Redox dynamics and surface structures of an active palladium catalyst during methane oxidation.

Catalysts based on palladium are among the most effective in the complete oxidation of methane. Despite extensive studies and notable advances, the nature of their catalytically active species and conceivable structural dynamics remains only partially understood. Here, we combine operando transmission electron microscopy (TEM) with near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and density functional theory (DFT) calculations to investigate the active state and catalytic function of Pd nanoparticles (NPs) under methane oxidation conditions. We show that the particle size, phase composition and dynamics respond appreciably to changes in the gas-phase chemical potential. In combination with mass spectrometry (MS) conducted simultaneously with in situ observations, we uncover that the catalytically active state exhibits phase coexistence and oscillatory phase transitions between Pd and PdO. Aided by DFT calculations, we provide a rationale for the observed redox dynamics and demonstrate that the emergence of catalytic activity is related to the dynamic interplay between coexisting phases, with the resulting strained PdO having more favorable energetics for methane oxidation.

Polyaniline-wrapped MnMoO4 as an active catalyst for hydrogen production by electrochemical water splitting.

Developing efficient, low-cost, and environment-friendly electrocatalysts for hydrogen generation is critical for lowering energy usage in electrochemical water splitting. Moreover, for commercialization, fabricating cost-efficient, earth-abundant electrocatalysts with superior characteristics is of urgent need. Towards this endeavor, we report the synthesis of PANI-MnMoO4 nanocomposites using a hydrothermal approach and an in situ polymerization method with various concentrations of MnMoO4. The fabricated nanocomposite electrocatalyst exhibits bifunctional electrocatalytic activity towards the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) at a lower overpotential of 410 mV at 30 mA cm-2 and 155 mV at 10 mA cm-2, respectively in an alkaline electrolyte. Furthermore, while showing overall water splitting (OWS) performance, the optimized PM-10 (PANI-MnMoO4) electrode reveals the most outstanding OWS performance with a lower cell voltage of 1.65 V (vs. RHE) at a current density of 50 mA cm-2 with an excellent long-term cell resilience of 24 h.

Strain in Silica-Supported Ga(III) Sites: Neither Too Much nor Too Little for Propane Dehydrogenation Catalytic Activity.

Well-defined Ga(III) sites on SiO2 are highly active, selective, and stable catalysts in the propane dehydrogenation (PDH) reaction. In this contribution, we evaluate the catalytic activity toward PDH of tricoordinated and tetracoordinated Ga(III) sites on SiO2 by means of first-principles calculations using realistic amorphous periodic SiO2 models. We evaluated the three reaction steps in PDH, namely, the C-H activation of propane to form propyl, the ß-hydride (ß-H) transfer to form propene and a gallium hydride, and the H-H coupling to release H2, regenerating the initial Ga-O bond and closing the catalytic cycle. Our work shows how Brønsted-Evans-Polanyi relationships are followed to a certain extent for these three reaction steps on Ga(III) sites on SiO2 and highlights the role of the strain of the reactive Ga-O pairs on such sites of realistic amorphous SiO2 models. It also shows how transition-state scaling holds very well for the ß-H transfer step. While highly strained sites are very reactive sites for the initial C-H activation, they are more difficult to regenerate. The corresponding less strained sites are not reactive enough, pointing to the need for the right balance in strain to be an effective site for PDH. Overall, our work provides an understanding of the intrinsic activity of acidic Ga single sites toward the PDH reaction and paves the way toward the design and prediction of better single-site catalysts on SiO2 for the PDH reaction.

Improving the optical and thermoelectric properties of Cs2InAgCl6 with heavy substitutional doping: a DFT insight.

The next-generation indium-based lead-free halide material Cs2InAgCl6 is promising for photovoltaic applications due to its good air stability and non-toxic behavior. However, its wide bandgap (>3 eV) is not suitable for the solar spectrum and hence reduces its photoelectronic efficiency for device applications. Here we report a significant bandgap reduction from 2.85 eV to 0.65 eV via substitutional doping and its effects on the optoelectronic and opto-thermoelectric properties from a first-principles study. The results predict that Sn/Pb and Ga and Cu co-doping will enhance the density of states significantly near the valence band maximum (VBM) and thus reduce the bandgap via shifting the VBM upward, while alkali metals (K/Rb) slightly increase the bandgap. A strong absorption peak near the Shockley-Queisser limit is observed in the co-doped case, while in the Sn/Pb-doped case, we notice a peak in the middle of the visible region of the solar spectrum. The nature of the bandgap is indirect with Cu-Ga/Pb/Sn doping, and a significant reduction in the bandgap, from 2.85 eV to 0.65 eV, is observed in the case of Ga-Cu co-doping. We observe a significant increase in the power factor (PF) (2.03 mW m-1 K-2) for the n-type carrier after Pb-doping, which is ∼3.5 times higher than in the pristine case (0.6 mW m -1 K-2) at 500 K.

