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2019


Effect of support oxygen storage capacity on the catalytic performance of Rh nanoparticles for CO2 reforming of methane


Yentekakis, IV; Goula, G; Hatzisymeon, M; Betsi-Argyropoulou, I; Botzolaki, G; Kousi, K; Kondarides, DI; Taylor, MJ; Parlett, CMA; Osatiashtiani, A; Kyriakou, G; Holgado, JP; Lambert, RM
Applied Catalysis B-Environmental, 243 (2019) 490-501

ABSTRACT

The effects of the metal oxide support on the activity, selectivity, resistance to carbon deposition and high temperature oxidative aging on the Rh-catalyzed dry reforming of methane (DRM) were investigated. Three Rh catalysts supported on oxides characterized by very different oxygen storage capacities and labilities (gamma-Al2O3, alumina-ceria-zirconia (ACZ) and ceria-zirconia (CZ)) were studied in the temperature interval 400-750 degrees C under both integral and differential reaction conditions. ACZ and CZ promoted CO2 conversion, yielding CO enriched synthesis gas. Detailed characterization of these materials, including state of the art XPS measurements obtained via sample transfer between reaction cell and spectrometer chamber, provided clear insight into the factors that determine catalytic performance. The principal Rh species detected by post reaction XPS was Rh, its relative content decreasing in the order Rh/CZ(100%) > Rh/ACZ(72%) > Fth/gamma Al2O3(55%). The catalytic activity followed the same order, demonstrating unambiguously that Rh is indeed the key active site. Moreover, the presence of CZ in the support served to maintain Rh in the metallic state and minimize carbon deposition under reaction conditions. Carbon deposition, low in all cases, increased in the order Rh/CZ < Rh/ACZ < Rh/gamma-Al2O3 consistent with a bi-functional reaction mechanism whereby backspillover of labile lattice O2- contributes to carbon oxidation, stabilization of Rh and modification of its surface chemistry; the resulting O vacancies in the support providing centers for dissociative adsorption of CO2. The lower apparent activation energy observed with CZ-containing samples suggests that CZ is a promising support component for use in low temperature DRM.


Abril, 2019 | DOI: 10.1016/j.apcatb.2018.10.048

UV and visible-light driven photocatalytic removal of caffeine using ZnO modified with different noble metals (Pt, Ag and Au)


Vaiano, V.; Jaramillo-Paez, C.A.; Matarangolo, M.; Navío, J.A.; Hidalgo, M.C.
Materials Research Bulletin, 112 (2019) 251-260

ABSTRACT

In this work, ZnO photocatalyst was modified with different noble metals (Pt, Ag and Au) through photodeposition method and then characterized by different techniques (XRD, XRF, BET, UV–vis DRS, FESEM, and XPS). The addition of noble metals produces important changes in the light absorption properties with a significant absorbance in the visible region due to the existence of surface plasmon resonance (SPR) observed at about 450 nm and 550 nm for ZnO modified with Ag and Au, respectively. The morphology of the samples was studied by TEM and the size ranges of the different metals were estimated. Noble metal nanoparticles were in every case heterogeneously deposited on the larger ZnO particles. All the prepared photocatalysts were tested in the photocatalytic removal of caffeine (toxic and persistent emerging compound) under UV and visible light irradiation. It was observed an enhancement of photocatalytic caffeine removal from aqueous solutions under UV light irradiation with the increase of metal content (from 0.5 to 1 wt %) for ZnO modified with Ag and Au (Ag/ZnO and Au/ZnO). In particular, Ag/ZnO and Au/ZnO with higher Ag and Au content (1 wt %) allowed to achieve the almost complete caffeine degradation after only 30 min and a TOC removal higher than 90% after 4 h of UV light irradiation. These two photocatalysts were investigated also under visible light irradiation and it was found that their photocatalytic performances were strongly enhanced in presence of visible light compared to unmodified ZnO. In particular, Ag/ZnO photocatalyst was able to reach the complete caffeine degradation and a TOC removal of about 70% after 4 h of visible light irradiation.


