7 Catalysis by Metal Nanoparticles Encapsulated …
229
of this solvent is less than the internal pore volume. Subsequently, after adsorption
of the precursor, the solid particles of the MOF have to be washed before proceeding
to generation of the metal NP by chemical reduction.
Besides adsorption from the gas or liquid phases, formation of metal NPs inside
MOFs has also been reported by adsorption in the solid phase. Thus, Haruta and coworkers have reported that grinding MOF-5 with a solid gold complex in a mortar
is sufficient to produce the incorporation inside the MOF of the gold complex that
subsequently can be reduced into Au NPs [34].
From the current state of the art, it can be summarized that there is a variety of
procedures that yield metal NPs and even small metal oxide NPs for those metals like
Zn and Ti that easily form metal oxides and that these procedures can be applied to
a large variety of MOF structures, including those like Zr MOFs and MIL-100/101
families that produce remarkably stable catalysts [26, 35, 36].
One issue of concern when preparing guests inside porous materials, and particularly inside MOFs, is to provide convincing evidence of the internal location of
the incorporated guest. This is always a very difficult task and probably the most
convincing technique to address this issue is high-resolution transmission electron
tomography. In this technique, a series of TEM images of the same MOF particle are
taken with different submicrometric increment in the vertical axis. These images are
later used to reconstruct a three-dimensional mapping of the crystallite, the process
being analogous to confocal optical microscopy widely used to determine internalization of particles inside living cells, except that the resolution in the case of electron
microscope is in the nanometer scale. Unfortunately, transmission electron tomography is not yet a routine technique, and therefore, it is not still widely available to
characterize samples of metal NPs inside MOFs on a regular basis.
For this reason, other methods should be implemented to provide some support to
the internal location of the metal NPs. In this context, conventional TEM, especially
in the dark field mode, should determine the average particle size and its distribution.
If the metal NPs are inside the pores their diameter has to be commensurate with
the dimension of the pores. In other words, large particles should not be present and
only particles with diameters smaller than the cavity size should be observed by TEM
images.
Besides electron microscopy, surface area measurements and catalytic activity
tests can also provide indirect support to the internal location of the metal NPs.
Thus, isothermal gas adsorption after incorporation of metal NPs should determine
a decrease in the internal surface area and pore volume. Similarly by testing the
catalytic activity of the materials for substrates with similar chemical reactivity,
but different molecular dimensions, smaller and larger than the pores of MOFs,
the contrasting activity of the metal NPs inside the MOF should reflect that only
those substrates that can diffuse inside the pores undergo chemical reaction, while
those others that cannot access the internal pores would not undergo chemical
transformation under same conditions.
The following section will describe selected examples of a variety of reactions
with the aim to illustrate the advantage and the potential of the use of metal NPs
229
of this solvent is less than the internal pore volume. Subsequently, after adsorption
of the precursor, the solid particles of the MOF have to be washed before proceeding
to generation of the metal NP by chemical reduction.
Besides adsorption from the gas or liquid phases, formation of metal NPs inside
MOFs has also been reported by adsorption in the solid phase. Thus, Haruta and coworkers have reported that grinding MOF-5 with a solid gold complex in a mortar
is sufficient to produce the incorporation inside the MOF of the gold complex that
subsequently can be reduced into Au NPs [34].
From the current state of the art, it can be summarized that there is a variety of
procedures that yield metal NPs and even small metal oxide NPs for those metals like
Zn and Ti that easily form metal oxides and that these procedures can be applied to
a large variety of MOF structures, including those like Zr MOFs and MIL-100/101
families that produce remarkably stable catalysts [26, 35, 36].
One issue of concern when preparing guests inside porous materials, and particularly inside MOFs, is to provide convincing evidence of the internal location of
the incorporated guest. This is always a very difficult task and probably the most
convincing technique to address this issue is high-resolution transmission electron
tomography. In this technique, a series of TEM images of the same MOF particle are
taken with different submicrometric increment in the vertical axis. These images are
later used to reconstruct a three-dimensional mapping of the crystallite, the process
being analogous to confocal optical microscopy widely used to determine internalization of particles inside living cells, except that the resolution in the case of electron
microscope is in the nanometer scale. Unfortunately, transmission electron tomography is not yet a routine technique, and therefore, it is not still widely available to
characterize samples of metal NPs inside MOFs on a regular basis.
For this reason, other methods should be implemented to provide some support to
the internal location of the metal NPs. In this context, conventional TEM, especially
in the dark field mode, should determine the average particle size and its distribution.
If the metal NPs are inside the pores their diameter has to be commensurate with
the dimension of the pores. In other words, large particles should not be present and
only particles with diameters smaller than the cavity size should be observed by TEM
images.
Besides electron microscopy, surface area measurements and catalytic activity
tests can also provide indirect support to the internal location of the metal NPs.
Thus, isothermal gas adsorption after incorporation of metal NPs should determine
a decrease in the internal surface area and pore volume. Similarly by testing the
catalytic activity of the materials for substrates with similar chemical reactivity,
but different molecular dimensions, smaller and larger than the pores of MOFs,
the contrasting activity of the metal NPs inside the MOF should reflect that only
those substrates that can diffuse inside the pores undergo chemical reaction, while
those others that cannot access the internal pores would not undergo chemical
transformation under same conditions.
The following section will describe selected examples of a variety of reactions
with the aim to illustrate the advantage and the potential of the use of metal NPs