nimgoal2o3andnimgo catalyzed sic foam absorbers for high temperature solar reforming of methane

7/28/2019 NiMgOAl2O3andNiMgO Catalyzed SiC Foam Absorbers for High Temperature Solar Reforming of Methane

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Ni/MgOeAl2O3 and NieMgeO catalyzed SiC foam absorbers

for high temperature solar reforming of methane

Nobuyuki Gokon a,*, Yuhei Yamawaki b, Daisuke Nakazawa b, Tatsuya Kodama b

a Center for Transdisciplinary Research & Department of Chemistry and Chemical Engineering, Niigata University, 8050 Ikarashi 2-nocho,

Nishi-ku, Niigata 950-2181, Japanb Graduate School of Science and Technology, Niigata University, 8050 Ikarashi 2-nocho, Nishi-ku, Niigata 950-2181, Japan

a r t i c l e i n f o

Article history:

Received 15 February 2010

Received in revised form

5 April 2010

Accepted 8 April 2010

Available online 3 June 2010

Keywords:

Solar heat

Hydrogen production

Reforming

Thermochemical processNi catalyst

Reticulated ceramic foam

a b s t r a c t

Ni catalyst supported on MgOeAl2O3 (Ni/MgOeAl2O3) prepared from hydrotalcite, and

NieMgeO catalyst are studied in regard to their activity in the CO2 reforming of methane at

high temperatures in order to develop a catalytically activated foam receivereabsorber for

use in solar reforming. First, the activity of their powder catalysts is examined. Ni/MgO-

eAl2O3 powder catalyst exhibits a remarkable degree of high activity and thermal stability

as compared with NieMgeO powder catalyst. Secondly, a new type of catalytically acti-

vated ceramic foam absorber e Ni/MgOeAl2O3/SiC e and NieMgeO catalyzed SiC foam

absorber are prepared and their activity is evaluated using a laboratory-scale recei-

verereactor with a transparent quartz window and a sun-simulator. The present Ni-based

catalytic absorbers are more cost effective than conventional Rh/g-Al2O3 catalyzed

alumina and SiC foam absorbers and the alternative Ru/g-Al2O3 catalyzed SiC foam

absorbers. Ni/MgOeAl2O3 catalyzed SiC foam absorber, in particular, exhibits superiorreforming performance that provides results comparable to that of Rh/g-Al2O3 catalyzed

alumina foam absorber under a high flux condition or at high temperatures above 1000 C.

Ni/MgOeAl2O3 catalyzed SiC foam absorber will be desirable for use in solar recei-

verereactor systems to convert concentrated high solar fluxes to chemical fuels via

endothermic natural-gas reforming at high temperatures.

2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction

Currently, the countries that have abundant natural gasreservoirs close to solar sites favorably set in a sun-belt

region that receives more than 1500 kWh/m2 a year enjoy

a great potential to generate hydrogen using renewable

energy sources. This can provide a convenient alternative to

natural gas power plants, whose main advantage is signifi-

cantly reduced CO2 emissions. The thermochemical conver-

sion of concentrated solar heat to chemical fuels has the

advantage of producing energy carriers for the storage and

transportation of solar energy from the sun-belt to remote

population centers [1e4]. The direct thermochemical

conversion of solar radiation energy is characterized by anideal high efficiency; its thermodynamic limit for enthalpy

storage is close to 100%. From the perspective of the chemical

pathway for this process, the solar reforming of natural gas

has been investigated as one of the most promising solar

thermochemical processes [5e23]. The following endo-

thermic reformings of natural gas are the basis for upgrading

the calorific value of the hydrocarbons which produce

syngas:

* Corresponding author. Tel./fax: 81 25 262 6820.E-mail address: [email protected] (N. Gokon).

A v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / h e

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 e n e r g y 3 5 ( 2 0 1 0 ) 7 4 4 1 e7 4 5 3

0360-3199/$ e see front matter 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

doi:10.1016/j.ijhydene.2010.04.040
mailto:[email protected]://www.sciencedirect.com/http://www.elsevier.com/locate/hehttp://www.sciencedirect.com/http://www.elsevier.com/locate/hemailto:[email protected]


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CH4 H2O(g) CO 3H2 DH298 K 206 kJ mol1, (1)

CH4 CO2 2CO 2H2 DH298 K 247 kJ mol1. (2)

In order to produce more hydrogen, solar-processed syngas

(CO and H2) is shifted via a water-shift reaction, in which

water catalytically reacts with CO in the following reaction:

CO H2O(g) CO2 H2 DH298 K 41 kJ mol1 (3)

The carbon dioxide product can then be separated, for

example, via a membrane, and sequestrated.In comparison to

the conventional reforming of natural gas, the solar reforming

of natural gas offers distinct advantages: (1) the utilization of

solar energy as a replacement for fossil fuels to obtain the

necessary process heat reduces fossil fuel consumption, (2)

concentrated solar radiation input theoretically upgrades the

reactants energy content by 22e28%, and (3) environmentally

hazardous pollutants and CO2 emissions are eliminated orreduced.

A project was proposed to develop a solar methanol

production system in the sun-belt [4]. In this project, the plan

is for methanol or DME to be produced from natural gas

(methane) and coal via methane reforming and coal gasifica-

tion using solar heat as the process heat. The liquid fuel that is

produced can then be transported overseas to Japan by

a modified oil tanker [4].

Concentrated solar radiation has the specific properties of

high density, heterogeneous distribution of thermal flux, and

frequent thermal transients due to the fluctuating insolation.

