UPD metal electrocatalysis of formic acid oxidation at teflon-bonded carbon substrate electrodes

Teflon-bonded porous electrodes have been prepared for the study of the upd metal effect on the electrocatalysis of formic acid oxidation. The influence of upd lead on finely divided platinum with porous carbon as substrate was tested in fuel cell anodes for the oxidation of formic acid in 1 M HClO₄ electrolyte.In order to investigate the increase in activity through the use of upd lead, the co-adsorption of lead and a strongly adsorbed intermediate was followed by a flux cell technique. From this study it follows that the third-body effect is of minor importance. Obviously upd lead itself catalyses the direct oxidation to CO₂.     

Electrochemical oxidation of formic acid at noble metals: Catalytic effects of foreign metal monolayers

The catalytic effects of a submonolayer of lead on noble metals have been shown and discussed in terms of a previously published model. A theoretical analysis of these effects is presented. It was demonstrated that foreign metal monolayers allow a determination of the true catalytic activity of electrodes in the case of self-poisoning reactions. A volcano-shaped curve for oxidation of formic acid on noble metal electrodes was obtained.     

钯纳米晶体在电催化甲酸氧化反应中的形貌效应

本文基于课题组前期工作,选用适当的金属前驱物、还原剂、稳定剂和保护剂,通过调控氧化刻蚀和反应动力学等,成功合成了形貌和尺寸均不相同的Pd纳米晶.经过认真的纳米粒子清洗和电极修饰组装,考察了它们在电催化甲酸氧化反应中的形貌与性能的关系.研究结果表明,Pd纳米晶样品的最大电流密度以纳米八面体（nanooctahedra）、纳米线（nanowires）、纳米立方体（nanocubes）、纳米瓜子（nanotapers）、凹面纳米立方体（concave nanocubes）的顺序递增,催化甲酸氧化反应的起始氧化电位均小于0.2V.研究结果印证了Pd纳米晶催化甲酸氧化反应的催化性能在尺寸效应上主要受活性表面积的影响,扣除表面积效应后的催化性能与其尺寸没有明确关系.该系列Pd纳米晶的催化性能主要取决于其表面结构,得出Pd纳米晶催化甲酸氧化反应遵循{111}晶面〈{100}晶面〈高指数晶面的性能活性顺序.综合最大电流密度和最小操作电位因素发现,Pd凹面纳米立方体和Pd纳米瓜子具有相对较好的商用价值.     

Structural effects of electrochemical oxidation of formic acid on single crystal electrodes of palladium

Structural effects on the rates of formic acid oxidation have been studied on Pd(111), Pd(100), Pd(110), and Pd(S)-[n(100) x (111)] (n = 2-9) electrodes in 0.1 M HClO4 containing 0.1 M formic acid with use of voltammetry. On the low index planes of Pd, the maximum current density of formic acid oxidation (jP) increases in the positive scan as follows: Pd(110) < Pd(111) < Pd(100). This order differs from that on the low index planes of Pt: Pt(111) < Pt(100) < Pt(110). Pd(S)-[n(100) x (111)] electrodes with terrace atomic rows n > or = 3 have almost the same jP as Pd(100), except Pd(911) n = 5. The value of jP on Pd(911) n = 5 is 20% higher than those of the other surfaces. Pd(311) n = 2, of which the first layer is composed of only step atoms, has the lowest jP in the Pd(S)-[n(100) x (111)] series. The adsorption geometry of the reaction intermediate (formate ion) is optimized by using density functional theory.     

The anodic oxidation of gold + palladium alloys

The anodic oxidation of Au+Pd alloys has been studied in solutions of 1 M H₂SO₄ and 0.1 M K₂SO₄ by voltammetric methods. A linear relationship between oxide reduction maximum and bulk alloy composition, often used to determine the surface composition of homogeneous alloys, could be shown to hold only for alloys up to 60 at% gold. At higher gold content the Au oxide peak must be additionally evaluated. With continuous cycling in acid solution the anodic dissolution of Pd, especially from gold-rich places, leads to a rather heterogeneous surface. The O--chemisorption is not governed by a transfer mechanism from Pd to Au surface atoms. The alloys are able to absorb the oxygen species generated in the positive potential region; however, this ability decreases with increase of the gold content.     

Photoelectrochemical behavior of nanostructured WO3 thin-film electrodes: The oxidation of formic acid.

