Ind. Eng. Chem. Res. 2010, 49, 5959–5968
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Cu-Zn-(Mn)-(Fe)-Al Layered Double Hydroxides and Their Mixed Metal Oxides: Physicochemical and Catalytic Properties in Wet Hydrogen Peroxide Oxidation of Phenol L. H. Zhang,*,†,‡ F. Li,*,‡ D. G. Evans,‡ and X. Duan‡ Department of Catalysis Science and Technology and Tianjin Key Laboratory of Applied Catalysis Science & Technology, School of Chemical Engineering, Tianjin UniVersity, Tianjin 300072, P.R. China, and State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, P.R. China
Cu-Zn-Mn-Fe-Al layered double hydroxides (LDHs) with Cu/Zn/Mn/Fe/Al atomic ratios of 1/1/0/0/1, 1/1/1/0.3/0.7, and 1/0.7/0.3/0.3/0.7, respectively, in synthesis mixture were prepared by coprecipitation under controlled conditions of temperature and pH. The mixed oxides were obtained by decomposition of the precursors at 500 °C in air. The characterization has been performed by XRD, ICP-ES, FT-IR, TG-DTA, TG-DTA-MS, TEM, XPS,O2-TPD, and H2-TPR. The objective is to investigate the effects of Mn and Fe partial substitution into Cu-Zn-Al mixed metal oxide derived from LDH precursor on the physicochemical and catalytic properties. The results show that the Fe and Mn substitutions decrease the structural stability of LDHs and improve the redox behavior of calcined LDHs. The activity of catalytic wet hydrogen peroxide oxidation of phenol is proportional to the content of surface metal ions and mainly Cu2+ centers linked to the surface lattice oxygen, and the degree of the deep oxidation of it is mainly related to the reactivity of the weakly bonded surface oxygen, depending on the nature of the transition metal ions in the structure. The preoxidation of phenol by surface •OH radical originating from hydrogen peroxide favors the next deep oxidation. 1. Introduction Layered double hydroxides (LDHs) belong to a large class of natural and synthetic anionic clays.1,2 They are less diffuse in nature than cationic clays, but they can be easily synthesized. Particularly, thermal treatments of these compounds lead to mixed oxides with higher surface area and higher dispersion and more homogeneous distribution of the metal cations than multimetallic mixed oxide obtained from conventional method, which are attractive properties for them as catalyts.3-5 Furthermore, the opportunity to control the composition of the hydroxide layers of the LDHs precursor allows tuning of the catalytic properties of the resulting catalyst.5-7 Cu-based mixed oxides systems are widely known as industrial catalysts for catalytic wet oxidation (CWO)8-11 and the production of methanol,12 higher alcohols,13 and the steam re-forming of methanol.14,15 A sort of synergic effect between the different components of the mixed oxides allows the catalytic activity to take place under relatively mild conditions (P ) 50-100 atm, T < 450 °C). It has been shown that the presence of Mn and Fe in some Cu-based catalysts results in better catalytic activity toward CWO.11,16 The Cu-, Mn-, and Fe-based oxide catalysts have been used individually or paired together in CWO, especially in the oxidation of phenol aqueous solutions.16-18 However, until now, no report is available on the synthesis and physicochemical and catalytic properties of a quinary mixed metal oxide containing Fe and Mn in the Cu-Zn-Al oxide system derived from a single LDH phase. * Corresponding authors. L.H.Z.: e-mail,
[email protected]; phone, +86-22-27405297. F.L.: e-mail,
[email protected]; phone, +8610-64451226. † Tianjin University. ‡ Beijing University of Chemical Technology.
