Ammonia decomposition over Ni catalysts supported on perovskite-type oxides for the on-site generation of hydrogen

Article abstract of DOI:10.1039/c8ra06100a

Ammonia decomposition has attracted increasing attention as a promising process for the on-site generation of hydrogen. In this study, Ni catalysts supported on perovskite-type oxides (ABO₃) were prepared and the activity for ammonia decomposit

Full text of DOI:10.1039/c8ra06100a

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Ammonia decomposition over Ni catalysts
supported on perovskite-type oxides for the on-site
Cite this: RSC Adv., 2018, 8, 32102
generation of hydrogen†
Kaname Okura, Kazunari Miyazaki, Hiroki Muroyama, * Toshiaki Matsui
and Koichi Eguchi*
Ammonia decomposition has attracted increasing attention as a promising process for the on-site
generation of hydrogen. In this study, Ni catalysts supported on perovskite-type oxides (ABO
prepared and the activity for ammonia decomposition was examined. The Ni/ANbO
Ni/AEMnO
3
) were
3
(A ¼ Na and K) and
3
(AE ¼ Ca, Sr, and Ba) catalysts were less effective for this reaction. Meanwhile, the Ni/REAlO
3
(RE ¼ La, Sm, and Gd) catalysts exhibited relatively high activity. For Ni/AETiO
3
and Ni/AEZrO
3
, the
performance strongly depended on the A-site element of the perovskite-type oxides, and the Sr and Ba
elements were more effective than the Ca one in the respective series. The catalytic activity for Ni/
3 3 3
AEZrO was higher than Ni/AETiO in the case of the same alkaline earth element, and Ni/BaZrO was
the most active among the samples investigated in this work. For these series, the order in the
performance corresponded well with that in the basic property. The nitrogen desorption profiles
revealed that the evolution of nitrogen atoms, which is one of the kinetically slow steps, effectively
Received 18th July 2018
Accepted 5th September 2018
proceeded for Ni/SrZrO
effect would be ascribed to the strong basic properties of the SrZrO
the high activity of Ni/SrZrO and Ni/BaZrO for ammonia decomposition.
3
and Ni/BaZrO
3
compared with the conventional Ni catalysts. This promotion
DOI: 10.1039/c8ra06100a
3
and BaZrO supports, resulting in
3
rsc.li/rsc-advances
3
3
hydrogen density (17.6 wt%) and energy density
Introduction
ꢀ
1
(
3000 W h kg ) as compared with other hydrogen carriers. In
Nowadays, the establishment of fuel cell systems is strongly addition, the liquefaction of ammonia proceeds under mild
ꢁ
required because of concerns about serious environmental conditions (ꢀ33.4 C at atmospheric pressure or 8.46 atm at 20
ꢁ
issues and the depletion of fossil fuels. Fuel cells are power
C). Moreover, the hydrogen generation via ammonia decom-
2
generation devices operating without harmful emissions, and position does not emit any CO and CO . The produced gas
their energy conversion efficiency is higher than that of from this reaction is preferable as a fuel for fuel cells operative
conventional internal-combustion engines. Hydrogen is at low temperatures such as polymer electrolyte fuel cells
utilized as a primary fuel source for fuel cells. However, the (PEFCs) since the Pt electrode is readily poisoned by CO.
direct storage and transportation of hydrogen are major Furthermore, the handling and the infrastructure for
obstacles for the commercialization of fuel cells due to its low ammonia are well established because it is one of the most
ꢀ
3
ꢁ
3–7
volumetric density (81.8 g m ) and boiling point (ꢀ252.8 C). fundamental industrial chemicals.
Thus, the feasibility of the compounds incorporating hydrogen
The ammonia decomposition reaction endothermically
as a hydrogen storage material has been discussed, and is called proceeds as described in eqn (1). According to thermodynamic
as a hydrogen carrier. These carriers can be transported to the calculation, the equilibrium ammonia conversion achieves
ꢁ
7
consumption area, and reformed or decomposed to generate above 99% at 400 C under a pressure of 1 atm.
1,2
hydrogen on the spot.
ꢁ
ꢀ1
NH
3
/ 1/2N
2
+ 3/2H
2
DH ¼ +46 kJ mol
(1)
Among the candidates for the hydrogen carriers, ammonia
has been a subject of attention owing to several interesting
features. Ammonia has signicantly high gravimetric
The catalytic ammonia decomposition involves the adsorp-
tion of ammonia, the stepwise hydrogen dissociation of adsor-
bed ammonia, and the desorption of nitrogen and hydrogen.