Abril, 2019 | DOI: 10.1016/j.materresbull.2018.12.034

Promoting effect of CeO2, ZrO2 and Ce/Zr mixed oxides on Co/gamma-Al2O3 catalyst for Fischer-Tropsch synthesis


Garcilaso, V; Barrientos, J; Bobadilla, LF; Laguna, OH; Boutonnet, M; Centeno, MA; Odriozola, JA
Renewable Energy, 132 (2019) 1141-1150

ABSTRACT

A series of cobalt-based catalysts have been synthesized using as support gamma-Al2O3 promoted by ceria/zirconia mixed oxides with a variable Ce/Zr molar ratio. The obtained catalysts demonstrated oxide promotion results in the protection of the major textural properties, especially for Zr-rich solids. Reducibility of cobalt species was enhanced by the presence of mixed oxides. The chemical composition of the oxide promoter influenced not only physicochemical properties of final catalysts but also determined their performance during the reaction. In this sense, Zr-rich systems presented a superior catalytic performance both in total conversion and in selectivity towards long chain hydrocarbons. The observed Zr-promotion effect could be explained by two significant contributions: firstly, the partial inhibition of Co-Al spinel compound formation by the presence of Zr-rich phases which enhances the availability of Co actives site and secondly, Zr-associate acidic sites promote higher hydrocarbons selectivity.


Marzo, 2019 | DOI: 10.1016/j.renene.2018.08.080

Microstructure, interfaces and properties of 3YTZP ceramic composites with 10 and 20 vol% different graphene-based nanostructures as fillers


Munoz-Ferreiro, C; Morales-Rodriguez, A; Rojas, TC; Jimenez-Pique, E; Lopez-Pernia, C; Poyato, R; Gallardo-Lopez, A
Journal of Alloys and Compounds, 777 (2019) 213-224

ABSTRACT

The graphene family comprises not only single layer graphene but also graphene-based nanomaterials (GBN), with remarkably different number of layers, lateral dimension and price. In this work, two of these GBN, namely graphene nanoplatelets (GNP) with n similar to 15-30 layers and few-layer graphene (FLG) with n < 3 layers have been evaluated as fillers in 3 mol% yttria stabilized tetragonal zirconia (3YTZP) ceramic composites. Composites with 10 and 20 vol% GNP or FLG have been fabricated by wet powder processing and spark plasma sintering (SPS) and the influence of the content and number of layers of the graphene-based filler has been assessed. For both graphene-based fillers, an intermediate zirconia oxycarbide has been detected in the grain boundaries. The lower stacking degree and much more homogeneous distribution of the FLG, revealed by transmission electron microscopy (TEM), can improve load transfer between the GBNs and the ceramic matrix. However, high FLG contents lower densification of the composites, due partly to the larger FLG interplanar spacing also estimated by TEM. The hardness (both Vickers and nanoindentation) and the elastic modulus decrease with increased GBN content and with improved graphene dispersion. The FLG greatly inhibit the crack propagation that occur perpendicular to their preferential orientation plane. The composites with thinner FLG have higher electrical conductivity than those with GNP. The highest electrical conductivity is achieved by composites with 20 vol% FLG in the direction perpendicular to the compression axis during sintering, sigma(perpendicular to) = 3400 +/- 500 Sm-1. 


Marzo, 2019 | DOI: 10.1016/j.jallcom.2018.10.336

Synthesis of sol-gel pyrophyllite/TiO2 heterostructures: Effect of calcination temperature and methanol washing on photocatalytic activity


El Gaidoumi, A.; Doña Rodríguez, J.M.; Pulido Melián, E.; González-Díaz, O.M.; Navío Santos, J.M.; El Bali, B.; Kherbeche, A.
Surfaces and Interfaces, 14 (2019) 19-25

