VOx-Pt/Al2O3 catalysts for hydrogen production

Article abstract of DOI:10.1016/j.cattod.2016.09.021

Platinum supported catalysts are promising systems to a wide range of catalytic reactions involved in the hydrogen production chain. The performance of these catalysts can be enhanced by designing properly their nature, composition and structure. In this context, the addition of a second metal oxide on a catalyst composed of a well dispersed Pt phase over a high surface area support may be a powerful strategy. In this work we impregnated VO_x species over Pt/Al₂O₃ and evaluated the catalysts performance for water-gas shift (WGS) and glycerol steam reforming (GSR) reactions. The catalysts characterization showed that VO_x species formed over the Pt/Al₂O₃ surface were not deeply affected by the loading in the range of 0.5–2.0?V atoms per nm² and were reduced at mild temperatures. In situ measurements during catalysts activation and WGS reaction showed that VO_x species presented mixed valence (V³⁺/V⁴⁺) while Pt was in metallic state. The addition of vanadium increased Pt/Al₂O₃ WGS activity; however, the improvement did not linearly correlate with the loading. Under GSR, the results indicated that the WGS step was favored by the presence of vanadium but the selectivity to H₂ decreased, which could be attributed to the parallel reactions enhanced by the acidic nature of the VO_x sites.

Full text of DOI:10.1016/j.cattod.2016.09.021

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Contents lists available at ScienceDirect
Catalysis Today
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VOx-Pt/Al2O3 catalysts for hydrogen production
Tathiana M. Kokumaia, Daniel A. Cantanea,1, Guilherme T. Meloa,b, Luigi B. Pauluccia,b
,
Daniela Zancheta,∗
a Institute of Chemistry, University of Campinas (UNICAMP), P.O. Box 6154, 13083-970 Campinas, SP, Brazil
b Faculty of Chemical Engineering, University of Campinas (UNICAMP), Av. Albert Einstein, 500, 13083-852 Campinas, SP, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 May 2016
Received in revised form 19 August 2016
Accepted 15 September 2016
Available online xxx
Platinum supported catalysts are promising systems to a wide range of catalytic reactions involved in the
hydrogen production chain. The performance of these catalysts can be enhanced by designing properly
their nature, composition and structure. In this context, the addition of a second metal oxide on a catalyst
composed of a well dispersed Pt phase over a high surface area support may be a powerful strategy. In this
work we impregnated VOx species over Pt/Al2O3 and evaluated the catalysts performance for water-gas
shift (WGS) and glycerol steam reforming (GSR) reactions. The catalysts characterization showed that
VOx species formed over the Pt/Al2O3 surface were not deeply affected by the loading in the range of
0.5–2.0 V atoms per nm2 and were reduced at mild temperatures. In situ measurements during catalysts
activation and WGS reaction showed that VOx species presented mixed valence (V3+/V4+) while Pt was in
metallic state. The addition of vanadium increased Pt/Al2O3 WGS activity; however, the improvement did
not linearly correlate with the loading. Under GSR, the results indicated that the WGS step was favored
by the presence of vanadium but the selectivity to H2 decreased, which could be attributed to the parallel
reactions enhanced by the acidic nature of the VOx sites.
Keywords:
Platinum catalysts
Vanadium oxides
Water-gas shift reaction
Glycerol steam reforming
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
with steam [6]. According to these steps, the H2 selectivity from
GSR reaction can be catalytic raised by favoring the WGS reaction
In the present energetic scenario, renewable energy sources are
not only strategic alternatives but are also becoming progressively
mandatory. Among the options, hydrogen has been considered
a promising carrier for different applications. Nowadays, steam
reforming of hydrocarbons (mostly methane from natural gas)
is still the main source of hydrogen at industrial scale [1], but
the reforming of more complex, oxygenated molecules, such as
bioethanol is becoming attractive [2,3]. The use of biomass deriva-
tives, however, end up adding complexity to the reaction pathways
and specific aspects in the catalyst design [4].
Among the oxygenated molecules that have been explored to
produce hydrogen is glycerol, a byproduct from biodiesel pro-
duction [5]. The glycerol steam reforming (GSR) reaction (Eq. (1))
involves two main reactions, the glycerol decomposition, produc-
ing H2 and CO (Eq. (2)), and the water-gas shift (WGS) reaction
(Eq. (3)), that produces additional H2 by the oxidation of CO to CO2
[5].
C3H8O3 + 3H2O ꢀ 7H2 + 3CO2 ꢀH = 128 kJ mol−1
(1)
(2)
(3)
C3H8O3 ꢀ 3CO + 4H2
CO + H 2O ꢀ CO 2 + H
ꢀH = 251 kJ mol−1
ꢀH = − 41 kJ mol− 1
2
The GSR reaction has been reported under several conditions,
mostly involving catalysts of Ni and Pt supported on oxides such as
Al2O3, SiO2, ZrO2, CeO2 and CeO2/ZrO2 [7–11]. Over the same sup-
port, Pt has been correlated to reduced methane formation when
compared to Ni; moreover, the support has shown to be a deter-
minant factor to favor H2 selectivity since it may participate in the
WGS step [6,7,11,12].
