Low-temperature aqueous-phase reforming of ethanol on bimetallic PdZn catalysts

Article abstract of DOI:10.1039/c4cy00914b

Bimetallic PdZn catalysts supported on carbon black (CB) and carbon nanotubes (CNTs) were found to be selective for CO-free H₂ production from ethanol at low temperature (250°C). On Pd, the H₂ yield was low (～0.3 mol H₂/mo

Full text of DOI:10.1039/c4cy00914b

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Low-temperature aqueous-phase reforming of
ethanol on bimetallic PdZn catalysts†
Cite this: DOI: 10.1039/c4cy00914b
Haifeng Xiong, Andrew DeLaRiva, Yong Wang and Abhaya K. Datye*a
a
a
bc
Bimetallic PdZn catalysts supported on carbon black (CB) and carbon nanotubes (CNTs) were found to be
selective for CO-free H
was low (~0.3 mol H /mol ethanol reacted) and the CH
Pd formed the intermetallic PdZn phase (atomic ratio of Zn to Pd is 1) with increased H
~1.9 mol H /mol ethanol reacted) and CH /CO ratio of <1. The higher H yield and low CH
formation was related to the improved dehydrogenation activity of the L1 PdZn phase. The TOF
2
production from ethanol at low temperature (250 °C). On Pd, the H
/CO ratio was high (~1.7). Addition of Zn to
yield
2
yield
2
4
2
β
2
(
2
4
2
2
4
0
β
increased with particle size and the CNTs provided the most active and selective catalysts, which may be
ascribed to pore-confinement effects. Furthermore, no significant changes in either the supports or the
PdZn
the carbon support are hydrothermally stable in the aqueous phase at elevated temperatures and pressures
>200 °C, 65 bar). No CO was detected for all the catalysts performed in aqueous-phase reaction, indicating
that both monometallic Pd and bimetallic PdZn catalysts have high water-gas shift activity during APR.
However, the yield of H is considerably lower than the theoretical value of 6 H per mole ethanol which
is due to the presence of oxygenated products and methane on the PdZn catalysts.
β
particles was found after aqueous-phase reforming (APR) indicating that the metal nanoparticles and
Received 13th July 2014,
Accepted 28th August 2014
(
DOI: 10.1039/c4cy00914b
2
2
www.rsc.org/catalysis
lower temperatures, and the conversion of oxygenated hydro-
1
. Introduction
carbons such as methanol, ethylene glycol and glycerol has
been reported, but there is little reported on the APR of bio-
H
2
is a clean energy carrier for future energy systems, espe-
1
8
cially for vehicles with fuel cells. The conventional approach
to produce H is steam reforming (SR) which is energy inten-
sive since it requires vaporization of water and the reaction is
ethanol. A major consideration for APR reactions is the lim-
2
ited hydrothermal stability of conventional supports in liquid
9
water at elevated temperatures (>200 °C). Roy et al. found by
2
carried out at high temperatures (>400 °C). The high tem-
XPS that a nickel oxide layer had formed on the nickel/alumina
perature SR process can also lead to the formation of coke on
2 3
catalyst and the γ-Al O support had transformed to boehmite
during ethanol APR. Likewise, Ravenelle and coworkers
3
,4
10
the catalyst, which results in catalyst deactivation. Aqueous-
phase reforming (APR), on the other hand, can be carried out
found that the transformation of γ-Al O into boehmite caused
2
3
5
at low temperatures (<300 °C). Compared to SR in the gas
the sintering of the supported nickel and Pt particles during
1
1
phase, APR exhibits several advantages such as reduced
energy consumption and low amounts of CO due to the water
aqueous phase reactions. The development of stable catalytic
1
2
materials for APR is therefore of great interest.
Carbon supports have been shown to be hydrothermally
6
gas shift (WGS) reaction.
1
3,14
In this study we focus on bioethanol which is a renewable
resource that can be obtained from biomass. There have been
numerous studies on gas phase ethanol steam reforming (SR),
showing that a high yield of hydrogen can be obtained, but
stable under conditions used for APR.
A recent study
using a carbon-supported ruthenium catalyst showed that the
Ru metal phase remained in the reduced state in sub- and
supercritical water and the catalyst was stable over long term
7
15
only at elevated temperatures (>400 °C). APR is conducted at
operation. The reaction products included CH , CO and
4
2
H
2
but the ratio of CH
yield of H . To achieve a high yield of H
suppress the formation of CH and enhance CO formation.
4
/CO
2
was 3 : 1 with a resulting low
a
2
2
it is necessary to
Department of Chemical & Biological Engineering and Center for Micro-engineered
Materials, University of New Mexico, Albuquerque, New Mexico 87131, USA.
E-mail: datye@unm.edu; Fax: +1 505 277 1024; Tel: +1 505 277 0477
4
2
Among the noble metal catalysts tested for APR of oxygenates,
b
Institute for Integrated Catalysis, Pacific Northwest National Laboratory,
it was found that Pt shows the highest reactivity, but Pd
Richland, WA 99352, USA
8,16
shows the best selectivity to CO formation.
