共查询到20条相似文献，搜索用时 31 毫秒
1.
The trapping-mediated dissociative chemisorption of three isotopes of propane (C(3)H(8), CH(3),CD(2)CH(3), and C(3)D(8)) has been investigated on the Pt(110)-(1 x 2) surface, and both the apparent activation energies and the preexponential factors of the surface reaction rate coefficients have been measured. In addition, the probabilities of primary and secondary C-H bond cleavage for alkane activation on a surface were evaluated. The activation energy for primary C-H bond cleavage was 425 calories per mole greater than that of secondary C-H bond cleavage, and the two true activation energies that embody the single measured activation energy were determined for each of the three isotopes. Secondary C-H bond cleavage is also preferred on entropic grounds, and the magnitude of the effect was quantified. 相似文献
2.
Nieto P Pijper E Barredo D Laurent G Olsen RA Baerends EJ Kroes GJ Farías D 《Science (New York, N.Y.)》2006,312(5770):86-89
The Born-Oppenheimer approximation of uncoupled electronic and nuclear motion is a standard tool of the computational chemist. However, its validity for molecule-metal surface reactions, which are important to heterogeneous catalysis, has been questioned because of the possibility of electron-hole pair excitations. We have performed experiments and calculations on the scattering of molecular hydrogen from a catalytically relevant metal surface, obtaining absolute probabilities for changes in the molecule's velocity parallel to the representative Pt(111) surface. The comparison for in-plane and out-of-plane scattering and results for dissociative chemisorption in the same system show that for hydrogen-metal systems, reaction and diffractive scattering can be accurately described using the Born-Oppenheimer approximation. 相似文献
3.
Scanning tunneling microscopy and monoenergetic molecular beams have been used to obtain real-space atomic images of the competition between abstractive and dissociative chemisorption. The size distribution of Si-F adsorbates on the Si(111)-(7x7) surface was examined as a function of the incident translational energy of the F(2) molecules. For F(2) molecules with 0.03 electron volt of incident energy, the dominant adsorbate sites were isolated Si-F species. As an F(2) molecule with low translational energy collides with the surface, abstraction occurs and only one of the F atoms chemisorbs; the other is ejected into the gas phase. For F(2) molecules with 0.27 electron volt of incident energy, many adjacent Si-F adsorbates (dimer sites) were observed because F(2) molecules with high translational energy collide with the surface and chemisorb dissociatively so that both F atoms react to form adjacent Si-F adsorbates. For halogens with very high incident energy (0.5-electron volt Br(2)), dissociative chemisorption is the dominant adsorption mechanism and dimer sites account for nearly all adsorbates. 相似文献
4.
A recently designed single-crystal surface calorimeter has been deployed to measure the energy difference between two solid surface structures. The clean Pt{100} surface is reconstructed to a stable phase in which the surface layer of platinum atoms has a quasi-hexagonal structure. By comparison of the heats of adsorption of CO and of C(2)H(4) on this stable Pt{100}-hex phase with those on a metastable Pt{100}-(1x1) surface, the energy difference between the two clean phases was measured as 20 +/- 3 and 25 +/- 3 kilojoules per mole of surface platinum atoms. 相似文献
5.
Stamenkovic VR Fowler B Mun BS Wang G Ross PN Lucas CA Marković NM 《Science (New York, N.Y.)》2007,315(5811):493-497
The slow rate of the oxygen reduction reaction (ORR) in the polymer electrolyte membrane fuel cell (PEMFC) is the main limitation for automotive applications. We demonstrated that the Pt3Ni(111) surface is 10-fold more active for the ORR than the corresponding Pt(111) surface and 90-fold more active than the current state-of-the-art Pt/C catalysts for PEMFC. The Pt3Ni(111) surface has an unusual electronic structure (d-band center position) and arrangement of surface atoms in the near-surface region. Under operating conditions relevant to fuel cells, its near-surface layer exhibits a highly structured compositional oscillation in the outermost and third layers, which are Pt-rich, and in the second atomic layer, which is Ni-rich. The weak interaction between the Pt surface atoms and nonreactive oxygenated species increases the number of active sites for O2 adsorption. 相似文献
6.
