Pt-Ni/ZnO-rod catalysts for hydrogen production by steam reforming of methanol with oxygen

Article abstract of DOI:10.1039/d0ra06181f

Ni, Pt and a mixture of Ni and Pt supported on ZnO-rods were evaluated in autothermal steam reforming of methanol (ASRM) for hydrogen production as a function of the reaction temperature. The catalytic materials were characterized by SEM-EDS, XRD, TEM, HR

Full text of DOI:10.1039/d0ra06181f

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Pt–Ni/ZnO-rod catalysts for hydrogen production
by steam reforming of methanol with oxygen†
Cite this: RSC Adv., 2020, 10, 41315
Ventura Rodrıguez Lugo,a Gilberto Mondragon-Galicia,b Albina Gutierrez-Martınez,b
´
´
´
´
´
Claudia Gutierrez-Wing,b Omar Rosales Gonzalez,a Pavel Lopez,b Pastora Salinas-
´
´
c
Hernandez,
Francisco Tzompantzi,d Marıa I. Reyes Valderramaa and Raul Perez-
´ ´ ´
´
b
*
Hernandez
´
Ni, Pt and a mixture of Ni and Pt supported on ZnO-rods were evaluated in autothermal steam reforming of
methanol (ASRM) for hydrogen production as a function of the reaction temperature. The catalytic materials
were characterized by SEM-EDS, XRD, TEM, HRTEM, TPR and BET. Analysis by SEM and TEM showed
structural modifications on the surface of the ZnO rods after Ni impregnation. The reactivity of the
catalytic materials in the range of 200–500 ꢀC showed that the bimetallic sample had better catalytic
activity among all the catalysts studied. This finding could be associated to PtZn and NiZn alloys present
in this catalyst, which were identified by XRD and HRTEM analyses. Catalyst characterization by XRD after
the catalytic testing showed that the intermetallic PtZn phase was stable during the reaction in the Pt/
ZnO-rod sample. The cubic Ni0.75–Zn0.25 structure identified in the Ni/ZnO-rod sample was transformed
to Zn0.1–Ni0.9–O and metallic Ni phases, respectively. On the bimetallic PtNi/ZnO-rod sample, the cubic
Ni0.75–Zn0.25 structure remained, although the tetragonal NiZn structure is unstable and was destroyed
during the ASRM reaction and then a new phase of Ni0.7Pt0.3 emerged. The promotion effect of Pt and/
or Ni on the ZnO-rod was clearly shown.
Received 16th July 2020
Accepted 3rd November 2020
DOI: 10.1039/d0ra06181f
rsc.li/rsc-advances
collaborators10,14 studied the ASRM reaction on bimetallic Cu/
Ni/ZrO2 catalysts, observing higher methanol conversion and
Introduction
selectivity toward H2 in NicoreCushell and Cucore–Nishell nano-
particles than in CuNi alloy catalysts. They suggested that the
crystalline anisotropy of the Cu and/or Ni had a benecial
inuence on H2 selectivity and methanol conversion. Moraes
et al.,15,16 observed that the presence of surface Pt on Ni particles
minimized the formation of nickel carbide while increasing the
stability of the catalyst at low temperature, during the SRE
reaction. Cesar et al.,17 showed that the Ni catalyst was more
efficient than the bimetallic PtNi catalyst in the aqueous phase
reforming of ethylene glycol reaction, but the bimetallic PtNi
catalyst showed a greater conversion and hydrogen selectivity in
The worldwide drive to access diverse, reliable, renewable
energy sources and attain desirable sustainability within a cyclic
economic context, commonly involves the use of new green
fuels. This has certainly attracted great interest because it
envisions a hydrogen-based society in the forthcoming future.
Thus, in the process of achieving the benets from a hydrogen
economy, it is preferable to produce H2 from non-fossil
resources, such as water or biofuels (methanol or ethanol).
