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Al2O3 system
Applied Catalysis A: General 241 (2003) 235–245
Effect of Li2 O-doping on surface and catalytic properties of Co3O4 –MoO3/Al2 O3 system G.A. El-shobaky a,∗ , A.M. Ghozza b , G.M. Mohamed a a
Lab. Surface Chemistry and Catalysis, National Research Center Dokki, Cairo, Egypt b Department of Chemistry, Faculty of Science, Zagazig University, Cairo, Egypt
Received 13 May 2002; received in revised form 18 June 2002; accepted 15 August 2002
Abstract The effect of Li2 O-doping on solid–solid interaction, surface and catalytic properties of Co3 O4 –MoO3 /Al2 O3 system were investigated using XRD, XPS, nitrogen adsorption at −196 ◦ C and oxidation of CO by O2 at 100–300 ◦ C. The nominal composition of the system was 0.2 Co3 O4 –0.05 MoO3 /Al2 O3 and the amount of dopant was varied between 0.14 and 0.57 wt.% Li2 O. The results revealed that Li2 O-doping inhibited the formation of CoAl2 O4 and CoMoO4 in the treated solids precalcined at 800 ◦ C and decreased the degree of crystallinity of Co3 O4 . Also, Li2 O-doping of the investigated system decreased the surface concentration of cobalt species and increased that of molybdenum species. The specific surface areas of the doped adsorbents precalcined at 600, 800 and 1000 ◦ C decreased progressively as a function of the amount of dopant added. The catalytic activities of pure and doped solids, in CO oxidation by O2 , decreased progressively by increasing the precalcination temperature in the range 600–1000 ◦ C. The doping process brought about a significant increase in the catalytic activities of the solids investigated due to an effective hindrance of formation of CoAl2 O4 and CoMoO4 which exhibit catalytic activities smaller than that of Co3 O4 . Lithia-doping of the investigated system did not modify the mechanism of the catalytic reaction but changed the concentration of catalytically active constituents without changing their energetic nature. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Li2 O-doping; Oxidation of CO by O2 ; CoAl2 O4 ; CoMoO4
1. Introduction Supported transition metals or oxides are commonly employed to catalyse several reactions including, oxidation–reduction, dehydration, dehydrogenation cracking, alkylation, . . . , etc. [1–12]. ␥-Al2 O3 is considered as the most convenient support for a big variety of catalysts especially those used in oxidation–reduction reactions. However, the catalytic activities of ␥-Al2 O3 supported solids de∗ Corresponding author. Fax: +20-2-3370-931. E-mail address: [email protected] (G.A. El-shobaky).
crease progressively as a function of time of use due to an enhanced conversion of active transition metal oxides into less active metal aluminates [13,14]. It has been reported [13,14] that the rate of reaction between transition metal oxide and Al2 O3 to produce metal aluminate decreases in the following order: Cu > Co > Ni > Fe. So, Al2 O3 support material should be treated with small amounts of certain foreign oxides in order to increase the service life time of the catalysts via hindering the formation of aluminate phases [15–17]. The surface properties of copper, nickel and cobalt oxides supported on ␥-Al2 O3 have been studied by
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several investigators [13,14,17,19–23]. The effect of foreign cations on the dispersion of catalytically active phases and on the solid–solid interaction between these phases and Al2 O3 support have been extensively investigated [18–22]. XPS, ISS, DRS and XRD analyses revealed that Li2 O-doping of Al2 O3 inhibited the formation of both cobalt and nickel aluminates due to the growth of a lithium aluminate phase which hindered the diffusion of cobalt and nickel ions into the ␥-Al2 O3 lattice during the calcination process [15–17,19]. Also, Be and Mg ions, like Li ions, inhibit the formation of cobalt aluminate and decrease the concentration of cobalt ions into the uppermost surface layers of the treated solids [23]. On the other hand, Na, K, Rb, Cs, Zn, Ca, Sr, Ba, Ga or Ge ions strongly stimulate the formation of surface epitaxial CoAl2 O4 by enhancing the diffusion of cobalt ions into tetrahedral sites of the ␥-Al2 O3 with subsequent increase in Co/Al ratio on the catalyst surface [15,17,20,24,25]. Indeed, Ca, Sr and Ba-doping of ␥-Al2 O3 catalyses the direct formation of CoAl2 O4 even at low temperature as 100 ◦ C [17]. Cobalt aluminate is generally undesirable component from the catalytic point of view due to its low reactivity towards oxidation–reduction and desulfurization reactions [14]. Nevertheless, in some cases the enhanced CoAl2 O4 formation was found to be connected with a large increase in the cobalt dispersion on the support surface [20]. Thus control of the active phase-carrier interactions to give optimum balance between the best state of dispersion and the minimum amount of CoAl2 O4 present, is important for maximising the catalytic activity. The present investigation is devoted to studying the effect of lithium-doping of the ␥-Al2 O3 support on the surface and catalytic properties of Co3 O4 –MoO3 supported catalysts. The techniques employed were XRD, XPS, nitrogen adsorption at −196 ◦ C and catalysis of CO oxidation by O2 at 100–300 ◦ C. 2. Experimental 2.1. Materials Pure Co3 O4 –MoO3 /Al2 O3 samples were prepared by the incipent wetness impregnation using Al(OH)3 solid, ammonium molybdate and cobalt nitrate. The
amounts of cobalt and molydenum oxides were fixed at 30.5 wt.% Co3 O4 and 4.5 wt.% MoO3 . The prepared solid samples were subjected to heat treatment for 5 h at 600, 800 and 1000 ◦ C. The Li2 O-doped samples were obtained by impregnating a known weight of Al(OH)3 solid with different proportions of LiNO3 dissolved in the minimum amount of distilled water making pastes. The pastes were dried at 100 ◦ C till constant weight then calcined at 500 ◦ C for 5 h. Then impregnating with calculated amount of ammoninum molybdate followed by drying and impregnation with a known amount of cobalt nitrate. The obtained solid samples were then calcined at 600, 800 and 1000 ◦ C for 5 h. The concentrations of dopant were 0.75, 1.5 and 3.0 mol% Li2 O which correspond to 0.14, 0.29 and 0.57 wt.% of Li2 O, respectively. The chemical employed were of analytical grade supplied by BDH and Prolabo companies. 2.2. Techniques The X-ray powder diffraction patterns of the various solids precalcined at 600 and 800 ◦ C were carried out using a Philips diffractometer (type PW 1390). The patterns were run with Fe-filtered cobalt radiation (λ = 1.7889 Å) at 30 kV and 10 mA with a scanning speed of 2◦ in 2θ min−1 . The surface concentration and binding energies of cobalt, molybdenum and aluminium ions were determined by a 550 ESCA/SAM spectrometer (Physical Electronics, USA) with a Mg anode X-ray tube (hν = 1253 eV). The pass energy of the analyser was 25 eV (E = 0.12 eV). The vacuum in the system was about 10−9 Torr. The energy of the scale was adjusted to match the Au 4f7/2 binding energy at 83.8 eV. All solid samples were finely powdered and spread on the sticky side of a thin film of Al foil. Small shifts in binding energy (BE) due to residual charging of the sample surface was corrected relative to C 1s electrons adsorbed hydrocarbons at BE of 284.6 eV. The relative atomic concentrations of Co, Mo and Al on the surface were determined by applying a computer program (PH 1 5267-YH V006B, Physical Electronics) that takes into consideration Scofield calculations photoionization cross-section energy dependence of electron escape depth and luminosity of the analyser. The surface properties, namely SBET , total pore volume (Vp ) and mean pore radius (¯r ) of different
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prepared solid samples were determined from nitrogen adsorption isotherms measured at −196 ◦ C using conventional volumetric apparatus. Before carrying out the measurements each sample was degassed under a reduced pressure of 10−5 Torr at 200 ◦ C for 2 h. The catalytic oxidation of CO by O2 on different catalyst samples was conducted at temperatures within the range 100–300 ◦ C using a static method. A mixture of carbon monoxide and oxygen at a pressure of 2 Torr was used. This method is simple and permits to follow up the kinetics of the catalytic reaction by measuring the drop in pressure of the reacting gases as a function of time till equilibrium was attained. A fresh 200 mg catalyst sample, placed in the hot zone of a Pyrex glass reactor was activated by heating under a reduced pressure of 10−6 Torr at 300 ◦ C for 2 h was employed in each kinetic experiment. Details of the apparatus used are given elsewhere [34].
