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Catalysis Letters ISSN 1011-372XVolume 143Number 1 Catal Lett (2013) 143:31-42DOI 10.1007/s10562-012-0937-7

Niobia-Supported Nanoscaled Bulk-NiOCatalysts for the Ammoxidation of Ethaneinto Acetonitrile

Elizabeth Rojas, M. Olga Guerrero-Pérez& Miguel A. Bañares

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Niobia-Supported Nanoscaled Bulk-NiO Catalystsfor the Ammoxidation of Ethane into Acetonitrile

Elizabeth Rojas • M. Olga Guerrero-Perez •

Miguel A. Banares

Received: 21 September 2012 / Accepted: 4 November 2012 / Published online: 27 November 2012

� Springer Science+Business Media New York 2012

Abstract Two series of supported nanoscaled NiO cata-

lysts have been prepared using Nb2O5 supports with dif-

ferent surface areas. NiO nanoparticles interacting with Ni–

O–Nb mixed phases are active and selective for ethane

ammoxidation; both are required in order to have an active

and selective catalyst. In this paper we report the dispersion

of NiO nanoparticles on two Nb2O5 supports in order to

generate an active phase for ethane ammoxidation. The use

of a support stabilizes NiO nanoparticles, which otherwise

would sinter. This work shows how the activity can be

modulated by the size of the NiO nanoparticles and by the

NiO/Ni–Nb–O ratio, which depends on the nickel coverage

and support surface area. The catalysts obtained are very

promising for ethane ammoxidation reaction.

Keywords Supported-nickel catalysts � Ammoxidation �Ethane � Acetonitrile � Niobic acid

1 Introduction

Acetonitrile is a saturated nitrile that finds many applica-

tions in the chemical industry, particularly the pharma-

ceutical industry, where the global demand accounts for

over 70 % of the total market [1]. It is used as a reactant in

chemical syntheses like the production of malononitrile,

pesticides or pharmaceuticals, and as a solvent in the

synthesis of pharmaceuticals and intermediates, oligonu-

cleotides, and peptides. High purity acetonitrile is also a key

solvent for HPLC analysis, due to its special properties.

Unlike other solvents, such as methanol, commercial ace-

tonitrile is not produced by a direct synthesis method, but it

is obtained as a by-product during the industrial-scale pro-

duction of acrylonitrile. Typically, 2–4 l of acetonitrile is

obtained for every 100 l of acrylonitrile produced. Aceto-

nitrile can also be produced by other methods, such as the

dehydration of acetamide, but these do not have commercial

relevance. Indeed, during the period 2008–2009, there was a

very limited production for acrylonitrile as a result of the

world economic downturn [2, 3]; the supply of acetonitrile

was reduced significantly, probably by at least 50 % in

2008, resulting in a six-fold increase in its price. Thus, there

is the need to develop a direct commercial process to pro-

duce this chemical. Since alkane feedstock’s are readily

available starting materials, the transformation of ethane

into acetonitrile is an attractive process for synthesizing this

chemical in one step.

In a previous work, we report that the bulk Ni–Nb–O

system is a promising catalytic system for such a process

[4]. There, we used a bulk Ni–Nb–O catalytic system;

however, nanostructured catalysts present valuable indus-

trial and academic advantages [5–7]. The catalytic prop-

erties of nanostructured materials can be more easily

modulated for than those of bulk materials; the higher

surface-to-volume ratios allow optimizing the loading and

maximize support effect. The use of a support significantly

helps improving mechanical properties. NiO is rather

inexpensive, but it exhibits a tendency to sinter when

nanoscaled. Supporting of nanoscaled particles provides a

convenient mean to stabilize them and, in turn, provides a

mean to further tune its properties [8–10]. It was reported

that NiO nanoparticles were dispersed on two different

E. Rojas � M. A. Banares

Catalytic Spectroscopy Laboratory, Instituto de Catalisis y

Petroleoquımica, CSIC, Marie Curie 2, 28049 Madrid, Spain

M. O. Guerrero-Perez (&)

Departamento de Ingenierıa Quımica, Universidad de Malaga,

29071 Malaga, Spain

e-mail: oguerrero@uma.es

123

Catal Lett (2013) 143:31–42

DOI 10.1007/s10562-012-0937-7

Author's personal copy

Nb2O5 materials and that efficient ethane ammoxidation

needs an interaction between NiO and Ni–Nb–O [11]. This

work reports the effect of NiO nanoparticles size and that

of the NiO/Nb–Ni–O ratio for the direct ammoxidation of

ethane into acetonitrile.