The activity
c
2
The Gene & Linda Voiland School of Chemical Engineering and Bioengineering,
and selectivity of these catalysts can be further improved
by alloying the noble metal with components that enhance
water gas shift activity. ZnO was found to be a selective
Washington State University, Pullman, WA 99164-2710, USA
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cy00914b
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7
support for SR of ethanol on cobalt catalysts, but it was not
clear whether an alloy was formed during reaction. In the
PdZn system it has been shown that the formation of the
To study the effect of particle size on the carbon nano-
tubes (CNTs) supported catalysts we prepared two bimetallic
PdZn catalysts with Pd loadings of 1 and 5 wt.% and a Zn to
Pd atomic ratio of 1. Carbon nanotubes were prepared by
chemical vapor deposition as described in a previous study.
The pore size of obtained CNTs is 5–15 nm and they were
intermetallic PdZn phase (Zn : Pd atomic ratio of 1) was criti-
cal for obtaining a high activity and selectivity for methanol
β
1
8
1
7
steam reforming to produce H
2
.
But the stability of the
PdZn intermetallic has not been tested under aqueous phase
conditions, hence the main objective of this study was to
investigate the behavior of the intermetallic PdZn phase for
functionalized in 50% HNO at 90 °C for 8 h. The preparation
3
procedure for these catalysts was similar to that for PdZn /CB,
1
1
described above. These catalysts were labeled as PdZn /CNT-1
H production during ethanol APR.
2
(1 wt.% Pd) and PdZn /CNT (5 wt.% Pd), respectively.
1
Two types of carbon supports were used, a commercial
carbon black and carbon nanotubes prepared in house. The
aim of this study was to: (I) investigate the effect of Zn on the
catalytic performance of Pd and (II) the effect of metal particle
size and support structure (carbon black and carbon nano-
tubes) on the performance of the PdZn bimetallic catalysts in
the APR of ethanol. We were especially interested in the yield
2
.2. Catalyst characterization
N2 physisorption experiments were performed using a
Quantachrome Autosorb-1 instrument. Prior to the experi-
ment, the sample was outgassed at 200 °C for 6 h. The
surface area was obtained using the BET method using
adsorption data over a relative pressure range from 0.05
to 0.30. The total pore volumes were calculated from the
of H
well as the formation of other oxygenated byproducts. As we
show in this paper, the addition of Zn enhanced the H yield
per mole of ethanol reacted and led to CH /CO ratios less
2 4 2
and the relative CH /CO selectivity of these catalysts as
amount of N adsorbed at a relative pressure of 0.99. X-ray
2
2
powder diffraction (XRD) patterns for the catalysts were
recorded with a Scintag Pad V diffractometer using Cu Kα
radiation and a Ni filter. For some of the catalysts we also
used a Rigaku Smart Lab XRD. Scanning transmission
electron microscopy (STEM) was carried out in a JEOL 2010F
microscope. The powders were deposited on holey carbon
support films after being dispersed in ethanol. An electron
probe, diameter of 0.2 nm, was scanned over the specimen,
and electrons scattered at high angles were collected to form
the image in HAADF (High Angle Annular Dark Field) mode.
The histogram of particle size distribution (Pd and PdZn) was
obtained by counting 200–300 particles in the STEM images.
4
2
than 1. Furthermore, large PdZn particles (>5 nm) provide
higher TOFs and carbon nanotubes provide the highest reac-
tivity. No CO is found during APR of ethanol, indicating these
catalysts have high WGS reactivity.
2
. Experimental
2
.1. Catalyst preparation
PdZn bimetallic catalysts supported on carbon black (CB)
with different Zn loadings were prepared by co-impregnation.
First, the commercial carbon black (Vulcan XC 72R) was
We tried to use H chemisorption at 100 °C to count the
2
functionalized in 50% HNO
filtration and washing in water till a pH = 7 was reached in
the filtrate. The carbon black was next dried at 120 °C for
3
at 90 °C for 14 h, followed by
number of surface Pd sites in these catalysts. The numbers
were not consistent with the dispersion obtained from TEM
images because of the weak chemisorption on the PdZn cata-
lysts. CO chemisorption is not recommended for Pd catalysts
because of variations in the ratio of bridge bonded and line-
arly adsorbed CO with particle size and Zn content. Since it
is difficult to count sites in Pd and PdZn catalysts via chemi-
sorption, we used CO oxidation reactivity to determine the
number of Pd sites in these catalysts. The product analysis
during CO oxidation was carried out on a Varian CP-4900
Micro-GC utilizing a TCD detector with 20 mg of sample
being used for each experiment. The sample was reduced at
1
2 h. The catalysts were prepared via incipient wetness
impregnation in two steps. The Pd and Zn nitrates were used
as catalyst precursors. The monometallic catalyst contained
5
wt.% Pd while the bimetallic catalyst contained the same
loading of Pd but we also added 3.1 wt.% Zn and 5.0 wt.%
Zn to study the role of Zn/Pd ratio (atomic ratio 1 : 1 and
1
: 1.6). Subsequently, the sample was kept at room tempera-
ture for 12 h in air, followed by drying at 120 °C for 12 h.
