Telomerization of 2-chloropropene with 1,1,1-trichlorethane

Conclusions When initiated with coordination initiators based on iron pentacarbonyl, the main telomerization product of 2-chloropropene with 1,1,1-trichloroethane is 2,2,4,4-tetrachloropentane. Small amounts of a telomer of structure CH₃CCl₂(CH₂CClCH₃)₂Cl, 1,1-(2,2-dichloropropyl)-ethylene, and chlorotri(2,2-dichloropropyl)methane are also formed.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2827–2828, December, 1978.     

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.     

Adsorbate conformation determines catalytic chemoselectivity: crotonaldehyde on the Pt(111) surface

Molecular orientation, which depends on surface coverage, determines whether or not catalytic hydrogenation is chemoselective.     

Preoxidation of CO on Os-modified Pt(111): a comparison with Ru-modified Pt(111)

The variation in CO adsorption structures during the preoxidation of CO on Os-modified Pt(111) (Pt(111)/Os) was investigated using cyclic voltammetry and electrochemical scanning tunneling microscopy. The spontaneous deposition of Os on Pt(111) resulted in randomly scattered islands with a coverage range of 0.13-0.54. During preoxidation on Pt(111)/Os, a phase transition from (2 × 2)-α to (√19 × √19) via the transient structures of (2 × 2)-β and (1 × 1) took place as on unmodified Pt(111). As the amount of Os increased, however, the transient structures of (2 × 2)-β and (1 × 1) appeared at lower potentials with higher populations. When the population of the transient structures was greater than 50%, an oxidative CO stripping process took place to the structure of (√19 × √19), completing the preoxidation. These observations strongly support the idea that the presence of Os increases the mobility of adsorbed CO by electronic modification of the Pt(111) surface (electronic effect). In addition, the results obtained with Pt(111)/Os were compared with those of Pt(111)/Ru.     

Coverage dependence and hydroperoxyl-mediated pathway of catalytic water formation on Pt (111) surface

Hydrogen oxidation on Pt (111) surface is modeled by density functional theory (DFT). Previous DFT calculations showed too large O2 dissociation barriers, but we find them highly coverage dependent: when the coverage is low, dissociation barriers close to experimental values (approximately 0.3 eV) are obtained. For the whole reaction, a new pathway involving hydroperoxyl (OOH) intermediate is found, with the highest reaction barrier of only approximately 0.4 eV. This may explain the experimental observation of catalytic water formation on Pt (111) surface above the H2O desorption temperature of 170 K, despite that the direct reaction between chemisorbed O and H atoms is a highly activated process with barrier approximately 1 eV as previous calculations showed.     

Molecular dynamics simulations of surface oxide-water interactions on Pt(111) and Pt/PtCo/Pt3Co(111)

Classical molecular dynamics simulations of the interactions of water with oxidized Pt(111) and Pt/PtCo/Pt(3)Co(111) surfaces are performed by modeling water with the CF1 central force model that allows molecular dissociation and therefore the presence of other intermediates of the oxygen reduction reaction different from atomic oxygen. It is found that the water-surface oxide interactions do not affect the overall structure of the catalyst represented by an extended periodic slab. However, such interactions are affected by changes in the electrochemical potential which are simulated by higher values of the surface and atomic oxygen charges at increased oxygen coverage. Thus, electrochemical potential as well as the presence of protons and anions products of acid dissociation define the identity and the amount of oxygen reduction reaction intermediates such as OH or H(3)O. We observe agglomerations of water molecules over regions of the surface and the presence of OH and H(3)O in their vicinity. Our simulation model is able to qualitatively reproduce features of the degradation of the catalyst surface after oxidation and reduction cycles.     

Selectivity control for the catalytic 1,3-butadiene hydrogenation on Pt(111) and Pd(111) surfaces: Radical versus closed-shell intermediates

The hydrogenation of 1,3-butadiene to different C4H8 species on both Pd(111) and Pt(111) surfaces has been studied by means of periodic slabs and DFT. We report the adsorption structures for the various mono- and dihydrogenated butadiene intermediates adsorbed on both metal surfaces. Radical species are more clearly stabilized on Pt than on Pd. The different pathways leading to these radicals have been investigated and compared to those producing 1-butene and 2-butene species. On palladium, the formation of butenes seems to be clearly favored, in agreement with the high selectivity to butenes observed experimentally. In contrast, the formation of dihydrogenated radical species seems to be competitive with that of butenes on platinum, which could explain its poorer selectivity to butenes and the formation of butane as a primary product.     

