Insights into the reduction of 4-nitrophenol to 4-aminophenol on catalysts

Highlights: • Water molecules not NaBH₄ are the hydrogen source for the reduced amino groups. • NaBH₄ could contribute to the ionization of H⁺ from water. • This reduction process is spontaneous to some extent. New insights for the mechanism of nitro to amino groups conversion have been revealed on both N-doped graphene and Ag nanoparticles catalysts, based on the paper assisted ultrasonic spray ionization mass spectrometry: (1) water molecules not NaBH₄ are the hydrogen source for the reduced amino groups, (2) NaBH₄ could contribute to the ionization of H⁺ from water, facilitating its adsorption on nitro groups, (3) six different intermediates have been detected to depict the whole catalytic process, and no condensed roots are involved in and (4) this reduction process is spontaneous to some extent, and even without catalysts it is not totally stopped as obtained from the spectral measurements.