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Materials Research Bulletin 40 (2005) 1065–1080 www.elsevier.com/locate/matresbu
Surface and catalytic properties of MoO3/Al2O3 system doped with Co3O4 A.A. Zahran a, W.M. Shaheen b, G.A. El-Shobaky b,* a
Chemistry Department, Faculty of Science, Ain Shams University, Cairo, Egypt b Department of Physical Chemistry, National Research Center, Cairo, Egypt
Received 28 October 2004; received in revised form 8 December 2004; accepted 8 April 2005
Abstract Thermal solid–solid interactions in cobalt treated MoO3/Al2O3 system were investigated using X-ray powder diffraction. The solids were prepared by wet impregnation method using Al(OH)3, ammonium molybdate and cobalt nitrate solutions, drying at 100 8C then calcination at 300, 500, 750 and 1000 8C. The amount of MoO3, was fixed at 16.67 mol% and those of cobalt oxide were varied between 2.04 and 14.29 mol% Co3O4. Surface and catalytic properties of various solid samples precalcined at 300 and 500 8C were studied using nitrogen adsorption at 196 8C, conversion of isopropanol at 200–500 8C and decomposition of H2O2 at 30–50 8C. The results obtained revealed that pure mixed solids precalcined at 300 8C consisted of AlOOH and MoO3 phases. Cobalt oxide-doped samples calcined at the same temperature consisted also of AlOOH, MoO3 and CoMoO4 compounds. The rise in calcination temperature to 500 8C resulted in complete conversion of AlOOH into very poorly crystalline g-Al2O3. The further increase in precalcination temperature to 750 8C led to the formation of Al2(MoO4)3, k-Al2O3 besides CoMoO4 and un-reacted portion of Co3O4 in the samples rich in cobalt oxide. Pure MoO3/Al2O3 preheated at 1000 8C composed of MoO3–aAl2O3 solid solution (acquired grey colour). The doped samples consisted of the same solid solution together with CoMoO4 and CoAl2O4 compounds. The increase in calcination temperature of pure and variously doped solids from 300 to 500 8C increased their specific surface areas and total pore volume which suffered a drastic decrease upon heating at 750 8C. Doping the investigated system with small amounts of cobalt oxide (2.04 and 4 mol%) followed by heating at 300 and 500 8C increased its catalytic activity in H2O2 decomposition. This increase, measured at 300 8C, attained 25.4- and 12.9fold for the solids precalcined at 300 and 500 8C, respectively. The increase in the amount of dopant added above this limit decreased the catalytic activity which remained bigger than those of un-treated catalysts. On the other hand, the doping process decreased the catalytic activity of treated solids in isopropanol conversion especially the
* Corresponding author. Tel.: +20 2 7494 265; fax: +20 2 3370 931. E-mail address: [email protected] (G.A. El-Shobaky). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.04.003
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catalysts precalcined at 300 8C. This treatment modified the selectivities of treated solids towards dehydration and dehydrogenation of reacted alcohol. The activation energies of H2O2 decomposition were determined for pure and variously doped solids. The results obtained were discussed in light of induced changes in chemical composition and surface properties of the investigated system due to doping with cobalt oxide. # 2005 Elsevier Ltd. All rights reserved.
1. Introduction The physicochemical, surface and catalytic properties of various transition metal oxides supported on g-Al2O3 made the object of several investigations carried out in our laboratory. These catalysts, include NiO/Al2O3, Co3O4/Al2O3, CuO/Al2O3, Mn2O3/Al2O3, Fe2O3/Al2O3 and Cr2O3/Al2O3 and were used in CO oxidation by O2 [1–10]. The catalytic activities of these solids have been found to increase by doping with certain foreign oxide as Li2O, K2O, MoO3, CeO2 or ZnO [11–14]. The alkali treatment of some of these systems increased their catalytic activities in alcohols conversion and H2O2 decomposition. Mo-based catalysts have been widely applied in many hydrotreating reactions due to their big activities for hydrodesulfurization, hydrodenitrogenation reactions and methane synthesis from hydrogenation of CO. Alkali treatment of these solids increased their activities in mixed alcohols synthesis [15–18]. g-Al2O3 has been regarded as a superior support for Mo-based catalysts due its ability to increase the degree of dispersion of Mo species increasing thus the concentration of catalytically active sites [19–23]. The present work reports the results of a study on the effects of Co3O4-doping of MoO3/Al2O3 system on its surface and catalytic properties. The role of calcinations condition and dopant concentration have been investigated. The techniques employed were XRD, nitrogen adsorption at 196 8C, H2O2 decomposition at 30–50 8C and isopropanol conversion at 200–500 8C.
2. Experimental details 2.1. Materials Pure MoO3/Al2O3 samples were prepared by treating a known mass of finely powdered aluminum hydroxide solid with fixed amount of ammonium molybdate dissolved in the least amount of distilled water making a paste. The proportion of ammonium molybdate was calculated so that the molar composition of calcined solids sample was 0.2 MoO3/Al2O3. The paste thus obtained was dried at 120 8C and then calcined for 5 h in air at 300, 500, 750 and 1000 8C. The mixed solids Co3O4–MoO3/ Al2O3 were prepared by treating a known mass of aluminum hydroxide with ammonium molybdate solution containing fixed amount of it, drying at 120 8C and then treating the dried solids with cobalt nitrate solution containing different proportions of it. The resulting materials were then dried at 120 8C and calcined for 5 h at 300, 500, 750 and 1000 8C. The concentration of cobalt nitrate solution employed corresponded to the addition of 2.04, 4, 7.69 and 14.29 mol% Co3O4 (with respect to the sum of MoO3 and Al2O3). All chemicals employed were of analytical grade and supplied by BDH company.
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2.2. Techniques An X-ray investigation of the thermal products of pure and variously doped mixed solids precalcined at 300, 500, 750 and 1000 8C was performed with a Philps diffractometer (type PW 1390). The patterns ˚ ) at 36 kV and 16 mA with scanning speed of were run with iron filtered cobalt radiation (l = 1.7889 A 28min1 in 2u. The specific surface areas, SBET, total pore volume, Vp and mean pore radius, r¯ of various solids were determined from N2-adsorption isotherms conducted at 196 8C using a conventional volumetric apparatus. Before carrying out the measurements, each sample was degassed under a reduced pressure of 105 Torr at 200 8C for 2 h. The catalytic activities of different catalyst samples were determined by studying the decomposition of H2O2 in their presence at temperatures within 30–50 8C using 100 mg of a given catalyst sample with 0.5 ml volume of H2O2 of known concentration diluted to 20 ml with distilled water. The reaction kinetics were monitored by measuring the volume of oxygen liberated at different time intervals until no further O2 was liberated. The catalytic activities of pure and variously doped catalyst samples were also determined by studying conversion of isopropanol over them. This reaction was carried out in contact with solids precalcined at 300 and 500 8C using micropulse technique under atmospheric pressure and different temperatures between 200–300 and 300–500 8C for the solids precalcined at 300 and 500 8C, respectively. A PerkinElmer Sigma 3B gas chromatograph was used with an OV1, 1 m and 1/8 in internal diameter, inj. 200 8C, oven 60 8C isotherm, FID 200 8C, 25 ml min1, helium as a carrier gas. Isopropanol was introduced in microquantities ((1–2) 103 cm3) with the aid of microsyringe in the form of pulse onto a small amount of catalyst sample (100 mg) the reaction products were transferred directly by the inert carrier gas (helium) to the gas chromatograph. 3. Results and discussion 3.1. XRD investigation of pure and variously doped solids precalcined at different temperatures XRD of pure and doped solids preheated in air at 300, 500, 750 and 1000 8C were measured. Figs. 1 and 2 show the XRD patterns of pure and variously doped solids preheated in air at 300 and 500 8C, respectively. Inspection of Fig. 1a revealed that: (i) All solids investigated precalcined at 300 8C composed of AlOOH as a major phase and MoO3 as a minor phase together with CoMoO4 in variously doped solids. (ii) The height of diffraction peaks of AlOOH decreased by increasing the amount of dopant added. This finding indicates that the dopant substrate enhanced the thermal decomposition of AlOOH into an amorphous alumina phase. (iii) The height of main diffraction peaks of CoMoO4 much increased upon doping with 4 mol% Co3O4 then decreased progressively upon increasing the amount of dopant added above this limit. (iv) The diffraction peaks of Co3O4 phase appeared in the XRD patterns of solids doped with 7.69 and 14.29 mol% Co3O4. These results showed that a portion of the dopant added 300 C interacted with MoO3 according to: Co3 O4 þ 3MoO3 ! 3CoMoO4 þ ð1=2ÞO2 and the other portion remained as a separate phase. The fact that the amount of cobalt oxide added was very small (2 mol%) and may be beyond the detection limit of X-ray diffractometer [24] suggested that most of cobalt added was retained on top surface layers of doped solids. This conclusion seems to be expected because the
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Fig. 1. X-ray diffractograms of pure and variously doped mixed solids precalcined at (a) 300 8C and (b) 500 8C.
