EMSL: Do Heui Kim's Publications

Kim DH, J Szanyi, JH Kwak, X Wang, JC Hanson, MH Engelhard, and CHF Peden. 2009. "Effects of sulfation level on the desulfation behavior of pre-sulfated Pt BaO/Al2O3 lean NOx trap catalysts: a combined H2 Temperature-Programmed Reaction, in-situ sulfur K-edge X-ray Absorption Near-Edge Spectroscopy, X-ray Photoelectron Spectroscopy, and Time-Resolved X-ray Diffraction Study." Journal of Physical Chemistry C 113(17):7336-7341. doi:10.1021/jp900304h Abstract Desulfation by hydrogen of pre-sulfated Pt(2wt%) BaO(20wt%)/Al2O3 with various sulfur loading (S/Ba = 0.12, 0.31 and 0.62) were investigated by combining H2 temperature programmed reaction (TPRX), x-ray photoelectron spectroscopy (XPS), in-situ sulfur K-edge x-ray absorption near-edge spectroscopy (XANES), and synchrotron time-resolved x-ray diffraction (TR-XRD) techniques. We find that the amount of H2S desorbed during the desulfation in the H2 TPRX experiments is not proportional to the amount of initial sulfur loading. The results of both in-situ sulfur K-edge XANES and TR-XRD show that at low sulfur loadings, sulfates were transformed to a BaS phase and remained in the catalyst, rather than being removed as H2S. On the other hand, when the deposited sulfur level exceeded a certain threshold (at least S/Ba = 0.31) sulfates were reduced to form H2S, and the relative amount of the residual sulfide species in the catalyst was much less than at low sulfur loading. Unlike samples with high sulfur loading (e.g., S/Ba = 0.62), H2O did not promote the desulfation for the sample with S/Ba of 0.12, implying that the formed BaS species originating from the reduction of sulfates at low sulfur loading are more stable to hydrolysis. The results of this combined spectroscopy investigation provide clear evidence to show that sulfates at low sulfur loadings are less likely to be removed as H2S and have a greater tendency to be transformed to BaS on the material, leading to the conclusion that desulfation behavior of Pt BaO/Al2O3 lean NOx trap catalysts is markedly dependent on the sulfation levels.

Kwak JH, JZ Hu, D Mei, CWW Yi, DH Kim, CHF Peden, L Allard, and J Szanyi. 2009. "Coordinatively unsaturated Al3+ centers as binding sites for active catalyst phases on γ-Al2O3." Science 325(5948):1670-1673. doi:10.1126/science.1176745 Abstract A combination of ultrahigh resolution spectroscopy and microscopy techniques (ultrahigh magnetic field solid state magic angle spinning nuclear magnetic resonance (MAS-NMR) and high-resolution scanning transmission electron microscopy (HR-STEM)) coupled with first principles DFT calculations reveal the nature of anchoring sites of a catalytically active phase onto the surface of γ-Al2O3. The results obtained unambiguously prove that coordinatively unsaturated penta-coordinate Al3+ (Al3+penta) centers present on the (100) facets of the γ-Al2O3 surface are the sites where the anchoring of Pt occurs. At low loadings, the active catalytic phase is atomically dispersed on the support surface (Pt/ Al3+penta=1), while two dimensional Pt rafts form at higher coverages.

Kwak JH, D Mei, CWW Yi, DH Kim, CHF Peden, L Allard, and J Szanyi. 2009. "Understanding the nature of surface nitrates in BaO/gamma-Al2O3 NOx storage materials: A combined experimental and theoretical study ." Journal of Catalysis 261(1):17-22. Abstract The special role of the interface between the active catalytic phase (metal or metal oxide) and the oxide support in determining the properties of practical catalysts has long been recognized; however, it is still very poorly understood in most systems

Lin SSY, DH Kim, and SY Ha. 2009. "Metallic phases of cobalt-based catalysts in ethanol steam reforming: The effect of cerium oxide." Applied Catalysis. A, General 355(1-2):69-77. Abstract The catalytic activity of cobalt in the production of hydrogen via ethanol steam reforming has been investigated in its relation to the crystalline structure of metallic cobalt. At a reaction temperature of 350 8C, the specific hydrogen production rates show that hexagonal close-packed (hcp) cobalt possesses higher activity than face-centered cubic (fcc) cobalt. However, at typical reaction temperatures (400– 500 8C) for ethanol steam reforming, hcp cobalt is transformed to less active fcc cobalt, as confirmed by in situ X-ray diffractometry (XRD). The addition of CeO2 promoter (10 wt.%) stabilizes the hcp cobalt structure at reforming temperatures up to 600 8C. Moreover, during the pre-reduction process, CeO2 promoter prevents sintering during the transformation of Co3O4 to hcp cobalt. Both reforming experiments and in situ diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that the surface reactions were modified by CeO2 promoter on 10% Ce–Co (hcp) to give a lower CO selectivity and a higher H2 yield as compared with the unpromoted hcp Co.

