Reduction kinetics of ceria surface by hydrogen

Materials Chemistry and Physics 86 (2004) 180–188

Reduction kinetics of ceria surface by hydrogen H.A. Al-Madfaa, M.M. Khader∗,1 Chemistry Department, College of Science, Qatar University, P.O. Box 2713, Doha, Qatar Received 31 December 2003; received in revised form 30 January 2004; accepted 1 March 2004

Abstract Sintered ceria pellets of porosity 16.4% and density 5.99 g cm−3 were treated in a hydrogen flow at 1 atm and various temperatures. The electrical conductivity was measured in situ while hydrogen gas was flowing over the CeO2 . The conductivity increased continuously during the hydrogen treatment due to the continuous generation of electron carriers. The conductivity–time relationship exhibits two distinct regions labeled as I and II. In the initial region there are two consecutive steps labeled as 1 and 2, during which the conductivity increased exponentially with time of hydrogen flow, however, with a change in the slope after a relatively short time. From the kinetic analysis region I it is suggested that the first step 1 is due to oxygen desorption, and that the second step 2 is due to surface reduction. The kinetics of steps 1 and 2 in both cases obey first-order rate law with activation energies of 86 and 115 kJ mol−1 for the first and the second step, respectively. These values of the activation energies from the conductivity measurements were further supported by one more value from thermogravimetry measurements. The activation energy of surface reduction from thermogravimetry was about an average value of the above two activation energies (95 kJ mol−1 ). Scanning electron microscopy (SEM) studies showed that surface grains were broken down into smaller ones due to reduction. These breakages did not extend towards the bulk of the pellet; revealing that reduction was limited to surface region. After completing the surface reduction, presumably, by the end of region I, the electrical conductivity subsequently increased slowly during region II. This region is assigned to a diffusion-controlled process during which the bulk of the pellet is reduced. © 2004 Elsevier B.V. All rights reserved. Keywords: Ceria; Surface reduction; Electrical conductivity

1. Introduction Ceria is an important catalyst for several reactions, which are of industrial and environmental interest. Recently, CeO2 and ceria mixed oxides have been employed in three-way catalysts (TWC) [1,2] simultaneously, to oxidize both CO and hydrocarbons to CO2 and reduce NOx to N2 [3]. The property of oxygen storage capacity (OSC) exhibited by ceria, i.e. the ability to act as an oxygen buffer, made this material an important component in auto catalytic converters [3]. The catalytic activity of ceria is due to a facile Ce4+ ⇔ Ce3+ interconversion. Ceria has been studied extensively, especially, its reduction [4–10] and oxygen diffusion properties [11]. The defect structure and electrical properties of a single crystal and polycrystalline ceria were also extensively studied [11–15]. The reduction of ceria has been studied by several techniques including temperature-programmed reduction (TPR) [16,17], magnetic susceptibility and Fourier transform infrared (FT-IR), ultraviolet and X-ray photo∗ Corresponding author. E-mail address: [email protected] (M.M. Khader). 1 On leave from Cairo University, Giza, Egypt.

0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.03.004

electron spectroscopic measurements (UPS and XPS) [9]. Nuclear magnetic resonance (NMR) was also employed to study ceria reduction [18]. Most of the previous works concerned with the study of the various reduction products and/or the nature of the defects rather than the nature of the reduction process itself. The present work aims to study the surface reduction kinetics of sintered pressed ceria pellets by electrical conductivity and the oxygen weight loss measurements. The experiments were carried out in a temperature range from 673 to 873 K in hydrogen flow. 2. Experimental 2.1. Sample preparation The ceria samples were prepared by precipitation of Ce(OH)3 from aqueous solutions of cereous nitrate, Ce(NO3 )3 [BDH] and ammonia solution. The hydroxide was repeatedly washed with triple distilled water then dried at 623 K for 3 h and then decomposed into the oxide by calcining the dried powder at 923 K for 4 h. Multi-point BET surface area measurements were performed using

H.A. Al-Madfaa, M.M. Khader / Materials Chemistry and Physics 86 (2004) 180–188 Table 1 The physical parameters related to the porosity of the samples Physical parameter

Sample Unsintered

Total pore area (m2 g−1 ) Median pore diameter, volume (␮m) Median pore diameter, area (␮m) Average pore diameter, 4V/A (␮m) Bulk density (g cm−3 ) Apparent skeletal density (g cm−3 ) Porosity percentage (%)

Sintereda

Reducedb,c

29.129 0.0122

7.642 0.0148

9.27 0.0141

0.0103

0.0089

0.0089

0.0123

0.0143

0.0137

4.3 7.1

6.0 7.2

5.7 6.9

39

16

18

a

The pellet was sintered at 1373 K for 24 h. b Reduction took place in hydrogen flow at 773 K for 3.5 h. c After reduction, the color of the pellet was pale blue but immediately changed to pale yellow on exposure to ambient atmosphere at room temperature. Also the room temperature conductivity was similar to that of unreduced pellet; revealing that the pellet was reoxidized before the porosity measurements were made.

