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)

Synthesis, characterization, and magnetic properties of monodisperse CeO2 nanospheres prepared by PVP-assisted hydrothermal method

Abstract Ferromagnetism was observed at room temperature in monodisperse CeO2 nanospheres synthesized by hydrothermal treatment of Ce(NO3)3·6H2O using polyvinylpyrrolidone as a surfactant. The structure and morphology of the products were characterized by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy, high-resolution transmission electron microscopy, and field-emission scanning electron microscopy (FE-SEM). The optical properties of the nanospheres were determined using UV and visible spectroscopy and photoluminescence (PL). The valence states of Ce ions were also determined using X-ray absorption near edge spectroscopy. The XRD results indicated that the synthesized samples had a cubic structure with a crystallite size in the range of approximately 9 to 19 nm. FE-SEM micrographs showed that the samples had a spherical morphology with a particle size in the range of approximately 100 to 250 nm. The samples also showed a strong UV absorption and room temperature PL. The emission might be due to charge transfer transitions from the 4f band to the valence band of the oxide. The magnetic properties of the samples were studied using a vibrating sample magnetometer. The samples exhibited room temperature ferromagnetism with a small magnetization of approximately 0.0026 to 0.016 emu/g at 10 kOe. Our results indicate that oxygen vacancies could be involved in the ferromagnetic exchange, and the possible mechanism of formation was discussed based on the experimental results.</jats:p

The role of Ce addition in catalytic activity enhancement of TiO2-supported Ni for CO2 methanation reaction

In this work, various amounts of Ce were added to TiO2 to form a mixed oxide support; CexTi1−xO2 (x = 0, 0.003, 0.05, 0.10 and 0.15) and then those synthesized supports were impregnated by 10 wt% Ni to produce a catalysts. The 10 wt% Ni–CexTi1−xO2 (x = 0, 0.003, 0.05, 0.10 and 0.15) catalysts were tested for CO2 methanation reaction by using a fixed-bed reactor in the temperature range of 100–500 °C. The sample was pretreated at 450 °C under H2 and then a mixed feed gas of CO2 and H2 was switched into the reactor to start the reaction. The results showed that 10 wt% Ni–Ce0.003Ti0.997O2 catalyst (the lowest Ce content) exhibited the highest CO2 conversion and CH4 yield. Moreover, 10 wt% Ni–Ce0.003Ti0.997O2 showed highly stable during the stability test (50 h.). The results indicated that upon addition of small amount of Ce into TiO2-supported Ni, the surface, structural, electrical and redox properties of the catalyst were improved to the extent that these properties can promote the catalytic activities for CO2 methanation. The Ce addition can improve the CO2 methanation catalytic activity by several ways. First, higher dispersion of Ni on catalysts surface upon addition of Ce was observed which resulted in higher adsorption rate of H2 on this metal active site. Second, formation of a larger amounts of oxygen vacancies as well as basicity improvement upon addition of Ce were occurred which can increase the CO2 adsorption on catalyst surface. Third, incorporation of Ce resulted in improving of a starting reduction temperature of Ni2+ to Ni0 for TiO2-supported Ni catalyst which can indicate that the reducibility of Ce-doped TiO2-supported Ni catalyst was enhanced and then alter its catalytic activity. However, increasing of Ce content led to lowering of CO2 methanation activities which resulted from increasing of basicity by Ce addition. The excess amounts of adsorbed CO2 would lead to competitive adsorption to H2 and then lead to a decrease of catalytic activity. Therefore, an appropriate amount of H2 and CO2 adsorption ability on catalyst surface was a prominent factor to dominate the catalytic activity

Tuning interactions of surface‐adsorbed species over Fe−Co/K−Al2O3 catalyst by different K contents: selective CO2 hydrogenation to light olefins

