The effect of doping CeO2 with zirconium in the oxidation of isobutane

~

A PP LE IY DSS CA TA L I A: GENERAL

ELSEVIER

Applied Catalysis A: General 139 (1996) 161 - 173

The effect of doping C e O 2 with zirconium in the oxidation of isobutane Carla de Leitenburg a A l e s s a n d r o Trovarelli a.* Jordi Llorca b Fabrizio C a v a n i c G i a n l u c a Bini c Dipartimento di Scienze e Tecnologie Chimiche, Universith di Udine, Via Cotonificio 108, 33100 Udine, Italy b Departament de Qufmica lnorghnica, Uni~'ersitat de Barcelona, Diagonal 647, 08028-Barcelona, Spain Dipartimento di Chimica lndustriale e dei Materiali, Viale Risorgimento 4, 40136 Bologna, ltalx

Received 4 August 1995; revised 30 November 1995; accepted 2 December 1995

Abstract

The preparation of a C e O 2 - Z r O 2 mixed oxide of composition Ceo 8Zro.20 2 with its characterization and its use as a catalyst in the oxidation of isobutane is reported, and compared with the reactivity of pure CeO 2. The formation of an homogeneous, fluorite-type solid solution is observed; the material is characterized by a higher reducibility and a higher capacity of oxygen uptake compared to pure CeO 2. The catalytic activity in the oxidation of isobutane is not greatly affected by the introduction of ZrO 2, but the selectivity to isobutene is significantly enhanced. This enhancement has been attributed to an increased oxygen mobility and to an increased activity for the C e 4 + / C e 3+ redox couple, occurring as a consequence of the creation of surface and bulk defects in the solid solution, induced by the introduction of the smaller Zr 4+ cation in the fluorite lattice. Keywords: Cerium oxide; Isobutane oxidation; Zirconium

1. Introduction The use of CeO2-based materials in catalysis has shown a rapid increase in the last years, owing to the utilization of CeO 2 as an active ingredient in the so called three-way catalysts for exhaust gas treatment from automobiles [1]. Particularly, CeO 2 has been found effective in the promotion of the water-gas * Corresponding author. Tel. ( + 39-432) 558886, fax. ( + 39-432) 558803 i Perviously submitted on 26 June 1995 0926-860X/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI O926-860X(95)O0334-7

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shift reaction [2] and in the catalytic oxidation of CO and hydrocarbons [3,4]; furthermore, the use of ceria in liquid-phase oxidation reactions [5,6], and in the removal of SO 2 from FCC plants [7] have also been reported. All these applications take advantages from the redox properties of CeO 2 and from its ability to store and release oxygen under net-oxidizing and reducing conditions, respectively. By introducing different elements in the CeO 2 lattice, an increase of the oxidation properties has also been observed. The presence of Gd203 was found effective in enhancing the catalytic properties of CeO2 in C O / N O reaction [8], while very recently it has been reported that formation of C e - H f and C e - Z r solid solutions leads to effective catalysts for the total oxidation of C H 4 [9]. Moreover, metal oxide composite catalysts for the total oxidation of CO and methane have recently been prepared by combining CeO 2 with transition metal like Cu and Au [10]. The common factor which is emphasized throughout all these applications is the participation of surface o x y g e n / o x y g e n vacancies to the catalytic reaction. It is very well known that CeO 2 can easily form oxygen vacancies and that both surface and bulk vacancies can be involved in the catalytic reaction [11]. This, in turn, implies that also oxygen mobility can have an important role to explain the catalytic properties of CeO2-based materials. It is known that by introducing different elements into the cubic lattice of a fluorite-structured material like C e O 2 , the transport properties of the corresponding doped oxide are greatly affected as a result of the creation of structural defects in the lattice [12]. This consideration has been used for example to explain the higher reducibility of CeO 2 and its oxygen storage capacity in CeO2-ZrO 2 mixed oxides, both in the presence [13] and in the absence of noble metals [14]. To address the above mentioned points and particularly to investigate on the role of doping CeOz with an isovalent element like Zr, we describe here the characterization of a fluoritestructured mixed-oxide, based on CeO 2, and its use in the oxidation of isobutane. There is a growing interest in catalytic systems able to activate paraffinic hydrocarbons, and selectively convert them to useful chemicals [15]. For example, oxidehydrogenation of paraffins to olefins is potentially an alternative process to energy-intensive dehydrogenation [16]. Therefore, the oxidation of isobutane has been used as a tool to characterize the reactivity of CeO2-ZrO 2 mixed oxide and of pure CeO 2 in hydrocarbons oxi-functionalization.

