Synthesis of mesoporous carbons with controllable N-content and their supercapacitor properties

A synthesis route to ordered mesoporous carbons with controllable nitrogen content has been developed for high-performance EDLC electrodes. Nitrogen-doped ordered mesoporous carbons (denoted as NMC) were prepared by carbonizing a mixture of two different carbon sources within the mesoporous silica designated by KIT-6. Furfuryl alcohol was used as a primary carbon precursor, and melamine as a nitrogen dopant. This synthesis procedure gave cubic Ia3d mesoporous carbons containing nitrogen as much as 13%. The carbon exhibited a narrow pore size distribution centered at 3-4 nm with large pore volume (0.6-1 cm g) and high specific BET surface area (700-1000 m g). Electrochemical behaviors of the NMC samples with various Ncontents were investigated by a two-electrode measurement system at aqueous solutions. At low current density, the NMC exhibited markedly increasing capacitance due to the increase in the nitrogen content. This result could be attributed to the enhanced surface affinity between carbon electrode and electrolyte ions due to the hydrophilic nitrogen functional groups. At high current density conditions, the NMC samples exhibited decreasing specific capacitance against the increase in the nitrogen content. The loss of the capacitance with the N-content may be explained by high electric resistance which causes a significant IR drop at high current densities. The present results indicate that the optimal nitrogen content is required for achieving high power and high energy density simultaneously

Ethanol-based synthesis of hierarchically porous carbon using nanocrystalline beta zeolite template for high-rate electrical double layer capacitor

A porous carbon with a mesopore-micropore hierarchy has been synthesized using ethanol as a carbon source within a beta zeolite template, which possesses mesopores and micropores in a hierarchical manner. Various compounds have been evaluated as the carbon source in the carbon synthesis. The result indicates that ethanol vapor is suitable for the hierarchical carbon synthesis, as compared with propylene or acetylene gas. The advantage of ethanol is attributed to the formation of water from the ethanol decomposition during carbonization process. The water seems to assist in delaying the deposition of carbons at the exterior (or the mesopore walls). The resultant carbon after removal of the template is composed of regular micropores of 1-nm diameter and irregular mesopores of 10-30 nm arranged in a hierarchical manner. The hierarchical carbon exhibits a very high electrical double-layer capacitance both at low and high discharge current densities in galvanostatic measurements, as compared to solely microporous or solely mesoporous carbons. The high performance is attributed to facile transport of electrolytes in the hierarchically porous structure

Ethanol-based synthesis of hierarchically porous carbon using nanocrystalline beta zeolite template for high-rate electrical double layer capacitor

A porous carbon with a mesopore–micropore hierarchy has been synthesized using ethanol as a carbon source within a beta zeolite template, which possesses mesopores and micropores in a hierarchical manner. Various compounds have been evaluated as the carbon source in the carbon synthesis. The result indicates that ethanol vapor is suitable for the hierarchical carbon synthesis, as compared with propylene or acetylene gas. The advantage of ethanol is attributed to the formation of water from the ethanol decomposition during carbonization process. The water seems to assist in delaying the deposition of carbons at the exterior (or the mesopore walls). The resultant carbon after removal of the template is composed of regular micropores of 1-nm diameter and irregular mesopores of 10–30 nm arranged in a hierarchical manner. The hierarchical carbon exhibits a very high electrical double-layer capacitance both at low and high discharge current densities in galvanostatic measurements, as compared to solely microporous or solely mesoporous carbons. The high performance is attributed to facile transport of electrolytes in the hierarchically porous structure.134341sciescopu

Physicochemical Stabilization of Pt against Sintering for a Dehydrogenation Catalyst with High Activity, Selectivity, and Durability

Suppressing irreversible catalyst deactivation is critical in heterogeneous catalysis. In particular, deactivation via sintering of active sites is a significant issue for reactions involving harsh reaction/regeneration conditions. In this work, we developed a PtGa/γ-Al₂O₃ alkane dehydrogenation catalyst with exceptionally high activity, selectivity, and long-term stability by markedly suppressing Pt sintering under harsh conditions (reaction/regeneration at >823 K). To stabilize Pt, physical and chemical stabilization strategies were synergistically combined. For the former, Pt was introduced during the synthesis of γ-Al₂O₃ via sol–gel chemistry, which can increase the interfacial contact between Pt and γ-Al₂O₃ due to the partial entrapment of Pt in γ-Al₂O₃. For the latter, atomically dispersed Ce was doped on γ-Al₂O₃, which can stabilize Pt via strong Pt–O–Ce interactions. Because of effective Pt stabilization, the catalyst showed remarkably steady activity and selectivity behaviors over the repeated reaction cycles, although the catalyst is regenerated via simple oxidation rather than industrially used oxychlorination. The Pt stabilization strategies reported in this work can be applied to other metal-catalyzed reactions that involve severe reaction/regeneration conditions

