Low temperature steam reforming of ethanol over cobalt doped bismuth vanadate [Bi4(V0.90Co0.10)2O11-δ (BICOVOX)] catalysts for hydrogen production

Article abstract of DOI:10.1016/j.jpcs.2020.109754

The atmospheric pressure low temperature steam reforming of ethanol over Bi₄(V_0.90Co_0.10)₂O_11-δ (BICOVOX) catalysts, synthesize by a solution combustion synthesis method and calcined at 400, 600 and 800 °C, has been investigated at different reactor temperatures, H₂O: EtOH molar ratios and feed flow rates. For fresh catalysts amount, crystallinity and particle size of pure γ-BICOVOX phase is observed to increase with increasing calcination temperature. Phase purity and crystallinity of the catalysts are almost retained till 30 h of activity study with some amount of carbon formation as derived from XRD, XPS, FESEM and simultaneous DTA-TGA study. Catalyst calcined at 600 °C (BICOVOX-600) exhibits the highest ethanol conversion (100%) with maximum H₂ selectivity (80%) under reaction conditions of 400 °C, 23:1H₂O: EtOH molar ratio and 0.35 cc min^?1 feed flow rate. The maximum O^2? vacancy present in lattice and lowest coke deposition could explain the best performance of BICOVOX-600 catalyst.

Full text of DOI:10.1016/j.jpcs.2020.109754

Journal of Physics and Chemistry of Solids 148 (2021) 109754
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Journal of Physics and Chemistry of Solids
journal homepage: http://www.elsevier.com/locate/jpcs
Low temperature steam reforming of ethanol over cobalt doped bismuth
vanadate [Bi4(V0.90Co0.10)2O11-δ (BICOVOX)] catalysts for
hydrogen production
,
*
Shweta Sharmaa, Shampa Aichb, Banasri Roya
a Department of Chemical Engineering, Birla Institute of Technology and Science (BITS), Pilani Campus, VidyaVihar, Pilani, 333031, Rajasthan, India
b Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur, West Bengal, 721302, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
The atmospheric pressure low temperature steam reforming of ethanol over Bi4(V0.90Co0.10)2O11-δ (BICOVOX)
catalysts, synthesize by a solution combustion synthesis method and calcined at 400, 600 and 800 ◦C, has been
investigated at different reactor temperatures, H2O: EtOH molar ratios and feed flow rates. For fresh catalysts
amount, crystallinity and particle size of pure γ-BICOVOX phase is observed to increase with increasing calci-
nation temperature. Phase purity and crystallinity of the catalysts are almost retained till 30 h of activity study
with some amount of carbon formation as derived from XRD, XPS, FESEM and simultaneous DTA-TGA study.
Catalyst calcined at 600 ◦C (BICOVOX-600) exhibits the highest ethanol conversion (100%) with maximum H2
selectivity (80%) under reaction conditions of 400 ◦C, 23:1H2O: EtOH molar ratio and 0.35 cc minꢀ 1 feed flow
rate. The maximum O2ꢀ vacancy present in lattice and lowest coke deposition could explain the best perfor-
mance of BICOVOX-600 catalyst.
Low temperature steam reforming
Ethanol
BICOVOX catalyst
Hydrogen production
Microstructural analysis
1. Introduction
steam reforming has shown great potential. High temperature steam
reforming, a well-known process, operates at 600–900 ◦C. In recent
Continuous growth in population, urbanization, and industrializa-
tion are major responsible factors for the constant consumption of fossil
fuel leading to their depletion and emission of greenhouse gases [1]. In
order to solve the energy & economy crises and environmental pollution
problems related to this, governments of different countries are looking
for the proper utilization of renewable energy resources. Hydrogen (H2)
is considered as one of the promising renewable energy carriers with
high heating value (34 kcalgmꢀ 1) to replace fossil fuels and fulfill the
requirements of the clean energy [2,3].
times, researchers have reported low temperature steam reforming
(200–600 ◦C) of hydrocarbons for H2 production. The success of this
technology would lead to energy savings. Additionally, the longevity of
the system is expected to be high, while reactor materials and opera-
tional costs would be lower compared to the conventional high tem-
perature steam reforming.
Many works on low temperature (200–600 ◦C) & atmospheric
pressure steam reforming of ethanol over inorganic catalyst powders
have been reported. Most of these are on unary (Al2O3, Y2O3, La2O3,
CeO2, etc.) or mixed ceramic oxide supported single (Ni, Pt, etc.) or
mixed metal (bi-, tri-etc.) catalysts [6,7]. Moraes et al. investigate low
temperature steam reforming (LTSR) of ethanol over Ni(10 wt%)/CeO2
and Pt(1 wt%)-Ni(10 wt%)/CeO2 catalyst at 400 ◦C, under atmospheric
pressure with H2O: EtOH molar ratio of 3:1 [8]. With only Ni, the EtOH
conversion is found to be 54% with high H2 (47–56%) and CO2
(10–22%) selectivity values, while, with the addition of Pt to Ni the
ethanol conversion increases to 69% with drops in H2 (41–44%) and CO2
(8–12%) selectivity values. Sahoo et al. perform steam reforming of
ethanol over Co (15 wt%)/Al2O3 catalyst, prepared by wet
Challenges regarding H2 production technology are lowering the cost
of production, at least by a factor of 3–4, and improving the production
rate. Ethanol has been chosen for the present study, because, it is less
toxic and can be produced from different renewable biomass waste
sources, bagasse or molasses (waste product after the extraction of sugar
from sugar cane), different agricultural wastes (rice husk & straw, wheat
straw, etc.) and forests debris like grass, wood, etc., by many diverse bio-
chemical processes [4,5].
Although there are several catalytic reactor systems (such as aqueous
phase reforming, combustion, partial oxidation, etc.) for H2 production,
* Corresponding author.
E-mail address: banasri.roy@pilani.bits-pilani.ac.in (B. Roy).
https://doi.org/10.1016/j.jpcs.2020.109754
Received 3 May 2020; Received in revised form 6 September 2020; Accepted 7 September 2020
Available online 12 September 2020
0022-3697/© 2020 Elsevier Ltd. All rights reserved.

S. Sharma et al.
Journal of Physics and Chemistry of Solids 148 (2021) 109754
impregnation, to investigate the effect of reaction temperature
(350–600 ◦C), contact-time (3–10 kg cat./(mol/s)) and H2O: EtOH
molar ratio (1:1–8:1) on H2 production. The maximum H2 (~78%) with
minimum CO (~5%) and CH4 (~2%) yield are achieved at 500 ◦C, H2O:
EtOH molar ratio = 3:1–5:1 and contact-time 15–17 kg cat./(mol/s) [9].
Banach et al. study the effect of potassium inclusion (1–2 wt%) on
Co–ZnO–Al2O3 catalysts for ethanol steam reforming [10]. The
reforming is carried out at 420 ◦C, atmospheric pressure, and H2O: EtOH
mole ratio 21:1, 15:1 and 12:1. The catalyst with 2 wt% potassium and
28 wt% Co shows the maximum H2 yield (5.5–5.3 mol H2/mol EtOH)
along with 1.8–1.7 mol CO2/mol EtOH.
observed to depend on the number of available active sites on the sur-
face of membranes [21,22].
Based on the above discussion, BICOVOX could be considered as a
catalyst for low temperature (200–400 ◦C) atmospheric pressure steam
reforming of ethanol. In our previous paper, limited study of BICOVOX
(Bi4(V0.90Co0.10)2O11-δ) powder, calcined at 800 ◦C, for steam reforming
of ethanol has been reported [23]. Here our interest is to present elab-
orate catalytic activity of BICOVOX powders (calcined at 400, 600 and
800 ◦C) at different reactor temperatures, feed concentrations and feed
flow rates and correlate the performance of the catalysts with their
physicochemical properties. X-ray diffraction (XRD) analysis has been
used to explain the phase composition, while an electron microscopy
technique with EDX is useful to understand the powder morphology and
particle size and elemental distribution of the samples. X-ray photo-
electron spectroscopy (XPS) helps to characterize the chemistry of the
powder surface. S-DTGA is used to study the thermal decomposition
behavior and Fourier transport infrared spectroscopy (FTIR) is utilized
for surface analysis of the samples.