2 Experimental Section

2.1 Catalyst Preparation

The ‘‘hydrated niobium pentoxide’’ support (Nb2O5�xH2O,

niobic acid) was supplied by Companhia Brasileira de

Metalurgia e Mineracao, Brazil (CBMM, batch AD/1426,

HY-340). This material generated two series of supported

catalysts were prepared and labeled as ‘‘NBO’’ and

‘‘Nb2O5’’. NBO series use the fresh hydrated niobium

pentoxide support material, while the Nb2O5-series uses

this support after activation in air at 550 �C for 12 h. These

supports were impregnated with a nickel acetate tetrahy-

drate (Ni(CH3CO2)2�4H2O, [99 %, Aldrich) solution; the

amount of nickel was calculated in order to obtain catalysts

with Ni contents in the 10–40 wt% range. This was kept

under stirring at 80 �C during 1 h to ensure complete dis-

solution and good mixing of the starting components. The

residual water was removed by evaporation in a rotary

evaporator at 60 �C at reduced pressure of 10–40 mmHg.

Finally, the resulting solid was dried at 120 �C for 12 h and

then calcined at 450 �C during 5 h, at a rate of 10 �C/min

in synthetic air. The calcined material was crashed and

sieved to 0.125–0.250 lm crystal size range. A reference

NiO sample was prepared by calcination in synthetic air of

nickel acetate tetrahydrate at 450 �C (10�/min) for 16 h in

synthetic air.

2.2 Characterization

Nitrogen adsorption isotherms were recorded with an

automatic Micromeritics ASAP-2000 apparatus. Prior to

the adsorption experiments, samples were outgassed at

140 �C for 2 h. BET areas were computed from the

adsorption isotherms (0.05 \ P/Po \ 0.27), taking a value

of 0.164 nm2 for the cross-section of the adsorbed N2

molecule at -196 �C. ICP analyses were made with a

PerkinElmer-3300 DV-Disgreggation. The samples were

disaggregated with a microwave oven using HNO3, HF and

HCl. The catalysts were analyzed by XRD in a Siemens

Krystalloflex D-500 diffractometer, with CuKa radiation, at

k = 1.5418 A, 40 kV, 30 mA, and scanning rate of 2�/min

for Bragg’s angles 2h from 5� to 90�.

The mean crystallite size D, is determined by the

Scherrer formula:

D ¼ KkB cos h

The size D as defined as (volume)1/3 leading to a value

K = 0.95 [12] when B is defined as the half-maximum line

width. The half-maximum line width from (111) and (200)

reflecting planes in the NiO was employed for determined

the NiO mean crystallite size.

Raman characterization of the catalysts was performed

in situ under dehydrated conditions. The spectra were

recorded with a Renishaw Micro-Raman System 1000

equipped with a cooled CCD detector and an Edge filter to

remove the elastic scattering. The samples were excited

with the 514 nm Ar line. X-ray photoelectron spectra were

recorded on a VG Escalab 200R electron spectrometer

provided with a hemispherical energy analyzer and an Mg

Ka X-irradiation source and a hemispherical electron

analyzer working at pass energy of 50 eV. Powdered

samples were pressed using small stainless-steel cylinders

and mounted on a standard sample probe, placed in a pre-

evacuation chamber up to ca. 10-5 Torr, before they were

moved into the main vacuum chamber. The residual pres-

sure in the turbo-pumped analysis chamber was kept below

7 9 l0-9 Torr during data collection. Each spectral region

was signal-averaged for a given number of scans to obtain

good signal-to-noise ratios. Although surface charging was

observed on all the samples, accurate binding energies

(BE) were determined by charge referencing with the C 1s

line at 284.6 eV. Data processing was performed with the

XPS peak program, the spectra were decomposed with the

least square fitting routine provided with the software with

Gaussian/Lorentzian (90/10) product function and after

subtracting a Shirly background.

X-ray absorption near-edge structure (XANES) spectra

at the Ni K-edge were measured at room temperature at the

BM-25A beamline of the European Synchrotron Radiation

Facility (ESRF) in Grenoble, France. A Si (111) mono-

chromator was used to select the X-ray beam from the

synchrotron light produced by the electron storage ring

operated at 1.37 GeV and a maximum current of 200 mA.

The Ni K-edge absorption spectra were recorded in the

transmission mode, in a photon energy range from 8.200 to

8.800 eV, using a CCD-based detector. Energy calibration

and normalization of XANES spectra were performed with

the open source software ATHENA-IFEFFIT.

2.3 Catalytic Activity Measurements

The kinetic measurements were carried out in a microre-

actor system. The gases used were helium (Air liquide,

99.999 % purity), oxygen (Air liquide, 99.999 % purity),

ethane (Air liquide, 99.96 % purity), ammonia (Air liquide,

32 E. Rojas et al.

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99.999 % purity). The remaining gases, except ethane,

were passed through a molecular sieve trap. Ethane was

used directly as supplied. The flow rates of the various

gases were monitored by pressure transducers (Bronk-

horst), which had been calibrated individually for each gas.

