A study to initiate development of sustainable Ni/γ-Al 2O3 catalyst for hydrogen production from steam reforming of biomass-derived glycerol

Article abstract of DOI:10.1039/c4ra01612b

Glycerol steam reforming, which is a potential technology for hydrogen production in fuel-cell applications, is of great interest to researchers in recent years. Using aqueous glycerol, which is a byproduct in biodiesel production, as a direct feed for st

Full text of DOI:10.1039/c4ra01612b

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A study to initiate development of sustainable
Ni/g-Al2O3 catalyst for hydrogen production from
steam reforming of biomass-derived glycerol
Cite this: RSC Adv., 2014, 4, 32429
Gullapelli Sadanandam, Kasala Ramya, Donthamsetti Bhanu Kishore,
*
Valluri Durgakumari, Machiraju Subrahmanyam and Komandur V. R. Chary
Glycerol steam reforming, which is a potential technology for hydrogen production in fuel-cell applications,
is of great interest to researchers in recent years. Using aqueous glycerol, which is a byproduct in biodiesel
production, as a direct feed for steam reforming is a promising method to produce hydrogen. Ni (5, 10, 15,
20 and 25 wt%) loaded on commercial g-Al2O3 by the impregnation method is used to study steam
reforming of glycerol. The catalyst was characterized by XRD, EDAX, BET surface area, TPR, NH3-TPD,
TEM, CHNS and Raman techniques. The catalysts are evaluated on time streams for the effect of Ni
loading, temperature, and glycerol-to-water mole ratio (GWMRs). Under the parameters investigated, 15
wt% Ni/g-Al2O3 was found to be the most promising catalyst in terms of glycerol conversion and
hydrogen production with minimum coking. The characterization of the catalysts clearly establishes that
interaction of Ni2+ with g-Al2O3 support, Ni2+ reducibility, particle size, and acidity of the support are
seen governing the stable activity of the catalysts. Thus, a structural activity correlation has been
established on the Ni/g-Al2O3 catalysts.
Received 24th February 2014
Accepted 30th June 2014
DOI: 10.1039/c4ra01612b
www.rsc.org/advances
The steam reforming of glycerol to produce hydrogen using
catalysts has been investigated in the recent decade.9–17 The
1 Introduction
steam reforming of glycerol by noble metal catalysts is expen-
sive, and the use of low-cost non-noble catalysts would be
advantageous from an economic standpoint. This reaction is
very interesting for its operational characteristics and greater
efficiency. Noble metal-supported catalysts are more active and
less susceptible to carbon deposition than non-noble
metals.11–13 At an industrial scale, the use of Ni-supported
catalysts for steam reforming is interesting because this catalyst
is inexpensive and more highly available than noble metals.
Both development and stability of Ni-supported catalysts are
subjects of investigation.14,15 Ni-based catalysts are used
because of their high efficiency for the cleavage of C–C, O–H,
and C–H bonds in hydrocarbons. Moreover, they catalyze the
water–gas shi reaction to remove adsorbed CO from the
surface of the catalysts.16 Ce, Mg, Zr, and La modifying Ni/Al2O3
enhanced the hydrogen selectivity with minimum coking.18
More recently, the glycerol steam reforming was evaluated on
Ni-supported CeO2, Al2O3, and CeO2-promoted Al2O3, and the
incorporation of low ceria loadings enhanced catalytic activity,
whereas increasing ceria contents reduced the capacity of the
catalyst to convert intermediate oxygenated hydrocarbons into
hydrogen.19
Because of the depletion of global fossil fuel resources, growing
environmental impact and energy security concerns, there is a
need to nd new alternative routes for hydrogen production.1–7
There is a signicant amount of research being pursued towards
