Adsorption Sites of Metal Atoms on MgO Thin Films and Rotational Quantum State Spectroscopy of Physisorbed H₂

The work presented in this thesis covers two different topics of surface science, both investigated with the scanning tunneling miscroscopy and spectroscopy at low temperatures (5~K). First, we are interested in the spectroscopic properties of physisorbed H2 (and D2) layers on metallic samples. The energy of the first rotational quantum state transition for H2 (D2) is characterized on a variety of metallic substrates. For homonuclear molecules, these rotational quantum states are directly dependent on the nuclear spin arrangement. Thus the rotational spectroscopy of molecular H2 (D2) allows for unprecedented spatial resolution of nuclear spin configurations. To isolate one physisorbed molecule from the surface, we grow Xe islands before exposing the sample to H2 (D2). Reducing the Xe islands size to support only one molecule would allow us to probe the rotational quantum state at the individual molecule level. We show that rotational spectroscopy of H2 (D2) on Xe islands is feasible. We find that this method is not viable to isolate a H2 (D2) molecule as small Xe islands become unstable in presence of them. The second topic deals with the adsorption of metallic atoms on thin MgO(100) films grown on a Ag(100) substrate. We use Ca-doping of the MgO layer to identify the Mg lattice positions. Subsequent adsorption of Ho allows the determination of their adsorption site, on top of the O and on the bridge site, halfway between two O (Mg) lattice positions. The abundance of the population over these sites differs with the MgO thickness. On the MgO monolayer, the Ho adatoms are distributed following the abundance of these sites, i.e., 1/3 and 2/3, respectively. On the MgO bilayer, exclusively the O site is populated. Atomic manipulation of the Ho adatoms with the STM tip permits the creation of a new species, adsorbing on the Mg site, which was correctly predicted by DFT. Adsorption sites for other metal atoms have further been measured. Dy and Er adsorb on the bridge and O site, with abundances depending upon the atomic species and the MgO thickness. Au adsorbs exclusively on bridge sites. Fe and Co populate uniquely the O site, independent of MgO thickness, while Tb adsorbs on all the three sites, i.e., O, bridge, and Mg. We finally investigated the Ho adsorption sites as a function of deposition and annealing temperature. Ho atoms on the O site become unstable at a temperature between 58 K and 77 K, while Ho atoms on the bridge site start to diffuse only at 125 K. This results in a hopping to the bridge site of all the Ho adatoms adsorbing on O between 58 K and 77 K. Additionally, we show that evaporation of Ho on the MgO bilayer held at 30 K gives rise to adsorption on almost uniquely bridge sites, in neat contrast to what is found for lower temperature (approximatively 10 K). We interpret this result as the action of entropy which changes the free energy of the two types of adsorption site as the temperature increases. Ideas for the source of entropy are given, but the amount of entropy needed to explain our observations is unrealistically large.