ABSTRACT

We successfully synthesized an efficient photoactive pyrophyllite/TiO2 heterostructures using a sol-gel route at ambient temperature. The samples were prepared by exfoliation of a pyrophyllite layered-type clay by TiO2. The prepared samples exhibited strong photocatalytic activity for the degradation of phenol. The heterostructure PTi750 (SBET = 16.58 m2/g) calcined at 750 °C, in which the mixed phases of anatase and rutile exist (52.2% anatase/10.7% rutile), showed the highest photocatalytic activity against commercial TiO2Aeroxide P25. The methanol washed PTi750 was 5 times faster than the corresponding unwashed sample; phenol was totally degraded with a TOC reduction of 89.2%. The materials have been characterized by: X-ray diffraction (XRD), Diffuse reflectance UV–vis spectrophotometry (UV–Vis DRS), scanning electron microscopy (SEM) and BET specific surface area.


Marzo, 2019 | DOI: 10.1016/j.surfin.2018.10.003

Multicycle CO2 capture activity and fluidizability of Al-based synthesized CaO sorbents


Azimi, B; Tahmasebpoor, M; Sanchez-Jimenez, PE; Perejon, A; Valverde, JM
Chemical Engineering Journal, 358 (2019) 679-690

ABSTRACT

CaO-based materials have been identified as promising sorbents for highly efficient pre-combustion and post-combustion CO2 capture in fluidized beds operated at high temperatures by means of the Calcium Looping (CaL) process. However, Ca-based sorbents suffer from a decline of the capture capacity over multiple sorption/desorption cycles, mainly due to sintering, and from a markedly heterogeneous fluidization behavior due to the strength of interparticle attractive forces as compared to particle weight. The present study is focused on the development of novel synthetic CaO/Al2O3 sorbents for CO2capture with enhanced CaL performance and fluidizability by dry mixing with flow conditioner nanopowders. The influence of initial precursors on the sorbents multicycle activity at realistic CaL conditions has been investigated. The formation of a stable Ca9Al6O18 mixed-phase during the preparation of the sorbents promotes the multicycle capture capacity. The type of Ca and Al precursors, either soluble or insoluble, can significantly affect the dispersion of this stabilizer (Ca9Al6O18) in the sorbent matrix and, consequently, may affect the carbonation activity of the materials. The sorbent prepared from soluble aluminum nitrate and calcium nitrate precursors by sol-gel method exhibits a very stable multicycle capture capacity with a capture capacity around 0.2 g of CO2/g of sorbent after 21 cycles keeping a 72% of its initial capture capacity. The fluidizability of this promising sorbent was also investigated as affected by the addition of three different flow conditioners. Fluidization experiments confirmed the positive effect of using hydrophilic alumina and hydrophobic silica nanoparticles on improving the fluidizability of the synthesized sorbents.


Febrero, 2019 | DOI: 10.1016/j.cej.2018.10.061

Photodegradation of 2,4-dichlorophenoxyacetic acid over TiO2(B)/anatase nanobelts and Au-TiO2(B)/anatase nanobelts


Chenchana, A.; Nemamcha, A.; Moumeni, H.; Doña Rodríguez, J.M.; Araña, J.; Navío, J.A.; González Díaz, O.; Pulido Melián, E.
Applied Surface Science, 467-468 (2019) 1076-1087

ABSTRACT

In this work, novel TiO2-based nanobelts with various phases were synthesized: biphasic TiO2(B)/anatase, pure TiO2(B) and pure anatase. These catalysts were obtained via hydrothermal reaction using two nanoparticulated TiO2 photocatalysts as precursors: Aeroxide TiO2 P25 (P25) and TiO2 synthesized via a sol-gel process (SG). In addition, the surface of the photocatalysts was modified with gold using a photodeposition method. A characterization study of the different photocatalysts was performed with X-ray diffraction analysis (XRD), UV–Vis diffuse reflectance spectra (DRS), scanning electron microscopy (SEM), X-ray photoelectron spectrum analysis (XPS) and Brunauer-Emmett-Teller measurements (BET). The photocatalytic reaction of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) was investigated under UVA irradiation. A toxicity analysis was performed with the marine bioluminescent bacteria Vibrio fischeri. The highest 2,4-D removal efficiency of 99.2% was obtained with the biphasic Au-TiO2(TiO2(B)/anatase) nanobelts with anatase as predominant phase. Toxicity was mainly due to the intermediate 2,4-dichlorophenol (2,4-DCP) which was eliminated in 4 h. The TiO2 nanobelt phase structure is shown to have a significant effect on photocatalytic activity.


