Ceria (CeO 2 ) has recently been found to catalyze the selective hydrogenation of alkynes, which has stimulated much discussion on the catalytic mechanism on various facets of reducible oxides. In this work, H 2 dissociation and acetylene hydrogenation on bare and Ni doped CeO 2 (110) surfaces are investigated using density functional theory (DFT). Similar to that on the CeO 2 (111) surface, our results suggest that catalysis is facilitated by frustrated Lewis pairs (FLPs) formed by oxygen vacancies (O v s) on the oxide surfaces. On bare CeO 2 (110) with a single O v (CeO 2 (110)–O v ), two surface Ce cations with one non-adjacent O anion are shown to form (Ce 3+ –Ce 4+ )/O quasi-FLPs, while for the Ni doped CeO 2 (110) surface with one (Ni–CeO 2 (110)–O v ) or two (Ni–CeO 2 (110)–2O v ) O v s, one Ce and a non-adjacent O counterions are found to form a mono-Ce/O FLP. DFT calculations indicate that Ce/O FLPs facilitate the H 2 dissociation via a heterolytic mechanism, while the resulting surface O–H and Ce–H species catalyze the subsequent acetylene hydrogenation. With CeO 2 (110)–O v and Ni–CeO 2 (110)–2O v , our DFT calculations suggest that the first hydrogenation step is the rate-determining step with a barrier of 0.43 and 0.40 eV, respectively. For Ni–CeO 2 (110)–O v , the reaction is shown to be controlled by the H 2 dissociation with a barrier of 0.41 eV. These barriers are significantly lower than that (about 0.7 eV) on CeO 2 (111), explaining the experimentally observed higher catalytic efficiency of the (110) facet of ceria. The change of the rate-determining step is attributed to the different electronic properties of Ce in the Ce/O FLPs – the Ce f states closer to the Fermi level not only facilitate the heterolytic dissociation of H 2 but also lead to a higher barrier of acetylene hydrogenation.
more »
« less
Electronic Structure of InAs and InSb Surfaces: Density Functional Theory and Angle‐Resolved Photoemission Spectroscopy
The electronic structure of surfaces plays a key role in the properties of quantum devices. However, surfaces are also the most challenging to simulate and engineer. Here, the electronic structure of InAs(001), InAs(111), and InSb(110) surfaces is studied using a combination of density functional theory (DFT) and angle‐resolved photoemission spectroscopy (ARPES). Large‐scale first principles simulations are enabled by using DFT calculations with a machine‐learned Hubbard U correction [npj Comput. Mater. 6, 180 (2020)]. To facilitate direct comparison with ARPES results, a “bulk unfolding” scheme is implemented by projecting the calculated band structure of a supercell surface slab model onto the bulk primitive cell. For all three surfaces, a good agreement is found between DFT calculations and ARPES. For InAs(001), the simulations clarify the effect of the surface reconstruction. Different reconstructions are found to produce distinctive surface states, which may be detected by ARPES with low photon energies. For InAs(111) and InSb(110), the simulations help elucidate the effect of oxidation. Owing to larger charge transfer from As to O than from Sb to O, oxidation of InAs(111) leads to significant band bending and produces an electron pocket, whereas oxidation of InSb(110) does not. The combined theoretical and experimental results may inform the design of quantum devices based on InAs and InSb semiconductors, for example, topological qubits utilizing the Majorana zero modes.