Nitrous Oxide Abatement over Fe-Zeolite

Bryan Bromley

The use of zeolites for catalytic reactions is a field in continuous development. Since it was demonstrated that metal-zeolites are efficient catalysts for NOx abatement, decomposition of nitrous oxide (N2O) over Fe-containing zeolites has attracted great interest by the scientific community mainly due: N2O is the third most important gas contributing to global warming, its concentration in atmosphere is still on the rise and direct N2O decomposition to N2 and O2 is a sustainable alternative. N2O interaction with Fe-zeolite leads to the formation of "special" adsorbed oxygen (often called α-oxygen) with great potential for selective oxidation. Although the focus of a number of studies, the nature of the actives sites for the formation of this adsorbed oxygen is still unclear. The main objectives of this work are: 1) study the decomposition mechanism of N2O over isomorpously substituted Fe-ZSM-5 zeolite aiming on catalyst optimization and 2) optimization and further use of structured Fe-zeolite in novel micro-structured reactor. A number of refined transient kinetics methods were intensively used such as transient response, Temperature Programmed Desorption (TPD) and Temporal Analysis of Product (TAP). Density Functional Theory (DFT) and in situ IR spectroscopy studies were conducted in collaboration and our results support the following: In chapter 3, quantitative measurements have been carrier out to analyze the effect of temperature on the activation (under helium flow) and the irreversible deactivations that can happen during the pretreatment. The results obtained indicate that high temperature treatment (1273 K) results in the formation of a high amount of α-sites. The addition of a very low fraction of oxygen (2%) in the feed demonstrates that the catalyst is sensitive to its presence. Certain sites can be irreversibly destroyed, a result ascribed to the oxidation of active Fe(II) to Fe2O3 clusters which contain inactive Fe(III). Catalyst treatment at high temperature (1273 K) in He for up to 3 hours, also reduced irreversibly the concentration of α-sites. TPD measurements have shown desorption of oxygen at 3 different temperatures. Desorption at the lowest temperature (650 K) was assign to the α-oxygen recombination. The oxygen which desorbs at 700 K was associated to the oxygen from the deactivated sites. Oxygen which desorbs at the highest temperature (730 K) is originated from the surface NOx that desorbed as NO and O2. After a comparison of the concentration of the molecules accumulated, we propose that NO2 is the adsorbed specie. The mechanism of N2O decomposition on the binuclear oxo-hydroxo bridged extraframework iron core site [FeII(μ-O)(μ-OH)FeII]+ inside the ZSM-5 zeolite has been studied by combining theoretical and experimental approaches (see chapter 4). Rate parameters computed using standard statistical mechanics and transition state theory reveal that elementary catalytic steps involved into N2O decomposition are strongly dependent on the temperature. This theoretical result was contrasted with the experimentally observed steady state kinetics of the N2O decomposition and TPD experiments. As predicted by the theoretical study, the switch of the reaction order with respect to N2O pressure (from zero to one) occurs at around 800 K which confirms a change of the rate determining step from the α-oxygen recombination to α-oxygen formation. The proposed mechanism was also confirmed by O2-TD which confirmed the viability of the binuclear complex. The α-oxygen recombination at low temperature (

