Sorption-Enhanced Steam Reforming of Glycerol over Ni–hydrotalcite: Effect of Promotion with Pt

Article abstract of DOI:10.1002/cctc.201600793

Sorption-enhanced steam reforming of glycerol (SESRG) is a promising method for the sustainable production of hydrogen (H₂). In this work, composites of Ni and cationic-modified hydrotalcite (HTlc) were promoted with Pt, thus resulting in two novel hybrid materials Pt-NiMgHTlc and Pt-NiCuHTlc. Activity trials for SESRG were performed in a fixed-bed reactor in the range 673–873 K and it was found that the promotion with Pt improved H₂ purity and multi-cycle durability. The best results were achieved when Pt-NiCuHTlc was employed at T=823 K: a H₂ concentration of 98.7 mol % and adsorption capacity of 1.34 mol CO₂/kg sorbent was achieved. When the multi-cycle performance was tested for 20 cycles, it was found that NiMgHTlc, NiCuHTlc, Pt-NiMgHTlc, and Pt-NiCuHTlc were stable for 5, 8, 13, and 18 cycles. Finally, a likely reaction pathway for SESRG over the investigated multifunctional materials was proposed.

Full text of DOI:10.1002/cctc.201600793

DOI: 10.1002/cctc.201600793
Full Papers
Sorption-Enhanced Steam Reforming of Glycerol over Ni–
hydrotalcite: Effect of Promotion with Pt
Karan D. Dewoolkar and Prakash D. Vaidya*[a]
Sorption-enhanced steam reforming of glycerol (SESRG) is
a promising method for the sustainable production of hydro-
gen (H2). In this work, composites of Ni and cationic-modified
hydrotalcite (HTlc) were promoted with Pt, thus resulting in
two novel hybrid materials Pt-NiMgHTlc and Pt-NiCuHTlc. Ac-
tivity trials for SESRG were performed in a fixed-bed reactor in
the range 673–873 K and it was found that the promotion
with Pt improved H2 purity and multi-cycle durability. The best
results were achieved when Pt-NiCuHTlc was employed at
T=823 K: a H2 concentration of 98.7 mol% and adsorption ca-
pacity of 1.34 molCO2/kg sorbent was achieved. When the
multi-cycle performance was tested for 20 cycles, it was found
that NiMgHTlc, NiCuHTlc, Pt-NiMgHTlc, and Pt-NiCuHTlc were
stable for 5, 8, 13, and 18 cycles. Finally, a likely reaction path-
way for SESRG over the investigated multifunctional materials
was proposed.
Introduction
Hydrogen (H2)—a clean energy carrier—can be used in fuel
cells for efficient electricity production. Its carbon-neutral pro-
duction from steam reforming of renewable resources such as
glycerol has been extensively studied.[1,2] In particular, catalysts
based on transition metals and noble metals are widely report-
ed.[3–8] However, the steam reforming process is energy-con-
suming and makes many unwanted byproducts. H2 productivi-
ty can be improved by sorption-enhanced steam reforming
(SESR), which combines the reforming process with the ad-
sorptive separation of byproduct CO2 in a single step.
they offer process simplicity, improved performance, and no
diffusion limitations.
As seen in our previous works on SESR of methane[26] and
ethanol,[27] bifunctional Ni catalysts derived from hydrotalcite-
like materials are stable and active and improve H2 production.
The high reforming activity of Ni, low cost, and ease of availa-
bility are renowned.[28] Hydrotalcite-like (HTlc) basic materials,
also known as layered double hydroxides, are good candidates
2+
for CO2 sorption. Their general structure is given by [M(1ꢀx)
Mx3+(OH)2]x+ (Anꢀ
)
x/n·mH2O, where M2+ =Mg2+, Mn2+, Fe2+
,
In SESR, pure H2 is produced at low temperature by using
mixtures of catalyst and chemisorbent. Thus, less energy is
consumed and catalyst sintering is lowered. Also, reactors
made of expensive materials are avoided. The chemisorbent
captures byproduct CO2 so that the water–gas shift (WGS) is fa-
vored, H2 selectivity is enhanced, and subsequent processing
for CO2 separation is precluded. The adsorbent is periodically
regenerated and reused. The process is cheap, simple, efficient,
and widely applicable.
