4.1.5. The Influence Factors on Catalytic Performance

In the real industrial process, a lot of factors influence the catalytic efficiency. Firstly, the kind of VOCs determines the selection of catalysts. For example, noble metal catalysts show the best VOCs eliminating efficiency, but they are not suitable for the destruction of chlorine and sulfur containing VOCs due to the poison of the catalysts. Secondly, the surface area of the catalysts is the main factor that influences the catalytic activity. Research shows that MnO<sup>2</sup> with a higher surface area showed much better catalytic performance than the one with a lower surface area [133]. Thirdly, the crystal type of catalysts also influences the catalytic performance of the catalysts with the same content. For instance, the catalytic performance of TiO<sup>2</sup> with different crystal types, namely rutile and anatase, showed a different catalytic activity [121,167]. The humidity is the common content in the industrial waste gas. In most of the reports, water molecules can suppress the catalytic activity due to the complete adsorption on active sites and destruction of catalysts [177]. In other researches, humidity plays a positive role in the oxidation. In the catalytic combustion of chlorobenzene over VOx/TiO2, VOx-WOx/TiO2, and VOx-MoOx/TiO2, water can remove the adsorbed Cl<sup>−</sup> from the catalysts surface and react with chlorine to produce HCl [134]. The water vapor can show a different effect on the same VOCs over different catalysts. Kullavanijayam et al. reported that water enhanced the catalytic oxidation of cyclohexene over the ceria–alumina supported Pt and Rh, but it had a negative effect on the oxidation of cyclohexane over the ceria–alumina supported Pd catalyst [178]. CO is another poison for precious metal catalysts, so during the catalytic combustion, enough air is needed to avoid the generation of CO. The life of catalysts is also important for catalytic combustion. The main obstacle for the development of catalysts for the catalytic combustion of halogen and sulfur containing VOCs is the short life of common catalysts. Although V oxides show less catalytic efficiency than other catalysts, it is still used in the elimination of halogen and sulfur containing VOCs due to its long catalytic life [152]. Since after the installation of catalytic combustion equipment, it will be in operation for a long time, therefore, the life of the catalysts determines the cost of catalytic combustion.

The catalytic combustion process has been well developed in recent years. The diversity of the catalysts has been investigated. However, there are still some obstacles on the way to the industrial application, such as short lifetime, high cost, and no universality to different kinds of VOCs. More work should be done on these problems.