Then the sample was pre-reduced at 500 °C for 2 h in a flow
of 7% H
2
/N
2
. The bimetallic catalysts are denoted as PdZn
1
250 °C in situ for 2 h in 6% H
2
/He followed by a cool down in
and PdZn1.6 indicating the atomic ratio of Zn/Pd in the cata-
lysts. The catalyst name also shows the support, CB – carbon
black and CNT – carbon nanotubes. To study the role of crys-
tallite size, we prepared 5 wt.% Pd/CB catalysts with different
Pd particle sizes. The 5 wt.% Pd/CB catalysts with small Pd
particle size (Pd/CB) was obtained by the impregnation of Pd
6% H /He to room temperature. The samples were then
2
−
1
exposed to CO oxidation conditions which used 1.5 mL min
−
1
−1
CO, 1 mL min
2
O , and 75 mL min He (~2% CO). The tem-
−
1
perature was ramped to 300 °C with a ramp rate of 2 °C min
and sampling performed every 3 min. The runs were repeated
to ensure reproducible behavior and the reactivity in the sec-
ond run was used to determine the number of exposed sur-
face Pd atoms as explained in more detail in the ESI.†
nitrate on carbon black, followed by H reduction at 250 °C
2
for 2 h in a flow of 7% H /N . The 5 wt.% Pd/CB catalyst
2
2
1
9
with large Pd particle size (Pd/CB–H) was obtained by a
reduction at 500 °C for 12 h in a flow of 7% H /N after the
In previous work, it was shown that PdZn catalysts show
higher specific reactivity than Pd caused by a weakening of
the CO bond due to added Zn. But these catalysts show rapid
2
2
impregnation.
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deactivation due to the surface oxidation of Zn in the PdZn
alloy. After reaching a steady state, the specific reactivity of
3
. Results and discussion
1
9
3.1. Catalyst characterization
Pd and PdZn was comparable. Our experimental procedure
involving two CO oxidation runs to 300 °C ensured that the
catalyst had reached its steady state behavior. Hence, we
could use the previously reported CO oxidation rates and
TOFs on Pd and PdZn to estimate the number of exposed Pd
The N2 physisorption data and the metal dispersion and
particle size for the as-prepared catalysts are shown in
Table 1. For the PdZn/CB catalysts with different Zn loadings,
the BET surface areas were in the range of 160–175 m g
and no significant change in the pore volumes of the cata-
lysts was found by varying the Zn loading. The PdZn /CNT
shows lower surface area and pore volume in comparison
with the bimetallic PdZn/CB catalysts due to the low surface
area of the CNT support. As explained in the experimental
section and the ESI,† the metal dispersion and particle size
was derived by using CO oxidation reactivity.
The XRD patterns of monometallic Pd/CB before and after
reaction are shown in Fig. 1. We also show the XRD pattern
of the Pd/CB–H catalyst (pretreated at 500 °C for 12 h in a
2
−1
1
9
atoms in our supported catalysts (see ESI† for further detail).
The estimates of numbers of surface atoms obtained from
CO oxidation reactivity agreed with the average particle diam-
eters obtained using TEM. Using these results, the turnover
1
−
1
frequency (TOF s ) for aqueous-phase reforming could then
be calculated. Furthermore, it should be pointed out that the
supports CB, CNT, ZnO/CB and ZnO/CNT are not active for
CO oxidation at T < 300 °C.
2
.3. Catalyst reactivity during aqueous-phase reforming of ethanol
Reaction studies for aqueous-phase reforming of ethanol
2 2
flow of H /N ) to increase the crystallite sizes, hence it shows
were conducted over a temperature range of 200–260 °C and
a sharp Pd diffraction peak (Fig. 1c). The broad peak seen in
the Pd/CB catalyst at 25° corresponds to the (002) plane of the
graphitic carbon. The peak at 40.2° comes from metallic Pd and
it is very broad due to the small particle size (Fig. 1a). Fig. 1
also shows the XRD pattern after reaction for the Pd/CB indi-
cating modest growth in particle size after reaction (Fig. 1b).
The XRD patterns of bimetallic PdZn catalysts are shown
6
5 bar pressure (controlled with a back pressure regulator) in
an up-flow tubular reactor. Before reaction measurements,
the catalyst (0.2 g) was reduced at 250 °C under flowing
3
−1
H
2
(20 cm (STP) min ) at atmospheric pressure for 4 h.
