Aerogel and xerogel catalysts based on θ-alumina doped with silicon for high temperature reactions

FULL PAPER

Aerogel and Xerogel Catalysts Based on θ-Alumina Doped with Silicon for

High Temperature Reactions

[a]

[b]

Aurelien Florin Popa, Laurence Courth e´ oux, Eric Gautron, Sylvie Rossignol,* and

Charles Kappenstein^[

b]

Keywords: Aerogel / Xerogel / Catalyst / Platinum / Alumina

Numerous materials (supports and catalysts) based on alu-

of the influence of the preparation mode on the properties of

mina have been prepared using the sol-gel process and car- platinum on alumina supported catalysts. The dispersion of

bon dioxide supercritical drying. In this work two types of

solids, i.e. xerogels and aerogels, were systematically com-

pared and a way of introducing platinum metal with a con-

tent of 5% percent by weight was examined. The structural

the metal phase is directly dependent on the specific surface

of the support. A significant value for the surface area implies

a large amount of centers for interaction with the metal pre-

cursor and, consequently, the appearance of more centres of

data, the surface area, Pt dispersion and catalytic activity for simultaneous germination. Although aerogels obtained by

the decomposition of the propellant were measured for the

various samples. The (Al 0.12 samples prepared

show very interesting porosity values, especially for the aero-

gel. For this reason, they were chosen as supports for the

synthesis of 5 wt% platinum on alumina catalysts. The results

presented in this work allowed us to obtain an overall view

carbon dioxide supercritical drying always show superior

properties compared with xerogels, for catalytic decomposi-

tions the xerogels still remain superior.

2

3 2

O )0.88(SiO )

Germany, 2005)

Introduction

barium or silicon) can be introduced during the sol-gel

[

6Ϫ10]

process.

Moreover, for catalytic applications, it is gen-

The sol-gel method represents a way of synthesising ma-

terials which display excellent properties suitable for many

applications such as catalysis, ceramics and glasses, and

nano-materials.^[Therefore, numerous catalyst supports or

catalysts have been prepared by this procedure which leads

to high purity, homogeneous materials with large surface

areas. These properties are key factors for the application

erally necessary to add an active phase (a metal phase such

as platinum, palladium etc.) to improve the properties.

Numerous authors have used alumina-supported plati-

num catalysts, prepared generally by the impregnation of

commercially available supports. The literature concerning

the sol-gel method was less important until the 1990Јs dur-

ing which time more papers appeared. Samples have been

generally prepared by conventional drying processes (293 to

1]

[

2,3]

of these materials.

Their properties are comparable and,

in some cases, superior to some catalysts prepared using

conventional procedures such as co-precipitation or con-

ventional ceramic methods. In particular, we have focussed

our interest in recent years on the preparation and evalu-

ation of alumina-based catalysts for applications concern-

ing the decomposition of monopropellants.^[For this

specific purpose, it is important to obtain catalytic materials

or supports displaying high thermal stability (i.e. at 1200

3

93 K) leading to xerogels. The use of supercritical drying,

generally in carbon dioxide, leads to aerogels but the corre-

sponding reports are much less numerous and focus mainly

on pure alumina supports. The first paper concerning a Pt

catalyst supported on alumina (aerogel and xerogel) and

prepared by the sol-gel method was published in 1993 by

Balakhrishnan et al.

aluminium tri-sec-butoxide Al(O-sec-C H ) (ATB) follow-

ing the work of Yoldas and the platinum was introduced

during the sol-gel transformation using H PtCl as a pre-

cursor. The supercritical conditions for obtaining the aero-

gels were T ϭ 295 °C and P ϭ 85 bar with methanol. The

4]

[11]

The alumina precursor used was

4

9 3

°

C or even higher). The selected supports for achieving this

[12]

[5]

purpose are aerogel aluminas or doped aluminas. In the

latter case, the doping element (lanthanum, praseodymium,

2

6

[

a]

Alexandru Ioan Cuza University, Faculty of Chemistry, samples were then treated at 773 K under helium. The main

Department of Inorganic Chemistry,

conclusions from this work were: (i) during the treatment,

6

600 Iasi, Romania

[

b]

University of Poitiers, LACCO UMR 6503, Laboratoire de a reduction of the surface area can be observed and (ii)

Catalyse par les M e´ taux,

aerogels display a lower Pt metal dispersion in comparison

4

0 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France

with xerogels of similar loading. In 1998, Cho et al.^[pre-

13]

Fax: (internat.) ϩ33-5-49454157

E-mail: sylvie.rossignol@univ-poitiers.fr

pared the same materials using aluminium tripropoxide Al-

Eur. J. Inorg. Chem. 2005, 543Ϫ554

DOI: 10.1002/ejic.200400657

543

A. F. Popa, L. Courth e´ oux, E. Gautron, S. Rossignol, C. Kappenstein

FULL PAPER

(

OC H ) (ATP) and H PtCl , with a final treatment at enable the determination of evolved water and thus the for-

3 7 3 2 6

773 K. They demonstrated that the sol-gel (xerogel) catalyst mula of the initially dried solids. At the beginning of the

displays a higher thermal stability of the metal particles measurements, the dried samples always contain an excess

than the conventionally impregnated catalyst. For the sol- of water and are known as hydrated boehmites. For both

gel sample, the exposed Pt metal particles are anchored to samples presented in Figure 2, the complete formulas are

the alumina surface by metalϪoxygen bonding. Other xero- AlOOH·0.94H O and (AlOOH)₀_.₉₄-(SiO ) ·1.0H O.

2

2 0.06

2

[

14]

gel Pt/Al O catalysts prepared by Castillo et al.

