Hydrogen production: via thermocatalytic decomposition of methane using carbon-based catalysts

Article abstract of DOI:10.1039/d0ra07440c

Thermocatalytic decomposition (TCD) of methane is one of the most effective methods for pure hydrogen production. Catalysts were selected for TCD of methane in this study to utilize biochar as a catalyst. Among these catalysts, two catalysts (named activated biochar (AB) and heat-treated biochar (HB)) were prepared from Douglas fir, whereas the other four were prepared using commercial activated carbon and zeolite with and without doping ruthenium metal. The catalysts were characterized using XRD, SEM imaging, TEM, H2-TPR, and BET specific surface area and pore size analysis. The Ru doped commercial activated carbon catalyst (Ru-AC) was deactivated continuously during a 60 h reaction run, whereas AB exhibited comparatively stable methane conversion up to 60 h. The methane conversion was 21% for Ru-AC and 51% for AB after 60 h of reaction time at 800 °C. The very high surface area of AB (～3250 m2 g-1) and its microporosity compared to other catalysts could have resulted in resistance against rapid deactivation. Furthermore, carbon nanotube by-products were observed in TEM images of solid residues that could form due to the presence of alkali metals in the biochar. Carbon nanotube formation could contribute significantly to the extended life of AB. This journal is

Full text of DOI:10.1039/d0ra07440c

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Hydrogen production via thermocatalytic
decomposition of methane using carbon-based
Cite this: RSC Adv., 2020, 10, 40882
catalysts†
Khalida Harun, Sushil Adhikari *ab and Hossein Jahromi
a
ab
Thermocatalytic decomposition (TCD) of methane is one of the most effective methods for pure hydrogen
production. Catalysts were selected for TCD of methane in this study to utilize biochar as a catalyst. Among
these catalysts, two catalysts (named activated biochar (AB) and heat-treated biochar (HB)) were prepared
from Douglas fir, whereas the other four were prepared using commercial activated carbon and zeolite with
2
and without doping ruthenium metal. The catalysts were characterized using XRD, SEM imaging, TEM, H -
TPR, and BET specific surface area and pore size analysis. The Ru doped commercial activated carbon
catalyst (Ru–AC) was deactivated continuously during a 60 h reaction run, whereas AB exhibited
comparatively stable methane conversion up to 60 h. The methane conversion was 21% for Ru–AC and
ꢀ
2
ꢂ1
5
1% for AB after 60 h of reaction time at 800 C. The very high surface area of AB (ꢁ3250 m
g ) and
Received 30th August 2020
its microporosity compared to other catalysts could have resulted in resistance against rapid
deactivation. Furthermore, carbon nanotube by-products were observed in TEM images of solid residues
that could form due to the presence of alkali metals in the biochar. Carbon nanotube formation could
contribute significantly to the extended life of AB.
Accepted 21st October 2020
DOI: 10.1039/d0ra07440c
rsc.li/rsc-advances
commercial production of hydrogen, a combination of steam
reforming reaction and partial oxidation of fossil fuels is used. A
1
Introduction
One of the signicant challenges of today's society is to mini- large amount of CO
(CO, CO ) is produced via reforming and
2
x
7
mize anthropogenic greenhouse gas emissions while meeting partial oxidation, and requires costly purication steps.
1
–5
the energy demand of the ever-increasing world population.
4
Methane (CH ) is the main component of natural gas, and
To address this issue, hydrogen fuel can be used as an alter- around 48% of world's total hydrogen production is coming
5
native to fossil fuel. Hydrogen produces only water during its from natural gas. Additionally, methane can be produced from
controlled oxidation in a fuel cell or in its direct combustion. It anaerobic digestion of various agricultural and food wastes.
is the most abundant element in the whole universe, but Since methane has the highest hydrogen carbon ratio (4 : 1), it
unfortunately, it is not available in its purest form on earth. gives the lowest CO
Hence, it is a secondary energy source, and it can be produced reforming and partial oxidation processes. To avoid the vast
while producing hydrogen in steam
x
5
from other sources such as natural gas, fossil fuel, biomass, and amount of CO
x
production, thermocatalytic decomposition
4
1,5,7,8
water.
(TCD) of methane has attracted researchers' attention.
Water electrolysis is normally used to produce high purity Besides the energy sector, hydrogen has several uses in elec-
1
–5,9
hydrogen; however, this process is uneconomical for industrial trical, glass, pharmaceutical, and fertilizer industries.
