Departamento De Química

The adsorption energies for iodine atom on the fcc, hcp, bridge, and atop sites of the Pt(1 1 1) surface were determined using ab initio DFT method in two different unit cells. A periodic slab model is used and the obtained energies are in agreement with the corresponding experimental values extrapolated at 0 K. The charge transfer is determined by the use of the Hirshfeld partitioning scheme, and the charge transfer values follow the adsorption energy trend for different sites of the Pt(1 1 1) surface. The results show that the plane-wave DFT approach correctly describes the adsorption of iodine on the Pt(1 1 1) surface and support the use of the Hirshfeld method in surface science problems.

In order to understand the well-ordered adsorbate-substrate systems at atomic level, a method is developed based on the simulation of packing arrangements for layers of unequal spheres, in three-dimensional space. The model, based on geometrical principles, is developed for fcc structure consisting of two hexagonal ordered layers. During simulation, adsorbate spheres were accommodated in different positions, forming a great variety of structures, in dependence of the intersphere distance of the upper layer spheres. Using the average height of the adsorbate layer on the flat substrate as a determinant parameter, several specific structures have been selected as the most probable: ͑ͱ3 ϫ ͱ 3͒R30°, ͑ͱ7 ϫ ͱ 7͒R19.1°, and ͑3 ϫ 3͒. Indeed, they correspond to typical accommodations of the iodine adatoms on the Pt(111) surface, earlier found in experimental studies, which clearly supports the validity of our model. The model developed in our study could completely and satisfactorily describe the accommodation process of the iodine adlayer on the Pt(111) surface. This methodology could be of great help for interpretation of scanning tunneling microscopy images, better understanding of adlayer structures, and design of adsorbate-substrate systems with exciting properties.

Catalytic behavior of platinum catalysts supported on Cs beta zeolite/␥-alumina, for n-heptane reforming reactions, was studied. Cs concentration in the beta zeolite varied from 0 to 0.84 wt.%, in order to neutralize Brönsted acid sites. Increase in Cs content leads to decrease cracking products, but without increase in the toluene selectivity. In the same study Cs beta zeolite catalysts were compared with pure ␥-alumina supported platinum catalyst, showing high toluene selectivity. Selectivity towards isomerization products is favored with catalysts containing Cs beta zeolite, suggesting a bifunctional behavior of such catalyst in spite of Cs content.

Deposition of submonolayer coverages of C on Si͑100͒ prior to Ge growth leads to the formation of Ge quantum dots below the critical thickness for Ge islanding on bare Si͑100͒. In situ scanning tunneling microscopy reveals a high density of irregularly shaped islands for Ge coverages from 2.5 to 4 ML. Island sizes are broadly distributed between 10 and 25 nm. Keeping the C coverage constant and increasing the Ge coverage from 2.5 to 4 ML, the islands increase in height but their density remains constant (ϳ10 11 cm Ϫ2 ). At a Ge coverage of 5.8 ML, formation of larger ͑105͒-faceted islands is observed. Their density is reduced by a factor of 4 compared to smaller Ge coverages. Transmission electron microscopy shows that the nonfaceted islands are preserved after Si capping.

