1 Introduction
Formaldehyde is an important chemical raw material and a common indoor volatile organic pollutant (VOCs), which mainly comes from artificial boards, coatings, thermal insulation materials and furniture used in house decoration
[1,2]. Formaldehyde has strong toxicity, which seriously endangers human health, and long-term exposure can even lead to cancer
[3⇓~5]. In 2004, formaldehyde was classified as the first carcinogen by the International Agency for Research on Cancer (IARC). Therefore, it is of great significance to develop a stable, efficient and safe degradation technology to reduce formaldehyde pollution in indoor air.
Based on the harmfulness of formaldehyde and the importance of removing indoor formaldehyde, researchers at home and abroad have done a lot of research work and developed a variety of technologies for removing indoor formaldehyde. The elimination methods can be divided into three aspects: source control, ventilation and terminal treatment
[6]. Among them, the use of metal catalysts to eliminate formaldehyde is one of the important methods in the terminal treatment. This process mainly includes thermal catalytic oxidation, photocatalytic oxidation and low-temperature plasma synergistic catalytic oxidation
[7][8][9]. Formaldehyde adsorbed on the surface of metal catalysts can be decomposed into CO
2 and water, which has attracted wide attention of researchers because of its high removal efficiency and stability. Noble metal catalysts (platinum, gold, palladium, silver) have excellent performance for formaldehyde removal, and the activity of noble metal catalysts can be further improved by metal doping, loading and other improved technologies to achieve a good effect of complete oxidation of formaldehyde at room temperature, but noble metal catalysts are expensive and difficult to be popularized and applied
[10][11][12][13][14,15].
Cheap metal (manganese, copper, iron, cobalt, titanium, etc.) Catalysts also have good formaldehyde removal effect, their price is relatively low, and their catalytic activity has the potential to improve, which has become a hot research topic in recent years and has been widely used in formaldehyde removal research
[16][17][18][19][20]. The activity of such catalysts at low temperatures remains to be improved compared with noble metal catalysts. Therefore, the researchers used metal doping, loading, alkali modification and other treatment technologies to further improve the activity of cheap metal catalysts, in order to reduce the temperature required for the complete oxidation of formaldehyde, and achieved good results
[21,22]. At present, the research summary of noble metal and cheap metal catalysts to remove formaldehyde at room temperature has been reported, but the professional review of supported cheap metal catalysts to remove formaldehyde at low temperature is rarely reported
[23,24]. In this paper, the research progress of supported cheap metal catalysts for formaldehyde removal in recent years is reviewed, including the application of these catalysts in thermal catalytic oxidation, photocatalytic oxidation and plasma synergistic catalytic oxidation of formaldehyde, and the influencing factors and reaction mechanism of formaldehyde removal are discussed.
2 Low-cost Supported Metal Catalysts and Their Performance for Formaldehyde Removal at Low Temperature
Manganese-based catalysts and titanium-based catalysts have been widely studied in the cheap supported metal catalysts for eliminating formaldehyde. Other cheap metals such as cobalt, copper and nickel are often used together with manganese and titanium to improve the activity of the catalyst through the synergistic effect between the metals. In addition, cheap metals can also be doped into noble metal catalysts, which can not only reduce the content of noble metals, but also improve the formaldehyde removal effect through the synergistic effect between metals. Porous materials, including activated carbon and porous minerals, are the main carriers for loading cheap metals
[25][26⇓~28]. In this section, the research progress of supported cheap metal catalysts for formaldehyde removal at low temperature is described from three aspects: thermal catalytic oxidation, photocatalytic oxidation and plasma synergistic catalytic oxidation.
2.1 Formaldehyde thermal catalytic oxidation
The reaction conditions and formaldehyde thermal catalytic effect of the supported cheap metal catalyst are shown in Table 1. A cheap metal catalyst load with a single component has high removal effect on low-concentration formaldehyde at room temperature, and that removal rate even reaches 100%. Formaldehyde with higher concentration (< 200 ppm) can be completely removed under the reaction condition higher than room temperature, but the removal temperature is generally not more than 180 ℃. Compared with the single-component cheap metal catalyst, the supported multi-component cheap metal catalyst has higher catalytic activity, and the doping of other metals significantly reduces the temperature required for the complete removal of formaldehyde
[36,37].
2.1.1 Carry single-component cheap metal catalyst
Manganese oxides (MnO
x) have been widely used in the removal of volatile organic compounds (VOCs), such as benzene and toluene, due to their low cost, high catalytic activity and abundant crystal morphology
[53][54]. In 2002, Sekine first compared the formaldehyde removal activity of different metal oxides at room temperature, and after 24 H of reaction, MnO
2 showed the best formaldehyde removal effect
[55]. This discovery has attracted the attention of researchers, making manganese the most widely used cheap metal in the field of formaldehyde catalytic oxidation.
The catalytic activities of MnO
x with different crystal structures for formaldehyde are quite different
[56]. Dai et al. Use KMnO
4 and MnSO
4 as manganese sources,Different crystal forms of MnO
2(α-MnO
2, γ-MnO
2, and δ-MnO
2) were successfully prepared by adjusting the reaction temperature and the ratio of KMnO
4/MnSO
4, and δ-MnO
2 was loaded onto activated carbon fiber (ACF)
[41]. The formaldehyde removal experiment showed that δ-MnO
2/ACF exhibited the highest catalytic activity, and the removal efficiency of formaldehyde (15 ppm) remained at 100% within 500 min. Miyawaki et al. showed that the use of Mn(CH
3COO)
2 as a manganese source resulted in the formation of Mn
3O
4 and a small amount of MnO on polyacrylonitrile-based activated carbon nanofibers (PAN-ACNF)
[49]. Subsequently, Wei et al. and Wang et al. used Mn(NO
3)
2(MN), Mn(CH
3COO)
2(MA), MnSO
4(MS), and KMnO
4(PP) as manganese sources, respectively,The catalysts of MnO
x/ halloysite (Hal) and MnO
x/ palygorskite (PG) were prepared by impregnation and in-situ oxidation-reduction methods
[35][28]. The results of MnO
x analysis showed that amorphous Mn
2O
3 and crystalline Mn
3O
4 were formed on the surface of the catalyst when MA was used. A β-MnO
2 is mainly generate using that MN; MS is adde to generate γ-MnO
2 with Mn
3O
4 and low crystallinity; Amorphous δ-MnO
2 containing K
+ can be formed on the surface of the load material by using the KMnO
4. Compared with other forms of MnO
2, the formaldehyde removal activity of the supported δ-MnO
2 catalyst is higher (Figure 1), which is due to the layered structure of δ-MnO
2, which can promote the combination of formaldehyde molecules and active sites, and accelerate the desorption of product CO
2 and water
[56].