2. Experimental C e O 2 w a s prepared by precipitation with ammonia from a solution of Ce(NO3) 3 • 6H20 (0.2 M) followed by washing, drying for 15 h at 373 K, and calcination at temperatures in the range 900-1200 K for 2 h. CeO2-ZrO 2 mixed oxide was prepared by coprecipitation, by adding dropwise an aqueous solution (0.2 M) of Ce(NO3) 3 - 6H20 and ZrO(NO3) 2 • x H 2 0 of the appropriate compo-

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163

sition to a solution of concentrated ammonia. After precipitation, the mixture has been filtered, washed, dried at 373 K for approximately 15 h, and then calcined in the temperature range 1100-1400 K for 2 h. X-ray diffraction patterns of powders were collected with an Inel Instrument, using Co radiation. To calculate the lattice parameters the four main reflections corresponding to (111), (200), (220) and (311) crystallographic planes have been considered. The position and the intensity of the diffraction lines, as well as line profile analysis has been calculated by fitting the signal with Voigt functions. Samples for TEM analysis were placed on carbon-coated copper grids from alcohol suspension. TEM-EDX studies were carried out with a HITACHI-H 800-MT microscope operating at 150 kV, coupled to a Kevex 8000 Quantum System. Quantitative temperature programmed reduction (TPR) was carried out in a U-shaped quartz reactor (i.d. 6 mm, l = 200 mm) with a 5% H 2 - 9 5 % Ar mixture. The gas flow through the sample was 35 m l / m i n and the sample was heated from 295 to 1400 K (heating rate 10 K/rain). Reduction of CuO to metallic Cu was used as a standard to calibrate for H 2 consumption. Peak analysis and integration have been made by using PeakFit graphical software. Oxygen storage capacity (OSC) experiments were performed in the same apparatus used for the TPR measurements. After reduction in H 2 at 700 K for 2 h, the samples were evacuated at the same temperature for an additional 2 h under inert flow; then they were taken to the reaction temperature and oxygen pulses (50 /xl) were injected in a He stream every 3 rain, until the exhaustion point was attained. The catalytic activity of C e O 2 and Ceo.sZro.202 mixed oxide for the gas-phase oxidation of isobutane was examined using a stainless steel flow reactor at atmospheric pressure, loaded with 1.0 cm 3 of catalyst, diluted with 2 cm 3 of inert material; a thermocouple was inserted in the catalytic bed to monitor the profile of temperature. The following conditions were employed: feed composition: 26% isobutane, 13% oxygen, 12% water, with He as balance: residence time 2 s. The temperature of reaction was varied in the range 573 to 673 K.

3. Results and discussion

3.1. Characterization of the material X-ray diffraction patterns of CeO 2 and CeO2-ZrO 2 are shown in Fig. 1. The diffraction analysis of the modified ceria confirms that the fluorite lattice structure has been preserved during the doping process. The position of the peaks is in accordance with the formation of a Ceo.sZr0.202 fluorite-structured solid solution, characterized by a face centered cubic (fcc) cell with a -- 5.364(4) A, in comparison to a --- 5.415(3) A calculated for pure CeO 2. The lower ionic

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C. de Leitenburg et al./Applied Catalysis A." General 139 (1996) 161-173

