Showing papers by "Fritz Haber Institute of the Max Planck Society published in 1994"

Multidimensional potential energy surface for H2 dissociation over Cu(111).

Abstract: We present ab initio density functional calculations within the generalized gradient approximation for ${\mathrm{H}}_{2}$ dissociating over Cu(111). The minimum barrier for dissociation is 0.5 eV and shows large corrugation within the unit cell and a strong dependence on the molecular orientation. Dissociation is predicted to depend strongly on translational, vibrational, and rotational degrees of freedom in accordance with experiment. We show that even for a noble metal, the $d$ electrons are important for the molecule-surface interaction.

Bifurcation to Traveling Spots in Reaction-Diffusion Systems

Abstract: A bifurcation leading to the onset of translational motion of localized particlelike structures (spots) in two-dimensional excitable media with long-range inhibition and global coupling is analytically and numerically investigated. Properties of slowly traveling spots and effects of collisions between these objects are studied.

Effects of boundaries on pattern formation: Catalytic oxidation of CO on platinum

Abstract: The effect of boundaries on pattern formation was studied for the catalytic oxidation of carbon monoxide on platinum surfaces. Photolithography was used to create microscopic reacting domains on polycrystalline foils and single-crystal platinum (110) surfaces with inert titanium overlayers. Certain domain geometries give rise to patterns that have not been observed on the untreated catalyst and bring to light surface mechanisms that have no analog in homogeneous reaction-diffusion systems.

Hydrogen adsorption on and desorption from Si: Considerations on the applicability of detailed balance.

Abstract: The translational energy of ${\mathrm{D}}_{2}$ desorbed from Si(100) and Si(111) surfaces was measured and found roughly equal to the thermal expectation at the surface temperature ${\mathit{T}}_{\mathit{s}}$. Combining these results with previously measured internal state distributions, the total energy of the desorbed molecules is approximately equal to the equilibrium expectation at ${\mathit{T}}_{\mathit{s}}$. Thus adsorption experiments, which suggest a large energetic barrier, are at variance with desorption experiments, which exhibit a trivial adsorption barrier, and the applicability of detailed balance for this system needs to be reexamined.

Chemical turbulence and standing waves in a surface reaction model: The influence of global coupling and wave instabilities

Abstract: Among heterogeneously catalyzed chemical reactions, the CO oxidation on the Pt(110) surface under vacuum conditions offers probably the greatest wealth of spontaneous formation of spatial patterns. Spirals, fronts, and solitary pulses were detected at low surface temperatures (T<500 K), in line with the standard phenomenology of bistable, excitable, and oscillatory reaction-diffusion systems. At high temperatures (T greater, similar 540 K), more surprising features like chemical turbulence and standing waves appeared in the experiments. Herein, we study a realistic reaction-diffusion model of this system, with respect to the latter phenomena. In particular, we deal both with the influence of global coupling through the gas phase on the oscillatory reaction and the possibility of wave instabilities under excitable conditions. Gas-phase coupling is shown to either synchronize the oscillations or to yield turbulence and standing structures. The latter findings are closely related to clustering in networks of coupled oscillators and indicate a dominance of the global gas-phase coupling over local coupling via surface diffusion. In the excitable regime wave instabilities in one and two dimensions have been discovered. In one dimension, pulses become unstable due to a vanishing of the refractory zone. In two dimensions, turbulence can also emerge due to spiral breakup, which results from a violation of the dispersion relation.

Direct identification of atomic and molecular adsorption sites using photoelectron diffraction

Abstract: THE determination of the adsorption geometry of molecules and molecular fragments on single-crystal metal surfaces is central to our understanding of heterogeneous catalysis. Adsorbate structures can be determined precisely by techniques such as low-energy electron diffraction and photoelectron diffraction. Such methods suffer, however, from the rather inefficient approach to data analysis: the diffracted intensities are compared with simulated data for a trial structure, which is successively modified by trial and error until good agreement is achieved. Following a suggestion by Barton that a photoelectron angular distribution may be regarded as a photoelectron hologram1,2, attention has been focused recently on simpler methods for adsorbate structure determination which give real-space information directly3–7. We demonstrate here a direct method for determining both the adsorption site and the adsorbate–substrate separation from photoelectron diffraction data8. We illustrate the method using two adsorption systems: CO on Cu(110) and OCH3 on Cu(111). Our results are consistent with those determined for the same systems by a related method7, which requires a considerably larger data set and provides bond lengths at lower precision. For adsorbates on simple surfaces, we therefore propose that our approach provides an accurate and easily implemented method for structure determination.

