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Review

A Review of Noble Metal Catalysts for Catalytic Removal of VOCs

by
Shaoqi Chu
1,2,
Enlai Wang
1,
Fushan Feng
1,
Changsheng Zhang
2,
Jie Jiang
2,
Qinqin Zhang
3,
Fang Wang
1,
Liancheng Bing
1,
Guangjian Wang
1 and
Dezhi Han
1,*
1
State Key Laboratory Base of Eco-Chemical Engineering, College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
2
SINOPEC Research Institute of Safety Engineering Co., Ltd., Qingdao 266000, China
3
Shandong Provincial Key Laboratory of Biochemical Engineering, College of Marine Science and Biological Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(12), 1543; https://doi.org/10.3390/catal12121543
Submission received: 28 October 2022 / Revised: 23 November 2022 / Accepted: 26 November 2022 / Published: 1 December 2022
(This article belongs to the Special Issue Synthesis and Application of Catalytic Materials in Energy and Environment)
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Abstract

:
Volatile organic compounds (VOCs) are important precursors for the formation of secondary pollutants, such as fine particulate matter (PM) and ozone (O3), which will lead to severe atmospheric environmental problems to restrict the sustainable development of the social economy. Catalytic oxidation is a safe, eco-friendly, and simple method for eliminating VOCs, which can be converted into CO2 and H2O without the generation of other harmful substances. The fabrication and development of catalysts are very crucial to enhance the catalytic oxidation efficiency of the removal of VOCs. The noble metal catalyst is one of the commonly used catalysts for the catalytic oxidation of VOCs because of the high reaction activity, good stability, poisoning-resistant ability, and easy regeneration. In this review, the research progress of noble metal (Pt, Pd, Au, Ag, and Ir) catalysts for the removal of VOCs in recent years was summarized with the discussion of the influence factors in the preparation process on the catalytic performance. The reaction mechanisms of the removal of VOCs over the corresponding noble metal catalysts were also briefly discussed.

Graphical Abstract

1. Introduction

With the fast development of the economy, society, and industry in recent years, environmental pollution has become an increasingly serious issue, such as the emission of particulate matter (PM), automobile/industrial exhaust gas, industrial sewage, etc. [1,2,3,4,5] Particularly, volatile organic compounds (VOCs) exhibit the characteristics of a pungent odor, toxicity, and carcinogenicity, which will result in a severe threat to the ecological system and human health (Table 1) [6,7]. As shown in Figure 1, the emission of VOCs from China’s petroleum industry increased by 24.08% from the year 2013 to 2019, which is much higher than that (4.07%) of the chemical industry [8,9]. Presently, there are some problems in the VOCs treatment technology, such as imperfect regulations and standards, inadequate environmental supervision, inefficient catalysts and processes, etc.
Generally, the effective methods to deal with VOCs are classified as membrane separation, adsorption, direct incineration, catalytic oxidation, etc. The membrane separation technology is one of the effective methods to remove low-concentration VOCs, which has been verified in the removal of formaldehyde (HCHO), benzene, n-hexane, and other VOCs [10]. However, the high cost, low stability, and low flux of separation membranes hinder its wide application in industry. Adsorption is also one of the common methods to remove VOCs, which may require the pretreatment of malodorants or the pre-removal of moisture. For example, Xiang et al. employed hickory wood to prepare biochar for adsorbing the VOCs [11]. The adsorption efficiency could be limited by the porosity of most traditional adsorbents. The direct incineration method is used to burn the VOCs at high temperatures (800–1400 °C), which is especially used to deal with the VOCs for poisoning the catalyst. The removal rate of VOCs in the modern incineration system can reach 95–99%, while the conversion of halogenated compounds may need additional control equipment [12,13,14].
Alternatively, the catalytic oxidation process (COP) can effectively treat VOCs at a low temperature with low emissions of toxic and harmful byproducts, in which the catalysts do not require a frequent regeneration [15]. The metal materials such as Pt, Pd, Ni, Cu, etc., were used as the catalysts, while air, oxygen, ozone, etc., were employed as the oxidants in the COP [16]. Except for the intensive application for the treatment of VOCs, the COP also is used to deal with oxygen-containing organic gas, sewage treatment, automobile exhaust, and so on [17,18]. One of the classic applications of the noble metal catalysts for the catalytic oxidation of VOCs is the three-way catalysts in the automotive industry, CO, NOx, and hydrocarbons, and other harmful substances are produced due to the insufficient combustion of fuel. Ternary catalysts usually contain Pt, Pd, Rh, and other noble metals to convert the emitted hydrocarbons into CO2 and H2O. Thus, the emitted vehicle exhaust becomes clean, and its negative impact on the environment and human health is reduced. Because engine emissions sometimes occur during a cold start, the improvement on the catalytic activity of noble metal catalysts at low temperatures has become a common concern of academia and industry [5,19,20,21]. Not only in the automobile industry but also in chemical and other industries, noble metal catalytic oxidation catalysts are also widely used.
The design, optimization, and preparation of catalysts are the key steps for enhancing their catalytic performance in COP. Catalytic oxidation catalysts are generally divided into noble metal catalysts and non-noble metal oxide catalysts, which can completely oxidize hydrocarbons to H2O and CO2 under relatively mild conditions [22]. Although the preparation cost of non-noble metal catalysts is cheap, their poisoning-resistant ability and stability are relatively low compared with those of noble metal catalysts [23,24]. The noble metal catalysts exhibited many advantages, such as a high activity at a low temperature, a good stability, and an easy regeneration and storage [22]. Moreover, some noble metal catalysts could be prepared under mild synthesis conditions via the simple synthesis methods, while the preparation cost of noble metal catalysts is relatively high. Therefore, the utilization efficiency of noble metal active components should be improved to reduce the preparation cost of noble metal catalysts.
Some previous reviews of the catalytic oxidation of VOCs summarized the removal mechanism of VOCs, the intrinsic properties of noble metal active sites and supports, catalyst deactivation mechanism, etc. [16,25,26]. The noble metal (Pt, Pd, Au, Ag, Ir, etc.) supported catalysts can achieve the complete oxidation of VOCs at lower temperatures [27,28,29,30]. Many factors can influence the catalytic oxidation performance of the obtained noble metal type catalyst, including the preparation methods, the physicochemical properties of the support and additives, and the size and chemical state of the noble metal [31,32,33,34,35]. In this review, the reaction mechanisms and the influence factors in the preparation process for the Pt-based and Pd-based catalysts are discussed firstly, and the other noble metal (Au, Ag, and Ir) catalysts are also briefly introduced.

