Article pubs.acs.org/cm
Core−Shell Metal−Ceramic Microstructures: Mechanism of Hydrothermal Formation and Properties as Catalyst Materials Jieun Kim† and Doohwan Lee*,† †
Catalysis and Nanomaterials Laboratory, Department of Chemical Engineering, University of Seoul, Siripdae-gil 13, Jeonnong-dong, Seoul 130-743, Republic of Korea S Supporting Information *
ABSTRACT: Unique metal−ceramic composites with core−shell microarchitecture (γ-Al2O3@Al and spinel-MeAl2O4@Al, Me = Zn, Ni, Co, Mn, and Mg) were obtained by a simple hydrothermal surface oxidation (HTSO) of Al metal particles in an aqueous solution of heterometal ions at elevated temperatures (393−473 K). The reactions afforded self-constructed core−shell microarchitecture with Al core encapsulated by the high-surface-area γ-Al2O3 or spinel metal aluminates (MeAl2O4) shell with various surface morphologies, compositions, and excellent physicochemical properties. Extensive experimental and theoretical investigation with period 3−6 metal elements (Na+, Ca2+, Sr2+, Ba2+, K+, Fe3+, Cu2+, Zn2+, Ni2+, Co2+, Mn2+, and Mg2+) at various metal concentrations and temperatures revealed that the heterogeneous self-construction of spinel-MeAl2O4@Al primarily depends on two intrinsic properties of the additive metal ions: (i) thermodynamic stability constant of the metal hydroxide complex and (ii) size of the metal ion. The spinel-MeAl2O4@Al microstructures formed with a limited number of hetero metal ions (Me = Zn2+, Ni2+, Co2+, Mn2+, and Mg2+) with (i) moderate rates of the hydroxide formation with compatible kinetics to the hydrolysis of aluminum on the Al surface and (ii) small size of additive metal ions enough for diffusion through the shell layer. As heterogeneous catalyst substrates, these metal−ceramic composites delivered markedly enhanced catalytic performance at intensive reaction conditions because of their facile heat transfer and superior physicochemical surface properties. The performance and effects of the core−shell metal−ceramic composites were demonstrated using Rh catalysts supported on MgAl2O4@Al. The Rh/MgAl2O4@Al catalyst was utilized for the endothermic glycerol stream reforming (C3H8O3 + 3H2O ⇄ 3CO2 + 7H2, ΔH0298 = 128 kJ mol−1), exhibiting markedly greater catalytic performance than the conventional Rh/MgAl2O4 under intensive reaction conditions attributed to significantly facilitated heat transport through the core−shell metal−ceramic microstructures.
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INTRODUCTION The performance of heterogeneous catalysts in practice is often governed by thermal and mass transport limitation, as the overall rate of catalytic reactions are coupled with intrinsic surface reaction kinetics, heat transport, and fluid dynamics of reactants/products through the catalysts. The thermal and mass transport properties of heterogeneous catalysts are the critical factors when the reactions are intensive with high rates and severe endothermic/exothermic reaction enthalpies.1−3 These factors are of particular importance for the microstructured chemical reactors and devices that demands unconventional catalytic performance under intensive reaction conditions, compact reactor integration, and rapid process dynamics: for example, the microreactors for fuel cells in the distributed and renewable production of fuels and electrical energy.4 However, the conventional heterogeneous catalysts are mostly constructed on ceramic substrates (Al2O3, SiO2, etc.) with low © XXXX American Chemical Society
thermal conductivities and high specific heat capacities; therefore, the structure is intrinsically unfavorable for facile heat transport and rapid load follow-up in dynamic system operation. In contrast, metal−ceramic composites can deliver various advantages overcoming the limitations of the conventional heterogeneous catalyst materials as described. A typical approach to construct metal−ceramic composites has been wash coating of porous metal oxides over metallic substrates.5 However, the resulting structure is prone to degradation at the interface, and this method is hardly applicable for small particles and pellets in the general shapes/forms of the heterogeneous catalysts utilized in industry. In our previous studies, we reported that the simple hydrothermal surface Received: February 8, 2016 Revised: April 1, 2016
