Kinetically Limited CO Adsorption: Spill-Over as a Highly Effective Adsorption Pathway on Bimetallic Surfaces

We demonstrate that the (local) adsorbed carbon monoxide, CO_ad, coverage on the Pt-free areas of bimetallic Pt/Ru(0001) surfaces (a Ru(0001) substrate partly covered by Pt monolayer islands) can be increased to ∼0.80 monolayers (ML), well above the established saturation CO_ad coverage of 0.68 ML, even under ultrahigh vacuum conditions by using spill-over of CO adsorbed on the Pt islands to the Ru areas as an highly effective adsorption channel. The apparent CO_ad saturation coverage of 0.68 ML on pure Ru(0001) is identified as due to kinetic limitations, hindering further uptake from the gas phase, rather than being caused by thermodynamic reasons. This spill-over mechanism is proposed to be a general phenomenon for adsorption on bimetallic surfaces.     

Carbon monoxide adsorption on Pt/Pt electrodes modified with silver adatoms

Adsorption of carbon monoxide on the Pt/Pt surface in the presence of preliminarily accumulated different amounts of UPD silver (_Ag) is studied by using voltammetry, electrode washings, and analytical techniques. It is found that carbon monoxide is adsorbed only on the sites unoccupied by Ag_ad and rather than displacing the latter atoms to the noticeable extent. The fraction of carbon monoxide chemisorbed in the linear form increases with _Ag. The shape of anodic voltammetric curves and the number of peaks in the region where the mixed layer CO_ads + Ag_ad is desorbed strongly depend on the CO_ads : Ag_ad ratio; at small and medium _Ag, three and more peaks are observed. Electroanalytic detection of silver passed to solution in various potential ranges in the course of the mixed-layer oxidation allowed us to identify the processes corresponding to different peaks.Translated from Elektrokhimiya, Vol. 41, No. 2, 2005, pp. 241–246.Original Russian Text Copyright © 2005 by Maksimov, Podlovchenko.This revised version was published online in April 2005 with corrections to the article note and article title and cover date.     

Pt/C电极表面活化处理对乙醇电催化氧化的影响

0引言直接甲醇燃料电池(DMFC)由于其燃料来源丰富、价格低廉、甲醇携带和储存安全方便等独特的优越性而越来越受到重视[1]。但DMFC中常用的阳极Pt催化剂对甲醇氧化的低的电催化活性及易于被甲醇氧化的中间体,吸附态的CO(COad)毒化的问题一直是制约DMFC走向实用化的关键问题[2,3]。由于甲醇分子小,在质子交换膜上有较大的透过作用,并且甲醇具有较高的毒性,所以寻求甲醇的替代燃料也是一直以来被广泛关注的问题[4,5]。乙醇是最简单的链醇分子,相对于甲醇来说,乙醇有很多优点,乙醇基本上没有毒性,来源丰富,价格可与甲醇竞争,能量密度高…     

Specific Features of the Adsorption of Carbon Monoxide on a Smooth Polycrystalline Platinum Electrode Modified with Adsorbed Silver Atoms

The adsorption of carbon monoxide at the surface of smooth polycrystalline platinum (smPt) is studied in conditions of a preliminary accumulation of various quantities of silver (θ_Ag) on the surface. A comparison with similar data obtained previously for Pt/Pt is conducted. It is discovered that on smPt, exactly as in the case of Pt/Pt, carbon monoxide undergoes adsorption at sites that are not occupied by adsorbed silver, without forcing the preliminarily adsorbed silver out. At small and intermediate Ag_ad, as opposed to Pt/Pt, a mere two peaks are observed in a voltametric curve in the region of electrodesorption of the mixed layer on smPt. It is shown that, in the region of potentials of the first peak, there occurs practically no transition of silver into solution in the course of oxidation of the mixed layer. Specific features that characterize the behavior of the CO_ads + Ag_ad mixed layer are discussed under the assumption about an “islet” character of the adsorption of silver.     