A solar-specific reactor for reforming was proposed on the

basis of a direct absorption concept in which the receiver andreformer comprise the same unit. This concept was realized in

a number of solar reforming systems such as the directly

irradiated volumetric reactors developed by the German

Aerospace Research Center (DLR) in Germany, Sandia

National Laboratories in the United States, and the Weizmann

Institute of Science (WIS) in Israel [7,9e11,13,16,17]. In this

concept, the concentrated solar radiation passes through

a transparent window and is absorbed by an absorber of

catalytically active, reticulated ceramic foam which is

mounted behind the window. In these volumetric reactors,

reticulated ceramic foam made of alumina and SiC, which

combines high gas permeability and turbulence of flow with

a geometry suitable for the effective and uniform absorptionof solar radiation, is considered preferable to conventional

honeycomb structures. The solar reforming of methane with

CO2 using the volumetric reactor system was first demon-

strated in DLR and Sandia National Laboratories in 1990 [7]. A

200e300 kW volumetric reactor was also demonstrated at the

WIS for the solar CO2 reforming of methane [17]. A prototype

was realized and tested as part of the SOLASYS (Solar

Upgrading of Fossil Fuels) project. The receiver has 400 kW of

thermal power absorbed by the gas and an operation pressure

of 10 bar. The project demonstrated the ability of the volu-

metric reactor receiver to generate processheat that could run

the chemical reaction at a temperature of about 900 C. The

syngas that was produced was used in a small gas turbine for

power generation. Further work in the ongoing SOLREF (Solar

Steam Reforming of Methane Rich Gas for Synthesis Gas

Production) aims to achieve a temperature higher than 900 C

in order to improve theefficiencyof the process and to make it

possible to couple the reactor with gas treatment for the

production of pure hydrogen.

The Commonwealth Science and Industrial Research

Organisation (CSIRO) in Australia operated a solar reformingprogram that used a solar dish concentrator from 1997 to 2002

[19]. Two reformer designs were tested: a multistraight tube

reformer consisting of six straight tubes connected in parallel,

and a single coiled reformer tube [19]. This successful opera-

tion demonstrated the production of proton exchange

membrane (PEM) fuel cell-quality H2. In 2004, CSIRO began

a new program to commercialize this technology by building

a single, small tower module of 500 kW solar capacity that

would demonstrate larger scale reforming [20,21]. A smaller

reformer has also been in operation for steam reforming

under solar irradiation [20,21].

Highly active catalysts based on metal Rh are frequently

used for the solar CO2 reforming of methane [5,8]. Rh/g-Al2O3-loaded alumina and SiC foam absorbers were extensively

tested for the solar reforming of methane by using solar

concentrating systemscapable of utilizing up to 100e300 kWthof solar power [7,9e11,13,17]. Ru catalysts have also been

proposed and demonstrated as highlyactivealternativesto Rh

catalysts for use in methane reforming. g-Al2O3-supported Ru

catalyst was examined for the CO2 reforming of methane at

a temperature of 550 C [24]. Berman and Epstein examined

the RueCe catalyst to improve the activity and thermal

stability of Ru/g-Al2O3 catalysts for the solar CO2 reforming of

methane [18]. Ru/(a-Al2O3MnOx) catalysts were studied for

use in solar reforming at temperatures of 500e900 C and

a total pressure of 1e7 atm [25].The present study introduces a new type of ceramic foam

absorber that is coated with cost effective and high-temper-

ature stable Ni/MgOeAl2O3 and NieMgeO catalysts. These

new products were prepared and their activity was tested

using a laboratory-scale receiverereactor with a transparent

quartz window and a sun-simulator.

2. Concept for development of new SiC foamabsorber activated by Ni-based catalyst

Ni-based catalysts are generally considered economically

suitable for solar reforming processes that producea synthetic gas and hydrogen. However, Ni-based catalysts

often cause carbon deposition on the surface of the catalyst

during the processes, resulting in a deactivation of the cata-

lyst. Many recent reports in the literature have indicated that

the activity and stability of Ni-based catalysts are greatly

improved by the use of rare-earth-based promoters or alka-

line-earth metal ions [26e37]. An Ni catalyst containing

MgOeCaO showed excellent activity and stability for the CO2reforming of methane [28]. A NickeleMagnesia solid solution,

Ni0.03Mg0.97O, provided ultrastable activity without carbon

deposition that sustained its desirable activity in the CO2reforming of methane for the long time of 100 days [34]. Stable

and cost effective NieMgeO was proposed as a catalyst for



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solar reforming and a solar receivereabsorber that used

alumina ceramic foam was prepared. The activity of the

absorber was studied for solar CO2 reforming in a laboratory-

scale volumetric receiverereactor under direct irradiation by

a sun-simulator [36]. The SiC foam, which was reported to be

more resistant to thermal shock at high temperatures in the

large-scale solar test of the Rh/g-Al2O3/SiC absorber [17], was

used instead of alumina foam as a matrix for the preparationof a catalytically activated Ru/NieMgeO absorber. The

reforming performance of the Ru/NieMgeO/SiC absorber was

investigated using a laboratory-scale volumetric recei-

verereactor and a sun-simulator [37]. However, as described

above, other active and thermally stable Ni-based absorbers

that do not use noble metals such as Pt, Rh and Ru will be

desirable for solar reforming at high temperatures.

Mixed oxides resulting from the thermal treatment of

hydrotalcites present several interesting properties [38].

Hydrotalcite-like compounds are layered double hydroxides.

The general formula of these compounds can be represented

as [M1 x(II) Mx

(III)(OH)2]x [Ax/n

n]mH2O where M(II) and M(III) are

divalent and trivalent cations and A is the compensationanion. This structure can accommodate a wide variety of the

different M(II) and M(III) metals, which in turn lend it different

properties. The thermal decomposition of these materials by

calcination results in the formation of homogeneous mixed

oxides with a high thermal stability and a larger surface area.

