Development of Cu foam-based Ni catalyst for solar thermal reforming of methane with carbon dioxide

Accepted Manuscript

Development of Cu foam-based Ni catalyst for solar thermal reforming of methane with carbon dioxide

http://www.journals.elsevier.com/ journal-of-energy-chemistry/

Jianzhong Qi , Yanping Sun , Zongli Xie , Mike Collins , Hao Du , Tianying Xiong PII: DOI: Reference:

S2095-4956(15)00070-4 10.1016/j.jechem.2015.10.001 JECHEM 47

To appear in:

Journal of Energy Chemistry

Received date: Revised date: Accepted date:

12 May 2015 7 July 2015 9 July 2015

Please cite this article as: Jianzhong Qi , Yanping Sun , Zongli Xie , Mike Collins , Hao Du , Tianying Xiong , Development of Cu foam-based Ni catalyst for solar thermal reforming of methane with carbon dioxide, Journal of Energy Chemistry (2015), doi: 10.1016/j.jechem.2015.10.001

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Graphical Abstract

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20Ni-γCu catalyst has higher activity as compared with the 20Ni-γSiSiC catalyst at 650 oC, similar activity at 720 oC and lower activity at 800 oC.

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ACCEPTED MANUSCRIPT Development of Cu foam-based Ni catalyst for solar thermal reforming of methane with carbon dioxide Jianzhong Qia, Yanping Sunb,*, [email protected], Zongli Xiec, Mike Collinsb, Hao Dua, Tianying Xionga Institute of Metal Research, Chinese Academy of Science, Shenyang 110016, Liaoning, China b

CSIRO Energy Flagship, PO Box 330, Newcastle, NSW 2300, Australia

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CSIRO Manufacturing Flagship, PO Box 10, Clayton South, VIC 3169, Australia *

Corresponding author. Tel: +61 2 4960 6106; Fax: +61 2 4960 6111;

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This work was supported by the CSIRO Energy Flagship and the Chinese Scholarship Council.

Abstract

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Using solar energy to produce syngas via the endothermic reforming of methane has been extensively investigated at the laboratory- and pilot plant-scales as a promising method of

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storing solar energy. One of the challenges to scaling up this process in a tubular reformer is to improve the reactor’s performance, which is limited by mass and heat transfer issues.

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High thermal conductivity Cu foam was therefore used as a substrate to improve the

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catalyst’s thermal conductivity during solar reforming. We also developed a method to coat the foam with the catalytically active component NiMg3AlOx. The Cu foam-based NiMg3AlOx

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performs better than catalysts supported on SiSiC foam, which is currently used as a substrate for solar-reforming catalysts, at high gas hourly space velocity (≥400,000 mL/(g·h)) or at low reaction temperatures (≤720 C). The presence of a γ-Al2O3 intermediate layer improves the adhesion between the catalyst and substrate as well as the catalytic activity.

Keywords: Cu foam-based Ni catalyst; Monolithic catalyst; Solar thermal reforming of methane 2

ACCEPTED MANUSCRIPT 1. Introduction Steam reforming of methane (SRM) and carbon dioxide reforming of methane (CDRM, also known as ‘dry reforming’), are promising solar thermochemical processes driven by concentrated solar energy. Both reactions can store solar energy in the form of syngas (a mixture of CO and H2) at a high storage density and with low thermal loss.

as follows:

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SRM (reaction 1) and CDRM (reaction 2) are reversible and highly endothermic reactions [1,2]

o H 25 = +206 kJ/mol o C

(1)

CH4 + CO2 ↔ 2CO + 2H2

o H 25 = +247 kJ/mol o C

(2)

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CH4 + H2O(g) ↔ CO + 3H2

The water–gas shift reaction (reaction 3), or its reverse, can also occur in conjunction with both reactions (1) and (2).

o H 25 = –41 kJ/mol o C

CO + H2O (g) ↔ H2 + CO2

(3)

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Reactions (1) and (2) occur at high temperature and, in the case of conventional reforming

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processes, the heat is generally provided by the combustion of additional methane. In solar reforming heat is supplied by concentrated solar energy. The stored solar energy can

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subsequently be recovered by conducting either the reverse of the reforming reactions (i.e., by the exothermic methanation process) or by utilising the syngas, produced with lower CO2

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emissions than in conventional reforming, in any one of the many applications requiring it as

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a feed stock.

