Full text of DOI:10.1002/cctc.201701892

Accepted Article
Title: Coupling of Methane to Ethane, Ethylene and Aromatics over
Nickel on Ceria-Zirconia at Low Temperatures
Authors: Chukwuemeka Okolie, Yimeng Lyu, Libor Kovarik, Eli
Stavitski, and Carsten Sievers
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ChemCatChem
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Coupling of Methane to Ethane, Ethylene and Aromatics over
Nickel on Ceria-Zirconia at Low Temperatures
[a]
[a]
[b]
[c]
[a]
Chukwuemeka Okolie, Yimeng Lyu, Libor Kovarik, Eli Stavitski, Carsten Sievers*
Abstract: Methane is converted over nickel on ceria zirconia (Ni/CZ)
natural gas conversion are performed through the indirect
route.[1, 3, 4, 11, 14] Some of the large volume products from
synthesis gas are methanol and longer chain hydrocarbons,
to ethane, aromatics and hydrogen at steady state up to the
o
o
o
thermodynamic limit at temperatures of 350 C to 500 C. At 450 C
o
[18-19]
and 500 C, traces of ethylene are also produced. Ni/NiO particles
which are produced through Fischer-Tropsch synthesis.
activate methane and couple the resulting surface species to
hydrocarbon products. Large Ni particles are responsible for the
formation of carbonaceous deposits. In addition, aromatics are
formed on these sites. Smaller Ni/NiO particles are responsible for the
formation of ethane and ethylene and appear to provide sustained
activity. On these sites, surface species couple into higher alkyl
chains, which can be desorbed as ethane. It is suggested that these
Ni sites are too small to assemble aromatic deposits and hence, they
remain active throughout the reaction.
Over the past years, several commercial plants for Fischer-
Tropsch synthesis (FTS) were built.[20] While this approach is in
principle feasible for upgrading methane, the initial methane
reforming step is extremely energy intensive, and FTS plants
require large capital investment and a significant scale to be
economically viable.[18] In particular, this limits the utilization of
remote and inconveniently located natural gas fields, which
accounts for a significant percentage of the world’s reserves.[
21]
A great motivation therefore still exists to develop processes for
direct conversion of methane to value-added products without
synthesis gas as an intermediate. For direct conversion of
methane to higher hydrocarbons, two main reaction pathways
have been considered in the literature, which are differentiated
by the presence or absence of an oxidant. Oxidative coupling of
methane (OCM) was intensely investigated in the mid-1980s to
the mid-1990s.[22-23] This reaction, although thermodynamically
favorable, suffers from selectivity issues due to over-oxidation to
Introduction
The world is continuing its reliance on hydrocarbons as
feedstocks for chemicals and fuels, especially with the discovery
of increasingly abundant sources of natural gas in shale and
offshore gas fields. Therefore, many efforts have been made to
utilize methane, which is the main constituent of natural gas.[
1-16]
CO
3
2
. No catalyst has been able to reach a C2+ yield higher than
0% and selectivity to C2+ hydrocarbons higher than 80%.[17, 23]
Methane is a very stable molecule with four equivalent C-H
bonds, which each have a bond energy of 438.8 kJ/mol.[17] The
C-H bonds in methane are stronger than the ones in the
possible products (hydrocarbons or oxygenates), which means
that the products will be more reactive than methane.[17]
Consequently, controlling the selectivity of methane conversion
is a much bigger challenge than achieving high reactivity.
An alternative strategy is the production of higher hydrocarbons
from methane in the absence of oxygen, so called non-oxidative
coupling of methane (NOCM).[2, 24-27] Since there is no oxidizing
agent in this reaction, over-oxidation is not an issue, and a high
selectivity to C2+ hydrocarbons could be attained. However, this
reaction is thermodynamically limited because it is missing the
coupled oxidation reaction as a driving force.
Several different approaches based on catalysis and reaction
engineering have been proposed and reported, which can be
grouped into two. The first one consists of indirect routes, in
which methane is reformed into synthesis gas, which is further
converted into useful chemicals and fuels like alkanes and
oxygenates.[1-4] The second type of process involves a direct
conversion of methane into C2+ hydrocarbons and oxygenates
Various attempts to accomplish the NOCM reaction used
classical and sometimes ill-defined heterogeneous catalysts in
multistep processes: (i) methane dissociation on Ru or Pt
particles by chemisorption (and stepwise surface carbon-carbon
bond formation) and (ii) liberation of products by
[25, 27]
hydrogenation.
Due to the thermodynamic constraints of
NOCM, a number of researchers used very high temperatures
[
5,
without the production of syn-gas as an isolated intermediate.
