Sb在Pt(100),Pt(110),Pt(111)及Pt(320)上不可逆吸附的电化学特性

研究了Sb在Pt(1 0 0 ) ,Pt(1 1 0 ) ,Pt(1 1 1 )和Pt(32 0 )单晶面上不可逆吸附的电化学特性 .发现当扫描电位的上限Eu≤ 0 .45V时 ,Sbad可以稳定地吸附在Pt(1 0 0 ) ,Pt(1 1 0 )和Pt(1 1 1 )表面 ,而Sbad在Pt(32 0 )表面稳定的电位较低 ,为Eu≤ 0 .40V .从饱和吸附Sb的铂单晶电极出发 ,通过改变电位扫描上限Eu 和电位扫描圈数可以获得不同Sb覆盖度 (θSb)的电极 .根据Sb和H在铂单晶电极表面共吸附的定量数据 ,对Sb在不同铂单晶面上饱和吸附的模型进行了初步探讨 .     

Studies of surface processes of electrocatalytic reduction of CO_2 on Pt(210), Pt(310) and Pt(510)

Surface processes of CO2 reduction on Pt(210), Pt(310), and Pt(510) electrodes were studied by cyclic voltammetry. Different surface structures of these platinum single crystal electrodes were obtained by various treatment conditions. The experimental results illustrated that the electrocatalytic activity of Pt single crystal electrodes towards CO2 reduction is decreased in an order of Pt(210)>Pt(310)>Pt(510), i.e., with the decrease of (110) step density on well-defined surfaces. When the surfaces were reconstructed due to oxygen adsorption, the catalytic activity of all the three electrodes has been enhanced to a cer- tain extent. Although the activity order remains unchanged, the electrocatalytic activity has been en- hanced more significantly as the density of (110) step sites is more intensive on the Pt single crystal surface. It has revealed that the more open the surface structure is, the more active the Pt single crystal electrode will be, and the easier for the electrode to be transformed into a surface structure that exhib- its higher activity under external inductions. However, the relatively ordered surfaces of Pt single crystal electrode are comparatively stable under the same external inductions. The present study has gained knowledge on the interaction between CO2 and Pt single crystal electrode surfaces at a micro- scopic level, and thrown new insight into understanding the surface processes of electrocatalytic re- duction of CO2.     

一种金属单晶电极制备方法的建立和Cu2+在铂单晶上UPD过程的研究

同种材料而表面结构不同的电极往往有完全不同的电化学性能.使用在原子水平上表面结构明确的单晶电极不仅有助于对电极表面吸脱附过程、电场作用下表面结构重组、双电层微观结构、分子水平上的反应机理等基础理论进行深入研究,且对高选择性、高效电催化剂的研制也有指导意义.单晶电化学研究的基础就是制备定向不同的单晶电极.本文建立了金属单晶电极制备方法,并报道了Cu²⁺在Pt单晶电极上UPD(欠电位沉积)过程的研究结果.     

Studies of surface processes of electrocatalytic reduction of CO<Subscript>2</Subscript> on Pt(210), Pt(310) and Pt(510)

Surface processes of CO2 reduction on Pt(210), Pt(310), and Pt(510) electrodes were studied by cyclic voltammetry. Different surface structures of these platinum single crystal electrodes were obtained by various treatment conditions. The experimental results illustrated that the electrocatalytic activity of Pt single crystal electrodes towards CO2 reduction is decreased in an order of Pt(210)＞Pt(310)＞Pt(510), i.e., with the decrease of (110) step density on well-defined surfaces. When the surfaces were reconstructed due to oxygen adsorption, the catalytic activity of all the three electrodes has been enhanced to a certain extent. Although the activity order remains unchanged, the electrocatalytic activity has been enhanced more significantly as the density of (110) step sites is more intensive on the Pt single crystal surface. It has revealed that the more open the surface structure is, the more active the Pt single crystal electrode will be, and the easier for the electrode to be transformed into a surface structure that exhibits higher activity under external inductions. However, the relatively ordered surfaces of Pt single crystal electrode are comparatively stable under the same external inductions. The present study has gained knowledge on the interaction between CO2 and Pt single crystal electrode surfaces at a microscopic level, and thrown new insight into understanding the surface processes of electrocatalytic reduction of CO2.     

