双金属合金团簇M12Ni(M=Pt,Sn,Cu)催化甲烷干法重整反应的理论研究

利用密度泛函理论(DFT)研究了M₁₂Ni(M=Pt, Sn, Cu) 3种双金属合金团簇的电子活性和结构稳定性, 并探讨了甲烷干法重整反应(DRM)在M₁₂Ni双金属团簇表面的反应能量变化情况. 经比较发现甲烷脱氢和二氧化碳活化过程在Pt₁₂Ni团簇表面进行需克服的活化能垒最低, 反应最易进行. Sn₁₂Ni团簇上生成碳需要较高的活化能, 说明Sn₁₂Ni团簇能够有效抑制焦碳的生成, 一定程度上克服了碳沉积导致的催化剂失活现象, 并且Sn₁₂Ni团簇在C ^*和CH ^*氧化过程中表现出最佳的催化活性. Cu₁₂Ni团簇仅在甲烷脱氢过程中表现出较为优异的催化活性.     

Density Functional Theory Study on the Adsorption of CN on Transition Metal M（100） （M = Ni,Pd, Pt,Cu, Ag and Au） Surfaces

The adsorption of cyanide on the top site of a series of transition metal M（100） （M = Cu, Ag, Au, Ni, Pd, Pt） surfaces via carbon and nitrogen atoms respectively, with the CN axis perpendicular to the surface, has been studied by means of density functional theory and cluster model. Geometry, adsorption energy and vibrational frequencies have been determined, and the present calculations show that the adsorption of CN through C-end on metal surface is more favorable than that via N-end for the same surface. The vibrational frequencies of CN for C-down configuration on surface are blue-shifted with respect to the free CN, which is contrary to the change of vibrational frequencies when CN is adsorbed by N-down structure. Furthermore, the charge transfer from surface to CN causes the increase of surface work function.     

One-step conversion of n-butane to isobutene over H-beta supported Pt and Pt,M (M = Cu, In, Sn) catalysts: An investigation on the role of the second metal

The one-step transformation of n-butane to isobutene was studied over H-beta zeolite supported Pt and Pt,M (M = Cu, In, Sn) catalysts. Catalytic performance of monometallic Pt/H-beta samples resulted to be affected by the support acidity, a lower number of acid sites leading to higher iso- and n-butenes selectivities and lower by-products formation. Addition of Cu, In or Sn to Pt enhanced both isobutene and n-butenes selectivities, which were in the order: Pt,In > Pt,Sn  Pt,Cu > Pt. All Pt,M samples exhibited also a higher stability than the corresponding monometallic Pt samples, the sequence of deactivation rates being: Pt > In,Pt > Cu,Pt ≈ Sn,Pt. On the basis of characterization results it was stated that the addition of Cu, In or Sn to Pt affects the n-butane dehydroisomerization modifying both the surface structure of Pt clusters and the support acidity. In particular the observed order of isobutene selectivity was related to the degree of Pt–M interaction leading to a dilution of Pt clusters, which inhibits hydrogenolysis reactions and enhances dehydrogenation processes. The decrease in the number of acid sites caused by addition of the second metal was instead accounted for the improved resistance to deactivation of Pt,M catalysts.     

Relativistic DFT studies of dehydrogenation of methane by Pt cationic clusters: cooperative effect of bimetallic clusters

The dehydrogenation reaction mechanisms of methane catalyzed by transition-metal clusters PtM(+) (M = Cu, Ag, Au) and Pt(n)(+) (n = 2-4) have been investigated theoretically. In the reactions of PtM(+) (M = Cu, Ag, Au) with CH(4), cleavage of the first C-H bond is quite facile without barrier. The second C-H bond activation and the release of H(2) from molecular complex are generally the rate-determining steps. In the reactions of platinum clusters Pt(n)()(+) (n = 2-4) with CH(4), the H(2) elimination from the dihydrogen complex is the rate-determining step. Spin crossover may occur in the reaction of Pt(2)(+) and CH(4). Pt(2)(+) and Pt(3)(+) can dehydrogenate methane efficiently due to remarkable thermodynamic stability of the products. The dehydrogenation of methane induced by Pt(4)(+) is less favored thermodynamically than Pt(n)()(+) (n = 1, 2, 3). On the basis of theoretical analyses, the differences in reactivity among the clusters and the nature of cooperative effect of the bimetallic cluster have been discussed. The calculated results provide a reasonable basis for understanding of experimental observations.     

