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Electrochemically-derived well-crystalline mesoporous silicon carbide (pSiC) was used as a host for cobalt nanoparticles to demonstrate superior catalytic performance during the CO hydrogenation according to Fischer-Tropsch. Colloidal Co nanoparticles (9 ± 0.4 nm) were prepared independently using colloidal recipes before incorporating them into pSiC and, for comparison purposes, into commercially available silica (Davisil) as well as foam-like MCF-17 supports. The Co/pSiC catalyst demonstrated the highest (per unit mass) catalytic activity of 117 µmol.g(CO)-1.g-1(Co).s-1 at 220 °C which was larger by about one order of magnitude as compared to both silica supported cobalt catalysts. Furthermore, a significantly higher C5+ hydrocarbons selectivity was observed for Co/pSiC. The stable performance of the catalyst is attributed to the high dispersion of the active phase and the use of pSiC acting as a thermally conductive and chemically inert mesoporous support., info:eu-repo/semantics/published

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.

The reconstruction of rhodium and platinum crystals of some tens of nanometres diameter was investigated during the ongoing hydrogenation of oxygen atoms resulting from the dissociation of O2 and NO2 species. Field ion and field emission electron microscopies (FIM and FEM) were used to characterise the apex of tip samples before, during and after the catalytic reactions. On rhodium samples, the exposure of less than 10 Langmuir of O2 is sufficient to induce significant morphological changes. At higher exposures, the presence of subsurface oxygen causes surface reconstructions illustrated with atomic resolution by FIM at 50 K. The same pattern is also visible at 505 K in the presence of H2 and O2 during water production. Upon the decrease of H2 pressure, surface oxidation shows a strong sensitivity to the local surface initiated along the 〈0 0 1〉 zone lines. On platinum, the kinetic instabilities of the NO2–H2 reaction are followed by FEM at 390 K starting from a hemispherical tip sample. The instabilities are expressed as surface explosions occurring randomly in time, but synchronised over {0 1 1} facets. These instabilities expand along the 〈0 0 1〉 lines over the (0 0 1) pole and exhibit self-sustained kinetic oscillations. The analysis of the tips by FIM after the reaction shows dark regions over the {1 1 3} facets, suggesting the extension of those to the detriment of vicinal ones. A well-controlled field evaporation procedure reveals that these regions appear dark due to the presence of surface oxygen. Structural reconstructions are observed but do not lead to the drastic morphological changes suggested by the FIM and FEM patterns. Nanoparticle dynamics must be accounted in models describing the non-linear features of catalytic reactions and more generally included in the description of catalytic properties of nanosized particles.

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.

The present paper overviews our recent experimental and theoretical results concerning the catalytic properties of nanosized rhodium and platinum facets in the presence of an external electric field. We first show that field ion microcopy (FIM) experiments along with corresponding density functional theory (DFT) calculations strongly indicate that a positive field value promotes the oxidation of a rhodium field emitter tip. We also demonstrate that the catalytic oscillatory nanopatterns, as observed when H2 and O2 are exposed on a rhodium field emitter tip, find their origin in the different catalytic properties of all the nanofacets that are simultaneously exposed at the tip’s surface. In the case of platinum field emitter tips, the dependence of dissociation barrier of NO on the external electric field is examined with DFT and compared to experimental results. Corresponding calculations on Pt are done for the (NO)2 dimer.

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.

Methods are developed to describe catalytic reactions on multi-facetted surfaces in high electric fields in the conditions of field emission microscopy. These methods are applied to the hydrogen–oxygen–rhodium system for which a mean-field kinetic model is established. This model is shown to reproduce not only the nonequilibrium regimes of bistability and oscillations and their nanopatterns, but also the temperature-programmed desorption spectra of hydrogen and oxygen on rhodium, as well as the equilibrium phase diagram of oxygen on rhodium with the transition to the trilayer surface oxide ORhO. The dependence of the kinetic constants on the surface orientation of the facets is taken into account by expanding them in kubic harmonics suitable for the fcc rhodium crystal. The electric field modifies the gas pressure as well as the activation energies of the different kinetic processes. The tip of the field emission microscope is shown to behave as a nanoreactor.

