Nanoporous gold modified by atomic layer deposition of titania was employed as a model, inverse supported precious metal catalyst to explore the oxidative reactivity of Au/TiO2 catalysts. The catalyst material is active for oxidation of CO using O from the titania lattice, even at low temperatures. The reactive lattice oxygen available for oxidative reactions was formed by thermal activation, with the total oxidative reactivity of the material showing a strong dependence on the annealing temperature prior to CO exposure. Finally, reduction of the titania via CO oxidation can be reversed under relatively mild conditions by exposure to O2, presenting a viable mechanism for CO oxidation in steady state catalytic conditions.

Understanding photochemical processes on nanomaterials is key to developing effective photocatalysts. Herein, methanol oxidation and reduction is used to probe the thermal and photochemical properties of rutile titania nanowires grown using a hydrothermal method. The presence of oxygen vacancy defects leads to methoxy formation and subsequent disproportionation to formaldehyde and methanol at 700 K. Methane and dimethyl ether are also produced in minor quantities. Oxygen adatoms enhance the formation of methoxy, which led to an increase in the disproportionation products and dimethyl ether at high temperature and a decreased amount of methane. The thermal reactivity of the nanowires parallels that of rutile TiO2(110) single crystals. Photo-oxidation of methoxy using UV light produced formaldehyde and methyl formate. These product yields were enhanced on nanowires with oxygen adatoms, but a majority of methoxy (∼70%) is not photoactive. In contrast, all methoxy is photo-oxidized on rutile TiO2(110) when O-adatoms are present. This difference indicates that holes created in the nanowires during UV excitation do not migrate to most of the methoxy—a required step for methoxy photo-oxidation. This lack of activity could be due to either trapping of holes in the material or different binding of the inactive methoxy. These studies demonstrate that while charge carriers can be efficiently created in nanowires differences in chemical properties can suppress photo-oxidation.

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.Keywords: alcohols; amides; coupling; esters; gold; heterogeneous catalysis; selective oxidation;

Enhancing the selectivity of catalytic processes has potential for substantially increasing the sustainability of chemical production. Herein, we establish relationships between reaction selectivity and molecular structure for a homologous series of key intermediates for oxidative coupling of alcohols on gold using a combination of experiment and theory. We establish a scale of binding for molecules with different alkyl structures and chain lengths and thereby demonstrate the critical nature of noncovalent van der Waals interactions in determining the selectivity by modulating the stability of key reaction intermediates bound to the surface. The binding hierarchy is the same for Au(111) and Au(110), which demonstrates a relative lack of sensitivity to the surface structure. The hierarchy of binding established in this work provides guiding principles for predicting how molecular structure affects the competition for binding sites more broadly. Besides the nature of the primary surface-molecule bonding, three additional factors that affect the stabilities of the reactive intermediates are clearly established: (1) the number of C atoms in the alkyl chain, (2) the presence of C–C bond unsaturation, and (3) the degree of branching of the alkyl group of the adsorbed molecules. We suggest that this is a fundamental principle that is generally applicable to a broad range of reactions on metal catalysts.

Here we demonstrate the gas-phase catalytic production of methyl acrylates by oxygen-assisted coupling of methanol with the unsaturated alcohols allyl alcohol and methylallyl alcohol over nanoporous gold (npAu) at atmospheric pressure. Analogous investigations on O-activated Au(110) exhibit the same pattern of reactivity and are used to establish that the competition between methoxy and allyloxy (or methallyloxy) reaction intermediates for adsorption sites, mediated by the reactants themselves, determines the selectivity of reaction. Our results clearly show that the C═C bond substantially increases the binding efficacy of the allyloxy (or methallyloxy), thus requiring extremely high methanol mole fractions (>0.99) in order to achieve comparable surface concentrations of methoxy and produce optimum yields of either methacrylate or methyl methacrylate. Allyloxy and methallyloxy were favored by factors of ∼100 and ∼450, respectively, vs methoxy. These values are more than 1 order of magnitude greater than those measured for competitive binding of ethoxy and 1-butoxy vs methoxy, demonstrating the strong effect of the carbon–carbon bond unsaturation. The 4.5-fold increase due to the addition of the methyl group in methylallyl alcohol vs allyl alcohol indicates the significant effect of the additional van der Waals interactions between the methyl group and the surface. Gas-phase acidity is also shown to be a good qualitative indicator for the relative binding strength of the alkoxides. This work provides insight into the control of reaction selectivity for coupling reactions and demonstrates the value of fundamental studies on single crystals for establishing key principles governing reaction selectivity. Notably, these oxygen-assisted coupling reactions occur without oxidation of the C═C bond.Keywords: esterification; green catalysis; methyl acrylate; methyl methacrylate; nanoporous gold; selective alcohol oxidation

