Jason F. Alvino

Chemically synthesised atomically precise gold clusters deposited and activated on titania. Part II

Synchrotron XPS was used to investigate a series of chemically synthesised, atomically precise gold clusters Au(n)(PPh3)y (n = 8, 9 and 101, y depending on the cluster size) immobilized on anatase (titania) nanoparticles. Effects of post-deposition treatments were investigated by comparison of untreated samples with analogues that have been heat treated at 200 °C in O2, or in O2 followed by H2 atmosphere. XPS data shows that the phosphine ligands are oxidised upon heat treatment in O2. From the position of the Au 4f(7/2) peak it can be concluded that the clusters partially agglomerate immediately upon deposition. Heating in oxygen, and subsequently in hydrogen, leads to further agglomeration of the gold clusters. It is found that the pre-treatment plays a crucial role in the removal of ligands and agglomeration of the clusters.

Phosphine-stabilised Au9 clusters interacting with titania and silica surfaces: The first evidence for the density of states signature of the support-immobilised cluster

Chemically made, atomically precise phosphine-stabilized clusters Au9(PPh3)8(NO3)3 were deposited on titania and silica from solutions at various concentrations and the samples heated under vacuum to remove the ligands. Metastable induced electron spectroscopy was used to determine the density of states at the surface, and X-ray photoelectron spectroscopy for analysing the composition of the surface. It was found for the Au9 cluster deposited on titania that the ligands react with the titania substrate. Based on analysis using the singular value decomposition algorithm, the series of MIE spectra can be described as a linear combination of 3 base spectra that are assigned to the spectra of the substrate, the phosphine ligands on the substrate, and the Au clusters anchored to titania after removal of the ligands. On silica, the Au clusters show significant agglomeration after heat treatment and no interaction of the ligands with the substrate can be identified.

Far-infrared absorption spectra of synthetically-prepared, ligated metal clusters with Au6, Au8, Au9 and Au6Pd metal cores

The far infra-red absorption spectra of a series of chemically synthesised, atomically precise phosphine-stabilised gold cluster compounds have been recorded using synchrotron light for the first time. Far-IR spectra of the Au6(Ph2P(CH2)3PPh2)4(NO3)2, Au8(PPh3)8(NO3)2, Au9(PPh3)8(NO3)3, and Pd(PPh3)Au6(PPh3)6(NO3)2 clusters reveal a complex series of peaks between 80 and 475 cm−1, for which all significant peaks can be unambiguously assigned by comparison with Density Functional Theory (DFT) geometry optimisations and frequency calculation. Strong absorptions in all spectra near 420 cm−1 are assigned to the P–Ph3 stretching vibrations. Distinct peaks within the spectrum of each specific cluster are assigned to the cluster core vibrations: 80.4 and 84.1 cm−1 (Au6) 165.1 and 166.4 cm−1 (Au8), 170.1 and 185.2 cm−1 (Au9), and 158.9, 195.2, and 206.7 cm−1 (Au6Pd). The positions of these peaks are similar to those observed to occur for the neutral Au7 cluster in the gas phase (Science, 2008, 321, 674–676). Au–P stretching vibrations only occur for Au6 near 420 cm−1, although they appear near 180 cm−1 for Au6Pd and involve gold core vibrations.

Mapping-Out Catalytic Processes in a Metal–Organic Framework with Single-Crystal X-ray Crystallography

Single-crystal X-ray crystallography is employed to characterize the reaction species of a full catalytic carbonylation cycle within a MnII -based metal-organic framework (MOF) material. The structural insights explain why the Rh metalated MOF is catalytically competent toward the carbonylation of MeBr but only affords stoichiometric turn-over in the case of MeI. This work highlights the capability of MOFs to act as platform materials for studying single-site catalysis in heterogeneous systems.