In certain applications CDRM has advantages over SRM due to its potential for a greater Lower Heating Value enhancement (up to 30% relative to the input methane) as a result of increased CO production relative to hydrogen [2]. Concentrated solar radiation has the specific properties of high energy intensity, variable thermal flux distribution, and frequent thermal transients due to the fluctuating insolation.

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ACCEPTED MANUSCRIPT Volumetric receiver-reactors and tubular reactors have been investigated for the solarreforming of methane. Both reactors have some disadvantages, as follows: 

A volumetric receiver-reactor is expensive and requires a quartz window, which may limit operating life and the potential for scale up.



A tubular reactor’s performance can be restricted by mass and heat transfer issues,

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due to the use of the reactor wall as the solar energy absorber and heat conductor to indirectly heat reactant gas and provide the heat of reaction [3].

One potential method for overcoming these limitations is to develop monolithic reforming catalysts with regular three-dimensional structure and high thermal conductivity that can

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improve mass and heat transfer in a tubular reformer. This could result in a more compact reformer with shorter gas residence time, and more efficient, less costly capture of solar energy in a thermochemical form [4].

Ceramic foam (e.g., SiC, SiSiC), which has high mechanical strength and thermal conductivity

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[5,6], has been used as catalyst substrate for solar methane reforming in volumetric

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receiver-reactors. However, metallic substrates, such as Cu foam, have better thermal conductivity and are less brittle than ceramic foam. Metallic foams could therefore minimise

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thermal transfer limitations in solar reforming. They could also prevent cracks caused by

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mechanical or thermal shock resulting from the temperature variations of fluctuating insolation and the daily starting up and down of the reformer [7].

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Interest is hence growing in using metallic foams as a catalyst substrate for solar reforming [8,9]. Roh et al. [10] investigated 2%Ru/Al2O3 catalyst-coated FeCralloy metal monolith for SRM in a tubular reactor at a temperature range of 577–727 oC. The monolithic catalyst had a higher bed thermal conductivity and greater heat transfer than Ru/Al2O3 pellet catalyst. They concluded that using a highly porous structured support with high thermal conductivity for these catalysts can improve heat exchange inside the reactor. Cybulski and Moulijn [11] modelled heat transfer in a FeCralloy monolith at a density of 62 cells/cm2 and air flow rate 4

ACCEPTED MANUSCRIPT of 0.2–0.7 kg/m2/s, as well as in a packed bed of 3.5-mm diameter pellets at the same flow rates and tube size. The overall heat transfer coefficients for the monolith were increased by about 15%. However, it is more difficult to coat catalysts onto metallic substrates than onto ceramic substrates. This is because both the smooth metallic surface and the difference in thermal

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expansion coefficient between the metal substrate and catalyst result in weak adhesion between them [12]. Hence, more research is required to improve the thermal conductivity of monolithic metallic supports, as well as the adhesion between substrate and catalyst, to improve heat transfer in monolithic catalysts.

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Lotus-type, porous Cu foams have recently attracted a great deal of attention. Their unique features include: (i) high thermal conductivity, (ii) controllable pore size and porosity, and (iii) high mechanical shock resistance. These metallic monoliths are expected to find uses in lightweight materials, catalyst supports, electrodes, vibration and acoustic energy damping

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materials, and impact energy absorption materials [13,14]. Figure 1 illustrates that Cu foam

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has the highest thermal conductivity among ceramic and metallic foams [15]. However, few

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studies to date have reported the use of Cu foam as a catalyst support.

Figure 1. Thermal conductivity of ceramic and metallic foam as a function of monolith void fraction. 5

ACCEPTED MANUSCRIPT Ni-based catalysts are used in catalyzing a wide range of reactions which includes dry reforming of methane [16], steam reforming of toluene [17] and water gas shift reaction [18]. Ni/Mg/Al catalysts prepared by co-precipitation show higher activity and stability for CDRM than catalysts prepared by conventional impregnation methods [19,20]. This is because the strong interaction between Ni and Mg-Al support improves Ni dispersion and inhibits Ni

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sintering, thereby increasing catalyst activity and stability. The objective of the paper is to investigate the catalyst Ni/Mg/Al metal oxide coated onto lotus-type porous Cu foam for solar CDRM applications. As part of this study, we developed an effective method to coat Ni/Mg/Al catalyst onto the Cu foam.

