共查询到20条相似文献,搜索用时 10 毫秒
1.
M. Ehsasi C. Scidel H. Ruppender W. Drachsel J. H. Block
K. Christmann
《Surface science》1989,210(3):L198-L208The oxidation of carbon monoxide on a Pd(110) single crystal plane has been studied using work function changes (Δφ) and mass spectrometric measurements. The rate of reaction showed oscillatory behaviour for oxygen pressures greater than 10−3 Torr. The existence region for oscillations was determined for pressures ranging from 10−3 to 1.0 Torr and depended on the pressures of oxygen and carbon monoxide and the sample temperature (Pco, PO2 T). Transitions from regular oscillation to chaos via period doubling have been observed in certain areas of the existence region. A comparison between Pd(110) and platinum single crystal surfaces that exhibit oscillations showed that similar but not identical oscillatory behaviour and existence regions exist in each case. Our results indicate that oscillations can occur on other metal single crystal surface that are less likely than platinum to reconstruct under reaction conditions. The extension of oscillations from UHV conditions to the 1.0 Torr pressure region indicates that the mechanism responsible for isothermal oscillations is basically independent of the reactant pressure up to several Torr. 相似文献
2.
A new feedback scheme for guided spatiotemporal pattern formation in reaction-diffusion systems is introduced. In contrast to previously established control methods, we present a coupling protocol that is sensitive to the presence of coherent structures in the medium. Applying this feedback to the catalytic CO oxidation on Pt(110) in both experiments and numerical simulations, we show that temporal evolution and spatial extension of self-organizing objects can be efficiently controlled. 相似文献
3.
M. Bowker R. P. Holroyd R. G. Sharpe J. S. Corneille S. M. Francis D. W. Goodman 《Surface science》1997,370(2-3):113-124
The adsorption and decomposition of ethanol on Pd(110) has been studied by use of a molecular beam reactor and temperature programmed desorption. It is found that the major pathway for ethanol decomposition occurs via a surface ethoxy to a methyl group, carbon monoxide and hydrogen adatoms. The methyl groups can either produce methane (which they do with a high selectivity for adsorption below 250 K) or can further decompose (which they do with a high selectivity for adsorption above 350 K) resulting in surface carbon. If adsorption occurs above 250 K a high temperature (450 K) hydrogen peak is observed in TPD, resulting from the decomposition of stable hydrocarbon fragments. A competing pathway also exists which involves C---O bond scission of the ethoxy, probably caused by a critical ensemble of palladium atoms at steps, defects or due to a local surface reconstruction. The presence of oxygen does not significantly alter the decomposition pathway above 250 K except that water and, above 380 K, carbon dioxide are produced by reaction of the oxygen adatoms with hydrogen adatoms and adsorbed carbon monoxide respectively. Below 250 K, some ethanol can form acetate which decomposes around 400 K to produce carbon dioxide and hydrogen. 相似文献
4.
5.
Clear polar and azimuth angle dependencies were found in rotational and vibrational energies of product CO2 in CO oxidation on Pd surfaces. On Pd(110)-(1x1), with increases in polar angle, both energies decreased in the [001] direction but remained constant in [110]. On the Pd(110) with missing rows, both energies increased in [001] but decreased in [110], indicating that the transition state changes with the geometry of the substrate. On Pd(111), the rotational energy greatly increased, but the vibrational energy decreased. Such angular dependence of internal energy provides new dimensions in surface reaction dynamics. 相似文献
6.
We present ab initio pseudo-potential plane-wave total-energy calculations for the geometric and electronic structure of the CO-covered Pd(110) surface. Our calculations were performed within the local-density approximation (LDA) of density functional theory (DFT). There has been some controversy as to whether CO prefers to adsorb at a bridge or on-top site when exposed to Pd(110). Total energy calculations for a CO monolayer adsorbed at the on-top and bridge adsorption sites revealed the bridge site adsorption to be favored by 0.59 eV per CO molecule. The preferential adsorption of CO to the bridge site was further corroborated by our band-structure calculations, with only the bridge site results being in good agreement with recent inverse photoemission experiments. 相似文献
7.