Febrero, 2019 | DOI: 10.1016/j.apsusc.2018.10.175

Insight into the BiFeO3 flash sintering process by in-situ energy dispersive X-ray diffraction (ED-XRD)


Perez-Maqueda, LA; Gil-Gonzalez, E; Wassel, MA; Jha, SK; Perejon, A; Charalambous, H; Okasinski, J; Sanchez-Jimenez, PE; Tsakalakos, T
Ceramics International, 45 (2019) 2828-2834

ABSTRACT

The sintering mechanism of BiFeO3 has been investigated in-situ by energy dispersive X-ray diffraction (ED-XRD) using a high-energy white collimated X-ray beam from the Advanced Photon Source (Argonne National Laboratories). Such radiation is very penetrating thereby allowing measurements of the sample even when placed inside the flash sintering set up. Additionally, the fast ED-XRD measurements permit monitoring the flash sintering process by providing information about phase composition and sample temperature in real time. Moreover, profile scans, obtained by moving the stage vertically while recording the ED-XRD spectra, permit investigating the homogeneity of the flash for the entire length of the sample. All experiments have been complemented by ex-situ studies. It has been concluded that flash sintering of BiFeO3 is a homogeneous process without any directionality effects. Furthermore, flash sintering takes place at quite low temperatures (below the Tc ≈ 830 °C), which may be related to the high quality of the samples, as pure, highly insulating ceramics without evidence of secondary phases with a homogenous nanostructured grain size distribution are obtained by this technique. Moreover, it is also evidenced that the rapid heating of the sample does not seem to justify, at least by itself, the densification process. Therefore, it appears that the electric current should play a role in the enhanced mobility during the sintering process.


Febrero, 2019 | DOI: 10.1016/j.ceramint.2018.07.293

Holmium doped fiber thermal sensing based on an optofluidic Fabry-Perot microresonator


Lahoz, F; Martin, IR; Soler-Carracedo, K; Caceres, JM; Gil-Rostra, J; Yubero, F
Journal of Luminescence, 206 (2019) 492-497

ABSTRACT

An optical temperature sensor suitable for label free liquid sensing has been designed and characterized. The sensor combines the photochemical stability of rare earth doped glasses and the high sensitivity of interferometric resonators. It is formed by a planar Eabry-Perot (FP) microcavity filled with the liquid to be monitored. A Ho3+ doped tapered optical fiber has been placed inside the microcavity surrounded by the fluid medium. An external laser is focused on the optical fiber inside the cavity to induce the luminescence of the Ho3+ ions, which couples to the FP optical resonances. The spectral position of the FP resonances is highly sensitive to the refractive index of the cavity medium. A second laser is co-aligned with the first one to locally heat the liquid medium around the optical fiber. An average blue shift of the FP resonances around 32 pm/degrees C is measured. The limit of detection of the laser induced heating of the liquid medium is about 0.3 degrees C in the biological temperature range. Alternatively, a hot-plate is used to heat the system. Interestingly, a red shift of the FP modes is observed with 75 pm/degrees C dependence and 0.12 degrees C limit of detection features.


