David Loffreda

Ecole normale supérieure de Lyon | ENS Lyon · Département de Chimie

Up to date, the influence of ambient air exposure on the energetics and stability of silver clusters has rarely been investigated and compared to clusters in vacuum. Silver clusters up to 3000 atoms in size, on an amorphous carbon film, have been exposed to ambient air and investigated by atomic-resolution imaging in the aberration-corrected Scanni...

Silver clusters up to 1000 atoms in size, on an amorphous carbon film, have been exposed to ambient air and investigated by atomic resolution imaging in the aberration-corrected Scanning Transmission Electron Microscope (ac-STEM). Ordered structures comprise more than half the population, the rest are amorphous. Of ordered isomer structures the mos...

The arduous modelling of reactions at heterogeneous catalysts is greatly simplified when adsorption-energy scaling relations between intermediates exist. The offset of these linear relations is structure-independent when the slope is unity and otherwise depends on the coordination number of the active sites. Here we examine the adsorption of *C, *C...

The metal cluster size in supported metal catalysts impacts the oxidation state of the metal atoms, coordination capability, and finally the catalytic activity—especially when the number of atoms becomes countable. The correlation between metal oxidation state and its catalytic consequences for ultrafine Pt was studied for photocatalytic overall wa...

Electrocatalysts are mainly characterized by their intrinsic adsorption properties. However, the observed electrocatalytic activity ultimately results from the interplay between such properties and various additional interactions within the electrified solid-liquid interface. One of such phenomena is solvation, which can substantially affect the st...

The role of the support anatase TiO2(100) and rutile TiO2(110) on the energetics, the shape and the charge transfer of deposited CuAu nanoclusters ( 1 nm) has been examined by using density functional theory calculations including Hubbard correction (DFT+U). Regular truncated octahedron clusters (rto), Cu(1-x)Au(x) (x being in the range 0.15-0.36)...

Statistical mechanics and transition-state theory have been used to investigate diffusion kinetics of gold and copper atoms on pristine and various reduced surfaces of rutile TiO2 (110). A DFT+U approach has been employed to calculate potential energy maps and to evaluate the required diffusion activation barriers. The role of the support reducibil...

Near-ambient pressure X-ray photoemission spectroscopy (NAP-XPS) [1] is a modification of XPS so that surfaces can be studied in presence of gases and not only in vacuum. This is necessary when you want to access chemical and electronic properties of catalytic surfaces in realistic reaction conditions (variable P, T). We have focused on the study o...

Experiments on model surfaces commonly help in identifying the structural sensitivity of catalytic reactions. Nevertheless, their conclusions do not frequently lead to devising superior “real-world” catalysts. For instance, this is true for single-crystal platinum electrodes and the oxygen reduction reaction (ORR), an important reaction for sustain...

Direct observation of the promoting effect of hydration on the nucleation of gold and copper nanoparticles supported on partially reduced rutile TiO2 (110) is achieved by combined scanning tunneling microscopy experiments and density functional theory calculations. The experiments show a clear difference between the two metals. Gold nanoparticles g...

Solvation can substantially modify the adsorption properties of heterogeneous catalysts. Although essential for achieving realistic theoretical models, assessing such solvent effects over nanoparticles is challenging from a computational standpoint due to the complexity of those liquid/metal interfaces. This effect is investigated by ab initio mole...

Polymer electrolyte membrane fuel cells are considered since more than two decades as promising clean energy converters for automotive applications. However, improving their durability and reducing the catalyst cost (platinum) are ongoing challenges. Alloys are often proposed as an alternative solution which combines a good catalytic activity towar...

Proton Exchange Membrane Fuel Cells (PEMFC) are energy efficient and environmentally friendly alternatives to conventional energy conversion systems in many yet emerging applications. In order to enable prediction of their performance and durability, it is crucial to gain a deeper understanding of the relevant operation phenomena, e.g., electrochem...