Nanostructured tungsten trioxide thin-film electrodes are prepared on conducting glass substrates by either potentiostatic electrodeposition from aqueous solutions of peroxotungstic acid or direct deposition of WO3 slurries. Once treated thermally in air at 450 degrees C, the electrodes are found to be composed of monoclinic WO3 grains with a particle size around 30-40 nm. The photoelectrochemical behavior of these electrodes in 1 M HClO4 apparently reveals a low degree of electron-hole recombination. Upon addition of formic acid, the electrode showed the current multiplication phenomenon together with a shift of the photocurrent onset potential toward less positive values. Photoelectrochemical experiments devised on the basis of a kinetic model reported recently [I. Mora-Seró, T. Lana-Villarreal, J. Bisquert, A. Pitarch, R. Gómez, P. Salvador, J. Phys. Chem. B 2005, 109, 3371] showed that an interfacial mechanism of inelastic, direct hole transfer takes place in the photooxidation of formic acid. This behavior is attributed to the tendency of formic acid molecules to be specifically adsorbed on the WO3 nanoparticles, as evidenced by attenuated total reflection infrared spectroscopy.     

Electrocatalysis by foreign metal monolayers: Oxidation of formic acid on palladium

Catalytic effects of foreign metal monolayers deposited at underpotentials have been found for the oxidation of formic acid on palladium electrode. Palladium is known to form no poisoning species in oxidation of HCOOH. However, the auto-inhibition of the reaction is found, caused most probably by a species _xC_x(OH)₂. Submonolayer amounts of Pb, Bi, Cd, Tl, Ag and Cu electrosorbed at underpotentials exhibited significant catalytic effects which could be explained by the so-called “third-body” effect in terms of a previously published semiquattitative model. The results have bearing on the understanding of the catalytic effects observed under comparable conditions on platinum and rhodium. Since these effects are orders of magnitude larger than those on palladium, the “third-body” effect probably plays only a minor role at the other two noble metals.     

High-efficiency palladium catalysts supported on ppy-modified C60 for formic acid oxidation

A facile preparation of polypyrrole-modified fullerene supported Pd nanoparticles catalyst is introduced; electrochemical measurements demonstrate that the obtained Pd/ppy-C(60) catalyst shows a good electrocatalytic activity and stability for the oxidation of formic acid.     

The formation and behavior of Ag adsorbates at Pd electrodes and their influence on electrocatalytic oxidation of formic acid at Pd

The electrochemical behavior of Ag ions at smooth Pd electrodes and Pd/Pt deposits is investigated. At adsorption potentials, which are more anodic than the reversible potential of the Ag⁺/Ag electrode, an underpotential deposition of Ag⁺ ions occur.We can distinguished two types of Ag ad-atoms at Pd. The first type of Ag is irreversibly adsorbed. The second, which exists only in the presence of Ag ions in the solution, is reversibly adsorbed.The influenced of various coverages of Ag ad-atoms at smooth Pd and Pd/Pt deposits on the electrocatalytical oxidation of formic acid was investigated. Even small coverages of Ag ad-atoms lower the rate of formic acid oxidation. With higher coverages this inhibiting influence grows continously.     

Facile synthesis of hollow palladium/copper alloyed nanocubes for formic acid oxidation

Hollow PdCu alloyed nanocubes were successfully synthesized by a novel one-pot template-free strategy through tuning the surface energy difference of the crystal planes by alloying. Compared with the solid nanoparticles, the hollow nanocubes exhibit superior electrocatalytic activity and stability for formic acid oxidation.     

Formic acid oxidation at platinized platinum electrodes: Part II. Effect of temperature on steady state oxidation process

A study has been made of the effect of temperature on the steady state oxidation of formic acid on platinized platinum electrodes in 0.5 M HCOOH+0.5 M H₂SO₄. The activation energy and entropy of activation for the anodic oxidation process decrease sharply with increase in electrode potential. The results provide additional evidence that the weakly adsorbed intermediate pathway proceeds through a rate-determining step involving the dissociation of weakly adsorbed formic acid molecule.     