The dissociation of carbonaceous molecules on the surface of a nano-sized Ni crystal induces deep morphological changes way below the melting temperature, as imaged by Field Ion Microscopy. In this Letter, classical Molecular Dynamics (MD) was used to demonstrate that carbon adsorption triggers Ni surface diffusion. More precisely, single adsorbed carbon atoms, preferentially bound to 5-fold surface sites, enhance thermally activated Ni diffusion. A mechanism is identified by which adsorbed carbon promotes the rupture of surface Ni atoms from their lattice position. This leads to a destabilization of edges between facets of the Ni crystal.

A comprehensive theory for the adsorption of H2 and O2 on a nanometric rhodium field emitter tip is developed to describe the equilibrium properties, the adsorption−desorption kinetics, as well as its observed nonlinear reaction behavior and oscillatory states. The basis is a kinetic mean-field model for hydrogen, oxygen, and subsurface oxygen which takes into account the anisotropy of the tip’s surface. The resulting model reproduces the correct anisotropy, period and form of the oscillations, as well as the bistability diagram for a varying temperature, hydrogen pressure, and external electric field as observed in a field ion microscope.

A dedicated three-dimensional atom-probe has been fitted with an in situ high pressure/temperature reaction cell. This was used for studying the interaction of gases with Pt-alloy surfaces typical of those employed in automotive exhaust catalysis. The reactions of nitric oxide (NO) and oxygen (O-2) on Pt and Pt-17.4 at.% Rh FIM specimens were explored, revealing a range of surface restructuring and segregation effects, which depend sensitively on catalyst composition, orientation, temperature and gas chemistry. Specifically, we observed that exposure of Pt-Rh to either NO or O-2 at 422-473 K and 10 mbar induced Rh surface segregation uniformly over the surface. However, on raising the temperature to 573 K or higher, the Pt-Rh{111} surface remained Rh-enriched but the Pt-Rh{011} surface became severely Rh-depleted. We attribute this observation to the possible diffusion of Rh species across the specimen apex, indicating that segregation occurs in directions both normal and parallel to the surface in Pt-Rh catalysts. Copyright (C) 2007 John Wiley and Sons, Ltd.

A combined experimental and theoretical study is presented of fluctuations observed by field ion microscopy in the catalytic reaction of water production on a rhodium tip. A stochastic approach is developed to provide a comprehensive understanding of the different phenomena observed in the experiment, including burst noise manifesting itself in a bistability regime, noisy oscillations, and nanopatterns with a cross-like oxidized zone separating the surface into four quadrants centered on the {111} facets. The study is based on a stochastic model numerically simulating the processes of adsorption, desorption, reaction, and transport. The surface diffusion of hydrogen is described as a percolation process dominated by large clusters corresponding to the four quadrants. The model reproduces the observed phenomena in the ranges of temperature, pressures, and electric field of the experiment.

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

A versatile gas reaction cell has been developed for a 3D atom probe system to provide a new method for studying the reaction mechanisms of heterogeneous catalysis. We describe the features and advantages of this new system. The reaction cell enables exposure of metal specimens in high temperature (≤873 K) and high pressure (≤1 bar) environments to a wide range of single gases or gas mixtures. Following treatment, specimens are transferred into the UHV analysis chamber of the 3D atom probe, which provides atomic-scale positional and chemical information for adsorbed gas and metal atoms/oxides from selected regions of the specimen apex. Results demonstrating the effects of heating alloy catalysts in vacuo and in air are presented, which validate the instrument design, alongside a sample of data for exposing Pt and Pt-Rh alloys to NO, revealing the power of the 3D atom probe technique in this field. © 2006 Elsevier B.V. All rights reserved.

We have studied the interaction of hydrogen with Rh tips under imaging conditions of field ion microscopy. In electric fields of 2.4 V/A, in the presence of hydrogen gas at 10 −5 mbar and at cryogenic temperatures, discrete events of field evaporation were found to form (1×2) patterns on Rh{1 1 0} due to the removal of Rh atoms along [0 1 1 ] the direction of densely packed atomic chains. This is different from the usual field evaporation mechanism which occurs by a shrinkage of layers from the edge to the center. Corroborating evidence for this interpretation is obtained from ab initio cluster calculations based on density functional theory.