Febrero, 2019 | DOI: 10.1016/j.jlumin.2018.10.103

Coupling of Ag2CO3 to an optimized ZnO photocatalyst: Advantages vs. disadvantages


P. Sánchez-Cid; C. Jaramillo-Páez; J.A. Navío; A.N. Martín-Gómez; M.C. Hidalgo
Journal of Photochemistry and Photobiology A: Chemistry, 369 (2019) 119-132

ABSTRACT

With the aim of improving the photocatalytic properties of a previously optimized zinc oxide photocatalyst, the effect of the incorporation of different amounts of Ag2CO3 on the aforementioned ZnO has been studied. For this purpose we report the synthesis, by means of simple precipitation procedures, of bare ZnO and Ag2CO3 samples as well as the coupled materials ZnO/Ag2CO3 (X) (where X = 1%, 2%, 4% and 5% in molar percentages). Both, single and coupled materials have been characterized by different techniques (XRD, XRF, N2-absorption, SEM, TEM, UV–vis/DRS and XPS). To assess the advantages or disadvantages that Ag2CO3 addition could have over the optimized ZnO, the photocatalytic properties have been established by following the photo-degradation of selected toxic molecules, both in the UV and in the visible, as well as using complementary techniques of liquid medium analyses (TOC and Atomic Emission Spectrometry with plasma ICP). Three selected substrates were chosen: Rhodamine B (RhB) as a dye, and phenol and caffeine as colourless recalcitrant toxic molecules.
Our results suggest that although the use of Ag2CO3 could be beneficial to implement the optical absorption towards the visible region, however, other effects have to be bore in mind, such as the photo-corrosion of Ag2CO3 and the chemical structure of the chosen substrate, to elucidate whether the addition of Ag2CO3 has beneficial or detrimental effects on the photocatalytic properties of the coupled ZnO/Ag2CO3 materials.


Enero, 2019 | DOI: 10.1016/j.jphotochem.2018.10.024



2018


New concept for old reaction: Novel WGS catalyst design


Garcia-Moncada, N; Gonzalez-Castano, M; Ivanova, S; Centeno, MA; Romero-Sarria, F; Odriozola, JA
Applied Catalysis A-General, 238 (2018) 1-5

ABSTRACT

The viability of water gas shift catalytic system for mobile application passes through obligatory reactor volume reduction, achieved normally by using less charge of more efficient catalyst. Completely new concept for catalyst design is proposed: a catalytic system including classically reported WGS catalysts of different nature or active phase (Cu, Pt or Au) mechanically mixed with an ionic conductor. The influence of the later on catalyst activity is studied and discussed, more precisely its effect on the rate of the reaction-limiting step and catalysts' efficiency. It is demonstrated with this study, that the presence of an ionic conductor in contact with a WGS catalyst is essential for the water supply (dissociation and transport), thereby potentiating the water activation step, whatever the mechanism and catalyst overall performance.


Diciembre, 2018 | DOI: 10.1016/j.apcatb.2018.06.068

CO/H-2 adsorption on a Ru/Al2O3 model catalyst for Fischer Trospch: Effect of water concentration on the surface species


Jimenez-Barrera, E; Bazin, P; Lopez-Cartes, C; Romero-Sarria, F; Daturi, M; Odriozola, JA
Applied Catalysis B-Environmental, 237 (2018) 986-995

ABSTRACT

Water presence and concentration strongly influence CO conversion and CS+ selectivity in the Fischer Tropsch reaction. In this work, the influence of the water concentration was investigated using a model Ru/Al2O3 (5 wt. %) catalyst. The surface species formed after CO and H-2 adsorption in dry and wet (different water concentrations) conditions were analyzed by FTIR. Firstly, water adsorption was carried out up to complete filling of the pores and then CO was put in contact with the catalyst. The absence of adsorbed CO species in these conditions evidences that CO diffusion in water controls the access of the gas to the active sites and explains the negative effect of high water concentrations reported by some authors. Moreover, the adsorption of a mixture of CO + H-2 + H2O, being the water concentration close to that needed to have a monolayer, and a dry mixture of CO + H-2 were carried out and compared. Results evidence that water in this low concentration, is able to gasify the surface carbon species formed by CO dissociation on the metallic sites. This cleaning effect is related to the positive effect of water on CO conversion detected by some authors.