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.     

Pt(111)单晶电极上乙二醇解离吸附反应动力学

运用电化学循环伏安和程序电位阶跃方法研究了乙二醇在Pt(111)单晶电极上的解离吸附过程.动力学研究的定量结果指出,乙二醇解离吸附反应的平均速率随电极电位变化呈火山型分布,其最大值在0.10 V(vs SCE)附近.测得在含2×10－3 mol•L－1乙二醇的溶液中,最大初始解离速率vi为4.35×10－12 mol•cm-－2•s－1.     

Surface X-ray scattering of stepped surfaces of platinum in an electrochemical environment: Pt(331) = 3(111)-(111) and Pt(511) = 3(100)-(111)

Real surface structures of the high-index planes of Pt with three atomic rows of terraces (Pt(331) = 3(111)-(111) and Pt(511) = 3(100)-(111)) have been determined in 0.1 M HClO(4) at 0.1 and 0.5 V(RHE) with the use of surface X-ray scattering (SXS). The surfaces with two atomic rows of terraces, Pt(110) = 2(111)-(111) and Pt(311) = 2(100)-(111) = 2(111)-(100), are reconstructed to a (1 × 2) structure according to previous studies. However, the surfaces with three atomic rows of terraces have pseudo-(1 × 1) structures. The interlayer spacing between the first and the second layers, d(12), is expanded 13% on Pt(331) compared to that of the bulk, whereas it is contracted 37% on Pt(511). The surface structures do not depend on the applied potential on either surface.     

Stripping voltammetry of carbon monoxide oxidation on stepped platinum single-crystal electrodes in alkaline solution

The electrochemical oxidation of a CO adlayer on Pt[n(111)x(111)] electrodes, with n = 30, 10, and 5, Pt(111), Pt(110) as well as a Pt(553) electrode (with steps of (100) orientation) in alkaline solution (0.1 M NaOH) has been studied using stripping voltammetry. On these electrodes, it is possible to distinguish CO oxidation at four different active oxidation sites on the surface, i.e. sites with (111), (110) and (100) orientation, and kink sites. The least active site for CO oxidation is the (111) terrace site. Steps sites are more active than the (111) terrace sites, the (110) site oxidizing CO at lower potential than the (100) site. The CO oxidation feature with the lowest overpotential (oxidation potential as low as 0.35 V vs. RHE) was ascribed to oxidation of CO at kink sites. The amount of CO oxidized at the active step or kink sites vs. the amount of CO oxidized at the (111) terrace sites depends on the concentration of the active sites and the time given for the terrace-bound CO to reach the active site. By performing CO stripping on the stepped surfaces at different scan rates, the role of CO surface diffusion is probed. The possible role of electronic effects in explaining the unusual activity and dynamics of CO adlayer oxidation in alkaline solution is discussed.     

Adsorption of formaldehyde on Pt(111) and Pt(100) electrodes: cyclic voltammetry and scanning tunneling microscopy

