Hydrogen production by methane decomposition on Pt/γ-alumina doped with neodymium catalysts and its kinetic study

Article abstract of DOI:10.1016/j.cattod.2018.05.024

This paper presents results for the catalytic decomposition of methane using Pt/γ-Al₂O₃ and Pt/γ-Al₂O₃-Nd₂O₃ catalysts for the hydrogen production. All catalysts were prepared by wet impregnation using Nd(NO₃)₃?6H₂O and H₂ PtCl₆.6H₂O as precursor salts. The concentrations of the catalysts were Pt, 1.0 wtpercent, and Nd, 1.0 and 10.0 wtpercent. The reaction was carried out from 400 to 750 °C. All the catalysts reduced methane decomposition temperature by 400–750 °C relative to noncatalytic thermal decomposition, which is carried out at 1300 °C. The Pt catalysts supported on γ-alumina doped with neodymium exhibited higher activity than Pt supported on alumina alone. All catalysts showed high activity and selectivity at 750 °C, with conversions around 57–80percentvol and hydrogen productions of 0.29, 0.27 and 0.19 mmoles of H₂/min*g_cat for PtANd10, PtANd1 and PtA catalysts respectively also were observed in the output stream, traces of C₂H₄ y C₂H₆ (less than 1percent) and unconverted methane. This work includes a kinetic study of methane cracking, for this purpose two experiments were carried out, the first one consisted of varying the amount of catalyst, keeping the feed flow constant, in the second experiment the flow rate was varied, keeping the mass of the catalyst constant. The activation energy of methane cracking is estimated at 35.5, 37.9 and 33 kJ/mol for the Pt/A, Pt/ANd1 and Pt/ANd10 catalysts, respectively. The characterization was done by X-ray diffraction, N₂ adsorption-desorption, H₂-TPR, FTIR of CO adsorption, FTIR of Pyridine adsorption, XPS, HRTEM, catalytic activity and TPO (Temperature programed Oxidation) analysis after reaction.

Full text of DOI:10.1016/j.cattod.2018.05.024

Accepted Manuscript
Title: Hydrogen production by methane decomposition on
Pt/␥-alumina doped with neodymium catalysts and its kinetic
study
´
Authors: M. Caballero, G. Del Angel, I. Rangel-Vazquez, L.
Huerta
PII:
S0920-5861(18)30592-3
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2018.05.024
CATTOD 11454
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
16-12-2017
5-5-2018
10-5-2018
´
Please cite this article as: Caballero M, Del Angel G, Rangel-Vazquez I,
Huerta L, Hydrogen production by methane decomposition on Pt/␥-alumina
doped with neodymium catalysts and its kinetic study, Catalysis Today (2010),
https://doi.org/10.1016/j.cattod.2018.05.024
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Hydrogen production by methane decomposition on Pt/γ-alumina doped with
neodymium catalysts and its kinetic study
M. Caballero1*, G. Del Angel1*, I. Rangel-Vázquez1, L. Huerta2
1Universidad Autónoma Metropolitana-Unidad Iztapalapa. Departamento de Química, Área
de Catálisis, Av. San Rafael Atlixco No. 186, C.P. 09340, A.P 55-534. México, Ciudad de
México.
2Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México,
Apartado Postal 70-360, 04510 México, D.F., México
* Corresponding author: phone number and e-mail marcabdi@yahoo.com.mx,
gdam@xanum.uam.mx, 52-55-58044668.
Graphical abstract
HIGHLIGHTS
 Methane decomposition on Pt/Nd2O3--Al2O3 catalysts showed 100% selectivity
to H2.
 Interaction between Pt and -Al2O3-Nd2O310%, favors the catalytic activity.
1

 The PtANd10 catalyst showed high conversion and yield.
 Weak interaction cause a spillover of the C from the platinum to the support
 Methane cracking is one kinetic order and the Ea is around 33-38 kJ/mol.
ABSTRACT:
This paper presents results for the catalytic decomposition of methane using Pt/-Al2O3 and
Pt/-Al2O3-Nd2O3 catalysts for the hydrogen production. All catalysts were prepared by wet
impregnation using Nd(NO3)36H2O and H2PtCl6.6H2O as precursor salts. The
concentrations of the catalysts were Pt, 1.0wt%, and Nd, 1.0 and 10.0 wt%. The reaction was
carried out from 400 to 750°C.
All the catalysts reduced methane decomposition temperature by 400-750°C relative to
noncatalytic thermal decomposition, which is carried out at 1300°C. the Pt catalysts
supported on γ-alumina doped with neodymium exhibited higher activity than Pt supported
on alumina alone. All catalysts showed high activity and selectivity at 750°C, with
conversions around 57-80 %vol and hydrogen productions of 0.29, 0.27 and 0.19 mmoles of
H2/min*gcat for PtANd10, PtANd1 and PtA catalyst respectively also were observed in the
output stream, traces of C2H4 y C2H6 (less than 1%) and unconverted methane.
This work includes a kinetic study of methane cracking, for this purpose two experiments
were carried out, the first one consisted of varying the amount of catalyst, keeping the feed
flow constant, in the second experiment the flow rate was varied, keeping the mass of the
catalyst constant. The activation energy of methane cracking is estimated at 35.5, 37.9 and
33 kJ/mol for the Pt/A, Pt/ANd1 and Pt/ANd10 catalysts, respectively. The characterization
2

