Materials Science and Engineering B 121 (2005) 216–223
Investigation of phase formation temperature of nano-sized solid solution of copper/cobalt molybdate and chromium–phosphate (M1xCr1−xMoxP1−xO4) [M1 = Co, Cu] T.K. Ghorai a , D. Dhak a , A. Azizan b , P. Pramanik a,b,∗ a b
Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India School of Material and Mineral Resources Engineering, University Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia
Received 30 November 2004; received in revised form 16 March 2005; accepted 28 March 2005
Abstract A series of nano-sized metal–chromium–molybdenum–phosphorous (M1 x Cr1−x Mox P1−x O4 ) [M1 = Co, Cu] oxides solid solutions have been prepared having different percentage of the constituting transition metals from their precursors metal molybdates and chromium phosphate using organic complexing agent TEA (triethanolamine). The variations of phase separation temperature with respect to the amount of different constituents have been studied. The materials are efficient catalyst for reduction of p-nitrophenol to p-aminophenol by sodium borohydride. © 2005 Elsevier B.V. All rights reserved. Keywords: Solid solution; Molybdate; Phosphate; Nano-size; Soft chemical method
1. Introduction Mixed metal oxides play a relevant role in many areas of modern technology and are used as catalysts in a large variety of commercial processes such as cracking, hydrogenation, dehydrogenation, oxidative dehydrogenation and regeneration in various chemical and petrochemical industries [1–3]. Copper chromite has wide applications as catalyst [4–6]. Cobalt chromite has got some excellent catalytic applications [7,8], such as adsorption of vinyl chloride for purification of gas ejection [8] or the combustion of chlorinated organic pollutants [7] has been done over this catalyst. Catalytic applications of Copper molybdates and cobalt molybdates have so far been studied [9–12]. Hydrogenation–dehydrogenation [9], oxidative dehydrogenation [10,11], decomposition [12], etc. can be done by these catalysts. Some mixed transition metal molybdates and chromites also have good catalytic properties [13,14]. Besides the catalytic properties of these ceramic oxides, other ∗
Corresponding author. Mobile: +60125343601; fax: +91 3222 255303. E-mail address: panchanan
[email protected] (P. Pramanik).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.03.031
properties like adsorption, electrical, dielectric, magnetic [15–17], photochemical properties are now widely used for various applications [18]. Copper chromite can be used as ceramic ink [19]. The molybdates of cobalt (II), copper (II) and manganese (II) are used in photoelectrochemical solar cell as semiconductor electrode [20]. Many properties and applications of these ceramics have been investigated [21,22]. The M1 x Cr1−x MoO4 (M1 = transition elements) possess interesting electrical and magnetic properties [23,24] and are used specially in catalytic applications [25]. All the properties of these mixed metal oxides are largely dependent on their microstructure. In the nanoparticle phase, the surface to volume ratio increases drastically and the surface atoms include an increasing fraction of the total particulate volume having high defect structures. Thus they are expected to show drastically improved catalytic properties. The authors have developed the chemical precursor method to prepare the nano-sized metal molybdate, metal tungstate, metal vanadate, and metal phosphate [26–28]. Attempts to prepare the solid solution of CrPO4 and M1 MoO4 in nano-size have been carried out, because the nano-size may stabilize the
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thermodynamically unstable solid solution. CrPO4 is used here to act as a diluent of molybdate salt.