EPFL2010

Heterogeneous kinetics of some atmospherically relevant reactions

A low pressure reactor (Knudsen cell) coupled to molecular beam modulated mass spectrometry was used to study the heterogeneous kinetics of the following three systems: (1) NO2 on amorphous carbon, (2) H2O vapor on ice and (3) gaseous HCl on ice. The first system was investigated at ambient temperature while the two last ones were investigated at temperatures typical of the stratosphere, that is roughly between 160 and 220K. The solid phase samples correspond to model surfaces whose physicochemical properties come very close to the ones of atmospheric particulates. Firstly, we studied the interaction of gaseous NO2 with three different commercial samples of amorphous carbon having widely differing physicochemical properties. Using in situ laser-induced fluorescence, the only major product was unambiguously determined to be NO with the oxidation product of carbon apparently remaining on the surface. Probing the gas phase with mass spectrometry (MS), both pulsed valve dosing and steady state experiments were performed and revealed a complex reaction mechanism for both the uptake as well as product formation. The initial uptake coefficient γo for NO2 was (6.4±2.0)·10-2 and proved to be identical within experimental uncertainty for all three types of amorphous carbon. The initial NO2 uptake rate was independent of the mass of the carbon sample and scaled with its external geometrical surface. Applying a simple chemical kinetic model to both pulsed dosing and steady state experiments resulted in an effective surface for uptake on amorphous carbon ranging between a factor of 2.8 to 8.4 larger than the geometrical surface area, but inversely proportional to the measured BET surface area of the three types of amorphous carbon. This means that the amorphous carbon with the highest BET surface (FW2) had the lowest external surface for NO2 adsorption. The chemical kinetic model included competing processes between Langmuir-type adsorption and inhibition controlling the adsorption kinetics of NO2 and revealed that the NO generation rate differed greatly between the three carbon samples examined. All samples showed saturation effects of differing degree that were partially reversible through prolonged pumping at 10-4 Torr and/or heating. Virgin amorphous carbon samples did not take up H2O vapor at 20 mTorr, and no HONO and/or HNO3 was detected in simultaneous NO2/H2O exposure experiments. CO and CO2 were detected when a sample previously exposed to NO2 was heated by an incandescent lamp. Moreover, upon heating such a sample, a MS signal m/e 62 originating from NO3 and/or N2O5 was detected. In addition to the experiments conducted on the three commercial carbon samples, we carried out uptake experiments of NO2 on acetylene soot. Originating from the flame of an acetylene burner, this soot was freshly deposited on glass dishes before each experiment. The initial uptake coefficient γo for NO2 was measured to be (3.0±1.1)·10-2 is smaller by a factor of two compared to γo observed in the uptake on commercial samples whose mass was a factor of 10 higher. In addition to NO, a net formation of HONO (m/e 47) was observed. This HONO formation is strongly dependent on the instantaneous water desorption from the sample as well as on the NO2 partial pressure in the reactor. On the other hand, it is not enhanced by an external water flow which did not show any interaction with the carbon sample. Upon heating the sample using an incandescent lamp, significant amounts of water desorbed, suggesting that the water vapor had condensed into the micropores of the soot during combustion. No quantitative results are presented yet. However, as no correlation between HONO formation and NO partial pressure has been observed so far, we suggest a reaction mechanism that only involves NO2 and H2O but no NO: 2 NO2 (g) + H2O HONO + HNO3 In a second study, the kinetics of condensation and evaporation of water on ice was determined in the temperature range of 170 to 220K. The rate of evaporation Fev corresponds to the evaporation of 70±10 monolayers of H2O per second at 200K. The condensation rate constant kc has a negative activation energy of -3.l±1.5kcal/mol which is significantly larger than the one recently measured by Haynes et al. We report the first real time kinetic measurements of water condensation on ice at stratospherically relevant temperatures. At temperatures above 160K, pulsed dosing experiments show a dose dependence of the condensation rate coefficient. For example, the condensation rate coefficient increases by a factor of five with a 1000 fold increase of the H2O dose from 1·1015 to 1·1018 molecules per pulse at 180K. This pressure dependence of the condensation rate constant may be explained by an autocatalytic mechanism involving the competition of two condensation channels. These two channels feed two different surface species, one of which leads to autocatalytic condensation. This mechanism gives insight into the manner in which a gas phase molecule is stepwise incorporated into tetrahedrally coordinated bulk ice. In a third and ongoing study, the uptake kinetics of HCl on ice has been investigated by pulsed valve experiments in the temperature range of 180 to 210K and are discussed as a function of the state of the surface. In fact, some of the pulsed valve experiments present a relaxation to a steady state level. It is shown that this level originates from the evaporation of a liquid solution corresponding to a temperature specific HCl concentration. The threshold of the injected dose leading to this solution layer is estimated to be 8·1014 molecules. This threshold corresponding to a fraction of a monolayer may be explained by the formation of small domains along grain boundaries or other lattice imperfections. Within detection limit, no difference in keff has been observed for pulses on the solution or on a previously exposed sample without formation of the solution. In the case of pulses forming a solution layer, a fraction of 20±20% of the injected molecules remains permanently adsorbed on the surface. In the temperature range from 210 to 180K, the pulsed valve experiments yield values of keff roughly between 5 and 25s-1 and an activation energy of -1.6±0.7kcal/mol which are consistent with the values observed by Flueckiger in steady state experiments using the 15mm escape aperture (kuni varying from 12 to 23s-1 and an activation energy of -1.9±0.25 kcal/mol).