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3. Results and discussion 3.1. XRD investigation of pure and Li2 O-doped samples X-ray diffractograms of pure and variously doped solids precalcined at 600 and 800 ◦ C were measured. The diffractograms of pure and doped samples precalcined at 600 ◦ C, not given, are similar to each other and consisted of all diffraction lines of Co3 O4 phase that having moderate degree of crystallinity and some lines of poorly crystalline ␥-Al2 O3 phase. Fig. 1 depicts the diffractograms of pure and doped solids samples precalcined at 800 ◦ C. Inspection of Fig. 1 showed the following: (i) The diffraction lines of CoMoO4 phase appeared in the diffractograms of pure sample and that doped with 0.14 wt.% Li2 O. The relative intensities of the diffraction lines of CoMoO4 phase decreased by doping with the smallest amount of Li2 O.
Fig. 1. X-ray diffractograms of pure and doped solids precalcined at 800 ◦ C.
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(ii) The diffraction lines of molybdate phase disappeared upon increasing the amount of dopant added to 0.29 or 0.57 wt.% Li2 O. (iii) The relative intensities of the diffraction lines of Co3 O4 or CoAl2 O4 phases decreased progressively as a function of the amount of Li2 O added. The heights of the diffraction lines (d = 2.86 Å) are 200, 178, 132 and 122 a.u. for pure sample and those treated with 0.14, 0.29 and 0.57 wt.% Li2 O, respectively. (iv) The average particle size of Co3 O4 or CoAl2 O4 phases was calculated for pure and doped samples and the data obtained are 157, 148, 145, and 135 Å for pure sample and the samples treated with 0.14, 0.29 and 0.57 wt.% Li2 O, respectively. The close similarity between the diffraction lines of Co3 O4 and CoAl2 O4 phases makes their distinction a difficult task. However, the relative intensity of the diffraction line located at 2.86 Å is 29 and 65% in the case of Co3 O4 and CoAl2 O4 phases, respectively [26]. This difference may be considered as a tool to differentiate between the two phases and to know which one of them exists as a major phase. This method was successfully used and the relative intensity of the diffraction line at d = 2.86 Å was 65, 64, 44, and 39% in the case of pure sample and those treated with 0.14, 0.29, and 0.57 wt.% Li2 O, respectively. These values suggested that CoAl2 O4 phase exists as the major phase in the case pure specimen and that doped with the smallest amount of dopant and calcined at 800 ◦ C. While Co3 O4 phase represents the major phase in the case of the other doped samples. This conclusion has been confirmed experimentally by comparing the colour of pure and doped samples. In fact, the pure sample and that treated with 0.14 wt.% Li2 O acquired blue
coloration (characteristic colour of CoAl2 O4 ) while the other doped samples were grey and dark grey indicating coexistence of cobaltic oxide and cobalt aluminate. These results clearly indicate that Li2 O-doping of the investigated system hindered the solid–solid interactions which yield CoAl2 O4 and CoMoO4 and the degree of hindrance increases by increasing the amount of dopant added. Also, Li2 O-treatment decreases the degree of crystallinity and particle size of Co3 O4 and CoAl2 O4 phases to an extent proportional to its amount present. 3.2. XPS investigation of pure and Li2 O-doped samples XPS investigation of pure sample and those doped with 0.29 and 0.58 wt.% Li2 O (1.5 and 3 mol%) precalcined at 800 ◦ C was carried out. This investigation enabled a precise determination of the concentration of Co, Al and Mo atoms on the top surface layers of the investigated solids. Also, the values of binding energies of cobalt, aluminium and molybdenum ions in different solids have been determined from the XPS investigation of these solids. Table 1 includes the results of XPS analysis of various samples. Inspection of the results given in Table 1 revealed that: (i) The concentration of cobalt and molybdenum species on the uppermost surface layers of the various investigated solids are greater than those in bulk of these solids. This finding suggested a preferential absorption of cobalt and molybdenum ions on the surfaces of catalyst samples in the course of impregnation of the support material with cobalt nitrate and
Table 1 Bulk and surface concentrations of Co, Al and Mo and their binding energies (BE) in the pure and doped solids precalcined at 800 ◦ C Dopant concentration (mol% Li2 O)
Bulk concentration of Co (at.%) Surface concentration of Co (at.%)a Bulk concentration of Al (at.%) Surface concentration of Al (at.%)a Bulk concentration of Mo (at.%) Surface concentration of Mo (at.%)a BE of Co (eV) BE of Al (eV) BE of Mo (eV) a
0
1.5
3.0
22.64 26.8 75.47 70.20 1.89 3.0 781 74.2, 73.5 231.4, 232.8
22.51 22.9 75.05 72.50 1.88 4.6 780.6 74.2, 73 232.8
22.39 21.23 74.63 74.17 1.87 4.6 779 74.3, 73.1 232.8
The surface concentrations of each species are relative atomic concentrations of Al, Co and Mo species.
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ammonium molybdate solutions. (ii) Li2 O-doping of the investigated system followed by heat treatment at 800 ◦ C led to a measurable progressive decrease in surface concentration of cobalt species. A decrease of 14.6 and 20.8% in surface concentration of cobalt species took place upon doping with 0.29 and 0.58 wt.% Li2 O, respectively. (iii) This treatment effected a significant increase in the concentration of molybdenum species in the uppermost surface layers of the treated samples. This increase attained more than 50%. (iv) The binding energy of cobalt ions (Co 2P3/2 ) in different investigated solids decreases by increasing the amount of Li2 O added. The value of binding energy of cobalt ion in the undoped sample is close to that of CoAl2 O4 compound [27–29] and this value in the case of the heavily doped sample is very close to that of Co3 O4 [27]. These results showed that Li2 O-doping of the investigated system hindered the solid–solid interaction between cobalt and aluminium oxides to yield cobalt aluminate. The degree of hindrance increases by increasing the amount of dopant added (x). The binding energy of molybdenum ions in pure sample varies between 231.8 and 232.8 eV. The first value corresponds to the value of BE of molybdenum species in cobalt molybdate and second value is indicative for BE of molybdenum ion in aluminium molybdate compounds [32]. On the other hand, the BE of molybdenum species in the two doped samples was 232.8 eV. This finding suggested that Li2 O-doping of the investigated system hinders the formation of cobalt molybdate and the presence of 0.29 wt.% Li2 O was enough to suppress completely the formation of CoMoO4 . So, the results of XRD investigation presented in the previous section of the present work have been confirmed by XPS investigation. The suppression of solid–solid interaction between cobalt and aluminium yielding CoAl2 O4 upon doping with Li2 O had been reported. In fact, it had been reported that Li2 O-doping of Co3 O4 /Al2 O3 much retarded CoAl2 O4 formation due to the growing up of LiAl5 O8 compound on the Al2 O3 surface that hindered the diffusion of Co2+ ions into tetrahedral sites of ␥-Al2 O3 support material [15–17,19]. Similar effect had been found in the case of doping the same system with Be2+ and Mg2+ species. Furthermore, the doping of NiO/Al2 O3 , Co3 O4 /Al2 O3 with Na+ , Zn2+ , Ga3+ and Ge4+ ions enhances the formation of surface aluminates produced via enhancing the diffusion
