Combustion Synthesis and Electrical Behavior of Nanometric β

Feb 19, 2010 - Metallic molybdates are of great importance for their potential use as catalysts of selective nature. The synthesis of the β-NiMoO4 ph...
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J. Phys. Chem. C 2010, 114, 4251–4257

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Combustion Synthesis and Electrical Behavior of Nanometric β-NiMoO4 B. Moreno,* E. Chinarro, M. T. Colomer, and J. R. Jurado Instituto de Cera´mica y Vidrio, CSIC, C/Kelsen 5, 28049, Cantoblanco, Madrid, Spain ReceiVed: August 14, 2009; ReVised Manuscript ReceiVed: February 5, 2010

Metallic molybdates are of great importance for their potential use as catalysts of selective nature. The synthesis of the β-NiMoO4 phase is not simple and has been approached employing different methods; however, combustion synthesis has never been proposed before. This work describes the synthesis of nanometric β-NiMoO4 powders from mixtures of Ni (II) and Mo (VI) acetylacetonates as cation precursors and urea as fuel. The characterization of the as-prepared combustion product showed that the combustion synthesis provides a straightforward method for the achievement of β-NiMoO4. This phase, prepared by this method, is stable at room temperature in the absence of NiO. The as-prepared powders are nanometric (∼6 nm as observed by TEM) and have a specific surface area of 31 ( 7 m2/g. Both parameters are crucial for an enhancement of its catalyst activity. As the β-NiMoO4 phase is not stable at room temperature its preparation is discussed taking into account its evolution with temperature and the synthesis conditions which promote metastable phases due to a high exothermic energy release followed by a rapid cooling. The electrical conductivity is determined in air as a function of temperature and discussed in relation with the phase transition that takes place promoted by temperature. Introduction Metal molybdates are of great importance for their potential use as catalysts with selective nature, particularly in the treatment of light olephins.1-5 The catalytic activity of nickel molybdates is large and concerns different reactions: hydrocarbon destilation as hydrodesulfuration and hydrodenitrogenation,6 water-gas shift,7 vapor reforming steps, hydrogenolysis, nbutane cracking reactions,8 and oxidative dehydrogenation of olefins.9,10 NiMoO4 exhibits three distinct crystalline phases, two of them, R and β, are stable in pressure standard conditions (1 atm). Both phases show monoclinic structure with spatial group C2/m; the R phase is described to be stable from room temperature to 600-720 °C1 being that the Mo atoms located in distorted octahedric sites. On the other hand, it has been stated that the high temperature β phase, which is isomorphous with R-MnMoO4, starts to crystallize on heating the R-phase above 600 °C and, once formed, is only stable at high temperatures. This phase shows a monoclinic structure as mentioned above but with Mo atoms situated in a distorted tetrahedric lattice site. On cooling, the β-phase is stable to 250 °C and at this temperature transforms again into the R-phase.11,12 The phase transition between R and β phases has been deeply studied finding to be strongly influenced by the conditions of the sample preparation.13 β-NiMoO4 shows stability up to the upper temperature limit of 730 °C.14 The synthesis of the β phase is rather complicated, although it remains interesting as it is generally assumed that this phase shows a better active and selective catalysis in some reactions, as the oxidative dehydrogenation reaction of propane.15 R and β-NiMoO4 have been synthesized using several methods; those which involve low temperatures or mild synthesis conditions as sol-gel,16 impregnation,17,18 freezedrying, or precipitation19 are the most popular and often reported. * Corresponding author. Tel: +34917355840, ext 1186. Fax: +34917355843. E-mail: [email protected].

