First-principles analysis of the effects of alloying Pd with Ag for the catalytic hydrogenation of acetylene-ethylene mixtures

The effect of alloying Pd with Ag on the hydrogenation of acetylene is examined by analyzing the chemisorption of all potential C(1) (atomic carbon, CH, methylene, and methyl) and C(2) (acetylene, vinyl, ethylene, ethyl, ethane, ethylidene, ethylidyne, and vinylidene) surface intermediates and atomic hydrogen along with the reaction energies for the elementary steps that produce these intermediates over Pd(111), Pd(75%)Ag(25%)/Pd(111), Pd(50%)Ag(50%)/Pd(111), and Ag(111) surfaces by using first-principle density functional theoretical (DFT) calculations. All of the calculations reported herein were performed at 25% surface coverage. The adsorption energies for all of the C(1) and C(2) intermediates decreased upon increasing the composition of Ag in the surface. Both geometric as well as electronic factors are responsible for the decreased adsorption strength. The modes of adsorption as well as the strengths of adsorption over the alloy surfaces in a number of cases were characteristically different than those found over pure Pd (111) and Ag (111). Adsorbates tend to minimize their interaction with the Ag atoms in the alloy surface. An electronic analysis of these surfaces shows that there is, in general, a shift in the occupied d-band states away from the Fermi level when Pd is alloyed with Ag. The s and p states also appear to contribute and may be responsible for small deviations from the Hammer-N?rskov model. The effect of alloying is more pronounced on the calculated reaction energies for different possible surface elementary reactions. Alloying Pd with Ag reduces the exothermicity (increases endothermicity) for bond-breaking reactions. This is consistent with experimental results that show a decrease in the decomposition products in moving from pure Pd to Pd-Ag alloys.(2-5) In addition, alloying increases the exothermicity of bond-forming reactions. Alloying therefore not only helps to suppress the unfavorable decomposition (bond-breaking) reaction rates but also helps to enhance the favorable hydrogenation (bond-forming) reaction rates.     

In situ soft X-ray studies of ethylene oxidation mechanisms and intermediates on the Pt(111) surface

In situ studies of ethylene oxidation on Pt(111) have been performed using a powerful combination of fluorescence yield soft X-ray methods for temperatures up to 600 K and oxygen pressures up to 0.01 Torr. Absolute carbon coverages have been determined both in steady-state and dynamic catalytic conditions on the Pt(111) surface. Fluorescence yield near-edge spectroscopy (FYNES) and temperature-programmed fluorescence yield near-edge spectroscopy (TP-FYNES) experiments above the carbon K edge were used to identify the structure and bonding of the dominant surface species during oxidation. TP-FYNES experiments of preadsorbed ethylene coverages in oxygen pressures up to 0.01 Torr indicate a stable intermediate is formed over the 215-300 K temperature range. By comparing the intensity of the C-H sigma resonance at the magic angle with the intensity in the carbon continuum, the stoichiometry of this intermediate has been determined explicitly. Based on calibration with known C-H stoichiometries, the intermediate has a C(2)H(3) stoichiometry for oxygen pressures up to 0.01 Torr, indicating oxydehydrogenation occurs before skeletal oxidation. FYNES spectra at normal and glancing incidences were performed to characterize the structure and bonding of this intermediate. Using FYNES spectra of ethylene, ethylidyne, and acetylene as reference standard, this procedure indicates the oxidation intermediate is tri-sigma vinyl. Thus, oxidation of ethylene proceeds through a vinyl intermediate, with oxydehydrogenation preceding skeletal oxidation.     

Geometric Requirements for Hydrocarbon Catalytic Sites on Platinum Surfaces

Vibrational spectroscopic measurements and density functional calculations were used to identify a preferential catalytic mechanism for the transformation of acetylene, HC? CH, to vinylidene, C? CH₂, on surfaces of Pt‐Sn ordered alloys. In this mechanism, two adjacent Pt atoms adsorb an acetylene molecule and a third neighboring Pt atom is required for stabilizing the reacting H atom during the transformation. Therefore, unlike a direct H shift along the C? C bond in organometallic compounds with a single transition‐metal atom, this mechanism has a geometric site requirement of three adjacent Pt atoms in the form of a three‐fold site. The same geometric site requirement is identified for preferential C? H bond cleavage of acetylene with the formation of adsorbed C? CH and H species. In the absence of three‐fold Pt sites, the reaction mechanism changes, and reactions of H transfer and C? H bond cleavage are suppressed.     

