The effect of ZnO addition on H2O activation over Co/ZrO2 catalysts

Article abstract of DOI:10.1016/j.cattod.2015.10.016

The effect of ZnO addition on the dissociation of H₂O and subsequent effects on cobalt oxidation state and ethanol reaction pathway were investigated over Co/ZrO₂ catalyst during ethanol steam reforming (ESR). Catalyst physical prope

Full text of DOI:10.1016/j.cattod.2015.10.016

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The effect of ZnO addition on H O activation over Co/ZrO catalysts
2
2
a
a,∗
a,b,∗
Stephen D. Davidson , Junming Sun , Yong Wang
a
The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99163, United States
Instiute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99354, United States
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The effect of ZnO addition on the dissociation of H O and subsequent effects on cobalt oxidation state
2
Received 1 August 2015
Received in revised form 1 October 2015
Accepted 12 October 2015
Available online xxx
and ethanol reaction pathway were investigated over Co/ZrO2 catalyst during ethanol steam reforming
(ESR). Catalyst physical properties were characterized by BET, XRD, and TEM. To characterize the catalysts
ability to dissociate H2O, Raman spectroscopy, H2O-TPO, and pulsed H2O oxidation coupled with H2-TPR
were used. It was found that the addition of ZnO to cobalt supported on ZrO2 decreased the activity for
H2O dissociation, leading to a lower degree of cobalt oxidation. The decreased H2O dissociation was also
found to affect the reaction pathway, evidenced by a shift in liquid product selectivity away from acetone
and towards acetaldehyde.
Keywords:
Cobalt
Ethanol reforming
ZnO
©
2015 Elsevier B.V. All rights reserved.
Water activation
Oxidation state
0
1
. Introduction
dehydrogenation reaction, Co is mainly responsible for C–C cleav-
age during ESR [12,13]. These model studies have been further
supported by recent work utilizing operando XAFS analysis to study
the oxidation state of cobalt under realistic reaction conditions [7].
Steam reforming is a well-established industrial process for
hydrogen production, currently ∼49% of world hydrogen pro-
duction is from steam reforming of methane [1]. Due to the
increasingly stringent limitations on CO2 emissions and increasing
energy demands, steam reforming of renewable feed stocks such
as methanol and ethanol has received extensive attention [2–5].
A wide range of catalysts have been investigated for ethanol
steam reforming (ESR) [2,4,5]. Among those studied, supported
cobalt catalysts have emerged as being particularly promising.
Noble metal catalysts have shown the highest activity for ethanol
conversion, however, they also have shown a high selectivity to
CH at lower temperatures (e.g., below 500 C) [6]. Ni and Co based
catalysts have comparable activity for C–C cleavage, but Ni has
demonstrated a higher selectivity to CH4 than Co [4].
From studies of Co-based catalysts, it has been found that both
cobalt oxidation state [7–14] and the reaction network [15–17] are
significantly influenced by either the reaction conditions [7,14,18]
or catalyst support [5,8,9,11,15–17,19–21], which eventually led
to different performances of the catalysts during ESR [5,22]. Over
model Co catalysts, it was found that while Co2+ is active for ethanol
2
+
0
Our recent work also revealed that both the ratio of Co /Co [10,11]
and reaction network [23,24] affected the selectivity to CH . For the
4
former case, higher Co2+/Co ratio was found to favor CO methana-
0
tion toward CH4 [10]. For the latter case, acetone was found to be a
key intermediate in ESR on Co/ZrO [16] and acetone steam reform-
2
ing resulted in the low CH4 selectivity (≤5%) [17,25]. Over Co/CNF
where acetaldehyde was identified as the only intermediate, a high
CH4 selectivity was observed (∼10%) [17].
The general trends of catalyst support material are well known
in regards to surface acidity and reducibility of the support
[2,15,19]. Acidic supports tend to dehydrate ethanol to form ethyl-
ene while basic supports favor not only dehydrogenation of ethanol
to acetaldehyde intermediate [2,15,26,27], but also further conden-
sation of acetaldehyde to acetone secondary intermediate [16,22].
◦
4
Reducible supports with high oxygen mobility (e.g., CeO ) tend to
give more stable performance in ESR, due to enhanced gasification
of CH * species, which limits both the quantity and stability of coke
deposited [28–30]. More recently, it was found that the oxidation
2
x
state of cobalt should be associated with the support chemistry
[
8,9,11]. Particularly, the addition of ZnO to the ZrO2 was found to
inhibit the metallic cobalt oxidation via water [11].
