Processing of catalysts by atomic layer epitaxy: modification of supports

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Applied Surface Science 121 / 122 (1997) 286-291

Processing of catalysts by atomic layer epitaxy: modification of supports Marina Lindblad

a,*,

Suvi Haukka a, Arla KytiSkivi a, Eeva-Liisa Lakomaa Aimo Rautiainen a, Tuomo Suntola a

b,l,

a Microchemistrv Ltd., P.O.Box 45, FIN-0215I Espoo, Finland b Neste Oy, P.O. Box 310, FIN-06101 Porvoo, Finland

Received I November 1996; accepted 12 February 1997

Abstract

Different supports were modified with titania, zirconia and chromia by the atomic layer epitaxy technique (ALE). In ALE, a metal precursor is bound to the support in saturating gas-solid reactions. Surface oxides are grown by alternating reactions of the metal precursor and an oxidizing agent. Growth mechanisms differ depending on the precursor-support pair and the processing conditions. In this work, the influences of the support, precursor and reaction temperature were investigated by comparing the growth of titania from T i ( O C H ( C H 3 ) 2 ) 4 on silica and alumina, titania from TiC14 and T i ( O C H ( C H 3 ) 2 ) 4 on silica, and zirconia from ZrCl 4 on silica and alumina. The modification of porous oxides supported on metal substrates (monoliths) was demonstrated for the growth of chromia from Cr(acac) 3. © 1997 Elsevier Science B.V. Keywords: Atomic layer epitaxy (ALE); Catalyst preparation; Surface modification; Gas-solid reactions: Surface saturation

1. Introduction

The preparation of well-defined active species has been central to the development of catalysts with improved properties. In addition to the effective deposition of an active metal component, improved catalytic activities, selectivities and stabilities often require a modification of the support material. One interesting approach to the preparation of tailored surface structures is chemical vapor deposition (CVD) (see e.g. [1-8]). We have adopted a slightly different approach, making use of saturating g a s -

* Corresponding author. Tel.: +358-204-505705; fax: +358204-505700; e-mail: [email protected]. Fax: +358-204-507113.

solid reactions in a technique known as atomic layer epitaxy (ALE) [9-11]. Already, catalysts active for a variety of reactions have been processed by ALE [12-14]. ALE has also been used in the controlled growth of metal oxides on alumina and silica supports as a means of modifying the chemical properties of the support surface [15-20]. Thorough knowledge of the mechanism of interaction between the precursor and support is needed for the preparation of well-defined surface structures. In ALE, saturating surface reactions are used to bind the precursor to the support. The chemistry of the precursor-support pair then controls the build-up of the surface structures and the saturation density of the surface species. A prerequisite for this self-controlling behavior is that the reaction temperature is

0169-4332/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S 0 1 6 9 - 4 3 3 2 ( 9 7 ) 0 0 3 0 7 - 3

M. Lindblad et al. / Applied Surface Science 121 / 122 (1997) 286-291

such that no decomposition or condensation of the precursor occurs. The strict conditions of ALE processing facilitate the study of interaction mechanisms and allow the build-up of predictable structures. The purpose of this paper is to show how the support, precursor and processing conditions affect the growth mechanisms of surface oxides in ALE. Our examples are the growth of zirconia and titania on alumina and silica and the growth of chromia on alumina and metal supported oxides.

2. Experimental Support materials in powder form were ",/-AI203 (Akzo Alumina 000-1.5E) and silica (Crosfleld EP 10 or Grace Davison) with surface areas of 190 and 300 m2/g, respectively, and pore volumes of 0.5 for y-Al203 and 1.75 or 1.6 cm3/g for silica. Metallic monoliths coated with alumina and an alumina based mixed oxide were obtained from Kemira Metalkat Oy. Alumina was supported on laboratory-scale monoliths ( 0 15 ram, length 75 mm) and the mixed oxide on larger-scale monoliths ( 0 95 mm, length 150 mm). ZrCl 4 (Fluka), T i ( O C H ( C H 3 ) 2 ) 4 ( = Ti(OPri)4, Merck), TiCI 4 (Merck), and Cr(acac) 3 (Riedel-de Haen) were used as reactants without further purification. The catalysts were processed in flow-type reactors with heated zones for the reaction chamber and for vaporizing solid and liquid reagents. Processing was done at a pressure of 5-100 kPa in nitrogen atmosphere. The processing procedure has been described in more detail earlier [10,11]. Table 1 summarizes