Microkinetic Modeling of Ethane Hydrogenolysis on Metals

The microkinetic analysis of ethane hydrogenolysis in the framework of the Sinfelt-Taylor mechanism is carried out using UBI-QEP data on the energetics of elementary steps. On single-crystalline surfaces of Pd(111), Pt(111), and Ni(111), the reaction predominantly occurs via ethyl formation in which the C-C bond is cleaved. The surface concentration of ethylidyne is very low, and ethyl has the highest concentration among all hydrocarbon fragments. For the conditions studied in this work, the activity in ethane hydrogenolysis increases in the series Pd(111) < Pt(111) < Ni(111). According to the results of kinetic modeling, the reaction occurs via quasi-equilibrium steps of adsorption and dissociation of hydrogen and ethane and further, practically irreversible formation of methane.     

Chemisorption of (CHx and C2Hy) hydrocarbons on Pt(111) clusters and surfaces from DFT studies

We used the B3LYP flavor of density functional theory (DFT) to study the chemisorption of all CH(x) and C(2)H(y) intermediates on the Pt(111) surface. The surface was modeled with the 35 atom Pt(14.13.8) cluster, which was found to be reliable for describing all adsorption sites. We find that these hydrocarbons all bind covalently (sigma-bonds) to the surface, in agreement with the studies by Kua and Goddard on small Pt clusters. In nearly every case the structure of the adsorbed hydrocarbon achieves a saturated configuration in which each C is almost tetrahedral with the missing H atoms replaced by covalent bonds to the surface Pt atoms. Thus, (Pt(3))CH prefers a mu(3) hollow site (fcc), (Pt(2))CH(2) prefers a mu(2) bridge site, and PtCH(3) prefers mu(1) on-top sites. Vinyl leads to (Pt(2))CH-CH(2)(Pt), which prefers a mu(3) hollow site (fcc). The only exceptions to this model are ethynyl (CCH), which binds as (Pt(2))C=CH(Pt), retaining a CC pi-bond while binding at a mu(3) hollow site (fcc), and HCCH, which binds as (Pt)HC=CH(Pt), retaining a pi bond that coordinates to a third atom of a mu(3) hollow site (fcc) to form an off center structure. These structures are in good agreement with available experimental data. For all species we calculated heats of formation (DeltaH(f)) to be used for considering various reaction pathways on Pt(111). For conditions of low coverage, the most strongly bound CH(x) species is methylidyne (CH, BE = 146.61 kcal/mol), and ethylidyne (CCH(3), BE = 134.83 kcal/mol) among the C(2)H(y) molecules. We find that the net bond energy is nearly proportional to the number of C-Pt bonds (48.80 kcal/mol per bond) with the average bond energy decreasing slightly with the number of C ligands.     

Transition state for beta-hydride elimination in alkyl groups on Pt(111)

