Recent Advances in the Catalytic Treatment of Volatile Organic Compounds: A Review Based on the Mixture Effect

Abstract

Catalytic total oxidation is an efficient technique for treating VOCs, which are mainly emitted by solvent-based industrial processes. However, studies of the catalytic oxidation of VOCs in combination with other pollutants are very limited, despite the fact that this is a key step of knowledge before industrial application. During the oxidation reaction, the behavior of a molecule may change depending on the reaction mixture. For the treatment of an effluent loaded with VOCs, it is necessary to carefully select not only the catalytic material to be used but also the reaction conditions. Indeed, the catalytic oxidation of a component in a VOCs mixture is not predicted solely from the behavior of individual component. Thus, the objective of this small review is to carry out a study on the effect observed in the case of the oxidation of a VOCs mixture or in the presence of water, NO_X or sulfur compounds.

1. Introduction

Air purification is a very important current topic and has become a public concern. In order to treat atmospheric pollution, a lot of research is being carried out to reduce or treat these pollutant emissions. Air pollutants generally fall into two categories: primary and secondary. The former are emitted directly from a given source (COx, NOx, SO₂) while the latter result from the chemical transformation of a substance in the atmosphere (O₃, H₂SO₄, HNO₃). Among air pollutants, volatile organic compounds (VOCs) are dominant pollutants which have the distinction of being both primary pollutants, precursors of secondary pollutants and themselves secondary pollutants. As a result, they will have several impacts at different scales on humans, the environment and the economy. In general, the terms “direct impacts” and “indirect impacts” are used to refer to these consequences at the level of the emission source and in the atmosphere. In fact, since the industrial revolution (from the 18th century), anthropogenic activities have caused a preponderant increase in the rate of VOCs emissions into the atmosphere. VOCs are a diverse group of chemical molecules with the unique property of being able to vaporize in ambient conditions. According to the Environmental Protection Agency, VOCs are defined as “any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates and ammonium carbonate, which participates in atmospheric photochemical reactions”. Thus, VOCs which possess characteristics of easy diffusivity, toxicity and volatility, released into our environment, are regarded as a critical factor in air pollution and give rise to serious damage to the eco-environment and human health [1,2,3,4,5,6,7,8,9,10,11,12].

VOCs are emitted from several sources, including outdoor sources (transportation, industrial and energetic processes) and indoor sources (household products, construction materials, furniture, consumer goods, combustion byproducts, etc.). VOCs emissions in the atmosphere are currently carefully restricted due to their hazardous properties. Catalytic total oxidation, which is an ecologically acceptable control technique, is one of the most promising solutions for VOCs remediation and has been extensively studied [5,6,7,8,9,10,11,12]. This technique requires a low working temperature (generally around 250–500 °C) compared to thermal oxidation processes (650–1100 °C). Moreover, the use of catalysts prevents the formation of toxic byproducts (NOx, dioxins) and drastically reduces the energetic cost of the process. According to the literature, noble metals, mixed metal oxides and perovskites are the most widely used for this application. Nevertheless, catalytic materials, allowing a VOCs total oxidation at lower temperatures, generally contain noble metals (Pt, Pd, Au…). However, they are expensive. In contrast, mixed metal oxide catalysts are cheaper and prevent the degradation and poisoning of the active materials. Another kind of catalyst is perovskites, in which many different cations can be incorporated, thereby increasing their thermal stability and activity [8,9,10,11,12].

However, although being an essential step toward industrial applications, research on the catalytic oxidation of VOCs molecules in mixtures with other contaminants is very limited. The authors typically observe inhibitory effects [13,14], which occur either by molecular competition or at the adsorption stage on the surface of the catalyst during the oxidation process (reaction with chemisorbed oxygen or lattice oxygen). These interactions can be influenced by different parameters, such as VOCs conformation and polarity, as well as the nature of the catalytic material. Moreover, some molecules have to be adsorbed on the catalytic surface before being oxidized, while others oxidize directly in contact with lattice oxygen in the gas phase. The oxidation of the adsorbed molecules will be strongly inhibited to the benefit of the other mechanisms, due to more favorable kinetics. Likewise, an adsorption competition can be observed when various compounds need to be adsorbed on the catalyst’s surface before being oxidized.

Thus, the aim of this small review is to understand the state of the art about the effect obtained during the catalytic oxidation of VOC in mixtures. The oxidation of representative mixtures of VOCs is discussed, as well as the effects of humidity, NOx and sulfur compounds.

2. Effect of Mixture on VOCs Oxidation

2.1. VOC in Mixture

A summary of several mixture effects described in the literature is presented in Figure 1 and

Table 1. Summary of catalytic effects reported for the oxidation of VOCs mixture.

