Thermal solid–solid interaction and physicochemical properties of CuO–Fe2O3 system

International Journal of Inorganic Materials 3 (2001) 1073–1081

Thermal solid–solid interaction and physicochemical properties of CuO–Fe 2 O 3 system a, b W.M. Shaheen *, A.A. Ali a

Physical Chemistry Department, National Research Center, Cairo, Egypt b Physics Department, Seoul National University, Seoul, South Korea Received 6 March 2001; accepted 14 August 2001

Abstract A series of single and mixed oxides of CuO–Fe 2 O 3 was prepared by thermal treatment of pure and mixed copper and ferric nitrate solids at 350–10008C. The thermal behaviour of the single and binary salts has been studied using the thermal analysis (TG-DTG) technique. The thermal products were characterised using the X-ray diffraction analysis. The results revealed that copper and ferric nitrates decomposed to CuO and Fe 2 O 3 at about 4008C, respectively, in the Cu–Fe mixtures. The presence of ferric oxide in the mixture prevents the reduction process of CuO to Cu 2 O. Crystalline copper ferrite phase was detected at 750 and 10008C as a result of solid–solid interaction between CuO and Fe 2 O 3 . The presence of excess of CuO in the mixture stimulates the formation of well crystalline copper ferrite phase while excess of Fe 2 O 3 hinders the previous process. The catalytic activity of the obtained pure and mixed oxides was measured using the decomposition of hydrogen peroxide at 30–508C. It was found that the mixed oxide solids have catalytic activity higher than single oxides preheated at 350 and 5508C. The rise in precalcination temperature to 750 and 10008C brought about a drastic decrease in the activity of mixed solids because of formation of copper ferrite phase and / or sintering process.  2001 Elsevier Science Ltd. All rights reserved. Keywords: CuO–Fe 2 O 3 ; Solid–solid interaction; Catalytic activity

1. Introduction A great deal of fundamental research has been done on mixed catalysts in the field of heterogeneous catalysis [1–3]. These mixed systems may be more active than their separate components [4,5]. In catalysis, it is known that the activity of oxide catalysts depends on many factors such as method of preparation and calcination conditions [6]. The so-called mixed catalysts are often produced by thermal decomposition of mixed components of transition metals [7]. The thermal treatment of mixed solid components may lead to the formation of new compounds as a result of solid–solid interactions between thermal products [8,9]. The results of many recent studies indicate that one of the important factors influencing the activity of oxide catalysts is explained in terms of change in oxidation–reduction state of the catalytic active component [10]. For instance, it was found that, the varying of oxidation state of the active *Corresponding author. E-mail address: [email protected] (W.M. Shaheen).

component of catalysts seems to be an important factor in the case of the more-component catalysts such as systems of Fe–Mo, Fe–Cr and Cu–Mo oxide catalysts [11–13]. The present investigation was devoted to study the effect of chemical composition and the calcination temperature on the mutual solid–solid interaction and physicochemical properties of CuO–Fe 2 O 3 system. The techniques employed were thermal analysis (TG-DTG) and X-ray diffraction (XRD) analysis. The catalytic activity of the pure and mixed oxide catalysts produced at temperature ranged between 350 and 10008C was measured in hydrogen peroxide decomposition reaction at 30–508C.

2. Experimental

2.1. Materials The starting materials were solids of copper nitrate hexahydrate, Cu(NO 3 ) 2 ?3H 2 O, and ferric nitrate hydrate, Fe(NO 3 ) 2 ?9H 2 O. The chemicals employed were analytical

1466-6049 / 01 / $ – see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S1466-6049( 01 )00165-9

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grade and supplied by Prolabo Company. Five mixtures of molar ratios; 3:1, 2:1, 1:1, 1:2 and 1:3 with respect to CuO–Fe 2 O 3 were prepared by thermal decomposition of the appropriate metal nitrate precursors [9]. The aqueous solutions of the metal salts were mixed in the stoichiometric proportions. The mixtures were evaporated to dryness on a water bath and the dried mass was precalcined at 3008C until the evolution of the nitrogen oxides was complete. The produced solids were thermally treated in air at 350, 550, 750 and 10008C for 4 h.

