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Applied Catalysis A: General 325 (2007) 220–226

HRTEM characterization of the nanostructural features formed in

highly active Ni–Nb–O catalysts for ethane ODH

E. Heracleous a, A. Delimitis b, L. Nalbandian b, A.A. Lemonidou a,*a Department of Chemical Engineering, Aristotle University of Thessaloniki and CERTH/CPERI, P.O. Box 1517,

University Campus, GR-54006 Thessaloniki, Greeceb Laboratory of Analysis & Characterization of Solids, Chemical Process Engineering Research Institute (CPERI/CERTH),

P.O. Box 361, GR-57001 Thessaloniki, Greece

Received 21 July 2006; accepted 7 February 2007

Available online 3 March 2007

Abstract

In this work, we report an in-depth structural characterization of pure NiO and Ni–Nb–O mixed oxide catalysts (Nb/Ni = 0–0.25), highly active

and selective materials for ethylene production via ethane ODH, using high resolution transmission electron microscopy (HRTEM) coupled with

energy dispersive X-ray analysis (EDS). This study led to the identification and investigation of the nanostructural features formed in the Nb-doped

NiO catalysts and their relation with the excellent catalytic functionality of the Ni–Nb–O materials. It was found that low-temperature treatment of

pure NiO leads to the formation of a non-stoichiometric oxidic phase with characteristic structural defects due to cationic deficiency, as

demonstrated by microscopy results. On the Nb-doped oxides, two distinct structural phases, formed via the reaction of the Nb cations with the

cationic vacancies, were identified: a NiO phase having Nb cations incorporated in the host lattice, which retains its initial cubic structure (Ni–Nb

solid solution), and a highly distorted Nb-rich phase, precursor for the formation of the mixed NiNb2O6 crystal compound. The reduction of the

structural defects in NiO via their interaction with the niobium ions was correlated with the extremely high selectivity of the Ni–Nb–O catalysts to

ethylene in the ethane ODH reaction, since these vacancies lead to the formation of strong oxidizing electrophilic oxygen species (O�), responsible

for the total oxidation of ethane to CO2.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Non-stoichiometric NiO; Ni–Nb–O mixed metal oxides; High-resolution transmission electron microscopy (HRTEM); Ethane oxidative

dehydrogenation; Ethylene

1. Introduction

Ethylene, one of the most important bulk petrochemicals, is

exclusively produced via steam cracking of various petroleum

fractions, mainly naphtha and ethane. With ethylene demands

constantly rising and the general claim for industry to convert to

clean energetically balanced processes in the frame of

environment protection and preservation of non-renewable

energy sources, the development of an alternative process for

ethylene production is becoming an increasingly urgent task.

Catalytic oxidative dehydrogenation (ODH) of ethane is an

attractive alternative route for the production of ethylene, which

overcomes the main disadvantages of the current conventional

* Corresponding author. Tel.: +30 2310 996273; fax: +30 2310 996184.

E-mail address: alemonidou@cheng.auth.gr (A.A. Lemonidou).

0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2007.02.030

method [1–5]. The key for a viable industrial application of this

process is the development of a highly active and selective

catalytic material, able to activate and convert the relatively

inert alkane to olefin at low temperature and minimize the

parallel and consecutive side reactions of both the alkane and

alkene to carbon oxides.

Our group has performed extensive investigations on

transition metal oxides catalysts for the oxidative dehydro-

genation of C2–C4 alkanes [6–12]. Recently, we have reported

[10–12] the high potential of a new class of catalytic materials

based on nickel for the oxidative dehydrogenation of ethane to

ethylene. The developed bulk Ni–Nb–O mixed nano-oxides

[11] exhibit high activity in ethane ODH at low reaction

temperature and very high selectivity, resulting in an overall

ethene yield of 46% at 400 8C. Varying the Nb/Ni atomic ratio

led to an optimum catalytic performance for catalysts with Nb/

Ni ratio in the range 0.11–0.18. Detailed characterization of the

E. Heracleous et al. / Applied Catalysis A: General 325 (2007) 220–226 221

as-synthesized materials with several techniques [11] showed

that the key component for the excellent catalytic behavior is

the Ni–Nb solid solution formed upon the introduction of

niobium in NiO, evidenced by the contraction of the NiO lattice

constant, since even small amounts of Nb effectively converted

NiO from a total oxidation catalyst (80% selectivity to CO2) to

a very efficient ethane ODH material. An upper maximum

dissolution of Nb5+ cations in the NiO lattice was attained for

Nb/Ni ratios �0.18, with higher Nb contents leading to

inhomogeneity and segregation of the NiO and Nb2O5 phases.

