Photochemistry and photocatalysis

Photocatalysis is an important branch of catalysis and much more than that. To understand the potential applications and the working mechanisms of photocatalysis, it is necessary to know some important concepts of photochemistry, the branch of science that deals with the interaction of light and matter: (1) light excitation with a photon of suitable energy promotes a molecule or a semiconductor from the ground state to an electronically excited state that exhibits its own chemical and physical properties; (2) the most relevant consequence from the viewpoint of photocatalysis is that the excited state is both a better oxidant and a better reductant than the ground state; (3) some molecules or semiconductors can serve as photosensitizers, i.e., they can absorb light and then make available the excited state energy to promote reactions of non-absorbing species. Photosensitization and photocatalysis play an important role in nature and technology and they may take place in homogeneous or heterogeneous phase. Such processes can use sunlight (1) to convert solar energy into chemical or electrical energy, (2) to perform organic synthesis that cannot be achieved by thermal activation, and (3) to remedy pollution. Water splitting using sunlight and suitable photosensitizers and catalysts (artificial photosynthesis) is perhaps one of the most thoroughly investigated chemical processes. Breakthrough in this area can contribute to solve the energy and climate crisis, but substantial technological development is still needed.