In addition, well-dispersed metallic particles are usually

obtained after a reduction treatment [38]. The preparation of

heterogeneous catalysts derived from MgeAl hydrotalcite-like

compounds containing Ni at the Mg sites as precursors, and

their utilization for the steam and dry reforming has been

reported [39e46]. Shishido et al. [40] tested Ni/MgeAl oxide

catalysts derived from MgeAl hydrotalcite precursor for the

CO2 reforming of methane under CH4/CO2 1:1 in a conven-tional flow reactor with a fixed bed quartz tubular reactor at

temperatures of 800 C at atmospheric pressure and a Gas

Hourly Space Velocity (GHSV) 51 Ndm3 gcat1 h1. It was found

that the Ni/MgeAl oxide catalysts exhibit higher activity than

those prepared by the conventional impregnation method,

such as Ni/a-Al2O3 and Ni/MgO. Tsyganok et al. [45] focused on

the deposition of coke onto the Ni catalyst surface, the spatial

distribution of supported Ni after the CO2 reforming of

methane, and the thermal stability of Ni catalyst supported on

MgeAl mixed oxide. Roh et al. [44] examined the combined

steam and CO2 reforming of methane using (H2O CO2)/CH4of 1.2 at 800 C under atmospheric pressure at

a GHSV 265 Ndm3 gcat1 h1. Ni/MgOeAl2O3 prepared from

hydrotalcite material revealed a high degree of activity and

stability in comparison to Ni/MgO, Ni/ZrO2, Ni/CeO2 and Ni/a-

Al2O3. Koo et al. [46] studied Ni catalyst supported MgOeAl2O3prepared from hydrotalcite material for the combined steam

and dry reforming of methane. Ni/MgOeAl2O3 prepared fromhydrotalcite material exhibited remarkable coke resistance,

but commercial Ni/MgeAl2O4 catalyst showed considerable

coke deposition during the combined reforming.

In the present work, Ni/MgOeAl2O3 catalyst derived from

hydrotalcite (Mg6Al2(OH)16CO34H2O) and NieMgeO catalyst,

which are much cheaper than Rh and Ru noble metal cata-

lysts, are first investigated in terms of their activity and the

thermal stability of the CO2 reforming of methane. Second,

a new type of catalytically activated ceramic foam absorberse

Ni/MgOeAl2O3 and NieMgeO catalyzed SiC foam absorbers e

were prepared and evaluated in terms of their activity and

thermal stability using a laboratory-scale receiverereactor

with a transparent quartz window under direct light irradia-tion by a sun-simulator.

3. Experimental procedure

3.1. Materials

The chemicals of hydrotalcite (Mg6Al2(OH)16CO34H2O) and Ni

(NO3)2$6H2O (purity 99.9%) were purchased from Wako Pure

Chemical Industries Ltd. MgO powder (Grade 500A, purchased

from Ube Material Industries Ltd.) was also used for prepara-

tion of the catalyst. SiC ceramic foam disks purchased from

Krosaki Harima Co. were used as a matrix for the preparationof foam absorbers. The disks had diameters of 20 and 30 mm,

a thickness of 10 mm, and cell size of 13 cpi (cells per linear

inch). Table 1 shows the foam absorbers prepared for the

present study.

3.2. Preparation of Ni/MgOeAl2O3 and NieMgeO

powder catalyst

For the preparation of Ni/MgOeAl2O3 catalyst, hydrotalcite

(Mg6Al2(OH)16CO3$4H2O) was used as a precursor. MgOeAl2O3

Table 1 e Catalytically activated foam absorbers used in this study.

Catalyst Foam matrix Porosity (cpi) Dimensions (mm)a Loading (wt%)

MgOb Nic NieMgeOb Ni/MgOeAl2O3b

Ni/MgO SiC 13 20 10 2e4 11

30 10 4 11

NieMgeO SiC 13 20 10 2e4

30 10 4

Ni/MgOeAl2O3 SiC 13 20 10 1e4

30 10 4

a Diameter and thickness.

b With respect to the mass of the foam matrix.

c With respect to the mass of the support.

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 e n e r g y 3 5 ( 2 0 1 0 ) 7 4 4 1 e7 4 5 3 7443


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support derived from hydrotalcite was prepared by pre-calci-

nation at various temperatures in the range of 800e1200 C for

6 h in air at a flow rate of 0.3 dm3 min1 in a normal state. The

calcined powder was suspended in a nickel nitrate solution

and was then left to evaporate until it was dry. The loading

amountof Nion the support was fixedat 12wt% [46]. Next, the

dried powder was calcined at 600e1000 C for 6 h in air at

a flow rate of0.3Ndm3 min1 and then reducedby a mixed gasof 50%Ar and 50% H2 at aflowrate of0.1 Ndm

3 min1 at 800 C

for 1 h.

NieMgeO catalyst was prepared as follows. The powder of

the MgO support was suspended in a nickel nitrate solution

that was then left to evaporate until it was dry. The dried

powder was then ground in a mortar and then calcined at

750e1300 C for 6 h in air (at a flow rate of 0.3 Ndm3 min1).

The loadingamount of Ni on theMgO support was fixed at 11%

by weight [37]. The calcined powder was reduced at 800 C for

1 h in a mixed gas of 50% Ar and 50% H2 at a flow rate of

0.1 Ndm3 min1.

The BET surface areas of the catalysts used were measured

by nitrogen adsorption (Shimadzu, Micromeritics Flow Sorb II2300) at 77 K. The catalyst powders were analyzed using XRD

with CuKa

radiation (MAC Science, MX-Labo) for identification

of the formed phases.

3.3. Preparation of Ni/MgOeAl2O3 and NieMgeO

catalyzed SiC foam absorber

The Ni/MgOeAl2O3 catalyzed SiC foam absorber was prepared

as follows. The SiC ceramic foam disks used in the experi-

ments were initially coated with Ni/MgOeAl2O3 particles by

the wash-coat method described below. The coating of the

foam disks was performed by soaking them in a well-stirred

aqueous slurry of fine Ni/MgOeAl2O3 particles. The Ni/MgO-eAl2O3-coated foam was dried at 100 C for 24 h then calcined

at 1000 C f o r 2 h i n a n N2 stream at a flow rate of

0.4 Ndm3 min1. It was necessary to keep the density of the

slurry (10e20 g dm3) sufficiently low in order to prevent the

pores of the foam structure from clogging. These Ni/MgO-

eAl2O3 coating processes were repeated until the loading

reached 1e4 wt%, with the Ni/MgOeAl2O3 loadings estimated

from the difference in their weight before and after the

coating process.

For the preparation of the NieMgeO catalyzed SiC foam

absorber, the SiC foam disk was initially coated with

NieMgeO particles by the wash-coat method described below.

The coating of the foam disks was performed by soaking themin a well-stirred aqueous slurry of fine NieMgeO particles.

