David A. Kriz

Understanding the Role of Gold Nanoparticles in Enhancing the Catalytic Activity of Manganese Oxides in Water Oxidation Reactions

The Earth-abundant and inexpensive manganese oxides (MnOx) have emerged as an intriguing type of catalysts for the water oxidation reaction. However, the overall turnover frequencies of MnOx catalysts are still much lower than that of nanostructured IrO2 and RuO2 catalysts. Herein, we demonstrate that doping MnOx polymorphs with gold nanoparticles (AuNPs) can result in a strong enhancement of catalytic activity for the water oxidation reaction. It is observed that, for the first time, the catalytic activity of MnOx/AuNPs catalysts correlates strongly with the initial valence of the Mn centers. By promoting the formation of Mn(3+) species, a small amount of AuNPs (<5%) in α-MnO2/AuNP catalysts significantly improved the catalytic activity up to 8.2 times in the photochemical and 6 times in the electrochemical system, compared with the activity of pure α-MnO2.

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.

Self-assembly of manganese oxide nanoparticles and hollow spheres. Catalytic activity in carbon monoxide oxidation

Reactions between MnSO(4) and KMnO(4) in the presence of carboxylic acids provide a facile, one-pot route to nanostructured manganese oxides with high surface areas. Acetic and propionic acid induce formation of hierarchical nanosphere morphologies whereas butyric acid promotes assembly of hollow spheres. The materials are active 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.

Water-gas-shift over metal-free nanocrystalline ceria: An experimental and theoretical study

A tandem experimental and theoretical investigation of a mesoporous ceria catalyst reveals the properties of the metal oxide are conducive for activity typically ascribed to metals, suggesting reduced Ce3+ and oxygen vacancies are responsible for the inherent bi‐functionality of CO oxidation and dissociation of water required for facilitating the production of H2. The degree of reduction of the ceria, specifically the (1 0 0) face, is found to significantly influence the binding of reagents, suggesting reduced surfaces harbor the necessary reactive sites. The metal‐free catalysis of the reaction is significant for catalyst design considerations, and the suite of in situ analyses provides a comprehensive study of the dynamic nature of the high surface area catalyst system. This study postulates feasible improvements in catalytic activity may redirect the purpose of the water‐gas shift reaction from CO purification to primary hydrogen production.

Fluoride additive in epoxide-initiated sol–gel synthesis enables thin-film applications of SnO2 aerogels

Aerogels of SnO2 were synthesized by an epoxide-initiated sol–gel method. Using ammonium fluoride in the precursor solution allowed for tunability of the aerogel morphology while no change in the conductivity was measured. In particular, aerogel shrinkage was decreased dramatically by the addition of the fluoride precursor. Unfluorinated aerogels showed severe shrinkage of 43% volume change upon supercritical drying compared to the original alcogel volume. Fluorinated samples exhibited a much less pronounced shrinkage at 7%. Multiple characterization methods converged to reveal the mechanism by which fluoride enables the morphological tunability. These findings enable the casting of SnO2 aerogels as thin films (which in the absence of fluoride these crack and delaminate due to shrinkage), opening potential uses in many optoelectronic devices including solar cells.

Partial Oxidation of Methane to Synthesis Gas Using Supported Ga‐Containing Bimetallic Catalysts and a Ti‐Promoter

A series of bimetallic Ga‐containing materials using TiO2 and TiO2‐promoted SiO2 supports have been prepared. Rhodium, palladium, and platinum have been used as additional metals in this system. The materials are characterized and used as catalysts for the partial oxidation of methane into synthesis gas (H2 and CO). The presence of a low quantity of titanium in the form of anatase TiO2 was shown to improve the overall activity of catalytic methane oxidation and to strongly increase the selectivity of partial oxidation products over the total oxidation of methane to carbon dioxide and water. Particular attention is paid to the formation of gallium‐metal alloys on the surface of the catalyst supports. Rh‐Ga‐Ti‐SiO2 was found to be the most active and selective catalyst, giving 89 % conversion of methane and 99 % selectivity to synthesis gas at 750 °C, as well as exhibiting catalytic activity and preferential conversion to partial oxidation products at temperatures as low as 350 °C.