Effect of lithium oxide doping on surface and catalytic properties of NiOFe2O3 solids

January 1995

Materials Letters 22 (1995) 39-45

ELSEVIER

Effect of lithium oxide doping on surface and catalytic properties of NiO/Fe203 solids G.A. El-Shobaky a*‘,A.N. Al-Noaimi b, A. Abd El-Aal ‘, A.M. Ghozza ’ aNational Research Center, Dokki, Cairo, Egypt b Chemistry Department, Faculty ofScience, Qatar University,Doha, Qatar c Chemistry Department, Faculty of Science, Zagazig University,Zagazig, Egypt Received 10 May 1994; in final form 8 August 1994; accepted 5 October 1994

The influence of Liz0 doping (0.75-3.0 mol%) on surface and catalytic properties of NiO/Fe20s, having a molar ratio of 1: 2, was investigated. Pure and doped mixed solids were heated in air at 700 and 800°C The surface characteristics of various solids were determined from N,-adsorption isotherms conducted at - 196°C and their catalytic activities were measured by following up the kinetics of CO oxidation by O2 at 300-400°C using a static method. The results revealed that the doping process brought about an increase in the specific surface area (Sa,) of mixed solids. A maximum increase of about two-fold was observed by treating with 1.5% LiaC). On the other hand, this treatment resulted in a drastic decrease (about 50%) in the catalytic activities of the doped catalysts. Lithium oxide doping altered the pre-exponential factor of the Arrhenius equation without changing the magnitude of the apparent activation energy of the catalytic reaction which might indicate that Liz0 did not change the mechanism of the catalytic reaction.

1. Introduction

Transition metal oxides are active catalysts for oxidation-reduction reactions [ 1-6 ] . Their activities could be modified by a variety of methods as: (i) Doping with certain. foreign oxides [2,5,8-l 11. (ii) Subjecting to ionizing radiations [ 12- 18 1. (iii) Loading on suitable support material [ 17-2 11. (iv) Mixing with other transition metal oxides [ 22,231. The oxidation of CO by O2 on different transition metal oxides has been made the object of several investigations with the aim of finding a correlation between catalytic properties and other physico-chemical properties [ l-6,1 1,12,15,16,2 11. Nickel oxide catalysts represent a focus of interest of a majority of

published works. On the other hand, ferric oxide and mixed nickel and iron oxides have received much less importance. The present investigation reports a study on the effect of lithium oxide doping on textural and catalytic properties of NiO/Fe203 binary oxides. The textural characteristics (surface properties), namely specific surface area, SBET,total pore volume, VP,and mean pore radius, f, were determined from nitrogen adsorption isotherms conducted at - 196°C. The catalytic activities of various solids were measured by following up the kinetics of CO oxidation by O2 at different temperatures between 300 and 400°C using a static method.

r The author to whom correspondence should be addressed. 0167-577x/95/$09.50 (4 1995 Elsevier Science B.V. All rights reserved. SSDIOl67-577x(94)00223-1

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G.A. El-Shobaky EI al. /Materials Leuers 22 fl99j) 1%45

2. Materials and experimental techniques 2.1. Materials

Hydrated ferric oxide was prepared by precipitation from ferric sulphate (BDH ) solution using a dilute NH,OH solution (0.2 M) at 70°C and pH=8. The gel obtained was carefully washed with bidistilled water till free from ammonium and sulphate ions then dried at 100°C to constant weight. The chemical and thermal analyses showed that the prepared solid has the formula FezOj. 1.5Hz0. Pure ferric oxide was obtained by heating the prepared hydrated oxide in air at 500°C for 5 h. XRD analysis revealed the formation of well crystallized a-FezO, phase. Basic nickel carbonate, NiCOJ.Ni(OH)2*4H20, of analytical grade was supplied by the Prolabor company. Preliminary experiments showed that the molar ratio of NiO: Fe203 of 1: 2 was in favour in nickel ferrite formation at 700 or 800°C. Pure nickel/iron mixed oxide solids having the nominal molar composition of NiO : 2Fe203 were obtained by mechanical mixing of finely powdered FeZOj at 500°C with calculated amount of basic nickel carbonate then heating in air at 700 and 800°C for 5 h. Lithium oxide doped mixed oxide solids were prepared by treating a known mass of FezOJNiCOJ mixed solids with solutions containing different proportions of LiN03 followed by drying at 100°C and calcination at 700 and 800°C for 5 h. The amounts of lithium, expressed as L&O were 0.75, 1.5 and 3 molW. 2.2. Techniques The surface characteristics, namely Sam, VPand f, of different solids were determined from N,-adsorption isotherms conducted at - 196°C using a conventional volumetric apparatus. Before carrying out the measurements each solid sample was degassed at 200°C for 2 h under a reduced pressure of 10e5 Torr. The catalytic oxidation of CO by O2 on different catalysts was carried out at 300-400°C using a static method. A fresh 200 mg catalyst sample was employed for each kinetic experiment and activated by