Structural transformations on elemental semiconductor surfaces typically occur above several hundred degrees Celsius, and the atomic motions involved are extremely rapid and difficult to observe. However, on the (111) surface of germanium, a few lead atoms catalyze atomic motions so that they can be observed with a tunneling microscope at temperatures below 80 degrees C. Mass transport and structural changes are caused by the creation and propagation of both vacancy-like and interstitial-like point defects within the crystal surface. The availability of dangling bonds on the surface is critical. A detailed atomic model for the observed motions has been developed and is used to explain the structural phase transition Ge(111)-c(2x8) <--> 1x1, which occurs near 300 degrees C. 相似文献
7.
The kinetics of photoinduced electron-transfer (ET) reactions have been measured in a series of synthetic donor-acceptor complexes. The electron donors are singlet or triplet excited iridium(I) dimers (Ir(2)), and the acceptors are N-alkylpyridinium groups covalently bound to phosphinite ligands on the Ir(2) core. Rate constants for excited-state ET range from 3.5 x 10(6) to 1.1 x 10(11) per second, and thermal back ET (pyridinium radical to Ir(2)(+)) rates vary from 2.0 x 10(10) to 6.7 x 10(7) per second. The variation of these rates with driving force is in remarkably good agreement with the Marcus theory prediction of a Gaussian free-energy dependence. 相似文献
8.
Generally, rotational isomerization about the carbon-carbon single bond in simple ethane derivatives in room-temperature solution under thermal equilibrium conditions has been too fast to measure. We achieved this goal using two-dimensional infrared vibrational echo spectroscopy to observe isomerization between the gauche and trans conformations of an ethane derivative, 1-fluoro-2-isocyanato-ethane (1), in a CCl4 solution at room temperature. The isomerization time constant is 43 picoseconds (ps, 10(-12) s). Based on this value and on density functional theory calculations of the barrier heights of 1, n-butane, and ethane, the time constants for n-butane and ethane internal rotation under the same conditions are approximately 40 and approximately 12 ps, respectively. 相似文献
9.
The structure of model silica-supported copper catalysts has been investigated with scanning tunneling microscopy (STM) and infrared reflection-absorption spectroscopy (IRAS). The IRAS studies of CO on the model silica-supported copper catalysts indicate that there are several types of copper clusters with surface structures similar to (111), (110), and other high-index planes of single-crystal copper. The STM studies show several types of copper clusters on silica and reveal images of metal clusters on an amorphous oxide support with atomic resolution. 相似文献
10.
Hanel R Conrath B Flasar M Herath L Kunde V Lowman P Maguire W Pearl J Pirraglia J Samuelson R Gautier D Gierasch P Horn L Kumar S Ponnamperuma C 《Science (New York, N.Y.)》1979,206(4421):952-956
Infrared spectra obtainedfrom Voyager 2 have provided additional data on the Jovian system, complementing those obtained from Voyager 1. The abundance ratio of ethane to acetylene in Jupiter's atmosphere appears to be about three times larger in the polar regions than at lower latitudes. A decidedly hemispherical asymmetry exists, with somewhat higher ratios prevailing in northern latitudes. An overall increase in the abundance ratio by a factor of about 1.7 appears to have occurred between the Voyager 1 and 2 encounters. Global brightness temperature maps of Jupiter at 226 and 602 cm(-1) exhibit a large amount of local- and planetary-scale structure, as well as temporal variability. Although heterogeneous cloud structure and ammonia concentration in the lower troposphere may contribute to the appearance of the 226-cm(-1) map, the detail in the 602-cm(-1) maps probably represents the actual horizontal thermal structure near the tropopause and suggests that dynamical heating and cooling processes are important. Low-latitude surface temperatures on the Galilean satellites rangefrom approximately 80 K on the dark sides to 155 K at the subsolar point on Callisto. Below a thin insulating layer, the thermal inertia of Callisto is somewhat greater than that of Earth's moon. Upper limits on the infrared optical depth of the Jovian ring rangingfrom approximately 3 x 10(-4) at 250 cm(-1) to 3 x 10(-3) at 600 cm(-1) have been found. 相似文献
11.
