Mechanochemical synthesis of nanocrystalline nickel molybdates

Journal of Alloys and Compounds 422 (2006) 53–57

Mechanochemical synthesis of nanocrystalline nickel molybdates D. Klissurski a , M. Mancheva a,∗ , R. Iordanova a , G. Tyuliev b , B. Kunev b a

Institute of General and Inorganic Chemistry, Bulgarian Academy of Science, “Acad. G. Bonchev” str., bl.11, 1113 Sofia, Bulgaria b Institute of Catalysis, Bulgarian Academy of Science, “Acad. G. Bonchev” str., bl.11, 1113 Sofia, Bulgaria Received 28 September 2005; received in revised form 26 November 2005; accepted 29 November 2005 Available online 23 January 2006

Abstract The aim of this study is to establish the possibilities of mechanochemical activation as a successful route for the preparation of NiMoO4 catalysts. A stoichiometric mixture of NiO and MoO3 in a 1:1 molar ratio was subjected to intense mechanical treatment in air using a planetary ball mill for different periods of time. The phase and structural transformations were monitored by X-ray diffraction (XRD) and infrared spectroscopy (IR). The obtained products were analyzed by X-ray photoelectron spectroscopy (XPS). It was found that 5 h milling of the reagents led to complete crystallization to single-phase ␣-NiMoO4 at room temperature. Mechanochemical activation for 2.5 h resulted in a high-temperature ␤-NiMoO4 phase under very mild experimental conditions. © 2005 Elsevier B.V. All rights reserved. Keywords: Mechanochemical activation; Nanopowders; XRD; IR; XPS

1. Introduction The AMoO4 (A = Ni, Co, Fe, Cu) family of materials possess various important catalytic properties. Nickel molybdates are basic components of industrial catalysts for partial oxidation of hydrocarbons and precursors in the synthesis of hydrodesulfurization (HDS) catalysts [1,2]. NiMoO4 may exist in several modifications: a low temperature one ␣-NiMoO4 [3,4], high temperature ␤-NiMoO4 [5–7], “high-pressure” modification (NiMoO4 -(II)) [8] and NiMoO4· nH2 O hydrate [9]. Both phases (␣ and ␤) are monoclinic with different space group I2/m (JCPDS card, 33-0948) and C2/m (JCPDS card, 45-0142). The compound ␣-NiMoO4 is isostuctural with ␣-CoMoO4 and ␣-FeMoO4 [3,4], while ␤-NiMoO4 is isostructural with ␣MnMoO4 and MgMoO4 [5–7]. The structural units building ␣-NiMoO4 are NiO6 and MoO6 octahedra, which share edges and form chains. The structure of ␤-NiMoO4 consists of molybdate tetrahedra, which share corners with four different nickel octahedra. The catalytic properties are closely related to their structure and depend on the method of preparation and on the thermal treatment. Usually single crystalline NiMoO4 can be obtained by solid state synthesis at high temperature [3,10], by

∗

Corresponding author. Tel.: +359 2 979 35 88; fax: +359 2 870 50 24. E-mail address: [email protected] (M. Mancheva).

0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.11.073

coprecipitation from aqueous solutions of soluble salts [11,12], by the sol-gel method [13,14] or by high-pressure methods [8]. This work deals with mechanochemical activation of NiO and MoO3 in order to obtain nanocrystalline and single-phase products. The advantages of this approach are that the use of voluminous solutions and complicated operations as well as the sintering of the final product can be avoided. 2. Experimental A stoichiometric mixture of NiO (BDH) and MoO3 (Reachim) in a 1:1 molar ratio was subjected to intense mechanical treatment in air for different times ranging from 2.5 to 5 h. The milling was performed in Fritsch No. 7 planetary ball mill, using a rotational speed of 450 rpm at a constant rotation direction and a ball to powder weight ratio of 10:1. The both container and balls were made of stainless steel. The sample mechanically activated for 2.5 h was annealed in the temperature range (350–450 ◦ C) below the eutectic point (610 ◦ C) according to the MoO3 –NiO phase diagram [15]. In order to accomplish the solid state reaction between NiO and MoO3 , a non-mechanically activated mixture was directly subjected to heat-treatment at 700 ◦ C [10]. The phase and structural transformation were monitored by X-ray diffraction (XRD) and infrared spectroscopy (IR). Powder XRD patterns were registered at room temperature with a TUR M62 diffractometer using Co K␣ radiation in the 5◦ < θ < 55◦ range. The crystallite sizes were calculated using the Scherrer formula for the (0 2 2) and (0 2 1) peaks of the ␣- and ␤-NiMoO4 sample, respectively. Infrared spectra were registered on a Nicolet-320 FTIR spectrometer using, the KBr pellet technique in the range 1200–400 cm−1 . The specific surface area of the ␣-NiMoO4 was measured using the BET method. The final products along with the NiO and MoO3 standard were analyzed by X-ray photoelectron