doped solids were prepared by wet impregnation method and most of cobalt ions were uptaken on the uppermost surface layers of impregnated solid samples. Examination of Fig. 1b showed that: (i) All diffraction peaks of AlOOH disappeared completely from the patterns of pure and doped solids upon heating at 500 8C yielding a poorly crystalline g-Al2O3. (ii) Pure solids consisted of MoO3 and very poorly crystalline g-Al2O3. (iii) Different doped solids consisted of MoO3 and CoMoO4 together with un-reacted portion of Co3O4 in the samples rich in cobalt oxide (7.69 and 14.29 mol%). (iv) Similar to doped solids precalcined at 300 8C, the height of diffraction peaks of CoMoO4 in the solids precalcined at 500 8C increases by increasing the amount of dopant reaching to a maximum limit at 4 mol% Co3O4 then decreases upon increasing the amount of dopant above this limit. The presence of free Co3O4 in the heavily doped solid samples preheated at 500 8C revealed that MoO3 interacted in part with cobalt oxide yielding cobalt molybdate and the remind portion remained as unreacted separate phase. Fig. 2a depicts the diffractograms of pure and variously doped solids precalcined at 750 8C. It is seen from this figure that: (i) Pure mixed solids consisted of Al2(MoO4)3 and k-alumina phases having an excellent degree of crystallinity. (ii) The doped mixed solids consisted of CoMoO4, free Co3O4, besides Al2(MoO4)3 and k-Al2O3 phases. (iii) The relative intensity of the diffraction lines of CoMoO4 and Co3O4 phases increases by increasing the amount of cobalt oxide dopant added. This finding indicates that the amounts of cobalt molybdate and Co3O4 present in various doped samples increased as a function
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Fig. 2. X-ray diffractograms of pure and variously doped mixed solids precalcined at (a) 750 8C and (b) 1000 8C.
of the amount of dopant added. (iv) The amount of Al2(MoO4) decreases by increasing the amount of cobalt oxide added as evidenced from the progressive decrease in the intensity of diffraction peaks of aluminum molybdate by increasing the amount of dopant oxide added. These results suggested that: (i) The solid–solid interaction between MoO3 and Co3O4 yielding CoMoO4 according to Co3 O4 þ 3MoO3 !300 C 3CoMoO4 þ ð1=2ÞO2 takes place more easily than the reaction taking place between MoO3 and Al2O3 yielding Al2(MoO4)3; the former takes place at 300 8C while latter occurs at 750 8C. (ii) So, Co3O4 added interacted with most of MoO3 present in the system investigated leaving a relatively small portion of molybdena to interact with Al2O3 yielding Al2(MoO4)3. Fig. 2b shows the diffractograms of pure and doped solids precalcined at 1000 8C. Pure mixed solids consisted of a-Al2O3 that have an excellent degree of crystallinity. This sample acquired a gray colour and not white colour characteristic for pure a-alumina. So, these coloured solids indicate the formation of MoO3–Al2O3 solid solution. In fact, it has been reported that a-Al2O3 dissolved about 40 wt.% MoO3 by heating mixed oxides sample at 1000 8C [25]. It can also be seen from Fig. 2b that the variously doped mixed solids consisted of CoMoO4, CoAl2O4 and a-Al2O3 phases. The amount of CoAl2O4 and CoMoO4 increases by increasing the amount of cobalt oxide added. The close similarity between the diffraction data of Co3O4 and CoAl2O4 makes the distinction between them a difficult task, which can be decided by an XPS investigation. However, the black and yellow colours of Co3O4 and CoAl2O4, respectively, have been followed up to distinguish between these two phases. In fact, the degree of yellow colouration of variously doped solids was found to increase by increasing the amount of dopant added. These results showed clearly an easy formation of CoMoO4, which remained stable even by heating at 1000 8C.
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Table 1 Surface characteristics of pure and variously doped MoO3/Al2O3 precalcined at different temperatures Dopant content, Co3O4 (mol%)
Calcination temperature (8C)
SBET (m2/g)
St (m2/g)
VP (cm3/g)
˚) r (A
BET-C constant
0.0 2.04 4.00 7.60 14.29
300
114 87 79 94 86
122 90 86 96 86
0.101 0.066 0.062 0.087 0.067
18 15 16 19 16
381 251 551 231 84
0.0 2.04 4.00 7.60 14.29
500
133 128 128 140 102
144 138 135 150 106
0.172 0.180 0.267 0.317 0.250
26 28 42 45 50
54 38 17 16 71
0.0 2.04 4.00 7.60 14.29
750
16 10 8 39 20
16 9 8 42 21
0.020 0.017 0.016 0.151 0.075
25 34 40 77 75
14 91 18 45 221
3.2. Surface properties of pure and doped solids The different surface characteristics of pure and doped MoO3–CoO4/Al2O3 system preheated at 300, 500 and 700 8C were determined from nitrogen adsorption isotherms measured at 196 8C. These characteristics include the specific surface areas (SBET and St), total pore volume (Vp) and mean pore radius (¯r). The adsorption isotherms, not given belong to type II of Brunauer classification [26]. The SBET of different adsorbents were computed from linear plots of BET equation. The data obtained is given in Table 1, which includes also the values of Vp and r¯. An additional set of specific surface areas St was obtained from volume–thickness curves (Vl–t plots). The Vl–t plots are similar for pure and variously doped adsorbents precalcined at 300, 500 and 750 8C. These plots were constructed using standard t-curves depending on the values of the BET-C constant of each adsorbent sample given in the last column of Table 1. The Vl–t plots of pure and doped solids precalcined at 300 8C showed downward deviation while the Vl–t plots of pure and doped solids precalcined at 500 and 750 8C exhibited upward deviation. Fig. 3 depicts the Vl–t plots of pure and doped solids precalcined at 300 and 500 8C, the Vl–l plots of the adsorbents precalcined at 700 8C, not given are similar to those of the solids precalcined at 500 8C. It is seen from Fig. 3 that the investigated solids precalcined at 300 8C contained narrow pores as dominant porosity, while the adsorbents precalcined at 500 and 750 8C contained mainly wide pores. However, all the investigated adsorbents are considered as ˚. mesoporous solids [27]. In fact, the r¯ values of all investigated adsorbents vary between 16 and 78 A Inspection of Table 1 showed that: (i) the values of SBET and St for the investigated solids are close to each other which justifies the correct choice of standard t-curves for pore analysis and indicates the absence of ultramicropores in these adsorbents; (ii) the rise in precalcination temperature of pure and doped adsorbents from 300 to 500 8C resulted in a measurable increase in their specific surface areas, which suffered a sudden drop upon increasing the precalcination temperature to 750 8C; (iii) cobalt oxidedoping of the investigated system (0–4 mol%) followed by precalcination at 300 or 750 8C brought about
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Fig. 3. Vl–t plots of pure and variously doped solids precalcined at 300 and 500 8C.
a measurable decrease in the BET-surface area, the decrease attained 30 and 50% for the solids precalcined at 300 and 750 8C, respectively; (iv) the doping process (0–4 mol%) followed by heat treatment at 500 8C did not much affect the specific surface areas of the treated solids; (v) the doping process followed by precalcination at 300 and 750 8C resulted in a significant pore widening of variously doped samples. The observed increase in the BET-surface areas of pure and variously doped samples due to increasing their precalcination temperature from 300 to 500 8C can be attributed to a complete dehydroxylation of AlOOH forming poorly crystalline g-Al2O3 (cf. Fig. 1). While the significant decrease in the specific surface areas of pure and variously doped solids due to increasing their precalcination temperature to 750 8C can be attributed to crystallization of alumina into k-Al2O3, widening of the pores and also to the formation of Al2(MoO4)3 phase (cf. Fig. 2). The different solid–solid interactions taking place in pure and variously cobalt oxide-doped MoO3/ Al2O3 solids and the observed changes in their surface properties are expected to modify their catalytic activities.
