Self-Activated Catalyst Layer for Partial Hydrogenation of 1,3-Butadiene on a Hydrogen-Precovered Pd(110) Surface

We have studied partial hydrogenation of 1,3-butadiene to butene on a hydrogen-precovered Pd(110) surface, focusing on how reactants were catalytically activated on a metal surface. Techniques of surface analysis, including temperature-programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), and scanning tunneling microscopy (STM), were utilized to identify chemisorbed states of the active species involved in hydrogenation. For 1,3-butadiene coverage of lower than 0.25 ML, 1,3-butadiene was in a strongly π-bonded state on H/Pd(110) and caused some of the precovered hydrogen atoms to move into the subsurface site. Under this condition, a hydrogen exchange reaction between the adsorbed 1,3-butadiene molecules and precovered hydrogen atoms occurred. At 1,3-butadiene coverage of 0.25 ML, the 1,3-butadiene molecules, which formed a c(4 × 2) layer accompanied with subsurface hydrogen atoms, were less active in hydrogen exchange and hydrogenation. In contrast, when 1,3-butadiene coverage exceeded 0.25 ML, the Pd surface was partially covered with weakly π-bonded 1,3-butadiene and became active in hydrogenation. The obtained results indicate that hydrogenation of 1,3-butadiene on Pd(110) requires the presence of both weakly π-bonded 1,3-butadiene and subsurface hydrogen atoms, and that it is important that the catalytic active species are produced by the reactants themselves. Here, we introduce a new concept of a self-activated catalyst layer for hydrogenation on Pd(110)

Fabrication of Interconnected 1D Molecular Lines along and across the Dimer Rows on the Si(100)−(2 × 1)−H Surface through the Radical Chain Reaction

To realize nanoscale wiring in two dimensions (2D), the fabrication of interconnected one-dimensional (1D) molecular lines through the radical chain reaction of alkene molecules (CH2CHR) on the H-terminated Si(100)−(2 × 1) surface have been investigated using scanning tunneling microscopy (STM) at 300 K. By the judicious choice of R in the CH2CHR molecule and by creating a dangling bond (DB) at a desired point using the STM tip, the perpendicularly connected allyl mercaptan (ALM) and styrene lines have been fabricated on the Si(100)−(2 × 1)−H surface. The continuous growth of the styrene line at the end DB of a growing ALM line (or vice versa) does not occur, perhaps, due to steric hindrance or/and interaction between adsorbed molecules

Partial Hydrogenation of 1,3-Butadiene on Hydrogen-Precovered Pd(110) in the Balance of π-Bonded C₄ Hydrocarbon Reactions

The hydrogenation and dehydrogenation of C4 hydrocarbon molecules (1,3-butadiene, 1-butene, trans-2-butene, cis-2-butene, and n-butane) on the hydrogen-precovered Pd(110) surface have been investigated by high-resolution electron energy loss spectroscopy (HREELS) and temperature-programmed desorption (TPD). 1,3-Butadiene was found to be adsorbed molecularly on the surface below 350 K. Further heating of the surface resulted in decomposition, forming hydrocarbons at 350 K and finally the graphite layer at 550 K. The butene isomers and n-butane adsorbed on the surface were, however, relatively unstable compared with 1,3-butadiene when heated. Some of the adsorbed butenes were desorbed, and the species that remained on the surface were dehydrogenated to 1,3-butadiene between 150 and 250 K. n-Butane on the surface showed similar reaction behavior except for the lower dehydrogenation and desorption temperature. Our findings indicate that the dehydrogenations of π-bonded C4 hydrocarbons on the Pd surface show significantly different pathways compared with those of the σ-bonded C4 hydrocarbon on Pt and Ru surfaces. Here, we discuss the selective partial hydrogenation of 1,3-butadiene on hydrogen-precovered Pd(110) in terms of the reactivity of the butenes and butanes

Single-Molecule Dynamics in the Presence of Strong Intermolecular Interactions

In contrast to conventional spectroscopic studies of adsorbates at high coverage that provide only spatially averaged information, we have characterized the laterally confined shuttling dynamics of a single molecule under the influence of intermolecular interactions by vibrational spectroscopy using a scanning tunneling microscope. The bridge sites on Pt(111) are only occupied by a CO molecule that is surrounded by four other CO molecules at on-top sites. The bridge-site CO undergoes laterally confined shuttling toward an adjacent on-top site to transiently occupy a metastable site, which is slightly displaced from the center of an on-top site through repulsive interaction with adjacent on-top CO molecules. Analysis of action spectra for the shuttling events reveals the C–O stretch frequency of the metastable CO. We also constructed a modified potential energy surface incorporating the intermolecular interaction, which reveals the underlying mechanism and provides a new way to experimentally determine detailed information on the energetics of the metastable state

Ligand Field Effect at Oxide–Metal Interface on the Chemical Reactivity of Ultrathin Oxide Film Surface

