Fe Clusters Research Articles

DFT analysis: Fe4 cluster and Fe(110) surface interaction studies with pyrrole, furan, thiophene, and selenophene molecules

DFT studies of both the Fe4 cluster and the Fe(110) surface interaction with pyrrole, furan, thiophene, and selenophene showed that selenophene forms a stabler adsorbate iron complex than the other heterocyclic molecules; this is consistent with the binding energy data that were calculated between the Fe cluster and the Fe(110) surface with the heterocycles. Furthermore, when the adsorption of the compounds with the iron cluster was analyzed by molecular orbital studies, the orbitals of selenophene overlapped more strongly with the Fe atom than that of the other molecules. In TD-DFT, the π → π* peak observed for the molecules disappeared when they formed complexes, and there appeared a charge transfer band (ligand to metal), thus confirming the coordination of these molecules with the cluster. The data suggest that the chemisorption is an exothermic process.

Adatoms and Clusters of3dTransition Metals on Graphene: Electronic and Magnetic Configurations

We investigate the electronic and magnetic properties of single Fe, Co, and Ni atoms and clusters on monolayer graphene (MLG) on SiC(0001) by means of scanning tunneling microscopy (STM), x-ray absorption spectroscopy, x-ray magnetic circular dichroism (XMCD), and abinitio calculations. STM reveals different adsorption sites for Ni and Co adatoms. XMCD proves Fe and Co adatoms to be paramagnetic and to exhibit an out-of-plane easy axis in agreement with theory. In contrast, we experimentally find a nonmagnetic ground state for Ni monomers while an increasing cluster size leads to sizeable magnetic moments. These observations are well reproduced by our calculations and reveal the importance of hybridization effects and intra-atomic charge transfer for the properties of adatoms and clusters on MLG.

Chemisorption of hydrogen on Fe clusters through hybrid bonding mechanisms

The interaction of H and Fe clusters of up to nine atoms were investigated within a density functional theory. Calculations indicate that one gas-phase Fe atom can absorb ten H atoms, an amount 2.5 times more than methane (CH4). The magnetic state of Fe atoms non-uniformly decrease by increasing the number of H. The bonding of Fe-H in FeH clusters consists of charge transfer and electron pairing. Thus, two types of bondings are involved. The bond mechanism is general in nature within transition metal clusters, bringing insight for the development of heterogeneous catalyst and hydrogen storage materials.

Effects of Mo promoters on the Cu–Fe bimetal catalysts for the DMC formation from CO2 and methanol

Abstract The Mo-promoted Cu–Fe bimetal catalysts were prepared and used for the formation of dimethyl carbonate (DMC) from CO2 and methanol. The catalysts were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), laser Raman spectra (LRS), energy dispersive spectroscopy (EDS) and temperature programmed desorption (TPD) techniques. The experimental results demonstrated that the Mo promoters can decrease the reducibility and increase the dispersion of Cu–Fe clusters. The concentration balance of base–acid sites can be readily adjusted by changing the Mo content. The moderate concentration balance of acid and base sites was in favor of the DMC formation. Under optimal experimental conditions, the highest methanol conversion of 6.99% with a DMC selectivity of 87.7% can be obtained when 2.5 wt% of Mo was loaded.

DFT Studies on Doping Effect of Al12X: Adsorption and Dissociation of H2O on Al12X Clusters

The adsorption and reaction of H2O molecule on neutral X-centered icosahedronal Al12X clusters (X = Al, Mg, Zn, Ga, Ni, Fe, B, C, Si, P) were investigated by PW91, PBE, and PWC methods. Reaction energies and reaction barriers were determined. The spin states and the doped atoms have important influences on the Al12X geometries, density, electronic properties, and energy density of reaction between Al12X with a single H2O molecule. The energies of the neutral X-centered Al12X are lower than that of surface X-replaced Al12X with the exception of Al12Mg. The H2O dissociation on the Al12X (X = Mg, Zn, Ga, Ni, Fe) clusters have relatively low activation barriers, but large activation barriers for Al12X (X = B, C, Si, P). The activation barrier of water dissociation on the singlet Al12Fe cluster is the lowest, whereas the highest barrier is with the Al12C. The reaction of H2O with Al12Fe is the most exothermic. The center-Fe atom can move out to the surface after the adsorption and dissociation of H2O with an energy barrier of 172 kJ/mol. The results showed that the water dissociation on the Al12X cluster can be tuned by controllable X doping.

