Bimetallic Pt-Ni composites on ceria-doped alumina supports as catalysts in the aqueous-phase reforming of glycerol

Article abstract of DOI:10.1039/c4ra00355a

Although Pt is the most appropriate catalyst for aqueous phase reforming (APR) of glycerol to generate H₂, it is expensive. We studied its possible minimisation to levels where acceptable H₂ yields are still maintained. When an additional catalytic metal, Ni, was introduced to our Pt/CeO₂-Al₂O₃ catalyst, the Pt content could be reduced from 3 to 1 wt%, with a slight increase in H₂ production. In this study, Pt and Ni in various ratios were supported on alumina doped with 3 wt% ceria, and the resulting materials were characterised and tested as catalysts for the APR of glycerol. Amongst the catalysts tested, bimetallic 1Pt-6Ni/3CeAl (containing 1 wt% Pt and 6 wt% Ni) gave the highest H₂ yield (86%) and gas-phase C yield (94%). Thus, although 1Pt-6Ni/3CeAl and our reported 3Pt/3CeAl catalyst produced almost same amount of H₂ (1.8 and 1.9 mmol, respectively) per gram of catalyst per hour, the latter produced three times as much H₂ per gram of Pt per hour (195 mmol); this measure is crucial to the competitiveness of a catalyst in large-scale H ₂ production. X-ray diffraction (XRD) patterns and thermogravimetric analyses of the spent catalysts showed no serious catalyst deactivation by carbon deposition after 30 h on stream, except in the case of Pt-free 6Ni/3CeAl, which ceased to produce H₂ after 15 h on stream. XRD and X-ray photoelectron spectroscopic analyses demonstrated that adding Ni impacted both the crystallite and electronic structure of Pt. These effects likely conspired to produce the high glycerol conversion and gas phase C yield and, ultimately, the high H₂ yield observed over 1Pt-6Ni/3CeAl.

Full text of DOI:10.1039/c4ra00355a

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PAPER
Bimetallic Pt–Ni composites on ceria-doped
alumina supports as catalysts in the aqueous-phase
Cite this: RSC Adv., 2014, 4, 18951
reforming of glycerol†
*
M. M. Rahman, Tamara L. Church, Meherzad F. Variava, Andrew T. Harris
*
and Andrew I. Minett
Although Pt is the most appropriate catalyst for aqueous phase reforming (APR) of glycerol to generate H2, it
is expensive. We studied its possible minimisation to levels where acceptable H2 yields are still maintained.
When an additional catalytic metal, Ni, was introduced to our Pt/CeO2–Al2O3 catalyst, the Pt content could
be reduced from 3 to 1 wt%, with a slight increase in H2 production. In this study, Pt and Ni in various ratios
were supported on alumina doped with 3 wt% ceria, and the resulting materials were characterised and
tested as catalysts for the APR of glycerol. Amongst the catalysts tested, bimetallic 1Pt–6Ni/3CeAl
(containing 1 wt% Pt and 6 wt% Ni) gave the highest H2 yield (86%) and gas-phase C yield (94%). Thus,
although 1Pt–6Ni/3CeAl and our reported 3Pt/3CeAl catalyst produced almost same amount of H2 (1.8
and 1.9 mmol, respectively) per gram of catalyst per hour, the latter produced three times as much H2
per gram of Pt per hour (195 mmol); this measure is crucial to the competitiveness of a catalyst in large-
scale H2 production. X-ray diffraction (XRD) patterns and thermogravimetric analyses of the spent
catalysts showed no serious catalyst deactivation by carbon deposition after 30 h on stream, except in
the case of Pt-free 6Ni/3CeAl, which ceased to produce H2 after 15 h on stream. XRD and X-ray
photoelectron spectroscopic analyses demonstrated that adding Ni impacted both the crystallite and
electronic structure of Pt. These effects likely conspired to produce the high glycerol conversion and gas
phase C yield and, ultimately, the high H2 yield observed over 1Pt–6Ni/3CeAl.
Received 14th January 2014
Accepted 10th April 2014
DOI: 10.1039/c4ra00355a
www.rsc.org/advances
CO2 via the water–gas shi (WGS) reaction (eqn (2)). The overall
APR of a mole of glycerol (eqn (3)) produces a maximum of
Introduction
seven moles of hydrogen (four from the reforming reaction and
three from the WGS reaction). Thus, a good APR catalyst must
catalyse both C–C bond cleavage and the WGS reaction9 without
promoting competing reactions such as C–O cleavage or
methanation (eqn (4)), which can greatly deteriorate the yield of
H2. Group VIII metals, particularly Pt, Pd and Ni, are especially
effective.10
Although hydrogen (H2) has the potential to become an envi-
ronmentally friendly energy carrier because of its high energy
density and lack of carbonaceous combustion products, its use
is currently problematic in that 95% of H2 is produced from
fossil fuels.1 Therefore, considerable research has focused on
producing H2 from renewable sources.2 Dumesic and co-
workers pioneered the catalytic aqueous phase reforming (APR)
of polyols (which can be derived from biomass) under relatively
ꢀ
C3H8O3 # 3CO + 4H2
3CO + 3H2O # 3CO2 + 3H2
C3H8O3 + 3H2O # 3CO2 + 7H2
4H2 + CO2 # CH4 + 2H2O
(1)
(2)
(3)
(4)
mild reaction conditions (200–250 C, 20–50 bar) to produce a
hydrogen-rich gas3,4 that contains less CO (<300 ppm) than the
product stream from conventional steam reforming does.5
Studies examining kinetics6 and catalyst design3,7,8 have
demonstrated that APR (decomposition into CO and H2 is
shown in eqn (1) for the case of glycerol) involves the cleavage of
C–C and C–H bonds to form metal-bound surface species,
especially CO, which can then react with H2O to form H2 and
We recently reported that Pt supported on alumina doped
with 3 wt% ceria gave signicantly higher H2 yield and selec-
tivity from the APR of glycerol than Pt on alumina.11 The
improved performance of these catalysts was attributed to their
higher coking resistance and oxygen storage capacity, as well as
to enhanced catalysis of the WGS reaction and lower metha-
nation activity under APR conditions. Although Pt catalysts are
School of Chemical and Bio-molecular Engineering, University of Sydney, Sydney,
NSW, 2006, Australia. E-mail: mrah7493@uni.sydney.edu.au; Andrew.minett@
sydney.edu.au
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra00355a
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highly active for APR,9 the high cost of Pt makes catalysts based
on non-precious metals desirable. Ni has shown initial APR
activity comparable to that of Pt, but was subject to signicant
deactivation.10 Thus, efforts have been made to improve the
catalytic activities of Ni catalysts by impregnating them with
other metallic elements.12
Results and discussion
Structural characteristics of synthesised catalysts
The textural properties of the catalysts and supports were
evaluated from nitrogen adsorption–desorption isotherms at
ꢁ196 ꢀC, and the results are shown in Fig. S1† and summarised
in Table 1. The support, composed of 3 wt% CeO2 in Al2O3, had
The activity of APR catalysts, as well as of other supported-
metal catalysts, can be enhanced by adding an additional metal.