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of Co2+ or Ni2+ ions to tetrahedral sites of top surface layers of ␥-Al2 O3 support [15,17,20,24,25]. However, the hindrance of solid–solid interaction between cobalt and molybdenum oxides to yield CoMoO4 upon treatment with Li2 O, to our knowledge, had not been cited in literature. The hindrance of solid–solid interactions between two oxides to produce a compound upon doping with Li2 O could be tentatively discussed in terms of: (i) A possible dissolution of some lithium ions in the lattices of the reacting oxides increasing their oxidation states which become unfavourable for the compound formation [30,31]. In the case of CoAl2 O4 and CoMoO4 compounds, the cobalt ions exist as divalent ions and doping of cobalt oxide by Li2 O resulted in conversion of some of host Co2+ ions into Co3+ ions which might not easily involved in formation of aluminate or molybdate compounds [30,31]. (ii) The dopant added might cover the surfaces of reacting oxides and acted as an energy barrier that hindered the diffusion of reacting cations involved the compounds formation. 3.3. Surface properties of pure and doped solids The effect of Li2 O-doping of the investigated system followed by heat treatment at 600, 800 and 1000 ◦ C on its BET surface area (SBET ), total pore volume (Vp ) and mean pore radius (¯r ) was investigated by nitrogen adsorption at −196 ◦ C over pure and doped samples. The computed values of SBET , Vp and r¯ for the samples investigated are given in Table 2. Another series of specific surface areas St was calculated from V1−t plots of the different investigated adsorbents. These plots were constructed using suitable standard t-curves depending on the value of the C-constant in BET equation given in the last column of Table 2. Inspection of Table 2 reveals that the values of SBET and St , calculated for the various investigated adsorbents, are close to each other, within the experimental error which indicates the correct choice of the reference t-curves for analysis and shows the absence of ultramicropores. Table 2 shows also that Li2 O-doping of the system investigated brought about a measurable decrease in its SBET to an extent proportional to the amount of dopant added. This treatment was accompanied by a progressive increase in the value of r¯ . It can also be seen from Table 2 that Li2 O-doping of Co3 O4 –MoO3 /Al2 O3 samples followed by calcination at 600 or 800 ◦ C resulted in a progressive increase the
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Table 2 Surface characteristics of pure and doped solids precalcined at different temperatures Dopant concentration (mol%)
Calcination temperature (◦ C)
SBET (m2 g−1 )
St (m2 g−1 )
Vp (cm3 g−1 )
r¯ (Å)
BET C-constant
0.0 0.75 1.50 3.0 0.0 0.75 1.50 3.0 0.0 0.75 1.50 3.0
600 600 600 600 800 800 800 800 1000 1000 1000 1000
251 248 228 213 164 152 143 126 107 103 87 85
250 232 229 215 155 144 140 114 100 102 85 82
0.334 0.379 0.382 0.449 0.348 0.418 0.410 0.402 0.306 0.289 0.286 0.279
33 38 42 53 53 69 72 80 71 70 82 82
21 20 23 27 20 21 22 23 19 21 23 27
value of Vp . The maximum decrease in SBET of the investigated samples due to treatment with Li2 O is 14.7, 23.2 and 17.5% for the samples precalcined at 600, 800 and 1000 ◦ C, respectively. The maximum increase in the value due to doping is 60.6, 34 and 17% for the samples preheated at 600, 800 and 1000 ◦ C, respectively. The observed decrease in the specific surface areas of the samples investigated could be attributed mainly to widening of pores present, especially the samples precalcined at 800 and 1000 ◦ C. The observed modifications of solid–solid interactions between the different catalyst components, changes in surface concentration of cobalt species and variations in different surface characteristics of the system investigated due to doping with Li2 O are expected to be accompanied by significant changes in their catalytic activities.
over various solids precalcined at 600, 800 and 1000 ◦ C. The reaction rate constant (k) was calculated for each sample at different reaction temperatures. Table 3 includes the values of k for different samples. It can be seen from Table 3 that Li2 O increases the catalytic activity of the investigated system to an extent proportional to its amount added. However, a maximum increase in the catalytic activity was attained in presence of 1.5 mol% Li2 O for the samples precalcined at 600 and 800 ◦ C. The maximum increase in the values of k measured at 150, 250 and 300 ◦ C for the catalyst samples calcined at 600, 800 and 1000 ◦ C was 108, 20 and 38%, respectively. It has been shown (Table 2) that Li2 O-doping of the investigated system followed by heat treatment at 600–1000 ◦ C brought about a progressive decrease in its BET surface area. This decrease is expected to be
3.4. Catalytic activity of pure and doped samples
Table 3 Effect of Li2 O content on the values of k measured at different temperatures over various catalysts calcined at 600, 800 and 1000 ◦ C
The catalytic oxidation of CO by O2 was conducted over pure and doped samples precalcined at 600, 800 and 1000 ◦ C. First-order kinetics were found in all cases, the slope of each first-order plots determines the values of the reaction rate constant (k). The linear plot of log P0 /P or ln P0 /P versus time (were P0 is the initial pressure of the reaction mixture at time t = 0 and P is the pressure of the reaction mixture at time equals t) is indicative for first-order kinetics. Fig. 2 shows representative first-order plots of CO oxidation by O2 carried out at different temperatures
Dopant concentration (mol%)
k150 ◦ C × 10−3 (min−1 )a
k250 ◦ C × 10−3 (min−1 )b
k300 ◦ C × 10−3 (min−1 )c
0.0 0.75 1.50 3.00
2.6 3.6 5.4 2.8
7.4 7.9 8.9 8.5
7.3 8.6 9.5 10.1
Calcination temperature: 600 ◦ C. Calcination temperature: 800 ◦ C. c Calcination temperature: 1000 ◦ C. a
b
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Fig. 2. First-order plots of CO oxidation by O2 conducted at different temperatures over various catalysts calcined at 600, 800 and 1000 ◦ C.
accompanied by a possible decrease in the catalytic activity of Li2 O-treated samples. So, the reaction rate ¯ was calculated for the constant per unit surface area (k) catalysed reaction conducted at different temperatures over various catalyst samples calcined at 600, 800 and 1000 ◦ C. The results obtained are graphically illustrated in Fig. 3. This figure shows that Li2 O-doping effected a progressive increase in the values of k¯ to an extent proportional to its amount present reaching to a maximum limit at 1.5 mol% Li2 O for the samples calcined at 600 ◦ C. The maximum increase in the values ¯ measured at 150, 250 and 300 ◦ C for the cataof k, lysts calcined at 600, 800 and 1000 ◦ C, respectively, was 127%, 45% and 74% for the previously mentioned
catalysts. The observed significant increase in the catalytic activity of the investigated system due to doping with Li2 O might reflect an effective increase in the concentration and/or nature of active sites contributing in chemisorption and catalysis of CO oxidation by O2 over pure and doped catalyst samples. 3.5. Activation energies of the catalytic reaction carried out over pure and doped solids Determination of the apparent activation energy (Ea ) for catalysis of CO oxidation by O2 in contact with pure and variously doped solids has shed some light on the possible change in the mechanism of
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G.A. El-shobaky et al. / Applied Catalysis A: General 241 (2003) 235–245 Table 4 Activation energies Ea , Ea∗ and frequency factor A of the catalytic oxidation of CO by O2 conducted over pure and Li2 O-doped catalysts calcined at different temperatures Dopant (mol%)
Calcination Ea temperature (kJ mol−1 )a (◦ C)
A (min−1 )
Ea∗ (kJ mol−1 )
0.0 0.75 1.50 3.0 0.0 0.75 1.50 3.00 0.0 0.75 1.50 3.0
600 600 600 600 800 800 800 800 1000 1000 1000 1000
3.10 0.58 2.00 2.60 1.57 10.1 3.04 8.3 25.7 51.7 3.7 3.1
23.9 23.5 22.8 23.8 22.7 23.3 22.8 22.7 38.2 39.3 37.5 37.5
23.9 17.7 21.3 23.4 22.7 30.7 25.1 29.5 38.2 40.4 27.8 27.4
a E values were calculated for the catalytic reaction carried a out at 100–200, 150–250 and 200–300 ◦ C over different catalysts calcined at 600, 800 and 1000 ◦ C, respectively.
