Nanosized Pt-containing Al2O3 as an efficient catalyst to avoid coking and sintering in steam reforming of glycerol

Article abstract of DOI:10.1039/c4ra09895a

The action of nanosized Pt-containing Al₂O₃ catalysts to avoid coking and sintering was studied in steam reforming of glycerol. The solids exhibited almost 100% conversion toward syngas produced at a suitable water to glycerol ratio. Depending on the promoter, a drastic drop in hydrogen yield was observed due to coking and sintering effects. Spent catalyst characterizations by Raman, HRTEM, XRD, TG and SEM-EDS as well as textural property techniques showed that coking, rather than sintering, was the main cause that determined the low hydrogen selectivity of nanosized Pt-containing Al₂O₃ with La₂O₃ or ZrO₂. In contrast, coking did not cover the active sites of Pt-containing Al₂O₃ with MgO or CeO₂. Thus, steam suppressed carbon deposition and improved the nanosized Pt/MgO-Al₂O₃ catalyst stability in the steam reforming of glycerol.

Full text of DOI:10.1039/c4ra09895a

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Nanosized Pt-containing Al2O3 as an efficient
catalyst to avoid coking and sintering in steam
Cite this: RSC Adv., 2014, 4, 61771
reforming of glycerol†
Davi C. Carvalho,a Helvio S. A. Souza,a Josue M. Filho,b Elisbete M. Assaf,c
´
Vivian V. Thyssen,c Adriana Campos,d E. Padron Hernandez,e Ramon Raudele
a
*
and Alcineia C. Oliveira
The action of nanosized Pt-containing Al2O3 catalysts to avoid coking and sintering was studied in steam
reforming of glycerol. The solids exhibited almost 100% conversion toward syngas produced at a suitable
water to glycerol ratio. Depending on the promoter, a drastic drop in hydrogen yield was observed due
to coking and sintering effects. Spent catalyst characterizations by Raman, HRTEM, XRD, TG and SEM-
EDS as well as textural property techniques showed that coking, rather than sintering, was the main
cause that determined the low hydrogen selectivity of nanosized Pt-containing Al2O3 with La2O3 or
ZrO2. In contrast, coking did not cover the active sites of Pt-containing Al2O3 with MgO or CeO2. Thus,
steam suppressed carbon deposition and improved the nanosized Pt/MgO–Al2O3 catalyst stability in the
steam reforming of glycerol.
Received 5th September 2014
Accepted 5th November 2014
DOI: 10.1039/c4ra09895a
www.rsc.org/advances
reaction, the amount of hydrogen produced and its selectivity
are less based on thermodynamic analyses.1–4
1. Introduction
So far, to take full advantage of the benets of hydrogen as a
clean energy carrier, economically viable and environmentally
friendly low carbon emitting, catalysts for producing hydrogen
are needed. In order to full this, detailed studies of glycerol
steam reforming using different catalysts in a xed-bed reactor
have been conducted.2–7 It should be highlighted that due to the
high water to organic ratio and the high specic heat of water,
the production of hydrogen is favoured. Moreover, upon using
steam-to-glycerol ratio in the range of 9–12 and reaction
temperature at around 600–700 ꢁC, the steam reforming of
glycerol is likely.1,5 Thus, nickel–cobalt-based catalysts and
noble metal ones are succeeded in steam reforming of glycerol
to catalyze most carbohydrates reforming reactions and
reforming of methanol, ethanol as well as glycerol, due to
chemical similarity of the feedstock.8–16 In general, one of the
reasons of choice between Pt- and Ni-based catalysts is the
resistance to phase transformation and tolerance against sulfur
compounds, rather than their catalytic activity. However, at
least in the case of steam reforming of glycerol, platinum-based
catalysts appear to be more efficient than Ni ones, which suffer
coking more than Pt-based ones and have shorter life.1,13
In this sense, glycerol has ability to decompose over the Pt
surface through O–H, C–C and C–H bond dissociations to
produce syngas.1 Indeed, Pt surface also has low activity toward
C–O bond dissociations, as observed throughout the Fischer–
Tropsch and methanation reactions to produce hydrogen. Pt
presence also avoids the water gas shi reaction because of the
lack of catalytic sites required to activate water.1,13
Glycerol is produced as a byproduct of biodiesel production.
Due to environmental concerns, it is highly desired to develop
alternatives for valorization of crude glycerol. Steam reforming
of glycerol to hydrogen or for syngas production is one of the
most attractive options to make use of glycerol, as it can stoi-
chiometrically produce up to 7 mol of hydrogen from 1 mol of
the trialcohol (eqn (1)).
C3H8O3 + 3H2O # 7H2 + 3CO2 DH298K ¼ +128 kJ molꢀ1 (1)
Although processes mainly based on pyrolysis, autothermal
reforming, supercritical water reforming, photo-reforming and
aqueous phase reforming of glycerol are potentially suitable for
hydrogen production over the conventional steam reforming
a
´
´
Universidade Federal do Ceara, Campus do Pici-Bloco 940, Fortaleza, Ceara, Brazil.
E-mail: alcineia@ufc.br; Fax: +55-85-3366-90-51 ext. 9982; Tel: +55-85-3366-90-51
ext. 9982
b
´
´
´
Universidade Federal do Ceara, Departamento de Fısica, Bloco 920, Fortaleza, Ceara,
Brazil. Fax: +55-85-3366-94-83; Tel: +55-85-3366-94-83
c
´
˜
˜
Instituto de Quımica de Sao Carlos, Universidade de Sao Paulo, Av. Trabalhador
˜
˜
Saocarlense, 400, 13560-970 Sao Carlos, SP, Brazil
d
´
CETENE, Cidade Universitaria, Av. Prof. Luiz Freire, Recife, Pernambuco, Brazil. Fax:
+55-81-33347224; Tel: +55-81-33347224
e
´
Universidade Federal de Pernambuco, Departamento de Fısica – CCEN, Cidade
´
Universitaria, Av. Professor Luiz Freire, S/N, 50670-901, Recife, PE, Brazil. Tel: +55-
81-91778360
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra09895a
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Concerning the carries, Pt supported on Al2O3 catalysts have catalysts were obtained. The nominal amount of Pt was about 1
been widely investigated in steam reforming reactions due to its wt%.
