Continuous microfluidic production of bimetallic nanoparticles stabilized on porous polystyrene microspheres for efficient 4-NP catalytic reduction

Well-defined porous microspheres exhibit great potential in catalysis as the catalytic substrate. Noble nanoparticle decoration is demonstrated to enhance catalytic performance significantly. In traditional synthetic methods, decorating noble metal nanoparticles on the catalytic substrate is complicated and time-consuming. Moreover, nanoparticles are distributed unevenly on the substrate and tend to aggregate into large size nanoparticles, which considerably reduce the surface area and reduce the catalytic activity. Additionally, batch-to-batch product variation greatly limits the downstream catalytic applications. In this work, a double-spiral microreactor was developed to produce porous polystyrene (PPS) microspheres and noble bimetallic nanoparticles (Ag-Au, Ag-Pt) loaded PPS microspheres in a controllable and efficient manner. The SEM characterization results demonstrated that the produced PPS microspheres exhibited highly ordered pores, which provided both competitive surface area (~80 m2·g-1) and confined microenvironment for 4-NP catalytic reaction. The ICP-OES measurement results demonstrated that 4.75 wt% Ag and 3.48 wt% Au were loaded on the PPS microspheres. For Ag-Pt@PPS microspheres, Ag and Pt species were determined as 4.86 wt% and 3.25 wt% by ICP-OES. The XPS analysis results revealed that Ag, Pt, Au components are present in the form of zero-valent metals, which is advantageous to the catalytic reaction. Then, the produced Ag-Au@PPS and Ag-Pt@PPS microspheres were respectively used to reduce p-Nitrophenol (4-NP) into 4-aminophenol (4-AP) in the presence of NaBH4. The results revealed that both Ag-Au@PPS and Ag-Pt@PPS microspheres exhibit significant catalytic activity for the reduction of 4-NP into 4-AP. In the presence of NaBH4, the catalytic reduction reaction follows first-order kinetics; the reaction rate constants are 0.007 s-1 and 0.006 s-1 for Ag-Au@PPS and Ag-Pt@PPS microspheres, respectively. The calculated catalytic activity parameters are 400 s-1·g-1 and 800 s-1·g-1, and the calculated turnover frequency (TOF) are 1210.0 h-1 and 2419.2 h-1 with Ag-Au@PPS and Ag-Pt@PPS microspheres served as catalysts, respectively. The catalytic performance for 4-NP reduction with Ag-Au@PPS and Ag-Pt@PPS microspheres significantly outperforms monometallic Ag@PPS microspheres. These results indicate that both Ag-Au@PPS and Ag-Pt@PPS microspheres possess remarkable catalytic performance, and the catalytic mechanism is proposed to elucidate the remarkable performance. More prominently, catalytic activity remains slightly change even after five cycles of reusability, suggesting that the catalyst is stable in the reaction system. In summary, this work not only develops a controllable, microfluidic continuous-flow strategy for the preparation of noble bimetallic nanoparticles loaded onto PPS microspheres but also provides an efficient strategy for the catalytic degradation of phenolic pollutants, offering significant advantages for water environment management.