use. On the other hand, biomass gasication can be used as an
5
Hydrogen produced via TCD of methane can be directly used
industrial source of hydrogen but gasication mostly produces in hydrogen fuel cell and ammonia production industries
syngas, which is used for producing electricity, wax, higher without further purications. Moreover, carbon produced by
6
hydrocarbon, and methanol but not pure hydrogen. In the this method can also be used for different purposes. Fibrous
carbon can be used in construction industries and polymer
10
additives industries. According to Lane and Spath, the selling
a
price of hydrogen would be $7–21 per GJ based on the price of
Department of Biosystems Engineering, Auburn University, Auburn, Alabama 36849,
USA. E-mail: sushil.adhikari@auburn.edu; Fax: +1 334 844 3530; Tel: +1 334 844 natural gas and by-product carbon. Steinberg and Chang esti-
3
543
mated that the selling price of hydrogen produced by TCD
b
Center for Bioenergy and Bioproducts, Auburn University, Auburn, Alabama 36849,
USA
3
reaction would be $58 per 1000 m as opposed to $67 per 1000
3
11
m via steam reforming of methane. The authors concluded
that TCD allows for easier separation of hydrogen and produces
solid carbon. Dufour et al. studied different hydrogen
†
Electronic supplementary information (ESI) available: Commercial catalyst
information, catalyst preparation and catalyst characterization methods. See
DOI: 10.1039/d0ra07440c
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ꢀ
production process using life cycle assessment tools and re- 10 h of reaction time at 850 C. Fibrous carbon is produced as
2
1
ported that TCD is the most environmentally friendly proce- by-product in the study.
12
dure.
3
The energy requirement for TCD reaction is
Douglas r is one of the widely harvested biomass in the
7.8 kJ mol H production, whereas 63 kJ mol H is needed northern part of the United States. Wood industries widely use
ꢂ1
ꢂ1
22
2
2
1
for the steam reforming reaction.
this biomass since it has the highest strength among all so-
However, TCD process needs a suitable and long-lasting woods available in the U.S. In the past, different types of char,
catalyst. Carbon deposition on the active surface of the cata- especially char made from charcoal was used in TCD of
lyst causes quick deactivation and makes this process unsuit- methane (as a catalyst for stable conversion). However, biochar
able for commercial applications. Recently, research efforts (by-product) produced during pyrolysis process has not been
were diverted towards guring out a long-lasting catalyst for this investigated in this regard. Azargohar et al. used fast pyrolysis
process. Catalysts used in TCD reaction can be divided into two biochar and activated that with KOH to produce activated bio-
23
major groups: metal and carbonaceous catalysts. Though metal char. According to them, activated carbon, which was
catalysts give very high initial conversion for TCD reaction, they produced by fast pyrolysis and went through KOH activation,
deactivate very fast with time. The formation of metal carbide is had a surface area 50 times higher than the starting material.
a major challenge for metal or metal-based catalysts. The Activated biochar produced from Douglas r had a large surface
carbonaceous catalyst overcame this problem and has attracted area and had different metal oxides (K
2
O, Na
2
O, CaO, MgO) in
1,4,13
researchers' attention because of better stability.
The its structure, which makes biochar an excellent catalyst for the
2
2
primary challenge with methane decomposition is sulfur decomposition of a stable compound. ZSM-5 (Zeolite Socony
content. Almost every reservoir of NG contains a small portion Mobil-5) is one of the widely used catalysts in different indus-
of the sulfur compounds. These sulfur compounds are trial applications. Its networking structure makes it unique to
poisonous to metal-based catalysts. Carbonaceous catalysts are perform as a catalyst. The presence of weak and strong acid sites
resistant to sulfur compounds and can absorb sulfur and large surface area and thermal stability makes zeolite
compounds from the feedstock. Activated carbon (AC), carbon perfect support for a high-temperature reaction. ZSM-5 was
black (CB), coal chars, glossy carbon, multi-walled nanotube used in non-oxidative methane conversion by several
2
4–26
(
MWNT), graphite, fullerenes have been used as a catalyst in researchers.
Nahreen et al. used 3% Ru–ZSM-5 catalyst for
14,15
TCD reaction.
Because of higher activity and better stability, non-oxidative methane conversion and obtained 44% conver-
AC and CB attracted researchers' attention the most. Muradov sion even aer 60 h of reaction at 800 C. On the other hand,
et al. studied thirty different carbonaceous materials and found Ru shows the highest sulfur resistance among group 8–10
that AC and CB are more active than the ordered ones. The metal, and Ru addition increases sulfur resistance of cata-
carbon catalyst activity has the following order: amorphous > lyst. According to Arcoya et al., group 8–10 metal catalysts
ꢀ
24
16
25
26
16
turbostractic > graphite. Amorphous carbon (AC and activated show the following order for sulfur resistance, Pt < Pd < Ni ꢃ Rh
25
26
biochar) is not well-ordered. An irregular array of carbon bonds < Ru. According to Ryu et al., the addition of Ru increases the
create free valences, surface defects, and dislocations. These catalyst resistance against sulfur poisoning. Stanley et al. used
energetic abnormalities increase high energy active sites for the Pt–Ru alloy to improve sulfur resistance for hydrogenation
27
reactant, and these catalysts will give better performance in catalysts.
TCD reaction.
