Mechanistic study for enhanced CO oxidation activity on (Mn,Fe) co-doped CeO2(111)

Owing to the unique properties such as facile redoxability and high stability, ceria has been used for a wide range of applications including automotive emission control, catalytic combustion, hydrocarbon reforming, and electrocatalytic reactions. It is well known that enhanced chemical reactivity can be achieved on transition metal (TM)-doped ceria nano-catalysts. In particular, co-doping of TM on CeO2 surface has recently opened a great potential to improve the catalytic activity compared to the single doped one. In this study, we performed OFT calculations to compare the activity of CO oxidation between Mn-, Fe-, and (Mn,Fe)-doped CeO2(111) via Mars-van Krevelen (MvK) mechanism. We firstly verified that a conventional linear relationship between oxygen vacancy formation energy and the catalytic activity of CO oxidation is also effective for the co-doped CeO2(111). It turns out that the energy required to create oxygen vacancy (E-vf), that is a key descriptor of the reactivity, will be extremely useful to rapidly screen the catalytic activity on co-doped oxide system. Then, we investigated the entire reaction profile of CO oxidation via the MvK mechanism on Fe-, Mn-and (Mn,Fe)-doped CeO2(111). Based on the results, we confirmed the improved activity of CO oxidation on the co-doped system, which was in good agreement with the prediction from E-vf. From this study, we believe that the co-doping of TM on oxide catalysts will be a noble strategy to enhance the catalytic activity. (C) 2016 Elsevier B.V. All rights reserved.11Nsciescopu

Effect of caffeic acid adsorption in controlling the morphology of gold nanoparticles: role of surface coverage and functional groups

Caffeic acid (CA) is well known for its strong adsorption on metal or metal oxide surfaces mostly due to the catecholic functional group. On the other hand, the detailed adsorption configurations and the effects of functional groups on molecular adsorption have not been clarified yet. In this study, first-principles calculations were implemented to elucidate the adsorption phenomena of CA and its deprotonated forms on Au(100), (110) and (111), and then predict the morphology of Au nanoparticles (AuNPs). The adsorption energetics and configurations were carefully examined by employing van der Waals interactions to take dispersion forces into consideration. It was found that the adsorption strengths and geometries of the adsorbates are significantly changed by the surface coverages, deprotonated forms, and bound functional groups. These changes in adsorption features induce changes in surface energies, thereby resulting in different morphologies of AuNPs. To accelerate the morphology prediction of AuNPs, we demonstrated that the adsorption energy of CA can be rapidly estimated by the sum of the adsorption energies of the effective functional groups. Our results provide not only fundamental information about the adsorption behaviors of organic molecules on metal surfaces, but also insights for application in the customized synthesis of nanoparticles.11Nsciescopu

First-Principles Study of Enhanced Oxygen Incorporation Near the Grain Boundary on Yttria-Stabilized Zirconia

The recently reported oxygen incorporation enhancement near the grain boundary (GB) of yttria-stabilized zirconia (YSZ) provided a potential to enable the low temperature solid oxide fuel cell. However, these empirical observations have not yet explained the detailed reaction mechanism. Here, we performed first-principles calculations to quantitatively access the mechanism that may govern the fast oxygen incorporation at the GB. We investigated the key elementary steps of oxygen incorporation onto both Sigma 5 (310)/[001] GB and (001) surfaces of YSZ at the atomic scale; yttrium dopant segregation, vacancy formation, and oxygen adsorption. Our results showed that the doped yttrium preferentially segregates toward the GB, inducing the easier formation of oxygen vacancy between the yttrium pair at the GB. After these steps, oxygen is favorably adsorbed near the oxygen vacancy accumulated at the GB, eventually incorporating into the vacancy site. On the basis of our results, we suggest the fast oxygen incorporation mechanism near the GB of YSZ, providing fundamental insight of oxygen surface kinetics at the interfaces of defected oxide materials.11Nsciescopu

Precise control of defects in graphene using oxygen plasma

The authors report on a facile method for introducing defects in graphene in a controlled manner. Samples were mounted face down between supports, and exposed to oxygen plasma in a reactive ion etching (RIE) system. Defect density and the rate of defect formation in graphene were analyzed according to the oxygen flow rates and power conditions, using Raman spectroscopy. The mechanism of defect formation was systematically investigated via both experiment and density functional theory (DFT) calculation. Based on our DFT results, sp(3) oxygen in the epoxide form would most likely be induced in pristine graphene after exposure to the oxygen plasma. Defect engineering through the fine tuning of the graphene disorder using a conventional RIE system has great potential for use in various graphene-based applications. (C) 2015 American Vacuum Society.11Nsciescopu