the development of H2 production technologies for fuel-cell
applications as fuel cells that consume hydrogen are environ-
mentally clean and highly efficient devices for electrical power
generation.8 The full environmental benet of generating power
from hydrogen fuel cells is achieved when hydrogen is produced
from renewable sources like solar power and biomass. In this
context, the conversion of cheap and available biomass or
biomass-derived byproducts for H2 production is considered to be
a promising approach to meet the requirement of H2 and realize
sustainable development. To date, glycerol has been considered
as an alternative fuel for hydrogen production because it is a
byproduct of biodiesel production, which uses vegetable oils or
fats as feedstock. Hence, in this context, hydrogen production
from steam reforming of glycerol appears to be a promising
alternative. The reaction that describes the production of
hydrogen from glycerol can be expressed as follows:
C3H8O3 + 3H2O / 3CO2 + 7H2
(1)
Ni/Al2O3 catalysts suffer deactivation during the steam
reforming of oxygenated hydrocarbons due to the formation
of carbonaceous deposits and the sintering of the metallic
phase. Coke formation is usually related to dehydration,
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology,
Hyderabad-500 607, India. E-mail: durgakumari@iict.res.in; Fax: +91-40-27160921;
Tel: +91-40-27193165
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cracking, and polymerization reactions taking place on the
acid sites of Al2O3,20 whereas the sintering of the metallic
phase can be associated with a transition of Al2O3 to the
crystalline phase during a reaction.21 Using supported cata-
lysts of Rh/Al2O3 and Ni on Al2O3, MgO, and CeO2, carbona-
ceous deposits were formed from olens, which were
produced by the glycerol thermal decomposition, and
temperatures above 650 ꢀC favoured the generation of
encapsulated carbon that decreases catalyst stability.22 Using
Ni/a-Al2O3 and Ni/a-Al2O3 modied with ZrO2 and CeO2,
Ni/a-Al2O3-modied CeO2 catalysts displayed great stability,
and its basic characteristic inhibits the reactions that
form carbonaceous deposits deactivating the catalyst.23 On
Ni-loaded Al2O3, ZrO2, and CeO2 catalysts, which are used in
glycerol for reforming the stable activity and maximum H2
selectivity, are observed on Al2O3, but the coke formation rate
is more on Al2O3.24 The effect of Ni precursors on Al2O3
catalysts used in the glycerol reforming reaction was studied,
and it was inferred that high Ni dispersion and small Ni
particle size promoted catalyst activity.25
Previous thermodynamic studies of glycerol-steam systems
indicate complete glycerol conversion with a high attainment of
H2 yield.26–30 These studies also suggest that carbon formation is
inhibited at high reaction temperatures (>900 K), low pressure
and high steam-to-glycerol ratio (STGR, 12 : 1). Adhikari et al.
considered the thermodynamic equilibrium analysis for glyc-
erol-steam reforming varying parameters, namely, pressure of
1–5 atm, temperatures at 600–1000 K, and STGR of 1 : 1–9 : 1.29
The best conditions for producing hydrogen are temperatures
higher than 900 K, atmospheric pressure, and a glycerol-to-
2 Materials and methods
2.1 Chemicals
g-Al2O3 extrudated (Engelhard corporation, AL-3996) with a
surface area of 192 m2 gꢂ1, Ni(NO3)2$6H2O (Sigma-Aldrich), and
glycerol (Qualigens Fine Chemicals Pvt. Ltd. (India)) were used.
2.2 Catalyst preparation
Nickel (5, 10, 15, 20 and 25 wt%) was loaded on g-Al2O3 (4 ꢁ 6
mm) by an impregnation method, which is generally used on
preformed supports for high dispersion and controlled size of
active sites. The method involved addition of g-Al2O3 to a
known amount of Ni(NO3)2$6H2O dissolved in distilled water.