Decisive Role of Perimeter Sites in Silica-Supported Ag Nanoparticles in Selective Hydrogenation of CO2 to Methyl Formate in the Presence of Methanol.

Methyl formate synthesis by hydrogenation of carbon dioxide in the presence of methanol offers a promising path to valorize carbon dioxide. In this work, silica-supported silver nanoparticles are shown to be a significantly more active catalyst for the continuous methyl formate synthesis than the known gold and copper counterparts, and the origin of the unique reactivity of Ag is clarified. Transient in situ and operando vibrational spectroscopy and DFT calculations shed light on the reactive intermediates and reaction mechanisms: a key feature is the rapid formation of surface chemical species in equilibrium with adsorbed carbon dioxide. Such species is assigned to carbonic acid interacting with water/hydroxyls on silica and promoting the esterification of formic acid with adsorbed methanol at the perimeter sites of Ag on SiO2 to yield methyl formate. This study highlights the importance of employing combined methodologies to verify the location and nature of active sites and to uncover fundamental catalytic reaction steps taking place at metal-support interfaces.

Nanostructured N-doped orthorhombic Nb2O5 as an efficient stable photocatalyst for hydrogen generation under visible light.

The synthesis of orthorhombic nitrogen-doped niobium oxide (Nb2O5-xNx) nanostructures was performed and a photocatalytic study carried out in their use in the conversion of toxic H2S and water into hydrogen under UV-Visible light. Nanostructured orthorhombic Nb2O5-xNx was synthesized by a simple solid-state combustion reaction (SSCR). The nanostructural features of Nb2O5-xNx were examined by FESEM and HRTEM, which showed they had a porous chain-like structure, with chains interlocked with each other and with nanoparticles sized less than 10 nm. Diffuse reflectance spectra depicted their extended absorbance in the visible region with a band gap of 2.4 eV. The substitution of nitrogen in place of oxygen atoms as well as Nb-N bond formation were confirmed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. A computational study (DFT) of Nb2O5-xNx was also performed for investigation and conformation of the crystal and electronic structure. N-Substitution clearly showed a narrowing of the band gap due to N 2p bands cascading above the O 2p band. Considering the band gap in the visible region, Nb2O5-xNx exhibited enhanced photocatalytic activity toward hydrogen evolution (3010 µmol h-1 g-1) for water splitting and (9358 µmol h-1 g-1) for H2S splitting under visible light. The enhanced photocatalytic activity of Nb2O5-xNx was attributed to its extended absorbance in the visible region due to its electronic structure being modified upon doping, which in turn generates more electron-hole pairs, which are responsible for higher H2 generation. More significantly, the mesoporous nanostructure accelerated the supression of electron and hole recombination, which also contributed to the enhancement of its activity.

Hybrid density functional based study on the band structure of trioctahedral mica and its dependence on the variation of Fe(2+) content.

A hybrid density functional based study of a phyllosilicate (PS) is presented here for the first time. Using all-electron electronic structure calculations with the B3LYP hybrid functional, we have investigated the electronic and structural properties of a series of trioctahedral 1M-polytype K-bearing micas starting from phlogopite (the Mg-end member), ending with the annite (the Fe-end member), and passing through the biotite (a solid solution of the end members). Electronic band gap is calculated for all the compositions and nature of the electronic transition is discussed with the aid of band structure and density of states plots. An excellent agreement with the available experimental data has been observed. An insulator to semiconductor transition is explained on the basis of orbital hybridization. A further comparison is made using the pure GGA functional. For the completeness of the study, the dielectric properties of phlogopite are calculated using the coupled perturbed Kohn-Sham scheme, as implemented within the CRYSTAL09 code.

A variant epidemic methicillin resistant Staphylococcus aureus-15 cavernous sinus thrombosis and meningitis: a rare occurrence with unusual presentation.

Septic cavernous sinus thrombosis (CST) is an uncommon clinical syndrome. Although Staphylococcus aureus (S aureus) is the most common bacterial pathogen causing CST, it is infrequent as a cause of meningitis. We report the first case of CST and meningitis from Bengaluru, Karnataka, caused by community-acquired epidemic methicillin resistant Staphylococcus aureus-15 (EMRSA-15), in a previously healthy individual without known risk factors; the patient recovered following treatment with vancomycin. The isolate was genotyped as belonging to staphylococcal cassette chromosome mec type IV and sequence type 22 and carried the panton-valentine leucocidin gene. It is the first Indian EMRSA-15 disease isolate from a case of meningitis. EMRSA-15 has been a major problem in hospitals in UK and it is a cause for great concern in Indian hospitals too.