Molecular Engineering of Electrode Surfaces for Electrocatalysis

Electrocatalysts play an important part on the route towards environmental friendly energy sources. They support modern technologies like fuel cells to move further away from the utilization of fossil fuels and the resulting CO2 emission into our atmosphere. The main principle of such energy conversion technologies is to store chemical energy within chemical bonds and to release it as electric energy to fulfill the energy demand of today's society. However energy conversion chemistry in such technologies requires catalysts in order to be effective by providing an alternative reaction pathway that lowers activation energies and maximizes the energy output. Providing suitable catalyst surfaces requires a fundamental knowledge of the interaction of reactants and reaction intermediates with the catalyst surfaces. This concern is addressed in this thesis by fabricating specifically tailored molecular components on metal surfaces, which act as organic/inorganic hybrid electrodes for various energy conversion reactions in electrocatalysis. Characterizing the electrode surface is crucial in order to identify active sites of the catalyst. The molecular structure is investigated by surface sensitive techniques, such as STM and XPS in combination with a home-build transfer system, which enables electrode fabrication in UHV and electrocatalytic experiments in a liquid environment. A detailed characterization of organic polymer components on electrode surfaces is obtained by combining experimental data with MD simulations. The morphology of polymer networks can be successfully mimicked and depending on the chemical composition and the structure, spatial correlations on short length scales are reported. Hybrid electrodes with organic polymer co-catalysts reveal an increased catalytic activity for the HER by providing suitable docking sites that act as catalytic active sites. Reactants and reaction intermediates are stabilized via hydrogen bonding, which results in a higher catalytic turnover. The specific designed structure of the organic co-catalyst helps to identify a fraction of the reaction mechanism and highlights the importance of molecular components in electrocatalysis. The utilization of organic units on electrocatalysts provides also an excellent opportunity in order to use single metal atoms as catalysts. Engineering large pi-systems has no significant impact on the ORR activity, while the distance between active sites and the underlying electrode are crucial for enhancing the catalytic activity. A significant factor in using molecular components in electrocatalysis is the stability of the organics. Using electrografting allows to design organic/inorganic hybrid electrodes with covalent bonds between the molecular component and the metal substrate. Attaching empty porphyrin molecules to substrates introduces a powerful technique in molecular engineering, since empty porphyrin cycles can be metallized with various metal centers that can act as electrocatalysts for different electrochemical reactions. In summary, this thesis provides multiple engineering aspects, which are useful in designing molecular components on metal electrodes for energy conversion. Such hybrid catalyst surfaces are capable of improving the catalytic activity and allow to study the mechanism of catalytic reactions. Implementing organic components provides new routes for tailoring novel materials, which deliver new insight into the complex field of electrocatalysis.