Mei D, Q Ge, JH Kwak, DH Kim, CM Verrier, J Szanyi, and CHF Peden. 2009. "Characterization of Surface and Bulk Nitrates of γ-Al2O3-Supported Alkaline Earth Oxides using Density Functional Theory." Physical Chemistry Chemical Physics. PCCP 11(18):3380-3389. doi:10.1039/b819347a Abstract “Surface" and "bulk" nitrates formed on a series of alkaline earth oxides (AEOs), AE(NO3)2, were investigated using first-principles density functional theory calculations. The formation of these surface and bulk nitrates was modeled by the adsorption of NO2+NO3 pairs on gamma-Al2O3-supported monomeric AEOs (MgO, CaO, SrO, and BaO) and on the extended AEO(001) surfaces, respectively. The calculated vibrational frequencies of the surface and bulk nitrates based on our proposed models are in good agreement with experimental measurements of AEO/gamma-Al2O3 materials after prolonged NO2 exposure. This indicates that experimentally observed "surface" nitrates are most likely formed with isolated two dimensional (including monomeric) AEO clusters on the gamma-Al2O3 substrate, while the "bulk" nitrates are formed on exposed (including (001)) surfaces (and likely in the bulk as well) of large three dimensional AEO particles supported on the gamma-Al2O3 substrate. Also in line with the experiments, our calculations show that the low and high frequency components of the vibrations for both surface and bulk nitrates are systematically red shifted with the increasing basicity and cationic size of the AEOs. The adsorption strengths of NO2+NO3 pairs are nearly the same for the series of alumina-supported monomeric AEOs, while the adsorption strengths of NO2+NO3 pairs on the AEO surfaces increase in the order of MgO < CaO < SrO ~ BaO. Compared to the NO2+NO3 pair that only interacts with monomeric AEOs, the stability of NO2+NO3 pairs that interact with both the monomeric AEO and the gamma-Al2O3 substrate is enhanced by about 0.5 eV. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.

Kim DH, YH Chin, JH Kwak, and CHF Peden. 2008. "Promotional Effects of H2O Treatment on NOx Storage over Fresh and Thermally Aged Pt-BaO/Al2O3 Lean NOx Trap Catalysts ." Catalysis Letters 124(1-2):39-45. doi:10.1007/s10562-008-9505-6 Abstract A simple liquid water treatment applied to fresh and thermally aged Pt(2wt%)-BaO(20wt%)/Al2O3 lean NOx trap catalysts at room temperature induces morphological and structural changes in the barium species as followed by XRD and TEM analysis. During the water treatment, liquid water sufficient to fill the catalyst pore volume is brought into contact with the samples. It was found that irrespective of the original barium chemical state (highly dispersed BaO or crystalline BaAl2O4), exposing the sample to this liquid water treatment promotes the formation of BaCO3 crystallites (about 15 – 25 nm of its size) without changing the Pt particle size. Such transformations of the barium species are found to significantly promote NOx uptake from 250 °C to 450 °C. The increase in the NOx uptake for the water-treated samples can be attributed to an enhanced Pt-Ba interaction through the redistribution of barium species. These results provide useful information for the regeneration of aged lean NOx trap catalysts since water is plentiful in the exhaust of diesel or lean-burn engines.

Kim DH, JH Kwak, J Szanyi, SJ Cho, and CHF Peden. 2008. "Roles of Pt and BaO in the Sulfation of Pt/BaO/Al2O3 Lean NOx Trap Materials: Sulfur K-edge XANES and Pt LIII XAFS Studies." Journal of Physical Chemistry C 112(8):2981-2987. doi:10.1021/jp077563i Abstract The roles of barium oxide and platinum during the sulfation of Pt-BaO/Al2O3 lean NOx trap catalysts were investigated by S K edge XANES (X-ray absorption near-edge spectroscopy) and Pt LIII XAFS (X-ray absorption fine structure). All of the samples studied (Al2O3, BaO/Al2O3, Pt/Al2O3 and Pt-BaO/Al2O3) were pre-sulfated prior to the X-ray absorption measurements. It was found that barium oxide itself has the ability to directly form barium sulfate even in the absence of Pt and gas phase oxygen. In the platinum-containing samples, the presence of Pt-O species plays an important role in the formation of sulfate species. Even if barium and aluminum sites are available for SO2 to form sulfate, for the case of the BaO(8)/Al2O3 sample, where the barium coverage is about 0.26 ML, S XANES spectroscopy results show that barium sulfates are preferentially produced over aluminum sulfates . When oxygen is absent from the gas phase, the sulfation route that involves Pt-O is eliminated after the initially present Pt-O species are completely consumed. In this case, formation of sulfates is suppressed unless barium oxide is also present. Pt LIII XAFS results show that the first coordination sphere around the Pt atoms in the Pt particles is dependent upon the redox nature of the gas mixture used during the sulfation process. Sulfation under reducing environments (e.g. SO2+H2) leads to formation of Pt-S bonds, while oxidizing conditions (e.g. SO2+O2) continue to show the presence of Pt-O bonds. In addition, the former condition was found to give rise to a higher degree of Pt sintering than the latter one. This result explains why samples sulfated under reducing conditions had lower NOx uptakes than those sulfated under oxidizing conditions. Therefore, our results provide needed information for the development of optimum practical operation conditions (e.g. sulfation or desulfation) for lean NOx trap catalysts that minimize deactivation by sulfur.