Ultrathin oxide film is currently one of the paramount candidates for a heterogeneous catalyst because it provides an additional dimension, i.e., film thickness, to control chemical reactivity. Here, we demonstrate that the chemical reactivity of ultrathin MgO film grown on Ag(100) substrate for the dissociation of individual water molecules can be systematically controlled by interface dopants over the film thickness. Density functional theory calculations revealed that adhesion at the oxide–metal interface can be addressed by the ligand field effect and is linearly correlated with the chemical reactivity of the oxide film. In addition, our results indicate that the concentration of dopant at the interface can be controlled by tuning the <i>drawing effect</i> of oxide film. Our study provides not only profound insight into chemical reactivity control of ultrathin oxide film supported by a metal substrate but also an impetus for investigating ultrathin oxide films for a wider range of applications

Self-Assembly of Meta-Aminobenzoate on Cu(110)

A variety of structures of meta-aminobenzoate molecules adsorbed on the Cu(110) surface have been characterized by scanning tunneling microscopy (STM) at a wide range of surface coverages, from a single molecule to saturated phases. At the start of molecular domain formation, individual molecules thermally diffuse to form chain structures via intermolecular hydrogen bonding. At higher surface coverages, there coexist three well-ordered phases, namely (12006) and chiral (6±5∓36) phases. The molecular orientation on the surface also varies with surface coverage. Flat-lying molecules are mainly observed at low surface coverage, while upright molecules start to appear as the surface becomes more highly covered. Our experimental findings and structural analysis are well supported by high-resolution STM images measured at 4.7 K and by molecular packing models with precise lattice parameters

Activation of Ultrathin Oxide Films for Chemical Reaction by Interface Defects

Periodic density functional theory calculations revealed strong enhancement of chemical reactivity by defects located at the oxide−metal interface for water dissociation on ultrathin MgO films deposited on Ag(100) substrate. Accumulation of charge density at the oxide−metal interface due to irregular interface defects influences the chemical reactivity of MgO films by changing the charge distribution at the oxide surface. Our results reveal the importance of buried interface defects in controlling chemical reactions on an ultrathin oxide film supported by a metal substrate

Different Adsorbed States of 1,4-Cyclohexadiene on Si(001) Controlled by Substrate Temperature

To elucidate the thermal chemical processes of 1,4-cyclohexadiene (C6H8) on Si(001), the adsorption states were characterized by temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), and high-resolution electron energy-loss spectroscopy (HREELS), in comparison with those for benzene (C6H6) and cyclohexene (C6H10). Consequently, three types of adsorption states, i.e., π-complex, single di-σ bonding, and double di-σ bonding species, were identified. At 85 K, all 1,4-cyclohexadiene molecules are adsorbed as π-complex species in the first layer. With increasing substrate temperature, above 150 K, these π-complex species chemically convert to single di-σ bonding species by [2 + 2] cycloaddition with a Si dimer. Upon further heating above 300 K, most single di-σ bonding species are dehydrogenated into benzene and then the benzene molecules desorb from the surface. In contrast, double di-σ bonding species are formed preferentially in low exposure at 300 K, and are dehydrogenated into benzene above 600 K

Long-Range Proton Transport for the Water Reaction on Si(001): Study of Hydrogen-Bonded Systems with a Model Liquid−solid Interface

The reaction pathways for water dissociation at a model liquid−solid interface have been investigated by a combination of experimental and theoretical approaches. By scanning tunneling microscopy (STM) and high-resolution electron energy-loss spectroscopy (HREELS), we revealed that the fragments of condensed water molecules, i.e., OH and H, efficiently terminate the isolated dangling bonds on a precovered Si(001) surface, in comparison with those of the isolated water molecules on the same surface. The most favorable reaction mechanism was predicted by first-principles calculations. At the first stage, the condensed water molecules create a new surface OH group at one of the isolated dangling bond sites. Simultaneously, counter fragment H and surrounding water molecules form a flexible hydronium complex along hydrogen bonds, because the fragment H takes a certain positive charge. Then, another dangling bond is terminated by a H fragment under the proton relay mechanism via the hydronium complex, in which a very low activation energy is expected because the hydronium complex near the surface is not sufficiently stabilized as in the case of aqueous liquid but is hindered in shallow potential energy surfaces. Since the spatial hindrance near solid surfaces is a common property, the characteristic proton pathway should appear at various aqueous liquid−solid interfaces and enhance the surface reactions involving proton transfer

Enhancement of Inelastic Electron Tunneling Conductance Caused by Electronic Decoupling in Iron Phthalocyanine Bilayer on Ag(111)

The effect of electronic decoupling on the inelastic electron tunneling process of iron phthalocyanine (FePc) molecules on Ag(111) was investigated using scanning tunneling microscopy (STM). A drastic difference in the inelastic electron tunneling to individual FePc molecules was found for the first and the second layer molecules grown on Ag(111). The spectrum of the first layer molecule is essentially structureless, whereas the second layer molecules provide giant conductance changes reaching several tens % due to the vibrational excitations. This is the first clear example to demonstrate, by using inelastic tunneling spectroscopy with STM, the enhancement of vibrational inelastic tunneling driven through the electronic decoupling of the molecules from the substrate