17
Considering all the advantages of Ru, ZSM-5, and activated
To utilize the advantages of both metal and carbonaceous carbon (commercial), the main objective of this study was to
catalysts, researchers are trying to add a small amount of metals examine the effect of activated biochar derived from Douglas r
on carbon materials. Carbon can be reductive at high temper- and compare the performance of ZSM-5 and activated carbon-
atures. This ability helps metal oxide for in situ reduction during supported Ru catalysts for TCD reaction. This study has the
18
the pretreatment step. A carbon catalyst with a small amount potential to nd an application of biochar (a by-product from
of metal on it (doped or genuinely present) increases its activity biomass thermochemical conversion) for hydrogen production
greatly. Small metal addition on amorphous carbon creates from greenhouse potent gas.
active high energy sites, which attract stable methane molecule
and increase conversion. Prasad et al. doped 5% and 10% Pd on
2
Experimental
activated carbon (AC). AC-Pd10 gave 50% conversion aer 4 h of
reaction time, whereas AC-Pd5 gave 30% conversion aer 4 h of 2.1 Catalyst preparation
19
reaction time. Zhang et al. doped Ni on coal and CLR (carbon-
3
wt% Ru was doped on ZSM-5 and activated carbon (AC)
loaded rubber) based carbon and obtained stable and consis-
(commercial) are denoted as Ru–ZSM-5 and Ru–AC, respectively
tent conversion than only metal, coal, and CLR based carbon at
in this paper. Activated biochar and heat-treated biochar
produced from Douglas r biochar are denoted as AB and HB.
Details of catalyst preparation are explained in ESI.†
ꢀ
8
50 C. Ni/Al O , Ni/SiO , and CLR gave 40%, 50%, and 20%
2
3
2
initial conversion, which decreased to 10% aer 3 h of reaction
ꢀ
20
at 850 C. However, Ni-doped on carbon gave 30% initial
conversion, which increased to 80% aer 8 h of reaction at
ꢀ
21
2.2 Experimental set-up and procedure
8
50 C. Hierarchical porous carbon (HPC) with Al O gave 27%
2 3
initial conversion and this conversion increased to 61% aer A xed bed reactor made of Inconel was used for the reaction.
Inconel shows better stability than stainless steel in high-
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temperature applications, and the reactor had 1/2 in (12.7 mm) AC, AC, AB, and HB) were analyzed by TEM (JEOL 200CX) using
outer diameter, 3/8 in (9.53 mm) inside diameter and 21 in 100 kV.
(533.5 mm) length.
TPR (temperature-programmed-reduction) analysis. Aer
Fig. 1 represents the schematic of methane decomposition calcination, metals in catalyst are present in their oxides form.
experimental set-up. Delta V operating system was used for To get active metal phase on catalysts, the in situ reduction was
controlling reactor temperature and reactants ow rate. A xed performed before every experiments. Reduction temperature
bed reactor (top-feed) was connected with feed gases cylinders. was varied from the catalyst to catalyst. TPR analysis uses
An electric heater was used to heat the reactor. Product gases hydrogen to determine the reduction temperature of catalysts.
were passed through a heat-exchanger, which was cooled by TPR analysis was done with TPR analyzer (Quantachrome,
circulating chilled ethylene glycol and water. The product gases ASIQC0YV200-4).
were passed through a micro GC to analyze their composition. A
Conversion calculation. Feed gas composition was measured
known amount of catalyst was placed in the reactor, and the at the beginning of the reaction, and product gas composition
catalysts were tested at 0.1 weight hourly space velocity (WHSV) was measured at the interval of every 30 min. Since the feed gas
using 50% N
2
4
and 50% CH , and product gases were analyzed at composition was measured in the beginning that passes
3
1
5
0 min internal. The catalysts were reduced owing hydrogen at through the system, and the product line from the reactor to GC
ꢂ1
0 mL min prior to the reaction. Ru–ZSM-5 was reduced at was quite long the product gas composition was collected only
ꢀ
ꢀ
00 C for 5 h; Ru–AC was reduced at 600 C for 5 h; and AC, AB aer 2 hours of starting the reaction. Feed gases ow rates were
ꢀ
and HB were reduced at 700 C for 30 min. Aer the catalyst controlled by Delta V operating system. The total product gas
reduction, the reactor was set to the desired temperature for ow rate was calculated using nitrogen gas balance. The
decomposition experiments. Details of the pyrolysis reactor and following procedure was used for calculating methane conver-
its schematic diagram are provided in ESI.†
sion and hydrogen yield.
From N balances:
2
V
1
ꢄ SN in ¼ V
2
ꢄ SN out
2.3 Products and catalyst characterization and analysis
2
2
Micro-GC. A micro-GC (Agilent 3000A) was used to analyze
gaseous product composition. The micro-GC was equipped with
four capillary columns which could identify hydrogen, satu-
rated, unsaturated hydrocarbon (C –C and C grouped peaks),
V
1
ꢄ SN in
2
ꢂ1
rV ¼
2
ml min
S
2
N out
1
5
6+
and xed gases (N
2
, O
2
, CO, CO
2
). Column A (Molsieve, 10 m ꢄ where, V ¼ total inlet ow rate; S
¼ nitrogen inlet concen-
1
2
N in
3
20 mm ꢄ 12 mm) used argon as a carrier gas. Column B tration; V ¼ total product ow rate; S
¼ nitrogen outlet
2
N out
2
(
3
PLOTU, 8 m ꢄ 320 mm ꢄ 30 mm), column C (Alumina, 10 m ꢄ concentration.