The incorporation of Nb in the NiO lattice, by substitution of

nickel atoms and/or filling of the cationic vacancies in the

defective non-stoichiometric NiO surface structure, led to a

reduction of the materials non-stoichiometry, as indicated by

TPD-O2, electrical conductivity measurements [11] and

isotopic oxygen (18O2) exchange [12] and consequently of

the electrophilic oxygen species (O�), which are abundant in

NiO and responsible for the total oxidation of ethane to carbon

dioxide. The Nb-induced effects were related to charge

compensation phenomena, as a consequence of the Ni2+

substitution by Nb5+ and due to the difference of ionic charge,

as discussed in detail in Ref. [11].

Therefore, the excellent catalytic behavior of the Ni–Nb–O

nano-oxides in ethane ODH is related to the electronic and

structural rearrangement induced by the incorporation of

niobium cations in the NiO lattice. This substitution-induced

enhancement in catalytic activity has been observed for several

mixed-metal oxides with different structures [13–15], with

however no concerted efforts to explore how the morphological

changes arising from cationic substitution, such as changes in

size, shape, surface area etc., are related or contribute to the

observed catalytic performance. In this work, we report an in-

depth characterization of the developed Ni–Nb–O mixed oxides

for ethane ODH (with Nb/Ni atomic ratios ranging from 0 to

0.25) using HRTEM observations, coupled with chemical

analysis by EDS, in order to study the nanostructural features

formed in the materials and their relation with the improved

catalytic results.

2. Experimental part

2.1. Catalyst preparation

The series of mixed Ni–Nb–O oxides with varying Nb/Ni

atomic ratio from 0 to 0.25 was prepared by the evaporation

method. Details on the preparation procedure followed can be

found in Ref. [11]. The catalysts are referred to as NixNby,

Table 1

Nomenclature and physicochemical characteristics of the catalysts [11]

Sample Nb/Ni atomic

ratio

% wt. NiO

loading

Surface area

(m2/g)

Crystalli

S–NiO 0 100 9.4 NiO

NS–NiO 0 100 16.7 NiO

Ni0.9Nb0.1 0.111 83.49 55.9 ‘‘NiO-lik

Ni0.85Nb0.15 0.176 76.10 85.1 ‘‘NiO-lik

Ni0.8Nb0.2 0.250 69.21 58.6 ‘‘NiO-lik

where x and y indicate the atomic content of Ni and Nb,

respectively, relative to the total metal content of the materials.

Non-stoichiometric (black) and stoichiometric (green) pure

nickel oxides phases (denoted as NS–NiO and S–NiO), used in

this study to explore the effect of non-stoichiometry, were

prepared by decomposition of Ni(NO3)2�6H2O (nickel nitrate

hexahydrate, >99%, Merck) precursor at 450 8C and 700 8C,

respectively, for 5 h in synthetic air.

2.2. HRTEM characterization

Samples for electron microscopy observations were

prepared by gently grinding the catalyst powders in high-

purity ethanol using an agate pestle and mortar. A drop of the

solution was subsequently deposited onto a lacey C-film

supported on a Cu grid and allowed to evaporate under

ambience conditions. Electron microscopy experiments were

carried out in a JEOL 2011 high resolution transmission

electron microscope, operating at 200 kV, with a point

resolution of 0.23 nm and Cs = 1.0 mm. The microscope is

also fitted with an Oxford Instruments INCAx-sight liquid

nitrogen cooled EDS detector with an Si(Li) window for

detailed elemental analysis of the catalysts. Processing of the

spectra was accomplished using the INCA Microanalysis Suite

version 4.02 software.

3. Results

The composition and main physicochemical characteristics

of the nickel oxide phases and the Ni–Nb–O mixed oxides, as

determined in Ref. [11], as well as the corresponding XRD

patterns are repeated in Table 1 and Fig. 1, respectively for the

reader’s convenience. A summary of the catalytic results of a

series of Ni–Nb–O mixed oxides with a Nb/Ni atomic ratio

ranging from 0 to 0.67, is presented in Table 2. The study of the

materials’ catalytic performance clearly demonstrated that

introducing even small amounts of Nb in NiO convert nickel

oxide from an effective total oxidation catalyst to an efficient

low-temperature with very high selectivity ODH catalyst [11].