Cu2+, Zn2+, or Ca2+, M3+ =Al3+, Fe3+, or Ga3+ and Anꢀ
=
[CO3]2ꢀ or Clꢀ.[29] They can be easily regenerated[30] and appro-
priately modified by the substitution of cations and anions. For
example, the presence of Cu in the HTlc structure is beneficial
because it promotes WGS and reduces carbon deposition,[31,32]
and it is not uncommon to employ such cationic-modified
hybrid materials for the SESR process.[27,33–35]
In this work, we applied basic composites of Ni and cationic-
modified (Mg and Cu) hydrotalcite (HTlc), that is, NiMgHTlc (or
HM1) and NiCuHTlc (or HM2) for SESRG. In addition, we pro-
moted such composites with Pt to yield two new materials, Pt-
NiMgHTlc (or Pt-HM1) and Pt-NiCuHTlc (or Pt-HM2). For the
first time, we employed these novel Pt-based materials for
SESRG. Pt is a popular steam reforming catalyst.[1,4,28,36] Its high
activity for CꢀC cleavage is well-known.[1,34] Here, the influence
of Pt promotion on the H2 purity, adsorption capacity, and
multi-cycle durability of the investigated composites was inves-
tigated in a fixed-bed reactor over a wide range of process
conditions. So far, there is only scarce information on HTlc-
based hybrid materials promoted with noble metals.[28,37–39]
Notably, SESR can be applied for H2 production from glycer-
ol.[9–11] So far, many aspects of sorption-enhanced steam re-
forming of glycerol (SESRG) such as thermodynamics,[12–14]
sorption kinetics,[15] and reactors[16] have been analyzed. In ad-
dition, several experimental investigations using catalyst–sorb-
ent mixtures[17–23] (e.g., Ni or Co–Ni catalysts mixed with Ca- or
Li-containing sorbents) and bifunctional catalytic materials[24,25]
have also been reported. Bifunctional materials, which merge
the catalytic and sorption features, are advantageous because
[a] K. D. Dewoolkar, Dr. P. D. Vaidya
Department of Chemical Engineering
Institute of Chemical Technology
Nathalal Parekh Marg
Matunga, Mumbai 400019 (India)
E-mail: pd.vaidya@ictmumbai.edu.in
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Results and Discussion
Table 1. Crystallite sizes and phases of hybrid materials.
Hybrid material
Pt-HM1
Phase
NiO
2q position
[8]
Crystallite size (d)
[nm]
Characterization of hybrid materials
All the characterization details for HM1 and HM2 are reported
in our previous work.[27] X-ray diffraction (XRD) profiles of the
Pt-promoted hybrid materials (Pt-HM1 and Pt-HM2) before re-
duction are shown in Figure 1. The presence of NiO in both
the hybrid materials is evident from the characteristic peaks at
2q=43.48 and 62.88 for Pt-HM1 and 2q=37.58 and 43.58 for
Pt-HM2. A typical peak representing the presence of HTlc is
seen at 2q=37.38 for Pt-HM1. The presence of CuAl2O4 as
a spinel phase was detected at 2q=58.68 and 638. CuO was
detected at 2q=39.18 and 49.38. Apart from these peaks, Pt-
HM2 also showed the presence of Al2O3, thus certifying that Al
remains in its mixed oxide phase. Both these hybrid materials
showed diffraction lines associated with platinum oxide at
2q=75.38 and 75.48, thereby suggesting the effective disper-
sion of Pt on the surface of the materials. The diffraction pat-
terns observed for Pt-HM1 and Pt-HM2 were similar to those
for HM1 and HM2 obtained in our previous work.[27] These re-
sults are also in good agreement with other works.[40,41] The
crystallite sizes of the several phases detected from the diffrac-
tion data were calculated from the Debye–Scherrer equation.
The phases and crystallite sizes of the various peaks are pre-
sented in Table 1. The surface areas of the hybrid materials,
43.4
62.8
75.3
37.3
37.5
43.5
75.4
30.5
35.9
68.4
39.1
49.3
58.6
63
10.4
8.9
6.6
Pt2O
HTlc
NiO
13.52
32.4
30.8
15.7
30.6
32.4
25.2
28.3
24.6
20.9
22
Pt-HM2
Pt2O
Al2O3
CuO
CuAl2O4
which influences activity and sorption capacity, are presented
in Table 2. Both Pt-HM1 and Pt-HM2 displayed higher surface
areas than HM1 and HM2. Also, the surface area of Pt-HM2
was much higher than that reported in a previous work.[40] The
pore volume and pore diameter of the unused and used
hybrid materials are also shown in Table 2.
Scanning electron microscope (SEM) images of fresh Pt-HM1
and Pt-HM2 are shown in Figure 2a,b. A highly porous and
Figure 1. XRD patterns for the hybrid materials. Top pattern: Pt-HM2, bottom pattern: Pt-HM1.
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Table 2. Surface area, pore volume, and pore diameter of the fresh and
used (after 20 cycles) hybrid materials.