The adsorption of formaldehyde (HCHO) on Pt(111) and Pt(100) electrodes was examined by cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM) in 0.1 M HClO(4). The extent of HCHO adsorption at both Pt electrodes was evaluated by comparing the CVs, particularly for the hydrogen adsorption and desorption between 0.05 and 0.4 V, obtained in 0.1 M HClO(4) with and without HCHO. The adsorption of HCHO on these Pt electrodes was significant only when [HCHO] >/= 10 mM. Adsorbed organic intermediate species acted as poisons, blocking Pt surfaces and causing delays in the oxidation of HCHO. Compared to Pt(111), Pt(100) was more prone to poisoning, as indicated by a 200 mV positive shift of the onset of HCHO oxidation. However, Pt(100) exhibited an activity 3 times higher than that of Pt(111), as indicated by the difference in peak current density of HCHO oxidation. Molecular resolution STM revealed highly ordered structures of Pt(111)-( radical7 x radical7)R19.1 degrees and Pt(100)-( radical2 x radical2) in the potential region between 0.1 and 0.3 V. Voltammetric measurements further showed that the organic poisons produced by HCHO adsorption behaved differently from the intentionally dosed CO admolecules, which supports the assumption for the formation of HCO or COH adspecies, rather than CO, as the poison. On both Pt electrodes, HCHO oxidation commenced preferentially at step sites at the onset potential of this reaction, but it occurred uniformly at the peak potentials.     

Borohydride electro-oxidation on Pt single crystal electrodes

The borohydride oxidation reaction on platinum single-crystal electrodes has been studied in sodium hydroxide solution using static and rotating conditions. The results show that borohydride electro-oxidation is a structure sensitive process on Pt surfaces. Significant changes in the measured currents are observed at low potentials. In this region, the Pt(111) electrode exhibits the lowest activity, whereas the highest currents are measured for the Pt(110) electrode. The behavior of the different electrodes is discussed taking as reference the observed behavior on the blank electrolyte and the possible formation of weakly adsorbed intermediates.     

Selective electrocatalysis of acetaldehyde oxime reduction on (111) sites of platinum single crystal electrodes and nanoparticles surfaces

The reduction of acetaldehyde oxime (AO) in acid medium on platinum surfaces is a structure sensitive reaction that takes place almost exclusively on (111) sites of Pt electrodes, and it is strongly inhibited on Pt(100) and Pt(110) surfaces. A study using stepped electrodes with (111) terraces and monoatomic steps either with (100) and (110) orientation shows that the activity of the electrode is also dependent on the terrace width, i.e., the wider the terrace is, the higher current density is recorded and the more positive the peak potential for AO reduction appears. Moreover, in the electrodes with (100) step sites, the reduction process appears at more negative potential than the electrodes with (111) step sites. Nanoparticles with some preferential orientations were also tested for the AO reduction reaction to check the presence of (111) ordered domains on the nanoparticles surface. Dedicated to Teresa Iwasita on the occasion of her retirement and for her contributions to Electrochemistry.     

Electrochemical behaviour of Pt(100), Pt(111) and Pt polycrystalline surfaces in hydrogencarbonate solution

It was demonstrated that adsorbed CO is obtained from the reduction of NaHCO₃ solution when Pt(100), Pt(110), disordered Pt(111) and polycrystalline electrodes are employed. Reduction of CO₂ coming from the dissociation of the hydrogencarbonate anion is proposed as the reaction that produces CO. By using Fourier transform infrared spectroscopy, linear and multi-bonded CO were detected on polycrystalline platinum electrodes. The shape of the band associated with linearly adsorbed CO is monopolar as a consequence of the partial overlapping, at lower wavenumbers, of the absolute bands at both potentials (0.05 and 0.35 V).     

Electrodeposition of Au monolayer on Pt(111) mediated by self-assembled monolayers