2. Pt-Based Catalysts

2.1. Reaction Mechanism of VOCs over Pt-Based Catalysts

The investigation of the reaction mechanisms of VOCs is particularly important for the design of high-performance Pt-based catalysts [36,37,38]. However, it is still difficult to provide a universal and unique reaction mechanism due to the different properties of VOCs under various reaction conditions. For instance, in the catalytic oxidation of toluene over the Pt/CeO2 catalyst, the CeO2 support could facilitate the dispersion of Pt species and promote the adsorption and activation of oxygen at the active sites [39]. Thus, the toluene was catalytically oxidized through the Langmuir Hinshelwood (L-H) kinetic model as follows, toluene → benzyl alcohol → benzaldehyde → benzoic acid → CO2 and H2O. Wen et al. prepared the meso-Pt/CeO2 catalyst with an ordered mesoporous structure for the catalytic oxidation of toluene (Figure 2) [40]. The toluene molecules were firstly adsorbed on the surface metal active centers and oxygen vacancies of the prepared catalyst. Then, the surface-active oxygen interacted with the C–H bond of the methyl group to form the benzyl group, and subsequently produce benzyl alcohol. The benzyl alcohol was further oxidized by the remaining active oxygen to intermediate products, such as benzoic acid, benzoate, and maleic anhydride, which finally were oxidized into CO2 and H2O.
Wang et al. prepared the Pt/TiO2 catalyst by the impregnation method for the catalytic oxidation of toluene [41]. When the reaction temperature was low, the disproportionation reaction of adsorbed toluene occurred to produce benzene and p-xylene. Additionally, then o-xylene was formed by the isomerization of p-xylene. The oxygen-containing intermediates with the benzene ring, such as benzyl alcohol, benzaldehyde, and benzoic acid, may be formed by the sequential oxidation of chemically adsorbed toluene molecules by surface active oxygen. On the other hand, the oxidation of o-xylene is most likely to form phthalic acid. With the increase in the reaction temperature, the phthalic acid and benzoic acid can be further oxidized to maleic anhydride and itaconic anhydride, which were transformed to the small acetone and acetic acid by a ring-opening reaction, respectively. Finally, these small molecules are completely oxidized to CO2 and H2O (Figure 3).
Wang et al. studied the oxidation mechanism of a mixed VOCs system over the Pt/TiO2 catalyst [42], revealing that the oxidation of mixed components did not generate new intermediates compared with the oxidation of single-component toluene or acetone (Figure 4). However, the research on the reaction mechanism of a mixed VOCs system still needs to be further explored.
In addition, in the case of the catalytic oxidation of HCHO, room temperature catalytic decomposition is considered to be the main method to continuously and effectively eliminate indoor HCHO [43]. Ye et al. proposed the possible oxidation mechanism of HCHO over the prepared Pt/Co3O4-NiO catalyst (Figure 5) [43]. They believed that HCHO was first converted into the intermediate species dioxymethylene (DOM), which was converted into formate. The formate species was finally decomposed into CO2 and H2O. In this process, the oxidation of DOM to formate was faster than the decomposition of formate, thus the decomposition of the formate species became the rate control step of the complete oxidation reaction of HCHO.
Zhang et al. proposed almost the same oxidation path for HCHO over noble metal catalysts using TiO2 as the support [44]. Additionally, the activity of the Pt-based catalyst was higher than that of other metal (Rh, Pd, and Au) catalysts.

2.2. Influence of Preparation Method

The interaction between the active component and the support is one of the key factors affecting the activity of the prepared catalysts, which is mainly subjected to the catalyst preparation methods.
Kondratowicz et al. prepared two different catalysts (Pt-I/@Zr and Pt-G/@Zr) for the catalytic oxidation of toluene, acetone, n-hexane, and other VOCs by the traditional wet impregnation and ethylene glycol reduction deposition methods, respectively [45]. The second approach was based on an introduction of metallic Pt nanoparticles prepared in the polyol process. The catalyst (Pt-G/@Zr) prepared by the ethylene glycol reduction deposition method has a better catalytic performance than the Pt-I/@Zr catalyst. When the average conversion rate of VOCs reached 90%, the reaction temperature of Pt-G/@Zr was about 100 ℃ lower than that (279 °C) of Pt-I/@Zr. It was found that Pt nanoparticles were mainly distributed on the outer surface of ZrO2 to form large particles over the Pt-G/@Zr catalyst. However, in the preparation of the Pt-I/@Zr catalyst by the traditional wet impregnation process, the Pt4+ cations were impregnated into the pores of the ZrO2 support, covering part of the active sites in the pores of the carrier, resulting in the low utilization efficiency of the active sites, and thus the low reaction activity of the obtained catalyst (Figure 6). Therefore, it necessary to develop the efficient preparation methods to tune the dispersion and location of the Pt active sites of the obtained catalysts in the COP of VOCs.
An et al. prepared a series of Pt/Fe2O3 catalytic oxidation catalysts for the low-temperature oxidation of HCHO using the impregnation method, coprecipitation method, and colloidal deposition method, respectively [35]. Among these catalysts, the Pt/Fe2O3-CD catalyst prepared by colloidal deposition exhibited the highest activity for the oxidation of HCHO due to the high dispersion of the Pt species (Figure 7), which could weaken the Fe–O bond nearby, enhance the migration rate of the active oxygen, and promote the adsorption and activation of the oxygen to participate in the oxidation of HCHO. Pt colloidal solution can be obtained via the colloid deposition method by adding NaOH into the Pt precursor solution, followed by mixing with the carrier, filtering, washing, and a calcination to prepare the final catalyst.