ability to preventing Pt from incorporation to g-Al2O3 phase,
The physicochemical properties of the aforesaid fresh cata-
stabilizing Pto particles and reducing support acidity.13 Never- lysts were deeply investigated in our previous work.17
theless, even with a better performance, there is always a need
for improving the Pt-based catalysts and also provide the
reasons for the deactivation of the solids. As steam reforming of
glycerol is endothermic and needs heat to work, this is a harsh
2.2. Characterizations of the spent solids
reaction condition to operate and would damage the catalysts.1 X-ray powder diffraction was used to identify the phases formed
It is also recognized that the catalytic activity may be enhanced aer the catalytic test. A from X'Pert ProMPD X-ray diffraction
by modifying Al2O3 with a small amount of La2O3, CeO2, MgO or equipment having an X-ray generator with CuKa radiation by
ZrO2.12 This can improve the catalytic activity and stability of the using 40 kV and 40 mA was used for the measurements. The
solids for steam reforming of glycerol reaction. However, coking solids were scanned in the range 2q ¼ 3–80ꢁ. XRD patterns were
blocks active sites and increases the amounts of light byprod- compared with those of JCPDS database.
ucts such as methane and ethylene, among others. In addition,
BET surface area, pore volume, and average pore diameter
supports possessing acid properties exhibit low activity to for spent solids were derived from nitrogen adsorption
gaseous products and form lateral products owing to dehydra- isotherms measured at ꢀ196 ꢁC in a Micromeritics ASAP 2000.
ꢁ
tion and condensation reactions. Whereas solids prepared Before the measurement, each sample was degassed at 120 C
through supports with neutral properties allow to obtain cata- under vacuum for 10 h to remove the adsorbed moisture on the
lysts with excellent activity levels to gaseous products in steam catalyst surface. The pore size distributions were calculated
reforming of glycerol, high selectivity to hydrogen and stability from the desorption branch of isotherms using Barrett–Joyner–
during long terms stability runs as well.1
Halenda method (BJH).
Recently, we have reported a low temperature process for the
Laser Raman spectroscopy measurements analyses were
dry reforming of methane (DRM) to synthesis gas over nano- performed at room temperature in a LabRam spectrometer
structured Pt-based catalysts.17 In fact, the deactivation of the (Jobin-Yvon). The emission line at 532 nm from an argon ion
catalysts in reason of carbonaceous residues deposition is laser was focused onto the samples using the macroscopic
inevitable in DRM reaction. However, the resistance exhibited conguration and samples were examined at low laser
by the nanostructured solids against sintering and phase power (2 mW) to avoid damage due to laser heating. The
transformation affords expectances to maximize hydrogen device was coupled to an Olympus confocal microscope
production with negligible coke deposition in steam reforming and a 100 times objective lens was used for simultaneous
of glycerol reaction.
illumination and collection. Additionally, the device was
Thus, the aim of this study is to examine hydrogen produc- equipped with a CCD with the detector cooled to about
tion from steam reforming of glycerol using nanostructured ꢀ10 ꢁC. Ten accumulated spectra were obtained in each
catalysts. The study comprises the role played by the structure of spectral range and the spectral resolution was 3 cmꢀ1 in the
the nanostructured oxides to avoid catalysts deactivation. Up to 5–2000 cmꢀ1 range.
now, it is possible for the reforming process to operate in
relatively low temperatures in order to reduce deactivation upon mission electron (TEM) micrographs of spent solids were
using such solids. obtained in a FEI Tecnai20 G2 200 kV electron microscope to
High resolution transmission electron (HRTEM) and trans-
The spent catalysts were characterized by the chemical observe morphology of carbon deposits and particle size.
analysis, nitrogen adsorption desorption measurements, XRD, Selected catalysts were ultrasonically dispersed in ethanol and a
Raman spectroscopy, HRTEM, TG, and SEM-EDS. The GC-MS drop of the suspension was then applied onto clean copper
are applied to analyze the products from steam reforming of grids and dried at room temperature.
glycerol.
Scanning electron microscopy (SEM) equipped with an
energy dispersive analysis system (EDS) measurements were
used to study the surface morphology and the element mapping
of surface composition of the catalysts. The measurements were
conducted on a TESCAN VEGA XMU electron microscope
2. Experimental
2.1. Catalyst preparation
Catalyst based on MeOx–Al2O3, in which Me ¼ MgO, CeO2, equipped with an EDS Bruker QUANTAX system coupled to the
La2O3 or ZrO2 were synthesized according to the previous SEM microscope, using an acceleration voltage of 20 kV. Previ-
works.14,15,17 The detailed procedure was described previously.17 ously, the solids were deposited on an aluminium sample
The MeOx/Al2O3 ratios were about 20 : 80 wt%.
holder and sputtered with gold and then SEM measurements
Briey, mesoporous MeOx–Al2O3 supports were impregnated were conducted.
with hexachloroplatinic acid (H2PtCl6) solution using 1 g of
Amounts of carbon deposits were obtained by thermogravi-
ꢁ
each support and the mixture was stirred at 40 C for 2 h. The metric analysis (TG) coupled to differential scanning calorim-
ꢁ
solid was subsequently dried and calcined for 3 h at 350 C in etry (DSC) apparatus in a Netzsch STA 409 PC/PG equipment
owing air (1 ꢁC minꢀ1). The mesoporous PtOx/MgO–Al2O3, coupled to a Bruker Tensor 27 IR instrument, under air stream
ꢀ1
ꢁ
ꢁ
PtOx/CeO2–Al2O3, PtOx/CeO2–La2O3 and PtOx/ZrO2–Al2O3 between 50 and 1000 C at a rate of 10 C min
.
