We report the size-dependent activity and stability of FTO-supported Pt1,4,7,8 for electrocatalytic hydrogen evolution reaction, and show that clusters are significantly more active than polycrystalline Pt, but also have stability under HER conditions that is size-dependent. To understand the mechanistic origin of the size effects, we carried out detailed DFT-based theory, accounting for the structural fluxionality under varying potentials. We show that, even under coverage of the simplest adsorbate H, the clusters can undergo drastic changes and populate a grand canonical ensemble of hydride states with diverse distributions of stoichiometry, structure, and thus reactivity. Both experiment and theory find that electrocatalysis mainly results from heavily hydrogenated states of the clusters (~2 H/Pt), and an ensemble-based kinetic model reproduces the experimental activity trend and provides detailed insights into the contributions from metastable configurations. The size-dependent stability trend is rationalized by chemical bonding analysis. The comparison of experiment and theory demonstrates the potential- and adsorbate-coverage-dependent fluxionality of subnano clusters of different sizes and offers a systematic modeling strategy to tackle the complexities.

A combination of density functional theory (DFT) and experiments with atomically size-selected Pt_n clusters deposited on indium-tin oxide (ITO) electrodes was used to examine the effects of applied potential and Pt_n size on the electrocatalytic activity of Pt_n (n = 1, 4, 7, and 8) for the hydrogen evolution reaction (HER). Activity is found to be negligible for isolated Pt atoms on ITO, increasing rapidly with Pt_n size such that Pt₇/ITO and Pt₈/ITO have roughly double the activity per Pt atom compared to atoms in the surface layer of polycrystalline Pt. Both the DFT and experiment find that hydrogen under-potential deposition (H_upd) results in Pt_n/ITO (n = 4, 7, and 8) adsorbing ∼2H atoms/Pt atom at the HER threshold potential, equal to ca. double the H_upd observed for Pt bulk or nanoparticles. The cluster catalysts under electrocatalytic conditions are hence best described as a Pt hydride compound, significantly departing from a metallic Pt cluster. The exception is Pt₁/ITO, where H adsorption at the HER threshold potential is energetically unfavorable. The theory combines global optimization with grand canonical approaches for the influence of potential, uncovering the fact that several metastable structures contribute to the HER, changing with the applied potential. It is hence critical to include reactions of the ensemble of energetically accessible Pt_nH_x/ITO structures to correctly predict the activity vs Pt_n size and applied potential. For the small clusters, spillover of H_ads from the clusters to the ITO support is significant, resulting in a competing channel for loss of H_ads, particularly at slow potential scan rates.

We report the size-dependent activity and stability of supported Pt_1,4,7,8 for electrocatalytic hydrogen evolution reaction, and show that clusters outperform polycrystalline Pt in activity, with size-dependent stability. To understand the size effects, we use DFT calculations to study the structural fluxionality under varying potentials. We show that the clusters can reshape under H coverage and populate an ensemble of states with diverse stoichiometry, structure, and thus reactivity. Both experiment and theory suggest that electrocatalytic species are hydridic states of the clusters (≈2 H/Pt). An ensemble-based kinetic model reproduces the experimental activity trend and reveals the role of metastable states. The stability trend is rationalized by chemical bonding analysis. Our joint study demonstrates the potential- and adsorbate-coverage-dependent fluxionality of subnano clusters of different sizes and offers a systematic modeling strategy to tackle the complexities.

A combination of density functional theory (DFT) and experiments with atomically size-selected Ptn clusters deposited on indium-tin oxide (ITO) electrodes was used to examine the effects of applied potential and Ptn size on the electrocatalytic activity of Ptn (n = 1, 4, 7, 8) for the hydrogen evolution reaction (HER). Activity is found to be negligible for isolated Pt atoms on ITO, increasing rapidly with Ptn size, such that Pt7/ITO and Pt8/ITO have roughly double the activity per Pt atom compared atoms in the surface layer of polycrystalline Pt. Both DFT and experiment find that hydrogen under-potential deposition (Hupd) results in Ptn/ITO (n = 4, 7, 8) adsorbing ~2 H atoms/Pt atom at the HER threshold potential, equal to ca. double the Hupd observed for bulk Pt or Pt nanoparticles. The cluster catalysts under electrocatalytic conditions are hence best described as a Pt hydride compound, significantly departing from a metallic Pt cluster. The exception is Pt1/ITO, where H adsorption at the HER threshold potential is energetically unfavorable. Theory combines global optimization with grand canonical approaches for the influence of potential, uncovering that several metastable structures contribute to HER, changing with the applied potential. It is hence critical to include reactions of the ensemble of energetically accessible PtnHx/ITO structures to correctly predict the activity vs. Ptn size and applied potential. For the small clusters, spillover of Hads from the clusters to the ITO support is significant, resulting in a competing channel for loss of Hads, particularly at slow potential scan rates.

Supported sub-nano clusters hold great promise as economical and highly active catalysts. However, they tend to deactivate rapidly by poisoning and sintering, impeding their widespread use. We find that self-limiting poisoning can stabilize and promote cluster catalysis, i.e., poisoning is not always detrimental, but can sometimes be exploited. Specifically, Pt-Ge alloy clusters supported on alumina undergo slow coking (carbon deposition) under conditions of thermal dehydrogenation, yet preserve strong binding sites. For the case of Pt4Ge/alumina, theory shows a number of thermally populated isomers, one of which catalyzes carbon deposition. Because the clusters are fluxional at high temperatures, this isomer acts as a gateway, slowly converting all the clusters to Pt4GeC2. The surprising result is that Pt4GeC2 is highly catalytically active and selective against further coking, i.e., coking produces functional, stable catalytic clusters. Ge and C2 have synergistic electronic effects, leading to efficient and highly selective catalytic dehydrogenation that stops at alkenes, and improving stability. Thus, under reaction conditions, the clusters develop into a robust catalyst, suggesting an approach to practicable cluster catalysis.

Supported sub-nano clusters hold great promise as economical and highly active catalysts. However, they tend to deactivate rapidly by poisoning and sintering, impeding their widespread use. We find that self-limiting poisoning can stabilize and promote cluster catalysis, that is, poisoning is not always detrimental but can sometimes be exploited. Specifically, Pt-Ge alloy clusters supported on alumina undergo slow and self-limiting coking (carbon deposition) under conditions of thermal dehydrogenation, modifying the cluster framework and electronic properties but preserving the Pt sites required for strong ethylene binding. For the case of Pt4Ge/alumina, theory shows a number of thermally populated isomers, one of which catalyzes carbon deposition. Because the clusters are fluxional at high temperatures, this isomer acts as a gateway, slowly converting all clusters to Pt4GeC2. The surprising result is that Pt4GeC2 is highly catalytically active and selective against further coking, that is, coking produces functional, stable catalytic clusters. Ge and C2 have synergistic electronic effects, leading to efficient and highly selective catalytic dehydrogenation that stops at alkenes and improving stability. Thus, under reaction conditions, the clusters develop into a robust catalyst, suggesting an approach to practicable cluster catalysis.