Langmuir 1991, 7, 1642-1646
1642
Effect of Spinel (Co#dl-,Fe204,0 d x d 1) Formation on the Kinetics of Catalytic Decomposition of Hydrogen Peroxide over Cobalt-Cadmium-Iron Mixed Oxide Systems R. M. Gabr,' M. M. Girgis, and A. M. El-Awad Chemistry Department, Faculty of Science, Assiut University, Assiut, Egypt Received June 27, 1990. In Final Form: February 15, 1991 A series of cobalt-cadmium-iron oxide mixtures as well as their spinels CozCdl,Fez04, with 0 d x d 1,were prepared by calcinationat temperatures between 400 and 850 "C. X-ray diffractionand IR spectral
studies were used to identify the different phases present in these oxide catalysts. The catalytic activity of these catalysts was investigated kinetically by using the decomposition of H202 as a model catalytic process. Maximum activity was observed at x = 1,where Con ions occupy ferrite octahedral lattice sites. For x < 1 values, the observed decrease in catalytic activity is explained in terms of the restricted redox couple represented by Mn/Mn-' in the electroniccomposition of the catalyst and possibly the absence of a more active ion Mn at x = 0 on the octahedral sites. Correlations were attempted between catalyst compositions and their catalytic activity. Introduction From earlier studies,lI2 it was found that the main interesting aspects controlling the catalytic decomposition of H202 is the occurrence of ions in variable valencies. Such a concept would indicate an activity of spinelcatalysts for redox reaction^.^^^ Metal-iron spinel oxides have attracted the attention of some investigators as catalysts for HzO2decompo~ition.~J3 However, the kinetic activities of C0~Fe-04 spinel were in~estigated.~ Moreover, little attention has been paid to establishing the significance of catalyst composition toward the enhancement of catalyst activity in the peroxide decomposition reaction.s*g According to Goldstein and Tseung, the mechanism of the high activity of cobalt ferrite catalysts is baaed on the presence of C$ ions on the octahedral sites which initiate a cyclic electron transfer process.10 The aim of the present article is to extend the applicability of this concept on CoXCdl,Fe2O~catalysts, thus correlating the intrinsic compositional factors of these catalysts and their activity toward the decomposition of H202. Experimental Section Materiala. Cobalt-cadmium-iron
oxide spinels (CozCdl,Fe2O& over the complete composition range of the system (0 i x i 1) were synthesized by using the corresponding metal nitrates." The appropriate amounts of the metal nitrates were admixed. Distilled water was added to form a paste, and drying WBB then achieved over a water bath with occasionalmixing. The mixed oxide catalysts were further dried in air at 110 O C . Oxide mixtures with values of x of 0.0, 0.1,0.3,0.5,0.7,0.9, and 1.0 were prepared. The parent mixtures were calcined in air at 400, 500, 600,and 850
grade.
OC for 5 h. All materials were of A.R.
(1)Schwab, G. M.; Kanungo, S.B. Z.Phys. Chem. (Munich) 1977,107,
109.
(2)Fahim, R. B.; Zaki, M. I.; El-Roudi, A.M.; Haeeaan, A. M. A.J. Res.
IMt. Catd. 1981 B,25.
(3)Bennet, D.E.R.; Roee, R. A. J. Catal. 1970,18,122.
(4) h n n a r d , R. J.; Kehl, W.L. J. Catal. 1971,21,282. (6) O n u c h u h , A. I. J. Chem. SOC., Faraday 7 " ~ 2 1984,80,1447. . (8) &+ndranathan, P.M. T.;Patil, K. C. Proc. Indian Acad. Sci.,
Chem. Scr. 1987 99.209. (7)O n u c h h , A.1:; Zuru, A. B.Mater, Chem. Phys. 1986,15,131. (8) Blaeee, C. Phrlrps Res. Rep. 1963,18,383. (9)Goldstein, J. R.; Tseung,A. C. C. J. Matter. Sei. 1972,7,1383. (10)Coldatein, J. R.; Tseung,A. C. C.J. Catal. 1974,32,452. (11) Rajarm,R. R.; Sermon, P.A. J. Chem. SOC., Faraday T r a ~I. 1986,81,2677.