NOVA-1000 gas sorption analyzer (Quanta Chrome Co.). In these measurements nitrogen was used as an adsorbate at 77 K. The BET surface area measurements were carried out on the calcined ceria powder, found to be 45 m2 g−1 , and also on powder of a grinded-sintered ceria pellet, where it was 38 m2 g−1 . The ceria powder was compressed in the form of pellets at a pressure of 10 t cm−2 ; the pellets had a diameter of 1.1 cm and a thickness of 0.1 cm. The pellets were sintered in air at 1373 K for 24 h. The porosity of the pellets, pore size, pore diameter and the density, were measured by a high-pressure mercury porosimeter “Micromeritics, USA” model 9320. Data are compiled in Table 1. The powder for thermogravimetry experiments was of a grinded-sintered pellet. 2.2. Electrical conductivity measurements The conductivity cell was made from circular-shaped platinum electrodes of the same diameter as the ceria pellet. These were made of platinum screens, to allow easy access of the hydrogen gas to the surface of the pellet. Platinum wire electrical leads were spot-welded to the screens. Tight contact between the electrodes and the pellet was maintained by pressing the assembly using two tungsten rods, each of 5 mm in diameter, and a pressure spring. A chromel–alumel thermocouple was used to measure the temperature, which was adjusted to ±5 K to the desired temperature. The sintered pellet was mounted between the two Pt electrodes of the conductivity cell and held tightly with the spring. The jacket of the cell was made of two quartz tubes, each containing one of the two electrodes. The two tubes were quick-fitted; forming the whole assembly. Within the quartz tubes were inlet and outlet connections for the gases. The conductiv-

181

ity cell has been described in details previously [19]. The cell was placed in a vertical tube furnace and a stream of oxygen was flowing over the sample during heating till the wanted temperature reached and stabilized for 30 min. Then a stream of argon gas was allowed to flow inside the cell before admitting hydrogen to pass over the pellet at a rate of 80 cm3 min−1 . The electrical conductivity of the pellets did not change during argon treatment; revealing that the samples were not reduced. The electrical conductivity was measured in situ during H2 flow by an EG&G Princeton applied research electrometer model 363 using the current–voltage technique. The electrical power on the sample was limited to 0.1 W cm−3 to prevent excessive joule heating. Hydrogen and argon gases (99.9%, Matheson Co.) were purified by passing over a series of columns filled with silica gel, anhydrous calcium chloride and copper turnings heated at 673 K. Oxygen gas (99.9%, Matheson Co.) was used without further purification. Room temperature conductivity was measured after exposing the pellets in the conductivity cell to argon flow at 393 K for 1 h. The pellets were cooled to room temperature before conductivity was measured. 2.3. Thermogravimetry and SEM measurements Thermogravimetry was performed in a conventional thermometric analyzer (Perkins-Elmer model TGA7). The TGA was equipped with Pyres software. A sintered pellet was ground into powder. Thermogravimetry was measured for 8 mg of the grinded powder. Before carrying out thermogravimetry experiment, the powder was treated in situ with an oxygen flow of a rate at 20 cm3 min−1 and 823 K for 2 h. Such a treatment was aimed to obtaining a fully oxidized and dry ceria sample. The flow was then switched to Ar for 10 min, and the temperature adjusted to the desired values prior to admitting hydrogen at a flow rate of 80 cm3 min−1 . SEM measurements were carried out on pellets before and after reduction.

3. Results and discussion 3.1. Porosity of the pellets Table 1 lists the various parameters related to the pore structure of a pellet before and after sintering and after reduction in H2 atmospheres, respectively. This table shows that the percentage porosity of the pellet decreased significantly by sintering and increased slightly upon reduction. Moreover, the density of the pellets increased by sintering and decreased slightly due to reduction. This is expected, as sintering aggregates both the small particles and small pores into bigger grains and bigger pores. The formation of bigger grains increased the density of the pellet as shown in Table 1. Reduction of the pellet took place at 773 K for a

