Sandwich-like N-C/Cu/N-C porous beads derived from alginate with enhanced catalytic activity and excellent recyclability for 4-nitrophenol reduction

•Sandwich-like N-C/Cu/N-C beads were constructed by a PDA coating method.•The PDA modification could optimize the carbon skeleton structure and restrict the agglomeration of Cu nanoparticles.•The PDA coating effectively improved the mechanical strength and enhanced the catalytic stability of the catalyst.•This work provides a robust route to design and synthesize metal nanoparticles supported on carbon-based materials.