Bimetallic Rh-Co/ZrO2 catalysts for ethanol steam reforming into hydrogen-containing gas

Article abstract of DOI:10.1134/S0023158410060157

The properties of supported bimetallic Rh-Co/ZrO₂ catalysts in ethanol steam reforming into hydrogen-containing gas were studied. The particles of Rh-Co solid solutions on the catalyst surface were prepared by the thermal decomposition of the double complex salt [Co(NH₃)₆] [Rh(NO₂)₆] and the solid solution Na ₃[RhCo(NO₂)₆]. It was found that the bimetallic Rh-Co/ZrO₂ catalysts exhibited high activity in the reaction of ethanol steam reforming. The equilibrium composition of reaction products was attained at 500-700°C and a reaction mixture space velocity of 10000 h ^-1. Pleiades Publishing, Ltd., 2010.

Full text of DOI:10.1134/S0023158410060157

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ISSN 0023ꢀ1584, Kinetics and Catalysis, 2010, Vol. 51, No. 6, pp. 893–897. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © E.M. Churakova, S.D. Badmaev, P.V. Snytnikov, A.I. Gubanov, E.Yu. Filatov, P.E. Plyusnin, V.D. Belyaev, S.V. Korenev, V.A. Sobyanin, 2010, published
in Kinetika i Kataliz, 2010, Vol. 51, No. 6, pp. 923–928.
YOUNG
SCIENTISTS’ SCHOOL
Bimetallic Rh–Co/ZrO2 Catalysts for Ethanol Steam
Reforming into HydrogenꢀContaining Gas
,
b
,
b
,
b
,
c
,c
E. M. Churakovaa , S. D. Badmaeva , P. V. Snytnikova , A. I. Gubanovb , E. Yu. Filatovb ,
,
c
,
b
,
c
,b
P. E. Plyusninb , V. D. Belyaeva , S. V. Korenevb , and V. A. Sobyanina
aBoreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia
bNovosibirsk State University, Novosibirsk, 630090 Russia
cNikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia
eꢀmail: sukhe@catalysis.ru
Received December 15, 2009
Abstract—The properties of supported bimetallic Rh–Co/ZrO2 catalysts in ethanol steam reforming into
hydrogenꢀcontaining gas were studied. The particles of Rh–Co solid solutions on the catalyst surface were
prepared by the thermal decomposition of the double complex salt [Co(NH3)6][Rh(NO2)6] and the solid
solution Na3[RhCo(NO2)6]. It was found that the bimetallic Rh–Co/ZrO2 catalysts exhibited high activity
in the reaction of ethanol steam reforming. The equilibrium composition of reaction products was attained
at 500–700°C and a reaction mixture space velocity of 10000 h–1.
DOI: 10.1134/S0023158410060157
INTRODUCTION
It has been shown [3, 7, 14] that a side reaction of ethꢀ
anol dehydration can occur on Al2O3; this reaction
leads to ethylene formation and, finally, catalyst carꢀ
bonization. Recently, ZrO2 and CeO2–ZrO2 have
been frequently used as supports. It was found [4, 9,
12, 13, 20] that these supports are characterized by
high thermal stability, and they provide high dispersity
of an active component to prevent its agglomeration.
Recently, attention has been focused on the develꢀ
opment of power plants based on various types of fuel
cells. This is due to the fact that fuel cells have a numꢀ
ber of advantages over traditional electric energy
sources. The most important of these advantages are a
high efficiency of the conversion of chemical fuel
energy into electrical energy, silence in operation,
modular construction, and high environmental charꢀ
acteristics.
Hydrogen or hydrogenꢀcontaining gas is a fuel for
fuel cells. However, the use of hydrogen for feeding
fuel cells is currently complicated because of the
absence of wellꢀdeveloped infrastructure and technolꢀ
ogies for its safe storage and transportation. To overꢀ
come these difficulties, it was proposed to use a fuel
processor—a device for the onꢀsite generation of
hydrogenꢀcontaining gas from various raw materials
(natural gas, gasoline, diesel fuel, bioethanol, methaꢀ
nol, dimethyl ether, etc.) using catalytic methods. Of
these raw materials, bioethanol is a renewable fuel. It
can be obtained by the biochemical conversion of varꢀ
ious agricultural commodities or wood and food
wastes. The resulting bioethanol is an aqueous soluꢀ
tion containing to 12–14 vol % ethanol.