Subsequently, a feed containing 10 wt.% ethanol in water
−
1
(
0.003 mL min ) was pumped through the reactor. Before
the back pressure regulator, we introduced a stream of
in Fig. 2. For PdZn
shows two diffraction peaks at 41° and 44° which correspond
to the intermetallic L1 PdZn phase (Zn : Pd = 1, ICDD
006-0620). Increasing the Zn loading to 5 wt.% (Zn : Pd = 1.6)
leads to the appearance of crystalline ZnO coexisting
1
/CB (Fig. 2a and insert), the XRD pattern
3
−1
Ar (10 cm (STP) min ) to sweep the gas products to the GC.
The effluent gas stream was composed of H , CO , CH
, C , CH CHO, C OH and was analyzed with an
2
2
4
,
0
β
2
0
C
2
H
4
2
H
6
3
2
H
5
online GC (VARIAN CP-3800 GC) equipped with a FID and a
TCD. A gas–liquid separation trap at room temperature
with the PdZn
surface area of the CNT support, the bimetallic PdZn
catalyst with 5 wt.% Pd had larger particles, hence well-
defined diffraction peaks of PdZn phase can be seen (Fig. 2e).
β
phase (Fig. 2b and insert). Due to the lower
(
in line before the back pressure regulator) was used to
1
/CNT
collect the liquid. The liquid included CH CH OH, CH CHO,
3
2
3
CH
3
COOH and CH
3
COCH
3
and the composition was quanti-
β
fied by the GC on the FID using a CP-PoraBOND Q capillary
column. All reactions were carried out for 100 h and no deac-
tivation was found over this period. The first data point was
recorded after 24 h of reaction by which time steady state
The higher crystallinity of the support leads to the sharper
graphite peaks in the CNT supported catalysts (Fig. 2d and e).
Since the average particle size was significantly greater than
the other catalysts, we also prepared a catalyst with 1 wt.%
Pd loading to provide particle sizes comparable to the other
behavior was obtained, and the H yield was stable after that.
2
The ethanol conversion (X), product selectivity (S) and H
production were calculated based on the following equations:
2
supports. The XRD pattern of this catalyst PdZn
/CNT-1
1
shows broad diffraction peaks of the PdZn phase (Fig. 2d)
β
moles of inlet ethanol  moles of outlet ethanol in liquid  moles of outlet ethanol in gas
X 
100%
moles of inlet ethanol
comparable to those seen on the CB support. The low Pd
loading enhances the relative intensity of the carbon peaks
from the CNT support, therefore we have drawn vertical lines
to show clearly the PdZn (111) and those from the CNT sup-
port (Fig. S4, ESI† provides a larger figure to show clearly the
CNT peaks in relation to PdZn). The XRD patterns of the
PdZn catalysts after reaction show very little change confirming
the stability of this catalyst (Fig. 2c and f).
moles of i produced
Si 
100%
moles of all products
moles of H produced
2
H production 
2
moles of ethanol converted
where i is CH , CO , C H , C H , CH CHO, CH COOH and
3.1.1. Catalyst morphology of the as-prepared catalysts
before aqueous-phase reforming. The TEM images along with
4
2
2
4
2
6
3
3
CH COCH .
3
3
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Table 1 Characterization of the as-prepared monometallic Pd and bimetallic PdZn catalystsa
Particle size (nm)
Surface area
Total pore volume
2
−1
3
−1
b
b
Sample
(m g
)
(cm g
)
Dispersion (%)
CO oxidation
XRD
TEM
Pd/CB
Pd/CB–H
175
170
161
156
95.4
98.1
0.72
0.7
0.78
0.64
0.29
0.32
37.6
16.0
41.0
33.5
4.0
2.7
6.3
2.4
3.0
25
4.1
8.7
3.8
4.5
30
2.5
4.7
2.9
3.1
8.5
2.1
1
PdZn /CB
PdZn1.6/CB
PdZn
PdZn
1
/CNT
1
/CNT-1
28.6
3.5
4.6
a
b
All catalysts had a loading of 5 wt.% Pd, except the catalyst in the last row which contained 1 wt.% Pd. The number of surface Pd sites was
obtained from CO oxidation at 185 °C (see ESI) and the particle size was estimated using the formula: d = 100/D, where d is particle size in nm
and D is dispersion.
reduced at 250 °C in H shows a TEM average particle dia-
2
meter of 2.5 nm (Fig. 3a and d), with the HRTEM image
showing lattice fringes of the (111) and (200) planes of fcc Pd
(
Fig. 3b). The XRD pattern of this catalyst shows very broad
XRD peaks due to the small particle size. The dispersion of
the metal phase in this catalyst as estimated from CO oxida-
tion reactivity (Table 1) is 32% which is consistent with the
TEM average particle size. After high temperature reduction
(
(
(
500 °C), we see an increase in particle size by TEM
Fig. 3c and d) and the XRD peaks become much sharper
Fig. 1c). The dispersion as measured from the CO oxidation
reactivity measurement shows correspondingly an increase in
estimated particle to 6.3 nm which is again consistent with
the TEM measurement reported in Fig. 3d.