Hybrid material
Surface area
Pore volume
Pore diameter
[nm]
[m2 gꢀ1
Fresh
]
[cm3 gꢀ1
Fresh
]
Used
Used
Fresh
Used
HM1
Pt-HM1
HM2
75.4
96.5
102
43.8
79.5
70.3
0.2
0.32
0.4
0.17
0.29
0.37
0.55
13.6
15.9
18.4
19.7
14
16.8
19.2
21.2
Pt-HM2
134.3
114.4
0.57
Figure 3. Glycerol conversion versus temperature plots for HM1 and HM2.
Reaction conditions: P=0.1 MPa, GHSV=3600 mLgꢀ1 hꢀ1, time on
stream=3 h.
the conversions reported in our work were higher than those
reported earlier.[33,34]
The influence of temperature on the gas-phase product dis-
tribution was studied and the results at a steady state in the
673–873 K range are shown in Figure 4. Similar to the results
of previous works at T>673 K,[7,23,42,43] four major products
were formed in the gas phase: H2, CH4, CO2, and CO. With the
rise in temperature, the H2 concentration increased whereas
the CH4 and CO2 content in the product decreased (see Fig-
ure 4a,b). This is possibly due to the predominance of CH4 re-
forming[44] and reverse water–gas shift (RWGS)[20] at higher
temperatures. Interestingly, both Pt-promoted hybrid materials
displayed improved H2 production compared with the un-pro-
moted materials (see Figure 4c,d), owing to the high reform-
ing and WGS activity of Pt. The merger of Pt, Ni, and Cu in
a single hybrid material (Pt-HM2) favored the WGS and steam
methane reforming, thereby resulting in the maximum concen-
tration of H2 (93.3 mole%).
Figure 2. SEM images of hybrid materials: a) Pt-HM1, b) Pt-HM2.
At T=873 K, H2 was formed in lower amounts, whereas the
CO formation increased. Such behavior can be ascribed to the
dominance of the RWGS reaction at high temperature.[20] No
carbon deposition was observed under the tested conditions,
owing to the high steam/carbon (S/C) ratios used. Also, the
presence of Cu and Mg induces an electronic effect on the
active phase of Ni, thereby avoiding carbon formation through
the Boudouard reaction.[31,34] In addition, the operating condi-
tions used in this work belong to the coke-free regime, accord-
ing to Da Silva and Muller.[14] Higher amounts of methane were
detected in the case of HM1 and HM2, thus suggesting CO
methanation. However, CH4 formation is not desired and a cata-
lyst with high WGS activity is preferential to reduce the CO
content of the product. In the liquid product, small amounts of
acetaldehyde and acetic acid were detected for the cases of
HM1 and HM2. This suggests that glycerol undergoes dehydra-
tion and subsequent dehydrogenation to form acetaldehyde,
which is further converted to acetic acid.[45] At T>673 K, acetic
acid may decompose over HM1 and HM2 to form CH4 and
CO2. In the case of the Pt-promoted hybrid materials, no liquid
granular structure was observed. The large size of the particles
and their aggregation is evident. These results are in line with
previous work.[34]
The chemical composition of the respective cations and
anions in the hybrid materials was found by using energy dis-
persive X-ray spectroscopy (EDX) analysis. The ratio of Mg
(63.5 mass%) to Al (23.8 mass%) in Pt-HM1 and Cu
(65.3 mass%) to Al (22.4 mass%) in Pt-HM2 was about 3. The
Ni content in all the samples was approximately 10 mass%.
Catalytic activity of the hybrid materials
The catalytic activity of HM1 and HM2 for glycerol conversion
was investigated in the range 673–873 K. The results are pre-
sented in Figure 3. Both materials facilitated glycerol reforming.
As expected, the increase in temperature resulted in increased
glycerol conversion. At T=873 K, conversion was maximized
(85.4 and 93.3% for HM1 and HM2). The higher efficacy of
HM2 was attributed to the additional presence of Cu. Notably,
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Figure 4. Product gas distribution (mol%) on a dry basis as a function of reaction temperature under steady-state conditions for the hybrid materials a) HM1,
b) HM2, c) Pt-HM1, d) Pt-HM2. Reaction conditions: P=0.1 MPa, GHSV=3600 mLgꢀ1 hꢀ1
.
products were detected thus suggesting the complete decom- are readily available for adsorption. CO2 formed during the
position of glycerol through CꢀC bond cleavage to form steam reforming and WGS reactions is effectively adsorbed on
syngas. Thus, the addition of Pt as promoter to the hybrid ma- the surface sites, thereby resulting in enhanced H2 production.
terials improves H2 production and avoids the formation of in- The highest concentrations of H2 are seen in this region owing
termediates. The various reactions that occur in the system to the shift in the equilibrium. As time progresses, a gradual
are:
transition into the breakthrough stage occurs. In this region,
the number of sites available for adsorption decreases, thus re-
sulting in the increase in CO2 levels. The breakthrough time of
the adsorbent (tb) is reached when CO2 is detected in the
outlet stream for the first time. As evident from Figure 5c,d,
both the Pt-promoted hybrid materials displayed longer break-
through times (60 min and 75 min). This performance was su-
perior to that reported in previous works.[27,34] Finally, as the
sorbent gets completely saturated, no further adsorption takes
place. In this post-breakthrough stage, the products reach
their equilibrium values and the system exhibits a catalyst-only
type behavior.