Researchers also found that when KMnO
4 was used as manganese source, the addition of different reducing agents would cause the morphological changes of δ-MnO
2 on the load material, which would lead to the difference of formaldehyde removal effect. Fang et al. Used methanol as a reducing agent to synthesize MnO
x/AC-methanol catalyst by in situ redox reaction of methanol and KMnO
4 on activated carbon (AC)
[25]. The formaldehyde removal results showed that MnO
x/AC-methanol had better catalytic activity than MnO
x/AC prepared without methanol, and the removal efficiency could be maintained at 100% within 700 min of reaction time (Fig. 2a). Although the specific surface area, pore volume and average pore size of the MnO
x/AC and the MnO
x/AC-methanol are substantially the same,However, the high-resolution transmission electron microscopy (HRTEM) images show that the δ-MnO
2 in the :MnO
x/AC-methanol are composed of thin nanosheets, while the δ-MnO
2 in the MnO
x/AC are granular. X-ray photoelectron spectroscopy (XPS) analysis showed that the high proportion of Mn
3+ and adsorbed oxygen on the MnO
x/AC-methanol was an important reason for the high catalytic activity of the MnO
x/AC-methanol. Further, Han et al. demonstrated that the MnO
x/ diatomite (DM) catalyst prepared using ammonium oxalate (AO) as the reducing agent had better formaldehyde removal effect and stability than that prepared using methanol (MT) as the reducing agent (Fig. 2b)
[26]. The morphology and structure of Mn20/DM-AO catalysts showed that the δ-MnO
2 particles in Mn20/DM-AO were small (50 ~ 100 nm) and had a moss-like loose structure, which indicated that ammonium oxalate could inhibit the aggregation of δ-MnO
2, improve the morphology of nano-δ-MnO
2 particles, and make the δ-MnO
2 better dispersed on the surface of DM.
The loading of active sites on the support material will change the specific surface area and adsorption properties of the catalyst. Different loading materials and pretreatment are also closely related to the removal efficiency of formaldehyde. Wang et al. compared the formaldehyde removal efficiency of catalysts using porous minerals (palygorskite (PG), montmorillonite (MT), diatomite (DT)) and γ-Al
2O
3 as the supporting materials
[27]. After 1500 min reaction, the removal rate of formaldehyde (1 ppm) was MnO
2/PG(100%)>MnO
2/MT(87%)>MnO
2/DT(74%)>MnO
2/Al
2O
3(59%) from large to small. The catalytic activity of MnO
2/ minerals is significantly higher than that of MnO
2/Al
2O
3, which is mainly due to the abundant Mn
3+/Mn
4+ and surface hydroxyl groups and higher chemical conductivity. Han et al. Treated diatomite with acid before loading MnO
x to remove impurities on the surface of diatomite, increase the number of pores in diatomite and provide good conditions for formaldehyde adsorption
[26]. Zhang et al. Found that the pre-oxidation treatment of activated carbon spheres not only made the activated carbon (ACS-O) produce a developed pore structure and significantly increased the adsorption capacity of the catalyst for formaldehyde, but also improved the dispersion of MnO
x on the porous carbon spheres and promoted the formation of surface adsorbed oxygen
[30]. After 30 H reaction at room temperature, the removal rate of formaldehyde (2.61 mg/m
3) by MnO
x/ACS-O was still more than 95%.
Averlant et al. Studied the catalytic activity of MnO
x/ mesoporous molecular sieve (SBA-15) under different parameters (manganese loading, calcination temperature)
[46]. The results show that the temperature of complete removal of formaldehyde (120 ppm) can be as low as 130 ℃ when the manganese loading is 40% and the calcination temperature is 200 ℃. Therefore, it is very important to explore the appropriate reaction conditions for the preparation of cheap supported metal catalysts for efficient removal of formaldehyde.
Powder and granular catalysts have the potential of dust pollution, which limits the application of these catalysts
[57]. In this regard, various inorganic and organic fibers have also been used to support MnO
x to improve catalyst activity and overcome the limitations of powdered catalysts in application
[33,40,44,50,58]. Su et al. Prepared manganese oxide octahedral molecular sieve (OMS-2)/SiO
2 nanofiber membrane catalyst by synchronous electrospinning spray method
[32]. The porous nanofiber membrane has connected pore structure, variable porosity and large specific surface area, which is beneficial to the adsorption and diffusion of formaldehyde molecules. Zhou et al. tested the catalytic activity of MnO
2/ cellulose and calculated the conversion rate of formaldehyde per milligram of MnO
2. The results showed that the conversion rate of formaldehyde per milligram of MnO
2 on MnO
2/ cellulose was about 17 times that of δ-MnO
2 at 60 ℃
[50]. Wang et al. Found that the growth of MnO
x on polyester fiber (PET) began with the hydrolysis of PET, and the surface partial hydrolysis of PET by alkali treatment could improve its hydrophilicity and promote the aggregation of MnO
x nanoparticles on the fiber surface
[44].
In addition to the elimination of single formaldehyde, supported MnO
x catalysts can achieve the triple functions of reducing O
3, enriching formaldehyde and degrading formaldehyde in the presence of O
3, and the O
3 molecules decompose on the surface of the catalyst to provide active oxygen to enhance the conversion efficiency of formaldehyde
[59]. Wang et al. considered that MnO on the surface of C @ MnO effectively promoted the synchronous decomposition of formaldehyde and O
3[60]. The catalyst can completely remove O
3(180 ppm) and formaldehyde (60 ppm) under the reaction condition of 30 deg C. In addition, the Mn-C interface formed between MnO and activated carbon is not only beneficial to formaldehyde adsorption and subsequent reaction, but also can protect the carbon from corrosion by O
3.
Cobalt oxide also has a good removal effect on formaldehyde, but also has a variety of morphological structures, which is a non-noble metal catalyst besides MnO
x[61]. Co @ N-doped carbon (NC) prepared by Zhu et al. Has excellent catalytic activity, and the removal rate of formaldehyde (100 ppm) is more than 80% within 60 min
[34]. The porous structure and pyridine nitrogen in Co @ NC are beneficial to the adsorption of formaldehyde and promote the subsequent catalytic oxidation reaction. Its study also showed that the catalyst calcined in N
2 atmosphere (Co @ NC) exhibited more excellent formaldehyde removal ability than calcined in air (Co-Co
3O
4@NC). Compared with Co-Co
3O
4@NC, Co @ NC has higher specific surface area and pyridine nitrogen, which makes Co @ NC have better adsorption performance for formaldehyde. Calcination in air also led to the oxidation of some of the best active sites Co to Co
3O
4, and the changes in structure and chemical composition were the important factors contributing to the difference in catalytic activity between the two catalysts.