TEM images of the bimetallic PdZn/CB catalysts are shown
Fig. 1 XRD patterns of monometallic Pd catalysts supported on
carbon black (CB) before and after ethanol APR reaction: (a) Pd/CB;
in Fig. 4. The average particle size for PdZn
1
/CB is 2.88 nm
(
Fig. 4a and b, insert) and is consistent with the broad XRD
(
2
b) Pd/CB after APR reaction; (c) Pd/CB–H aged in H at 500 °C.
peaks (Fig. 2a) and with the dispersion from CO oxidation
which was 41%. The HRTEM image of this sample can
be indexed to the (200) plane of the PdZn phase (Fig. 4b).
β
Keeping the Pd loading at 5 wt.% but increasing the Zn load-
ing to 5 wt.% (PdZn1.6/CB) caused only a small increase in
particle size to 3.1 nm (Fig. 4c). The HRTEM image indexes
to the (111) and (200) planes of PdZnβ in this catalyst
(Fig. 4c, insert). The slight increase in particle size is consistent
with a small drop in the dispersion (Table 1). The majority
of the particles are smaller than 4 nm in these two catalysts
prepared on the carbon black support.
We also prepared catalysts with similar weight loading
with the CNT support. However, because of the lower surface
area, the average PdZn
that on the carbon black. For the PdZn
wt.% Pd loading, the STEM image shows that the PdZnβ
β
particle size was much larger than
1
/CNT catalyst with
5
particle sizes were in the range of 2–30 nm (Fig. 4d, average
size = 8.5 nm), which is consistent with the XRD result show-
ing sharp, well defined peaks of the intermetallic PdZnβ
Fig. 2 XRD patterns of bimetallic PdZn catalysts supported on
different carbons before and after aqueous-phase reforming (APR)
of ethanol: (a) PdZn
d) PdZn /CNT-1 before APR; (e) PdZn
after APR, and the zoom of patterns a–c at 40–45° (insert).
1
/CB; (b) PdZn1.6/CB; (c) PdZn
1
/CB after APR;
phase. We found that the PdZn
inside the CNT pores in this PdZn
particles were also located
/CNT catalyst (Fig. 4e
β
(
1
1
/CNT before APR; (f) PdZn
1
/CNT
1
and S5, ESI†). The estimated dispersion based on CO oxida-
tion reactivity suggests a particle size of 25 nm which is
much greater than the TEM estimate. We attribute the
lower dispersion due to the presence of PdZn particles inside
the pores, and the large size of the particles may lead to
the XRD patterns provide complementary information on the
metal phase in these catalysts. The monometallic Pd catalyst
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Fig. 3 Representative TEM/STEM images of Pd/CB catalysts with different Pd particle sizes: (a) and (b) Pd/CB; (c) Pd/CB–H (pretreated at 500 °C
for 12 h in a flow of 7% H /N mixture); (d) particle size distribution diagrams.
2
2
some pore blocking leading to poor accessibility during CO
oxidation. Since the particle sizes are much larger for this
CNT supported catalyst than the catalysts on the CB support,
we also prepared a catalyst with 1 wt.% Pd loading. The
shown in Fig. 5. The XRD pattern of the spent Pd/CB
(Fig. 1b) showed that Pd peak was sharper than that of Pd/CB
before APR reaction, indicating some particle growth after
the APR reaction. The spent monometallic Pd/CB catalyst
shows Pd particles with average size of 3.1 nm (Fig. S6, ESI†),
which is only slightly higher than the size of the catalyst
before reaction (2.51 nm, Fig. 3a and b). The number average
does not capture fully the difference seen by XRD, since the
major difference is the appearance of some larger particles.
For the bimetallic particles the reaction caused very little
change in the particle size distribution and only the slightest
sharpening of the XRD peaks (see Fig. 2). This suggests that
1
STEM image of this PdZn /CNT-1 catalyst shows highly-
dispersed bimetallic PdZnβ particles (Zn : Pd = 1) with an
average size of 2.1 nm (Fig. 4f, insert). The particle size distri-
bution (Fig. 4f, insert) for this catalyst is similar to that of
the carbon black supported catalyst shown in Fig. 4a and c
and is also consistent with the dispersion shown in Table 1.
3
.1.2. Catalyst morphology after the aqueous-phase
reforming reaction. The phase and crystallinity of the spent
Pd and bimetallic PdZn catalysts after ethanol APR were
investigated via XRD as shown in Fig. 1 and 2. The micro-
structure of the spent catalysts after APR was also investi-
gated by electron microscopy and representative images are
the intermetallic PdZn
hydrothermally stable. The spent PdZn
β
phase and the carbon supports, are
/CB bimetallic cata-
1
lysts shows PdZn particles with an average size of 3.0 nm
β
after APR reaction (Fig. 5a), which is also slightly larger than
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Fig. 4 Representative TEM/STEM images of bimetallic PdZn catalysts supported on carbon black and carbon nanotubes: (a) and (b) PdZn
1
/CB;
(
(
c) PdZn1.6/CB; (d) PdZn
f) STEM image and particle size distribution of PdZn
1
/CNT; (e) STEM image of PdZn
/CNT-1 with 1 wt.% Pd loading.