Direct Synthesis of Iron(0) N-Heterocyclic Carbene Complexes by Using Fe3(CO)12 and Their Application in Reduction of Carbonyl Groups

Iron Fe(NHC)(CO)4 complexes were formed by direct reaction of Fe3(CO)12 with equimolecular amounts of NHC imidazolium halide precursors; addition of base was not needed in this reaction. When excess (9:1 ratio) 1,3-dimesitylimidazolium chloride is reacted with the iron cluster Fe3(CO)12, a mixture of Fe(IMes)(CO)4 and Fe(IMes)2(CO)3 is obtained. Single crystals of Fe(IMes)(CO)4 and crystals resulting from the cocrystallization of Fe(IMes)(CO)4 and Fe(IMes)2(CO)3 have been studied by X-ray diffraction. These iron(0) complexes were found to catalyze the reduction of benzaldehydes.

Size and Shape Dependence of Thermal Spin Transitions in Nanoislands

Theoretical calculations of thermal spin transitions in nanoscale clusters on a surface are presented. The mechanisms for magnetization reversal are identified and the activation energy and pre-exponential factor for the rate are evaluated using a recently developed harmonic transition state theory and a Heisenberg-type Hamiltonian. A maximum is found in the pre-exponential factor as a function of cluster size corresponding to a crossover from a uniform rotation mechanism to temporary domain wall formation. As the islands grow, the energy barrier increases up to a limit where the domain wall is fully established. For larger islands, the minimum energy path becomes flat resulting in a significant recrossing correction to the transition state theory estimate of the rate. The parameters of the Hamiltonian are chosen to mimic Fe clusters on a W(110) surface, a system that has previously been studied extensively in the laboratory and the calculated results are found to be in close agreement with the reported measurements.

Characterization of Fe-Exchanged BEA Zeolite Under NH3 Selective Catalytic Reduction Conditions

Structural changes to Fe3+ cationic species in Fe-exchanged zeolite BEA during the selective catalytic reduction (SCR) of NOx with NH3 were probed by UV–vis spectroscopy. The distribution between Fe2+ and Fe3+ species was characterized by IR spectroscopy of adsorbed CO. Upon heating to 723 K, some of the Fe cations formed Fe–O–Fe bonds that underwent reversible structural transformation under NH3–SCR conditions. The in situ formed Fe oxide clusters could be dissociated to isolated Fe cations at 423 K, while at higher temperatures O-bridged Fe clusters were again formed. The structure of the Fe cluster is related to the Al distribution in the zeolite probed by Co2+ ion exchange. We propose here that two Fe cations bound within one six-membered ring containing an Al pair form hydroxylated dimeric Fe–O–Fe in the zeolite. This was supported by a structure simulation of a binuclear [HO-Fe(III)–O–Fe(III)–OH] model.

The stabilization of Fe, Ru, and Os clusters upon hydrogenation

The interaction of H2 and small Fe, Ru, and Os clusters of up to nine atoms was investigated through the density functional theory. The ground state structures of H2 adsorbed Fe, Ru, and Os clusters were explored by using the basin-hopping algorithm. Calculations indicate that Os clusters show relatively high stability and reactivities upon H2 adsorption while bare and hydrogenated Fe clusters show relatively high magnetic moments. The thresholds upon stability change between bare and hydrogenated clusters were also discovered.