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2. Experimental

2.1. Foam catalyst preparation 2.1.1. Catalyst substrates

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Lotus-type porous Cu foam with a pore size of 420 μm was selected as a monolithic catalyst substrate. Substrates were fabricated using a unidirectional solidification method described

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in our previous work [21]. In a standard preparation, high-purity copper (99.99 wt%) was

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melted in a crucible by middle-frequency heating after the chamber was evacuated to 5.0 Pa. When the temperature reached 1250 oC, which was monitored by a W-5Re/W-26Re

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thermocouple, certain pressure high-purity hydrogen and argon were introduced into the chamber respectively. To dissolve the hydrogen uniformly in the liquid copper, the pressure

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was maintained at 5.0 Pa for 1800 s at the selected temperature. Finally, the melt was poured into the mould and solidified under the same pressure. SiSiC foam (pore size of 1000 μm, provided by Erbicol SA) was used as a reference to evaluate the performance of the Cu foam-based catalysts. To eliminate traces of grease and other contaminants, all substrates were degreased using an ultrasonicator in acetone for 20 min, and then washed with distilled water three times. The Cu foam was washed further

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ACCEPTED MANUSCRIPT using an inorganic acid solution to improve its surface roughness and coating adhesion. Preparation of the acid solution and the pre-treatment method are described in a US patent [22]. Briefly, the Cu foam was dipped into a mixed acid aqueous solution containing 40 g/L H2O2, 90 g/L H2SO4 and 4 g/L 5-aminotetrazole and etched for 3 min at room temperature. The surface of the Cu foam then became roughened.

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As an alternative approach to improving adhesion between the catalyst powder and metallic substrate [23], a γ-Al2O3 layer was first coated on the substrate before coating it with the active catalyst phase. The γ-Al2O3 layer was prepared as follows: a boehmite γ-AlO(OH) mineral) solution was first prepared by dispersing 10% (w/w) of commercial, high-purity,

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dispersible alumina (Disperal23N4-80, Sasol) in a 0.4% (w/w) HNO3 solution. After 10 min of mixing, a stable dispersion of boehmite was obtained. The substrate was then dipped into the boehmite solution and removed at a controlled speed of 3 cm/min, and dried at room

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temperature for 30 min. A well-adhered layer formed on the substrate surface. The catalyst powder was then washcoated on the γ-Al2O3 layer coated substrate described above.

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2.1.2. Ni-containing foam catalysts

NiMg3AlOx was selected as a CDRM catalyst in this work, because the catalyst formulation

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showed the highest performance among a series of NiMgbAlcOx catalysts in our previous

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work [24]. NiMg3AlOx powder was prepared by the co-precipitation method reported [22]. Ni-containing monolithic catalysts were prepared by depositing NiMg3AlOx catalyst powder

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onto substrates by the following washcoat technique. The NiMg3AlOx catalyst powders were first wet-milled in a laboratory vibration mill (typical milling condition: 10 g catalyst powder + 100 g and 800 μm of ZrO2 milling media + 70 g deionised water, milled for 2 h. The pretreated catalyst substrate was dipped into the milled slurry, and excess slurry was removed by applying a gentle flow of air through the void of the substrates. After air drying at room temperature overnight, the resulting washcoated catalysts were dried at 100 oC for 3 h in an oven and then at 300 oC in air in a muffle furnace. The washcoating procedure was repeated 7

ACCEPTED MANUSCRIPT until an ideal amount of catalyst was coated on the substrate. Finally, the coated substrate was dried at 300 oC for 3 h. The catalyst loading (wt%) was expressed as the weight gain between the bare substrate and the washcoated substrate because there is no water of hydration remaining on the catalyst after this treatment. We coated 10, 15 and 20 wt% NiMg3AlOx on the selected substrates to examine the effect of Ni loading on catalyst activity.

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Figure 2 shows the two fresh substrates and corresponding 20 wt% Ni-foam catalysts with γAl2O3 layer. The compositions of the Ni/Al2O3-coated foam catalysts tested in this work are

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summarised in Table 1.

Figure 2. Photographs of (a) fresh Cu foam (420 µm), (b) 20 wt% Ni/Al2O3-coated Cu foam

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catalyst, (c) fresh SiSiC foam (1000 µm), (d) 20 wt% Ni/Al2O3-coated SiSiC foam catalyst.