Pollutant 1	Pollutant 2	Effect 1 on 2	Effect 2 on 1
Toluene	m-xylene	Not affected [15]	Inhibition [15]
	Ethyl-acetate	Inhibition [16,17,18,19]/Promotion [20]	Not affected [17]/Inhibition [16,18,20]/Promotion [19]
	Acetone	Inhibition [16,21]/Promotion [18]	Inhibition [16,21]/Promotion [18]
	Butyl acetate	Inhibition [16]	Inhibition [16]
	n-hexane	Inhibition [22,23]	Not affected [22]/Inhibition [23]
	CO	-	Promotion [14,24]
	Ethylene	Inhibition [25]	Not affected [25]
	Propylene	Inhibition [25]	Not affected [25]
	Ethanol	Inhibition [17,20,26]	Promotion [17]/Inhibition [20,26]
	2-propanol	Inhibition [27]	Not affected [27]
	Benzene	Inhibition [19,22,28]/Promotion [29,30]	Inhibition [19,22,28]
	Butanone	Inhibition [13,31]/Promotion [32]	Inhibition [13,31]/Promotion [13]
Benzene	Ethyl acetate	Not affected [33]/Promotion [34,35]/Inhibition [19]	Inhibition [19,33,34]
	n-butanol	Promotion [36]/Inhibition [36]	Inhibition [35]
	n-hexane	Inhibition [22,35,37]	Not affected [22,37]
	Ethanol	Promotion [30]	-
Ethyl acetate	Acetone	Promotion [15]/Not affected [15]	Not affected [15]/Inhibition [15]
	m-xylene	Promotion [15]	Inhibition [15]
	n-hexane	Inhibition [16,18,35]	Not affected [16]/Inhibition [18]/Promotion [35]
	Ethanol	Inhibition [17,20]	Inhibition [17,20]
	Butyl acetate	Inhibition [38]	Inhibition [38]
Methyl isobutyl ketone	o-xylene	Not affected [39]	Inhibition [39]
Butanone	2-propanol	-	Not affected [27]
n-hexane	Acetone	Inhibition [16]	Inhibition [16]
	Butyl acetate	Not affected [16]	Inhibition [16]
Carbon monoxide	Methanol	Not affected [40]	Inhibition [40]
	Dimethyl ether	Not affected [40]	Inhibition [40]
Methane	Propane	-	Inhibition [41]
	Formaldehyde	-	Inhibition [41]
	Methanol	-	Inhibition [41]
Ethanol	Acetic acid	-	Inhibition [42]
Propene	Acetic acid	Inhibition [43]	Inhibition [43]
	Acetaldehyde	Inhibition [43]	Inhibition [43]
Ethylene	Propylene	Not affected [25]	Not affected [25]

. This table shows the effect(s) of one pollutant compared with others, such as inhibition or promotion effects or no effects.

The presence of cyclic VOCs generally causes an inhibitory effect in the oxidation reaction of all the others. No effect is also reported, as in the case of the effect of benzene over ethyl acetate [33] or the toluene over m-xylene [15]. In some cases, both promoting and inhibiting effects are reported: the effect of benzene over n-butanol oxidation [36], the effect of toluene over butanone oxidation [13,31,32], the effect of toluene on acetone oxidation [16,18,21] and benzene oxidation [19,22,28,29,30]. In the first example, the difference of the effect is attributed to the nature of the active phase used. The Pd based catalyst presented the promotional effect while using a Pt based catalyst allows an inhibition effect of n-butanol oxidation. For the second one, the inhibition effect of butanone oxidation in the presence of toluene can be detected with Pd/γ-Al₂O₃ and CoAlCeO catalysts [13] while a promotion is observed with the Pd/V-TiO₂ catalyst [13]. Colman et al. [31] also studied the mixture effect with four different pollutants (toluene, xylene, CHCl₃ and MEK) over a Pt/Mn monolith catalyst. The authors present that the conversions of pollutants are similar whether alone or in a mixture. However, the authors reveal a mutual inhibition effect in a butanone/toluene mixture.

Concerning oxygenated derivatives, only a few rare cases of promoting effects are reported. The presence of ethyl-acetate can promote the toluene [19] and m-xylene [15] oxidations. Ethanol allows the oxidation of toluene at lower temperatures [17], such as carbon monoxide [14,24] or butanone in the presence of CoAlCeO [13]. Concerning the effect of butanone over toluene oxidation, inhibition is also reported in the presence of Pd/γ-Al₂O₃ [13]. This difference can be explained by the facility of CoAlCeO to oxidize by-products at lower temperatures than Pd/γ-Al₂O₃.

For hydrocarbons, only one article reported a promoting effect corresponding to the benefit of n-hexane over ethyl acetate oxidation [35]. In all other cases, the hydrocarbon presence has no effect or inhibits the oxidation of the other pollutant.

2.2. Toluene in Mixture

Among all papers cited in this section, toluene is the most reported when authors studied the catalytic oxidation of mixtures. Toluene is a model VOC molecule that is present in our indoor environment as well as in our industrial environment. It is consistently the most studied concerning the catalytic oxidation of VOCs. A detailed description of the effect of toluene in a mixture with various compounds (as reported in Table 1) will be discussed here.

First of all, the oxidation of toluene was studied in the presence of similar molecules such as benzene [19,22,28,29,30,44] or m-xylene [15].

The effect of different concentrations of binary mixtures of VOCs (benzene, toluene, n-hexane) was investigated in the presence of Pt/γ-Al₂O₃ catalyst by S. Ordóñez et al. [22]. The authors reported that n-hexane had a negligible effect on benzene or toluene oxidation while hexane oxidation is inhibited by the presence of one of them. Concerning benzene and toluene mixtures, they present a mutual inhibition as reported by S. Kim et al. in the presence of Ca doped Mn₃O₄ catalyst [28]. The authors studied the kinetic model based on the Mars–Van–Krevelen mechanism to explain the inhibition phenomena observed. The first model able to explain this inhibition is the competitive model where both cyclic hydrocarbons are adsorbed on the same active site and, consequently, the oxidation rate of each compound is reduced. The second model used is the non-competitive model where cyclic hydrocarbons react with oxidized sites of the catalyst. A good correlation between the experimental results and the calculated competitive model has been found by authors.

An Au-Pd alloy supported on α-MnO₂ nanotubes was synthetized by Y. Xia et al. [15] for the catalytic oxidation of toluene, m-xylene, ethyl acetate and acetone alone and in a binary mixture. The better catalytic performance of 0.91 wt% Au-Pd/α-MnO₂ catalyst in any oxidation reaction of VOC alone can be explained by a significant improvement in the reactivity of the lattice oxygens. Concerning the catalytic oxidation of mixture, the authors reported that the toluene oxidation is inhibited by the presence of m-xylene but the oxidation of m-xylene is not affected.

The effect of toluene on the oxidation of other VOCs is also well reported, such as its mixture with oxy-derivate VOCs such as ethyl-acetate [16,17,18,19,20], acetone [16,18,21], butyl acetate [16], ethanol [17,20,26], 2-propanol [27] or butanone [13,31,32].

The catalytic oxidation of various VOCs mixtures (toluene, acetone, ethyl acetate, butyl acetate and n-hexane) was studied in the presence of monolithic perovskite catalyst [16]. A. Musialik-Piotrowska et al. compared the different T₅₀ and T₉₀ of VOCs during oxidation alone or in a mixture (

Table 2. Temperature (°C) of 50% and 90% conversion efficiencies of selected compounds oxidized individually and in the mixtures, data from [16].

