Surface characteristics and electrochemical capacitances of carbon aerogels obtained from resorcinol and pyrocatechol using boric and oxalic acids as polymerization catalysts

Carbon aerogels were prepared by carbonizing (at 500–1500 °C) organic aerogels obtained from the polymerization reaction of resorcinol and/or pyrocatechol with formaldehyde using boric and oxalic acids as polymerization catalysts. Prepared samples were characterized by different techniques to ascertain their composition, surface chemistry, morphology, and surface physics, determining their electrochemical capacitances in acidic medium. The use of pyrocatechol yielded carbon aerogels that were micro–mesoporous, showing Type IV N₂ adsorption isotherms with Type H2 hysteresis cycles. The volume and size of mesopores depended on the acid catalyst used and the temperature at which the carbon aerogel was obtained. Conversely, the sample prepared with resorcinol and boric acid as catalyst was micro–macroporous and that obtained with a resorcinol–pyrocatechol mixture was micro–mesoporous but with large mesopores. Most of the boric acid used was lost during the exchange of water with acetone in the organic hydrogels before their supercritical CO₂ drying. Carbon aerogels obtained at 900 °C and using boric acid as polymerization catalyst showed a capacitance between 17 and 24 μF/cm². Boron influenced the capacitance because it increased the oxygen content. Sample synthesized using pyrocatechol, formaldehyde, and oxalic acid and heat-treated at 900 °C had the highest capacitance, 34 μF/cm².     

Unified study of flow regimes and gas holdup in the presence of positive and negative surfactants in a non-uniformly aerated bubble column

In this paper, a study on the global gas holdup and hydrodynamic flow regimes developed in a partially aerated bubble column at variable air superficial velocities (U_G) in the presence of positive and negative surfactants is presented. According to the results obtained, despite the different liquid phase properties variation caused by the presence of positive (alcohols) and negative (electrolytes) surfactants, both reduce coalescence and the effect in the gas holdup (ε_G) is equivalent: it increases with the surfactant concentration (C) but only when the (C/C_t) ratio is clearly above 1, being C_t the transition concentration. Contrary to the results obtained for totally aerated bubble columns, for lower values of the (C/C_t) ratio, the holdup remains practically invariable. Considering the crucial role that C and C_t play in the resulting ε_G, a new prediction equation for ε_G accounting for the ratio (C/C_t) and U_G is presented and its performance for both types of surfactants validated. Additionally, visual and wall pressure fluctuations studies reveal that the vortical flow (VF), characterized by an oscillating bubble plume, prevails in ultrapure water (UPW) but results destabilized in the presence of surfactants. This destabilization results in an evolution to a pseudo-steady flow regime, the double cell turbulent flow regime (DCTF), characterized by a quasi-static bubble jet, located at the column centerline that determines the appearance of two static symmetrical vortices     

Miniaturized ionization gas sensors from single metal oxide nanowires

Gas detection experiments were performed with individual tin dioxide (SnO2) nanowires specifically configured to observe surface ion (SI) emission response towards representative analyte species. These devices were found to work at much lower temperatures (T≈280 °C) and bias voltages (V≈2 V) than their micro-counterparts, thereby demonstrating the inherent potential of individual nanostructures in building functional nanodevices. High selectivity of our miniaturized sensors emerges from the dissimilar sensing mechanisms of those typical of standard resistive-type sensors (RES). Therefore, by employing this detection principle (SI) together with RES measurements, better selectivity than that observed in standard metal oxide sensors could be demonstrated. Simplicity and specificity of the gas detection as well as low-power consumption make these single nanowire devices promising technological alternatives to overcome the major drawbacks of solid-state sensor technologies.     

NMR relaxation reveals modifications in rubber phase dynamics during long-term degradation of high-impact polystyrene (HIPS)

The microstructure of rubber-modified polystyrene after thermal ageing at 90 °C and multiple extrusion was analyzed by time-domain nuclear magnetic resonance (TD-NMR) in a non-destructive manner. The transverse magnetization decay behaviour observed in TD-NMR was related to the total rubber fraction and its cross-linking density. The data reveal different mechanisms of long-term rubber degradation in high-impact polystyrene (HIPS) during thermo-oxidation and multiple processing: Multiple processing causes a slight increase in the cross-linking density of the rubber phase, without appreciably altering the total amount of rubber fraction. Thermo-oxidation is accompanied by a significant overall decrease of the rubber fraction, an increase of the cross-linking density, and a pronounced increase of the non-crosslinked fraction (chain ends and fragmented segments). The NMR results correlate well with spectroscopic observations and moderately with macroscopic mechanical properties.     