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Table 1. Composition of Ni/Al2O3-coated foam catalysts

Catalyst code 10Ni-γCu 10Ni-Cu 15Ni-γCu 15Ni-Cu 15Ni-γSiSiC 15Ni-SiSiC 20Ni-γCu 20Ni-Cu 20Ni-γSiSiC 20Ni-SiSiC

Composition (wt%Ni) 10%Ni/Mg3AlOx-γ-Al2O3 layer-Cu foam 10%Ni/Mg3AlOx-Cu foam 15%Ni/Mg3AlOx-γ-Al2O3-Cu foam 15% Ni/Mg3AlOx-Cu foam 15%Ni/Mg3AlOx-γ-Al2O3-SiSiC foam 15%Ni/Mg3AlOx-SiSiC foam 20%Ni/Mg3AlOx-γ-Al2O3-Cu foam 20%Ni/Mg3AlOx-Cu foam 20%Ni/Mg3AlOx-γ-Al2O3-SiSiC foam 20%Ni/ Mg3AlOx-SiSiC foam

2.2. Catalyst characterisation 8

ACCEPTED MANUSCRIPT The particle size distribution of powder and slurry samples was determined using laser particle size analysis on a Malvern Mastersizer 2000 equipped with a Hydro 2000G, based on laser diffraction. Typical suspension concentrations ranged between ca. 0.002 and 0.02 wt%.

H2-temperature programmed reduction (TPR) of the calcined catalysts was performed using

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a Micromeritics Chemisorb 2750 instrument. The experimental procedure is described in our previous paper [25]. Briefly, about of 0.12 g of catalyst sample was loaded in a U-shaped glass tube and placed in an electric furnace. The sample was first treated by passing 10% O2 in He through the tube for 2 h at 400 oC to be sure that the sample was oxidised. The sample

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was cooled down room temperature and then heated to 1000 oC at a linear programmed rate of 10 oC/min at atmospheric pressure, in a reducing gas stream of 10% H2 in Ar at a flow

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rate of 50 mL/min. The TPR profile was recorded using an online data acquisition system.

The surface area of monolithic catalyst supports was determined using a Poremaster-60

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(Quantachrome) mercury porosimeter operating up to 206.85 MPa.

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X-ray diffraction (XRD) analysis of all samples was carried out using a D/Max-2500PC

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(RIGAKU) goniometer operated at 45 kV and 30 mA using Cu Kα radiation. All samples were run in a step mode at 0.04-step intervals with a 5-s counting time per step over an angular

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range of 2–90. Peaks were identified using Bruker Eva search/match software. 2.3. Catalyst performance evaluation NiMg3AlOx-coated metallic and SiSiC foams were evaluated for their CDRM activity at 720 oC and atmospheric pressure in a fixed-bed quartz reactor (i.d. 8 mm) contained within a stainless steel tube. The diagram of the rig and the details of the testing procedure were described in our previous paper [25]. A piece of catalyst sample was loaded onto a quartz frit located at the centre of the furnace. Novel reforming catalysts were reduced in situ at 700 oC 9

ACCEPTED MANUSCRIPT in 100% H2 for 2 h prior to each run. The gas mixture used to test the performance of the catalysts consisted of the premixed gas of 80% CH4 and 20% N2 (99.99%, BOC) as well as CO2 gas (food grade, BOC) with the molecular ratio of CH4:CO2=1:1.2. The feed and product gases were analysed by an online gas chromatograph (Varian CP4900 Micro-GC, USA). The chromatograph was equipped with a TCD using a PoraPLOT U column and a Molecular Sieve

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5A PLOT column for complete separation and analysis of the gaseous components. To compare each catalyst’s activity, CH4 conversion was controlled to keep it far from equilibrium, with a high gas hourly space velocity of 195,000 mL/(g∙h). The conversion of CH4

flows and compositions as follows:

YH 2 (%) 

FCH4 ,in

100

FH2 ,out 2( FCH4,in  FCH4 ,out )

FCO,out

(5)

(6)

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FH2 ,out

100

(4)

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H 2 /CO 

FCH4 ,in  FCH4 ,out

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XCH 4 (%) 

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(X), yield of H2 (Y) and H2/CO ratio were determined from the measured input and outlet

