Guesmi Hazar

Sorbonne University | UPMC · Laboratoire de Réactivité de Surface (LRS) (UMR 7197)

The synthesis of supported multielement transition metal phosphides (TMPs) to exploit the synergistic interplay between electronic and geometric effects resulting from the presence of different metals in the material and the arrangement of heterogeneous atoms is pivotal for reducing metal content while offering multiple active sites. However, the i...

Platinum−yttrium alloys are considered promising candidates to satisfy the challenging requirements for the cathodic oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Nevertheless, the practical structure−activity11 stability trends of these electrocatalysts in the form of carbon supported nanostructures are poorly un...

Single atom alloy (SAA) clusters formed by anchoring single atoms in small supported host clusters are emerging as catalysts with high performance. In this work, density functional theory (DFT) calculations and ab‐initio molecular dynamics (AIMD) simulations are performed to study the stability and the structure evolution of γ‐alumina‐supported pla...

Supported gold catalysts have been reported to exhibit exceptional selectivity towards the desirable product, an alkene, in chemoselective hydrogenation of highly unsaturated alkynes and alkadienes. However, insights on the interactions between the gold nanoparticles and the 1,3‐butadiene adsorbates are lacking. In this work, using atomic scale in...

We combine electron microscopy measurements of the surface compositions in Cu-Au nanoparticles and atomistic simulations to investigate the effect of gold segregation. While this mechanism has been extensively investigated within Cu-Au in the bulk state, it was never studied at the atomic level in nanoparticles. By using energy dispersive x-ray ana...

Platinum-rare earth nanoalloys have been predicted to be promising proton exchange membrane fuel cell (PEMFC) electrocatalysts for the cathodic oxygen reduction reaction (ORR). However, their implementation in PEMFCs is limited by the challenge of their preparation as carbon-supported nanostructures. Consequently, the practical structure–activity–s...

In this work, we combine electron microscopy measurements of the surface compositions in Cu-Au nanoparticles and atomistic simulations to investigate the effect of gold segregation. While this mechanism has been extensively investigated within Cu-Au in the bulk state, it was never studied at the atomic level in nanoparticles. By using energy disper...

Embedding energetically stable single metal atoms in the surface of Pt nanocatalysts exposed to varied temperature (T) and hydrogen pressure (P) could open up new possibilities in selective and dynamical engineering of alloyed Pt catalysts, particularly interesting for hydrogenation reactions. In this work, an environmental segregation energy model...

The Cover Feature shows NO molecules adsorbed on four different M13 clusters prior and after NO dissociation. Doping Pt13 with Ru, Rh or Ir stabilizes NO adsorption and lowers activation energies of the DeNOx process, pictorially represented by mountainous terrain surrounding the process. Cover design by Dr. Jelle Vekeman. More information can be f...

The dissociation of environmentally hazardous NO through dissociative adsorption on metallic clusters supported by oxides, is receiving growing attention. Building on previous research on monometallic M13 clusters [The Journal of Physical Chemistry C 2019, 123 (33), 20314–20318], this work considers bimetallic Pt12M (M=Rh, Ru or Ir) clusters. The a...

In a recent work [A. Nassereddine et al., Small 2021, 17, 2104571] we reported the atomic-scale structure and dynamics of sub-4 nm sized Au nanoparticles (NPs) supported on titania in H2 at atmospheric pressure obtained by using aberration-corrected environmental transmission electron microscopy (ETEM), density functional theory (DFT) optimizations...

Machine learning approaches can drastically decrease the computational time for the predictions of spectroscopic properties in materials, while preserving the quality of the computational approaches. We studied the performance of kernel-ridge regression (KRR) and gradient boosting regressor (GBR) models trained on the isotropic shielding values, co...

In this contribution, we would like to underline the peculiar chemical properties of nanometer scale metallic particles. To attain this goal, we select the case of DeNox catalysis (NOx reduction to nitrogen molecule) for which such nanomaterials play a crucial role. Experimental data as well as recent theoretical calculation through density functio...

The focus of this short review is directed towards investigations of the dynamics of nanostructured metallic heterogeneous catalysts and the evolution of interfaces during reaction—namely, the metal–gas, metal–liquid, and metal–support interfaces. Indeed, it is of considerable interest to know how a metal catalyst surface responds to gas or liquid...

The enhancement of the catalytic activity of gold nanoparticles with their decreasing size is often attributed to the increasing proportion of low‐coordinated surface sites. This correlation is based on the paradigmatic picture of working gold nanoparticles as perfect crystal forms having complete and static outer surface layers whatever their size...

