Cédric Barroo

Mapping reactive flow patterns in monolithic nanoporous catalysts

Abstract The development of high-efficiency porous catalyst membranes critically depends on our understanding of where the majority of the chemical conversions occur within the porous structure. This requires mapping of chemical reactions and mass transport inside the complex nanoscale architecture of porous catalyst membranes which is a multiscale problem in both the temporal and spatial domains. To address this problem, we developed a multiscale mass transport computational framework based on the lattice Boltzmann method that allows us to account for catalytic reactions at the gas–solid interface by introducing a new boundary condition. In good agreement with experiments, the simulations reveal that most catalytic reactions occur near the gas-flow facing side of the catalyst membrane if chemical reactions are fast compared to mass transport within the porous catalyst membrane.

Catalytic reduction of NO2 with hydrogen on Pt field emitter tips: kinetic instabilities on the nanoscale.

The catalytic reduction of NO(2) with hydrogen on a Pt field emitter tip is investigated using both field electron microscopy (FEM) and field ion microscopy (FIM). A rich variety of nonlinear behavior and unusually high catalytic activity around the {012} facets are observed. Our FEM investigations reveal that the correlation function exhibits damped oscillations with a decaying envelope, showing that molecular noise will influence the dynamics of the oscillations. The dependence of the oscillatory period on the P(H(2))/P(NO(2)) pressure ratios is analyzed. Similar patterns are reported under FIM conditions. Corresponding density functional theory (DFT) calculations for the adsorption of NO(2) on Pt{012} in the presence of an external electric field are performed in order to gain an atomistic understanding of the underlying nonlinear phenomena.

Fluctuating Dynamics of Nanoscale Chemical Oscillations: Theory and Experiments

Chemical oscillations are observed in a variety of reactive systems, including biological cells, for the functionality of which they play a central role. However, at such scales, molecular fluctuations are expected to endanger the regularity of these behaviors. The question of the mechanism by which robust oscillations can nevertheless emerge is still open. In this work, we report on the experimental investigation of nanoscale chemical oscillations observed during the NO2 + H2 reaction on platinum, using field electron microscopy. We show that the correlation time and the variance of the period of oscillations are connected by a universal constraint, as predicted theoretically for systems subjected to a phenomenon called phase diffusion. These results open the way to a better understanding, modeling, and control of nanoscale oscillators.

Influence of Step Geometry on the Reconstruction of Stepped Platinum Surfaces under Coadsorption of Ethylene and CO

We demonstrate the critical role of the specific atomic arrangement at step sites in the restructuring processes of low-coordinated surface atoms at high adsorbate coverage. By using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), we have investigated the reconstruction of Pt(332) (with (111)-oriented triangular steps) and Pt(557) surfaces (with (100)-oriented square steps) in the mixture of CO and C2H4 in the Torr pressure range at room temperature. CO creates Pt clusters at the step edges on both surfaces, although the clusters have different shapes and densities. A subsequent exposure to a similar partial pressure of C2H4 partially reverts the clusters on Pt(332). In contrast, the cluster structure is barely changed on Pt(557). These different reconstruction phenomena are attributed to the fact that the 3-fold (111)-step sites on Pt(332) allows for adsorption of ethylidyne-a strong adsorbate formed from ethylene-that does not form on the 4-fold (100)-step sites on Pt(557).

Hydrogenation of NO and NO2 over palladium and platinum nanocrystallites: case studies using field emission techniques

In this work, we investigate the catalytic hydrogenation of NO over palladium and platinum and of NO2 over platinum surfaces. Samples are studied using field emission techniques including field emission/ion microscopies (FEM/FIM). The aim of this study is to obtain detailed information on the non-linear dynamics during NOx hydrogenation over nanocrystallites at the atomic scale. The interaction between Pd and pure NO has been studied between 450 K and 575 K and shows the dissociative adsorption of NO. After the subsequent addition of hydrogen in the chamber, a surface reaction with the oxygen-adlayer can be observed. This phenomenon is reversible upon variation of the H2 pressure, exhibits a strong hysteresis behaviour but does not show any unstable regime when control parameters are kept constant. On platinum, NO is dissociated and the resulting O(ads) layer can also react with H2. Although occurring on both Pd and Pt metals, the reaction mechanism seems to be different. On palladium, NO dissociation takes place on the whole visible surface area leading to a “surface oxide” that can be reacted off by raising the H2 pressure whereas on Pt, the catalytic reaction is self-sustained and restricted to 〈001〉 zone lines comprising {011} and {012} facets and where self-triggered surface explosions are observed. Two kinetic phase diagrams were established for the NO–H2 reaction over palladium and platinum samples under similar experimental conditions. Their shapes reflect a different chemical reactivity of metal surfaces towards oxygen species resulting from the dissociation of NO. NO2 hydrogenation is followed over Pt samples and shows self-sustained kinetic instabilities that are expressed as peaks of brightness that are synchronized over the whole active area (corresponding to the 〈001〉 zone lines as in the NO case) within 40 ms, the time resolution of the video-recorder used for this work.

Imaging and chemically probing catalytic processes using field emission techniques: a study of NO hydrogenation on Pd and Pd–Au catalysts

Nitric oxide hydrogenation is investigated on palladium and gold–palladium alloy crystallites, i.e. the extremity of sharp tip samples aimed at modelling a single catalytic grain. Field ion microscopy and field emission microscopy are used to monitor adsorption and reaction in real time. One-dimensional atom probe and atom probe tomography are used on the same samples to unravel the surface composition of the adsorbed layers and the composition of the very first atomic layers of the Pd–Au surface. At constant NO pressure and at 450 K, the surface composition of the adsorbed layer on Pd samples shows a strong hysteresis behavior when H2 gas is varied. Under oxidizing conditions, N2O is formed via the occurrence of surface (NO)2 dimers. In the presence of Pd–Au alloys, the NO–H2 interaction comprises a simple NO dissociation causing the formation of surface NO2 species. On Pd–Au tip samples, atom probe tomography proves the occurrence of significant surface enrichment of palladium atoms in the presence of NO gas, but it is not sufficient to drift the behavior of the surface to that of pure palladium. Accordingly, external control parameters could be changed to tune the surface composition of Pd–Au catalysts and thus their activity and/or selectivity.

Nonlinear Dynamics of Reactive Nanosystems: Theory and Experiments

Reactive systems are known to give birth to complex spatiotemporal phenomena, when they are maintained far enough from their equilibrium state.

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.