Boris Kiefer

Low-temperature carbon monoxide oxidation catalysed by regenerable atomically dispersed palladium on alumina

Catalysis by single isolated atoms of precious metals has attracted much recent interest, as it promises the ultimate in atom efficiency. Most previous reports are on reducible oxide supports. Here we show that isolated palladium atoms can be catalytically active on industrially relevant γ-alumina supports. The addition of lanthanum oxide to the alumina, long known for its ability to improve alumina stability, is found to also help in the stabilization of isolated palladium atoms. Aberration-corrected scanning transmission electron microscopy and operando X-ray absorption spectroscopy confirm the presence of intermingled palladium and lanthanum on the γ-alumina surface. Carbon monoxide oxidation reactivity measurements show onset of catalytic activity at 40 °C. The catalyst activity can be regenerated by oxidation at 700 °C in air. The high-temperature stability and regenerability of these ionic palladium species make this catalyst system of potential interest for low-temperature exhaust treatment catalysts.

Density functional theory calculations of XPS binding energy shift for nitrogen-containing graphene-like structures

Our results validate the use of independent DFT predicted BE shifts for defect identification and constraining ambient pressure XPS observations for Me-Nx moieties in pyrolyzed carbon based ORR electrocatalysts. This supports the understanding of such catalysts as vacancy-and-substitution defects in a graphene-like matrix.

Crystal structure of graphite under room-temperature compression and decompression

Recently, sophisticated theoretical computational studies have proposed several new crystal structures of carbon (e.g., bct-C4, H-, M-, R-, S-, W-, and Z-carbon). However, until now, there lacked experimental evidence to verify the predicted high-pressure structures for cold-compressed elemental carbon at least up to 50 GPa. Here we present direct experimental evidence that this enigmatic high-pressure structure is currently only consistent with M-carbon, one of the proposed carbon structures. Furthermore, we show that this phase transition is extremely sluggish, which led to the observed broad x-ray diffraction peaks in previous studies and hindered the proper identification of the post-graphite phase in cold-compressed carbon.

Finite element simulations of the laser-heated diamond-anvil cell

Axial and radial temperature gradients in the laser-heated diamond-anvil cell are examined using finite element simulations. Calculations are carried out for an optically thin silicate or oxide sample separated from the diamonds by an insulation medium and heated by a TEM00 mode from an infrared laser. The peak temperature of the simulations was chosen to be a representative value (2200K) and sample dimensions are typical for experiments in the 20–50‐GPa range. The distance between the anvils is 30μm. The total temperature drop across the sample in the axial direction is controlled by two parameters: the filling fraction (thickness of sample∕distance between anvils) and the ratio of thermal conductivity between the sample and insulator (kS∕kI). The results of the numerical calculations agree well with a one-dimensional numerical model. For a sample filling fraction of 0.5, the axial temperature drop will range from about 1000K (>45%) for a thermal conductivity ratio of 1 to about 200K (<10%) for a conduct...

Rietveld structure refinement of MgGeO3 post-perovskite phase to 1 Mbar

Abstract Using the CaIrO3-type structure model (space group Cmcm), lattice parameters and atomic positions of the MgGeO3 post-perovskite (pPv) phase were determined based on Rietveld refinements at 78-109 GPa and first-principles calculations based on density functional theory. The reproducibility of structural parameters obtained for different samples, consistency with theoretical calculations, and good agreement with expected bond lengths based on structurally similar materials all provide evidence for both validity of CaIrO3-type structure model for the pPv phase in MgGeO3 exceeding 1 Mbar and reliability of structural parameters obtained by Rietveld refinements approaching 1 Mbar. The MgGeO3 pPv phase exhibits strong anisotropy in axial compressibility, with the b-axis being most compressible. The polyhedral bulk modulus for the GeO6 octahedron is 1.9× larger than that for the MgO8 hendecahedron. Examination of neighboring O-O distances shows that the O-O distance aligned along the a direction is one of the longest and that aligned along c is one of the shortest, and these may be related to the lower compressibility along c compared with a. Comparison of structural features of MgGeO3 pPv with those for MgSiO3, NaMgF3, and CaIrO3 pPv show that MgSiO3 pPv has more similarity with NaMgF3 and MgGeO3 pPv than with CaIrO3 pPv in such parameters as degree of octahedral distortion, implying that both NaMgF3 and MgGeO3 pPv are better analogs to MgSiO3 pPv than CaIrO3 pPv.

