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6                           jour nal o f e nvironmental sciences 92 (2020) 1 e10

          electrode potential (E 0 ) for PNP/PAP ¼0.76 V and H 3 BO 3 /  deprotonated 4-nitrophenolate. The zeta potential of the as-
          BH 4 ¼1.33 V versus the normal hydrogen electrode (NHE).  synthesized products followed the order of Ag/Fe 2 O 3 NPs
          However, kinetically, it was restricted without an appropriate  (15 mV) > Ag/Fe 2 O 3 NSs (17 mV) > Ag/Fe 2 O 3 NCs (22 mV).
          catalyst (Comotti et al., 2006). In the presence of Ag/Fe 2 O 3  The negatively charged reactant absorbed on the Ag/Fe 2 O 3
          composites, the PNP solution turned to greenish yellow from  NPs more readily than Ag/Fe 2 O 3 NSs and Ag/Fe 2 O 3 NCs, due to
          light yellow after NaBH 4 was added. And accordingly, the peak  a weaker repulsive force. The point of zero charge (PZC) of Ag/
          at 317 nm represent PNP was shifted to 400 nm on account of  Fe 2 O 3 was then calculated after various zeta potentials at
          the generation of the deprotonated PNP under alkaline con-
          dition (Jiang et al., 2013; Fan et al., 2016). The deprotonated
          PNP gradually disappearanced, along with the appearance of a
          new peak at 298 nm (PAP) in the mixture, and the isosbestic
          points revealed a single product produced during the reaction
          (Ye et al., 2016). The UV-Vis spectrum displayed in Fig. 5 sug-
          gests that the reactions completed in different time over
          various catalysts. Precisely, the reactions were completed in 5
          and 8 min for Ag/Fe 2 O 3 NPs and Ag/Fe 2 O 3 NSs, respectively. By
          contrast, the reaction still continued in 120 min for Ag/Fe 2 O 3
          NCs. The model reduction was regarded as a pseudo-first-
          order reaction, when the use of NaBH 4 was excessive rela-
          tive to PNP. The kinetic equation of the reaction could be
          expressed as follows:
          lnðC t = C 0 Þ¼ lnðA t =A 0 Þ¼kt             (1)
          where C t and C 0 represent the concentration of PNP at reaction
          time t and initial time 0, A t and A 0 were the absorbance of PNP
          corresponding to different time t and initial time 0, respec-
          tively, and k was the rate constant which could be calculated
          from the fitted kinetic curves of ln(C t /C 0 ) versus time. It is
          worth mentioning that the k value of Fe 2 O 3 NPs (0.863 min )
          was 1.6 times and 12 times higher than Ag/Fe 2 O 3 NSs
          (0.547 min ) and Ag/Fe 2 O 3 NCs (0.071 min ), respectively.
          2.3.   Mechanism

          It has been previously reported that the size and content of the
          noble metal was crucial to the catalytic activity. The above
          TEM data showed that the size of silver nanoparticles was
          similar. The accurate amount of silver in three samples was
          determined by ICP-OES. The atomic concentration for Ag/
          Fe 2 O 3 NSs, Ag/Fe 2 O 3 NCs, and Ag/Fe 2 O 3 NPs, was 4.3%, 4.6%,
          and 4.0%, respectively. Though the results varied slightly with
          different samples, there was no significant difference in the
          content of silver. In our work, the size and content of Ag in
          three catalysts were very close. Then, other factors would
          determine their catalytic performance. Heterogeneous catal-
          ysis reactions occurred on the surface of catalysts (Aditya
          et al., 2015). Thus, the surface area of the catalysts could
          directly influence the reaction rate. It should be noted that the
          BET surface area followed a decreasing order of Ag/Fe 2 O 3
          NSs > Ag/Fe 2 O 3 NCs > Ag/Fe 2 O 3 NPs, which was not consistent
          with its catalytic activity. Unexpectedly, Ag/Fe 2 O 3 NPs with
          the lowest surface area performed the highest activity.
          Therefore, other factors should be highlighted, and studied
             Further measurements about surface charge were carried
          out, as the surface charge effected the electrostatic interaction
          between the catalyst and substance, and thus affected its
          catalytic activity (Larsen et al., 2016; Aditya et al., 2017). Under  Fig. 6 e Point of zero charge (PZC) of Ag/Fe 2 O 3 NPs, Ag/
          the alkaline condition, PNP was converted to anionic  Fe 2 O 3 NSs, and Ag/Fe 2 O 3 NCs.
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