NO Chemisorption on Pt(111), Rh/Pt(111), and Pd/Pt(111)

The chemisorption of NO on clean Pt(111), Rh/Pt(111) alloy, and Pd/Pt(111) alloy surfaces has been studied by first principles density functional theory (DFT) computations. It was found that the surface compositions of the surface alloys have very different effects on the adsorption of NO on Rh/Pt(111) versus that on Pd/Pt(111). This is due to the different bond strength between the two metals in each alloy system. A complex d-band center weighting model developed by authors in a previous study for SO2 adsorption is demonstrated to be necessary for quantifying NO adsorption on Pd/Pt(111). A strong linear relationship between the weighted positions of the d states of the surfaces and the molecular NO adsorption energies shows the closer the weighted d-band center is shifted to the Fermi energy level, the stronger the adsorption of NO will be. The consequences of this study for the optimized design of three-way automotive catalysts, (TWC) are also discussed.     

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.     

Thermal chemistry of C4 hydrocarbons on Pt(111): Mechanism for double-bond isomerization

The thermal chemistry of a number of C4 hydrocarbons (1,3-butadiene, 1-bromo-3-butene, 1-bromo-2-butene, trans-2-butene, cis-2-butene, 1-butene, 2-iodobutane, 1-iodobutane, and butane) was investigated on clean and hydrogen- and deuterium-predosed Pt(111) single-crystal surfaces by temperature-programmed desorption and reflection-absorption infrared spectroscopy. A combination of rapid beta-hydride eliminations from alkyls to olefins and the reverse insertions of those olefins into metal-hydrogen bonds explains the hydrogenation, dehydrogenation, and H-D exchange products that desorb from the surface. A preference for hydrogenation at the end carbons and dehydrogenation from the inner carbons also explains the extent of the isotope exchange and the preferential isomerization of 1-butene to 2-butene observed on this Pt(111) surface. The reactions of more dehydrogenated C4 species is also discussed.     

噻吩在M(111)(M=Pd,Pt,Au)表面的吸附(英文)

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.     

DFT characterization of adsorbed NH(x) species on Pt(100) and Pt(111) surfaces

Periodic density functional theory (DFT) calculations using plane waves have been performed to systematically investigate the adsorption and relative stability of ammonia and its dehydrogenated species on Pt(111) and Pt(100) surfaces. Different adsorption geometries and positions have been studied, and in each case, the equilibrium configuration has been determined by relaxation of the system. The vibrational spectra of the various ammonia fragments have been computed, and band assignments have been compared in detail with available experimental data. The adsorption of NH3 (on top) and NH2 (bridge) is more favorable on Pt(100) than on Pt(111), while similar adsorption energies were computed for NH (hollow) and N (hollow) on both surfaces. The remarkably lower adsorption energy of NH2 over Pt(111) as compared with Pt(100) (the difference being approximately 0.7 eV) can be related to different geometric and electronic factors associated with this particular intermediate. Accordingly, the type of platinum surface determines the most stable NH(x) fragment: Pt(100) has more affinity for NH2 species, whereas NH species are preferred over Pt(111).     

Origin of the selectivity for trans-to-cis isomerization in 2-butene on Pt(111) single crystal surfaces

DFT calculations were performed to explain the preference for the conversion of trans-2-butene to its less stable cis isomer on Pt(111) surfaces reported previously. The results indicate that the main factors controlling the direction of this reaction are the lesser degrees of molecular rearrangement and surface reconstruction required in the adsorption of the cis isomer on the hydrogen-covered Pt surface. This is a case where hydrogen coverage and atomic restructuring not only dominate the energetics of adsorption but also define reaction selectivity.     