The reactor consisted of a quartz tube of 29 cm long and

9 mm OD. To prevent the participation of homogeneous

reactions, the reactor was designed to minimize gas-phase

activation of ethane (no void volume). The axial temper-

ature profile was monitored by a thermocouple sliding

inside a quartz tube inserted into the catalytic bed. Heating

of the reactor was done by a cylindrical ceramic heating

furnace which encircled the reactor. Tests were made using

the following reaction feed composition (vol% ): 25 % O2,

9.8 % ethane, 8.6 % NH3 in helium. The total flow rate was

20 ml/min, corresponding to a gas hourly-space velocity

(GHSV) of ca. 3,000 h-1. The quantity of catalyst and total

flow were determined in order to avoid internal and

external diffusion limitations. The reactor outlet is con-

nected to an on-line gas chromatograph (Varian CP-3800)

equipped with flame ionization and thermal-conductivity

detectors, for product analysis. The accuracy of the ana-

lytical determinations was checked for each test by verifi-

cation that the carbon balance (based on the ethane

converted) was within the cumulative mean error of the

determinations (±10 %). Yields and selectivities in prod-

ucts were determined on the basis of the moles of ethane

feed and products, considering the number of carbon atoms

in each molecule. The turnover frequency (TOF) data were

calculated by considering the amount of acetonitrile pro-

duced per Ni atom incorporated in the catalysts

formulation.

3 Results

The composition, BET surface area, NiO average crystal

size and evolution of mixed phase of Nb–Ni–O of refer-

ences materials and catalysts (NBO-series and Nb2O5-ser-

ies) are listed in Table 1. The NBO support presents a high

surface area (197.0 m2/g) that decreases after impregnation

and calcination in the corresponding supported samples

(NBO-series). The NiO average crystal size (calculated by

XRD) increase with NiO coverage for the NBO series. The

Nb2O5-series presents lower BET area values, since the

Nb2O5 support possesses lower specific area than NBO

support. The trends for the both series are quite similar,

decreasing with NiO coverage.

The XRD patterns of the supports and for the fresh and

used samples are shown in Fig. 1. The pattern of pure NiO

sample shows reflections at 2h = 37.2�(110), 43.2�(200),

62.7�(220), 75.4�(311) and 79.4�(222) with a lattice con-

stant of a = 4.1694 A, which is consistent with the cubic

rock salt structure with nickel ions in octahedral sites and

very close to the reference pattern (JCPDS: 4–835) [13].

Figure 1a shows the diffraction patterns corresponding to

the NBO series. NBO support pattern presents two broad

peaks near 27� and 55�, attributed to an amorphous

Table 1 Surface area, lattice

constant and NiO average

crystal of references materials

and of all catalysts

a Nb2O5�xH2O (niobic acid)b (200) Peak higher intensityc Calculated from more

representative peaks of Ni–Nb–O

and NiO phase on XRD

Catalyst Composition

(ICP)

(wt%) Ni

SBET

(m2/g)

NiO average

crystal

size (nm)b

INb–Ni–O/INiOc

Fresh Used

References materials

NBOa – 197.0 – – –

NiO 100 19.7 48 – –

Nb2O5 – 37.9 – – –

NBO-series

10Ni/NBO 9.9 69.1 – – –

20Ni/NBO 18.5 57.8 15 2.42 3.09

25Ni/NBO 26.4 54.9 16 1.09 1.24

30Ni/NBO 30.8 47.8 20 0.88 1.28

35Ni/NBO 33.3 48.3 29 0.62 0.60

40Ni/NBO 41.3 48.5 38 0.35 0.49

Nb2O5-series

10Ni/Nb2O5 9.2 36.9 8 – –

20Ni/Nb2O5 17.1 37.1 16 – 0.14

25NiNb2O5 23.7 33.5 20 0.13 0.19

30Ni/Nb2O5 26.1 34.9 24 0.13 0.12

35Ni/Nb2O5 32.9 35.4 26 0.11 0.20

40Ni/Nb2O5 36.0 29.9 33 0.12 0.13

Nanoscaled Bulk-NiO Catalysts 33

123

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niobium phase [14, 15], these bands are also visible in the

NBO-supported catalysts. The NiO diffraction pattern is

evident at NiO loading above 20 %. Both fresh and used

NBO-samples exhibit additional peaks at 2h = 26.6�,

35.8�, 38.5�, 40.8�, 53.5�, which have been detected also in

bulk Ni–Nb–O catalytic materials [13, 15], and assigned to

a mixed Nb–Ni–O phase. In a previous work the same

pattern was found for Ni–Nb–O bulk catalytic materials,

the HRTEM study showed that it corresponds to the

presence of two different Nb–Ni–O structures, with dif-

ferent crystal size and Nb/Ni atomic ratio [15]. That study

shows that one structure is characterized by two Raman

bands near 850 and 800 cm-1, which corresponds to a Nb-

poor mixed phase with crystal size in the 20–50 nm range.