Methanol is a promising candidate due to its easy handling,
high H/C ratio and low price. Several ways to produce H2 from
methanol through different processes have been reported, such
as steam reforming-SRM,1–3 partial oxidation-POM4,5 or auto-
ꢀ
SR at 450 C. It was reported elsewhere that catalysts based on
thermal steam reforming-ASRM.6–14 R. Perez-Hernandez and
zinc oxide have high methanol conversion and selectivity
toward CO2 and H2 in the SRM reaction18 with low selectivity to
CO and methane.1 It was reported also that Pt1 and Au1 single-
´
´
a
´
ꢀ
´
´
Universidad Autonoma del Estado de Hidalgo, Area Academica de Ciencias de la
Tierra y Materiales, Carr. Pachuca – Tulancingo km. 4.5, C.P. 42184 Pachuca,
Hidalgo, Mexico. E-mail: raul.perez@inin.gob.mx
atom catalysts (SACs) supported on ZnO {1010} nanofacets
promote the electron transfer between Pt1 and Au1 atoms to
ꢀ
ZnO {1010} surfaces, causing that Pt1 and Au1 atoms become
bInstituto Nacional de Investigaciones Nucleares, Carr. Mexico-Toluca S/N, La
´
positively charged. This charge transfer is proposed to facilitate
the intermediaries adsorption reaction during SRM and thus
modify the reaction pathways.19 High selectivity for crotyl
alcohol (CH3CH]CHCH2OH) was observed when a PtZn alloy
was formed in the Pt/ZnO catalyst, during the crotonaldehyde
reaction.20 In the same way, Iwasa et al.,21 observed high
Marquesa, Ocoyoacac, Edo. de Mexico C. P. 52750, Mexico
c
´
Instituto de Estudios de la Energıa, Universidad del Istmo-Campus Tehuantepec,
Santo Domingo Tehuantepec, Oaxaca, C.P. 70760, Mexico
dDepto. de Quımica, Area de Catalisis, Universidad Autonoma Metropolitana-
´
Iztapalapa, Av. San Rafael Atlixco No. 189, Iztapalapa, CDMX, 09340, Mexico
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra06181f
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selectivity during methanol steam reforming on PdZn and PtZn porous structure promotes the deposit of the active phase
alloys that were formed by reduction of Pd/ZnO and Pt/ZnO onto the surface of the rod, avoiding the diffusion of the active
catalysts. Among metals supported on ZnO, Pd has performed phase inside the support. In this case, all the active phase is
better than other metals such as Co, Ni, Pt, Ru and Ir.21 Changes fully exposed for the catalytic reaction. This characteristic
in CO production were attributed to the presence of an inter- makes the ZnO rods-based catalyst suitable for this and other
metallic PdZn compound in the Pd/ZnO sample.22 When Pt was reactions. Another concomitant advantage is the fact that ZnO
supported on ZnO, methanol conversion was 28% lower than in rods have different exposed planes, which can contribute to
the Pd/ZnO sample, and high selectivity toward CO was reactivity in different ways, in contrast to those structures which
observed.23 The selectivity towards CO in the Pt/ZnO catalyst was do not have a specic shape as observed on Pt/ZnO-powders.
attributed to the PtZn alloy formation.23,24 The SRM reaction was Hexagonal ZnO prismatic rods with six smooth planes
studied in presence and absence of O2 on Pd/ZnO and Pt/ZnO throughout their length were observed by SEM technique.
catalysts composed by PdZn and PtZn alloys. In the same While, catalysts characterization showed rough surface along
study it was reported that the formation of a NiZn alloy was not the ZnO-rods only in Ni-based samples (ESI 1†).
favored. On this sample, the decomposition of methanol occurs
The XRD powder patterns of the synthesized bare ZnO rods,
in the steam reforming reaction. In the presence of oxygen, the and the ZnO rod-based catalysts (Pt/ZnO, Ni/ZnO and PtNi/ZnO
oxidation of methanol occurs predominantly and the hydrogen respectively), are presented in Fig. 1. Diffraction peaks of the
production is not improved, although the total conversion of hexagonal zincite crystalline structure (according to the JCPDS
methanol is increased.25 Wang et al.,26 claimed that particle size 04-003-2106) were identied in all samples. The strong and well-
of the active phase did not have inuence on the crotonalde- dened peaks reveal highly crystalline samples. No diffraction
hyde hydrogenation reaction on the Pt–ZnO catalyst, instead, peaks of metallic zinc or another impurity element were found
electronic effects are believed to play essential roles. Higher in the ZnO-rods (Fig. 1a). XRD powder patterns of reduced
reactivity and selectivity were observed in ZnNiAl catalysts on samples before the catalytic reaction, showed characteristic
the hydrogenolysis of glycerol reaction toward 1,2-propanediol diffraction peaks of ZnO–zincite structure. Additionally, low
due to the formation of Ni–Zn alloys.27 Catalysts were studied intensity diffraction peaks ca. 41ꢀ, 44ꢀ, 47ꢀ and 51ꢀ were
for many reactions using zinc oxide catalysts; a few reports observed. High resolution (HR) X-ray diffraction analysis from
using ZnO-rod samples on the ASRM reaction were reported
lately.1,12,13,28,29 Hu et al.,30 found that ZnO-rods exhibit a high
production rate of 1,2-propandiol in the reforming hydro-
genolysis on non-polar planes. In this way, the use of new
materials with catalytic applications is necessary in the industry
of high-quality fuels transportation. In this context, the devel-
opment of one-dimensional (1D) catalysts such as rods, with
large surface-to-volume ratio and dimension, are especially
sensitive to surface-interactions of the active phase and mole-
cules, to produce hydrogen. Based on all the above, the goal of
this work was to evaluate the catalytic properties of Pt and Ni
supported on ZnO rods in the ASRM reaction. The new 1D-rod
catalysts obtained were characterized by SEM, EDS, XRD,
TEM, HRTEM and TPR. The catalytic properties of the PtNi/
ZnO-rod catalysts were evaluated for the autothermal steam
reforming methanol reaction.
