Time-Resolved XANES Studies on Used Silica Supported Cobalt Catalysts

In order to investigate the phases of used Co/SiO2 catalysts, one of the most commonly used catalysts for Fischer-Tropsch synthesis, time-resolved X-ray absorption near edge structure (TR-XANES) technique was introduced. The catalysts were prepared by incipient wetness impregnation method with %Co loading of 15% and 20% and used for Fischer-Tropsch synthesis at the reaction temperature of 190oC and the pressure of 10 and 20 bar, called 15%Co/Aerosil_wi_used_10_bar, 15%Co/Aerosil_wi_used_20_bar, and 20%Co/Aerosil_wi_used_20_bar. TR-XANES showed the edge energy of 7721 eV for all used catalysts and by looking at the feature of their spectra, the results implied that the major phase was CoO. To further investigate their ability of being oxidized at elevated temperatures, the catalysts were oxidized by heating from ambient to 450oC with the heating rate of 8oC/min at the pressure of 1 bar with the O2:N2 flow rate of 70:30 ml/min. Once reaching 450oC, the temperature was held at 450oC for 90 min before cooling down to room temperature. During heating, holding, and cooling, the catalyst properties were measured by TR-XANES. When all catalysts heating up from 300 to 400oC, the edge energy of 15%Co/Aerosil_wi_used_10_bar, 15%Co/Aerosil_wi_used_20_bar, and 20%Co/Aerosil_wi_used_20_bar at 7719, 7717, and 7718 eV, respectively, showed the mixed phases of Co3O4 and CoO. There were no significant changes in phase while holding the temperature at 450oC. Once cooling from 450oC to room temperature, the edge energy of 15%Co/Aerosil_wi_used_10_bar at 7724 eV showed the main Co3O4 phase, the ones of 15%Co/Aerosil_wi_used_20_bar and 20%Co/Aerosil_wi_used_20_bar at 7717 and 7718 eV showed the mixed phase of Co3O4 and CoO. All results would be confirmed by further studies on temperature programmed oxidation (TPO)

Three-Dimensional Hierarchical Porous TiO2 for Enhanced Adsorption and Photocatalytic Degradation of Remazol Dye

Three-dimensional hierarchical mesoporous structures of titanium dioxide (3D-HPT) were synthesized by self-assembly emulsion polymerization. Polymethyl methacrylate (PMMA) and pluronic 123 (P123) were used as the soft templates and co-templates for assisting the formation of hierarchical 3D porous structures. The TiO2 crystal structure, morphology, and Remazol red dye degradation were investigated. The 3D-HPT and normal three-dimensional titanium dioxide (3D-T) presented the good connection of the nanoparticle-linked honeycomb within the form of anatase. The 3D-HPT structure showed greatly enhanced adsorption of Remazol dye, and facilitated the efficient photocatalytic breakdown of the dye. Surprisingly, 3D-HPT can adsorb approximately 40% of 24 ppm Remazol dye in the dark, which is superior to 3D-T and the commercial anatase at the same condition (approx. 5%). Moreover, 3D-HPT can completely decolorize Remazol dye within just 20 min, which is more than three folds faster than the commercial anatase, making it one of the most active photocatalysts that have been reported for degradation of Remazol dye. The superior photocatalytic performance is attributed to the higher specific surface area, amplified light-harvesting efficiency, and enhanced adsorption capacity into the hierarchical 3D inverse opal structure compared to the commercial anatase TiO2

Rate Enhancements in Structural Transformations of Pt–Co and Pt–Ni Bimetallic Cathode Catalysts in Polymer Electrolyte Fuel Cells Studied by in Situ Time-Resolved X‑ray Absorption Fine Structure

In situ time-resolved X-ray absorption fine structure spectra of Pt/C, Pt₃Co/C, and Pt₃Ni/C cathode electrocatalysts in membrane electrode assemblies (catalyst loading: 0.5 mg_metal cm^–2) were successfully measured every 100 ms for a voltage cycling process between 0.4 and 1.0 V. Systematic analysis of in situ time-resolved X-ray absorption near-edge structure and extended X-ray absorption fine structure spectra in the molecular scale revealed the structural kinetics of the Pt and Pt₃M (M = Co, Ni) bimetallic cathode catalysts under polymer electrolyte fuel cell operating conditions, and the rate constants of Pt charging, Pt–O bond formation/breaking, and Pt–Pt bond breaking/re-formation relevant to the fuel cell performances were successfully determined. The addition of the 3d transition metals to Pt reduced the Pt oxidation state and significantly enhanced the reaction rates of Pt discharging, Pt–O bond breaking, and Pt–Pt bond re-forming in the reductive process from 1.0 to 0.4 V

Kinetics and Mechanism of Redox Processes of Pt/C and Pt₃Co/C Cathode Electrocatalysts in a Polymer Electrolyte Fuel Cell during an Accelerated Durability Test

The degradation of Pt electrocatalysts in membrane electrode assemblies (MEAs) of polymer electrolyte fuel cells under working conditions is a serious problem for their practical use. Here we report the kinetics and mechanism of redox reactions at the surfaces of Pt/C and Pt₃Co/C cathode electrocatalysts during catalyst degradation processes by an accelerated durability test (ADT) studied by operando time-resolved X-ray absorption fine structure (XAFS) spectroscopy. Systematic analysis of a series of Pt L_III-edge time-resolved XAFS spectra measured every 100 ms at different degradation stages revealed changes in the kinetics of Pt redox reactions on Pt/C and Pt₃Co/C cathode electrocatalysts. In the case of Pt/C, as the number of ADT cycles increased, structural changes for Pt redox reactions (charging, surface, and subsurface oxidation) became less sensitive because of the agglomeration of catalyst particles. It was found that their rate constants were almost constant independent of the agglomeration of the Pt electrocatalyst. On the other hand, in the case of Pt₃Co/C, the rate constants of the redox reactions of the cathode electrocatalyst gradually reduced as the number of ADT cycles increased. The differences in the kinetics for the redox processes would be differences in the degradation mechanism of these cathode electrocatalysts