The transition state for beta-hydride elimination in alkyl groups on the Pt(111) surface has been probed by studying the effects of fluorine substitution on the barriers to beta-hydride elimination, DeltaE++(betaH). Four different fluoroalkyl groups have been formed on the Pt(111) surface by dissociative adsorption of four fluoroalkyl iodides: RCH(2)CH(2)-I (R = CF(3), CF(3)CH(2), and CF(3)CF(2)) and (CF(3))(2)CHCH(2)-I. In the absence of preadsorbed hydrogen, fluoroalkyl groups on the Pt(111) surface dehydrogenate via beta-hydride elimination to form unsaturated fluorocarbons and deposit hydrogen atoms onto the surface. Those hydrogen atoms then hydrogenate the remaining fluoroalkyl groups to produce fluoroalkanes that desorb rapidly from the surface. The kinetics of hydrogenation and fluoroalkane desorption are rate limited by the beta-hydride elimination step and thus serve as measures of the kinetics of beta-hydride elimination. The field effects of the fluorinated substituents increase the barriers to beta-hydride elimination with a reaction constant of rho(F) = 19 +/- 2 kJ/mol. The interpretation of this effect is that the beta-carbon atom in the transition state is cationic, [RC(delta+...)H]++, with respect to the reactant. The field effect of the fluorinated substituent energetically destabilizes the electron deficient beta-carbon atom in the transition state. This is consistent with observations made on the Cu(111) surface; however, the substituent effect is significantly smaller on the Pt(111) surface. On the Pt(111) surface, the transition state for beta-hydride elimination is less polarized with respect to the initial state alkyl group than on the Cu(111) surface.     

Methane oxidation mechanism on Pt(111): a cluster model DFT study

The electronic energy barriers of surface reactions pertaining to the mechanism of the electrooxidation of methane on Pt (111) were estimated with density functional theory calculations on a 10-atom Pt cluster, using both the B3LYP and PW91 functionals. Optimizations of initial and transition states were performed for elementary steps that involve the conversion of CH(4) to adsorbed CO at the Pt/vacuum interface. As a first approximation we do not include electrolyte effects in our model. The reactions include the dissociative chemisorption of CH(4) on Pt, dehydrogenation reactions of adsorbed intermediates (*CH(x) --> *CH(x-1) + *H and *CH(x)O --> *CH(x-1)O + *H), and oxygenation reactions of adsorbed CH(x) species (*CH(x) + *OH --> *CH(x)OH). Many pathways were investigated and it was found that the main reaction pathway is CH(4) --> *CH(3) --> *CH(2) --> *CH --> *CHOH --> *CHO --> *CO. Frequency analysis and transition-state theory were employed to show that the methane chemisorption elementary step is rate-limiting in the above mechanism. This conclusion is in agreement with published experimental electrochemical studies of methane oxidation on platinum catalysts that have shown the absence of an organic adlayer at electrode potentials that allow the oxidation of adsorbed CO. The mechanism of the electrooxidation of methane on Pt is discussed.     

Enhanced reactivity of Pt nanoparticles supported on ceria thin films during ethylene dehydrogenation

The adsorption and reaction of ethylene on Pt/CeO(2-x)/Cu(111) model catalysts were studied by means of high resolution photoelectron spectroscopy (HR-PES) in conjunction with resonant photoemission spectroscopy (RPES). The dehydrogenation mechanism is compared to the HR-PES data obtained on a Pt(111) single crystal under identical conditions. It was found that the Pt nanoparticle system shows a substantially enhanced reactivity and several additional reaction pathways. In sharp contrast to Pt(111), partial dehydrogenation of ethylene on the supported Pt nanoparticles already starts at temperatures as low as 100 K. Similar to the single crystal surface, dehydrogenation occurs via the isomer ethylidene (CHCH(3)) and then mainly via ethylidyne (CCH(3)). In the temperature region between 100 and 250 K there is strong evidence for spillover of hydrocarbon fragments to the ceria support. In addition, splitting of ethylene to C(1) fragments is more facile than on Pt(111), giving rise to the formation of CH species and CO in the temperature region between 250 and 400 K. Upon further annealing, carbonaceous deposits are formed at 450 K. By heating to 700 K, these carbon deposits are completely removed from the surface by reaction with oxygen, provided by reverse spillover of oxygen from the ceria support.     

Ruthenium-catalyzed hydration of 1-alkynes to give aldehydes: insight into anti-Markovnikov regiochemistry.