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ndings states that the theoretical ratios for glycerol steam
reforming is close to 2.3 (mol/mol).1,7 Thus, the H2/CO2 ratio
obtained are similar and agree with those predicted theoreti-
cally and it may suggest an optimal behaviour for steam
reforming of glycerol. On the other hand, values of H2/CO ratio
between 5.0 and 7.0 indicate the presence of side reactions such
as glycerol decomposition to hydrogen and carbon and reverse
water gas shi (RWGS) reaction, among others, as shown
elsewhere.18
2.3. Activity testing
Catalytic tests in steam reforming of glycerol were performed in
a quartz xed-bed reactor with 39 cm and 1 cm of inner diam-
eter. Blank runs were performed prior to the catalytic reactions
and conversion of glycerol values was negligible. Typically, 150
mg catalysts were sandwiched by quartz wool and packed in the
reactor and activated in situ under a gas mixture of hydrogen
and nitrogen at 600 ꢁC for 1 h with hydrogen ow of 30 cm3
minꢀ1. Thereaer, the system was purged owing nitrogen
while it reached the reaction temperature. A thermocouple was
inserted into the centre of the catalytic bed to monitor the
reaction temperature. A mixture of water to glycerol ratio of 9 : 1
was fed into the reactor through a high-precision pump. The
cold trap contained the liquid products which were analysed
without preliminary extraction or separation using a gas chro-
matograph (Shimadzu) equipped with a FID detector and HP5
capillary column and operating between 35 and 250 ꢁC with
hydrogen as carrier gas.
Gaseous samples were analyzed using gas chromatograph
Varian Chrompack GC-3800 using hydrogen as carrier gas. The
chromatograph was equipped with two packed columns,
namely Porapak-N ꢁand Molecular Sieve 13X and operates
between 40 and 80 C with a carrier gas of He and N2, respec-
tively, owing at 10 mL minꢀ1. The carbon balances were about
5% in errors.
Fig. 1 shows plots of yields and selectivity of the products
obtained versus time on stream.
Both gaseous and liquid products are obtained by chro-
matogram analyses over all catalysts. The hydrogen yield is
about 7.0 within ca. 5 h over PtOx/ZrO2–Al2O3 due to the
complete steam reforming (Fig. 1a) leading to similar produc-
tion of hydrogen, as that observed for PtOx/MgO–Al2O3. Similar
results are seen for gas compositions over the aforesaid catalyst
at space velocity as low as 0.25 hꢀ1 (Fig. 1b). The elevated
hydrogen production is believed to be from the well dispersion
of Pt on the support, as previously observed.17 However, sin-
tering of particles from PtOx/ZrO2–La2O3 and PtOx/MgO–Al2O3
at the expense of faster coking rates can further explain these
results. Moreover, relatively low amounts of CO2 and CO are
produced over these solids, owing to the reverse water gas shi
(RWGS) reaction occurrence whereas CH4 production is
negligible.
Hydrogen yields for CeO2-containing catalysts achieve values
below to 5 comparing with the other catalysts. Such an effect
can be expected to be resulted from the different types of
interaction of CeO2 with La2O3 and Al2O3. The role of CeO2-in
tuning structural properties could be a result of presence of a
stable phase e.g., CeAlO3 combined with its high surface area,
which promote the dispersion of the Pto nanoparticles, as
observed for dry reforming of methane.17 This not the case of
the interaction between CeO2 with La2O3, whose a huge
mismatch between fast carbon deposition (latter shown in
spent catalysts characterizations) and reduction/oxidation
cycles of CeO2 as well as a mobile surface oxygen species
transferred to Pt are observed.19 The yields of CO and CO2 are
rather limited to the impurity amounts.
3. Results and discussion
3.1. Catalytic performances of the solids in steam reforming
of glycerol
Characterizations of the fresh catalysts have been described in
detail in our previous paper that contains a summary of the
main features of the catalysts screened.17
Conversion values in steam reforming of glycerol are shown
in Table 1.
The catalysts exhibit high efficiency in steam reforming of
glycerol with optimized reaction conditions along of 300 h time
on stream. According to the table, glycerol conversion is
maximum for the PtOx/MgO–Al2O3. It is evident that the other
catalysts show signicant performances with respect to that
PtOx/MgO–Al2O3, except for PtOx/CeO2–La2O3 whose perfor-
mance is the lowest among the solids studied. The H2/CO2
ratios are indicative of the yields of gaseous products. The
Although the conversions of glycerol over PtOx/ZrO2–Al2O3
and PtOx/MgO–Al2O3 are slightly higher than those Ce-
containing catalysts, the formers have very close H2 selectivity
(about 55%, Fig. 1a). The hydrogen selectivity decreases with
Table 1 Catalytic activity of the solids in steam reforming of glycerol reaction. Reaction conditions: reaction temperature at 600 ꢁC, water to
glycerol ratio of 9 : 1 and catalyst mass of 0.15 g during 5 h of reaction. The physicochemical characterizations of the catalysts studied after the
aforesaid catalytic test conditiona
Glycerol
conversion (%)
H2/CO
ratio
Phases detected aer
the catalytic test
Particle
Sgc
Vpc
Dpc
(nm)
Catalyst
H2/CO2 ratio
sizesb (nm)
(m2 gꢀ1
)
(cm3 gꢀ1
)
PtOx/CeO2–Al2O3
PtOx/ZrO2–Al2O3
PtOx/MgO–Al2O3
PtOx/CeO2–La2O3
97.0
97.0
100
2.8
2.9
2.7
2.6
6.3
5.2
7.5
7.0
CeO2; g-Al2O3; CeAlO3; C
ZrO2; g-Al2O3
MgO; g-Al2O3; MgAl2O4; C
CeO2; g-Al2O3; La2O3; C
5
35
15
30
83
73
152
103
0.11
0.11
0.20
0.14
4.5
2.8
4.0
3.1
96.0
a
XRD. b TEM. c Nitrogen adsorption–desorption measurements.
