Chang Won Yoon

Carbon dioxide mediated, reversible chemical hydrogen storage using a Pd nanocatalyst supported on mesoporous graphitic carbon nitride

Reversible, carbon dioxide mediated chemical hydrogen storage was first demonstrated using a heterogeneous Pd catalyst supported on mesoporous graphitic carbon nitride (Pd/mpg-C3N4). The Pd nanoparticles were found to be uniformly dispersed onto mpg-C3N4 with an average size of 1.7 nm without any agglomeration and further exhibit superior activity for the dehydrogenation of formic acid with a turnover frequency of 144 h−1 even in the absence of external bases at room temperature. Initial DFT studies suggest that basic sites located at the mpg-C3N4 support play synergetic roles in stabilizing reduced Pd nanoparticles without any surfactant as well as in initiating H2-release by deprotonation of formic acid, and these potential interactions were further confirmed by X-ray absorption near edge structure (XANES). Along with dehydrogenation, Pd/mpg-C3N4 also proves to catalyze the regeneration of formic acid via CO2 hydrogenation. The governing factors of CO2 hydrogenation are further elucidated to increase the quantity of the desired formic acid with high selectivity.

Ultrasmall palladium nanoparticles supported on amine-functionalized SBA-15 efficiently catalyze hydrogen evolution from formic acid

The success of the so-called “hydrogen economy” for large-scale applications will ultimately depend on efficient and sustainable production, storage and distribution of hydrogen. Owing to its low toxicity, high volumetric H2 storage capacity and availability both from renewable resources (e.g., biomass) and non-renewable resources (e.g., fossil fuel feedstocks), formic acid is one of the most favorable chemical hydrogen storage media for large-scale energy storage applications. However, for FA to become a viable hydrogen storage medium, efficient catalysts that enable it to release H2 at low cost are necessary. Herein we report a facile synthetic route to amine-functionalized nanoporous silica-supported ultrasmall Pd nanoparticles (SBA-15-Amine/Pd) that are highly active catalysts for formic acid dehydrogenation, producing hydrogen at ambient temperature with a high turn-over-frequency (TOF) of 293 h−1, which is among the highest TOFs ever reported for the reaction by a heterogeneous catalyst. We also show that the material is easily recyclable multiple times, without losing its catalytic activity. So, the catalyst we developed can be expected to be part of the solutions of our sustainability challenges.

Efficient catalytic conversion of ammonia borane to borazine and its use for hexagonal boron nitride (white graphene)

Nickel nanoparticles (NiNPs) prepared in tetraglyme (TG) efficiently catalyzed the conversion of ammonia borane (AB, NH3BH3) to borazine (B3N3H6). Under the optimized conditions, 3 mol% of the NiNPs were introduced into a 1.5 M AB solution in TG and held at 80 °C for 6 h under a dynamic vacuum that was maintained at 30 torr. Borazine was isolated through a series of −45 °C, −78 °C, and −196 °C traps to give (−78 °C trap) pure borazine in 53% yield. The borazine produced was then utilized as a molecular precursor for high quality h-BN (white graphene) and large area h-BN sheets were prepared by applying low pressure chemical vapor deposition (LPCVD). Ultra-thin (single to few layers) h-BN was synthesized on Ni foil at the optimal ratio between borazine and NH3, and the number of layers was tuned by varying the NH3 partial pressure.

Metal-free, polyether-mediated H2-release from ammonia borane: roles of hydrogen bonding interactions in promoting dehydrogenation.

Polyetheral additives were found to be efficient promoters to enhance the rate of H2-release from ammonia borane (AB) at various temperatures. In particular, tetraethylene glycol dimethyl ether (T4EGDE, 29 wt% relative to AB + T4EGDE) exhibited significantly improved activities for AB dehydrogenation, with the material-based hydrogen storage capacity of 10.3 wt% at 125 °C within 40 min. In situ FT-IR spectroscopy indicated the formation of B-(cyclodiborazanyl)amino-borohydride (BCDB), borazine, and μ-aminodiborane as gaseous byproducts. In addition, (11)B nuclear magnetic resonance (NMR) spectroscopy further revealed that diammoniate of diborane (DADB) was initially formed to give polyaminoborane as liquid and/or solid spent-fuel, consistent with previous reports. Density Functional Theory (DFT) calculations suggested that hydrogen bonding interactions between AB and a polyetheral promoter initially played an important role in increasing the reactivity of B-H bonds of AB by transferring electron density from oxygen atoms of the promoter into B-H bonds of AB. These partially activated, hydridic B-H bonds were proposed to help promote the formation of diammoniate of diborane (DADB), which is considered as a reactive intermediate, eventually enhancing the rate of H2-release from AB. In addition, our in situ solid state (11)B magic angle spinning (MAS) NMR measurements further confirmed that the rate of DADB formation from AB with a small quantity of T4EGDE was found to be much faster than that of pristine AB even at 50 °C. This metal-free method for H2-release from AB with an added, small quantity of polyethers would be helpful to develop feasible hydrogen storage systems for long-term fuel cell applications.

