3.1. X-ray Diffraction Powder Analysis
XRD pattern of product A is shown in Fig. 1. The peaks are identified and indexed by comparing with XRD standard pattern using MATCH 3.0 software. The peaks are identified at specific 2θ values. The peaks are observed at 2-theta 10.14o, 19.14o, 29.14o 38.77o, 48.79o and 67.48o which are indexed to (110), (200), (310), (211), (600) and (541) miller indices respectively (JCPDS # 44–141). This set of peaks and miller indices indicate that synthesized product A is consist of α- MnO2. Few additional peaks are also observed at 2-theta 28.14o, 33.98o and 59.62o which are indexed to (110), (101) and (220) miller indices respectively (JCPDS # 24–73). These peaks are characteristic to β-MnO2. Moreover, the peaks are observed at 20.99o, 32.22o, 57.40o, 62.20o which are indexed to (120), (131), (160) and (421) respectively (JCPDS # 16–644) which are characteristic to γ-MnO2. This indicates that synthesized product A is consist of three phases of MnO2. No extra peak is observed in pattern . So the product is pure. All the peaks are very sharp indicate that the product is highly crystalline. It is difficult to stabilize multiple phases at same time using chemical synthesis approach . Bitter apple contains verity of compounds like phenols, flavonoids, phytochemicals etc. which act as stabilizing agents. The stabilizing/capping property of all these compounds are different from each other. So presence of variety of biomolecules help in stabilization of these three phases (α, β and γ) of MnO2 . The prepared product belongs to space group P 12/m (10) monoclinic system. No atom is present at center of unit cell. Four atoms of manganese and four atoms of oxygen are present per unit cell. Two Mn-O are present as each face . The distance between two Mn-O present in same face is same on both faces.
3.2. Effect of Plant Extract on the Morphology of Products
Product A is synthesized by green method and its SEM images are shown in Fig. 2(a-d) at magnification 50,000X, 100,000X and 200,000X. Small size of nanoparticles is observed from Fig. 2a. Some particles are observed semispherical at higher magnification. Some particles of irregular shape of average size 65 nm are also observed at higher magnification. The morphology of these particles is heterogeneous semispherical with distinct boundaries . Uneven morphology with wide size distribution may be attributed to cause particle agglomeration . Product is highly aggregated even at higher magnification (200000X). STEM mode images of α/β/γ-MnO2 are shown in Fig. 2(Ha, #132) at various magnification and morphology is not seen clearly. STEM images also show that boundaries are not regular and particles are fused with each other. The boundaries are fused, that’s why shape is not clearly observed .
SEM images of product B synthesized by chemical method are shown in Fig. 3(a-b). Some particles are in semispherical shape and some are in irregular shape. The particles possess irregular morphology and the boundaries are not clear even at higher magnification (Fig. 3(c-d)) . Average size of these particles is around ~ 20 nm. STEM images of product B synthesized by chemical method is shown in Fig. 3(e-f). Figure 3e represents the overall view of the product. Figure 3f shows that the product consists of fused particles and boundaries are not defined clearly. Morphology of the product is not clear, even at higher magnification the particles show agglomeration as it can be seen from the Fig. 3g-3h. Size distribution histogram of both products are given as Fig. 4 (a-b). Size of particles is calculated from high magnification SEM and STEM images. Particles size of product A is ranged from 10–60 nm while the size of majority of particles is in between 20–50 nm (Fig. 4a). The size of particles of product B is ranged from 5 to 35 nm. The majority of particles are lied around 20–25 nm (Fig. 4b). Both the graphs do not follow Gaussian Bell-shape distribution.
3.3. Catalytic Application of Products A and B
Organic dyes and nitro-compound are reduced by using α/β/γ-MnO2 nanoparticles as catalyst in the presence of sodium borohydride (NaBH4). The absorbance of substrate is observed at their λmax after regular time intervals. The absorbance is decreased with time and graph become parallel to x-axis which indicates that reaction has been completed and solution become colorless . It follows the pseudo-first order reaction kinetics and values of kapp is calculated from slope. The wavelength of maximum absorbance of substrates such as CV, MB, CR, MR, MG, EBT, RB, PA, 2-NP, 2,4-DNP, and 4-NP are 590, 670, 495 430, 617, 510, 595, 414, 400, 520 and 400 nm respectively. Electrons are transferred from BH4‾ to substrate molecule via surface of α/β/γ-MnO2 nanoparticle and substrates are reduced into corresponding amino products. Plots of ln(At/Ao) as a function of time are given as Fig. 5. Plot of reduction of substrates by catalyst A are shown in Fig. 5(a-b) while plots of reduction of substrates by catalyst B are shown in Fig. 5(c-d). Initial decrease in value of ln(At/Ao) with time is showed that reduction is in progress and concentration of substrates are decreased with time (according to Beer-Lambert’s law, absorbance is directly proportional to the concentration). The slope of all of these plots is different from each other which indicates that reduction of few substrate is fast while few substrates are reduced slowly .