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3. Results and discussion

3.1. Particle size measurement

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The adhesion between the catalyst layer and substrate is a major challenge facing the development of monolithic catalysts [25]. Adhesion primarily forms through a mechanical mechanism, such as anchoraging and interlocking of the washcoat particles with surface irregularities on the substrate [26]. The adhesion of the catalyst layer mainly depends on the particle size of deposited catalyst powders: the smaller the particle size, the stronger the adhesion [27,28]. A small, uniform size of coating catalyst powder could also improve catalyst distribution, and performance [9]. One widely used method to obtain a uniform 10

ACCEPTED MANUSCRIPT slurry for preparation of the catalyst layer precursor is wet treatment of powders in a ball mill [29,30]. Milled slurry containing particles with d0.9 (90% of particles) of <10 μm and d0.5 (mean particle size) of < 5 μm could achieve desirable adhesion [23]. Figure 3 shows a typical particle size distribution of 20%Ni/Mg3AlOx catalyst before and after wet milling (also presented in Table 2). Wet milling reduces the mean particle size and

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narrows the particle size distribution. A similar finding has been reported by Jia et al. [23], who indicate that d0.9 should be <10 μm for a desirable washcoat. Compared with large diameter particles, smaller diameter particles have a higher percentage of corresponding agglomerates, which leads to tighter particle packing. The tighter packing improves adhesion,

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because neighbouring particles are subject to stronger mechanical or interfacial forces acting between them.

Table 2. Particle size comparison of catalyst 20%Ni/Mg3AlOx

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20%Ni/Mg3AlOx before milling (𝜇𝑚) 2.643 9.609 27.011

20%Ni/Mg3AlOx after milling (𝜇𝑚) 2.375 4.985 9.776

(a)

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Analysis results d(0.1) d(0.5) d(0.9)

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(b)

Figure 3. Particle size distribution of 20%Ni/Mg3AlOx (a) after and (b) before milling.

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3.2. H2-temperature programmed reduction

H2-TPR experiments were performed to determine the reducibility and optimum reduction temperature of NiMg3AlOx-coated Cu foam and SiSiC foam catalysts. They were also used to

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examine the effect of catalyst support and Ni loading on catalyst reduction. Figure 4 shows the TPR profiles of 20%Ni/Mg3AlOx-coated Cu foam catalyst with and without

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a γ-Al2O3 layer, compared with a 20%Ni/Mg3AlOx catalyst powder. The TPR profile of the Cu foam catalyst without the γ-Al2O3 layer shows a sharp peak with high intensity at 218 oC, and

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a broad peak from 280–400 oC. The first peak could be ascribed to the reduction of CuO, but

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shfted to lower temperatures compared to the reduction peak of the pure CuO appears at around 275 oC [31,32]. The second peak, which is smaller than the reduction peak of pure

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NiO at 490 oC [33], might be assigned to the interaction between Cu2+ and Ni2+ species due to the lower reduction temperature of CuO.

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Figure 4. H2-temperature programmed reduction profiles of 20%Ni loading

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on Cu foam with and without γ-Al2O3 layer as well as 20%Ni/Mg3AlOx catalyst powder.

The interaction between Cu2+ and Ni2+ species can significantly decrease the reduction temperature of Ni2+ species [34], compared with the reduction peak observed at 850 oC in

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the TPR profile of 20%Ni/Mg3AlOx catalyst powder which attributed to the reduction of Ni2+ species due to the formation of a solid solution of Ni with catalysts support Mg3AlOx [22].

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On the contrary, in the TPR profile of 20%Ni/Mg3AlOx-γ-Al2O3-Cu foam catalyst, the reduction peak of Cu2+ species becomes smaller at about 220 oC and shifts to a higher

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temperature compared with the TPR spectrum of its corresponding Cu foam-based Ni

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catalyst without the γ-Al2O3 layer. However, the combined reduction peak of mixed Cu2+ and Ni2+ species at 300–600 oC is broader and stronger than the other two catalysts. This means

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that the presence of γ-Al2O3 layer could interact with Cu2+ and Ni2+ species, decreasing their reducibility. The TPR profile of 20Ni-γSiSiC is shown in Figure 5. The low intensity peak at about 350 oC could be ascribed to the reduction of pure NiO. The broad peak at about 780 oC is assigned to the reduction of Ni-Mg-Al-O solution formed in the two monolithic catalysts, compared with the TPR profile of 20%Ni/Mg3AlOx catalyst powder. The reduction temperatures of Ni2+

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it is easier to reduce Ni2+ species coated on Cu foam than that on SiSiC foam.