碱性介质中CO对甲醇在PtAu(111)和Pt(111)表面氧化生成甲酸的影响的第一性原理研究

采用密度泛函理论计算研究了碱性介质中甲醇在清洁的PtAu(111)和Pt(111)表面、及有CO存在的PtAu(111)和Pt(111)表面的氧化。计算结果表明,在碱性介质中,预吸附的CO促进了甲醇在PtAu(111)和Pt(111)表面氧化的每一步反应,这与其在Au(111)表面的作用相似。究其原因,是由于CO的吸附增强了OH的稳定性和碱性,从而增强了OH夺取氢原子的能力。     

Adsorption and reaction of 1-epoxy-3-butene on Pt(111): implications for heterogeneous catalysis of unsaturated oxygenates

High-resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD), and density functional theory (DFT) calculations were used to study the adsorption and reaction of 1-epoxy-3-butene (EpB) on Pt(111). These investigations were conducted to help elucidate mechanisms for improving olefin hydrogenation selectivity in reactions of unsaturated oxygenates. EpB dosed to Pt(111) at 91 K adsorbs molecularly on the surface through the vinyl group with apparent rehybridization to a di-sigma-bound state. By 233 K, however, EpB undergoes epoxide ring opening to form an aldehyde intermediate, which further decomposes upon heating to yield gas phase products CO, H2, and propylene. Comparison of the HREELS and TPD data to experiments performed with 2-butenal (crotonaldehyde) shows that EpB and 2-butenal decompose through related pathways. However, the EpB-derived aldehyde intermediate clearly has a unique structure, features of which have been elucidated by DFT calculations. In conjunction with previous surface science studies of EpB chemistry, these results can help explain selectivity trends for reactions of EpB on Pt catalysts and bimetallic PtAg catalysts, with indications that the enhanced olefin hydrogenation selectivity of PtAg catalysts likely originates from a bifunctional effect.     

Adsorption of oxygenates on alkanethiol-functionalized Pd(111) surfaces: mechanistic insights into the role of self-assembled monolayers on catalysis

Recent work shows that coating a supported palladium catalyst with a self-assembled monolayer (SAM) of alkanethiols can dramatically improve selectivity in the hydrogenation of 1-epoxy-3-butene (EpB) to 1-epoxybutane. Here, we present the results of surface-level investigations of the adsorption of EpB and related molecules on SAM-coated Pd(111), with an aim of identifying mechanistic explanations for the observed catalytic behavior. Alkanethiol SAM-covered Pd(111) surfaces were prepared by conventional techniques and transferred to ultrahigh vacuum, where they were characterized using Auger electron spectroscopy (AES) and temperature-programmed desorption (TPD) of EpB and other probe molecules. Whereas previous studies have shown that EpB undergoes rapid decomposition via epoxide ring opening on uncoated Pd(111), TPD studies show that EpB does not undergo substantial ring opening on SAM-covered surfaces but rather desorbs intact at temperatures less than 300 K. Systematic comparisons of EpB desorption spectra to spectra for other C(4) oxygenates suggest that the SAM creates a kinetic barrier to epoxide ring-opening reactions that does not exist on the uncoated surface. The EpB desorption spectra as a function of exposure show behavior similar to the desorption of olefins from Pd(111), indicating that the binding of the olefin functionality, in contrast to that of the epoxide ring, is not significantly perturbed. EpB desorption spectra from surfaces with less well-ordered SAMs show the presence of weakly bound states not observed on well-ordered SAM surfaces. The lower activity observed on catalysts covered with less well-ordered SAMs is hypothesized to occur due to partial confinement of adsorbates into these weakly bound, less active states.     

Chiral recognition of PVBA on Pd(111) and Ag(111) surfaces

An asymmetric planar molecule, 4-trans-2-(pyrid-4-yl-vinyl) benzoic acid (PVBA), has been used to establish the organic chiral recognition on fcc(111) metal surfaces. The strong correlation between the orientation and chiral recognition of PVBA on both Ag(111) and Pd(111) guides the choice of a model potential, which determines the relative binding energy of PVBA on fcc(111). An angle-dependent calculation of relative binding energy reproduces the experimental observation of the chiral recognition of PVBA on Ag(111) but not on Pd(111).     

