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Catalytic partial oxidation of methane over nanosized Rh supported on Fecralloy foams
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 9 ( 2 0 1 4 ) 1 1 4 7 3 e1 1 4 8 5
Available online at www.sciencedirect.com
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Catalytic partial oxidation of methane over nanosized Rh supported on Fecralloy foams E. Verlato a, S. Barison a, S. Cimino b,*, F. Dergal b,1, L. Lisi b, G. Mancino b, zquez-Go mez a M. Musiani a, L. Va a b
Istituto per l’Energetica e le Interfasi IENI e CNR, Corso Stati Uniti 4, 35127 Padova, Italy Istituto di Ricerche sulla Combustione IRC e CNR, P.le V. Tecchio 80, 80125 Napoli, Italy
article info
abstract
Article history:
Structured catalysts for the partial oxidation of methane were prepared by supporting Rh
Received 15 January 2014
nanoparticles onto Fecralloy foams at relatively low precious metal loadings. The inves-
Received in revised form
tigation was focused mainly on an innovative and straightforward preparation procedure
17 April 2014
consisting in the direct cathodic electrodeposition of Rh onto foam samples. For the sake of
Accepted 7 May 2014
comparison, other Rh-based catalysts were prepared with a more traditional approach, by
Available online 18 June 2014
using the same foams and an AlPO4 washcoat layer. The catalysts were characterized by SEM-EDS, XRD and cyclic voltammetry, to assess the Rh surface area, and tested in the CPO
Keywords:
of methane to syngas under self-sustained high temperature conditions at short-contact-
Electrodeposition
time. During prolonged CPO tests the performance of electrochemically prepared catalysts
Rh nanoparticles
underwent a progressive decline, as compared to stable operation of AlPO4 washcoated
Structured catalyst
catalysts, which was mainly ascribed to sintering of Rh nanoparticles, negatively affecting
Syngas
the activity for methane steam reforming.
Steam reforming
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The Catalytic Partial Oxidation of hydrocarbons in shortcontact-time autothermal reactors represents a promising technology for syngas or H2 production in small scale applications such as on board fuel reformers or catalytic burners [1e9]. Rh-based catalysts are recognized as the most active and selective during the CPO of hydrocarbons from methane up to diesel and jet-fuels [1e5]. However rhodium is among the scarcest and most expensive precious metals,
having mainly no viable substitute for the reduction of NOx in three-way automotive catalysts [10e11]. Therefore its effective usage is of vital importance in terms of cost reduction. In fact many authors have used relatively high Rh loadings (from 2 to 15% by weight [2,3,5e7,12e14], generally referred to the total weight of the structured reactor including the substrate) to counterbalance possible activity losses during high temperature CPO, due for instance to thermal sintering of the metal clusters [13] or volatilization of the active species [12].
* Corresponding author. E-mail addresses: [email protected], [email protected] (S. Cimino). 1 Permanent address: Centre de Recherche Scientifique et Technique en Analyses Physico-chimiques, (C.R.A.P.C.), BP 248, Alger RP 16004 Alger, Algeria. http://dx.doi.org/10.1016/j.ijhydene.2014.05.076 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Among structured substrates for CPO reactors foams with high density of pores are generally preferred [14] to honeycombs because of their outstanding gas-to-solid heat- and mass-transfer characteristics and high specific geometric surface areas, at the expense of only a small increase in pressure drops [15]. Fecralloy metallic foams have outstanding resistance to oxidative degradation and maximum operating temperatures around 1250 C, and allow to design lightweight reactors preshaped in almost any geometry, with low thermal inertia and fast transient response during start-ups and load variations, while avoiding thermal-shock limitations of ceramic materials [15e18]. Those substrates generally serve as structures for highsurface-area ceramic supports such as g-Al2O3 which are used to disperse and stabilize the active metal phase. However, the deposition of a thin uniform washcoat layer of refractory oxides, firmly anchored to the metal struts, which is of paramount importance in order to guarantee long catalyst durability and avoid pore blocking and additional pressure drops [17,18], is not a trivial matter. Therefore, an innovative approach for the catalysts preparation was explored. Thanks to the metallic nature of Fecralloy foams and their high electrical conductivity, electrochemical methods can be used to disperse noble metals directly on the carrier [18,19], in order to obtain technological catalysts with high noble metal dispersion (i.e. surface area) and relatively low loadings. When the noble metal deposits thus formed are in electrical contact with the metallic foam [19], electrochemical methods can be used also to measure their surface area. In order to assess the validity of this new preparation method, the electrochemically prepared catalysts were compared with other Rh-based catalysts comprising an AlPO4 washcoat layer over the same Fecralloy foam, prepared with a more traditional approach and tested under identical conditions, and with results in the literature.
Experimental Catalyst preparation and characterization Fecralloy foams used in this work were cut out of a single commercial panel (Porvair, Metpore 50 pores per inch, ppi) with average composition: Fe 70.1; Cr 20.8; Al 9.1% (mass) [19,20]. Table 1 reports the main geometrical features of the foam: the average diameter of the struts (ds) and pores (dp) were estimated by SEM (respectively at 1 102 cm and 5.5 102 cm). The specific surface area of the metal foam (Sv), evaluated by EIS measurements as described in Ref. [20], was 100 ± 5 cm1. This value, which also includes roughness caused by features with a characteristic dimension much smaller than the average strut or pore diameters, can be regarded as an upper limit. The Sv predicted for the same foam by a cubic cell model made of smooth cylindrical struts [[15], and ref. therein] is 31 cm1 (Table 1), suggesting that this is possibly an oversimplified description of the reticulated structure. Parallelepipeds (0.4 1.0 1.0 cm; electrode volume Vfoam ¼ 0.4 cm3) were used in most electrochemical experiments. Catalysts for partial oxidation tests were prepared from Fecralloy cylinders (1.8 cm diameter 0.96 cm height, Vfoam ¼ 2.44 cm3). Before use in spontaneous or electrochemical deposition the foam samples were successively washed with dichloromethane, acetone and water; they were then used as such or after etching in acid or basic media. The solutions used for either spontaneous or electrochemical deposition of Rh were prepared by dissolving 0.001 M Na3RhCl6 in 1 M NaCl and adding the HCl necessary to achieve pH 2.0. These solutions were deaerated and kept at 25 C with a thermostat. Since the formation kinetics of chlorocomplexes of Rh is known to be slow [21], and their speciation may evolve during several days, only solutions aged
Table 1 e Morphological properties of Rh-based catalysts with structured substrates used in this and in previous studies. Active phase
Support
Strutb Poreb
Sv
Washcoat s.a.
LRh
am Rh
dRhe
aRh
mg/cm3
m2/g
nm
cm2/cm3
Ref.
ds
dp
cm
cm
cm2/cm3
mm
m2/g
31f (100) 31 (100) 31 (100) 35.8 35.8 28 28
e
e
0.93
1.08
30
16
325
this work
e
e
0.93
1.40
30
16
420
this work
4.5
16
0.90
0.18
97
5.0
170
this work
31 18 e 5.0
122 122 e 120
0.75 0.80 0.70
1.48 0.94 50 50
2.9 2.1 3300 380
2515 2175 75 635
Rh
Fecralloy foam 50 ppi
0.01
0.055
Rh
Fecralloy foam 50 ppi
0.01
0.055
Rh/AlPO4
Fecralloy foam 50 ppi
0.01
0.055
Rh/LAa Rh-P/LAa Rh Rh/g-Al2O3
Honeycomb 600 cpsi Honeycomb 600 cpsi a-Al2O3 foam 80 ppi a-Al2O3 foam 80 ppi
0.076 0.076
0.096 0.096 0.060 0.060
t
c
Voidd 3
170 232 0.15 1.30
[4] [4] [3] [3]
LA: 3% La-g-Al2O3 (Sasol LCF140L3). For honeycombs: ds ¼ wall thickness, dp ¼ side of square channel. c Thickness calculated assuming uniform washcoat deposition above the geometric surface area and apparent density rw ¼ 1.5 g/cm3. d Fecralloy foam by cubic cell model: (1 3 ) ¼ ¾p ((ds þ 2t)/(dp 2t))2; honeycomb: 3 ¼ ((dp 2t)/(dp þ ds))2; a-Al2O3 foam by pycnometry [3]. e 3 Rh particles supposed spherical, dRh ¼ 6=rRh am Rh , rRh ¼ 12.4 g/cm . f Cubic cell model Sv ¼ 4 (1 3 )/ds; values in parenthesis were measured by EIS measurements [20] and include small-scale surface roughness of Fecralloy. a
b
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during a week or more were used. The duration of the spontaneous deposition reaction was varied between 1 and 24 h. Electrodeposition was performed in different twocompartment cells, depending on the size of the foam electrode, so as to have a ratio ca. 50 between the solution volume and the foam volume. The Fecralloy foam working electrode, washed but not etched, and a Pt wire counter electrode were placed in the main compartment; a Saturated Calomel Electrode (SCE) placed in the lateral compartment was used as reference. Rh was deposited under potentiostatic control, in the potential range 0.4 to 0.55 V (vs. SCE), with deposition charges QDEP varying between 0.5 and 8.5 C for parallelepipeds (Vfoam ¼ 0.4 cm3), and between 10 and 30 C for cylinders (Vfoam ¼ 2.44 cm3). The mass of Rh deposited onto Fecralloy samples was evaluated by measuring the decrease in the concentration of Rh(III) complexes caused by each deposition experiment, using ion chromatography [22]. The Rh loading, denoted LRh, is henceforth expressed in milligrams of Rh per unit volume of the foam. A further type of catalyst was prepared with a more traditional approach by applying an active washcoat overlayer onto the same Fecralloy foam. In particular amorphous aluminum phosphate (AlPO4) was selected as a support material to disperse Rh particles according to recent literature reports showing optimal metal-support interactions that can reduce significantly Rh loading in automotive three way catalysts [11]. The AlPO4 support was prepared in powder form, according to the coprecipitation route reported in Refs. [11,23], and calcined in air at 900 C for 3 h. Fecralloy foam discs were dip-coated employing a water slurry of AlPO4 powder and a dispersible pseudobohemite binder (Disperal, Sasol), after ball milling with ZrO2 spheres (in a Retch Planetary Miller PM100). The final washcoat loading (measured after air calcination at 800 C for 3 h) was limited to ca. 5% by weight of the foam in order to prevent pore blocking: assuming a uniform deposition of the washcoat (apparent density rw ¼ 1.5 g cm3) over the estimated geometric foam area (31 cm1), a theoretical thickness of 4.5 mm can be calculated. Rh was dispersed onto washcoated foams by 2 consecutive impregnation cycles with an aqueous acid solution of Rh(NO3)3 followed by calcination in air at 350 C. A reference powder system was prepared following an identical procedure starting from the dried slurry and used for characterization. The catalysts were finally reduced in 2% H2/N2 at 900 C for 2 h. LRh was 0.18 mg cm3, corresponding to ca. 0.86% by weight of the AlPO4 support, as measured by ICP-MS analysis (Agilent 7500) after microwaveassisted digestion of samples in nitric/hydrochloric acid solution. Voltammetric experiments aimed at assessing the Rh surface area were performed using the Rh-modified Fecralloy foams as working electrodes, a Pt wire as counter-electrode and a Hg/HgO/ 1 M KOH reference electrode, located in a separate compartment connected to the main one through a Luggin capillary. However, throughout this paper, potential is referred to SCE. Cyclic voltammograms were recorded in 1 M KOH, over a potential range where the H adsorption/desorption charge could be measured, with only a minor interference by redox processes due to Fecralloy. As reported in a previous paper [24], the H desorption charge measured with Rh-
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Fecralloy samples, Q foam, was compared to that measured with a carefully polished Rh disc electrode, Q disc, so as to obtain aRh, the surface area of the Rh deposits per unit volume of the foam (expressed in cm1, i.e. cm2/cm3) according to: aRh ¼
Q foam Adisc Vfoam Q disc
(1)
where Adisc is the disc geometric area, assumed identical to the true one. By taking the ratio between aRh and LRh the Rh surface area per unit Rh mass (henceforth denoted am Rh and expressed in m2 g1) was calculated. The specific surface area of the AlPO4 washcoat was measured with BET method by N2 adsorption at 77 K performed in a Quantachrome Autosorb 1-C. The exposed surface area of Rh dispersed on the AlPO4 support (am Rh ) was estimated by CO chemisorption measurements performed in the same instrument at room temperature according to the double isotherm method, as described in Ref. [4]. Electrochemical experiments were performed with Autolab PGSTAT 302N or Autolab PGSTAT 100 potentiostats. Na3RhCl6 solutions were analysed with a Metrohm 850 Professional IC liquid chromatographer equipped with a Metrohm 887 UVevisible detector and a METROSEP A SUPP 15e50 anionic column. SEM images and EDS analyses were obtained with a Zeiss SIGMA instrument, equipped with a field emission gun, operating in high vacuum conditions. Accelerating voltages variable between 5 and 30 keV were used, depending on the observation needs. X-Ray diffraction (XRD) patterns were obtained by using a Philips X-PERT PW3710 diffractometer with a Bragg-Brentano geometry, employing a CuKa source (40 kV, 30 mA). Before recording the diffractograms, the foams were compressed to obtain flat, ca. 0.5 mm thick samples. The crystallite dimension was estimated by Rietveld analysis, performed through the Maud software [25].