287

the processing parameters selected for the different metal precursor and support pairs in the growth of surface oxides. The vaporized reactants were led through the solid support bed (5-12 g) or monolith (coated with 2 or 200 g of the porous oxides) and stabilized at the selected reaction temperature. The amount of vaporized reactants was kept high enough to ensure saturation of the surface. Water or synthetic air was used as ligand removal agent, leaving the metal in oxide form. Increasing metal oxide concentrations were obtained by repeating the reaction cycle of metal compound binding and ligand removal. To remove unreacted reactants, each reaction step was followed by a nitrogen purge at the reaction temperature. Metal concentrations were determined by XRF (Zr), instrumental neutron activation analysis, INAA (Zr, Ti, Cr) and U V / V I S spectrophotometry (Ti). Carbon was determined with a Leco CR 12 carbon analyser and chloride by potentiometric titration. Some of the samples have also been characterized in more detail by BET, XRD, SEM-EDS, FTIR, OH MAS NMR and LEIS measurements, as described in previous papers [ 17,18].

3. Results and discussion In the following, we first discuss our investigation of interaction mechanisms for different precursors and supports as a function of reaction temperature. We then briefly survey the factors influencing the growth rates. We have earlier shown that the success of the ALE technique is independent of the shape of

Table 1 Support preheating, precursor reaction and ligand removal temperatures used for different precursor-support pairs in the growth of surface oxides Precursor

Support

Support preheating (°C)

Precursor reaction (°C)

Ligand removal (°C)

ZrCI 4 ZrC14 Ti(OPr i )4 Ti(OPri)4 TiCl 4 Cffacac) 3 Cr(acac) 3

AI 203 S i02 A12O 3 SiO2 SiO 2 A1203 ~ mixed oxide b

600 300 600 450 450 600 600

300 300 100 160 450 200 200

H 20 / 6 0 0 H: 0 / 3 0 0 air/450 air/450 H 20/450 air/600 --

Alumina in powder form or supported on a laboratory-scale metallic monolith. Alumina based mixed oxide supported on a large-scale metallic monolith.

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the support, i.e. homogeneous distributions have been obtained on powders as well as on extrudates [11]. This was now confirmed for supports in monolith form.

3.1. Interaction mechanisms precursor-support pairs

for

different

(a)

4

~ 3

c k- 2

3.1.1. TiO 2 grown on silica and alumina from cycles of Ti(OPri)4 and air TiO 2 was grown on alumina and silica by sequential reactions of vaporized titanium isopropoxide, Ti(OPrS)~, and air. To study the interaction of Ti(OPr~)4 with these supports, the surface density of titanium and the C / T i atomic ratio were determined for the binding of Ti(OPr~)4 on the pure supports at various reaction temperatures (Fig. 1). The results revealed very different behavior of Ti(OPri)4 on alumina and silica. Whereas both quantities remained constant with increasing temperature on silica, they changed dramatically on alumina, suggesting a more active participation of alumina in the interaction. The increase in the density of titanium and the simultaneous decrease in the C / T i ratio on alumina were earlier observed to be due to the decomposition of Ti(OPrl)4 [11]. According to XRD measurements, however, the reaction of Ti(OPr~)4 on alumina in the temperature range 100-200°C gave rise to well-dispersed titanium oxide species. The XRD spectrum of the sample prepared at 200°C containing 6 a t / n m 2 titanium was identical with the spectrum of the pure support, indicating that no crystalline or amorphous agglomerates were present on the surface. At such high titanium concentrations, any amorphous agglomerates would have been seen as a very broad signal or at least as a lift in the baseline, The good dispersion, the thermal stability of the precursor at the temperatures used [21], and the fact that no decomposition was observed on silica rules out the possibility that the decomposition occurred in the gas phase. The decomposition of Ti(OPri)4 on alumina is thus proposed to be a surface-induced oligomerization of adsorbed Ti(OPrS)x species, forming a network of T i - O - T i bridged surface species, with a simultaneous release of hydrocarbon products into the vapor phase. Further oligomerization may occur during the subsequent air treatment through dehydroxylation of the Ti-OH groups formed.

