Artículos SCI

Abstract

Catalytic epoxidation of alkenes plays an essential role in the transformation and synthesis of many organic chemicals. Ti atoms grafted on mesoporous silica, silylated on the surface, is considered the most active and selective catalyst for these reactions. However, the durability and robustness of the active centers remain as the main drawback in industry. In this paper, the characterization of industrial samples is combined with DFT calculations to rationalize the deactivation process of the catalyst and improve its performance. Silylating agents are characterized by experimental and simulated 29Si-NMR and their role in the catalytic mechanism is analysed. Potential deactivation processes are identified before, during and after the reaction. Modifications of the silylating agents and of the active center are proposed to improve the durability of the catalyst.

Abstract

Plant cuticles have been used as models to produce hydrophobic films composed of sodium alginate, the fatty acid fraction of tomato pomace agrowaste, and beeswax. The fabrication process consisted of the blending of components in green solvents (water and ethanol) and a subsequent thermal treatment (150 degrees C, 8 h) to polymerize unsaturated and polyhydroxylated fatty acids from tomato pomace. When sodium alginate and tomato pomace fatty acids were blended, free-standing films were obtained. These films were characterized to evaluate their morphological (SEM), chemical (solid-state NMR, ATR-FTIR), mechanical (tensile tests), thermal (TGA), and hydrodynamic (water contact angle, uptake, and permeability) properties. A comparison between nonpolymerized and polymerized samples was carried out, revealing that the thermal treatment represents a sustainable route to create structured, composite networks of both components. Finally, beeswax was added to the blend with the same amounts of sodium alginate and tomato pomace fatty acids. The presence of the wax improved the hydrophobicity and the mechanical and water barrier properties as well as decreased the water uptake. These results indicate that polymerized plant cuticle-like films have valuable potential for packaging applications.

Abstract

The measured Yb 4d(3/2) intensity is larger than the Yb 4d(5/2) in X-ray photoelectron (XPS) emission of metallic Yb, which is unexpected. The shape and intensity of photoelectron peaks are strongly affected by extrinsic excitations due to electron transport out of the surface (including bulk and surface effects) and to intrinsic excitations due to the sudden creation of the static core hole. To quantitatively extract from experimental XPS the primary excitation spectrum (ie, the initial excitation process) of the considered transition, these effects must be included within the theoretical description. The combined effect of both extrinsic and intrinsic excitations can be described by an effective energy-differential inelastic electron scattering cross section for XPS evaluated by a dielectric response model with the dielectric function as only input. Then, using this cross section, a direct evaluation of the primary excitation spectrum is performed by standard peak shape analysis for thick homogeneous samples. We use this approach in the present paper to determine the Yb 4d photoemission spectrum for metallic Yb. We show that the unexpected larger intensity of Yb 4d(3/2) compared to 4d(5/2) can be fully accounted for by our model and that the total spectrum consists of a sum of symmetric primary excitation peaks.

Abstract

Grain growth is a ubiquitous phenomenon in all materials, and it affects both structural and functional properties. Despite its intrinsic importance, a full comprehension of grain growth from a fundamental point of view-i.e., from the nanoscale to the macroscale-is still a pending issue. In practical terms, our knowledge relies on the classical kinetic laws reported sixty years ago. 

This paper reports the violation of such classical laws in boron carbide ceramics consolidated by spark plasma sintering. The conjunction of high temperature gradients with large compressive stress when a pulse electric current passes through the ceramic powders gives rise to an intense twinning-detwinning formation. These forming steps at the grain boundaries change the grain mobility drastically. Therefore, a new 'exotic' law for grain-growth kinetics is found and validated at different temperatures and dwell times.