Therefore, the design of high performance catalysts for H2 pro-
duction from GSR reaction is a challenging task and many systems
and additives have been explored. In this context, vanadium oxide
is an effective promoter in several studies especially focused on the
transformation of hydrocarbons to olefins, by dehydrogenation, or
to COx and H2 through oxidation reactions [13–15]. For instance,
when VOx/Pt/Al2O3 catalysts were applied to propane total oxida-
tion, they showed higher catalytic activity compared to Pt/Al2O3
[16]. Moreover, when a WGS stage was integrated to ethanol oxi-
∗
Corresponding author.
E-mail address: daniela@iqm.unicamp.br (D. Zanchet).
Present address: Itaipu Technological Park , P.O. Box 2039, 85867-900, Foz do
1
Iguac¸ u, PR, Brazil.
http://dx.doi.org/10.1016/j.cattod.2016.09.021
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: T.M. Kokumai, et al., VOx-Pt/Al2O3 catalysts for hydrogen production, Catal. Today (2016),
http://dx.doi.org/10.1016/j.cattod.2016.09.021

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dation, the V addition to the WGS Pt/CeO2 catalyst enhanced the
activity [17]. The effect of vanadium as promoter on catalytic activ-
ity has demonstrated to be dependent on several factors. One of
them is the support nature, as observed for methanol oxidation
reaction to produce formaldehyde. Vining et al. [18] compared VOx
grafted on SiO2 and CeO2/SiO2 and observed a remarkable increase
in methanol conversion for the VOx/CeO2/SiO2 catalyst. Further-
more, the promoting effect of vanadium seems to rely on the VOx
species formed over the support surface; Kilos et al. [19] showed
that when tridimensional structures were created, they affected the
accessibility of ethanol to the active sites of VOx/Al2O3 catalysts,
thus decreasing the turnover rates for ethanol oxidative dehydro-
genation to acetaldehyde. Moreover, the presence of vanadium in
CeO2 catalysts has increased the stability towards Cl poisoning
when applied to the combustion of chlorobenzene [20].
Thereby, the deposition of dispersed vanadium species over
metal oxides can play an interesting role in glycerol decomposition,
since these species are active for selective dehydrogenation of alco-
hols. In addition, VOx species have showed increased WGS activity
when added to ZrO2 and CeO2 supported Pt catalysts through the
increased reducibility of the support [21]. Hence, in this work the
impact of vanadium loading in VOx-Pt/Al2O3 catalysts applied to
WGS and GSR has been evaluated, showing that while the WGS
step was favored with vanadium addition, the selectivity to H2 in
GSR reaction decreased, suggesting the competitive role of the acid
and redox nature of vanadium sites.
4200 by N2 adsorption at – 196 ◦C, after pre-treatment at 150 ◦C by
24 h.
Temperature programmed reduction (TPR-H2) was conducted
in a Micromeritics AutoChem 2920 equipped with a U-tube reac-
tor. Thirty mg of catalyst were treated under N2 flow for 30 min
at 200 ◦C, cooled to room temperature and heated to 1000 ◦C, at
10 ◦C min−1 under 30 mL min−1 of 10% H2/He. The H2 consumption
was quantified by a thermal conductivity detector (TCD) detector
and a calibration curve. The signal obtained for the bare alumina
support was subtracted prior the analysis.
In situ X-ray absorption fine structure (XAFS) spectra at V K-
edge (5465 eV) and Pt L3-edge (11564 eV) were obtained at XAFS2
beamline at the Brazilian Synchrotron Light Laboratory (LNLS),
Campinas, Brazil. The samples packed in pellets were mounted in
a home-made furnace connected to a gas flow system, working in
transmission mode. Catalyst activation was conducted under 2.5%
H2/He flow, 100 mL min−1 and the WGS reaction was conducted
using CO:H2O vol. ratio of 1:3 and total flow of 100 mL min−1. To
help the identification of the intermediate V species, bulk V2O5
reduction was conducted and the X ray absorption near edge struc-
ture (XANES) data was analyzed by the methodology described by
Wong et al. [23]. Briefly, the spectra obtained at selected tempera-
tures were compared to V references compounds. After attributing
the selected spectra to a known vanadium oxide phase (V2O5, V4O7,
V2O4, V2O3 and VO), the identification of V species in the xVPtAl
catalysts was done. X ray absorption fine structure (EXAFS) at Pt L3-
edge were used to estimate the Pt metallic dispersion, through the
analysis of the coordination numbers obtained for the first nearest
neighbors of Pt, assuming a spherical nanoparticle model. The XAFS
data was analyzed using Demeter software package [24].
2. Experimental details
Diffuse reflectance spectroscopy in the ultraviolet-visible range
(UV–vis DRS) was performed using an Agilent Cary 5000 UV–vis-
NIR spectrophotometer. The catalyst was finely ground and the
absorbance was measured in the 200–800 nm range at ambient
conditions. All spectra were obtained with hydrated samples and
the reference compound was BaSO4. The Kubelka-Munk function,
2.1. Catalyst preparation
The chemicals were used as received, without any pre-
treatment. The support ␥-Al2O3 was synthetized by the sol-gel
method previously described [22]. Briefly, aluminium tri-sec
buthylate (Merck, 97%) was stirred with ethanol and water at 80 ◦C
for 1 h, then a certain amount of 0.1 mol L−1 HNO3 solution was
added and the system was kept under reflux for 14 h. The formed
gel was dried at 110 ◦C overnight and calcined under synthetic air
flow at 500 ◦C, 4 h.
F(R ), was obtained from the absorbance.
