Sun Hee Choi

Thermal crosslinking of PBI/sulfonated polysulfone based blend membranes

Crosslinked polybenzimidazole (PBI) membranes are most often obtained by reacting the nitrogen atoms of PBI with an alkylating agent. These links can be attacked by nucleophiles at elevated temperatures. To avoid N–CH2-links we introduce a new method to crosslink PBI, starting from ionically crosslinked acid/base blend membranes. By heating them to temperatures above say 200 °C, a Friedel–Crafts reaction between sulfonic acid groups and electron rich phenyl groups covalently crosslinks the acid and base components in the blend by chemically stable aromatic sulfone bonds. According to the literature pure PBI can also be cured and a radical mechanism involving air was suggested. We show that PBI can also be cured in an inert atmosphere. We propose that the thermal curing of pure PBI, which necessitates slightly higher temperatures than blend membranes, proceeds via hydrolysis of imidazole to –COOH and diamine, followed by a Friedel–Crafts reaction of the acid. While crosslinks cannot be directly analysed by nmr or IR, our data support the mentioned mechanism. We show the effect of curing temperature and time on membrane properties like solubility, phosphoric acid uptake and mechanical properties, and test a membrane in a fuel cell, proving that the membranes are gas tight and show a good performance.

Experimental and computational studies of formic acid dehydrogenation over PdAu: influence of ensemble and ligand effects on catalysis

The critical role of the ligand effect and ensemble effect in enhancing formic acid (FA) dehydrogenation over PdAu catalysts was highlighted by both experimental and theoretical studies. FA dehydrogenation energy was calculated by DFT on PdAu model catalysts of different surface atomic arrangements. The Pd3Au1 surface exhibited the lowest reaction energy and kinetic barrier for FA dehydrogenation among four different PdAu surfaces. The Pd trimer played a critical role in stabilizing reaction intermediates. The experimental FA dehydrogenation activity of three different PdAu catalysts supported the theoretical results. In addition, the electronic interaction between the surface and subsurface layers also proved to contribute to the improved catalytic activity of PdAu catalysts via modification of Pd d states.

Atomically dispersed Cu on Ce1-xRExO2-δ nanocubes (RE = La and Pr) for water gas shift: influence of OSC on catalysis

To elucidate the effect of CeO2 shape and doping on activity, Cu0.02Ce0.98O2−δ and Cu0.02Ce0.86RE0.12O2−δ (RE = La and Pr) were synthesized by a molecular precursor approach. The materials showed distinct activities depending on the shape and composition of CeO2, which was well correlated with their different oxygen storage capacities.

Hydrodesulfurization of Diesel for Molten Carbonate Fuel Cell Applications

Hydrogen production from commercial diesel fuels is an attactive option for energy generation purpose due to the low cost and good availability of diesel fuels. However, in order to utilize commercial diesel fuels, the sulfur contents must be removed down to approximately 0.1 ppm level to protect the fuel cell catalysts from poisoning. Commercial catalysts CoMo/Al2O3 and NiMo/Al2O3 were tested for HDS (Hydrodesulfurization) of model diesel and commercial diesel. The experimental conditions were 250 - 400 °C and LHSV (Liquid Hourly Space Velocity) 0.27 - 2.12 hr -1 . NiMo/Al2O3 was found to be more effective than CoMo/Al2O3 in removing sulfur from model diesel. Based on the experimental results of model diesel, commercial diesel fuel purchased from a local petrol station was tested for HDS using NiMo/Al2O3. The GC-SCD (Gas Chromatography Sulfur Chemiluminescence Detector) results showed that the DMDBT (Dimethyldibenzothiophene) derivatives were fully removed from the commercial diesel fuel proving that HDS with NiMo/Al2O3 is technically feasible for industrial applications.