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Fig. 1 Short-term catalytic runs over the catalysts studied. (a) Yield of gaseous products formed (b) Selectivity to the products obtained. Reaction
conditions: fixed temperature (600 ꢁC), time of 5 h, water to glycerol ratio of 9 and space velocity of 0.25 hꢀ1 (c) yield of gaseous products formed
and selectivity to the products obtained. Reaction conditions: fixed temperature (600 ꢁC), time 5 h and water to glycerol ratio of 9 and space
velocity of 0.5 hꢀ1 (d) long term stability runs over the PtOx/MgO–Al2O3 and PtOx/CeO2–Al2O3 at 600 ꢁC, water to glycerol ratio of 9 : 1 and
space velocities of 0.25 and 0.5 hꢀ1 as well.
time on stream in reason of hydrogen in water favouring the
Both PtOx/CeO2–Al2O3 and PtOx/CeO2–La2O3 demonstrate
hydrogen production via Water Gas Shi (WGS) reaction over that the H2 selectivity has a plateau by ca. 54% within ca. 5 h on
PtOx/ZrO2–Al2O3 at the beginning of the reaction and a further stream whereas CO2 and CO selectivities are 20%. Despite the
stabilization of hydrogen production is observed. The same low hydrogen yield, PtOx/CeO2–La2O3 has elevated selectivity to
linearity is shown for PtOx/MgO–Al2O3 and hydrogen selectivity the gaseous products. This important aspect implies the CeO2
is always 55%, suggesting that the mechanisms of hydrogen combined with La2O3 facilitate the formation of CO2 and water
production are the same for these two catalysts. Meanwhile, the by WGS reaction providing a pathway for removing adsorbed
selectivities of CO2 and CO experience an increase at the CO from the Pt surface and thus inhibiting reforming reactions
beginning of the reaction and subsequently drop, being quite by blocking surface sites.20 In addition, the elevated C2H4
low at the end of the catalytic runs. This observation also selectivity indicates that coking caused by polymerization is not
suggests that possible phase transformations may be occurred benecial for hydrogen production.
over PtOx/ZrO2–Al2O3 while MgO modication on PtOx/Al2O3,
In case of condensate liquid effluents, analyses show that the
led to similar changes in H2 due to sintering (further shown by small amounts of byproducts are formed. There are formic and
TEM and XRD). Moreover, very low detectable amounts of CO, propionic acids, acetone, diols, acetone, ethanol, glycidol,
CH4 and CO2 (below 20%) and undesired C2H4 can be observed sorbitol, propylene glycol and dioxolanes, which are detected in
in the short-term stability run over both PtOx/ZrO2–Al2O3 and the condensate.
PtOx/MgO–Al2O3, demonstrating that the hydrogen production
is facilitated over these solids.
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Traces of higher molecular weight condensation products
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are also found over PtOx/CeO2–Al2O3 and PtOx/CeO2–La2O3,
conrming the existence of coke precursor over these solids.
Studies lead on PtOx/CeO2–Al2O3 and PtOx/MgO–La2O3
reveal that the yield of the products and selectivity decay for
much longer space velocity (up to 0.25 hꢀ1, Fig. 1c). Compari-
sons between Fig. 1b and c show the effect of space velocity
process variable over product distribution and glycerol conver-
sion. As it can be seen, lower glycerol conversion and hydrogen
yield as well as change of CO product distribution (H2/CO2 ratio
of 6.0) are observed with increase in space velocity. Besides, the
yields of methane and CO2 are also negligible under these
conditions. These behaviors suggest that CO seems to be
formed by consecutive reactions such as RWGS reaction.
Thermodynamic analyses for steam reforming of glycerol based
on non-stoichiometric approach shows that the hydrogen yield
value at equilibrium is found to be 6.86 with dilute feed system
containing 10 wt% of glycerol.9 Thus, the hydrogen yield is far
from equilibrium using space velocity higher than 0.25 hꢀ1 and
lower glycerol feed concentration (i.e. 10 wt% glycerol).
Extended tests over longer periods up to 12 h results are
shown in Fig. 1d. The hydrogen production on PtOx/MgO–Al2O3
is particularly high, showing approximately 6 mol molꢀ1 of
glycerol supplied greater yield than on PtOx/CeO2–Al2O3 and the
H2/CO ratio is ca. 7.5 as well. Following the same trends,
conversion of glycerol remains nearly 100% and 97% for PtOx/
MgO–Al2O3 and PtOx/CeO2–Al2O3, respectively. In addition, the
production of methane, carbon monoxide and carbon dioxide is
negligible over both catalysts studied. Besides, the yield of H2
and glycerol conversion becomes small over PtOx/CeO2–Al2O3
by using a space velocity of 0.5 hꢀ1 while no change of the
products distribution is observed.
Fig. 2 XRD patterns of the spent catalysts after 5 h of time on stream in
glycerol steam reforming.
indicate that metal particles are too big to produce the aforesaid
reections. This observation is in agreement with the metal
particle size of 35 nm for (002) plane of t-ZrO2 as calculated from
Scherer equation and also suggest sintering of the solid. No
other characteristic peaks of platinum oxide phases eventually
formed are detected owing to the superimposition with the
aforesaid phases.