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period of 3.5 h. At the end of reduction the color of the pellet changed from pale yellow to pale blue. When this pellet was cooled to room temperature in a hydrogen flow, it preserved its blue color, but changed back to pale yellow when exposed to ambient atmosphere. The color change is due to the partial reduction of CeO2 to a lower oxide that is not stable in ambient conditions [18]. Such a lower oxide is not expected to be the trivalent Ce2 O3 , because Ce2 O3 is a stable pale yellow oxide. The pale blue oxide that is formed during the conductivity measurements could be of an oxidation state between that of CeO2 and Ce2 O3 [18], i.e. could have the formula CeO2−x with 2 > x > 1.5. The X-ray diffraction pattern of ceria, which was measured in ambient atmosphere before and after reduction produced typical diffraction patterns; again, revealing that ceria was partially reduced to the lower oxide CeO2−x which is unstable in ambient atmosphere [18]. Moreover, the room temperature conductivities of the pellet before reduction, and that exposed to ambient conditions after reduction were both similar (∼8 × 10−7 −1 cm−1 ). All three previous results indicate that ceria was reduced partially to a lower stoichiometric product CeO2−x rather than the fully reduced oxide, i.e. Ce2 O3 . Upon reduction, the first row in Table 1 shows about 20% increase in the total pore area of the reduced pellet compared to that before reduction. Moreover, results (Table 1) show that the percentage porosity increased from 16% for the sintered pellet to 18% for the reduced pellet. The increase in the total pore area and the percentage porosity might be attributed to a break down of bigger grains into smaller particles. Cracking of the ceria grains could take place due to the transformation of the fluorite structure of CeO2 into another structure of CeO2−x upon reduction according to the reaction: CeO2 + xH2 ⇔ Ce2 O2−x + xH2 O

(1)

Scanning electron microscopy study was carried out with the objective of finding out effects of reduction on surface morphology. The scanning electron micrographs (SEM) of Fig. 1(a) and (b) are for a sintered ceria pellet before and after reduction, respectively. Fig. 1(b) shows that surface grains were broken down into uniform smaller grains. It is obvious from Fig. 1 that the new formed surface grain boundaries did not extend within the bulk of the pellet. It can thus be concluded that reduction was limited to surface region. This is further proof that the product of reduction was the partially reduced oxide, i.e. Ce2 O2−x rather than the fully reduced one, i.e. Ce2 O3 . Furthermore, surface breakage can also account for the increased porosity as shown from Table 1. 3.2. The electrical conductivity measurements Fig. 2 shows the variation of the electrical conductivity with the length of time of the hydrogen gas flow over the ceria pellets of the same thickness at different temperatures.

It is clear from Fig. 2 that there are two distinct regions; the one at lower reduction time is labeled region I and the other at higher reduction time labeled as region II. What follows is a detailed kinetic analysis for first region and a brief analysis for the second region. 3.2.1. Region I Within this region the conductivity increased exponentially with time. It is known that CeO2 is an n-type semiconductor [20]. Therefore, the increase in the electrical conductivity corresponds to a subsequent increase in the density of the conducting electrons. The relation between the conductivity and electron density is σ(t) = enµ

(2)

where e is the electronic charge, µ the electronic mobility and n is the electron density. During hydrogen treatment, oxygen from the surface of the oxide as well as from its bulk is removed as water. The bulk oxygen is always in the form of oxide ions, O2− and/or O− . On the other hand, surface oxygen could be lattice oxide ions as well as chemisorbed oxygen, usually in the form of supper oxide O2 − [21,22]. Normally, each surface cerium ion is surrounded by eight groups. Four of there are directed to the bulk and are lattice oxygen ions. The other four groups are directed perpendicular to the surface; they are, usually, chemisorbed oxygen in the form of the supper oxide, coordinately adsorbed H2 O, and surface hydroxyls. During the oxygen treatment process, which was usually carried out prior to each reduction, the coordinately adsorbed water as well as the water formed due to the condensation of surface hydroxyls are expected to desorb from the surface and be replaced by chemisorbed oxygen in the form of the supper oxides [17,21,22]. Prior to hydrogen reduction, the surface layer is thus expected to have lattice surface oxide ions O2− as well as chemisorbed supper oxide ions O2 − [21,22]. The reduction of ceria surface is expected to proceed via two steps; an initial step, during which the supper oxide is removed, followed by a second step where the surface oxide is reduced. Indeed, in the study of Morris and coworkers [18] by the magic angle spinning proton nuclear magnetic resonance (1 H MAS-NMR), they observed two peaks, both due to surface hydroxyls. They assigned one due to hydrogen adsorption on chemisorbed supper oxide sites, the other surface hydroxyl was assigned due to adsorbed hydrogen on surface lattice oxide. These observations are in good agreement with the present results from the electrical conductivity measurements. Whether the chemisorbed supper oxide or the surface oxide ion is removed during H2 -reduction, conducting electrons will be left behind without molecular orbitals to accommodate them. These electrons are transferred into the conduction band. They will cause increases in the electrical conductivity, however with different magnitudes depending on whether they originated from chemisorbed oxygen sites or from surface oxide ions. Indeed, the conductivity transients of Fig. 2 in the first region I show there are two steps