∗ Corresponding authors at: The Gene and Linda Voiland School of Chemical
Engineering and Bioengineering, Washington State University, Pullman, WA 99163,
United States. Tel.: +1 5093716273.
It is apparent that water must play a key role in oxidizing the
active metal and carbon species as part of the redox cycle to min-
imize the deposition of coke on the catalyst. Unfortunately, there
are relatively few studies correlating the dissociation of water with
E-mail addresses: junming.sun@wsu.edu (J. Sun), yong.wang@pnnl.gov
(
Y. Wang).
http://dx.doi.org/10.1016/j.cattod.2015.10.016
920-5861/© 2015 Elsevier B.V. All rights reserved.
0
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the catalytic performances in ESR. In this work, we investigated
the impact that water dissociation has on both the cobalt oxidation
state and the ESR reaction network, particularly the selectivity to
acetaldehyde and acetone, and subsequent C1 products.
Pulsed water oxidation and water TPO (H O-TPO) experiments
2
were performed on a Micromeritics AutoChem 2920. 50 mg of cata-
lyst was loaded into a U-shaped quartz tube, sandwiched between
two pieces of quartz wool. Outlet gases were monitored by both
thermal-conductivity detector (TCD) and a Pfeiffer ThermoStar
quadrupole mass-spectrometer (QMS). For pulsed water oxidation,
◦
◦
2
. Experimental methods
the catalyst was first reduced at 450 C (5 C/min ramp rate) for 2 h
◦
under 10%H /Ar (50 SCCM), and then purged at 450 C under He
2
2
.1. Catalyst synthesis
(50 SCCM) for 30 min. ∼10%H2O/He was generated by bubbling He
◦
(
50 SCCM) through DI H O in a flask heated to 55 C followed by
2
◦
◦
All catalysts were prepared via incipient wetness impregnation.
a reflux at 45 C. H2O was then pulsed over the catalyst at 450 C,
and pulses were repeated until the catalyst surface was saturated
monitored by TCD. QMS was used to monitor the effluent gases, par-
ticularly hydrogen. Following water pulses the catalyst was again
The ZrO2 support was synthesized by calcining Zr(OH) (MEL Cat
XZO1247/01, 97+% purity) at 400 C (3 C/min) for 3 h, then ramping
to 500 C at 5 C/min and holding for another 5 h. ZnO support was
4
◦
◦
◦
◦
◦
made by decomposing Zn(CH COO) ·2H O (Alfa Aesar 11559, 98+%
purged under He (50 SCCM) at 450 C for 30 min, then cooled to
35 C. To confirm degree of oxidation, H2-TPR was run and H2
3
2
2
◦
◦
◦
purity) at 500 C (10 C/min) for 3 h; prior to the decomposition, the
◦
◦
Zn(CH COO) ·2H O precursor was dried at 125 C (10 C/min) for
uptake was quantified. Percentage oxidized by both H2O pulses and
H2-TPR were calculated by taking the ratio of Co loaded and either
H2O taken up or H2 consumed respectively. For H2O-TPO, the cat-
3
2
2
2
9
h. Metal precursors used were Co(NO ) ·6H O (Aldrich 239,267,
3
2
2
8+% purity) and Zn(NO ) ·6H O (Aldrich 228,737, 98+% purity).
3
2
2
◦
◦
◦
Before impregnation, the ZrO2 or ZnO was pre-dried at 80 C for
24 h to remove the majority of adsorbed water. A calculated
alyst was first reduced at 450 C (5 C/min ramp rate) for 2 h under
◦
∼
10%H2/Ar (50 SCCM). The catalyst was then cooled to 35 C under
amount of metal precursor was dissolved in a given amount of
He (50 SCCM). Next, the gas flow was switched to ∼10%H2O/He
◦
◦
deionized (DI) H O. The total volume of the metal precursor solu-
(50 SCCM) and the temperature was ramped to 500 C at 10 C/min.
2
tion was set to reach the incipient wetness of the support surface.
After the impregnation, the obtained wet powders were dried at
◦
ambient temperature for ∼24 h and then at 80 C for 12 h and then
2.3. Catalyst activity evaluation
◦
at 120 C for 12 h, after that the temperature was then ramped to
50 C at 10 C/min for 5 h to complete the calcination. For ZnO
doped catalysts cobalt and zinc precursors were dissolved into a
single solution and the same process was followed.