Compound	T50 (°C)	T90 (°C)
Toluene	250	360
With acetone	280	410
With ethyl acetate	275	400
With butyl acetate	265	400
n-hexane	255	425
With acetone	270	>500
With ethyl acetate	290	500
With butyl acetate	270	500
Acetone	200	235
With toluene	230	300
With n-hexane	<200	250
Ethyl acetate	205	300
With toluene	250	345
With n-hexane	215	300
Butyl acetate	<200	280
With toluene	200	325
With n-hexane	200	280

). These data indicate that the presence of oxy-derivatives compounds inhibits the oxidation of hydrocarbons. The presence of toluene inhibited the oxy-derivatives reactions while increasing the concentration of incomplete oxidation by-products such as acetaldehyde.

The mixture effects on the oxidation of toluene, ethyl acetate and ethanol were studied over a cryptomelane catalyst [17]. Different mixture effects were observed by V.P. Santos et al. during their study. The presence of toluene inhibits both ethyl acetate and ethanol oxidation while the toluene oxidation is only slightly inhibited by the presence of ethyl acetate and is promoted by the presence of ethanol. A different effect is reported concerning the oxidation of ethanol and toluene alone or in a mixture over a Pt/TiO₂ catalyst [26]. Ethanol and toluene present mutual inhibition in a mixture. This mutual inhibition can be explained by the adsorption competition of oxygen atoms chemisorbed on the Pt particles.

The oxidation of a VOCs mixture (2-propanol, toluene and methyl-ethyl-ketone (MEK) was studied by N. Burgos et al. over the Pt/Al₂O₃ catalyst [27]. For toluene and MEK, both complete oxidations are not affected by the presence of other VOCs in the gas flow. The authors suggest that they are oxidized directly from the gas phase with oxygen atoms chemisorbed on Pt.

The influence of a toluene–butanone oxidation mixture was studied by J. Brunet et al. in the presence of a benchmark catalyst (Pd/γ-Al₂O₃) and a CoAlCeO catalyst [13]. Depending on the catalyst, different effects have been highlighted. For Pd/γ-Al₂O₃, a mutual inhibition of toluene and butanone oxidation is observed (Figure 2). An increase of the T₅₀ of both pollutants is reported (36 °C and 63 °C) for toluene and butanone oxidation, respectively. In contrast, in the presence of the CoAlCeO catalyst, butanone oxidation is inhibited (an increase of about 30 °C of T₅₀ between the test alone and the test in a mixture) and toluene oxidation is promoted (T₅₀ reaches 30 °C earlier in the presence of a mixture). This difference can be explained by the facility of CoAlCeO to oxidize by-products at lower temperatures than Pd/γ-Al₂O₃. These two catalysts were also tested for the abatement of an industrial’s mixture simulation of ten different VOCs, and CoAlCeO presented the best abatement properties.

T. Barakat et al. observed the reverse effect over Pd supported on vanadium doped TiO₂ catalyst [32]. A beneficial effect of toluene on butanone oxidation is observed. The authors explained, supported by DRIFT analysis, that in the presence of toluene, butanone has a higher affinity for the catalyst surface, resulting in a faster oxidation process. They also showed that the vanadium doping limits the adsorption on the surface and limits the formation of by-products.

The catalytic performances of three mesoporous manganese oxides (Mn₂O₃, Mn₃O₄ and Mn_xO_y) were studied by M. Piumetti et al. for total oxidation of toluene and hydrocarbons alone (ethylene, propylene) and in a ternary mixture [25]. The oxidation of toluene is not affected by the propylene and ethylene presence in the reaction mixture.

The effect of Ce addition into a MgAl mixed oxide catalyst obtained via a hydrotalcite method was studied by E. Genty et al. for the catalytic oxidation of toluene and carbon monoxide alone or in a mixture [24]. The Ce increased the redox properties of the catalyst, resulting in increased catalytic properties in both oxidation reactions. The authors reported a promotional effect of carbon monoxide for toluene oxidation reaction probably caused by the exothermicity of CO oxidation, which increased local temperature.

The same authors studied the catalytic properties of mixed oxide (CoAlCeO) obtained via a hydrotalcite method for toluene oxidation alone or in the presence of carbon monoxide [14]. Again, there is a promotional effect of carbon monoxide on the oxidation reaction of toluene and the authors proposed that this is related to the observed by-product difference. Benzaldehyde, the by-product formed in the presence of CO is easier to oxidize into CO₂ than into benzylic alcohol, the by-product formed during the oxidation of toluene alone. Those catalysts also presented good stability.

F. Aguero et al. [20] proposed that the total conversion temperature of a single component in a mixture is determined by the temperature at which the most difficult molecule is oxidized. That usually conducts to an inhibition of ethyl acetate and ethanol oxidation in the presence of toluene. In these articles, it is admitted that the presence of toluene inhibits the oxidation of the second pollutant. However, by using non-thermal plasma catalysis, J. Karuppiah et al. [45] present a conversion improvement of a mixture of VOCs and a better selectivity to the total oxidation of the mixture. This result presents an interesting alternative to the thermal catalytic oxidation of a VOC mixture.

2.3. Benzene in Mixture

Benzene is included in the family of aromatic hydrocarbons, such as toluene, and its oxidation in mixtures is also reported. It has already been discussed that it presents a mutual inhibition with toluene [22] or that it inhibits the oxidation of n-hexane while n-hexane does not affect benzene oxidation.

The catalytic performance of Pt/TiO₂ doped with W⁶⁺ was studied in benzene and ethyl acetate oxidation alone or in a mixture [33]. The authors proposed that W⁶⁺ doping allows better activity on VOC oxidation than a traditional Pt/γ-Al₂O₃ catalyst caused by complementary bifunctional steps on the Pt and support sites. The authors reported that benzene oxidation is strongly inhibited in the presence of ethyl acetate and water.

