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Bimetallic Pd1Nix alloys supported on nitrogen-doped carbon (Pd1Nix/N–C, x = 0.37, 1.3 and 3.6) exhibit higher activities than Pd/N–C towards dehydrogenation of formic acid (HCO2H, FA). Density functional theory (DFT) calculations provided electronic and atomic structures, energetics and reaction pathways on Pd(111) and Pd1Nix(111) surfaces of different Pd/Ni compositions. A density of states (DOS) analysis disclosed the electronic interactions between Pd and Ni revealing novel active sites for FA dehydrogenation. Theoretical analysis of FA dehydrogenation on Pd1Nix(111) (x = 0.33, 1 and 3) shows that the Pd1Ni1(111) surface provides optimum H2-release efficiency via a favorable ‘HCOO pathway’, in which a hydrogen atom and one of the two oxygen atoms of FA interact directly with surface Ni atoms producing adsorbed CO2 and H2. The enhanced efficiency is also attributed to the blocking of an unfavorable ‘COOH pathway’ through which a C–O bond is broken and side products of CO and H2O are generated.

Experimental and first-principles studies were conducted to understand the adsorption mechanism of elemental mercury on FeCl3- and CuCl2-impregnated activated carbons. Activated carbon was impregna...

Hydrogen energy is a potential next-generation energy source for fossil fuel replacement. The development of high-efficiency materials and catalysts for storage and transportation of hydrogen energy must be achieved to realize hydrogen economy. Recently, catalyst systems such as Pd nanoclusters (Pd NCs) supported on nickel hydroxide (Ni(OH)2) have been reported to have advantages, including effective suppression of CO production and efficiency enhancement of HCOOH dehydrogenation. However, the reaction mechanism and multi-metallic interface system design of such systems have not been elucidated. Therefore, various Ni(OH)2 surfaces supported on a graphene system were designed through density functional theory calculations, and the support material was combined with Pd38NC (Pd38NC/Ni(OH)2-G). Subsequently, the adsorption behavior of HCOOH dehydrogenation intermediates was analyzed. We found a new adsorption configuration in which HCOOH* (where * and a single underline indicates the adsorbed species and adsorbed atom, respectively) was adsorbed in a more stable manner (adsorption energy, Eads= −1.22eV) on the system than HCOOH* (Eads=−1.10eV) owing to the presence of Ni(OH)2-G. This affected the next step in HCOOH dehydrogenation, i.e., formation of HCOO* species, and showed a positive effect on the HCOOH dehydrogenation. To fundamentally understand this phenomenon, electronic structure (d-band center and density of states) and stability (vacancy formation energy) analyses were performed.

Reversible formate (HCOO–) dehydrogenation and bicarbonate (HCO3–) hydrogenation would be desirable for the utilization and storage of hydrogen (H2) as an effective energy carrier. Carbon-supported Pd-based nanoparticles demonstrated enormous competitive advantages for these reactions. However, the fundamental mechanisms underlying these reversible reactions have not yet been elucidated. Herein, we report the reaction pathways for reversible reactions on a Pd-based catalyst using density functional theory (DFT) calculations and propose key factors for improving the reaction efficiency. As the first essential step, the difficulty in the conventional DFT modeling, that is simulation of an anion environment caused by HCOO–, was overcome by designing two-sided Pd12 nanoclusters supported on graphene (Pd12NC-G) with extra electrons. Using Pd12NC-G, we demonstrated that the key factor determining the potential limiting steps for the reversible reaction was desorption of hydrogen in HCOO– dehydrogenation (1.24 e...