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spectroscopy (XPS). The XPS measurements were carried out in the UHV chamber of an ESCALAB-MkII (VG Scientific) electron spectrometer using Mg K␣ excitation with total instrumental resolution of −1 eV. Energy calibration was performed, taking the C 1s line at 285 eV as a reference. Surface atomic concentrations were evaluated using Scofield’s ionization cross-sections with no corrections for λ (the mean free path of photoelectrons) and analyzer transmission function.

3. Results and discussion 3.1. XRD analysis The phase identification was carried out using powder Xray diffraction. Fig. 1 shows the XRD patterns of the initial mixture before and after mechanical treatment and solid state synthesis of ␣-NiMoO4 . The initial XRD patterns exhibit all peaks corresponding to NiO and MoO3 precursors (Fig. 1a). After a milling time of 2.5 h, partial amorphization is observed, the principal peaks of MoO3 being broadened and their intensity decreased (Fig. 1b). This is an indication that the longrange order of MoO3 is destroyed and the structure of MoO3 is strongly modified. In contrast, the NiO with its highly symmetrical structure is rather resistant with respect to ball milling. In the same XRD data (Fig. 1b) three new diffraction peaks (110, 220 and 022 plane) appear which are the principal peaks ˚ JCPDS card, of the ␣-NiMoO4 phase (d = 6.19, 3.71 and 3.09 A 33-0948). This is an indication that the reaction between NiO and MoO3 starts after a milling time of 2.5 h. The all diffrac-

Fig. 2. X-Ray diffraction patterns of: (a) the sample after 2.5 h milling time, (b) the sample obtained after 2.5 h milling time and heat-treatment at 350 ◦ C for 3 h, (c) ␤-NiMoO4 obtained after 2.5 h milling time and heat-treatment at 400 ◦ C for 3 h, (d) ␤-NiMoO4 obtained after 2.5 h milling time and heat-treatment at 400 ◦ C for 20 h.

Fig. 1. X-ray diffraction patterns of the: (a) non-activation sample, (b) sample after 2.5 h mechanocliemical treatment, (c) ␣-NiMoO4 obtained by 5 h mechanochemical treatment, (d) ␣-NiMoO4 obtained by solid state synthesis.

tion lines of the ␣-NiMoO4 (Fig. 1c) were observed after 5 h milling time. The XRD pattern shows an increased diffraction scattering background as compared to the X-ray diffractogram of solid state synthesized ␣-NiMoO4 (Fig. 1d). This is a result of the lower crystallinity of the mechanochemical synthesized sample. The calculated crystallite sizes of ␣NiMoO4 obtained by mechanochemical treatment and solid state reaction is 30 and 34 nm, respectively. The specific surface area of the ␣-NiMoO4 obtained by 5 h milling time is 8 m2 /g. In order to obtain the ␤-NiMoO4 phase, the 2.5 h mechanically activated sample was subjected to heat-treatment at 350 and at 400 ◦ C. Annealing of the sample at 350 ◦ C led to no noticeable phase changes. The X-ray patterns shows principle peaks of initial oxides and ␣-NiMoO4 (Fig. 2b). Heat-treatment at 400 ◦ C for 3 h (Fig. 2c) leads to the appearance of several new diffraction lines on the 0 0 1, 2 0 1 and 0 0 2 planes, which are assigned ˚ [7]. All diffraction peaks to ␤-NiMoO4 (d = 6.70, 3.78, 3.331 A) of ␤-NiMoO4 are observed after calcination at 400 ◦ C for 20 h (Fig. 2d). Traces of initial oxides and probably of ␣-NiMoO4 are also detected. The assignment of the reflection of this XRD

D. Klissurski et al. / Journal of Alloys and Compounds 422 (2006) 53–57

Fig. 3. X-ray diffraction pattern of: (a) ␣-NiMoO4 obtained by 5 h mechanochemical treatment, (b) ␣-NiMoO4 obtained by 5 h milling time and heat-treatment at 400 ◦ C for20 h.

