Electrochemical and FTIRS characterisation of NO adlayers on cyanide-modified Pt(111) electrodes: the mechanism of nitric oxide electroreduction on Pt

We report here a study, using cyclic voltammetry and FTIRS, of NO irreversibly adsorbed on a cyanide-modified Pt(111) electrode. NO adlayers were formed by immersion of the cyanide-modified Pt(111) electrode in an acidic solution of KNO(2). The behaviour of NO adsorbed on the cyanide-modified electrode is very similar to that of NO on the clean Pt(111) surface, suggesting that adsorbed cyanide (saturation coverage theta(CN) = 0.5) behaves simply as a third body, blocking some of the surface sites but leaving the free Pt sites unaffected. Comparison of the voltammetric profile for NO electroreduction on Pt(111) and on cyanide-modified Pt(111) electrodes has allowed us: (i) to confirm that the reduction of three-fold hollow NO and atop NO on Pt(111) electrodes occurs in two distinct reduction peaks, as previously proposed by Rosca et al. (Langmuir, 2005, 21, 1448); (ii) to suggest that the reduction of irreversibly adsorbed NO layers on Pt electrodes can proceed through two possible paths, one involving an EE mechanism in which the rate-determining step (rds) is an Eley-Rideal reaction, with a direct proton transfer from the solution to adsorbed NO, and the other involving an EC mechanism in which the rds is a Langmuir-Hinshelwood reaction of adsorbed NO with adsorbed H. The availability of adsorbed hydrogen determines which path is followed by the reaction; (iii) to identify the smallest atomic ensemble for the reduction of NO on Pt as being composed of two adjacent Pt atoms.     

A rotating disc electrode study of oxygen reduction at platinised nickel and cobalt coatings

Platinized nickel and cobalt coatings, Pt(Ni) and Pt(Co), have been prepared on glassy carbon, GC, rotating disc electrode substrates by a two-step room temperature procedure that involved the electrodeposition of nickel and cobalt layers and their spontaneous partial replacement by platinum (“transmetalation”) when immersed into a chloroplatinic acid solution. By tuning the quantity of initially deposited nickel and cobalt, Pt(Ni) and Pt(Co) bimetallic coatings having a 26% atom Ni and 30% atom Co composition have been prepared. For both materials typical Pt surface electrochemistry was recorded during fast voltammetry in deaerated acid, pointing to the existence of a continuous Pt skin over a Pt–Ni and Pt–Co core. Oxygen reduction at the Pt(Ni)/GC and Pt(Co)/GC electrodes was studied by means of steady-state voltammetry at a rotating disc electrode and the construction of Tafel plots from corresponding voltammetric data. It was found that, when the initial potential of the voltammetric sweep allowed the formation of a complete Pt oxide monolayer, then oxygen reduction was hindered for low overpotentials at Pt(Ni) and Pt(Co), compared to pure bulk Pt. On the other hand, when the initial potential was less positive (thus leading to the formation of a fraction of surface oxide monolayer) the presence of Ni and Co enhanced the kinetics of oxygen reduction. The former behaviour is attributed to a decrease in oxide reduction ability of Pt in the presence of Ni and Co, while the latter to an increase in dissociative oxygen chemisorption due to Ni and Co.     

Electrocatalytic aspects of electrogenerated polymer layers

Two main aspects of electrocatalysis of electrogenerated polymer layers (polypyrrole in this case) are presented here. The first aspect concerns the electrogenerated conducting polypyrrole layer as such; i.e. its preparation with built-in electrocatalysts, like Pt or metal-organic complexes. Its properties are discussed in terms of their real electrocatalytic behaviour, viz. their turn-over numbers. The second part deals with specific studies like: - the anodic oxidation of hydrogen with built-in Pt, and, - the cathodic reduction of oxygen with Co and/or Fe-phthalocyanines. The theoretical model predicts rather well the experimental behaviour.     

