Probing the Electronic Effect of Carbon Nanotubes in Catalysis: NH₃ Synthesis with Ru Nanoparticles

Abstract

Carbon nanotubes (CNTs) have been shown to modify some properties of nanomaterials and to modify chemical reactions confined inside their channels, which are formed by curved graphene layers. Here we studied ammonia synthesis over Ru as a probe reaction to understand the effect of the electron structure of CNTs on the confined metal particles and their catalytic activity. The catalyst with Ru nanoparticles dispersed almost exclusively on the exterior nanotube surface exhibits a higher activity than the CNT-confined Ru, although both have a similar metal particle size. Characterization with TEM, N(2) physisorption, H(2) chemisorption, temperature-programmed reduction, CO adsorption microcalorimetry, and first-principles calculations suggests that the outside Ru exhibits a higher electron density than the inside Ru. As a result, the dissociative adsorption of N(2), which is an electrophilic process and the rate-determining step of ammonia synthesis, is more facile over the outside Ru than that over the inside one.

The Effects of Confinement inside Carbon Nanotubes on Catalysis

“…Take filling of CNTs with Ru particles as an example, a RuCl 3 /acetone solution was employed. TEM indicated that around 80% of Ru particles were distributed inside the channels following drying and reduction in H 2 , 20 which was confirmed by three-dimensional tomography. 21 The particles had a rather narrow size distribution with 90% falling in the range of 2À4 nm.…”

“…20 Figure 6 shows the differential electron density isosurfaces of a Ru 6 cluster inside (Ru 6 -in) and outside (Ru 6 -out) of a SWCNT (10,10). 20 Mulliken population analysis indicated that the Ru 6 -out cluster donated 1.66 electrons to the CNT convex surface whereas the inside cluster transferred 2.41 electrons to the concave surface. This is consistent with the electron structure of CNTs.…”

Carbon nanotubes prepared by anodic aluminum oxide template method

“…Compared with the conventional synthesis methods [95][96][97], the AAO template approach shows unique advantage for hetero atom doping in CNTs [93,[98][99][100][101]. The heteroatoms can be selectively doped at the inner or outer surface of CNTs, which is very useful in many applications requiring different polar nature and chemical or physical reaction activity, such as bioseparations and catalyst supports [14,39]. This unique advantage originates from the synthesis procedure of AAO-CNTs.…”

“…Structural parameters of CNTs such as inner diameter, wall thickness, length, crystallinity, and electronic structure have a great influence on the performance of the supported particles or loaded matters [12][13][14][15]. Especially, the size of the CNT hollow core and electron density plays an important role in determining the property of the filled matters owing to the confinement effect and the difference in electron density between the exterior and interior CNT surface due to their different graphene curvature [14,16]. Some new phenomena were reported for the materials confined in CNT nanochannels, which have never been observed for their bulk counterparts [17][18][19].…”

In Situ Solution Process for Fabricating Thermally and Mechanically Stable Highly Conductive ZnO-CNT Fiber Composites

“…Furthermore, CNT fiber devices offer low efficiency photoelectric conversion and energy storage properties [5,13]. The deposition of a variety of metal oxides (MnO 2 [14], NiO [15], RuO 2 [16], V 2 O 5 [17], TiO 2 [3], ZnO [18]) or metal nanoparticles (Pt [19], PtRu [20], PtSn [21], Pd [22], Au [22], Ru [23], Ag [24]) onto the CNT fiber surfaces has been reported to improve the electrochemical and optical properties of CNT fibers. Metal oxides or metal nanoparticles are typically deposited randomly and inhomogeneously onto CNT fibers.…”