The present work on the Cu-Zn-Mn-Fe-Al system is aimed at the synthesis, characterization, and preliminary evaluation of catalytic performance of well-dispersed Cu-Zn-Al, Cu-Zn-Fe-Al, and Cu-Zn-Mn-Fe-Al mixed oxides obtained by thermal treatment of LDHs precursors. The Cu2+/ M2+/M3+ atomic ratio of 1/1/1 has been maintained in all the samples. This is because an earlier study4 has demonstrated that a Cu-Zn-Al catalyst containing ca. 33 mol % Cu was the most active for the oxidation of aqueous phenol solutions reactions. A comparative study of partial substitution of Fe for Al and Mn for Zn in the Cu-Zn-Al system on their physicochemical and catalytic properties was performed by employing various spectroscopic techniques and activity tests. This work is a part of an ongoing research project which aims to improve the performance of Cu-based oxide catalysts for oxidation of aqueous phenol solutions by hydrogen peroxide. 2. Experimental Section 2.1. Preparation of Samples. Three LDH-type precursors with Cu/Zn/Mn/Fe/Al atomic ratios in synthesis mixture of 1/1/ 0/0/1, 1/1/1/0.3/0.7, and 1/0.7/0.3/0.3/0.7 were prepared by a coprecipitation method. An alkaline solution of NaOH and Na2CO3 {[CO32-] ) 2[M3+], [OH-] ) 2(2[M2+] + 3[M3+])} was added dropwise with vigorous stirring into an appropriate metal salt aqueous solution with (Cu2+ + Zn2+ + Mn2+)/ (Fe3+ + Al3+) atomic ratio equal to 2 and a total cation concentration of 1.2 mol/L until the solution pH was 10.0. The resulting slurry was aged at 60 °C for 10 h with stirring, filtered, and washed with deionized water several times until the pH of the filtrate was around 7. For the Mn-containing sample, these processes were manipulated under nitrogen atmosphere, and even the deionized water was also treated by nitrogen. The filter cake was dried at 60 °C in an air oven overnight. Their
10.1021/ie9019193 2010 American Chemical Society Published on Web 06/07/2010
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corresponding calcined products were obtained by calcining at 500 °C for 3 h in air. The precursors obtained in this way were denoted by acronyms with representative elemental composition and layered double hydroxide. For example, precursor with Cu/Zn/Mn/Fe/ Al atomic ratios of 1/1/0/0/1 can be written as Cu-LDH, the corresponding calcined sample is marked as Cu-CLDH. 2.2. Characterization Techniques. Power X-ray diffraction (XRD) patterns were obtained with a Shimadzu XRD-6000 X-ray diffractometer using 40 kV, 30 mA, Ni-filtered Cu KR radiation (λ ) 1.5406 Å with a scan speed of 2θ ) 5°/min). Elemental analysis was performed by an inductively coupled plasma emission spectrometer (ICP-ES) (Shimadzu ICPS-7500 model). 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). The Fourier transform infrared (FT-IR) spectra were recorded on a Bruker vector-22 Fourier transform infrared spectrometer in the 4000-400 cm-1 wavenumber range using pressed KBr pellets. The thermogravimetric combined with mass spectrometric (TG-DTA-MS) measurements of the samples (ca. 10 mg) were carried out using a Seiko 6300 TG/DTA/DSC synchronization thermal analyses apparatus from room temperature to the setting temperature. The heating rate of 10 °C/min in nitrogen with flow rate of 200 mL/min was used. The gases that evolved during the thermal decomposition process were continuously monitored with a quadrupole mass spectrometer (ThermoStar QMS 200) connected online to the microbalance by a quartz capillary transfer line at 190 °C to prevent the condensation of evolved gases. Transmission electron microscopy (TEM) studies were performed using a Hitachi H-800 model machine for highresolution observation. The accelerating voltage applied was 100 kV. Specimens for TEM were prepared by standard techniques. O2 temperature programmed desorption (O2-TPD) experiments were performed in a continuous flow reactor system, operated at atmospheric pressure with online TPX-MT24 thermal analytic system analysis of desorption products. Briefly, 50 mg of catalyst was loaded into a quartz reactor tube and supported on quartz sieve plate. The loaded reactor tube was then positioned in the heating zone of a temperature programmable furnace. A K-type thermocouple embedded in the catalyst bed was used to monitor catalyst temperature. Before the experiments, catalyst samples were purge at 200 °C in flowing N2 for 1 h to remove surface impurities (water and possible hydrocarbon). Unless removed, the surface impurities reacted with oxygen at the catalyst surface during the TPD run, thereby complicating the analysis. TPD experiments were then carried out as follows: Catalyst samples were exposed to O2 (20 mL/ min) for 1 h at 450 °C. Following treatment, samples were cooled to room temperature under oxygen. TPD data were then obtained by heating the catalyst from room temperature to ca. 850 at 20 °C/min under a dry N2 purge (35 mL/min). The experimental curve fitted with a program that made use of Lorentzian lines after having subtracted an L-shaped background. The relative amount of desorbed oxygen was expressed as the relative values of peak areas for corresponding TPD spectra. The redox 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 up to 600 °C and held at this temperature for
Figure 1. The XRD patterns of CuZnMnFeAl-LDH with different ratios: (a) Cu-LDH, (b) CuFe-LDH, and (c) CuMnFe-LDH.
