Unraveling the effects of Ni particle size and facet on CH4 activation: From cluster to nanoparticle

Methane activation is a key step in the application of natural gas converting into high-value added chemicals via methane steam/dry reforming reactions. Ni element was found to be the most widely used catalyst for these reforming processes. In this work, methane activation on Ni surface from cluster to nanoparticle has been studied through detailed DFT calculations. Ni13, Ni55 and Ni116 show different ability for C–H bond activation of CH4 molecule. Ni13 is considered as a typical cluster model in the surface reaction, and proved to exhibit unique performance for CH4 activation due to its size effect. C–H bond activation on Ni13 requires to overcome a relatively low energy barrier of 0.8 eV, indicating it is favored to carry out. With the increase of Ni particle from cluster to nanoparticle, it shows the change of Ni surface structure, which will affect the catalytic performance for CH4 activation. Ni55 and Ni116 mainly consist of Ni(111), Ni(100) and step/edge surface. Step/edge surface shows a promising ability for C–H bond activation with an energy barrier around 1.0 eV, however, it accounts for the smallest proportion in Ni55 and Ni116 particles, compared to the Ni(111) and Ni(100). It suggests that flat site is dominant on Ni55 and Ni116 nanoparticles, compared to the step/edge site. On the other hand, the CH4 activation on the Ni(111) and Ni(100) of nanoparticle needs to overcome an energy barrier over 2.0 eV, which is much larger than that on the Ni13 cluster and step/edge surface of Ni nanoparticle. As a result, step/edge site of Ni particle presents the best performance for C–H activation, while flat surface shows weak ability. Thus, to enhance the catalytic ability for CH4 activation, the particle of Ni should be limited in the cluster-size instead of nano-size. This paper could provide insights into understanding size effect of Ni particle on CH4 activation, and proposed results would give some guidance for the development of high-efficient Ni reforming catalysts.