Preparation and characterization of a praseodymium oxide to be used as a catalytic support

Journal of AUoys and Compounds, 180 (1992) 271-279 JAL 8080

271

P r e p a r a t i o n a n d c h a r a c t e r i z a t i o n of a p r a s e o d y m i u m oxide to be u s e d ~s a catalytic s u p p o r t S. Bernal, F. J. B o t a n a , G. Cifredo, J. J. Calvino, A. J o b a c h o a n d J. M. R o d r i g u e z - I z q u i e r d o Departamento de Quimica Inorgdnica, Facultad de Ciencias, Universidad de Cddiz, Apartado 40, Puerto Real, 11510 Cddiz (Spain)

Abstract This work reports on the preparation and characterization of a praseodymia sample to be used as a support for highly dispersed metals. The oxide sample was prepared by decomposing in a flow of helium, at 973 K, the phase obtained by precipitation with ammonia from a solution of Pr(N03)3. The preparation conditions were established after investigating the thermal evolution of the precursor phase by thermogrammetric analysis, temperature-programmed decomposition-mass spectrometry, Fourier transform IR spectroscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Data corresponding to the textural and chemical characterization studies carried out on the oxide are also presented and discussed.

1. I n t r o d u c t i o n R a r e - e a r t h - o x i d e - s u p p o r t e d ( p r o m o t e d ) m e t a l s c o n s t i t u t e a relatively n e w c l a s s o f m u l t i c o m p o n e n t catalytic s y s t e m s s h o w i n g r a t h e r u n c o n v e n t i o n a l b e h a v i o u r s [ 1 - 4 ] . In p a r t i c u l a r , c e r i a - c o n t a i n i n g p h a s e s h a v e r e c e i v e d v e r y m u c h a t t e n t i o n b e c a u s e o f t h e i r u s e as t h r e e - w a y c a t a l y s t s [5, 6]. T h e c h a r a c t e r i z a t i o n o f c e r i a - s u p p o r t e d m e t a l c a t a l y s t s c a n also b e c o n s i d e r e d a m a j o r scientific challenge. In effect, it is g e n e r a l l y a c k n o w l e d g e d t h a t t h e c h e m i s t r y o f c e r i a a g a i n s t s o m e o f t h e classic p r o b e m o l e c u l e s s u c h a s CO a n d H2 is r a t h e r c o m p l e x [ 7 - 1 2 ] . Likewise, s e v e r a l w o r k e r s h a v e reported the occurrence of strong metal support interaction phenomena on a n u m b e r o f M - C e O 2 c a t a l y s t s [ 1 3 - 1 6 ] . At p r e s e n t , t h e a c t u a l n a t u r e of this i n t e r a c t i o n r e m a i n s u n c l e a r [17, 18]. C o n t r a r y t o t h e c a s e o f M - C e O 2 catalysts, the literature c o n c e r n i n g s o m e o t h e r r e d u c i b l e r a r e e a r t h o x i d e s s u c h as p r a s e o d y m i a is r a t h e r s c a r c e [19, 20]. In s p i t e o f this, it is o u r o p i n i o n t h a t t h e i n v e s t i g a t i o n o f M - P r O ~ s y s t e m s , in a d d i t i o n t o its intrinsic scientific interest, m i g h t well c o n t r i b u t e to a better understanding of the characterization problems and metal-support i n t e r a c t i o n p h e n o m e n a o b s e r v e d o n c e r i a c o n t a i n i n g catalysts. This p r o m p t e d u s t o initiate t h i s r e s e a r c h p r o j e c t t h e first r e s u l t s o f w h i c h a r e p r e s e n t e d here. This work reports on the preparation and characterization of a praseod y m i u m o x i d e to b e u s e d a s t r a n s i t i o n m e t a l s u p p o r t . In a c c o r d a n c e w i t h

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272

the application that we have foreseen for the oxide, we have tried to prepare a sample with a mean surface area value, good textural stability under the reducing conditions used during the preparation of the supported metal catalysts, and an anionic impurity content as low as possible, particularly with respect to carbonates.