Excess water was evaporated to dryness with constant stirring
and slow heating. The dried sample was calcined at 500 ꢀC/5 h
in air. The catalysts with 0, 5, 10, 15, 20 and 25 (wt%) of nickel-
loaded g-Al2O3 were labelled as A, 5NA, 10NA, 15NA, 20NA and
25NA. The Ni-loaded g-Al2O3 catalysts were used in glycerol
steam reforming at glycerol-to-water mole ratio (GWMRs) of
1 : 9 and 650 ꢀC, and the used catalysts were labelled as 5NA-1-9,
10NA-1-9, 15NA-1-9, 20NA-1-9 and 25NA-1-9. A nickel-loaded
g-Al2O3 catalyst (15 wt%) was used in glycerol steam reforming
ꢀ
at different temperatures (500, 550, 600 and 650 C using glyc-
erol-to-water mole ratio of 1 : 9) and different GWMRs (1 : 3,
ꢀ
1 : 6 and 1 : 9 at 650 C) and were labelled as 15NA-500, 15NA-
550, 15NA-600, 15NA-650, 15NA-1-3, 15NA-1-6 and 15NA-1-9,
respectively.
2.3 Catalyst characterization
water mole ratio of 1 : 9 as CH4 production is minimized and The X-ray diffraction (XRD) patterns of the Ni/g-Al2O3 fresh and
carbon formation is thermodynamically inhibited under these used catalysts were recorded with Rigaku Miniex diffractom-
conditions.
eter with a nickel-ltered Cu Ka radiation (l ¼ 0.15406 nm)
In our previous work, we have reported the effect of from 2q ¼ 5ꢀ–80ꢀ with beam voltage and beam currents of 40 kV
catalyst size in glycerol steam reforming for H2 production and 100 mA, respectively. The crystallite sizes of the catalysts
over Ni/SiO2, where 2 ꢁ 2 and 2 ꢁ 4 mm catalysts promote were calculated using the Debye-Scherrer equation (Kl/b cos q).
coke formation at high temperatures (600 ꢀC). The higher Elemental analysis was carried out using Link, ISIS-300, Oxford,
size of 3 ꢁ 5 mm improves the catalytic performance and energy-dispersive analysis of X-ray spectroscopy (EDAX). The
minimizes coke formation.31 The present investigation is a Brunauer–Emmett–Teller (BET) surface areas of fresh and used
part of the ongoing activity on catalyst development for samples were measured by N2 adsorption at ꢂ196 ꢀC in an
hydrogen production from biomass-derived glycerol for 2–3 Autosorb-I (Quantachrome) instrument. Transmission electron
kW PEMFC system to meet the immediate requirement of microscopy (TEM) studies were conducted on a TECHNAI 20B2
alternate clean energy-based back-up power supply for tele- S-Twin unit operated at 120 kV with a lament current of 28 mA.
com towers, which were sponsored by the Ministry of New & The carbon contents were estimated using an Elementar vario
Renewable Energy (MNRE), Government of India. There is no Microcube (Germany) CHNS analyser and calibrated with sul-
commercial catalyst available for glycerol reforming, and phanilic acid using samples in duplicate. The sample was
IICT has initiated catalyst development based on the dropped into the combustion tube automatically and subjected
previous experience from the development of methanol to combustion temperatures of up to 1200 ꢀC. Complete
reforming.32 Ni/g-Al2O3 is a fundamentally well-established combustion of all samples was ensured with a special oxygen jet
system as a catalyst for methane reforming, and glycerol injection of tungsten oxide (catalyst for oxidation). The He
reforming is altogether a new activity with more number of carrier gas transfers the combustion gaseous products into the
carbon atoms and high coking rate. The present investigation copper tube where nitrogen oxide is reduced to molecular N2 at
details the preparation, characterization and time on stream 850 ꢀC. The mixture of helium, CO2, H2O, and SO2 are guided to
activity of Ni/g-Al2O3 catalysts. Moreover, an understanding specic adsorption traps and measurements, and nitrogen
of the stable and sustainable activity of these catalysts is travels to the TC detector. Confocal micro-Raman spectra were
discussed in terms of acidity, reducibility, Ni crystallite size recorded at room temperature in the range of 1000–2000 cmꢂ1
and coking rate.