was done by X-ray diffraction, N2 adsorption-desorption, H2-TPR, FTIR of CO adsorption,
FTIR of Pyridine adsorption, XPS, HRTEM, catalytic activity and TPO (Temperature
programed Oxidation) analysis after reaction.
Keywords: Rare earth, TPO determination, XPS study, particle size, acidity, activation
energy.
1.
INTRODUCTION
The current need for energy is provided by the combustion of non-renewable energy sources,
i.e. fossil fuels, and is associated with the release of large amounts of greenhouse gases
(GHGs), especially carbon dioxide (CO2) and other harmful gases released into the
atmosphere.
The use of hydrogen as a fuel has been the subject of several investigations in recent years.
Extensive studies have been conducted for the development of safe, economical, and efficient
hydrogen production to meet the global energy demand [1]. The main reason behind such
interest is the significant decrease in greenhouse gas emission.
Another incentive to use hydrogen as a fuel is the higher efficiency achieved from its
utilization in fuel cells compared to the traditional methods of electrical energy production
from fossil fuels [2,3]. Although different substances can be used for producing hydrogen,
the most attractive and accessible one in terms of economics, environment and technological
feasibility for the near to medium term future is methane. This gas can be obtained from two
major sources, namely natural gas, which is one of the most abundant fossil fuels, and biogas
[4].
3

Hydrogen does not occur naturally in nature, but is combined with other elements such as
oxygen and carbon, ie water or hydrocarbons, so these substances must be decomposed /
reformed to obtain H2 [3].
Currently, the main process for producing hydrogen is steam reforming of natural gas [5].
An attractive alternative to this method, which has recently received increased attention, is
the decomposition of methane [6]. One of the advantages of this method over steam
reforming is the removal of carbon dioxide from the by-products, which simplifies the
process by eliminating the requirements for the separation of carbon dioxide from hydrogen
and its subsequent sequestration. An overall comparison between these two processes shows
that the decomposition of methane is a potential alternative to steam reforming and
investigation about this process is valuable for the hydrogen economy [7]. Steam reforming
is a multiple stage process.
In addition to steam reforming, partial oxidation is also used to generate hydrogen from fossil
fuels [8], but the produced hydrogen is still mixed with CO and CO2, which again needs a
complicated separation process as in the steam reforming case.
Increasing demand for CO-free hydrogen has increased interest in the direct catalytic
cracking of natural gas [9], described by Equation 1. The two reaction products are hydrogen
and carbon, the latter being essentially in the form of filamentous carbon or carbon nanotubes
[10].
CH4
C(s) + 2H2
ΔH° = 75.6 KJ/mol … (1)
As a result of methane cracking, only hydrogen is produced as a gaseous product in a mixture
with unreacted methane. Separation of methane and hydrogen can be achieved easily by
absorption or membrane separation to produce a stream of 99% by volume hydrogen, which
4

is much simpler than the need for further complicated separation processes that deal with
CO2 or CO [8,11].
Unlike the steam reforming process, the catalytic decomposition of methane does not include
water gas shift and preferential oxidation of CO, which considerably simplifies the process
and may reduce the hydrogen production costs [11]. The energy required for methane
catalytic cracking is nearly one half of that required for steam reforming per mole of methane
decomposed (for steam reforming ΔH°298 = +253.2 kJ/mol, for methane cracking ΔH°298
=
+74.8 kJ/mol) [9,11–13]. The energy requirement is 37.4 kJ/mol H2, in methane catalytic
cracking compared to 63.3 kJ/mol H2 in the steam reforming process [13]. In addition to the
lower energy demand for methane catalytic cracking compared to steam reforming, there is
no need for additional energy for steam generation or gas treatment.
The heat requirement for catalytic cracking can be covered by burning ~15-20% of the
hydrogen produced, which further reduces CO2 emissions, because CO2 is not produced in
this reaction [6].
Thermal methane cracking is not feasible at moderate temperatures. To achieve a reasonable
yield, a temperature higher than 1200°C is required [14]. An active catalyst is required to
obtain high methane conversion at reasonable temperatures: 500-700ºC for nickel-based
catalysts, 700-950ºC for iron-based catalysts, 850-950ºC for carbon-based catalysts, and 700-
1000ºC for Co, Pd, Pt, Cr, Ru, Mo, W catalysts [14,15].
Supported metal catalysts can be used to catalytically decompose hydrocarbons to produce
hydrogen at more moderate temperatures. Nickel is particularly active for hydrocarbon
cracking, especially for methane but present fast deactivation [16]. Cobalt and iron can also
be used to catalyze methane cracking but their carbon/active site capacities are much lower
than that of nickel, with additional problems in the case of cobalt, which are associated with
5

its higher cost and toxicity [17]. Metal catalysts are usually deposited on supports such as
SiO2 or Al2O3 and the performance of the catalyst depends to some extent on the combination
of metal and support [18].
Rare earth oxides have various fields of application, for example, in catalysis they are
recognized as the most active and selective catalysts for the coupling of methane oxidation
to form higher hydrocarbon products (mainly ethane and ethylene) [19]. Nd2O3 has high
basicity and contains medium and strong basic sites, this characteristic result in a unique
catalytic performance when used as active components or as supports of catalyst.
In rare earth‐noble metal catalysts, rare earth materials also play an important role improving
the activity, selectivity, and catalyst stability; increasing the dispersion of the active metal,
and decreasing the amount of noble metal to reduce catalyst costs; also to improve poison
tolerance, prolong the catalyst life and enhancing the catalyst coke‐resistance [20,21].
In the literature was observed that the methane decomposition reaction showed wide ranges
of activation energy (Ea) for different catalysts. Holmen et al. [22] reported an Ea value for
methane decomposition, using a tubular reactor without a catalyst, of 370 kJ/mol over the
temperature range of 1773−2273 K at 0.1 atm of pressure of methane. Steinberg et. al. [23]
reported an Ea value of 131 kJ/mol for methane decomposition over the temperature range
of 973−1173 K and around 28−56 atm of pressure, which was found to be substantially lower
than the value for methane decomposition without a catalyst. Kuvshinov et al. [24] observed
an Ea value of 97 kJ/mol for methane decomposition with a mixture of hydrogen over nickel
catalysts. The catalyst function is to reduce the activation energy required for methane
decomposition, leading to lower operating temperatures.
6

