Equivalent Monolayers Research Articles

WO3–CeO2 and Pd/WO3–CeO2 as Potential Catalysts for Reforming Applications: I. Physicochemical Characterization Study

WO3–CeO2 (9.1 wt% WO3) and Pd/WO3–CeO2 materials were prepared with a tungsten loading corresponding to a tungsten coverage lower than one theoretical equivalent monolayer. Physicochemical characterizations (N2 adsorption, X-ray diffraction, Raman spectroscopy, and TPR experiments) allowed us to show that the technique used to prepare the catalyst favors the formation of a dispersed W phase containing tetrahedral tungsten species. We identified the important role of the Na remaining from the tungstate precursor (Na2WO4) on the structural evolution of W species. Na+ strongly bound to the support would prevent the condensation of monomeric WO42−, avoiding the formation of polymeric species. We have proposed the existence of various WO42− species (monomeric or dimeric), as a function of the adsorption mode for which the tungsten is maintained in the +VI oxidation state. Dehydration treatment leads to WO42− species linked to a pair of hydroxyl groups which induces structure distortions. Some WO3 crystallites are observed after a reduction at 350°C of the WO3–CeO2 material which are no longer observed after calcination. The tungstate species are strongly bound to the support and thus hardly reducible. They are reduced around 900°C in one step giving tungsten metal.

Characterization of La 2O 3-TiO 2 and V 2O 5/La 2O 3-TiO 2 catalysts and their activity for synthesis of 2,6-dimethylphenol

The La 2O 3-TiO 2 (1:5 molar ratio) mixed oxide was prepared by a co-precipitation method with in situ generated ammonia and was impregnated with various amounts of vanadia (4–12 wt.%). The La 2O 3-TiO 2 and the V 2O 5/La 2O 3-TiO 2 catalysts were subjected to thermal treatments from 773 to 1073 K and were investigated by X-ray diffraction, FT-infrared, BET surface area, and O 2 chemisorption methods to establish the effects of vanadia loading and thermal treatments on the surface structure of the dispersed vanadium oxide species and the temperature stability of these catalysts. Conversion of cyclohexanol to cyclohexanone/cyclohexene was investigated as a model reaction to assess the acid–base properties of these materials. The catalytic property was evaluated for a single step synthesis of 2,6-dimethylphenol from cyclohexanone and methanol in the vapor phase at normal atmospheric pressure. Characterization results suggest that the co-precipitated La 2O 3-TiO 2 when calcined at 773 K is in X-ray amorphous state and exhibits reasonably high specific surface area. The amorphous La 2O 3-TiO 2 is converted into crystalline compounds La 2Ti 2O 7 and La 4Ti 9O 24 at 873 and 1073 K, respectively. The La 2O 3-TiO 2 also accommodates a monolayer equivalent of V 2O 5 (12 wt.%) in a highly dispersed state on its surface. The V 2O 5/La 2O 3-TiO 2 catalyst is thermally quite stable up to 873 K calcination temperature. When subjected to thermal treatments beyond 873 K, the dispersed vanadium oxide selectively interacts with La 2O 3 portion of the mixed oxide and forms a LaVO 4 compound. The remaining TiO 2 appears in the form of anatase phase. The La 2O 3-TiO 2 and the V 2O 5/La 2O 3-TiO 2 differ in terms of acid–base properties and the 12% V 2O 5/La 2O 3-TiO 2 catalyst provided a maximum yield of 2,6-dimethylphenol among various catalysts investigated.

Stability of an ultra-thin Fe film on a Cu(1 0 0) surface studied by positron-annihilation induced Auger electron spectroscopy (PAES)

The stability and intermixing of ultra-thin Fe layers grown on a Cu(1 0 0) surface has been studied as a function of film thickness, growth temperature and annealing history using positron-annihilation induced Auger electron spectroscopy (PAES). Theoretical calculations and previous experimental PAES studies indicate that when a complete metal layer is formed on top of a metal substrate almost of all the PAES signal originates from the overlayer. However, in the research reported here, it was found that even after depositions in excess of 1 equivalent monolayer (eq. ML) of Fe at 173 K and 3 eq. ML at 303 K, a significant fraction of the Cu PAES signal was observed. These result are consistent with the formation of Fe islands leaving regions of exposed Cu surface or with Cu mixing with or moving on top of the deposited Fe. The increase in the fraction of Cu in the top most layer observed as the sample was heated is consistent with a thermally induced increase in island height or an increase in the diffusion of Cu into or on top of the surface layer.