2. Experimental The synthesis of M1 x Cr1−x Mox P1−x O4 (M1 = Co, Cu) has been performed by two steps. First step involved the synthesis of starting materials, cobalt and copper molybdates and chromium phosphate which one shown in Fig. 1. Cobalt and copper molybdates have synthesized by precipitating the corresponding metal nitrate solutions by solution of ammonium molybdates (MERCK) adjusted pH at about 9 using aqueous ammonia. Chromium phosphates has also been prepared in situ taking stoichiometric amount of analytical grade ammonium dichromate ((NH4 )2 Cr2 O7 ) (MERCK) and ammonium di-hydrogen phosphate (NH4 H2 PO4 ) (MERCK) with molar ratio 1:2. For this synthesis, ammonium dichromate ((NH4 )2 Cr2 O7 ) has been reduced to Cr (III) in warm condition adding a few drops of dilute nitric acid and ethanol. After the complete conversion of chromium from (+VI) state to (+III) state, stoichiometric amount of ammonium di-hydrogen phosphate (NH4 H2 PO4 ) has been added to prepare mother solution for formation of CrPO4 . In the second step of this procedure as presented in Fig. 2, the already prepared M1 MoO4 (M1 = Co, Cu) are dissolved separately in dilute nitric acid and mixed well to the mother solution of chromic phosphate (CrPO4 ) in nitric
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acid maintaining different values of x obeying the formula (M1 MoO4 )x (CrPO4 )1−x which is equivalent to M1 x Cr1−x Mox P1−x O4 . After that requisite amount of triethanolamine (TEA) was added to this homogeneous solution in such a way that the total metal ions (CoII , CrIII and MoVI ) to TEA mole ratio is maintained at 1:4. The ultimate solution has been maintained acidic in the range of pH 3–4 to avoid the precipitation of hydroxides of the metal ions. The resulting clear solution of TEA complexed metal ions was then evaporated on plate at about 200 ◦ C with constant stirring. The continuous heating of the solution caused foaming and puffing. The evaporation of the nitrate ions provided an in situ oxidizing environment for TEA, which converted the hydroxyl groups of TEA to carboxylic acids. After the completion of dehydration, the solution becomes more viscous but without the visible formation of any precipitate or turbidity. Finally it decomposed to a voluminous, organic black fluffy powder. It was then homogenized. The powder obtained in this way referred to as “precursor”. The “precursor” was then calcined at various temperatures to get a series of M1 x Cr1−x Mox P1−x O4 (M1 = Co, Cu) alloy powders; x ranging from 0.15 to 0.40. The composition and corresponding heat treatment temperature has been shown in Tables 6 and 7. The heat treatment of the precursor materials (in air, for 2 h) had been facilitated at 700–900 ◦ C at a heating rate of 8–10 ◦ C/min. These powders were characterized by X-ray diffraction, transmission electron microscopy (TEM), and TG–DTA and EDS X-ray study.
Fig. 1. Flow-chart for the preparation of MMoO4 (M = Cu, Co) and CrPO4 .
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Fig. 2. Flow-chart for the preparation of various composition of (M1 x Cr1−x Mox P1−x O4 ) [M1 = Co, Cu] compounds.
For analysis of composition of samples by EDS-X-ray method, we have calibrated the machine with the known compositions. This is required because in presence of Cr3+ , Mo6+ always show the lower value of concentration due to excitation-energy transfer from Mo6+ to Cr3+ . These compositions are further verified with the analysis of Co, Cu, Cr and Mo by atomic absorbance spectroscopy. The solution of sample has been prepared in dilute nitric acid (10% w/w). It should be mentioned here that there is no possibility of missing ions in any of the steps of the preparative process.
without catalyst takes 1.5 h for decolourization, and their corresponding molybdates (CoMoO4 and CuMoO4 ) and phosphates (CrPO4 ) takes more time for decolourization, which is represented in Table 1. M1 x Cr1−x Mox P1−x O4 [M1 = Co, Cu] also give much faster reaction kinetics than that of their corresponding molybdates and phosphates. The solution of the above two sets containing the catalysts show a continuous increase in absorption at ∼295 nm which indicates the formation of 4-aminophenol. 4. Results and discussion
