3 research outputs found
Promoting Strong Metal Support Interaction: Doping ZnO for Enhanced Activity of Cu/ZnO:M (M = Al, Ga, Mg) Catalysts
The promoting effect of Al, Ga, and
Mg on the support in Cu/ZnO
catalysts for methanol synthesis has been investigated. Different
unpromoted and promoted ZnO supports were synthesized and impregnated
with Cu metal in a subsequent step. All materials, supports, and calcined
and activated catalysts were characterized by various methods, including
contactless (microwave) conductivity measurements under different
gas atmospheres. Small amounts of promoters were found to exhibit
a significant influence on the properties of the oxide support, concerning
textural as well as electronic properties. We found correlations between
the conductivity of the ZnO support and the activity of the catalyst
in the reverse water-gas shift reaction (rWGS) as well as in methanol
synthesis. In rWGS the activation energy and reaction order in H<sub>2</sub> are decreased upon promotion of the ZnO support with the
trivalent promoters Al<sup>3+</sup> and Ga<sup>3+</sup>, indicating
an electronic promotion. In methanol synthesis, results point to a
structural promotion by Al<sup>3+</sup> and Ga<sup>3+</sup>. A detrimental
effect of Mg<sup>2+</sup> doping was observed in both reactions. This
effect is discussed in the context of the reducibility of ZnO under
reaction conditions, which can be tuned by the promoter in different
ways. The reducibility is seen as a critical property for the dynamic
metal support interaction of the Cu/ZnO system
Ambient-Pressure Soft X‑ray Absorption Spectroscopy of a Catalyst Surface in Action: Closing the Pressure Gap in the Selective <i>n</i>‑Butane Oxidation over Vanadyl Pyrophosphate
In order to close the pressure gap
in the investigation of catalyst surfaces under real operation conditions
we have developed a variable-pressure soft X-ray (<i>h</i>ν ≤1.5 keV) absorption cell coupled to a gas analysis
system to study the pressure dependency of the electronic and catalytic
properties of catalyst surfaces in reactive atmospheres at elevated
temperatures. With this setup we investigated the vanadium L<sub>3</sub>-edge and catalytic performance of polycrystalline vanadyl pyrophosphate
in the selective oxidation of <i>n</i>-butane to maleic
anhydride between 10 and 1000 mbar at 400 °C. As a result, major
gas phase and pressure dependent spectral changes are observed at
energies attributed to V 2p-3d<sub><i>z</i><sup>2</sup></sub> excitations assigned to vanadium atoms square-pyramidally coordinated
to oxygen atoms. This can be interpreted in terms of a shortened vanadyl
bond (VO) and an increased vanadium oxidation state with higher
pressures. Since this is accompanied by an increasing catalytic activity
and selectivity, it indicates that vanadyl oxygen is actively involved
in the selective oxidation of the alkane
Promoted Ceria: A Structural, Catalytic, and Computational Study
The role of trivalent (La, Sm, Gd,
and Y) and tetravalent (Hf,
Zr, and Ti) dopants in the catalytic, structural, and electronic properties
of ceria was investigated. Promoted ceria catalysts were synthesized
by coprecipitation with ammonia and tested in HCl and CO oxidation.
Ceria catalysts exhibit a medium high reactivity and excellent stability
in HCl oxidation. The intrinsic reactivity of ceria in HCl oxidation
can be improved by a factor of 2 when doping with Hf and Zr in appropriate
quantities, whereas trivalent dopants are detrimental. Although both
oxidation reactions rely on the existence of oxygen vacancies, the
order of reactivity in HCl and CO oxidation is not completely parallel.
The effects of promoters on the electronic conductivity and the vacancy
formation energy were studied by contactless conductivity experiments
using the microwave cavity perturbation technique and by density functional
theory calculations. Furthermore, transport properties were also assessed
on the basis of theoretical calculations. The order of oxygen vacancy
formation energy follows well the order of conductivity (polaron mobility)
(trivalent > tetravalent > undoped) observed under inert and
oxidizing
conditions. This implies that none of these properties correlates
with the reactivity. On the other hand, reducing conditions strongly
enhanced the conductivity of Hf- and Zr-doped ceria. In HCl oxidation,
only the balanced reduction of both Cl and O vacancy formation energies
allows for an enhanced reactivity. Promoters give rise to lattice
contraction–expansion modifying vacancy formation energies,
adsorption properties, and surface coverages