(
> 800 °C) to perform a direct conversion of methane to higher
1
1, 12, 16, 17]
Today, all large-scale commercial processes for
[2, 24]
alkanes and aromatics.
Isolated iron sites embedded in a
silica matrix[ and molybdenum oxide nanostructures in
zeolites[24] were found to be the active sites for this reaction.
However, the energy requirements for such processes are
substantial, and it is challenging to avoid the formation of
carbonaceous deposits and agglomeration of a highly dispersed
active phase at such high temperatures. Another study
demonstrated that a silica-supported tantalum hydride catalyst
can directly couple methane into ethane at low temperatures up
2]
[a]
Dr. C. Okolie, Y. Lyu, Dr. C. Sievers
School of Chemical & Biomolecular Engineering
Georgia Institute of Technology
311 Ferst Drive NW, Atlanta, GA, 30332, United States
E-mail: carsten.sievers@chbe.gatech.edu
Dr. Libor Kovarik
Environmental Molecular Sciences Laboratory
Pacific Northwest National Laboratory
[
b]
c]
3335 Innovation Blvd., Richland, WA, 99354, United States
[26]
to the thermodynamic limit of the reaction. The complex
[
Dr. E. Stavitski
synthesis protocol to make the catalyst coupled with the
unpredictable trend in tantalum price,[28] makes this material not
very desirable for the preparation of industrial catalysts.
National Synchrotron Light Source II
Brookhaven National Laboratory
Upton, NY, 11973, United States
Supporting information for this article is given via a link at the end of
the document
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Moreover, a steady decline in methane conversion was
observed affording only a limited turnover number. Therefore,
the development of a catalyst for non-oxidative coupling of
methane with commercial viability still remains as a challenge.
whereas a concentration of 80 μmol.g-1 was observed for NiO/CZ,
indicating that the NiO clusters are Lewis acidic.[4] The
characteristic band of pyridinium ions protonated on Brønsted
acid sites was not observed for either sample.
Catalysts comprised of nickel particles on pure or doped ceria
supports have been extensively used in the literature for the
purpose of methane activation and reforming reactions.[
This is because ceria and ceria-zirconia can stabilize small
clusters of group 8, 9 and 10 metal nanoparticles with high
thermal stability.[32] Typically, the supported metal nanoparticles
act as the active sites for the activation of C-H bonds of
hydrocarbons.[ A study of the effect of the ceria to zirconia
The EDX maps of NiO/CZ revealed small and well-dispersed
NiO particles on CZ (Figure 1). Specifically, NiO mainly existed
as clusters below the detection limit of ~2-3 nm. The size of
smaller particles cannot be determined reliably due to the limited
stability of NiO clusters under high electron beam currents of
modern EDX instruments. Nevertheless, the apparently uniform
distribution of the Ni signal indicated well-dispersed NiO
particles on the entire surface of the ceria-zirconia support,
demonstrating that NiO clusters may be significantly smaller and
that clusters containing only a few Ni atoms may be present. In
addition to the small particles of NiO, some larger particles of up
to 10 nm were observed. It is suggested that CZ stabilizes very
small, well-dispersed nickel oxide clusters, which are capable of
activating methane.[4] Previous studies showed that pure ceria
can stabilize similar non-metallic Pt and Au species, which are
very active for the water-gas shift reaction.[48]
29-32]
32]
x 2
ratio on the performance of Ni/Ce Zr1-xO for steam reforming of
methane revealed that higher thermal stability, higher redox
properties, and smaller Ni crystallite size were some of the
important descriptors for the performance of the catalysts.[
33]
In this study, nickel supported on ceria-zirconia was used for
methane activation and subsequent coupling to higher
hydrocarbons and aromatics at temperatures not exceeding 500
˚
C. Continuous production of ethane up to the thermodynamic
limit was observed over 2 wt.% Ni/CZ. Some ethylene and
aromatics were also continuously produced.
Reactivity for Coupling of Methane. Previous studies showed
that methane can be activated over NiO/CZ to form surface
methyl groups, which can be further coupled to higher alkyl
groups on the surface.[4] Based on this, it was hypothesized that
higher hydrocarbons could be formed from methane over this
catalyst. It was observed that these surface alkyl groups formed
Results and Discussion
o
on NiO/CZ can be removed in high vacuum at 250 C within 1 h
Structural and Textural Properties of the Catalysts. The
fractions of ceria and zirconia as determined by ICP-OES were
as has been reported recently.[4] A similar desorption step
should occur more readily at higher temperatures. However, the
formation of higher alkanes (e.g., ethane) is thermodynamically
limited (Table 2).
0.83 and 0.17, respectively. The actual content of nickel on ceria-
zirconia (NiO/CZ) as determined by ICP-OES was 1.96 wt.%
which is very close to the nominal value (Table 1).
