NiO diluted in high surface area TiO2 as an efficient catalyst for the oxidative dehydrogenation of ethane

Article abstract of DOI:10.1016/j.apcata.2017.02.012

Catalysts consisting of NiO diluted in high surface area TiO₂can be as efficient in the oxidative dehydrogenation of ethane as the most selective NiO-promoted catalysts reported previously in the literature. By selecting the titania matrix and

Full text of DOI:10.1016/j.apcata.2017.02.012

Accepted Manuscript
Title: NiO diluted in high surface area TiO2 as efficient
catalysts for the oxidative dehydrogenation of ethane
Authors: R. Sanchis, D. Delgado, S. Agouram, M.D. Soriano,
´
´
´
´
M.I. Vazquez, E. Rodrıguez-Castellon, B. Solsona, J.M.Lopez
Nieto
PII:
S0926-860X(17)30067-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.02.012
APCATA 16148
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
17-10-2016
20-1-2017
13-2-2017
Please cite this article as: R.Sanchis, D.Delgado, S.Agouram, M.D.Soriano,
´
´
´
´
M.I.Vazquez, E.Rodrıguez-Castellon, B.Solsona, J.M.Lopez Nieto, NiO diluted in high
surface area TiO2 as efficient catalysts for the oxidative dehydrogenation of ethane,
Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2017.02.012
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NiO diluted in high surface area TiO2 as efficient catalysts for the
oxidative dehydrogenation of ethane
1
2
3
R. Sanchis , D. Delgado , S. Agouram , M.D. Soriano 2,4, M.I. Vázquez 1, E.
Rodríguez-Castellón 4, B. Solsona 1*, J.M. López Nieto 2*
1 Department of Chemical Engineering, Universitat de València, Av. Universitat s/n,
46100 Burjassot-Valencia, Spain. E-mail: Benjamin.Solsona@uv.es
2 Instituto de Tecnología Química, Universitat Politècnica de València-Consejo Superior
de Investigaciones Científicas, Avenida de los Naranjos s/n, 46022 Valencia, Spain
E-mail: jmlopez@itq.upv.es
3 Department of Applied Physics and Electromagnetism, Universitat de València,
C/Dr. Moliner 50, 46100 Burjassot, Valencia, Spain
4 Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de
Málaga, 29071 Málaga, Spain
1

Graphical abstract
95
85
75
OPTIMUM
5-10 theoretical layers of NiO
High surface area titania (50-85 m2g- 1 )
65
55
0.00
0.02
0.04
0.06
0.08
0.10
gNiO m-2
2

Highlights
-
-
-
-
-
NiO diluted in high surface area TiO2 is highly efficient in ODH of ethane.
Optimal nickel oxide loading depends on the surface area of the support.
Intermediate nickel oxide loadings are the optimal.
High yield and productivity to ethylene and low overoxidation.
Yield to ethylene higher than 40%.
Abstract
Catalysts consisting of NiO diluted in high surface area TiO2 can be as efficient in the
oxidative dehydrogenation of ethane as the most selective NiO-promoted catalysts
reported previously in the literature. By selecting the titania matrix and the NiO loading,
yields to ethylene over 40% have been obtained. In the present article, three different
titanium oxides (TiO2) have been employed as supports or diluters of nickel oxide and
have been tested in the oxidative dehydrogenation of ethane to ethylene. All TiO2 used
present anatase as the main crystalline phase and different surface areas of 11, 55 and 85
m2 g-1. It has been observed that by selecting an appropriate nickel loading and the
titanium oxide extremely high selectivity towards ethylene can be obtained. Thus,
nickel oxide supported on TiO2 with high surface areas (i.e. 55 and 85 m2 g-1) have
resulted to give the best catalytic performance although the optimal nickel loading is
different for each case. The optimal catalyst has been obtained for NiO-loadings up to
5-10 theoretical monolayers regardless of the TiO2 employed. Free TiO2 is inactive
whereas unsupported NiO is active and unselective (forming mainly carbon dioxide)
and, therefore, unmodified NiO particles have to be avoided in order to obtain the
optimal catalytic performance. The use of low surface area titania (11 m2 g-1) have led
to the lowest selectivity to olefin due to the presence of an excess of free NiO particles.
3

Keywords: Ethylene; Oxidative dehydrogenation of ethane (ODH); Surface coverage;
Titanium oxide; Nickel oxide.
4