220 311

degree ( 2 0 )

Fig. 1. X-ray powder diffraction pattern of CeO 2 (upper) and CeO; -ZrO 2 (lower).

radius of Z r 4+ (0.84 ,~ compared to 0.97 A for C e 4+ [17])justifies the decrease in the value of the cell parameter as predicted by the Vegard law. To evaluate the change of the lattice parameter of CeO 2 solid solutions as a function of the guest metal content it is possible to use an empirical formula given by Kim [18]: dce =

0.5414 + ~k (0.0220A r k + 0.00015A

zk)m~

where d (in nanometers) is the lattice parameter of the fluorite structured solid solution at room temperature, Ar k is the difference in ionic radius ( r ~ - r h) between the kth dopant (r~) and the host cation (rh), Azk is the valency difference and m k is the mole percent of the kth dopant. By using only ZrO 2 as dopant the former equation reduces to: d c e = 0.5414 + 0.0220(rzr - r c e ) m Introducing the r values and the molar content m of ZrO 2 we obtained a theoretical value of 5.365 ,~ for d, which is in agreement with the experimental lattice parameter. This result is an indirect evidence that we are dealing with a pure fcc phase of Ceo.sZr0.202 and that no free CeO z and ZrO 2 domains are present. A direct experimental proof for the formation of a solid solution comes from a combined TEM-EDX analysis. Fig. 2a shows a transmission electron micrograph of Ce0.sZr0.202 particles. As can be seen the sample is characterized by regular, rounded shape particles with size around 5 nm, and with a very narrow distribution around this value. EDX analysis were performed using a 5 nm probe on individual particles (Fig. 2b). It indicates that bimetallic particles are obtained; in no case either Zr or Ce particles alone have been observed. The atomic ratios Z r / C e are found to be constant, within experimental error, from one particle to the other and correspond well to the starting composition Ceo.sZr0.202. The BET surface areas obtained after different calcination temperatures are reported in Fig. 3. As can be seen there is a marked difference in the textural properties between CeO 2 and CeO2-ZrO 2 prepared by this method. The

C. de Leitenburg et al. /Applied Catalysis A: General 139 (1996) 161-173

165

CeL~l

CeLl],1 OKc~

CuKoc

, 2

, 4

, G

,

, ,o

, ,2

, ,4

, 1G

,

Energy (KEY)

Fig. 2. (A) TEM micrographs of CeO 2 - z r O 2 mixed oxide particles. (B) EDX pattern from a representative particle of (A).

introduction o f Zr into ceria lattice significantly e n h a n c e s the surface area o f the p o w d e r , m o r e o v e r it stabilizes the material against the drop in surface area o b s e r v e d o v e r pure ceria in the range 9 0 0 - 1 1 0 0 K, in a g r e e m e n t with recent findings [19]. T h e thermal stability o f d o p e d ceria has b e e n recently investigated both theoretically and e x p e r i m e n t a l l y [20], and it has been f o u n d that the rate o f

lOO 9o 80

70 60 0o 09 ~9 40

oo 20 10

07oo

800

900

lOOO 1100 1200 1300

, 1400

Temperature (K)

Fig. 3. Effect of calcination temperature on surface area: CeO 2 ( • ); CeO 2 - Z r O 2 ( O ) .

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C. de Leitenburg et al. / Applied Catalysis A: General 139 (1996) 161-173

g

"2

8

300

500

700

900

1100

1300

Temperature (K) Fig. 4. TPR profiles of CeO 2 and Ceo.sZro.202 samples: CeO 2 22 m 2 / g (a); sample a after an oxidation following the first TPR (a'), Ceo.sZro.202, 47 m 2 / g , (b); Ceo.sZro.202, 29 m 2 / g , (c); sample c after an oxidation following the first TPR (c'). Shaded area: see text.