Catalyst testing Rh-Fecralloy and Rh/AlPO4-Fecralloy foam catalysts were tested for the CPO of methane to syn-gas in a lab scale reactor operated at nearly atmospheric pressure, under selfsustained short contact time conditions using air as the oxidant [4,26,27]. The former, containing only metallic Rh, were tested directly, without further conditioning; on the other hand Rh/AlPO4eFecralloy catalyst was preliminary reduced at 900 C, in order to form and stabilize Rh metal particles. The catalytic foams were stacked between two mullite foam heat shields (45 ppi, L ¼ 12 mm), wrapped with ceramic paper and fitted in a quartz tube reactor (I.D. ¼ 20 mm) which was inserted in an electric tubular furnace used for pre-heating (fixed at 230 C). High-purity gases (CH4, O2, N2: >99.995%) calibrated via Brooks 5850Mass Flow Controllers, were pre-mixed and fed to the reactor at a gas hourly space velocity (GHSV) comprised between 3.2 and 10.5 104 h1 (based on Vfoam). The molar feed ratio CH4/ O2 was varied in the range 1.7e2.6, so as to include ratios both lower and higher than 2, the stoichiometric value for the overall CPO reaction: CH4 þ 1=2O2 4CO þ 2H2
DH25 r
C
¼ 35:6 kJ=mol
(2)
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A complete cycle lasted ca. 4 h. Reactor temperatures were measured by means of K-type thermocouples (d ¼ 0.5 mm) placed in contact with the front face of the catalytic foam (Tin, an average between the relatively cold feed gas and the hot catalyst [3,28]), and at several positions downstream of it, inside the back heat shield: in fact the temperature of the gas leaving the catalyst (Tg out) was estimated by a linear regression in the post reaction zone. Product gases were continuously analysed (by a GEIT Syngas analyzer) to measure the concentrations of CO, CO2 and CH4 (non-dispersive IR), O2 (ECD), and H2 (TCD, with cross sensitivity corrections). For all CH4/O2 feed ratios, O2 was always completely converted under steady state operation, being consumed both in CPO, reaction (2), and in CH4 total oxidation, reaction (3): ¼ 802 kJ mol
(3)
Thus, H2O production was calculated from the O-balance. Carbon and hydrogen balances were closed within ±1% and ±3%, respectively. Methane conversion, yields and selectivities to CO and H2 were calculated according to the following definitions based on measured dry molar fractions:
OUT
CO CHOUT þ COOUT þ COOUT 4 2
YH2
YCO xCH4
(4)
YH2 xCH4
1.0
a 0.01
0.8
! (5)
0.00
(6)
100 HOUT 2 $ ¼ OUT 2 CH4 þ COOUT þ COOUT 2
SH2 ¼ 100$2$
!
0.6
!
-0.01
0.4
-0.02
0.2
(7) -0.8
-0.6
-0.4
0.0 0.0
-0.2
E vs SCE / V (8)
Equilibrium calculations with constant enthalpy and pressure were performed using CHEMKIN 4.1.1 software, excluding solid carbon formation, which was never observed experimentally.
2.5 b 2.0 -3
YCO ¼ 100$
SCO ¼ 100$
CHOUT 4 CHOUT þ COOUT þ COOUT 4 2
LRh / mg cm
xCH4 ¼ 100$ 1
Rh cathodic deposition has been described by several authors. Deposition baths with commercial significance are sulphate, phosphate or mixed sulphate-phosphate solutions [29e31], but also chloride baths have been investigated [32,33]. In all cases a low pH is required to prevent Rh(III) ion hydrolysis. As a consequence, H2 evolution is a significant side reaction which severely limits the current efficiency. We used, as electrodeposition baths, solutions identical to those employed in spontaneous deposition, and similar to the chloride solution studied by Pletcher and Urbina [32], because obtaining smooth and bright Rh deposits is not only unnecessary, but may be even detrimental in the preparation of heterogeneous catalysts for which an extended surface area is desirable.
Current efficiency
C
Preparation of Rh-modified Fecralloy by electrodeposition
I / A
CH4 þ 2O2 4CO2 þ 2H2 O DH25 r
irreproducible and therefore unsatisfactory results, ascribed to an inhomogeneous reactivity of the Fecralloy foam surface, which acid or basic etching was not apt to remove, Rh electrochemical deposition was considered as an alternative method for Rh-Fecralloy preparation.
1.5 1.0
Results and discussion 0.5
Preparation of Rh-modified Fecralloy by spontaneous deposition Several Rh-modified Fecralloy samples were prepared by spontaneous deposition, by varying the immersion time of the Fecralloy foam in a 0.001 M Na3RhCl6 þ 1 M NaCl solution at pH ¼ 2.0. Their LRh values were found to be heavily scattered both when the foam samples were simply washed with dichloromethane, acetone and water, and when they were also etched in HCl or KOH solutions. Most measurements of Rh surface area per unit Rh mass yielded am Rh values in the range 10e20 m2 g1, but values as low as 5 m2 g1 and as high as 35 m2 g1 were occasionally obtained. Owing to these
0.0 0
1
2
3
4
5
6
7
8
9
QDEP / C
Fig. 1 e (a) Cyclic voltammogram (red solid line) recorded with a Fecralloy foam electrode (Vfoam ¼ 0.4 cm3) in 0.001 M Na3RhCl6 þ 1 M NaCl solution at pH ¼ 2.0, with a 50 mV s¡1 scan rate and current efficiency values (full points) measured at selected potentials for a 5 C deposition charge. (b) Dependence of the Rh loading on the deposition charge. The dashed lines are an aid for the eye. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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The continuous curve in Fig. 1a is a cyclic voltammogram recorded with a Fecralloy foam electrode (Vfoam ¼ 0.4 cm3) in 0.001 M Na3RhCl6 þ 1 M NaCl solution at pH ¼ 2.0. The cathodic peak at E ¼ 0.54 V is due to overlapping Rh deposition and hydrogen evolution [32]; the anodic peak at E ¼ 0.28 V is ascribed to the oxidation of molecular H2, since it is known that Rh anodic stripping does not occur [32,34]. The current efficiencies, measured for a constant 5 C reduction charge and shown in Fig. 1a as full symbols joined by a dashed line, decrease as the potential becomes more negative, as expected. Therefore, a deposition potential E ¼ 0.40 V was preferentially used in further experiments, so as to have reasonably
high deposition rate and current efficiency. Fig. 1b shows that, at E ¼ 0.40 V, LRh increases less than linearly with the deposition charge QDEP. This behaviour results from a progressive loss in current efficiency due to an enhancement of the H2 evolution reaction as the Rh deposition proceeds. Fig. 2 shows SEM images of as-prepared Rh-Fecralloy catalysts. Comparison of samples a and b, both prepared by electrodeposition, shows that upon increasing QDEP the coverage of the foam by Rh particles markedly increases, but the size of the particles is not very strongly affected. The insets with higher magnification images, placed in the upper right corner of Fig. 2a and b, allow to appreciate that most particles have “diameters” 150 nm and appear to consist of aggregated smaller crystallites. The third sample, prepared by spontaneous deposition (c), has a similar aspect and a surface coverage by Rh particles intermediate between those of electrodeposited samples. To achieve such coverage, the spontaneous deposition had to be prolonged for 24 h, a time long enough to cause some deterioration of the mechanical properties of the foam. SEM-EDS analyses were used to assess how homogeneously the Rh particles were distributed inside the foam. To this aim, a 2.44 cm3 cylinder was cut so as to obtain a 1.6 0.96 0.46 cm pseudo-parallelepiped (as schematically
Fig. 2 e SEM images of as-prepared Rh-Fecralloy samples (Vfoam ¼ 0.4 cm3) obtained by electro-deposition with QDEP ¼ 1 C (a) or 5 C (b) or spontaneous deposition during 24 h (c).
Fig. 3 e Schematic representation of the sample used to assess the homogeneity of Rh distribution in Rh-Fecralloy foams via EDS analyses. The table in the bottom part of the figure reports local compositions.
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shown in Fig. 3). EDS analyses were then performed by sampling 0.05 mm2 areas located at the positions denoted A to G. The Rh local amount, calculated as
Fig. 4 e Cyclic voltammograms recorded with Fecralloy and Rh-Fecralloy electrodes (Vfoam ¼ 0.4 cm3) in 1 M KOH solution. The Rh deposition charges and Rh loadings are indicated on the figure. Scan rate ¼ 50 mV s¡1.
aRh / cm
500 400 300 Edep= -0.4V
200
Edep= -0.45V Edep= -0.5V
100 0 0.0
Edep= -0.55V
0.5
1.0
1.5
2.0
2.5
-3
LRh / mg cm
b
35
-1
30 25
2
(where NMe is the number of atoms of each metal Me) is shown in the bottom part of Fig. 3. Although these analyses were devoid of a rigorous quantitative character, since Rh was not present as an alloy component but in the form of particles sticking on the walls of the foam cells, the following conclusions may be drawn: (i) comparable Rh amounts were present in different parts of the foam; (ii) the local Rh loading at peripheral positions was somewhat higher than in the centre of the sample, probably due to some current distribution and/or different Rh(III) ions transport rates. Fig. 4 shows cyclic voltammograms measured with Fecralloy and Rh-Fecralloy electrodes in 1 M KOH solution. Hydrogen adsorption/desorption currents, observed in the potential range negative of E ¼ 0.7 V increase with the deposition charge. Since the current measured with an unmodified Fecralloy foam electrode, in the same potential range, is comparatively very small, Qfoam may be reliably measured and used to calculate aRh and am Rh values as described in the experimental section. The dependence of these quantities on LRh is shown in Fig. 5a and b, respectively. The Rh specific surface area of samples prepared at E ¼ 0.4 V (full points in Fig. 3a) increases linearly with the loading, and becomes larger than the specific area of Fecralloy (100 cm1) for LRh > 0.3 mg cm3. A linear aRh vs. LRh dependence is in agreement with the progressive formation of an increasing
-1
600
(9)
m
NRh 100 NFe þ NCr þ NAl þ NRh
a 700
aRh / m g
Rh at% ¼
800
20 15 10
Edep= -0.4V
5
Edep= -0.5V
0 0.0
Edep= -0.45V Edep= -0.55V
0.5
1.0
1.5
2.0
2.5
-3
LRh / mg cm
Fig. 5 e Dependence of the Rh specific area aRh (a) and Rh surface area per unit Rh mass am Rh (b) on the Rh loading LRh. Rh was electrodeposited at the potentials indicated on the figures. In part (a) the best fitted straight line relevant to E ¼ ¡0.4 V is shown.