Electrocatalytic oxidation of ammonia on Pt(111) and Pt(100) surfaces

The electrocatalytic oxidation of ammonia on Pt(111) and Pt(100) has been studied using voltammetry, chronoamperometry, and in situ infrared spectroscopy. The oxidative adsorption of ammonia results in the formation of NH(x) (x = 0-2) adsorbates. On Pt(111), ammonia oxidation occurs in the double-layer region and results in the formation of NH and, possibly, N adsorbates. The experimental current transients show a hyperbolic decay (t(-1)), which indicates strong lateral (repulsive) interactions between the (reacting) species. On Pt(100), the NH(2) adsorbed species is the stable intermediate of ammonia oxidation. Stabilization of the NH and NH(2) fragments on Pt(111) and Pt(100), respectively, is in an interesting agreement with recent theoretical predictions. The Pt(111) surface shows extremely low activity in ammonia oxidation to dinitrogen, thus indicating that neither NH nor N (strongly) adsorbed species are active in dinitrogen production. Neither nitrous oxide nor nitric oxide is the product of ammonia oxidation on Pt(111) at potentials up to 0.9 V, as deduced from the in situ infrared spectroscopy measurements. The Pt(100) surface is highly active in dinitrogen production. This process is characterized by a Tafel slope of 30 mV decade(-1), which is explained by a rate-determining dimerization of NH(2) fragments followed by a fast decay of the resulting surface-bound hydrazine to dinitrogen. Therefore, the high activity of the Pt(100) surface for ammonia oxidation to dinitrogen is likely to be related to its ability to stabilize the NH(2) adsorbate.     

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.     

CO preoxidation on Ru-modified Pt(111)

Electrochemical scanning tunneling microscopy was used to study the structural evolution of adsorbed CO during preoxidation on Pt(111) modified with spontaneously deposited Ru. During the preoxidation process, a phase transition was observed from (2 × 2)-3CO-α to (√19 × √19)R23.4°-13CO via the transient structures (2 × 2)-3CO-β and (1 × 1)-CO. A comparison of these structural changes with those that occur on unmodified Pt(111) revealed that the presence of Ru resulted in higher populations of transient structures at lower potentials and a cathodic shift in the potential at which preoxidation is complete. These observations are discussed in terms of increased mobility of adsorbed CO in the presence of Ru.     

n-alkanes on Pt(111) and on C(0001)Pt(111): chain length dependence of kinetic desorption parameters

We have measured the desorption of seven small n-alkanes (C(N)H(2N+2), N=1-4,6,8,10) from the Pt(111) and C(0001) surfaces by temperature programed desorption. We compare these results to our recent study of the desorption kinetics of these molecules on MgO(100) [J. Chem. Phys. 122, 164708 (2005)]. There we showed an increase in the desorption preexponential factor by several orders of magnitude with increasing n-alkane chain length and a linear desorption energy scaling with a small y-intercept value. We suggest that the significant increase in desorption prefactor with chain length is not particular to the MgO(100) surface, but is a general effect for desorption of the small n-alkanes. This argument is supported by statistical mechanical arguments for the increase in the entropy gain of the molecules upon desorption. In this work, we demonstrate that this hypothesis holds true on both a metal surface and a graphite surface. We observe an increase in prefactor by five orders of magnitude over the range of n-alkane chain lengths studied here. On each surface, the desorption energies of the n-alkanes are found to increase linearly with the molecule chain length and have a small y-intercept value. Prior results of other groups have yielded a linear desorption energy scaling with chain length that has unphysically large y-intercept values. We demonstrate that by allowing the prefactor to increase according to our model, a reanalysis of their data resolves this y-intercept problem to some degree.     

Interaction of SO3 with c-ZrO2(111) films on Pt(111)

Single-crystalline sulfated c-ZrO2(111) films of the cubic (c) type have been prepared by reactive deposition of Zr onto Pt(111) in an O2 atmosphere and subsequent exposition to a SO3 atmosphere. The morphology, atomic structure, and composition have been examined by scanning tunneling microscopy, low-energy electron diffraction (LEED), Auger electron spectroscopy, and density functional theory (DFT) calculations. The clean c-ZrO2(111) films display a (2x2) surface structure. During SO3 exposure at room temperature, a clear (radical3xradical3)R30 degrees structure develops. At about 700 K, the SO3-induced (radical3xradical3)R30 degrees structure disappears and the bright (2x2) LEED pattern of the clean ZrO2 films reappears. The energies of plausible c-ZrO2(111)/SO3 structures have been examined by DFT. The (radical3xradical3)R30 degrees structure found in the experiments turned out to be the most stable one for temperatures below 700 K. At temperatures around 700 K, a disordered low coverage structure may exist, which can not be observed by conventional LEED. A comparison of cubic zirconia surfaces with the alternative tetragonal system yields similar results for the SO3 adsorption in the DFT calculations and shows that c-ZrO2 surfaces are good models for the industrial used tetragonal ZrO2 supports.     

