Low-Temperature Dissociation of CO₂ on a Ni/CeO₂(111)/Ru(0001) Model Catalyst

The adsorption of CO₂ on CeO_2‑<i>x</i>(111) and Ni/CeO_2‑<i>x</i>(111)/Ru­(0001) surfaces has been studied with reflection absorption infrared spectroscopy (RAIRS) and X-ray photoelectron spectroscopy (XPS). On the maximal-oxidized CeO₂(111) surface physisorbed linear CO₂ and a CO₂^– species are identified at 97 K. The reduced CeO_2‑<i>x</i>(111) surface exhibits higher reactivity toward adsorbed CO₂, which leads to higher coverages of CO₂^– and promotes CO₂ dissociating into CO and an active oxygen species at higher temperature, reoxidizing the reduced CeO_2‑<i>x</i>(111) films. Deposition of Ni on the maximal-oxidized CeO₂ thin films leads to slight reduction of ceria films. Adsorption of CO₂ on Ni/CeO_2‑<i>x</i>(111) films causes dissociation at 97 K and leads to Ni-CO adsorbates plus partial oxidation of Ni nanoparticles. This process is inhibited when Ni nanoparticles on CeO₂ are fully oxidized. In contrast to the results reported for CO₂ adsorption on Ni single-crystals, where the dissociation temperature was found to be higher than 240 K, the much lower dissociation temperature (∼97 K) for CO₂ on Ni nanoparticles supported on CeO₂(111) suggests that the Ni/CeO₂ catalyst exhibits high activity toward CO₂ activation

Structural and Optical Interplay of Palladium-Modified TiO₂ Nanoheterostructure

The electronic structure and optical properties of Pd-modified TiO₂ nanotubes (NTs) with a vertically aligned nanotubular structure grown by a two-step electrochemical anodization method have been studied using X-ray spectroscopy. X-ray absorption near-edge structure (XANES) at the Ti L_3,2- and O K-edges was used to investigate the TiO₂ NTs before and after Pd modification. It was found that Pd nanoparticles (NPs) are uniformly coated on the NT surface. The Pd L₃-edge of the deposited Pd NPs shows a more intense whiteline and a blue shift for the Pd L₃-edge absorption threshold relative to Pd metal, indicating charge depletion from the Pd 4d orbital as a result NP formation. The lattice of Pd is slightly contracted upon NP formation, although it remains fcc as revealed by extended X-ray absorption fine structure (EXAFS) analysis at the Pd K-edge. X-ray-excited optical luminescence (XEOL) together with XANES with element and site specificity was used to study the optical luminescence from TiO₂ NTs. It was found that the defect-originated XEOL intensity dropped noticeably in the Pd NP-coated NTs, suggesting a Pd NP–TiO₂-interaction-mediated reduction in the radiative recombination of electrons and holes. Further evidence is provided by Ti 2p resonant inelastic X-ray scattering (RIXS), in which no low-energy loss features (d–d transitions) were observed. The implications of these results are discussed

Traces of Potassium Induce Restructuring of the Anatase TiO₂(001)-(1×4) Surface from a Reactive to an Inert Structure

Reconstruction of solid surfaces is generally accompanied by changes in surface activities. Here, via a combined experimental and theoretical study, we successfully identified that a trace amount of potassium dopant restructures the mineral anatase TiO2(001) single-crystal surface from an added molecule (ADM) termination to an added oxygen (AOM) one without changing the (1×4) periodicity. The anatase TiO2(001)-(1×4)-ADM surface terminated with 4-fold coordinated Ti4c and 2-fold coordinated O2c sites is (photo)catalytically active, whereas the anatase TiO2(001)-(1×4)-AOM surface terminated with O2c and inaccessible 5-fold coordinated Ti5c sites is inert. These results unveiled a mechanism of dopant-induced transformation from a reactive to an inert TiO2(001)-(1×4) surface, which unifies the existing arguments about the surface structures and (photo)catalytic activity of anatase TiO2(001)-(1×4)

