The photochemical reactor is a processing technology that has only emerged in the past 20 years. With sufficient reaction time, it can usually mineralize organic matter into simple inorganic substances such as CO2 and H2O, avoiding secondary pollution. The photochemical reactor is simple and has promising development prospects. Due to the problem of catalyst separation and recovery in the photocatalytic oxidation method using titanium dioxide powder as a catalyst, the practical application of this technology has been affected. Therefore, the technology of fixing photochemical reactors on certain carriers to avoid or make it easier to separate and recover has aroused widespread interest among scholars at home and abroad.

There are many differences between photochemical reactions and general thermochemical reactions, mainly manifested in: when heating activates molecules, the distribution of molecular energy in the system follows a Boltzmann distribution; When molecules are activated by light, selective excitation can be achieved in principle, and the distribution of molecular energy in the system belongs to non-equilibrium distribution. So the pathway and products of photochemical reactions are often different from those of ground state thermochemical reactions. As long as the wavelength of light is appropriate and can be absorbed by the substance, photochemical reactions can still proceed even at very low temperatures.

The primary process of photochemistry is the absorption of photons by molecules to excite electrons, and the molecules are elevated from the ground state to the excited state. The electronic, vibrational, and rotational states in molecules are quantized, meaning that the energy changes between adjacent states are discontinuous. Therefore, when the initial state and termination state of molecular excitation are different, the required photon energy is also different, and the energy values of the two are required to match as much as possible.

Due to the fact that molecules are in a stable state with low energy under general conditions, it is called the ground state. After being exposed to light, if molecules can absorb electromagnetic radiation, they can be elevated to a higher energy state, called an excited state. If molecules can absorb electromagnetic radiation of different wavelengths, they can achieve different excited states. According to their energy levels, they are referred to as the excited state, the second excited state, and so on, from the ground state upwards; And all excited states above the excited state are collectively referred to as highly excited states.

The lifetime of excited state molecules is generally short, and the higher the excited state, the shorter their lifetime, so there is no time for chemical reactions to occur. Therefore, photochemistry is mainly related to low excited states. The electromagnetic radiation absorbed by molecules during excitation has two main dissipation pathways: one is the merging with the thermal effect of photochemical reactions; The second is to convert it into other forms of energy through photophysical processes.

The photophysical processes can be divided into radiative relaxation processes and non radiative relaxation processes. Radiation relaxation process refers to the process of dissipating all or part of excess energy in the form of radiation energy, causing molecules to return to their ground state, such as emitting fluorescence or phosphorescence; The non radiative relaxation process refers to the process in which excess energy is dissipated in the form of heat and molecules return to their ground state.

Determining the true pathway of a photochemical reaction often requires establishing several hypothetical models corresponding to different mechanisms, identifying the kinetic equations between each model system and concentration, light intensity, and other relevant parameters, and then examining which one has the highest degree of agreement with experimental results to determine which one is the most likely reaction pathway.

The commonly used experimental methods for studying reaction mechanisms in photochemistry, in addition to the tracer atom labeling method, the quenching method, which was first adopted in photochemistry, is still a very effective method. This method studies the mechanism of photochemical reactions by measuring the kinetics of fluorescence emitted by excited molecules and quenching by other molecules. It can be used to determine the acidity of molecules in an electronically excited state, the reaction rate of molecular dimerization, and the long-range energy transfer rate.