The purpose of the present study was to investigate the influence the addition of Nd (1 and
10 wt%) on the Pt/Al2O3 catalysts in the activity, selectivity and kinetic parameters for the
CH4 decomposition. The reaction was carried out at temperatures from 400 to 750°C.
The characterization by X-ray diffraction, N2 adsorption-desorption, H2-TPR, FTIR of CO
adsorption, FTIR Pyridine of adsorption, XPS, HRTEM, Catalytic activity and TPO analysis
after reaction, was performed.
2.
MATERIAL AND METHODS
2.1
Experimental Procedure. Supports and catalysts preparation
-Al2O3 support was prepared from Boehmita Catapal B (CONDEA, high purity 99,999%,
74% AlOOH, 26% de H2O). Firstly the Boehmite was dried to 120° C for 12 hours, then the
solid was calcined in air flow of 60 mL/min for 24 h using a ramp of temperature from 25°C
to 650°C. The -Al2O3-Nd2O3 (loaded with 1 and 10 wt% neodymium) mixed oxides, were
prepared by wet impregnation of the Boehmite with the necessary quantity of
Nd(NO3)3·6H2O (Strem Chemicals, 99.99 %), the mixture was maintained in stirring for 3h.
Then, the solids were dried in an oven to 120°C for 12 h, after that, samples were calcined at
650°C in airflow for 24 h.
The Pt catalysts were prepared by wet impregnation of the -Al2O3 and -Al2O3-Nd2O3
supports with the necessary quantity of H2PtCl6.6H2O (Aldrich 99.9 %) to obtain 1wt% of
Pt. The solids were left in stirring for 3 hours, and then, the water is evaporated using a
vacuum evaporator bath. Subsequently the solids were dried in an oven at 120°C, for 12
hours. The catalysts were calcined at 500°C under airflow for 5 h, finally reduced in H2 flow
at 500°C for 5 h. The Pt real percentage on the catalysts was obtained by atomic absorption
7

technique. Catalysts were labeled as: PtANdX, where: Platinum, as Pt, alumina as A,
neodymium as Nd, X is the concentration of Neodymium in wt%, AR to the catalysts after
reaction and ST stability test.
2.2. Catalysts Characterization
The X-ray diffraction patterns of the supports were obtained using a D-2 phaser desktop X-
ray powder diffractometer from Bruker AXS equipped with a Cu Kα radiation anode in
Bragg-Brentano geometry. The detection was carried out using a LYNXEYETM detector,
which is a linear-type. Intensity data was measured in continuous mode through the 2θ ranges
between 10° to 70° with a 2θ step of 0.02°.
The characterization of the textural properties of the catalysts was obtained by the adsorption
of N2 at 77 K. The BET surface area of the catalysts was measured using a Quantachrome
Multistation Autosorb 3B analyzer. Nitrogen was used as the measuring gas at -196°C.
Before adsorption, the sample (100 mg) was outgassed at 300ºC for 24 h. The specific surface
areas were calculated with the BET equation and the mean pore size by the BJH method.
The TPR determinations were carried out in a Chembet-3000 (Quantachrome Co) apparatus
using 0.2 g of catalyst by means of the following protocol: samples were heated at 300°C
under nitrogen flow for 30 min. Then, the samples were cooled dawn to room temperature
and a mixed gas flow (5% H2/95% N2) was passed through the cell. The TPR profiles were
recorded by heating the sample from room temperature up to 500°C at a rate of 10°C/min.
X-ray photoelectron spectroscopy analyses were performed in an ultra-high vacuum (UHV)
system Scanning XPS microprobe PHI 5000 VersaProbe II. with a Al K X-ray source (hʋ=
8

1486.6 eV ), and a MCD analyzer. The surface of the samples were etched for 5 min with 1
kV Ar+ at 0.04 µA mm-2. The XPS spectra were obtained at 45º to the normal surface in the
constant pass energy mode (CAE), E0 = 100 and 10 eV for survey surface and high-resolution
narrow scan, respectively. The binding energies (BE) were referenced to the Al 2p peak, the
BE of which was fixed at 73.5 eV for -Al2O3. In this case, the position of the C 1s line was
284.8 eV. The intensities and integration of the peaks for each of the elements were estimated
by subtracting the Shirley type background and peaks adjustment performed using a mixture
of Lorentzian/Gaussian curves. The XPS spectra were fitted using the SDP v 4:1 program.
Transmission electron microscopy (TEM) study was performed on an electron microscope
of field emission that operates at 200 kV. The microscope is equipped with a field of Schottky
type emission cannon and a configuration of ultra-high resolution (UHR) (Cs = 0.5 mm; Cc
1.1 mm; point to point resolution, 0.19 nm). The samples were also characterized by high
angle annular dark field detector (HAADF) used when the microscope became operate in
STEM mode known as “Z contrast”. The samples were powdered in an agate mortar and
were suspended in ethanol at room temperature and dispersed in an ultrasonic bath for two
minutes, then one drop of the solution was deposited on a carbon copper grid. It is important
to note that the samples were transported in a plastic bag filled with argon in order to prevent
the contamination of O2. The mean Pt crystallite size for all catalysts were determined by ds
= Σ nidi3/nidi2 equation, where ds, is the mean crystallite size of Pt, ni, the number of Pt
crystallite diameter di, measured directly on the micrographs, at least 150 Pt particles were
counted.
The high Resolution Transmission Electron Microscopy was determined in a HRTEM-2010F
JEOL equipped with a field emission source and an acceleration of 200 kV. The sample was
9

