How the hydrogen sorption properties of palladium are modified through interaction with iridium

Abstract

Hydrogen sorption (adsorption/absorption) in metals, in the form of thin films or nanoparticles, is a key process in the fields of energy storage and heterogeneous catalysis. Atomic hydrogen dissolved in the subsurface of a metal affects its surface atomic and electronic structures, and thereby its surface reactivity and catalytic properties. In addition, alloy effects modify both catalytic and hydrogen sorption phenomena. In order to rationalize recent experimental results showing the negative impact of hydrogen absorption on catalysis, the present article proposes an insight into structure-reactivity relationships through computational simulations, using density functional theory, of hydrogen sorption in the near-surface region of palladium atomic layers interacting with an iridium substrate. A detailed analysis of the electronic structure using local projected densities of states (PDOS) and crystal orbital overlap population (COOP) curves was carried out. It is found that the Pd/Ir system, with respect to pure Pd surfaces, keeps acceptable adsorption properties for surface reactions while preventing hydrogen penetration. The results of electronic structure calculations show that the most important difference between Pd and Ir is related to the strong anti-bonding character of the 1s-H/5p-Ir interaction, leading to the non-bonding character of the sp-Ir interaction with hydrogen. Thus, increasing the Ir concentration in a Pd-based system increases the anti-bonding contribution, which strongly weakens the overall metal-hydrogen interaction.

Restructuring effects of the chemical environment in metal nanocatalysis and single-atom catalysis

“…Beyond downsizing, nanoalloying is an additional way of boosting the hydrogen absorption capacity, as shown for Pd-Pt and Ag-Rh systems [160]. In contrast, we have found that nanoalloying Pd with Ir strongly decreases the hydrogen absorption capacity of Pd and suppresses β hydriding [161,162] through the formation of a core-shell Ir@Pd structure (Figure 7, left-hand side) [163,164]. The decreased hydrogen capacity is correlated to a strongly increased preferential CO oxidation activity in the presence of H 2 (PROX process) with respect to pure Pd and Ir counterparts [161,165].…”

Synthesis, Structure, Properties, and Applications of Bimetallic Nanoparticles of Noble Metals

“…[ 84 ] A DFT analysis in the Pd‐Ir system was performed to describe the hydrogen‐metal interaction which was weakened by the presence of iridium. [ 85 ] The influence of the particle structure and internal elemental distribution was approached both experimentally and theoretically (by DFT simulations) on Pt‐Au nanoparticles (7 nm), and compositions with a maximum catalytic efficiency were identified on the basis of the surface atom arrangement. [ 86 ] The strain inside nanoparticles was decisive for the catalytic performance in electrocatalysis.…”

Synthesis of carbon-supported bimetallic palladium–iridium catalysts by microemulsion: characterization and electrocatalytic properties

“…Thus, hydrogen bonding is weaker in our bimetallic PdIr/C catalysts than in the Pd/C. The recent DFT studies found a significant weakening of the strength of hydrogen bonding in the model system consisting of the Pd atomic layer deposited on the Ir(1 1 1) substrate ascribed to a downshift of the palladium d-band center due to the Ir-species [62]. For the pseudomorphic Pd monolayer on the crystalline Ir substrate, Kibller et al [64] measured negatively shifted hydrogen desorption peak potential by ca.…”

“…Computational studies [61][62][63] have predicted that a Pd overlayer or Pd alloyed into the first layer of Ir(1 1 1) will show a decreased d-band center energy by ca. 0.4-0.73 eV, relative to pure Pd, which resulted in the experimentally determined weaker adsorption strength of small molecules like CO, H 2 [64].…”