Sulfide minerals are important in immobilizing toxic contaminants in reducing environments. Iron sulfide (FeS) is ubiquitous in anoxic conditions and is a good scavenger of various organic and redox sensitive contaminants and heavy metals. Despite its contaminant removal capabilities, FeS has not been used as a practical adsorbent in contaminant removal due to its rapid oxidation under atmospheric conditions. To increase its applicability, we developed a method of modifying FeS by the addition of NaBH4 to form the less oxygen-sensitive NaBH4–FeS. We conducted oxidation tests using laboratory batch testing and real-time synchrotron X-ray absorption spectroscopy (XAS). The Fe K-edge XAS results showed that the oxidation rate of NaBH4–FeS was eight times slower than that of unmodified FeS while maintaining comparable contaminant removal capacities for Cr(VI) and As(III). The results of mechanistic density functional theory (DFT) calculations demonstrated that the oxidation of FeS occurred through electron transfer from sulfur of FeS to an oxidizing agent of oxygen, and that the hydride ion provided by NaBH4 retarded electron transfer from the FeS surface.

In the attempt to reduce the high-cost and improve the overall durability of Pt-based electrocatalysts for the oxygen reduction reaction (ORR), density-functional theory (DFT) calculations have been performed to study the energetics of the elementary steps that occur during ORR on TiN(100)- and TiC(100)-supported single Pt atoms. The O2 and OOH* dissociation processes on Pt/TiN(100) are determined to be non-activated (i.e. “barrier-less” dissociation) while an activation energy barrier of 0.19 and 0.51 eV is found for these dissociation processes on Pt/TiC(100), respectively. Moreover, the series pathway (which is characterized by the stable OOH* molecular intermediate) on Pt/TiC(100) is predicted to be more favorable than the direct pathway. Our electronic structure analysis supports a strong synergistic co-operative effect by these non-oxide supports (TiN and TiC) on the reduced state of the single-atom Pt catalyst, and directly influences the rudimentary ORR steps on these single-atom platinized supports.

First-principles study explores the catalytic performance of the Cu(1 1 1) and Cu/G catalysts for the electrochemical CO2 reduction using computational hydrogen electrode model. Two different pathways, determined based on the initially generated intermediates (COOH* and HCOO*), as well as successive intermediates and the end products, were investigated. Although the two catalysts promote both pathways, Cu(1 1 1) surface favors “COOH pathway” and Cu/G catalyst strongly promotes the “HCOO pathway” with an free energy barrier of 0.83 eV (CO* → CHO*) and 0.87 eV (HCOO* → H2COO*). Electronic properties and binding nature of the final intermediate (CH3O*) on both catalysts sheds light on the fundamental reason for the selective end products (CH4 on the Cu(1 1 1) surface and CH3OH on the Cu/G catalyst). CH3OH is predominantly produced with the Cu/G catalyst because the C O bond of CH3O* is stronger than the bond between the single Cu atom and O of CH3O*, given that the energy of the d-band center of the single Cu atom is relatively highly negative. The current study explores the different electrochemical reduction pathways on the Cu(1 1 1) and Cu/G catalyst, providing insight for the design of highly efficient catalysts that afford selective end products.

The conversion of carbon dioxide (CO2) into usable hydrocarbon fuels is important for recycling carbon resources and mitigating environmental problems. However, converting CO2, which is a stable compound, requires a high additional energy. Therefore, it is essential to understand the electrochemical reduction mechanisms of CO2 and develop more efficient catalysts. In this study, density functional theory calculations were performed to examine electrochemical CO2 reduction on copper nanoclusters (NCs) (Cu13 NCs and Cu55 NCs) and the Cu(1 1 1) surface to verify the effect of the surface geometry and size of the NCs on the conversion of CO2 into CH4. The highest energy barriers to CO2 reduction (i.e., the potential-limiting step) on the Cu13 NCs (0.64 eV), Cu55 NCs (0.83 eV), and Cu(1 1 1) surface (0.86 eV) lie in the CO∗ → CHO∗ step. The formation of an adsorbed CHO intermediate depending on the catalyst surface geometry may significantly influence the energy barrier, as demonstrated by analyses of the electronic properties, such as the density of states, charge density difference, and highest occupied molecular orbital and lowest unoccupied molecular orbital band gap.

