Urea-Based Combustion Process for the Synthesis of Nanocrystalline NiLa-FeO Catalysts

Nanocrystalline Ni-La-Fe-O catalysts having the general formula NiLaxFe2−xO4 (0.00 ≤ x ≤ 2.00) were synthesized by the combustion route employing urea as a combustion fuel. The calcination process was affected at 500◦C. The structural properties of the obtained catalysts were systematically investigated by X-ray powder diffraction (XRD), scanning electronic microscopy (SEM), energy-dispersive X-ray spectra (EDX), and nitrogen adsorption at−196◦C. Crystalline NiFe2O4 and La2NiO4 phases were detected for the catalysts having x = 0.00 and 2.00, respectively, as a result of solid-solid interaction between mixtures precursors. The activity of the obtained catalysts was checked for hydrogen peroxide decomposition at 35–55◦C. A synergic effect was observed for the catalysts having x-value of 1.00 and 1.50. Such effect was attributed to the increase in the number of the active constituents involved in the catalytic decomposition of H2O2.


Introduction
Nickel ferrite, NiFe 2 O 4 , is one of the most important ferromagnetic materials which is known to exhibit low conductivity and thus lower eddy current losses, high electrochemical, thermal and chemical stability, abundance in nature, and so forth [1,2].NiFe 2 O 4 has an inverse spinel structure in which the tetrahedral (A) sites are occupied by Fe 3+ ions and the octahedral (B) sites are occupied by Ni 2+ and Fe 3+ ions in the spinel formula AB 2 O 4 [3].It has been widely used for various applications such as ferrofluids, catalysts, microwave devices, magnetic materials, gas sensors, high-density information storage, and as adsorbent to treat wastewater [4][5][6].
Lanthanum-nickelate-(La 2 NiO 4 -) based materials have attracted much attention in the past few years, as highly efficient electrochemical systems, including solid oxide fuel cells and ceramic membranes for oxygen separation and partial oxidation of hydrocarbons [7].The La 2 NiO 4 structure consists of alternating LaNiO 3 perovskite layers and LaO rock-salt layers with excess oxygen atoms occupying the interstitial sites between the LaO layers [7].La 2 NiO 4 exists over a broad range of oxygen nonstoichiometry and its structural, electrical, and magnetic properties are very sensitive to the amount of oxygen present [8].
The conventional ceramic method which involves the solid state reaction between the metal oxides, requiring a working temperature above 1000 • C for several days, was commonly used for the preparation of NiFe 2 O 4 [9].Employing such high operating temperature lead to the formation of inhomogeneity, poor stoichiometry, and higher crystallite size NiFe 2 O 4 spinel [10].In agreement, it was reported that high temperature, 1100-1400 • C or higher, is required for the preparation of La 2 NiO 4 from its precursors oxides employing the ceramic method [11,12].
Soft chemical processes such as sol-gel, precipitation, and combustion methods represent other alternative methods for the preparation of powder materials.Among the wet chemical methods, combustion process is known to be simple and cost effective and small crystallite size of the resultants, latter of which may have an important influence on the properties of the materials prepared [8,13].Its basic principle is to distribute metal ions throughout the polymeric network and to inhibit their segregation and precipitation.Moreover, it involves an exothermic, generally very fast and self-sustaining chemical reaction between the desired metal salts and a suitable organic or inorganic fuel, which is ignited at temperatures much lower than the actual phase formation temperature [13].
To our knowledge in the open literature there is one paper dealing with the preparation and characterization of nano-crystalline NiFe 2−x La x O 4 , where the x-value was only limited to 0.09, which were synthesized by using metal nitrate and egg-white extract in aqueous medium [14].Therefore, the present contribution was focused on the preparation and characterization of a series of nanocrystalline Ni-La-Fe-O catalysts via combustion synthesis.Five mixtures having the general formula NiLa x Fe 2−x O 4 (x = 0.00, 0.50, 1.00, 1.50, and 2.00) were prepared using urea as a combustion fuel.The molar ratio of urea/nitrate was adjusted to be 1.Calcination was affected, for 1 h, in static air atmosphere at 500 • C. The obtained solids were characterized for their structure and surface morphology by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive Xray spectra (EDX), and nitrogen adsorption at −196 • C techniques.The crystallite size was calculated using XRD data and Scherrer's formula.The activity of the obtained catalysts towards H 2 O 2 decomposition was tested.Five mixtures having the general formula NiLa x Fe 2−x O 4 (x = 0.00, 0.50, 1.00, 1.50, and 2.00) were prepared using urea as a combustion fuel.The molar ratio of urea/nitrate was adjusted to be 1.Prior to the calcination, the appropriate amounts of the reactants, with little added distilled water, were first mixed in a small porcelain crucible, then heated in an oven at 90 • C. Finally, after the solution was converted to a viscous gel it was calcined, for 1 h, in air at 500 • C, and then quenched to room temperature.During the first few minutes of the calcination process ignition took place with a rapid evolution of large amounts of gases.Therefore, only small portions of the gels were calcined.