20 min. Hydrogen consumption due to the reduction of Cucontaining phases was monitored continuously by a gas chromatograph. The X-ray photoelectron spectroscopy (XPS) results were carried out with a V.G. Scientific ESCALAB Mark II system. Mg KR (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 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 Cu2+ species. All binding energy (BE) values were charge-corrected to the C1s signal, which was 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 a 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. Activity Tests. The catalytic oxidation experiments were carried out in a three-neck glass reactor (250 mL). Hydrogen peroxide (1 mL, 30% w/v) was added at once to the stirred solution of phenol (100 mL, 100 mg/L) containing catalyst (0.2 g) kept at room temperature for 60 min. The concentrations of phenol and those of the oxidized products were analyzed by Table 1. Analytical and Structural Data for as-Synthesized Samples sample Cu:Zn:Mn:Fe:Al atomic ratioa M2+/M3+ atomic ratioa XRD phaseb obtained d003/Å for LDH d006/Å for LDH d009/Å for LDH d110/Å for LDH fwhm[003] for LDH fwhm[006] for LDH lattice parameterc,a (Å) lattice parameter,d c (Å) Le(nm)
Cu-LDH
CuFe-LDH
CuMnFe-LDH
1:1:0:0:0.80 1:1.03:0:0.27:0.55 1:0.72:0.29:0.27:0.58 2.50 LDH 7.58 3.77 2.59 1.53 0.37 0.29 3.07 22.75 21.89
2.48 LDH 7.50 3.76 2.58 1.53 0.53 0.31 3.07 22.49 14.74
2.36 LDH 7.46 3.75 2.59 1.53 0.45 --3.06 22.40 17.00
a Chemical analyses results obtained from ICP. b LDH ) layered double hydroxide. c a ) 2d110. d Average value calculated from (003), (006), and (009) reflections. e Value in crystallite size in c direction calculated from the Scherrer equation calculated from the fwhm of (003) and (006) reflections (see the text).
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Figure 2. The FT-IR spectra of CuZnMnFeAl-LDH with different ratios: (a) Cu-LDH, (b) CuFe-LDH, and (c) CuMnFe-LDH. Table 2. FT-IR Band Assignments (cm-1) for CuZnMnFeAl-LDH sample 2- a
ν3(CO3 ) ν(OH)b δ(H2O)c
Cu-LDH
CuFe-LDH
CuMnFe-LDH
1360 3426 1629
1365 3444 1626
1364 3442 1629
a The ν3 stretching vibration of the interlayer CO32- anion. b The overlapping stretching vibration of the hydroxyl (OH) groups arising from metal-hydroxyl groups in the layers and hydrogen-bonded interlayer water molecules. c The deformation mode of water molecules.
HPLC. Aliquots of 5 µL were injected into a reverse-phase C-18 column, with a mixture of 30% methanol and 70% redistilled water as a mobile phase at a total flow rate of 0.8 mL/min. The absorbance at 280 nm was used to measure the concentration of phenol. 3. Results and Discussion 3.1. Structure, Composition and Morphology of the Precursors. The powder XRD patterns of the as-synthesized materials with different composition are shown in Figure 1, and the chemical compositions and some structural properties of these samples are summarized in Table 1. It is seen that the XRD patterns of these precursors are similar to those typically reported in the literature for brucite-like LDHs materials.1 The LDH phase (JCPDS file no. 38-0487) is observed as the only crystalline phase in all the samples. In each case, the XRD
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patterns give a series of (00l) peaks indexed as (003), (006), and (009), appearing as narrow symmetric lines at low angle, corresponding to the basal spacing and higher order reflections. Furthermore, we found that the diffraction intensities of pure LDH phase of Cu-LDH are higher than those of other samples, which is due to the higher Al content being helpful to the growth of crystal. There is an obvious shift of (003) diffraction toward high 2θ angles from Cu-LDH to CuFe-LDH, denoting a decrease in the basal spacing. In addition, Figure 2 presents the FT-IR spectra of all samples, and the typical band position and assignment are listed in Table 2. The results reveal bands characteristic of LDHs containing CO32- as the counteranions in the interlayer.1,19 It is well-known that LDHs belongs to the hexagonal system, where the lattice parameter a ()2d110)20 is a function of the average radii of the metal cations in the layers and reflects the density of metal ions stacking in the (003) crystal plane.1 Generally, there are two reasons for the increase of the lattice parameter a. The first is that the decrease of positive charge density in the layers with the increase of M2+/M3+ ratio makes the M2+-M3+-O octahedron larger. The second is that the bigger the radius of metal cations, the larger the distance of metal cations in the layers and vice versa. The lattice parameter c ()(d003 + 2d006 + 3d009))20 is a measure of the thickness of the crystal cell. It depends on the charge density in the layers as well as the nature of the interlayer anion and the water content, and hence no pattern is apparent.1 Usually, the higher the charge density in the layers or the smaller the effective radius of the interlayer anions, the smaller the lattice parameter c. As shown in Table 1, the lattice parameter a of Cu-LDH and CuFe-LDH is identical, in line with the similar value of the actual M2+/M3+ ratio and the radius between Al3+ hexacoordinate (0.54 Å, Shannon ionic radii21) and Fe3+ hexacoordinate (0.55 Å, Shannon ionic radii21), while the slightly lower a value of CuMnFe-LDH is attributed to the lower actual M2+/M3+ ratio rather than the former’s and the substitution of the bigger Zn2+ hexacoordinate (0.74 Å, Shannon ionic radii21) by the smaller Mn2+ hexacoordinate (0.67 Å, Shannon ionic radii21) in the layers. It can be found that the lattice parameter c has some concomitant relation with the actual M2+/M3+ ratio, namely, the higher the charge density in the layers, the smaller the lattice parameter c. A sharper decrease in c parameter is observed from Cu-LDH to CuFe-LDH than that from CuFe-LDH to CuMnFeLDH, suggesting that the decrease is more due to the depletion of Al3+ and less due to the incorporation of Mn2+. The average crystallite size in the c direction size (the stacking direction, perpendicular to the layers) may be estimated from
Figure 3. TEM images of CuZnMnFeAl-LDH with different ratios: (a) Cu-LDH, (b) CuFe-LDH, and (c) CuMnFe-LDH.