number of particles which retain essentially the same shape and size, suggested by the SEM images in Fig. 2. Samples prepared at E < 0.4 V (empty points in Fig. 5a) show significantly, but not dramatically, lower aRh values which also depend almost linearly on LRh. A linear aRh vs. LRh means that am Rh values are independent of LRh. For samples prepared at 2 1 E ¼ 0.4 V, am Rh is 30 ± 5 m g , Fig. 5b, a value only slightly lower than those measured for Rh spontaneously deposited onto Ni foams [27] and much larger than those relevant to Pd spontaneously deposited onto Fecralloy [19,20]. Fig. 6 compares the X-ray diffractograms of three RhFecralloy samples: (a) as-deposited, (b) after 44 h on stream during CPO tests, and (c) after thermal treatment at 1000 C in 5% H2/N2 atmosphere for 5 h. In the diffractogram of the asprepared sample, only broad low-intensity peaks which can be ascribed to Rh [35] are visible, besides the reflections due to the Fecralloy substrate. The Rh crystallites are estimated to have a characteristic size of 25 nm, significantly lower than the average particle size, and in line with the average diameter of metal clusters (16 nm) estimated from the corresponding value of am Rh (Table 1). The diffractograms in Fig. 6b and c are discussed below, after the CPO tests.
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Preparation of supported RheAlPO4 on Fecralloy
Fig. 6 e X-ray diffractograms of Rh-Fecralloy catalyst (LRh ¼ 1.4 mg cm¡3): (a) as-deposited, (b) after 44 h on stream during CPO tests, (c) after thermal treatment at 1000 C in H2 atmosphere for 5 h. The peaks labelled with Miller indexes are ascribed to Rh, in part a, or Al-poor AlRh alloys, in parts (b) and (c).
The Rh/AlPO4 active layer anchored onto the Fecralloy foam had an overall BET surface area of 16 m2 g1 after reduction at 900 C (Table 1), which is significantly lower than the value of 80 m2 g1 previously reported by Machida et al. [11]. In fact, the wet ball milling step in the preparation of the slurry for dipcoating was found to induce the collapse of pores of the amorphous AlPO4 support, whose initial surface area was as high as 87 m2 g1 after reduction in H2/N2 at 900 C. CO chemisorption measurements for Rh/AlPO4 catalyst 2 1 yielded a am Rh ¼ 97 m g , i.e. three times larger than for Rh/ Fecralloy counterparts without washcoat layer. This corresponds to an average diameter of 5 nm for Rh nanoparticles (assumed spherical) dispersed on amorphous AlPO4. Table 1 also reports relevant features of two Rh-based honeycomb catalysts characterized by a metal loading LRh in the same range as Rh-Fecralloy foams. For their preparation, commercial honeycomb substrates (Cordierite, 600 square channels per square inch, cpsi, NGK) were washcoated with a La-g-Al2O3 (LA), which was selected in order to improve and stabilize Rh dispersion under high temperature CPO conditions [4,26]. Accordingly, the average dimension of metal aggregates dispersed on g-alumina (Table 1) was ca 2.9 nm (Rh/ LA), and it further decreased down to ca. 2.1 nm due to doping with phosphorous [4], corresponding to am Rh values from 5 to 8 times larger than for Rh-Fecralloy samples. As shown in Table 1, the cordierite honeycomb substrate has a specific geometric area similar to that of the Fecralloy foam; however its almost twice larger and straight channels strongly facilitate the anchoring of a thicker catalytic layer by conventional dipcoating methods, without the risk of pore blocking, typically encountered with the tortuous paths of interconnected struts in foams [17]. The catalytic activity of those Rh-based honeycomb catalysts was studied in the same experimental rig,
Fig. 7 e CPO of methane over Rh-based catalysts indicated on the figure: (a) methane conversion, (b) exit gas temperature, (c) selectivity to CO, (d) selectivity to H2 as a function of the CH4/O2 feed ratio. Dashed lines represent thermodynamic equilibrium values.
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Table 2 e Performances of Rh-based catalysts in CPO of CH4. Catalyst
Support
V
Rh-Fecr1 Rh-Fecr2 Rh/AlPO4 Rh/LA Rh-P/LA Rha Rh/g-Al2O3a
3
2
F
XCH4
YCO
YH2
cm
mg/cm
cm
Sl/h
%
%
%
2.44 2.44 2.44 2.12 2.24 2.50 2.50
1.08 1.40 0.18 1.48 0.94 50 50
790 1025 420 5335 4870 190 1590
135 135 135 135 135 300 300
62.0 64.5 63.3 78.9 78.9
51.4 54.3 56.7 69.5 69.0
49.7 54.0 50.0 73 73.6
eq
YH2 =YH2
Tg out
0.63 0.69 0.64 0.93 0.94 0.83 0.99
Ref.
C
838 802 814 688 691
this work this work this work [4] [4] [3] [3]
Preheating 150 C; thermal reactor efficiency ca.1.
Fig. 7 presents the results of CPO tests in terms of methane conversion, gas temperatures recorded at the exit of the catalyst and selectivities to CO and H2, as a function of the CH4/O2 feed ratio. Catalytic performances at stoichiometric conditions for syngas production (CH4/O2 ¼ 2) are reported in Table 2, together with flow conditions and the main catalyst features including the total exposed Rh area in the CPO reactor (ARh ¼ aRh $V). In agreement with many previous reports, Fig. 7 shows that methane conversion over each of the foam catalysts increased almost linearly with decreasing the feed ratio from 2.6 to 1.7, in line with the progressively higher availability of oxygen, the limiting reactant, which was always completely converted. This agrees with the trend predicted by adiabatic equilibrium (dashed lines in Fig. 7), but methane conversion remained as much as 15e25% points below its corresponding equilibrium value. Fuel conversion slightly increased with metal loading for Rh-Fecralloy foams. The Rh/AlPO4-Fecralloy foam, displayed intermediate performance between the two Rh-Fecralloy counterparts (Table 2) in spite of its significantly lower LRh, confirming the effectiveness of increasing the noble metal dispersion with the use of an appropriate washcoat layer [3,14]. As expected, the temperature of the gas leaving the catalyst (Fig. 7b, Table 2) increased with decreasing the feed CH4/
90
900
80 850
70 60
800
Tg out °C
CH4 CPO tests
O2 ratio, according to progressively higher temperatures on the catalyst (not shown). Tg out was constantly above the corresponding adiabatic equilibrium value, although the thermal efficiency of the reactor under those conditions was estimated around 0.85 [19]. Moreover, the temperature was higher for the catalyst with lower activity (i.e. lower methane conversion). Rh-Fecralloy and Rh/AlPO4-Fecralloy catalysts were tested with different feed flow rates (F) at fixed feed ratio, finding that, in both cases, increasing F above ca 100 Sl h1 and up to 250 Sl h1 did not alter significantly their catalytic performance (see Fig. 8). This generally corresponds to the balancing between an increase in the thermal efficiency of the CPO reactor (translating into higher operating temperatures) [9] and a lower contact time, which can adversely affect the contribution from slower reactions, in particular steam reforming [5,27,28]. Note that methane dry reforming, reaction (11), was shown not to occur at significant rate on Rh catalysts under CPO conditions [3,28,36].
CH4 Conversion %
under almost identical conditions, as reported in details in previous works [4,26]. Considering their higher metal dispersion and similar metal loadings, the active surface area per unit volume of catalyst aRh for honeycombs is larger than for foams by as much as one order of magnitude (Table 1). Finally, for comparison purposes, Table 1 reports the main morphological properties of two Rh foam catalysts reported by Donazzi et al. [3], who impregnated Rh onto a-Al2O3 foam supports (with a nominal density of 80 ppi) either with or without an intermediate washcoat layer of g-Al2O3. With respect to our metallic foam the ceramic structure was characterized by similar dimension of pores and geometric surface area (Sv), in spite of a lower void fraction due to larger struts. Rh loading was the same for both samples and was significantly larger than for Rh on Fecralloy counterparts (from 35 to 230 times); on the other hand, the specific metal area am Rh was estimated to be as low as 0.15 m2 g1 and 1.30 m2 g1, respectively for samples without or with the intermediate gAl2O3 washcoat. The latter had roughly the same nominal thickness than the AlPO4 layer on our Fecralloy foams.
50 40 60
120
100
Selectivity %
a
Fecralloy foam 50 ppi Fecralloy foam 50 ppi Fecralloy foam 50 ppi honeycomb 600 cpsi honeycomb 600 cpsi a-Al2O3 foam 80 ppi a-Al2O3 foam 80 ppi
ARh
LRh 3
180
750 300
240
Feed, Sl/h
90 CO
80 H2
70 60 50 60
120
180
240
300
Feed, Sl/h Fig. 8 e Effect of feed flow rate during the CPO of methane over Rh-Fecralloy catalyst (LRh ¼ 1.4 mg cm¡3) at fixed feed ratio CH4/O2 ¼ 2.2: (a) methane conversion and exit gas temperature, (b) selectivity to CO and H2.
