Methanol Tolerant Oxygen Reduction Reaction electrocatalysis using Size-Specific Triphenylphosphine-Ligated Gold Nanoclusters

Version of record online: June 7, 2022The development of a stable and efficient Oxygen Reduction Reaction (ORR) electrocatalysts with high methanol tolerance is crucial for Direct Methanol Fuel Cells (DMFCs). Herein, triphenylphosphine (PPh3)-ligated gold nanoclusters and complexes (AuNCs), Au101(PPh3)21Cl5, Au9(PPh3)8(NO3)3, and Au1(PPh3)Cl supported on reduced graphene oxide (rGO) have been explored as methanol tolerant ORR electrocatalysts. Electrocatalytic performance of each AuNCs-rGO was determined through linear sweep voltammograms (LSV) and cyclic voltammetry (CV) and compared with Pt/C. Size-dependent ORR activity was observed which followed the size trend of Au101NC-rGO>Au9NC-rGO>Au1NC-rGO. Repeated LSV and chronoamperometry measurements revealed that the long-term stability over 24 hours followed the trend Au101NC-rGO>Au9NC-rGO∼Au1NC-rGO and all were more stable than Pt/C. The methanol tolerance of each AuNCs-rGO was also evaluated via LSV and CV. Size-independent methanol tolerance with no noticeable change in ORR performance of AuNCs-rGO was observed in the presence of methanol. AuNCs-rGO nanocomposites are promising cathode electrocatalysts for DMFCs.Hanieh Mousavi, Shailendra K. Sharma, Vladimir Golovko, Cameron J. Shearer, and Gregory F. Meth

Graphene Bridge for Photocatalytic Hydrogen Evolution with Gold Nanocluster Co-Catalysts

Published: 17 October 2022Herein, the UV light photocatalytic activity of an Au101NC-AlSrTiO3-rGO nanocomposite comprising 1 wt% rGO, 0.05 wt% Au101(PPh3)21Cl5 (Au101NC), and AlSrTiO3 evaluated for H2 production. The synthesis of Au101NC-AlSrTiO3-rGO nanocomposite followed two distinct routes: (1) Au101NC was first mixed with AlSrTiO3 followed by the addition of rGO (Au101NC-AlSrTiO3:rGO) and (2) Au101NC was first mixed with rGO followed by the addition of AlSrTiO3 (Au101NC-rGO:AlSrTiO3). Both prepared samples were annealed in air at 210 °C for 15 min. Inductively coupled plasma mass spectrometry and high-resolution scanning transmission electron microscopy showed that the Au101NC adhered almost exclusively to the rGO in the nanocomposite and maintained a size less than 2 nm. Under UV light irradiation, the Au101NC-AlSrTiO3:rGO nanocomposite produced H2 at a rate 12 times greater than Au101NC-AlSrTiO3 and 64 times greater than AlSrTiO3. The enhanced photocatalytic activity is attributed to the small particle size and high loading of Au101NC, which is achieved by non-covalent binding to rGO. These results show that significant improvements can be made to AlSrTiO3-based photocatalysts that use cluster co-catalysts by the addition of rGO as an electron mediator to achieve high cluster loading and limited agglomeration of the clusters.Hanieh Mousavi, Thomas D. Small, Shailendra K. Sharma, Vladimir B. Golovko, Cameron J. Shearer and Gregory F. Meth

Investigating Pt atom addition to gas phase Ce(2)O(n) clusters using photoionization efficiency spectroscopy and density functional theory

Photoionization efficiency spectra for PtCe₂O(n) (n = 0–1) clusters were recorded and compared with simulations based on density functional theory to determine adiabatic ionization energies (IE) and verify cluster structures. Calculated structures were also presented for PtCe₂O₂–₅, though no comparison with experiment was made due to nondetection of these species in the mass spectrum. The IEs of PtCe₂ and PtCe₂O were determined to be 4.59 and 4.55 eV, respectively. The calculated IEs for PtCe₂O₂–₅ sharply rise to >5.95 eV, coinciding with the HOMO of the clusters being lower energy Pt 6s-like, Ce 4f-like, and Pt 5d-like orbitals, from the higher energy Ce 6s-like HOMOs of PtCe₂O₀,₁. The sharp increase in IE also explains the lack of PtCe₂O₂–₅ ionization products observed in the experiment, though photodissociation is found to be a less likely possibility. As the level of oxidation in the clusters increased the charge on the Pt atom in the cluster became more positive, representing a transition from Ce → Pt to Pt → Ce charge transfer. This transition coincided with the formation of the first Pt–O–Ce structural motif and the sharp decrease in O bond dissociation energy in PtCe₂O₃. PtCe₂O₄ and PtCe₂O₅ each possess a second Pt–O–Ce site formed with the latter also having a sharp increase in calculated IE to 7.71 eV due to the low-energy Pt localized 5d-like HOMO.Aidan M. Karayilan and Gregory F. Meth

