Adsorbate dynamics on iron oxide surfaces by scanning tunneling microscopy

Epitaxial Fe3O4(111) films were studied by scanning tunneling microscopy (STM). Atomic resolution STM images exhibit a hexagonal lattice of protrusions with a 6 Å periodicity which are assigned to the topmost Fe cations. In contrast to the bias polarity independent STM images obtained on the clean surface, the adsorbed species are imaged as protrusions at positive bias and as depressions at negative sample bias. They occupy top and 3-fold hollow sites of the topmost Fe-layer, whereas no adsorbates were found on Fe vacancy sites, at step edges and on the surface domains that are FeO(111) in nature. The data were explained by assuming the dissociative water adsorption to occur on the regular Fe3O4(111) surface, which exposes both acidic Fe and basic O sites. The diffusion of adsorbed species was studied by consecutive STM images. Both translational and rotational moving of “dimer”-species is observed

Hexagonal hetero-layer(s) on a square lattice: a combined STM and LEED study of FeO(111) on Pt(100)

Formation of the epitaxial iron oxide monolayer on the Pt(100)-hex substrate was studied by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). High resolution STM images reveal a sinusoidal height modulation of the top atomic rows along the [011] direction of the original Pt(100)-hex substrate. This modulation is assigned to the buckling of the top oxygen layer due to the interaction with Pt substrate atoms. Two superstructures described as FeO(111)/Pt(100)-c(2x10) and -(2x9) coincidence structure coexist on the surface. The latter structure results in a much lower Pendry R-factor in dynamical LEED analysis than earlier reported for a c(2x10) structure. Numerous islands, having the same surface structure as the terraces, develop on the dense FeO overlayer. They are assigned to the Pt(100)-(1x1) islands formed during (hex)®(1x1) reconstruction of the Pt substrate underneath the FeO(111) bilayer. The islands are rectangular and elongated in the direction of hex-reconstruction on the original Pt(100). Combined STM and LEED data clearly indicate that anisotropy in the substrate reconstruction leads to anisotropy of the oxide overlayer

Surface-bonded precursor determines particle size effects for alkene hydrogenation on palladium

Size matters: The adsorption of trans-2-pentene on Pd/Al2O3 model catalysts exhibits site-specific behavior, which results in a strong increase in hydrogenation activity within the 1-5-nm particle size range, in contrast to ethene hydrogenation (see figure). The size effects are explained by the reactions proceeding via di--bonded pentene, which is favored on the terrace sites of large particles, and -bonded ethene

Alkene chemistry on the palladium surface; nanoparticles vs. single crystals

The adsorption of trans-2-pentene, cis-2-pentene, and 1-pentene on Pd(111) and Pd/Al2O3 model catalysts has been studied using temperature-programmed desorption (TPD). Each molecule reacts in an identical manner on the Pd(111) surface. Three distinct molecular adsorption states are observed, which have been assigned to multilayer, π-bonded pentene and interchanging di-σ-bonded pentene/pentyl groups. The latter species undergo coverage-dependent stepwise dehydrogenation. For trans-2-pentene on D2 preadsorbed Pd(111), a H–D exchange reaction occurs, resulting in D-substituted pentene, which molecularly desorbs or dehydrogenates on heating similar to nonexchanged pentene. Pentane is not formed as a hydrogenation product on Pd(111). On Pd nanoparticles, dehydrogenation proceeds more readily than on Pd(111). In addition, the extent of the H–D exchange reaction is considerably greater on particles. In contrast to Pd(111), the hydrogenation reaction occurs on the Pd particles. Data show that di-σ-bonded pentene is the precursor for both the H–D exchange reaction and the pentane formation, with each occurring via a pentyl group. This pentyl group reacts either by β-H elimination to form pentene or by reductive elimination to form pentane. The results for pentenes are compared with those for ethene. We have found that, under the low-pressure conditions studied, alkene hydrogenation only occurs in the presence of subsurface hydrogen. The accessibility of the subsurface hydrogen atoms is enhanced on the particles, due to the nanoscale dimensions, relative to that on crystals. The results are rationalized on the basis of a model of overlapping desorption states, which may predict both the feasibility of alkene hydrogenation on Pd catalysts and the active species involved in the reaction