Abstract

The effect of single walled carbon nanotubes (SWNT) at zirconia grain boundaries on the stability of a tetragonal zirconia polycrystalline matrix has been explored in as-sintered composites and after low temperature hydro thermal degradation (LTD) experiments. For this purpose, highly-dense 3 mol% Y2O3-doped tetragonal zirconia polycrystalline (3YTZP) ceramics and SWNT/3YTZP composites were prepared by spark plasma sintering (SPS). Quantitative X-ray diffraction analysis and microstructural observations point out that an increasing amount of well-dispersed SWNT bundles surrounding zirconia grains decreases the metastable tetragonal phase retention in the ceramic matrix after sintering. In contrast, the tetragonal ceramic grains in composites with SWNTs are less sensitive to the presence of water, i.e. to undergo a martensitic transformation under LTD conditions, than monolithic 3YTZP ceramics. The SWNT incorporation diminishes micro-cracking due to tetragonal to monoclinic ZrO2 phase transformation in the composites.

Abstract

Geopolymers are a new class of non-Portland cements produced using an alumino-silicate material and an activating solution, which is mainly composed of sodium or potassium and waterglass to be subsequently cured at relatively low temperatures. Those can be formulated by adding natural minerals, waste and/or industrial by-products. The study investigates the microstructural properties of geopolymers synthesized from metakaolin (MK) and the admixture of fly ash (FBA) and aluminium industry slags (AIS) at different ages of curing. Five different geopolymer compositions were prepared and characterized by XRD, ATR-FTIR and SEM/EDS. The study revealed that geopolymeric gels are identified, which show mainly glassy microstructures, in agreement with the X-ray amorphous diffraction patterns, broad FTIR features and confirmed by SEM/EDS, with promising results prior to an industrial scale.

Abstract

Gilding threads collected from Spanish and Portuguese palaces and from the embroideries and adornments of sculptures of the Virgin and Christ that form part of Sevillian Holy Week were analyzed and compared (20 artifacts were evaluated). The study covered a broad time period with examples from the 13th to 14th centuries, 18th to 20th centuries, and also including modern embroideries. A combination of scanning electron microscopy and energy-dispersive X-ray spectroscopy was used. The knowledge of the layered structures of the threads has provided very valuable information regarding the manufacturing techniques. The different metal threads found in the embroidery studied consisted of gold, silver, copper, and alloys of these metals and aluminium. The fabrication procedures often differed in the different workshops and changed with time. In the modern embroideries, a decrease of precious metal concentration was detected. The threads were wound around a core of silk threads.

Abstract

Nanolayered CrAIN and CrAIYN/CrAIN (average contents of Al approximate to 25 at.% and Y approximate to 1.6 at. %) coatings are deposited on M2 and 316 steel substrates and heated to 1000 degrees C in air for 2 h to study their oxidation mechanism, the thermal stability and the reactive element (RE) effect of yttrium. CrAIN on M2 develops a Cr2O3/Al2O3 passivation layer that preserves in high degree the fcc-CrAIN structure however iron ions leave the substrate and travel to the surface along the column boundaries. The CrAIYN/CrAIN coatings deposited on steels are not stable at 1000 degrees C, and the initial fcc-CrAIN phase is partially transformed to hcp-Al(O)N and Cr-Fe phases (M2) and Cr2N and Al2O3 (316). The addition of Y changes the predominant scale growth direction. Inward oxygen diffusion becomes dominant but a reduction of the oxide scale thickness as compared to CrAIN is not observed. The advanced microstructural analysis made by transmission electron microscopy combined with electron energy loss spectroscopy determined that yttrium migrates mainly to the oxide scale (forming mixed oxides with substrate elements - V and Mo, either as dispersed particles or segregated at the grain boundaries) in M2, and to the oxide interface and column boundaries (forming Al-Y oxides and YN, respectively) in 316 steel. The benefits of addition of Y in improving the oxidation resistance are discussed comparatively with literature data. The RE effect of yttrium is thus observed to be dependent on the substrate, film architecture and composition.