Pt/MO2/CNTs催化剂上乙醇电氧化研究

应用循环伏安法研究了几种催化剂Pt/MO2/CNTs（M = Sn, Ti, Ce）和Pt/CNTs对乙醇在H2SO4溶液中的电催化氧化过程。结果表明,金属氧化物的加入有利于乙醇的电氧化,其中,Pt/CeO2/CNTs对乙醇电氧化中间态产物具有显著的氧化性能。综合初始氧化电位、峰电流、总氧化峰面积等参数可以得出Pt/SnO2/CNTs催化剂性能最佳。     

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.     

Adsorption and dissociation of H2O2 on Pt and Pt-alloy clusters and surfaces

The adsorption of H(2)O(2) on Pt and Pt-M alloys, where M is Cr, Co, or Ni, is investigated using density functional theory. Binding energies calculated with a hybrid DFT functional (B3PW91) are in the range of -0.71 to -0.88 eV for H(2)O(2) adsorbed with one of the oxygen atoms on top Pt positions of Pt(3), Pt(2)M, and PtM(2), and enhanced values in the range of -0.81 to -1.09 eV are found on top Ni and Co sites of the Pt(2)M clusters. Adsorption on top sites of Pt(10) yields a weaker binding of -0.48 eV, whereas on periodic Pt(111) and Pt(3)Co(111) surfaces, H(2)O(2) generally dissociates into two OH radicals. On the other hand, attempts to attach H(2)O(2) on bridge sites cause spontaneous dissociation of H(2)O(2) into two adsorbed OH radicals, suggesting that stable adsorptions on bridge sites are not possible for any of the clusters or extended surfaces that are being studied. We also found that the water-H(2)O(2) interaction reduces the strength of the adsorption of H(2)O(2) on these clusters and surfaces.     

Experimental and DFT studies of the conversion of ethanol and acetic acid on PtSn-based catalysts

Reaction kinetics studies were conducted for the conversions of ethanol and acetic acid over silica-supported Pt and Pt/Sn catalysts at temperatures from 500 to 600 K. Addition of Sn to Pt catalysts inhibits the decomposition of ethanol to CO, CH4, and C2H6, such that PtSn-based catalysts are active for dehydrogenation of ethanol to acetaldehyde. Furthermore, PtSn-based catalysts are selective for the conversion of acetic acid to ethanol, acetaldehyde, and ethyl acetate, whereas Pt catalysts lead mainly to decomposition products such as CH4 and CO. These results are interpreted using density functional theory (DFT) calculations for various adsorbed species and transition states on Pt(111) and Pt3Sn(111) surfaces. The Pt3Sn alloy slab was selected for DFT studies because results from in situ (119)Sn M?ssbauer spectroscopy and CO adsorption microcalorimetry of silica-supported Pt/Sn catalysts indicate that Pt-Sn alloy is the major phase present. Accordingly, results from DFT calculations show that transition-state energies for C-O and C-C bond cleavage in ethanol-derived species increase by 25-60 kJ/mol on Pt3Sn(111) compared to Pt(111), whereas energies of transition states for dehydrogenation reactions increase by only 5-10 kJ/mol. Results from DFT calculations show that transition-state energies for CH3CO-OH bond cleavage increase by only 12 kJ/mol on Pt3Sn(111) compared to Pt(111). The suppression of C-C bond cleavage in ethanol and acetic acid upon addition of Sn to Pt is also confirmed by microcalorimetric and infrared spectroscopic measurements at 300 K of the interactions of ethanol and acetic acid with Pt and PtSn on a silica support that had been silylated to remove silanol groups.     

