A substance can be thought to be a catalyst when it accelerates a chemical reaction
without being consumed as a reactant; that is to say, it appears in the rate expression
describing a thermal reaction without appearing in the stoichiometric equation [49].
A catalyst is a compound that lowers the free activation enthalpy of the reaction.
Then, photocatalysis can be defined as the acceleration of a photoreaction by the
presence of a catalyst [38, pp. 362–375]. This definition, as pointed out in [29,
pp. 1–8], includes photosensitization, a process by which a photochemical alteration
occurs in one molecular entity as a result of initial absorption of radiation by
another molecular entity called the photosensitizer [13], but it excludes the photoacceleration
of a stoichiometric thermal reaction irrespective of whether it occurs
in homogeneous solution or at the surface of an illuminated electrode. Otherwise,
any photoreaction would be catalytic [29, pp. 1–8]. Depending on the specific photoreaction,
the catalyst may accelerate the photoreaction by interaction with the
substrate in its ground or excited state and/or with a primary photoproduct.
When the light is adsorbed by the catalyst C, the system represents a sensitised
photoreaction which may occur through two different ways: