Abstract
The spillover mechanism of molecular hydrogen on carbon nanotubes in the presence of catalytically active platinum clusters was critically and systematically investigated by using density-functional theory. Our simulation model includes a Pt4 cluster for the catalyst nanoparticle and curved and planar circumcoronene for two exemplary single-walled carbon nanotubes (CNT), the (10,10) CNT and one of large diameter, respectively. Our results show that the H2 molecule dissociates spontaneously on the Pt4 cluster. However, the dissociated H atoms have to overcome a barrier of more than 2 eV to migrate from the catalyst to the CNT, even if the Pt4 cluster is at full saturation with six adsorbed and dissociated hydrogen molecules. Previous investigations have shown that the mobility of hydrogen atoms on the CNT surface is hindered by a barrier. We find that instead the Pt4 catalyst may move along the outer surface of the CNT with activation energy of only 0.16 eV, and that this effect offers the possibility of full hydrogenation of the CNT. Thus, although we have not found a low-energy pathway to spillover onto the CNT, we suggest, based on our calculations and calculated data reported in the literature, that in the hydrogen-spillover process the observed saturation of the CNT at hydrogen background pressure occurs through mobile Pt nanoclusters, which move on the substrate more easily than the substrate-chemisorbed hydrogens, and deposit or reattach hydrogens in the process. Initial hydrogenation of the carbon substrate, however, is thermodynamically unfavoured, suggesting that defects should play a significant role.
Highlights
The results show that the process is rather endothermic, with E of 2.0, 3.4, and 1.9 eV for two H atoms attached to the large diameter carbon nanotubes (CNT) model in ortho, meta, and para positions, respectively
Becher et al (2003) concluded that reversible hydrogen storage at ambient temperature is unlikely based on the endothermic adsorption energies, which range from 2–3 eV
We have investigated the individual steps of hydrogen storage by the spillover mechanism and complemented our calculations with data from the literature
Summary
Hydrogen spillover has been proposed as a promising mechanism for hydrogen storage (Lueking and Yang, 2004; Marella and Tomaselli, 2006; Li and Yang, 2006a,b; Chen and Huang, 2007, 2008; Liu et al, 2007; Zacharia et al, 2007; Zieliñski et al, 2007; Bhowmick et al, 2011). Carbon-based materials, such as carbon nanofibres (Lueking and Yang, 2004; Marella and Tomaselli, 2006), graphite (Mitchell et al, 2003a,b), and carbon nanotubes (CNTs) (Lueking and Yang, 2003, 2004; Nikitin et al, 2005, 2008; Zacharia et al, 2007; Chen and Huang, 2008; Yang and Wang, 2009; Bhowmick et al, 2011), are some of the common materials proposed as storage substrates. In two comprehensive reviews and an experimental work, Becher et al (2003), Meregalli and Parrinello (2001) and Tibbetts et al (2001) criticized earlier reports of hydrogen storage capacities of carbon-based materials greater than 1 wt % at ambient temperature
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