Ensemble Effect Evidenced by CO Adsorption on the 3-Fold PdGa Surfaces

The atomic structure and composition of a catalyst's surface have a major influence on its performance regarding activity and selectivity. In this respect, intermetallic compounds are promising future catalyst materials, as their surfaces exhibit small and well-defined ensembles of active metal atoms. In this study, the active adsorption sites of the 3-fold-symmetric surfaces of the PdGa interrnetallic compound were investigated in a combined experimental and computational approach using CO as a test molecule. The PdGa(111) and (-1-1-1) surfaces exhibit very similar electronic structures, but have Pd sites with very different, well-defined atomic coordination and separation. They thereby serve as prototypical model systems for studying ensemble effects on bimetallic catalytic surfaces. Scanning tunneling microscopy and Fourier transform infrared spectroscopy show that the CO adsorption on both surfaces is solely associated with the topmost Pd atoms and Ga acts only as an inactive spacer. The different local configurations of these Pd atoms dictate the CO adsorption sites as a function of coverage. The experimental results are corroborated by density functional theory and illustrate the site separation and ensemble effects for molecular adsorption on intermetallic single crystalline surfaces.

Design of Surface-Supported Bio-inspired Networks for CO2 and O2 Activation at Room Temperature

Daniel Eduardo Hurtado Salinas

The structure of a biological system defines its function. Therefore, nature has developed sophisticated systems that catalyze chemical reactions that are vital for life on earth. For instance, the photosynthesis is a biological process that is partially regulated by the metallo-enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase), whose function is to catalyze the capture and transfer of CO2 into organic molecules. This carboxylation reaction occurs at the active site, where a magnesium ion (Mg2+) is bound to organic molecules with carboxylate (COO-) and amine (NH2) functional groups. In the last decades, supramolecular chemistry has proven to be a valid approach to replicate the structural characteristics of these natural metal-organic systems. Metal atoms and organic molecules are used as building blocks to form two-dimensional (2D) metal-organic networks (MONs) on metal surfaces, through a self-assembly process that occurs spontaneously. In these systems the molecules bind to the metal centers and control their oxidation states that, as found in metallo-enzymes, is crucial to perform a catalytic task. Therefore, these supramolecular assemblies show promising features to be explored for heterogeneous catalysis. The main objective of this thesis is to reverse the structure-function equation, that is: instead of finding specific functions of well-known 2D structures for future applications, 2D structures are designed to reproduce the specific functions of well-known bio-systems. Thus, inspired in the structure of the active site of RuBisCO, 2D Mg-based supramolecular assemblies are rationally designed on Cu(100) or Mg(0001) surfaces at room temperature (RT) as a platform for CO2 adsorption. This work is the first study of self-assembly of alkaline earth metal atoms (Mg) with organic molecules ((1,4-benzenedicarboxylic acid (TPA) and 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT)) on metal surfaces. The first part of this thesis investigates the self-assembly of TAPT molecules. Scanning Tunneling Microscopy (STM) and High Resolution X-ray Photoemission Spectroscopy (HR-XPS) demonstrate that the molecules typically semi-deprotonate to form MONs with the adatoms from the host substrates. The exposure of these structures to CO2 results in the carboxylation of the amino groups. The second part proves that the COOH moieties of TPA molecules fully-deprotonate to form homo-molecular structures. The incorporation of Mg adatoms result in ionic MONs, which reproduce the carboxylate environment of the active site of RuBiSCO. STM, XPS and Density Functional Theory (DFT) analyze the structural changes, the chemical reactions and charge transfer during either the CO2 or O2 adsorption on the Mg2+ centers. The last part focusses on the co-deposition of both molecules to form 2D metallic-hetero-molecular networks with Mg2+ centers. The role of the Mg2+ centers as active sites for molecular adsorption and model systems for gas storage is studied. STM reveals that CO2 partially modifies the network towards a configuration that resembles the active site of RuBisCO. The formation of 3D structures to enhance CO2 adsorption is also evaluated.

EPFL2018

Adsorption of Small Hydrocarbons on the Three-Fold PdGa Surfaces: The Road to Selective Hydrogenation

Harald Brune, Daniele Passerone, Jan Prinz, Roland Widmer

Intermetallic compounds are a promising class of materials as stable and selective heterogeneous catalysts. Here, the (111) and (-1-1-1) single crystal surfaces of the PdGa intermetallic compound were studied as model catalysts with regard to the selective hydrogenation of acetylene (C2H2) to ethylene (C2H4). The distinct atomic surface structures exhibit isolated active centers of single atomic and three atomic Pd ensembles, respectively. For the two prototypal model catalyst surfaces, the adsorption sites and configurations for hydrogen (H-2), acetylene, and ethylene were investigated by combining scanning tunneling microscopy, temperature-programmed desorption, and ab initio modeling. The topmost Pd surface atoms provide the preferred adsorption sites for all studied molecules. The structural difference of the Pd ensembles has a significant influence on the adsorption energy and configuration of C2H2, while the influence of the ensemble structure is weak for C2H4 and H-2 adsorption. To approach the question of catalytic performance, we simulated the reaction pathways for the heterogeneous catalytic hydrogenation of acetylene on the two surfaces by means of density functional theory. Due to the geometrical separation of the Pd sites on the surfaces, the steric approach of the reactants (H and C2Hx) was found to be of importance to the energetics of the reaction. The presented study gives a direct comparison of binding properties of catalytic Pd on-top sites vs three-fold Pd hollow sites and is therefore of major relevance to the knowledge-based design of highly selective hydrogenation catalysts.

American Chemical Society2014

Magnetism of individual atoms adsorbed on surfaces

Harald Brune, Pietro Gambardella

Magnetic impurities in solids cause manifold changes in their macroscopic properties, such as anomalous low temperature resistance due to Kondo screening, reduction of the superconducting transition temperature due to local suppression of the order parameter, they create magnetic signatures in semiconductors, and lead to inelastic spin excitations in tunnel junctions. In the present paper we review what has been learnt about these effects from a surface science approach. Placing the magnetic impurities at well defined adsorption sites on single crystal surfaces makes their effect on the host, as well as their own magnetic properties better accessible to experiments, and also better understandable since the atomic environment of the impurity is exactly known lending comparison with theory more direct. After an introduction we discuss X-ray magnetic circular dichroism measurements which are spatially averaging and therefore report on ensemble properties. One of the recent progresses achieved in surface science is the preparation of well defined ensembles, such as surfaces with only single adatoms, each of them in an identical atomic environment and with sufficient mutual distance to exclude interactions. Due to this approach we can now determine the electronic configuration of individual adatoms, their hybridization with the host, and quantify their spin and orbital moments, as well as the spin–orbit induced magneto-crystalline anisotropy, which can be orders of magnitude larger than thin film and bulk values. In the second part we review recent progress in revealing the magnetic properties of individual atoms with the scanning tunneling microscope (STM). With this technique the spatial extent of the Kondo screening cloud and of subgap excitations in the superconductor quasiparticle density of states became apparent. We outline the first pioneering experiments measuring transport through reversible atomic point contacts containing magnetic atoms and measurements using the subgap features caused in superconducting STM tips to detect the magnetism of individual atoms. We then describe experiments using inelastic spin excitation spectroscopy to pin down the magnetic ground state and anisotropy energy of magnetic impurities. We continue with spin-polarized STM experiments reporting magnetization curves of individual magnetic adatoms and finish by a description of the most recent spin-excitation experiments revealing the necessary anisotropy environment for a high spin impurity to display the Kondo effect.