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Ammonia decomposition activity over Ni/SiO2

catalysts with different pore diameters

Ryosuke Atsumi a, Reiji Noda b, Hideyuki Takagi c, Luigi Vecchione d,Andrea Di Carlo d, Zaccaria Del Prete d, Koji Kuramoto c,*

a Department of Chemical and Environmental Technology, Faculty of Engineering, Gunma University, 1-5-1,

Tenjincho, Kiryu, Gunma 376-8515, Japanb Division of Environmental Engineering Science, Faculty of Science and Engineering, Gunma University, 1-5-1,

Tenjincho, Kiryu, Gunma 376-8515, Japanc Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, 16-1,

Onogawa, Tsukuba, Ibaraki 305-8569, Japand Department of Mechanical and Aerospace Engineering, SAPIENZA University of Rome, 00184 Rome, Italy

a r t i c l e i n f o

Article history:

Received 13 March 2014

Received in revised form

11 June 2014

Accepted 1 July 2014

Available online 1 August 2014

Keywords:

Hydrogen production

Ammonia

Ni-loaded catalysts

Pore diffusion

Kinetic study

Knudsen diffusion

* Corresponding author. Tel.: þ81 29 861 807E-mail address: koji-kuramoto@aist.go.jp

http://dx.doi.org/10.1016/j.ijhydene.2014.07.00360-3199/Copyright © 2014, Hydrogen Energ

a b s t r a c t

Ammonia decomposition over Ni-loaded SiO2 catalysts (Ni/SiO2) was observed in a fixed-

bed reactor at different temperatures (ranging from 773 to 973 K) and ammonia feeding

rates (ranging from 1200 to 18,000 h�1). As support materials, several porous and inert SiO2

particles with different mean pore diameters ðdÞ ranging from 7.7 to 34.8 nm were used to

clarify the effect of pore diameter on the kinetic parameters for catalytic ammonia

decomposition. The Ni/SiO2 catalyst with the smallest pores, d ¼ 7.7 nm, showed the

highest activity at temperatures below 923 K, while the activity of this catalyst at 973 K was

lower than that of catalysts with larger pores. Kinetic analysis indicated that the activation

energy for d ¼ 7.7 nm was significantly decreased at higher temperatures, suggesting the

occurrence of strong diffusion resistance of ammonia molecules in the pores. Our exper-

iments also confirmed that almost complete decomposition of ammonia could be achieved

over Ni/SiO2 with d ¼ 26.7 nm at 973 K and a gas hourly space velocity as high as 42,000 h�1.

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

Introduction

Hydrogen possesses the potential to provide environmentally

friendly energy for internal combustion engines and fuel cells

[1e3], and can be produced from a variety of available energy

resources, including fossil fuels and renewable sources [4].

However the volumetric and gravimetric energy density of

6; fax: þ81 29 861 8209.(K. Kuramoto).03y Publications, LLC. Publ

hydrogen is inherently low, which presents a crucial obstacle

impeding hydrogen's use as a fuel [5]. For a practical hydrogen

filling station, hydrogen can be stored in cylinders or con-

tainers in compressed or liquefied form. The volumetric

densities of 70-MPa compressed hydrogen and liquefied

hydrogen are 0.039 and 0.070 kg/L, respectively. To efficiently

store liquefied hydrogen, practical problems such as the en-

ergy loss associated with the hydrogen liquefaction process,

ished by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 3 9 5 4e1 3 9 6 1 13955

boil-off during storage and delivery, and tank cost must be

addressed. To introduce hydrogen as an efficient energy

source, several chemical compositions for storage (e.g., hy-

drocarbons, boron hydrides, ammonia-boron, and ammonia)

and efficient transportation techniques have been investi-

gated [6]. More specifically, hydrocarbons such as methanol

and ethanol can produce hydrogen via steam re-forming in

conjunction with partial oxidation [7e10], which inherently

causes energy loss during steam re-forming. Ammonia (NH3)

is another candidate for efficient hydrogen storage, because

the hydrogen mass density of liquefied NH3 is 17.75 wt%,

which is higher than that of other liquid hydrocarbons such as

methanol (12.3 wt%) and ethanol (13.0 wt%). In addition,

contrary to pure hydrogen, NH3 can be liquefied under rela-

tively mild conditions (0.8 MPa, 298 K).