Results and discussion
Surface area of the ZnO rods and that of the commercial ZnO
before impregnation was ca. 1.5 and 1.2 m2 gꢁ1, respectively.1
These surface area values are close to those of the Ni/ZnO, PtNi/
ZnO and Pt/ZnO synthesized catalysts, which were 1.3, 1.0 and
1.2 m2 gꢁ1 respectively. Some catalysts with high specic
surface area have shown low catalytic activity, which has been
attributed to the low diffusivity of the reactants toward the
active sites inside the micropores of the sample. Also, high
surface area in the support promotes high dispersion of the
active phase in the catalyst, obtaining small particle sizes of this
phase, which increases the metal–support interaction,
Fig. 1 (a) XRD patterns of bare ZnO-rod, PtNi/ZnO-rod catalysts, (b)
decreasing the activity. Contrary to high surface area-catalysts,
the low surface area of the synthesized ZnO rods and its non- zinc oxide. Dash line-Zn and continuous line Pt-(in (a)).
high resolution XRD respectively (zone in the rectangle in (a)) *nickel
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39–55ꢀ (2 theta) was performed (Fig. 1b) in order to identify
these peaks and are reported in Table 1. It is clear that the
intensity of the diffraction peak at 43.56ꢀ is slightly higher in the
bimetallic sample than the one at 43.73ꢀ on the monometallic
Ni/ZnO-rod catalyst. This nding indicates that the double
thermal treatment during the catalyst preparation process,
promotes the segregation of Pt and Ni phases, and the NiZn or
PtZn alloys along the ZnO-rod were obtained. As well, the
tetragonal PtZn phase was favored instead of the cubic Ni0.75
Zn0.25 phase.
–
No evidence of diffraction peaks corresponding to PtO or
NiO phases were observed on the fresh PtNi/ZnO-rod sample.
This result indicates that a complete reduction of the active
phase of the catalyst has taken place, and most of the metallic
nickel and platinum form NiZn or PtZn alloys during the
reduction step. A similar behavior was reported with the Pd/ZnO
catalyst prepared either with commercial ZnO or ZnO rods as
supports.31 Iwasa et al.,25 reported the presence of the PdZn and
PtZn alloys obtained by the reduction of Pd/ZnO and Pt/ZnO,
but no NiZn alloy was reported in the Ni/ZnO catalyst.
Catalysts characterization aer the catalytic test are shown in
Fig. 1b. No changes in the XRD pattern of the Pt/ZnO-rod cata-
lyst was observed, indicating that the tetragonal PtZn phase is
stable under the reaction conditions used in this research. For Ni/
ZnO-rod sample, the cubic Ni0.75–Zn0.25 phase was transformed to
Zn0.1–Ni0.9–O (PDF 04-013-0897) and metallic cubic Ni (PDF 04-
010-6148) respectively. This result showed that the intermetallic
cubic Ni0.75–Zn0.25 phase present in the as-prepared Ni/ZnO-rod
sample, is unstable during the reaction. Analysis of the bime-
tallic PtNi/ZnO-rod catalyst aer the catalytic reaction shows
Ni0.75–Zn0.25 and Ni0.7Pt0.3 (PDF 04-018-6851) cubic phases.
TEM characterization of raw ZnO-rods, showed that ZnO
grows with star-like morphologies ca. 3 mm long (ESI 2†). TEM
image of Pt/ZnO rod catalyst is shown in Fig. 2. Nanoparticles
with sizes between 4–13 nm distributed along the ZnO rod were
observed (Fig. 2a). Inset in Fig. 2a shows an electron diffraction
Fig. 2 (a) TEM image of the Pt/ZnO-rod catalyst, inset electron
diffraction pattern in [1100] direction indicated that the rod grows in
ꢀ
the h0002i direction and particle size distribution, (b) HRTEM image of
the PtZn nanoparticle on the ZnO rods, inset image shows the FFT
pattern of the PtZn alloy, (c) Ni/ZnO-catalyst with large nanoparticles
distributed along the rod and (d) Ni/ZnO, electron diffraction pattern
and EDS chemical analysis from a nanoparticle (inset) respectively.
the XRD pattern of this sample. The PtZn alloy particle size
distribution (ESI 3†), indicating an average particle size of
6.5 nm. The analysis was based on the measurement of more
than 1300 particles. Hyman et al.,32 impregnated Pd on the ZnO
(1010) and ZnO (0001) facets of a single crystal, observing the
formation of a PdZn alloy on the ZnO (0001) polar facets. Zhang
et al.,31 observed a stable formation of the PdZn alloy on polar ZnO
facets in Pd/ZnO catalyst. In agreement with literature, we observed
nanoparticles of the intermetallic PtZn alloy distributed over the
surface of the polar and non-polar facets of the ZnO-rods support.
Ni/ZnO-rod catalyst showed nanoparticles with sizes ranging
from 20 to 120 nm along the ZnO-rod (Fig. 2c). However, these
nanoparticles apparently were released from the rod surface.
The EDS chemical analysis of these nanoparticles indicated the
presence of Ni, Zn, and O at 77.56, 19.47 and 2.97 wt%
respectively. No HRTEM images were taken from this sample
due to detection limit of the TEM equipment, and the magnetic
character of the sample that causes distortion in the image.