more » « less- NSF-PAR ID:
- 10469048
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Quantum Technologies
- Volume:
- 5
- Issue:
- 3
- ISSN:
- 2511-9044
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Global perspectives of the bulk electronic structure of URu 2 Si 2 from angle-resolved photoemissionmore » « less
Abstract Previous high-resolution angle-resolved photoemission (ARPES) studies of URu2Si2have characterized the temperature-dependent behavior of narrow-band states close to the Fermi level (
E F) at low photon energies near the zone center, with an emphasis on electronic reconstruction due to Brillouin zone folding. A substantial challenge to a proper description is that these states interact with other hole-band states that are generally absent from bulk-sensitive soft x-ray ARPES measurements. Here we provide a more globalk -space context for the presence of such states and their relation to the bulk Fermi surface (FS) topology using synchrotron-based wide-angle and photon energy-dependent ARPES mapping of the electronic structure using photon energies intermediate between the low-energy regime and the high-energy soft x-ray regime. Small-spot spatial dependence,f -resonant photoemission, Si 2p core-levels, x-ray polarization, surface-dosing modification, and theoretical surface slab calculations are employed to assist identification of bulk versus surface state character of theE F-crossing bands and their relation to specific U- or Si-terminations of the cleaved surface. The bulk FS topology is critically compared to density functional theory (DFT) and to dynamical mean field theory calculations. In addition to clarifying some aspects of the previously measured high symmetry Γ,Z andX points, incommensurate 0.6a * nested Fermi-edge states located alongZ –N –Z are found to be distinctly different from the DFT FS prediction. The temperature evolution of these states aboveT HO, combined with a more detailed theoretical investigation of this region, suggests a key role of theN -point in the hidden order transition. -
more » « less
Abstract Solid–gas interactions at electrode surfaces determine the efficiency of solid‐oxide fuel cells and electrolyzers. Here, the correlation between surface–gas kinetics and the crystal orientation of perovskite electrodes is studied in the model system La0.8Sr0.2Co0.2Fe0.8O3. The gas‐exchange kinetics are characterized by synthesizing epitaxial half‐cell geometries where three single‐variant surfaces are produced [i.e., La0.8Sr0.2Co0.2Fe0.8O3/La0.9Sr0.1Ga0.95Mg0.05O3−δ/SrRuO3/SrTiO3(001), (110), and (111)]. Electrochemical impedance spectroscopy and electrical conductivity relaxation measurements reveal a strong surface‐orientation dependency of the gas‐exchange kinetics, wherein (111)‐oriented surfaces exhibit an activity >3‐times higher as compared to (001)‐oriented surfaces. Oxygen partial pressure ()‐dependent electrochemical impedance spectroscopy studies reveal that while the three surfaces have different gas‐exchange kinetics, the reaction mechanisms and rate‐limiting steps are the same (i.e., charge‐transfer to the diatomic oxygen species). First‐principles calculations suggest that the formation energy of vacancies and adsorption at the various surfaces is different and influenced by the surface polarity. Finally, synchrotron‐based, ambient‐pressure X‐ray spectroscopies reveal distinct electronic changes and surface chemistry among the different surface orientations. Taken together, thin‐film epitaxy provides an efficient approach to control and understand the electrode reactivity ultimately demonstrating that the (111)‐surface exhibits a high density of active surface sites which leads to higher activity.
-
Tuning the electronic properties of oxide surfaces is of pivotal importance, because they find applicability in a variety of industrial processes, including catalysis. Currently, the industrial protocols for synthesizing oxide surfaces are limited to only partial control of the oxide's properties. This is because the ceramic processes result in complex morphologies and a priori unpredictable behavior of the products. While the bulk doping of alumina surfaces has been demonstrated to enhance their catalytic applications ( i.e. hydrodesulphurization (HDS)), the fundamental understanding of this phenomenon and its effect at an atomic level remain unexplored. In our joint experimental and computational study, simulations based on Density Functional Theory (DFT), synthesis, and a variety of surface characterization techniques are exploited for the specific goal of understanding the structure–function relationship of phosphorus-doped γ-Al 2 O 3 surfaces. Our theoretical calculations and experimental results agree in finding that P doping of γ-Al 2 O 3 leads to a significant decrease in its work function. Our computational models show that this decrease is due to the formation of a new surface dipole, providing a clear picture of the effect of P doping at the surface of γ-Al 2 O 3 . In this study, we uncover a general paradigm for tuning support–catalyst interactions that involves electrostatic properties of doped γ-Al 2 O 3 surface, specifically the surface dipole. Our findings open a new pathway for engineering the electronic properties of metal oxides’ surfaces.more » « less
-
more » « less
Abstract Fullertubes, that is, fullerenes consisting of a carbon nanotube moiety capped by hemifullerene ends, are emerging carbon nanomaterials whose properties show both fullerene and carbon nanotube (CNT) traits. Albeit it may be expected that their electronic states show a certain resemblance to those of the extended nanotube, such a correlation has not yet been found or described. Here it shows a scanning tunneling microscopy (STM) and spectroscopy (STS) characterization of the adsorption, self‐assembly, and electronic structure of 2D arrays of [5,5]‐C90fullertube molecules on two different noble metal surfaces, Ag(111) and Au(111). The results demonstrate that the shape of the molecular orbitals of the adsorbed fullertubes corresponds closely to those expected for isolated species on the grounds of density functional theory calculations. Moreover, a comparison between the electronic density profiles in the bands of the extended [5,5]‐CNT and in the molecules reveals that some of the frontier orbitals of the fullertube molecules can be described as the result of the quantum confinement imposed by the hemifullerene caps to the delocalized band states in the extended CNT. The results thus provide a conceptual framework for the rational design of custom fullertube molecules and can potentially become a cornerstone in the understanding of these new carbon nanoforms.