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3.3. Catalytic activities of pure and variously doped catalysts The catalytic activities of pure and variously Co3O4-doped MoO3/Al2O3 solids precalcined at 300 and 500 8C were determined using H2O2 decomposition and conversion of isopropanol. 3.3.1. H2O2 decomposition over pure and doped solids Preliminary experiments showed that Al2O3 support material precalcined at 300 and 500 8C exhibited no measurable catalytic activity in H2O2 decomposition even when measured at 50 8C. So, the catalytically active constituents of the investigated system are MoO3, Co3O4, CoAl2O4 and CoMoO4 while Al2O3 acts as an inactive support material for H2O2 decomposition. The kinetics of H2O2 decomposition in the presence of pure and variously doped catalyst samples precalcined at 300 and 500 8C were monitored by measuring the volume of oxygen liberated at different time intervals at 30, 40 and 50 8C, respectively, until no further evolution of oxygen takes place. The catalytic decomposition reaction was found to follow first order-law in all cases as demonstrated by the plots obtained whose slopes allowed a ready determination of the reaction rate constant, k, measured at a given temperature over a given catalyst. The first order-plots of the catalytic reaction, not given, conducted at 30, 40, 50 8C over pure and doped catalyst samples precalcined at 300 and 500 8C allowed k values to be determined from the slopes of these plots. The computed k values measured at different reaction temperatures were graphically represented as a function of dopant concentration in Figs. 4 and 5 for the catalysts precalcined at 300 and 500 8C, respectively. Figs. 4 and 5 show that: (i) the curves relating k and dopant concentration show maxima found at 2.04 and 4 mol% Co3O4 for the catalysts precalcined at 300 and 500 8C, respectively; (ii) the rise of precalcination temperature of all solids investigated from 300 to 500 8C brought about a significant decrease (more than 50%) in their catalytic activities; (iii) the maximum increase in the catalytic activity of MoO3/Al2O3 precalcined at 300 8C due to doping with 2.04 mol% Co3O4 attained 1900, 250 and 139% for the catalytic reaction conducted at 30, 40 and 50 8C, respectively; (iv) the maximum increase in the catalytic activity of the solids investigated precalcined at 500 8C due to doping with 4 mol% Co3O4 attained 1266, 247 and 122% for the catalytic reaction carried out at 30, 40 and 50 8C, respectively. The increase in precalcination temperature of pure and doped solids from 300 to 500 8C brought about a measurable increase in their specific surface areas (cf. Table 1). This treatment, however, decreased their catalytic activities. This finding reflects a measurable decrease in the concentration of catalytically active constituents taking part in the catalytic reaction and could be attributed a possible decrease in the specific surface areas of active constituents, CoMoO4 and MoO3 via sintering process. The observed significant increase in the catalytic activities of the solids investigated due to doping with cobalt oxide (2.04 and 4 mol%) can be discussed in terms of conversion of MoO3, devoted with small catalytic activity into an active CoMoO4 compound. The total amount of cobalt oxide present as a dopant is relatively small, however its amount retained on the uppermost surface layers of the treated solids should be much greater than that nominally added. This conclusion is based on XPS measurements carried out before on the same system [14]. Treatment of MoO3/Al2O3 with small amounts of cobalt oxide followed by calcination at 300 or 500 8C which led to conversion of some of MoO3 into CoMoO4 and led also to creation of Co2+– Mo6+ and Co3+–Mo6+ ion pairs which act as active sites for H2O2 decomposition. The presence of maxima in the curves relating the catalytic activity and dopant concentration had been reported in the case of oxidation of CO by O2 and H2O2 decomposition carried out over MoO3 doped Co3O4/Al2O3 solids [28]. The descending part of curves relating k versus the amount of cobalt oxide added to
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Fig. 4. Effect of Co3O4 doping on catalytic activity of MoO3/Al2O3 precalcined at 300 8C at reaction termperatures 30, 40 and 50 8C.
MoO3/Al2O3 system (Figs. 4 and 5) could be attributed to formation of well ordered and big sized CoMoO4 and Co3O4 phases. Determination of the apparent activation energy (DEa) for the catalytic decomposition of H2O2 in the presence of pure and doped MoO3/Al2O3 solids has shed some light on the possible changes in the mechanism of the catalyzed reaction. Thus, the values of k measured at 30, 40 and 50 8C over pure and variously doped solids have allowed (DEa) to be obtained via direct application of the Arrhenius equation. The values of DEa thus obtained are listed in Table 2. Also, included in Table 2 are the values of log A (frequency factor of the Arrhenius equation) calculated for variously investigated solids. Inspection of the data listed in Table 2 revealed that DEa values calculated for pure and doped solids precalcined at 300 and 500 8C suffer fluctuation, i.e. both decrease and increase upon increasing the amount of dopant added. This finding did not run parallel to the observed changes in the catalytic activity of doped solids. It can also be seen from Table 2 that same fluctuation in the values of log A is also observed. The recalculation of DEa values for the reaction carried out over pure and variously doped solids adopting the values of log A for the pure specimens precalcined at 300 and 500 8C to the other doped samples precalcined at the same temperatures, DEa are thus computed and given in the
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Fig. 5. Effect of Co3O4 doping on catalytic activity of MoO3/Al2O3 precalcined at 500 8C at reaction termperatures 30, 40 and 50 8C.
last column of Table 2. The comparison of DEa for pure and doped samples showed that the values of 122 1 and 117 kJ mol1 stand for the activation energies of H2O2 decomposition conducted over pure and doped solids precalcined at 300 and 500 8C, respectively. In other words, Co3O4-doping of MoO3/Al2O3 followed by precalcination at 300 and 500 8C did not modify the mechanism of catalytic decomposition of H2O2 but increased the concentration of active sites taking part in the catalytic reaction. This conclusion is also, further supported by an analysis based on the dissipation function for the energy of the active sites arising from surface heterogeneity ƒ(Ei) = a exp(hEi), where Ei is the interaction energy of site i with substrate. This equation may be converted into the form [29,30]: A = a exp(hDEa). Suggesting that a plot of log A versus (DEa) for the variously doped catalyst samples should give a straight line whose slope and intercept would allow the evaluation the constants h and a, respectively. Fig. 6 depicts the variation of DEa as a function of log A for the catalytic decomposition of H2O2 carried out over pure and doped solids, precalcined at 300 and 500 8C, respectively. The computed values of h were 0.377 and 0.378 mol/kJ min for cobalt oxide-doped solids precalcined at 300 and 500 8C, respectively. The calculated values of a were 0.30 and 0.14 min1 for pure and doped solids preheated at 300 and 500 8C.
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Table 2 Effect of Co3O4 doping of MoO3/Al2O3 system precalcined at 300 and 500 8C on the reaction rate constant (k0 ), activation energies (DEa, DEa ) and frequency factor (log A) for H2O2 decomposition conducted at 30, 40 and 50 8C Dopant concentration (mol%)
Calcination temperature (8C)
k0 , 30 8C (103 min1 m2)
k0 , 40 8C (103 min1 m2)
k0 , 50 8C (103 min1 m2)
DEa (kJ mol1)
Log A
DEa (kJ mol1)
0.00 2.04 4.00 7.69 14.29
300
6.1 160.9 46.8 69.1 53.5
52.6 241.4 121.5 138.3 127.9
100.1 316.1 201.3 210.6 211.6
123 28 53 46 45
45.88 10.10 19.39 16.83 16.41
123 121 122 121 122
0.00 2.04 4.00 7.69 14.29
500
2.3 12.5 32.0 20.7 20.6
14.3 28.9 51.6 40.0 47.1
33.8 51.6 78.1 65.7 23.3
118 73 48 55 75
43.87 26.74 17.34 19.98 27.68
118 117 117 117 117
Fig. 6. Relationship between activation energy and frequency factor for the catalytic reaction conducted over pure and doped solids precalcined at 300 and 500 8C.
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Table 3 Dependence of catalytic activity and selectivity of pure and variously doped MoO3/Al2O3 system precalcined at 300 8C on reaction temperature and amount of Co3O4 dopant in isopropanol conversion reaction Selectivity for propene formation (%)
Selectivity for acetone formation (%)
Dopant concentration (mol%)
Reaction temperature (8C)
Isopropanol conversion (%)
0.00 2.04 4.00 7.69 14.29
300
100.00 57.96 16.60 38.65 25.20
4.24 0.00 0.00 0.00 0.00
95.76 100.00 100.00 100.00 100.00
0.00 2.04 4.00 7.69 14.29
280
80.00 38.85 9.55 23.94 16.22
31.8 0.00 0.00 0.00 0.00
68.12 100.00 100.00 100.00 100.00
0.00 2.04 4.00 7.69 14.29
260
75.34 23.76 5.70 15.58 8.91
38.09 0.00 0.00 0.00 0.00
61.91 100.00 100.00 100.00 100.00
0.00 2.04 4.00 7.69 14.29
240
61.00 16.00 3.51 12.25 5.75
48.28 0.00 0.00 0.00 0.00
51.72 100.00 100.00 100.00 100.00
0.00 2.04 4.00 7.69 14.29
220
23.12 11.84 1.96 6.11 3.31
69.39 0.00 0.00 0.00 0.00
30.61 100.00 100.00 100.00 100.00
0.00 2.04 4.00 7.69 14.29
200
10.14 7.23 1.20 3.44 1.99
100.00 0.00 0.00 0.00 0.00
0.00 100.00 100.00 100.00 100.00
