3.1 X-ray diffraction analysis
The prepared compositions were characterized by X-ray powder diffraction. The X-ray diffraction patterns were recorded at room temperature with a 2θ scanning angle ranging from 20° to 70°.
Figure 2 shows the Rietveld refinement of the XRD pattern for the Co1-XNiXAl2O4 (x = 0.0, 0.25, 0.50, and 0.75) system performed using Fullprof software. The major Bragg reflections were seen at 2θ values of 31.25, 36.14, 38..36, 44.20, 55.92, and 65.51 corresponding to corresponding to the [220], [311], [400], [422], [511] and [440] diffraction planes and were seen to be in accordance with JCPDS data file 44-0160 indicating the formation of pure spinel phase belonging to Fd3m space group without any impurity [19,20]. The goodness of refinement quality is evident from the refinement parameters (c2, Rwp, Rexp, RBragg, and RF) listed in table 1.
Table 1 Refinement parameters obtained Rietveld analysis
Material composition
|
c2
|
RWP
|
REXP
|
RBRAGG
|
RF-FACTOR
|
CoAl2O4
|
1.38
|
81
|
68.7
|
57.3
|
39.3
|
Co0.75Ni0.25Al2O4
|
1.42
|
54.2
|
45.4
|
17.4
|
14.3
|
Co0.50Ni0.50Al2O4
|
1.24
|
57.5
|
51.7
|
27.9
|
20.6
|
Co0.25Ni0.75Al2O4
|
1.59
|
83.2
|
65.8
|
63.2
|
44.2
|
The variation of lattice constant ‘a’, x-ray density, and cell volume as a function of Ni+2 concentration are shown in Figure 3 (a,b) The lattice constant was seen to decrease initially for x=0.0 to x=0.25 and further showed a marginal rise with increasing Ni+2 at the tetrahedral site. The initial decrease in lattice constant can be attributed to smaller ionic radii of Ni+2 ions (0.83Å) replacing the larger Co+2 ions with ionic radii of 0.88Å [21]. For the compositions beyond x=0.25 i.e. for x=0.50 and 0.75, the increase in lattice constant could be due to electrostatic shielding caused by 3d electrons of Ni+2 ions diluting the nuclear dominance and causing marginal expansion in the tetrahedral environment [22]. A trend similar to that of lattice constant variation was observed in the values of cell volume while x-ray density was seen to vary inversely (Figure 2b).
3.2 FTIR spectra analysis
Figure 4 shows IR absorption bands for citrate precursor and Co1-XNiXAl2O4 system, in the range of 4000-500 cm-1. In general, FTIR spectra of all the prepared spinels show weak absorption bands at about 3450, 1630, and 1385 cm-1. The characteristic vibrational frequencies of CoAl2O4 aluminate spinel appeared in the range of 750-400 cm-1. The absorption band around 3400 to 3500 cm-1 was assigned to stretching vibrations and 160-1400cm-1 is assigned to bending vibration respectively which is due to adsorbed moisture. The band observed at 750-500 cm-1can be attributed to the symmetric stretching (ʋ1), bending (ʋ2), and asymmetric stretching (ʋ3) modes of M-O-Al, M-O, and Al-O bonds at tetrahedral and octahedral sites in CoAl2O4 lattice [23]. The broad absorption bands observed at 670 and 560 cm-1 are the characteristic vibrational bands representing a typical pattern of CoAl2O4 normal spinel structure, in agreement with the literature [24]. These bands correspond to the AlO6 units, which is the mainframe of the CoAl2O4 crystal, and ensure the formation of spinel structure. The absorption frequencies observed between 1800-1000 cm-1 were assigned to the deformation mode of Al-OH and Co-OH, which is typical of this class of materials [25]. In the Co0.25Ni0.75Al2O4 nanoparticle spectra (e), the prominent vibrational peaks observed at around 600, 530, and 495 cm-1, the absence of a strong absorption band at 560 cm-1, and the appearance of a stronger frequency band at around 598 cm-1 indicate the substitution of Ni atoms has taken place replacing Co atoms in the lattice. The first two absorption peaks are attributed to the intrinsic stretching vibrations of the M-O at tetrahedral sites, while the lower mode is assigned to the stretching vibration of the M-O at the octahedral site[23-27]. No other impurity or organic matter absorption bands were observed indicating the purity of the materials, in agreement with the results obtained by XRD. FTIR measurements were also helpful to illustrate the formation of spinel from the precursor.
3.3 VSM data analysis
The magnetic hysteresis loops obtained at room temperature for Co1-xNixAl2O4 nano-powders are shown in Figure 5
The samples were seen to exhibit paramagnetic behavior [1]. The values of saturation magnetization (MS), coercive field (HC), and remnant magnetization (MR) are listed in Table 3.