It has been suggested that adding noble metal promoters to a Ni
catalyst for dry methane reforming may reduce coke deposition
and therefore provide stability.13 Relevant to C–C bond cleavage,
adding Pd to a Ni/SiO2 catalyst increased the amount of gas
produced from cellulose pyrolysis; this was attributed to greater
tar-cracking activity.14 The Pt–Ni system in particular has been
extensively studied in a range of applications because of its
synergetic catalytic effect.15 Kunkes et al. reported the conver-
sion of glycerol by APR over carbon-supported Pt (5 wt%) and
Pt–Re catalysts. The addition of Re led to an increase in the
production of H2, CO, CO2, and light alkanes (primarily
methane) and, ultimately, to better hydrogen selectivity.16 Wang
et al. showed that adding Co to a Pt-based (8 wt%) APR catalyst
signicantly increased its activity without impacting the selec-
tivity for H2.17 Manfro et al.18 added Cu to a Ni catalyst and
obtained decreased CH4 formation, which increased H2 selec-
tivity. Ko et al.19 showed that, under the same pretreatment
conditions, Pt–Ni bimetallic catalysts had more active sites than
monometallic Pt or Ni catalysts. Tupy et al.20 found that, aer
24 h on-stream in the APR of ethylene glycol, a supported Pt–Ni
(2.7 wt%) catalyst was more active than a Pt catalyst with the
same Pt content because Ni segregation occurred, producing a
Ni-enriched surface. Huber et al.5 suggested that the activity of
Pt-based (3 wt%) catalysts for APR could be increased by alloy-
ing Pt with Ni or Co, which would decrease the strength with
which CO and H2 interact with the surface, thereby increasing
the fraction of catalytic sites available to react with ethylene
glycol. Therefore, we investigated the addition of Ni to our Pt
catalysts supported on 3 wt%-ceria-doped alumina. The ratio of
Pt to Ni on the support was optimised and the catalyst charac-
terised to better understand the system.
SBET ¼ 162 m2 gꢁ1
.
11 Adding 6 wt% Ni lowered the surface area
to 125 m2 gꢁ1, whereas adding Pt (1 or 3 wt%) caused a smaller
loss of surface area, to ꢂ150 m2 gꢁ1
. As Ni was added to 1Pt/
11
3CeAl, SBET and Vp gradually decreased. Dp decreased signi-
cantly when 12 or 18 wt% Ni was present.
The XRD patterns of the catalysts aer calcination and
reduction are shown in Fig. 1 and 2, respectively. As expected,
the characteristic peaks of NiO in the patterns of the calcined
catalysts intensied and became sharper as the Ni content
increased from 3 to 18 wt%, indicating that both the relative
amount of crystalline NiO and its crystallite size increased with
increasing Ni content. At higher Ni loading ($12 wt%), three
clear diffraction lines of the NiO uorite structure were
observed at 2q ¼ 37.2, 43.3, and 62.9ꢀ, representing the (111),
(200), and (220) planes, respectively;25,26 the last one in partic-
ular was difficult to discern at lower Ni loadings. Two clear
diffraction peaks representing the (111) and (200) planes,
respectively, of metallic platinum27 were observed at 2q ¼ 39.9
and 45.9ꢀ. Neither PtO (JCPDS 43-1100) nor PtO2 (JCPDS 23-
1306) were present in signicant amounts. Three broad peaks at
2q ¼ 38, 46, and 67ꢀ in the XRD pattern indicated the presence
of g-Al2O3,28 and those at 2q ¼ 29, 33, and 56ꢀ represented the
uorite-structured CeO2.29
Fig. 2 shows the XRD patterns of the catalysts following
reduction at 800 ꢀC. These demonstrated that NiO was
completely reduced to Ni0, with diffraction peaks at 2q ¼ 44.5
and 51.8ꢀ corresponding to the (111) and (200) planes, respec-
tively.25 The peak intensities, and thus the amount of detectable
crystalline Ni0, increased with increasing Ni content. The Ni0
crystallite sizes for 1Pt–12Ni/3CeAl and 1Pt–18Ni/3CeAl were
calculated based upon the peak at 51.8ꢀ, and were larger on the
latter sample (see Table 1). The Ni0 peaks in the XRD patterns of
Table 1 Textural properties of catalystsa
Particle size
SBETb (m2 gꢁ1
)
Vp (cm3 gꢁ1
c
)
Dpd (nm)
Nie (nm)
Pte (nm)
Mdisp (%)
f
Supports/catalysts
3CeO2–Al2O3 (3CeAl)
1Pt/3CeAl
6Ni/3CeAl
1Pt–3Ni/3CeAl
1Pt–6Ni/3CeAl
1Pt–12Ni/3CeAl
1Pt–18Ni/3CeAl
162
149
125
139
120
116
109
0.28
0.25
0.22
0.23
0.20
0.19
0.18
4.9
4.8
4.9
4.9
4.9
4.3
4.3
NAg
NAg
NAg
11
NAg
10
h
—
NAg
8.8
4.6
8.1
6.7
NAg
13
h
—
h
—
25
14
17
12
21
a
Measured by N2 adsorption–desorption at ꢁ196 ꢀC. Prior to measurement, samples were calcined in air at 600 ꢀC for 6 h. b Specic surface area
(SBET) was determined from the linear portion of the isotherm (P/P0 ¼ 0.05–0.35).21 Pore volume (Vp) was calculated at P/P0 ¼ 0.995. d Predominant
c
e
pore size (Dp volume basis) was calculated from the adsorption isotherm using the Barrett–Joyner–Halenda (BJH) formula.22 Calculated by
applying the Scherrer equation23 to the XRD peak generated from the (200) plane of Ni or the (111) plane of Pt in the reduced catalysts (Fig. 2).
