Oh-Shim Joo

Migration and reduction of formate to form methanol on CuZnO catalysts

Abstract Temperature-programmed decomposition (TPD) and temperature-programmed hydrogenation (TPH) experiments were performed on copper and zinc formates. Formic acid was found to be the only organic product from the copper formate reactions. Methanol and methyl formate were found to be the major organic products from the zinc formate reactions. When ZnO was mixed with copper formate, the yields of methanol and methyl formate increased. Temperature-programmed IR (TPIR) spectroscopy of the copper formate/ZnO indicated the migration of the formate from copper to ZnO in the temperature range below 430 K at which the decomposition of the copper formate began. The experimental results elucidated the synergistic effect between copper and ZnO; formate migration onto ZnO and its hydrogenation to methanol on ZnO. Based on the synergistic effect, a reaction mechanism for methanol synthesis from the hydrogenation of carbon oxides in Cu ZnO catalysts was proposed.

Effect of the Si/TiO2/BiVO4 Heterojunction on the Onset Potential of Photocurrents for Solar Water Oxidation

BiVO4 has been formed into heterojunctions with other metal oxide semiconductors to increase the efficiency for solar water oxidation. Here, we suggest that heterojunction photoanodes of Si and BiVO4 can also increase the efficiency of charge separation and reduce the onset potential of the photocurrent by utilizing the high conduction band edge potential of Si in a dual-absorber system. We found that a thin TiO2 interlayer is required in this structure to realize the suggested photocurrent density enhancement and shifts in onset potential. Si/TiO2/BiVO4 photoanodes showed 1.0 mA/cm(2) at 1.23 V versus the reversible hydrogen electrode (RHE) with 0.11 V (vs RHE) of onset potential, which were a 3.3-fold photocurrent density enhancement and a negative shift in onset potential of 300 mV compared to the performance of FTO/BiVO4 photoanodes.

Structural change of Cu/ZnO by reduction of ZnO in Cu/ZnO with methanol

The reducibility of ZnO was investigated in the temperature range of 523–623 K in a stream of a reducing agent such as H2, CO, and methanol. ZnO was reduced only in the presence of copper in the vicinity of ZnO with CO and methanol, but it was not reduced with H2. Methanol was a stronger reducing agent in the reduction of ZnO than CO, while CO was stronger in the reduction of CuO than methanol. Two types of brass were observed resulting from the reduction of ZnO in the Cu/ZnO sample by XRD. Zanghengite brass started to be formed at 573 K in addition to α-brass which was observed at the temperature above 523 K in the temperature range of 523–623 K during the ZnO reduction with methanol. The carbon monoxide chemisorption showed that the copper surface areas decreased during the reduction of ZnO with methanol.

Low temperature chemically synthesized rutile TiO2 photoanodes with high electron lifetime for organic dye-sensitized solar cells

Electron lifetime in mesoporous nanostructured rutile TiO2 photoanodes, synthesized via a simple, cost-effective, low temperature (50-55 °C) wet chemical process, annealed at 350 °C for 1 h and not employing any sprayed TiO2 compact layer, was successfully tailored with 0.2 mM TiCl4 surface treatment that resulted in light to electric power conversion efficiency up to 4.4%.

Cobalt Ferrite Nanocrystallites for Sustainable Hydrogen Production Application

Cobalt ferrite, CoFe2O4, nanocrystalline films were deposited using electrostatic spray method and explored in sustainable hydrogen production application. Reflection planes in X-ray diffraction pattern confirm CoFe2O4 phase. The surface scanning microscopy photoimages reveal an agglomeration of closely-packed CoFe2O4 nanoflakes. Concentrated solar-panel, a two-step water splitting process, measurement technique was preferred for measuring the hydrogen generation rate. For about 5 hr sustainable, 440 mL/hr, hydrogen production activity was achieved, confirming the efficient use of cobalt ferrite nanocrystallites film in hydrogen production application.

Facile preparation of nanostructured α-Fe2O3 thin films with enhanced photoelectrochemical water splitting activity

