Al2O3 solids as being influenced by Li2O and K2O-doping

Colloids and Surfaces A: Physicochemical and Engineering Aspects 178 (2001) 287 – 296 www.elsevier.nl/locate/colsurfa

Surface and catalytic properties of Fe2O3Cr2O3/Al2O3 solids as being influenced by Li2O and K2O-doping Gehan A. Fagal, Gamil A. El-Shobaky *, Sahar M. El-Khouly Department of Physical Chemistry, Laboratory of Surface Chemistry and Catalysis, National Research Center, Dokki, Cairo, Egypt Received 9 November 1999; accepted 11 July 2000

Abstract The effects of Li2O (0.15–0.60 wt.%) and K2O (0.45 – 1.80 wt%)-doping on surface and catalytic properties of Fe2O3Cr2O3/Al2O3 solids were studied using nitrogen adsorption at − 196°C and oxidation of CO by O2 at 225–300°C. The results showed that Li2O (0.6%) and K2O (1.8%) doping followed by precalcination at 700°C resulted in an important increase in the BET- surface areas (about one-fold) of the treated solids. The precalcination of variously doped solids at 400°C exerted no significant change in the specific surface areas of the doped adsorbents. The catalytic activity increased by increasing the amount of Li2O added to the solids preheated at 400 and 700°C. On the other hand, K2O-doping conducted at 400°C, which resulted in a considerable increase in the catalytic activity of the treated solids, much decreased their catalytic activity upon precalcination at 700°C. The doping process did not modify the mechanism of catalytic oxidation of CO by O2, but changed the concentration of catalytically-active component (surface Fe2O3 crystallites) without modifying their energetic nature. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Surface and catalytic properties; Fe2O3Cr2O3/Al2O3; Doping with Li2O and K2O; Oxidation of CO by O2

1. Introduction The catalytic activity and selectivity of a large variety of catalysts can be modified by various methods such as loading on a finely divided support [1–6], subjecting to ionizing radiations [7– 15] and doping with certain foreign oxides [16–22]. The loading on suitable support material results in an increase in the concentration of catalytically active constituents via increasing * Corresponding author. Fax: +-20-2-3370931.

their dispersity and hindering their grain growth. The ionizing radiations might also increase the number of active components by splitting them into small-sized particles besides modifying their acidic properties [12,13]. The doping with certain foreign oxides such as Li2O, K2O or ZnO hinders the metal oxide-support interactions thus increasing the stability of catalytically active constituents [16–23]. The catalytic oxidation of CO by O2 is of environmental and industrial importance and is being utilized in an increasing number of practical

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applications [24 – 28]. The effect of gamma-irradiation on the surface properties and catalytic activity, in CO oxidation by O2, of Fe2O3Cr2O3/Al2O3 was the object of our recent investigation [15]. The present work reports a study on the influence of Li2O and K2O-doping on the surface and catalytic properties of Fe2O3Cr2O3/Al2O3 using adsorption of N2 at − 196°C and catalysis of CO oxidation by O2 at different temperatures over variously doped solids.

2. Experimental

2.1. Materials Aluminum hydroxide was prepared from an aluminum nitrate (BDH) solution containing 10.2 gl − 1 of aluminium using a NH4OH solution (0.2 N) at 70°C and a pH value of 8. The gel obtained was washed with double distilled water until free from ammonium and nitrate ions, and then dried at 100°C to constant weight. Pure iron, chromium and aluminum mixed oxide specimens were prepared by mixing a known mass of finely powdered Al(OH)3 with a calculated amount of CrO3, then impregnating the obtained solid with a solution containing a given amount of Fe(NO3)3 (AR grade, supplied by Prolabo). The impregnated solid sample was dried at 120°C and then heated in air at 400 and 700°C for 4 h. The nominal molar composition of the calcined mixed solids sample was 0.125 Fe2O3 0.025 Cr2O3/Al2O3. Doped solid specimens were prepared by treating a known mass of finely powdered Al(OH)3 with solutions containing calculated amounts of LiNO3 or KNO3 (BDH) prior to treatment with CrO3 and ferric nitrate solution. The solids obtained were dried at 120°C and then calcined at 400 and 700°C for 4 h. The concentrations of dopants were 0.15, 0.3, 0.6 wt% Li2O and 0.45, 0.9 and 1.8 wt% K2O.

