Formation of Bimetallic Pt-Rh Surface with Nanosize Metal Domains

An approach to produce a bimetallic Pt-Rh model catalyst has been developed. It includes vacuum deposition of platinum on a Rh₂O₃ film, followed by the oxide reduction with hydrogen. The thermal stability range of the resulting bimetallic structure has been determined.     

Carbon monoxide oxidation on Pt/TiO2 sol-gel catalysts

Comparative results (specific area, metallic dispersion, and activity in the carbon monoxide oxidation) on sol-gel and impregnated Pt/TiO₂ catalysts are presented. In order to explain the important differences between the two preparations, among them high resistivity of sol-gel catalyst to sintering, the formation of anchored and/or partially buried particles into the support is proposed.     

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.     

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.     

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.     

Real‐Space Observation of Atomic Site‐Specific Electronic Properties of a Pt Nanoisland/Au(111) Bimetallic Surface by Tip‐Enhanced Raman Spectroscopy

Resolving atomic site‐specific electronic properties and correlated substrate–molecule interactions is challenging in real space. Now, mapping of sub‐10 nm sized Pt nanoislands on a Au(111) surface was achieved by tip‐enhanced Raman spectroscopy, using the distinct Raman fingerprints of adsorbed 4‐chlorophenyl isocyanide molecules. A spatial resolution better than 2.5 nm allows the electronic properties of the terrace, step edge, kink, and corner sites with varying coordination environments to be resolved in real space in one Pt nanoisland. Calculations suggest that low‐coordinate atomic sites have a higher d‐band electronic profile and thus stronger metal–molecule interactions, leading to the observed blue‐shift of Raman frequency of the N≡C bond of adsorbed molecules. An experimental and theoretical study on Pt(111) and mono‐ and bi‐atomic layer Pt nanoislands on a Au(111) surface reveals the bimetallic effect that weakens with the increasing number of deposited Pt adlayer.     

Density Functional Theory Study of the Adsorption of C2H2 on the Cu/Pt(111) Bimetallic Surfaces

We applied periodic density-functional theory to investigate the adsorption of C2H2 on the Cu/Pt bimetallic and monometallic surfaces, including Cu-Pt-Pt and Pt-Cu-Pt representing the monolayer Cu on the Pt surface and subsurface Cu in the Pt surface, respectively. For the Pt(111) and Pt-Cu-Pt surfaces, C2H2 is preferentially a 3-fold "parallel-bridge" configuration, and a "μ-bridge" structure exists above the Cu(111) and Cu-Pt-Pt surfaces. The adsorption energy of C2H2 on these surfaces decreases in the order Pt(111) > Cu-Pt-Pt > Pt-Cu-Pt > Cu(111). The analysis of density of states, charge, and vibrational frequencies showed obviously weakening of the adsorbed C-C bond and high sp2 character on the carbon atom. Furthermore, when the top-layer compositions are equal, the nearer the EF d-band center is, the larger the C2H2 adsorption energy will be.     

ChemInform Abstract: Structure and Reactivity of Binary Au12Pt Cluster.

An all‐electron scalar relativistic calculation on the geometrical structure and reactivity of Au₁₂Pt cluster is performed by DFT with the generalized gradient approximation at PW91 level.     

负载型Au-Pd双金属催化剂的制备及其对CO氧化的催化活性

采用改性的等体积浸渍法制备了SiO2负载的Au-Pd双金属催化剂,考察了催化剂的焙烧温度对CO氧化反应活性的影响.与623,723和773K的条件下焙烧的催化剂相比,673K焙烧的催化剂具有良好的催化CO氧化活性,CO完全转化温度低于398K.应用N2物理吸附、X射线衍射、程序升温还原、CO程序升温脱附及X射线光电子能谱等技术对催化剂进行了表征.结果表明,673K焙烧的催化剂具有最大的比表面积和最小的孔径,存在Au0,Pd0和PdO相,AuxPdy合金相很少;而773K焙烧的催化剂上除了含有Au0,Pd0和PdO相外,还存在明显的AuxPdy合金相.具有大比表面积,小孔径,Au0,Pd0和PdO多相共存的催化剂可使CO的吸附量增加,催化活性提高;而AuxPdy合金相的生成并不能提高催化剂的催化活性.     