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DOI: 10.1021/acs.chemmater.6b00582 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials
under vigorous stirring. The pH of this mixture was continuously adjusted to 9.5 by dropwise addition of an aqueous ammonia solution (8.0 M). The solid precipitates were collected, washed, and treated under the same conditions applied for the core−shell composites. Rh (1, 3, and 5 wt % loading) catalysts supported on these magnesium aluminates were prepared by the incipient wetness impregnation under the same condition used for the Rh/MgAl2O4@Al catalysts. Characterizations. The surface morphologies and cross-sectional features of the Al2O3@Al and MeAl2O4@Al core−shell microstructures were characterized by scanning electron microscopy (SEM, XL-30 FEG, FEI). The surface composition of the shell was obtained by energy-dispersive X-ray spectroscopy (EDX) equipped with SEM. The powder X-ray diffraction (XRD, X’Pert PW 3040, Philips) patterns of the samples were obtained at 40 kV and 30 mA using monochromic Cu−Kα radiation with a scan speed of 1 s per step (0.05°). The phase content of Al metal core and metal oxide shell of the sample was estimated from the XRD peak area ratio with comparison to the reference values predetermined using known amounts of Al and crystalline TiO2 (internal standard) mixtures.24 The atomic contents and bonding states of metal oxide shell were characterized by X-ray photoelectron spectroscopy (XPS, Axis ultra DLD, Kratos). The surface area and pore size distribution of the samples were measured by Brunauer−Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) methods, respectively, using N2 adsorption−desorption isotherms obtained in a volumetric unit (NOVA 2200e, Quantachrome). The thermal conductivity of the Al2O3@Al metal−ceramic composite and Al2O3 samples were obtained from their thermal diffusivities and specific heat capacities measured using a laser flash apparatus (LFA, LFA457, NETZCH)25 and a differential scanning calorimeter (DSC, DSC 200F3, NETZSCH), respectively. The Rh contents of the supported Rh catalysts were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES, JY-70Plus, Jobin-Yvon). Tempeature-programmed reduction (TPR) of the catalysts were carried out in an automated volumetric unit (AutoChem 2920, Micromeritics) equipped with a thermal condutivity detector under 10 mol % H2/Ar flow at a ramping rate of 10 K min−1 from room temperature to 873 K. The dispersion of Rh on the catalyst was estimated from the amount of CO chemisorption and H2 chemisorption measured using a pulse chemisorption analyzer (AutoChem 2920, Micromeritics) at 308 K after reduction of the sample in H2 at 823 K. The average size of dispersed Rh particles was estimated assuming the hemisphere geometry of the particles. The surface structure of the catalysts was further characterized by highresolution transmission electron microscopy (HR-TEM, JEM-3010, JEOL) operated at 300 kV. The size distribution of Rh clusters on these catalysts were obtained using image-pro-plus software (Media Cybernetics). Glycerol Steam Reforming. GSR was conducted in a fixed-bed quartz glass reactor (OD = 10, ID = 8, L = 450 mm). The details experimental setup can be found elsewhere.8 The catalysts pellets (250−425 μm) were packed in the middle of the reactor using a fritted disk and quartz wool. The reactor temperature was measured using a K-type thermocouple attached to the outside reactor wall of the catalyst bed with PID control. All the gases introduced to the reactor were controlled using a mass flow controller (5850E, Brooks). The catalysts were reduced in H2 (50 mL min−1) at 823 K for 0.5 h and subsequently purged in flowing Ar. GSR was conducted by introducing a glycerol−water reactant mixture (H2O/C3H8O molar ratio = 4.5) using a HPLC pump (Series II, LabAlliance). The reaction was carried out at two different feed conditions of 870 and 1,740 mmol gcat−1 h−1. The liquid reactant feed was evaporated through a preheater (603 K) under Ar carrier gas flow (1 mol %-N2 internal standard balanced with Ar). The condensable effluents were collected by a condenser maintained at 269 K and analyzed separately using a flame ionization detector (FID) equipped gas chromatograph (7890A, Agilent). The gaseous products were analyzed online using a thermal conductivity detector (TCD) equipped gas chromatograph (7890A, Agilent). The effects of support composition for the glycerol reforming reaction were investigated using Rh catalysts supported on magnesium aluminates of
oxidation (HTSO) of Al metal particles in aqueous solution of heterometal precursors afforded unique shell@core microarchitectures with Al metal core encapsulated by densely grown porous metal oxide shells of various crystal structures, compositions, and physicochemical properties.6−9 The selfconstruction of these core−shell metal−ceramic microstructures by HTSO is of particular interest considering that these unique layered composites can be obtained by a simple synthesis protocol with superior physicochemical properties, great scalability, and broad applicability. Herein, we first report thorough studies on the formation mechanism and characteristics of the spinel-MeAl2O4@Al (Me = Zn, Ni, Co, Mn, and Mg) core−shell metal−ceramic microarchitectures with an extensive experimental and theoretical investigation with various period 3−6 metal elements (Na, Ca, Sr, Ba, K, Fe, Cu, Zn, Ni, Co, Mn, and Mg). The spinel bimetal−aluminates are important materials in electrodes, sensors, and catalysts and are generally synthesized by sol−gel,10,11 coprecipitation,12,13 solid-state reaction,14,15 hydrothermal,16,17 and combustion18,19 methods. The spinelMeAl2O4@Al structures self-constructed by heterogeneous HTSO of aluminum in this study delivered superior material properties with high thermal conductivity, high surface area, macro-to-micro hierarchical porosity, and unique surface morphologies. The Rh catalysts supported on MgAl2O4@Al core−shell composite exhibited marked catalytic performance for glycerol steam reforming (GSR, C3H8O3 + 3H2O ⇄ 3CO2 + 7H2, ΔH0298 = 128 kJ mol−1). The reaction produces hydrogen utilizing the glycerol byproduct from biodiesel synthesis20,21 and therefore can deliver synergistic advantages for hydrogen fuel cell and biofuel areas.22,23 The GSR is endothermic and thus is favored by high reaction temperature and facile heat flux; therefore, the core−shell composite structures provided significant constructive effects for the reaction.
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EXPERIMENTAL SECTION
Hydrothermal Synthesis of the Core−Shell Metal−Ceramic Composite. Core−shell MeAl2O4@Al metal−ceramic composites were prepared by the hydrothermal surface oxidation of Al metal particles (Ø ≤ 25 μm, Goodfellow) in an aqueous solution of heterometal precursors.8 The size distribution of the Al precursor particles characterized by SEM is shown in Figure S1. The metal precursors used were Na(NO3), Ca(NO3)2·4H2O, Sr(NO3)2, Ba(NO3)2, K(NO3), Fe(NO3)3·9H2O, Cu(NO3)2·3H2O, Mg(NO3)2· 6H2O, and Mn(NO3)2·6H2O (these chemicals were purchased from Junsei Chem.), and Ni(NO3)2·6H2O (Alfa-Aesar), Zn(NO3)2·6H2O (Sigma-Aldrich), Co(NO3)2·6H2O (Alfa-Aesar). In a typical synthesis, 2.0 g of Al metal powder was placed in a Teflon-lined autoclave, and 40 mL of aqueous metal precursor solution (0.93 M) was added. The hydrothermal synthesis was conducted at various temperatures in the range 393−473 K for 3 h under autogenous pressure condition. The inner reactor temperature was constantly maintained by proportionalintegral-derivative (PID) control. The resulting samples were washed several times with deionized water (DI), filtered, dried at 393 K for 12 h, and calcined at 823 K for 4 h (ramp = 10 K min−1). Catalyst Preparation. Rhodium (1, 3, and 5 wt % loading) catalysts supported on the MgAl2O4@Al core−shell microstructure were prepared by incipient wetness impregnation with an aqueous solution of RhCl3·xH2O (Alfa-Aesar). The resulting samples were dried at 393 K for 12 h and calcined at 823 K in air for 4 h (ramp = 10 K min−1). For comparisons, magnesium aluminates were synthesized at various Al/Mg atomic ratios (Al/Mg = 2−10) by the coprecipitation method. Al(NO3)3·9H2O (Junsei Chem.) and Mg(NO3)2·6H2O (Junsei Chem.) precursors at various stoichiometric ratios were homogeneously dissolved in DI water in a beaker at room temperature B