Rencently, electrochemical synthesis of molybdates has been also reported.20 In the present study combustion synthesis is proposed as a novel method, not reported before to the best of our knowledge, for preparing β-NiMoO4 in a fast, reliable, and straighforward way. Briefly, the combustion synthesis technique consists of heating a mixture of the component metal salts and the suitable organic fuel, until it ignites and a self-sustaining and rather fast combustion reaction takes place. The resulting oxide powder is usually crystalline and finely divided. On heating, the metal nitrates can decompose into the melt oxides. A constant external heat supply, a simply hot plate which reaches 300 °C, is necessary to maintain the system at the temperature required to accomplish the reaction between the oxide components. In combustion synthesis the energy released from the exothermic reaction between the nitrates and the fuel can rapidly heat the system to an elevated temperature and sustain it long enough for reaction to occur.21 In this work the synthesis has been approached replacing nitrates by Ni (II) and Mo (VI) acetylacetonate precursors, to generate a higher exothermic energy release, responsible of the achievement of nanoparticulate β-NiMoO4 single phase. For this reason, ammonium nitrate is used in the reaction to balance the excess of reducing valences that comes from the acetylacetonates. These adjustments performed in the method provide the thermodynamic conditions required for the stabilization at room temperature of this unstable phase in one step and apparently, in the absence of secondary phases used to stabilize it. The asprepared powders were characterized employing different techniques, i.e., X-ray diffraction (XRD), scanning electron microscopy, energy dispersion X-rays (SEM-EDX), and Fourier transformed infrared spectroscopy (FTIR), which confirmed that the as-prepared powders are β-NiMoO4 as single phase. 2. Experimental Methods 2.1. Synthesis of β-NiMoO4. Batches were calculated to attain 0.5 g of NiMoO4 using as precursors Mo (VI) acetylacetonate (97% ABCR, Germany) and Ni (II) acetylacetonate

10.1021/jp907870a  2010 American Chemical Society Published on Web 02/19/2010

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(97% Aldrich, Germany). Urea, (NH2)2CO (98% Aldrich, Germany), was used as fuel and NH4NO3 (98% Aldrich, Germany) as a nitrate or oxidizer source required to achieve a stoichiometric ratio of the fuel/oxidizer mixture. β-NiMoO4 was synthesized using the following molecular ratios: (Ni:Mo: CO(NH2)2) (1:1:24) and (CO(NH2)2:NH4NO3) (1:1). The reactants were first heated at 75 °C and stirred on a hot plate, then the basin was heated up to 200 °C employing a heating mantle inside a fume-cupboard. The whole procedure was performed under ventilation. Soon after the liquid began frothing, the temperature was raised to 300 °C, and in a few seconds, ignition took place as observed by a flame wave that lasted 1 min approximately. The as-prepared powders were grinded in an agate mortar and then sieved at 63 µm. Calcination of the as-prepared combustion powders were performed in air at a heating and cooling rate of 5°/min. Calcinations were carried out every 100 °C in the range from 200 to 800 °C and the samples were held at each calcination temperature for 5 h. In addition, three temperatures, 650, 750, and 850 °C were chosen for rapid cooling after calcination (quenching). The as-prepared powders were calcined at 10 °C/ min and maintained for 2 h when the set up temperature was reached, the samples were then quenched to room temperature. 2.2. Characterization of β-NiMoO4 Powders. The combustion powders were characterized using different analysis techniques. XRD was used for the phase analysis of the as-prepared and calcined powders, after the cooling of the samples. A Siemens D5000 difractometer, operating at 50 kV and 30 mA using the Cu KR radiation and a Ni-filter in the range of 2θ ) 10-70° was used. The scanning step was 0.05°, the time/step 1.5s and the rotation speed used was 15 rpm. Crystallite size was measured from the XRD patterns obtained using the DebyeSherrer equation (L ) (0.94λ)/B(2θ) cos θ).22 A Jeol Superprobe (JXA-8900H-WD/ED Combined microanalyzer, Japan) scanning electron microscope (SEM) was used for the microstructural characterization of the as-prepared and calcined powders. TEM was also employed for the microstructural characterization of the as-prepared powders with a JEM-2000FX (Japan) microscope. Thermogravimetry and differential thermal analysis (DTA-TG) were employed to study the thermal behavior of the sample in air up to 800 °C with a SDT Q600 analyzer from TA Instruments (USA), being that the heating and cooling rates were of 2 °C/min. A FTIR spectrometer, NICOLET 6700 from Thermo Scientific (USA), was connected online to the outlet of the TG analyzer. FTIR spectra of the gases produced were acquired during the thermal analysis. The as-prepared and calcined powders were characterized by ATR, in a FTIR Spectrum 100 spectrometer (Perkin-Elmer, USA). Specific surface area was determined by the BET method in a Monosorb Analyzer MS-13 QuantaChrome (USA), particle size distribution was analyzed with a Mastersizer (Malvern Instruments, UK) equipment. X-ray photoelectron spectroscopy (XPS) analyses were performed using a Fisons ESCALAB mag 200R spectrometer (Canada). A Digital pyrometer Impac Infratherm IGA-5 MB-18 (Germany) was employed for measuring the temperature during the combustion reaction. Electrical conductivity was measured employing AC electrochemical impedance spectroscopy (EIS) with a two points holder, following the procedure described elsewhere.23 To perform the EIS measurements the powders were pressed as a pellet and placed into a Pt crucible, which was used for the electrical contact, while a Pt foil placed in the opposite face of the pellet was used as the second electrode. The measurements were acquired with a HP Agilent 4294A impedancimeter (USA) dynamically from 25 to 800 °C,

