Solar Energy Harvesting on S- and N-doped nanoporous Carbons

Nowadays heteroatom-containing carbonaceous materials such as graphene or CNT have gained more and more attention of the scientists searching for inexpensive substitutes of the catalysts for energy related applications such an oxygen reduction reactions. Discovery of graphene and an extensive characterization of its electronic properties caused that the surface of traditional activated carbon has been viewed from other, unexplored before, angles. The main advantage of activated or nanoporous carbons, within the family of carbonaceous materials, is their porosity where the confined pore space effect can be utilized. Recently we have shown that specific nanoporous carbons obtained from commodity polymers can catalyze oxygen evolution reactions1, oxygen reduction reaction2 and exhibit photoluminescence properties3. This behavior was attributed to the specificity of surface microstructure, texture, and chemistry. It was found that the carbons obtained at relatively low temperature (800 oC) contain 10 nm graphic units enhancing their DC conductivity. They have also rich surface chemistry based on sulfur, nitrogen and oxygen containing groups. Even though small sp2 clusters should be important to affect the width of the band gap, the sulfur and nitrogen containing groups are hypothesized to act as chromophores/antenna accepting visible light energy. Electron deficiency on them promotes water splitting in small pores. These groups also change the electronic structure of the carbons surface and bring some level hydrophobicity to it. These features were found as important for oxygen reduction reactions4. These reactions enhance the performance of carbons as supercapcitors when the process takes place in the visible light5. References 1. Ania, C.O.; Seredych, M.; Rodriguez-Castellon, E.; Bandosz, T.J. Visible light driven photoelectrochemical water splitting on metal free nanoporous carbon promoted by chromophoric functional groups. Carbon 79 (2014) 432–441. 2. Seredych, M.; Idrobo, J-C.; Bandosz, T.J. Effect of confined space reduction of graphite oxide followed by sulfur doping on oxygen reduction reaction in neutral electrolyte. J. Mater. Chem. A. 1 (2013) 7059-7067. 3. Bandosz, T.J.; Rodriguez-Castellon, E.; Montenegro J.M.; Seredych, M. Photoluminescence of nanoporous carbons: Opening a new application route for old materials.. Carbon 77 (2014) 651–659. 4. Confined space reduced graphite oxide doped with sulfur as metal-free oxygen reduction catalyst. Seredych, M.; Rodriguez-Castellon, E.; Bandosz, T.J. Carbon 66 (2014) 227-233. 5. Seredych, M.; Rodriguez-Castellon, E.; Biggs, MJ. Skinner, W.l Bandosz, T.J. Effect of visible light and electrode wetting on the capacitive performance of S- and N-doped nanoporous carbons: Importance of surface chemistry. Carbon 78 (2014) 540–558.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Revisiting the chemistry of graphite oxides and its effect on ammonia adsorption

Graphite oxide (GO) was synthesized using two different methods: one with sulfuric acid as part of the oxidizing mixture (Hummers-Offeman method) and another one without the sulfur-containing compound involved in the oxidation process (Brodie method). They were both tested for ammonia adsorption in dynamic conditions, at ambient temperature, and characterized before and after exposure to ammonia by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, potentiometric titration, energy-dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS) and elemental analysis. Analyses of the initial materials showed that besides epoxy, hydroxyl and carboxylic groups, a significant amount of sulfur is incorporated as sulfonic group for GO prepared by the Hummers-Offeman method. The process of ammonia adsorption seems to be strongly related to the type of GO. For GO prepared by the Brodie method, ammonia is mainly retained via intercalation in the interlayer space of GO and by reaction with the carboxylic groups present at the edges of the graphene layers. On the contrary, when GO prepared by the Hummers method is used, the ways of retention are different: not only is the intercalation of ammonia observed but its reaction with the epoxy, carboxylic and sulfonic groups present is also observed. In particular, during the ammonia adsorption process, sulfonic groups are converted to sulfates in the presence of superoxide anions O2-*. These sulfates can then react with ammonia to form ammonium sulfates. For both GOs, an incorporation of a significant part of the ammonia adsorbed as amines in their structure is observed as a result of reactive adsorption. © 2009 The Royal Society of Chemistry

Building MOF Nanocomposites with Oxidized Graphitic Carbon Nitride Nanospheres: The Effect of Framework Geometry on the Structural Heterogeneity