Energy and dose dependence of proton-irradiation damage in graphene

Monolayer graphenes were irradiated with 5-15 MeV high-energy protons at various doses from 1 x 10(16) to 3 x 10(16) cm(-2), and their characteristics were systematically investigated using micro-Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). As the energy and dose of the proton irradiation increased, the defects induced in the graphene layers also increased gradually. The average defect distances of 10 MeV proton-irradiated graphene decreased to 29 +/- 5 nm at a dose of 3 x 10(16) cm(-2). The defect formation energies for various types of defects were compared by using density functional theory calculation. After proton irradiation, the results of micro-Raman scattering and XPS indicated p-doping effects due to adsorption of environmental molecules on the damaged graphene. Our results show a direct relationship between the defect formation of the graphene layers and the energy/dose of the proton irradiation.11Nsciescopu

Mechanistic insights into the phase transition and metal ex-solution phenomena of Pr0.5Ba0.5Mn0.85Co0.15O3-delta from simple to layered perovskite under reducing conditions and enhanced catalytic activity

We use density functional theory calculations and in situ X-ray diffraction spectroscopy experiments on Co-doped Pr0.5Ba0.5MnO3-delta (PBMCO) to understand how the phase transition from PBMCO to layered PBMCO occurred. The role of Co dopants for both the phase transition and the ex-solution is also elucidated. It turns out that the selective formation of oxygen vacancies at the Pr layer plays a key role in the phase transition to layered perovskite. The ex-solved Co nanoparticles showed higher catalytic activity than the doped one for CO oxidation. These results can guide the design of highly-active perovskite-based redox catalysts

Sr Segregation in Perovskite Oxides: Why It Happens and How It Exists

Among the phenomena related to the surface rearrangement of cations in perovskite-based oxides, A-site cation enrichment, Sr in particular, near the surface has been frequently observed. Upon annealing in an oxidizing atmosphere, Sr is often enriched on the surface as compared with the bulk composition of the material, which eventually forms Sr-rich phases or rearranges the crystal structure of the surface. This Sr segregation changes the structure and composition of the perovskite surfaces and thus affects the stability of the materials and the reactivity with gas phases. In this regard, many studies have been carried out in the field of solid oxide electrochemical cells (SOCs). In this review, we summarize the latest research efforts on Sr segregation in perovskite-based SOC O2 electrodes, with a focus on how excess Sr is present. We then discuss the origins of Sr segregation and suggest strategies for suppressing it to realize high-performance perovskite-based O2 electrodes.11Nsciescopu

Concentration Effect of Reducing Agents on Green Synthesis of Gold Nanoparticles: Size, Morphology, and Growth Mechanism

Under various concentration conditions of reducing agents during the green synthesis of gold nanoparticles (AuNPs), we obtain the various geometry (morphology and size) of AuNPs that play a crucial role in their catalytic properties. Through both theoretical and experimental approaches, we studied the relationship between the concentration of reducing agent (caffeic acid) and the geometry of AuNPs. As the concentration of caffeic acid increases, the sizes of AuNPs were decreased due to the adsorption and stabilizing effect of oxidized caffeic acids (OXCAs). Thus, it turns out that optimal concentration exists for the desired geometry of AuNPs. Furthermore, we investigated the growth mechanism for the green synthesis of AuNPs. As the caffeic acid is added and adsorbed on the surface of AuNPs, the aggregation mechanism and surface free energy are changed and consequently resulted in the AuNPs of various geometry.11Nsciescopu

Different catalytic behaviors of Pd and Pt metals in decalin dehydrogenation to naphthalene

The catalytic dehydrogenation from decalin to tetralin to naphthalene is usually performed over supported Pd or Pt catalysts at a high temperature due to the endothermic nature of the reaction. However, the mechanistic studies of the catalytic activity and selectivity are not still sufficient to understand the dehydrogenation reaction on these metal surfaces. In this study, we mechanistically investigated the dehydrogenation reaction of decalin to tetralin to naphthalene on Pd and Pt catalysts using density functional theory (DFT) calculations combined with experimental validation. We firstly explored the relative energy profile of the entire elementary steps of the dehydrogenation reaction. Our theoretical results demonstrate that the conversion of decalin to tetralin on the Pt catalyst is energetically more preferred to that on Pd. On the other hand, Pd exhibits an energetically more favored reaction pathway in the conversion of tetralin to naphthalene than Pt. It is found that the difference in the catalytic activity and selectivity between Pd and Pt originates from the different structural and chemical characteristics of the metals. Our experimental results also support that decalin is more easily dehydrogenated over Pt/C while the dehydrogenation of tetralin is more facile over Pd/C.11Nsciescopu

A new etching process for zinc oxide with etching rate and crystal plane control: experiment, calculation, and membrane application

The etching process can be a useful method for the morphology control of nanostructures. Using precise experiments and theoretical calculations, we report a new ZnO etching process triggered by the reaction of ZnO with transition metal cations and demonstrate that the etching rate and direction could be controlled by varying the kind of transition metal cation. In addition, the developed etching process was introduced to form a thin and uniform ZnO layer, which was utilized for the fabrication of an improved propylene-selective ZIF-8 membrane via conversion seeding and secondary growth.11Nsciescopu