The other Nb–Ni–O phase is richer in Nb-content, exhibits

a Raman band near 800 cm-1 and possesses smaller crystal

size.

The diffraction peaks at 2h = 22.6�(001), 28.3�(180),

28.8�(200), 36.4�(181), 36.9�(201), 46.0�(002), 50.6�(380),

54.9�(182) dominate the XRD patterns of the Nb2O5-series

(Fig. 1b); these correspond to the hexagonal TT-Nb2O5

phase (JCPDS: 7–61) [14]. The pattern of NiO is also

apparent in the fresh and used catalysts; as expected, the

intensity of its peaks increases with NiO loading. The

peaks associated with a Nb–Ni–O phase in the NBO series

are also visible in fresh and used catalysts in Nb2O5-series

at loading values higher than 20 % (Fig. 1b).

The intensity of the Nb–Ni–O mixed phase diffraction

peaks is weaker in the Nb2O5-series than in the NBO-ser-

ies. A Nb–Ni–O/NiO population ratio was estimated using

the most intense diffraction peaks of both phases (Table 1).

The Nb–Ni–O/NiO ratio is higher in the NBO series than in

the Nb2O5 one; such a ratio decreases upon use in ethane

ammoxidation. The Nb–Ni–O/NiO ratio in the Nb2O5-

series does not show a clear tendency upon use in reaction.

Thus, nickel and niobium oxides further blend during

ethane ammoxidation into Nb–Ni–O mixed phase, which is

consistent with operando Raman-GC observations of this

system during such ammoxidation reaction [16].

The in situ Raman spectra of the supports and of the cat-

alysts under dehydrated conditions (synthetic air, 200 �C)

are shown in Fig. 2. Figure 2a shows the spectra corre-

sponding to the NBO series, which exhibits two broad bands

near 798 and 849 cm-1, in addition to those of the support

(914, 667 and 230 cm-1), which are visible for coverage’s up

to 30 %. The intensity of these two broad bands near 798 and

20 40 60 8020 40 60 80

♦

***

♦♦

♦♦♦

♦

♦♦ ♦

♦♦

♦♦

♦

♦♦♦

♦*

**

***

*

*

*

NiO

*

*

*NiO

40Ni/NBO used

40Ni/NBO fresh

35Ni/NBO used

35Ni/NBO fresh

30Ni/NBO used

30Ni/NBO fresh

25Ni/NBOused

25Ni/NBO fresh

20Ni/NBO used

20Ni/NBO fresh

10Ni/NBO used

NBO

10Ni/NBO fresh

*♦

*

*

*

*

∗

2 theta / º

♦♦♦

♦♦♦

♦♦♦

♦

**

**

**

**

∗

∗

∗

∗∗

♦♦

**

**

*

♦*

∗

∗

∗

∗∗

♦♦

♦

♦

♦

♦♦♦

♦Nb-Ni-O phase

∗

*

***

*

*

*

Nb2O5

* NiO

Ni-Nb-O phase

Inte

nsi

ty (

u.a

.)

2 theta / º

Inte

nsi

ty (

u.a

.)

a

Fig. 1 XRD patterns of fresh and used catalysts of a NBO-series and b of Nb2O5-series

34 E. Rojas et al.

123

Author's personal copy

849 cm-1 increases with the increasing NiO loading. Such

bands have been detected in bulk Ni–Nb–O catalysts [15].

These bands are related with the diffraction peaks at

2h = 26.6�, 35.8�, 38.5�, 40.8�, 53.5� (Fig. 2a). A previous

study combining HRTEM, XRD and Raman spectroscopy

[15] assigns these to the presence of two different Nb–Ni–O

mixed phases: one that present a low Nb content and a

smaller crystal size (4–6 nm), as shown by HRTEM [15];

and another with higher niobium content and larger crystal

sizes (20–50 nm). These same Raman bands are visible in

Nb2O5 series at loading values higher than 20 %, in addition

to those of the support (707, 313 and 245 cm-1) with

TT-Nb2O5 phase (Fig. 2b) being it’s a more ordered struc-

ture than the NBO support.

