61 research outputs found
In situ characterization of mesoporous Co/CeO2 catalysts for the high-temperature water-gas shift
Mesoporous Co/CeO2 catalysts were found to exhibit significant activity for the high-temperature water-gas shift (WGS) reaction with cobalt loadings as low as 1 wt %. The catalysts feature a uniform dispersion of cobalt within the CeO2 fluorite type lattice with no evidence of discrete cobalt phase segregation. In situ XANES and ambient pressure XPS experiments were used to elucidate the active state of the catalysts as partially reduced cerium oxide doped with oxidized cobalt atoms. In situ XRD and DRIFTS experiments suggest facile cerium reduction and oxygen vacancy formation, particularly with lower cobalt loadings. In situ DRIFTS analysis also revealed the presence of surface carbonate and bidentate formate species under reaction conditions, which may be associated with additional mechanistic pathways for the WGS reaction. Deactivation behavior was observed with higher cobalt loadings. XANES data suggest the formation of small metallic cobalt clusters at temperatures above 400 °C may be responsible. Notably, this deactivation was not observed for the 1% cobalt loaded catalyst, which exhibited the highest activity per unit of cobalt.Peer ReviewedPostprint (author's final draft
Microscopic Relaxation Channels in Materials for Superconducting Qubits
Despite mounting evidence that materials imperfections are a major obstacle
to practical applications of superconducting qubits, connections between
microscopic material properties and qubit coherence are poorly understood.
Here, we perform measurements of transmon qubit relaxation times in
parallel with spectroscopy and microscopy of the thin polycrystalline niobium
films used in qubit fabrication. By comparing results for films deposited using
three techniques, we reveal correlations between and grain size, enhanced
oxygen diffusion along grain boundaries, and the concentration of suboxides
near the surface. Physical mechanisms connect these microscopic properties to
residual surface resistance and through losses arising from the grain
boundaries and from defects in the suboxides. Further, experiments show that
the residual resistance ratio can be used as a figure of merit for qubit
lifetime. This comprehensive approach to understanding qubit decoherence charts
a pathway for materials-driven improvements of superconducting qubit
performance
Distinguishing electronic contributions of surface and sub-surface transition metal atoms in Ti-based MXenes
MXenes are a rapidly-expanding family of 2D transition metal carbides and nitrides that have attracted attention due to their excellent performance in applications ranging from energy storage to electromagnetic interference shielding. Numerous other electronic and magnetic properties have been computationally predicted, but not yet realized due to the experimental difficulty in obtaining uniform surface terminations (Tx), necessitating new design approaches for MXenes that are independent of surface terminations. In this study, we distinguished the contributions of surface and sub-surface Ti atoms to the electronic structure of four Ti-containing MXenes (Ti2CTx, Ti3C2Tx, Cr2TiC2Tx, and Mo2TiC2Tx) using soft x-ray absorption spectroscopy. For MXenes with no Ti atoms on the surface transition metal layers, such as Mo2TiC2Tx and Cr2TiC2Tx, our results show minimal changes in the spectral features between the parent MAX phase and its MXene. In contrast, for MXenes with surface Ti atoms, here Ti3C2Tx and Ti2CTx, the Ti L-edge spectra are significantly modified compared to their parent MAX phase compounds. First principles calculations provide similar trends in the partial density of states derived from surface and sub-surface Ti atoms, corroborating the spectroscopic measurements. These results reveal that electronic states derived from sub-surface M-site layers are largely unperturbed by the surface terminations, indicating a relatively short length scale over which the Tx terminations alter the nominal electron count associated with Ti atoms and suggesting that desired band features should be hosted by sub-surface M-sites that are electronically more robust than their surface M-site counterparts
Chemical profiles of the oxides on tantalum in state of the art superconducting circuits
Over the past decades, superconducting qubits have emerged as one of the
leading hardware platforms for realizing a quantum processor. Consequently,
researchers have made significant effort to understand the loss channels that
limit the coherence times of superconducting qubits. A major source of loss has
been attributed to two level systems that are present at the material
interfaces. We recently showed that replacing the metal in the capacitor of a
transmon with tantalum yields record relaxation and coherence times for
superconducting qubits, motivating a detailed study of the tantalum surface. In
this work, we study the chemical profile of the surface of tantalum films grown
on c-plane sapphire using variable energy X-ray photoelectron spectroscopy
(VEXPS). We identify the different oxidation states of tantalum that are
present in the native oxide resulting from exposure to air, and we measure
their distribution through the depth of the film. Furthermore, we show how the
volume and depth distribution of these tantalum oxidation states can be altered
by various chemical treatments. By correlating these measurements with detailed
measurements of quantum devices, we can improve our understanding of the
microscopic device losses
Avoiding CO2 Improves Thermal Stability at the Interface of Li7La3Zr2O12 Electrolyte with Layered Oxide Cathodes
Observation of Tunneling in the Hydrogenation of Atomic Nitrogen on the Ru(001) Surface to Form NH
The kinetics of NH and ND formation and dissociation reactions
on Ru(001) were studied using time-dependent reflection absorption
infrared spectroscopy (RAIRS). Our results indicate that NH and ND
formation and dissociation on Ru(001) follow first-order kinetics.
