Browsing by Author "Machado, Bruno"
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- Amphiphilic carbon nanotubes for catalytic wet peroxide oxidation of 4-nitrophenolPublication . Roman, Fernanda; Sanches, Flávia Kim; Díaz de Tuesta, Jose Luis; Marin, Pricila; Machado, Bruno; Serp, Philippe; Silva, Adrián; Faria, Joaquim; Gomes, HelderCarbon nanotubes (CNTs) were synthesized via chemical vapor deposition (CVD) over an AlCoFeO4 catalyst by a sequential feed of ethylene (E, as carbon source) and acetonitrile (A, as nitrogen source). The resulting samples were noted E20 (hydrophobic), E10A10 (amphiphilic), and A20 (hydrophilic), the number referring to the feed time (minutes) of each precursor, as reported elsewhere1. These materials were tested in the catalytic wet peroxide oxidation (CWPO) of 4-nitrophenol (4-NP). The reaction was monitored by HPLC (to determine the concentration of 4-NP and respective intermediates), TOC analyzer, and UV-vis spectrophotometry (to quantify H2O2) (Figure 1). After 8 h of reaction, A20 led to the highest consumption of H2O2 (90%), followed by E10A10 (61%) and E20 (52%). On the other hand, the highest degradation of 4-NP was observed with the amphiphilic E10A10 material (98%) followed by E20 (95%), whereas A20 only led to a removal of 69%. Similar behavior was found when analyzing the formation of reaction intermediates (data not shown), i.e., while A20 resulted in the accumulation of 4-nitrocatechol (4-NTC) and hydroquinone (HQ) E10A10 and E20 led to the total conversion of formed 4-NTC and HQ. This resulted in a lower TOC removal for A20 (37%) than to E10A10 and E20 (53%). Therefore, the amphiphilic E10A10 material is a promising catalyst for the CWPO of 4-NP.
- Carbon aerogel supported platinum catalysts for selective hydrogenation of cinnamaldehydePublication . Machado, Bruno; Morales-Torres, Sergio; Gomes, Helder; Pérez-Cadenas, A.F.; Maldonado-Hódar, F.J.; Carrasca-Marín, F.; Figueiredo, José; Faria, JoaquimThis paper describes the preparation and characterization of 1% wt. Pt catalysts supported in carbon aerogels for the application in the liquid-phase selective hydrogenation of cinnamaldehyde. Carbon aerogel supports with different textures were activated with hydrogen peroxide and ammonium peroxydisulfate leading to large amounts of surface groups but keeping unchanged their textural properties. After introducing Pt, the surface chemistry and morphology of the catalysts was characterized by analytical techniques like SEM, TPD, N2 adsorption isotherms, mercury porosimetry and H2 chemisorption. Catalysts prepared with activated aerogels exhibited good selectivity towards the desired product, cinnamyl alcohol. A high temperature post-treatment of the catalysts failed to enhance their selectivity, mainly leading to hydrocinnamaldehyde production.
- Carbon nanotube catalysts for the cwpo of 2-nitrophenol in biphasic systems: kinetic insightsPublication . Díaz de Tuesta, Jose Luis; Machado, Bruno; Serp, Philippe; Silva, Adrián; Faria, Joaquim; Gomes, HelderCarbon nanotubes (CNTs) were used as catalysts in the removal of 2-nitrophenol (2-NP) by catalytic wet peroxide oxidation (CWPO). Different CNTs were synthesized by catalytic chemical vapour deposition (CVD) using Fe/γ-Al2O3, as described elsewhere (Purceno et al., 2015 and Martin-Martinez et al., 2016). A fluidized-bed reactor was employed by feeding a gas mixture of hydrogen and: i) ethylene alone for 30 min (sample E30); ii) acetonitrile for 20 min, followed by ethylene for 20 min (sample A20E20); iii) acetonitrile for 20 min, followed by ethylene for 10 min (sample A20E10); iv) ethylene for 10 min, followed by acetonitrile for 20 min (sample E10A20); and v) acetonitrile alone for 30 min (sample A30). The CNTs were tested for the oxidation of 2-nitrophenol (2-NP) in aqueous-phase (Martin-Martinez et al., 2016) and in biphasic conditions (cyclohexane-water) simulating oily wastewater effluents as described in previous works (Diaz de Tuesta et al., 2018). Both systems were modelled by using kinetic power-law equations, following a methodology presented previously (Diaz de Tuesta et al., 2017).
- Carbon nanotubes as catalysts for wet peroxide oxidation: structure-reactivity relationshipsPublication . Ribeiro, Rui; Martin-Martinez, Maria; Machado, Bruno; Serp, Philippe; Morales-Torres, Sergio; Silva, Adrián; Figueiredo, José; Faria, Joaquim; Gomes, HelderMagnetic neat and N-doped carbon nanotubes with different properties have been synthesized by chemical vapour deposiüon and tested in the catalytic wet peroxide oxidation of 4-nitrophenol solutions (5 g L') at relatively mild operating conditions (atmospheric pressure, T = 50 °C, pH = 3)~using a catalyst load = 2.5 g L-' and [H202]o = 17.8 g L-1. The results demonstrate that the catalyst hydrophobicity/ hydrophilicity is a detenninant property in the CWPO reaction, since it affects the rate ofH202 decomposition. The controlled formation ofreactive radicais (HO* and HOO*) at hydrophobic surfaces avoids the formation of non-reactive species (02 and H20), increasing.