In-situ scanning tunneling microscopy (STM), cyclic voltammetry (CV), and infrared reflection-adsorption spectroscopy (IRRAS) have been used to examine the electrodeposition of gold onto Pt(111) electrodes modified with benzenethiol (BT) and benzene-1,2-dithiol (BDT) in 0.1 M HClO4 containing 10 microM HAuCl4. Both BT and BDT were attached to Pt(111) via one sulfur headgroup. STM and IRRAS results indicated that the other SH group of BDT was pendant in the electrolyte. Both BT and BDT formed (2 x 2) structures at the coverage of 0.25, and they were transformed into (square root(3) x square root(3))R30 degrees as the coverage was raised to 0.33. These two organic surface modifiers resulted in 3D and 2D gold islands at BT- and BDT-coated Pt(111) electrodes, respectively. The pendant SH group of BDT could interact specifically with gold adspecies to immobilize gold adatoms on the Pt(111) substrate, which yields a 2D growth of gold deposition. Molecular resolution STM revealed an ordered array of (6 x 2 square root(13)) after a full monolayer of gold was plated on the BDT/Pt(111) electrode. Since BDT was strongly adsorbed on Pt(111), gold adatoms only occupied free sites between BDT admolecules on Pt(111). This is supported by a stripping voltammetric analysis, which reveals no reductive desorption of BDT admolecules at a gold-deposited BDT/Pt(111) electrode. It seems that the BDT adlayer acted as the template for gold deposit on Pt(111). In contrast, a BT adlayer yielded 3D gold deposit on Pt(111). This study demonstrates unambiguously that organic surface modifiers could contribute greatly to the electrodeposition of metal adatoms.     

阴离子特性吸附和Pt(111)电极表面结构对乙二醇解离吸附动力学的影响

运用电化学循环伏安和程序电位阶跃方法研究了阴离子特性吸附和Pt(111)电极表面结构对乙二醇解离吸附反应动力学的影响. 结果表明, 阴离子特性吸附显著影响乙二醇的解离吸附, 在高氯酸介质中(无特性吸附)测得乙二醇解离吸附反应的初始速率vi以及解离吸附物种(DA)的饱和覆盖度均明显大于硫酸溶液(发生SO2-4/HSO-4特性吸附)中的相应值; 其平均速率v随电极电位的变化呈类似火山型分布, 最大值位于0.22 V(vs SCE)附近. 还发现通过不同处理获得的Pt(111)电极的不同表面结构对这一表面过程也具有显著的影响.     

The effect of the cooling atmosphere in the preparation of flame-annealed Pt(111) electrodes on CO adlayer oxidation

The effect of the cooling atmosphere on the rate of CO adlayer oxidation on flame-annealed Pt(111) has been studied. Cooling of a flame-annealed Pt(111) electrode in air results in a higher amount of crystalline defects compared to Pt(111) cooled in a hydrogen–argon stream. Although the blank profiles in 0.5 M H₂SO₄ of Pt(111), cooled in air and under oxygen exclusion, are virtually identical, CO adlayer oxidation occurs at significantly lower overpotentials on the former electrode. Three voltammetric peaks are observed for subsaturated CO adlayer oxidation on Pt(111), cooled in Ar+H₂ mixture, while only two peaks develop in the case of a Pt(111) surface cooled in air. Random crystalline defects, introduced via cooling of a flame-annealed Pt(111) in air, enhance CO adlayer oxidation, and apparently also suppress the third high-potential peak observed on a quasi-perfect (111) surface. The high sensitivity of the saturated CO adlayer oxidation to the presence of crystalline defects on Pt(111) can hence be used as a straightforward, sensitive, though qualitative method to assess the degree of crystalline order of the electrode.     

In situ FTIRS studies of kinetics of HCOOH oxidation on Pt(110) electrode modified with antimony adatoms

The electrochemical behavior of irreversibly adsorbed antimony on a Pt(110) electrode (Pt(110)/Sb) with various coverages was studied using cyclic voltammetry. The kinetics of HCOOH oxidation via reactive intermediates on Pt(110)/Sb were investigated quantitatively by employing the potential step technique and in situ FTIR spectroscopy. The results demonstrated that Sb adatoms were stable on Pt(110) when the electrode potential was below 0.45 V (SCE). It has been revealed that the dissociative adsorption of formic acid can be inhibited by the presence of Sb_ad on the Pt(110) surface. The electrocatalytic effects of Sb_ad towards HCOOH oxidation consist in a negative shift of the oxidation potential (about 350 mV) and the enhancement of the oxidation current. Based on the data processing method of integration transform developed in our previous papers, the kinetics of HCOOH oxidation on Pt(110)/Sb electrodes of different θ_Sb have been investigated quantitatively, and both the rate constant k_f and the transfer coefficient β were determined and reported.     