Tracking the Local Effect of Fluorine Self-Doping in Anodic TiO₂ Nanotubes

We report herein a study in which we reveal the role of F^– incorporated in the very anodic TiO₂ nanotubes prepared electrochemically from a Ti foil using a fluoride based electrolyte. X-ray absorption near edge structure (XANES), resonant X-ray emission spectroscopy (RXES), and X-ray photoelectron spectroscopy (XPS) have been used to examine the as-prepared and the annealed TiO₂ nanotubes. It is found that the additional electron resulting from the substitution of O^2– by self-doped F^– in the TiO₂ lattice is localized in the t_2g state. Consequently, a localized Ti³⁺ state can be tracked by a d–d energy loss peak with a constant energy of 1.6 eV in the RXES, in contrast to TiO₂ nanostructures where this peak is hardly noticeable when F^– is driven out of the lattice upon annealing

Traces of Potassium Induce Restructuring of the Anatase TiO₂(001)-(1×4) Surface from a Reactive to an Inert Structure

Reconstruction of solid surfaces is generally accompanied by changes in surface activities. Here, via a combined experimental and theoretical study, we successfully identified that a trace amount of potassium dopant restructures the mineral anatase TiO2(001) single-crystal surface from an added molecule (ADM) termination to an added oxygen (AOM) one without changing the (1×4) periodicity. The anatase TiO2(001)-(1×4)-ADM surface terminated with 4-fold coordinated Ti4c and 2-fold coordinated O2c sites is (photo)catalytically active, whereas the anatase TiO2(001)-(1×4)-AOM surface terminated with O2c and inaccessible 5-fold coordinated Ti5c sites is inert. These results unveiled a mechanism of dopant-induced transformation from a reactive to an inert TiO2(001)-(1×4) surface, which unifies the existing arguments about the surface structures and (photo)catalytic activity of anatase TiO2(001)-(1×4)

Confined Synthesis of Organometallic Chains and Macrocycles by Cu–O Surface Templating

The bottom-up construction of low-dimensional macromolecular nanostructures directly on a surface is a promising approach for future application in molecular electronics and integrated circuit production. However, challenges still remain in controlling the formation of these nanostructures with predetermined patterns (such as linear or cyclic) or dimensions (such as the length of one-dimensional (1D) chains). Here, we demonstrate that a high degree of structural control can be achieved by employing a Cu(110)-(2×1)O nanotemplate for the confined synthesis of organometallic chains and macrocycles. This template contains ordered arrays of alternating stripes of Cu–O chains and bare Cu, the widths of which are controllable. Using scanning tunneling microscopy and low-energy electron diffraction, we show that well-defined, ordered 1D zigzag organometallic oligomeric chains with uniform lengths can be fabricated on the Cu stripes (width >5.6 nm) of the Cu(110)-(2×1)O surface. In addition, the lengths of the <i>meta</i>-terphenyl (MTP)-based chains can be adjusted by controlling the widths of the Cu stripes within a certain range. When reducing the widths of Cu stripes to a range of 2.6 to 5.6 nm, organometallic macrocycles including tetramer (MTP-Cu)₄, hexamer (MTP-Cu)₆, and octamer (MTP-Cu)₈ species are formed due to the spatial confinement effect and attraction to the Cu–O chains. An overview of all formed organometallic macrocycles on the Cu stripes with different widths reveals that the origin of the formation of these macrocycles is the <i>cis</i>-configured organometallic dimer (MTP)₂Cu₃, which was observed on the extremely narrow Cu stripe with a width of 1.5 nm

Surface-Assisted Formation, Assembly, and Dynamics of Planar Organometallic Macrocycles and Zigzag Shaped Polymer Chains with C–Cu–C Bonds