20 mm ꢄ 8 mm), and column D (OV1, 14 m ꢄ 150 mm ꢄ 20 mm)
used He as a carrier gas.
Now, methane flow in ¼ V
1
ꢄ SCH in
4
SEM (scanning electron microscope). SEM (Zeiss, EVO 50,
UK) was used to get the microscopic image of carbon product.
Before taking SEM images, the samples were coated with gold
Methane flow out ¼ V ꢄ S
2
4
CH out
where SCH in ¼ methane inlet concentration; SCH out ¼ methane
4
4
(Au) to make samples conductive.
outlet concentration
TEM (transmission electron microscope). Product carbon
ZSM-5 and Ru–ZSM-5) were analyzed in TEM (Zeiss EM10)
(
methane flow in ꢂ methane flow out
%
conversion ¼
ꢄ 100%
using 80 kV to determine product carbon identity (nanotube or
methane flow in
nanober or amorphous). In the same way, spent catalyst (Ru–
Fig. 1 Schematic of experimental set-up for methane decomposition. (a) Nitrogen gas cylinder, (b) hydrogen gas cylinder, (c) methane gas
cylinder, (d) fixed bed reactor, (e) electric heater, (f) heat exchanger, (g) chiller, (h) micro-GC.
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V
1
ꢄ SCH in ꢂ V
2
ꢄ SCH out
4
4
¼
V
1
ꢄ SCH in
4
V
1
ꢄ SN in
2
V
1
ꢄ SCH in
ꢂ
ꢄ SCH out
4
4
S
2
N out
¼
V
1
ꢄ SCH in
4
V
1
ꢄ SCH in ꢄ SN out ꢂ V
1
ꢄ SN in ꢄ SCH out
4
2
2
4
¼
V
1
ꢄ SCH in ꢄ SN out
4
2
S
CH in ꢄ SN out ꢂ SN in ꢄ SCH out
4
2
2
4
¼
S
CH in ꢄ SN out
4
2
and,
So,
ꢂ1
H
2
flow out ¼ V
2
ꢄ SH out ml min
2
V
2
ꢄ SH out
2
ꢄ 1000 mmol minꢂ1
H
2
production ¼
ð1000 ꢄ 22:4Þ
where SH out ¼ hydrogen outlet concentration.
2
3
Results and discussions
3.1 Catalyst characterizations
XRD gives an idea about the structure of the catalyst (amor-
phous and crystalline). Fig. 2(a) represents the XRD pattern for
Ru–ZSM-5 and ZSM-5, and sharp peaks in the gure represent
Fig. 2 XRD pattern of (a) ZSM-5-based, (b) AC-based, and (c) biochar-
based catalysts.
ꢀ
the crystalline structure of ZSM-5. The ZSM-5 peaks at 13.59 ,
ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ
1
5.14 , 15.91 , 23.38 , 24.16 , 25.63 , and 30.18 that are asso-
ciated with (102), (131), (022), (051), (313), (323), and (062)
planes, respectively. The diffraction peaks matched very well
with those reported in the literature and were completely
indexed to the ZSM-5 zeolite structure, corresponding to the
JCPDS card no. 89-1421. Besides, there are three extra peaks for
ꢀ ꢀ ꢀ ꢀ
4 to 26 and a broad peak from 40 to 50 indicated the
2
30,31
amorphous structure of AB.
Chemical activation (KOH)
eliminated most of the quartz from AB and hence, no quartz
peak was observed for AB.
ꢀ
ꢀ
ꢀ
Fig. 3 represents the TPR proles of different catalysts. There
Ru–ZSM-5 at 28 , 35 and 54 , as these three peaks represent the
ꢀ
2
4,28
is a small hump at 130 C for Ru–ZSM-5 which represents the
presence of RuO
2
on the catalyst surface.
In the case of
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
reduction temperature of RuO
4
. The sharp peak at 189
C
Fig. 2(b), broad peaks from 20 to 30 and 40 to 50 indicate the
29–31
represents the reduction temperature of RuO
2
. The TPR prole
amorphous structure of AC and Ru–AC.
Ru doping did not
change the structure of the catalyst, but it just reduced the
intensity of the peak. Fig. 2(c) represents the XRD pattern of
biochar, HB, and AB. In the XRD pattern for biochar, broad
ꢀ
ꢀ
ꢀ
ꢀ
peaks from 20 to 28 and 35 to 50 ensure the amorphous
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
structure. Sharp peaks at 21 , 26 , 50 , 60 , and 68 indicate the
presence of turbostratic crystalline carbon and quartz (SiO ) on
2
biochar. A similar pattern was observed for HB with more sharp
peaks. Douglas r biochar has alkali, alkaline earth metals and
22
2
quartz (SiO ) in its structure. The volatile compounds present
in biochar were responsible for the reduced height of the peaks.
In the case of HB, heat treatment process eliminated all the
ꢀ
ꢀ
volatile compounds. Therefore, extra sharp peaks at 36 , 39 ,
ꢀ
and 43 were observed. All of these peaks indicated the presence
of quartz (SiO
2
). On the other hand, a small wide peak from Fig. 3 TPR pattern for different catalysts.