To release hydrogen from NH3, we can simply apply a

thermochemical cracking process with a catalyst. NH3 can be

decomposed over a metal-loaded catalyst to generate

hydrogen (H2) along with nitrogen (N2):

NH3/0:5N2 þ 1:5H2; DH0 ¼ 46 kJ mol�1 (1)

This NH3 decomposition process produces no COx, SOx, or

NOx, which can poison the anode catalysts of proton-

exchange membrane fuel cells (PEMFCs1); steam reforming

process of hydrocarbons can produce COx, NOx from trace

compounds in fossil fuels and SOx from methyl mercaptan in

city gas.

To use NH3-derived hydrogen as a fuel for PEMFCs, it is

necessary to decompose NH3 completely, because the poly-

mer electrolyte membrane as well as anode catalysts can be

damaged by trace levels of remaining NH3 in the fuel gas, as

reported by Soto et al. [11] and by Halseid et al. [12]. Thus we

need to develop a high-performance catalyst that can

decompose NH3 completely within a wide spaceevelocity

range to ensure the stable and safe operation of PEMFCs with

NH3-derived hydrogen.

A technical survey [13] of the catalytic activity of various

metals (Ru, Ir, Ni, Rh, Pt, Pd, Fe) and catalyst supports (SiO2,

TiO2, Al2O3, zeolite, MgO, activated carbon, and carbon

nanotubes [CNTs]) for NH3 decomposition found that Ru-

loaded CNTs are the most active catalyst for NH3 decompo-

sition, which can be attributed to a high rate of electron

transfer from the CNTs to Ru. Choudhary et al. [14] conducted

a systematic study on the activity of Ru-, Ir-, and Ni-loaded

SiO2 catalysts, and compared their turnover frequencies.

Their results indicated that the activities for NH3 decomposi-

tion over these metal surfaces decreased in the order

Ru > Ir > Ni: the Ru-loaded catalyst showed the highest ac-

tivity for temperatures ranging from 773 to 973 K. Papapoly-

merou and Bontozoglou [15] investigated the activity of Ir, Rh,

Pt, and Pd, and found that the NH3 decomposition activity of Ir

is the highest among the selected metals. Yin et al. [16]

examined the effects of some support materials (CNTs, MgO,

TiO2, activated carbon, ZrO2, Al2O3) on catalytic NH3 decom-

position with Ru-loaded catalysts in the temperature range

1 Abbreviations: CNTs, carbon nanotubes; GHSV, gas hourlyspace velocity; PEMFCs, proton-exchange membrane fuel cells;PFR, plug-flow reactor.

from 623 to 773 K, and found that Ru-loaded CNTs were the

most active of the group.

As described above, most of the previous studies focused

on noble-metal-loaded catalysts such as Ru-loaded catalysts

because these catalysts can achieve higher performance for

NH3 decomposition than other metal-loaded catalysts. How-

ever, noble metals are extremely expensive, and thus the

development of noble-metal-free, highly active catalysts as a

cost-efficient substitute for Ru-loaded catalysts is needed.

Toward this goal, we investigated Ni as a loading metal for

catalytic NH3 decomposition, since Ni is the most active

transition metal. Many researchers studied effects of various

supportmaterials on Ni catalysts [17e20]. Murayama et al. [21]

systematically investigated the performance of Ni-loaded

catalysts for NH3 decomposition, and they examined the ef-

fects of ceramic support materials (Al2O3, La2O3, SiO2, MgO,

Ce2O, TiO2, and ZrO2), Ni loading amount (for 10e70 wt% Ni),

and catalyst preparation method on the performance of Ni-

loaded catalysts. They reported that Ni/Al2O3 catalyst exhibi-

ted the highest NH3 decomposition rate, and suggested that

the high surface area of Al2O3 facilitated the dispersion of Ni

particles.