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, The density functional theory (DFT) calculations and kinetic
Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan. E-mail: eguchi@scl.kyoto-u. analysis supposed that the rst and second dehydrogenation of
ac.jp; muroyama.hiroki.5c@kyoto-u.ac.jp
ammonia and/or the combinative desorption of nitrogen atoms
were the kinetically slow steps.
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ra06100a
8–13
1
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The previous studies have demonstrated that various metals mixed and ground in ethanol overnight. Aer the evaporation of
1
4–28
15
14,15,28–39
14
14
14
40–42
14,43
ꢁ
(
(
Ru,
Ir, Ni,
Rh, Pt, Pd, Co,
and Fe ), alloys ethanol, the obtained powder was calcined at 1100 C for 5 h in
44
45
46
47–50
49
Mo–Fe, Ni–Fe, and Ni–Pt ), nitrides (MoN ,
FeMoN ,
air. In the case of the citric acid complex method, the corre-
x
x
4
8,49,51
49,50,52,53
54
NiMoN ,
and CoMoNx
), and carbides (WC and sponding nitrate salts were employed as the starting
x
55
VC
Among them, the Ru metal is widely known as the most active $6H
catalyst. Until now, much effort has been devoted for the Mn(NO
x
) served as the catalysts for ammonia decomposition. compounds; Ca(NO
O, Sm(NO $6H
$6H O, and ZrO(NO
However, the Industries, Ltd.). The metal nitrates with stoichiometric ratios
3
)
2
$6H
O, Gd(NO
$2H
2
O, Sr(NO
3
)
2
, Ba(NO
3
)
2
, La(NO
3
3
) -
O,
2
3
)
3
2
3
)
3
$6H O, Al(NO
O (Wako Pure Chemical
2
3
)
3
$9H
2
3
)
2
2
3
)
2
2
16–28
development of the highly active Ru catalysts.
utilization of precious metal catalyst is unfavorable due to its were dissolved in distilled water and the solution was stirred at
ꢁ
limited availability. Thus, in recent years, the Ni metal has ca. 60 C for 1 h. Then, citric acid (Wako Pure Chemical
attracted much interest as the alternative catalysts owing to its Industries, Ltd.) was added to this solution at a 1.5 molar times
low cost and the highest activity for ammonia decomposition of total metal cations. Subsequently, the pH in the solution was
4–7
among non-noble metals.
3
adjusted to 8.0 by the 28% NH aqueous solution (Wako Pure
Various materials have been investigated as supports of Ni Chemical Industries, Ltd.). Aer the evaporation of water at
ꢁ
ꢁ
catalyst for ammonia decomposition. The supports with high 90 C, the precursor was heated at 350 C for 3 h. The resulting
30
38
ꢁ
surface area such as mesoporous silica, Ce0.8Zr0.2
O
2
,
and powder was calcined at 1100 C for 5 h in air.
multi-wall carbon nanotube (MWCNT) were preferable for the The supported Ni catalysts were prepared by the impregna-
Ni-based catalytic system. In our study, the rare-earth oxides tion method. As the support material, Nb O , TiO , Al O (AKP
39
2
5
2
2 3
were effective as the support materials although their surface G-015, Sumitomo Chemical Co., Ltd.), MnO2 (Wako Pure
2
ꢀ1 36
areas were signicantly small (<10 m g ). This was because Chemical Industries, Ltd.), ZrO (Sigma-Aldrich, Co.), and the
2
3 2 2
hydrogen, which is well known as the inhibitive species for prepared perovskite-type oxides were used. Ni(NO ) $6H O
ammonia decomposition, readily desorbed from the Ni metal (Wako Pure Chemical Industries, Ltd.) was employed as a nickel
supported on these oxide materials. Moreover, the promotion source. The nickel nitrate was dissolved in the distilled water
effect of the additive species has been also studied for Ni cata- and mixed with the support oxide. Aer the nickel nitrate
ꢁ
lysts. The modication by Sr and Ba species or rare-earth oxides solution was evaporated on a steam bath at 80 C, the obtained
32–35,37
ꢁ
improved the performance of supported Ni catalysts.
precursor was calcined at 600 C for 5 h in air. The Ni metal
Based on these results, the presence of basic components is loading was xed at 40 wt%.
expected to promote ammonia decomposition over Ni catalysts.