Elucidation of Reaction Behaviors in Sonocatalytic Decolorization of Amaranth Dye in Water Using Zeolite Y Co-Incorporated with Fe and TiO<sub>2</sub>

The Zeolite Y as a support was modified by the incorporating of Fe and TiO2 in one single step using impregnation method. The synergistic effects between those components enhance the catalytic activity for the degradation of amaranth dye under ultrasonic irradiation with output power of 50 W and 40 kHz frequency. Different characterization techniques were used to elucidate the physical and chemical properties of the produced catalysts. The XRD results indicated that the type of titanium precursor significantly effects on the crystallinity of 0.4% Fe/15%TiO2-NaY catalyst. The AFM results detected that the TiO2 formed a layer covered the surface of zeolite while Fe clusters were located close to TiO2. The influence of reaction parameters such as TiO2 and Fe content, pH values, amount of hydrogen peroxide used, catalyst loading and the initial dye concentration were investigated for the decolorization efficiency of amaranth. The maximum decolorization efficiency for 0.4%Fe/15%TiO2-NaY was 100% after 120 min of reaction time with an initial dye concentration of 30 mg/L, 2 g/L of catalyst loading, natural pH about 5.5 and 0.65 mM H2O2.

A density functional theory study of oxygen reduction reaction on Me–N4 (Me = Fe, Co, or Ni) clusters between graphitic pores

In this study, we performed first-principles density functional theory (DFT) calculations to investigate the reaction pathway of oxygen reduction reaction (ORR) on Me–N4 (Me = Fe, Co, or Ni) catalytic clusters formed between pores in graphene supports. Our DFT results indicate that formation of Me–N4 clusters at the edges of graphitic pores is energetically feasible. The catalytic mechanism of the Me–N4 clusters for ORR was elucidated by calculating the binding energies of ORR intermediates O2, O, OH, OOH, HOOH, and H2O on these clusters. It was found that O2 could be chemisorbed on the Co–N4 and Fe–N4 clusters but not on the Ni–N4 clusters. Thus, the Co–N4 and Fe–N4 clusters are predicted to be active for catalyzing ORR. Moreover, we observed that the O–O bond in HOOH was broken in the lowest-energy adsorption configuration of HOOH on the Fe–N4 and Co–N4 clusters. This process of breaking the O–O bond suggests that the Fe–N4 and Co–N4 clusters between graphitic pores could promote 4e− reduction of O2 with a single active site. Here, our theoretical study provides an explanation to the experimental findings of 4e− ORR on nitrogen-derived transition metal/carbon electrocatalysts. Furthermore, we illustrated that the ORR activity of the Me–N4 clusters could be gauged by an electronic descriptor related to the d-electron orbitals of the transition metal atom.

In situ observations of meteor smoke particles (MSP) during the Geminids 2010: constraints on MSP size, work function and composition

Abstract. We present in situ observations of meteoric smoke particles (MSP) obtained during three sounding rocket flights in December 2010 in the frame of the final campaign of the Norwegian-German ECOMA project (ECOMA = Existence and Charge state Of meteoric smoke particles in the Middle Atmosphere). The flights were conducted before, at the maximum activity, and after the decline of the Geminids which is one of the major meteor showers over the year. Measurements with the ECOMA particle detector yield both profiles of naturally charged particles (Faraday cup measurement) as well as profiles of photoelectrons emitted by the MSPs due to their irradiation by photons of a xenon-flash lamp. The column density of negatively charged MSPs decreased steadily from flight to flight which is in agreement with a corresponding decrease of the sporadic meteor flux recorded during the same period. This implies that the sporadic meteors are a major source of MSPs while the additional influx due to the shower meteors apparently did not play any significant role. Surprisingly, the profiles of photoelectrons are only partly compatible with this observation: while the photoelectron current profiles obtained during the first and third flight of the campaign showed a qualitatively similar behaviour as the MSP charge density data, the profile from the second flight (i.e., at the peak of the Geminids) shows much smaller photoelectron currents. This may tentatively be interpreted as a different MSP composition (and, hence, different photoelectric properties) during this second flight, but at this stage we are not in a position to conclude that there is a cause and effect relation between the Geminids and this observation. Finally, the ECOMA particle detector used during the first and third flight employed three instead of only one xenon flash lamp where each of the three lamps used for one flight had a different window material resulting in different cut off wavelengths for these three lamp types. Taking into account these data along with simple model estimates as well as rigorous quantum chemical calculations, it is argued that constraints on MSP sizes, work function and composition can be inferred. Comparing the measured data to a simple model of the photoelectron currents, we tentatively conclude that we observed MSPs in the 0.5–3 nm size range with generally increasing particle size with decreasing altitude. Notably, this size information can be obtained because different MSP particle sizes are expected to result in different work functions which is both supported by simple classical arguments as well as quantum chemical calculations. Based on this, the MSP work function can be estimated to lie in the range from ~4–4.6 eV. Finally, electronic structure calculations indicate that the low work function of the MSP measured by ECOMA indicates that Fe and Mg hydroxide clusters, rather than metal silicates, are the major constituents of the smoke particles.