The constants h and a values indicate that Co3O4 doping of MoO4-doping of MoO3/Al2O3 system did not modify the dissipation of active sites, i.e. character of surface heterogeneity. In other words, this treatment did not change the energetic nature of active sites but increased their concentration. 3.3.2. Conversion of isopropanol over pure and doped solids Preliminary experiments showed that Al2O3 support material precalcined at 300 or 500 8C exhibited relatively high catalytic activity in alcohol conversion yielding propene only and acting thus as a selective dehydration catalyst. The catalytic conversion of isopropanol was conducted over pure and cobalt oxidedoped MoO3/Al2O3 solids precalcined at 300 and 500 8C. The reaction temperature was varied between
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Table 4 Dependence of catalytic activity and selectivity of pure and variously doped MoO3/Al2O3 system precalcined at 500 8C on reaction temperature and amount of Co3O4 dopant in isopropanol conversion reaction Selectivity for propene formation (%)
Selectivity for acetone formation (%)
Dopant concentration (mol%)
Reaction temperature (8C)
Isopropanol conversion (%)
0.00 2.04 4.00 7.69 14.29
500
100.00 100.00 100.00 100.00 100.00
22.44 81.63 58.56 33.39 21.97
77.56 18.37 41.44 66.61 78.03
0.00 2.04 4.00 7.69 14.29
480
100.00 100.00 100.00 100.00 100.00
24.25 87.31 61.64 39.00 27.27
75.75 12.69 38.36 61.00 72.73
0.00 2.04 4.00 7.69 14.29
460
100.00 100.00 100.00 100.00 100.00
25.71 89.33 63.93 44.30 32.51
74.29 10.67 36.07 55.70 67.49
0.00 2.04 4.00 7.69 14.29
440
100.00 100.00 100.00 100.00 100.00
28.87 91.01 67.56 50.26 34.64
71.13 8.99 32.44 49.74 65.36
0.00 2.04 4.00 7.69 14.29
420
100.00 100.00 100.00 85.91 81.22
32.38 91.51 69.79 54.40 37.71
67.62 8.49 30.21 45.46 62.29
0.00 2.04 4.00 7.39 14.29
400
92.25 100.00 100.00 78.53 78.17
36.47 93.49 71.11 67.87 39.31
63.63 6.51 28.89 32.13 60.69
0.00 2.04 4.00 7.69 14.29
350
46.07 100.00 100.00 59.84 35.22
81.05 95.40 81.04 72.29 44.47
18.95 4.60 18.96 27.71 55.53
0.00 2.04 4.00 7.69 14.29
300
5.00 35.56 38.42 31.37
100.00 100.00 88.62 77.24
0.00 0.00 11.38 22.76
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200 and 300 8C for the catalysts preheated at 300 8C and between 300 and 500 8C for the solids preheated at 500 8C. The chromatographic investigation of the reaction products over various catalysts for the reaction carried out at different temperatures permitted the determination of activity and selectivity of various catalysts as being influenced by doping, reaction temperature and calcination temperature of solids investigated. The activity and selectivity towards propene formation (dehydration) and acetone (dehydrogenation) were calculated for different solids preheated at 300 and 500 8C and the data obtained are listed in Tables 2–4. Inspection of Tables 2–4 showed the following: (i) The catalytic activity of solids investigated precalcined at 300 8C decreases progressively by increasing the amount of dopant added. (ii) The addition of small amounts of cobalt oxide dopant (2.04–14.29 mol%) to the system investigated followed by heating at 300 8C turned these solids from dehydration to dehydrogenation catalysts for alcohol conversion carried out at 200–300 8C. (iii) The dehydrogenation selectivity for various solids calcined at 300 or 500 8C increases by increasing the reaction temperature. (iv) Opposite trend is observed for the selectivity in dehydration reaction. (v) The addition of small amounts of cobalt oxide (2.04, 4 mol) to MoO3/Al2O3 solids followed by calcination at 500 8C resulted in an effective increase in their catalytic activity measured at 300–400 8C. (vi) The selectivity of doped solids precalcined at 500 8C towards propene formation (dehydration) decreases progressively by increasing the amount of dopant added within 2.04–14.29 mol%. (vii) The dehydrogenation selectivity (acetone formation) of these solids increases as a function of the amount of dopant. (viii) The selectivities of pure and variously doped solids precalcined at 500 8C towards dehydrogenation of alcohol increases by increasing the reaction temperature while that of dehydration reaction decreases as a function of reaction temperature. (ix) The rise in precalcination temperature of pure and doped solid from 300 to 500 8C brought about a measurable decrease in their catalytic activity measured at 300 8C. The decrease was, however, more pronounced for pure solid catalysts. The results obtained revealed that Al2O3 support material and MoO3/Al2O3 solids precalcined at 300 and 500 8C behave as dehydration catalysts for alcohol conversion carried out at 200 and 300 8C for the solids precalcined at 300 and 500 8C, respectively.
4. Conclusions The results obtained in the present investigation permitted to draw the following conclusions: 1. Heat treatment of MoO3–Co3O4/Al2O3 system at 300 8C led to the formation of CoMoO4, which remained stable even by heating at 1000 8C. Solid–solid interaction between Al2O3 and MoO3 yielding Al2(MoO4)3 took place by heating at 750 8C. The produced aluminum molybdate decomposed at 1000 8C giving MoO3 and a-Al2O3. A portion of produced molybdena sublimed and the other portion dissolved in the alumina to give MoO–Al2O3 solid solution, which acquired a grey colour. 2. Doping of MoO3/Al2O3 system with small amount of Co3O4 followed by heating at 300–750 8C brought about measurable changes in its surface characteristics. Both increase and decrease in the BET-surface areas were observed depending on calcination temperature and the amount of dopant added.
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3. The doping process of the investigated system with small amounts of Co3O4 (2.04, 4 mol%) effected a considerable increase in its catalytic activity in H2O2 decomposition. The increase was, however, more pronounced for the catalysts precalcined at 300 8C. The maximum increase in the catalytic activity, expressed as reaction rate constant per unit surface area for the reaction conducted at 30 8C over pure and doped solids attained 96- and 13-fold for the catalysts precalcined at 300 and 500 8C, respectively. 4. Treating MoO3/Al2O3 solids with small amounts of Co3O4 followed by calcination at 300 8C brought about a progressive decrease in their catalytic activities in isopropanol conversion conducted at 200– 300 8C. This treatment, on the other hand, increased the selectivity of the treated solids towards dehydrogenation of reacting alcohol yielding acetone. Doping MoO3/Al2O3 system with Co3O4 > 2.04 mol% conducted at 500 8C lead to a progressive decrease in its selectivity towards dehydration of isopropanol (yielding propene). 5. The doping process of MoO3/Al2O3 system with Co3O4 did not modify the mechanism of catalytic decomposition of H2O2 but changed the concentration of catalytically active sites without changing their energetic nature.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
[25] [26]
G.A. El-Shobaky, A.N. Al-Noaimi, Appl. Catal. 29 (1987) 235. G.A. El-Shobaky, A.N. Al-Noaimi, T.M.H. Saber, Bull. Soc. Chim. France 6 (1988) 930. G.A. El-Shobaky, G.A. Fagal, T.M.H. Saber, Bull. Soc. Chim. France 4 (1987) 547. G.A. El-Shobaky, G.A. Fagal, Egypt. J. Chem. 31 (5) (1988) 331. A.M. Youssef, S.A. El-Hakam, G.A. El-Shobaky, Radiat. Phys. Chem. 40 (1992) 575. G.A. El-Shobaky, Th. El-Nabarawy, G.A. Fagal, A.S. Ahmed, Mater. Lett. 17 (1993) 297. G.A. El-Shobaky, G.A. Fagal, A.M. Ghozza, M.A. Shouman, Mater. Lett. 19 (1994) 225. G.A. El-Shobaky, A.M. Ghozza, S. Hammad, Adsorpt. Sci. Technol. 12 (2) (1995) 119. G.A. El-Shobaky, A.M. Ghozza, S.H. El-Khouly, Adsorpt. Sci. Technol. 13 (1996) 153. G.A. El-Shobaky, A.M. Ghozza, G.M. Mohamed, Adsorpt. Sci. Technol. 16 (1998) 415. H.G. El-Shobaky, M. Mokhtar, G.A. El-Shobaky, Appl. Catal. A 180 (1999) 335. A.M. Ghozza, G.A. El-Shobaky, G.M. Mohamed, Adsorpt. Sci. Technol. 18 (2000) 561. N. Radwan, G.A. Fagal, G.A. El-Shobaky, Colloids Surf. A 178 (2001) 277. G.A. El-Shobaky, A.M. Ghozza, A.A. El-Warraky, G.M. Mohamed, Colloids Surf. A 219 (2003) 97. G.-Z. Biana, L. Fanb, Yi-Fua, K. Fujimotob, Appl. Catal. A 170 (1998) 255. G.J. Quarderer, K.A. Cochran, European Patent 119,609 (1984). R.R. Stevens, European Patent 172,431 (1986). Y. Xie, B.M. Naasz, G.A. Somorjai, Appl. Catal. 27 (1986) 233. C.B. Murchison, M.M. Conway, R.R. Stevens, G.J. Quarderer, in: M.J. Philips, M. Ternan (Eds.), Proceedings of the 9th International Congress on Catalalysts, vol. II, Calgary, Ottawa, Canada, 1988, p. 626. T. Tatsmi, A. Muramatsu, H. Tominaga, Appl. Catal. 34 (1987) 77. J. Zhang, Y. Wang, L. Chang, Appl. Catal. A 126 (1995) 205. M. Jiang, G.Z. Bian, Y.L. Fu, J. Catal. 146 (1994) 144. Y. Avila, C. Kappenstein, S. Pronier, J. Bronier, J. Barrault, Appl. Catal. A 132 (1995) 97. (a) E. Swanson, C.F. Fuyat, Nat. Bur. Stand. Circ. II (1953) 38; (b) E. Swanson, C.F. Fuyat, Powder Diffraction File (JCPOS), International Center Diffraction Data, Swarthmore, PA, 1979. G.A. El-Shobaky, F.H.A. Abdalla, A.M. Ghozza, K.A. Khalil, Thermochim. Acta 275 (1996) 235. S. Brunauer, L.S. Deming, W.E. Deming, E.E. Teller, J. Am. Chem. Soc. 62 (1940) 1723.
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[27] J. Rouquerol, D. Avnir, C.W. Fairbridge, D.H. Evett, G.H. Haynes, N. Pernicone, J.D.F. Ramsay, K.S.W. Sing, K.K. Unger, Pure Appl. Chem. 66 (1994) 1739. [28] G.A. El-Shobaky, S.A. El-Molla, G.M. Mohamed, A.A. Zahran, Mater. Lett. 57 (2003) 1612. [29] M. Lo Jacono, M. Schiavello, V.H.J. Dee Beer, G. Minerlli, Am. Chem. Soc. 81 (1977) 1583. [30] A.A. Balandin, Dokl. Akad. Nauk. SSSR 93 (1953) 55.