2.3. Influence of Chemical Valence of Pt

The chemical valence of Pt is one of the important factors affecting the catalytic activity of Pt-based catalysts for COP of VOCs. An et al. prepared supported Pt catalysts by the impregnation method using three silica materials, namely gas phase SiO2 (f-SiO2), porous particle SiO2 (g-SiO2), and mesoporous SBA-15, for the catalytic oxidation of HCHO [46]. It is found that the Pt/f-SiO2 catalyst exhibited the highest activity with a 100% conversion of HCHO at room temperature, which was much lower than that of 50 and 110 °C over the Pt/SBA-15 and Pt/g-SiO2, respectively. This is due to the high proportion of Pt0 in the Pt/f-SiO2 catalyst according to the analysis of the XPS results (Table 2). The appropriate proportion of Pt0/Pt2+ is the key factor to improve the catalytic ability of the prepared catalyst for activating oxygen molecules at a low temperature. A similar result was also reported by Kim et al. in the study of the effect of a H2 high-temperature (300 °C) reduction in the catalytic performance of the Pt/TiO2 catalyst for the oxidation of gaseous benzene [47]. The analysis of the XPS results indicated that the content of the Pt0 particles in the Pt/TiO2-R catalyst with a H2 reduction was about 2.8 times higher than that of the Pt/TiO2 catalyst without a H2 reduction (Figure 8). Correspondingly, the Pt/TiO2-R catalyst possessed the high conversion (90%) of gaseous benzene at 167 °C.

2.4. Influence of the Support Type

For Pt-based catalysts, the catalyst activity is closely related to the types of support. Peng et al. prepared Pt-based catalysts by using CeO2 nanorods (CeO2-r), CeO2 nanoparticles (CeO2-p), and CeO2 nanocubes (CeO2-c), respectively [48]. When the toluene conversion was 90%, the reaction temperature of CeO2-r was only 150 °C, which was much lower than that of 175 °C and 190 °C of CeO2-p and CeO2-c, respectively.
Avila et al. prepared Pt-based catalysts with CeO2, TiO2, and Al2O3 as the support, respectively, for the catalytic oxidation of propylene and propane [49]. The order of activity over the three catalysts in the propane oxidation is Pt/TiO2 > Pt/CeO2 > Pt/Al2O3. It was found that the propane oxidation occurred not only at the Pt0 site but also at the Pt0-Ce3+ site, which provides a new way for the propane oxidation reaction. In addition, the propane adsorption capacity of Pt/TiO2 is 5.5 times that of Pt/CeO2, which is the reason for the high catalytic activity of the Pt/TiO2 catalyst. In the case of a propylene oxidation, the reaction order of propylene is negative, while the reaction order of oxygen is positive, proving that propylene is easier to form an adsorption state than oxygen in the propylene oxidation reaction. The Pt/CeO2 catalyst with the best propylene oxidation performance could provide more oxygen vacancies to accelerate the reaction rate.

2.5. Influence of Pt Particle Size

The conversion of some VOCs is sensitive to the Pt particle size of the prepared Pt-based catalysts [33,34,50].
Zeolite is considered to be one of the excellent supports for loading active components due to its large specific surface area and unique pore structure [51,52,53,54]. Khawaja et al. prepared the Pt/NaY catalysts using modified Y zeolite through three steps: dealumination, desiliconization, and an acid treatment [55]. The characterization results demonstrated that the Pt-NaYmod catalyst with a mesoporous structure exhibited a higher activity in the catalytic oxidation of toluene than the Pt-NaY catalyst with only a pure microporous structure, which is due to the reduced Pt particle size in the NaYmod catalyst (Figure 9). Moreover, the existence of mesopores in the prepared NaYmod catalyst also had the positive effect on the reduction in the catalyst carbon’s deposition and the improvement in the catalyst’s stability.
Wang et al. employed different zeolites (ZSM-5, S-1, Beta, and USY) as supports to prepare Pt-based catalysts with a micro-/meso-porous composite structure through an organic alkali TPAOH treatment [32]. The results showed that the Pt/ZSM-5 catalyst with a high dispersion and small particle size (ds = 3.5 nm) had a low T90% of 130, 150, 210, and 180 °C for the conversion of benzene, n-hexane, acetonitrile, and ethyl acetate, respectively (Figure 10).
Huang et al. synthesized a series of Pt/TiO2 catalysts with different Pt particle sizes [56]. It was found that the obtained catalyst with a Pt particle size of about 10 nm exhibited a superior catalytic performance in the oxidation of HCHO (Figure 11).
The reaction conditions and activities of some Pt-based catalysts for the removal of VOCs were listed in Table 3.