Fig. 3. Variation of k as a functions of dopant content for catalytic reaction conducted at different temperatures over various catalysts calcined at 600, 800, and 1000 ◦ C.
the catalysed reaction and given useful information about the possible change in the concentration and nature of catalytically active constituents taking part in the catalytic reaction. The variation of k measured at different temperatures over the variously doped solids precalcined at 600–1000 ◦ C have enabled Ea to be calculated by direct application of the Arrhenius
equation. The calculated values of Ea are listed in Table 4 which also includes the values of the pre-exponential factor (A) for the Arrhenius equation. Inspection of Table 4 reveals that Li2 O-doping of various solids followed by calcination at 600–1000 ◦ C resulted in fluctuation in the values of Ea . This finding did not run parallel to the induced increase in their catalytic activities. However, a same fluctuation in the values of A was observed in the cases of different investigated samples indicating that a compensation effect might be responsible for the observed changes in the values of Ea due to Li2 O-doping. This assumption which reflects the heterogeneity of the surfaces of the treated solid. The activation energies for the catalytic reaction were calculated adopting the values of A for the untreated catalyst samples precalcined at 600, 800 and 1000 ◦ C, respectively for the other doped samples precalcined at the same temperatures. The resulting (Ea∗ ) values obtained were virtually the same (within the experimental error) for the pure and variously doped samples precalcined at the same temperatures. This indicates that Li2 O-doping of the investigated system followed by calcination at 600–1000 ◦ C did not change the mechanism of the catalytic reaction but rather increased the concentration of the catalytically active sites on the catalyst surface upon doping
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with lithia. 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 [22] F (Ei ) = a exp(h Ei ) where Ei is the interaction energy of site i with the substrate. This equation may be converted to the following: A = a exp(h E) Suggesting that a plot of ln A versus E for the variously doped catalyst samples should give a straight line whose slope and intercept would allow the evaluation of the constants “h” and “a”, respectively. Fig. 4 depicts the variation of E and ln A for the catalytic oxidation of CO by O2 carried out in contact with pure and doped catalyst samples precalcined at 600, 800 and 1000 ◦ C, respectively. The computed values of “h” were 0.14, 0.21, 0.21 and 0.21 mol kJ−1 min−1 for pure and variously doped catalysts precalcined at 600, 800 and 1000 ◦ C, respectively. The computed values of the constant a were 0.14, 0.22 and 0.22 min−1 for Li2 O-doped samples precalcined at 600, 800 and 1000 ◦ C, respectively. The identical values of the contestants “h” and “a” for pure and different doped solids calcined at one and the same temperature might indicate that the doping process did not change the nature of catalytically active sites involved in the catalytic reaction. The values of “h” and “a” indicate that Li2 O-doping of the investigated system followed by calcination at 600, 800 and 1000 ◦ C did not change the dissipation of active sites on the surfaces of variously doped catalysts, i.e. the nature of the surface heterogeneity, but led to changes (increase in their concentration). The observed changes in the catalytic activity of the investigated system due to doping with Li2 O conducted at 600–1000 ◦ C could be discussed in terms of (i) Dissolution of some of dopant ions in the lattice of Co3 O4 in the catalyst samples with subsequent transformation of some of host Co2+ into Co3+ ions. The creation of trivalent cobalt ions due to doping with Li2 O might take place according the following mechanism, which can be simplified by adopting Kroger’s notions in the following manner [33]: Li2 O + 21 O2 (g) → Li(Co2+ ) + 2Co3+
Fig. 4. Relationship between Ea and ln A for pure and Li2 O-doped catalysts precalcined at different temperatures.
where Li(Co2+ ) are monovalent lithium ions located in the positions of host Co2+ ions in the Co3 O4 solid and Co3+ are trivalent cobalt ions (lattice defects) created in the doped solids. The dissolution of Li2 O in Co3 O4 lattice according to the above mentioned
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mechanism is followed by fixation of atmospheric oxygen with subsequent conversion of some of Co2+ into Co3+ ions. The created trivalent cobalt ions act as active sites for chemisorption and catalysis of CO oxidation by O2 . As CO acts as electron donor gas its chemisorption could be favoured by creation of Co3+ ions that act as electron acceptor sites. (ii) The observed decrease in the concentration of cobalt species on the top surface layers of doped samples cf. Table 1. (iii) Hindrance of formation CoAl2 O4 and CoMoO4 compounds in the investigated system as a result of doping with small amounts of Li2 O (cf. Fig. 1 and Table 1). The fact that Co3 O4 solid exhibits higher catalytic activity than those of CoAl2 O4 and CoMoO4 compounds, the suppression of cobalt aluminate and molybdate due to Li2 O-treatment might lead to an increase in the catalytic activity of the doped solids. The observed limited decrease in the concentration of surface cobalt species (14.6–20.8%, cf. Table 1) in doped samples precalcined at 800 ◦ C might be accompanied by decrease in their catalytic activity. The detected measurable increase in the catalytic activity of the investigated system due to treatment with small amounts of Li2 O might suggest the domination of both creation of Co3+ ions and suppression of solid–solid interactions between Co3 O4 and Al2 O3 and MoO3 that yield CoAl2 O4 and CoMoO4 , respectively on the observed limited decrease in the concentration of surface cobalt species.
4. Conclusions These are the main conclusions that may be drawn from the results obtained in the present investigation: 1. Doping of Co3 O4 –MoO3 /Al2 O3 system with Li2 O (0.75–3 mol%) followed by heat treatment at 600–1000 ◦ C hindered the solid–solid interaction between Co3 O4 and each of Al2 O3 and MoO3 that yield CoAl2 O4 and CoMoO4 . The degree of hindrance ran parallel to the amount of dopant added. 2. This treatment led to a limited decrease in the concentration of cobalt species present on the outermost surface layers of the treated solids. 3. The doping process of the investigated system resulted in a measurable decrease in its specific surface area.
4. The catalytic activity in CO oxidation by O2 , of the investigated system was found to increase by increasing the amount of lithia added. The maximum increase in the catalytic activity, expressed ¯ as reaction rate constant per unit surface area (k) ◦ measured at 150, 250, and 300 C for the catalysts calcined at 600, 800 and 1000 ◦ C attained 127, 45 and 74%, respectively. This significant increase in the values of k¯ resulted mainly from an effective increase in the oxidation state of Co3 O4 via conversion of some of its host Co2+ into Co3+ ions and suppression of formation of cobalt aluminate and molybdate. 5. The doping process did not change the mechanism of the catalytic reaction but rather increase the concentration of catalytically active sites without changing their energetic nature. References [1] N.Z. Muradov, Int. J. Hydrogen Energy 18 (1993) 211. [2] A.C.B. dos Santos, W.B. Kover, A.C. Faro, Appl. Catal. A 153 (1997) 83. [3] T. Zhang, M.D. Amiridis, Appl. Catal. A 167 (1998) 161. [4] S. Velu, C.S. Swamy, Appl. Catal. A 162 (1997) 81. [5] G.A. El-Shobaky, A.N. Al-Noaimi, Appl. Catal. A 29 (1987) 235. [6] M.F. Luo, Y.-J. Zhong, X.-X. Yuan, X.-M. Zheng, Appl. Catal. A 162 (1997) 121. [7] G.A. El-Shobaky, A.S. Ahmed, M. Mokhtar, J. Radioanal. Nucl. Chem. Articles 219 (1997) 89. [8] A. Golchet, J.M. White, J. Catal. 53 (1978) 266. [9] G.A. El-Shobaky, N.H. Amin, N.M. Deraz, S.M. El-Molla, Adsorp. Sci. Technol. 19 (2001) 45. [10] A.M. Youssef, S.A. El-Hakam, G.A. El-Shobaky, Phys. Radiat. Chem. 40 (1992) 575. [11] F.M.T. Mendes, M. Schmal, Appl. Catal. A 163 (1997) 153. [12] H. Li, J.F. Ding, Appl. Catal. A 193 (2000) 9. [13] A. Molt, Collect. Chem. Commun. 51 (8) (1986) 1571. [14] P.H. Bolt, M.E. Van Ipenburg, J.W. Geus, F.H.P.M. Habraken, Mat. Res. Soc. Symp. Proc. 344 (1994) 15. [15] A. Lycourghitios, C. Defosse, F. Delannay, B. Delmon, J. Chem. Soc. Faraday Trans. 1 (76) (1980) 1677. [16] M. Houlla, J. Lemaitre, B. Delmon, J. Chem. Soc. Faraday Trans. 1 (78) (1982) 1389. [17] A. Lycourghitios, D. Vattis, N.A. Katsanos, Zeit. Phys. Chem. 125 (1981) 239. [18] G.A. El-Shobaky, A.N. Al-Noaimi, Surf. Technol. 26 (1985) 235. [19] A. Lycourghitios, D. Vattis, G. Karaiskis, N.A. Kantsanos, J. Less Common Met. 86 (1982) 137. [20] A. Lycourghitios, C. Deffose, B. Delmon, Rev. Chim. Miner. 16 (1979) 473.
