4.1.2. Non-Noble Metal Catalysts

To address the cost of noble metals, non-noble metal oxides catalysts were developed for the abatement of VOCs. The materials which have been studied as non-noble metals include the derivatives of transition metals and rare earth elements, such as Ti, Cu, Mn, Al, Ce, Co, Fe, Cr, and V (Table 6) [1,126–129]. Although transition metal oxides catalysts generally showed a lower catalytic activity than noble metal catalysts for the oxidation of VOCs, they have many advantages, such as a resistance to chlorine and sulfur poisoning, tunable material properties, low cost, long on-stream lifetime, easy regeneration, and low environmental impact. The non-noble metal oxides catalysts applied and studied in VOCs abatement include CuOx, MnO2, FeOx, NiOx, CrOx, and CoOx.

The non-noble metal oxides catalyst systems which we will discuss include ones in which the metal oxide is both supported or unsupported. Due to the presence of mobile oxide species in lattice, Co2O<sup>3</sup> displays excellent reduction and oxidation abilities. As such, studies have shown Co2O<sup>3</sup> to be one of the best catalysts used for the combustion of benzene, toluene, propane, 1,2-dichloroethane, and 1,2-dichlorobenzene [130–133]. The catalytic activity of Co2O<sup>3</sup> is determined by the method of preparation, treatment conditions, and surface area.

MnO<sup>2</sup> is another commonly used metal oxide catalyst, which has been applied and studied in the abatement of n-hexane, acetone, benzene, ethanol, toluene, propane, trichloroethene, ethyl acetate, and NO<sup>x</sup> [133–139]. The catalytic activity can be tuned by the preparation method and depends on the structure, surface area, support materials, and oxidation states of catalysts. In the catalytic combustion of ethyl acetate and hexane, MnO<sup>2</sup> even achieved a better activity than Pt/TiO<sup>2</sup> [140]. Yonghui Wei et al. removed the La atoms from the LaMnO<sup>3</sup> perovskite to prepare MnO<sup>2</sup> with a high surface area (>150 m<sup>2</sup> /g), upon which it showed excellent catalytic activity in the oxidation of toluene [96]. Zhang Kai et al. synthesized the nano-cubic MnO<sup>x</sup> which has a large specific surface area, many oxygen vacancies, and good low temperature reducibility. The conversion of toluene via combustion was more than 90% at 350 ◦C [95]. Xueqin Yang et al. found that the acid treatment did not change the morphology of the catalyst, but could improve the oxidation ability of the catalyst by increasing the number of Mn4<sup>+</sup> species and structural defects on the surface of the catalyst [69]. A common theme throughout the implementation of Mn-based metal oxide catalysts for VOC oxidation is the availability and mobility of oxygen within the MnOx, which is attributed to the oxidation and reduction ability of the Mn afforded by the multiple oxidation states in which it can exist.

Copper oxides are another kind of efficient catalysts used in total oxidation of methane, methanol, ethanol, and acetaldehyde [141,142]. The main factors which influence the catalytic activity are the Cu oxidation state and the availability of lattice oxygen. The addition of other metal oxides, such as CeO2, can enhance the catalytic ability noticeably [133].

Chromium oxides are also promising oxidation catalysts, especially for the combustion of halogenated VOCs [124,133,143]. For chromium oxides, highly crystalline samples showed a better catalytic activity than amorphous ones [144]. Rotter et al.'s research showed that, when using TiO<sup>2</sup> as a support material, chromium oxides achieved higher catalytic oxidation of trichloroethylene than manganese oxide, cobalt oxide, and iron oxide [82]. Chromium has also been successfully supported on silica, alumina, porous carbon, and clay to eliminate pollutants such as carbon tetrachloride, chloromethane, trichloroethylene, ethyl chloride, chlorobenzene, and perchloroethylene [142]. However, chromium oxides also suffer deactivation due to the reaction between chromium and chlorine to form Cr2Cl<sup>2</sup> [145,146].

