Performance of NiO and Ni–Nb–O active phases during the ethane ammoxidation into acetonitrile

CatalysisScience & Technology

PAPER

a Catalytic Spectroscopic Laboratory, Instituto de Catálisis y Petroleoquímica,

CSIC Marie Curie 2, E-28049-Madrid, SpainbDepartamento de Ciencia de los Materiales e Ingeniería Metalúrgica y Química

Inorgánica, Universidad de Cádiz, E-11510-Puerto Real, Spainc Departamento de Ingeniería Química, Universidad de Málaga, E-29071-Málaga,

Spain. E-mail: [email protected]

Catal. Sci. Technol.This journal is © The Royal Society of Chemistry 2013

Cite this: Catal. Sci. Technol., 2013, 3,3173

Received 17th June 2013,Accepted 30th August 2013

DOI: 10.1039/c3cy00415e

www.rsc.org/catalysis

Performance of NiO and Ni–Nb–O active phases duringthe ethane ammoxidation into acetonitrile

Elizabeth Rojas,a Juan J. Delgado,b M. Olga Guerrero-Pérezc and Miguel A. Bañaresa

There is a need to develop catalysts for the direct ammoxidation of ethane to acetonitrile. This work reports a

series of bulk Ni–Nb–O catalysts with increasing niobium content, analysing the structure and the effect on acidity

and redox properties and how these relate to Ni–Nb interaction. Niobium doping affects NiO lattice leading to

progressively smaller unit cell sizes. The Nb/Ni atomic ratio determines the formation of two Nb–Ni–O mixed

phases identified by HRTEM, and which Raman bands are identified near 850 cm−1 and 800 cm−1 for a Nb-poor

phase and near 800 cm−1 for a Nb-rich Ni–Nb–O one.

Introduction

Nitriles, like acrylonitrile and acetonitrile, are useful chemi-cal intermediates.1 Acetonitrile is used as solvent in manyindustrial processes, such as high performance liquid chro-matography or for the butadiene extraction in hydrocarbonstreams. It is also used in several organic and inorganic syn-theses, such as the synthesis of flavones and flavonol pig-ments. Acetonitrile can also be used to make acrylonitrile byreaction with methane2 or methanol.3 Currently, acetonitrileis obtained as a by-product during the propylene ammoxida-tion process to obtain acrylonitrile, delivering 2–4 L ofacetonitrile per 100 L of acrylonitrile produced. Acetonitrilecan also be produced by other methods, such as the dehydrationof acetamide,4 but these do not have commercial relevance atthis moment in time.

Currently, there is no method for the direct commercialsynthesis for acetonitrile, which creates a need for such adirect process, being ethane a most attractive raw material.

Alkane feedstocks are currently produced by steam crack-ing of various petroleum fractions, delivering an easy supplyof ethane, which, in addition, is the second major compo-nent in natural gas.5 Alkane ammoxidation reactions, inwhich the hydrocarbon is oxidized in the presence of ammo-nia, are increasingly attracting interest as a route to obtainnitriles from alkane feedstocks.1,6,7 In recent years, propaneammoxidation has attracted much attention and efficient cat-alysts have been developed.9–13 Such technology is now avail-able at a commercial scale until very recently Asahi Kasei

Chemical opened a 200 000 tonne per year propane ammoxi-dation plant in Thailand.14 Propane-based technology wouldreplace the propylene-based technology. But the reaction condi-tions to activate alkanes are more energy demanding thanthose of olefins,8 which has a negative effect on selectivity.

Unlike propane ammoxidation, ethane ammoxidation is notthat extensively studied, in fact, there are just few studies deal-ing with the ethane ammoxidation reaction.15–19 Since Sb–V–Ocatalytic system has demonstrated to be active and selective forthe propane ammoxidation route,1 we also investigated thepossibility of using those catalysts for the ethane ammoxida-tion process, but they proved to be not efficient. The combina-tion of theoretical and experimental data demonstratedthat adsorbed C2 molecules are displaced by much strongeradsorption of ammonia molecule on both, antimony andvanadium sites at the surface of the vanadium antimonite.20–22

Those data confirm that another catalytic system must bedeveloped for the ethane ammoxidation process. Aliev andSokolovskii studied the ethane ammoxidation to acetonitrileover Cr–Nb–Mo–O catalyst at 350–500 °C, achieving 10%selectivity to acetonitrile at 18% ethane conversion.15 Later,Catani and Centi investigated ethane ammoxidation overalumina supported Nb–Sb oxides; reporting a selectivity closeto 50% for conversions of 40% at 530 °C.23 More recently,better performances have been described with cobalt ionexchanged zeolites. Y-zeolite and dealuminated USY showed ayield to acetonitrile close to 15% at 24% ethane conversion at450 °C.24 Li and Armor reported significantly improved yieldsto acetonitrile, near 26%, at 47% ethane conversion at 475 °Cover a catalyst based on Co exchanged into Beta zeolite.18

Lemonidou and co-workers reported that niobium-promoted NiO catalysts are able to activate the ethanemolecule during the oxidative dehydrogenation reaction.25,26

Our group reported some preliminary results showing thatNi–Nb–O bulk oxide catalysts are also promising during the

, 2013, 3, 3173–3182 | 3173

Catalysis Science & TechnologyPaper

direct ammoxidation of ethane to acetonitrile.19 Since thatstudy indicated that NiO catalyst was not able to incorporatethe nitrile group into ethane molecule in absence of niobiumin the catalyst formulation, we investigated supportedNiO/Nb2O5 catalysts for ethane ammoxidation.27,28 Twodifferent synthesis methods for those supported catalystswere studied, a conventional impregnation,27 and a noveldry mixing method.28 In both cases, these studies show thatNiO oxide phase is required along with the presence of a mixedNi–Nb–O phase to obtain efficient ethane ammoxidation. Thus,the objectives of this paper are to investigate the performanceof bulk Ni–Nb–O catalysts with respect to that of supportedNiO/Nb2O5 catalyst series to uncover the nature and character-ize the two active phases which coexistence appear necessaryfor ethane ammoxidation, NiO and Ni–Nb–O mixed phase.