2.2. Techniques TG-DTG of pure copper and ferric nitrates and their mixtures with molar ratios (3:1,1:1 and 1:3) were investigated using Shimadzu DT-40 thermal analyser. The samples were studied under air atmosphere with flowing rate of 30 ml / min. The rate of heating was fixed at 108C min 2l . XRD patterns were obtained at room temperature using a Philips X-ray diffractometer (PW 1390), employing CuKa radiation as the X-ray source. The X-ray tube was

Fig. 1. TG-DTG of (a) pure Cu(NO 3 ) 2 ?3H 2 O; (b) pure Fe(NO 3 ) 2 ?9H 2 O.

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Fig. 2. TG-DTG of mixed solids with molar ratio (a) 3Cu:1Fe; (b) 1Cu:1Fe; (c) 1Cu:3Fe.

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Fig. 3. X-ray diffractograms of mixed solids (Cu–Fe) precalcined in air at (a) 3508C; (b) 5508C.

operated at 36 kV and 16 mV. Samples were finely ground and packed in a plastic holder. The diffraction angle of 2u was scanned at rate of 2 min 21 . The decomposition of hydrogen peroxide in aqueous solution at 30–508C was used as a model reaction [1] for determining the catalytic activity of various (single and mixed) solids precalcined at temperature ranges between 350 and 10008C.

and Fe(NO 3 ) 2 ?9H 2 O has been carried out and represented on Fig. 1a and b. It was found that, pure copper nitrate hexahydrate decomposed completely to copper oxide at 3008C with total mass loss of 67%. At temperature above 9308C, copper oxide reduced to cuprous oxide with total mass loss of 70.5%. On the other hand, pure ferric nitrate decomposed to ferric oxide at 3008C with total mass loss of 80% and the produced oxide was thermally stable up to 10008C.

3. Results and discussion

3.2. Thermal analysis of Cu( NO3 )2?3 H2 O and Fe( NO3 )2? 9 H2 O mixtures

3.1. Thermal analysis of copper and ferric nitrate solids The thermal analysis of pure solids of Cu(NO 3 ) 2 ?3H 2 O

Fig. 2a and c illustrates the thermal decomposition steps of the mixed solids of copper and ferric nitrates with molar

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ratios 3:1, 1:1 and 1:3, respectively. The first thermal decomposition step for all the mixtures starts at about 858C and represents the removal of water of crystallisation and nitrogen oxides from both salts, with DTG maxima at about 80 and 1608C. The second thermal step starts at temperatures .2008C, reaching constant mass losses of 74, 77and 79%, respectively, at about 4258C. This step corresponds to the complete decomposition of pure salts to CuO and Fe 2 O 3 . The thermograms of all the Cu–Fe mixtures did not show any noticeable mass loss from 500 to 9008C, which might reflect a probability of solid–solid interaction between the produced oxides to form copper ferrite [5] according to the following equation CuO 1 Fe 2 O 3 → CuFe 2 O 4

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The comparison of thermograms of pure and mixed copper and ferric nitrate salts shows that the presence of iron oxide in the Cu–Fe mixture prevents the reduction of copper oxide to cuprous oxide. Also, the presence of both copper and ferric nitrate salts together in the mixture retarded their decomposition to CuO and Fe 2 O 3 .

3.3. XRD characterisation of mixtures of Cu( NO3 )2 ? 3 H2 O and Fe( NO3 )2 ?9 H2 O thermally treated at 350 – 10008 C Figs. 3 and 4 depict the results of X-ray diffractograms of the mixtures of copper and ferric nitrates salts having molar ratios; 3Cu:1Fe, 2Cu:1Fe, 1Cu:1Fe, 1Cu:2Fe and

Fig. 4. X-ray diffractograms of mixed solids (Cu–Fe) precalcined in air at (a) 7508C; (b) 10008C.

