3.2.1. FT-IR study
Figure 4 shows the FT-IR spectra of CNM (Fig. 4a) and CNR (Fig. 4b) catalysts. It can be seen from the figures that the FT-IR spectra of both catalysts are perfectly matched. The broad absorption band which appears at 3200–3600 cm− 1 is due to the stretching frequencies of adsorbed water molecules. The prominent band observed at 1620 cm− 1 is due to the bending frequency of adsorbed water molecules. The characteristic peak at 732 cm− 1 is assigned to Al–O stretching vibration bond [27]. Absorption bands at 553 and 467 cm− 1 are attributed to the Cu–‐O and Na–O stretching modes, respectively.
3.2.2. XRD data
The crystal structure and crystal phase of CNM and CNR was characterized by X-ray powder diffraction (XRD) and the result is presented in Fig. 5. The CNM and CNR catalysts show the diffraction peaks that matched well to pure of CuO (JCPDS No. 89-5895), space group C2/c of a monoclinic system with cell constants of a = 4.6820, b = 3.4240, and c = 5.1270 Å. The diffraction lines of CuO was observed at 2θ = 32.61°, 35.63°, 38.81°, 48.90°, 53.56°, 58.33°, 61.65°, 65.81°, 68.13°,72.45°and 75.1°corresponded to (110), (002), (111), (202), (020), (202), (113), (311), (113), (311) and (004) [28]. The XRD patterns of CNM and CNR show the diffraction lines at 2θ = 25.23°, 34.13°, 43.15°, 52.11°, 57.27° and 66.29°, which can be attributed to α-Al2O3 according to the standard JCPDS card (No: 42-1468). Both catalysts show diffraction lines at 28.02, 32.24, 54.91 and 57.29o which indexed for Na2O. The particle sizes of CuO and Na2O in CNM catalyst were calculated by the Scherrer equation (Eq. 1) and are found to be 32.4 and 27.9 nm, respectively. The average crystalline sizes of CuO and Na2O in CNR catalyst were estimated to be about 53.5 and 46.6 nm, respectively.
D = Kλ/ (βcosθ) (1)
D is the mean crystal size (nm), K is the Scherrer constant (shape factor) (0.89), λ is the X-ray wavelength (0.154056 nm), β is the full width of the peak at half maximum of diffraction peak and θ is the Bragg diffraction angle.
3.1.4. Scanning electron microscopy
Figure 6 (a-c) shows the SEM images of CNM catalyst. It can be seen that the prepared catalyst has truncated octahedral morphology with a smooth surface. Note that the distributions of particle size are not uniform. Figure 6d shows the SEM image of the CNR catalyst. The micrographs reveal that the particles of CNR are irregular and bigger in comparison with CNM catalyst.
3.2.3. Textural properties
Table 1 shows the BET surface area, pore-volume and average pore diameter of CNM and CNR catalysts. The results show that the surface area of CNM is higher compared to CNR. These results are in agreement with the result of SEM and XRD analysis which can be related to the preparation method.
Table 1
Properties of CNM and CNR catalysts in this work.
Catalyst | BET surface area (m2/g) | Pore volume (cm3/g) | Average pore diameter (nm) |
CNM | 145 | 0.521 | 11.2 |
CNR | 86 | 0.231 | 10 |