Studies of the electrochemical oxidation of a series of straight-chain terminal alkenoic acids adsorbed at a Pt(111) electrode surface are reported. Compounds adsorbed were: propenoic acid (acrylic acid, PPA); 3-butenoic acid (vinylacetic acid, 3BTA); 4-pentenoic acid (allylacetic acid, 4PTA); 6-heptenoic acid (6HPA); and 10-undecenoic acid (10UDA). Vibrational spectra of adsorbed layers were obtained by use of electron energy-loss spectroscopy (EELS). Molecular packing densities were measured by use of Auger spectroscopy. Electrochemical oxidation of each adsorbed layer was explored by means of cyclic voltammetry in aqueous inert electrolyte (KF/HF). As the analogous aliphatic acids are not chemisorbed at Pt under the same conditions, the alkenoic acids evidently adsorb at Pt(lll) predominantly through the C=C double bond. Molecular packing densities indicate that the carboxylic acid moiety is in contact with the Pt surface only in the case of PPA. EELS spectra also indicate that the carboxylate groups (other than in PPA) are present as pendants. The carboxylic acid O-H stretching bands of most of the adsorbed acids are red-shifted and broadened, evidently due to extensive intermolecular hydrogen bonding; the exceptions are PPA, for which the interaction is primarily with the Pt surface, and 3BTA, for which intermolecular interaction between the carboxylic acid pendants is apparently prevented by steric considerations. The surface-attached carboxylic acid moieties react with KOH solution, leading to retention of K + ions, detected by Auger spectroscopy, and to changes in the vibrational spectra indicative of carboxylate anions; reactivity toward KOH decreases with chain length. Adsorbed alkenoic acids at Pt(lll) surfaces are stable in water and in vacuum. Oxidation of the adsorbed short-chain acids PPA and 3BTA proceeds to completion, forming CO 2 as the principal product. Oxidation of the adsorbed long-chain acids converts the C=C moiety to 2CO2, and transforms the remainder of the molecule to an unadsorbed diacid (likely possibilities are malonic acid from 4PTA; glutaric acid from 6HPA; and heptane-l,7-dioic acid from 10UDA). Electrochemical oxidation, alkenoic acids on platinum.

This work presents the characterization of carbon on deactivated Pt-Sn/Al 2 O 3 industrial catalysts, with Pt-Sn (0.3-0.3 wt.%) and g-Al 2 O 3 (with 0.9 wt.% Cl). The samples were deactivated in different cycles of industrial operation and compared with a catalyst deactivated in methylcyclopentane (MCP) reaction under laboratory conditions. Different sets of samples were used: (a) PS0 (deactivation under laboratory conditions); (b) PS1, with one cycle of industrial operation; and (c) PS2, with 106 cycles of industrial operation. Deactivated catalysts were analyzed and characterized using a combination of different techniques: elemental analysis, atomic force microscopy (AFM), temperatureprogrammed oxidation (TPO) and 13 C nuclear magnetic resonance spectroscopy for solids (NMR). The 13 C-CP/MAS NMR spectra were very similar, indicating the presence of aromatic carbons. TPO profiles showed differences between the samples deactivated during industrial operation and that deactivated in the MCP reaction. In the case of industrial catalysts, one type of carbon was detected. In contrast, the catalyst deactivated under laboratory conditions showed two types of carbon or two types of adsorption sites. AFM images revealed two types of coke morphology, granulate and laminate, which were characterized at the nanometer scale. Our results show that the characteristics of the coke depend on the deactivation process, and that coke on the industrial catalyst could be successfully described and defined. However, to correlate all the data obtained from different techniques and to understand the complete mechanism of coke formation, more detailed analysis should be performed.

The copper electrodeposition process was studied onto different gold substrates, single crystal (1 1 1) and polycrystalline, using electrochemical techniques. It was found, from the analysis of the experimental current density transients, that the potentiostatic formation of a full copper monolayer onto the gold electrode under UPD conditions follows the same mechanism, regardless of the crystallinity of the substrate. The mechanism involved the simultaneous presence of an adsorption process and of two 2D nucleation processes, progressive and instantaneous, respectively.