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the values of the full width at half-maximum (fwhm) of the (003) and (006) diffraction peaks by means of the Scherrer equation [L ) 0.89λ/β(θ) cos θ, where L is the crystallite size, λ is the wavelength (0.1542 nm) of the radiation used, θ is the Bragg diffraction angle, and, β(θ) is the fwhm].1 The values given in Table 1 (average value of L obtained from the data for the (003) and (006) diffraction peaks) show that the crystallite sizes in the c direction decrease as the M2+/M3+ ratio increases. The presence of a larger number of Al3+ 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 (110) peak,22 the low intensity of this peak coupled with the approximations inherent in the Scherrer equation introduce a large uncertainty into the calculated value. The modification of lattice parameters suggests a distortion of the LDHs network induced in the process of Zn2+ and Al3+ being partially substituting by Mn2+ and Fe3+, respectively. This result can be attributed not only to the different ionic radius of the new cations but also to the different electronic interactions that act inside the network from Cu-LDH to CuFe-LDH and to CuMnFe-LDH. ICP results, together with the structural changes that are revealed by XRD analysis, demonstrate the incorporation of Fe and Mn in the LDHs network. The TEM micrographs in Figure 3 show that the expected hexagonal platelike nature of the crystallites is clearly apparent in each case. The average diameter of the platelets is smaller than 100 nm. Obviously some fragments occur when Fe3+ is introduced into the layers, and platelet particles break into more small pieces for the Mn-containing sample. This result is consistent with the above XRD analysis for the crystallinity superior sequence of three samples being as follow: Cu-LDH > CuFe-LDH > CuMnFe-LDH. 3.2. Thermal Stability of the Precursors. The structure stability of LDHs mainly includes two aspects: the first is the stability of layered structure; the second is the structural stability between interlayer anions and water molecules. The stability of the layered structure is mainly the difficulty in removing the intralayer hydroxyl groups, which is closely correlated to its polarity. The stability of interlayer ions is mainly affected by the nature of ions and the chemical environment provided by the layers. The incorporation of Fe3+ and Mn2+ in the LDHs matrix is clearly seen from the changes in the band position (Table 2). The absorption bands of both ν3(CO32-) and ν(OH) are shifted toward high frequency when Fe3+ and Mn2+ are introduced, presumably due to a decrease in electron density around OH groups resulting in consequent reduction of H-bond attractive force between the intralayer M-OH groups and the interlayer CO32- anions, associated with the increasing bond energy of M-O23 (Cu-O approximately 255 kJ/mol, Fe-O approximately 398 kJ/mol, and Mn-O approximately 402 kJ/mol24) coordinated to OH groups. When the metal ion with higher M-O bond energy is localized in octahedral, the polarity of intralayer OH linked to it can be reinforced. It means the H-bond attractive force between the layers and the interlayer anions and water molecules. Therefore, it is concluded that the interlayer CO32- anions and the intralayer OH groups become easy to remove, and thermal stability is reduced. The structural stability of LDHs was also examined by simultaneous TG-DTA experiments. Partial substitution of Al3+ and Zn2+ cations respectively with Fe3+ and Mn2+ cations resulted in the different thermal decomposition pattern. The results of the TG-DTA analysis of LDHs precursors are
Figure 4. The TG-DTA patterns of CuZnMnFeAl-LDH with different ratios: (a) Cu-LDH, (b) CuFe-LDH, and (c) CuMnFe-LDH.