1
/CNT showing a significant fraction of the PdZn particles located inside the CNT pores;
β
1
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Fig. 5 Representative STEM images and particle size histograms of spent Pd and PdZn catalysts after ethanol aqueous-phase reforming:
a) PdZn /CB; (b) PdZn1.6/CB; (c) PdZn /CNT-1; (d) PdZn /CNT.
(
1
1
1
that on PdZn /CB before reaction. We did not observe signifi-
cant change for the particle size of the spent PdZn1.6/CB,
PdZn /CNT-1 and PdZn /CNT (Fig. 5b–d), as compared to that
1 1
of the catalysts before reaction. Moreover, the STEM-EDX
results did not show significant change for the Pd/Zn ratios
on the bimetallic catalysts after APR reaction, compared to
the catalysts before APR reaction (see Fig. S7 in ESI†).
form simultaneously. Because of the low activity, we present
data for reactions performed at 250 °C. For both monometallic
Pd and bimetallic PdZn catalysts, we did not observe any for-
mation of CO in the effluent gas and the main reaction prod-
1
ucts were H
2 4 2
, CH , CO , acetaldehyde, acetic acid, methanol,
C
2
hydrocarbons and acetone. The mass balances for all the
components have been calculated and the experimental errors
are less than ±5% (see discussion below and Fig. S8, ESI†).
Although ZnO has been reported to be active in vapor
ethanol steaming reforming, the temperatures used were high
3
.2. Aqueous-phase reforming of ethanol
.2.1. Catalytic performance and reaction pathway. The
3
2
1
aqueous-phase reforming (APR) of ethanol on Pd and bime-
tallic PdZn catalysts was carried out at 200–260 °C and 65 bar.
It was found that for both monometallic Pd and bimetallic
PdZn catalysts, the ethanol conversion was very low and only
(≥350 °C). We tested the supports used (CB, CNT, ZnO/CB
and ZnO/CNT) and found them to show negligible activity in
ethanol APR at the temperatures studied (200–260 °C). The
results of APR of ethanol for the supported catalysts at 250 °C
are shown in Table 2.
2
small amounts of H , acetaldehyde and acetone were produced
when the reaction was performed at 200 °C. With the increase
of reaction temperature to 220 °C, CH and CO were found to
Effect of Zn addition to Pd. The Pd/CB catalyst showed a
CH
4
selectivity of 55.6% and CO
2
selectivity of 32.2% with
4
2
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very low H2 yield (<0.4 mol per mole ethanol reacted).
Pd/CB. The ethanol APR results for these two catalysts are
comparable indicating that the effect of particle size on the
monometallic catalyst is not significant. But on the bimetallic
catalysts, we see a significant particle size effect. The catalyst
Addition of Zn and the formation of the PdZn phase led to
β
4 2 2
decreased CH selectivity, increased CO selectivity and the H
yield increased up to 1.9 moles per mole of ethanol consumed.
The bimetallic PdZn/CB catalysts showed higher amounts of
oxygenated products in the liquid phase, especially acetic
acid. The results indicate that the PdZnβ phase is able to
dehydrogenate ethanol while the monometallic Pd leads to the
decomposition of ethanol. The high CH content is also a
4
result of the hydrogenation of CO or CO2 to CH4 on Pd,
which does not occur on the PdZn catalysts.
Effect of support. We studied two different morphologies
of carbon support: the carbon black contains graphitized
spherical particles while the carbon nanotubes have well
defined cylindrical pores with pore sizes of 5–15 nm. The
with the large PdZn particles exhibited a significantly higher
β
specific reactivity than smaller PdZn . Our TEM results revealed
β
that all the PdZn particles exhibit a spherical morphology
β
with well-defined facets visible only on the larger particles.
The larger particles are located in the pores of the carbon
and could lead to pore blocking and errors in counting sites.
However, since we find that the total reactivity per gram cata-
lyst (and % conversion) is much higher on the catalyst with
larger particles. Therefore the larger particles of PdZn must
be more reactive. A similar particle size effect has been
2
3
suggested previously for methanol steam reforming.
bimetallic PdZn particles supported on CNTs show higher
Reaction pathways. The observed product distributions
β
conversion and higher specific reactivity than the particles on
the CB support. We ascribe the higher specific reactivity for
bimetallic PdZn/CNT catalysts to a confinement effect since
significant fraction of the particles are located within the
suggest two limiting behaviors, that of monometallic Pd
which provides very low yield of H
2 2
(~0.2 mole H per mole
of ethanol reacted) and CH /CO ~1.5 and bimetallic PdZn
4
2
which leads to higher yields of H
ethanol reacted) and CH /CO
exhibit intermediate behavior (~1 mol H per mole ethanol
2 2
(~2 mole H per mole
2
2
CNT pores (Fig. 4e and S5, ESI†). It is known that carbon
black contains small amounts of S, however the small
amounts of S in the CB did not influence the catalytic
4
2
≤1. Some of the catalysts
2
reacted). Based on our observed product distributions, we
propose the following series of steps during aqueous-phase
reforming of ethanol over Pd and PdZn bimetallic catalysts
(Scheme 1).