Oxidation of D2 and CO on oxygen pre-exposed 200 nm thick Pd films, epitaxially grown on MgO(100), MgO(110) and MgO(111), has been investigated in the temperature range 100–300°C. Oxygen initial sticking coefficients have been determined to be close to 1 for the 100 and 110 films, and around 0.8 for the 111 film. The sticking coefficient and reactive sticking coefficient for CO oxidation on Pd/MgO(100) is also close to 1, and the maximum reactive sticking coefficient for hydrogen oxidation is determined to be around 0.9 at temperatures above 200°C. It is shown that the reactivities for the different surfaces vary strongly with surface and oxygen coverage, and the consequence of this for supported particle catalysts is pointed out. 相似文献
8.
The decomposition of N2O was studied on Pd(110) through analysis of the angular and velocity distributions of desorbing products by means of angle-resolved thermal desorption combined with time-of-flight techniques. Both desorption and decomposition of N2O(a) were completed below 170 K, simultaneously emitting N2. N2 showed two desorption peaks, β1-N2 at 152 K and β2-N2 at 134–140 K. The former revealed an inclined emission at ±43° off the normal in a plane along the (001) direction and a hyper-thermal translational energy, whereas the latter showed a cosine angular distribution without an excess translational energy. Different desorption channels were proposed to yield these desorption. 相似文献
9.
LEED studies indicate that both CO and carbon adsorb differently on Pd(001) surfaces with and without steps. However, electron energy loss spectra for Pd(001) samples with and without ordered steps show no detectible surface structural sensitivity to CO adsorption. For both samples an additional loss feature at 13.5 eV appears upon CO adsorption. This feature is identified as a (1π?5σ) to 2π★ type intramolecular electronic excitation, and it appears to be a universal feature of molecular CO adsorption on metals. A second loss expected in the 6–7 eV range associated with a d to 2π★ type charge-transfer excitation was not detected. Either the transition probability is low for this excitation or hybridization with surface plasmons may obscure its identification. 相似文献
10.
L. Vattuone A. Gerbi L. Savio D. Cappelletti F. Pirani M. Rocca 《The European Physical Journal B - Condensed Matter and Complex Systems》2010,75(1):81-87
We report on stereodynamical effects in heterogeneous catalytic reactions as measured by molecular beam-surface experiments. Specifically for CO oxidation on Pd(100) we find that the rotational alignment of the incoming O2 at low (Θ = 0.04 ML) and at intermediate (ΘCO = 0.17 ML) CO pre-coverage, causes a higher reactivity of molecules in high and in low helicity states, respectively (corresponding to helicoptering and cartwheeling motion of O2). In first approximation, at low CO pre-coverage the difference in reactivity is determined by the different location of the O atoms generated in the dissociation process by the different parent molecules, while at intermediate CO pre-coverage the reactivity is influenced also by the different ability of cartwheeling and helicoptering O2 to penetrate through the CO adlayer. In accord with this the total amount of CO2 produced is always largest for helicopters which generate supersurface O atoms at least in the low CO pre-coverage limit. A deeper inspection of the data indicates, however, that the dynamics is more complex, two different pathways being present for the reaction with O generated by helicopters and one for O generated by cartwheels. Moreover, cartwheels generated oxygen influences the reactivity of subsequently arriving helicopters. 相似文献
11.
Hörnis H West JR Conrad EH Ellialtioglu R 《Physical review. B, Condensed matter》1993,47(19):13055-13058
12.
A variety of jumps has in the past been identified in diffusion of atoms on 1D channeled surfaces. To establish the jump processes important in diffusion on a 2D surface, the movement of individual Pd atoms has been examined on W(110). From the distribution of displacements of Pd at high temperatures, double jumps are found along the close-packed <111>. For the first time, sizable differences are also observed between the mean-square displacements along x and y, which demonstrate unexpected contributions from jumps along <110>, but not along <001>. These jumps proceed over activation barriers higher than for single jumps, under conditions predicted from previous work with Pd on the channeled W(211). 相似文献
13.
《Surface science》1989,215(3):L307-L315
Oscillations in the low-pressure oxidation of CO on Pt(110) were monitored simultaneously at different parts of the surface by means of two Kelvin probes. The oscillations in both regions were found to always exhibit the same frequency, however, in general with non-vanishing phase difference as well as different shapes and amplitudes. The results are interpreted in terms of spatial coupling through the gas phase between regions whose autonomous oscillatory properties are somewhat at variance to each other because of differences in the surface defect structure. 相似文献
14.