◦
◦
5
ESR activity tests were performed in a fixed-bed, single-pass
quartz tube reactor (I.D., 6.5 mm). ESR reactions were carried out
◦
at 450 C under atmospheric pressure. 50–100 mg of the cata-
Nomenclature used in this paper is as follows, 10 wt.% Co sup-
ported on ZrO will be written as 10Co/ZrO , 10 wt.% Co with 1 wt.%
lyst (60–100 mesh) diluted 9× in SiC (70 mesh) was loaded and
2
2
held in place using quartz wool. Prior to the reaction, the cat-
◦
◦
Zn supported on ZrO will be written as 10Co1Zn/ZrO , and 10 wt.%
alyst was reduced in situ at 450 C (5 C/min ramp rate) for 2 h
2
2
Co supported on ZnO will be written as 10Co/ZnO.
under 10%H /Ar (100 SCCM), followed by a N2 purging (20 SCCM)
2
for 60 min. An aqueous solution of H O and ethanol, 10:1 molar
2
◦
ratio, was fed to the evaporator (175 C) using a Cole Parmer
2.2. Catalyst characterization
syringe pump. The vaporized H O/EtOH mixture was then car-
2
ried to the reactor by flowing N . The ethanol partial pressure
2
Nitrogen physisorption experiments were performed on a
(PEtOH) in the reactant gas was 7.4 kPa and WHSV was varied
◦
−1
−1
−1
−1
Micromeritics 3020 TriStar II physisorption analyzer at −196 C.
from 9.3 gEtOH
g
h
to 18.5 gEtOH
g
h
to control the
Catalyst
Catalyst
◦
The fresh catalysts were degassed under vacuum at 300 C for 1 h
◦
ethanol conversion. A downstream cold trap (−5 C) was used to
prior to measurement. Catalyst surface area was calculated using
the Brunauer–Emmett–Teller (BET) model.
capture the condensable components (e.g., unreacted H O/EtOH,
2
acetaldehyde) in the effluent gas stream.
XRD patterns were collected using
a Rigaku Mini-Flex
The dry gases (e.g., N , H , COx, CH ) were analyzed with an
2
2
4
X-ray powder diffractometer with a Cu-K␣ (incident wave-
on-line Agilent CP490 micro gas chromatograph equipped with
length = 0.15406 nm) radiation source at 40 kV and 15 mA.
˚
four parallel columns (5 A molecular sieve, PPQ, Al O and SiO2)
2
3
◦
Diffraction patterns were collected from 20 to 80 (2ꢀ) using contin-
and thermal conductivity detectors (TCD) for quantifications. N2
was used as an internal standard. Liquid products collected by the
cold trap were analyzed by an Agilent 7890A gas chromatograph
with a 30 m DB-FFAP column and flame ion detector (FID), ace-
◦
◦
uous accumulation with 0.01 step size at a rate of 1.5 2ꢀ/min. For
reduced sample measurements diffraction patterns were collected
◦
◦
from 40 to 55 (2ꢀ) using continuous accumulation with 0.01
◦
step size at a rate of 0.25 2ꢀ/min. Crystallite sizes were calculated
tonitrile was used as an internal standard. H -yield was calculated
2
from XRD patterns using the Scherer equation. Preparation for the
using equation (1).
◦
◦
reduced samples consisted of reduction at 450 C (5 C/min) under
0% H /N (50 SCCM) for 2 h, followed by passivation at ambient
ꢀ
ꢁ
1
2
2
H molar flow
6 · EtOH molar feed rate)
2
◦
H yield =
· 100
(1)
(2)
(3)
temperature (25 C) under 0.1% O /N (50 SCCM) for 2 h.
2
2
2
(
TEM images were collected on a Philips CM-200 equipped with
a LaB6 filament. An acceleration voltage of 200 kV was used during
imaging. Catalysts were dispersed in ethanol using an ultra-sonic
bath, the suspension was then loaded onto a copper grid coated
with ultrathin lacy carbon for TEM imaging.
For particle size analysis, spherical particles were assumed and
ImageJ software was used to calculate the diameter of the parti-
cles [31]. For statistical analysis over 100 particles were measured
for each sample. From analysis using Origin software, the popula-
tion distributions were determined to be log-normal, based on this
mean and variance were calculated accordingly.