3.4. kapp of Catalyst A and B
The values of the kapp of substrates by using α/β/γ-MnO2 nanoparticles (Product A) as a catalyst is shown in Fig. 6a. The trend of kapp of the substrates is PA > MB > MR > MG > 2-NP > EBT > CV > RB > CR > 2,4-DNP. The value of kapp of PA is highest among substrates because it has three nitro-groups and accessibility easy towards active site of catalyst and Greater affinity between catalyst and substrate favors its test reduction while in the case of methyl blue it acts as electrophile in aqueous solution with respect to catalyst and accept electron from the reducing agent the nucleophile BH4 can offer electrons to the catalyst and catalyst acts as electron relay for MB reduction in NaBH4 solution that’s why the kapp is larger than other remaining substrates of and 2, 4-DNP has lowest value of kapp among all the substrate. Values of the kapp of substrates by α/β/γ-MnO2 -nanoparticles as a catalyst is shown in Fig. 6a. The trend of kapp of the substrates is MB > 2,4-DNP > RB > CV > MG > CR > PA > MR > EBT > 2-NP. The trend of substrate is not same for catalyst B because this product is synthesized by chemical method and no plants extract is added in it. For example, in for product A PA has highest kapp while in the case of product B MB has highest value and for product A 2,4-DNP has lowest kapp and for product B there is 2-NP has lowest kapp.
3.5. Reduction time of catalyst A and B
Histogram of substrate versus reduction time by using product A as a catalyst is given as Fig. 6b. The trend of reduction time is 2,4-DNP > PA > CV > MR > EBT > CR > MG > RB > 2-NP > MB. The reduction time of nitrocompounds are comparatively greater to other dyes because of more contact time and high adsorption rate of nitro compounds on the surface of manganese-based-nanoparticles catalyst. Reduction time of 2,4-DNP is highest among other substrates because two nitro-groups are present one is present at ortho which form hydrogen bonding with hydroxyl group of phenolic part of the compound which stabilizes the structure, due to which adsorption of substrates over catalyst takes more time that’s why substrate takes longer time to reduce. Reduction time of MB is lowest because nitrogen of azo group is present at the outside of the ring and accessibility of the catalyst is increase towards the substrate and reactivity is enhanced therefore its reduction time is lowest . The plot of reduction time versus substrates by using Product B as a catalyst is given in Fig. 6b. The trend for reduction time is MB > 2-NP > MR > 2,4-DNP > PA > MG > EBT > CR > CV > RB. MB shows stability because it has two N, N-dimethyl groups that cause steric hindrance that’s why MB highest reduction time. RB shows least stability because it contains SO3 group due to which the accessibility of substrate towards the catalyst is increased and it also contain azo and hydrazine group which triggers the reactivity of RB . The trend is not same for both products e.g. in Product A 2,4-DNP has greater reduction time while in the case of Product B MB has greater reduction time this trend indicates that there is irregular trend of reduction time for both products for example in the case of product A 2,4-DNP has highest reduction time while for product B 2,4 –DNP forth highest value. In the case of product B MB has highest reduction time while in the case of product A it in last. RB has same reduction time in both product A and B and PA also have same reduction time in both catalysts .
3.6. Percentage reduction of Product A and B
The percentage reduction of all substrate is shown in Fig. 6c. The trend of %reduction is PA > MG > CV > MB > EBT > 2-NP > RB > 2,4-DNP > CR. PA shows the higher percentage reduction because three nitro groups are present in the PA which increases the accessibility of the active sites of the catalyst for the substrate, therefore the % reduction of the PA is highest among other substrate. Among all the nitrocompounds 2,4-DNP has lowest % reduction because intramolecular hydrogen bonding stabilizes the structure of 2,4-DNP. MG shows greater % reduction among all the organic dyes because methyl group is present outside the ring which has electron donating effect while CR have lowest percentage reduction among all the substrate because there are two phenyl rings are joined by diazo linkage and two sulphonic groups are present in the structure of the CR and the reaction is kinetically less favorable as compared to the other substrates due to which CR reduced slowly and show lowest reduction potential nanoparticles as catalyst. The percentage reduction of all substrate is shown in Fig. 6c. The trend of %reduction is RB > CR > PA > CV > MR > EBT > 2-NP > 2,4-DNP > MG > MB. RB shows least stability because it contains SO3 group due to which the accessibility of substrate towards the catalyst is increased and it also contain azo and hydrazine group which triggers the reactivity of RB . Therefore, the % reduction of the RB is higher than the other substrate. MB shows stability because it has two N, N-dimethyl groups that cause steric hindrance that’s why MB shows greater stability and least reactive. The % reduction of substrates by using both catalyst is different in product A PA has highest % reduction while in Product B RB has highest % reduction .
3.7. Reduced Concentration by Product A and B
Trend of reduced concentration by both catalysts are plotted in Fig. 6d. The trend of reduce by product A concentration for MB the reduced concentration is 0.075 for CR 0.091, EBT 0.083, For MR 0.043 for MG 0.078, for 2-NP 0.082, 2,4-DNP 0.08 and PA 0.095. in the case of product B reduced concentration for MB the reduced concentration is 0.035 for CR 0.0935, EBT 0.062, For MR 0.052, for MG 0.065, for 2-NP 0.062, 2,4-DNP 0.063 and PA 0.017. The values of substrates reduced by both catalysts are different for example MB is reduced by the green catalyst has different value while MB reduced by the Product has different and same is the case of other substrates which are reduced by the both product have different reduced concentration .