Oxidation of formic acid on the platinum catalyst supported on high area carbon was investigated by potentiodynamic and quasi-steady-state polarization measurements. It was found that the poisoning ...of the reaction occurred both in the hydrogen region and in the double-layer region, but poisons were formed faster at lower potentials. Kinetics of the reaction was consistent with the dual path mechanism. At lower potentials HCOOH was oxidized to CO
2 at the Pt sites uncovered by CO
ads. If high coverage by the poisoning species was attained, the reaction reached the limiting current plateau and further increase of the current densities started at the potential of CO
ads oxidation. Kinetic parameters of the HCOOH oxidation suggested that the rate determining step was the transfer of the first electron from HCOOH
ads, which was adsorbed under the Temkin conditions. Oxidation of formic acid became pH-dependent reaction in the electrolytes of pH
<
1 with the reaction order with respect to H
+ ions of about −0.8.
The Pd and three AgPd alloy layers (AgPd1, AgPd2 and AgPd3) were electrodeposited onto Au disc electrodes from the solution containing high concentration of chloride ions (>12 M). All coatings were ...investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), anodic linear sweep voltammetry (ALSV), while their surface composition was investigated by X-ray photoelectron spectroscopy (XPS). The AgPd1 and AgPd2 samples were electrodeposited at different constant current densities (−0.178 mA cm−2 and -0.415 mA cm−2 respectively) to the charge of −0.2 C cm−2 (thickness ∼ 0.18 μm) at a stationary disc electrode, while the sample AgPd3 was electrodeposited to the charge of −3.0 C cm−2 (thickness ∼ 2.8 μm) at a constant current density of −7.0 mA cm−2 under the conditions of convective diffusion. Samples AgPd1 and AgPd2 had similar morphologies of low roughness, while the morphology of AgPd3 was characterized by large crystals and higher roughness. The most active and the most poisoning tolerant coatings for ethanol oxidation reaction (EOR) are the AgPd3 and AgPd1 alloy samples, containing 72.6 at.% Ag – 27.4 at.% Pd and 84.7 at.% Ag – 15.2 at.% Pd respectively (XPS analysis). In this study, we demonstrated for the first time that the activity for the EOR at AgPd alloys was closely related to the amount of non-reduced Ag2O (most probably as Ag – hydroxide). Accordingly, all AgPd alloy samples had to be cycled in the potential region of Ag2O formation and reduction before the investigation of the EOR, in order to provide their catalytic activity towards the EOR.
•Electrodeposited AgPd alloy coatings were tested as the ethanol oxidation reaction catalysts.•The highest activity was obtained for AgPd alloy, containing 76 at.% Ag – 24 at.% Pd.•Presence of non-reduced Ag2O at the surface is necessary for efficient ethanol oxidation reaction.•Ag2O was found to facilitate the ethanol oxidation kinetics through bifunctional mechanism.
Electrodeposited PdNi and Pd samples were tested for methanol and ethanol oxidation reaction (MOR and EOR) in alkaline solution using cyclic voltammetry (CV) and chronoamperometric (CA) measurements. ...Among the CVs for the MOR, all alloy samples showed higher current densities than pure Pd. Also the peak current densities for MOR of PdNi coatings are 5 to 8 times higher than that obtained on Pd coating depending on composition. Based on the ratio of the forward peak current to the backward peak current the tolerance of the catalysts to accumulation of carbonaceous species is determined. Besides this ratio increased with extending the anodic potential limit. The most tolerant catalyst is found to be Pd
0.74
Ni
0.28
in methanol oxidation. Comparative study of methanol and ethanol electrooxidation in the alkaline media reveals that Pd
0.74
Ni
0.26
alloy has a higher catalytic activity, as well as a better tolerance to accumulation of carbonaceous species.
•Platinum coated Bi deposits on GC electrode prepared by a two-step process.•Electrochemical treatment creates shell composed of Pt and Bi-oxide.•Pt(Bi)/GC compare to Pt/GC exhibit enhanced activity ...in formic acid oxidation.•Improved activity in formic acid oxidation with increasing Bi content.
