Recent Advances in Formaldehyde Catalytic Oxidation Catalysts

Abstract

Formaldehyde (HCHO), a colorless gas, is currently a toxic gas that seriously endangers human health and the environment. To effectively remove formaldehyde, catalytic oxidation is considered to be the most promising, widely studied, and applied method. This method utilizes a catalyst to promote the reaction of HCHO with O₂, converting it into harmless CO₂ and H₂O. In recent years, researchers have developed various catalysts, including noble metal catalysts (such as Pt, Pd) and transition-metal catalysts (such as Co₃O₄, MnO₂), to improve the efficiency of formaldehyde oxidation. In experimental studies, by optimizing the composition, structure, and reaction conditions of the catalyst, the conversion rate and selectivity of formaldehyde can be significantly increased. This article reviews the current research status of noble metal catalysts and transition metal catalysts in the field of formaldehyde catalytic oxidation, discusses the main factors affecting the efficiency of formaldehyde catalytic oxidation in experimental studies, and finally explores the overall reaction mechanism of formaldehyde catalytic oxidation. In summary, formaldehyde catalytic oxidation technology has broad application prospects in indoor air purification, industrial waste gas treatment, etc.

1. Introduction

Formaldehyde (HCHO or CH₂O), also known as formic aldehyde, is a typical VOC pollutant. It generally has two states: gaseous and liquid. In a liquid state, a 35–40% formaldehyde water solution is the well-known formalin solution. In the gaseous state, low-concentration formaldehyde is a colorless and odorless gas, while high-concentration formaldehyde has a strong, pungent, and suffocating smell, which can irritate the eyes, nose, etc. It primarily originates from furniture, decoration materials, textiles, and fabric products, among others [1,2,3]. HCHO is seriously harmful to human health and the ecosystem, and long-term exposure to HCHO can cause headaches, pneumonia, and even cancer [4]. The International Agency for Research on Cancer classifies HCHO as a Group I carcinogen [5]. So, how to effectively remove formaldehyde has become extremely urgent. In recent years, various methods for removing formaldehyde have been reported, such as plant adsorption [6], physical and chemical absorption [7,8,9], photocatalytic oxidation [10,11,12,13,14,15,16], thermal catalytic oxidation [17,18,19,20,21,22,23,24,25,26,27], and photothermal catalytic oxidation [28,29].

The adsorption method for removing formaldehyde involves using adsorbents to adsorb formaldehyde from the air onto the surface of the adsorbents, thereby reducing the concentration of formaldehyde in the air. Common adsorbents include activated carbon, new porous nanomaterials, and molecular sieves [30,31,32,33,34,35]. The adsorption method is simple to operate, has a low cost, and causes no secondary pollution. It is suitable for indoor air purification in places such as homes and offices, and can be used to treat formaldehyde pollution in newly renovated houses. It can also serve as an auxiliary method for other treatment approaches. However, it has the issue of adsorption saturation, and the adsorbents need to be replaced or regenerated regularly; otherwise, it may cause secondary pollution. Photocatalytic oxidation is an advanced technology for removing formaldehyde. The core of photocatalytic oxidation technology lies in the photocatalyst. Under the irradiation of light (usually ultraviolet or visible light), the catalyst is excited and generates active oxygen substances with strong oxidizing properties, such as hydroxyl radicals and superoxide radicals. These active oxygen substances can oxidize and decompose formaldehyde into carbon dioxide and water. The commonly used catalysts include nano-titanium dioxide (TiO₂) [36,37,38], heterojunction photocatalyst [39], and metal-organic frameworks (MOFs) [40], etc. Photocatalytic oxidation technology is an efficient and environmentally friendly method for removing formaldehyde, featuring a high removal rate, thorough oxidation, a wide application range, and low energy consumption. Although photocatalytic oxidation technology has many advantages, it also has some limitations. The photocatalyst has stability issues; its activity may decline after long-term use. Thermal catalytic oxidation is a mature technology for removing formaldehyde. In thermal catalytic oxidation, energy is supplied through heating, and a catalyst is used to oxidize and decompose volatile organic compounds (VOCs) such as formaldehyde into harmless carbon dioxide and water, thereby achieving air purification. The catalyst can significantly reduce the activation energy of the reaction, enabling the oxidation reaction to proceed efficiently at relatively mild temperatures. Thermal oxidation catalytic technology has the advantages of high efficiency, selectivity, and stability, wide treatment objects, less secondary pollution, and high market recognition. In recent years, significant progress has been made in catalyst research and development, which can be divided into two major categories: noble metal catalysts and non-noble metal catalysts. Noble metal catalysts such as Pt [41,42,43], Ag [44,45,46,47], Pd [48,49], and Au [50,51], etc., are widely used due to their high activity and stability, but they are costly; while non-noble metal catalysts such as MnO_X [52,53,54], and CoO_X [55,56], etc., have lower costs, but their performance needs to be further optimized according to specific pollutants and operating conditions. With the development of materials science, the performance of catalysts has been continuously improved. For example, through doping and composite methods, the activity and stability of catalysts can be enhanced, allowing them to maintain efficient catalytic performance over a wider temperature range.

