Size- and support-dependency in the catalysis of gold

Although homogeneous gold catalysis was known previously, an exponential growth was only induced 12 years ago. The key findings which induce that rise of the field are discussed. This includes early reactions of allenes and furanynes and intermediates of these conversions as well as hydroarylation reactions. Other substrate types addressed are alkynyl epoxides and N-propargyl carboxamides. Important vinylgold intermediates, the transmetalation from gold to other transition metals, the development of new ligands for gold catalysis, and significant contributions from computational chemistry are other crucial points for the field highlighted here.

Gold-Catalyzed Organic Reactions

Gold octamers (Au8) bound to oxygen-vacancy F-center defects on Mg(001) are the smallest clusters to catalyze the low-temperature oxidation of CO to CO2, whereas clusters deposited on close-to-perfect magnesia surfaces remain chemically inert. Charging of the supported clusters plays a key role in promoting their chemical activity. Infrared measurements of the stretch vibration of CO adsorbed on mass-selected gold octamers soft-landed on MgO(001) with coadsorbed O2 show a red shift on an F-center-rich surface with respect to the perfect surface. The experiments agree with quantum ab initio calculations that predict that a red shift of the C-O vibration should arise via electron back-donation to the CO antibonding orbital.

https://science.sciencemag.org/content/sci/307/5708/403.full.pdf

Charging Effects on Bonding and Catalyzed Oxidation of CO on Au8 Clusters on MgO

Artificial Molecular Rotors

Despite occasional references in the older literature to the ability of gold to catalyze certain reactions, the metal has until recently had the reputation of being one of the least catalytically useful. The recent discovery that some supported gold catalysts can affect the oxidation of carbon monoxide at or below ambient temperature has, however, focused attention on the metal's ability in this respect. For oxidation of carbon monoxide at low temperature, catalysts comprising small (<5 nm) gold particles supported preferably on an oxide of the first transition series (e.g., TiO2, α-Fe2O3) are needed. Deposition–precipitation and coprecipitation are better methods than impregnation for this purpose and provide the desired intimacy of contact between metal and support. High activity may well originate at sites at the gold–support interface, with the support making a vital contribution. Stable activity can result by optimizing aging in solution during the preparation, and low calcination temperatures are ge...

Catalysis By Gold

The isolation of C{sub 60} in preparatively useful quantities has stimulated intensive efforts to generate and characterize functional derivatives. Many reactions of C{sub 60} furnish complex, inseparable mixtures of products, and only a few discrete, monofunctional congeners have been described to date. We now report that photooxidation of C{sub 60} in benzene furnishes a single monooxide, C{sub 60}O, as the sole isolable product. Moreover, we have obtained the same species as a byproduct of the synthesis of C{sub 60}. Spectroscopic analysis strongly supports the epoxide structure. It is demonstrated that C{sub 60}O is efficiently converted to C{sub 60} in a 91% yield during chromatography of neutral alumina. The widespread use of alumina for purification of the fullerenes may explain why the fullerene oxide was not isolated before.

Synthesis and characterization of C60O, the first fullerene epoxide

We have examined the reactivity and saturation of small gold clusters (cations, neutrals and anions) towards several molecules and find that specific small gold clusters exhibit a pronounced variation in their reactivity towards hydrogen, methane and oxygen. The reactivity not only depends strongly on cluster size but also on the cluster charge state. For example, small (n<15) gold cations react readily with D2, but no evidence of reaction is observed for the anions under our experimental conditions. Similar behavior is seen for methane. With oxygen only even atom (odd electron) anions are reactive, and Au
 10
 +
 is the only cation which exhibits evidence of reaction. The global features (small cluster cations reactive towards H2, CH4, but large ones not reactive, odd electron anions reactive towards O2) are qualitatively explained by appealing to a simple frontier orbital picture. The uptake of deuterium and methane on gold clusters also exhibits a pronounced size dependence with D/Au varying from a high of 3 for the dimer to zero for clusters containing more than 15 Au atoms. Comparison of the methane and deuterium saturation behavior leads us to suggest that methane is dissociated and bound as CH3 and H.

Gold clusters: reactions and deuterium uptake

Coherent quasielastic neutron scattering has been used to investigate the character of the rotational dynamics in the high-temperature solid phase of ${\mathrm{C}}_{60}$. The observed scattering can be described by a model in which each molecule undergoes rotational diffusion which is uncorrelated with the motions of adjacent molecules. The rotational diffusion constant ${\mathit{D}}_{\mathit{R}}$ is (1.4\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}${10}^{10}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ at 260 K and (2.8\ifmmode\pm\else\textpm\fi{}0.8)\ifmmode\times\else\texttimes\fi{}${10}^{10}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ at 520 K. The temperature dependence of ${\mathit{D}}_{\mathit{R}}$ is consistent with a thermally activated process having an activation energy of 35\ifmmode\pm\else\textpm\fi{}15 meV.

Coherent quasielastic neutron scattering study of the rotational dynamics of C60 in the orientationally disordered phase.

X-ray and neutron-diffraction measurements have been used to study the evolution of the lattice parameter of solid C{sub 60} through the orientational-ordering transition. The lattice parameter jumps by +0.044{plus minus}0.004 A on heating, indicating a strongly first-order transition. The average isobaric volume thermal-expansion coefficient both above and below the transition is 6.2{plus minus}0.2{times}10{sup {minus}5} K{sup {minus}1}. We observe phase coexistence over a 5-K range, but little if any hysteresis.

Discontinuous volume change at the orientational-ordering transition in solid C60.

The chemical reactions of size selected Si+n (n=3–24) clusters with C2H4 have been studied at thermal energies using selected ion drift tube techniques. Except for Si+3 (which dehydrogenates C2H4 to yield Si3C2H+2) the dominant products arise from adsorption of C2H4 onto the silicon clusters. The reaction rates show large variations with cluster size. Si+13 and Si+14 were found to be particularly unreactive. The reactions were studied over a temperature range of 143–298 K. The reactivity increases as the temperature is lowered indicating that variation in the rates observed for the different cluster sizes is not due to an activation barrier, but reflects the stability of the SinC2H+4 adducts. Statistical phase space theory was used to model the reactions of Si+4–Si+10 and provide an estimate of the binding energy of the first C2H4 to these clusters. The binding energies vary between 0.8 and 2.0 eV. Binding energies of this magnitude are too small to be accounted for by strong di‐σ bonding, suggesting that the C2H4 molecule is bound to the silicon clusters by a weaker π‐bonding interaction. Kinetic evidence for the presence of structural isomers was found for several clusters. With Si+9 the relative abundance of an unreactive isomer could be changed from ∼1% up to 17% by changing the source conditions. More than one C2H4 molecule will adsorb onto the silicon clusters (up to eight adsorb on Si+4). Rate constants for the adsorption of the first few (up to seven) C2H4 molecules on Si+n (n=4–10) have been determined. They show large variations with the number of adsorbed C2H4 molecules.

Kathleen M. Creegan

Papers

Synthesis and characterization of C60O, the first fullerene epoxide

Gold clusters: reactions and deuterium uptake

Coherent quasielastic neutron scattering study of the rotational dynamics of C60 in the orientationally disordered phase.

Discontinuous volume change at the orientational-ordering transition in solid C60.

Chemistry of semiconductor clusters: A study of the reactions of size selected Si+n (n=3–24) with C2H4 using selected ion drift tube techniques