不同pH酸性溶液中铂电极上氢析出和二氧化碳还原的电化学研究

本文利用循环伏安法和电化学原位红外光谱的联用,研究了Pt(111)和Pt膜电极在CO₂饱和的酸性溶液中氢析出和CO₂还原的竞争. 发现：(i)在pH>2的溶液中,主要反应是氢析出,界面pH值随着氢析出突然增加;(ii)通过红外光谱检测,CO_ad是CO₂还原过程中唯一的吸附中间体;(iii)CO_ad生成速率随着欠电位沉积氢(UPD-H)覆盖的增加而增大,并在氢析出的起始电位达到最大值;(iv)在氢析出时,CO_ad的减少与CO₂吸附和还原所必需的的中间产物(H_ad)有限的可用位点和停留时间相关.     

硫酸(氢)根吸附层对Pd(111)电极上甲酸氧化反应的影响研究

本文使用循环伏安法和电势阶跃法分别研究了添加和不添加Na₂SO₄的0.1 mol/LH₂SO₄+0.1 mol/LHCOOH溶液中Pd(111)电极上甲酸氧化反应(FAO)的动力学行为,并与同样条件下0.1 mol/LHClO₄中的动力学行为进行比较. 加入0.05 mol/L或者0.1 mol/LNa₂O₄后,在相同的电位下负向扫描的FAO电流比正向扫描的显著减小. 本文推测在(SO₄^*_ad)_m+[(H₂O)_n-H₃O⁺]或(SO₄^*_ad)_m+[Na⁺(H₂O)_n-H₃O⁺]吸附层相转变电势以正的电位, 这个吸附层的结构可能随着电位的增加或Na₂SO₄的加入变得更加致密和稳定. 因此,破坏或者脱附致密的硫酸(氢)根吸附层变得更加困难,使得FAO 动力学在较高电位和随后的负扫电位受到明显的抑制.     

[Co(CO2)n]+团簇的红外解离光谱实验和理论研究

本文利用红外光解离光谱研究了一价钴阳离子与二氧化碳之间的相互作用. 通过密度泛函理论计算得到[Co(CO₂)_n]⁺团簇的几何结构,并且模拟了它们的振动光谱与实验数值进行比较. 研究结果表明,在[Co(CO₂)_n]⁺(n=2∽6)团簇中,钴阳离子通过电四极矩静电作用以端点结合的方式与二氧化碳中的氧原子结合在一起. 团簇的红外光谱都集中在二氧化碳反对称伸缩的波数附近,并且随着团簇尺寸的变化出现蓝移,最后把[Co(CO₂)_n]⁺的红外光解离光谱与稀有气体贴附的[Co(CO₂)_n]⁺-Ar的红外光解离光谱进行了比较.     

Pt(111)电极上O2与OHad的吸脱附及氧还原反应动力学 (cited: 1)

研究了Pt(111)电极在0.1 mol/L HClO₄溶液中O₂吸附与OH_ad脱附及氧还原反应的动力学．研究发现OH_ad的可逆吸脱附速率很快;在氧还原的动力学或动力学与传质混合控制区,恒电位下氧还原的电流随反应时间缓慢衰减,在转速较大,扫速较慢的情形下正向扫描过程中氧还原的电流总是明显低于逆向扫描的电流;Pt/0.1 mol/L HClO₄从无O₂切换到O_{2相似文献}   

利用原位红外光谱与微分电化学质谱联用技术研究在Pt以及PtRu电极上发生的甲醇氧化反应

利用电化学衰减全反射原位傅里叶变换红外光谱与微分电化学质谱联用技术,在流动电解池环境以及恒电位条件下研究了Pt电极和Pt电极通过表面电沉积Ru形成的PtRu电极(Pt_xRu_y)上发生的甲醇氧化反应(反应电解质溶液为0.1 mol/L HClO₄+0.5 mol/L MeOH). 在0.3～0.6 V(参比电极为可逆氢参比)实验用到的所有电极上,CO是唯一能从红外光谱观察到的与甲醇相关的表面吸附物;在Pt_0.56Ru_0.44电极上可以观察到CO吸附在Ru原子形成的岛上和CO线式吸附在Pt电极表面红外波段,而其他电极上只能观察到Pt表面上线式吸附的CO;甲醇氧化活性按Pt_0.73Ru_0.27>Pt_0.56Ru_0.44>Pt_0.83Ru_0.17>Pt的顺序递减;在0.5 V 时,甲醇在Pt_0.73Ru_0.27电极上的氧化反应的CO₂电流效率达到了50%.     