Composite of two MOFs, copper-based Cu-BTC (HKUST-1) and zirconium-based Zr-BDC (UiO-66), with oxidized graphitic carbon nitride nanospheres were synthesized. For comparison, pure MOFs were also obtained. The surface features were analyzed using x-ray diffraction (XRD), sorption of nitrogen, thermal analysis, and scanning electron microscopy (SEM). The incorporation of oxidized g-C3N4 to the Cu-BTC framework caused the formation of a heterogeneous material of a hierarchical pores structure, but a decreased surface area when compared to that of the parent MOF. In the case of UiO-66, functionalized nanospheres were acting as seeds around which the crystals grew. Even though the MOF phases were detected in both materials, the porosity analysis indicated that in the case of Cu-BTC, a collapsed MOF/nonporous and amorphous matter was also present and the MOF phase was more defectous than that in the case of UiO-66. The results suggested different roles of oxidized g-C3N4 during the composite synthesis, depending on the MOF geometry. While spherical units of UiO-66 grew undisturbed around oxidized and spherical g-C3N4, octahedral Cu-BTC units experienced geometrical constraints, leading to more defects, a disturbed growth of the MOF phase, and to the formation of mesopores at the contacts between the spheres and MOF units. The differences in the amounts of CO2 adsorbed between the MOFs and the composites confirm the proposed role of oxidized g-C3N4 in the composite formation

Highly mesoporous carbons obtained using a dynamic template method

New nanoporous carbons with extremely high mesopore volumes and surface areas were obtained using mesoporous silica with a 3-D wormhole porous framework as templates. Mesoporous silica was synthesized following the literature described methods. Polystyrene sulfonic acid-based organic salts were used as carbon precursors. To evaluate the effect of sodium on porosity development silica matrices with various thicknesses of pore walls were synthesized. Prior to carbonization, in order to increase surface heterogeneity, the precursor chemistry was modified by cation exchange with catalytically active metals (i.e., copper, nickel, cobalt). Carbonization followed by HF etching of silica templates generated mesoporous carbons with large surface areas and high pore volumes, which is accompanied by high dispersion of catalytically active metals on the carbon surface. Sodium present in the carbonaceous precursor causes in the dynamic template effect via its reactions with a silica matrix during carbonization. This, along with reactive gases evolved during heating lead to the expansion of the carbonaceous structure, and thus to the unique wide mesopore size distributions of the templated carbons with pore sizes between 10 and 50 nm and their volume exceeding 2.5 cm3 g−1.The financial support for this research, provided by FICYT and PSC CUNY (PSC CUNY 66382-0035) is gratefully acknowledged. We thank Dr. Pis and Dr. Beguin for kindly providing SEM and XRD, and TEM, respectively. Thomas Cacciaguerra is also acknowledged for assistance in TEM analysis.Peer reviewe

Microcalorimetric insight into the analysis of the reactive adsorption of ammonia on Cu-MOF and its composite with graphite oxide

The determination of reactive adsorption mechanisms on metal–organic frameworks remains largely unexplored and knowledge in that field would provide an important stepping stone in enhancing the performance of these materials for gas separation. In this study, the mechanisms of ammonia adsorption on HKUST-1 and its composite with graphite oxide (GO) were analyzed using microcalorimetry, and the results were compared to those derived from other characterization tools. The key to this study lies in conducting measurements at very low surface coverage in order to define the most energetic adsorption phenomena. It was found that ammonia reacted with the Cu sites and then with the ligands causing MOF to collapse. On the other hand, GO enhanced the heats of adsorption owing to its additional reactive sites

Sodium on the Surface of Activated Carbons as a Factor Enhancing Reactive Adsorption of Dibenzothiophene

On the basis of the results described in the literature on the reactivity of sodium toward DBT and its derivates, the effect of sodium dispersed on the surface of a carbonaceous support on the removal of DBT from hexane solution at ambient conditions has been investigated. Since the main objective of this paper is to demonstrate the effect of sodium on the reactive adsorption of DBT from an organic solvent, hexane was chosen as a model solvent. As adsorbents, two carbons with and without sodium on the surface were investigated. The carbons are obtained by carbonization of polystyrene−sulfonic acid−co-maleic acid in sodium salt at 800 °C in nitrogen. The results show that sodium present on the surface of carbon takes part in the reaction with DBT in which sodium metalates and sodium sulfides are formed.This work was supported by FICYT and PSC CUNY (PSC CUNY 66382-0035). The authors thank Dr. Martin Gorbaty of ExxonMobil for inspiration and fruitful discussion. Dr. Alvarez is kindly acknowledged for his help in SEM.Peer reviewe

Comparing Strategies to Prevent Stroke and Ischemic Heart Disease in the Tunisian Population: Markov Modeling Approach Using a Comprehensive Sensitivity Analysis Algorithm.

