Search

Metallic gold has emerged as a highly successful catalyst for selective oxygen-assisted coupling reactions of alcohols and amines to yield industrially important classes of molecules, including esters and amides. In expanding the substrate scope of this class of reactions, fundamental mechanistic principles determined for simple model systems have been used to predict new reactive pathways for other more complex molecular transformations. Given the importance of these fundamental reaction paradigms, this review aims to consolidate the understanding of oxygen-assisted coupling mechanisms on metallic gold catalysts across a broad range of reported studies, including gas-phase and liquid-phase systems. Furthermore, the review indicates areas where additional understanding is still needed, and where collaboration between the gas-phase and liquid-phase catalysis communities would be instrumental in the elucidation of detailed reaction mechanisms.

Nanoporous gold is a complex material comprised of a small amount of silver that is the residual from the dealloying process used in its formation. This material activates dioxygen and selectively self-couples methanol. The dissociative adsorption of O2 and the subsequent reaction of methanol with the adsorbed atomic oxygen are critical steps in this selective oxidation. The density of sites for O2 dissociation was determined to be 0.1% of the total surface (3 × 1012 per cm2) using both transient and steady flow measurements. The activation energy for O2 dissociation was measured to be 5.0 kcal/mol and is similar in magnitude to that on metallic Ag and much lower than expected for Au surfaces. The area-averaged dissociation probability of O2 at 423 K is ∼1 × 10−7, commensurate with the active site density and the activation barrier to dissociation. The reactive oxygen is immobile under reaction conditions. The collisional reaction probability of methanol striking an adsorbed O atom is 10−4–10−5, which corresponds well with the measured turnover frequency for methanol conversion to form methylformate of ∼160 s−1 at 423 K. Taken together, these results strongly indicate that Ag is an integral part of the active site for O2 activation and the subsequent activation of methanol.

Achieving high selectivity for high volume chemical synthesis is important for lowering energy consumption through reduction in waste. We report the selective synthesis of methyl esters—methyl acetate and methyl butyrate—through catalytic O2-assisted cross-coupling of methanol with ethanol or 1-butanol using activated, support-free nanoporous gold (npAu). Both well-controlled studies on ingots in UHV and experiments under ambient pressure catalytic conditions on both ingots and microspherical hollow shell catalysts reveal guiding principles for controlling selectivity. Under UHV conditions, the ester products of the cross-coupling of methanol with both ethanol and 1-butanol evolve near room temperature in temperature-programmed reaction studies, indicating that the reactions occur facilely. Under steady-state catalytic operation, high stable activity was observed for cross-coupling in flowing gaseous reactant mixtures at atmospheric pressure and 423 K with negligible combustion. Optimum selectivity for cross-coupling is obtained in methanol-rich mixtures due to a combination of two factors: (1) the relative coverage of the respective alkoxys and (2) the relative facility of their β-H elimination. The relative coverage of the alkoxys is governed by van der Waal’s interactions between the alkyl groups and the surface; here, we demonstrate the importance of these weak interactions in a steady-state catalytic process.

We report a new method for facile and reproducible activation of nanoporous gold (npAu) materials of different forms for the catalytic selective partial oxidation of alcohols under ambient pressure, steady flow conditions. This method, based on the surface cleaning of npAu ingots with ozone to remove carbon documented in ultrahigh vacuum conditions, produces active npAu catalysts from ingots, foils, and shells by flowing an ozone/dioxygen mixture over the catalyst at 150 °C, followed by a temperature ramp from 50 to 150 °C in a flowing stream of 10% methanol and 20% oxygen. With this treatment, all three materials (ingots, foils, and shells) can be reproducibly activated, despite potential carbonaceous poisons resulting from their synthesis, and are highly active for the selective oxidation of primary alcohols over prolonged periods of time. The npAu materials activated in this manner exhibit catalytic behavior substantially different from those activated under different conditions previously reported. On...

Not touching but sticking: By using cationic surfactant micelles as depletants, a directional entropic force approach (DEFA) assembles anisotropic nanoparticles into superlattices in solution. The micelles induce the face-to-face stacking of the nanoparticles to maximize the system's entropy. The shape of the nanoparticles determines the symmetry of the superlattice, the interparticle spacing is determined by the charged surfactant.

The article of record as published may be found at http://dx.doi.org/10.1021/nl3032235 We report a simple and general strategy for selectively exposing and functionalizing the sharp corners of concave nanocubes, which are the SERS hot spots for such structures. This strategy takes advantage of the unique shape of the concave cubes by coating the particles with silica and then etching it away to expose only the corner regions, while maintaining the silica coating in the concave faces. These corner regions can then be selectively modified for improved enhancement and signal response with SERS AFOSR National Science Foundation NERC (DOE) Funded by Naval Postgraduate School. Awards No. N00244-09-1-0012 and N00244-09-1-0071 (NPS) Award no. FA9550-09-1-0294 (AFOSR) DMR-0520513 and DMR-01121262 (NSF) Award no. DE-SC0000989 (NERC)

Two {110}-faceted gold nanostructures--rhombic dodecahedra and obtuse triangular bipyramids--have been synthesized via a Ag-assisted, seed-mediated growth method. The combination of a Cl(-)-containing surfactant with a low concentration of Ag(+) plays a role in the stabilization of the {110} facets. To the best of our knowledge, this is the first reported synthesis of a {110}-faceted bipyramid structure.

