Exploring the synergistic effect of palladium nanoparticles and highly dispersed transition metals on carbon nitride/super-activated carbon composites for boosting electrocatalytic activity.

In the present work, multifunctional electrocatalysts formed by palladium nanoparticles (Pd NPs) loaded on Fe or Cu-containing composite supports, based on carbon nitride (C3N4) and super-activated carbon with a high porosity development (SBET 3180 m2/g, VDR 1.57 cm3/g, and VT 1.65 cm3/g), were synthesised. The presence of Fe or Cu sites favoured the formation of Pd NPs with small average particle size and a very narrow size distribution, which agreed with Density Functional Theory (DFT) calculations showing that the interaction of Pd clusters with C3N4 flakes is weaker than with Cu- or Fe-C3N4 sites. The electroactivity was also dependent on the composition and, as suggested by preliminary DFT calculations, the Pd-Cu catalyst showed lower overpotential for hydrogen evolution reaction (HER) while bifunctional oxygen reduction reaction/ oxygen evolution reaction (ORR/OER) behaviour was superior in Pd-Fe sample. The Pd-Fe electrocatalyst was studied in a zinc-air battery (ZAB) for 10 h, showing a performance similar to a commercial Pt/C + RuO2 catalyst with a high content of precious metal. This study demonstrates the synergistic effect between Pd species and transition metals and shows that transition metals anchored on C3N4-based composite materials promote the electroactivity of Pd NPs in HER, ORR and OER due to the interaction between both species.

Metal oxide Perovskite-Carbon composites as electrocatalysts for zinc-air batteries. Optimization of ball-milling mixing parameters.

Zn-air batteries (ZABs) are promising electrochemical devices to store energy. Metal oxide perovskites mixed with carbon materials are highlighted as interesting materials for this application because of their appropriate bifunctional performance in oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The interaction between both components of the electrocatalyst is important in the bifunctional electrocatalytic activity, and the mixing method plays an important role in this interaction. Then, different mixing methods have been studied in this work (ball-milling, mortar and manual shaking). The use of different physicochemical techniques such as temperature programmed desorption (TPD), temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) in the materials characterization, allows us to conclude that the mixing method strongly influences the particle size and the interaction between both components, which determine the final electrocatalytic activity. The materials prepared by ball-milling displayed the best performance. Herein, the experimental conditions were optimized to obtain electrocatalysts with enhanced electrocatalytic activity for ORR and OER. Low rotating speed, air atmosphere and low ball-milling time generate electrocatalysts with a small nanoparticle size, more homogeneous and with a higher interaction between both components, which enhances electron transfer, and consequently, the overall oxygen-involved reactions. The best electrocatalyst obtained was studied as air-electrode in a Zn-air battery and it was compared to a commercial Pt/C electrocatalyst, obtaining higher cyclability (55.2 vs 51.7 %) for 30 h, and higher energy density at 5 mA/cm2 (764 mAh/g vs 741 mAh/g).

Transition metal oxides with perovskite and spinel structures for electrochemical energy production applications.

Transition metal oxide-based materials are an interesting alternative to substitute noble-metal based catalyst in energy conversion devices designed for oxygen reduction (ORR), oxygen evolution (OER) and hydrogen evolution reactions (HER). Perovskite (ABO3) and spinel (AB2O4) oxides stand out against other structures due to the possibility of tailoring their chemical composition and, consequently, their properties. Particularly, the electrocatalytic performance of these materials depends on features such as chemical composition, crystal structure, nanostructure, cation substitution level, eg orbital filling or oxygen vacancies. However, they suffer from low electrical conductivity and surface area, which affects the catalytic response. To mitigate these drawbacks, they have been combined with carbon materials (e.g. carbon black, carbon nanotubes, activated carbon, and graphene) that positively influence the overall catalytic activity. This review provides an overview on tunable perovskites (mainly lanthanum-based) and spinels featuring 3d metal cations such as Mn, Fe, Co, Ni and Cu on octahedral sites, which are known to be active for the electrochemical energy conversion.

Electrocatalytic activity of calcined manganese ferrite solid nanospheres in the oxygen reduction reaction.

In this study, we synthesized MnFe2O4 solid nanospheres (MSN) calcined at different temperatures (200-500 °C) and MSN-based materials mixed with carbon black, for their use as electrocatalysts in the oxygen reduction reaction (ORR) in alkaline medium (0.1 M KOH). It was demonstrated that the calcination temperature of MSN material determined its chemical surface composition and microstructure and it had an important effect on the electrocatalytic properties for ORR, which in turn was reflected in the performance of MSN/CB-based electrocatalysts. The study revealed that the presence of Mn species plays a key role in the ORR activity. Among tested, MSN200/CB and MSN350/CB exhibited the best electrochemical performances together with outstanding stability.