Although the effects of support material composition on

the activity of Ni catalysts have been investigated, the specific

effects of support materials' porosity or pore structure on Ni-

catalyzed decomposition mechanisms are unclear. To clarify

the effect of pore diameter of supports and determine an

optimized value, a porousmaterial that is inert to Ni should be

used to examine the pore diffusion process. We examined

porous SiO2 particles with different pore diameters as the

support material, and prepared Ni-loaded SiO2 catalysts

(designated Ni/SiO2) by wet impregnation to examine the ef-

fect of the SiO2 particles' pore diameter on the extent of NH3

decomposition. Previous research has examined catalysts at

low NH3 flow rates, but NH3 decomposition behavior under

high gas hourly space velocity (GHSV) conditions remains

unclear. We applied our Ni/SiO2 catalysts to NH3 decomposi-

tion tests under different temperatures and GHSV conditions,

and we discuss the correlation betweenmass transport in the

porous support and catalytic activity based on a simplified

kinetic model utilizing a plug-flow reactor (PFR).

Materials and methods

Catalyst synthesis

SiO2-supported Ni catalysts were prepared by a wet impreg-

nation method with a nominal Ni loading of 10 wt%. We

selected the porous silica particles CARiACT Q-3, Q-15, Q-30,

and Q-50 (Fuji Silysia, Ltd., Kasugai, Aichi, Japan) as support

materials; these particles possessed mean pore diameters

ranging from 3.7 to 19.9 nm, which is nearly equal to themean

free path of an NH3 molecule. Pore diameters smaller than

mean free path would have decreased the apparent activation

energy for NH3 decomposition [22]. Nickel(II) nitrate hexahy-

drate (Ni(NO3)2$6H2O, Wako Pure Chemical Industries, Ltd.,

Chuo-ku, Osaka, Japan) was used as a precursor. The Ni pre-

cursor and SiO2 were dissolved in ion-exchanged water. The

slurry was stirred for 24 h and dried using a rotary evaporator

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 3 9 5 4e1 3 9 6 113956

at 383 K. The supported Ni catalyst was calcined at 973 K for

1 h under flowing Ar and was then reduced at 973 K for 2 h

under flowing H2.

Catalyst testing

NH3 decomposition tests were conducted in a fixed-bed

reactor. Fig. 1 shows a schematic diagram of the fixed-bed

reactor system. The reactor consisted of a stainless steel

(SUS316) tube (inner diameter ¼ 1 in.) with an internal K-type

thermocouple. The catalyst bed was supported on a bed of

quartz glasswool in themiddle of the reactor, and 0 or 0.25 g of

Ni/SiO2 was used in each experiment. In the case of the 0 g of

catalyst, no peaks of hydrogen and nitrogen were detected.

Thus NH3 conversions which was measured by GC-TCD is

attributed precisely the activity of catalysts. The catalyst bed

was heated with an electric furnace, and the temperature of

the furnace wall was monitored with a K-type thermocouple.

Prior to the NH3 decomposition reaction the catalyst was

reduced with dry H2 at a flow rate of 100 mL/min for 1 h at

973 K. Following reduction treatment, the catalyst bed was

flushed with Ar gas and the reaction temperature was

adjusted to 773, 823, 873, 923, or 973 K. For the NH3 decom-

position tests, pure NH3 (99.999% purity, Tomoe Shokai Co.,

Ltd., Ota-ku, Tokyo, Japan) at a flow rate ranging from 150 to

1000mL/min (or GHSV range 1200 to 18,000 h�1) was fed to the

catalyst bed. The inlet gas flow rates of Ar, H2, and NH3 were

controlled with mass flow controllers. Product gas analysis

was carried out with an on-line gas chromatograph (GC-14A,

Shimadzu Ltd., Nakagyo-ku, Kyoto, Japan) equipped with a

thermal conductivity detector, and the NH3 decomposition

rate was estimated from the measured H2 and N2 concentra-

tions. Notably, NH3 damaged the SUS materials and thermal

conductivity detector under sufficiently high temperatures.

