Víctor Eduardo Comparán Padilla

Context BaTiO₃ is one of the most important ferroelectric oxides in electronic applications. Also, it has attractive properties for catalysis that could be used for reducing contamination levels, especially carbon monoxide, CO. CO is one of the main gaseous pollutants generally released from the combustion of fossil fuel. In this work, the CO transformation on pristine and Au-modified BaTiO₃ perovskite for H₂CO obtention is studied. The CO adsorption and hydrogenation on pristine BaTiO₃ leads to formaldehyde synthesis as the most stable product through two possible routes. Furthermore, hydrogenation stages are less probable on pristine BaTiO₃. On Au-modified BaTiO₃ formaldehyde is the principal product too but Au adatom generates H₂CO competition with HCOH. After BaTiO₃ modification with Au unpaired electrons were generated. These unpaired electrons are related to the adatom reactivity. According to the obtained results, pristine and Au-modified BaTiO₃ can adsorb and hydrogenate CO generating formaldehyde as the principal product. BaTiO₃ modifications with Au increase the reactivity of the perovskite in the CO hydrogenation reactions. CO hydrogenation process on Au suggests that further hydrogenation stages beyond formaldehyde are possible. Methods The study was performed through ab initio calculations using the periodic spin-polarized Density Functional Theory (DFT) as implemented in Quantum ESPRESSO. DFT calculations were carried out using the Plane Wave self-consistent field (PWscf). Spin density difference allows us to identify reactive regions related to dangling bonds and unpaired electrons. A plane wave basis set was used to represent the electron states. Vanderbilt pseudopotentials with nonlinear core correction were used to model the ionic cores and valence electrons interaction. Exchange-correlation energies were treated within the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) parameterization.

Context. BaTiO₃ is one of the most important ferroelectric oxides in the electronic applications. Also, it has attractive properties for catalysis that could be used for reducing the contamination levels especially carbon monoxide, CO. CO is one of the main gaseous pollutants generally released from the combustion of fossil fuel. In this work, the catalytic properties of pristine and Au-modified BaTiO₃ perovskite for CO hydrogenation are studied. The CO adsorption and hydrogenation on pristine BaTiO₃ leads to formaldehyde synthesis as the most stable product through two possible routes. Further hydrogenation stages are less probable. On Au-modified BaTiO₃ formaldehyde is the principal product too but Au adatom induces selectivity making just one reaction route as the most stable. After BaTiO₃ modification with Au unpaired electrons were generated. These unpaired electrons increased the adatom reactivity. According to the obtained results, pristine and Au-modified BaTiO₃ can adsorb and hydrogenate CO generating formaldehyde as the principal product. BaTiO₃ modifications with Au increases the reactivity and selectivity of the perovskite in the CO hydrogenation reactions. Methods. The study was performed through ab initio calculations using the periodic Density Functional Theory (DFT) as implemented in Quantum ESPRESSO. DFT calculations were carried out using the Plane Wave self-consistent field (PWscf). Spin density was considered for systems with unpaired electrons. Plane wave basis set was used to represent the electron states. Vanderbilt pseudopotentials with nonlinear core correction were used to model the interaction of ionic cores and valence electrons. Exchange-correlation energies were treated within the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) parameterization.

Copolymerizations of ethylene and alfa-olefins, using Ziegler–Natta or metallocene catalysts, testing two methods of co-monomer addition, through batch or dossing mode during the reactions, are reported in this work. Copolymerizations are monitored by in line Raman spectroscopy, comparing the effect of the kind of catalyst and the co-monomer addition modes on the chemical composition of the copolymers produced. The global co-monomer composition is determined by ¹³C NMR spectroscopy, compared with the monitoring by Raman spectroscopy along the reactions, where it is possible to define homogeneous or heterogeneous co-monomer distributions. Batch addition achieves higher incorporations of co-monomers, compared to dosed addition, where it is possible to determine the maximal co-monomer addition without affecting activities by transfer reactions. The incorporation mode of alfa-olefins in this type of reaction has been little reported, and until it is known, there is no rapid technique available to determine the uniformity of the co-monomer incorporations in real time. Copolymerization kinetics are also reported here and correlated to the addition method of the comonomers in both kinds of reactions. Homogeneous and heterogeneous co-monomer incorporations promoted by a single site catalyst (metallocene) or multisite system (Ziegler–Natta) is related to the homogeneous or heterogeneous co-monomer distributions detected by Raman spectroscopy, using each kind of catalytic system.

