected samples.

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Fig. 7. XPS of Cu2p3/2 (a) and Zn2p (b) regions of the calcined CuZnAl-LDHs. Table 2 XPS characteristics of Cu2p3/2 region for the calcined CuZnAl-LDHs Sample CLDH-3/1 2+

Binding energy (eV)

CuB CuA 2+ CuB 2+ sat. CuA 2+ sat.

Peak intensity (%)

I(CuB 2+ )b I(CuA 2+ )c

(2.7)a

933.2 934.7 (3.1) 941.2 (3.3) 943.2 (3.0) 61.7 38.3

CLDH-2/1

CLDH-1/1

933.4 934.8 941.2 943.4

933.3 934.7 941.0 943.4

(2.7) (3.1) (3.4) (3.2)

56.5 43.5

(2.7) (3.1) (3.4) (3.2)

55.6 44.4

a

Number in parentheses refer to FWHM in eV. Intensity of the CuB 2+ peaks (main peak and sat.) in % of the total Cu2p3/2 area. c Intensity of the Cu 2+ peaks (main peak and sat.) in % of the total Cu2p A 3/2 area. b

which can be attributed to the fact that the external electrons have formed the molecular orbital and hence the degree of non-localization increases. The above conclusions are also consistent with the formation of a great amount of composite metal oxide when the Cu2+ /Zn2+ atomic ratio decreases. Fig. 8 shows the TPR profiles of the calcined LDHs, where the total hydrogen consumption corresponds to the reduction of Cu2+ to Cu0 . All the calcined LDHs exhibit two fitting reduction peaks in the temperature range of 200–400 ◦ C with a higher temperature reduction peak for the composite metal oxide phase containing copper and a lower temperature one for free CuO phases. Three different Cu species were observed in the CS sample, where a large reduction peak at ca. 282 ◦ C mainly presented as isolated copper ions, while a smaller shoulder was observed at ca. 252 ◦ C, and copper was mainly present as small copper clusters, the origin of the third small peak at high temperature has been explained in terms of composite oxide containing copper [40,41]. The TPR and BET data for the all samples are summarized in Table 3 . Obviously, with decreasing Cu2+ /Zn2+ ratio the proportion of composite oxide phases in the calcined LDHs and the BET surface areas increase, and the reduction peaks also shift towards lower temperature simul-

taneously. On the other hand, although the total TPR peak area of the CLDH-1/1 is the same as that of the CS sample, there are twice as Cu2+ centers per unit area in CS as that in the CLDH-1/1 according to the relative values of BET sur-

Fig. 8. TPR curves of the samples.

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Table 3 TPR and textural properties for the samples Sample

TPR peak Relative area

CS CLDH-3/1 CLDH-2/1 CLDH-1/1 a b

1.0 1.8 1.4 1.0

TPR peak intensity (%) Peak temperature (◦ C) CuO

Composite oxide

282 (252) 320 282 266

302 337 300 281

I(CuO)a

I(composite oxide)b

91.7 60.7 52.5 31.1

8.3 29.3 47.5 68.9

Intensity of the CuO phase peak in % of the total TPR peak area. Intensity of the composite metal oxide peak in % of the total TPR peak area.

Fig. 9. SEM–EDS images of some representative samples.

Surface area (m2 g−1 )

35.7 35.7 47.4 71.8

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Table 4 Catalytic activity of the samples Sample

LDH-1/1 CS CLDH-1/1 CLDH-2/1 CLDH-3/1

Product distribution (wt.%)

Conversion of PhOH (%)

HHQ

HQ

CTC

Others

7.8 40.9 4.0 0 0

0 4.3 26.1 48.8 54.1

0 0 9.1 20.1 15.3

92.2 54.8 60.8 31.1 30.6

face area, suggesting a great amount of copper centers can cluster on the limited surface area and decrease the effective number of active Cu2+ centers in the CS. The distribution of copper centers on the surface of the calcined LDHs was characterized by SEM–EDS as shown in Fig. 9, where the left diagrams are back scattered electron images of the whole chemical element, and the bright spots in the right diagrams represent distributions of copper elements. It can be found that the distribution of copper centers on the surface in the calcined LDHs is homogeneous whilst a significant number of the copper centers in the CS sample have clustered. With regard to the calcined LDHs, the dispersion of copper centers with low Cu2+ /Zn2+ ratio in bulk is high. The above results show out that the chemical states of metal species on the surface of the samples changes significantly with the copper content and preparation method. 3.3. Catalytic activity Oxidation of aqueous phenol solution with hydrogen peroxide over calcined LDHs was investigated at room temperature. Analysis of the oxidation products by HPLC indicated the presence of phloroglucinol (HHQ), hydroquinone (HQ) and catechol (CTC) as well as deep oxidation organic products such as acetic acid and acetone. According to the values of total organic carbon (TOC) of the representative reactants