Abstract

An optimized fluorinated TiO2 catalyst with high {001} facet exposure loaded with platinum (TiO2-PtFAC) was tested in the photocatalytic hydrogen production from glycerol solution under UV light irradiation. The samples were synthesized by direct hydrothermal treatment starting from two different types of precursors that are titanium tetraisopropoxide (I) or titanium butoxide (B), while platinisation was performed by photodeposition method. The obtained catalysts were characterised by different techniques (XRD, FESEM, TEM, BET, UV–vis DRS, XRF and XPS) and the results evidenced that anatase is the only crystalline phase present in all TiO2 samples. The morphology of the samples was seen as rectangular platelets particles where Pt particles were was observed all over the surface. The presence of Pt and F in the platinised samples was also confirmed by XRF and XPS analysis. The photocatalytic results have shown that the presence of Pt on TiO2{001}facet surface remarkably enhanced the hydrogen production from aqueous solution at 5 wt % of glycerol. Comparing the results obtained from the photocatalysts prepared by the two different precursors, it was found that the best performances in terms of H2 production was achieved with TiO2-PtFAC(I) (about 13 mmol L−1 after 4 h of irradiation time), while the H2 production was lower for TiO2-PtFAC(B) (about 9 mmol L−1 after 4 h of irradiation time). The effect of the operating conditions using TiO2-PtFAC(I) evidenced that the highest H2 production was obtained with a photocatalyst dosage equal to 1.5 g L−1, initial glycerol concentration at 5 wt% and a pH value equal to 7. Finally, a photocatalytic test was also performed on glycerol solution prepared with a real water matrix. Despite the presence of ions scavengers (chlorides and carbonates) in solution, TiO2-PtFAC(I) was able to reach a photocatalytic H2production of about 6 mmol L−1 after 4 h of UV light irradiation.

Abstract

3 mol% yttria tetragonal zirconia polycrystalline (3YTZP) ceramic composite powders with 10 vol% nominal content of graphene nanoplatelets (GNPs) were prepared using four different homogenization routines: dispersion of the powder mixture by ultrasonication in isopropyl alcohol, homogenization in a high-energy planetary ball mill in wet or dry conditions after ultrasonication, and milling of the powders in a high-energy planetary ball mill in dry conditions. A significant effect of the homogenization routine on the powders particle size distribution was revealed by laser granulometry and Raman spectroscopy. Highly densified composites were obtained after spark plasma sintering (SPS) and remarkable differences on the GNP size, shape and distribution throughout the ceramic matrix and also in the electrical conductivity were observed in the four different composites. The composite with the best performance in terms of electrical conductivity was the one prepared after planetary ball milling of the powders in dry conditions as a consequence of the reduced dimensions of the GNPs and their excellent distribution throughout the ceramic matrix. 

Abstract

In this work, we studied the thermal decomposition of a widely employed silicone elastomer, polydimethylsiloxane, in an inert atmosphere. This silicone elastomer has several applications due to its high thermal stability such as MEMS (microelectromechanical systems) precursors, microfluidic components, adhesives, lubricants, and precursors for non-porous ceramics. Therefore, a reliable description of the thermal decomposition kinetics is important to prevent or control the decomposition in such applications. While the decomposition has been amply reported as a complex process, most kinetic studies published on this system use simplified methods that avoid the fact that the entire process cannot be described by a single kinetic triplet. Here, we have studied the decomposition process by first separating the overall reaction into its three constituent steps which were subsequently analysed independently. The deconvolution was carried out using Fraser-Suzuki function that is capable of fitting an asymmetric peak fitting function. The resulting kinetic parameters proved to be able to reconstruct the original experimental curves but are also capable of producing accurate predictions of curves recorded at heating schedules different from those employed to record the experimental data used in the kinetic analysis. Finally, it was found that the rate limiting step of all stages is the diffusion of the gases released during the polymer decomposition through the transforming polymeric matrix.

Abstract

Here, a kinetic study of the thermal decomposition of synthesized hydrotalcite, Mg-6 Al-2 CO3(OH)(16)center dot H2O, has been carried out using thermogravimetric experiments in air atmosphere. It is shown that the thermal decomposition occurs in two well differentiated stages. The first one is a single-step dehydration process that comprises the release of four water molecules. On the other hand, the second stage is complex and corresponds to both dehydroxylation and decarbonation processes which occur simultaneously. The kinetic parameters describing all processes were calculated by means of a combined approach comprising isoconversional, model-fitting and deconvolution methods. It was concluded that dehydroxilation and decarbonization cannot be separated by TG experiments and the two stages contributing to the complex process do not apparently match the expected stoichiometry of the process. Therefore, it is proposed that such stages mark a change on the reaction mechanism due to the structural collapse of the laminar double hydroxide.