∞
2.3. Catalytic activity
WGS reaction tests were performed in a fixed bed quartz reactor
(i.d. 9 mm) operating at atmospheric pressure. The catalyst (75 mg)
was diluted in 125 mg of ground quartz and reduced in situ at 500 ◦C,
10 ◦C min−1 for 1 h in 35 mL min−1 of H2. The catalyst was cooled
at 250 ◦C and the WGS reaction was performed with 1:3 CO:H2O
(v/v) feed ratio and total flow of 100 mL min−1. The CO conversion
(%) was followed by gas chromatography in an Agilent CG 7890
equipped with a TCD detector, at temperatures of 250, 300, 350,
400 and 450 ◦C, with 5 measurements at each temperature. The CO
conversion was obtained by % COconv = [CO2/(CO + CO2)] × 100. The
specific rates were estimated by r = FCO (mol s−1) × % COconv × MM
(g mol−1)/(100 × mcatalyst (g) × tm × D), where FCO CO molar flux;
MM = Pt molecular mass; tm = metal loading and D = metal disper-
sion (estimated by EXAFS data).
The Pt/Al2O3 catalyst (1% wt) was synthetized by wet impreg-
nation. One gram of the ␥-Al2O3 support was added to an ethanolic
solution containing 0.025 g of H2PtCl6·H2O (Umicore, 40% Pt basis)
and the dispersion was stirred in a rotary evaporator for 4 h. The
ethanol was evaporated in vacuum; the solid was dried at 110 ◦C
overnight and then calcined again under synthetic air flow at 500 ◦C,
4 h. The catalyst was labeled PtAl.
The VOx-Pt/Al2O3 catalysts were prepared by wet impregnation
of PtAl by adding 41, 82 and 163 mg of NH4VO3 (Aldrich, >99%) dis-
solved in 80 mL of deionized water, to result in three catalysts with
different loadings of vanadium (1.8, 2.5 and 6.6% V wt/gcat, respec-
tively). After stirring and solvent evaporation, the solids were dried
and calcined as before. The final catalysts were labeled 05VPtAl,
1VPtAl and 2VPtAl, and generally addressed as xVPtAl. For com-
parison, a 1VAl sample was prepared in similar way without adding
the Pt precursor.
GSR reaction was performed in the same catalytic unit using
100 mg of catalyst and 400 mg of diluent. The catalytic test was
performed at 400 ◦C for 2 h, with in situ reduction of the catalyst at
500 ◦C for 1 h under H2. A glycerol aqueous solution (3.3 mol L−1
)
2.2. Catalyst characterization
was fed into the reactor by a Cole Parmer 74900 syringe pump
(at a rate of 1.9 mL h−1) and vaporized before reaching the cat-
alyst bed, using He as carrier gas (30 mL min−1). The total liquid
fraction was collected in a condenser flask and the gaseous prod-
ucts H2, CO, CO2 and CH4 were analyzed by an Agilent GC7890A
gas chromatograph, using a TCD and He as carrier gas. Glycerol
total conversion was obtained by detection of unreacted glycerol
in the liquid fraction in an Agilent GC7890A using a Flame Ioniza-
Powder X-ray diffraction (XRD) was measured in a Shimadzu
XRD7000 equipped with Cu target (K␣ = 1.5406 Å), operating
at 40 kV and 30 mA. Vanadium and platinum loadings were
obtained by X-ray fluorescence (XRF) in a Shimadzu XRF1800.
The surface area of the PtAl catalyst was obtained by the
Brunauer–Emmet–Teller (BET) method in a Quantachrome Nova
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3
2VPtAl
1VPtAl
05VPtAl
PtAl
Al2O3
Fig. 2. Illustration of V structures as a function of V loading over alumina surface.
The S symbol stands for the alumina support.
V2O5 standard
also confirmed by visible Raman spectroscopy (not shown) that did
not present the characteristic intense band around 994 cm−1 of the
crystalline V2O5 phase [15,21,26,29].
20
30
40
50
60
70
80
2 Theta (°)
One of the interesting and challenging aspects of vanadium
is that it presents a variety of oxidation states and oxide struc-
tures. When dealing with vanadium oxides dispersed on alumina,
it has been shown that the main species depend on the V loading
[26–28]. At low loadings, these species can be monomeric, having
Fig. 1. Diffraction profiles of xVPtAl catalyst and ␥-Al2O3 support; V2O5 standard
(PDF 041–146) is shown for comparison.
Table 1
Chemical composition obtained by XRF and vanadium surface coverage (V atoms
per nm2).
V
O bonds with the support, and polymeric, with bridging V O V
bonds, as depicted in Fig. 2. It is also important to consider the
existence of hydroxyl species due to hydrated conditions [27].