2. E x p e r i m e n t a l d e t a i l s The praseodymium oxide was prepared from Pr(NO3)3"6H20, 99.9% pure, supplied by Fluka. The concentrated aqueous ammonia used here was from Merck. The thermogravimetric analysis (TGA) experiments were performed in a Mettler modelTA-H 20 balance. Our temperature-programmed decomposition (TPD) or reduction studies with analysis of the evolved gases by mass spectrometry (MS) were carried out under the following conditions: flow rate of either helium o r H2 through the reactor, 1 c m 3 s - l ; heating rate, 10 K rain -1. Details of the experimental set-up have been given elsewhere [21]. X-ray diffraction (XRD) patterns were recorded with a Siemens model D-500 instrument interfaced to a microcomputer. The radiation was Cu Ka, and a filter of nickel was used. The IR spectra were recorded with a Fourier transform IR (FTIR) instrument from Philips, model PU9800. The sample disks were prepared in a KBr matrix (95% KBr and 5% sample). The transmission electron microscopy (TEM) images were obtained with a JEOL model 2000 EX instrument, equipped with a top-entry specimen holder and ion pump. The scanning electron microscopy (SEM) study was performed with a JEOL model 820 instrument. The Brunauer-Emmett-Teller (BET) surface area of both the precursor and the final oxide phases were determined by nitrogen adsorption at 77 K.

3. R e s u l t s a n d d i s c u s s i o n The procedure followed by us to prepare the praseodymium oxide consisted of the thermal decomposition, in a flow of helium, of a precursor phase obtained by adding an excess of concentrated ammonia to an aqueous solution of Pr(NO3)s. The precipitate was washed several times with distilled water, dried in an oven at 383 K and further stored with no special precautions. The specific conditions used for the preparation of the oxide sample were established after investigating the thermal evolution of the precursor phase by TGA, TPD-MS, l~rIR spectroscopy, SEM and TEM. Earlier studies [22l dealing with the preparation of rare earth oxides by means of a method similar to that used here suggest that, in the present case, the precursor phase would consist of partially nitrated and carbonated

273

praseodymium(HI) hydroxide. This suggestion is confirmed by the XRD and l~'rIR characterization studies carried out on the precursor. According to the TPD-MS diagrams for the precursor phase given in Fig. 1, the main decomposition processes, those involving the elimination of H20 and NO=, take place below 773 K. This is also confirmed by TGA. It should be noted, however, that in accordance with the TPD diagram for C02 (re~e, 44), the complete elimination of carbonates requires much higher decomposition temperatures. Taking also into account the trace for m/c = 32, which gives information about the evolution of 02, i.e. on the thermal stability of the different non-stoichiometric praseodymia phases, the following preparation conditions were established. The precursor phase was calcined in a flow of helium (1 cm 3 s - l ) , at a heating rate of 5 K min -I, up to 973 K; the sample was further held at 973 K for 1 h, and then it was cooled to room temperature, always in a flow of inert gas. The XRD pattern for the recently prepared praseodymium oxide (Fig. 2(a)) fits well that reported in the ASTM file for Pr6On. The praseodymia sample was also studied by TPD-MS (Fig. 3). As in the case of the remaining characterization studies carried out on the final oxide phase, this TPD-MS experiment was performed within the first 48 h after its preparation. Regarding Fig. 3, some interesting aspects can be outlined. The signals for H20 and CO2 are much less intense than those reported for the precursor in Fig. 1. In particular, the trace for water shows a first peak, the most intense one, at around 400 K, which might well correspond to weakly adsorbed molecular water; a second peak occurs at around 525 K, and several other small features spread over a wide range of temperatures. These latter might reasonably be assigned to dissociatively

/ ~

18

4

3O

32 2.8 10 .9 I

7.6 10 -9

273

T(K)

1671°8

,,

1273 273

T(K}

1273

Fig. 1. TPD-MS study of the p r e c u r s o r phase. Traces corresponding to the evolution of CO2 (m/e=44), H20 (m/e=18), NO (m/e=30) a n d 02 (m/e=32). The ordinate scale, in Torrs, for each of t h e signals is indicated o n the figure.

274

._9 I

I

i

7B

46

14

(a)

l

ifo t

!