In order to better understand the commonly used catalyst systems in the field of formaldehyde catalytic oxidation, as well as the influencing factors during the reaction process and the reaction mechanism—thereby improving and optimizing the formaldehyde catalytic oxidation reaction. This work reviews the research progress of noble metal catalysts and non-noble metal catalysts in detail, and also discusses the main factors affecting the efficiency of formaldehyde oxidation and the corresponding reaction mechanism. Compared with previous research work, we have found that in recent years, different catalysts have similar reaction mechanisms. The main outline of this review is displayed in Figure 1.

2. Noble-Metal Catalysts

In recent years, precious metal catalysts have attracted the interest of many researchers due to their excellent low-temperature oxidation activity and high selectivity. Due to the sintering and oxidation of precious metals, they are usually loaded on certain oxide carriers with excellent thermal stability, which makes HCHO easy to convert at low temperatures. Nonetheless, the particular catalytic characteristics are dictated by multiple factors, including the type and quantity of the precious metal, as well as the nature and morphological configuration of the carrier. The carriers of noble metal catalysts used in the oxidation reaction of formaldehyde can be classified into two types. The first type of carrier materials has a large specific surface area, which is favorable for the adsorption of formaldehyde molecules, such as SiO₂ [57,58], carbon materials [24,59], some molecular sieves [60,61], and other carrier materials [18,25,62]. These are common catalyst carriers and have already been commercialized. The second type is single or mixed metal oxides, such as CeO₂ [63], CoO_x [64], ZrO₂ [65], and MnO_x [66], etc. They possess poor activity for HCHO oxidation before supporting noble metal; these are common metal oxide carriers. Furthermore, based on these common catalysts, the development of catalysts with different morphologies is currently a hot research topic [67,68]. Although noble metal catalysts have significantly enhanced the activity of the catalysts, during the research process, it was discovered that the dispersion degree of noble metal atoms can greatly affect the catalytic activity. In response to this phenomenon, many researchers adopted the method of adding alkaline metal additives such as Na⁺ [69] and K⁺ [70], and this strategy achieved satisfactory results.

Table 1. Summary of the catalytic oxidation of formaldehyde using precious metal catalysts.

Catalysts	Reaction Conditions	Conversion	Ref.
Supports with large specific surface area
Pt-CeO2/N-rGO	100 ppm HCHO, GHSV = 80,000 mL/(g·h), RH = 35%	100% at R.T.	[71]
Ag/Meso-ZSM-5	90 ppm HCHO, 20 vol.% O2, total flow rate = 50 mL/min RH = 65%	80% at 250	[72]
ZSM-5-Ag/SBA-15	100 ppm HCHO, 20 vol.% O2, GHSV = 36,000 mL/(g·h)	100% at 65	[73]
Pd/USY	150 ppm HCHO, 20 vol.% O2, WHSV = 150,000 mL/(g·h), RH = 35%	100% at 25	[74]
Ag-K-Al2O3	100 ppm HCHO, 30 mL/min 21% O2, WHSV = 36,000 mL/(g·h), RH = 10/30/50%	100% at 40	[70]
Pt@S-1	120 ppm HCHO, 20 vol.% O2, GHSV = 180,000 mL/(g·h), RH = 0–98%	100% at R.T.	[75]
FeNi@NC/Pt	100 ppm HCHO, 20 vol.% O2, GHSV = 60,000 mL/(g·h), RH = 30%	100% at R.T.	[76]
Pd-Ba/TiO2	150 ppm HCHO, 20 vol.% O2, total flow rate = 100 mL/min RH = 35%	100% at R.T.	[77]
Ag-AC	100 ppm HCHO, total flow rate = 500 mL/min	100% at RT	[78]
Pd/DZ200	80 ppm HCHO, Air, WHSV = 120,000 mL/(g·h), RH = 50%	100% at RT	[61]
Pd/SiO2(H2)-R	150 ppm HCHO, 20 vol.% O2, total flow rate = 100 mL/min RH = 35%	100% at 25	[79]
Ag/γ-Al2O3	100 ppm HCHO, 19.5 vol.% O2, GHSV = 84,000 mL/(g·h)	100% at 125	[80]
Metal oxide supports with high-temperature oxidation activities
Pt/ZrO2	100 ppm HCHO, 21 vol.% O2, GHSV = 60,000 mL/(g·h), RH = 30%	95.3% at 20	[81]
Ag5-LCO-I	320–350 mg/m3 HCHO, Air	52% at 30	[82]
Ag/MnOx-0.5	1 ppm HCHO, 21 vol.% O2, GHSV = 150 L/(g·h), RH = 55%	>80% at 25	[83]
Pt/kit-CeO2	20 ppm HCHO, 20 vol.% O2, WHSV = 200,000 mL/(g·h), RH = 30%	~95% at 30	[27]
4-Ag/MnOX	20 ppm HCHO, Air, WHSV = 200,000 mL/(g·h)	94% at R.T.	[84]
Metal oxide supports with special morphologies
0.3PdLN	50 ppm HCHO, GHSV = 24,000 mL/(g·h)	100% at R.T.	[85]
Au@Co3O4	75 ppm HCHO, GHSV = 300,000 mL/(g·h), RH = 4/45/76%	~95% at 30	[86]
8AgMCL-H	35 ppm HCHO, RH = 50%, GSHV = 22,200 mL/(g·h)	100% at 30	[87]
3Ag-5K/Co3O4-MnO2 spheres	150 ppm HCHO, 21 vol% O2, RH = 30%, WHSV = 36,000 mL/(g·h)	100% at 50	[88]

The “100% at 30” in the table indicates the 100% conversion rate at 30 °C. “RH” stands for relative humidity. rGO stands for reduced Graphene Oxide, USY stands for Ultra-Stable Y zeolite, Meso-ZSM-5 stands for mesopore-modified zeolite, AC stands for activated carbon, and DZ200 stands for desilicated zeolite.

presents a summary of the research on the use of precious metal oxide catalysts for the catalytic oxidation of formaldehyde.