The Effectiveness of Single Oxidants and AOPs in the Degradation of Emerging Contaminants in Waters: A Comparison Study

The effectiveness of single oxidants and several AOPs was studied for the degradation of five selected emerging contaminants: Benzotriazole, N,N-diethyl-m-toluamide or DEET, Chlorophene, 3-Methylindole and Nortriptyline HCl. First-order rate constants and half-life times for the degradation of each compound in ultra-pure water were deduced and compared. The AOPs were later applied to the degradation of these ECs present in three real waters: public reservoir water, and two secondary effluents from municipal wastewater plants. The effect of the variables on the ECs elimination was established. Finally, a cost estimation based on the operating costs was established for the degradation of 3-Methylindole by the single oxidants and AOPs tested.     

Degradation of an Azo Dye (Ponceau 4R) and Treatment of Wastewater from a Food Industry by Ozonation

Experiments for degradation of the extensively marketed Ponceau 4R dye in aqueous solution and for oxidation of raw wastewater from a confectionary industry have been carried out by using ozone. All the experiments were performed in a cylindrical semi-batch reactor at approximately 20 ^oC for 7200 s. A mass flow rate of 1.158?×?10^?6 kg s^?1 of ozone was continuously fed in the reactor. The pH of the azo dye aqueous solution (distilled water + Ponceau 4R) was always kept at approximately 5.8, while in the case of the raw wastewater the same factor was changed from 4.7 to 9.4 in two different experimental runs. Absorbance measurements at 508 nm show that the investigated azo dye found in the azo dye aqueous solution was completely degraded after only 600 s. At this initial period a substantial fall of TOC (Total Organic Carbon) (up to 45%) was noticed, but the rate was exponentially decreased at longer reaction times up to a TOC removal no higher than 60%. The ozonation was also responsible for reducing the apparent color of the raw wastewater to almost 10% of its initial value at the optimum pH (9.4 ± 1.5). The effect of pH was important on apparent color, but it had absolutely no influence on the kinetics results of COD (Chemical Oxygen Demand), which were kept constant over the entire period of reaction.     

Expansion of core–shell PS/PMMA particles

Structured micrometric polystyrene/poly(methyl methacrylate) particles were obtained by suspension polymerization and their expansion behavior was investigated using n‐pentane as blowing agent. The expanded particles presented two distinct microstructures with an outer region (PMMA‐rich shell) composed by cells of about 10 µm while the center of the particle (PS‐rich core) had much larger cells (50–100 μm). The core–shell particles did not expand at 100°C meaning that the PMMA shell hindered the expansion of the particles. Maximum expansion was dependent on the PMMA concentration and also on the heating temperature and the increase in the PMMA molar mass led to a delay in the onset of the process. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4521–4527, 2013     

Nanoparticle Shape Selectivity in Catalysis: Butene Isomerization and Hydrogenation on Platinum

New advances in colloidal and other self-assembly synthetic methods have afforded the controlled growth of nanoparticles with well-defined sizes and shapes. Recently, the catalysis community has been trying to capitalize on this knowledge for the design of new catalytic processes. In particular, the use of metal nanoparticles with specific shapes has been explored in several instances as a way to control reaction selectivity. Here we review the results from our efforts to use platinum nanoparticles dispersed on high-surface-area supports to perform selective olefin conversions. Emphasis is given to the surface-science experiments and quantum-mechanics calculations that led us to identify potential variations in selectivity in carbon–carbon double-bond isomerization and hydrogenation reactions with the structure of the metal surface. Temperature programmed desorption (TPD) and reflection–absorption infrared spectroscopy data for 2-butenes adsorbed on Pt(111) single-crystal surfaces highlighted the relative higher stability of adsorbed cis-2-butene compared to trans-2-butene and the preference for the promotion of trans-to-cis conversions on that surface. It was also determined that coadsorbed hydrogen plays a key role in defining the relative stabilities of the adsorbates, favoring pi rather than di-sigma bonding and reversing the higher stability of the trans adsorbates seen on clean Pt(111). DFT calculations suggested that such unique results may be accounted for by the need for extensive surface reconstruction to accommodate the adsorbates on such flat planes, a requirement that appears to be less severe with the cis isomer. TPD experiments on stepped Pt(557) surfaces pointed to the minimal importance of steps in promoting these isomerization reactions, although they do seem to help with the full hydrogenation to the alkanes. More extensive olefin adsorption destabilization with hydrogen coadsorption and faster alkane production was seen on Pt(100), but selectivity towards the cis isomer was still identified. On the more open (2 × 1)-reconstructed Pt(110) surface, on the other hand, trans-2-butene is the most stable of the two isomers. It was finally shown that these surface-science results translate into changes in selectivity in real catalysts with platinum nanoparticle shape. Catalysts were prepared by using colloidal Pt nanoparticles with tetrahedral, cubic, and rounded shapes, and unique selectivity toward cis-2-butene formation was measured on the first of those samples. It appears that the (111) facets exposed by the tetrahedral Pt nanoparticles do show the same trans-to-cis conversion preference in catalysis seen in the surface-science studies carried out with single-crystal surfaces and under ultrahigh vacuum conditions.