C1 product yield was calculated using equation (2)
ꢀ
ꢁ
(
CO + CO + CH molar flows)
2
4
C1 yield =
· 100
(
2 · EtOH molar feed rate)
Ethanol conversion was calculated using equation (3)
ꢀ
ꢁ
(
EtOH in product liquid)
EtOH in feed liquid)
EtOH conversion =
· 100
(
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Table 1
Physical properties of catalyst.
2
Oxide crystallite sizea [nm]
Oxide particle sizeb [nm]
Metal crystallite sizea [nm]
Sample
BET surface area [m /g]
ZrO2
ZnO
51
13
41
35
9
–
–
18
14
19
–
–
18
18
26
1
1
1
0Co/ZrO2
0Co1Zn/ZrO2
0Co/ZnO
8
8
15
a
Calculated from XRD using Scherrer equation.
Estimated from TEM using log-normal distribution.
b
Product selectivity was calculated using equation (4), integra-
tion is over the run time of the experiment to get total C of the
product over the entire experiment.
ꢂ
ꢃ
ꢅ
(
molar C flow of product)
Selectivity =
ꢄ
· 100
(4)
(
all C in products over experiment)
2
.4. In situ Raman characterization of cobalt oxidation by H O
2
In situ Raman spectra characterizing cobalt oxidation by H O
2
were collected on a Horiba LabRAM HR Raman/FTIR microscope
spectrometer, equipped with a 532 nm diode laser source (Ventus
LP 532), a Synapse CCD (Charge Coupled Device), and an in situ cell
◦ ◦
(
Linkam CCR1000). Samples were first reduced at 450 C (5 C/min
ramp rate) for 2 h under 10%H /He (50 SCCM) then cooled to ambi-
2
ent temperature. H O oxidation experiment was performed by
2
bubbling He (10 SCCM) through H O at ambient temperature while
2
the sample was heated to the target temperature and held for
3
0 min. Following the H O pretreatment, the sample was purged in
2
flowing He (50 SCCM) for 30 min and then cooled down to ambient
temperature where Raman spectra were collected.
Fig. 1. XRD patterns for (a) 10Co/ZrO2; (b) 10Co1Zn/ZrO2; (c) 10Co/ZnO. Peak icons:
␣ = Co O ; ␥ = ZrO ; ␨ = ZnO.
3
4
2
3
3
3
. Results and discussion
was used to estimate the cobalt peak area due to the strong overlap
with ZrO2.
.1. Catalyst characterization
TEM data, shown in Fig. 2, indicated that the particle size distri-
bution for all catalysts was log-normal, also summarized in Table 1.
Over 10Co/ZrO , the average Co O particle size was found to be
.1.1. Effect of ZnO on physical properties
The results of physical properties are shown in Table 1. The ZrO
2
3
4
2
1
8.2 nm. The addition of Zn (10Co1Zn/ZrO ) did not significantly
2
2
support was found to have a surface area of ∼51 m /g and the ZnO
change the average particle size, 17.2 nm. It was observed that the
addition of Zn did help to minimize the frequency of occurrence of
2
support to have a surface area of ∼13 m /g. For both ZrO and ZnO,
2
the addition of 10 wt.% Co (10Co/ZrO and 10Co/ZnO) decreased
2
larger particles (Fig. 2B). Over 10Co/ZnO, the average Co O4 parti-
2
2
3
the surface area to 41 m /g and 9 m /g respectively. 10Co1Zn/ZrO
showed a further decrease of surface area to 35 m /g, possibly due
to a strong interaction between the ZnO and the ZrO2.
2
cle size was found to be 26.1 nm. While it is not typical to identify
the average particle size as being larger with TEM than with XRD,
there are two interpretations for the observation in this work. (1),
2
Fig. 1 shows the XRD of the supported cobalt catalysts. Over
The contrast between the Co O4 and both ZrO2 and ZnO is not
3
1
2
0Co/ZrO2 (Fig. 1a), and 10Co1Zn/ZrO2 (Fig. 1b), strong peaks at
4.3, 28.3, 31.6, 34.4, 35.4, 41.0, 49.3, and 50.3, with minor peaks
very well pronounced, as they have similar diffraction patterns and
atomic densities. Only the large particles are differentiated from
the support material by TEM. (2), XRD provides information about
the crystallite size, not directly the particle itself; the big particles
observed by TEM could be secondary particle of aggregated smaller
Co O crystallites.
at 38.7, 45.0, 45.4, 54.2, 55.6, 57.4, 58.0, 60.2, 61.8, 62.9, 65.7, 71.3,
and 75.3 corresponding to typical peaks of the tetragonal phase
of ZrO2 (JCDPDS PDF card 00-036-0420). In addition, one strong
Co O peak at 36.9 can be clearly resolved. Both 10Co/ZrO2 and
3
4
3
4
1
0Co1Zn/ZrO showed shifts in the 45.4 and 60.2 peaks, most likely
2
due to overlap with the Co O4 peaks at 44.8 and 59.2 respectively.