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Figure 1. XRD of the β-NiMoO4 powders as-prepared. The only phase presented is the β-phase; its main reflection is indicated in the figure.

every 50 °C, in a frequency range from 40 to 107 Hz; the heating and cooling ramps during the measurement were of 10 °C/min, the same rate used in the calcination and quenching experiments performed with the as-prepared powders. When reaching 800 °C the temperature was maintained and the measurements were obtained every 10 min, during 1 h. 3. Results and Discussion The maximum average temperature reached in the combustion synthesis was 830 °C. The combustion reaction lasted no more than 180 s, being the reaction mixture exposed to the maximun combustion temperature for an average time of 50 s. The fastness and the energy release of this reaction are the parameters exploited in this work to achieve the nanometric β-NiMoO4 phase at room temperature (RT). Furthermore, the use of acetylacetonates as metal cation precursors provides an additional fuel source, which in combination with the exothermic decomposition of urea generates a higher amount of gases that acts directly on the flame temperature. This excess of fuel in the reaction explains the use of ammonium nitrate as an oxidizer source free of metallic cations. This compound is used to balance the oxidizer and reductant valence equilibrium, namely the equivalence ratio,24 producing the ignition of the reaction mixture. The theoretical total reaction could be R1, as is written below. The acetylacetonate enthalpy data are not available in the literature, for this reason the total enthalpy involved in the reaction could not be calculated. Ni[CH3COCH ) C(O-)CH3]2(s) + MoO2[CH3COCH ) C(O-)CH3]2(s) + xCO(NH2)2(s) + yNH4NO3(s) + (22 + 3x/2 - y/2)O2(g) f β-NiMoO4(s) + (20 + x)CO2(g) + (14 + 2x + 2y)H2O(g) + (x + y)N2(g)

(R1) One of the features of the combustion synthesis is the obtention of powders with a high specific surface area. The specific surface area of the as-prepared powders was 31 ( 7 m2/g. According to the XRD analysis the combustion route promotes directly the synthesis of β-NiMoO4 as a single phase. The most intense reflection located at 2θ ) 26.7° (Figure 1) was ascribed to the high temperature phase β-NiMoO4, whereas the R-NiMoO4 phase was not detected by XRD in the asprepared powders. Nickel oxide, frequently used for the stabilization of β-NiMoO4,25 was not detected in the as-prepared powders either by XRD (Figure 1) or by FTIR (Figure 7). It is

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Figure 3. XRD patterns of the as-prepared powders quenched at 650, 750, and 850 °C and quenching of the as-prepared combustion powders. (0) β-NiMoO4 phase and (1) R-NiMoO4 phase.