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1Cu:3Fe; precalcined in air at 350, 550, 750 and 10008C. The relationship between the degree of crystallinity (intensity of common X-ray patterns) and the composition of the Cu–Fe mixtures is represented in Fig. 5. Inspection of the above figures shows that, well crystalline phases of CuO (monoclinic) and a-Fe 2 O 3 (rhombohedral) were detected for all the mixtures thermally treated at 3508C, except the mixture rich in iron (1Cu–3Fe) shows only well crystalline ferric oxide phase. The disappearance of all the diffraction lines of CuO phase

suggested its presence in small-sized crystallites that can be detected by XRD investigation. This behaviour expected to be accompanied by an increase in the degree of dispersion of CuO and consequent increase in its catalytic activity. The increasing of treatment temperature up to 5508C was followed by a significant increase in the intensities of the X-ray patterns of ferric oxide phase. In contrast, the thermal treatment of the Cu–Fe mixtures at 7508C showed progressive decrease in the intensities of the X-ray patterns of both Fe 2 O 3 and CuO phases. The ferric

Fig. 5. Dependence of degree of crystallinity (intensity of X-ray patterns) on composition of the Cu–Fe mixture precalcined at (a) 3508C; (b) 5508C; (c) 7508C and (d) 10008C.

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oxide phase was not detected for the mixtures rich in copper and calcined at 7508C, while the copper oxide phase disappeared for the mixture rich in iron precalcined at the same temperature. On the other hand, the thermal treatment of all the Cu–Fe mixtures at 7508C resulted in the formation of well crystalline copper ferrite phase [14] as a result of solid–solid interaction between the copper and ferric oxides as previously mentioned. The crystalline copper ferrite phase was detected as a major phase together with free copper oxide phase in the mixture containing equimolar proportion, 1Cu:1Fe; and also those containing an excess of copper oxide phase. In the case of the mixed solids containing an excess amount of iron heated at 7508C, crystalline copper ferrite was detected as a major phase beside ferric oxide phase. Also, Fig. 5 shows that the presence of excess of copper oxide stimulated and favoured the solid–solid interaction process and the formation of the copper ferrite phase. On the other hand, ferric oxide hindered the formation of the copper ferrite phase at 7508C to an extent proportional to its amount in excess of stoichiometry. When the treatment temperature was further increased to 10008C, the crystalline CuO phase was detected only for the mixtures rich in copper, while the crystalline Fe 2 O 3 phase was detected for the mixture rich in iron. Beside these phases, well crystalline copper ferrite phase was detected as a major phase for all the Cu–Fe mixtures preheated at 10008C. From the above results it can be concluded that the intensity and formation of the copper ferrite phase affected by the calcination temperature and composition of the mixtures under study was as follows: (i) maximum formation of copper ferrite for the mixture (2Cu–1Fe) precalcined at 750 and 10008C, (ii) an increase in calcination temperature from 750 to 10008C was accompanied by an increase in the intensity of the copper ferrite phase and (iii)

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the presence of excess free Fe 2 O 3 in the Cu–Fe mixture hindered the formation of CuFe 2 O 4 .

