Gihan Kwon

Size-dependent subnanometer Pd cluster (Pd4, Pd6, and Pd17) water oxidation electrocatalysis

Water oxidation is a key catalytic step for electrical fuel generation. Recently, significant progress has been made in synthesizing electrocatalytic materials with reduced overpotentials and increased turnover rates, both key parameters enabling commercial use in electrolysis or solar to fuels applications. The complexity of both the catalytic materials and the water oxidation reaction makes understanding the catalytic site critical to improving the process. Here we study water oxidation in alkaline conditions using size-selected clusters of Pd to probe the relationship between cluster size and the water oxidation reaction. We find that Pd4 shows no reaction, while Pd6 and Pd17 deposited clusters are among the most active (in terms of turnover rate per Pd atom) catalysts known. Theoretical calculations suggest that this striking difference may be a demonstration that bridging Pd-Pd sites (which are only present in three-dimensional clusters) are active for the oxygen evolution reaction in Pd6O6. The ability to experimentally synthesize size-specific clusters allows direct comparison to this theory. The support electrode for these investigations is ultrananocrystalline diamond (UNCD). This material is thin enough to be electrically conducting and is chemically/electrochemically very stable. Even under the harsh experimental conditions (basic, high potential) typically employed for water oxidation catalysts, UNCD demonstrates a very wide potential electrochemical working window and shows only minor evidence of reaction. The system (soft-landed Pd4, Pd6, or Pd17 clusters on a UNCD Si-coated electrode) shows stable electrochemical potentials over several cycles, and synchrotron studies of the electrodes show no evidence for evolution or dissolution of either the electrode material or the clusters.

Solution Structures of Highly Active Molecular Ir Water-Oxidation Catalysts from Density Functional Theory Combined with High-Energy X-ray Scattering and EXAFS Spectroscopy

The solution structures of highly active Ir water-oxidation catalysts are elucidated by combining density functional theory, high-energy X-ray scattering (HEXS), and extended X-ray absorption fine structure (EXAFS) spectroscopy. We find that the catalysts are Ir dimers with mono-μ-O cores and terminal anionic ligands, generated in situ through partial oxidation of a common catalyst precursor. The proposed structures are supported by (1)H and (17)O NMR, EPR, resonance Raman and UV-vis spectra, electrophoresis, etc. Our findings are particularly valuable to understand the mechanism of water oxidation by highly reactive Ir catalysts. Importantly, our DFT-EXAFS-HEXS methodology provides a new in situ technique for characterization of active species in catalytic systems.

Size- and support-dependent evolution of the oxidation state and structure by oxidation of subnanometer cobalt clusters.

Size-selected subnanometer cobalt clusters with 4, 7, and 27 cobalt atoms supported on amorphous alumina and ultrananocrystalline diamond (UNCD) surfaces were oxidized after exposure to ambient air. Grazing incidence X-ray absorption near-edge spectroscopy (GIXANES) and near-edge X-ray absorption fine structure (NEXAFS) were used to characterize the clusters revealed a strong dependency of the oxidation state and structure of the clusters on the surface. A dominant Co(2+) phase was identified in all samples. However, XANES analysis of cobalt clusters on UNCD showed that ∼10% fraction of a Co(0) phase was identified for all three cluster sizes and about 30 and 12% fraction of a Co(3+) phase in 4, 7, and 27 atom clusters, respectively. In the alumina-supported clusters, the dominating Co(2+) component was attributed to a cobalt aluminate, indicative of a very strong binding to the support. NEXAFS showed that in addition to strong binding of the clusters to alumina, their structure to a great extent follows the tetrahedral morphology of the support. All supported clusters were found to be resistant to agglomeration when exposed to reactive gases at elevated temperatures and atmospheric pressure.

Catalytic oxidation of cyclohexane by size-selected palladium clusters pinned on graphite

We report the catalytic oxidation of cyclohexane to CO and CO2 over size-selected palladium clusters (Pd N clusters, N = 10–120) supported on graphite as a function of cluster size. The stability of the pinned clusters (nanoparticles) under reaction conditions is investigated by scanning tunnelling microscopy measurement both before and after reaction. Temperature-programmed reaction experiments at 800 Torr show that the turnover rates (per surface Pd atom) for both CO and CO2 increase significantly as cluster size decreases and correlate with the number of Pd perimeter atoms at the graphite interface. Under oxygen-rich conditions, the activity of the clusters increases by a factor of 3 while the product ratio CO:CO2 rises by an order of magnitude.