1. INTRODUCTION
Ethylene, the main building block used in petrochemistry, has not been affected by the
international crisis of the last decade. In fact, a continuous increase of the global
capacity for ethylene production has been observed [1,2] However, steam cracking,
which is the current industrial process to produce ethylene, presents some drawbacks,
such as low energetic efficiency [3,4] and low selectivity to the desired compounds.
This has meant that many research groups have directed their efforts to find other
alternatives. Among them we can highlight the oxidative dehydrogenation (ODH) of
ethane, an exothermic reaction with just a few by-products easy to be separated [5,6,7,8]
At the moment the ODH of ethane has not been implemented in industry. However,
there are promising results at lab- and pilot plant scale which suggest that this process
can be developed in the mid-term. These results have been mainly obtained using either
promoted nickel oxide [9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24] or Mo-V-O-
based catalysts [25,26,27,28,29,30,31]
In the case of nickel oxide, the addition of promoters with certain characteristics is
necessary
to
achieve
high
yields
to
ethylene
[9,10,11,12,13,14,15,1,6,17,18,19,20,21,22,23,24,[32]In this way, the presence of
vacancies in NiO crystals have been proposed to be responsible for changes in the
catalytic performance as it has been theoretically demonstrated by McFarland and Metiu
[32].
Otherwise, in unpromoted NiO catalyst, the formation of CO2 is remarkable and the
selectivity to ethylene low [9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,32]If basic
dopants (elements with low valence, e.g. Na+, K+, Cs+) are incorporated as metal oxides,
the valence state of the host element pushes towards its highest accessible valence. In
5

contrast, acidic dopants (elements with high valence state, e.g., W6+, Nb5+, Sn4+) will
push the valence state of the host metal towards its lowest available valence. Thus, the
addition of alkali elements increases the concentration of Ni3+ species while W6+, Nb5+
or Sn4+ tends to maintain Ni2+ species [17]. By analyzing both the physico-chemical and
the catalytic properties of NiO-based catalysts, it can be seen that non-stoichiometric
nickel sites, Ni3+, seems to be involved in non-selective reactions (COx formation), due
to the stabilization of electrophilic oxygen species. Conversely, the elimination of these
species means an enhancement in the ethylene formation[16,20,21,22]
Therefore, the nature of the promoter (with moderate acid characteristics and/or high
oxidation state) has to be controlled in order to decrease the concentration of unselective
non-stoichiometric Ni3+ species. In this way, Nb5+ and Sn4+ seem to be the most
promising dopants.
Another possibility to improve the catalytic performance is by dispersing or diluting
nickel oxide on a support. In this way a better dispersion of the active phase could be
achieved when enhancing the number of available active sites. Among supports/diluters
reported in the literature for NiO in ODH of ethane we can highlight MgO [12]zeolites
[33,34] and Al2O3 [35,36,37,38,39,40]
The appropriate loading of diluted/supported NiO catalysts is different to that of
promoted catalysts. In this regard NiO-loading must be remarkably lower in the case of
diluted/supported catalysts, in order to achieve a Ni-diluter/promoter interaction similar
to that obtained in NiO-promoted catalysts (usually prepared by co-precipitation)
[40,41]. Unfortunately the catalytic performance of supported catalysts does not seem
to be as good as that of promoted catalysts. Recently it has been reported that porous
clays heterostructures can be efficient supports for NiO in the oxidative
dehydrogenation of ethane, especially if these clays present columns made of titania
6

[41]. Nanosized Ti–Ni–O catalysts prepared by a modified sol–gel method have also
been investigated [42]. Different compositions were studied with the optimal
composition showing yields to ethylene not higher than 12%. Recently, Zhu et al. [9]
used a sol-gel method to synthesize NiO based catalysts promoted with W or Ti (with
low amount of promoter, i.e. 5 wt% of metal promoter) observing high yields to
ethylene, remarkably higher than those achieved over unpromoted NiO. Thus the
incorporation of a second metal oxide to NiO drastically changes the structure and the
catalytic properties.
In the present work several titanium oxides have been chosen as supports/diluters for
nickel oxide. TiO2 has been selected because it presents acidic characteristics and an
oxidation state higher than +3 (i.e. Ti4+) which have been proposed to be beneficial for
an optimal catalytic performance. The effect of the TiO2 support, especially of the
surface area, and the nickel oxide loading has been studied in detail.
2. EXPERIMENTAL
2.1. Preparation of catalysts
Three different titanium oxides have been used as diluters of nickel oxide: i) Pure
anatase (Sigma-Aldrich), with low surface area (ca. 10 m2 g-1), and called as A; ii) TiO2
(Degussa P25), mainly anatase (with low proportion of rutile), which presents medium
surface area of ca. 55 m2 g-1, and called as B; and iii) Nanocrystalline anatase (Chempur
APS 5 nm/ssa), with a surface area of 150 m2 g-1, was calcined in air at 500ºC, then
decreasing its surface area to 85 m2 g-1. This titania has been called C. Different
amounts of nickel oxide have been deposited on each titanium oxide (Table 1).
Diluted NiO/TiO2 catalysts were prepared through the evaporation at 60 ºC of a stirred
ethanolic solution of nickel nitrate, Ni(NO3)2•6H2O (Sigma-Aldrich) and oxalic acid
7