2.2. Techniques X-ray investigations of pure and variously doped mixed solids preheated in air at 400 and 700°C were carried out using a Philips diffrac-

tometer (type PW 1390). The patterns were run with Fe-filtered cobalt radiation (l= 1.7889A, ) at 30 kV and 10 mA with a scanning speed of 2° in 2U min − 1. The surface properties, i.e. SBET, Vp and r¯ of the different treated samples were determined from nitrogen adsorption isotherms measured at − 196°C using a conventional volumetric apparatus. Before carrying out the measurements, each sample was degassed under a reduced pressure of 10 − 5 torr for 2 h at 200°C. Catalytic oxidation of CO by O2 over the various catalysts was carried out at temperatures in the range of 225–300°C using a static method. A stoichiometric mixture of CO and O2 at a pressure of 2 torr was used. A fresh catalyst sample (200 mg) was employed for each kinetic experiment and was activated by heating under a reduced pressure of 10 − 6 torr for 2 h at 350°C. The CO2 produced was removed from the reaction system by freezing in a liquid nitrogen trap. The kinetics of the catalytic reaction were monitored by measuring the pressure of the reacting gases at different time intervals until equilibrium was attained. The percentage drop in pressure at a given time determines the percentage reaction at that time.

3. Results and discussion

3.1. XRD analysis of pure and doped solids X-ray diffractograms of pure and variously doped solids precalcined at 700°C (not depicted here) showed that they consisted of poorly crystalline gamma-Al2O3 and a-Fe2O3 phases. However, the diffraction lines of a-Fe2O3 phase were not observed in the patterns of Li2O-doped solids (treated with 0.3 or 0.6 wt%) precalcined at 700°C. This finding might indicate that Li2Otreatment of Fe2O3Cr2O3/Al2O3 system followed by precalcination at 700°C resulted in an increase in the degree of dispersion of Fe2O3 phase yielding small sized ferric oxide crystallites that could not be detected by XRD. The X-ray analysis of pure and doped solids precalcined at 400°C showed the amorphous nature of these solids.

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3.2. Surface properties of the 6ariously doped adsorbents The surface characteristics of pure and doped mixed oxide samples calcined in air at 400 and 700°C were determined from nitrogen adsorption isotherms conducted at −196°C. The isotherms obtained belong to type II of the Brunauer’s classification [29]. Fig. 1 shows representative isotherms for pure and Li2O-doped solids preheated at 700°C, which showed hysteresis loops of an area that increased by increasing the amount of dopant added. The different surface characteristics, i.e. SBET, Vp and r¯ obtained for the various adsorbents are listed in Table 1. Another series of specific surface areas St were calculated from the volume thickness (Vl − t ) plots of the various adsorbents. The obtained Vl − t plots were constructed using convenient standard t-curves. The plots obtained for the various treated adsorbents precalcined at 400 and 700°C were similar to each other. Fig. 2 shows representative Vl − t plots of pure and K2O-doped samples preheated at 700°C.

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These plots indicate that the investigated solids consisted mainly of wide pores (mesopores). The computed values of St are given in Table 1. It is observed from Table 1 that the SBET and St are close to each other, thereby justifying the correct choice of standard t-curves and showing the absence of ultramicropores in tested samples. Inspection of the results in Table 1 revealed that: (i) Li2O and K2O doping followed by precalcination at 700°C resulted in a progressive increase of the BET-surface area of the treated mixed oxide solids; (ii) the observed increase of the BET-surface area due to doping attained 100 and 93% in the presence of 0.6 mol% Li2O and 1.8 wt% K2O, respectively; (iii) Li2O-treatment followed by precalcination at 700°C effected a progressive decrease of the total pore volume, Vp (12–39%), of the treated samples; (iv) treatment of the mixed solids with the smallest amounts of dopant (0.15 wt% Li2O and 0.45 wt% K2O) followed by heat treatment at 400°C resulted in a decrease in the SBET (23 and 27%) for Li2O and K2O, respectively; (v) the heavily doped solids (0.6 and 1.8

Fig. 1. N2-adsorption isotherms for pure and different doped samples calcined at 400 and 700°C.