Pd修饰Cu电极的电化学CO2还原

利用微分电化学质谱和电化学原位衰减全反射红外光谱技术探究了Cu和CuPd催化剂上CO₂和CO的电化学还原行为. 红外光谱观察到了生成甲醇、甲烷与乙烯的CH_x中间物种. 在CuPd电极CO₂还原过程中，红外光谱的CO吸附峰起始电位比Cu正移大约300 mV，说明CuPd能够有效促进CO₂还原；CO饱和溶液中，Cu和CuPd电极CO起始吸附电位基本相同；两电极上CO谱带出现的电位与CO₃^2-的谱带降低的电位基本相同，说明CO的吸附需要CO₃^2-的脱附. 利用电化学在线质谱发现在CuPd电极上CO还原产生CH₄和CH₃OH的起始电位比Cu电极正移约200 mV. 推测催化活性的提升可能是由于Pd的引入改变了Cu的d能带，且Pd吸附更多的H，从而促进CO₂还原，使CO能够与H结合并被深度还原.     

非水体系中甲酸在铂电极上催化氧化的表面增强拉曼光谱研究

采用共焦显微拉曼系统研究了甲酸在非水体系中的纯多晶铂电极上的表面增强拉曼光谱,实验发现电位较低时甲酸首先在粗糙铂电极表面生成CO中间体,当电位逐渐变正并高于0.1 V时CO开始氧化,但是此时新生成的CO足以弥补其氧化的消耗,表现在Raman强度和一定覆盖度下的耦合作用并没有减少。当电位达到约0.6 V时CO的氧化速度进一步加剧并完全氧化为最终产物CO₂。     

第三族金属氧化物离子对二氧化碳转化的红外光谱研究

本文利用红外光解离光谱研究了第三族金属氧化物离子对二氧化碳分子的转化机制. 研究表明，对于[ScO(CO₂)_n]⁺体系，在n≤4时，形成了溶剂化结构；在n=5时，形成了碳酸盐结构，实现了二氧化碳的转化. 对于[YO(CO₂)_n]⁺体系，需要4个二氧化碳分子就可以实现二氧化碳的转化. 而在[YO(CO₂)_n]⁺体系中，只发现了溶剂化结构，没有观察到碳酸盐结构. 理论计算表明，[YO(CO₂)_n]⁺体系拥有最小的溶剂化结构向碳酸盐结构转化能垒，[LaO(CO₂)_n]⁺体系拥有最大的溶剂化结构向碳酸盐结构转化能垒. 本文从分子水平揭示了不同金属氧化物离子对二氧化碳分子转化的影响规律.     

重新审视桥式吸附甲酸根在铂电极上电催化氧化甲酸过程中的作用 (cited: 2)

基于以前报道的电化学原位ATR-FTIRS数据(Langmuir 22,10399 (2006)和Angewa. Chem. Int. Ed., 50,1159 (2011)),详细讨论了甲酸在铂电极上电催化氧化机理及动力学过程．提出了直接反应路径的动力学模型,即甲酸吸附(同时C-H键活化)作为此反应的决速步骤,此反应路径贡献甲酸氧化反应的大部分电流．该动力学模型可以很好地拟合在无CO毒化影响和浓度在0.1 mol/L以下的红外光谱结果．这种机理预测了甲酸氧化直接途径可能只需要一个Pt原子作为反应位点,甲酸根阻碍活性位点,并非为反应中间物．另外还详细检验了之前其他小组曾提出的甲酸根途径(一级或二级反应)为甲酸氧化直接途径,并指出了引起分歧的原因．     