Instead, the electron diffraction pattern (EDP) of some nano-
particles was taken. Fig. 2d shows a Ni/ZnO-rod image. The EDP
in [011] zone axes (inset) taken from a nanoparticle of 120 nm in
diameter, indicates that it corresponds to the cubic crystalline
ꢀ
pattern of the rod in the [1100] zone axes, indicating that its
growth took place in the h0002i direction. The EDS chemical
analysis of the nanoparticles observed on the surface of the Pt/
ZnO rods catalyst, indicated the presence of Pt, Zn, and O at
4.27, 76.41 and 19.32 wt% respectively. Fig. 2b shows a HRTEM
image of a nanoparticle observed over a Pt/ZnO-rod. The FFT
(Fast Fourier Transform, inset image) analysis of this nano-
particle shows that it corresponds to the tetragonal phase of the
intermetallic PtZn alloy, with a ¼ 0.2858 nm and c ¼ 0.3465 nm
lattice parameters (04-016-2848), associated to the (110) and
ꢀ
(110) planes respectively, in the [001] zone axes. This agrees with
Table 1 High resolution (HR) X-ray diffraction analysis from 39–55ꢀ (2 theta) of the reduced samplesa
Sample
Phase 1
Phase 2
Phase 3
Ni/ZnO
Pt/ZnO
PtNi/ZnO
*(111) and (200) planes, (43.73ꢀ and 50.94ꢀ)
**(111) plane, (40.62ꢀ)
—
—
—
—
**(111) plane, (40.64ꢀ)
*(200) plane, (50.94ꢀ)
***(101) and (110) planes, (43.56ꢀ, 46.81ꢀ)
a
*cubic Ni0.75–Zn0.25 phase (PDF-04-004-4485), **tetragonal PtZn phase (04-016-2848), ***tetragonal NiZn (PDF-00-006-0672).
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phase of the intermetallic Ni0.75Zn0.25 alloy with lattice param- showing that a small section of the surface zinc oxide was
eter a ¼ 0.3582 nm (PDF 04-004-4485), which corresponds to reduced. The TPR prole of the Ni/ZnO rod sample was
ꢀ ꢀ
ꢀ
(111), (111) and (200) planes respectively. This nding is described by two hydrogen consumption peaks and a shoulder
consistent with the XRD pattern obtained from this sample.
at 190, 310 and 405 ꢀC, respectively. The rst hydrogen
PtNi/ZnO-rod catalyst showed the same morphology as the consumption peak was attributed to the reduction of large NiO
Ni/ZnO-rod sample. Irregular edges and detached material were particles that have weak interaction with the support, thus
observed when Pt and Ni were impregnated on ZnO-rods (Fig. 3a). making the nickel particles easy to be reduced. The second
The size of the nanoparticles distributed along the rod ranges from hydrogen consumption peak could be assigned to the reduction
3 to 140 nm. Fig. 3b shows a HRTEM image of a nanoparticle of of NiO in intimate contact with the support. In addition, the
10 nm in size. According to FFT and IFFT analysis, its lattice existence of bulk NiO, delays the H2 diffusion into the NiO
parameters correspond to the intermetallic PtZn phase (PDF 04- particles to complete its reduction. This causes the broadening
016-2848) with a ¼ 0.2858 nm and c ¼ 0.3465 nm. The identied of the reduction peak at high temperature,6,10,11,13,14,33 since large
planes of the tetragonal PtZn alloy are the (110) and (101) in the particles tend to be reduced slower than the small particles, due
ꢀ
[111]. The cubic NiZn phase (PDF 04-004-4485) on the bimetallic to their relatively smaller surface area exposed to hydrogen. The
catalyst was identied by EDP and EDS chemical analysis. Fig. 3c Pt/ZnO-rod sample showed two low intensity reduction peaks.
shows an intermetallic NiZn nanoparticle of about 80 nm in The rst peak is attributed to the reduction of Zn–PtOx oxidized
diameter. The EDP (inset) along the [011] zone axis indicates the species that can be formed by the metal–support interaction,34
ꢀ
(111) and (200) planes respectively.
the second reduction peak observed at 355 C is attributed to
The average size of the PtZn alloy and the NiZn particles on the surface reduction of the ZnO generated by the interaction
the PtNi/ZnO catalyst was 6.26 nm and 54.22 nm respectively, as with the Pt particles. The hydrogen consumption of the PtNi/
determined by the analysis of more than 2300 particles in the ZnO-rod sample showed a reduction peak at 425 ꢀC and is
sample by TEM (ESI 3b and c†). The PtZn alloy identied in the higher in comparison to the monometallic samples. This
bimetallic PtNi/ZnO-rod sample, showed (101) and (110) planes, nding could be attributable to sintering of the active phase,
while in the monometallic Pt/ZnO-rod catalyst, the exposed planes since this catalyst was exposed to a two stages-calcination.
were {110}. This nding might have inuenced the catalytic During the reduction of the PtNi/ZnO-rod catalyst, the PtO is
properties of the bimetallic sample. The NiZn nanoparticles reduced before the NiO (Fig. 4). During the TPR experiments,
showed the same particle size and zone axis as those nanoparticles the hydrogen is rst activated on Pt (or Ni) surface, then the
identied in the monometallic Ni/ZnO-rod sample, implying active hydrogen is transferred to the surface ZnO by the
a structural similarity between the two samples.
hydrogen spillover, reducing it. Aer this, most of the metallic
The TPR proles of calcined Pt/ZnO-rod, Ni/ZnO-rod, and Ni (or Pt) and Zn atoms react between them to form the inter-
PtNi/ZnO-rod catalytic materials are presented in Fig. 4. Bare metallic NiZn or PtZn alloys,35 eq. I. The H2 consumption (M/H2)
ZnO-rods without Ni, Pt or PtNi showed a hydrogen consump- was 0.83, 1.13 and 0.93 for Ni/ZnO, PtNi/ZnO and Pt/ZnO
tion before 100 ꢀC which increases as a function of temperature, respectively. Monometallic samples clearly exceed the H2
consumption in comparison with the bimetallic catalyst.
