Cynthia M. Friend

Nanoporous Gold Catalysts for Selective Gas-Phase Oxidative Coupling of Methanol at Low Temperature

Methanol Coupling Catalyzed with Gold Gold surfaces can be effective catalysts for partial oxidation reactions, in part because lower interaction strengths of molecules absorbed on gold allow products to desorb before further unwanted oxidations occur. One challenge in these reactions is the low rate of formation of reactive atomic surface oxygen. Wittstock et al. (p. 319; see the Perspective by Christensen and Nørskov) created high–surface area gold catalysts by leaching silver from gold-silver alloys. This material proved to be an effective catalyst for partial oxidative coupling of methanol, yielding methyl formate. Residual silver appears to play a key role in activating the dissociation of molecular oxygen. Leaching of gold-silver alloys creates a highly active catalyst for partial oxidation reactions. Gold (Au) is an interesting catalytic material because of its ability to catalyze reactions, such as partial oxidations, with high selectivities at low temperatures; but limitations arise from the low O2 dissociation probability on Au. This problem can be overcome by using Au nanoparticles supported on suitable oxides which, however, are prone to sintering. Nanoporous Au, prepared by the dealloying of AuAg alloys, is a new catalyst with a stable structure that is active without any support. It catalyzes the selective oxidative coupling of methanol to methyl formate with selectivities above 97% and high turnover frequencies at temperatures below 80°C. Because the overall catalytic characteristics of nanoporous Au are in agreement with studies on Au single crystals, we deduced that the selective surface chemistry of Au is unaltered but that O2 can be readily activated with this material. Residual silver is shown to regulate the availability of reactive oxygen.

Infrared absorption by conical silicon microstructures made in a variety of background gases using femtosecond-laser pulses

We show that the near-unity infrared absorptance of conical microstructures fabricated by irradiating a Si(111) surface with 100 fs laser pulses depends on the ambient gas in which the structures are formed. SF6 produces an absorptance of 0.9 for radiation in the 1.2–2.5 μm wavelength range, higher than any of the other gases. Use of Cl2, N2, or air produces surfaces with absorptances intermediate between that for microstructures formed in SF6 and that for flat crystalline silicon, for which the absorptance is roughly 0.05–0.2 for a 260 μm thick sample. Secondary ion mass spectrometry shows that elements from the ambient gas are incorporated into the silicon surface in high concentration.

Vapour-phase gold-surface-mediated coupling of aldehydes with methanol

Selective coupling of oxygenates is critical to many synthetic processes, including those necessary for the development of alternative fuels. We report a general process for selective coupling of aldehydes and methanol as a route to ester synthesis. All steps are mediated by oxygen-covered metallic gold nanoparticles on Au(111). Remarkably, cross-coupling of methanol with formaldehyde, acetaldehyde, benzaldehyde and benzeneacetaldehyde to methyl esters is promoted by oxygen-covered Au(111) below room temperature with high selectivity. The high selectivity is attributed to the ease of nucleophilic attack of the aldehydes by the methoxy intermediate-formed from methanol on the surface-which yields the methyl esters. The competing combustion occurs via attack of both methanol and the aldehydes by oxygen. The mechanistic model constructed in this study provides insight into factors that control selectivity and clearly elucidates the crucial role of Au nanoparticles as active species in the catalytic oxidation of alcohols, even in solution.

Selectivity Control in Gold-Mediated Esterification of Methanol†

The Midas touch: The low-temperature transformation of methanol to methyl formate, formaldehyde, and formic acid is promoted by atomic oxygen adsorbed on metallic gold (see picture). The reactions occur with O-containing Au nanoparticles formed on Au(111) upon oxidation with ozone at 200 K; the facile esterification to methyl formate occurs well below room temperature.

Formation of regular arrays of silicon microspikes by femtosecond laser irradiation through a mask

We report fabrication of regular arrays of silicon microspikes by femtosecond laser irradiation of a silicon wafer covered with a periodic mask. Without a mask, microspikes form, but they are less ordered. We believe that the mask imposes order by diffracting the laser beam and providing boundary conditions for capillary waves in the laser-melted silicon.

Achieving Selective and Efficient Electrocatalytic Activity for CO2 Reduction Using Immobilized Silver Nanoparticles

Selective electrochemical reduction of CO2 is one of the most sought-after processes because of the potential to convert a harmful greenhouse gas to a useful chemical. We have discovered that immobilized Ag nanoparticles supported on carbon exhibit enhanced Faradaic efficiency and a lower overpotential for selective reduction of CO2 to CO. These electrocatalysts were synthesized directly on the carbon support by a facile one-pot method using a cysteamine anchoring agent resulting in controlled monodispersed particle sizes. These synthesized Ag/C electrodes showed improved activities, specifically decrease of the overpotential by 300 mV at 1 mA/cm(2), and 4-fold enhanced CO Faradaic efficiency at -0.75 V vs RHE with the optimal particle size of 5 nm compared to polycrystalline Ag foil. DFT calculations enlightened that the specific interaction between Ag nanoparticle and the anchoring agents modified the catalyst surface to have a selectively higher affinity to the intermediate COOH over CO, which effectively lowers the overpotential.