Figure 5. H2-temperature programmed reduction profiles of 20%Ni loading on different

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substrates with γ-Al2O3 layer as well as 20%Ni catalyst powder.

Figure 6 illustrates that increasing Ni loading has a slight effect on the reduction of Cu2+

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species, but little effect on the reduction of Ni2+ species. The reduction temperature of Cu2+ species decreases from 228 to 219 oC when Ni loading is increased from 10 to 20 wt%. The

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temperature difference might stem from the different strength of the interaction between

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Cu2+ and Ni2+ species. Saw et al. [35] comprehensively investigated Ni-Cu/CeO2 catalyst for water-gas shift reaction by using XRD, H2-TPR, XPS and EXAFS. Their results revealed that

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there is the interaction between Ni and Cu, which could form Ni-Cu alloy.

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Figure 6. H2 temperature-programmed reduction profiles of different Ni loading

3.3. Surface area analysis

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on Cu foam with γ-Al2O3 layer.

Table 3. Physical properties of substrates and catalyst 20Ni-γCu 2

Surface area (cm /g) 0.61 1.38 3.88 4.62

Pore diameter (nm) 12.6 11.9

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Pore volume (cm /g) 0.0213 0.0648 0.0119 0.0141

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Samples SiSiC foam Cu foam γ-Al2O3/Cu 20Ni-γCu

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Table 3 shows that the specific surface area of Cu foam is larger than SiSiC foam. Moreover, the surface area of γ-Al2O3 coated Cu foam is much larger than bare Cu foam. Its surface

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increases when 20%NiMg3AlOx catalyst was loaded on the γ-Al2O3 coated Cu foam. 3.4. X-ray diffraction

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Figure 7 shows XRD profiles of Ni/Mg/Al catalyst before and after calcination. Figure 7(a) shows that the former is composed of well-crystallized double hydroxides in carbonate form which act as catalyst precursors. No other phases were detected, suggesting that both Ni2+ and Al3+ have isomorphically replaced Mg2+ cations in the brucite-like layers [36].

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Figure 7. X-ray diffraction spectrums of Ni/Mg/Al catalyst (a) before and (b) after

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calcinations.

Figure 7(b) shows that the Ni/Mg/Al catalyst after calcination mainly consists of magnesium nickel oxides (magnesium oxide) in a Ni-Mg-O solid solution which is confirmed by the TPR

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pattern of 20%Ni/Mg3AlOx powder in Figure 4 and by Sun et al. [25]. The strong interaction

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between Ni and support material could result in the formation of stable Ni species (NiAl2O4 and Ni-Mg-O solid solution), which could improve Ni dispersion but also retards the

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reducibility of Ni2+ species [37].

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3.5. Catalyst performance

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3.5.1. Effect of γ-Al2O3 layer on catalyst activity The effect of the γ-Al2O3 layer on the catalytic activity of 15%Ni/Mg3AlOx catalysts was examined at a gas hourly space velocity (GHSV) of 195,000 mL/(g·h) at 720 oC (Table 4). The γ-Al2O3 layer improves the catalytic activity of 15%Ni/Mg3AlOx catalysts coated on the two foam substrates. The effect of the γ-Al2O3 layer on the Cu foam is stronger than on the SISIC foam.

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ACCEPTED MANUSCRIPT Table 4. γ-Al2O3 effect on catalytic performance of 15%Ni catalyst on different substrates (T=720 oC, CH4:CO2=1:1.2, GHSV=195,000 mL/(g·h), after 1 h reaction). CH4 conversion (%) with γ-Al2O3 without γ-Al2O3

Substrates Cu foam SiSiC foam

63.1 63.7

53.1 60.6

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Possible explanations for the effect of γ-Al2O3 layer on catalytic activity are:

(a) The monolithic surface area of the Cu foam can be increased from 1.38 to 3.88 cm2/g (Table 3) after coating high-surface-area γ-Al2O3 (200 m2/g) on the Cu foam. The increased specific surface area results in wider dispersion of catalyst on the substrate,

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and thus could improve catalyst activity.

(b) Sasol-dispersible γ-Al2O3 is easily dispersed in HNO3 aqueous solution to form colloidal sols, which are used to coat a transition γ-Al2O3 layer on the substrates. Moreover, the

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thermal expansion of the coating is similar to Ni/Mg3AlOx catalyst powder, which would

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improve the adhesion between catalyst and substrate.