The oxidation of the VOC in low concentrations was carried out in the presence of group VII metals supported on alumina [36]. The case of the benzene–butanol mixture was also discussed and revealed that the oxidation of benzene was totally suppressed as long as the butanol was still present in the reaction mixture. However, a benzene promoting effect on butanol oxidation was observed in the presence of a Pd catalyst, rather than a Pt catalyst, which presented a benzene inhibiting effect on butanol oxidation.

2.4. Oxi-Derivated in Mixture

2.4.1. With Oxi-Derivated VOCs

Au-Pd alloy supported on α-MnO₂ nanotube was synthetized by Y. Xia et al. for the catalytic oxidation of toluene, m-xylene, ethyl acetate and acetone alone and in a bi-mixture [15]. Concerning the catalytic oxidation of the mixture, the authors reported that the oxidation of acetone and ethyl acetate presented a minor mutual inhibition effect, and the oxidation of m-xylene was promoted in the presence of ethyl acetate. In the absence of a precious metal, the oxidation of acetone was inhibited by ethyl acetate while the oxidation of ethyl acetate was unaffected by the presence of acetone.

The presence of ethyl acetate and ethanol in the flow results in a mutual inhibition effect [17].

The mixing effect of ethyl acetate and butyl acetate was studied over a cryptomelane catalyst and revealed a mutual inhibition effect on their respective oxidations [38]. By comparing the inhibition effect with different concentrations of VOCs, the authors supposed that this inhibition was caused by adsorption competition of VOCs. The authors also studied the addition of water in the catalytic flow. They reported an inhibition of VOCs oxidation probably caused by adsorption competition between water vapor and VOCs blocking active sites on the catalyst’s surface.

Catalytic oxidation of ethanol and acetic acid alone or in a mixture was studied on MnO_x-CeO₂ mixed oxide, obtained using combustion methods [42]. Results showed that the presence of acetic acid suppressed the ethanol conversion, probably because the acetic acid was more adsorbed on the catalyst than ethanol.

The impact of precious metal (Au on Al₂O₃) was investigated for the catalytic oxidation of a VOCs mixture (carbon monoxide, methanol, dimethyl ether) by T. Tabakova et al. [40]. In any case, the carbon monoxide oxidation was inhibited by the presence of methanol or dimethyl ether while the presence of carbon monoxide did not have an impact on the methanol or dimethyl ether oxidation. In the mixture, in the presence of Au materials, the total oxidation of carbon monoxide and methanol occurred at the same temperature. The authors hypothesized that these two molecules are oxidized on the same active site. Whereas, in the presence of Cu-Mn, the oxidation of methanol has shifted to a lower temperature; in this case, the authors proposed that carbon monoxide and methanol were competitively adsorbed. The authors concluded that the beneficial effect of Au promotion is related to the strong capacity of Au NPs to adsorb and activate carbon monoxide molecules and its ability to improve Cu-Mn redox properties.

2.4.2. With Hydrocarbon VOCs

The catalytic oxidation of various VOCs mixtures was studied in the presence of a monolithic perovskite catalyst [16], results were previously presented in the part 2.2 (Table 2). These data reveal that the presence of toluene inhibits the oxy-derivative oxidation reactions while increasing the concentration of incomplete oxidation by-products such as acetaldehyde.

The oxidation reaction of a hydrocarbons mixture (ethylene, propene, ethane, propane) and CO was investigated in the presence of α-Cr₂O₃ by Y.F.Y. Yao [43]. The author showed that when two hydrocarbons or CO were in the reaction mixture, a mutual inhibition was observed in any combination. The author added other compounds in the mixture. In the case of the addition of ethanol or acetaldehyde in small quantities, the oxidation of propene in a reversible way is completely suppressed. With the progressive addition of SO₂ in the mixture stream (O₂ + propene + He), the reaction rate progressively decreases. The addition of ammonia recessively suppresses the oxidation of the propene while transforming into N₂O instead of NO if it was alone. The presence of propene inhibits the oxidation of ammonia into NO to promote N₂O formation.

The partial oxidation of methane in a mixture with ethylene, formaldehyde, methanol or halogens (chloromethane, dichloromethane) has been studied over a palladium catalyst by C.F. Cullis [41]. With the addition of other VOCs, the partial oxidation of methane is inhibited in any case. By the addition of halogens, the oxidation rate of methane is also reduced and the formation of formaldehyde is favored with high selectivity. This suggests that the presence of halogen modifies the catalytic surface, which inhibits the oxidation of the initial product of methane oxidation.

2.5. Hydrocarbons in Mixture

The reactivity of the mixture of C₅ and C₆ hydrocarbons was studied over a Pt/zeolite catalyst [46]. By comparing the reaction rate, a mutual inhibition is observed between the hydrocarbons. The catalytic performance of mesoporous manganese oxides was studied for the total oxidation of VOCs alone (ethylene, propylene, toluene) or in a ternary mixture [25]. The oxidation of toluene is the most difficult alone or in a mixture. However, the total VOCs mixture conversion into CO₂ is similar to that of toluene conversion into CO₂ alone. It could probably be caused by the highest absorbance capacity of toluene compared to propylene and ethylene. The authors also proposed that their catalyst has a higher relative performance caused by the presence of a higher amount of electrophilic oxygens on the surface, Brønsted and Lewis acidic sites. Those properties may be responsible for the adsorption/desorption rates of reactant and product and allow the activation of hydrocarbons.

2.6. Conclusion

It can be concluded that, in general, the presence of other molecules inhibits the oxidation of VOCs, which can be explained by the adsorption competition between various VOC components and reaction intermediates on active sites. Few studies reported a beneficial effect but it depends enormously on the nature of the reaction mixture and the catalyst active sites. Thus, special attention should be paid in order to be able to determine how a VOC can affect the reactivity of another according to the kind of catalysts used but also according to the nature of the VOC mixture and the operating conditions.