Transition metal nitrides possessing superior electrical conductivity and outstanding oxidation and corrosion resistance have been described as good substitutes for carbon support materials which are vulnerable during proton exchange membrane fuel cell (PEMFC) operation due to corrosion and poor life cycles. A closer theoretical inspection of the stability and electronic properties of titanium nitride-supported Pt in comparison with carbon-supported Pt (using graphite and graphene) has been conducted using density functional theory calculations. A single Pt atom adsorbed more strongly to the TiN surface than to both graphite and graphene, causing a larger degree of charge transfer between Pt and TiN.

Conversion of carbon dioxide (CO 2 ) into methane (CH 4 ) on copper (Cu) surfaces were investigated to understand the fundamental mechanism of CO 2 reduction, and to suggest a key factor for designing promising catalysts for CO 2 conversion into hydrocarbon fuels. The density functional theory calculations revealed the lowest-energy reaction pathways on Cu(1 0 0), Cu(1 1 0), and Cu(1 1 1) planes, and determined that the potential limiting step for CO 2 reduction lies between the reaction intermediates CO ∗ and CHO ∗ ( ∗ denotes the state adsorbed on the catalyst surface). The energy barrier to the potential limiting step is lowered in the following order: Cu(1 1 0) 2 reduction by designing a Cu thin film on a supporting material with larger lattice constant than Cu. To demonstrate this, we evaluated the energy barriers to the potential limiting step on a single Cu thin layer supported on both palladium (Pd) and silver (Ag), and confirmed that the Pd supporting material can enlarge the interatomic distance of the single Cu(1 1 1) layer by 8.7% and thereby lower the energy barrier from 0.97 to 0.63 eV.

Demand on the practical synthetic approach to the high performance electrocatalyst is rapidly increasing for fuel cell commercialization. Here we present a synthesis of highly durable and active intermetallic ordered face-centered tetragonal (fct)-PtFe nanoparticles (NPs) coated with a "dual purpose" N-doped carbon shell. Ordered fct-PtFe NPs with the size of only a few nanometers are obtained by thermal annealing of polydopamine-coated PtFe NPs, and the N-doped carbon shell that is in situ formed from dopamine coating could effectively prevent the coalescence of NPs. This carbon shell also protects the NPs from detachment and agglomeration as well as dissolution throughout the harsh fuel cell operating conditions. By controlling the thickness of the shell below 1 nm, we achieved excellent protection of the NPs as well as high catalytic activity, as the thin carbon shell is highly permeable for the reactant molecules. Our ordered fct-PtFe/C nanocatalyst coated with an N-doped carbon shell shows 11.4 times-higher mass activity and 10.5 times-higher specific activity than commercial Pt/C catalyst. Moreover, we accomplished the long-term stability in membrane electrode assembly (MEA) for 100 h without significant activity loss. From in situ XANES, EDS, and first-principles calculations, we confirmed that an ordered fct-PtFe structure is critical for the long-term stability of our nanocatalyst. This strategy utilizing an N-doped carbon shell for obtaining a small ordered-fct PtFe nanocatalyst as well as protecting the catalyst during fuel cell cycling is expected to open a new simple and effective route for the commercialization of fuel cells.

The competitive adsorption between phosphate and arsenic (As) in natural soil and mineral systems is important for controlling subsurface contaminants. Phosphate is known to adsorb more strongly than As in Fe(III) oxide-based mineral systems. Here, X-ray absorption spectroscopy (XAS) and density functional theory (DFT) were employed to understand the fundamental reaction mechanisms of As and phosphate on an FeS surface. The competitive effect of phosphate in As(V)-contaminated soil systems differed with changing FeS concentrations. In soil batches with high FeS contents, As(V) was more strongly adsorbed than phosphate, and no additional As release was observed in the aqueous phase, in the presence of phosphate. Through the XAS measurement of laboratory batch samples, the reaction mechanisms could be comprehensively understood, including the reduction of As(V) to As(III), the secondary precipitation of sulfide minerals, and the adsorption mechanism of As(V) on the surface of FeS. The DFT calculations revealed the relative adsorption strengths on the FeS surface were As(V) > phosphate > As(III). Comparing the adsorption energies revealed that As(V) reacted more strongly with FeS than phosphate and As(III), and that the effect of phosphate co-occurrence was negligible in the FeS–As system, in line with the experimental measurements.