The nucleation and growth of gold on silica

The nucleation and growth of Au clusters supported on SiO 2/Mo(1 1 0) has been studied by X-ray photoelectron spectroscopy (XPS), low energy ion scattering (LEIS), and temperature programmed desorption (TPD). Stoichiometric ultra-thin SiO 2 films were synthesized and characterized by Auger electron spectroscopy (AES) and XPS. At 300 K, the growth mode of Au at fractional monolayer coverages is quasi-two-dimensional (2D); at higher coverages, three-dimensional (3D) growth was found with no evidence of a significant chemical interaction between gold and silica. Annealing Au/SiO 2/Mo(1 1 0) to 1000 K led to sintering of the gold clusters. The desorption activation energies for Au coverages <0.2 monolayer equivalents were determined by TPD to be significantly lower than the sublimation energies found for higher coverages of Au clusters and for bulk gold.

Organic Matter-Surface Area Relationships in Acid Soils

Soil organic matter (OM) and mineral surfaces are intimately related, affecting the dynamics of each and their reactivity with many environmentally important substances. We examined the coverage of mineral surfaces by OM in acid soils of Massachusetts. Specific surface areas are controlled by a combination of clay and sesquioxide contents. Subsurface horizons, especially C horizons with pH 4.6 to 4.8, contained a phase with significant microporosity (pores <2 nm) that could be eliminated by 350°C muffling. Organic C (OC) concentrations in surficial (A, O) horizons have surface area-normalized loadings usually above the monolayer-equivalent (ME) level (1 mg OC m -2 ), while B and C horizons usually have loadings at this level. Surface area-normalized loadings are inversely related to pH for each horizon type. Samples with high loadings show occlusion of the bulk of mineral surface area by OM, as evidenced by release of significant surface area after OM removal. However, a new method of assessing OM coverage of exposed surfaces, using the energetics of gas adsorption, indicate that the bulk of surface area exposed in most untreated samples consists of mineral rather than organic material. The data are consistent with a model in which the occluding OM is present in a low-surface area configuration, such as organoclay aggregates, rather than as dispersed coatings on mineral grains.

The effect of the duration of n-butane/air pretreatment on the morphology and reactivity of (VO)2P2O7 catalysts

Three vanadium pyrophosphate catalysts have been prepared by calcining vanadium hydrogen phosphate hemihydrate (VOHPO4⋅0.5H2O, prepared in an organic medium) for different lengths of time (40, 100 and 132 h) in a n-butane (0.75%)/air mixture at 473 K. The catalysts were designated VPO40, VPO100 and VPO132. Increasing the duration of reaction with n-butane/air mixture led to an increase in the total surface area from 21.3 m2 g−1 (VPO40) to 24.9 m2 g−1(VPO100) and to 27.0 m2 g−1(VPO132). It also led to the complete removal of the VOPO4 phase from catalysts VPO100 and VPO132, this VOPO4 phase having seen as a minor component of catalyst VPO40. Scanning electron microscopy showed that longer periods of pretreatment in the n-butane/air mixture produced catalysts with increasing amounts of a characteristic rosette-type of agglomerate. Temperature-programmed reduction with H2 resulted in the removal of ∼11 monolayers equivalent of oxygen from all three of these catalysts at a peak maximum temperature of ∼1000 K with the development of a second reduction peak at ∼1100 K which increases with increasing time of n-butane/air pretreatment. The morphology produced by extended pretreatment in the n-butane/air mixture at 673 K is therefore predisposed to reaction with H2 (and probably with n-butane). Apparently paradoxically, increasing the duration of n-butane/air pretreatment results in catalysts which on temperature-programmed desorption desorb less oxygen.