Characterization of the high-temperature treated NiO

sample (S–NiO) by HRTEM indicates that pure NiO calcined

at high temperature is structurally uniform and consists of well-

defined crystallites [Fig. 2(a)] in the 50–200 nm range, with

average size at �60 nm. The inset shows a selected area

electron diffraction (SAD) pattern, obtained from the single

crystalline particle in Fig. 2(a). SAD analysis reveals that it can

be indexed to a fcc NiO structure along its [1 1 0] zone axis,

ne phases NiO average crystal

size (by XRD) (nm)

NiO lattice constant

(by XRD) (A´

)

61.7 4.1760

31.8 4.1762

e’’ phase, Nb-rich phase 21.4 4.1751

e’’ phase, Nb-rich phase 16.7 4.1725

e’’ phase, NiNb2O6 14.3 4.1722

Fig. 1. X-ray diffraction patterns of the Ni–Nb–O catalysts (Nb/Ni atomic ratio:

0–0.25) and pure stoichiometric and non-stoichiometric NiO [11].

Fig. 2. (a) Bright field (BF) TEM image of a platelet in the S–NiO catalyst,

viewed slightly off its [1 1 0] projection, as shown in the SAD pattern inset. (b)

HRTEM image of the same platelet near the surface area, revealing that the S–

NiO catalyst is free from any extended structural defects. The (1 1 1) lattice

fringes of NiO are clearly resolved.

E. Heracleous et al. / Applied Catalysis A: General 325 (2007) 220–226222

with a lattice parameter of a = 0.4176 nm, in agreement with

reported data (JCPDS 4-0835) and the analysis of the X-ray

diffraction pattern of the above-mentioned sample (see

Table 1). The clearly visible (1 1 1) lattice planes of the NiO

phase, illustrated in the HRTEM image of Fig. 2(b), confirm the

long-range order crystallization of the stoichiometric sample.

Investigation of the non-stoichiometric NiO sample (NS–

NiO) shows that as the calcination temperature decreases to

450 8C, a morphological change and a distortion of the nickel

oxide crystal structure occurs. The HRTEM image in Fig. 3

demonstrates the presence of extended structural defects

(arrowed), which are identified as stacking faults (SFs), while

the distorted lattice planes can be clearly seen in the same

image. The inset [1 1 0] SAD pattern confirms that the SFs run

along the two (1 1 1) and (1 1 1) crystal planes, which is

commonly found for cubic structures. Energy-dispersive X-ray

(EDS) analysis on several parts of the investigated sample

reveals a systematic excess of oxygen compared to the nickel

content, in contrast to the S–NiO sample where the atomic ratio

of Ni:O was always in the range of 1:1. The detected planar

defects (SFs) are homogeneously distributed in the individual

mono-crystallites, however it should be pointed out that the

amount of defects differs throughout the sample with areas of

Table 2

Catalytic performance of the Ni–Nb–O mixed oxides in the ethane oxidative dehy

Sample Reaction temperature = 300 8Ca

C2H6 conversion (%) C2H4 selectivity (%) CO2 selectivity

NS–NiO 3.5 12.4 87.6

Ni0.9Nb0.1 7.3 77.8 22.2

Ni0.85Nb0.15 10.5 86.2 13.8

Ni0.8Nb0.2 7.0 80.9 19.1

Ni0.7Nb0.3 5.8 77.2 22.8

Ni0.6Nb0.4 2.0 80.2 19.8

a Reaction conditions: W/F = 0.54 g s/cm3, C2H6/O2 = 1/1.

higher and lower density of cationic vacancies. Consequently,

characterization by transmission electron microscopy provides

clear evidence that NiO calcined at low temperatures is a highly

defective, non-stoichiometric oxide with cationic deficiency

(Ni1�xO), in accordance with the general consensus in literature

about the non-stoichiometric nature of NiO [16,17].