PtnCum(n+m=4)对甲醇第一步脱氢催化性能的理论研究

采用密度泛函理论（DFT）中的B3PW91/LANL2DZ（ECP）方法在Gaussian09程序包中计算了二元合金催化剂Pt_nCu_m（n+m=4）催化甲醇第一步脱氢反应的相关几何参数,主要研究了Pt_nCu_m（n+m=4）在甲醇分子表面的吸附和脱氢反应的机理。通过比较E_ads和脱氢能垒等发现,Pt_nCu_m（n+m=4）催化甲醇脱氢的最优反应路径为甲醇分子中的甲基氢吸附在该二元金属催化剂Pt位点上导致的C-H断裂。并对比了Pt_nCu_m（n+m=4）中Pt与Cu比例对甲醇催化脱氢活性的影响,结果表明,当二元合金催化剂中Pt与Cu的比例为1：1时,催化活性最高。     

Comprehensive mechanism and structure-sensitivity of ethanol oxidation on platinum: new transition-state searching method for resolving the complex reaction network

Ethanol oxidation on Pt is a typical multistep and multiselectivity heterogeneous catalytic process. A comprehensive understanding of this fundamental reaction would greatly benefit design of catalysts for use in direct ethanol fuel cells and the degradation of biomass-derived oxygenates. In this work, the reaction network of ethanol oxidation on different Pt surfaces, including close-packed Pt{111}, stepped Pt{211}, and open Pt{100}, is explored thoroughly with an efficient reaction path searching method, which integrates our new transition-state searching technique with periodic density functional theory calculations. Our new technique enables the location of the transition state and saddle points for most surface reactions simply and efficiently by optimization of local minima. We show that the selectivity of ethanol oxidation on Pt depends markedly on the surface structure, which can be attributed to the structure-sensitivity of two key reaction steps: (i) the initial dehydrogenation of ethanol and (ii) the oxidation of acetyl (CH3CO). On open surface sites, ethanol prefers C-C bond cleavage via strongly adsorbed intermediates (CH2CO or CHCO), which leads to complete oxidation to CO2. However, only partial oxidizations to CH3CHO and CH3COOH occur on Pt{111}. Our mechanism points out that the open surface Pt{100} is the best facet to fully oxidize ethanol at low coverages, which sheds light on the origin of the remarkable catalytic performance of Pt tetrahexahedra nanocrystals found recently. The physical origin of the structure-selectivity is rationalized in terms of both thermodynamics and kinetics. Two fundamental quantities that dictate the selectivity of ethanol oxidation are identified: (i) the ability of surface metal atoms to bond with unsaturated C-containing fragments and (ii) the relative stability of hydroxyl at surface atop sites with respect to other sites.     

Influence of coadsorbed hydrogen on ethylene adsorption and reaction on a (radical3 x radical3)R30 degrees-Sn/Pt(111) surface alloy

The effect of surface-bound hydrogen adatoms on adsorption, desorption, and reaction of ethylene (CH(2)=CH(2)) on a (radical3 x radical3)R30 degrees-Sn/Pt(111) surface alloy with theta(Sn) = 0.33 was investigated by using temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). Preadsorbed H decreased the saturation coverage of chemisorbed ethylene and less H was required to completely block ethylene chemisorption on this alloy than that on Pt(111). This is also the first report of extensive H site-blocking of ethylene chemisorption on Pt(111). Preadsorbed H also decreased the desorption activation energy of ethylene on the alloy surface. The reaction chemistry of ethylene on this Sn/Pt(111) alloy is dramatically different than on the Pt(111) surface: the H-addition reaction channel taking ethylene to ethane on Pt(111) is totally inhibited on the alloy. This is important information for advancing understanding of the surface chemistry involved in hydrogenation and dehydrogenation catalysis.     