Nevertheless, the loading range that dictates the prevalence of
one or another structure and which one would render the support
a higher reducibility depend on several factors, such as the vana-
dium precursor [29] and the oxide support. For instance, according
to Nguyen-Thanh et al. [21] the predominant species over ZrO2 at
low V loadings (up to 2.8 nm−2) are monomeric VOx (having at
Sample
Pt (wt%)
V (wt%)
V nm−2
PtAl
0.55
0.43
0.44
0.49
–
–
05VPtAl
1VPtAl
2VPtAl
1.18
2.51
4.79
0.5
1.0
1.9
tion Detector (FID). The liquid components were identified by an
Agilent GC7890A using a mass spectrometer detector quadrupole
5978C inert XL NSD Agilent.
least one V
O Zr bond with the support). At higher loadings, the
amount of these isolated species increases to the limit in which the
formation of polymeric VOx and a monolayer are favored, which
3. Results and discussion
possess more V O V bridging bonds, therefore interacting less
with the support. The authors then correlated the high concentra-
tion of monomeric units to the increase in the support reducibility,
which in turn was attributed as the main factor to the enhanced
catalytic activity observed for WGS reaction in Pt-VOx/ZrO2 cata-
lysts. In contrast, Wu et al. [25,26] investigated the surface of Al2O3
after impregnation with vanadium and suggested that the pres-
ence of dispersed polymeric units rendered facilitated reducibility
in comparison with the monomeric ones. This observation was in
agreement with the work by Ballarini et al. [28] in samples contain-
ing about 2% wt of V2O5 impregnated over ␥-Al2O3 (0.7 V nm−2),
which reported the reduction of isolated VOx species at higher tem-
peratures. Samples with higher V loadings (7 and 10% wt) presented
predominance of polymeric species and bulk vanadium oxide that
reduced at lower temperatures.
To compare the samples concerning the geometric and elec-
tronic structure of vanadium over xVPtAl catalysts, XANES spectra
of the fresh samples (as-prepared) were collected at V K- and Pt
L3-edges and are shown in Fig. 3a and b, respectively. Fig. 3a shows
that at room temperature all xVPtAl catalysts, in average, are com-
posed by similar species in respect to their electronic and geometric
nature. It is also evident that the dominant species are not similar
to bulk V2O5, although the presence of the pronounced pre-edge
peak around 5470 eV is consistent with a similar local environment,
with V5+ species in a non-centrosymmetric coordination with oxy-
gen atoms [23]. The XANES data is in accordance to the works by
Tanaka et al. [30] and Ruitenbeek et al. [31], who identified V species
impregnated over alumina as VO4 units having V5+ ions in distorted
tetrahedral coordination, as illustrated in Fig. 2. These results are
also in agreement with the absence of V2O5 crystalline phase on
The diffraction profiles of the catalysts and the bare ␥-Al2O3
support are shown in Fig. 1. The diffraction pattern of PtAl catalyst
is similar to the ␥-Al2O3 support and the absence of peaks corre-
sponding to crystalline phases of Pt and/or PtO2 indicates that the
metallic phase is well dispersed over the alumina surface. For the
xVPtAl catalysts, an increase in the diffracted intensity can be seen
below 2␪–30◦. In fact, for the V2O5 standard, the three main peaks
are found at 2␪ equal to 20.28◦, 26.15◦ and 31.01◦ and correspond
to the (001), (110) and (031) reflections of the ␣-phase, respec-
tively (PDF 041-146). Therefore, the observed slight increase in the
intensity of the signal at this 2␪ region for the xVPtAl catalysts sug-
gests the absence of a highly ordered V2O5 phase. Table 1 shows
the the Pt and V wt% obtained by XRF. Considering these values
and the specific surface area obtained for the PtAl catalyst (BET
300 m2/g), it was possible to estimate the vanadium surface cover-
age (V atoms per nm2) for each catalyst (Table 1). At these loadings,
below 2 V nm−2, the formation of V2O5 nanoparticles, not detected
by XRD, was indeed not expected [25–28]. For example, Carrero
et al. [29] found that V2O5 nanoparticles were formed when using
ammonium metavanadate as precursor for coverages of 6.1 V nm−2
and greater, which is about three times higher than the maximum
V loading of our samples. For other precursors the threshold was
even higher (V loading necessary to form a monolayer over alumina
was about 8 V nm−2). Similar results were found by Wu et al. [26]
that detected that crystalline V2O5 becomes the dominant species
only at much higher coverage, i.e., 14.2 V nm−2, although it started
to be formed at coverages of 4.4 V nm−2. The absence of nanocrys-
talline V2O5 nanoparticles in our samples indicated by XRD were
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(a)
V K - edge
2VPtAl
1VPtAl
05VPtAl
V2O5
5440
5460
5480
5500
5520
5540
5560
Energy (eV)
Fig. 4. UV-VIS DRS spectra of the fresh samples.
(b)
V2O5
Pt L3 - edge
0.005
2VPtAl
1VPtAl
05VPtAl
PtAl
0.000
400
500
600
700
800
900
2VPtAl
Temperature (°C)
1VPtAl
05VPtAl
PtO2
Pt foil
1VAl
PtAl
100 200 300 400 500 600 700 800 900
Temperature (°C)
11540
11560
11580
Energy (eV)
11600
Fig. 5. TPR-H2 profiles of xVPtAl samples. The inset shows the TPR-H2 profile of
bulk V2O5.
Fig. 3. XANES spectra of the fresh samples at (a) V K-edge and (b) Pt L3-edge. Bulk
V2O5, Pt0 and PtO2 are shown for comparison.
the surface of the catalysts. In the case of Pt, the XANES spectra for
all catalyst (Fig. 3b) indicate the presence of oxidized Pt species;
however, the spectra do not match the PtO2 standard indicating
the formation of a highly dispersed PtOx phase.
To gain more insights about the evolution of VOx species as a
function of V loading, the samples were analyzed by UV–vis DRS
and the profiles are shown in Fig. 4. It is worth to point out that
the 1VPtAl and 2VPtAl samples may have sufficient V to present
polymeric VOx species, following the work of Nguyen-Thanh et al.