Experimental proceduresPreparation of samples

Bulk Ni–Nb–O catalysts were prepared by evaporation of solu-tions of the corresponding precursors. Ammonium niobiumsoluble complex (CBMM-Niobium Products) was added to anaqueous solution of nickel acetate tetrahydrate (>99%,Aldrich) in the appropriate proportion depending the desiredNi/Nb atomic ration. This solution was kept under stirring at80 °C during 1 h to ensure complete dissolution and goodmixing of the starting compounds. The resulting solutionwas dried in a rotatory evaporator at 80 °C under a reducedpressure of 10–40 mmHg. The resulting solid was dried at120 °C for 24 h and then calcined at 450 °C during 5 h, at aheating rate of 5 °C min−1 in a flow of synthetic air and thencooled to ambient temperature inside the oven. Bulk NiO wasprepared by calcination in synthetic air of nickel acetate at450 °C for 16 h and bulk Nb2O5 used as reference was pre-pared by calcinations in synthetic air of hydrated niobiumpentoxide at 400 °C by 12 h. The nomenclature of catalystswas as follows: NiyNbx; where y and x indicate the atomiccontent of Ni and Nb, respectively.

Characterization

Nitrogen adsorption isotherms (−196 °C) were recorded by anautomatic Micromeritics ASAP-2000 apparatus. Prior to theadsorption experiments, samples (ca. 0.2 g) were outgassedunder vacuum of 10−3 Pa at 140 °C during 2 h to removeadsorbed species. BET areas were computed from the adsorp-tion isotherms (0.05 < P/Po < 0.27), taking a value of 0.164 nm2

for the cross-section of the adsorbed N2 molecule. ICP elemen-tal chemical analysis of the niobium-promoted NiO catalystswas made on a Perkin-Elmer-3300 DV-Disgreggation using Nband Ni elemental standards (Alfa Aesar). Around 0.1 g ofsample were dissolved with nitric, chloride, and fluoride acidsin a microwave milestone MLS 1200 MEGA with a maximumpower of 650 W.

X-ray diffraction patterns were recorded on a SiemensKrystalloflex D-500 diffractometer using Cu Kα radiation(λ = 0.15418 nm) and a graphite monochromator. Working

3174 | Catal. Sci. Technol., 2013, 3, 3173–3182

conditions were 40 kV, 30 mA and scanning rate of 2° min−1

for Bragg's angles (2θ) from 10° to 90°. The crystallite size ofthe synthesized nickel catalysts, Dc, was calculated from themajor diffraction peaks of the base of (200) using the Scherrerformula: Dc = (Kλ)/(Bcosθ); where K is a constant (ca. 0.9)assuming particles spherical; λ is the X-ray wavelength used inXRD (λ = 1.5418 Å); θ is the Bragg angle; B is the purediffraction broadening of a peak at half-height, that is, broaden-ing due to the crystallite dimensions.29

Ammonia chemisorption experiments were performedusing an ASAP-2000 apparatus. The catalysts (0.2 g) werepretreated at 250 °C for 0.5 h and then cooled to 100 °C underHe flow to remove adsorbed species. The pretreated sampleswere exposed to 5% NH3/He, with subsequent flushing withhelium at 100 °C for 1 h to remove the physisorbed ammonia.The acidity values were determined from the differencebetween ammonia adsorption and desorption isotherms.

X-ray photoelectron spectra were recorded on a VG Escalab200R electron spectrometer using a Mg Kα X-Ray excitingsource (1254.6 eV, 10 kV, 100 W) of a twin anode in the con-stant analysed energy mode, with a pass energy of 25 eV forgeneral spectra and 75 eV for acquisition of spectral regions.Spectra were recorded for Ni 2p, Nb 3d, C 1s and O 1sregions. The accuracy of the BE was ±0.1 eV. The samples inthe form of self-supporting wafers of 10 mm diameter wereoutgassed overnight in the preparation chamber of the spec-trometer and subsequently transferred to the analysis cham-ber. The pressure of the main chamber during spectraacquisition was maintained at ca. 1 × 10−5 Torr. The residualpressure in the turbo-pumped analysis chamber was keptbelow 7 × l0×9 Torr during data collection. Each spectralregion was signal-averaged for a given number of scans toobtain good signal-to-noise ratios. Although surface chargingwas observed on all the samples, accurate binding energies(BE) were determined by charge referencing with the C 1sline at 284.6 eV. Data processing was performed with the XPSpeak program, the spectra were decomposed with the leastsquare fitting routine provided with the software withGaussian/Lorentzian (90/10) product function and aftersubtracting a Shirley type background. Atomic surface ratioswere obtained using the area under the curve of Ni 2p,Nb 3d, C 1s and O 1s core-level photoemission spectra withthe atomic sensitivity factors reported by Wagner et al.30

Raman spectra were run with a single-monochromatorRenishaw System 1000 equipped with a cooled CCD detector(−73 °C) and an Edge filter, which removes the elastic scatter-ing. The samples were excited with the line from an Ar+ laser(488 nm) operating at 10 mW; spectral resolution was ca. 4 cm−1;spectra acquisition consisted of 10 accumulations of 30 s.The signal was collected by using a microscope Ramanspectrometer (Renishaw Micro-Raman System 1000) in the100–1500 cm−1 Raman shift range. The spectra were obtainedunder dehydrated conditions (50 ml min−1 synthetic air,ca. 200 °C) in a hot stage (Linkam TS-1500).

