Atomically precise, inherently charged Au(25) clusters are an exciting prospect for promoting catalytically challenging reactions, and we have studied the interaction between CO(2) and Au(25). Experimental results indicate a reversible Au(25)-CO(2) interaction that produced spectroscopic and electrochemical changes similar to those seen with cluster oxidation. Density functional theory (DFT) modeling indicates these changes stem from a CO(2)-induced redistribution of charge within the cluster. Identification of this spontaneous coupling led to the application of Au(25) as a catalyst for the electrochemical reduction of CO(2) in aqueous media. Au(25) promoted the CO(2) → CO reaction within 90 mV of the formal potential (thermodynamic limit), representing an approximate 200-300 mV improvement over larger Au nanoparticles and bulk Au. Peak CO(2) conversion occurred at -1 V (vs RHE) with approximately 100% efficiency and a rate 7-700 times higher than that for larger Au catalysts and 10-100 times higher than those for current state-of-the-art processes.

No abstract

Exposure of Pd-based hydrogen purification membranes to H,S. a common contaminant in coal gasification streams, can cause membrane performance to deteriorate, either by deactivating surface sites required for dissociative H, adsorption or by forming a low-permeability sulfide scale. In this work. the composition, structure, and catalytic activity of Pd4S, a surface scale commonly observed in Pd-membrane separation of hydrogen from sulfur-containing gas streams, were examined using a combination of experimental characterization and density functional theory (DFT) calculations. A Pd,S sample was prepared by exposing a 100 f1m Pd foil to H2S at 908 K. Both X-ray photoemission depth profiling and low energy ion scattering spectroscopic (LEISS) analysis reveal slight sulfur-enrichment of the top surface of the sample. This view is consistent with the predictions of DFT atomistic thermodynamic calculations. which identified S-terminated Pd,S surfaces as energetically favored over corresponding Pd-terminated surfaces. Activation barriers for H2 dissociation on the Pd,S surfaces were calculated. Although barriers are higher than on Pd(lll). transition state theory analysis identified reaction pathways on the S-terminated surfaces for which hydrogen dissociation rates are high enough to sustain the separation process at conditions relevant to gasification applications.

Atomically precise gold nanoclusters provide opportunities for correlating the structure and electrocatalytic properties at the atomic level. Here, we report the single-atom doping effect on CO2 reduction by comparing monopalladium-doped Pd1Au24 and homogold Au25 nanoclusters (both protected by thiolates) that share an identical core structure. Experimental results show that single Pd-substitution drastically inhibits H2 evolution at large currents; thus, Pd1Au24 can convert CO2 to CO with ∼100% faradaic efficiency ranging from −0.6 (onset) to −1.2 V (vs RHE), while Au25 starts to decline at −0.9 V. Theoretical simulations reveal that the Pd dopant influences the Au nanocluster properties through a unique mechanism different from that in conventional alloy nanoparticles. The surface S atoms of the thiolate ligand are identified as the active sites (with the Au13 core as the electron reservoir) for selective CO2 reduction, whereas undercoordinated Au atom active sites are predicted to favor H2 evolution. Thermodynamic analysis of the ligand removal process predicts that Pd1Au24 should retain a larger population of S atom active sites under cathodic potentials compared with Au25, which extends the potential range for selective CO2 reduction. Our results demonstrate that single-atom substitution can substantially improve the CO2 reduction selectivity of gold nanoclusters at large potentials. The dopant-induced ligand stability may serve as a design strategy to modify the stability of catalytic active sites under harsh conditions.

Thiolate‐protected gold nanoclusters (NCs) are promising catalytic materials for the electrochemical CO2 reduction reaction (CO2RR). In this work an atomic level modification of a Au23 NC is made by substituting two surface Au atoms with two Cd atoms, and it enhances the CO2RR selectivity to 90–95 % at the applied potential between −0.5 to −0.9 V, which is doubled compared to that of the undoped Au23. Additionally, the Cd‐doped Au19Cd2 exhibits the highest CO2RR activity (2200 mA mg−1 at −1.0 V vs. RHE) among the reported NCs. This synergetic effect between Au and Cd is remarkable. Density‐functional theory calculations reveal that the exposure of a sulfur active site upon partial ligand removal provides an energetically feasible CO2RR pathway. The thermodynamic energy barrier for CO formation is 0.74 eV lower on Au19Cd2 than on Au23. These results reveal that Cd doping can boost the CO2RR performance of Au NCs by modifying the surface geometry and electronic structure, which further changes the intermediate binding energy. This work offers insights into the surface doping mechanism of the CO2RR and bimetallic synergism.