[21] and Ballarini et al. [28].
According to several authors [26,32,33], the absorption bands
of the UV–vis DRS spectra around 240 nm can be assigned to
monomeric species of vanadium, while the bands at higher wave-
lengths (near 300–400 nm) would be related to polymeric VOx.
Crystalline V2O5 would present a broad absorption extending to
longer wavelengths (> 450 nm). A more quantitative analysis based
on the linear relationship between the energy edge determined by
as a function of the loading (especially by observing the shift to
higher wavelengths from 05VPtAl to 2VPtAl and the raise in the
signal around 330 and 400 nm). These results are in accordance
with previous works on bare alumina up to the monolayer coverage
[26,34,35].
So far, the fresh catalysts were characterized and they are com-
posed of dispersed PtOx and monomeric and polymeric VOx species
over the surface, with the latter probably increasing with V load-
ing. Nevertheless, prior the reaction the catalysts were activated
under H2 up to 500 ◦C for 1 h and Fig. 5 shows the modification of
the catalyst during TPR-H2. The reduction profiles provide informa-
tion about the reducibility of species as well as indirect information
about the interaction between the components of the catalysts (V-
Pt).
Due to the small Pt loading (0.5% wt), the H2 consumption cor-
responding to PtOx reduction could not be detected in the PtAl
catalyst, in agreement with the work by Garcia et al. [32]. On the
other hand, clear H2 consumption is observed for all xVPtAl cat-
alysts and the 1VAl support (prepared in a similar way to 1VPtAl
but without Pt), indicating that the reduction of the VOx species
could be detected. The inset in Fig. 5 shows the reduction profile
of bulk V2O5, which exhibits a broad peak centered at about 670 ◦C
[26]. No H2 consumption at higher temperatures was found for all
catalysts, which further corroborates the absence of a crystalline
UV–vis DRS and the number of V
O V bonds found in polymeric
species was proposed [34]. In our xVPtAl catalysts, the absorp-
tion bands are broad and due to the presence of Pt a quantitative
evaluation is difficult. Nevertheless, it seems a consensus that the
bands at longer wavelengths can be attributed to polymeric enti-
ties (once V2O5 NPs are excluded) and as a consequence, Fig. 4
indicates that the amount of polymeric species seems to increase
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5
21 °C
Pt L3 - edge
2VPtAl
49 °C
66 °C
108 °C
1VPtAl
148 °C
200 °C
243 °C
288 °C
335 °C
377 °C
422 °C
467 °C
512 °C
556 °C
602 °C
645 °C
650 °C
05VPtAl
PtAl
Pt foil
a)
5440
5460
5480
5500
5520
5540
5560
Energy (eV)
11540
11560
11580
Energy (eV)
11600
22.5
23.0
Fig. 6. In situ XANES spectra at Pt L3-edge of the catalysts after TPR-H2. The Pt
standard is shown for comparison.
34.5
Table 2
36.5
Structural parameters obtained from the EXAFS analysis at Pt L3-edge of catalysts
after the reduction, at room temperature.
650 °C: V3+ - V2O3
602 °C: V4+/3+ - V4O7
512 °C: V4+ - V2O4
3.0
26.0
2
22.0
22.0
38.0
Sample
NPt-Pt
RPt-Pt (Å)
␴
(Å2)
Pt Dispersion (%)
Pt-Pt
PtAl
8.5
9.8
9.7
9.9
2.75
2.75
2.75
2.75
0.007
0.007
0.008
0.007
79.9
48.5
50.6
47.3
4.5
4.5
5.0
29.0
42.0
05VPtAl
1VPtAl
2VPtAl
V2O5 phase. Comparing to the 1VAl support, the H2 consumption
peak shifts the center to about 500 ◦C, indicating that VOx species
dispersed on the alumina surface are reduced more easily than bulk
V2O5, in accordance with the literature [26,28,35]. Comparing the
1VAl support and 1VPtAl catalyst, it is clear that the presence of Pt
shifts further the reduction towards lower temperatures (around
300 ◦C). This effect has been associated to H2 spillover promoted
by Pt over VOx species [32,36], suggesting some degree of Pt-V
interaction. By comparing 1VPtAl with 05VPtAl and 2VPtAl cat-
alysts, the H2 consumption is in line with the V loading increase.
The similar reduction profiles of 1VPtAl and 2VPtAl suggest the
predominance of similar VOx species, in agreement with the DRS
UV–vis and V K-edge XANES data. Therefore, the TPR-H2 profiles
indicate that the interaction with alumina and the presence of
Pt facilitate the reducibility of the VOx species independently of
the vanadium content, as also observed by Zheng et al. [37]. This
property has been pointed out as an important factor in catalytic
processes such as propane total oxidation [32] and WGS reaction
[38]. The support reducibility can induce the formation of active
OH species and/or promote their mobility over the catalyst surface
[21], which are important steps in the WGS and steam reforming
mechanisms [6,38].
As an attempt to clarify the catalysts changes during the reduc-
tion process, in situ XANES experiments at Pt L3- and V K-edges
were performed. In the case of Pt, the results show the evolution
of PtOx species to metallic Pt, as evidenced in Fig. 6. The analysis of
the EXAFS data are shown in Table 2 and indicates that xVPtAl sam-
ples undergo a decrease in Pt dispersion compared to PtAl, which
was possibly caused by the additional calcination performed after
vanadium impregnation.
