Kinetic Isolation between Turnovers on Au₁₈ Nanoclusters: Formic Acid Decomposition One Molecule at a Time

Abstract

Formic acid (HCOOH or FA) is a clean, safe, and renewable hydrogen storage material. Although Au catalysts decompose vapor-phase FA with high activity and selectivity towards hydrogen, the active site and reaction mechanism remain unclear. Here, we show that the subnanometric Au18 cluster (0.8 nm in diameter) is likely the active species for FA decomposition. We performed coverage self-consistent, density functional theory-based kinetic Monte Carlo simulations of FA decomposition on gas-phase Au18 clusters, predicting 100% selectivity towards hydrogen and turnover frequencies close to experimentally determined values. The active site is made up of a triangular ensemble of three atoms each possessing a coordination number of 5. Although there are two active site ensembles on the Au18 cluster, their occupations are strongly correlated due to strong, stabilizing, interactions between pairs of open-shell adsorbates mediated by the superatomic nature of the cluster. Because the occupation of the active sites blocks the dissociation of additional HCOOH molecules, there is kinetic isolation between turnovers: only one HCOOH molecule can dissociate on the cluster at a time. This explains the extraordinary, experimentally-2 observed selectivity of Au catalysts towards HD during decomposition of HCOOD and DCOOH. Our work offers nanoscale insights into the reaction mechanisms of FA decomposition over Au. To our knowledge, this represents the first example of heterogeneous catalysis by a cluster that catalyzes reactants one molecule at a time. Our work on Au18 thus sheds light on how the unique electronic properties of subnanometric clusters can be used to design quasi-molecular catalysts with high activity and selectivity.

Zeolite‐Encaged Pd–Mn Nanocatalysts for CO2 Hydrogenation and Formic Acid Dehydrogenation

“…The other carboxyl (COOH) pathway leads to the formation of CO, [19] which is against the experimental observations. All energetics were evaluated at infinite separation; although previous studies showed that lateral interactions between adsorbates may significantly affect the energetics of FA decomposition on small clusters, [20] coadsorption of the intermediates does not significantly impact the trends identified in our work (Figure S29). The resulting potential energy diagrams are shown in Figure 7 c; energies of the studied intermediates and transition states are tabulated in Table S5.…”

Zeolite‐Encaged Pd–Mn Nanocatalysts for CO2 Hydrogenation and Formic Acid Dehydrogenation

“…Next, we probed the reaction energetics of FA decomposition over both Pd 13 and Pd 8 Mn 5 O(OH) 13 .Out of the two main FA decomposition pathways,w eo nly considered the formate (HCOO À )p athway.T he other carboxyl (COOH) pathway leads to the formation of CO, [19] which is against the experimental observations.A ll energetics were evaluated at infinite separation;a lthough previous studies showed that lateral interactions between adsorbates may significantly affect the energetics of FA decomposition on small clusters, [20] coadsorption of the intermediates does not significantly impact the trends identified in our work ( Figure S29). The resulting potential energy diagrams are shown in Figure 7c; energies of the studied intermediates and transition states are tabulated in Table S5.…”

Combining Computational Modeling with Reaction Kinetics Experiments for Elucidating the In Situ Nature of the Active Site in Catalysis

“… ChenB. W. J.StamatakisM.MavrikakisM. Chen, B. W. J. Stamatakis, M. Mavrikakis, M. 2019994469457 …”

“…Combination of DFT-derived mean-field MKMs and reaction kinetics experiments on Au/SiC, led to the conclusion that Au 18 might resemble the active site the best. Kinetic Monte Carlo (KMC) study on Au 18 revealed that a pair of adjacent Au 3 ensembles (coordination number (CN) of 5), can be the active site for FA decomposition on Au/SiC (ref ).…”

“…Here, we provide an account of the methodology developed in our group for formulating high-fidelity MKMs. − ,,− Specifically, we present our algorithmic approach for identifying active sites and reaction mechanisms in the context of the formic acid (FA) decomposition reaction (Scheme ), a chemistry which we have studied extensively, both computationally and experimentally, on supported Au − and Pt catalysts (details regarding our assumptions and reaction engineering aspects of our MKM can be found in the corresponding references). Through these examples, we strive to convey the advantages of a systematic and integrated approach of combining DFT calculations, reaction kinetics experiments, and microkinetic modeling to iteratively refine the atomic-scale picture of the active site and the reaction mechanism.…”