3. Effect of Moisture

Among all different possible mixtures with VOCs, the effect of water is the most reported in the literature. Indeed, this effect on VOCs oxidation is very important to understand for many reasons. It is always a product of the total oxidation of VOCs and it is really often present in industrial air (or indoor air) that needs to be treated. The effect is generally negative for the total oxidation of VOCs such as those reported for the oxidation of methane [47], styrene [48], benzene [49], hexane [49] and ethyl acetate [49]. In this part, the effect of water on different model VOCs, such as formaldehyde, toluene or CO, will be summarized.

3.1. Moisture Effect over HCHO Oxidation

The studies are mainly performed with noble metals such as Pt, Pd, Au or Ag but the case of manganese oxide is also reported. The detail of the articles presenting the HCHO oxidation in the presence of water is summarized in

Table 3. Summary of HCHO oxidation catalytic properties in the presence of water reported.

Catalyst	Conditions	Water Content in Relative Humidity	Activity	Ref.
0.5 Pd/γ-Al2O3	GHSV = 125,000 h−1 T = 23–25 °C Activities are measured after 15 h	20%	50%	[50]
		30%	50%
		40%	45%
		49%	43%
		61%	44%
		80%	40%
		87%	37%
Fe2O3-MnO2		20%	6%
		50%	5%
		80%	5%
CuO-MnO2		20%	15%
		50%	20%
		74%	22%
MnO2	[HCHO] = 150–200 ppm Face velocity = 50 cm s−1 Activities are measured after 100 h	>90%	~80%	[51]
		25–30%	85–90%
MnOx-CeO2	GHSV = 21,000 mL·gcat−1 h−1 [HCHO] = 580 ppm T = 100 °C	0%	100%	[52]
		92%	95%
MnO2 Birnessite	GHSV = 180,000 h−1 [HCHO] = 10 ppm mcata = 0.05 g Rate flow = 300 mL·min−1 T = 25 °C	0%	-	[53]
		33%	85%
		65%	85%
		92%	65%
1% Pt/TiO2 1% Na—1% Pt/TiO2 2% Na—1% Pt/TiO2	GHSV = 120,000 h−1 [HCHO] = 600 ppm Flow rate = 50 mL·min−1 T = 25 °C	50%	19% 98% 100%	[54]
0.1% Pt/TiO2	GHSV = 80,000 h−1 [HCHO] = 10 ppm T = 25 °C Activities are measured after 10 h	0%	45.2%	[55]
		25%	~100%
		50%	~100%
		75%	~97%
		97.5%	95.6%
0.1% Pd-TiO2/DP	GHSV = 120,000 mL·gcat−1 h−1 [HCHO] = 10 ppm mcata = 0.5 g Rate flow = 1 L min−1 T = Room temperature Activities are measured after 10 h	0%	54.5%	[56]
		25%	78.5%
		50%	>95%
		75%	>95%
		97.5%	>95%
HZ	[HCHO] = 70 ppm T = 25 °C	0	<0.1	[57]
		50%	<0.1
Pt/HZ		0	60
		50%	100
Pd/HZ		0	20
		50%	96
Ag/HZ		0	<0.1
		50%	<0.1
Pt/SiO2		0	90
		50%	100
Pt/Fe2O3	GHSV = 60,000 h−1 [HCHO] = 400 ppm mcata = 0.2 g T = 25 °C	0%	85%	[58]
		3%(water vapor)	100%
Au/FeOx-C200	GHSV = 34,000 h−1 [HCHO] = 80 ppm mcata = 0.19 g Rate flow = 100 mL min−1 T = 25 °C Activities are expressed in µmol·s−1·gAu.s−1	0%	0.21	[59]
		25%	7.55
		50%	10.78
		75%	9.7
δ-MnO2/PET	GHSV = 17,000 h−1 [HCHO] = 0.6 mg·m−3 mcata = 0.5 g Flow rate = 1 L·min−1 T = 25 °C Activities are measured after 10 h	0%	65%	[60]
		30%	75%
		50%	94%
		80%	89%
MnO2 birnessite	[HCHO] = 40 ppm mcata = 0.1 g Flow rate = 200 mL·min−1 T = 30–140 °C	80%	T50 = 53 °C	[61]
K-Birnessite	GHSV = 1,200,000 h−1 [HCHO] = 200 ppm mcata = 50 mg T = room temperature Activities are measured after 10 h	48%	40%	[62]
Mg-Birnessite			0%
Ca-Birnessite			30%
Fe-Birnessite			5%
Mn0.75Co2.25O4	GHSV = 60,000 h−1 [HCHO] = 80 ppm mcata = 0.150 g Flow rate = 100 mL/min T = 70 °C Activities are measured after 30 h	0%	30%	[63]
		50%	70%
MnO2	GHSV = 240,000 mL·g−1.h−1 [HCHO] = 480 ppm	48%	T50 = 85 °C T90 = 110 °C	[64]
MnO2-P1			T50 = 74 °C T90 = 97 °C
MnO2-P2			T50 = 62 °C T90 = 86 °C
MnO2/SBA-15	GHSV = 30,000 h−1 [HCHO] = 100 ppm mcata = 0.1 g T = 130 °C Activities are expressed in relative activity after 60 h	50%	0.9	[65]
Mn3O4/SBA-15			0.5

.

3.1.1. Adsorption Competition between Water and HCHO

The impact of moist air in formaldehyde oxidation was studied by Z. Wang et al. [50]. In the case of Pd/γ-Al₂O₃, conversion into CO₂ decreased with the increase of relative humidity. The authors supposed that this inhibition can be explained by the hydrophilic properties of γ-Al₂O₃. The authors also presented the effect of water on different transition metal based catalysts such as Fe₂O₃-MnO₂ and CuO-MnO₂. In any case, catalyst activity decreased with time in the presence of water. The CuO-MnO₂ catalyst presents high catalytic activity at high relative humidity and Fe₂O₃-MnO₂ presents an optimal activity at 50% of relative humidity. This difference can be explained by their hydrophobic or hydrophilic properties but the mechanism remains difficult to understand for now.