2.1.2 Upported multi-component cheap metal catalyst
By adding other cheap metals, the distribution of active sites on the surface of the supported catalyst can be improved, and the removal efficiency of formaldehyde can be improved through the synergistic effect between metals. Liu et al. found that the activity of MnO
x was higher than that of CuO
x on palygorskite (PG), but the composite catalyst of Mn and Cu showed a more efficient removal effect
[37]. The addition of Cu could not only inhibit the aggregation of MnO
x particles, but also improve the reducibility and formaldehyde adsorption performance of the composite due to the strong metal synergistic effect of the tetrahedrally coordinated binary oxide (CuMn
2O
4) formed by the combination of Cu and Mn. In 2018, Wang et al. Found that the removal efficiency of formaldehyde by CeO
2/PG at 180 ℃ was only 6.9%, but the introduction of Ce into the MnO
x/PG could significantly reduce the average particle size of MnO
x-CeO
2 and enhance its dispersion, thus improving the removal efficiency of formaldehyde
[36]. Researchers believe that CeO
2 do not directly participate in the catalytic reaction of formaldehyde in MnO
x-CeO
2/PG, but act as O
2 carriers, which can significantly improve the mobility of O
[62]. Huang et al. Also proved that the addition of Co brought high catalytic activity to the Co
xMn
yO
4/ carbon fiber
[43]. The complete removal of formaldehyde (50 ppm) can be achieved by Co
xMn
yO
4/ carbon fibers at 95 ℃.
2.1.3 Cheap metal-noble metal supported catalyst
The introduction of cheap metals into noble metal catalyst system can reduce the total cost of the catalyst and effectively improve its catalytic activity. Qu et al believed that the electron transfer between Ag and Co would form a strong intermetallic interaction, which would make the catalyst have a high Co
3+/Co
2+ ratio and good low temperature reducibility
[47]. In addition, the porous structure of silica (APTES @ MCM-41) was not destroyed after the introduction of Ag-Co, which made AgCo/APTES @ MCM-41 have higher adsorption performance for formaldehyde. Cui et al. Also proved that the addition of Fe can adjust the redox characteristics of Pt catalysts, especially the Pt-O-Fe located at the Pt-FeO
x boundary shows a strong O
2 activation ability
[45]. Yan et al. Used CeO
2/ to support Pt, and tested the formaldehyde catalytic activity of the catalyst. The experimental results showed that the formaldehyde concentration decreased from 230 ppm to 19. 0 ppm after 60 min of reaction
[63]. The introduction of CeO
2 into the catalyst system, due to the high oxygen storage capacity of CeO
2, Pt/CeO
2/AlOOH the formation of more surface active oxygen on the surface of the catalyst, which is an important reason for the improvement of catalytic activity. Kharlamova et al. compared the effects of Mn and Ce addition on the catalytic performance of Ag/TiO
2[52]. The results showed that the removal efficiency of formaldehyde by Ag/CeO
2/SiO
2 was better than that by Ag/MnO
x/SiO
2. By comparing the catalytic activity of CeO
x/TiO
2 and MnO
x/TiO
2, it was found that the conversion of formaldehyde was higher in CeO
x/TiO
2 at 180 ℃.The selectivity of CO
2 is also better, while the product of MnO
x/TiO
2 for formaldehyde under the same conditions is mainly formate, and no gaseous CO
2 is formed. The different catalytic activities of Mn and Ce oxides resulted in different formaldehyde removal efficiencies of Ag/CeO
2/SiO
2 and Ag/MnO
x/SiO
2.
Zhang et al. added Fe to Pt/α-AlOOH, and after analysis, it was speculated that there were two types of hydroxyl groups (AlOOH and FeOOH) in the catalyst, and it was considered that the abundant hydroxyl groups on the surface of the Pt/Fe
x/α-AlOOH promoted the adsorption and conversion of formaldehyde
[38]. Similarly, Yang et al. suggested that the introduction of Ni
2+ would form PtNi(OH)
x nanoparticles and increase the concentration of hydroxyl groups in the vicinity of Pt particles
[39]. The associated hydroxyl groups near the Pt active sites can oxidize formic acid and thus promote the conversion of formaldehyde.
Ye et al. loaded Pt/MnO
2 nanosheets using 3D-structured carbon foam (CF) with Pt content as low as 0.3 wt%
[29]. The hierarchical porous structure of the composite catalyst is conducive to the diffusion of formaldehyde molecules, and due to the synergistic effect of Pt-MnO
2, there are abundant oxygen vacancies on the surface of the catalyst, thus producing a large number of active oxygen. Yu et al. Supported Au
0.5Pt
0.5/MnO
2 on cotton, which not only improved the catalytic activity, but also overcome the problem of powder pollution of the catalyst
[48]. The synergistic effect of Au, Pt, MnO
2, and cotton allowed formaldehyde (460 ppm) to be completely converted to CO
2 and water at 120 ° C.
2.2 Photocatalytic oxidation of formaldehyde
The photocatalytic effect of the supported cheap metal on formaldehyde is shown in Table 2. The light source used in photocatalysis is divided into ultraviolet light and visible light, and the catalytic effect of ultraviolet light is better than that of visible light. Considering the high energy consumption of ultraviolet light, the photocatalytic removal of formaldehyde under visible light has more application prospects.
2.2.1 Carry single component cheap metal photocatalyst
TiO
2 is widely used in the removal of formaldehyde and other VOCs because of its abundant reserves, low price, good stability and excellent photocatalytic effect
[64⇓~66]. As mentioned above, loading the active sites on the support can change the dispersion of the active sites and improve the formaldehyde adsorption performance of the catalyst. The specific surface area of acid-activated sepiolite (AAS) loaded TiO
2,TiO
2/AAS used by Zhang et al. Was significantly improved compared with that of AAS, which provided favorable conditions for the adsorption and oxidation of formaldehyde
[67]. In the study of Kibanova et al., the formaldehyde adsorption capacity of TiO
2/ hectorite was 4.1 times that of TiO
2[68]. In another study, Zhang et al. Suggested that TiO
2/ diatomite has higher formaldehyde adsorption capacity than TiO
2 due to the abundant hydroxyl groups on the surface of diatomite (Fig. 3)
[69].
