Guowen Peng

Alkali-Stabilized Pt-OHx Species Catalyze Low-Temperature Water-Gas Shift Reactions

Substituting Salt for Cerium Oxide The water-gas shift reaction converts carbon monoxide and water to hydrogen and carbon dioxide. Catalysts that operate at lower temperatures will be useful in fuel cells. Nanoparticles of platinum adsorbed on reducible oxides, such as ceria, can stabilize catalytically active Ptoxygen species. Zhai et al. (p. 1633) now show that, when alkali atoms are added, atomically dispersed Pt can be an active catalyst for the water-gas shift reaction at ∼100°C, even on simple oxides such as alumina and silica. The formation of hydrogen from carbon monoxide and water is catalyzed by the formation of oxidized platinum atoms. We report that alkali ions (sodium or potassium) added in small amounts activate platinum adsorbed on alumina or silica for the low-temperature water-gas shift (WGS) reaction (H2O + CO → H2 + CO2) used for producing H2. The alkali ion–associated surface OH groups are activated by CO at low temperatures (~100°C) in the presence of atomically dispersed platinum. Both experimental evidence and density functional theory calculations suggest that a partially oxidized Pt-alkali-Ox(OH)y species is the active site for the low-temperature Pt-catalyzed WGS reaction. These findings are useful for the design of highly active and stable WGS catalysts that contain only trace amounts of a precious metal without the need for a reducible oxide support such as ceria.

Improving Electrocatalysts for O2 Reduction by Fine-Tuning the Pt−Support Interaction: Pt Monolayer on the Surfaces of a Pd3Fe(111) Single-Crystal Alloy

We improved the effectiveness of Pt monolayer electrocatalysts for the oxygen-reduction reaction (ORR) using a novel approach to fine-tuning the Pt monolayer interaction with its support, exemplified by an annealed Pd(3)Fe(111) single-crystal alloy support having a segregated Pd layer. Low-energy ion scattering and low-energy electron diffraction studies revealed that a segregated Pd layer, with the same structure as Pd (111), is formed on the surface of high-temperature-annealed Pd(3)Fe(111). This Pd layer is considerably more active than Pd(111); its ORR kinetics is comparable to that of a Pt(111) surface. The enhanced catalytic activity of the segregated Pd layer compared to that of bulk Pd apparently reflects the modification of Pd surfaces electronic properties by underlying Fe. The Pd(3)Fe(111) suffers a large loss in ORR activity when the subsurface Fe is depleted by potential cycling (i.e., repeated excursions to high potentials in acid solutions). The Pd(3)Fe(111) surface is an excellent substrate for a Pt monolayer ORR catalyst, as verified by its enhanced ORR kinetics on PT(ML)/Pd/Pd(3)Fe(111). Our density functional theory studies suggest that the observed enhancement of ORR activity originates mainly from the destabilization of OH binding and the decreased Pt-OH coverage on the Pt/Pd/Pd(3)Fe(111) surface. The activity of Pt(ML)/Pd(111) and Pt(111) is limited by OH removal, whereas the activity of Pt(ML)/Pd/Pd(3)Fe(111) is limited by the O-O bond scission, which places these two surfaces on the two sides of the volcano plot.

Magnetism in BN nanotubes induced by carbon doping

We performed ab initio calculation on the pristine and carbon-doped (5,5) and (9,0) BN nanotubes. It was found that carbon substitution for either a single boron or a single nitrogen atom in the BN nanotubes can induce spontaneous magnetization. Calculations based on density functional theory with the local spin density approximation on the electronic band structure revealed a spin polarized, dispersionless band near the Fermi energy. The magnetization can be attributed to the carbon 2p electron. Compared to other theoretical models of light-element or metal-free magnetic materials, the carbon-doped BN nanotubes are more experimentally accessible and can be potentially useful.

Water-mediated proton hopping on an iron oxide surface

Water-Assisted Proton Diffusion Proton diffusion on metal oxide surfaces can play an important role in many catalytic processes. The presence of water is thought to accelerate proton diffusion. Merte et al. (p. 889) used high-speed, high-resolution scanning tunneling microscopy to study proton diffusion on an iron oxide. On oxygen-terminated FeO monolayer films formed on Pt, molecular water accelerated proton diffusion. Density function theory calculations implicated a H3O+ transition state in the diffusion process. The presence of adsorbed water enhances proton diffusion, likely through a hydronium ion transition state. The diffusion of hydrogen atoms across solid oxide surfaces is often assumed to be accelerated by the presence of water molecules. Here we present a high-resolution, high-speed scanning tunneling microscopy (STM) study of the diffusion of H atoms on an FeO thin film. STM movies directly reveal a water-mediated hydrogen diffusion mechanism on the oxide surface at temperatures between 100 and 300 kelvin. Density functional theory calculations and isotope-exchange experiments confirm the STM observations, and a proton-transfer mechanism that proceeds via an H3O+-like transition state is revealed. This mechanism differs from that observed previously for rutile TiO2(110), where water dissociation is a key step in proton diffusion.