Surface-Mediated Self-Coupling of Ethanol on Gold

The transformation of ethanol to its carbonyl compounds, namely acetaldehyde, ethyl acetate, acetic acid, and ketene, occurs on Au(111) with O-containing Au nanoparticles formed as a result of Au atom release upon ozone exposure. The product distribution strongly depends on the surface oxygen coverage. Ethoxy and acetate are identified as two key reaction intermediates during the oxidation of ethanol. The formation of acetaldehyde is due to the deprotonation of ethoxy, which can be further oxidized into acetate. The low-temperature formation of the ester, ethyl acetate, proceeds via the coupling of acetaldehyde with excess surface ethoxy. These reaction pathways appear relevant to heterogeneous processes catalyzed by supported gold nanoparticles, thus providing further insight into the mechanistic origin of gold-mediated oxidation of alcohols.

A density functional study of clean and hydrogen-covered α-MoO3(010): Electronic structure and surface relaxation

We report extensive density functional theory calculations, using pseudopotentials with a plane-wave basis, for the properties of the (010) face of molybdenum trioxide (α-MoO3). The surface is modeled by a one-layer slab. Calculated bond lengths compare favorably with experimental measurements. The bonding of the different oxygen species to molybdenum is analyzed using the crystal orbital overlap population. This analysis indicates that the bonding is a combination of ionic and covalent character for all oxygen species. The terminal oxygen exhibits covalent bonding to Mo which is stronger than either of the two bridging oxygens. We also study the adsorption of hydrogen on this surface. Hydrogen is most strongly adsorbed over the terminal oxygen, followed by the asymmetric bridging oxygen, and then the symmetric bridging oxygen. This trend is explained in terms of simple chemical concepts. The inclusion of full surface relaxation is important for even a qualitative description of adsorbate bonding.

Sequential Photo-oxidation of Methanol to Methyl Formate on TiO2(110)

Methyl formate is produced from the photo-oxidation of methanol on preoxidized TiO(2)(110). We demonstrate that two consecutive photo-oxidation steps lead to methyl formate using mass spectrometry and scanning tunneling microscopy. The first step in methanol oxidation is formation of methoxy by the thermal dissociation of the O-H bond to yield adsorbed CH(3)O and water. Formaldehyde is produced via hole-mediated oxidation of adsorbed methoxy in the first photochemical step. Next, transient HCO is made photochemically from formaldehyde. The HCO couples with residual methoxy on the surface to yield methyl formate. Exposure of the titania surface to O(2) is required for these photo-oxidation steps in order to heal surface and near-surface defects that can serve as hole traps. Notably, residual O adatoms are not required for photochemical production of methyl formate or formaldehyde. All O adatoms react thermally with methanol to form methoxy and gaseous water at rt, leaving a surface devoid of O adatoms. The mechanism provides insight into the photochemistry of TiO(2) and suggests general synthetic pathways that are the result of the ability to activate both alkoxides and aldehydes using photons.

Oxygen-mediated coupling of alcohols over nanoporous gold catalysts at ambient pressures.

Heterogeneous catalysis is a key technology to address the ever-increasing demand for the cost-effective and environmentally friendly production of commodity and fine chemicals. A major challenge is the development of catalytic processes that operate at low temperatures with high conversion and high selectivity for the desired product. Achieving this goal would be facilitated by the rational design of catalysts based on a molecular-level understanding of the reactive processes that occur on the catalyst surface. However, the transfer of insights from model studies at low pressure to the complex environment of practical catalytic systems—the so-called “material” and “pressure gaps”— often proves challenging. For catalysis based on metallic gold such a correlation is feasible, opening the possibility of the directed preparation of catalysts. This is because most molecules, including H2O, bind weakly to metallic gold. Hence, the steady-state concentrations of surface species remain low—conditions that can be mimicked in ultrahigh vacuum (UHV) on metallic gold. This strong correspondence between model studies at very low pressure and the actual working catalyst was previously demonstrated for the selective oxidative coupling of methanol to give methyl formate, which was studied on metallic Au(111) containing atomic O and over nanoporous Au (npAu) catalysts using O2 as an oxidant. [4] These experiments demonstrated an astonishing correlation between the two regimes, thus making it possible to understand the reactivity and selectivity of npAu on a molecular level. In the present study, we pursued the question of whether the transferability of UHV-based insight is more universal and can be extended to the oxidation of other alcohols as well. The structure of npAu consists of a three-dimensional network of gold ligaments in the range of a few tens of nanometers (typically 30 to 50 nm, Figure 1) with a high surface area of approximately 10 m g . The surprising