using a Horiba Jobin-Yvon Lab Ram HR spectrometer with a
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À
Á
À
17 mW internal He–Ne (helium–neon) laser source of excitation
with a wavelength of 632.8 nm. The catalyst sample in powder
form (about 5–10 mg) was usually spread onto a glass slide
below the confocal microscope for measurements. Tempera-
ture-programmed reduction (TPR) was carried out in a quartz
microreactor interfaced to gas chromatography with a thermal
conductivity detector (GC with TCD) unit. For the TPR analysis,
a catalyst sample of about 50 mg was loaded in an isothermal
zone of a quartz reactor (i.d. ¼ 6 mm, length ¼ 30 cm) heated by
an electric furnace at a rate of 10 ꢀC minꢂ1 to 300 ꢀC in owing
helium gas at a ow rate of 30 ml minꢂ1, which facilitates the
desorption of physically adsorbed water. Then, aer the sample
was cooled to room temperature, helium was switched over to
30 ml minꢂ1 reducing gas of 5% H2 in argon, and the temper-
ature was increased to 1000 ꢀC at a rate of 5 ꢀC minꢂ1. Hydrogen
consumption was measured by means of a thermal conductivity
detector. The steam formed during the reduction was removed
by a molecular sieve trap prior to detection. The acidity of the
catalysts was measured by temperature-programmed desorp-
tion of ammonia (NH3-TPD). In a typical experiment, 0.1 g of
Gly in ꢂ Gly out
Xgly % ¼
ꢁ 100
(2)
(3)
Gly in
À
Á
Á
Xgas ml
Xgas % ¼
ꢁ 100
Total gasðmlÞ
where Xgly (%) ¼ glycerol conversion.
Xgas (%) ¼ gas (%), where X is H2, CO2, CO and CH4.
In all the glycerol steam reforming reactions, 6 ml of feed
was used. Prior to the reaction, catalysts were reduced using
ꢀ
10% H2/N2 at 550 C/5 h. The evaluation studies were carried
out for 25 h time on stream-varying parameters.
3 Results and discussion
3.1 Catalyst characterization
3.1.1 XRD
Fresh catalysts. X-ray diffraction patterns of the catalysts with
different loadings of Ni on g-Al2O3 calcined at 500 ꢀC are shown
in Fig. 1A. The characteristic peaks of NiO are seen at 2q of
37.32ꢀ, 43.36ꢀ, 62.99ꢀ, 75.56ꢀ and 79.56ꢀ, corresponding to
(111), (200), (220), (311), and (222) planes of NiO (JCPDS # 75-
0197).33 XRD patterns clearly explain that with an increase in Ni
loading on g-Al2O3, the NiO crystallite size increases and
calculated from the X-ray line broadening of NiO peak (2q ¼
43.36ꢀ) using the Scherrer equation and values are shown in
Table 1. The NiO particle size varied from 8.5 nm (10NA) to 13.1
ꢀ
catalyst was loaded and pretreated in He gas at 300 C for 2 h.
Aer pretreatment, the temperature was brought to 100 ꢀC, and
the adsorption of NH3 was carried out by passing a mixture of
10% NH3-balanced He gas over the catalyst for 1 h. The catalyst
surface was ushed with He gas at the same temperature for 2 h
to remove the physisorbed NH3. TPD of NH3 was carried out
with a temperature ramp of 10 ꢀC minꢂ1, and the desorption of
ammonia was monitored using a thermal conductivity detector
(TCD) of a gas chromatograph.
2.4 Catalyst evaluation
Glycerol steam reforming reactions were carried out using a
xed-bed, tubular downow quartz reactor 18 mm in diameter
with a thermocouple 2 mm in width. The reactor was provided
with a pre-heater, a syringe pump, a cold condenser and gas
ow meter. The catalyst (2 g) was loaded in the middle of the
reactor. The reactor was placed in a tubular furnace with an
inner diameter of 25 mm. The feed mixture (1 : 3 to 1 : 9 mole
ratio of glycerol to water) was fed into the vaporizer using a
syringe (B. Braun) pump. The feed entering the pre-heater was
maintained at 500 ꢀC before reaching the catalyst bed. A Nippon
(NC-2538) temperature controller was used for maintaining the
temperature of the pre-heating zone and catalyst bed of the
reactor. The conversion of glycerol was calculated from the
volume of the condensate. The total gas is measured to under-
stand the glycerol to gaseous product, and the main products
H2, CO2, CO and CH4 are analysed by gas chromatography
(Shimadzu GC-2014) using a thermal conductivity detector
(TCD), Carboxen 1000 column and helium as a carrier gas. The
other gas products observed in trace quantities are C2 products.
