Density functional study of hydrogen adsorption on Pt-Ni nanoclusters
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Elektronirakenteen tiheysfunktionaaliteoriaan perustuvat simulaatiot vedyn adsorptiolle Pt-Ni nanoklustereiden pinnoilla
The development of affordable and earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) has attracted great attention in recent years. The effective production of molecular hydrogen is of crucial importance, because it could provide a practical solution to the energy storage problem of renewable energy. The platinum group metals (PGMs) are the best HER catalysts, but their large-scale applicability is limited by their high cost, scarcity, and high supply risk. Therefore, finding alternative HER catalysts or reducing the amount of PGMs in catalysts is essential. In the present work, density functional theory calculations were performed on small 55-atom Pt-Ni nanoclusters to study their activity towards HER. Two compositions, Pt12Ni43 and Pt20Ni35, were studied in the gas phase and on a gamma-Al2O3 support. The critical hydrogen coverages were determined using the method based on the differential free energy of hydrogen adsorption. The free energy of hydrogen adsorption was used as a descriptor of the HER activity. This descriptor has been successful in predicting the HER activity of various non-noble materials, such as molybdenum disulfide (MoS2) edges and transition metal phosphides. The Pt12Ni43 and Pt20Ni35 clusters are icosahedral in the gas phase and they were observed to retain their shape when adsorbed on the alumina support. The calculated excess energies indicate that the formation of the bimetallic clusters is energetically favorable. Bader charge analysis revealed that the support donates electron density to the clusters. Ni3 hollow sites were discovered to be strongly binding sites for hydrogen. The clusters adsorb approximately four H atoms per facet before the adsorption becomes endergonic, therefore the critical hydrogen coverages on the clusters are high. The obtained descriptor values are smaller than 0.1 eV in the gas phase and on the support, suggesting that the clusters are catalytically active towards HER.