I review a concept that models heterogeneous catalysis based on a surface‐science approach. It is shown that models catching part of the complexity of the real system, which is connected with the ...finite size of active components and the flexibility of the arrangement of atoms in the active component, play an important part in determining the activity and selectivity of the system. I have chosen several examples from our own laboratory to elaborate the details and will put those into perspective with respect to the literature. I will show that Pd nanoparticles in hydrogenation incorporate hydrogen, which turns out to be crucial for the actual hydrogenation step. Another example correlates the structure of vanadia monolayer catalysts with its reactivity in methanol oxidation. With a third example we address the question of charge on Au nanoparticles when anchored to an oxide surface, a problem heavily discussed in the literature. Further examples refer to ultrathin oxide film catalysts in which the oxide metal interface controls either the charge state of Au particles grown on the film, and, in a last example, the oxide film itself exhibits remarkable CO‐oxidation activity, which can be traced to a reactive intermediate structure of the ultrathin film.
Scratching the surface: In this Review models for heterogeneous catalysis are discussed based on a surface‐science approach (see figure). It is shown that models catching part of the complexity of the real system, which is connected with the finite size of active components and the flexibility of the arrangement of atoms in the active component, play an important part in determining the activity and selectivity of the system.
Cholinergic neurons of the medial forebrain are considered important contributors to brain plasticity and neuromodulation. A reduction of cholinergic innervation can lead to pathophysiological ...changes of neurotransmission and is observed in Alzheimer's disease. Here we report on six patients with mild to moderate Alzheimer's disease (AD) treated with bilateral low-frequency deep brain stimulation (DBS) of the nucleus basalis of Meynert (NBM). During a four-week double-blind sham-controlled phase and a subsequent 11-month follow-up open label period, clinical outcome was assessed by neuropsychological examination using the Alzheimer's Disease Assessment Scale-cognitive subscale as the primary outcome measure. Electroencephalography and (18)F-fluoro-desoxyglucose positron emission tomography were, besides others, secondary endpoints. On the basis of stable or improved primary outcome parameters twelve months after surgery, four of the six patients were considered responders. No severe or non-transitional side effects related to the stimulation were observed. Taking into account all limitations of a pilot study, we conclude that DBS of the NBM is both technically feasible and well tolerated.
The morphology of ceria films grown on a Ru(0001) substrate was studied by scanning tunneling microscopy in combination with low-energy electron diffraction and Auger electron spectroscopy. The ...preparation conditions were determined for the growth of nm-thick, well-ordered CeO2(111) films covering the entire surface. The recipe has been adopted from the one suggested by Mullins et al. D.R. Mullins, P.V. Radulovic, S.H. Overbury, Surf. Sci. 429 (1999) 186 and modified in that significantly higher oxidation temperatures are required to form atomically flat terraces, up to 500A in width, with a low density of the point defects assigned to oxygen vacancies. The terraces often consist of several rotational domains. A circular shape of terraces suggest a large variety of undercoordinated sites at the step edges which preferentially nucleate gold particles deposited onto these films. The results show that reactivity studies over ceria and metal/ceria surfaces should be complemented with STM studies, which provide direct information on the film morphology and surface defects, which are usually considered as active sites for catalysis over ceria.
Ultrathin FeO(1
1
1) films grown on Pt(1
1
1) unexpectedly showed high activity towards CO oxidation. The reaction proceeds through the formation of a well-ordered, oxygen-rich FeO
x
(1
<
x
<
2) ...film. In CO-rich ambient the film dewets the Pt surface, ultimately resulting in highly dispersed iron oxide particles on Pt(1
1
1).
CO oxidation on a clean Pt(1
1
1) single crystal and thin iron oxide films grown on Pt(1
1
1) was studied at different CO:O
2 ratios (between 1:5 and 5:1) and partial pressures up to 60
mbar at 400–450
K. Structural characterization of the model catalysts was performed by scanning tunnelling microscopy, low energy electron diffraction, Auger electron spectroscopy and temperature-programmed desorption. It was found that monolayer FeO(1
1
1) films grown on Pt(1
1
1) were much more active than clean Pt(1
1
1) and nm-thick Fe
3O
4(1
1
1) films at all reaction conditions studied. Post-characterization of the catalysts revealed that at CO:O
2
>
1 the FeO(1
1
1) film dewets the Pt surface with time, ultimately resulting in highly dispersed iron oxide particles on Pt(1
1
1). The film dewetting was monitored
in situ by polarization-modulated infrared reflection absorption spectroscopy. The reaction rate at 450
K exhibited first order for O
2 and non-monotonously depended on CO pressure. In O
2-rich ambient the films were enriched with oxygen while maintaining the long-range ordering. Based on the structure-reactivity relationships observed for the FeO/Pt films, we propose that the reaction proceeds through the formation of a well-ordered, oxygen-rich FeO
x
(1
<
x
<
2) film that reacts with CO through the redox mechanism. The reaction-induced dewetting in fact deactivates the catalyst. The results may aid in our deeper understanding of reactivity of metal particles encapsulated by thin oxide films as a result of strong metal–support interaction.