Table 1. Physicochemical properties of ceria-zirconia based catalysts.
Catalyst
Abbreviation
Metal
Surface area
(m g )
Average
pore
Pore volume
(cm g )
LAS Conc.
(μmol.g )
Dispersionf
2.
-1
b
3. -1
d
-1
e
loading (wt.
a
%
)
diameter
c
(Å)
Ce0.83Zr0.17
O
2
CZ
N/A
85
78
66
0.097
0.098
3
N/A
NiO/Ce0.83Zr0.17
O
2
NiO/CZ
1.96
69
80
0.26
2
a Determined by ICP-OES analysis b Calculated from N physisorption by BET method c calculated by BJH adsorption method d Determined by BJH
cumulative pore volume e Determined by pyridine adsorption followed by FTIR spectroscopy f Dispersion of metal oxide determined from pyridine
adsorption on Lewis acidic metal oxides
During the conversion of methane at 450 and 500 °C, two
distinct regimes were observed with a transition occurring after
approximately 4 h. After activation in nitrogen, the conversion of
methane at 350 – 500 °C initially increased with increasing time
on stream. At 450 and 500 °C, it reached a maximum at 0.58 ±
0.02% and 1.19 ± 0.01%, respectively (Figure 2a). The
The X-ray diffraction patterns of CZ and NiO/CZ contained four
prominent diffraction peaks associated with the (111), (200),
42-46]
(
220), and (311) planes of CZ (Figure S1).[
As in our previous
work, all peaks in the diffractograms were attributed to the cubic
fluorite phase of ceria-zirconia, indicating that no crystalline
impurities were present.[35, 47] No peaks representative of the
additional metal oxides were seen in the XRD patterns of NiO/CZ.
This means that the NiO on CZ was well-dispersed. The crystallite
size calculated by the Scherrer equation were approximately 8
nm for pure CZ as well as for NiO/CZ. The concentration of Lewis
acid sites (LAS) on CZ as determined by pyridine adsorption
conversion then declined and approached the thermodynamic
limit for the conversion of methane to ethane. Specifically, the
o
o
conversions of methane after 480 min at 500 C, 450 C and
o
3
50 C were 0.39 ± 0.01%, 0.26 ± 0.04% and 0.11 ± 0.02%,
respectively, which is slightly lower than the equilibrium
conversion for the reaction of methane to ethane (Table 2).
-1
followed by FTIR spectroscopy was very low (3 μmol.g ),
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The increase in methane conversion at the beginning is
probably due to activation of the catalyst by progressive
reduction of the NiO particles to metallic Ni (vide infra). The
maximum of methane conversion in the earlier stages of the
reactions at 450 and 500 °C is attributed to conversion of
methane into aromatics and carbonaceous deposits, which were
quantified based on the carbon balance and the oxidative
formation of aromatics (Figure 2 b-d). The yield of hydrogen also
went through a maximum in a similar fashion as the conversion
of methane (Figure 2e) and the yield of carbon products (Figure
S2), indicating that dehydrogenation of methane occurred as
part of the formation of solid carbonaceous deposits (Table 2
Line 4), aromatics (Table 2, Line 5) and ethane (Table 2 Line 1).
Multiple studies reported the formation of carbon
nanotubes/nanofibers by methane dehydrogenation and C-C
coupling over Ni based catalysts around 450 ˚C.[49-51] It is
suggested that in the beginning of the reaction, the formation of
the solid carbonaceous deposits can occur on Ni particles that
are big enough for them to assemble, while smaller aromatic
may also form on smaller NiO/Ni particles. An aromatic ring has
a diameter of 0.45 nm,[52] but the minimum domain size for
aromatization might be even larger than that. The formation of
aromatic products alongside the carbonaceous deposits
probably results from the coupling of methane derivatives during
dehydrogenation (Scheme 1). The thermodynamic data
provided in Table 2 (Lines 5 and 6) shows that aromatization is
much more favorable when some of the hydrogen atoms are
oxidized to water. A certain amount of active oxygen from the
redox active CZ support can be supplied for this purpose (vide
infra). The formation of aromatics continued after oxygen from
CZ was depleted but to a lower extent in the non-oxidative
environment.
R
R
-
H
2
coupling
+O
CH4
CH
x
C H
+ H O
2
2
y
+H
2
Coke
Coke
Scheme 1. Mechanism for the formation of aromatics and coke in methane
53
aromatization.