Table 3 Values of magnetic parameters obtained for Co1-xNixAl2O4
Sample
|
Saturation
Magnetization
(MS) emu/g
|
Coercive
field
(HC) Oe
|
Remnant
Magnetization
(HR) emu/g
|
Squareness
|
Magnetic
Moment
m (Bohr Magneton)
|
CoAl2O4
|
0.56
|
16.04
|
0.05
|
0.083
|
0.018
|
Co0.75Ni0.25Al2O4
|
0.51
|
24.06
|
0.03
|
0.058
|
0.016
|
Co0.50Ni0.50Al2O4
|
0.72
|
72.18
|
0.04
|
0.061
|
0.022
|
Co0.25Ni0.75Al2O4
|
0.66
|
24.34
|
0.04
|
0.051
|
0.020
|
The MS values were seen to vary with Ni+2 content and showed a maximum value of 0.72emu/g for x=0.5 and a minimum of 0.51emu/g for x=0.25 while the remnant magnetization was seen to remain almost constant with values ranging between 0.03emu/g to 0.05emu/g. The squareness values calculated using the following equation (1) were found to be very low confirming the paramagnetic behavior of the nano-powders. However, these values were much higher than the reported squareness limit for superparamagnetic materials [28-30]. The magnetic moment m calculated for all the samples using equation (2) was seen to remain in the range of 0.016 to 0.022 Bohr magneton with increasing Ni concentration in the spinel lattice [31-35].
Where Mx is the molecular weight of the composition.
The anisotropy constant was calculated using equation 3 given below showed a trend similar to that of HC. The plot of variation in anisotropy constant ‘K’ with increasing Ni concentration is shown in figure 6.
3.4 TEM image analysis
The transmission electron micrographs obtained on CoAl2O4 and Co0.25Ni0.75Al2O4 nanocrystalline powders along with particle size distribution histograms are shown in Figure 7. The observed particle size of these nanocrystalline spinel aluminates was in the range of 20 nm to 47 nm.
3.5 The Bignelli reaction as a model catalytic application
The as-synthesized spinel Co-Ni aluminate compositions were explored for catalytic efficiency of synthesizing dihydropyrimidinone derivative as a model test reaction as shown in Figure 8. A solution of Benzaldehyde (10 mmol, 1.06 g), ethylaceto acetate (13mmol, 1.69 g), and urea (15 mmol, 0.90g) was refluxed at 85-90°C in ethanol in the presence of materials (0.2g) under investigation, for 3 hours. On completion of the reaction, the catalyst was filtered off from the mixture, and the filtrate was collected in crushed ice. The product obtained was recrystallized using ethyl acetate. The synthesized product was identified by comparison of melting point (mp) and the spectral data (FTIR).
The catalytic product was confirmed by melting point (mp) and FTIR spectroscopy. Table 4 summarises the yield obtained (%) for the dihydropyrimidinone product. The results obtained using Ni substituted cobalt ferrite nanopowders as catalyst are comparable to those obtained by Ezzat Rafiee et. al. in which dihydropyrimidinone was reported to synthesize using heteropoly acids such as H3PW12O40, H3PMo12O40 [35]. The efficiency nanopowders of Co1-XNiXAl2O4 can be enhanced to produce higher yield by increasing the Ni concentration in the composition as these nanopowders possess higher surface area in comparison to that of heteropoly acids.
Table 4 Yield of dihydropyrimidinone and their melting points.
Nanocatalysts
|
% Yield
|
Physical constant (mp)
|
In the absence of a catalyst
|
30%
|
207°C
|
CoAl2O4
|
45.91%
|
205°C
|
Co0.75Ni0.25Al2O4
|
46.34%
|
205°C
|
Co0.50Ni0.50Al2O4
|
49.79%
|
207°C
|
Co0.25Ni0.75Al2O4
|
51.93%
|
206°C
|
Figure 9 shows the FTIR spectra of the product and the characteristic absorbance peaks which are summarised in Table.5, confirming the formation and purity of dihydropyrimidinone The melting point of the purified product was found to be ranging between 205oC to 207 OC which again indicated the purity of the reaction product [36-40].
Table 5 Characteristic absorption peaks of dihydropyrimidinone.
Observed absorption bands (cm-1)
|
Type of stretching
|
3337.39
|
N-H stretch
|
3115.53
|
N-H stretch
|
2979.46
|
C-H stretch
|
1725.29
|
C=O ester stretch
|
1702.88
|
C=O amide stretch
|
1650.29
|
C=C stretch
|
1089.21
|
Monosubstituted aromatic ring
|
As the atomic concentration of Al remains practically the same in all the Co1-xNixAl2O4 spinel nanomaterials, it may have an indirect role in the activity. The observed increase of 30 to 51 % in the yield percentage with the Ni substitution and its atomic concentration in CoAl2O4 crystal structure, may be attributed due to atomic size variation and synergistic effect of both Co+2 and Ni+2 ions.