Geunjae Kwak

Wettability control of ZnO nanoparticles for universal applications.

Herein, a facile approach for the fabrication of a superhydrophobic nanocoating through a simple spin-coating and chemical modification is demonstrated. The resulting coated surface displayed a static water contact angle of 158° and contact angle hysteresis of 1°, showing excellent superhydrophobicity. The surface wettability could be modulated by the number of ZnO nanoparticle coating cycles, which in turn affected surface roughness. Because of its surface-independent characteristics, this method could be applicable to a wide range of substrates including metals, semiconductors, papers, cotton fabrics, and even flexible polymer substrates. This superhydrophobic surface showed high stability in thermal and dynamic conditions, which are essential elements for practical applications. Furthermore, the reversible switching of wetting behaviors from the superhydrophilic state to the superhydrophobic state was demonstrated using repeated chemical modification/heat treatment cycles of the coating films.

Chemically Modified Superhydrophobic WOx Nanowire Arrays and UV Photopatterning

A facile route is reported for the fabrication of superhydrophobic tungsten oxide (WO(x)) nanowire surfaces through the chemical adsorption of alkyltrichlorosilane with a static water contact angle (CA) of 163.5 degrees. It is confirmed that CAs on the superhydrophobic surface decreased gradually under UV illumination because of the UV-assisted decomposition of alkyltrichlorosilane chemically adsorbed onto the surface. Superhydrophobic-superhydrophilic switching is also demonstrated by alternating self-assembled monolayer deposition and UV irradiation on the photopatterned nanowire surfaces. Furthermore, the superhydrophobic surface could be transformed selectively into a hydrophilic state by simply exposing the surface to UV through a shadow mask. These studies provide a relatively simple strategy for the design of superhydrophobic surfaces.

Multifunctional transparent ZnO nanorod films.

Transparent ZnO nanorod (NR) films that exhibit extreme wetting states (either superhydrophilicity or superhydrophobicity through surface chemical modification), high transmittance, UV protection and antireflection have been prepared via the facile ammonia hydrothermal method. The periodic 1D ZnO NR arrays showed extreme wetting states as well as antireflection properties due to their unique surface structure and prevented the UVA region from penetrating the substrate due to the unique material property of ZnO. Because of the simple, time-efficient and low temperature preparation process, ZnO NR films with useful functionalities are promising for fabrication of highly light transmissive, antireflective, UV protective, antifogging and self-cleaning optical materials to be used for optical devices and photovoltaic energy devices.

Impact dynamics of water droplets on chemically modified WOx nanowire arrays

The effects of surface energy on the wetting transition for impinging water droplets were investigated on the chemically modified WOx nanowire surfaces. We could modify the surface energy of the nanowires through chemisorption of alkyltrichlorosilanes with various carbon chain lengths and also by the ultraviolet-enhanced decomposition of self assembled monolayer molecules. Three surface wetting states could be identified through the balance between antiwetting and wetting pressures. This approach establishes a simple strategy for design of the water-repellent surface to impinging droplets.

Wettability control and water droplet dynamics on SiC-SiO2 core-shell nanowires.

We present a simple method for fabricating superhydrophobic SiC-SiO(2) core-shell nanowire surfaces via the facile dip-coating of alkyltrichlorosilanes. Water droplets displayed a variety of shapes with varying surface energies on the nanowire surfaces, which could be modified through chemisorption of alkyltrichlorosilanes with variable carbon chain length. The effects of UV irradiation on the superhydrophobic nanowire arrays were also investigated. UV light efficiently decomposed the chemisorbed molecules, and the superhydrophobic surface gradually converted into a hydrophilic surface with increasing UV exposure. The water droplet impact behavior on the modified surfaces was studied to test the stability of the superhydrophobicity under dynamic conditions.

Enhanced catalytic activity of cobalt catalysts for Fischer–Tropsch synthesis via carburization and hydrogenation and its application to regeneration