2.1. Metal Oxide Supports

Precious metals are loaded onto metal oxide supports for HCHO catalytic oxidation through impregnation methods, one-pot methods, or other preparation techniques. Here is an introduction to researchers’ findings. In Liu’s work [79], after pre-treating SiO₂, Pd was loaded onto it through the impregnation method. After the acid-base etching and H₂ treatment, as can be seen from Figure 2a, the broad diffraction peak centered at 22° is an amorphous silica structure, and the two diffraction peaks at 40.1° and 46.7° correspond to the metal Pd species. Figure 2b shows that the oxygen vacancies increase, and Figure 2c–f shows that the defect anchoring inhibits sintering and promotes the high dispersion of Pd. This study has, for the first time, revealed the water inhibition phenomenon: the traditional understanding has been overturned, indicating that what determines the catalytic activity is the ability to generate hydroxyl groups, rather than the total amount of hydroxyl groups. As shown in Figure 3, after pretreatment of the SiO₂ support with H₂ reduction, HNO₃, or NH₃·H₂O solution, the catalytic performance of all Pd/SiO₂ catalysts in formaldehyde oxidation was significantly enhanced. Specifically, the removal efficiencies were 4% for Pd/SiO₂-R, 59% for Pd/SiO₂(HNO₃)-R, 94% for Pd/SiO₂(NH₃·H₂O)-R, and 100% for Pd/SiO₂(H₂)-R. Among them, Pd/SiO₂(H₂)-R demonstrated the highest activity for HCHO oxidation.

Chen et al. [77] prepared the 1Pd-Ba/TiO₂ catalyst using the impregnation method. As can be seen from the Figure 4, after adding the alkaline earth metal Ba, both the conversion removal efficiency of formaldehyde (13.8% to 100% at 25 °C) and the yield of carbon dioxide (0 to 100% at 25 °C) increased significantly, there are a large number of single-atom Pd (marked in red in Figure 5g), which is attributed to the fact that Ba inhibited sintering. This is a significant improvement for noble metal catalysts. From Figure 6, it can be observed that new Pd^δ− (334.5 eV, 51.2%) has been formed, which is due to the electron transfer from Ba-O-Pd and a significant increase in chemisorbed oxygen (22.5–35.4%). Alkali metals (such as Na⁺ and K⁺) have been proven to enhance activity through the electron donor effect. However, the mechanism of action of alkaline earth metals (such as Ba and Ca) in the oxidation of formaldehyde remains unexplored. In the future, research can be conducted on how to utilize alkaline earth metals (Ba) as electronic auxiliaries to regulate the electronic structure of noble metal atoms, stabilize single atoms/tuples, and optimize the O₂ activation pathway, thereby achieving efficient room-temperature formaldehyde oxidation without increasing the number of noble metals.

Peng et al. [81] prepared Pt/ZrO₂ by a modified P123-assisted hydrothermal method [89]. P123 is the abbreviation of polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer (PEO₂₀-PPO₇₀-PEO₂₀), which is a nonionic surfactant commonly used as a template for the synthesis of mesoporous molecular sieves (such as SBA-15). Figure 7 indicates that the optimized Pt-V_O-ZrO₂ with Pt contents of 0.87 wt.% could display HCHO removal and HCHO conversion (>95%) for 720 min at 20 °C, higher than that of Pt-ZrO₂ and ZrO₂. Moreover, the concentration of HCHO could be reduced from about 1 ppm to below 0.05 ppm after 580 min. This work first proposed the oxygen vacancy (V_O)-controlled surface metal-support interaction (SMSI) interface (Pt-V_O-ZrO₂) for room-temperature formaldehyde oxidation. Furthermore, it was found that V_O on the surface of ZrO₂ captures and activates O₂, generating O* (with an activation energy of only 0.3 eV). O* acts as an electron transport channel, accelerating the oxidation path of HCHO → formate → CO₂.

Zhang et al. [82] prepared the Li-Ag-CoO_x catalyst by the co-precipitation method and investigated the effects of Li⁺, Na⁺, and K⁺ on the catalytic oxidation of formaldehyde by Ag-CoO_x. It was found that Li⁺ was the only significant alkaline metal that enhanced the activity (Na⁺ inhibited, K⁺ had a limited effect) of Li-5Ag-Co₃O₄ from Figure 8d. Therefore, alkali metal additives do not necessarily enhance the performance of all catalysts. The specific situation must be explored through experiments.

Tang et al. [83] prepared the Ag/MnO_X catalyst by a one-step method at room temperature. By adjusting the molar ratio of Ag/Mn (0.1–1), they optimized the contents of Mn⁴⁺ and Ag⁰, thereby regulating the active sites. As shown in Figure 9, the Ag/MnO_X-0.5 removal rate reaches 100% within 3 h.