presented in Figure 4, while the evolved gaseous products of their thermal decomposition are shown in Figure 5. Combined with Figures 4 and 5, three stages can be divided for the mass loss of samples. The first mass loss, from room temperature to approximately 200 °C, is mainly assigned to the removal of the interlayer and weakly adsorbed water (detected as m/z 18, due to H2O); the second gradual mass loss, which occurs in the temperature range 200-300 °C, is mainly ascribed to the dehydroxylation of the lattice; the evolution of CO2, which was formed by thermal decomposition of interlayer CO32-anions, is observed mainly in the 500-700 °C range, and small amounts of CO2 also occur in the temperature range of the first and
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Figure 5. Evolution of the gaseous products of thermal decompositions of CuZnMnFeAl-LDH with different ratios: (a) Cu-LDH, (b) CuFe-LDH, and (c) CuMnFe-LDH.
second weight loss. For sample CuMnFe-LDH, another peak at around 352 °C is exhibited as another distinct evolution stage of CO2, which suggests that the structure of CuMnFe-LDH is more complicated and its mass loss more facile than that of the others. As seen from the peak temperature marked in the DTG curves in Figure 4, the peak positions for the removal of interlayer water, dehydroxylation of the lattice, and decomposition of the interlayer CO32- anions are shifted toward lower temperature with the introduction of Fe3+ and Mn2+, indicating a slight and gradual decrease of the thermal stability of LDHs on going from Cu-LDH to CuFe-LDH and to CuMnFe-LDH. This is also indicative of a decrease in the strength of hydrogen bonds between water molecules and interlayer anions and thus a reduced electrostatic interaction between the layers and the anions, which facilitates the process for mass loss, thus the structural transformation.23 The result coincides with the FTIR data discussed above. 3.3. Structure of the Calcined Productions. Figure 6 shows the XRD patterns of Cu-LDH, CuFe-LDH, and CuMnFe-LDH calcined at 500 °C for 3 h in air. It can be seen that the hydrotalcite layered structure has completely collapsed due to
the removal of structural H2O and CO2 from the interlayer. The composite metal oxide phase containing some metal cations appears and becomes more and more perfect with the continuous introduction of transition metal ions. Even for CuMnFe-CLDH, a well-defined composite metal oxide Cu1.5Mn1.5O4 phase (copper manganese oxide, JCPDS file no. 35-1172) is formed (see Figure 6c). But other amorphous phases containing the metal cations are most likely to be present that cannot be detected by XRD due to their poor and highly disordered crystallization. The crystallinity of the calcined samples is influenced by the structure and composition of the as-synthesized LDHs. 3.4. Copper and Oxygen Species in the Calcined Products. XPS Characterization. The XPS surface analysis was employed to predict the chemical states and the relative concentration of Cu and O element on the surface of calcined samples. Figure 7A presents the main peak at ca. 934.0 eV and shake up satellite peak in the range of 940-945 eV of Cu2p3/2 core level spectra of the calcined LDHs. According to the literature,25,26 we can identify that the copper is present in the +2 oxidation state. The broad main Cu2p3/2 peak and its satellite peak in the range of 930-945 eV can be deconvoluted into
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Figure 6. The XRD patterns of CuZnMnFeAl-CLDH with different ratios: (a) Cu-CLDH, (b) CuFe-CLDH, and (c) CuMnFe-CLDH.
two peaks, respectively. That is to say, there exist two copper species on the surface of the calcined LDHs. The BE and peak intensity of every assigned peak for the samples are summarized in Table 3. The high binding energy (BE) contribution is assigned to Cu2+ in a composite metal oxide phase (CuA2+), and the low one is related to Cu2+ in the CuO phase (CuB2+).27,28 A slightly increased BE value for CuA2+ and CuB2+ in Table 3 can be observed, in accordance with the order of Cu-CLDH, CuFe-CLDH, and CuMnFe-CLDH. This is due to an increased surface interaction between Cu2+ ions and other near groups, which can make a partial composite metal oxide phase change into a spinel-like structure, as has been discussed in the above XRD results and will be verified below by O2TPD and TPR profiles. It can be predicted that the spinel-like structure can enhance the stability of the active center, decrease the leaking metal ions concentration resulting from external condition, and prolong the operational life span of catalysts. But generally, there are few surface defects for the regular spinel-like structure, which will work against the process of the catalytic reaction. Furthermore, according to the proportions of the two chemical states of the Cu2+, as determined from the main peaks as well as their satellite lines in the Cu2p3/2 regions, it is observed that the partial substitution of Fe and Mn for Al and Zn favors thus CuB2+ rather