Copper nanoparticles (CuNP) were obtained by a green synthesis method using cotton textile fibers and water as solvent, avoiding the use of toxic reducing agents. The new synthesis method is environmentally friendly, inexpensive, and can be implemented on a larger scale. This method showed the cellulose capacity as a reducing and stabilizing agent for synthetizing Cellulose–Copper nanoparticles (CCuNP). Nanocomposites based on CCuNP were characterized by XRD, TGA, FTIR and DSC. Functional groups present in the CCuNP were identified by FTIR analysis, and XRD patterns disclosed that nanoparticles correspond to pure metallic Cu°, and their sizes are at a range of 13–35 nm. Results demonstrated that CuNPs produced by the new method were homogeneously distributed on the entire surface of the textile fiber, obtaining CCuNP nanocomposites with different copper wt%. Thus, CuNPs obtained by this method are very stable to oxidation and can be stored for months. Characterization studies disclose that the cellulose crystallinity index (CI) is modified in relation to the reaction conditions, and its chemical structure is destroyed when nanocomposites with high copper contents are synthesized. The formation of CuO nanoparticles was confirmed as a by-product, through UV spectroscopy, in the absorbance range of 300–350 nm.

Copper nanoparticles (CuNPs) functionalized with polyethyleneimine (PEI) and 4-aminobutyric acid (GABA) were used to obtain composites with isotactic polypropylene (iPP). The iPP/CuNPs composites were prepared at copper concentrations of 0.25–5.0 wt % by melt mixing, no evidence of oxidation of the CuNP was observed. Furthermore, the release of copper ions from iPP/CuNPs composites in an aqueous medium was studied. The release of cupric ions was higher in the composites with 2.5 and 5.0 wt %. These composites showed excellent antibacterial activity (AA) toward Pseudomona aeruginosa (P. aeruginosa) and Staphylococcus aureus (S. aureus). The incorporation of CuNP into the iPP polymeric matrix slightly decreased the thermal stability of the composite material but improved the crystallinity and the storage modulus. This evidence suggests that CuNPs could work as nucleating agents in the iPP crystallization process. The iPP/CuNPs composites presented better AA properties compared to similar composites reported previously. This behavior indicates that the new materials have great potential to be used in various applications that can be explored in the future.

In‐situ polymerization of ethylene with graphene has been previously studied; however, the reported nanocomposites exhibit poor filler dispersion, even at low graphene loadings. In the present work, well dispersed and homogeneous graphene nanoplatelets (GNP)‐high density polyethylene (HDPE) nanocomposites were obtained in up to 12.5% weight content of graphene through an in‐situ polymerization of ethylene in graphene suspensions. The composites' potential as masterbatches was evaluated by melt mixing them with commercial HDPE. The obtained materials exhibited important physical‐mechanical properties increases when compared with those of commercial HDPE. To our knowledge, our materials' GNP concentration (≤0.82 wt.%) is the lowest reported, with the composites showing a 27% increase in their maximum tensile strength and a 41% increase in their flexural modulus when compared with blank HDPE. In addition, Raman spectroscopy and scanning electron microscope (SEM) analyses give evidence of the higher exfoliation degree of GNP obtained in the in‐situ ethylene polymerizations.

Studies of hydrogen sulfide (H₂S) and ammonia (NH₃) adsorption on phosphorus (P) and silicon (Si) doped graphene are performed by ab initio calculations using the periodic density functional theory (DFT). The P and Si incorporation in graphene distorts the unit cell altering the bond lengths and angles. Unlike the pristine, the phosphorus-doped graphene shows a metallic behavior, and the silicon-doped graphene is a semiconductor with an energy gap of 0.25 eV. Moreover, the electronic properties of phosphorus-doped graphene may change with the adsorption of hydrogen sulfide and ammonia. However, the silicon-doped graphene only shows changes with the adsorption of hydrogen sulfide. In addition, the silicon-doped graphene exhibits chemisorption when interacting with ammonia. According to the obtained results, phosphorus-doped graphene is suitable as a gas sensor of hydrogen sulfide and ammonia, in contrast with the silicon-doped structure, which may be used as a sensor of hydrogen sulfide.

Zirconocene aluminohydride complexes (e.g. ⁿBuCp₂ZrH₃AlH₂) afford higher MW polyethylene at significantly higher activity than their zirconocene dichloride counterparts (ⁿBuCp₂ZrCl₂) when activated by methylaluminoxane (MAO). In this work, supported catalysts with different Al:Zr ratios on SiO₂, modified with MAO were prepared, and evaluated for their tendency towards leaching in ethylene polymerization, using additional MAO or other alkylaluminum compounds. The extent of catalyst leaching was estimated by comparing kinetic data of filtered versus unfiltered catalysts during ethylene polymerization. Polydispersities and molecular weights of the HDPE obtained using filtered versus unfiltered catalysts were also compared under similar polymerization conditions. Polyethylene morphologies obtained in homogeneous and heterogeneous phase were compared by SEM. Bulk densities of the HDPE were evaluated and related to the leaching in the polymerizations.