DOI: 10.1021/acs.chemmater.6b00582 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 1. (a) Thermal conductivity and specific heat capacity of various metals and metal oxides [*] (inset: schematic of the core−shell metal− ceramic microarchitecture with highly heat conducting Al metal core encapsulated by high-surface-area porous metal oxide shell), (b) SEM micrograph of Al−Al2O3 cermet consisting of a number of aggregated Al2O3@Al microparticles self-constructed by HTSO of Al at 473 K (inset: high-resolution cross-sectional feature of γ-Al2O3@Al), (c) thermal properties of the Al2O3−Al cermet and γ-Al2O3 pellet (specimen, 10 × 10 × 3 mm3) experimentally measured at 298 K. various Al/Mg atomic ratios (Al/Mg = 2−10) prepared by the coprecipitation method. The effects of Rh particle size on the reaction rate and selectivity were investigated with Rh/MgAl2O4 catalysts of various Rh dispersions. These were conducted after high dilution of the catalysts with inert MgAl2O4 powder (4−10 times by weight) to eliminate internal heat and mass transfer limitation on the overall reaction kinetics measurement.
reveals a nicely developed Al2O3@Al core−shell architecture with the γ-Al2O3 crystallites densely developed over the Al metal core via the heterogeneous hydrothermal growth of boehmite crystallites on the Al surface followed by dehydration into the rhombic shape γ-Al2O3 crystallites after calcination. The growth kinetics of the alumina shell was primarily governed by the diffusion of the oxidants through the thickening Al hydroxide overlayer, whereas the resulting shell morphology largely depended on the hydrothermal temperature and pH condition.7 This Al2O3@Al core−shell composite can provide 4−7-fold greater thermal conductivity than γ-Al2O3 depending on the shell thickness as confirmed from the experimental measurement and theoretical calculation (inset table in Figure 1). The hydrothermal reaction medium had strong effects on the resulting shell structure and composition. The HTSO of Al particles in aqueous solution of various period 3−6 heterometal ions (Me = Na+, Ca2+, Sr2+, Ba2+, K+, Fe3+, Cu2+, Zn2+, Ni2+, Co2+, Mn2+, and Mg2+) under similar reaction condition resulted in markedly different structures, which can be categorized into three groups: (i) γ-Al2O3@Al, (ii) γ-Al2O3@ Al with heterometal oxide precipitates (MeOx), and (iii) spinelMeAl2O4@Al. Figure 2 shows the XRD results of the resulting structures obtained by HTSO in various metal precursor solutions; the XRD pattern of Al2O3@Al prepared in DI water is also shown for comparison. The XRD spectra of the samples prepared in
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RESULTS AND DISCUSSION Figure 1a displays the intrinsic thermal conductivities and heat capacities of various metals and metal oxides obtained from the literature,26 in which the superior heat transport properties of metals contributed from their high thermal conductivities and low specific heat capacities are apparent. For example, Al has ∼10-fold greater thermal conductivity with 1/4 of heat capacity than γ-Al2O3, being able to facilitate the heat flux with a low amount of specific thermal energy storing in the structure. The core−shell metal−ceramic microarchitectures (the schematic in Figure 1a) consisting of a porous metal oxide shell and a highly heat conductive Al metal core can deliver significantly enhanced thermal conductivity, high surface area, and hierarchical macroto-micropore structure favorable for catalyst materials. As shown in Figure 1b, the foam-like composite obtained by the hydrothermal surface oxidation (HTSO) of Al metal particles exhibits a number of Al2O3@Al core−shell microspheres aggregated into mechanically strong cermet, where macropores developed between these primary core−shell particles are clearly observed. The inset SEM micrograph (Figure 1b) C
