This document summarizes research on using various catalysts to promote the dehydrogenation of cyclohexane to produce hydrogen gas. Key findings include: - Monometallic silver (Ag) catalysts supported on activated carbon cloth showed increasing hydrogen evolution rates with increasing Ag loading up to 10 wt%, but rates decreased at 15 wt% loading likely due to poorer dispersion. - Bimetallic catalysts with 1 wt% noble metals (platinum, palladium, rhodium) promoted on 10 wt% Ag/ACC showed enhanced hydrogen evolution rates compared to the monometallic Ag catalyst. In particular, a 10 wt% Ag-1 wt% Pt catalyst produced hydrogen at twice the rate of the 10 wt%

1. Catalytic dehydrogenation of cyclohexane over Ag-M/ACC
catalysts for hydrogen supply
Jayshri V. Pande, Anshu Shukla, Rajesh B. Biniwale*
National Environmental Engineering Research Institute (NEERI), Council of Scientific and Industrial Research, Nagpur 440020, India
a r t i c l e i n f o
Article history:
Received 5 October 2011
Received in revised form
12 January 2012
Accepted 18 January 2012
Available online 14 February 2012
Keywords:
Dehydrogenation
Bimetallic catalyst
Cyclohexane
Pulse spray reactor
Hydrogen storage
a b s t r a c t
Dehydrogenation of cyclohexane to benzene has been carried out over Ag supported on
activated carbon cloth (Ag/ACC) catalysts using a spray- pulse reactor. Hydrogen evolution
was studied for hydrogen storage and supply system applications. The maximum rate of
hydrogen evolution rate using monometallic Ag/ACC catalysts was 6.9 mmol/gmet/min for
Ag loading of 10 wt%. An enhanced hydrogen evolution was observed by adding a small
amount of noble metal (1 wt% Pt, Pd, Rh) to the Ag based catalysts. A synergistic effect was
observed in the case of the Pt promoted catalysts on the hydrogen production were twice as
compared to 10 wt% Ag catalyst only.
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1. Introduction
Hydrogen-based energy systems are being pursued widely
across many countries as cleaner energy options [1]. The
transporation of hydrogen from hydrogen production facility
and delivery to fueling station is one of the important chal-
lenges in case of implementation of hydrogen-based energy
[2]. Liquid organic hydrides such as cycloalkanes are highly
potential candidates for hydrogen storage and supply; it holds
high hydrogen content about 6e8% on weight basis and
60e65 kg/m3
on the volume basis at atmospheric temperature
and pressure. Furthermore, hydrogen can be easily stored and
extracted from the liquid organic hydrides using a catalytic
process [3]. Therefore, it is convenient to use cycloalkanes for
storage and supply of hydrogen. Further, the system does not
produce any by-products such as CO or CO2 [4].
A wide variety of monometallic and bimetallic catalysts
has been reported in literature for high selectivity towards
dehydrogenation of cycloalkane to extract hydrogen [5e9]. Pt
based catalysts have been reported by various researchers for
selective dehydrogenation of cycloalkanes and seasonal
storage of energy as hydrogen. Kobayashi and coworkers have
used an unsteady-state spray pulse mode reactor for dehy-
drogenation of 2-propanol on Pt/-Al2O3 [10]. Literature reports
indicate that bimetallic catalysts have profound role in cata-
lytic reactions and in enhancing activity as compared to
monometallic catalysts in many reactions. Activity of bime-
tallic catalyst is highly dependent on electronic effect, CeH
bond cleavage and hydrogen recombination ability of metal
[11]. Bimetallic catalysts containing a small amount of Pt have
been shown promise as potential cost-effective catalysts. In
such cases, the second metal acts as a promoter and enhances
* Corresponding author. Tel.: þ91(712) 2249885x410, þ91-9822745768(Mobile); fax: þ91(712) 2249900.
E-mail address: rb_biniwale@neeri.res.in (R.B. Biniwale).