Thus, in this work, we focused on the perovskite-type oxides
including alkaline, alkaline earth, and rare-earth elements, and
Characterization
employed them as the support material to develop highly active X-ray diffraction (XRD) measurement was carried out for the as-
Ni catalysts. The perovskite-type oxide is generally expressed as calcined and reduced catalysts on X-ray diffractometer (Rigaku,
3
a chemical formula of ABO , in which two types of sites, A site Ultima IV) at a setting of 40 kV and 40 mA with a scanning rate
ꢁ
ꢀ1
and B site, are occupied by cations. These materials are known of 2 min . The BET surface area of the support materials was
ꢁ
to exhibit interesting properties as the support of catalysts for examined by N physisorption at ꢀ196 C (BEL Japan, Bellsorp-
2
various chemical reactions. Wang et al. reported that the miniII). Before the measurement, the samples were treated at
ꢁ
BaZrO
3
-supported Ru catalyst showed the outstanding perfor- 300 C for 30 min under vacuum. The Ni surface area was
56
mance for ammonia synthesis. Urasaki et al. showed that the estimated by the CO pulse measurement (BEL Japan, BELCAT-
Ni catalysts supported on the perovskite-type oxides had the B) based on the assumption that CO molecules adsorbed on
superior long-term stability as compared with the conventional Ni surface atoms in a ratio of 1 : 1. Prior to the CO adsorption,
57,58
ꢁ
catalysts in the steam reforming of hydrocarbons.
In this the catalysts were reduced at 600 C for 2 h. To clarify the basic
study, the effective species in the perovskite-type oxides for property of the Ni catalysts, the CO temperature-programmed
2
ammonia decomposition were discussed by systematically desorption (CO -TPD) measurement was performed (BEL
2
changing the A-site and B-site elements.
Japan, BELCAT-B). The catalyst was pretreated in pure O at
2
ꢁ
8
00 C for 1 h, and subsequently in pure H
2
at the same
ꢁ
temperature for 2 h. Aer the exposure to CO at 50 C for 1 h,
2
Experimental
Sample preparation
the samples were heated in the He ow at a heating rate of
ꢁ
ꢀ1
ꢀ1
5
2
C min (ow rate: 30 ml min ). The desorbed CO was
The perovskite-type oxides were fabricated by the solid state conrmed by a thermal conductive detector (TCD). To discuss
reaction method or the citric acid complex method. ANbO
Na and K) and AETiO
(AE ¼ Ca, Sr, and Ba) were prepared by the ammonia temperature-programmed surface reaction (NH
the former method, while REAlO
AEMnO , and AEZrO by the latter one. In the solid state reac- The pretreatment condition was the same as that in the CO
3
(A ¼ the desorption process of nitrogen in ammonia decomposition,
3
3
-
3
(RE ¼ La, Sm, and Gd), TPSR) measurement was carried out (BEL Japan, BELCAT-A).
3
3
2
-
tion method, the following carbonates or metal oxides were TPD measurement except for the reactant adsorption step.
ꢁ
used as the starting materials; Na CO , K CO , CaCO , SrCO , Ammonia was supplied at 50 C for 1 h, followed by the gas
2
3
2
3
3
3
BaCO
3
, Nb
2
O
5
(Wako Pure Chemical Industries, Ltd.), and TiO
2
2
replacement by Ar for 5 h. The desorption behavior of N in
ꢁ
ꢀ1
(Sigma-Aldrich, Co.). These compounds were stoichiometrically a heating process at a rate of 5 C min in Ar (ow rate: 30
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ꢀ
1
ml min ) was monitored by an on-line mass spectrometer
(Pfeiffer Vacuum, OmniStar GSD320).
The Ni particle size distribution and the morphology were
examined by a transmission electron microscope (TEM, JEM-
100F, JEOL) equipped with an energy dispersive X-ray spec-
2
trometer (EDS, JED-2300T, JEOL). Prior to the analysis, the Ni
ꢁ
catalysts were reduced at 600 C for 2 h in 50% H
2
/Ar. The
average Ni particle size of each sample was estimated from 150–
80 particles.