Cu-doped GaN: A dilute magnetic semiconductor from first-principles study

First-principles calculations based on spin density functional theory are performed to study the spin-resolved electronic properties of GaN doped with 6.25% of Cu. The Cu dopants are found spin polarized and the calculated band structures suggest a 100% polarization of the conduction carriers. The Cu-doped GaN favors ferromagnetic ground state which can be explained in terms of p‐d hybridization mechanism, and a Curie temperature around 350K can be expected. These results suggest that the Cu-doped GaN is a promising dilute magnetic semiconductor free of magnetic precipitates and may find applications in the field of spintronics.

Ferromagnetism in Mg-doped AlN from ab initio study

Ab initio calculations based on spin density functional theory were carried out to investigate Mg-doped AlN as a possible dilute magnetic semiconductor. It was found that both Al vacancy and substitutional Mg impurity in AlN lead to spin-polarized ground states. However, sufficient Al vacancy concentration may be difficult to achieve under thermal equilibrium because of the high formation energy of Al vacancy. On the other hand, formation energy of Mg defect is fairly low and the authors’ calculations predict a ferromagnetic coupling among MgN4 tetrahedra. Based on the analysis on Cu-doped ZnO [L. H. Ye et al., Phys. Rev. B 73, 033203 (2006)], room temperature ferromagnetism can be expected in AlN doped with 7% of Mg which can be incorporated at a growth temperature of 2000K under N-rich condition.

Low-Temperature CO Oxidation on Ni(111) and on a Au/Ni(111) Surface Alloy

From an interplay between scanning tunneling microscopy, temperature programmed desorption, X-ray photoelectron spectroscopy, and density functional theory calculations we have studied low-temperature CO oxidation on Au/Ni(111) surface alloys and on Ni(111). We show that an oxide is formed on both the Ni(111) and the Au/Ni(111) surfaces when oxygen is dosed at 100 K, and that CO can be oxidized at 100 K on both of these surfaces in the presence of weakly bound oxygen. We suggest that low-temperature CO oxidation can be rationalized by CO oxidation on O(2)-saturated NiO(111) surfaces, and show that the main effect of Au in the Au/Ni(111) surface alloy is to block the formation of carbonate and thereby increase the low-temperature CO(2) production.

Electronic structure of germanium nitride considered for gate dielectrics

First-principles calculations based on density-functional theory and the local-density approximation have been used to investigate structural, electronic, and optical properties of α, β, and γ phases of germanium nitride (Ge3N4). β-Ge3N4 was found to be the most stable among the three structures, and it has a very small lattice mismatch with Ge, which indicates that it could be grown epitaxially on Ge. The calculated band gaps of α-, β-, and γ-Ge3N4 are about 3.15, 3.07, and 2.33eV, and the corresponding static dielectric constants are 4.70, 4.74, and 6.27, respectively, within local-density approximation. Results of our calculations indicate that the band gap and static dielectric constants of Ge3N4, as well as Si3N4, could satisfy the requirements of gate dielectrics for Ge-based field effect transistors.

Quantum Tunneling Enabled Self-Assembly of Hydrogen Atoms on Cu(111)

Atomic and molecular self-assembly are key phenomena that underpin many important technologies. Typically, thermally enabled diffusion allows a system to sample many areas of configurational space, and ordered assemblies evolve that optimize interactions between species. Herein we describe a system in which the diffusion is quantum tunneling in nature and report the self-assembly of H atoms on a Cu(111) surface into complex arrays based on local clustering followed by larger scale islanding of these clusters. By scanning tunneling microscope tip-induced scrambling of H atom assemblies, we are able to watch the atomic scale details of H atom self-assembly in real time. The ordered arrangements we observe are complex and very different from those formed by H on other metals that occur in much simpler geometries. We contrast the diffusion and assembly of H with D, which has a much slower tunneling rate and is not able to form the large islands observed with H over equivalent time scales. Using density functional theory, we examine the interaction of H atoms on Cu(111) by calculating the differential binding energy as a function of H coverage. At the temperature of the experiments (5 K), H(D) diffusion by quantum tunneling dominates. The quantum-tunneling-enabled H and D diffusion is studied using a semiclassically corrected transition state theory coupled with density functional theory. This system constitutes the first example of quantum-tunneling-enabled self-assembly, while simultaneously demonstrating the complex ordering of H on Cu(111), a catalytically relevant surface.

Possible graphitic-boron-nitride-based metal-free molecular magnets from first principles study

We perform first principles calculations based on density functional theory with the generalized gradient approximation (GGA) on the electronic and magnetic properties of carbon dopant in graphitic boron nitride (graphitic-BN) by the supercell method. It is found that carbon substitution for either boron or nitrogen atoms in graphitic-BN favours spin polarization over non-spin-polarization by 0.1 eV. The calculated electronic band structures show a spin-polarized, dispersionless band near the Fermi energy. The spin polarization is found to originate from the carbon dopant and can be attributed to the carbon 2p electron. The lower energies and the resistivity to curvature effect of the spin polarization suggest a possibility of ferromagnetic ordering of the carbon dopants in many realistic BN-based nanostructures which possess a hexagonal network such as nanotubes. Energies required for carbon substitution turn out to be within experimental access. The random substitution and the low doping energy indicate that graphitic-BN-based nanostructures can be viewed as candidates for metal-free magnet applications.