(oxalic acid/Ni molar ratio of 3) to which the corresponding titanium oxide was added.
The solids obtained were dried overnight at 120 ºC and finally calcined for 2 h at 500
ºC. The catalysts have been named as xNi-Y, x being the theoretical NiO wt% loading
and Y the diluter employed (i.e. A, B or C).
2.2. Characterization of the catalysts
Surface areas were determined by multi-point N2 adsorption at −196 °C. The data were
treated in accordance with the BET method.
Powder X-ray diffraction (XRD) was used to identify the crystalline phases present in
the catalysts. An Enraf Nonius FR590 sealed tube diffractometer, with a monochromatic
CuKα1 source operating at 40 kV and 30 mA was used.
Morphological, compositional and structural analysis the samples were performed by
high resolution transmission electron microscopy (HRTEM) with a field emission gun
TECNAI G2 F20 microscope operating at 200 kV, having the capabilities of selected
area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDX). The
elemental composition and distribution of nickel have been determined using EDX-
mapping. In order to prepare the TEM specimens, powdered samples were sonicated in
absolute ethanol for several minutes. A drop of the resulting suspension was deposited
onto a holey carbon film supported on a copper grid, which was subsequently dried.
Temperature-programmed reduction (H2-TPR) was carried out in a Micromeritics
Autochem 2910 equipped with a TCD detector, in which the reducing gas was 10% H2
in Ar (total flow rate of 50 ml min−1). The temperature range explored was from room
temperature to 800 °C. The heating rate was maintained at 10 °C min−1.
The chemical characterisation of the surface of the samples was performed by XPS
analysis. A Physical Electronics spectrometer (PHI 5700) with X-ray Mg K radiation
8

(300W, 15 kV, 1253.6 eV) as the excitation source was used for high-resolution record.
Measurements were performed by a concentric hemispherical analyser operating in the
constant pass energy mode at 29.35 eV, using a 720 m diameter analysis area. Under
these conditions, the Au 4f7/2 line was recorded with 1.16 eV FWHM at a binding
energy of 84.0 eV. The spectrometer energy scale was calibrated using Cu 2p3/2, Ag
3d5/2 and Au 4f7/2 photoelectron lines at 932.7, 368.3 and 84.0 eV, respectively. Charge
referencing was done against adventitious carbon at 284.8 eV. Each spectral region was
scanned performing several sweeps, until a good signal to noise ratio was observed. The
pressure in the analysis chamber was maintained lower than 5×10−6 Pa. PHI ACCESS
ESCA-V6.0 F software package was used for acquisition and data analysis. A Shirley-
type background was subtracted from the signals. Recorded spectra were always fitted
using Gauss–Lorentz curves and following the methodology described in detail
elsewhere [23,43]. Atomic concentration percentages of the characteristic elements of
the surfaces were determined taking into account the corresponding area sensitivity
factor [43] for the different measured spectral regions.
Diffuse reflectance UV–Vis spectra were collected on a UV-2600 Shimadzu equipped
with a “Praying Mantis” attachment from Harrick. The sample cell was equipped with a
heater unit, a thermocouple, and a gas flow system for in situ measurements. The
samples were dehydrated in situ in dry air at 150ºC for 30 min.
2.3. Catalytic test for ethane oxydehydrogenation
The catalytic tests in ethane oxidation were carried out in a tubular isothermal flow
reactor in the 300–500 ºC temperature range. The feed mainly corresponds to a mixture
consisting of C2/O2/He with a molar ratio of 3/1/26, although C2/O2/He molar ratio of
3/3/24 was also used in a few cases. Both the catalyst amounts loaded and the total
9