Figure 2. XRD patterns obtained in the as-prepared powders calcinations in air at different temperatures. The reflections were indexed as follows, (0) β-NiMoO4 phase, (1) R-NiMoO4 phase, and (O) NiO.

well-known that NiO main reflections appear at 2θ ) 37.2° and 43.5°, and none of them appears in the XRD pattern of Figure 1. The crystallite size resulted calculated employing the Debye-Sherrer equation22 and was 6 nm. The as-prepared powders were calcined in air at different temperatures up to 800 °C to study the phase evolution versus the calcination temperature. It is noted that β phase crystallizes when R-NiMoO4 is heated, although this β-phase is not stable during cooling and from 250 °C to room temperature transforms again into R-NiMoO4, according to data reported in the literature.26 As it is mentioned above, combustion synthesis promotes the stabilization of β phase at room temperature. The XRD pattern for the powders after several calcinations and cooling down to room temperature, shows a mixture of R and β phases (Figure 2). When the stabilized β-NiMoO4 is heated and then cooled down to room temperature, βfR phase transition takes place during cooling, although not all the particles transformed to R (stable thermodynamically at RT) and some remained as β-NiMoO4. Figure 2 shows that β (metastable) phase remains stable after cooling on those calcinations carried out up to 600 °C, whereas at higher temperatures, the XRD pattern shows only R-NiMoO4 after cooling. It is also remarkable that at 800 °C this β-NiMoO4 phase does not decompose. Rodriguez et al.14 published an upper temperature limit of 730 °C for the stabilization of this material; these authors detected by high temperature XRD the formation of NiO and concluded that volatilization of MoO3 causes the decomposition of the β-NiMoO4 mixed oxide to NiO. Nevertheless, in our case the asprepared sample calcined at 800 °C did not decompose and, after cooling, the powders were constituted only by R-phase. Moreover, the quenching carried out at different temperatures (650, 750, and 850 °C) showed that the powders (at RT) were a mixture of both polymorphs (R and β-NiMoO4) which remained stable at least up to 850 °C. The XRD patterns of Figure 3 shows mainly R-NiMoO4, and a small reflection of

Figure 4. DTA-TG in air of β-NiMoO4 phase prepared by combustion synthesis.

β-NiMoO4, which was present probably due to the rapid cooling of the sample. The DTA-TG curves of the as-prepared β-NiMoO4 nanopowders were measured in air. In Figure 4, the slight weight loss (∼2 wt %) up to 150 °C was associated with the loss of physisorbed water. The exothermic peaks detected at 215, 383, and 432 °C in the DTA curve, labeled as B, C and D were ascribed to the decomposition of unreacted Ni (II) and Mo (VI) acetylacetonates. These assumptions are based on the thermal study (DTA-TG) of these cation precursors, in which both acetylacetonates exhibit high exothermic peaks when decompose around these temperatures, Figure 5, panels a and b, respectively. These exothermic peaks found in Figure 4 are shifted to higher temperatures than those found in Figure 5, probably due to exothermic reactions that could take place during the measurement as Biamino et al. suggested.27 The presence of precursors unreacted, specially Ni (II) acetylacetonate, can be the reason for the stabilization of β-phase, taking into account that NiO is not present in the as-prepared powders. This oxide could be in solid solution with the β-NiMoO4, since nickel ions (Ni2+(IV) ) 0.69 Å) can replace molybdenum ions (Mo6+(IV) ) 0.55 Å) in the crystal lattice stabilizing this phase.1 Moreover, at 500 °C there is another sharp exothermic peak (labeled as E in Figure 4) which does not have any weight loss associated, that could be related with the crystallization of the β phase.28 The FTIR spectra acquired at the outlet of the TG confirms the DTA-TG results. Figure 6 shows the FTIR spectra of the gases produced during heating the sample. Six different spectra namely A-F are plotted in Figure 6. Each spectrum corresponds to significant points of the DTA curve. CO2 is detected in all of the spectra; nevertheless, the intensity of the CO2 bands (2640 and 664 cm-1) increases as the temperature increases, since the precursors that remain unreacted in the as-prepared powders are eliminated. Small bands located at 2357 and 2350 cm-1 indicate the

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Figure 7. FTIR spectra obtained for (a) the β-NiMoO4 as-prepared and (b) the as-prepared powders calcined at 700 °C/12 h and composed by R-NiMoO4.