Liquid products like acetaldehyde, acetone, methanol, and
acroline were analysed by GC-MS. Carbon deposition is
obtained by weighing the catalysts at the end of the reaction and
comparing with carbon from the CHNS analysis.
Glycerol conversion and gas composition are calculated as
per the following equations:
Fig. 1 XRD of Ni/g-Al2O3 (A) calcined catalysts and (B) used catalysts
at 650 ꢀC, GWMRs ¼ 1 : 9.
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nm (25NA). On the other hand, the diffraction peaks at
2q ¼ 39.2ꢀ, 46.2ꢀ and 66.9ꢀ conrm presence of g-Al2O3. The
characteristic peaks of NiAl2O4 spinel (2qꢀ ¼ 37.32ꢀ, 46.2ꢀ, and
66.9ꢀ) are very close to that of NiO and g-Al2O3 and cannot be
distinguished.
Used catalysts. Diffraction patterns of different Ni-loaded
g-Al2O3 catalysts studied for 25 h time on stream at a reaction
temperature of 650 ꢀC, and GWMRs (1 : 9) are presented in
Fig. 1B. The XRD of all ve catalysts show peaks at 44.38ꢀ,
51.72ꢀ, and 76.2ꢀ, and these reections are assigned to metallic
Ni crystallites. Characteristic peaks of g-Al2O3 (2q ¼ 39.2ꢀ, 46.2ꢀ,
and 66.9ꢀ) are also detected in all of the catalysts. The XRD
patterns of the 15NA (15 wt% Ni/g-Al2O3) catalyst evaluated in
glycerol reforming at different temperatures are presented in
Fig. 2A. The characteristic peaks seen at 44.38ꢀ, 51.72ꢀ, and
76.2ꢀ assigned to metallic Ni crystallites are increasing with
temperature and the crystallite size is calculated from X-ray line
broadening of the Ni peak (2q ¼ 44.38ꢀ), and the values are
shown in Table 2.31,33 The 15NA catalyst is further studied using
different GWMRs at 650 ꢀC for 25 h and the diffraction patterns
are shown in Fig. 2B. With an increase in steam ratio, a nominal
increase in Ni crystalline size is seen. These studies show that
the effect of temperature on Ni sintering is more pronounced
compared to steam. The reducible NiO phase is observed as the
metallic Ni peak in the XRD of all used catalysts. No carbon
peak is observed in the XRD of all used catalysts as the carbon
deposition is below the XRD detection range. Note that the
carbon deposition is detected by CHNS and Raman.
Fig. 2 XRD of 15NA catalyst used (A) at different temperatures (ꢀC) at
1 : 9 GWMRs, and (B) different GWMRs at 650 ꢀC.
3.1.2 EDAX. The EDAX analysis of fresh samples was
carried out to determine nickel content on g-Al2O3, and the data
is shown in Table 1. The composition of the Ni loaded on
g-Al2O3 catalysts evaluated in glycerol reforming is shown in
Table 2. EDAX analysis of Ni-loaded g-Al2O3 used catalysts
shows low Ni content compared to calcined catalysts due to
carbon deposition. The carbon deposition decreased up to
15NA catalyst, and at higher loadings (20NA and 25NA), carbon
deposition increased. When 15NA is studied under different
GWMR ratios, the Ni% decreased with decreasing steam. The
carbon deposition is also seen as increasing with decreasing
steam. The observed decrease in Ni content is due to an increase
in the carbon on surface layers, which may change the ratios.
