Won Choon Choi

Modification of proton conducting membrane for reducing methanol crossover in a direct-methanol fuel cell

Abstract In order to reduce methanol cross-over to the positive electrode in a direct-methanol fuel cell (DMFC), the concept of modifying the morphology of the proton-conducting membrane is proposed. The method involves using plasma etching and palladium-sputtering on a Nafion™ polymer membrane. Methanol permeability tests were conducted in a specially-designed cell. Plasma etching of Nafion™ membrane increases the roughness of the membrane surface and decreases the methanol permeability. The sputtering of palladium on the plasma-etched Nafion™ further decreases the methanol cross-over. Improved DMFC performance curves are obtained in a single cell which contains the modified Nafion™.

Quaternary Pt-based electrocatalyst for methanol oxidation by combinatorial electrochemistry

Abstract The demonstration to apply the combinatorial method using a repeated cyclic voltammetry is reported to find the anodic material for DMFC that shows a higher electrocatalytic activity and that can replace a portion of precious metals with cheap ones. The activity of newly found electrocatalyst whose composition was determined through high-throughput screening was compared with that of commercially available Johnson–Matthey Pt(50)Ru(50). It was found Pt(77)Ru(17)Mo(4)W(2) was more active and stable than Pt(50)Ru(50) in methanol electro-oxidation. The repeated cyclic voltammetry makes the combinatorial method expand into a screening tool to find the electrocatalyst that not only showing an initial excellent performance but also being active in the long-run reaction.

Balancing acidity and basicity for highly selective and stable modified MgO catalysts in the alkylation of phenol with methanol

Abstract The alkylation of phenol was investigated over magnesium oxide catalysts modified with the addition of small amount of vanadium, manganese or other components. The catalytic performance was strongly dependent on the kinds of dopants due to changes in the acid–base properties. The modified MgO catalysts showed an activity higher than pure MgO catalyst. V–Mn–MgO catalyst was found to be more active and selective than that of V–Mgo or Mn–MgO catalyst for 2,6-xylenol. Among the modified catalysts, V–Mn–MgO catalyst was excellent in both phenol conversion and the selectivity to 2,6-xylenol. The alkylation of phenol in ortho position was explained in the acid–base property of mixed oxide. Phenol conversion and 2,6-xylenol selectivity increased with acidity, whereas the selectivity to o-cresol decreased because of the nature of the series reaction (phenol→o-cresol→2,6-xylenol→2,4,6 - trimethyl phenol (2,4,6-TMP)). Na–Mn–MgO catalyst prepared to achieve a balance between acidity and basicity was not deactivated and highly selective to o-cresol and 2,6-xylenol. Reaction temperature, feed composition and space velocity also significantly influenced the conversion and the selectivity in the alkylation of phenol.

Development of the Al2O3-supported NaNO3-Na2Mg(CO3)2 sorbent for CO2 capture with facilitated sorption kinetics at intermediate temperatures

For the development of a dry solid sorbent having quite fast CO2 sorption kinetics in an intermediate temperature range of 245–300 °C to be applicable to a riser-type fluidized bed carbonator, samples of Al2O3-supported MgCO3 (1.2 mmol/g) promoted with different molar amounts of Na2CO3 (1.2, 1.8 mmol/g) and/or NaNO3 (0.6 mmol/g) were prepared by incipient wetness pore volume impregnation. For a reference, an unsupported bulk phase sorbent of NaNO3-Na2Mg(CO3)2 was also prepared. From the sorption reaction using a gas mixture containing CO2 by 2.5–10% at 1 bar for the sorbents after their activation to MgO, Al2O3-supported sorbents were featured by their rapid carbonation kinetics in contrast to the unsupported sorbent showing a quite slow carbonation behavior. The addition of Na2CO3 to the MgCO3/Al2O3 sorbent made MgO species more reactive for the carbonation, bringing about a markedly enhanced kinetic rate and conversion, as compared with the unpromoted MgCO3/Al2O3 sorbent having a small negligible reactivity. The addition of NaNO3 to MgCO3/Al2O3 or to Na2CO3-MgCO3/Al2O3 induced the same promotional effects, but to a lesser magnitude, as observed for the Na2CO3 addition. It was also characteristic for all these MgCO3-based sorbents that initial carbonation conversions with time appeared as sigmoid curves. For the Al2O3-supported sorbent comprised of NaNO3, Na2CO3, and MgCO3 by 0.6, 1.8, and 1.2 mmols, respectively, per gram sorbent, showing the best kinetic performance, a kinetic equation capable of reflecting such sigmoid conversion behavior was established, and its applicability to a riser carbonator was examined throughout a simple model calculation based on the kinetics obtained.

Effects of crystallinity of ZSM-5 zeolite on para -selective tert-butylation of ethylbenzene

Abstract Highly crystalline ZSM-5 zeolites are important for para-selective alkylation of alkyl aromatics, because they carry few external acid sites for isomerization of p-dialkyl products. Such zeolites (Si/Al = 25, 50, and 75) were synthesized in a fluoride medium between pH 4 and 6. Their crystallinities, crystal sizes, and surface areas were higher than those of a commercial ZSM-5 zeolite. Their para selectivities in alkylation were tested for vapor-phase tert-butylation of ethylbenzene between 200 and 400 °C. As expected, all the catalysts showed more than 90% para selectivity. At 300 °C, ethylbenzene conversion decreased in the order ZSM-5(25, commercial) > ZSM-5(25) > ZSM-5(50) > ZSM-5(75). The catalysts had weak, medium, and strong acid sites, but all the acid sites of ZSM-5(75) were weaker than those of ZSM-5(25) and ZSM-5(50). The high activity of commercial ZSM-5 was caused by its strong acid sites being stronger than those of the synthesized zeolites. Although the activity of the commercial catalyst was higher than those of the present catalysts, the selectivity for 4-t-butylethylbenzene (4-t-BEB) was low. The optimum feed ratio (ethylbenzene:t-butyl alcohol) was 2:1 and the feed rate was 1.65 h−1 for high ethylbenzene conversion and 4-t-BEB selectivity. Time-on-stream studies showed slow catalyst deactivation. Highly crystalline ZSM-5 zeolites are therefore better than a commercial zeolite for para-selective alkylation of alkyl aromatics. They do not require much post-modification for high para selectivity. A fluoride medium is therefore better than an alkaline medium for obtaining highly crystalline para-selective ZSM-5 zeolites.

86 Improving direct methanol fuel cell performance by using combinatorial electrochemistry and highly porous carbon support

In order to improve the performance of direct-methanol fuel cell (DMFC) performance, the combinatorial method using a repeated cyclic voltammetry was used to optimize the metal composition for the anodic material and the highly porous carbon. which showed the higher Pt dispersion, was investigated using various characterization tools. The activity of newly-developed electrocatalyst, Pt(77)Ru(17)Mo(4)W(2), whose composition was determined through high-throughput screening was more active and stable than Pt(50)Ru(50) in methanol electrooxidation. The Pt particles supported on highly porous carbons (Cgl/USY-1 and Csu/MCM-48-2) synthesized with USY zeolite and MCM-48 silica as a template showed the larger electrochemical active surface areas and the higher Pt dispersions, compared to Pt particles supported on Vulcan XC-72R carbon.