Studies of chemisorption phenomena, the cornerstone of heterogeneous catalysis, have become the central part of contemporary surface science. As a result of the great variety of the available experimental techniques, a backlog of information, some of which conflicts with current theoretical constructs, has accumulated. New models that combine analytical and computational facets have now begun to appear, revealing intrinsic relations among seemingly disparate chemisorption phenomena. Among the major findings are (i) the crucial role of antibonding adsorbate orbitals in bond activation and in the heat of chemisorption, (ii) adsorbate-induced surface polarization leading to a decrease of the metal work function and to an increase of the surface core binding energy, and (iii) important differences between atomic and molecular adsorbate modes of bonding and surface migration. 相似文献
12.
Keays RR Ganapathy R Laul JC Anders E Herzog GF Jeffery PM 《Science (New York, N.Y.)》1970,167(3918):490-493
Lunar soil and type C breccias are enriched 3-to 100-fold in Ir, Au, Zn, Cd, Ag, Br, Bi, and Tl, relative to type A, B rocks. Smaller enrichments were found for Co, Cu, Ga, Pd, Rb, and Cs. The solar wind at present intensity can account for only 3 percent of this enrichment; an upper limit to the average proton flux during the last 4.5 x 109 years thus is 8 x 10(9) cm(-2) yr(-1). The remaining enrichment seems to be due to a 1.5 to 2 percent admixture of carbonaceous-chondritelike material, corresponding to an average influx rate of meteoritic and cometary matter of 2.9 x 10(-9) g cm(-2) yr(-1) at Tranquility Base. This is about one-quarter the terrestrial rate. Type A, B rocks are depleted 10-to 100-fold in Ag, Au, Zn, Cd, In, Tl, and Bi, relative to terrestrial basalts. This suggests loss by high-temperature volatilization, before or after accretion of the moon. Positron activities due mainly to (22)Na and (26)Al range from 90 to 220 beta(+) min(-1) kg(-1) in five small rocks or fragments (9 to 29 g). The higher activities presumably indicate surface locations. Th and U contents generally agree with those found by the preliminary examination team. 相似文献
13.
Sundström G Mowat JR Danared H Datz S Broström L Filevich A Källberg A Mannervik S Rensfelt KG Sigray P Ugglas MA Larsson M 《Science (New York, N.Y.)》1994,263(5148):785-787
Knowledge of the abundance of H(3)(+) is needed in interstellar and planetary atmospheric chemistry. An important destruction mechanism of H(3)(+) is low-energy electron impact followed by dissociation, but estimates of the reaction rate span several orders of magnitude. As an attempt to resolve this uncertainty, the cross section for dissociative recombination of vibrationally cold H(3)(+) has been measured with an ion storage ring down to collision energies below 1 millielectron volt. A rate coefficient of 1.15 x 10(-7) cubic centimeters per second at 300 kelvin was deduced. The cross section scaled with collision energy according to E(-1.15), giving thee rate a temperature dependence of T(-0.65). 相似文献
14.
The molecular volume of crystals depends on their isotopic masses. This influence originates from the zero-point motion and the resulting small differences in lattice constants. This effect was measured with high precision by using an x-ray standing wave. The standing wave is generated during Bragg reflection and thus is in phase with the planes of the substrate crystal, which is covered with a homoepitaxial film that has a different isotopic composition than the substrate. The positions of the surface planes of the film with respect to the substrate planes are revealed by the photoelectrons excited by the maxima of the standing wave. For germanium-76 on natural germanium(111), a difference in lattice constant of -1.1 x 10(-5) and -2.5 x 10(-5) at 300 and 54 kelvin, respectively, was found. The results are in good agreement with theoretical predictions. 相似文献
15.