Experimental evidence for an oxidation/reduction mechanism in rate oscillations of catalytic co oxidation on pt/sio2

Abstract: In situ X-ray diffraction experiments have been conducted during rate oscillations of catalytic CO oxidation on a supported Pt catalyst, EuroPt-1. The measurements which were carried out at atmospheric pressure with flow rates of ∼ 200 ml/min showed that the non-isothermal oscillations in the reaction rate were accompanied by periodic intensity variations of a Bragg peak. A Debye function analysis of beam profiles recorded at the two extrema of the oscillations revealed that the Pt catalyst undergoes a periodic oxidation and reduction during rate oscillations. The diffraction experiments are therefore considered to be the first experimental proof that the oxide model proposed originally by Sales, Turner and Maple to explain rate oscillations in the CO + O2 reaction at atmospheric pressure is in fact correct.

On the mechanism of the selective oxidation of methanol over elemental silver

Abstract: We analysed previously the interaction of silver with oxygen and characterised three different atomic oxygen species. The present communication uses TPRS data to assign a chemical function to each of these three species prepared on a sample of practical electrolytic silver particles. With stationary and instationary conversion experiments close to practical conditions we confirmed the conclusions from the TPRS data also to hold qualitatively for the stationary operating catalyst. Surface oxygen was found to react in an oxydehydrogenation reaction with adsorbed methanol with a significant selectivity to total oxidation. “Sub-surface” oxygen catalyses the dehydrogenation of the adsorbed methanol with no selectivity to total oxidation. Dissolved atomic oxygen from the bulk replenishes both surface species via “sub-surface” oxygen. The interconversion of all three species at the high reaction temperatures required to overcome the barriers for the formation and motion of the various atomic oxygen species limits the overall selectivity of the formaldehyde production.

Zeolite MCM-22: Synthesis, Dealumination and Structural Characterization

Abstract: Zeolite MCM-22 was synthesized and characterized by selected physico-chemical methods. MAS NMR spectroscopic studies reveal that at least five crystallographically non-equivalent T-sites exist in the structure of MCM-22 and that they are affected to different degrees by dealumination with Sicl 4 vapors. From IR and TPDA experiments it can be concluded that the acidic properties of HMCM-22 are very similar to HZSM-5. A Spaciousness Index of ca. 8 is indicative of an effective pore width in the intermediate region between 10- and 12-membered ring zeolites.

Empirical one-electron model of optical transitions in Cu-doped ZnS and CdS

Abstract: Optical absorption and emission processes in the paradigmatic wide-gap II-VI compounds ZnS: Cu and CdS: Cu find an unconstrained interpretation in a plain strong-field model of light-induced electronic processes which depicts the charge states Cu2+ (d9) and Cu+ (d10), viz. their internal and charge transfer transitions. A variety of observed spectroscopic features between the UV and the IR regions are included, supplemented by some tentative assignments and/or predictions. Among the phenomena covered are the blue (B-Cu), green (G-Cu) and orange-red (R-Cu) emission bands of ZnS and their homologues in CdS. Novel near-infrared luminescence bands in ZnS peaking near 7200 and 9200 cm-1 are tentatively assigned to radiative hole-recombination at a Cu acceptor state and to the t52e3 → t42e4 internal transitions of Cu3+ (d8), respectively. Participation of a native donor turns out to be essential for some of the processes covered. Formation of deep-acceptor-bound excitons is demonstrated for several absorptive and emissive optical transitions.

Atomic and magnetic structure of fcc Fe/Cu(100).

Abstract: The atomic and magnetic structure of fcc Fe grown on Cu(100) are determined using spin-density-functional theory and the full-potential linear muffin-tin orbital method. Different types of interlayer and intralayer magnetism are investigated. For films of four up to eleven adsorbed Fe layers our ab initio total-energy calculations always give a ferromagnetic intralayer spin ordering of the Fe atoms. The surface and first subsurface layers couple ferromagnetically, whereas the deeper ones show interlayer antiferromagnetic coupling. This magnetic ordering is accompanied by a 3.9% expansion of the first layer spacing and a 1% contraction of the second, which agree well with low-energy electron diffraction analyses. We link the results to ground-state calculations for fcc iron.

Adsorbate-induced electronic modification of alkali-metal overlayers.

Abstract: For submonolayer coverages of Cs on Ru(0001) surfaces, the Cs-Ru vibration is observed at 7.8 meV (63 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) by means of high-resolution electron-energy-loss spectroscopy. At Cs coverages near the complete monolayer the adlayer becomes metallic as indicated by screening of the Cs-Ru vibration and the occurrence of an electronic excitation (plasmon) at 580 meV. Coadsorption of CO as well as of oxygen leads to the reappearance of the Cs-Ru vibration and the disappearance of the electronic excitation which is interpreted as a demetallization through chemical interaction with the coadsorbed species.