Surface and catalytic properties of MoO3/Al2O3 system doped with Co3O4 A.A. Zahran a, W.M. Shaheen b, G.A. El-Shobaky b,* a
Chemistry Department, Faculty of Science, Ain Shams University, Cairo, Egypt b Department of Physical Chemistry, National Research Center, Cairo, Egypt
Received 28 October 2004; received in revised form 8 December 2004; accepted 8 April 2005
Abstract Thermal solid–solid interactions in cobalt treated MoO3/Al2O3 system were investigated using X-ray powder diffraction. The solids were prepared by wet impregnation method using Al(OH)3, ammonium molybdate and cobalt nitrate solutions, drying at 100 8C then calcination at 300, 500, 750 and 1000 8C. The amount of MoO3, was fixed at 16.67 mol% and those of cobalt oxide were varied between 2.04 and 14.29 mol% Co3O4. Surface and catalytic properties of various solid samples precalcined at 300 and 500 8C were studied using nitrogen adsorption at 196 8C, conversion of isopropanol at 200–500 8C and decomposition of H2O2 at 30–50 8C. The results obtained revealed that pure mixed solids precalcined at 300 8C consisted of AlOOH and MoO3 phases. Cobalt oxide-doped samples calcined at the same temperature consisted also of AlOOH, MoO3 and CoMoO4 compounds. The rise in calcination temperature to 500 8C resulted in complete conversion of AlOOH into very poorly crystalline g-Al2O3. The further increase in precalcination temperature to 750 8C led to the formation of Al2(MoO4)3, k-Al2O3 besides CoMoO4 and un-reacted portion of Co3O4 in the samples rich in cobalt oxide. Pure MoO3/Al2O3 preheated at 1000 8C composed of MoO3–aAl2O3 solid solution (acquired grey colour). The doped samples consisted of the same solid solution together with CoMoO4 and CoAl2O4 compounds. The increase in calcination temperature of pure and variously doped solids from 300 to 500 8C increased their specific surface areas and total pore volume which suffered a drastic decrease upon heating at 750 8C. Doping the investigated system with small amounts of cobalt oxide (2.04 and 4 mol%) followed by heating at 300 and 500 8C increased its catalytic activity in H2O2 decomposition. This increase, measured at 300 8C, attained 25.4- and 12.9fold for the solids precalcined at 300 and 500 8C, respectively. The increase in the amount of dopant added above this limit decreased the catalytic activity which remained bigger than those of un-treated catalysts. On the other hand, the doping process decreased the catalytic activity of treated solids in isopropanol conversion especially the
* Corresponding author. Tel.: +20 2 7494 265; fax: +20 2 3370 931. E-mail address: [email protected] (G.A. El-Shobaky). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.04.003
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catalysts precalcined at 300 8C. This treatment modified the selectivities of treated solids towards dehydration and dehydrogenation of reacted alcohol. The activation energies of H2O2 decomposition were determined for pure and variously doped solids. The results obtained were discussed in light of induced changes in chemical composition and surface properties of the investigated system due to doping with cobalt oxide. # 2005 Elsevier Ltd. All rights reserved.
1. Introduction The physicochemical, surface and catalytic properties of various transition metal oxides supported on g-Al2O3 made the object of several investigations carried out in our laboratory. These catalysts, include NiO/Al2O3, Co3O4/Al2O3, CuO/Al2O3, Mn2O3/Al2O3, Fe2O3/Al2O3 and Cr2O3/Al2O3 and were used in CO oxidation by O2 [1–10]. The catalytic activities of these solids have been found to increase by doping with certain foreign oxide as Li2O, K2O, MoO3, CeO2 or ZnO [11–14]. The alkali treatment of some of these systems increased their catalytic activities in alcohols conversion and H2O2 decomposition. Mo-based catalysts have been widely applied in many hydrotreating reactions due to their big activities for hydrodesulfurization, hydrodenitrogenation reactions and methane synthesis from hydrogenation of CO. Alkali treatment of these solids increased their activities in mixed alcohols synthesis [15–18]. g-Al2O3 has been regarded as a superior support for Mo-based catalysts due its ability to increase the degree of dispersion of Mo species increasing thus the concentration of catalytically active sites [19–23]. The present work reports the results of a study on the effects of Co3O4-doping of MoO3/Al2O3 system on its surface and catalytic properties. The role of calcinations condition and dopant concentration have been investigated. The techniques employed were XRD, nitrogen adsorption at 196 8C, H2O2 decomposition at 30–50 8C and isopropanol conversion at 200–500 8C.
2. Experimental details 2.1. Materials Pure MoO3/Al2O3 samples were prepared by treating a known mass of finely powdered aluminum hydroxide solid with fixed amount of ammonium molybdate dissolved in the least amount of distilled water making a paste. The proportion of ammonium molybdate was calculated so that the molar composition of calcined solids sample was 0.2 MoO3/Al2O3. The paste thus obtained was dried at 120 8C and then calcined for 5 h in air at 300, 500, 750 and 1000 8C. The mixed solids Co3O4–MoO3/ Al2O3 were prepared by treating a known mass of aluminum hydroxide with ammonium molybdate solution containing fixed amount of it, drying at 120 8C and then treating the dried solids with cobalt nitrate solution containing different proportions of it. The resulting materials were then dried at 120 8C and calcined for 5 h at 300, 500, 750 and 1000 8C. The concentration of cobalt nitrate solution employed corresponded to the addition of 2.04, 4, 7.69 and 14.29 mol% Co3O4 (with respect to the sum of MoO3 and Al2O3). All chemicals employed were of analytical grade and supplied by BDH company.
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2.2. Techniques An X-ray investigation of the thermal products of pure and variously doped mixed solids precalcined at 300, 500, 750 and 1000 8C was performed with a Philps diffractometer (type PW 1390). The patterns ˚ ) at 36 kV and 16 mA with scanning speed of were run with iron filtered cobalt radiation (l = 1.7889 A 28min1 in 2u. The specific surface areas, SBET, total pore volume, Vp and mean pore radius, r¯ of various solids were determined from N2-adsorption isotherms conducted at 196 8C using a conventional volumetric apparatus. Before carrying out the measurements, each sample was degassed under a reduced pressure of 105 Torr at 200 8C for 2 h. The catalytic activities of different catalyst samples were determined by studying the decomposition of H2O2 in their presence at temperatures within 30–50 8C using 100 mg of a given catalyst sample with 0.5 ml volume of H2O2 of known concentration diluted to 20 ml with distilled water. The reaction kinetics were monitored by measuring the volume of oxygen liberated at different time intervals until no further O2 was liberated. The catalytic activities of pure and variously doped catalyst samples were also determined by studying conversion of isopropanol over them. This reaction was carried out in contact with solids precalcined at 300 and 500 8C using micropulse technique under atmospheric pressure and different temperatures between 200–300 and 300–500 8C for the solids precalcined at 300 and 500 8C, respectively. A PerkinElmer Sigma 3B gas chromatograph was used with an OV1, 1 m and 1/8 in internal diameter, inj. 200 8C, oven 60 8C isotherm, FID 200 8C, 25 ml min1, helium as a carrier gas. Isopropanol was introduced in microquantities ((1–2) 103 cm3) with the aid of microsyringe in the form of pulse onto a small amount of catalyst sample (100 mg) the reaction products were transferred directly by the inert carrier gas (helium) to the gas chromatograph. 3. Results and discussion 3.1. XRD investigation of pure and variously doped solids precalcined at different temperatures XRD of pure and doped solids preheated in air at 300, 500, 750 and 1000 8C were measured. Figs. 1 and 2 show the XRD patterns of pure and variously doped solids preheated in air at 300 and 500 8C, respectively. Inspection of Fig. 1a revealed that: (i) All solids investigated precalcined at 300 8C composed of AlOOH as a major phase and MoO3 as a minor phase together with CoMoO4 in variously doped solids. (ii) The height of diffraction peaks of AlOOH decreased by increasing the amount of dopant added. This finding indicates that the dopant substrate enhanced the thermal decomposition of AlOOH into an amorphous alumina phase. (iii) The height of main diffraction peaks of CoMoO4 much increased upon doping with 4 mol% Co3O4 then decreased progressively upon increasing the amount of dopant added above this limit. (iv) The diffraction peaks of Co3O4 phase appeared in the XRD patterns of solids doped with 7.69 and 14.29 mol% Co3O4. These results showed that a portion of the dopant added 300 C interacted with MoO3 according to: Co3 O4 þ 3MoO3 ! 3CoMoO4 þ ð1=2ÞO2 and the other portion remained as a separate phase. The fact that the amount of cobalt oxide added was very small (2 mol%) and may be beyond the detection limit of X-ray diffractometer [24] suggested that most of cobalt added was retained on top surface layers of doped solids. This conclusion seems to be expected because the
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Fig. 1. X-ray diffractograms of pure and variously doped mixed solids precalcined at (a) 300 8C and (b) 500 8C.
