甲醇在Pt-Mo(111)/C表面上的吸附

采用密度泛函理论和周期平板模型相结合的方法, 对CH3OH分子在Pt-Mo(111)/C表面的顶位、穴位和桥位共计9种吸附模型进行了构型优化、能量计算和频率分析, 结果表明top-Pt位是较有利的吸附位. Mo掺杂后价带与导带位置均有不同程度的降低, 电子结构的变化使得Pt-Mo(111)/C的催化活性提高. 并且在考虑催化剂抗中毒性能时发现: CO在Pt(111)/C面上的吸附能比甲醇吸附能要高, CO在Pt-Mo(111)/C上的吸附能比甲醇的要低, 说明CO在Pt(111)/C面上的吸附会阻碍甲醇的吸附, 并影响催化过程的进行, 而Pt-Mo(111)/C的抗CO中毒化能力增强, 是催化氧化甲醇较好的催化剂.     

甲醇在Pt-Fe(111)/C表面吸附的理论研究

采用密度泛函理论和周期平板模型相结合的方法, 对CH3OH分子在Pt-Fe(111)/C表面top, fcc, hcp和bridge位的吸附模型进行了构型优化、能量计算, 结果表明bridge位是较有利的吸附位. 掺杂后费米能级的位置发生了右移, 价带和导带均增宽, 极利于电子-空穴的迁移, 这对提高催化活性是非常有利的. 考察抗中毒性发现: CO在Pt(111)/C面上的吸附能比甲醇吸附能要高, CO在Pt-Fe(111)/C的吸附能比甲醇吸附能要低, 可说明CO在Pt(111)/C面上有中毒效应, 而Pt-Fe(111)/C的抗CO中毒能力增强, 是催化氧化甲醇良好的催化剂.     

Pt（111）和Pt3Ni（111）表面上CO催化氧化反应的密度泛函理论研究

采用密度泛函理论,对Pt(111)和Pt3Ni(111)表面上CO和O的单独吸附、共吸附以及CO的氧化反应进行了系统的研究. 结果表明, Pt3Ni(111)表面上CO的吸附弱于Pt(111)表面, O的吸附明显强于Pt(111)表面. 两个表面表现出相似的CO催化氧化活性. 表面Ni的存在不但稳定了O的吸附,同时也降低了过渡态O的能量.     

CO在Pt(111)和Pt-M(111)(M=Ni,Mg)表面吸附的理论研究

采用密度泛函理论与周期性平板模型相结合的方法,对CO在Pt(111)表面top,fcc,hcp和bridge 4个吸附位和Pt-M(111)(M=Ni,Mg)表面h-top,M-top,Pt(M)Pt-bridge,Pt(M)M-bridge,Pt(Pt)M-bridge,M(Pt)M-bridge,Pt1M2-hcp...     

CO在CeO2(111)表面的吸附与氧化

采用密度泛函理论计算了CO在CeO2(111)表面的吸附与氧化反应行为. 结果表明, O2在洁净的CeO2(111)表面为弱物理吸附, 而在氧空位表面是强化学吸附, 且O2分子活化程度较大, O—O键长为0.143 nm. CO在CeO2(111)表面吸附行为的研究表明, CO在洁净表面及氧空位表面上为物理吸附, 吸附能均小于0.42 eV; 当表面氧空位吸附O2后, CO可吸附生成二齿碳酸盐中间体或直接生成CO2, 与原位红外光谱结果相一致. 表面碳酸盐物种脱附生成CO2的能垒仅为0.28 eV. 计算结果表明, 当CeO2表面存在氧空位时, Hubbard参数U对CO吸附能有一定的影响. CeO2载体在氧化反应中可能的催化作用为, 在氧气氛下, CeO2表面氧空位吸附O2分子, 形成活性氧物种, 参与CO催化氧化反应.     

三种Au(111)催化水煤气变换反应机理的比较

采用密度泛函理论对三种水煤气变换反应(WGSR)机理(氧化还原机理、羧基机理、甲酸基的生成机理)在Au(111)面上的反应历程进行详细讨论.通过对表面吸附物种(H2O、CO、OH、O、H、CO2、COOH、HCOO)的吸附行为进行研究,得到最佳活性吸附中心.对三种机理中的14个基元反应的活化能进行分析,得出WGSR在Au(111)上按照羧基机理和氧化还原机理进行的可能性较大,按照甲酸基的生成机理进行的可能性较小.相比较羧基机理和氧化还原机理,反应更有可能按照羧基机理进行,最佳反应途径为H2O-H→OH+CO→COOH+OH→CO2.     