G.A. El-shobaky et al. / Applied Catalysis A: General 241 (2003) 235–245 [21] M. Houlla, B. Delmon, C. R. Acad. Sci. Paris 290 (1980) 301. [22] R.B. Shalovy, P.J. Reucroft, J. Electr. Spectrosc. Relat. Phenom. 12 (1981) 95. [23] A. Lycourghitios, A. Tsiatsios, N.A. Katsanos, Zeit. Phys. Chem. 126 (1981) 95. [24] A. Lycourghitios, D. Vattis, Ph. Aroni, Zeit. Phys. Chem. 120 (1980) 2. [25] C. Deffose, M. Houlla, A. Lycourghitios, in: Proceedings of the 7th International Congress Catalysis, Tokyo, July 1980, Paper A4. [26] Powder Diffraction File (JCPDS), International Center for Diffraction Data, Swarthmore, PA, 1979. [27] D.M. Hercules, J. Phys. Chem. 80 (1976) 2095.
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[28] J.P. Bonnelle, J. Grimblot, A.D. D’Huysser, J. Electr. Spectrosc. 7 (1975) 151. [29] T.A. Patterson, J.C. Carver, D.E. Leyden, D.M. Hercules, J. Phys. Chem. 80 (1976) 1702. [30] G.A. El-Shobaky, N.M. Ghoneim, I.M. Morsi, Thermochim. Acta 70 (1983) 325. [31] G.A. El-Shobaky, N.R.E. Radwan, F.M. Radwan, Thermochim. Acta 380 (2001) 27. [32] A.A. Balandin, Dokl. Akad. Nauk. USSSR 93 (1953) 55. [33] F.A. Kroger, Chemistry of Imperfect Crystals, North Holland, Amsterdam, 1964. [34] M. Mokhtar, Ph.D. Thesis, Faculty of Science, Cairo University, Egypt, 1997.
Effect of Li2 O-doping on surface and catalytic properties of Co3O4 –MoO3/Al2 O3 system G.A. El-shobaky a,∗ , A.M. Ghozza b , G.M. Mohamed a a
Lab. Surface Chemistry and Catalysis, National Research Center Dokki, Cairo, Egypt b Department of Chemistry, Faculty of Science, Zagazig University, Cairo, Egypt
Received 13 May 2002; received in revised form 18 June 2002; accepted 15 August 2002
Abstract The effect of Li2 O-doping on solid–solid interaction, surface and catalytic properties of Co3 O4 –MoO3 /Al2 O3 system were investigated using XRD, XPS, nitrogen adsorption at −196 ◦ C and oxidation of CO by O2 at 100–300 ◦ C. The nominal composition of the system was 0.2 Co3 O4 –0.05 MoO3 /Al2 O3 and the amount of dopant was varied between 0.14 and 0.57 wt.% Li2 O. The results revealed that Li2 O-doping inhibited the formation of CoAl2 O4 and CoMoO4 in the treated solids precalcined at 800 ◦ C and decreased the degree of crystallinity of Co3 O4 . Also, Li2 O-doping of the investigated system decreased the surface concentration of cobalt species and increased that of molybdenum species. The specific surface areas of the doped adsorbents precalcined at 600, 800 and 1000 ◦ C decreased progressively as a function of the amount of dopant added. The catalytic activities of pure and doped solids, in CO oxidation by O2 , decreased progressively by increasing the precalcination temperature in the range 600–1000 ◦ C. The doping process brought about a significant increase in the catalytic activities of the solids investigated due to an effective hindrance of formation of CoAl2 O4 and CoMoO4 which exhibit catalytic activities smaller than that of Co3 O4 . Lithia-doping of the investigated system did not modify the mechanism of the catalytic reaction but changed the concentration of catalytically active constituents without changing their energetic nature. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Li2 O-doping; Oxidation of CO by O2 ; CoAl2 O4 ; CoMoO4
1. Introduction Supported transition metals or oxides are commonly employed to catalyse several reactions including, oxidation–reduction, dehydration, dehydrogenation cracking, alkylation, . . . , etc. [1–12]. ␥-Al2 O3 is considered as the most convenient support for a big variety of catalysts especially those used in oxidation–reduction reactions. However, the catalytic activities of ␥-Al2 O3 supported solids de∗ Corresponding author. Fax: +20-2-3370-931. E-mail address: [email protected] (G.A. El-shobaky).
crease progressively as a function of time of use due to an enhanced conversion of active transition metal oxides into less active metal aluminates [13,14]. It has been reported [13,14] that the rate of reaction between transition metal oxide and Al2 O3 to produce metal aluminate decreases in the following order: Cu > Co > Ni > Fe. So, Al2 O3 support material should be treated with small amounts of certain foreign oxides in order to increase the service life time of the catalysts via hindering the formation of aluminate phases [15–17]. The surface properties of copper, nickel and cobalt oxides supported on ␥-Al2 O3 have been studied by
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several investigators [13,14,17,19–23]. The effect of foreign cations on the dispersion of catalytically active phases and on the solid–solid interaction between these phases and Al2 O3 support have been extensively investigated [18–22]. XPS, ISS, DRS and XRD analyses revealed that Li2 O-doping of Al2 O3 inhibited the formation of both cobalt and nickel aluminates due to the growth of a lithium aluminate phase which hindered the diffusion of cobalt and nickel ions into the ␥-Al2 O3 lattice during the calcination process [15–17,19]. Also, Be and Mg ions, like Li ions, inhibit the formation of cobalt aluminate and decrease the concentration of cobalt ions into the uppermost surface layers of the treated solids [23]. On the other hand, Na, K, Rb, Cs, Zn, Ca, Sr, Ba, Ga or Ge ions strongly stimulate the formation of surface epitaxial CoAl2 O4 by enhancing the diffusion of cobalt ions into tetrahedral sites of the ␥-Al2 O3 with subsequent increase in Co/Al ratio on the catalyst surface [15,17,20,24,25]. Indeed, Ca, Sr and Ba-doping of ␥-Al2 O3 catalyses the direct formation of CoAl2 O4 even at low temperature as 100 ◦ C [17]. Cobalt aluminate is generally undesirable component from the catalytic point of view due to its low reactivity towards oxidation–reduction and desulfurization reactions [14]. Nevertheless, in some cases the enhanced CoAl2 O4 formation was found to be connected with a large increase in the cobalt dispersion on the support surface [20]. Thus control of the active phase-carrier interactions to give optimum balance between the best state of dispersion and the minimum amount of CoAl2 O4 present, is important for maximising the catalytic activity. The present investigation is devoted to studying the effect of lithium-doping of the ␥-Al2 O3 support on the surface and catalytic properties of Co3 O4 –MoO3 supported catalysts. The techniques employed were XRD, XPS, nitrogen adsorption at −196 ◦ C and catalysis of CO oxidation by O2 at 100–300 ◦ C. 2. Experimental 2.1. Materials Pure Co3 O4 –MoO3 /Al2 O3 samples were prepared by the incipent wetness impregnation using Al(OH)3 solid, ammonium molybdate and cobalt nitrate. The