Abstract

The most representative of greenhouse-crop plant biomass residues of tomatoes (Solanum lycopersicum L.) were selected for this study by using X-ray fluorescence spectrometry (XRF) and X-ray powder diffraction (XRD). The heating dynamics in air in the 600-1150 degrees C range of these residues for the production of renewable energy and the resultant ashes have been investigated. A total of 11 elements were determined by XRF in the biomass ashes and some minor elements. The content of alkaline elements and chlorides decreased as increasing heating temperature and disappeared at 1150 degrees C. Alkaline salts, NaCl and KCl, were volatilized by heating since 800 degrees C. The total contents of S and P in the biomass ashes were associated to CaSO4, and a complex phosphate identified by XRD. CaCO3 present at 600 degrees C was decomposed to CaO with disappearance at 1000 degrees C. By heating, new silicates were formed by solid-state reactions in the biomass residue. The minor elements have been found in a relative proportion lower than 0.9wt.% and they characterized the obtained ashes, with potential use as micronutrients.

Abstract

This work reports about the sensing and biosensing applications of a novel screen printed electrode (SPE) modified by nanostructured nickel oxide thin films obtained by reactive magnetron sputtering under an oblique angle configuration. Using these films as electrodes we demonstrate their ability to detect hydrogen peroxide under neutral pH conditions. Furthermore, as a proof-of-concept, NiO-modified SPEs have been developed and their cholesterol biosensing properties determined by cyclic voltammetry and chronoamperometry.

Abstract

Under the framework of circular economy, agricultural wastes are an interesting carbon-based feedstock for thermal energy and power generation. Their use could extend the availability of biomass-based fuel and, at the same time, would reduce negative environmental effects. However, depending on the residues' characteristics, their direct combustion in boilers presents some challenges which could be overcome with a carbonization pretreatment. In this paper, the main mechanisms of thermochemical transformation of an abundant agricultural waste, olive stone, into biochar products via slow carbonization are analyzed, with emphasis on the effect of peak carbonization temperature. Thermogravimetric and differential scanning calorimetry analysis are used to evaluate the performance of the resulting biochars compared to raw olive stone in combustion processes and to assess the correlation between the peak carbonization temperature and compositional and fuel properties. Results show that with a prior treatment up to an optimum temperature of 800 degrees C the energy density is increased up to three times compared to the raw material. These findings suggest that carbonization of olive stones reduces the barriers to their direct use in current biomass boiler technology.

Abstract

The use of allotropic phases of carbon (i.e. nanotubes, graphene or carbon nanofibers) as second phases to design ceramic composites is a hot topic at present. Researchers try to provide a remarkable improvement of the parent ceramic assuming that some of the outstanding mechanical properties of these phases migrate to the resultant composite. This reasonable idea has been questioned severely in the case of nanotubes addition but there is not any analysis for the other two phases cited previously. To elucidate this question, zirconia was selected as a model ceramic. This paper reports the mechanical properties of zirconia composites reinforced either with graphene or carbon nanofibers, with special emphasis on the high-temperature plasticity.

Abstract

For Pt catalysts which have demonstrated great activity for the WGS reaction, the activation of water is described as the rate-limiting step. Such limitation could be overcome through the design of supports able to supply water. In this study, the hexagonal and monoclinic phases of CePO4 have been evaluated as supports for Pt WGS catalysts. The hexagonal structure presents channels containing water, absent in the monoclinic structure. The presence of these channels in the hexagonal phase increases the interaction with the water molecules, leading to an enhancement of the WGS catalytic performance. DRIFTS results showed that dissociation of water does not occur on these supports, while calculated apparent activation energies present values similar to those reported in the literature for the dissociation of water in Pt (111). These results suggest that cerium phosphates act as water suppliers, increasing the number of available species to be dissociated on the Pt surface.

