II International Symposium on Carbon for Catalysis ABSTRACTS

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THEORY OF MECHANICAL BEHAVIOR OF MICROPOROUS CARBON
CATALYSTS
Rusanov А.I.
Mendeleev Center of St. Petersburg State University, St. Petersburg, Russia
e-mail: rusanov@AR1047.spb.edu
Researchers dealing with microporous carbon sorbents observed the sorption-striction
phenomenon (a change in the sorbent volume), which can take different forms depending on
the external conditions. When a sorbate enters a sorbent, the latter can expand at a high
temperature, but also can contract at a sufficiently low temperature and a low amount of the
sorbate with subsequent transition to expansion at higher sorbate amounts. Naturally, the
phenomenon is determined by molecular interactions of the sorbate matter with the micropore
walls. A special case is chemisorption that is typical for catalysis.
The theory of mechanical behavior of microporous carbon bodies reported in this
presentation includes three parts. The first part contains the calculation of the pressure tensor
inside a micropore where there cannot be the sorbate bulk phase, and the pressure tensor
structure is determined by the micropore shape. Calculations have been carried out for
spherical cavities, cylindrical pores, and flat (slit-like) pores. Since carbon interacts with
dispersion forces, all calculations were made on the basis of the pair potential
−λ
∅ () r =− A r
(1)
ij
( λ )
ij
where r is the distance between molecules of sorts i and j,
( )
A λ
ij
is a constant of interaction, λ
= 6 for the ordinary van der Waals forces (at r < 12 nm) and λ = 7 for the van der Waals
forces with electromagnetic retardation (at r > 50 nm). As Equation (1) itself, the whole
computational scheme is based on the asymptotic theory of dispersion interactions (presuming
the pore diameter to be considerably larger than the molecular size). The calculation yields a
negative value for the normal (to the pore walls) pressure (leading to the self-contraction of a
porous body) in vacuum and establishes conditions when the interaction with the first portion
of a sorbate provokes an additional contraction (the most probable in the case of
chemisorption). This additional contraction is shown to be attained easier the smaller is the
pore size and is best realizable for nanoporous solids.
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