Structural, bonding, and magnetic properties of Fen–xSix (n, x ⩽ 6) clusters: Theoretical investigation based on density functional theory

The structural, bonding, and magnetic properties of clusters of Fen, Sin, and Fen–xSix (n=2–6, x=1–6) optimized at the B3LYP/LanL2DZ level with symmetry constraints are determined. The effects of the substitution of Si in Fen on the properties of Fen are identified and discussed.The cohesion of the Fe cluster is nonmonotonically increased by the successive Si substitutions. In the mixed Fe–Si clusters, the Fe–Fe bond lengths are not affected by the Si substitutions and are similar to those in Fe clusters as well as the Fe–Si bond lengths in mixed clusters. Most of the Si–Si bond lengths in mixed clusters are similar to those in Si clusters, while a few Si–Si bond lengths characterized by the hybridization with high s-character are smaller than or equal to that in Si2. Several distances between neighboring atoms at equator in the mixed clusters are found to be remarkably large, and there is the lack of bonding between them. The average magnetic moments of mixed clusters decrease nonmonotonically with the number of Si atoms, and approach those of Si clusters. There are large amount of spin on Fe atoms and small amount of spin on Si atoms in the mixed clusters. Most of the local atomic moments on Si atoms align antiferromagnetically with respect to those on Fe atoms.The properties of Fe clusters with Si substitutions result from the electron transfer from Fe to its surrounding Si atoms. The electron transfer is closely connected with the properties of Fe–Si alloys.

Radical-translocation intermediates and hurdling of pathway defects in "super-oxidized" (Mn(IV)/Fe(IV)) Chlamydia trachomatis ribonucleotide reductase.

A class I ribonucleotide reductase (RNR) uses either a tyrosyl radical (Y(•)) or a Mn(IV)/Fe(III) cluster in its β subunit to oxidize a cysteine residue ∼35 Å away in its α subunit, generating a thiyl radical that abstracts hydrogen (H(•)) from the substrate. With either oxidant, the inter-subunit "hole-transfer" or "radical-translocation" (RT) process is thought to occur by a "hopping" mechanism involving multiple tyrosyl (and perhaps one tryptophanyl) radical intermediates along a specific pathway. The hopping intermediates have never been directly detected in a Mn/Fe-dependent (class Ic) RNR nor in any wild-type (wt) RNR. The Mn(IV)/Fe(III) cofactor of Chlamydia trachomatis RNR assembles via a Mn(IV)/Fe(IV) intermediate. Here we show that this cofactor-assembly intermediate can propagate a hole into the RT pathway when α is present, accumulating radicals with EPR spectra characteristic of Y(•)'s. The dependence of Y(•) accumulation on the presence of substrate suggests that RT within this "super-oxidized" enzyme form is gated by the protein, and the failure of a β variant having the subunit-interfacial pathway Y substituted by phenylalanine to support radical accumulation implies that the Y(•)(s) in the wt enzyme reside(s) within the RT pathway. Remarkably, two variant β proteins having pathway substitutions rendering them inactive in their Mn(IV)/Fe(III) states can generate the pathway Y(•)'s in their Mn(IV)/Fe(IV) states and also effect nucleotide reduction. Thus, the use of the more oxidized cofactor permits the accumulation of hopping intermediates and the "hurdling" of engineered defects in the RT pathway.

