Natalia Gilis

Chiral adsorption studied by field emission techniques: the case of alanine on platinum

Chirality at surfaces has become an active research area targeting possible applications of enantioselective separation or detection. Here, we propose a promising route for obtaining fundamental understanding of the enantiospecific interaction of chiral molecules on metal surfaces using field emission techniques, i.e. field ion microscopy (FIM) and field electron microscopy (FEM). These techniques have been chosen for their particular advantages in exposing a wide range of structurally different facets in one atomically resolved picture. This diversity allows the study of interactions between a chemical species and a number of facets during the adsorption process on the same sample. In the present study, we focused on the adsorption of alanine on platinum surfaces modelled as sharp tips and imaged by FIM and FEM. Our results show a clear preference of the alanine to adsorb on chiral facets. Although the 20 A resolution of the FEM does not allow the edges of the facets of interest to be unraveled, the net images after exposure to one enantiomer of alanine show the occurrence of enantioselective adsorption over the sector of the same chiral symmetry. The results show that L-alanine has a strong tendency to adsorb onto R facets. Conversely, D-alanine adsorbs onto S facets.

Nanoscale Chiral Recognition Using Field Ion and Field Emission Microscopy

Chirality at surfaces has become an active research area targeting possible applications of enantioselective separation or detection. In this context, significant success has been achieved these past decades by developing new methods for a better understanding of enantiospecific interactions of chiral adsorbate with surfaces. Here, we propose a promising route for obtaining fundamental understanding of enantiospecific interaction of chiral molecules on metal surfaces using field emission based techniques. This technique has been chosen for its particular advantage to expose a wide range of structurally different facets in one atomically resolved picture. This diversity allows us to screen with one sample the interactions between a chemical species and a number of facets during the adsorption process.In the present study, we envisage the adsorption of alanine on platinum surfaces modelled as sharp tip using field emission and field microscopy along with theoretical studies using density functional theory.In order to observe the adsorption pattern of the adsorption, the Pt surface is kept at temperatures between 150 and 300K, and then exposed to vapors of D or L-alanine. The in-situ FEM is filmed with a high-speed camera. The whole process is also performed in absence of alanine molecules to perform reference experiments. The subtraction of the results before and after the adsorption gives us a net image of the adsorption sites. Our results show a clear preference of the alanine to adsorb on chiral facets. Although the 20 A resolution of the FEM does not allow to unravel the edges of the facets of interest, the net images after exposures to one enantiomer of alanine show the occurrence of an enantioselective adsorption over sector of the same chiral symmetry. The results show that L-alanine has a strong tendency to adsorb onto R facets. Conversely, D-alanine adsorbs onto the S facets.

Field Emission Microscopy to Study the Catalytic Reactivity of Binary Alloys at the Nanoscale

Catalysis plays a crucial role in modern industrial applications. The aim in every process involving catalysis is to obtain a high and sustainable conversion along with a high selectivity towards the desired product(s). In the case of heterogeneous catalysis, one of the ways to reach this goal is to design tailored nanoparticles that present a specific composition, shape and morphology. Such engineering of catalysts only works if one understands how the reaction proceeds on different morphologies and how the reaction may induce structural changes. Another way to improve the efficiency relies in the control of the catalytic reaction. For this, the study of the dynamics occurring at the surface of the catalyst is used to determine the reaction mechanism with better accuracy, which in turn opens the way to a rationale for assessing the reproducibility, the predictability and the controllability of the reaction. To improve a catalytic process, a fundamental understanding of the catalytic behavior of the active materials is thus required. Surface science studies had, and still have, a great impact on the understanding of catalytic systems. These studies are mainly performed on catalytic reactions occurring at the surface of pure metals. There is, however, an increasing interest in using alloy catalysts in industrial applications, which calls for in situ studies providing a fundamental understanding of the properties of alloy catalysts.

Nanoscale Imaging of Subsurface Oxygen Formation on Rhodium Catalysts

During a catalytic process, a catalyst may undergo changes of its structure or morphology, as well as modifications of its local composition, which is due to oxidation/reduction processes, surface segregation, or even the presence of subsurface species [1]. All these modifications may affect the activity and the selectivity of the catalyst and contribute to the ageing of the catalyst. To develop catalysts with improved efficiency, a fundamental understanding of the catalytic process is needed. The shape of the nanoparticle, its size, its local chemical composition and the synergistic influences of these features on the catalytic activity must be determined, down to the molecular level, to unravel the details of this reaction. Such studies gain significance if they are performed during the ongoing process so as to highlight transient behaviors that cannot be observed before and after reactions.