In the case of V K-edge, the analysis is more complex. Wong et al.
[23] analyzed the main oxide phases and several other compounds
with known structure and coordination. Among the most impor-
tant vanadium oxide structures are V2O5, V2O4, V4O7, V2O3 and VO;
21 °C: V5+ - V2O5
b)
-40
-20
0
20
40
60
80
Energy (eV)
Fig. 7. (a) In situ XANES at V K-edge spectra for bulk V2O5 during TPR-H2; (b) identifi-
cation of V oxide phases formed during TPR-H2 and the corresponding temperatures.
their V oxidation states varies from +5 to +2 and their coordination
evolves from distorted square pyramid (V2O5) to distorted octahe-
dral and finally to regular octahedral (VO). For a better comparison
of the samples, the authors took the zero of energy with respect to
the first maxima of the first derivative of V reference foil spectra (at
5465.0 eV) and the position of all peaks was then given respective
to this energy. To support the analysis of the catalysts, we first per-
formed the in situ reduction of bulk V2O5 and followed the changes
of V K-edge spectra. In this way, the position of the features of the
selected spectra measured here was compared to the data pub-
lished by Wong et al. [23] and the intermediate vanadium oxides
formed on the catalysts during reduction were assigned. Fig. 7a
presents the results for bulk V2O5 under TPR-H2. The intense pre-
edge peak at 5470.0 eV in V2O5 spectra (spectrum taken at room
temperature) is characteristic of a non-symmetric local coordina-
tion, as in the distorted square pyramid, and correspond to the
dipole-forbidden 1s → 3d transition that becomes partially allowed
due to the mixing of 3d-4p metal orbitals and overlap from V 3d
orbitals with O 2p [23]. When the structure evolves from V2O5 to VO
(regular octahedral) the symmetry increases and therefore the pre-
edge peak decreases. The decrease in the V oxidation state from +5
to +2 also shifts the absorption edge to lower energies. Therefore, by
heating under H2 the V K-edge XANES spectra evolves (Fig. 7a), with
a progressive decrease in the pre-edge peak intensity and energy
and a rise of the white line intensity. The corresponding assign-
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05VPtAl
1VPtAl
2VPtAl
05VPtAl
1VPtAl
2VPtAl
b)
a)
1
0
1
05VPtAl
1VPtAl
2VPtAl
05VPtAl
1VPtAl
2VPtAl
d)
c)
V2O3 (650 °C)
V4O7 (602 °C)
0
5440
5520
5440
5480
5520
5560
5440
5480
5520
5560
Energy (eV)
Fig. 8. Selected XANES spectra of xVPtAl catalysts obtained at V K-edge during TPR-H2 (a) room temperature, (b) 300 ◦C, (c) 400 ◦C and (d) 500 ◦C. The inset in (d) shows the
spectra corresponding to the V4O7 and V2O3 phases (see Fig. 7b) for comparison.
ments of the intermediate vanadium oxides are shown in Fig. 7b.
PtAl
Accordingly, at 512 ◦C the structure of V2O4 is formed, with V4+ in
70
05VPtAl
distorted octahedral coordination. At 602 ◦C the V4O7 phase can be
1VPtAl
2VPtAl
identified, with V3+ and V4+ ions in distorted octahedral coordina-
tion. Finally, at 650 ◦C the spectra is similar to the one attributed
to V2O3. Comparing to conventional TPR-H2 (inset, Fig. 5) for bulk
V2O5, the H2 consumption up to 650 ◦C can be associated to the
reduction of V2O5 to V2O3, which is probably reduced further to
VO up to 800 ◦C.
60
50
40
30
20
10
0
After the successful assignment of the intermediary vanadium
oxides formed during bulk V2O5 reduction, xVPtAl catalysts were
also investigated by in situ XANES at V K-edge. The corresponding
spectra obtained at selected temperatures are shown in Fig. 8.
Comparing the samples under reduction, above 400 ◦C all three
catalyst have similar profiles that did not change at 500 ◦C. The
main reduction of vanadium species up to 400 ◦C was in accor-
dance with conventional TPR-H2 results (Fig. 5), which showed no
H2 consumption above this temperature for xVPtAl samples. The
similarities of the spectra at 500 ◦C for the three catalysts and V4O7
and V2O3 bulk phases suggest the presence of V3+ and V4+ ions
coordinated in a more centrosymmetric configuration, consistent
with distorted VO6 species. The small discrepancies with the bulk
phases are likely originated by the dispersed nature of the species
in the catalysts [23,31]. The presence of mixed valence (V3+ and
V4+) species after reduction in H2 was also found by Wu et al. [25]
on VOx/Al2O3 samples. Therefore, the results suggest that all three
xVPtAl samples after reduction, including 05VPtAl, present mixed
valence (V3+ and V4+) VOx species on the surface that would be the
species available for the reaction after catalysts activation.
Fig. 9 shows the performance of the catalysts for the WGS
reaction and it became evident that the presence of VOx species
significantly increases the CO conversion in comparison with PtAl
catalyst, especially above 300 ◦C. Table 3 shows the specific rate
obtained at 250 and 300 ◦C. Interestingly, a clear correlation with
V loading was not found.
250
300
350
400
450
Temperature (°C)
Fig. 9. CO conversion (%) for VPtAl catalysts as a function of temperature.