超氧自由基电化学发生体系的研究

超氧自由基是人体内氧代谢的重要产物 [1 ] .迄今为止 ,超氧自由基的研究方法有电子自旋共振法、光度法和电化学法等 [2～ 5] .其中 ,通过电化学还原氧产生超氧自由基是一种简便快捷的方法 [6,7] .本文使用长链表面活性剂 DSAB作为疏水分子 ,比较了强碱性介质中氧在汞电极和 4种非汞电极上的电化学还原行为 ,建立了超氧自由基在非汞电极 (铂电极 )上电化学发生新体系 .通过石英晶体微天平和计时电量法进一步证实了超氧自由基的产生 ,并用循环伏安法比较了抗坏血酸和半胱氨酸两种抗氧化剂对超氧自由基的清除能力 .1　实验部分1.1　试剂与仪…     

Role of acidity on the electrochemistry of Prussian Blue at plain and Nation filmcoated electrodes

Prussian Blue (PB) coated on plain platinum (Pt) shows a redox wave at 0.44V vs SCE in addition to the two usual redox waves at 0.82 and 0.12 V vs SCE when the electrodes were dipped in acidic KC1 solution. PB incorporated into Nation film-coated on Pt electrode exhibited the same behaviour even in the presence of neutral KG solution. In acidic KC1, the additional redox wave observed for PB incorporated into Nation film shifted positively to 0.39V vs SCE and the peak separation was reduced to 30mV. The observed additional redox wave for PB coated on plain Pt electrode and incorporated into Nafion film-coated Pt electrodes was assigned to the partial reduction of PB occurring due to the insertion of protons into the film. The effect of various cations on the electrochemistry of PB incorporated into Nafion film-coated electrode was also studied.     

Catalytic reduction of U(VI) to U(IV) using hydrogen with platinum loaded on alumina and silica

During the reprocessing of spent nuclear fuel, uranium (U) and plutonium (Pu) are together extracted by employing tri-n-butyl phosphate (TBP)/dodecane mixture and their partitioning is achieved by adding uranous nitrate. The partitioning agent, uranous is conventionally produced by the electrolytic reduction of uranyl nitrate. An alternate route for the reduction of U from (VI) to (IV) using hydrogen (H₂) as reductant was developed using platinum (Pt) based catalyst. Improvements in the development of the catalyst have been carried out in order to reduce the requirement of Pt without affecting the reduction performance. Experiments using 2 wt% Pt loaded on alumina beads and alumina powder have been performed and results are discussed. As the catalyst supported on alumina was found to be unstable in acidic environment, Pt loaded on silica powder has also been developed. Pt loaded on alumina and silica substrates have been tried to envisage the reduction behaviour using H₂ as reductant in presence of hydrazine nitrate which acts as U(IV) stabiliser as well as reductant. Parametric studies have been carried out to optimise the process parameters namely pressure, temperature, U concentration, free acidity, hydrazine concentration and catalyst to U (C/U) ratio. 2 wt% Pt loaded on silica has been selected for further scale up studies for making uranous.     

碳化钨在对硝基苯酚电还原过程中的电催化行为

以碳化钨(WC)粉末为电催化材料制成了碳化钨粉末微电极(WC-PME). 采用循环伏安和线性扫描等方法研究了酸性溶液中对硝基苯酚(PNP)在WC-PME上的电还原行为. 研究表明, 在相同测试条件下, PNP在WC-PME上电还原的电位比Cu-Hg微电极正得多；WC-PME对氢具有较强的吸附能力, 有利于有机物的电还原反应. PNP在WC-PME上和Pt微电极上的还原电位相近, 但在WC-PME上的峰电流比在Pt微电极上高5倍多, 这主要与WC粉末的结构形貌有关.     