3.1.3 BET surface area. The surface area obtained of
g-Al2O3 and both fresh and used Ni/g-Al2O3 catalysts are
shown in Table 1 and 2. The surface area of g-Al2O3 is around
192 m2 gꢂ1, which decreased with an increase in Ni loading.34
The surface area of the used catalysts also decreased, which may
be due to some factors like reaction temperature and carbon
deposition. The alumina structure undergoing transition at
higher temperatures may reduce the surface area. However, in
the interacted aluminas, transitions are slow, and the corre-
sponding textural changes are not observed in XRD. The coke
deposited on the surface of the catalyst may block the pores and
result in decreased surface areas.31,35
3.1.4 CHNS analysis. CHNS analysis was carried out to
determine the amount of carbon deposition studied under
different reaction conditions, and the data is shown in Table 2.
Carbon deposition is more at lower loadings of Ni (5NA
and 10NA) due to acid sites available on the g-Al2O3 surface
Table 1 Physical characteristics of Ni/g-Al2O3-calcined catalysts
NiO crystallite
size (nm) from XRD
Ni (wt%)
by EDAX
BET surface area
H2 consumption (mmol gcatꢂ1)/Ni
reducibilitya (%)
(m2 gꢂ1
)
Acidityb (mmol gcat
)
ꢂ1
Catalysts
A
—
—
8.5
10.1
12.3
13.1
—
4.9
10.2
16.3
21.1
24.2
192
185
179
175
170
162
—
—
300
—
160
—
85
5NA
10NA
15NA
20NA
25NA
0.55/65
1.14/67
1.73/68
2.41/71
2.98/70
a
H2 consumption/Ni reducibility (%) from TPR analysis. b Acidity from NH3-TPD analysis.
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Table 2 Physical characteristics of Ni/g-Al2O3-used catalysts
Elemental
composition
from EDAX
Ni crystallite
Catalysts
size (nm) from XRD
Ni%
C%
Carbon (%) from CHNS analysisa
BET surface areab (m2 gꢂ1
)
5NA-1-9
9.5
9.8
3.9
8.4
2.5
2.0
1.5
2.5
3.5
3.0
3.8
1.6
1.8
2.0
4.2
3.8
3.1
4.8
5.9
7.6
9.7
3.2
3.4
3.7
165
150
150
140
125
135
120
—
10NA-1-9
15NA-1-9
20NA-1-9
25NA-1-9
15NA-1-6
15NA-1-3
15NA-600
15NA-550
15NA-500
10.6
13.1
14.9
10.4
10.1
10.5
10.3
10.1
14.1
18.6
21.5
12.3
11.2
13.8
13.4
13.0
—
—
a
Standard deviation of carbon ¼ ꢃ0.1%. b Standard deviation of BET surface area ¼ ꢃ5.
(Table 1). With increasing Ni content, the acid sites decreased, heterogeneous distribution of the Ni2+ species. The reduction
and less coking is observed on 15NA. However, with the further peak appearing at higher temperatures may be attributed to the
increase in nickel, coking rate increased. Of all the catalysts reduction of Ni2+ ions from non-stoichiometric nickel-alumi-
studied, the carbon deposition observed is low on the 15NA nate species.16,38
catalyst, which was studied further at different temperatures,
3.1.6 NH3-TPD. The ammonia adsorption–desorption
and GWMRs showed more carbon deposition at low tempera- technique usually enables one to determine the strength of acid
ture and low steam.36 The reasons are discussed in catalyst sites present on the surface of catalyst. The TPD of NH3 proles
evaluation section.