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Table 1 Surface characteristics of pure and doped Fe2O3Cr2O3/Al2O3 adsorbents precalcined at 400 and 700°C Adsorbent Fe2O3Cr2O3/Al2O3 +0.15 wt% Li2O +0.30 wt% Li2O +0.60 wt% Li2O +0.45 wt% K2O +0.90 wt% K2O +1.80 wt% K2O Fe2O3Cr2O3/Al2O3 +0.15 wt% Li2O +0.30 wt% Li2O +0.60 wt% Li2O +0.45 wt% K2O +0.90 wt% K2O +1.80 WT% K2O

Calcination temperature (°C)

SBET a(m2g−1) St (m2g−1)

Vp b(cm3g−1)

r¯ c(A, )

400°C

331 255 310 373 241 300 351

327 268 307 357 235 280 300

0.691 0.533 0.502 0.495 0.553 0.542 0.498

52 52 40 33 57 45 35

73 39 71 18 19 38 24

700°C

180 249 301 361 237 300 347

173 243 296 356 214 300 321

0.545 0.477 0.433 0.331 0.511 0.432 0.518

75 48 36 23 44 36 37

65 36 70 21 43 39 27

BET-C

a

Specific surface area. Total pore volume. c Mean pore radius. b

wt% K2O) precalcined at 400°C measured specific surface areas slightly higher than that measured for pure sample calcined at 400°C. Similar results have been reported in the case of CuO/Al2O3, NiOFe2O3/Al2O3, CuO-ZnO/Al2O3, NiOFe2O3, Co3O4, V2O5/Al2O3, and Cr2O3/Al2O3 systems being doped with Li2O or Na2O, or K2O and subjected to heat treatment at 500–800°C [30–34,20–22]. The observed significant increase of the specific surface areas of mixed oxide solids treated either with Li2O or with K2O followed by precalcination at 700°C can be attributed to pore narrowing process accompanying the doping process. In fact, the mean pore radius (r¯ ) values for the Fe2O3Cr2O3/Al2O3 system decreased from 75 to 23 A, and from 75 to 37 A, upon doping with 0.6 and 1.8 wt% K2O followed by heating in air at 700°C, respectively. However, the increase induced in the SBET values of mixed oxide solids as a result of doping with Li2O or K2O followed by heat treatment in air at 700°C can, also, be attributed to the creation of pores produced from liberation of gaseous nitrogen oxides during the thermal decomposition of LiNO3 or KNO3 during the thermal treatment of doped solids at 700°C [20–22,30–34].

3.3. Catalytic oxidation of CO by O2 o6er pure and doped solids The catalytic oxidation of CO by O2 on pure and doped mixed solids precalcined at 400 and 700°C was carried out at 225–300°C. The results showed that the catalysis of CO oxidation by O2 follows first-order kinetics over pure and doped catalyst samples. Figs. 3 and 4 show kinetic curves and first-order plots for the catalytic reaction conducted at 300°C over pure and doped samples precalcined at 400 and 700°C, respectively. The slope of the first-order plots determines the magnitude of the reaction rate constant (k) of the catalysed reaction carried out at a given temperature. It is seen from Fig. 3 that k increases progressively on increasing the Li2O or K2O contents in the mixed oxide solids precalcined at 400°C. The maximum increase due to doping with 0.60 wt% Li2O and 1.8 wt% K2O was 120 and 170%, respectively. Fig. 4 shows that doping of Fe2O3Cr2O3/Al2O3 system with Li2O followed by precalcination at 700°C resulted in a progressive increase in the k values for the reaction carried out at 300°C. The treatment of mixed solids with 0.3 wt% Li2O at 700°C effected an increase of 126% in the value of