increase of crystallite size is strongly dependent upon dopant concentration, being higher for pure, undoped ceria. The result is a surface area stabilization due to the introduction of dopants (like Zr 4÷) into the CeO 2 lattice. Quantitative temperature programmed reduction of CeOe and of Ce0.8Zro.202 are reported in Figs. 4 and 5: Fig. 4 gives the hydrogen consumption, while Fig. 5 reports the reduction extent of Ce 4+ to Ce 3+ as a function of the temperature. The TPR profile of CeO 2 consists of two peaks centered at around 770 and 1100 K. The presence of two peaks in CeO 2 has been associated with a stepwise reduction of the oxide; the second signal is mainly due to bulk oxygen removal

1 ¸. 60

/J

:d/ /"

/.

40

i /'

0

500

700

/

'

900

lJ

(!!.' J,' ,"

1100

1300

Temperature (K) Fig. 5. Reduction extent (% reduction of CeO 2) of sample a ( . . . . . .

c'( ..... ).

), a' ( - - -), b(

), c (---),

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167

while the first peak has been related to easily reducible surface Ce a+ [21], although formation of partially reduced CeO x suboxides cannot be excluded [22]. These features are maintained also with Ce0.sZr0.202; in this case, however, the relative intensity and position of the two peaks are different. The first peak is centered at around 820 K and is more intense than the second broad peak, centered at around 1050-1100 K. A different behavior is observed if the compounds are subjected to repeated oxidation/reduction treatments, as evidenced by profile a' and c' of Fig. 4, which are obtained by running a second TPR on samples a and c. In these conditions the first peak of CeO 2 almost disappears, due to extensive sintering of ceria following a high temperature treatment, while the second peak remains almost unaffected. In the case of Ceo.sZr0.202 the effect of redox cycling is less dramatic and even after a second TPR run, the extent of reduction at low temperature remains high, although the surface area drops from 29 to 2 m 2 / g after a TPR run. There are two factors that can affect the reduction extent of C e 4+ in these compounds: (i) the surface area, which is an important factor in determining the reduction at lower temperatures; the higher is the surface area, the larger is the surface available to H 2 for C e 4+ reduction process; (ii) oxygen mobility, which determines the kinetics of reduction of bulk Ce 4+ atoms. At low temperatures, where oxygen mobility in CeO 2 is low, surface reduction is favored, while on increasing temperatures also bulk reduction occurs. Fig. 5 shows that the isovalent substitution of Zr in the fcc cell of CeO~ promotes the reduction extent in almost all the temperature ranges investigated. The difference in surface area alone is not sufficient to account for such different behavior (both sample a' and c' have low surface area). The total amount of H 2 consumed for Ce 4+ reduction in each Ce0.aZro.20 2 sample corresponds to the reduction of approximately 65% (+_ 5%) of the CeO 2 present, leading to the formation of Ceo.8Zr0.zOx with x varying from 1.72 to 1.76. It is difficult to quantify the contribution of H 2 consumption for surface and bulk reduction; however, an approximate evaluation can be given according to the model developed by Johnson and Mooi [23]. By applying this model to the samples of Fig. 4, the H 2 consumption for surface reduction in CeO 2 and CeO2-ZrO 2 system can be estimated. These values are indicated in Fig. 4 by evidencing the area under the TPR curve, up to the point where surface reduction is complete. For CeO2-ZrO 2 samples, hydrogen consumption for removal of surface oxygens is restricted to the initial part of the first TPR peak. This behavior, which is much more evident for the low surface area sample (sample c'), indicates that most of the low temperature reduction in these systems is associated to reduction of the internal CeO 2 layers, suggesting that removal of 02 from bulk Ce0.sZro.202 takes place also under the low temperature signal. This fact has been recently observed with Rh supported on low surface areas C e - Z r solid solutions [13] and has been attributed to the increased oxygen mobility in the defective fluorite structure generated by introduction of

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Table 1 02 uptake measurements Sample (m2/g)

CeOz (6)