CeO<sup>2</sup> is a widely used catalyst in oxidation reactions due to its strong interactions with other metals, high oxygen storage capacity, and ready shuttling between the Ce3<sup>+</sup> and Ce4<sup>+</sup> states [147–149]. Dai et al. compared the removal of chlorinated alkanes and alkylenes over CeO<sup>2</sup> [83]. The results showed that CeO<sup>2</sup> is more efficient when it comes to oxidizing chlorinated alkanes than chlorinated alkylenes. CeO<sup>2</sup> also faced the deactivation problem due to the absorption of Cl<sup>2</sup> and HCl on the surface [150], so the design of chlorine resistance metal oxide catalysts is still a challenge which must be overcome.

Vanadium oxides were also developed to decompose chlorinated VOCs, such as polychlorinated pollutants and dichlorobenzene due to its tolerance of chlorine and sulfur compounds [151]. The presence of water can enhance and suppress the catalytic activity of V2O<sup>5</sup> via the removal of surface absorbed chlorine and reduction of active sites, respectively [152]. Other non-noble metal oxides were also investigated for abatement of VOCs, such as NiO and FeOx, which require further improvement of catalytic efficiency [153].

The above discussion suggests that the use of a single metal oxide as VOC oxidation catalysts is too often plagued by either a low catalytic activity or catalyst poisoning. Thus, focus has shifted to the development of mixed metal oxide catalysts such as Mn-Ce, Mn-Cu, Co-Ce, Sn-Ce, Mn-Co, and Ce-Cu oxides [119,154–157]. The logic here is that combining two metal oxides with different materials and catalytic properties allows for a synergistic enhancement in performance. The previous studies showed that the rate determining step of VOC catalytic combustion was the oxygen removal from the catalysts lattice [119], so the goal of mix-metal oxides catalysts design was the enhancement of the lattice oxygen species availability.

The addition of copper into CeO<sup>2</sup> can promote the catalytic efficiency due to a synergistic effect, leading to a highly efficient decomposition of ethyl acetate, ethanol, propane, benzene, and toluene [158–160]. MnOx-CeO<sup>2</sup> has been applied for the destruction of ethanol, formaldehyde, hexane, phenol, ethyl acetate, and toluene [155,161–164]. Mn-Co oxides catalysts also showed improved catalytic activity relative to either MnO<sup>x</sup> or Co2O<sup>3</sup> in the combustion of ethyl acetate and n-hexane [100]. CeO2-CrO<sup>x</sup> showed excellent catalytic activity for the decomposition of chlorinated VOCs [165], while the removal of chlorobenzene over MnOx-TiO<sup>2</sup> and MnOx-TiO2-SnO<sup>x</sup> showed much better catalytic performance than not only the individual oxides, but also achieved removal efficiencies on par with noble metal catalysts [166]. A three-dimensional ordered mesoporous material of mixed cerium-manganese oxide was prepared for the efficient catalytic combustion of chlorine-containing VOCs due to its large specific surface, enriched Ce3<sup>+</sup> content, oxygen vacancies, active oxygen species, and acidic sites. It showed good water resistance and high airspeed applicability. However, catalyst deactivation caused by inorganic chlorine adsorption still occurred [90]. Layered copper manganese oxide has been prepared for the catalytic combustion of CO and VOCs, which showed efficient activity due to the interfacial structure of mixed phases and the formation of the Cu2+-O2−-Mn4<sup>+</sup> entity [58]. Acidic sites can be provided by support to prevent the decrease of catalytic activity. CeO2@SiO<sup>2</sup> was prepared to catalyze the combustion of 1,2-dichloroethane. SiO<sup>2</sup> can provide weak acid sites, as well as promote the adsorption and activation of 1,2-dichloroethane and the desorption of generated HCl [85].

A vast number of preparation methods exist for the synthesis of mixed metal oxide catalysts, including thermal decomposition, impregnation, co-precipitation, and the sol-gel method [167–169]. The selection of preparation methods depends on the properties of catalysts and the application situation. Furthermore, as discussed above, complex ordered microporous, multilayer or core-shell structures have recently been applied to catalytic oxidation processes to access properties which come from having a highly controlled particle composition and morphology. The chance to alter not only the metal centers present within mixed metal oxide systems, but also the relative metal ratios.


**Table 6.** Non-noble metal catalysts for catalytic combustion of VOCs.