Chemical and electrochemical promotion of supported rhodium catalyst

Olena Baranova

The chemical and electrochemical promotion of highly dispersed nanofilm Rh catalysts (dispersion: about 10 %, film thickness: 40 nm) has been investigated for the first time. To this end Rh metal was sputter-deposited, either on a purely ionic conductor (8 % Y2O3-stabilized ZrO2) or on a mixed ionic-electronic conductor (TiO2), the latter being a highly dispersed layer of TiO2 (4 µm) deposited on YSZ. These catalysts are designated as Rh/YSZ and Rh/TiO2/YSZ, respectively. It was established analytically that both in the Rh/YSZ and in the Rh/TiO2/YSZ system, the catalyst films have a nanoparticle-size grain structure. The Rh supported on titania is rather porous, exhibiting a higher dispersion and surface area than Rh on YSZ. Both after reduction (H2, T=400 °C) and after oxidation (O2, T=400 °C), Rh supported on TiO2 was found to be in a more highly reduced state than Rh on YSZ. After reducing treatment, the Rh/TiO2/YSZ samples contain a larger amount of weakly bonded oxygen, which can be attributed to oxygen "backspillover" from the TiO2. A major feature of this research was the electrochemical characterization of the oxygen/Rh/solid electrolyte three-phase boundary by steady-state polarization measurements and by impedance spectroscopy. These are powerful techniques for extracting experimental trends and details that are useful for an understanding of the electrochemical promotion principles. It was shown that the exchange current densities at the Rh/solid electrolyte interface are lower on account of the TiO2 layer. The exchange current densities are more than twice lower at Rh/TiO2(4 µm)/YSZ than at Rh/YSZ, demonstrating that the former interface is much more polarizable. The mechanism of oxygen exchange occurring close to equilibrium (O2/O2- couple) was investigated for the first time at Rh catalyst electrodes interfaced with solid electrolyte. The processes of cathodic oxygen reduction and anodic oxygen evolution are not symmetric, but they are similar in the two systems, Rh/YSZ and Rh/TiO2(4 µm)/YSZ. The cathodic process consists of three steps: dissociative adsorption of oxygen at the gas-exposed Rh surface, atomic oxygen diffusion to the electrochemical reaction sites (ERS), and a two-electron transfer to this oxygen on the ERS. The cathodic process is limited by interfacial diffusion of oxygen atoms from the gas-exposed metal surface to the ERS. The anodic process includes two steps: two-electron transfer reaction, which is the rate-determining step, and oxygen desorption to the gas phase. With data obtained from impedance spectroscopy at the equilibrium potential, it was possible to confirm the reaction scheme proposed. It was demonstrated that Rh nanofilm catalysts interfaced with YSZ or TiO2/YSZ can be electrochemically promoted for the reaction of ethylene oxidation. Small anodic currents cause periodic oscillations in catalytic rate and potential of the Rh/YSZ catalyst, while at Rh/TiO2/YSZ they give rise to a stable and reversible rate enhancement by up to a factor 80. The increase in ethylene oxidation rate is up to 2000 times larger than the electrochemical rate, I/2F, of O2- oxidation. The pronounced electrochemical promotion behavior that has been observed is due to the anodically controlled migration of O2- species from the electrolyte to the Rh/gas interface. At the Rh surface, these species destabilize the formation of rhodium surface oxide (Rh2O3). The existence of backspillover oxygen species has been confirmed by impedance measurements under positive applied potential. Another significant result for heterogeneous catalysis is the finding that thick as well as thin films of Rh/TiO2/YSZ catalyst are open to chemical promotion of ethylene oxidation and partial methane oxidation, ie, they offer a higher catalytic activity and stability than the Rh/YSZ catalysts. The modification of catalytic activity observed for Rh/TiO2/YSZ was attributed to either a "long-range" electronic-type SMSI mechanism or to a self-driven electrochemical promotion mechanism. In both cases, the ultimate cause of promotion are different work functions of catalyst and support. Equilibration of the work functions of two solids in contact induces surface charging, a migration of O2- ions to the catalyst/gas interface, and a weakening of the Rh-O chemisorptive bonds. It facilitates reduction of oxidized surface sites. Another important achievement of the present work was that of exploring and confirming the possibilities of current-assisted activation of Rh/TiO2/YSZ catalysts. In partial methane oxidation using close to stoichiometric gas compositions (CH4 : O2 = 2 : 1) at 550 °C, the inactive (oxidized) Rh/TiO2/YSZ catalyst was successfully activated by applied currents, either positive or negative. This phenomenon is an example of "permanent" electrochemical promotion furnishing a permanent rate enhancement ratio of γ = 11. The activation by negative currents is explained in terms of an electrochemical reduction of rhodium surface oxide, while the activation by positive currents can be explained by the mechanism of electrochemical promotion.

