Ni/H-ZSM-5 as a stable and promising catalyst for COx free H2 production by CH4 decomposition

Article abstract of DOI:10.1039/c6ra03857c

Catalytic decomposition of methane for CO_x free hydrogen production is carried out over Ni supported on H-ZSM-5 catalysts with different Si/Al ratios (i.e. 40, 150, 300 and 485) at 550 °C and atmospheric pressure. Methane decomposition activity of Ni/H-ZSM-5 is decreased with time on stream and finally deactivated completely. The fresh and reduced catalysts are characterized by BET-SA, XRD, FT-IR, UV-DRS, TPR, pulse chemisorption of H₂ and N₂O and some of the used catalysts are characterised by CHNS, SEM, TEM and Raman spectroscopy. Raman spectra of the used catalysts showed both ordered and disordered carbon at 1580 cm^-1 and 1320 cm^-1. The 20 wt% Ni/H-ZSM-5 (Si/Al = 150) exhibited a higher H₂ production rate over the other Ni loadings. The superior performance of 20 wt% Ni/H-ZSM-5 (Si/Al = 150) is rationalized by the physico-chemical properties of the various Ni loaded H-ZSM-5 catalysts.

Full text of DOI:10.1039/c6ra03857c

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Ni/H-ZSM-5 as a stable and promising catalyst for COx free H2 production by CH4
decomposition
Chatla Anjaneyulua, Gutta Naresha,c, Velisoju Vijay Kumara,c, Aytam Hari Padmasrib, James
Tardioc, Suresh Kumar Bhargavac and Akula Venugopal*,a
a
Catalysis Laboratory, Inorganic & Physical Chemistry Division, CSIRꢀIndian Institute of
Chemical Technology, Tarnaka ꢀ 500007, Hyderabad, Telangana, India. *Corresponding
author; Eꢀmail: akula@iict.res.in Tel: +91ꢀ40ꢀ27191720/3510; Fax: +91ꢀ40ꢀ27160921
b
Department of Chemistry, University College for Women Osmania University, Koti,
Hyderabad ꢀ 500 095, Telangana, India.
c
Centre for Advanced Materials & Industrial Chemistry (CAMIC), School of Applied
Sciences, RMIT University, GPO BOXꢀ2476, Melbourne 3001, Australia.
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Abstract
Catalytic decomposition of methane for COx free hydrogen production is carried out over Ni
supported on HꢀZSMꢀ5 catalysts with different Si/Al ratio (i.e. 40, 150, 300 and 485) at 550
°C and atmospheric pressure. Methane decomposition activity of Ni/HꢀZSMꢀ5 is decreased
with time on stream and finally deactivated completely. The fresh and reduced catalysts are
characterized by BETꢀSA, XRD, FTꢀIR, UVꢀDRS, TPR, pulse chemisorption of H2 and N2O
and some of the used catalysts are characterised by CHNS, SEM, TEM and Raman
spectroscopy. Raman spectra of the used catalysts showed both ordered and disordered
carbon at 1580 cmꢀ1 and 1320 cmꢀ1. The 20 wt% Ni/HꢀZSMꢀ5 (Si/Al = 150) exhibited higher
H2 production rate over the other Ni loadings. The superior performance of 20wt% Ni/Hꢀ
ZSMꢀ5 (Si/Al = 150) is rationalized by physicoꢀchemical properties of the various Ni loaded
HꢀZSMꢀ5 catalysts.