TPR experiments were carried out on a TPD/TPR 2900(Micromeritics) instrument. In a typical experiment ca. 20 mg

This journal is © The Royal Society of Chemistry 2013

Catalysis Science & Technology Paper

of oven-dried samples (110 °C for 12 h) was taken in aU-shaped quartz tube (i.d. 6 mm). Prior to TPR studies, thecatalyst sample was pretreated in an inert gas (He, 50 ml min−1)at 300 °C for 2 h. After pretreatment, the sample was cooledto ambient temperature and the carrier gas consisting of 5%hydrogen balance argon (30 ml min−1), was allowed to passover the sample and the temperature was increased from ambi-ent temperature to 1000 °C at a heating rate of 10 °C min−1.Hydrogen concentration in the effluent stream was monitoredwith a thermal conductivity detector.

High-Resolution Transmission Electron Microscopy (HRTEM)and High-Angle Annular Dark Field-Scanning TransmissionElectron Microscopy (HAADF-STEM) images were recordedon a JEOL-2010F instrument. The spatial resolution atScherzer defocus conditions in HRTEM mode is 0.19 nm,while the HAADF-STEM studies were performed using anelectron probe of 0.7 nm of diameter and a diffraction cameralength of 10 cm. It should be pointed that the chemicalcomposition of the sample was studied in STEM mode usingan Energy-dispersive X-ray spectrometer (Oxford Instrument,Inca Energy-200).

Catalytic measurements

Activity tests were performed with a tubular quartzmicroreactor (OD = 9 mm, ID = 6.8 mm, length = 290 mm)coupled to an online gas chromatograph (Varian CP-3800)equipped with both flame ionization and thermal conductiv-ity detectors. The GC columns consisted of a molecular sieve5A and a Porapak Q. To prevent participation of homoge-neous reactivity, the reactor was designed to minimize gas-phase activation of propane, minimizing void volume beforeand after the catalytic bed. The catalytic tests were madeusing 0.2 g of powder sample with particle dimensions in the0.250–0.125 mm range. The catalytic bed (ca. 1 cm) waslocated between two quartz wool beds and pretreated in airflow at 200 °C for 1 h. The axial temperature profile wasmonitored by a thermocouple sliding inside a quartz tubeinserted into the catalytic bed. Tests were made using the fol-lowing reaction feed composition (% volume): 25% O2, 9.8%ethane, 8.6% NH3 in balance helium. The total flow rate was20 ml min−1, corresponding to a gas hourly-space velocity(GHSV) of ca. 3000 h−1. The quantity of catalyst and total flowwere determined in order to avoid internal and external diffu-sion limitations.31 The accuracy of the analytical determina-tions was checked for each test by verification that thecarbon balance (based on the ethane converted) was withinthe cumulative mean error of the determinations (±10%).Yields and selectivities of the reaction products were deter-mined on the basis of the moles of ethane feed and products,considering the number of carbon atoms in each molecule.

Ethane conversion, selectivities, and yields are defined asfollows:

conversion of C2H6: X ¼

X

i

yini

yEnEX

i

yini


selectivity of product Pi: Si ¼ yiniX

i

yini

yield of product Pi: Yi ¼ X * Si100

where yi and yE are the mole fractions of products Pi andC2H6, respectively; ni and nE are the number of carbon atomsin each molecule of product Pi and C2H6, respectively, and allthe terms were evaluated for the exit stream. The major prod-ucts of ethane ammoxidation reaction over niobium-promoted NiO catalysts are C2H3N, C2H4, CO2. The turnoverfrequency (TOF) data were calculated by considering theamount of acetonitrile produced per Ni atom (determined inthe XPS analyses). Since this is closer to the concentration ofNi on the surface of the catalysts and, thus, accessible toreactants. We consider it more accurate to determine theTOF value based on XPS than on ICP results, which indicatethe bulk Ni content.

Results

Fig. 1 shows the XRD patterns of fresh and used catalysts. Alldiffractograms present peaks at 37.31°, 43.47°, 62.98°, 75.61°,and 79.59°, which correspond to the (111), (200), (220), (311),(222) crystalline planes of NiO with cubic rock salt structureand a lattice constant of a = 4.1694 Å (Table 1)(JCPDS 4-835).26

This NiO pattern is detected for all the fresh samples,being sharper as the niobium content increases. Ni0.7Nb0.3,Ni0.6Nb0.4, Ni0.4Nb0.6 and Ni0.35Nb0.65 samples exhibitadditional diffraction peaks after ammoxidation reaction,uncovering some structural arrangement induced by catalyticreaction. The intensity of this reaction-induced phase dif-fraction peak increases with Nb content (Fig. 1). Millet'sgroup also detected those peaks in NiO-Nb2O5 catalysts withNb contents above 15 wt% and identified it as NiNb2O6.