Lu et al. [88] synthesized the hollow sea urchin-shaped Ag-5K/Co₃O₄-MnO₂ through template self-assembly and an impregnation method, as shown in Figure 10a. It achieved a complete conversion of 100 ppm HCHO at 50 °C, and it can maintain a 100% conversion rate even after continuous operation for 100 h. This study achieved efficient catalytic oxidation of formaldehyde (50 °C, 100% conversion) with low Ag loading (3 wt.%) through the design of a hollow sea urchin structure, K regulation of Ag electronic state, and Co-Mn synergy. However, as can be seen from Figure 10b–e, under other reaction conditions, Ag-5K/Co₃O₄-MnO₂ does not have priority. Therefore, the additional amount of the alkali metal catalyst needs to be explored under specific experimental conditions.

2.2. Non-Metal Oxide Supports

Compared with metal oxide carriers, non-metal oxide carriers have the advantages of high specific surface area, high thermal stability, and low cost of raw materials. They can provide an ideal platform for precious metals with low loading, high activity, and long lifespan. They have also been widely used in the oxidation of formaldehyde reactions. Liu et al. [74] prepared a Pd/USY catalyst with a large specific surface area using the acid treatment and impregnation methods. As can be seen from Figure 11, after being treated with 0.2 M HCl, the Pd/USY catalyst shows a smaller Pd particle size than the others. The Figure 12 shows that the catalyst treated with 0.2 M HCl can completely oxidize formaldehyde into carbon dioxide and water with 6 h under the conditions of 25 °C, 150 ppm HCHO, 20% O₂, 35% RH, and WHSV = 150,000 mL/(g·h). This work is the first to apply the acid washing, aluminum removal, and hydroxylation strategy to USY molecular sieves, achieving efficient formaldehyde oxidation at room temperature. In the future, it can be extended to other molecular sieves (Beta, ZSM-5) or metal oxides (TiO₂, CeO₂) to verify its universality.

Guo et al. [71] employed a one-step solvent heat, impregnation, and calcination combined process to prepare a Pt-CeO₂/N-rGO composite catalyst. As shown in Figure 13, the conversion rate of 100 ppm HCHO by Pt-CeO₂/N-rGO reached 100% within 1 h at 30 °C and RH of 35. After continuous operation for 48 h, the activity still remained above 80%. This work achieves the synergistic goal of complete oxidation at room temperature and no secondary pollution through a three-pronged strategy of defect engineering, electronic coupling, and microstructure regulation, with low content (0.7%) of precious metals.

Nguyen et al. [72] used Ag nanoparticles as the active phase to systematically investigate the key role of mesoporous scale regulation in the oxidation of formaldehyde (HCHO) using ZSM-5 molecular sieve as the carrier. Through a three-step method of alkali de-silication, ion exchange, and chemical reduction, they constructed the Ag/Meso-ZSM-5 and Ag/ZSM-5 control systems. Figure 14 shows the relationship between the complete conversion rate of HCHO and temperature in the Ag/ZSM-5 and Ag/Meso-ZSM-5 catalysts under a relative humidity of 65%. In both Ag/Meso-ZSM-5 and Ag/ZSM-5, the catalyst activity was not observed until the temperature exceeded 120 °C. When the temperature exceeded 120 °C, Ag/Meso-ZSM-5 exhibited better catalytic performance than Ag/ZSM-5. Particularly at 200 °C, the proportion of HCHO converting to CO₂ on Ag/Meso-ZSM-5 reached 70%, while the conversion rate shown by Ag/ZSM-5 was only 20%. This work first quantitatively correlated diffusion limitation with adsorption-desorption behavior, providing a reference structure-activity relationship for catalytic reactions of HCHO at low- and medium-temperature. Therefore, it can be seen that for catalyst carriers of the molecular sieve type, pre-treatment can significantly improve the catalyst’s activity.

Li et al. [73] broke away from the traditional single active site-surface L-H mechanism approach and for the first time systematically applied the concept of tandem catalysis to the oxidation of low-concentration HCHO, using acidic ZSM-5 zeolite as the upstream activator and Ag/SBA-15 as the downstream oxidizer, as show in the Figure 15a, achieving a 100% conversion rate at 65 °C, with a 50-fold increase in activity compared to a single-functional Ag catalyst, and a significant reduction in the number of precious metals used, and the Figure 15c shows that the optimal mass ratio of ZSM-5 zeolite to Ag/SBA-15 is 1:4. This work provides a new paradigm for the treatment of low-temperature HCHO from the four aspects of design strategy, mechanism discovery, structure-activity relationship, and theoretical verification.

3. Transition/Rare Earth Metal Catalysts

Although precious metal catalysts have high activity and selectivity at specific temperatures, their own prices are very high. Moreover, under certain harsh reaction conditions, such as high temperature, high pressure, and strong corrosive reaction environments, the active components of precious metal catalysts may undergo phenomena like sintering, poisoning, or loss, resulting in the deactivation of the catalyst. For instance, in a sulfur-containing environment at high temperature, sulfur atoms may combine with the active components of the precious metal catalyst, causing poisoning of the catalyst and reducing its catalytic performance. In contrast, transition metal catalysts are inexpensive, abundantly available, easy to prepare, highly stable, and have wide applications in VOC treatment. Usually, researchers first examine the catalytic performance of a single transition metal oxide catalyst. If the performance is not satisfactory, they then attempt to incorporate other metal elements such as copper (Cu), zirconium (Zr), cerium (Ce), etc., onto this single metal oxide. Furthermore, some studies have also utilized rare earth metal oxides such as cerium oxide (CeO₂) for the catalytic oxidation of formaldehyde.