In situ FTIR spectroscopic studies of CO adsorption on electrodes with nanometer-scale thin films of ruthenium in sulfuric acid solutions

A nanometer-scale thin film of ruthenium supported on glassy carbon (nm-Ru/GC) was prepared by electrochemical deposition under cyclic voltammetric conditions. Scanning tunneling microscopy (STM) was used to investigate the structure and to measure the thickness of the thin film. It has been found that the Ru thin film is composed of layered Ru crystallites that appear in a hexagonal form with dimensions of about 250 nm and thickness around 30 nm. In situ FTIR spectroscopic studies demonstrated that such a nanostructured Ru thin film exhibits abnormal infrared effects (AIREs) for CO adsorption (G.Q. Lu et al., Langmuir 16 (2000) 778). In comparison with CO adsorbed on a massive Pt electrode, the IR absorption of CO_ad on nm-Ru/GC was significantly enhanced. Moreover, the direction of CO_ad bands is inverted and the full width at half maximum of CO_ad bands is increased. It has been revealed that the enhancement factor of IR absorption of CO adsorbed on nm-Ru/GC electrodes depends strongly on the thickness of the Ru film. An asymmetrical volcano relationship between the enhancement factor and the thickness of the Ru film has been obtained. The maximum value of the enhancement factor was measured as 25.5 on a nm-Ru/GC electrode of Ru film thickness around 86 nm. The present study has contributed to exploration of the particular properties of nanostructured Ru film material and to the origin of the abnormal infrared effects.     

The selective oxidation of ethanol to CO2 at Ptpc/Ir/Pt metallic multilayer nanostructured electrodes

In this work, Pt_pc/Ir/Pt metallic multilayer nanostructured electrodes were prepared. The composition and number of the constituent metal layers were varied and the number of Ir and Pt layers studied were: 1.5:1.5, 1.5:10, 10:1.5, 10:10 and 250:250 Ir and Pt monolayers. The ethanol electrooxidation reaction and its products was studied using electrochemical in situ FTIR technique and could be observed as a selective cleavage of the ethanol CC bond in acidic electrolyte. Neither acetaldehyde nor acetic acid IR band could be observed for ethanol electrooxidation at 1.5 V vs. RHE over Pt_pc/Ir₂₅₀/Pt₂₅₀ metallic multilayer electrodes. Also, the enhancement on CO₂ production over this electrode was more than six times the amount observed using the Pt_pc electrodes. Thus, the complete CC cleavage bond in ethanol molecule was observed, leading only CO₂ as reaction product.     

Spontaneous Bi-modification of polycrystalline Pt electrode: fabrication, characterization, and performance in formic acid electrooxidation

Spontaneous modification of polycrystalline Pt by irreversibly adsorbed bismuth was performed in BiCl₃ solution in concentrated hydrochloric acid under open-circuit conditions. After spontaneous modification, followed by extensive rinsing with water and drying, the surface was characterized using X-ray photoelectron spectroscopy and electrochemistry. Bi-oxy(chloride), oxide species, and metallic Bi were found at a submonolayer coverage on the Pt surface after spontaneous modification. The electrochemical response of Bi-modified polycrystalline Pt electrode in sulfuric acid solution exhibits a complex multi-peak feature, which is resulting in about constant redox charge (Bi species coverage) in the potential region from 0 to 0.9 V (vs. a standard hydrogen electrode). The spontaneously Bi-modified Pt catalyst in model studies exhibits a superior activity towards formic acid oxidation at fuel cell anode relevant potentials. The catalytic effect of bismuth oxy-species is explained in terms of both inhibition of CO_ad formation and oxidation of CO_ad in reaction with Bi-oxy-species.     

A spectroscopic proof of a surface equilibrium between on top and bridge bonded CO at Pt(1 1 0) in acid solution

According to most of works in the literature, adsorbed carbon monoxide at Pt(1 1 0) electrodes in acid media presents only linear bonded (CO_L) so-called, atop geometry. In the present work, the formation of bridge bonded carbon monoxide (CO_B) is shown via in situ infrared FT spectra, measured on a Pt(1 1 0) electrode covered with 25% CO, in HClO₄ solutions. For the first time, the inter conversion between atop and bridge bonded CO at potentials in the hydrogen adsorption region is reported in acid solution. Band intensity and band center frequency indicate dipole–dipole coupling effects in spite of the low CO total coverage.     

