Ni-Cu bimetallic catalytic membranes for continuous nitrophenol conversion

Bimetallic nanocatalysts are of great interest due to their greater activity, selectivity, and chemical and electrochemical stability, compared to their monometallic counterparts. Bimetallic nanocatalysts formed from abundant and inexpensive elements provide greater opportunities for applications over noble metal catalysts. In this study, inexpensive Ni-Cu bimetallic catalytic membrane microreactors (CMMRs) were synthesized in a simple two-step process to catalytically degrade the environmental pollutant, 4-nitrophenol (4-NP), and produce the valuable feedstock, 4-aminophenol (4-AP). Ni-Cu nanoparticles were either produced by a replacement reduction reaction or a co-reduction reaction, producing either bimodal or integrated nanostructures, respectively, as demonstrated by transmission electron microscopy (TEM), while the electronic reconfiguration between bimetallic systems was verified by X-ray photoelectron spectroscopy (XPS). Compared to 4-NP batch conversion, flow-through reactions demonstrated enhanced mass transfer contributing to 2-fold higher conversion (>99%), 30-fold higher processing capacity (0.95 mol∙m−2∙h−1) and co-reduced Ni-Cu CMMRs boasted a reaction rate constant of 725.03 min−1. 4-NP conversion on the Ni-Cu catalysts in the presence of NaBH4 followed the Langmuir-Hinshelwood (L-H) mechanism, and the conversion efficiency was highly dependent on flow rate, representing an optimization trade-off critical for CMMR applications. The polydopamine-assisted fabrication and the tortuous membrane pore structure contributed to the CMMRs’ stability with <5% metal loss during operation. The enhanced activity was attributed to the synergistic electronic effects of the Ni-Cu bimetallic structure, the metal-polydopamine interactions, and the catalysts’ unique structure and high surface area.