## 3 Results and Discussion
### Catalytic Hydrogenation of Nitroaromatics
The obtained pseudo-first order plot is displayed in Figure 6.  
The Re@NCat led to a \(>\)90% reduction of 4-NA within 8 min with the apparent rate constant \(k_{1}\) = 0.360 min\({}^{-1}\) and the mass normalized rate constant \(k_{m1}\) = 7.2 \(\times\) 10\({}^{-3}\) min\({}^{-1}\) mg\({}^{-1}\). These values were approximately 20% greater as compared to the corresponding rate constants obtained for the catalytic hydrogenation of 4-NP. However, they were smaller than those obtained for the homogenous catalyst containing ReNPs loaded onto carbon nanostructures (\(k_{1}\) = 8.72 min\({}^{-1}\)) [26]. However, the heterogeneous Re@NCat material offered more facilitated 4-NA hydrogenation as compared to polymer-encapsulated ReNPs  
Figure 6: The first-order kinetics plot for catalytic hydrogenation of 4-NA using 50 mg of the Re@NCat. used as the homogenous catalyst for this process [25]. In this case, although it was expected that the heterogeneous Re@NCat should have smaller \(k_{1}\) values, it outperformed the large size (\(\approx\)1.7 nm) and small size (\(\approx\)0.7 nm) ReNPs that led to reduction of 4-NA with rate constants \(k_{1}\) = 0.152 and 0.0758 min\({}^{-1}\), respectively [25]. When considering PGMs NPs, the Re@NCat had also the greater catalytic activity compared to that found for the trimetallic FeAgPt homogenous catalyst, i.e., the rate constant \(k_{1}\) = 0.1 min\({}^{-1}\) for 4-NA reduction [45]. The Re@NCat revealed the greater catalytic activity in the case of 4-NA reduction when also compared to other metallic NPs, which were recently reported as heterogeneous NCats. This includes AgNPs loaded into crosslinked polymeric hydrogels (\(k_{1}\) = 0.0115-0.4692 min\({}^{-1}\)) [46] and AgNPs immobilized onto PU/keratin nanofibrous mats (\(k_{1}\) = 0.072-0.114 min\({}^{-1}\)) [47]. Based on these results and the literature review, it was established that the Re-based heterogeneous Re@NCat revealed the superior catalytic activity in the hydrogenation of 4-NP and 4-NA, even though the concentration of active sites (Re) in the polymer was \(\approx\)1%. This suggests a prospective use of nanomaterials containing ReNPs in different catalytic systems. Despite the price of Re, its catalytic activity linked with its trace amounts required to achieve the desired effect (efficient reduction of NARs), it appears that the Re@NCat could be a real alternative toward already reported solutions. The synthesized NCat showed yet another significant advantage. Its morphology, arising from the polymeric matrix (the anion exchange resin based on the suspension copolymer), enabled easily separating and re-using the catalytic material [15]. Hence, the Re@NCat was used in the catalytic reduction of 4-NA over 11 subsequent catalytic cycles, within which the maximum conversion in time defined as 8 min and the \(k_{1}\) value were estimated (see Figure 7). Based on these results, it could be stated that the initial catalytic activity (>90% of 4-NA conversion, \(k_{1}\cong\) 0.32 min\({}^{-1}\)) was retained over two initial catalytic cycles. After this, the reduction yield (%) dropped and remained in the range of 44-56%. Simultaneously, values of the first-order rate constant (\(k_{1}\)) decreased. After the initial period (two cycles), \(k_{1}\) values were in the range of 0.57-0.88 min\({}^{-1}\) (Figure 7). Although efficiency of the Re@NCat decreased over the cycle number, it must be remembered that to re-use it, it had to be just simply filtrated and directly re-introduced into a next run of the tested catalytic reaction. Figure 7: First-order rate constants (\(k_{1}\)) and maximum conversion values of 4-NA to 1,4- phenylenediamine within 8 min and using 50 mg of the Re@NCat. This makes the proposed NCat unique among others that often require (when it is possible) supplying intensive methods for their separation [48]. Despite this, minimal \(k_{1}\) values obtained for an 8-min reduction of 4-NA (Figure 7) are still better than those achieved for the other NCats discussed and compared above.