Loading...
3 results
Search Results
Now showing 1 - 3 of 3
- Biomass-derived nanoporous carbons as electrocatalysts for oxygen reduction reactionPublication . Fernandes, Diana M; Mestre, Ana S.; Martins, Angela; Nunes, Nelson; Carvalho, A. P.; Freire, CristinaElectrocatalysts (ECs) for the oxygen reduction reaction (ORR) are crucial in fuel cells and for this reason developing cost-effective metal-free ECs with high electrocatalytic activity and high-volume production remains a huge challenge. Herein, we report the application as ORR electrocatalysts of a series of high grade nanoporous carbons, prepared by chemical activation of acid-chars obtained from the H2SO4 digestion and polycondensation (acid-mediated carbonization) of a biomass residue (Agave sisalana). All the nanoporous carbons presented good ORR electrocatalytic activities in alkaline medium. The AC(1) carbon exhibited the most promising ORR performance with E-onset = 0.84 vs. RHE, j(L, 0.26 V, 1600 rpm) = -3.12 mA cm(-2) and n(O2) = 3.6 electrons. The Tafel slopes of all carbons varied between 47 mV dec(-1) (AC(3)) and 250 mV dec(-1) (AC(1)). Furthermore, the carbons revealed superior tolerance to methanol when compared with commercial Pt/C and a competitive long-term electrochemical stability, with current retentions of 75-85 % after 20,000 s. The results obtained in this work suggest a promising method based on sustainable and economical biomass residues towards the development and engineering of novel value-added biomass-derived carbons as effective metal-free electrocatalysts for alkaline fuel cells.
- Friedel-crafts acylation reaction over hierarchical Y zeolite modified through surfactant mediated technologyPublication . Martins, Angela; Neves, Vera; Moutinho, João; Nunes, Nelson; Carvalho, AnaFriedel-Crafts acylation reaction was studied under mild conditions using hierarchical HY zeolite samples prepared through surfactant mediated technology, in the presence of NH4OH, using CTAB or DTAB as surfactants and changing the duration of treatment from 6 to 48 h. The materials were characterized by powder X-ray diffraction, low temperature N2 adsorption isotherms, SEM microscopy and pyridine adsorption followed by FTIR. The catalytic behaviour was studied in the acylation of furan by acetic anhydride. The catalytic results reflect the role of the duration of the treatment as well as the surfactant molecule used. As the time of treatment increases, the enlargement of the pores leads to an increase of the rate constant and turnover frequency (TOF), except for sample modified during longer time, 48 h, due to the occurrence of secondary reactions that produce larger products or reaction intermediates that become trapped inside the pores. The role of the surfactant molecule is also relevant since the sample modified in the presence of the larger surfactant molecule, CTAB, a significant increase in product yield and rate constant is obtained when compared with the sample prepared in the same conditions using DTAB. However, in that case, by prolonging the treatment a substantial decrease in the same parameters occurs due to the occurrence of deactivation phenomena, pointing out that the optimized modification of porosity needs to be customized according to the needs of each catalytic system.
- Eco-friendly cyclohexane oxidation by a V-scorpionate complex immobilized at hierarchical MOR zeolitePublication . Ottaviani, Davide; Van-Dúnem, Vanmira; Carvalho, Ana; Martins, Angela; Martins, LuisaThe hydrotris(3,5-dimethylpyrazol-1-yl)borate dioxido-vanadium(V) complex [VO2{HB(3,5-Me(2)pz)(3)}] (V, pz= pyrazol-1-yl) immobilized at hierarchical MOR zeolite modified through surfactant mediated technology using NaOH as alkaline agent, V@MOR_NaOH, provides an efficient (TON up to 5.2x10(2)) catalyst for the peroxidative oxidation of cyclohexane with tert-butyl hydroperoxide to KA oil (52% yield), at room temperature, and is easily recovered and reused up to four consecutive catalytic cycles with no significant leaching of vanadium. The textural modifications performed on the zeolitic matrix produced an effective support for the anchorage of the vanadium complex allowing an improved catalytic performance and reusability. This work constitutes an unprecedented use of supported vanadium B-scorpionate complexes as catalysts for the oxidation of alkanes.