3
3
.1.2. Effect of ZnO on H O dissociation
These observations suggest the formation of Co O species. Over
2
3
4
To study the ability of the catalyst to dissociate H O, Raman
Co/ZnO, the strong peaks at 31.8, 34.5, 36.3, 47.6, 56.6, 62.9, and
8.0, as well as minor peaks at 66.4, 69.1, 72.6, and 77.0, suggest
the Wurtzite structure of the ZnO support (JCPDS PDF card 00-036-
451). Additional peaks at 31.2, 38.4, 44.8, 55.6, 59.2, and 65.2, along
2
0
spectroscopy of H O oxidation of Co , pulsed H O oxidation with
TPR, and H O-TPO were used. A summary of the results is shown
in Table 2.
6
2
2
2
1
with the formation of a shoulder at 36.6 reveal the formation of
Co O species (JCPDS PDF card 00-042-1467). Co O4 peak at 36.3
3.1.2.1. Operando Raman study of cobalt oxidation by H O. Raman
3
4
3
2
was used for 10Co/ZrO2 and 10Co1Zn/ZrO2 crystallite calculations,
while Co O peak at 44.8 was used for 10Co/ZnO to accommodate
spectra for the oxidation of Co over 10Co/ZrO2 and 10Co1Zn/ZrO2
are shown in Fig. 3A and B. Fresh catalysts for both 10Co/ZrO2
3
4
for peak overlaps. For the reduced samples (supporting information
Fig. S1) the metallic cobalt peak at 44.2 was used to estimate the
crystallite size of the cobalt particles. Peak deconvolution software
and 10Co1Zn/ZrO2 showed features assigned to ZrO support
2
−
1
at 175/184, 325, 366, 468, 515, and 612 cm
and cobalt oxide
−
1
species at 485 and 674 cm [32,33]. The strong ZrO2 doublet at
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Fig. 2. TEM images and particle size distributions from TEM images for A) 10Co/ZrO2; B) 10Co1Zn/ZrO2; C) 10Co/ZnO. For each analysis, at least 100 particles were measured.
Yellow circles were added to help identify Co particles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the
article.)
Table 2
Comparison of H2O dissociation over different supports.
Sample
Raman spectra
H2O pulse oxidation
H2O-pulsea [% Ox.]
H2O-TPO
◦
H2-TPRb [% Ox.]
◦
Ox. Onset [ C]
Ox. peak [ C]
1
1
1
0Co/ZrO2
0Co1Zn/ZrO2
0Co/ZnO
30
30
50
12.8
12.1
6.4
16.7
9.4
9.2
49
49
51
a
Calculated from H2O uptake from m/z = 18.
Calculated from calibrated TCD signal.
b
−
1
−1
◦
1
84 and 175 cm
is most likely masking the 195 cm
peak of
following 2 h at 450 C, some surface Co remained in the oxi-
dation state, possibly due to strong interaction with the ZrO2
support, consistent with our previous studies [16]. In addition,
features ascribed to tetragonal ZrO2 appeared, evidenced by the
cobalt oxides. The ZrO2 support, 10Co/ZrO , and 10Co1Zn/ZrO2
all displayed Raman bands typical of monoclinic ZrO2 in the
fresh catalyst evidenced by the typical peaks at 175/184 cm and
peaks at 325, 366, 468, 515, and 612 cm
2
−
1
−1
−1
−1
[23,34]. This is con-
peaks at 147 and 249 cm , the leading edge of the 325 cm
−
1
−1
trast to the XRD pattern shown in Fig. 1, where the tetragonal
ZrO2 was observed. Kim et al. found that because the monoclinic
ZrO2 has much stronger Raman activity that Raman spectroscopy
can provide a better metric for measuring the ratio of mono-
clinic/tetragonal ZrO2 when the monoclinic fraction is small [34].
Additionally, from Li’s study of the phase transformations of ZrO2
it appears that monoclinic ZrO2 will form on the surface first,
while the bulk of ZrO2 remains in the tetragonal phase [23].