f
g
h
Mdisp ¼ metal dispersion of Pt and Pt–Ni, calculated according to eqn (5).24 Not applicable. Peak was too small and broad to be measured
reliably.
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representing the Pt (111) reection occurred at higher 2q in all
Pt–Ni samples than in monometallic 1Pt/3CeAl, as shown in
Fig. 2 (inset). This type of peak shi, which has also been
observed by Tegou et al. for Pt–Ni particles (Pt/Ni ꢂ 4)30 and by
Fu et al.31 for Ni@Pt core–shell nanoparticles at high Ni/Pt
ratios, can indicate the formation of a solid solution, i.e., an
alloy. Two thermodynamic alloys of these metals, NiPt and
Ni3Pt, are known, and can be produced in ordered form at 645
and ꢂ580 ꢀC, respectively;32 thus both could have formed under
the catalyst-reduction conditions used here. Diffraction peaks
at 2q ¼ 41.1 or 41.6ꢀ have been assigned to the (111) reection of
NiPt;33,34 whereas a peak at 2q ¼ 40.1ꢀ has been assigned to
Ni3Pt.34 Though neither of these alloys appeared as bulk phases
in any of our samples (see Fig. 2, inset), near-surface alloys of Ni
and Pt may have produced the observed shi in 2q for Pt (111),30
and would be expected to impact the reactivity of the metal.35–37
In addition to a shi in its position, the Pt (111) peak varied
in breadth among the XRD patterns of our supported Pt–Ni
materials. The mean diameter of the Pt (or Pt–Ni) crystallites
was calculated by applying the Scherrer equation to this peak,
and the results are shown in Table 1, together with the corre-
sponding Pt (or Pt–Ni) dispersions. Notably, all of the bimetallic
Pt–Ni catalysts bore smaller crystallites than the monometallic
Pt catalyst with the same Pt loading; thus, adding Ni to the
catalysts reduced the crystallite size from 11 nm for 1Pt/3CeAl to
as low as 4.6 nm for 1Pt–6Ni/3CeAl. Larger amounts of Ni (12
and 18 wt%) did not promote further reduction in Pt–Ni crys-
tallite sizes. As the catalyst with 1 wt% Pt and 6 wt% Ni showed
the greatest peak width at half height for Pt (111), it had the
greatest calculated metal dispersion (Mdisp ¼ 25%, Table 1).
Auspiciously, this value was signicantly higher than that for
the Pt-only catalyst (Mdisp ¼ 10%). Even in the case that these Pt
particles contained dissolved Ni atoms, Pt atoms are expected to
form the surface layer of a solid solution of Ni in Pt that is
produced under H2,35 so greater dispersion implies that a
greater fraction of the Pt atoms in the sample existed on the
particle surfaces.
Fig. 1 X-ray diffraction (XRD) patterns of xPt–yNi/3CeAl catalysts that
had been calcined at 600 ꢀC for 6 h under air (heating rate 1.5 ꢀC
minꢁ1).
the remaining catalysts were not sufficiently intense to permit
reliable calculations of particle size, but were qualitatively
broad, suggesting smaller metal particles. The diffraction peak
As shown in Fig. 3a, the Pt 4f7/2 XPS peaks of Pt–Ni catalysts
occurred at higher binding energies than that of 1Pt/3CeAl;
thus, the electronic environment of Pt was modied when Ni
was introduced. A similar effect has been observed in the X-ray
photoelectron spectra of core–shell Pt-coated Au nano-
particles.38 On the other hand, nanoparticles of Pt–Ni alloys
have actually shown lower Pt 4f7/2 binding energies than Pt
nanoparticles synthesized according to the same methods.31,34
Among our 1Pt–yNi/3CeAl samples, Ni addition had the greatest
impact on the Pt 4f7/2 binding energy in 1Pt–3Ni/3CeAl and 1Pt–
6Ni/3CeAl (BEPt
¼ 71.29 and 71.35 eV, respectively; cf.
4f(7/2)
71.09 eV for 1Pt/3CeAl), suggesting that the electronic impacts
of Ni–Pt interactions were most signicant in those samples.