doped solids were prepared by wet impregnation method and most of cobalt ions were uptaken on the uppermost surface layers of impregnated solid samples. Examination of Fig. 1b showed that: (i) All diffraction peaks of AlOOH disappeared completely from the patterns of pure and doped solids upon heating at 500 8C yielding a poorly crystalline g-Al2O3. (ii) Pure solids consisted of MoO3 and very poorly crystalline g-Al2O3. (iii) Different doped solids consisted of MoO3 and CoMoO4 together with un-reacted portion of Co3O4 in the samples rich in cobalt oxide (7.69 and 14.29 mol%). (iv) Similar to doped solids precalcined at 300 8C, the height of diffraction peaks of CoMoO4 in the solids precalcined at 500 8C increases by increasing the amount of dopant reaching to a maximum limit at 4 mol% Co3O4 then decreases upon increasing the amount of dopant above this limit. The presence of free Co3O4 in the heavily doped solid samples preheated at 500 8C revealed that MoO3 interacted in part with cobalt oxide yielding cobalt molybdate and the remind portion remained as unreacted separate phase. Fig. 2a depicts the diffractograms of pure and variously doped solids precalcined at 750 8C. It is seen from this figure that: (i) Pure mixed solids consisted of Al2(MoO4)3 and k-alumina phases having an excellent degree of crystallinity. (ii) The doped mixed solids consisted of CoMoO4, free Co3O4, besides Al2(MoO4)3 and k-Al2O3 phases. (iii) The relative intensity of the diffraction lines of CoMoO4 and Co3O4 phases increases by increasing the amount of cobalt oxide dopant added. This finding indicates that the amounts of cobalt molybdate and Co3O4 present in various doped samples increased as a function
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Fig. 2. X-ray diffractograms of pure and variously doped mixed solids precalcined at (a) 750 8C and (b) 1000 8C.
of the amount of dopant added. (iv) The amount of Al2(MoO4) decreases by increasing the amount of cobalt oxide added as evidenced from the progressive decrease in the intensity of diffraction peaks of aluminum molybdate by increasing the amount of dopant oxide added. These results suggested that: (i) The solid–solid interaction between MoO3 and Co3O4 yielding CoMoO4 according to Co3 O4 þ 3MoO3 !300 C 3CoMoO4 þ ð1=2ÞO2 takes place more easily than the reaction taking place between MoO3 and Al2O3 yielding Al2(MoO4)3; the former takes place at 300 8C while latter occurs at 750 8C. (ii) So, Co3O4 added interacted with most of MoO3 present in the system investigated leaving a relatively small portion of molybdena to interact with Al2O3 yielding Al2(MoO4)3. Fig. 2b shows the diffractograms of pure and doped solids precalcined at 1000 8C. Pure mixed solids consisted of a-Al2O3 that have an excellent degree of crystallinity. This sample acquired a gray colour and not white colour characteristic for pure a-alumina. So, these coloured solids indicate the formation of MoO3–Al2O3 solid solution. In fact, it has been reported that a-Al2O3 dissolved about 40 wt.% MoO3 by heating mixed oxides sample at 1000 8C [25]. It can also be seen from Fig. 2b that the variously doped mixed solids consisted of CoMoO4, CoAl2O4 and a-Al2O3 phases. The amount of CoAl2O4 and CoMoO4 increases by increasing the amount of cobalt oxide added. The close similarity between the diffraction data of Co3O4 and CoAl2O4 makes the distinction between them a difficult task, which can be decided by an XPS investigation. However, the black and yellow colours of Co3O4 and CoAl2O4, respectively, have been followed up to distinguish between these two phases. In fact, the degree of yellow colouration of variously doped solids was found to increase by increasing the amount of dopant added. These results showed clearly an easy formation of CoMoO4, which remained stable even by heating at 1000 8C.
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Table 1 Surface characteristics of pure and variously doped MoO3/Al2O3 precalcined at different temperatures Dopant content, Co3O4 (mol%)
Calcination temperature (8C)
SBET (m2/g)
St (m2/g)
VP (cm3/g)
˚) r (A
BET-C constant
0.0 2.04 4.00 7.60 14.29
300
114 87 79 94 86
122 90 86 96 86
0.101 0.066 0.062 0.087 0.067
18 15 16 19 16
381 251 551 231 84
0.0 2.04 4.00 7.60 14.29
500
133 128 128 140 102
144 138 135 150 106
0.172 0.180 0.267 0.317 0.250
26 28 42 45 50
54 38 17 16 71
0.0 2.04 4.00 7.60 14.29
750
16 10 8 39 20
16 9 8 42 21
0.020 0.017 0.016 0.151 0.075
25 34 40 77 75
14 91 18 45 221
3.2. Surface properties of pure and doped solids The different surface characteristics of pure and doped MoO3–CoO4/Al2O3 system preheated at 300, 500 and 700 8C were determined from nitrogen adsorption isotherms measured at 196 8C. These characteristics include the specific surface areas (SBET and St), total pore volume (Vp) and mean pore radius (¯r). The adsorption isotherms, not given belong to type II of Brunauer classification [26]. The SBET of different adsorbents were computed from linear plots of BET equation. The data obtained is given in Table 1, which includes also the values of Vp and r¯. An additional set of specific surface areas St was obtained from volume–thickness curves (Vl–t plots). The Vl–t plots are similar for pure and variously doped adsorbents precalcined at 300, 500 and 750 8C. These plots were constructed using standard t-curves depending on the values of the BET-C constant of each adsorbent sample given in the last column of Table 1. The Vl–t plots of pure and doped solids precalcined at 300 8C showed downward deviation while the Vl–t plots of pure and doped solids precalcined at 500 and 750 8C exhibited upward deviation. Fig. 3 depicts the Vl–t plots of pure and doped solids precalcined at 300 and 500 8C, the Vl–l plots of the adsorbents precalcined at 700 8C, not given are similar to those of the solids precalcined at 500 8C. It is seen from Fig. 3 that the investigated solids precalcined at 300 8C contained narrow pores as dominant porosity, while the adsorbents precalcined at 500 and 750 8C contained mainly wide pores. However, all the investigated adsorbents are considered as ˚. mesoporous solids [27]. In fact, the r¯ values of all investigated adsorbents vary between 16 and 78 A Inspection of Table 1 showed that: (i) the values of SBET and St for the investigated solids are close to each other which justifies the correct choice of standard t-curves for pore analysis and indicates the absence of ultramicropores in these adsorbents; (ii) the rise in precalcination temperature of pure and doped adsorbents from 300 to 500 8C resulted in a measurable increase in their specific surface areas, which suffered a sudden drop upon increasing the precalcination temperature to 750 8C; (iii) cobalt oxidedoping of the investigated system (0–4 mol%) followed by precalcination at 300 or 750 8C brought about
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Fig. 3. Vl–t plots of pure and variously doped solids precalcined at 300 and 500 8C.
a measurable decrease in the BET-surface area, the decrease attained 30 and 50% for the solids precalcined at 300 and 750 8C, respectively; (iv) the doping process (0–4 mol%) followed by heat treatment at 500 8C did not much affect the specific surface areas of the treated solids; (v) the doping process followed by precalcination at 300 and 750 8C resulted in a significant pore widening of variously doped samples. The observed increase in the BET-surface areas of pure and variously doped samples due to increasing their precalcination temperature from 300 to 500 8C can be attributed to a complete dehydroxylation of AlOOH forming poorly crystalline g-Al2O3 (cf. Fig. 1). While the significant decrease in the specific surface areas of pure and variously doped solids due to increasing their precalcination temperature to 750 8C can be attributed to crystallization of alumina into k-Al2O3, widening of the pores and also to the formation of Al2(MoO4)3 phase (cf. Fig. 2). The different solid–solid interactions taking place in pure and variously cobalt oxide-doped MoO3/ Al2O3 solids and the observed changes in their surface properties are expected to modify their catalytic activities.