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 6 7 5 6 e6 7 6 3
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doi:10.1016/j.ijhydene.2012.01.069

2. the activity as also selectivity of the catalyst. Promoting effect
of second metal has been reported for Pt-M catalysts wherein
M ¼ W, Re, Rh, and Ir for dehydrogenation of cyclohexane. It
has been well reported by Kariya et al., that the increase in the
catalytic activity in terms of increased hydrogen evolution
rate may result due to the electronic effect of adjacent second
metal, which improves the CeH bond breaking ability,
stability of intermediate and desorption of aromatic product
[10]. Ichikawa and coworkers have reported the dehydroge-
nation of cyclic hydrocarbons over Pt and PteM (M ¼ Re, Rh,
Pd) catalysts supported on activated carbon under non steady
spray pulse reaction conditions [10,12].
From the economic point of view, we have a specific
interest to minimize the use of Pt or find the alternate catalyst
composition with improved or equal activity and selectivity as
that of Pt catalysts. In an attempt to reduce the amount of Pt,
we have earlier reported Ni/C catalyst and synergistic effect of
addition of Pt for dehydrogenation of cyclohexane [7].
Hodoshima et al. had reported Ni/C, Ru/C and NieRu/C cata-
lysts for dehydrogenation of tetralin for efficient hydrogen
supply [13].
It is evident from the above discussions that use of liquid
cycloalkane is a potential option for storage and supply of
hydrogen. Endothermic reaction of dehydrogenation of
cycloalkanes on solid catalysts is facilitated under unsteady-
state conditions. It has been reported that dehydrogenation
of cyclohexane by feeding liquid reactant in pulse, creating an
alternate wet-dry condition on the heated catalyst surface,
improves hydrogen evolution rates for the highly endo-
thermic dehydrogenation reaction [7]. Hodoshima et al. have
reported the use of a superheated liquid film reactor for
enhancing the hydrogen evolution during dehydrogenation of
decalin [14].
In the present study, we have reported relatively high
selectivity towards hydrogen during dehydrogenation of
cyclohexane over monometallic Ag/ACC (Ag supported on
activated carbon cloth) under spray pulse mode. Effect of
addition of a small amount of noble metal to the Ag based
catalysts was also studied. It has been reported that, presence
of the group Ib (group 11) metals (Ag, Au, Cu) decreases the
extent of hydrogenolysis in alkane isomerisation reaction [15].
This effect of group Ib metals is due to their property of not
promoting CeC bond cleavage. The bimetallic catalysts re-
ported in this study include Ag-M/ACC wherein M ¼ Pt, Pd and
Rh. The catalysts were characterized using XRD, SEM and XPS.
2. Experimental
2.1. Catalyst preparation
Activated Carbon Cloth (ACC) with surface area of 800 m2
/g
was used as support for loading metal catalysts. Monometallic
Ag catalysts used in the present study were 5, 10 and 15 wt%
Ag/ACC. The Ag loading was achieved by adsorption method
using solution of AgNO3 in acetone. Activated carbon cloth
was stirred with stoichiometric amount of AgNO3 corre-
sponding to Ag content of ca. 5, 10 and 15 wt% and subse-
quently dried in an oven at 100
C. Bimetallic catalysts, Ag-M/
ACC with second metal were synthesized by wet-
impregnation method for loading of 1 wt% of Pt, Pd and Rh
over Ag/ACC base catalyst with subsequent drying at 100
C.
2.2. Characterization of catalysts
The catalysts were characterized using XRD, SEM and XPS
after reduction under the hydrogen atmosphere. X-ray
diffraction patterns for various catalysts in this study were
obtained using Rigaku Miniflex II, Dekstop X-ray Diffractom-
eter with Cu Ka radiation (l ¼ 1.5405). The oxidation state of
surface metal was identified by X-ray photoelectron spec-
troscopy technique (XPS). The XPS analysis of all mono-
metallic and bimetallic catalysts were carried out before and
after the reaction. XPS measurements were performed on
a Vacuum Generator Microtech photoelectron spectropho-
tometer (model ESCA 3000) by using monochromatized MgKa
radiation (l ¼ 1253.6). The pressure of the XPS analysis
chamber during the measurement was 1 Â 10À9
Pa. All the
spectra were corrected by subtracting a Shirley type back-
ground. Binding energies were corrected with respect to C1s
binding energy of 285 eV.