Apparatus and Technique. X-ray diffractionpatternswere recorded with a Philip diffractometer (type PW 1010). A Philips generator operated at 35 kV and 20 mA provided a source of Cu Ka radiation (Ni filtered). The diffraction patterns were matched with ASTM cards.I2 The BET specific surface areas were measured by wing nitrogen adsorption at -196 "C, after initial degassing of the catalysts at 150 "C for an hour. Infrared spectra for the solid oxide catalysts were recorded using the KBr technique in the range of 1 W 2 0 0 cm-1 with the aid of a Perkin-Elmer infrared spectrophotometer, Model 699B. The kinetic activities of the mixed oxide/apinel catalysts toward B O 2 decomposition were followed by using the gasometric technique proposed by Deren et al.'a A constant catalyst weight (50 mg)was injected into a thermostated reaction veaael containing 5 cms of HpO2 (35.5%). The oxygen evolution rate was monitored for a given temperature at atmospheric preaaure, the oxygen displacing water from a Bunte gas buret. The timedependent volume of evolved oxygen was monitored at 30-8 intervals in all cases studied. The results were corrected for the self-decompositionof H202.10J4 The oxygen evolution rate WBB found to be independent of stirringand directly proportional to the catalyst mass used in the reaction mixture. The amounts of surfaceexcess oxygen of all catalystsampleswere determined by the hydrazine method.15
Results (A)Characterization. To ensure that the spinel or any other phase has been formed, the catalyst samples were subjected to X-ray analysis. The values obtained for the d-plane spacings and their relative intensities of reflection were compared with relevant ASTM cards in order to determine the major phases present. Diffraction patterns for the different catalysts calcined at temperatures between 500 and 860 OC for 5 h are shown in Figure 1. The XRD patterns for the composition x = 1 (Figure 1) show the diffraction lines of Cos04 and cr-FezO3a t temperatures up to 600"C, whereas the pattern for the sample calcined at 850 "C indicates the formation of the inverse spinel CoFe204 as a major phase. On the other hand (at (12)Powder Diffraction (InorganicCompoundr); McClune,W.F., Ed.; JCPOS: Swarthmore, PA, 1978. (13)Deren, J.; Haber, J.; Podgorrecka,A.;Bursyk, J. J. Catal. 1968, 2,161. (14)Keating, K.B.;Rojner, A. G. J. Phys. Chem. 1966,69,5668. (15)Baeeett, J.; Denney, R. C.; Jeffery, C. H.; Mendham, J. Vogers Textbook of Quontitative Inorganic Analysis; Longman: London and New York. 1978.
0743-7463/91/2407-1642$02.50/00 1991 American Chemical Society
Effect of Spinel Formation on HOOp Decomposition
Langmuir, Vol. ?, No. 8, 1991 1643
t
1 :trace
S5dC
Cd -Fe oxide q s k m
I II
60;C
1.
I
500°C
I.
1
b f e oxide system
500% 1 1403
85iC
( , I
I
I
wavenumber ,
I
t
I
I
iaao
600 (
I
1
m
cm-')
Figure 2. Infrared absorption spectra of Co,-Cdl,-FQ oxide
systems: x: = 1.0; 0.5; 0.0. 606C
IIIII
I.