So far, most of the catalysts (including those mentioned above) used
for steam reforming are in reduced form, in which the active phases are
metal. However, few researchers have used simple and complex oxide
catalysts for H2 production without reduction as well. Ethanol steam
reforming over CeNixOy (0 < x < 5) mixed oxide catalysts has been
demonstrated at 200–480 ◦C, atmospheric pressure and H2O: EtOH
molar ratio of 3:1 by Duhamel et al. [11]. Catalyst without reduction
shows the production of H2 (70% selectivity) with 15% EtOH conversion
at 300 ◦C. In the case of reduced catalyst, 62% H2 selectivity and 17%
EtOH conversion at the same reaction temperature have been reported.
Lee et al. investigate the ethanol steam reforming over unreduced
MgAl2O4 and Zn0.3Mg0.7Al2O4 oxide catalysts, at 300–600 ◦C, atmo-
spheric pressure, and gas hourly space velocity (GHSV) of 6000 hꢀ 1
[12]. For MgAl2O4 catalyst the occurrence of H2 (50% selectivity)
happens at 350 ◦C and maximum H2 selectivity (78%) with 89% EtOH
conversion is obtained at 550 ◦C. In case of Zn0.3Mg0.7Al2O4 catalyst, H2
(58%) starts to be produced at 300 ◦C and the maximum H2 selectivity of
88% with 94% EtOH conversion is achieved at 550 ◦C.
2. Experimental
2.1. Catalyst preparation
BICOVOX catalysts are synthesized by a self-propagating solution
combustion synthesis (SCS) method. Stoichiometric (O:F = 1:1) amounts
of bismuth nitrate [Bi(NO3)3.5H2O, > 99%; Rankem, Gurgaon, India],
vanadium oxide [V2O5, >98%; Himedia, Mumbai, India], cobalt nitrate
[(Co(NO3)2.6H2O, > 99%; Rankem, Gurgaon, India] and glycine
[C2H5NO2, >99%; Rankem, Gurgaon, India] are mixed thoroughly with
10 ml ethanol in a mortar with a pestle for about 30 min. The mixture is
transferred to a crystallization disk, allowed to air dry for overnight, and
ignited on a hot plate at ~300 ◦C inside a fume-hood. The tip of a
sheathed type-K thermocouple is placed just on the top of the mixture in
an attempt to measure the temperature change with time during com-
bustion. The mixture is kindled at a spot and the flame wave propagates
throughout the volume instantaneously. Thus, a transient reddish flash
appears over the entire area of the disk and a voluminous black smoke is
produced. After cooling down, the foamy and porous mass is ground to
obtain the powder sample, which is washed four to five times with
deionized water to remove unwanted impurities. The powder sample is
dried at 60 ◦C for a period of 12 h and named as ‘BICOVOX-wash’. For
the catalytic study, the powder is heat-treated in a muffle furnace at 400,
600 and 800 ◦C for a period of 2 h each, and then is designated as
‘BICOVOX-400_F’, ‘BICOVOX-600_F’ and ‘BICOVOX-800_F’ catalysts,
respectively [24].
Clearly, the development of efficient and stable catalysts, which can
lead to high H2 yield and ethanol conversion, is a critical step [4]. It has
been found in the literature that supports with higher O2ꢀ ion conduc-
tivity may play a very important role in the reforming. A comparative
study shows the higher activity of Ni/CeO2 system over Ni/Al2O3 for
aqueous-phase reforming of n-BuOH and this behavior is explained on
the basis of higher O2ꢀ ion mobility through the CeO2 lattice [13,14].
Oxygen mobility through ceria and other oxide related catalysts is dis-
cussed by many other researchers [15,16].
The metal-doped bismuth vanadate (BIMEVOX) systems could be
considered as potential catalysts (or supports for catalysts) for H2 pro-
duction due to high O2ꢀ ion conductivity. The Bi4V2O11 (BIVOX) is the
parent compound, where, V5+ ion is partially substituted (10–15 at%)
with other metallic ions (Me = Li+, Cu2+, Co2+, Ni2+, Zn2+, Fe3+, Al3+
,
Ti4+, Sn4+, Pb4+, Nb5+, etc.) to acquire BIMEVOX.
α (face-centered
orthorhombiczh), β (orthorhombic), and γ (tetragonal), the three main
polymorphs of BIVOX are shown below with sequential phase transition
temperatures [17].
2.2. Characterization of the catalysts
≈450∘C
≈570∘C
≈890∘C
α
β
γ
Liquid
(1)
̅̅̅̅̅̅→ ̅̅̅̅̅̅→ ̅̅̅̅̅̅→
Simultaneous thermo-gravimetric and differential thermal analysis
are carried out (DTG-60H, SHIMADZU) under atmospheric air, from 50
to 800 ◦C with a ramping rate of 10 ◦C minꢀ 1 to understand the
decomposition behavior of the catalyst. The DTA data is also collected
during cooling from 800 to 200 ◦C in order to study the phase stability.
The subvalent substitution of vanadium may stabilize the highest
conductive γ-phase and leads to the formation of additional O2ꢀ va-
cancies in the significantly deformed vanadate octahedral, giving rise to
enhanced oxygen ion mobility at low temperature [17,18].
XRD (Rigaku miniflex II system with CuKα radiation, operated at 30
Some of the BIMEVOX systems are used as catalysts in a fixed bed
reactor and in a dense membrane catalytic reactor for the oxidation of
hydrocarbons. Chetouani et al. report the application of BICOVOX and
BICUVOX for vapor phase oxidation of propene in a stainless-steel fixed
kV and 15 mA) study is accomplished over the 2-theta range from 10 to
90◦ at a scan rate of 0.5◦/minute in order to characterize the phase
content/composition and crystalline nature of the powder samples.
The BICOVOX catalysts are investigated using the IR probe in the
mid-infrared region (Frontier, PerkinElmer, India) equipped with
attenuated total reflection (ATR) attachment (GladiATR, PIKE Tech-
nologies, Inc.) to identify the presence of different chemical bonds and
groups. A diamond crystal is used as an internal reflection element. The
data is averaged over 20 scans.
◦
bed reactor in the temperature range of 300–550 C. For both of the
catalysts, up to 450 ◦C, propene (C3H6) is mostly converted to CO2, while
◦
from 450 to 550 C zone, oxidative dimerization of C3H6 to 1,5-hexa-
diene (C6H10) is increased at the expense of CO2. The detailed study
shows that BICOVOX is more active and selective to the formation of 1,5-
¨
hexadiene than BICUVOX [19,20]. Lofberg et al. use polycrystalline
The particle size distribution, morphology, and microstructure of the
washed catalyst powder are studied with a field emission scanning
electron microscope (FESEM Hitachi S-800, Krefeld, Germany),
BICOVOX and BICUVOX membranes in a catalytic dense membrane
reactor for the oxidation of propene and propane. The conversion rate is
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S. Sharma et al.
Journal of Physics and Chemistry of Solids 148 (2021) 109754
◦
operated at 15 kV with an energy-dispersive x-ray spectroscopy (EDX)
detector (Inca Penta Fetx3). For other powders, the FESEM instrument
from Carl Zeiss Merlin with EDX detector (EDX Ametek) is used. In order
to prepare the FE-SEM specimen, a pinch of powder is sprinkled over
carbon tape mounted on top of an aluminum stub.
preheated at 110 C and N2 gas (>99.99%) is swept through the pre-
heater to carry the vapor mixture to the reactor. The tubing connect-
ing the preheater to the reactor is heated at 110 ◦C to prevent conden-
sation. The effluent from the reactor is passed through an ice-cooled
phase separator in order to separate the gaseous and liquid phase
products. The gaseous products from the phase separator are analyzed
by using gas chromatography (SHIMADZU-2014) instrument equipped
with a thermal conductivity detector (TCD) and liquid product is
analyzed with a headspace gas chromatography (HSGC; Shimadzu-
HS10) equipped with a flame ionization detector (FID). The schematic
of the LTSR experimental set-up is shown in the supplementary file (S-
Fig. 1).