The reaction of hydrogen/oxygen gas mixtures with rhodium single crystals was studied using video-FIM (Field Ion Microscopy) at temperatures between 350 and 550 K and up to 2×10 −2 Pa total pressure. Imaging at 500 K in a hydrogen rich gas mixture (H 2 : O 2 =9) revealed considerable morphological changes of the (0 0 1)-oriented field emitter tip, i.e. the growth of low-index at the expense of high-index planes and strong crystal coarsening. Decreasing the hydrogen partial pressure led to chemical and structural changes of the Rh sample. Starting on the {1 1 0} planes a surface oxide formed, which spread anisotropically across the surface until it finally covered the whole visible surface area. The transformation was reversible upon increasing the hydrogen pressure back to its initial value. However, a hysteresis behavior was observed, i.e. a larger hydrogen partial pressure was found to be necessary to re-establish the initial patterns of a reactive O ad /H ad layer. By varying the temperature from 400 to 500 K a phase diagram was established for the O ad /H ad system. Increasing the electric field proved to shift the phase diagram towards higher H 2 pressures. At 550 K self-sustained kinetic oscillations with a cycle time of ∼40 s could be observed.

The interaction of small gold crystal tips with oxygen gas and CO/O2 gas mixtures was studied by means of field ion microscopy (FIM). High-resolution FIM-images of clean tips were obtained with hydrogen and neon as imaging gas. At temperatures between 300 and 450 K the exposure of a clean Au sample to O2 gas at 100–1000 mbar, in the absence of an electric field, led to oxygen chemisorption and formation of a “surface oxide”. The presence of an electric field of 12–15 V/nm was found to enhance the oxidation process. Exposure to CO gas at 300 K led to the removal of the surface oxide. This was associated with the occurrence of a wave front which started in the apex centre and extended to the outskirts of the tip sample. The build-up of the surface oxide and its titration by carbon monoxide was completely reversible. Our results strongly suggest that pure gold crystals are active catalysts for the CO oxidation at 300 K.

Field ion microscopy (FIM) was used to study the reaction of nitric oxide (NO) with silver (Ag) field emitter tips. The clean and almost hemispherical shape of the Ag tip apex was found to undergo restructuring during field-free interaction with 150 L NO (1 L = 1.3 × 10 -4 Pa.s) at 400 K. Imaging at 57 K revealed an enlarged central (111) pole and a smaller number of layers leading to peripheral planes. Thus, crystal coarsening occurred during reaction with NO gas. Moreover, planes lying along the zone lines appeared dark and could only be recovered by field evaporation of a large number of surface layers. Areas of the former {012} planes appeared enlarged. These structural changes are interpreted as being caused by oxygen atoms released during NO decomposition. Cleaning the oxygen-covered tip by field evaporation and heating it to 400 K reproduced this reconstruction form. This finding is explained by the formation of subsurface oxygen and its resegregation to the surface. However, if the sample, either clean or oxygen-covered, was sputter-cleaned and subsequently heated to 400 K, only slight changes of the surface structure were found. These changes involved increased local disorder and somewhat enlarged low-index planes as compared to the field-evaporated clean Ag specimen.

The interaction of oxygen/hydrogen gas mixtures with Rh crystals was studied by means of video-FIM (field ion microscopy) at temperatures between 400 and 500 K and total pressures up to 10 -2 Pa. Rh samples were found to suffer considerable morphological changes during the reaction at 505 K and relative gas amounts of H 2 /O 2 = 0.1…9. Strong faceting of planes lying along the zone lines was seen and interpreted as being oxygen-driven. The surface structures changed while removing hydrogen from the gas mixture. In situ video-FIM allowed us to monitor the time dependence of changes. Accordingly, planes lying along the zone lines transformed first from O ad /H ad into O ad -covered The process started in {011 } planes and finally reached the (001) pole. A cross-like image feature thus appeared and divided the apex surface of the Rh crystal into four equivalent quadrants with {111 } planes in their center. These quadrants reacted subsequently in an independent manner, that is, the transformation from an O ad /H ad mixed layer into an O ad layer occurred with a delay period of up to several minutes while decreasing the hydrogen gas pressure. The structural changes observed were reversible upon increasing the hydrogen gas pressure.