甘氨酸修饰的Pt(111)电极上的氧还原

利用单晶旋转圆盘电极技术（Hanging Meniscus Rotating Disk Electrode,　HMRD）在硫酸和高氯酸溶液中分别研究了甘氨酸修饰的Pt(111)电极表面氧分子的电催化还原反应. 实验发现：在硫酸溶液中,经甘氨酸修饰的Pt(111)电极表面的氧还原活性明显提高,其中氧还原的半波电位与Pt(111)电极的相比正移约0.1 V,而在高氯酸溶液中,甘氨酸修饰的Pt(111)电极的活性几乎没有发生变化. 该实验结果表明：甘氨酸修饰的Pt(111)电极一方面抑制了SO₄^2-在电极表面的吸附,另一方面又能在电极表面提供相邻的空位供氧分子吸附. 通过与文献中报道的CN^-修饰的Pt(111)电极上的氧还原结果的对比,可以推测甘氨酸修饰的Pt(111)电极表面氧还原活性提高是由于甘氨酸在Pt(111)表面可能先被氧化成CN^-后吸附在电极表面,进而促进了氧分子的电催化还原反应.     

不同介质中甲醛在Aucore@Ptshell/Pt电极上氧化的原位SERS研究

应用化学还原法合成了Au_core@Pt_shell纳米粒子, 并用扫描电子显微镜(SEM)和能量色散光谱(EDS)对其进行了表征; 采用电化学原位表面增强拉曼散射(SERS)光谱技术研究了不同介质中甲醛在Au_core@Pt_shell/Pt电极上的电催化氧化行为, 获得了不同介质中甲醛在Au_core@Pt_shell/Pt电极上电催化氧化行为的原位SERS谱. 研究结果表明, 不论在酸性、中性还是碱性介质中, 甲醛均能在Au_core@Pt_shell/Pt电极上自发氧化解离出强吸附中间体CO, 只是在碱性介质中桥式吸附CO的比例明显增大, 且桥式吸附比线形吸附CO更易被氧化, 使CO在碱性介质中基本氧化完毕的电位比在中性及酸性介质中提前了约950 mV. 分子水平的研究结果表明, CO和甲醛在碱性介质中比在中性和酸性介质中更易被氧化.     

甲醇在欠电位沉积Sn/Pt电极上催化氧化

在欠电位沉积（upd）锡修饰的铂电极（upd-Sn/Pt）上,对甲醇电化学催化氧化过程进行了研究.发现当Pt表面upd-Sn的覆盖率在20％附近时,对甲醇的催化氧化的增强作用最为明显；在电位低于0.35 V （vs RHE）时,甲醇在Pt与upd-Sn/Pt电极上氧化只进行到脱氢生成CO的步骤;在0.35 V以后,表面Sn－OH形成,反应Sn－OH＋COads=Sn＋CO2＋H＋＋e有利于表面CO的去除;而Pt电极上,只有0.6 V以后,才有反应Pt－OH＋COads=Pt＋CO2＋H＋＋e发生.因此,Sn的存在有利于甲醇在较低的电位下氧化; Pt电极上CH3OH脱氢并释放出电子的过程是一个快速的过程,表面CO的去除是甲醇氧化过程的控制步骤;甲醇氧化产生的表面吸附态CO 以线式吸附为主,少量的桥式吸附态CO在反应初期即达到吸附饱和; Pt表面上upd-Sn表现的催化增强作用,在光亮铂电极和在高分散铂黑电极上是一致的.     