- Carbon nanotubes as catalysts for wet peroxide oxidation: the effect of surface chemistryPublication . Martin-Martinez, Maria; Machado, Bruno; Serp, Philippe; Morales-Torres, Sergio; Silva, Adrián; Figueiredo, José; Faria, Joaquim; Gomes, HelderThree magnetic carbon nanotube (CNT) samples, named A30 (N-doped), E30 (undoped) and E10A20 (selectively N-doped), synthesized by catalytic chemical vapor deposition, were modified by introducing oxygenated surface groups (oxidation with HNO3, samples CNT-N), and by heat treatment at 800 °C for the removal of surface functionalities (samples CNT-HT). Both treatments lead to higher specific surface areas. The acid treatment results in more acidic surfaces, with higher amounts of oxygenated species being introduced on Ndoped surfaces. Heat-treated samples are less hydrophilic than those treated with nitric acid, heat treatment leading to neutral or basic surfaces, only N-quaternary and N-pyridinic species being found by XPS on N-doped surfaces. These materials were tested in the catalytic wet peroxide oxidation (CWPO) of highly concentrated 4-nitrophenol solutions (4-NP, 5 g L−1) at atmospheric pressure, T=50 °C and pH=3, using a catalyst load of 2.5 g L−1 and the stoichiometric amount of H2O2 needed for the complete mineralization of 4-NP. The high temperature treatment enhanced significantly the activity of the CNTs towards CWPO, evaluated in terms of 4-NP and total organic carbon conversion, due to the increased hydrophobicity of their surface. In particular, E30HT and E10A20HT were able to remove ca. 100% of 4-NP after 8 h of operation. On the other hand, by treating the CNTs with HNO3, the activity of the less hydrophilic samples decreased upon increasing the concentration of surface oxygen-containing functionalities, whilst the reactivity generated inside the opened nanotubes improved the activity of the highly hydrophilic A30 N.
- Carbon nanotubes supported catalysts for selective hydrogenations of unsaturated aldehydesPublication . Machado, Bruno; Gomes, Helder; Serp, Philippe; Kalck, Philippe; Faria, Joaquim
- Carbon supported noble metal catalysts prepared by photochemical depositionPublication . Faria, Joaquim; Machado, Bruno; Gomes, Helder; Serp, Philippe; Kalck, PhilippePhotochemical deposition of noble metals in different supports is gaining importance because of its simplicity and advantages. Its main advantage is the ability of spreading very effectively the metal throughout the support, thus leading to very high dispersions, resulting in higher molecular control, with a positive effect on both activity and selectivity. This type of catalysts is important for industrial preparation of fine chemicals. A common synthetic route in these processes is the selective catalytic hydrogenation of organic substrates containing unsaturated functional groups, like steroids or α,β-unsaturated aldehydes.
- Carbon xerogel as catalytic support for noble metal based selective hydrogenation reactionsPublication . Machado, Bruno; Gomes, Helder; Serp, Philippe; Kalck, Philippe; Figueiredo, José; Faria, JoaquimNitric acid treated carbon xerogel was used as support to prepare Pt, Ir and Ru monometallic catalysts. The introduction of oxygen surface groups was important to increase metal dispersion but proved to limit selectivity to cinnamyl alcohol during the hydrogenation of cinnamaldehyde. After an activation treatment to remove unwanted surface groups, Pt catalysts exhibited the highest selectivity, 73%, followed by Ir with 65% and finally Ru with only 32% (measured at 50% conversion).
- Carbon xerogel supported noble metal catalysts for fine chemical applicationsPublication . Machado, Bruno; Gomes, Helder; Serp, Philippe; Kalck, Philippe; Figueiredo, José; Faria, JoaquimCarbon xerogels are mesoporous materials obtained upon pyrolysis of the dried gels resulting from polycondensation of resorcinol and formaldehyde. Treatment with nitric acid under severe conditions introduces high amounts of oxygen containing functional groups onto the surface of thematerial, leading however to the collapse of its porous structure. The resulting material is then used to support 1 wt.% Pt, Ir and Ru monometallic catalysts by wet impregnation using organometallic precursors. The catalysts are characterized by N2 adsorption–desorption isotherms at 77 K, temperature programmed desorption coupled with mass spectrometry, scanning and transmission electron microscopy, and H2 chemisorption. The liquid-phase selective hydrogenation of cinnamaldehyde to cinnamyl alcohol is used in order to assess the catalytic performance of the prepared materials. Pt and Ru catalysts are initially very selective towards the hydrogenation of the olefinic double bond, while Ir is mostly selective towards the carbonyl group. After a thermal post-reduction treatment at 973 K, selectivity towards cinnamyl alcohol is significantly improved regardless of the metal nature. The Pt catalyst exhibits the best behavior, a complete shift in C=C to C=O hydrogenation being detected. The improvement in selectivity is rationalized in terms of both an increase in metal particle size and a modification in the surface chemistry of the catalyst after the post-reduction treatment.
- Carbon xerogel supported noble metal catalysts for fine chemical applicationsPublication . Machado, Bruno; Gomes, Helder; Serp, Philippe; Kalck, Philippe; Figueiredo, José; Faria, JoaquimCarbon xerogel, a mesoporous material, was produced by polycondensation of resorcinol and formaldehyde. A concentrated nitric acid solution was subsequently used to introduce high amounts of oxygenated groups on the surface. The carbon xerogel served as a support for Pt, Ir and Ru monometallic catalysts by using organometallic precursors. The catalysts were tested in the liquid-phase selective hydrogenation of cinnamaldehyde to cinnamyl alcohol. The introduction of surface groups was important to increase metal dispersion but proved to limit selectivity towards the unsaturated alcohol. After a thermal treatment at 973 K the catalysts showed excellent thermal stability and a narrow metal size distribution. Regarding the catalytic results measured at 50% conversion, Pt catalysts exhibited the highest selectivity to cinnamyl alcohol (73 %) followed by Ir with 65 % and finally Ru with only 32 %.