Selected areas of a clean or Li-doped Rh field emitter tip were studied by low-temperature O + 2 -FIM and Li + -FDM in order to reveal the promoting influence of Li on the early stages of the oxidation process. The experiments were conducted by graphitizing a Rh tip specimen in a first step and reacting off subsequently the surface carbon to create two single “windows” with a clean Rh surface in the region of (1 0 0) and (1 2 3). Both windows were imaged by Li + -FDM. At high field strengths Li + formation was restricted to the border between the clean surface areas of the windows and the graphite-like overlayer, i.e. ring-like ion images appeared on an otherwise dark background. Since the Li source was originally present on the shank of the Rh tip it must be concluded that Li atoms penetrate the graphite-like layer in order to form an intercalation structure ensuring high mobility to sustain steady supply for image formation in Li + -FDM. Field strengths and temperatures could also be adjusted so that the initially clean Rh windows were precovered by Li without desorption occurring. This enabled us to study the influence of Li on the rate of surface oxidation while imaging in O + 2 -FIM at 78 K. We found an enhancement of this rate (as compared to the clean surface) and explain this observation by an increase of the sticking probability of O 2 on the Li-covered surface. In another experiment, we found that O + 2 -FIM of Rh surfaces covered by CO ad yield higher brightness than those covered by O ad .

The adsorption and reaction of NO on silver-field emitter tips was studied using FEM at low temperatures. Exposing an either [1 1 1] or [0 0 1] oriented clean tip to a single dose of NO gas (50 L whereby 1 L=1.3×10 −4 Pa s) at sample temperatures between 50 and 61 K under zero-field conditions was found to strongly increase the work function of the Ag specimen. Upon heating to 66 K a wave-like pattern of high brightness could be observed starting at the shank and moving toward the center of the tip apex. The pattern occurred in an explosive manner by spreading over the whole surface during ∼2 s and was associated with an exponential increase of the FEM brightness. The phenomenon is not reversible, i.e. on recooling to 50 K neither the explosive pattern formation nor the initial darkness of the FEM image as obtained after NO adsorption were observed. We suggest that the occurrence of a surface explosion during heating is associated with the desorption of multilayer (NO) 2 dimers. The model is advanced that the self-accelerating pattern evolution is associated with the thermal dissolution of a field-oriented dipole structure, starting at the low-field outskirts of the tip and propagating toward the apex.

The reaction of oxygen with Rh single-crystal tips was investigated using field ion microscopy (FIM). Emphasis was laid on revealing the atomic structure of individual tip surface planes along with their influence on the early stages of the reaction, i.e., chemisorption and nucleation. Under field-free conditions, the interaction of oxygen with a (001) oriented Rh tip was found to lead to major reconstructions at temperatures above 400 K. While the original shape of the tip was nearly hemispherical before the reaction, it was polyhedral thereafter. In particular, the {011} and {113} planes were seen to adopt a “missing row” type reconstruction in the presence of oxygen adsorbate. On the Rh{113} plane, a (1 × 3) reconstruction prevailed for oxygen exposures larger than 60 L (1 L = 1.3 × 10-4 Pa·s) and temperatures T ≥ 550 K. In this type of missing-row reconstruction two adjacent dense-packed chains of atoms are absent. Surface oxidation was found to be promoted by the presence of an external electric fiel...

The adsorption of NO on Au 3D hemispherical crystals (“field emitter tips”) has been studied by means of pulsed field desorption mass spectrometry (PFDMS) under dynamic gas flow conditions and at 300 K. Local chemical probing of ~200 Au sites in the stepped surface region between the central (111) pole and the peripheral (001) plane leads to the detection of NO+, N2O+ and (NO) species. Obviously, molecular NO adsorption on stepped Au surfaces can lead to dimerization. Nitrous oxide formation probably occurs via the dimer, (NO)2.

The adsorption of NO and its reaction with H2 over Pd tips were investigated by means of field ion microscopy (FIM) and pulsed field desorption mass spectrometry (PFDMS) in the 10-3 Pa pressure range and at sample temperatures between 400 and 600 K. By varying the H2 partial pressure while keeping the other control parameters constant, the NO + H2 reaction over Pd crystallites is shown to exhibit a strong hysteresis effect. The hysteresis region narrows with increase in temperature and the H2 pressures delimiting this hysteresis decrease as well. Abrupt transformations of the micrographs are observed by FIM from bright to dark patterns and vice versa. These transformations define the hysteresis region. The collected data allow establishing a novel kinetic phase diagram of the NO+H2/Pd system within the range of temperatures and pressures indicated. The observed features are correlated with a local chemical analysis by means of field pulses. NO+ seems to be the dominating imaging species under all conditions. At high relative H2 pressures (the "hydrogen-side" ), H atoms seem to diffuse subsurface. This process is blocked at lower H2 pressure (the "NO-side" ) due to NOad and Oad accumulation on the surface. Probe-hole measurements with field pulses indicate that the Pd surface undergoes oxidation as revealed by the occurrence of PdO2 species in the mass spectra. © 2010 Elsevier B.V., SCOPUS: ar.j, info:eu-repo/semantics/published

