H₂ Adsorption on Cu_4-xM_x (M = Au, Pt; x = 0–4) Clusters: Similarities and Differences As Predicted by Density Functional Theory

Abstract

The active search for alternatives to current fossil fuels via energy conversion mechanisms has bolstered the study of the adsorption of molecules on surfaces and clusters as a fundamental step before moving into complex heterogeneous catalysis scenarios. In this respect, intensive investigations have been thoroughly performed to obtain cheaper and more efficient catalysts, such as in the hydrogen evolution reaction (HER) in the electrocatalysis field. In this paper, we present systematic density functional theory simulations of H 2 adsorption on bimetallic Cu 4−x M x (M = Pt, Au; x = 0−4) neutral clusters. The molecular and dissociative adsorption mechanisms are explored, and the occurrence of one or the other is explained in terms of the reactivity of the host clusters. We begin with a global exploration of the potential energy surface to obtain the ground-state structures. The effects of H 2 adsorption and host cluster composition on the interatomic distances are investigated. A detailed analysis of H 2 adsorption on the Cu 4−x Pt x family shows that dissociative adsorption occurs for x = 1−4, that is, for all clusters containing Pt, while nondissociative molecular adsorption is obtained in Cu 4-x Au x clusters with x = 0−3, that is, for all clusters containing Cu. This is consistent with the values of the electronic HOMO−LUMO gap, which are predicted to be small (high reactivity) and large (small reactivity) for bimetallic Cu−Pt and Cu−Au clusters, respectively. The adsorption energy increases with the Pt load in Cu−Pt clusters, while it is almost constant in the case of Cu−Au clusters.

Molecular Adsorption of H2 on Small Neutral Silver–Copper Bimetallic Nanoparticles: A Search for Novel Hydrogen Storage Materials

“…Different structural shapes for the different stoichiometry of copper and silver in a particular cluster size can be observed, and Cu atoms reside in such a way that they can possess the optimal coordination number. As shown in Figure a, the mixed clusters follow similar structural patterns of Ag n and Cu n clusters observed in this study as well as in previous work. ,,, The dimer of the Cu m Ag n cluster is a 1-D structure, whereas the trimer to the hexamer of Cu m Ag n clusters possess planar 2-D structure patterns except for the 3-D structure of CuAg 5 . Cu m Ag n clusters above the hexamer are in 3-D arrangements (Figure b).…”

“…These observations can be summarized as the adsorption of the H 2 molecule noticeably favors the sites, where the most positively charged environment persists. H 2 molecule adsorbs onto either the most positively charged copper atoms (or reaches onto the most positive silver atom in Ag n monometallic clusters) at the vertices of the cluster or the site which has the most positive charge environment avoiding the negatively charged atoms in the vicinity as also observed in previous studies, especially that the H 2 adsorption occurred on top of the most positively charged three-coordinated Cu atom in the small diagonal of Cu 4 rhombus. , With the adsorption, the positive charge density of the adsorbed atom or the atom closer to the adsorbed site decreases, acquiring more negative charge density. We will discuss the electron distributions further in the natural bond orbital (NBO) analysis below.…”

“…In addition to these minimum energy structures, we examined the other higher energy structures (low-lying isomers) of Cu m Ag n clusters for m + n = 3–8, and the structures of those isomers with energies relative to the lowest energy geometry are given in Figure S2a–f in the Supporting Information. A detailed comparison of low-lying energy structures is beyond the scope of this article and interested readers can find a detailed comparison in the literature. ,− …”

“…Fang et al studied H 2 adsorption on platinum-doped gold clusters . A density functional study of molecular adsorption on small gold–copper binary clusters was also reported in 2015 by Zhao et al Very recently, Gálvez-González et al reported H 2 adsorption on Au- and Pt-doped copper clusters with the size of four atoms . The research focusing on the potential use of mixed transition-metal nanostructured materials for hydrogen storage is limited, and this study aims to explore a copper–silver mixed system in all compositions (Cu m Ag n for m + n ≤ 8) to find the most potential heterogeneous system within the recommended physisorption limit.…”

Structure, Energetics, and Thermal Behavior of Bimetallic Re–Pt Clusters

“…In this respect, computational studies mostly based on density functional theory (DFT) calculations have contributed extensively to the understanding of the effects of the interaction between Pt-based bimetallic clustersboth gas-phase and supportedwith small adsorbates such as O 2 , CO, CO 2 , ethylene, and ethanol, for example. − Some of these previous studies have remarked the importance of the putative global minimum structure and low-lying isomers exhibiting structural changes in both, the cluster and molecule, the former showing structural interconversion and the latter, the existence of molecular activation or even dissociation. Despite many other examples which can be found in the literature regarding the computational modeling of bimetallic clusters, combined with the experimental work carried out on Re–Pt systems over the years, it is worth noticing that just a few DFT-based theoretical studies have actually dealt with rhenium clusters.…”