3.4. Catalytic activity measurements of single and mixed CuO–Fe2 O3 obtained at 350 – 10008 C The experimental results obtained revealed that the decomposition of H 2 O 2 over pure and mixed CuO–Fe 2 O 3 obtained by the treatment of single and mixed salts at 350, 550 and 7508C followed first order-kinetics. Table 1 depicts the values of reaction rate constant k?min 21 measured at reaction temperatures 30, 40 and 508C. The values of rate constants were obtained from the plots of ln (a /a 2 x) against time where a is the initial concentration of H 2 O 2 and x is the amount of evolved oxygen (see Fig. 6a). From Fig. 6b and Table 1 it can be concluded that single solids of CuO and Fe 2 O 3 obtained by thermal treatment of their salts at 350, 550 and 7508C exhibit a very small catalytic activity in the H 2 O 2 decomposition reaction [15,16]. On the other hand, the catalytic activity of mixed CuO–Fe 2 O 3 obtained by thermal treatment of the Cu– Fe mixtures at the same calcination temperatures was found to be greater than that of single oxides. This can be interpreted in terms of the concept of bivalent catalytic centres [10,17], by assuming that for CuO the centres consist of Cu 21 –Cu 1 ion pairs and for Fe 2 O 3 they consist of Fe 31 –Fe 21 ion pairs. The higher catalytic activity of the two component oxides may be due the fact that beside the one component sites Cu 21 –Cu1, Fe 31 –Fe 21 , there will be also the mixed sites Cu 21 –Fe 1 and / or Cu 1 –Fe 21 ion pairs as a result of mutual charge interaction. In other words, the increasing activity of mixed oxides might be attributed to increase in the concentration of active sites via creation of new ion pairs. From the above, the mechanism of H 2 O 2 decomposition

Table 1 Dependence of rate constant (k?10 min 21 ) on catalyst composition, calcination temperature and reaction temperature (k?10) min 21

DE (kJ mol 21 )

Catalyst from

Calcination temp. (8C)

308C

408C

508C

Cu(NO 3 ) 2 ?6H 2 O 3Cu:1Fe 2Cu:1Fe 1Cu:1Fe 1Cu:2Fe 1Cu:3Fe Fe(NO 3 ) 2 ?9H 2 O

350

0.07 0.15 0.10 0.08 0.12 0.17 0.06

0.17 0.37 0.24 0.20 0.30 0.40 0.14

0.39 0.90 0.69 0.46 0.75 1.00 0.26

71 76 80 76 76 74 139

Cu(NO 3 ) 2 ?6H 2 O 3Cu:1Fe 2Cu:1Fe 1Cu:1Fe 1Cu:2Fe 1Cu:3Fe Fe(NO 3 ) 2 ?9H 2 O

550

0.03 0.06 0.08 0.10 0.05 0.04 0.02

0.10 0.16 0.20 0.34 0.14 0.12 0.06

0.29 0.40 0.53 0.65 0.36 0.31 0.19

95 79 78 77 82 84 94

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Fig. 6. (a) Dependence of ln (a /a 2 x) vs. time for catalyst (3Cu–1Fe) precalcined at (a) 3508C and 5508C; (b) Dependence of the rate constant (k min 21 ) on catalyst composition (Cu–Fe) precalcined at (a) 3508C, (b) 5508C.

on (CuO–Fe 2 O 3 ) mixed oxide catalysts can be summarized as follows [11,12]: → H 1 1 HO 22 H2O2 ← 2 HO 2 → HO 2? 1 e Cu 1 e → Cu Fe 31 1 e → Fe 21 Cu 1 1 Fe 31 → Cu 21 1 Fe 31 21

1

In this case, the presence of reduced copper [18] in the system is capable to reduce Fe 31 ion (major catalytic

centers) partly to ions of lower oxidation state (Fe 21 ) minor centers being more active species in hydrogen peroxide decomposition. This means that the observed changes of the catalytic activity are probably due to the changes of the valence state of the catalytically active components of the catalyst [19]. Table 1 and Fig. 6b also show that the catalytic activity of the Cu:Fe system is a distinct nonmonotonic function of their composition. The catalytic activity drops with increasing content of Fe 2 O 3 to a minimum at the stoichiometric mixture, 1Cu:1Fe, precalcined at 3508C. The cata-