Abstract

In the field of hybrid organic-inorganic perovskite based photovoltaics, there is a growing interest in the exploration of novel and smarter ways to improve the cells light harvesting efficiency at targeted wavelength ranges within the minimum volume possible, as well as in the development of colored and/or semitransparent devices that could pave the way both to their architectonic integration and to their use in the flowering field of tandem solar cells. The work herein presented targets these different goals by means of the theoretical optimization of the optical design of standard opaque and semitransparent perovskite solar cells. In order to do so, we focus on the effect of harmless, compatible and commercially available dielectric inclusions within the absorbing material, methylammonium lead iodide (MAPI). Following a gradual and systematic process of analysis, we are capable of identifying the appearance of collective and hybrid (both localized and ex tended) photonic resonances which allow to significantly improve light harvesting and thus the overall efficiency of the standard device by above 10% with respect to the reference value while keeping the semiconductor film thickness to a minimum. We believe our results will be particularly relevant in the promising field of perovskite solar cell based tandem photovoltaic devices, which has posed new challenges to the solar energy community in order to maximize the performance of semitransparent cells, but also for applications focusing on architectonic integration. 

Abstract

ZnO nanoparticles have been previously synthesized by a facile precipitation procedure by mixing aqueous solutions of Zn(II) acetate and dissolved Na2CO3 at pH ca. 7.0 without the addition of a template. The as-prepared ZnO material was anealed at 400 °C in air for 2 h. The Pt-ZnO catalysts (0.5 or 1.0 Pt wt.%) were obtained by photochemical deposition method on the surface of the prepared ZnO sample, using hexachloroplatinic acid (H2PtCl6). It has been shown that Zn2+ is lost from the photocatalyst to the medium and a replacement of the cationic vacancies of Zn2+ by Pt4+ cations occurs during the platinization process of the ZnO samples, regardless of whether the platinum metal photodeposition process. The as-prepared catalysts were characterized by XRD, BET, FE-SEM, TEM, XPS and diffuse reflectance spectroscopy (DRS). Three different probe molecules were used to evaluate the photocatalytic properties under UV-illumination: Methyl Orange and Rhodamine B were chosen as dye substrates and Phenol as a transparent substrate. High conversion values (ca. 100%) and a total organic carbon (TOC) removal of 90–96%, were obtained over these photocatalysts after 160 min of UV illumination. In general, it was observed that the presence of Pt on ZnO affects the lattice parameters and the crystallite size. Although ZnO can completely degrade RhB, MO and Phenol totally in ca. 60 min, the process is more efficient for Pt–ZnO photocatalysts.

Abstract

The rapid mechanochemical synthesis of nanocrystalline CuFeS2 particles prepared by high-energy milling for 60 min in a planetary mill from copper, iron and sulphur elements is reported. The CuFeS2 nanoparticles crystallize in tetragonal structure with mean crystallite size of about 38 ± 1 nm determined by XRD analysis. HRTEM study also revealed the presence of nanocrystals with the size of 5–30 nm with the tendency to form agglomerates. The Raman spectrum confirms the chalcopyrite structure. Low temperature magnetic data for CuFeS2 support the coexistence of antiferromagnetic and paramagnetic spin structure. Moreover, the hysteresis loops taken at temperatures from 5 K to 300 K revealed a presence of very small amount of ferromagnetic phase, which seems to be associated with the non-consumed elemental Fe in as-prepared nanoparticles. The optical band gap of CuFeS2nanoparticles has been detected to be 1.05 eV, larger than band gap of the bulk material. The wider gap possibly resulted from the nano-size effect. Photoresponses of CuFeS2nanoparticles were confirmed by I-V measurements under dark and light illumination. It was demonstrated that mechanochemical synthesis can be successfully employed in the one step preparation of nanocrystalline CuFeS2 with good structural, magnetic, optical and electrooptical properties.