Unraveling the Atomic Structure of Ultrafine Iron Clusters

Unraveling the atomic structures of ultrafine iron clusters is critical to understanding their size-dependent catalytic effects and electronic properties. Here, we describe the stable close-packed structure of ultrafine Fe clusters for the first time, thanks to the superior properties of graphene, including the monolayer thickness, chemical inertness, mechanical strength, electrical and thermal conductivity. These clusters prefer to take regular planar shapes with morphology changes by local atomic shuffling, as suggested by the early hypothesis of solid-solid transformation. Our observations differ from observations from earlier experimental study and theoretical model, such as icosahedron, decahedron or cuboctahedron. No interaction was observed between Fe atoms or clusters and pristine graphene. However, preferential carving, as observed by other research groups, can be realized only when Fe clusters are embedded in graphene. The techniques introduced here will be of use in investigations of other clusters or even single atoms or molecules.

Hybridization and magnetism in small FePt alloy clusters

Magnetic properties of small, mass selected, Fe and FePt alloy clusters deposited on a remanently magnetized Ni/Cu(001) substrate have been investigated using x-ray magnetic circular dichroism. Strong size- and stoichiometry-dependent magnetic properties of Fe and FePt clusters are found, together with enhanced orbital moments compared to bulk and nanoparticles. Compared to pure Fe clusters, FePt alloy clusters have lower orbital and higher spin moments. In addition, they show a higher branching ratio of Br = L3/(L2 + L3) due to stronger spin–orbital coupling and hybridization.

Theoretical study on the encapsulation of Pd3-based transition metal clusters inside boron nitride nanotubes

Chemical functionalization of the boron nitride nanotube (BNNT) allows a wider flexibility in engineering its electronic and magnetic properties as well as chemical reactivity, thus making it have potential applications in many fields. In the present work, the encapsulation of 13 different Pd(3)M (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Pd, Pt, and Au) clusters inside the (10, 0) BNNT has been studied by performing comprehensive density functional theory (DFT) calculations. Particular attention is paid to searching for the stable configurations, calculating the corresponding binding energies, and evaluating the effects of the encapsulation of Pd(3)M cluster on the electronic and magnetic properties of BNNT. The results indicate that all the studied Pd(3)M clusters can be stably encapsulated inside the (10, 0) BNNT, with binding energies ranging from -0.96 (for Pd(3)Sc) to -5.31 eV (for Pd(3)V). Moreover, due to a certain amount of charge transfer from Pd(3)M clusters to BNNT, certain impurity states are induced within the band gap of pristine BNNT, leading to the reduction of the band gap in various ways. Most Pd(3)M@BNNT nanocomposites exhibit nonzero magnetic moments, which mainly originate from the contribution of the Pd(3)M clusters. In particular, the adsorption of O(2) molecule on BNNT is greatly enhanced due to Pd(3)M encapsulation. The elongation of O-O bonds of the adsorbed O(2) molecules indicates that Pd(3)M@BNNT could be used to fabricate the oxidative catalysis.

Anoxia-Induced Release of Colloid- and Nanoparticle-Bound Phosphorus in Grassland Soils

Particle-facilitated transport is a key mechanism of phosphorus (P) loss in agroecosystems. We assessed contributions of colloid- and nanoparticle-bound P (nPP; 1-415 nm) to total P released from grassland soils receiving biannual poultry litter applications since 1995. In laboratory incubations, soils were subjected to 7 days of anoxic conditions or equilibrated at pH 6 and 8 under oxic conditions and then the extract was size fractionated by differential centrifugation/ultrafiltration for analysis of P, Al, Fe, Si, Ti, and Ca. Selected samples were characterized by transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS) and field flow fractionation (FFF-ICP-MS). Particles released were present as nanoaggregates with a mean diameter of 200-250 nm, composed of ~50-nm aluminosilicate flakes studded with Fe and Ti-rich clusters (<10 nm) that contained most of the P detected by EDS. Anoxic incubation of stimulated nPP release with seasonally saturated soils released more nPP and Fe(2+)(aq) than well-drained soils; whereas, nonreductive particle dispersion, accomplished by raising the pH, yielded no increase in nPP release. This suggests Fe acts as a cementing agent, binding to the bulk soil P-bearing colloids that can be released during reducing conditions. Furthermore, it suggests prior periodic exposure to anoxic conditions increases susceptibility to redox-induced P mobilization.