H2 a

3.0

Ce0.8Zro.202(4.5)

10.9

Ce08 Zr0.202 (29)

12.1

T (K)

500 600 700 400 450 500 600 400 450 500 550 600

0 2 uptake ml/g

/xmol 0 2 //zmol Ce

/xl/m 2

0.28 0.69 0.83 1.48 2.27 2.77 3.67 3.49 4.53 5.52 5.99 5.95

0.0020 0.0049 0.0060 0.013 0.019 0.024 0.032 0.031 0.040 0.050 0.054 0.053

47 115 138 329 504 615 815 120 156 190 206 205

H 2 consumed (ml/g) for the reduction preceding the OSC experiments.

the Zr cation. In the present case a similar hypothesis can be put forth, although the role of the surface area adds to the complexity of the system. A further proof of the increased activity of the r e d / o x couple Ce4+/Ce 3+ in CeO2-ZrO 2 mixed oxides comes from oxygen uptake measurements that have been performed to evaluate the relative oxygen storage capacity of the samples and their oxygen mobility. 02 uptake measurements have been carried out at different temperatures in the range 400-700 K and the results are reported in Table 1. As can be seen, 02 uptake is an activated process; when oxygen mobility controls the process, the activation energy can be calculated by using the procedure applied by Cho [8] and described by Tuller and Nowick [24]. For pure ceria a value of 4 . 9 6 + 0 . 1 5 kcal/mol has been obtained, while for Ce0.sZr0.202 the activation energy was 2.56 ___0.13 kcal/mol for a sample having a surface area of 29 m 2 / g and 3.11 + 0.13 kcal/mol for a sample having a surface area of 4.5 m2/g. The values have been calculated by considering the slopes of the lines of ln(NT) vs. 1/T as reported in Fig. 6, where N represents the number of 02 pulses completely adsorbed, and T stands for temperature. Once again, in the case of CeO2-ZrO > the influence of surface area on the dynamic of oxygen adsorption is rather limited. Since the ultimate oxygen uptake depends only on the extent of reduction of the sample [8], which in turn depends on the properties of the material and the pretreatment conditions, the 02 uptakes measured in these experiments are only a fraction of the total oxygen storage capacity of the material. This fraction depends on both the exposed surface area and oxygen mobility, which in turn depends on temperature. As can be seen from Table 1, the total oxygen uptake of Ceo.~Zro202 is more than one order of magnitude larger than 02 uptake in pure ceria; the

C de Leitenburg et al./Applied Catalysis A: General 139 (1996) 161-173

3

1

169

b

z. 0,~3

0,0bJ7 0,din 0,c~,:~ l/T

~,0013 0,,~b~7 0,~i~2~ 0,00Z5 IFF

Fig. 6. (a) Total oxygen uptake for CeO 2 (11), Ceo 8Zr0.202, 29 m2/g (©) and Cell,sZr0.202, 4.5 m~-/g (O). (b) Activation energy for oxygen uptake.