Han et al. Immobilized carbon-doped TiO
2(TiO
2-C) and carbon-nitrogen co-doped TiO
2(TiO
2-NC) on polyester fibers (PET) by spraying, respectively, and used them to study the photocatalytic removal of formaldehyde under visible light
[74]. The results show that the photocatalytic activity of TiO
2-C/PET is higher than that of TiO
2-CN/PET, which is due to the narrow band gap of TiO
2-C and the high utilization of visible light. The introduction of carbon into the photocatalyst can not only promote the generation and transfer of photogenerated electron holes, but also improve the light absorption of TiO
2. The results of Liu et al showed that C-Ti in TiO
2/ activated carbon promoted the photoelectron transfer and enhanced the absorption of visible and ultraviolet light
[71]. Liu et al. speculated that graphite-like carbon nitride (g-C
3N
4) could transfer photogenerated electrons to TiO
2 quickly, hindering the recombination of photogenerated electrons and holes
[76]. Hu et al. Found that the addition of hydroxyapatite (HAP) would reduce the light absorption capacity of TiO
2/HAP composites and have a negative impact on the photodegradation of formaldehyde, but the catalytic activity of TiO
2/HAP was improved due to the strong adsorption capacity of HAP on formaldehyde
[58]. Different support materials will change the light absorption of the catalyst in different degrees, affecting the catalytic activity. Therefore, while ensuring a high degree of dispersion of the catalyst, the selection of support materials that can expand the light absorption can further improve the photocatalytic performance of formaldehyde.
Cui et al. Loaded TiO
2 on polyester-cotton fabrics, and studied the formaldehyde removal effect of the catalyst and the toxicity of TiO
2 particles to cells
[73]. The degradation rate of formaldehyde (1.5 mg/m
3) of TiO
2/ polyester cotton is 77% under visible light irradiation and 97% under ultraviolet light irradiation. The catalyst was found to be free of TiO
2 nanoparticles (~ 0.0%) from the fabric after a standard friction test and was non-toxic to macrophages as well as human hepatocytes and kidney cells, so it has high safety and practical value. In the study of Curcio et al., TiO
2/ polypropylene (PP) showed excellent stability, and no deactivation of the photocatalyst was observed after 360 H of reaction
[77]. In addition, Chen et al. Used melamine-formaldehyde resin to load TiO
2-SiO
2 aerogel on decorative paper to achieve dual functions of formaldehyde degradation and smoke suppression
[78]. The SiO
2-TiO
2 can form a stable and closed barrier layer during combustion, and the porous structure of the SiO
2 can absorb harmful volatiles generated by wood combustion, thereby reducing smoke emissions and improving the practical application performance of the catalyst.
Besides Ti oxides, Bi-based photocatalytic materials such as BiVO
4 and BiOX (X = Cl, Br, and I) have also become the research focus of formaldehyde photocatalysis. Yang et al. successfully loaded BiVO
4(BVO) quantum tube (QT) on reduced graphene (rGO) aerogel to prepare a composite photocatalyst
[79]. The test results showed that the BiVO
4-QT/rGO reduced formaldehyde from 1.0 ppm to 0.4 ppm in 15 min. Compared with BVO, the size of BVO-QT nanoparticles is as small as several nanometers, which can significantly increase the band gap width and generate high-energy photogenerated electrons under visible light excitation
[80,81]. Wang et al. prepared a 3D crown-shaped BiOCl
0.5Br
0.5/GO catalyst, and the 3D layered structure helps to increase the adsorption sites on the surface and improve the visible light absorption
[82]. High photocurrent density can be generated by adding high conductivity material graphene, so that photoelectrons can directly combine with O
2 in the air through graphene to generate active oxygen, which inhibits the recombination of electrons and holes and is conducive to enhancing the activity of photocatalysts
[76,79].
2.2.2 Upported multi-component cheap metal photocatalyst
Huang et al. prepared Zr-TiO
2/ glass fiber (GF) composite photocatalyst (ZT/GF) by sol-gel method
[72]. The glass fib is helpful for dispersing that Zr-TiO
2 particle and obtaining more active sites. Zr
4+ can replace Ti
4+ in the lattice of TiO
2, which is beneficial to the formation of anatase TiO
2 and the reduction of calcination temperature. The formaldehyde degradation experiment also showed that the formaldehyde removal rate of ZT/GF was as high as 95. 14%, which was better than that of TiO
2. Hu et al. Have shown that the combination of various metals can enhance the photocatalytic activity
[70]. Under the irradiation of simulated sunlight and visible light, the formaldehyde adsorption and decomposition capacity of BiOCl/TiO
2/ sepiolite composite was significantly higher than that of TiO
2, BiOCl and TiO
2/BiOCl. The enhanced photocatalytic activity is mainly attributed to the synergistic effect of TiO
2, BiOCl and sepiolite, which provides more reactive sites and has stronger photoresponse ability. After coupling of TiO
2, BiOCl and sepiolite, the absorption edge of the composite shifted to the long wavelength range (red shift) and showed stronger absorption intensity in the UV and visible range (Fig. 4). Liu et al. And Zhang et al. Also observed that the absorption spectrum of TiO
2/ sepiolite has an obvious red-shift phenomenon
[83][67]. Huang et al also pointed out that with the increase of Zr
4+ concentration, the absorption spectrum of ZT/GF also showed a red-shift phenomenon
[72].
2.3 Low-temperature plasma synergistic catalytic oxidation of formaldehyde
Low-temperature plasma can produce high-energy electrons to ionize the molecules in the air and form active species to degrade pollutants, but it also produces by-products such as O
3 and NO
x in the process of plasma generation. The combination of the catalyst and the plasma can reduce the formation of reaction byproducts. Table 3 shows the low-temperature plasma synergistic catalytic effect of the supported catalyst on formaldehyde. Under the pulsed discharge plasma field, the MnO
2 can decompose the O
3 into active oxygen, and then participate in the catalytic process of formaldehyde
[84⇓~86].
Dong et al. Used TiO
2/ zeolite to remove formaldehyde under the condition of pulsed discharge. The results showed that TiO
2/ zeolite had higher formaldehyde catalytic activity than TiO
2. Plasma and ultraviolet light generated in the discharge process could stimulate the activity of TiO
2 and promote the removal of formaldehyde
[87]. Zhao et al. Reported that AgCu/zeolite (HZSM-5) catalyst has excellent performance of adsorbing formaldehyde in humid air, and can completely oxidize the adsorbed formaldehyde into CO
2 in the subsequent discharge stage
[88]. However, when air is used as the discharge gas, besides the CO
2, a small amount of N
2O and nitric oxide are also produced. Although plasma and catalyst can cooperate to remove formaldehyde and reduce the production of by-products, the existence of by-products and energy consumption are still the main problems faced by this technology.