grinded and dispersed on a holey carbon film grid of 300 meshes, and the high-resolution
images were obtained from the transmission of electrons that go through the floating sample.
The measure of the crystallographic planes was done by using a Digital Micrograph program
by the Gatan Software team.
FT-IR of CO adsorption spectra was determined at room temperature by using a FTIR Nicolet
170-SX apparatus, with a resolution of 2 cm-1. The sample pressed in thin wafers were placed
in a Pyrex glass cell, equipped with CaF2 windows, coupled to a vacuum system and gas lines
supplied. The samples were maintained under vacuum (10-3 Torr) at 400°C for 30 min. Then,
the cell was cooled to room temperature and the CO (Praxair UHP) admission of 20 Torr was
carried out. The CO excess was evacuated during 30 min, after the CO adsorbed FTIR spectra
were recorded.
FT-IR of pyridine adsorption spectra was determined at room temperature by using a FTIR
Nicolet 170-SX apparatus, with a resolution of 2 cm-1. The catalyst is sprayed in an agate
mortar and is pressed in thin wafers, which is placed in a cell, which is coupled to vacuum
lines and gas lines. It is then subjected to a vacuum treatment for 30 min. (1x10-3 Torr) at
400°C. The temperature is then lowered to 25°C and pyridine added at a pressure of 20 Torr.
Subsequently, excess pyridine is evacuated under vacuum for 30 min to obtain the FTIR
spectra.
The TPO study after reaction was performed to determine the amount of carbon deposited in
the catalyst after reaction. The TPO determinations were carried out in a CHEMBET-3000
apparatus using a thermal conductivity detector (TCD), and 0.1 g of catalyst. In these
experiments, the coke formed during the reactions was oxidized. A flow rate of 10 mL min−1
10

of the 5% O2/95 % He mixture was passed on the sample at a heating rate of 10◦C min−1.
Then, the spectra were recorded from room temperature to 750°C and keeping it at this
temperature for one hour. The equipment was calibrated by sending pulses of O2 diluted in
He
2.3. Catalytic tests.
The activity determinations were carried out in a fixed bed quartz micro-reactor (i.d. = 1.2
cm and length = 56 cm) at atmospheric pressure and catalyst load of 0.05 g. The reaction
temperatures varied from 400 to 700°C, the increase of temperature was on steps of 100ºC
and the last at 750ºC with a rate of 5ºC min-1. Experiments consisted in a previous thermal
treatment to the catalysts with flowing nitrogen for 15 min at room temperature. Then, a pure
CH4 (99.999 %) feed steam (2 mL min-1) was introduced into the reactor. The inlet and outlet
effluents were monitored on line gas chromatograph, Shimadzu GC-2014 equipped with 6
packed columns and TCD and FID detectors. Which allows the measurement of inorganic
gases (H2, CO and CO2) and organic hydrocarbons as CH4, C2H4, and C2H6. The
identification of reaction products were accomplished by the quantification of them, which
were carried out with calibration curves of products reaction using the appropriate software.
The methane conversion was calculated as the amount of methane converted throughout the
experiment over the amount admitted during that step. The selectivity was the amount of
each product H2, C2H4, or C2H6 collected at reactor outlet over the methane converted. The
yield was calculated with (% Conversion * % Selectivity H2).
2.4. kinetic Study.
A kinetic study of the decomposition reaction of methane in the temperature range of 400 -
750°C under atmospheric pressure was also carried out. A fixed bed quartz reactor was used
11

to investigate the kinetics and to evaluate the parameters of the model, for this purpose two
experiments were carried out, the first consisted of varying the amount of catalyst, keeping
the feed flow constant, in the second experiment the flow rate was varied, keeping the mass
of the catalyst constant.
3. Results and Discussion
3.1 Catalysts Characterization
The XRD patterns of the Pt catalysts after reduction at 500°C for 5 h are showed in Fig. 1.
The catalysts show the characteristic peaks of -alumina phase (JCPDS PDF 056-0457
Quality: Rietveld). The lines associated with PtO (PDF 42-0866) were not observed in this
sample, neither signals corresponding to Pt crystals (JCPDS 04-0802) were detected in any
of the catalysts studied. This is explained by a low Pt concentration and small Pt particles,
which could not be detected by the equipment.
Also no peaks for Nd(OH)3 (PDF 06-0601) nor Nd2O3 (1 and 10 wt%) (JCPDS PDF 01-079-
9858 Quality: Star (*)) were detected. The diffraction pattern of PtANd1 catalyst does not
show notable difference in respect to that observed in PtA catalyst. However, in PtANd10
catalyst a modification of the diffraction pattern is observed. The alumina peaks in that
catalyst shows lower intensity that in the PtA and PtANd1 catalysts, which indicate low
crystallinity of the PtANd10 catalyst. The presence of neodymium at 10 wt. % inhibited the
growth of the alumina crystals [25]. In Fig. 1 is plotted an inset where is observed the peaks
of the alumina having a maxima at around 67° in 2. The increase in the broad is related with
the reduction of the crystallite size as the concentration of Nd2O3 increases.
12