5oooc
IIIII
I
I
'
, J
'
J
Figure 1. XRD for Co,-Cdl,Fe2 oxide systems: x = 1.0; 0.5; 0.0. Table I. Variation of Specific Surface Area of Co=Cdl-,Fer Oxide Systems (0 5 I I1) with the Calcination Temperature surface area, m2 gl Catalyst with x value 4W°C wO°C 600°C 850OC 0 8.7 7.3 3.3 1.6 0.1 5.6 4.3 2.3 1.1 0.3 4.5 3.1 2.2 1.1 0.5 2.3 2.4 1.2 0.9 0.7 3.8 3.3 2.0 1.1 0.9 7.1 5.0 2.3 1.3 1.0 7.7 5.1 4.0 1.2
x = 01,CdO and a-Fe20g are present at temperatures up to 600OC, whereas the 850OC sample represents CdFezO4 spinel structure (Figure 1). On the contrary, the patterns for x = 0.5 samples indicate that the spinel phase (C0o.sC&@e204) started at 600 OC and completely formed at 850 "C (Figure 1). The BET specific surface areas of the studied catalysts as a function of x and Tc are listed in Table I. The recorded IR spectra for the oxide/spinel catalysts calcined a t 500 and 850 OC are shown in Figure 2. The absorption spectra for the ferrite catalyst being calcined at 850 "C have a strong broad frequency band sited in the region of 500-610 cm-l ( u d and a medium frequency band sited at 370-450 cm-l1* (uz), depending on the x values. At 260 cm-l, a small band (ug) was assigned. Waldronl' attributed the u1 band to the intrinsic vibration of the tetrahedral groups and ug to the octahedral ones. Preud(16) Nek"ta K.Infrared and Raman Spectra of Inorganic and Coordination Compoundr, 3rd. d.; John Wiley & SOIM:London, 1978. (17) Waldron, R. D. Phyu. Rev. 1966,gS, 1727.
homme and Tartels reported that US depends on the mass of bivalent tetrahedral cations. The u2 band was reporteds to be assigned to the vibrations at the bivalent metal ion oxygen complex. Examination of the recorded IR spectra of the catalyst samples calcined at 500 OC indicates that the observed absorption bands correspond to that of the corresponding metal oxides;lQcf. Figure 2. (B)Determination of the Kinetic Activity of the Peroxide Decomposition Reaction. The analysis of experimental data hae been carried out on the assumption" that the decomposition of H202 is a fmt-order process. As the maximal conversion observed after 30 min never exceeded0.05, it may be assumed in the f i t approximation that the reaction runs at the constant concentration of H202. At such conditionsthe integration of the first-order equation gives that V = VO+ K t , where V represents the volume of oxygen evolved and VO,the volume of oxygen evolved to the moment at which the time measurements started. The rate constants have been computed from the slope of V vs t plots. Figure 3 shows V-t plots for the catalysts calcined at 500OC. The initial rate constant ( K ) per second per gram of catalyst at 34.8 OC is plotted as a function of the catalyst composition ( x ) (Figure 4A). The maximum error check of K values is *5 % and reproducibility was good. The rate constants ( K ) increased with temperatures between 20 and 40 OC in accordance with the Arrhenius equation. The decomposition rate constant dependence on both the surface excess oxygen and the catalyst mass is shown in Figure 5. Figures 6 and 7 depict the influence of the catalyst composition and calcination temperature, respectively, on the Arrheniue activation energy calculated by the means square root method for the different catalyst samples. The error in activation energy is f2.1 kJ mol-'. The decomposition activation parameters AH*,AG*,and AS*were evaluated by using (18)Preudhomme, J.; Tarte, P;Spectmchim. Acta, Part A 1971,27, 1817. (19) Nyguist, R. A.; Kagel, R. 0. Infrared Spectra of Inorganic Compounde; Academic Press: London and New York, 1971. (20)Fahim, R. B.;Zaki, M.I.; Gabr, R.M.J. Surf. Technol. 1981, So, 105.
Gabr et al.
1644 Langmuir, Vol. 7, No.8, 1991
Figure3. V-t plots for H101decompositionat 34.8 O C by 50 mg of the Co,-Cdl,-Fez oxide systems calcined at 500 O C for 5 h. Values of x are 1.0(l),0.9 (2), 0.7 (3),0.5 (4),0.3 (5), 0.1 (6),and 0.0 (7).
1
--.-.-----*3
Eyring equation7 and are given in Table 11. The variation of entropy of activation with x is shown in Figure 8. Changes in activation enthalpy and entropy evaluated in this study were subject to an experimental error of f1.5 kJ mol-'.