Key words: Ni/HꢀZSMꢀ5, H2 production, H2 pulse chemisorption, CNTs, Ni metal surface
area, N2O decomposition
1. Introduction
Hydrogen is a versatile and efficient fuel, which undergoes a clean combustion as
water is the only product generated [1]. Hence it is regarded as a promising energy vector in
both transport and power generation sectors. Nowadays, most of the hydrogen is produced by
steam reforming of methane, which is accompanied by large emissions of CO2 [2]. Thus, all
the carbon contained in the raw methane ends up as CO2 a green house gas. There are several
processes for the production of hydrogen and two of them are steam and auto thermal
reforming of natural gas. Although these processes are mature technologies, but they are
somewhat complex and contain multistep. In addition, CO free H2 is highly desirable for use
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in PEM fuel cells as CO poisons the Pt electrode. The direct cracking of methane over
supported nickel catalysts is an alternative route for the production of hydrogen from natural
gas [3, 4]. Methane decomposition reaction is a moderately endothermic process. The energy
required per mole of hydrogen produced (45.0 kJ/mol H2 at 600 °C) is less than that for the
steam reforming of methane (48.0 kJ/mol H2). Unlike the steam reforming process, the
catalytic decomposition of methane (CDM) process does not include water gas shift and
preferential oxidation of CO, which considerably simplifies the process and cost of hydrogen
production [5–7].
CH4→ C + 2H2; ∆H° = 75.6 kJ mol−1
This hydrogen can be used as a clean energy for fuelꢀcells and the produced carbon
can be used as functional material, such as fibres, graphite, carbon black, composites and
catalyst supports. Here it is noteworthy to mention that carbon alone can be used as a catalyst
for CDM but only at high temperatures (> 750 °C) [8]. Nickel is a common transition metal
used for the conversion of methane [9ꢀ16]. The most important factors that influence the
carbon deposits during methane decomposition are the particle size, dispersion and
stabilization of the metallic nickel particle by selecting appropriate support. Michalkiewicz
and coꢀworkers critically reviewed the effect of reaction temperature on the carbon growth
due to the Ni loading and also the nature of support that would strongly influence the H2
yields as well as the quality of the CNTs formed during the CDM process [17ꢀ19]. Takenaka
et al. [20] reported that a typical 40% Ni/SiO2 catalyst with the nickel particle size of 60–100
nm that could give the carbon yield of as high as 491 gC (gNi)−1 during methane
decomposition at 550°C. Furthermore, Ermakova et al. [21] reported the 90 wt% Ni/SiO2
catalyst with the nickel particle of 10ꢀ40 nm size provided carbon yields in the order of 385 g
C (gNi)−1 at 550°C. Other supports, such as TiO2, MgO, ZrO2 and Al2O3 gave relatively
lower carbon yields [22]. Though the activated carbon and carbon blacks were explored for
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CDM, due to their lower activity, metal catalysts particularly Ni based catalysts were found
to be better than the other transitional metal catalysts [23ꢀ26]. A recent review emphasized
the importance of modelling studies and regeneration of the catalysts in the catalytic
decomposition of CH4 [27]. Choudary et al observed the formation of small amount of CO
during CH4 decomposition over Ni/HꢀZSMꢀ5 catalyst [28]. We believe that CO formation is
occurring due to incomplete reduction of catalyst during the reductive pretreatment (Figure
S1). Presence of ionic Ni species may be a reason for the formation of CO during the CDM
process. In the present study Ni supported on HꢀZSMꢀ5 with varied Si/Al ratio samples were
reduced at 550 C for 2 h prior to the reaction; surprisingly formation of COx compounds was
absent during the CDM reaction.
Hence the present work is undertaken to explore the catalytic performance of Ni
supported on HꢀZSMꢀ5 with four different Si/Al ratios i.e. 40, 150, 300 and 485 for methane
decomposition at reaction temperature of 550 ºC. The activity and stability of catalysts is
discussed based on the Ni metal surface area in conjunction with Raman spectra of
deactivated samples.