Table 2. Summary of the catalytic oxidation of formaldehyde using transition-metal oxide catalysts.

Catalysts	Reaction Conditions	Conversion	Ref.
NiTi-LDH	10 ppm HCHO, 20 vol.% O2, WHSV = 75,000 mL/(g·h), RH = 35%	99.2% at RT	[90]
MnO2·MnO	100 ppm HCHO, 21 vol% O2, GHSV =120 L/(g·h), RH = 50%	100% at 70	[20]
Ce0.2Mn-P	310–325 mg/m3 HCHO, RH = 20–23%	~83% at RT	[26]
3D-δ-MnO2	100 ppm HCHO, RH = 55%, 21% O2, GHSV = 90 L/(g·h)	100% at RT	[91]
K0.02Co0.98Cr2O4	200 ppm HCHO, 20 vol% O2, WHSV = 60,000 mL/(g·h)	100% at 150	[92]
Rb-MnO2	~200 ppm HCHO	~97% at 36	[93]
R-Bir	10 ppm HCHO, GHSV = 60 L/(g·h)	100% at RT	[94]
MnOx-S-A	∼1 ppm HCHO, GHSV = 150 L/(g·h), RH = 55%	>95% at RT	[95]
1-MnCo2O4	50 ppm HCHO, flow rate = 50 mL/min, 21 vol. % O2	100% at 90	[96]
1-CuMn2O4	50 ppm HCHO, flow rate = 50 mL/min,	100% at RT	[97]
Co3O4	10–12 ppm HCHO, GHSV = 60,000 mL/(g·h)	>99% at RT	[98]
Am-MnO2-AC	100 ppm HCHO	100% at 100	[99]
MnOx/Sep-H	100 ppm HCHO, RH = 50%, GHSV = 6000 mL/(g·h)	100% at 85	[100]
δ-MnO2@GO-RT	100 ppm HCHO, RH =50%, GHSV =72 L/(g·h)	100% at RT	[101]
Co-N/C-1000	100 ppm HCHO, GHSV = 72,000 mL/(g·h)	92.8% at RT	[102]
CeO2@MnO2-5	300 ppm HCHO, 21% O2, RH = 30%, WHSV = 36,000 mL/(g·h)	100% at 40	[103]
MnO2@Co3O4-45	200 ppm HCHO, 21 vol% O2, RH = 30%, WHSV = 36,000 mL/(g·min)	100% at 60	[104]
MnO2-3K	500 ppm HCHO, 20 vol% O2, RH = 70%, GHSV =60,000 mL/(g·h)	100% at 90	[105]
1.0Co3O4@NC-HC	150 ppm HCHO, RH = 50%	96.5% at RT	[106]

NiTi-LDH stands for Nickel titanium layered double hydroxide, R-Bir stands for Reduced Birnessite, Am-MnO2-AC stands for Amorphous Manganese Dioxide Loaded on Activated Carbon, δ-MnO2@GO-RT stands for δ-MnO₂ Nanosheets Anchored on Graphene Oxide.

presents a summary of the research on the use of transition-metal oxide catalysts for the catalytic oxidation of formaldehyde. However, it cannot be concluded whether the catalytic activity of the composite metal oxide is definitely better than that of the single metal.

3.1. Single Metal Oxide Catalysts

Under what circumstances should a single metal oxide catalyst be selected? The answer is high temperature (>80 °C), short operating period (<24 h), and high concentration (instantaneous concentration 200–1000 ppm). Jiao et al. [98] employed a two-step method of electrospinning and temperature-controlled sintering to prepare a Co₃O₄ porous nanofiber monolithic catalyst (labeled as Co-300 °C). As shown in Figure 16a,b, Co-300 °C exhibited the highest catalytic oxidation activity for HCHO, achieved a removal rate of 99% within 60 min, and maintained a conversion rate of 81.6% within 1600 min.

He et al. [95] conducted a systematic study on how to achieve efficient adsorption and catalytic oxidation of formaldehyde at room temperature by regulating the specific surface area and surface-active oxygen content of MnOx. As shown in Figure 17a,c, MnOx-S-A completely removes HCHO (~1 ppm) within 1.25 h, and after 5 cycles, it still maintains a 100% removal rate. Figure 7b shows that the adsorption capacity of MnOx-S-A is as high as 75% in the initial 0.5 h, which is much better than other samples. In addition, Figure 17d underwent dynamic testing, the result shows that MnOx-S-A maintains a removal rate of >95% for 9 consecutive hours under GHSV = 150 L/(g · h) and ~1 ppm HCHO; In contrast, the activity of other catalysts rapidly decreases. In this article, it is proposed to induce the oxidation-reduction cycle of Mn³⁺/Mn²⁺ through acid treatment, simultaneously enhancing the density of oxygen vacancies, surface reactive oxygen species, and the stability of the mesoporous structure. As shown in Figure 16, MnO_x-S-A performed the best under all test conditions, featuring high activity at room temperature, high stability, and high adsorption capacity. MnO_x-S-A is a type of high specific surface area manganese oxide that regulates oxygen vacancies and reactive oxygen content through acid treatment. It possesses extremely strong adsorption capacity, room-temperature catalytic activity, and long-term stability, and has the potential for large-scale application in air purifiers, household filters, and vehicle deodorization modules. These three catalysts are all based on MnO₂-KL and synthesized using different ratios and hydrothermal temperatures.