Catalysts Characterization.
XRD patterns of the calcination products were recorded using the powder diffraction pattern technique with 2θ ranging between 4 and 80 • , with the aid of a Philips model PW 2103/00 diffractometer.The Philips generator, operated at 35 kV and 20 mA, provided a source of CuKα radiation.The FTIR spectra of the calcination products were recorded using the KBr disk technique in the range 4000-400 cm −1 using a Thermo-Nicolet-6700 FTIR spectrophotometer.Surface areas were determined by BET analysis of the corresponding nitrogen adsorption isotherms (at −196 • C).The morphology of the samples was analyzed by field-emission scanning electron microscope (FE-SEM) on a JEOL model JSM-7600F microscope.The compositions were examined by energy-dispersive X-ray spectroscopy (EDX) in the SEM.Counts (a.u.)

Activity Measurements.
The measurements of the kinetics of catalytic decomposition of hydrogen peroxide have been carried out in a glass volumetric system.The measurements were conducted at 35-55 • C temperature range.A constant catalyst weight 0.1 g was added to a thermostated reaction vessel containing 5 mL of hydrogen peroxide solution (30%, w/v).The analysis of the experimental data has been carried out on the assumption that the decomposition of H 2 O 2 is a zero-order process.The pseudo-homogeneous zero-order rate constants, k hom , have been calculated according to where V is the volume of oxygen evolved at time t and V 0 is the volume of oxygen evolved to the moment at which the measurements started.The heterogeneous rate constant, k het , is then calculated from k hom as follows: where w cat is the weight of catalyst used and S cat is the specific surface area of the catalyst.The crystallite sizes of the obtained catalysts were determined using the well-known Scherrer's formula on the basis of the full width of the diffraction line at half the maximum (FWHM) intensity measured in the most intense peak: where λ is the X-ray wavelength, θ isthe Bragg's angle and β is the full width of the diffraction line at half the maximum intensity.The obtained values are listed in Table 1.Santos et al. [15] have reported an average crystallite size of 29 nm for nanosized NiFe 2 O 4 powders being prepared by the combustion synthesis.Close value, 28 nm, was reported for sample synthesized via the thermal plasma method [16].A value of 22 nm was reported for NiFe 2 O 4 synthesized using sol-gel autocombustion [17] and ball milling [18] routes.Comparing the result of crystallite size for NiFe 2 O 4 (around 10 nm) obtained in this work with the former reported values manifests the high efficiency of the combustion synthesis, employing urea as a fuel, in the preparation of nanosized