The type of isotherms of adsorption-desorption presented by the catalysts are shown in Figure
1, are classified according to the IUPAC as type IV [26], which is characteristic of
mesoporous materials. On the other hand, the hysteresis type for the catalysts is type H1 [27].
The BET specific surface areas of the supports and fresh catalysts are reported in Table 1.
The increased surface area in the ANd1 support relative to -alumina can be attributed to
the neodymium which completely blocks the pores of -alumina and for ANd10 support
neodymium is not fully dispersed and partially blocks the pores of the -alumina (see table
1). The incorporation of platinum (1wt. % nominal) to the aforementioned materials, the area
decreases in all cases respect to the supports. However, the pore volume decreases by about
5% for catalysts with neodymium; this indicates that platinum contributes to the reduction in
size of the pore in a consistent manner for both materials (see inset figure 1). The pore volume
and pore diameter of PtA catalyst (Table 1) remained constant with respect to bare γ-alumina
support (0.54 nm and 12.3 nm respectively). That means that platinum is over the γ-alumina
surface, therefore there are no changes in the distribution and volume of pore.
The concentrations of Pt on the catalysts were determined by atomic absorption and reported
in Table 1. The nominal concentration of Pt was 1.0wt%, the real concentrations are 0.51,
0.60 and 0.83 wt%, and these values can be considered valid, in the error range.
The TPR profile of PtA, PtANd1 and PtANd10 catalysts are showed in Fig.2. At least three
temperature peak profiles were detected. The first one (from low to higher temperatures),
corresponding to the reduction of PtO2 species [28]. The second one arises from the reduction
of oxy- or hydroxychlorinated Pt species [28], and the third one, it has been assigned to the
bulk phase of the PtOx and to highly dispersed particles with different degree of interaction
on the support, as previously reported [29,30].
13

The PtA and PtANd1 present similar broad temperature profile of 156 and 157°C for the first
signal while 302 and 300°C respectively for the second. Whereas for the third, the
temperatures signal specie are 434 and 432°C respectively, in both cases the temperature is
analogous although the peak in PtA is slightly more intense than in PtANd1 catalyst. In this
last, the hydrogen consumption is smaller. The effect of the addition of 1wt% of Nd is notable
mainly in the low amount of hydrogen for the reduction of the PtOx specie, which represent
a higher size of particle and can be conclude that in these two catalysts, the interaction Pt-
Al2O3-N2O3 is high. On PtANd10 catalyst, the temperature for the first reduction specie is
155°C in agreement to the PtA and PtANd1 catalysts. However the temperature of reduction
for the second and third specie are shifted at lower temperatures 268 and 428°C than the
aforementioned catalysts, indicating that in PtANd10 catalyst the interaction of Pt in surface
is weak and mainly with neodymium oxide, Pt-Nd2O3, which concentration is in great
proportion on the surface, favoring the reduction of the Pt species.
The XPS technique was used to determine the binding energies (Table 2) of the chemical
species of the elements and the proportions on the surface of the fresh catalysts, after reaction
(PtANd10AR) and stability test (PtANd10ST). To determine the stability test of the PtANd10
catalyst, the sample, was maintained in time on stream for 10 h at 750°C.
Literature [31,32] reports an overlap of the Al 2p core level of the support with the most
intense lines of the active phase of platinum Pt 4f core level. Therefore, the study of platinum
in this region is complicated for the allocation of the currents and the binding energies.
Therefore, another platinum signal (Pt 4d core level) is chosen for the study. This lower-
intensity line no longer overlap with another spectral line of another compound.
In Figure 3, the high-resolution XPS spectra of platinum corresponding to the core level
spectra of Pt 4d5/2 in the range of 305.00-340.00 eV are shown. In the deconvolution of the
14

platinum spectra, it was observed two components in the fresh samples, (Table 2). The first
component with a binding energy of 313.40 eV is correlated to reduced platinum Pt°,
according to that reported by several authors [31,33,34]. The following binding energy at
315.60 eV is associated with the PtO specie [33,35]. An explanation of the presence of the
PtO species is given by the authors [33] that by reducing the catalyst at 500°C, they observe
that the platinum maintains a + character, which indicates that there is a strong interaction
between the metal and the support. It has been reported that by impregnating platinum and
calcined it in different supports such as Al2O3, CeO2 and La2O3 [31] the proportions of the
reduced and oxidized platinum species were affected. This causes the platinum has a positive
character with respect to the support (electron deficiency) and was correlated by the charge
transfer of the platinum to the support.
Although the catalysts of PtA and PtANdX were reduced to 500°C for 5 hours in hydrogen
flow, platinum particles in the oxidized state prevailed (Table 2). However, by incorporating
+
different proportions of neodymium (1 and 10 wt%) the ratio of species Pt°/Pt decreases
and the PtO species is found in greater proportion in PtANd1 with respect to the PtA
reference, this effect was correlated by the charge transfer of the metal to the support as
mentioned above.
In figure 3, the high resolution spectra of core level Nd 4p3/2 are shown where two spectral
lines were observed, whose binding energies are placed in 229.00 and 224.50 eV
corresponding to Nd2O3 and Nd (OH)3 respectively. The addition of 1% of neodymium to the
+
PtANd1 catalyst present a Pt° specie diminution, favoring the oxidized Pt specie (44%).
Higher concentration of neodymium (10%) in the PtANd10 catalyst, the Pt° species increases
at 72.7%. The Nd2O3 and Nd(OH)3 are species present in both PtANd1 and PtANd10
15