3. Pd-Based Catalysts

3.1. Reaction Mechanism of VOCs over Pd-Based Catalysts

In the reaction mechanism of the Pd-based catalyst, Wei et al. studied the catalytic oxidation of benzene on Pd particles in detail [57]. It was found that C6H6 was mainly dehydrogenated to C6H through dehydrogenation oxidation (Figure 12a–f), and then was oxidized to C6HO (Figure 12g), which was finally decomposed into smaller molecules (Figure 12h). In this process, the steps from C6H6 to C6H3 were the rate-controlling steps, and the C6H3O and C6H2O may be the important intermediates.
Aguirre et al. believed that the catalytic oxidation of toluene over a Pd-based catalyst was similar to the Pt-based catalysts [58]. The toluene was dehydrogenated on the surface of the Pd/CeO2 catalyst to form a benzyl species, and then gradually oxidized to benzyl alcohol (Figure 13). Subsequently, benzyl alcohol was further oxidized to the aldehyde species, then the formate species, which would generate the final CO2 product. The conversion of the formate species to CO2 was regarded as the rate control step of the whole reaction process. Additionally, the benzoic acid intermediates can also be oxidized to anhydride species (maleic anhydride or succinic anhydride), which accumulated on the catalyst surface to form CO2 with a slow reaction rate.

3.2. Influence of Acidity of the Supports

The acidity of the support would significantly influence the catalytic performance of the Pd-based catalysts in COP of VOCs. Compared with the neutral or basic support, Pd nanoparticles are easily oxidized on the acidic support because of their electrophilic properties to form electron-deficient Pd atoms [59,60].
Wu et al. believed that the high content of the medium-strength acidity of the support was one of the reasons for the high activity of the Pd/Silicalite-1-H catalyst in the oxidation of toluene [61]. Li et al. prepared Pd-based catalysts by using γ-Al2O3, ZSM-5, and SiO2 for the oxidation of 1,2-dichlorobenzene [62]. The order of the activity of these catalysts was as follows: Pd/ZSM-5(25) > Pd/γ-Al2O3 > Pd/ZSM-5(200) > Pd/SiO2. The Lewis (L) acid sites facilitated the adsorption of chlorinated organic molecules on the catalyst surface [63], while the Brønsted (B) acid could promote the breaking of C–C bonds and accelerate the oxidation reaction [64,65], which was responsible for the superior activity over the Pd/ZSM-5 (25) catalyst. (Figure 14 and Table 4).
Jabłońska et al. prepared a series of Pd-based catalysts by using HY, NaY, and γ-Al2O3 as the support for the catalytic oxidation of methanol, demonstrating that the HY with an appropriate acidity could dramatically enhance the dispersion Pd species to promote the catalytic oxidation performance of the corresponding catalyst [66]. He et al. also believed that the acidity of the catalyst could promote the dispersion and oxidation of Pd particles, thus leading to the high activity of Pd/SBA-15 for the toluene oxidation reaction [67]. Deng et al. pretreated the ceramic and glass fiber with water, sulfuric acid, and nitric acid to fabricate the Pd-based catalysts for the catalytic oxidation of benzene [68]. The ceramic fiber with a relatively strong acidity facilitated the dispersion of the Pd particles, thus enhancing the efficiency of the benzene conversion.
However, Yue et al. found that the strong acid sites of the Pd/ZSM-5 catalyst would lead to the generation of more by-products during the catalytic oxidation of butyl acetate [69]. Additionally, the strong acidity may also lead to the carbon deposition for deactivating the catalyst, especially in the catalytic oxidation reaction of carbon-containing VOCs. Therefore, the regulation of the support acidity is critical for tuning the activity and stability of the catalysts in the removal of VOCs.

3.3. Influence of Support Type

Perovskite oxides are widely used as the catalyst support in the catalytic oxidation of VOCs due to their superior thermal stability and high oxygen migration rate. Yi et al. prepared the Pd-CeMnO3 perovskite catalysts by the sol-gel method and impregnation method [70]. The high efficiency of benzene complete oxidation at 200 °C was achieved due to the high dispersion of Pd particles on the perovskite surface, and the enhanced interaction between Ce ions and Pd to accelerate the electron transfer for promoting the production of active oxygen.
He et al. prepared 3D@2D constructed Al2O3@CoMn2O4 microspheres to load the Pd nanoparticles for the catalytic oxidation of VOCs [71]. The obtained catalysts with the hollow hierarchical structure possessed the large specific surface area, more accessible active sites, and promoted the catalytic activity, especially the 1 wt% Pd/h-Al@4CoMn2O4 catalyst (the molar rate of Al and CoMn2O4: 4:1) with the best catalytic activity and high stability for the catalytic oxidation of VOCs (Figure 15).
Moreover, the metal organic framework (MOF) is considered to be an excellent support to fabricate the Pd-based catalysts for the catalytic oxidation of VOCs [72]. Li et al. prepared the Pd/MOF catalysts by impregnating metal precursors in pre-fabricated MOFs (Figure 16) [73]. The obtained metal multifunctional organic compounds integrate a unique multifunctional organic structure to provide adsorption sites for VOCs, and highly dispersed metal nanoparticles provide a large number of catalytic active sites to efficiently catalyze the oxidation of VOCs.
In recent years, single transition metal oxides, rare earth oxides, and mixed metal oxides have been employed as a catalyst support in the conversion of VOCs. The catalysts prepared with rare earth oxides as the support exhibited an excellent activity in the application of environmental catalysis [74]. For instance, the CeO2 with abundant oxygen vacancies, a strong interaction with metals, and variable exposed crystal planes is one of the important catalyst supports in heterogeneous catalytic oxidation [75]. Gil et al. fabricated the Pd catalysts supported on CeO2, TiO2, and Al2O3, respectively, for the catalytic oxidation of propene [76]. It was found that the Pd metal had a stronger interaction with CeO2 than other oxides, which was responsible for the enhanced catalytic oxidation activity.