The XPS data are collected by a Kratos Axis Ultra DLD X-ray photo-
electron spectrometer with monochromatic Al Kα source operating at 13
kV AC voltage and 9 mA emission current. The operating pressure is
maintained at ~10ꢀ 9 torr. Charge compensation is achieved by the
technique of charge neutralization using low energy electrons. After a
wide-range survey of each, high-resolution C (1s), V (2p), Bi (4f), Co (2p)
and O spectra of the samples are acquired in order to quantify the C/
coke deposition and to identify the oxidation states of the different
metal/metal-oxides. Both BICOVOX_F and BICOVOX_30Hrs_U (after
using a catalyst for 30 h in the reactor at best operating parameters)
catalysts are examined to understand the catalyst stability and changes
with use. The deconvolution of all spectra is performed with the
XPSPEAK 4.1 program using a Voigt function.
In the present study, ethanol conversion (XEtOH (%)) and the selec-
tivity of C-containing products are calculated by using the following
equations [14]:
XEtOH (%) = [(m ol EtOHin ꢀ m ol EtOHout) /m ol EtOHin] * 100
(2)
S (%) = [(C in CO, CO2 or CH4)/(total C present in the out flow gas)] * 100
(3)
2.3. Catalyst activity test
H2 selectivity is calculated as the number of moles of H2 present in
the outflow gas, normalized by the number of moles of H2 that would be
formed if each mole of C in ethanol is considered to participate in the
reforming reaction ideally to give 3 mol of H2 according to equation (4)
[14]:
Catalytic activity is performed by using a custom-designed contin-
uous flow fixed bed stainless still reactor placed in a muffle furnace
under atmospheric pressure. 2 gm batch of the BICOVOX catalyst,
sandwiched in between two layers of quartz wool, is loaded in the
reactor and preheated at 150 ◦C for 30 min with flowing nitrogen (10 ml
minꢀ 1) for the removal of the adsorbed moisture from the surface. The
performance of the catalyst is evaluated at atmospheric pressure, for
23:1 and 2.5:1H2O: EtOH molar ratios, reactor temperature range of
200–400 ◦C, and 0.1 and 0.35 cc minꢀ 1 feed flow rates (GHSV =
5097.9–17,318 hꢀ 1 at STP). The liquid mixture of ethanol and water is
SH (%) = [ (m ol% H2 in out flow gas)/(3* m ol% CO2 in out flow gas)] *100
2
(4)
The selectivity of CH3CHO, CH3OH, and CH3COCH3, in the liquid
effluent, is calculated similarly to equation (3):
Fig. 1. XRD spectra of the (a) BICOVOX-wash and
BICOVOX_F catalysts calcined at 400, 600, and 800
◦C, (b) BICOVOX catalysts calcined at 400, 600, and
800 ◦C and used in reactor for 30 h at 400 ◦C, 0.35 cc
minꢀ 1 feed flow rate, and 23:1H2O: EtOH molar ratio.
The symbols * and $ represent tetragonal (amcsd-
0010069) and monoclinic (amcsd-0011925) Bi2O3,
and # represents (amcsd-0014281) BiVO4 -related
phases, respectively. FTIR spectra of (c) BICOVOX-
wash and BICOVOX_F catalysts calcined at 400,
600, and 800 ◦C, and (d) BICOVOX catalysts calcined
at 400, 600, and 800 ◦C and used in reactor for 30 hrs
at 400 ◦C, 0.35 cc min-1 feed flow rate, and 23:1 H2O:
EtOH molar ratio.
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S. Sharma et al.
Journal of Physics and Chemistry of Solids 148 (2021) 109754
S (%) = [(C in CH3CHO, CH3OH, or CH3COCH3)/(total C present in liquid products)] * 100
(5)
to be mostly pure γ-BICOVOX (around 93 wt% with average crystallite
size 20.4 nm) phase with the presence of the small amount of tetragonal-
BiVO4 (7.2 wt%; peaks at ~ 18.8, 28.9 and 30.5◦) [17,25]. For
BICOVOX-400_F (around 75 wt% pure γ-BICOVOX with crystallite size
45 nm) and BICOVOX-600_F (pure γ-BICOVOX around 83.0 wt% with
crystallite size 46.8 nm) samples, little amount of BiVO4 phase is
observed (around 15 and 7 wt%, respectively) and two more extra
peaks, at ~ 28.0 and ~ 46.3◦, indicate the presence of tetragonal β-Bi2O3
related secondary phases. However, in BICOVOX-600_F, besides β-Bi2O3
3. Results
BICOVOX catalysts have been testes at different reactor tempera-
tures, feed flow rates, and feed concentrations. Among all, the best ac-
tivity is obtained at 400 ◦C, 0.35 cc minꢀ 1, and 23:1H2O: EtOH molar
ratio. Accordingly, the physicochemical characterization of the catalysts
corresponding to this reaction condition is performed in detailed and
reported here.
monoclinic (~1 wt%)
observed [24]. After use in a reactor, for BICOVOX-600_30Hrs_U sample,
amount of -Bi2O3 increases slightly, while for BICOVOX-400_30Hrs_U
sample a new -Bi2O3 is appeared (Fig. 1(b)). No phase change occurs
α
-Bi2O3 peaks at ~25.8 and ~27.4◦ is also
3.1. Microstructural and related analysis
α
α
XRD spectra (Fig. 1(a) & (b)), shown in layer lines, illustrate the
effects of heat treatment and reactor conditions on phase composition
and crystallinity of the BICOVOX fresh and used catalysts. BICOVOX-
Wash sample (Fig. 1(a)) shows a multiphase combination, containing
tetragonal β-Bi2O3 (*; American mineralogist crystal structure data base
in BICOVOX-800_30Hrs_U catalyst and it retains good crystallinity. XRD
patterns for parent compound BIVOX (wash and calcined at 600 ◦C)
depicts the presence of more amount of secondary phases even after
calcination (supplementary file (S-Fig. 2)).
FT-IR spectra (Fig. 1(c) & (d)) of washed, fresh and used catalysts
reveal a broad and strong band at ~710 cmꢀ 1 which could be assigned to
V–O stretching mode, corresponds to the variation of the V–O bond
length in the range of 1084–1079 Å for VO4 tetrahedra and/or VO6
0010069 or amcsd-0010069), monoclinic α-Bi2O3 ($; amcsd-0011925),
BiVO4 (#; amcsd-0014281) along with the desired pure γ-BICOVOX
(amcsd-0016748). The fresh sample calcined at 800 ◦C, can be indexed
Fig. 2. DTA/TGA spectra for (a) BICOVOX-wash, (b) BICOVOX-400_30Hrs_U, (c) BICOVOX-600_30Hrs_U and (d) BICOVOX-800_30Hrs_U catalysts used in reactor at
400 ◦C, 0.35 cc minꢀ 1 feed flow rate, and 23:1H2O: EtOH molar ratio.
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S. Sharma et al.
Journal of Physics and Chemistry of Solids 148 (2021) 109754
octahedra [25]. In addition, the Bi–O symmetric vibrations due to β-
Bi2O3 are observed at 609, 510 and 418 cmꢀ 1 [26]. Peaks captured in
the region of 477 to 440 cmꢀ 1 could be ascribed to doping metal-oxygen
bond Co–O [27]. Additionally, an extra peak at 823 cmꢀ 1 could be
assigned to asymmetric V–O stretching [25]. BICOVOX-600_F and
BICOVOX-400_30Hrs_U show additional bands in the region of 516–422
literature [30,31].
S-DTGA profiles of BICOVOX-wash and BICOVOX_30Hrs_U catalysts
are unveiled in Fig. 2. The DTA profile of the BICOVOX-wash catalyst
(Fig. 2(a)) contains two peaks-the first one, endothermic at ~ 270 ◦C and
corresponds to weight loss of ~1.7%, could be attributed to chemisorbed
moisture and ethanol, carbon left from glycine, and/or some Bi–V–O
cmꢀ 1, which could be attributed to
α
- Bi2O3 [26,28], and supported by
(such as α-Bi2O3 to β-Bi2O3) related phase change. The second one,
XRD results. In the case of all BICOVOX_30Hrs_U catalysts one peak at
exothermic in nature at ~650 ◦C and observes only during heating
~1369 cmꢀ 1 signifies the presence of C C stretching and confirms the
(without weight loss), could be due to the Bi–V–O (secondary phases
–
–
carbon deposition (Fig. 1(d)) [29]. One band is observed (only in used
samples) at 1204 cmꢀ 1, corresponds to an asymmetric V–O stretching
[30]. Our observations are consistent with the results reported in the
(α-Bi2O3 and β- Bi2O3) to pure γ-BICOVOX) related phase transformation
[32]. The exothermic DTA peak, at ~600 ◦C for BICOVOX-400_30Hrs_U
and BICOVOX-600_30Hrs_U catalysts (Fig. 2(b–c)) with ~0.3% weight
Fig. 3. FESEM images with particle size distribution and EDX spectrum for (a) BICOVOX-wash. Catalysts calcined at 400 ◦C (b) & (c), 600 ◦C (d) & (e), and 800 ◦C (f)
& (g) show the variation of particle size and elemental distribution for catalysts in fresh stage and after use in reactor for 30 h at 400 ◦C, 0.35 cc minꢀ 1 feed flow rate,
and 23:1H2O: EtOH molar ratio, respectively.