Based on this literature, Raman spectroscopy identifying mono-
clinic ZrO2 on the surface and XRD identifying tetragonal ZrO2 in
the bulk is not surprising. Following reduction, the cobalt oxide
peak, and the shift up of the 612 cm peak to 622 cm [23,34].
It has been observed that under reducing conditions the tetragonal
phase of ZrO2 can be stabilized despite the monoclinic phase typ-
ically being more stable [35]. Following water exposure at 30 C,
the cobalt oxide features at 485 and 674 cm
◦
−
1
were observed
to increase again, suggesting that oxidation is occurring. Looking
−
1
closely at the cobalt oxide peak intensity at 674 cm and the adja-
cent ZrO2 peak intensity at 612 cm−1, it was found that 10Co/ZrO2
had a ratio of 1.22 and 10Co1Zn/ZrO2 had a ratio of 1.02. This
suggests that more oxidation occurred over 10Co/ZrO2 than over
10Co1Zn/ZrO2. In addition, following H2O exposure the features for
tetragonal ZrO2 again begin to diminish. The decrease of tetrago-
nal ZrO2 is most likely due to the reforming of monoclinic ZrO2,
−1
features at 485 and 674 cm were greatly diminished, however,
they were not completely removed. This suggested that even
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which has been observed to be enhanced in the presence of water
24,36]. As temperature increases (Supporting Fig. S2A and B), the
[
cobalt oxide features continued to grow, suggesting continued oxi-
dation. Similarly, the features ascribed to tetragonal ZrO continued
2
to decrease while those features associated with monoclinic ZrO2
increased.
Over 10Co/ZnO, the features of cobalt oxide were observed on
the fresh sample (Fig. 3C). In addition to the peaks at 482 and
− −1
1
6
74 cm , the peak at 190 cm was also visible. It is of note that
the cobalt oxide peaks were not as well defined over 10Co/ZnO as
they were for either 10Co/ZrO2 or 10Co1Zn/ZrO . This may be due
2
to the strong interaction between ZnO and CoO and the possibility
of a solid oxide solution of ZnO and CoO forming. Peaks assigned
to ZnO were also observed at 95, 316, 430, 519, and a very broad
−
1
peak at 1090 cm [37,38]. Following reduction, the cobalt oxide
features were again diminished but not completely removed. Low
◦
temperature H O exposure at 50 C showed no change in the cobalt
2
oxide features, suggesting that no additional oxidation occurred.
◦
With H O exposure at 150 C growth was observed in the cobalt
2
oxide features, suggesting that oxidation had begun.
To summarize the data from Raman spectroscopy, all catalysts
showed strong Raman peaks for cobalt oxide over the fresh sam-
ple. Following reduction, the intensity of the cobalt oxide peaks
was greatly diminished but not eliminated. Following H O expo-
2
◦
sure at 30 C, only 10Co/ZrO2 and 10Co1Zn/ZrO2 showed growth
of the cobalt oxide features. Following H O exposure above 50 C
◦
2
1
0Co/ZnO showed growth of the cobalt oxide features as well. Of
the catalysts investigated, 10Co/ZnO appeared to have the low-
est activity for H O dissociation leading to cobalt oxidation, again
2
matching well with literature [39,40]. The comparison of 10Co/ZrO2
and 10Co1Zn/ZrO was more challenging as both showed oxidation
2
at the lowest temperature H O exposure investigated. Examining
2
−
1
the ratio of the cobalt oxide peak intensity at 674 cm and the adja-
cent ZrO2 peak intensity at 612 cm−1, it was found that 10Co/ZrO2
had a ratio of 1.22 and 10Co1Zn/ZrO had a ratio of 1.02, suggesting
2
that more oxidation occurred over 10Co/ZrO2.
3
.1.2.2. Pulsed H O oxidation/H -TPR. In order to further confirm
2 2
the results from Raman spectroscopy, pulsed H O oxidation was
2
also used to quantify the activity of the catalysts for H O dis-
2
sociation and oxidation. By pulsing H O over reduced catalysts
2
◦
at reaction temperature (450 C) the uptake of H O and the pro-
2
duction of H2 can be tracked. Fig. 4A shows the peak areas
for H2 (m/z = 2) produced during H O pulsing. 10Co/ZrO2 and
2
1
0Co1Zn/ZrO2 show very similar profiles with a lower initial peak
followed by an increase to the highest peak area for the sec-
ond pulse and peak areas gradually decreasing with subsequent
pulses. 10Co/ZnO showed a slightly different profile with a high
initial peak followed by rapid decrease of H2 produced. From
0
H O uptake, 12.8% of the Co on 10Co/ZrO2 was oxidized and
2
0
1
2.1% of the Co on 10Co1Zn/ZrO2 was oxidized, for 10Co/ZnO
0
only 6.4% of the Co was calculated to be oxidized (Table 2).