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lytic activity is maximum for the mixtures rich in copper and iron species (3Cu:1Fe and 1Cu:3Fe) precalcined at 3508C. In this case, the excess of CuO and Fe 2 O 3 plays as promoters to each other leading to increasing the concentration of catalytically active ion pairs species as previously mentioned. Furthermore, Fig. 6a and Table 1 illustrate that increasing of calcination temperature from 3508C to 5508C was accompanied by a progressive decrease in the catalytic activity of mixed solids. This because of increasing the degree of crystallinity of oxide phases which was accompanied by increasing their particle size and consequently decreasing the surface areas of the above phases leading to decrease their catalytic activity. Table 1 also shows that the values of activation energy, DE, of the mixed solids precalcined at 350 and 5508C are clustered in the range of 74–84 kJ mol 21 . The constancy of the values of activation energy points to the similar nature of active centres being different from that of single copper or ferric nitrates precalcined at the same temperatures. On increasing the calcination temperature of mixed solids to 7508C, no measurable catalytic activity showed because of the formation of the spinel form of copper ferrite (CuFe 2 O 4 ). The poor performance of mixed solids precalcined at 7508C is explained in terms of the restricted redox couple represented by M n / M n 21 in the electronic composition of the catalysts and, possibly, the absence of M n on the octahedral lattice site which may initiate the cyclic electron-transfer process on the catalyst surface 0 [20,21]. On the other hand, there is no noticeable catalytic activity for the solids preheated at 10008C because of the sintering process [22].

4. Conclusions From the above results, we can conclude that pure copper nitrate decomposed to CuO at 3008C and to Cu 2 O at temperatures above 9508C. Pure ferric nitrate decom-

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posed to Fe 2 O 3 at 3008C and was thermally stable up to 10008C. Moreover, the precalcination of mixed solids at 750 and 10008C showed the formation of well crystalline copper ferrite phase as a result of the solid–solid interaction between the corresponding oxides. The XRD intensities for the copper ferrite phase were affected by the composition (Cu–Fe mixture) and the calcination temperature. Mixed oxide solids obtained at 350 and 5508C had catalytic activities higher than those of pure oxides obtained at the same temperature. There was no measurable catalytic activity for solids precalcined at 750 and 10008C because of the formation of inactive copper ferrite phase and / or sintering process.

References [1] Shaheen WM, Selim MM. Thermochim Acta 1998;322:117–28. [2] Reddy BM, Reddy EP, Srinivas ST. J Catal 1992;136:50–8. [3] Kapetien F, Rodrigvez-Mirosol J, Moulizn JA. Appl Catal B 1996;9:25–64. [4] Mucka V. Z Phys Chem Leipzig 1989;270(2):246–56. [5] Youssef NA, Selim MM, Kamel ES. Bull Soc Chem Fr 1991;128:648–53. [6] El-Shobaky GA, Fagal GA, Ghozza MA, Mokhtar M. Colloids Surf 1998;142:17–25. [7] Brynowicz LG. Thermochim Acta 1994;239:181–99. [8] Shaheen WM, Selim MM. J Thermal Anal Calor 2000;59:961–70. [9] Sundararjan R, Srinvason V. Appl Catal 1991;73:165–71. [10] Mucka V. Collect Czech Chem Commun 1984;49:2231–40. [11] Wassel MA, Zidan FJ, El-Wardani M. J Serb Chem Soc 1989;54(8):425–33. [12] Mucka V. Radiat Phys Chem 1996;48(3):309–13. [13] Lonyi F, Valyon J. J Thermal Anal 1996;46:211–8. [14] S Prasad SV, Sitakara RS. Adv Catal 1985;7:241–8. [15] Selim MM, Khalil LB. Afinidad XLVIII 1991;433:167–72. [16] Shaheen WM, Selim MM. Afinidad LVL 1999;480:129–34. [17] Mucka V. Collect Czech Chem Commun 1986:1874–2240. [18] Youssef NA, Farid T, Selim MM. Afinidad L 1993;443:47–50. [19] Onuchukwu IW. J Chem Soc Faraday Trans 1 1984;80(6):1447–56. [20] Mucka V. Collect Czech Chem Commun 1967;41(6):1717–26. [21] Mucka V. Lang Collect Czech Chem Commun 1988;41(8):1836–46. [22] Selim MM, Hassan NA, Mokhtar M, Attia AA. Bull NRC Egypt 1996;21(1):1–9.