Manasilp et al.

and Therefore, in the low temperature range (20Ϫ800 °C), the

using the same precursors (ATB-ATP, behaviour of the doped samples is similar to that of pure

H PtCl ) and treated at 823 K and 773 K, respectively, boehmites. On the other hand, the TDA curves reveal an

2

3

[

15]

2

6

demonstrate that the co-gelation of alkoxides and noble me- important difference at high temperature: the exothermic

tals induces the stabilisation of the metallic phase and its peak at about 1200 °C which characterises the transform-

incorporation into the alumina network producing better ation into alpha alumina is clearly visible in the case of the

metal-support interactions. Recently, using ATB with ace- pure boehmites but is shifted to a higher temperature for

[16]

tylacetonate and LiBH as precursors, Sault et al.

dem- the doped samples. In this case, the boehmite is transformed

4

onstrated that after calcination at 873 K, the metal particles into transition alumina but its structure is more stable and

are not encapsulated or occluded into the support. All these leads to alpha alumina at much higher temperatures (Ͼ1350

observations confirm that the metal-support interactions °C). This thermal behaviour is the same for the aerogel and

strongly depend on the precursors used, on the thermal xerogel samples. Consequently, we expected that the doped

treatment and on the method of drying. In order to extend samples would not be transformed into alpha alumina, even

the use of such samples to high temperature applications, after a hard thermal treatment (5 h calcination at 1200 °C).

it seemed important to study the influence of the drying

conditions and to then compare xerogels and aerogels.

In this paper, we present our results on the sol-gel prep-

aration of alumina or alumina doped with silica as a sup-

port. A systematic comparison has been made between sup-

port samples dried under conventional conditions (xerogels)

and under supercritical conditions using CO (aerogels) in-

2

cluding the behaviour after the thermal treatment. The in-

troduction of platinum was carried out using the conven-

tional impregnation of dried or calcined sol-gel supports or

during the sol-gel process. Powder X-ray diffraction (XRD),

transmission electron microscopy (TEM) and thermal

analyses (TDA-TGA) were used to determine the structural

properties. To determine the effects of the synthesis on the

catalytic activity, an evaluation of the properties of the cata-

lyst towards the propellant decomposition has been per-

formed with aqueous solutions of hydroxylammonium ni-

ϩ

Ϫ

trate NH OH NO (HAN). The preliminary results are

3

Figure 1. X-ray diffraction patterns of pure and doped boehmites,

xerogels and aerogels (lines: PDF file 21Ϫ1307).

presented here.

Results and Discussion

A. Supports

The diffraction patterns of the four different types of sup-

ports, after drying, are presented in Figure 1. They are all

composed of nanocrystalline hydrated boehmite γ-

[

17]

AlO(OH)·nH O, see PDF file 21Ϫ1307 mentioned in ref.

2

The peak positions are essentially identical for all samples

which also show the same cell parameters. The shift of the

[

18]

first diffraction peak has been discussed previously.

The thermal analyses curves (Figure 2) indicate two

major weight losses accompanied by endothermic effects

which correspond firstly to the dehydration of the hydrated

boehmite and then to the transformation of boehmite into

gamma alumina. Assuming that at the end of the analyses

Figure 2. Thermal analyses of dried samples of AlO(OH) (x) and

the final product is the anhydrous α-alumina, the TG curves AlO(OH)-Si12 (x)

Eur. J. Inorg. Chem. 2005, 543Ϫ554

Aerogel and Xerogel Catalysts Based on Doped θ-Alumina

FULL PAPER

The diffraction patterns of the calcined supports (Fig- thermore, the X-ray mapping revealed the atomic distri-

ure 3) confirm this fact, revealing that the transformation bution of Al and Si across the samples. For the sake of

into alpha alumina is delayed by doping. After the thermal clarity, we present a previously published diagram (Fig-

[

17]

treatment, the structure of the Al O Si12 xerogel or aerogel ure 4).

For ASi12 (a) (aerogel sample) we can see that

2

3

corresponds to θ alumina (monoclinic, PDF file 23Ϫ1009) the distribution of both aluminium and silicon atoms is

with a well ordered structure. By comparison, the non- homogeneous over the entire zone of the sample which was

doped aerogel (Al O ) is entirely transformed into alpha analysed. Other explored zones show that this homogeneity

2

3

alumina after the same treatment (Figure 7).

is a characteristic of the aerogel samples (see 2 in Figure 4).

Xerogel sample ASi12 (x) contains some zones showing ob-

vious microscale heterogeneity (see 1 Figure 4). This latter

sample contains domains where the silicon concentration is

enhanced. From EDX analyses, the Si and Al atomic com-

positions were quantified. For the aerogel, the atomic Si:Al

ratio of 6:94 is in excellent agreement with the expected

value (6.3:93.7). For the xerogel, this ratio strongly depends

on the analysed zone (200 nm). In Figure 4 (1), the analysed

zone exhibits a very important excess of silicon: Si:Al ϭ

33:67. In fact, the large (200 nm) dark particle clearly vis-

ible on the image of the xerogel (see 2 in Figure 4) contains

only silicon and oxygen atoms which correspond to pure

amorphous silica. The aluminium content is smaller in the

zones where this silicon agglomeration appears. In compen-

2 3

Figure 3. X-ray diffraction patterns of doped samples of Al O -

Si12 (a) and (x) and nondoped Al

2

O

3

(a), calcined at 1200 °C for sation, other zones are reduced in their silicon content lead-

5

h (θ Al

2

O

3

, PDF: 23Ϫ1009; α Al

2

O

3

, PDF: 46Ϫ1212)

ing to a higher content of aluminium in comparison with

the calculated value. In these zones, the aluminium content

can reach values as high as 95.7%.