Diciembre, 2018 | DOI: 10.1016/j.apcatb.2018.06.053

Understanding the Role of the Acid Sites in 5-Hydroxymethylfurfural Oxidation to 2,5-Furandicarboxylic Acid Reaction over Gold Catalysts: Surface Investigation on CexZr1-xO2 Compounds


Megias-Sayago, C; Chakarova, K; Penkova, A; Lolli, A; Ivanova, S; Albonetti, S; Cavani, F; Odriozola, JA
ACS Catalysis, 8 (2018) 11154-11164

ABSTRACT

A series of CexZr1-xO2 supports with different Ce/Zr molar ratios were utilized for the preparation of gold catalyst used in the selective oxidation of 5-hydroxymethyl-2-furfural to 2,5-furandicarboxylic acid. The used method of gold deposition allows the preparation of gold particles with homogeneous size and shape distribution, a formulation very useful for studies dedicated to revealing the support participation in the reaction. The supports are characterized by Fourier transform infrared spectroscopy using CO as probe molecule, and the sample catalytic activity is thereafter correlated to the support acid site distribution. The possible participation of its Lewis/Bronsted acidity in the reaction mechanism is also proposed.


Diciembre, 2018 | DOI: 10.1021/acscatal.8b02522 DEC 2018

Revealing the substitution mechanism in Eu3+:CaMoO4 and Eu3+,Na+:CaMoO4 phosphors


Becerro, AI; Allix, M; Laguna, M; Gonzalez-Mancebo, D; Genevois, C; Caballero, A; Lozano, G; Nunez, NO; Ocana, M
Journal of Materials Chemistry C, 6 (2018) 47

ABSTRACT

Eu3+-Doped calcium molybdate is an excellent phosphor for lighting and display devices due to the very intense pure red emission after UV excitation. It has been reported in the literature that the CaMoO4 unit cell volume expands after Eu3+ doping, in spite of the smaller Eu3+ ionic radius compared with Ca2+. Likewise, several studies found that the emission intensity of the phosphor could be improved by codoping with alkaline ions like Li+, Na+ or K+. None of these studies correlated the apparent volume expansion and luminescence enhancement with the crystal structural details. This paper analyses the aliovalent substitution mechanism and crystal structure of Eu3+:CaMoO4 and Eu3+,Na+:CaMoO4 phosphors using complementary techniques like Raman spectroscopy, EXAFS and SPD. We found that the substitution mechanism was different for both systems, with Ca site vacancies forming in the Eu3+:CaMoO4 phosphors and leading to Ca1-3xEu2xxMoO4 compositions, while the Eu3+,Na+:CaMoO4 phosphors formed Ca1-2xEuxNaxMoO4. SPD showed that the cell volume expansion observed with increasing Eu3+ content is related to the increase of the Mo-O bond distance due to the higher electronegativity of Eu3+ compared with Ca2+. Finally, it was shown that the luminescence properties, i.e. lifetime values and quantum yields (the latter reported here for the first time), do not depend on the presence of monovalent ions in the crystal structure but, exclusively, on the Eu3+ content of the phosphor. The integral and detailed analysis of the materials presented in this paper, ranging from crystal structure to luminescent properties including elemental composition, allows a full picture of the structure-property relationships that had never been addressed before for CaMoO4-based phopshors.


Diciembre, 2018 | DOI: 10.1039/c8tc04595j

Role of calcium looping conditions on the performance of natural and synthetic Ca-based materials for energy storage


Sarrion, B; Perejon, A; Sanchez-Jimenez, PE; Perez-Maqueda, LA; Valverde, JM
Journal of CO2 utilization, 28 (2018) 374-384

