Environmental Engineering Reference
In-Depth Information
In the present chapter, we will discuss two types of nanoporous solids: nanoporous car-
bon and nanoporous hydrogels. Additionally, those porous materials could be functional-
ized by loading with nanoparticles made of metal, metal oxides, or conducting polymers.
Therefore, we will briely examine the literature on those materials.
2.1.2.1 Nanoporous Carbon
Besides activated carbon, it is possible to synthesize nanoporous carbon by a variety of
methods [40]. Inorganic nanoporous solids (e.g., silica) can be used as a hard template by
covering the pore surface with a thin layer of carbon [41]. This is achieved by the formation
of an organic polymer (e.g., phenol-formaldehyde) by sequential precursor adsorption, fol-
lowed by reaction [42]. The polymer is then carbonized by heating in an inert atmosphere
[43]. Then, the inorganic template is removed by dissolution [44]. The inorganic template
should have an interpenetrated pore topology [45]. Otherwise, only carbon nanoparticles
will be obtained. Inorganic nanoparticles could also be used as hard templates of porous
carbon. To do that, the nanoparticles are aggregated into opals and an organic precursor is
iniltrated in the interstitial space between particles [46]. The silica nanoparticles have to be
in contact with each other; otherwise, the template cannot be removed from inside the solid.
Organic species could be used as soft templates for porous carbon synthesis, among them
are molecular micelles [47] and polymeric micelles [48]. We have extensively studied the syn-
thesis of porous carbons by pyrolysis of porous polymeric (resorcinol-formaldehyde [RF])
resins [49]. The resin gels maintain nanoporosity during conventional drying through sta-
bilization of resin nanoparticles by cationic supramolecular species. The carbon source
(precursor) is subjected to a heat treatment (pyrolysis) at high temperature in the absence
of oxygen to produce the carbon. Since the main research goal was to produce electrode
materials for supercapacitors [50], both capacitance and response rate should be maxi-
mized. All porous carbons can be surface functionalized by linking covalent groups with
the native groups present in the carbon surface [51]. Additionally, synthetic porous carbon
can be produced with other elements (N, S, P) in the structure by using different precur-
sors [52-54].
Porous materials (e.g., carbon) are usually characterized by measuring the adsorption
isotherm of inert gases (e.g., N
2
). By modeling the adsorption data, the surface area, pore
volume, and pore size distribution can be evaluated. However, in electrochemical applica-
tions, only the surface area accessible to the electrolyte is important. It is well known that
small micropores cannot be illed with electrolyte and do not contribute to the electro-
chemical active area but are measured by the gas adsorption isotherms [55].
Therefore,
in situ
measurements of ion adsorption phenomena such as differential capacitance [56]
or probe beam delection (PBD) [57] render more useful data of the carbon texture. PBD
has shown to be very useful for the study of porous materials [58,59], including carbon
aerogels [60].
2.1.2.2 Nanoporous Hydrogels
Cross-linked polymers constitute three-dimensional networks containing nanopores. The
mean size of the pores is directly related to the cross-linking degree [61]. Hydrogels are
cross-linked networks bearing hydrophilic groups that promote gel swelling in water [62].
The mean pore size can be calculated from the swelling capacity by using the Flory-Rehner
theory [63]. Soluble species could dissolve in the water pool of the pores or interact directly
with the polymer chains. Such materials have been used to remove toxic ions (cyanide,