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C
¼ þ247 kJ=mol
(10) (11)
Under identical operating conditions, the two Rh-based honeycombs (Fig. 7, Table 2), having similar LRh but a much higher available metal area ARh than Rh-Fecralloy foams, closely approached equilibrium. In fact, the slight lack of methane conversion and gas temperatures below the predicted values are motivated by the heat losses from the experimental reactor. Some significant differences among Rh catalysts were found in the selectivity to partial oxidation products. CO was formed with very high selectivity (approaching equilibrium) on the Rh/AlPO4-Fecralloy, as well as on both honeycomb catalysts with Rh/La-g-Al2O3 washcoat (Fig. 7c). In contrast, Rh-Fecralloy foams without intermediate washcoat layer showed systematically lower CO selectivities (by as much as 4e6 points %), which only marginally increased with Rh loading. This implies that more CO2 was formed when Rh was in direct contact with the metal foam, suggesting a strong noble metalemetal support interaction (see paragraph 3.5). Under high temperature reaction conditions the surface of Fecralloy becomes covered by an inert alumina overlayer; therefore the contribution of the bare support to the methane total oxidation is generally negligible [19]. Regarding H2 production, it can be observed in Fig. 7d that highly dispersed Rh honeycomb catalysts converted methane with almost equilibrium selectivity to H2. On the other hand, Rh/AlPO4-Fecralloy and Rh-Fecralloy foams showed lower values of H2 selectivity, which increased with LRh. The presence of the AlPO4 washcoat layer on the Fecralloy foam was not as able to guarantee H2 selectivities comparable with equilibrium values as in the case of CO production, suggesting a kinetic limitation in H2 production. All those observations can be rationalized considering that oxygen consuming reactions (2,3) are so fast on Rh catalysts to proceed under full mass transfer control in a thin front zone of the reactor [3,28,36]. Steam reforming, reaction (10), is a much slower, strongly endothermic reaction, which prevails in the post oxidation zone (i.e. after molecular O2 has been completely consumed), and proceeds under kinetic control even at the very high temperatures of the CPO reactor [6,28,36]. In fact, detailed experimental temperature and species profile along CPO monoliths have shown that Rh catalysts ensure better reactor performance with respect to different noble metals (i.e. Pt-based systems) [36,37] due to the high intrinsic catalytic reforming activity of Rh. Similar conclusions have also been drawn from the observation of the poisoning effects of sulphur, which partly covers the surface of Rh under CPO conditions, and selectively inhibits steam reforming [4,26,37]. Accordingly, the ranking of catalysts in terms of their hydrogen yield and approach to equilibrium (Table 2) generally follows the trend of the total surface area of Rh available for steam reforming (ARh). However, if we limit the comparison to metal foam catalysts, it appears that RheAlPO4 produced the same amount of H2, though its ARh
value was ca. ½ of that nominally available for the RhFecralloy foam samples. Rh-Fecralloy foam and Rh/AlPO4-Fecralloy catalysts were submitted to consecutive CPO tests in which the CH4/O2 feed ratio was varied between 1.7 and 2.6, each cycle lasting ca. 4 h. Fig. 9 shows that the CPO performance of Rh-Fecralloy foam samples decreased progressively with time on stream. Methane conversion and hydrogen selectivity decreased, CO selectivity was constant, and the exit temperatures increased. On the contrary, the addition of the AlPO4 washcoat preserved constant catalytic activity during up to 32 h on stream. To understand the reasons for their declining performance, the Rh-Fecralloy foam catalysts were further characterized after use.
Characterization of Rh-Fecralloy catalysts after CPO tests Visual inspection of both Rh-Fecralloy catalysts used in CPO tests (Fig. 10a) reveled that the front face of the foam was partially melted, so that mullite fragments (from the front heat shield) strongly adhered to it. This suggests that, under reaction conditions, surface temperatures well above 1200 C were reached close to the entrance section of the reactor, although not captured by thermocouple measurement (Tin 1080 C). On the other hand, no sign of damages due to catalyst overheating could be seen on the Rh/AlPO4-Fecralloy catalyst after CPO tests under identical conditions (Fig. 10b). The same holds for Rh/LA catalysts with cordierite honeycomb substrates (melting temperature ca. 1240 C), which in
90 1
80 70
3
CO
1 2 3
60 50 40 1.6
100
1.8
3 2 1
2.0
2.2
2.4
2.6 900
CH4/O2
90
80
800
1 2 3
700
70
Tg out °C
DH25 r
¼ þ206 kJ=mol
CH4 conv.; CO sel. %
CH4 þ CO2 42CO þ 2H2
C
DH25 r
H2 selectivity %
CH4 þ H2 O4CO þ 3H2
600 1.6
1.8
2.0
2.2
2.4
2.6
CH4/O2 Fig. 9 e Evolution of the CPO performance of Rh-Fecralloy foam catalyst (LRh ¼ 1.4 mg cm¡3) during successive tests in which the CH4/O2 feed ratio was varied between 1.7 and 2.6.
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fact were tested under more severe conditions (CH4/O2 down to 1.4, Fig. 10c) [26]. Rh-Fecralloy catalysts used in CPO tests were also reexamined by cyclic voltammetry, SEM and X-ray diffraction.
Fig. 11 e SEM images of a Rh-Fecralloy catalysts (LRh ¼ 1.4 mg cm¡3) after 44 h on stream during CPO tests.
Fig. 10 e Images of the front face of Rh-based catalysts after CH4 CPO tests: (a) Rh-Fecralloy (LRh ¼ 1.4 mg cm¡3); (b) Rh/ AlPO4-Fecralloy; (c) Rh/LA with cordierite honeycomb substrates [26].
Attempts to quantify Rh surface area by cyclic voltammetry were unsuccessful because no current due to hydrogen adsorption/desorption was measured with used catalysts. Etching procedures which had proved effective in restoring the electrochemical activity of RheNi foams [27] and PdFecralloy foams heated at T 600 C [19] were also unsuccessful. Two SEM images of a used Rh-Fecralloy foam catalyst are shown in Fig. 11. The image recorded with the lower magnification (Fig. 11a), fully comparable with those in Fig. 2a and b, shows that prolonged exposure to high temperature reaction conditions caused a significant increase in the Rh particle size, which is the most probable reason for the declining CPO performance of Rh-Fecralloy foam catalyst during prolonged tests (Fig. 9). Due to sintering of Rh particles, which may have started to occur early after light-off of the reactor (during the stabilization phase), the ARh values reported in Table 2 for RhFecralloy are likely to be overestimated and not to provide an accurate assessment of the real Rh surface area active in CPO tests. The effects shown in Figs. 9 and 10 can be related to the loss of steam reforming activity due to active metal sintering. If less CH4 is converted by steam reforming (reaction 10), the yield to CO (eq. (5)) drops, but CO selectivity (eq. (6)) remains constant; at the same time, the temperature level increases proportionally to the drop in conversion due to the
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endothermic nature of that reaction [26,28]. Fig. 11b shows that, after the CPO tests, the Fecralloy surface was strongly modified, being coated by scales which, on the basis of local EDS analyses, and of results in the literature [38e40], were identified as Al2O3 which is known to form when Fecralloy is heated at temperatures above 900 C. The X-ray diffractogram of a used Rh-Fecralloy catalyst (Fig. 6b) was also substantially different from that of an asprepared sample for the presence of two sets of reflections. Those marked with an empty circle can be ascribed to aAl2O3 [41]. Thus, SEM and XRD evidences suggest that, although still present in the foam, Rh particles were no longer in electrical contact with it, due to the presence of an insulating, rather thick Al2O3 layer. Etching of the used catalysts was either unable to remove Al2O3, or caused detachment of the Rh particles (or both). In all cases, Rh particles were no longer electrochemically active. The other reflections, seen in Fig. 6b at 2q ¼ 41.24, 48.005, 70.275 and 84.92 degrees, are located at angles close to those expected for metallic Rh, but systematically higher than them, the difference between the expected and the observed angle progressively increasing with 2q. Such a result might suggest that the crystal lattice of the Rh, better developed than in the as-prepared sample, is compressed or a solid solution is formed between Rh and one or more of the Fecralloy components which have lower atomic dimensions than Rh. The crystallites of this modified Rh phase are estimated to have an average size of 270 nm, comparable with the average particle size observed in used catalysts, and about 10 times larger than that determined for as-prepared samples. The diffractogram of the Rh-Fecralloy sample heated in H2 atmosphere, Fig. 6c, shows the peaks marked with an empty triangle, in addition to those of Fecralloy, Al2O3 and “modified Rh”. Their positions correspond to those of an AleRh alloy with composition close to 1:1 [42]. One may speculate that, in the reducing H2 atmosphere, no Al2O3 is formed, and so the Al may interact with Rh forming this Al-rich alloy. In the slightly more oxidizing CPO conditions, extensive AleRh alloy formation is prevented by Al2O3 formation and only minor amounts of Al are incorporated into Rh particles. All these data point out that temperature control inside the reactor is a major issue when Rh-Fecralloy foams are used. Although steam reforming is the predominant reaction in the post-oxidation zone of the reactor, it does contribute also in the first oxidation zone, where oxygen coverage of the catalytic surface is zero due to mass transfer limitations [28]. This circumstance has very important implications for the heat management of the reactor, since the endothermic steam reforming can effectively consume part of the heat generated by catalytic fuel oxidation, mitigating the hot spot that forms on the catalyst close to its front face [12,28]. Rh sintering due to high temperatures adversely impacted on the steam reforming activity of Rh catalyst, whereas it did not affect significantly the oxidation rates. Under such conditions the surface temperature can increase further with a self-promoting effect on catalyst deactivation [12], eventually causing permanent material failure due to the loss of chemical quenching. Some signs of such a failure were observed with Rh-Fecralloy catalysts for which the situation was further complicated by their above mentioned higher intrinsic selectivity to form CO2 rather than CO, implying that more
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heat is produced for each mole of oxygen converted in the first part of the reactor. Obviously, the risk of overheating was strongly reduced when the CPO reactor was run at lower temperatures using CH4/O2 feed ratios above 2. Significantly higher noble metal loadings on Fecralloy foams can contrast the effect of metal sintering (as in the case of Rh-alumina foams), but this is not economically feasible for practical applications. On the other hand, the application of a suitable washcoat layer on the foam is very effective in stabilizing metal aggregates during reaction, but the drawback comes from the maximum amount of catalyst which can be anchored on the support without affecting fluid-dynamics and durability. Attempts to stabilize the relatively small Rh nanoaggregates electrochemically formed on Fecralloy supports are in-progress. Possible strategies comprise the deposition on the foam surfaces of species containing elements such as Mg, Al [18] or P, and the formation of the superficial alumina layer on the Fecralloy foam after Rh electrodeposition and before exposure to the reaction atmosphere.
Conclusions Electrodeposition of Rh nanoparticles onto Fecralloy foams has been studied as a new method for the preparation of structured catalysts for the partial oxidation of CH4. It has been shown that a fairly homogeneous distribution of Rh may be achieved in rather large foam samples (2.44 cm3 cylinders appropriate for CPO tests). In as-prepared Rh-Fecralloy samples, the Rh surface area per unit Rh mass is of the order of 30 m2 g1, essentially independent of loading, and the typical Rh crystallites dimensions are ca. 25 nm. The performance of Rh-Fecralloy sample catalysts in methane CPO is comparable with that of Rh/AlPO4-Fecralloy catalysts with lower Rh loading and higher dispersion onto the intermediate washcoat layer, in terms of CH4 conversion, exit temperature and H2 selectivity, but somewhat worse in terms of CO selectivity (4e6% lower), probably due to Rh interaction with the metallic foam support. Prolonged CPO tests at high temperature modify the catalysts by inducing: (i) an increase in the dimension of both Rh particles and Rh crystallites which become as large as 270 nm, with the probable formation of AlRh alloys; (ii) the growth of an insulating Al2O3 layer underneath the Rh particles, which prevents them from being electrochemically active, but does not interfere with their CPO activity. Due to the growth of the Al2O3 layer, the Rh-Fecralloy samples are converted to catalysts more similar to the ordinary ones prepared by washcoating methods. However, sintering of Rh particles in direct contact with the Fecralloy foam due to high temperatures negatively affects the catalytic activity for steam reforming, which declines during successive cycles of test runs by varying the CH4/O2 feed ratio, whereas it does not affect significantly the oxidation rates. Under such conditions the surface temperature can increase further with a self-promoting effect on catalyst deactivation due to the loss of chemical quenching by steam reforming, eventually leading to melting of the Fecralloy foam. Rh nanoparticles dispersed on the intermediate washcoat layer (AlPO4 as well as La-g-Al2O3) guarantee
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rather stable CPO performance with time on stream: the higher selectivity to CO compared to Rh-Fecralloy counterparts also helps to reduce the risk of catalyst overheating in the entrance zone of the reactor.