EPFL2005

Single-molecule-level characterization of supramolecular systems at surfaces

Dietmar Payer

This thesis deals with the self-organization of individual building blocks, specifically organic molecules and metal atoms, into supramolecular structures on metal surfaces under ultra high vacuum conditions (UHV). Self-organization of supramolecular systems is a promising candidate for a bottom-up approach in the fabrication of complex structures with specific properties. This process is steered by interactions between functional groups of the individual building blocks and is of apparent interest to gain a detailed understanding of the influence and the behavior of these functional groups enabling intermolecular binding. In the first part of the thesis, we investigate hydrogen bonded structures on metal surfaces. Of especial interest is the possibility of the formation of ionic hydrogen bonds, a special class of hydrogen bond which connects a charged donor (acceptor) and a neutral acceptor (donor) and is important in biological systems. By combined Scanning Tunneling Microscopy (STM), X-ray Photoelectron Spectroscopy (XPS) and Near Edge X-ray Absorption Fine Structure (NEXAFS) measurement we show the deprotonation of carboxylic functions in the molecules, forming carboxylate groups involved in intermolecular binding. With our complementary experimental and theoretical examinations we clearly show that the carboxylate groups are indeed still charged although adsorbed onto a conducting metal surface and that the observed structures can be computationally reproduced by using these charges in classical molecular dynamic calculations. Our investigations clearly reveal that all requirements for the formation of ionic hydrogen bonds in these samples at metal surfaces under solvent free conditions are fulfilled. In the second part, the confinement of surface state electrons in self-assembled hydrogen-bonded structures is investigated. The examined structure contains two-dimensional cavities, with different inner diameters in the periodic arrangement and in domain boundaries. Inside the cavities we detect a spatially localized modification in the local density of states (LDOS) of the Ag(111) surface state electrons. The energy of the peak correlates with the cavity area as predicted by a simple "particle in a box" model. This clearly shows that the surface state electrons form a confined state inside the cavities. Furthermore we examine self-assembled periodic structures as templates for binding single macrocyclic molecules. The macrocyclic molecules can be deposited by sublimation, as STM-topographs with submolecular resolution reflect the shape of the intact molecules and reveal a coverage dependent aggregation behavior. We show that it is possible to fabricate a structure in which at most one macrocyclic molecule per pore can be found in a defined binding position. In the last part of the thesis we investigate the influence of individual metal atoms on the chemical activity and the self-assembly process of molecules on metal surfaces. First, we demonstrate the high chemical reactivity of these metal atoms. In our experiments we address the chemical reactivity of static active sites, like kinks and step edges, and of dynamic active sites, like the metal atoms, separately. Doing this we show that a non-negligible amount of the chemical reactivity of a metal surface is due to this dynamic adatom gas. Furthermore the influence of metal atoms on the structure formation of organic molecules is investigated. We show that two-dimensional structures based on a complex formation between metal centers and ligands can be formed and that the coordination number is independent of the substrate symmetry. Furthermore, we show the possibility that on metal surfaces one can prepare three-fold coordinated Fe-centers, a coordination number which is rarely seen in three dimensional chemistry. In the last part we study catenanes, which consist of two interlocked macrocyclic molecules. Main interest is on the confirmation of a structural change of the catenanes adsorbed on a metal substrate. For our experiments we use the fact that the catenanes are designed to "trap" a Cu-atom by complex formation. As this reaction is associated with a structural change and a change in intermolecular interaction, we can verify a reaction of catenanes adsorbed onto a surface with Cu adatoms and therefore a structural alteration by simply imaging the formed structures. Our measurements show the possibility of depositing large molecules like catenanes by sublimation and of their potential for use as molecular machines adsorbed onto metal surfaces.

EPFL2007

Two-Dimensional Metal-Organic Networks as a New Class of Electrocatalysts

Benjamin Wurster

This thesis deals with the electrocatalytic properties of two-dimensional metal organic coordination networks (2D-MOCNs) self-assembled on electrode surfaces and catalytic activity of 2D-MOCNs is demonstrated for the first time. The low-coordinated metal centers within these supramolecular structures resemble the catalytically active sites of metallo-enzymes. The activity of the metal centers is determined by the type of the metal and the coordination environment; both parameters can be carefully adjusted in 2D-MOCNs. In order to study and correlate electrocatalytic properties with the atomic structure of 2D-MOCNs, a scanning tunnelling microscope operating in ultra-high vacuum (UHV) was combined with an electrochemical (EC) setup. Sample preparation and scanning tunneling microscopy (STM) were conducted under UHV conditions. By STM the composition and structure of the networks were controlled and characterized prior to EC experiments. A transfer system between UHV and EC instrumentation was constructed to keep the sample at all times in a clean and controlled environment. It is demonstrated that the mechanism of the oxygen reduction reaction (ORR) can be influenced by the type of the metal center in the network. The networks formed by benzene-tricarboxylic acid (TMA) with either Fe or Mn atoms are structurally identical, but their electrochemical signal differs significantly. Both networks catalyze the reduction of O2 to H2O, but on different pathways. These results emphasize the key role of the unsaturated metal centers in the electrocatalytic reduction. The electrocatalytic response in the ORR can also altered by the distinct coordination environment; this is shown for Fe atoms nitrogen-coordinated by either tetracyanoquinodimethane (TCNQ), bis-pyridyl-bipyrimidine (PBP) or phythalocyanine (Pc). Depending on the specific coordination environment formed by the different ligands the the final product (H2O2 or H2O) and the mechanism of the ORR is changed. In the last part of this thesis the two metal binding sites of 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP) are used to selectively incorporate different metal centers in a fixed organic environment and create homo- and hetero-bimetallic networks. The coupling of the two metals centers promoted by the organic environment modifies the electrocatalytic response. The correct combination of two metal centers in the right positioning makes an efficient catalyst. In the ORR the peak and onset potential is varied depending on the metal combination. In the case of the oxygen evolution reaction a non-linearly increased catalytic activity by the cooperative bimetallic effect is obtained. The results presented in this thesis demonstrate the high potential of 2D-MOCNs for heterogeneous catalytic chemical conversions. Engineering of the network structure and the distinct coordination environment of the metal centers offers the possibility to tune their catalytic activity. This opens up a new route for the design of a new class of nanocatalyst materials.

EPFL2015