The mechanism of hydrogen storage by atomic hydrogen spillover on a Pt-doped graphite (0001) surface was studied by means of density functional theory. The coronene molecule and a Pt 4 cluster were used as primary models for the carbon surface and the metal nanoparticles, respectively. It was found that H 2 dissociates spontaneously on a Pt cluster, but the dissociated H atoms have to overcome excessively large energy barriers (>60 kcal/mol) to migrate from Pt to the graphite surface. H atoms on a graphite (0001) surface can be either chemisorbed or physisorbed. The transition from the chemisorbed to the physisorbed state happens at sufficiently high rates. In the physisorbed state, H atom diffusion is essentially free of energy barriers. Physisorbed H atoms readsorb selectively adjacent to other chemisorbed H atoms. Our results indicate that H atom migration from a transition metal to the graphite surface is rate-limiting in the overall spillover process. The implications of the findings are discussed.
DFT modeling was used to understand the role of epoxide (C-O-C) and hydroxyl (C-OH) functional groups on the spillover mechanism for hydrogen storage on graphite oxide and oxygen-modified carbons. A primary spillover model was used, consisting of a Pt(4) cluster, a graphite substrate model, and O and OH functional groups adsorbed on graphite. The spillover mechanism was found to proceed via the migration of dissociated hydrogen atoms from the Pt cluster to epoxide groups adjacent to the cluster (to form OH), followed by H migration by hopping on the adsorbed O atoms. The low energy barriers required for the relevant elementary steps indicate that the spillover process is facile when the carbon substrate is decorated with oxygen functionalities, leading to enhanced hydrogen uptake and faster charge/discharge kinetics. However, a reaction path was also identified, in which surface OH groups can react to form water, which can have adverse consequences for hydrogen storage on oxygenated carbons via spillover.
The electronic energy barriers of surface reactions pertaining to the mechanism of the electrooxidation of methane on Pt (111) were estimated with density functional theory calculations on a 10-atom Pt cluster, using both the B3LYP and PW91 functionals. Optimizations of initial and transition states were performed for elementary steps that involve the conversion of CH(4) to adsorbed CO at the Pt/vacuum interface. As a first approximation we do not include electrolyte effects in our model. The reactions include the dissociative chemisorption of CH(4) on Pt, dehydrogenation reactions of adsorbed intermediates (*CH(x) --> *CH(x-1) + *H and *CH(x)O --> *CH(x-1)O + *H), and oxygenation reactions of adsorbed CH(x) species (*CH(x) + *OH --> *CH(x)OH). Many pathways were investigated and it was found that the main reaction pathway is CH(4) --> *CH(3) --> *CH(2) --> *CH --> *CHOH --> *CHO --> *CO. Frequency analysis and transition-state theory were employed to show that the methane chemisorption elementary step is rate-limiting in the above mechanism. This conclusion is in agreement with published experimental electrochemical studies of methane oxidation on platinum catalysts that have shown the absence of an organic adlayer at electrode potentials that allow the oxidation of adsorbed CO. The mechanism of the electrooxidation of methane on Pt is discussed.
In this feature article, the atomic-scale understanding of the hydrogen spillover mechanism for hydrogen storage in metal-doped carbon materials and metal-organic frameworks is discussed by critically assessing recent computational and experimental studies. It is argued that the spillover mechanism involves: (a) the generation and desorption of mobile H atoms on the metal nanoparticles (b) the diffusion of H atoms in weakly-bound states on the support (c) the sticking and immobilization of H atoms at preferential locations of the receptor where barriers to sticking are decreased, and, (d) the Eley-Rideal recombination of the adsorbed H atoms with diffusing mobile H atoms to form H(2). The implications and open questions on the mechanism and effectiveness of hydrogen storage by spillover are critically assessed.
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