At 350 oC, the conversion of methane did not go through a
maximum but continuously approached 0.118 ± 0.002%, the
thermodynamic limit for the conversion of methane to ethane in
the absence of an oxidant (Table 2, Line 1). Previous studies
showed that the reduction of surface ceria in CZ starts around
Figure 1. EDX maps of 2 wt.% NiO/CZ and plot showing the spectra at the
marked points
o
[35]
o
300 C, but at 350 C, only a very small fraction of cerium at
the surface is reduced. Hence, the amount of oxygen released at
this temperature was significantly smaller than the amount
o
The carbon containing products of the reactions at 500 C, 450
oC and 350 oC were ethane, benzene, toluene and traces of
o
o
released at 450 C and 500 C. As seen in Figure 2d, the
selectivity to aromatics at 350 ˚C was very low compared to 450
oC (Figure 2c) and 500 ˚C (Figure 2b), which might be partially
related to the limited oxygen supply from the reduction of ceria at
this low temperature.
o
o
ethylene (at 500 C and 450 C) and some heavier aromatics such
as ethylbenzene, styrene, isopropyl benzene and n-propyl
benzene. At the start of the reaction, aromatics were the main
products. The selectivities to these aromatic products
continuously declined, while the selectivity to ethane increased
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o
o
and reached 80 ± 1% at 500 C, 81.4 ± 1.6% at 450 C and 96.3±
0
Ce L
3
and Ni K edges indicate that easily reducible ceria at or
o
.7% at 350 C, all on a carbon atom basis. The selectivity to
ethylene also increased during the reaction and reached 4.2 ±
.2 % at 500 oC and 1.3 ± 0.2% at 450 oC. No ethylene was
near the surface can supply oxygen to NiO clusters to keep
them oxidized until the active oxygen species in CZ are
depleted.[49] During the surface reduction with methane, oxygen
atoms are removed and steam is formed. Hence, the reduction
of ceria and eventually nickel oxide at the initial stage of the
reaction supplies oxygen, which can promote methane
aromatization for about the first 4 h on stream (Table 2, Line 6)
(Scheme 1). After the reduction of NiO to Ni after 4 h, the
formation of ethane becomes the dominant reaction path.
0
o
observed at 350 C. The yields of hydrogen at steady state was
o
o
o
higher in the order of 500 C > 450 C > 350 C, which is expected
due to the endothermic nature of dehydrogenation reaction. The
ratios of ethylene and hydrogen to ethane formed at 450 °C and
5
00 °C were slightly below the ratios expected under
thermodynamic control. The decreasing selectivities of ethane
coupled with an increase in the selectivity of ethylene at higher
temperatures indicates that ethane is either undergoing a
dehydrogenation reaction to form ethylene or that the surface
species are undergoing dehydrogenation before desorption to
form ethylene.
Analysis of Carbonaceous Deposits. To gain insight into the
nature of the carbonaceous deposits, catalysts were analyzed by
C1s XPS after reacting with methane for 4 h (Figure 4). The XP
spectrum showed a C=C peak at 284.9 eV indicating that carbon
was present in graphitic form.[54-55] Another peak at 283.9 eV
suggested the presence of a hydrogen-poor sp-type of pre-
Table 2. Thermodynamics data for gas phase reactions of methane.
.
-1
ΔG /kJ mol
X %
CH4 /
o
o
o
Reaction
350 C
450 C
500 C
350
oC
450
oC
500
oC
2CH4(g) ↔ C
2
H
6(g) + H2(g)
76
77
78
0.13
0.32
0.45
2
CH4(g) ↔ C
CH4(g) ↔ C
2
H
H
4(g) + 2H2(g)
2(g) + 3H2(g)
137
241
26
126
220
18
121
209
14
0.019
7.56.10-06
13
0.11
0.23
2.56.10-04
38
2
2
9.31.10-05
28
CH4(g) ↔ C(s) + 2H2(g)
6
CH4(g) ↔ C
6
H
6(g) + 9H2(g)
335
303
288
0.13
100
0.53
100
0.94
100
6CH4(g) + 4.5O2(g) ↔ C
6
H
6(g) + 9H
O
2 (g)
-1592
-1583
-1579
XCH4 represents equilibrium conversion of methane.
4
Redox Activity of NiO/CZ during Reaction with CH . In-situ
graphitic carbon.[56] No carbon species bound to oxygen were
detected. Combustion analysis of the spent catalyst revealed that
3.3 wt.%, 4.6 wt.% and 7.6 wt.% of carbon and hydrogen were
deposited during the reactions at 350 oC, 450 oC and 500 oC,
respectively, which was in good agreement with TPO in a
thermogravimetric analyzer (Figure 5a).