50

100

150

200

250

Reaction temperature (°C)

._o

~ 6

._

o

o

o

o

o o •

o

o

5 4 3 2 1 50

100

150

200

250

Reaction temperature (°C)

Fig. 1. Effect of reaction temperature on (a) Ti concentration and (b) C/Ti atomic ratio, when Ti(OPri)4 is bound to alumina (O) and silica (©).

On silica, surface species formed after the first Ti(OPri)4 reaction probably are isolated ( - S i - O - ) 2 Ti(OPrS)2 groups, which would not oligomerize into bridged networks until the air treatment. For the further growth of TiO 2 on alumina and silica, reaction temperatures of 100 and 160°C, respectively, were selected for the Ti(OPri)4 reaction. At these temperatures, similar surface densities of Ti and similar C / T i ratios were obtained in the reaction with the pure supports. The growth rates were similar on the two supports, resulting in surface densities of about 5 T i / n m 2 in five reaction cycles. After five reaction cycles on alumina, no TiO 2 crystallites were observed by XRD. The amorphous surface species were thermally stable up to 800°C, suggesting a

M. Lindblad et al./ Applied Surface Science 121 / 122 (1997) 286-291

strong interaction with the support which prevented the formation of crystalline phases. When TiO 2 was grown on silica, a small TiO2-anatase signal was observed by XRD after four reaction cycles. Probably this was due to a weaker interaction with silica, which allowed the TiO, surface species to agglomerate during the air treatments carried out at 450°C.

/o.

H

H

289 H

[

J

CLz;Cl l

•~ •~b.-

•

Ti(OPr)4

e

TiCI4

2

0 1

2

3

4

5

Number of reaction cycles

Fig. 2. Increase in Ti concentration with n u m b e r of reaction cycles w h e n Ti(OPri)4 and TiC14 are used as precursors.

H

+ ZrCI4

3.1.2. TiO 2 grown on silica f r o m cycles o f TiCI 4 and water or Ti(OPri)4 and air

Different surface structures are obtained from TiC14 and Ti(OPrl)4 precursors in the growth of TiO 2 on silica. TiCI 4 reacts in a similar way to ZrC14 (see Section 3.1.3), i.e. through exchange reactions with OH groups forming isolated TiCI x species or through direct chlorination of the surface with simultaneous formation of plate-like crystalline agglomerates [17,18]. Reaction temperatures as high as 550°C have been used with TiC14, but for the thermally less stable Ti(OPri)4 the highest temperature used was 200°C. As proposed in the preceding section, the interaction of Ti(OPr~)4 with silica in the reaction temperature interval 100-200°C probably proceeds through isolated titanium surface species. Fig. 2 shows the growth rate of mainly crystalline TiO 2 from reaction cycles of TiCI 4 and water carried out at 450°C [19], and the growth rate of mainly amorphous TiO 2 from reaction cycles of Ti(OPri)4 and air carried out at 160 and 450°C, respectively. A corresponding controlled formation of crystalline TiO 2 from surface reactions of Ti(OPri)4 is probably not possible as the precursor will decompose thermally at < 350°C, i.e. at temperatures lower than the formation of crystalline TiO 2 phases requires. The

H

? ? 9 /o~ ? ?

J + H20

H

H

O,z~,O

[

J

Fig. 3. Schematic representation of b o n d i n g sites on a l u m i n a and silica and of zirconium surface species during the first reaction cycle of ZrCI 4 and water vapor.

surface densities of titanium obtained in the first reaction cycle were 1.0 and 1.4 T i / n m 2 with TiC14 and Ti(OPri)4, respectively. But then, even though the ligand removal temperatures were the same, the growth rate obtained with TiC14 became markedly slower, perhaps due to only minor participation of the crystalline agglomerates in the further growth of the oxide. SEM investigations have shown the agglomerates to grow laterally with increasing number of reaction cycles. 3.1.3. ZrO 2 grown on silica and alumina f r o m cycles o f ZrCl 4 and water

The growth of ZrO 2 at different reaction temperatures demonstrates the influence of the processing conditions on growth mechanisms. The growth mechanism of ZrO 2 on alumina and silica in sequential saturating reactions of ZrCI 4 and water vapor has already been reported [15,16]. Fig. 3 summarizes the proposed surface species on the support surface after the first ZrC14 and water reactions. In the reaction of ZrC14 at 300°C, isolated ZrC1 x species were formed as main surface species in the exchange reaction with OH groups on the surface. At higher reaction temperatures, direct chlorination of the surface was facilitated leading to the formation of crystalline ZrO 2 agglomerates. The extent of agglomeration was restricted by the amount of OH groups on the surface. After water treatment the