In Fischer–Tropsch synthesis (FTS), cobalt carbide (Co2C) is not a catalytically active material, but rather an undesired cobalt phase associated with low catalytic performance. It is known that Co2C can be easily transformed back to metal cobalt in a H2 environment at 220 °C. The transformed metal cobalt (hcp phase) even shows higher catalytic activity in low-temperature FTS, compared with the reduced cobalt metal from the cobalt oxide species. In this study, to obtain Co2C with high catalytic activity in FTS, we determined the optimum conditions for effective metal cobalt carburization and Co2C hydrogenation by monitoring the phase transformation of cobalt using X-ray absorption spectroscopy (XAS) and temperature-programmed hydrogenation (TPH). We also verified that the transitions effectively occur under the same conditions as those for FTS (2.0 MPa, 220 °C). Based on the conditions determined for the transitions, the deactivated cobalt catalyst can be completely regenerated in the FT reactor by simply altering the injected gases from syngas to CO and then H2. Moreover, the regenerated catalyst shows enhanced catalytic performance compared with the fresh catalyst. The selective formation of hcp cobalt metal via carburization and hydrogenation of the spent catalyst was found to be the key for both the improved catalytic activity and the effective regeneration in situ. As a result, the formation of Co2C, which is mainly considered a nuisance, could provide valuable applications in investigations into catalyst activation and regeneration in FTS.

A study on the dynamic behaviors of water droplets impacting nanostructured surfaces

We have investigated the influence of impact velocity and intrinsic surface wettability of nanostructures on the impact dynamic behaviors of water droplets on nanostructure surfaces. Nanowires array surfaces with tunable wettabilities ranging from superhydrophilic to superhydrophobic were fabricated by the deposition of surface modifiers differing in alkyl chain length. The transition criteria of rebound/wetting state and rebound/splashing state based on the relationship between the Webber (We) number and the surface free energy were determined. We have confirmed that the critical We number that determines the transition of the rebound/wetting increased as surface energy decreased. Additionally, the We number at which fragmentation occurred on our superhydrophobic surface was relatively low compared to previously reported values.

Coke-Tolerant Gadolinium Promoted HZSM-5 catalyst for Methanol Conversion into Hydrocarbons

The role of Gd as a coke inhibitor was studied for the methanol‐to‐hydrocarbons reaction. It was revealed that thin films of gadolinium oxide covered the ZSM‐5 crystals and that the Gd atoms that were uniformly dispersed on the surface enhanced the basicity of the parent ZSM‐5. These synergetic effects improved the durability of the acidic function and diminished the growth of coke precursors, which resulted in an increased lifetime of the catalyst.

Direct confinement of Ru nanoparticles inside nanochannels of large pore mesoporous aluminosilicate for Fischer–Tropsch synthesis

Metal/ordered mesoporous aluminosilicates (OMAS) have received great attention as bifunctional Fischer–Tropsch (FT) catalysts that directly convert syngas into liquid fuels. However, both synthesis of OMAS with large pores and efficient pore confinement of metal nanoparticles still remain challenging. Here, we report a simple method to synthesize Ru nanoparticles confined in the nanochannels of OMAS (Ru@OMAS). This method eliminates laborious multi-processes that are typically required for pore confinement of metal nanoparticles. We prepare three types of Ru@OMAS with different Si/Al molar ratios (denoted as Si/Al-x, x = 10, 30, and 50) having the same large pore size (∼30 nm) and Ru NP loading (3 wt%). Changing the Si/Al ratio strongly affects the number/strength of acid sites and the metal–support interaction, thereby mediating the catalytic activity and product selectivity. With increasing Al content (decreasing Si/Al ratio), supports acidity and metal–support interactions increase, whereas the reducibility of Ru decreases significantly. As a consequence, among the Si/Al-x catalysts, the Si/Al-50 shows the highest selectivity (63.6%) for liquid fuels (C5–C20) and excellent FT activity (CO conversion of 47.8%) due to its mild acidity and relatively good reducibility.

Maximum production of methanol in a pilot-scale process

Mathematical models for both bench- and pilot-scale methanol synthesis reactors were developed by estimating the overall heat transfer coefficients due to different heat transfer characteristics, while the effectiveness factor was fixed because the same catalysts were used in both reactors. The overall heat transfer coefficient of a pilot-scale reactor was approximately twice that of a bench-scale reactor, while the estimate from the correlation reported for the heat transfer coefficient was 1.8-times higher, indicating that the values determined in the present study are effective. The model showed that the maximum methanol production rate of approximately 16 tons per day was achievable with peak temperature maintained below 250 °C in the open-loop case. Meanwhile, when the recycle was used to prevent the loss of unreacted gas, peak temperature and production rate decreased due to low CO and CO2 fraction in the recycled stream at the same space velocity as the open-loop operation. Further analysis showed that, since the reaction was in the kinetic regime, the production rate could be maximized up to 18.7 tons per day by increasing the feed flowrate and inlet temperature despite thermodynamically exothermic reaction.