4.1.1. Noble Metal Catalysts

The general consensus of previous studies is that noble metal catalysts show the best catalytic performance in the combustion of non-halogenated VOCs. The noble catalysts which have been investigated include platinum, palladium, ruthenium, iridium, gold, and silver (Table 5) [2,41–44]. Due to their size-dependence catalytic properties and high price, noble metal catalysts are often supported on porous supports, such as γ-Al2O3, SiO2, zeolite, and other non-metal oxides to increase the dispersion of noble metal nanoparticles and surface area, which can improve the catalytic efficiency of noble metal catalysts [106–111]. Catalysts are often supported on the substrate, which is in the form of a monolith or honeycomb material, such as cordierite, aluminum, and stainless steel [112]. Noble catalysts display high efficiencies for VOC removal at a lower temperature than other kinds of catalysts. Pt shows the best catalytic combustion of VOCs, exhibiting equal removal efficiencies at operating temperatures of up to 100 K lower than those used for other noble metals [42,44]. Two key factors were found to influence the catalytic performance of alumina supported Pt, dispersion and loading amount, with an increase in either of these properties associated with an obvious improvement of catalytic activity [106,113,114]. The metal particle size is also an important factor that influences the catalytic activity. Changes to the active metal particle size can also enhance catalytic performance. For instance, when the crystalline size of alumina supported Pt increased from 1.0 to 15.5 nm, the oxidation rate was found to increase by a factor of 10 [115]. The same phenomenon was also found in the oxidation of propylene over alumina supported Pt and Pd catalysts.

An important factor in the performance of noble metal catalysts for VOC combustion is specificity; in other words, the target VOCs molecules. For instance, P. Papaefthimiou et al. found that Pt and Pd supported on alumina showed good performance for the oxidation of benzene and butanol, but not ethyl acetate, with Pd generally outperforming Pt [57]. M. J. Patterson et al. discovered that alumina supported rhodium is the most active noble metal for 1-hexene, but not for aromatics, while benzene can be decomposed the most easily on platinum, and palladium showed the best catalytic performance in abatement of toluene [116]. The impact of certain non-VOC species in the waste stream can also impact performance, where these species can potentially 'poison' the catalyst or cause side reactions to occur. One example is the presence of CO, which was found to have little effect on the performance of Pd catalysts, but significantly inhibit the activity over Pt [117].


Transition metal oxides can be utilized as both supports and promoters for noble metal catalysts. Pt, Pd, Ru, Au supported on MgO, SnO2, Co3O4, NiO, TiO2, CeO2, La2O3, ZrO<sup>2</sup> or PrO<sup>2</sup> have been explored for the oxidation of toluene, benzene, xylene, propene, light alkane, ethanol, propanol, butanol, formaldehyde, acetone, and acetic acid, respectively [42]. The role of the transition metal oxides is not only to supply a large surface area to disperse the noble metal particles, but in some cases, it can also improve the catalytic performance of noble metal particles by enhancing the mobility of lattice oxygen species. Previous studies showed that active oxygen species formed on cobalt oxide spinel-type crystallites can enhance the catalytic oxidation over PdO supported on alumina [118,119].

The reduction properties also influence the oxidation ability of supported noble metal catalysts. T. Mitsui et al. prepared SnO2, CeO2, and ZrO<sup>2</sup> supported Pt and Pd catalysts for the abatement of acetaldehyde [120]. These prepared catalysts were treated in an H2/N<sup>2</sup> flow and calcined in the atmosphere. The results showed that the SnO<sup>2</sup> supported Pt and Pd showed the best catalytic performance among the calcined catalysts in the atmosphere, while after treatment in an H2/N<sup>2</sup> flow, the catalytic activity of SnO<sup>2</sup> supported Pt and Pd decreased due to the formation of inactive inter-metallic phases (PtSn and Pd3Sn2). In contrast, CeO<sup>2</sup> and ZrO<sup>2</sup> supported catalysts showed the improved catalytic activity after reduction. In the elimination of formaldehyde over a TiO2-supported catalyst, Pt/TiO<sup>2</sup> showed a superior catalytic performance to Rh/TiO2, Pd/TiO2, Au/TiO2, and neat TiO<sup>2</sup> [121]. Other research showed that a series of supported Pt, Pd, and Au catalysts can even partially eliminate formaldehyde at room temperature [122].

Recently, the use of MnO<sup>x</sup> based materials as catalyst supports has gained attention. In one instance, supporting Ag on NiO-doped MnO<sup>2</sup> showed a high activity towards the combustion of o-xylene [89]. The improvement in xylene oxidation was attributed to the enhanced oxygen activation and mobility afforded to the catalyst support via the addition of NiO and Ag. Wenbo Pei et al. explored the use of ordered, mesoporous Mn2O<sup>3</sup> supports with embedded Pt particles for the catalytic combustion of toluene [66]. They found that the strong interaction between Pt and Mn2O<sup>3</sup> in the ordered structure improves the activity and stability of the catalyst.

The single noble metal catalysts cannot satisfy the requirements of VOCs combustion. Therefore, some mixed noble metal catalysts have been developed to combine the advantages of different noble metal catalysts, such as Pt-Au, Cu-Au, and Pd-Au. T. Tabakova et al. found that the Pd deposition on the deposited gold showed the best catalytic performance for benzene combustion, which was totally eliminated at 200 ◦C. It also showed good stability [57]. M. Hosseini et al. showed that the deposition of palladium on aurum supported on TiO<sup>2</sup> (Pd(shell)-Au(core)/TiO2) can significantly improve the catalytic activity for oxidation of toluene and propylene [67]. Der Shing Lee et al. deposited Au-Pd bimetallic nanoparticles on CeO<sup>2</sup> for toluene degradation, which showed a much better catalytic performance than Au/CeO<sup>2</sup> and Pd/CeO<sup>2</sup> catalysts due to the synergistic effect of gold and palladium [68]. The addition of non-noble metals also can improve the catalytic activity of noble metal catalysts. Roberto Fiorenza et al. prepared Au-Ag/CeO<sup>2</sup> and Au-Cu/CeO<sup>2</sup> bimetallic catalysts for alcohol oxidation and CO oxidation. These two catalysts showed higher selectivity for intermediate products higher CO conversion at a low temperature (100 ◦C) than Au/CeO<sup>2</sup> [93]. The addition of Au also can improve the performance of Al2O<sup>3</sup> supported Cu-Pt catalysts in DMDS oxidation [81].

Noble metal catalysts showed a high catalytic activity and remarkable thermal stability in catalytic elimination of VOCs. However, the use of noble metal catalysts is also associated with distinct disadvantages. Firstly, the high cost of noble metal limits their application in the industrial abatement of VOCs. Secondly, the presence of chlorine, sulfur, CO, and water can suppress the catalytic performance significantly [42,123–125]. The regeneration and recycling of noble metal catalysts poisoned by Cl and S is difficult, so they are not suited to the treatment chlorine and sulfur-containing VOCs. In fact, the release of chlorine and sulfur containing VOCs is particularly common in pharmaceutical production processes, so chlorine- and sulfur-containing VOCs need to be removed prior to the treatment by noble metal catalysts, which will further increase the cost of waste gas purification.