Kim DH, JH Kwak, X Wang, J Szanyi, and CHF Peden. 2008. "Sequential high temperature reduction, low temperature hydrolysis for the regeneration of sulfated NOx trap catalysts." Catalysis Today 136(1-2):183-187. doi:doi:10.1016/j.cattod.2007.12.134 Abstract We describe a new method that minimizes irreversible Pt sintering during the desulfation of sulfated Pt/BaO/Al2O3 lean NOx trap (LNT) catalysts. While it is known that the addition of H2O to H2 promotes desulfation, we find that the significant and irreversible Pt sintering arising from the presence of water is unavoidable. Control of precious metal sintering is considered to be one of the critical issues in the development of durable LNT catalysts. The new method described here is a sequential desulfation process: the first step is to reduce the sulfates with hydrogen only at higher temperatures to form BaS, followed by a treatment of the thus reduced sample with water at low to moderate temperatures to convert BaS to BaO and H2S. The data showed that Pt sintering was significantly inhibited due to the absence of H2O during the desulfation at high temperatures, and also demonstrates the similar NOx uptake with the desulfated sample cooperatively with H2 and H2O. Therefore, the sequential desulfation process may find applications in realistic systems to inhibit the irreversible sintering of the Pt in the lean NOx trap catalyst, leading to a longer catalyst life.

Kwak JH, DH Kim, J Szanyi, and CHF Peden. 2008. "Excellent Sulfur Resistance of Pt/BaO/CeO2 Lean NOx Trap Catalysts." Applied Catalysis. B, Environmental 84(3-4):545-551. doi:10.1016/j.apcatb.2008.05.009 Abstract In this work, we investigated the NOx storage behavior of Pt-BaO/CeO2 catalysts, especially in the presence of SO2. High surface area CeO2 (~ 110 m2/g) with a rod like morphology was synthesized and used as a support. The Pt-BaO/CeO2 sample demonstrated slightly higher NOx conversion in the entire temperature range studied compared with Pt-BaO/γ-Al2O3. More importantly, this ceria-based catalyst showed higher sulfur tolerance than the alumina-based one. The time of complete NOx uptake was maintained even after exposing the sample to ~3 g/L of SO2. The same sulfur exposure, on the other hand, eliminated the complete NOx uptake time on the alumina-based NOx storage catalysts. TEM images show no evidence of either Pt sintering or BaS phase formation during reductive de-sulfation up to 600°C on the ceria based catalyst, while the same process over the alumina-based catalyst resulted in both a significant increase in the average Pt cluster size and the agglomeration of a newly-formed BaS phase into large crystallites. XPS results revealed the presence of about 5 times more residual sulfur after reductive de-sulfation at 600°C on the alumina based catalysts in comparison with the ceria-based ones. All of these results strongly support that, besides their superior intrinsic NOx uptake properties, ceria based catalysts have a) much higher sulfur tolerance and b) excellent resistance against Pt sintering when they are compared to the widely used alumina based catalysts.

Kwak JH, JZ Hu, AC Lukaski, DH Kim, J Szanyi, and CHF Peden. 2008. "The Role of PentaCoordinated Al3+ Ions in the High Temperature Phase Transformation of γ-Al2O3." Journal of Physical Chemistry C 112(25):9486–9492. doi:10.1021/jp802631u Abstract In this work, the structural stability of gamma-alumina (γ-Al2O3) was investigated by a combination of XRD and high resolution solid state 27Al MAS NMR at an ultra-high magnetic field of 21.1 tesla. XRD measurements show that γ-Al2O3 undergoes a phase transition to θ-Al2O3 during calcination at 1000oC for 10hr. The formation of the θ-Al2O3 phase is further confirmed by 27Al MAS NMR; additional 27Al peaks centered at 10.5 and ~78 ppm were observed in samples calcined at this high temperature. Both the XRD and NMR results indicate that, after calcination at 1000°C for 10 hrs, the ratio of the θ-Al2O3 phase to the total alumina in samples modified by either BaO or La2O3 is significantly reduced in comparison with γ-Al2O3. 27Al MAS NMR spectra revealed that the reduction in the extent of θ-Al2O3 formation was highly correlated with the reduction in the amount of penta-coordinated aluminum ions, measured after 500°C calcination, in both BaO- and La2O3-modified γ-Al2O3 samples. These results strongly suggest that the penta-coordinated aluminum ions, present exclusively on the surface of γ-Al2O3, play a critical role in the phase transformation of γ-Al2O3 to θ-Al2O3. The role of the modifiers, in our case BaO or La2O3, is to convert the penta-coordinated aluminum ions into octahedral ones, thereby improving the thermal stabilities of the samples. Oxide additives, on the other hand, had no beneficial effect on preventing the specific surface area reduction that occurred during high temperature (≤1000°C) calcination.