Methane activation on Pt and Pt4: a density functional theory study

The activation mechanisms of a methane molecule on a Pt atom (CH4-Pt) and on a Pt tetramer (CH4-Pt4) were investigated using density functional theory (B3LYP and PW91) calculations. The results from these two functionals are different mostly in predicting the reaction barrier, in particular for the CH4-Pt system. A new lower energy pathway was identified for the CH4 dehydrogenation on a Pt atom. In the new pathway, the PtCH2 + H2 products were formed via a transition state, in which the Pt atom forms a complex with carbene and both dissociated hydrogen atoms. We report here the first theoretical study of methane activation on a Pt4 cluster. Among the five single steps toward dehydrogenation, our results show that the rate-limiting step is the third step, that is, breaking the second C-H bond, which requires overcoming an energy barrier of 28 kcal/mol. On the other hand, the cleavage of the first C-H bond, that is, the first reaction step, requires overcoming an energy barrier of 4 kcal/mol.     

In situ time-resolved XAFS study on the structural transformation and phase separation of Pt3Sn and PtSn alloy nanoparticles on carbon in the oxidation process

The dynamic behavior and kinetics of the structural transformation of supported bimetallic nanoparticle catalysts with synergistic functions in the oxidation process are fundamental issues to understand their unique catalytic properties as well as to regulate the catalytic capability of alloy nanoparticles. The phase separation and structural transformation of Pt(3)Sn/C and PtSn/C catalysts during the oxidation process were characterized by in situ time-resolved energy-dispersive XAFS (DXAFS) and quick XAFS (QXAFS) techniques, which are element-selective spectroscopies, at the Pt L(III)-edge and the Sn K-edge. The time-resolved XAFS techniques provided the kinetics of the change in structures and oxidation states of the bimetallic nanoparticles on carbon surfaces. The kinetic parameters and mechanisms for the oxidation of the Pt(3)Sn/C and PtSn/C catalysts were determined by time-resolved XAFS techniques. The oxidation of Pt to PtO in Pt(3)Sn/C proceeded via two successive processes, while the oxidation of Sn to SnO(2) in Pt(3)Sn/C proceeded as a one step process. The rate constant for the fast Pt oxidation, which was completed in 3 s at 573 K, was the same as that for the Sn oxidation, and the following slow Pt oxidation rate was one fifth of that for the first Pt oxidation process. The rate constant and activation energy for the Sn oxidation in PtSn/C were similar to those for the Sn oxidation in Pt(3)Sn/C. In the PtSn/C, however, it was hard for Pt oxidation to PtO to proceed at 573 K, where Pt oxidation was strongly affected by the quantity of Sn in the alloy nanoparticles due to swift segregation of SnO(2) nanoparticles/layers on the Pt nanoparticles. The mechanisms for the phase separation and structure transformation in the Pt(3)Sn/C and PtSn/C catalysts are also discussed on the basis of the structural kinetics of the catalysts themselves determined by the in situ time-resolved DXAFS and QXAFS.     

噻吩在M(111)(M=Pd,Pt, Au)表面的吸附

采用密度泛函理论(DFT), 选取DMol³程序模块, 对噻吩在M(111) (M=Pd, Pt, Au)表面上的吸附行为进行了探讨. 通过对噻吩在不同底物金属上的吸附能、吸附构型、Mulliken 电荷布居、差分电荷密度以及态密度的分析发现, 噻吩在Pd(111)面上的吸附能最大, Pt(111)面次之, Au(111)面最小. 吸附后, 噻吩在Au(111)面上的构型几乎保持不变, 最终通过S端倾斜吸附于top 位; 噻吩在Pd(111)及Pt(111)面上发生了折叠与变形, 环中氢原子向上翘起, 最终通过环平面平行吸附于hollow 位. 此外, 噻吩环吸附后芳香性遭到了破坏, 环中碳原子发生sp³杂化, 同时电子逐渐由噻吩向M(111)面发生转移, M(111)面上的部分电子也反馈给了噻吩环中的空轨道, 这种协同作用最终导致了噻吩分子稳定吸附于M(111)面.     

