In Situ Processing of a Porous Calcium Zirconate/Magnesia Composite with Platinum Nanodispersion and Its Influence on Nitric Oxide Decomposition

Article abstract of DOI:10.1111/j.1151-2916.2001.tb01079.x

A porous CaZrO₃/MgO composite with ∞1% nanodispersed platinum was synthesized in air using several in situ reactions, including the pyrolysis of PtO₂. The composite had a uniformly open, three-dimensional pore structure (porosity of 56%), with a narrow pore-size distribution. Catalytic NO decomposition to N₂ and O₂ and NO reduction by C₂H₄ were investigated up to 900°C. In the absence of oxygen, the NO conversion rate reached ～52% for the direct decomposition and ～100% for the reduction by C₂H₄. The results suggested the possibility of the porous composite as a multifunctional filter for simultaneous filtering of hot gas and removal of NO_x.

Full text of DOI:10.1111/j.1151-2916.2001.tb01079.x

J. Am. Ceram. Soc., 84 [11] 2713–15 (2001)
journal
In Situ Processing of a Porous Calcium Zirconate/Magnesia Composite with
Platinum Nanodispersion and Its Influence on
Nitric Oxide Decomposition
Yoshikazu Suzuki,* Hae Jin Hwang,* Naoki Kondo,* and Tatsuki Ohji*
†
Synergy Materials Research Center, National Institute of Advanced Industrial Science and Technology (AIST),
Nagoya 463-8687, Japan
A porous CaZrO /MgO composite with ϳ1% nanodispersed
platinum was synthesized in air using several in situ reactions,
PtO during in situ processing. PtO can thermally decompose to
2 2
metallic platinum under ordinary air atmosphere without emission
of toxic gas species.
3
including the pyrolysis of PtO . The composite had a uniformly
2
open, three-dimensional pore structure (porosity of 56%), with
a narrow pore-size distribution. Catalytic NO decomposition
to N and O and NO reduction by C H were investigated up
The pyrolysis of a commercial PtO powder (99.9%, Kojundo
2
Chemical Laboratory Co., Ltd., Sakado, Saitama, Japan) was
studied using thermogravimetry and differential thermal analysis
(TG-DTA) at 10°C/min up to 1200°C in a static air atmosphere.
2
2
2
4
to 900°C. In the absence of oxygen, the NO conversion rate
reached ϳ52% for the direct decomposition and ϳ100% for
the reduction by C H . The results suggested the possibility of
X-ray diffractometry (XRD; CuK at 40 kV and 100 mA) was used
␣
2
4
to characterize the PtO powder as well as the composite produced.
2
1
3
the porous composite as a multifunctional filter for simulta-
neous filtering of hot gas and removal of NOx.
High-purity natural CaMg(CO ) (Ͻ75 ␮m ), undoped ZrO
3 2 2
(99.9%, Sumitomo Cement Co., Tokyo, Japan), PtO , and LiF
2
(99.9%, Wako Pure Chemical Industries, Osaka, Japan) powders
were used as starting materials: 50:50 (mol%) of CaMg(CO ) and
3
2
I. Introduction
ZrO2 with PtO2 adjusted to give a composition of CaZrO3/
MgO:Pt ϭ 99:1 (vol%) after sintering. LiF (0.5 wt% of total
starting powders) was added to form a liquid phase during
OT-GAS cleaning is a key issue for various combustion and
1
H
power applications, for example, in automobiles, advanced
9
sintering to enhance reactivity and neck growth. The powders
2
3,4
biomass-based gasifier engines, and coal-fueled gas turbines.
were wet-ball-milled in ethanol for 6 h in a planetary ball-mill
3
–5
Porous ceramics are important as the physical filters of soot, as
(acceleration of 6g). The mixed slurry was dried, subsequently
6,7
8
catalysts themselves, and as supports for the chemical decom-
position of toxic species (e.g., NO or SO ).
dry-ball-milled for 24 h, and sieved through a 100 mesh screen.
The mixed powder was cold isostatically pressed at 200 MPa after
mold-pressing. The green compacts (15 mm in diameter and ϳ5
mm in thickness) were sintered in air at 1100°C for 2 h to obtain
the porous composite.
The microstructure was characterized using scanning electron
microscopy (SEM). The porosity and the pore-size distribution
were determined using mercury porosimetry (Model Poresizer
x
x
Recently, the present authors developed uniformly porous
CaZrO /MgO composites having a three-dimensional network
The composites were synthesized using reactive
sintering of equimolar dolomite and zirconia mixed powders with
LiF additive. During and after the formation of the network
structure, CO evaporated to form a homogeneous open-porous
structure. The pore-size distribution was very narrow (with pore
size of ϳ1 ␮m), and the porosity was controllable (typically
ϳ30%–50%) by changing the sintering temperature.
3
9
,10
structure.
2
9
320, Micromeritics Instrument Corp., Norcross, GA). Mercury
intrusion was conducted at pressures between 0 and 207 MPa.
NO decomposition was examined up to 900°C at atmospheric
pressure in a quartz microreactor. Gas mixtures of 1000 ppm NO
balanced by helium (with or without 500 ppm C H as a reducing
9
In this study, we have dispersed nanoscale particles of platinum
in a similarly produced CaZrO /MgO composite and have mea-
3
2
4
sured the catalytic activity of NO decomposition and reduction by
C H .
agent) were passed through the microreactor containing 3 g of the
2
4
porous CaZrO /MgO/Pt composite (containing 141 mg of plati-
3
num) at a flow rate of 50 mL/min. NO decomposition was detected
using a chemiluminescent NO–NO analyzer (Model BSU-100uH,
x
II. Experimental Procedure
There are several possible routes to disperse nanosized platinum
Best Instrument, Kyoto, Japan) and was confirmed using gas
chromatography (Model CP-2002, Varian Chrompack Interna-
tional, B.V., Middelburg, The Netherlands).
into a ceramic matrix, e.g., electroless plating using a platinum
1
1
12
salt, metal–organic precursor decomposition, and coating after
sintering. In the present work, however, we use the pyrolysis of
III. Results and Discussion
Figures 1(a) and (b) show the XRD pattern and TG-DTA
diagrams of the PtO2 powder. Figure 1(a) shows the powder
consisted of hexagonal ␣-PtO with some amorphous PtO . The
A. Searcy—contributing editor
2
2
color of the as-received PtO powder was dark-brown-gray, which
2
suggested the co-existence of dihydrate (PtO ⅐2H O, brown) and
2
2
1
4
monohydrate (PtO ⅐H O, black) as well as anhydride (black).
2
2
Manuscript No. 187754. Received April 18, 2001; approved July 10, 2001.
Supported by METI, Japan, as part of the Synergy Ceramics Project.
Thus, in Fig. 1(b), the first weight loss (and the corresponding
endothermic peak) was due to the desorption of the adsorbed water
and the dehydration of crystalline water of the dihydrate (i.e.,
PtO ⅐2H O 3 PtO ⅐H O ϩ H O). The second weight loss (at
Preliminary part of this work was presented at the 25th Annual International
Conference on Advanced Ceramics and Composites, Cocoa Beach, FL, 2001.
*
Member, American Ceramic Society.
AIST incorporates former NIRIN since April 1, 2001.
†
2
2
2
2
2
2
713