Hua et al. [107] investigated the influence of the hydrothermal synthesis temperature on the performance of Co₃O₄ in catalyzing the oxidation of gaseous formaldehyde. As shown in Figure 18, the activity of the catalyst was tested under different reaction conditions. The Co₃O₄-RT catalytic activity is the best in an environment with low HCHO concentration, low flow rate, and no water. The Co₃O₄ synthesized at room temperature (Co₃O₄-RT) exhibits excellent catalytic oxidation performance for formaldehyde at low temperatures due to its high Co³⁺ content, abundant oxygen vacancies, good oxygen migration property, and exposed (111) active crystal faces. Moreover, it is cost-effective and has practical application potential.

Chang et al. [108] prepared three different morphologies of cerium oxide (CeO₂) by the hydrothermal method and systematically studied their performance differences in the catalytic oxidation of formaldehyde (HCHO). As shown in Figure 19, the CeO₂-S achieved the highest conversion rate 87% at 120 °C, demonstrating the optimal activity. The CeO₂ spherical nanosheet aggregates (CeO₂-S) exhibit a high specific surface area, abundant oxygen vacancies, and surface hydroxyl groups. At 120 °C, they achieve an 87% conversion rate of formaldehyde and can be considered suitable for various scenarios of low-temperature formaldehyde purification.

3.2. Composite Metal Oxide Catalysts

The composite metal oxide outperforms the single metal oxide in terms of three key indicators: low-temperature activity, long-term stability, and resistance to water poisoning. It is the preferred solution for catalytic oxidation of formaldehyde at room temperature. Xu et al. [109] synthesized a series of La_1-xMnO₃ (x = 0–0.4) perovskites via the sol-gel method, introducing A-site cation vacancies (V_La) by adjusting the La/Mn ratio. In Figure 20 and Figure 21, EPR spectra and XPS spectra confirmed that the concentration of defect-induced oxygen vacancies (V_O) increases linearly with the increase of La defects (reaching a peak at x = 0.3), but excessive defects (x = 0.4) lead to structural collapse and phase separation of Mn₃O₄. This study elucidates that A-site cation defects synergistically enhance the low-temperature oxidation activity of La_1-xMnO₃ perovskite towards HCHO by regulating the concentration of oxygen vacancies, electronic structure, and surface-active oxygen species, providing a new paradigm of defect engineering for the design of low-cost and high-efficiency air purification catalysts. Future efforts are needed to further optimize the low-temperature activity (<100 °C) and large-scale preparation process.

Yan et al. [110] adopted a three-step method involving co-precipitation, calcination, and NaBH₄ reduction to construct Mn-doped cobalt oxide (R-Mn_xCo_3−xO₄). Figure 22 shows that Mn ions entered the Co₃O₄ lattice to replace Co sites, inducing lattice distortion. Figure 23 confirmed that Mn doping reduction increased the Vo concentration to 5.77% (atomic ratio) and the proportion of surface adsorbed oxygen (O_ads) to 42.2% (compared to only 15.1% in Co₃O₄). This study elucidated the intrinsic correlation between oxygen vacancy concentration, electronic structure, and catalytic activity, proposing a synergistic strategy of “atomic doping reduction defect engineering”. This provides a universal design approach for developing non-precious metal, low-cost, high-stability catalysts for low-temperature HCHO purification, combining theoretical value with industrialization potential.

Yu et al. [111] employed a hydrothermal co-precipitation-in-situ redox method to anchor MnO_x (δ-MnO₂, Mn₃O₄) onto MgAl-LDH layered supports with varying Mg/Al ratios (1:1, 3:1, 5:1). As shown in the Figure 24, the 10Mn/Mg₃Al₁-LDH achieved 100% HCHO removal and a 45% CO₂ yield at 80 °C, outperforming the 5 wt.% and 20 wt.% samples. 10Mn/Mg₃Al₁-LDH, with hydrotalcite carrier-manganese oxide-hydroxyl synergy at its core, exhibits excellent HCHO oxidation performance under low temperature, high humidity, and high space velocity conditions, combining low cost with scalability potential. In the future, further optimization of carrier lattice defects and MnO_X-carrier interface electronic coupling is needed to reduce the complete oxidation temperature to <60 °C, and to expand to the synergistic purification of other VOCs.

Zheng et al. [112] employed a hydrothermal-impregnation method to anchor active components with varying Mn/Ce ratios (1:1–5:1) onto anatase TiO₂ with exposed {001} (Ti-NS) and {101} (Ti-NP) crystal faces. Figure 25 revealed the following activity sequence: 5Mn1Ce/Ti-NP > 5Mn1Ce/Ti-NS, achieving a HCHO conversion rate of 98.5% at 127 °C with the lowest activation energy (80.55 kJ/mol). Figure 26 and Figure 27 indicate that the Ti-NP-supported catalyst possessed higher surface chemisorbed oxygen (28.25%) and stronger Lewis acidic sites, facilitating HCHO adsorption and the stepwise oxidation of dioxymethylene (DOM) to formate to CO₂. The study elucidated that the crystal faces of the support dominate the reaction pathway and energy barriers by regulating the electronic structure of the active components, oxygen vacancy density, and surface acidity. It proposed the counterintuitive concept that “the low-activity {101} crystal face actually constructs a high-activity Mn-Ce interface,” offering a universal strategy for optimizing non-precious metal catalysts through crystal face engineering, combining theoretical value with practical prospects.

