Photoelectrochemical reactions adsorption control

Surface effects and adsorption equilibria thus will significantly influence the course of photoelectrochemical transformations since they will effectively control the movement of reagents from the electrolyte to the photoactivated surface as well as the desorption of products (avoiding overreaction or complete mineralization). The stability and accessibility toward intermolecular reaction of photogenerated intermediates will also be controlled by the photocatalyst surface. Since diffusion and mass transfer to and from the photocatalyst surface will also depend on the solvent and catalyst pretreatment, detailed quantitative descriptions will be difficult to transfer from one experiment to another, although qualitative principles governing these events can be easily recognized. 

In principle, like all electrochemical reactions initiated by the transfer of an electron across an electrode-electrolyte interface, photoelectrochemical transformations offer the possibility of more precise control than can be attained with reactions that take place in homogeneous solution [62, 63]. This better selectivity derives from three features associated with reactions that take place on surfaces, and hence with the photoelectrochemical event the applied potential (allowing for specific activation of a functional group whose oxidation potential is higher, even in a multifunctional molecule) the chemical nature of the electrode surface (and hence of the adsorption equilibrium constant of a specific molecule present in the double layer) and, finally, control of current flow (and hence a constraint on the number of electrons passed to an adsorbed reactant). 

Most reactions on surfaces are complicated by variations in mass transfer and adsorption equilibrium [70], It is precisely these complexities, however, that afford an additional means of control in electrochemical or photoelectrochemical transformations. Not only does the surface assemble a nonstatistical distribution of reagents compared with the solution composition, but it also generally influences both the rates and course of chemical reactions [71-73]. These effects are particularly evident with photoactivated surfaces the intrinsic lifetimes of both excited states and photogenerated transients and the rates of bimolecular diffusion are particularly sensitive to the special environment afforded by a solid surface. Consequently, the understanding of surface effects is very important for applications that depend on chemical selectivity in photoelectrochemical transformation. 

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