flows used were largely varied in order to get different contact times and finally to
achieve different ethane conversions at a given reaction temperature. Samples were
introduced in the reactor diluted with silicon carbide in order to keep a constant volume
in the catalytic bed. Reactant and products were analyzed by gas chromatography using
two packed columns: (i) molecular sieve 5 Å (2.5 m); and (ii) Porapak Q (3 m). Blank
runs showed no conversion in the range of reaction temperatures employed.
3. RESULTS AND DISCUSSION
3.1. Catalytic results in the oxidative dehydrogenation of ethane
NiO supported or diluted in TiO2 has been tested in the oxidation of ethane and the
catalytic results obtained indicate that these catalysts are very efficient for the
production of ethylene although the catalytic results depend on both the nature of the
titania used and the loading of nickel oxide (Table 1). In all cases, ethylene has been the
main reaction product and CO2 was also detected as minority. However, neither CO nor
other partially oxygenated compounds were identified.
In this article we want to study not only the influence of the diluter but also the
influence of the nickel loading on the catalytic performance. Thus, Figure 1a shows the
selectivity to ethylene obtained at 450 ºC for an ethane conversion of 10% (using
different contact times but keeping the C2/O2/He ratio constant) for the different
catalysts studied. In all cases, the selectivity to ethylene over NiO-TiO2 catalysts is
remarkably higher than that achieved over pure NiO (ca. 45%). However, selectivity to
ethylene values highly vary depending on the characteristics of catalysts (i.e. Ni-loading
and/or support). As it can be seen the optimal selectivity is obtained at low and
especially at intermediate NiO contents although it is observed that the preferred
10

amount of NiO depends also on the diluter, in a way that the higher the surface area of
the diluter the higher the optimal Ni-loading is.
The overall highest selectivity to ethylene has been obtained using titanium oxide of
both intermediate (Ni-B series) and high (Ni-C series) surface area, reaching in the best
cases 90% selectivity to ethylene at 10% ethane conversion. This value has been
obtained by catalysts with NiO contents of ca. 20-50 wt% for Ni-B series and ca. 50
wt% for Ni-C series. On the other hand, the most selective catalyst using the low
surface area support (Ni-A series) presents a NiO-loading from 4 to 20 wt% NiO,
although the selectivity to ethylene is only 65-70%.
Figure 1b shows the evolution of the selectivity to ethylene with the ethane conversion
at 450ºC for some representative catalysts focusing on those with 20 wt% NiO. As can
be seen the decomposition of ethylene is moderate for all NiO/TiO2 catalysts tested.
The catalytic activity was also very different for the catalysts tested. Table 1 shows the
ethane conversion obtained with these catalysts. It can be seen that the catalytic activity
for a fixed nickel loading varies for the different titania supports. In spite of the fact that
the amount of the supposedly active sites (NiO species) was the same in all cases the
conversions achieved are different. Thus, the most active catalyst was the one based on
the low surface area (A support) and the catalytic activity decreases with increasing the
surface area of the support.
Data shown until now in the present work have been obtained in an O-poor atmosphere
(C2/O2/He = 3/1/26; O2/C2=1/3 molar ratio). Therefore the ethane conversion is highly
limited as we would run out of oxygen and this would modify the nature and oxidation
state of nickel. Due to this we also used O-richer feed (C2/O2/He = 3/3/24; O2/C2=1/1
molar ratio) to be able to reach higher conversions [44]. This way, we have achieved an
ethylene yield of ca. 41% (ethane conversion = 55.2%, selectivity to ethylene = 73.9%)
11

at 450 ºC on 50Ni-B catalyst, at 450ºC and using a contact time, W/F, of 16 gcat h-1
(molC2H6)-1. By optimizing the catalytic conditions this value could be improved.
3.2. Characterization results of NiO/TiO2 catalysts
Since these catalytic results could be related to the higher/lower concentration of NiO
crystallites which are not interacting with titania supports, the catalysts have been
characterized by several physicochemical techniques in order to explain the catalytic
results. Table 2 shows some characteristics of these catalysts.
Figure 2 presents the XRD powder patterns of the most representative NiO/TiO2
catalysts (the remaining XRD results are shown, as supplementary information, in Figs.
S1 to S3, for A-, B- and C-series of catalysts, respectively). TiO2 anatase was only
observed in the Ni-A- and Ni-C-series supported catalysts whereas TiO2 anatase, with
low amount of TiO2 rutile, was observed in catalysts of Ni-B-series. As expected, the
relative intensity of the crystalline titanium oxides phases compared to NiO decreased
when increasing the Ni-content of the catalysts. Regarding to the Ni-species detected,
NiO crystallites were the only one observed. No mixed Ni-Ti-O phases are apparent.
For a more comprehensive knowledge about the influence of the morphology and
crystallite size on the catalytic performance, a detailed microscopy study has been
conducted. Figure 3 presents transmission electron micrographs of some representative
supported NiO samples on different TiO2 materials. In general, as we can observe in
Figure 3, all samples consist of nearly spherical nanoparticles although with variable
crystal sizes, mainly ranging from 4 to 50 nm. The changes in crystal grain sizes and
morphology of samples were related to both the Ni-content and the nature of the TiO2
matrix.
12

As shown in TEM image (Fig. 3a), 4Ni-A sample is composed of small NiO
nanoparticles (NPs) with size ranging from 4 to 12 nm which are spread over the
surface of large titania particles of 40 to 250 nm. However, an increase in the density of
the NiO nanoparticles as well as a small increase in grain size can be observed when the
Ni-loading increases (20Ni-A sample, Fig. 3b); the measured diameter of NiO NPs was
found to be in the 5-15 nm range. In 20Ni-A the area of the TiO2 matrix covered by NiO
NPs is higher than that of 4Ni-A. Moreover, in 20Ni-A sample, it is also seen that there
are a few aggregations of NiO nanoparticles which are not in contact with TiO2.
If the surface area of the TiO2 matrix increases a different trend is observed. Thus, for
20Ni-B (Fig. 3c) an increase in size and density of NiO nanoparticles is observed
(which is also accompanied with a decrease in the area of free titania matrix). It seems
that the size of TiO2 crystals in the case of support A was about 40-250 nm while in the
case of support B the size of TiO2 crystals was reduced (ranging from 20 and 100 nm).
In the case of the sample 20Ni-C (with the matrix presenting the highest surface area), it
is observed that the size of NPs increases (ranging from 10 to 33 nm). These domains
mainly present a core-shell structure in which the TiO2 matrix is covered by NiO.
Moreover, some uncovered or only partly covered TiO2 particles were also observed.
The EDX analysis shows an irregular composition and, depending on the analyzed area,
a richer or poorer Ni concentration is observed, which is in agreement with the
corresponding TEM images. In the case of the catalyst with higher Ni-loading, i.e.
sample 50Ni-C, a clear increase in NPs size which are in the 20-35 nm range is
observed (Fig. 3d). Moreover, the NPs density drastically increases, resulting in a total
coverage of TiO2 matrix by NiO particles presenting a higher thickness than that
observed in 20Ni-C. The NiO NPs distribution was not homogenous and the Ni content
measured by EDX, depending of the analyzed area varied between 30 and 100 at %.
13

This is in accordance with the images obtained in which a few agglomerations of NiO
particles which are not in contact with the TiO2 matrix are also observed. At higher Ni-
loading, 80Ni-C, a similar scenario was observed but with a higher concentration of
agglomerations of TiO2-free NiO particles. Fig. 3f shows a representative EDX
spectrum of a single NiO nanoparticle on titania matrix, in which Ti, Ni and O atoms
are the only elements detected (apart from copper and carbon from copper grid
employed). In order to determine the Ni-O crystalline phases of the sample; SAED
patterns were recorded and indexed from prepared samples. SAED patterns (Fig. 3e)
show several concentric rings formed by clear small spots; which indicates that these
samples consist of small NiO crystalline nanoparticles. The measured interplanar
distance from diffraction rings were about 0.243, 0.209, 0.149, 0.127 and 0.122 nm and
could, respectively, be attributed to (111), (200), (220), (311) and (222) planes of NiO.
SAED patterns of NiO were indexed to Face-centered cubic phase structure space group
Fm3m of NiO (JCPDS: 01-78-0643).
On the other hand, and in order to confirm the size and distribution of NiO
nanoparticles, additional measurements in Dark field (DF-TEM) mode were made
(Figs. S4, Supporting Information). Conventional DF-TEM imaging was achieved by
selecting and inserting a small objective aperture around (111) and (002) diffraction
spots from SAED pattern. It can be seen that, in the case of 4Ni-A sample, NiO
nanoparticles are well dispersed with sizes ranging from 4 to 12 nm, as mentioned
above (Figs. S4, Supporting Information).
Figure 4 shows a high resolution TEM image of NiO nanoparticles with size 4-10 nm in
4NiA sample. The measured interplanar distance HRTEM micrograph were about 0.241
nm and 0.208 nm corresponding to the (111) and (200) planes respectively. The FFT
pattern (not shown) was indexed to face centered cubic phase structure of NiO, which is
14

in good agreement with the results obtained by SAED and confirms the nature of nickel
oxide nanoparticles. Modifications in crystal lattice parameter could give some
indications about crystal size and structure disruptions. In order to check this, the mean
value of the a-lattice parameter of NiO was determined for some representative catalysts
from (111), (200) and (220) orientations (Table 2). As we can observe some variations
in the lattice parameter are observed which could be related to the crystallite size of NiO
NPs, as well as to the distribution over the TiO2 matrix.
Catalysts in C-series as well as samples with NiO content of ca. 20 wt% (i.e. 20Ni-A,
20Ni-B, 20Ni-C) were analyzed by XPS (Figure 5) and their main surface features are
summarized in Table 3. As it can be noted, the resulting surface composition is higher
than that given by the bulk chemical composition of the catalysts, which would result in
a Ni/(Ni+Ti) ratio of 0.20 (Table 3). In this regard, 20Ni-A catalyst leads to a surface
Ni/(Ni+Ti) ratio of 0.28, while samples 20Ni-B and 20Ni-C present 0.46 and 0.54 ratios
respectively. These results are in agreement with TEM studies performed on the
samples, in which NiO species are worse dispersed over the low surface area support A,
which also presents not covered TiO2 particles, facts that would lead to a lower Ni
concentration at the surface. This effect is even higher for catalysts in C-series, which
present the highest surface area support. Accordingly 10Ni-C sample shows the triple
Ni concentration at the surface than that expected from its chemical composition.
Ni 2p core level spectra show a main line together with a satellite structure (SII) ca. 7
eV higher BE, which has been associated to a ligand-metal charge transfer (the shake-up
satellite) [45,46,47]. Also a peak broadening over the main line can be observed, fact
that suggests the presence of another satellite peak (SI) at ca. 1.5 eV higher BE, which
has been ascribed to the presence of several defects and Ni species such as, Ni2+
vacancies [46], Ni2+-OH [47], Ni3+ ions [48] or Ni2+ surface interactions [49]. Another
15

weak intensity line at lower BE attributed to differential charging of the samples is
present in the examined materials (asterisk in Figure 6). Ni 2p3/2 main peak is in the
range 853.6-854.3 eV which corresponds to a Ni2+ oxidation state in an environment
ranging from pure NiO to Ni(OH)2 [50]. On the other hand, some differences on the
satellite/main line intensity ratios can be noted (Table 3). Not significant changes have
been observed in the SI/main peak ratio, however SII/main peak ratio increases on
samples 20Ni-B and 20Ni-C (Table 3). These differences can be related to an
enhancement in the non-local screening process, associated to the concentration and the
number of surface cation vacancies, to particle size and structural transitions [45].
Figure 6 shows H2-TPR profiles of the NiO-TiO2 catalysts, whereas Table 2
summarizes both the H2-uptake and temperature of the maximum H2 consumption
(TMC) in the H2-TPR experiments. For a suitable assignment of the reduction bands,
TPR experiments were also conducted on the pure titania supports. No reduction band
was observed and the consumption of hydrogen detected was negligible. Accordingly,
the reductions observed in NiO-TiO2 catalysts are assigned to nickel species. It can be
seen that as nickel content increases in each series the temperature of the maximum H2
consumption (TMC) slightly shifts to higher temperature values, regardless of the
nature of the titania support used. Generally NiO TPR-profiles consist of two reduction
bands, corresponding to two types of NiO-species that reduce to metallic nickel; either
particles which present different oxide-support interaction, or Ni species with different
oxidation state [51,52,53]. Taking into account that no clear evidences of the presence
of Ni3+ were obtained by XPS, the signal appearing at lower temperatures (250-300 ºC)
can be attributed to NiO particles which present low dispersion over the support, while
the signal appearing at higher temperatures (300-500 ºC) can be assigned to better
dispersed NiO species [53]. Therefore, for a fixed NiO content (20% wt) the TMC shifts
16

