Background: There is considerable interest in using transition metal catalysts in place of rare and expensive precious metal catalysts in fixed bed catalysis. The use of hierarchically porous materials potentially provides a method of improving mass transport and decreasing sintering and metal particle growth that leads to decreased catalytic activity.
Methods: Composites consisting of nickel nanoparticles incorporated in a hierarchically porous carbon monolith (NC/Ni), with varying Ni loading were synthesized by a one-pot, nanocasting pathway in which the carbon and nickel were produced in a single step, and characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), nitrogen adsorption for surface area determination, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and Raman spectroscopy. Catalysis of the reduction of p-nitrophenol (p-NP) to p-aminophenol (p-AP) was carried out to evaluate the catalytic activity.
Results: TEM and SEM images of the composites show well dispersed nanoparticles in the carbon matrix with small nanoparticles of about 4-12 nm located within the mesopores and larger nanoparticles (20-60 nm) located in the macropores. The nanoparticles were unevenly distributed on the mm scale within the monoliths. The surface area and the mesopore volume of the carbon decreased with increasing metal loading. XRD suggested the nanoparticles were metals at zero oxidation state but XPS showed the presence of Ni(OH)2. Raman spectra showed a significant increase in carbon ordering with increasing metal loading. The composites were found to be catalytically active for the reduction of p-NP and the intermediate p-quinoimine (p-QI) was identified by GC-MS.
Conclusions: The one-pot nanocasting method is able to produce catalytically active hierarchically porous carbon monoliths incorporating nickel nanoparticles. However, the size and distribution of the nanoparticles is not yet well controlled.
Keywords: Nanocasting, monolith, nickel, mesoporous carbon, 4-nitrophenol, reduction, p-quinoimine.