Kinetics of dissociative adsorption of formic acid on Pt(100), Pt(610), Pt(210) and Pt(110) single-crystal electrodes in perchloric acid solutions

The kinetics of dissociative adsorption of HCOOH on Pt(100), Pt(610), Pt(210) and Pt(110) single-crystal electrodes has been investigated. The oxidation of dissociative adsorbate (DA) derived from the dissociative adsorption of HCOOH was used as a probe reaction together with the potential-step technique of short time windows. The quantity Q_ad of DA produced at a given potential E_ad and in a defined time window t_ad of adsorption has been determined systematically. At fixed t_ad peaked curves of Q_ad versus E_ad in the potential range between −0.25 and 0.25 V/SCE have been obtained on all four electrodes. The maximum rate of dissociative adsorption of HCOOH decreases in the order Pt(110) Pt(210) Pt(610) Pt(100).     

Electrochemistry of platinum single crystal surfaces: Part II. Structural effect on formic acid oxidation and poison formation on Pt (111), (100) and (110)

The geometrical arrangement of sites favourable for formic acid oxidation and the poison formation reaction is determined using low index platinum single crystal planes. For this determination, the least number of sites required for the reactions to occur, which was obtained in the study of electrocatalysis by adatoms, was used, that is three adjacent sites are required for formic acid oxidation and four adjacent sites are required for poison formation.The triplet of sites on a unit lattice of Pt (111) and that on a unit lattice of Pt (100) plane are equally very favourable for the main oxidation reaction, but that on a unit lattice of Pt (110) is not so favourable as those on the former two planes. The oxidation rate is more than one order of magnitude lower on the latter than on the former triplets.The poison formation reaction proceeds at a very high rate on the (100) and the (110) planes. The geometrical arrangement of four sites on a square unit lattice of the (100) plane and on a rectangular unit lattice of the (110) plane are favourable for the poison formation reaction, but that on a hexagonal unit lattice of the (111) plane is not so favourable as the former two.     

Co-adsorption of CO onto a Ag-modified Pt(111)--restructuring of a Ag UPD layer monitored by EC-STM

The effect of co-adsorption of CO on an underpotentially deposited (UPD) silver monolayer on a Pt(111) single crystal electrode in 0.05 M sulfuric acid is investigated for the first time by means of electrochemical scanning tunneling microscopy (EC-STM). Pure electrochemical experiments suggest that the co-adsorption of CO onto Pt single crystal electrodes previously modified by a monolayer of Ag, forces Ag atoms of the first UPD monolayer into a second adlayer. The present EC-STM studies reveal the formation of a large-area Ag network after the co-adsorption of CO. The resulting Ag nanostructures formed on wide Pt(111) terraces are approximately 0.5 nm high and 10 nm wide. The desorption of the newly formed second Ag adlayer, the oxidation of CO and the desorption of Ag atoms from the first adlayer are monitored by EC-STM and simultaneously detected in the corresponding CVs in three different oxidation peaks. EC-STM images recorded afterwards show the unchanged Pt surface. The presence of Ag on the surface leads to a downward shift of the onset of oxygen adsorption on the Pt(111) surface.     

铂单晶电极表面下不可逆反应动力学(III)

在本系列研究论文Ⅰ[1]和Ⅱ[2]报导的结果基础上，进一步发展动力学数据解析方法．通过改变反应体系的温度，首次获得甲酸在Pt（100），Pt（110），Pt（111）电极上直接氧化反应的表观活化Gibbs自由能（△G≠0）．在实验选定的标准状态下，即0．0V／SCE、298．15K和1．013×105Pa下，来算得到。还发现传递系数β在实验温度范围（283．15～303．15K）不随温度变化，但随电极表面原子排列结构变化，大小次序与△G≠0的变化一致．