Imaging ultrathin Al 2O 3 films with scanning tunneling microscopy

Reproducible scanning tunneling microscopic (STM) images were obtained from ultrathin Al 2O 3 films epitaxially grown on Re(0 0 0 1) . Initially, the oxide films grow two-dimensionally in a layer-by-layer fashion with well-ordered surface morphologies. As the oxide film thickens to ca. 9 monolayer equivalents (MLE), the surface roughens and becomes more disordered yet still exhibits significant long-range, hexagonal periodicity as indicated by low energy electron diffraction (LEED). Because of limited conductivity, films thicker than ca. 9 MLE could not be imaged.

Protein interactions with polyelectrolyte multilayers: interactions between human serum albumin and polystyrene sulfonate/polyallylamine multilayers.

The interactions between polystyrenesulfonate (PSS)/polyallylamine (PAH) multilayers with human serum albumin (HSA) were investigated by means of scanning angle reflectometry (SAR). We find that albumin adsorbs both on multilayers terminating with PSS (negatively charged) or PAH (positively charged) polyelectrolytes. On films terminating with PSS only, an albumin equivalent monolayer is found whereas when PAH constitutes the outer layer, albumin interacts with the multilayer in such a way as to form a protein film that extends over thicknesses that can be as high as four times the largest dimension of the native albumin molecule. Once the protein film is formed, it is found that when the albumin solution is replaced by a pure buffer solution of same ionic strength as the adsorption solution almost no desorption takes place. On the other hand, when a buffer solution of higher ionic strength is brought in contact with the albumin film, a significant amount of adsorbed proteins is released. One also observes that, for albumin solutions of a given protein concentration, the adsorbed protein amount depends on the ionic strength of the adsorption solution. On surfaces terminating with PAH, the adsorbed protein amount first increases rapidly but passes through a maximum and decreases with the ionic strength. The ionic strength corresponding to the maximum of the adsorbed albumin amount itself depends on the albumin concentration. On the other hand, on films terminating with PSS the adsorbed amount increases with the salt concentration before leveling-off. These results show that the underlying complexity of concentration and pH dependent adsorption/desorption equilibria often simply termed "protein adsorption" is the result of antagonist competing interactions that are mainly of electrostatic origin. We also propose two microscopic models, that are compatible with our experimental observations.

Atomic layer doping of SiGe – fundamentals and device applications

The paper reviews results of atomic layer processing for P and B doping of SiGe and for SiGe:C epitaxy using LP(RT)CVD. Atomic layer processing for the base doping of SiGe:C heterojunction bipolar transistors (HBTs) is demonstrated for the first time. P atomic layer doping is self-limiting. The process is controlled by the dissociative adsorption of PH3 with an activation energy of 0.3 eV. We find no self-limitation of diborane adsorption for B atomic layer doping. At low diborane partial pressures, the doping is dominated by the dissociative adsorption of diborane at Si and Ge surface sites. B doses below 1 ML can be deposited. At high partial pressures of diborane the diborane is adsorbed at B occupied surface sites, resulting in several monolayer equivalents of B. The SiGe:C atomic layer epitaxy is found to be controlled by the dissociative adsorption of methylsilane with an activation energy of 0.36 eV. Transit frequencies of about 45 GHz have been obtained for B atomic layer doped SiGe:C HBTs. The HBT results demonstrate the capability of atomic layer processing for doping of advanced devices with critical requirements for dose and location control.

CO-free production of hydrogen via stepwise steam reforming of methane

CO-free hydrogen is produced by the reversible cyclic stepwise steam reforming of methane for use in fuel cells and other processes that are sensitive to CO poisoning. The process consists of two steps involving the decomposition of methane in a first step followed by steam gasification of the surface carbon in a second step. The ease of carbon removal in step II is strongly dependent on temperature and surface coverage of carbon. To optimize the process conditions, we have investigated this cyclic methane steam-reforming process as a function of temperature and surface coverage of carbon. These studies show that it becomes increasingly difficult to remove the surface carbon at higher temperatures and coverage. Since higher temperatures favor methane conversion and hydrogen production in step I, it is essential to delicately optimize the temperature conditions for the process. The hydrogen produced in step I of the cyclic process in the optimum range of temperature conditions (≤673 K) is CO-free (<20 ppm).The amount of H2 produced in step I varies from 1.0 to 1.3 mol/mol methane consumed. The optimum process conditions for carrying out the two-step process are temperatures of 648–673 K and a carbon surface coverage of 0.10–0.20 monolayer equivalents (MLEs).