The formation, structure, and dynamics of planar organometallic macrocycles (<i>meta</i>-terphenyl-Cu)_<i>n</i> and zigzag-shaped one-dimensional organometallic polymers on a Cu(111) surface were studied with scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Vapor deposition of 4,4″-dibromo-<i>meta</i>-terphenyl (DMTP) onto Cu(111) at 300 K leads to C–Br bond scission and formation of C–Cu–C bonds, which connect neighboring <i>meta</i>-terphenyl fragments such that room-temperature stable macrocycles and zigzag chains are formed. The chains self-assemble to form islands, which are elongated in the direction of the chains. If DMTP is deposited onto Cu(111) held at 440 K, the island size is drastically increased (>200 × 200 nm²). STM sequences show the formation of ordered structures through reversible scission and reformation of the C–Cu–C bonds. The cyclic organometallic species such as the hexamer (<i>meta</i>-terphenyl-Cu)₆ may represent intermediates in the surface-confined Ullmann synthesis of hydrocarbon macrocycles such as the recently discovered hyperbenzene

Interaction of Au with Thin ZrO₂ Films: Influence of ZrO₂ Morphology on the Adsorption and Thermal Stability of Au Nanoparticles

The model catalysts of ZrO₂-supported Au nanoparticles have been prepared by deposition of Au atoms onto the surfaces of thin ZrO₂ films with different morphologies. The adsorption and thermal stability of Au nanoparticles on thin ZrO₂ films have been investigated using synchrotron radiation photoemission spectroscopy (SRPES) and X-ray photoelectron spectroscopy (XPS). The thin ZrO₂ films were prepared by two different methods, giving rise to different morphologies. The first method utilized wet chemical impregnation to synthesize the thin ZrO₂ film through the procedure of first spin-coating a zirconium ethoxide (Zr­(OC₂H₅)₄) precursor onto a SiO₂/Si­(100) substrate at room temperature followed by calcination at 773 K for 12 h. Scanning electron microscopy (SEM) investigations indicate that highly porous “sponge-like nanostructures” were obtained in this case. The second method was epitaxial growth of a ZrO₂(111) film through vacuum evaporation of Zr metal onto Pt(111) in 1 × 10^–6 Torr of oxygen at 550 K followed by annealing at 1000 K. The structural analysis with low energy electron diffraction (LEED) of this film exhibits good long-range ordering. It has been found that Au forms smaller particles on the porous ZrO₂ film as compared to those on the ordered ZrO₂(111) film at a given coverage. Thermal annealing experiments demonstrate that Au particles are more thermally stable on the porous ZrO₂ surface than on the ZrO₂(111) surface, although on both surfaces, Au particles experience significant sintering at elevated temperatures. In addition, by annealing the surfaces to 1100 K, Au particles desorb completely from ZrO₂(111) but not from porous ZrO₂. The enhanced thermal stability for Au on porous ZrO₂ can be attributed to the stronger interaction of the adsorbed Au with the defects and the hindered migration or coalescence resulting from the porous structures

Unravelling the Mechanism of Glaser Coupling Reaction on Ag(111) and Cu(111) Surfaces: a Case for Halogen Substituted Terminal Alkyne

The mechanisms of Glaser coupling reaction on metal surfaces have been poorly understood. Herein, we propose a reaction pathway toward surface-confined Glaser coupling which is initiated by single-molecule dehydrogenation of terminal alkyne. This is inspired by our experimental observations of alkynyl–Ag–alkynyl and alkynyl–Cu–alkynyl type organometallic intermediates in the coupling reaction of 1,1′-biphenyl,4-bromo-4′-ethynyl (BPBE) on Ag(111) and Cu(111), respectively. Theoretical calculations reveal that the dehydrogenation process of terminal ethynyl of BPBE is most likely catalyzed by a stray H adatom on Ag(111) but by a Cu adatom on Cu(111), followed by the formation of the organometallic intermediates. After the release of interstitial metal adatoms, the final C–C coupling occurs easily on Ag(111) but shows extremely low efficiency on Cu(111), due to the too strong interaction between ethynylene and the Cu(111) substrate