Table 3
Specific rate calculated at 250 and 300 ◦C.
specific rate (s−1
250 ◦
)
Sample
C
300 ◦
C
PtAl
0.001
0.005
0.005
0.006
0.002
0.011
0.012
0.015
05VPtAl
1VPtAl
2VPtAl
previous results, the nature of VOx species over the surface of the
catalysts may be very similar in spite of the increased V content.
To understand the nature of V and Pt species under WGS reaction
conditions, in situ XANES at V-K and Pt-L3 edges were measured.
The results for 1VPtAl catalysts are shown in Fig. 10; the other ones
(not shown) have similar profiles. The behavior of Pt and V species
were not altered under WGS conditions, while Pt was in metallic
form as after the reduction step, the vanadium remained in mixed
V3+/V4+ oxidation state in a distorted octahedral coordination.
The fivefold increase in the specific rate at both temperatures
for xVPtAl catalysts compared to PtAl evidences the promoting
effect of vanadium; however, the results suggest that the enhance-
ment was not significantly affected by the V loading. Based on the
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Hence, the increase in the CO conversion of the xVPtAl samples
when compared to the unpromoted PtAl catalyst can be thought
considering the important role of the support in the WGS reaction
mechanism. The addition of other oxides in Pt-based catalysts can
modify the catalytic activity in several ways, such as by improving
the dispersion and stabilization of the metal phase over the surface
of the support, changing the reducibility of the support, altering
the electronic state of metal phase, by creation of new active sites
at the metal-support interface, among others [15,41,42]. Accord-
ingly, the results presented in this work indicate that the enhanced
catalytic activity observed for V promoted catalysts may be asso-
ciated to the increase in the reducibility of the support and/or the
creation of more active interface sites. The presence of polymeric
VOx could lead to the formation of a higher amount of OH species
on the catalyst surface, compared to PtAl, and these species would
react to produce the intermediates as formate [36]. It has been pro-
posed that the polymeric moieties could also increase the mobility
of these OH species through the support, favoring the reaction [21].
It is interesting to note, however, that the lack of correlation with
V content, suggests that the further addition of V in 1VPtAl and
2VPtAl creates additional VOx species that do not significantly par-
ticipate in the reaction. This behavior is different from the one
found for model Pt/Al2O3 catalysts promoted with cerium, where
a correlation with CeO2 loading was found [42] suggesting that an
increase in the promoter content increases the probability of form-
ing metal-oxide interfacial sites. In the case of vanadium, however,
the interaction with alumina and the tendency to form polymeric
species spread on its surface [43] might frustrate the odds to form
new Pt-VOx interfacial sites with increasing loading. It is interest-
ing to note that in the 2VPtAl there are 30 times more V atoms than
Pt ones. The importance of interfacial sites has been demonstrated
in other systems. For example, Sener et al. [44], applying controlled
surface reactions to produce Pt/Mo bimetallic catalysts, reported
that the turnover frequency for WGS reaction exhibited a linear
correlation with the amount of MoOx species in close contact with
Pt metallic sites. The authors then suggested that the interfacial Pt-
Mo sites would be the active ones for the reaction, and the MoOx
promoter would have the role to stabilize water and/or OH groups,
thus increasing their coverages, facilitating the reaction. DFT (den-
sity functional theory) has also helped to highlight the important
role played by the Pt-metal oxide interfacial sites, revealing a com-
plex framework depending on the system [45]. In this sense, for the
xVPtAl catalysts the interfacial Pt-VOx sites may be the responsi-
ble for the 5-fold enhancement of the activity for WGS reaction
compared to the PtAl catalyst.
(a)
1VPtAl
Pt L3- edge
400 °C, WGS
200 °C, WGS
500 °C, H2
25 °C
PtO2
Pt foil
11540
11560
Energy (eV)
11580
11600
(b)
1VPtAl
V K - edge
450 °C, WGS
200 °C, WGS
500 °C, H2
25 °C
V2O5
5440
5460
5480
5500
5520
5540
5560
Energy (eV)
Fig. 10. In situ XANES of 1VPtAl at (a) Pt L3-edge and (b) of V K-edge. Spectra corre-
sponding to fresh catalyst (at 25 ◦C); after activation and under H2 at 500 ◦C; under
WGS reaction at 200 ◦C and 400/450 ◦C. Pt0, PtO2 and V2O5 references are shown for
comparison.
In the light of these results, consideration about the WGS reac-
tion mechanism is useful to shed light in the promoting effect of VOx
species in this set of catalysts. Over bifunctional catalysts, there is a
consensus that the dominant mechanism depends on the catalyst
nature and reaction conditions, and there are several proposals in
literature [38]. In systems composed of a noble metal (Pt) supported
in a non-reducible oxide such as Al2O3 it is generally accepted that
the WGS reaction proceeds by the associative mechanism, in which
CO would adsorb on Pt while H2O would be activated on the sup-
port, forming H and OH species. These species would react with
adsorbed CO and then produce intermediates such as formates, car-
bonates and bicarbonates, that would decompose to form CO2 and
H2. The correct assignment of the sites in which the formation of
these intermediates would occur and how it would happen (migra-
tion of OH species to the metal or of CO to the support, or yet to
the interface) remains a challenge [39]. The importance of the sup-
port has also been extensively reported in WGS reaction studies
and in the presence of reducible oxides such as CeO2, TiO2, Co3O4,
the “redox” mechanism, involving the participation of oxygen from
the support becomes favored [40]. This process would provoke the
reduction of the support and the formation of an oxygen vacancy
which would be regenerated with the adsorption and dissociation
of a water molecule preferentially on these sites, therefore forming
H2 [39].