amounts of cobalt and molydenum oxides were fixed at 30.5 wt.% Co3 O4 and 4.5 wt.% MoO3 . The prepared solid samples were subjected to heat treatment for 5 h at 600, 800 and 1000 ◦ C. The Li2 O-doped samples were obtained by impregnating a known weight of Al(OH)3 solid with different proportions of LiNO3 dissolved in the minimum amount of distilled water making pastes. The pastes were dried at 100 ◦ C till constant weight then calcined at 500 ◦ C for 5 h. Then impregnating with calculated amount of ammoninum molybdate followed by drying and impregnation with a known amount of cobalt nitrate. The obtained solid samples were then calcined at 600, 800 and 1000 ◦ C for 5 h. The concentrations of dopant were 0.75, 1.5 and 3.0 mol% Li2 O which correspond to 0.14, 0.29 and 0.57 wt.% of Li2 O, respectively. The chemical employed were of analytical grade supplied by BDH and Prolabo companies. 2.2. Techniques The X-ray powder diffraction patterns of the various solids precalcined at 600 and 800 ◦ C were carried out using a Philips diffractometer (type PW 1390). The patterns were run with Fe-filtered cobalt radiation (λ = 1.7889 Å) at 30 kV and 10 mA with a scanning speed of 2◦ in 2θ min−1 . The surface concentration and binding energies of cobalt, molybdenum and aluminium ions were determined by a 550 ESCA/SAM spectrometer (Physical Electronics, USA) with a Mg anode X-ray tube (hν = 1253 eV). The pass energy of the analyser was 25 eV (E = 0.12 eV). The vacuum in the system was about 10−9 Torr. The energy of the scale was adjusted to match the Au 4f7/2 binding energy at 83.8 eV. All solid samples were finely powdered and spread on the sticky side of a thin film of Al foil. Small shifts in binding energy (BE) due to residual charging of the sample surface was corrected relative to C 1s electrons adsorbed hydrocarbons at BE of 284.6 eV. The relative atomic concentrations of Co, Mo and Al on the surface were determined by applying a computer program (PH 1 5267-YH V006B, Physical Electronics) that takes into consideration Scofield calculations photoionization cross-section energy dependence of electron escape depth and luminosity of the analyser. The surface properties, namely SBET , total pore volume (Vp ) and mean pore radius (¯r ) of different
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prepared solid samples were determined from nitrogen adsorption isotherms measured at −196 ◦ C using conventional volumetric apparatus. Before carrying out the measurements each sample was degassed under a reduced pressure of 10−5 Torr at 200 ◦ C for 2 h. The catalytic oxidation of CO by O2 on different catalyst samples was conducted at temperatures within the range 100–300 ◦ C using a static method. A mixture of carbon monoxide and oxygen at a pressure of 2 Torr was used. This method is simple and permits to follow up the kinetics of the catalytic reaction by measuring the drop in pressure of the reacting gases as a function of time till equilibrium was attained. A fresh 200 mg catalyst sample, placed in the hot zone of a Pyrex glass reactor was activated by heating under a reduced pressure of 10−6 Torr at 300 ◦ C for 2 h was employed in each kinetic experiment. Details of the apparatus used are given elsewhere [34].
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3. Results and discussion 3.1. XRD investigation of pure and Li2 O-doped samples X-ray diffractograms of pure and variously doped solids precalcined at 600 and 800 ◦ C were measured. The diffractograms of pure and doped samples precalcined at 600 ◦ C, not given, are similar to each other and consisted of all diffraction lines of Co3 O4 phase that having moderate degree of crystallinity and some lines of poorly crystalline ␥-Al2 O3 phase. Fig. 1 depicts the diffractograms of pure and doped solids samples precalcined at 800 ◦ C. Inspection of Fig. 1 showed the following: (i) The diffraction lines of CoMoO4 phase appeared in the diffractograms of pure sample and that doped with 0.14 wt.% Li2 O. The relative intensities of the diffraction lines of CoMoO4 phase decreased by doping with the smallest amount of Li2 O.
Fig. 1. X-ray diffractograms of pure and doped solids precalcined at 800 ◦ C.
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(ii) The diffraction lines of molybdate phase disappeared upon increasing the amount of dopant added to 0.29 or 0.57 wt.% Li2 O. (iii) The relative intensities of the diffraction lines of Co3 O4 or CoAl2 O4 phases decreased progressively as a function of the amount of Li2 O added. The heights of the diffraction lines (d = 2.86 Å) are 200, 178, 132 and 122 a.u. for pure sample and those treated with 0.14, 0.29 and 0.57 wt.% Li2 O, respectively. (iv) The average particle size of Co3 O4 or CoAl2 O4 phases was calculated for pure and doped samples and the data obtained are 157, 148, 145, and 135 Å for pure sample and the samples treated with 0.14, 0.29 and 0.57 wt.% Li2 O, respectively. The close similarity between the diffraction lines of Co3 O4 and CoAl2 O4 phases makes their distinction a difficult task. However, the relative intensity of the diffraction line located at 2.86 Å is 29 and 65% in the case of Co3 O4 and CoAl2 O4 phases, respectively [26]. This difference may be considered as a tool to differentiate between the two phases and to know which one of them exists as a major phase. This method was successfully used and the relative intensity of the diffraction line at d = 2.86 Å was 65, 64, 44, and 39% in the case of pure sample and those treated with 0.14, 0.29, and 0.57 wt.% Li2 O, respectively. These values suggested that CoAl2 O4 phase exists as the major phase in the case pure specimen and that doped with the smallest amount of dopant and calcined at 800 ◦ C. While Co3 O4 phase represents the major phase in the case of the other doped samples. This conclusion has been confirmed experimentally by comparing the colour of pure and doped samples. In fact, the pure sample and that treated with 0.14 wt.% Li2 O acquired blue
coloration (characteristic colour of CoAl2 O4 ) while the other doped samples were grey and dark grey indicating coexistence of cobaltic oxide and cobalt aluminate. These results clearly indicate that Li2 O-doping of the investigated system hindered the solid–solid interactions which yield CoAl2 O4 and CoMoO4 and the degree of hindrance increases by increasing the amount of dopant added. Also, Li2 O-treatment decreases the degree of crystallinity and particle size of Co3 O4 and CoAl2 O4 phases to an extent proportional to its amount present. 3.2. XPS investigation of pure and Li2 O-doped samples XPS investigation of pure sample and those doped with 0.29 and 0.58 wt.% Li2 O (1.5 and 3 mol%) precalcined at 800 ◦ C was carried out. This investigation enabled a precise determination of the concentration of Co, Al and Mo atoms on the top surface layers of the investigated solids. Also, the values of binding energies of cobalt, aluminium and molybdenum ions in different solids have been determined from the XPS investigation of these solids. Table 1 includes the results of XPS analysis of various samples. Inspection of the results given in Table 1 revealed that: (i) The concentration of cobalt and molybdenum species on the uppermost surface layers of the various investigated solids are greater than those in bulk of these solids. This finding suggested a preferential absorption of cobalt and molybdenum ions on the surfaces of catalyst samples in the course of impregnation of the support material with cobalt nitrate and
Table 1 Bulk and surface concentrations of Co, Al and Mo and their binding energies (BE) in the pure and doped solids precalcined at 800 ◦ C Dopant concentration (mol% Li2 O)
Bulk concentration of Co (at.%) Surface concentration of Co (at.%)a Bulk concentration of Al (at.%) Surface concentration of Al (at.%)a Bulk concentration of Mo (at.%) Surface concentration of Mo (at.%)a BE of Co (eV) BE of Al (eV) BE of Mo (eV) a
0
1.5
3.0
22.64 26.8 75.47 70.20 1.89 3.0 781 74.2, 73.5 231.4, 232.8
22.51 22.9 75.05 72.50 1.88 4.6 780.6 74.2, 73 232.8
22.39 21.23 74.63 74.17 1.87 4.6 779 74.3, 73.1 232.8
The surface concentrations of each species are relative atomic concentrations of Al, Co and Mo species.