Acknowledgements The authors acknowledge the financial support of the Italian Ministry for Economic Development (MiSE) and MiSE-CNR Agreement on National Electrical System, and the assistance of Dr. F. Agresti and Dr. R. Gerbasi (IENI, CNR) in recording and analyzing XRD data. Mr. F. Dergal gratefully acknowledges the financial support from the Italian Ministry of Foreign Affairs (MAE).
references
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Available online at www.sciencedirect.com
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Catalytic partial oxidation of methane over nanosized Rh supported on Fecralloy foams E. Verlato a, S. Barison a, S. Cimino b,*, F. Dergal b,1, L. Lisi b, G. Mancino b, zquez-Go mez a M. Musiani a, L. Va a b
Istituto per l’Energetica e le Interfasi IENI e CNR, Corso Stati Uniti 4, 35127 Padova, Italy Istituto di Ricerche sulla Combustione IRC e CNR, P.le V. Tecchio 80, 80125 Napoli, Italy
article info
abstract
Article history:
Structured catalysts for the partial oxidation of methane were prepared by supporting Rh
Received 15 January 2014
nanoparticles onto Fecralloy foams at relatively low precious metal loadings. The inves-
Received in revised form
tigation was focused mainly on an innovative and straightforward preparation procedure
17 April 2014
consisting in the direct cathodic electrodeposition of Rh onto foam samples. For the sake of
Accepted 7 May 2014
comparison, other Rh-based catalysts were prepared with a more traditional approach, by
Available online 18 June 2014
using the same foams and an AlPO4 washcoat layer. The catalysts were characterized by SEM-EDS, XRD and cyclic voltammetry, to assess the Rh surface area, and tested in the CPO
Keywords:
of methane to syngas under self-sustained high temperature conditions at short-contact-
Electrodeposition
time. During prolonged CPO tests the performance of electrochemically prepared catalysts
Rh nanoparticles
underwent a progressive decline, as compared to stable operation of AlPO4 washcoated
Structured catalyst
catalysts, which was mainly ascribed to sintering of Rh nanoparticles, negatively affecting
Syngas
the activity for methane steam reforming.
Steam reforming
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The Catalytic Partial Oxidation of hydrocarbons in shortcontact-time autothermal reactors represents a promising technology for syngas or H2 production in small scale applications such as on board fuel reformers or catalytic burners [1e9]. Rh-based catalysts are recognized as the most active and selective during the CPO of hydrocarbons from methane up to diesel and jet-fuels [1e5]. However rhodium is among the scarcest and most expensive precious metals,
having mainly no viable substitute for the reduction of NOx in three-way automotive catalysts [10e11]. Therefore its effective usage is of vital importance in terms of cost reduction. In fact many authors have used relatively high Rh loadings (from 2 to 15% by weight [2,3,5e7,12e14], generally referred to the total weight of the structured reactor including the substrate) to counterbalance possible activity losses during high temperature CPO, due for instance to thermal sintering of the metal clusters [13] or volatilization of the active species [12].
* Corresponding author. E-mail addresses: [email protected], [email protected] (S. Cimino). 1 Permanent address: Centre de Recherche Scientifique et Technique en Analyses Physico-chimiques, (C.R.A.P.C.), BP 248, Alger RP 16004 Alger, Algeria. http://dx.doi.org/10.1016/j.ijhydene.2014.05.076 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Among structured substrates for CPO reactors foams with high density of pores are generally preferred [14] to honeycombs because of their outstanding gas-to-solid heat- and mass-transfer characteristics and high specific geometric surface areas, at the expense of only a small increase in pressure drops [15]. Fecralloy metallic foams have outstanding resistance to oxidative degradation and maximum operating temperatures around 1250 C, and allow to design lightweight reactors preshaped in almost any geometry, with low thermal inertia and fast transient response during start-ups and load variations, while avoiding thermal-shock limitations of ceramic materials [15e18]. Those substrates generally serve as structures for highsurface-area ceramic supports such as g-Al2O3 which are used to disperse and stabilize the active metal phase. However, the deposition of a thin uniform washcoat layer of refractory oxides, firmly anchored to the metal struts, which is of paramount importance in order to guarantee long catalyst durability and avoid pore blocking and additional pressure drops [17,18], is not a trivial matter. Therefore, an innovative approach for the catalysts preparation was explored. Thanks to the metallic nature of Fecralloy foams and their high electrical conductivity, electrochemical methods can be used to disperse noble metals directly on the carrier [18,19], in order to obtain technological catalysts with high noble metal dispersion (i.e. surface area) and relatively low loadings. When the noble metal deposits thus formed are in electrical contact with the metallic foam [19], electrochemical methods can be used also to measure their surface area. In order to assess the validity of this new preparation method, the electrochemically prepared catalysts were compared with other Rh-based catalysts comprising an AlPO4 washcoat layer over the same Fecralloy foam, prepared with a more traditional approach and tested under identical conditions, and with results in the literature.
Experimental Catalyst preparation and characterization Fecralloy foams used in this work were cut out of a single commercial panel (Porvair, Metpore 50 pores per inch, ppi) with average composition: Fe 70.1; Cr 20.8; Al 9.1% (mass) [19,20]. Table 1 reports the main geometrical features of the foam: the average diameter of the struts (ds) and pores (dp) were estimated by SEM (respectively at 1 102 cm and 5.5 102 cm). The specific surface area of the metal foam (Sv), evaluated by EIS measurements as described in Ref. [20], was 100 ± 5 cm1. This value, which also includes roughness caused by features with a characteristic dimension much smaller than the average strut or pore diameters, can be regarded as an upper limit. The Sv predicted for the same foam by a cubic cell model made of smooth cylindrical struts [[15], and ref. therein] is 31 cm1 (Table 1), suggesting that this is possibly an oversimplified description of the reticulated structure. Parallelepipeds (0.4 1.0 1.0 cm; electrode volume Vfoam ¼ 0.4 cm3) were used in most electrochemical experiments. Catalysts for partial oxidation tests were prepared from Fecralloy cylinders (1.8 cm diameter 0.96 cm height, Vfoam ¼ 2.44 cm3). Before use in spontaneous or electrochemical deposition the foam samples were successively washed with dichloromethane, acetone and water; they were then used as such or after etching in acid or basic media. The solutions used for either spontaneous or electrochemical deposition of Rh were prepared by dissolving 0.001 M Na3RhCl6 in 1 M NaCl and adding the HCl necessary to achieve pH 2.0. These solutions were deaerated and kept at 25 C with a thermostat. Since the formation kinetics of chlorocomplexes of Rh is known to be slow [21], and their speciation may evolve during several days, only solutions aged
Table 1 e Morphological properties of Rh-based catalysts with structured substrates used in this and in previous studies. Active phase
Support
Strutb Poreb
Sv
Washcoat s.a.
LRh
am Rh
dRhe
aRh
mg/cm3
m2/g
nm
cm2/cm3
Ref.
ds
dp
cm
cm
cm2/cm3
mm
m2/g
31f (100) 31 (100) 31 (100) 35.8 35.8 28 28
e
e
0.93
1.08
30
16
325
this work
e
e
0.93
1.40
30
16
420
this work
4.5
16
0.90
0.18
97
5.0
170
this work
31 18 e 5.0
122 122 e 120
0.75 0.80 0.70
1.48 0.94 50 50
2.9 2.1 3300 380
2515 2175 75 635
Rh
Fecralloy foam 50 ppi
0.01
0.055
Rh
Fecralloy foam 50 ppi
0.01
0.055
Rh/AlPO4
Fecralloy foam 50 ppi
0.01
0.055
Rh/LAa Rh-P/LAa Rh Rh/g-Al2O3
Honeycomb 600 cpsi Honeycomb 600 cpsi a-Al2O3 foam 80 ppi a-Al2O3 foam 80 ppi
0.076 0.076
0.096 0.096 0.060 0.060
t
c
Voidd 3
170 232 0.15 1.30
[4] [4] [3] [3]
LA: 3% La-g-Al2O3 (Sasol LCF140L3). For honeycombs: ds ¼ wall thickness, dp ¼ side of square channel. c Thickness calculated assuming uniform washcoat deposition above the geometric surface area and apparent density rw ¼ 1.5 g/cm3. d Fecralloy foam by cubic cell model: (1 3 ) ¼ ¾p ((ds þ 2t)/(dp 2t))2; honeycomb: 3 ¼ ((dp 2t)/(dp þ ds))2; a-Al2O3 foam by pycnometry [3]. e 3 Rh particles supposed spherical, dRh ¼ 6=rRh am Rh , rRh ¼ 12.4 g/cm . f Cubic cell model Sv ¼ 4 (1 3 )/ds; values in parenthesis were measured by EIS measurements [20] and include small-scale surface roughness of Fecralloy. a
b
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 9 ( 2 0 1 4 ) 1 1 4 7 3 e1 1 4 8 5
during a week or more were used. The duration of the spontaneous deposition reaction was varied between 1 and 24 h. Electrodeposition was performed in different twocompartment cells, depending on the size of the foam electrode, so as to have a ratio ca. 50 between the solution volume and the foam volume. The Fecralloy foam working electrode, washed but not etched, and a Pt wire counter electrode were placed in the main compartment; a Saturated Calomel Electrode (SCE) placed in the lateral compartment was used as reference. Rh was deposited under potentiostatic control, in the potential range 0.4 to 0.55 V (vs. SCE), with deposition charges QDEP varying between 0.5 and 8.5 C for parallelepipeds (Vfoam ¼ 0.4 cm3), and between 10 and 30 C for cylinders (Vfoam ¼ 2.44 cm3). The mass of Rh deposited onto Fecralloy samples was evaluated by measuring the decrease in the concentration of Rh(III) complexes caused by each deposition experiment, using ion chromatography [22]. The Rh loading, denoted LRh, is henceforth expressed in milligrams of Rh per unit volume of the foam. A further type of catalyst was prepared with a more traditional approach by applying an active washcoat overlayer onto the same Fecralloy foam. In particular amorphous aluminum phosphate (AlPO4) was selected as a support material to disperse Rh particles according to recent literature reports showing optimal metal-support interactions that can reduce significantly Rh loading in automotive three way catalysts [11]. The AlPO4 support was prepared in powder form, according to the coprecipitation route reported in Refs. [11,23], and calcined in air at 900 C for 3 h. Fecralloy foam discs were dip-coated employing a water slurry of AlPO4 powder and a dispersible pseudobohemite binder (Disperal, Sasol), after ball milling with ZrO2 spheres (in a Retch Planetary Miller PM100). The final washcoat loading (measured after air calcination at 800 C for 3 h) was limited to ca. 5% by weight of the foam in order to prevent pore blocking: assuming a uniform deposition of the washcoat (apparent density rw ¼ 1.5 g cm3) over the estimated geometric foam area (31 cm1), a theoretical thickness of 4.5 mm can be calculated. Rh was dispersed onto washcoated foams by 2 consecutive impregnation cycles with an aqueous acid solution of Rh(NO3)3 followed by calcination in air at 350 C. A reference powder system was prepared following an identical procedure starting from the dried slurry and used for characterization. The catalysts were finally reduced in 2% H2/N2 at 900 C for 2 h. LRh was 0.18 mg cm3, corresponding to ca. 0.86% by weight of the AlPO4 support, as measured by ICP-MS analysis (Agilent 7500) after microwaveassisted digestion of samples in nitric/hydrochloric acid solution. Voltammetric experiments aimed at assessing the Rh surface area were performed using the Rh-modified Fecralloy foams as working electrodes, a Pt wire as counter-electrode and a Hg/HgO/ 1 M KOH reference electrode, located in a separate compartment connected to the main one through a Luggin capillary. However, throughout this paper, potential is referred to SCE. Cyclic voltammograms were recorded in 1 M KOH, over a potential range where the H adsorption/desorption charge could be measured, with only a minor interference by redox processes due to Fecralloy. As reported in a previous paper [24], the H desorption charge measured with Rh-
11475
Fecralloy samples, Q foam, was compared to that measured with a carefully polished Rh disc electrode, Q disc, so as to obtain aRh, the surface area of the Rh deposits per unit volume of the foam (expressed in cm1, i.e. cm2/cm3) according to: aRh ¼
Q foam Adisc Vfoam Q disc
(1)
where Adisc is the disc geometric area, assumed identical to the true one. By taking the ratio between aRh and LRh the Rh surface area per unit Rh mass (henceforth denoted am Rh and expressed in m2 g1) was calculated. The specific surface area of the AlPO4 washcoat was measured with BET method by N2 adsorption at 77 K performed in a Quantachrome Autosorb 1-C. The exposed surface area of Rh dispersed on the AlPO4 support (am Rh ) was estimated by CO chemisorption measurements performed in the same instrument at room temperature according to the double isotherm method, as described in Ref. [4]. Electrochemical experiments were performed with Autolab PGSTAT 302N or Autolab PGSTAT 100 potentiostats. Na3RhCl6 solutions were analysed with a Metrohm 850 Professional IC liquid chromatographer equipped with a Metrohm 887 UVevisible detector and a METROSEP A SUPP 15e50 anionic column. SEM images and EDS analyses were obtained with a Zeiss SIGMA instrument, equipped with a field emission gun, operating in high vacuum conditions. Accelerating voltages variable between 5 and 30 keV were used, depending on the observation needs. X-Ray diffraction (XRD) patterns were obtained by using a Philips X-PERT PW3710 diffractometer with a Bragg-Brentano geometry, employing a CuKa source (40 kV, 30 mA). Before recording the diffractograms, the foams were compressed to obtain flat, ca. 0.5 mm thick samples. The crystallite dimension was estimated by Rietveld analysis, performed through the Maud software [25].
