Environmental Engineering Reference
In-Depth Information
FIGURE 2.3
Scanning electron micrograph of a hierarchical carbon produced by carbonization RF resin adsorbed on a cel-
lulose open cloth.
precursor materials are organic polymers containing hydrogen and oxygen besides carbon
(approx. formula C
22
H
22
O
10
), the pyrolysis to give pure carbon involves mass loss and volume
contraction. Soft templates like polymers and surfactants are spontaneously eliminated dur-
ing burning of the carbon material; therefore, posttreatments are not necessary. On the other
hand, hard templates like silica and metal oxides survive to high temperatures and must be
removed after pyrolysis by chemical etching. In this process, the space initially occupied by
the templates is transformed into the pores in the resulting carbon materials. The whole pro-
cess results in a reverse copy of the template [106]. To produce carbon with an open structure,
we used silica nanoparticles as a hard template. The volume contraction during the pyrolysis
of RF resin is easily observable by comparison between the piece size before and after the
thermal decomposition of the precursor. However, when the carbon sample contains a rigid
template, the observed contraction in the macroscale is very low owing to the presence of a
noncompressible skeleton. In our case, this rigid support is composed by an array of nanoscale
SiO
2
nanoparticles in close contact between them. The scanning electron microscopy (SEM)
image of these SiO
2
nanoparticles reveals the existence of almost monodisperse spheres. As
shown in Scheme 2.3, the hard template can be eliminated before or after pyrolysis. We have
shown [19]
that elimination before pyrolysis produces a carbon with one main pore size (1PSC),
which is determined by the template nanoparticle size but takes into account the 20%-30%
contraction of the material upon pyrolysis. However, the presence of the silica nanoparticles
seems to stabilize the microporosity/mesoporosity since relatively large surface areas are
measured (~150 m
2
/g). It should be noted that a porous matrix with holes of ~400 nm will have
~6 m
2
/g of speciic surface area.
On the other hand, if the hard template is left during pyrolysis, the contraction of the solid
material has to take place in the interstitial spaces between particles. In that way, the solid
becomes microfractured and the carbon has at least two pore sizes (2PSC), one directly related
to the hard template size (e.g., 400 nm) and another much smaller in the order of mesopores
(<50 nm). Additionally, the carbon presents the sintered beads of carbon linked into a matrix
through necks, which has been proposed as the building block of the RF resin. Those beads,
after precipitation, form the condensed monoliths [107,108] (Scheme 2.5).
In the lat focused ion beam (FIB) cut, it is possible to observe the open three-dimensional
nature of the carbon (Figure 2.4). The surface area is of ~650 m
2
/g, revealing that micro-
fractures contribute greatly to the surface area.
