Strained few-layer MoS2 with atomic copper and selectively exposed in-plane sulfur vacancies for CO2 hydrogenation to methanol.

In-plane sulfur vacancies (Sv) in molybdenum disulfide (MoS2) were newly unveiled for CO2 hydrogenation to methanol, whereas edge Sv were found to facilitate methane formation. Thus, selective exposure and activation of basal plane is crucial for methanol synthesis. Here, we report a mesoporous silica-encapsulated MoS2 catalysts with fullerene-like structure and atomic copper (Cu/MoS2@SiO2). The main approach is based on a physically constrained topologic conversion of molybdenum dioxide (MoO2) to MoS2 within silica. The spherical curvature enables the generation of strain and Sv in inert basal plane. More importantly, fullerene-like structure of few-layer MoS2 can selectively expose in-plane Sv and reduce the exposure of edge Sv. After promotion by atomic copper, the resultant Cu/MoS2@SiO2 exhibits stable specific methanol yield of 6.11 molMeOH molMo-1 h-1 with methanol selectivity of 72.5% at 260 °C, much superior to its counterparts lacking the fullerene-like structure and copper decoration. The reaction mechanism and promoting role of copper are investigated by in-situ DRIFTS and in-situ XAS. Theoretical calculations demonstrate that the compressive strain facilitates Sv formation and CO2 hydrogenation, while tensile strain accelerates the regeneration of active sites, rationalizing the critical role of strain.

Controlling surface cation segregation in a double perovskite for oxygen anion transport in high temperature energy conversion devices.

Double perovskite materials have shown promising applications as an electrode in solid oxide fuel cells and Li-air batteries for oxygen reduction, evolution, and transport. However, degradation of the material due to cation migration to the surface, forming secondary phases, poses an existential bottleneck in materials development. Herein, a theoretical approach combining density functional theory and molecular dynamics simulations is presented to study the Ba-cation segregation in a double perovskite NdBaCo2O5+δ. Solutions to circumvent segregation at the molecular level are presented in two different forms by applying strain and introducing dopants in the structure. On applying compressive strain or Ca as a dopant in the NBCO structure, segregation is estimated to reduce significantly. A more direct way of estimating cation segregation is proposed in MD simulations, wherein the counting of the cations migrating from the sub-surface layers to the surface provided a reliable theoretical assessment of the level of cation segregation.

Genesis of Active Pt/CeO2 Catalyst for Dry Reforming of Methane by Reduction and Aggregation of Isolated Platinum Atoms into Clusters.

Atomically dispersed metal catalysts offer the advantages of efficient metal utilization and high selectivities for reactions of technological importance. Such catalysts have been suggested to be strong candidates for dry reforming of methane (DRM), offering prospects of high selectivity for synthesis gas without coke formation, which requires ensembles of metal sites and is a challenge to overcome in DRM catalysis. However, investigations of the structures of isolated metal sites on metal oxide supports under DRM conditions are lacking, and the catalytically active sites remain undetermined. Data characterizing the DRM reaction-driven structural evolution of a cerium oxide-supported catalyst, initially incorporating atomically dispersed platinum, and the corresponding changes in catalyst performance are reported. X-ray absorption and infrared spectra show that the reduction and agglomeration of isolated cationic platinum atoms to form small platinum clusters/nanoparticles are necessary for DRM activity. Density functional theory calculations of the energy barriers for methane dissociation on atomically dispersed platinum and on platinum clusters support these observations. The results emphasize the need for in-operando experiments to assess the active sites in such catalysts. The inferences about the catalytically active species are suggested to pertain to a broad class of catalytic conversions involving the rate-limiting dissociation of light alkanes.

Engineering nanoscale H supply chain to accelerate methanol synthesis on ZnZrOx.

Metal promotion is the most widely adopted strategy for enhancing the hydrogenation functionality of an oxide catalyst. Typically, metal nanoparticles or dopants are located directly on the catalyst surface to create interfacial synergy with active sites on the oxide, but the enhancement effect may be compromised by insufficient hydrogen delivery to these sites. Here, we introduce a strategy to promote a ZnZrOx methanol synthesis catalyst by incorporating hydrogen activation and delivery functions through optimized integration of ZnZrOx and Pd supported on carbon nanotube (Pd/CNT). The CNT in the Pd/CNT + ZnZrOx system delivers hydrogen activated on Pd to a broad area on the ZnZrOx surface, with an enhancement factor of 10 compared to the conventional Pd-promoted ZnZrOx catalyst, which only transfers hydrogen to Pd-adjacent sites. In CO2 hydrogenation to methanol, Pd/CNT + ZnZrOx exhibits drastically boosted activity-the highest among reported ZnZrOx-based catalysts-and excellent stability over 600 h on stream test, showing potential for practical implementation.

Controlling surface cation segregation in a nanostructured double perovskite GdBaCo2O5+δ electrode for solid oxide fuel cells.