Gas phase photocatalytic water splitting of moisture in ambient air: toward reagent-free hydrogen production

We report our recent results investigating photocatalytic water-splitting using water vapour. In this case we use well-defined photocatalysts made up of Al-doped SrTiO3 (SrTiO3:Al) loaded with a RhCrOx and CoOy co-catalyst in a batch phase reactor containing either water vapour dosed into N2 gas or real air samples. The catalyst samples are deposited onto an inert substrate and irradiated under variable intensity UV LED. Both hydrogen and oxygen are produced in approximately 2:1 stoichiometric ratio over a duration of over 6 h. Importantly, the back reaction of H2O formation from H2 and O2 gas is suppressed, even in air which is a high O2 environment. The production rates are found to be highly sensitive to the water vapour concentration and temperature of the reactor, as has been found previously for other types of photocatalysts. The photocatalysts are shown to be robust enough in high oxygen environment to actively produce hydrogen using water sourced from ambient air (without any conditioning). These findings suggest photocatalysis may be used as a portable, self-contained hydrogen source for energy in remote locations.Cameron J. Shearer, Takashi Hisatomi, Kazunari Domen, Gregory F. Meth

A systematic density functional theory study of the complete de-ligation of Ru(3)(CO)(12)

First published: 1 May 2016Ru3(CO)12 was systematically de-ligated computationally and the energetic minima for all structures were calculated using density functional theory at the M06/SDD/cc-pvdz level of theory. The lowest energy electronic configuration of the clusters Ru3(CO)12 to Ru3(CO)3 was found to be a singlet state, while for Ru3(CO)2, Ru3(CO)1 and Ru3 the minimum spin state was predicted to be quintet, septet, and nonet, respectively. The CO binding energies for all structures was found to increase approximately linearly with de-ligation from Ru3(CO)12 to Ru3(CO)5, whereupon the binding energy decreased from this structure to Ru3CO. The average bond length of terminally bound CO ligands on each cluster was found to be larger than free carbon monoxide in all cases, and increased from 1.147 Å for Ru3(CO)12, to 1.161 Å for Ru3CO.Reuben White, Trystan Bennett, Vladimir Golovko, Gunther G. Andersson, and Gregory F. Meth

Self-sorting of porous Cu(4)L(2)L'(2) metal-organic cages composed of isomerisable ligands

We report the self-sorting of a dynamic combinatorial library (DCL) of metal–organic cages composed of a rotationally isomerisable ligand. Convergence of the DCL occurs upon crystallisation and leads to low-symmetry Cu₄L₂L′₂ cages that display differing porosities based on their overall shape and ligand configuration.Adrian W. Markwell-Heys, Matthew L. Schneider, Jenica Marie L. Madridejos, Gregory F. Metha and Witold M. Bloc

Investigation of phosphine ligand protected Au(13) clusters on defect rich titania

Atomically precise chemically synthesized gold clusters protected by diphenylphosphine ligands [(Au₁₃(dppe)₅Cl₂)Cl₃] (dppe = 1,2-bis(diphenylphosphino)ethane) were deposited onto a defect-rich, atomic layer deposited titania surface prepared by heating and subsequently sputtering the titania under ultrahigh vacuum (UHV). The gold clusters deposited onto the pretreated titania were heated at 200 °C for 20 min under UHV with the aim to remove the ligands protecting the gold core of the clusters. The change in the electronic structure and the chemical composition after deposition after aforementioned heat treatment was investigated using metastable induced electron spectroscopy (MIES) and X-ray photoelectron spectroscopy (XPS). The MIES data allow identification of the density of states (DOS) of the deligated Au₁₃ clusters on the titania to be made. The DOS of the Au₁₃ are found to be similar to those previously found for Au₉ clusters on sputtered ALD titania but have shifted binding energies. The differences in DOS of Au₁₃ and Au₉ are discussed.Gowri Krishnan, Namsoon Eom, Ryan M. Kirk, Vladimir B. Golovko, Gregory F. Metha, and Gunther G. Andersso