Background. Mathematical models offer the potential to analyze and compare the effectiveness of very different interventions to prevent future cardiovascular disease. We developed a comprehensive Markov model to assess the impact of three interventions to reduce ischemic heart diseases (IHD) and stroke deaths: (i) improved medical treatments in acute phase, (ii) secondary prevention by increasing the uptake of statins, (iii) primary prevention using health promotion to reduce dietary salt consumption. Methods. We developed and validated a Markov model for the Tunisian population aged 35–94 years old over a 20-year time horizon. We compared the impact of specific treatments for stroke, lifestyle, and primary prevention on both IHD and stroke deaths. We then undertook extensive sensitivity analyses using both a probabilistic multivariate approach and simple linear regression (metamodeling). Results. The model forecast a dramatic mortality rise, with 111,134 IHD and stroke deaths (95% CI 106567 to 115048) predicted in 2025 in Tunisia. The salt reduction offered the potentially most powerful preventive intervention that might reduce IHD and stroke deaths by 27% (−30240 [−30580 to −29900]) compared with 1% for medical strategies and 3% for secondary prevention. The metamodeling highlighted that the initial development of a minor stroke substantially increased the subsequent probability of a fatal stroke or IHD death. Conclusions. The primary prevention of cardiovascular disease via a reduction in dietary salt consumption appeared much more effective than secondary or tertiary prevention approaches. Our simple but comprehensive model offers a potentially attractive methodological approach that might now be extended and replicated in other contexts and populations

Robust graphene-based monoliths of homogeneous ultramicroporosity

Graphite oxide (GO) and graphene monoliths were prepared using the unidirectional freezing of GO water suspension. These materials were saturated with a poly(ammonium-4-styrene sulfonate) water soluble polymer and then carbonized at 1123 K. This process increases significantly the materials strength and density. A uniform deposition of the polymer-derived carbon on the external layers of the graphene sheets of the monolith was found. The carbon from polymer not only provided more contact between the graphene sheets but also apparently increased the overall graphitization level (based on Raman spectra). The modification decreased the electrical resistance by one order of magnitude compared to that of the graphene monolith. N-2 adsorption at 77 K showed that the thus-treated graphene monoliths have quite homogenous pores with the pore width of 0.7 nm. These pores, combined with large transport pores, and conductive properties make the monoliths tested the promising materials for separation, energy storage, and/or gas sensing. The tunability of the properties and pore structure of the robust graphene ultramicroporous monolith through the control of chemistry of the initial GO monolith was shown. (C) 2015 Elsevier Ltd. All rights reserved.ArticleCARBON. 87:87-97 (2015)journal articl

CO2 interactions with porous carbons: Is the surface stable at ambient conditions?

Interactions of CO2 with polymer derived carbon/rGO composites at ambient conditions were studied. Both, dynamic adsorption tests and equilibrium adsorption measurements were analyzed. The samples differed in the porosity, oxidation level and speciation of sulfur on the surface. Even though more CO2 was adsorbed on the oxidized sample than in the unmodified one, the surface chemistry of the latter was found as having more pronounced effect on attracting CO2 to the pore system. The results showed the marked changes in S-doped nanoporous carbon composite surface chemistry upon CO2 adsorption at ambient conditions. The changes were more pronounced for carbon with higher density of sulfur in thiophenic configurations on the surface emphasizing the role of these species in CO2 reduction. Even though CO could not be the target of our detection, identification of water, SO and SO2 as products of surface reactions supports our hypothesis that CO2 adsorption was accompanied by some extent of its reduction to CO. CO is formed in the process of electron transfer from thiophenes to CO2 in which the former are oxidized forming sulfones and sulfonic acids. Those species are likely thermodynamically unstable and decompose forming SO/SO2 and water providing additional electrons for CO2 reduction. Conductivity of carbon matrix and the local increase in this feature owing to the presence of the graphene-based phase facilitate this process. Based on the results collected, it is recommended that the stability of carbons towards carbon dioxide should be evaluated before it is used as CO2 sequestration medium

Efficient Air Desulfurization Catalysts Derived from Pig Manure Liquefaction Char

Biochar from the liquefaction of pig manure was used as a precursor of H2S desulfurization adsorbents. In its inorganic matter, it contains marked quantities of calcium, magnesium and iron, which are known as hydrogen sulfide oxidation catalysts. The char was used either as-received or mixed with 10% nanographite. The latter was added to increase both the content of the carbon phase and conductivity. ZnCl2 in two different ratios of char to an activation agent (1:1 and 1:2) was used to create the porosity in the carbon phase. The content of the later was between 18–45%. The activated samples adsorbed 144 mg/g H2S. Sulfur was the predominant product of reactive adsorption. Its deposition in the pore system and blockage of the most active pores ceased the materials’ activity. The presence of the catalytic phase was necessary but not sufficient to guarantee good performance. The developed porosity, which can store oxidation products in the resulting composite, is essential for the good performance of the desulfurization process. The surface of the composite with nanographite showed the highest catalytic activity, similar to that of the commercial Midas® carbon catalyst. The results obtained indicate that a high quality reactive adsorbent/catalyst for H2S removal can be obtained from pig manure liquefaction wastes