EPFL1996

TCNQ-based Supramolecular Architectures at Metal Surfaces

Tzu-Chun Tseng

Molecular self-assembly at metal surfaces has been recognized as an efficient strategy to create supramolecular nanoarchitectures with promising functionalities. The focus of this thesis lies on the strong electron accepting molecule 7,7,8,8-tetracyanoquinodimethane (TCNQ). The self-assembly of TCNQ and metal-TCNQ coordination networks on metal surfaces were investigated. The structural formation is directed by non-covalent interactions. By controlling fabrication parameters, surface coordination nanostructures with different chemical composition or molecular packing have been synthesized and characterized by scanning tunneling microscopy (STM). The electronic and magnetic properties of the surface organic or metal-organic nanostructures were studied by a combination of x-ray absorption spectroscopy (XAS) and x-ray photoelectron spectroscopy (XPS). The thesis is organized into three parts. In the first part, the self-assembly of TCNQ molecules on Cu(100) and Ag(100) surfaces will be discussed. The two substrates provide different chemical environments as exhibited by the surface mobility of TCNQ. Charge transfer from either Cu(100) or Ag(100) substrate induces conformational bending of the -C≡N cyano groups of TCNQ, which consequently facilitates the bonding of molecules to surfaces via nitrogen lone pairs. As shown by density functional theory (DFT) calculations, on Cu(100), the substrate-cyano interaction leads to local substrate reconstructions at the proximity of cyano groups. These charge-transfer induced structural modifications at the TCNQ/Cu(100) interface and the electrostatic interaction between molecules determine the self-assembly of TCNQ into long-range ordered domains. The adsorption and self-assembly of TCNQ on Ag(100) is presumably governed by a similar mechanism as concluded from the molecular packing and charge transfer phenomenon. In the second part of this thesis, the fabrication of MTCNQx (M = Mn, Fe, Co, and Ni) coordination complexes on metal surfaces is presented. The molecular adsorption geometry in all structures indicates the participation of the cyano-substrate interaction in determining the final structures, except the x = 1 case in which all cyano groups are coordinated to metal adatoms. The competition between the coordination of TCNQ to metal adatom and TCNQ to substrate is shown by the growth phenomena on the Cu(100) and Ag(100) substrates. The weaker substrate interaction provided by the Ag(100) substrate results in multiple ordered phases (x = 1, 2, or 4), in contrast to the single ordered x = 2 phase on Cu(100). The MTCNQ2 networks synthesized on both substrates are isostructural apart from a rotation of the network orientation to adapt the substrate periodicities. This highlights the importance of the substrate lattice in guiding the growth of supramolecular networks as well as the robust intra-network interaction to obtain a stable structure. The inherent chemical reactivity of Mn, Fe, Co and Ni atoms, which can be further modified upon adsorption due to substrate hybridization, strongly influences the resultant coordination structures. In addition to metal coordination, hydrogen bonding also plays a role in the stabilization of metal-TCNQ networks. Adjusting the substrate temperature has been an important and effective method to tune the interplay between adsorbate-adsorbate and adsorbate-substrate interactions. These experiments provide a systematic investigation on the formation of metal-TCNQ coordination structures on metal surfaces. In the last part of this thesis, the electronic and magnetic properties of the MTCNQx complexes on metal surfaces will be discussed. XPS measurements and DFT calculations show that both the substrate and metal coordination atoms contribute to the electron donation to TCNQ molecules. The magnetic properties of the metal coordination centers were studied in x-ray magnetic circular dichroism (XMCD) measurements. The Mn centers are found to be paramagnetic from 300 K to 8 K. The Mn-coordinated networks represent ordered arrays of magnetic atoms carrying a high spin moment of S = 5/2. In the case of Ni-coordinated networks, the magnetism of Ni centers is induced by TCNQ coordination and dehybridization from the substrate. These results show the interesting physical phenomena emerging in two-dimensional metal-organic coordination systems.

EPFL2010

Heterogeneous kinetics of some atmospherically relevant reactions

EPFL1996

TCNQ-based Supramolecular Architectures at Metal Surfaces

Tzu-Chun Tseng

EPFL2010

Nitrous Oxide Abatement over Fe-Zeolite

Bryan Bromley

EPFL2010