3.4. Influence of Promoter

The catalyst promoter could generate the synergistic effect with the active component, change the active component–support interaction, and tune the acid property of the support for improving the catalyst activity in the catalytic removal of VOCs.
Zhao et al. prepared the Mn-modified Pd/TiO2 catalyst by the simple impregnation method for the acetone oxidation reaction [77]. The high content of Pd2+, abundant adsorbed oxygen, and low-temperature reducibility of Pd0.01Mn0.2/TiO2 catalysts with a large particle size (d = 2.4 nm) could be responsible for the enhanced catalytic performance (Figure 17).
Similarly, Taylor et al. improved the activity of the Pd/TiO2 catalyst by adding Nb with different loading amounts in the impregnation step of a catalyst preparation [78]. The propane oxidation activity of the prepared catalysts increased with the increase in the Nb loading amount due to the enhanced migration rate of oxygen. Gil et al. modified the Pd/Al2O3 catalyst by adding Fe, Ce, Ti, Mn, etc., for the catalytic oxidation of propylene [76]. The results showed that the addition of Ce and Fe improved the catalyst activity of the corresponding catalysts. Aznárez et al. also studied the effect of a Ce addition on the properties of the Pd/Al2O3 catalyst for the propylene conversion, indicating that the addition of Ce can effectively provide more strong acid sites of the prepared catalyst [79]. He et al. added the Mo promoter to the Pd/Al2O3 catalyst for the catalytic oxidation of benzene [80]. The addition of Mo could enhance the Pd dispersion (Figure 18), change the oxidation state of the Pd species, and promote the oxygen adsorption on the catalyst surface, thus effectively improving the activity and stability of the prepared Pd/Al2O3 catalyst.
The reaction conditions and activities of some Pd-based catalysts for the removal of VOCs were listed in Table 5.

4. Other Noble Metal Catalysts

4.1. Au-Based Catalysts

Au nanoparticles exhibit the unique performance in the catalytic oxidation of VOCs, which has attracted great interest from researchers [81,82]. Although the catalytic activity of Au-based catalysts is lower than that of Pt- and Pd-based catalysts, the highly dispersed Au on the oxide support with the high specific surface area can lead to the improved catalytic activity of the catalytic oxidation of VOCs, and the higher CO2 selectivity in comparison with other noble-based catalysts [83]. The performance of Au-based catalysts for the catalytic oxidation of VOCs is highly dependent on the size of Au particles and the physicochemical characteristics of the supports, which are dramatically influenced by the preparation methods and pretreatment conditions [82,84,85].
Solsona et al. prepared a series of Au-based catalysts by three different methods (the coprecipitation method, deposition precipitation method, and impregnation method) for the catalytic oxidation of alkanes [86]. It was found that the Au/CoOx catalyst prepared by the coprecipitation method was effective for the total combustion of propane with the reaction temperature of 200 °C, in which the Au could promote the reducibility of the support to increase the catalyst reactivity.
CeO2, as an excellent oxygen storage material, is often used as the support for preparing Au-based catalysts. The CeO2 can enhance the dispersion of Au particles with a small and stable particle size, leading to the elevated catalytic oxidation performance of the obtained Au-based catalysts. For instance, Jia et al. compared the catalytic oxidation activity of HCHO over the TiO2 and CeO2 supported Au catalysts, demonstrating that the Au/CeO2 catalyst showed a higher activity with T100% of 80 °C for the HCHO oxidation [87]. Shen et al. also prepared a set of Au/CeO2 catalysts with a low Au content (≤0.85 wt%) and small Au crystals (<2–3 nm) by the coprecipitation method, revealing that the calcination temperature was crucial to achieving a highly dispersed gold species for an effective HCHO oxidation. The 0.78Au catalyst calcined at 300 °C could realize the complete oxidation of HCHO at only 107 °C (Figure 19) [88].
Therefore, the choice of suitable supports is very critical to realize the high dispersion of the Au species, thus improving the catalytic oxidation performance of the corresponding catalysts in the removal of VOCs. Moreover, the bimetallic catalysts containing Au particles usually exhibit a better catalytic performance than the single metal catalysts, which also has attracted interest from researchers recently [89,90]. For example, Hosseini et al. studied the catalytic performance of bimetallic Pd-Au catalysts deposited on a mesoporous TiO2 support [91]. It was found that the Pd(shell)-Au(core)/TiO2 catalyst possessed a higher catalytic activity for the oxidation of toluene and propylene than that of the catalysts with the alloy or Au (shell)-Pd (core) morphology (Figure 20).

4.2. Ag-Based Catalysts

Compared to the Pt, Pd, and Au active metals, Ag is relatively cheap in the preparation of the catalysts for the removal of VOCs. Moreover, Ag could generate the synergistic effect with metal oxide supports, and enhance the formation of oxygen active species during the catalytic oxidation of VOCs, leading to the improved catalytic activity of the obtained catalysts [92]. The catalytic activity of Ag-based catalysts is closely related to the characteristics of the supports [93]. In recent years, transition metal oxides have attracted much attention due to their low price and acceptable catalytic activity [94,95,96].
Tang et al. prepared Ag-based catalysts supported by three different metal oxides for toluene oxidation [97]. The reaction activity order was Ag-Mn-O > Ag-Co-O > Ag-Ce-O. In addition, the Ag-Mn-O catalyst also showed a high stability at room temperature. Bhat et al. prepared the Ag/TiO2 catalyst by encapsulating the Ag species in porous reducible TiO2 support, demonstrating that the encapsulation method can prevent the sintering of the active metal Ag to improve the activity of the prepared catalyst compared with the traditional impregnation method [96].
In terms of the CeO2 support, Wang et al. prepared a series of Ag/CeO2 catalysts by different methods for the catalytic oxidation of toluene, indicating that the preparation methods had a great impact on the physicochemical properties of the fabricated Ag-based catalysts [93]. The catalyst (Ag-Ce-BTC-C) prepared by the one-pot solvothermal method exhibited the optimum performance due to the abundant active Ag species, more concentration of oxygen vacancies, and the large molar ratio of Oads/Olat, Ce3+/Ce4+, and Ag0/Agδ+ (Figure 21).
Benaissa et al. studied the influence of three preparation methods (impregnation reduction, urea precipitation, and wet impregnation) on the performance of the Ag/CeO2 catalyst in the total oxidation of propylene [95]. Among these catalysts, the catalyst (Ag loading of 4 wt%) prepared by impregnation reduction exhibited the highest catalytic activity due to the presence of the Ag2+, Ag+, and Ag0 species to allow an electronic interaction among the different redox couples.
Dong et al. employed the polyvinyl alcohol-protected reduction method to prepare the layered MnO2-supported Ag catalysts with the reduced graphene oxide (rGO) as the promoter for the oxidation of ethyl acetate [94]. The 1.0Ag-1.0rGO/δ-MnO2 catalyst showed a superior catalytic performance in ethyl acetate oxidation (Ea = 39.8 kJ/mol, T90% = 160 °C), which is due to the enhanced interaction between Ag and MnO2, high Oads/Olatt molar ratio, and good low-temperature reducibility (Figure 22).