Solid-state ion exchange in zeolites. Part 5.—NH4-Y–iron(II) chloride

Abstract: Solid-state ion-exchange reactions proceeding upon grinding of hydrated NH4-Y zeolite and FeCl2·4H2O in air and subsequent heating in vacuum have been followed by X-ray diffraction (XRD) and temperature-programmed evolution of volatile reaction products (H2O, NH3 and HCl) monitored by mass spectrometry (MS). Changes in valence state and coordination environment of the exchanged iron ions upon heat treatment up to 720 K in vacuum and air, as well as in hydrogen after treatment in air, were studied by Mossbauer spectroscopy. Partial ion exchange and oxidation to cationic hydroxy-iron(III) species proceed upon mere grinding. Heat treatment in vacuum results in partial autoreduction and characteristic environmental changes of di- and tri-valent Fe species, especially in the temperature range between 620 and 720 K. In an oxidizing atmosphere, autoreduction is suppressed; however, at temperatures >520 K, iron(III) oxide is formed in increasing amounts. Reduction by hydrogen subsequent to heat treatment in air at 720 K results in the formation of a magnetite-like oxide phase and some metallic α-iron.

The dissociative adsorption of N2 on a multiply promoted iron catalyst used for ammonia synthesis: a temperature-programmed desorption study

Abstract: The temperature-programmed desorption (TPD) of N2 from a multiply promoted iron catalyst used for ammonia synthesis has been studied in a microreactor system at atmospheric pressure. From TPD experiments with various heating rates a preexponential factorA = 2 × 109 molecules/site s and an activation energyE = 146 kJ/mol was derived assuming second-order desorption. The observed dependence of the TPD peak shapes on the heating rates indicated the influence of readsorption of N2 in agreement with the results obtained for various initial coverages. Simulating the N2 TPD curves using the model by Stoltze and Norskov revealed that the calculated TPD curves were not influenced by the molecular precursor to desorption. However, the calculated rate of readsorption was found to be overestimated at high coverage compared with the experimental results. A coverage-dependent net activation energy for dissociative chemisorption (E*) was introduced as the simplest assumption rendering the dissociative chemisorption of N2 activated at high coverage. The best fit of the experimental data yieldedE* = (−15+30θ) kJ/mol using only a single type of atomic nitrogen species. These findings are in satisfactory agreement with the parameters underlying the Stoltze-Norskov model for the kinetics of ammonia synthesis as well as with the data reported for Fe(111) single crystal surfaces.

Influence of the initial- and final-state configuration interaction on the anisotropy of the resonant Auger decay of Kr 3d-15p and Xe 4d-16p states.

Abstract: The anisotropy of resonant Auger decay of photo-excited Kr 3${\mathit{d}}_{3/2,5/2}^{\mathrm{\ensuremath{-}}1}$5p and Xe 4${\mathit{d}}_{3/2,5/2}^{\mathrm{\ensuremath{-}}1}$6p states has been studied by multiconfiguration Dirac-Fock method. The calculations account for the configuration interaction both in the initial and in the final states of the Auger decay. For the nonresolved ${\mathit{nd}}^{\mathrm{\ensuremath{-}}1}$(n+2)p(J=1) resonances (n=3 for Kr and n=4 for Xe) the average intensities and anisotropies of Auger lines were calculated by weighing each partial rate by the pertinent Dirac-Fock photoexcitation probabilities. Our results show that, in addition to the initial- and final-state correlation, both the relaxation and the exchange interaction have a substantial effect on the anisotropy of these Auger spectra. For most Auger lines there is good agreement between our calculated \ensuremath{\beta} parameters and experimental values for the Kr and Xe ${\mathit{nd}}_{5/2}^{\mathrm{\ensuremath{-}}1}$(n+2)p photoexcitation resonances; there is also satisfactory agreement for the Kr 3${\mathit{d}}_{3/2}^{\mathrm{\ensuremath{-}}1}$5p and Xe 4${\mathit{d}}_{3/2}^{\mathrm{\ensuremath{-}}1}$6p resonant Auger spectra. However, the remaining notable discrepancies between theory and experiment indicate that important correlation effects are still omitted in our calculations. Discrepancies between different experimental results stress the need for further improvements on the experimental side.

Nonadiabatic processes during the oxidation of Li layers

Abstract: The first stages of oxidation of thin Li films (leading to the formation of ${\mathrm{Li}}_{2}$O) were found to be accompanied by emission of ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ ions as well as electrons, reflecting the participation of electronically highly excited states in this reaction. A model is proposed and supported by the results of local-spin-density-approximation cluster calculations whereby electron transfer from the metal onto the impinging ${\mathrm{O}}_{2}$ molecule leads to formation of a transient ${\mathrm{O}}_{2}$ $^{2\mathrm{\ensuremath{-}}}$ species. This species dissociates without a noticeable activation barrier and there is a finite (but rather low) probability that one of the ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ fragments formed near the surface is ejected into the gas phase. The ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ species at the Li surface forms, on the other hand, a hole state, which subsequently transforms into the ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$ ground state. For excitations larger than the work function, the energy associated with its decay (g2 eV) may be released in an Auger process associated with electron emission. The yield of light emission was found to be below the detection limit of about ${10}^{\mathrm{\ensuremath{-}}10}$ photons per reacting ${\mathrm{O}}_{2}$ molecule and indicates a short lifetime (100 fs) of the ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ species at a Li surface.