1
Catalytic activity
The catalytic ammonia decomposition reaction was performed
in a xed-bed ow reactor. The catalyst (10–18 mesh, 300 mg)
ꢁ
was placed in the center of quartz tube, and reduced at 600 C
ꢀ
1
for 2 h in 50% H
down to 350 C in Ar, pure NH
(
2
/Ar (ow rate: 80 ml min ). Aer the cooling
was supplied at the weight/ow
ꢁ
3
ꢀ
3
W/F) ratio of 0.60 g s cm under an atmospheric pressure. The
catalytic activity was evaluated in a heating process. The
ꢁ
stability test was carried out for 35 h at 550 C and W/F ¼ 0.18 g s
ꢀ
3
cm . The ow rate of outlet gas was measured by a soap-lm

ow meter (HORIBA STEC) aer removing unconverted
ammonia by diluted sulfuric acid. The conversion of ammonia
was calculated by the following equation with an assumption
that the ratio of produced nitrogen to hydrogen was 1 : 3.
NH
3
conversion/% ¼ (Fout/2Fin) ꢂ 100
(2)
where Fin and Fout are the ow rates of inlet ammonia gas and
outlet hydrogen and nitrogen gases, respectively.
Results and discussion
Perovskite-type oxide support
The crystal structure of the Ni catalysts supported on the
perovskite-type oxide was examined by XRD analysis. As the
representative samples in each system, the XRD patterns of the
as-calcined and reduced Ni/NaNbO
3 3 3
, Ni/LaAlO , Ni/CaMnO ,
Ni/SrTiO , and Ni/BaZrO catalysts are shown in Fig. 1(a) and
3
3
(b), respectively. For the as-calcined Ni catalysts, the charac-
teristic peaks assigned to the phases of the corresponding
support oxide and nickel oxide were conrmed. Aer the
ꢁ
reduction treatment at 600 C, nickel oxide was changed to
metallic nickel. As for the perovskite-type oxides, NaNbO
LaAlO , SrTiO , and BaZrO were stable under the reducing
atmosphere, while CaMnO decomposed to various
compounds. The similar tendency was observed in the XRD SrTiO
3
,
3
3
3
3
Fig. 1 XRD patterns of 40 wt% Ni/NaNbO
3
, Ni/LaAlO
3
, Ni/CaMnO
3
, Ni/
ꢁ
3
, and Ni/BaZrO
3
(a) after calcination at 600 C and (b) after
ꢁ
subsequent reduction at 600 C.
patterns of the other Ni/ANbO , Ni/REAlO , Ni/AEMnO , Ni/
3
3
3
AETiO
This result suggests that the AEMnO
as the support material for ammonia decomposition.
3
, and Ni/AEZrO
3
catalysts (see Fig. S1 and S2 in ESI†).
oxides were inappropriate surface area should lead to the low dispersion of Ni particles.
Actually, in most of the prepared Ni catalysts, the Ni surface area
3
The physical properties of the Ni catalysts were studied. estimated by the CO pulse chemisorption measurement was
Table 1 summarizes the BET surface area of each support oxide signicantly low (see Table 1). This suggests that the Ni particle
determined by N
specic surface area of all the perovskite-type oxides was of CO adsorption on Ni metal by the strong metal-support
2
adsorption/desorption measurement. The size for these samples was highly large. Besides, the inhibition
2
ꢀ1
considerably small (<10 m g ) probably because the sintering interaction (SMSI) effect would be partly responsible for this
59
3
of support material remarkably proceeded during the calcina- result. For example, it has been reported that BaTiO and
ꢁ
56,60
3
tion at 1100 C in the preparation step. The support with small BaZrO were the SMSI oxides.
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Table 1 Physical properties for Ni catalysts supported on various oxides
a
2
sampleꢀ1
b
2
sampleꢀ1
Catalyst
Support surface area (m g
)
Ni surface area (m g )
Ni/Nb
Ni/NaNbO
2
O
5
7.4
4.1
8.4
130
9.9
8.1
4.6
2.4
6.6
7.9
7.2
12
6.5
5.5
4.9
15
6.0
6.1
7.4
Trace
Trace
Trace
4.9
0.83
1.4
3
Ni/KNbO
Ni/Al
Ni/LaAlO
3
2 3
O
3
Ni/SmAlO
Ni/GdAlO
Ni/MnO
Ni/CaMnO
Ni/SrMnO
Ni/BaMnO
Ni/TiO
Ni/CaTiO
Ni/SrTiO
Ni/BaTiO
3
3
1.3
2
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
3
3
3
2
3
3
3
Ni/ZrO
Ni/CaZrO
Ni/SrZrO
Ni/BaZrO
2
3
3
3
a
b
ꢁ
Determined by N
2
physisorption measurement. Determined by CO pulse measurement aer reduction at 600 C.