32 Itshould be noted that these signals are not present in the cat-alysts with the highest niobium content (Ni0.2Nb0.8 used). Infresh and used samples with Nb contents higher than 15–20%,it appears a broad band near 27° that have been attributedto an amorphous Nb-rich phase by Lemonidou et al.26,33

The intensity of such band increases with Nb content. Noneof the diffraction pattern peaks of crystalline Nb2O5 oxideare detected in any sample; this is indicative that niobiumoxide would remain as an amorphous phase or that it wouldbe present in very small crystalline domains, not large enoughto generate a diffraction pattern (ca. <4 nm).

The NiO lattice constant values and NiO average crystalsizes are listed in Table 1. Pure NiO crystallizes in the cubicrock salt structure, with a lattice parameter of a = 4.1694 Å ́.The niobium incorporation progressively contracts the latticeparameter; NiO and Ni0.9Nb0.1 exhibit the largest NiO crystalcell size. The BET area obtained for NiO oxide is quite low(20 m2 g−1) but increases upon niobium incorporation,reaching a maximum for Ni0.6Nb0.4 sample. On the other hand,NiO crystallite size in the mixed oxides is smaller than in thepure oxide (Table 1). This may reflect the incorporation of Nbcations into the NiO lattice structure. This is possible since

Catal. Sci. Technol., 2013, 3, 3173–3182 | 3175

Fig. 1 XRD patterns of fresh and used catalysts.

Table 1 Physicochemical characteristics and composition of pure NiO and Ni–Nb–O niobium-promoted NiO catalysts

Catalyst Nb/Ni atomic ratio SBET m2 g−1)a NiO lattice constant (by DRX)b NiO average crystal size, Dc (nm) Total acidity (μmol NH3 m

−2)

NiO 0 20 4.1694 48 1.9Ni0.9Nb0.1 0.111 57 4.1692 35 2.0Ni0.85Nb0.15 0.176 65 4.1602 17 2.3Ni0.8Nb0.2 0.250 65 4.1666 17 2.8Ni0.7Nb0.3 0.428 66 4.1664 20 3.8Ni0.6Nb0.4 0.666 97 4.1634 19 3.1Ni0.4Nb0.6 1.500 94 4.1572 17 3.5Ni0.35Nb0.65 1.857 84 4.1514 11 3.5Ni0.2Nb0.8 4 46 4.1444 15 3.3Nb2O5 68 0.4

a SBET: surface area. b Determined considering the (200) peak higher intensity.


the ionic radii of Ni2+ and Nb5+ cations are quite similar (0.69 Åand 0.64 Å, respectively). The acidity of the samples calculatedfor ammonia adsorption are shown in Table 1, as expected,acidity increases with progressive Nb incorporation.

Fig. 2 shows the TPR profiles of reference materials and offresh catalysts. The TPR profile of nickel oxide exhibits areduction peak near 335 °C with a shoulder near 266 °C. Thisreduction peak has been attributed to the Ni2+→Ni0 reduc-tion step, whereas the shoulder at lower temperature hasbeen related to the reduction of Ni3+ to Ni2+, which may bepresent in non-stoichiometric NiO.34–36 The shoulder around266 °C is not visible in the samples after niobium


incorporation. The fact that XPS data (Table 3) that indicatedthat Ni3+ species population decreases with niobium content,suggests that the Ni3+ sites present at the surface of NiOwould reaction with niobium into Ni2+ phase. A new reduc-tion peak at 370 °C becomes increasingly apparent withniobium addition. This new reduction peak must to berelated to the reduction of Nb-containing NiO domains, prob-ably to the mixed Nb–Ni–O phases detected by XRD (Fig. 1).Such phases are already apparent to Raman spectroscopy atvery low niobium loading (Fig. 3). The additional weak asym-metric reduction peak near 855 °C, in catalysts with high Nbcontents (Ni0.35Nb0.65 and Ni0.2Nb0.8), has been assigned to the


Fig. 3 Raman spectra of fresh (left) and used (rig

Fig. 2 Temperature-programmed reduction profiles catalysts and NiO and Nb2O5

reference materials.



reduction of the Nb5+ cations in Nb2O5,35 these must be very small

domains since they do not generate X-ray diffraction pattern.Fig. 3 shows the Raman spectra of fresh and used catalysts.

The Raman bands near 504 and 1050 cm−1 are assigned to NiOphase and, as expected, their intensity decreases with niobiumcontent. The Raman bands near 910, 664 and 248 cm−1, belongto Nb2O5 oxide, and are visible in fresh and used Ni0.35Nb0.65and Ni0.2Nb0.8 samples. Nb2O5 was not detected by XRD,indicating that their crystal sizes are not big enough to gene-rate a diffraction pattern. Two additional bands are detectednear 797 and 854 cm−1, which are more intense in usedniobium-containing samples and grow stronger as niobiumcontent increases. These bands have to be due to Nb–O–Nimixed phases, in line with XRD data (Fig. 1). Only one Ramanband near 800 cm−1 is detected in this region at higher Nbcontents (Ni0.2Nb0.8). These Raman bands in the 790–850 cm−1

range have been assigned to vibrations of bridging Nb–O–Nibonds.37 Thus, in the Ni–Nb–O vibrations range (790–850 cm−1),only two Raman bands are detected for samples with lowniobium contents; whereas only one is apparent at higherniobium contents. This trend may indicate that two differentNb–Ni–O phases form.

ht) catalysts. a (NiO), b (Nb2O5), c (Ni–Nb–O).