of typical Ni/g-Al2O3 catalysts (5NA, 15NA and 25NA) are shown
3.1.5 TPR. TPR analyses were performed to investigate the in Fig. 4. Total acidity (300–700 ꢀC) on these decreased with an
reducibility of Ni2+ species present in the calcined catalysts, and increase in Ni loading (Table 1).20
the results are shown in Fig. 3 and Table 1. TPR proles of
3.1.7 TEM. The morphology of deposited carbon is studied
different Ni-loaded g-Al2O3 catalysts show two distinct reduc- by TEM and photographs of representative samples are shown
tion processes. A less intense peak is seen between 300 and 350 in Fig. 5. On the 15NA-1-9 sample (Fig. 5a and b) Ni metal
ꢀC is due to the reduction of Ni2+ species that are in weak particles are observed as dark spots on the surface of g-Al2O3
interaction with the support. A broad high temperature peak in support. At higher Ni loadings, i.e. on 20NA-1-9 and 25NA-1-9,
the region of 450–800 ꢀC with Tmax of reduction around 650 ꢀC TEM images show formation of more carbon laments with
that may be seen due to strong interaction Ni2+ with g-Al2O3 dispersed nickel particles (Fig. 5c and d). The glycerol steam
support.37,38 At higher Ni loadings (20NA and 25NA), a broad reforming reaction over Ni/g-Al2O3 catalyst leads to the forma-
signal is shied to a lower temperature (550 ꢀC), indicating the tion of graphitic ake-like carbon (lamentous carbon or
presence of Ni2+ species with decreased strength of interaction. carbon nanotubes) and amorphous carbon, and the results
This indicates that at higher Ni loadings, the support is with obtained are in conrmation with the reported literature.39–41
Fig. 4 NH3-TPD analysis of Ni/g-Al2O3 catalysts: (a) 5NA, (b) 15NA and
Fig. 3 TPR analysis of Ni-loaded g-Al2O3 catalysts.
(c) 25NA.
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Fig. 5 TEM images of (a) used Ni/g-Al2O3 catalysts and (b) 15NA-1-9,
(c) 20NA-1-9, and (d) 25NA-1-9.
3.1.8 Raman spectra. Raman spectroscopy is used to
characterize post-reaction catalysts because it is a powerful
technique for characterizing the structure of carbonaceous
materials. On all catalysts, Raman spectra revealed two broad
bands around 1335 cmꢂ1 (D band) and 1591 cmꢂ1 (G band)
(Fig. 6). The former is ascribed to the disordered carbon
(amorphous) and the latter is attributed to in-plane carbon–
carbon stretching vibrations (E2g) of the graphitic
carbon.39,40,42–43 In all cases, the D band is more intense than the
G band, indicating a predominance of disordered carbon. The
degree of graphitic carbon deposits can be estimated by the
ratio of the area of the D band to that of the G band (ID/IG). Note
that a higher degree of graphitization produces a lower ID/IG
ratio. The results indicated that on 15NA disordered carbon and
graphitic carbon are minimum as shown in Fig. 6a. The 15NA
catalysts are further studied at different GWMRs, and the used
catalysts are subjected to Raman spectra as (Fig. 6b). These
spectra clearly explain that by decreasing steam, the formation
of both types of carbon is seen increasing.4
Fig. 6 Raman spectra of used Ni/g-Al2O3 catalysts: (a) different Ni
loading at 1 : 9 GWMRs and 650 ꢀC; (b) 15NA catalyst at different
GWMRs and 650 ꢀC.
production rate, are showing decreased tendency. On 5NA and
10NA catalysts, activity decreased with time on stream that may
be seen as due to the acid sites of alumina available on these
catalysts forming coke with time and deactivating the cata-
lyst.20,25 At a given metal loading, the number of active sites in a
catalyst is a function of the metal dispersion. The surface
saturation of Ni over g-Al2O3 may be seen around 15% of Ni. The
3.2 Catalyst evaluation
3.2.1 Effect of Ni loading on g-Al2O3. Glycerol steam
reforming reaction on Ni/g-Al2O3 catalysts is studied at 1 : 9
ꢀ
GWMRs and 650 C, and the results are shown in Fig. 7. The
glycerol conversion and H2 production rates on 5NA and 10NA
catalysts decrease with time on stream.44 On 15NA catalyst,
steady glycerol conversion activity is observed for 25 h. Note that
100% glycerol conversion and 67.5% of H2 in the gas stream
with minimum coking are achieved on this catalyst. At higher Ni
loadings (20NA and 25NA) also steady conversion is observed
Fig. 7 Effect of Ni (wt%) on g-Al2O3 catalysts in glycerol reforming at
for 15 h, aer which the conversion, as well as the H2 1 : 9 GWMRs and 650 ꢀC. Inset: product gas distribution.