Discussion Cobalt-cadmium-iron oxide mixtures, as well as their corresponding spinels ( C O ~ C ~ I - ~ 0FI ~ x~ I O 11, ~ , calcinated at different temperatures were synthesized. Characterization of these catalysts by X-ray diffraction and IR spectral data reveals that the spinel oxides prepared were impurity free. The composition (cation di~tribution)~ and kinetic properties of the spinel samples are represented in Table 111. As has been reported,lo in the decomposition of H202 by the Co,Fes-,O4 system, the major controlling factor affecting the intrinsic activities order is the chemical and electronic structure of the catalyst. The occurrence of CoXron octahedral sites of the spinel, with its ready conversion to Coni, allows a highly effective redox system to be set up, initiated by COncenters.l0 Thusthe significant objectiveof our work in the analysis of the reaction kinetics is to establish an intrinsic order of catalyst activities for the C0,Cdl-~FezO4system in H202 decomposition that is dependent on catalyst composition rather than on microstructural differences. Figure 4A shows the dependence of the rate constants (K,8-1 g-l) of H202 decomposition on the catalyst composition ( x ) at calcination temperatures (T,) ranging from 400 to 850 "C. The results compiled in this figure show that the rate constant increases at first gradually until x = 0.5 and then sharply at x > 0.7 to a maximum value at x = 1.0. Moreover, it can be seen that raising the calcination temperature of the catalyst series is accompanied by a decrease in the reaction rate. When the decomposition rate constants for the studied catalysts were normalized to unit BET specific surface area, it was found that the normalizing procedurehad left largely unaffected the activity order (see Figure 4). Therefore the activity order is far more dependent on catalyst composition than on microstructural differences. These results along with the structure and composition information obtained by correlating the results of XRD and IR analysis suggest that compositional effects operate within the catalyst series. A radical mechanism was suggested13 for the decomposition of HzO2 on semiconductor surfaces, that must present two kinds of active centers-donors and
I
24t
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0
0.2
00
04
0.6
08
1.0
X-
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I
1
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0
0.2
0.6
0.4
08
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Figure 4. Plot of K (8-l g-l (A) and s-l cm* (B))againat compositionx at 34.8O C forCoz4dl,-Feooxideaystema calcined at 400 "C (l),600 "C (2), 600 O C (31, and 850 OC (4). acceptors-to be active. In the catalyst samples being investigated, however, there is only one type of divalent ion readily promoted to the trivalent state, i.e. Con ions. Hence we would expect that Con may act as a catalytic center for H202 decomposition. In this respect the electronic structure of the normal spinel Cos04 was ~ ] , O ~those ~ , of the reportedlo as ( C O ~ ) ~ ~ [ C O ~ whereas inverse spinels, CoFe204, C00.&&,$e204, and CdFe201, were characterizedlO as (Fe1rr)~t[Co11-Fe111]oct042-, ( Fe I ) [ C oIIo.r,- C d I I o , 5- F e I I 1 J 0 4 2-, a n d (Fen1)bt[Cd'LFem]oct042-,respectively; cf. Table III. It is worth mentioning that the most active center is (Con), and this configuration afforded somewhat greater activity
Langmuir, Vol. 7, No.8, 1991 1646
Effect of Spinel Formation on Ha00Decomposition
0
40
80
Catalyst mass 1 mg
4 '20
-
Surface excess mygen x 10
160
IQ'I - a
b
Figure 5. (a) Relation between the surface excess oxygen and the catalytic activity at 34.8 "C for Co,-Cdl,-Fe2 oxide systems calcined at 400 "C for 5 h. Values of x are 1.0 (11,O.g (2), 0.7 (3), 0.5 (41,0.3 (5), 0.1 (6), and 0.0 (7). (b) Relation between the catalyst mass and the catalytic activity at 30 O C for the Co-Fe2 oxide system calcined at 400 "C for 5 h.
400
800
600 Tc,oc-
Figure 7. Plot of the activation energy (All') for Ha03 decompositionvs calcination temperature (TJ.Values of x are 1.0 (11,0.6 (21, and 0.0 (3).