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. Once activated in this manner, they can be stored and easily reactivated by flow of reactant gases at 150 °C for a few hours. They possess improved selectivity for the coupling of higher alcohols, such as 1-butanol, and are not active for carbon monoxide oxidation. This ozone-treated npAu is a functionally new catalytic material.Keywords: activation; energy-efficient catalysis; nanoporous gold; ozone; selective oxidation

Nanoporous gold (np-Au), a three-dimensional nanoporous bulk material, is made by selective corrosion of Ag from Ag–Au alloys, a technique already applied by the pre-Columbian cultures of South America. Nanoporous gold is actually a Au-rich Ag–Au alloy which, specifically the Ag0.03Au0.97 composition, combines high reactivity and selectivity for a wide variety of oxidation reactions, from simple CO oxidation to complex oxygen-assisted coupling reactions. Its catalytic reactivity is surprising because np-Au is a nonsupported Au catalyst with relatively large feature sizes on the order of tens of nanometers, thus breaking the generally accepted notion that gold must be in the form of small particles (about a few nanometers) to be an active catalyst. The ease of sample preparation in combination with high reactivity, selectivity, and long-term stability suggests that nanoporous gold has the potential to bring Au catalysis closer to practical applications. In this perspective, we provide a critical review of the current understanding of the origin of the high catalytic activity of nanoporous gold in context of morphology and surface composition.Keywords: activation; energy-efficient catalysis; nanoporous gold; ozone; selective oxidation

The oxidative coupling reaction of aldehydes with methanol occurs in the vapor phase over a support-free nanoporous gold (npAu) catalyst over a wide pressure range—from 10−9 Torr to 1 atm. The dependence of the aldehyde-to-ester reaction rate on the oxygen, methanol and aldehyde partial pressures suggests that the rate-limiting step for coupling is the reaction of the aldehyde with surface sites saturated with adsorbed methoxy. Stable catalyst activity is achieved for aldehyde–methanol coupling in flowing reactant mixtures at 70 °C. While the conditioned npAu catalyst exhibits high selectivity for methanol–aldehyde coupling, its activity for the self-coupling reaction of methanol to methyl formate is reduced by the exposure to the alcohol–aldehyde mixture in a manner that is consistent with the buildup of spectator species. The activity for methanol self-coupling can be regenerated by extended exposure to flowing methanol, CO and O2 at 70 °C. Overall, the observed catalytic esterification is consistent with model studies of both the npAu catalyst and single crystal gold in ultrahigh vacuum.

A dilute Ag alloy of nanoporous Au (npAu) has been shown to self-couple methanol with 100 % selectivity and high conversion under catalytic flow conditions. However, because prior studies in flow reactors showed difficulty in self-coupling ethanol and 1-butanol over npAu in flow reactors, the inherent capability on npAu for self-coupling of ethanol and 1-butanol was examined under ultrahigh vacuum conditions on identical npAu catalysts. This study shows that the oxygen-covered Ag-modified npAu does efficiently effect the self-coupling of ethanol and 1-butanol under UHV conditions. The coupling is initiated by adsorbed atomic oxygen formed from O2 dissociation via a chemisorbed molecular state. The amount of ester formed increases with the degree of oxygen precoverage at the expense of aldehyde production. Repeated annealing of the catalyst above 550 K for temperature programmed reaction changes the ligament and pore sizes, affecting the product distribution, but high reactivity is sustained over many heating cycles.

The oxidative coupling reaction of aldehydes with methanol occurs in the vapor phase over a support-free nanoporous gold (npAu) catalyst over a wide pressure range—from 10−9 Torr to 1 atm. The dependence of the aldehyde-to-ester reaction rate on the oxygen, methanol and aldehyde partial pressures suggests that the rate-limiting step for coupling is the reaction of the aldehyde with surface sites saturated with adsorbed methoxy. Stable catalyst activity is achieved for aldehyde–methanol coupling in flowing reactant mixtures at 70 °C. While the conditioned npAu catalyst exhibits high selectivity for methanol–aldehyde coupling, its activity for the self-coupling reaction of methanol to methyl formate is reduced by the exposure to the alcohol–aldehyde mixture in a manner that is consistent with the buildup of spectator species. The activity for methanol self-coupling can be regenerated by extended exposure to flowing methanol, CO and O2 at 70 °C. Overall, the observed catalytic esterification is consistent with model studies of both the npAu catalyst and single crystal gold in ultrahigh vacuum.