CO和H在Pt/WC(0001)表面的吸附

采用密度泛函理论(DFT)和周期平板模型,研究两种WC(0001)表面的几何结构和表面能,并对Pt原子单层(PtML)在两种WC(0001)表面的高对称性吸附位上的吸附能和分离功进行计算.结果发现,终止于W原子的WC(0001)为最稳定的WC(0001)表面,Pt原子单层以hcp位的方式吸附于W终止的WC(0001)表面是PtML/WC(0001)体系最稳定的几何构型.在此基础上研究了CO分子和H原子分别在PtML/WC(0001)表面和具有相似表面结构的Pt(111)表面的吸附行为.在0.25 ML(monolayer)低覆盖度下,与在Pt(111)表面相比,在PtML/WC(0001)表面上的Pt—C间距明显拉长和CO分子吸附能减少,说明PtML/WC(0001)表面抗CO中毒能力比Pt(111)表面高;态密度分析进一步解释了CO分子与不同表面Pt原子的成键机理.在同一覆盖度下,H原子在PtML/WC(0001)表面的最大吸附能等于甚至略高于在Pt(111)表面的,表明Pt/WC对氢气氧化反应具有良好的催化活性,是一种很有前途的质子交换膜燃料电池(PEMFC)阳极催化剂.     

Decomposition pathways of methanol on the PtAu(111) bimetallic surface: a first-principles study

Systematic density functional studies revealed that PtAu(111) significantly alters the reaction kinetics of methanol oxidation from that on Pt(111). PtAu(111) facilitates the reaction path that starts from cleavage of the OH bond. Furthermore, it prevents CH(2)O from immediate decomposition as on the clean Pt(111) surface. This indicates that proper arrangement of Au and Pt sites offers great opportunities for non-CO(ad) paths for high H productivity in fuel cells.     

Methanol oxidation on a Pt(111)-OH/O surface

The methanol oxidation on a hydroxylated Pt (Pt(111)-OH) surface has been investigated by means of infrared reflection absorption spectroscopy (IRAS) in ultra-high vacuum (UHV) and in acidic solution. The Pt(111)-OH surface in UHV was prepared by introducing water molecules on a Pt(111)-(2 x 2)-O surface and annealed at temperature higher than 160 K. Methanol was then, introduced to the Pt(111)-OH surface to show the dependence of the reaction intermediate on the annealing temperature. At an annealing temperature below 160 K, IR bands assignable to methanol overlayer were observed and no detectable intermediates, such as CO, formaldehyde and formate, were formed, suggesting that methanol molecules remain stable on Pt(111) surface without dissociation at this temperature region. At an annealing temperature above 160 K, on the other hand, CO and formate were observed. In addition, the oxidation of CO on Pt(111)-OH showed no sign of formate formation, indicating that formate is not derived from CO, but from a direct oxidation of methanol. Methanol oxidation was carried out in 0.1 mol dm(-3) HClO(4) solution on Pt(111) with a flow cell configuration and showed the formation of formate. These results indicate that the formate is the dominant non-CO intermediate both in UHV and in acidic solution, and the preadsorbed oxygen-containing species, in particular OH adsorbates, on Pt(111) surface plays a very important role in the formate formation process in methanol oxidation reaction.     

Methanol dehydrogenation and oxidation on Pt(111) in alkaline solutions

Adsorption, dehydrogenation, and oxidation of methanol on Pt(111) in alkaline solutions has been examined from a fundamental mechanistic perspective, focusing on the role of adsorbate-adsorbate interactions and the effect of defects on reactivity. CO has been confirmed as the main poisoning species, affecting the rate of methanol dehydrogenation primarily through repulsive interactions with methanol dehydrogenation intermediates. At direct methanol fuel cell (DMFC)-relevant potentials, methanol oxidation occurs almost entirely through a CO intermediate, and the rate of CO oxidation is the main limiting factor in methanol oxidation. Small Pt island defects greatly enhance CO oxidation, though they are effective only when the CO coverage is 0.20 ML or higher. Large Pt islands enhance CO oxidation as well, but unlike small Pt islands, they also promote methanol dehydrogenation. Perturbations in electronic structure are responsible for the CO oxidation effect of defects, but the role of large Pt islands in promoting methanol dehydrogenation is primarily explained by surface geometric structure.     