Figure 5. DTA-TG in air of (a) Ni (II) acetyl acetonate and Mo(VI) acetyl acetonate used as precursors in the synthesis of β-NiMoO4 phase.

Figure 8. (a) SEM of the β-NiMoO4 as-prepared powders, (b) mapping of Mo, and (c) mapping of Ni.

Figure 6. FTIR spectra of the subproducts generated during the DTATG analysis shown in Figure 4. A, B, C, D, E and F indicates the spectra at certain temperatures, labeled in Figure 4. (*) νas (CO2), (**) δ(CO2), (+) ν(CO) and (left pointing triangle) ν(CdO) + ν(CdC).

formation of CO from 215 °C, as a subproduct of the combustion of Ni(II) and Mo(VI) acetylacetonates. The FTIR spectra acquired at 500 °C show the presence of CO and CO2 as well. At this temperature, the peak present in the DTA curve, which does not have any weight loss associated in the TG, has been ascribed to the β phase crystallization; as was mentioned above, the lower intensity bands in Figure 6 are due to the features of the analysis which is carried out online and thus detects residual gases from the exothermic combustions that took place at lower temperatures. In this sense, the spectrum F acquired at 600 °C does show similar bands. Taking into account the XRD, DTA-TG, and FTIR results, the presence in the asprepared powders of Ni (II), in the nickel acetyl acetonate could be the explanation of the stabilization of β-phase at low temperatures, when the temperature is slightly increased the β phase transforms to R phase. FTIR was also employed to asses the presence of R or β oxides in the β-NiMoO4 prepared by combustion, on the basis of the structural differences between the two isomorphs of Nimolibdate.29 Figure 7 shows the spectra of the (a) as-prepared β-NiMoO4 and (b) the previous material calcined at 700 °C in air and cooled down slowly to RT. These latter powders are

constituted only by R-NiMoO4. The spectrum of the as-prepared powders, Figure 7a, is characterized by the presence of bands at low wavenumber, which are useful in determining the presence of Mo with octahedrical or tetrahedrical coordination index, in the R and β-NiMoO4 polymorphs, respectively.1 The β-NiMoO4 phase depicts a band located at 946 cm-1 and two new bands at 870-892 cm-1 that do not appear in the R-NiMoO4. The spectrum in Figure 7a shows also some strong intensity bands in the range of 1300-1700 cm-1. These bands (1300, 1470, and 1640 cm-1) have been identified as ν(C-O), δ(C-H), and ν(CdC) bonds, respectively, which confirm the presence in the as-prepared powders of unreacted acetylacetonates. On the other hand, the spectrum of the heat treated sample in Figure 7b corresponds to the R-NiMoO4 phase and shows the characteristic bands at 922 and 950 cm-1 (pointed out in the figure). From SEM characterization it was observed that combustion powders exhibited the classical morphology of the materials synthesized by the combustion route. The as-prepared powders have a foamy appearance with an agglomerate average size of 1 µm, Figure 8a. These agglomerates were mainly formed by rounded particles of much smaller average size (∼10 nm) as shown in TEM images of Figure 9. Mapping showed (Figure 8) that panels b and c indicate that both Ni and Mo elements, respectively, are homogeneously and uniformly distributed throughout the sample. From TEM observations of the as-prepared powders (Figure 9a) foam-like agglomerates of high porosity were also found.

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Figure 9. (a) TEM micrograph of β-NiMoO4 as prepared powders and (b) calcined at 700 °C/12 h.