In our reaction temperature range (320â390 K for NH and 340â390
K for ND), the apparent activation energies for NH and ND formation
were found to be 72.2 ± 1.9 and 87.1 ± 1.8 kJ/mol, respectively,
while NH and ND dissociation reactions between 370 and 400 K have
apparent activation barriers of 106.9 ± 4.1 and 101.8 ±
4.8 kJ/mol, respectively. The lower apparent activation energy for
NH formation than that for ND as well as the comparison between experimentally
measured isotope effects with theoretical results strongly indicates
that tunneling already starts to play a role in this reaction at a
temperature as high as 340 K
Stabilized CoâFree LiâRich Oxide Cathode Particles with An Artificial Surface Prereconstruction
© 2020 Wiley-VCH GmbH Li-rich metal oxide (LXMO) cathodes have attracted intense interest for rechargeable batteries because of their high capacity above 250 mAh gâ1. However, the side effects of hybrid anion and cation redox (HACR) reactions, such as oxygen release and phase collapse that result from global oxygen migration (GOM), have prohibited the commercialization of LXMO. GOM not only destabilizes the oxygen sublattice in cycling, aggravating the well-known voltage fading, but also intensifies electrolyte decomposition and Mn dissolution, causing severe full-cell performance degradation. Herein, an artificial surface prereconstruction (ASR) for Li1.2Mn0.6Ni0.2O2 particles with a molten-molybdate leaching is conducted, which creates a crystal-dense anion-redox-free LiMn1.5Ni0.5O4 shell that completely encloses the LXMO lattice (ASR-LXMO). Differential electrochemical mass spectroscopy and soft X-ray absorption spectroscopy analyses demonstrate that GOM is shut down in cycling, which not only stabilizes HACR in ASR-LXMO, but also mitigates the electrolyte decomposition and Mn dissolution. ASR-LXMO displays greatly stabilized cycling performance as it retains 237.4 mAh gâ1 with an average discharge voltage of 3.30 V after 200 cycles. More crucially, while the pristine LXMO cycling cannot survive 90 cycles in a pouch full-cell matched with a commercial graphite anode and lean (2 g Aâ1 hâ1) electrolyte, ASR-LXMO shows high capacity retention of 76% after 125 cycles in full-cell cycling
Investigation of Water Dissociation and Surface Hydroxyl Stability on Pure and Ni-Modified CoOOH by Ambient Pressure Photoelectron Spectroscopy
Water adsorption and reaction on
pure and Ni-modified CoOOH nanowires
were investigated using ambient pressure photoemission spectroscopy
(APPES). The unique capabilities of APPES enable us to observe water
dissociation and monitor formation of surface species on pure and
Ni-modified CoOOH under elevated pressures and temperatures for the
first time. Over a large range of pressures (UHV to 1 Torr), water
dissociates readily on the pure and Ni-modified CoOOH surfaces at
27 °C. With an increase in H<sub>2</sub>O pressure, a greater
degree of surface hydroxylation was observed for all samples. At 1
Torr H<sub>2</sub>O, ratios of different oxygen species indicate a
transformation of CoOOH to CoO<sub><i>x</i></sub>H<sub><i>y</i></sub> in pure and Ni-modified CoOOH. In temperature dependent
studies, desorption of weakly bound water and surface dehydroxylation
were observed with increasing temperature. Larger percentages of surface
hydroxyl groups at higher temperatures were observed on Ni-modified
CoOOH compared to pure CoOOH, which indicates an increased stability
of surface hydroxyl groups on these Ni-modified surfaces
Spectroscopic Identification of Surface Intermediates in the Dehydrogenation of Ethylamine on Pt(111)
Reflection absorption infrared spectroscopy,
temperature-programmed
desorption, and density functional theory (DFT) have been used to
study the surface chemistry and thermal decomposition of ethylamine
(CH<sub>3</sub>CH<sub>2</sub>NH<sub>2</sub>) on Pt(111). Ethylamine
adsorbs molecularly at 85 K, is stable up to 300 K, and is partially
dehydrogenated at 330 K to form aminovinylidene (CCHNH<sub>2</sub>), a stable surface intermediate that partially desorbs as acetonitrile
(CH<sub>3</sub>CN) at 340â360 K. DFT simulations using various
surface models confirm the structure of aminovinylidene. Upon annealing
to 420 K, undesorbed aminovinylidene undergoes further dehydrogenation
that results in the scission of the remaining CâH bond and
the formation of a second surface intermediate called aminoethynyl
with the structure CCNH<sub>2</sub>, bonded to the surface through
both C atoms. The assignment of this intermediate species is supported
by comparison between experimental and simulated spectra of the isotopically
labeled species. Further annealing to temperatures above 500 K shows
that the CâN bond remains intact as the desorption of HCN is
observed
Spectroscopic Identification of Surface Intermediates in the Decomposition of Methylamine on Ru(001)
The thermal decomposition
of methylamine on Ru(001) was studied
with reflection absorption infrared spectroscopy (RAIRS) and temperature
programmed reaction spectroscopy (TPRS). After the multilayer methylamine
desorbs at 150 K, the RAIR spectra of the remaining monolayer methylamine
undergo small changes due to structural rearrangements but do not
indicate any chemical changes until 250 K. The results are in agreement
with recent theoretical investigations indicating that CH<sub>3</sub> dehydrogenation to produce H<sub><i>x</i></sub>CNH<sub>2</sub> species occurs before NâH and CâN bond scission.
Experimental spectra of <sup>13</sup>C- and <sup>15</sup>N-substituted
methylamine combined with DFT calculations of H<sub><i>x</i></sub>CNH<sub><i>y</i></sub> fragments attached to a Ru<sub>19</sub> cluster to simulate the RAIR spectra, including isotopic
shifts, clarified the spectral assignments
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