11 research outputs found
Adsorption and Reactions of Carbon Monoxide and Oxygen on Bare and Au-Decorated Carburized W(110)
Adsorption and coadsorption of carbon
monoxide and oxygen on different
types of Au clusters on R(15 × 3)ÂC/W(110) and R(15 × 12)ÂC/W(110),
respectively, are studied with respect to the catalytic behavior for
oxidation of CO as well as of surface carbon. Carburization of the
W(110) surface results in a weakening of the adsorption bond for molecularly
adsorbed CO. Dissociation of carbon monoxide, which occurs on W(110),
is reduced on the low-carbon coverage R(15 × 12) surface and
completely suppressed on the carbon-saturated RÂ(15 × 3) phase.
Deposition of gold results in a blocking of adsorption sites for molecularly
adsorbed CO and reopening of the dissociation channel. Probably the
latter is associated with the existence of double-layer gold clusters
and islands. At room temperature the gold clusters on both carburized
templates are stable in CO atmosphere as shown by in-situ STM measurements.
In contrast, exposure to oxygen alters the clusters on the R(15 ×
12) surface, implying dissociation of oxygen not only on the substrate
but also on or in immediate vicinity of the gold clusters. On the
Au-free carburized templates oxygen adsorbs dissociatively and is
released as CO at temperatures beyond 800 K due to reaction with carbon
atoms from the templates. Deposition of gold enhances the desorption
rate of the formed CO at the low-temperature end of the recombinative
CO desorption range, indicating a promoting effect of gold for oxidation
of surface carbon. In contrast, low-temperature CO oxidation catalyzed
by the deposited Au clusters is not observed. Two reasons could be
identified: (1) weakly bound CO with desorption temperatures between
100 and 200 K (as reported for other related systems) is not observed,
and (2) oxygen atoms are bonded too strongly to the templates
Alloying and Structure of Ultrathin Gallium Films on the (111) and (110) Surfaces of Palladium
Growth, thermal stability, and structure of ultrathin gallium films
on Pd(111) and Pd(110) are investigated by low-energy ion scattering
and low-energy electron diffraction. Common to both surface orientations
are growth of disordered Ga films at coverages of a few monolayers
(<i>T</i> = 150 K), onset of alloy formation at low temperatures
(<i>T</i> ≈ 200 K), and formation of a metastable,
mostly disordered 1:1 surface alloy at temperatures around 400–500
K. At higher temperatures a Ga surface fraction of ∼0.3 is
slightly stabilized on Pd(111), which we suggest to be related to
the formation of Pd<sub>2</sub>Ga bulk-like films. While on Pd(110)
only a Pd-up/Ga-down buckled surface was observed, an inversion of
buckling was observed on Pd(111) upon heating. Similarities and differences
to the related Zn/Pd system are discussed
Noble-metal nanostructures on carburized W(110)
Noble metal nanostructures of Au, Ag and Cu were prepared on two types of carbon-modified W(110) surfaces—R(15 × 12) and R(15 × 3)—and investigated by means of scanning tunneling microscopy. For all deposited metals qualitatively the same behaviour is observed: On the R(15 × 12)-template always isotropic clusters are formed. In contrast, on the R(15 × 3)-substrate the anisotropy of the nanostructures can be tuned from clusters at low temperatures via thin nanowires to thicker nanobars at high deposition temperatures. At intermediate temperatures on the R(15 × 3) the anisotropic Au nanowires arrange themselves into straight lines along domain boundaries induced by deposition of the Au metal. Similarities and differences to Au nanostructures as recently reported by Varykhalov et al. [A. Varykhalov, O. Rader, W. Gudat. Physical Review B 77, 035412 (2008).] are discussed
In situ XPS study of methanol reforming on PdGa near-surface intermetallic phases
In situ X-ray photoelectron spectroscopy and low-energy ion scattering were used to study the preparation, (thermo)chemical and catalytic properties of 1:1 PdGa intermetallic near-surface phases. Deposition of several multilayers of Ga metal and subsequent annealing to 503-523 K led to the formation of a multi-layered 1:1 PdGa near-surface state without desorption of excess Ga to the gas phase. In general, the composition of the PdGa model system is much more variable than that of its PdZn counterpart, which results in gradual changes of the near-surface composition with increasing annealing or reaction temperature. In contrast to near-surface PdZn, in methanol steam reforming, no temperature region with pronounced CO2 selectivity was observed, which is due to the inability of purely intermetallic PdGa to efficiently activate water. This allows to pinpoint the water-activating role of the intermetallic/support interface and/or of the oxide support in the related supported PdxGa/Ga2O3 systems, which exhibit high CO2 selectivity in a broad temperature range. In contrast, corresponding experiments starting on the purely bimetallic model surface in oxidative methanol reforming yielded high CO2 selectivity already at low temperatures (similar to 460 K), which is due to efficient O-2 activation on PdGa. In situ detected partial and reversible oxidative Ga segregation on intermetallic PdGa is associated with total oxidation of intermediate C-1 oxygenates to CO2. (c) 2012 Elsevier Inc. All rights reserved
Subsurface-Controlled CO2 Selectivity of PdZn Near-Surface Alloys in H2 Generation by Methanol Steam Reforming
More than skin deep: In spite of their identical 1:1 surface composition, the geometric and electronic structures of a multilayer and monolayer PdZn surface alloy are different, as are their catalytic selectivities. The CO2 selective multilayer alloy features surface ensembles of PdZn exhibiting a Zn-up/Pd-down corrugation (see picture). These act as bifunctional active sites both for water activation and for the conversion of methanol into CO2. On the monolayer alloy CO and not CO2 is produced