Of the test systems, Coꢀ and Rhꢀcontaining cataꢀ
lysts are considered most promising [18]. Rhodium
catalysts are stable and highly active [7, 8, 14, 15, 18,
19]. It was found that the activity of Rh catalysts is due
to the ability of Rh to cleave the C–C bond in the ethꢀ
anol molecule [11, 14]. Cobaltꢀcontaining catalysts
are active in ethanol steam reforming; however, they
undergo carbonization, and their productivity is lower
than that of rhodium catalysts [3, 10, 15, 18, 21]. In
addition, it was noted [2, 4] that Coꢀcontaining cataꢀ
lysts are prone to the formation of byꢀproducts such as
acetaldehyde.
To increase catalyst activity and to decrease the
concentration of expensive noble metals in catalysts
for ethanol steam reforming remain a problem of conꢀ
siderable current interest. This problem can be solved
by using more complex bimetallic catalysts. Thus,
Steam reforming is the most efficient process for Pereira et al. [21] studied the reaction of ethanol steam
producing hydrogenꢀcontaining gas from ethanol; it reforming on Co/SiO2, Rh/SiO2, СоRh/SiO2,
results in a maximum yield of hydrogen. A great numꢀ Ru/SiO2, and CoRu/SiO2 catalysts with the addition
ber of supported catalysts based on both common of oxygen to the reaction mixture. They found that the
metals (Cu, Co, and Ni) and noble metals (Pd, Pt, Ru, CoRu/SiO2 and СоRh/SiO2 catalysts were more active
Rh, and Ir) have been proposed to perform this reacꢀ in this process than the monometallic catalysts. In the
tion [1–19]. The nature of the support has a considerꢀ course of reaction, all of the catalysts were carbonized
able effect on the reaction of ethanol steam reforming. and their activity decreased. The presence of oxygen in
893

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894
CHURAKOVA et al.
Characteristics of the support and bimetallic Rh–Co catalysts
Metal content, wt %
were held in a flow of a mixture of 5 vol % H2
+
95 vol % He at 600°С for 2 h. Henceforth, the catalyst
thus prepared is referred to as Rh–Co(SS)/ZrO2.
The second method consisted in the use of aqueous
solutions of Na3[Rh(NO2)6] and [Co(NH3)6]Cl3 comꢀ
plex salts. As a result of mixing these salts, the double
complex salt [Co(NH3)6][Rh(NO2)6] was formed, the
thermal decomposition of which led to the formation
of RhCo solid solution particles. The resulting samples
Sample
S
BET, m2/g
Co
Rh
ZrO2
Rh–Co(DCS)/ZrO2
Rh–Co(SS)/ZrO2
35
23
23
–
0.59
0.59
–
1.01
1.05
were held in a flow of a mixture of 5 vol % H2
+
the reaction mixture resulted in the oxidation of Co
particles to CoO; this is undesirable because CoO is
active in the reaction of ethanol dehydrogenation to
acetaldehyde. In the CoRh and CoRu catalysts, the
platinum group metals prevented the oxidation of
cobalt by the formation of bimetallic Co–Rh or Co–
Ru particles. This makes it possible to regenerate a
carbonized catalyst by oxidative treatment with the
retention of its initial catalytic properties.
In this work, we studied the reaction of ethanol
steam reforming into hydrogenꢀcontaining gas on
bimetallic Rh–Co/ZrO2 catalysts. The main goal was
to develop the targetꢀoriented synthesis of bimetallic
Rh–Co particles on the support surface. Double comꢀ
plex salts (compounds containing a complex cation
and a complex anion, where different metals (Rh and
Co) are central atoms) and isostructural Rh and Co
salts that form solid solutions were used as the precurꢀ
sors of catalyst active components. The advantage of
the above compounds is that the stoichiometry of a
complex precursor is responsible for the composition
of the resulting polymetallic phase; this allowed us to
expect the formation of bimetallic particles with a
specified composition in the catalyst.
95 vol % He at 600°С for 2 h. Henceforth, the catalyst
thus prepared is referred to as Rh–Co(DCS)/ZrO2.
Using these methods, we obtained catalyst samples
with the atomic ratio Rh : Co = 1 and a total metal
content of ~1.6 wt % (see the table).
The catalytic properties of the synthesized bimetalꢀ
lic Rh–Co/ZrO2 systems in the reaction of ethanol
steam reforming were compared with the properties of
preciously studied monometallic Rh/ZrO2 [19] and
Co/ZrO2 catalysts [22].