The Ni 2p3/2 regions of the XPS spectra also varied among
catalysts (Fig. 3b). Although the impact of Pt : Ni ratio on the Ni
2p3/2 XPS regions of alloyed Pt–Ni particles has been studied
quantitatively,34 the lower concentrations of Ni on our sup-
ported catalysts did not produce XPS signals of sufficient quality
for quantitative analysis. However, both Ni0 (main peak at 852.6
eV39) and Ni2+ (main peak at 854.6 eV,39 present as Ni(OH)2 and
Fig. 2 X-ray diffraction patterns of catalysts that had been reduced in
flowing H2 (50 vol% in N2) at 800 ꢀC for 60 min (heating rate 1.5 ꢀC
minꢁ1). 1Pt/3CeAl was reduced at 500 ꢀC. Inset shows the Pt (111) peak
region.
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Fig. 3 The Pt 4f and Al 2p regions of the X-ray photoelectron spectra
of xPt–yNi/3CeAl catalysts that had been reduced in flowing H2 (50 vol
% in N2) at 800 ꢀC for 60 min (heating rate 1.5 ꢀC minꢁ1). 1Pt/3CeAl was
reduced at 500 ꢀC.
Fig. 4 Effect of Ni content in xPt–yNi/3CeAl catalysts on yields,
selectivity and glycerol conversions in the aqueous phase reforming of
glycerol (240 ꢀC, 40 bar, 1 wt% glycerol, 0.05 mL minꢁ1, 250 mg
catalyst; data are mean values over t ¼ 5–20 h). Error bars indicate one
standard deviation; each bar is the average of $2 experiments. Mix cat.
¼ mixture of separate Pt/3CeAl and Ni/3CeAl catalysts with a total of 1
wt% Pt and 6 wt% Ni.
possibly also as NiO) were clearly visible in the spectra of all Ni-
containing catalysts except 1Pt–3Ni/CeAl, albeit in varying
relative amounts. The nickel hydroxides and oxides likely
formed on the surface of the Ni0 particles upon air exposure of
the reduced catalysts prior to analysis. Although we cannot
exclude the possibility that some Ni2+ remained following the
treatment in H2, neither NiO nor Ni(OH)2 (JCPDS 14-0117) were
evident in the XRD patterns of the reduced catalysts, supporting
the notion that they were minor contaminants. The Ni 2p3/2
signal in the X-ray photoelectron spectrum of 1Pt–3Ni/3CeAl
was too weak to be interpreted reliably. In the future, EXAFS or
XANES studies may shed further light on the nature of the
interactions between Ni and Pt on CeO2–Al2O3 supports;20,40
however, it is clear from the XRD and XPS evidence that adding
Ni impacted both the electronic and crystallite structure of Pt.
Further, energy-dispersive spectroscopic analysis of 1Pt–18Ni/
3CeAl conrmed that Ni and Pt co-existed in some areas on that
material (Fig. S8†).
concentration in the product gas was below the GC detection
limit (i.e. [CO] # 100 ppm) in all reactions.
The H2 yields (Fig. 4) and concentrations in the gaseous
products (Fig. 5) from glycerol reforming over three of the
nickel-containing catalysts, 1Pt–6Ni/3CeAl, 1Pt–12Ni/3CeAl and
1Pt–18Ni/3CeAl, were similar to those obtained over our
reported 3Pt/3CeAl catalyst (H2 yield ¼ 78%; [H2] in the gaseous
products ¼ 69%),11 despite that these Pt–Ni catalysts contained
one third as much Pt. Among these three best nickel-containing
catalysts, the H2 yield decreased with increasing Ni loading.
Thus the highest H2 yield (86%) and selectivity (83%) were
observed for APR over 1Pt–6Ni/3CeAl. The lowest H2 yield (13%)
and H2 selectivity (57%) among any of the catalysts was
observed over Pt-free 6Ni/3CeAl. The H2 selectivity obtained
from 1Pt–6Ni/3CeAl was quite similar to those reported by
Lehnert and Claus41 for 3 wt% Pt catalysts supported on
alumina (highest H2 selectivity, 85%, obtained at 250 ꢀC/20 bar,
10 wt% glycerol owing at 0.5 mL minꢁ1) and by Cortright et al.3
Catalytic tests
An aqueous solution of 1 wt% glycerol was used to evaluate the for 3 wt% Pt catalysts supported on nanobers of g-alumina
performance of the catalysts. All reactions were performed (highest H2 selectivity was 75%, obtained at 225 ꢀC/29 bar,
using the optimised reaction conditions determined for our 10 wt% glycerol owing at 0.06 mL minꢁ1). Moreover, the APR
3Pt/3CeAl catalyst,11 i.e. at 240 C, 40 bar, and with a feed ow of glycerol over 1Pt–6Ni/3CeAl, 1Pt–12Ni/3CeAl and 1Pt–18Ni/
ꢀ
rate of 0.05 mL minꢁ1, irrespective of the catalyst used. The 3CeAl produced higher H2 selectivity than that over the C-sup-
reaction data presented in Fig. 4 and 5 show that the aqueous- ported Pt and Pt–Re catalysts reported by King et al.,42 who
phase reforming of glycerol over any of the studied catalysts obtained 56% selectivity for H2 when owing a 10 wt% glycerol
indeed led to a hydrogen-rich gas phase. Alkanes larger than solution through 200 mg catalyst at 225 ꢀC and 30 bar. Notably,
methane (i.e. ethane) were only detected in trace amounts and the APR of glycerol over 1Pt–6Ni/3CeAl, 1Pt–12Ni/3CeAl and
were not quantied. No CO was detected, indicating that CO 1Pt–18Ni/3CeAl produced more CO2 than that over 1Pt/3CeAl
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active for the APR of glycerol. In our case, the 1Pt–6Ni/3CeAl
catalyst, which had the smallest Pt metal particles (4.6 nm) and
highest dispersion (25%), showed the highest H2 selectivity
(83%) and yield (86%).