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Ni particle formed at this coverage appears to be more suitable
for stable activity in glycerol reforming. Above 15% Ni loading,
although there is no much difference observed in the particle
size of Ni, the TPR clearly shows that the Ni particle formed at
higher loading (20NA and 25NA) is reduced relatively at low
temperature compared to 15NA. This shows that on 15NA Ni
particle is obtained by reducing the interacted Ni (Ni–O–Al).
Above this loading, the possibility of formation of free NiO is
seen, which may accelerate the Ni particle size affecting the
stable activity.45 Thus, in 15NA, the catalyst is further studied.
3.2.2 Effect of temperature. Glycerol conversion and
product gas distribution on 15NA are shown as a function of
temperature in Fig. 8. The results show that at 500–600 ꢀC,
activity is observed decreasing slightly with time. Below 600 ꢀC
due to dehydrogenation and dehydration, the formation
byproduct is observed and the side reactions of these byprod-
ucts may deposit carbon with time. This is possibly minimised
at higher temperatures due to the complete decomposition of
glycerol to CO and H2. Fig. 8 (inset) clearly explains the product
gas distributions on 15NA as a function of temperature at 1 : 9
Fig. 9 Time on steam activity on the 15NA catalyst in glycerol steam
reforming at 1 : 9 GWMRs and 650 ꢀC.
are depicted in Fig. 10. As the GWMRs increased (steam
decreased), the glycerol conversion decreased with time due to
carbon deposition. The gaseous product distribution also
changes, wherein the products H2 and CO2 are decreased, and
CO and CH4 are increased due to the absence of a water–gas
shi reaction and methane steam reforming.25,33,44
3.2.4 Coke formation. Coke formation during steam
reforming causes rapid deactivation of catalysts and thereby
results in low durability. Therefore, it is interesting to deter-
mine the reaction conditions to minimize coke formation for
the design of efficient carbon resistant catalysts. Fig. 11 repre-
sents the carbon formation as a function of Ni loading and
GWMRs in glycerol steam reforming.
ꢀ
GWMRs. With an increase in temperature from 500 to 650 C,
the glycerol conversion and hydrogen percentages are increased
due to the endothermic nature of glycerol steam reforming.19,44
With an increase in temperature, CH4 is decreased and CO is
increased due to methane steam reforming is favoured at high
temperatures.25 Fig. 9 clearly explains the glycerol conversion
and gas product distribution with time on stream on the 15NA
catalyst. No change in glycerol conversion is observed for 25 h.
However, the decrease in H2 production, for example 1–2%, a
decrease in CO2 up to 3% and increases in CO up to 6% are
observed. The aforementioned results explain that water gas
shi reaction decreased with time and building CO concentra-
tions. This may be seen as the loss of active sites due to coking
with time.25,40,46
The carbon deposition is calculated with the following
formula:
Cdeposition (mg gꢂ1 cat. hꢂ1) ¼ (MT ꢂ Mcat)/(Mcat ꢁ T),
3.2.3 Effect of GWMRs. Analysing the effect of the reaction
temperature on the catalytic performance of 15NA, a tempera-
ture of 650 ꢀC was chosen to study the effect of GWMRs.
Moreover, steam reforming is an energy-intensive process, and
GWMRs optimization is necessary to minimize the process cost.
Thus, glycerol content in the feed was changed, and the results
where MT is the total mass of the catalyst and carbon produced
aer 25 h of reaction, Mcat is the mass of the catalyst before
reaction, and T is total reaction time (25 h). With an increase of
Ni on g-Al2O3, the carbon formation decreases (1.9 to 1.3 mg
carbon per g cat. per h); up to 15% Ni and beyond, carbon
formation increased to 3.3 mg carbon per g cat. per h. 5NA and
Fig. 8 Effect of temperature on 15NA in glycerol steam reforming; Fig. 10 Effect of GWMRs on the 15NA catalyst in glycerol steam
GWMRs ¼ 1 : 9, inset: product gas distribution.
reforming at 650 ꢀC; inset is gaseous product distribution.
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