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ammonium molybdate solutions. (ii) Li2 O-doping of the investigated system followed by heat treatment at 800 ◦ C led to a measurable progressive decrease in surface concentration of cobalt species. A decrease of 14.6 and 20.8% in surface concentration of cobalt species took place upon doping with 0.29 and 0.58 wt.% Li2 O, respectively. (iii) This treatment effected a significant increase in the concentration of molybdenum species in the uppermost surface layers of the treated samples. This increase attained more than 50%. (iv) The binding energy of cobalt ions (Co 2P3/2 ) in different investigated solids decreases by increasing the amount of Li2 O added. The value of binding energy of cobalt ion in the undoped sample is close to that of CoAl2 O4 compound [27–29] and this value in the case of the heavily doped sample is very close to that of Co3 O4 [27]. These results showed that Li2 O-doping of the investigated system hindered the solid–solid interaction between cobalt and aluminium oxides to yield cobalt aluminate. The degree of hindrance increases by increasing the amount of dopant added (x). The binding energy of molybdenum ions in pure sample varies between 231.8 and 232.8 eV. The first value corresponds to the value of BE of molybdenum species in cobalt molybdate and second value is indicative for BE of molybdenum ion in aluminium molybdate compounds [32]. On the other hand, the BE of molybdenum species in the two doped samples was 232.8 eV. This finding suggested that Li2 O-doping of the investigated system hinders the formation of cobalt molybdate and the presence of 0.29 wt.% Li2 O was enough to suppress completely the formation of CoMoO4 . So, the results of XRD investigation presented in the previous section of the present work have been confirmed by XPS investigation. The suppression of solid–solid interaction between cobalt and aluminium yielding CoAl2 O4 upon doping with Li2 O had been reported. In fact, it had been reported that Li2 O-doping of Co3 O4 /Al2 O3 much retarded CoAl2 O4 formation due to the growing up of LiAl5 O8 compound on the Al2 O3 surface that hindered the diffusion of Co2+ ions into tetrahedral sites of ␥-Al2 O3 support material [15–17,19]. Similar effect had been found in the case of doping the same system with Be2+ and Mg2+ species. Furthermore, the doping of NiO/Al2 O3 , Co3 O4 /Al2 O3 with Na+ , Zn2+ , Ga3+ and Ge4+ ions enhances the formation of surface aluminates produced via enhancing the diffusion
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of Co2+ or Ni2+ ions to tetrahedral sites of top surface layers of ␥-Al2 O3 support [15,17,20,24,25]. However, the hindrance of solid–solid interaction between cobalt and molybdenum oxides to yield CoMoO4 upon treatment with Li2 O, to our knowledge, had not been cited in literature. The hindrance of solid–solid interactions between two oxides to produce a compound upon doping with Li2 O could be tentatively discussed in terms of: (i) A possible dissolution of some lithium ions in the lattices of the reacting oxides increasing their oxidation states which become unfavourable for the compound formation [30,31]. In the case of CoAl2 O4 and CoMoO4 compounds, the cobalt ions exist as divalent ions and doping of cobalt oxide by Li2 O resulted in conversion of some of host Co2+ ions into Co3+ ions which might not easily involved in formation of aluminate or molybdate compounds [30,31]. (ii) The dopant added might cover the surfaces of reacting oxides and acted as an energy barrier that hindered the diffusion of reacting cations involved the compounds formation. 3.3. Surface properties of pure and doped solids The effect of Li2 O-doping of the investigated system followed by heat treatment at 600, 800 and 1000 ◦ C on its BET surface area (SBET ), total pore volume (Vp ) and mean pore radius (¯r ) was investigated by nitrogen adsorption at −196 ◦ C over pure and doped samples. The computed values of SBET , Vp and r¯ for the samples investigated are given in Table 2. Another series of specific surface areas St was calculated from V1−t plots of the different investigated adsorbents. These plots were constructed using suitable standard t-curves depending on the value of the C-constant in BET equation given in the last column of Table 2. Inspection of Table 2 reveals that the values of SBET and St , calculated for the various investigated adsorbents, are close to each other, within the experimental error which indicates the correct choice of the reference t-curves for analysis and shows the absence of ultramicropores. Table 2 shows also that Li2 O-doping of the system investigated brought about a measurable decrease in its SBET to an extent proportional to the amount of dopant added. This treatment was accompanied by a progressive increase in the value of r¯ . It can also be seen from Table 2 that Li2 O-doping of Co3 O4 –MoO3 /Al2 O3 samples followed by calcination at 600 or 800 ◦ C resulted in a progressive increase the
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Table 2 Surface characteristics of pure and doped solids precalcined at different temperatures Dopant concentration (mol%)
Calcination temperature (◦ C)
SBET (m2 g−1 )
St (m2 g−1 )
Vp (cm3 g−1 )
r¯ (Å)
BET C-constant
0.0 0.75 1.50 3.0 0.0 0.75 1.50 3.0 0.0 0.75 1.50 3.0
600 600 600 600 800 800 800 800 1000 1000 1000 1000
251 248 228 213 164 152 143 126 107 103 87 85
250 232 229 215 155 144 140 114 100 102 85 82
0.334 0.379 0.382 0.449 0.348 0.418 0.410 0.402 0.306 0.289 0.286 0.279
33 38 42 53 53 69 72 80 71 70 82 82
21 20 23 27 20 21 22 23 19 21 23 27
value of Vp . The maximum decrease in SBET of the investigated samples due to treatment with Li2 O is 14.7, 23.2 and 17.5% for the samples precalcined at 600, 800 and 1000 ◦ C, respectively. The maximum increase in the value due to doping is 60.6, 34 and 17% for the samples preheated at 600, 800 and 1000 ◦ C, respectively. The observed decrease in the specific surface areas of the samples investigated could be attributed mainly to widening of pores present, especially the samples precalcined at 800 and 1000 ◦ C. The observed modifications of solid–solid interactions between the different catalyst components, changes in surface concentration of cobalt species and variations in different surface characteristics of the system investigated due to doping with Li2 O are expected to be accompanied by significant changes in their catalytic activities.
over various solids precalcined at 600, 800 and 1000 ◦ C. The reaction rate constant (k) was calculated for each sample at different reaction temperatures. Table 3 includes the values of k for different samples. It can be seen from Table 3 that Li2 O increases the catalytic activity of the investigated system to an extent proportional to its amount added. However, a maximum increase in the catalytic activity was attained in presence of 1.5 mol% Li2 O for the samples precalcined at 600 and 800 ◦ C. The maximum increase in the values of k measured at 150, 250 and 300 ◦ C for the catalyst samples calcined at 600, 800 and 1000 ◦ C was 108, 20 and 38%, respectively. It has been shown (Table 2) that Li2 O-doping of the investigated system followed by heat treatment at 600–1000 ◦ C brought about a progressive decrease in its BET surface area. This decrease is expected to be
3.4. Catalytic activity of pure and doped samples
Table 3 Effect of Li2 O content on the values of k measured at different temperatures over various catalysts calcined at 600, 800 and 1000 ◦ C
The catalytic oxidation of CO by O2 was conducted over pure and doped samples precalcined at 600, 800 and 1000 ◦ C. First-order kinetics were found in all cases, the slope of each first-order plots determines the values of the reaction rate constant (k). The linear plot of log P0 /P or ln P0 /P versus time (were P0 is the initial pressure of the reaction mixture at time t = 0 and P is the pressure of the reaction mixture at time equals t) is indicative for first-order kinetics. Fig. 2 shows representative first-order plots of CO oxidation by O2 carried out at different temperatures
Dopant concentration (mol%)
k150 ◦ C × 10−3 (min−1 )a
k250 ◦ C × 10−3 (min−1 )b
k300 ◦ C × 10−3 (min−1 )c
0.0 0.75 1.50 3.00
2.6 3.6 5.4 2.8
7.4 7.9 8.9 8.5
7.3 8.6 9.5 10.1
Calcination temperature: 600 ◦ C. Calcination temperature: 800 ◦ C. c Calcination temperature: 1000 ◦ C. a
b
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241
Fig. 2. First-order plots of CO oxidation by O2 conducted at different temperatures over various catalysts calcined at 600, 800 and 1000 ◦ C.
