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 $T_1$ 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 $T_1$ 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 $T_1$ 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

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 (320–390 K for NH and 340–390 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 Co‐Free Li‐Rich 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 g−1. 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 g−1 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 A−1 h−1) 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 H₂O pressure, a greater degree of surface hydroxylation was observed for all samples. At 1 Torr H₂O, ratios of different oxygen species indicate a transformation of CoOOH to CoO_<i>x</i>H_<i>y</i> 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 (CH₃CH₂NH₂) 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 (CCHNH₂), a stable surface intermediate that partially desorbs as acetonitrile (CH₃CN) at 340–360 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 C–H bond and the formation of a second surface intermediate called aminoethynyl with the structure CCNH₂, 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 C–N 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 CH₃ dehydrogenation to produce H_<i>x</i>CNH₂ species occurs before N–H and C–N bond scission. Experimental spectra of ¹³C- and ¹⁵N-substituted methylamine combined with DFT calculations of H_<i>x</i>CNH_<i>y</i> fragments attached to a Ru₁₉ cluster to simulate the RAIR spectra, including isotopic shifts, clarified the spectral assignments