Mitigating carbon dioxide (CO2) emissions is one of today's most important scientific challenges. Electrochemical conversion of CO2 into industrially relevant chemicals is a leading strategy because it would allow sustainable production of commodity chemicals. In this review, we outline the current progress in nanostructuring gold, copper, and alloy catalysts for the electrochemical CO2 reduction reaction. In general, Au catalysts show structure and voltage dependent CO selectivity alongside the H2 evolution reaction. The ability to tune CO to H2 product distributions is appealing for downstream processing into a variety of industrially relevant chemicals. Cu catalysts produce a wider range of products, and current efforts focus on controlling the product distribution by tuning the catalyst size, structure, oxidation state, and crystallographic orientation. Finally, we discuss the emerging field of computational electrocatalysis with emphasis on the computational hydrogen electrode method. The combination of experiment and computation is important because it provides fundamental insight into chemical processes driving catalytic CO2 conversion. Continued work will help to tune catalyst structure and create next‐generation materials with high catalytic activity and desirable product selectivity.

The electrochemical oxygen evolution reaction (OER) is an important anodic process in water splitting and CO2 reduction applications. Precious metals including Ir, Ru. and Pt are traditional OER catalysts, but recent emphasis has been placed on finding less expensive, earth-abundant materials with high OER activity. Ni-based materials are promising next-generation OER catalysts because they show high reaction rates and good long-term stability. Unfortunately, most catalyst samples contain heterogeneous particle sizes and surface structures that produce a range of reaction rates and rate-determining steps. Here we use a combination of experimental and computational techniques to study the OER at a supported organometallic nickel complex with a precisely known crystal structure. The Ni6(PET)12 (PET = phenylethyl thiol) complex out performed bulk NiO and Pt and showed OER activity comparable to Ir. Density functional theory (DFT) analysis of electrochemical OER at a realistic Ni6(SCH3)12 model determined the Gibbs free energy change (ΔG) associated with each mechanistic step. This allowed computational prediction of potential determining steps and OER onset potentials that were in excellent agreement with experimentally determined values. Moreover, DFT found that small changes in adsorbate binding configuration can shift the potential determining step within the OER mechanism and drastically change onset potentials. Our work shows that atomically precise nanocatalysts like Ni6(PET)12 facilitate joint experimental and computational studies because experimentalists and theorists can study nearly identical systems. These types of efforts can identify atomic-level structure–property relationships that would be difficult to obtain with traditional heterogeneous catalyst samples.

Recent synthetic advances have produced very small (sub-2 nm), ligand-protected mixed-metal clusters. Realization of such clusters allows the investigation of fundamental questions: (1) Will heteroatoms occupy specific sites within the cluster? (2) How will the inclusion of heteroatoms affect the electronic structure and chemical properties of the cluster? (3) How will these very small mixed-metal systems differ from larger, more traditional alloy materials? In this report we provide experimental and computational characterization of the ligand-protected mixed-metal Au 25−x Ag x (SC 2 H 4 Ph) 18 cluster (abbreviated as Au 25−x Ag x , where x = 0−5 Ag atoms) compared with the unsubstituted Au 25 (SC 2 H 4 Ph) 18 cluster (abbreviated as Au 25 ). Density functional theory analysis has predicted that Ag heteroatoms will preferentially occupy sites on the surface of the cluster core. X-ray photoelectron spectroscopy revealed Au−Ag state mixing and charge redistribution within the Au 25−x Ag x cluster. Optical spectroscopy and nonaqueous electrochemistry indicate that Ag heteroatoms increased the cluster lowest unoccupied molecular orbital (LUMO) energy, introduced new features in the Au 25−x Ag x absorbance spectrum, and rendered some optical transitions forbidden. In situ spectroelectrochemical experiments revealed charge-dependent Au 25−x Ag x optical properties and oxidative photoluminescence quenching. Finally, O 2 adsorption studies have shown Au 25−x Ag x clusters can participate in photomediated charge-transfer events. These results illustrate that traditional alloy concepts like metal-centered state mixing and internal charge redistribution also occur in very small mixed-metal clusters. However, resolution of specific heteroatom locations and their impact on the cluster's quantized electronic structure will require a combination of computational modeling, optical spectroscopy, and nonaqueous electrochemistry.