TABLE 1: Binding Energies for r,β-NiMoO4 Analyzed by XPS and Ni/Mo Atomic Surface Ratio sample β-NiMoO4 (as-prepared) R-NiMoO4 (β-NiMoO4 calcined 700 °C/12 h)

Mo 3d5/2 (eV)

Ni 2p3/2 (eV)

O 1s (eV)

Ni/Mo at

232.8 232.4

856.2 856.2

531.0 530.8

0.349 0.701

The particle size was calculated from the micrographs. TEM observations confirm the low particle size (6-10 nm) measured from the XRD pattern of β-NiMoO4 phase. From Figure 9a it is clear that β-NiMoO4 phase attained by combustion is achieved with a nanocrystalline structure and a small particle size (lower than 10 nm), being both facts relevant for an effective catalytic activity and for the stabilization at room temperature of the β-phase. The powder calcined at 700 °C, Figure 9b, exhibits particles with a higher size (20-30 nm), with a polygonal shape and less porous appearance comparing with the as-prepared material, this morphology was also observed by other authors30 although they used other synthesis routes, and corresponds to the R-NiMoO4 phase. Conclusively, the SEM and TEM observations are in agreement with the analysis of XRD, which seems to imply that when the particle size is higher than 20 nm the R-phase evolves to β-NiMoO4, hence a critical particle size (∼20 nm) may exits for the R f β NiMoO4 transformation. Below that size R-NiMoO4 would be favored, and R f β transformation would take place when the particle size exceeds that value. Finally, the XPS analysis of the as-prepared and calcined sample in air (700 °C/2 h) was useful in the determination of the surface composition and in the discrimination of metal oxides such as MoO3 or NiO. In Table 1 the binding energies (B.E.) obtained by means of the deconvolution of the band are listed. As it can be seen, there are not significant differences between the two samples studied. From the XRD and FTIR measurements it was concluded that the as-prepared sample was composed by β-NiMoO4 stabilized by the formation of a solid solution with NiO. The sample calcined to 700 °C/2 h and then cooled slowly to room temperature was composed of only R-NiMoO4. Comparing the data with those reported in the literature, the B.E. measured can be associated with the presence of NiMoO4 (using this technique is not possible to discriminate between R and β phase). The Mo d5/2 binding energy has a similar value for Mo in both the NiMoO4 crystallographic environment and in the case of MoO3. In the case of Ni2p3/2, its B.E. differs when Ni (II) is in the lattice of NiMoO4, being the B.E. 856.2 eV, as shown in Table 1, shifted from the Ni (II) B.E. value when is into the lattice of NiO, B.E.) 853.8 eV.1

Figure 10. Curves of the conductivity vs temperature of the β-NiMoO4 phase. The measurements were performed in air. Insert: Comparison of the variation of conductivity with temperature on the cooling and heating ramps.

The differences in both samples are found in the Ni/Mo ratio, since the calcined sample is Ni enriched comparing with the as-prepared sample. That fact is a consequence of the thermal treatment that induces Ni segregation to the surface; the differences in the Ni/Mo ratio on the surface composition between both samples will determine the catalytic behavior of the powders. β-NiMoO4 as-prepared powders show semiconductor behavior, and the conductivity values are in the range of 10-8 to 10-5 S/cm, in the temperature interval from 25 to 800 °C. Figure 10 shows the trend of the conductivity in air with the temperature during heating and cooling steps. The electrical response of the sample is in accordance with the DTA-TG curves shown in Figure 4. Stable β-NiMoO4 exhibits a maximum conductivity value of 4.4 × 10-6 S/cm at 250 °C. This expected increase in conductivity could be ascribed to the presence of organic residuals in the sample. As long as these organic compounds are eliminated (∼450 °C, as shown in the DTA-TG curve, Figure 4) the conductivity of the sample decreases up to 5.6 × 10-8 S/cm. From 450 °C, the conductivity shows a slight increase with a second maximum of 3.5 × 10-7 S/cm located at 600 °C, Figure 10. The latter conductivity value achieved is higher than the conductivity found by other authors for β-NiMoO4 and cesium doped NiMoO4 catalysts.31 The difference in the conductivity values is related with the phase transition that takes place in the powders around 500 °C. From 500-800 °C the conductivity of the powders continues increasing and reaches a value of 9 × 10-7 S/cm at 800 °C. Apparently, the presence of the β-phase increases the conductivity of the powders. Similar results are achieved during the cooling ramp of the sample, Figure 10b, although the conductivity values are lower than those expected. From 800 to 250 °C the powders are believed to be only composed by the β-phase; however, as the conductivity values are lower than those obtained during the heating ramp, it is reasonable to consider that, during these measurements, the transition R f β was not fully accomplished. A second measurement was acquired in the same conditions. By that time the powders of β-NiMoO4 were previously calcined in air at 800 °C for 4 h. With this thermal treatment, the organic residuals are completely eliminated and the powders are only composed, at room temperature, by R-NiMoO4. Figure 11 shows the electrical conductivity changes that occur while temperature varies. During the heating ramp, the conductivity does not show any significant change from 550 to 650 °C. It is assumed that the sample is still composed by a mixture of R and β-NiMoO4 within this range. From 700 °C, the conductivity starts increasing