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183

Fig. 1. Scanning electron micrographs of a sintered pellet: (a) before reduction; (b) after reduction at 773 K for 1 h.

increasing the conductivity, however with different slopes. The reduction steps 1 and 2, of region I, are shown more clearly in Fig. 3(a)–(c) at reduction temperatures of 673, 773 and 823 K, respectively. At higher temperatures the increase in the conductivity was so fast that no clear-cut distinction could be observed between steps 1 and 2 of region I. It is plausible to account for the initial increase in the conductivity, step 1 of region I, as being due to the removal of

the chemisorbed oxygen and step 2 to surface reduction. As shown in Fig. 2 and more clearly in Fig. 3, the conductivity during step 1 started at t = 0 and extended exponentially with time; this is due to the removal of chemisorbed oxygen which, as mentioned above, leaves behind conducting electrons. After a relatively short time, the slope of the conductivity transient changes, presumably due to the beginning of step 2 during which the surface is reduced. Each of steps 1

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Fig. 2. Conductivity transients at various temperatures. Ceria pellets were reduced in hydrogen flow of 80 cm3 min−1 at a constant pressure of 1 atm.

and 2 consume short time that depends on the temperature at constant H2 flow. The rate of increase in conductivity in both steps can thus be regarded as the rate of oxygen removal. The rate for the removal of chemisorbed oxygen can be expressed as dndes = kPH2 [O2 − ] = kdes ndes dt

(3)

where kdes = kPH2 [O2 − ] is the specific rate constant of desorbing oxygen, and ndes is the density of the conducting electrons generated due to the oxygen desorption. Similarly, the rate of surface reduction can be expressed as dnred (4) = kPH2 [O2− ] = kred nred dt where nred is density of the conducting electrons that are generated due to the surface reduction and kred = kPH2 [O2− ] is the specific rate constant of surface reduction. The exponential increase in conductivity with reduction time within region I for both steps 1 and 2 predicts that the rates of both steps obey first-order kinetics. Therefore, the density of the conducting electrons generated in each step can be related to reduction time by the following equivalent first-order equations: ndes(t) = n0 exp(−kdes t)

(5)

nred(t) = n0 exp(−kred t)

(6)

where n(t) is the number of electrons generated at time t of hydrogen flow and n0 is the number of free electrons in the stoichiometric sample. From Eqs. (2), (5) and (6), the conductivity during the two steps can be given by σdes(t) = σ0 exp kdes t

(7)

σred(t) = σ0 exp kred t

(8)

When the data from steps 1 and 2 of region I of Fig. 2 were plotted according to Eqs. (7) and (8), i.e. with ln ␴red(t) and ln ␴red(t) , both values versus time (t) straight lines relationships were obtained. The slopes of these straight lines are the rate constants kdes and kred , respectively. These rate constants are calculated and tabulated in Table 2. The activation energies, Ea(des) and Ea(red) of oxygen desorption and surface reduction, respectively, can be obtained from the Arrhenius equations: ln kdes = ln A −

Ea(des) RT

(9)

ln kred = ln A −

Ea(red) RT

(10)

where A is a pre-exponential constant. Fig. 4 shows the Arrhenius plots of oxygen desorption and surface reduction. These plots are satisfactory straight lines with negative slopes, which give activation energies of 86 and 112 kJ mol−1 for oxygen desorption and surface Table 2 The specific rate constants of oxygen desorption and surface reduction at different temperatures T (K)

kdes a (s−1 )

673 698 723 773 823 873

6.1 1.4 2.3 8.0 1.2 2.3

× × × × × ×

10−5 10−4 10−4 10−4 10−3 10−3

kred b (s−1 ) 1.2 1.5 2.1 1.3 3.7 7.7

× × × × × ×

10−5 10−5 10−5 10−4 10−4 10−4

a The values of k des are the slopes of the straight line relations between ln σ (t) and the time of surface reduction, these data were taken from step 1, region I of Fig. 1. b The values of k red are the slopes of the straight line relations between ln σ (t) and the time of surface reduction, these data were taken from step 2, region I of Fig. 1.