Formic acid oxidation was studied on platinum-coated bismuth deposits on glassy carbon substrate. The catalyst was prepared by a two-step process using chronocoulometry, i.e. controlled amount of Bi was electrodeposited onto glassy carbon followed by electrodeposition of Pt layer. The amount of Pt was constant while the amount of Bi vary to correspond the molar ratio of Pt:Bi=1:0.1 or 1:1 or 1:10. AFM characterization of the electrode surface indicates that Pt is deposited preferentially on previously formed Bi particles, but cyclic voltammetry revealed Bi leaching meaning that Bi was not completely occluded by Pt. In order to obtain stable electrode surface, deposits were subjected to potential cycling up to 1.2V vs. SCE in supporting electrolyte, prior to use as catalysts for formic acid oxidation. On this way the electrodes composed of Bi core occluded by Pt and Bi-oxide surface layers were obtained. The Pt(Bi)/GC electrodes exhibit enhanced electrocatalytic activity in comparison to Pt/GC for formic acid oxidation which depends on composition and surface morphology. High currents and onset potential shifted to negative values indicate a significant increase in direct path what is explained through ensemble effect induced by Bi-oxide species interrupting Pt domains. Electronic modification of Pt both by surface and sub-surface Bi can play some role as well. Significantly prolonged potential cycling in supporting electrolyte of previously stabilized Pt(Bi)/GC electrodes by Bi oxide, led to considerably lower Bi leaching accompanied by dissolution and redeposition of Pt and the outcome of this treatment was Pt shell over Bi core. These Pt@Bi/GC catalysts also exhibit higher activity for HCOOH oxidation in comparison to Pt/GC depending on the quantity of Bi remained under Pt shell, but in this case the improvement is induced solely by electronic effect of under-laying Bi.
Electrochemical oxidation of HCOOH in H2SO4 and HClO4 solutions was examined on thin film Pt2Ru3/C electrode. XRD pattern revealed that Pt2Ru3 alloy consisted of the solid solution of Ru in Pt and ...the small amount of Ru or solid solution of Pt in Ru. According to STM images, Pt2Ru3 particles size was between 2 and 6nm. It was established that electrochemical oxidation of HCOOH commenced at -0.1V versus SCE at Pt sites in the catalyst. Kinetic parameters indicated that dehydrogenation path was predominant. Dehydration occurs in parallel, but without significant poisoning by COad owing to oxidative removal by OH species on Ru atoms. The coverage of Pt2Ru3 surface by CO preadsorbed from the solution was found to be 24% lower when the surface was modified by irreversibly adsorbed Bi. Modification by Bi also shifted the onset potential for HCOOH oxidation for about 50mV towards more negative values and consequently, increased the reaction rate for a factor of two. It was proposed that Ru acts through bifunctional mechanism, i.e. OH species adsorbed on Ru oxidizes COad from Pt sites, while Bi hinders the adsorption of CO on Pt sites via electronic and/or ensemble effects.
The Ag, Pd and AgPd alloys of different morphologies and compositions were electrodeposited onto Au and glassy carbon (GC) disc electrodes from the solution containing 0.001 M PdCl2 + 0.04 M ...AgCl + 0.1 M HCl + 12 M LiCl under the conditions of non-stationary (RPM = 0, samples AgPd1 and AgPd2) and convective diffusion (RPM = 1000, sample AgPd3). Electrodeposited alloy layers were characterized by the scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). Oxygen reduction reaction (ORR) was investigated on all coatings in 0.1 M NaOH solution saturated with oxygen. The j-E curves for the ORR were recorded by two procedures: (1) samples were cycled with 5 mV s−1 from open circuit potential (OCP) to −0.8 V for Ag and AgPd alloys (or −0.6 V for pure Pd) and back; (2) samples were cycled with 5 mV s−1 from open circuit potential to 0.45 V (formation of Ag2O, in the case of Pd formation of PdO and PdO2), from 0.45 V to −0.60 V and back to the OCP. Significant catalytic activity for the Ag and AgPd alloys was detected after cycling electrodes in the potential region of Ag2O formation and reduction. Increase of the catalytic activity for AgPd alloys was, for the first time in the literature, ascribed to the presence of a certain amount of Ag2O which could not be completely reduced during the reverse sweep from 0.45 V to −0.6 V. Catalytic activity of AgPd alloys was found to be closely related to the amount of non-reduced Ag2O (most probably in the form of Ag-hydroxide). In the absence of such treatment, the catalytic activity for the ORR on electrodeposited Ag and AgPd alloy coatings was not detected.