2.3. Catalytic Reaction
Dehydrogenation reaction was carried out in a stainless steel
reactor with a spray pulse mode injection of the cyclohexane
feed. The average feed rate of cyclohexane was 2.4 mmol/min.
The details of experimental setup are reported in an earlier
article [9]. Catalyst was pre-treated in nitrogen flow at 300
C.
The catalyst was activated in flow of nitrogen (100 ml/min)
and hydrogen (50 ml/min) following a specific heating cycle up
to 400
C. Hydrogen evolution rate was studied by monitoring
the concentration of outlet gas. Analysis of hydrocarbons
(benzene and unreacted cyclohexane) in condensed product
was carried out by GC-FID (Shimadzu GC-2014). Hydrogen
concentration in product gas was monitored using GC-TCD
(Shimadzu GC- 2014). The packed column (porapack-Q) was
used in GC-TCD for hydrogen separation from sweep gas N2.
3. Result and discussion
3.1. Catalyst characterization
Fig. 1 depicts XRD patterns for the monometallic and bime-
tallic catalysts. The peaks in XRD pattern can be attributed to
metallic silver particles, which was confirmed by JCPDS card
no. 89-3722. This confirms the crystalline phase of Ag. The size
of the metal particles was derived from the Debye- Scherer’s
formula. The estimated size of Ag was in the range of
8e10 nm. In case of the promoted catalyst, XRD pattern did
not show any additional peak than peaks for Ag. This may be
due to relatively lower concentration of the second metal,
however, shift in the peak position was observed. All the
peaks were attributed to silver.
SEM micrograph of Ag/ACC given in Fig. 2 revealed that the
Ag particles were well dispersed on the carbon fibres.
However, agglomerates were also observed.
The survey analysis of the catalyst during XPS confirms the
presence of various elements present on carbon cloth. The
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3. binding energy values of XPS spectra for all monometallic and
bimetallic catalysts for both reduced and after exposure to
cyclohexane (after reaction) are reported in Table 1. XPS
analysis of reduced sample of Ag/ACC catalyst confirms that
metal present on the support was in zero valent state. Fig. 3
represent XPS spectrum for Ag catalyst before and after
exposure to cyclohexane. In case of fresh catalyst, peak was
observed for the binding energy of Ag 3d5/2 and Ag 3d3/2 was at
about 368 eV and 374 eV, which confirms the presence of Ag in
zero oxidation state [16]. The peaks for 10 wt% Ag/ACC cata-
lysts after exposure to cyclohexane were observed at binding
energy of 370 eV and 376 eV. Shift in the peak position was
attributed to the change in the oxidation state of catalyst over
the support from metallic silver to Agþ1
[17,18]. Monometallic
Pt catalysts exhibit a single peak at 74.5 eV, which shifted to
75.1 eV after the reaction. This indicates that platinum was
fully reduced to metallic state after treatment at 400
C in
hydrogen flow. Wherein, after the reaction shift at higher
binding energy assigned to change in the oxidation state [19].
Similarly, XPS spectrum of all bimetallic catalysts reduced
under hydrogen flow corresponds to zero valent state of
metal. Binding energy values of 10 wt% Ag-1 wt% Pd/ACC
catalysts were observed to be 368.6 eV, 374.5 eV and 335.2 eV
corresponding to Ag 3d5/2, Ag 3d3/2 and Pd 3d respectively. For
10 wt% Ag-1 wt% Rh/ACC catalyst binding energies were
368.4 eV, 374.4 eV and 306.6 eV corresponding to Ag 3d5/2, Ag
3d3/2 and Rh 3d5/2 respectively. All these values represent zero
valent state of metals [8,20]. XPS spectrum of reduced 10 wt%
Ag -1 wt% Pt/ACC catalyst also indicates zero valent state of Ag
Pt. After reaction, in the case of 10 wt% Ag-1 wt % Pd/AACC
catalyst the binding energy of Ag was almost same as reduced
catalysts but the binding energy of Pd was shifted towards
higher value reveals change in oxidation state of Pd. This
result is in good agreement with the experimental data.
