posted on 2015-01-14, 00:00authored byJianping Xiao, Xiulian Pan, Shujing Guo, Pengju Ren, Xinhe Bao
An
increasing number of experimental studies have demonstrated
that metal or metal oxide nanoparticles confined inside carbon nanotubes
(CNTs) exhibit different catalytic activities with respect to the
same metals deposited on the CNT exterior walls, with some reactions
enhanced and others hindered. In this article, we describe the concept
of confinement energy, which enables prediction of confinement effects
on catalytic activities in different reactions. Combining density
functional theory calculations and experiments by taking typical transition
metals such as Fe, FeCo, RhMn, and Ru as models, we observed stronger
strains and deformations within the CNT channels due to different
electronic structures and spatial confinement. This leads to downshifted
d-band states, and consequently the adsorption of molecules such as
CO, N2, and O2 is weakened. Thus, the confined
space of CNTs provides essentially a unique microenvironment due to
the electronic effects, which shifts the volcano curve of the catalytic
activities toward the metals with higher binding energies. The extent
of the shift depends on the specific metals and the CNT diameters.
This concept generalizes the diverse effects observed in experiments
for different reactions, and it is anticipated to be applicable to
an even broader range of reactions other than redox of metal species,
CO hydrogenation, ammonia synthesis and decomposition discussed here.