DOI: 10.1021/acs.chemmater.6b00582 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials
Figure 2. Powder X-ray diffraction (XRD) spectra of the structures obtained by the HTSO of Al metal particles (393 K for 3 h) in various metal ion precursor solution (0.1 M) followed by calcination at 823 K for 4 h: (a) HTSO of Al formed γ-Al2O3 or γ-Al2O3@Al with the heterometal ions remained unreacted in the solution or homogeneously precipitated as metal oxide powder and (b) HTSO of Al formed spinel-MeAl2O4@Al core− shell microstructures. The metal oxides are indicated as A (Al), AO (Al2O3), ZA (ZnAl2O4), NA (NiAl2O4), CA(CoAl2O4), GA (MnAl2O4), MA (MgAl2O4), FO (Fe2O3), and CO (CuO).
Figure 3. SEM micrographs of the core−shell microstructures. These samples were obtained by HTSO of Al metal particles in (a) DI water, (b) zinc nitrate, (c) nickel nitrate, (d) cobalt nitrate, (e) manganese nitrate, and (f) magnesium nitrate solution (0.1 M) at 393 K for 3 h followed by calcination at 823 K for 4 h. Atomic contents of the shell obtained by energy dispersive X-ray spectroscopy (EDX) indicates the formation of metal aluminate shell (the Al content in the shell was higher than the stoichiometric value because of the interference from the Al metal core).
D
DOI: 10.1021/acs.chemmater.6b00582 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials Zn2+, Ni2+, Co2+, Mn2+, and Mg2+ solutions clearly revealed the formation of spinel-MeAl2O4 (Me = Zn, Ni, Co, Mn, Mg) crystalline shell along with the strong characteristic diffraction peaks of the Al metal core. However, the XRD patterns of the structures prepared in Na+, Ca2+, Sr2+, Ba2+, and K+ solutions exhibited γ-Al2O3 and Al peaks, indicating the formation of γAl2O3@Al core−shell structure without incorporation of the heterometal ions into the shell structure. These metal ions remained in the solution without reactions with the Al species or homogeneous precipitation. Differently, the hydrothermal reactions in Fe3+ and Cu2+ solutions under the same reaction condition resulted in homogeneous precipitation of these additive meal ions dominantly into monometallic hydroxides, forming Fe2O3 and CuO after calcination treatment. The Al2O3@Al structure formed independently in these solutions was shadowed in the XRD results by the dominant amount of Fe2O3 and CuO precipitates presented as a mixture in the resultants. Figure 3 displays the SEM micrographs of the γ-Al2O3@Al and the spinel-MeAl2O4@Al (Me = Zn, Ni, Co, Mn, Mg) core−shell microstructures, exhibiting the crystalline porous metal oxide shell constructed densely on the Al metal core. The spinel-MeAl2O4 shell morphologies show unique petal-like structures consisted of a number of vertically grown thin and curved platelets (Figure 3b−f). The EDX analysis confirmed the incorporation of heterometal species into the structure resulting in the spinel-MeAl2O4 shell. The XPS analysis on the binding energy and composition of Me (Me = Zn, Ni, Co, Mn, and Mg) and Al atoms (Figure S2) also showed excellent agreements with the above results, confirming the formation of a bimetallic spinel-MeAl2O4 shell. Figure 4 shows the nitrogen
Table 1. Al Metal Core Content, Surface Area, Pore Size, and Pore Volume of the Metal−Ceramic Core−Shell Microstructures Obtained by HTSO of Al Particles (393 K for 3h, calcination at 823 K for 4 h) structure Al2O3@Al ZnAl2O4@Al NiAl2O4@Al CoAl2O4@Al MnAl2O4@ Al MgAl2O4@ Al
Al core content (wt %)a
BET surface area (m2 g−1)b
BJH average pore size (nm)
total pore volume (cm3 g−1)b
55 78 57 50 45
121 (269) 38 (176) 111 (257) 92 (186) 103 (188)
5.0 5.0 3.6 5.1 3.6
0.20 0.15 0.23 0.22 0.15
35
118 (182)
6.0
0.29 (0.45)
(0.44) (0.69) (0.53) (0.44) (0.27)
a Calculated from the XRD result. bThe values in parentheses are the net properties of the metal oxide shell only.