Here we report the preparation of hollow microspheres with a thin shell composed of mixed cobalt nitride (Co–N) and cobalt oxide (Co–O) nanofragments encapsulated in thin layers of nitrogen-doped carbon (N–C) nanostructure (Co–N/Co–O@N–C) arrays with enhanced bifunctional oxygen electrochemical performance. The hybrid structures are synthesized via heat treatment of N-doped hollow carbon microspheres with cobalt nitrate, and both the specific ratio of these precursors and the selected annealing temperature are found to be the key factors for the formation of the unique hybrid structure. The as-obtained product (Co–N/Co–O@N–C) presents a large specific surface area (493 m2 g−1), high-level heteroatom doping (Co–N, Co–O, and N–C), and hierarchical porous nanoarchitecture containing macroporous frameworks and mesoporous walls. Electronic interaction between the thin N–C layers and the encapsulated Co–N and Co–O nanofragments efficiently optimizes oxygen adsorption properties on the Co–N/Co–O@N–C and thereby triggers bifunctional oxygen electrochemical activity at the surface. The Co–N/Co–O@N–C nanohybrid exhibited a high onset potential of 0.93 V, and a limiting current density of 5.6 mA cm−2 indicating 4-electron oxygen reduction reaction (ORR), afforded high catalytic activity for the oxygen evolution reaction (OER) and even exceeded the catalytic stability of the commercial precious electrocatalysts; furthermore, when integrated into the oxygen electrode of a regenerative fuel cell device, it exhibited high-performance oxygen electrodes for both the ORR and the OER.

The mobility of heavy metalloids including As, Sb, Mo, W, and Cr in soil was investigated under both reducing and oxidizing conditions. The effects of soil mineralogy and the presence of competitive anions were studied as important factors affecting the mobility of these contaminants. Batch experiments conducted with the addition of oxidized and fresh FeS exhibited enhanced sorption rates for As and W under oxidizing conditions, and for Mo under reducing conditions. The inhibitory effect of phosphate on the sorption rates was most apparent for As and Mo under both oxidizing and reducing conditions, while only a small phosphate effect was observed for Sb and W. For Sb and W mobility, pH was determined to be the most important controlling factor. The results of long-term batch experiments revealed that differences in the mobility of metalloids, particularly As, were also influenced by microbial activity in the oxidizing and reducing conditions.

Transition metals, such as iron (Fe)- or cobalt (Co)-based nanomaterials, are promising electrocatalysts for oxygen reduction reactions (ORR) in fuel cells due to their high theoretical activity and low cost. However, a major challenge to using these metals in place of precious metal catalysts for ORR is their low efficiency and poor stability, thus new concepts and strategies should be needed to address this issue. Here, we report a hybrid aciniform nanostructures of Fe nanofragments embedded in thin nitrogen (N)-doped graphene (Fe@N-G) layers via a heat treatment of graphene oxide-wrapped iron oxide (Fe2O3) microparticles with melamine. The heat treatment leads to transformation of Fe2O3 microparticles to nanosized zero-valent Fe fragments and formation of core-shell structures of Fe nanofragments and N-doped graphene layers. Thin N-doped graphene layers massively promote electron transfer from the encapsulated metals to the graphene surface, which efficiently optimizes the electronic structure of the graphene surface and thereby triggers ORR activity at the graphene surface. With the synergistic effect arising from the N-doped graphene and Fe nanoparticles with porous aciniform nanostructures, the Fe@N-G hybrid catalyst exhibits high catalytic activity, which was evidenced by high E1/2 of 0.82 V, onset potential of 0.93 V, and limiting current density of 4.8 mA cm−2 indicating 4-electron ORR, and even exceeds the catalytic stability of the commercial Pt catalyst.