Other studies have been performed with manganese oxide by M.A. Sidheswaran et al. [51], and with manganese-cerium mixed oxide by X. Tang et al. [52]. In both cases, the authors showed an inhibition of oxidation catalytic properties in the presence of a high concentration of water (relative humidity higher than 90%), which is also attributed to the adsorption competition between water and HCHO described above. This deactivation at high relative humidity is also described in the presence of MnO₂ birnessite [53].

3.1.2. Beneficial Effect of the Water

H. Chen et al. [66] presented that the monolith-like Pt/Ti nanotube was an effective catalyst for the removal of HCHO under mild conditions and suggested an oxidation mechanism in two steps:

• Storage of HCHO as CO form by oxidation at the surface of Pt;
• Oxidation step of CO possible by external O₂ or Pt–O sites to form CO₂.

M. Flytzani et al. [54] suggested another mechanism, which involved the oxidation of adsorbed reaction intermediate (formate) at the surface by surface hydroxide (–HCOO + –OH → H₂O + CO₂). This oxidation method would be easier than the mechanism proposed by H.Chen et al. The creation of such a hydroxyl is possible by the addition of water in the gas flow and the presence of surface oxygen [O]s ([O]s + H₂O → 2 –OH). The continuous creation of this hydroxyl allows a good stability of the catalyst over time. M. Flytzani et al. show the importance of surface hydroxyl and the beneficial effect of water on HCHO oxidation.

Different authors confirm these results in the case of Pt/TiO₂ [55], Pd/TiO2 [56], Pt or Pd/HZ zeolite [57], Pt/Fe₂O₃ [58], Au/FeO_x [59], supported manganese oxide on polyethylene terephthalate (PET) [60], or cation-doped birnessite [61,62].

In the case of the manganese oxide catalyst, another mechanism is proposed by J. Wang et al. [53] concerning the oxidation of formaldehyde in the presence of water. This mechanism consists in four steps:

• HCHO adsorption on birnessite favored by hydrogen bonds between HCHO and surface hydroxyl;
• Adsorbed HCHO oxidation into formate or carbonate by hydroxyl;
• Regeneration of surface hydroxyl via the reaction between active oxygen and water vapor (O₂⁻, O⁻ + H₂O → 2 –OH);
• Carbonate and formate desorption into CO₂ stimulated by the adsorption competition with water.

An optimum of around 50% of relative humidity seems to be a good compromise between the beneficial effect of hydroxyl formation and the inhibition effect of adsorption competition [53,55,56,59,60,63]. This is why relative humidity of 50% is usually used for HCHO oxidation [54,62,64,65].

The addition of water during the catalytic oxidation of formaldehyde can present an inhibition at high relative humidity (>90%), attributed to the adsorption competition between formaldehyde and water. However, at the optimum of 50% of relative humidity, water can take part in the oxidation by continuously creating surface hydroxyl compounds that are normally consumed during the reaction for the oxidation of intermediate such as formate and carbonate.

3.2. Moisture Effect over Toluene Oxidation

The case of the toluene–water mixture is less discussed in the literature [67,68,69,70,71]. The authors agree and describe an inhibition effect of water vapor on the catalytic oxidation of toluene attributed to the adsorption competition between water and toluene on active sites (

Table 4. Summary of moisture effect over toluene oxidation.

Catalyst	Conditions	Water Content	Activity		Ref.
La0.9SR0.1CoO3	200 mg—[Toluene] = 1000 ppm in air—230 °C	0	93%	Toluene conversion into CO2	[67]
		50% R.H.	58%
CuMnO	180 mg—[Toluene] = 1000 ppm in air	0	100%	Toluene conversion at 230 °C	[68]
		2.3 vol%	97%
		0	96.6%	Toluene conversion at 220 °C
		2.3 vol%	86.9%
		0	62.5%	Toluene conversion at 210 °C
		2.3 vol%	58.3%
Au/Mn2O3	50 mg—[Toluene] = 1000 ppm in air—33.4 mL·min−1—40 000 mL·g−1·h−1	0	260 °C	90% Toluene conversion	[69]
		1 vol%	280 °C
Pd/Mn2O3		0	235 °C
		1 vol%	240 °C
Au-Pd/Mn2O3		0	165 °C
		0.5 vol%	160 °C
		1 vol%	150 °C
		2 vol%	160 °C
		5 vol%	180 °C
Cu-Mn/γ-Al2O3	[Toluene] = 1200 ppm in air—15,000 h−1	0	225 °C	90% Toluene conversion	[70]
		3.8 vol%	270 °C
Cu-Mn/TiO2		0	225 °C
		3.8 vol%	255 °C
Cu-Mn/cordierite		0	255 °C
		3.8 vol%	280 °C
V2O5/TiO2	P toluene = 0.0128 atm in air—240 °C	0	0.2	mmol.gcat−1·h−1 after 200 min	[72]
		0.2 atm P H2O	1.15

).

J. Hu et al. present the effect of water during toluene oxidation over CuMnO nanowires at different temperatures [68]. The authors show that the inhibition effect of water is more and more pronounced when the temperature decreases and suggest not to use a temperature below the temperature of 100% conversion of toluene.

A study of toluene oxidation on noble metals supported over manganese oxide was reported by S. Xie et al. [69]. An inhibition effect was described using Au/Mn₂O₃ and Pd/Mn₂O₃ catalysts but for the Au–Pd/Mn₂O₃ catalyst, beneficial effects were reported with an optimal concentration around 1% volume of water. The authors proposed that the good oxygen activation adsorption and the strong interaction between the noble metal NPs and the metal oxide support were the origin of the excellent performance of the Au–Pd/Mn₂O₃ catalyst in the presence of water.

X. Li et al. [70] present the catalytic oxidation of toluene over a copper–manganese based catalyst in the presence of 3.78% of water. As mentioned above, an inhibition effect is observed but the proportions of the inhibition differ from the catalyst. This difference has been attributed to the difference in the adsorption ability of the material to water vapor. Using H₂O-TPD, the authors show that the material with the highest interaction strength between water molecules and the surface had the lowest durability towards water vapor.