Figure 3 shows the XPS spectra of fresh catalysts for

NBO- and Nb2O5-series, respectively. In all samples, the

BE obtained for niobium are characteristic of Nb5? species

[14]. The BE in NiO show at 855.0 and 865.9 eV (Ni2? and

Ni3?, respectively) along with the corresponding satellites

at 862.2 and 867 eV [17]. This indicates that both oxidation

20 40 60 80 20 40 60 80

*

* ∗∗ ∗ ∗∗

∗∗∗

∗∗

∗∗∗

∗ ∗

∗∗∗∗∗

∗∗

10Ni/Nb2O5 used

10Ni/Nb2O5 fresh

2 theta / º

Nb2O5

**

∗

∗∗∗∗∗∗

∗∗

∗

∗

25Ni/Nb2O5 fresh

20Ni/Nb2O5 fresh

***** ∗∗

∗∗

∗∗∗∗∗

25Ni/Nb2O5 used

20Ni/Nb2O5 used

♦♦♦

*

****

*

***

*∗∗ ∗∗ ∗ ∗

∗∗

∗

∗*

Nb2O5Nb2O5NiO

♦♦♦

♦

♦♦

Nb-Ni-O phaseNb-Ni-O phase

∗*

***

*

∗∗∗ ∗ ∗∗∗∗

∗∗

∗∗∗∗∗∗∗

∗∗

∗

♦♦♦

♦

NiO*∗

∗∗∗∗∗∗∗

∗∗

∗

***

*

*

*

**

*

*

*

♦

♦♦♦

♦♦♦

Inte

nsi

ty (

u.a

.)

Inte

nsi

ty (

u.a

.)

∗∗∗∗∗∗

∗∗

∗

∗∗∗

∗∗∗∗∗

∗**

*

**

***

*

NiO

40Ni/Nb2O5 used

40Ni/Nb2O5 fresh

35Ni/Nb2O5 used

35Ni/Nb2O5 fresh

30Ni/Nb2O5 used

30Ni/Nb2O5 fresh

2 theta / º

♦♦♦

*

*

♦♦♦∗

∗∗∗∗∗∗

∗∗∗

***

*

♦♦♦

∗∗

∗∗∗∗∗∗

∗

∗

∗

∗

∗∗∗∗

∗

***

**

***

*

bFig. 1 continued

Nanoscaled Bulk-NiO Catalysts 35

123

Author's personal copy

states are present for Ni species on all samples, and the

catalysts with lower coverage’s (10–20 wt%) exhibit higher

Ni2? population. The analysis of the XANES region pro-

vides additional information on the symmetry and the oxi-

dation state of the Ni. Figure 4 shows the Ni K-edge

XANES profiles of some representative supported catalysts,

and that of NiO reference. As expected, all samples exhibit

strong lines, similar to NiO. The spectra exhibit two

regions: the pre-edge peak (*8,328 eV) corresponding to

transitions of 1s photoelectron to 3d orbitals and the main

edge peak (*8,361 eV), which corresponds to transitions

of 1s to p symmetry levels [17, 18].The main edge energy in

all catalysts were not shifted significantly from that of NiO

indicating that the dominant oxidation state of Ni in all

metal loadings were ?2 [17].

Figure 5 shows ethane conversion and the yield values

to main products delivered versus reaction temperature by

both series during ethane ammoxidation. These results

show that acetonitrile is the main product in all the cases.

The activity of NBO series is quite low at low loadings,

with ethane conversion below 10 % (Fig. 5a), whereas the

Nb2O5-series delivers higher ethane conversions and,

subsequently, higher acetonitrile yields (Fig. 5b). The

activity is higher on NBO series at higher loadings,

affording 17.2 % yield at 425 �C. Such value is very close

to that reported for the bulk supported catalysts at higher

temperature, indicative that the supported catalytic mate-

rials reported here perform better [15]. This trend is also

apparent in Fig. 6, which shows acetonitrile selectivity

versus ethane conversion profiles by both series. The

selectivity to acetonitrile is higher for the catalysts with

higher NiO loading for NBO-series (Fig. 6a). Acetonitrile

selectivity remains quite high (up to 60 %) in the conver-

sions range studied, and higher that those reported for the

bulk series [15]. Acetonitrile selectivity is lower for the

samples with low coverage in the NBO series, while the

Nb2O5 series delivers lower selectivity (30–35 wt%) at

high NiO loading values (Fig. 6b). Figure 7 shows the

acetonitrile produced versus NiO coverage for both series,

expressed as activity per nickel site (TOF values, Fig. 7a)

20040060080010002004006008001000

10Ni/NBO-used

10Ni/NBO-fresh

NBO

ccc b

bb

Raman shift (cm-1)

25Ni/NBO-fresh

20Ni/NBO-used

20Ni/NBO-fresh

40Ni/NBO-used

NiO

40Ni/NBO-fresh

35Ni/NBO-used

35Ni/NBO-fresh

30Ni/NBO-used

30Ni/NBO-fresh

25Ni/NBO-used

b

a

a

Inte

nsi

ty (

u.a

.)