catalysts with Nd2O3 percentages of 84.1% and 90.6% respectively and the rest corresponds
to Nd(OH)3 specie. The high abundance of Nd2O3 on the surface of the catalyst, with the
highest concentration of this compound (PtANd10) favors the presence of Pt° species.
The increase of the platinum oxidized species on the PtANd1 catalyst after the treatment in
H2 flow at 500 ° C can be attributed to the transfer of charge of platinum to neodymium,
keeping platinum in a deficient state of electrons [36]. This change observed can be attributed
to a weakening of the bonding force Nd-O in the presence of Pt, which leads to a high
mobility of reticular oxygen in PtANd1. While in the catalyst PtANd10 with greater amount
of Nd2O3, this specie present greater stability, decreasing the transfer of change between Pt
and Nd [36]. That explain the return of the original value of Pt° specie in the PtANd10
catalyst
The oxygen spectrum (O 1s) shows for the PtA catalyst a binding energy of 530.74 eV,
however, when adding neodymium at different proportions this energy is shifted to lower
energies (530.53 – 530.47 eV) this could be due to that neodymium and produces in the
alumina structural and textural changes favoring a greater interaction between both
compounds [37].
The histograms of the distribution of particle size determined by TEM (Fig. 4), for PtA and
PtANdX fresh catalysts. The PtA catalyst was observed platinum highly dispersed, with
mean particle diameter around 1.6 nm, the dispersion corresponding to this value was 71%.
The catalysts with neodymium (1 and 10 wt%) had an important fraction of particles with
mean diameter greater than 2 nm, with dispersions of 54 and 48% respectively, the mean Pt
particle sizes measured by FTIR-CO, were confirmed with the values obtained by TEM, Z
contrast (Table 1).
16

The HRTEM image of the PtANd10 catalyst is shown in Figure 5, which was examined by
several zones with boxes a), b), c), and d). Later these zones were worked using the FFT-
HRTEM for each of the boxes. In the right part of Figure 5 the FFT of the examined areas
showed several interplanar distances, these were assigned to different crystalline phases in
the following way. The distance 1.31 and 2.28 Å correspond to the crystallographic planes
(442) and (222) belonging to -alumina with card number JCPDS 00-056-0457 Quality:
Rietveld. The distance 1.96 Å corresponds to the plane (200) of metallic platinum with card
number JCPDS 04-0802. The distances 1.06 and 1.91 Å correspond to the planes (213) and
(110) correspond to the numbered Nd2O3 of card JCPDS 01-079-9858 Quality: Star (*).
Finally the distances 1.30 and 1.84 Å were found, these correspond to the planes (302) and
(300) of the crystalline Nd(OH)3 phase with PDF card number 06-0601 and the distances
1.34 and 2.17 Å corresponding to the planes (202) and (110) of the crystalline PtO phase
with PDF card number 42-0866.
The adsorption of CO as a test molecule is a technique by which Pt catalysts supported on γ-
alumina can be characterized by FTIR. The deconvolution of FTIR-CO spectra for platinum
monometallic catalysts (1 wt% Pt) supported on γ-Al2O3 and γ-Al2O3 modified with 1 and
10 wt% neodymium are shown in Figure 6. For the FTIR-CO spectrum of PtA, PtANd1 and
PtANd10 different bands were detected between 1930-1700 cm-1. This region was assigned
to the multiple bond of the CO molecule adsorbed on platinum [38], other authors consider
the weak band in 1850 cm-1 corresponds to the bridged link Pt0-CO-Pt0 [39] and the 1784
cm-1 band corresponds to CO adsorbed is triply bounded to Pt [40].
The region located between 2010-1990 cm-1, is correlated with the linearly bound CO species
in metallic platinum in an isolated corner (Pt0-CO isolated corners). The next region in the
range of 2080-2040 cm-1, is attributed to the linear carbonyl bond of CO with metallic
17

platinum [41]. Other authors correlate it with adsorption of linear CO in the platinum metal
particle in a corner (Pt0-CO corners) in the range of 2066-2041 cm-1 and the region latter
localed between 2162-2158 cm-1 is correlated to the oxidized platinum species (Pt2+-CO)
which occurs at 2134 cm-1 [42].
The Figure 6 shows slight displacement in the CO absorbed species in the Pt metallic, which
can be due to the presence of neodymium. In addition, it was observed that with the increase
in the neodymium load, there was an increase in the IR intensity of the linear CO adsorption
band that could be related to the Pt crystallite size increase (Table 1) said catalysts present
suitable sites for a greater adsorption of methane.
The results obtained by FT-IR spectroscopy-pyridine adsorption, allowed elucidating the
amount of Brönsted acid and Lewis acid sites present in the samples. It has been proposed
that the metal–support interaction is associated with the acidity/alkalinity of the support
[43,44]. With increasing support alkalinity, electron transfer between the support oxygen
atoms and the nearby metal particles can increase the electron density on the metal particles.
The peaks at 1448 and 1614 cm−1 showed the existence of Lewis acid sites, while no
adsorption band characteristic of Brönsted acid sites at 1540 cm−1 was observed. Considering
the high basicity of the neodymium respect to the alumina, it can be inferred that the decrease
of the acidic sites in the materials with neodymium can be due to the high dispersion of this
on the surface of the γ-Al2O3 blocking the acid sites (Table 1). In the case of Al2O3 doped
with Nd2O3 (Figure not showed) the intensity bands associated with Lewis acidity were much
lower, this result indicated that Nd2O3 decreased the number of Lewis acid sites. The fact
that the acidity decreases could indicate that the additive (neodymium), selects very acid sites
in the support to be deposited as it has mentioned in its study Oudet et. al. [45].
18