32
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Table 1 Results of chemisorption analysis
Specic active
metal
Average
Name of
catalysts
Percent of metal
dispersion (%)
surface area
crystallite
size (nm)
2
ꢂ1
(m g
)
Ru–ZSM-5 2.94 ꢅ 0.08
Ru–AC
3.16 ꢅ 0.04
1.07 ꢅ 0.03
1.15 ꢅ 0.01
44.64 ꢅ 1.31
42.46 ꢅ 0.50
2
4,31
of Ru-oxide was also matched with the literature,
and the
Fig. 4
2
N adsorption/desorption profile for different catalysts.
ꢀ
reduction was performed at 500 C for Ru–ZSM-5 catalyst. There
is a big sharp peak at 214 C and a small sharp peak at 327 C for
ꢀ
ꢀ
Ru–AC. These two peaks represent the reduction temperature of
ZSM-5 and AC were doped with 3% Ru. The chemisorption
was performed to determine the average crystal size of Ru on
support. Table 1 represents the chemisorption results for Ru–
ZSM-5 and Ru–AC. Ru on AC had slightly higher dispersion,
higher specic active surface area and smaller crystallite size.
These could be attributed to greater contribution of micropores,
mesopores, and macropores to total pore volume in Ru–AC than
Ru–ZSM-5 catalyst as discussed later in this document.
Elemental analysis and ash content data are presented in
Table 2. In general, biochar and activated biochar have much
more carbon than biomass. Around 3.9 ꢅ 0.3%, ash content in
AB indicated that chemical activation (KOH) could not elimi-
nate all the alkali and alkaline earth metals. The ash content of
HB (7.6 ꢅ 0.3%) was higher than the ash content of biochar (4.8
ꢅ 0.6). Heat-treatment eliminated volatile components and
increased the percent of ash content in HB.
ꢀ
RuO
4
and RuO
2
on AC, respectively. A broad peak at 520 C
represents the presence of impurities on support AC or
31
unknown Ru species. To compare this, TPR was performed on
AC as well. In order to make sure complete reduction of catalyst
occurred prior to methane decomposition, reduction was per-
ꢀ
ꢀ
formed at 600 C for Ru–AC and at 700 C for AC. Biochar
derived catalysts have ash contents as impurities and some
oxygen containing functional groups, and TPR was performed
on AB and HB to determine the reduction temperature of those
ꢀ
impurities. Ash contents of AB were reduced at 600 C, whereas
ash contents of HB were reduced at 380 C, 450 C, and 650 C
very small humps). The reduction was performed at 700 C for
ꢀ
ꢀ
ꢀ
ꢀ
(
both of the catalysts (AB and HB). However, carbon catalysts
AC, AB, and HB) showed very small hydrogen consumption.
(
Additionally, small hydrogen consumption for AC, AB and HB
could be due to the reduction of oxygen functional groups in
those catalysts.
Surface area plays an important role in catalysis. Higher
surface area gives higher space for methane molecules to get
a
Table 2 Elemental analysis of biomass, biochar, AB, and HB
Name
C (%)
H (%)
N (%)
S (%)
Ash (%)
AB
HB
Biochar
Biomass
AC
84.70 ꢅ 0.5
81.9 ꢅ 0.4
84.0 ꢅ 1.0
50.1 ꢅ 0.4
90.1 ꢅ 0.3
2.80 ꢅ 0.10
2.40 ꢅ 0.01
3.70 ꢅ 0.04
7.80 ꢅ 0.05
1.60 ꢅ 0.10
0.30 ꢅ 0.01
0.30 ꢅ 0.01
0.12 ꢅ 0.00
0.02 ꢅ 0.01
0.30 ꢅ 0.10
0.03 ꢅ 0.02
ND
ND
ND
0.01 ꢅ 0.00
3.9 ꢅ 0.3
7.6 ꢅ 0.3
4.8 ꢅ 0.6
0.4 ꢅ 0.3
4.1 ꢅ 0.4
a
ND ¼ not detectable.
Table 3 Specific surface area, pore volume, and average pore size of different catalysts
3
ꢂ1
Pore volume (cm g
)
Specic surface
area (BET) (m g
2
ꢂ1
Name of catalysts
)
Micropore
Mesopore
Macropore
Total pore volume
Average pore size (nm)
a
a
d
d
d
b
b
d
d
d
b
b
c
c
Ru–ZSM-5
ZSM-5
Ru–AC
AC
AB
HB
297 ꢅ 0.10
363 ꢅ 11.30
693 ꢅ 3.40
776 ꢅ 18.60
3256 ꢅ 42.40
109 ꢅ 4.20
0.19 ꢅ 0.00
0.23 ꢅ 0.02
0.32 ꢅ 0.01
0.35 ꢅ 0.01
1.30 ꢅ 0.03
—
0.07 ꢅ 0.01
0.06 ꢅ 0.01
0.26 ꢅ 0.01
0.34 ꢅ 0.01
0.29 ꢅ 0.03
—
0.04 ꢅ 0.01
0.27 ꢅ 0.01
3.65 ꢅ 0.01
c
c
0.04 ꢅ 0.01
0.32 ꢅ 0.01
3.56 ꢅ 0.01
c
c
—
—
—
—
0.65 ꢅ 0.03
1.88 ꢅ 0.03
c
c
0.75 ꢅ 0.01
1.89 ꢅ 0.02
c
c
1.78 ꢅ 0.01
1.10 ꢅ 0.01
c
c
0.09 ꢅ 0.00
1.64 ꢅ 0.06
a
b
c
d
Satio–Foley method (SF). Dollimore–Heal method (DH). Brunauer–Emmett–Teller (BET). Density functional theory method (DFT).