patterns is complicated because of overlapping of the diffraction peaks of ␣ and ␤-NiMoO4 . In order to verify whether ␣ → ␤ transformation proceeds under these conditions mechanochemically synthesized ␣NiMoO4 has been subjected to heating at 400 ◦ C for 20 h (Fig. 3b). XRD data show that phase transformations do not take place. Hence, the formation of ␤-NiMoO4 is a result of the reaction between mechanically activated NiO and MoO3 . The calculated crystallite size of ␤-NiMoO4 is 32 nm. 3.2. Infrared analysis The formation of ␣- and ␤-NiMoO4 has been confirmed by IR spectroscopy. In the IR spectrum of the non-activated sample there are bands characteristic of MoO6 and NiO6 polyhedra building MoO3 and NiO oxides, respectively (Fig. 4a). The band at 990 cm−1 corresponds to the stretching modes of the Mo O terminal bond present in each octahedron of MoO3 . The bands at 870 and 820 cm−1 are assigned to stretching vibrations of the OMo2 (Mo–O–Mo) entity. The absorption bands below 600 cm−1 are a superposition of stretching vibrations of OMo3 units and NiO6 polyhedra [16–18]. After mechanical activation

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Fig. 4. IR spectra of: (a) non-activated sample, (b) the sample after 2.5 h milling time, (c) ␣-NiMoO4 obtained by 5 h milling time, (d) ␣-NiMoO4 obtained by solid state synthesis, (e) ␤-NiMoO4 obtained by 2.5 h milling time and heattreatment at 400 ◦ C for 20 h.

(2.5 h), the band at 990 cm−1 is transformed into a shoulder, the bands at 870 and 820 cm−1 are broadened and their intensity decreases (Fig. 4b). This fact is a result of destruction of the long range order and partial amorphization of the reagents. The decrease in intensity of the band around 600 cm−1 versus the intensity of the bands at 880 cm−1 is an indication that mechanical activation leads to a decreased number of edge shared MoO6 octahedra (i.e. OMo3 units). Fig. 4c presents the characteristic IR spectrum of ␣-NiMoO4 prepared by mechanochemical treatment. The bands at 960 and 930 cm−1 are due to activation of the ν1 vibration of the distorted MoO6 octahedra present in the crystalline ␣-NiMoO4 . The band at 650 cm−1 is assigned to ν3 vibrations of the same units. The low-frequency bands (450 and 420 cm−1 ) are a superposition of ν4 and ν5 of MoO and ν3 of NiO6 groups building ␣-NiMoO4 [7,19,20]. Fig. 4d presents the IR spectrum of ␣-NiMoO4 prepared by solid state synthesis. The absorption bands are more intense in comparison with the absorption bands of the mechanochemical synthesized sample. This indicates the increase of crystallinity of the solid state synthesized sample. The typical IR bands of ␤-NiMoO4 are observed at 960, 940, 880, 800, 700 and 440 cm−1 (Fig. 4e). The bands at 960 and 940 cm−1 are due to activation of the ν1 vibration of highly

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Table 1 Binding energy (eV) of O 1s, Mo 3d5/2 and Ni 2p3/2 for different NiMoO4 sample and MoO3 and NiO standard Sample

O 1s

MoO3 (Reachim) ␣-NiMoO4 obtained by 5 h milling time ␣-NiMoO4 obtained by solid-state synthesis ␤-NiMoO4 obtained by 2.5 h milling time and heated at 400 ◦ C for 20 h NiO (BDH)

O2−

HBE (eV) (%)

Mo 3d5/2

Ni 2p3/2

Mo 4p3/2

Ni 3s

530.8 530.8 530.8 530.8 529.4

532.7 (2.5) 532.5 (20) 532.6 (11) 532.9 (8) 531.0 (76)