Considering the positive impact of vanadium in the WGS reac-
tion, the catalysts were tested for the GSR reaction, aiming to
maximize H2 production through the WGS step. Fig. 11 shows
the gaseous products distribution (CO2, H2, CH4 and CO) obtained
under GSR reaction.
The results showed that for all samples the major gaseous prod-
ucts were CO and H2, followed by CO2 and a small fraction of CH4.
The product distribution was consistent with glycerol decomposi-
tion reaction (Eq. (2)); however, the presence of CO2 indicated that
WGS reaction (Eq. (3)) was also occurring. The CO2 fraction was
higher for the xVPtAl catalysts and increased with the V loading, in
agreement with the presence of VOx species favoring the WGS reac-
tion. However, an opposite behavior for H2 was found; the xVPtAl
catalysts showed lower H2 fraction compared to PtAl. This suggests
that the H2 produced by WGS reaction may have been consumed
by parallel reactions. According to Pompeo et al. [8], Pt-Al2O3 cat-
alysts in GSR reaction favor the dehydration pathway of glycerol
molecule, due to the acid nature of alumina, forming CH4 and CO2,
by the following overall reaction:
C3H8O3 → CO2 + CO + CH4 + 2H2
(4)
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Fig. 11. Distribution of gaseous products CO2, H2, CH4 and CO as a function of time on stream during GSR reaction at 400 ◦C for xVPtAl samples.
The authors reported that this reaction would proceed along-
side the decomposition reaction (Eq. (2)), leading to the additional
formation of CO and H2. Taking this reaction into account and the
formation of the small amount of CH4, it is suggested that this
step may occur as a minor route. Moreover, the higher fraction of
CH4 and the reduced amount of H2 for xVPtAl suggest that parallel
reactions are enhanced by the vanadium presence.
trated the powerful tuning of the catalysts properties, as observed
by their performance on WGS reaction. The promoted samples
showed a five-fold increase in the WGS activity when compared to
PtAl, therefore demonstrating the success of this approach. How-
ever, the increase of V content did not clearly impact in the WGS
activity, which might be related to the formation of polymeric VOx
species interacting with alumina surface instead of new interfacial
Pt-VOx sites that would favor the reaction. The in situ characteriza-
tion of the catalysts showed that metallic Pt and V3+/V4+ species are
present during WGS reaction regardless of V loading. When applied
to GSR reaction, the presence of VOx species favored the WGS step;
however, the H2 fraction did not increased as expected, probably
due to its consumption in parallel reactions and/or due to the acidic
nature of VOx species over the surface that would lead to the forma-
tion of byproducts. Therefore, the overall result for both reactions
reinforces the necessity of finding the right tuning of several aspects
to optimize the catalyst activity and selectivity.
The total liquid fraction of the products, collected at the end of
the reactions, indicated no residual glycerol within the range of the
calibration curve (up to 95%), showing that the reaction conditions
led to total conversion of glycerol. The major products detected
in the liquid fraction for all catalysts were ethanol, 2-propenal
(acroleyn), 1-hydroxy-2-propanone, propanal and phenol, which
strongly evidences the presence of glycerol dehydration reactions
in acid sites of the support (Al2O3 and VOx/Al2O3) [8,46]. In fact,
it has been reported the increase in support acidity with the addi-
tion of VOx species over Al2O3 within monolayer coverage regime
[35]. Furthermore, the lower selectivity for H2 observed for xVPtAl
catalysts are consistent with the reported activity of vanadium cat-
alysts for the dehydration reactions of acetaldehyde and glycerol
[27,47], that would be promoted by the acid sites of VOx, therefore
leading to the formation of the aforementioned byproducts.
In summary, although the V promoted catalysts exhibited higher
CO conversions in WGS reaction when compared to PtAl sample,
under GSR the presence of vanadium seems to favor the formation
of byproducts, decreasing the fraction of H2, likely due to the acidic
nature of VOx species.
Acknowledgements
The authors are grateful for the financial support of CNPq
(Conselho Nacional de Desenvolvimento Científico e Tecnológico,
Procs. 406879/2013-3 and 309373/2014-0). T.M.K was supported
by CAPES (Coordenac¸ ão de Aperfeic¸ oamento de Pessoal de Nível
Superior) scholarship, D.A.C. by FAPESP (Fundac¸ ão de Amparo
à Pesquisa do Estado de São Paulo, 2013/177720-6) fellowship
and G.T.M and L.B.P by CNPq scholarships. The authors gratefully
acknowledge LNLS (Brazilian Synchrotron Light Laboratory) and
LNNano (Brazilian Nanotechnology National Laboratory) at CNPEM
(Centro Nacional de Pesquisas em Energia e Materiais) and Institute
of Chemistry at UNICAMP (University of Campinas) for the access
to the facilities.
4. Conclusions
Vanadium oxides are interesting and challenging structures to
apply in catalysis and their addition on Pt/Al2O3 catalysts illus-
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Please cite this article in press as: T.M. Kokumai, et al., VOx-Pt/Al2O3 catalysts for hydrogen production, Catal. Today (2016),
http://dx.doi.org/10.1016/j.cattod.2016.09.021