Spin-polarized density functional theory studies of hydrogen sulfide (H2S) adsorption and decomposition on Ni(100) and Ni3Al(100) surfaces were conducted to understand the aluminum (Al) alloying effect on H2S dissociation. For such purpose, we first determined the near surface structure of fully ordered Ni3Al alloy along the [100] direction by calculating the Al segregation energy to the surface and then examined the adsorption energies of the adsorbates (H2S, HS, S, and H) and the activation barriers for the H2S and HS decomposition by using Climbing Image-Nudged Elastic Band method. We found that regardless of the way to terminate the surface, Al atom in bimetallic Ni3Al(100) tends to exist in the first surface layer, rather than in the second or third layer, and the Ni3Al(100) surface can substantially retard the H2S decomposition by reducing the adsorption energy of sulfur compounds compared to the pure Ni(100) case. Finally, we presented how the Al in Ni3Al modifies the activity of surface Ni atoms toward the sulfur compounds by calculating the local density of states and charge distribution in alloying components. This work hints the importance of knowing how to properly tailor the reactivity of Ni based materials to enhance the resistance for sulfur poisoning.

Described herein is the development of a novel Co-based oxygen electrode catalyst coupled with unique carbon structures. The present carbon shell coated Co nanoparticles of which the surface composites are modified by phosphorus incorporation, exhibit efficient oxygen reduction activities as well as oxygen evolving properties.

Edge-exposed MoS2 nano-assembled structures are designed for high hydrogen evolution reaction activity and long term stability. The number of sulfur edge sites of nano-assembled spheres and sheets is confirmed by Raman spectroscopy and EXAFS analysis. By controlling the MoS2 morphology with the formation of nano-assembled spheres with the assembly of small-size fragments of MoS2, the resulting assembled spheres have high electrocatalytic HER activity and high thermodynamic stability.

The structural and electronic properties of Hg, SO(2), HgS, and HgO adsorption on Au(111) surfaces have been determined using density functional theory with the generalized gradient approximation. The adsorption strength of Hg on Au(111) increases by a factor of 1.3 (from -9.7 to -12.6 kcal/mol) when the number of surface vacancies increases from 0 to 3; however, the adsorption energy decreases with more than three vacancies. In the case of SO(2) adsorption on Au(111), the Au surface atoms are better able to stabilize the SO(2) molecule when they are highly undercoordinated. The SO(2) adsorption stability is enhanced from -0.8 to -9.3 kcal/mol by increasing the number of vacancies from 0 to 14, with the lowest adsorption energy of -10.2 kcal/mol at 8 Au vacancies. Atomic sulfur and oxygen precovered-Au(111) surfaces lower the Hg stability when Hg adsorbs on the top of S and O atoms. However, a cooperative effect between adjacent Hg atoms is observed as the number of S and Hg atoms increases on the perfect Au(111) surface, resulting in an increase in the magnitude of Hg adsorption. Details of the electronic structure properties of the Hg-Au systems are also discussed.

Preventing the release of mercury from coal-fired power plants continues to be a challenge. The design of effective and affordable control strategies depends upon the speciation of mercury from the high temperature region of the boiler to the lower temperature environment of the stack. Both homogeneous and heterogeneous oxidation pathways play a role in determining mercury's speciation over the temperature range of coal-fired flue gas. This review explores the current state of knowledge associated with the kinetically-limited homogeneous reaction pathways in addition to the complexities associated with heterogeneous oxidation processes. In particular, oxidation pathways associated with selective catalytic reduction and precious metal catalysts are considered. In addition, adsorption mechanisms on various materials are discussed, including fly ash and activated carbon for flue gas applications and precious metals for fuel gas applications.