Inte

nsi

ty (

u.a

.)

b - Nb-Ni-O phasec - Nb

2O

5

a - NiOb - Nb-Ni-O phasec - Nb

2O

5

a - NiO

Raman shift (cm-1)

aFig. 2 In situ Raman spectra of

dehydrated fresh and used

catalysts of a NBO-series and

b Nb2O5-series

36 E. Rojas et al.

123

Author's personal copy

and per gram of catalysts (Fig. 7b). In both cases it can be

appreciated how the Nb2O5-series is more active below

25 wt% NiO loading, while the NBO-series is more effi-

cient at high NiO loading.

Figure 8 shows the XRD Ni–Nb–O/NiO intensity ratio

(Table 1) versus the yield to acetonitrile in the NBO-series;

these data were not plotted for the Nb2O5-series due to the

low intensity of these peaks. The best performance values

correspond to the samples with NiO loading in the

30–40 wt% range, which corresponds to a Ni–Nb–O/NiO

XRD intensity ratio near 0.5. These results are in line with

those reported previously [11] and confirm that the pres-

ence of NiO sites along with Ni–Nb–O is necessary for

efficient ethane ammoxidation. In addition, the catalytic

data reported here indicate that NiO and Ni–Nb–O phases

are more active when they are present with an adequate

crystal size. In order to have a high amount of dispersed

NiO nanoparticles, it is more convenient the use of a

support with high surface area (NBO series).

4 Discussion

NiO coverage and the nature of niobia support have a clear

effect on catalytic performance, which appears to be related

to the nature of Nb–Ni interaction, since the size and nature

of the active phase can be modulated by the use of the

appropriate support and NiO loading (Scheme 1). High-

20040060080010002004006008001000

b - Nb-Ni-O phase

c - Nb2O

5

a - NiO

bb

cc

c

cc

c

a

b - Nb-Ni-O phase

c - Nb2O

5

a - NiO

40Ni/Nb2O

5used

40Ni/Nb2O

5 fresh

35Ni/Nb2O

5 used

35Ni/Nb2O

5 fresh

30Ni/Nb2O

5 used

30Ni/Nb2O

5 fresh

25Ni/Nb2O

5used

25Ni/Nb2O

5 fresh

20Ni/Nb2O

5 used

20Ni/Nb2O

5 fresh

10Ni/Nb2O

5 used

10Ni/Nb2O

5 fresh

Nb2O

5

Raman shift (cm-1)Raman shift (cm-1)

NiO

Inte

nsi

ty (

u.a

.)

Inte

nsi

ty (

u.a

.)

bFig. 2 continued

Nanoscaled Bulk-NiO Catalysts 37

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Author's personal copy

surface area niobia support (NBO) favors NiO particles

dispersion, which size, as calculated by XRD, remains low

(Table 1). In addition, NBO support is amorphous (Fig. 1a),

and it possesses a more flexible structure, easier to interact

with other molecules, such as, in this case, nickel oxide

particles [19]. Both factors favor the Ni–Nb interfaces and

thus the formation of Nb–Ni–O phases. At low coverage on

NBO, the extent of Nb–Ni–O active phase formation is

850855860865870 204206208210212

Binding energy (eV)

35Ni/NBO

30Ni/NBO

25Ni/NBO

20Ni/NBO

Nb3d

Ni2+

Ni3+

Ni2p3/2

10Ni/NBO

NiO

Binding energy (eV)

850855860865870 204206208210212

NiO

35Ni/Nb2O

5

Nb 3d

30Ni/Nb2O

5

25Ni/Nb2O

5

Ni 2p3/2

10Ni/Nb2O

5

20Ni/Nb2O

5

Ni3+

Ni2+

Bending energy (eV)Bending energy (eV)

a b

Fig. 3 a XP spectra of catalysts of NBO-series in the Ni 2p3/2 and Nb3d regions. b XPS spectra of catalysts of Nb2O5-series in the Ni 2p3/2 and

Nb3d regions

0.0

0.4

0.8

1.2

1.6

8361

No

rmal

ized

χμ(

Ε)

Energy (eV)

30Ni/NBO 35Ni/NBO 40Ni/NBO NiO

8328

a

8300 8320 8340 8360 8380 8400 8300 8320 8340 8360 8380 84000.0

0.4

0.8

1.2

1.6

8361

8328

b

No

rmal

ized

χμ(

Ε)

Energy (eV)

25Ni/Nb2O

5

30Ni/Nb2O

5

35Ni/Nb2O

5

NiO

Fig. 4 XANES spectra of a NBO-series and b Nb2O5-series

38 E. Rojas et al.

123

Author's personal copy

significant, minimizing the presence of remaining NiO

nanoparticles; being the activity of these catalysts not sat-

isfactory (Fig. 8). This confirms the relevance of coexis-

tence of Ni–Nb interaction along with NiO domains for

ethane ammoxidation. Activity results suggest that Nb–Ni

interaction steers selectivity towards nitrile insertion, but

the coexistence of NiO is also necessary. XRD diffraction

intensities have been used to estimate an optimal Ni–Nb–O/

NiO ratio, which value is near 0.5.