Catalytic activity
As mentioned in Fig. 1, the AEMnO oxides decomposed to
3
ꢁ
various species under the reducing atmosphere. A collapse of
the support structure would give rise to the sintering of the
Fig. 2 depicts the ammonia conversion at 550 C for the Ni
catalysts prepared in this study. In the Ni/niobate series, the
loaded metal. Additionally, a part of the Ni/AETiO
3
and Ni/
performances of Ni/Nb
almost the same. Among the samples investigated, the conver-
sions for these catalysts were considerably low. The Ni/REAlO
2 5 3 3
O , Ni/NaNbO , and Ni/KNbO were
AEZrO , their performances strongly depended on the A-site
3
element of the perovskite-type oxides. In series, the Sr and Ba
elements were more effective than the Ca one. Hereaer, the
support effect of AETiO and AEZrO was discussed in detail.
3 3
3
catalysts exhibited relatively high activity, and were more active
than the Ni/Al O catalyst. As reported in the previous litera-
2
3
32–36
tures,
enhancement of performance for ammonia decomposition. The
activity of Ni/REAlO was scarcely affected depending on the
rare-earth element. The conversions of Ni/AEMnO were higher
the presence of rare-earth elements led to the
Ni/titanate and Ni/zirconate
As mentioned above, the support effect of AETiO
3
3
and AEZrO
3
3
than those of Ni/ANbO
3
3
, while lower than those of Ni/REAlO .
depended on the alkaline earth element of the perovskite-type
oxides. The temperature dependencies of ammonia conver-
sion for the Ni/titanate and Ni/zirconate series are indicated in
Fig. 3 and 4, respectively. The decomposition reaction started
ꢁ
from ca. 350 C and the ammonia conversion increased with
a rise in reaction temperature. In the Ni/titanate series, Ni/
SrTiO
CaTiO
3
and Ni/BaTiO
3
were more active than Ni/TiO
2
and Ni/
ꢁ
3
, and the reaction almost completed at ca. 600 C over
the former catalysts. On the other hand, in the case of the Ni/
zirconate catalysts, the order of the performance in this
temperature range was as follows; Ni/BaZrO > Ni/SrZrO > Ni/
3
3
CaZrO > Ni/ZrO . The behavior of activity against the difference
3
2
in the alkaline earth element was similar in both series; the Sr
and Ba species were more preferable as the constituent
elements of support material. Besides, the Ni/AEZrO
exhibited higher ammonia conversion than the Ni/AETiO
for the same AE element, and the Ni/BaZrO catalyst was the
3
catalysts
3
ones
3
most active in this work. As for Ni/BaZrO
3
, the catalytic stability
ꢁ
ꢀ3
was studied at 550 C and W/F ¼ 0.18 g s cm . As shown in
Fig. 5, the ammonia conversion at this reaction condition was
ca. 62% and remained unchanged over 30 h. This result claried
ꢁ
Fig. 2 Ammonia conversion for ammonia decomposition at 550
C
for the Ni catalysts supported on various oxides. Reaction condition;
ꢀ3
3
NH : 100 vol%, W/F ¼ 0.60 g s cm
.
3
that the Ni/BaZrO catalyst was highly stable.
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Fig. 3 Ammonia conversion for ammonia decomposition against
ꢁ
Fig. 5 Stability test over 40 wt% Ni/BaZrO
3
at 550 C. Reaction
reaction temperature over 40 wt% Ni/TiO
2
, Ni/CaTiO
3
, Ni/SrTiO
3
, and
ꢀ3
3
condition; NH : 100 vol%, W/F ¼ 0.18 g s cm
.
ꢀ
3
Ni/BaTiO . Reaction condition; NH
3
3
: 100 vol%, W/F ¼ 0.60 g s cm
.
respectively. In the case of Ni/BaZrO
3
, the ratio of Ni particles
above 100 nm was high, and the average Ni particle size was
99.2 nm. In comparison with Ni/Al
2
O
3
(average Ni size; 23.7
nm), the Ni particles for the Ni/AEZrO
3
catalysts were signi-
cantly larger and the particle size distribution range was wider
regardless of alkaline earth elements in the perovskite-type
oxides. These results implied that the AEZrO supports were
3
ineffective for the dispersion of Ni particles, leading to the
smaller Ni surface area. These dispersion states should be
ascribed to the small surface area of the zirconate supports.
