Hydrogen spillover in the context of hydrogen storage using solid-state materials

Abstract

Hydrogen spillover has emerged as a possible technique for achieving high-density hydrogen storage at near-ambient conditions in lightweight, solid-state materials. We present a brief review of our combined theoretical and experimental studies on hydrogen spillover mechanisms in solid-state materials where, for the first time, the complete mechanisms that dictate hydrogen spillover processes in transition metal oxides and nanostructured graphitic carbon-based materials have been revealed. The spillover process is broken into three primary steps: (1) dissociative chemisorption of gaseous H 2 on a transition metal catalyst; (2) migration of H atoms from the catalyst to the substrate and (3) diffusion of H atoms on substrate surfaces and/or in the bulk materials. In our theoretical studies, the platinum catalyst is modeled with a small Pt cluster and the catalytic activity of the cluster is examined at full H atom saturation to account for the essentially constant, high H 2 pressures used in experimental studies of hydrogen spillover. Subsequently, the energetic profiles associated with H atom migrations from the catalyst to the substrates and H atom diffusion in the substrates are mapped out by calculating the minimum energy pathways. It is observed that the spillover mechanisms for the transition metal oxides and graphitic carbon-based materials are very different. Hydrogen spillover in the transition metal oxides is moderated by massive, nascent hydrogen bonding networks in the crystalline lattice, while H atom diffusion on the nanostructured graphitic carbon materials is governed mostly by physisorption of H atoms. The effects of carbon material surface curvature on the hydrogen spillover as well as on hydrogen desorption dynamics are also discussed. The proposed hydrogen spillover mechanism in carbon-based materials is consistent with our experimental observations of the solid-state catalytic hydrogenation/dehydrogenation of coronene.

Is Spillover Relevant for Hydrogen Adsorption and Storage in Porous Carbons Doped with Palladium Nanoparticles?

“…[18][19][20][21][22][23][24][25][26] The metals mostly used in those investigations are alkali metals 18,19 and some transition metals like vanadium, palladium, and platinum. [22][23][24][25][26][27] To explain the observed enhancement of the hydrogen adsorption, a mechanism of spillover 28,29 has generally been assumed. [22][23][24][25][26][27]30,31 In that mechanism, the hydrogen molecules are first adsorbed and dissociated on the surface of the metallic nanoparticles, and are next transported onto the surface of the substrate carbon material.…”

Hydrogen Storage for Energy Application

“…Thus the process of physisorption results in the H2 molecule remaining intact. And, in opposite, chemisorption leads to H2 bond dissociation, with the resulting H atoms forming chemical bonds with the storage substrate [49]. Physisorption can occur as a preliminary state to chemisorption.…”

“…It has been observed that, for instance, by heating Pt dispersed on carbon at 623 K, Pt/Al2O3 at 473-573 K, Pd/C at 473 K, and Pt/WO3, under hydrogen pressure the amount of H2 absorbed is in excess of the known H2-sorption capacity of the metal alone. While the concept of hydrogen spillover is normally associated with solid-state materials such as activated carbon (or transition metal oxides), there have been a small number of reports of the solid-state hydrogenation of organic compounds that appear to implicate hydrogen spillover as the mechanism for hydrogenation [49].…”

“…Subsequently, H atoms on the catalyst surfaces migrate to the substrate in the vicinity of the catalyst particles. Finally, H atoms near the catalysts diffuse freely to the surface or bulk sites far from where the catalyst particles reside [49].…”

Templated synthesis of nickel nanoparticles: Toward heterostructured nanocomposites for efficient hydrogen storage

“…9 Hydrogen spillover can occur between a metal and a metal oxide 18 or between two metals 26 and is believed to occur through three principal steps (Scheme 2): (1) Dissociative chemisorption of hydrogen gas on a metal surface, (2) migration of hydrogen atoms to the metal-substrate interface, (3) and diffusion of hydrogen atoms across the metal-substrate interface. 27 Often times, the rate-limiting step and key species in hydrogen spillover are surface diffusion of hydrogen atoms across the metal-substrate interface, or more generally the activator-acceptor interface ( Figure 4). 22,28 The mobility of hydrogen atoms on the acceptor surface gives rise to catalytic behavior and also "cleans" the surface of the metal catalyst, allowing another hydrogen molecule to undergo dissociative chemisorption.…”