The NieMgeO-coated foam was subsequently dried at 100 C

for 24 h then calcinedat 700 C for 2 h in an N2 streamat a flow

rate of 0.4 Ndm3 min1. These NieMgeO coating processes

were repeated until the loading reached 2e4 wt%, with the

NieMgeO loadings estimated from their difference in weight

before and after the coating process.

For comparison, a SiC foam absorber activated with Ni/

MgO was also prepared using the same SiC foam disk,

according to the procedure used in a previous work [37]. The

SiC foam disk was first wash-coated with MgO slurry solution,

then dried at room temperature overnight, and then calcined

at 1000 C for 1 h in air. This MgO coating processwas repeated

until the MgO coating reached the desired degree of MgO

loading on the foam. The MgO loadings were calculated from

the masses of coated and uncoated foams. After the MgO

coating process, an Ni2 was applied at 11 wt% Ni with respect

to the mass of the magnesia support. An Ni(NO3)2 ethanol

solution was added dropwise to the MgO-coated ceramic foam

disk, then allowed to dry at room temperature overnight, and

thencalcined at 1000 C in air for 3 h. Hereinafter, theSiC foamabsorber activated with Ni/MgO is referred to in this paper as

Ni/MgO/SiC foam absorber.

The absorbers of the Ni/MgOeAl2O3e, NieMgeOe, and Ni/

MgO-catalyzedSiCfoamthatwerepreparedarelistedin Table1.

3.4. Activity testing of Ni/MgOeAl2O3 and NieMgeO

powder catalyst

The experimental setup is illustrated in Fig. 1(a). The powder

catalyst (0.0725 g) was packed in the reactor of a transparent

quartz tube with an inner diameter of 23 mm. The thickness of

the catalystbed was set to1 mm. The powdercatalystwas put on

a floccose quartz and sandwiched between SiC foam for fixationof the powder catalyst inside the quartz tube while the alumina

tubeswere insertedfor thepreheatingof thereactant gasmixture

beneath the lower SiC foam. The quartz tube reactor was placed

in an electric furnace and insulated with refractory bricks. A

CH4eCO2 gas mixture (pCH4 50% andpCO2 50%) was fed into

the reactor ata flow rateof 0.2 N dm3 min1 at Gas Hourly Space

Velocity(GHSV)165Ndm3 gcat1 h1 at1atm.Thepowdercatalyst

in the reactor was then heated at 900 C in order to carry out the

methane reforming. The temperature of the catalyst bed was

measured using a K-type thermocouple placed at the center of

the catalyst bed that was in contact with it and that was packed

inside the reactor. The effluent gases were analyzed by gas

chromatography equipment (Shimadzu, GC-8A) with a thermalconductivity detector (TCD) to determine the gas composition.

Ni/MgOeAl2O3 powder catalyst was tested at a flow rate of

1.4 N dm3 min1 (Gas Hourly Space Velocity

(GHSV) 1155 Ndm3 gcat1 h1), which was 7 times higher than

that which was used testing the NieMgeO powder catalyst.

Because it displayed extremely high activity, the methane

conversion reached 100% under all testing conditions at a flow

rate of 0.2 Ndm3 min1.

3.5. Activity testing of catalytically activated SiC foam

absorbers by Xe-light irradiation

A double-walled quartz reactor was used for the catalyticactivity tests of the prepared absorbers. The experimental

setup is illustrated in Fig. 1(b). The inner diameter of the

outer quartz reactor was 39 mm, while the inner quartz tube

had an inner diameter of 31 mm, and the thickness of both

quartz tubes was about 2 mm. The absorber disk was

attached to a porous quartz plate inside the inner tube and

the reactor was insulated with refractory bricks. The reaction

feed gas was fed into the outer annulus and was allowed to

flow through the foam absorber disk into the inner tube of

the reactor. A 50% CH4/50% CO2 gas mixture was fed into the

reactor at a flow rate of 1.34 dm3 min1 (GHSV of 25,000 h1)

for the foam with a diameter of 20 mm and 1.50 dm 3 min1

(GHSV of 25,000 h1) for the foam with a diameter of 30 mm,



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after which the absorber was irradiated with solar-simulated

Xe light in order to carry out the CO2 reforming of methane. A

Xe-arc lamp house set (CINEMECCANICA, 5 kW ZX8000H,

Milan, Italy) was used to simulate concentrated solar radia-

tion. The energy flux distribution of the simulated incident

light on the front surface of the absorber was measured

previously using a heat flux transducer with a sapphire

window attachment (Medherm, 64-1000-21). The tempera-

ture of the absorber was measured by a K-type thermocouple

shielded with a glossy metal cover at the center of the front

surface of the absorber. The steam in the effluent gas mixture

from the reactor was condensed in a cooling trap connectedto the outlet of the reactor, after which the composition of

the gas mixture was analyzed using a gas chromatograph

(Shimadzu, GC-8A) equipped with a TCD detector.

3.6. Calculation of efficiencies

The methane conversion (X ) was estimated from the

following equation:

X yCO yH2

4yCH4 yCO yH2(4)

where YCH4 , yCO, and YH2 are the respective mole fractions of

CH4, CO, and H2 in the effluent gas [36,37].

Next, the conversion from light to chemical energy, i.e., the

chemical storage efficiency (hchem), was estimated as follows.

First, the overall reaction occurring in the gas phase was

determined from the experimental data. The CO2 reforming of

methane, as described in Eq. (2), is frequently associated with

a reverse wateregas shift reaction

H2 CO2 CO H2O(g) H298 K 41 kJ mol1 (5)

and the H2/CO ratio in the product gas becomes lower than

the stoichiometric ratio of 1. In this case, the overall reaction

in the gas phase is written as:

CH4 (1 x)CO2 (2 x)CO (2 x)H2 xH2O(g) H298 K

(overall) 247 41x kJ mol1 (6)

where x indicates the contribution of the reverse wateregas

shift reaction. The value of x was determined from the

experimental H2/CO ratio in the product gas. Thus, the power

stored as chemical enthalpy by the overall reaction,

Wchem(kW), can be experimentally estimated from:

Wchem fCH4$X$DH298Koverall (7)

where fCH4 (mol s1) indicates the molar flow rate of methane

into the reactor inlet. The chemical storage efficiency, hchem,of the incident light with respect to the chemical enthalpy is

defined as:

hchem Wchem=Winc (8)

where Winc (kW) is the total power of the incident light on the

absorber.