Glycerol decomposition : C3H8O3 3 COþ4 H
ð1Þ
ð2Þ
ð3Þ
ð4Þ
ð5Þ
!
2
Methanation reaction : CO þ 3 H2 3 CH þH O
!
4
2
Water ꢀ gas shift : CO þ H O
H
þ CO2
!
2
2
Steam methane reforming : CH4 þ H O CO þ 3 H
!
2
2
Overall SESRG : C H O þ 3 H O 3 CO2þ7 H2
!
3
8
3
2
SESRG studies
The efficacy of the hybrid materials can be evaluated in
Breakthrough studies were performed to gain insights into the terms of the adsorption capacity, which was in the order
SESRG process. The dependency of concentrations of H2 and Pt-HM2> Pt-HM1> HM2> HM1 (see Table 3). The results ob-
CO2 on time at T=773 K and S/C=9 molmolꢀ1 was studied. tained clearly suggest that Cu-based hybrid materials exhibit
The results are represented in Figure 5.
superior adsorption characteristics and produce high amounts
Figure 5a–d shows the breakthrough curves for the hybrid of H2. In particular, Pt-HM2 gave the highest adsorption capaci-
materials. Sorbent behavior is represented by breakthrough ty of 1.1 molCO2/kg sorbent in comparison to Pt-HM1
time (tb) and adsorption capacity (Qads). The surface area and (0.84 molCO2/kg sorbent). These adsorption capacities are
the presence of specific cations in the hybrid material influ- higher than those reported in our previous work.[27] Hybrid ma-
ence the adsorption capacities. Three separate regions are evi- terials retain their sorption capacity until the breakthrough
dent, that is, pre-breakthrough, breakthrough, and post-break- time (see Table 3). After that, this capacity is reduced. The CO2
through. In the pre-breakthrough stage, the adsorbent sites productivity of Pt-HM1 and Pt-HM2 was 0.016 and
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Figure 5. Breakthrough curves for hybrid materials a) HM1, b) HM2, c) Pt-HM1, d) Pt-HM2. Reaction conditions: P=0.1 MPa, T=773 K, steam/carbon ratio=9
molmolꢀ1, GHSV=3600 mLgꢀ1 hꢀ1, time on stream=3 h.
Effect of temperature on SESRG
Table 3. Adsorption capacities of hybrid materials.[a]
The effect of temperature on the performance of Pt-HM1 and
Hybrid material
Breakthrough time (tb)
[min]
Qads
[molCO2 kgꢀ1
]
Pt-HM2 was investigated in the range 673–873 K. The other re-
action variables were chosen to be: P=0.1 MPa and
S/C=9 molmolꢀ1. At first, the H2 concentration in the product
stream increased with a rise in temperature, owing to the ef-
fective separation of CO2 from the product (see Figure 6). The
maximum value was reached at T=773 K (89.6 mol%) for Pt-
HM1 and T=823 K (96 mol%) for Pt-HM2. After that, the H2
concentration decreased with a further increase in tempera-
ture, which was possibly due to the inhibition of carbonation
reactions.[27,46] Thus, the preferred operating temperatures for
optimum H2 production are 773 and 823 K for Pt-HM1 and Pt-
HM2, respectively.
sorbent
HM1
HM2
Pt-HM1
Pt-HM2
15
30
60
75
0.36
0.52
0.84
1.1
[a] P=0.1 MPa, T=773 K, S/C ratio=9 molmolꢀ1, GHSV=3600 mLgꢀ1 hꢀ1
time on stream=3 h.
,
0.011 molgꢀ1 hꢀ1, respectively. The H2 productivity of Pt-HM1
and Pt-HM2 was higher (0.13 and 0.16 molgꢀ1 hꢀ1) than that of
HM1 and HM2 (0.07 and 0.09 molgꢀ1 hꢀ1). The enhancement in
H2 production and adsorption capacity can be attributed to
the high surface area, which in turn leads to a higher number
of adsorption sites for Pt-HM2. Also, the synergistic effect of
the active metals (Pt, Ni, and Cu) along with the smaller crystal
sizes resulted in higher activity and enhanced H2 production.
