M.E. Gálvez

Fast and Reversible Direct CO2 Capture from Air onto All-Polymer Nanofibrillated Cellulose—Polyethylenimine Foams

Fully polymeric and biobased CO2 sorbents composed of oxidized nanofibrillated cellulose (NFC) and a high molar mass polyethylenimine (PEI) have been prepared via a freeze-drying process. This resulted in NFC/PEI foams displaying a sheet structure with porosity above 97% and specific surface area in the range 2.7-8.3 m(2)·g(-1). Systematic studies on the impact of both PEI content and relative humidity on the CO2 capture capacity of the amine functionalized sorbents have been conducted under atmospheric conditions (moist air with ∼400 ppm of CO2). At 80% RH and an optimum PEI content of 44 wt %, a CO2 capacity of 2.22 mmol·g(-1), a stability over five cycles, and an exceptionally low adsorption half time of 10.6 min were achieved. In the 20-80% RH range studied, the increase in relative humidity increased CO2 capacity of NFC/PEI foams at the expense of a high H2O uptake in the range 3.8-28 mmol·g(-1).

Design Principles of Perovskites for Thermochemical Oxygen Separation

Abstract Separation and concentration of O2 from gas mixtures is central to several sustainable energy technologies, such as solar‐driven synthesis of liquid hydrocarbon fuels from CO2, H2O, and concentrated sunlight. We introduce a rationale for designing metal oxide redox materials for oxygen separation through “thermochemical pumping” of O2 against a pO2 gradient with low‐grade process heat. Electronic structure calculations show that the activity of O vacancies in metal oxides pinpoints the ideal oxygen exchange capacity of perovskites. Thermogravimetric analysis and high‐temperature X‐ray diffraction for SrCoO3−δ, BaCoO3−δ and BaMnO3−δ perovskites and Ag2O and Cu2O references confirm the predicted performance of SrCoO3−δ, which surpasses the performance of state‐of‐the‐art Cu2O at these conditions with an oxygen exchange capacity of 44 mmol O 2 mol SrCoO 3−δ −1 exchanged at 12.1 μmol O 2 min−1 g−1 at 600–900 K. The redox trends are understood due to lattice expansion and electronic charge transfer.

Towards new generation fuel cell electrocatalysts based on xerogel–nanofiber carbon composites

Xerogel–nanofiber carbon composites (XNCCs) have been easily synthesized by using a Ni catalyst supported on carbon xerogel (CXG), growing randomly oriented carbon nanofibers (CNFs) within the coralline-like structure of the xerogel (CXG). This novel composite combines the advantages of xerogel and fiber nanostructures. The interactions between these phases as well as their effect as a support on Pt electrocatalysts for the oxygen reduction reaction (ORR) have been investigated. Platinum catalysts supported on different XNCCs (varying in terms of CXG and CNF contents) as well as on bare CXG and CNFs have been synthesized using a microemulsion route. They have been characterized in terms of structure, morphology and porosity and investigated for the ORR in a half-cell configuration. The catalyst supported on the XNCC with a 44% CNF content shows the best electrochemical behavior. This catalyst formulation leads to a catalytic activity 5 times higher than that obtained on a Vulcan-based catalyst at low overpotential and 2.5 times higher at large overpotential. Accelerated degradation tests also show better stability for the composite support-based catalyst. Compared to bare CNF and CXG supports, a stabilization effect is envisaged by the presence of highly graphitic CNFs within the composite structure.

Platinum Ruthenium Catalysts Supported on Carbon Xerogel for Methanol Electro‐Oxidation: Influence of the Catalyst Synthesis Method

A high surface area, highly mesoporous carbon xerogel was synthesised and used as a support in the preparation of platinum–ruthenium catalysts by different synthetic routes. The platinum–ruthenium carbon xerogel catalysts were physico‐chemically characterised and used for the chemical electro‐oxidation of methanol. The synthetic routes pursued included: 1) impregnation with metal chloride precursors and reduction with two different reducing agents: sodium borohydride and formic acid; 2) a microemulsion‐based method and 3) a sulfite complex method, which led to catalysts with different physico‐chemical features that strongly influence their catalytic behaviour towards methanol oxidation. The electro‐oxidation of methanol was found to depend on both the crystal size and the extent of active phase reduction as well as on the platinum concentration on the catalyst surface, which were maximised for the impregnation method and reduction with formic acid.

N-Doped Carbon Xerogels as Pt Support for the Electro-Reduction of Oxygen

Durability and limited catalytic activity are key impediments to the commercialization of polymer electrolyte fuel cells. Carbon materials employed as catalyst support can be doped with different heteroatoms, like nitrogen, to improve both catalytic activity and durability. Carbon xerogels are nanoporous carbons that can be easily synthesized in order to obtain N-doped materials. In the present work, we introduced melamine as a carbon xerogel precursor together with resorcinol for an effective in-situ N doping (3–4 wt % N). Pt nanoparticles were supported on nitrogen-doped carbon xerogels and their activity for the oxygen reduction reaction (ORR) was evaluated in acid media along with their stability. Results provide new evidences of the type of N groups aiding the activity of Pt for the ORR and of a remarkable stability for N-doped carbon-supported Pt catalysts, providing appropriate physico-chemical features.