In case of PtOx/MgO–Al2O3, the broad diffraction peaks
ascribable to cubic MgO (JCPDS 4-829) are present with reex-
ions at 2q ¼ 42 and 62ꢁ, respectively corresponding to (220) and
(200) planes together with those of g-Al2O3. Moreover, weak
diffraction peaks of PtOx species may be suggested, but they are
at the same position of the MgAl2O4 at about 2q ¼ 20, 31, 37, 45,
Nevertheless, the activity of the catalysts differs depending
upon whether more carbon deposition would form with
increasing the time on stream. Thereby, it seems that unless
coking can be removed on catalyst surface, a lesser hydrogen
production would be observed on the solids. These results
indicate superior catalytic properties in PtOx/MgO–Al2O3 and
appear to be attributed primarily to the ability of avoiding hard
coking deposition. This data are further conrmed by spent
catalysts characterization results. Moreover, the conversion of
the solids is higher than that of unprompted 1 wt% Pt/Al2O3
catalyst.1
ꢁ
ꢀ
56, 60 and 65 , corresponding to the reexions of a Fd3m cubic
spinel oxide (JCPDS-21-1152). Another possibility is that Pt
particles are highly dispersed with the crystallite size beyond
the detection limit of XRD. The ndings states that during
reforming of hydrocarbons reactions lead with Pt supported on
MgAl2O4 catalysts, phase transformations of MgAl2O4 to MgO
and g-Al2O3, decrease the catalytic performance of the solid.14 In
addition, no diffraction peaks of metallic platinum are
discernable over PtOx/ZrO2–Al2O3 and PtOx/MgO–Al2O3, indi-
cating that Pt could be highly dispersed. Graphite peak is also
detected in small intense peaks around 2q equal to 26 (002) and
44ꢁ (101). These assumptions suggest that sintering and phase
transformations are the leading causes of the decrease of the
catalytic performance of PtOx/ZrO2–Al2O3 at the beginning of
the reaction; but this factor may not cause the deactivation of
the solid. For PtOx/MgO–Al2O3, all of these factors together with
coking contribute do not decrease the hydrogen production.
Spent PtOx/CeO2–Al2O3 yields a broad diffraction pattern
containing peaks at 2q ¼ 34 (111) and 55ꢁ (220) belonging to the
3.2. Characterizations of spent catalysts
3.2.1. XRD and Raman measurements. XRD patterns of the
spent catalysts are shown in Fig. 2. All catalysts displays the
main peaks at approximately 2q equal to 33, 37, 49 and 68ꢁ,
which corresponded, respectively to the (111), (220), (311), (400)
and (440) planes of the R3C type structure of g-Al2O3 (JCPDS 46-
1131). For PtOx/ZrO2–Al2O3, the diffraction pattern contains any
peaks of ZrO2 with tetragonal symmetry, belonging to P42/nmc
space group at around 2q ¼ 30, 35, 50, 60 and 74ꢁ regions of the
XRD pattern. These peaks are indexed to be from (101), (002),
(112), (211) and (200) reexions and can be overlapped with
those of alumina. Furthermore, the intensities of the peaks
ꢀ
Fd3m of cubic CeO2. This pattern closely match the pattern for
cubic uorite type structure of CeO2,15,19 besides the aforesaid
peaks of g-Al2O3. Also, this pattern suggests the presence of an
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interaction between CeO2 and Al2O3 to produce R3-C cubic
CeAlO3 structure with the typical peak at 37ꢁ (110),15 with the
absence of unalloyed Pt and support. The presence of graphite
phase at 2q ¼ 25 and 43ꢁ is consistent with the (002) (101)
planes, which predicts the coexistence of carbonaceous
deposits over the spent solid. Based on the Scherrer equation
and the most intensive CeO2 (111) diffraction peak, the average
crystallite size of magnetite was estimated to be ca. 10 nm.
Platinum peaks are not visible in the diffraction patterns
probably due to the superimposition of the peaks. These facts
accounts for a minor performance concerning to hydrogen yield
of PtOx/CeO2–Al2O3, as compared to those of PtOx/ZrO2–Al2O3
and PtOx/MgO–Al2O3.
The addition of La2O3 instead of Al2O3 on PtOx/CeO2–La2O3
seems to not change the XRD pattern comparing with spent
PtOx/CeO2–Al2O3. This feature is attributed to the incorporation
of the CeO2 on lanthana framework forming a solid solution.
The presence of the diffraction peaks at 2q ¼ 35 and 55ꢁ may be
assigned to the (111) and (220) planes of CeO2. Lanthana
presence could be suggested at 2q ¼ 26 and 30ꢁ, which is
assigned to hexagonal La2O3 (JCPDS 74-2430) but these reex-
ions appear in the same region of ceria; thereby, it is not
possible to ascertain the presence of this oxide. Platinum pha-
ses are not seen, indicating a well-dispersed Pt structure on the
surface of the support.
Crystallite size is calculated from X-ray line broadening of
CeO2 (111) using the Scherrer equation and the value is not
obtained for PtOx/CeO2–La2O3 due to the broadness of the dif-
fractogram. The slight intensity of the graphite observed for
PtOx/CeO2–La2O3 at about at 2q ¼ 26 and 42ꢁ suggests that hard
coking happened instead of crystal growth.
In addition, according to the JCPDS card number 01-1190,
metallic platinum is commonly observed at around 2q ¼ 39.8
(111), 46.5 (200) and 67.8ꢁ (220). However, these peaks are not
visible for all spent solids due to either overlap Pt peaks with
those of the other phases or even the possible dispersion of Pt
particles to undetectable sizes.
Glycerol has lower potential to form solid carbon in steam
reforming reaction.1 However, studies on Raman spectroscopy
suggest that carbonaceous deposition is induced by some type
of catalysts. To conrm this hypothesis, Raman spectra of the
spent solids are shown in Fig. 3.
The shape and the broadness of the spectra of PtOx/CeO2–
La2O3, PtOx/CeO2–Al2O3 and PtOx/MgO–Al2O3, especially for
vibrational modes at high wavenumbers conrm that carbo-
naceous deposits are formed during the catalytic runs. This
interpretation is consistent with the fact that other studies have
shown coking in steam reforming of hydrocarbons reaction over
Pt or Ni-based catalysts.21–23
Fig. 3 Raman spectra of spent catalysts after 5 h of time on stream.