We studied structure and reactivity of ZnO(0001) ultrathin films grown on Ag(111) and Cu(111) single crystal surfaces. Structural characterization was carried out by scanning tunneling microscopy, ...Auger electron spectroscopy, low-energy electron diffraction, and temperature programmed desorption. The CO oxidation behavior of the films was studied at low temperature (450 K) at near atmospheric pressures using gas chromatography. For ZnO/Cu(111), it is shown that under reaction conditions ZnO readily migrates into the Cu crystal bulk, and the reactivity is governed by a CuO
x
oxide film formed in the reaction ambient. In contrast, the planar structure of ZnO films on Ag(111) is maintained, similarly to the previously studied ZnO films on Pt(111). At sub-monolayer coverages, the “inverse” model catalysts are represented by two-monolayer-thick ZnO(0001) islands on Pt(111) and Ag(111) supports. While the CO oxidation rate is considerably increased on ZnO/Pt(111), which is attributed to active sites at the metal/oxide boundary, sub-monolayer ZnO films on Ag(111) did not show such an effect, and the reactivity was inhibited with increasing film coverage. The results are explained by much stronger adsorption of CO on Pt(111) as compared with Ag(111) in proximity to O species at the oxide/metal boundary. In addition, the water–gas shift and reverse water–gas shift reactions were examined on ZnO/Ag(111), which revealed no promotional effect of ZnO on the reactivity of Ag under the conditions studied. The latter finding suggests that wetting phenomena of ZnO on metals does not play a crucial role in the catalytic performance of ZnO-based real catalysts in those reactions.
Graphical Abstract
The atomic structure and reactivity of zinc oxide ultrathin films on Pt(111) was examined by the CO oxidation reaction at near-atmospheric pressures as a function of the film thickness and coverage. ...The observed structure-reactivity relationships directly show that the boundary between a metal and a two-layers-thick oxide (see figure) provides the most active sites for this reaction. Display omitted
► Thin films of ZnO were grown on Pt(111) in a layer-by-layer mode. ► The CO oxidation reaction at near-atmospheric pressures as a function of the film thickness and coverage was studied. ► Two-monolayers-thick ZnO(0001) islands dominate the surface of the most active catalysts. ► Direct proof for the oxide/metal boundary providing the active sites.
Well-ordered ultrathin ZnO(0001) films were grown on Pt(111) in a layer-by-layer mode. The reactivity of the films as a function of the film thickness and coverage was examined by the CO oxidation reaction at near-atmospheric pressures. At low temperatures (∼450K), CO2 production is found to be much higher on the films of partial coverage than on dense ZnO(0001) films and bare Pt(111). Under reaction conditions, monolayer islands and an entire monolayer film transform into two-monolayers-thick islands, which dominate the surface of the active catalysts. The results provide an adequate structural model for elucidating the reaction mechanism on the oxide/metal boundary at technologically relevant conditions.
Thin SiO₂ films were grown on a Ru(0001) single crystal and studied by photoelectron spectroscopy, infrared spectroscopy and scanning probe microscopy. The experimental results in combination with ...density functional theory calculations provide compelling evidence for the formation of crystalline, double-layer sheet silica weakly bound to a metal substrate.
Anthropogenic increases in atmospheric greenhouse gas
concentrations are the main driver of current and future climate change. The
integrated assessment community has quantified anthropogenic ...emissions for
the shared socio-economic pathway (SSP) scenarios, each of which represents
a different future socio-economic projection and political environment.
Here, we provide the greenhouse gas concentrations for these SSP scenarios
– using the reduced-complexity climate–carbon-cycle model MAGICC7.0. We
extend historical, observationally based concentration data with SSP
concentration projections from 2015 to 2500 for 43 greenhouse gases with monthly and latitudinal resolution. CO2 concentrations by 2100 range
from 393 to 1135 ppm for the lowest (SSP1-1.9) and highest (SSP5-8.5)
emission scenarios, respectively. We also provide the concentration
extensions beyond 2100 based on assumptions regarding the trajectories of fossil
fuels and land use change emissions, net negative emissions, and the
fraction of non-CO2 emissions. By 2150, CO2 concentrations in the
lowest emission scenario are approximately 350 ppm and approximately plateau
at that level until 2500, whereas the highest fossil-fuel-driven scenario
projects CO2 concentrations of 1737 ppm and reaches concentrations
beyond 2000 ppm by 2250. We estimate that the share of CO2 in the total
radiative forcing contribution of all considered 43 long-lived greenhouse
gases increases from 66 % for the present day to roughly 68 % to 85 % by
the time of maximum forcing in the 21st century. For this estimation,
we updated simple radiative forcing parameterizations that reflect the Oslo
Line-By-Line model results. In comparison to the representative concentration pathways (RCPs), the five main SSPs
(SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5) are more evenly spaced
and extend to lower 2100 radiative forcing and temperatures. Performing two
pairs of six-member historical ensembles with CESM1.2.2, we estimate the
effect on surface air temperatures of applying latitudinally and seasonally
resolved GHG concentrations. We find that the ensemble differences in the
March–April–May (MAM) season provide a regional warming in higher northern
latitudes of up to 0.4 K over the historical period, latitudinally averaged
of about 0.1 K, which we estimate to be comparable to the upper bound
(∼5 % level) of natural variability. In comparison to the
comparatively straight line of the last 2000 years, the greenhouse gas
concentrations since the onset of the industrial period and this studies'
projections over the next 100 to 500 years unequivocally depict a
“hockey-stick” upwards shape. The SSP concentration time series derived in
this study provide a harmonized set of input assumptions for long-term
climate science analysis; they also provide an indication of the wide set of
futures that societal developments and policy implementations can lead to –
ranging from multiple degrees of future warming on the one side to
approximately 1.5 ∘C warming on the other.
PDF