Cyclic voltammetry, chronoamperometry and in situ electrochemical scanning tunneling microscopy were used to study the kinetics of nucleation and crystal growth during the initial stages of copper overpotential deposition (OPD) on a previously iodine-modified Au(1 1 1) electrode, from an aqueous solution 10 -3 M CuSO 4 in 0.05 M H 2 SO 4 . The starting potential during step experiments was chosen in the region where the gold electrode was completely free of the copper deposit. The recorded current transients for copper deposition onto the iodine-modified Au(1 1 1) electrode surface appear to be very complex, with the unusual presence of two or more current maxima. A new method was used for quantitative evaluation of current transients that involves the transition UPD-OPD, developed by our group [M. Palomar-Pardavé, I. González, N. Batina, J. Phys. Chem. B 104 (2000) 3545], was used for the quantitative interpretation. Our results show that, within a single current transient, copper adsorption and two types of nucleation process: two-dimensional (2D) and three-dimensional (3D) limited by lattice incorporation of copper adatoms and diffusion of Cu(II) ion, respectively, take place simultaneously. STM images revealed the enhanced growth of 3D copper on edge of I-Au(1 1 1) during the early stages of deposition. Moreover, our results strongly suggest that the iodine adlayer is constantly present, even after the striping Cu that was overpotential deposited.

Studies of pollution due to hexavalent chromium are important because it represents a risk to human health and the environment. Hexavalent chromium is a toxic substance and has been found to be carcinogenic. Fortunately, its reduced form, Cr(III), is much less toxic. This paper discusses the remediation Cr(VI) by its reduction in aqueous media by polypyrrole (PPy)-coated reticulated vitreous carbon substrates (RVC). The study is focused on the effect of PPy-film electropolymerization conditions and its efficiency toward the reduction of toxic chromate. We have studied parameters such as the influence of the polymerization electrolyte, the scan rate, the potential limits, and the polymerization cycles concerning the reduction capability of the film. Key results obtained in the present study show that in the presence of different anions during formation of the PPy film, the efficiency of chromate reduction depends on the nature of the anion. We found that the films prepared and treated in the presence of KI performed much better and lasted much longer than those prepared in the presence of KF. Films synthesized at lower scan rates, and higher positive potential limits showed higher chromate reduction efficiencies. Some of our results show reduction of ca. 100% of Cr(VI) from a 10 mg/L solution, even after using the film in 20 contacts with the chromium solution, with films synthesized in KBr and KI electrolytes, at a scan rate of 20 mV/s, between-0.3 and 0.9 V and 40 polymerization cycles. Our study shows that remediation of Cr(VI) by reduction with PPy-coated substrates could be developed into an effective method toward the solution of this problem.

Scanning electron microscopy (SEM) is employed to monitor the surface morphology of polypyrrole films (PPy) grown on different working electrodes (i.e., vitreous carbon and Au (111)) under diverse experimental conditions (i.e., dynamic vs. static potential protocols) and anion dopants (i.e., I- and F-). The morphology of the electrosynthesized films includes rings (doughnuts) and microcontainers, and depends on the synthesis parameters such as the electropolymerization method, the nature of the substrate, the anion dopant, and the sequence of sandwich composite growth. The formation of well-defined rings and microcontainers is attributed to overoxidation occurring during the formation of F--doped PPy. It is possible to design microcontainers by controlling the overoxidation and degradation of the polymer surface.

Unequal-sphere packing model is applied for the simulation of large number of hexagonal adlayer structures with surface coverage between = 1 3 and = 1 on the hexagonal substrate, with atomic radius of the adsorbate and substrate atoms as the only input. Each structure is characterized with respect to collective adlayer properties: the average adlayer height and the adlayer roughness. The distribution of hexagonal arrangements is presented in a special plot, which can be used for identification and characterization of hexagonal adlayers of different surface coverages and atomic registries. The most likely structures are related to the extreme values of our model parameters. The usefulness of this methodology is successfully demonstrated by comparison with some real adsorbate-substrate systems, i.e., halogens and rare gases adsorbed on ͑111͒ surface. Besides the agreement with experimental results, our model offers new insight into the formation of atomic adlayers and detailed analysis of the atomic registry. We believe that our approach will be of use for identification of probable structures among the large number of combinatorial possibilities in theoretical studies and for better interpretation of experimental results ͑i.e., scanning-tunneling microscopy images of atomic adlayers͒.