Reducing the Cost and Preserving the Reactivity in Noble‐Metal‐Based Catalysts: Oxidation of CO by Pt and Al–Pt Alloy Clusters Supported on Graphene

The oxidation mechanisms of CO to CO₂ on graphene‐supported Pt and Pt‐Al alloy clusters are elucidated by reactive dynamical simulations. The general mechanism evidenced is a Langmuir–Hinshelwood (LH) pathway in which O₂ is adsorbed on the cluster prior to the CO oxidation. The adsorbed O₂ dissociates into two atomic oxygen atoms thus promoting the CO oxidation. Auxiliary simulations on alloy clusters in which other metals (Al, Co, Cr, Cu, Fe, Ni) replace a Pt atom have pointed to the aluminum doped cluster as a special case. In the nanoalloy, the reaction mechanism for CO oxidation is still a LH pathway with an activation barrier sufficiently low to be overcome at room temperature, thus preserving the catalyst efficiency. This provides a generalizable strategy for the design of efficient, yet sustainable, Pt‐based catalysts at reduced cost.     

银/铂双金属中空纳米颗粒的制备及光学性质研究

通过以Ag纳米颗粒为模板的置换和沉积反应,制备了Ag/Pt双金属复合纳米颗粒.用透射电子显微镜(TEM)对颗粒的形貌、尺寸和结构进行了表征,发现复合颗粒具有中空结构.紫外可见吸收光谱(UV-Vis)研究表明,Ag/Pt双金属中空复合纳米颗粒具有单峰的表面等离子共振吸收特征,随着反应溶液中氯铂酸和硝酸银摩尔比的增加,吸收峰先红移后蓝移.表面增强拉曼光谱实验结果表明,Ag/Pt双金属复合纳米颗粒对吡啶分子具有较好的增强效果.     

The Synergistic Effect of a Bimetallic Catalyst for the Synthesis of Carbon Nanotube Aerogels and their Predominant Chirality

Synthesis of continuous spinnable carbon nanotube (CNT) fibers is the most promising method for producing CNT fibers for commercial applications. The floating-catalyst chemical vapor deposition (FC-CVD) method is a rapid process that achieves catalyst formation, CNT nucleation and growth, and aerogel-like sock formation within a few seconds. However, the formation mechanism is unknown. Herein, the progress of CNT fiber formation with bimetallic catalysts was studied, and the effect of catalyst composition to CNT fiber synthesis and their structural properties was investigated. In the case of bimetallic catalysts, the carbon source rapidly decomposes and generates various secondary hydrocarbon species, such as CH₄, C₂H₄, C₂H₂, C₃H₆, and C₄H₁₀ whereas monometallic catalysts generate only CH₄ and C₂H₄ on decomposition. CNT fiber formation with Fe₁Ni₀ begins about 400 mm from the reactor entrance, whereas CNT formation with Fe_0.8Ni_0.2 and Fe_0.5Ni_0.5 begins at about 500 and 300 mm, respectively. The formed CNT bundles and individual CNTs are oriented along the gas flow at these locations. The enhanced rate of fiber formation and lowering of growth temperature associated with bimetallic catalysts is explained by the synergistic effects between the two metals. The synthesized CNTs become predominantly semiconducting with increasing Ni contents.     

