The electrocatalytic properties of the oxides of noble metals in the electrooxidation of methanol and formic acid

Sequential electrodeposition of Cu-Pt bimetallic nanocatalysts on boron-doped diamond electrodes for the simple and rapid detection of methanol

In this work, a novel electrochemical sensor for methanol determination was established by developing a bimetallic catalyst with superiority to a monometallic catalyst. A Cu–Pt nanocatalyst was proposed and easily synthesized by sequential electrodeposition onto a boron-doped diamond (BDD) electrode. The successful deposition of this nanocatalyst was then verified by scanning electron microscopy and energy dispersive spectroscopy. The electrodeposition technique and sequence of metal deposition significantly affected the surface morphology and electrocatalytic properties of the Cu–Pt nanocatalyst. The presence of Cu atoms reduced the adsorption of other species on the Pt surface, consequently enhancing the long-term stability and poisoning tolerance of Pt nanocatalysts during the methanol oxidation process. This advanced sensor was also integrated with sequential injection analysis to achieve automated and high-throughput analysis. This combination can significantly improve the detection limit of the developed sensor by approximately 100 times compared with that of the cyclic voltammetric technique. The limit of detection of this sensor was 83 µM (S/N = 3), and wide linearity of the standard curve for methanol concentrations ranging from 0.1 to 1000 mM was achieved. Finally, this proposed sensor was successfully applied to detect methanol in fruit and vegetable beverage samples.

Gold nanocrystal arrays as electrocatalysts for the oxidation of methanol and ethanol

A general difficulty in the comparison of catalysts regarding their electrochemical activities is the significant dependency on the electrode preparation method. In addition to single-crystal, thin-film, and polycrystalline electrodes, most electrocatalysts contain a physical mixture of catalytically active nanocrystals (NCs), conductive carbon support, and binding agent. This type of preparation makes the agglomeration of NCs to larger entities inevitable and simultaneously decreases the catalytically active surface area. In this work, electocatalysts based on two-dimensional arrays of self-assembled monodisperse Au NCs with a particle size of 8 nm have been fabricated. Their electrocatalytic performance in the electrochemical oxidation reaction of methanol and ethanol was investigated for different pH values. The self-assembly of Au NCs into two-dimensional arrays enables to fabricate electrocatalysts with a high mass activity in alkaline electrolytes for alcohol oxidation reactions.

In situ surface-enhanced Raman spectroscopic study of formic acid electrooxidation on spontaneously deposited platinum on gold

Present formic acid fuel cell efficiency is limited by low kinetics at the anode, indicating the need for effective catalysts to improve the formic acid oxidation. As a prerequisite, the nature of adsorbed species and specifically the reaction intermediates formed in this process needs to be examined. This work focuses on the electrooxidation of formic acid and the nature of the intermediates at a platinum-modified gold surface prepared through spontaneous deposition using a combination of electrochemistry and in situ surface enhanced Raman spectroscopy (SERS). This Pt-modified gold electrode surface assists in oxidizing formic acid at potentials as low as 0.0 V vs. Ag/AgCl which is 0.15 V more negative than a bare Pt surface. The oxidation current obtained on the Pt-modified gold electrode is 72 times higher than on a bare Au surface and 5 times higher than on a bare Pt surface at the same potential. In situ SERS has revealed the involvement of formate at a low frequency as the primary intermediate in this electrooxidation process. While previous studies mainly focused on the formate mode at ca. 1322 cm(-1), it is the first time that a formate peak at ca. 300 cm(-1) was observed on a Pt or Pt-associated surface. A unique relationship has been observed between the formic acid oxidation currents and the SERS intensity of this formate adsorbate. Furthermore, the characteristic Stark effect of the formate proves the strong interaction between the adsorbate and the catalyst. Both electrochemical and spectroscopic results suggest that the formic acid electrooxidation takes place by the dehydrogenation pathway involving a low frequency formate intermediate on the Pt-modified gold electrode catalyst.

Theoretical Study of Methanol Decomposition Mediated by Au3+, Au3 and Au3−: Mechanism and Effect of Charge State of Gold on its Catalytic Activity

By performing density functional theory calculations, we studied methanol decomposition promoted by neutral, anionic and cationic Au trimers, which represent the three simplest prototypes of Au-cluster-based catalysts with different charge states. The results show that the Au3– and Au3+-mediated reactions proceed via four successive single dehydrogenation steps, CH3OH → CH3O → CH2O → CHO → CO, while the Au3–-mediated reaction occurs through two double dehydrogenation steps, CH3OH → CH 2O → CO. The additional negative charge reduces remarkably the binding capability of CO (the completely dehydrogenated product of methanol) on the cluster, and is thus beneficial in reducing poisoning of the catalyst by CO. In contrast, the neutral and positively charged clusters present strong interactions with CO, making the catalyst readily poisoned by CO. Furthermore, the reaction promoted by the cationic cluster shows a much higher energy barrier than those by the neutral and anionic clusters. So selecting suitable substrates that make Au nanoparticles negatively charged may be a promising strategy for promoting methanol oxidation.

Electrochemical and STM studies of 1-thio-β-D-glucose self-assembled on a Au(111) electrode surface

In this study, a Au(111) electrode is functionalized with a monolayer of 1-thio-β-D-glucose (β-Tg), producing a hydrophilic surface. A monolayer of β-Tg was formed on a Au(111) surface by either (1) potential-assisted deposition with the thiol in a supporting electrolyte or (2) passive incubation of a gold substrate in a thiol-containing solution. For each method, the properties of the β-Tg monolayer were investigated using cyclic voltammetry (CV), differential capacitance (DC), and chronocoulometry. In addition, electrochemical scanning tunneling microscopy (EC-STM) was used to obtain images of the self-assembled monolayer with molecular resolution. Potential-assisted assembly of β-Tg onto a Au(111) electrode surface was found to be complicated by oxidation of β-Tg molecules. The EC-STM images revealed formation of a passive layer containing honeycomb-like domains characteristic of a formation of S(8) rings, indicating the S-C bond may have been cleaved. In contrast, passive self-assembly of thioglucose from a methanol solution was found to produce a stable, disordered monolayer of β-Tg. Since the passive assembly method was not complicated by the presence of a faradaic process, it is the method of choice for modifying the gold surface with a hydrophilic monolayer.