Pt/C和Pt/CNTs电极的电化学稳定性研究

采用恒电位氧化法研究了Pt/C和Pt/CNTs电极的电化学稳定性. 相同条件下, Pt/C电极的氧化电流大约为Pt/CNTs电极的2倍; 120 h氧化后, Pt/C电极Pt的电化学表面积下降了21.3%, 而Pt/CNTs电极仅下降了7.6%, 表明Pt/CNTs电极性能衰减较慢. X射线光电子能谱(XPS)分析表明, Pt/C的载体碳黑表面氧增加量大于Pt/CNTs中碳纳米管(CNTs)表面氧的增加量, 说明碳黑的被氧化程度较高, 电化学稳定性差; Pt的表面化学状态没有发生变化; 碳纳米管本身的抗电化学氧化性也大于碳黑. 所以, 载体的被氧化程度不同是两种电极性能衰减不同的主要原因之一, 并且排除了Pt表面状态的影响.     

Insights into the enhanced tolerance to carbon monoxide on model tungsten trioxide-decorated polycrystalline platinum electrode

Polycrystalline platinum decorated by WO₃ nanoparticles (WO₃/Pt_pc) is used as a model electrode to gain insights into the enhanced tolerance to carbon monoxide (CO) observed on such composite materials. Bifunctional-type reactions involving WO₃ and Pt active sites are observed, such as hydrogen spill-over or the electrooxidation of CO molecules adsorbed on Pt sites neighboring the WO₃ nanoparticles. The resulting CO_ad-free Pt sites are active for the hydrogen oxidation reaction (HOR), thereby enhancing the HOR activity for WO₃/Pt_pc electrode relatively to bare Pt_pc in 300 ppm CO/H₂ saturated HClO₄ electrolyte. However, this bifunctional effect occurs exclusively for CO molecules weakly adsorbed on Pt, i.e. only for a small fraction of the CO_ad fully covering the Pt surface.     

Adsorption of CO on Ru and Pt blacks and catalysts and the possibility of its utilization for the determination of the ruthenium-free surface

Electrochemical voltammetric curves on Ru and Pt blacks of a different surface area were measured in potential intervals 0.05–1.05 V in pure 0.5 M H₂SO₄ and after CO adsorption. It was proved that after the CO adsorption, the outset of ruthenium oxidation is shifted by about 150 mV towards the positive potentials, e.g. to the region of oxidation of adsorbed CO. This fact made possible the determination of a double-layer charging current of Ru electrodes and, subsequently, also the determination of the amount of adsorbed hydrogen on the Ru surface. An evaluation of the amount of CO and hydrogen adsorption showed that the ratio of adsorbed CO:H on the Pt surface was about 1:1, while on Ru electrodes this ratio was around twice as large. The amount of hydrogen adsorbed on Ru blacks depends on the preliminary preparation of the electrodes. The CO adsorption could also be employed in the determination of a charging current of electrode double-layers during voltammetric oxidation of adsorbed hydrogen on ruthenium supported on Al₂O₃, SiO₂, or TiO₂ carriers. However, a similar determination of hydrogen adsorbed on the tin-modified Ru catalysts is not very reliable.     

Structural and electrocatalytic features of Pt/C catalysts fabricated in supercritical carbon dioxide

Pt/carbon black samples fabricated from dimethyl (1,5-cyclooctadiene) platinum(II) in supercritical CO₂ are characterized in relation to possible applications in methanol fuel cell. The problem of precise material characterization is addressed in frames of X-ray diffractometry, transmission electron microscopy, and electrochemical techniques of the true surface area determination. The catalysts with Pt loading of 20–40 wt.% consist of nm-size particles, with the lattice defectiveness dependent on the fabrication mode. To check the effect of support, various types of carbon blacks (Vulcan XC72R and acetylene black AC-1) are used. In contrast to commercial HiSpec catalysts, no pronounced increase of particle size with Pt loading is found. Specific steady-state activity towards methanol oxidation appears to be essentially higher than for commercial catalysts, mostly because the self-poisoning effects are less pronounced. As for poisoning of Pt with organic species (resulting from the ligand of precursor), its effects are demonstrated to be minor after CO or methanol adsorption accompanied by desorption of contaminating by-product.     

