Atmospheric contamination with organic compounds is undesired in industry and in society because of odor nuisance or potential toxicity. Resistive gas sensors made of semiconducting metal oxides are effective in the detection of gases even at low concentration. Major drawbacks are low selectivity and missing sensitivity toward a targeted compound. Acetaldehyde is selected due to its high relevance in chemical industry and its toxic character. Considering the similarity between gas-sensing and heterogeneous catalysis (surface reactions, activity, selectivity), it is tempting to transfer concepts. A question of importance is how doping and the resulting change in electronic properties of a metal-oxide support with semiconducting properties alters reactivity of the surfaces and the functionality in gas-sensing and in heterogeneous catalysis. A gas-phase synthesis method is employed for aerogel-like zinc oxide materials with a defi ned content of aluminum (n-doping), which were then used for the assembly of gas sensors. It is shown that only Al-doped ZnO represents an effective sensor material that is sensitive down to very low concentrations (<350 ppb). The advance in properties relates to a catalytic effect for the doped semiconductor nanomaterial.
Nanoporous monoliths with hierarchical nanostructure are prepared via in situ assembly of template and carbon precursor gel by controlled ultracentrifugation experiments. Benefits of the gradient porosity are demonstrated for the Li-O battery.
Processing of substrates with different solvent compatibility is a persistent problem in homogeneous catalysis, in particular when one starting compound is water soluble and the other is not. A promising concept reported in the literature is micellar catalysis. However, the process of developing catalysts that are surfactants at the same time is still in its early stages. We report the synthesis of a new surfactant system with an N‐heterocyclic carbene (NHC) moiety as a head group. Characteristic surfactant properties such as the formation of micelles or liquid crystals is documented. The new surfactant ligand forms coordination compounds with various metals, most importantly Pd2+, in square planar geometry. In addition, the Pd‐NHC compound shows surfactant features, and can be used successfully for C−C cross‐coupling reactions (Suzuki, Heck). The boost in catalytic activity by one order of magnitude compared to analogous but non‐amphiphilic species is reported.
The
maximization of activity is a general aim in catalysis research.
The possibility for light-triggered enhancement of a catalytic process,
even if the process is not photochemical in nature, represents an
intriguing concept. Here, we present a novel system for the exploration
of the latter idea. A surfactant with a catalytically active head
group, a protonated polyoxometalate (POM) cluster, is attached to
the surface of a gold nanoparticle (Au NP) using thiol coupling chemistry.
The distance of the catalytically active center to the gold surface
could be adjusted precisely using surfactants containing hydrocarbon
chains (Cn) of different lengths (n = 4–10). Radiation with VIS-light has no effect
on the catalytic activity of micellar aggregates of the surfactant. The
situation changes, as soon as the surfactants have been attached to
the Au NPs. The catalytic activity could almost be doubled. It was
proven that the effect is caused by coupling the surface plasmon resonance
of the Au NPs with the properties of the POM head group. The improvement
of activity could only be observed if the excitation wavelength matches
the absorption band of the used Au NPs. Furthermore, the shorter the
distance between the POM group and the surface of the NP, the stronger
is the effect. This phenomenon was explained by lowering the activation
energy of the transition state relevant to the catalytic process by
the strong electric fields in the vicinity of the surfaces of plasmonic
nanoparticles. Because the catalytic enhancement is wavelength-selective,
one can imagine the creation of complex systems in the future, a system
of differently sized NPs, each responsible for a different catalytic
step and activated by light of different colors.
Gaining external control over self‐organization is of vital importance for future smart materials. Surfactants are extremely valuable for the synthesis of diverse nanomaterials. Their self‐assembly is dictated by microphase separation, the hydrophobic effect, and head‐group repulsion. It is desirable to supplement surfactants with an added mode of long‐range and directional interaction. Magnetic forces are ideal, as they are not shielded in water. We report on surfactants with heads containing tightly bound transition‐metal centers. The magnetic moment of the head was varied systematically while keeping shape and charge constant. Changes in the magnetic moment of the head led to notable differences in surface tension, aggregate size, and contact angle, which could also be altered by an external magnetic field. The most astonishing result was that the use of magnetic surfactants as structure‐directing agents enabled the formation of porous solids with 12‐fold rotational symmetry.
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