4.3. Ir-Based Catalysts

Although the reaction rate of Ir-based catalysts is generally lower than that of commonly used Pt-based and Pd-based catalysts, they exhibit the high anti-sintering ability in the oxygen-containing atmosphere during the catalytic oxidation of VOCs [98,99].
Wang et al. fabricated the Ir/Ce/Al2O3 catalysts by confining the CeO2 particles’ size for the complete oxidation of toluene, in which the size of CeO2 nanoparticles and loaded ultrafine Ir0 nanoparticles was <5 nm and <2 nm, respectively (Figure 23) [100]. The strong IrO2-CeO2 interaction was responsible for the optimized Ir0/Irδ+ ratio in the H2-reduced Ir/Ce(A)/Al2O3-H2 catalyst (with the addition of acetic acid), resulting in the improvement of the catalyst activity with T90% of 290 °C for the toluene conversion, which was lower than that of 306 and 318 °C over the H2-reduced Ir/Al2O3-H2 and Ir/Ce(W)/Al2O3-H2 (without the addition of acetic acid), respectively.
Schick et al. investigated the relationship between the Ir particle size and the total oxidation activity of short-chain alkanes over the Ir/SiO2 catalysts [99]. The Ir particle size in the prepared catalysts could be tuned by changing the calcination temperature. A low-temperature calcination (350 °C) would facilitate the formation of small Ir particles (ca. 5 nm), leading to an increase in the defective Ir3+ species and a high proportion of highly reducible iridium species for enhancing the catalyst activity. The similar structure–performance relationship was also found in a methane oxidation over the Ir-based catalysts [99].
Thus, the activity of Ir-based catalysts for the catalytic oxidation of VOCs can be effectively improved by using appropriate supports and adjusting the structure and chemical state of Ir particles.

5. Conclusions and Prospect

To reduce the emission of VOCs, high-efficiency catalytic oxidation catalysts have been fabricated and developed with a high stability and poisoning-resistant ability in recent years. Noble metal catalysts, such as Pt-, Pd-, Au-, Ag-, Ir-, etc., based catalysts were intensively used in the removal of VOCs from industry and academia due to their advantages of a high activity, good stability, and easy regeneration. The catalytic performance of these catalysts is determined by many factors, for instance, the particle’s size, dispersion degree, and the valence of noble metal as well as the metal-support interaction, etc. Therefore, many studies mainly focus on the effects of the types or properties of supports and metals, the catalyst preparation methods, and catalyst promoters on the catalytic performance during the removal of VOCs. Due to the high preparation cost of noble metal catalysts, it is also very important to reduce the loading amount of noble metal to prepare the catalysts with an acceptable activity and stability. The stability and lifetime of the noble metal catalysts should be further improved. Since most catalytic oxidation reactions for the removal of VOCs are the oxidation of carbon-containing compounds at high temperatures, it is also very important to improve the anti-sintering and anti-carbon deposition properties of the catalysts. Moreover, the reaction and deactivation mechanism of the catalytic oxidation still need further investigation due to the complexity of the various catalysts and different VOCs. Additionally, the influence of the reaction conditions, such as the addition of water vapor or sulfide on the catalytic performance, can be further optimized in the subsequent studies. The regeneration and recycling of the deactivated catalysts were also critical to reduce the preparation and operation costs in the removal of VOCs.

Author Contributions

Conceptualization, S.C. and D.H.; methodology, C.Z.; writing—original draft preparation, S.C., E.W., and F.F.; writing—review and editing, D.H., Q.Z.; supervision, J.J.; project administration, G.W.; funding acquisition, D.H., F.W. and L.B.; All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Natural Science Foundation of Shandong Province (ZR2022MB019, ZR2021MB134), the Talent Fund for Province and Ministry Co-construction Collaborative Innovation Center of Eco-chemical Engineering (STHGYX2220), National Natural Science Foundation of China (22008131).

Data Availability Statement

Data sharing not applicable.