Hydrogen abstraction was carried out on Ag sites, addition of
palladium improved the stability of the catalysts and had no
considerable effect on dehydrogenation activity. Peaks ob-
tained from XPS analysis of 10 wt% Ag-1 wt% Rh/ACC catalysts
showed the shift towards higher binding energy than the
reduced catalyst indicating the change in the oxidation state.
Binding energy values of 10 wt% Ag-1 wt% Pt/ACC catalyst
remained unchanged even after exposure to cyclohexane.
This indicates that the addition of platinum facilitates the
stabilization of Ag-Pt catalyst.
3.2. Influence of catalyst metal content on hydrogen
evolution rate
The hydrogen evolution rate during dehydrogenation of
cyclohexane over monometallic Ag catalysts at 300
C and
with cyclohexane feed rate of 2.4 mmol/min (pulse injection
frequency of 0.33 Hz and pulse width of 10 ms), are shown in
Fig. 4. The loading of Ag was varied as 5, 10 and 15 wt% on
activated carbon cloth. The reactions were carried out as
described in experimental section. The product gas was
monitored using TCD GC with a run time of about 3 min. The
concentration of hydrogen monitored was not the instanta-
neous value after pulse injection. Rather the prevailing bulk
concentration of hydrogen under the flow of nitrogen (as
sweep gas) and pulse injection of cyclohexane was observed.
The cyclic behaviour of the hydrogen concentration data is
due to the variation in bulk concentration under these
conditions. Therefore, the time interval for cyclic behaviour of
hydrogen concentration observed was different from the
pulse injection interval. With the change in the Ag loading,
there is a considerable change in hydrogen evolution rates.
Hydrogen evolution rate for 5 wt% Ag/ACC was observed to be
3.4 mol/gmet/min and for 10 wt% Ag/ACC 6.9 mmol/gmet/min at
30 min. The maximum conversion and hydrogen evolution
rate was observed for 10 wt% Ag/ACC catalyst at 300
C. It was
observed that hydrogen evolution rate increases with the
increase in metal loading. On the contrast, further increase in
the metal loading to 15 wt% Ag/ACC catalytic activity
decreases to 2.85 mol/gmet/min. At higher Ag loading catalysts
activity was relatively lower may be because of the lower
dispersion of the catalysts [9]. The optimal loading of Ag for
Fig. 1 e XRD patterns for monometallic and bimetallic
catalysts (a) Ag/ACC (b) AgePt/ACC (c) AgePd/ACC (d)
AgeRh/ACC.
Fig. 2 e SEM image of 10 wt% Ag/ACC catalyst.
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4. dehydrogenation of cyclohexane was therefore considered as
10 wt%. This was also confirmed by calculating average crys-
talline size (estimated from XRD data) of Ag as 3.01 and 3.8 nm
for 10 wt% Ag/ACC and 15 wt% Ag/ACC respectively.
In case of monometallic Ag/ACC catalyst, the reaction
products contained benzene and hydrogen. There were no
partial dehydrogenation (cyclohexene) or hydrogenolysis
products (such as CH4) observed. Therefore, the selectivity
towards the hydrogen formation was nearly 100%.
The temperatures mentioned in this manuscript are the set
temperatures for the heater. Whereas, the catalyst surface
temperature may vary under the spray pulse feed conditions.
The variations in the surface temperature have been reported
in our previous article regarding thermal profile of the catalyst
surface [6]. Actual surface temperature of the catalyst may be
lower by 50e75
C approximately under the wet condition
during spray injection of cyclohexane.
3.3. Influence of reaction temperature under the spray
pulsed condition on the hydrogen evolution rate
Reaction has been carried out over three different tempera-
tures viz. 250, 300 and 350
C. The hydrogen evolution rates
with respect to temperature for dehydrogenation of cyclo-
hexane over 10 wt% Ag/ACC catalyst have been observed as
depicted in Fig. 5. The reactant feed has been kept constant,
with injection frequency of 0.33 Hz and pulse width of 10 ms.