Saturation, defined as peak areas being less than 10% different
from the average final peak area, occurred after 11 pulses for
1
1
0Co/ZrO , 10 pulses for 10Co1Zn/ZrO , and only 4 pulses for
0Co/ZnO. To help confirm the extent of oxidation via pulsed
2
2
H O oxidation, H -TPR was run following pulsed H O oxidation
2
2
2
and a purge to remove excess H O (Fig. 4B). TPR for 10Co/ZrO
2
2
◦
showed two primary reduction peaks centered at 288 and 402 C.
10Co/ZrO2 also showed the highest fraction of high tempera-
ture reduction species. 10Co1Zn/ZrO2 also showed two primary
Fig. 3. Plot A – 10Co/ZrO2; Plot B – 10Co1Zn/ZrO2; Plot C – 10Co/ZnO. Raman spectra
◦
for: ␣ – fresh catalyst; ␤ – reduced catalyst; ␹ – H2O exposure at 30 C; ␦ – H2O
◦
◦
◦
exposure at 50 C; ␧ – H2O exposure at 150 C; ␾ – H2O exposure at 250 C; ␬ –
◦
reduction peaks centered at 268 and 400 C. 10Co/ZnO showed mul-
◦
H2O exposure at 350 C. Cobalt oxide peaks highlighted by dashed line. Cobalt oxide
tiple reduction peaks over a wide temperature range. 10Co/ZnO
also showed the highest fraction of low temperature reduction
species. From these profiles it was calculated 16.7%, 9.4%, and 9.2%
of the Co0 had been oxidized in 10Co/ZrO , 10Co1Zn/ZrO , and
peaks highlighted by dashed line.
2
2
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Fig. 5. QMS m/z = 2 signal for H2O-TPO A – 10Co/ZrO2; B – 10Co1Zn/ZrO2; C –
0Co/ZnO.
1
Fig. 4. Plot A – H2 (m/z = 2) peak areas during H2O pulsed oxidation; plot B – TPR
profiles following H2O pulsed oxidation. ␣ – 10Co/ZrO2, reduction peaks marked
by dash line; ␤ – 10Co1ZnO/ZrO2, reduction peaks marked by dash-dot line; ␹ –
Fig. 6. ESR at 450 ◦C, H O:EtOH = 10:1 molar, P
EtOH
= 7.4 kPa, WHSV was varied from
2
9.3 gEtOH g−1
−
1
−1
−1
h
to 18.5 gEtOH
g
h
to control the total ethanol between
Catalyst
Catalyst
40% and 50%. Other products are the cumulative selectivity of ethylene, ethane,
1
0Co/ZnO, reduction peaks marked by dot line.
ethyl acetate, and acetic acid.
1
0Co/ZnO respectively (Table 2). It further confirms that 10Co/ZrO2
some of the more active ZrO2 surface with less active ZnO. These
results also matched well with the following ESR selectivity to
was the most active catalyst for H O dissociation and oxidation,
2
followed by 10Co1Zn/ZrO , and 10Co/ZnO had the lowest activity
acetaldehyde and acetone, as dissociation of H O is critical to the
2
2
for H O dissociation and oxidation.
conversion of acetaldehyde to acetone.
2
3
.1.2.3. H O-TPO. Additionally, H O-TPO was also used to inves-
3.2. Effect of ZnO on ethanol steam reforming
2
2
tigate the activity of the catalysts to dissociate H O and oxidize
2
0
Co . H O-TPO profiles of the H (m/z = 2) signal from on line QMS
The ESR activity of the catalysts is shown in Fig. 6. The 10Co/ZrO2
catalyst showed a 12.2% C1 product yield and 13.5% H2 yield
at a total ethanol conversion of 41.6%. The gas products were
predominantly C1 products with CO2 being the most abundant
at 19.3% selectivity, followed by CO and CH4 at 5.3% and 1.5%
selectivity respectively. A small amount of ethylene was observed
with selectivity of 2.7%, ethane was also detected, however, the
selectivity was <0.5% and was not considered significant. The
majority of products produced were collected in the liquid phase.