2
714
Communications of the American Ceramic Society
Vol. 84, No. 11
Fig. 1. Characterization of a PtO powder: (a) XRD pattern and (b) TG-DTA diagrams, indicating pyrolysis behavior in air atmosphere.
2
ϳ100°–620°C) was attributed to the dehydration of crystalline
PtO powder used could be nominally expressed as PtO ⅐0.26H O
2
2
2
water of the monohydrate as well as the partial decomposition of
from the TG-DTA analysis.
Figures 2(a)–(d) show the XRD pattern, pore-size distribution,
1
5,16
the dioxide. Platinum has several lower oxidation states.
The
third significant weight loss and the large endothermic peak were
and SEM micrographs of the porous CaZrO /MgO/1-vol%-Pt
3
attributed to the pyrolysis of PtO . The deoxidation continued at
composite sintered at 1100°C. XRD analysis revealed that the
2
temperatures higher than ϳ680°C, and it was completed at
ϳ1100°C. The weight loss from 100° to 1150°C was 16.4%,
composite consisted of CaZrO , MgO, and metallic platinum. The
3
CaZrO /MgO/Pt composite (porosity of 56%) had a narrow size
3
which agreed well with that in the ideal reduction, PtO 3 Pt ϩ
distribution with a mean cross section of ϳ1 ␮m (Fig. 2(b)) and a
well-developed three-dimensional network similar to that of the
2
O . Without distinction of the adsorbed and crystalline water, the
2
Fig. 2. Characterization of the porous CaZrO /MgO/1-vol%-Pt composite (porosity of 56%) sintered at 1100°C: (a) XRD pattern, (b) pore-size distribution,
3
(c) SEM photograph (low magnification), and (d) SEM photograph (high magnification).

November 2001
Communications of the American Ceramic Society
2715
composite had a narrow pore-size distribution with a mean pore
size of ϳ1 ␮m. Because the composite decreased NO concentra-
x
tion, it may be applicable as a filter material with catalytic
functions.
Acknowledgments
This work has developed out of earlier work and useful discussions with Dr. Peter
E. D. Morgan at Rockwell Science Center. The authors also thank Dr. M. H. Zahir at
Fine Ceramics Research Association for the helpful discussion.
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3
Ⅺ