A. Heinen et al. study the competitive adsorption between water and toluene over activated carbon supports [71]. The authors demonstrated that the toluene adsorption is not influenced by the presence of water, but only by the pore volume available for adsorption. It should be noted that these materials exhibited a really high pore volume (between 0.6 and 1.3 mL.g⁻¹) compared to the previous study.

In the case of the V₂O₅/TiO₂ catalyst described by J. Zhu and S. Andersson [72], when water is added to the feed, the oxidation rate is more than doubled and the deactivation is much lower compared to the test without water. The oxidation of toluene under a moist atmosphere conducted the formation of benzoic acid. The authors proposed that a high degree of hydroxylation on the surface is possible with water and facilitated the reaction between the surface benzoate intermediate and surface hydrogens.

3.3. Conclusion

The effect of water on the oxidation of different VOCs may be beneficial or inhibitory. This depends on different factors such as the affinity (of water and the VOC) with the material, the absorbance capacity of materials, the facility to form surface hydroxyl, and the possibility to oxidize different intermediates which usually inhibit the reaction or block active sites. Moreover, the effect of water may be promotional or inhibitory according to its content as revealed by S. Xie et al. on the Au–Pd/Mn₂O₃ catalyst [69]. The recent new formulations based on transition metal oxides seem to favor the oxidation of VOCs under a moist atmosphere and allow catalytic oxidation under more realistic conditions.

4. VOCs Oxidation in Presence of NO_X

Although VOCs and NOx are both precursors of ground-level ozone [8] and are often emitted together by industry, the abatement of VOCs in the presence of NOx is rarely discussed in the literature. The articles which present a simultaneous catalytic elimination of these two pollutants focus especially on three-way automotive catalytic converters for vehicles and some articles focus on the treatment of gases resulting from the combustion of coal [73] or industrial processes and mainly the incineration of municipal and medical waste [74]. The elimination of VOCs is therefore often studied in conjunction with a selective catalytic reduction (SCR) system. As the conditions are specific to mobile sources, we will only focus on articles dealing with the treatment of VOCs in the presence of NOx with different types of catalysts for stationary sources.

Firstly, it is interesting to see the study by Xiao et al. [75] about the oxidation of methanol in the presence of NOx for theoretical and experimental studies without a catalyst. The methanol oxidation begins with the partial oxidation to formaldehyde, then formaldehyde transforms to carbon oxides. They showed that the presence of NOx caused a promoter effect on VOC oxidation. Actually, the initiating temperature for deep oxidation of methanol was about 500 °C in the absence of NO and only about 400 °C in the presence of NO (600 ppm).

The studied catalysts are mainly metal oxides or supported metal oxides. Some of them are issued from copper based catalysts, as Cu/ZSM5 was found to be powerful for selective catalytic reduction by hydrocarbons (HC-SCR) [76]. In the article by Motak et al. [77] Cu/aluminosilicates have been used for ethanol oxidation both in the presence and absence of NOx. In this study, they showed that the conversion of VOC on the modified layered aluminosilicates that decreased slightly in the presence of NOx, whereas Karthic et al. [78] considered the effect of a mixture of acetone and NO on Cu/MCM41 and the supply of catalyst and NOx (Figure 3) clearly presents the added value of NOx for the acetone oxidation.

Aissat et al. [79] studied the effect of Cs on Cu/zirconium oxide for the oxidation of toluene with NOx. They partly explained the low activity of the sample Csx–Cuy/ZrO_2, having high amounts of Cs, by NOx adsorbing strongly in nitrate form, lowering the toluene conversion. For the catalysts with lower Cs content (Cs_0.015–Cuy/ZrO₂), the activity is high for toluene oxidation due to NOx acting as an oxidizing agent. The dispersed Cu(II) species interacting with Cs would be responsible for the activity but also for the transformation of NO into NO₂ preceding the toluene oxidation. Copper oxide is therefore interesting and this is what Mrad et al. [80] also put forward, with the oxidation of propene in the presence of a CuMgAlFeOx catalyst from the hydrotalcite route.

Many works are also focused on Vanadium based catalysts. In the article by L. Chen et al. [73], VW on titanium doped with Ce/Mo catalysts showed good efficiency for the simultaneous removal of a benzene–toluene mixture with NO for coal combustion exhausts. However, the development of this type of catalyst was rather applied to preventing the emission of chlorinated organics and NOx from industrial or incinerator sources. Studies concerning the oxidation of chlorobenzene in the presence of NO on catalysts based on VOx/TiO₂ showed that NO oxidized to NO₂ assisted O₂ for the reoxidation of vanadium oxide after oxidation of the VOC by VOx phase (Mars Van Krevelen mechanism) [81,82]. They showed that this doped phase with WOx or MoOx is particularly interesting for enhanced treatment. Li et al. [74] also used a VOx/TiO₂ catalyst for the same treatment, but doped with Pd, and showed the roles of NO₂, Brønsted acidic site and oxygen vacancy. The latest, even more promising, work now focuses on MnO₂-based catalysts. Fan et al. [83] showed the relationship between lattice distortion and catalytic redox properties Fe doped α-MnO₂ nanorods for chlorobenzene oxidation with NO.

To conclude this section, we have seen that many works have raised the existing competition for the adsorption of VOC and NO. However, this competition seemed to be negligible at high temperatures, where both toluene and NO attained high conversion efficiencies in the work of Shao et al. [84]. These findings provide a feasible way for the simultaneous removal of NOx and VOCs using catalytic processes with the selection of an optimal temperature window.

5. VOCs Oxidation in Presence of Sulfur

Unlike other pollutants, the impact of sulfur during the total oxidation of VOCs has been little studied. In fact, studies carried out until the 1990s showed that sulfur compounds brought about catalyst deactivation, whether bulk or supported, with noble metals or transition metals. This concept has been validated by the entire scientific community, so most of the current studies have focused more on the design of catalytic materials resistant to sulfur compounds than on the oxidation of VOCs in the presence of sulfur compounds.