1
4
reactivity for the dehydration of n-butanol in previous work.
Hence we conclude that the higher reactivity of CNT supports
observed in this study may be related to the pore confine-
ment effects. We also see a particle size effect as explained in
the next section.
The bimetallic PdZn catalysts show the maximum observed
yield of 2 moles of H per mole of ethanol converted. The over-
2
It is also noteworthy that the CH
4
/CO
2
ratio for the
2
all reaction stoichiometry to get this yield of H is shown in
PdZn/CNT is <1, which is lower than that for those on the CB
eqn (1). Mechanistically, the first step is the dehydrogenation
of ethanol to acetaldehyde to yield the first mole of H2.
Decomposition of the acetaldehyde leads to formation of CO
supports (Table 2). An explanation is that the acetone pro-
4
2
duced can be reformed to produce CO during the ESR.
This is consistent with the lower acetone selectivity for the
PdZn/CNT catalyst, as seen in Table 2. Another explanation
and CH
with water due to the high water gas shift activity of these
catalysts, yielding CO and the second mole of H . On PdZn
catalysts we see significant amounts of oxygenated products
in the liquid phase, hence we infer that the acetaldehyde can
4 4
yielding a 1 : 1 ratio of CH and CO. The CO reacts
for the lower CH
intermediate acetaldehyde on the PdZn/CNT catalysts.
4
/CO
2
is due to the decarboxylation of surface
2
2
1
5
Particle size effect. The average particle sizes of Pd and
bimetallic PdZn catalysts obtained from CO oxidation, XRD
and TEM are summarized in Table 1. It can be seen that
a good agreement was found for the average particle sizes
obtained from different techniques, which allowed us to
discuss the particle size effect in APR reaction. Based on
the data, the Pd/CB–H catalyst has larger Pd particles than
also dehydrogenate to acetic acid, leading to additional H
2
formation. However, none of these steps would explain the
observed CO /CH ratio being greater than 1, which is only
2
4
1
5
possible if a decarbonylation step is also involved. Hence,
we indicate the path leading to acetone formation, which
then allows us to account for all of the products observed
Table 2 Product distribution during aqueous-phase reforming of ethanol for Pd and PdZn catalysts pretreated in H a
2
Product distribution (mol%)
OH converted) CH CO Alkanes CH
TOF
X (%) (min
H
2
production
−1
−1
Sample
)
(mol mol
C
2
H
5
4
2
3
3 3 3 3
CHO CH COOH CH OH CH COCH
Pd/CB
Pd/CB–H
16.9
15.5
10.8
8.8
1.24
0.23
0.37
1.1
1.7
1.9
55.6 32.2 2.9
54.8 35.1 1.9
33.3 31.3 3.98
40.6 35.5 1.02
23.8 34.6 4.32
2.9
3.2
1.5
6.6
0.22
0.1
0.58
0.8
22.9
15.2
36.8
28.5
2.9
1.0
1.0
0.5
0.11
0
2.9
3.5
5.97
0.51
0.22
0.1
2.67
0.72
0.72
23.3
2.45
1
PdZn /CB
PdZn1.6/CB
PdZn
PdZn
1
/CNT
29.5
1
/CNT-1 14.7
1.0
29.6 37
4.7
a
−1
Reaction conditions: 250 °C, 65 bar, ethanol/water = 10%, pump rate = 0.003 mL min , catalytic performance data were collected at steady
state (after 24 h).
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Scheme 1 Proposed APR mechanism of ethanol on monometallic Pd and bimetallic PdZn catalysts.
during the APR of ethanol. Other pathways leading to the
minor products such as ethane and methanol are also shown
in Scheme 1. The overall C and H balance is consistent with
this mechanistic picture (Fig. S8, ESI†).
When excess Zn was present as in the PdZn1.6/CB catalyst we
see the expected behavior of the PdZn phase. Another cata-
lyst that shows intermediate behavior is the lower weight
loading PdZn/CNT-1 catalyst which has very small particles.
β
In contrast to the bimetallic catalyst, the monometallic
We also saw previously that very small particles of PdZn
were not selective to CO formation during methanol steam
reforming which might be related to altered electronic properties.