Enhancement of Activity and Durability through Cr Doping of TiO2 Supports in Pt Electrocatalysts for Oxygen Reduction Reactions

A challenging issue in the commercialization of fuel cells is to improve the kinetics of the sluggish oxygen reduction reaction (ORR) and durability of the cathode electrocatalyst under corrosive ORR conditions. In this paper, we report a promising approach to address these two major issues by Cr doping of TiO2 supports in Pt‐based electrocatalysts. It was clearly revealed that Cr doping led to a marked enhancement of ORR kinetics, which was attributed to the compressive strain in the Pt lattice as well as the increased electronic conductivity of the Cr–TiO2 supports. Furthermore, Pt/Cr–TiO2 demonstrated a far superior durability to that of conventional Pt/C, which was assessed by accelerated durability tests (ADT) and in situ X‐ray absorption near‐edge structure studies. The specific activity of Pt/C decreased by 43 % after the ADT (141 μA cm−2 and 82 μA cm−2 before and after ADT, respectively), whereas that of Pt/Cr–TiO2 was merely reduced by 13 % (472 μA cm−2 and 409 μA cm−2).

Water Oxidation by Size-Selected Co27 Clusters Supported on Fe2O3

The complexity of the water oxidation reaction makes understanding the role of individual catalytic sites critical to improving the process. Here, size-selected 27-atom cobalt clusters (Co27 ) deposited on hematite (Fe2 O3 ) anodes were tested for water oxidation activity. The uniformity of these anodes allows measurement of the activity of catalytic sites of well-defined nuclearity and known density. Grazing incidence X-ray absorption near-edge spectroscopy (GIXANES) characterization of the anodes before and after electrochemical cycling demonstrates that these Co27 clusters are stable to dissolution even in the harsh water oxidation electrochemical environment. They are also stable under illumination at the equivalent of 0.4 suns irradiation. The clusters show turnover rates for water oxidation that are comparable or higher than those reported for Pd- and Co-based materials or for hematite. The support for the Co27 clusters is Fe2 O3 grown by atomic layer deposition on a Si chip. We have chosen to deposit a Fe2 O3 layer that is only a few unit cells thick (2 nm), to remove complications related to exciton diffusion. We find that the electrocatalytic and the photoelectrocatalytic activity of the Co27 /Fe2 O3 material is significantly improved when the samples are annealed (with the clusters already deposited). Given that the support is thin and that the cluster deposition density is equivalent to approximately 5 % of an atomic monolayer, we suggest that annealing may significantly improve the exciton diffusion from the support to the catalytic moiety.

Oxyanion induced variations in domain structure for amorphous cobalt oxide oxygen evolving catalysts, resolved by X-ray pair distribution function analysis

Amorphous thin-film oxygen evolving catalysts, OECs, of first-row transition metals show promise to serve as self-assembling materials in solar-driven, photoelectrochemical ‘artificial leaf’ devices. This report demonstrates the ability to use high-energy X-ray scattering and atomic pair distribution function analyses, PDF, to resolve structure in amorphous metal oxide catalysts films, and is applied here to resolve domain structure differences induced by oxyanion substitution during the electrochemical assembly of amorphous cobalt oxide catalyst films.

Resolution of Electronic and Structural Factors Underlying Oxygen-Evolving Performance in Amorphous Cobalt Oxide Catalysts

Non-noble-metal, thin-film oxides are widely investigated as promising catalysts for oxygen evolution reactions (OER). Amorphous cobalt oxide films electrochemically formed in the presence of borate (CoBi) and phosphate (CoPi) share a common cobaltate domain building block, but differ significantly in OER performance that derives from different electron-proton charge transport properties. Here, we use a combination of L edge synchrotron X-ray absorption (XAS), resonant X-ray emission (RXES), resonant inelastic X-ray scattering (RIXS), resonant Raman (RR) scattering, and high-energy X-ray pair distribution function (PDF) analyses that identify electronic and structural factors correlated to the charge transport differences for CoPi and CoBi. The analyses show that CoBi is composed primarily of cobalt in octahedral coordination, whereas CoPi contains approximately 17% tetrahedral Co(II), with the remainder in octahedral coordination. Oxygen-mediated 4 p-3 d hybridization through Co-O-Co bonding was detected by RXES and the intersite dd excitation was observed by RIXS in CoBi, but not in CoPi. RR shows that CoBi resembles a disordered layered LiCoO2-like structure, whereas CoPi is amorphous. Distinct domain models in the nanometer range for CoBi and CoPi have been proposed on the basis of the PDF analysis coupled to XAS data. The observed differences provide information on electronic and structural factors that enhance oxygen evolving catalysis performance.