Pt (or Ni)/ZnO (s) + H2 / ZnPt (or NiZn) (s) + H2O. (l)
The overall activity as a function of the reaction temperature
is shown in Fig. 5. Methanol conversion of the support is
included for purposes of comparison. The methanol conversion
on the catalysts and the support starts at about 200 ꢀC. The bare
Fig. 3 (a) Nanoparticles distributed along the ZnO rod in the PtNi/
ZnO-rod catalyst, (b) a PtZn nanoparticle identified by HRTEM analysis
and (c) NiZn identified by EDP and EDS analysis.
Fig. 4 TPR profiles of the bare ZnO rod and catalysts.
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ꢀ
ZnO-rod and Ni/ZnO-rod catalyst showed low methanol
The bimetallic catalyst achieves 100% conversion at 400 C.
conversion at the beginning of the reaction. ZnO-rod support The better catalytic properties of this sample, could be associ-
reaches its maximum methanol conversion of nearly 76% at ated to different crystalline phases on the surface of the catalyst
500 ꢀC, whereas, the Ni/ZnO-rod achieved a 100% methanol (Table 1). This kind of active phase on the bimetallic PtNi/ZnO-
conversion at the end of the studied reaction temperature.
rod catalyst had a cooperative effect on the methanol conver-
The catalytic activity of bare ZnO-rod support was higher sion. However, the bimetallic sample aer the catalytic test,
than that reported by Koga et al.,36 and Mo et al.,37 where no showed that PtZn and NiZn phases were not present, instead,
catalytic activity was observed when they used ZnO whiskers the Ni0.7Pt0.3 phase merged and only the cubic Ni0.75–Zn0.25
and commercial ZnO powders for SRM and POM reaction, phase remained. This indicates that the tetragonal NiZn and
respectively. But the methanol conversion is close to that re- PtZn phase are not stable during the ASRM reaction and these
ported previously in ref. 1. Hu et al.,30 used ZnO with different were transformed into intermetallic Ni0.7Pt0.3 and ZnO phases,
morphologies as co-catalysts in combined reforming hydro- respectively. So, aer catalytic reaction, metallic Ni was obtained in
genolysis of glycerol, where they observed a positive effect in the the monometallic catalyst, while in the bimetallic sample Ni
1,2-propendiol production rate and its dependence on their interacted with Pt to obtain the Ni0.7Pt0.3 phase. Palma and
crystal habits. This is, more abundant nonpolar planes improve colleagues,39,40 prepared a NiPt/CeO2 catalyst by adding rst Ni to
selectivity to hydrogenolysis products and production rate of ceria, then the Pt was impregnated. They observed higher activity
1,2-propendiol. The methanol conversion aer 400 ꢀC on the and selectivity. However, carbon formation was observed on the
catalyst containing both Ni and ZnO-rod, is better than on the low temperature steam reforming (LTSR) of ethanol. The studied
ZnO-rod. The methanol conversion of Pt/ZnO-powder synthe- catalysts showed the following behavior to achieve maximum
sized with ZnO-Sigma-Aldrich powder (0.5 wt% of Pt) was methanol conversion: PtNi/ZnO-rod > Pt/ZnO-rod > Ni/ZnO-rod [
included to compare both Pt-based catalysts. The catalytic Pt/ZnO powder y ZnO-rod.