Acetaldehyde showed a selectivity of 27.4% while acetone had
a selectivity of 39.4%. Ethyl acetate and acetic acid were also
observed in the liquid products with 0.9% and 3.3% selectivity
respectively. Acetone, acetaldehyde, ethyl acetate, and acetic acid
have all been reported previously intermediates of the ESR reaction
2
2
are shown in Fig. 5. Both 10Co/ZrO2 and 10Co1Zn/ZrO2 show an
◦
initial oxidation peak at 47 C. In addition, however, a broad shoul-
◦
der extending up to 100 C was observed on 10Co/ZrO , suggesting
2
more oxidation at low temperatures. Although 10Co1Zn/ZrO also
2
◦
showed a broad shoulder from 50 to 100 C, the intensity is sig-
nificantly smaller than that of Co/ZrO . Over 10Co/ZnO the initial
2
◦
oxidation peak temperature was at 51 C. There was no shoulder
on the initial oxidation peak. The onset of oxidation observed here
matched well with those observed during Raman spectroscopy.
In summary, 10Co/ZrO2 showed the highest extent and activity
0
for oxidation of Co in the presence of H O while 10Co/ZnO showed
2
the lowest. 10Co1Zn/ZrO displayed an intermediate degree of H O
2
2
dissociation and oxidation, as would be expected from coating
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pathway [17,26,41,42]. Additionally, the requirement of water for
conversion of ethanol to acetone via acetaldehyde has also been
reported [43–45]. The high selectivity to acetone, especially being
higher than the selectivity to acetaldehyde suggests that the con-
version of acetaldehyde to acetone is very facile over the 10Co/ZrO2.
acetone over 10Co/ZrO2 to acetaldehyde over 10Co/ZnO. Based on
this and prior work it appears that the dissociation of H O on the
2
catalyst surface can be tuned by adding a promoting material, in
this case ZnO, to adjust the product distribution and the oxidation
stability of cobalt. With a higher degree of H O dissociation, the
2
The addition of a small amount of ZnO, 10Co1Zn/ZrO , increased
catalyst will favor acetone formation from acetaldehyde, which can
lead to more complete steam reforming; however, the oxidation of
cobalt is also more facile, leading to a decrease in the C–C cleav-
2
the C product yield to 12.7%, H yield to 15.3%, and overall ethanol
1
2
conversion to 51.1%. The selectivity to gas products, however,
decreased. CO , CO, and CH selectivity all decreased to 17.5%, 3.9%,
age. With lower degree of H O dissociation, the catalyst will favor
2
4
2
and 1.2% respectively. Ethylene selectivity also decreased to 1.4%,
ethane selectivity remained <0.5%. While the gas product distri-
butions remained relatively similar, the liquid product selectivity
changed significantly. The selectivity to acetaldehyde increased to
acetaldehyde formation and acetaldehyde steam reforming, which
can limit the extent of steam reforming; the oxidation of cobalt is
also less facile, helping to increase C–C cleavage.
3
6.9% while the selectivity to acetone decreased to 33.2%, ethyl
Acknowledgments
acetate selectivity remained at 0.9% and acetic acid selectivity
increased slightly to 4.5%. The large shift in acetaldehyde selectivity
can be attributed to a combination of effects. First is that the dehy-
drogenation reaction converting ethanol to acetaldehyde is more
active over basic catalysts than over acidic catalysts, ZnO has higher
We acknowledge the financial support from the US Department
of Energy (DOE), Office of Basic Energy Sciences, Division of Chem-
ical Sciences, Geosciences, and Biosciences (DE-FG02-05ER15712).
The WSU Franceschi Microscopy Center and Dr. Knoblauch for use
of the TEM.
basicity than ZrO [46]. Second, that the conversion of acetaldehyde
2
to acetone requires H O dissociation [17,44], which is suppressed
2
by the addition ZnO on the ZrO2 support as evidenced by the afore
mentioned Raman, H O uptake, and H O-TPO data. A decrease in
Appendix A. Supplementary data
2
2
H O dissociation is further supported by the increase in CO selec-
2
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2015.10.
tivity. The water–gas shift (WGS) reaction is cited as operating in
parallel with, or as part of, the ESR reaction mechanism, the WGS
reaction is also cited as being more dependent on H O and H O
0
16.
2
2
dissociation than it is on CO adsorption [15,26,39,40,47].
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