accompanied by a possible decrease in the catalytic activity of Li2 O-treated samples. So, the reaction rate ¯ was calculated for the constant per unit surface area (k) catalysed reaction conducted at different temperatures over various catalyst samples calcined at 600, 800 and 1000 ◦ C. The results obtained are graphically illustrated in Fig. 3. This figure shows that Li2 O-doping effected a progressive increase in the values of k¯ to an extent proportional to its amount present reaching to a maximum limit at 1.5 mol% Li2 O for the samples calcined at 600 ◦ C. The maximum increase in the values ¯ measured at 150, 250 and 300 ◦ C for the cataof k, lysts calcined at 600, 800 and 1000 ◦ C, respectively, was 127%, 45% and 74% for the previously mentioned
catalysts. The observed significant increase in the catalytic activity of the investigated system due to doping with Li2 O might reflect an effective increase in the concentration and/or nature of active sites contributing in chemisorption and catalysis of CO oxidation by O2 over pure and doped catalyst samples. 3.5. Activation energies of the catalytic reaction carried out over pure and doped solids Determination of the apparent activation energy (Ea ) for catalysis of CO oxidation by O2 in contact with pure and variously doped solids has shed some light on the possible change in the mechanism of
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G.A. El-shobaky et al. / Applied Catalysis A: General 241 (2003) 235–245 Table 4 Activation energies Ea , Ea∗ and frequency factor A of the catalytic oxidation of CO by O2 conducted over pure and Li2 O-doped catalysts calcined at different temperatures Dopant (mol%)
Calcination Ea temperature (kJ mol−1 )a (◦ C)
A (min−1 )
Ea∗ (kJ mol−1 )
0.0 0.75 1.50 3.0 0.0 0.75 1.50 3.00 0.0 0.75 1.50 3.0
600 600 600 600 800 800 800 800 1000 1000 1000 1000
3.10 0.58 2.00 2.60 1.57 10.1 3.04 8.3 25.7 51.7 3.7 3.1
23.9 23.5 22.8 23.8 22.7 23.3 22.8 22.7 38.2 39.3 37.5 37.5
23.9 17.7 21.3 23.4 22.7 30.7 25.1 29.5 38.2 40.4 27.8 27.4
a E values were calculated for the catalytic reaction carried a out at 100–200, 150–250 and 200–300 ◦ C over different catalysts calcined at 600, 800 and 1000 ◦ C, respectively.
Fig. 3. Variation of k as a functions of dopant content for catalytic reaction conducted at different temperatures over various catalysts calcined at 600, 800, and 1000 ◦ C.
the catalysed reaction and given useful information about the possible change in the concentration and nature of catalytically active constituents taking part in the catalytic reaction. The variation of k measured at different temperatures over the variously doped solids precalcined at 600–1000 ◦ C have enabled Ea to be calculated by direct application of the Arrhenius
equation. The calculated values of Ea are listed in Table 4 which also includes the values of the pre-exponential factor (A) for the Arrhenius equation. Inspection of Table 4 reveals that Li2 O-doping of various solids followed by calcination at 600–1000 ◦ C resulted in fluctuation in the values of Ea . This finding did not run parallel to the induced increase in their catalytic activities. However, a same fluctuation in the values of A was observed in the cases of different investigated samples indicating that a compensation effect might be responsible for the observed changes in the values of Ea due to Li2 O-doping. This assumption which reflects the heterogeneity of the surfaces of the treated solid. The activation energies for the catalytic reaction were calculated adopting the values of A for the untreated catalyst samples precalcined at 600, 800 and 1000 ◦ C, respectively for the other doped samples precalcined at the same temperatures. The resulting (Ea∗ ) values obtained were virtually the same (within the experimental error) for the pure and variously doped samples precalcined at the same temperatures. This indicates that Li2 O-doping of the investigated system followed by calcination at 600–1000 ◦ C did not change the mechanism of the catalytic reaction but rather increased the concentration of the catalytically active sites on the catalyst surface upon doping
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with lithia. 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 [22] F (Ei ) = a exp(h Ei ) where Ei is the interaction energy of site i with the substrate. This equation may be converted to the following: A = a exp(h E) Suggesting that a plot of ln A versus E for the variously doped catalyst samples should give a straight line whose slope and intercept would allow the evaluation of the constants “h” and “a”, respectively. Fig. 4 depicts the variation of E and ln A for the catalytic oxidation of CO by O2 carried out in contact with pure and doped catalyst samples precalcined at 600, 800 and 1000 ◦ C, respectively. The computed values of “h” were 0.14, 0.21, 0.21 and 0.21 mol kJ−1 min−1 for pure and variously doped catalysts precalcined at 600, 800 and 1000 ◦ C, respectively. The computed values of the constant a were 0.14, 0.22 and 0.22 min−1 for Li2 O-doped samples precalcined at 600, 800 and 1000 ◦ C, respectively. The identical values of the contestants “h” and “a” for pure and different doped solids calcined at one and the same temperature might indicate that the doping process did not change the nature of catalytically active sites involved in the catalytic reaction. The values of “h” and “a” indicate that Li2 O-doping of the investigated system followed by calcination at 600, 800 and 1000 ◦ C did not change the dissipation of active sites on the surfaces of variously doped catalysts, i.e. the nature of the surface heterogeneity, but led to changes (increase in their concentration). The observed changes in the catalytic activity of the investigated system due to doping with Li2 O conducted at 600–1000 ◦ C could be discussed in terms of (i) Dissolution of some of dopant ions in the lattice of Co3 O4 in the catalyst samples with subsequent transformation of some of host Co2+ into Co3+ ions. The creation of trivalent cobalt ions due to doping with Li2 O might take place according the following mechanism, which can be simplified by adopting Kroger’s notions in the following manner [33]: Li2 O + 21 O2 (g) → Li(Co2+ ) + 2Co3+
Fig. 4. Relationship between Ea and ln A for pure and Li2 O-doped catalysts precalcined at different temperatures.
where Li(Co2+ ) are monovalent lithium ions located in the positions of host Co2+ ions in the Co3 O4 solid and Co3+ are trivalent cobalt ions (lattice defects) created in the doped solids. The dissolution of Li2 O in Co3 O4 lattice according to the above mentioned
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mechanism is followed by fixation of atmospheric oxygen with subsequent conversion of some of Co2+ into Co3+ ions. The created trivalent cobalt ions act as active sites for chemisorption and catalysis of CO oxidation by O2 . As CO acts as electron donor gas its chemisorption could be favoured by creation of Co3+ ions that act as electron acceptor sites. (ii) The observed decrease in the concentration of cobalt species on the top surface layers of doped samples cf. Table 1. (iii) Hindrance of formation CoAl2 O4 and CoMoO4 compounds in the investigated system as a result of doping with small amounts of Li2 O (cf. Fig. 1 and Table 1). The fact that Co3 O4 solid exhibits higher catalytic activity than those of CoAl2 O4 and CoMoO4 compounds, the suppression of cobalt aluminate and molybdate due to Li2 O-treatment might lead to an increase in the catalytic activity of the doped solids. The observed limited decrease in the concentration of surface cobalt species (14.6–20.8%, cf. Table 1) in doped samples precalcined at 800 ◦ C might be accompanied by decrease in their catalytic activity. The detected measurable increase in the catalytic activity of the investigated system due to treatment with small amounts of Li2 O might suggest the domination of both creation of Co3+ ions and suppression of solid–solid interactions between Co3 O4 and Al2 O3 and MoO3 that yield CoAl2 O4 and CoMoO4 , respectively on the observed limited decrease in the concentration of surface cobalt species.
4. Conclusions These are the main conclusions that may be drawn from the results obtained in the present investigation: 1. Doping of Co3 O4 –MoO3 /Al2 O3 system with Li2 O (0.75–3 mol%) followed by heat treatment at 600–1000 ◦ C hindered the solid–solid interaction between Co3 O4 and each of Al2 O3 and MoO3 that yield CoAl2 O4 and CoMoO4 . The degree of hindrance ran parallel to the amount of dopant added. 2. This treatment led to a limited decrease in the concentration of cobalt species present on the outermost surface layers of the treated solids. 3. The doping process of the investigated system resulted in a measurable decrease in its specific surface area.
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