2. Experimental
2.1 Catalysts preparation
The Ni/HꢀZSMꢀ5 samples were prepared by a simple incipient wet impregnation
method. Here we have chosen HꢀZSMꢀ5 as a support with different Si/Al ratio (i.e. Si/Al
ratio = 40, 150, 300 and 485 mole ratio). The zeolite support is commercially available; Hꢀ
ZSMꢀ5 (Si/Al = 40, surface area = 600 m2 g−1, Si/Al = 150 surface area = 470 m2gꢀ1, Si/Al =
300 surface area= 340 m2gꢀ1 and (Si/Al = 485; surface area = 450 m2 gꢀ1 prepared by us, see
SI) supplied by M/s. Sud Chemie India Private Limited. In a typical impregnation method
the required amount of nickel nitrate [Ni(NO3)2·6H2O] was taken to give required Ni loading
with known amount of water in a 100 ml beaker and mixed with the requisite amount of
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support material to yield the respective Ni wt% needed. These solutions were dried with
constant stirring at 100 ºC until the sample gets dried and then kept for drying at 100 ˚C for
about 24 h, followed by calcination in air at 550 °C for 5 h. Thus, prepared catalysts were
used for evaluating CDM activities.
2.2. Catalysts Characterisation
The XRD patterns of the samples were obtained on a Rigaku miniflex Xꢀray
diffractometer using Ni filtered Cu Kα radiation (λ = 1.5406 A°) from 2θ = 10 to 80º at a scan
rate of 2º/min with the beam voltage and a beam current of 30 kV and 15 mA respectively.
The surface areas of the fresh catalysts were measured by N2 physical adsorption at ꢀ196 ºC
in an AutosorbꢀI (Quantachrome) instrument. The specific surface area was calculated
applying the BET method. The SEM images of the fresh and used catalysts were recorded
using a JEOLꢀJSM 5600 instrument. For TEM analysis the samples dispersed in methanol
solution and suspended on a 400ꢀmesh, 3.5mm diameter Cu grid and images were taken using
JEOL JEM 2010 highꢀresolution transmission electron microscope. The Raman spectra for
the structural characterization of deposited carbon in deactivated catalysts have been acquired
with a Horiba JobinꢀYvon lab ram HR spectrometer using a laser beam excitation of λ =
632.81nm. The infrared spectra were recorded between 4000 and 400 cmꢀ1 at room
temperature in a Agilent cary 600 FTꢀIR spectrometer. The carbon contents in deactivated
catalysts are measured with VARIO EL analyser instrument. The UV–Vis spectra were
recorded at room temperature using a UVꢀ2000, Shimadzu Spectrophotometer equipped with
a diffuse reflectance attachment with an integrating sphere containing BaSO4 as a reference.
The spectra were recorded in the range between 190 and 800 nm with sampling interval 0.5
nm and slit width 2 nm and spectra were converted to KubelkaꢀMunk function.
The TPR analysis was carried out in a quartz microꢀreactor interfaced to GC with
TCD unit. About 50 mg of sample was loaded in an isothermal zone of a quartz reactor (i.d =
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0.6 cm, length = 30 cm) heated by an electric furnace at a rate of 10 ºC minꢀ1 to 300 ºC in
flowing helium gas at a flow rate of 30 mL minꢀ1. After degassing the sample was cooled to
room temperature then the helium gas switched to 5% H2 in argon (30 mL minꢀ1) and the
temperature was increased to 700 ºC at a ramping rate of 5 ºC/min. Hydrogen consumption
measured by analyzing effluent gas by means of thermal conductivity detector and the H2
uptakes were estimated with a calibration TPR of Ag2O under similar protocol. Hydrogen
chemisorption measurements were carried out on a Varian 3800 instrument. Prior to H2
adsorption ~100 mg of the sample was reduced in a flow of hydrogen (30 mL minꢀ1) at 550
°C for 2 h and flushed out in a pure argon (Ar) gas flow (purity 99.995%) for an hour at 550
°C. The sample was subsequently cooled to 60 °C temperature in Ar gas stream. Hydrogen
uptake was determined by injecting pulses of 5% hydrogen balance argon from a calibrated
loop connected to onꢀline sampling valve in Ar stream passing the over the reduced sample.