Both HM1 and HM2 showed lower adsorption capacities (0.36
and 0.52 molCO2/kg sorbent). This suggests that Pt addition to
the hybrid materials results in improved H2 production. Despite
its low surface area, Pt-HM1 adsorbs more CO2 than HM2,
which is possibly due to its improved stability in the presence
of Pt. Next, the influence of reaction variables on the per-
formance of the Pt-promoted hybrid materials was studied.
Effect of S/C ratio on SESRG
The S/C ratio is a crucial parameter that influences the eco-
nomics of SESRG. The role of a high S/C ratio in diminishing
carbon formation is renowned; however, the excess use of
steam results in increased operating costs. Thus, developing
hybrid materials that resist coking even at low values of S/C
appears preferential. The influence of this parameter was inves-
tigated in the range 3–7.5 molmolꢀ1 at T=773 K (for Pt-HM1)
and T=823 K (for Pt-HM2). The other parameter values were:
P=0.1 MPa and GHSV=3600 mLgꢀ1 hꢀ1. The increase in S/C
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Figure 7. Effect of S/C ratios on SER for hybrid materials a) Pt-HM1, b) Pt-
HM2. Reaction conditions: P=0.1 MPa, T=773 K (Pt-HM1) or 823 K (Pt-HM2),
GHSV=3600 mLgꢀ1 hꢀ1, time on stream=2.5 h.
Figure 6. Effect of temperature on SER for hybrid materials a) Pt-HM1, b) Pt-
HM2. Reaction conditions: P=0.1 MPa, steam/carbon ratio=9 molmolꢀ1
GHSV=3600 mLgꢀ1 hꢀ1, time on stream=3 h.
,
ratio resulted in increased H2 concentration in the product gas.
The highest H2 purity was 96.4 mol% H2 (S/C=6 molmolꢀ1) for
Pt-HM1 and 98.7 mol% H2 (S/C=4.5 molmolꢀ1) for Pt-HM2.
These results are represented in Figure 7a,b. Notably, H2 pro-
duction decreased at higher S/C ratios for both materials. Such
behavior is ascribed to the reduced CO2 partial pressure at
high values of S/C, thus resulting in reduced adsorption capaci-
ty.[30] Even so, it can be concluded that the Pt-promoted hybrid
materials are efficient for H2 production at lower S/C ratios.
Figure 8. Effect of sorbent mass fraction on SESRG for hybrid materials. Re-
action conditions: P=0.1 MPa, T=773 K (Pt-HM1) or 823 K (Pt-HM2), steam/
carbon ratio=6 molmolꢀ1 (for Pt-HM1) and 4.5 molmolꢀ1 (for Pt-HM2), time
on stream=2.5 h.
Effect of sorbent mass fraction on SESRG
Knowledge of the optimum sorbent mass fraction for SESRG is
essential. Both adsorption capacity and H2 yield are dependent
on the mass of hybrid material. The effect of sorbent mass
fraction on H2 yield was investigated in the range 0.09–
0.27 kghmolꢀ1 at P=0.1 MPa. Pt-HM1 was tested at 773 K
(S/C=6 molmolꢀ1) whereas Pt-HM2 was studied at 823 K
(S/C=4.5 molmolꢀ1). As shown in Figure 8, the highest H2 pro-
ductivity was obtained at 0.27 kghmolꢀ1. This corresponds to
3 g of hybrid material inside the reactor. This behavior is in line
with previous works.[27,30]
Cyclic stability tests
The development of highly stable sorbents is a prerequisite for
the successful applications of SESRG. Sorbents that display
multi-cycle durability and ease of regeneration are desirable. In
previous work,[26] we showed that HTlc-like hybrid materials
can be more easily regenerated than Ca-based materials. Treat-
ment with excess steam at the reaction temperature is
a straightforward method for the regeneration of HTlc-like
hybrid materials.[27,47] The addition of steam regenerates the
basic HTlc structure and also inhibits coke deposition.[47]
To investigate the cyclic stability of the hybrid materials, 20
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cycles of reaction and regeneration were performed. The reac-
tions were carried out at the optimal conditions for the hybrid
materials. In the case of our hybrid materials, the regeneration
was carried out by passing excess steam (1.5 to 2 times the re-
quired flow rate of steam) at the reaction temperature for
20 min. The complete regeneration of CO2 was ascertained by
its presence in the exit stream from GC analysis.
promoted materials. Our results were in line with previous
works.[30,35,48] The cyclic stability of our Pt-promoted hybrid ma-
terials was higher than those reported previously.[23]
Towards the end of 20 cycles, the hybrid materials showed
a decrease in H2 concentrations. This reduction in H2 produc-
tion can be attributed to the sharp decline in the surface area
(see Table 2). A plausible reason for the decrease in surface
area may be metal sintering (see Table 4) and reduced porosity.
