Chapter 7
SUMMARY
This study was directed towards the development of the heat resistant catalyst material for high temperature catalytic com
bustion. In particular, as mentioned in the Introductory Survey (Chapter 1), the present study has been made in order to study following two problems for the practjcal application of combus
tion catalysts: ( 1) the d0velopment of large surface area sup
ports with excellent thermal stability, and (2) the catalyst design suitable for catalytic activity and thermal stability.
From the viewpoint of materials design, these problems contain two types of trade-off relations between large surface area and sinterability, and between catalytic activity and slnterability.
As to the first one of Lhcsc problem, the results of inves-tigation were divided into mate rials, their preparation and their
characterization as summarized in Chapters 2, 3 and 4, respec
tively. The material screening in Chapter 2 clearly revealed that a large surface area was always retained at high temperatures when the additives to alumina lead to the formation of hexaalumi-nate structure. This is a new type of stabilizing effect of alumina support and, without doubt, has made valuable contribu
t ions to the materjal design of catalysts submitted to various high-temperature processes. Thermal stability of hexaaluminate is much higher than those of conventional support oxides. For in-stance, the surface area of ca. 11 m2/g after calcination at
1600 � can not be attained so far. This is because the hexaalu-minate is an equilibrium crystal phase at each compositions,
- 123
-Chapter 7
whereas any other alumina-additive systems reported so far are in metastable states.
Since the hexaaluminate phase is the origin of the large surface area retention, the preparation of fine particles of this phase was subsequently studied in Chapter 3. The Large surface area hexaaluminate was obtained by using hydrolyzed alkoxides as a precursor, being one of the most successful example of the sol-gel preparation of ceramic materials. Local analysis by analytical electron microscopy was employed on the solid state reaction mechanism to elucidate the increase in surface area.
The result demonstrated that excellent features of the alkoxide process, i.e., high chemical homogeneity at a nanometer level and small particle size of precursors, promote the formation of hexaaluminate at low temperatures (ca.llOO �). The effect of preparation conditions in the alkoxide process was also discussed in this Chapter. Especially, the amount of water added to alkox
ide affected the surface area of resultant hexaaluminate. This is because the chemic al structure of hydrolyze d p recursors i s strongly dependent o n the water/alkoxide ratio. Homogeneous
mixing of constituent cations at an intraparticle level seems to play a key role in deriving the large surface area hexaaluminate.
In Chapter 4, characterization of hexaaluminate particles was examined to clarify the crystallographjc origin of large sur-face area retention. High resolution electron microscopy re-vealed the planar morphology of hcxaaluminate of which facets are correctly normal to c axis. This is the result of anisotropic crystal growth normal to c axis and such a large aspect ratio particle is quite effective in maintaining a large specific sur
face area. The anisotropic crystal growth corresponds to the difference of surface microstructure of basal and side plane of
124
-Summary
planar particles and to the anisotropic solid state oxygen-diffu
sion in hexaaluminate. For the anisotropic diffusion, secondary ion mass spectroscope analysis showed that monoatomic layers between spinel blocks of hexaaluminate is the preferential mass transport and enhance anisotropic crystal growth in the sintering process. Of course, the result of these crystallographic charac
terization is generally applicable not only to hexaaluminate
related compounds but also to other isomorphs, which show large surface area retention. In other words, the present study has also made valuable contributions to elucidation of structure
crystal growth relationship in anisotropic compounds, which was interested in the area of solid state chemistry.
For the development of a heat resistant catalyst, which is the second object of this study, the catalyst design based on the hexaaluminate structure was investigated in Chapter 5. Catalytic activity and thermal stability were not achieved simultaneously by conventional impregnated catalysts due to sinterlng and so id state reaction. On the other hands, cation-substitution, proposed in this Chapter, is effective in introducing active sp cles into the hexaaluminate structure without decreasing the thermal sta
bility. Deactivation due to solid state reaction can be also neglected in such a single phase catalyst. Thus, the concept of
cation-substituted hexaaluminates is expected to be suitable for design of high temperature catalyst. This chapter also presents the first description on the catalytic property of hexaaluminate compounds. The catalytic activity is closely related to the reduction-oxidation property of substituents in the hexaaluminate lattice. Therefore, the catalytic activJty of cation-substituted hexaaluminate can be explained by the heat of formation of subsi
tituent oxides. Among the transition elements in the first row,
125
-Chapter 7
manganese, which plays the reversible reduction-oxidation cycle, showed the highest catalytic activity in the hexaaluminate lat-tice.
The latter half part of this Chapter was devoted to further improvement of catalytic activity of manganese-substituted hexaa
luminate by the structural modification. In particular, mixing cations in the mirror plane significantly enhanced the surface area and the catalytic activity as shown in the Srl-xLaxMn
Al11o19_a system. The increase in the surface area by Sr-substi
tution results from the decrease in aspect ratio of hexaaluminate facets. The substitution also increases the oxidation state of Mn species and thus enhances the amount of oxygen desorption at the reaction temperatures. These results of first systematic study on the catalytic property of hexaaluminate compounds will provide the valuable knowledge on controlling their catalytic properties.
The val i d i ty of the catalyst design proposed in this study was confirmed by the catalytic reaction under combustor operation conditions in Chapter 6. The honeycomb-shaped hexaalum1nate cata
lyst showed the excellent performance for high-temperature uses, which could not be achjeved by conventional catalyst materials.
It is easily understood that the results of the present study and the proposed catalyst design are very valuable for the develop
ment of catalytic combustion.
Consequently, the present study includes valuable knowledge on development of a new mate rials for catalytic combustion effective for low NOx emi ss ions. The author believe that the result of this study make a very valuable contribution not onlY to a progress in solid state chemistry but also to energy indus
tries and, finally, the protection of global environment.
126
-Acknowledgment
The author wishes to express grateful acknowledgment to professor H. /\rai for continuj ng instruction and encouragement throughout the present work. The author is grateful to professor A.Kato, professor N.Yamazoe, and professo r K.Morinaga for pre
cious suggestion.
The author thanks Dr. K.Eguchi for valuable discussion over ye ar s . Gr a te f u l t h ank s a r e ex ten d e d t o D r . H. H a n eda a n d Dr.Y.Shimizu for the help in SIMS measurement. Thanks are also d ue to Mr. H.Ka wasaki and Mr.T.Shiomitsu for their val uable experimental assistance. The experiment described in Chapter 6 was partially supported by Catalysts and Chemicals, Inc., far East, and Toshiba Corp.