Methanol oxidation was studied in 0.1 M NaOH at supported Pt electrodes, and compared with the single crystal Pt electrodes, Pt(1
1
1), Pt(1
1
0) and Pt(3
3
2), chosen as model systems, and with a ...polycrystalline Pt electrode. The supported Pt electrodes were obtained by chemical (Pt–C/GC) and electrochemical (Pt/GC) deposition of the catalyst layer on glassy carbon resulting in the same metal loading of 20 μg
Pt
cm
−2. Using STM in air, the average particle size distributions, 3–6 nm at Pt–C/GC and 4–32 nm at Pt/GC were determined. Both supported Pt catalysts were less active than polycrystalline Pt. Negligible differences in the kinetics observed between Pt–C/GC and Pt(1
1
0) and also Pt/GC and Pt(1
1
1) suggested that the activities of supported Pt electrodes could be correlated with the activities of single crystal Pt electrodes oriented as the sites dominating in the Pt particles in catalyst deposits. The electrocatalytic activities of the electrodes studied increase in the sequence: Pt(3
3
2)
>
polycrystalline Pt
>
Pt–C/GC
∼
Pt(1
1
0)
>
Pt/GC
∼
Pt(1
1
1). On the basis of diagnostic criteria obtained, the chemical reaction between HCO
ad and OH
ad giving formate was proposed as the rate limiting step in methanol oxidation.
Esomeprazole is the most effective of the proton-pump inhibitors for the acid-related diseases and at first was examined for the electroanalytical purposes. The drug standard and as a content of ...injection powder was investigated by cyclic voltammetry (CV) and quantitatively determined using square wave voltammetry (SWV) via its electrooxidation at Au electrode in 0.05M NaHCO3. SWV showed a linear dependency of the anodic peak currents vs. esomeprazole standard concentrations in the range from 3.0 to 500μgmL−1 with the values of limit of detection (LOD) and limit of quantification (LOQ): 1.4 and 4.6μgmL−1, respectively. Using the constructed and validated calibration curve, the values of unknown esomeprazole concentrations in injection powder and in human serum spiked with standard were determined. Before the electrochemical oxidation, it was shown by atomic force microscopy (AFM) that the small esomeprazole islands formed inside holes were visible and their diameter was about 200nm attributed to physico-chemical characteristics of esomeprazole. After the electrochemical oxidation, the morphology of esomeprazole standard on Au surface was completely changed and composed of spherical particles in a diameter between 200 and 600nm. With esomeprazole suspended in human serum, the process of crystallization partly occurred in the form of spherical grains with the average size of these grains was about 4μm. The analysis at the macro level done by the optical microscopy (OM) confirmed this opinion.
The study of esomeprazole degradation showed that at Au electrode, after 3h of cycling, a neglectable amount of the esomeprazole was changed. Using IrOx electrode under directed stress conditions, its almost complete degradation was realized after 3h confirmed by high performance liquid chromatography (HPLC). Total organic carbon (TOC) analysis showed that 95% of esomeprazole was mineralized. The HPLC and Liquid chromatography-mass spectrometry (LC-MS) study revealed the formation of 4-hydroxy omeprazole sulphide, 4-hydroxy omeprazole sulphone, esomeprazole sulphone and methylated esomeprazole.
Display omitted
•SWV method for esomeprazole determination using Au electrode was developed.•Method was applied for esomeprazole determination in human serum/injection powder.•Morphology of Au electrode surface during study was characterized by AFM and OM.•Esomeprazole degradation at IrOx electrode led to 95% of its mineralization.•Several degradation products were detected by HPLC and LCMS analysis.
PDF