The liquid phase from the reaction over each of the catalysts
was also analysed in order to quantify glycerol consumption and
examine the formation of larger byproducts. Apart from
unreacted glycerol, we identied traces of other compounds,
particularly ethanol, propylene glycol, and acetic acid. These
were not quantied. The APR of 1 wt% glycerol over catalysts
1Pt–6Ni/3CeAl, 1Pt–12Ni/3CeAl and 1Pt–18Ni/3CeAl produced
similar conversions and gas phase carbon yields as that over
3Pt/3CeAl, though both quantities decreased as the Ni content
increased from 6 to 18 wt%.
In order to evaluate the activity and efficiency of each cata-
lyst, the rates of H2 formation were normalised to the mass of
catalyst or Pt used (Fig. 6). Catalysts 3Pt/3CeAl and 1Pt–6Ni/
3CeAl produced almost the same amount of H2 per gram of
catalyst per hour, despite that the latter contained less Pt. Thus
the amount of expensive metal could be reduced threefold by
adding Ni, and with a slight improvement in H2 production.
Conversely, 1Pt/3CeAl and 1Pt–6Ni/3CeAl contained the same
amount of Pt, but the latter produced H2 approximately twice as
quickly. Overall, 1Pt–6Ni/3CeAl combined the highest glycerol
conversion with the greatest rate of H2 production and H2
selectivity, which could make it competitive for large-scale H2
production.
Fig. 5 Effect of Ni content in xPt–yNi/3CeAl catalysts on the distri-
bution of gaseous products from the aqueous phase reforming of
glycerol (240 ꢀC, 40 bar, 1 wt% glycerol, 0.05 mL minꢁ1, 250 mg
catalyst; data are mean values over t ¼ 5–20 h). Error bars indicate
one standard deviation; each bar is the average of $2 experiments.
Mix cat. ¼ mixture of separate Pt/3CeAl and Ni/3CeAl catalysts with a
total of 1 wt% Pt and 6 wt% Ni.
Based upon the characterisation and catalytic data, a few
inferences can be drawn regarding the mechanism(s) by which
(Fig. 4), or even 3Pt/3CeAl, which produced 62% CO2 yield.11 the addition of 6 wt% Ni enhanced catalyst activity. First, Ni
This is consistent with the higher activity of Ni as a WGS (eqn
(2)) catalyst,43 and may help to explain why similar H2 yields
were generated by these 1Pt–xNi/3CeAl catalysts and by 3Pt/
3CeAl, despite the much lower Pt content of the bimetallic
catalysts. However, the increased WGS activity that was
provided upon Ni addition was not sufficient to explain the high
H2 yield and selectivity of the best catalyst, 1Pt–6Ni/3CeAl, as a
mixture of separate Pt/3CeAl and Ni/3CeAl catalysts with a total
of 1 wt% Pt and 6 wt% Ni did not perform as well. Despite that
Ni also favors methanation,44 the APR of glycerol over 1Pt–6Ni/
3CeAl, 1Pt–12Ni/3CeAl and 1Pt–18Ni/3CeAl showed similar CH4
yields to the reaction over 1Pt/3CeAl (which gave a CH4 yield of
20%). This could be due to an interaction between Pt and Ni; an
interaction between Pt and Cu has been credited for lowering
methane production in the APR of glycerol over catalysts sup-
ported on magnesium/aluminium oxides.40 Nevertheless, the
highest fraction of CH4 in the gas product (32%) was obtained
using 6Ni/3CeAl as the catalyst (Fig. 5).
Some authors have correlated the activity and H2 selectivity
of supported-metal APR catalysts with metal particle size and
dispersion. Wawrzetz et al.45 showed that H2 formation from the
APR of glycerol decreased with increasing Pt particle size. On
Fig. 6 Rate of H2 production from the APR of glycerol (240 ꢀC, 40 bar,
the other hand, Lehnert and Claus41 showed that bigger Pt
1 wt% glycerol, 0.05 mL minꢁ1, 250 mg catalyst; data are mean values
particles produced higher H2 selectivity (but similar activity),
and concluded that the adsorption of glycerol occurred prefer-
entially at face positions of the metal crystallite. Iriondo et al.46
over t ¼ 5–20 h), normalised to the mass of catalyst or Pt used. Error
bars indicate one standard deviation; each data point is the average of
$2 experiments. Mix cat. ¼ mixture of separate Pt/3CeAl and Ni/3CeAl
observed that less-dispersed Ni and PtNi catalysts were more catalysts with a total of 1 wt% Pt and 6 wt% Ni.
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itself contributed to the H2 yield by catalysing the WGS reaction; deactivated; it ceased to produce a detectable H2 peak aer 15 h
however, this was insufficient to explain the exceptional activity on-stream. In a longer test, H2 formation over the most active
of 1Pt–6Ni/3CeAl (see above). Further, 1Pt–6Ni/3CeAl bore the catalyst, 1Pt–6Ni/3CeAl, occurred at a relatively constant rate
smallest and most-dispersed Pt nanoparticles, and thus had over 85 h.
more Pt atoms located at the particle surfaces than the Ni-free
Two main causes of catalyst deactivation in APR are carbon
catalyst with the same Pt loading. Shabaker et al. have also deposition and the sintering of the active metal,12 and we
noted a correlation between Pt dispersion and the apparent therefore examined the catalysts for signs of these problems.
