1 3
Topics in Current Chemistry (2019) 377:11
is larger than that of M 2 , it can be shown that: (a) the ratio of partial coordination
numbers of the two metals is in direct relationship with the ratio of the mole fractions, viz. x(M 1 )/x(M 2 ) = N(M 2 − M 1 )/N(M 1 − M 2 ); (b) the total coordination number
of one metal must be equal to that of the other, i.e., N(M 1 − M 1 ) + N(M 1 − M 2 ); and
(c) the distances between the two metal atoms M 1 − M 2 should be between that of
M 1 − M 1 and M 2 − M 2 [102].
The relationship between coordination numbers and the interatomic distances can
be also derived for the two heterogeneous mixtures in a similar manner [103, 104].
In core–shell nanoparticle, an atom of the core metal (M core ) is surrounded mostly
with atoms of the same type, whereas the M shell atom feels the presence of atoms
of the same type that form the shell, as well as that of the other type of atoms forming the core; however, atoms in the shell always have lower coordination numbers.
In addition, for a core–shell nanoparticle of a certain size, the total coordination
number of the core atoms should be similar to the average coordination number for
the nanoparticle of the same size composed of core atoms only, N(M core_nanoparticle ).
Lastly, the distance between two atoms in the core should be similar to their bulk
interatomic distance R(M core_bulk ), while the distance between two atoms in the shell
can be either smaller or larger than bulk interatomic distance R shell_bulk , depending
on the relative sizes of the two metals comprising the core–shell particle.
Aggregate mixtures consist of two nanoparticles that are completely separated
from one another, each consisting of one type of metal atoms only. Therefore, the
coordination numbers N 1 − N 2 and N 2 − N 1 should be close to zero, and the distances
between atoms of the same type should be close to that in the bulk [104].
4.1 Pt/Rh/SnO 2 Ternary Catalyst for Ethanol Oxidation
The ternary Pt–Rh–SnO 2 electrocatalyst that effectively splits the C–C bond at room
temperature without production of adsorbed CO was prepared by synthesizing SnO 2
particles and absorbing them on carbon support, followed by deposition of Pt and
Rh on top of the SnO 2 surface from their solutions by chemical displacement of previously deposited Pb. The diameters of the nanoclusters of SnO 2 and Pt–Rh clusters
were measured by TEM and found to be larger than 10 nm and between 1 and 3 nm,
respectively [23].
Operando XANES and EXAFS studies during the course of ethanol oxidation
has shown only minor change in the Sn spectra collected from 0.21 to 0.91 V, i.e.,
between potentials at which no oxidation of ethanol is expected to that at which ethanol oxidation is in full progress. As seen in Fig. 10, EXAFS Sn K-edge spectrum
showed a slight increase in intensity of the Sn–O scattering path. Electrical potential
seems to have little influence on the behavior of the nonconducting SnO 2 surface, as
its electrical state is controlled mostly by the solution’s pH and its interaction with
the environment is mediated by the “carpet” of OH groups resulted by the dissociative adsorption of water molecules [105, 106]. This effect probably shifts the onset
of surface oxidation of both Rh and Pt to positive potentials, thus keeping Pt–Rh
nanoparticles in zero-valent state in the potential region of practical interest for fuel
cell technology (0 < E < 0.6 V).
Reprinted from the journal
19
Topics in Current Chemistry (2019) 377:11
is larger than that of M 2 , it can be shown that: (a) the ratio of partial coordination
numbers of the two metals is in direct relationship with the ratio of the mole fractions, viz. x(M 1 )/x(M 2 ) = N(M 2 − M 1 )/N(M 1 − M 2 ); (b) the total coordination number
of one metal must be equal to that of the other, i.e., N(M 1 − M 1 ) + N(M 1 − M 2 ); and
(c) the distances between the two metal atoms M 1 − M 2 should be between that of
M 1 − M 1 and M 2 − M 2 [102].
The relationship between coordination numbers and the interatomic distances can
be also derived for the two heterogeneous mixtures in a similar manner [103, 104].
In core–shell nanoparticle, an atom of the core metal (M core ) is surrounded mostly
with atoms of the same type, whereas the M shell atom feels the presence of atoms
of the same type that form the shell, as well as that of the other type of atoms forming the core; however, atoms in the shell always have lower coordination numbers.
In addition, for a core–shell nanoparticle of a certain size, the total coordination
number of the core atoms should be similar to the average coordination number for
the nanoparticle of the same size composed of core atoms only, N(M core_nanoparticle ).
Lastly, the distance between two atoms in the core should be similar to their bulk
interatomic distance R(M core_bulk ), while the distance between two atoms in the shell
can be either smaller or larger than bulk interatomic distance R shell_bulk , depending
on the relative sizes of the two metals comprising the core–shell particle.
Aggregate mixtures consist of two nanoparticles that are completely separated
from one another, each consisting of one type of metal atoms only. Therefore, the
coordination numbers N 1 − N 2 and N 2 − N 1 should be close to zero, and the distances
between atoms of the same type should be close to that in the bulk [104].
4.1 Pt/Rh/SnO 2 Ternary Catalyst for Ethanol Oxidation
The ternary Pt–Rh–SnO 2 electrocatalyst that effectively splits the C–C bond at room
temperature without production of adsorbed CO was prepared by synthesizing SnO 2
particles and absorbing them on carbon support, followed by deposition of Pt and
Rh on top of the SnO 2 surface from their solutions by chemical displacement of previously deposited Pb. The diameters of the nanoclusters of SnO 2 and Pt–Rh clusters
were measured by TEM and found to be larger than 10 nm and between 1 and 3 nm,
respectively [23].
Operando XANES and EXAFS studies during the course of ethanol oxidation
has shown only minor change in the Sn spectra collected from 0.21 to 0.91 V, i.e.,
between potentials at which no oxidation of ethanol is expected to that at which ethanol oxidation is in full progress. As seen in Fig. 10, EXAFS Sn K-edge spectrum
showed a slight increase in intensity of the Sn–O scattering path. Electrical potential
seems to have little influence on the behavior of the nonconducting SnO 2 surface, as
its electrical state is controlled mostly by the solution’s pH and its interaction with
the environment is mediated by the “carpet” of OH groups resulted by the dissociative adsorption of water molecules [105, 106]. This effect probably shifts the onset
of surface oxidation of both Rh and Pt to positive potentials, thus keeping Pt–Rh
nanoparticles in zero-valent state in the potential region of practical interest for fuel
cell technology (0 < E < 0.6 V).
Reprinted from the journal
19