Electrochemical regeneration of spent activated carbon from drinking water treatment plant at different scale reactors.

The electrochemical regeneration of real spent activated carbons (AC) used in drinking water treatment plants was studied at different reactor scales. The electrochemical regeneration was carried out in a 6 g filter-press cell and a 3.5 kg batch reactor, allowing the scaling-up of the process between the two electrolytic reactors. The effect of the electrolyte, the divided/undivided compartment configuration and the current density were studied in the filter-press cell. The effect of compartment configuration and the influence of the regeneration time were studied in the scaled-up reactor. A current density of 0.025 A cm-2 was used and the electrodes were Pt/Ti as anode and Pt/Ti and stainless-steel as cathode. The ACs were characterized by N2 adsorption isotherms to analyse the recovery of porosity and TPD-MS to analyse the AC surface after the electrochemical treatment. In filter-press cell, a recovery of the surface area of 96% was achieved after 8 h of treatment, by introducing the AC in the cathodic compartment using 0.05 M H2SO4 solution as electrolyte. In the 3.5 kg electrochemical reactor, 95% of the pristine AC surface area was recovered. Thus, electrochemical methods can provide a green alternative to the regeneration of spent AC.

Structural and morphological alterations induced by cobalt substitution in LaMnO3 perovskites.

The chemical composition of a LaMnO3 perovskite was modified sequentially by an improved sol-gel method to include cobalt centers in some B sites formerly occupied by Mn. In this way, a representative set of materials of general formula LaMn1-xCoxO3 was obtained whose composition extends from LaMnO3 to LaCoO3. These perovskites, as promising materials for oxygen reduction or oxygen evolution reactions, were characterized by several imaging (SEM), spectroscopic (XPS, EDX) and diffraction (XRD) techniques to elucidate their structure and to demonstrate the existence of composition differences between the catalytic surface and the bulk material. Specifically, it was found that lanthanum ions prevail at the surface of the catalyst but high cobalt-substitution levels stimulate the surface enrichment in B cations in their respective higher oxidation states (Mn4+ and Co3+ against Mn3+ and Co2+). This phenomenon opens the possibility of tuning their electrocatalytic properties and to synthesize suitable materials for electrochemical reactions involving molecular oxygen.

Synthesis of robust hierarchical silica monoliths by surface-mediated solution/precipitation reactions over different scales: designing capillary microreactors for environmental applications.

A synthetic procedure to prepare novel materials (surface-mediated fillings) based on robust hierarchical monoliths is reported. The methodology includes the deposition of a (micro- or mesoporous) silica thin film on the support followed by growth of a porous monolithic SiO2 structure. It has been demonstrated that this synthesis is viable for supports of different chemical nature with different inner diameters without shrinkage of the silica filling. The formation mechanism of the surface-mediated fillings is based on a solution/precipitation process and the anchoring of the silica filling to the deposited thin film. The interaction between the two SiO2 structures (monolith and thin film) depends on the porosity of the thin film and yields composite materials with different mechanical stability. By this procedure, capillary microreactors have been prepared and have been proved to be highly active and selective in the total and preferential oxidation of carbon monoxide (TOxCO and PrOxCO).

Optimizing the performance of catalytic traps for hydrocarbon abatement during the cold-start of a gasoline engine.

A key target to reduce current hydrocarbon emissions from vehicular exhaust is to improve their abatement under cold-start conditions. Herein, we demonstrate the potential of factorial analysis to design a highly efficient catalytic trap. The impact of the synthesis conditions on the preparation of copper-loaded ZSM-5 is clearly revealed by XRD, N2 sorption, FTIR, NH3-TPD, SEM and TEM. A high concentration of copper nitrate precursor in the synthesis improves the removal of hydrocarbons, providing both strong adsorption sites for hydrocarbon retention at low temperature and copper oxide nanoparticles for full hydrocarbon catalytic combustion at high temperature. The use of copper acetate precursor leads to a more homogeneous dispersion of copper oxide nanoparticles also providing enough catalytic sites for the total oxidation of hydrocarbons released from the adsorption sites, although lower copper loadings are achieved. Thus, synthesis conditions leading to high copper loadings jointly with highly dispersed copper oxide nanoparticles would result in an exceptional catalytic trap able to reach superior hydrocarbon abatement under highly demanding operational conditions.

On the origin of the high capacitance of nitrogen-containing carbon nanotubes in acidic and alkaline electrolytes.

The synthesis of nitrogenated carbon nanotubes (N-CNTs) with up to 6.1 wt% N, via the use of pyridine as the nitrogen containing carbon precursor, can provide a facile route to significantly enhance the low intrinsic specific capacitance of carbon nanotubes. The nitrogen functionalities determine this, at least, five-fold increase of the specific capacitance.