The values found for the BET surface area of the samples

Table 1) confirm that the doping retards the transform-

ation into alpha alumina. The doped xerogel retains a good

(

2

Ϫ1

specific surface area (65 m ·g ) whereas the aerogel has a

2

Ϫ1

much better value of 120 m ·g

(compared with 1Ϫ3

2

Ϫ1

m ·g for the nondoped alumina). This value is similar to

that published by Mizushima,^[⁸^,⁹^](114 m ·g ) for a 10%

doped sample prepared in an alcoholic medium. The po-

rous volume is slightly larger for the ASi12 (a) sample (0.24

2

Ϫ1

3

Ϫ1

3

Ϫ1

cm ·g ) than for the ASi12 (x) sample (0.16 cm ·g ). The

mean pore sizes of the two doped aluminas are comparable

at 9.5 and 9.7 nm, respectively.

The transmission electron microscopy images enabled us

to analyse the morphology of these calcined supports. Fur- for the: (1) Al O -Si12 (x) and (2) Al O Si12 (a) samples

2 3 2 3

Figure 4. Electron microscopy (X-ray mapping and EDX analyses

Table 1. Samples prepared, mode of platinum introduction, thermal treatment, nomenclature, specific surface area (BET) and experimen-

tal platinum load (expected value 5 wt.-%)

Sample

aerogel (a); xerogel (x)

Pt introduction

impregnation sol-gel

Nomenclature

BET

[m /g]

Pt

[%]

Pt size

[nm]

2

Al

2

O

3

(a)

(x)

1

A (a)

A (x)

ASi12 (a)

ASi12 (x)

SG-Pt-B-c (a)

SG-Pt-B-c (x)

SG-Pt-BSi12-c (a)

SG-Pt-BSi12-c (x)

Pt-A-r (a)

Pt-A-r (x)

Pt-ASi12-r (a)

Pt-ASi12-r (x)

3

1

3

(

Al

2

O

3

)

0.88(SiO)0.12 (a)

0.88(SiO)0.12 (x)

120

65

41

44

130

41

3

1

120

68

2

3

5

% Pt/Al

% Pt/(Al

% Pt/Al

2

O

3

(a)

(x)

SG-1

Imp-2

3.0

3.0^[

3.5

2.6

2.2^[

2.6

2.4

3.5

7.0

11.0

5.6

4.3

2.6

3.0

a]

2

3

2

O

3

)

0.88(SiO)0.12 (a)

0.88(SiO)0.12 (x)

2

3

2

O

3

(a)

(x)

2

3

% Pt/(Al

2

O

3

)

0.88(SiO)0.12 (a)

0.88(SiO)0.12 (x)

2

3

3.8

[

a]

Values obtained by XRD quantification. 1. calcination at 1200 ° (5 h); 2. reduction at 400 ° (3 h).

Eur. J. Inorg. Chem. 2005, 543Ϫ554

www.eurjic.org

545

A. F. Popa, L. Courth e´ oux, E. Gautron, S. Rossignol, C. Kappenstein

FULL PAPER

The silica segregation can explain the lower specific sur-

face area (Table 1) of the xerogel sample 1-Asi12(x) after

the thermal treatment at high temperature. During the

transformation of alumina into the alpha form, small sites

are first formed by a nucleation process. The corundum for-

mation then continues by a growing and sintering process

which requires contact between the former crystallites and

migration of cations or cationic vacancies from the alumina

particles of the previous transition.^[²⁵^]

The stabilising effect of silicon occurs at the surfaces of

the crystallites. When the silicon distribution is homo-

geneous, the silicon atoms are bonded tetrahedrally through

µ-oxo bridges AlϪOϪSiϪOϪAlϪ and cannot take part at

the sintering process because the α-Al O structure contains

2 3

Figure 5. Enlargement of the diffraction patterns of the Al O Si12

samples X: (a) ϭ aerogel, (x) ϭ xerogel

2

3

the cations in octahedral sites. It is much more difficult for

these sites to be occupied by silicon. The surface silicon

layer behaves as a barrier between the growing areas and

limits the transformation. Another explanation for the de-

This cell parameter difference does not exist for the non-

lay of the transformation into alpha alumina is related to calcined samples displaying the boehmite pattern (Fig-

the crystalline structure of the transition alumina. Beyond ure 1). No displacements can be seen, in agreement with the

600Ϫ700 °C, the quantity of surface OH groups contained boehmite structure which displays only octahedral alu-

in alumina is no longer sufficient to maintain the stability minium sites. These sites are able to host aluminium atoms

of the structure. The removal of these hydroxyl groups leads but not silicon atoms. During the sol synthesis, boehmite

to the formation of tetrahedral vacancies which destabilise nanocrystallites are formed first by hydrolysis of the alu-

the structure. In the presence of well distributed silicon minium precursor. After addition of the silicon precursor,

atoms, the influence of these tetrahedral vacancies is low- hydrolysis occurs later and produces silicon species which

ered and the transformation is delayed.^[

19Ϫ22]

may be bonded to the surface aluminium atoms of the

The diffraction peaks of the two samples after the high boehmite particles by a µ -oxo bridge, this being a conse-

2

temperature treatment have been enlarged to focus on the quence of an oxolation reaction:ϪSiϪOH ϩ HOϪAlϪ Ǟ

peak displacement (Figure 5). We observed a systematic ϪSiϪOϪAlϪ ϩ H O

2

and slight but visible displacement of the aerogel peaks to

Thus, before calcination, silicon atoms are located mainly

higher 2θ values. This displacement varies between 0.09 and at the surface of the crystallites and only during the trans-

0

.29° and can be explained by the cell parameters shrinking formation of boehmite into γ alumina can they participate

due to the replacement of aluminium by silicon atoms in in the transition γ-alumina structure by substituting tetra-

the structure. Silicon atoms are slightly smaller than Al hedral aluminium atoms. In Figure 6, which shows a pos-

˚

atoms (bond length SiϪO ϭ 1.91 A compared with sible structural rearrangement during the transformation of

˚

[23]

AlϪO ϭ 2.03 A).

