Valerio Paolini

Highly-Ordered Covalent Anchoring of Carbon Nanotubes on Electrode Surfaces by Diazonium Salt Reactions

Since their discovery in 1991, carbon nanotubes (CNTs) have attracted growing interest in view of their unique mechanical and physicochemical properties, and have successfully been applied in microelectronics such as memory devices, switches, supercapacitors, and in the biomedical area as drug-delivery devices and biosensors. The oriented assembly of CNTs on an electrode surface offers further possibilities with respect to both electrochemical biosensors because of the high electron-transfer rate along the tube, and the attachment of redoxactive molecules (or redox enzymes) to the ends of the CNTs. Electrodes modified with vertically aligned CNTs have been accomplished through direct growth or self-assembly of CNTs. Although different procedures can be followed to achieve a covalent attachment of molecules, the most applied method to anchor CNTs is perhaps that based on the formation of amide bonds from the reaction between the amines located on the modified electrode and the carboxylic groups at the ends and side-wall defects of the nanotubes. Since 1992, when an aryl diazonium salt was used for the covalent functionalization of a carbon electrode, this approach has been applied to a variety of surfaces such as carbon-based materials, metals, and semiconductors. The method has also been used for the derivatization of CNTs, that is, multi-walled carbon nanotubes (MWCNTs) or single-walled carbon nanotubes (SWCNTs). To the best of our knowledge, no report of CNTanchoring on a surface by the use of diazonium salt reactions has been published to date. The aim of the present work is to develop a new approach based on diazonium salt reactions that provides a simple, stable, and well-organized assembly of CNTs for a wide range of substrates. To chemically modify an electrode surface, a diazonium salt is reduced to the corresponding aryl radical, which binds to the surface rapidly. The process is promoted by a variety of experimental conditions, and can even be used in combination with click chemistry. Herein, the procedures adopted for the derivatization of a glassy carbon electrode (GCE) are shown in Figure 1. For the immobilization of unfunctionalized SWCNTs on the chemically modified GCE surface, the p-nitro diazonium ions, which are formed in situ from p-nitroaniline (see Experimental Section for details), bind to the CGE surface (Figure 1a). The amine groups obtained by electrochemical reduction of nitro groups are converted into the corresponding diazonium functionalities, which covalently bind SWCNTs. The incorporation of phenyl groups to form a mixed monolayer, which is anticipated to prevent the diazo coupling, a reaction that occurs in a densely packed p-aniline monolayer. Indeed, the electrophilic attack of the diazonium functionality at the ortho position of a vicinal p-aniline (diazo coupling) or to the vicinal amine group, avoids CNTs immobilization. Alternatively, SWCNTs are first functionalized with p-nitrobenzene groups, then the amines formed by reduction of the nitro groups are converted into diazonium ions, which covalently bind SWCNTs on a clean GCE surface (Figure 1b). The orientation of SWCNTs attached to a GCE surface was investigated by scanning electron microscopy (SEM). A typical SEM image, taken from a 458 tilted view, is shown in Figure 1c; the image clearly shows that SWCNTs are arranged vertically on the surface. The obtained arrangement is independent of the reaction route. TEM images clearly show that SWCNTs are perpendicularly oriented with only one end anchored on the CGE surface (Figure 1d,e). The SWCNTs attached to the GCE were 80–120 nm in length, with a diameter of 2–8 nm. Cyclic voltammograms (CVs) were recorded in a solution of K3[Fe(CN)6] in water (2 10 3 molL ) using bare GCEs or GCEs modified with SWCNTs according to the procedure described above (Figure 2). The assembly of CNTs on the electrode leads to a 1.5-fold increase of the current intensity, which results from the increased active electrode surface; the double-layer capacitance increases accordingly. Sonication of the modified electrode slightly decreases the current intensity, thus suggesting that some SWCNTs may be adsorbed on the surface. Also, the electrode modification slightly improves the reversibility, as indicated by the peak separation values. The electron-transfer rate constants (kET) determined for both bare and SWCNT-modified GCEs, were calculated from the peak separation assuming the [Fe(CN)6] 3 /4 diffusion coef[*] Prof. T. Ferri, V. Paolini Department of Chemistry, “Sapienza” University of Rome Piazzale Aldo Moro 5, 00185 Rome (Italy) E-mail: tommaso.ferri@uniroma1.it

Biogas cleaning and upgrading with natural zeolites from tuffs.