CH3O decomposition on PdZn(111), Pd(111), and Cu(111). A theoretical study

Methanol steam re-forming, catalyzed by Pd/ZnO, is a potential hydrogen source for fuel cells, in particular in pollution-free vehicles. To contribute to the understanding of pertinent reaction mechanisms, density functional slab model studies on two competing decomposition pathways of adsorbed methoxide (CH(3)O) have been carried out, namely, dehydrogenation to formaldehyde and C-O bond breaking to methyl. For the (111) surfaces of Pd, Cu, and 1:1 Pd-Zn alloy, adsorption complexes of various reactants, intermediates, transition states, and products relevant for the decomposition processes were computationally characterized. On the surface of Pd-Zn alloy, H and all studied C-bound species were found to prefer sites with a majority of Pd atoms, whereas O-bound congeners tend to be located on sites with a majority of Zn atoms. Compared to Pd(111), the adsorption energy of O-bound species was calculated to be larger on PdZn(111), whereas C-bound moieties were less strongly adsorbed. C-H scission of CH(3)O on various substrates under study was demonstrated to proceed easier than C-O bond breaking. The energy barrier for the dehydrogenation of CH(3)O on PdZn(111) (113 kJ mol(-)(1)) and Cu(111) (112 kJ mol(-)(1)) is about 4 times as high as that on Pd(111), due to the fact that CH(3)O interacts more weakly with Pd than with PdZn and Cu surfaces. Calculated results showed that the decomposition of methoxide to formaldehyde is thermodynamically favored on Pd(111), but it is an endothermic process on PdZn(111) and Cu(111) surfaces.     

Adsorption and Decarbonylation Reaction of Furfural on the Pd(111) and M/Pd(111)(M=Ni,Cu and Ru) Surfaces

By performing with density functional theory(DFT) method, the detailed adsorption process and the catalytic decarbonylation mechanisms of furfural over Pd(111) and M/Pd(111)(M = Ni, Cu, Ru) surfaces toward furan were clarified. The results of atomic size factor, formation energy and d-band center showed that Ru/Pd(111) surface was the most stable and active. The adsorption energies of furfural on the different surfaces followed the order Ru/Pd(111) Cu/Pd(111) Pd(111) Ni/Pd(111). After analyzing Mulliken atomic charge population and the deformation density, we can find that on Ru/Pd(111) surface, the number of charge transfer was the most and the interaction was the strongest. Therefore, its adsorption energy was the highest. Furthermore, the furfural decarbonylation pathway is more kinetically feasible on bimetallic surface, and the reaction is the most likely to occur on Ru/Pd(111).     

Crystalline ice growth on Pt(111) and Pd(111): nonwetting growth on a hydrophobic water monolayer

The growth of crystalline ice films on Pt(111) and Pd(111) is investigated using temperature programed desorption of the water films and of rare gases adsorbed on the water films. The water monolayer wets both Pt(111) and Pd(111) at all temperatures investigated [e.g., 20-155 K for Pt(111)]. However, crystalline ice films grown at higher temperatures (e.g., T>135 K) do not wet the monolayer. Similar results are obtained for crystalline ice films of D2O and H2O. Amorphous water films, which initially wet the surface, crystallize and dewet, exposing the water monolayer when they are annealed at higher temperatures. Thinner films crystallize and dewet at lower temperatures than thicker films. For samples sputtered with energetic Xe atoms to prepare ice crystallites surrounded by bare Pt(111), subsequent annealing of the films causes water molecules to diffuse off the ice crystallites to reform the water monolayer. A simple model suggests that, for crystalline films grown at high temperatures, the ice crystallites are initially widely separated with typical distances between crystallites of approximately 14 nm or more. The experimental results are consistent with recent theory and experiments suggesting that the molecules in the water monolayer form a surface with no dangling OH bonds or lone pair electrons, giving rise to a hydrophobic water monolayer on both Pt(111) and Pd(111).     