3.5.2. Effect of Ni loading on catalyst activity

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Figure 8 shows the effect of Ni loading on the performance of Ni/Mg3AlOx coated on the Cu

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foam with γ-Al2O3 layer for CDRM, in terms of CH4 conversion and CH4 reaction rate.

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Figure 8. Catalytic performance as a function of Ni loading on Cu foam with γ-Al2O3 layer

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(T=720 oC, CH4:CO2=1:1.2, GHSV=195,000 mL/g·h).

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The 20%Ni-loading catalyst has the greatest activity of three different Ni-loading catalysts. CH4 conversion falls with decreasing Ni loading in the order of: 20Ni-γCu(73.8%) > 15Ni-γCu

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(63.1%) > 10Ni-γCu(54.0%). No catalyst deactivation was observed for these monolithic

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catalysts during the testing period.

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3.5.3. Effect of temperature Figure 9 illustrates the effect of reaction temperature on the performance of 20Ni-γCu and 20Ni-γSiSiC for CDRM. For both catalysts, CH4 conversion and CH4 reaction rate increased as the temperature rose from 650 to 720 to 800 oC, and both remained stable at these temperatures. The equilibrium CH4 conversion at the three temperatures, was also plotted in Figure 9 for a comparison. For 20Ni-γCu, CH4 conversion increased from 50% (650 oC) to

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ACCEPTED MANUSCRIPT 72% (720 oC) to 91% (800 oC). The CH4 reaction rate increased from 10.75×10-5 mol/(cm2·s)

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(650 oC) to 15.55×10-5 mol/(cm2·s) (720 oC) to 19.75×10-5 mol/(cm2·s) (800 oC).

Figure 9. Performance of 20Ni-γSiSiC and 20Ni-γCu catalysts as a function of temperature

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(CH4:CO2=1:1.2, GHSV=195,000 mL/(g·h)) with equilibrium CH4 conversion

For 20Ni-γSiSiC, CH4 conversion increased from 39% (650 oC) to 70% (720 oC) to 100% (800 C), while the CH4 reaction rate increased from 10.77×10-5 mol/(cm2·s) (650 oC) to 19.3×10-

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mol/(cm2·s) (720 oC) to 27.82×10-5 mol/(cm2·s) (800 oC). Compared with the 20Ni-γSiSiC

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catalyst, the 20Ni-γCu catalyst has higher activity at 650 oC, similar activity at 720 oC and

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lower activity at 800 oC. A possible explanation is that the Cu foam has much higher thermal conductivity than the SiSiC foam, which could improve the heat transfer of the monolithic

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catalyst and increase catalyst activity, especially at low temperatures. When reaction temperatures rise, the positive effect of the Cu foam is not obvious, because a larger temperature difference could also improve these catalysts’ heat transfer rates. Therefore, the two catalysts show a similar activity at 720 oC, although their catalyst substrates are different. At 800 oC, Cu foam could start to sinter, which might reduce its activity compared with the SiSiC foam-based catalyst. However, further study is required to investigate the

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ACCEPTED MANUSCRIPT mechanism of the decreased performance of the Cu foam-based Ni catalyst at high temperatures.

3.5.4. Gas hourly space velocity effect on catalyst activity To further investigate the efficacy of Cu foam as a catalyst support for CDRM, the GHSV was

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increased from 200,000 to 500,000 mL/(g·h) for CDRM over 20Ni-γCu and 20Ni-γSiSiC catalysts at 720 oC. Figure 10 shows that the catalytic activity of 20Ni-γCu and 20Ni-γSiSiC decreases with increasing GHSV in terms of CH4 conversion and CH4 reaction rate, although it remains constant at each fixed GHSV. Moreover, the activity of 20Ni-γCu catalyst is similar to

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that of 20Ni-γSiSiC with 70% CH4 conversion at 200,000 mL/(g·h) and 60% of CH4 conversion at 300,000 mL/(g·h). However, with further increase in GHSV from 400,000 to 500,000 mL/(g·h), 20Ni-γCu activity is higher than that of 20Ni-γSiSiC by 7% in CH4 conversion at 400,000 mL/(g·h) and 5% at 500,000 mL/(g·h), respectively.