The metal-support interaction on the chemical reactivity is illustrated here by density functional theory calculations for the hydrogenation of crotonaldehyde on Pt13 clusters deposited on CeO2(1 1 1). Adsorption of crotonaldehyde on the supported cluster is globally strengthened, compared to Pt(1 1 1), both on top of the cluster and at the interfa...

Accounting for surface coordination The exploration of heterogeneous catalysts using first-principles calculations can be daunting because the large number of atoms and possible surface geometries. Calle-Vallejo et al. describe a simpler metric for assessing optimal reactivity: a weighted average of surface coordination that includes second-nearest...

The search for improved heterogeneous catalysts is an important but difficult task. Scaling relations between the adsorption energies of reaction intermediates greatly facilitate the computational design of catalysts. However, this methodology does not currently incorporate structure sensitivity and hence cannot describe adequately the overall acti...

Understanding the selectivity of oxygen reduction reaction, and more especially the formation of water versus hydrogen peroxide in fuel cells, is an ongoing challenge in electrochemistry, surface science and catalysis. In this study, we propose a comprehensive thermodynamic analysis of the reaction intermediates for the formation of water on Pt(111...

Predicting the reaction mechanism of water and hydrogen peroxide formation on a platinum catalyst is a crucial step toward the understanding of the corresponding selectivity in polymer electrolyte membrane fuel cells. In this perspective, the environment of the catalytic active site should play an important role; however, its explicit description a...

This combined experimental and theoretical study aims at understanding why surface hydroxyl groups may enhance catalytic reactivity of MgO surfaces in basic catalysis, whereas hydroxyls are weakly deprotonating groups. We investigated that reactivity enhancement in the catalytic conversion of 2-methyl-but-3-yn-2-ol (MBOH). Reaction kinetics was exp...

Platinum nanoparticle catalysts are used in a myriad of gas-phase, liquid-phase, and electrochemical reactions. Although a high catalytic activity is paramount, stability must also be guaranteed, especially when the nanoparticles are in contact with strongly bound adsorbates. Therefore, it is crucial to be able to accurately calculate adsorption-en...

Platinum is a prominent catalyst for a multiplicity of reactions because of its high activity and stability. As Pt nanoparticles are normally used to maximize catalyst utilization and to minimize catalyst loading, it is important to rationalize and predict catalytic activity trends in nanoparticles in simple terms, while being able to compare these...

Density Functional Theory (DFT) calculations show on two examples how catalytic reactivity qualitatively depends on the environment of the active sites. The example of butadiene selective hydrogenation to butenes on Pt or Pd first illustrates how alloying this catalytically active metal with a non- or less reactive elements (Sn or Au, respectively)...

The knowledge of the catalyst active phase on the atomic scale under realistic working conditions is the key for designing new and more efficient materials. In this context, the investigation of CO oxidation on the bimetallic Pt 3 Sn(111) surfaces by near-ambient-pressure X-ray photoelectron spectroscopy and density functional theory calculations i...

Probing the nature of the adhesion of an organic liquid on a metal surface is of high importance for understanding the stability and the chemical catalytic reactivity at these interfaces. However, to date, the morphology, the layering and the chemical properties of these systems are still insufficiently known. Using semi-empirical dispersion correc...

The growth of platinum on non-reduced CeO(2) (111) surface is studied by means of calculations based on the density functional theory. Particles of increasing size are formed on the oxide surface by incorporating the platinum atoms one by one until multilayer films are obtained. The main conclusion is that platinum atoms tend to maximize the number...

The knowledge of the catalyst active phase on the atomic scale under realistic working conditions is the key for designing new and more efficient materials. In this context, the investigation of CO oxidation on the bimetallic Pt3Sn(111) surfaces by near-ambient-pressure X-ray photoelectron spectroscopy and density functional theory calculations ill...

In this work we present a multiscale theoretical methodology that scales up ab initio calculated data into elementary kinetic models in order to simulate Polymer Electrolyte Membrane Fuel Cells (PEMFC) transient operation. Detailed Density Functional Theory (DFT) calculations are performed on a model Pt(1 1 1) surface to determine the elementary ki...