Experimental and theoretical study of reactivity trends for methanol on Co/Pt(111) and Ni/Pt(111) bimetallic surfaces

Methanol was used as a probe molecule to examine the reforming activity of oxygenates on NiPt(111) and CoPt(111) bimetallic surfaces, utilizing density functional theory (DFT) modeling, temperature-programmed desorption, and high-resolution electron energy loss spectroscopy (HREELS). DFT results revealed a correlation between the methanol and methoxy binding energies and the surface d-band center of various NiPt(111) and CoPt(111) bimetallic surfaces. Consistent with DFT predictions, increased production of H2 and CO from methanol was observed on a Ni surface monolayer on Pt(111), designated as Ni-Pt-Pt(111), as compared to the subsurface monolayer Pt-Ni-Pt(111) surface. HREELS was used to verify the presence and subsequent decomposition of methoxy intermediates on NiPt(111) and CoPt(111) bimetallic surfaces. On Ni-Pt-Pt(111) the methoxy species decomposed to a formaldehyde intermediate below 300 K; this species reacted at approximately 300 K to form CO and H2. On Co-Pt-Pt(111), methoxy was stable up to approximately 350 K and decomposed to form CO and H2. Overall, trends in methanol reactivity on NiPt(111) bimetallic surfaces were similar to those previously determined for ethanol and ethylene glycol.     

Characterization and methanol electrooxidation studies of Pt(111)/Os surfaces prepared by spontaneous deposition

Catalytic activity of the Pt(111)/Os surface toward methanol electrooxidation was optimized by exploring a wide range of Os coverage. Various methods of surface analyses were used, including electroanalytical, STM, and XPS methods. The Pt(111) surface was decorated with nanosized Os islands by spontaneous deposition, and the Os coverage was controlled by changing the exposure time to the Os-containing electrolyte. The structure of Os deposits on Pt(111) was characterized and quantified by in situ STM and stripping voltammetry. We found that the optimal Os surface coverage of Pt(111) for methanol electrooxidation was 0.7 +/- 0.1 ML, close to 1.0 +/- 0.1 Os packing density. Apparently, the high osmium coverage Pt(111)/Os surface provides more of the necessary oxygen-containing species (e.g., Os-OH) for effective methanol electrooxidation than the Pt(111)/Os surfaces with lower Os coverage (vs e.g., Ru-OH). Supporting evidence for this conjecture comes from the CO electrooxidation data, which show that the onset potential for CO stripping is lowered from 0.53 to 0.45 V when the Os coverage is increased from 0.2 to 0.7 ML. However, the activity of Pt(111)/Os for methanol electrooxidation decreases when the Os coverage is higher than 0.7 +/- 0.1 ML, indicating that Pt sites uncovered by Os are necessary for sustaining significant methanol oxidation rates. Furthermore, osmium is inactive for methanol electrooxidation when the platinum substrate is absent: Os deposits on Au(111), a bulk Os ingot, and thick films of electrodeposited Os on Pt(111), all compare poorly to Pt(111)/Os. We conclude that a bifunctional mechanism applies to the methanol electrooxidation similarly to Pt(111)/Ru, although with fewer available Pt sites. Finally, the potential window for methanol electrooxidation on Pt(111)/Os was observed to shift positively versus Pt(111)/Ru. Because of the difference in the Os and Ru oxophilicity under electrochemical conditions, the Os deposit provides fewer oxygen-containing species, at least below 0.5 V vs RHE. Both higher coverage of Os than Ru and the higher potentials are required to provide a sufficient number of active oxygen-containing species for the effective removal of the site-blocking CO from the catalyst surface when the methanol electrooxidation process occurs.     

CO and methanol adsorption on (2 × 1)Pt(110) and ion‐eroded Pt(111) model catalysts

Methanol adsorption on ion‐sputtered Pt(111) surface exhibiting high concentration of vacancy islands and on (2 × 1)Pt(110) single crystal were investigated by means of photoelectron spectroscopy (PES) and thermal desorption spectroscopy. The measurements showed that methanol adsorbed at low temperature on sputtered Pt(111) and on (2 × 1)Pt(110) surfaces decomposed upon heating. The PES data of methanol adsorption were compared to the data of CO adsorbed on the same Pt single crystal surfaces. In the case of the sputtered Pt(111) surface, the dehydrogenation of H_xCO intermediates is followed by the CO bond breakage. On the (2 × 1)Pt(110) surface, carbon monoxide, as product of methanol decomposition, desorbed molecularly without appearance of any traces of atomic carbon. By comparing both platinum surfaces we conclude that methanol decomposition occurs at higher temperature on sputtered Pt(111) than on (2 × 1)Pt(110). Copyright © 2010 John Wiley & Sons, Ltd.     