Ag on Pt(111): Changes in Electronic and CO Adsorption Properties upon PtAg/Pt(111) Monolayer Surface Alloy Formation

The electronic and chemical (adsorption) properties of bimetallic Ag/Pt(111) surfaces and their modification upon surface alloy formation, that is, during intermixing of Ag and Pt atoms in the top atomic layer upon annealing, were studied by X‐ray photoelectron spectroscopy (XPS) and, using CO as probe molecule, by temperature‐programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS), respectively. The surface alloys are prepared by deposition of sub‐monolayer Ag amounts on a Pt(111) surface at room temperature, leading to extended Ag monolayer islands on the substrate, and subsequent annealing of these surfaces. Surface alloy formation starts at ≈600–650 K, which is evidenced by core‐level shifts (CLSs) of the Ag(3d_5/2) signal. A distinct change of the CO adsorption properties is observed when going to the intermixed PtAg surface alloys. Most prominently, we find the growth of a new desorption feature at higher temperature (≈550 K) in the TPD spectra upon surface alloy formation. This goes along with a shift of the CO_ad‐related IR bands to lower wave number. Surface alloy formation is almost completed after heating to 700 K.     

All Kinds of Reactivity: Recent Breakthroughs in Metal‐Catalyzed Alkyne Chemistry

Alkynes of reactions : Recent breakthroughs in metal‐catalyzed alkyne reactions, which expand the synthetic utility of alkynes, have been achieved. These approaches broaden the range of alkynes that are accessible by C? N and C? C bond‐forming reactions and demonstrate that the use of bifunctional heterobimetallic catalysts can lead to new reactivity and excellent enantioselectivity (see scheme).

     

Pt-Ni/γ-Al2O3 双金属催化剂上1,3-环己二烯的低温加氢和脱氢反应

 采用等体积浸渍法制备了 γ-Al2O3 负载的 Pt 和/或 Ni 双金属催化剂或单金属催化剂, 测定了它们的 CO 化学吸附量, 并在原位红外间歇反应装置上评价了其催化 1,3-环己二烯 (1,3-CHD) 的低温 (308 K) 加氢和脱氢性能. 结果表明, Pt-Ni/γ-Al2O3 催化剂性能优于 Pt/γ-Al2O3 或 Ni/γ-Al2O3. 结合密度泛函理论计算的不同催化剂上 1,3-CHD 的表面吸附能, 验证了具有较弱环烯烃吸附能的双金属催化剂加氢活性较高.     

Effect of the Pt/Pt Electrode Roughness on the Interaction of CO with Oads

Transients of potential E during the CO-O_ads interaction on Pt/Pt electrodes of different roughness (f = 20-1200), measured in 0.5 M H₂SO₄ under open-circuit conditions, slow down with increasing f. Dependence of specific activity of Pt/Pt on f cannot be described by one CO-O_ads interaction mechanism in a wide range of f. At f < 100, more acceptable is the mechanism suggested earlier for polycrystalline (smooth) Pt, and at larger f—mechanism of conjugated reactions. Presumably, increasing f at small f reduces intrinsic electrocatalytic activity of the Pt surface, while at large f inner-diffusion limitations can arise.     

Coordination Chemistry of Cyclopropenylidene‐Stabilized Phosphenium Cations: Synthesis and Reactivity of Pd and Pt Complexes

A straightforward synthesis of cyclopropenylidene‐stabilized phosphenium cations 1 a – g through the reaction of [(iPr₂N)₂C₃⁺Cl]BF₄ with secondary phosphines is described. Their donor ability was evaluated by analysis of the CO stretching frequency in Rh complexes [RhCl(CO)L₂](BF₄)₂ and electrochemical methods. The cyclopropenium ring induces a phosphite‐type behavior that can be tuned by the other two substituents attached to the phosphorus atom. Despite of the positive charge that they bear, phosphenium cations 1 a – g still act as two‐electron donor ligands, forming adducts with Pd^II and Pt^II precursors. Conversely, in the presence of Pd⁰ species, an oxidative insertion of the Pd atom into the C_carbene–phosphorus bond takes place, providing dimeric structures in which each Pd atom is bonded to a cyclopropenyl carbene while two dialkyl/diaryl phosphide ligands serve as bridges between the two Pd centers. The catalytic performance of the resulting library of Pt^II complexes was tested; all of the cationic phosphines accelerated the prototype 6‐endo‐dig cyclization of 2‐ethynyl‐1,1′‐biphenyl to afford pentahelicene. The best ligand 1 g was used in the synthesis of two natural products, chrysotoxene and epimedoicarisoside A.