Catalyst testing Rh-Fecralloy and Rh/AlPO4-Fecralloy foam catalysts were tested for the CPO of methane to syn-gas in a lab scale reactor operated at nearly atmospheric pressure, under selfsustained short contact time conditions using air as the oxidant [4,26,27]. The former, containing only metallic Rh, were tested directly, without further conditioning; on the other hand Rh/AlPO4eFecralloy catalyst was preliminary reduced at 900 C, in order to form and stabilize Rh metal particles. The catalytic foams were stacked between two mullite foam heat shields (45 ppi, L ¼ 12 mm), wrapped with ceramic paper and fitted in a quartz tube reactor (I.D. ¼ 20 mm) which was inserted in an electric tubular furnace used for pre-heating (fixed at 230 C). High-purity gases (CH4, O2, N2: >99.995%) calibrated via Brooks 5850Mass Flow Controllers, were pre-mixed and fed to the reactor at a gas hourly space velocity (GHSV) comprised between 3.2 and 10.5 104 h1 (based on Vfoam). The molar feed ratio CH4/ O2 was varied in the range 1.7e2.6, so as to include ratios both lower and higher than 2, the stoichiometric value for the overall CPO reaction: CH4 þ 1=2O2 4CO þ 2H2
DH25 r
C
¼ 35:6 kJ=mol
(2)
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A complete cycle lasted ca. 4 h. Reactor temperatures were measured by means of K-type thermocouples (d ¼ 0.5 mm) placed in contact with the front face of the catalytic foam (Tin, an average between the relatively cold feed gas and the hot catalyst [3,28]), and at several positions downstream of it, inside the back heat shield: in fact the temperature of the gas leaving the catalyst (Tg out) was estimated by a linear regression in the post reaction zone. Product gases were continuously analysed (by a GEIT Syngas analyzer) to measure the concentrations of CO, CO2 and CH4 (non-dispersive IR), O2 (ECD), and H2 (TCD, with cross sensitivity corrections). For all CH4/O2 feed ratios, O2 was always completely converted under steady state operation, being consumed both in CPO, reaction (2), and in CH4 total oxidation, reaction (3): ¼ 802 kJ mol
(3)
Thus, H2O production was calculated from the O-balance. Carbon and hydrogen balances were closed within ±1% and ±3%, respectively. Methane conversion, yields and selectivities to CO and H2 were calculated according to the following definitions based on measured dry molar fractions:
OUT
CO CHOUT þ COOUT þ COOUT 4 2
YH2
YCO xCH4
(4)
YH2 xCH4
1.0
a 0.01
0.8
! (5)
0.00
(6)
100 HOUT 2 $ ¼ OUT 2 CH4 þ COOUT þ COOUT 2
SH2 ¼ 100$2$
!
0.6
!
-0.01
0.4
-0.02
0.2
(7) -0.8
-0.6
-0.4
0.0 0.0
-0.2
E vs SCE / V (8)
Equilibrium calculations with constant enthalpy and pressure were performed using CHEMKIN 4.1.1 software, excluding solid carbon formation, which was never observed experimentally.
2.5 b 2.0 -3
YCO ¼ 100$
SCO ¼ 100$
CHOUT 4 CHOUT þ COOUT þ COOUT 4 2
LRh / mg cm
xCH4 ¼ 100$ 1
Rh cathodic deposition has been described by several authors. Deposition baths with commercial significance are sulphate, phosphate or mixed sulphate-phosphate solutions [29e31], but also chloride baths have been investigated [32,33]. In all cases a low pH is required to prevent Rh(III) ion hydrolysis. As a consequence, H2 evolution is a significant side reaction which severely limits the current efficiency. We used, as electrodeposition baths, solutions identical to those employed in spontaneous deposition, and similar to the chloride solution studied by Pletcher and Urbina [32], because obtaining smooth and bright Rh deposits is not only unnecessary, but may be even detrimental in the preparation of heterogeneous catalysts for which an extended surface area is desirable.
Current efficiency
C
Preparation of Rh-modified Fecralloy by electrodeposition
I / A
CH4 þ 2O2 4CO2 þ 2H2 O DH25 r
irreproducible and therefore unsatisfactory results, ascribed to an inhomogeneous reactivity of the Fecralloy foam surface, which acid or basic etching was not apt to remove, Rh electrochemical deposition was considered as an alternative method for Rh-Fecralloy preparation.
1.5 1.0
Results and discussion 0.5
Preparation of Rh-modified Fecralloy by spontaneous deposition Several Rh-modified Fecralloy samples were prepared by spontaneous deposition, by varying the immersion time of the Fecralloy foam in a 0.001 M Na3RhCl6 þ 1 M NaCl solution at pH ¼ 2.0. Their LRh values were found to be heavily scattered both when the foam samples were simply washed with dichloromethane, acetone and water, and when they were also etched in HCl or KOH solutions. Most measurements of Rh surface area per unit Rh mass yielded am Rh values in the range 10e20 m2 g1, but values as low as 5 m2 g1 and as high as 35 m2 g1 were occasionally obtained. Owing to these
0.0 0
1
2
3
4
5
6
7
8
9
QDEP / C
Fig. 1 e (a) Cyclic voltammogram (red solid line) recorded with a Fecralloy foam electrode (Vfoam ¼ 0.4 cm3) in 0.001 M Na3RhCl6 þ 1 M NaCl solution at pH ¼ 2.0, with a 50 mV s¡1 scan rate and current efficiency values (full points) measured at selected potentials for a 5 C deposition charge. (b) Dependence of the Rh loading on the deposition charge. The dashed lines are an aid for the eye. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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The continuous curve in Fig. 1a is a cyclic voltammogram recorded with a Fecralloy foam electrode (Vfoam ¼ 0.4 cm3) in 0.001 M Na3RhCl6 þ 1 M NaCl solution at pH ¼ 2.0. The cathodic peak at E ¼ 0.54 V is due to overlapping Rh deposition and hydrogen evolution [32]; the anodic peak at E ¼ 0.28 V is ascribed to the oxidation of molecular H2, since it is known that Rh anodic stripping does not occur [32,34]. The current efficiencies, measured for a constant 5 C reduction charge and shown in Fig. 1a as full symbols joined by a dashed line, decrease as the potential becomes more negative, as expected. Therefore, a deposition potential E ¼ 0.40 V was preferentially used in further experiments, so as to have reasonably
high deposition rate and current efficiency. Fig. 1b shows that, at E ¼ 0.40 V, LRh increases less than linearly with the deposition charge QDEP. This behaviour results from a progressive loss in current efficiency due to an enhancement of the H2 evolution reaction as the Rh deposition proceeds. Fig. 2 shows SEM images of as-prepared Rh-Fecralloy catalysts. Comparison of samples a and b, both prepared by electrodeposition, shows that upon increasing QDEP the coverage of the foam by Rh particles markedly increases, but the size of the particles is not very strongly affected. The insets with higher magnification images, placed in the upper right corner of Fig. 2a and b, allow to appreciate that most particles have “diameters” 150 nm and appear to consist of aggregated smaller crystallites. The third sample, prepared by spontaneous deposition (c), has a similar aspect and a surface coverage by Rh particles intermediate between those of electrodeposited samples. To achieve such coverage, the spontaneous deposition had to be prolonged for 24 h, a time long enough to cause some deterioration of the mechanical properties of the foam. SEM-EDS analyses were used to assess how homogeneously the Rh particles were distributed inside the foam. To this aim, a 2.44 cm3 cylinder was cut so as to obtain a 1.6 0.96 0.46 cm pseudo-parallelepiped (as schematically
Fig. 2 e SEM images of as-prepared Rh-Fecralloy samples (Vfoam ¼ 0.4 cm3) obtained by electro-deposition with QDEP ¼ 1 C (a) or 5 C (b) or spontaneous deposition during 24 h (c).
Fig. 3 e Schematic representation of the sample used to assess the homogeneity of Rh distribution in Rh-Fecralloy foams via EDS analyses. The table in the bottom part of the figure reports local compositions.
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shown in Fig. 3). EDS analyses were then performed by sampling 0.05 mm2 areas located at the positions denoted A to G. The Rh local amount, calculated as
Fig. 4 e Cyclic voltammograms recorded with Fecralloy and Rh-Fecralloy electrodes (Vfoam ¼ 0.4 cm3) in 1 M KOH solution. The Rh deposition charges and Rh loadings are indicated on the figure. Scan rate ¼ 50 mV s¡1.
aRh / cm
500 400 300 Edep= -0.4V
200
Edep= -0.45V Edep= -0.5V
100 0 0.0
Edep= -0.55V
0.5
1.0
1.5
2.0
2.5
-3
LRh / mg cm
b
35
-1
30 25
2
(where NMe is the number of atoms of each metal Me) is shown in the bottom part of Fig. 3. Although these analyses were devoid of a rigorous quantitative character, since Rh was not present as an alloy component but in the form of particles sticking on the walls of the foam cells, the following conclusions may be drawn: (i) comparable Rh amounts were present in different parts of the foam; (ii) the local Rh loading at peripheral positions was somewhat higher than in the centre of the sample, probably due to some current distribution and/or different Rh(III) ions transport rates. Fig. 4 shows cyclic voltammograms measured with Fecralloy and Rh-Fecralloy electrodes in 1 M KOH solution. Hydrogen adsorption/desorption currents, observed in the potential range negative of E ¼ 0.7 V increase with the deposition charge. Since the current measured with an unmodified Fecralloy foam electrode, in the same potential range, is comparatively very small, Qfoam may be reliably measured and used to calculate aRh and am Rh values as described in the experimental section. The dependence of these quantities on LRh is shown in Fig. 5a and b, respectively. The Rh specific surface area of samples prepared at E ¼ 0.4 V (full points in Fig. 3a) increases linearly with the loading, and becomes larger than the specific area of Fecralloy (100 cm1) for LRh > 0.3 mg cm3. A linear aRh vs. LRh dependence is in agreement with the progressive formation of an increasing
-1
600
(9)
m
NRh 100 NFe þ NCr þ NAl þ NRh
a 700
aRh / m g
Rh at% ¼
800
20 15 10
Edep= -0.4V
5
Edep= -0.5V
0 0.0
Edep= -0.45V Edep= -0.55V
0.5
1.0
1.5
2.0
2.5
-3
LRh / mg cm
Fig. 5 e Dependence of the Rh specific area aRh (a) and Rh surface area per unit Rh mass am Rh (b) on the Rh loading LRh. Rh was electrodeposited at the potentials indicated on the figures. In part (a) the best fitted straight line relevant to E ¼ ¡0.4 V is shown.