If the structure of θϪΑl Ο can be boehmite, the surface Si atom is linked to a surface HO

2 3

viewed as a distorted spinel structure,^[we can approxi- group of boehmite and to an external hydroxyl group. The

mate the peak displacement on the basis of a cubic struc- transformation of the hydroxyl group and a slight silicon

ture. Assuming that all the silicon atoms are located in the displacement leads to the position of silica in a tetrahedral

tetrahedral sites of the θϪΑl Ο structure, we can estimate site of the transition alumina. The presence of these silicon

24]

2

3

the 2θ displacement from the bond length difference to be atoms stabilises the alumina defect structure and can ex-

of the order of 0.2 to 0.5°. The experimental shifts between plain the delay in the transformation into corundum.

the aerogel and the xerogel peak positions range from 0.1

The results presented in this section show that the drying

to 0.3°, indicating that the silicon content incorporated into technique has an important influence on the features of

the structure is far superior in the case of the aerogel (of doped alumina. The morphology, thermal stability, homo-

the order of 50%).

geneity, fine structure and the textural properties are depen-

Figure 6. Model of silicon insertion into the gamma alumina structure during the boehmite transformation

46 www.eurjic.org Eur. J. Inorg. Chem. 2005, 543Ϫ554

5

Aerogel and Xerogel Catalysts Based on Doped θ-Alumina

FULL PAPER

dent on this. The ASi12 samples prepared show very inter- 2. Physico-Chemical Characterisation

esting porosity values, especially for the aerogel. For this

reason, they were chosen as supports for the synthesis of 5

wt.-% platinum on alumina catalysts.

a) Catalysts on Nondoped Supports

The diffraction patterns of both aerogel catalysts (sol-gel

and impregnated) are presented in Figure 7. As expected,

the structure of the support is α-Al O and the metal phase

2

3

appears as large sized crystallites. The average size values

calculated from peak breadths are around 70 nm for the

sol-gel catalyst and 26 nm for the impregnated one. For the

xerogel sample, the respective values are of the same order

B. Catalysts

1. Effective Catalyst Metal Load

(

Table 1). The sintering is slightly more pronounced for the

For a selection of catalysts (6 samples), the platinum con-

sol-gel samples compared with the samples prepared by im-

pregnation.

tent was analysed in order to establish precisely the effective

metal load after the thermal treatment. For two other

samples, SG-Pt-B-c (x) and Pt-A-r (a), the metal load was

estimated from a quantitative analysis of the X-ray diffrac-

For the Pt-A-r (a) sample, transmission electron mi-

croscopy agrees well with this average size. The image re-

veals a rather large distribution of the crystallite sizes, rang-

ing from a few tenths to several hundred angstroms. We

present two different characteristic zones of the sample in

Figure 8.

tion patterns with respect to their homologues SG-Pt-B-c

25]

(

a) and Pt-A-r (x).^[The results are presented in Table 1.

The metal content is between 47 and 71% of the calculated

value, so that a quantity of platinum is lost during the prep-

aration of the catalysts. This loss probably takes place dur-

ing the thermal treatment processes (calcination and re-

[

26]

duction).

During the introduction of the platinum pre-

cursor, the anions present in solution ([PtCl₆]²

Ϫ

,

Ϫ

[

Pt(H O)Cl ] or others) are adsorbed into the surfaces of

2 5

the crystallites and this fixation is mainly due to electro-

static interactions between negative complexes and surface

ϩ

OH₂groups of the support present in acidic solutions or

Ϫ

to ligand substitution around the platinum atoms (Cl by

surface OH groups). The fraction of platinum atoms which

is not firmly anchored on the surface can become detached

Figure 8. Two transmission electron microscopy images of the Pt-

and leave during the thermal treatments, probably in the A-r (a) sample

form of more volatile oxychloride species. Nevertheless,

hexachloroplatinic acid remains the best precursor because

it allows the preparation of catalysts with high metal disper-

b) Catalysts on Doped Supports

[

11,27]

sions.

We expect also a lower metal percentage for the

Catalysts Prepared by Sol-Gel Synthesis: The diffraction

patterns indicate the presence of θ alumina for the support

and the same large metal crystallites (Figure 9) of average

size 43 nm for the xerogel and 56 nm for the aerogel. This

is the only case when the aerogel crystallites are larger than

the xerogel ones. This fact is probably related to the particu-

lar texture of the sample of SG PtϪBSi12-c (a). In fact, the

precursor anions can be adsorbed onto the boehmite sites

concurrently with the silicon species. As a consequent, the

materials displaying the α-Al O structure due to the strong

decrease of the BET surface area (Table 1).

For the samples supported on doped alumina, this plati-

num content decrease could be related to the surface

2

3

SiϪOH groups which can prevent the metal from bonding

_f_i_r_m_l_y_t_o_t_h_e_s_u_r_f_a_c_e_.[28]

Figure 7. Diffraction patterns of catalysts on nondoped supports:

(

1) SG Pt-B-c (a) sample; (2) Pt-A-r (a) sample. (PDF: Pt: Figure 9. Diffraction patterns of (1) SG Pt-BSi12-c (a) and (2) SG

0

4Ϫ0802; αAl

2

O

3

: 46Ϫ1212)

2 3

Pt-BSi12-c (x) samples (θ Al O , PDF: 23Ϫ1009)

Eur. J. Inorg. Chem. 2005, 543Ϫ554

www.eurjic.org

547

A. F. Popa, L. Courth e´ oux, E. Gautron, S. Rossignol, C. Kappenstein

FULL PAPER

surface nature and the interactions between crystallites may ticle, revealed a better silicon repartition compared with the

be different from the case of a pure boehmite sol. xerogel. Instead, the signal due to the platinum atoms is

After the thermal treatment at 1200 °C, the sample SG practically invisible over the zone showing that the metal is

Pt-BSi12-c (a) exhibits the largest specific surface area (130 present only as isolated crystallites.