difference is much more evident by considering the 0 2 uptake per mol of Ce only. By comparing the H 2 consumed in the reduction preceding the OSC experiments with 02 uptake, it is evidenced that, within the experimental error, oxygen consumption in Ce0.sZro.202 (29 m2/g) at temperatures greater than 550 K corresponds to the stoichiometric H 2 consumed in the preceding reduction (12.1 m l / g of H 2 for the reduction, and to 5.9-6.0 m l / g of 02 for the oxidation), indicating that all Ce 3+ can be oxidized to Ce 4+. This is clearly seen in Fig. 6a, in which a change of slope in the line relative to the Ce0.sZr0.202 is observed at temperatures higher than 550 K (accordingly, the corresponding point has not been taken into consideration in the calculation of the activation energy). This indicates that at T > 550 K, for Ce0.sZr0.eO 2 only, oxygen storage capacity is not dependent on temperature but only on the number of total oxygen vacancies. The measured OSC in this case equals the total OSC, indicating that oxygen mobility at higher temperature is not limiting the process. Under the same conditions 02 uptake on CeO 2 is limited by oxygen mobility and the total 02 consumed is, by far, less than the stoichiometric hydrogen used for reduction. An intermediate situation is observed with low surface area CeO2-ZrO 2, in which, although oxygen mobility is higher than in ceria, oxygen consumption at 600 K is slightly less than that required for the complete filling of oxygen vacancies. Although the difference in surface area between the samples is also responsible for the increase in the 02 uptake on Ceo.8Zro.202, the activation energy values and the data reported in the Table as ttl O 2 / m 2 evidence that also the increase of 02 mobility plays an important role in this process, although a correct estimation of the extent of the two contributions on total OSC is not possible. To summarize, the results show that 02 uptake is greatly enhanced by introduction of Zr 4+ into the CeO 2 lattice; a relevant part of this enhancement is due to oxygen mobility which is affected by the creation of defects on the surface and in the bulk.

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3.2. Catalytic activity in isobutane oxidation Four samples were used for the catalytic studies. Two low surface area samples (CeO 2 6 m2/g and CeO2-ZrO 2 4.5 mZ/g) and two samples having a higher surface area (CeO 2 22 m2/g and CeO2-ZrO 2 29 mZ/g). The catalytic activity has been carried out to study the effect of isovalent Z r 4+ substitution in C e O 2 lattice on the catalytic performance in the oxidation of isobutane. The results at different temperatures are reported in Table 2 for the four samples examined. All the results were collected under stationary conditions, and no deactivation phenomena were observed during the time-on-stream (approximately 300 h for each catalyst). The following indications arise: 1. the prevailing products are isobutene and CO 2. Other by-products, formed in minor amounts, are CO, propylene and traces of oxygenated compounds (methacrolein and methacrylic acid); 2. data measured in the range of temperature 550-680 K indicate that introduction of Zr does not markedly affect catalytic activity. In both doped and undoped samples the most active samples are those with the higher surface area, for which total conversion of oxygen (the limiting reactant) is achieved at temperatures below 610 K; 3. the product distribution is heavily modified by introduction of Zr. The selectivity to isobutene increases from 10-15% with pure ceria (in both low-surface-area and high-surface area samples) up to 25-40% with the CeO2-ZrO 2 systems. The improvement in the selectivity to isobutene occurs at the expense of CO 2, which correspondingly decreases. Table 2 Reactivity of CeO 2 and CeO 2 - Z r O 2 in the oxidehydrogenation of isobutane T (K)

i-C4Hlo conv. (%)

O z conv, (%)

CeO2, surface area 6 m 2 / g 585 0.13 1.6 605 1.1 11 629 3 29 CeO2, surface area 22 m 2 / g 584 9 80 604 11 89 629 12 96 CeO 2 - Z r O 2, surface area 4.5 m 2 / g 629 0.8 6 658 1.3 10 680 2.8 32 CeO 2 -ZrO2, surface area 29 m 2 / g 566 5.1 41 590 10 91 611 10.8 97 632 11.5 98

Sel. i-C4H 8 (%)

Sel. C O 2 (%)

Sel. CO (%)

Sel. others (%)

11 11 12

81 82 81

5 6 6

3 1 1

12 13 12

82 78 78

4 5 5

2 4 5

40 36 32

55 61 65

1 1 1

4 2 2

26 25 23 24

64 67 72 71

5 4 3 3

5 4 2 2

C. de Leitenburg et al. / Applied Catalysis A: General 139 (1996) 161-173

171

8O

g ~ 4

60

"6

>

§

40

m

§

~2

2o g I----~= 0

r-'

.

i

i

i

i

i

0,4

0,8

1,2

1,6

2

0

2,4

residence time, s Fig. 7. Effect of residence time on isobutane conversion ( • ) , on oxygen conversion ( • ) , on selectivity to isobutene (O), to CO 2 ('k), to CO ( v ) and to others ([2) (others: propylene, methacrylic acid and methacrolein), for the high-surface area CeO 2-ZrO 2 mixed oxide. Reaction conditions: temperature 566 K, feedstock composition: 26 mol-% i-C4Hl0 , 13% 02, 12% H20, balance He.