Electrochemical and physical characterization of Pt–Ru alloy catalyst deposited onto microporous–mesoporous carbon support derived from Mo2C at 600 °C

The electrochemical reduction of oxygen on binary Pt–Ru alloy deposited onto microporous–mesoporous carbon support was studied in 0.5 M H₂SO₄ solution using cyclic voltammetry, rotating disk electrode (RDE), and impedance method. The microporous–mesoporous carbon support C(Mo₂C) with specific surface area of 1,990 m²?g^?1 was prepared from Mo₂C at 600 °C using the chlorination method. Analysis of X-ray diffraction, photoelectron spectroscopy, and high-resolution transmission electron microscopy data confirms that the Pt–Ru alloy has been formed and the atomic fraction of Ru in the alloy was ～0.5. High cathodic oxygen reduction current densities (?160 A?m^?2 at 3,000 rev?min^?1) have been measured by the RDE method. The O₂ diffusion constant (1.9?±?0.3?×?10^?5?cm²?s^?1) and the number of electrons transferred per electroreduction of one O₂ molecule (～4), calculated from the Levich and Koutecky–Levich plots, are in agreement with literature data. Similarly to the Ru/RuO₂ system in H₂SO₄ aqueous solution, nearly capacitive behavior was observed from impedance data at very low ac frequencies, explained by slow electrical double-layer formation limited by the adsorption of reaction intermediates and products into microporous–mesoporous Pt–Ru–C(Mo₂C) catalyst. All results obtained for C(Mo₂C) and Pt–Ru–C(Mo₂C) electrodes have been compared with corresponding data for commercial carbon VULCAN® XC72 (C(Vulcan)) and Pt–Ru–C(Vulcan) electrodes processed and measured in the same experimental conditions. Higher activity for C(Mo₂C) and Pt–Ru–C(Mo₂C) has been demonstrated.     

化学镀制备高性能直接乙醇燃料电池(DEFC)阳极催化剂PtRu/C和PtRuSn/C

应用化学镀方法,以活化-敏化处理的活性炭作载体,制备高分散催化剂PtRu/C和PtRuSn/C.XRD、TEM及电化学测试表明,该催化剂Pt、Ru、Sn形成合金.金属颗粒的平均粒径约为3 nm.PtRu/C和PtRuSn/C二者对乙醇的阳极氧化都具有良好的催化活性和稳定性.     

The electro-oxidation of carbon monoxide and ethanol on supported Pt nanoparticles: the influence of the support and catalyst microstructure

The sluggish kinetics of ethanol oxidation on Pt-based electrodes is one of the major drawbacks to its use as a liquid fuel in direct ethanol fuel cells, and considerable efforts have been made to improve the reaction kinetics. Herein, we report an investigation on the effect of the Pt microstructure (well-dispersed versus agglomerated nanoparticles) and the catalyst support (carbon Vulcan, SnO₂, and RuO₂) on the rate of the electrochemical oxidation of ethanol and its major adsorbed intermediate, namely, carbon monoxide. By using several structural characterization techniques such as X-ray diffraction, X-ray absorption spectroscopy, and transmission electron microscopy, along with potentiodynamic and potentiostatic electrochemical experiments, we show that by altering both the Pt microstructure and the support, the rate of the electrochemical oxidation of ethanol can be improved up to a factor of 12 times compared to well-dispersed carbon-supported Pt nanoparticles. As a result of a combined effect, the interaction of Pt agglomerates with SnO₂ yielded the highest current densities among all materials studied. The differences in the activity are discussed in terms of structural and electronic properties as well as by mass transport effects, providing valuable insights to the development of more active materials.     

NiO增强Pt/C催化剂催化氧化甲醇活性的研究

本文基于NiO作为Pt催化甲醇助催化剂的思路,通过Pt纳米颗粒担载在NiO修饰的碳材料载体上制备了Pt/NiO-C催化剂,系统地研究了不同的NiO/C热处理温度对Pt粒径的影响,并重点探讨了Pt对NiO的质量比对催化氧化甲醇的影响。X射线衍射分析结果显示NiO和Pt均为立方晶系,且NiO的加入有利于主催化剂Pt形成较小的粒径,且经400℃热处理NiO修饰的C材料作为载体有利于Pt的有效分散。所获得的Pt/NiO-C催化剂的电化学活性在甲醇酸性溶液中通过循环伏安法(CV)和计时电流法(CA)进行性能测试。CV测试结果显示以Pt/NiO重量比为4∶1的催化剂其电氧化甲醇活性最大,其峰值氧化电流密度达806 mA/mgPt,是Pt/C催化剂的1.64倍。CA测试结果显示Pt/NiO-C比Pt/C具有更好的抗CO中毒性能和稳定性。     