CO oxidation on Pt-modified Rh(111) electrodes.

The CO electro-oxidation reaction was studied on platinum-modified Rh(111) electrodes in 0.5 M H2SO4 using cyclic voltammetry and chronoamperometry. The Pt-Rh(111) electrodes were generated during voltammetric cycles at 50 mV s(-1) in a 30 microM H2PtCl6 and 0.5 M H2SO4 solution. Surfaces generated by n deposition cycles were investigated (Ptn-Rh(111) with n=2, 4, 6, 8, 10, and 16). The blank cyclic voltammograms of these surfaces are characterized by a pronounced sharpening of the hydrogen/(bi)sulfate adsorption/desorption peaks, typical for Rh(111), and the appearance of contributions between 0.1 and 0.4 V, which were ascribed to hydrogen/(bi)sulfate adsorption/desorption on the deposited platinum. At higher potentials, the surface oxidation of Rh(111) is enhanced by the presence of platinum. The structure of the Pt-modified electrodes was investigated by STM imaging. At low Pt coverages (Pt2-Rh(111)), monoatomically high islands are formed, which grow three dimensionally as the number of deposition cycles increases. After eight cycles, the monolayer islands have grown in diameter and range from mono- to multiatomic height. At even higher Pt coverage (Pt16-Rh(111)), the islands grow to particles of approx. 10 nm in diameter, which are 5-6 atoms high. The CO stripping voltammetry on these surfaces is characterized by two peaks: A low-potential, structure-insensitive peak, ascribed to CO reacting at the platinum monolayer islands, whose onset is shifted 150, 250, and 100 mV negatively with respect to pure Rh(111), Pt(111), and polycrystalline Pt, respectively, indicating the enhanced CO electro-oxidation properties of the Pt overlayer system. A peak at higher potentials displays strong structure sensitivity (particle-size effect) and was ascribed to CO reacting on the islands of multiatomic height. Current-time transients recorded on the surface with the highest amount of monolayer islands (Pt4-Rh(111)) also indicate enhanced CO-oxidation kinetics. Comparison of the Pt4-Rh(111) current-time transients recorded at 0.635, 0.675, and 0.750 V versus RHE (reversible hydrogen electrode) with those of pure Rh(111) and Pt(111) shows greatly reduced reaction times. A Cottrellian decay at long times indicates surface-diffusion-limited CO oxidation on the bare Rh(111) surface, while the peak visible at short times is indicative of CO reacting at the monolayer platinum islands. The results presented here show that, as indicated by density functional theory (DFT) calculations, the CO-adlayer oxidation for this system is enhanced compared to both pure Rh and Pt.     

Pt（111）表面上一氧化碳的吸附与氧化反应

Ｐｔ（１１１）表面上一氧化碳的吸附与氧化反应１）刘金尧（清华大学一碳化工国家重点实验室北京１０００８４）ＸｕＭＺａｅｒａＦ（ＤｅｐａｒｔｍｅｎｔｏｆＣｈｅｍｉｓｔｒｙＵｎｉｖｅｒｓｉｔｙｏｆＣａｌｉｆｏｒｎｉａＲｉｖｅｒｓｉｄｅＣＡ９２５２１）关键词...     

Electrocatalytic oxidation of formic acid and methanol on Pt deposits on Au(111)

This work presents characteristics of Pt deposits on Au(111) obtained by the use of spontaneous deposition and investigated by electrochemical scanning tunneling microscopy (EC-STM). On such prepared and STM characterized Au(111)/Pt surfaces, we studied electrocatalytic oxidation of formic acid and methanol. We show that the first monatomic layer of Pt displays a (square root 3 x square root 3)R30 degrees surface structure, while the second layer is (1 x 1). After prolonged deposition, multilayer Pt deposits are formed selectively on Au(111) surface steps and are 1-20 nm wide and one to five layers thick. On the optimized Au(111)/Pt surface, formic acid oxidation rates are enhanced by a factor of 20 compared to those of pure Pt(111). The (square root 3 x square root 3)R30 degrees-Pt yields very low methanol oxidation rates, but the rates increase significantly with further Pt growth.