Modulating local environment of Ni with W for synthesis of carbon nanotubes and hydrogen from plastics

Abstract

The strategy of thermochemical conversion of plastic waste into carbon nanotubes (CNTs) and hydrogen makes both environmental and economic sense. Exploring high-performance catalysts is an essential prerequisite of converting plastics by pyrolysis and in-line catalysis technology. To improve the catalytic performance of Ni–Mg system, high-valency and refractory tungsten as a promoter was introduced. A series of W–Ni–Mg oxides with various W/Ni atomic ratios (0, 1/15, 1/7, 1/3) were synthesized and their catalytic performances were evaluated. The sample with W/Ni atomic ratio of 1/7 exhibited the mixed phase of MgNiO2 and MgWO4 and showed the highest yields of CNTs and hydrogen. Furthermore, the effect of tungsten on the modulating the local coordination and electronic structure of active Ni was explored. The addition of tungsten facilitated the reduction of Ni and maintained it as solid state, not traditional quasi-liquid state, to alleviate the aggregation and migration of Ni nanoparticles. Thus, the CNT growth over W–Ni–Mg oxide sample predominantly followed vapor−solid−solid mechanism, while vapor−liquid−solid mechanism could still occur as the secondary growth of CNTs.