Abstract

Graphitized carbon materials from biomass resources were successfully synthesized with an iron catalyst, and their electrochemical performance as anode materials for lithium-ion batteries (LIBs) was investigated. Peak pyrolysis temperatures between 850 and 2000 degrees C were covered to study the effect of crystallinity and microstructural parameters on the anodic behavior, with a focus on the first-cycle Coulombic efficiency, reversible specific capacity, and rate performance. In terms of capacity, results at the highest temperatures are comparable to those of commercially used synthetic graphite derived from a petroleum coke precursor at higher temperatures, and up to twice as much as that of uncatalyzed biomass-derived carbons. The opportunity to graphitize low-cost biomass resources at moderate temperatures through this one-step environmentally friendly process, and the positive effects on the specific capacity, make it interesting to develop more sustainable graphite-based anodes for LIBs.

Abstract

A Cu/TiO2 photocatalyst has been synthesised by reducing a Cu precursor with NaBH4 onto the surface of a sulphate pretreated TiO2 obtained by a sol-gel procedure. The catalyst, that shows a clearly defined anatase phase with high crystallinity and relatively high surface area, and contains Cu2O and CuO deposits on its surface, has been used to produce hydrogen in a solar driven pilot plant scale photocatalytic reactor. Different electron donor aqueous solutions (methanol, glycerol, and a real municipal wastewater treatment plant influent) have been tested showing similar or even higher energy efficiency than those obtained using more expensive noble metal based photocatalytic systems. The glycerol solutions have provided the best reactive environments for hydrogen generation.

Abstract

Automatic lay-up and in-situ consolidation with thermoplastic composite materials is a technology under research for its expected use in the profitable manufacturing of structural aeronautical parts. This study is devoted to analysing the possible effects of thermal degradation produced by this manufacturing technique. 
Rheological measurements showed that there is negligible degradation in PEEK for the temperatures reached during the process. Thermogravimetric analysis under linear heating and constant rate conditions show that thermal degradation is a complex process with a number of overlapping steps. A general kinetic equation that describes the degradation of the material with temperature has been proposed and validated. Attenuated total reflectance Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy confirmed that there is no remarkable degradation. The use of a combination of in-situ and ex-situ experimental techniques, including kinetic modelling, not only provides reliable information about degradation but also allows setting optimal processing conditions.

Abstract

Biodeterioration of stone monuments is estimated to be as high as 20-30% of the total degradation suffered by Cultural Heritage constructions. With regard to this problem, bactericidal treatments are mainly based on cleaning. These processes, while effective in the short term, require frequent reapplications increasing potential damages to the monument. Silver nanoparticles offer many advantages over traditionally employed products, such as their prolonged biocide efficacy and their low toxicity to humans and environment. The aim of this study was to evaluate the applicability and effectiveness of seven nanocomposite treatments based on titanium dioxide and/or silver nanoparticles to prevent biodeterioration of limestone monuments. These nanocomposites were characterized by UV Visible spectrophotometry, Dynamic Light Scattering and Electron Microscopy. To assess their bactericidal activity, accelerated weathering tests were performed on limestones from the quarry of Utrera, a source widely employed in such iconic monuments as the Cathedral of Seville (Spain). Furthermore, the samples of biopatina employed in our assays stemmed from the fa ades of historical buildings from Seville. Our results show that silver and titanium dioxide nanocomposites stabilized by citrate achieve a high biocide effect while maintaining color alterations at a low level.

Abstract

The development of Deep Geological Repositories (DGP) to the storage of high-level radioactive waste (HLRW) is mainly focused in systems of multiple barriers based on the use of clays, and particularly bentonites, as natural and engineered barriers in nuclear waste isolation due to their remarkable properties. 
Due to the fact that uranium is the major component of HLRW, it is required to go in depth in the analysis of the chemistry of the reaction of this element within bentonites. The determination of uranium under the conditions of HLRW, including the analysis of silicate matrices before and after the uranium-bentonite reaction, was investigated. The performances of a state-of-the-art and widespread radiochemical method based on chromatographic UTEVA resins, and a well-known and traditional method based on solvent extraction with tri-n-butyl phosphate (TBP), for the analysis of uranium and thorium isotopes in solid matrices with high concentrations of uranium were analysed in detail. 
In the development of this comparison, both radiochemical approaches have an overall excellent performance in order to analyse uranium concentration in HLRW samples. However, due to the high uranium concentration in the samples, the chromatographic resin is not able to avoid completely the uranium contamination in the thorium fraction.