An experimental evaluation, under realistic conditions, of the catalytic properties of a Pt3Sn(111) model catalyst in CO oxidation under excess of hydrogen is reported in this study. From a structural point of view, two different surface terminations are available in this alloy, viz. (2×2) and (3×3)R30°, differing by their tin concentration and Pt–...

Understanding the nature of the adhesion of an organic liquid on a metal surface is of paramount importance for elucidating the stability and chemical reactivity at these complex interfaces. However, to date, the morphology, layering and chemical properties at organic liquid metal interfaces have been rarely known. Using semi-empirical dispersion c...

The prediction of a reaction mechanism and the identification of the corresponding chemical intermediates is a major challenge in surface science and heterogeneous catalysis, due to a complex network of elementary steps and surface species. Here we demonstrate how to overcome this difficulty by tracking the temperature dependent formation of the in...

Reaction pathways have been explored with periodic DFT calculations in order to understand the origin of the high selectivity for the hydrogenation of 1,3-butadiene on the Pt2Sn/Pt(1 1 1)-(√3 × √3)R30° surface alloy. The adsorption structures of butadiene, 1- and 2-butenes and all the intermediate species have been studied. Compared to the referenc...

The activation of oxygen reduction reaction fascinates numerous scientists in the fields of heterogeneous catalysis, electrochemistry and surface science. Indeed, oxygen dissociation is considered usually as the rate determining step of several reactions on conventional platinum catalysts. From combined experimental and theoretical approaches, we d...

In this study, density functional theory (DFT) calculations are presented for molecular and atomic oxygen adsorption on the Pt(111) surface at low coverage. The final goal is the atomistic description of oxygen dissociation kinetics in order to feed a multiscale kinetic model which simulates the Polymer Electrolyte Membrane Fuel Cell (PEMFC) electr...

The hydrogenation of unsaturated molecules is a key reaction in heterogeneous catalysis, with very broad applications in chemistry. The accepted scheme today is that the double bond is first chemisorbed on the catalyst’s surface, such as platinum, and that hydrogen is transferred on the adsorbed molecule by a concerted mechanism through a triangula...

Recently we presented high-resolution electron energy loss spectroscopy (HREELS) and temperature-programmed desorption (TPD) studies on the adsorption of prenal (3-methyl-2-butenal) and crotonaldehyde (2-butenal) on Pt(111) and on ultrathin Pt3Sn/Pt(111) and Pt2Sn/Pt(111) surface alloys. Vibrational studies employing HREELS have been analyzed with...

Figuring it out: Linear relations are found when the energies of the transition states () are plotted against those of the precursor states () for the hydrogenation of unsaturated aldehydes on platinum (see picture, FS=final state). This relation allows a quick and accurate determination of the energy barriers of the reaction when changing the chem...

Brønsted-Evans-Polanyi-Beziehungen sind im Bereich der heterogenen Katalyse bekannt für die schnelle Vorhersage von Aktivierungsbarrieren allein mit Kenntnis der Reaktionsenthalpien. Dieses Modell wurde nun von D. Loffreda et al. auf die Hydrierung ungesättigter Aldehyde an Platin erweitert. In ihrer Zuschrift auf S. 9140 ff. finden die Autoren auf...

Figuring it out: Linear relations are found when the energies of the transition states (ioads) are plotted against those of the precursor states (EcoadsIS) for the hydrogenation of unsaturated aldehydes on platinum (see picture, FS = final state). This relation allows a quick and accurate determination of the energy barriers of the reaction when ch...

By employing high-resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED), we have studied the adsorption and thermal decomposition of crotonaldehyde (2-butenal) on Pt(111) as well as Pt 3 Sn/Pt(111) and Pt 2 Sn/Pt(111) surface alloys. In order to understand the adso...

On the basis of density functional theory calculations and an original use of a generalized Brönsted–Evans–Polanyi relationship, the key question of the change of selectivity has been solved for hydrogenation of three unsaturated aldehydes (acrolein, crotonaldehyde and prenal) on a Pt(111) surface. This study supports the idea that the selectivity...

