Carbon monoxide adsorption on palladium electrodes modified with silver adatoms

Adsorption of carbon monoxide on palladium deposits and smooth polycrystalline palladium is studied in conditions of preliminary accumulation of various amounts of adsorbed silver (?_Ag) on them. It is discovered that carbon monoxide undergoes adsorption on palladium as it does on platinum, i.e. at the sites free of adsorbed silver, virtually without forcing out preliminarily adsorbed silver. At small and medium values of ?_Ag in the region of electrodesorption of a mixed layer on palladium, as opposed to Pt/Pt, only two peaks are observed in an anodic voltammetric curve. The nature of the processes that are responsible for the presence of two peaks pertaining to the oxidation of a mixed adsorption layer is discussed.     

Electrolytic palladium deposits: Dependence of adsorption properties and texture on the deposition potential

Specific features of the copper and oxygen adsorption on electrolytic palladium deposits obtained from chloride solutions at different deposition potentials are considered. It is shown that, in sulfuric acid solutions, the palladium dissolution, which accompanies the oxygen adsorption, obscures the adsorption behavior of the deposits with respect to oxygen. Assumptions are made about the predominant crystallographic orientation (100) of the surface of some palladium deposits being dependent on the deposition potential and about the presence of regions whose adsorption properties are anomalous with respect to copper. The assumptions are based on a comparative analysis of the copper adsorption data and X-ray diffraction patterns. In addition to sites of crystalline palladium, the deposits are found to have disordered areas as well. Dedicated to the ninetieth anniversary of Ya.M. Kolotyrkin’s birth.     

Electrooxidation of the hypophosphite ion on a palladium electrode

Anodic oxidation of sodium hypophosphite on smooth Pd and Pd/Pt electrodes and a Pd membrane is studied. Thei vs.E curves for the Pd electrode exhibit two anodic current peaks. One is caused by oxidation of H₂PO ₂ ^- , and the other, by simultaneous ionization of Pd and oxidation of H₂PO ₂ ^- . The hypophosphite ion adsorbed on the Pd surface hinders the formation of the passive film. This brings about a rapid dissolution of Pd in the oxygen region and its subsequent deposition with the formation of palladium black. The oxidation probably includes a slow heterogeneous chemical reaction, specifically, a cleavage of the P-H bond of the hypophosphite ion. The change in the reaction stoichiometry following an increase in solution pH and in anodic polarization is probably due to changing conditions of the H₂PO ₂ ^- adsorption and the number of adsorption sites occupied by H₂PO ₂ ^- on the surface. Following an increase in polarization, the phosphite ion may undergo oxidation to phosphate. Deceased.     

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.     

The study of electrochemical palladium behavior using the quartz crystal microbalance

The electrochemical quartz crystal microbalance (EQCMB) method has been used to evaluate the processes which occur in/on the palladium electrode in basic solutions. Hydrogen electrosorption in palladium is accompanied by an additional frequency shift that can be attributed to the stresses generated inside the Pd metal. A non-linear dependence between the mass change and the charge consumed during hydrogen oxidation in the Pd electrode has been found for hydrogen absorbed in the α- and β-phases. This effect precludes the objective estimation of the amount of hydrogen absorbed inside the Pd electrode. The EQCMB method has been used, however, for studying the surface electrode processes on the Pd electrode, i.e. specific anion adsorption, surface oxidation and dissolution. Also, the structure of the palladium oxide formed on the Pd surface during electrochemical oxidation is discussed in this paper and the effect of the anodic limiting potential on the oxide structure is reported. Received: 10 August 1999 / Accepted: 24 September 1999     

Hydrogen Storage Mediated by Pd and Pt Nanoparticles

The hydrogen storage properties of metal nanoparticles change with particle size. For example, in a palladium–hydrogen system, the hydrogen solubility and equilibrium pressure for the formation of palladium hydride decrease with a decrease in the particle size, whereas hydrogen solubility in nanoparticles of platinum, in which hydrogen cannot be stored in the bulk state, increases. Systematic studies of hydrogen storage in Pd and Pt nanoparticles have clarified the origins of these nanosize effects. We found a novel hydrogen absorption site in the hetero‐interface that forms between the Pd core and Pt shell of the Pd/Pt core/shell‐type bimetallic nanoparticles. It is proposed that the potential formed in the hetero‐interface stabilizes hydrogen atoms rather than interstitials in the Pd core and Pt shells. These results suggest that metal nanoparticles a few nanometers in size can act as a new type of hydrogen storage medium. Based on knowledge of the nanosize effects, we discuss how hydrogen storage media can be designed for improvement of the conditions of hydrogen storage.     

