Effects of Oxygen Adsorption on the Optical Properties of Ag Nanoparticles

Plasmonic metal nanoparticles are efficient light harvesters with a myriad of sensing- and energy-related applications. For such applications, the optical properties of nanoparticles of metals such as Cu, Ag, and Au can be tuned by controlling the composition, particle size, and shape, but less is known about the effects of oxidation on the plasmon resonances. In this work, we elucidate the effects of O adsorption on the optical properties of Ag particles by evaluating the thermodynamic properties of O-decorated Ag particles with calculations based on the density functional theory and subsequently computing the photoabsorption spectra with a computationally efficient time-dependent density functional theory approach. We identify stable Ag nanoparticle structures with oxidized edges and a quenching of the plasmonic character of the metal particles upon oxidation and trace back this effect to the sp orbitals (or bands) of Ag particles being involved both in the plasmonic excitation and in the hybridization to form bonds with the adsorbed O atoms. Our work has important implications for the understanding and application of plasmonic metal nanoparticles and plasmon-mediated processes under oxidizing environments.

Nanoporous Gold: From Structure Evolution to Functional Properties in Catalysis and Electrochemistry

Nanoporous gold (NPG) is characterized by a bicontinuous network of nanometer-sized metallic struts and interconnected pores formed spontaneously by oxidative dissolution of the less noble element from gold alloys. The resulting material exhibits decent catalytic activity for low-temperature, aerobic total as well as partial oxidation reactions, the oxidative coupling of methanol to methyl formate being the prototypical example. This review not only provides a critical discussion of ways to tune the morphology and composition of this material and its implication for catalysis and electrocatalysis, but will also exemplarily review the current mechanistic understanding of the partial oxidation of methanol using information from quantum chemical studies, model studies on single-crystal surfaces, gas phase catalysis, aerobic liquid phase oxidation, and electrocatalysis. In this respect, a particular focus will be on mechanistic aspects not well understood, yet. Apart from the mechanistic aspects of catalysis, best practice examples with respect to material preparation and characterization will be discussed. These can improve the reproducibility of the materials property such as the catalytic activity and selectivity as well as the scope of reactions being identified as the main challenges for a broader application of NPG in target-oriented organic synthesis.

Dilute Alloys Based on Au, Ag, or Cu for Efficient Catalysis: From Synthesis to Active Sites

The development of new catalyst materials for energy-efficient chemical synthesis is critical as over 80% of industrial processes rely on catalysts, with many of the most energy-intensive processes specifically using heterogeneous catalysis. Catalytic performance is a complex interplay of phenomena involving temperature, pressure, gas composition, surface composition, and structure over multiple length and time scales. In response to this complexity, the integrated approach to heterogeneous dilute alloy catalysis reviewed here brings together materials synthesis, mechanistic surface chemistry, reaction kinetics, in situ and operando characterization, and theoretical calculations in a coordinated effort to develop design principles to predict and improve catalytic selectivity. Dilute alloy catalysts─in which isolated atoms or small ensembles of the minority metal on the host metal lead to enhanced reactivity while retaining selectivity─are particularly promising as selective catalysts. Several dilute alloy materials using Au, Ag, and Cu as the majority host element, including more recently introduced support-free nanoporous metals and oxide-supported nanoparticle "raspberry colloid templated (RCT)" materials, are reviewed for selective oxidation and hydrogenation reactions. Progress in understanding how such dilute alloy catalysts can be used to enhance selectivity of key synthetic reactions is reviewed, including quantitative scaling from model studies to catalytic conditions. The dynamic evolution of catalyst structure and composition studied in surface science and catalytic conditions and their relationship to catalytic function are also discussed, followed by advanced characterization and theoretical modeling that have been developed to determine the distribution of minority metal atoms at or near the surface. The integrated approach demonstrates the success of bridging the divide between fundamental knowledge and design of catalytic processes in complex catalytic systems, which can accelerate the development of new and efficient catalytic processes.

Gold-silver nanoparticles modified electrochemical sensor array for simultaneous determination of chromium(III) and chromium(VI) in wastewater samples

The oxidation state of ions is a crucial aspect that often has been overlooked when determining the toxicity of chromium (Cr) species in environmental samples. In this study, a novel electrochemical sensor array based on gold-silver nanoparticles modified electrodes was developed for simultaneous determination of the two main chromium species (Cr(III) and (VI)). Specifically, the working electrodes of screen-printed carbon electrodes (SPCEs) were modified with silver-gold bimetallic nanoparticles through electrochemical deposition for detection of Cr(VI). The silver-gold bimetallic nanoparticles were further oxidized to form stable silver-gold bimetallic oxide nanoparticles for the detection of Cr(III). The results showed that the addition of silver with a theoretical value of 1% of gold could contribute to the formation and stabilization of oxides on the surface of gold nanoparticles. After characterization, the two kinds of electrodes were integrated as an electrochemical sensor array for selective and sensitive detection of Cr(VI) and Cr(III). The linear range and limit of detection (LOD, identified by three times of signal-to-noise ratio) were found to be 0.05-5 ppm and 0.1 ppb for Cr(VI), and 0.05-1 ppm and 0.1 ppb for Cr(III), respectively. Finally, the electrochemical sensor array was proven for successful detection of Cr(VI) and Cr(III) in tap water, artificial saliva and artificial sweat samples, and monitoring of Cr(VI) and Cr(III) in chromium-containing wastewater treatment process. Combined with a handheld dual-channel electrochemical device, the simultaneous determination of Cr(VI), Cr(III) and total chromium contents can be easily achieved for various samples.