异构化能量分解分析Ni2CO5的(18，18)与(18，16)构型竞争

本文研究了中性Ni₂CO₅配合物的势能图,发现一个C_2v双桥羰基新构型,与以前报道的三羰基桥联的D_3h构型竞争Ni₂CO₅的基态结构. 尽管三桥式异构体拥有更有利的(18,18)电子构型,即双金属中心都满足18电子规则,中性的Ni₂CO₅$配合物更倾向于(18,16)电子结构的双桥式几何. 异构化能量分解分析表明,这样的结构优选是静电作用和轨道相互作用极大化的结果.     

Surface (electro-)chemistry on Pt(1 1 1) modified by a Pseudomorphic Pd monolayer

The formic acid and methanol oxidation reaction are studied on Pt(1 1 1) modified by a pseudomorphic Pd monolayer (denoted hereafter as the Pt(1 1 1)-Pd_1 ML system) in 0.1 M HClO₄ solution. The results are compared to the bare Pt(1 1 1) surface. The nature of adsorbed intermediates (CO_ad) and the electrocatalytic properties (the onset of CO₂ formation) were studied by FTIR spectroscopy. The results show that Pd has a unique catalytic activity for HCOOH oxidation, with Pd surface atoms being about four times more active than Pt surface atoms at 0.4 V. FTIR spectra reveal that on Pt atoms adsorbed CO is produced from dehydration of HCOOH, whereas no CO adsorbed on Pd can be detected although a high production rate of CO₂ is observed at low potentials. This indicates that the reaction can proceed on Pd at low potentials without the typical “poison” formation. In contrast to its high activity for formic acid oxidation, the Pd film is completely inactive for methanol oxidation. The FTIR spectra show that neither adsorbed CO is formed on the Pd sites nor significant amounts of CO₂ are produced during the electrooxidation of methanol.     

The decomposition of formic acid on Ni(100)

The decomposition of HCOOD was studied on Ni(100). Low temperature adsorption of HCOOD resulted in the desorption of D₂O, CO₂, CO, and H₂. The D₂O was evolved below room temperature. CO₂ and H₂ were evolved in coincident peaks at a temperature above that at which h₂ desorbed following H₂ adsorption and well above that for CO₂ desorption from CO₂ adsorption; CO desorbed primarily in a desorption limited step. The decomposition of formic acid on the clean surface was found to yield equal amounts of H₂, CO, and CO₂ within experimental error. The kinetics and mechanism of the decomposition of formic acid on Ni (110) and Ni(100) single crystal surfaces were compared. The reaction proceeded by the dehydration of formic acid to formic anhydride on both surfaces. The anhydride intermediate condensed into islands due to attractive dipole-dipole interactions. Within the islands the rate of the decomposition reaction to form CO₂ was given by:

$\text{Rate = 6 × 10}{}^{15}\text{exp{?[25,500 + ω(}\text{c}\text{c}{}_{\mathrm{sat}}\text{)]/RT} × c}$

, where c is the local surface concentration, c_sat is the saturation coverage for the particular crystal plane, and ω is the interaction potential. The interaction potential was determined to be 2.7 kcal/mole on Ni(110) and 1.4 kcal/mole on Ni(100); the difference observed was due to structural differences of the surfaces relating to the alignment of the dipole moments within the islands. These attractive interactions resulted in an autocatalytic reaction on Ni(110), whereas the interaction was not strong enough on Ni(100) to sustain the autocatalytic behavior. Formic acid decomposition oxidized the Ni(100) surface resulting in the formation of a stable surface oxide. The buildup of the oxide resulted in a change in the selectivity reducing the amount of CO formed. This trend indicated that on the oxide surface the decomposition proceeded via a formate intermediate as on Ni(110) O.     