Understanding the surface structure of metal nanocrystals with specific facet indices is important due to its impact on controlling nanocrystal shape and functionality. However, this is particularly challenging for halide-adsorbed nanocrystals due to the difficulty in analysing interactions between metals and light halides (for example, chloride). Here we uncover the surface structures of chloride-adsorbed, silver-coated gold nanocrystals with {111}, {110}, {310} and {720} indexed facets by X-ray absorption spectroscopy and density functional theory modelling. The silver–chloride, silver–silver and silver–gold bonding structures are markedly different between the nanocrystal surfaces, and are sensitive to their formation mechanism and facet type. A unique approach of combining the density functional theory and experimental/simulated X-ray spectroscopy further verifies the surface structure models and identifies the previously indistinguishable valence state of silver atoms on the nanocrystal surfaces. Overall, this work elucidates the thus-far unknown chloride–metal nanocrystal surface structures and sheds light onto the halide-induced growth mechanism of anisotropic nanocrystals., Nanocrystal surface structure affects many properties but is tough to determine for halide-adsorbed materials. Here, the authors combine X-ray absorption measurements and computational modelling to elucidate the chloride metal surface structures for silver-coated gold nanocrystals with controlled shapes.

A new class of gold nanostructures, concave nanocubes, enclosed by 24 high-index {720} facets, have been prepared in a monodisperse fashion by a modified seed-mediated synthetic method. The Cl(-) counterion in the surfactant plays an essential role in controlling the concave morphology of the final product. The concave nanocubes exhibit higher chemical activities compared with low-index {111}-faceted octahedra.

Guanosine derivatives with a nucleophilic group at the 5' position (G-5') are oxidized by the Pt (IV) complex Pt( d, l)(1,2-(NH 2) 2C 6H 10)Cl 4 ([Pt (IV)(dach)Cl 4]). The overall redox reaction is autocatalytic, consisting of the Pt (II)-catalyzed Pt (IV) substitution and two-electron transfer between Pt (IV) and the bound G-5'. In this paper, we extend the study to improve understanding of the redox reaction, particularly the substitution step. The [Pt (II)(NH 3) 2(CBDCA-O,O')] (CBDCA = cyclobutane-1,1-dicarboxylate) complex effectively accelerates the reactions of [Pt (IV)(dach)Cl 4] with 5'-dGMP and with cGMP, indicating that the Pt (II) complex does not need to be a Pt (IV) analogue to accelerate the substitution. Liquid chromatography/mass spectroscopy (LC/MS) analysis showed that the [Pt (IV)(dach)Cl 4]/[Pt (II)(NH 3) 2(CBDCA-O,O')]/cGMP reaction mixture contained two Pt (IV)cGMP adducts, [Pt (IV)(NH 3) 2(cGMP)(Cl)(CBDCA-O,O')] and [Pt (IV)(dach)(cGMP)Cl 3]. The LC/MS studies also indicated that the trans, cis-[Pt (IV)(dach)( (37)Cl) 2( (35)Cl) 2]/[Pt (II)(en)( (35)Cl) 2]/9-EtG mixture contained two Pt (IV)-9-EtG adducts, [Pt (IV)(en)(9-EtG)( (37)Cl)( (35)Cl) 2] and [Pt (IV)(dach)(9-EtG)( (37)Cl)( (35)Cl) 2]. These Pt (IV)G products are predicted by the Basolo-Pearson (BP) Pt (II)-catalyzed Pt (IV)-substitution scheme. The substitution can be envisioned as an oxidative addition reaction of the planar Pt (II) complex where the entering ligand G and the chloro ligand from the axial position of the Pt (IV) complex are added to Pt (II) in the axial positions. From the point of view of reactant Pt (IV), an axial chloro ligand is thought to be substituted by the entering ligand G. The Pt (IV) complexes without halo axial ligands such as trans, cis-[Pt(en)(OH) 2Cl 2], trans, cis-[Pt(en)(OCOCF 3) 2Cl 2], and cis, trans, cis-[Pt(NH 3)(C 6H 11NH 2)(OCOCH 3) 2Cl 2] ([Pt (IV)(a,cha)(OCOCH 3) 2Cl 2], satraplatin) did not react with 5'-dGMP. The bromo complex, [Pt (IV)(en)Br 4], showed a significantly faster substitution rate than the chloro complexes, [Pt (IV)(en)Cl 4] and [Pt (IV)(dach)Cl 4]. The results indicate that the axial halo ligands are essential for substitution and the Pt (IV) complexes with larger axial halo ligands have faster rates. When the Pt (IV) complexes with different carrier ligands were compared, the substitution rates increased in the order [Pt (IV)(dach)Cl 4][Pt (IV)(en)Cl 4][Pt (IV)(NH 3) 2Cl 4], which is in reverse order to the carrier ligand size. These axial and carrier ligand effects on the substitution rates are consistent with the BP mechanism. Larger axial halo ligands can form a better bridging ligand, which facilitates the electron-transfer process from the Pt (II) to Pt (IV) center. Smaller carrier ligands exert less steric hindrance for the bridge formation.

Decreasing energy consumption in the production of platform chemicals is necessary to improve the sustainability of the chemical industry, which is the largest consumer of delivered energy. The majority of industrial chemical transformations rely on catalysts, and therefore designing new materials that catalyse the production of important chemicals via more selective and energy-efficient processes is a promising pathway to reducing energy use by the chemical industry. Efficiently designing new catalysts benefits from an integrated approach involving fundamental experimental studies and theoretical modelling in addition to evaluation of materials under working catalytic conditions. In this review, we outline this approach in the context of a particular catalyst—nanoporous gold (npAu)—which is an unsupported, dilute AgAu alloy catalyst that is highly active for the selective oxidative transformation of alcohols. Fundamental surface science studies on Au single crystals and AgAu thin-film alloys in combination with theoretical modelling were used to identify the principles which define the reactivity of npAu and subsequently enabled prediction of new reactive pathways on this material. Specifically, weak van der Waals interactions are key to the selectivity of Au materials, including npAu. We also briefly describe other systems in which this integrated approach was applied.