4. Main Factors Affecting HCHO Oxidation Efficiency

The catalytic oxidation performance of formaldehyde is influenced by the preparation method of the catalyst, microscopic morphology and structure, active sites, surface oxygen species, and experimental conditions such as reaction temperature, humidity, and gas flow rate. In particular, humidity has a significant impact on the catalytic oxidation of formaldehyde.

4.1. Effect of Preparation Method

The catalysts prepared by different methods have different morphologies (exposing different crystal planes), and therefore have different catalytic activities. In the oxidation of formaldehyde, the catalytic activity is the combined effect of multiple physical and chemical properties of the catalyst, such as surface oxygen vacancies and specific surface area.

Li et al. [101] prepared MnO_X/Sep-I, MnO_X/Sep-H, and MnO_X/Sep-P by the impregnation, hydrothermal, and precipitation methods. It is a material synthesis method that is widely used nowadays. As shown in Figure 28a, the comparison of catalyst activity among the three different preparation methods reveals that Sep-H has the best activity (100% at 85 °C). Figure 28b–d indicate the catalytic activity of Sep-H under different concentrations, humidity, and space velocities. The data show that as the concentration, humidity, and space velocity increase, a higher temperature is required to achieve 100% conversion. Figure 29 represent the H₂-TPR, O₂-TPD, and CO₂-TPD of three catalysts. From the H₂-TPR profiles in Figure 29a, the low-temperature peak of Sep-H is the lowest (202 °C), indicating that its surface oxygen is the easiest to be reduced, which is beneficial for the oxidation of HCHO. The O₂-TPD spectrum in Figure 28b shows that Sep-H has a lower-temperature peak, indicating that its surface oxygen activation ability is the strongest and it is the key active site for the reaction. The CO₂-TPD spectrum in Figure 28c shows that the peak temperature of Sep-H is the lowest (201 °C) and the intensity is the highest, which is conducive to the desorption of CO₂ products and prevents the blocking of active sites, thereby improving stability. Figure 30a represents the XPS full spectrum of the catalyst, which is used to confirm all the elements contained in the catalyst. Figure 30b shows that the three oxygen species are lattice oxygen, adsorbed oxygen, and hydroxyl; Figure 30c is the Mn 3s spectrum, which is used to calculate the active oxygen species and obtain the data Sep-H as the highest 2.31, thereby generating more Mn vacancies; Figure 30d is the Mn 2p spectrum, indicating the ratio of Mn³⁺/Mn⁴⁺, and from the fitted data it is concluded that Sep-H’s Mn³⁺/Mn⁴⁺ = 1.67:1, indicating that a high proportion of Mn³⁺ can promote the formation of oxygen vacancies.

It happens that there is a similar case, Li et al. [102] prepared δ-MnO₂ by room-temperature co-precipitation, room-temperature in-situ growth method, and hydrothermal method. The highlight of this article is that by introducing GO, the agglomeration of MnO₂ is effectively suppressed, resulting in a uniform dispersed nanorod structure, thereby maximizing the exposure of catalytic sites. Moreover, the 2D structure shortens the diffusion path of HCHO molecules and reduces the diffusion resistance, thereby enhancing the reaction mass transfer efficiency. Additionally, the synergistic effect between GO and MnO₂ promotes the activation of O₂ and H₂O, thereby enabling the catalyst to have abundant surface reactive oxygen (ROS) and hydroxyl (-OH). Figure 31a,b shows that δ-MnO₂@GO-RT has the best activity, maintaining a 100% conversion rate for over 400 min and with stable CO₂ production. However, the conversion rates of other samples decrease significantly over time and have poor stability. Figure 31c,d indicates that δ-MnO₂@GO-RT performs best at a relative humidity of 50%. This is because too low a humidity (<50%) will cause insufficient surface-OH, while too high a humidity (>75%) will lead to competition for adsorption by water molecules and occupation of active sites, both of which will result in poor performance. Figure 32a shows that all samples exhibit the δ-MnO₂ (birnessite) characteristic peaks (12.3°, 36.8°, 65.7°). However, the peaks of δ-MnO₂-RT and δ-MnO₂@GO-RT are weaker, indicating an amorphous structure and more defects. Figure 32b shows that all surface samples exhibit Mn-O vibration peaks in the range of 499–637 cm⁻¹, and the peak intensity of δ-MnO₂@GO-RT is the weakest and red-shifted, indicating that the Mn-O bond is weaker and more oxygen vacancies are formed. The Mn 2p spectra in Figure 32c,d show that Mn²⁺, Mn^3+, and Mn⁴⁺ coexist, and the proportion of Mn³⁺ in δ-MnO₂@GO-RT is the highest (0.48), confirming the maximum number of oxygen vacancies and the strongest activity. As shown in Figure 33a, the reduction peak temperature of δ-MnO₂@GO-RT is the lowest (276 °C), indicating that it is the most easily reducible and the active oxygen species are more easily released; and Figure 33b also shows that the oxygen desorption temperature of δ-MnO₂@GO-RT is the lowest (150 °C), with the most surface active oxygen species (O₂⁻, O⁻); Figure 33c indicates that the proportion of adsorbed oxygen (Oads) of δ-MnO₂@GO-RT is the highest (0.48), confirming that it has the most oxygen vacancies; Figure 33d shows the adsorption ability of formaldehyde with the catalyst, and the desorption temperature of HCHO from δ-MnO₂@GO-RT is the highest (107 °C), indicating that the adsorption capacity of HCHO is the strongest.

