Role of the Sulphur Source in the Solvothermal Synthesis of Ag-CdS Photocatalysts: Effects on the Structure and Photoactivity for Hydrogen Production

© 2020 by the authors.The aim of this work is to study the influence of the sulphur source (elemental sulphur, thiourea and L-cysteine) in the solvothermal synthesis of Ag-CdS over its growth, structuration and state of Ag and how these changes influence on its photoactivity. The differences in the generation rate of the S2− from the sulphur sources during the solvothermal synthesis determine the nucleation and growth pathways of CdS affecting to the silver state and its incorporation into the CdS lattice. The hydrogen production on Ag-CdS photocatalysts decreases according the sequence: thiourea > elemental sulphur >> L-cysteine. The changes in the photoactivity of Ag-CdS samples are analysed in terms of the differences in the insertion of Ag+ into the CdS lattice, the formation of composites between CdS and Ag2S and the formation of CdS crystalline domains with strong confinement effect derived from the different sulphur source used in the solvothermal synthesis.The present work was performed within the research program supported by MINECO (Spain) under the projects CTQ2016-76505-C3-1-R and PID2019-1112119RB-100 and CAM within the BIOTRES-CM (S2018/EMT-4344) project. E. Soto would like to acknowledge MINECO for the FPI research grant.Peer reviewe

Nature of the mixed-oxide interface in ceria-titania catalysts: Clusters, chains, and nanoparticles

The ceria-titania mixed metal oxide is an important component of catalysts active for the production of hydrogen through the water-gas shift reaction (CO + H2O → H2 + CO2) and the photocatalytic splitting of water (H2O + hv → H2 + 0.5O 2). We have found that ceria-titania catalysts prepared through wet chemical methods have a unique hierarchal architecture. Atomic resolution imaging by high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) reveals that ceria supported on titania exhibits a range of morphologies. One can clearly identify ceria structures involving clusters, chains, and nanoparticles, which are distributed inhomogeneously on the titania support. These structures are often below the sensitivity limit of techniques such as X-ray diffraction (XRD), which in this case identifies the average particle size of the ceria and titania nanoparticles (via the Debye-Scherer equation) to be 7.5 and 36 nm, respectively. The fluorite-structured ceria grows epitaxially on the anatase-structured titania, and this epitaxial growth influences the morphology of the nanoparticles. The presence of defects in the ceria - such as dislocations and surface steps - was routinely observed in HAADF STEM. Density functional theory (DFT) calculations indicate an energetic preference for the formation of O vacancies and the corresponding Ce 3+ sites at the ceria-titania interface. Experimental corroboration by soft X-ray absorption spectroscopy (SXAS) does suggest the presence of Ce3+ sites at the interface. © 2013 American Chemical Society.The research carried out at the Center for Functional Nanomaterials and the Chemistry Department of Brookhaven National Laboratory was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. The theoretical studies were funded by the Ministry of Economy and Competitiveness (Spain, grants MAT2012-31526 and CSD2008-0023) and EU FEDER. Computational resources were provided by the Barcelona Supercomputing Center/Centro Nacional de Supercomputación (Spain).Peer Reviewe

Remembering professor Jose Luis García Fierro (1948–2020)

On 3 February, Professor Jose Luis Garcia Fierro died in Madrid, Spain, at the age of 71. Professor Fierro studied Chemistry at the University of Oviedo (Spain) where in 1973 he graduated with honours. In 1976, he received his PhD Degree in Chemistry from the Complutense University of Madrid (Spain). As post-doctoral researcher, he expanded his scientific experience working in prestigious laboratories of University Pierre et Marie Curie (Paris, France), University College of Cork (Ireland) and Université Catholique de Louvain (Louvain-la-Neuve, Belgium). Since 1974 and untill end of his life, he worked at the Spanish National Council for Scientific Research (CSIC) where he developed his passion for scientific research.Peer reviewe

From Nanorods to Nanowires of CdS Synthesized by a Solvothermal Method: Influence of the Morphology on the Photoactivity for Hydrogen Evolution from Water