The chemically absorbed power density, Pd (kW m2), of

the absorber was defined by the power absorbed in the

chemical reaction, Wchem, divided by the irradiated surface

area, S, of the absorber.

The power of the reformed gas (CO and H 2), HHVreformed gas(kW), was estimated from the following equation:

HHVreformed gas Fout$PCO$DHCO Fout$PH2$DHH2 (9)

where Fout, PCO, PH2 , DHCO, and DHH2 are the molar flow rate of

the outlet gas (mol s1), the partial pressure of CO and of H 2(%), and the combustion heat of CO and of H 2 (kJ mol

1),

respectively. The combustion heat of CO and H 2 are written as

follows:

CO 1/2O2 (g) CO2 DHCO 282.98 kJ mol1, (10)

H2 1=2O2 H2Ol DHH2 285:83 kJ mol1 (11)

The molar flow rate of the outlet gas,Fout, is expressed using

the molar feed rate of CH4,fCH4 , and CO2, fCO2 , (mol s1),

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Fig. 1 e Schematics of the experimental setup for testing the activity of (a) powder catalyst in a tubular quartz reactor and (b)

absorber by solar-simulated Xe-light irradiation.



6/13

methane conversion, X, and H2/CO ratio in the outlet gas as

follows:

Fout fCH4 fCO2 2X$fCH4

2 1 H2=CO

1 H2=CO

!$X$fCH4 (12)

4. Results and discussion

4.1. Activity of Ni/MgOeAl2O3 and NieMgeO powder

catalyst

The XRD patterns of the original hydrotalcite and the hydro-

talcite that was pre-calcined at various temperatures ranging

from 800to 1200 C are shown in Fig. 2(a). Hydrotalcite powder

was transformed after pre-calcination at 800 C into an

MgAlO4 (MgOeAl2O3)-like phase with broad peaks. After pre-

calcination at 1000 C, obvious MgOeAl2O3 peaks appeared in

the pattern. In addition, the intensity of the MgOeAl2O3 peaks

increased due to crystalline growth at increasing pre-calci-

nation temperature ranging from 800 to 1200 C. Fig. 2(b)

shows XRD patterns of Ni-loaded MgOeAl2O3 (Ni/MgOeAl2O3)

powder catalyst calcined at various temperatures ranging

from 600 to 1000 C. The loading amount of Ni catalyst on the

MgOeAl2O3 support was fixed at 12 wt%. Strong peaks due to

metallic Ni were not observed, but a very weak peak was

found at a diffraction angle 2q of around 50 in all the XRD

patterns. The results indicate that well-distributed fine Ni

particles are loaded on the MgOeAl2O3, although MgOeAl2O3powder after Ni impregnation was calcined at high tempera-

tures in the rangeof 600e1000 C. Fig. 3 shows XRD patterns of

Ni/MgOeAl2O3 and NieMgeO powder catalysts before use in

activity tests. As seen in Fig. 3(a), the peak intensities due to

metallic Ni for Ni/MgOeAl2O3 catalyst were not enhanced

after H2 reduction at 800 C. Thus, the Ni/MgOeAl2O3 powder

catalyst for use in the activity test was prepared at a pre-

calcination temperature of 1000 C forhydrotalciteregent, and

was subsequently treated at 800 C after Ni impregnation by

the wash-coating method. Ni/MgOeAl2O3/SiC foam absorber

was also prepared using the powder catalyst. On the other

hand, for NieMgeO catalyst, the peaks due to metallic Ni were

not observed in the XRD pattern (Fig. 3(b)). The high

0

2000

4000

6000

8000

10000

80706050403020

eticlatordyhlanigirO

C008

C0001

C0011

C0021

C006

C007

C009

C0001

2 ( K-uC

seerged/)

Intensity/cps

Intensity/cps

2 ( K-uC

seerged/)

C008

0

1000

2000

3000

4000

5000

6000

80706050403020

lAgM 2O4

gM 6 lA 2 )HO( 61 OC 3 H4 2O

iN

lAgM 2O4

a

b

Fig. 2 e XRD patterns of (a) original and pre-calcined hydrotalcite at various temperatures of 800e1200 C and (b) Ni-loaded

MgOe

Al2O3 (Ni/MgOe

Al2O3) powder catalyst pre-calcined at various temperatures of 600e

1200 C.



7/13

temperature calcinations of 750e1300 C in the preparation

make it possible that the Ni ions were incorporated into the

lattice of the MgO support to form an NieMgeO solid solution.

The BET surface areas of the NieMgeO powder catalyst that

was calcined at 750e1300 C are listed in Table 2. The surface

areas of the NieMgeO catalyst were at a maximum value at

1200 C calcination. XRD patterns of the NieMgeO catalyst

calcined at 750e1300 C were observed in addition to the

measurement of BET surface area. The XRD peaks due to

NieMgeO particles broadened after the calcination at 1200 C

in comparison to the other calcination temperatures. This

result means that the calcination at 1200 C caused a forma-

tion of fine particles of an NieMgeO solid solution.

The catalytic activities of the NieMgeO powders calcined

at various temperatures ranging from 750 to 1300 C were

examined at a GHSV of 165 Ndm3 gcat1 h1 in the time span

from 60 to 130 min. The activity tests were performed under

homogeneous heating with an electric furnace. The time

variations of methane conversion for the NieMgeO powders

calcined at 750e1300 C are presented in Fig. 4. The NieMgeO

catalyst after calcination at 1200 C exhibited the greatest

activity among those calcined at temperatures of 750e1300 C.

In addition, the NieMgeO catalyst with H2 reduction (solid

circles) showed activity that was substantially higher than

that without the H2 reduction (open circles). Fig. 4 also

represents time variations of methane conversion for Ni/

MgOeAl2O

3powder catalyst during 70 min of reforming (open

squares). The Ni/MgOeAl2O3 powder catalyst sustained

a 100% methane conversion at a GHSV of 165 Ndm3 gcat1 h1

through the time period of reforming, and showed a higher

degree of methane conversion than NieMgeO powder

catalysts.