Atomically resolved structure of ligand-protected Au9 clusters on TiO2 nanosheets using aberration-corrected STEM

Triphenylphosphine ligand-protected Au9 clusters deposited onto titania nanosheets show three different atomic configurations as observed by scanning transmission electron microscopy. The configurations observed are a 3-dimensional structure, corresponding to the previously proposed Au9 core of the clusters, and two pseudo-2-dimensional (pseudo-2D) structures, newly found by this work. With the help of density functional theory (DFT) calculations, the observed pseudo-2D structures are attributed to the low energy, de-ligated structures formed through interaction with the substrate. The combination of scanning transmission electron microscopy with DFT calculations thus allows identifying whether or not the deposited Au9 clusters have been de-ligated in the deposition process.

Identification of the vibrational modes in the far-infrared spectra of ruthenium carbonyl clusters and the effect of gold substitution.

High-quality far-IR absorption spectra for a series of ligated atomically precise clusters containing Ru3, Ru4, and AuRu3 metal cores have been observed using synchrotron radiation, the latter two for the first time. The experimental spectra are compared with predicted IR spectra obtained following complete geometric optimization of the full cluster, including all ligands, using DFT. We find strong correlations between the experimental and predicted transitions for the low-frequency, low-intensity metal core vibrations as well as the higher frequency and intensity metal-ligand vibrations. The metal core vibrational bands appear at 150 cm(-1) for Ru3(CO)12, and 153 and 170 cm(-1) for H4Ru4(CO)12, while for the bimetallic Ru3(μ-AuPPh3)(μ-Cl)(CO)10 cluster these are shifted to 177 and 299 cm(-1) as a result of significant restructuring of the metal core and changes in chemical composition. The computationally predicted IR spectra also reveal the expected atomic motions giving rise to the intense peaks of metal-ligand vibrations at ca. 590 cm(-1) for Ru3, 580 cm(-1) for Ru4, and 560 cm(-1) for AuRu3. The obtained correlations allow an unambiguous identification of the key vibrational modes in the experimental far-IR spectra of these clusters for the first time.

Grouping and aggregation of ligand protected Au9 clusters on TiO2 nanosheets

Atomically precise chemically synthesised Au clusters, in the form of [Au9(PPh3)8](NO3)3, were deposited onto titania nanosheets after UV pre-treatment of the substrate and examined with scanning tunneling microscopy (STM), atomic force microscopy (AFM) and synchrotron X-ray photoelectron spectroscopy (XPS) before and after heat treatment. The STM, AFM and XPS results complement each other. AFM was performed to determine the height of the deposited species and their dispersion on titania nanosheets. STM shows groups of clusters that at least partially consist of individual clusters both before and after annealing. STM cannot exclude the existence of individual clusters on the titania nanosheets outside the groups. XPS shows that before annealing the Au clusters are attached to the titania surface as individual clusters thus as clusters with non-agglomerated cluster cores. After annealing, both individual and agglomerated clusters are found on the surface. The combination of AFM, STM and XPS shows that the groups formed by the clusters consist of individual and agglomerated clusters.

Apparatus for the investigation of high-temperature, high-pressure gas-phase heterogeneous catalytic and photo-catalytic materials

A high-temperature, high-pressure, pulsed-gas sampling and detection system has been developed for testing new catalytic and photocatalytic materials for the production of solar fuels. The reactor is fitted with a sapphire window to allow the irradiation of photocatalytic samples from a lamp or solar simulator light source. The reactor has a volume of only 3.80 ml allowing for the investigation of very small quantities of a catalytic material, down to 1 mg. The stainless steel construction allows the cell to be heated to 350 °C and can withstand pressures up to 27 bar, limited only by the sapphire window. High-pressure sampling is made possible by a computer controlled pulsed valve that delivers precise gas flow, enabling catalytic reactions to be monitored across a wide range of pressures. A residual gas analyser mass spectrometer forms a part of the detection system, which is able to provide a rapid, real-time analysis of the gas composition within the photocatalytic reaction chamber. This apparatus is ideal for investigating a number of industrially relevant reactions including photocatalytic water splitting and CO2 reduction. Initial catalytic results using Pt-doped and Ru nanoparticle-doped TiO2 as benchmark experiments are presented.