Synthesis of Supported Nickel Nanoparticles via a Nonthermal Plasma Approach and Its Application in CO2 Reforming of Methane

A microwave plasma treatment was applied to obtain not only a desired strong metal-support interaction but also well-dispersed nickel nanoparticles supported on ceria. The catalytic properties of these supported nanoparticles were tested in CO2 reforming of methane. The plasma-treated Ni/CeO2 catalysts showed enhanced turnover frequencies (TOFs), normalized by Ni on the surface, as compared with the thermally calcined samples. The Ni/CeO2 treated under plasma with low Ni loading gave an enhanced TOF of 9.5 s(-1) (700 degrees C, 50% CH4 and 50% CO2, and 1 atm) as compared with the thermally calcined catalyst (8.7 s(-1)). Increasing the Ni loading on the plasma-treated Ni/CeO2 catalysts gave an improved TOF (10.4 s(-1)) which was stable with time, while the TOF was observed to drop by a factor of 2 relative to the optimal TOF on the thermally calcined catalyst after 5 h. For the plasma-treated samples, concurrent treatment of both the uncalcined ceria support and the loaded metal precursor generated strong metal-support interaction and formation of well-dispersed Ni particles, resulting in a superior and stable TOF with time. In the case of thermally calcined catalysts, the weak metal-support interaction and the agglomeration of Ni clusters together with the migration of the Ni particles into the ceria support hindered the accessibility of active nickel sites, resulting in deactivation of the materials during reaction. Moreover, high-resolution transmission electron microscopy, high-angle annular dark-field scanning transmission electron microscopy, H-2 temperature-programmed reduction, and X-ray photoelectron spectroscopy yielded a clear picture of the impact of microwave plasma treatment on the nickel particle size, shape, distribution, and interaction with the ceria support.