An improved methodology of the Zr specimen preparation was developed which allows fabrication of stable Zr nanotips suitable for FIM and AP applications. Initial oxidation of the Zr surface was studied on a Zr nanotip by FIM and on a polycrystalline Zr foil by XPS, both at low oxygen pressure (10−8–10−7 mbar). The XPS data reveal that in a first, fast stage of oxidation, a Zr suboxide interlayer is formed which contains three suboxide components (Zr+1, Zr+2 and Zr+3) and is located between the Zr surface and a stoichiometric ZrO2 overlayer that grows in a second, slow oxidation stage. The sole suboxide layer has been observed for the first time at very early states of the oxidation (oxygen exposure ≤4 L). The Ne+ FIM observations are in accord with a two stage process of Zr oxide formation.
In heterogeneous catalysis research, the reactivity of the individual nanofacets of single particle is typically not resolved. We applied in situ field electron microscopy (FEM) to the apex of a curved rhodium crystal (radius of 650 nanometers), providing high spatial (~2 nanometers) and time resolution (~2 ms) of oscillatory catalytic hydrogen oxidation, imaging adsorbed species and reaction fronts on the individual facets. Using ionized water as imaging species, the active sites were directly imaged by field ion microscopy (FIM). The catalytic behavior of differently structured nanofacets and the extent of coupling between them were monitored individually. We observed limited interfacet coupling, entrainment, frequency-locking, and reconstruction-induced collapse of spatial coupling. The experimental results are backed-up by microkinetic modelling of time-dependent oxygen species coverages and oscillation frequencies.
Self-sustained oscillations
in H
2
oxidation on a Rh
nanotip mimicking a single catalytic nanoparticle were studied by
in situ
field emission microscopy (FEM). The observed spatio-temporal
oscillations result from the coupling of subsurface oxide formation/depletion
with reaction front propagation. An original sophisticated method
for tracking kinetic transition points allowed the identification
of local pacemakers, initiating kinetic transitions and the nucleation
of reaction fronts, with much higher temporal resolution than conventional
processing of FEM video files provides. The pacemakers turned out
to be specific surface atomic configurations at the border between
strongly corrugated Rh{973} regions and adjacent relatively flat terraces. These
structural ensembles are crucial for reactivity: while the corrugated
region allows sufficient oxygen incorporation under the Rh surface,
the flat terrace provides sufficient hydrogen supply required for
the kinetic transition, highlighting the importance of interfacet
communication. The experimental observations are complemented by mean-field
microkinetic modeling. The insights into the initiation and propagation
of kinetic transitions on a single catalytic nanoparticle demonstrate
how
in situ
monitoring of an ongoing reaction on
individual nanofacets can single out active configurations, especially
when combined with atomically resolving the nanoparticle surface by
field ion microscopy (FIM).
The use of semiconductor materials as photocatalysts for cleaning gas processes has received increasing interest in the last decade. In order to make more active catalysts, it is worthwhile to investigate the main processes influencing photocatalytic reactions in more detail. One of these is the process of coadsorption of the reaction components on the catalyst surface under irradiation. It is shown that photoassisted NO adsorption can serve as a model system in order to investigate the influence of irradiation intensity and temperature on the adsorption isotherm, respectively. This advantage stems from the fact that NO is a radical, offering the possibility to stabilize electrons as well as holes on the TiO2 surface. This results in the formation of NO/NO pairs. The proposed adsorption model, however, does not only consider this pair formation but, in addition, the adsorption of NO molecules on charged sites while the complementary charged sites are stabilized by traps present on the surface. The proposed adsorption isotherm is supported by experimental results.
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