Figure 6. Plot of the activation energy (All*) for Hz02 decomposition vs x in Co,-Cdl,-Fez oxide systsms. than the case of (CoII)bt due to the effective isolation of tetrahedral sites.' These findings explain the maximum activity observed at x = 1(Figure 4) for all catalyst series and agree with the results obtained by Goldstein,lo since the greatest availability of octahedral CoI1 occurs at this composition. Moreover, at x values less than 1 (Le. increasing of Cd content), the expected substitution of (Con)& centers by Cd" ions explains the observed continuous decrease in the rate constant until x = 0, where we reach a minimum activity. In this respect, Royz1correlated the H202 decomposition activity of several transition-metal oxides with their measured redox potentials. The dual valence oxideswith standard redox potentials considerably above the standard reduction potential for H202 oxidation, 0 2 + 2H+ + 2e- +H202, were effective catalysts, whereas those oxides with low redox potentials were much less active. Hence, the lower performance of the cadmiumcontaining series cannot be explained by a consideration of microstructural defects but rather by the restricted redox couple represented by Mnf Mn-*in the electronic (21) Roy, C. B.J. Catal. 1969,12,129.
composition of the catalyst and, possibly, the absence of a more active ion (Mn) at high composition on the octahedral lattice sites which can initiate the cyclic electron-transfer process on the catalyst surface. Consequently the minimum activity observed in the case of CdFe204 ( x = 0)can be explained tentatively in terms of the absence of a redox-couple mechanism. Figure 5a shows the effect of the surface excess oxygen on the catalytic decomposition rate. It is clear that a higher surface excess oxygen value has a profound effect on the activity of H202 decomposition, while at lower values its effect on the initial rate of decomposition is less pronounced. A reasonable interpretation for the origin of such an effect can be given in terms of the following two main points (a) nonstoichiometry in metal oxides leads to an oxygen excess or deficiency and p- or n-type semiconducting properties, moreover, oxygen uptake is more extensive on p- than n-type semiconductingoxides;ll (b) in general p-type semiconductingoxides are more active catalysts than n-type oxides for H202 decomposition.% Such correlations were confirmed in the present study. On the basis of the foregoingconsiderations,the catalytic centers in the C0,Cdl-~Fe204system can be arranged in order of decreasing intrinsic activity as follows: ( F e 1 1 1 ) t , t [ C o 1 1 - F e 1 1 1 ] o c ~ 0 4( 2x - = 1) > (Fe111)t,t[Co110.s-Cd110.~-Fe111]o,t042( x = 0.5) > (Fe111)~t[Cd"-Fem]~042(x = 0). One possible approach to ascertaining the intrinsic activity is to make use of a kinetic parameter that is effectively independent on the catalyst microstructure. A suitable choice of such a parameter would be the activation energy and entropy of the peroxide decomposition reaction, which are dependent on the catalyst composition rather than its surface morphology.= The trend of variation of the activation energy with both the catalyst composition ( x ) (Figure 6 ) and calcination temperature (T,) (Figure 7) is compatible with the increase of the octahedral active centers being investigated, on increasing x and Tovalues, since the catalyst activities may be compared on the basis that the most effective H2Oz decomposition catalyst possesses the lowest activation energy. Consequently, the (22) Mouea, M. A. h o c . Pak. Acad. Sei. 1987, !24,3W. (23) Tseung, A.c. C.; Goldstein,J. R.J. Phye. Chem. 1972,76,3648.
1646 Langmuir, Val. 7, No.8,1991
Gabr et al.
Table 11. Rate Conrtants (R), Activation Energiem (Am), Preexponential Factors (A), and Thermodynamic Parameten of Activation for HaOt Decomposition on CoXdl-Ai'et Odder calcination tamp catalyst with lop K,8-1 g' AE*,a loglo A,a AH*: kJ AG*: kJ A s , * b kJ (Tc,OC/5 h) II value 20 OC 25 OC 30 "C 35 OC 40 OC kJ mol-* kJ mol-' mol-' (303 K) mol-' (303 K) mol-' (303 K)
SC
H202
8dC
11
- 120
0
I
I
H20
+
'1202
(Figure 5b). Since the diffusion kinetics would be largely independent on catalyst composition, the "differences" in the activation energies would be small as observed. Moreover the values of the activation entropy (ASo) in
