The non-oxidative dehydrogenation of ethanol to acetaldehyde has long been considered as an important method to produce acetaldehyde and clean hydrogen gas. Although monometallic Cu nanoparticles have high activity in the non-oxidative dehydrogenation of ethanol, they quickly deactivate due to sintering of Cu. Herein, we show that adding a small amount of Ni (Ni 0.01 Cu-Ni 0.001 Cu) into Cu to form highly dilute NiCu alloys dramatically increases the catalytic activity and increases their long-term stability. The kinetic studies show that the apparent activation energy decreases from ~70 kJ/mol over Cu to ~45 kJ/mol over the dilute NiCu alloys. The improved performance is observed both for nanoparticles and nanoporous NiCu alloys. The improvement in the long-term stability of the catalysts is attributed to the stabilization of Cu against sintering. Our characterization data show that Ni is atomically dispersed in Cu. The comparison of the catalytic performance of highly dilute alloy nanoparticles with nanoporous materials is useful to guide the design of novel mesoporous catalyst architectures for selective dehydrogenation reactions.
Understanding photochemical
processes on nanomaterials is key to
developing effective photocatalysts. Herein, methanol oxidation and
reduction is used to probe the thermal and photochemical properties
of rutile titania nanowires grown using a hydrothermal method. The
presence of oxygen vacancy defects leads to methoxy formation and
subsequent disproportionation to formaldehyde and methanol at 700
K. Methane and dimethyl ether are also produced in minor quantities.
Oxygen adatoms enhance the formation of methoxy, which led to an increase
in the disproportionation products and dimethyl ether at high temperature
and a decreased amount of methane. The thermal reactivity of the nanowires
parallels that of rutile TiO2(110) single crystals. Photo-oxidation
of methoxy using UV light produced formaldehyde and methyl formate.
These product yields were enhanced on nanowires with oxygen adatoms,
but a majority of methoxy (∼70%) is not photoactive.
In contrast, all methoxy is photo-oxidized on rutile TiO2(110) when O-adatoms are present. This difference indicates that
holes created in the nanowires during UV excitation do not migrate
to most of the methoxya required step for methoxy photo-oxidation.
This lack of activity could be due to either trapping of holes in
the material or different binding of the inactive methoxy. These studies
demonstrate that while charge carriers can be efficiently created
in nanowires differences in chemical properties can suppress photo-oxidation.
Many application-relevant properties of nanoporous metals critically depend on their multiscale architecture. For example, the intrinsically high step-edge density of curved surfaces at the nanoscale provides highly reactive sites for catalysis, whereas the macroscale pore and grain morphology determines the macroscopic properties, such as mass transport, electrical conductivity, or mechanical properties. In this work, we systematically study the effects of alloy composition and dealloying conditions on the multiscale morphology of nanoporous copper (np-Cu) made from various commercial Zn-Cu precursor alloys. Using a combination of X-ray diffraction, electron backscatter diffraction, and focused ion beam cross-sectional analysis, our results reveal that the macroscopic grain structure of the starting alloy surprisingly survives the dealloying process, despite a change in crystal structure from body-centered cubic (Zn-Cu starting alloy) to face-centered cubic (Cu). The nanoscale structure can be controlled by the acid used for dealloying with HCl leading to a larger and more faceted ligament morphology compared to that of HPO. Anhydrous ethanol dehydrogenation was used as a probe reaction to test the effect of the nanoscale ligament morphology on the apparent activation energy of the reaction.
In situ and ex situ X-ray photoelectron spectroscopy and electron-microscopy reveal that the stability of nanoporous NiCu alloy catalysts for non-oxidative ethanol dehydrogenation improves by generating kinetically trapped Ni2+ subsurface states.
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