Doping NiO with niobium results in significant changes in

the crystalline morphology. The HRTEM image of the sample

Ni0.9Nb0.1 (Nb/Ni ratio = 0.111) in Fig. 4 demonstrates a clear

reduction of the crystallite size, while two relatively well-

defined areas are recognized: a well-crystallized cubic phase of

ordered structure (marked as I on Fig. 4) and a highly distorted

phase II, which appears in small concentration throughout the

drogenation reaction [11]

Reaction temperature = 400 8Ca

(%) C2H6 conversion (%) C2H4 selectivity (%) CO2 selectivity (%)

28.6 22.7 77.3

55.1 66.9 33.1

66.7 67.8 32.2

61.1 64.0 36.0

45.3 64.7 35.3

25.3 73.8 26.2

Fig. 3. HRTEM image from a platelet in the NS–NiO catalyst, viewed along the

[1 1 0] projection. A significant density of SFs lying on {1 1 1} planes, depicted

with white arrows, dominate the platelets in this material. Moire fringes, due to

local NiO lattice distortions are also revealed.

E. Heracleous et al. / Applied Catalysis A: General 325 (2007) 220–226 223

sample. EDS analysis on each of the two distinct morphologies

(included as insets in Fig. 4) shows that phase I is comprised of

Nb and Ni in an atomic ratio of�0.1, while phase II is enriched

in niobium with Nb and Ni ranging between 1:1 and 2:1 ratio.

Characterization with XRD, shown in Fig. 1, shows that the

mixed Ni–Nb–O oxides with Nb content up to 15% relative to

the total metal content exhibit diffraction lines corresponding to

a crystalline ‘‘NiO-like’’ phase, as well as a broad band

centered at 2u 278 attributed to highly disordered niobium

phase. Calculation of the lattice parameters of this ‘‘NiO-like’’

phase from the XRD patterns reveals a contraction of the NiO

lattice constant. Given that the ionic radii of Ni2+ and Nb5+ are

0.69 A and 0.64 A, respectively, a substitution between the two

cations is possible and would lead to a reduction of the lattice

constant, consistent with the experimental XRD observations.

Based on the above, the ‘‘NiO-like’’ phase detected in the

Fig. 4. Typical BF image from the Ni0.9Nb0.1 catalyst, where the two distinct morpho

of particles, shown in the same image, revealed that the Nb content is significantly

solution particles (type I), compared with the NS–NiO catalyst, is also apparent.

diffractograms of the Ni–Nb–O mixed oxides is identified as a

Ni–Nb solid solution. By combining the HRTEM with the XRD

and the EDS results, it could be inferred that the structural

feature detected in the Ni0.9Nb0.1 sample (denoted as I) indeed

corresponds to this Ni–Nb solid solution, formed via the

substitution of the host cations (Ni2+) and/or filling of cationic

vacancies of the non-stoichiometric NiO lattice by the Nb5+

cations. The reaction of the niobium cations with the structural

defects in the NiO lattice disorders the crystal structure and

induces a rupture of the larger platelets into smaller ones, thus

causing the reduction of crystallite size observed in the Nb-

doped catalytic sample.

Further evidence for the nature of the formed Ni–Nb solid

solution is provided by the high-resolution TEM micrographs

of the sample with slightly higher Nb-content, Ni0.85Nb0.15 (Nb/

Ni ratio = 0.176), illustrated in Fig. 5(a) and (b), which again

consists of the two aforementioned structural features. EDS

analysis on phase I particles reveals an atomic ratio of Nb/

Ni � 0.179, while the corresponding SAD pattern [inset in

Fig. 5(b)] reveals strong rings, indicative of a nanostructural

phase, demonstrating the significant decrease of the crystal size

compared to undoped NiO. Estimation of the interplanar

spacings from the SAD pattern shows that the structure of phase

I corresponds to the NiO crystal structure. This confirms the

conclusions derived from the XRD analysis and demonstrates

that this nano-phase I observed in the HRTEM images of the

Ni–Nb–O mixed oxides can be identified as a Ni–Nb solid

solution, formed by the incorporation of a small amount of Nb

in the NiO lattice structure. Consequently, this Nb-induced

increase in the crystallographic disorder inhibits the wide range

crystallization of NiO and ruptures the larger NiO crystallites,

leading to the formation of significantly smaller size

nanocrystals (consistent with a significant surface area increase

of the Nb/NiO sample compared to undoped NiO [11], see

Table 1), which however retain the NiO crystal structure, as

both the SAD and XRD results proved.

As mentioned earlier, both samples contain a second Nb-rich

disordered phase II in small amounts. However, even though

logies (I and II) of particles are seen. Elemental analysis by EDS from both types

higher in the type II particles. A great reduction in the size of the Ni–Nb solid

Fig. 5. HRTEM images from the Ni0.85Nb0.15 catalyst, showing the two distinct

morphologies, I and II. The highly disordered structure of the NixNbyO phase is

revealed in (a), whereas both the {1 1 1} lattice fringes resolved and the spotty

ring SAD pattern inset in (b) show the single crystalline nature of the nanosized

Nb-doped NiO particles.