Single-atom alloys (SAAs) are emerging materials containing isolated metal atoms dispersed on host metal surfaces, exhibiting unique reactivity compared with the corresponding monometallic counterparts. However, the stability of the isolated atoms in the host metal has hardly been studied, although, metal segregation has been commonly observed in b...

The selective shortening of gold nanorods (NRs) is a directional etching process that has been intensively studied by UV-Vis spectroscopy because of its direct impact on the optical response of these plasmonic nanostructures. Here, liquid-cell transmission electron microscopy is exploited to visualize this peculiar corrosion process at the nanoscal...

Revealing the Dynamics of Functional Nanomaterials in Their Formation and Application Media with Liquid and Gas-phase TEM - Damien Alloyeau, Abdelali Khelfa, Kinan Aliyah, Adrian Chmielewski, Jun Meng, Hakim Amara, Hazar Guesmi, Jaysen Nelayah, guillaume Wang, Cyrille Hamon, Doru Constantin, Christian Ricolleau

This contribution aims at rationalizing the observations made by in situ IR spectroscopy during CO adsorption over ca. 1.8 nm Pt-Sn nanoparticles by Density Functional Theory (DFT) calculations and describing the environment and electronic properties of surface Pt atoms modified by Sn. Pt surface enrichment upon CO exposure was observed by diffuse...

Electrosynthesis, a historically powerful tool for the production of a number of industrial-scale inorganic or organic materials, has experienced a renaissance over the last ten years with research efforts seeking...

With the aim to rationalize the reactivity of transition metal (TM) clusters, the model reaction of NO dissociation on TM13 clusters was investigated on a set of seven TMs. The adsorption energy and the activation energy for NO dissociation was calculated for each TM and discussed within the findings of Boudart in his article “Model catalysts: redu...

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)...

The electrocatalytic production of hydrogen from methanol dehydrogenation successfully uses platinum catalysts. However, they are expensive and Pt has the tendency to be poisoned from the intermediate compounds, formed during the methanol oxidation reaction (MOR). For these two reasons, there has been active research for alternative bi- and tri-com...

Despite intensive research efforts, the nature of the active sites for O2 and H2 adsorption/dissociation by supported gold nanoparticles (NPs) is still an unresolved issue in heterogeneous catalysis. This stems from the absence of a clear picture of the structural evolution of Au NPs at near reaction conditions, i.e. at high pressures and high temp...

The stability of Ag-Pt systems: 13-atoms icosahedral cluster model (Ag13-nPtn) and 54-atoms (111) surface slab model (45Ag9Pt) have been systematically investigated by using Density Functional Theory within the generalized gradient approximation (DFT–GGA). Experimentally tested resistance of AgPt alloy nanoparticles supported on silica to decay upo...

In this work first principles density-functional-theory (DFT) calculations and Monte Carlo (MC) simulations are used to study the stability of the new surface phases formed under CO gas pressure on Au-Pd (100). The segregation isotherms reveal the apparition at CO=0.5 monolayer (ML) of two exotic ordered phase structures over the (100) facet, depe...

The reactivity of various Pd ensembles on the Au—Pd(100) alloy catalyst toward CO oxidation was investigated by using density functional theory (DFT). This study was prompted by the search for efficient catalysts operating at low temperature for the CO oxidation reaction that is of primary environmental importance. To this aim, we considered Pd mod...

We identified new magic compositions of Pd-Au nanolloys of simple symmetries according to the icosahedral and the cubic truncated octahedral structures. Chemically ordered structures as well as segregated ones are characterized using Monte Carlo simulations in semi-grand canonical ensemble and tight-binding semi-empirical potential fitted to ab ini...

The reaction mechanism for the formation of alkyl thiol SAM on Au(111) is still not clearly understood. Especially, the role of defects on the chemisorption process is an important goal to be addressed. In this work, different minimum energy reaction paths of R-SH dissociation of thiols (with long and short chains and di-thiol species) adsorbed on...

Hydrogen interaction with bimetallic Au(Pd) and Au(Rh) systems are studied with the density functional theory (DFT)-based periodic approach. Several bimetallic configurations with varying concentrations of Pd and Rh atoms in the under layer of a gold surface(111) were considered. The reactivity of the doped Au(111) toward hydrogen adsorption and ab...

The surface composition of bimetallics can be strongly altered by adsorbing molecules where the metal with the strongest interaction with the adsorbate segregates into the surface. To investigate the effect of reactive gas on the surface composition of Au−Cu alloy we examined by means of Density Functional Theory (DFT) study the segregation behavio...

A DFT based characterization of tungsten oxide supported on amorphous hydroxylated silica is presented. The different molecular organizations are investigated on the surface topology and tungsten oxygen coordination. The presence of mono- and di-grafted species is discussed and rationalized, using an atomistic thermodynamic approach. The presence o...