The SEM images of the hybrid materials after 20 cyclic tests are
shown in Figure 11a–d. It is noteworthy that our lab-made
hybrid materials displayed encouraging performance in contin-
uous operation cycles.
The concentrations of H2 and CO2 over 20 cycles are report-
ed (see Figure 9a–d). It is evident that both the Pt-promoted
hybrid materials exhibit superior stability of up to 13 (Pt-HM1)
and 18 (Pt-HM2) cycles. Contrarily, the un-promoted hybrid
materials remained stable for five (HM1) and eight (HM2)
cycles, respectively. The resilience and ease of regeneration of
our materials make them suitable for real applications. The
multi-cycle durability can be attributed to a strong metal–sup-
port interaction, improved CO2 adsorption, and enhanced sta-
bility as a result of the presence of Pt. As evident from XRD
analysis (Figure 10), the HTlc structure was largely retained
after the cyclic tests. The marginal rise in crystallite sizes (see
Table 4) suggested that the sintering effect on the Ni and Cu
particles in the promoted materials was lowered. This suggests
the role of Pt as a textural promoter in enhancing stability.
Low intensity peaks for MgAl2O4 were observed for HM1 and
Pt-HM1. A phase transformation from alumina to boehmite
(AlOOH) was seen for all hybrid materials after the cyclic tests.
In addition, distinct peaks for Cu and CuO along with spinel
CuAl2O4 were detected for HM2 and Pt-HM2. Reduced Pt0
species with increased crystallite size were obtained for both
Reaction pathway
A schematic representation depicting the plausible SESRG reac-
tion pathway over the investigated hybrid materials is pro-
posed (see Figure 12).
The un-promoted hybrid materials HM1 and HM2 are as-
sumed to follow the first pathway. In this route, the adsorbed
glycerol molecule undergoes subsequent dehydration and de-
hydrogenation reactions. This leads to the formation of acetal-
dehyde. Then, the adsorbed acetaldehyde undergoes reactions
of hydration–dehydrogenation to form acetic acid. This may be
due to the fact that the hybrid material with basic features
promotes fast dehydrogenation.[30] Acetic acid undergoes pos-
sible CꢀC bond cleavage to form CH4, CO, CO2, and H2. The
presence of Ni as an active agent effectively promotes CꢀC
Figure 9. Cyclic stability studies for hybrid materials a) HM1, b) Pt-HM1, c) HM2, d) Pt-HM2. Reaction conditions: P=0.1 MPa, T=773 K (HM1 and Pt-HM1) or
823 K (HM2 and Pt-HM2), steam/carbon ratio=6 molmolꢀ1 (HM1 and Pt-HM1) or 4.5 molmolꢀ1 (HM2 and Pt-HM2), GHSV=3600 mLgꢀ1 hꢀ1
.
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Figure 10. XRD patterns for the hybrid materials post cyclic tests.
Table 4. Crystallite sizes and phases of hybrid materials after cyclic tests.
Hybrid material
HM1
Phase
Ni
2q position
[8]
Crystallite size (d)
[nm]
43.2
53.5
37.3
14.4
28.1
38.2
36.1
44.4
43.3
28.4
37.2
52.4
75.4
35.7
48.4
63
43.5
53.7
37.5
61.8
75.5
32.8
66.5
39
68.2
43.1
37.1
78.8
56.6
74.9
35.6
36.3
62.5
47.6
68
52.6
20.6
48.5
72.9
43.8
40.4
39
HTlc
AlOOH
MgAl2O4
32
HM2
Ni
AlOOH
33.7
65.9
45.5
27.5
26.6
54.4
33.4
24.4
34.7
26.3
41.3
27.7
18.9
52.6
15.1
34.8
17.2
36.7
41.7
24.6
27.2
25.1
24
Cu
CuO
CuAl2O4
Ni
Pt-HM1
HTc
AlOOH
Figure 11. SEM images of hybrid materials after 20 cyclic tests: a) HM1,
b) HM2, c) Pt-HM1, d) Pt-HM2.
MgAl2O4
Pt0
bond rupture.[36] The products CO and CH4 subsequently un-
dergo WGS [Eq. (3)] and steam reforming [Eq. (4)] reactions to
form CO2 and H2. In the case of HM1, there is a possibility for
methanation of CO [Eq. (2)], leading to higher amounts of CH4
being formed. Finally, the CO2 formed gets adsorbed and pure
H2 is released.
Pt-HM2
Ni
AlOOH
Cu
CuO
45
CuAl2O4
Pt0
26.6
30.1
25.4
For the case of the Pt-promoted hybrid materials, the
second pathway seems feasible. In this pathway, fast
decomposition involving CꢀC bond rupture of glycerol occurs
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Figure 12. Proposed reaction pathways for SESRG.