number of particles which retain essentially the same shape and size, suggested by the SEM images in Fig. 2. Samples prepared at E < 0.4 V (empty points in Fig. 5a) show significantly, but not dramatically, lower aRh values which also depend almost linearly on LRh. A linear aRh vs. LRh means that am Rh values are independent of LRh. For samples prepared at 2 1 E ¼ 0.4 V, am Rh is 30 ± 5 m g , Fig. 5b, a value only slightly lower than those measured for Rh spontaneously deposited onto Ni foams [27] and much larger than those relevant to Pd spontaneously deposited onto Fecralloy [19,20]. Fig. 6 compares the X-ray diffractograms of three RhFecralloy samples: (a) as-deposited, (b) after 44 h on stream during CPO tests, and (c) after thermal treatment at 1000 C in 5% H2/N2 atmosphere for 5 h. In the diffractogram of the asprepared sample, only broad low-intensity peaks which can be ascribed to Rh [35] are visible, besides the reflections due to the Fecralloy substrate. The Rh crystallites are estimated to have a characteristic size of 25 nm, significantly lower than the average particle size, and in line with the average diameter of metal clusters (16 nm) estimated from the corresponding value of am Rh (Table 1). The diffractograms in Fig. 6b and c are discussed below, after the CPO tests.
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Preparation of supported RheAlPO4 on Fecralloy
Fig. 6 e X-ray diffractograms of Rh-Fecralloy catalyst (LRh ¼ 1.4 mg cm¡3): (a) as-deposited, (b) after 44 h on stream during CPO tests, (c) after thermal treatment at 1000 C in H2 atmosphere for 5 h. The peaks labelled with Miller indexes are ascribed to Rh, in part a, or Al-poor AlRh alloys, in parts (b) and (c).
The Rh/AlPO4 active layer anchored onto the Fecralloy foam had an overall BET surface area of 16 m2 g1 after reduction at 900 C (Table 1), which is significantly lower than the value of 80 m2 g1 previously reported by Machida et al. [11]. In fact, the wet ball milling step in the preparation of the slurry for dipcoating was found to induce the collapse of pores of the amorphous AlPO4 support, whose initial surface area was as high as 87 m2 g1 after reduction in H2/N2 at 900 C. CO chemisorption measurements for Rh/AlPO4 catalyst 2 1 yielded a am Rh ¼ 97 m g , i.e. three times larger than for Rh/ Fecralloy counterparts without washcoat layer. This corresponds to an average diameter of 5 nm for Rh nanoparticles (assumed spherical) dispersed on amorphous AlPO4. Table 1 also reports relevant features of two Rh-based honeycomb catalysts characterized by a metal loading LRh in the same range as Rh-Fecralloy foams. For their preparation, commercial honeycomb substrates (Cordierite, 600 square channels per square inch, cpsi, NGK) were washcoated with a La-g-Al2O3 (LA), which was selected in order to improve and stabilize Rh dispersion under high temperature CPO conditions [4,26]. Accordingly, the average dimension of metal aggregates dispersed on g-alumina (Table 1) was ca 2.9 nm (Rh/ LA), and it further decreased down to ca. 2.1 nm due to doping with phosphorous [4], corresponding to am Rh values from 5 to 8 times larger than for Rh-Fecralloy samples. As shown in Table 1, the cordierite honeycomb substrate has a specific geometric area similar to that of the Fecralloy foam; however its almost twice larger and straight channels strongly facilitate the anchoring of a thicker catalytic layer by conventional dipcoating methods, without the risk of pore blocking, typically encountered with the tortuous paths of interconnected struts in foams [17]. The catalytic activity of those Rh-based honeycomb catalysts was studied in the same experimental rig,
Fig. 7 e CPO of methane over Rh-based catalysts indicated on the figure: (a) methane conversion, (b) exit gas temperature, (c) selectivity to CO, (d) selectivity to H2 as a function of the CH4/O2 feed ratio. Dashed lines represent thermodynamic equilibrium values.
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Table 2 e Performances of Rh-based catalysts in CPO of CH4. Catalyst
Support
V
Rh-Fecr1 Rh-Fecr2 Rh/AlPO4 Rh/LA Rh-P/LA Rha Rh/g-Al2O3a
3
2
F
XCH4
YCO
YH2
cm
mg/cm
cm
Sl/h
%
%
%
2.44 2.44 2.44 2.12 2.24 2.50 2.50
1.08 1.40 0.18 1.48 0.94 50 50
790 1025 420 5335 4870 190 1590
135 135 135 135 135 300 300
62.0 64.5 63.3 78.9 78.9
51.4 54.3 56.7 69.5 69.0
49.7 54.0 50.0 73 73.6
eq
YH2 =YH2
Tg out
0.63 0.69 0.64 0.93 0.94 0.83 0.99
Ref.
C
838 802 814 688 691
this work this work this work [4] [4] [3] [3]
Preheating 150 C; thermal reactor efficiency ca.1.
Fig. 7 presents the results of CPO tests in terms of methane conversion, gas temperatures recorded at the exit of the catalyst and selectivities to CO and H2, as a function of the CH4/O2 feed ratio. Catalytic performances at stoichiometric conditions for syngas production (CH4/O2 ¼ 2) are reported in Table 2, together with flow conditions and the main catalyst features including the total exposed Rh area in the CPO reactor (ARh ¼ aRh $V). In agreement with many previous reports, Fig. 7 shows that methane conversion over each of the foam catalysts increased almost linearly with decreasing the feed ratio from 2.6 to 1.7, in line with the progressively higher availability of oxygen, the limiting reactant, which was always completely converted. This agrees with the trend predicted by adiabatic equilibrium (dashed lines in Fig. 7), but methane conversion remained as much as 15e25% points below its corresponding equilibrium value. Fuel conversion slightly increased with metal loading for Rh-Fecralloy foams. The Rh/AlPO4-Fecralloy foam, displayed intermediate performance between the two Rh-Fecralloy counterparts (Table 2) in spite of its significantly lower LRh, confirming the effectiveness of increasing the noble metal dispersion with the use of an appropriate washcoat layer [3,14]. As expected, the temperature of the gas leaving the catalyst (Fig. 7b, Table 2) increased with decreasing the feed CH4/
90
900
80 850
70 60
800
Tg out °C
CH4 CPO tests
O2 ratio, according to progressively higher temperatures on the catalyst (not shown). Tg out was constantly above the corresponding adiabatic equilibrium value, although the thermal efficiency of the reactor under those conditions was estimated around 0.85 [19]. Moreover, the temperature was higher for the catalyst with lower activity (i.e. lower methane conversion). Rh-Fecralloy and Rh/AlPO4-Fecralloy catalysts were tested with different feed flow rates (F) at fixed feed ratio, finding that, in both cases, increasing F above ca 100 Sl h1 and up to 250 Sl h1 did not alter significantly their catalytic performance (see Fig. 8). This generally corresponds to the balancing between an increase in the thermal efficiency of the CPO reactor (translating into higher operating temperatures) [9] and a lower contact time, which can adversely affect the contribution from slower reactions, in particular steam reforming [5,27,28]. Note that methane dry reforming, reaction (11), was shown not to occur at significant rate on Rh catalysts under CPO conditions [3,28,36].
CH4 Conversion %
under almost identical conditions, as reported in details in previous works [4,26]. Considering their higher metal dispersion and similar metal loadings, the active surface area per unit volume of catalyst aRh for honeycombs is larger than for foams by as much as one order of magnitude (Table 1). Finally, for comparison purposes, Table 1 reports the main morphological properties of two Rh foam catalysts reported by Donazzi et al. [3], who impregnated Rh onto a-Al2O3 foam supports (with a nominal density of 80 ppi) either with or without an intermediate washcoat layer of g-Al2O3. With respect to our metallic foam the ceramic structure was characterized by similar dimension of pores and geometric surface area (Sv), in spite of a lower void fraction due to larger struts. Rh loading was the same for both samples and was significantly larger than for Rh on Fecralloy counterparts (from 35 to 230 times); on the other hand, the specific metal area am Rh was estimated to be as low as 0.15 m2 g1 and 1.30 m2 g1, respectively for samples without or with the intermediate gAl2O3 washcoat. The latter had roughly the same nominal thickness than the AlPO4 layer on our Fecralloy foams.
50 40 60
120
100
Selectivity %
a
Fecralloy foam 50 ppi Fecralloy foam 50 ppi Fecralloy foam 50 ppi honeycomb 600 cpsi honeycomb 600 cpsi a-Al2O3 foam 80 ppi a-Al2O3 foam 80 ppi
ARh
LRh 3
180
750 300
240
Feed, Sl/h
90 CO
80 H2
70 60 50 60
120
180
240
300
Feed, Sl/h Fig. 8 e Effect of feed flow rate during the CPO of methane over Rh-Fecralloy catalyst (LRh ¼ 1.4 mg cm¡3) at fixed feed ratio CH4/O2 ¼ 2.2: (a) methane conversion and exit gas temperature, (b) selectivity to CO and H2.
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C
¼ þ247 kJ=mol
(10) (11)
Under identical operating conditions, the two Rh-based honeycombs (Fig. 7, Table 2), having similar LRh but a much higher available metal area ARh than Rh-Fecralloy foams, closely approached equilibrium. In fact, the slight lack of methane conversion and gas temperatures below the predicted values are motivated by the heat losses from the experimental reactor. Some significant differences among Rh catalysts were found in the selectivity to partial oxidation products. CO was formed with very high selectivity (approaching equilibrium) on the Rh/AlPO4-Fecralloy, as well as on both honeycomb catalysts with Rh/La-g-Al2O3 washcoat (Fig. 7c). In contrast, Rh-Fecralloy foams without intermediate washcoat layer showed systematically lower CO selectivities (by as much as 4e6 points %), which only marginally increased with Rh loading. This implies that more CO2 was formed when Rh was in direct contact with the metal foam, suggesting a strong noble metalemetal support interaction (see paragraph 3.5). Under high temperature reaction conditions the surface of Fecralloy becomes covered by an inert alumina overlayer; therefore the contribution of the bare support to the methane total oxidation is generally negligible [19]. Regarding H2 production, it can be observed in Fig. 7d that highly dispersed Rh honeycomb catalysts converted methane with almost equilibrium selectivity to H2. On the other hand, Rh/AlPO4-Fecralloy and Rh-Fecralloy foams showed lower values of H2 selectivity, which increased with LRh. The presence of the AlPO4 washcoat layer on the Fecralloy foam was not as able to guarantee H2 selectivities comparable with equilibrium values as in the case of CO production, suggesting a kinetic limitation in H2 production. All those observations can be rationalized considering that oxygen consuming reactions (2,3) are so fast on Rh catalysts to proceed under full mass transfer control in a thin front zone of the reactor [3,28,36]. Steam reforming, reaction (10), is a much slower, strongly endothermic reaction, which prevails in the post oxidation zone (i.e. after molecular O2 has been completely consumed), and proceeds under kinetic control even at the very high temperatures of the CPO reactor [6,28,36]. In fact, detailed experimental temperature and species profile along CPO monoliths have shown that Rh catalysts ensure better reactor performance with respect to different noble metals (i.e. Pt-based systems) [36,37] due to the high intrinsic catalytic reforming activity of Rh. Similar conclusions have also been drawn from the observation of the poisoning effects of sulphur, which partly covers the surface of Rh under CPO conditions, and selectively inhibits steam reforming [4,26,37]. Accordingly, the ranking of catalysts in terms of their hydrogen yield and approach to equilibrium (Table 2) generally follows the trend of the total surface area of Rh available for steam reforming (ARh). However, if we limit the comparison to metal foam catalysts, it appears that RheAlPO4 produced the same amount of H2, though its ARh
value was ca. ½ of that nominally available for the RhFecralloy foam samples. Rh-Fecralloy foam and Rh/AlPO4-Fecralloy catalysts were submitted to consecutive CPO tests in which the CH4/O2 feed ratio was varied between 1.7 and 2.6, each cycle lasting ca. 4 h. Fig. 9 shows that the CPO performance of Rh-Fecralloy foam samples decreased progressively with time on stream. Methane conversion and hydrogen selectivity decreased, CO selectivity was constant, and the exit temperatures increased. On the contrary, the addition of the AlPO4 washcoat preserved constant catalytic activity during up to 32 h on stream. To understand the reasons for their declining performance, the Rh-Fecralloy foam catalysts were further characterized after use.