3. Catalytic test Catalytic activity of the above mixed oxide solid solutions (M1 x Cr1−x Mox P1−x O4 ) [M1 = Co, Cu] are substantiated in the following way. In six sets of 100 ml beaker, 50 ml of aqueous solution of 4-nitrophenol (0.1 mmol l−1 ) is taken and a freshly prepared aqueous solution of NaBH4 (0.529 mol dm−3 ) is introduced in each of the set. Then, 0.1 g of each of the M1 x Cr1−x Mox P1−x O4 [M1 = Co, Cu] is added to the mixture and is stirred occasionally. In another 100 ml beaker is taken just without the catalyst solid solutions keeping the other conditions same. The result shows that yellow colour of 4-nitrophenol disappears just after 4 min in presence of the catalyst for both the cases; while only NaBH4 Table 1 Time required in different conditions for complete decolourization of yellow colour of 4-nitro phenol Reaction compositions (1) M1 x Cr1−x Mox P1−x O4 [M1 = Cu, x = 0.30] + NaBH4 + 4-nitro phenol (2) M1 x Cr1−x Mox P1−x O4 [M1 = Co, x = 0.30] + NaBH4 + 4-nitro phenol (3) NaBH4 + 4-nitro phenol (4) CrPO4 + NaBH4 + 4-nitro phenol (5) CoMoO4 + NaBH4 + 4-nitro phenol (6) CuMoO4 + NaBH4 + 4-nitro phenol
Time (min) 4 4 90 60 25 20
Thermogravimetric and differential thermal analysis (TG–DTA) (Model DT-40, Shimadzu Co., Kyoto, Japan) has been performed in air at a heating rate of 5 ◦ C/min. The DTA curves in Figs. 3 and 4 indicate that the M1 x Cr1−x Mox P1−x O4 precursor powder decomposes exothermally, with a sharp peak at 437.5 ◦ C for M = Co and at 450 ◦ C for M = Cu. This exotherm can be assigned to major decomposition of metal (TEA) complexes. The small exothermic peak for M = Co at 325 ◦ C is due to the oxidation of residual pyrolyzed amines obtained from TEA. From the graph it is also observed that the metal-TEA precursor powder exhibits weight loss up to 590 ◦ C and above 590 ◦ C the weight becomes almost constant for both the cases. This indicates that below 590 ◦ C, a metal complex along with unreacted amine decomposes. The whole thermal process is associated with the evolution of a large amount of gases (such as CO, CO2 , NH3 , NO2 and water vapor) that is reflected by the total weight loss of 70% in the TGA curve. The solubility CoMoO4 or CuMoO4 in CrPO4 is poor. They are not miscible in any reasonable molar ratio. However in nano-structured form they can form solid solution, which has been manifested in these studies. The nano-structured compounds have been examined to find the atomic level dispersion through EDS X-ray study in Tables 2–5. The X-ray diffractograph in Figs. 5 and 6 show that those are amorphous below a certain temperature of heat treatment. For both the
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Fig. 3. DTA–TG and DTG of Cux Cr1−x Mox P1−x O4 precursor.
Fig. 4. DTA–TG and DTG of Cox Cr1−x Mox P1−x O4 precursor.
cases (M = Cu and Co), the phase separation temperature decreases with increasing the mole percent of CuMoO4 and CoMoO4 respectively. Tables 6 and 7 show the temperatures above which phase separations occur. The XRD peaks of Table 2 EDS X-ray data of Cu0.30 Cr0.70 Mo0.30 P0.70 O4 Element
Atomic % (theoretical)
PK Cr K Cu K Mo L Total
35.18 (35) 35.36 (35) 15.18 (15) 14.28 (15) 100.00
the newly formed compounds reveal that most of the major peaks are either the XRD pattern of copper–molybdate or cobalt–molybdate. The sizes of crystallites separated from various compositions at their phase separation temperatures have been evaluated from Scherrer equation. The data of crystallite sizes Table 4 EDS X-ray data of CuMoO4 Element
Atomic % (theoretical)
Cu K Mo K Total
52.67 (50) 47.33 (50) 100.00
Table 3 EDS X-ray data of Co0.30 Cr0.70 MoP0.70 O4 Element
Atomic % (theoretical)
PK Cr K Co K Mo L Total
35.28 (35) 35.26 (35) 15.08 (15) 14.38 (15) 100.00
Table 5 EDS X-ray data of CoMoO4 Element
Atomic % (theoretical)
Co K Mo k Total
55.22 (50) 44.78 (50) 100.00
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Fig. 5. XRD (using Cu K␣ radiation) pattern of various composition of Cux Cr1−x Mox P1−x O4 calcined at various temperatures.
obtained from the diffractographs have been presented in Tables 8 and 9. It is noted that the lowest crystallite sizes of the CuMoO4 or CoMoO4 are around 20 nm. It is probably due to higher reactivity for growth of crystallites below of 20 nm. During the evaporation of precursor solution triethanolamine and its complexes generate mesoporous phase in the intermittent temperature. The BET surface area