XANES spectroscopy was used to probe the oxidation states of
ceria and nickel during 5 h of exposure to methane at 450 °C in
the absence of any oxidant (Figure 3). No shifts of the ceria or
nickel edges were observed during activation in nitrogen. During
the first 4 h of exposure to methane, a continuous shift of the
3
position of the Ce L edge to a lower energy indicated a
4
+
3+
In-situ IR spectra during exposure of NiO/CZ to methane at 450
C (Figure S5) contained several peaks corresponding to C=C
reduction of some Ce species to Ce (Figure 3a). The position
and white line intensity at the Ni K edge remained unchanged
for the first 4 h of exposure to methane, indicating that oxidation
state of nickel oxide remained approximately constant (Figure
o
-
1
bonds of aromatics and coke were also seen between 1653 cm
-1
and 1502 cm , but the accumulation of these species was
relatively limited compared to their intensity after 1 h. It is
suggested that some of these aromatic species desorb from the
surface in the form of benzene, toluene and traces of heavier
aromatics, such as styrene, ethylbenzene and propylbenzene, as
4b). The reduction of big NiO particles that was proposed based
on the conversion over time profile (Figure 2a, vide supra) was
not observed in the XANES spectra. It is suggested that only a
small fraction of Ni is present in such particles, so that their
reduction has an insignificant effect on the spectrum. The white
line then disappeared within 30 min, and the edge position
shifted to lower energy, which shows that nickel oxide was
reduced to metallic nickel. The combined observations from the
-
seen on the reactivity results (Figure 2). A peak around 1580 cm
1
[63-64]
was also observed and is assigned to graphitic coke.
The
intensity of this peak increased slowly and approached an
asymptotic limit. This indicates that most of the graphitic coke was
formed within the first hour on stream.
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9
0
o
(
a)
500 C
(b)
1
1
0
.2
.0
.8
o
80
70
4
50 C
o
3
50 C
Ethane
Ethylene
Benzene
Toluene
Heavy Products
C Deposit
60
50
40
30
20
10
0
0.6
0.4
0.2
0.0
0
100
200
300
400
500
0
100
200
300
400
500
Time /min
Time /min
90
100
(c)
(d)
9
8
7
0
0
0
8
0
Ethane
Benzene
Toluene
Heavy Products
C Deposit
7
0
0
Ethane
6
Ethylene
Benzene
Toluene
Heavy Products
C Deposit
60
50
40
30
5
0
0
4
30
20
10
2
0
0
1
0
0
0
100
200
300
400
500
0
100
200
300
400
500
Time /min
Time /min
0.3
o
(e)
500 C
o
4
50 C
o
3
50 C
0.2
0.1
0.0
0
100
200
300
400
500
Time /min
o
o
o
Figure 2. (a) Conversion of methane over NiO/CZ at different temperatures (b) Selectivity of carbon products over NiO/CZ at 500 C (c) 450 C (d) 350 C (e)
Yields of hydrogen at different temperatures.
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8
7
6
5
4
3
2
1
0
1
1
0
0
.5
.0
.5
(a)
(
a)
CHN Analysis
TPO using TG analysis
.0
5
h 4 h 3 h 2 h 1 h 10 mins 0 h
5
700
5720
5740
5760
Photon Energy /eV
3
50 ˚C
450 ˚C
500 ˚C
1
1
0
0
.5
(b)
0
h
1
0 mins
D
(b)
1
h
.0
.5
G
2
3
h
h
4 h
4
5
o
5
00 C
h 10 mins
h
.0
o
4
3
50 C
8320
8330
8340
8350
8360
Photon Energy /eV
o
50 C
o
Figure 3. In-situ XANES of NiO/CZ during exposure to methane at 450 C (a)
Ce L
3
-edge (b) Ni K-edge.
1
000 1200 1400 1600 1800 2000
Raman Shift /cm-1
Pre-graphitic
Original Trace
Fitted Trace
2
2
83.9 eV
84.9 eV
Figure 5. (a) Amount of carbon deposit on spent catalysts after reaction at
different temperatures measured by combustion analysis and TPO using TG
analysis (b) Raman spectra of spent catalysts after reactions at different
temperatures.
Graphitic
The Raman spectra of the spent catalysts showed signals at 1350
-1
and 1590 cm , which represent “D” (disorder) and “G” (graphite)
bands, respectively (Figure 5b).[57-58] However, these bands were
deconvoluted to obtain detailed structural information (Figure S6).
The two bands were fitted with four Lorentzian-shaped bands (D1,
-1
D2, D4 and G) at ν = 1350, 1600, 1210 and 1575 cm ,
2
83
284
285
286
-
respectively and one Gaussian-shaped band (D3) at 1530 cm
Binding Energy /eV
1
[59-60]
.