Table 2 and Fig. 4 show the XPS and the bulk (ICP) atomiccompositions. Niobium surface concentrations are higherthan the corresponding bulk values, indicating that the sur-face is niobium-enriched. Table 3 shows the binding energy(BE) values. Ni 2p3/2 in NiO possesses two bands at 853.8 and855.7 eV, which correspond to Ni2+ and Ni3+, respectively,along with two satellites at 862.2 and 867 eV.38 The peak areavalues of these two peaks are listed in Table 3, illustrating anevolution of the Ni2+ and Ni3+near-surface populations. Thesedata indicate a progressive depletion of Ni3+ species by niobium,

Table 3 XPS binding energy data and contribution from the total area of thepeaks

Catalysts Ni 2p3/2 O.S.a Nb5+ (3d5/2) O 1s

NiO 855.7 (56.8%)a 3+ — 529.1 (64.2%)853.8 (43.2%) 2+ — 530.3 (35.8%)856.1 (45.4%) 3+ — 529.3 (54.5%)

Ni0.9Nb0.1 fresh 854.1 (54.6%) 2+ 206.8 530.9 (45.5%)

Ni0.9Nb0.1 used 855.4 (44.8%) 3+ 207.1 529.4 (100%)853.4 (55.1%) 2+

Ni0.6Nb0.4 fresh 856.1 (25.0%) 3+ 206.5 529.2 (66.4%)853.8 (75.0%) 2+ 530.5 (33.5%)

Ni0.6Nb0.4 used 856.3 (30.6%) 3+ 206.4 529.4 (100%)854.5 (69.3%) 2+

Ni0.4Nb0.6 fresh 856.2 (36.2%) 3+ 206.6 529.3 (69.1%)854.4 (63.8%) 2+ 530.8 (30.9%)

a O.S.: oxidation state.

Table 2 Bulk atomic (by ICP) and surface atomic compositions (by XPS) forfresh and used catalysts

Catalyst

Atomic composition(by ICP) (%)

Surface composition(by XPS) (%)

Ni Nb O Ni Nb O

NiO 50 — 50 43.3 — 56.7

Ni0.9Nb0.1 fresh 57.7 1.8 40.5 42.6 2.7 54.8Ni0.9Nb0.1 used 39.0 5.4 55.5








Fig. 4 XPS fresh samples (pure NiO and Ni0.9Nb0.1, Ni0.6Nb0.4 and Ni0.4Nb0.6catalysts) for A) Nb 3d and B) Ni 2p3/2 lines.


probably associated with the formation of Nb–Ni–O structures;such phases must essentially be constituted by Ni2+ and Nb5+

ions (such as NiNb2O6, Ni4Nb2O9, Ni3Nb2O8, etc.). The BE valuesobtained for niobium species are characteristic of Nb5+

(Fig. 4).37–39

HRTEM images of Ni0.9Nb0.1 corroborate the presence oflarge nanoparticles, which digital diffraction pattern (DDP) ofthe selected area corresponds to that of NiO (Fig. 5A). Itshould be noted that these particles show a slightly smallersize than those present in the pure nickel oxide. NiO particleswere found in all the samples and their particle sizedecreases with increasing niobium loading; this is consistentwith the surface area and XRD trends (Table 1). On the otherhand, HRTEM (Fig. 5B) shows that some areas of the freshsamples, including those with low niobium content, are cov-ered by an amorphous phase, being increasingly evident athigher niobium loadings. Fig. 5C shows a typical HAADF-STEM image of sample Ni0.9Nb0.1, where larger (20–50 nm)and smaller particles (4–6 nm) coexist. According to XRD pat-terns (Fig. 1), no Nb2O5 particles big enough to give a diffrac-tion pattern are detected by HRTEM. All samples exhibit asimilar trend but the amount and size of the larger particlesdecreases with increasing niobium content. Chemical analy-ses of selected areas (0.5 nm) were performed by taking


Fig. 5 HRTEM images of fresh samples Ni0.9Nb0.1 (A) and Ni0.2Nb0.8 (B), as well as

a low magnification HAADF image of Ni0.9Nb0.1 (C). HRTEM micrographs of samples

Ni0.9Nb0.1(D) and Ni0.2Nb0.8 (E) after the catalytic run and (F) X-EDS spectrum of the

marked areas.

Fig. 6 A) Ethane conversion vs. temperature, B) yield to acetonitrile vs. reactiontemperature. Reaction conditions: total flow, 20 ml min

−1; feed composition

(% volume), C3H8/O2/NH3 (9.8/25/8.6), 200 mg of catalyst.


advantage of the possibility to control the beam positionoperating in STEM mode. Thus, Fig. 5F includes X-EDS spec-trum of the areas marked in Fig. 5C. According to theseresults, there is an enrichment of niobium in the smaller par-ticles, which is consistent with the results previously reportedby Heracleous et al.40 Interestingly, the samples used in thecatalytic run show an increase in the crystallinity of the nio-bium amorphous layer. Fig. 5D and E clearly show this evolu-tion. Although commonly the Nb-rich samples still shows anill-defined structure (Fig. 5E), some well-defined crystals canbe found. In both cases, the elemental analyses confirm nio-bium enrichment in those areas. The DDP in Fig. 5 is consis-tent with that of NiNb2O6 phase (JCPDS 31-906, ICDS 37212)along with the [2 5 1] zone axis. The poor crystallinity of thesample in Fig. 5E, makes it difficult to precisely identify aphase; the observed spacing (2.8–2.9 Å ́) cannot be assigned toNiO but it may be related to the presence of [0 2 0] reflectionsof the NiNb2O6 phase (JCPDS 31-906).