Convenient metal embedment into mesoporous silica channels for high catalytic performance in AB dehydrogenation

The infiltration of palladium nanoparticles (PdNPs) into the channels of SBA-15 was conveniently achieved via an incipient wetness procedure employing a tetraglyme solution. Electron tomography demonstrated that PdNPs were outgrown preferentially from the channels. The resultant Pd/SBA-15 showed high performance in the dehydrogenation kinetics of ammonia borane.

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.

Ru-N-C Hybrid Nanocomposite for Ammonia Dehydrogenation: Influence of N-doping on Catalytic Activity

For application to ammonia dehydrogenation, novel Ru-based heterogeneous catalysts, Ru-N-C and Ru-C, were synthesized via simple pyrolysis of a mixture of RuCl3·6H2O and carbon black with or without dicyandiamide as a nitrogen-containing precursor at 550 °C. Characterization of the prepared Ru-N-C and Ru-C catalysts via scanning transmission electron microscopy, in conjunction with energy dispersive X-ray spectroscopy, indicated the formation of hollow nanocomposites in which the average sizes of the Ru nanoparticles were 1.3 nm and 5.1 nm, respectively. Compared to Ru-C, the Ru-N-C nanocomposites not only proved to be highly active for ammonia dehydrogenation, giving rise to a NH3 conversion of >99% at 550 °C, but also exhibited high durability. X-ray photoelectron spectroscopy revealed that the Ru active sites in Ru-N-C were electronically perturbed by the incorporated nitrogen atoms, which increased the Ru electron density and ultimately enhanced the catalyst activity.

Amine/Hydrido Bifunctional Nanoporous Silica with Small Metal Nanoparticles Made Onsite: Efficient Dehydrogenation Catalyst

Multifunctional catalysts are of great interest in catalysis because their multiple types of catalytic or functional groups can cooperatively promote catalytic transformations better than their constituents do individually. Herein we report a new synthetic route involving the surface functionalization of nanoporous silica with a rationally designed and synthesized dihydrosilane (3-aminopropylmethylsilane) that leads to the introduction of catalytically active grafted organoamine as well as single metal atoms and ultrasmall Pd or Ag-doped Pd nanoparticles via on-site reduction of metal ions. The resulting nanomaterials serve as highly effective bifunctional dehydrogenative catalysts for generation of H2 from formic acid.

Development of a solvent-free hydrogen storage and release system based on semi-solid-state ammonia borane (AB) fuel: high gravimetric capacity and feasibility for practical application

Ammonia borane (AB), with high hydrogen contents and favorable dehydrogenation properties, is receiving intensive attention for its potential as a hydrogen storage material. In this study, we demonstrate a new type of solvent-free AB fuel system to obtain a high hydrogen systemic gravimetric capacity needed for practical fuel cell application. The new storage material constitutes AB soaked in tetraethylene glycol dimethyl ether (TEGDE) with catalytic amounts of palladium nanoparticles. Notably, TEGDE is very essential for the successful preparation of AB fuel system in a semi-solid state. The use of a minimum amount of TEGDE in this system allows the hybrid AB catalytic system to be fabricated as an efficient solvent and catalytic reaction medium, enabling a high gravimetric and volumetric capacity. For practical applications, AB pellets with spherical shapes have been manufactured by the co-precipitation of AB/TEGDE/PdNPs, followed by the compression of semi-solid AB fuel mixture for fuel transfer from the fuel tank to the hydrogen generator. Consequently, this hybrid semi-solid state catalytic system exhibits a high gravimetric capacity of hydrogen [10.01 material weight%]. With a high hydrogen capacity, a high performance dehydrogenation is obtained because of the synergistic effects facilitated by the highly active PdNPs well-dispersed in a TEGDE medium.

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.