Ignazio
Renato Bellobono
Centro di Ricerca per l'Ambiente e l'Impresa, Università degli Studi
di Milano;
via C.Golgi, 19 - I-20133 Milano.
1. Presentation of the technology
The semiconductor-mediated photocatalytic oxidation of organic compounds in
waste waters and in polluted gaseous emissions has attracted a great deal of
attention in recent years, as documented by fact that much over 2500 papers,
and a remarkable number of patents have been published on this topic since 1981.
It is outside the scope of this report to overview, even by rapid citations,
the field of semiconductor photocatalysis the extent of which has been recently
summarized, with ample references to existing literature reviews and to newly
acquired experimental facts [1] (see also review papers cited in Ref. [2]).
Most of the work has exploited the favourable electronic and mechanical characteristics
of titanium dioxide. This semiconductor has a band gap corresponding to integral
photon absorption for wavelengths less than 380 nm, so that it can be pumped
with inexpensive UV lamps. The oxidation potential of a photogenerated hole
exceeds 3.0 V. The solid is mechanically robust, chemically stable and resists
both thermal and photochemical degradation. For industrial applications, however,
the almost general use of nanopowders, which has been made in the literature,
is unacceptable. Due to the almost generalized use of photocatalyst suspensions,
the development of a reliable knowledge base for technology has been until now
in its initial stages, in which problems related to catalyst preparation and
photoactivation, chemical and kinetic networks of pollutant oxidation, inhibition
by degradation products, effects of irradiance levels, photocatalytic reactor
design, and optical properties of suspended photocatalyst represent some of
the main weaknesses. One of the most relevant problems, when using suspended
semiconductor in liquid media, to drive the complex electrochemical and catalytic
phenomena, has been that of the radiation distribution in heterogeneous systems
in which absorption and scattering are coupled with chemical reactions. Furthermore,
the separation of a colloidal photocatalyst at the end of the process, is an
uneconomical other than insurmountably difficult complication. When operating
in gaseous media, such as could be for the application of these processes to
air purification and simultaneous debacterization in air conditioning equipments,
the release of nanopowders in the gaseous stream represents a serious drawback.
Consequently and unfortunately, low efficiencies have limited up to now the
possibilities for economic scale-up in applications. This has led to a number
of attempts to anchor titanium dioxide on supports, including glass beads, fibre
glass, silica, electrodes, clay, and zeolites (see Ref. [2], Ref. [3], and Refs.
cited therein). So far these efforts have not produced materials which meet
all desiderata of photocatalytic activity. Supported titanium dioxide is commonly
reported to be less photoactive than the corresponding "free" unsupported
semiconductor.
Industrial applicability of these photocatalytic processes has lately been shown
as effective, by immobilizing conveniently photocatalytic systems in membranes,
prepared photochemically by a very fast method based on photografting (for recent
reviews, see Refs. [4-5]) . On one side, by this method (PHOTOPERM® process),
full permeability through the supporting structure could be maintained, while
keeping completely, or nearly completely, active the surface area of photocatalysts.
On the other side, immobilization of semiconductor, and promoting photocatalysts
(able to enhance remarkably the activity of semiconductor photocatalyst) as
well, in a reliable optically absorbing configuration was able to remove all
difficulties and drawbacks of conventional homogeneous or heterogeneous catalysis,
by exploiting simultaneously all known advantages of membrane processes, such
as continuous separation or concentration of solutions being processed, or,
during the treatment of gaseous streams, a firm attachment of the catalyst on
to the support, accompanied by elimination of any danger due to release of catalyst
nanopowders, as well as the most convenient modelling of photoreactor, able
to choose the appropriate membrane geometry, and taking benefit from the very
high surface-volume ratio in membrane reactors.
In a recent paper [6] a research study has been reported, based on the use of
the PHOTOPERM® technology, as applied to the treatment of gaseous emissions
and air. This investigation has been carried out on trichloroethene, as a model
molecule of air contaminants in industrial atmospheres, and, at the same time,
a model molecule of the particularly toxic chloro-derivatives. Notwithstanding
trichloroethene, like other chloroaliphatics, is notoriously one of the most
difficult compounds to degrade, the cited research work has shown that integral
oxidation and photodegradation of trichloroethene in air is possible, provided
convenient promoting species are added to titanium dioxide immobilized in the
catalytic membranes, and relative humidity of air kept high. An advantageous
method for air purification may thus be based on this technology, which meets
the requirements of safety, because no toxic degradation intermediate, such
as phosgene or carbon monoxide, which had been observed in previous studies
[7,] withstands degradation in the presence of suitable promoting photocatalysts,
enhancing the activity of titanium dioxide. At this time, this process, contrarily
to the classical carbon adsorption, does not need any regeneration of the reactive
medium, because photoactivity is kept indefinitely. Furthermore, contrarily
to the well known drawback, which accompanies the use of active carbon, i.e.
the proliferation of bacteria on the absorbing bed, in the photocatalytic process,
due to the presence of UV radiation, a simultaneous and continuous debacterization
is assured.
Research has been extended successfully to other substrates, different from
trichloroethene, such as methanol, acetone, ethyl acetate [8], thus showing
the almost general applicability of this technique.
Further research, of course, appears to be necessary, particularly to extend
the laboratory - scale, which has been practised in the cited works [6,7], to
other molecules of interest, in order to be able to scale-up the process at
a pilot plant - scale level.
2. References
[1] A. Mills, S. Le Hunte, J. Photochem. Photobiol., A:Chem., 108, 1-35 (1997).
[2] Y. Xu, C.H. Langford, J. Phys. Chem. B, 101, 3115-3121 (1997).
[3] Y. Xu, C.H. Langford, J. Phys. Chem., 99, 11501-11507 (1995).
[4] I.R. Bellobono, L. Righetto, in J.P. Fouassier, J.F. Rabek (Eds.), Radiation
Curing in Polymer Science and Technology, vol.4, ch.8, Elsevier Applied Science,
London, UK, 1993, pp. 151-177.
[5] I.R. Bellobono, in A. Caetano, M.N. De Pinho, E. Drioli, H. Muntau (Eds.),
Membrane Technology: Application to Industrial Wastewater Treatment, Kluwer
Academic Publishers, Dordrecht, NL, 1995, pp. 17-24.
[6] I.R. Bellobono, Life Chem. Repts., 13 (1995) 249.
[7] K.I. Suzuki, in D.F. Ollis, H. Al-Ekabi (Eds.), The First International
Conference on TiO2. Photocatalytic Purification and Treatment of Water and Air,
Elsevier, Amsterdam, NL, 1993, pp. 421-434.
[8] I.R. Bellobono, in A. Mantovani (Ed.), Inquinamento dell'Aria e Tecniche
di Riduzione, Isatec, Padova, I, 1994, pp. 476-485.