The structural and electronic properties of Pt13 nanoparticles adsorbed on monovacancy defective graphene have been determined to understand oxygen adsorption on Pt nanoparticles based upon density functional theory predictions using the generalized gradient approximation. We demonstrate that a monovacancy site of graphene serves a key role as an anchoring point for Pt13 nanoparticles, ensuring their stability on defective graphene surfaces and suggesting their enhanced catalytic activity toward the interaction with O2. Strong hybridization of the Pt13 nanoparticle with the sp2 dangling bonds of neighboring carbon atoms near the monovacancy site leads to the strong binding of the Pt13 nanoparticle on defective graphene (−7.45 eV in adsorption energy). Upon both adsorption of the Pt13 nanoparticle on defective graphene and O2 on Pt13–defective graphene, strong charge depletion of the Pt atom at the interfaces of Pt–C and Pt–O2 is observed. Pt13 nanoparticles are able to donate charge to both defective grap...

To date, carbon-based materials including various carbon nanostructured materials have been extensively used as an electrocatalyst support for proton exchange membrane fuel cell (PEMFC) applications due to their practical nature. However, carbon dissolution or corrosion caused by high electrode potential in the presence of O2 and/or water has been identified as one of the main failure modes for the device operation. Here, we report the first TiN nanofiber (TNF)-based nonwoven structured materials to be constructed via electrospinning and subsequent two-step thermal treatment processes as a support for the PEMFC catalyst. Pt catalyst nanoparticles (NPs) deposited on the TNFs (Pt/TNFs) were electrochemically characterized with respect to oxygen reduction reaction (ORR) activity and durability in an acidic medium. From the electrochemical tests, the TNF-supported Pt catalyst was better and more stable in terms of its catalytic performance compared to a commercially available carbon-supported Pt catalyst. For example, the initial oxygen reduction performance was comparable for both cases, while the Pt/TNF showed much higher durability from an accelerated degradation test (ADT) configuration. It is understood that the improved catalytic roles of TNFs on the supported Pt NPs for ORR are due to the high electrical conductivity arising from the extended connectivity, high inertness to the electrochemical environment and strong catalyst-support interactions.

The role of samarium (Sm) 4f states and Sm-perturbed O 2p states in determining the sulfur tolerance of Sm-doped CeO2 was elucidated by using the density functional theory (DFT) + U calculation. We find that the sulfur tolerance of Sm-doped CeO2 is closely related to the modification of O 2p states by the strong interaction between Sm 4f and O 2p states. In particular, the availability of unoccupied O 2p states near the Fermi level is responsible for enhancing the sulfur tolerance of Sm-doped CeO2 compared to the pure CeO2 by increasing the activity of the surface lattice oxygen toward sulfur adsorption, by weakening the interaction between Sm–O, and by increasing the migration tendency of the subsurface oxygen ion toward the surface.

Density functional theory studies demonstrate that defective graphene-supported Cu nanoparticles can modify the structural and electronic properties of copper for enhancing electrochemical reduction of carbon dioxide (CO2) into hydrocarbon fuels (CH4, CO, and HCOOH). We not only provide improved understanding of CO2 conversion mechanisms on both Cu and the Cu nanoparticle system, but also explain a key factor for enhanced CO2 conversion. A promising catalytic material for CO2 conversion into hydrocarbon fuels may allow for geometry flexibility upon interaction with a key intermediate of CHO*.

Benzaldehyde (I) forms a Pt-bound pyrylium with PtCl2 that undergoes intermolecular [3 + 2] cycloaddition with alkenes to form oxygen-bridged intermediates with good stereoselectivity.

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.

2-(3-Benzyloxy)prop-1-ynyl)benzaldehyde with PtCl(2) in toluene would form Pt-pyryliums that underwent [3+2] cycloaddition with alkenes to the oxygen-bridged (5H-benzo[7]annulen-5-ylidene)platinum(ii) intermediates with good stereoselectivities. Their tandem rearrangement afforded diverse types of polycycles depending on the electronic nature of the alkenes.