These components are assigned as graphene edges,
graphene sheets, polyenes, graphitic carbon and amorphous
carbon respectively. It has been suggested that the ratio ID1/(I
D1 + ID2) is a measure of the degree of graphitization of
carbonaceous materials in which I is the integral of the fitted
peak.[60] The intensity ratio of the bands ID1/(I
+ ID1 + ID2)
G
+
Figure 4. X-ray photoelectron spectra in the C1s region of NiO/CZ after 4 h of
methane exposure at 450 C.
o
I
G
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increased from 0.67 at 350 °C to 0.71 at 450 °C and 0.73 at
Aldrich. Gases (methane and nitrogen) with ultra-high purity
(UHP Grade 5) were purchased from Airgas. Dry air for
calcination was generated in our labs using a Parker Balston
Gas Generator 1000. Deionized water was obtained from a
Barnstead NANOpure ultrapure water system which was purified
to 18.2 MΩ/cm.
500 °C, which shows that the coke became more graphitic with
increasing temperature. The amount of carbon deposit increased
when the reaction was run at higher temperatures in agreement
with the CH and TPO results.
Strategy for Improved Performance. The present contribution
shows that Ni/CZ is capable of activating methane at low
temperatures and coupling surface methyl groups to ethane.
However, this endothermic reaction is thermodynamically limited
to 0.45% conversion at 500 °C (Table 1). To improve the
conversion to an acceptable level for industrial application without
increasing the operating temperatures, these catalysts could be
incorporated into membranes that can selectivity remove
hydrogen to drive the reaction in the forward direction. Several
studies in the literature have successfully demonstrated the use
of such membranes on systems with either catalysts that are
significantly more expensive than and not as active as the catalyst
in this study or for non-oxidative methane aromatization at high
temperatures (700 – 800 ˚C).[27, 65]
Catalyst Synthesis. The ceria zirconia support was prepared
by coprecipitation of the precursors of ceria and zirconia.[
Cerium nitrate hexahydrate and zirconyl nitrate hydrate were
dissolved in deionized water to form a 0.1 M solution. The
coprecipitation was modified by adding the 0.1 M precursor
solution drop-wise to an aqueous ammonium hydroxide solution
while stirring continuously. The precipitate was then filtered,
rinsed with deionized water, and dried in an oven overnight at
373 K. The catalyst was then calcined for 4 h in 200 mL/min
34-35]
o
o
zero grade air at 450 C with a ramp rate of 5 C/min. Metal
oxide clusters of Ni was deposited on the CZ support using
conventional dry impregnation or pore volume impregnation.[
36]
For this purpose, the nickel precursor for 2 wt.% nickel loading
was dissolved in deionized water equal to the pore volume of
the ceria zirconia support, as determined by nitrogen
Conclusions
physisorption. The resulting solution was added dropwise to the
support and collected in a beaker at room temperature. It was
Nickel particles on ceria zirconia catalysts are active for the
conversion of methane into ethane, ethylene and aromatics
such as benzene, toluene, ethylbenzene, styrene and
o
mixed thoroughly for 30 minutes, dried at 100 C for 5 h and
o
o
then calcined for 4 h at 450 C with a ramp rate of 10 C/min.
propylbenzene. The nickel sites are responsible for the
activation of methane. Coupling of activated methane groups
resulted in the formation of different products. Two types of Ni
particles are present on CZ and play different roles in the
conversion of methane based on their particle size. Bigger Ni
particles, which are at least larger than 0.45 nm, are responsible
for the formation of carbonaceous deposits because they have
enough surface sites for aromatic species to assemble.
Oxidation of surface hydrogen with active oxygen from CZ can
promote the formation of aromatics. Ni particles that are smaller
than a benzene ring (0.45 nm) are not involved in the formation
of carbonaceous deposits. When the temperature is high
enough, ethane can either be dehydrogenated to ethylene or the
surface alkyl groups dehydrogenate before desorption. After 8 h
on stream, the catalyst remains active for methane coupling to
aromatics and higher hydrocarbons. Oxygen from CZ facilitates
the initial formation of aromatics, while the formation of ethane
dominates after this oxygen source is depleted. Thus, Ni/CZ is
an efficient and cheap catalyst for converting methane into
useful hydrocarbons in the absence of an oxidant within the
thermodynamic limit at reasonably low temperatures. By
selectively removing hydrogen from the product mixture in a
membrane reactor the yields of higher hydrocarbons from
methane could be improved in a non-oxidative environment.
Characterization. Nitrogen physisorption measurements of CZ
and Ni/CZ were performed using a Micromeritics ASAP 2020
o
physisorption analyzer. The catalysts were degassed at 200 C
for 4 h prior to measurement. Surface areas and pore volumes
were calculated based on the BET method[37] and BJH
method,[38] respectively.
To determine the amount of ceria, zirconia, and nickel on the
catalyst, ceria zirconia and NiO on ceria zirconia were sent to
Galbraith Laboratories for inductively coupled plasma-atomic
emission spectroscopy (ICP-AES) analysis.