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k measured at 300°C. Fig. 4 shows that K2O doping of mixed solids followed by heat treatment in air at 700°C brought a significant decrease in the catalytic activities of the treated solids to an extent proportional to the amount added. A decrease of 57% in the k300°C value was found upon doping mixed solids with 1.8 wt% K2O. The doping of Fe2O3Cr2O3/Al2O3 mixed solids at 700°C resulted in an increase in their specific surface areas (31–100%). This increase in SBET is expected to be followed by an increase and not a decrease in the catalytic activity. In order to account for the increase in SBET induced as a result of doping with Li2O and K2O at 700°C,the values of k( (reaction rate constant per unit surface area) were calculated for pure and doped solids. The values of k( 300°C were 15.8 × 10 − 5, 21.4 ×10 − 5 and 3.6 × 10 − 5 min − 1m − 2 for the catalytic reaction conducted at 300°C over the pure catalyst sample precalcined at 700°C and those doped with 0.3 wt% Li2O and 1.8 wt% K2O, respectively. So, doping of mixed oxide solids with 0.3 wt% Li2O followed by precalcination at 700°C affected an increase of 35% in k( value measured for the catalysed reaction carried out at 300°C. On the

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other hand, doping of mixed solids with K2O (1.8 wt%) followed by heat treatment brought about a decrease of 77% in the k( value measured at 300°C. These results clearly indicate the role of the nature of the dopant and the temperature of the heat treatment of mixed oxide solids in modifying their catalytic activities towards CO oxidation by O2. These findings also showed that other parameters besides surface characteristics determine the catalytic activity of Fe2O3Cr2O3/Al2O3 systems. These parameters include the concentration of catalytically active constituents on the top surface layers of treated solids and the possible metal oxide–support interaction. Treatment of supported mixed oxide solids with doping with certain foreign oxides or subjecting to ionizing radiations have been reported to induce significant changes in their catalytic activities [7–15]. These treatments might change both the concentration of the catalytically active components on the catalyst’s surface and the metal oxide-support interactions thereby modifying effectively their catalytic activities [16–20,23]. The doping process may affect the number of active sites on the catalyst’s surface contributing in chemisorption

Fig. 2. Volume–thickness plots (V1 − t ) of the pure and doped samples calcined at 400 and 700°C.

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Fig. 3. Kinetic curves and first-order plots of CO-oxidation conducted at 300°C over pure and doped solids calcined at 400°C.

and catalysis of CO-oxidation by O2. The energetic nature of these sites could also be influenced by doping. Furthermore, the mechanism of the catalytic reaction could be changed by doping.

3.4. Effects of Li2O and K2O doping on DE of the catalysed reaction Determination of the apparent activation energy (DE) for the catalysis of CO oxidation by O2 over pure and variously doped solids precalcined at different temperatures sheds some light on the change in the mechanism of the catalysed reaction and hence gives useful information about the possible alteration in the concentration and nature of catalytically-active constituents. The values of k measured at temperatures varying between 225 and 300°C over the variously doped solids enable DE to be calculated by direct application of the Arrhenius equation. The calculated values of DE are listed in Table 2, which includes the values of the pre-exponential factor (A) of the Arrhenius equation. Table 2 shows that A changes with doping, which may be an indication of the heterogenity of the catalyst surface. It

can be seen from the results in Table 2 that doping of Fe2O3Cr2O3/Al2O3 solids with Li2O and K2O followed by precalcination at 400°C brought about a decrease in DE values to an extent proportional to their amounts added. These results expressed the observed increase in the catalytic activities of the treated catalyst samples. It is observed from the results given in Table 2 that Li2O and K2O doping carried out at 700°C brought about a fluctuation in DE values, i.e. both increases and decreases were found. However, the increase in the value of DE was accompanied by a corresponding increase in the value of log A and vice versa. Hence, the observed change in DE value due to Li2O and K2O doping of Fe2O3Cr2O3/Al2O3 solids may result from the change in the magnitude of the frequency factor. In fact, the activation energy of the catalytic reaction over pure and doped solids was calculated with the values of A for pure solids calcined at 400 and 700°C being adopted for the variously doped catalysts precalcined at the same temperatures, i.e. ln A values of −0.525 and 2.863 found for pure solids calcined at 400 and 700°C, respectively, were adopted for the different Li2O and