activation energy in APR, albeit on a range of different Fig. 8 presents the XRD patterns of the fresh and spent 1Pt–6Ni/
supports.6 Finally, the XRD pattern and X-ray photoelectron 3CeAl catalysts; patterns for the remaining catalysts are shown
spectrum of 1Pt–6Ni/3CeAl pointed to interactions between Pt in Fig. S11.† No carbon formation48 (expected at 2q ¼ 25.5ꢀ) or
and Ni (see above), including the dissolution of some Ni atoms NiO (2q ¼ 37.2 and 62.9ꢀ)25 was observed on the spent 1Pt–6Ni/
in the Pt (i.e. surface, though not bulk, alloy formation); both 3CeAl catalyst aer 85 h on-stream. Rather, the only difference
computational35 and experimental36,37 studies have demon- observed was in the widths of the Pt0 and Ni0 peaks, which were
strated that surface Pt–Ni alloys bind H2 less strongly than Pt0. slightly sharper in the XRD pattern of spent 1Pt–6Ni/3CeAl. This
The importance of H2 binding strength has been demonstrated could have indicated a small amount of particle agglomeration
by Shabaker et al.,47 who showed that hydrogen inhibits the APR (based upon the Pt (111) peak, calculated particle sizes were 4.6
of oxygenated hydrocarbons on Pt catalysts, likely by occupying and 4.7 nm, respectively, for the fresh and spent catalysts). The
and thus blocking Pt active sites. Similarly, Huber et al.5 spec- X-ray photoelectron spectrum of spent 1Pt–6Ni/3CeAl showed a
ulated that supported Pt–Ni and Pt–Co catalysts, which out- signal for the Pt 4f7/2 electrons (Fig. S14a†) that was similar to
performed a supported Pt catalyst, had lower heats of H2 and the one for the fresh catalyst, though with a slightly higher
CO adsorption than pure Pt, and thus more unoccupied active energy (BEPt 4f(7/2) ¼ 71.45 eV). This energy shi could have been
sites accessible to reactants. Thus the exceptional activity of due to increased interaction with Ni; however, because of its
1Pt–6Ni/3CeAl was likely caused by a conuence of factors; in small magnitude (DBEPt 4f(7/2) ¼ 0.1 eV), the effects of charge
particular, it offered the best balance of advantageous Ni–Pt compensation cannot be ruled out. The Ni 2p3/2 signals of the
interactions and high Pt dispersion/small Pt particle size.
fresh and spent catalysts (Fig. S14b†) were also similar, though
One of the major problems related to the operation of the latter showed higher relative intensity at lower binding
heterogeneous catalysis is the loss of catalytic activity, i.e. energy. This would seem to indicate that the Ni0/Ni2+ ratio was
“deactivation”, over time, and Ni catalysts are generally more greater on the surface of the spent than the fresh catalyst;
susceptible than noble-metal catalysts. Fig. 7 shows the stability however, as both catalysts were exposed to air prior to
of our ceria–alumina-supported catalysts in the APR of glycerol measurement, and as Ni surfaces can be oxidised in air,
over 30 h on-stream. Only catalyst 6Ni/3CeAl was severely conclusions based upon this comparison would be dubious.
Nevertheless, consistent with the XRD results, the spent catalyst
Fig. 7 Variation of H2 formation rate with time on-stream in the APR Fig. 8 XRD patterns of 1Pt–6Ni/3CeAl catalyst freshly reduced in
of glycerol (240 ꢀC, 40 bar, 1 wt% glycerol, 0.05 mL minꢁ1, 250 mg flowing H2 (50 vol% in N2) at 800 ꢀC for 60 min, and spent after 30 h
catalyst). Each value is the average of $2 experiments. The best on-stream (240 ꢀC, 40 bar, 1 wt% glycerol, 0.05 mL minꢁ1, 250 mg
catalyst, 1Pt–6Ni/3CeAl, was tested in an extended run.
catalyst).
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clearly bore Ni0. The liquid products of the APR of glycerol over
On the other hand, the TGA of 6Ni/3CeAl aer a 30 h use
1Pt–6Ni/3CeAl were analysed for metal content using induc- (shown, along with TGA curves for the other spent catalysts, in
tively coupled plasma/mass spectrometry (ICP-MS). A small Fig. S12†) revealed that combustible material, presumably
amount (<1 ppm) of Ni was present, representing a loss of carbon (greatest weight loss occurred over T ¼ 500–600 ꢀC),
<0.001% of the Ni in the catalyst, but no Pt could be detected. made up >4 wt% of the sample, and the XPS scan showed that
The XPS analysis of spent 1Pt–6Ni/3CeAl also indicated that its the surface of spent 6Ni/3CeAl bore twice as much C as the fresh
surface contained signicantly less carbon than the fresh catalyst. NiO peaks were clearly observed in the XRD of spent
catalyst (1.8 vs. 8.1 at% based on detection-sensitivity-corrected 6Ni/3CeAl (Fig. S11†), and neither XRD nor XPS (Fig. S14b†)
areas; see Fig. S13†); thus adventitious carbon was likely the showed evidence of Ni0 in that sample. Thus both modes of
main source of surface carbon. Finally, as amorphous carbon catalyst inactivation, i.e. metal oxidation and carbon deposi-
deposition would not have been seen in the XRD pattern, we tion, contributed to the failure of 6Ni/3CeAl aer only 15 h on-
also performed a thermogravimetric analysis (TGA) of spent stream. Moreover, ICP-MS analysis of the liquid products
1Pt–6Ni/3CeAl in air (Fig. 9). No evidence of weight loss due to evinced signicant nickel loss (1300 ppm, representing 0.8% of
carbon combustion was observed, supporting the idea that no the Ni metal in the catalyst) from 6Ni/3CeAl following 30 h on-
signicant carbon deposition occurred on 1Pt–6Ni/3CeAl.
stream. The spent sample of a catalyst with intermediate
stability, 1Pt–18Ni/3CeAl, was also studied. No evidence of NiO
formation was observed in the XRD pattern of that catalyst
(Fig. S11†), the Ni 2p3/2 signals in the XPS of the fresh and spent
samples were virtually identical (Fig. S14b†), and the losses of
Ni and Pt to solution were below the limits of detection.