The mechanism of the selective conversion of 1-alkynes to aldehydes by hydration was investigated by isolating organic and organometallic byproducts, deuterium-labeling experiments, and DFT calculations. The D-labeled acetylenic hydrogen of 1-alkyne was found exclusively in the formyl group of the resulting aldehydes. After the reaction, the presence of metal-coordinated CO was confirmed. All of the experimental results strongly suggest the involvement of a metal-acyl intermediate with the original acetylenic hydrogen also bound to the metal center as a hydride, with the next step being release of aldehyde by reductive elimination. Theoretical analyses suggest that the first step of the catalytic cycle is not oxidative addition of acetylene C [bond] H or tautomerization of eta(2)-alkyne to a vinylidene complex, but rather protonation of the coordinated 1-alkyne at the substituted carbon to form a metal-vinyl intermediate. This cationic intermediate then isomerizes to Ru(IV)-hydride-vinylidene via alpha-hydride migration of the vinyl group to the metal center, followed by attack of the vinylidene alpha-carbon by OH(-) to give the metal-hydride-acyl intermediate.     

Methane dissociative adsorption on the Pt(111) surface over the 300-500 K temperature and 1-10 Torr pressure ranges

The dissociative adsorption of methane on the Pt(111) surface has been investigated and characterized over the 1-10 Torr pressure and 300-500 K temperature ranges using sum frequency generation (SFG) vibrational spectroscopy and Auger electron spectroscopy (AES). At a reaction temperature of 300 K and a pressure of 1 Torr, C-H bond dissociation occurs in methane on the Pt(111) surface to produce adsorbed methyl (CH(3)) groups, carbon, and hydrogen. SFG results suggest that C-C coupling occurs at higher reaction temperatures and pressures. At 400 K, methyl groups react with adsorbed C to form ethylidyne (C(2)H(3)), which dehydrogenates at 500 K to form ethynyl (C(2)H) and methylidyne (CH) species, as shown by SFG. By 600 K, all of the ethylidyne has reacted to form the dissociation products ethynyl and methylidyne. Calculated C-H bond dissociation probabilities for methane, determined by carbon deposition measured by AES, are in the 10(-8) range and increase with increasing reaction temperature. A mechanism has been developed and is compared with conclusions from other experimental and theoretical studies using single crystals.     

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.     

Radical processes in the pyrolysis of acetylene

The literature on the pyrolysis of acetylene below 1000 K is reviewed and it is shown that the reaction is a radical chain polymerization initiated by H atoms. The induction period is examined and shown to arise from a rapid autocatalysis produced by the very first product, vinyl acetylene. Data from studies of acetylene polymerization induced by vinyl acetylene and styrene are examined and shown to be in good qualitative agreement with the radical chain process presented. Measured and where needed, estimated rate constants are given to produce numerical estimates of chain length, rates of polymerization, induction period, and rates of initiation. It is further shown that all the products of the polymerization are capable of efficiently initiating chains in the system. An examination of the possible kinetics of vinylidene shows that it does not play any role in acetylene chemistry.     

Identification and hydrogenation of C2 on Pt(111)

Reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) were used to identify the molecular species formed upon the reaction of hydrogen with surface carbon that is deposited by exposing acetylene to a Pt(111) surface held at 750 K. At this temperature, the acetylene is completely dehydrogenated and all hydrogen is desorbed from the surface. Upon subsequent hydrogen exposure at 85 K followed by sequential annealing to higher temperatures, ethylidyne (CCH3), ethynyl (CCH), and methylidyne (CH) are formed. The observation of these species indicates that carbon atoms and C2 molecules exist as stable species on the surface over a wide range of temperatures. Through a combination of RAIRS intensities, hydrogen TPD peak areas, and Auger electron spectroscopy, quantitative estimates of the coverages of the various species were obtained. It was found that 79% of the acetylene-derived carbon was in the form of C2 molecules, with the remainder in the form of carbon atoms. Essentially all of the acetylene-derived carbon could be hydrogenated. In contrast, 85% of an equivalent coverage of carbon deposited by ethylene exposure at 750 K was found to be inert toward hydrogenation.     