Revisiting side-chain alkylation of toluene to styrene: Critical role of microporous structures in catalysts

Due to potential advantages such as large availability and low cost of reactant feedstocks, the side-chain alkylation of toluene with methanol has been investigated as an alternative pathway for producing styrene. In this study, the effects of acid-base properties and microporous structures of zeolite catalysts were comprehensively investigated. The results showed that the main active sites for this reaction are base sites, which are required for activating the methyl group in toluene and converting methanol into HCHO, an alkylating agent. On the other hand, the role of acid sites is to stabilize toluene via Lewis acid-base interactions. Our results indicated that a significant dilemma exists in the design of the catalyst. Catalysts having sufficient basicity generally have limited Lewis acidity, which is ineffective for stabilizing toluene under the reaction conditions at elevated temperatures. Fortunately, the micropores of zeolites (e.g., X zeolites) provide secondary interactions such as van der Waals forces to toluene, which substantially enhance toluene stabilization, and thereby, the alkylation activity. In contrast, the model catalysts synthesized using mesoporous A zeolite showed no toluene adsorption at all due to the absence of any accessible microporosity. Consequently, even though the A zeolite catalysts have similar acid-base properties to those of the X zeolite catalysts, they showed no detectable toluene alkylation activity and produced only the decomposition products of methanol. The results clearly showed the importance of secondary interactions (or solvation effects) in zeolite catalysis, and explained why basic zeolites are superior to the conventional basic metal oxides that lack microporosity. © 2019 Elsevier In

2,6-Di-tert-butylpyridine sorption approach to quantify the external acidity in hierarchical zeolites

This work was aimed to evidence that substituted pyridine, 2,6-di-tert-butylpyridine, is a suitable probe for the quantitative investigation of the external acidity in hierarchically structured zeolites. The 2,6-di-tert-butylpyridine was too large to enter the micropores, even in wide pore zeolites, and nearly no sites in nonmesoporous zeolites were available. Accessibility studies of acid sites in zeolites TNU-9 and BEA involving quantitative IR measurements with hindered 2,6-di-tert-butylpyridine as a probe were performed. The extinction coefficients of the 1615 cm-1 diagnostic bands of 2,6-di-tert-butylpyridine interacting with Brønsted acid sites were determined. Lewis acid sites were not detected with the probe. The accessibility factor (AF) for the 2,6-di-tert-butylpyridine probe molecule was defined as the number of sites detected by adsorption of the dTBPy (external sites) divided by the total amount of acid sites in the studied zeolites as quantified by pyridine sorption. Upon desilication resulting in the fabrication of the secondary mesopores, the enhanced accessibility of the protonic sites was observed. In comparison to the mesoporous zeolites with the secondary system of mesopores generated by alkaline leaching, considerably higher accessibility of protonic sites was evidenced in both ultrathin ZSM-5 and delaminated ITQ-2 zeolite

Synergistic Integration of Ion-Exchange and Catalytic Reduction for Complete Decomposition of Perchlorate in Waste Water

Ion-exchange has been frequently used for the treatment of perchlorate (ClO₄^–), but disposal or regeneration of the spent resins has been the major hurdle for field application. Here we demonstrate a synergistic integration of ion-exchange and catalytic decomposition by using Pd-supported ion-exchange resin as an adsorption/catalysis bifunctional material. The ion-exchange capability of the resin did not change after generation of the Pd clusters via mild ethanol reduction, and thus showed very high ion-exchange selectivity and capacity toward ClO₄^–. After the resin was saturated with ClO₄^– in an adsorption mode, it was possible to fully decompose the adsorbed ClO₄^– into nontoxic Cl^– by the catalytic function of the Pd catalysts under H₂ atmosphere. It was demonstrated that prewetting the ion-exchange resin with ethanol significantly accelerate the decomposition of ClO₄^– due to the weaker association of ClO₄^– with the ion-exchange sites of the resin, which allows more facile access of ClO₄^– to the catalytically active Pd-resin interface. In the presence of ethanol, >90% of the adsorbed ClO₄^– could be decomposed within 24 h at 10 bar H₂ and 373 K. The ClO₄^– adsorption-catalytic decomposition cycle could be repeated up to five times without loss of ClO₄^– adsorption capacity and selectivity

Quantitative aspects of the identification of Fe(II) moieties in ZSM-5 zeolites with various pore hierarchies

This work was attempted in order to provide a new and well-established approach for the quantification of Fe2+ sites in zeolites. The experimental procedure was based on carbon monoxide and nitrogen monoxide sorption in Fe-exchanged ZSM-5 zeolites with different pore hierarchies, which was followed by IR spectroscopy. The values of the absorption coefficients of the respective monocarbonyl Fe2+(CO) and mononitrosyl Fe2+(NO) IR bands were obtained. In turn, the concentrations of Fe(II) moieties of various kinds were calculated and discussed with regard to the composition (Si/Al and Fe/Al ratios) of the zeolites studied