Aerobic oxidation of alkanes on icosahedron gold nanoparticle Au55

Seeing an Unobservable Fe(III)/Fe(IV) Redox Process of the Nonheme Iron N4Py Complex by High-Speed Surface-Enhanced Raman Spectroelectrochemistry

High-valent iron oxido species, central to many enzymatic and biomimetic catalyzed organic oxidative transformations, can be generated by direct electrochemical oxidation, circumventing high-energy O atom donor reagents. Electrochemical generation necessitates knowledge of the redox potentials involved, which is hindered by the lack of well-defined Fe(III)/Fe(IV) redox waves in the voltammetry of many iron-based catalysts. Hence, other approaches including chemical oxidation and bulk spectro(electro)chemical methods need to be taken. In the case of the well-studied oxidation catalyst, [ ( N 4 P y ) F e ( I I ) O H 2 ] 2 + , where N4Py is 1,1-bis(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine, estimates of the Fe(III/IV) redox potentials range from 0.4 to 1.3 V vs SCE. Here, we show that electrochemical surface-enhanced Raman scattering spectroscopy reveals "hidden" redox waves, and hence redox potentials, when coupled with cyclic voltammetry. Rapid spectral acquisition (>2 Hz) of surface-enhanced Raman spectra at electrochemically roughened gold electrodes enables real-time spectral acquisition during cyclic voltammetry. We show that the Fe(III)/Fe(IV) redox potential of [ ( N 4 P y ) F e ( I I ) O H 2 ] 2 + is close to that determined earlier by chemical redox titrations (0.85 V vs SCE). Furthermore, comproportionation and adsorption processes are shown to impact the rates of electron transfer observed, which rationalizes the absence of a distinct Fe(III)/Fe(IV) redox wave in its cyclic voltammetry.

Adsorption of O2 on the Preferred -O-Au Sites of Small Gold Oxide Clusters: Charge-dependent Interaction and Activation

Decades of research have illuminated the significant roles of gold/gold oxide clusters in small molecule catalytic oxidation. However, many fundamental questions, such as the actual sites to adsorb and activate O2 and the impact of charge, remain unanswered. Here, we have utilized an improved genetic algorithm program coupled with the DFT method to systematically search for the structures of Au1-5Ox-/+/0 (x = 1-4) and calculated binding interactions between Au1-5Ox-/+/0 (x = 1-2) and O2, aiming to determine the active sites and to elucidate the impact of different charge states in gold oxide systems. The results revealed that the reactivity of all three kinds of small gold oxide clusters toward O2 is strongly site-dependent, with clusters featuring an -O-Au site exhibiting a preference for adsorption. The charges on small gold oxide clusters significantly impact the interaction strength and the activation degree of adsorbed O2: in the case of anionic cluster, the interaction between O2 and the -O-Au sites leads to a chemical reaction involving electron transfer, thereby significantly activating O2; in neutral and cationic clusters, the adsorption of O2 on their -O-Au sites can be viewed as an electrostatic interaction. Pointedly, for cationic clusters, the highly concentrated positive charge on the Au atom of the -O-Au sites can strongly adsorb but hardly activate the adsorbed O2. These results have certain reference points for understanding the gold oxide interfaces and the improved catalytic oxidation performance of gold-based systems in the presence of atomic oxygen species.

Supported Gold Nanoparticles as Catalysts for the Oxidation of Alcohols and Alkanes

Supporting gold nanoparticles have shown to be extremely active for many industrially important reactions, including oxidations. Two representative examples are the oxidation of alcohols and alkanes, that are substrates of industrial interest, but whose oxidation is still challenging. This review deals with these reactions, giving an insight of the first studies performed by gold based catalysts in these reactions and the most recent developments in the field.

Synthesis of Bimetallic Gold-Silver (Au-Ag) Nanoparticles for the Catalytic Reduction of 4-Nitrophenol to 4-Aminophenol

Bimetallic gold-silver nanoparticles as unique catalysts were prepared using seed colloidal techniques. The catalytic capabilities of the nanoparticles were ascertained in the reduction of 4-nitrophenol to 4-aminophenol in the presence of sodium borohydride. Our results clearly showed that the rate of 4-NP reduction to 4-AP increased with a corresponding decrease in the diameter of the bimetallic NPs. The Au-Ag nanoparticles prepared with 5.0 mL Au seed volume indicated higher reduction activity, which was approximately 1.2 times higher than that of 2.0 mL Au seed volume in the reductive conversion of 4-NP to 4-AP. However, the monometallic NPs showed relatively less catalytic activity in the reductive conversion of 4-NP to 4-AP compared to bimetallic Au-Ag nanoparticles. Our studies also reinforced the improved catalytic properties of the bimetallic Au-Ag nanoparticles structure with a direct impact of the size or diameter and relative composition of the bimetallic catalytic nanoparticles.