Catalytic Experiments
Ethanol steam reforming was studied in a quartz
flow reactor at atmospheric pressure over the temperꢀ
ature range of 300–700°С. The reactor was a
Uꢀshaped tube with an inner diameter of 4 mm. The
experiments were performed using a reaction mixture
containing 16 vol % С2Н5ОН, 64 vol % Н2О, and
20 vol % N2 at a space velocity of 10000 h–1. Nitrogen
was used as an internal standard in chromatographic
analysis.
The concentrations of reactants and reaction prodꢀ
ucts were determined on two Tsvetꢀ500 chromatoꢀ
graphs equipped with thermalꢀconductivity detectors.
Ethanol, water, acetaldehyde, ethylene, and carbon
dioxide were separated at 125°С on a column with
Porapak T; hydrogen, nitrogen, carbon monoxide, and
methane were separated at room temperature on a colꢀ
umn with NaX molecular sieves. Argon was used as a
carrier gas.
EXPERIMENTAL
Catalyst Preparation
The ZrO2 support was prepared by precipitation
from an aqueous solution of zirconyl nitrate at a conꢀ
stant value of pH 10. A 5% aqueous solution of ammoꢀ
nia was used as a precipitating agent. The resulting
precipitate was filtered off and washed on filter with
water and then with ethanol. The resulting mass was
dried in air for two days; thereafter, it was calcined at
500°C in air for 4 h.
Before the experiments, the catalysts were reduced
in a flow of a mixture of 5 vol % H2 + 95 vol % N2 at
400°С and a flow rate of 90 ml/min for 2 h.
Investigation Techniques
The bimetallic rhodium–cobalt catalysts were preꢀ
pared by impregnating the ZrO2 support with aqueous
solutions of complex compounds containing Rh and Co.
For the Xꢀray diffraction (XRD) analysis of support
and catalyst samples, diffraction patterns were
The first method consisted in the use of aqueous obtained on an HZGꢀ4C apparatus (Cu
K radiation;
α
solutions of isostructural Na3[Rh(NO2)6] and graphite monochromator in a reflected beam). The
Na3[Со(NO2)6] salts. The Na3[RhxCo1 – x(NO2)6] diffraction patterns were measured by pointꢀbyꢀpoint
solid solution can be obtained upon mixing depending scanning over the angle range of
2
θ
= 10°–90° at a
on the initial ratio between the components. The therꢀ step of 0.05 and a 10ꢀs accumulation time at a point.
°
mal decomposition of this solid solution results in the The qualitative phase composition of the samples was
formation of RhCo solid solution particles. In particꢀ determined using the JCPDS data cards [23]. The
ular, we used an equimolar ratio between the Rh and sizes of coherentꢀscattering regions (CSRs) were calꢀ
Co salts in this study to obtain the culated from the broadening of diffraction peaks due
Na3[Rh0.5Co0.5(NO2)6] complex. The resulting samples to the detected phases.
KINETICS AND CATALYSIS Vol. 51
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2010

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BIMETALLIC Rh–Co/ZrO2 CATALYSTS FOR ETHANOL STEAM REFORMING
895
The XRD analysis of [Rh(NO2)6][Co(NH3)6] and
Na3[Rh0.5Co0.5(NO2)6] polycrystals and their therꢀ
molysis products (after treatment with a mixture of
5 vol % H2 + 95 vol % He at 400°С for 2 h) was perꢀ
formed on DRONꢀ3M and DRONꢀRM4 instruments
(а)
ZrO2
(R = 192 mm; CuK radiation; Ni filter; scintillation
α
detector with amplitude discrimination) over the angle
range of = 5°–90°. The samples were prepared as a
2
θ
Rh–Co
thin layer on a polished side of a standard quartz cell.
An analogously prepared Si sample was used as an
external standard.
Rh–Co
The highꢀresolution transmission electron microsꢀ
copy (HR TEM) images were obtained on a JEMꢀ2010
electron microscope (with a grid resolution of 0.14 nm
at an accelerating voltage of 200 kV). Before measureꢀ
ments, the catalyst samples were supported onto copꢀ
per gauze coated with a perforated carbon film using a
UZDNꢀ144.2 ultrasonic disperser. The highꢀresoluꢀ
tion images of periodic structures were analyzed by the
Fourier method. The energy dispersive Xꢀray
microanalysis (EDX) of samples was performed using
characteristic energy dispersive Xꢀray spectroscopy on
an EDAX spectrometer (EDAX) equipped with a
Si(Li) detector with an energy resolution of 130 eV.