3 3
Considering that Ni/SrZrO and Ni/BaZrO were highly active,
these supports would remarkably enhance the turnover
frequency (TOF). Although the Ni surface and/or Ni/support
material interface serve as the reaction sites in ammonia
decomposition, in this catalyst system the differences in the
activity could not be explained in terms of the size and surface
area of Ni particle. Thus, the chemical properties of support
surface and the electronic state of nickel species could be ex-
pected to affect the catalytic activity.
Fig. 4 Ammonia conversion for ammonia decomposition against
reaction temperature over 40 wt% Ni/ZrO , Ni/CaZrO , Ni/SrZrO , and
Ni/BaZrO . Reaction condition; NH
: 100 vol%, W/F ¼ 0.60 g s cm
2
3
3
ꢀ3
3
3
.
Next, the basic property of the Ni/AETiO
catalysts was evaluated by the CO -TPD measurement since it is
known that the catalysts with the strong basic sites promote
3 3
and Ni/AEZrO
2
1
9
The microstructure and the average Ni particle size of the Ni/ ammonia decomposition. Fig. 7 describes the CO desorption
2
ꢁ
AEZrO series were examined by TEM observation to evaluate proles for the samples reduced at 800 C. In the Ni/AETiO
the inuence of alkaline earth element on the morphology of series, the CO desorption peak initiated from ca. 50 C and the
3
3
ꢁ
2
catalysts. Fig. 6 illustrates the TEM images and the Ni particle peak shape was similar to each other. On the other hand, the
size distributions for Ni/ZrO
BaZrO
of Ni/Al
for Ni/Al
2
, Ni/CaZrO
3
, Ni/SrZrO
3
, and Ni/ evolution of CO was detected at relatively high temperatures
2
ꢁ
3
aer the reduction at 600 C. For comparison, the result for Ni/AEZrO as compared with Ni/AETiO at the same A-site
3
3
2
O
3
is also depicted. It is expected that the Ni dispersion element. This means that the basic property of Ni/AEZrO was
is relatively high judging from the large Ni surface stronger than that of Ni/AETiO . In particular, Ni/SrZrO and Ni/
3
2
O
3
3
3
area as listed in Table 1. In all the images, some Ni particles are BaZrO should possess highly strong basic sites judging from
3
ꢁ
indicated by white arrows based on the result of EDS analysis. the desorption above 700 C. The CO desorption amount per
2
ꢁ
The enormous particles (>ca. 1000 nm) observed in the Ni/ the sample weight in the range of 50–800 C is presented in
zirconate series were assigned to the support oxides. The Fig. 8. From the desorption amount for Ni/AETiO3 and Ni/
average Ni particle size was 37.4 nm for Ni/ZrO
and Ni/SrZrO catalysts had larger Ni particles as compared with basic sites was larger for the latter series. The electronegativity
Ni/ZrO , and the average Ni size was 48.4 nm and 57.6 nm, of Zr smaller than that of Ti might be one of the reasons for this
2
. The Ni/CaZrO
3
3
AEZrO in the case of the same A-site element, the amount of
3
2
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2 3 3 3 2 3
Fig. 6 TEM images and Ni particle size distribution histograms of 40 wt% (a) Ni/ZrO , (b) Ni/CaZrO , (c) Ni/SrZrO , (d) Ni/BaZrO , and (e) Ni/Al O .
result. Moreover, when the B-site element was xed, the CO
2
AE element, and that the Sr and Ba species were more effective
desorption amount was larger for the Sr and Ba species than the than the Ca species in the respective series. This trend corre-
Ca one in both series. This result indicated that the Sr or Ba sponded to that of basic property in Ni/AETiO and Ni/AEZrO .
3
3
components were more preferable for an increase in the basic Therefore, the strong basic property of the perovskite-type oxide
sites. The catalytic test for Ni/AETiO
3
and Ni/AEZrO
3
(see Fig. 2– would be responsible for the high catalytic activity for ammonia
4) revealed that the latter sample was more active for the same decomposition.
