Revisiting the activity of Ru-hcp(001) and Ru-fcc(111) for alkaline HER: A DFT study

We present a comprehensive density-functional-theory (DFT) study of the water dissociation reaction on Ru-hcp(001) and Ru-fcc(111). The motivation stems from experiments showing that supported fcc-structured Ru nanoparticles outperform hcp-structured ones as catalysts for the hydrogen evolution reaction (HER) in alkaline media, as well as from prior combined experimental and computational work (Zheng et al., 2016) attributing this difference to intrinsic activity of the two facets derived from DFT calculations. By systematically varying slab thickness, supercell size, and water models, and by rigorously evaluating vibrational contributions to the Gibbs energy, we demonstrate that reaction Gibbs energies strongly depend on model details, whereas activation Gibbs energies are less sensitive. Importantly, when vibrational contributions are treated rigorously, water dissociation is predicted to exhibit slightly lower activation Gibbs energy on Ru-hcp(001), suggesting that it should be somewhat more active than Ru-fcc(111). This finding contrasts with the earlier aforementioned study, whose reported data reveal that Ru-fcc(111) is more active due to favorable vibrational contributions. This discrepancy stems primarily from the incomplete treatment of vibrational contributions in the earlier work and from the omission of corrections for low-frequency modes. Our results highlight the need to consider additional factors—such as support effects and the relative abundance and intrinsic activity of different facets present on Ru nanoparticles—to explain the experimentally observed superior alkaline HER activity of fcc-structured Ru nanoparticles. They further underscore the critical role of accurately evaluating vibrational Gibbs energy contributions in computational heterogeneous catalysis. • Activity of Ru-hcp(001) and Ru-fcc(111) for alkaline HER evaluated by DFT. • Reaction Gibbs energies are more sensitive than activation Gibbs barriers to model details. • Rigorous vibrational analysis shows Ru-hcp(001) slightly more active than Ru-fcc(111). • This finding contrasts with DFT conclusions from an earlier experimental–DFT study. • Superiority of Ru-fcc cannot be explained by the activity of close-packed Ru-hcp(001) and Ru-fcc(111).