Tuning the redox activity of encapsulated metal clusters via the metallic and semiconducting character of carbon nanotubes

Abstract

We demonstrate that reactions confined within single-walled carbon nanotube (SWCNT) channels are modulated by the metallic and semiconducting character of the hosts. In situ Raman and X-ray absorption near-edge structure spectroscopies provide complementary information about the electronic state of carbon nanotubes and the encapsulated rhenium species, which reveal electronic interactions between encapsulated species and nanotubes. More electrons are transferred from metallic tubes (m-SWCNTs) to oxidic rhenium clusters, leading to a lower valence state rhenium oxide than that in semiconducting tubes (s-SWCNTs). Reduction in 3.5% (vol/vol) H 2 /Ar leads to weakened host-guest electronic interaction. The high valence state Re within s-SWCNTs is more readily reduced when raising the temperature, whereas only a sluggish change is observed for Re within m-SWCNTs. Only at 400°C does Re reach a similar electronic state (mixture of Re 0 and Re 4+ ) in both types of tubes. Subsequent oxidation in 1% O 2 /Ar does not show changes for Re in s-SWCNTs up to 200°C. In comparison, m-SWCNTs facilitate the oxidation of reduced rhenium (160°C). This can be exploited for rational design of active catalysts with stable species as a desired valence state can be obtained by selecting specific-type SWCNTs and a controlled thermal treatment. These results also provide a chemical approach to modulate reversibly the electronic structure of SWCNTs without damaging the sidewalls of SWCNTs.confined catalysis | confinement effect A n increasing number of studies reveal that confinement of metal or metal oxide nanoparticles inside carbon nanotubes (CNTs) often leads to significantly enhanced catalytic activity with respect to the same bare metal nanoparticles or those deposited on the outer walls of CNTs (1-3). Such a different behavior originates from both the physical (spatial restriction of the channels) and chemical factors (electronic interaction of confined species with the curved graphene walls) inside CNTs (1-5). In this sense, single-walled carbon nanotubes (SWCNTs) are very interesting because they have a higher degree of uniformity and smaller channel compared with multiwalled CNTs (6). More interestingly, they exhibit intrinsically either metallic or semiconducting properties with different electronic structure around the Fermi level. Metallic and semiconducting SWCNTs (m-SWCNTs and s-SWCNTs, respectively) have been demonstrated to vary significantly in reactivity toward covalent and noncovalent functionalization of their sidewalls (7-9). For example, diazonium salts (10), nitronium ions (NO 2 + ) (11), and OsO 4 (12) reacted selectively with metallic tubes, which was attributed to their higher electron density near the Fermi level. Metallic tubes were also reported preferentially etched away by water and methane plasma because of their higher reactivity (13,14). On the other hand, H 2 O 2 (15), azomethineylides (16), and gaseous SO 3 (17) preferentially attacked s-SWCNTs, and recently s-SWCNTs were also shown to be preferentially...

Effective Doping of Single-Walled Carbon Nanotubes with Polyethyleneimine

“…However, taking into account the diameter distribution of the material (1.8 ± 0.4 nm [31]), metallic SWCNTs should also be excited at the selected wavelength according to the Kataura plot [40]. This fact justified why the shape of the G − band was somewhat smeared on the lower wavenumber side, which could be expected for the material containing metallic SWCNTs [41].…”

Interface confined hydrogen evolution reaction in zero valent metal nanoparticles-intercalated molybdenum disulfide

“…The confined space within the adjacent MoS 2 layers provides an intriguing environment for catalysis 15 39 40 . First, the narrow space limits the growth of the Pt nanoparticles, and dispersion within the MoS 2 layers prevents aggregation.…”

Recent Advances on the Design of Group VIII Base-Metal Catalysts with Encapsulated Structures

“…As mentioned in the introduction section, this fine-tuning effect of metal NPs' catalytic properties is termed as the confinement effect in catalysis literature. 73,84,197,198 Many efforts and progress have been made to discover and understand the difference between redox and catalytic properties of Fe-based NPs inside and outside CNTs by Bao and co-workers. 66,75,76,[199][200][201] The different redox properties between iron oxide inside and outside CNTs were observed and confirmed by in situ TEM, CO temperatureprogrammed desorption (TPD) and XRD.…”