Nanoparticle growth in supported nickel catalysts during methanation reaction - Larger is better

Abstract

A major cause of supported metal catalyst deactivation is particle growth by Ostwald ripening. Nickel catalysts, used in the methanation reaction, may suffer greatly from this through the formation of [Ni(CO)4]. By analyzing catalysts with various particle sizes and spatial distributions, the interparticle distance was found to have little effect on the stability, because formation and decomposition of nickel carbonyl rather than diffusion was rate limiting. Small particles (3-4 nm) were found to grow very large (20-200 nm), involving local destruction of the support, which was detrimental to the catalyst stability. However, medium sized particles (8 nm) remained confined by the pores of the support displaying enhanced stability, and an activity 3 times higher than initially small particles after 150 h. Physical modeling suggests that the higher [Ni(CO)4] supersaturation in catalysts with smaller particles enabled them to overcome the mechanical resistance of the support. Understanding the interplay of particle size and support properties related to the stability of nanoparticles offers the prospect of novel strategies to develop more stable nanostructured materials, also for applications beyond catalysis. Mind the breakage: The conversion of CO and H2 to CH 4 over Ni catalysts suffers from particle growth through [Ni(CO) 4]-mediated Ostwald ripening. By varying the size and distance of the Ni particles, the size was found to be key: Small 3-4 nm particles grow to large inactive particles, breaking the pore structure of the silica support, while medium 8-9 nm particles remain confined by the pores resulting in stable catalysts.