Acknowledgments

We thanks for the partial financial support from the SINOPEC Research Institute of Safety Engineering Co., Ltd.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Time variation trend of emissions of VOCs from China’s petroleum and chemical industries from 2013 to 2019 [8].
Figure 1. Time variation trend of emissions of VOCs from China’s petroleum and chemical industries from 2013 to 2019 [8].
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Catalysts 12 01543 g002 550
Figure 2. The possible reaction pathway and mechanism on meso-Pt/CeO2 Catalyst [40].
Figure 2. The possible reaction pathway and mechanism on meso-Pt/CeO2 Catalyst [40].
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Catalysts 12 01543 g003 550
Figure 3. The possible catalytic mechanism for toluene oxidation over 0.2 wt% Pt/TiO2 [41].
Figure 3. The possible catalytic mechanism for toluene oxidation over 0.2 wt% Pt/TiO2 [41].
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Catalysts 12 01543 g004 550
Figure 4. Mixed reaction path of toluene and acetone over Pt/TiO2 catalyst [42].
Figure 4. Mixed reaction path of toluene and acetone over Pt/TiO2 catalyst [42].
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Catalysts 12 01543 g005 550
Figure 5. HCHO oxidation mechanism on Pt/Co3O4-NiO catalyst [43].
Figure 5. HCHO oxidation mechanism on Pt/Co3O4-NiO catalyst [43].
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Catalysts 12 01543 g006 550
Figure 6. Pt particle distribution of catalysts prepared by different methods (left) and different VOCs removal effects (right) [45].
Figure 6. Pt particle distribution of catalysts prepared by different methods (left) and different VOCs removal effects (right) [45].
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Catalysts 12 01543 g007 550
Figure 7. TEM image of (A) Pt/Fe2O3-CD, (B) Pt/Fe2O3-CP, (C) Pt/Fe2O3-IMP (Pt particles are dark dots in the figure) [35].
Figure 7. TEM image of (A) Pt/Fe2O3-CD, (B) Pt/Fe2O3-CP, (C) Pt/Fe2O3-IMP (Pt particles are dark dots in the figure) [35].
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Catalysts 12 01543 g008 550
Figure 8. XPS spectra of Pt/TiO2 without H2 reduction and Pt/TiO2-R with H2 reduction [47].
Figure 8. XPS spectra of Pt/TiO2 without H2 reduction and Pt/TiO2-R with H2 reduction [47].
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Catalysts 12 01543 g009 550
Figure 9. TEM images and size distribution of (a) Pt/NaY, (b) Pt/NaYmod [55].
Figure 9. TEM images and size distribution of (a) Pt/NaY, (b) Pt/NaYmod [55].
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Catalysts 12 01543 g010 550
Figure 10. The catalytic activity of (a) benzene, (b) n-hexane, (c) acetonitrile, and (d) ethyl acetate oxidation over the Pt/zeolite catalysts. T90%: temperature corresponding to reactant conversion = 90% [32].
Figure 10. The catalytic activity of (a) benzene, (b) n-hexane, (c) acetonitrile, and (d) ethyl acetate oxidation over the Pt/zeolite catalysts. T90%: temperature corresponding to reactant conversion = 90% [32].
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Catalysts 12 01543 g011 550
Figure 11. TOFs as a function of Pt particle size of Pt/TiO2 catalyst for HCHO oxidation reaction [56].
Figure 11. TOFs as a function of Pt particle size of Pt/TiO2 catalyst for HCHO oxidation reaction [56].
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Catalysts 12 01543 g012 550
Figure 12. The oxidation process (a–h) of C6H6 on palladium nanoparticles [57].
Figure 12. The oxidation process (a–h) of C6H6 on palladium nanoparticles [57].
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Catalysts 12 01543 g013 550
Figure 13. Mechanism of toluene oxidation on Pd/CeO2 catalyst [58].
Figure 13. Mechanism of toluene oxidation on Pd/CeO2 catalyst [58].
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Catalysts 12 01543 g014 550
Figure 14. 1,2-dichlorobenzene conversion and selectivity of CO2 and CO [62].
Figure 14. 1,2-dichlorobenzene conversion and selectivity of CO2 and CO [62].
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Catalysts 12 01543 g015 550
Figure 15. Schematic diagram of synthesis and structure of Pd/h-Al@4CoMn2O4 catalyst [71].
Figure 15. Schematic diagram of synthesis and structure of Pd/h-Al@4CoMn2O4 catalyst [71].
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Catalysts 12 01543 g016 550
Figure 16. The schematic description of the preparation methods of metal/MOF NPs catalysts [73].
Figure 16. The schematic description of the preparation methods of metal/MOF NPs catalysts [73].
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Catalysts 12 01543 g017 550
Figure 17. HRTEM results of (a,c) Pd0.01/TiO2 and (b,d) Pd0.01Mn0.2/TiO2 catalysts for acetone oxidation reaction [77].
Figure 17. HRTEM results of (a,c) Pd0.01/TiO2 and (b,d) Pd0.01Mn0.2/TiO2 catalysts for acetone oxidation reaction [77].
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Catalysts 12 01543 g018 550
Figure 18. HAADF-STEM and EDS of 1.0%Pd/Al2O3 (a) and 1.0%Pd-5%Mo/Al2O3 (b) catalyst [80].
Figure 18. HAADF-STEM and EDS of 1.0%Pd/Al2O3 (a) and 1.0%Pd-5%Mo/Al2O3 (b) catalyst [80].