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K2O-doped solids precalcined at the same temperatures. The recalculated values of the activation energy DE* for the various solids investigated are listed in the last column of Table 2. The comparison of DE* for pure and variously doped catalysts revealed that doping of Fe2O3Cr2O3/Al2O3 mixed solids either with Li2O or with K2O followed by precalcination at 400 and 700°C did not much modify the activation energy of CO oxidation by O2 over different solids; DE values of 209 2 and 309 2 kJmol − 1 were found for pure and variously doped solids precalcined at 400 and 700°C, respectively. These results clearly indicate that Li2O and K2O doping of Fe2O3Cr2O3/Al2O3 solids at 400 and 700°C did not modify the mechanism of the catalysed reaction but changed the concentration of catalytically active constituents without altering their energetic nature. This conclusion finds an additional evidence from the plot of the equation: A = a exp h D E, derived on the basis of the dissipation function of active sites by their energy

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as a consequence of surface heterogeneity [35]. F(Ei)= a exp h Ei where Ei is the energy of interaction of site ‘i’ with the substrate [35]. The plot of ln A versus DE for doped solids precalcined at a given temperature give a straight line where its slope and intercept determine the values of h and a, respectively. Fig. 5 depicts the variation of ln A as a function of DE for Fe2O3Cr2O3/ Al2O3 solids doped with Li2O and K2O and precalcined at 400 and 700°C. The computed values of the constant h are 0.35, 0.25, 0.32 and 0.15 mol/(kJ min − 1) for Li2O-doped solids calcined at 400 and 700°C and for K2O-doped solids calcined at 400 and 700°C, respectively. The calculated values of the constant a are 2.48 × 10 − 4, 8.6 × 10 − 3, 2.48 × 10 − 4 and 0.2 min − 1 for the previously tested mixed oxide solids. The constants h and a, indicate that the doping of Fe2O3Cr2O3/ Al2O3 either with Li2O or with K2O followed by precalcination at 400 and 700°C did not change the energetic nature of the active sites but changed their concentration on the top surface layers of

Fig. 4. Kinetic curves and first-order plots of CO-oxidation conducted at 300°C on pure and doped solids precalcined at 700°C.

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Table 2 Activation energy (DE and DE*) and frequency factor for the catalytic oxidation of CO by O2 over pure and doped solids Solid Fe2O3Cr2O3/Al2O3 +0.15 wt% Li2O +0.30 wt% Li2O +0.60 wt% Li2O +0.45 wt% K2O +0.90 wt% K2O +1.80 wt% K2O Fe2O3Cr2O3/Al2O3 +0.15 wt% Li2O +0.30 wt% Li2O +0.60 wt% Li2O +0.45 wt% K2O +0.90 wt% K2O +1.80 wt% K2O

Calcination temperature (°C)

DE (kJmol−1)