Fig. 1 e Illustration of experimental se

Because NH3 is a corrosive gas, a Porapak-Q column and four

solenoid valves in a constant-temperature oven were used to

remove unreacted NH3 from the produced gas mixture and to

automatically send the NH3-free gas sample to the gas chro-

matograph for analysis.

Catalyst characterization

The crystallite diameter of Ni nanoparticles was determined

by an X-ray diffractometer (Rint-2000, Rigaku Ltd., Akishima,

Tokyo, Japan) operated at 30 kV, 20mAwith a scanning rate of

0.1� min�1. Crystallite sizes were estimated using the Scherrer

equation, and we evaluated the difference in crystallite

diameter between as-prepared and spent catalysts. The spe-

cific surface area of each catalyst was measured by the Bru-

nauereEmmetteTeller (BET) method with N2 adsorption at

77 K. Pore structure was determined by means of a mercury

intrusion technique (Autopore IV 9520, Micrometrics Ltd.,

Norcross, Georgia, United States). The characteristics of the

Ni/SiO2 catalysts are presented in Table 1: in comparison to

the bare supports, the mean pore diameters of all Ni/SiO2

catalysts increased upon loading Ni on the support surface.

Results and discussion

The extent of NH3 conversion is plotted against GHSV in Fig. 2.

NH3 conversion decreased with increasing GHSV over the

entire temperature range examined, since an increase in

GHSV decreased the residence time of NH3 in the reactor. In

contrast, the NH3 decomposition activity increased as the re-

action temperature increased, because NH3 decomposition is

an endothermic reaction [see Eq. (1)]. Although silica-

supported catalysts with different SiO2 pore diameters

tup for NH3 decomposition tests.

Fig. 2 e NH3 conversion against GHSV over Ni/SiO2 catalysts

Table 1 e Characteristics of Ni/SiO2 catalysts.

d [nm] SBET [m2 g�1] DNi [nm] Vpore [mL g�1]

7.7 410 13.8 0.51

16.6 160 26.7 2.34

26.7 100 30.9 2.26

34.8 60 24.9 2.45

d: mean pore diameter, SBET: BET surface area,DNi: crystallite size of

loaded Ni, Vpore: pore volume of catalyst.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 3 9 5 4e1 3 9 6 1 13957

exhibited different activities, there was only poor correlation

between pore diameter and catalytic activity. At reaction

temperatures of 773, 823, 873, and 923 K, the activity for the

smallestmean pore diameter, d¼ 7.7 nm, was the highest (see

Fig. 1a, b, c, and d). To clarify the relationship between crys-

tallite sizes of Ni nanoparticles (DNi) and catalytic activity, DNi

was estimated by means of X-ray diffraction (see Table 1).

Fig. 2 and Table 1 indicate that the catalytic activity increased

as DNi decreased for temperatures lower than 873 K. Smaller

pores provided a higher surface area and thus an increased

at (a) 773 K, (b) 823 K, (c) 873 K, (d) 923 K, and (e) 973 K.

Fig. 3 e Plots of 2 ln½1=ð1� XNH3 Þ� � XNH3 vs. t over Ni/SiO2

for d ¼ 7.7 nm.

Fig. 4 e Arrhenius plot of Ni/SiO2 catalysts from 773 to

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 3 9 5 4e1 3 9 6 113958

dispersion of Ni. We concluded that this was the reason why

the highest catalytic activity was observed for Ni supported on

the smallest silica particles. These results illustrate that

dispersion of loaded metal enhances catalytic activity. How-

ever, at 973 K, the activity forDNi¼ 13.8 nm and d¼ 7.7 nmwas

found to be slightly lower. Therefore, we concluded that the

pore diffusion resistance decreased NH3 conversion for this

catalyst at 973 K. Almost complete decomposition of NH3 was

achieved at 973 K and GHSV of 36,000, 32,000, 42,000, and

33,000 for d ¼ 7.7, 16.6, 26.7, and 34.8 nm, respectively; there-

fore, we concluded that the Ni/SiO2 catalyst with d ¼ 26.7 nm

was the most active from the view point of efficient hydrogen

production.