A simple unequal-sphere packing model is applied to study the iodine ͑3 ϫ 3͒ adlayer on the Pt͑111͒ surface. By using a newly introduced parameter, defined as the average adsorbate height, three characteristic adlattices, ͑3 ϫ 3͒-sym, ͑3 ϫ 3͒-asym, and ͑3 ϫ 3͒-lin, have been selected and characterized in great detail, including the exact adatom registry. The ͑3 ϫ 3͒-sym iodine adlattice, observed in many experimental studies, appears to be, on average, the closest one to the substrate surface. A special contour plot of average adsorbate height vs X and Y positions of the ͑3 ϫ 3͒ iodine unit cell indicates the existence of two local minima, which are related to preferential formation of ͑3 ϫ 3͒-sym and ͑3 ϫ 3͒-asym iodine adlattices. Our model gives good agreement with experimental findings, and explains the mechanism of preferential appearance of ͑3 ϫ 3͒-sym and ͑3 ϫ 3͒-asym structures.

In this work, we present a new approach using analytical and optical techniques in order to determine the physical and chemical properties of silica gel, as well as the measurement of the pore size in the network of the gel by scanning electron microscopy. The gel acupuncture technique developed by GarcmH a-Ruiz et al. (Mater. Res. Bull 28 (1993) 541) GarcmH a-Ruiz and Moreno (Acta Crystallogr. D 50 (1994) 484) was used throughout the history of crystal growth. Several experiments were done in order to evaluate the nucleation control of model proteins (thaumatin I from Thaumatococcus daniellii, lysozyme from hen egg white and catalase from bovine liver) by the porous network of the gel. Finally, it is shown how the number and the size of the crystals obtained inside X-ray capillaries is controlled by the size of the porous structure of the gel.

We performed an electrochemical study of silver electrodeposition from an electrolytic bath containing 1 M NH OH and 1 M KNO 4 3 Ž. Ž. y4 pH s 11 , over a Ag I concentration range of 10 to 0.3 M, and identified silver nucleation processes on the vitreous carbon surface. Ž. Depending upon the silver concentration in the deposition bath, growth occurred either two-dimensionally 2D , controlled by adatom Ž. incorporation, or three-dimensionally 3D , controlled by diffusion or lattice incorporation. Qualitative and quantitative characterization of the observed nucleation processes were based on the results of cyclic voltammetry and chronoamperometry analysis. For quantitative Ž. characterization, different theoretical models related to electrocrystallization processes were used. Atomic force microscopy AFM was employed to probe the surface morphology of the silver deposit. AFM images revealed that silver deposits formed from ammonia baths Ž. with different concentrations of Ag I ion possess different morphological characteristics. Indeed, many of the surface characteristics clearly corroborated the mechanism of silver deposition proposed by electrochemical analysis. q 1998 Elsevier Science S.A.

The adsorption of (3-mercaptopropyl)trimethoxysilane (MPS) has been studied on iodine coated gold electrodes. The MPS adsorption from alcoholic solution on Au(111) and iodine coated Au(111) was studied by atomic force microscopy (AFM) and Xray photoelectron spectroscopy (XPS). The electrochemical formation of MPS monolayers was studied by cyclic voltammetry on polycrystalline uncoated and coated gold electrodes with different MPS pre-treatment conditions. Lead electrochemical deposition was used to probe the defect sites of the surfaces created. The MPS-over-iodine coated gold surface produces a lower-density monolayer than the MPS over pure gold. The MPS monolayer formed electrochemically on the iodine coated gold is chemically equal to its counterpart after the iodine desorption. The MPS adsorption occurs via an Au Ã/S bond, after the partial reductivedesorption of the iodine monolayer from the iodine coated gold electrode, and produces an ordered composite monolayer of MPS/ iodine. The size of the defects can be controlled by varying the electrochemical preparation conditions, using the following reaction: AuÃI (ads) 'MPSÃH (ac) 'e ( l AuÃMPS (ads) 'H ' (ac) 'I ( (ac) :/