Mechanistic studies, utilizing molecular dynamics (MD) and density functional theory (DFT) calculations, were undertaken to provide a molecular level explanation of Ba cation segregation in double perovskite GdBaCo2O5+δ (GBCO) electrodes. The energy (γ) of the terminal surface having only Ba cations, indicated the surface to be the most stable (γ = 6.7 kJ mol-1Å-2) as compared to the other surfaces. MD simulations elaborated on the cation disorder in the near surface region where Ba cations in the subsurface region were observed to migrate towards the surface. This led to a disruption in cation ordering with a propensity to form multiphases in the near surface region. In the near surface zone, oxygen anion diffusivity was observed to be reduced by an order of magnitude (D = 1.6 × 10-11 cm2 s-1 at 873 K) as compared to the bulk oxygen anion diffusivity value (D = 1.96 × 10-10 cm2 s-1 at 873 K). A novel idea was then proposed to control the degree of surface segregation of Ba cations by applying nanostructuring of the GBCO material in the form of nanoparticles. MD simulations elucidated that the near surface region having a high degree of cation disorder in the nanostructured GBCO may regain back the oxygen anion diffusivity value (D = 3.98 × 10-10 cm2 s-1, at 873 K) comparable to the bulk core region (D = 2.51 × 10-10 cm2 s-1, at 873 K). A proof of concept experiment was setup to test this hypothesis. The electrochemical performance of the electrode, fabricated using GBCO nanoparticles, was measured to improve by 15% as compared to the electrode synthesized with a bulk size GBCO material. This was attributed to the control in Ba-cation segregation, obtained on nanostructuring which resulted in higher oxygen anion transport in the near-surface region of the electrode material. XPS characterization of the surface of the nanostructured GBCO materials supported this assertion.

Identifying the Origin of the Limiting Process in a Double Perovskite PrBa0.5Sr0.5Co1.5Fe0.5O5+δ Thin-Film Electrode for Solid Oxide Fuel Cells.

Oxygen reduction reaction in a double perovskite material, PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF), was studied for application as a cathode in a solid oxide fuel cell (SOFC). Electrochemical measurements were performed on a geometrically well-defined dense thin-film (0.8-2 µm thickness) electrode, fabricated as a symmetric cell. In combination with density functional theory (DFT) and molecular dynamics (MD) simulations, experiments provided an insight into the operating mechanism of the SOFC material tested at an open-circuit voltage. The dense thin-film electrode of PBSCF showed a thickness-dependent electrochemical performance, suggesting bulk diffusion limitation. To understand the origin of this diffusion-limiting electrochemical performance, DFT calculations were utilized to calculate the surface (γ) and oxygen vacancy formation (EOV) energies. For example, EOV in the Pr plane (190 kJ/mol) of PBSCF was measured to be lower than that of the BaSr plane (EOV = 297 kJ/mol). In addition, oxygen vacancies were difficult to be created in the BaSr/CoFe terminal surface (EOV = 341.6 kJ/mol) as compared to other terminal surfaces. MD simulations further elaborated on the nature of cation disordering in the surface and subsurface regions, consequently leading to the preferential segregation of the Ba cations to the surface, which is a known phenomenon in such double perovskite materials. Because of cation disordering and segregation of Ba species, the oxygen anion diffusivity (∼10-12 cm2 s-1), calculated from MD, in the near-surface region was observed to be 2 orders of magnitude lesser than that of the bulk (D = 2.98 × 10-10 cm2 s-1) of the material at 973 K. Surface characterization of the thin-film electrode using X-ray photoelectron spectroscopy was indicative of a nonperovskite Ba2+ phase on the electrode surface. The segregation of Ba cations was linked with the transport of oxygen anions, which was limiting the electrochemical performance of the electrode.

Characteristic symptoms and adaptive behaviors of children with autism.

OBJECTIVE: To determine the characteristic symptoms and adaptive behaviors of children with autism, as well as the distribution of autism severity groups across gender. STUDY DESIGN: Cross-sectional observational study. PLACE AND DURATION OF STUDY: Special Education Schools of Rawalpindi and Islamabad, from September 2011 to January 2012. METHODOLOGY: Thirty nine children of either gender, aged 3 - 16 years and enrolled in special education schools, fulfilled the DSM-IV-TR criteria of autism. Among those, were identified as meeting the criteria of autism. The childhood autism rating scale-2 (CARS-2) was used to study the characteristics and severity of symptoms of autism. Later, adaptive behavior scale (school edition: 2) ABS-S: 2, was administered on children (n=21) to formulate the level of adaptive functioning. RESULTS: There were 15 boys and 8 girls with mean age of 10.6 ± 2.97 years. They showed marked impairment in verbal communication (mean=3.17 ± 0.90) followed by relating to people (mean=2.75 ± 0.83) and general impression (mean=2.73 ± 0.7). Most of the children showed average to below average adaptive behaviors on number and time (n=19, 90.5%), independent functioning (n=17, 81.0%), self direction (n=17, 81.0%), physical development (n=13, 61.9%), responsibility (n=12, 57.1%) and socialization (n=13, 61.9%) as well as poor to very poor adaptive behaviors on prevocational skill (n=15, 71.4%), language development (n=13, 61.9%) and economic development (n=13, 61.9%). The frequency of boys with autism was more towards moderate to severely impaired spectrum, without gender differences in any symptom associated with autism. CONCLUSION: Comprehension of the presentation of characteristic symptoms of children with autism will be helpful in devising the indigenous intervention plans that are congruent with the level of adaptive functioning.