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

Abstract

Experiments have shown that the efficiency of nanoporous carbons to store hydrogen becomes enhanced by doping the material with metallic nanoparticles. In particular, doping with palladium has been used with success. The hypothesis to justify the enhancement has been that the Pd nanoparticles dissociate the hydrogen molecules and then the hydrogen atoms spill over the carbon substrate, where the hydrogen is retained. To test this hypothesis we have performed ab initio molecular dynamics simulations of the deposition of molecular hydrogen on Pd nanoparticles (Pd 6 and Pd 13 ) supported on graphene, which is a good model for the wall of a carbon nanopore. Three channels have been identified in the simulations: bouncing off the molecule, molecular adsorption, and dissociation of the molecule in two H atoms. The relative percentage of those channels is sensitive to the size of the Pd particle. Dissociation occurs more frequently on Pd 13 and it generally takes place on the lateral regions of the Pd particles.However, in our simulations we have not found a single case of H atoms or H 2 molecules spilling over the carbon substrate. We have also tested the situation when several H atoms are preadsorbed on the Pd 6 and Pd 13 particles and found that not a single dissociation event occurs on these H-saturated nanoparticles. These results lead us to cast strong doubts on the validity of the spillover mechanism for explaining the enhancement of hydrogen adsorption on porous carbons doped with transition metal nanoparticles.2

Theoretical study of the adsorption of hydrogen on cobalt clusters

“…The structure is similar to that found in Pd13 . 20 The distances between neighbor Co atoms vary between 2.30 Å y 2.40 Å, the binding energy is 3.47 eV per atom, and the magnetic moment is µ = 27 µB (2.08 µB per atom). Co13 has two low-lying isomers, icosahedron and cuboctahedron, both with Eb = 3.33 eV/atom, a bit lower than that of the ground state.…”

“…[15][16][17] For this reason, the interaction of H2 with metal clusters and nanoparticles, in particular those of the 4d transition metals, has been investigated. Some theoretical calculations have focused on palladium, [18][19][20] concluding that the mechanism responsible for the enhancement of the storage capacity in the doped carbons is still not well understood. 20 The interaction between hydrogen and transition metal clusters is also relevant in the operation of the fuel cell.…”

“…Some theoretical calculations have focused on palladium, [18][19][20] concluding that the mechanism responsible for the enhancement of the storage capacity in the doped carbons is still not well understood. 20 The interaction between hydrogen and transition metal clusters is also relevant in the operation of the fuel cell. Transition metal clusters embedded in the anode of the fuel cell catalyze the dissociation of molecular hydrogen in protons (which go through a proton exchange membrane) and electrons (which go through an external electric circuit), that recombine with oxygen at the cathode of the fuel cell.…”

Free and graphene supported metallic nanoparticles and their interaction with hydrogen

“…A theoretical description of the spillover process is highly challenging since the computational model must account for the effects of the metal catalyst used to split hydrogen molecules into H atoms, the substrate on which the H atoms should be able to diffuse freely on the carbon surface and the interplay at the catalyst-substrate interface. Many of the spillover mechanism steps have been studied but it seems that the picture might not be as simple at it appears [85,109].…”

“…[Here we do not refer to the spillover mechanism in the contexts of heterogeneous catalysis on metal catalysts supported on metal-oxide surfaces where this mechanisms has been extensively proven]. Blanco et al have shown that hydrogen spillover mechanism in Pd 6 and Pd 13 clusters supported on pristine graphene is a endothermic process with very high activation barriers [109]. Nonetheless, vacancies and defects of the graphene layer might facilitate the spillover mechanism.…”