Structural features of Y-saturated and supersaturated grain boundaries in alumina

Grain boundary segregation of Y in α-Al 2O 3 and evolution of the structural environment around the Y atoms have been investigated using high resolution STEM and EXAFS. The stages of incorporation of Y atoms by α-Al 2O 3 grain boundaries, on average, are characterized by three composition regimes: (I) dilute to saturated; (II) supersaturated [where the degree of supersaturation is determined by the nucleation barrier for Y 3Al 5O 12 (YAG)]; and (III) equilibrium with YAG precipitates. The average Y grain boundary concentration in equilibrium with YAG precipitates has been determined to be ∼1/4 equivalent monolayer, and the maximum supersaturation concentration has been determined to be ∼1/2 equivalent monolayer. EXAFS revealed that accompanying the supersaturation of grain boundaries with Y is an increasing Y–O nearest neighbor coordination number and, simultaneously, a significantly increased degree of ordering of Y with respect to Al ions beyond nearest neighbor O. This Y–Al distance is the same as that for Y absorbed on the free surface of α-Al 2O 3, and the same as that expected for the Y–Al distance when Y substitutes for Al with the Y–O distance relaxed to that in Y 2O 3. This compositional and structural information has led to a clearer picture of how the grain boundary segregated Y concentration influences grain boundary structure. For dilute Y concentrations, Y ions preferentially fill sites in the grain boundary core which have well defined order only within the nearest neighbor shell of oxygens. As the Y concentration increases, Y begins to occupy near-boundary sites, forming two near-boundary layers, each adjacent to a grain surface. The near-boundary layer has nearest neighbor ordering extending at least to nearest neighbor cations. Nucleation of the YAG phase leads to the depletion of Y from these partially ordered layers.

Two-dimensional Mn–Co surface alloy on [formula omitted

Surface alloy formation of Mn on Co(001) was detected by low-energy electron diffraction and in situ magneto-optic Kerr effect experiments. Here we discuss the electronic and magnetic properties of an equivalent Mn monolayer (ML) at the surface of Co(001) using the tight-binding linear muffin-tin orbitals method. Magnetic configurations and stability of ordered 2D Mn–Co alloy 2 ML thick vs Mn/Co(001) and Co/Mn/Co(001) have been considered. Multiple magnetic configurations have been always obtained but a comparison between the ground states display an instability of the perfect Mn ML on Co(001). Total energy differences are considerably reduced in the case of spin-polarization calculations as compared with the non-magnetic one.

Structure, microstructure and electronic characterisation of the Al 2O 3/SiO 2 interface by electron spectroscopies

Al 2O 3 has been deposited by evaporation and post-oxidation of Al on a flat SiO 2 substrate. The obtained films have been analysed by X-ray photoemission spectroscopy (XPS) and reflection electron energy loss spectroscopy (REELS). It has been found that the modified Auger parameter of Al varies by ∼2.2 eV between a low coverage situation (below an equivalent monolayer) and the bulk material. This change has been attributed to variations in the coordination state of the aluminium atoms. Some minor contributions due to bonding and polarisation effects are also detected. Valence band photoemission and REELS have shown that the band gap energy of the deposited Al 2O 3 increases as the coverage decreases. These effects are discussed in terms of the bonding interactions between the SiO 2 and Al 2O 3 at the interface between the two oxides. Sandwich structures formed by approximately one equivalent monolayer of Al 2O 3 intercalated between SiO 2 present Auger parameter and band gap energy values which are larger than those of a monolayer-like Al 2O 3 thin film supported on SiO 2. Also, it has been shown that the energy of the plasmon losses behind the Al KLL Auger peak can be used as a fingerprint to discriminate between supported Al 2O 3 thin films or layers of this material embedded within a sandwich structure of two SiO 2 layers.