activity of Pt/ZnO-powder and Pt/ZnO-rod samples showed
The products obtained from the ASRM reaction on the Ni/
differences in methanol conversion. The methanol conversion ZnO-rod, Pt/ZnO-rod and PtNi/ZnO-rod samples were H2, CO
of Pt/ZnO-powder and Pt/ZnO-rod catalysts was 14% and 26% and CO2, Fig. 6a–c. The selectivity towards CO2 on the ASRM
respectively at the beginning of catalytic test. The Pt/ZnO- reaction decreases up to 22% near 350 ꢀC due to H2 and CO
powder sample exhibited a methanol conversion close to that formation, although beyond this temperature, it remains
of bare ZnO-rod. This nding clearly demonstrated the bene- constant in all catalysts. Iwasa et al.21 mentioned that Pt/ZnO
cial effect of the ZnO-rods as support for Pt instead of, the ZnO- catalysts exhibit higher activity and selectivity towards CO2
powder for the ASRM reaction. The Pt/ZnO-rod catalyst achieved production compared with the Pt/SiO2 sample in methanol
100% of methanol conversion at 450 ꢀC, while the monometallic reforming. It is clear that CO selectivity in the bimetallic catalyst
Pt/ZnO-powder only reached 72% conversion at 500 ꢀC. On these is slightly higher up to 300 ꢀC, above this temperature ZnO has
samples, the tetragonal PtZn phase identied by XRD technique a greater selectivity towards this product respect to other
remained aer the catalytic reaction on the Pt/ZnO-rod catalyst, samples. The Ni/ZnO-rod catalyst had high selectivity toward H2
while in the Pt/ZnO-powder, a mixture of the PtZnO and tetragonal and CO2 and is practically CO free (Fig. 6b). This suggests that
PtZn phase were identied by XRD technique (gure not shown). the interaction between the active phase and the ZnO-rod plays
This result might explain the better catalytic performance observed an important role in decreasing the CO concentration. In
in the Pt/ZnO-rod than in the Pt/ZnO-powder catalysts. That is, the addition, the ZnO support is better to produce low CO selectivity
nonpolar planes of the ZnO-rod, stabilized the PtZn phase, that are and high selectivity for H2 production, ESI 4.† The same
benecial for the catalytic reaction1 than on the polycrystalline behavior was observed on the Pd/ZnO catalyst which exhibited
commercial ZnO. Ito et al.,38 used different alloys with Pt supported high selectivity to CO2 with low CO selectivity. This was attrib-
on activated carbon, and found that the PtZn alloy had the best utable to formation of the PdZn alloy (Pd/Zn ¼ 1 : 1) via
catalytic behavior in methanol reforming.
reduction of Pd/ZnO in H2 at >350 ꢀC.41 Zhang et al.,31 observed
differences in the CO selectivity when Pd was supported on ZnO
rods or commercial ZnO, without any dominant facets. They
associated the higher CO selectivity at lower Pd loading to the
formation of Pd-rich PdxZny preferentially on ZnO nonpolar facets
in the SRM reaction. Low CO selectivity was observed on the Ag/
ZnO-rod catalysts during SMR reaction.1 On the Pd/ZrO2, Pd/TiO2
and Pd/ZrO2–TiO2 catalysts tested in the SRM reaction, the H2
selectivity was reported in the range of 60 to 70% and 25 to 35% for
CO, respectively. In our case, the selectivity towards CO is much
lower, being benecial since it means a lower production of this
gas. The monometallic Pt/ZnO-rod catalyst was slightly better in H2
production than the other catalysts tested, Fig. 6c. However, the
bimetallic sample showed better performance in methanol
conversion than the other samples. This nding is attributed to
the presence of the stable tetragonal PtZn phase of this sample, as
Fig. 5 Temperature dependence of ARM activity of catalysts.
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Fig. 7 MS profiles during temperature-programmed of ASRM reaction
of the PtNi/ZnO-rod catalyst.
real-time analysis of catalytic reaction. Since CH3OH and O2 had
the same molecular mass, we followed the CH3O instead of the
CH3OH mass signal in order to differentiate these compounds
during the TP-ASRM reaction.
Fig. 7 shows the CH3O, H2O, H2, CO2, CO and CH4 mass
signals monitored by MS of bimetallic PtNi/ZnO catalyst. The
CH3O mass signal prole showed a decrease, while the mass signal
of the other reaction products increased as reaction temperature
rose. This behavior agrees with methanol conversion and selec-
tivity as reported above, suggesting that the catalyst is stable
during the reaction, because no further changes in the outlet
signals on the real-time monitoring analysis by MS were observed.
So, if there were any intermediates during the reaction, they could
be released to the effluent and be determined by MS.
These species can furnish valuable insights into a catalytic
process. At the beginning of the reaction, the mass signal of CO2
increases slightly; this result indicates a complete combustion
of methanol. Then, the selectivity of the catalyst changes, the
CO production begins, and the H2 mass signal rises. The
maximum CO production resulted at 325 ꢀC, then it diminishes,
and the mass signal of CO2 grows, in addition, the CH4 mass
signal slightly increases. The following sequence of reactions
can be summarized during autothermal steam reforming
reaction over this PtNi/ZnO catalyst, according with mass
spectrometry (MS) results:
Fig. 6 Reaction products of ASRM reaction (a) CO2, (b) CO and (c) H2.
Equilibrium (red lines) distribution as a function of reaction tempera-
ture on PtNi/ZnO-rod catalysts respectively.
CH3OH + 3/2O2 / CO2 + 2H2O
CH3OH + H2O / CO2 + 3H2
CO2 + H2 ¼ H2O + CO (RWGSR)
CO + H2O / CO2 + H2 (WGSR)
2CO + 2H2 / CH4 + CO2
discussed in the XRD section. In addition, this PtZn phase was
more effective for H2 production, than the Niꢀ, Ni0.7Pt0.3 alloy and
NiZnO observed on the monometallic Ni/ZnO-rod and bimetallic
NiPt/ZnO samples respectively, as discussed above.
Temperature-programmed of ASRM reaction (TP – CH3OH +
H2O + O2)
Based on our GC results it can be seen that the PtNi/ZnO-rod
sample was the most suitable catalyst in terms of methanol
conversion. A detailed study was carried out with this catalyst.