The effect of temperature and water/thiourea ratio on the growth, crystallinity and morphological characteristics of CdS nanostructures synthetized by a solvothermal method using ethylenediamine as solvent were studied. The temperature and water/thiourea ratio used in the synthesis determine the surface area, shape, length and degree of crystallinity of the CdS nanostructures obtained. Nanowires of high crystallinity and length were obtained when the solvothermal synthesis was performed at 190 °C, while nanorods with lower length and crystallinity were obtained as the solvothermal temperature decreased to 120 °C. The change in the water/thiourea ratio affects the crystallinity and length of the CdS nanostructures to a lesser extent than temperature. Nevertheless an increase in the water/thiourea ratio used during the solvothermal synthesis resulted in CdS nanorods with higher crystallinity, lower aspect ratio and lower specific surface area. Textural, structural and surface properties of the prepared CdS nanostructures were determined and related to the activity results in the production of hydrogen from aqueous solutions containing SO32− + S2− under visible light.The present work was performed within the research programs supported by the Secretaría de Estado de Investigación, Desarrollo e Investigación and CAM (Spain) under projects CTQ2013-48669-P and S2013/MAE-2882 respectively. Fernando Vaquero would like to acknowledge the Secretaria de Estado de Investigación, Desarrollo e Innovación for the FPI research grant.We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI)

Controlling the impregnation of nickel on nanoporous aluminum oxide nanoliths as catalysts for partial oxidation of methane

Three different conventional impregnation methods were studied in order to elucidate the effect of impregnation variables on the homogeneity in the incorporation of Ni species into the nanoporous structure of the aluminum oxide nanoliths. From results obtained in this study, two factors seem to control the Ni incorporation onto the aluminium oxide nanostructure: (i) the methodology used in the impregnation: incipient wetness impregnation, wet impregnation, or forced wet impregnation, and (ii) the Ni precursor concentration used during the impregnation process. Both the nickel loading and the nickel distribution entities inside the porous structure of aluminium oxide nanoliths depend on the method and concentration of the Ni impregnating solution used in the impregnation. The distribution of the NiO loaded on nanoliths was not uniform as SEM analyses demonstrated. Segregation of NiO entities on surface of AOO nanolith was observed in all samples. For impregnation with Ni solution of low concentration, only incipient wetness impregnation and forced wet impregnation allows the deposit of nickel entities on the porous structure of the AAO nanoliths. By increasing the Ni concentration in the impregnating solution, a larger degree of pore coverage was obtained for all impregnation methods used.The present work was performed within the research project supported by the MEyC (Spain) under Project ENE2010-21198-C04-01.Peer Reviewe

Unravelling the Structural Modification (Meso-Nano-) of Cu/ZnO-Al2O3 Catalysts for Methanol Synthesis by the Residual NaNO3 in Hydroxycarbonate Precursors

The effects of residual NaNO3 on the modification of Cu/ZnO-Al2O3 catalysts have been extensively documented, but the modification mechanism is so far unclear. This work studies in detail the influence of the residual sodium nitrate present in the hydroxycarbonate precursors on their decomposition during calcination and how it affects to the formation and configuration of the final active sites of the Cu/ZnO-Al2O3 catalysts. Different samples with varying sodium content after washing (from 0.01 to 7.3 wt%) were prepared and studied in detail after calcination and reduction steps. The results of this work demonstrated that NaNO3 affects the decomposition mechanism of the hydroxycarbonate precursors during calcination and produces its decarbonation at low temperature. The enhancement of the decarbonation by NaNO3 leads to segregation and crystallization of CuO and ZnO with loss of mesostructure and surface area in the calcined catalysts. The loss of mesostructure in calcined catalysts affects the subsequent reduction step, decreasing the reducibility and damaging the nanostructure of the reduced catalysts forming large Cu particles in poor contact with ZnOx that results in a significant decrease in the intrinsic activity of the copper active sites for methanol synthesis

Producción de hidrógeno mediante un ciclo termoquímico de disociación de agua utilizando óxidos redox

La presente invención se refiere a un procedimiento para la producción de hidrógeno mediante ciclos termoquímicos que comprende el uso de óxidos activos, tales como el los óxidos de cerio modificados. Estos sistemas permiten la producción de hidrógeno puro a baja temperatura, de manera cíclica, mediante la utilización de un sistema sencillo y de fácil operación. Siendo un procedimiento para la producción de hidrógeno de forma renovable y fuera del ciclo del carbono.NoEndesa Generación S.A.B1 Patente sin examen previ

Direct Synthesis of Dimethyl Ether from CO2: Recent Advances in Bifunctional/Hybrid Catalytic Systems