Fig. 6. HRTEM image and SAD pattern (inset) from agglomerated particles of

the highly disordered NixNbyO phase. The fringes resolved may be attributed to

crystal planes of either the NiNb2O6 or Ni2Nb4O9 phases.

Fig. 7. HRTEM image from the Ni0.8Nb0.2 catalyst, revealing the increasing

crystallisation of the NixNbyO phase with increased Nb content. The two sets of

fringes mainly resolved correspond to the (4 0 0) and (0 2 0) planes of the

NiNb2O6 phase.

E. Heracleous et al. / Applied Catalysis A: General 325 (2007) 220–226224

this phase does not give clear diffraction peaks in the XRD

patterns of the catalysts, it cannot be classified as entirely

amorphous since highly distorted lattice planes with average

spacing of 0.335 nm can be detected in the HRTEM images, as

shown in Fig. 6. In order to elucidate the nature of this phase, a

selected area electron diffraction pattern of the area was acquired

and analyzed. Based on the SAD pattern (inset in Fig. 6), the d-

spacing of the Nb-rich phase amounts to �0.335 nm and

therefore cannot be attributed to the NiO structure, but rather to a

precursor phase for the formation of mixed Ni–Nb crystal

structures, such as NiNb2O6 and Ni2Nb4O9 (JCPDS 31-906 and

JCPDS 46-525). Furthermore, based on the d-spacing value that

was calculated, this Nb-rich phase should appear on the XRD

pattern at 2u � 25–288. This is in perfect agreement with the

diffractograms of the Ni–Nb–O mixed oxides with Nb atomic

content<15% where a broad band is detected at 2u � 278, which

as shown by the HRTEM analysis can undoubtedly be attributed

to the highly distorted Nb-rich matrix.

Characterization by XRD also shows that a slight increase in

the Nb atomic content from 15% to 20% (catalyst Ni0.8Nb0.2)

leads to the appearance of well-defined reflections correspond-

ing to the mixed crystal NiNb2O6 phase. Indeed, HRTEM-

investigations of the Ni0.8Nb0.2 sample indicate the presence of

three types of structures: the Ni–Nb solid solution, the distorted

Nb-rich phase and a third crystal phase (III) which was

recognized as NiNb2O6 in accordance with the XRD results.

EDS analysis on the area III under study shows that the

concentration of niobium is two times that of nickel,

conforming to the molecular formula of the NiNb2O6

compound. Furthermore, as shown in the HRTEM image of

Fig. 7, the clearly resolved lattice fringes of this phase confirm

the ordered crystal structure, while the calculated interplanar

spacings of 0.351 nm and 0.284 nm correspond to the (4 0 0)

and (0 2 0) planes of the NiNb2O6 phase (JCPDS 31-906).

4. Discussion—correlation of nanostructural features

with catalytic performance

By combining the present HRTEM investigations and all

characterization results reported in Ref. [11] on pure NiO and

E. Heracleous et al. / Applied Catalysis A: General 325 (2007) 220–226 225

Ni–Nb–O mixed oxides, the following can be deduced: pure

NiO calcined at high temperatures is stoichiometric, structu-

rally uniform and consists of long-range order, well-defined

cubic microcrystallites. Calcination at low temperatures leads

to the formation of a highly non-stoichiometric oxidic phase,

with visible extensive structural defects due to cationic

vacancies (i.e., nickel deficiency). These defects are inhomo-

geneously distributed throughout the sample, with areas of

higher and lower concentration of vacant sites. Addition of

niobium leads to the reaction of the foreign species (Nb5+) with

these NiO-defects leading to the formation of two distinct

structural features. On the areas of low-density population of

cationic vacancies, a small amount of Nb is incorporated in the

NiO host lattice, causing distortion of the crystallographic

planes and rupture of the larger crystallites that consequently

leads to significantly lower crystal size (in the nanorange) and

contributes to the higher surface area (see Table 1) of the Ni–

Nb–O catalysts. The SAD patterns indicate that the NiO lattice

structure is retained and therefore, this nano-phase can be

attributed to a Ni–Nb solid solution (designated as phase I). On

the areas of high-density population of defects, the amount of

Nb inserted in NiO exceeds the maximum capacity of foreign

species that nickel oxide lattice is capable of accommodating

and preserving at the same time its crystallographic structure.