Phenomenological aspects related to the electrochemical behaviour of smooth palladium electrodes in alkaline solutions

The voltammetric behaviour of smooth palladium electrodes in 1 M NaOH is studied in the potential range related to the thermodynamic stability of water. The electrosorption of H atoms on bulk Pd appears as a reversible reaction coupled to a diffusion process which occurs within bulk Pd. The voltammetric electrodesorption of H from bulk Pd is a process under mixed control, i.e. the diffusion from the bulk and the surface oxidation of H atoms. Fast pseudocapacitive reactions are detected in the range 0.2–0.4 V associated with the adsorption of H atoms at the submonolayer level. The initial stages of Pd oxide layer formation, at ca. 0.68 V, involves two reversible stages. The Pd oxide monolayer formation is achieved at 1.25 V/RHE and is followed by the formation of a third reversible system. This system is enhanced by an excursion in the potential range of the oxygen evolution reaction. This reversible system is probably a redox system involving Pd(II)/Pd(IV) species. The voltammetric electroreduction of the Pd oxide film shows rather irreversible behaviour. Inhibition effects on the reversible adsorption of H atoms due to residual oxide species were observed as well as inhibition on loading the Pd electrode with hydrogen to form the (α + β)-PdH phase. Rotating ring-disc experiments demonstrate that Pd electrodissolution in basic solutions is much smaller than in acid solutions. However, soluble palladium species are detected, especially during the formation of the fast redox systems, in the potential range related to Pd oxide layer growth.     

Correction to: The effect of small silver inclusions on the palladium activity in formicacid oxidation reaction and corrosion stability

A PdAg deposit containing ~ 25 at.% Ag is obtained by the electrochemical codeposition from an aqueous solution of Pd and Ag sulfates (Au support, 0.5 M H₂SO₄). The deposit is characterized by means of various physical, physicochemical, and electrochemical methods. The PdAg deposit demonstrates the ~ 2 times higher specific activity (per the electrochemically active surface area (EASA) of Pd) in the formic acid oxidation reaction (FAOR) as compared with the individual Pd deposit prepared under the same conditions. The effect of silver additions on the palladium activity depends on many factors. The corrosion stability of PdAg is studied in 0.5 M H₂SO₄ solution based on the overall cyclic voltammograms (CVAs) and also on anodic and cathodic half-cycles in the region E = 0.3 − 1.25 V (vs. reversible hydrogen electrode (RHE)). The electrochemical estimates are compared with the results of direct analytical determination of dissolution products in solution after anodic polarization of deposits. The total amounts of Pd dissolved substantially increase with incorporation of Ag, which is associated, first of all, with the considerable increase in the EASA; at the same time, the specific dissolution of Pd also substantially increases. The possible factors determining the active dissolution of PdAg deposits are discussed; in particular, the specific mechanism of their dissolution via silver adatoms is proposed.

     

CeO_2修饰炭载体负载PdPt二元合金催化剂及其在甲酸氧化电催化中的应用

用CeO2修饰炭粉做载体,使用有机溶胶法还原PdPt二元合金的方法制备了一系列PdPt/CeO2-C催化剂.借助电化学测试,探讨催化剂中不同Pd与Pt原子比例的PdPt二元合金和不同含量的CeO2对于甲酸电氧化催化活性的影响.不断减少PdPt合金中Pt的比例可以促使甲酸氧化的起始电位前移,当Pd：Pt=15：1时氧化电流出现极值;同时,随着催化剂中CeO2含量的增加,催化剂对于HCOOH氧化的电流密度增加,当含量为15%时达到最大值.相对于Pd/C催化剂,在Pd15Pt1/15CeO2-C催化剂表面的甲酸氧化反应起始电位负移至少0.1V,氧化的电流密度提高60%以上.结合X射线衍射（XRD）,X射线光电子能谱（XPS）,透射电镜（TEM）和热重（TG）等测试数据可以发现,当极少量的Pt与Pd形成合金,Pt与Pd之间产生电子效应,使得合金表面HCOOH氧化的过电位降低;而CeO2的添加不仅有助于PdPt二元合金的分散,更有可能改变甲酸在PdPt表面的氧化反应路径,发挥双功能机理.     