Examining the redox and formate mechanisms for water–gas shift reaction on Au/CeO2 using density functional theory

We perform DFT calculations to investigate the redox and formate mechanisms of water–gas-shift (WGS) reaction on Au/CeO₂ catalysts. In the redox mechanism, we analyze all the key elementary steps and find that the OH cleavage is the key step. Three possible pathways of OH cleavage are calculated: (1) OH_ad^″+*^′→H_ad^′+O_ad^″; (2) H_ad^′+OH_ad^″→H₂(g)+O_ad^″+*^′; and (3) OH_ad^″+OH_ad^″→2O_ad^″+H₂(g) (*′: the free adsorption sites on the oxides; ad′: adsorption on the metal; ad″: adsorption on the oxide, respectively). In the formate mechanism, we identify all the possible pathways for the formation and decomposition of surface formates in the WGS reaction. It is found that there is a shortcoming in the redox and formate mechanisms which is related to surface oxygen reproduction. Four possible pathways for producing surface oxygen are studied, and all the barriers of the four pathways are more than 1 eV. Our results suggest that the processes to reproduce surface oxygen in the reaction circle are not kinetically easy.     

The decomposition of formic acid on Ru(101̄0)

The decomposition of formic acid was studied on a clean Ru(101̄0) surface adsorption temperature between 100 and 460 K by means of flash thermal desorption. The decomposition products observed were H₂, CO₂, H₂O and CO. HCOOH itself was also desorbed, although at low exposures no formic acid was observed. The H₂ and CO₂ products were desorbed in identical first order peaks, with a peak temperature of 395 K. The H₂O product desorbed in a second order peak at 813 K, in contrast to H₂O desorption from low coverage H₂O adsorption which occurs in two peaks in the region of 220 and 265 K. The CO product desorbed in a first order peak at 488 K, identical to CO from CO adsorption. The dependence of the product peaks on adsorption temperature of the Ru surface was also studied. These results suggest a model involving the formation and decomposition of a surface intermediate species.     

Reaction intermediates of methanol synthesis and the water–gas-shift reaction on the ZnO(0001) surface

The polar Zn-ZnO(0001) surface is involved in the catalysis of methanol synthesis and the water–gas-shift reaction. We use density functional theory calculations to explore the favorable binding geometries and energies of adsorption of several molecular species relevant to these reactions, namely carbon monoxide (CO), carbon dioxide (CO₂), water (H₂O) and methanol (CH₃OH). We also consider several proposed reaction intermediates, including hydroxymethyl (CH₂OH), methoxyl (CH₃), formaldehyde (CH₂O), methyl (CH₃), methylene (CH₂), formic acid (HCOOH), formate (HCOO), formyl (HCO), hydroxyl (OH), oxygen (O) and hydrogen (H). For each, we identify the preferred binding geometry at a coverage of 1/4 monolayers (ML), and report calculated vibrational frequencies that could aid in the identification of these species in experiment. We further explore the effects on the binding energy when the adsorbate coverage is lowered to 1/9 and 1/16 ML.     

二甲基乙醚低温氧化中关键中间体(HOOCH2OC(=O)H)的分解反应机理

在QCISD(T)/CBS//B3LYP/6-311++G(d,p)方法的基础上计算了DME(二甲基乙醚)低温氧化的中间体HPMF(HOOCH₂OC(=O)H)的分解路径, 并且结合变分过渡态理论及RRKM/主方程求解了温度以及压力依赖的动力学速率. 计算结果表明,除了脱OH的直接解离路径之外,HPMF将经过异构分解生成甲酸及CH₂OO中间体. 新的动力学数据的结果证实了CH₂OO 路径是不容忽略的. 这条分解路径将会降低整个模型的低温氧化活性. 同时从化学本质上去分析了HPMF的化学结构以及电子效应,进一步解释及验证了计算结果的合理性.     

