Altug S. Poyraz

A general approach to crystalline and monomodal pore size mesoporous materials

Mesoporous oxides attract a great deal of interest in many fields, including energy, catalysis and separation, because of their tunable structural properties such as surface area, pore volume and size, and nanocrystalline walls. Here we report thermally stable, crystalline, thermally controlled monomodal pore size mesoporous materials. Generation of such materials involves the use of inverse micelles, elimination of solvent effects, minimizing the effect of water content and controlling the condensation of inorganic frameworks by NO(x) decomposition. Nanosize particles are formed in inverse micelles and are randomly packed to a mesoporous structure. The mesopores are created by interconnected intraparticle voids and can be tuned from 1.2 to 25 nm by controlling the nanoparticle size. Such phenomena allow the preparation of multiple phases of the same metal oxide and syntheses of materials having compositions throughout much of the periodic table, with different structures and thermal stabilities as high as 800 °C.

Heterogeneous acidic TiO2 nanoparticles for efficient conversion of biomass derived carbohydrates

Selective conversion of biomass derived carbohydrates into fine chemicals is of great significance for the replacement of petroleum feedstocks and the reduction of environmental impacts. Levulinic acid, 5-hydroxymethyl furfural (HMF) and their derivatives are recognized as important precursor candidates in a variety of different areas. In this study, the synthesis, characterization, and catalytic activity of acidic TiO2 nanoparticles in the conversion of biomass derived carbohydrates were explored. This catalyst was found to be highly effective for selective conversion to value-added products. The nanoparticles exhibited superior activity and selectivity towards methyl levulinate from fructose in comparison to current commercial catalysts. The conversion of fructose to methyl levulinate was achieved with 80% yield and high selectivity (up to 80%). Additionally, conversions of disaccharides and polysaccharides were studied. Further, the production of versatile valuable products such as levulinic esters, HMF, and HMF-derived ethers was demonstrated using the TiO2 nano-sized catalysts in different solvent systems.

Facile Synthesis of Co3O4@CNT with High Catalytic Activity for CO Oxidation under Moisture-Rich Conditions

The catalytic oxidation reaction of CO has recently attracted much attention because of its potential applications in the treatment of air pollutants. The development of inexpensive transition metal oxide catalysts that exhibit high catalytic activities for CO oxidation is in high demand. However, these metal oxide catalysts are susceptible to moisture, as they can be quickly deactivated in the presence of trace amounts of moisture. This article reports a facile synthesis of highly active Co3O4@CNT catalysts for CO oxidation under moisture-rich conditions. Our synthetic routes are based on the in situ growth of ultrafine Co3O4 nanoparticles (NPs) (∼2.5 nm) on pristine multiwalled CNTs in the presence of polymer surfactant. Using a 1% CO and 2% O2 balanced in N2 (normal) feed gas (3-10 ppm moisture), a 100% CO conversion with Co3O4@CNT catalysts was achieved at various temperatures ranging from 25 to 200 °C at a low O2 concentration. The modulation of surface hydrophobicity of CNT substrates, other than direct surface modification on the Co3O4 catalytic centers, is an efficient method to enhance the moisture resistance of metal oxide catalysts for CO oxidation. After introducing fluorinated alkyl chains on CNT surfaces, the superhydrophobic Co3O4@CNT exhibited outstanding activity and durability at 150 °C in the presence of moisture-saturated feed gas. These materials may ultimately present new opportunities to improve the moisture resistance of metal oxide catalysts for CO oxidation.

Crystalline Mesoporous K2–xMn8O16 and ε-MnO2 by Mild Transformations of Amorphous Mesoporous Manganese Oxides and Their Enhanced Redox Properties

Synthesis of crystalline mesoporous K(2-x)Mn8O16 (Meso-OMS-2), and ε-MnO2 (Meso-ε-MnO2) is reported. The synthesis is based on the transformation of amorphous mesoporous manganese oxide (Meso-Mn-A) under mild conditions: aqueous acidic solutions (0.5 M H(+) and 0.5 M K(+)), at low temperatures (70 °C), and short times (2 h). Meso-OMS-2 and Meso-ε-MnO2 maintain regular mesoporosity (4.8-5.6 nm) and high surface areas (as high as 277 m(2)/g). The synthesized mesoporous manganese oxides demonstrated enhanced redox (H2-TPR) and catalytic performances (CO oxidation) compared to nonporous analogues. The order of reducibility and enhanced catalytic performance of the samples is Commercial-Mn2O3 < nonporous-OMS-2 < Meso-Mn2O3 < Meso-OMS-2 < Meso-ε-MnO2 < Meso-Mn-A.