Total oxidation of ethanol and toluene over ceria—zirconia supported platinum catalysts

The effect of platinum loading (0.09–1.00 mass %) on the performance of ceria-zirconia supported catalysts in the total oxidation of ethanol and toluene in air was investigated. The introduction of platinum promoted the reduction of surface cerium and decreased the acidity of the catalysts. In ethanol oxidation, the temperature of 50 % conversion decreased with increasing platinum content. This increase in catalytic performance was more pronounced for the catalysts with 0.59 mass % and 1.00 mass % Pt. On the other hand, higher amount of by-products (mainly acetaldehyde) was observed at increased platinum loadings. For all catalysts, a correlation between their H₂-TPR profiles and catalytic performance was revealed. In toluene oxidation, only the catalysts with 0.59 mass % and 1.00 mass % Pt exhibited a lower temperature of 50 % conversion than pristine ceria-zirconia. The effect of platinum loading was less pronounced than in ethanol oxidation and a correlation between reduction behaviour and catalytic performance was not observed. The superior catalytic performance of the catalysts with 0.59 mass % and 1.00 mass % of Pt in both ethanol and toluene oxidation was ascribed to the presence of large platinum nanoparticles, which were not observed at lower Pt loadings.     

Formation mechanism of Pt particles by photoreduction of Pt ions in polymer solutions

The formation mechanisms of metal particles (platinum (Pt) particles) in an aqueous ethanol solution of poly(N-vinyl-2-pyrrolidone) (PVP) by the photoreduction method have been studied by transmission electron microscopy (TEM) and in situ and ex situ X-ray absorption fine structure (XAFS) analysis. The average diameter of the dilute and concentrated Pt particles in the PVP solution is estimated from TEM to be 2.0 and 2.5 nm, respectively. XAFS analysis was performed for the reduction process of Pt4+ ions to metallic Pt particles for the Pt L3 edge of the colloidal dispersions of the concentrated Pt solutions. The photoreduction process proceeds by the following steps: (1) reduction of PtCl6(2-) to PtCl4(2-), (2) dissociation of Cl from PtCl4(2-), followed by reduction of Pt2+ ionic species to Pt0, (3) formation of a Pt0-Pt0 bond and particle growth by the association of Pt0-Pt0. The reduction of PtCl4(2-) to Pt0 is a slower process, compared with the reduction of PtCl6(2-) to PtCl4(2-). There is a delay between the disappearance of PtCl4(2-) and the formation of Pt0-Pt0 clusters.     

Mutual indirect probing of platinized platinum/tungstate nanostructural features

The comparative study of isopolytungstates immobilized on smooth Pt and platinized Pt (Pt/Pt) provides a possibility to separate several specific effects on H- and O-upd at an external platinum surface and inside the pores of Pt/Pt. In the former case, a solid rechargeable product is formed (nonstoichiometric tungsten bronze), which an inherent porous structure allows solution penetration and does not prevent charging of the Pt/solution interface. Electrochemical responses of smooth Pt give more precise information on the film behaviour, when adsorption phenomena are clarified on the basis of data for Pt/Pt. Electrocatalytic tests (nitrate reduction) confirm unambiguously that the major part of the internal Pt/Pt surface area is modified by adsorbed polytungstate. This result gives, in its turn, new understanding of the size of Pt/Pt pores on the nanometre scale.

金属-半导体催化剂的研究Pt/SnO2上氢的室温溢流

本工作研究了Pt/SnO_2与气体分子H_2、CO之间的相互作用. 用溢流吸附反应确证室温下氢在Pt/SnO_2表面会发生一次溢流和二次溢流. 溢流氢可改变Pt/SnO_2的电导甚至能改变担体阳离子的价态.     

Evaluation of Pt anode stability in repeated electrochemical oxide reduction reactions for pyroprocessing

The widespread use of Pt anodes in pyroprocessing for oxide reduction inspired us to investigate the stability of a representative Pt anode in repeated UO₂ reduction experiments. The formation and subsequent exfoliation of an oxide layer on the Pt surface was shown to result in anode degradation, decreasing the electrode thickness from 2 to 1.22 mm and reducing its volume by 42.4% after 33 experiments. Although the above degradation exhibited almost no effect on the UO₂ reduction performance of the Pt anode, Pt degradation during long-term oxide reduction was demonstrated to be an important factor for estimating the cost of a pyroprocessing facility.     