from 328 to 291 ºC, as the surface area of the support increases (Fig.6.b, d, g). The same
trend is observed at lower NiO contents, in which reduction temperature shifts from 308
to 266 ºC (Fig.6, patterns a, c and f).
DR-UV-Vis. spectra were conducted on representative catalysts in order to observe the
degree of dispersion of Ni2+ species on titania (Fig. S5, supplementary information).
The first observation of the spectra lead us to the conclusion that a high intensity band
between 220 and 400 nm is detected for all catalysts, and the size of this band decreases
when the nickel-loading increases. Thus, it indicates that this band is mainly linked to
the presence of TiO2 anatase, specifically due to the charge transfer from O 2p valence
band to Ti 3d conduction band, as reported elsewhere [54]. Bands at 450 and 515 nm
have been related to the presence of Ni-Ti-O species (charge transfer from Ni2+ to Ti4+
[55]) but in none of the catalysts tested here have been clearly appreciated. However, a
band at ca. 560 nm and a shoulder at ca. 640 nm which can be related to less dispersed
NiO on the titania are observed in some of the catalysts, but not in the pure supports.
Finally a band at ca. 720 nm which is due to the presence of bulk NiO particles [56] is
present in all the Ni-containing samples and increases with the nickel loading.
Unfortunately, not an accurate information can be drawn from the DR-UV-Vis. study
since in the 220-450 nm range the absorption of Ni species are overlapped by more
intense bands related to the TiO2 support.
3.3. General remarks
In this work it has been observed that the nature of the titania employed as a support of
nickel oxide defines the catalytic performance. Thus, it seems that medium and high
surface area titania are the appropriate supports whereas low surface area titania leads to
considerably less selective catalysts. On the other hand, if high NiO loading is present
17

in the catalysts a decrease in the olefin formation is also observed compared to the
optimal catalysts. Therefore, two factors must be controlled, i.e. the nature of titania and
the nickel loading.
The low selectivity obtained by the catalysts supported on the low surface area titania
(support A) must be due to the low amount of domains in which titania and NiO are in
contact. According to the TEM results these catalysts are formed to a high extent by free
TiO2 particles and free NiO crystallites. Thus the amount of unselective NiO crystallites
which are not modified by titania is high, and consequently the selectivity to ethylene
low. The microscopy results can be confirmed by XPS. Thus, for the catalyst with 20
wt.% NiO on the low surface area titania (20Ni-A) the surface Ni/(Ni+Ti) atomic ratio
is 0.28 which is only slightly higher than that of the bulk composition (0.21). However,
in the catalysts with the same NiO loading but with the supports with higher surface
area (i.e. 20Ni-B and 20Ni-C catalysts) the Ni/(Ni+Ti) atomic ratio increases up to
0.46-0.50, respectively, demonstrating higher interaction/dispersion of nickel oxide with
the titania.
The same reasoning can be applied to the catalysts with high Ni-loadings. Thus, if the
NiO loading is too high the olefin formation decreases and this must be due, as detected
by TEM analyses, to the presence of unmodified NiO which alone presents a low
selectivity to the olefin.
Figure 7 shows the variation of the selectivity to ethylene (at fixed conversions and
reaction temperature) with the theoretical coverage of the titania. The theoretical
coverage of NiO (Table 2) has been calculated from reference [57], in which the
monolayer of NiO has been estimated to be 0.12 g / 100 m2 of support. In all cases a
similar trend is observed, with the optimal performance obtained at until 5-10
monolayers. Thus NiO catalysts diluted in TiO2, presenting NiO crystallite with some
18

interaction with TiO2 particles, are highly efficient, whenever an excess of unmodified
NiO is not achieved.
Anyhow, according to TEM results (and depending on both nickel oxide loading and
the TiO2 support), the distribution of nickel oxide and titanium oxide is not that of a
typical TiO2-supported NiO catalyst (i.e. TiO2 support and, over it, a variable number of
nickel oxide layers). In fact we can see a heterogeneous mixture of: i) core/shell
agglomerations with TiO2 particles inside and NiO particles outside; ii) free NiO
particles; iii) free TiO2 particles; and iv) some agglomerations of particles with Ni
species spread over the titania.
The optimal supported NiO/TiO2 catalysts studied in the present article have a catalytic
performance at the same level that the most efficient promoted NiO catalysts reported in
the literature for the oxidative dehydrogenation of ethane [16, 20]. It has been
demonstrated that using a diluter as high surface area TiO2 it is possible to achieve
similar performance than promoted NiO catalysts. However, the amount of promoter
(i.e. Nb2O5, SnO2) or diluter (TiO2) which leads to the optimal performance is very
different. Thus for promoted NiO-catalysts prepared by co-precipitation just a low
amount of promoter is required whereas a higher amount of diluter/support is required
in the diluted/supported catalysts prepared by wet impregnation.
Moreover, we have observed that these catalysts present a good stability. A
representative catalyst, 20Ni-B, was studied for 26 h using invariable reaction
conditions. Thus, after a slight decrease in the ethane conversion for the first 2-3 h the
activity remained stable for the following 20 h. Regarding to the selectivity to ethylene,
just a subtle increase was observed for the first hours of experiment and then no
significant changes were appreciated (Fig. S6, supplementary information). The surface
area of the catalyst used in the stability test was measured, slightly decreasing from 50.4
19