Ackermann MD Pedersen TM Hendriksen BL Robach O Bobaru SC Popa I Quiros C Kim H Hammer B Ferrer S Frenken JW 《Physical review letters》2005,95(25):255505
We present the first structure determination by surface x-ray diffraction during the restructuring of a model catalyst under reaction conditions, i.e., at high pressure and high temperature, and correlate the restructuring with a change in catalytic activity. We have analyzed the Pt(110) surface during CO oxidation at pressures up to 0.5 bar and temperatures up to 625 K. Depending on the pressure ratio, we find three well-defined structures: namely, (i) the bulk-terminated Pt(110) surface, (ii) a thin, commensurate oxide, and (iii) a thin, incommensurate oxide. The commensurate oxide only appears under reaction conditions, i.e., when both and CO are present and at sufficiently high temperatures. Density functional theory calculations indicate that the commensurate oxide is stabilized by carbonate ions (CO3(2-)). Both oxides have a substantially higher catalytic activity than the bulk-terminated Pt surface. 相似文献
15.
Deexcitation of metastable He1 21S (excitation energy E1 = 20.6 eV) or 23S (E* =19.8 eV) atoms at a clean Pd(110) surface proceeds through a two-stage process (resonance ionization + Auger neutralization, RI + AN). The measured electron energy distribution reflects the self-convolution of the local density of states of the outmost atomic layer. A CO adlayer suppresses the RI step and the spectra are caused by Auger deexcitation (Penning ionization). Comparison with corresponding UPS data allows identification of the valence orbitals of the adsorbate. Emission up to the Fermi level is ascribed to contributions from the 5σ level. The effectively available excitation energy in front of the adlayer is lowered by 0.5 eV. Extensive data on the variation of the intensities from the adsorbate valence levels with angle of incidence as well as of emission are presented and are analyzed in terms of an empirical model. 相似文献
16.
We have studied the adsorption of CO on Pd(2 1 0) by performing density functional theory (DFT) calculations within the generalized gradient approximation. We find a relatively small corrugation in the CO adsorption energies with the two bridge sites being energetically almost degenerate. CO is furthermore known as a strong poison in heterogeneous catalysis. We have therefore also addressed the coadsorption of CO with atomic hydrogen. There is a significant inhibition of the hydrogen adsorption due to the presence of CO which is analysed in terms of the electronic structure of the adsorbate system. 相似文献
17.
Guowen Peng S.J. Sibener George C. Schatz Manos Mavrikakis 《Surface science》2012,606(13-14):1050-1055
Hydrogen (H) in the subsurface of transition-metal surfaces exhibits unique reactivity for heterogeneously catalyzed hydrogenation reactions. Here, we explore the potential of subsurface H for hydrogenating carbon dioxide (CO2) on Ni(110). The energetics of surface and subsurface H reacting with surface CO2 to form formate, carboxyl, and formic acid on Ni(110) is systematically studied using self-consistent, spin-polarized, periodic density functional theory (DFT-GGA-PW91) calculations. We show that on Ni(110), CO2 can be hydrogenated to formate by surface H. However, further hydrogenation of formate to formic acid by surface H is hindered by a larger activation energy barrier. The relative energetics of hydrogenation barriers is reversed for the carboxyl-mediated route to formic acid. We suggest that the energetics of subsurface H emerging to the surface is suitable for providing the extra energy needed to overcome the barrier to formate hydrogenation. CO2 hydrogenation to formic acid could take place on Ni(110) when subsurface H is available to react with CO2. Additional electronic-structure based dynamic calculations would be needed to elucidate the detailed reaction paths for these transformations. 相似文献
18.
《Surface science》1988,202(3):L555-L558
The absolute surface coverages of CO and O on Pd(110) have been measured by nuclear reaction analysis (NRA) using the 12C(d, p)13C and 16O(d, p1)17O1 reactions. The CO coverages of the (2 × 1) and (4 × 2) phases of CO on Pd(110) are 1.00 ±0.05 and 0.73 ±0.05 ML (1 ML = 1 monolayer = 9.4 × 1014 CO molecules cm−2) respectively. The oxygen coverage in the c(4 × 2) phase of O on Pd(110) is 0.50 ±0.05 ML. 相似文献
19.
Belorussian State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 35, No. 2, pp. 103–129, February, 1992. 相似文献
20.