© 2021 by the authors.Dimethyl ether (DME) is a versatile raw material and an interesting alternative fuel that can be produced by the catalytic direct hydrogenation of CO2. Recently, this process has attracted the attention of the industry due to the environmental benefits of CO2 elimination from the atmosphere and its lower operating costs with respect to the classical, two-step synthesis of DME from syngas (CO + H2). However, due to kinetics and thermodynamic limits, the direct use of CO2 as raw material for DME production requires the development of more effective catalysts. In this context, the objective of this review is to present the latest progress achieved in the synthesis of bifunctional/hybrid catalytic systems for the CO2-to-DME process. For catalyst design, this process is challenging because it should combine metal and acid functionalities in the same catalyst, in a correct ratio and with controlled interaction. The metal catalyst is needed for the activation and transformation of the stable CO2 molecules into methanol, whereas the acid catalyst is needed to dehydrate the methanol into DME. Recent developments in the catalyst design have been discussed and analyzed in this review, presenting the different strategies employed for the preparation of novel bifunctional catalysts (physical/mechanical mixing) and hybrid catalysts (co-precipitation, impregnation, etc.) with improved efficiency toward DME formation. Finally, an outline of future prospects for the research and development of efficient bi-functional/hybrid catalytic systems will be presented.The present investigation was performed within the research programs CTQ2016-76505-C3-1 and PID2019-111219RB-100 supported by the Spanish Ministry of Science, Innovation and Universities. The Autonomous Community of Madrid (CAM) is also gratefully acknowledged for funding the BIOTRES-CM (P2018/EMT-4344) project. Elena Millán would like to acknowledge the FPI program from the Spanish Ministry of Science, Innovation and Universities for the research grant

Rh/Al2O3–La2O3 catalysts promoted with CeO2 for ethanol steam reforming reaction

Rh-based catalysts supported on γ-Al2O3–La2O3 modified with CeO2 promoter were prepared in the present work. The objective was to study the promotional effect of the addition of different CeO2 contents and 15 wt% La2O3 on the stability of Rh/γ-Al2O3 catalysts for ethanol steam reforming reaction. The physico-chemical characteristics of the catalysts, activity and the deposition of carbon on the catalyst surface were surveyed by NH3-TPD, XRD, TPR, XPS, DRIFT, TPO-TG and Raman. The stability of Rh/γ-Al2O3 catalyst was effectively improved by the addition of 15 wt% La2O3 and 5 wt% CeO2, as the carbon deposition rate was reduced significantly (from 2.3 to 0.16 mgcoke gcat−1 h−1). The role of the promoter was to decrease the selectivity to ethylene by modification of Lewis acid sites on the support and the removal of carbon deposits during the reaction. This is achieved through the formation of lanthanum oxycarbonate from La2O3 and higher amount of mobile lattice oxygen induced by the CeO2 promoter. This catalyst exhibited good selectivity to H2, with no presence of C2 intermediates above 723 K; in addition this catalyst was stable up to 48 h of time on stream at 773 K. Furthermore, the original activity of the catalyst was recovered through regeneration by the removal of some carbon deposits which are potential causes of deactivation.Authors thank CONICYT for financial support FONDECYT postdoctoral GRANT 3140130 and P. Osorio-Vargas thanks to Red Doctoral REDOC and CTA MECESUP for doctoral fellowship.Peer Reviewe

Improved ethanol steam reforming on Rh/Al2O3 catalysts doped with CeO2 or/and La2O3: Influence in reaction pathways including coke formation

In this paper, Rh catalysts supported over bare γ-Al2O3 and γ-Al2O3 modified with different promoters (15 wt%La2O3 and/or 2.5 wt%CeO2) were reported. The catalysts were prepared by successive impregnation: first the rare earth oxide promoter and then Rh. Hydrogen production by ethanol steam reforming (ESR) over these catalysts was investigated. Among the catalysts, Rh/La2O3–CeO2–γ-Al2O3 catalyst shows the highest hydrogen yield as well as the best stability during the reaction. Reaction pathways of the ESR over these Rh catalysts are investigated from the analysis of the adsorbed species and gas phase products formed during temperature programmed desorption of ethanol and ethanol steam reforming reaction. The differences in the reaction pathways and products distribution are as a result of the modification of the stability of some reaction intermediates caused by the presence of a small quantity of CeO2 and La2O3 on the support surfaces: in effect, acetaldehyde was formed as an intermediate over the modified-γ-Al2O3 supported catalysts while in the case of the bare Al2O3 support the main route of the steam reforming reaction is the direct ethanol decomposition. Analysis of the used catalysts by XPS, TGA and RAMAN showed that changes in reaction pathways associated to the presence of promoters in the Al2O3 support also modifies the extension and type of carbon deposited on catalyst surfaces.Authors thank CONICYT for financial support FONDECYT postdoctoral GRANT 3140130. P. Osorio-Vargas is thankful to Red Doctoral REDOC, MINEDUC Project UCO 1202 and CTA MECESUP for doctoral fellowship.Peer Reviewe