Hyung Chul Ham

Communication: Enhanced oxygen reduction reaction and its underlying mechanism in Pd-Ir-Co trimetallic alloys

Based on a combined density functional theory and experimental study, we present that the electrochemical activity of Pd3Co alloy catalysts toward oxygen reduction reaction (ORR) can be enhanced by adding a small amount of Ir. While Ir tends to favorably exist in the subsurface layers, the underlying Ir atoms are found to cause a substantial modification in the surface electronic structure. As a consequence, we find that the activation barriers of O/OH hydrogenation reactions are noticeably lowered, which would be mainly responsible for the enhanced ORR activity. Furthermore, our study suggests that the presence of Ir in the near-surface region can suppress Co out-diffusion from the Pd3Co substrate, thereby improving the durability of Pd-Ir-Co catalysts. We also discuss the relative roles played by Ir and Co in enhancing the ORR activity relative to monometallic Pd catalysts.

Effect of gold subsurface layer on the surface activity and segregation in Pt/Au/Pt3M (where M = 3d transition metals) alloy catalyst from first-principles

The effect of a subsurface hetero layer (thin gold) on the activity and stability of Pt skin surface in Pt3M system (M = 3d transition metals) is investigated using the spin-polarized density functional theory calculation. First, we find that the heterometallic interaction between the Pt skin surface and the gold subsurface in Pt/Au/Pt3M system can significantly modify the electronic structure of the Pt skin surface. In particular, the local density of states projected onto the d states of Pt skin surface near the Fermi level is drastically decreased compared to the Pt/Pt/Pt3M case, leading to the reduction of the oxygen binding strength of the Pt skin surface. This modification is related to the increase of surface charge polarization of outmost Pt skin atoms by the electron transfer from the gold subsurface atoms. Furthermore, a subsurface gold layer is found to cast the energetic barrier to the segregation loss of metal atoms from the bulk (inside) region, which can enhance the durability of Pt3M based catalytic system in oxygen reduction condition at fuel cell devices. This study highlights that a gold subsurface hetero layer can provide an additional mean to tune the surface activity toward oxygen species and in turn the oxygen reduction reaction, where the utilization of geometric strain already reaches its practical limit.

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.

Highly selective, facile NO2 reduction to NO at cryogenic temperatures on hydrogen precovered gold.

We have discovered that NO(2) is reduced to NO at 77 K by hydrogen precovered gold in vacuum. Here, we investigate the partial reduction of NO(2) to NO on an atomic-hydrogen populated model gold catalyst for a more fundamental understanding of the surface chemistry of hydrogenation. Gold-based catalysts have been found to be active for many hydrogenation reactions, but few related fundamental studies have been conducted. Our experimental results reveal a high catalytic activity for gold: indeed, NO(2) is reduced to NO with 100% conversion and 100% selectivity at temperatures lower than 120 K. Density functional theory calculations and reflection-absorption infrared spectroscopy measurements indicate that HNO(2) and N(2)O(3) are intermediates which are highly dependent on surface hydrogen concentrations; subsequent hydrogenation of HNO(2) and dissociation of N(2)O(3) upon annealing induces the production of NO and H(2)O.

Development of Reinforced Matrix for Molten Carbonate Fuel Cells by Using Sintering Aids

In order to reinforce a matrix for molten carbonate fuel cell (MCFC), sintering additives having low melting points such as Al [660oC], B2O3 [450oC] and Al(OH)3 powders [320oC] have been included into conventional LiAlO2 green sheets by tape casting method. The mechanical strength was clearly increased by the addition of B2O3 into the LiAlO2 matrix. The enhancement of mechanical strength in the B2O3 -included matrix was mainly attributed to the Li2AlBO4 formation by the reaction between B2O3 and LiAlO2.

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.

Transition metal alloying effect on the phosphoric acid adsorption strength of Pt nanoparticles: an experimental and density functional theory study

The effect of alloying with transition metals (Ni, Co, Fe) on the adsorption strength of phosphoric acid on Pt alloy surfaces was investigated using electrochemical analysis and first-principles calculations. Cyclic voltammograms of carbon-supported Pt3M/C (M = Ni, Co, and Fe) electrocatalysts in 0.1 M HClO4 with and without 0.01 M H3PO4 revealed that the phosphoric acid adsorption charge density near the onset potential on the nanoparticle surfaces was decreased by alloying with transition metals in the order Co, Fe, Ni. First-principles calculations based on density functional theory confirmed that the adsorption strength of phosphoric acid was weakened by alloying with transition metals, in the same order as that observed in the electrochemical analysis. The simulation suggested that the weaker phosphoric acid adsorption can be attributed to a lowered density of states near the Fermi level due to alloying with transition metals.

Anomalous in situ Activation of Carbon-Supported Ni 2 P Nanoparticles for Oxygen Evolving Electrocatalysis in Alkaline Media

Electrochemical water splitting is one of the most promising systems by which to store energy produced from sustainable sources, such as solar and wind energy. Designing robust and stable electrocatalysts is urgently needed because of the relatively sluggish kinetics of the anodic reaction, i.e. the oxygen evolution reaction (OER). In this study, we investigate the anomalous in situ activation behaviour of carbon-supported Ni2P nanoparticles (Ni2P/C) during OER catalysis in alkaline media. The activated Ni2P/C shows an exceptionally high activity and stability under OER conditions in which the overpotential needed to achieve 10 mA cm−2 was reduced from approximately 350 mV to approximately 300 mV after 8,000 cyclic voltammetric scans. In situ and ex situ characterizations indicate that the activity enhancement of Ni2P catalysts is due to a favourable phase transformation of the Ni centre to β-NiOOH, including increases in the active area induced by structural deformation under the OER conditions. These findings provide new insights towards designing transition metal/phosphide-based materials for an efficient water splitting catalyst.