Nanosized palladium on holey graphene sheets incorporating PxOy for effective formic acid oxidation

A stack of basic structural carbon units incorporating phosphorus oxides (P_xO_y) at edges and internal defects is obtained and used as a support material for Pd nanoparticles, with the aim of creating an efficient electrocatalyst for HCOOH oxidation. In contrast to the lack of activity displayed by Pd supported on lyophilized and vacuum-annealed graphene oxides, Pd on holey graphene sheets incorporating P_xO_y (Pd/HGP_xO_y) shows efficient activity and stability in HCOOH oxidation. Surface analysis of fresh and reacted catalysts reveals that HCOOH oxidation is favored by Pd/HGP_xO_y due to the decreased Pd electron density caused by electron transfer from Pd to HGP_xO_y.     

Ethylenediamine tetramethylene phosphonic acid assisted synthesis of palladium nanocubes and their electrocatalysis of formic acid oxidation

The synthesis of Pd nanocrystals of controlled size and morphology has drawn enormous interest due to their catalytic activity. We report a new and efficient strategy for the one-step synthesis of monodispersed Pd nanocubes with ethylenediamine tetramethylene phosphonate (EDTMP) as a complex-forming and capping agent. The morphology, structure, and growth mechanism of the Pd nanocubes were fully characterized via selected area electron diffraction (SAED), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). It was found that the morphology of the Pd nanocrystals in the proposed EDTMP–PdCl₂ system could be changed from octahedrons to nanocubes simply by adjusting the amount of iodide used during synthesis. After UV/ozone and electrochemical cleaning, the as-prepared Pd nanocubes demonstrated excellent electrocatalytic activity and stability during formic acid oxidation, owing to their abundant {100} facets and small particle size.     

Regularities of formic acid oxidation in the presence of porous silicon nanocomposites with palladium

Nanocomposites demonstrating high catalytic activity in the oxidation of formic acid were obtained by depositing palladium on porous silicon support. The palladium samples on porous silicon are superior to commercial samples on carbon black in many characteristics. A decrease in the size of palladium nanoparticles favors an increase in the catalytic activity of the palladium/porous silicon nanocomposites.     

Structural effects on the oxidation of formic acid on the high index planes of palladium

Electrochemical oxidation of formic acid has been studied on the stepped and kinked-stepped surfaces of Pd in 0.1 M HClO₄ containing 0.1 M formic acid with the use of voltammetry. The surfaces examined are Pd(S)-[n(1 0 0) × (1 1 0)], Pd(S)-[n(1 1 1) × (1 0 0)] and Pd(S)-[n(1 1 1) × (1 1 1)] series (n = 2–9). The results are compared with those of Pd(S)-[n(1 0 0) × (1 1 1)] series reported previously. All the electrodes give maximum currents of formic acid oxidation j_P between 0.5 and 0.8 V (RHE). The values of j_P plotted against the density of step (kink) atoms d_S depend on the surface structure remarkably. Pd(S)-[n(1 1 1) × (1 0 0)] surfaces provide maximum of j_P at n = 5, whereas Pd(S)-[n(1 0 0) × (1 1 0)] and Pd(S)-[n(1 1 1) × (1 1 1)] do not give maximum of j_P. The values of j_P have the following order: Pd(S)-[n(1 1 1) × (1 1 1)] < Pd(S)-[n(1 1 1) × (1 0 0)] < Pd(S)-[n(1 0 0) × (1 1 0)] < Pd(S)-[n(1 0 0) × (1 1 1)]. The anodic current at more negative potential 0.20 V (RHE) shows different activity series: Pd(1 1 1) and Pd(1 1 0) have the highest rate for formic acid oxidation at 0.20 V (RHE).     

Electrochemical fabrication of multiplicate palladium hierarchical architectures and their electrocatalysis toward oxidation of formic acid

Dendritic, cactoid, splintery flowers-like and spinous flowers-like micro/nano-Pd hierarchical architectures were successfully deposited on the conductive substrates without assistance of any templates. Distinct from other general electrodeposition at a constant potential or cyclic potential, we utilized pulse potentials as deposition and dissolution potential, which were controlled by a simple and convenient electrochemical method—differential pulse amperometry. It was found that the morphologies of these novel micro/nanoparticles could be regulated with different pulse potentials. The resulting nanostructures were characterized by scanning electron microscopy and X-ray diffractometry. The results show that series of Pd micro/nanoparticles were bounded on the different index facets. It means that the growth direction could be effectively controlled by regulating the pulse potentials. Moreover, the as-synthesized Pd micro/nanoparticles also exhibited strikingly difference in catalytic activity toward electrooxidation of formic acid.