Magnetically stabilized Mn–Co surface alloys on Co(001)

Choi et al. (Phys. Rev. B 58 (1998) 5166) using low-energy electron diffraction recently displayed an ordered Mn–Co alloy at the surface of Co for a 0.3–0.8 equivalent monolayer of Mn on Co(001). From magneto optic Kerr effect and X-ray magnetic circular dichroism experiments it was concluded that the coupling between Mn and Co in these surface alloys is ferromagnetic in type. Using the tight binding linear muffin tin orbitals scheme within an atomic sphere approximation and an local density approximation (LDA) in the film geometry we conclude that Mn in the ordered 1 ML thick Mn 0.5Co 0.5 surface alloy is in a high spin state of ca. 3.2 μ B. Both ferromagnetic and antiferromagnetic couplings between Mn and Co are obtained but the antiferromagnetic coupling seems to be the preferred type. Going beyond LDA and considering a slightly disordered surface alloy should, therefore, be worthwhile.

AFM and RHEED study of Ge/Si(001) quantum dot modification by Si capping

Atomic force microscopy (AFM) images of 6 Ge equivalent monolayers (EML) deposited on Si(001) at 500°C by solid source molecular beam epitaxy reveal the formation of ∼2 nm high islands with a height/base dimension ratio close to 1/10. Clear differences are observed, by both reflection high-energy electron diffraction and AFM, according to the Si capping conditions of such hut clusters. At room temperature the impinging Si adatoms homogeneously cover the Ge clusters due to very low Si and Ge surface diffusions nearly preserving the corrugation of uncovered Ge clusters. In contrast, at 500°C and a low Si deposition rate (∼2 EML min −1), a surface smoothing occurs after deposition of a thickness as low as 5 Si EML. A Si adatom diffusion from the apex towards the bottom of the islands concomitant with a Ge lateral segregation lead to a rapid collapse of the buried islands. This scenario is supported by the observation of a (2×9) superstructure, characteristic of the formation of SiGe alloy.

A study of gold ultrathin film growth on yttria-stabilized ZrO 2(100)

The initial stages, up to five equivalent monolayers, of Au growth on yttria-stabilized ZrO 2 (100) at 680 K were studied by X-ray photoelectron and X-ray induced Auger electron spectroscopy (XPS/XAES). The XPS results indicate that Au grows as three-dimensional clusters on the substrate. The coverage-dependent average coordination number for Au atoms is estimated from the observed Au 5d 5/2–5d 3/2 splitting. Analysis of the XPS and XAES results indicate that Au does not interact substantially with the substrate. The initial state contribution to the binding energy shift, as obtained by an Auger parameter analysis, and its relation with Au cluster size is discussed.

The heterogeneous reaction of ozone with soot aerosol

The loss of ozone on carbon soot aerosol was investigated in the temperature range 238–330 K for reaction times up to 72 h: 200 μg m −3 ozone in dry synthetic air is depleted with an initial reaction probability γ=1.2×10 −6 at 296 K. The reaction probability decreases with a characteristic time τ of about 12 h (1/ τ= k 3=(2.3±0.6)×10 −5 s −1) due to surface passivation. The reaction rate has a positive temperature dependence, and a complex negative dependence on the ozone concentration. A minimum set of four quasi-elementary reactions is required to model the observed concentration and time dependencies of the ozone loss rate: rapid destruction of one monolayer equivalent of ozone on pristine surface sites (SS): SS+O 3→SSO+O 2 (I), ozone-induced recovery of reactive sites: SSO+O 3→SS+2O 2 (or SS'+O 2+CO 2) (IIa,b), spontaneous recovery of reactive sites: SSO→SS'+CO, and spontaneous site passivation: SSO→SS p (III). The kinetic parameters γ 2ab, k 2c , and k 3, which were determined in 11 independent experiments, lead to the following analytical expression for the effective reaction probability γ eff which applies when the fast initial surface oxidation has reached a steady state: γ eff =[2γ 2ab+2.6×10 15k 2c/([ O 3]〈c〉)] exp (−k 3t). This equation is valid over a wide range of atmospheric conditions. It implies that ozone depletion on dry soot aerosol is negligible both in the troposphere and lower stratosphere.