The samples after reaction were characterized by Fourier-pyridine transform spectroscopy;
the results show that the Lewis-type acidity is maintained. However, the total acidity of the
fresh PtA catalyst (258 μmol/gcat) decreases considerably after the reaction (92 μmol/gcat) and
for the fresh PtANd10 material its value is 213 μmol/gcat and after reaction gives 74 μmol/gcat.
In both cases the acidity decreases to one third of the original value, this may be due to the
fact that coke forms is anchored in the active sites of the metal.
3.2. Activity, Selectivity and coke deposited.
Methane dehydrogenation was conducted at temperatures of 400, 500, 600, 700 and 750ºC
with undiluted methane flow and a space velocity of 6 L h-1g-1. Figure 7 shows the profiles
of methane conversion as function of temperature. It can be seen that the conversion of
methane increases as the temperature increases from 400ºC to a maximum conversion at
750ºC. The PtA catalyst present a conversion of 64 % at 750°C and activity per site (TOF)
of 34.2 h-1 with the crystallite size of 1.6 nm, this catalyst shows a production of hydrogen
of 0.19 mmolH2/min*gcat. While the PtANd1 catalyst exhibits a behavior similar to the PtA
catalyst, with a conversions of 56.8% a 750°C, and TOF of 33.6 h-1 with the crystallite size
of 2.1 nm, this catalyst shows the highest molar flow of 0.27 mmolH2/min*gcat. The PtANd10
catalyst, presents the highest production at 750°C, with a conversion of 80% of methane
(Table 3), crystallite size of 2.4 nm, TOF of 38.6 h-1 and one molar flow of 0.29 mmol
H2/min*gcat (Figure 8).
The presence of a large proportion of oxidized species of Pt affected the catalytic activity of
the PtA and PtANd1 catalysts due to a strong interaction between the metal and the support.
On the contrary, in the PtANd10 catalyst an increase on the catalytic activity was observed
due to the presence of neodymium, which favors a lower metal support interaction, leading
19

to a rapid desorption of the reaction products. As it was observed in the characterization by
H2-TPR and XPS of these catalysts.
On the other hand, the determination of the particle size for the PtANd10_AR catalyst, by
TEM using the contrast Z method, showed that the particles size increased notably from 2.4
nm (fresh catalyst), to 10.5 nm (after reaction) a sintering of the catalyst was observed by a
temperature effect. The particle size raise, then the structure and morphology of the Pt
particles changed at high temperature (750°C). This is explained by lower interaction of the
Pt nanoparticles on Al2O3-Nd2O3 support, which favors the sintering of Pt particles leading
to formation of large Pt particles at 750°C.
Comparing the conversion at 500°C (50.5 vol%) and at 750°C (80.0 vol%) is observed that
large Pt particles plays an important role in CH4 conversion. The values of activity and
selectivity (see Table 3) increase with the increase of particle size, then the reaction of
methane dehydrogenation was sensitive to particle size [46].
In previous studies, it was analyzed that chlorine is retained in the alumina support surface
in Pt catalysts with low metal loading after reduction under hydrogen flow at 400 °C, and
that the amount of residue remaining in the catalyst after reduction depends largely on the
choice of support material. There is a good consensus that platinum does not coordinate
directly with chlorine in the post-reduction state.
Chloride ions from the precursor at a short distance from the active metal particles hinder the
adsorptive and catalytic properties of the metal. For this reason, catalyst precursor has an
important effect in catalyst activity, e.g. in the hydrogenation and oxidation of hydrocarbons
[47]. In our case, the catalysts were calcined in air at 500°C under airflow for 5 h and finally
reduced in H2 flow at 500°C for 5 h, after the treatment the amount of residual chlorine was
20

found around 0.3% at. By analysis of EDS and XPS, said percentage remained constant
before and after the reaction. In our case, we detected that the chlorine residual improve the
activity for the cracking of methane reaction.
The carbon contents of the used Pt catalysts obtained by TPO analysis are showed in Figure.
9, noble metal catalysts are more tolerant toward coking than conventional nickel catalysts.
Higher activity together with the better resistance toward coking extends catalyst lifetime,
making the use of noble metals feasible [48].
Temperature-programmed oxidation (TPO) has been widely used to determine the amount
of carbon deposited on the catalyst after reaction [49,50]. In the present case it was
investigated the differences in the reactivity of coke formed on Pt catalysts by through
oxidation of the coke on the catalyst surface followed by TPO. The temperature at which
oxidation occurs is determined by the location and chemical nature of the coke species. Since
metals catalyze oxidation, the location is represented by the distance from the active metal.
Moreover, the more graphitic forms of coke oxidize at high temperatures [51]. Different
forms of coke can thus be classified with respect to their chemical nature and location on the
catalyst [41,50].
The area under a TPO profile (Figure 9) represents, the total amount of CO2 formed in the
course of one TPO experiment. The total amount of CO2 was used to calculate the relative
amount of coke deposited on the catalysts during the reaction (Table 4).
In the TPO profiles, a shift to lower oxidation temperatures is observed, as the neodymium
content was increased. In the TPO profiles for the coke formed in the reaction until 750°C,
coke oxidation begins around 80°C, coke that oxidized at temperatures  200 °C is assumed
to be located on the active metal since metals catalyze oxidation. The CO2 maxima peaks
21