The nickel metal surface area was calculated assuming a stoichiometry of one hydrogen
molecule per two surface nickel atoms using an atomic crossꢀsectional area of 6.49 x 10ꢀ20
m2/Ni atom. The H2 adsorption was deemed to be complete after at least three successive
peaks showed the similar GC areas [29]. N2O pulse titration experiments were performed to
analyze the Ni dispersion, size and Ni metal surface area [30, 31] using Varian 3800 series
system equipped with a thermal conductivity detector (TCD). The catalysts of 50−100 mg
were placed in the quartz tube and reduced to 550 °C for 1 h in 5% H2/Ar prior to the test. He
gas was used as carrier gas at 30 mL min−1, and the successive pulses of pure N2O gas were
subsequently introduced into He stream by means of a calibrated sampling valve at a
temperature of 90°C. The product gases (N2 and N2O) were analyzed by Porapak N column
using GC with TCD.
2.3 Methane decomposition activity measurements
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The catalytic activities were performed at 550 °C at atmospheric pressure in a fixedꢀ
bed vertical quartz reactor (i.d = 1 cm, length = 46 cm) operated in a down flow mode heated
by an electric furnace. Methane supplied by Bhuruka gases limited (99.99%) was used
directly without further purification. Many of the experimental conditions adopted were same
as reported earlier [8] and some of the details were: a catalyst charge of ~20 mg, with
methane flow rate = 30 mL min−1, i.e. GHSV of 90 L hꢀ1 (gcat)−1. Prior to the reaction, the
catalyst was reduced at 550 °C with 5% H2 in Ar for 2 h. The reaction was continued until all
the catalysts were deactivated. Then, obtained catalysts were weighed and after each
experiment. The outflow gas was analyzed online by gas chromatography equipped with a
carbosphere column and TCD detector using N2 as a carrier. The conversion of methane was
determined using calibrated data. The first analysis was done 10 min after methane was
passed over the catalyst.
3. Results
3.1 FT-IR analysis
The 30wt% Ni supported on zeolite with different Si/Al mole ratios are analysed by
FTIR spectroscopy (see SI). The absorption bands near 788, 1084 and 1218 cmꢀ1 are
characteristic of SiO4 tetrahedron units [32, 33]. The strong absorption band in the region
1000ꢀ1200 cmꢀ1 has been assigned to the internal vibration of SiO4, AlO4 tetrahedra for ZSMꢀ
5 [34]. The band around 1084 cmꢀ1 is attributed to the internal asymmetric stretching
vibration of SiꢀOꢀSi linkage and is found to be shifted towards higher wave number with
increasing Si/Al ratio. The band near 788 cmꢀ1 is assigned to the symmetric stretching of the
external linkages and the strong band near 543 cmꢀ1 is ascribed to the double fiveꢀring lattice
vibration of the external linkages [35] (Fig. S2A). The vibrational band around 450 cmꢀ1 is
due to the TꢀO bending vibrations of the SiO4 and AlO4 internal tetrahedra. The absorption
bands around 543 and 450 cmꢀ1 are characteristic of the ZSMꢀ5 crystalline structure [36]. The
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sample with various Ni loadings on HꢀZSMꢀ5 (Si/Al = 150) (Fig. S2B) exhibited bands at
1230 cmꢀ1, 1100 cmꢀ1 and 796 cmꢀ1 that are attributed to the SiꢀOꢀSi asymmetric and
symmetric stretching frequencies as well as internal stretching frequencies of HꢀZSMꢀ5. The
band near 3429 cmꢀ1 indicates that there are plenty of –OH groups on Ni/HꢀZSMꢀ5 surface.
The band at 1625 cmꢀ1 is attributed to adsorbed water, and these results are similar to the
earlier reports [37].
