DFT-Based Global Optimization of Sub-nanometer Ni–Pd Clusters

10.1021/acs.jpcc.9b0597

Competition between Palladium Clusters and Hydrogen to Saturate Graphene Vacancies

Producción CientíficaDoping with palladium has been proposed as a means to enhance the hydrogen storage capacity of nanoporous carbon materials. Palladium atoms and clusters attach strongly to defects on the walls of nanoporous carbons, which can be mimicked as graphene layers with vacancies. On the other hand, atomic hydrogen also binds strongly to the dangling bonds of defects and edges of graphitic carbon. Therefore, hydrogen adsorbed on Pd-doped nanoporous carbons could compete with the Pd dopant to saturate the vacancies. In this work we have performed density functional calculations to investigate the competition between palladium atoms and clusters, on one hand, and hydrogen, on the other hand, to saturate graphene vacancies. We find that palladium binds stronger than hydrogen to graphene vacancies and, therefore, hydrogen can not replace the palladium atoms or clusters attached to the vacancies. Instead, hydrogen adsorbs on the palladium. Thus, hydrogen adsorption on Pd-doped carbons does not destroy the stability of the material. Moreover, our study shows that graphene vacancies decorated with Pd just on one side of the graphene layer are not fully saturated. The other side of the vacancy remains quite reactive and therefore Pd atoms and clusters can be attached, simultaneously, to both sides of the vacancy. Interestingly, the hydrogen adsorption mechanisms and energies do not depend on whether Pd atoms and clusters are decorating one side or both sides of the vacancies

Dynamics of Cluster Isomerization Induced by Hydrogen Adsorption

Ab initio dynamical simulations based on the density functional formalism have been performed for molecular hydrogen impinging on a Pd6 cluster anchored to a vacancy defect in graphene. Under the conditions assumed in the simulations, most H2 molecules rebound after colliding with the Pd6 cluster, but a number of molecules stay adsorbed on its surface. Depending on whether the substrate is initially at 0 or 300 K, either one-third or one-half of those adsorbed molecules later on dissociate on the cluster, leading to two chemisorbed H atoms. For both substrate temperatures, dissociation of H2 triggers a transition from the original octahedral structure of the anchored Pd6 to an incomplete pentagonal bypiramid structure, which is essentially produced by a severe elongation of the distance between two particular neighbor Pd atoms. Interestingly, no such structural changes were previously observed for Pd6 adsorbed on pristine graphene. Although this new result comes for a specific reaction—this one occurring, for instance, in the anode of hydrogen fuel cells—we anticipate that the observation of a structural change, which means that the cluster structure is not immune to the reaction taking place on its surface, can be relevant for many catalytic processes occurring on the surface of small metal particles

Interaction of Hydrogen with Graphitic Surfaces, Clean and Doped with Metal Clusters

Producción CientíficaHydrogen is viewed as a possible alternative to the fossil fuels in transportation. The technology of fuel-cell engines is fully developed, and the outstanding remaining problem is the storage of hydrogen in the vehicle. Porous materials, in which hydrogen is adsorbed on the pore walls, and in particular nanoporous carbons, have been investigated as potential onboard containers. Furthermore, metallic nanoparticles embedded in porous carbons catalyze the dissociation of hydrogen in the anode of the fuel cells. For these reasons the interaction of hydrogen with the surfaces of carbon materials is a topic of high technological interest. Computational modeling and the density functional formalism (DFT) are helping in the task of discovering the basic mechanisms of the interaction of hydrogen with clean and doped carbon surfaces. Planar and curved graphene provide good models for the walls of porous carbons. We first review work on the interaction of molecular and atomic hydrogen with graphene and graphene nanoribbons, and next we address the effects due to the presence of metal clusters on the surface because of the evidence of their role in enhancing hydrogen storage.Ministerio de Economía, Industria y Competitividad (Grant MAT2014-54378-R

Dynamics of cluster isomerization induced by hydrogen adsorption

Ab initio dynamical simulations based on the density functional formalism have been performed for molecular hydrogen impinging on a Pd cluster anchored to a vacancy defect in graphene. Under the conditions assumed in the simulations, most H molecules rebound after colliding with the Pd cluster, but a number of molecules stay adsorbed on its surface. Depending on whether the substrate is initially at 0 or 300 K, either one-third or one-half of those adsorbed molecules later on dissociate on the cluster, leading to two chemisorbed H atoms. For both substrate temperatures, dissociation of H triggers a transition from the original octahedral structure of the anchored Pd to an incomplete pentagonal bypiramid structure, which is essentially produced by a severe elongation of the distance between two particular neighbor Pd atoms. Interestingly, no such structural changes were previously observed for Pd adsorbed on pristine graphene. Although this new result comes for a specific reaction - this one occurring, for instance, in the anode of hydrogen fuel cells - we anticipate that the observation of a structural change, which means that the cluster structure is not immune to the reaction taking place on its surface, can be relevant for many catalytic processes occurring on the surface of small metal particles

Absence of spillover of hydrogen adsorbed on small palladium clusters anchored to graphene vacancies

Experimental evidence exists for the enhancement of the hydrogen storage capacity of porous carbons when these materials are doped with metal nanoparticles. One of the most studied dopants is palladium. Dissociation of the hydrogen molecules and spillover of the H atoms towards the carbon substrate has been advocated as the reason for the enhancement of the storage capacity. We have investigated this mechanism by performing ab initio density functional molecular dynamics (AIMD) simulations of the deposition of molecular hydrogen on Pd6 clusters anchored on graphene vacancies. The clusters are initially near-saturated with atomic and molecular hydrogen. This condition would facilitate the occurrence of spillover, since our energy calculations based on density functional theory indicate that migration of preadsorbed H atoms towards the graphene substrate becomes exothermic on Pd clusters with high hydrogen coverages. However, AIMD simulations show that the H atoms prefer to intercalate and absorb within the Pd cluster rather than migrate to the carbon substrate. These results reveal that high activation barriers exist preventing the spillover of hydrogen from the anchored Pd clusters to the carbon substrate.The authors acknowledge financial support by the Gobierno Vasco-UPV/EHU Project No. IT1246-19, the Spanish Ministerio de Ciencia e Innovación [Grants No. PID2019-107396 GB-I00/AEI/10.13039/501100011033 and PID2019-104924RB-I00], Junta de Castilla y León [Grant VA021G18], and University of Valladolid (Grupo de Física de Nanoestructuras). A. G. acknowledges a predoctoral fellowship from Junta de Castilla y León.Peer reviewe