Results and Discussion
Volume adsorbed (cc/g @ STP) 0 5 cc NiFe 2 O 4 powders.In agreement, Vivekanandhan et al. [10] have reported a value of 14 nm for their NiFe 2 O 4 being prepared via combustion process with metal nitrates as Ni and Fe ion sources and polyacrylic acid.
SEM micrographs of NiLa x Fe 2−x O 4 catalysts are depicted in Figure 2. The surface of the NiFe 2 O 4 (x = 0.00), Figure 2(a), consists of a network of spherical particles seems to be practically uniform with average size 20-35 nm.The FE-SEM image of the NiLaFeO 4 catalyst, Figure 2(b), clarifies that this catalyst has bigger spheres like particles having a size in the range 25-100 nm.Moreover, irregular holes distributed among the various particles without a characteristic size or shape can be seen.Figure 2(c) indicates that the La 2 NiO 4 catalyst consists of larger particles having irregular shape.SEM results suggest that the combustion technique, employing urea as fuel, is effective in terms of the preparation of nanocrystalline NiLa x Fe 2−x O 4 catalysts especially those having lower x-value with uniform structural properties.The EDX patterns of the synthesized NiLa x Fe 2−x O 4 catalysts were carried out to screen the composition of the metals.EDX analysis on several crystals revealed constancy of compositions.EDX analysis of the catalysts having x = 0.00 and 2.00 (not shown) indicates that the nanoparticles are composed of Ni and Fe for NiFe 2 O 4 (x = 0.00) and Ni and La for La 2 NiO 4 (x = 2.00).In addition, the atomic ratio of Fe/Ni V a (cc/g) t ( Å) Adsorption-desorption isotherms of nitrogen, measured at −196 • C, over NiLa x Fe 2−x O 4 catalysts (x = 0.00, 0.50, 1.00, 1.50, and 2.00) are shown in Figure 3. On analyzing these isotherms it is possible to drive the specific area (S BET ), external surface area (S t ), the total pore volume (V p ), and the average pore diameter of each catalyst, as given in Table 2.The obtained isotherms are generally Type I according to Brunauer's classification [19] at low pressure values and a little of type II features at higher P/P 0 values.Moreover, the different catalysts exhibit a hysteresis loop nearly belongs to type H4 [19].Furthermore, the closure point of the hysteresis loops for all the samples is approximately at P/P 0 = 0.1, which indicates either a strong affinity of adsorbate towards the surface or the existence of ultramicropores [20].The specific surface areas were calculated by applying the BET equation, in its normal range of applicability, whereas S t values were calculated using the V a−t plots of de Bore [19].
The S BET value of NiFe 2 O 4 , Table 2, is 19.73 m 2 /g.It is evident that, increasing x-value leads to a continuous decrease of the S BET value till x = 1.50, then it shows a slight increase on further x-value increase (x = 2.00).The obtained S t values follow approximately similar trend.It is worth   mentioning that, the obtained S BET value of NiFe 2 O 4 in this work is higher than that, 6.16 m 2 /g, reported by Hou et al. [6].Also, the obtained value for our La 2 NiO 4 , 14.29 m 2 /g, is higher than that, 6.7 m 2 /g, reported by Ramesh et al. [21].The v a−t plots characterizing the different NiLa x Fe 2−x O 4 catalysts are shown in Figure 4.It is evident that, both NiFe 2 O 4 and La 2 NiO 4 catalysts exhibit a mild positive deviation (upward deviation).This, in turn, indicates the mesoporous nature of both catalysts.At higher P/P 0 values the two curves show a negative deviation (downward deviation).Such behavior suggests the presence of microporous of both catalysts.The catalysts having x = 0.50, 1.00, and 1.50 exhibit microporous nature only as indicated by the downward deviations in the relevant v a−t plots of these catalysts.