than CuA2+. It is very important to realize oxygen species from O1s information in metal oxides by XPS analysis. Figure 7B displays the O1s core level spectra from the calcined products. It is evident that the signal has been split into two fitted peaks, representing two different kinds of surface species. The first peak in lower BE could be attributed to the lattice oxygen (OI) bound to metal cations of the structure, while the second peak in higher BE could be appearing due to a weak-bonded surface oxygen (chemisorbed oxygen, OII), including adsorbed oxygen species and mainly hydroxyl groups and O- (or O22-).27,29-31 Due to the higher mobility of OII compared to those oxygen bound with lattice cations, the OII is often recognized as the active oxygen species and plays an important role in the complete oxidation reaction of organic substances even at very low temperatures.8,31 The relative proportions of OI and OII species obtained by XPS are presented in Table 3 along with the corresponding BE value. It could be observed that the incorporation of transition metal ions gives rise to that BE shift
slightly toward low value, indicating that the mobility of OI and OII increase from Cu-CLDH to CuFe-CLDH and to CuMnFe-CLDH, which are also proved by O2-TPD and TPR below. As it is clearly found in Table 3, the increase of the relative ratio of OI goes from Cu-CLDH to CuFe-CLDH, and then a sudden decrease appears as a result of the introduction of Mn. Meanwhile, the change of OII shows a reverse trend. The jump of the relative proportion of OI and OII species in samples might be due to the Fe-containing sample having lower electron-donating ability to oxygen, and Mn-doping greatly enhances the electron-donating ability, which results in the changes of amount of oxygen vacancy. Moreover, in contact with the catalytic activity, those results will further show the significance of the proportion of lattice oxygen species and adsorbed oxygen species on the surface of such calcined LDHs. It is noticeable that the variation of the amount and reactivity of surface metal ions is consistent with that of surface lattice oxygen OI. O2-TPD Analysis. O2-TPD analysis was used to identify the oxygen species formed during the catalyst exposure to O2 at appointed temperatures and to characterize the redox properties of oxide catalysts. Figure 8a shows the O2-TPD curve obtained following CuCLDH exposure to O2 for 1 h at 450 °C. The curve contains two major peaks with maxima at 456 and 651 °C, labeled R and β, respectively. It is worth noting that the β peak is narrow and symmetrical, while the broad R peak can be fitted into three peaks, such as R1, R2 and R3. The dotted lines are the results of fitting and the respective peak areas are given in Table 4. Upon the basis of TPD data reported in the literature32,33 and XPS spectra presented above, the low-temperature R1 peak can be attributed to the desorption of weak-bonded surface oxygen, but excluding hydroxyl groups due to the condensate treatment. The next R2 and R3 peaks are likely related to the release of surface lattice oxygen, while the high-temperature β peak centered at 651 °C is associated with the release of lattice oxygen bound to metal cations in the bulk of the catalyst. The existence of R3 peak reflects the diversity of the surface lattice oxygen. Certainly, the removal of lattice oxygen must involve the reduction of metal cations.33,34 Figure 8b,c shows the O2-TPD profiles taken following the same condition as Figure 8a. The β peak shifts about 18 °C toward the slightly lower temperature and its intensity decreases, while its width increases. Even in Figure 8c, the incorporation of Mn makes the β peak split into two peaks at 633 and 760 °C. The appearance of different migration mechanisms of bulk oxygen32 or the existence of different oxygen-coordinated metal ions35 can be used to explain the broadening and split of the β peak with the introduction of Fe and Mn element into the CuCLDH. Simultaneously, the decrease of the β peak maximum implies that the mobility or activity of the bulk oxygen can be improved by Fe and further Mn doping. In addition, it is remarkable that the maximum peak of surface lattice oxygen in every case shifts from 456 to 379 °C with the addition of Fe and further Mn element to the structure. The decrease of the R peak temperature is associated with the increase of the interaction among metal ions. Just this interaction weakens the M-O bond and thereby increases the mobility or reactivity of the R oxygen. In the reduction process, the more active the oxygen ion, the more easily the hydrogen attacks it and reduces the oxide catalyst, which is further proved by H2-TPR below. Except the peak temperature, the amount of oxygen released also varies from Cu-CLDH to CuFe-CLDH and to CuMnFeCLDH, as shown in Table 4. Compared to Cu-CLDH, a
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Figure 7. XPS of the Cu2P3/2 (A) and O1s (B) regions of the calcined samples: (a) Cu-CLDH, (b) CuFe-CLDH, and (c) CuMnFe-CLDH.