The surface restructuring of Pt(3)Sn(111) induced by oxygen chemisorption is examined by means of density-functional theory calculations. Molecular and atomic oxygen chemisorption is investigated on the two available terminations of the bulk alloy--(2 x 2) and (square root(3) x square root(3))R30 degrees--these two surfaces differing by the tin con...

In heterogeneous catalysis, bond activation caused by hybridization of the adsorbed reactants is a crucial phenomenon. It is commonly believed that the hybridization of adsorbed molecules, which is closely connected to the reactivity, is directly related to the heat of adsorption on alloy surfaces. We have investigated the adsorption of two prototy...

The adsorption of CO on the two terminations, p(2 × 2) and (√3 × √3) R30°, of the bulk Pt3Sn(111) alloy is studied by high pressure polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The influence of O2 is also investigated. While no differences are observed when CO is chemisorbed alone, the behavior of the two terminat...

The adsorption and thermal decomposition of the R,-unsaturated aldehyde prenal (3-methyl-2-butenal) have been studied on Pt(111), the Pt 3 Sn/Pt(111) and Pt 2 Sn/Pt(111) surface alloys, and the corresponding terminations of the Pt 3 Sn(111) bulk alloy by means of high-resolution electron energy loss spectroscopy (HREELS), temperature-programmed des...

We have used spin-polarized density functional theory calculations to investigate the structure of the Pd8Ni92(110) alloy surface, which has been extensively studied for its excellent catalytic properties. Our calculations have corroborated that this alloy has a quite complex (N×1, N∼6) structure, so-called “sawtooth” (ST) reconstruction. Moreover,...

Periodic density functional calculations are reported for NO dissociation on the Rh(221) and RhCu(221) surfaces. The analysis of the reaction pathways indicates that the presence of steps results not only in new adsorption sites for NO adsorption but also on the appearance of a completely new reaction pathway with activation energies which are smal...

Vibrational insights into the CO adsorption states on the (2 × 2) surface of the Pt3Sn(111) bulk alloy are reported for the first time by a systematic comparison between high-resolution electron energy loss spectroscopy (HREELS) measurements and density functional theory (DFT) anharmonic frequency calculations. CO adsorbs preferentially in top conf...

A complementary experimental and theoretical study of the alloying effects of Sn on the molecule–surface bonding of crotonaldehyde on Pt(1 1 1) is presented. By comparing high-resolution electron energy loss spectroscopy (HREELS) data and density-functional theory (DFT) calculations of vibrational spectra from a complete set of possible adsorption...

Adsorptionsstrukturen multifunktioneller Moleküle: Nach einer DFT-Studie für vier konjugierte Moleküle auf sechs Metallen korrelieren die Molekül-Metall-Schwingungen mit der Stärke und Struktur der Adsorption (siehe Bild). Die η2μ1-Struktur auf Ag und Au ist schwach adsorbiert und hat weichere Schwingungen, dagegen wird die η4μ3-Struktur auf Ni, Pd...

The efficiency of PtSn alloy surfaces toward CO oxidation is demonstrated from first-principles theory. Oxidation kinetics based on atomistic density-functional theory calculations shows that the Pt3Sn surface alloy exhibits a promising catalytic activity for fuel cells. At room temperature, the corresponding rate outstrips the activity of Pt(111)...

Adsorption thermodynamics based on density functional theory (DFT) calculations are exposed for the interaction of several multifunctional molecules with Pt and Au(110)-(1×2) surfaces. The Gibbs free adsorption energy explicitly depends on the adsorption internal energy, which is derived from DFT adsorption energy, and the vibrational entropy chang...

The usual empirical rule stating that the C=C bond is more reactive than the C=O group for catalytic hydrogenations of unsaturated aldehydes is invalidated from the present study. Density functional theory calculations of all the competitive hydrogenation routes of acrolein on Pt(111) reveals conversely that the attack at the C=O bond is systematic...

Competitive ethene oxidation pathways are presented for a p(4 x 4) surface-oxide phase on Ag(111) obtained from density functional theory (DFT) calculations. Both parallel routes are found to proceed from a common oxametallacycle intermediate (OMME) in agreement with previous mechanistic studies on low coverage O adatom phase, although acetaldehyde...