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S. Sharma et al.
Journal of Physics and Chemistry of Solids 148 (2021) 109754
loss, could be attributed to both C deposited on samples and phase
transformation [32]. However, for BICOVOX-800_30Hrs_U catalyst
(Fig. 2(d) small weight loss (due to deposited carbon to CO2 formation),
without any DTA peak (indicating no phase decomposition during
reforming) is observed. The featureless DTA profile during cooling
shows the phase stability of the catalysts. The TGA spectra of
BICOVOX-wash shows that most of the residual carbon burned out
within 400 ◦C. In used samples, the TGA spectra extended till 700–750
◦C implies that the nature of C formed during sample preparation
(amorphous) is different from the C-deposited (mixed amorphous and
graphitic nature) in the course of reforming reaction. These results are in
accordance with the XRD and FTIR analysis. The endothermic peak at
~623 ◦C in the DTA cooling spectrum of BIVOX-wash powder (supple-
mentary file; S-Fig. 3) suggests the reverse transition of γ-BIVOX which
indicates the instability of the undoped parent phase. Here, it is
important to mention that DTGA is a standard method for detecting C
deposited on used catalysts [32,33].
are calibrated by carbon i.e. 284.4 eV as it is found to be the maximum
intensity peak for C1s.
High resolution XPS spectra of the V2p, for fresh and used catalysts
calcined at different temperatures, are depicted in Fig. 4(a–c). For all the
BICOVOX_F catalysts, the deconvoluted V2p3/2 peak at ~517.0 eV,
corresponds to the V5+ oxidation state and the V2p3/2 peak at ~516.2 eV
could be attributed to V4+ oxidation state [37]. After catalytic activities,
the intensity and area of the lower binding energy peak (for V4+
oxidation state) is found to reduce and intensity of high binding energy
peaks, indicating the presence of V5+ state, are found to increase [38].
The
V
5+:V4+ ratio for BICOVOX-400_F, BICOVOX-600_F and
BICOVOX-800_F is calculated to be 0.65, 0.33 and 0.63, respectively
which reflects that BICOVOX-600_F catalyst contains the maximum
amount of V4+ oxidation state. Clearly, Co dopant incorporated in
structure is responsible for the formation of dual oxidation states, as
supported by the literature. Kim et al. and Khaerudini et al. study and
report the V2p spectra for Bi4V2O11
, Bi4(V0.90Co0.10)2O11-δ and
The porous and flaky network of interconnected particles is a com-
mon morphology for the solution combustion synthesized BICOVOX
powders and well evident in the FESEM images (Fig. 3 (a), (b), (d) and
(f)) along with the particle size distribution and EDX spectra of the wash
and fresh catalysts. EDX data show the presence of Bi, V, Co, C and O
elements. Table 1 includes the particle size distribution and elemental
analysis, obtained from FESEM-EDX, for wash, fresh, and 30 h used
BICOVOX catalysts.The average particle size for the fresh catalysts in-
creases with calcination temperature, as expected, and some particle
agglomeration is observed after using these catalyst for 30 h in the
reactor (Fig. 3(c), (e) and (g)). No clear carbon structure is evident in the
FESEM images but EDX indicates coke deposition on the used catalysts.
Which implies the formation of a thin layer of carbon. EDX shows some
what higher amount of carbon deposition on the BICOVOX-800_30Hrs_U
catalyst compared to the others.
Bi4(V0.90Nb0.10)2O11-δ powder, prepared by SSR method [37,39].
Bi4V2O11 shows only V5+ oxidation state, however, both V5+ and V4+
oxidation states are seen for the other two, reflecting the formation of
more anionic vacancies.
The high-resolution C1s core level XPS data, as shown in Fig. 4(d–f),
indicates the variation of coke present on the surface of the fresh and used
catalysts. For all fresh catalysts, C1s profile comprises adventitious carbon
at ~284.0 and ~285.6 eV ascribe to sp2 and sp3 hybridized C–C bonding,
–
respectively with C O at ~288 eV. It is observed that peak area and in-
–
tensity reduces with calcination temperature, from BICOVOX-400_F to
BICOVOX-800_F catalysts, as expected [40,41]. C1s spectrum of all used
catalysts is noticed to increase in intensity and area of all the peaks, indi-
cating deposition of C during reforming. Calculated fresh:30Hrs_U carbon
ratio is 1:2.2, 1:1.9 and 1:2.8 for BICOVOX-400, BICOVOX-600, and
BICOVOX-800 catalysts, respectively. The maximum amount of carbon is
seen for BICOVOX-800_30Hrs_U catalyst, and it is supported by FESEM
results [40].
As reported, most of the research works on BIMEVOX powders use
the standard solid state reaction (SSR) technique for the sample prepa-
ration and average particle size of the synthesized power lies in the
The high resolution Bi4f spectra of BICOVOX-wash and all BICO-
VOX_F catalysts (Fig. 5(a–c)) exhibit two peaks centered at ~164.2 and
~159.1 eV, which might be ascribed to Bi4f5/2 and Bi4f7/2 of Bi3+ in
Bi2O3 [42]. Two more peaks at ~163.7 and ~158.4 eV, corresponding to
the Bi4f5/2 and Bi4f7/2 of Bi metal (Bi0), are observed in general, except
for BICOVOX-800_F catalyst [42]. After using the catalysts in the
reactor, intensity and area of the lower binding energy peaks are
observed to increase, suggesting the reduction of Bi3+ to lower oxidation
states Bi0 [42]. Obtain an order of reduction (Bi3+ to Bi0) is
range of ~15–25
μm [34,35]. The particle size distribution of the fresh
catalysts (0.1–13.6
μ
m) indicates that SCS could be a useful method for
the preparation of BIMEVOX powder of smaller particle size leading to
highly effective materials of application. Roy et al. synthesize the
Bi4(V0.85Cu0.15)2O11-δ catalyst by SCS and calcined at 600, 700 and 800
◦C [24]. Reported particle sizes for as-combusted, 600, 700 and 800
calcined catalysts are 0.1–1, 0.2–5.8, 0.5–6.5 and 1–10 μm, respectively
which is in the same line of our results.
BICOVOX-400_30Hrs_U
30Hrs_U.
>
BICOVOX-800_30Hrs_U>BICOVOX-600_
3.2. Surface chemistry analysis
Fig. 5(d–f), representing the high resolution XPS spectra for O1s,
indicates the presence of two sites of oxygen. The deconvoluted peaks at
~529.2 and ~531.2 eV confirm the presence of lattice oxygen (OL) and
O2ꢀ vacancies (OV) with ꢀ 2 and +2 oxidation state, respectively, in all
catalysts [43]. The OL: OV ratio for BICOVOX-400_F, BICOVOX-600_F,
and BICOVOX-800_F are calculated to be 5.7, 1.7 and 2.7, respectively
which reflects that BICOVOX-600_F catalyst contains the maximum
amount of O2ꢀ vacancies. Spectra of Co2p for all BICOVOX catalysts are
depicted in Fig. 5(g–i). The five deconvoluted peaks at ~780.0(Co3+),
Wide range XPS spectra of the washed and fresh (supplementary file
(S-Fig. 4 (a))) and used (supplementary file (S-Fig. 4 (b))) catalysts,
presented in layer lines, show the presence of all essential elements; Bi,
V, O, and Co, along with adventitious C. The peaks at ~27.0, 93.0,
163.0, 440.0, 460.0, 680.0, and 940.0 eV could be assigned to Bi5d,
Bi5p, Bi4f, Bi4d5/2, Bi4d3/2, Bi4p, and Bi4s, respectively [36,37]. Peaks
at ~284.0, ~530.0, ~515.0, and ~779.0 eV correspond to C1s, O1s,
V2p, and Co2p, respectively [36]. Binding energies of all the elements
Table 1
Particle size distribution from FESEM and elemental analysis from EDX and XPS of wash, fresh, and 30 h used BICOVOX catalysts.