located between 500-600°C, suggests that, the coke was located on the metal or metal-
support interface, and that the nature of the carbonaceous species was uniform, therefore, the
deposits are polyaromatic and their composition depends very much on the reactant. Their
formation involves hydrogen transfer (acid catalysts) and dehydrogenation; and coke that
oxidized at high temperatures (˃700°C) is considered to be more graphitic and located on the
support rather than on the active metal [41,52]. The coke formation was greater on the PtA
catalyst with 4.77%, followed by PtANd10 catalyst with 3.51%, and the catalyst with less
coke formation was PtANd1 catalyst with 2.84%, due to the low conversion of methane in
this catalyst. The PtANd1 catalyst showed lower amount of coke deposited, since the
conversion of methane was also lower at all temperatures of reaction with respect to the PtA
and PtNd10 catalysts. Methane cracking and hydrogen yield increased significantly with
increasing reaction temperature from 400 to 750 ° C. The maximum hydrogen yield of 79.9%,
63.9% and 56.7% was achieved on the catalysts PtANd10, PtA and PtANd1 respectively, at
750 ° C, showing the same tendency at all reaction temperatures.
During the temperature-programmed oxidation, CO2 was detected, and the temperature for
CO2 evolution over the PtANd10 catalyst is much lower than those over the PtANd1 and PtA
catalysts. This suggested that the carbon species deposited on the PtANd10 catalyst is more
active than those on the PtANd1 and PtA catalysts and that were more near the metal [52].
Table 2 e Total amount of coke after ATR of n-hexadecane
3.3. Kinetics of the reaction, pseudo-order of reaction and activation energies
22

The rate of decomposition of methane was strongly dependent on the temperature, the
methane flow and the mass of the catalyst. By increasing the flow rate in the fixed bed, the
contact efficiency between methane and catalyst particles decreased, and therefore methane
conversion decreased [53,54].
Assuming a plug flow for the gaseous phase flowing through the catalyst bed, integration of
the differential mass balance conducted on the catalyst yielded the following equation:
푋
푊
푑푥
퐶퐻4
푐푎푡
퐶퐻4
=
… (2)
∫
0
퐹
퐶퐻40
푟
퐶퐻4
Substitution of 푟퐶퐻4 = 퐾푛 * 퐶퐶푛퐻4 y 퐹퐶퐻40= 푣0* 퐶퐶퐻40 in equation (2)
푋
푊
푑푋
퐶퐻4
퐶퐻4
푐푎푡
=
… (3)
∫
푛
0
푣 ∗퐶
퐾 ∗ 퐶
푛
0
퐶퐻40
퐶퐻4
In addition, for gas phase expansion reaction the relationship between 퐶퐶퐻4 and initial
methane concentration 퐶퐶퐻40 is defined as:
1−푋
퐶퐻4
퐶퐶퐻4 = 퐶퐶퐻40
…. (4)
퐶퐻4
1+휀푋
The result of the equation is as follows:
푛
1−푛
[1+휀푋
]
4
푋
푊
퐶
퐶퐻40
퐶퐻
퐶퐻
푐푎푡
4
=
{
} 푑푋퐶퐻
]
퐶퐻
4
… (5)
∫
4
0
푣
퐾
푛
[1−푋
0
where 푣0 is the methane flow in L/min, n is the order of the methane decomposition reaction
and Kn is the kinetic constant (L(gcat min)-1(mol L-1)1-n and ε is the expansion factor equal to
1 according to the equation (1).
Equation (5) was used to determine the pseudo-order n and the kinetic constant 퐾푛 by plotting
푊
푐푎푡
of
versus the value of the integral shown in the right-hand side of equation (5). These
푣
0
were carried out to the experimental data reported in Table 5 for experiments with variation
of catalyst mass (Fig. 10) and for experiments with variation in methane feed flow (Fig. 11),
separately. For that purpose, various values of n were tried (n = 0, 0.5, 1, 1.5 and 2) and best
23

fits were obtained with n = 1 (correlation coefficient > 0.99 and >0.97) for experiments with
variation of catalyst mass (Fig. 10) and (correlation coefficient > 0.99 and >0.98) for
experiments with variation in methane feed flow (Fig. 11). The value of kn was calculated
from the slope 4.1 - 4.75 (L/gcat min).
In order to find kn at temperatures of 400, 500, 600, 700 and 750°C, the value of the integral
of equation (5) was found according to the data of each experiment and then the value of kn
was calculated.
The activation energy was calculated by using an Arrhenius plot of ln k against 1/T (Fig. 12)
and the slope of the linear plot is (-Ea/R). The activation energy, the value of the pre-
exponential factor and the Arrhenius equation reads in the Table 6:
4. Conclusions
A larger metal particle size, lower acidity and a surface rich on neodymium oxide of
PtANd10 catalyst favored a high catalytic activity.
A surface rich on neodymium oxide leads to a weak surface interaction between Pt and
Nd2O3--Al2O3 support, improving the catalytic activity due to an easy desorption of the
products. That weak interaction can also cause a spillover of the carbon obtained after
reaction from the platinum to the support.
It is interesting to note that the selectivity to hydrogen in all the catalysts was 100%.
The apparent kinetics shows that the reaction order is 1 and activation energy is 33 kJ/mol
for PtANd10 catalyst, 37.9 kJ/mol for PtANd1 catalyst and 35.5 kJ/mol for PtA catalyst;
concluding that the best catalyst was PtANd10.
24

Acknowledgements
We acknowledge to CONACYT for the support provided to the project SEP-CONACYT
CB-2013-01-220191 and Marina Caballero Diaz thanks to the CONACYT for the grant
awarded No 387137/255851.
25

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