heating at 300°C for 2 h under a reduced pressure of 10m6Torr. X-ray investigation of pure and doped mixed oxide solids preheated in air at 700 and 800°C was carried out using a Philips diffractometer (type PW 1390). The patterns were run with iron-filtered cobalt radiation (A= 1.7889 A) at 30 kV and 10 mA with a scanning speed of 2” in 20 min-‘.

3. Results and discussion 3.1. XRD investigation ofpure and doped solids preheated in air at 700 and 800°C

X-ray diffractograms (Figs. 1 and 2) of pure and doped mixed solids heated in air at 700 and 800°C revealed the presence of well crystallized a-FeZ03 and NiO phases together with small amounts of NiFezO., and LiFeO, phases. The magnetite phase Fe904 has not been detected in all investigated solids preheated at 700 and 800°C. Its formation needs heating at temperatures above this limit. The complete conversion of NiO into nickel ferrite requires a prolonged heating of the mixed oxides at > 1000°C [ 23 1. 3.2. Surface properties of pure and doped mixed solia? Various surface characteristics of pure and doped mixed oxides precalcined in air at 700 and 800°C were determined from nitrogen adsorption isotherms at - 196°C. The results obtained are given in Table 1. This table includes also another series of specific surface areas S, which were calculated from I’_, plots of various adsorbents. The I’,_,plots were constructed using convenient t-curves. These plots were similar for all investigated solids and are given in Fig. 3. It is seen from Fig. 3 that pure and doped mixed solids constitute of wide pores as dominant porosity. However, the increase in the amount of Liz0 present resulted in a decrease in deviation from linearity in the V,_,plots which might reflect the role of lithia in decreasing the pore size of the treated mixed solids. In fact, the mean pore radii (i) of all doped solids are quite smaller than those found for pure solids (c.f. Table I, last column). It is clear from Table 1 that most of the values of S,, and S, are close to each

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G.A. El-Shobakyet al. /Materials Letters 22 (1995) 39-45

-1::::lr & 3

3

0

oh20

1

Fig. I. XRD diffractograms of pure and doped mixed solids preheated in air at 700°C: ( 1) a-Fe,03, (2) NiO, ( 3) NiFe204.

23

2 1

1

F:/ 14

Ad

2 1

ti I

70

I

60

I

&O

50

30

2ecS)

Fig. 2. XRD diffractograms of pure and doped mixed solids precalcined in air at 800°C: ( 1) a-FezO,, (2) NiO, (3) NiFe204, (4) flLiFeO,.

other which justifies the correct choice of standard tcurves and indicates the absence of ultramicropores in the investigated adsorbents. However, Sn, and S, for pure mixed solids are different from each other probably due to the presence of ultramicropores in these solids. It can also be seen from Table 1 that lithium oxide doping brought about an increase in specific surface

areas of NiO/Fez03 mixed solids. The increase was, however, more pronounced (about twofold) by treating with the smallest amount of Liz0 (0.75%). The doping process also resulted in a significant decrease in the mean pore radius of treated solids. It is known that NiO can dissolve about 16 molW Liz0 without producing any lithium nickel compound as lithium nicklate [ 241. The dissolution pro-

G.A. El-Shobaky et al. /Materials Letters 22 (199s) 39-45

42

Table 1 Some surface characteristics of L&O doped NiO: 2Fe203 preheated at 700 and 800°C Liz0 (mol%)

Calcination temperature(Y)

s (Zg)

S* (m*/g)

0.00 0.75 1.50 3.00

700 700 700 700

24 39 50 53

35 39 50 52

0.07 0.06 0.07 0.08

68 40 33 40

0.00 0.75 1.50 3.00

800 800 800 800

22 40 45 36

30 42 44 37

0.05 0.06 0.06 0.05

60 40 30 38

f (A)

simplified by the use of Kroger’s [ 271 notions in the following manner: Li,O+fO,(g)+:!