Online mass spectrometry (MS) was used to monitor the prod-
Conclusions
ucts of the reaction from this catalyst on the autothermal steam Zinc oxide synthesized by a simple hydrothermal method,
reforming of methanol reaction. The steam sample was taken results in ower shaped structures that provide high interaction
immediately at the outlet of the reactor as a means of enabling with Pt and Ni atoms on the surface of the polar and non-polar
41320 | RSC Adv., 2020, 10, 41315–41323
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facets of the ZnO-rods support, increasing the bifunctional
The XRD patterns were acquired on a Bruker D8 Advance X-
behavior and thermal stability of samples. All catalysts showed ray diffractometer using Cu Ka radiation (l ¼ 0.15418 nm), with
high methanol conversion and selectivity to H2 and CO2, a step size of 0.02515, from 5ꢀ to 80ꢀ, 2q. ZnO-1D morphology
avoiding CO formation. Which is highly desirable in catalytic was analyzed by Field Emission Scanning Electron Microscopy
reactions for the production of H2 as fuel for PEM cells, where (FESEM) JEOL JSM – JSM7800FEG, equipped with an OXFORD
CO is highly damaging. It was observed that the sample that microprobe for chemical analysis, using energy dispersive X-ray
showed the highest methanol conversion on ASRM reaction was spectroscopy (EDS). HRTEM and TEM analyses of the samples
the PtNi/ZnO-rod catalyst, in comparison with the mono- were performed in a JEOL JEM-2010HT transmission electron
metallic Pt/ZnO-rod and Ni/ZnO-rod samples respectively. microscope with a spatial resolution of 0.19 nm point to point
However, H2 selectivity of Pt/ZnO-rod sample was much higher equipped with a ThermoNoran EDS microprobe for chemical
than on PtNi/ZnO-rod catalyst. This difference is attributed to analysis. The reducibility of the catalysts was analyzed by
the crystalline structure on each catalyst. In addition, the high Temperature-Programmed Reduction (TPR) experiments in an
degree of selectivity is attributed to a bifunctional behavior automatic multitask unit BELCAT-B (Bel Japan Inc.) equipped
between the carrier and the PtZn and NiZn alloys, where the with a Thermal Conductivity Detector (TCD) coupled to a mass
ZnO-rod provides adsorption sites for intermediate reactions, spectrometer (BELMass) with output to a computer. The fresh
whereas intermetallic PtZn and NiZn alloy particles facilitate sample (0.050 g) was placed in the “U” type reactor. For TPR
the mobility and hydrogen utilization. However, these inter- measurements, a H2 (5%)/Ar gas mixture was used (40
metallic phases that were observed in the Ni/ZnO-rod and PtNi/ mL minꢁꢁ1)1. The temperature was increased at a rate of
ZnO-rod samples, are unstable during the reaction and are 10 ꢀC min from room temperature to 600 ꢀC. The effluent gas
destroyed, yielding new phases. Our results demonstrate that from the reactor was passed through silica gel to remove water
the catalytic activity and selectivity are not related to the specic before measuring the amount of H2 consumed by the oxidized
surface area, but rather appears to be determined by the catalyst catalyst during the reduction by the TCD. Surface area was
composition and crystalline structure, indicating that the determined (in the same equipment) by N2 adsorption–
reaction is structure-sensitive.
desorption using a gas mixture 30% N2 in 70% He. The
measurement was carried out at ꢁ196 ꢀC (liquid nitrogen
temperature).
The catalytic properties of samples were determined in the
ASRM reaction. The analysis was carried out using a xed ow
Experimental
The ZnO rods were synthesized by the hydrothermal method. A quartz reactor (8 mm i.d.) with dynamic ow in a RIG-100 ISR
0.2 M of Zn(NO3)2$6H2O and 1 M of NaOH solutions were Inc. automatic multitask unit. 0.1 g of the catalyst diluted in
prepared separately. At room temperature, these solutions were 0.5 g of SiC was used for the analysis at a temperature range of
mixed and stirred for 10 min. The obtained mixture was 200 to 500 ꢀC, at atmospheric pressure, with steps of 50 ꢀC and
transferred into a Teon-lined stainless-steel autoclave (1 L) and a stabilization time of 7 h in each temperature. A K-type ther-
heated for 6 h at 100 ꢀC. Then, the autoclave was cooled to room mocouple in contact with the catalyst bed to measure and
temperature and the obtained precipitate was ltered and control the catalyst bed temperature. The sample was acti-
washed several times with deionized water until a neutral pH vated prior the test using a H2 ow (60 mL minꢁ1) at 500 C,
ꢀ
was reached. Next, the powder was dried at 90 ꢀC for 24 h. The 1 h. The sample was brought up to the reaction temperature
solid was calcined at 600 ꢀC for 3 h using a heating ramp of in He and the reaction mixture was introduced. For the
ꢁ1
catalytic test, 30 mL minꢁ1 of the O2 (5%)–He mixture was fed
ꢀ
10 C min
.
