## 3 Results and Discussion
### Catalytic Hydrogenation of Nitroaromatics  
The synthesized Re@NCat material was used as a new heterogeneous catalyst for the hydrogenation of 4-NP and 4-NA. First, the catalyst mass taken for these tests was optimized. To achieve thereof, 5 and 50 mg of the Re@NCat were used for 4-NP reduction. In parallel, the same masses of the anion exchange resin (without ReO4\({}^{-}\) ions or ReNSs) were tested for the same catalytic reactions as appropriate controls. Figure 5 displays pseudo-first-order kinetics plots for this process. Based on collected UV-Vis spectra (results not shown here), the anion exchange resin itself was established to be unable to catalyze both studied reactions because no effect at all, even at prolonged contact times with 4-NP and 4-NA, was observed. Then, it was established that the Re@NCat material led to 87-94% hydrogenation of 4-NP within 64 min when its 5 mg was applied and within 10 min when its 50 mg was used. Simultaneously, pseudo-first-order rate constants (\(k_{1}\)) assessed in these conditions were 0.035 and 0.282 min\({}^{-1}\) for 5 and 50 mg of the Re@NCat, respectively (Figure 5). The greater mass of the Re@NCat consequently led to the greater \(k_{1}\) value; however, the actual catalyst efficiency was confirmed by the value of the mass-normalized rate constant (\(k_{m1}\)) that represents the catalyst activity with respect to its mass applied within the process. In this context, while 50 mg of the Re@NCat had the \(k_{m1}=\) 5.63 \(\times\) 10\({}^{-3}\) min\({}^{-1}\) mg\({}^{-1}\), its 5 mg portion revealed the outstanding \(k_{m1}=\) 6.94 \(\times\) 10\({}^{-3}\) min\({}^{-1}\) mg\({}^{-1}\). At the moment, there is no other example of the Re-based heterogeneous NCat that was applied for the hydrogenation of NARs. However, to the best of our knowledge, there is one report focusing on homogenous catalysts based on ReNPs that were used for the  
Figure 5: First-order kinetics plots for the catalytic hydrogenation of 4-NP using 5 and 50 mg of the Re@NCat. catalytic hydrogenation of 4-NP [26]. In this particular case, ReNPs were loaded into highly porous carbon nanostructures and revealed a greater \(k_{1}\) value for the catalytic reduction of 4-NP, i.e., 8.99 min\({}^{-1}\), vs. 0.282 min\({}^{-1}\) obtained in the present work. However, it must be noticed that in the latter case, the concentration of Re in the synthesized heterogeneous Re@NCat was at a trace level (\(\approx\)1% (\(w/w\)). In this context, it can be recognized as a more economically viable material along with its convenience arising from heterogeneous catalysis, i.e., its ease of operation and stability, which will be highlighted further in the manuscript. Comparing the Re@NCat with other recently reported heterogeneous NCats with AuNPs, the obtained \(k_{m1}\) value of 4-NP hydrogenation was higher as compared to those assessed for AuNPs loaded on a mung bean starch (\(k_{m1}\) = 0.4 \(\times\) 10\({}^{-3}\) min\({}^{-1}\) mg\({}^{-1}\)) [40] or an amino-functionalized olive biomass (\(k_{m1}\) = 5.7 \(\times\) 10\({}^{-3}\) min\({}^{-1}\) mg\({}^{-1}\)). Although Re does not formally belong to PGMs, its ionic properties (ability to form stable oxoanions) allow grouping it with these metals. As such, the obtained \(k_{1}\) values are much greater than those assessed for a PdNPs homogenous NCat (\(k_{1}\) = 0.0125 min\({}^{-1}\)) [41], or NCats being Pt-Au and Pt dendrimers immobilized on graphene oxide (\(k_{1}\) = 0.041 min\({}^{-1}\) and 0.057 min\({}^{-1}\), respectively) [42]. The catalytic reduction of 4-NP was considered as a model reaction, which is convenient for assessing the catalyst activity of the new Re@NCat. Another reaction that was selected, although much less popular, was the catalytic reduction of 4-NA to 1,4-phenylenediamine [43; 44]. Therefore, the Re@NCat was also applied for the catalytic hydrogenation of 4-NA. Since 50 mg of the catalyst was responsible for the fastest reduction of 4-NP, it was also applied in the present process.