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La(NO3)3
Co(NO3)2
Citric acid
PdCl2
Dispersion in distillated water
Dissolution in distillated water
Stirring for 24 h
at room temperature
Evaporation to dryness
Dissolution in distillated water again
Dip-coating on metal plate
Filtration
PdCl2 filtration liquid
Scheme 2. Preparation of PdCl2 filtration liquid.
Calcination
follows: metal ion nitrates included in the perovskite type oxide
were dissolved in distilled water, then citric acid was added to the
solution at the 1/3 molar ratio of total metal ions/citric acid. It was
then evaporated to form a powder at 353 K for 10 h. Following that,
the powder was dissolved in H2O again and a viscid gel solution
was obtained. The adherence of the perovskite-type oxide onto the
star-shaped metal substrate (20 mm × 30 mm length) was carried
out by immersion of the substrate in the viscid gel solution. The
metal substrate was stainless steel with a 100 m thickness. After
the gel coating, calcination of the coated substrate was performed
at 673 K for 2 h. Finally, the substrate was calcined at 873 K for 10 h.
The prepared catalyst in this way was represented as the structured
LaCoO3 catalyst.
(673 K, 2h followed by 873 K, 10 h)
Structured LaCoO3 catalyst
Scheme 1. Preparation of the structured LaCoO3 catalyst.
A preliminary study showed that the reduction temperature of
the catalyst could influence the catalytic performance of the WGS
reaction [23,24]. A precious metal supported on a perovskite-type
oxide has the unique property; the reduction temperature of the
perovskite-type oxide is significantly reduced by supporting novel
metals such as platinum (Pt) and palladium (Pd). The reduction by
the loading of Pd or Pt resulted in an increased of WGS reaction
activity, in other words, the easier release of lattice oxygen, more
CO was oxidized to CO2 by the lattice oxygen in the LaCoO3, and also
accompanied by a change in the produced hydrogen. The mobility
of the lattice oxygen in the catalyst was found to be a key point for
achieving a high activity.
For the application for the perovskite-type catalyst to a fixed-
bed reaction system, the lattice oxygen in the catalyst at the center
of the reactor is not effectively utilized, compared to the catalyst
in the vicinity of the reaction wall because a severe temperature
gradient is generated from the reactor wall toward the center. For
effectively utilizing the lattice oxygen in the catalyst, we focused
on a plate type catalyst on a metal substrate [25–27]. The plate
type catalyst, which is commonly called a structured catalyst, is
composed of a regularly arranged catalyst on the metal plate. By
placing the catalyst on the metal plate, an effective heat transfer to
the reaction field could be achieved due to the conductional heat
transfer. Furthermore, such a catalyst could overcome the disad-
vantage of a poor heat conductivity of the perovskite-type oxides,
and high mobility of the lattice oxygen would be acquired at a low
temperature due to the effective utilization for the external heat
energy.
2.2. Loading method of Pd on the structured LaCoO3 catalyst
The Pd loading was performed by immersing the structured
LaCoO3 catalyst in an aqueous solution of the Pd precursor salt of
Pd(NH3)4Cl2 or Pd(ethylenediamine)2Cl2 (denoted as Pd(en)2Cl2)
for 2 min. The Pd-dipped catalyst was then dried at room temper-
ature for 30 min. These operations were repeated until a 1 wt% Pd
loading was achieved. In addition, Pd(acetylacetonate)2 (denoted as
Pd(acac)2), which was dissolved in an acetone solution, was used as
the precursor. PdCl2 was also used as the precursor, however, the
PdCl2 hardly dissolved in the H2O and organic solvents, unlike the
the PdCl2 reagent was dispersed in H2O (denoted as the slurry) and
the slurry was stirred for 24 h at room temperature. Filtration of
the slurry was then performed, and the filtration liquid was used
as the Pd precursor solution. Scheme 2 shows steps in the prepa-
ration of this PdCl2 filtration liquid. The pH of this filtration liquid
was 2.5. After the Pd loading, the substrate was calcined at 823 K
for 2 h.
The washing procedures for the highly active catalyst were
applied in order to remove the chloride ligand from the Pd com-
plexes loaded on the support. The washing agents were water, an
aqueous solution of NH3 or NaOH at pH 11.5. The washing proce-
dure was employed by immersion of the Pd-supporting structured
catalyst without calcination in each solution for 30 min without
stirring. After immersion in each solution, the structured catalyst
was washed with distilled water for 2 min. The catalyst was then
calcined at 823 K for 2 h.
In this study, a plate-like LaCoO3 catalyst was developed on
the metal substrate by the following procedures; a viscid perov-
skite precursor solution was coated on a stainless steel substrate,
then the coated substrate was calcined several times. The WGS
performance of the plate-like LaCoO3 catalyst was then investi-
gated. Furthermore, for enhancing the WGS activity, the effects of
Pd supported on the structured LaCoO3 catalyst were examined by
changing the types of Pd precursors.
2.3. WGS performance
2. Experimental
The WGS performance test of the prepared catalyst was per-
formed at 573 K in a conventional flow reactor system. After setting
the prepared catalyst in the reactor, the reactants of CO and H2O
were supplied at 12.2 and 24.4 ml min−1, respectively. The ratio of
2.1. Preparation of the structured LaCoO3 catalyst
Scheme 1 shows steps in the preparation of the structured
LaCoO3 catalyst. The preparation procedure of the catalyst was as