Mesoporous Co3O4 nanostructured material synthesized by one-step soft-templating: A magnetic study

A combined magnetization and zero-field 59Co spin-echo nuclear magnetic resonance (NMR) study has been carried out on one member of a recently developed class of highly ordered mesoporous nanostructured materials, mesoporous Co3O4 (designated UCT-8, University of Connecticut, mesoporous materials). The material was synthesized using one-step soft-templating by an inverse micelles packing approach. Characterization of UCT-8 by powder x-ray diffraction and electron microscopy reveals that the mesostructure consists of random close-packed Co3O4 nanoparticles ≈ 12 nm in diameter. The N2 sorption isotherm for UCT-8, which is type IV with a type H1 hysteresis loop, yields a 134 m2/g BET surface area and a 7.7 nm BJH desorption pore diameter. The effect of heat treatment on the structure is discussed. The antiferromagnetic Co3O4 nanoparticles have a Neel temperature TN = 27 K, somewhat lower than the bulk. A fit to the Curie-Weiss law over the temperature range 75 K ≤ T ≤ 300 K yields an effective magnetic moment of μeff = 4.36 μB for the Co2+ ions, indicative of some orbital contribution, and a Curie-Weiss temperature Θ = −93.5 K, consistent with antiferromagnetic ordering. The inter-sublattice and intra-sublattice exchange constants for the Co2+ ions are J1/kB = (−)4.75 K and J2/kB = (−)0.87 K, respectively, both corresponding to antiferromagnetic coupling. The presence of uncompensated surface spins is observed below TN with shifts in the hysteresis loops, i.e., an exchange-bias effect. The 59Co NMR spectrum for UCT-8, which is attributed to Co2+ ions at the tetrahedral A sites, is asymmetrically broadened with a peak at ≈55 MHz (T = 4.2 K). Since there is cubic symmetry at the A-sites, the broadening is indicative of a magnetic field distribution due to the uncompensated surface spins. The spectrum is consistent with antiferromagnetically ordered particles that are nanometer in size and single domain.

Effective recycling of manganese oxide cathodes for lithium based batteries

While rechargeable lithium ion batteries (LIBs) occupy a prominent consumer presence due to their high cell potential and gravimetric energy density, there are limited opportunities for electrode recycling. Currently used or proposed cathode recycling processes are multistep procedures which involve sequences of mechanical, thermal, and chemical leaching, where only the base material is recovered and significant processing is required to generate a recycled electrode structure. Another significant issue facing lithium based batteries is capacity fade due to structural degradation of the electroactive material upon extending cycling. Herein, inspired by heterogeneous catalyst thermal regeneration strategies, we present a new facile cathode recycling process, where previously used cathodes are removed from a cell, heat treated, and then inserted into a new cell restoring the delivered capacity and cycle life. An environmentally sustainable manganese based material is employed, where binder-free self-supporting (BFSS) electrodes are prepared using a fibrous, high aspect ratio manganese oxide active material. After 200 discharge–charge cycles, the recycled BFSS electrodes display restored crystallinity and oxidation state of the manganese centers with the resulting electrochemistry (capacity and coulombic efficiency) reminiscent of freshly prepared BFSS cathodes. Notably, the BFSS electrode structure is robust with no degradation during the cell disassembly, electrode recovery, washing, and heat treatment steps; thus no post-processing is required for the recycled electrode. This work shows for the first time that a thermal regeneration method previously employed in catalyst systems can fully restore battery electrochemical performance, demonstrating a novel electrode recycling process which could open up new possibilities for energy storage devices with extended electrode lifecycles.

Bimodification of Mesoporous Silicon Oxide by Coupled “In Situ Oxidation at the Interface and Ion Exchange” and its Catalytic Activity in the Gas-Phase Toluene Oxidation

A bimodification synthesis method—“in situ oxidation at the interface (IOI) coupled with an ion exchange”—has been developed for the internal surface modification of mesoporous silicon oxide (MPS) templates. First, manganese oxide was formed at the internal surface of the MPS template through IOI. In the IOI method, high‐valent oxo‐anions of manganese (MnO4−) were used for the selective oxidation of poly(ethylene oxide) (PEO) groups of the Pluronic P123 (PEO20PPO70PEO20; PPO=poly(propylene oxide)) surfactant and they formed manganese oxide at the organic–inorganic (corona) interface. The oxide formation was restricted at the corona interface by a positively charged CTA+ (cetyltrimethylammonium) head group of the cationic surfactant CTABr. Then, the second modification of the MPS template was also performed by introducing promoter cations (Cs+, K+, or H+) through an ion exchange reaction between the cations and CTA+. The bimodified MPSMnX (X=Cs, K, or H) samples preserved the mesoporosity and high surface area of the MPS template. The bimodified MPSMnX samples were found to be more active, selective, and stable than the singly modified MPSMn sample for the gas‐phase oxidation of toluene.

