order rate constant. The observed rate constants were calculated by linear fit to the logarithmic plot for each reaction. Crystallite size dependent reaction time and rate constants for NiSe_BM10, NiSe_BM5, NiSe_BM3, NiSe_BM1.6 are shown in Fig. 10, which clearly indicates that smaller crystallite size favours quicker reactions with corresponding increase in rate constant (Fig. 10). In these studies, the best rate constants were obtained for the compound NiSe_BM1.6 with k=4.31.0×10−3 s−1 (for the reduction of PNA and k=5.00×10−3 s−1) for the reduction of PNP. These values are better than the best catalytic activity values reported for noble metal catalysts [67] and related compounds except NiS (8.94×10−3 s−1) [16], Ni/MC750 (6.26×10−3 s−1) [68] (Tables S2 and S3). Comparison of specific rate constants for the reduction of PNP and PNA over NiSe compounds with different crystallite size were tabulated
in the Tables S4 and S5. The surface area of the NiSe compounds increases with the decrease in crystallite size of the particles, as result increase in the respective specific rate constant was observed. The NiSe_BM1.6 with crystallite size of 7 nm exhibits higher specific rate constant of 7.72×10−4 and 8.96×10−4 g/m2 s for the reduction of PNP and PNA respectively, compared to the NiSe compounds of higher crystallite size of 9 and 11 nm.