2

Ϫ1

2

Ϫ1

m ·g ), compared with only 41 m ·g for its xerogel hom-

Catalysts Prepared by Impregnation on Doped Alumina:

ologue SG Pt-BSi12-c (x). The porous volume is 0.45 The diffraction patterns of the catalysts impregnated on

3

Ϫ1

cm ·g , five times larger than for the xerogel (0.09 doped alumina are presented in Figure 12. In the case of the

3

Ϫ1

cm ·g ) and the mean pore sizes are 14 and 7 nm, respec- aerogel, the peaks corresponding to platinum are practically

tively. Consequently, the platinum crystallites located inside indiscernible from the background indicating a very small

the aerogel pores can develop and reach larger sizes.

crystallite size. For the xerogel, the platinum peaks are weak

The TEM pictures (Figure 10) show, for the xerogel but visible. By comparing the intensity of the isolated (311)

sample, some zones with a normal aspect of the porous sup- peak (36 counts degree) with the SG-Pt-BSi12 c (x) sample

port and metal crystallites with sizes ranging from 7 to (which has practically the same metal load, 304 counts de-

100 nm and some other singular zones particles with differ- gree) we can estimate that only about 10 wt% of the plati-

ent textures can be observed. In order to identify the nature num contributes to the diffraction peaks in the Pt-ASi12-r

of the particles visible on the B image, we recorded three (x) sample. By using the width of the (311) peak, which

EDX spectra, shown in Figure 11. The first (a) corresponds appears well isolated at 2θ ϭ 81.3°, we can estimate a size

to a homogeneous zone. We could establish the presence of of 40 nm. In fact, for a large size distribution, the mean

aluminium and silicon but not platinum. The second reveals value calculated on the basis of peak’s width is largely over-

platinum crystallites and, finally, the third (c) indicates a estimated since the small crystallites make very little contri-

large particle containing only aluminium atoms. The ap- bution and the peak profile is mainly influenced by the

[

27,29]

pearance of such particles can be explained by the presence large crystallites.

We can therefore assume that a large

of the metal precursor in the boehmite sol in the earlier part of the crystallites are small in size and only a small

stages of the synthesis. The competition of adsorption on other part of the better developed crystallites will be re-

boehmite crystallites between the platinum and silicon spec- sponsible for the diffraction signal.

ies leads to crystallites covered by more platinum and less

silicon. The X-ray analysis reveals that a small fraction of

platinum atoms is rather well dispersed over the sample but

the other fraction exists as isolated crystallites. In the same

way as observed for the supports, some zones with a silicon

agglomeration can be identified.

Figure 12. Diffraction patterns of the (1) Pt-ASi12-r (a) and (2) Pt-

2 3

ASi12-r (x) samples (the circles correspond to θ Al O , PDF:

23Ϫ1009)

2

Ϫ1

The specific surface areas of these catalysts are 68 m ·g

2

Ϫ1

for the xerogel and 120 m ·g for the aerogel. This is prac-

Figure 10. Two transmission electron microscopy images for the tically the same as in the supports and indicates no pore

sample SG Pt-BSi12-c (x)

blocking which is in agreement with the small sizes of most

of the crystallites.

For the aerogel, we observed similar behaviour with plati-

Transmission electron microscopy confirmed these re-

num isolated crystallites and α-Al O particles. The X-ray sults. For the aerogel, almost all the platinum particles are

2

3

mapping, performed on a zone around the α alumina par- smaller than 4 nm (Figure 13, left). The xerogel exhibits a

Figure 11. EDX spectra corresponding to different zones of the sample SG Pt-BSi12-c (x)

548

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Eur. J. Inorg. Chem. 2005, 543Ϫ554

Aerogel and Xerogel Catalysts Based on Doped θ-Alumina

FULL PAPER

larger size distribution with crystallites smaller than 4 nm Table 2. Comparison between average crystallite sizes determined

but with larger particles (Figure 13, right). Both histograms by transmission electron microscopy or by metal dispersion meas-

urements

are shown in Figure 14 as well as the average crystallite

sizes. From the metal dispersion measurements, an average

surface crystallite size can be determined (Table 2). Two

models may be used used, a cubic model with one face in

contact with the support and a hemispherical model. The

average sizes refer to the cube parameter or the diameter.

The results for the aerogel are in good agreement with the

average sizes determined from the electron microscopy his-

togram. For the xerogel, this agreement is poorer because

of the larger size distribution revealed by electron mi-

croscopy and X-ray diffraction (90% of the platinum ap-

pears as crystallites with average sizes of about 4 nm, i.e.

10% larger and visible by XRD). It is also possible that a

part of the metal crystallites, located in closed pores, is not

accessible for the hydrogen adsorption.^[

30,16]

C. Discussion

. Influence of the Preparation Mode by Impregnation

1

versus the One-Step Method (Sol-Gel)

For the samples prepared by the sol-gel method, the cal-

cination at 1200 °C induces an important sintering of the

metal phase, whatever the nature of the support (xerogel or

aerogel). The SG Pt-BSi12-c (a) sample is of particular

note, displaying a very large specific surface area of 130

m ·g . This important difference is due to the specific syn-

thetic conditions (simultaneous presence of silicon and

platinum species in the sol) as well as to the supercritical

drying which allows the retention of the gel morphology.