3 Influential factors
The removal efficiency of formaldehyde by cheap metal catalysts is affected by many factors, and the introduction of carrier materials makes the role of influencing factors more complex. First, the influence of reaction conditions, such as humidity, light source, discharge voltage and pulse frequency, on the removal effect; The second is the influence of preparation conditions of the catalyst, such as calcination temperature, loading and doping amount; The third is the influence of the carrier on the catalytic effect. The influence of carrier materials on formaldehyde removal efficiency of cheap supported metal catalysts was emphatically introduced.
3.1 Influence of reaction conditions
3.1.1 Relative humidity
The change of humidity has an important influence on the thermal catalytic effect of formaldehyde. Cui et al. Found that under dry conditions, the formaldehyde conversion rate was only 76. 2% at 25 ℃, while under humid conditions (RH = 30% or 55%)Pt-FeO
x/Al
2O
3), the catalytic activity was significantly improved, and the formaldehyde conversion rate was close to 100%. When the humidity continued to increase to 80%, the formaldehyde removal rate decreased to 97. 2%
[45]. Researchers speculate that water molecules can be converted into active hydroxyl groups by interacting with Pt-FeO
x on the surface of the catalyst to compensate for the hydroxyl groups consumed in the decomposition of formaldehyde, thus maintaining high catalytic activity. Zhang et al. Also considered that the oxidation of formaldehyde or its intermediate consumes the hydroxyl groups on the surface of the catalyst, but the water produced during the reaction can compensate for the consumed hydroxyl groups at the same time
[30]. Proper moisture is beneficial to the removal of formaldehyde, but too high humidity will hinder the adsorption of formaldehyde on the surface of the catalyst. Miyawaki et al. Showed that the formaldehyde removal performance of the (RH=90%)MnO
x@PAN-ACNF decreased under humid conditions because the high concentration of water molecules in the inlet gas would compete with formaldehyde and adsorb on the active sites, resulting in catalyst deactivation
[49][89].
Humidity has a similar effect on photocatalysis and plasma co-catalysis of formaldehyde. In 2011, Wan et al. Reported that properly increasing the relative humidity could improve the effect of plasma synergistic catalytic removal of formaldehyde, and also reduce the generation of by-product O
3[86]. In the photocatalytic process, water molecules can combine with the photogenerated hole h
+ to produce hydroxyl radicals, which participate in the reaction of formaldehyde
[90]. Therefore, appropriate humidity can enhance the photocatalytic activity of formaldehyde, but too high humidity will cause the competitive adsorption of water molecules and formaldehyde on the active sites, resulting in the reduction of catalyst activity
[74].
3.1.2 Light source type and intensity
The light source used for formaldehyde photocatalysis is divided into visible light and ultraviolet light. Due to the different response of photocatalyst to these two kinds of light, the formaldehyde removal effect of supported photocatalyst is more excellent under ultraviolet radiation
[73]. Light intensity is also one of the key factors affecting the photocatalytic reaction rate of formaldehyde. Generally speaking, when the light intensity increases, more photons will be produced, thus providing more energy, and the activity of the supported photocatalyst will also be enhanced
[70]. The experimental results of Zhang et al show that when the light intensity is 8 W, the photons used in the photocatalytic process are limited, and the removal rate of formaldehyde is 55.33%
[69]. When the light intensity increased to 40 W, the removal rate of formaldehyde increased to 78. 18%. The photocatalyst produced more electron-hole pairs under higher light intensity, which was beneficial to the photocatalytic oxidation of formaldehyde. In the study of Han et al., the removal efficiency of formaldehyde increased linearly with the increase of visible light intensity (4000 ~ 14 000 lux)
[74]. Qu et al. Also found that the removal rate of formaldehyde increased linearly from 25.5% to 59.6% with the increase of light intensity (0 ~ 30 000 lux), and the promotion effect of formaldehyde catalytic removal tended to be stable when the light intensity continued to increase
[91]. Increasing the light intensity can promote the photocatalytic oxidation of formaldehyde, but also increase the energy consumption. Therefore, it is of great significance to optimize the reaction conditions and balance the energy consumption and removal effect for the application of photocatalytic removal of formaldehyde.
3.1.3 Discharge mode and intensity
The discharge mode of plasma can be divided into alternating current and direct current. When AC power is used, the discharge voltage and pulse frequency have an important influence on the removal efficiency of formaldehyde. The increase of voltage/frequency can improve the removal efficiency of formaldehyde to a certain extent, and with the further increase of voltage/frequency, the improvement of reaction efficiency will tend to be flat
[87]. Wan et al. Found that when DC power supply was used, the removal efficiency of formaldehyde increased with the increase of specific energy density, and the concentration of O
3 also increased accordingly
[86]. In addition, the results also show that the positive DC corona discharge can remove formaldehyde more effectively than the negative DC corona discharge, and also produce more O
3. Therefore, it is very important to optimize the reaction conditions of plasma synergy and adjust the balance of catalytic effect, energy consumption and by-products. The effects of other factors such as discharge polarity, discharge electrode configuration, and the composition of the reaction gas flow in the plasma synergistic catalysis process have been described in detail, and will not be described in detail here
[92,93].
3.2 Influence of support material
Support materials generally have a large specific surface area, and when the active center is loaded on the support, the dispersion is better, and even a larger specific surface area than the original metal oxide can be obtained
[37,60]. Generally speaking, larger specific surface area can produce more active sites and improve the reaction efficiency
[82]. Because the reaction of the supported catalyst involves many aspects, Zhang et al believed that although the formaldehyde removal efficiency was affected by physical factors such as specific surface area and pore volume, the chemical properties of the catalyst played a decisive role
[30].
Adsorption on the surface of catalyst is the premise of gas-solid phase reaction, and the adsorption performance of catalyst for formaldehyde will significantly affect the removal efficiency. Porous materials, such as activated carbon and mineral stone, have high specific surface area and pore volume. As cheap metal loading materials, they are conducive to the adsorption of formaldehyde and the transfer of products, and accelerate the conversion of formaldehyde
[49][37,46][29]. The abundant hydroxyl groups on the surface of the support material can also adsorb formaldehyde molecules through hydrogen bonds. Li et al loaded MnO
x-CeO
2 onto TiO
2 colloid, and the generated hydrophilic —OH/TiO
2 interface could improve the adsorption of formaldehyde at low humidity, so that the catalyst showed stable catalytic performance in the environment with large humidity changes
[94]. In addition, the surface hydroxyl groups can participate in the oxidation process of formaldehyde
[95]. Fang et al pointed out that the further oxidation of the intermediate formate was the rate-limiting step in the whole catalytic process
[25]. Accumulation of formate and carbonate at the active sites leads to deactivation of the catalyst
[30,89,96]. The reaction of hydroxyl groups on the surface of the supported material with formaldehyde can accelerate the oxidation process of formaldehyde and avoid the accumulation of intermediate products, thus improving the stability of the catalyst.