Density functional theory study of the oxygen reduction reaction mechanism in a BN co-doped graphene electrocatalyst

Density functional theory calculations were performed to explore the stability and chemistry of active sites, and the mechanism of the ORR in a metal free BN co-doped graphene electrocatalyst. The results show that formation of graphitic G-BCxNy defects is energetically favorable than vacancy induced V-BCxNy defects in graphene. We find O2 physisorption on G-BC3, G-BC2N and G-BCN2 defects. Thus these defects are unlikely sites that initiate the ORR. In contrast, the chemisorption of ORR species O2, OOH and O, and the downhill energy landscape of the ORR on G-BN3 sites show that G-BN3 sites are active for the complete 4e− reduction of O2 to 2H2O. We furthermore explore the catalytic activity of vacancy induced V-BCxNy defects for the ORR. Much stronger adsorption of O2 and OH on V-BCxNy sites compared to G-BN3 sites indicates that V-BCxNy sites would likely be blocked by OH and the catalytic activity is limited to G-BN3 sites. Thus, an enhancement in catalytic activity and selectivity of BN co-doped graphene for a net 4e− complete O2 reduction can be achieved by increasing the concentration of G-BN3 defects.

P–V equation of state for Fe2P and pressure-induced phase transition in Fe3P

As part of our ongoing investigations of elasticity and high-pressure stability in the Fe–P system, we have measured the room-temperature bulk modulus (K 0T) of Fe2P, barringerite, to 8 GPa using in situ synchrotron X-ray diffraction and diamond anvil cells. A second-order fit (i.e. dK/dP fixed at 4) to our experimental data using the Birch–Murnaghan equation of state produces a K 0T of 165±3 GPa. This value is ∼4% less than the experimental values for Fe3P. For comparison with the experimental data, we have also performed first-principle theoretical calculations on this phase. For ferromagnetic Fe2P at zero pressure, we find that the magnetic moments increase rapidly for a Hubbard U>1 eV and are significantly higher than observed experimentally. Thus, our results support previous findings that magnetism in Fe2P is largely itinerant with at most a minor component due to on-site correlation in the iron-3d shell. Additionally, we present new high-pressure diffraction data for a natural Fe3P, schreibersite, sample which conclusively demonstrate that a first-order phase transformation occurs between 15 and 20 GPa.

Determining the high-pressure phase transition in highly-ordered pyrolitic graphite with time-dependent electrical resistance measurements

Long-duration, high-pressure resistance measurements on highly-ordered pyrolytic graphite in a diamond-anvil cell show a sluggish phase transition occurring at ∼19 GPa, as evidenced by the time-dependent behavior of the sample resistance. The instantaneous resistance response to pressure adjustment shows a ∼10 GPa hysteresis that has been observed previously, rendering the conjectured direct relationship between resistance and phase-transition tentative. In contrast, the evolution of the resistance with time after the instantaneous response shows a systematic, reproducible, and distinct behavior, which allows reducing the uncertainty in transition pressure to ∼2 GPa. This largely reduced hysteresis shows explicitly that the phase transition is directly related to changes in electronic structure and resistance and establishes consistency with other commonly used experimental techniques to explore phase transitions at high pressures. We augment our experiments with first-principle density-functional theory ...

Elastic moduli and hardness of highly incompressible platinum perpnictide PtAs2

PtAs2 appears to be the least compressible known arsenide with a bulk modulus of 220(5) GPa and a shear modulus of between 64 and 77 GPa. PtAs2 has a hardness of 11(1) GPa, which is remarkably high for an arsenide. These elastic and mechanical properties in combination with the known chemical inertness and the small indirect band gap add interest to the use and occurrence of PtAs2 at Pt-GaAs contacts in transistors. We note the modest fracture toughness of 1.1–1.6 MPa m1/2 of PtAs2.

Defects on Graphene with and without Nitrogen

We have used density functional theory calculations to study the stability of high density bulk defects in carbon with and without nitrogen. It was found that all the structures are energetically unstable, and the stability of the defects decreases with increasing number of nitrogen atoms independent of synthesis route and the carbon reference state. The results also highlight the significance of stabilizing factors for these defects, such as transition metals which are often used during synthesis of the carbon supports.