Therefore, on these Nb-rich areas a non-well crystallized

structural feature forms (designated as phase II), which

however exhibits highly distorted lattice planes with interplanar

spacings that correspond to mixed Ni–Nb phases, such as

NiNb2O6 and Ni2Nb4O9, as demonstrated by the SAD, HRTEM

and EDS results. Phase II has an extremely small particle size

(less than 10 nm) and is probably the main contributor to the

high surface area recorded for the Ni–Nb–O catalysts.

Additionally, this distorted phase constitutes the precursor

for formation of the mixed NiNb2O6 crystal phase (designated

as phase III), which gives clear reflections on the XRD pattern

of the intermediate Nb-loading catalyst (Ni0.8Nb0.2) and

conforms to the observed longer-range order structure in the

HRTEM images of the sample. Further increase of the Nb/Ni

ratio results in extended segregation and separation of the pure

NiO and Nb2O5 phases [11].

Moving on to the catalysis aspects of the Ni–Nb–O mixed

oxides, this new class of catalytic materials, as recently

demonstrated in a previous work [11], exhibits high potential

for the low-temperature oxidative dehydrogenation of ethane to

ethylene (see Table 2 for a summary of catalytic performance).

This was related to the reaction of the Nb cations with the

cationic defects and their insertion in the NiO lattice, leading to

the formation of a Ni–Nb solid solution. This conclusion is

nicely confirmed and complimented by the HRTEM investiga-

tions presented in this work, where a nanostructural Ni-rich

phase, which however retains the NiO crystal structure, was

observed in the Ni–Nb–O mixed oxide catalysts. The reaction

of Nb with the cationic vacancies leads to rupture of larger

crystals into smaller ones, thus increasing the surface area and

consequently the per weight activity of the samples in the

ethane ODH reaction. The elimination and/or reduction of the

vacant sites on NiO via their reaction with the foreign cations is

responsible for the high selectivity of the catalysts to ethylene

during ethane ODH. According to literature [11,18,19], the

non-stoichiometry of NiO renders the surface able to

accommodate large amounts of excess electrophilic oxygen

species (O�) on the surface, strong oxidizing species

responsible for the total oxidation of ethane to CO2. Therefore,

the reduction of non-stoichiometry leads to elimination of the

unselective oxygen species, resulting in the high ethylene

selectivity and enhanced ethane ODH activity observed over

the Ni–Nb–O mixed oxides. The contribution of the second Nb-

rich structural feature to the catalytic performance still remains

unclear, since it is almost impossible to isolate this phase and

evaluate independently its catalytic function. The nanoparticles

of this phase certainly are the main contributors to the increased

surface area (and thus to the per weight activity) of the

catalysts, which significantly drops when the mixed NiNb2O6

phase acquires a well-crystallized structure (Ni0.8Nb0.2

sample). Further studies are in progress in order to fully

comprehend and interpret the catalytic action of the individual

structures in the ethane oxidative dehydrogenation reaction

over Ni–Nb–O mixed metal oxide catalysts.

5. Conclusions

In-depth structural characterization of pure NiO and Ni–Nb–

O mixed oxide catalysts (highly active and selective materials

for the production of ethylene via ethane oxidative dehydro-

genation) by HRTEM led to the identification of the

nanostructural features formed upon doping of NiO with Nb

cations. Low-temperature treatment of NiO causes the

formation of highly reactive stacking faults due to cationic

deficiency. The reaction of the foreign species (Nb) with these

cationic vacancies leads to the formation of two distinct

structural phases: a Ni-rich crystallized phase with Nb cations

incorporated in the NiO lattice, which retains its original

crystallographic structure, and a highly distorted Nb-rich phase,

precursor for the formation of the mixed NiNb2O6 crystal

compound. In terms of catalytic performance, the elimination

of the NiO structural defects by niobium is responsible for the

high selectivity of the Ni–Nb–O catalysts in the ethane ODH

reaction to ethylene, since these vacancies lead to the formation

of strong oxidizing electrophilic oxygen species (O�)

responsible for the total oxidation of ethane to CO2.

Furthermore, the insertion of Nb significantly reduces the

crystallite size and consequently increases the surface area and

the per weight activity of the materials.

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