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adsorbed on the catalyst surface. Surface area, pore volume, and became difficult. Therefore, desorption of N molecules fol-
average pore size of six catalysts were measured, and presented lowed different paths at high P/P and created a hysteresis
o
30
in Table 3. ZSM-5 and Ru–ZSM-5 showed high micropore loop. AC and Ru–AC have micropores and mesopores in their
structures. Both of the catalysts exhibited type IV isotherm with structures, and type IV isotherms were observed for both of the
a hysteresis loop at higher relative pressure (Fig. 4). Initially, catalysts. AB displayed type I isotherm as it has high micropo-
capillary condensation of N
oporous structures of ZSM-5 and Ru–ZSM-5. At high relative
pressure (P/P ), N desorption from the mesoporous structures for AB. Adsorption and desorption isotherms did not match
2
molecules took place on the mes- rous structure.
Chemical treatment (KOH) created a microporous structure
o
2
Fig. 5 SEM image of fresh catalysts.
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Table 4 EDS results for Ru–ZSM-5
Element
Weight%
Atomic%
O
Al
Si
58.09 ꢅ 0.12
2.09 ꢅ 0.06
36.05 ꢅ 0.14
3.01 ꢅ 0.02
71.50 ꢅ 0.10
2.09 ꢅ 0.06
25.42 ꢅ 0.12
0.59 ꢅ 0.01
Ru
with each other for HB. This type of isotherm indicates that
there is no dened pore structure. The reason behind this type
ꢀ
Fig. 6
H
2
production results for different catalysts at 800 C and 0.1
of isotherm is that it was easy for N
HB surface at high P/P . At the same time, it was difficult for N
molecules to desorb from the nonporous structure of HB.
Therefore, N desorption isotherm followed a different path is noticeable that white dots are uniformly distributed on Ru–
from the adsorption isotherm. Normally, biochar shows this ZSM-5, indicate Ru-metal was uniformly distributed on ZSM-5.
2
molecules to adsorb on the WHSV.
o
2
2
33,34
type of isotherm.
Heat-treatment did not make any changes Normally, backscatter electrons are used to distinguish two
in N adsorption/desorption isotherm for HB. Table 3 presents phases in SEM image. A secondary electron detector was used to
2
physical properties of the catalysts tested in this study. Micro, capture the images of fresh AC and Ru–AC catalysts. Since
meso, and macropore volumes of different catalysts were carbon is a very small molecule, the energy released by back-
calculated by DFT (Density Functional Theory), SF (Satio–Foley), scatter electron cannot be captured on the detector. Therefore,
and DH (Dollimore–Heal) methods where appropriate. Micro- it was not possible to separate Ru and AC phases on SEM. There
pore and mesopore volumes were subtracted from the total pore is no major difference between the SEM images of fresh AC, Ru–
volume to calculate macropore volume. Typically, pore diameter AC, AB, and HB. Particle sizes are bigger for AC and Ru–AC. EDS
less than 2 nm considers as micropore, from 2 nm to 50 nm (energy dispersion spectroscopy) analysis was performed to
considers as mesopore and greater than 50 nm considers as estimate the composition of different catalysts. Tables 4 and 5
35
macropore. ZSM-5 and Ru–ZSM-5 catalysts showed mesopore represent the composition of Ru–ZSM-5 and Ru–AC. Results
and micropore. Therefore, the DFT method did not give any from EDS analysis ensured that around 3% Ru was on both
results for zeolite catalysts, and the SF method was used to catalysts.