We report on the use of a facile electrospray technique for the synthesis of α-Fe2O3 thin films on a FTO substrate for photoelectrochemical (PEC) water splitting. The effect of synthesis parameters such as substrate temperature, discharge potential and post-heat treatment on morphology, particle size and PEC performance of α-Fe2O3 films were investigated. With an increase in substrate temperature, the surface morphology of the α-Fe2O3 film was altered from a packed worm-like surface to highly porous nanostructures. XRD analysis revealed that the (110) grain orientation of the film was transformed to the (104) grain orientation at 300 °C, due to the oxidation of the precursor at the surface of the substrate. Raman spectroscopy and XPS analysis indicated the presence of highly pure α-Fe2O3 in the film. By changing the discharge potential, the size of particles in the film was reduced to a minimum of 23 nm. Under optimized conditions the nanostructured α-Fe2O3 films showed a water splitting photocurrent of ∼0.6 mA cm−2 at 1.23 V versus RHE under standard illumination conditions (AM 1.5 G 100 mW cm−2), and an incident photon to current efficiency (IPCE) of 13% at 350 nm (at 1.4 V versus RHE) which are among the best results obtained for undoped α-Fe2O3 photoanodes. This enhanced PEC performance can be attributed to the efficient charge separation at the α-Fe2O3-electrolyte interface due to the larger interfacial area of small-sized particles in the film. This study thus provides a simple route for the synthesis of highly active α-Fe2O3 thin films that can be extended to metal doped films such as Ti-doped α-Fe2O3.

Construction of efficient CdS–TiO2 heterojunction for enhanced photocurrent, photostability, and photoelectron lifetimes

The photoefficiency of CdS/TiO2 electrodes can be enhanced by employing efficient method of CdS sensitization from which, the contact area, thickness of CdS layer, and the recombination of photoelectrons with electrolyte can be controlled. Here, we demonstrate a simple solvothermal approach of CdS quantum dots (QDs) sensitization on TiO2 nanoparticle (NP) film coated on FTO. Our new approach prevents the clogging of CdS QDs and promotes uniform deposition of QDs throughout the mesoporous TiO2 NP film. The sensitization of CdS can be controlled by the reaction time and the concentration of the precursors. The solvothermally sensitized photoanodes exhibit enhanced photocurrents and fill factors and improved photostability in aqueous solution compared to the one prepared by a conventional SILAR method. Open-circuit potential decay measurement under shutting off illumination shows that the lifetime of photoelectron is extended with solvothermally prepared CdS layer, indicating efficient suppression of recombination of the accumulated electron in TiO2 to the electrolyte. This methodology can be applied in making more efficient heterojunctions consisting of CdS and other wide band gap oxide semiconductors which could improve charge separation and mitigate charge recombination for photoelectrochemical applications.

Deactivation of Cu/ZnO catalyst during dehydrogenation of methanol

The deactivation of Cu/ZnO catalyst during methanol dehydrogenation to form methyl formate has been studied. The Cu/ZnO catalyst was seriously deactivated under the reaction conditions: various temperatures of 493, 523 and 553 K, atmospheric pressure and methanol GHSV of 3000 ml (STP)/g-cat h. The weight loss due to reduction of ZnO in the Cu/ ZnO catalyst was monitored by a microbalance. X-ray induced Auger spectroscopy of Zn(L3M4,5M4,5) showed the increase in the concentration of metallic Zn on the catalyst surface after the reaction. Temperature-programmed reduction (TPR) of the Cu/ZnO catalyst with methanol demonstrated that the reduction of ZnO in Cu/ ZnO was suppressed by introduction of CO2 into the stream of helium-methanol. As the concentration of CO2 in the feed gas increased, the weight loss of the Cu/ZnO catalyst due to the reduction of ZnO decreased. The deactivation of the Cu/ZnO catalyst in the methanol dehydrogenation was also retarded by the addition of CO2. In particular, oxygen injection into the reactant feed regenerated the Cu/ ZnO catalyst deactivated during the reaction. Based on these observations, the cause of deactivation of the Cu/ZnO catalyst has been discussed.

Carbon Nanotube RLC Circuits

The present invention relates to carbon nanotube Resistance Inductance Capacitance (hereinafter referred as to “RLC”) circuits. More particularly, the present invention is to provide the carbon nanotube prepared by chemical vapor deposition (hereinafter referred as to “CVD”) on a surface of inorganic substrate to have advantages in: (i) its use for resistance, inductance and capacitance elements, (ii) the formation of micro circuits loaded with RLC characters and different inductor from the inductor used ferrite core and coil, (iii) heat resistance and impact resistance because it is made of carbon/inorganic composite materials, and (iv) the formation of nanotubes unlike conventional chip inductor.

Preparation of Cu/ZnO/M2O3 (M = Al, Cr) catalyst to stabilize Cu/ZnO catalyst in methanol dehydrogenation

ZnO reducibility in three component catalysts was monitored in a methanol stream by a microbalance. The ZnO in the Cu/ZnO is reduced in the methanol stream from ca 500 K, but the addition of a third component to Cu/ZnO can suppress the reduction of ZnO. The Cu/ZnO/Cr2O3 prepared from a hydrotalcite-like precursor shows high stability in methanol dehydrogenation. The high stability of Cu/ZnO/Cr2O3 is due to the stabilization of ZnO by formation of ZnCr2O4 even in the reduced condition.