For catalysts with high coverage, the trend is the

opposite to that observed at low loadings (Fig. 7), in this

case the NiO particles dispersion is lower in both supports,

and the crystal size in the case of the catalysts prepared

with the high surface area support (NBO) is higher, and

present better performances with respect NBO support

(Figs. 7, 8). For the same NiO contents and using the

support with low surface area, the NiO particles seems to

be agglomerated, since for Nb2O5 series the XRD patterns

indicate that the structure is more close to a mixture of

Nb2O5 and NiO oxides than a supported catalysts (Sche-

me 1b), thus, in this case the NiO crystal sizes found are

smaller than for the NBO series for the same NiO contents

(Table 1). Although in this case the amount of both Ni and

Nb species is high, the characterization data indicate that

the extent of formation of Nb–Ni–O phases is not favored

in this case (Nb2O5 support at high loading), due to the

poor dispersion and the big size of the NiO particles.

The catalysts with 40 % NiO content on the NBO sup-

port present remarkable ethane ammoxidation results

(Fig. 7b), with a high acetonitrile production per gram of

0

10

20

30

40

50

0

10

20

30

40

50

0

10

20

30

40

50

0

10

20

30

40

50

0

10

20

30

40

50

350 375 400 425

0

10

20

30

40

50

Eth

ane

con

vers

ion

or

Yie

ld (

%)

Temperature (ºC)

BA

Eth

ane

con

vers

ion

or

Yie

ld (

%)

Temperature (ºC)

DC

Temperature (ºC)Temperature (ºC)

Eth

ane

con

vers

ion

or

Yie

ld (

%)

Temperature (ºC)

Eth

ane

con

vers

ion

or

Yie

ld (

%)

Temperature (ºC)

FE

Eth

ane

con

vers

ion

or

Yie

ld (

%)

Eth

ane

con

vers

ion

or

Yie

ld (

%)

Ethane conversion CO

2

Ethylene Acetonitrile Acetaldehyde Methane

350 375 400 425

350 375 400 425 350 375 400 425

350 375 400 425 350 375 400 425

aFig. 5 a Ethane conversion and

yield profiles for products

different versus temperature on;

(A) 10Ni/NBO, (B) 20Ni/NBO,

(C) 25Ni/NBO, (D) 30Ni/NBO,

(E) 35Ni/NBO, (F) 40Ni/NBO.

b Ethane conversion and yield

profiles for products different

versus temperature on;

(A) 10Ni/Nb2O5, (B) 20Ni/

Nb2O5, (C) 25Ni/Nb2O5,

(D) 30Ni/Nb2O5, (E) 35Ni/

Nb2O5

Nanoscaled Bulk-NiO Catalysts 39

123

Author's personal copy

0

10

20

30

40

50

0

10

20

30

40

50

0

10

20

30

40

50

0

10

20

30

40

50

350 375 400 425 350 375 400 425

0

10

20

30

40

50

Eth

ane

conv

ersi

on o

r yi

eld

(%)

Temperature (ºC)

Ethane conversion CO

2

Ethylene Acetonitrile Methane

A

C

B

Eth

ane

conv

ersi

on o

r yi

eld

(%)

Temperature (º C)

Eth

ane

conv

ersi

on o

r yi

eld

(%)

Temperature (º C)

E

D

Eth

ane

conv

ersi

on o

r yi

eld

(%)

Temperature (º C)

Eth

ane

conv

ersi

on o

r yi

eld

(%)

Temperature (º C)

350 375 400 425 350 375 400 425

350 375 400 425

bFig. 5 continued

0

20

40

60

80

100

Sel

ectiv

ity to

ace

toni

trile

(%

)

Ethane conversion (%)

NBO 10Ni/NBO 20Ni/NBO 25Ni/NBO 30Ni/NBO 35Ni/NBO 40Ni/NBO NiO

0 5 10 15 20 25 30 35 400

20

40

60

80

100

Sel

ectiv

ity to

ace

toni

trile

(%

)

Ethane conversion (%)

Nb2O

5

10Ni/Nb2O

5

20Ni/Nb2O

5

25Ni/Nb2O

5

30Ni/Nb2O

5

35Ni/Nb2O

5

0 5 10 15 20 25 30 35 40

a bFig. 6 Acetonitrile selectivity

as a fuction of ethane

conversion for a NBO-series

and b Nb2O5-series

40 E. Rojas et al.

123

Author's personal copy

catalysts. Raman spectroscopy (Fig. 2a) shows that with

this synthesis method the Nb–Ni–O mixed phase forms,

delivering a catalyst selective for acetonitrile formation.