36.9 36.9 57.0 33.8 33.3

TOC (mg l−1 ) Reactants

Products

– 78.9 78.9 – –

– 77.8 80.8 – –

and products (see Table 4), the mass balance of the reaction could be well verified. It implies that the reactants and products have not been adsorbed on the surface of catalyst. Moreover, CLDH-1/1 sample was also tested for the adsorption of phenol on its surface in the absence of H2 O2 under the same reaction conditions. The result in Fig. 10 shows that in the absence of H2 O2 phenol is never adsorbed on the CLDH-1/1 sample before contact time of 5 h, further confirming that the conversion of phenol is undoubtedly due to its oxidation in the presence of H2 O2 . The conversion of phenol (PhOH) and product distribution obtained with different samples is summarized in Table 4. It can be seen that the conversion of phenol increases with decreasing Cu2+ /Zn2+ atomic ratio in the calcined LDHs, and hence the CLDH-1/1 sample has the highest catalytic activity. According to the structural characteristics of the calcined LDHs, it can be concluded that high catalytic activity of the CLDH-1/1 may be relative to the formation of a great amount of composite metal oxide in the CLDH-1/1 sample and high dispersion of active copper centers on the surface of the CLDH-1/1 sample. In addition, compared to the calcined CLDH-1/1 sample, the LDH-1/1 precursor has a lower catalytic activity for oxidation of aqueous phenol solution. This is because catalytic active centers on the surface of the Cu-containing LDHs are different from those on the surface of the calcined LDHs, and essentially come from the surface HO–Cu2+ –OH functions [42].

4. Conclusions

Fig. 10. Relationship between the amount of adsorbed phenol on CLDH-1/1 and contact time in the absence of H2 O2 .

LDH carbonates with Cu2+ /Zn2+ /Al3+ atomic ratios from 1:1:1 to 3:1:1 were synthesized by coprecipitation method. The atomic ratios in the synthesis mixture have a significant influence on the properties of the LDHs and their calcined products at 773 K. With decreasing Cu2+ /Zn2+ atomic ratio, crystallinity and purity of the synthesized LDHs phase increase, but the thermal stability decreases. With regard to the calcined LDHs, as Cu2+ /Zn2+ atomic ratio decreases, the amount of composite metal oxide and the dispersion property of copper centers on the surface increase. The calcined LDH with Cu2+ /Zn2+ /Al3+ atomic ratio of 1:1:1 has the highest catalytic activity for oxidation of phenol, which may be related to the formation of a great amount of composite metal oxide and high dispersion of active copper centers present on the surface.

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Acknowledgements We gratefully acknowledge the financial support from the National Natural Science Foundation of China (no. 20306003) and the Beijing Nova Program (no. 2003B10). References [1] Fortuny, C. Bengoa, J. Font, F. Castells, A. Fabregat, Catal. Today 53 (1999) 107. [2] S. Hamoudi, A. Sayari, K. Belkacemi, F. Larachi, Catal. Today 62 (2000) 379. [3] St.G. Christoskova, M. Stoyanova, M. Georgieva, Appl. Catal. A Gen. 208 (2001) 243. [4] A. Pintar, J. Levec, Chem. Eng. Sci 47 (1992) 2395. [5] F. Luck, Catal. Today 27 (1996) 195. [6] J. Barrault, C. Bouchoule, K. Echachoui, N. Frini-Srasra, M. Trabelsi, F. Bergaya, Appl. Catal. B Environ. 15 (1998) 269. [7] A. De Roy, C. Forano, K.E. Malki, J.-P. Besse, in: M.L. Occelli, H.E. Robson (Eds.), Synthesis of Microporous Materials, Vol. 2, Expanded Clays and Other Microporous Systems, Van Nostrand Reinhold, New York, 1992, p. 108. [8] F. Cavani, F. Trifiro, A. Vaccari, Catal. Today 11 (1991) 173. [9] S. Miyata, Clays Clay Miner. 23 (1975) 369. [10] W.T. Reichle, J. Catal. 94 (1985) 547. [11] J.G. Numan, P.B. Himelfarb, R.G. Herman, K. Klier, C.E. Bogdan, G.W. Simmons, Inorg. Chem. 28 (1989) 3868. [12] M.A. Aramendý’a, V. Borau, C. Jime’nez, J.M. Marinas, F.J. Romero, J.R. Ruiz, J. Solid State Chem. 131 (1997) 78. [13] J.M. Ferna’ndez, C. Barriga, M.A. Ulibarri, F.M. Labajos, V. Rives, J. Mater. Chem. 4 (1994) 1117. [14] S. Miyata, Clays Clay Miner. 28 (1980) 50. [15] A. Ookubo, K. Ooi, H. Hayashi, Langmuir 9 (1993) 50. [16] E.W. Serwicka, P. Nowak, K. Bahranowski, W. Jones, F. Kooli, J. Mater. Chem. 7 (1997) 1937. [17] T. Kwon, G.A. Tsigdinos, P. Pinnavaia, J. Am. Chem. Soc. 110 (1988) 3653.