Reaction conditions: fixed temperature (600 ꢁC) and water to glycerol
ratio of 9.
band) assigned to phonons of E2g and A1g symmetry, respec-
tively. The D band is attributed to be caused by defects and
disorder in the graphene-like structure whereas the G band is
associated with the rst-order scattering of the E2g mode of in-
plane sp2 carbon single bond stretching vibrations from
graphite sheets.11,22,24,25 These modes denitely prove that
disordered carbonaceous deposits always form on catalyst
surface from C–C bond breakage of glycerol and their byprod-
ucts. Moreover, PtOx/CeO2–La2O3 spectrum shows the D0 band
at about 1605–1620 cmꢀ1, which is due to the stretching of C]C
bonds carbon of less structured coke or graphite-like struc-
tures.24,25 In addition, two distinct broad modes with maxima at
around 2652 and 2930 cmꢀ1 are attributed to presence of
defects, including the edges of the nanotubes in the so called G0
(or 2D) band.26
A comparison between PtOx/CeO2–Al2O3 and PtOx/CeO2–
La2O3 spectra show distinct features concerning the intensity,
position, and width of the modes. This suggest that deforma-
tion taking place in the crystal structure due to the larger ionic
radius of La3+ compared to Ce4+ as well as the lattice expansion
and the presence of oxygen vacancies over PtOx/CeO2–La2O3, as
for Ce–La based catalysts applied to steam reforming of
The appearance of a strong mode at around 460 cmꢀ1 in
PtOx/CeO2–Al2O3 means the F2g mode of uorite CeO2 is clearly
visible. In addition, the defects modes at around 610 cmꢀ1 of
Ce-based oxides14,15 could not be detected due to its low inten-
sity. This indicates that the phases such as CeO2 and CeAlO3 are
present in the solid, in agreement with XRD results. At high
frequencies, the Raman ngerprint of disordered Csp2-based
materials are found at around 1350 (D band) and 1585 cmꢀ1 (G
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ethanol.27 Literature reports that modes at around 350–400
cmꢀ1 region are assumed to be from fundamental modes of La–
O.28 However, overlapping of the modes with those of CeO2 does
not allow lanthanum oxides identication. Besides, a hard
carbon deposit on catalyst surface is considered a main cause of
catalyst deactivation of PtOx/CeO2–La2O3, as already predicted
by XRD.
Of similar features is the PtOx/MgO–Al2O3 spectrum, which
the G and D modes dominate the Raman spectrum together
with an additional broad mode emerges at around 3227 cmꢀ1
due to water presence.
The structural characterization of the deposited coke is
taken from the intensity ratio of the ID/IG bands. According the
ndings, defects formation increases the ID/IG ratio but
increases the I2D/IG ratio.26 Noticeably, the ratio between the
areas of the D and G bands of ca. 0.32 from PtOx/MgO–Al2O3
indicates that the degree of graphitization of carbonaceous
deposits greater than the other samples. Conversely, the D-to-G
intensity ratio increases for 0.49 over PtOx/CeO2–Al2O3 in line
with deposition of both low-graphitic coke with only few
structural defects and amorphous carbonaceous deposits
formation. Instead, deposition of amorphous carbon is
observed for PtOx/CeO2–La2O3 with the ID/IG ratio of 0.57.
Regarding the these results, the harsh coking from graphite
type-carbon deposits accounts for the decrease performance of
PtOx/MgO–Al2O3 and PtOx/CeO2–La2O3 catalysts whereas
correspondingly amorphous coke formation may be smoothly
removed by steam and this not diminish PtOx/MgO–Al2O3
performance. It may be possible that this kind of coke may be
from the formation of olenic chains and aromatic rings, as for
magnesia and alumina-based catalysts tested in glycerol steam
reforming.21
Raman spectrum of PtOx/ZrO2–Al2O3 displays modes at
around 319 (Eg), and 634 cmꢀ1 (B1g) due to tetragonal (P42/nmc
(D4h15) space group) and monoclinic (P21/c (C2h5) space group)
phases of ZrO2.29 This means that the typical of both tetragonal
and monoclinic ZrO2 phases are present in the in segregated
phases conrming the hypothesis of sintering of the solid aer
the catalytic test. A strong luminescence caused by alumina
impedes the other modes identication. It is most likely that
PtOx/ZrO2–Al2O3 is deactivated by phase transformation and
sintering instead of carbon deposition, since carbon modes are
not visible by Raman spectroscopy.
3.2.2. TEM and Surface texture of the solids. TEM images
of the spent catalysts are shown in Fig. 4.
Fig. 4 (a) TEM and HRTEM images of the spent catalysts, after 5 h on
stream in steam reforming of glycerol and water to glycerol molar ratio
of 9 at 600ꢁ C.
It can be seen that the spent PtOx/CeO2–Al2O3 (Fig. 4a1)
exhibits the characteristic rod-like carbon morphology, similar
to that of carbon nanotubes. The value of d spacing of ca 0.225
nm, from (111) plane as well as dispersed black spots, conrm
the Pt nanoparticles remains intact aer the catalytic test and
possess varying between 5 to 12 nm. In addition, these nano-
particles are in a well dispersed fashion and the nanoparticles
sizes values close to that obtained XRD results. It is further
illustrated the presence of typical carbonaceous deposits
formation in Fig. 4b1. Filamentous and carbon nanotubes
formation are likely on the spent solid. Moreover, nanoscale Pt
metal particles possessing diameters ranging from 0.5 to 2 nm
are very evident inside the tubes along with amorphous carbon
(Fig. 4a1). It might be possible that on PtOx/CeO2–Al2O3 surface,
the decomposition of glycerol and the subsequent layers of
carbonaceous species deposition start to form on Pt sites of
active metals for shorter duration; however, a slight hindrance
of the activity of the solid is observed due to carbon gasication
towards CO2 formed during the reaction, as observed in the
catalytic results.