233.10 233.00 233.05 233.10 –

– 856.3 856.1 856.2 854.2

40.7 40.7 40.8 40.6 –

– 113.7 113.8 113.6 112.3

distorted MoO4 tetrahedra building ␤-NiMoO4 . The absorption bands between 880 and 700 cm−1 are assigned to ν3 vibration of the same group. The low-frequency band at 440 cm−1 is a superposition of ν4 of MoO4 and ν3 of NiO6 units present in the crystalline structure of ␤-NiMoO4 [7,21]. The broad band at about 600 cm−1 can be assigned to the vibrations of the OMo3 entity of MoO6 octahedra building MoO3 and ␣-NiMoO4 . 3.3. XPS analysis The Mo 3d, Ni 2p and O 1s spectra for all samples including standards are presented in Fig. 5. The O 1s, Mo 3d5/2 , Ni 2p3/2 , Mo 4p3/2 , Ni 3s binding energy values for ␣-NiMoO4 , ␤-NiMoO4 , NiO and MoO3 along with the surface atomic concentrations evaluated from the peak intensities are presented in Tables 1 and 2. Within the accuracy of the energy scale, there is no difference in the O 1s, Mo 3d5/2 and Ni 2p3/2 .peak positions. Similar observations are reported in a previous work [14,22]. The main Ni 2p3/2 line for all NiMoO4 samples exhibits a single peak with no low binding energy shoulder in contrast to that of NiO (Fig. 5). Having in mind that the Ni 2p3/2 main line shape is sensitive to the near neighbor environment, one can conclude, that nickel ions are well dispersed into the oxide matrix and there is no extra NiO phase on the surface. Considering the surface compositions evaluated from the XPS peak intensities, it is reasonable to assume some nickel depletion in the topmost layer of NiMoO4 samples. In order check this a combination of peaks with different binding energies are used, i.e. Ni 2p, Mo 3d and Ni 3s, Mo 4p. As one can see from Table 2, the

Fig. 5. Binding energy of: (a) NiO standard, (b) ␤-NiMoO4 obtained by 2.5 h milling time ant heat-treatment at 400 ◦ C for 20 h, (c) ␣-NiMoO4 obtained by solid state synthesis, (d) ␣-NiMoO4 obtained by 5 h mechanochemical treatment, (e) MoO3 standard.

nickel concentration increases by 50% when using Ni 3s line instead of Ni 2p, although it remains below the stoichiometric one. The higher amount of Mo as revealed by XPS can also be explained by some extra MoO3 on the sample surface. It is worth to mention the different quantity of high binding energy shoulder of the O 1s line for different samples (values in the parentheses in column O 1s, Table 1). The highest amount (20%) is observed for the ␣-NiMoO4 sample prepared by 5 h mechanical treatment.

Table 2 Surface composition (at. %) estimated from the XPS intensities Sample

O (%) (from O 1s)

Mo (%) (from Mo 3d)

MoO3 (Reachim)

68 –

– 66

32 –

– –

␣-NiMoO4 obtained by 5 h milling time

65.5 –

– 59

22 –

12.5 –

– 23.5

– 17.5

␣-NiMoO4 obtained by solid-state synthesis

64.5 –

– 59

21 –

14.5 –

– 21

– 20

␤-NiMoO4 obtained by 2.5 h milling and heated at 400 ◦ C for 20 h

64.5

–

23.5

12

–

–

–

57

–

–

24

19

– 41.5

– –

52.5 –

– –

NiO (BDH)

47.5 –

Ni (%) (from Ni 2p)

Mo (%) (from Mo 4p) – 34

Ni (%) (from Ni 3s) – –

– 58.5

D. Klissurski et al. / Journal of Alloys and Compounds 422 (2006) 53–57

4. Conclusion It is established that mechanochemical treatment of NiO and MoO3 is a very appropriate method for synthesis of nickel molybdates. Mechanochemical activation provides precursors of high reactivity. This is the main reason for direct mechanochemical synthesis of ␣-NiMoO4 as nanosized crystallites. High-temperature ␤-NiMoO4 was obtained by heattreatment of mechanically activated initial oxides. The synthesis was performed at 400 ◦ C, which is considerably lower as compared to the temperatures needed for the traditional solid state reaction (700 ◦ C). References [1] J.L. Brito, A.L. Barbosa, A. Albornoz, F. Severino, J. Laine, J. Catal. Lett. 26 (1994) 329–334. [2] R.A. Madeley, S. Wanke, Appl. Catal. 39 (1988) 295–298. [3] G.W. Smith, Acta Cryst. 15 (1962) 1054–1057. [4] G.W. Smith, J.A. Ibers, Acta Cryst. 19 (1965) 269–274. [5] S.C. Abrahams, J.M. Reddy, J. Chem. Phys. 43 (1965) 2533–2538. [6] A.W. Sleight, B.L. Chamberland, Inorg. Chem. 7 (1968) 1672–1675. [7] L.M. Plyasova, I.Yu. Ivanchenko, M.M. Andrushkevich, R.A. Buyanov, I. Sh. Itenberg, G.A. Khramova, L.G. Karakchiev, G.N. Kustova, G.A. Stepanov, A.L. Tsailingol’d, F.S. Pilipenko, Kinet. Catal. 14 (1973) 1010–1014.

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