Carbon Corrosion in Proton-Exchange Membrane Fuel Cells: Spectrometric Evidence for Pt-Catalysed Decarboxylation at Anode-Relevant Potentials

The carbon oxidation reaction (COR) is a critical issue in proton-exchange membrane fuel cells (PEMFCs), as carbon in various forms is the most used electrocatalyst support material. The COR is thermodynamically possible above the C/CO₂ standard potential, but its rate becomes significantly important only at high overpotential (e. g. PEMFC cathode potential). Herein, using on-line differential electrochemical mass spectrometry, we show that oxygen-containing carbon surface groups present on high-surface aera carbon, Vulcan XC72 or reinforced graphite are oxidized at PEMFC anode-relevant potential (E=0.1 V vs. the reversible hydrogen electrode, RHE), but not at E=0.4 V vs. RHE. We rationalized our findings by considering a Pt-catalysed decarboxylation mechanism in which Pt nanoparticles provide adsorbed hydrogen species to the oxygen-containing carbon surface groups, eventually leading to evolution of carbon dioxide and carbon monoxide. These results shed fundamental light on an unexpected degradation mechanism and facilitate the understanding of the long-term stability of PEMFC anode nanocatalysts.     

Abnormally fast mobility of CO at electrochemical interfaces

A very fast rate of CO transport at the Pt/electrolyte interface is reported. The mass transport was monitored by following the change of the electrode capacitance caused by H_ad displacement during carbon monoxide adsorption at potentials in the hydrogen adsorption region. A diffusion parameter was calculated assuming the validity of Fick’s law. No dependence was found on the surface structure.     

The influence of Pt-activation on the corrosion of carbon in gas diffusion electrodes—A dems study

Investigations of the dependence on the potential of the anodic oxidation of carbon electrodes using differential electrochemical mass spectroscopy (DEMS) show that pure carbon is oxidized only at potentials higher than 0.9 V (RHE) (with CO₂ and, to a lesser extent, CO being the main products), and that Pt activation catalyzes the oxidation of a CO_surf surface layer to CO₂ at potentials between 0.6 and 0.8 mV (RHE), with the CO_surf being formed on the carbon at E>0.3 V (RHE).It is assumed that the Pt-induced carbon corrosion occurs in the neighbourhood of the Pt-sites, thus damaging the Pt to carbon contact. Surface segregation of Pt-clusters and a loss of catalytic activity is the result.     

Adsorption and oxidation of ethanol on colloid-based Pt/C, PtRu/C and Pt3Sn/C catalysts: in situ FTIR spectroscopy and on-line DEMS studies

The interaction of colloid-based, carbon supported Pt/C (40 wt%), PtRu/C (45 wt%) and Pt3Sn/C (24 wt%) catalysts with ethanol and their performance for ethanol electrooxidation were investigated in model studies by electrochemical, in situ infrared spectroscopy and on-line differential electrochemical mass spectrometry measurements. The combined application of in situ spectroscopic techniques on realistic catalysts and under realistic reaction (DEMS, IR) and transport conditions (DEMS) yields new insight on mechanistic details of the reaction on these catalysts under the above reaction and transport conditions. Based on these results, the addition of Sn or Ru, though beneficial for the overall activity for ethanol oxidation, does not enhance the activity for C-C bond breaking. Dissociative adsorption of ethanol to form CO2 is more facile on the Pt/C catalyst than on PtRu/C and Pt3Sn/C catalysts within the potential range of technical interests (<0.6 V), but Pt/C is rapidly blocked by an inhibiting CO adlayer. In all cases acetaldehyde and acetic acid are dominant products, CO2 formation contributes less than 2% to the total current. The higher ethanol oxidation current density on the Pt3Sn/C catalyst at these potentials results from higher yields of C2 products, not from an improved complete ethanol oxidation to CO2.