3.2 XRD analysis of fresh calcined and reduced catalysts
The powder XRD patterns of 30wt% Ni loaded on HꢀZSMꢀ5 (with Si/Al = 40, 150,
300 and 485) fresh samples showed the reflections at 2θ = 37.28, 43.3, 62.9° and 75.3°
pertaining to crystalline NiO phase [ICDS # 01ꢀ1239], along with minor diffraction lines of
HꢀZSMꢀ5 phase (Fig. S3A). The (Ni loaded) fresh calcined 10ꢀ50 wt% Ni/HꢀZSMꢀ5 (Si/Al
ratio = 150) samples also confirmed both NiO and HꢀZSMꢀ5 phases. Increase in Ni loading,
intensity of NiO diffraction lines are increased due to formation of larger crystallites (Fig.
S3A). The XRD patterns of 30wt% Ni/HꢀZSMꢀ5 samples (reduced) displayed the metallic
Ni phase [ICDS# 87ꢀ0712] (Fig. S4A & Fig. S4B).
3.3 H2-TPR analysis
The TPR patterns of Ni loaded HꢀZSMꢀ5 samples presented in Fig. 1A, and the H2
uptakes are reported in Table 1. The 30 wt% Ni/HꢀZSMꢀ5 exhibited the Tmax 540 ºC ascribed
to reduction of bulk NiO species. The H2 uptakes are more or less same, over the 30wt%
Ni/HꢀZSMꢀ5 (with varied Si/Al ratio), whereas a linear increase (Table 1) with increase in Ni
loading is found. This suggests the bulk property of H2ꢀTPR. Fig. 1B, exhibit broad curves
due to NiO in two stage reduction. The low temperature peak (380ꢀ450 °C) is assigned to
dispersed and the high temperature (~575 °C) peak is mainly due to large sized NiO clusters.
It is also observed that at higher Ni loadings the reduction peak is sharpened [50wt%
Ni/HꢀZSMꢀ5 (Si/Al = 150)]. XRD analysis of these samples demonstrated the increase in
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NiO domain size (Table 1) at higher Ni loadings. Based on these results it is concluded that
reduction peak at 450 °C and 575 °C are due to NiO species that are located at outer surface
and sodalite cages of zeolite crystals respectively [38].
3.4 UV-DRS analysis
As shown in Fig. 2A the UVꢀDR spectra were nearly the same, irrespective of the
varied Si/Al mole ratio in zeolite. The absorption bands at 240, 310 nm are originating from
O → Ni charge transition are stronger (Fig. 2B) [39]. The bands at 715 nm and 377 nm
represent the octahedrally coordinated Ni2+ species in NiO lattice, while the absorption signal
in the range of 600–645 nm assigned to the tetrahedrally coordinated Ni2+ species [40]. It is
clearly evident that the band intensity of 720nm peak is increased upon increasing the Ni
loading (Fig. 2B).
3.5 Influence of Si/Al ratio on CDM activity of Ni/H-ZSM-5
The influence of Si/Al ratio is examined keeping 30wt% nickel (arbitrarily fixed) in
the Ni/HꢀZSMꢀ5 catalysts, and the TOS analysis of these catalysts is reported in Fig. 3. It
shows that all the catalysts were deactivated after certain time. In the comparative analysis
30wt% Ni/HꢀZSMꢀ5 with Si/Al ratio = 150 displayed better activity and longevity of 17 h
with a H2 yield of 105.2 Nm3/molNi. The yields were estimated from the kinetic curves of the
methane conversion to carbon and hydrogen proceeded stoichiometrically. The high H2 yield
(105.2 Nm3/molNi) on 30wt%Ni/HꢀZSMꢀ5 (Si/Al ratio = 150) catalyst is substantiated the
sustainability and longevity compared to 30wt% Ni supported on Si/Al with 40, 300 and 485
HꢀZSMꢀ5 samples (Fig. 4). It should be mentioned here that none of these samples showed
the formation of carbon oxides (CO and CO2) unlike Choudhary et al who found certain
amount of CO during the CH4 decomposition over Ni/HꢀZSMꢀ5 catalysts [28]. Hence various
Ni loadings on HꢀZSMꢀ5 (with Si/Al = 150) samples are evaluated for CDM process.
