Promising Stability of Gold-Based Catalysts Prepared by Direct Anionic Exchange for DeNO x Applications in Lean Burn Conditions

Supported gold catalysts on γ-Al₂O₃ have been investigated in the catalytic reduction of NO _x in simulated Diesel exhaust gas conditions. Different parameters have been examined essentially the mode of gold incorporation via classical deposition–precipitation and anionic exchange methods and the nature of the pre-activation thermal treatment. The resistance to thermal ageing under reactive conditions at 500 °C was found completely different with a significant rate enhancement on anionic-exchange samples. Further comparisons also show that the nature of the pre-activation thermal treatment influences the extent of surface reconstructions during thermal ageing with a detrimental effect of reductive pre-treatment on the catalytic performances.     

Theoretical study on strain-induced variations in electronic properties of monolayer MoS2

Ultrathin MoS₂ sheets and nanostructures are promising materials for electronic and optoelectronic devices as well as chemical catalysts. To expand their potential in applications, a fundamental understanding is needed of the electronic structure and carrier mobility as a function of strain. In this paper, the effect of strain on electronic properties of monolayer MoS₂ is investigated using ab initio simulations based on density functional theory. Our calculations are performed in both infinitely large two-dimensional (2D) sheets and one-dimensional (1D) nanoribbons which are theoretically cut from the sheets with semiconducting \( [\bar{1}100] \) (armchair) edges. The 2D crystal is studied under biaxial strain, uniaxial strain, and uniaxial stress conditions, while the 1D nanoribbon is studied under a uniaxial stress condition. Our results suggest that the electronic bandgap of the 2D sheet experiences a direct-indirect transition under both tensile and compressive strains. Its bandgap energy (E _g) decreases under tensile strain/stress conditions, while for an in-plane compression, E _g is initially raised by a small amount and then decreased as the strain varies from 0 to ?6 %. On the other hand, E _g at the semiconducting edges of monolayer MoS₂ nanoribbons is relatively invariant under uniaxial stretches or compressions. The effective masses of electrons at the conduction band minimum (CBM) and holes at the valence band maximum (VBM) are generally decreased as the in-plane extensions or compressions become stronger, but abrupt changes occur when CBM or VBM shifts between different k-points in the first Brillouin zone.     

Low temperature preparation of stabilized zirconia

The preparation of stabilized zirconia by thermal decomposition of metal alkoxides is reported. Formation of stabilized zirconia takes place at 400° C. The a.c. conductivity of the samples has been measured from 400 to 1000°C. The best conductivity is found in ZrO₂doped with 15 per cent CaO, which at 400° C is 2.37×10⁻⁶ Ω⁻¹ cm⁻¹ and at 1000°C is 1.26×10⁻² Ω⁻¹ cm⁻¹, with an activation energy of 1.16eV. Transport number measurements show that stabilized zirconia prepared by this method is purely an oxygen ion conductor.     

Role of nanoscale Cu/Ta interfaces on the shock compression and spall failure of nanocrystalline Cu/Ta systems at the atomic scales

Molecular dynamics (MD) simulations are used to investigate the role of size and distribution of nanoscale Cu/Ta interfaces on the nucleation and evolution of defects during shock loading and spall failure of nanocrystalline (nc) Cu/Ta alloys. Cu/Ta interfaces are introduced through the embedding of Ta clusters in nc-Cu matrix. The phase stability of the embedded Ta clusters either as FCC or BCC clusters is first investigated and reveals that the FCC Ta clusters have a lower energy for diameters less than 4 nm, whereas the BCC Ta clusters have a lower energy for the larger diameters. The shock simulations are then carried out for Ta clusters with an average diameter of 1 and 3 nm and concentrations of 3.0, 6.3 and 10.0% to investigate the role of size and distribution of Cu/Ta interfaces (due to presence of clusters) on the nucleation and evolution of dislocations as well as the spall strength of the alloy. The MD simulations indicate that the Cu/Ta interfaces reduce the capability of nc-Cu to accommodate plasticity through nucleation of dislocations and create void nucleation sites during spallation. The MD simulations further reveal that the impact strengthening effects due to the presence of nanoscale Cu/Ta interfaces are strongly dependent upon the size and distribution of Ta clusters, as well as the grain size of Cu matrix. Smaller size of interfaces (cluster size), higher concentration of Ta (smaller spacing between interfaces) and larger matrix grain size render higher spall strengths of nc-Cu/Ta microstructures.     

Role of $${\varvec{\alpha}} \to {\varvec{\varepsilon}} \to {\varvec{\alpha}}$$ α → ε → α phase transformation on the spall behavior of iron at atomic scales

Journal of Materials Science - Shock compression of iron microstructures above a threshold stress results in a $$\alpha \left( {BCC} \right) \to \varepsilon \left( {HCP} \right)$$ transformation,...     

Decomposition of NO over Cu-AITS-1 zeolites

H-AITS-1 zeolite with Si/Ti = 50 and Si/Al = 50 was employed in preparing catalyst samples by ion-exchange and impregnation with a copper nitrate solution to obtain 0.24–1.15 wt.% and 1.5, 2 and 2.5 wt.% Cu loading, respectively. The catalytic properties for the NO decomposition were compared with that of Cu-ZSM-5 (Si/Al = 25 with 2 wt.% Cu loading) and similarity was found between the AITS-1 based samples and Cu-ZSM-5. Due to the higher acidity, the activity at 500°C per total copper atoms (an apparent turnover frequency, TOF) was significantly higher over Cu based AITS-1 samples being 2–3 × 10⁻³ s⁻¹ as compared to 1 × 10⁻³ s⁻¹ measured on Cu-ZSM-5. For the ion-exchanged Cu-AITS-1 there was an increase in TOF with increasing copper content, whereas on the impregnated samples a decrease in TOF was found. On all catalysts there was a maximum in the NO conversion at 500–550°C. The amount of NO per copper atom measured by temperature programmed desorption (TPD) was about the same as that on Cu-ZSM-5 and the features of the TPD were also similar. At the first contact of the catalyst at 500°C with the 2 vol% NO/Ar gas a transient N₂O formation and a considerable delay in the O₂ formation was observed. This could, however, be reproduced only on fresh catalyst, while all further transients showed different but reproducible features using the same sample.     

Effect of the Strain Rate and Microstructure on Damage Growth in Aluminum

Materials used in soldier protective structures, such as armor, vehicles and civil infrastructures, are being improved for performance in extreme dynamic environments. Nanocrystalline metals show significant promise in the design of these structures with superior strengths attributed to the dislocation-based and grain-boundary-based processes as compared to their polycrystalline counterparts. An optimization of these materials, however, requires a fundamental understanding of damage evolution at the atomic level. Accordingly, atomistic molecular dynamics simulations are performed using an embedded-atom method (EAM) potential on three nano-crystalline aluminum atom systems, one a Voronoi-based nano-crystalline system with an average grain size of 10 nm, and the other two single crystals. These simulations are performed under the condition of uniaxial expansion at several strain rates ranging from 10⁶s^-1 to 10¹⁰s^-1. Results for the effective stress are discussed with the aim of establishing the role of the strain rate and microstructure on the evolution of the plastic strain and void volume fraction and the eventual loss of stress carrying capability of the atom systems.