25

fo)

I

I

I

I

31

2 o

Fig. 2. XRD diffraction patterns of the praseodymia sample prepared in this work: (a) recently prepared; Co) aged in air, at 298 K, for 6 months.

chemisorbed water. The trace for CO2 in Fig. 3 is about three times less intense than that shown in Fig. 1. In spite of its complexity, two main regions can be distinguished in it. Peaks appearing in the lower temperature region, i.e. below 873 K, can be assigned to surface carbonate species formed upon exposure of the oxide to air, at room temperature; the feature peaking at about 1073 K, on the contrary, might well correspond to residual inner carbonate species not d e c o m p o s e d dining the calcination treatment, leading to preparation of the oxide. In other words, the high temperature peak can be used as a rough indication of the carbonate content intrinsically associated with the preparation procedure followed by us. The residual content of nitrate species was found to be negligible; the signal reported in Fig. 3 is about 200 times less intense than that obtained for the precursor in Fig. 1; furthermore, the temperature at which the peak is observed in Fig. 3, 725 K, would indicate that it can p r o c e e d from surface contamination occurred after the preparation. Regarding the evolution of oxygen (m/e=32), Fig. 3 shows the occurrence of a narrow peak at 800 K, well below the temperature reached during the calcination treatment (973 IO. This suggests that, upon

275

18

44

\

72I

6 10-11 "

6.6 10"91

J

273

T(K)

1273 273

,

h

,

T(K)

,

,

,

1273

Fig. 3. TPD--MS study of the recently prepared praseodymia sample. Traces corresponding to the evolution of CO2 (m/e=44), H20 (m/e=18), NO (re~e=30) and 02 (m/e=32). The ordinate scale, in Torrs, for each of the signals is indicated on the figure.

exposure to air at room temperature, some sample reoxidation does occur. This prompted us to investigate the effects on the praseodymium oxide prepared by us of the long-term exposure to air, at r o o m temperature. Figure 2(b) depicts the XRD pattern recorded for the praseodymia sample after exposure to air for approximately 6 months. Compared with the XRD diagram for the recently prepared sample (Fig. 2(a)), that for the oxide aged in air shows a broadening of its main diffraction lines. Finally, after aging for several months, the broadened peaks b e c o m e s,,fficiently resolved as to suggest the coexistence of two different praseodymia phases. In accordance with the data reported in the ASTM file, the phase resulting from aging might well be PRO2. In addition to the segregation of a phase richer in oxygen, the XRD for the aged phase shows some weak diffraction lines which can be assigned to praseodymium(III) hydroxide. This proposal is also supported by FTIR spectroscopy. In effect, the spectrum reported in Fig. 4, corresponding to the praseodymia sample which had been aged in air shows, in addition to a series of bands at 1496, 1391 and 849 c m - ' typically associated with carbonate species, two features at 3607 and 664 cm -1 which in accordance with the literature [23] can be interpreted as due to praseodymium(III) hydroxide. The observation of at least two well-defined phases, PrO2 and Pr(OH)3, suggests that the aging process can induce the disproportionation of the mixed-valence praseodymium oxide prepared by us. The occurrence of solvolytic disproportionation of praseodymia phases under mild conditions has recently been discussed in ref. 24. We have also investigated by TPR-MS the behaviour of the recently prepared praseodymia sample under reducing conditions (Fig. 5). First of all, the trace for water shows a major very intense peak centred at 673 K. This feature can be assigned to the reduction of the starting oxide to Pr208,

276

T~)

70

5O !

400O

!

3O0O

2OOO

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I ooo

WAVENUMBER

Fig. 4. FrIR spectrum for the praseodymia sample after being exposed to air, at 298 K, for 6 months.

15

,

18

8.5 10 "11

30

44

1.2 10 "11

1.6 10 "9

J, 273

i

T(K)

1273 273

i

,

. . . . .

TiK)

,

1273

Fig. 5. TPR-MS study of the recently prepared praseodymia sample. Traces corresponding to the evolution of CH4 ( m / e l l 5 ) , H20 (m/e=18), COz (m/effi44) and NO (m/e=30). The ordinate scale, in Torrs, for each of the signals is indicated on the figure.

the final oxide phase that we have found to occur. This suggests that, in contrast with that observed in the TPD-MS experiments, where the evolution of oxygen takes place through several well-resolved steps (Fig. 3); when we operate in a flow of H2, the reduction to the sesquioxide phase occurs through a single step at a fairly low temperature. The trace for CO~. in Fig. 5 is quite different from that depicted in Fig. 3. It can be noted that at 673 K the signal for m/e---44 in the TPR experiment abruptly falls to the background level, which would suggest that, from this temperature upwards, reduction of the carbonate species does occur. This is confirmed by the trace for m~ e----15, in accordance with which CH4 is formed in the high temperature range of the TPR spectrum.