Trends in the magnetic properties of Fe, Co, and Ni clusters and monolayers on Ir(111), Pt(111), and Au(111)

We present a detailed theoretical investigation on the magnetic properties of small single-layered Fe, Co and Ni clusters deposited on Ir(111), Pt(111) and Au(111). For this a fully relativistic {\em ab-initio} scheme based on density functional theory has been used. We analyse the element, size and geometry specific variations of the atomic magnetic moments and their mutual exchange interactions as well as the magnetic anisotropy energy in these systems. Our results show that the atomic spin magnetic moments in the Fe and Co clusters decrease almost linearly with coordination on all three substrates, while the corresponding orbital magnetic moments appear to be much more sensitive to the local atomic environment. The isotropic exchange interaction among the cluster atoms is always very strong for Fe and Co exceeding the values for bulk bcc Fe and hcp Co, whereas the anisotropic Dzyaloshinski-Moriya interaction is in general one or two orders of magnitude smaller when compared to the isotropic one. For the magnetic properties of Ni clusters the magnetic properties can show quite a different behaviour and we find in this case a strong tendency towards noncollinear magnetism.

Trigonal Mn3 and Co3 clusters supported by weak-field ligands: a structural, spectroscopic, magnetic, and computational investigation into the correlation of molecular and electronic structure.

Transamination of divalent transition metal starting materials (M(2)(N(SiMe(3))(2))(4), M = Mn, Co) with hexadentate ligand platforms (R)LH(6) ((R)LH(6) = MeC(CH(2)NPh-o-NR)(3) where R = H, Ph, Mes (Mes = Mesityl)) or (H,Cy)LH(6) = 1,3,5-C(6)H(9)(NHPh-o-NH(2))(3) with added pyridine or tertiary phosphine coligands afforded trinuclear complexes of the type ((R)L)Mn(3)(py)(3) and ((R)L)Co(3)(PMe(2)R')(3) (R' = Me, Ph). While the sterically less encumbered ligand varieties, (H)L or (Ph)L, give rise to local square-pyramidal geometries at each of the bound metal atoms, with four anilides forming an equatorial plane and an exogenous pyridine or phosphine in the apical site, the mesityl-substituted ligand ((Mes)L) engenders local tetrahedral coordination. Both the neutral Mn(3) and Co(3) clusters feature S = (1)/(2) ground states, as determined by direct current (dc) magnetometry, (1)H NMR spectroscopy, and low-temperature electron paramagnetic resonance (EPR) spectroscopy. Within the Mn(3) clusters, the long internuclear Mn-Mn separations suggest minimal direct metal-metal orbital overlap. Accordingly, fits to variable-temperature magnetic susceptibility data reveal the presence of weak antiferromagnetic superexchange interactions through the bridging anilide ligands with exchange couplings ranging from J = -16.8 to -42 cm(-1). Conversely, the short Co-Co interatomic distances suggest a significant degree of direct metal-metal orbital overlap, akin to the related Fe(3) clusters. With the Co(3) series, the S = (1)/(2) ground state can be attributed to population of a single molecular orbital manifold that arises from mixing of the metal- and o-phenylenediamide (OPDA) ligand-based frontier orbitals. Chemical oxidation of the neutral Co(3) clusters affords diamagnetic cationic clusters of the type [((R)L)Co(3)(PMe(2)R)(3)](+). Density functional theory (DFT) calculations on the neutral (S = (1)/(2)) and cationic (S = 0) Co(3) clusters reveal that oxidation occurs at an orbital with contributions from both the Co3 core and OPDA subunits. The predicted bond elongations within the ligand OPDA units are corroborated by the ligand bond perturbations observed by X-ray crystallography.