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TEM micrograph of PtOx/CeO2–La2O3 (Fig. 4a2) illustrates deposits formation on solid surface and thus, affecting the
domains possessing a high degree of crystallinity resulting in a textural properties.
needle-like structure and a corrugated surface with repeating
Contrary to the abovementioned expectation, the unchanged
ridges. HRTEM image (Fig. 4b2) displays transparent carbon surface area of ca. 83 m2 gꢀ1 and pore volume of ca.0.12 cm3 gꢀ1
sheets with crumpled silk veil waves and the rumples are registered for PtOx/CeO2–Al2O3 may be attributed to a lesser
involved by the Pt nanoparticles, which has an average size of intensity of the catalytic deactivation, probably. This implied
approximately 6–18 nm. In addition, fringes of several crystal- that carbon entities formed on spent catalyst neither occupy the
lites in different orientations can be seen from an electron active sites nor cover the Pt site, consequently these fact did not
diffraction pattern including the layered carbon nano- impair the activity of the solid. BET surface, pore volume and
structures. Also, the crystal lattice fringes reveal that these pore size of the other samples decrease aer catalytic evalua-
particles are from highly crystalline features of Pt. These tion. It is noted that surface area is reduced from 143 m2 gꢀ1 for
features suggest that deactivation of PtOx/CeO2–La2O3 is mainly the fresh PtOx/ZrO2–Al2O3 to 73 m2 gꢀ1, which is considered to
due to coking in agreement with previous Raman and XRD be a drastic reduction due to sintering effects, as observed
results. The Ce–La solid solution formation is formed during previously by the other characterization techniques. In case of
the catalytic test consisting with the results based on litera- PtOx/MgO–Al2O3, the textural properties remain almost unal-
ture.27 This might be induced further a crystal growth and tered, thereby, it can be regarded that coking is eliminated on
thereby, formation of the aforesaid crystal type-structure results the solid surface as soon as it is formed and this accounts for
in a low performance of the solid in the steam reforming of the best catalytic properties of the solid. For PtOx/CeO2–La2O3, a
glycerol reaction. Additionally, the ndings states that carbon drop of the BET surface area and pore volume is observed
species formed on metallic sites may be partially removed by compared to the fresh solid, suggesting the coke is not vanished
oxygen species originating from lanthanum carbonate species from solid surface resulting in a low catalytic performance of
such as La2O2CO3.7 However, this phenomenon is not observed the solid. The pore diameter of the spent catalyst is adequate
for PtOx/CeO2–La2O3, judging from its low performance in the (2.8–4.5 nm) to allow the reforming reaction occurring inside
reaction.
the inner channels because it was much greater than the size of
It is clearly seen that the TEM image of PtOx/ZrO2–Al2O3 glycerol molecule which is around 0.62 nm3. Even if the pores of
(Fig. 3a3) exhibits large particles occupying almost of the entire PtOx/CeO2–Al2O3 and PtOx/ZrO2–Al2O3 experience a decreased
of solid surface. These particles are indeed densely aggregated due to sintering, it is likely that deactivation may be not due to
with fairly even distribution (Fig. 3b3), which indicate the strong the diffusion limitation of the reactants or products.
interaction between the Pt nanoparticles and the ZrO2–Al2O3.
3.2.3. SEM-EDS and TG analyses. Fig. 5 exhibits the SEM-
The characteristics of a crystalline Pt face centred cubic struc- EDS micrographs of the spent solids. It seems that the
ture is conrmed by d-spacings of the lattice fringes at around morphology of spent PtOx/CeO2–La2O3 (Fig. 5a1) consists of
0.225 nm, which can be indexed to as the (111) crystalline plane coke deposits in which nanobers are noticed. Elemental maps
of Pt.30 The lattice distances of the Al2O3 obtained from high- of the main elements such as Pt, Al, Ce, La and C from the
resolution TEM (HRTEM) image (e.g., 0.7 nm) and t-ZrO2 sample area suggest a heterogeneous distribution of carbon on
(belonging to the P42/nmc space group possessing d ¼ 0.284 nm solid surface. According to the above Raman, TEM and XRD
from (111) planes with JCPDS number card 37-1484) are clearly predictions, the carbonaceous deposits (both amorphous and
visible. The particles particle sizes obtained are estimated to be graphitic) on solid surface is the main cause of the lesser
39 nm, in average and this is in coherence with the data performance of PtOx/CeO2–La2O3 to hydrogen production from
obtained from XRD patterns. From these results, it can be steam reforming of glycerol. SEM micrograph of PtOx/CeO2–
conrmed that the cause of deactivation of PtOx/ZrO2–Al2O3 Al2O3 suggests the formation of aggregates of nanoparticles,
catalyst is sintering of Pt particles and phase transformations. probably from the segregated CeOx island. Nevertheless, this
Based on TEM measurements of PtOx/MgO–Al2O3 (Fig. 4a4), could be a moderate accumulation of CeOx particles, as proved
agglomeration of Pt particles in random regions with an average previously by TEM image of the solid. The corresponding EDS
particle size of 10–30 nm is observed. The edges of the particles spectrum (Fig. 5a2), a quite uniform distribution of Pt and Al is
area are all surrounded by carbonaceous species, which are observed, besides the expected carbon presence, which tends to
supposed to be from carbon whiskers bers formed during the be accumulated on spent PtOx/CeO2–Al2O3 surface.
glycerol steam reforming tests (Fig. 4b4). Although some metal
As seen from SEM image of PtOx/MgO–Al2O3, the solid
particles encapsulated within the laments could help the side surface is composed of very bulky spherical-like particles. These
WGS reaction and carbon formation with an auto consumption particles are coated by a carbon layer in which carbon bers and
process during reforming process,31 coke formation together carbon nanotubes are observed, as indicated previously by
with sintering contributes to decrease the catalytic properties of Raman spectroscopy. As EDS spectrum suggests an homoge-
the solid, which is consistent with the XRD and Raman results. neous distribution of Al, Mg and Pt elements, it would be
Table 1 displays the textural properties of spent catalysts. indicated that carbon deposition does not cover the active sites
The fresh catalysts are mesoporous materials that possess and the morphological and textural properties of the solid
elevated textural properties.17 It seems that some of the catalyst remains unaltered. This is also observed for Ni-based catalysts
pores might have suffered by sintering and carbonaceous in the glycerol steam reforming31,32 and coking do not damage
the catalytic performance of the solid.
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the latter is too low and any kind of heterogeneity of the
composition is denitely shown.