calculate micropore volume for ZSM-5 and Ru–ZSM-5, as SF
method only works for micropore. Mesopore and macropore
volume of ZSM-5 and Ru–ZSM-5 were calculated by DH method
3.2 Reaction results
since DH method is widely accepted for calculating mesopore Fig. 6 represents the total amount of hydrogen produced in the
and macropore volume. On the other hand, Ru–AC, AC, and AB reaction. Hydrogen production graph followed the same
showed only micropore and mesopore. Therefore, the DFT sequence as the conversion graph. Initial hydrogen yields were
1
method was used for the pore size distribution analysis. HB did low in every case. According to literature, most of the activated
not have any dened pore structure. Micropore and mesopore carbon has carbon–oxygen groups in structure. Product
volume could not be calculated for this catalyst. Among the six hydrogen gas might have been consumed by these functional
2
ꢂ1
catalysts, HB gave the lowest surface area (110 ꢅ 5 m g ), group in rst few hours of reaction although very little hydrogen
2
ꢂ1
whereas AB had the highest surface area (3260 ꢅ 40 m g ). was consumed by the carbon based catalysts as seen from the
ꢂ1
Loading of Ru reduced the surface areas of ZSM-5 and AC. Ru– TPR results. Ru–ZSM-5 gave 0.14 mmol min of hydrogen
2
ꢂ1
ꢂ1
ZSM-5 had 297 ꢅ 1 m g surface area whereas ZSM-5 gave 360 production, whereas ZSM-5 gave 0.07 mmol min of hydrogen
2
ꢂ1
2
ꢂ1
ꢅ 10 m g . On the other hand, Ru–AC had 690 ꢅ 5 m g
2
ꢂ1
surface area, and AC had 780 ꢅ 20 m g surface area.
Fig. 5 represents SEM image of fresh catalysts. SEM image of
fresh ZSM-5 and fresh Ru–ZSM-5 catalysts were captured by
using backscatter electrons. By comparing these two images, it
Table 5 EDS results for Ru–AC
Element
Weight%
Atomic%
C
O
Ru
86.69 ꢅ 1.16
10.44 ꢅ 1.14
2.90 ꢅ 0.07
91.37 ꢅ 0.94
8.27 ꢅ 0.93
ꢀ
0.36 ꢅ 0.01 Fig. 7 Catalytic behavior of Ru–AC and AB at 800 C and 0.1 WHSV in
6
0 h reaction run.
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ꢀ
Fig. 8 SEM image of spent catalysts at 800 C and 0.1 WHSV: (a) spent ZSM-5 after 8 h, (b) spent Ru–ZSM-5 after 8 h, (c) spent AC after 8 h, (d)
spent Ru–AC after 60 h, (e) spent AB after 60 h, (f) spent HB after 8 h.
ꢀ
production aer 8 h of reaction time. Ru–AC produced 0.26
Methane conversion for different catalysts at 800 C and 0.1
ꢂ1
mmol min
of hydrogen whereas AC produced 0.18 WHSV is presented in ESI (Fig. S9†). Blank run indicates the
of hydrogen aer 8 h of reaction time. AB reaction without any catalyst at 800 C in an empty reactor.
produced 0.23 mmol min of hydrogen production aer 8 h of Around 2–3% methane was converted in a blank run. Normally,
the reaction. In the case of HB, it was 0.14 mmol min , same as methane should not be converted at that temperature without
ꢂ1
ꢀ
mmol min
ꢂ1
ꢂ1
4
Ru–ZSM-5 aer 8 h of the reaction. Hydrogen yields were above any catalyst. However, the reactor has Ni and Fe as its
ꢀ
9
0% for all of the catalysts at 800 C and 0.1 WHSV.
components and that might have caused a small conversion of
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methane. ZSM-5 gave 21% conversion aer 8 h of reaction time. Table 6 BET analysis results for fresh and spent AB catalyst
Aer being doped with Ru, ZSM-5 gave 40% conversion at the
3
ꢂ1
Pore volume (cm g ) (DFT)
same time of reaction. Ru enhanced the catalytic activity of
ZSM-5 by 20%. Ru metal particles give electron to zeolite surface
sites, and zeolite channels conned with small particles are Name
electron decient. This electrophilic character enhance the
Specic surface
area (BET) (m g
Total
pore
2
ꢂ1
)
Micropore
Mesopore
Fresh AB
Aer 8 h
Aer 60 h
3256
1893
746
1.30
0.74
0.29
0.29
0.22
0.10
1.59
0.96
0.39
36
activity of zeolite supported catalyst. On the other hand, AC
alone gave 51% conversion aer 8 h of reaction time, whereas
Ru–AC gave 73% conversion at the same amount of reaction
time. Methane conversion for AB was 69% aer 8 h of reaction
time. The high surface area and the presence of ash contents on
the structure of AB were probably the reasons for this higher
conversion. Just HB gave 41% conversion aer the same time of
reaction time. HB has a low surface area compared to Ru–ZSM-5
and ZSM-5. Additionally, the inuence of feeding rate at 0.4
WHSV suggested good methane conversion and hydrogen
production on AB and Ru–AC performed the best (see Fig. S9 in
ESI†).
2
ꢂ1
the surface area of AB was reduced to 1893 m g (Table 6).
2
ꢂ1 ꢂ1
Surface area reduction by 8 h of reaction was 170 m g
whereas surface area reduction by rest of 52 h of reaction was 22
h
2
ꢂ1 ꢂ1
m g
h . Aer 60 h of reaction, AB still had higher surface
2
ꢂ1
2
ꢂ1
area (746 m g ) than fresh Ru–AC (693 m g ). Product CNTs
have catalytic ability for methane conversion. Several
researchers had used CNTs as catalyst for methane conver-
1
,4,16,43
sion.
Product carbon create new active sites for TCD by
1
Among six different catalysts, AB and Ru–AC exhibited the
highest conversions aer 8 h of reaction time. Therefore, these
catalysts were tested to determine long-term catalytic stability.