To clarify the correlation between NH3 decomposition

rates and the pore diffusion process, a kinetic study was car-

ried out with the design equation of a plug-flow reactor [22]:

t ¼ZCNH3

CNH3 ;0

dCNH3

�rNH3

; (2)

where t is the residence time of gases [s], CNH3is the con-

centration of NH3 [mol m�3], CNH3 ;0 is the concentration of

NH3 in the inlet gas, and rNH3is the decomposition rate of NH3

[mol s�1]. As NH3 is decomposed according to Eq. (1), the total

gas volume increases in a constant-pressure system. Thus we

defined the fractional change in volume (V) of the system,

εNH3, as

εNH3¼ VX¼1 � VX¼0

VX¼0¼ ð0:5þ 1:5Þ � 1

1¼ 1; (3)

where X is the NH3 conversion [e]. Using εNH3and CNH3

for NH3

conversion, X can be obtained:

CNH3¼

�εNH3

� X�

�εNH3

þ X�C0;NH3

¼ ð1� XÞð1þ XÞC0;NH3

: (4)

Assuming a first-order reaction, the decomposition rate

can be expressed as a function of a kinetic constant, k, by

Eq. (5):

rNH3¼ �k$ð1� XÞ

ð1þ XÞ C0;NH3: (5)

Eq. (4) is inserted into Eq. (2) to obtain Eq. (6):

kt ¼ 2 ln1

1� X� X: (6)

Fig. 3 shows plots of 2 ln[1/(1 � X)] � X vs. t for d ¼ 7.7 nm.

Good linearity was obtained in these plots. Though not shown

here, we also obtained good linearity for other Ni/SiO2 cata-

lysts with different d values. This linearity ensures that a first-

order reaction is an applicable assumption. The kinetic con-

stant (k) for each temperature can be calculated from the

slopes of the plots in Fig. 3.

Kinetic constants obtained from Fig. 3, and from the plots

not shown, are plotted against the inverse of temperature in

Fig. 4 to yield the Arrhenius plot for each Ni catalyst. The

activation energy and frequency factor obtained from Fig. 4

are summarized in Table 2 and Fig. 5. We found that the

activation energy and frequency factor for d ¼ 7.7 nm are

lower than those for the other Ni/SiO2 catalysts, which all had

similar activation energies and frequency factors. We

concluded that the high diffusion resistance of the small pores

must have decreased the apparent activation energy and the

NH3 conversion of Ni/SiO2 for d ¼ 7.7 nm at 973 K. From

Fig. 2(e) it is shown that Ni/SiO2 for this condition show lower

activity than other catalysts at high GHSV conditions due to

increasing of the diffusion resistance.

It is necessary to estimate the pore diffusion resistance

affecting the catalytic activity for d ¼ 7.7 nm. Fig. 6 shows the

973 K.

Table 2 e Activation energy and frequency factor of Ni/SiO2 catalysts.

d [nm] Activation energy[kJ mol�1]

Frequency factor [s�1]

7.7 108 17.6

16.6 133 20.9

26.7 136 21.4

34.8 131 20.9

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 3 9 5 4e1 3 9 6 1 13959

pore distribution of the Ni/SiO2 catalysts. The value of d for

each catalystwas calculated from Fig. 6, andwe found that the

Ni/SiO2 catalyst with d ¼ 7.7 nm hadmany pores smaller than

10 nm in diameter, whereas the others had a defined peak

above 10 nm. Note that there are peaks at 40 mm for each pore

distribution, however these peaks derived from voids of

catalyst particles and didn't use to evaluate mean pore di-

ameters. We concluded that the decreasing of the activation

energy for d ¼ 7.7 nm must have been due to the small pore

size of that catalyst's support material.

Using d and themean free path of the NH3 molecule (L), the

Knudsen number (Kn) for each pore diameter can be calcu-

lated, which is a dimensionless number defined as

Kn ¼ L.d: (7)

When the mean free path is greater than 10 times the pore

diameter (Kn�1 < 0.1), collisions of the molecule with the pore

wall dominate [23]. This diffusion regime is different from

molecular diffusion, and is known as Knudsen diffusion. L for

NH3 was calculated from the following equation:

L ¼ kBTffiffiffi2

pps2P

; (8)

where kB is the Boltzmann constant, T is 978 K, s is the mo-

lecular diameter of NH3, and P is 0.1 MPa. For simplicity, we

will discuss the diffusion of only NH3 molecules.