TG analysis of PtOx/CeO2–La2O3 aer catalytic run reveals
the occurrence of a mass loss in the rst step (25–150 ꢁC), which
is assigned to removal of adsorbed water. A subsequent signif-
icant mass loss is observed in the 150–350 ꢁC is due to the
elimination of interstitial water and combustion of amorphous
carbonaceous species originated from glycerol decomposition
to other organic by-products that can be adsorbed on the
surface covering the active sites, and in some extent blocking
the catalyst pores.4 The IR curves from TG experiments are
included in ESI.† According to the ndings, amorphous carbon
oxidizes below 550 ꢁC whereas graphitic/lamentous carbon
does at higher temperature.33 The third weight loss event, with a
highest mass loss is observed at around 400–800 ꢁC, being
ascribed to the graphitic carbon elimination at elevated
temperatures. As reported in the literature,33 under steam
reforming conditions, coking gasication is promoted by water
in steam reforming reactions. This could indeed result in
structural defects, such as vacancies, face edges and among
others defects on the solid. With this in mind, the weight loss of
ca. 1% on PtOx/CeO2–La2O3 might be result of previous coking
removal by steam and oxidation of CeO2 to create such defects
could be likely.
A comparison between PtOx/CeO2–La2O3 and PtOx/CeO2–
Al2O3 curves shows that the weight loss of the latter is divided in
two events. The rst one is observed at around 25–350 ꢁC
temperature range, which is ascribed to the removal of inter-
stitial water and/or the oxidation of the amorphous carbona-
ceous species, as previously suggested by Raman spectroscopy.
The second event peak is very wide and oxidation of carbon
deposits take place in the 300–700 ꢁC, and overlaps graphitic
carbonaceous species oxidation and re-oxidation oxidation of
CeO2 and Pt atoms. The 2% amount of coke deposited on the
solid is a result of the formation rate minus the gasication
rate, which accounts for the lower coke amount found on PtOx/
CeO2–Al2O3, in agreement with SEM-EDS results.
TG curve of spent PtOx/MgO–Al2O3 shows only one weight
loss event that occurred at around 25–400 ꢁC is due to the
formed coke elimination at lower temperatures. It has being
stated that Mg and Ca incorporation to catalysts can enhance
the steam gasication of coke by activating water adsorption33
and thus, this accounts for the better performance of this solid,
even if graphite carbon is majoritary formed during SR (Raman
spectroscopy). The TG curve of PtOx/ZrO2–Al2O3 does not seem
to undergo any difference concerning the unique event
observed. However, it might be a result of the evaporation of
occluded water and the oxidation of the aggregated particle,
since a low amount of coke is deposited on the solid surface.
These results are consistent with those obtained by SEM-EDS
and Raman analyses.
Fig. 5 (a) SEM-EDS of the solids after the catalytic test (b) TG of spent
solids.
SEM image of PtOx/ZrO2–Al2O3 shows the sintering of the
particles of aer reaction, since aggregation of particles is
noted. The EDS spectrum of the solid displays the signals of the
main elements such as Zr, Al, Pt and C, in which the content of
4. Conclusions
Pt-containing MeOx–Al2O3 catalysts were evaluated in glycerol
steam reforming. Using MgO and CeO2 promoters, the solids
had elevated glycerol conversions and selectively produced
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hydrogen with low yields to byproducts whereas La2O3 and ZrO2 14 H. S. A. de Sousa, A. N. da Silva, A. J. R. Castro, A. Campos,
promoters were found to be lesser selectives to hydrogen
production. Spent catalysts characterizations by Raman, TEM,
J. M. Filho and A. C. Oliveira, Int. J. Hydrogen Energy, 2012,
37, 12281.
XRD, TG and SEM-EDS as well as textural properties techniques 15 F. F. de Sousa, H. S. A. de Sousa, A. C. Oliveira,
showed that coking, rather than sintering is the main cause that
determine the low hydrogen selectivity of the solids. The coking
did not cover the active sites of Pt/MgO–Al2O3 and Pt/CeO2–
M. C. C. Junior, A. P. Ayala, E. B. Barros, B. C. Viana,
J. M. Filho and A. C. Oliveira, Int. J. Hydrogen Energy, 2012,
37, 3201.
Al2O3 and steam suppress carbon deposition, and thus improve 16 K. Seung-hoon, J. Jae-sun, Y. Eun-hyeok, L. Kwan-Young and
the catalyst stability in steam reforming of glycerol.
M. D. Ju, Catal. Today, 2014, 228, 145.
17 D. C. Carvalho, H. S. A. De Souza, J. M. Filho, A. C. Oliveira,
´
´
A. Campos, E. R. C. Milet, F. F. de Sousa, E. P. Hernandez
and A. C. Oliveira, Appl. Catal., A, 2014, 473, 132.
18 C. A. Franchini, W. Aranzaez, A. M. Duarte de Farias,
G. Pecchi and M. A. Fraga, Appl. Catal., B, 2014, 147, 193.
19 P. Pantu and G. R. Gavalas, Appl. Catal., A, 2002, 223, 253.
20 J. W. Shabaker, R. R. Davd, G. W. Huber, R. D. Cortright and
J. A. Dumesic, J. Catal., 2003, 215, 344.
Acknowledgements
The nancial supports of this work by CNPq with contract no.
490162/2011-8 and Funcap (areas estrategicas 2011 project) are
gratefully acknowledged. We would like to thank CETENE-INT
for TEM measurements as well as Dr A. J de Paula for SEM-
EDS analyses at Central Analitica da UFC.
21 B. Valle, B. Aramburu, A. Remiro, J. Bilbao and A. G. Gayubo,
Appl. Catal., B, 2014, 147, 402.
22 M. El Doukkali, A. Iriondo, P. L. Arias, J. F. Cambra,
I. Gandarias and V. L. Barrio, Int. J. Hydrogen Energy, 2012,
37, 8298.
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This journal is © The Royal Society of Chemistry 2014