The average rate of hydrogen evolution at 250
C was found
to be 2.3 mmol/gmet/min. With increase in the temperature to
300
C the average rate of hydrogen evolution was 6.8 mmol/
gmet/min. It was about three times higher than rate of
hydrogen production at 250
C. With further increase in the
temperature to 350
C, the average hydrogen evolution rate
decreased to 4.4 mol/gmet/min. At the set temperature of
250
C rate of hydrogen evolution was relatively lower due to
decrease in the catalyst surface temperature. This resulted in
decreasing the dehydrogenation rate. The dehydrogenation of
cyclohexane is a highly endothermic reaction and is therefore
favoured at higher temperatures [21]. The temperature of the
catalyst surface plays an important role in order to maintain
close contact of reactant and catalysts surface. However, as
the temperature increases residence time of the reactant on
the catalyst surface decreases, which is unfavourable in the
dehydrogenation reaction [4].
The optimum temperature of 300
C facilitates the proper
reactant-catalyst contact and provides the required energy for
endothermic dehydrogenation reaction. At this temperature
hydrogen and aromatics are removed rapidly from the cata-
lyst surface during the dry conditions (interval between pulse
injections). Clean and regenerated catalyst surface is there-
fore available for further reaction, which suppress the reverse
reaction and results in the high hydrogen evolution rate [21].
The reactant after injection as an atomized spray for
a controlled period (pulse width) reaches the catalyst surface
and evaporates on the heated catalyst surface. This forms the
dense vapor phase in the vicinity to the catalysts surface
facilitating better contact between reactant and catalysts.
3.4. Effect of addition of second metal
We have studied the interaction of AgePt, AgePd and AgeRh
on dehydrogenation of cyclohexane for hydrogen evolution
rates. Bimetallic catalysts exhibited higher activity in terms of
increased hydrogen evolution rates. Furthermore, bimetallic
catalysts exhibited higher stability and yield in comparison to
the monometallic catalysts. The improved catalytic activity
was observed for Ag catalysts promoted by addition of 1 wt%
Pt, Pd and Rh as shown in Fig. 6. Addition of a second metal to
Ag, improved the hydrogen evolution due to synergistic effect
of second metal for CeH bond cleavage, high hydrogen-
reverse-spill over or the hydrogen-recombination [22e24]
abilities of the catalysts.
The reactions were carried out under a spray- pulsed
injection of feed. The fluctuating hydrogen evolution rate was
Table 1 e Binding energy values of fresh and used samples characterized by XPS.
Element Reported values Observed values Reference
Zero valent
metal
Higher oxidation
state
Before reaction
(reduced)
After reaction
(used)
Ag 368.3 370 368.0 370 [15e17]
374.5 376 374.0 376
Pt 70.6 73.1 70.7 71.6 [18]
73.9 76.4 74.5 75.1
AgePt
Ag 368.3 370 368.5 368.5 [15e17]
374.5 376 374.6 374.6
Pt 70.6 73.1 71.4 71.3 [18]
73.9 76.4 74.5 73.7
AgePd
Ag 368.3 370 368.6 368.5 [15e17]
374.5 376 374.5 374.4
Pd 335.2 337 335.3 345.4 [7]
AgeRh
Ag 368.3 370 368.7 367.4 [15e17]
374.5 376 374.5 373.4
Rh 306.6 309 306.8 307.3 [19]
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5. observed for monometallic catalysts, wherein the activity was
relatively lower. In case of bimetallic catalysts activity was
relatively higher in terms of hydrogen evolution rates the
product gas has a more homogenous concentration of
hydrogen. The activity of bimetallic catalysts was higher in
terms of the hydrogen evolution rate as compare to mono-
metallic 10 wt% Ag/ACC. The highest catalytic activity was
observed for 10 wt% Ag þ 1 wt% Pt/ACC catalyst with
hydrogen evolution rate of 14.2 mol/gmet/min. In case of 10 wt
% Ag/ACC monometallic catalyst hydrogen evolution rate was
varying in the range of 6.8 to 2.5 mol/gmet/min. Hydrogen
evolution rate was constant at about 14.2 mmols/gmet/min for
bimetallic catalysts. The improved activity of bimetallic
catalyst may be attributed to the higher hydrogen recombi-
nation ability of Pt. As it is reported by Kariya et al., that the
promotion of activity may be due to the electronic effect of
adjacent second metal, which improves the CeH bond
breaking ability of Pt [12]. Leiske et al., reported that the
electronically modified platinum crystal does not adsorb coke
precursors on its surface [25]. These results signify that 10 wt%
Ag-1 wt% Pt/ACC catalyst show higher activity than mono-
metallic Ag/ACC, and among all bimetallic Ag-M/ACC catalyst
selected for dehydrogenation of cyclohexane at 300
C.