In the photocatalytic process of formaldehyde, the carrier can also affect the light absorption of the catalyst. The results of Liu et al. Show that a large number of hydroxyl groups on the surface of activated carbon not only enhance the adsorption of formaldehyde on the catalyst, but also enhance the absorption of ultraviolet radiation
[71]. After Hu et al coupled TiO
2, BiOCl and sepiolite, the absorption band edge of the composite material was red-shifted, and the light absorption in the ultraviolet and visible light range was stronger
[70]. However, related reports also show that the addition of the carrier will have a negative impact on the light absorption of the composite catalyst
[58,69].
3.3 Influence of preparation conditions of catalyst
3.3.1 Loading and doping amount
The loading amount of the active component is an important factor affecting the activity of the supported catalyst. Averlant et al considered that the surface area and pore volume of MnO
x/ mesoporous molecular sieve (SBA-15) decreased with the increase of manganese content, and there was a linear relationship between them
[46]. Overloading of MnO
2 may lead to agglomeration of nanoparticles, a similar phenomenon was also found by Miyawaki et al
[49]. Zhang et al considered that the increase of TiO
2 content in TiO
2/ sepiolite (AAS) composite was beneficial to the degradation of formaldehyde, but with the further increase of titanium content, the photocatalytic activity of TiO
2/AAS decreased, and the aggregation of TiO
2 particles was an important reason for this phenomenon
[67].
Metal doping can significantly improve the activity of the catalyst, but excessive doping will reduce the activity of the catalyst. The study of Huang et al. showed that the catalyst activity was affected by the change of Co content. As the Co content increased to 21%, the formaldehyde conversion increased significantly (Fig. 5), and as the Co content further increased, the conversion decreased to 69.9%
[43]. The X-ray photoelectron spectroscopy results show that the Co
3+/Co
2+ and the ratio of adsorbed oxygen to lattice oxygen first increase and then decrease with the increase of Co content. Zhang et al. found that the catalytic activity of Pt/Fe
x/α-AlOOH decreased when the loading of Fe exceeded 3.8 wt%, which was caused by the corrosion of α-AlOOH by high-concentration Fe(NO
3)
3 solution
[38].
3.3.2 Calcination temperature
Zhang et al. reported that the formaldehyde degradation efficiency increased from 87.0% to 93.1% when the calcination temperature was increased from 200 ℃ to 300 ℃, and decreased when the treatment temperature was further increased to 400 ℃ and 500 ℃
[67]. Liu et al, Hu et al and Averlant et al also found similar phenomena in related studies
[83][70][46]. Averlant et al. Found that the average oxidation state of Mn was strongly affected by the calcination temperature. At 200 ℃ and 400 ℃, the average oxidation state of Mn was about 3.9. With the increase of calcination temperature, the proportion of MnO in the MnO
x increased, and the mean oxidation state decreased to 3.2
[46]. Hu et al. believed that an appropriate calcination temperature was conducive to the formation of anatase TiO
2 nanoparticles and the elimination of impurities (such as Cl
-, organic residues, etc.) In sepiolite, resulting in a larger specific surface area, while too high a temperature would destroy the ordered layered structure of sepiolite, resulting in a decrease in pore volume and specific surface area
[70]. Excessive calcination temperature will also remove the bound water in the catalyst and reduce the surface hydroxyl content, thus affecting the chemical adsorption of formaldehyde on the catalyst.
Calcination temperature also affects the activity of the deactivated catalyst after regeneration. Liu et al. Calcined the deactivated CuMn/Pal catalyst in air at 250, 300, 350 and 400 ℃, respectively, and tested its formaldehyde activity
[96]. When the calcination temperature increased to 400 ℃, the formaldehyde removal activity of the regenerated catalyst also increased. Formaldehyde conversion intermediate (formate) accumulated on the surface of the deactivated catalyst will be eliminated at higher temperature, and surface oxygen will be replenished under the action of O
2. However, the specific surface area of CuMn/Pal decreased after calcination at 500, 600 and 700 ℃, respectively, resulting in the decrease of catalytic activity. In addition, if the calcination temperature is too high, the silicon atoms in the support may migrate to the active sites, reducing the number of active surface sites and reducing the reaction efficiency
[97].
4 Reaction Mechanism
Understanding the mechanism of formaldehyde oxidation on cheap metals is of great importance for the development of catalysts with high efficiency, low cost and good stability. At present, the reaction mechanism of formaldehyde on cheap metal catalysts has not been accurately concluded, and doping other metals or adding other carriers increases the complexity of the reaction process of formaldehyde. In this section, we will briefly introduce the reaction pathway and intermediate products of formaldehyde from three aspects of thermal catalysis, photocatalysis and plasma synergistic catalysis of formaldehyde on supported cheap metal catalysts, focusing on the role of hydroxyl groups in the catalytic process of formaldehyde.
4.1 Mechanism of thermal catalytic reaction of formaldehyde
The oxidation of formaldehyde on metal catalysts is generally considered to be completed with the participation of surface active oxygen and hydroxyl. The O
2 adsorbed on the surface of the catalyst forms active oxygen species (O
-,
) and surface hydroxyl groups with water molecules. These reactive species first oxidize formaldehyde to formate, which is then further oxidized to CO
2 and water
[25,30,34]. Zhu et al. considered that formaldehyde and O
2 molecules were first adsorbed on Co @ NC
[34]. The O
2 adsorbed on the surface of CO
0 decomposes into active oxygen, which oxidizes the adjacent formaldehyde to form formate. Finally, the adsorbed formate was further oxidized to adsorbed CO
2 and water and released from the Co surface. In addition to the formate mentioned above, the intermediate products of formaldehyde oxidation on cheap metal catalysts include methylenedioxy (DOM), carbonate and methanol. Formaldehyde adsorbed on the catalyst will combine with active oxygen to form DOM, which will be rapidly converted into formate, and finally decomposed into CO
2 and water under the action of surface active oxygen
[26,28,52,98]. According to Wang et al., at low concentration (1 ppm), the intermediate formate is directly converted into CO
2 and water under the action of active oxygen, while at higher concentration (1200 ppm), the formate is not completely oxidized and is partially converted into carbonate
[28]. Kharlamova et al. Found that the reaction of high concentration (8000 ppm) formaldehyde on Ag/CeO
2/SiO
2 produced a small amount of methanol in addition to formate
[52]. Liu et al. Studied the deactivation mechanism of Cu-Mn/Pal, and the results showed that when the temperature was less than 200 ℃, the removal of formaldehyde on the catalyst surface was not synchronized with the formation of CO
2, but the conversion efficiency of formaldehyde increased with the increase of temperature, and reached 100% at 250 ℃
[96]. Therefore, the researchers believe that the incomplete conversion of intermediate products such as formate and carbonate will lead to the deactivation of the catalyst when formaldehyde reacts at lower temperatures
[25,26,95].