However, the spent sample did lose mass upon combustion
(Fig. S12†), indicating that although metal oxidation was not a
signicant problem, some carbon was deposited on that cata-
lyst, and was presumably responsible for the minor decline in
its activity aer 20 h on-stream (Fig. 7).
Conclusion
We have previously shown that a 3Pt/3CeAl catalyst was more
active and more selective towards H2 production than 3Pt/Al2O3
in the APR of glycerol.11 Pt catalysts are highly active for APR,
but Pt is expensive, making catalysts containing little or no Pt
desirable. Bimetallic Pt–Ni catalysts active for the APR of glyc-
erol to H2 were developed in this work. Bimetallic 1Pt–6Ni/
3CeAl showed the highest H2 yield and gas phase C yield, and
produced three times as much H2 per gram of Pt as 3Pt/3CeAl.
The favourable characteristics of 1Pt–6Ni/3CeAl could not be
attributed to a single factor, but rather appeared to stem from
smaller crystallite size (4.6 nm), higher metal dispersion (25%)
and greater degree of electronic interaction (BEPt
¼
4f(7/2)
71.35 eV) between the metals, likely as surface alloy formation.
Further, the 3 wt%-CeO2-doped Al2O3 support enhanced both
the activity and selectivity towards H2 production. As a result
1Pt/3CeAl showed higher glycerol conversion and H2 yield than
the benchmark catalyst, 3Pt/Al2O3.11 Future work will study APR
over 1Pt–6Ni/3CeAl at higher glycerol concentrations and on
larger scales.
Experimental section
Catalyst preparation
The 3 wt% CeO2–Al2O3 supports were prepared by impregnating
2.0 g of dried (120 ꢀC overnight) g-Al2O3 (Sigma-Aldrich) with a
solution prepared by dissolving 197 mg of (NH4)2[Ce(NO3)6]
(99%, Sigma-Aldrich) in 10 mL deionised water in a 100 mL
glass vial. The mixture was stirred overnight at room tempera-
ture, and the water was allowed to evaporate. The sample was
Fig. 9 Thermogravimetric analysis of fresh (reduced: 50 vol% H2 in N2,
800 ꢀC, 60 min) and spent (after reaction: 240 ꢀC, 40 bar, 1 wt%
glycerol, 0.05 mL minꢁ1, 85 h) 1Pt–6Ni/3CeAl. Samples were heated at
10 ꢀC minꢁ1 in instrument air.
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then dried in air at 120 ꢀC for 12 h and calcined under owing mA and 15.2 kV) and under a vacuum of #2 ꢃ 10ꢁ9 mbar.
air at 600 ꢀC for 3 h (heating rate 1.5 ꢀC minꢁ1). [Pt(NH3)4](NO3)2 Binding energies were referenced to the adventitious hydro-
(Strem Chemical) and Ni(NO3)2$6H2O (Sigma-Aldrich) were carbon C 1s signal at 285.0 eV. For spent samples, the binding
dissolved, individually or together, into a minimum amount of energies were adjusted to give the same Al 2p binding energy as
deionised water to make monometallic or bimetallic catalysts, in the corresponding fresh samples. The Ce 3d and Pt 4d5/2
respectively. These were deposited on 3 wt% CeO2–Al2O3 regions of the XPS spectra are shown in Fig. S9 and S10,†
supports (3CeAl) using a conventional impregnation technique. respectively. Note that the characterisation of reduced catalysts
Specically, to prepare 1Pt–3Ni/3CeAl, 2.205 g of calcined occurred ex situ (see “Catalyst preparation”); therefore, mate-
3CeO2–Al2O3 support was impregnated with a solution prepared rials were exposed to ambient conditions prior to and, in
by dissolving 43.1 mg [Pt(NH3)4](NO3)2 and 324.5 mg of the case of XRD, during data collection. Thermogravimetric
Ni(NO3)2$6H2O in 10 mL of deionised water in a 100 mL glass analysis of the spent catalysts was measured on a TA Instru-
vial. The mixture was then stirred overnight at room tempera- ments Q500 analyser under a ow of 40 mL minꢁ1 instrument
ꢁ1
ꢀ
ꢀ
ture, and the water was allowed to evaporate. The sample was air. Samples were heated at 10 C min to 1000 C.
then dried in air at 120 ꢀC for 12 h and calcined under owing
air at 600 ꢀC for 6 h (heating rate 1.5 ꢀC minꢁ1). Catalysts were
reduced ex situ in owing hydrogen (50 mL minꢁ1) at 800 ꢀC for
Catalytic test
60 min (heating rate 1.5 ꢀC minꢁ1) at atmospheric pressure and The APR of glycerol was studied in a continuous-ow xed-bed
stored under vacuum prior to use. For comparison, a mixed reactor system. The catalyst (250 mg) was loaded into a 5 mm
“1Pt/3CeAl + 6Ni/3CeAl” catalyst was also prepared. In order to i.d. stainless steel tubular reactor and held in position with
prepare this physical mixture of catalysts with a total of 1 wt% Pt quartz wool plugs. Reaction temperature was measured by a K-
and 6 wt% Ni, catalysts 2Pt/3CeAl and 12Ni/3CeAl were inde- type thermocouple that was placed inside the reactor, very close
pendently prepared, then mixed in a 1 : 1 ratio by mass; this was to the catalyst bed. The reactor was mounted in a tube furnace
denoted as “Mix cat.”