The improvement in the catalytic activity of Ag-M/ACC as
compared to Ag/ACC catalysts was observed to be dependent
on the promoter metal selected. The improvement in the
activity by addition of 1 wt% of promoters has been found in
the following order;
Pt Rh Pd
While, for improving the stability of Ag based catalysts the
addition of noble metal promoters has been found in the order
of as;
Pt Pd Rh
0
2000
4000
6000
8000
10000
12000
0 400 800 1200
B.E. (eV)
Intensity(a.u.)
8000
12000
16000
20000
364 366 368 370 372 374 376 378 380
i
ii
B.E. (eV)
Intensity(a.u.)
a
b
Fig. 3 e XPS spectrum of (a) 10 wt % Ag/ACC fresh catalyst
for survey analysis, (b) Comparison of 10 wt % Ag/ACC (i)
fresh (ii) used catalysts.
0
1
2
3
4
5
6
7
8
0 20 40 60 80 100 120
H2evolutionrate(mmol/gmet/min)
Time (min)
(a)
(b)
(c)
Fig. 4 e Effect of metal content on hydrogen evolution rate
at 300
C over (a) 5 wt % Ag/ACC (b) 10 wt % Ag/ACC and (c)
15 wt % Ag/ACC catalysts.
0
1
2
3
4
5
6
7
8
0 20 40 60 80 100 120
H2evolutionrate(mmols/gmet/min)
(b)
(c)
(a)
Time (min)
Fig. 5 e Effect of temperature on hydrogen evolution rates
over 10 wt % Ag/ACC catalyst (a) 250
C (b) 300
C and (c)
350
C.
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6. Hydrogen evolution rate for AgePd catalyst was ca.
7.5 mmol/gmet/min and it was stable up to 120 min. Kariya
et al., reported that Pd has a good hydrogen spillover ability
but has no activity for dehydrogenation of cycloalkanes.
Addition of Pd has no significant effect on the activity, but it
improves the stability of the catalyst [12]. The average crys-
talline size for Ag-M/ACC bimetallic catalyst was in the range
of 1.73e1.86 nm.
The yield of hydrogen production was calculated using
following equation (1):
%yield of H2 ¼
moles of H2 formed
moles of CH converted
 100 (1)
The yield of hydrogen was calculated as 65% for 10 wt% Ag-
1 wt% Pt/ACC catalyst at 120 min. The hydrogen yield for 10
wt% Ag-1wt% Pd/ACC, 10 wt% Ag- 1 wt% Rh/ACC and 10 wt%
Ag/ACC catalyst at 120 min were calculate to be 41, 39 and
17.5% respectively. It is reported that bimetallic catalyst
exhibits a synergistic effect on the hydrogen evolution rate by
addition of a small amount of noble metal. Cyclohexane was
efficiently dehydrogenated over 10 wt% Ag/ACC, 1 wt% Pt/
ACC, 10 wt% Ag- 1 wt % Pt/ACC catalysts shown in Fig. 7.