significantly diminished β oxygen desorption together with an enhanced release of surface lattice oxygen for CuFe-CLDH and CuMnFe-CLDH can be attributed to the migration of the β oxygen from the bulk to the surface,32,36 just which replenishes and enhances the surface lattice oxygen during the hightemperature desorption processes. H2-TPR Study. The reducibility of the calcined products was investigated by temperature-programmed reduction (TPR) experiments, and the profiles are displayed in Figure 9. All the samples exhibit a reduction profile together with shoulders in the temperature range 200-400 °C. Both the content of the
aluminum and the type and amount of the transition metal have an important influence on the reducibility of the catalyst. The peak at about 350 °C, which is related to the reduction of Cu2+ to Cu0 in the composite metal oxide phase containing copper37-39 in Cu-CLDH (Figure 9a), shifts toward lower temperature with decreasing Al content in the structure, and even the reduction peak of CuMnFe-CLDH is lower by about 16 °C compared to the CuFe-CLDH. The reason for this is that the interaction between Cu and Mn makes Cu2+ more active and easy to reduce in the reduction process. Obviously, the frontal peak intensity gradually increases with the increase of component number, but
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Table 3. XPS Characteristics of Cu2p3/2 and O1s Regions for the Calcined CuZnMnFeAl-LDHs sample binding energy (eV)
CuB2+ CuA2+ CuB2+ sat. CuA2+ sat. OI OII I(CuB2+)b I(CuA2+)b I(OI)c I(OII)c
Cu2P3/2
O1s peak intensity (%)
Cu2P3/2 O1s
a Number in parentheses refer to fwhm in eV. peaks as percent of the total O1s area.
b
Cu-CLDH 933.1 (2.5)a 934.5 (3.2) 940.6 (3.5) 943.2 (3.5) 529.8 (2.5) 531.3 (2.8) 51.1 48.9 58.6 41.4
CuFe-CLDH 933.2 (2.5) 934.7 (3.1) 940.2 (3.2) 942.5 (3.0) 529.7 (2.5) 531.2 (2.5) 53.1 46.9 60.3 39.7
CuMnFe-CLDH 933.4 (2.4) 934.8 (3.2) 940.5 (2.8) 942.9 (3.0) 529.5 (2.0) 531.0 (2.6) 56.0 44.0 48.6 51.4
Intensity of the peaks (main peak and sat.) as percent of the total Cu2p3/2 area.
c
Intensity of the
Table 4. Relative O2-TPD Area for the Calcined Samples sample
R1
R2 + R3
β
Cu-CLDH CuFe-CLDH CuMnFe-CLDH
1 1.266 1.200
1 1.576 1.280
1 0.598 0.735
like solid solutions41 in the Cu-Zn-Mn-Fe-Al-O system obtained by the strong interaction between Cu and Mn. It is worth mentioning that the transformation of small amounts of Fe3+ to Fe2+ and Mn4+ to Mn3+ in the CuFe-CLDH and CuMnFe-CLDH catalyst may take place in the same temperature range.42 In any case, based on the above O2-TPD and H2-TPR results, we can draw a conclusion that the redox properties of the active Cu2+ centers in the Cu-CLDH can be improved by the partial substitution of Fe for Al and further Mn for Zn. Meanwhile, the TPR result can definitely explain the reactivity of the weakly bonded surface oxygen and lattice oxygen in the system.43,44 Under the same conditions, the lower the reduction temperature, the stronger the reactivity of oxygen species is. It is clear that the peak maximum for TPR profiles decreases in the following order Cu-CLDH > CuFe-CLDH > CuMnFeCLDH, indicating that the mobility or reactivity of the oxygen follows an increasing trend in accordance with the order from Cu-CLDH to CuFe-CLDH and to CuMnFe-CLDH. 3.5. 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
Figure 8. O2-TPD curves of CuZnMnFeAl-CLDH with different ratios: (a) Cu-CLDH, (b) CuFe-CLDH, and (c) CuMnFe-CLDH.
the peak position stays dependent on the chemical states of Cu2+. This result combined with XRD and XPS analysis indicates that the frontal peak can be attributed to the reduction of the isolate microcrystal CuO40 and the weak-bonded surface oxygen. For the Mn-doped sample, besides the two reduction peaks, there is a weak reduction peak at high temperature (Figure 9c), arising from the reduction of a very small amount of spinel-
Figure 9. TPR curves of CuZnMnFeAl-CLDH with different ratios: (a) Cu-CLDH, (b) CuFe-CLDH, and (c) CuMnFe-CLDH.