Factors influencing catalytic activity of size-specific triphenylphosphine-ligated gold nanoclusters in the electrocatalytic hydrogen evolution reaction

Hydrogen production via electrocatalytic water splitting has attracted growing attention as an alternative renewable and clean energy source. Size-specific gold nanoclusters and complexes (AuNCs) can serve as models for investigating the catalytic behavior toward the hydrogen evolution reaction (HER) at the atomic level. This work is focused on exploring the factors influencing the catalytic activity of phosphine-ligated AuNCs as electrocatalysts for improving HER performance using Au101(PPh3)21Cl5, Au9(PPh3)8(NO3)3, and Au1(PPh3)Cl supported on reduced graphene oxide (rGO). Production of AuNC–rGO nanocomposites without agglomeration of the AuNCs was confirmed by transmission electron microscopy, X-ray photoelectron spectroscopy, and visible light absorbance. The weight loading of gold in the nanocomposite material was confirmed to be ≈5 wt % by thermogravimetric analysis and inductively coupled plasma mass spectrometry. Electrocatalytic performance of the AuNCs was determined through linear sweep voltammograms in 0.5 M sulfuric acid. Greater performance was observed for Au101NC–rGO, while Au9NC–rGO and Au1NC–rGO showed similar performance. The stability of each AuNC was determined through extended chronoamperometry experiments, and negligible reduction in performance was observed for Au101NC–rGO and Au9NC–rGO, while Au1NC–rGO was less stable. The variation in performance was attributed to a range of factors including catalyst size, electronic structure, and ligand density. This work provides guidelines to design highly efficient electrocatalysts using ligated metal clusters.Hanieh Mousavi, Yanting Yin, Shailendra Kumar Sharma, Christopher T. Gibson, Vladimir Golovko, Gunther G. Andersson ... et al

Optical properties of the atomically precise C(4) core [Au(9)(PPh(3))(8)](3+) cluster probed by transient absorption spectroscopy and time-dependent density functional theory

Structural isomerism of [Au9(PPh3)8]3+ has been studied experimentally, mostly concerning the symmetry of the Au9 core. Recently, the C4 isomer of [Au9(PPh3)8]3+ has been shown to exist in solution phase while the D2h isomer is present in the solid state [Inorg. Chem.2017, 56, 8319–8325]. In this work, geometric, electronic, and optical properties of C4 [Au9(PPh3)8]3+ are investigated by using the combined second-order density-functional tight-binding (DFTB2) method and time-dependent density functional theory (TD-DFT) calculations with spin–orbit coupling. Additionally, the excited-state relaxation dynamics of the [Au9(PPh3)8]3+ cluster in dichloromethane and methanol solutions are studied using femtosecond transient absorption spectroscopy. [Au9(PPh3)8]3+ is optically pumped to different excited states by using 432, 532, and 603 nm light. For all three pump wavelengths, the photoexcitation event induces an excited-state absorption (ESA) band centered at 600 nm with decay time constants of 2.0 and 45 ps, which are attributed to intersystem crossing and nonradiative relaxation of [Au9(PPh3)8]3+, respectively. On the other hand, optical pumping of [Au9(PPh3)8]3+ using 432 nm light gives rise to an additional ESA band at 900 nm. This band exhibits fast relaxation through internal conversion with a time constant of ∼0.3 ps. Our combined computational and experimental study reveals that the excitation wavelength-dependent relaxation dynamics of the [Au9(PPh3)8]3+ cluster are related to the different electron densities of the excited states of [Au9(PPh3)8]3+, consistent with it possessing molecular-like electronic states.Jenica Marie L. Madridejos, Takaaki Harada, Alexander J. Falcinella, Thomas D. Small, Vladimir B. Golovko, Gunther G. Andersson ... et al