H.A. Al-Madfaa, M.M. Khader / Materials Chemistry and Physics 86 (2004) 180–188

Fig. 3. Details of the conductivity transient of a ceria pellet reduction by hydrogen at (a) 673 K, (b) 773 K and (c) 823 K. 185

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Fig. 4. Arrhenius plots of oxygen desorption and surface reduction. The specific rate constants of oxygen desorption and surface reduction at various temperatures, as determined from conductivity measurements are tabulated in Table 2.

reduction, respectively. The value of 86 kJ mol−1 is within the range of activation energies of chemical adsorption processes. On the other hand, the higher value of 112 kJ mol−1 is a typical value for the activation energy of a chemical reaction. Further investigation of surface reduction was carried out by thermogravimetry. In these experiments, the time was recorded as a function of oxygen weight loss during hydrogen reduction. In order to correlate the oxygen weight loss to surface reduction, the following assumptions were proposed: (1) the surface is composed of a layer of oxide ions on top of cerium ions; (2) the density of the oxide ions on ceria surface is calculated taking into consideration that the (1 1 1) phase is the predominate phase of ceria and the cross-sectional area of the oxide ion 196 pm−2 . The oxide ion density is thus 1.6 × 1015 ions cm−2 of ceria surface; (3) the surface reduction is completed by the end of the duration time of region I in the conductivity transients of Fig. 2. One can thus calculate the mass of oxygen necessary to cover one monolayer on the surface and thus determine the time (tm ) needed to loose this mass. To make the results of thermogravimetry comparable to those of the electrical conductivity measurements, powder for thermogravimetry was obtained from grinding a sintered-pellet. The surface area of grinded-ceria was 38 m2 g−1 . From the previous assumptions 1 and 2, the mass of surface oxygen covering a monolayer g−1 of CeO2 is 0.016, i.e. 1.6% of a grinded-sintered ceria sample. Thermogravimetry was performed in flowing hydrogen at various temperatures with the objective of determining tm which is the time necessary to loose 1.6 mass% of ceria during its reduction. The values of tm at various temperatures are tabulated in Table 3. These values of tm

are in good agreement with the time at which region I of the conductivity transient (Fig. 2) was ended. Therefore, to a good approximation, one can predict that tm coincided with the loss of surface oxygen solely rather than the loss of both surface and bulk oxygen. The time of surface reduction (tm ) is inversely proportional to the specific rate constant of surface reduction, which was defined before as kred . Therefore, the previous Arrhenius equation (10) for surface reduction can be re-expressed as
 Ea 1 = ln A − (11) ln tm RT The relation between ln(1/tm ) and the reciprocal of the reduction temperature, in accordance of Eq. (11), is presented in the straight line relationship of Fig. 5. The activation energy of surface reduction is 95 kJ mol−1 . This value is close to the previous value of surface activation energy, which was determined, obtained by the conductivity measurements. However, the activation energy from conductivity measurement should be the more accurate. Table 3 Time (tm ) of surface reductiona at various temperatures T (K)

tm (s)

873 823 773 723

1100 3050 7100 15900

at m was determined from thermogravimetry as the time needed to lose one monolayer of oxygen from ceria surface.

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Fig. 5. Arrhenius plot of ceria surface reduction from thermogravimetry data. The rate is proportional to 1/tm . The values of tm is the time by which one monolayer of surface oxygen was removed during the hydrogen reduction. The values of tm at various temperatures are listed in Table 3.

3.2.2. Region II As mentioned before, region II occurs at a relatively long time and high conductivities. It is characterized by a rather slow increase in the electrical conductivity with reduction time; which suggests the onset of the diffusion control of the reduction process [23–25]. As the reduction proceeds, more conducting electrons are generated due to the removal of atomic oxygen from the bulk of the pellet, which results in an increase in conductivity according to Eq. (2). Within step II, the conductivity change of the sample is caused by either the inward diffusion of hydrogen gas into the bulk of the pellet or the outward diffusion of the oxide ion from the bulk of the pellet towards its surface. Due to the relatively high porosity of the pellets (16.4%), application of the diffusion theory to study bulk kinetics is not adequate to describe the behavior within these pellets [26].

4. Conclusions Ceria was reduced partially by hydrogen into lower oxides that were not stable at ambient conditions. As a result of reduction, surface grains were broken down into smaller granules that increased porosity of the samples. Kinetics of surface reduction of ceria pellets was studied by the electrical conductivity and thermogravimetry techniques. The electrical conductivity transients signified two surface processes: oxygen desorption and surface reduction. Both of oxygen desorption and surface reduction obeyed first-order kinetics with activation energies of 95 and 115 kJ mol−1 , respectively. The activation energy of surface reduction was also determined from the oxygen mass loss by thermogravimetry and found to be about an average value of the previous two values.

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