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zirconium surface species consisted of Zr-OH groups formed in the hydrolysis of isolated ZrC1x species and Z r O 2 agglomerates. Chlorinated OH groups of the original supports were also released. Both exchange reactions and agglomeration are possible during subsequent reaction cycles. The extent of agglomeration can be controlled by the reaction temperature. When five reaction cycles of ZrCl 4 and water vapor were carried out on silica and alumina, an average density of 5 Z r / n m 2 was achieved. This corresponds to or exceeds the estimated monolayer density of ZrO 2. However, due to the formation of agglomerates the surface coverage was incomplete. Characterization by LEIS showed that only 50% of the silica surface was covered by the modifying oxide.

3.1.4. Chromia grown on alumina from cycles of Cr(acac) 3 and air ALE reaction cycles of Cr(acac) 3 and air were earlier used to grow chromia on alumina powders [14]; here growth was on alumina supported on a laboratory scale metallic monolith. The growth rate was linear in both cases, with a saturation level of 1.3 wt% on the powder and 0.8 wt% on the monolith in each cycle. The different metal concentrations were due to the different surface properties of the aluminas. The homogeneity of the distribution of chromium was tested by binding Cr(acac) 3 on a large-scale monolith coated with an alumina based mixed oxide. Samples were taken at three different radial positions and two different axial positions of the monolith and these, totally six samples, gave an average of 1.0 + 0.1 wt% chromium. To our knowledge, this is the first time metal compounds have successfully been introduced from vapor phase onto supports in the form of monoliths. 3.2. Factors influencing growth rates When support surfaces are modified by repeated reaction cycles, the relative numbers of different surface sites change with each cycle and thus a linear growth rate would not necessarily be expected. Linear rates have, nevertheless, been observed in the growth of NiO from Ni(acac) 2 and chromia from

Cr(acac) 3 on alumina [13,14]. In these cases the steric hindrance of the acac ligands, and not the limited number of bonding sites, determined the saturation density. OH groups of the original support or the modifying oxide are the most frequently used bonding sites. The number of OH groups depends on the extent of dehydroxylation, i.e. the temperature used in ligand removal. In addition to dehydroxylation, the number of available bonding sites may be restricted by an ineffective ligand removal. Even small ligand residues may interfere with the reaction of the precursor with the surface, either through sterical hindrance or by occupying part of the bonding sites. Often a slightly curvilinear growth rate is obtained with repeated reaction cycles, reflecting the gradual changes in the surface sites.

4. Conclusions Various metal oxides have successfully been deposited on alumina and silica supports by ALE. Knowledge of the interaction between the precursor and support has facilitated the choice of appropriate processing conditions to obtain a specific surface structure. The growth of metal oxides depends on the mechanism of interaction between the precursor and support and the conditions selected for the ligand removal reaction. As shown here for the TiCI 4 and ZrC14 systems, the reaction temperature has a significant effect on the relative amounts of amorphous and crystalline surface species. This in turn determines the properties of the modified surface. Whereas the reactions of ZrC14 were about the same on alumina and silica, this was not the case for Ti(OPri)4: oligomerization of Ti(OPri)~ species increased with reaction temperature on alumina, but not on silica. Through careful choice of processing conditions, similar growth of TiO 2 was nevertheless achieved on the two supports. Different surface structures were formed when TiC14 and Ti(OPri)4 were selected as precursor for the growth of TiO 2 on silica. The growth rate was independent of the shape of the support as shown for the growth of chromia on alumina in powder and monolith forms. The favorable features of ALE - - providing homoge-

M. Lindblad et al./ Applied Surface Science 121 / 122 (1997) 286 291

neous distributions and permitting easy scale-up - were demonstrated for the first time for the growth of chromia on a large-scale monolith.

Acknowledgements P~iivi Jokimies, Petri Laiho and Mirja Rissanen are thanked for the ALE processing. The Catalyst Research Group of Kemira Metalkat Oy is thanked for providing the monoliths, and the:Department of Analytical Research of the Neste Co. and the Technical Research Centre of Finland are thanked for the analysis of the samples.

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