The impregnation of the active phase was performed using into a stainless-steel saturator that contained methanol. 30
the incipient impregnation method. Pt (0.5 wt%) was impreg- mL minꢁ1 of He was fed to the stainless-steel saturator that
nated using Pt-acetylacetonate (Sigma-Aldrich, 97%) in an contained water and 140 mL minꢁ1 of He was used as
acetone solution. Another Pt/ZnO-powder was prepared using a diluent. A total ow rate of 200 mL minꢁ1 (38 197 hꢁ1) was
commercial ZnO (Sigma-Aldrich) to compare the effect of the maintained. The amount of methanol, water and oxygen in
ZnO-rod versus the ZnO-powder. Ni (5 wt%) was impregnated the feed was 1.0, 3.4 and 0.1 mol hꢁ1 respectively. The gases
using a Ni(NO3)2$6H2O (Meyer, purity 98%) in ethanol solution. from the quartz reactor were examined in a gas chromato-
ꢀ
˚
The remaining liquid was removed at 50 C with constant stir- graph equipped with a TCD and two columns system (5 A
ring. The obtained solid were calcined at 400 C for 1 h. The molecular sieve operated at 45 ꢀC and Porapack Q, working at
ꢀ
bimetallic PtNi/ZnO-rod catalyst was synthesized by successive 110 ꢀC) and two injectors controlled by Clarity soware V
impregnations with Pt (0.5 wt%) and Ni (5 wt%) respectively. 7.2.0.73 (ESI 5†). The following equations were employed to
Briey, an aliquot of the Ni/ZnO-rod sample previously calculate the activity during methanol conversion (eqn (1))
synthesized and calcined was impregnated with
a Pt- and selectivity for CO2, CO, and H2 (eqn (2)–(4) respectively).
acetylacetonateꢀsolution in acetone. The remaining liquid was The effluent gases from the catalytic reactor (RIG-100) of the
removed at 50 C under constant stirring. The resulting solid ASRM reaction were monitored in real-time from 200 to
was calcined at 400 ꢀC for 1 h. All the samples were reduced at 500 ꢀC by MS, using a BELMass benchtop-type quadrupole
500 C in H2 stream (40 mL minꢁ1) 1 h before the character- mass spectrometer.
ꢀ
ization, except for the TPR analysis.
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for hydrogen production over Cu/CeO2-ZrO2 catalysts,
Energy Mater., 2008, 3, 152–157.
Cin ꢁ Cout
X ð%Þ ¼
ꢂ 100
(1)
ꢂ 100 (2)
ꢂ 100 (3)
ꢂ 100 (4)
Cin
´
´
´
´
9 R. Perez-Hernandez, A. Gutierrez-Martınez, J. Palacios,
nCO2
out
´
´
M. Vega-Hernandez and V. Rodrıguez-Lugo, Hydrogen
production by oxidative steam reforming of methanol over
Ni/CeO2-ZrO2 catalysts, Int. J. Hydrogen Energy, 2011, 36,
6601–6608.
SCO ð%Þ ¼
2
nCO2 out þ nCOout þ nCH4 out þ nH2
out
out
out
nCOout
SCO ð%Þ ¼
nCO2 out þ nCOout þ nCH4 out þ nH2
´
10 P. Lopez, et al., Hydrogen production from oxidative steam
reforming of methanol: Effect of the Cu and Ni
impregnation on ZrO2 and their molecular simulation
studies, Int. J. Hydrogen Energy, 2012, 37, 9018–9027.
nH2
out
SH ð%Þ ¼
2
nCO2 out þ nCOout þ nCH4 out þ nH2
C is the initial (in) and nal (out, at Ti – temperature i) methanol
concentration respectively and n is the number of moles
produced.
11 R. Perez-Hernandez, D. Mendoza-Anaya, A. Gutierrez and
A. Gomez-Cortes, Catalytic Steam Reforming of Methanol
to Produce Hydrogen on Supported Metal Catalysts, in
Hydrogen Energy - Challenges and Perspectives, ed. D. Minic,
InTech, 2012, vol. 149–174, DOI: 10.5772/49965.
Conflicts of interest
´
´
12 R. Perez-Hernandez, et al., Ag nanowires as precursors to
synthesize novel Ag-CeO2 nanotubes for H2 production by
methanol reforming, Catal. Today, 2013, 212, 225–231.
There are no conicts to declare.
´
´
´
13 R. Perez-Hernandez, G. Mondragon-Galicia, A. Allende
Maravilla and J. Palacios, Nano-dimensional CeO2
nanorods for high Ni loading catalysts: H2 production by
autothermal steam reforming of methanol reaction, Phys.
Chem. Chem. Phys., 2013, 15, 12702.
Acknowledgements
This work has been supported by the CONACYT-SENER (grant
No. 226151) and ININ (grant CA-607). The authors thanks Dr M.
A. Romero-Romo for his valuable support in the revision of the
manuscript.
´
´
´
´
14 R. Perez-Hernandez, A. Gutierrez-Martınez, M. E. Espinosa-
Pesqueira, M. L. Estanislao and J. Palacios, Effect of the
bimetallic Ni/Cu loading on the ZrO2 support for H2
production in the autothermal steam reforming of
methanol, Catal. Today, 2015, 250, 166–172.
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