ABSTRACT

In this work, the multicycle activity of natural CaO precursors (limestone and dolomite) and Ca-based composites (Ca3Al2O6/CaCO3 and ZrO2/CaCO3 mixtures) has been studied for Thermochemical Energy Storage (TCES) in Concentrated Solar Power (CSP) plants by means of the Calcium-Looping process (CaL), using two integration schemes proposed elsewhere that differ in the calcination stages. Under CSP-He conditions, calcination for CaO regeneration is performed under pure He at low temperatures (725 degrees C) while under CPS-CO2 conditions calcination is carried out under pure CO2 at high temperatures (950 degrees C). The latter avoids the use of selective membranes to separate He from CO2 even though it requires the use of more expensive materials for solar receptors. Carbonation/calcination conditions drastically affect the multicycle CO2 uptake of the materials tested. Effective multicycle conversion is higher in CSP-He tests due to the mild conditions employed for calcination, which mitigates CaO sintering. On the other hand, the harsh calcination conditions used in CSP-CO2 tests enhance sintering of CaO derived from limestone and the Ca3Al2O6/CaCO3 composite due to the low Tammann temperature of Ca3Al2O6. CaO sintering is hindered by the presence of inert oxides with high Tammann temperatures, such as ZrO2 in the ZrO2/CaCO3 composite and MgO in dolomite. Dolomite derived CaO shows high effective conversion values along the carbonation/calcination cycles when tested under both types of conditions, as compared to limestone and the composites, which suggests that the integration scheme based on CSP-CO2 conditions would be a feasible alternative to CSP-He if natural dolomite were used as CaO precursor.


Diciembre, 2018 | DOI: 10.1016/j.jcou.2018.10.018

Molten carbonate salts for advanced solar thermal energy power plants: Cover gas effect on fluid thermal stability


Fereres, S; Prieto, C; Gimenez-Gavarrell, P; Rodriguez, A; Sanchez-Jimenez, PE; Perez-Maqueda, LA
Solar Energy Materials and Solar Cells, 188 (2018) 119-126

ABSTRACT

The eutectic mixture Li2CO3-Na2CO3-K2CO3 is investigated as a high temperature heat transfer fluid and storage medium alternative for molten salt solar thermal power plants. This salt has an operating temperature range of 400–700 °C, enabling the use of higher temperature/efficiency power cycles. However, this carbonate mixture is known to thermally decompose in air. This study evaluates the thermal stability of the salt mixture under different cover gases: air, nitrogen, carbon dioxide, and an 80/20 carbon dioxide/air mixture. Initial characterization is performed through thermogravimetric analysis (TGA), followed by larger scale testing in a custom-made reactor to simulate conditions closer to its practical use. The results show improved thermal stability with a CO2 atmosphere. The decomposition kinetics under different cover gases are estimated from TGA data. However, larger-scale, longer duration experiments show much slower decomposition rates compared to the classical TGA approach. These findings indicate that the main contribution to mass loss in TGA is due to vaporization rather than thermal decomposition. Thus, a proper evaluation of the molten salt´s thermal stability can only be obtained from reactor experiments where vaporization is inhibited. Very long induction periods (of the order of days) are observed, suggesting that the kinetic decomposition mechanism is a nucleation and growth type. Other considerations for future plants incorporating these high temperature salts are discussed.


Diciembre, 2018 | DOI: 10.1016/j.solmat.2018.08.028

Improving the Bulk Emission Properties of CH3NH3PbBr3 by Modifying the Halide-Related Defect Structure


Tiede, David O.; Calvo, Mauricio E.; Galisteo-Lopez, Juan F.; Miguez, Hernan
Journal of Physical Chemistry C, 122 (2018) 27250-27255

ABSTRACT

The peculiar defect chemistry of hybrid organic–inorganic lead halide perovskites is believed to be partially responsible for the outstanding performance of this solution-processed material in optoelectronic devices. While most effort has been put on the management of halide defects (the ones presenting the highest mobility) for CH3NH3PbI3, its bromide counterpart has not been so widely studied. Although the former is the material of choice for photovoltaics, the latter is present in most light-emitting applications. Here, we report how the exposure of CH3NH3PbBr3 single crystals to a bromine atmosphere strongly affects its emission properties. Such improvement takes place in the absence of apparent signs of degradation and remains for tens of hours. We propose an explanation based on the defect structure for this material where bromine-related defects can act as deep or shallow traps. These results are of relevance for a material expected to be present in a new generation of solution-processed light-emitting devices.


