compounds using sulphides generates substantial environmental waste. Therefore catalytic reduction has been considered as an environment friendly method for the large-scale production of many amines from the corresponding aromatic nitro compounds. Here, we have selected the catalytic reduction of two aromatic nitro compounds; PNA (1 mL) and PNP (1 mL) using NiSe nanoparticle as catalyst (0.1 mg) using NaBH4 as the reducing agent. Since, several noble metals based bimetallic and alloys were used for the reduction of PNA [26,51–54] and PNP [55] to confirm the catalytic activity of these materials. The step wise reaction process for both the reduction reactions are represented schematically in Figs. S2 and S4. The reduction processes of these reactions were monitored through UV– Vis absorption spectrometer (Figs. 5–8). PNA and PNP in aqueous phase exists as nitro groups with corresponding absorbance maximum (λmax) at 380 and 317 nm respectively, under neutral or acidic condition [37,56]. In PNP reduction, the addition of NaBH4 enhances the basic condition of the solution and in alkaline medium phenolate ions are predominating with absorbance maximum shifts to 400 nm, which can be observed by change in the colour intensity of the solution. The aqueous solution of PNA is bright yellow in colour (Fig. S2), which is directly related to the presence of absorption maxima at 380 nm in UV–Vis spectrum. The addition of aqueous solution of NaBH4 to PNA did not change in colour of the solution (Fig. S2). The peak at 380 nm remains unchanged in the presence of NaBH4 (Fig. S3a) and in the absence of the catalyst for a period of 15 min of the reaction time (Fig. S3b) indicate that the reduction of aromatic nitro group under mild conditions does not occur in the absence of any catalyst. It is well known that reduction of nitro group to amino group by using NaBH4 is
a kinetically slow process [57]. Upon the addition of the NiSe nanoparticles, the colour of the solution gradually changed from yellow to colourless (Fig. S2) and two new peaks were observed at 305 and 240 nm in the absorption spectra, which corresponds to formation of PPD [58]. The intensity of the absorption peak at 380 nm dramatically decreases with a proportional increase of the new peaks at 305 and 240 nm confirming the conversion of PNA–PPD (Figs. 5 and 6). As shown in Fig. 5, the compound NiSe_BM1.6 is the most active catalyst for ball milling sample favouring, the complete reduction of PNA–PPD within 3 min (with the highest rate constant k=5.0×10−3 s−1) as compared to the samples NiSe_BM10 (26 min), NiSe_BM5 (10 min) and NiSe_BM3 (6 min). The particle size and deficiency of nickel are playing crucial role in the reduction process. It is well known fact that the reduction of the particle size increases the surface to volume ratio leading to an increase in the number of active sites [59–61]. It is also known that, the creation of deficiency at active site also favours better catalytic activity [62]. We also checked the catalytic activity of NiSe_PN and NiSe_PP for the reduction of PNA–PPD (Fig. 6). NiSe_PN favours the reaction faster (8 min reduction time and rate constant k=4.22×10−3 s−1) compared to NiSe_PP (16 min reduction time and rate constant of k=3.09×10−3 s−1) clearly showing the reduction of activity in the presence of surfactant due to blocking of substrate molecules. On the other hand, an aqueous solution of PNP is light yellow in colour, which shows absorption maximum at 317 nm under neutral or acidic conditions [37]. The addition of the aqueous solution of NaBH4 to PNP cause a change in colour of the solution from light to deep yellow (Fig. S4). This colour change can be attributed to an increase in the alkalinity of the solution, which results in the formation of the paranitrophenolate ion [37]. The absorption maximum of the para-nitrophenolate ion observed at 400 nm (Fig. S5a) remains unchanged in the presence of NaBH4, even up to 15 min of the reaction (Fig. S5b). This also indicates the reduction of PNP–PAP does not occur without the help of catalyst. Upon the addition of NiSe catalyst nanoparticles, the colour of the solution changed and the intensity of the absorption peak at 400 nm (corresponding to PNP) gradually decreased and a new peak was observed at 310 nm, which corresponds to PAP. Similar to the previous reaction, among the samples obtained from the ball milling method, NiSe_BM1.6 shows the highest activity or rate constant compared to other samples (Fig. 7) with the rate constant of k=4.31×10−3 s−1. As expected, NiSe_PN (Fig. 8a) favours the complete reduction within 12 min (k=3.352×10−3 s−1) compared to 24 min required for NiSe_PP (Fig. 8b) to complete the reaction with almost half rate constant of k=1.74×10−3 s−1. This increased catalytic activity is due to the presence of small crystallite size of NiSe_PN and absence of any surfactant. In order to check the catalytic properties of NiSe nanoparticles counterparts, we have performe