Molecular lateral diffusion in model membrane systems

The effect of both interface type and temperature on the lateral diffusion coefficient ( D) of the fluorescent probe NBD-PE in egg PC films of different types has been studied by FRAP. Three model membrane systems (monolayer, black lipid membrane (BLM) and foam films (FF) of different thickness’) have been investigated. The monolayer at an air/water interface has been taken as a point of reference for the other two types of interface, probing the effect of introducing a second monolayer in close proximity to the first one. The results on monolayers showed an exponential relation between the area and diffusion. In the case of FFs going from thinner to thicker films saw a corresponding increase in the probe D, ultimately approaching its characteristic value in monolayers of similar free volume, at a macroscopic air/water interface above an infinitely thick subphase. The fact that the diffusion is clearly a function of thickness of FFs indicates a complex regime of modal coupling in which the headgroup interaction plays an important role. The diffusion coefficients in BLMs were found to be higher than those in the equivalent monolayers and FFs. A comparative analysis of D in monolayers, FFs of different thickness and BLMs has demonstrated the importance of monolayer orientation in the different model systems for the rate of molecular mobility in the plane of model interfaces. It also highlights the role of headgroup–headgroup interactions normal to the plane of the film which are predominant in FF systems.

Structure and morphology of the Ag/MgO(001) interface duringin situgrowth at room temperature

The structure and morphology of Ag deposits grown at room temperature on high-quality MgO(001) surfaces have been investigated in situ, from 0.2 to 300 equivalent monolayers (ML) of Ag deposited. Surface x-ray diffraction and grazing incidence small angle x-ray scattering parallel and perpendicular to the surface were combined. Nucleation, growth, and coalescence of islands are found from 0.2 ML. The average in-plane width, height, and in-plane separation of growing islands are deduced and are found to reproduce well the Mg $1s$ x-ray photoemission spectroscopy spectrum previously reported by other authors. The height over width ratio of the islands is $\ensuremath{\sim}0.37\ifmmode\pm\else\textpm\fi{}0.05$ at all stages of the deposit. Ag grows in cube-on-cube epitaxy with respect to the MgO(001) substrate. A very unusual evolution of the state of strain in Ag with increasing amount of Ag deposited is observed. Below 4--6 ML (island width smaller than 90 \AA{}), the small Ag islands are coherent with the MgO. Below 1 ML (island width smaller than 35 \AA{}), they have their bulk lattice parameter, and between 1 and 4 ML they become more and more strained by the MgO substrate, with an average lattice parameter intermediate between those of Ag and of MgO. Around 4--6 ML, the islands reach a critical size and misfit dislocations are introduced at the edges. Above 30 ML, the film is almost continuous, and the interfacial misfit dislocations reorder to form a square network, oriented along 〈110〉 directions. Stacking faults appear in Ag at this stage. A small amount of twinned Ag also starts to grow around 4 ML. This unusual evolution of the strain in the Ag islands and the following introduction of misfit dislocations are interpreted on the basis of a one-dimensional Frenkel-Kontorova model involving a very weak Ag-MgO interaction and a weak corrugation of the interatomic potential. Quantitative measurements and analysis of the MgO crystal truncation rods (CTR's) during growth were shown to provide different structural parameters of the interface that are important for theoretical calculation, especially the epitaxial site, above oxygen atoms of the substrate, and the interfacial distance $(2.52\ifmmode\pm\else\textpm\fi{}0.1\AA{}).$ The origin of the interference along the CTR's is discussed according to the strain state of the epitaxial Ag.

Vanadium oxides thin films grown on rutile TiO 2(110)-(1×1) and (1×2) surfaces

Vanadium oxides on rutile TiO 2(110)-(1×1) and (1×2) surfaces have been prepared under ultrahigh vacuum conditions by evaporation of vanadium in background O 2 and characterized by various vacuum surface analytical techniques. Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS) including core-level and X-ray-induced valence band, indicate the formation of V 2O 3 on both substrates at vanadia coverages greater than one monolayer equivalent (MLE). The dispersion of vanadia is higher on the (1×2) surface than on the (1×1) surface. At lower coverages <1 MLE, the interface shows two chemical states of vanadium, V 3+ and V 4+.