Aromatic, olefinic, and acetylenic hydrocarbons, but especially
saturated hydrocarbons belong to persistent pollutants difficult
to eliminate from the troposphere. The only exceptions are some
aromatic compounds that undergo direct photodegradation in
result of solar irradiation. Alkanes are undoubtedly much less
reactive than other organic compounds including unsaturated
hydrocarbons, surely because they are more completely
saturated and their activation involves cleavage of the relatively
strong C H bond (e¼415 kJ).
The CH bond can be activated by a metal complex, particu-
larly when the complex plays the role of catalyst or
photocatalyst. The reactions of hydrocarbons with metal
complexes occur at low temperatures and can be selective. There
are different pathways for C H bond activation: (i) by low-
valence metal complexes, (ii) by high-valent metal-oxo com-
pounds, (iii) by molecular oxygen and oxygen atom donors, (iv)
by biological oxidation, or (v) by photocatalytic enhancement
( 21 ).
The cleavage of the C H bond by direct participation of a
transition metal ion proceeds via an oxidative addition mecha-
nism or an electrophilic substitution mechanism. Metals in low
oxidation states undergo oxidative addition while high oxidation
state metals take part in electrophilic substitutions. Another
function of the metal complex in these reactions consists of
abstracting an electron or a hydrogen atom from the hydrocar-
bon, RH. The RHþradical ions or Rradicals which are formed
then interact with other species, such as molecular oxygen which
is present in the solution or in one of the ligands of the metal
complex ( 21 ).
To the main metal-containing systems which are capable to
react with hydrocarbons and other C H compounds belong also
those producing the OHradicals, as Fenton and photo-Fenton
reagents (see Section V.A.1). Other strong enough oxidizers
may be generated as result of sunlight excitation of some transi-
tion metal complexes. For example, cyclohexene (C 6 H 10 ) was
reported to undergo photochemical oxygenation by Ru
(II)–porphyrin complexes chemically activated upon visible light
irradiation ( 22 ).
Some diverse VOCs (halocarbons, isoprene (CH 2 C(CH 3 )
CH 2 CH 2 ), monoterpenes, ethanol, and methyltert-butyl ether,
(CH 3 ) 3 COCH 3 )) were found to be photooxidized efficiently on
solid aerosols. Solid photocatalyst particles, such as TiO 2 , ZnO,
and Fe 2 O 3 , were here of special importance, but the VOC oxida-
tion was photoassisted also by dessert sand, volcanic ash, or even
by chalk particles( 23 – 25 ). Similarly, sulfur dioxide was found to
METAL COMPLEXES AS SOLAR PHOTOCATALYSTS 301