Enhanced hydrogen evolution rate has been observed by
promoting Ag catalyst with 1 wt% Pt. The average hydrogen
evolution rate for 10 wt % Ag/ACC was about 6.9 mmol/gmet/
min whereas it was almost twice for 10 wt% Ag- 1 wt% Pt/
ACC catalyst at about 14.2 mmol/gmet/min. As compared to
1 wt% Pt catalyst AgePt bimetallic catalyst exhibit four times
higher hydrogen evolution rate. Using AgePt bimetallic
catalyst stability was also improved. When 10 wt% Ag-1wt%
Pt/ACC was tested for 300 min, the hydrogen evolution was
observed to be stable with nearly 14 mmol/gmet/min. The
hydrogen production rate decreased to 2.8 mmol/gmet/min as
the metal loading of Pt was increased from 1 wt% to 3 wt% as
shown in Fig. 8. In case of bimetallic catalyst 10 wt% Ag-1wt%
Pt/ACC the average particle size estimated by XRD was
6e10 nm and 15 nm for 10 wt% Ag-3 wt% Pt/ACC. The
relatively higher activity of 10 wt% Ag-1 wt% Pt/ACC catalysts
can be attributed to the smaller particle size and well
dispersed metal.
The Ag-M/ACC catalysts have been compared with NiePt
catalyst as reported earlier as detailed in Table 2 [6]. Ag-M/ACC
catalysts used in the present study exhibit the similar activity
in terms of hydrogen evolution rate as compared to NiePt
catalyst. However, no CH4 observed indicating only dehydro-
genation reaction was prevailing. The selectivity of the cata-
lyst for dehydrogenation of cyclohexane to hydrogen and
benzene was observed to be as high as 100%.
0
2
4
6
8
10
12
14
16
0 20 40 60 80 100 120
H2
evolutionrate(mmol/gmet/min)
Time (min)
(b)
(c)
(d)
(a)
Fig. 6 e Hydrogen evolution rate of cyclohexane over
monometallic and bimetallic catalysts at 300
C (a) 10 wt %
Ag/ACC (b) 10 wt % Ag D 1 wt % Pt/ACC (c) 10 wt %
Ag D 1 wt % Pd/ACC and (d) 10 wt % Ag D 1 wt % Rh/ACC.
0
2
4
6
8
10
12
14
16
0 20 40 60 80 100 120
H2evolution(mmol/gmet/min)
Time (min)
Fig. 7 e Hydrogen evolution rates over (a) 10 wt % Ag/ACC, (b)
1 wt % Pt/ACC and (c) 10 wt % Ag D 1 wt % Pt/ACC at 300
C.
0
2
4
6
8
10
12
14
16
0 20 40 60 80 100
evolutionrate(mmol/gmet/min)
Time (min)
(a)
(b)
Fig. 8 e Hydrogen evolution rates over (a) 10 wt %
Ag D 1 wt % Pt/ACC and (b) 10 wt % Ag D 3 wt % Pt/ACC
catalysts at 300
C.
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7. 3.5. Influence of pulse frequency and pulse width for
reactant feed over 10 wt% Ag-1 wt% Pt/ACC catalyst
As the rate of hydrogen production depends on the rate of
reactant feed, beyond the optimal rate of feeding, reactant
pool forms over the catalyst surface. This results in the
decrease of surface temperature and consequent decrease in
the hydrogen production rate [6]. Before the reaction start, the
catalyst surface is dry. The catalysts experience an alternate
wet-dry condition when the reactant strikes over the catalyst
surface through the nozzle in spray pulse injection mode. This
alternate wet-dry condition is advantageous for the dehy-
drogenation reaction as it results in higher temperature of the
catalyst surface. Rapid desorption of the hydrogen and
aromatic product prevents the reverse reaction [21].
Fig. 9 depicts the influence of pulse injection frequency of
cyclohexane over the catalyst at 300
C. The phenomenon of
variation in pulse width and pulse frequency is well explained
by Biniwale et al. [2]. The pulse width remained constant as
10 ms and pulse frequency was varied from 0.5 s to 3 s. Shown
in Fig. 9 at 3s pulse frequency and 10 ms pulse width was an
optimal amount of cyclohexane feeding, at this point the
highest hydrogen evolution rate of 14.2 mmol/gmet/min was
observed. The highest hydrogen evolution rate for 5 s and 1 s
was observed to be 7.2 and 7.5 respectively.