Selective CO2 hydrogenation to light olefins over Fe−Co/K−Al2O3 catalysts was enhanced by tuning bonding strengths of adsorbed species by varying the content of the K promotor. Increasing the K/Fe atomic ratio from 0 to 0.5 increased the olefins/paraffins (O/P) ratio by 25.4 times, but then slightly raised upon ascending K/Fe to 2.5. The positive effect of K addition is attributed to the strong interaction of H adsorbed with the catalyst surface caused by the electron donor from K to Fe species. Although the Fe−Co/K−Al2O3 catalyst with K/Fe=2.5 reached the highest O/P ratio of 7.6, the maximum yield of light olefins of 16.4 % was achieved by the catalyst promoted with K/Fe of 0.5. This is explained by the considerable reduction of amount of H2 adsorbed on the catalyst surface with K/Fe=2.5

Electrochemical determination of diethylstilbestrol by using a magnetic nanoparticle/graphene composite film electrode

Abstract The magnetic nanoparticle-graphene modified magnetic carbon electrode (MCE) was fabricated and the electrochemical effect of diethylstilbestrol (DES) on this electrode was studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The factors that affected the performance of the sensor were also optimized. The experimental results exhibited good electrocatalytic activity and a fast response for the electrochemical oxidation of DES. The linear range for the determination of diethylstilbestrol was 1 to 100 μM and the detection limits were 1.06 × 10−7 M based on S/N = 3.</jats:p

Disposable Electrochemical Sensor for Food Colorants Detection by Reduced Graphene Oxide and Methionine Film Modified Screen Printed Carbon Electrode

A facile synthesis of reduced graphene oxide (rGO) and methionine film modified screen printed carbon electrode (rGO-methionine/SPCE) was proposed as a disposable sensor for determination of food colorants including amaranth, tartrazine, sunset yellow, and carminic acid. The fabrication process can be achieved in only 2 steps including drop-casting of rGO and electropolymerization of poly(L-methionine) film on SPCE. Surface morphology of modified electrode was studied by scanning electron microscopy (SEM). This work showed a successfully developed novel disposable sensor for detection of all 4 dyes as food colorants. The electrochemical behavior of all 4 food colorants were investigated on modified electrodes. The rGO-methionine/SPCE significantly enhanced catalytic activity of all 4 dyes. The pH value and accumulation time were optimized to obtain optimal condition of each colorant. Differential pulse voltammetry (DPV) was used for determination, and two linear detection ranges were observed for each dye. Linear detection ranges were found from 1 to 10 and 10 to 100 µM for amaranth, 1 to 10 and 10 to 85 µM for tartrazine, 1 to 10 and 10 to 50 µM for sunset yellow, and 1 to 20 and 20 to 60 µM for carminic acid. The limit of detection (LOD) was calculated at 57, 41, 48, and 36 nM for amaranth, tartrazine, sunset yellow, and carminic acid, respectively. In addition, the modified sensor also demonstrated high tolerance to interference substances, good repeatability, and high performance for real sample analysis.</jats:p

Disposable Electrochemical Sensor for Food Colorants Detection by Reduced Graphene Oxide and Methionine Film Modified Screen Printed Carbon Electrode

A facile synthesis of reduced graphene oxide (rGO) and methionine film modified screen printed carbon electrode (rGO-methionine/SPCE) was proposed as a disposable sensor for determination of food colorants including amaranth, tartrazine, sunset yellow, and carminic acid. The fabrication process can be achieved in only 2 steps including drop-casting of rGO and electropolymerization of poly(L-methionine) film on SPCE. Surface morphology of modified electrode was studied by scanning electron microscopy (SEM). This work showed a successfully developed novel disposable sensor for detection of all 4 dyes as food colorants. The electrochemical behavior of all 4 food colorants were investigated on modified electrodes. The rGO-methionine/SPCE significantly enhanced catalytic activity of all 4 dyes. The pH value and accumulation time were optimized to obtain optimal condition of each colorant. Differential pulse voltammetry (DPV) was used for determination, and two linear detection ranges were observed for each dye. Linear detection ranges were found from 1 to 10 and 10 to 100 µM for amaranth, 1 to 10 and 10 to 85 µM for tartrazine, 1 to 10 and 10 to 50 µM for sunset yellow, and 1 to 20 and 20 to 60 µM for carminic acid. The limit of detection (LOD) was calculated at 57, 41, 48, and 36 nM for amaranth, tartrazine, sunset yellow, and carminic acid, respectively. In addition, the modified sensor also demonstrated high tolerance to interference substances, good repeatability, and high performance for real sample analysis