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3.3. Catalytic activities of pure and variously doped catalysts The catalytic activities of pure and variously Co3O4-doped MoO3/Al2O3 solids precalcined at 300 and 500 8C were determined using H2O2 decomposition and conversion of isopropanol. 3.3.1. H2O2 decomposition over pure and doped solids Preliminary experiments showed that Al2O3 support material precalcined at 300 and 500 8C exhibited no measurable catalytic activity in H2O2 decomposition even when measured at 50 8C. So, the catalytically active constituents of the investigated system are MoO3, Co3O4, CoAl2O4 and CoMoO4 while Al2O3 acts as an inactive support material for H2O2 decomposition. The kinetics of H2O2 decomposition in the presence of pure and variously doped catalyst samples precalcined at 300 and 500 8C were monitored by measuring the volume of oxygen liberated at different time intervals at 30, 40 and 50 8C, respectively, until no further evolution of oxygen takes place. The catalytic decomposition reaction was found to follow first order-law in all cases as demonstrated by the plots obtained whose slopes allowed a ready determination of the reaction rate constant, k, measured at a given temperature over a given catalyst. The first order-plots of the catalytic reaction, not given, conducted at 30, 40, 50 8C over pure and doped catalyst samples precalcined at 300 and 500 8C allowed k values to be determined from the slopes of these plots. The computed k values measured at different reaction temperatures were graphically represented as a function of dopant concentration in Figs. 4 and 5 for the catalysts precalcined at 300 and 500 8C, respectively. Figs. 4 and 5 show that: (i) the curves relating k and dopant concentration show maxima found at 2.04 and 4 mol% Co3O4 for the catalysts precalcined at 300 and 500 8C, respectively; (ii) the rise of precalcination temperature of all solids investigated from 300 to 500 8C brought about a significant decrease (more than 50%) in their catalytic activities; (iii) the maximum increase in the catalytic activity of MoO3/Al2O3 precalcined at 300 8C due to doping with 2.04 mol% Co3O4 attained 1900, 250 and 139% for the catalytic reaction conducted at 30, 40 and 50 8C, respectively; (iv) the maximum increase in the catalytic activity of the solids investigated precalcined at 500 8C due to doping with 4 mol% Co3O4 attained 1266, 247 and 122% for the catalytic reaction carried out at 30, 40 and 50 8C, respectively. The increase in precalcination temperature of pure and doped solids from 300 to 500 8C brought about a measurable increase in their specific surface areas (cf. Table 1). This treatment, however, decreased their catalytic activities. This finding reflects a measurable decrease in the concentration of catalytically active constituents taking part in the catalytic reaction and could be attributed a possible decrease in the specific surface areas of active constituents, CoMoO4 and MoO3 via sintering process. The observed significant increase in the catalytic activities of the solids investigated due to doping with cobalt oxide (2.04 and 4 mol%) can be discussed in terms of conversion of MoO3, devoted with small catalytic activity into an active CoMoO4 compound. The total amount of cobalt oxide present as a dopant is relatively small, however its amount retained on the uppermost surface layers of the treated solids should be much greater than that nominally added. This conclusion is based on XPS measurements carried out before on the same system [14]. Treatment of MoO3/Al2O3 with small amounts of cobalt oxide followed by calcination at 300 or 500 8C which led to conversion of some of MoO3 into CoMoO4 and led also to creation of Co2+– Mo6+ and Co3+–Mo6+ ion pairs which act as active sites for H2O2 decomposition. The presence of maxima in the curves relating the catalytic activity and dopant concentration had been reported in the case of oxidation of CO by O2 and H2O2 decomposition carried out over MoO3 doped Co3O4/Al2O3 solids [28]. The descending part of curves relating k versus the amount of cobalt oxide added to
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Fig. 4. Effect of Co3O4 doping on catalytic activity of MoO3/Al2O3 precalcined at 300 8C at reaction termperatures 30, 40 and 50 8C.
MoO3/Al2O3 system (Figs. 4 and 5) could be attributed to formation of well ordered and big sized CoMoO4 and Co3O4 phases. Determination of the apparent activation energy (DEa) for the catalytic decomposition of H2O2 in the presence of pure and doped MoO3/Al2O3 solids has shed some light on the possible changes in the mechanism of the catalyzed reaction. Thus, the values of k measured at 30, 40 and 50 8C over pure and variously doped solids have allowed (DEa) to be obtained via direct application of the Arrhenius equation. The values of DEa thus obtained are listed in Table 2. Also, included in Table 2 are the values of log A (frequency factor of the Arrhenius equation) calculated for variously investigated solids. Inspection of the data listed in Table 2 revealed that DEa values calculated for pure and doped solids precalcined at 300 and 500 8C suffer fluctuation, i.e. both decrease and increase upon increasing the amount of dopant added. This finding did not run parallel to the observed changes in the catalytic activity of doped solids. It can also be seen from Table 2 that same fluctuation in the values of log A is also observed. The recalculation of DEa values for the reaction carried out over pure and variously doped solids adopting the values of log A for the pure specimens precalcined at 300 and 500 8C to the other doped samples precalcined at the same temperatures, DEa are thus computed and given in the
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Fig. 5. Effect of Co3O4 doping on catalytic activity of MoO3/Al2O3 precalcined at 500 8C at reaction termperatures 30, 40 and 50 8C.
last column of Table 2. The comparison of DEa for pure and doped samples showed that the values of 122 1 and 117 kJ mol1 stand for the activation energies of H2O2 decomposition conducted over pure and doped solids precalcined at 300 and 500 8C, respectively. In other words, Co3O4-doping of MoO3/Al2O3 followed by precalcination at 300 and 500 8C did not modify the mechanism of catalytic decomposition of H2O2 but increased the concentration of active sites taking part in the catalytic reaction. This conclusion is also, further supported by an analysis based on the dissipation function for the energy of the active sites arising from surface heterogeneity ƒ(Ei) = a exp(hEi), where Ei is the interaction energy of site i with substrate. This equation may be converted into the form [29,30]: A = a exp(hDEa). Suggesting that a plot of log A versus (DEa) for the variously doped catalyst samples should give a straight line whose slope and intercept would allow the evaluation the constants h and a, respectively. Fig. 6 depicts the variation of DEa as a function of log A for the catalytic decomposition of H2O2 carried out over pure and doped solids, precalcined at 300 and 500 8C, respectively. The computed values of h were 0.377 and 0.378 mol/kJ min for cobalt oxide-doped solids precalcined at 300 and 500 8C, respectively. The calculated values of a were 0.30 and 0.14 min1 for pure and doped solids preheated at 300 and 500 8C.
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Table 2 Effect of Co3O4 doping of MoO3/Al2O3 system precalcined at 300 and 500 8C on the reaction rate constant (k0 ), activation energies (DEa, DEa ) and frequency factor (log A) for H2O2 decomposition conducted at 30, 40 and 50 8C Dopant concentration (mol%)
Calcination temperature (8C)
k0 , 30 8C (103 min1 m2)
k0 , 40 8C (103 min1 m2)
k0 , 50 8C (103 min1 m2)
DEa (kJ mol1)
Log A
DEa (kJ mol1)
0.00 2.04 4.00 7.69 14.29
300
6.1 160.9 46.8 69.1 53.5
52.6 241.4 121.5 138.3 127.9
100.1 316.1 201.3 210.6 211.6
123 28 53 46 45
45.88 10.10 19.39 16.83 16.41
123 121 122 121 122
0.00 2.04 4.00 7.69 14.29
500
2.3 12.5 32.0 20.7 20.6
14.3 28.9 51.6 40.0 47.1
33.8 51.6 78.1 65.7 23.3
118 73 48 55 75
43.87 26.74 17.34 19.98 27.68
118 117 117 117 117
Fig. 6. Relationship between activation energy and frequency factor for the catalytic reaction conducted over pure and doped solids precalcined at 300 and 500 8C.
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Table 3 Dependence of catalytic activity and selectivity of pure and variously doped MoO3/Al2O3 system precalcined at 300 8C on reaction temperature and amount of Co3O4 dopant in isopropanol conversion reaction Selectivity for propene formation (%)
Selectivity for acetone formation (%)
Dopant concentration (mol%)
Reaction temperature (8C)
Isopropanol conversion (%)
0.00 2.04 4.00 7.69 14.29
300
100.00 57.96 16.60 38.65 25.20
4.24 0.00 0.00 0.00 0.00
95.76 100.00 100.00 100.00 100.00
0.00 2.04 4.00 7.69 14.29
280
80.00 38.85 9.55 23.94 16.22
31.8 0.00 0.00 0.00 0.00
68.12 100.00 100.00 100.00 100.00
0.00 2.04 4.00 7.69 14.29
260
75.34 23.76 5.70 15.58 8.91
38.09 0.00 0.00 0.00 0.00
61.91 100.00 100.00 100.00 100.00
0.00 2.04 4.00 7.69 14.29
240
61.00 16.00 3.51 12.25 5.75
48.28 0.00 0.00 0.00 0.00
51.72 100.00 100.00 100.00 100.00
0.00 2.04 4.00 7.69 14.29
220
23.12 11.84 1.96 6.11 3.31
69.39 0.00 0.00 0.00 0.00
30.61 100.00 100.00 100.00 100.00
0.00 2.04 4.00 7.69 14.29
200
10.14 7.23 1.20 3.44 1.99
100.00 0.00 0.00 0.00 0.00
0.00 100.00 100.00 100.00 100.00