2

4

6

0

10

12

t(A)

Fig. 3. Volume-thickness curves ( Vkl plots) of pure and doped NiO/Fe,O, mixed oxides preheated in air at 700°C.

cess takes place, mainly, via substitution of some host Ni*+ ions in the NiO lattice by Li+ ions with subsequent creation of lattice defects ( Ni3+ ) [ 25,26 1. The incorporation of Li+ ions in the NiO lattice could be

Li+(Ni*+)+2P.

Li+ (Ni*’ ) is a monovalent ion located in the position of a host cation (Ni*+ ) of the NiO lattice and P is a created positive hole localized on an Ni*+ ion giving rise to Ni3+. Conversion of some of Ni*+ ions into Ni3+ ions in the NiO lattice should be accompanied by a decrease in the specific surface area of the treated solid [ 28,3]. Fe203 can dissolve a small amount of lithium oxide via substituting some of host Fe3+ ions and/or location in interstitial position in the Fe203 lattice. However, the dissolved amount of Li20 might be very small due to the ability of formation of lithium ferrites [ 23 1. The observed modifications in surface characteristics of mixed oxides due to Li20 doping can be discussed in terms of: (i) Pore narrowing process, a decrease in P value by doping. (ii) Dissolution of Liz0 in the NiO lattice. (iii) Creation of new pores during liberation of nitrous oxides during the thermal decomposition of LiN03 in the mixed solids [ 111. The narrowing process and creation of new pores are expected to result in an increase in specific surface areas of the treated adsorbents. On the other hand, dissolution of Liz0 in the NiO lattice might be accompanied by a decrease in S,, of doped solids. The observed net increase in the Su, of treated solids might indicate the dominance of the pore narrowing process and creation of new pores.

G.A.El-Shobabet al. /MaterialsLetters22 (1995)39-45

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3.3. Catalytic activities of pure and doped solids

The catalysis of CO oxidation by O2 over pure and doped solids was carried out at temperatures between 300 and 400°C. First-order kinetics were found in all cases. Fig. 4 shows representative kinetic curves and first-order plots of the catalytic reaction conducted at 325, 350 and 400°C over the 0.75OhL&Odoped solid preheated in air at 700°C. The slopes of these plots determine the magnitudes of the reaction rate constant (k) of the catalytic reaction carried out at different temperatures. The effect of doping on the catalytic activity of NiO/Fe203 mixed oxide catalysts is better investigated by plotting k as a function of Liz0 content for the reaction carried out at different temperatures. In order to account for the increase in S,,, of investigated solids due to treating with LizO, the reaction rate constant per unit surface area (&) was calculated and the data obtained are graphically represented in Fig. 5 for the reaction conducted at different temperatures over various solids preheated at 800°C. Fig. 5 shows that both k and k; mea-

0.5

1.o

1.5 20 L 120 mole %

25

30

Fig. 5. Variation of k and Efor CO oxidation reaction as function of Liz0 content for solids preheated in air at 800°C.