In order to study how adsorption of CO molecules changes the surface composition of AuPd alloys, we develop a theoretical methodology which is able to take this effect into account. An Ising model based on density functional theory calculations is derived to define interatomic potentials that describe metal–metal, metal–CO, and CO–CO interactions....

In this paper, the size-dependent changes in energetic, vibrational, and electronic properties of C–O gas molecule interacting with surface Pd atom of a variety of AuPd nanoalloy structures are investigated by means of first principles calculations. The variation in C–O adsorption energies, C–O vibration frequencies (νC−O), and Pd d-bond centers (ε...

Gas or liquid phase transesterification reactions are used in the field of biomass valorization to transform some platform molecules into valuable products. Basic heterogeneous catalysts are often claimed for these applications but the role of basicity on the reaction mechanism depending on the operating conditions is still under debate. In order t...

The crossovers among the most abundant structural motifs (icosahedron, decahedron and truncated octahedron) of Pd-Au nanoalloys are determined theoretically on a size range between 2 and 7 nm and for three compositions equivalent to Pd$_3$Au, PdAu and PdAu$_3$. The chemical ordering and segregation optimisation are performed with Monte Carlo simula...

A systematic study on the magnetic property variations of Pd13 particles as a function of particle sizes and structures, and of Pd13 interactions with the (100) TiO2 anatase surface, was carried out with standard density functional theory (DFT) based method with periodic boundary conditions. The biplanar Pd13 structure with the spin state quintet i...

Abstract Bimetallic catalysts have manifold technological applications; their reactivity can greatly exceed that of the original single metals. In this work, we investigate an ideal model-system consisting of a complete monolayer of Pd underneath the surface of Au(111). First, we investigate the stability of this system, and then its interaction wi...

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...

This paper reports a systematic study of the effect of CO gas on the chemical composition at the surface of gold-based alloys. Using DFT periodic calculations in presence of adsorbed CO the segregation behavior of group 9-10-11 transition metals (Ag, Cu, Pt, Pd, Ni, Ir, Rh, Co) substituted in semi-infinite gold surfaces is investigated. Although, C...

Periodic DFT calculations have been performed on molybdenum(vi) oxide species supported on the hydroxylated amorphous silica surface. The Mo grafting site has been investigated systematically for the type of silanol (geminate, vicinal, isolated or in a nest) accessible on the surface, as well as its effect on H-bond formation and stabilization, wit...

Alloy catalysts typically operate under high-pressure and high-temperature conditions, and these reactive environments may substantially influence the alloy surface composition. Theoretical studies of catalytic properties are often investigated on model systems where no account is taken for the possibility that the surface composition can be modifi...

Combined Density Functional Theory (DFT) calculations and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) were performed to study the distribution of Pd atoms in bimetallic AuPd nanoparticles in the presence of adsorbed CO. Compared to vacuum condition, the results showed evidence of Pd surface enrichment where both Pd monomers...

Solid materials possessing basic properties are naturally covered by carbonates and hydroxyl groups. Those natural adsorbates modify their chemical reactivity. This article aims to specifically evidence the role of surface carbonates and hydroxyls in basic heterogeneous catalysis on MgO. It compares the catalytic behaviors of hydroxylated or carbon...

In the case of fully potassium exchanged X and LSX faujasites, it was reported from a single crystal X-ray study on zeolite X and preliminary powder X-ray diffraction (PXRD) study on zeolite LSX that K+ ions can be found concomitantly in both sites I and I′ with partial or full occupancy of site I inside the hexagonal prism depending on the Si/Al r...

Combined experimental infrared (IR) and theoretical approaches have been carried out in an attempt to specify the actual structure of the CO2 species adsorbed on the magnesium oxide surface. The interaction of CO2 with regular sites of the MgO(100), (111), and (110) surfaces as well as MgO(100) defect sites (steps, corners, kinks, and divacancies)...

The Cr/SiO2 system is investigated using periodic DFT. The model represents the amorphous character of the silica surface and allows the investigation of the effect of hydration on the Cr(VI) monomers. First, the geometry and energetics are discussed and compared with experimental data. The phase diagram plotted from an atomistic thermodynamics mod...

Segregation phenomena of group 10 (M = Ni, Pd, Pt) transition-metals substituted in Au (1 1 1) surface and sub-surface layers are investigated by DFT periodic calculations in presence of adsorbed atomic and molecular oxygen. In contrast with vacuum conditions, where the metal impurities M prefer to be in the bulk of gold, in the presence of adsorbe...