In order to optimize a preparation of Ni/MgOeAl2O3powder catalyst and evaluate the superiority of its catalytic

activity to NieMgeO powder catalyst, the catalytic activity of

Ni/MgOeAl2O3 powder was examined using MgOeAl2O3powder pre-calcined at various temperatures ranging from

800 to1200 C. The activity tests were performed usinga GHSV

of 1155 Ndm3 gcat1 h1, which is 7 times higher than was used

in the reforming test at a GHSV of 165 Ndm3 gcat1 h1, as shown

in Fig. 4. Fig. 5(a) shows the average methane conversion

during the time period of reforming and the BET surface area

of Ni/MgOeAl2O3 powder catalyst after the activity test.

Nevertheless, methane conversion and the BET surface area

reached their highest values of 72% and 60 m2 g1, respec-

tively, at a calcination temperature of 1000 C. Next, the

catalytic activity of Ni/MgOeAl2O3 powder after Ni

0

1000

2000

3000

4000

5000

6000

7000

0

200

400

600

800

1000

1200

1400

1600

1800

80706050403020

2 ( K-uC seerged/)

80706050403020

2 ( K-uC seerged/)

Intensity/cps

Intensity/cps

OgMiN

lAgM O

lA-OgM/iN2O

3 O-gM-iNa b

Fig. 3 e XRD patterns of (a) Ni/MgOeAl2O3 and (b) NieMgeO powder catalysts before activity testing. The Ni/MgOeAl2O3powder catalyst was prepared using the calcined temperature of 1000 C for original hydrotalcite and at 800 C for Ni

loading. The NieMgeO powder catalyst was prepared using calcination temperature of 1200 C.

Table 2 e BET surface area of NieMgeO powder catalystafter H2 reduction.

Calcination temperature (C) 750 1000 1100 1200 1300

BET surface area (m2/g) 32.9 14.4 12.4 20.5 0.2

Methaneco

nversion/%

Time / min

C057

C0001

C0011

C0021

Htuohtiw(C0021 2 )noitcuder

C0031

0

20

40

60

80

100

140120100806040200

lA-OgM/iN 2O3 )C0001(

Fig. 4 e Time variations of methane conversions for the Ni/

MgOeAl2O3 and NieMgeO powder catalysts at GHSV of

165 Ndm3 gcatL1 hL1.



8/13

impregnation (Ni 12 wt%) calcined at various temperatures

ranging from 600 to 1000 C was examined. Fig. 5(b) shows the

time variations of methane conversion for the Ni/MgOeAl2O3powder catalyst. After Ni impregnation, the maximum

methane conversion was obtained at a calcination tempera-

ture of 800 C. Therefore, hydrotalcite was calcined at 1000 C,

and the obtained mixed oxide of MgOeAl2O3 was subse-

quently treated for Ni impregnation at 800 C. The resulting

Ni/MgOeAl2O3 powder catalyst was used for irradiation

testing of the Ni/MgOeAl2O3/SiC foam absorber.

4.2. Activity of absorbers by solar-simulated Xe-light

irradiation

Photographs of the original SiC foam (non-coated) and three

kinds of catalytically activated SiC foam absorbers before the

0

20

40

60

80

1200110010008000 0

20 20

40 40

60 60

80 80

C/erutarepmetnoitaniclaC

BETsurfacearea/(m2/g)

Averagemethaneconversion/%

6040200

Methaneconversion/%

Time / min

C006C007

C008

C009

C0001

a b

Fig. 5 e Reforming performances for Ni/MgOeAl2O3 powder catalyst at GHSV of 1155 Ndm3 gcatL1 hL1. (a) Average methane

conversion during reforming time of period and BET surface area after the activity test. The hydrotalcite calcined at various

temperatures of 800e1200 C was used for a preparation of the catalyst. (b) Time variations of methane conversion. The

powder catalysts after Ni loading were calcined at various temperatures of 600e1000 C.

Fig. 6 e Photographs of (a) original SiC foam and (bed) the prepared catalytically activated foam absorbers. (b) Ni/MgO, (c)

NieMgeO and (d) Ni/MgOeAl2O3-activated SiC foam absorbers before activity testing.



9/13

Xe-light irradiation test are shown in Fig. 6. The color of the

original SiC foam was gray, and this did not change after

application of the Ni/MgO, NieMgeO, and Ni/MgOeAl2O3powder catalysts. In addition, many obvious open pores on

the prepared SiC foam absorbers were observed that were not

clogged by agglomeration of their powder catalysts. Thus,

their powder catalysts were successfully well-distributed and

loaded onto the SiC foam.

Fig. 7 shows the energy flux distribution of the incident

solar-simulated Xe light on the irradiated surface of the

absorbers. The total power input (Winc) and average flux

density of the incident Xe light input into absorbers that haddifferent diameters of 2 and 3 cm are shown in Table 3. The

total power input (Winc) of the incident Xe light into the

absorbers was 0.120e0.204 kW, and the peak flux density of

the incident Xe light on the exposed surface of the absorbers

was 547 kW m2. The average flux density (r) on the exposed

surface of the absorbers was 289e383 kW m2, which is

defined as r Winc/S, where S is the surface area of the irra-

diated absorbers.

The catalytic activity of the Ni/MgOeAl2O3/SiC, NieMgeO/

SiC, and Ni/MgO/SiC foam absorbers was examined at a GHSV

of 25,000 h1(1.34 Ndm3 min1) during Xe-light irradiation for

60 min. The irradiation tests were performed under the above-

mentioned irradiation condition (Fig. 7). Fig. 8(a) shows the

time variations of methane conversion for the Ni/MgOeAl2O3/

SiC foam absorber with a diameter of 2 cm. The loading

amount of Ni/MgOeAl2O3 catalyst varied from 1 to 4 wt% withrespectto the mass of theSiC foam matrix. As seen in Fig. 8(a),

the methane conversion gradually increased with increasing

irradiation time for all the foam absorbers, and almost pla-

teaued at an irradiation time of 60 min. The 2 wt% Ni/MgO-

eAl2O3/SiC foam absorber provided the highest degree of

methane conversion among them, which was twice as high as

that of the NieMgeO/SiC foam absorber, as described below.