[Eq. (1)]. This leads to the formation of syngas, which can be
confirmed by the absence of liquid byproducts. Such absence
of liquid intermediates in the SER of glycerol is not uncom-
mon.[21] The adsorbed CO molecule then undergoes a WGS re-
action to form CO2 and H2. The role of Ni, Mg, and Cu in the
promotion of dehydrogenation reactions and CꢀC bond cleav-
age is renowned. In addition, the presence of Pt, which has
high WGS activity and enhanced stability, is beneficial. As no
significant intermediate byproducts are present, high concen-
trations of H2 are obtained. In the end, the CO2 thus formed
gets effectively adsorbed, releasing H2 in a way similar to the
previous pathway. In conclusion, the hybrid material design
plays a critical role in the formation of different products in-
volved in the SESRG process.
Conclusions
Pt-promoted cationic-modified HTlc-based hybrid materials
were investigated for effective in situ CO2 removal and im-
proved H2 production. Both the Pt-promoted hybrid materials
possessed higher adsorption capacities and cyclic stability in
comparison to the un-promoted materials. Optimization of the
SESRG process was done by evaluating the effect of various re-
action variables on the performance of the Pt-promoted
hybrid materials. Amongst all the hybrid materials, Pt-HM2
showed great potential for enhanced H2 production. It gave
the highest H2 concentrations (above 98 mol%) and adsorption
capacities (1.34 molCO2/kg sorbent) at its optimal conditions.
A reaction pathway demonstrating the role of the various
metals in the hybrid materials was suggested. Such novel
multi-functional materials are promising for improved H2
production.
General Remarks
Enhanced H2 production along with satisfactory multi-cycle
performance was achieved by using our Pt-promoted hybrid
materials. From the optimization studies, we can determine
the maximum adsorption capacities for the hybrid materials.
Pt-HM1 gives a maximum adsorption capacity of 1.12 molCO2/
kg sorbent at its optimal conditions, that is, T=773 K,
P=0.1 MPa, S/C=6 molmolꢀ1, tb =70 min. The breakthrough
time and adsorption capacity for Pt-HM1 was higher than that
reported in previous works.[27,47] Importantly, Pt-HM2 exhibited
the highest adsorption capacity of 1.34 molCO2/kg sorbent at
its optimal conditions, that is, T=823 K, P=0.1 MPa, S/C=
4.5 molmolꢀ1, tb =90 min. This adsorption capacity was much
higher in comparison to an earlier work.[32] The adsorption ca-
pacities of the Pt-promoted materials were much superior to
those for the un-promoted materials. HM1 and HM2 gave ad-
sorption capacities of 0.65 and 0.9 molCO2/kg sorbent, respec-
tively. The addition of a noble metal such as Pt to the hybrid
materials proved successful for enhancing the adsorption ca-
pacity. However, there is scope for improving the multi-cycle
performance by modifying the anions or addition of suitable
metals to the HTlc structure.
Experimental Section
Materials
Nickel nitrate, magnesium nitrate, copper nitrate, aluminium ni-
trate, sodium carbonate, sodium hydroxide, and chloroplatinic acid
hexahydrate used in the synthesis of the HTlc-based hybrid materi-
als were procured from Sigma–Aldrich Pvt. Ltd., Mumbai. Glycerol
(purity 99%), used in all experiments, was also purchased from
Sigma–Aldrich Pvt. Ltd., Mumbai. Gases such as air, H2, and nitro-
gen (N2) in cylinders (purity 99.995%) were acquired from Industrial
Oxygen Company Ltd., Mumbai. Standard calibration gas mixtures
containing CO, CO2, and CH4 were procured from Chemtron Labo-
ratory Pvt. Ltd, Mumbai.
Hybrid material synthesis
A co-precipitation technique was used to synthesize the HTlc
hybrid materials (NiMgHTlc and NiCuHTlc) according to a method
previously reported in the literature.[49] Nitrates of the respective
aforesaid metal components were used in appropriate amounts.
The ratio of M2+/M3+ was selected as 3 so as to have a higher ba-
sicity in the material, thereby leading to effective adsorption of
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CO2. An aqueous solution of the metal nitrates was slowly added
into a beaker containing an aqueous solution of sodium carbonate
under continuous stirring at room temperature and constant pH of
10. The pH of the solution was maintained by using an aqueous
solution of sodium hydroxide (2m). The resulting suspension was
then kept at room temperature for 24 h. The precipitate thus ob-
tained was filtered and washed several times with de-ionized
water to remove any traces of nitrates; this was further confirmed
by the brown ring test. The resulting material was dried at 383 K
overnight and ground into a fine powder and calcined at 1073 K
for 5 h in air. The addition of platinum (2.5 wt%) to the prepared
hybrid materials was carried out by the incipient wetness impreg-
nation technique. An aqueous solution of chloroplatinic acid hexa-
hydrate was impregnated over the HTlc material. The resulting ma-
terial was dried at 383 K for 2 h and calcined at 873 K for 4 h. The
material was then crushed and sieved to fine particles with 30–
60 mesh size (0.3–0.6 mm). In all the hybrid materials, 10 wt%
nickel loading was maintained. The hybrid materials were designat-
ed as NiMgHTlc (HM1), Pt-NiMgHTlc (Pt-HM1), NiCuHTlc (HM2), and
Pt-NiCuHTlc (Pt-HM2).