Reduction of NO adlayers on Pt(110) and Pt(111) in acidic media: evidence for adsorption site-specific reduction

We present a combined in situ Fourier transform infrared reflection-absorption spectroscopy and voltammetric study of the reduction of saturated and subsaturated NO adlayers on Pt(111) and Pt(110) single-crystal surfaces in acidic media. The stripping voltammetry experiments and the associated evolution of infrared spectra indicate that different features (peaks) observed in the voltammetric profile for the electrochemical reduction of NO adlayers on the surfaces considered are related to the reduction of NO(ads) at different adsorption sites and not to different (consecutive) processes. More specifically, reduction of high- and intermediate-coverage (ca. 0.5-1 monolayers (ML)) NO adlayers on Pt(110) is accompanied by site switching from atop to bridge position, in agreement with the ultra-high-vacuum data. On Pt(111) linearly bonded (atop) NO and face-centered cubic 3-fold-hollow NO species coexist at high coverages (0.25-0.5 ML) and can be reduced consecutively and independently. On Pt(111) and Pt(110) electrodes, linearly bonded NO species are more reactive than multifold-bonded NO species. Both spectroscopic and voltammetric data indicate that ammonia is the main product of NO(ads) reduction on the two surfaces examined.     

Structure of activated complex of CO2 formation in a CO + O2 reaction on Pd(110) and Pd(111)

Examinations of CO2 formed during steady-state CO oxidation reactions were performed using infrared (IR) chemiluminescence. The CO2 was formed using a molecular-beam method over Pd(110) and Pd(111). The CO2 formation rate is temperature dependent under various partial pressure conditions. The temperature of the maximum formation rate is denoted as TSmax. Analyses of IR emission spectra at surface temperatures higher than TSmax showed that the average vibrational temperature (TVAV) is higher for Pd(111) than for Pd(110). The antisymmetric vibrational temperature (TVAS) is almost equal on both surfaces. These results suggest that the activated CO2 complex is more bent on Pd(111) and straighter on Pd(110). Furthermore, the difference in the TVAV value was small for surface temperatures less than TSmax. The TVAS value was much higher than TVAV on both surfaces. These phenomena were observed only when the surface temperature was lower than TSmax: they became more pronounced at lower temperatures, suggesting that the activated complex of CO2 formation is much straighter on both Pd surfaces than that observed at higher surface temperatures. Combined with kinetic results, the higher CO coverage at the lower surface temperatures is inferred to be related to the linear activated complex of CO2 formation.     

镍和铂单晶(111)面上氢解离的比较研究

镍和铂单晶（１１１）面上氢解离的比较研究周鲁，孙本繁，吕日昌，唐向阳，滕礼坚（中国科学院大连化学物理研究所分子反应动力学国家重点实验室，大连１１６０２３）关键词镍晶面，铂晶面，氢解离吸附，位能面，分子催化过渡金属镍和铂是催化加氢、脱氢以及临氢重整的重...     

Surface X-ray scattering of stepped surfaces of platinum in an electrochemical environment: Pt(331) = 3(111)-(111) and Pt(511) = 3(100)-(111)

Real surface structures of the high-index planes of Pt with three atomic rows of terraces (Pt(331) = 3(111)-(111) and Pt(511) = 3(100)-(111)) have been determined in 0.1 M HClO(4) at 0.1 and 0.5 V(RHE) with the use of surface X-ray scattering (SXS). The surfaces with two atomic rows of terraces, Pt(110) = 2(111)-(111) and Pt(311) = 2(100)-(111) = 2(111)-(100), are reconstructed to a (1 × 2) structure according to previous studies. However, the surfaces with three atomic rows of terraces have pseudo-(1 × 1) structures. The interlayer spacing between the first and the second layers, d(12), is expanded 13% on Pt(331) compared to that of the bulk, whereas it is contracted 37% on Pt(511). The surface structures do not depend on the applied potential on either surface.     