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Fig. 9
SrZrO
N
2
desorption profiles for 40 wt% Ni/ZrO
2 3
, Ni/CaZrO , Ni/
Fig. 7 CO
2 3 3 3
–TPD profiles for 40 wt% Ni/CaTiO , Ni/SrTiO , Ni/BaTiO ,
3
, and Ni/BaZrO obtained by NH –TPSR measurement in Ar
3
3
Ni/CaZrO
3
, Ni/SrZrO
3
, and Ni/BaZrO
3
in He under a heating rate of
ꢁ
ꢀ1
.
ꢁ
ꢀ1
flow under a heating rate of 5 C min
5
C min
.
ZrO
2 3
and Ni/CaZrO were ineffective for the nitrogen evolution.
On the other hand, in the proles of Ni/SrZrO
the nitrogen desorption started at ca. 120–130 C, and termi-
3
and Ni/BaZrO
3
,
ꢁ
nated at relatively low temperatures as compared with Ni/ZrO2
and Ni/CaZrO . These onset desorption temperatures were
3
signicantly low as compared with those of various Ni cata-
3
5–37
lysts.
supports were effective for the nitrogen desorption, leading to
the higher performance of Ni/SrZrO and Ni/BaZrO . Consid-
ering that the electron donation from the support to active
3 3
This result means that the SrZrO and BaZrO
3
3
61
metal could promote the nitrogen desorption step, this
support effect would be mainly ascribed to the strong basic
property of SrZrO and BaZrO .
3
3
ꢁ
Fig. 8 CO
CO
BaTiO
2
desorption amount in the range of 50–800 C obtained by
, Ni/ Conclusions
2
–TPD measurement in Fig. 7 for 40 wt% Ni/CaTiO
, Ni/CaZrO , Ni/SrZrO , and Ni/BaZrO
3
, Ni/SrTiO
3
3
3
3
3
.
The ammonia decomposition was investigated over the Ni
catalysts supported on the perovskite-type oxides. The Ni/
In previous literatures, the electron donation from the
additive or support to the active metal promoted the nitrogen
ANbO
reaction, while the Ni/REAlO
activity. For Ni/AETiO and Ni/AEZrO
3
and Ni/AEMnO
3
catalysts were less effective for this
ones exhibited relatively high
, the performance
3
19,37,61
desorption step, which is kinetically slow.
3
Thus, the NH –
3
3
TPSR measurement was carried out to discuss the nitrogen
strongly depended on the alkaline earth metal in the perovskite-
type oxides, and the Sr and Ba species were superior to the Ca
species in the respective series. The activity was higher for the
desorption behavior for the Ni/AEZrO series. The NH3
3
ꢁ
adsorption was conducted at 50 C. Fig. 9 shows the nitrogen
(
m/z ¼ 28) desorption behavior for the Ni/ZrO
2 3
and Ni/AEZrO
Ni/AEZrO
3
catalyst than the Ni/AETiO
3
one in the case of the
catalyst was the most
catalysts. In the case of Ni/ZrO , which was low active for
2
same AE element, and the Ni/BaZrO
3
ammonia decomposition, the nitrogen desorption was
active among the samples investigated. In the Ni/AETiO
Ni/AEZrO series, the trend in the catalytic activity corre-
sponded with that in the basic property. The NH -TPSR
measurement for the Ni/AEZrO catalysts revealed that the
3
and
ꢁ
conrmed above ca. 500 C. For Ni/CaZrO
3
, the desorption
3
ꢁ
initiated at ca. 180 C and continuously proceeded even above
3
ꢁ
ꢁ
600 C. Considering that the peak area at ca. 550 C was large,
3
the ratio of nitrogen atoms strongly-interacting with Ni metal
would be relatively high. These desorption temperature range
were higher than those for the conventional Ni catalysts sup-
nitrogen desorption effectively proceeded over Ni/SrZrO and
3
Ni/BaZrO . This promotion effect would be ascribed to the
3
strong basic property of SrZrO
to the high activity of Ni/SrZrO
3
and BaZrO
and Ni/BaZrO .
3
supports, and lead
35–37
ported on alumina or rare-earth oxide,
suggesting that Ni/
3
3
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3 W. Rar ´o g-Pilecka, D. Szmigiel, Z. Kowalczyk, S. Jodzis and
J. Zielinski, J. Catal., 2003, 218, 465–469.
Conflicts of interest
There are no conicts to declare.
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