Pd8簇催化氢化乙炔的反应机理

采用密度泛函理论(DFT)中的B3PW91方法研究了过渡金属Pd₈簇催化氢化乙炔的反应机理. 研究表明: H₂进入Pd₈簇后解离成H原子, 并且只有当H原子吸附在Pd₈簇表面上时, 催化氢化反应才能发生. 过渡金属Pd₈簇催化氢化乙炔的反应机理的研究证实该催化反应从两种反应物出发经过两条不同的反应途径完成催化氢化反应, 两种反应物分别为吸附在Pd₈簇上的乙炔(Pd₈(2H)－CH＝CH)和其同分异构体亚乙烯基吸附物(Pd₈(2H)－C＝CH₂). 两条途径均为多步连续的加氢反应, 不同之处在于从Pd₈(2H)－CH＝CH出发的为单一路径, 解离后的H原子分步依次加成到吸附在Pd₈簇上的乙炔中的C原子上, 直到反应完成生成乙烷. 而从Pd₈(2H)－C＝CH₂出发的路径较为复杂, 分别经过两个不同的过渡态和中间体生成次乙基中间体, 该过程相对应的反应位垒相差约12.552 kJ·mol^-1, 说明这两个过渡态同时存在, 无先后次序. 然后继续加成H原子直到生成乙烷完成反应. 同时, 两条路径分别形成一系列具有应用价值的C₂有机化合物中间体, 其中一些中间体通过分子内质子转移相互转化, 使得原本独立的两条反应路径联系在一起, 成为网状路径.     

Gold-Catalyzed Annulation of 1,8-Dialkynylnaphthalenes: Synthesis and Photoelectric Properties of Indenophenalene-Based Derivatives

A simple gold-catalyzed annulation of 1,8-dialkynylnaphthalenes utilizing a cationic gold catalyst was developed. Such a peri-position of two alkynyl substituents has not been studied in gold catalysis before. Dependent on the substrate, the reactions either follow a mechanism involving vinyl cation intermediates or involve a dual gold catalysis mechanism which in an initial 6-endo-dig-cyclization generates gold(I) vinylidene intermediates that are able to insert into C−H bonds. Indenophenalene derivatives were obtained in moderate to high yields. In addition, the bidirectional gold-catalyzed annulation of tetraynes provided even larger conjugated π-systems. The optoelectronic properties of the products were also investigated.     

Electrochemical and FTIRS characterisation of NO adlayers on cyanide-modified Pt(111) electrodes: the mechanism of nitric oxide electroreduction on Pt

We report here a study, using cyclic voltammetry and FTIRS, of NO irreversibly adsorbed on a cyanide-modified Pt(111) electrode. NO adlayers were formed by immersion of the cyanide-modified Pt(111) electrode in an acidic solution of KNO(2). The behaviour of NO adsorbed on the cyanide-modified electrode is very similar to that of NO on the clean Pt(111) surface, suggesting that adsorbed cyanide (saturation coverage theta(CN) = 0.5) behaves simply as a third body, blocking some of the surface sites but leaving the free Pt sites unaffected. Comparison of the voltammetric profile for NO electroreduction on Pt(111) and on cyanide-modified Pt(111) electrodes has allowed us: (i) to confirm that the reduction of three-fold hollow NO and atop NO on Pt(111) electrodes occurs in two distinct reduction peaks, as previously proposed by Rosca et al. (Langmuir, 2005, 21, 1448); (ii) to suggest that the reduction of irreversibly adsorbed NO layers on Pt electrodes can proceed through two possible paths, one involving an EE mechanism in which the rate-determining step (rds) is an Eley-Rideal reaction, with a direct proton transfer from the solution to adsorbed NO, and the other involving an EC mechanism in which the rds is a Langmuir-Hinshelwood reaction of adsorbed NO with adsorbed H. The availability of adsorbed hydrogen determines which path is followed by the reaction; (iii) to identify the smallest atomic ensemble for the reduction of NO on Pt as being composed of two adjacent Pt atoms.     