Synthesis of cryptomelane type α-MnO2 (KxMn8O16) cathode materials with tunable K+ content: the role of tunnel cation concentration on electrochemistry

The role of tunnel cations in the electrochemistry of α-MnO2 materials has been long discussed and demands investigation as the electrochemistry of α-MnO2 materials is strongly dependent on the material specific properties (i.e. morphology, surface area, crystallite size, and chemical composition). Here, we systematically synthesized a series of α-MnO2 samples with differing K+ content but similar physicochemical and morphological properties allowing direct investigation of the role of tunnel cation (K+) on the lithium ion electrochemistry of α-MnO2 cathodes. The nanofibrous α-MnO2 materials have a chemical composition of KxMn8O16·yH2O, where 0 ≤ x ≤ 0.75 and 0.53 ≤ y ≤ 0.81. The α-MnO2 materials have similar morphology, crystallite size (17–19 nm), surface area (66–76 m2 g−1), and tunnel water content (0.53–0.81). The electrochemistry of the α-MnO2 materials was evaluated using cyclic voltammetry, galvanostatic cycling, and galvanostatic intermittent titration type tests. The α-MnO2 materials with 0 to 0.32 K+ content showed discharge curves with higher voltage, higher specific energies, and improved capacity retention compared to the 0.75 K+ containing α-MnO2 material. Fewer structural distortions were observed in lithiated samples with lower K+ content through modelling of X-ray absorption spectroscopy data indicating improved structural stability of those samples which positively impacted the electrochemistry.

Silver-Containing α-MnO2 Nanorods: Electrochemistry in Na-Based Battery Systems

Manganese oxides are considered attractive cathode materials for rechargeable batteries due to the high abundance and environmental friendliness of manganese. In particular, cryptomelane and hollandite are desirable due to their ability to host cations within their octahedral molecular sieve (OMS-2) α-MnO2 structure. In this work, we investigate silver containing α-MnO2 structured materials (AgxMn8O16, x = 1.22, L-Ag-OMS-2 or 1.66, H-Ag-OMS-2) as host materials for Li ion and Na ion insertion/deinsertion. The results indicate a significant difference in the lithiation versus sodiation process of the OMS-2 materials. Initial reduction of Ag1.22Mn8O16 to 1.0 V delivered ∼370 mAh/g. Cycling of Ag1.22Mn8O16 between voltage ranges of 3.8-1.7 V and 3.8-1.3 V in a Na battery delivered initial capacities of 113 and 247 mAh/g, respectively. In contrast, Ag1.66Mn8O16 delivered only 15 mAh/g, ∼ 0.5 electron equivalents, to 1.7 and 1.3 V. Study of the system by electrochemical impedance spectroscopy (EIS) showed a significant decrease in charge transfer resistance from 2029 Ω to 594 Ω after 1.5 electron equivalents per Ag1.22Mn8O16 formula unit of Na ion insertion. In contrast, both Ag1.22Mn8O16 and Ag1.66Mn8O16 exhibited gradual impedance increases during lithiation. The formation of silver metal could be detected only in the sodiated material by X-ray diffraction (XRD). Thus, the impedance of Ag-OMS-2 decreases upon sodiation coincident with the formation of silver metal during the discharge process, consistent with the more favorable formation of silver metal during the sodiation process relative to the lithation process.

Lithiation Mechanism of Tunnel‐Structured MnO2 Electrode Investigated by In Situ Transmission Electron Microscopy

Manganese oxide (α-MnO2 ) has been considered a promising energy material, including as a lithium-based battery electrode candidate, due to its environmental friendliness. Thanks to its unique 1D [2 × 2] tunnel structure, α-MnO2 can be applied to a cathode by insertion reaction and to an anode by conversion reaction in corresponding voltage ranges, in a lithium-based battery. Numerous reports have attributed its remarkable performance to its unique tunnel structure; however, the precise electrochemical reaction mechanism remains unknown. In this study, finding of the lithiation mechanism of α-MnO2 nanowire by in situ transmission electron microscopy (TEM) is reported. By elaborately modifying the existing in situ TEM experimental technique, rapid lithium-ion diffusion through the tunnels is verified. Furthermore, by tracing the full lithiation procedure, the evolution of the MnO intermediate phase and the development of the MnO and Li2 O phases with preferred orientations is demonstrated, which explains how the conversion reaction occurs in α-MnO2 material. This study provides a comprehensive understanding of the electrochemical lithiation process and mechanism of α-MnO2 material, in addition to the introduction of an improved in situ TEM biasing technique.