(MTI GSL-1100X). A backpressure regulator (0–68 bar, Swagelok)
attached to a pressure gauge was used to pressurise the system
with N2 to 40 bar. A schematic of the reactor can be found in
Fig. S4.† A 1 wt% glycerol solution was introduced using a
digital hplc pump (Waters 510) at a rate of 0.05 mL minꢁ1, and
heating of the catalyst bed was initiated. When the reactor
reached 240 ꢀC, N2 ow was set at 50 sccm using a Bronkhorst
mass ow controller. The system was allowed to stabilise for
about 2 h before analysis of the reaction products began.
Gas products were analysed at 25 min intervals using an
online gas chromatograph (Varian CP-3800) equipped with one
Hayesep N, 60/80 Mesh, 5 m ꢃ 1/800 SST column and one Mol-
Catalyst characterisation
The textural properties of the catalysts were measured by N2
adsorption–desorption at liquid nitrogen temperature
(ꢁ196 ꢀC) using an Autosorb-iQ apparatus (Quantachrome).
Prior to analysis, the samples were outgassed for 12 h at 140 ꢀC.
The specic surface areas were determined from the linear
portions of the adsorption isotherms (P/P0 ¼ 0.05–0.35) using
the Brunauer–Emmett–Teller method,21 and the pore volumes
were calculated at P/P0 ¼ 0.995. The pore-size distributions were
calculated from the adsorption isotherms using the Barrett–
Joyner–Halenda (BJH) formula.22 Isotherms are displayed in the
ESI (Fig. S1†). The crystalline structures of the supported cata-
lysts were determined by X-ray diffractometry using Cu Ka
radiation (l ¼ 0.1542 nm) and a graphite monochromator
(Shimadzu S6000). The instrument was operated at 40 kV and
40 mA. Scans were recorded over the range 2q ¼ 10–75ꢀ in steps
of 0.01ꢀ, and data for each point was collected for 1 s. The mean
Pt crystallite sizes were calculated by applying the Scherrer
equation23 to the Pt (111) peak, and the corresponding Pt (or Pt–
Ni) dispersion, Mdisp, was estimated according to eqn (5):24
00
˚
sieve 5 A, 60/80 Mesh, 1 m ꢃ 1/8 column, connected in series.
Thermal conductivity (TCD) and ame ionisation (FID) detec-
tors, in series, were used to analyse H2 and hydrocarbons,
respectively. The GC was calibrated using highly pure gases
(grade 5.0) from Coregas. For each reading, ten successive
injections were made and the relative standard deviations were
measured. The calibration curves (Fig. S5†) were developed, and
ꢀ
the samples were analysed, with the oven at 80 C, the TCD at
200 ꢀC and the FID at 300 ꢀC. A representative GC curve
(Fig. S6†) shows only four peaks, representing H2, N2, CH4 and
CO2, respectively, for each injection of product gas.
The liquid products of the APR reaction were collected in a
condenser downstream of the reactor bed (refer to Fig. S4†), and
Mdisp ¼ 6V/dA
(5)
here, V is the Pt atomic volume (0.0151 nm3), d is the crystallite aliquots of the condensed liquid were analysed with a Shimadzu
size (nm) and A is the surface area of a single Pt atom HPLC, comprising a degasser (DGU-20A5), a pump (LC-20AD),
(0.080 nm2). Scanning electron microscopy (FESEM, Zeiss an autosampler (SIL-20A HT), an oven (CTO-20A), and a
Ultra+) was used to examine the morphology of fresh and spent refractive index detector (RID-10A).
A Rezex RCM-Mono-
catalysts (Fig. S2†) and transmission electron microscopy (TEM, saccharide column (300 ꢃ 7.8 mm) was used for analyte sepa-
Philips CM120 BioFilter) was employed to gain insight into the ration. Ultrapure DI water (ow rate 0.5 mL minꢁ1) was used as
inner pore structure and the distribution of metal sites the eluent. A representative HPLC trace and a calibration curve
(Fig. S3†). X-ray photoelectron spectra were recorded on an for glycerol are shown in Fig. S7.† As the intercept of the cali-
ESCALAB250Xi (Thermo Scientic, UK) using
a mono- bration curve was non-zero, glycerol concentration will be
chromated Al Ka source (1486.68 eV) operating at 164 W (10.8 overestimated when the conversion is high, and thus
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conversion will be underestimated under those circumstances. 11 M. M. Rahman, T. L. Church, A. I. Minett and A. T. Harris,
The catalysts were evaluated on the bases of H2, CO2 and CH4 ChemSusChem, 2013, 6, 1006–1013.
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`
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to:
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moles of x produced experimentally
yield of species x ¼
ꢃ 100%
moles of x produced theoretically
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Catal., 1986, 101, 169–177.
moles of C in the gas products
C conversion to gas ¼
ꢃ 100%
moles of C fed into the reactor
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L. E. Murillo, J. G. Chen and D. J. Buttrey, J. Catal., 2008,
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2 ꢃ moles of H2 produced experimentally
H2 selectivity¼
ꢃ100%
total H atoms in the gas products
h
i
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moles of glycerol in liquid products
moles of glycerol in feed
glycerol conversion ¼ 1 ꢁ
ꢃ100%
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M.M.R is grateful to the University of Sydney for a postgraduate
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for Microscopy & Microanalysis (ACMM) at the University of
Sydney for SEM and TEM facilities and to Mr V. Lo for his
assistance with SEM and TEM analysis, to Dr B. Gong from the
University of New South Wales Surface Analytical Centre for his
assistance with XPS analysis, to Ms D. Yu from the University of
New South Wales for assistance with ICP-MS, and to Dr J. Shi
from the School of Chemical and Biomolecular Engineering,
University of Sydney, for assistance with N2 adsorption
measurements.
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