Methanol oxidation on the PtPd(111) alloy surface: A density functional theory study

The mechanisms of methanol (CH₃OH) oxidation on the PtPd(111) alloy surface were systematically investigated by using density functional theory calculations. The energies of all the involved species were analyzed. The results indicated that with the removal of H atoms from adsorbates on PtPd(111) surface, the adsorption energies of (i) CH₃OH, CH₂OH, CHOH, and COH increased linearly, while those of (ii) CH₃OH, CH₃O, CH₂O, CHO, and CO exhibited odd‐even oscillation. On PtPd(111) surface, CH₃OH underwent the preferred initial C H bond scission followed by successive dehydrogenation and then CHO oxidation, that is, CH₃OH → CH₂OH → CHOH → CHO → CHOOH → COOH → CO₂. Importantly, the rate‐determining step of CH₃OH oxidation was found to switch from CO → CO₂ on Pt(111) to COOH → CO₂ + H on PtPd(111) with a lower energy barrier of 0.96 eV. Moreover, water also decomposed into OH more easily on PtPd(111) than on Pt(111). The calculated results indicate that alloying Pt with Pd could efficiently improve its catalytic performance for CH₃OH oxidation through altering the primary pathways from the CO path on pure Pt to the non‐CO path on PtPd(111).     

Synergetic effect of surface and subsurface Ni species at Pt-Ni bimetallic catalysts for CO oxidation

Various well-defined Ni-Pt(111) model catalysts are constructed at atomic-level precision under ultra-high-vacuum conditions and characterized by X-ray photoelectron spectroscopy and scanning tunneling microscopy. Subsequent studies of CO oxidation over the surfaces show that a sandwich surface (NiO(1-x)/Pt/Ni/Pt(111)) consisting of both surface Ni oxide nanoislands and subsurface Ni atoms at a Pt(111) surface presents the highest reactivity. A similar sandwich structure has been obtained in supported Pt-Ni nanoparticles via activation in H(2) at an intermediate temperature and established by techniques including acid leaching, inductively coupled plasma, and X-ray adsorption near-edge structure. Among the supported Pt-Ni catalysts studied, the sandwich bimetallic catalysts demonstrate the highest activity to CO oxidation, where 100% CO conversion occurs near room temperature. Both surface science studies of model catalysts and catalytic reaction experiments on supported catalysts illustrate the synergetic effect of the surface and subsurface Ni species on the CO oxidation, in which the surface Ni oxide nanoislands activate O(2), producing atomic O species, while the subsurface Ni atoms further enhance the elementary reaction of CO oxidation with O.     

Vinyl acetate formation by the reaction of ethylene with acetate species on oxygen-covered Pd(111)

The reaction pathway of vinyl acetate synthesis is scrutinized by reacting gas-phase ethylene (at an effective pressure of 1 x 10-4 Torr) with eta2-acetate species (with a coverage of 0.31 +/- 0.02 monolayer) on a Pd(111)-O(2x2) model catalyst surface in ultrahigh vacuum. It is found that the 1414 cm-1 infrared feature due to the symmetric OCO stretching mode of the acetate species decreases in intensity due to reaction with gas-phase ethylene, while temperature-programmed desorption experiments demonstrate that vinyl acetate is formed. The formation of ethylidyne species is detected when almost all of the acetate species have been removed. The experimental removal kinetics are reproduced by a model in which adsorbed acetates react with an ethylene-derived (possibly ethylene or vinyl) species, where ethylene adsorption is blocked by the acetate present on the surface.     

Theoretical elucidation of the competitive electro-oxidation mechanisms of formic acid on Pt(111)

The mechanisms of formic acid (HCOOH) oxidation on Pt(111) under electrochemical conditions have been studied using density functional theory and then compared with the analogous gas-phase reaction. Results show that HCOOH oxidation under a water-covered surface behaves substantially differently than in the gas phase or using a solvation model involving only a few water molecules. Using these models, we evaluated the detailed reaction process, including energies and geometric structures of intermediates and transition states under the influence of different solvation models and electrode potentials. Our calculations indicate that this potential-dependent electrochemical oxidation proceeds via a multipath mechanism (involving both the adsorbed HCOOH and HCOO intermediates), a result succinctly rationalizing conflicting experimental observations. Moreover, this study highlights how subtle changes in electrochemical reaction environments can influence (electro)catalysis.