The metal dispersion is not related to the porosity of the

support because the mean size of the platinum crystallites

for the SG Pt-BSi12-c (a) sample is still greater than 50 nm.

Therefore, the metal dispersion is mainly determined by the

growth and sintering of the particles.

Figure 13. Transmission electron microscopy photos of the (left)

Pt-ASi12-r (a) and (right) Pt-ASi12-r (x) samples

2

Ϫ1

In contrast, the catalysts prepared by impregnation on

calcined alumina contain smaller metal particles because

sintering cannot occur during the reduction process. In this

case, the dispersion of the metal phase is directly dependent

on the specific surface of the support. A large value of the

surface area implies a large amount of adsorption centers

within the metal precursor (i.e. H PtCl ) and, consequently,

2

6

the formation of numerous centers for germination of plati-

num particles during the reduction. The catalysts supported

Figure 14. Histogram of the size repartition of the platinum crystal- on doped alumina also exhibit special characteristics, es-

lites for the (1) Pt-ASi12-r (a) and (2) Pt-ASi12-r (x) samples (Tv,

pecially for the aerogel sample Pt-ASi12-r (a), namely a

Ts: see Table 2)

2

Ϫ1

specific surface area higher than 120 m ·g after thermal

treatment, good metal dispersion (Ͼ 50%) and a homo-

geneous distribution of silicon and platinum atoms over the

sample. As a consequence, this sample could be tested as

an interesting catalytic material for applications requiring

1) high temperatures.

(

Nevertheless, it must be stated that the great difference

in dispersion between sol-gel and impregnated catalysts is

related to the temperature difference applied to the plati-

Eur. J. Inorg. Chem. 2005, 543Ϫ554

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549

A. F. Popa, L. Courth e´ oux, E. Gautron, S. Rossignol, C. Kappenstein

FULL PAPER

num precursor. For the sol-gel samples, the platinum pre- platinum content is also reported. The decomposition of

cursor is decomposed to platinum crystallites during the the HAN₇₉_%/water mixture alone appears at a temperature

high temperature treatment (1200 °C, 5 h) in an oxidising of 165 °C.

atmosphere leading to a strong sintering effect. In the other

All the supports (without the active phase) display a cata-

case, the impregnated sample is reduced at low tempera- lytic effect. This effect is more pronounced in the case of

tures (see Exp. Sect.) and the maximum temperature experi- the xerogel samples (106 °C for A (x) and 96 °C for ASi12

enced by the platinum particles is only 400 °C. To confirm (x)). For the aerogels, the decomposition temperatures are

the critical role of the temperature, the xerogel sample (Pt- higher and similar for each sample (140 °C). The very

ASi12-r (x)) was treated with the same high temperature strong difference in the surface area between nondoped and

conditions after reduction at 400 °C. A small decrease in Si-doped alumina seems to have a limited influence on this

2

Ϫ1

the surface area could be observed from 68 m ·g to 43 reaction. The introduction of platinum leads to a further

2

Ϫ1

m ·g , accompanied by a strong sintering of the platinum catalytic effect in the case of the impregnated catalysts. A

particles. The average size was between 4 nm and 30 nm further decrease in the decomposition temperature, in com-

before treatment and 40 nm after treatment. It must be said parison with the support, is of the order of 25 °C except

that the real catalytic conditions used will not be so drastic, for the Si-doped aerogel which display a decrease of 60 °C.

the high temperatures being reached over very short time For the sol-gel catalysts, the effect of platinum is much

ranges.

more limited.

With regard to the supports, the effect of the surface area

remains very limited for the catalysts except for the impreg-

nated aerogel samples Pt-ASi12-r (x) which display an im-

portant decomposition temperature decrease (from 125 °C

2

. Influence of the Drying Mode: Xerogel versus Aerogel

The main advantage of the supercritical drying is the pos-

sibility of maintaining the microporosity of the wet gel and

obtaining materials with high specific surfaces and porosit-

ies. Indeed, the specific surface area of the aerogel supports

and catalysts is about 120 m ·g even after a thermal treat-

ment at 1200 °C (5 h). By comparison, the xerogel catalysts

reveal a specific surface area respectively for the sol-gel, 41

2

Ϫ1

to 85 °C) related to the surface area increase (1 m ·g to

2

Ϫ1

6

8 m ·g ). For both samples the platinum is of the same

2

Ϫ1

order. The higher decomposition temperatures of the sol-

gel catalysts compared with the impregnated catalysts are

mainly due to the platinum particles sintering during the

calcinations leading to a very low platinum accessibility.

This effect is clearly observed for the samples Pt-ASi12-r

(x) and SG-Pt-BSi12-c (x) which have the same effective

metal content (3.5%) but exhibit a decomposition tempera-

ture difference of 20 °C. This indicates the importance of

the preparation mode.

2

Ϫ1

2

Ϫ1

m ·g and for the impregnated, 68 m ·g . Those values

are related to the high porosity of the aerogel materials

3

Ϫ1

3

Ϫ1

(0.24 cm ·g for the doped alumina and only 0.16 cm ·g

for the corresponding xerogel). An other important aspect

is the platinum size distribution which is shorter for the

catalyst prepared by impregnation on doped aerogel than

for the xerogel catalyst. Furthermore, the platinum particles

are more homogeneously distributed on the aerogel sup-

port. All these remarks display the importance of the drying

mode for the final properties of the materials.

The comparison between aerogel and xerogel supports

and catalysts shows that the xerogel samples are always

more efficient (Figure 15). A possible explanation for this

difference can be related to the higher insulation properties

[

31]

of the aerogels.