The surface composition of an Au-62 at% Pd alloy has been studied by means of a catalytic atom probe (CAP) before and after exposures to nitric oxide (NO) at temperatures ranging from 300 to 573 K for 20 min. Subsequent CAP analysis at 100 K revealed a considerable surface enrichment in Pd (to ∼80 at%) after exposure at 573 K. This is correlated with the occurrence of NO dissociation, and the formation of strong Pd–O bonds at the surface. Blank experiments in ultra-high vacuum reflect the surface composition of the bulk material, in excellent agreement with electron microprobe analysis. At 573 K, no detectable surface segregation occurs in the absence of NO adsorption for the times and temperatures studied. However, classical Metropolis Monte-Carlo simulations performed with a semi-empirical potential on the Au 40 Pd 60 (1 1 1), (1 1 0) and (1 0 0) systems show surface enrichment of gold at equilibrium. This suggests that the temperatures of the clean surface segregation experiments are too low to reach equilibrium within times of the order of hours.

We present a study of the early stages of carbon nanotubes nucleation in CVD synthesis by combining field ion/electron emission microscopy (FIM/FEM) and atom-probe investigation (AP) of the nickel-carbon interaction. Acetylene decomposition on Ni tips at 873K is observed to induce additional step formation on an initially facetted (polyhedral) crystal. Carbon-enriched steps are then observed to act as preferential nucleation centers of graphene sheets formation. Atom-probe experiments reveal C(2) and C(3) species and frequency dependent studies demonstrate that the origin of these species is different from C(1). Experiments provide clear evidence for the crucial role of carbon-enriched steps as nucleation sites of graphene sheets on the Ni surface.

We report a study of the adsorption and reaction of CO on a gold nanotip in high electrostatic fields. Field ion microscopy is used to investigate the emergence of a Au-carbonyl wave that is made visible with oxygen as the imaging gas. We set up a simple kinetic model that reproduces the adsorption wave and confirms that the presence of oxygen merely serves as an imaging gas and does not lead to field-induced oxidation of CO.

International audience; We have studied oxygen interaction with Au crystals (field emitter tips) using time-resolved (atom-probe) field desorption mass spectrometry. The results demonstrate no adsorption to take place on clean Au facets under chosen conditions of pressures (p < 10(-4) mbar) and temperatures (T=300-350 K). Steady electric fields of 6 V/nm do not allow dissociating the oxygen molecule. The measured O-2(+) intensities rather reflect ionization of O-2 molecules at critical distances above the Au tip surface. Certain amounts of Au-O-2 complex ions can be found at the onset of Au field evaporation. Calculations by density functional theory (DFT) show weak oxygen end-on interaction with Au-10 clusters (Delta E=0.023 eV) and comparatively stronger interaction with Au-1/Au(100) model surfaces (Delta E=0.25 eV). No binding is found on \210\ facets. Including (positive) electric fields in the DFT calculations leads to an increase of the activation energy for oxygen dissociation thus providing an explanation for the absence of atomic oxygen ions from the field desorption mass spectra. (c) 2006 American Institute of Physics.

The effect of atom coordination and influence of an external electrostatic field on binding and dissociation energies of molecular oxygen on a nano modified Au surfaces are investigated. Weak O and O 2 adsorption are found in our models. A considerable decrease of the dissociation energy of O 2 was found when adsorbed on small gold clusters or gold pyramids. It is concluded that in the presence of an electric field no dissociation of O 2 will occur in our model. This result is in agreement with field ion microscopy (FIM) and field electron microscopy (FEM) experiments, where no dissociation of O 2 is encountered.

International audience; To improve mass transport in the catalytic layers of proton exchange membrane fuel cells, the usual Pt catalyst support (carbon blacks) can be advantageously replaced by carbon aerogels or xerogels. The pore texture of such materials can indeed be tailored, which enables choosing an adequate pore texture minimizing diffusional limitations within the catalytic layers.