Abstract

The design of hybrid mesoporous TiO2-SiO2(TS1) materials decorated with Ag and Pt nanoparticles was performed. The photocatalytic degradation of phenol under artificial solar irradiation was studied and the activity and selectivity of the intermediate products were verified. TiO2-SiO(2)was prepared by sol-gel method while Ag- and Pt-based photocatalysts (TS1-Ag and TS1-Pt) were prepared by photodeposition of the noble metals on TS1. Two series of photocatalysts were prepared varying Ag and Pt contents (0.5 and 1.0 wt%). An increase in the photocatalytic activity up to two and five times higher than TS1 was found on TS1-Ag-1.0 and TS1-Pt-1.0, respectively. Changes in the intermediate products were detected on Ag- and Pt-based photocatalysts with an increase in the catechol formation up to 3.3 and 6.6 times higher than that observed on TS1, respectively. A two-parallel reaction mechanism for the hydroquinone and catechol formation is proposed. A linear correlation between the photocatalytic activity and the surface concentration of noble metals was found indicating that the electron affinity of noble metals is the driven force for both the increase in the photoactivity and for the remarkable changes in the selectivity of products.

Abstract

Different carbon supported metal catalysts were synthesized, and characterized with various physico-chemical methods and tested in vanillin hydrodeoxygenation under 30 bar total pressure in water as a solvent at 100 degrees C. The catalysts exhibited high specific surface area and the metal dispersion decreased in following order: Pt/ C > Pd/C > Au/C > Rh/C > Ru/C. The most active catalyst was Pd/C followed by Ru/C. Vanillin hydrodeoxygenation proceeded via hydrogenation forming vanillyl alcohol further to its hydrogenolysis forming p-creosol. Both hydrogenation and hydrogenolysis were promoted by Pd/C, which exhibited rather high dispersion. The highest selectivity to p-creosol, 95% at complete vanillin conversion, was obtained with Pd/C. Kinetic modelling of vanillyl alcohol selectivity as a function of vanillin conversion was performed.

Abstract

Mullite precursors were prepared using kaolin waste, sericite clay containing kaolinite and industrial kaolin with addition of alumina in a wet medium to synthesize mullite (72 wt% Al2O3 and 28 wt% SiO2). Uniaxial pressed bars of the powdered mullite precursors were fired in the range 1400-1600 degrees C with soaking times 30-120 min. The resultant materials were studied by XRD and SEM-EDX. Bulk densities, apparent porosities and flexural strengths in four points were determined in the fired bars at 1500, 1550 and 1600 degrees C. It was concluded that the thermal behaviour of these mullite precursors was influenced by the presence of impurities in the raw materials. These impurities originate a liquid phase forming a glassy phase which produces a progressive and enhanced densification of the mullite materials by reaction sintering at 1500-1600 degrees C. The technical properties were also influenced by the relative proportion of vitreous phase. The microstructure of characteristic mullite crystals was revealed by SEM. It was emphasized the use of kaolin waste by-products of mining and sericite clays as valuable raw materials for mullite preparation.

Abstract

Herein we present a combined study of the evolution of both the photoluminescence (PL) and the surface chemical structure of organic metal halide perovskites as the environmental oxygen pressure rises from ultrahigh vacuum up to a few thousandths of an atmosphere. Analyzing the changes occurring at the semiconductor surface upon photoexcitation under a controlled oxygen atmosphere in an X-ray photoelectron spectroscopy (XPS) chamber, we can rationalize the rich variety of photophysical phenomena observed and provide a plausible explanation for light-induced ion migration, one of the most conspicuous and debated concomitant effects detected during photoexcitation. We find direct evidence of the formation of a superficial layer of negatively charged oxygen species capable of repelling the halide anions away from the surface and toward the bulk. The reported PL transient dynamics, the partial recovery of the initial state when photoexcitation stops, and the eventual degradation after intense exposure times can thus be rationalized.