Ind. Eng. Chem. Res., Vol. 49, No. 13, 2010 Table 5. Activity and Selectivity for the Calcined Samples product distribution (%) sample
PhOH conversion (%)
HHQ
HQ
CTC
others
Cu-CLDH CuFe-CLDH CuMnFe-CLDH
72.7 100.0 14.6
6.7 0 0
3.7 5.0 0
10.6 0 0
79.0 95.0 100.0
Scheme 1
products (others) such as acetic acid and acetone. The conversion of phenol (PhOH) and the product distribution obtained with different samples is summarized in Table 5. The partial replacement of Al by Fe gives rise to a marked increase in the conversion of phenol and selectivity of the deep oxidation products. The further partial replacement of Zn by Mn also modifies the selectivity to the deep oxidation products, but decreases conversion of phenol to the lowest value. The differences of performance between catalysts originate from the differences of its structure. According to the structural characteristics of the calcined LDHs, it can be found that the differences in activities are correlated with the relative content of surface metal ions, and mainly Cu2+ centers linked to the surface lattice oxygen OI, while the degree of deep oxidation of phenol, namely the selectivity of others, follows the mobility of surface oxygen species OII, depending on the interaction strength between Cu2+ ions and other transition metal ions in the calcined products. On the basis of the above analysis and some proposed schemes about similar reaction system,8,45 a schematic representation of all the important steps in the oxidation of aqueous phenol solutions by hydrogen peroxide is constructed in Scheme 1. In the proposed scheme, the reaction is initiated by the adsorption of hydrogen peroxide on the catalyst surface and the simultaneous liberating of •OH radicals by oxidizing the surface metal ions (Mn+), especially active Cu2+ centers, to +(n + 1) ions. Obviously, the amount of •OH radicals generated correlates positively with the content of surface metal ions. Next, the •OH radicals will participate in the partial oxidation of phenol to produce diphenols. This explains why the phenol conversion hinges on the amount of surface metal ions. After hydroxylation, these diphenols are further oxidized and even broken down into small molecular substances by the weakly bonded surface oxygen OII. The higher mobility of OII, the easier the deep oxidation of diphenols, the higher the selectivity of others. Certainly, the enhancement of the redox properties of active Cu2+ centers by adding Fe and even Mn ions may improve the selectivity of others and the formation rate of •OH radicals as well as the initial activity of phenol hydroxylation. The results indicate that the combined analysis helps in correlating the structure-property relationship with end use properties. In the heterogeneous chemical processes occurring on the solid/liquid interface, other factors influencing the catalytic
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properties of the wet hydrogen peroxide oxidation of phenol, such as the adsorption of reactant phenol molecules, the leaching of active Cu2+ and other metal ions, the recovery of the layered structures, and so on, will be discussed in another paper. 4. Conclusions In our study, it was found that Fe and Mn were promising promoters and changed the structure and performance of Cu-Zn-Al oxide catalyst. In conclusion, we would like to stress the following points. (i) Three single-phase layered double hydroxides (LDHs), containingCu-Zn-Al,Cu-Zn-Fe-Al,andCu-Zn-Mn-Fe-Al, respectively, with well-crystallized hydrotalcite-like layered structure were obtained by coprecipitation method and no excess phases can be detected by X-ray diffraction. (ii) The introduction of transition metal ions in the synthesis mixture decreases the structural stability of LDHs, going from Cu-LDH to CuFe-LDH and to CuMnFe-LDH. (iii) The redox properties of the calcined LDHs can be successively enhanced by partial substitution of Al ions by Fe ions and further partial substitution of Zn ions by Mn ions. (iv) The products calcined at 500 °C consist mainly of two kinds of copper-bearing species, depending on the pristine nature of the starting LDHs precursors, that is, composite metal oxide together with isolate CuO. (v) All calcined samples exhibit two types of oxygen species, one is for the lattice oxygen including surface and bulk lattice oxygen, the other is for the weak-bonded surface oxygen, namely active oxygen. (vi) The Fe-substituted CuFe-CLDH increases the conversion of phenol and the simultaneous substitution of Fe for Al and Mn for Zn in oxide catalyst containing Cu, Zn, and Al decreases it. However, the ability to carry out the deep oxidation can be enhanced by introduction of transition metal ions in proportion to the catalyst system. (vii) The reactivity of surface chemisorbed oxygen as well as the synergy between Cu and adjacent transition metal atoms is responsible for the deep oxidation activity enhancement, while the conversion of phenol is proportional to the content of surface metal ions and mainly Cu2+ centers linked to the surface lattice oxygen. (viii) A new reaction pathway is proposed, namely, the phenol is preferentially oxidized by •OH radical breaking away from HO-OH, which is accompanied by the redox of the surface metal ions. After that, the partial oxidation products are further oxidized by surface oxygen being weakly bound to the metal oxide. It will be of great practical importance to revisit the three catalysts and research the fundamental cause of the high conversion and deep oxidation of phenol in an aqueous medium by hydrogen peroxide using more methods. Acknowledgment We gratefully thank the financial support from the National Natural Science Foundation of China (Grant No. 20901056) and 973 Program (2009CB939802). Literature Cited (1) Cavani, F.; Trifiro`, F.; Vaccari, A. Hydrotalcite-type anionic clays: Preparation, properties and applications. Catal. Today 1991, 11, 173–301. (2) Evans, D. G.; Slade, R. C. T. In Layered Double Hydroxides sStructure and Bonding 119; Mingos, D. M. P.; Duan, X.; Evans, D. G., Eds.; Springer-Verlag: Berlin, 2006; pp 1-87.
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ReceiVed for reView December 4, 2009 ReVised manuscript receiVed May 7, 2010 Accepted May 26, 2010 IE9019193