Calculated phase diagrams for the electrochemical oxidation and reduction of water over Pt(111)

Ab initio density functional theory is used to calculate the electrochemical phase diagram for the oxidation and reduction of water over the Pt(111) surface. Three different schemes proposed in the literature are used to calculate the potential-dependent free energy of hydrogen, water, hydroxyl, and oxygen species adsorbed to the surface. Despite the different foundations for the models and their different complexity, they can be directly related to one another through a systematic Taylor series expansion of the Nernst equation. The simplest model, which includes the potential only as a shift in the chemical potential of the electrons, accounts very well for the thermochemical features determining the phase-diagram.     

Sb在Pt(100),Pt(110),Pt(111)及Pt(320)上不可逆吸附的电化学特性

研究了Sb在Pt(1 0 0 ) ,Pt(1 1 0 ) ,Pt(1 1 1 )和Pt(32 0 )单晶面上不可逆吸附的电化学特性 .发现当扫描电位的上限Eu≤ 0 .45V时 ,Sbad可以稳定地吸附在Pt(1 0 0 ) ,Pt(1 1 0 )和Pt(1 1 1 )表面 ,而Sbad在Pt(32 0 )表面稳定的电位较低 ,为Eu≤ 0 .40V .从饱和吸附Sb的铂单晶电极出发 ,通过改变电位扫描上限Eu 和电位扫描圈数可以获得不同Sb覆盖度 (θSb)的电极 .根据Sb和H在铂单晶电极表面共吸附的定量数据 ,对Sb在不同铂单晶面上饱和吸附的模型进行了初步探讨 .     

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.     

Electrochemical behaviour of Pt(100), Pt(111) and Pt polycrystalline surfaces in hydrogencarbonate solution

It was demonstrated that adsorbed CO is obtained from the reduction of NaHCO₃ solution when Pt(100), Pt(110), disordered Pt(111) and polycrystalline electrodes are employed. Reduction of CO₂ coming from the dissociation of the hydrogencarbonate anion is proposed as the reaction that produces CO. By using Fourier transform infrared spectroscopy, linear and multi-bonded CO were detected on polycrystalline platinum electrodes. The shape of the band associated with linearly adsorbed CO is monopolar as a consequence of the partial overlapping, at lower wavenumbers, of the absolute bands at both potentials (0.05 and 0.35 V).     

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).     

Adsorption and desorption of HCl on Pt(111)

The adsorption and desorption of HCl on Pt(111) is investigated by temperature programmed desorption, infrared reflection absorption spectroscopy, and low energy electron diffraction. Five peaks are identified in the desorption spectra prior to the onset of multilayer desorption. At low coverage ( < 0.25 monolayers (ML)), desorption peaks at approximately 135 and 200 K are observed and assigned to recombinative desorption of dissociated HCl. At higher coverages, desorption peaks at 70, 77, and 84 K are observed. These peaks are assigned to the desorption of molecularly adsorbed HCl. The infrared spectra are in agreement with these assignments and show that HCl deposited at 20 K is amorphous but crystallizes when heated above 60 K. Kinetic analysis of the desorption spectra reveals a strong repulsive coverage dependence for the desorption energy of the low coverage features ( < 0.25 ML). The diffraction data indicate that at low temperature the adsorbed HCl clusters into ordered islands with a (3 x 3) structure and a local coverage of 4/9 with respect to the Pt(111) substrate.     

Stereochemistry of 1,2-dichloroethane adsorbed on Pt(111)

The rotational isomerism of 1,2-dichloroethane (DCE, CH2ClCH2Cl) adsorbed on Pt(111) was studied in the temperature range of 35-100 K using high-resolution electron energy loss spectroscopy and metastable atom electron spectroscopy. In the coverage below monolayer the physisorbed and chemisorbed species coexist at 35 K in the gauche and slightly distorted trans form, respectively. Owing to the direct Pt-Cl interactions, the nonbonding Cl 3p states of the chemisorbed DCE are split off, giving rise to degradation in symmetry from the pure trans form (C2h). The physisorbed gauche conformers are arranged with the C2 axis parallel (or heavily tilted) to the substrate and converted irreversibly to the pseudo-trans form by heating at 70 K. In the multilayer, the trans and gauche conformers exist at 35 K, where the former population is increased with increasing layer thickness. Upon annealing the bilayer at 80 K, the irreversible conversion takes place to yield a higher population of the gauche conformer in the topmost layer. The conformational stabilities and mutual changes of DCE adsorbed on a metal surface are discussed in terms of intramolecular rotational potential.