The location of the divalent nickel cation exchanged in low amount in dehydrated NaX is identified by a combination of experimental (XAS) and theoretical (DFT) approaches. The Ni–O and Ni–T distances as well as the coordination numbers evaluated from experiments are in good agreement with the predicted models obtained from calculations. After dehyd...

Reliable experimental IR and theoretical approaches, both investigating CO adsorption on NaY faujasites, are supporting that CO capture occurs through the completion of the vacant coordination of Na(+) cations located in the accessible S(II) sites. As a result, carbonyl adsorbed species are formed by the capture of one, two or three CO molecules an...

The location and coordination of nickel cations inside the hexagonal prism of dehydrated sodium faujasites have been investigated as a function of the framework Al content and distribution by density functional theory (DFT) calculations. Three different Si/Al ratios have been considered in order to represent the Na−LSX (Si/Al = 1), Na−X (Si/Al = 1....

Segregation energies of isolated Pd in PdAu(111) alloy are investigated by DFT calculations as a function of the oxygen coverage. Segregation of Pd is found to be oxygen-coverage dependent. While for ‘clean’ PdAu(111), Pd prefers to be in the bulk, in the presence of more than 1/3 ML of oxygen, Pd meanly segregates to the surface. The analysis of P...

The oxidative power toward CO of α-oxygen formed upon N2O dissociation over isolated and binuclear Fe/ZSM-5 zeolite is investigated by means of DFT calculations. The two α-sites [Fe–O]+ and [Fe–(μO)–(μOH)–Fe]+ exchanged in ZSM-5 were considered since their activity in the N2O decomposition was recently shown. Computed electronic properties, charge...

The characterization of Fe/ZSM5 zeolite materials, the nature of Fe-sites active in N(2)O direct decomposition, as well as the rate limiting step are still a matter of debate. The mechanism of N(2)O decomposition on the binuclear oxo-hydroxo bridged extraframework iron core site [Fe(II)(mu-O)(mu-OH)Fe(II)](+) inside the ZSM-5 zeolite has been studi...

The reaction mechanism for nitrous oxide (N2O) direct decomposition into molecular nitrogen and oxygen was studied on binuclear iron sites in Fe-ZSM-5 zeolite using the density functional theory (DFT). Starting from the hydroxylated bi-iron complex [HOFeIII(μ-O)(μ-OH)FeIIIOH]+, a reductive dehydroxylation pathway was proposed to justify the formati...

Elementary reaction mechanisms for nitrous oxide (N2O) dissociation were studied on [FenII(μ-O)(μ-OH)FeII]+ exchanged in ZSM-5, using density functional theory (DFT). The effect of the cluster size on the energetics and on the reaction routes of N2O dissociation were investigated over di-iron core inserted inside two different Z cluster (Z−) and (Z...

The kinetic and thermodynamic properties of adsorption and desorption of Sb onto a Si(111) surface are analyzed by mass spectrometry and ab initio calculations. Two domains of temperature are evidenced. At T<600 °C there is an irreversible adsorption involving a subtle competition between sticking, adsorption, then dissociation of Sb4 tetramers ass...

The reaction mechanism for nitrous oxide (${N}_{2}O$) direct decomposition into molecular nitrogen and oxygen was studied on binuclear iron sites in Fe-ZSM-5 zeolite using the density functional theory (DFT). Starting from the hydroxylated bi-iron complex ${\left}^{+}$, a reductive dehydroxylation pathway was proposed to justify the formation of th...

We have studied by Spot Profile Analysis Low Energy Electron Diffraction (SPA-LEED) the growth of gold particles on the N/Cu(001) self-organized surface. This template consists of nitrogen islands separated by bare Cu lines and forming a regular 2D array of period 5nm. When Au is evaporated onto this surface, it mainly grows at the intersections be...

Mme. Marie-José CASANOVE (présidente) Mme. Sylvie ROUSSET (examinatrice) M. Stéphane ANDRIEU (rapporteur) M. Bernard LEGRAND (rapporteur) M. Pierre MÜLLER (directeur de thèse) M. Guy TRÉGLIA (directeur de thèse)

The experimental study of the thermodynamic and kinetic properties of the Sb/Si(111) interface reveals a surprising behavior: a 2D phase condensates when the Sb coverage increases, indicating strong attractive Sb-Sb interactions, whereas the isotherms present a quasi-Langmuir shape, suggesting that these interactions should be negligible. Ab initio...

In this paper, we use mass spectrometry (MS) and reflection high-energy electron (RHEED) to study the kinetics of adsorption of Sb on Si(1 1 1) surface and its relation to the corresponding surface structure. At high temperature (T > 800 degreesC) all the impinging Sb-4 molecules completely dissociate at the silicon surface and a 2D gas of Sb monom...