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Figure 11. Conductivity vs temperature of β-NiMoO4 phase. The measurements were performed in air during the cooling and heating ramps while the temperature was maintained at 800 °C during 1 h.

Figure 12. log(σ) vs 1000/T Arrhenius plots for the β-NiMoO4 phase during the heating (9) and the cooling ramp (0).

exponentially while the R phase continues to be slowly transformed into the β-phase, although both phases are still coexisting at that temperature. The sample was then maintained at 800 °C for 60 min, and every 10 min the conductivity was measured showing a significant increase from 1 × 10-5 to 6 × 10-5 S/cm. We concluded that during those 60 min the phase transition is completely accomplished, and thus, the final value can be attributed to the real conductivity of the β-NiMoO4 as a single phase as confirmed by XRD. It should be noticed that this datum is remarkable higher than that provided by other authors.32 Cooling the sample, the conductivity starts to decrease exponentially with temperature. In any case, the values achieved for each temperature show a hysteresis in comparison with the conductivity values obtained during the heating ramp, and only when 550 °C are reached, the conductivity values are comparable. For the R-NiMoO4 phase calcined (800 °C), the log(σ) versus 1/T Arrhenius plot shows a discontinuity at 650 °C during the heating ramp (Figure 12). This fact can be associated with the phase transition that starts to take place around this temperature. Two activation energies (Ea) can be calculated for the electronic conduction mechanism in this sample. Both the R and β phases show almost the same values, R (1.03 eV) and β (0.97 eV). Differences in the coordination of Mo in the lattice for both polymorphs are not enough to promote a change which allows an increase of electron hopping (n-type). However, when both phases coexists (600-800 °C, during the heating process, Figure 12) the mentioned differences can be observed for the Ea (1.98 eV). It could be associated with an electron blocking effect observed by the formation of energy barriers which can forbid the electron movement; however, the measurements needed to corroborate this hypothesis are now in progress. 4. Conclusions It has been established that the combustion synthesis can be used as a novel method for the straightforward synthesis of