size, and total pore volume of the core−shell structures; the Al content of the samples varied with the shell thickness in the range of 35−78 wt % and can be modulated by reaction temperature and time. The BET surface area of the samples was in the range 170−270 m2 g−1 depending on the structure and composition, and the average pore size was 3.6−6.0 nm on these samples. The cross-sectional SEM micrograph of MgAl2O4@Al and the pore size distribution analysis results on the core−shell microstructures can be found in Figures S3 and S4. The extensive and controlled synthesis experiments with the period 3−6 metal elements under various reaction conditions led to the conclusion that the characteristics of heterometal additives and hydrothermal temperature are the major factors determining the self-construction of core−shell microstructures and their physicochemical properties. It was found that only a limited number of metals (Zn, Ni, Co, Mn, and Mg) in the period 3−6 elements formed the spinel-MeAl2O4 shell via heterogeneous HTSO of Al, primarily attributed to two critical parameters: (i) the thermodynamic stability constant of the metal hydroxide complex and (ii) the size of metal ions. The thermodynamic stability constant of metal hydroxide complex is defined by eq 1, reflecting the propensity of metal ions for hydroxide formation M(OH)n − 1 + OH ↔ M(OH)n , n ≥ 1
K=
[M(OH)n ] [M(OH)n − 1][OH]
(1)
in which M is metal cation. The size of metal ions determines their diffusivity through the growing shell layer to form the metal aluminate moieties during the HTSO of Al surface. Therefore, excessively fast hydrolysis and condensation of heterometal ions lead to their homogeneous precipitation without enough probability to react with Al species on the interface, whereas low diffusion rates of heterometal ions through the shell limit their chance to participate in the bimetal oxide formation with atomic level homogeneity. Figure 5 shows the characteristic properties of the period 3−6 metal ions as a function of their size and the stability constant of metal hydroxide complexes obtained from the literature.29,30 Based on these quantities and the resulting structures constructed by HTSO of Al, the metal ion elements could be categorized into three different groups. The group A metal ions (Na+, Ca2+, Sr2, Ba2+, and K+) with the large atomic sizes (>1 Å) and the low
Figure 4. Nitrogen adsorption−desorption isotherm on γ-Al2O3@Al and Spinel−MeAl2O4@Al (Me = Zn, Ni, Co, Mn, and Mg) core−shell microstructures.
adsorption−desorption isotherms of the samples, indicating the development of highly porous shell structures. The N 2 isotherms on these samples showed a forced closure of the hysteresis loop at p/po of ∼0.38, suggesting that a minor fraction of the pore volume interconnected to the outer space through the opening size smaller than ∼3.5−4 nm existed on the primary core−shell particles.27,28 To avoid artifacts incurring from the nonequilibrium desorption process, the BJH pore size distributions on these samples were obtained using the adsorption branch of the N2 isotherm. Table 1 summarizes the Al core content, BET surface area, average pore E
DOI: 10.1021/acs.chemmater.6b00582 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials
Figure 5. Effects of heterometal ion properties and the mechanism of hydrothermal self-construction of the core−shell metal−ceramic microarchitectures.