Pt／YSZ固体电解质界面的电化学动力学研究

用循环伏安，线性扫描等电化学方法研究Ｐｔ／ＹＳＺ固体电解质界面的电化学行为，发现氧在Ｐｔ上的电化学吸附过程是Ｔｅｍｋｉｎ吸附过程，吸附量随时间对数增加，吸附氧的阴极还原是不可逆的过程，通过研究获得吸附氧阴极还原过电荷传递系数，Ｔｅｍｋｉｎ系数等重要参数。     

DFT studies of oxidation routes for Pd9 clusters supported on γ-alumina

This research is focused on the analysis of adsorbed bare and oxidized Pd(9) nanoparticles supported on γ-alumina. From first-principle density functional theory calculations, several configurations, charge transfer and electronic density of states have been analyzed in order to determine feasible paths for the oxidation process. Studies of Pd/PdO nanoparticles prove that they are stable at γ-alumina supports. It is shown that the Pd(9) nanoparticle favors dissociative adsorption of oxygen molecules. The most energetically preferable sites for adsorption are close to the contact between the cluster and the support, where one oxygen atom interacts with a 5-coordinated aluminium atom, and the remaining oxygen is in contact with the closest palladium atom. After first dissociation, one oxygen atom creates a bridge between the palladium atom and the 5-coordinated aluminium atom and the second oxygen atom moves to the top of the Pd(9) cluster, making a bridge between two palladium atoms. Subsequent dissociations arise analogously, with the difference that oxygen atoms in the second layer of the palladium cluster occupy hollow sides of the cluster. Investigation of the charge distribution in each oxidation step reveals that charge transfer increases towards the Pd/PdO nanoclusters. The electronic density of states indicates that gradual oxygen molecule adsorption and dissociation shift the highest states of the Pd/PdO nanoparticles in different ways. The overall investigation is found to be beneficial for studying methane oxidation.     

Determination of platinum, palladium and rhodium in Pt-catalysts by high performance thin layer chromatography using densitometry

The determination of palladium and platinum in alumina-supported bimetallic Pt-Re, Pd and Pt catalysts at the 0.03 and 0.3% (m/m) levels, respectively, and of rhodium and platinum in platinum sieve catalysts at the 0.04% (m/m) level and in the 4.75–19.75% (m/m) range, respectively, is described. The platinum group metals in fresh or spent catalysts are determined chromatographically using densitometry by forming theirN,N-diethyl-N-benzoylthiourea complexes. The precision of the method, %RSD, is 0.4–3.5, 2.2–6.7, and 4.2–6.0 for Pt, Pd and Rh, respectively. The accuracy was tested using alumina-based and active carbon platinum and palladium standards. The results were also compared with those obtained by FAAS. There was at most 8% difference between the results obtained with these methods, except for one active carbon based Pd standard.     

Investigation of concentration hysteresis in methane oxidation on bimetallic Pt–Pd/Al2O3 catalyst by in situ XPS and mass spectrometry

A combination of in situ XPS and mass spectrometry was used to investigate concentration hysteresis in the low- temperature oxidation of methane over a bimetallic Pt–Pd catalyst. It was shown that a decrease in the oxygen concentration leads to a sharp increase in the catalytic activity of the sample under study. The transition of the catalyst to a high/low activity state occurs due to the partial reduction/oxidation of the active component, with both platinum and palladium participating in the methane oxidation reaction.     

Pt/Pd/C catalysts with ultra low platinum content for oxygen reduction reaction

The activity of Pt/Pd/C ETEK catalysts of the core-shell type with an ultralow content of platinum (0.5–15 μg cm^?2) based on a commercial palladium catalyst is shown to exceed the activity of commercial Pt/C ETEK catalysts in the oxygen reduction reaction. The activity sharply increases with the decrease in the platinum content down to values corresponding to monolayer and submonolayer of platinum on palladium. This dependence wasn’t observed for the same amounts of platinum deposited on the carbon support Vulcan XC-72. This makes it possible to conclude that the most probable factor responsible for the high catalytic activity of Pt/Pd/C ETEK is the effect of palladium on the electronic properties of platinum rather than the effect of structural modification of the platinum deposit induced by the decrease in the platinum amount deposited on a foreign metal or a carbon support.     

氢原子在钯低指数表面上的吸附和扩散

采用氢－钯相互作用的五参数Ｍｏｒｓｅ势，用对势方法研究了氢原子在Ｐｄ（１００）、Ｐｄ（１１１）和Ｐｄ（１１０）低指数平坦表面上的吸附和扩散，得到氢原子在三个表面上的吸附位、吸附几何、结合能和本征振动等数据，计算结果和实验结果符合得很好。在此基础上，系统地研究了三个系统的吸附扩散势能面结构。     

Surface modification of phosphate ion to promote photocatalytic recovery of precious metals

Photocatalytic recovery,a novel precious metal recycling technology,dedicates to solving the environmental and energy consumption problems caused by traditional technologies.The activation of molecular oxygen (O₂) is one of the most critical steps in the whole process.Herein,we regulated the different adsorption intensity of oxygen on the surface by designing phosphate (PO₄^3-) modified titanium oxide(Ti O₂).The results show that the adsorption of oxyge...     