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At low GHSV (up to 300,000 mL/(g·h)), the reactants (CH4, CO2) have a longer residence time

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inside the reactor, thus having enough time to react with the catalysts which resulting that both catalysts have similar performance. In this case, the heat transfer of the catalysts might

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not be a key factor affecting the performance. However, at high GHSV (>400,000 mL/(g·h)), the residence time is shorter, so the reactants have less time in contact with the catalysts. In

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this case, the high thermal conductivity of the 20Ni-γCu catalyst gives it an advantage and

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could reduce the heat transfer limitation, thus allowing it to perform better than 20Ni-γSiSiC. Ryu et al. [9] examined a FeCralloy monolith-based Ni catalyst for steam reforming of methane. They found that the metallic monolith Ni catalyst has the same CH4 conversion as its corresponding powder catalyst at low GHSVs. At high GHSVs, however, the former has a 10% higher CH4 conversion than the latter, due to higher heat transfer capacity. Their findings are consistent with our results.

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ACCEPTED MANUSCRIPT The residence time is one of the important reaction parameters in this reaction system. Reactant gas, passing through the absorber, diffuses to the catalyst layer. Gas diffusion on the catalyst layer generally depends on the retention time due to transport resistance. For this reason, when GHSV was increased, methane conversion was decreased. Because space velocity is inversely proportional to the residence time, the methane conversion is strongly

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dependent on the residence time. As shown in Fig.10, different from the variation tendency of the CH4 conversion, the CH4 react rate increases with the increasing GHSV. The same phenomenon was observed on different Ni loading catalysts by Lemonidou et al. [38] who investigated the CO2/CH4

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reforming on Ni/CaO-Al2O3 catalyst. They reported the relation between react rate of CH4/CO2 and the pressure of CH4/CO2. They found that the reaction rate of CH4/CO2

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increased with increasing pressure of CH4/CO2.

Figure 10. Catalytic performance of 20Ni-γSiSiC and 20Ni-γCu as a function of GHSV, mL/(g·h) (T=720 oC, CH4:CO2=1:1.2). 3.5.5. Long-term stability test 21

ACCEPTED MANUSCRIPT We tested 20Ni-γCu and 20Ni-γSiSiC monolithic catalysts for CDRM at a GHSV of 195,000 mL/(g·h) at 720 oC for 3–4 thermal cycles of about 7 h each. The catalysts underwent rapid thermal cycling in each run to simulate what would occur when they are

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operated under real solar conditions.

Figure 11. Long-term stability test for 20Ni-γCu and 20Ni-γSiSiC for CO2 reforming of

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methane (T=720 oC, CH4:CO2=1:1.2, GHSV=195,000 mL/(g·h)).

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Figure 11 illustrates that both monolithic catalysts have relatively stable and similar catalytic

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activity throughout the entire period of 30 h. Catalyst deactivation after 30 h was 6% for 20Ni-γCu and 7% for 20Ni-γSiSiC. More research is therefore required to determine the deactivation mechanism so that the stability of the Cu foamed Ni catalyst can be improved.

4. Conclusions Lotus-type porous Cu foam used as catalyst support was coated with NiMg3AlOx catalyst for solar-driven CDRM. The Cu foam-based NiMg3AlOx performed better than SiSiC foam-based

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ACCEPTED MANUSCRIPT NiMg3AlOx at high GHSVs (≥400,000 mL/(g·h)) and at low reaction temperatures (≤720 oC). The presence of a γ-Al2O3 intermediate layer improved the adhesion between the catalyst and substrate, thus increasing catalytic activity. The activity of Ni-Cu foam-based catalysts with γ-Al2O3 intermediate layer increases with the increasing Ni loading in the following order: 20%Ni>15%Ni>10%Ni. High performance of thes Ni-Cu based catalysts could result

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from high thermal conductivity of Cu, nano-particle sized catalysts and the interaction between Ni and Cu foam. Lotus-type porous Cu foam is therefore promising for use as a monolithic catalyst support for reforming reactions at temperatures below 720 oC. In this context they may have potential for being used in low temperature membrane reformers

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which are being investigated for operation at temperatures in the range 500–600 oC.

5. Acknowledgments

This work was supported by the CSIRO Energy Flagship. Jianzhong Qi gratefully

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acknowledges the one-year scholarship provided by the Chinese Scholarship Council, which

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enabled him to work on the project at CSIRO’s Energy Centre in Newcastle, Australia.

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