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For methane, steam reforming occurs in two stages:
2 a first conversion by a highly endothermic reaction
CH 4 + H 2 O m CO + 3H 2
2 a second slightly exothermic step called the “water gas shift”
CO + H 2 O m CO 2 + H 2
These reactions occur at high temperature (800 °C).
Stoichiometry (departure from) – Consider an ionic crystal with the formula M a X b
resulting from the equilibrium
aM + b
2
X 2 m M a X b
In reality, the composition of this crystal varies with temperature and its surroundings and the formula may be written in the following different forms:
M a X b−β , M a X b +β , M a+α X b , M a −α X b
where α and β represent the departure from stoichiometry. In M a X b − β and
M a − α X b , the crystal is deficient in X and M, respectively. In M a X b + β and
M a + α X b , the crystal has excess X and M, respectively. The departure from
stoichiometry is manifested by the presence of point defects (vacancies,
interstitial ions) whose role is primordial in electrical conductivity, ionic
diffusion, …
Structure defect – A site filled with an anomaly with respect to the perfect crystal.
In the Kröger and Vink notation, it is defined by its site, the chemical species
that occupies the site, and its effective charge.
Structure element – In an ionic crystal, a structure element denotes a chemical
entity defined by its site, the element that occupies the site, and the effective
charge of the entity. It is said to be normal if it corresponds to the perfect
crystal and is otherwise referred to as a defect. For example, in the solid
solution CaF 2 -KF where KF is the dopant, potassium substitutes for calcium.
The species Ca
#
Ca and F
#
F are normal structure elements whereas the species
K ′
Ca and V
•
F are structure defects.
Substitution position – An atom is said to occupy a substitution position in a solid
when it occupies a site normally occupied in the ideal structure by an atom
of a different chemical nature.
Tafel equation – Expresses the overpotential η of an electrode as a function of the
current density in the electrode as follows: η = a + b log | i|
a and b are constant that give access, in particular, to the exchange-current
density i 0 . This law is verified for slow electronic transfer reactions, when
the electrode is subjected to a strong polarization.
Solid-State Electrochemistry
For methane, steam reforming occurs in two stages:
2 a first conversion by a highly endothermic reaction
CH 4 + H 2 O m CO + 3H 2
2 a second slightly exothermic step called the “water gas shift”
CO + H 2 O m CO 2 + H 2
These reactions occur at high temperature (800 °C).
Stoichiometry (departure from) – Consider an ionic crystal with the formula M a X b
resulting from the equilibrium
aM + b
2
X 2 m M a X b
In reality, the composition of this crystal varies with temperature and its surroundings and the formula may be written in the following different forms:
M a X b−β , M a X b +β , M a+α X b , M a −α X b
where α and β represent the departure from stoichiometry. In M a X b − β and
M a − α X b , the crystal is deficient in X and M, respectively. In M a X b + β and
M a + α X b , the crystal has excess X and M, respectively. The departure from
stoichiometry is manifested by the presence of point defects (vacancies,
interstitial ions) whose role is primordial in electrical conductivity, ionic
diffusion, …
Structure defect – A site filled with an anomaly with respect to the perfect crystal.
In the Kröger and Vink notation, it is defined by its site, the chemical species
that occupies the site, and its effective charge.
Structure element – In an ionic crystal, a structure element denotes a chemical
entity defined by its site, the element that occupies the site, and the effective
charge of the entity. It is said to be normal if it corresponds to the perfect
crystal and is otherwise referred to as a defect. For example, in the solid
solution CaF 2 -KF where KF is the dopant, potassium substitutes for calcium.
The species Ca
#
Ca and F
#
F are normal structure elements whereas the species
K ′
Ca and V
•
F are structure defects.
Substitution position – An atom is said to occupy a substitution position in a solid
when it occupies a site normally occupied in the ideal structure by an atom
of a different chemical nature.
Tafel equation – Expresses the overpotential η of an electrode as a function of the
current density in the electrode as follows: η = a + b log | i|
a and b are constant that give access, in particular, to the exchange-current
density i 0 . This law is verified for slow electronic transfer reactions, when
the electrode is subjected to a strong polarization.
Solid-State Electrochemistry