Although both active oxygen and hydroxyl are involved in the catalytic oxidation of formaldehyde, it is difficult to distinguish them in practical studies. Zhang et al. First proposed the role of hydroxyl in the oxidation of formaldehyde. The abundant hydroxyl groups on the surface of Pt/TiO
2 can react with formate, an intermediate of formaldehyde, to form CO
2 and water
[99]. Wang et al. Used density functional theory to study the catalytic oxidation mechanism of formaldehyde under the different effects of active oxygen and hydroxyl, and found that the energy barrier (0.837 eV) for the synergistic oxidation of formaldehyde by active oxygen and hydroxyl was lower than that by active oxygen (1.119 eV) or hydroxyl (1.424 eV) alone
[100]. Researchers have increased the number of hydroxyl groups in the catalyst by adding alkali metals or alkali treatment, thereby improving the catalytic activity
[95,101]. In addition, some loading materials such as minerals, zeolites and TiO
2 have abundant surface hydroxyl groups, which can adsorb formaldehyde through hydrogen bonds
[102,103]. Su et al. propose that formaldehyde can be captured by that hydroxyl group on the catalyst surface and subsequently oxidized to CO
2 and water by active oxygen (fig. 6)
[32]. Wei et Al. Also proved that formate was produced on halloysite by in-situ infrared analysis, which was considered to be the chemical adsorption of hydroxyl groups (Si-O-OH, Si-OH and Al-OH) on formaldehyde
[35]. Through X-ray photoelectron spectroscopy analysis, Yan et al. speculated that the value of the third peak of O S1 would decrease after AlOOH was loaded with Au, and considered that the peak at 532.2 ~ 532.4 eV corresponded to two components: water, hydroxyl group formed by the interaction of AlOOH and Au — (OH) Au
[104]. The results of in situ infrared analysis also showed that there was the consumption of hydroxyl groups during the oxidation of formaldehyde. Therefore, it is speculated that there are two kinds of hydroxyl groups involved in the oxidation of formaldehyde, namely, the hydroxyl groups on the support AlOOH combine with formaldehyde to form formate species, and — (OH) Au has high oxidation activity and can oxidize formate. Therefore, the hydroxyl on the surface of the supported catalyst not only adsorbs formaldehyde, but also participates in the oxidation process of formaldehyde, which simplifies the reaction process and improves the reaction efficiency. The types of hydroxyl groups in cheap metal catalysts, especially in supported cheap metal catalysts, have not been distinguished in detail in the study of formaldehyde catalytic mechanism, and the types of hydroxyl groups in cheap metal catalysts can be further explored in future studies.
For supported multi-component metal catalysts, the oxidation of formaldehyde also involves the synergy between metals. Wang et al. Considered that formaldehyde was first oxidized to formate by surface active oxygen attack, and then further oxidized to CO
2 and water, which involved the redox cycle of Ce
4+/Ce
3+ and Mn
4+/Mn
3+[36]. According to Yan et al., CeO
2 has a high oxygen storage capacity, which can interact with oxygen molecules in the air through the redox cycle of Ce
4+/Ce
3+ to generate additional surface oxygen species, thus improving the catalytic activity
[63].
In short, the reaction process of formaldehyde on cheap metals is relatively complex, and there is no exact reaction pathway. In addition, there are many kinds of cheap metals, and it is more difficult to determine the catalytic pathway of formaldehyde when combined with other metals, so further research is needed.
4.2 Photocatalytic reaction mechanism of formaldehyde
When the light energy absorbed by the TiO
2 is greater than or equal to its band gap width (3. 2 eV), the photogenerated electrons in the valence band will jump to the conduction band, and holes will be formed in the valence band
[90]. Photogenerated electrons and holes h
+ combine with oxygen molecules and water to form · OH,
and H
2O
2, respectively, which have strong oxidizing properties
[105]. The above strong oxidation species formed in the photocatalytic process will participate in the oxidation process of formaldehyde, but researchers still have different views on which oxide plays a leading role
[74]. Zhang et al. proposed that formaldehyde is oxidized to CO
2 and water under the action of hydroxyl
[67]. The TiO
2 mixed phase (anatase-rutile) may contribute to the catalytic activity of the TiO
2/ acid-activated sepiolite (AAS) composite. The rutile TiO
2 accepts holes h
+, while the anatase TiO
2 accepts electrons (fig. 7), extending the lifetimes of the photogenerated electrons and holes and effectively suppressing their recombination. Liu et al believed that although both
and · OH would participate in the reaction of formaldehyde, · OH was the main species of the reaction, and
could react with H
2O
2 to convert into · OH
[83]. Different from the UV photocatalysis with hydroxyl as the main oxidation species, Han et al considered that the hole h
+ and
generated under visible light were the main reasons for the degradation of formaldehyde
[74]. Hu et al. Considered that the photogenerated hole h
+ was the main active species through the analysis of free radical scavenger test and electron spin resonance (ESR) spectrum results
[70]. Different active species produced under different light conditions have different contributions to the decomposition of formaldehyde. Similar to the thermal catalysis of formaldehyde, the hydroxyl groups on the surface of the catalyst also participate in the photocatalytic oxidation of formaldehyde. The combination of photogenerated holes and H
2O/
-OH is one of the ways to produce · OH, so the loading materials rich in surface hydroxyl groups can improve the · OH content
[71].
Because TiO
2 can only be excited under ultraviolet light, the efficiency of visible light utilization is limited. Hu et al. Found that BiOCl/TiO
2/ sepiolite has a narrow band gap (2. 64 eV) and can be excited under visible light irradiation
[70]. The TiO
2 conduction band edge potential is low (-0.20 eV), so that the photoelectrons generated on the TiO
2 conduction band quickly migrate to the BiOCl conduction band, and at the same time, the holes on the BiOCl valence band can be transferred to the TiO
2 valence band along the BiOCl/TiO
2 interface (Figure 8). The redistribution of charge creates a built-in electric field at the heterojunction interface. Electron-hole migration facilitates the efficient separation of photoelectron-hole pairs in the ternary coupled system. The photoelectrons accumulated in the conduction band of BiOCl and the adsorbed O
2 molecules combine to form
. The hole h
+ and
attack the formaldehyde molecule and gradually decompose it into CO
2 and water. In addition, the holes in the valence band of TiO
2 can also convert — OH and H
2O into · OH for the oxidation of formaldehyde.