Small-diameter (1 to 7 nanometers) silicon nanowires (SiNWs) were prepared, and their surfaces were removed of oxide and terminated with hydrogen by a hydrofluoric acid dip. Scanning tunneling microscopy (STM) of these SiNWs, performed both in air and in ultrahigh vacuum, revealed atomically resolved images that can be interpreted as hydrogen-terminated Si (111)-(1 x 1) and Si (001)-(1 x 1) surfaces corresponding to SiH3 on Si (111) and SiH2 on Si (001), respectively. These hydrogen-terminated SiNW surfaces seem to be more oxidation-resistant than regular silicon wafer surfaces, because atomically resolved STM images of SiNWs were obtained in air after several days' exposure to the ambient environment. Scanning tunneling spectroscopy measurements were performed on the oxide-removed SiNWs and were used to evaluate the electronic energy gaps. The energy gaps were found to increase with decreasing SiNW diameter from 1.1 electron volts for 7 nanometers to 3.5 electron volts for 1.3 nanometers, in agreement with previous theoretical predictions. 相似文献
16.
The platinum-rhodium tip of a scanning tunneling microscope that operates inside of an atmospheric-pressure chemical reactor cell has been used to locally rehydrogenate carbonaceous fragments deposited on the (111) surface of platinum. The carbon fragments were produced by partial dehydrogenation of propylene. The reactant gas environment inside the cell consisted of pure H(2) or a 1:9 mixture of CH(3)CHCH(2) and H(2) at 300 kelvin. The platinum-rhodium tip acted as a catalyst after activation by short voltage pulses. In this active state, the clusters in the area scanned by the tip were reacted away with very high spatial resolution. 相似文献
17.
The interface between a two-dimensional (2D) molecular gas and a 2D molecular solid has been imaged with a low-temperature, ultrahigh-vacuum scanning tunneling microscope. The solid consists of benzene molecules strongly bound to step edges on a Cu{111} surface. Benzene molecules on the Cu{111} terraces move freely as a 2D gas at 77 kelvin. Benzene molecules transiently occupy well-defined adsorption sites at the 1D edge of the 2D solid. Diffusion of molecules between these sites and exchange between the two phases at the interface are observed. On raised terraces of the copper surface, the 2D gas is held in a cage of the solid as in a 2D nanometer-scale gas bulb. 相似文献
18.
The scanning tunneling microscope (STM) can be used to select a particular adsorbed molecule, probe its electronic structure, dissociate the molecule by using electrons from the STM tip, and then examine the dissociation products. These capabilities are demonstrated for decaborane(14) (B(10)H(14)) molecules adsorbed on a silicon(111)-(7 x 7) surface. In addition to basic studies, such selective dissociation processes can be used in a variety of applications to control surface chemistry on the molecular scale. 相似文献
19.
Photochemical production rates and steady-state concentrations of hydroxyl radicals (.OH) were measured in sunlight-irradiated seawater. Values ranged from 110 nanomolar per hour and 12 x 10(-18) molar in coastal surface water to 10 nanomolar per hour and 1.1 x 10(-18) molar in open ocean surface water. The wavelengths responsible for this production are in the ultraviolet B region (280 to 320 nanometers) of the solar spectrum. Dissolved organic matter (DOM) appears to be the main source for .OH over most of the oceans, but in upwelling areas nitrite and nitrate photolysis may also be important. DOM in the deep sea is degraded more readily by .OH (and its daughter radicals), by a factor of 6 to 15, than is DOM in open-ocean surface water. This finding may in part bear on major discrepancies among current methods for measuring dissolved organic carbon in seawater. 相似文献
20.
The various products from the reaction of chlorine (Cl) with the adatom layer of the Si(111)-(7x7) surface have been identified with scanning tunneling microscopy (STM). Initially, a single Cl atom reacts with the adatom dangling bond. At higher surface coverage, additional Cl atoms insert themselves into the Si-Si backbonds between the adatom and rest-atom layers, producing adatoms that have reacted with two or three Cl atoms. These products are characterized by different registries with respect to the underlying rest layer and appear in STM images as adatoms of different sizes, consistent with the breaking of Si-Si backbonds and the formation ofnew Si-Cl bonds. 相似文献