A First‐Principle Calculation of Sulfur Oxidation on Metallic Ni(111) and Pt(111), and Bimetallic Ni@Pt(111) and Pt@Ni(111) Surfaces

Sulfur, a pollutant known to poison fuel‐cell electrodes, generally comes from S‐containing species such as hydrogen sulfide (H₂S). The S‐containing species become adsorbed on a metal electrode and leave atomic S strongly bound to the metal surface. This surface sulfur is completely removed typically by oxidation with O₂ into gaseous SO₂. According to our DFT calculations, the oxidation of sulfur at 0.25 ML surface sulfur coverage on pure Pt(111) and Ni(111) metal surfaces is exothermic. The barriers to the formation of SO₂ are 0.41 and 1.07 eV, respectively. Various metals combined to form bimetallic surfaces are reported to tune the catalytic capabilities toward some reactions. Our results show that it is more difficult to remove surface sulfur from a Ni@Pt(111) surface with reaction barrier 1.86 eV for SO₂ formation than from a Pt@Ni(111) surface (0.13 eV). This result is in good agreement with the statement that bimetallic surfaces could demonstrate more or less activity than to pure metal surfaces by comparing electronic and structural effects. Furthermore, by calculating the reaction free energies we found that the sulfur oxidation reaction on the Pt@Ni(111) surface exhibits the best spontaneity of SO₂ desorption at either room temperature or high temperatures.     

Highly Dense Isolated Metal Atom Catalytic Sites: Dynamic Formation and In Situ Observations

Atomically dispersed noble‐metal catalysts with highly dense active sites are promising materials with which to maximise metal efficiency and to enhance catalytic performance; however, their fabrication remains challenging because metal atoms are prone to sintering, especially at a high metal loading. A dynamic process of formation of isolated metal atom catalytic sites on the surface of the support, which was achieved starting from silver nanoparticles by using a thermal surface‐mediated diffusion method, was observed directly by using in situ electron microscopy and in situ synchrotron X‐ray diffraction. A combination of electron microscopy images with X‐ray absorption spectra demonstrated that the silver atoms were anchored on five‐fold oxygen‐terminated cavities on the surface of the support to form highly dense isolated metal active sites, leading to excellent reactivity in catalytic oxidation at low temperature. This work provides a general strategy for designing atomically dispersed noble‐metal catalysts with highly dense active sites.     

Methanol Conversion into Dimethyl Ether on the Anatase TiO2(001) Surface

Exploring reactions of methanol on TiO₂ surfaces is of great importance in both C1 chemistry and photocatalysis. Reported herein is a combined experimental and theoretical calculation study of methanol adsorption and reaction on a mineral anatase TiO₂(001)‐(1×4) surface. The methanol‐to‐dimethyl ether (DME) reaction was unambiguously identified to occur by the dehydration coupling of methoxy species at the fourfold‐coordinated Ti⁴⁺ sites (Ti_4c), and for the first time confirms the predicted higher reactivity of this facet compared to other reported TiO₂ facets. Surface chemistry of methanol on the anatase TiO₂(001)‐(1×4) surface is seldom affected by co‐chemisorbed water. These results not only greatly deepen the fundamental understanding of elementary surface reactions of methanol on TiO₂ surfaces but also show that TiO₂ with a high density of Ti_4c sites is a potentially active and selective catalyst for the important methanol‐to‐DME reaction.     

Influence of coadsorbates on the NO dissociation on a rhodium(311) surface.

Density functional theory (DFT) studies were performed to investigate the influence of coadsorbates on the nitrogen oxide dissociation on the vicinal rhodium(311) surface. This study amplifies prior studies on the dissociation of oxygen and nitrogen oxide on the (111) facet of rhodium. The influence of coadsorbates on the kinetic parameters and thermochemistry of the NO dissociation on Rh311 was studied. In addition, the activation energy and thermochemistry of this reaction were determined as a function of oxygen preoccupation/initial coverage. Steric and electronic effects and their influence on the dissociation reaction were examined. The results are discussed in the face of an NOx dissociation catalyst system proposed by Nakatsuji.     

Structure of Ag clusters Grown on Fs-Defect Sites of an MgO(1 0 0) surface

The structure of AgN clusters (N=1-4, 6, 8, 10), both in the gas phase and grown on the MgO(1 0 0) surface containing Fs-defects, has been investigated by a density functional basin-hopping (DF-BH) approach. In analogy with what observed in the case of gold clusters, it is found that the presence of the defect implies a double frustration and a cylindrical invariance of the metal-surface interaction, causing small Ag clusters growing around the Fs defect to be highly fluxional. Nevertheless, two different structural crossovers are found to be induced by the metal-defect interaction for the adsorbed clusters such that: 1) planar structures prevail for Nor=7), prevail for N=6 and N=8; 3) distorted face-centered cubic (fcc) structures grown pseudomorphically on the defected surface prevail for N=10. The transition from fivefold to fcc motifs is rationalized in terms of the double-frustration effect, which increases the bond strain of the noncrystalline structures. Detrapping energies from the defect were also calculated; the lowest energy pathway corresponds to the detachment of a dimer.     

DFT Study of Thiophene Adsorption on the Pd（111） and Pt（111） Surfaces

Thiophene adsorption on the(111) surfaces of Pd and Pt have been investigated by density functional theory.The results indicate that the adsorption at the hollow sites is the most stable.To our interest,the molecular plane of thiophene ring is distorted with C=C bond being elongated to 1.450  and C-C bond being shortened to 1.347 ,and the C-H bonds tilt 13.91～44.05o away from this plane.Furthermore,analysis on population and density of states verified the calculated adsorption geometries.Finally,charge analysis suggests that thiophene molecule is an electron acceptor,reflecting the interaction between the lone pair of sulfur and the d-orbitals of metal.     