Heterogeneous TiO2–Fe-plate catalyst for the discoloration and mineralization of aqueous solutions of cationic and anionic dyes

AbstractA novel structured TiO2–Fe-plate catalyst was prepared by immobilization of iron and TiO2 on a natural clay plate. This catalytic system allows effective mineralization of wastewaters under a wide range of operation conditions. Upon UVA irradiation and in the presence of several oxidant mixtures (H2O2 or/and K2S2O8), this catalyst was found to be highly effective in the mineralization of malachite green and red congo in water solution. The effectiveness of this catalyst was assigned to a synergy between its photo-catalytic and photo-Fenton activities. Activity was enhanced in the presence of small amounts of K2S2O8 and verified over a wide range of pH.

Titanium Dioxide Supported on Different Porous Materials as Photocatalyst for the Degradation of Methyl Green in Wastewaters

TiO2 nanoparticles were immobilized on two porous materials used as catalyst supports, namely, activated carbon (AC) and natural clay (NC), through an impregnation process using TiO2 (P25) as precursor. The so-prepared composite materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transition electron microscopy (TEM), and nitrogen physisorption, that is, Brunauer-Emmett-Teller (BET) surface area determination. SEM and TEM observation evidenced that TiO2 was deposited on AC and NC surface. XRD results confirmed that TiO2 existed in a mixture of anatase and rutile phases. The specific surface area of photocatalysts decreased drastically in comparison with the original materials. The photocatalytic activity of these materials was assayed in the oxidation of Methyl Green (MG) dye in aqueous medium under UV irradiation. TiO2/AC exhibited higher photocatalytic oxidation activity than TiO2 at neutral pH. Total mineralization of MG was confirmed by means of COD analysis, pointing to these materials as an efficient, cost-effective, and environment friendly alternative for water treatment.

Nitrogen Doped and Functionalized Carbon Materials as Supports for Catalysts in Electro-Oxidation of Methanol

The objective of this work is to study the behavior of Nitrogen-doped carbons as supports of catalysts for the electro-oxidation of methanol. Two carbon materials have been considered: a) carbon xerogels (CXG), highly mesoporous, whose porosity and pore size distribution are easily performed during the synthesis method; b) carbon nanofibers (CNF), which have a high electrical conductivity, good behavior in high temperature conditions and resistance to acid/basic media. Meanwhile, a commercial carbon black (Vulcan XC72R) which is commonly used in manufacturing of electrocatalysts fuel cells was used for comparison. Nitrogen was introduced into the CXG during the synthesis process, what is commonly referred as doping, by including melamine as a reactant. In contrast, N-groups were created over CNF by post-treatment with: ammonia (25%), urea (98%), melamine (99%) and ethylenediamine (99.5%), with a carbon: nitrogen molar ratio 1:0.6. N-containing carbon materials were characterized by elemental analysis, nitrogen adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), SEM-EDX and TEM to determinate the amount and forms of nitrogen introduced. Pt-catalysts were prepared by the microemulsion method. The influence of the nitrogen doping and functionalization on the catalytic behavior in the electrochemical oxidation of methanol was evaluated by different physicochemical and electrochemical analysis.

Mg-promotion of Ni natural clay-supported catalysts for dry reforming of methane

Mg-promotion of natural clay based Ni-catalysts was considered, as a way of boosting the dry reforming of methane (DRM) activity of these materials. The results of the DRM experiments performed at temperatures from 600 °C to 850 °C evidenced much higher methane and CO2 conversions for the Mg-promoted catalysts. Mg-promotion led of course to a significant increase of CO2-adsorption ability (basicity). However, the increased catalytic activity of the Mg-promoted materials was rather linked to increased Ni-dispersion and Ni0 crystallite size. Indeed, independent of the physico-chemical properties of the support, the presence of Mg led to the formation of a MgNiO2 mixed phase that, upon reduction, resulted in the formation of metallic Ni clusters having sizes around 7–9 nm, considerably smaller than in any of the non-promoted catalysts. Carbon formation was found to take place to a greater extent in the presence of the Mg-promoted catalysts, due to C–H bond activation leading also to favored direct methane decomposition (DMD). In spite of this, the activity of the Mg-promoted catalysts was well maintained over 5 hour DRM experiments performed at 750 °C.

Experimental investigation on the influence of the presence of alkali compounds on the performance of a commercial Pt–Pd/Al 2 O 3 diesel oxidation catalyst

Experimental investigation was conducted on the influence of the presence of alkali compounds such as K and Na present in biofuels on catalytic behaviors of a commercial diesel oxidation catalyst (Pt/Pd/Al2O3) in the monolith form. Doping of different alkali metal components on carrots of monolith was performed. These carrots were physicochemically characterized, and the catalytic tests consisted of series of temperature-programmed surface reactions with representative exhaust gas mixtures from diesel combustion. The aim of the present study is to reveal the effect of the alkali metal on overall catalytic activity of diesel oxidation catalyst (DOC) and more particularly to show their influence on the reactions involving CO, hydrocarbons, NO, and NO2. Potassium and sodium lead to different catalytic properties. A promotion effect was found in the presence of K, whereas an inhibiting effect was evidenced in the presence of Na or when both Na and K were doped onto the DOC.