Due to its lower thermal conductivity

and higher porosity, the temperature within the aerogel is

lower than the measured crucible temperature, leading to a

temperature difference higher than for xerogels and thus to

a higher measured decomposition temperature. Moreover,

when the exothermic decomposition starts on the external

surface of the catalyst powder, the release energy propagates

more slowly to the centre of the aerogel powder resulting in

a delay in the decomposition reaction of the monopropell-

ant.

D. Catalytic Tests

The decomposition temperatures of the HAN/water mix-

tures, with and without the presence of supports or catalysts

are reported in Figure 15. For the catalysts, the effective

Catalysts supported on silicon-doped alumina are pro-

mising for high temperature applications. Moreover, xerogel

catalysts have been revealed to be better than aerogel cata-

lysts despite the heterogeneous dispersion of silicon. To

confirm this hypothesis, studies must be carried out on the

decomposition characteristics, i.e. the temperature increase,

pressure increase and ignition delay.

Conclusion

Figure 15. Decomposition temperatures of HAN/water mixtures

with and without supports or catalysts and metal content for aero-

gels and xerogels materials

We have synthesised sol-gel materials based on pure or

doped alumina using classical and supercritical drying con-

550

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Eur. J. Inorg. Chem. 2005, 543Ϫ554

Aerogel and Xerogel Catalysts Based on Doped θ-Alumina

FULL PAPER

ditions. For the supports, the morphology, thermal stability, stirred at 60 °C for 1 h. Concentrated hydrochloric acid (0.28 mL,

Ϫ3

Ϫ1

3

(

.3 10

mol, 12 mol·L ) and, if necessary, tetraethoxysilane

homogeneity and the structure depend strongly on the dry-

ing technique used. The ASi12 sample displays a high

parosity and it was selected as support of catalyst.

A general characteristic of all catalysts is that a fraction

of the platinum is lost during the preparation of the cata-

lysts and the real metal content is between 47 and 71% of

the calculated value (5 wt.-%).

Ϫ3

0.51 mL, 2.24 10 mol, 98%, Aldrich) as a doping precursor were

then added and the temperature was raised and maintained at 80

C for 2 h.

The xerogel samples were obtained after drying overnight at 120

C in a ventilated oven. For the aerogel samples, about 75% of the

water content evaporated at 80 °C over 3 h affording a viscous sol

which was poured in the drying mould (stainless steel, d ϭ 3.6 cm,

°

The comparison between the aerogel and xerogel samples h ϭ 3.5 cm). A 5-mm hole in the bottom of the mould enables the

displays, respectively, a fine distribution on the surface and solvent exchange. After 15 h, the solvent exchange started: water

a heterogeneous distribution of the metallic phase. This fea- was replaced first by acetone (complete miscibility) and then ace-

2

tone was replaced by CO just before the supercritical drying. The

ture can be linked to the larger specific surface area of the

aerogels.

The introduction mode of platinum by impregnation

leads to smaller particles which are well dispersed. On the

other hand, the sol-gel method results in the formation of

large particles.

solvent exchange procedure was as follows: the wet gel was covered

with acetone (about 25 mL) and the solvent was evacuated through

the bottom hole before the addition of acetone (25 mL, three times

a day over 5 days). The mould was then introduced into the auto-

clave (300 mL, Parr Instruments) and acetone (100 mL) was added.

The autoclave was equipped with two entries (liquid CO

2

and ar-

Finely, from our evaluation experiments on HAN solu-

gon) and two outflows (gas and liquid) and the temperature and

tions, the best catalytic activity is displayed by platinum the pressure can be controlled as shown in Figure 17.

catalysts prepared by impregnation on Si-doped xerogel al- The reactor was filled with liquid carbon dioxide using several fill-

umina.

ing and purging steps in order to quantitatively evacuate the ace-

tone and the air contained in the autoclave. Upon completely filling

the autoclave with liquid CO

tained between the bottle (room temperature) and the autoclave

water cooling system). The temperature difference (about 4Ϫ5 °C)

was sufficient to push liquid CO into the reactor because the press-

2

, a temperature difference was ob-

Experimental Section

(

2

A. Preparations

ure in the bottle (55 bar at 20 °C) exceeded the equilibrium pressure

in the reactor (49 bar at 14 °C). After complete filling, the pressure

was about 55 bar (Figure 18, point 1). The reactor was isolated and

the temperature increased from 15 °C to 40 °C which led to an

important pressure increase (step 1Ϫ2) up to 140Ϫ150 bar (inside

the supercritical domain). After 1 h, the supercritical fluid was

evacuated over 1 h at the same temperature and the pressure was

decreased to 1 bar (step 2Ϫ3). The heating was then turned off and

argon was flushed through the reactor (step 3Ϫ4). The aerogel was

then taken out of the reactor.

a) Support: Pure Alumina and Doped Alumina

The preparation procedure for pure alumina and Si-doped alumina

2 3 2

[(Al O )0.88(SiO )0.12] by the sol gel method has been described in

[6]

previous work and is shown in Figure 16. The ASi12 label for

this material (Table 1) indicates the final molar composition (after

drying and firing) as SiO /(SiO ϩAl O ) ϭ 0.12. The silica weight

2 2 2 3

percent in the dried sample is 7.4%, corresponding to an atomic

ratio Si:Al ϭ 6.3:93.7.

Pure boehmites and Si-doped boehmites (aerogels and xerogels)

Ϫ1

were calcined at 1200 °C under air for 5 h (ramp of 5 °C·min ) to

obtain pure or doped aluminas.