Moreno et al. β-NiMoO4 nanopowders. The fastness and the energy release of the combustion reaction adjusted by the precursors and the NH4NO3 used, are the parameters exploited in this work to obtain the nanometric β-NiMoO4 phase. The characterization of the powders attained reveals the absence of NiO, currently used to stabilize the high temperature phase, that is β-NiMoO4. Nevertheless, the presence of Ni (II) in the acetylacetonate precursor observed in the DTA-TG and FTIR analyses can be related with the stabilization of the β-phase. The as-prepared powders are nanometric with an average particle size of 6-10 nm observed by TEM, and a surface area of 31 ( 7 m2/g. After calcination at 600 °C the particle size grows up and is higher than 20 nm, then the β-phase evolves to R-NiMoO4, thus a critical particle size may exits for the β to R transformation (i.e., > 20 nm). Electrical measurements were explained in relation with the structural evolution of the sample with temperature, finding a semiconductor behavior with higher conductivity values than those reported in the literature for the β-phase. The activation energies calculated in this work indicate that both R and β phases exhibit a similar conduction mechanism although the mixture of both phases could produce the formation of energy barriers that forbid the movement of electrons. Acknowledgment. The authors are grateful to the APOLLON B (NMP/STREP (FP6), NMP3-CT-033228) and MEC Plan Nacional I+D+I (8ENE2005-09124-C04-00/01) projects for the financial support. Thanks to Prof. Garcia Fierro for the XPS measurements, Dr. Fausto Rubio for the DTA-TG and FTIR combine measurements, and Marcos Borro for technical support. References and Notes (1) Kaddouri, A.; Tempesti, E.; Mazzocchia, C. Comparative study of β-nickel molybdate phase obtained by conventional precipitation and the sol-gel method. Mater. Res. Bull. 2004, 39, 695–706. (2) Harlin, M. E.; Backman, L.; Krause, A. O. I.; Jylha¨, O. J. T. Activity of Molybdenum Oxide Catalyst in the Dehydrogenation of n-Butane. J. Catal. 1999, 183, 300–313. (3) Erdo¨helyi, A.; Ma´te´, F.; Solymosi, F. Partial oxidation of ethane over silica-supported alkali metal molybdate catalysts. J. Catal. 1992, 135, 563–575. (4) Stern, D. L.; Graselli, R. K. Reaction Network and Kinetics of Propane Oxydehydrogenation over Nickel Cobalt Molybdate. J. Catal. 1997, 167, 560–569. (5) Kaddouri, A.; Anouchinsky, R.; Mazzocchia, C.; Madeira, L. M.; Portela, M. F. Oxidative dehydrogenation of ethane on the R and β phases of NiMoO4. Catal. Today 1998, 40, 201–206. (6) Vasudevan, P. T.; Fierro, J. L. G. A Review of Deep Hydrodesulfurization Catalysis. Catal. ReV. Sci. Eng. 1996, 38, 161–188. (7) Andreev, A. A.; Kafedjiysky, V. J.; Edreva-Kardjieva, R. M. Appl. Catal. A: Gen. 1999, 179, 223. (8) Borowiecki, T.; Giecko, G.; Panczyk, M. Effects of small MoO3 additions on the properties of nickel catalysts for the steam reforming of hydrocarbons: II. Ni-Mo/Al2O3 catalysts in reforming, hydrogenolysis and cracking of n-butane. Appl. Catal. A: Gen 2002, 230, 85–97. (9) Centi G.; Cavani F.; Trifiro F. SelectiVe oxidation by heterorgeneous Catalysis: Kluwer Academy/Plenum Publishers: New York, 2001. (10) Madeira, L. M.; Portela, M. F. Kinetics and mechanism of the selective oxidation and degradation of n-butane over nickel molybdate catalysts. Stu. Surf. Sci. Cat. 1998, 119, 611–616. (11) Madeira, L. M.; Portela, M. F.; Mazzocchia, C. Nickel Molybdate Catalysts and Their Use in the Selective Oxidation of Hydrocarbons. Catal. ReV. 2004, 46, 53–110. (12) Abdel, D.; Ayem, H. M.; Ruiz, P. Stud. Surf. Sci. Catal. 2001, 138, 363. (13) Di Renzo, C.; Mazzocchia, C. How thermal treatment influences the phase transition of NiMoO4. Thermochim. Acta 1985, 85, 139–142. (14) Rodriguez, J. A.; Chatuverdi, S.; Hanson, J. C.; Albornoz, A.; Brito, J. L. Electronic Properties and Phase Transformations in CoMoO4 and NiMoO4: XANES and Time-Resolved Synchrotron XRD Studies. J. Phys. Chem. B 1998, 102, 1347–1355. (15) Mazzocchia, C.; Tempesti, E.; Aboumrad, C. Fr. Patent 89-00522, 1989.

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