Fig. 6A shows ethane conversion values vs. reaction tem-perature in the all catalysts. Ni0.6Nb0.4, Ni0.7Nb0.3, Ni0.8Nb0.2,Ni0.85Nb0.15 and Ni0.9Nb0.1 samples are more active than pureNiO for the studied temperature range; conversely, the sam-ples with higher niobium content (Ni0.35Nb0.65 and Ni0.2Nb0.8)present lower activity. Fig. 6B shows the yield to acetonitrilevs. reaction temperature. The performance of Ni0.9Nb0.1 isquite different than that of the rest of the catalysts,presenting the highest yield to acetonitrile. Table 4 shows theselectivities to main products along with the TOF values, cal-culated per Ni site assuming that all Ni sites visible to XPSare exposed. These values also show that Ni0.9Nb0.1 catalystexhibits the highest TOF value. Thus, the incorporation ofsmall niobium quantities into the catalyst formulationenhances both activity and performance to acetonitrile, whileit would also increase the activity per nickel site; affordingyield values close to 19.1%, which is a promising value. Thepromoting effect of niobium appears limited to low loadings.


These results are in line with those reported previously withsupported NiO/Nb2O5 catalysts,27,28 that showed that bothNiO and mixed Ni–Nb–O mixed phase are required in orderto deliver efficient ethane ammoxidation.

The stability vs. time-on-stream of the best performingNi0.9Nb0.1 and Ni0.6Nb0.4 catalysts has been also investigated(Fig. 7). The data show that ethane conversion values forNi0.9Nb0.1 catalyst remain essentially constant during morethan two days of operation at 450 °C. In the case of Ni0.6Nb0.4catalysts, some activation occurs during the first hours ofreaction, this should reflect the extensive rearrangement ofNb–Ni–O species into Nb–Ni–O mixed oxide during reaction;in line with the Raman spectra and XRD characterizationdata of the used samples.

Discussion

Raman spectroscopy evidences two Nb–Ni–O structures, char-acterized by modes near 850 and 800 cm−1; both bands aredetected at low niobium contents, while the former red shiftsand is the only present at higher niobium contents (Fig. 3),indeed, at high niobium loadings, only the band near800 cm−1 is apparent after reaction for all samples. The


Table 4 Ethane conversion, selectivity to the different products, TOF and productivity of catalysts. 425 °C

CatalystNb/Niatomic ratio

Ethaneconversion (%)

Selectivity (%)

TOF (s−1)aProductivity(gACE/gcat h)CO2 Ethylene ACN Methane

NiO 0 41.9 31.6 57.4 10.7 0.3 0.06 0.26Ni0.9Nb0.1 0.111 55.1 50.2 14.5 35.3 0.1 0.16 0.68Ni0.85Nb0.15 0.176 79.8 45.2 38.3 16.2 0.3 0.13 0.48Ni0.8Nb0.2 0.25 75.9 36.7 47.5 15.7 0.1 0.10 0.37Ni0.7Nb0.3 0.428 73.2 30.9 56.0 13.0 0.1 0.13 0.34Ni0.6Nb0.4 0.666 69.3 25.0 62.7 12.2 0.1 0.12 0.33Ni0.4Nb0.6 1.5 62.7 21.7 72.8 5.5 0.0 0.07 0.14Ni0.35Nb0.65 1.857 49.4 5.9 91.5 2.6 0.1 0.02 0.04Ni0.2Nb0.8 4 27.2 4.3 91.7 4.0 0.0 0.05 0.04Nb2O5 — 1.0 6.8 64.0 28.3 0.9 — 0.02

a TOF (Turnover Frequency): acetonitrile produced per surface Ni atom as calculated by XPS.


relative intensities of these bands evolve with niobium con-tent. Supported catalysts, in which Nb–Ni–O forms at theinterphase between the active phase and the support,28

exhibit only the phase low-Nb contents phase. In addition toRaman spectroscopy, HRTEM trends along with the evolutionof the XPS Ni2+/Ni3+ ratio (Table 3) indicate also that differentNb–Ni–O structures form, depending on the niobium con-tent. Also all characterization data uncover a rearrangementof Nb–Ni–O structures upon use in reaction (cf. Fig. 1 and 3).Since only the band near 800 cm−1 is visible at high Nb con-tents, the 800 cm−1 Raman band should correspond to theNb-rich Nb–Ni–O mixed phase. According to the HRTEMresults, two distinct Nb–Ni–O phases exist in the catalystseries; these exhibit different sizes. One is in the 20–50 nmrange and the other in the 4–6 nm range. The one withsmaller size range is richer in niobium; this is consistentwith the results reported by Heracleous et al.33 Thus, theRaman band near 800 cm−1 must correspond to the mixedphase with smallest particle size.