277

An important property of the oxides to be used in catalytic applications is their textural stability. Table 1 reports the BET surface area of both the precursor phase and the praseodymium oxide submitted to a number of different treatments similar to those usually applied for the preparation of supported metal catalysts. According to Table 1, the preparation procedure used here does not induce strong surface area reduction of the sample, compared with that of the precursor phase. This rather modest effect of the calcination treatment at 973 K on the textural properties of the sample can reasonably be understood by comparing the SEM and low magnification TEM images obtained by us for both precursor and final oxide phases. Figure 6 shows an example of our TEM study. It can be deduced from the analysis of these two figures that, under the conditions used here, decomposition of the precursor phase takes place without significant morphological modifications of the material. This means that the method that we have followed leads to a praseodymium oxide with a mean surface area value very convenient

Fig. 6. TEM images corresponding to (A) the precursor phase and (B) the recently prepared praseodymium oxide.

278 TABLE 1 Textural stability of the praseodymia sample prepared by us using the Brunauer-Emmett-Teller surface area data after a number of different treatments Sample

Treatment

Surface area (m2 g-l)

Atmosphere

Temperature (k~

Time (h)

Precursor Precursor PreOn Pr6Ol~ Pr6Ol~

He Vacuum

973 623

2 1

46 32 31

Vacuum Vacuum

773 973

1 1

34 31

Pro011

H2

PrsOn Pr80 n

Hz H~

623 773 995

1 1 1

33 30 25

f o r t h e specific a p p l i c a t i o n f o r e s e e n f o r it -- t h e use as a s u p p o r t o f highly d i s p e r s e d transition m e t a l catalysts. R e g a r d i n g t h e effect o n t h e s u r f a c e a r e a o f the o x i d e o f f u r t h e r t h e r m a l t r e a t m e n t s c a r r i e d o u t u n d e r h i g h v a c u u m a n d h y d r o g e n , the results given in T a b l e 1 indicate t h a t t h e t e x t u r a l stability o f o u r s a m p l e is v e r y good. In effect, o u r s t u d y s h o w s that, e v e n u n d e r s t r o n g r e d u c i n g conditions, at a fairly h i g h t e m p e r a t u r e ( 9 9 5 K) t h e s u r f a c e a r e a o f o u r p r a s e o d y m i a s a m p l e d e c r e a s e s v e r y slightly.

4. Conclusions

To s u m m a r i z e , w e h a v e c a r r i e d o u t t h e p r e p a r a t i o n as well as the chemical a n d t e x t u r a l c h a r a c t e r i z a t i o n o f a p r a s e o d y m i a s a m p l e to b e u s e d as s u p p o r t o f highly d i s p e r s e d m e t a l catalysts. Upon, e x p o s u r e t o air at r o o m t e m p e r a t u r e , t h e p r a s e o d y m i a s a m p l e p r e p a r e d b y u s u n d e r g o e s significant d i s p r o p o r t i o n a t i o n into PrO2 a n d Pr(OI-1)a. This c a n b e a n i m p o r t a n t f a c t o r in d e t e r m i n i n g t h e catalytic b e h a v i o u r o f t h e starting o x i d e p h a s e . T h e t e x t u r a l p r o p e r t i e s o f t h e o x i d e t h a t w e h a v e p r e p a r e d are also v e r y interesting f o r its applications as a s u p p o r t . Its r e s i s t a n c e to s i n t e r i n g c o u l d b e v e r y useful for investigating t h e actual n a t u r e o f t h e so-called s t r o n g m e t a l - s u p p o r t i n t e r a c t i o n p h e n o m e n o n [25[, n o t a b l y d i s t u r b e d b y this side effect in t h e c a s e o f M-CeO2 s y s t e m s [17I.

Acknowledgments

This w o r k h a s r e c e i v e d financial s u p p o r t f r o m t h e Comisidn Interministerial d e Ciencia y T e c n o l o g i a u n d e r the c o n t r a c t P B 8 7 - 0 9 6 1 . W e wish to ack n o w l e d g e P r o f e s s o r L. E y r i n g f o r his material help a n d stimulating discussions.

279

We also thank Dr. A. Justo and Mrs. G. Blanco for recording respectively the XRD diagrams and F]?IR spectrum included in this work.

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