Time(min.1

Fig. 4. Kinetic curves and first-order plots of CO oxidation by O2 at different temperatures over mixed oxides doped with 0.75 mol% Liz0 and heated in air at 700°C.

sured at 325 and 35O”C,decreased drastically in the presence of the smallest amount of dopant oxide (0.75%) then suffered further slight change by increasing the lithium oxide content above this limit. The observed decrease in the catalytic activity of NiO/FezOa mixed solids due to doping could be discussed in the light of (i) the effect of L&O doping in catalytic activities of NiO and Fez03 present as sep arate phases; (ii) the effect of Liz0 doping on solidsolid interactions between NiO and Fe203. It has been shown by one of the authors [ 23 ] that Liz0 doping enhances the interaction between NiO and Fez03 to produce NiFe*O+ due to increasing the mobility of Ni2+ ions especially those in the outermost surface layers of NiO lattice. Liz0 doping of NiO is known to result in a decrease of its catalytic activity in CO

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G.A. El-Shobaky et al. /Materials Letters 22 (1995) 39-45

oxidation by O2 [ 29-3 I]. The conversion of some of NiO to NiFezOd might be expected to be followed by a decrease in the catalytic activity of the investigated solid. To our knowledge, the effect of Liz0 doping on the catalytic activity of FeZ03, in CO oxidation by OZ, has not been studied. On this basis, the dissolution of Liz0 in the NiO lattice and enhancement of nickel ferrite formation can account for the observed decrease in the catalytic activity of NiO/Fez03 mixed solids due to doping with lithium oxide. The determination of the apparent activation energy (A,??)of catalysis of CO oxidation over pure and doped mixed oxides can throw light on the possible change in the mechanism of catalytic reaction. The data of k measured at different temperatures over various solids permitted the calculation of AE by direct application of the Arrhenius equation. The calculated values of AE are given in Table 2. Included also in Table 2 are the values of pre-exponential factor (A) of the Arrhenius equation. It can be seen from Table 2 that A changes with doping due to heterogeneity of catalyst’s surface. The change in A value has been reported in the case of CO oxidation by O2 over Co304/A1203 treated with different doses of gamma rays [ 161. The magnitudes of the apparent activation energy of the catalytic reaction were recalculated (AE*) for different doped catalysts adopting the A value of pure mixed solids. The computed AE* values are given in the last column of Table 2. It is clearly seen from Table 2 that AE* values are almost identical in the case of pure and doped solids preheated at 700°C and are slightly different in the case of catalysts heated at 800°C. These results indicate that doping of NiO/Fe203 at 700°C did not

modify the mechanism of catalysis of CO oxidation by O2 but brought about a decrease in the concentration of catalytically active constituents of the mixed solids which are Ni’+, Fe3+ ions present in the outermost surface layers of mixed oxide solids. On the other hand, doping at 800°C might change not only the number of active sites but also their energetic nature.

4. Conclusions The main conclusions derived from the results can be summarized as follows: ( 1) Heating of pure and doped NiO/FezOS solids in air at 700 and 800°C led to the formation of small amount of NiFe204 phase and the majority of NiO and Fez03 remained as separate phases. (2) Lithium oxide doping of nickel/iron mixed oxides increased the specific surface areas of the treated adsorbents to an extent proportional to the amount of dopant oxide present. This increase resulted mainly from narrowing of pores present and creation of new pores in the course of heating process due to liberation of nitrogen oxides from thermal decomposition of lithium nitrate. (3 ) The doping process resulted in a drastic decrease in the catalytic activities of the treated solids. The doping process did not change the magnitude of activation energy of the catalytic reaction but altered the frequency factor of the Arrhenius equation. This indicates that the catalysis of CO oxidation reaction by O2 proceeds, in contact with pure and doped solids, according to the same mechanism.

Table 2 Effect of Liz0 doping on the magnitudes of the apparent activation energy of the CO oxidation reaction by 02 Liz0 content ( mol% )

Calcination temperature (‘C)

AE (kcal mol-‘)

Temperature range (“C)

A

AE*

(min-‘)

(kcal mol-‘)

0.00

700 700 700 700

13 16 9 13

300-350 325-400 325-400 325-400

1.58x 10’ 5.01 x 102 0.16~102 3.63x 10’

13.0 13.9 13.3 13.0

800 800

6.40 12.5 12.00 22.00

325-400 325-400 325-400 350-400

0.01 x 0.63x 0.07x 6.30x

0.75 1.50 3.00 0.00

0.75 1.50 3.00

800 800

lo* lo* lo2 10’

6.4 7.2 7.2 8.2

G.A. El-Shobakyet al. /Materials Letters22 (1995) 39-45

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