X-ray diffraction patterns were obtained using a Phillips X’Pert
diffractometer equipped with an X’celerator module using Cu Kα
radiation. Diffractograms were collected at incident angles from
2θ = 5 to 90° with a step size of 0.0167°.
Pyridine adsorption followed by FTIR spectroscopy was
performed using a Nicolet 8700 FTIR spectrometer with an
MCT/A detector. Each spectrum was recorded with 64 scans at
-1
a resolution of 4 cm . Each sample was pressed into a
translucent self-supported wafer and loaded into a vacuum FTIR
o
transmission cell. The sample was activated at 450 C for 1 h
o
under high vacuum and cooled to 150 C. A background
spectrum was taken. The chamber was dosed with 0.10 mbar of
pyridine for 30 mins or until adsorption equilibrium of pyridine
was reached. Subsequently, the cell was evacuated for 1 h to
remove physisorbed pyridine and a spectrum was taken. After
each experiment, the density of the wafer was determined by
using a circular stamp of 6.35 mm to cut a disc of specific size
from the wafer. The concentration of Lewis and Brønsted acid
Experimental Section
Materials. Cerium (III) nitrate hexahydrate (99% trace metals
basis), zirconyl (IV) oxynitrate hydrate (99% trace metals basis),
nickel (II) nitrate hexahydrate, and ammonium hydroxide (A.C.S.
3
regent grade, 28–30% NH content) were purchased from Sigma
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ChemCatChem
10.1002/cctc.201701892
FULL PAPER
sites were determined by the Beer-Lambert law using the
integral of the peaks at 1445 cm-1 and 1540 cm , respectively.
Extinction coefficients were used as reported by Tamura et al.[39]
sensitivity factors contained in the ULVAC-PHI, Inc. MultiPak
V9.5.0.8 version date of 10-30-2013 software. Peak area
intensities required for quantification were calculated after
applying a Shirley background subtraction. These quantification
results include the instrument transmission function, source
angle, and asymmetry corrections. Typical XPS analysis depth
for ceria based materials using inelastic mean free path
calculation is up to 3 nm depending on the kinetic energy of the
detected electron.[40] The majority of the electrons detected are
coming from the first atomic layer.
-1
3
In-situ XANES Ce L -edge and Ni K-edge were collected at
beamline 9-BM-C at the Advanced Photon Source of Argonne
National Laboratories (Proposal GUP-43235). A Si(1 1 1) double
monochromator was used to select the beam energy for all
measurements. The beam size was 800 x 1000 μm. Samples
were pressed into wafers and placed in a cubic reactor. The
beam was internally calibrated with a metallic cerium reference
foil for Ce L
3
-edge and a metallic nickel reference foil for Ni K-
EDX map was collected with a Scanning Transmission electron
microscopy (STEM) (FEI Titan 80-300). The FEI Titan is
equipped with CEOS GmbH double-hexapole aberration
corrector for the probe-forming lens, which allows imaging with
sub Angstrom resolution in STEM mode. EDX mapping was
edge. Spectra were then collected in fluorescence mode. A
spectrum was first taken of each of the fresh samples. The
samples were then activated in helium at 450 C for 1 h in the
cubic reactor while continuously taking spectra. The gas was
then switched to 5% methane and balance helium and spectra
were taken continuously. Separate experiments were performed
o
R
performed with FEI ChemiSTEM , which comprise of four
symmetrically arranged windowless silicon drift detectors (SDD)
with solid angle of ~0.8 sr. Acquisition and evaluation of the
spectra was performed with Bruker Espirit software package. In
general, the STEM sample preparation involved mounting
powder samples on copper grids covered with lacey carbon
support films, and then immediately loading them into the STEM
airlock to minimize an exposure to atmospheric O2.
3
under the same conditions for both Ce L -edge and Ni K-edge.
XPS measurements were performed with a Physical Electronics
Quantera Scanning X-ray Microprobe. This system uses a
focused monochromatic Al Kα X-ray (1486.7 eV) source for
excitation and a spherical section analyzer. The instrument has
a 32 element multichannel detection system. The 83 W X-ray
beam focused to 100 μm diameter was rastered over a 1.1 mm
x 0.1 mm rectangle on the sample was used for the analysis.
The X-ray beam is incident normal to the sample and the
photoelectron detector is at 45° off-normal. High energy
resolution spectra were collected using a pass-energy of 69.0
eV with a step size of 0.125 eV. For the Ag 3d5/2 line, these
conditions produced a FWHM of 1.07 eV. The sample
The carbon content on the spent catalysts were determined
using a TA instruments SDT Q600 TGA. An empty alumina
crucible was used to tare the TGA before each analysis.