Pt／Al_2O_3，Pd／Al_2O_3催化剂上CO氧化与表面氧脱出－恢复性能

Ｐｔ／Ａｌ_２Ｏ_３，Ｐｄ／Ａｌ_２Ｏ_３催化剂上ＣＯ氧化与表面氧脱出－恢复性能周仁贤，郑小明（杭州大学催化研究所，杭州３１００２８）关键词铂，钯，氧化铝，氧化锆，负载型催化剂，一氧化碳，氧化，氧脱附ＡＩ。Ｏ。负载的贵金属（Ｐｔ，Ｐｄ或Ｒｈ等）催化剂对有...     

Mechanism of nitrate electroreduction on Pt(100)

Kinetics and mechanism of nitrate anion reduction on the Pt(100) electrode in perchloric and sulfuric acid solutions are studied. Analysis of the results of electrochemical measurements (combination of potentiostatic treatment and cyclic voltammetry) and the data of in situ IR spectroscopy allow suggesting the following scheme of the nitrate reduction process on Pt(100) differing from that in the literature. If the potential of 0.85 V is chosen as the starting potential for a clean flame-annealed electrode surface and negativegoing (cathodic) potential sweep is applied, then an NO adlayer with the coverage of about 0.5 monolayer is formed on Pt(100) in the nitrate solution already at 0.6 V. The further decrease in the potential results in NO reduction to hydroxylamine or/and ammonia, desorbing products vacate the adsorption sites for nitrate and hydrogen adatoms. At E < 0.1 V, adsorbed hydrogen is mostly present on the surface. During positive-going (anodic) potential sweep, the process of nitrate reduction starts after partial hydrogen desorption, the cathodic peak of nitrate reduction to hydroxylamine or ammonia is observed at 0.32 V on cyclic voltammograms. The process of nitrate anion reduction continues up to 0.7 V; at higher potentials, the surface redox process with participation of hydroxylamine or ammonia (the anodic peak at 0.78 V) and nitrate (the cathodic peak at 0.74 V is due to nitrate reduction to NO on the vacant adsorption sites) occurs.     

Enhanced electro-oxidation of formic acid by a PdPt bimetallic catalyst on a CeO2-modified carbon support

PdPt bimetallic catalysts that employ CeO2-modified carbon black as a support have been prepared using an organic colloidal method.PdPt/CeO2-C shows excellent performance toward the anodic oxidation of formic acid.The effects of varying both Pd to Pt ratio and CeO2 content have been investigated.The optimal Pd to Pt atomic ratio is 15,indicating that addition of small amounts of Pt can significantly enhance the activity of the catalyst.When the CeO2 content in the catalyst reaches as high as ～15 wt.%,the catalyst shows the maximum activity.Adding CeO2 not only enhances the catalytic activity of the material,but may also change the mechanism of its catalysis of the anodic oxidation of formic acid.Pd15Pt1/15CeO2-C exhibited 60% higher activity than Pd/C,and had a negative shift in onset potential of more than 0.1 V.Based on characterization by X-ray diffraction,X-ray photoelectron spectroscopy,thermogravimetric analysis and transmission electron microscopy,the interactions between the components are revealed and discussed in detail.     

Durable High-Temperature Proton Exchange Membrane Fuel Cells Enabled by the Working-Temperature-Matching Palladium-Hydrogen Buffer Layer

The durability degradation during stack-operating conditions seriously deteriorates the lifetime and performance of the fuel cell. To alleviate the rapid potential rise and performance degradation, an anode design is proposed to match the working temperature of high-temperature proton exchange membrane fuel cells (HT-PEMFCs) with the release temperature of hydrogen from palladium. The result is significantly enhanced hydrogen oxidation reaction (HOR) activity of Pd and superior performance of the Pd anode. Furthermore, Pd as hydrogen buffer and oxygen absorbent layer in the anode can provide additional in situ hydrogen and absorb infiltrated oxygen during local fuel starvation to maintain HOR and suppress reverse-current degradation. Compared with the traditional Pt/C anode, the Pd/C also greatly improved HT-PEMFCs durability during start-up/shut-down and current mutation. The storage/release of hydrogen provides innovative guidance for improving the durability of PEMFCs.