Electrochemical behaviour of oxocarbons on single crystal platinum electrodes: Part V. Tetrahydroxy-p-benzoquinone in 0.5 M sulphuric acid medium

The electrochemical behaviour of tetrahydroxy-p-benzoquinone (THQ) on Pt(111), Pt(100) and Pt(110) surfaces has been studied in sulphuric acid solutions by cyclic voltammetry. The experimental results show that THQ is the oxocarbon having a lesser tendency to form CO-like species after adsorption on platinum surfaces. THQ is directly adsorbed on Pt(111) and Pt(100), although it is not stable and undergoes a slow reaction to CO adspecies on Pt(100) surfaces. Clear evidence of CO adspecies has been found only on Pt(100) electrodes.     

Shape- and size-controlled synthesis in hard templates: sophisticated chemical reduction for mesoporous monocrystalline platinum nanoparticles

Here we report a novel hard-templating strategy for the synthesis of mesoporous monocrystalline Pt nanoparticles (NPs) with uniform shapes and sizes. Mesoporous Pt NPs were successfully prepared through controlled chemical reduction using ascorbic acid by employing 3D bicontinuous mesoporous silica (KIT-6) and 2D mesoporous silica (SBA-15) as a hard template. The particle size could be controlled by changing the reduction time. Interestingly, the Pt replicas prepared from KIT-6 showed polyhedral morphology. The single crystallinity of the Pt fcc structure coherently extended over the whole particle.     

PDDA修饰的碳纳米管载铂电催化剂的合成及其在碱性条件下的氧气还原反应催化性能

采用长链聚合物聚二烯丙基二甲基氯化铵(PDDA)对多壁碳纳米管(MWCNTs)进行修饰,并将采用胶体法还原出的铂(Pt)纳米粒子通过静电作用担载于PDDA修饰的多壁碳纳米管上,从而制备出Pt/PDDA/MWCNTs复合电催化剂.透射电镜(TEM)与X射线衍射(XRD)测试结果表明, Pt纳米粒子均匀地分布在MWCNTs的表面,其平均粒径约为3.6 nm.热失重分析显示催化剂的实际负载量为36%(w).旋转圆盘电极测试结果表明, Pt/PDDA/MWCNTs催化剂对碱性条件下的氧气还原反应(ORR)具有优异的催化活性.与负载量为40%(w)的商业Pt/C催化剂相比, Pt/PDDA/MWCNTs催化剂的氧气还原反应的起始电位和半波电位均正移约30 mV,其质量比活性更大.动力学研究结果进一步证实Pt/PDDA/MWCNTs催化剂比负载量为40%(w)的商业Pt/C催化剂在碱性条件下对氧气还原反应具有更优异的催化活性.     

Reduction of NO adlayers on Pt(110) and Pt(111) in acidic media: evidence for adsorption site-specific reduction

We present a combined in situ Fourier transform infrared reflection-absorption spectroscopy and voltammetric study of the reduction of saturated and subsaturated NO adlayers on Pt(111) and Pt(110) single-crystal surfaces in acidic media. The stripping voltammetry experiments and the associated evolution of infrared spectra indicate that different features (peaks) observed in the voltammetric profile for the electrochemical reduction of NO adlayers on the surfaces considered are related to the reduction of NO(ads) at different adsorption sites and not to different (consecutive) processes. More specifically, reduction of high- and intermediate-coverage (ca. 0.5-1 monolayers (ML)) NO adlayers on Pt(110) is accompanied by site switching from atop to bridge position, in agreement with the ultra-high-vacuum data. On Pt(111) linearly bonded (atop) NO and face-centered cubic 3-fold-hollow NO species coexist at high coverages (0.25-0.5 ML) and can be reduced consecutively and independently. On Pt(111) and Pt(110) electrodes, linearly bonded NO species are more reactive than multifold-bonded NO species. Both spectroscopic and voltammetric data indicate that ammonia is the main product of NO(ads) reduction on the two surfaces examined.     