Time variations of methane conversion for the Ni/MgO/SiC

prepared by the previous method [37] and the NieMgeO/SiC

foam absorbers prepared by the presentstudy were examined,

and a comparison on the basis of their activity is presented in

Fig. 8(b). According to the previous method that employed two

wash-coatings for MgO loading and subsequent Ni loading,the methane conversion initially increased but then gradually

decreased with increasing irradiation time. In addition, the

methane conversion was hardly enhanced at all, although the

loading amount of Ni/MgO catalyst was increased. On the

contrary, in the case of the present study that used a wash-

coating of prepared NieMgeO catalyst, the methane conver-

sion increased with an increasing loading amount of

NieMgeO catalyst, and deactivation was not observed for the

foam absorbers. It can thus be expected that much further

loading of NieMgeO catalyst would enhance methane

conversion.

Fig. 9 shows time variations of temperature and methane

conversion for the Ni/MgOeAl2O3/SiC and NieMgeO/SiC foamabsorbers when SiC foam with a diameter of 3 cm was used as

a foam matrix. The testing of catalytic activity involved

examination at a GHSV of 25,000 h1 (1.34 Ndm3 min1) during

Xe-light irradiation of 60 min. The temperature was measured

in the center of the irradiated surface of the foam absorber.

Both foam absorbers displayed nearly constant temperatures

of 1070e1080 C for the NieMgeO/SiC foam and 1000e1010 C

for the Ni/MgOeAl2O3/SiC foam during the time period of

0

100

200

300

400

500

600

700

20151050-5-10-15-20

)mm(sixa-X

Fluxdens

ity/kW-m-2

maofmm02

maofmm03

Fig. 7 e Energy flux distribution of incident Xe light on the

irradiated surface of the absorber. The total power and flux

density for the Xe-light irradiation condition are shown in

Table 3.

Table 3e

Results for CO2 reforming with catalytically activated foam absorbers used in this study under the irradiation byhigh flux Xe light.

Foamabsorber

Dimensions(mm)

Total powerinput(kW)

Average fluxdensity

(kW m2)a

Peak fluxdensity

(kW m2)b

GHSV(104 h1)

Temperature(C)c

Power ofreformedgas (kW)

Maximummethane

conversion(%)

Ni/MgO-

Al2O3/SiC

20 10 0.120 383 547 2.5 995e1033 0.21 37

30 10 0.204 289 547 2.5 1002e1008 0.44 35

NieMgeO/

SiC

20 10 0.120 383 547 2.5 1030e1084 0.11 19

30 10 0.204 289 547 2.5 1067e1085 0.30 23

a Mean flux density of solar-simulated Xe light for irradiation.

b Peak (or central) flux density of solar-simulated Xe light for irradiation.

c Temperature at the center of the irradiated surface of the absorber.



10/13

irradiation. It should be noted that the foam temperature for

the Ni/MgOeAl2O3/SiC foam was lower than for the NieMgeO/

SiC foam under the same irradiation condition. The resulting

methane conversion was 1.5 times higher for the Ni/MgO-

eAl2O3/SiC foam than for the NieMgeO/SiC foam during the

2-h period of reforming. These results indicate that Ni/MgO-

eAl2O3/SiC foam exhibits stable and high catalytic activity

during direct irradiation with concentrated Xe light

throughout an entire period of at least 2 h. Thus, the Ni/

MgOeAl2O3-activated SiC foam absorber is preferable for use

at a relatively high average flux. In the next step of this study,the irradiation time period is much longer, and the thermal

endurance, deactivation, and local-overheating of the foam

absorber is necessary to be studied for long-term light irradi-

ation at the level of 100 h.

The power stored as chemical enthalpy by the overall

reaction (Wchem) is plotted against the irradiation time period

for the SiC foam absorbers activated with Ni/MgOeAl2O3 and

NieMgeO catalysts. The results are shown in Fig. 10. The

Wchem values for the Ni/MgOeAl2O3/SiC foam absorber were

1.8 timesgreater in the 2-cm foam and 2 times greater in the 3-

cm foam than those for the NieMgeO/SiC foam absorber.

Thus, the higher catalytic activity of the Ni/MgOeAl2O3/SiC

foam absorber makes it preferable for use at relatively high

values of average flux (r 289e383 kW m2).

Fig. 11 shows a comparison of the chemically absorbed

power density(Pd) of the SiC foam absorbers activated with Ni/

MgOeAl2O3 and NieMgeO catalysts. The Pd value was plotted

against the irradiation time period. The CH4eCO2 mixture was

passed through the absorbers at a GHSV of 25,000 h1

. The Pdvalues reached 82e97 kW m2 for the NieMgeO/SiC foam

absorbers and 145e151 kW m2 for the Ni/MgOeAl2O3/SiC

foam absorbers. The Pd values for the NieMgeO/SiC foam

absorbers were 1.8 times greater in the 2-cm foam and 1.5

times in the 3-cm foam than those for the NieMgeO/SiC foam

absorber. In order to evaluate the catalytic activity of the

present foam absorbers, the Pd values and the light-to-

chemical energy conversion efficiency, hchem, were compared

0 0

10

10

20

20

30

3040

6050403020100

lA-OgM/iN2O

3

Methaneconversion/%

Time / min

6050403020100

Time / min

%tw1

%tw2

%tw4

%tw3

Methaneconversion/%

)ydutssuoiverp(CiS/OgM/iN

)ydutstneserp(CiS/O-gM-iN

O-gM-iN

%tw2

%tw2

%tw4%tw4

%tw3 %tw3

a b

Fig. 8 e Time variations of methane conversion for (a) the Ni/MgOeAl2O3/SiC foam absorber and (b) the NieMgeO/SiC foam

absorber with diameter 20 mm. The loading amount for the powder catalysts varied in 1e4 wt% and 2e4 wt% with respect to

the mass of SiC foam matrix.

