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W. Piskorz and F. Zasada
In most cases, the oxygen atom transfer from the catalyst to the adsorbed molecule,
along the Mars–van Krevelen (MvK [82]) mechanism, leads to the oxygen vacancy
formation. In order to close the catalytic cycle and restore the initial stoichiometry of
the catalyst, the reaction must be carried out under oxygen (or other oxidising agent,
e.g. N 2 O) pressure so that gas-phase oxygen species can interact with the surface,
dissociate, and eventually refill the vacancy. To assess the origin of the oxidiser—to
distinguish between the solid O donor or the gas-phase O donor (in other words, to
scrutinise the mechanism to tell the MvK from the reactive surface oxygen speciesbased one)—the isotopic measurements are performed [83].
The reducible oxides can form bulk materials, nanocrystals, and nanostructured
or functionalised surfaces. They are versatile systems whose unique properties stem
from the wealth of the oxidation states of metal cations and from easy transformation
between them [84].
The reducible oxides tend to fulfil the Grasselli’s [85] canonical properties (“seven
pillars”) of a well-performing and efficient redox catalyst, namely as originally enumerated: “lattice oxygen, metal–oxygen bond strength, host structure, redox, multifunctionality of active sites, site isolation, and phase cooperation”. One postulate,
however, seems to be worth adding: the mutual electronic communication of the
sites which allows for the catalytic cycle to close in the larger scale, both temporary
and spatially, thus facilitating the redox properties in the more flexible way. In other
words, the catalytic cycle does not necessarily have to close upon each and every
catalytic act (“differentially”) but only on average, “integrally”. One site can be, e.g.,
an electron donor for more than one reacting molecule while the needed electrons
can be supplied by other sites and delivered through the bulk by, e.g., polaron mechanism. Such electron shuttling requires, however, the cooperation of the conductive
bulk system.
3.1.1 Ceria CeO 2
Although cerium is the lanthanide metal, it will be discussed together with transition
metals since its oxide’s redox chemistry is actually very close to that of TM oxides.
Ceria, CeO 2 , is one of the most abundant redox metal oxide catalysts and definitely
the most common lanthanide oxide in the redox catalysis [86] for many reasons.
Ceria acts as an oxygen reservoir in both simple and composite catalysts [87, 88]
in automotive technology (the most successful, ceria-utilising three-way catalysts,
TWC [89, 90]), water-gas shift, WGS [91, 92] (CO + H 2 O → CO 2 + H 2 ), in fuel
cells technology [93, 94], and also in organic catalysis [95]. It has also a prominent
usage in the petrochemical industry in hydrocarbon reforming [96] and oxidation
[97].
Its reduction and reoxidation during catalytic process are crucial in pure ceria,
which was devoted a number of applied [98] and fundamental experimental [99,
100] and theoretical [81, 101–103] studies, in doped ceria [104], and in metal–ceria
composite system catalysis. Ceria, possessing a complex electronic structure [105,
106], is particularly interesting due to its unique properties: pronounced structure
W. Piskorz and F. Zasada
In most cases, the oxygen atom transfer from the catalyst to the adsorbed molecule,
along the Mars–van Krevelen (MvK [82]) mechanism, leads to the oxygen vacancy
formation. In order to close the catalytic cycle and restore the initial stoichiometry of
the catalyst, the reaction must be carried out under oxygen (or other oxidising agent,
e.g. N 2 O) pressure so that gas-phase oxygen species can interact with the surface,
dissociate, and eventually refill the vacancy. To assess the origin of the oxidiser—to
distinguish between the solid O donor or the gas-phase O donor (in other words, to
scrutinise the mechanism to tell the MvK from the reactive surface oxygen speciesbased one)—the isotopic measurements are performed [83].
The reducible oxides can form bulk materials, nanocrystals, and nanostructured
or functionalised surfaces. They are versatile systems whose unique properties stem
from the wealth of the oxidation states of metal cations and from easy transformation
between them [84].
The reducible oxides tend to fulfil the Grasselli’s [85] canonical properties (“seven
pillars”) of a well-performing and efficient redox catalyst, namely as originally enumerated: “lattice oxygen, metal–oxygen bond strength, host structure, redox, multifunctionality of active sites, site isolation, and phase cooperation”. One postulate,
however, seems to be worth adding: the mutual electronic communication of the
sites which allows for the catalytic cycle to close in the larger scale, both temporary
and spatially, thus facilitating the redox properties in the more flexible way. In other
words, the catalytic cycle does not necessarily have to close upon each and every
catalytic act (“differentially”) but only on average, “integrally”. One site can be, e.g.,
an electron donor for more than one reacting molecule while the needed electrons
can be supplied by other sites and delivered through the bulk by, e.g., polaron mechanism. Such electron shuttling requires, however, the cooperation of the conductive
bulk system.
3.1.1 Ceria CeO 2
Although cerium is the lanthanide metal, it will be discussed together with transition
metals since its oxide’s redox chemistry is actually very close to that of TM oxides.
Ceria, CeO 2 , is one of the most abundant redox metal oxide catalysts and definitely
the most common lanthanide oxide in the redox catalysis [86] for many reasons.
Ceria acts as an oxygen reservoir in both simple and composite catalysts [87, 88]
in automotive technology (the most successful, ceria-utilising three-way catalysts,
TWC [89, 90]), water-gas shift, WGS [91, 92] (CO + H 2 O → CO 2 + H 2 ), in fuel
cells technology [93, 94], and also in organic catalysis [95]. It has also a prominent
usage in the petrochemical industry in hydrocarbon reforming [96] and oxidation
[97].
Its reduction and reoxidation during catalytic process are crucial in pure ceria,
which was devoted a number of applied [98] and fundamental experimental [99,
100] and theoretical [81, 101–103] studies, in doped ceria [104], and in metal–ceria
composite system catalysis. Ceria, possessing a complex electronic structure [105,
106], is particularly interesting due to its unique properties: pronounced structure