m2 g-1 in the fresh sample to 48.2 m2 g-1 (see Table S2). This drop can be related, at least
partly, to the decrease of the catalytic activity observed. An analysis of the carbon
present in the catalyst shows that the amount of C on the surface is very low for both
fresh and used catalysts. Thus, the deposition of carbon on the surface does not seem to
be related to this deactivation.
As mentioned above the catalytic performance in terms of conversion/selectivity to
ethylene of these NiO/TiO2 catalysts is in the range of the best materials reported.
Additionally, the productivity to ethylene per gram of catalyst achieved is high but the
productivity per gram of nickel is more notorious, as in this case our catalysts present a
high proportion of support/diluter, in contrast with more recently promoted NiO
catalysts, which are predominantly nickel oxide with low amounts of the promoter
oxide [10-24]. Thus, in Table 4 catalytic data in the ODH of ethane of some
representative catalysts reported in the literature based on NiO are shown. As it can be
seen 20Ni-B (with only 20 wt% NiO) achieves productivity towards ethylene of ca.
2500 gC2H4 kgNi-1 h-1, remarkably higher than those of the other catalysts shown. It must
be noted that our data has been drawn at 450ºC which is a higher temperature than in
most of the other data presented in Table 4.
Conclusions
NiO diluted in TiO2 can be as efficient in the oxidative dehydrogenation of ethane as the
most selective NiO-promoted catalysts reported previously in the literature, with the
advantage of requiring lower amount of nickel in the catalyst composition, which could
mean lower sintering and better physical properties. In this work, yields to ethylene over
40% have been achieved, which could be improved by selecting suitable reaction
conditions. Over NiO diluted in TiO2 catalysts selectivity to ethylene over 90% can be
20

achieved at 10% conversion and that selectivity decreases only slightly by increasing
the ethane conversion. A selection of both the nickel loading and the titanium oxide
support is required to obtain the optimal catalytic results. Thus, TiO2 with high surface
areas (55 and 85 m2/g) have resulted to give the best catalytic performance although the
optimal nickel loading is different for each case. The design of the catalysts must be
done in a way that the presence of unmodified NiO sites is minimal in order to achieve
the best catalytic results.
Acknowledgments
The authors would like to acknowledge the DGICYT in Spain CTQ2012-37925-C03-2,
CTQ2015-68951-C3-1-R, CTQ2015-68951-C3-3-R and SEV-2012-0267 Projects) for
financial support. D.D. also thanks Severo Ochoa Excellence fellowship (SVP-2014-
068669). We also thank the University of Valencia (UV-INV-AE-16-484416 project)
and SCSIE-UV for assistance.
21

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Caption to figures
Fig. 1. Catalytic performance of NiO/TiO2 catalysts: a) Variation of selectivity to
ethylene (at ethane isoconversion of 10%) with the NiO-loading for catalysts of Ni-A-,
Ni-B- and Ni-C-series; b) Variation of selectivity to ethylene with ethane conversion at
-1
different contact times, W/F, in the range of 1-12 gcat h-1 molC2
.
Fig. 2. XRD patterns of representative NiO/TiO2 catalysts.
26

Fig 3: Representative TEM images of (a) 4Ni-A; (b): 20Ni-A; (c) 20Ni-B; and (d)
50Ni-C. Photographies (e) and (f) correspond to SAED pattern and EDX spectrum,
respectively, recorded from 20Ni-C sample. Scale bar is 100 nm.
27

Fig. 4. High resolution TEM image of NiO nanoparticles in (a) 4Ni-A and (b) 20Ni-B
samples.
Fig. 5. Ni 2p core level spectra of the 20% wt. NiO-TiO2 catalysts: a) 20Ni-A, b) 20Ni-
B and c) 20Ni-C.
28

Fig. 6. H2-TPR profiles of catalysts for Ni-A- (patterns a and b), Ni-B- (patterns c to e)
and Ni-C-series (patterns f to h) series: a) 4Ni-A; b) 20Ni-A; c) 2Ni-B; d) 20Ni-B; e)
50Ni-B; f) 10Ni-C; g) 20Ni-C; h) 50Ni-C.
Fig. 7. Variation of the selectivity to ethylene with the theoretical NiO coverage (fixed
conversion ≈ 10%). Note: Reaction temperature = 450ºC, C2H6/O2/He with a molar ratio
of 3/1/26.
29