800

900

1000

1100

1200

1201008060402000

10

20

30

40

50

120100806040200

erutarepmeT noisrevnocenahteM

lA-OgM/iN 2O3 CiS/

CiS/O-gM-iN

CiS/O-gM-iN

Time / min Time / min

Temperature/C

Methaneconversion/% lA-OgM/iN 2O3 CiS/

a b

Fig. 9 e Time variations of (a) temperature and (b) methane conversion for the Ni/MgOeAl2O3/SiC and the NieMgeO/SiC

foam absorbers with diameter of 30 mm.



11/13

with those reported elsewhere. The results for the Pd and

hchem values were plotted together with those from thereferences shown in Fig. 12. The result by Buck et al. [7] was

reported in regard to the CAESAR project that was selected as

a referencefor evaluation of the present results. In most of the

literature, the solar demonstration of catalytically activated

ceramic absorbers was carried out in much larger volumetric

reactors and at higher solar input levels. Thus, their Pd values

and hchem values cannot be directly compared with the current

results. However, according to the report [7], in the CAESAR

project, an Rh/g-Al2O3-activated ceramic absorber with

a diameter of 64 cm was mounted in a large-scale volumetric

receiverereactor and tested with a higher solar input level of

74e115 kW. Both uniform and non-uniform ceramic absorbers

were tested using the absorber design that modeleda flat platefoam disk. The results obtained by the uniform absorber were

selected and used for comparison in the present paper. As

seen in Fig. 12(a), the Pd values for the Ni/MgOeAl2O3/SiC

activated ceramic absorber were comparable with those of the

CAESER absorber. On the other hand, those of the NieMgeO/

SiC activated ceramic absorber were relatively lower than

those of the CAESER absorber at a high average flux density ofirradiation. However, the Pd value could be significantly

improved by the Ru/NieMgeO/SiC activated ceramic absorber

inthecasethatanNieMgeO/SiC absorber loads Ru catalyst on

the surface [37]. The chemical storage efficiencies, hchem, for

the Ni/MgOeAl2O3/SiC and NieMgeO/SiC activated ceramic

absorbers were compared with those of the CAESER absorber.

As seen in Fig. 12(b), the hchem values for both absorbers

increased with an increase in the foam size of the absorber. In

addition, the hchem values were 50% for the Ni/MgOeAl2O3/SiC

and 34% for the NieMgeO/SiC activated ceramic absorber

with the larger foam absorber (Table 3). The hchem value of 50%

is comparable with those reported for the CAESER absorber.

The present Ni-based catalytic absorbers are more costeffective than conventional Rh/g-Al2O3 and Ru/g-Al2O3 cata-

lyzed SiC foam absorbers. The Ni/MgOeAl2O3 catalyzed SiC

foam absorber, in particular, will be desirable for use in solar

receiverereactor systems for the conversion of concentrated

high solar fluxes to chemical fuels via endothermic natural-

gas reforming at high temperatures.

0

0.02

0.04

0.06

0.08

0.1

0.12

150100500

)mc3(CiS/O-gM-iN

)mc2(CiS/O-gM-iN

lA-OgM/iN O )mc2(CiS/

lA-OgM/iN O )mc3(CiS/

nim/emiT

Powerstoredasa

chemicalenthalpybythe

overallreaction(W

)/kW

Fig. 10 e Chemically absorbed power (Wchem) as a function

of irradiation time for the Ni/MgOeAl2O3/SiC and the

NieMgeO/SiC foam absorbers.

0

50

100

150

200

150100500PowerdensityabsorbedInthechemical

reaction(P)/kWm

nim/emiT

)mc3(CiS/O-gM-iN

)mc2(CiS/O-gM-iN

lA-OgM/iN O )mc2(CiS/lA-OgM/iN O )mc3(CiS/

Fig. 11 e Power density absorbed in the chemical reaction

(Pd) as a function of irradiation time for the Ni/MgOeAl2O3/

SiC and the NieMgeO/SiC foam absorbers. The CH4eCO2mixture was passed through the absorbers at a GHSV of

25,000 hL1.

0

50

100

150

200

200

250

250 450 450300 300400 400350 350

foytisnedxulfegarevA mWk/noitaidarri

Powerdensityabsorb

edinthe

chemicalreaction(P)/kWm-2

0

20

40

60

80

250200

ytisnedxulfegarevA mWk/noitaidarrifo

Light-to-chemicale

nergy

conversionefficien

cy/%

lA-OgM/iN(ydutstneserP O )CiS/

)CiS/O-gM-iN(ydutstneserP

/hR(RASEAC lA- O )animulA/

CiS/OgM-iN/uR

lA-OgM/iN(ydutstneserP O )CiS/

)CiS/O-gM-iN(ydutstneserP

/hR(RASEAC lA- O )animulA/

CiS/OgM-iN/uR

a b

Fig. 12 e Comparison of (a) chemically absorbed power densities of Pd and (b) chemical storage efficiencies ofhchem.

Symbols: Solid squares and circles are respectively for the Ni/MgOeAl2O3/SiC foam absorber and the NieMgeO/SiC foam

absorber in the present study. Asterisk is for Ru/NieMgeO/SiC foam absorber in the previous study [37]. Triangles are for the

Rh/g-Al2O3-activated alumina foam absorber in the CAESAR project, estimated from data in [7].



12/13

5. Summary

For the absorption and chemical conversion of high energy

fluxes into fuels, Ni/MgOeAl2O3 and NieMgeO activated SiC

absorbers were tested by the application of concentrated Xe-

light radiation (around 280 and 380 kW m2 of average flux

density) and examined as to their reforming performance. TheNi/MgOeAl2O3 catalyzed SiC foam absorber is comparable in

its chemically absorbed power densities (Pd) and chemical

storage efficiencies (hchem) to that of the conventional Rh/g-

Al2O3. Furthermore, the new absorber is more cost effective

than the Ru/NieMgeO catalyzed SiC absorber reported in the

previous paper. Ni/MgOeAl2O3 catalyzed SiC foam absorber is

found to be a promising solar absorber for a volumetric

receiverereactor or reformer.

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nimgoal2o3andnimgo catalyzed sic foam absorbers for high temperature solar reforming of methane

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