with a TCD. A Hayesep DB column (length 3.6 m) was used for de-
tecting H2, CH4, CO2, and CO. In addition, the liquid products were
periodically collected and analyzed by GC employing Tenax
column (length 1.9 m). Standard calibration gas and liquid mixtures
were used to determine the product composition. The steam-to-
carbon ratio was equal to 9 molmolꢀ1 unless stated otherwise. The
liquid feed rate was varied from 0.25 to 1.25 mLminꢀ1
.
The conversion of glycerol was defined as follows:
½Glycerolꢂin ꢀ ½Glycerolꢂout
ð6Þ
Glycerol conversion ð%Þ ¼
ꢃ 100
½Glycerolꢂin
where [Glycerol]in and [Glycerol]out represent the inlet and outlet
molar flow rate (molhꢀ1) of glycerol. The H2 yield was defined as:
½H2ꢂout
ð7Þ
H2 yield ð%Þ ¼
ꢃ 100
7 ꢃ ½Glycerolꢂin
where [H2]out represents the outlet molar flow rate (molhꢀ1) of H2.
The gas hourly space velocity (GHSV) was defined as:
GHSV ¼ Inlet gas flow ðmL minꢀ1Þ=Weight of material ðgÞ
Characterization of hybrid materials
ð8Þ
The synthesized hybrid materials were characterized by using sev-
eral physico-chemical techniques. Crystallinity and textural patterns
were predicted from XRD (X-ray diffraction) data, which were re-
corded by using a Rigaku–miniflex powder diffractometer with CuK
(1.54 ꢁ) radiation. Nitrogen adsorption–desorption techniques (Mi-
cromeritics ASAP2010) were used to find the BET (Brunauer–
Emmet–Teller) surface area, pore volume, and pore diameter by
BJH (Barrett–Joyner–Halenda) and multipoint BET method. SEM
(scanning electron microscope) micrographs and EDX (energy dis-
persive X-ray spectroscopy) data were obtained by using a JEOL-
JSM 6380 LA instrument.
The adsorption capacity of the adsorbents was calculated by using
the relationship:
½F ꢃ C0 ꢃ tbꢂ
ð9Þ
Qads
¼
W
where F is the total molar flow of the feed gas (molminꢀ1), C0 is
the initial CO2 concentration (mole%), W is the mass of the solid
adsorbent loaded on the column (g), and tb is the stoichiometric
time in minutes, determined from the breakthrough curve.
Acknowledgments
Experimental setup
A SS-316 tubular fixed-bed down-flow reactor (inner diameter
19 mm, outer diameter 25.4 mm, and length 560 mm), purchased
from Chemito Technologies Pvt. Ltd., Mumbai, was used in all ex-
periments. The setup was supplied with a control panel, tempera-
ture-controlled furnace, gas chromatograph (GC 8610) unit, and
a data acquisition system (Proficy HMI/SCADA-ifax software). The
temperature was controlled by using a temperature controller
(West, Germany). A pressure transducer (0–100 bar) enabled the
measurement of pressure with an uncertainty of ꢁ0.1 bar.
K.D.D. is thankful to the Department of Science and Technology,
Government of India, New Delhi, for providing financial assis-
tance (DST-INSPIRE [IF140201]). P.D.V. is grateful to the Technical
Education Quality Improvement Program (TEQIP) Phase II—
Center of Excellence for Process Intensification (CoEPI) at ICT,
Mumbai.
Keywords: glycerol · hydrogen · sorption-enhanced reforming
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Received: June 30, 2016
Published online on && &&, 0000
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K. D. Dewoolkar, P. D. Vaidya*
&& – &&
Sorption-Enhanced Steam Reforming
of Glycerol over Ni–hydrotalcite:
Effect of Promotion with Pt
Sorption-enhanced steam reforming:
Two novel Pt-promoted cationic modi-
fied hydrotalcite-based hybrid materials
were synthesized and tested for the
sorption-enhanced steam reforming
(SESR) of glycerol. The hybrid materials
demonstrated excellent activity and sta-
bility with high purity hydrogen (>95
mol%).
&
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!