Catalysts
Particle size Dist. (FESEM)
Elemental Analysis-EDX (at%)
Elemental Analysis-XPS (at%)
Bi
V
Co
C
O
Bi
V
Co
C
O
BICOVOX-WASH
0.1–1.1
0.2–4.9
0.3–8.3
0.4–7.8
0.5–10.9
0.4–13.6
0.7–15.8
30.8
34.7
35.9
35.9
38.5
36.9
35.2
14.9
16.6
16.4
16.8
16.8
17.5
15.8
1.7
1.5
1.9
1.9
1.9
1.9
1.9
15.5
12.2
16.5
10.5
14.7
8.5
37.1
35.0
29.3
34.9
28.1
35.2
28.4
13.3
20.0
12.9
15.4
11.7
16.2
12.0
6.6
9.8
6.3
7.5
5.5
8.1
5.8
0.7
1.1
0.7
0.8
0.6
0.9
0.6
47.1
43.2
49.9
39.6
48.7
33.9
52.3
32.3
25.9
30.2
36.7
33.5
40.9
29.3
BICOVOX-400_F
BICOVOX-400_30HRS_U
BICOVOX-600_F
BICOVOX-600_30HRS_U
BICOVOX-800_F
BICOVOX-800_30HRS_U
18.7
6

S. Sharma et al.
Journal of Physics and Chemistry of Solids 148 (2021) 109754
Fig. 4. High resolution XPS spectra of V for the catalysts calcined at (a) 400 ◦C, (b) 600 ◦C, (c) 800 ◦C, and C for the catalysts calcined at (d) 400 ◦C, (e) 600 ◦C, and
(f) 800 ◦C showing the variation of oxidation states for fresh, and 30 Hrs_U BICOVOX catalysts.
~782.6(Co2+), 786.5(satellite), 797.2(Co2+) and 804.8(satellite) eV
denote the presence of Co in the lattice in Co3+ and Co2+ oxidation states
[44]. However, no certain trend of doping metal oxidization state with
heat treatment or reactor conditions are observed. High resolution XPS
spectra of V2p and O1s for BIVOX-600 (fresh and used) are reported in
the supplementary file (S-Figure 4 (c) and (d)). Calculated V5+:V4+ (2.4)
and OL: OV (8.6) ratio for fresh catalyst shows a small amount of O2ꢀ
vacancy and for used catalyst, only V5+ oxidation states and OL are
detected.
selectivity reduces with time, attain the steady-state within 5–6 h and
remain stable till ~30 h. Fig. 6 shows an example, where the activity is
carried out at 400 ◦C, 23:1H2O: EtOH molar ratio and 0.35 cc minꢀ 1 feed
flow rate. For BIVOX steady state is obtained within 6 h, but H2 and CO2
selectivity values (~12.0 and ~5%, respectively) detected to be much
smaller compared to BICOVOX catalysts (supplementary file; S-Fig. 5).
In the following sections, the effect of temperature, flow rate, and
feed concentrations on steady-state EtOH conversion, C-in gas, the
selectivity of gaseous and liquid products of BICOVOX catalysts are
discussed.
Table 1 shows the elemental analysis of BICOVOX fresh and used
catalysts obtained from XPS profile. Atomic % of the elements is
determined using CasaXPS-2.3.17 software and considering the core
shell peaks for each element. The Bi: V and V: Co atomic ratios, calcu-
lated from both EDX and XPS data, are ~2 and ~9 (taking ±0.5 error)
for all BICOVOX catalysts, which suggest that proper stoichiometry is
maintained in the compositions.
3.4. Effect of temperature
EtOH conversion, C in the gas phase and selectivity values of H2 and
CO2 increase with temperature irrespective of feed concentration and
flow rate, while CO and CH4 show an opposite trend as shown in Fig. 7
(a–f) and Table 2. Maximum EtOH conversion of 100% with highest H2
(80%), CO2 (72%) selectivity, and C in gas phase (97%), and lowest CO
(0%) and CH4 (28%) selectivity is obtained for BICOVOX-600 catalyst at
3.3. Catalytic activity study
400 C, 0.35 cc minꢀ 1, and 23:1H2O: EtOH molar ratio. H2 and CO2
◦
The catalytic activity is presented in terms of ethanol conversion (%),
C in the gas product (%), selectivity (%) of gaseous (H2, CO2, CO, and
CH4) and liquid (CH3CHO, CH3OH, and CH3COCH3) products for
various temperatures, feed concentrations and flow rates.
selectivity of BICOVOX-600 are noticed to be 30.0 and 23.6% higher
with respect to BICOVOX-400 and 23.7 and 12.5% higher with respect
to BICOVOX-800 catalysts, respectively. The temperature at which 1st
CO2 is formed, called crossover point. Only CO is detected below 200 ◦C,
in general and CO2 is started to form (crossover point) at ~200 ◦C for the
catalysts calcined at 600 and 800 ◦C and for the catalysts calcined at 400
Irrespective of reactor temperature, feed concentration, and flow
rate, for all the BICOVOX catalysts, EtOH conversion, C in the gas phase
and selectivity values of H2 and CO2 increases and CO and CH4
7

S. Sharma et al.
Journal of Physics and Chemistry of Solids 148 (2021) 109754
Fig. 5. High resolution XPS spectra of the elements present in the BICOVOX catalysts calcined at 400 ◦C (a) Bi, (d) O, and (g) Co; 600 ◦C (b) Bi, (e) O, and (h) Co; and
800 ◦C (c) Bi, (f) O, and (i) Co show the variation of oxidation states and chemistry in fresh state and after use in reactor for 30 h at 400 ◦C, 0.35 cc minꢀ 1 feed flow
rate, and 23:1H2O: EtOH molar ratio.
◦C crossover point is at ~225 ◦C. Fig. 7(g–i) show that selectivity of
CH3CHO and CH3COCH3 increases with an increase in reaction tem-
perature and CH3OH selectivity shows an opposite trend. Similar trends
have been observed in our last paper [23].
Steam reforming of ethanol over BIMEVOX catalysts has not been
reported till date. Research works reported in the scientific literature for
LTSR over various other catalysts have been considered for analyzing
our results. Dai et al. perform LTSR over Pt nano-particles encapsulated
8

S. Sharma et al.
Journal of Physics and Chemistry of Solids 148 (2021) 109754
Fig. 6. Variation of EtOH conversion, and selectivity of the gaseous products as a function of time under steam reforming conditions of 400 ◦C, atmospheric pressure,
23:1H2O:EtOH molar ratio and 0.35 cc minꢀ 1 feed flow rate of (a) BICOVOX-400, (b) BICOVOX-600, and (c) BICOVOX-800 C catalysts.
in hollow zeolite microreactor catalyst at atmospheric pressure, weight
hourly space velocity (WHSV) of 8.5 hꢀ 1, H2O: EtOH mole ratio of 4:1
selectivity increases from 0.0 to 13.0% and 28.0 to 33.0%, respectively
with increasing EtOH concentration, at 400 ◦C, and 0.35 cc minꢀ 1 for
BICOVOX-600 catalyst. Obtain trends for liquid products shows that
selectivity of CH3COCH3 and CH3OH increases and CH3CHO decreases
with increasing EtOH concentration from 23:1 to 2.5:1H2O: EtOH molar
ratio. In our last paper we have also seen the similar trends [23]. These
are consistent with the effects of feed concentration on product distri-
bution reported by other researchers. Comas et al. perform ethanol
steam reforming over Ni/Al2O3 catalyst at 1 atm, 400 ◦C, and H2O: EtOH
molar ratio varies from 6:1 to 1:1 [47]. Results show that with increasing
EtOH concentration, the H2 and CO2 selectivity reduces 90.0 to 55.0%
and 42.0 to 30.0%, respectively and CO and CH4 selectivity increases
from 15.0 to 38.0% and 21.0 to 23.0%, respectively. Mulewa et al. shows
that EtOH conversion, H2 and CO2 yield reduces from 98.9 to 88.9%,
54.6 to 28.8% and 21.4 to 13.3%, respectively, with the increase in feed
concentration from 10:1 to 1:10 H2O: EtOH molar ratio at 1 atm, 400 ◦C
and GHSV = 13200 ml/gcat⋅h [48]. On the contrary, CO (2.5–14.3%)
and CH4 (12.6–13.3%) yield increases with an increase in feed
concentration.