Pt及其修饰电极上乙醇吸附和氧化的CV和EQCM研究

运用电化学循环伏安和石英晶体微天平研究了乙醇在Pt电极和以Sb,S吸附原子修饰的Pt(Pt/Sbad和Pt/Sad)电极上的吸附和氧化过程。结果表明乙醇的氧化与电极表面氧物种有着极其密切的关系。Pt电极表面Sb吸附原子能在较低的电位下吸附氧,可显著提高乙醇电催化氧化活性。相反,Pt电极表面S吸附原子的氧化会消耗表面氧物种,抑制了乙醇的电氧化。本文从表面质量变化提供了吸附原子电催化作用的新数据。     

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.     

Sn/Pt纳米立方体对乙醇电催化氧化的原位FTIR光谱研究

采用循环伏安电沉积方法制备了Sn修饰的铂纳米立方体,并研究其乙醇的电催化氧化. 结合传统的电化学技术和原位电化学红外光谱技术研究了Sn的作用机理. 循环伏安研究表明,Sn修饰后使其乙醇氧化的起始电位显著负移,Sn覆盖度~ 0.90时乙醇氧化的起始电位为-0.1 V. 原位红外光谱结果表明,修饰Sn后极大地促进了乙酸的生成,更利于乙醇的直接氧化途径,但对乙醇的C—C键断裂促进作用不大.     

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.     

糠醛在Pt(111)表面的吸附和脱碳反应

采用广义梯度近似的密度泛函理论并结合平板模型的方法, 优化了糠醛分子在Pt(111)面的吸附模型,并探究了糠醛脱碳反应形成呋喃的机理. 结果表明: 吸附后糠醛分子环上的C―H(O)键及支链―CHO相对于金属表面倾斜上翘, 分子平面被扭曲, 易于呋喃的形成; 同时, 糠醛分子向Pt表面转移电子0.765e, 环中的大π键与Pt(111)表面的d轨道发生较强的相互作用, 使得糠醛的芳香性被破坏, 环上的碳原子呈现准sp³杂化. 此外, 对糠醛脱碳反应中的各反应步骤进行过渡态搜索, 通过比较各步骤的活化能, 得出糠醛更易先失去支链上的H形成酰基中间体(C₄H₃O)CO, 中间体继续脱碳加氢形成产物呋喃. 该过程的控速步骤为(C₄H₃O)CO^*+^*→C₄H₃O^*+CO^* (^*为吸附位),活化能为127.65 kJ·mol^-1.     

氢原子在Pt及Pt系双金属催化表面吸附的密度泛函理论研究

采用密度泛函理论(dFT)考察了Pt(100)、(110)、(111)三种表面氢原子的吸附行为, 计算了覆盖度为0.25 ML时氢原子在Pt 三种表面和M-Pt(111)双金属(M=Al, Fe, Co, Ni, Cu, Pd)上的最稳定吸附位、表面能以及吸附前后金属表面原子层间弛豫情况. 分析了氢原子在不同双金属表面吸附前后的局域态密度变化以及双金属表面d 带中心偏离费米能级的程度并与氢吸附能进行了关联. 计算结果表明, 在Pt(100), Pt(110)和Pt(111)表面, 氢原子的稳定吸附位分别为桥位、短桥位和fcc 穴位. 三种表面中以Pt(111)的表面能最低, 结构最稳定. 氢原子在不同M-Pt(111)双金属表面上的最稳定吸附位均为fcc 穴位, 其中在Ni-Pt 双金属表面的吸附能最低, Co-Pt 次之. 表明氢原子在Ni-Pt 和Co-Pt 双金属表面的吸附最稳定. 通过对氢原子在M-Pt(111)双金属表面吸附前后的局域态密度变化的分析, 验证了氢原子吸附能计算结果的准确性. 掺杂金属Ni、Co、Fe 的3d-Pt(111)双金属表面在吸附氢原子后发生弛豫, 第一层和第二层金属原子均不同程度地向外膨胀. 此外, 3d金属的掺入使得其对应的M-Pt(111)双金属表面d带中心与Pt 相比更靠近费米能级, 吸附氢原子能力增强, 表明3d-Pt系双金属表面有可能比Pt具有更好的脱氢活性.