ZrO2
100 nm
ZrO2
Intensity, arb. units
Intensity, arb. units
Cok
290
261
290
(b)
(c)
261
232
203
174
145
RhL
232
203
174
Cok
145
ZrL
116
87
58
29
0
116
RhL
87
58
ZrL
29
The specific surface area (SBET) was determined on
a TriStar 3000 instrument using the full isotherms of
lowꢀtemperature nitrogen adsorption (–196°С).
0
0
1.6 3.2 4.8 6.4 8.0
keV
0
1.6 3.2 4.8 6.4 8.0
keV
E,
E,
The concentrations of metals in the catalysts were
determined by Xꢀray fluorescence analysis on a
VRAꢀ30 instrument with a Cr anode in the Xꢀray tube
using an external standard technique.
Fig. 1. (a) TEM micrograph of the Rh–Co(DCS)/ZrO
catalyst and (b, c) the EDX spectra of marked regions.
2
tively. The XRD analysis of the thermal decomposition
products of these complex compounds revealed the
formation of a mixture of Rh0.65Co0.35 and Rh0.55Co0.45
solid solutions with CSR sizes of ~90 Å. These results
suggest that Rh–Co solid solutions can also be formed
on the support surface in the course of the preparation
of bimetallic Rh–Со/ZrO2 catalysts.
RESULTS AND DISCUSSION
Physicochemical Properties
An analysis of the diffraction pattern of the syntheꢀ
sized ZrO2 demonstrated that the support consisted of
two zirconium oxide phases: monoclinic mꢀZrO2
(90 wt %) and tetragonal tꢀZrO2 (10 wt %) structures
with CSR sizes of 150 and 100 Å, respectively. The unit
cell parameters of the zirconia phases were consistent
with standard values [23].
Unfortunately, the XRD analysis of the Rh–
Co/ZrO2 catalysts did not allow us to identify the forꢀ
mation of bimetallic phases. This was due to the fact
that the main reflections of Rh and Co coincided with
the reflections of the ZrO2 support and the integrated
intensity of metal peaks was small because of low metal
contents of the catalyst.
To study in more detail the morphology and strucꢀ
ture of the Rh–Co/ZrO2 catalysts, we used HR TEM
and EDX. Particles of sizes from 20 to 100 nm as larger
agglomerates were observed in a micrograph of the
Rh–Co(DCS)/ZrO2 catalyst (Fig. 1a). As an examꢀ
ple, Figs. 1b and 1c show the EDX spectra that charꢀ
acterize the elemental compositions of marked regions
in the micrographs. The spectra contained lines due to
Zr, Rh, and Co; in this case, the ratio between the
integral intensities of Rh and Co corresponding to the
atomic ratio between the elements was 1.
At the same time, the XRD analysis of the samples
Interplanar distances for ZrO2 support particles
obtained after drying an equimolar mixture of the and bimetallic Rh–Co particles were determined
aqueous solutions of Na3[Rh(NO2)6] and using the Fourier transform. The observed interplanar
Na3[Со(NO2)6] complex salts, as well as of the precipꢀ distances of 3.61 and 3.05 Å correspond to distances
itate obtained upon mixing the aqueous solutions of between the (011) and (111) planes of the monoclinic
Na3[Rh(NO2)6] and [Co(NH3)6]Cl3 complex salts, phase of ZrO2 (PCPDF ICSD no. 371484). The
demonstrated the formation of singleꢀphase comꢀ observed interplanar distances of 2.74 and 3.38 Å corꢀ
pounds: a Na3[Rh0.5Co0.5(NO2)6] solid solution and a respond to the tetragonal phase of ZrO2 (PCPDF
[Co(NH3)6][Rh(NO2)6] double complex salt, respecꢀ ICSD no. 170923). A reflection corresponding to the
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896
Conversion, vol %
CHURAKOVA et al.
in the ethanol molecule, as compared with monomeꢀ
tallic cobalt systems.
100
1
Catalytic Activity
80
2
Figure 2 compares the conversion of ethanol on the
Rh–Co(DCS)/ZrO2 and Rh–Co(SS)/ZrO2 catalysts
and the ZrO2 support. It can be seen that the bimetallic
catalysts exhibited high activity. The conversion of
ethanol into hydrogenꢀcontaining gas on these cataꢀ
lysts was as high as 100% even at 450°С, and it then
remained unchanged.