The constants h and a values indicate that Co3O4 doping of MoO4-doping of MoO3/Al2O3 system did not modify the dissipation of active sites, i.e. character of surface heterogeneity. In other words, this treatment did not change the energetic nature of active sites but increased their concentration. 3.3.2. Conversion of isopropanol over pure and doped solids Preliminary experiments showed that Al2O3 support material precalcined at 300 or 500 8C exhibited relatively high catalytic activity in alcohol conversion yielding propene only and acting thus as a selective dehydration catalyst. The catalytic conversion of isopropanol was conducted over pure and cobalt oxidedoped MoO3/Al2O3 solids precalcined at 300 and 500 8C. The reaction temperature was varied between
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Table 4 Dependence of catalytic activity and selectivity of pure and variously doped MoO3/Al2O3 system precalcined at 500 8C on reaction temperature and amount of Co3O4 dopant in isopropanol conversion reaction Selectivity for propene formation (%)
Selectivity for acetone formation (%)
Dopant concentration (mol%)
Reaction temperature (8C)
Isopropanol conversion (%)
0.00 2.04 4.00 7.69 14.29
500
100.00 100.00 100.00 100.00 100.00
22.44 81.63 58.56 33.39 21.97
77.56 18.37 41.44 66.61 78.03
0.00 2.04 4.00 7.69 14.29
480
100.00 100.00 100.00 100.00 100.00
24.25 87.31 61.64 39.00 27.27
75.75 12.69 38.36 61.00 72.73
0.00 2.04 4.00 7.69 14.29
460
100.00 100.00 100.00 100.00 100.00
25.71 89.33 63.93 44.30 32.51
74.29 10.67 36.07 55.70 67.49
0.00 2.04 4.00 7.69 14.29
440
100.00 100.00 100.00 100.00 100.00
28.87 91.01 67.56 50.26 34.64
71.13 8.99 32.44 49.74 65.36
0.00 2.04 4.00 7.69 14.29
420
100.00 100.00 100.00 85.91 81.22
32.38 91.51 69.79 54.40 37.71
67.62 8.49 30.21 45.46 62.29
0.00 2.04 4.00 7.39 14.29
400
92.25 100.00 100.00 78.53 78.17
36.47 93.49 71.11 67.87 39.31
63.63 6.51 28.89 32.13 60.69
0.00 2.04 4.00 7.69 14.29
350
46.07 100.00 100.00 59.84 35.22
81.05 95.40 81.04 72.29 44.47
18.95 4.60 18.96 27.71 55.53
0.00 2.04 4.00 7.69 14.29
300
5.00 35.56 38.42 31.37
100.00 100.00 88.62 77.24
0.00 0.00 11.38 22.76
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200 and 300 8C for the catalysts preheated at 300 8C and between 300 and 500 8C for the solids preheated at 500 8C. The chromatographic investigation of the reaction products over various catalysts for the reaction carried out at different temperatures permitted the determination of activity and selectivity of various catalysts as being influenced by doping, reaction temperature and calcination temperature of solids investigated. The activity and selectivity towards propene formation (dehydration) and acetone (dehydrogenation) were calculated for different solids preheated at 300 and 500 8C and the data obtained are listed in Tables 2–4. Inspection of Tables 2–4 showed the following: (i) The catalytic activity of solids investigated precalcined at 300 8C decreases progressively by increasing the amount of dopant added. (ii) The addition of small amounts of cobalt oxide dopant (2.04–14.29 mol%) to the system investigated followed by heating at 300 8C turned these solids from dehydration to dehydrogenation catalysts for alcohol conversion carried out at 200–300 8C. (iii) The dehydrogenation selectivity for various solids calcined at 300 or 500 8C increases by increasing the reaction temperature. (iv) Opposite trend is observed for the selectivity in dehydration reaction. (v) The addition of small amounts of cobalt oxide (2.04, 4 mol) to MoO3/Al2O3 solids followed by calcination at 500 8C resulted in an effective increase in their catalytic activity measured at 300–400 8C. (vi) The selectivity of doped solids precalcined at 500 8C towards propene formation (dehydration) decreases progressively by increasing the amount of dopant added within 2.04–14.29 mol%. (vii) The dehydrogenation selectivity (acetone formation) of these solids increases as a function of the amount of dopant. (viii) The selectivities of pure and variously doped solids precalcined at 500 8C towards dehydrogenation of alcohol increases by increasing the reaction temperature while that of dehydration reaction decreases as a function of reaction temperature. (ix) The rise in precalcination temperature of pure and doped solid from 300 to 500 8C brought about a measurable decrease in their catalytic activity measured at 300 8C. The decrease was, however, more pronounced for pure solid catalysts. The results obtained revealed that Al2O3 support material and MoO3/Al2O3 solids precalcined at 300 and 500 8C behave as dehydration catalysts for alcohol conversion carried out at 200 and 300 8C for the solids precalcined at 300 and 500 8C, respectively.
4. Conclusions The results obtained in the present investigation permitted to draw the following conclusions: 1. Heat treatment of MoO3–Co3O4/Al2O3 system at 300 8C led to the formation of CoMoO4, which remained stable even by heating at 1000 8C. Solid–solid interaction between Al2O3 and MoO3 yielding Al2(MoO4)3 took place by heating at 750 8C. The produced aluminum molybdate decomposed at 1000 8C giving MoO3 and a-Al2O3. A portion of produced molybdena sublimed and the other portion dissolved in the alumina to give MoO–Al2O3 solid solution, which acquired a grey colour. 2. Doping of MoO3/Al2O3 system with small amount of Co3O4 followed by heating at 300–750 8C brought about measurable changes in its surface characteristics. Both increase and decrease in the BET-surface areas were observed depending on calcination temperature and the amount of dopant added.
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3. The doping process of the investigated system with small amounts of Co3O4 (2.04, 4 mol%) effected a considerable increase in its catalytic activity in H2O2 decomposition. The increase was, however, more pronounced for the catalysts precalcined at 300 8C. The maximum increase in the catalytic activity, expressed as reaction rate constant per unit surface area for the reaction conducted at 30 8C over pure and doped solids attained 96- and 13-fold for the catalysts precalcined at 300 and 500 8C, respectively. 4. Treating MoO3/Al2O3 solids with small amounts of Co3O4 followed by calcination at 300 8C brought about a progressive decrease in their catalytic activities in isopropanol conversion conducted at 200– 300 8C. This treatment, on the other hand, increased the selectivity of the treated solids towards dehydrogenation of reacting alcohol yielding acetone. Doping MoO3/Al2O3 system with Co3O4 > 2.04 mol% conducted at 500 8C lead to a progressive decrease in its selectivity towards dehydration of isopropanol (yielding propene). 5. The doping process of MoO3/Al2O3 system with Co3O4 did not modify the mechanism of catalytic decomposition of H2O2 but changed the concentration of catalytically active sites without changing their energetic nature.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
[25] [26]
G.A. El-Shobaky, A.N. Al-Noaimi, Appl. Catal. 29 (1987) 235. G.A. El-Shobaky, A.N. Al-Noaimi, T.M.H. Saber, Bull. Soc. Chim. France 6 (1988) 930. G.A. El-Shobaky, G.A. Fagal, T.M.H. Saber, Bull. Soc. Chim. France 4 (1987) 547. G.A. El-Shobaky, G.A. Fagal, Egypt. J. Chem. 31 (5) (1988) 331. A.M. Youssef, S.A. El-Hakam, G.A. El-Shobaky, Radiat. Phys. Chem. 40 (1992) 575. G.A. El-Shobaky, Th. El-Nabarawy, G.A. Fagal, A.S. Ahmed, Mater. Lett. 17 (1993) 297. G.A. El-Shobaky, G.A. Fagal, A.M. Ghozza, M.A. Shouman, Mater. Lett. 19 (1994) 225. G.A. El-Shobaky, A.M. Ghozza, S. Hammad, Adsorpt. Sci. Technol. 12 (2) (1995) 119. G.A. El-Shobaky, A.M. Ghozza, S.H. El-Khouly, Adsorpt. Sci. Technol. 13 (1996) 153. G.A. El-Shobaky, A.M. Ghozza, G.M. Mohamed, Adsorpt. Sci. Technol. 16 (1998) 415. H.G. El-Shobaky, M. Mokhtar, G.A. El-Shobaky, Appl. Catal. A 180 (1999) 335. A.M. Ghozza, G.A. El-Shobaky, G.M. Mohamed, Adsorpt. Sci. Technol. 18 (2000) 561. N. Radwan, G.A. Fagal, G.A. El-Shobaky, Colloids Surf. A 178 (2001) 277. G.A. El-Shobaky, A.M. Ghozza, A.A. El-Warraky, G.M. Mohamed, Colloids Surf. A 219 (2003) 97. G.-Z. Biana, L. Fanb, Yi-Fua, K. Fujimotob, Appl. Catal. A 170 (1998) 255. G.J. Quarderer, K.A. Cochran, European Patent 119,609 (1984). R.R. Stevens, European Patent 172,431 (1986). Y. Xie, B.M. Naasz, G.A. Somorjai, Appl. Catal. 27 (1986) 233. C.B. Murchison, M.M. Conway, R.R. Stevens, G.J. Quarderer, in: M.J. Philips, M. Ternan (Eds.), Proceedings of the 9th International Congress on Catalalysts, vol. II, Calgary, Ottawa, Canada, 1988, p. 626. T. Tatsmi, A. Muramatsu, H. Tominaga, Appl. Catal. 34 (1987) 77. J. Zhang, Y. Wang, L. Chang, Appl. Catal. A 126 (1995) 205. M. Jiang, G.Z. Bian, Y.L. Fu, J. Catal. 146 (1994) 144. Y. Avila, C. Kappenstein, S. Pronier, J. Bronier, J. Barrault, Appl. Catal. A 132 (1995) 97. (a) E. Swanson, C.F. Fuyat, Nat. Bur. Stand. Circ. II (1953) 38; (b) E. Swanson, C.F. Fuyat, Powder Diffraction File (JCPOS), International Center Diffraction Data, Swarthmore, PA, 1979. G.A. El-Shobaky, F.H.A. Abdalla, A.M. Ghozza, K.A. Khalil, Thermochim. Acta 275 (1996) 235. S. Brunauer, L.S. Deming, W.E. Deming, E.E. Teller, J. Am. Chem. Soc. 62 (1940) 1723.
1080
A.A. Zahran et al. / Materials Research Bulletin 40 (2005) 1065–1080
[27] J. Rouquerol, D. Avnir, C.W. Fairbridge, D.H. Evett, G.H. Haynes, N. Pernicone, J.D.F. Ramsay, K.S.W. Sing, K.K. Unger, Pure Appl. Chem. 66 (1994) 1739. [28] G.A. El-Shobaky, S.A. El-Molla, G.M. Mohamed, A.A. Zahran, Mater. Lett. 57 (2003) 1612. [29] M. Lo Jacono, M. Schiavello, V.H.J. Dee Beer, G. Minerlli, Am. Chem. Soc. 81 (1977) 1583. [30] A.A. Balandin, Dokl. Akad. Nauk. SSSR 93 (1953) 55.