The hydrogen production rate significantly depends on
frequency of reactant sprayed over the catalyst surface. At
lower pulse frequency (10 ms pulse frequency and 1 s pulse
width) a liquid rich condition may form over the surface of the
catalyst due to continuous spraying of reactant, which
decreases the hydrogen evolution rate. As liquid phase
reduces the surface temperature and inhibits desorption of
product and unreacted reactant. This would result in the
occurrence of reverse reaction and decline of the conversion
[20]. At higher pulse frequency (10 ms pulse frequency and 5 s
pulse width) reactant feeding was relatively small, and the
reactant would evaporate rapidly before reaching to the
catalyst surface. The residence time of the reactant over the
0
2
4
6
8
10
12
14
16
18
20
0 10 20 30 40 50 60
)nim/temg/lomm(etarnoitulove2
H
Time(min)
(b)
(c)
(a)
Fig. 9 e Hydrogen evolution rate with variation in pulse
frequency over 10 wt% Ag-1 wt% Pt/ACC catalysts at
reaction temperature of 300
C (a) interval of 1s and pulse
witdh of 10 ms (b) interval of 3s and pulse width of 10 ms
(c) interval of 5s and pulse width of 10 ms.
Table 2 e Hydrogen evolution rates reported for dehydrogenation of cyclohexane over various catalysts (Pt and other
bimetallic) using different reactor systems.
Sr no. Catalysts Temperature (
C) Highest hydrogen
evolution (mmol/gmet/min)
Reactor System Ref.
Pt based catalysts
1. 5 wt % Pt/AC 82 0.034 Batch [10]
2. 3.82 wt% Pt/AC 300 1800 Batch [12]
3. 2 wt% Pt/Alumina 300 910 Batch [12]
4. 2 wt% Pt/Alumina 315 29 Flow system [12]
5. 10 wt% Pt/ACC 260 98 Flow system [12]
6. 10 wt% Pt/ACC 330 510 Flow system [12]
7. 11 wt% Pt-Re/ACC 328 550 Flow system [12]
8. 12 wt% Pt-Rh/ACC 331 520 Flow System [12]
9. 5 wt% Pt/AC 235 98 Spray-pulsed
system
[7]
10. 0.5 wt % Pt/ACC* 300 0.22 Spray-pulsed
system
[7]
11. 1 wt% Pt/ACC 300 2.49 e This study
Non Pt monometallic and Bimetallic with lower amount of Pt Catalysts
12. 10 wt% Ni/ACC 300 7.1 Spray-pulsed
system
[7]
13. 20 wt% Ni/ACC 300 8.5 Spray-pulsed
system
[7]
14. 20 wt% Ni À0.5 wt % Pt/ACC 300 13.1 Spray-pulsed
system
[7]
15. 10 wt% Ag/ACC 6.9 10 This study
16. 10 wt% Ag-1 wt% Pd/ACC 7.5 8 This study
17. 10 wt% Ag-1 wt% Rh/ACC 12.34 8 This study
18. 10 wt% Ag-1 wt% Pt/ACC 13.36 6 This study
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 6 7 5 6 e6 7 6 36762

8. catalyst surface would be too small to come in contact with
catalyst completely. Hence, hydrogen evolution rate
decreases at increased feed rate. Thus, 10 ms pulse frequency
and 3 s pulse width was observed to be optimum feeding rate
of reactant to strike on the catalyst surface.
4. Conclusions
Dehydrogenation of cyclohexane was successfully carried out
over Ag based catalysts. The addition of a second metal to Ag
catalysts has exhibited a synergistic effect resulting in rela-
tively higher hydrogen evolution rates. Particularly, 10 wt%
Ag-1 wt% Pt and 10 wt% Ag-1 wt% Rh supported on activated
carbon cloth are potential catalysts. The probable reason for
higher activity in bimetallic catalyst can be attributed to role
of noble metal such as platinum to maintain Ag in zero valent
state. Ag based catalysts with addition of 1 wt % of noble metal
promoters are potential candidate for dehydrogenation of
cyclohexane.
Acknowledgement
This work was financially supported by Ministry of New and
Renewable Energy (MNRE), New Delhi. Senior Research
Fellowships awarded to Ms. Jayshri Pande and Ms. Anshu
Shukla by CSIR, New Delhi are gratefully acknowledged.
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