Table 2. Physicochemical and Catalytic Properties of Rh Catalysts (1.0 wt % Rh Loading) Supported on Magnesium Aluminates of Various Al/Mg Ratios, MgO, γ-Al2O3, and MgAl2O4@Al Core−Shell Microstructurea catalyst properties
catalyst Rh/MgxAlyOz
Rh/MgO Rh/Al2O3 Rh/MgAl2O4@Al a
selectivity (%)
Al/Mg atomic ratio
CO uptake (μmol g−1)
metal dispersion (%)
metal diameter (nm)
conversion (%)
conversion TOF (s−1)
H2
CO
CO2
CH4
C2H4
C2H6
2 3 5 10
49 49 41 46 11 11 57
51.0 50.9 42.2 47.6 11.5 11.8 59.0
2.2 2.2 2.6 2.3 9.6 9.3 1.9
74 77 74 53 12 12 85
0.71 0.68 0.78 0.50 0.47 0.46 0.70
59.3 59.4 59.4 59.7 51.5 54.3 58.3
20.1 14.7 18.0 20.4 31.9 33.6 20.4
17.1 20.8 17.8 16.6 12.5 8.0 16.9
2.9 4.8 4.4 2.7 1.5 1.8 3.8
0.5 0.3 0.3 0.5 2.4 2.1 0.3
0.2 0.1 0.1 0.1 0.2 0.2 0.2
2
The GSR was conducted at 823 K (H2O/C3H8O3 = 4.5, F/W = 870 mmol g cat−1 h−1).
of these MeAl2O4@Al structures were 2−3 μm (HTSO at 423−473 K for 3 h) and exhibited nearly no further growth even after extended hydrothermal reaction times over 24 h. This agree with our previous studies that the growth of metal oxide shell on Al substrate is limited by the diffusion of the oxidants through the dense shell layer; therefore, the shell growth rate significantly decreased with a gradual increase in the shell thickness.7 The effects and performance of the core−shell metal− ceramic composites were investigated using Rh catalysts supported on spinel-MgAl2O4@Al for glycerol steam reforming to hydrogen (GSR, C3H8O3 + 3H2O ⇄ 3CO2 + 7H2, ΔH0298 = 128 kJ mol−1), which is endothermic favoring high reaction temperature and facile heat flux. The results were compared with those of the Rh catalysts supported on conventional MgAl2O4 support at the same reaction condition. The effects of support composition for glycerol reforming reaction were investigated with Rh (1.0 wt %) catalysts supported on magnesium aluminates of various Al/Mg atomic ratios (Al/ Mg = 2−10) prepared by the coprecipitation method. Table 2 displays the physicochemical properties (CO uptake, dispersion, and size of Rh particles) and performance of the Rh catalysts for GSR at 823 K (F/W = 870 mmol gcat−1 h−1, H2O/
stability constant (log K < 2) did not form Me−Al bimetal oxide shell nor homogeneously precipitated heterometal oxide (MeOx) species, likely because of their limited inner-diffusion through the shell layer and unfavorable properties for the formation of metal hydroxide complexes. Consequently, only the Al2O3@Al core−shell structure was hydrothermally constructed, whereas the heterometal ions mostly remained in the solution being unreacted. In contrast, the group B metal ions (Fe2+, Cu2+, and Al3+) with the high stability constants for metal hydroxide complexes easily precipitated into their hydroxides via homogeneous hydrolysis and condensation reactions in the solution, being unable to form the bimetal hydroxides on the Al surface; therefore, the Al2O3@Al structure was obtained with the abundant concurrent formation of Fe2O3 and CuO precipitates even at relatively low HTSO temperatures (