Molecular Dynamics and Machine Learning in Catalysts

Given the importance of catalysts in the chemical industry, they have been extensively investigated by experimental and numerical methods. With the development of computational algorithms and computer hardware, large-scale simulations have enabled influential studies with more atomic details reflecting microscopic mechanisms. This review provides a comprehensive summary of recent developments in molecular dynamics, including ab initio molecular dynamics and reaction force-field molecular dynamics. Recent research on both approaches to catalyst calculations is reviewed, including growth, dehydrogenation, hydrogenation, oxidation reactions, bias, and recombination of carbon materials that can guide catalyst calculations. Machine learning has attracted increasing interest in recent years, and its combination with the field of catalysts has inspired promising development approaches. Its applications in machine learning potential, catalyst design, performance prediction, structure optimization, and classification have been summarized in detail. This review hopes to shed light and perspective on ML approaches in catalysts.

Direct and Broadband Plasmonic Charge Transfer to Enhance Water Oxidation on a Gold Electrode

Plasmonic photocatalysis via hot charge carriers suffers from their short lifetime compared with the sluggish kinetics of most reactions. To increase lifetime, adsorbates on the surface of a plasmonic metal may create preferential states for electrons to be excited from. We demonstrate this effect with O adsorbates on a nanoporous gold electrode. Nanoporous gold is used to obtain a broadband optical response, to increase the obtained photocurrent, and to provide a SERS-active substrate. Only with adsorbates present, we observe significant photocurrents. Illumination also increases the adsorbate coverage above its dark potential-dependent equilibrium, as derived from a two-laser in situ SERS approach. Density functional theory calculations confirm the appearance of excitable states below the Fermi level. The photocurrent enhancement and broadband characteristics reveal the potential of the plasmonic approach to improve the efficiency of photoelectrochemical water splitting.

Aerobic Methanol Oxidation over Unsupported Nanoporous Gold: The Influence of an Added Base

We studied the aerobic oxidation of methanol over nanoporous gold catalysts under neutral and alkaline conditions. We find that under neutral conditions the catalyst has an activation period of about 10 h while upon addition of a base the catalyst becomes active right away. After this activation period, however, the activity of the catalyst is in both cases similar. Moreover, the selectivity was not affected by the base. We tested different bases and found the largest effect when adding OH−. The cation, however, does not play a role. We conclude that it is OH−, which is impacting the reaction and propose a mechanism for the suppression of the activation period. While the catalytic cycle, i.e., the reaction of methanol on the catalyst surface seems unaffected, the transient adsorption of OH− onto the surface can facilitate the activation of molecular oxygen by donating electrons to the surface. Due to the intermediate formation of oxidic Ag species, an effective segregation of surface-near Ag can be induced, which increases the abundance of Ag being essential for the activation of oxygen at the surface. In this way, a more efficient pathway for the generation of active oxygen is opened, allowing the reaction to set in faster.

Independent control over residual silver content of nanoporous gold by galvanodynamically controlled dealloying

A new procedure was developed and characterized for the galvanodynamically controlled dealloying (GCD) of AuxAg100-x alloys to obtain nanoporous gold (npAu) mainly as an unsupported catalyst material for partial oxidation of alcohols. Such catalysts require residual Ag content of less than 1 at%. GCD was compared to the preparation of npAu by potentiostatically controlled deallyoing (PCD) and free corrosion (FC). The main advantage of GCD is the ability to obtain npAu with a predetermined residual Ag content including residual Ag contents below 1 at% while retarding the coarsening of the ligaments. For PCD and FC, there is a strong increase of ligament size with decreasing residual Ag content because the longer times required for dealloying unavoidably lead to coarsening of the npAu structure. On the other hand, GCD also prevented too high initial current density that leads to cracking of the samples and prevents formation of mechanically stable monoliths. GCD tolerates different compositions of the starting alloy for AuxAg100-x within the tested composition range (20 at% ≤ xAu ≤ 30 at%). The samples obtained by GCD were tested for methanol and ethanol oxidation and showed favorable characteristics for partial oxidation of methanol to methyl formate and of ethanol to ethyl acetate.

CO adsorption and oxygen activation on group 11 nanoparticles – a combined DFT and high level CCSD(T) study about size effects and activation processes

The focus of this study lies in the activation of molecular oxygen and reaction with CO within density functional theory (DFT) and high level CCSD(T) calculations.

Electrocatalytic methanol oxidation with nanoporous gold: microstructure and selectivity

The properties of Nanoporous Gold (NPG) obtained by the selective dissolution of Ag from an Au-Ag alloy can be tuned by the details of its fabrication, and specifically the residual Ag content is correlated to the ligament size of the material. We link this correlation to methanol electro-oxidation. Specifically, two different NPG types (obtained by potentiostatic dealloying) are compared with one obtained by free corrosion. They show remarkable differences in activity. Quantitative product analysis reveals that NPG shows nearly selective oxidation of CH₃OH to HCOO^- when NPG is used as an active electrode in contrast to planar Au. This trend can further be enhanced when applying finer nanoporous structures that are linked to a higher Ag content. X-ray photoelectron spectroscopy (XPS) reveals changes in the nature of residual Ag from which we conclude that Ag is not a passive component in the methanol oxidation process.