Fig. 7 shows methane conversion reaction results on AB and
Ru–AC for 60 h. Methane conversion was higher for Ru–AC than
AB for the rst 14 h. However, the conversion was decreasing
steadily with time and aer 60 h, the methane conversion was
growing outside of catalyst pore. Product CNTs took part in
auto catalysis.
growth in AB could contribute to a stable conversion pattern in
the long reaction run. Ru–AC showed 50% micropore and 50%
mesopore in its structure whereas mainly micropores were
20,21
The initial high surface area and the CNTs
38,39
contributed for methane conversion.
larger pores block with product carbon without using the inside
Sometimes, mouth of
42
2
1%. Although the methane conversion was decreasing rapidly
surface area. Though Ru–AC had comparable methane
conversion in the beginning, the effect of Ru addition dimin-
ished in the long run. Because of lower conversion, production
of CNTs were also low and those CNTs were attached with
catalyst pores (blocked active pore of catalyst). Therefore, auto
catalysis reaction rate was comparatively low for Ru–AC.
for the rst 10 h with AB, the conversion remained pretty stable
aer that and the conversion was 51% at the end of 60 h run.
Metal–support interaction plays an important role in CNT
growth as well as catalyst deactivation. If metal–support inter-
action is weak, then metal particles detach from the support.
Detach metal particles localize at the tip of the nanotubes. This
type of phenomena causes separation of the metal particles
from support and catalyst deactivates with time. For strong
3.3 Carbon produced in reaction
metal–support interaction, the metal particle does not separate Carbon produced in the reactions were analyzed by SEM and
37
from support and the deactivation rate is slow. Since Ru–AC TEM. Fig. 8 represents the SEM image of different spent cata-
ꢀ
catalyst was prepared by the wet impregnation method, weak lysts aer reaction at 800 C. A big tube structure with lots of
metal–support interaction presented between Ru and AC. small tube structures were observed in this SEM image. Fig. 9
Production of nanotube (Fig. 8) caused separation of Ru parti- represents the TEM image of different spent catalysts aer
ꢀ
cles from AC. With time, Ru particles were covered by carbon. reaction at 800 C. Aer analyzing SEM (Fig. 8(a) and (b)) and
All these phenomena were responsible for deactivation in the TEM (Fig. 9(a) and (b)) image, it was conrmed that carbon
long run. Ash contents are part of biochar structure. In case of nano-tubes (CNTs) were producing with ZSM-5 and Ru–ZSM-5
AB, strong metal–support interaction presented between ash catalysts. For the TEM image of these two catalysts, produced
content and carbon. CNTs growth (Fig. 8) could not separate the carbons were separated from the catalysts and crushed to ne
metal particle from carbon support. As a catalyst, Ru always powder. In zeolite, metal ions are impregnated only in ion
exhibits good conversion but Ru–AC has a comparatively low exchange and are accessible only for the small molecule, which
2
ꢂ1
2
surface area (693 m g ). AB has a large surface area (3256 m
g
has the diameter less than pore diameter and the only product
ꢂ1
). This large surface area accommodated product carbon for with the small diameter less than pore diameter can exit pore
the long reaction time. AB has 82% microporous structure. channel of the catalyst. So, CNTs growth can be possible only on
According to Krzy ˙z y n´ ski et al. higher surface area and higher the outer surface metal particle of zeolites. That is why zeolite
38
pore volume give higher resistant to catalyst deactivation.
gives less conversion as well as less CNTs. Because catalyst
Ashok et al. and Dhunker and Ortmann concluded that surface is easily blocked with CNTs and become non-accessible
39–41
44,45
methane decomposition mainly occur at micropores.
within few hours of reactions.
Aer analyzing SEM (Fig. 8(c)
According to Kim et al., methane decomposition rate is higher and (d)) and TEM (Fig. 9(c) and (d)) images, it can be concluded
for smaller particles and it occurs on outer shell of catalyst that carbon produced by AC had a turbostratic structure
42
particles. For larger particles, pore mouth blocked by carbon whereas 3% Ru doping on AC caused CNT production. For TEM
deposition while inner area remain intact. Aer 8 h of reaction, analysis, spent catalysts were crushed to ne powder, and from
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Fig. 9 TEM image of spent catalysts: (a) spent ZSM-5 after 8 h, (b) spent Ru–ZSM-5 after 8 h, (c) spent AC after 8 h, (d) spent Ru–AC after 60 h,
and (e) spent AB after 60 h.
SEM (Fig. 8(e)) and TEM (Fig. 9(e)) image of AB, it can be
concluded that CNTs were produced by AB. The similar SEM
Acknowledgements
image was observed for HB (Fig. 8(f)) as observed for Ru–AC This work was supported by the Alabama Agricultural Experi-
(Fig. 8(d)). However, it was not possible to observe any CNTs in ment Station and the Hatch Program (ALA014-1-19068) of the
the TEM image.
National Institute of Food and Agriculture, U.S. Department of
Agriculture.
4
Conclusions
ꢀ
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