In addition, studies have mainly been carried out on the treatment of methane and butanal in the presence of sulfur compounds. Methane is primarily emitted from agriculture (by ruminants and cultivations), whereas non-methane VOCs (or NMVOCs) are mainly emitted by transports, industrial processes and the use of organic solvents. As this review focuses on the total oxidation of industrial VOCs, the oxidation of methane will not be discussed.

Only one paper deals with the deactivation of Pt catalyst in the presence of a sulfur component. D. Pope et al. [85] have shown that the addition of sulfur compounds (methyl mercaptan, H₂S, carbonyl sulfide) causes a delay (up to 100 °C) in the light-off curves of the oxidation of butanal using a Pt-honeycomb catalyst. However, the total oxidation of the latter occurs, at the same time, as if there were no sulfur compound.

Due to the concept of the deactivation of noble metals by sulfur, more studies have been conducted on transition metal catalysts. Firstly, regarding transition metal bulk, Christopher J. Heyes et al. [86] compared the deactivation of simple oxide catalysts (Co₃O₄, MnO₂, CuO) for the oxidation of butanal in the presence of methyl mercaptan at a temperature of 400 °C, at which butanal and methyl mercaptan are completely oxidized. Their results obtained after 150 h are summarized in Figure 4 below.

In the absence of methyl mercaptan, the conversion of butanal was completely and totally directed towards CO₂ whereas in the presence of this component we can note selectivity shared between CO and CO₂. The presence of the sulfur component considerably affects the Co₃O₄ and MnO₂ materials whereas the CuO one seems to be less affected. Indeed, for the cobalt and manganese catalysts, the conversion and the selectivity towards CO₂ have a significant decrease, compared to the copper one where only the formation of CO₂ decreases in favor of the formation of CO but less than the other two catalysts.

However, for all catalysts, there is a significant difference between the converted butanal and the CO + CO₂ produced, which can be attributed to coke formation on the catalyst.

Secondly, on supported transition metals, two studies related the effect of sulfur components on manganese supported on alumina. Andrey N. Zagoruiko et al. [87] studied the influence of the presence of H₂S, performing 150 cycles (catalytic test then regeneration), during the oxidation of a mixture of VOCs (styrene, methyl ethyl ketone, acetic acid) with the supported catalyst, MnOx/Al₂O₃. They observed an almost complete deactivation if the catalyst was activated at 500 °C. Jiaming Shao et al. [88] studied the influence of SO₂ during the oxidation of toluene using a manganese catalyst supported on alumina. They proceeded by making time cycles with SO₂ free periods and periods with SO₂ at 120 °C. In the absence of SO₂, toluene is converted to 70% in CO₂ and 25% in CO + 5% (methanol, formic acid). In the presence of SO₂, there is a decrease in the conversion of toluene, the extent of deactivation depends on the sulfur content (more sulfur leads to highest deactivation). This deactivation is accompanied by an incomplete oxidation reaction because they observed a decrease in selectivity for CO₂ (60%) and an increase in selectivity for CO (35%) + 5% of by-products (methanol, formic acid, acetic acid).

The deactivation of catalysts is explained by a large accumulation of sulfur species adsorbed on the surface of the catalyst [85,86,87,88]. This metal sulfate formation blocks the access of VOCs to the active site of the catalyst, which annihilates their oxidation.

Despite the deactivation of the catalysts due to the metal sulfate formation, the authors provide some interesting ways to restore good activity.

Pope et al. [85] observe that, even if there is a deactivation in the light-off curve, the total oxidation occurs at nearly the same temperature. They explained this phenomenon by a complete decomposition of the sulfur component at temperatures above 300 °C, avoiding the formation of sulfate on platinum surfaces.

Zagoruiko et al. [87] and Shao et al. [88] have shown that regeneration at very high temperatures, or when the arrival of sulfur compounds in the gas is stopped, allows the obtaining of a total oxidation of VOCs, which suggests that the adsorption of metal sulfate on catalysts is not irreversible. Even if these results are promising, the authors remain cautious and recommend carrying out tests/cycles over longer periods to ensure that the catalysts do not always deactivate.

To conclude, the presence of sulfur compounds during the oxidation of VOCs therefore prevents the catalytic oxidation reaction by the absorption of sulfur on the surface of the catalytic materials. However, after regeneration at high temperatures, certain materials regain their catalytic activity. However, the total oxidation of VOCs in the presence of sulfur compounds is no longer really studied and has been replaced by the partial oxidation of VOCs in the presence of sulfur compounds to produce molecules of interest [89,90].

6. Conclusions

Catalytic oxidation is a promising technology for the treatment of waste gases containing VOCs. For this reason, numerous studies are reported in the literature, in particular concerning the development of suitable catalytic materials. In general, the study of these different catalytic materials proves their effectiveness in the case of an oxidation reaction of a model VOC and only few studies exist on the treatment of VOCs mixtures. During the oxidation reaction, the behavior of a molecule may change depending on the reaction mixture. For the treatment of an effluent loaded with VOCs, it is necessary to carefully select not only the catalytic material to be used but also the reaction conditions, and to have the best knowledge of the nature and composition of the VOCs mixtures since the catalytic oxidation of a component in a VOCs mixture cannot be predicted solely from the behavior of individual components. Thus, the objective of this article is to carry out a small review of the state-of-the-art of the different effects observed in the case of the oxidation of a mixture of VOCs or in the presence of water, NO_X or sulfur compounds. It can be observed that, in general, the presence of other molecules inhibits the oxidation of VOCs, which can be explained by the competition between various VOC components and reaction intermediates at the adsorption site. Few studies have found a beneficial effect but it depends enormously on the nature of the reaction mixture, the type of catalysts used and the operating conditions. Due to the complexity of the process that occurs during the oxidation of VOC mixtures, it is difficult to predict the behavior of the catalyst. For a better understanding of these mixing effects, many studies remain to be done, in particular by ensuring that the oxidation process does not transform the pollutant to be treated when it is in a mixture into a more toxic compound.