Characterization of Rh-Fecralloy catalysts after CPO tests Visual inspection of both Rh-Fecralloy catalysts used in CPO tests (Fig. 10a) reveled that the front face of the foam was partially melted, so that mullite fragments (from the front heat shield) strongly adhered to it. This suggests that, under reaction conditions, surface temperatures well above 1200 C were reached close to the entrance section of the reactor, although not captured by thermocouple measurement (Tin 1080 C). On the other hand, no sign of damages due to catalyst overheating could be seen on the Rh/AlPO4-Fecralloy catalyst after CPO tests under identical conditions (Fig. 10b). The same holds for Rh/LA catalysts with cordierite honeycomb substrates (melting temperature ca. 1240 C), which in
90 1
80 70
3
CO
1 2 3
60 50 40 1.6
100
1.8
3 2 1
2.0
2.2
2.4
2.6 900
CH4/O2
90
80
800
1 2 3
700
70
Tg out °C
DH25 r
¼ þ206 kJ=mol
CH4 conv.; CO sel. %
CH4 þ CO2 42CO þ 2H2
C
DH25 r
H2 selectivity %
CH4 þ H2 O4CO þ 3H2
600 1.6
1.8
2.0
2.2
2.4
2.6
CH4/O2 Fig. 9 e Evolution of the CPO performance of Rh-Fecralloy foam catalyst (LRh ¼ 1.4 mg cm¡3) during successive tests in which the CH4/O2 feed ratio was varied between 1.7 and 2.6.
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fact were tested under more severe conditions (CH4/O2 down to 1.4, Fig. 10c) [26]. Rh-Fecralloy catalysts used in CPO tests were also reexamined by cyclic voltammetry, SEM and X-ray diffraction.
Fig. 11 e SEM images of a Rh-Fecralloy catalysts (LRh ¼ 1.4 mg cm¡3) after 44 h on stream during CPO tests.
Fig. 10 e Images of the front face of Rh-based catalysts after CH4 CPO tests: (a) Rh-Fecralloy (LRh ¼ 1.4 mg cm¡3); (b) Rh/ AlPO4-Fecralloy; (c) Rh/LA with cordierite honeycomb substrates [26].
Attempts to quantify Rh surface area by cyclic voltammetry were unsuccessful because no current due to hydrogen adsorption/desorption was measured with used catalysts. Etching procedures which had proved effective in restoring the electrochemical activity of RheNi foams [27] and PdFecralloy foams heated at T 600 C [19] were also unsuccessful. Two SEM images of a used Rh-Fecralloy foam catalyst are shown in Fig. 11. The image recorded with the lower magnification (Fig. 11a), fully comparable with those in Fig. 2a and b, shows that prolonged exposure to high temperature reaction conditions caused a significant increase in the Rh particle size, which is the most probable reason for the declining CPO performance of Rh-Fecralloy foam catalyst during prolonged tests (Fig. 9). Due to sintering of Rh particles, which may have started to occur early after light-off of the reactor (during the stabilization phase), the ARh values reported in Table 2 for RhFecralloy are likely to be overestimated and not to provide an accurate assessment of the real Rh surface area active in CPO tests. The effects shown in Figs. 9 and 10 can be related to the loss of steam reforming activity due to active metal sintering. If less CH4 is converted by steam reforming (reaction 10), the yield to CO (eq. (5)) drops, but CO selectivity (eq. (6)) remains constant; at the same time, the temperature level increases proportionally to the drop in conversion due to the
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endothermic nature of that reaction [26,28]. Fig. 11b shows that, after the CPO tests, the Fecralloy surface was strongly modified, being coated by scales which, on the basis of local EDS analyses, and of results in the literature [38e40], were identified as Al2O3 which is known to form when Fecralloy is heated at temperatures above 900 C. The X-ray diffractogram of a used Rh-Fecralloy catalyst (Fig. 6b) was also substantially different from that of an asprepared sample for the presence of two sets of reflections. Those marked with an empty circle can be ascribed to aAl2O3 [41]. Thus, SEM and XRD evidences suggest that, although still present in the foam, Rh particles were no longer in electrical contact with it, due to the presence of an insulating, rather thick Al2O3 layer. Etching of the used catalysts was either unable to remove Al2O3, or caused detachment of the Rh particles (or both). In all cases, Rh particles were no longer electrochemically active. The other reflections, seen in Fig. 6b at 2q ¼ 41.24, 48.005, 70.275 and 84.92 degrees, are located at angles close to those expected for metallic Rh, but systematically higher than them, the difference between the expected and the observed angle progressively increasing with 2q. Such a result might suggest that the crystal lattice of the Rh, better developed than in the as-prepared sample, is compressed or a solid solution is formed between Rh and one or more of the Fecralloy components which have lower atomic dimensions than Rh. The crystallites of this modified Rh phase are estimated to have an average size of 270 nm, comparable with the average particle size observed in used catalysts, and about 10 times larger than that determined for as-prepared samples. The diffractogram of the Rh-Fecralloy sample heated in H2 atmosphere, Fig. 6c, shows the peaks marked with an empty triangle, in addition to those of Fecralloy, Al2O3 and “modified Rh”. Their positions correspond to those of an AleRh alloy with composition close to 1:1 [42]. One may speculate that, in the reducing H2 atmosphere, no Al2O3 is formed, and so the Al may interact with Rh forming this Al-rich alloy. In the slightly more oxidizing CPO conditions, extensive AleRh alloy formation is prevented by Al2O3 formation and only minor amounts of Al are incorporated into Rh particles. All these data point out that temperature control inside the reactor is a major issue when Rh-Fecralloy foams are used. Although steam reforming is the predominant reaction in the post-oxidation zone of the reactor, it does contribute also in the first oxidation zone, where oxygen coverage of the catalytic surface is zero due to mass transfer limitations [28]. This circumstance has very important implications for the heat management of the reactor, since the endothermic steam reforming can effectively consume part of the heat generated by catalytic fuel oxidation, mitigating the hot spot that forms on the catalyst close to its front face [12,28]. Rh sintering due to high temperatures adversely impacted on the steam reforming activity of Rh catalyst, whereas it did not affect significantly the oxidation rates. Under such conditions the surface temperature can increase further with a self-promoting effect on catalyst deactivation [12], eventually causing permanent material failure due to the loss of chemical quenching. Some signs of such a failure were observed with Rh-Fecralloy catalysts for which the situation was further complicated by their above mentioned higher intrinsic selectivity to form CO2 rather than CO, implying that more
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heat is produced for each mole of oxygen converted in the first part of the reactor. Obviously, the risk of overheating was strongly reduced when the CPO reactor was run at lower temperatures using CH4/O2 feed ratios above 2. Significantly higher noble metal loadings on Fecralloy foams can contrast the effect of metal sintering (as in the case of Rh-alumina foams), but this is not economically feasible for practical applications. On the other hand, the application of a suitable washcoat layer on the foam is very effective in stabilizing metal aggregates during reaction, but the drawback comes from the maximum amount of catalyst which can be anchored on the support without affecting fluid-dynamics and durability. Attempts to stabilize the relatively small Rh nanoaggregates electrochemically formed on Fecralloy supports are in-progress. Possible strategies comprise the deposition on the foam surfaces of species containing elements such as Mg, Al [18] or P, and the formation of the superficial alumina layer on the Fecralloy foam after Rh electrodeposition and before exposure to the reaction atmosphere.
Conclusions Electrodeposition of Rh nanoparticles onto Fecralloy foams has been studied as a new method for the preparation of structured catalysts for the partial oxidation of CH4. It has been shown that a fairly homogeneous distribution of Rh may be achieved in rather large foam samples (2.44 cm3 cylinders appropriate for CPO tests). In as-prepared Rh-Fecralloy samples, the Rh surface area per unit Rh mass is of the order of 30 m2 g1, essentially independent of loading, and the typical Rh crystallites dimensions are ca. 25 nm. The performance of Rh-Fecralloy sample catalysts in methane CPO is comparable with that of Rh/AlPO4-Fecralloy catalysts with lower Rh loading and higher dispersion onto the intermediate washcoat layer, in terms of CH4 conversion, exit temperature and H2 selectivity, but somewhat worse in terms of CO selectivity (4e6% lower), probably due to Rh interaction with the metallic foam support. Prolonged CPO tests at high temperature modify the catalysts by inducing: (i) an increase in the dimension of both Rh particles and Rh crystallites which become as large as 270 nm, with the probable formation of AlRh alloys; (ii) the growth of an insulating Al2O3 layer underneath the Rh particles, which prevents them from being electrochemically active, but does not interfere with their CPO activity. Due to the growth of the Al2O3 layer, the Rh-Fecralloy samples are converted to catalysts more similar to the ordinary ones prepared by washcoating methods. However, sintering of Rh particles in direct contact with the Fecralloy foam due to high temperatures negatively affects the catalytic activity for steam reforming, which declines during successive cycles of test runs by varying the CH4/O2 feed ratio, whereas it does not affect significantly the oxidation rates. Under such conditions the surface temperature can increase further with a self-promoting effect on catalyst deactivation due to the loss of chemical quenching by steam reforming, eventually leading to melting of the Fecralloy foam. Rh nanoparticles dispersed on the intermediate washcoat layer (AlPO4 as well as La-g-Al2O3) guarantee
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rather stable CPO performance with time on stream: the higher selectivity to CO compared to Rh-Fecralloy counterparts also helps to reduce the risk of catalyst overheating in the entrance zone of the reactor.
Acknowledgements The authors acknowledge the financial support of the Italian Ministry for Economic Development (MiSE) and MiSE-CNR Agreement on National Electrical System, and the assistance of Dr. F. Agresti and Dr. R. Gerbasi (IENI, CNR) in recording and analyzing XRD data. Mr. F. Dergal gratefully acknowledges the financial support from the Italian Ministry of Foreign Affairs (MAE).
references
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