ln A

400°C

22 19 15 13 17 15 12

−0.525 −1.513 −3.080 −3.762 −2.405 −3.004 −4.135

22 21 20 19 20 18 19

700°C

30 27 40 26 36 27 32

2.863 2.203 5.585 1.785 3.925 2.505 3.250

30 28 28 28 31 32 32

the treated catalysts. In other words, the catalysis of CO oxidation by O2 over the Fe2O3Cr2O3/ Al2O3 system is not structure dependent. The changes in the concentration of catalytically active sites (surface F2O3 crystallites) due to doping of Fe2O3Cr2O3/Al2O3 solids by Li2O and K2O could be discussed in terms of: (i) metal oxide-support interactions as being influenced by the doping process; (ii) a possible change in degree of dispersion of Fe2O3 crystallites due to treatment with dopant oxides; and (iii) pore narrowing that might decrease the accessibility of reacting substances. The last parameter might exhibit a minor role in the case of CO oxidation reaction by O2 where no bulky molecules are involved in the catalytic process. It has been reported that the reaction between the transition metal oxides and Al2O3 to produce metal aluminate is strongly dependent upon the nature of the transition metal element. The rate of reaction between the metal oxide and Al2O3 deceases in the following order: Cu\Co \ Ni Fe [24]. So, the metal oxide – support interaction in the case of the Fe2O3/Al2O3 system exerted no significant role in changing its catalytic activity. However, K2O could interact with Fe2O3 and Cr2O3 in the course of precalcination at 700°C of the treated solids to yield potassium ferrites and potassium chromate [36]. The significant decrease in the catalytic activ-

DE* (kJmol−1)

ity due to K2O-treatment of mixed solid catalysts followed by precalcination at 700°C might be attributed to a significant decrease in the concentration of Fe2O3 crystallites on the suppermost surface layers of the treated solids due to conversion of some of them into potassium ferrites. The observed increase in the catalytic activity of Fe2O3Cr2O3/Al2O3 system due to doping with Li2O followed by precalcination at 400 and 700°C and K2O-doping at 400°C might be interpreted as a result of an effective increase in the degree of dispersion of catalytically active constituents (surface Fe2O3 crystallites) due to such treatment. The confirmation of this assumption requires an accurate determination of Fe3 + ions on the outermost surface layers of the treated mixed oxide samples by XPS analysis. However, the disappearance of the diffraction lines of Fe2O3 phase from the XRD patterns of Li2O-treated solids subjected to heat treatment at 700°C could stand as a quantitative evidence for an induced increase in the concentration of Fe2O3 crystallites via increasing their degree of dispersion.

4. Conclusions The main conclusion that may be drawn from the results obtained can be summarized as follows:

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1. Pure and doped Fe2O3Cr2O3/Al2O3 solids precalcined at 700°C consisted of poorly crystalline gama-Al2O3 and a-Fe2O3 phases. However, the addition of Li2O (0.3, 0.6 wt%) resulted in complete disappearance of the diffraction lines of Fe2O3 phase which indicates an effective increase in its degree of dispersion yielding small-sized crystallites. 2. Doping the mixed oxide solids either with Li2O or with K2O followed by precalcination at 700°C brought about an increase in their BET-surface areas to an extent proportional to their amounts added. The increase attained 100 and 93% in the presence of 0.6 wt% Li2O and 1.8 wt% K2O, respectively and resulted mainly from an effective narrowing of the

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pores, r¯ decreased from 75 to 23 and 37 A, due to this treatment. On the other hand, the treatment of mixed oxide solids with small amounts of Li2O or K2O followed by precalcination at 400°C effected a decrease in their SBET, which increased upon increasing the amounts of dopants reaching values slightly higher than that measured for the untreated sample. 3. The catalytic activity, in CO oxidation by O2 of Fe2O3Cr2O3/Al2O3 system was found to increase by increasing the amounts of dopant added for Li2O-treated solids precalcined at 400 and 700°C. By contrast, K2O-doping followed by precalcination at 400°C, which resulted in a considerable increase in the

Fig. 5. Relationship between DE and frequency factor ln A for the catalytic reaction conducted over different doped catalysts calcined at 400 and 700°C.

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catalytic activity of the treated solids, much decreased their catalytic activity upon precalcination at 700°C. 4. Li2O and K2O-doping of the investigated mixed oxide solids did not modify the mechanism of catalytic oxidation of CO by O2 carried out at 225 – 300°C over various solids, but changed the concentration of catalytically active constituents (surface Fe2O3 crystallites) without modifying their energetic nature.

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