3.6 Influence of Ni loading on CDM activity
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Fig. 5 shows in methane conversion (%) as a function of time (h) during the catalytic
decomposition of methane at 550°C over Ni/HꢀZSM (Si/Al = 150) catalysts with different Ni
loadings from 10 to 50 wt%. The activity measurements were carried out under similar
reaction conditions in order to optimize Ni loading on HꢀZSMꢀ5 (Si/Al = 150) zeolite. It is
clear from Fig. 5 that methane conversion is found to be high in the initial stages and
decreased rapidly or gradually with time on stream as a result of carbon deposition. The
catalyst deactivation is attributed due to the deposition of carbon on the catalyst
surface/active sites or accumulated at the entrance of the pores by pore blockage. The
reaction run time until complete deactivation of the catalysts are in the order 50 < 10 < 30 <
20 wt% of Ni over HꢀZSMꢀ5 zeolite. Therefore the 20wt% Ni is optimum for HꢀZSMꢀ5
(Si/Al = 150) that showed a better H2 yield of 171 Nm3/molNi and longevity compared to
other catalysts [12].
It was reported that CH4 decomposition takes place on surface Ni sites followed by
deposition of carbon by generating gaseous H2. According to earlier reports, carbon
deposition on Ni surface and diffusion of deposited carbon through Ni particle to the interface
where it gets precipitated in the form of graphitic carbon as CNTs and/or CNFs occurs
simultaneously. The carbon accumulation would be high when the rate of carbon deposition
on the Ni surface is lower than the rate of carbon diffusion through Ni particle. If the reverse
is true then the catalyst deactivates rapidly consequently lower H2 yields are observed [41].
The typical volcano shape relationship obtained (Fig. 6) with carbon formed per gNi (C/Ni)
of the catalyst with increase Ni loading, indicates how critical is the role of metal loading on
the catalyst performance during the CDM process. The H2 yields are increased in the order of
50 < 10 < 30 < 20 wt% Ni/HꢀZSMꢀ5 (Si/Al = 150). A similar observation of volcano type
tendency on H2 yields over Ni/SiO2 catalysts during the catalytic decomposition of methane
is reported [12].
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4. Discussion
The Ni metal surface areas of Ni/HꢀZSMꢀ5 samples measured by H2 pulse
chemisorption and the results are reported in Table 2 [26]. Fig. 7 demonstrated a linear
relationship between the Ni metal surface area and the H2 yields obtained from CDM results.
Thus it is concluded that the H2 production rate is dependent up on the Ni metal surface area
of Ni/ HꢀZSMꢀ5 (Si/Al ratio = 150) catalysts. It has also been observed that the H2 rates are
in good correlation with the Ni metal surface areas of the catalysts measured by N2O
decomposition studies (Table 2). High dispersion of Ni over 20wt%Ni/HꢀZSMꢀ5 could be a
possible for the higher H2 yields than the other Ni loaded samples.
Fig. 8 reveals the XRD patterns of deactivated 30wt%Ni/HꢀZSMꢀ5 (Si/Al = 40, 150,
300 and 485) catalysts recovered after CDM reaction. The irreversible carbon that is
deposited on the catalyst appeared as graphitic carbon in XRD analysis. The strong
reflections at 2θ = 26.1°, 42.9°, 53.8° and 77.7° are attributed to the graphitic carbon [ICSD
#01ꢀ0640] and weak diffraction lines due to metallic Ni at 2θ = 44.2˚, 51.7˚ and 76.4˚.
The nature of deposited carbon is confirmed by Raman spectra of the Ni/HZSM5
catalysts and results are presented in Fig. 9A and Fig. 9B. The Raman spectra displayed two
distinct bands centered at around 1320 cmꢀ1 (Dꢀband) attributed to structural imperfection of
graphite or the presence of carbon nanoparticles and amorphous carbon. The band at 1580
cmꢀ1 (Gꢀband) ascribed to the inꢀplane carbon–carbon stretching vibrations rather ordered
structure carbon [18, 42, 43].