With all systems C O 2 formation accounts for the majority of products, while the amount of CO is surprisingly low, considering the range of temperature employed. This feature may be related to the excellent property of CeO2-based materials in the catalytic combustion of hydrocarbons [9,10]. It is also well known that CeO 2 has an excellent activity in the water-gas shift reaction [2]; this might account for the low amount of CO which is formed under our reaction conditions. However, H a was not detected among the reaction products, and this allows to exclude the presence of the WGS reaction as a major contribution to CO 2 formation. Fig. 7 shows the effect of the residence time on the catalytic performance of the CeOz-ZrO 2 compound with high surface area. The isobutane and oxygen conversions increased when the residence time was increased; this allowed to establish that the reaction was not controlled by boundary-layer diffusion phenomena. The selectivity to the products was constant all over the examined range of residence time, indicating that the reaction network is constituted of parallel reactions for the formation of the various products, and that no consecutive over-oxidation reactions are present under the conditions employed for catalytic tests. This means that the very low formation of CO is a typical feature of these ceria-based compounds, and is not due to the presence of consecutive oxidation reactions of CO transformation. In addition, since the selectivity to isobutene is not markedly affected by the reaction temperature (see Table 2), these parallel reactions are characterized by comparable values of apparent activation energy. This suggests that a single type of active center is responsible for the formation of the various products. Literature data on isobutane oxidehydrogenation are scarce, and mainly focused with metal phosphates as catalysts [25-29]; these systems operate at temperatures in the range 720 to 770 K. The rate of isobutane depletion on the

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high-surface-area Ceo.8Zr0.20 2 compound at 588 K (11.2 mmol i-C4Hl0 g - i min- ~) is comparable to that of the catalysts described in the literature, but with respect to these the temperature is approximately 150 degrees lower. The better selectivity of CeO2-ZrO 2 for partial oxidation products at these relatively low reaction temperatures can be explained by the presence of a low concentration of reactive surface oxygen species (O-, 0 2, 022-) which may favor non-selective oxidation by interaction with hydrocarbon fragments [30]. In contrast, surface lattice oxygen species (0 2- ) minimize non-selective surface reactions [31,32]. Thus, CeO2-ZrO 2 with the high mobility of oxygen in both low and high surface area samples, has a high ability of transforming any surface oxygen species into lattice oxygen, thereby reducing the nonselective surface reactions caused by interaction of hydrocarbon fragments with weakly adsorbed oxygen species. This finding may contrast with the known ability of doped CeO 2 to catalyze high-temperature combustion reactions. However, it should be noted that the relatively low temperature at which this reaction occurs, and the composition of the reaction mixture, have a strong effect on the oxygen mobility and on the extent of Ce4+/Ce 3+ reduction, thus strongly affecting catalytic performance. Annihilation of oxygen vacancies with formation of oxygen-hole centres have been recently invoked to elucidate the role of defects and oxygen ion migration in the catalytic activity of doped lanthanum oxide in oxidative coupling of methane [33], while the role of 0 2 mobility and oxide reducibility in the kinetics of surface reduction was recently pointed out in the conversion of methane to CO x over CaO-CeO 2 mixed oxide catalysts [34]. In conclusion, the introduction of Zr 4+ in CeO 2 lattice leads to significant variations in the chemical physical features of ceria, and improves the selectivity to isobutene in the oxidative dehydrogenation of isobutane. The possibility of modifying the lattice oxygen mobility and the kinetics of Ce4+/Ce 3+ reduction by introduction of isovalent elements represents a tool for the design of oxidation catalysts.

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