Since this sample possesses a high amount of NiO nano-

particles with the adequate size (20–50 nm) and approxi-

mately an Nb–Ni–O/NiO XRD intensity ratio of 0.5

(Scheme 1a). Mikolajska et al. [20] identified two different

mixed phases in these catalytic materials by HRTEM and

studied their catalytic properties during the ethane ODH

reaction. One is a Ni-rich crystalline phase with some Nb

cations incorporated in the NiO lattice, and the other is a

highly distorted Nb-rich NiO-phase. We have also

observed by HRTEM the presence of such structures and

assigned the corresponding Raman bands [15], assessing

their performance for ethane ammoxidation; better per-

formances were obtained with the Ni- rich catalyst. The

40Ni/NBO catalyst presents the Nb–Ni–O/NiO XRD

intensity ratio closest to 0.5 (Fig. 8). The data studied in

the present paper indicate that the impregnation method on

the support with high surface area (NBO) promotes the

Nb–Ni interaction to a higher extent than on Nb2O5-series,

as confirmed by Raman and XRD data. This would account

for the better performance of the NBO-series.

The catalytic performance improves when the active

phase (NiO) is nanoscaled and stabilized on an adequate

support since data suggest that NBO support would be most

suitable than Nb2O5-series, since it is not calcined. It

possesses lower crystal size and it is essentially present as

niobic acid, which is structurally more reactive than upon

calcination (which generates the Nb2O5-series). This rele-

vance of nanoscaled nature of the supported phase is in line

with previous results for several supported oxide catalysts,

such as Sb–V–O, VPO, or Mo–V–Nb–Te–O [8, 10, 21].

0.0

0.4

0.8

1.2

1.6

TOF

(s

-1)

% wt. NiO

NBO - series Nb

2O

5-series

a

10 20 30 40 10 20 30 400.0

0.4

0.8

1.2

1.6b

% wt. NiO

Pro

duct

ivity

(g A

CE/g

cat*h

)

NBO - series Nb

2O

5-series

Fig. 7 a Acetonitrile produced per Ni site (TOF values) and b productivity in both NBO and Nb2O5 series: reaction conditions; feed composition

(% volume), C3H8/O2/NH3/He (9.8/25/8.6/x), 200 mg of catalyst, T = 425 �C

0.0 0.5 1.0 1.5 2.0 2.50

4

8

12

16

20 NBO - series40 % 35 %

30 %

25 %

Yie

ld t

o a

ceto

nit

rile

(%

)

INb-Ni-O

/INiO

20 %

Fig. 8 Intensity ratio of Nb–Ni–O/NiO phases on function of yield to

acetonitrile for catalysts of NBO-series. Calculated with the more

representative peaks of Nb–Ni–O and NiO phase on XRD

Scheme 1 NiO and Nb–Ni–O nanoparticles on the two different

supports (NBO and Nb2O5)

Nanoscaled Bulk-NiO Catalysts 41

123

Author's personal copy

5 Conclusions

Both NiO and Ni–Nb–O mixed phase are required to

deliver efficient ethane ammoxidation. The results pre-

sented in this paper, indicate that the best catalytic results

were found when the Nb–Ni–O/NiO peak XRD intensity

ratio is near 0.5. This work shows that both NiO crystal

size and Nb–Ni–O/NiO population ratio can be modulated

with the use of the appropriate support and with NiO

coverage. This would deliver supported catalysts using an

economic and simple preparation procedure that performs

better than their bulk counterparts.

Supporting nanoscaled bulk catalysts improves their

cost and mechanical properties; in addition, they increase

the exposure of the active site, triggering the activity per

gram of active component. In addition, supporting nano-

scaled oxide particles would enable easier modulation of

the activity by controlling coverage and support interac-

tion. Such control can be made by the use of the adequate

support and with the appropriate synthesis parameters.

Acknowledgments The Ministry of Science and Innovation (Spain)

funded this study under project CTQ2011-25517. E. Rojas thanks

(Mexico) and ICyTDF (Mexico) for her PhD program fellowship. We

acknowledge the European Synchrotron Radiation Facility for pro-

vision of synchrotron radiation facilities (Spanish CRG X-ray

beamline, Grenoble, France), and also to German Castro and Ivan da

Silva, Spline staff members, for their help in XANES measurements.

The authors thank R. Lopez-Medina for his help with catalytic tests.

The authors thank CBMM, Brazil, for providing the hydrated pent-

oxide sample.

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