(

2)

b) Supported Platinum Catalysts (5 wt.-%)

The catalysts were prepared by two procedures: (i) a one step pro-

cedure using the sol-gel method or (ii) an impregnation procedure

on a calcined support (pure or doped alumina).

2 6

Sol-Gel Procedure: The platinum precursor solution (H PtCl ) was

introduced during the preparation of the support in the sol with

the hydrochloric acid (see Figure 16). The preparation procedure

follows the same steps used for the support (drying, calcination at

1

200 °C). The high temperature calcination induces the formation

[1]

of metal by a redox reaction:

(

3)

Figure 16. Scheme of the sol-gel preparation procedure

To prepare 2 g of support, aluminium tri-sec-butoxide (8.48 g,

Impregnation Procedure: A volume of the metallic solution

PtCl ) corresponding to the porous volume of the support

0.0334 mol, 97%, Aldrich) and pure water (60.1 g, 3.34 mol) were (H

2

6

Eur. J. Inorg. Chem. 2005, 543Ϫ554

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551

A. F. Popa, L. Courth e´ oux, E. Gautron, S. Rossignol, C. Kappenstein

FULL PAPER

Figure 17. Diagram of the supercritical drying apparatus

Figure 18. Diagram of the pressure and temperature for supercritical drying

(

1.5 mL·g^Ϫ¹) was added to the alumina (wetness impregnation).

B. Characterisations

The concentration of the solution was then calculated giving a me-

tal content of 5 wt.-%. The catalysts were dried overnight at 120

The effective metal load was determined from the CNRS analyses.

The specific surface areas of the samples were determined by the

BET method from the nitrogen adsorption isotherms at Ϫ196 °C

in an automated Micromeritics ASAP 2000 apparatus after evacu-

ation for 1 h 30 min at 350 °C. The method used is a simple BET

Ϫ1

°

C, calcined at 400 °C for 3 h under air (20 mL·min ) and reduced

Ϫ1

under H

2

(20 mL·min ) for 1 h at 200 °C and 2 h at 400 °C to

obtain platinum supported alumina.

7

0

points procedure with P/P values between 0.05 and 0.25. The

(

4)

complete adsorption and desorption isotherm was obtained for a

few samples leading to the porous volume values.

Whatever the procedure, the introduction of the metallic precursor

did not affect the acidity (pH near 1).

Differential thermal analyses (DTA) and thermogravimetric analy-

ses (TGA) were recorded in order to characterise the supports after

552

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Eur. J. Inorg. Chem. 2005, 543Ϫ554

Aerogel and Xerogel Catalysts Based on Doped θ-Alumina

FULL PAPER

drying. TDA-TGA experiments were carried out between 25 and

due to the high exothermicity of the decomposition which destroys

a small amount of catalyst.

1

350 °C using a thermal analyst 2100 TA instrument with platinum

crucibles. The samples were heated in nitrogen flow

100 mL·min^Ϫ¹) with a ramp of 10 °C·min^Ϫ¹

a

(

.

Diffraction patterns and particles sizes were determined by X-ray

diffraction (XRD) experiments performed on a Siemens D 5005

powder θ-θ diffractometer using the Cu-Kα radiation (λKα

ϭ

0.15186 nm) and a graphite monochromator. The diffraction pat-

terns were obtained under the following conditions: dwell time: 2

s, step: 0.04°, constant divergence slit: 1°.

Crystalline phases were identified by comparison with PDF stand-

ards (Powder Diffraction Files) from ICDD. (AlOOH: 21Ϫ1307,

2 3 2 3

α-Al O : 46-1212, θ-Al O : 23Ϫ1009, Pt: 04Ϫ0802)

The average crystallite sizes (D) were determined from the Scherrer

equation: D ϭ (0.9·λ)/(B·cos θ), λ ϭ 0.15186 nm, B is the width at

half maximum and θ the Bragg angle for the peak in question (rad).

For boehmite, the peak (020) was used without any correction due

to the very small size of the particles. For platinum, the peaks (311)

and (222) were used and a width correction was introduced, i.e.

2

2 1/2

B

cor ϭ (Bexp Ϫ Binst

)

. To determine Binst, the analysis of stand-

ard LaB

6

powder was performed under the same conditions. The

Figure 19. Example of the catalytic decomposition temperature of

a HAN/water mixture on Pt-ASi-r (x)

widths of the peaks were obtained by profile analyses using a

pseudovoigt function from the diffract AT software (socabim,

France).

The structural pictures were obtained using the software CaRine

[

32]

Crystallography.

The platinum dispersion was obtained by hydrogen chemisorption

at room temperature with a lab-build apparatus using pressure

measurements. The samples (300 to 500 mg) were reduced at 400

Acknowledgments

The authors thank all the people working in the ‘‘service de Mesure

Physique’’ of the LACCO.

°

C over 1 h under a static pressure of hydrogen, degassed at this

temperature over 1 h and cooled to ambient temperature. The ad-

sorption isotherms enabled the determination of reversibly and ir-

reversibly adsorbed hydrogen. The dispersion was calculated using

the stoichiometric ratio H/Pt ϭ 1.^[

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at 1.486 keV (Al) and Kα at 1.739 keV (Si), respectively.

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Ϫ1

under an argon flow (100 cm ·min ). An example of the catalytic

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[

18]

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2

Received July 28, 2004

Early View Article

Published Online December 6, 2004

9

554

www.eurjic.org

Eur. J. Inorg. Chem. 2005, 543Ϫ554

Article Doi

Aerogel and xerogel catalysts based on θ-alumina doped with silicon for high temperature reactions

DOI: 10.1002/ejic.200400657

Source and publish data:

Authors:

Article abstract of DOI:10.1002/ejic.200400657

Full text of DOI:10.1002/ejic.200400657

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