We
report on the activation of CO
2
on Ni single-atom
catalysts. These catalysts were synthesized using a solid solution
approach by controlled substitution of 1–10 atom % of Mg
2+
by Ni
2+
inside the MgO structure. The Ni atoms
are preferentially located on the surface of the MgO and, as predicted
by hybrid-functional calculations, favor low-coordinated sites. The
isolated Ni atoms are active for CO
2
conversion through
the reverse water–gas shift (rWGS) but are unable to conduct
its further hydrogenation to CH
4
(or MeOH), for which Ni
clusters are needed. The CO formation rates correlate linearly with
the concentration of Ni on the surface evidenced by XPS and microcalorimetry.
The calculations show that the substitution of Mg atoms by Ni atoms
on the surface of the oxide structure reduces the strength of the
CO
2
binding at low-coordinated sites and also promotes
H
2
dissociation. Astonishingly, the single-atom catalysts
stayed stable over 100 h on stream, after which no clusters or particle
formation could be detected. Upon catalysis, a surface carbonate adsorbate-layer
was formed, of which the decompositions appear to be directly linked
to the aggregation of Ni. This study on atomically dispersed Ni species
brings new fundamental understanding of Ni active sites for reactions
involving CO
2
and clearly evidence the limits of single-atom
catalysis for complex reactions.
We
report on a combined quantitative charge carrier and catalytic
activity analysis of Cu/ZnO(:Al) model catalysts. The promoting effect
of Al3+ on the ZnO support for CO2 activation
via the reverse water–gas-shift reaction has been investigated.
The contact-free and operando microwave Hall Effect technique is applied
to measure charge carriers in Cu/ZnO(:Al) based model catalysts under
reverse water–gas shift reaction conditions. This method allows
us to monitor the electrical conductivity, charge carrier mobility,
and absolute number of charge carriers. An increase in charge carrier
concentration with increasing Al3+ content and its direct
correlation with the catalytic activity for CO formation is found.
We conclude that the increased availability of charge carriers plays
a key role in CO2 activation and CO formation, which finds
additional support in a concurrent decrease of the apparent activation
energy and increase in the reaction order of CO2. In combination
with comprehensive DFT calculations, the impact of the interfacial
charge transfer, coupled to oxygen defect sites in ZnO and CO2 adsorption properties, is elucidated and highlighted. In
conclusion, the results from this operando investigation combined
with DFT calculations demonstrate the importance of charge transfer
processes as decisive descriptors for understanding and explaining
catalytic properties.
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