measurements by nitrogen adsorption–desorption isotherm in BECKMAN COULTER SA3100 shows that powders produced at their calcination temperatures for M1 x Cr1−x Mox P1−x O4 (M1 = Co, Cu), has effective surface area around 60–71 m2 /g; 61.736 m2 /g for Cux Cr1−x Mox P1−x O4 (x = 0.30) produced at 800 ◦ C and 70.844 m2 /g for Cox Cr1−x Mox P1−x O4 (x = 0.30) produced at 850 ◦ C. The attempts made to make such solid solution by co-precipitation
Table 6 Cux Cr1−x Mox P1−x O4 showing the temperatures at which the amorphous and crystalline structures are obtained Sample number
Values of ‘x’
Cux Cr1−x Mox P1−x O4
Heat treatment temperature (◦ C) in air
Amorphous structure
1
0.15
Cu0.15 Cr0.85 Mo0.15 P0.85 O4
800 850 900
* *
2
0.20
Cu0.20 Cr0.80 Mo0.20 P0.80 O4
700 800 850
* *
700 750 800
* *
700 750 800
* *
3
4
0.30
0.40
Cu0.30 Cr0.70 Mo0.30 P0.80 O4
Cu0.40 Cr0.60 Mo0.40 P0.60 O4
Crystalline structure
*
*
*
*
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Fig. 6. XRD (using Cu K␣ radiation) pattern of various composition of Co1 x Cr1−x Mox P1−x O4 calcined at various temperatures. Table 7 Cox Cr1−x Mox P1−x O4 showing the temperatures at which the amorphous and crystalline structures are obtained Sample number
Values of ‘x’
Cox Cr1−x Mox P1−x O4
Heat treatment temperature (◦ C) in air
Amorphous structure
1
0.15
Co0.15 Cr0.85 Mo0.15 P0.85 O4
900 950 1000
* *
750 800 850
* *
2
0.20
Co0.20 Cr0.80 Mo0.20 P0.80 O4
Crystalline structure
*
*
3
0.30
Co0.30 Cr0.70 Mo0.30 P0.70 O4
750 800 850
* *
4
0.40
Co0.40 Cr0.60 Mo0.40 P0.60 O4
700 800 850
* *
*
*
Table 8 Summary of the particle sizes obtained from X-ray diffraction studies of Cux Cr1−x Mox P1−x O4
Table 9 Summary of the particle sizes obtained from X-ray diffraction studies of Cox Cr1−x Mox P1−x O4
Cux Cr1−x Mox P1−x O4
Phase separation temperature (◦ C)
Crystallite size (nm)
Cox Cr1−x Mox P1−x O4
Phase separation temperature (◦ C)
Crystallite size (nm)
Co0.15 Cr0.85 Mo0.15 P0.85 O4 Cu0.20 Cr0.80 Mo0.20 P0.80 O4 Cu0.30 Cr0.70 Mo0.30 P0.70 O4 Cu0.40 Cr0.60 Mo0.40 P0.60 O4
850 800 750 750
27 26 21 19
Co0.15 Cr0.85 Mo0.15 P0.85 O4 Co0.20 Cr0.80 Mo0.20 P0.80 O4 Co0.30 Cr0.70 Mo0.30 P0.70 O4 Co0.40 Cr0.60 Mo0.40 P0.60 O4
950 800 800 800
23 21 21 20
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Table 10 Tap density measurements of Cux Cr1−x Mox P1−x O4 Cux Cr1−x Mox P1−x O4
Tap density (g/cm3 )
Cu0.15 Cr0.85 Mo0.15 P0.85 O4 Cu0.20 Cr0.80 Mo0.20 P0.85 O4 Cu0.30 Cr0.70 Mo0.30 P0.70 O4 Cu0.40 Cr0.60 Mo0.40 P0.60 O4
0.46 0.48 0.55 0.58
Table 11 Tap density measurements of Cox Cr1−x Mox P1−x O4 Cox Cr1−x Mox P1−x O4
Tap density (g/cm3 )
Co0.15 Cr0.85 Mo0.15 P0.85 O4 Co0.20 Cr0.80 Mo0.20 P0.80 O4 Co0.30 Cr0.70 Mo0.30 P0.70 O4 Co0.40 Cr0.60 Mo0.40 P0.60 O4
0.33 0.40 0.52 0.55
were unsuccessful. Tap densities of final powders have been measured and presented in Tables 10 and 11. The finer details of the particles and their morphologies have been investigated by TEM (Model Philips TM30, Philips Research Laboratories). The bright-field (BF) electron micrograph of the Cux Cr1−x Mox P1−x O4 powder
(x = 0.30) produced at 800 ◦ C (2 h) reflects narrow distribution of particles, with an average particle diameter of 71 ± 15 nm which is shown in Fig. 7. The bright-field (BF) electron micrograph of Cox Cr1−x Mox P1−x O4 powder (x = 0.30) calcined at 850 ◦ C has also been taken from the smallest visible particles with the amorphous and/or their aggregates shown in Fig. 8 which shows an average particle diameter 53 ± 15 nm.
5. Conclusion The following conclusions can be drawn from the experiments cited in this paper. • The meta stable solid solutions with high surface areas are possible to prepare by this soft chemical method. • The compound prepared with such an excellent surface area may be useful for catalytic reactions. • The crystallites of MI MoO4 (MI = Cu, Co) appear first from the amorphous phases during phase separation.
Acknowledgement The authors are grateful to the Council for Scientific and Industrial Research (CSIR), New Delhi, India for the financial grant offered in support of this work.
References
Fig. 7. Bright-field TEM of Cu0.30 Cr0.70 Mo0.30 P0.70 O4 calcined at 800 ◦ C.
Fig. 8. Bright-field TEM of Co0.30 Cr0.70 Mo0.30 P0.70 O4 calcined at 850 ◦ C.
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