[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]

[30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42]

L. Raki, D.G. Rancourt, C. Detellier, Chem. Mater. 7 (1995) 221. N.T. Whilton, P.J. Vickers, S. Mann, J. Mater. Chem. 7 (1997) 1623. K.R. Franklin, E. Lee, C.C. Nunn, J. Mater. Chem. 5 (1995) 565. C.P. Kelkar, A.A. Shutz, Micropor. Mater. 1 (1993) 33. H.C.B. Hansen, C.B. Koch, Clays Clay Miner. 42 (1994) 170. E. Lo’pez-Salinas, Y. Ono, Micropor. Mater. 10 (1997) 163. F. Basile, L. Basini, M.D. Amore, G. Fornasari, A. Guarinoni, D. Matteuzzi, D. Piero, F. Trifiro, A. Vaccari, J. Catal. 173 (1998) 247. S. Kannan, Appl. Clay Sci. 13 (1998) 347. M.J.H. Hermandez-Moreno, M.A. Ulibarri, J.I. Rendon, C.J. Serna, Phys. Chem. Miner. 12 (1985) 34. A. Alejandre, F. Medina, X. Rodriguez, P. Salagre, Y. Cesteros, J.E. Sueiras, Appl. Catal. B Environ. 30 (2001) 195. C. Busetto, G. Del Piero, G. Mamara, F. Trifir‘o, A. Vaccari, J. Catal. 85 (1984) 260. P. Gherardi, O. Ruggeri, F. Trifirò, A. Vaccari, G. Del piero, G. Manara, B. Notari, in: G. Poncelet, P. Grange, P.A. Jacobs (Eds.), Preparation of Catalysts III, Preparation of Cu–Zn–Al Mixed Hydroxycarbonates Precursors of Catalysts for the Synthesis of Methanol at Low Pressure, Elsevier, Amsterdam, 1983, p. 723. A. Alejandre, F. Medina, X. Rodriguez, P. Salagre, J.E. Sueiras, J. Catal. 188 (1999) 311. F. Millange, R.I. Walton, D. O’Hare, J. Mater. Chem. 10 (2000) 1713. R.D. Shanon, Acta Crystallogr. A 32 (1976) 751. Z.P. Xu, H.C. Zeng, J. Phys. Chem. B 105 (2001) 1743. D.L. Bish, G.W. Brindley, Am. Miner. 62 (1977) 458. J. Töpfer, A. Feld, D. Gräf, B. Hackl, L. Raupach, P. Weissbrodt, Phys. Stat. Sol. A 134 (1992) 405. J. Haber, T. Machej, L. Ungier, J. Ziolkowski, J. Solid State Chem. 25 (1978) 207. P.F. Schoæeld, C.M.B. Henderson, S.A.T. Redfern, G. Van der Laan, Phys. Chem. Miner. 20 (1993) 375. R. Tavares Figueiredo, A. Mart´ınez-Arias, M. López Granados, J.L.G. Fierro, J. Catal. 178 (1998) 146. W.H. Huang, J. Tianjin, Univ. Commerce 2 (1998) 13. L.J. Kundakovic, M. Flytzani-Stephanopoulos, Appl. Catal. A Gen. 171 (1998) 13. W. Liu, M. Flytzani-Stephanopoulos, J. Catal. 153 (1995) 317. K. Zhu, C. Liu, X. Yue, Y. Wu, Appl. Catal. A Gen. 168 (1998) 365.