β
Pd/CB shows a significantly higher CH
4
/CO
2
ratio and a very
2
23
low yield of H per mole ethanol. This indicates that the H
2
2
is being consumed to produce CH
revealed in eqn (2) and (3). The hydrogenation of CO
has been suggested to occur during ethanol aqueous-phase
reforming on a Ru/C catalyst where the resulting CH /CO
ratio and the low
4
on the Pd catalyst, as
The ordered intermetallic tetragonal PdZn has very different
local density of states near the Fermi level, resembling Cu
rather than Pd. But the fcc PdZn alloy shows very different
selectivity. In summary, large, crystalline particles of the
2
to CH
4
4
2
1
5
ratio was 3 : 1. The observed CH
4
/CO
2
PdZn
β
phase yield the highest specific reactivity and selectivity
yield of H on the monometallic Pd can be accounted by a
for APR of ethanol.
2
combination of eqn (1) and (3).
4
. Conclusions
CH CH OH + H O → CO + 2H + CH
(1)
(2)
(3)
3
2
2
2
2
4
The performance of monometallic Pd and bimetallic PdZn
catalysts supported on both carbon black (CB) and carbon
nanotubes (CNTs) was evaluated for H production during
aqueous-phase reforming (APR) of ethanol at low tempera-
ture (250 °C). All of the Pd and bimetallic PdZn catalysts
showed high water-gas shift activity since no CO was detected
in the products. The CB and CNTs were found to be hydro-
thermally stable under the liquid phase hydrothermal condi-
tions used for these experiments (250 °C and 65 bar). The
Hydrogenation: CO + 3H
CO + 4H → CH
2
→ CH
4
+ H
2
O
2
2
2
4
+ 2H
2
O
The intermediate behavior (between that of monometallic
Pd and bimetallic PdZn ) of some of the catalysts (yield of
1 mol H per mole ethanol reacted) can result from altered
β
~
2
selectivity due to depletion of the Zn from the near surface
region of the PdZn nanoparticles, as seen during methanol
PdZn particles were also stable, with minimal growth in crys-
tallite size. The TOF for ethanol steam reforming and the
product H was affected by the PdZn particle size, with the
β
24
steam reforming. We attribute the intermediate H
2
yield to the
inability to form the ordered intermetallic tetragonal L10 phase,
2
β
but rather Zn alloyed randomly within fcc Pd (the α phase).
most effective catalysts being those where the nanoparticles
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were located in the pores of the carbon nanotubes. It is possi-
ble that pore-confinement within the CNT plays a role since a
8 G. W. Huber and J. A. Dumesic, Catal. Today, 2006, 111,
119–132.
large fraction of the large PdZn
the pores of the nanotubes. Among the catalysts studied, we
β
particles were located within
9 H. N. Pham, A. E. Anderson, R. L. Johnson, K. Schmidt-Rohr
and A. K. Datye, Angew. Chem., Int. Ed., 2012, 51,
13163–13167.
conclude that PdZn in carbon nanotubes provide an excel-
β
lent catalyst for the aqueous phase reforming of ethanol,
10 B. Roy, K. Artyushkova, H. N. Pham, L. Li, A. K. Datye and
C. A. Leclerc, Int. J. Hydrogen Energy, 2012, 37, 18815–18826.
11 R. M. Ravenelle, J. R. Copeland, W.-G. Kim, J. C. Crittenden
and C. Sievers, ACS Catal., 2011, 1, 552–561.
yielding CO-free H at low temperatures (250 °C) with a yield
2
of 2 moles of H2 per mole of ethanol reacted. However,
the selectivity towards H is considerably lower than the theo-
2
retical value of 6 H per mole ethanol converted. This is a
12 A. L. Jongerius, J. R. Copeland, G. S. Foo, J. P. Hofmann,
P. C. A. Bruijnincx, C. Sievers and B. M. Weckhuysen, ACS Catal.,
2013, 3, 464–473.
2
limitation of the low temperature route which does not allow
for complete oxidation of both carbon atoms in the ethanol
molecule and results in the formation of oxygenated products.
13 H. Xiong, H. N. Pham and A. K. Datye, J. Catal., 2013, 302,
9
3–100.
1
1
1
4 H. Xiong, M. Nolan, B. H. Shanks and A. K. Datye, Appl.
Catal., A, 2014, 471, 165–174.
5 S. Rabe, M. Nachtegaal, T. Ulrich and F. Vogel, Angew.
Chem., Int. Ed., 2010, 49, 6434–6437.
6 P. Ferrin, D. Simonetti, S. Kandoi, E. Kunkes, J. A. Dumesic,
J. K. Nørskov and M. Mavrikakis, J. Am. Chem. Soc.,
Acknowledgements
This work is supported by DOE grant DE-FG02-05ER15712
and the Center for Biorenewable Chemicals (CBiRC) supported
by NSF under no. EEC-0813570. We thank Eric Peterson for
help with XRD, Jay McCabe for CO oxidation reactivity and
2
009, 131, 5809–5815.
2
Dr. H. Pham for assistance with N physisorption measurements.
1
7 B. Halevi, E. J. Peterson, A. Roy, A. DeLariva, E. Jeroro,
F. Gao, Y. Wang, J. M. Vohs, B. Kiefer, E. Kunkes,
M. Hävecker, M. Behrens, R. Schlögl and A. K. Datye,
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