60
3
40
On the ZrO2 support, 100% ethanol conversion was
reached at 600°С; in this case, the main products were
acetaldehyde and ethylene formed in the following
reactions:
20
0
С2Н5ОН
С2Н5ОН
С2Н4 + Н2О;
300
400
500
600
700
СН3СОН + Н2.
Temperature, °C
The temperature dependences of the concentraꢀ
tions of the reaction products of ethanol steam
reforming on the Rh–Co(DCS)/ZrO2 and Rh–
Co(SS)/ZrO2 catalysts were found identical. In the
temperature range of 300–650°С, the main products
were H2, CO, CO2, and CH4. Acetaldehyde and ethylꢀ
ene occurred in trace amounts (<0.1 vol %) only at
Fig. 2. Dependence of the conversion of ethanol on temꢀ
perature on the ( ) Rh–Co(SS)/ZrO and ( ) Rh–
Co(DCS)/ZrO catalysts and (
1
2
2
3
) ZrO .
2
2
interplanar distance
d = 2.11 Å was observed in the
Fourier diffraction pattern of a metal particle; this disꢀ
300–400°С
.
tance is inconsistent with both Co (Fm3m d111
2.0467 Å, PCPDF ICSD no. 150806; P63/mmc d100
2.17026 Å, PCPDF ICSD no. 52935) and Rh (d111
2.1960 Å, PCPDF ICSD no. 050685). A comparison
with EDX data in the same particle region allowed us
to conclude that this interplanar distance corresponds
to the interplanar distance of a solid solution with the
=
=
=
As an example, Fig. 3 shows the dependence of the
concentrations of H2, CO, CO2, and CH4 on temperꢀ
ature in the course of reaction on Rh–
Co(DCS)/ZrO2. Figure 3 also shows the equilibrium
concentrations of ethanol steam reforming products,
which were calculated based on the assumption that
the following reactions occurred in the system:
ratio Rh : Co = 1 : 1 (Fm3m
ICSD no. 102635).
d111 = 2.1188 Å, PCPDF
С2Н5ОН = СО + Н2 + СН4;
CO + Н2О = СО2 + Н2;
СН4 + Н2О = СО + 3Н2.
The results of the analysis allowed us to state that
dark spots of size 20–50 nm, which are observed in
Fig. 1a, are bimetallic Rh–Co solid solutions with the
ratio Rh : Co = 1 : 1. Analogous data were also
obtained for the Rh–Co(SS)/ ZrO2 sample.
It can be seen that the concentrations of all reacꢀ
tion products are close to equilibrium values in the
temperature range of 500–650°С. At low temperaꢀ
tures (300–400°С), СО2 was almost not observed in
Thus, the use of the double complex salt
[Co(NH3)6][Rh(NO2)6] and the solid solution
Na3[Rh0.5Co0.5(NO2)6] as precursor compounds
resulted in the formation of bimetallic Rh–Co partiꢀ
cles in the catalyst. As found in the studies of the cataꢀ
lysts after the reaction of ethanol steam reforming, no
detectable changes occurred in the particles of Rh–Co
solid solutions (particle size or composition) under
reaction conditions. The formation of carbon on the surꢀ
face of the bimetallic catalyst did not exceed 0.5 wt %, as
determined by the combustion of the samples after
catalytic experiments; at the same time, the amount of
carbon on the monometallic cobalt catalyst was about
10 wt % [22]. The lower carbonization of the Rh–
Co/ZrO2 catalyst was due to the formation of bimetalꢀ
lic Rh–Co particles, which resulted in changes in the
catalytic properties. It is likely that these bimetallic
the reaction products; the main products were H2
,
CO, and CH4 in the molar ratio H2 : CO : CH4 = 2 : 1 : 1.
As the temperature was increased to 650°С, the conꢀ
centration of methane decreased to ~1 vol %; in this
case, the concentrations of Н2 and СО2 increased to
~45 and 13 vol %, respectively.
Thus, to obtain a maximum yield of hydrogen in
the absence of methane from the reaction products,
the reaction of ethanol steam reforming on the Rh–
Co/ZrO2 catalysts should be performed in the temperꢀ
ature range of 600–700 С. In this case, the concentraꢀ
°
tion of CO in the mixture is at a level of 10 vol %. This
mixture is suitable for feeding highꢀtemperature fuel
cells [18, 24].
The Rh–Co/ZrO2 samples are similar to rhodium
particles enhanced the ability to cleave the C–C bond catalysts in catalytic characteristics [19]; they contain
KINETICS AND CATALYSIS Vol. 51
No. 6
2010