FTIR study of the effects of water pretreatment on the acid sites and the dispersion of metal particles in Y zeolites and mordenites

FTIR results on zeolite-supported Pt and Pd show that the presence of water during metal reduction modifies both the acid and the metal functions of these catalysts. The water treatment eliminates Lewis acid sites and lowers the concentration of Brønsted acid sites by partial dealumination. At low temperature CO is adsorbed on Brønsted sites; the position of the corresponding FRIR band indicates that for Pt/HMOR, (MOR = mordenite) unlike Pt/HY, the intrinsic acid strength of these sites is increased by the wet reduction procedure. FTIR spectra after CO adsorption at room temperature show that wet reduction markedly improves the dispersion of Pt in HMOR; this effect is weaker for HY and absent for the Na forms of the zeolites. Bands of gem-Pt(CO)₂, which are indicative of very small, possibly electron-deficient Pt clusters, are detected in Pt/HMOR.     

Preparation of nanoparticles by reducing intermediate radicals formed in sonolytical pyrolysis of surfactants

Metal nanoparticles with a narrow size distribution could be prepared by sonolysis of aqueous solutions of metal cations in the presence of surfactants such as sodium dodecyl sulfate, polyethylene glycol monostearate, etc. The role of the surfactans is not only to stabilize formed particles, but also to produce reductive radicals in pyrolysis or hydrogen abstraction of OH radicals from surfactants. Particles with a smaller size could be obtained in a faster reduction rate with dilute metal cations concentration. Pt(IV) is consecutively reduced in two steps to Pt(0)via Pt(II). By comparing the sonolytical reduction withγ-ray radiolysis, two kinds of organic reducing radicals are proposed to contribute to the reduction. One (R_ab) is an intermediate radical which is produced by hydrogen abstraction of OH radical from surfactant and effective only on the reduction of Pt(II) to Pt(0). The other (R_py) is also an intermediate radical which is produced by thermal decomposition of surfactant at the interface between the cavity and bulk solution and effective on the reduction of Pt(IV) to Pt(II).     

Nucleation and aggregative growth process of platinum nanoparticles studied by in situ quick XAFS spectroscopy

The early stage in the nucleation and subsequent aggregative particle growth of the colloidal platinum (Pt) dispersions produced by photoreduction in an aqueous ethanol solution of poly(N-vinyl-2-pyrrolidone) (PVP) was quantitatively investigated by means of in situ quick XAFS (QXAFS) measurements. The stages of the reduction-nucleation and the association process (aggregative particle growth and Ostwald ripening) of Pt atoms to produce Pt nanoparticles was successfully discriminated in course of the photoreduction time. The present QXAFS analysis indicated that Pt nuclei (i.e., (Pt(0))(m) nucleates approximately m = 4) were continuously produced in the reduction-nucleation process at the early time, followed by the aggregative particle growth with the autocatalytic reduction of Pt ionic species on the surface of Pt nuclei to produce Pt nanoparticles. Subsequently the particle growth proceeded via Ostwald ripening, resulting in the production of larger Pt nanoparticles at a later time. It was also found that the aggregative particle growth follows a sigmoidal profile well described either by the solid-state kinetic model or by the chemical-mechanism-based kinetic model, specifically the Avrami-Erofe'ev or Finke-Watzky models. The difference in terms of the formation mechanism was observed between the reduction of Pt(IV)Cl(6)(2-) and Pt(II)Cl(4)(2-) as a source material. Also presented is that the addition of the photoactivator such as benzoin, benzophenone, and acetophenone in the system is very effective to enhance the rate for the formation of Pt nanoparticles.     

A Study of Pt4+-Adsorption and Its Reduction by Bacillus Megaterium D01

The properties of Pt4+-adsorption and its reduction by Bacillus megaterium D01 were studied by means of ICP,anode-stripping voltammetry,TEM,IR and XPS.The results of ICP analyses showed that the Pt4+-adsorptive efficiency of the strain D01 was as high as 94.3% under the conditions of 100 mg Pt4+/L,1 g biomass/L,pH 3.5 and at 30 ℃ for 24 h.Moreover,it was confirmed from anode stripping voltammetry that the strain D01 possessed a strong reducibility.The TEM analysis indicated that the strain D01 was able to adsorb and reduce Pt4+ to Pt0,small particles.The XPS result further supported the reduction of Pt4+ to Pt2+,followed by the further recuction to Pt0.The IR spectrum implied that D01 biomass adsorption of Pt4+ may result in the complexation of the C=O bond to the Pt species.