The degree of crystallinity of deposited carbon can be obtained from the frequency
̉
̉
̉
and intensity of Raman ‘D’ and ‘G’ bands. The ratio of peak area of the ‘D’ band to that of
the ‘G’ band i.e. ID/IG is considered as an index for the crystalline order of graphite which is
inversely proportional to the average plane size of the perfect graphenes; that is the
graphitization degree of carbon is higher at lower ID/IG values (Fig. 10) [19,44].
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The morphology of deposited carbon can be determined using SEM analysis. The
SEM images of 30wt%Ni/HꢀZSMꢀ5 with Si/Al = 40, 150 and 300 (Fig. S5) showed
agglomerated clusters (Fig. S5A), spherical (Fig. S5B) and large size clusters of Ni particles
(Fig. S5C). Formation of carbon nanotubes is evident in their respective deactivated samples.
It appears that the deposited carbon is in nanosize and few micrometers in length of
filamentous carbon. On the other hand, the size and length of deposited filaments varied with
variation in Si/Al ratios. The nature of filamentous carbon is further evidenced from TEM
image of deactivated Ni/HꢀZSMꢀ5 samples (Si/Al = 150) (Fig. 11). It is also found that the Ni
metal is occupied at the tip of the carbon filament. The size of the produced filament is more
or less similar to Ni particle size (~30 nm) occupied at the tip of the carbon nanofiber [17,45,
46]. These results are in good agreement with the mechanisms proposed earlier for
filamentous carbon growth, according to which the diameter of carbon nanofibers must be
approximately equal to the size of Ni particle at the tip of the carbon nanofiber [47].
5.0 Conclusions
The 30wt%Ni loaded on HꢀZSMꢀ5 zeolite with varied Si/Al ratios were examined for
the COx free hydrogen production. The optimized Si/Al = 150 (HꢀZSMꢀ5) has been further
explored with different Ni loadings. The catalyst with 20wt% Ni supported on HꢀZSMꢀ5 with
Si/Al = 150 demonstrated tremendously higher H2 yields than the other compositions under
identical experimental condition. The SEM and TEM images revealed the presence of carbon
nanofibers in all the deactivated samples. The normalized intensities of Raman spectra with
ID/IG vs Ni loadings defined the higher amount of ordered carbon over 20wt%Ni/HꢀZSMꢀ5
(Si/Al = 150) thus produced higher H2 yields (171 Nm3/molNi). The superior performance of
this catalyst can also be explained by the higher Ni metal surface area of 20wt% Ni/HꢀZSMꢀ5
(Si/Al = 150) catalyst. A direct correlation between Ni loading vs H2 yields and Ni metal
surface area is observed. Further studies are in progress on the differences in morphology of
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various Si/Al ratios of HꢀZSMꢀ5 for Ni in the COx free hydrogen production by catalytic
decomposition of methane.
ACKNOWLEDGMENT
The authors CA thank CSIR New Delhi, VVK and GN thanks CSIRꢀIICT for providing
necessary facilities.
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Figure captions:
Figure 1: (A) TPR profile of (a) 30 wt% Ni/HꢀZSMꢀ5 (Si/Al = 40), (b) 30wt%Ni/HꢀZSMꢀ5
(Si/Al = 150) and (c) 30wt%Ni/HꢀZSMꢀ5 (Si/Al = 300) and (d) 30wt%Ni/HꢀZSMꢀ5 (Si/Al =
485) catalysts, (B) Ni loading on HꢀZSMꢀ5 (Si/Al=150) catalysts (a) 10, (b) 20, (c) 30 and (d)
50wt%.
Figure 2: (A) UVꢀVis diffuse reflectance spectra of calcined 30wt% Ni supported on HꢀZSMꢀ
5 with (a) Si/Al = 40, (b) Si/Al = 150, (c) Si/Al = 300 and Si/Al = 485 catalysts, (B) Ni
loaded on HꢀZSMꢀ5 (Si/Al = 150) catalysts, (a) 10, (b) 20, (c) 30 and (d) 50wt%.