(Chemical Equation Presented) Coordinating the attack: A theoretical approach based on first-principles calculations and a kinetic model has shown that the partial hydrogenation of the C=O or C=C bonds in acrolein on a Pt(111) catalyst is controlled by the balance between the hydrogenation steps on the surface and the desorption of the partially hy...

A CO stretching frequency analysis is presented for the adsorption of CO on various Au(110) surfaces from density functional theory calculations. The structure sensitivity of the adsorption has been studied by considering the unreconstructed (1 x 1) surface, the missing-row reconstructed (1 x 2) surface, the vicinal stepped (20) surface, and the ad...

Adsorption thermodynamics of acrolein (CH2CHHCO) on Pt (111) is investigated with adsorption free energy simulations as a function of temperature and pressure. This atomistic thermodynamic approach based on density-functional theory energies shows how adsorption modes and coverage are modified in realistic experimental conditions. The various adsor...

Using in situ polarization modulation infrared reflection absorption spectroscopy and density functional theory calculations, a new high-coverage monomeric NO adsorption state on Pd(111) was observed and proposed to have a (3 x 3)-7NO structure. Formation of this high coverage NO phase was found to take place only at elevated pressure and temperatu...

Through an interplay between atomistic thermodynamics, isotherm models, and kinetics, all based on density functional theory calculations, we propose a mechanism for the removal of the (1×2) missing-row reconstruction on Au(110) under high CO pressure. Elevating the pressure from ultrahigh vacuum to one atmosphere causes the emergence of adsorption...

The adsorption of carbon monoxide on the Au(1 1 1) single-crystal surface has been investigated in the [10−3–103 Torr] range at room temperature (RT). Using scanning tunneling microscopy (STM), a CO-induced modification of the surface morphology (step edge roughening) and terrace structure (22×√3→1×1 transition) is evidenced. Reflection absorption...

A combination of GGA total-energy calculations and vibrational spectra simulations with HREELS experiments is reported to obtain new insights in the stable adsorption modes of the acrolein molecule on the Pt(111) surface. The simulations of the EELS spectra have allowed us to interpret the evolution of the experimental spectra, as a function of gas...

A new method has been recently developed for the hydrodesulfurization of gasoils based on the formation of charge transfer complexes. This method is highly selective towards the removal of alkyldibenzothiophenes from other aromatic compounds present in gasoils. Total energy calculations based on density-functional theory have been performed to stud...

Reaction mechanisms and activation energies for the complete conversion of ethene to ethene epoxide on two recently characterized oxidized Ag{111} surfaces have been determined from density functional theory. On both surfaces, epoxidation proceeds through a two-step nonconcerted mechanism via an oxametallacycle intermediate. The key implications ar...

The vibrational modes of oxygen on Ag(0 0 1) are studied both theoretically, by density functional perturbation theory (DFPT) within the local density approximation, and experimentally, by means of high resolution electron energy loss spectroscopy (HREELS). The frequencies of the O–Ag stretch mode and of the parallel modes are calculated for the p(...

We present a comparative density-functional theory study of the chemisorption and the dissociation of the NO molecule on the close-packed (111), the more open (100), and the stepped (511) surfaces of palladium and rhodium. The energetic and kinetic properties of the reaction pathways are reported. The structure sensitivity is correlated to the cata...

We present a theoretical study––based on first principles calculations––aimed at characterizing the surface reconstructions which occur at the Ag(0 0 1) surface when oxygen is dosed on it. We first model this system at different coverages using (1×1), c(2×2), and p(2×2) structures of oxygen atoms adsorbed on the hollow sites of the Ag(0 0 1) surfac...

In the present work different adsorption modes of trichloroethene on the PdCu alloy have been investigated systematically. With application of ab initio periodic density functional theory, some insights about these adsorption modes have been revealed. The two different (110) terminations of the Cu3Pd alloy have been employed as models for the surfa...