Activity Measurements.
The kinetics of the catalytic decomposition of hydrogen peroxide was conducted at 35-55 • C temperature range over the different NiLa x Fe 2−x O 4 catalysts calcined at 500 • C. The treatment of the experimental data has been carried out on the assumption that the decomposition of H 2 O 2 is a zero-order reaction.Thus, the volume of the evolved oxygen was recorded as a function of time.Figure 5 depicts the variation of the volume of oxygen evolved as a function of time at 45 • C over all the catalysts.Straight lines were obtained and from the slope of these lines the relevant values of k hom were obtained.In order to account for the induced changes in the specific surface area as a result of x-value change, the values of k hom were converted to k het for each catalyst at each reaction temperature and the obtained values were plotted as k het versus x-value at different reaction temperatures as shown in Figure 6.From the inspection of this figure, it is obvious that increasing the temperature leads to a continuous increase in the obtained rate constant values for all the catalysts.Moreover, increasing the x-value is accompanied by an activity increase giving a maxima at x = 1.5.In other words, a synergic effect can be observed which is more pronounced for the catalyst with the composition x = 1.50.
Single transition metal oxides like NiO or Fe 2 O 3 were reported to exhibit low activity towards hydrogen peroxide decomposition [22,23].On the other hand, higher activity patterns were reported for mixed transition metal oxides which are influenced by the ratio of the metal oxides in their mixtures as well as the presence of dopants.In this context, it was shown that the H 2 O 2 decomposition activity of Cu : Fe mixed oxide varies in a nonmonotonic way with their composition [24].The H 2 O 2 decomposition activity, for the 350 • C precalcined catalysts, was maximum for the mixtures rich in copper and iron species (3Cu : 1Fe and 1Cu : 3Fe).High H 2 O 2 decomposition activity was reported over a series of Ag/Fe x Al 2−x O 3 catalysts, being calcined at 300-700 • C temperature range [13].Irrespective of the calcination or the reaction temperatures, the highest activity was exhibited by catalyst having x = 1.5, that is, Ag/Fe 1.5 Al 0.5 O 3 catalyst.The activity of the Mn-oxide/Al 2 O 3 catalysts, being calcined at 400-800 • C, was greatly enhanced upon doping with Fe 2 O 3 reaching a maximum value at 1.96%, then sharply decreases with further increase in iron content [22].Concurrently, it was demonstrated that the addition of a very small amount of ZnO to the Co 3 O 4 /Al 2 O 3 system led to an enhancement of its catalytic activity towards H 2 O 2 decomposition [25].x-value The activity of mixed oxide catalysts during hydrogen peroxide decomposition is usually interpreted in terms of the concept of bivalent catalytic centres [14,[22][23][24][25].In this way, for NiO/MgO doping with Fe 2 O 3 it was suggested that, the doping effect did not modify the mechanism of H 2 O 2 decomposition but rather formation of new active sites contributing in reaction.Such sites were believed to be ion pairs (Ni 2+ -Fe 3+ , Mg 2+ -Fe 3+ ) [23].For CuO-Fe 2 O 3 catalysts, the higher catalytic activity of the two-component oxides was correlated, in addition to the one-component sites Cu 2+ -Cu + and Fe 3+ -Fe 2+ ions, to the newly formed mixed sites Cu 2+ -Fe + and/or Cu + -Fe 2+ ion pairs as a result of mutual charge interaction [24].The H 2 O 2 decomposition activity of mixed Fe 2 O 3 -MoO 3 catalyst, obtained by thermal treatment of the Fe-Mo mixtures at the same calcination temperature, was found to be greater than that of single oxides [26].Such behavior was interpreted, also, in terms of the concept of bivalent catalytic centers.Thus, the higher catalytic activity of the two-component oxides was ascribed to the fact that beside the one-component sites Fe 3+ -Fe 2+ and Mo 6+ -Mo 5+ , there will also be the mixed sites Fe 3+ -Mo 5+ and/or Fe 2+ -Mo 6+ ion pairs as a result of mutual charge interactions [26].
Thus, in agreement with the above-mentioned literature data, the observed activity of NiFe 2 O 4 and La 2 NiO 4 catalysts, during H 2 O 2 decomposition could be attributed to the mixed sites Ni 2+ -Fe +3 and Ni 2+ -La 3+ ion pairs as a result of  mutual charge interaction.Moreover, the synergic effect of mixing NiFe 2 O 4 and La 2 NiO 4 , during H 2 O 2 decomposition might be attributed to the increase in the concentration of active sites via creation of new ion pairs, probably Fe 2+ -La 3+ ion pair.Figure 7 depicts the Arrhenius plots; ln k is related to the reciprocal absolute temperature, for H 2 O 2 decomposition for the different NiLa x Fe 2−x O 4 catalysts.For the different catalysts, good linearity was obtained with correlation coefficients higher than 0.99.The obtained activation energy values were 71.8, 60.1, 66.8, 62.0, and 70.2 kJ/mol for the catalysts having x = 0.00, 0.50, 1.00, 1.50, and 2.00, respectively.The constancy of the obtained activation energy values suggests the similarity in nature of active centres over such catalysts series.Similar argument was suggested for H 2 O 2 decomposition over other catalytic systems [24][25][26].

Conclusions
The results presented in this work showed that combustion synthesis, employing urea as combustion fuel, is a suitable and alternate method to prepare NiLa x Fe 2−x O 4 (0.00 ≤ x ≤ 2.00) catalysts at temperature as low as 500 • C. NiFe 2 O 4 and La 2 NiO 4 represent the major phases for the catalysts having x = 0.00 and 2.00, respectively.The rest of the catalyst was found to be composed of a mixture of these two phases.However, impurities of iron oxide and lanthanum carbonate were detected.Kinetic studies of H 2 O 2 decomposition reaction on this series of catalysts revealed a gradual activity increase accompanying the x-value increase passing a maxima at x = 1.5.In other words, a synergic effect was observed which is more pronounced for the catalysts having x = 1.00 and 1.50.Such effect could be anticipated to increase in the concentration of active sites throughout the formation of new ion pairs.

Figure 7 :
Figure 7: Arrhenius plots for H 2 O 2 decomposition for the different NiLa x Fe 2−x O 4 catalysts calcined at 500 • C.

Table 2 :
Textural data for the different NiLa x Fe 2−x O 4 catalysts.