Figure 3: Methane conversion (%) with time (h) over 30wt% Ni/HꢀZSMꢀ5 (Si/Al = 40, 150,
300 and 485) catalysts at 550 °C temperature.
Figure 4: Hydrogen yields during methane decomposition over 30 wt% Ni /HꢀZSMꢀ5 with
varying Si/Al mole ratio.
Figure 5: Methane conversions (%) with time (min) over Ni loading on Ni/HꢀZSMꢀ5 (Si/Al
ratio = 150) catalysts at 550 °C temperature.
Figure 6: Carbon yields during methane decomposition as a function of Ni loading over
Ni/HꢀZSMꢀ5 (Si/Al ratio = 150) catalysts.
Figure 7: Relationship between Ni metal surface area and H2 yields vs Ni loading on HꢀZSMꢀ
5 (Si/Al = 150).
Figure 8: XRD patterns of deactivated 30wt%Ni/HꢀZSMꢀ5 with Si/Al ratio (a) 40, (b) 150,
(c) 300 and (c) 485 catalysts.
Figure 9: Raman spectra of deactivated (A) different Si/Al ratio Ni/HꢀZSMꢀ5 (Si/Al: 40, 150,
300 and 485) and (B) Ni loadings of Ni/HꢀZSMꢀ5 (Si/Al ratio=150) catalysts.
Figure 10: The ID/IG ratio and H2 yields against various Ni loaded HꢀZSMꢀ5 (Si/Al = 150)
deactivated catalysts.
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Figure 11: TEM images of spent (a) 30wt%Ni/HꢀZSMꢀ5 (Si/Al = 150); and (b) 20wt%Ni/Hꢀ
ZSMꢀ5 (Si/Al = 150) catalysts.
Tables:
Table1: Physicochemical properties of Ni supported on HꢀZSMꢀ5 catalysts.
Table2: CDM activity data and Ni metal surface areas of Ni/HꢀZSMꢀ5 with varied Si/Al
ratio.
Table 1
:
Ni/HꢀZSMꢀ5
Si/Al
ratio
BETꢀSA
(m2/g)
aNiO
bNi
cH2 uptake
(µmol/gcat)
crystallite domain
size (nm) size (nm)
30wt%Ni
30wt%Ni
30wt%Ni
30wt%Ni
10wt%Ni
20wt%Ni
50wt%Ni
40
220
396
255
326
432
406
265
29
21
25
27
12
16
23
25
29
27
31
22
25
32
2121
2058
2035
2023
525
150
300
485
150
150
150
985
2482
aFrom XRD patterns of calcined samples
bXRD patterns of reduced samples
cH2 uptakes measured from TPR analysis with a calibrated Ag2O TPR.
Table 2:
Ni/HꢀZSMꢀ5
Si/Al
ratio
40
150
300
485
150
150
150
H2 uptake SNi
N2O uptake SNi
H2 yield
(µmol/gcat)a (m2/gcat)a (µmol/gcat)b (m2/gcat)b (Nm3/molNi)c
30wt%Ni
30wt%Ni
30wt%Ni
30wt%Ni
10wt%Ni
20wt%Ni
50wt%Ni
45
85
63
59
75
124
70
3.4
6.4
4.7
4.1
5.6
9.3
5.2
39.8
101.9
56.0
47.4
83.4
120.2
94.9
1.6
4.0
2.2
1.9
3.3
4.8
3.7
18.4
105.2
31.8
28.2
75.6
171.0
40.6
aH2 pulse chemisorption, bN2O titration, cCDM activity
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Figure 8: XRD patterns of deactivated 30wt%Ni/H-ZSM-5 with Si/Al ratio (a) 40, (b) 150, (c) 300 and (c)
485 catalysts.
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