◦
and temperature range from 200 to 400 C [45]. Catalytic activity re-
sults show the increase in EtOH conversion from 68 to almost 100% and
an increase in H2 and CO2 selectivity from ~42.0 to 72.0% and
~15.0–23.0%, respectively on increasing temperature from 200 to 400
◦C. CO and CH4 selectivity show an opposite trend with temperature. Lee
et al. perform ethanol steam reforming over mesoporous
Sn-incorporated SBA-15 catalyst at atmospheric pressure, GHSV 6600
hꢀ 1, H2O: EtOH molar ratio of 1:1, and temperature varying from 200 to
400 ◦C [46]. The maximum EtOH conversion and H2 selectivity of 92.0
and 70.0%, respectively, are achieved at 400 ◦C. Results reported in the
literature with respect to the effect of temperature on product distri-
bution and EtOH conversion, are in the same line of our experimental
outcomes.
3.4.1. Effect of ethanol concentration in the feed
The effect of feed concentration on the catalytic activity is reported
in Fig. 7 and (supplementary file (S-Fig. 6)). As a general trend, ethanol
conversion, C in gaseous products, and H2 & CO2 selectivity reduce and
on the contrary CO and CH4 selectivity increase with increasing EtOH
concentration from 23:1 to 2.5:1H2O: EtOH molar ratio for all the cat-
alysts. EtOH conversion, H2 and CO2 selectivity reduces from 100 to
86%, 80.0 to 66.0% and 72.0 to 54.0%, respectively and CO and CH4
3.4.2. Effect of feed flow rate
In general, EtOH conversion, CO and CH4 selectivity reduce with an
increase in flow rate from 0.1 to 0.35 cc minꢀ 1, but H2 and CO2 selec-
tivity show an opposite trend, as shown in Fig. 7. H2, CO2 selectivity
9

S. Sharma et al.
Journal of Physics and Chemistry of Solids 148 (2021) 109754
Fig. 7. Steady state variation of (a) EtOH conversion,(b) C in gaseous phase, selectivity of the gas products; (c) H2, (d) CO2, (e) CO, (f) CH4, and liquid products; (g)
CH3CHO, (h) CH3OH, and (i) CH3COCH3 as a function of reactor temperature, for H2O:EtOH molar ratio of 23:1 and feed flow rates 0.1 & 0.35 cc minꢀ 1 over the
BICOVOX catalysts calcined at different temperatures. Legend is represented as: catalyst calcination temperature –H2O: EtOH molar ratio-feed flow rate.
increases from 73.0 to 80.0%, and 63.0 to 72.0% respectively and CO
and CH4 selectivity reduces from 2.0 to 0.0% and 35.0 to 28.0%,
respectively with increasing flow rate from 0.1 to 0.35 cc minꢀ 1 at
23:1H2O: EtOH molar ratio and 400 ◦C for BICOVOX-600 catalyst. EtOH
conversion reaches 100.0% at 400 ◦C for BICOVOX-600 catalyst and
remains unaffected with the change in flow rate further. The selectivity
of CH3COCH3 and CH3OH reduces and CH3CHO increases with an in-
crease in flow rate. Hongbo et al. perform ethanol steam reforming over
Ni/Y2O3–Al2O3 catalyst at H2O:EtOH = 13:1 molar ratio, temperature
300–500 ◦C , and feed flow rate varies from 0.01 to 0.3 ml minꢀ 1 [49].
At 400 ◦C, complete conversion of EtOH happens and remains unaf-
fected as feed flow rate changes from 0.01 to 0.3 ml minꢀ 1, and H2 yield
increases from 32.1 to 38.5% with increasing feed flow rate, which is in
the same line with our results. The effect of feed concentration and flow
rate on gaseous and liquid products at 400 ◦C is reported in Table 2 for
all catalysts.
10

S. Sharma et al.
Journal of Physics and Chemistry of Solids 148 (2021) 109754
Table 2
Steady state catalytic activity data for LTSR of EtOH over BICOVOX catalysts at 400 ◦C reactor temperature under different feed concentration and flow rates.
Catalysts
Feed flow rate (cc minꢀ 1
BICOVOX-400
0.1
BICOVOX-600
0.1
BICOVOX-800
0.1
)
0.35
23:1
0.35
23:1
0.35
23:1
H2O:EtOH (molar ratio)
23:1
2.5:1
2.5:1
23:1
2.5:1
2.5:1
23:1
2.5:1
2.5:1
EtOH conv. (%)
C in gas (%)
83
83
50
48
16
36
50
31
19
71
78
38
38
28
34
35
40
25
78
85
56
55
10
35
57
28
15
68
80
44
42
22
36
44
35
21
100
90
73
63
2
96
79
59
48
19
33
51
25
24
100
97
80
72
0
86
82
66
54
13
33
60
20
20
88
90
55
55
13
32
59
23
18
75
75
43
46
16
38
44
30
26
83
96
61
63
8
69
83
51
50
14
36
52
25
23
H
2 selectivity (%)
CO2 selectivity (%)
CO selectivity (%)
CH4 selectivity (%)
CH3CHO selectivity (%)
CH3OH selectivity (%)
35
65
16
19
28
75
15
10
29
65
20
15
CH3COCH3 selectivity (%)
4. Discussion
Several research works have been published in the field of ethanol
steam reforming over various catalysts with the aim of improving the
catalytic activity and less carbon formation [50–55]. Martinelli et al.
study the effect of Na (2.5 wt%) addition on the performance of Pt(2.0
wt%)/ZrO2 catalyst for ethanol steam reforming at 250–400 ◦C, atmo-
spheric pressure, and GHSV of 381000 hꢀ 1. Na enhances the CO2
selectivity and H2 selectivity by promoting C–C scission. The maximum
H2 and CO2 selectivity are obtained to be 68 and 47%, respectively, with
almost 100% ethanol conversion [50]. Dai et al. investigate low tem-
perature ethanol steam reforming over dragon fruit-like engineered
structure Pt–Cu (Pt/Cu = 2.5/1)/mesoporous SiO2 catalyst at 250–450
◦C, and atmospheric pressure. Almost 100% ethanol conversion with
maximum H2 and CO2 selectivity values of 71 and 22%, respectively, are
achieved at 450 ◦C [51]. Riani et al. compare the activity of unsupported
Co nanoparticles prepared by borohydride reducing route with that of
prepared via thermal decomposition for ethanol steam reforming at
250–500 ◦C. Co nanoparticles prepared from the borohydride reducing
route, contain borate and boride impurities, show 82% H2 yield, 77%
CO2 selectivity, and almost 100% ethanol conversion at 500 ◦C.
Whereas, the Co nanoparticles prepared via thermal decomposition and
free form boron show lower catalytic activity; maximum 51% H2 yield,
61% CO2 selectivity, and 77% ethanol conversion at 500 ◦C [52].
Gharahshiran et al. develop yttria and/or zirconia promoted meso-
porous carbon-Ni-Co catalysts for ethanol steam reforming at 200–400
◦C, H2O:EtOH molar ratio 3:1 and atmospheric pressure [53]. Addition
of Y2O3 reduces the active phase particle size compare to that of the
ZrO2. The Y2O3-promoted catalyst show 39% H2 yield, 49% CO2 selec-
tivity, and 58% ethanol conversion, where as the ZrO2- promoted
catalyst show 29% H2 yield, 60% CO2 selectivity with 40% ethanol
conversion [54]. Hence, it is clear that the activity results obtained in the
present study for BICOVOX catalyst are consistent and comparable with
those reported recently in the literature.
Fig. 8. (a) Possible reaction path ways and (b) possible mechanisms for the
LTSR reaction over the BICOVOX catalyst. Numbers are showing the possible
reaction step sequentially. The arrows are showing the incoming reactants
(arrow tail) and outgoing products (arrow head). where shows the oxy-
gen vacancies.