The niobium-rich Nb–Ni–O phase is not active for ethaneammoxidation (cf. Fig. 6), while those catalysts with low nio-bium loading are efficient for this reaction. Ni0.9Nb0.1delivers the best performances for ethane transformation

Fig. 7 Ethane conversion and yield to acetonitrile as a function of time-on-stream

for Ni0.9Nb0.1 and Ni0.6Nb0.1 catalysts. (Reaction conditions: total flow, 20 ml min−1;

feed composition (% volume), C2H6/O2/NH3/He (9.8/25/8.6/56.6), 200 mg of cata-

lyst, T = 450 °C).


into acetonitrile, outperforming the rest of the series(cf. Fig. 6 and Table 4), although the selectivity to COx is high.Thus, niobium incorporation into the NiO lattice deliverslarger particles (20–50 nm) (detected by HRTEM) andenhances both the activity and the selectivity to acetonitrilesince the NiO is not selective for acetonitrile formation. This isthe Nb–Ni–O mixed phase detected in NiO/Nb2O5 supportedcatalysts. These data show that extensive addition of niobiumgenerates smaller niobium-rich particles that are not active forethane ammoxidation.

The amount of Ni–Nb–O particles with small size (Ramanband near 800 cm−1) is higher at higher niobium contents;Nb2O5 oxide is also detected by Raman spectroscopy (cf. Fig. 3)and TPR analyses (cf. Fig. 2). The crystal size of theseNb2O5 particles is small, in line with the lack of X-ray dif-fraction pattern (cf. Fig. 1), and HRTEM images, whichshow the presence of an amorphous Nb2O5 layer, whichcrystallinity increases upon use in reaction. Thus, smallNb–Ni–O particles are detected at high niobium contents(Ni0.35Nb0.65 and Ni0.2Nb0.8); the BET area values decrease,as well as the NiO average crystal size, and the formation ofsegregated Nb2O5 oxide is detected. Ammonia chemisorp-tion data show that the samples with high niobium contentpresent higher acidity (cf. Table 1), as expected.

The changes in the catalysts morphology and compositionupon niobium incorporation correlate with changes in thecatalytic performance. The catalytic-promoting effect of nio-bium is limited to low loading and is lost when well definedniobium-containing phases form.41 At low loadings, mixedNi–Nb oxide phases are beginning to form; niobium modu-lates redox properties and increases acidity (cf. Table 1), ren-dering a catalytic system that is more efficient for theammoxidation reaction. Ethane ammoxidation need sites toactivate both, ethane and ammonia, and such sites must benearby for an efficient formation of the nitrile molecule.

The acidity is related with the ability of the catalysts toadsorb ammonia during reaction. However, high aciditywould lead to a very strong adsorption of ammonia, limitingits reactivity. This would explain why the samples with high Nbcontents are not selective to acetonitrile formation (cf. Table 4),although they are highly selective to ethylene. The progressive



increase of acidity upon addition of niobium would accountfor a better activation of ammonia, necessary for nitrile forma-tion. However, it may also limit the presence of nearbyadsorbed hydrocarbon molecules, necessary to combine withammonia into the nitrile.

Nb–Ni–O phases induce the formation of Ni2+ speciesremoving non-stoichiometric Ni3+ ions that appear to belocated at the surface of NiO particles (cf. XPS analyses).Selective removal of Ni3+ ions by stabilization into Ni2+uponniobium doping would progressively decrease redox proper-ties in NiO system thus modulating its oxidation capacity. Anincipient phase (i.e., a phase that is beginning to develop)between nickel and niobium oxides would thus modulateredox properties. In such a sense, the relevance of highly-defective Nb–Ni–O phases would be critical to facilitate theappropriate redox cycle of nickel ions. Such a phase wouldpresent properties distinct from those of pure NiO or Nb2O5,but still not those of a fully developed mixed Ni–Nb–O phase.Both oxidation states would be involved in the reaction. Sucha trend would be consistent with the promotion of oxygenexchange reported by Lemonidou et al. in this system usingisotopic labelling.33

Niobium doping increases acidity while modulates redoxproperties. This would lead to an efficient activation ofammonia minimizing the risk to oxidize it to nitrogen. Thus,there is a need for a balance between redox and acidic sites,which appears to exist at low niobium content.

Conclusions

Niobium-containing NiO catalysts are an efficient formula-tion for the direct ammoxidation of ethane to acetonitrile.Two different mixed Nb–Ni–O phases have been identified,with different Nb/Ni atomic ratios. A Nb-poor Nb–Ni–Ophase, characterized by two Raman bands near 850 cm−1 and800 cm−1, which corresponds to a mixed phase with particlesize in the 20–50 nm range; and a Nb-rich Nb–Ni–O phasewith, presents smaller particle size and characterized by asingle broad Raman band near 800 cm−1. The catalytic datashow that the Nb-poor phase is efficient for ethane ammoxi-dation, while the Nb-rich one is not efficient for nitrile forma-tion, but delivers very good performance to ethylene. Theincorporation of a small amount of niobium into the NiO lat-tice induces important changes into the oxide structure thatenhances the activity and the selectivity for ethane ammoxi-dation to acetonitrile. This appears related to an increase inacidity and modulation of redox properties due to an interac-tion between Ni and Nb through the formation of mixedNi–Nb–O phases.

Acknowledgements

The Spanish Ministry of Science and Innovation (CTQ2011-25517-E and MAT2008-00889) is acknowledged for the finan-cial support. E. R. thanks CONACYT (Mexico) and ICyTDF


(Mexico) for her predoctoral fellowship. The authors thank R.López-Medina for his help with catalytic tests.