First Principles Calculations for Hydrogenation of Acrolein on Pd and Pt: Chemoselectivity Depends on Steric Effects on the Surface

The chemoselective hydrogenation of acrolein on Pt(111) and Pd(111) surfaces is investigated employing density functional theory calculations. The computed potential energy surfaces together with the analysis of reaction mechanisms demonstrate that steric effects are an important factor that governs chemoselectivity. The reactions at the C=O functionality require more space than the reactions at the C=C functionality. Therefore the formation of allyl alcohol is more favorable at low coverage, while the reduction of the C=C bond and the formation of propanal becomes kinetically more favorable at higher coverage. The elementary reaction steps are found to follow different reaction mechanisms, which are identified according to terminology typically used in organometallic catalysis. The transition state scaling (TSS) relationship is demonstrated and the origin of multiple TSS lines is linked to variation of an internal electronic structure of a carbon skeleton.     

Reconstruction of Supported Metal Nanoparticles in Reaction Conditions

Metal nanoparticles (NPs) dispersed on a high‐surface‐area support are normally used as heterogeneous catalysts. Recent in situ experiments have shown that structure reconstruction of the NP occurs in real catalysis. However, the role played by supports in these processes is still unclear. Supports can be very important in real catalysis because of the new active sites at the perimeter interface between nanoparticles and supports. Herein, using a developed multiscale model coupled with in situ spherical aberration‐corrected (Cs‐corrected) TEM experiments, we show that the interaction between the support and the gas environment greatly changes the contact surface area between the metal and support, which further leads to the critical change in the perimeter interface. The dynamic changes of the interface in reactive environments can thus be predicted and be included in the rational design of supported metal nanocatalysts. In particular, our multiscale model shows quantitative consistency with experimental observations. This work offers possibilities for obtaining atomic‐scale structures and insights beyond the experimental limits.     

How Absorbed Hydrogen Affects the Catalytic Activity of Transition Metals

Heterogeneous catalysis is commonly governed by surface active sites. Yet, areas just below the surface can also influence catalytic activity, for instance, when fragmentation products of catalytic feeds penetrate into catalysts. In particular, H absorbed below the surface is required for certain hydrogenation reactions on metals. Herein, we show that a sufficient concentration of subsurface hydrogen, H^sub, may either significantly increase or decrease the bond energy and the reactivity of the adsorbed hydrogen, H^ad, depending on the metal. We predict a representative reaction, ethyl hydrogenation, to speed up on Pd and Pt, but to slow down on Ni and Rh in the presence of H^sub, especially on metal nanoparticles. The identified effects of subsurface H on surface reactivity are indispensable for an atomistic understanding of hydrogenation processes on transition metals and interactions of hydrogen with metals in general.     

The role of water in the adsorption of oxygenated aromatics on Pt and Pd

Catalytic processing of biomass‐derived oxygenates to valuable chemical products will contribute to a sustainable future. To provide insight into the conversion of processed sugars and lignin monomers, we present density functional theory studies of adsorption of phloroglucinol, a potentially valuable biomass derivative, on Pt(111) and Pd(111) surfaces. A comprehensive study of adsorption geometries and associated energies indicates that the bridge site is the most preferred adsorption site for phloroglucinol, with binding energies in the range of 2–3 eV in the vapor phase. Adsorption of phloroglucinol on these metal surfaces occurs via hybridization between the carbon p_z orbitals and the metal d and d_yz orbitals. With explicit solvent, hydrogen bonds are formed between phloroglucinol and water molecules thereby decreasing binding of phloroglucinol to the metal surfaces relative to the vapor phase by 20–25%. Based on these results, we conclude that solvent effects can significantly impact adsorption of oxygenated aromatic compounds derived from biomass and influence catalytic hydrogenation and hydrodeoxygenation reactions as well. © 2012 Wiley Periodicals, Inc.     

甲烷在清洁Pd(111)及氧改性的Pd(111)表面解离的密度泛函理论研究

 采用广义梯度近似 (GGA) 的密度泛函理论 (DFT) 并结合平板模型, 研究了 CH4 在清洁 Pd(111) 及 O 改性的 Pd(111) 表面发生 C朒 键断裂的反应历程. 优化了裂解过程中反应物、过渡态和产物的几何构型, 获得了反应路径上各物种的吸附能及反应的活化能. 结果表明, CH4 采用一个 H 原子指向表面的构型在 Pd(111) 表面的顶位吸附, CH3 的最稳定的吸附位置为顶位, OH, O 和 H 的最稳定吸附位置均为面心立方. CH4 在清洁 Pd(111) 表面裂解的活化能为 0.97 eV, 低于它在 O 原子改性 (O 没有参与反应) 的 Pd(111) 表面的活化能 1.42 eV, 说明表面氧原子抑制了 CH4 中 C朒 键的断裂. 当亚表面 O 原子和表面 O 原子 (O 参与反应) 共同存在时, C朒 键断裂的活化能为 0.72 eV, 低于只有表层氧存在时的活化能 (1.43 eV), 说明亚表面的 O 原子对 CH4 分子的活化具有促进作用. CH4 在 O 原子改性的 Pd(111) 表面裂解生成 CH3 和 H, 以及生成 CH3 和 OH 的反应活化能分别为 1.42 和 1.43 eV, 说明 CH4 在 O 原子改性的 Pd(111) 表面发生这两种反应的难易程度相当.