ABSTRACT CO2 adsorption on synthetic zeolites has become a consolidated approach for biogas upgrading to biomethane. As an alternative to synthetic zeolites, tuff waste from building industry was investigated in this study: indeed, this material is available at a low price and contains a high fraction of natural zeolites. A selective adsorption of CO2 and H2S towards CH4 was confirmed, allowing to obtain a high-purity biomethane (CO2 <2 g m−3, i.e. 0.1%; H2S <1.5 mg m−3), suitable for injection in national grids or as vehicle fuel. The loading capacity was found to be 45 g kg−1 and 40 mg kg−1, for CO2 and H2S, respectively. Synthetic gas mixtures and real biogas samples were used, and no significant effects due to biogas impurities (e.g. humidity, dust, moisture, etc.) were observed. Thermal and vacuum regenerations were also optimized and confirmed to be possible, without significant variations in efficiency. Hence, natural zeolites from tuffs may successfully be used in a pressure/vacuum swing adsorption process.

Volatilization and oxidative artifacts of PM bound PAHs collected at low volume sampling (1): Laboratory and field evaluation

Laboratory and field studies were carried out to assess the effects of oxidative degradation and volatilization on PM10 bound polycyclic aromatic hydrocarbons (PAHs), collected at low volume condition according to the EU sampling reference method EN12341:2014 (flow rate 2.3 m3 h-1), on 47 mm quartz filters. For the laboratory experiments, pairs of twin samples were collected in field and, after treatments favoring decomposition or/and volatilization of PAHs on one sample, the PAH amount was compared with that of the corresponding untreated sample. Ozone exposure caused a general PAHs decay with more marked effects on benzo [a]pyrene, perylene and benz [a]anthracene; these compounds showed, similarly to benzo [ghi]perylene, correlations between ozone dose and losses. Treatments with zero air exhibited losses due to volatilization even for 5-ring PAHs up to benzo [a]pyrene, whereas a linear dependence was observed between filter PAH load and losses for benzo [a]anthracene, chrysene and benzofluoranthenes. Concentrations on samples collected simultaneously over 48, 24, 12 and 6 h were compared. Results confirmed a lack of temporal auto-consistency in the PAHs sampling methodology here adopted. In particular higher atmospheric PAH concentrations were ascertained on samples constituted by cumulative filters exposed over shorter sampling times. When 24-h and 2 × 12-h samples were compared, comparable losses were evaluated in the hot and cold seasons. This finding shows that, although in summer meteorology conditions favor sampling artifacts, the effectiveness of these phenomena continue in the winter, probably due to the larger amount of PAH available on the sampling filter (total PAHs ∼ 10 vs 0.5 ng m-3).

Environmental impact of biogas: A short review of current knowledge

ABSTRACT The social acceptance of biogas is often hampered by environmental and health concerns. In this study, the current knowledge about the impact of biogas technology is presented and discussed. The survey reports the emission rate estimates of the main greenhouse gases (GHG), namely CO2, CH4 and N2O, according to several case studies conducted over the world. Direct emissions of gaseous pollutants are then discussed, with a focus on nitrogen oxides (NOx); evidences of the importance of suitable biomass and digestate storages are also reported. The current knowledge on the environmental impact induced by final use of digestate is critically discussed, considering both soil fertility and nitrogen release into atmosphere and groundwater; several case studies are reported, showing the importance of NH3 emissions with regards to secondary aerosol formation. The biogas upgrading to biomethane is also included in the study: with this regard, the methane slip in the off-gas can significantly reduce the environmental benefits.

Characterisation and cleaning of biogas from sewage sludge for biomethane production

This study investigates the conversion of sewage sludge from wastewater treatment plants (WWTP) into biomethane for automotive fuel or grid injection. A prototype plant was monitored in Northern Italy, based on vacuum swing adsorption (VSA) on synthetic zeolite 13×: this biogas upgrading method is similar to pressure swing adsorption (PSA) and commonly used for other kinds of biomass. Measurements of biogas inlet, biomethane outlet and off-gas were performed including CH4, CO2, CO, H2, O2, N2, HCl, HF, NH3, H2S and volatile organic compounds (VOCs). Critical levels were observed in the biogas for of H2S and HCl, whose concentrations were 1570 and 26.8 mg m-3, respectively. On the other hand, the concentration of halogenated VOCs (including tetrachloroethylene and traces of perfluoroalkilated substances, PFAS) and mercaptans were relatively low. A simultaneous and reversible adsorption on 13× zeolite was achieved for H2S and CO2, and carbon filters played a minor role in desulfurisation. The presence of HCl is due to clarifying agents, and its removal is necessary in order to meet the required biomethane characteristics: an additional carbon-supported basic adsorbent was successfully used to remove this contaminant. This study also highlights the interference of CO2 towards HCl if sampling is performed in compliance with the new EU standard for biomethane. High total volatile silicon (TVS) was confirmed in sewage sludge biogas, with a major contribution of siloxane D5: the suitability of this compound as an indicator of total siloxanes is discussed. Results demonstrate that volatile methyl siloxanes (VMS) do not represent a critical issue for the VSA upgrading methodology.