Electrochemical performance of hierarchical porous carbon materials obtained from the infiltration of lignin into zeolite templates.

Hierarchical porous carbon materials prepared by the direct carbonization of lignin/zeolite mixtures and the subsequent basic etching of the inorganic template have been electrochemically characterized in acidic media. These lignin-based templated carbons have interesting surface chemistry features, such as a variety of surface oxygen groups and also pyridone and pyridinic groups, which results in a high capacitance enhancement compared to petroleum-pitch-based carbons obtained by the same procedure. Furthermore, they are easily electro-oxidized in a sulfuric acid electrolyte under positive polarization to produce a large amount of surface oxygen groups that boosts the pseudocapacitance. The lignin-based templated carbons showed a specific capacitance as high as 250 F g(-1) at 50 mA g(-1) , with a capacitance retention of 50 % and volumetric capacitance of 75 F cm(-3) at current densities higher than 20 A g(-1) thanks to their suitable porous texture. These results indicate the potential use of inexpensive biomass byproducts, such as lignin, as carbon precursors in the production of hierarchical carbon materials for electrodes in electrochemical capacitors.

CuH-ZSM-5 as hydrocarbon trap under cold start conditions.

Cold start tests are carried out to evaluate the performance of copper-exchanged zeolites as hydrocarbon traps under simulated gasoline car exhaust gases, paying special attention to the role of copper in the performance of these zeolites. It is concluded that the partial substitution of the protons in the parent H-ZSM-5 zeolite is highly beneficial for hydrocarbon trapping due to the formation of selective adsorption sites with specific affinity for the different exhaust components. However, it is also observed that uncontrolled exchanging process conditions could lead to the presence of CuO nanoparticles in the zeolite surface, which seem to block the pore structure of the zeolite, decreasing the hydrocarbon trap efficiency. Among all the zeolites studied, the results point out that a CuH-ZSM-5 with a partial substitution of extra-framework protons by copper cations and without any detectable surface CuO nanoparticles is the zeolite that showed the best performance under simulated cold start conditions due to both the high stability and the hydrocarbon retaining capacity of this sample during the consecutive cycles.

Molecular simulation design of a multisite solid for the abatement of cold start emissions.

A highly effective hydrocarbon (HC) trap for the abatement of cold start HC emissions with specific adsorption sites for the different molecules present in the exhaust gases has been designed by means of molecular simulation tools, and later synthesized.

Arsenic species interactions with a porous carbon electrode as determined with an electrochemical quartz crystal microbalance.

The interactions of arsenic species with platinum and porous carbon electrodes were investigated with an electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry in alkaline solutions. It is shown that the redox reactions in arsenic-containing solutions, due to arsenic reduction/deposition, oxidation/desorption, and electrocatalyzed oxidation by Pt can be readily distinguished with the EQCM. This approach was used to show that the arsenic redox reactions on the carbon electrode are mechanistically similar to that on the bare Pt electrode. This could not be concluded with just classical cyclic voltammetry alone due to the obfuscation of the faradaic features by the large capacitative effects of the carbon double layer.For the porous carbon electrode, a continual mass loss was always observed during potential cycling, with or without arsenic in the solution. This was attributed to electrogasification of the carbon. The apparent mass loss per cycle was observed to decrease with increasing arsenic concentration due to a net mass increase in adsorbed arsenic per cycle that increased with arsenic concentration, offsetting the carbon mass loss. Additional carbon adsorption sites involved in arsenic species interactions are created during electrogasification, thereby augmenting the net uptake of arsenic per cycle.It is demonstrated that EQCM, and in particular the information given by the behavior of the time derivative of the mass vs. potential, or massogram, is very useful for distinguishing arsenic species interactions with carbon electrodes. It may also prove to be effective for investigating redox/adsorption/desorption behavior of other species in solution with carbon materials as well.

Comparative characterization study of microporous carbons by HRTEM image analysis and gas adsorption.

The present work presents a useful comparison of micropore size distributions (MPSDs) obtained from gas adsorption and image analysis of high-resolution transmission electron micrographs. It is shown that the MPSD obtained for a chemical activated carbon is concordant with that obtained from CO2 adsorption. In addition, this technique has allowed us to obtain the MPSD of a carbon molecular sieve (CMS) prepared in our laboratory by a copyrolysis process, which could only be characterized by CO2 adsorption at 273 K (not by N2 adsorption at 77 K due to diffusional problems). The MPSD obtained by high-resolution transmission electron microscopy (HRTEM) for the CMS is wider than that obtained by CO2 adsorption, suggesting that HRTEM is detecting the closed porosity existing in this sample, which is not accessible to gas adsorption. The existence of closed porosity in the CMS is explained considering the preparation method used. Thus, HRTEM combined with image analysis seems to be useful for structural analysis of narrow micropores including closed porosity.