Fig. 5 e Activation energy and frequency factor plotted

against mean pore diameter of Ni/SiO2 catalysts.

Fig. 6 e Pore distribution of Ni/SiO2 catalysts.

Fig. 8 e Illustration of pore diffusion mechanisms for NH3

molecules diffusing in Ni/SiO2 catalysts with different pore

diameters.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 3 9 5 4e1 3 9 6 113960

Fig. 7 shows a plot of the activation energy against the in-

verse of the Knudsen number for each Ni/SiO2 catalyst. The

activation energy is almost constant in the Kn�1 above ca. 0.1,

but decreases sharply beneath this value. The activation en-

ergy of the surface reaction on each Ni/SiO2 catalyst should be

the same independent of pore structure. However, if d is too

small to cause strong pore diffusion resistance, the apparent

activation energy of the catalyst can be lower than that of

others with larger pores. Fig. 7 suggests that Knudsen diffu-

sion increased the diffusion resistance, and that the apparent

activation energy was decreased as a result. Fig. 8 shows the

diffusion regimes of NH3molecules below or above d¼ 7.7 nm.

We concluded that the Ni/SiO2 catalysts with d ¼ 16.6 nm or

larger had high NH3 decomposition activity owing to the

support's low diffusion resistance. Furthermore, Ni/SiO2 with

d ¼ 26.7 nm can decompose NH3 completely at the highest

value of GHSV examined, 42,000 h�1 at 973 K.

Comparing the activity of catalysts in this study with the

catalysts Goodman et al. prepared [14], the activation energy

of Ni/SiO2 which was Goodman et al. prepared is ca. 91 kJ/

mol, and that of we prepared show slightly higher value,

107 kJ/mol for d ¼ 7.7 nm or ca. 130 kJ/mol for other pore

diameter. While Goodman et al. didn't reported the pore

structure of SiO2, assuming that they use SiO2 particle with

small pore, the activation energy for d ¼ 7.7 nm in this study

is nearly equal to that they obtained. In the view point of

activation energy, the kinetic constant of Ni/SiO2 which

Goodman et al. prepared should be compared with the cat-

alysts with d ¼ 7.7 nm in this study. For instance, the kinetic

constant at 873 K in their study is ca. 4.0 s�1 and that in this

study is 6.7 s�1. Thus we considered that the activity of Ni/

SiO2 with d ¼ 7.7 nm shows almost same as that of Good-

man's Ni/SiO2. However Ni/SiO2 with d ¼ 26.7 nm shows the

highest activity at 973 K (see Fig. 4). Thus we success that

preparing Ni/SiO2 catalyst with higher activity at above 973 K

by controlling pore diameter.

Fig. 7 e Activation energy plotted against the inverse of the

Knudsen number of each catalyst at 973 K.

Conclusion

The effect of pore diameter on the NH3 decomposition rate

over Ni/SiO2 catalysts with mean pore diameters ðdÞ of 7.7,

16.6, 26.7, and 34.8 nm was investigated. The Ni/SiO2 catalyst

with the smallest pore diameter (7.7 nm) showed the highest

activity at temperatures at or below 923 K. However, at 973 K,

Ni/SiO2 with d ¼ 26.7 nm decomposed NH3 completely at the

highest value of GHSV examined, 42,000 h�1. The Knudsen

number (Kn) for each catalyst was calculated, and the inverse

of this value for Ni/SiO2 with d ¼ 7.7 nm is the lowest, which

indicates that Knudsen diffusion was the dominating diffu-

sionmechanism for that catalyst.We concluded that a Ni/SiO2

catalyst with a high surface area and small pore diameter

would exhibit high catalytic activity below 923 K; however, we

emphasize that Ni/SiO2 with large pores exhibited high ac-

tivity as well, and that in terms of complete decomposition of

NH3, the Ni/SiO2 catalyst with d¼ 26.7 nm showed even higher

activity.

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