4.3 Mechanism of Plasma Synergistic Catalytic Reaction of Formaldehyde
Plasma synergetic catalytic removal of formaldehyde can be achieved in two ways: one is that formaldehyde reacts directly with active species produced in the plasma discharge process, such as O, · OH and high-energy electrons, to produce CO
2 and water, but Zhu et al believe that high-energy electron collision only has a weak impact on the reaction of low concentration formaldehyde
[106]; The other is the cooperation of various active substances and catalysts produced in the discharge process to remove formaldehyde. Fan et al. Showed that although the O
3 produced by plasma discharge itself could not directly react with the formaldehyde in the intake air, when combined with MnO
x/Al
2O
3, it could produce active oxygen species, improve the conversion rate of formaldehyde and reduce the reaction by-product O
3[84]. The thermal catalysis of formaldehyde is also found in the plasma synergistic catalysis, but because the plasma synergistic catalysis reaction is carried out at room temperature, the thermal catalytic activity is not high, and the influence of thermal catalysis can be ignored
[106]. In addition, the plasma not only produces high-energy particles, but also produces ultraviolet light, which can be absorbed by photocatalyst to remove formaldehyde
[93]. Dong and Lan found that TiO
2/ zeolites can absorb the ultraviolet rays generated in the process of pulsed discharge and lead to the photocatalytic degradation of formaldehyde. Pulsed discharge can also prevent the recombination of photogenerated electrons and holes and improve the catalytic efficiency
[87]. In addition to the discharge process, photocatalysis and thermocatalysis are also involved in the plasma synergistic catalysis process of loaded cheap metals, and the reaction process is relatively complex, so the reaction mechanism needs to be further explored.
5 Conclusions and outlook
As one of the main indoor volatile organic pollutants, formaldehyde can seriously endanger human health. As one of the common ways to eliminate gaseous pollutants, catalytic oxidation can be used in thermal catalysis, photocatalysis and plasma synergistic catalysis of formaldehyde. Noble metal catalysts have excellent catalytic effect, but their high price limits their large-scale application. Some cheap metal catalysts (manganese, titanium, copper, iron, etc.) Can also be used to eliminate formaldehyde, but compared with noble metal catalysts, the main problem faced by cheap metal catalysts is their low catalytic activity. Loading cheap metal catalysts on the support can properly improve the activity of the catalyst and improve its practical application performance.
In this paper, the progress of supported cheap metal catalysts in formaldehyde thermal catalysis, photocatalysis and plasma co-catalysis is introduced, the effect of support on formaldehyde catalysis is discussed, and the mechanism of formaldehyde thermal catalysis, photocatalysis and plasma co-catalysis is briefly introduced. For thermal catalysis, the loading method can effectively improve the reaction efficiency of the catalyst and reduce the formaldehyde catalytic temperature. The loading materials can not only improve the dispersion and adsorption performance of the catalyst, but also participate in the oxidation process of formaldehyde. For the photocatalytic oxidation of formaldehyde, in addition to the above effects, the support materials and surface hydroxyl groups will change the absorption of the catalyst to light, resulting in red shift or blue shift. For plasma synergistic catalytic oxidation, the synergistic effect of plasma and catalyst can improve the catalytic effect on the one hand, and reduce the generation of reaction by-products on the other hand.
To sum up, cheap supported metal catalysts have made great progress in formaldehyde removal at low temperature, but they still face problems and challenges, which need to be paid attention to in future research. First of all, further improving the catalytic activity of cheap metal catalysts at room temperature is still the focus of future research on formaldehyde catalysis. When the concentration of formaldehyde is low, the supported cheap metal catalyst can completely remove formaldehyde in a long time, but compared with the noble metal catalyst, the thermal catalytic activity is low. Therefore, the catalyst activity can be further improved by means of catalyst modification, addition of other metals, and the like. For the photocatalysis of formaldehyde, most of the photocatalysts used at present are TiO2, and the photocatalytic activity of formaldehyde under UV light is higher than that under visible light, but the use of UV light is limited in practical application. Therefore, it is possible to explore other photocatalysts, such as Bi-based catalysts, in combination with TiO2, to further improve the visible-light photocatalytic activity of photocatalysts. In addition, the combination of photocatalysis and room temperature thermocatalysis is a predictable way to remove formaldehyde. Considering the practical application scenarios of catalysts, the combination of photocatalysis and thermocatalysis can complement each other, reduce energy consumption and make full use of visible light resources. For plasma co-catalysis, exploring the appropriate discharge conditions to reduce energy consumption and further reduce the reaction by-products is still the focus of future research.
At present, most of the supported cheap metal catalysts exist in the form of powder, which will cause potential pollution problems in practical application. Therefore, exploring suitable supporting materials and supporting methods should be one of the future research directions. While making full use of the loading materials, we should also consider the possible harm of the catalyst to the human body, which can not eliminate one pollutant while bringing another health threat. From the above, it can be seen that the relative humidity will have a significant impact on the catalytic effect of formaldehyde. In practical applications, the relative humidity in different seasons and different locations will change, so improving the stability of the catalyst in different humidity is also the focus of future research.
While improving the efficiency of formaldehyde elimination, the reaction mechanism of formaldehyde needs to be further explored. Although the active species on the surface of the catalyst play an important role in the catalytic process of formaldehyde, the contribution of each active species in the catalytic process is still unclear. Although researchers believe that the hydroxyl groups in the active center and the supporting materials can be adsorbed and participate in the formaldehyde catalysis process, the role of different types of hydroxyl groups in formaldehyde catalysis is less studied and needs further analysis. For photocatalysis and plasma co-catalysis, there are few studies on the intermediate products and reaction pathways of formaldehyde reaction, which need to be paid attention to by researchers.
National Natural Science Foundation of China(21203093)
Key Research and Development Program of Jiangsu Province(BE2018718)
Cooperation Fund of Energy Research Institute, Nanjing Institute of Technology(CXY201911)
Cooperation Fund of Energy Research Institute, Nanjing Institute of Technology(CXY201923)
Scientific Research Fund of Nanjing Institute of Technology(YKJ2019110)
Scientific Research Fund of Nanjing Institute of Technology(YKJ2019111)
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