Individual samples were then loaded on the crucible and put in
o
the TGA for analysis. The samples were ramped to 800 C at 20
oC/min in a 100 mL/min flow of dry air.
experienced variable degrees of charging. Low energy electrons
at ~1 eV, 20μA and low energy Ar ions were used to minimize
Spent catalysts were also sent to Atlantic Microlab for CHN
analysis.
+
this charging. The catalyst powder was pressed into 4 mm x 1
mm diameter holes machined into a custom 13 mm diameter SS
sample stub designed for use with a custom modified ULVAC
PHI XPS sample platen. The custom sample platen and sample
stub including the fresh catalysts were placed into the XPS
vacuum introduction system and pumped to <1x10-7 Torr using
a turbomolecular vacuum pumping system prior to introduction
into the XPS spectrometer ultrahigh vacuum system. The
spectrometer vacuum system pressure was maintained at
Raman spectroscopy was also performed on the spent
catalysts. For this purpose, the samples were loaded on
microscope slides. The microscope was then focused on the
sample region of interest at 20x resolution. Using a WITEC
spectrometer with a 532 nm laser, the structural fingerprint of
the samples were measured to determine the molecules
present. Three individual measurements were then taken with
an integration time of 10 seconds each which were averaged. A
smooth function was used to remove noise and/or interference.
<1x10-9 Torr during analysis and pumped using a series of
sputter ion and turbomolecular vacuum pumps. After XPS
analysis of the fresh catalysts, the sample stub was transferred
to an attached catalytic reaction side chamber also pumped by a
series of sputter ion and turbomolecular vacuum pumps. The
sample stub was then transferred from the catalytic side
chamber into an attached high vacuum tube furnace. The
pressure of the tube furnace was increased ~760 Torr using 5%
methane/He at a flow rate of 100 SCCM. The catalyst was
heated to 450 C at a rate of 10 C/minute and held for 4 hours.
After the catalyst was cooled to room temperature, the gas was
quickly removed using a turbomolecular vacuum pump attached
to the tube furnace. The stub was then transferred into the UVH
catalytic chamber and then into the XPS spectrometer vacuum
for analysis. Quantification was performed using standard
In-Situ FTIR Spectroscopic Study during Reaction with
Methane at 450 C over NiO/CZ. 100 mg of the catalyst was
o
pressed into a circular self-supported wafer with a diameter of 2
cm. The wafer was loaded into the sample holder, and it was
assembled as part of the FTIR cell together with the cell body
and heating block. The cell was built based on a design reported
in the literature.[41] The assembled FTIR cell was placed into a
Nicolet 8700 FTIR spectrometer. Each spectrum was recorded
with 64 scans at a frequency of one per minute for 12 h during
exposure to 5% methane and balance inert at 100 ccm total flow
o
o
o
rate at 450 C and atmospheric pressure.
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ChemCatChem
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FULL PAPER
Reactivity Studies. Reactivity experiments for the coupling of
methane were performed in a packed bed reactor. The studies
were performed using 200 mg of catalyst sieved to 75 µm.
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[
[
1
[
[
1
[
1
[
ꢂꢃꢄꢅ,ꢆꢇ − ꢂꢃꢄꢅ,ꢈꢉꢊ
Conversion: ꢀ [%] =
x 100
ꢁ
ꢂꢃꢄꢅ,ꢆꢇ
[
1
[
ꢍꢌ . ꢂꢌ,ꢈꢉꢊ
C Product Selectivity: ꢋ [%] =
x 100
ꢌ
ꢂꢃꢄꢅ,ꢆꢇ− ꢂꢃꢄꢅ,ꢈꢉꢊ
[
ꢂꢄꢎ,ꢈꢉꢊ
H
2
yield: [%] =
x 100
[
ꢎ.ꢂꢃꢄꢅ,ꢆꢇ
[
p
Where ν is the number of carbon atoms in the compound and
[
F
x
is the molar flow rate of the gas in question.
[
1
[
Acknowledgements
[
The work was performed with funds from the Petroleum
Research Fund of the American Chemical Society (grant
number: 53873) and the U.S. Department of Energy (DOE)
Office of Basic Energy Science under Contract No. DE-
SC0016486. The XAS data was obtained at beamline 9-BM of
the Advanced Photon Source, a U.S. Department of Energy
2
2
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[
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Ethane, Ethylene and Aromatics
are produced continuously over
nickel on ceria-zirconia in a
non-oxidative environment at
temperatures not exceeding
Chukwuemeka Okolie, Libor
Kovarik, Eli Stavitski, Carsten
Sievers*
Page No. – Page No.
500 ˚C.
Non-Oxidative Coupling of
Methane to Ethane, Ethylene
and Aromatics over Nickel on
Ceria-Zirconia
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