Diciembre, 2018 | DOI: 10.1021/acs.jpcc.8b09315 DEC 6 2018

Selective CO methanation with structured RuO2/Al2O3 catalysts


Munoz-Murillo, A; Martinez, LM; Dominguez, MI; Odriozola, JA; Centeno, MA
Applied Catalysis B-Environmental, 236 (2018) 420-427

ABSTRACT

Active and selective structured RuO2/Al2O3 catalysts for CO methanation using a flow simulating CO2-rich reformate gases from WGS and PROX units (H-2 excess, CO2 presence and 300 ppm CO concentration) were prepared. Both, the RuO2/Al2O3 powder and the slurry prepared from it for its structuration by washcoating of the metallic micromonolithic structure, were also active and selective. Both the slurry (S-RuAl) and micro monoliths (M-RuAl) were able to completely and selectively methanate CO at much lower temperatures than the parent RuAI powder. The optimal working temperature in which the CO conversion is maximum and the CO2 conversion is minimized was determined to be from 149 degrees C to 239 degrees C for S-RuAl and from 165 degrees C to 232 degrees C for M-RuAl, whilst it was from 217 degrees C to 226 degrees C for RuAI powder. TPR, XRD and TEM measurements confirmed that the changes in the activity and selectivity for CO methanation among the considered catalysts can be related with modifications in the surface particle size of ruthenium and its reducibility. These were ascribed to the metallic substrate, the presence of PVA and colloidal alumina in the slurry preparation, the aqueous and acidic media and the thermal treatment used, resulting in a more active and selective catalysts than the parent powder.


Noviembre, 2018 | DOI: 10.1016/j.apcatb.2018.05.020

Nanoporous Pt-based catalysts prepared by chemical dealloying of magnetron-sputtered Pt-Cu thin films for the catalytic combustion of hydrogen


Giarratano, F; Arzac, GM; Godinho, V; Hufschmidt, D; de Haro, MCJ; Montes, O; Fernandez, A
Applied Catalysis B-Environmental, 235 (2018) 168-176

ABSTRACT

In this work, we prepared SiC-supported Pt-Cu thin films by magnetron sputtering for use as catalysts for the combustion of hydrogen under oxidizing conditions. We tested the catalysts as prepared and after chemical dealloying. A methodology is presented to fabricate catalytic thin films of a desired composition with tailored magnetron targets with lower Pt consumption. The deposition gas was changed to prepare columnar (Ar-deposited) and closed-porous (He-deposited) films to study the effect of the microstructure on the activity. The effect of composition was also studied for the columnar samples. The as-prepared Pt-Cu thin films showed significant activity only at temperatures higher than 100 °C. Dealloying permitted an increase in the activity to achieve near room-temperature activity. The dealloyed closed-porous He-deposited sample was the most active, being able to convert as much as 13.15 LH2·min−1 gPt−1 at 70 °C (Ea = 1 kJ mol−1). This sample was preferentially dealloyed on the surface, yielding an almost pure Pt shell (96% at. Pt) and a Cu-depleted interior (71% at. Pt). This compositional inhomogeneity enabled the sample to achieve enhanced activity compared to the Ar-deposited columnar sample (with similar initial composition, but uniformly dealloyed), probably due to the compressive surface lattice strain. The dealloyed closed-porous He-deposited sample was shown to be durable over five cycles.


Noviembre, 2018 | DOI: 10.1016/j.apcatb.2018.04.064

Sustainable Fabrication of Plant Cuticle-Like Packaging Films from Tomato Pomace Agro-Waste, Beeswax, and Alginate


Tedeschi, G; Benitez, JJ; Ceseracciu, L; Dastmalchi, K; Itin, B; Stark, RE; Heredia, A; Athanassiou, A; Heredia-Guerrero, JA
ACS Sustainable Chemistry & Engineering, 6 (2018) 14955-14966

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.


Noviembre, 2018 | DOI: 10.1021/acssuschemeng.8b03450

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