4.1. Reaction pathway
Pathway 1: reaction step 2: C–C bond breakage in acetaldehyde
A set of possible reaction pathways, which can be drawn based on the
gas and liquid phase products acquired, is schematically presented in
Fig. 8 (a) and described below. Here, the standard heat of reactions is
calculated by considering the gaseous state of the reactant and products
as these are present in gaseous form at the reaction temperature. The
overall steam reforming reaction is
CH3CHO(g)⇆ CH4(g) + CO(g); ΔH0298K = ꢀ 19 kJm olꢀ 1
Reaction step 3: water-gas shift reaction (WGSR)
CO(g) + H2O(g)⇆ CO2(g) + H2(g); ΔH0298K = ꢀ 41.2 kJm olꢀ 1
(8)
(9)
Reaction Steps 4 and 5: Formation of methane via Fischer-Tropsch
C2H5OH(g) + 3H2O(g)⇆2CO2(g) + 6H2(g); ΔH0298K = 174 kJm olꢀ 1
(6)
reactions (FTR)
Reaction step 1: dehydrogenation leading to the formation of
CO(g) + 3H2(g)⇆ CH4(g) + H2O(g); ΔH0298K = ꢀ 205.8 kJm olꢀ 1
(10)
(11)
CH3CHO
C2H5OH(g)⇆ CH3CHO(g) + H2(g); ΔH0298K = 68.9 kJm olꢀ 1
(7)
CO
2(g) + 4H2(g)⇆ CH4(g) + 2H2
O(g); ΔH0 = ꢀ 165 kJm olꢀ 1
298K
CH3CHO can undergo to various pathways:
11

S. Sharma et al.
Journal of Physics and Chemistry of Solids 148 (2021) 109754
Reaction Steps 6 and 7: Steam reforming of methane (either created
high amount of carbon formation [59].
by C–C bond breaking or via FTR)
The BICOVOX catalysts are organized on the basis of EtOH conver-
sion, amount of C in gas, and the selectivity of H2 at the best operating
parameters (400 ◦C, 23:1H2O: EtOH molar ratio, atmospheric pressure,
and 0.11 cc minꢀ 1 feed flow rate) as follow:
CH4(g) + H2O(g)⇆CO(g) + 3H2(g); ΔH0298K = 205.8 kJm olꢀ 1
(12)
(13)
CH4(g) + 2H2O(g)⇆CO2(g) + 4H2(g); ΔH0298K = 165 kJm olꢀ 1
Reaction step 8: formation of CO via reverse WGSR
CO2(g) + H2(g)⇆CO(g) + H2O(g); ΔH0298K = 41.2 kJm olꢀ 1
Reaction step 9: formation of CH3OH
The decreasing order of the EtOH conversion is: BICOVOX-
600>BICOVOX-800> BICOVOX-400; The decreasing order of the
Amount of C in gas: BICOVOX-600>BICOVOX-800> BICOVOX-400.
The decreasing order of the H2 selectivity is: BICOVOX-
600>BICOVOX-800> BICOVOX-400
(14)
(15)
CO2(g) + 3H2(g)⇆CH3OH(g) + H2O(g); ΔH0298K = ꢀ 48.9 kJm olꢀ 1
4.2. Possible mechanisms over oxide catalyst
Pathway 2: reaction step 10: formation of CH3COCH3
CH3CHO(g) + CH3OH(g)⇆CH3COCH3(g) + H2O(g); ΔH2098K
= ꢀ 93.4 kJm olꢀ 1
Most of the steam reforming catalysts are based on metal-related
active centers on ceramic oxide supports. The ethanol or feed elec-
tronically attaches to the active center and high oxygen ion mobility of
the support helps in WGSR. Only a few works have been reported on the
mechanism of ethanol steam reforming over unreduced oxide catalysts.
Lee et al. perform ethanol steam reforming over unreduced Zn-doped
(Zn0.3Mg0.7Al2O4) and Zn–Pb doped (Pd0.01Zn0.29Mg0.7Al2O4) MgAl2O4
catalysts. The proposed mechanism path of reaction indicates attach-
ment of ethanol molecules on the basic active sites of Zn–O–Mg surfaces
to produce intermediate CH3CHO, followed by decarboxylation on
Mg–O–Al acidic surfaces to give CO and CH4 [12]. Further, the CH4
reforming reaction takes place over Mg–O–Al/Zn–O–Al–O–Mg surfaces
to give H2 and CO. At this point, the exact reaction mechanism for LTSR
of ethanol on BIMEVOX is not clear, but a possible process could be that
similar. This is proposed and represented in Fig. 8(b) as ‘Possible
Mechanism-I’. Accordingly, ethanol might be activated sequentially on
the acidic or basic sites of oxide catalyst (electronegativity difference
between the metals present in the oxide could create acidic and basic
active sites) to produce the intermediate products. Ethanol would be
attached to the localized basic sites to form acetaldehyde by C–H bond
cleavage which can undergo further decomposition through C–C bond
cleavage on the acidic sites of catalyst to produce CH4 and CO species
[57,58]. ’Later, oxygen mobility on V(Co)–O(⃞ ) layer (⃞ indicates
O2ꢀ vacancy) is expected to act as a promoter in CH4 reforming and
WGSR [60]. The creation of O2ꢀ vacancies in the V2O5 layer doped with
a Co metal ion could be represented through Kroger-Vink equation as
[61]:
(16)
Pathway 3: reaction step 11: hydration of CH3CHO to CH3COOH
CH3CHO(g) + H2O(g)⇆ CH3COOH(g) + H2(g); ΔH0298K = ꢀ 30.1 kJm olꢀ 1
(17)
Reaction step 12: C–C bond breakage in acetic acid to form CO2
CH3COOH(g)⇆ CO2(g) + CH4(g); ΔH0298K = ꢀ 29.9 kJm olꢀ 1
(18)
However, in our case CH3COOH is not comprehended in liquid
product, presumably because the amount present could very small or the
survival time of the intermediate is too short as both the reaction steps
11 and 12 are exothermic with almost same amount of heat of reaction
and therefore this step-in Fig. 8 (a) is showed with dash line.
At low temperatures, below 250 ◦C, along with high EtOH concen-
tration in the feed mixture, the predominant reaction could be the
decomposition of ethanol [56].
C2H5OH(g)⇆ CH4(g) + CO(g) + H2(g); ΔH0298K = 50 kJm olꢀ 1
(19)
At lower temperatures with small amounts of water in the feed, the
contribution of the WGSR (path 3, equation (9)) is small. Therefore, no
CO2 is observed at a temperature of 200 ◦C and 2.5:1H2O: EtOH molar
ratio for BICOVOX-800. The presence of large amount of CO and CH4
indicates the dehydrogenation of ethanol to CH3CHO (path 1, equation
(7)) as the most probable, which is followed by C–C bond breakage (path
2, equation (8)) along with the formation methane via FTR (path 4 and
5, equations (10) and (11)). An increase in temperature, ethanol
reforming reaction (equation (6)) and the WGSR (path 3, equation (9))
become dominant and lead to an increase in the selectivity of H2 and
CO2. Meanwhile, the low selectivity of by-products (CO and CH4) are
also exhibited in the gas products, suggesting that the acetaldehyde
decomposition reaction (path 1, equation (7) followed by path 2,
equation (8)) is occurred in parallel with the methane steam reforming
(path 6 and 7, equations (12) and (13)) and WGSR (path 3, equation
(9)), [57].
V2O5 → 2V′ + VO⋅⋅ + 4OOX
(20)
V2O5
2CoO
2Co′′′ + 3VO⋅⋅ + 2OOX
(21)
̅̅̅̅̅→
V
Where 2Co′V′′ represents the occupancy of a V5+ site by a Co2+ with three
negative charge, VO⋅⋅ represents positively charged O2ꢀ vacancies, and OXo
represents neutral lattice oxygen. Singh et al., explains the ion conduc-
tion mechanism of vanadium polyhedral in BIMEVOX, starting with 3:1
tetrahedral to octahedral ratio in the initial stage [61]. Oxide ion leaves
the original position in vanadium octahedral (coordination number 6) to
occupy equatorial vacancy. The coordination number of vanadium
octahedra changes from 6 to 5 thereby creating an intermediate state of
two 4 coordination number and two 5 coordination vanadium poly-
hedra. Finally, second oxide ion from 5 coordination vanadium poly-
hedra leaves to occupy another vacancy resulting to the starting
situation of 3:1 tetrahedra to octahedral. The vanadium octahedral and
tetrahedral is in the changing state leading to the mobility of oxide ion in
the vanadate layer.