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.     

Au/Pd(111)双金属表面催化噻吩加氢脱硫的反应机理

采用密度泛函理论(DFT)计算了Pd(111)表面含有N(N=1-4)个Au原子数目时的表面形成能,选取最优构型进一步研究了噻吩在Au/Pd(111)双金属表面的吸附模式及加氢脱硫反应过程.结果表明:当Pd(111)表面含有1个Au原子时,其形成能最低.在Au/Pd(111)双金属表面噻吩初始吸附于Pd-Hcp-30°位时,其构型最稳定.在各加氢脱硫过程中,反应总体均放出热量.对于直接脱硫机理,其所需活化能较低,但脱硫产物较难控制;对于间接脱硫机理,反应最有可能按照顺式加氢方式进行,C―S键断裂开环时所需活化能最高,是反应的限速步骤.此外,与单一Au(111)面及Pd(111)面相比,Au/Pd(111)双金属表面限速步骤的反应能垒最低,表明AuPd双金属催化剂比Au、Pd单金属催化剂更有利于噻吩加氢脱硫反应的进行.     

Ultrathin TiO(x) films on Pt(111): a LEED, XPS, and STM investigation

Ultrathin ordered titanium oxide films on Pt(111) surface are prepared by reactive evaporation of Ti in oxygen. By varying the Ti dose and the annealing conditions (i.e., temperature and oxygen pressure), six different long-range ordered phases are obtained. They are characterized by means of low-energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS), and scanning tunneling microscopy (STM). By careful optimization of the preparative parameters, we find conditions where predominantly single phases of TiO(x), revealing distinct LEED pattern and STM images, are produced. XPS binding energy and photoelectron diffraction (XPD) data indicate that all the phases, except one (the stoichiometric rect-TiO2), are one monolayer thick and composed of a Ti-O bilayer with interfacial Ti. Atomically resolved STM images confirm that these TiO(x) phases wet the Pt surface, in contrast to rect-TiO2. This indicates their interface stabilization. At a low Ti dose (0.4 monolayer equivalents, MLE), an incommensurate kagomé-like low-density phase (k-TiO(x) phase) is observed where hexagons are sharing their vertexes. At a higher Ti dose (0.8 MLE), two denser phases are found, both characterized by a zigzag motif (z- and z'-TiO(x) phases), but with distinct rectangular unit cells. Among them, z'-TiO(x), which is obtained by annealing in ultrahigh vacuum (UHV), shows a larger unit cell. When the postannealing of the 0.8 MLE deposit is carried out at high temperatures and high oxygen partial pressures, the incommensurate nonwetting, fully oxidized rect-TiO2 is found The symmetry and lattice dimensions are almost identical with rect-VO2, observed in the system VO(x)/Pd(111). At a higher coverage (1.2 MLE), two commensurate hexagonal phases are formed, namely the w- [(square root(43) x square root(43)) R 7.6 degrees] and w'-TiO(x) phase [(7 x 7) R 21.8 degrees]. They show wagon-wheel-like structures and have slightly different lattice dimensions. Larger Ti deposits produce TiO2 nanoclusters on top of the different monolayer films, as supported both by XPS and STM data. Besides the formation of TiO(x) surfaces phases, wormlike features are found on the bare parts of the substrate by STM. We suggest that these structures, probably multilayer disordered TiO2, represent growth precursors of the ordered phases. Our results on the different nanostructures are compared with literature data on similar systems, e.g., VO(x)/Pd(111), VO(x)/Rh(111), TiO(x)/Pd(111), TiO(x)/Pt(111), and TiO(x)/Ru(0001). Similar and distinct features are observed in the TiO(x)/Pt(111) case, which may be related to the different chemical natures of the overlayer and of the substrate.