Understanding Oxygen Activation on Nanoporous Gold

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.

Activity origin of boron doped carbon cluster for thermal catalytic oxidation: Coupling effects of dopants and edges

Catalytic oxidation plays important roles in energy conversion and environment protection. Boron-doped crystalline carbocatalyst has been demonstrated effective; however, the application potential of boron-doped amorphous carbocatalyst remains to be explored. For amorphous carbon material, finite-sized carbon clusters are the basic structural units, which exhibit unique activity due to edge and size effect. Herein, using sulfur dioxide (SO₂) and carbon monoxide (CO) oxidation as probe thermal-catalysis reactions, we found the distribution and reactivity of active sites in boron-doped carbon clusters are simultaneously determined by dopants and edges. According to comparisons of oxygen (O₂) chemisorption energy at different sites of symmetric and non-symmetric carbon cluster, the most active site is found to be the edge carbon atom with high electron donation ability, which can be accurately identified by electrophilic Fukui function. More importantly, the reactivity of boron-doped cluster is simultaneously influenced by doping configuration and the type of edge, based on which -O-B-O- configuration embedded into K-region edge (isolated carbon-carbon double bonds that do not belong to Clar sextet) is predicted to exhibit the highest reactivity among various boron doping configurations. This work clarifies unique activity origin of heteroatom-doped amorphous carbon materials, providing new insights into designing high-performance carbocatalysts.

Electrochemical Molecular Conversion of α-Keto Acid to Amino Acid at a Low Overpotential Using a Nanoporous Gold Catalyst

A nanoporous gold (NPG) electrode prepared through a facile anodization technique was employed in the electrochemical reductive amination of biomass-derivable α-keto acids in the presence of a nitrogen source to produce the corresponding amino acids. NPG showed a clear reductive current in the presence of α-keto acid and NH₂OH, and the electrolysis experiments confirmed the production of L-amino acid. A reductive voltammetric signal at the NPG electrode appeared at a more positive potential by 0.18-0.79 V, compared with those at the planar-gold electrode without anodization and other previously reported electrode systems, indicating the high activity of the prepared nanostructure for the electrochemical reaction. Maximum Faradaic efficiencies (FEs) of 74-93% in the reductive molecular conversion to amino acids of Ala, Asp, Glu, Gly, and Leu were obtained under the optimized conditions. The FE values were strongly dependent on the applied potential in the electrolysis, suggesting that the hydrogen evolution reaction at the electrode surface was more significant as the applied potential became more negative. The effect of potential at the NPG was lower than that at the planar-gold electrode. These results indicate that nanostructurization decreases the overpotential for the electrochemical reductive amination, resulting in high FE.

First-principles-based kinetic Monte Carlo simulations of CO oxidation on catalytic Au(110) and Ag(110) surfaces

At the core of the development of more efficient and reliable fuel cells (FCs), there are several essential chemical reactions, namely carbon monoxide (CO) oxidation. This reaction is a keystone in the cleaning of hydrogen fuel used in fuel cells due to strong poisoning by this species of the platinum catalyst used in these devices. The present work aims to provide insight regarding the activation of CO oxidation by gold or silver microfacets possessing low coordinated atoms. To achieve this, density functional theory (DFT) quantum calculations, which determined two competing reaction pathways for CO oxidation, i.e., by molecularly adsorbed oxygen, and by dissociated oxygen, are combined with first-principles kinetic Monte Carlo (1p-kMC) simulations, which employed the resulting DFT parameters in order to address the effect of temperature and partial pressures and the interplay of the elementary reaction events. The use of 1p-kMC is a step further from available works regarding the CO oxidation on gold- and silver-based catalysts for cleansing of hydrogen that is used as a fuel in FCs. Indeed, this research contributes to the conclusion that CO oxidation should preferentially occur on silver microfacets, while the obtained turnover frequencies (TOFs) reinforced such a conclusion.

In-situ reconstruction of non-noble multi-metal core-shell oxyfluorides for water oxidation

The electrochemical anodic behavior of transition metal compounds plays an undeniably non-negligible role across many electrooxidation reactions. In this work, a chronopotentiometric technique was employed to activate the multicomponent non-noble metal oxyfluorides in-situ for oxygen evolution reaction (OER). It is interesting to unravel that the increasing applied current density helps to reconstruct the catalyst into nanoporous core-shell structure and introduce metal oxyhydroxide on the surface, which guarantees more channels for efficient ion/mass transportation and thus contributes to exposing more active sites for catalytic reaction. The activated five-membered oxyfluoride shows the best catalytic activity with overpotential of 348 ± 2 mV to achieve the current density of 10 mA/cm² and a Tafel slope of 110.3 ± 0.1 mV/dec, in contrast with the pristine one (532 ± 2 mV & 240.2 ± 0.1 mV/dec). It still maintains high stability after long time OER measurement, making it a promising succedaneum for noble metal catalysts. The high-entropy effect, amorphous state and high active sites density jointly contribute to its enhanced OER performance. This work provides new ideas for realizing the potential of inactive elements via entropy engineering and using electrochemical self-reconstruction to modify semiconductors for advanced water oxidation.