Cobalt electrodeposition onto a stainless steel substrate from 1.17 M Co(II) aqueous solution containing 0.98 M H 2 SO 4 , 0.56 M KCl, and 0.2 M H 3 BO 3 was evaluated in the absence (i) and presence (ii) of 0.1 M KNO 3 . Cobalt electrodeposited from the electrolytic bath (i) was white-gray colored, whereas deposition from bath (ii) formed a black-colored surface. SEM-WDX, AFM, and XRD analysis of the steel surfaces covered with these two deposits revealed distinct characteristics for black and white cobalt films. Although both deposits were composed of metallic cobalt, the white cobalt deposit was a smooth, 2D film while the black deposit consisted of many dispersed, nano-sized clusters of 150 to 250 nm in diameter. Analysis of potentiostatic current transients (I-t curves) indicated that formation of white cobalt was carried out by multiple 3D nucleation limited by lattice incorporation of cobalt adatoms to the growth centers. Formation of black cobalt was shown to involve the simultaneous processes of 3D nucleus formation and growth, limited by mass transfer, and the reduction of nitrates in the medium onto the surfaces of these nuclei. It is shown that, beside this cobalt-nitrate interaction, NO 3 Ϫ ions in solution can block active sites for cobalt reduction and the effect of this phenomenon strongly depends on the nitrate concentration. These facts could explain the observed dispersion of the black cobalt coating.

The mechanism and progress of oxidation of the pearlite phase of 1018 carbon steel in contact with a borate medium was monitored and characterized using the in situ electrochemical scanning tunneling microscopy ͑ECSTM͒ technique. The pearlite phase was identified by characteristic microstructure using a previous experimental methodology. The progress of the pearlite oxidation due to differently imposed anodic potential was monitored via change of the electrode surface morphology. ECSTM images indicate two types of the iron oxide formation on the pearlite surface, with different and distinguished morphology. Regarding previously reported data about the oxide formation, these two types of iron oxides were associated to magnetite ͑internal oxide͒ and maghemite ͑external oxide͒. The needle-like features, as a genuine characteristic of the pearlite phase, are visible at the internal oxide layer and even at the maghemite layer. The cross-sectional analysis of the obtained images clearly shows detailed mechanism and morphology characteristics of the oxide layer development in both cases. Based on these observations, a detailed model of the pearlite surface modification during the oxidation was proposed. The model describes pearlite oxidation involving internally and externally formed iron oxides via several consecutive stages.

A series of chemically amplified resists based on polymers of 4-hydroxystyrene, 2-ethyl-2-adamantyl methacrylate and a monomer-bound anionic photoacid generator (PAG) were prepared and characterized. Specifically, the following PAGs were separately incorporated into the main-chain of the polymers: the isomers triphenylsulfonium salt 2-(methacryloxy)-4-trifluoromethyl benzenesulfonate and triphenylsulfonium salt 4-(methacryloxy)-2-trifluoromethyl benzenesulfonate (CF3 PAG); triphenylsulfonium salt 4-(methacryloxy)-3-nitro-benzenesulfonate (NO2 PAG); and triphenylsulfonium salt of 1,1,2-trifluorobutanesulfonate methacrylate (MTFB PAG). Triphenylsulfonium salt 4-(methacryloxy)-2,3,5,6-tetrafluorobenzenesulfonate (F4 PAG) was used as the reference PAG. The intrinsic lithography performance of these polymer-bound PAG resists showed sub-50-nm half-pitch resolution and <5 nm LER (3s) under 100 keV electron beam patterning. The relative sensitivity of these materials under 100 keV e-beam exposure was MTFB PAG $ F4 PAG > CF3 PAG > NO2 PAG. Resolved pattern sizes of 40 and 32.5 nm half-pitch were obtained for fluorinated PAGs (such as MTFB PAG and F4 PAG) bound polymer resists under EUV interference lithography. The surface roughness was inspected with AFM.

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