Notes and references

1 M. O. Guerrero-Pérez, J. L. G. Fierro and M. A. Bañares,
J. Catal., 2002, 206, 339.
2 K. E. Khcheian, O. M. Revenko, A. N. Shatalova and
E. G. Gelperina, US Patent 3 751 44, 1973 .
3 W. Ueda, T. Yokoyama, Y. Moro-Oka and T. Ikawa, Ind. Eng.
Chem. Prod. Res. Dev., 1985, 24, 340.
4 M. Subrahmanyam, S. J. Kulkarni, B. Srinivas and
A. R. Prasad, J. Indian Chem. Soc., 1992, 69, 681.
5 J. F. Brazdil, Top. Catal., 2006, 38, 289.
6 F. Cavani and J. H. Teles, ChemSusChem, 2009, 2, 508. 7 F. Cavani, J. Chem. Technol. Biotechnol., 2010, 85, 1175. 8 G. Centi, D. Pesheva and F. Trifiro, Appl. Catal., 1987, 33, 343. 9 Y. C. Kim, W. Ueda and Y. Moro-ok, Catal. Today, 1992, 13, 673.
10 G. Centi, R. K. Grasselli and F. Trifiro, Catal. Today,
1992, 13, 661.
11 R. K. Grasselli, Catal. Today, 1999, 49, 141.
12 M. O. Guerrero-Pérez, J. N. Al-Saeedi, V. V. Guliants and
M. A. Bañares, Appl. Catal., A, 2004, 260, 93.13 M. O. Guerrero-Pérez, J. L. Rivas-Cortés, J. A. Delgado-Oyagüe,

J. L. G. Fierro and M. A. Bañares, Catal. Today, 2008, 139, 202.14 Chemical Week, www.chemweek.com, February 14, 2013.
15 S. M. Aliev and V. D. Sokolovskii, React. Kinet. Catal. Lett.,
1978, 9, 91.16 Y. Li and J. N. Armor, Chem. Commun., 2013, 1997.
17 Y. Li and J. N. Armor, J. Catal., 1998, 173, 511. 18 Y. Li and J. N. Armor, Appl. Catal., A, 1999, 183, 107. 19 E. Rojas, M. O. Guerrero-Pérez and M. A. Bañares, Catal.
Commun., 2009, 10, 1555.20 E. Rojas, M. Calatayud, M. O. Guerrero-Pérez and

M. A. Bañares, Catal. Today, 2010, 158, 178.21 E. Rojas, M. Calatayud, M. O. Guerrero-Pérez and

M. A. Bañares, Catal. Today, 2012, 187, 212.22 E. Rojas, M. Calatayud, M. A. Bañares and M. O. Guerrero-Pérez,

J. Phys. Chem. C, 2012, 116, 9132.23 R. Catani and G. Centi, J. Chem. Soc., Chem. Commun.,

1991, 1081.24 B. Wichterlová, Z. Sobalik, Y. Li and J. N. Armor, Stud. Surf.

Sci. Catal., 2000, 130, 869.25 E. Heracleous and A. A. Lemonidou, J. Catal., 2010, 270, 67.
26 E. Heracleous and A. A. Lemonidou, J. Catal., 2006, 237, 162. 27 E. Rojas, M. O. Guerrero-Pérez and M. A. Bañares, Catal.
Lett., 2012, 143, 31.28 F.Rubio-Marcos,E.Rojas,R.López-Medina,M.O.Guerrero-Pérez,

M. A. Bañares and J. F. Fernandez, ChemCatChem, 2011, 3, 1637.29 M. Salavati-Niasari, F. Davar, M. Mazaheri and M. Shaterian,

J. Magn. Magn. Mater., 2008, 320, 575.30 C. D. Wagner, L. E. Davis, M. V. Zeller, J. A. Taylor,

R. H. Raymond and L. H. Gale, Surf. Interface Anal.,1981, 3, 211.

31 M. O. Guerrero-Pérez, M. A. Peña, J. L. G. Fierro and
M. A. Bañares, Ind. Eng. Chem. Res., 2006, 45, 4537.


32 B. Savova, S. Loridant, D. Filkova and J. M. M. Millet, Appl.
Catal., A, 2010, 148–157.
33 Z. Skooufa, E. Heracleous and A. A. Lemonidou, Catal.
Today, 2012, 192, 169.
34 B. Solsona, J. M. López Nieto, P. Concepción, A. Dejoz and
F. Ivars, J. Catal., 2011, 280, 28.
35 B. Solsona, P. Concepción, S. Hernández, B. Demicol and
J. M. López Nieto, Catal. Today, 2012, 180, 51.
36 N. K. Kotsev and L. I. Ilieva, Catal. Lett., 1993, 18, 173.


37 I. Nowak and M. Ziolek, Chem. Rev., 1999, 99, 3603.
38 P. Salagre, J. L. G. Fierro, F. Medina and J. E. Sueriras,
J. Mol. Catal. A: Chem., 1996, 106, 125.39 R. López-Medina, J. L. G. Fierro, M. O. Guerrero-Pérez and

M. A. Bañares, Appl. Catal., A, 2010, 375, 55.40 E. Heracleous, A. Delimitis, L. Nalbandian and

A. A. Lemonidou, Appl. Catal., A, 2007, 325, 220.41 M. O. Guerrero-Pérez and M. A. Bañares, Catal. Today,

2009, 142, 245–251.


Performance of NiO and Ni–Nb–O active phases during the ethane ammoxidation into acetonitrile

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