As the amount of water increases, water inhibits the ethanol
decomposition reaction by channeling ethanol reactivity towards the
main ethanol steam reforming reaction (equation (6)) [58]. An increase
in the amount of water also has the effect of increasing the extents of the
WGSR (path 3, equation (9)) and methane steam reforming reactions
(path 6 and 7, equations (12) and (13)) and consequently, a reduction in
the amounts of the undesired products, CO and CH4 which leads to
increasing overall H2 selectivity [47].
Ethanol conversion is found to decrease with increasing flow rate, as
flow rate increases, residence time reduces. High residence time allows
subsequent promotion of all possible and feasible reactions along with
enough time for complete conversion of ethanol and vice versa. On
contrary to this, longer residence time (low flow rate) decreases H2 and
CO2 selectivity due to methane formation reaction between CO2 and H2
(path 4 and 5, equations (10) and (11)), which further responsible for a
Zhu et al. study the role of O2ꢀ vacancy in methane reforming over
◦
unreduced CeO2–Fe2O3 catalyst at 850 C [13]. Under a reductive at-
mosphere in the methane-conversion (CH4 to CO/CO2) step, Fe2O3
partially reduced to metal/oxide (Fe/FeO) system and CeFeO3 is formed,
via 3CeO2 + Fe2O3 + Fe → Ce4+Fe2+O3 and/or CeO2 + FeO →
Ce4+Fe2+O3 reactions. CeFeO3 formation causes the generation of
additional O2ꢀ vacancies, which enhances the WGSR and reduces the
12

S. Sharma et al.
Journal of Physics and Chemistry of Solids 148 (2021) 109754
carbon deposition. It is also seen in the literature that O2ꢀ vacancies are
able to accept hydride species and enhances the ability to abstract H2
conversion, C in the gas phase, and H2 and CO2 selectivity is found to
increase with increasing reactor temperature and decreasing EtOH
concentration in the feed for all the BICOVOX catalysts. Selectivity for
CO and CH4 shows an opposite trend with increasing temperature and
decreasing EtOH concentration. With increasing flow rate EtOH con-
version is found to decrease, however, C in the gas phase, and H2, CO2,
CO and CH4 selectivity values are observed to increase. Maximum H2
selectivity of 80% for BICOVOX-600, with almost 100% EtOH conver-
from ethanol during dehydrogenation step (O2–Mn+
☐
+
H2→OH–Mn+Hꢀ with ☐ anionic vacancy) [11]. Thus, O2ꢀ vacancies
present in BICOVOX catalyst favor the CH4 reforming and WGSR as
mentioned in ‘Possible Mechanism-I’.
Another probable mechanism for ethanol steam reforming over
BICOVOX oxide catalyst could involve the reduced bismuth metal
(generated in the reducing atmosphere (CH4, CO, H2, N2) during the
reforming reactions) as the active metal center. XPS (XPS analysis, Fig. 5
(a–c)) also shows a small amount of Bi metal on the surface of the fresh
catalysts. Mutual interaction between ethanol and basic sites of catalysts
could create acetaldehyde as an intermediate product, which undergoes
the decarbonylation reaction over Bi-metal to form CO and CH4 as
depicted in Fig. 8 (b) as ‘Possible Mechanism-II’. A similar reaction has
already been reported in the literature majorly in the field of oxidative
C–C cleavage of epoxides over Bi-mandelate catalysts to give carboxylic
acid [62,63]. Later, oxygen mobility, via O2ꢀ vacancies, plays its role in
CH4 reforming and WGSR as discussed above. However, more detailed
experimental and/or modeling work need to be conducted to realize the
actual path way of the LTSR of ethanol on BIMEVOX.
◦
sion at reactor conditions of 400 C, 23:1H2O: EtOH molar ratio and
0.35 cc minꢀ 1 feed flow rate is observed.
XRD study reveals that the BICOVOX-400_F and BICOVOX-600_F
samples contain higher amount of secondary phases
(α-Bi2O3,
β-Bi2O3, and
α-BiVO4) compare to BICOVOX-800_F catalyst, as it shows
mostly pure γ-BIMEVOX phase.
FESEM analysis shows the growth in particle size with calcination
temperature from 400 to 800 ◦C. After using a catalyst for 30 h in the
reactor, the scale of agglomeration is increased. XPS spectra analysis of
the BICOVOX catalysts show maximum V4+:V5+ ratio for the BICOVOX-
600_F sample, indicating higher oxygen mobility in BICOVOX-600
catalyst compared to BICOVOX-800_F and BICOVOX-400_F catalysts.
XPS data confirms the carbon deposition in used catalysts and it also
confirms the deposition of the maximum amount of C in used BICOVOX-
800 catalyst. The carbon deposition during reaction arises question
regarding the stability of catalysts. The study on the catalyst stability
and carbon deposition is on progress and the following part of this paper
will present it elaborately.
4.3. Effect of heat treatment on the catalysts
The lower catalytic activity of the powders heat treated at 400 and
800 ◦C compared to that of the BICOVOX-600 sample could be explained
in terms of phase composition, particle size distribution, and coke
deposition on the surface, as studied by XRD, DTGA, FESEM, EDX, and
XPS. Correspondingly, these catalysts are arranged in decreasing order
as follows:
CRediT authorship contribution statement
Shweta Sharma: Methodology, Data curation, Writing - original
draft, Writing - review & editing. Shampa Aich: Methodology, Visual-
ization, Investigation. Banasri Roy: Conceptualization, Methodology,
Supervision, Resources, Project administration, Funding acquisition.
Phase composition (amount of γ-phase, both in fresh and used):
BICOVOX-800 > BICOVOX-600 > BICOVOX-400.
Particle size distribution (largest to smallest, both in fresh and used):
BICOVOX-800 > BICOVOX-600 > BICOVOX-400.
Coke deposition (highest to lowest, in used): BICOVOX-800 >
BICOVOX-400 > BICOVOX-600.
Declaration of competing interest
While a higher amount of γ-phase could enhance the activity, larger
particle size reduces WGSR and increases C deposition in BICOVOX-800
[11,12,48,57,58]. Reportedly, during high-temperature steam reform-
ing, carbon may enwrap the surface as filamentary strands, surface
films, or graphitic species and reduces the catalytic activity [64,65].
Rate of WGS reaction and coke deposition phenomena are inter-
connected and significantly depend on the particle size distribution of
the catalysts. Zhou et al. investigate the behavior of decreasing CeO2
particle size and report that O2ꢀ vacancies increased by 2 orders of
magnitude when particle size is reduced from 60 to 4 nm [66]. Addi-
tionally, large particles could decrease the density of active centers and
be responsible for reduced overall activity.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgments
The authors gratefully thank the Centre for Nano Science and Engi-
neering (CeNSE)-IISc, Bangalore, for XPS characterization, Department
of Physics, BITS Pilani, Rajasthan, India for XRD characterization,
Department of Chemical Engineering, BITS Pilani, Rajasthan, India for
providing experimental facility and support of this research. The first
author (S. Sharma) would also like to thank DST for providing INSPIRE
scholarship
The BICOVOX-400_F contains the smallest particle size, which could
stabilize the disordered γ-phase, lower the activation energy for oxygen
ion transition and helps in high ionic conductivity [17]. But the least
amount of γ-phase and presence of a maximum amount of carbon in the
fresh catalyst deteriorate its performance.
This research did not receive any specific grant from funding
agencies in the public, commercial, or not-for-profit sectors.
‘There are no conflicts to declare.’
In terms of the amount of γ-phase and average particle size
BICOVOX-600 is in the middle position, but contains the maximum
amount of V4+ oxidation state (i.e. more amount of O2ꢀ vacancies), as
unveiled from XPS analysis. This, besides particle size, could help to
minimize the C deposition on the catalysts and leads to maximum
performance.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.jpcs.2020.109754.
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