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Catalysts 12 01543 g019 550
Figure 19. Effect of calcination temperature on oxidation of HCHO over Au/CeO2 catalyst [88].
Figure 19. Effect of calcination temperature on oxidation of HCHO over Au/CeO2 catalyst [88].
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Catalysts 12 01543 g020 550
Figure 20. (a) Pd(shell)-Au(core)/TiO2; (b) Au(shell)-Pd(core)/TiO2; (c) Pd-Au(alloy)/TiO2 [91].
Figure 20. (a) Pd(shell)-Au(core)/TiO2; (b) Au(shell)-Pd(core)/TiO2; (c) Pd-Au(alloy)/TiO2 [91].
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Catalysts 12 01543 g021 550
Figure 21. XPS spectra of the supported Ag catalysts: (A) full spectra, (B) O 1s, (C) Ce 3d, and (D) Ag 3d [93].
Figure 21. XPS spectra of the supported Ag catalysts: (A) full spectra, (B) O 1s, (C) Ce 3d, and (D) Ag 3d [93].
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Catalysts 12 01543 g022 550
Figure 22. Oxidation model of ethyl acetate over 1.0Ag-1.0rGO/δ-MnO2 catalyst [94].
Figure 22. Oxidation model of ethyl acetate over 1.0Ag-1.0rGO/δ-MnO2 catalyst [94].
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Catalysts 12 01543 g023 550
Figure 23. HAADF-STEM images and the corresponding particle size distribution of Ir: (a) Ir/Al2O3-H2, (b) Ir/Ce(W)/Al2O3-H2, and (c) Ir/Ce(A)/Al2O3-H2 [100].
Figure 23. HAADF-STEM images and the corresponding particle size distribution of Ir: (a) Ir/Al2O3-H2, (b) Ir/Ce(W)/Al2O3-H2, and (c) Ir/Ce(A)/Al2O3-H2 [100].
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Table
Table 1. Effects of some VOCs on the environment and human health [7].
Table 1. Effects of some VOCs on the environment and human health [7].
PollutantsEffect on the EnvironmentEffect on the HumanSource of Emission
FormaldehydeIndoor air pollutionSkin allergy, nasopharynx discomfort, cough, acute and chronic bronchitis, and other respiratory diseasesIncomplete combustion of interior decoration, fuel, and tobacco
BenzeneWater and air pollution and photochemical smogLong-term inhalation will damage the nervous system of people, and acute poisoning will cause nervous spasm, even coma, and deathGases volatilized from automobile exhaust, paint, adhesives, and furniture
TolueneWater and air pollutionProblems while pregnancy: retardation, premature delivery, congenital malformationsVolatilization of perfume, detergent, wallpaper, adhesive, paint, and burning of tobacco
AcetoneStrong ecological toxicity, water and air pollutionAdverse effects on the nervous systemBurning waste materials, the exhaust of automobiles, tobacco smoke, landfills
Table
Table 2. Summary of XPS data of three catalysts for HCHO oxidation [46].
Table 2. Summary of XPS data of three catalysts for HCHO oxidation [46].
CatalystBinding EnergyPt 4f7/2 (eV)Pt0/(Pt0 + Pt2+)
Pt0Pt2+
Pt/f-SiO271.272.60.48
Pt/SBA-1571.472.40.33
Pt/g-SiO271.472.50.30
Table
Table 3. Pt-based catalysts for the removal of VOCs.
Table 3. Pt-based catalysts for the removal of VOCs.
CatalystType of VOCsContent of VOCs
(ppm)		T
(℃)		Conversion
(%)		Ref.
Pt0.008-G/@ZrC7H8100017290[45]
Pt0.002/CeO2-rC7H8100015090[48]
Pt0.005-NaYmodC7H8100013090[55]
Pt0.01/TiO2-RC6H610016790[47]
Pt0.005/ZSM-5C6H6100013090[32]
Pt0.01/TiO2-R-120HCHO402581.3[56]
Pt0.01/f-SiO2HCHO30025100[46]
Pt0.05/CeO2C3H6800019050[49]
Pt0.005/TiO2C3H8800033050[49]
Table
Table 4. Catalyst acidity measured by pyridine FT-IR [62].
Table 4. Catalyst acidity measured by pyridine FT-IR [62].
CatalystExtremely Weak AcidWeak AcidMedium Strength AcidStrong Acid
CBCLCBCLCBCLCBCL
Pd/γ-Al2O3--0.54-0.31-0.19-0.12
Pd/ZSM-5 (25)1.790.471.420.210.990.170.390.11
Pd/ZSM-5 (200)0.170.280.090.050.080.04--
Pd/SiO2--0.19-0.01----
CB: concentration of B acid sites (mmol/g-cat.); CL: concentration of L acid sites (mmol/g-cat.).
Table
Table 5. Pd-based catalysts for the removal of VOCs.
Table 5. Pd-based catalysts for the removal of VOCs.
CatalystType of VOCsContent of VOCs
(ppm)		T
(℃)		Conversion
(%)		Ref.
Pd0.005/SBA-15C7H8100022790[67]
Pd0.0028/Silicalite-1-HC7H8100018990[61]
Pd0.05/h-Al@4CoMn2O4C7H850165100[71]
Pd0.005-CeMnO3C6H6500200 100[70]
Pd0.01-Mo0.05/Al2O3C6H6200019090[80]
Pd/CeO2C3H61000213 80[76]
Pd0.005-Nb0.06/TiO2C3H8500037090[78]
Pd0.005-Ce0.01/Al2O3C3H6500030390[79]
Pd0.01Mn0.2/TiO2C3H6O1000259 95[77]
Pd0.015/HYCH3OH40,00012090[66]
Pd0.005/ZSM-5 (25)C6H4Cl245047490[62]
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Chu, S.; Wang, E.; Feng, F.; Zhang, C.; Jiang, J.; Zhang, Q.; Wang, F.; Bing, L.; Wang, G.; Han, D. A Review of Noble Metal Catalysts for Catalytic Removal of VOCs. Catalysts 2022, 12, 1543. https://doi.org/10.3390/catal12121543

AMA Style

Chu S, Wang E, Feng F, Zhang C, Jiang J, Zhang Q, Wang F, Bing L, Wang G, Han D. A Review of Noble Metal Catalysts for Catalytic Removal of VOCs. Catalysts. 2022; 12(12):1543. https://doi.org/10.3390/catal12121543

Chicago/Turabian Style

Chu, Shaoqi, Enlai Wang, Fushan Feng, Changsheng Zhang, Jie Jiang, Qinqin Zhang, Fang Wang, Liancheng Bing, Guangjian Wang, and Dezhi Han. 2022. "A Review of Noble Metal Catalysts for Catalytic Removal of VOCs" Catalysts 12, no. 12: 1543. https://doi.org/10.3390/catal12121543

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