Controllable synthesis of yolk–shell nickel phyllosilicate for CO2 methanation: Identifying effect of pore structure of silica sacrificial template

Abstract

A group of yolk-shell nickel phyllosilicate catalysts were synthesized via the hydrothermal reaction of fibrous silica (KCC-1) and nickel nitrate for CO2 methanation. For the sake of revealing the effect of the pore structure of silica material on the Ni-phyllosilicate morphology, a mesoporous MCM-41 material with a smooth outer surface was used as a sacrificial template for comparison. After the hydrothermal reaction at 180 °C for 36 h, the KCC-1-dervied Ni-phyllosilicate (Ni/K-36) displayed a typical yolk-shell nanosphere morphology covered by nanosheets, while MCM-41-derived Ni-phyllosilicate (Ni/M-36) obtained nanosheets-wrapped sphere morphology with core-shell structure. This result showed that the pore structure of silica material played an important role in the formation of Ni-phyllosilicate with different structures. The fibrous, drive-through channels of KCC-1 enhanced pore accessibility and mass transfer for the simultaneous growth of Ni-phyllosilicate on its external and internal section, and the rapid accumulation of Ni-phyllosilicate outside as “shell” could slow down the further hydrothermal reaction of internal KCC-1, resulting in the shell of Ni-phyllosilicate and yolk of unreacted KCC-1. The chamber between yolk and shell could be expanded by increasing hydrothermal time; however, too long hydrothermal time was adverse for the catalytic activity because of the increased Ni particle size. On the other hand, the gradual etching of MCM-41 sphere from the outer surface resulted in the external accumulation of Ni-phyllosilicate as “shell” and internally unreacted MCM-41 as “core”. The yolk-shell Ni/K-36 exhibited higher catalytic activity than the core-shell structural Ni/M-36 for CO2 methanation owing to high Ni content and similar Ni particle size. In situ DRIFTS analysis showed that the existence of more active formate and absorbed CO intermediates resulted in the high activity of Ni/K-36 for CO2 methanation.