CO2 hydrogenation to formic acid on Ni(110)

Hydrogen (H) in the subsurface of transition-metal surfaces exhibits unique reactivity for heterogeneously catalyzed hydrogenation reactions. Here, we explore the potential of subsurface H for hydrogenating carbon dioxide (CO₂) on Ni(110). The energetics of surface and subsurface H reacting with surface CO₂ to form formate, carboxyl, and formic acid on Ni(110) is systematically studied using self-consistent, spin-polarized, periodic density functional theory (DFT-GGA-PW91) calculations. We show that on Ni(110), CO₂ can be hydrogenated to formate by surface H. However, further hydrogenation of formate to formic acid by surface H is hindered by a larger activation energy barrier. The relative energetics of hydrogenation barriers is reversed for the carboxyl-mediated route to formic acid. We suggest that the energetics of subsurface H emerging to the surface is suitable for providing the extra energy needed to overcome the barrier to formate hydrogenation. CO₂ hydrogenation to formic acid could take place on Ni(110) when subsurface H is available to react with CO₂. Additional electronic-structure based dynamic calculations would be needed to elucidate the detailed reaction paths for these transformations.     

Interaction of O2, CO2, CO,C2H4 and C2H4O with Ag(110)

The interaction of O₂, CO₂, CO, C₂H₄ AND C₂H₄O with Ag(110) has been studied by low energy electron diffraction (LEED), temperature programmed desorption (TPD) and electron energy loss spectroscopy (EELS). For adsorbed oxygen the EELS and TPD signals are measured as a function of coverage (θ). Up to θ = 0.25 the EELS signal is proportional to coverage; above 0.25 evidence is found for dipole-dipole interaction as the EELS signal is no longer proportional to coverage. The TPD signal is not directly proportional to the oxygen coverage, which is explained by diffusion of part of the adsorbed oxygen into the bulk. Oxygen has been adsorbed both at pressures of less than 10^-4 Pa in an ultrahigh vacuum chamber and at pressures up to 10³ Pa in a preparation chamber. After desorption at 10³ Pa a new type of weakly bound subsurface oxygen is identified, which can be transferred to the surface by heating the crystal to 470 K. CO₂ is not adsorbed as such on clean silver at 300 K. However, it is adsorbed in the form of a carbonate ion if the surface is first exposed to oxygen. If the crystal is heated this complex decomposes into O_ad and CO₂ with an activation energy of 27 kcal/mol(1 kcal = 4.187 kJ). Up to an oxygen coverage of 0.25 one CO₂ molecule is adsorbed per two oxygen atoms on the surface. At higher oxygen coverages the amount of CO₂ adsorbed becomes smaller. CO readily reacts with O_ad at room temperature to form CO₂. This reaction has been used to measure the number of O atoms present on the surface at 300 K relative to the amount of CO₂ that is adsorbed at 300 K by the formation of a carbonate ion. Weakly bound subsurface oxygen does not react with CO at 300 K. Adsorption of C₂H₄O at 110 K is promoted by the presence of atomic oxygen. The activation energy for desorption of C₂H₄O from clean silver is ～ 9 kcal/mol, whereas on the oxygen-precovered surface two states are found with activation energies of 8.5 and 12.5 kcal/mol. The results are discussed in terms of the mechanism of ethylene epoxidation over unpromoted and unmoderated silver.     

Electron energy loss spectroscopy of the decomposition of formic acid on Ru(001)

Electron energy loss spectroscopy has demonstrated the existence of both a monodentate and a symmetric bidentate bridging formate as stable intermediates in the decomposition of formic acid on the Ru(001) surface. The monodentate formate converts upon heating to the bidentate formate which decomposes via two pathways: CH bond cleavage to yield CO₂ and adsorbed hydrogen; and CO bond cleavage to yield adsorbed hydrogen, oxygen and CO. Thermal desorption spectra demonstrate the evolution of H₂,H₂O, CO and CO₂ as gaseous products of the decomposition reaction. The observation of this product distribution from Ru(100), Ni(100) and Ni(110) had prompted the proposal of a formic anhydride intermediate, the existence of which is rendered questionable by the spectroscopic results reported here.