Wang et al. [113] compared the performance differences in the complete oxidation of formaldehyde (HCHO) at low temperatures between MnO_X-CeO₂ catalysts prepared by the two-step hydrothermal (HT) method and the deposition-precipitation (DP) method. As shown in Figure 34a,b, the MnCe (HT) prepared by the hydrothermal method can achieve 100% HCHO conversion at 90 °C. Its activity is 6 times that of the DP method, and it maintains no deactivation during continuous operation for 30 h at 75 °C, demonstrating excellent stability. Figure 34c shows that when the humidity is 5%, the activity of MnCe (HT) decreases by 10% at 75 °C, and there is almost no effect at 100 °C, indicating that the deactivation is reversible and is mainly due to competitive adsorption of water vapor. Figure 34d indicates the regeneration ability of the catalyst. After running at 50 °C for 10 h, MnCe (HT) can fully restore its activity when heated to 300 °C. Thus, it can be concluded that the MnCe (HT) catalyst prepared by the hydrothermal method has low-temperature high activity, moisture resistance, and reversible regeneration capability. As shown in Figure 35a,b, the peak intensity ratio I₆₄₆/I₄₅₈ of Mn-O (646 cm⁻¹) to Ce-O (458 cm⁻¹) for MnCe (HT) and MnCe (DP) is 1.22 and 0.85, respectively. This indicates that the surface of MnCe (HT) has a higher content of Mn-O and more active sites; subsequently, Figure 35c,d confirmed that the MnO_X-CeO₂(HT) sample exhibits a higher content of surface-active oxygen and a lower lattice oxygen reduction temperature (352 °C vs. 425 °C), indicating stronger oxygen migration capability.

Figure 36 indicates that the MnO_X-CeO₂(HT) sample has a higher content of Mn⁴⁺ on its surface (49.3% vs. 22.5%), providing more active centers for HCHO oxidation; a higher proportion of Ce³⁺ (44.9% vs. 20.5%), which induces a large number of oxygen vacancies and chemically adsorbed oxygen (O_α, 58.6% vs. 48.1%) through the synergistic effect of Ce⁴⁺ + Mn³⁺ → Ce³⁺ + Mn⁴⁺.

4.2. Effect of Reaction Conditions

Different experimental conditions (temperature and humidity, space velocity or flow rate, and the initial HCHO concentration) can lead to different conversion rates. The following provides a summary.

4.2.1. Temperature and Humidity

On one hand, the change in temperature directly regulates the migration ability of surface lattice oxygen and the reaction energy barrier; on the other hand, humidity can either promote the activation of HCHO through hydroxylation or inhibit the performance due to competitive adsorption and pore blockage. Therefore, it is crucial to systematically clarify the effect of temperature-humidity coupling on the catalytic oxidation efficiency of formaldehyde. Xie et al. [114,115] studied the activity of different catalysts within the humidity range of 10–100%. The results (Figure 37a,b) show that increasing the RH from 10–100% and 50–75% improved removal efficiency, which is because surface hydroxyl promotes the activation of HCHO. However, when RH > 50%, the HCHO removal efficiency slightly decreases, which could be attributed to the excessive production of OH groups that partially covered the catalyst surfaces, resulting in less HCHO adsorption/activation. From another perspective, as can be seen from Figure 37, when the humidity remains constant, the HCHO removal efficiency increases as the temperature rises. Chen [90] studied the HCHO removal efficiency at humidity ranges from 35–75% over NiTi-LDH (shown in Figure 37c). Their results show that a HCHO removal efficiency of over 90% can still be maintained at 30–75% humidity (99.2% at 35% RH, 97.4% at 60% RH, 93.2% at 75% RH). Gao [92] demonstrated the effect of temperature on the catalytic process. As shown in Figure 37d, the results showed that the HCHO removal efficiency increased with the rise in temperature until complete removal was achieved. In conclusion, generally, the catalytic activity increases with the rise in temperature and also increases with the increase in humidity within a certain humidity range.

4.2.2. Space Velocity or Total Flow Rate

In the large-scale application of HCHO catalytic oxidation in a fixed bed, the gas space velocity (GHSV) or flow rate often becomes the key factor determining the mass transfer-reaction equilibrium, if the space velocity is too low, the reactants will have a longer residence time on the catalyst surface, which is beneficial for deep oxidation but results in an increase in side reactions; if the space velocity is too high, due to the shortened contact time and increased external diffusion resistance, the penetration phenomenon is prone to occur, and the activity will rapidly decline. Hua [96] studied the effect of flow rate on the HCHO conversion rate (flow rates of 50, 100, 200, 300, and 400 mL/min) over 1-MnCo₂O₄, Figure 38a show that the conversion rate of formaldehyde over 1-MnCo₂O₄ at room temperature is as follows: 100% at 50 mL/min (7022 mL/(g·h)) > 31% at 100 mL/min (14,043 mL/(g·h)) > 21% at 200 mL/min (28,087 mL/(g·h)) > 15% at 300 mL/min (42,130 mL/(g·h)) > 14% at 400 mL/min (56,173 mL/(g·h)). Li [101] studied the effect of different GHSV(3000, 6000, 12,000, 30,000 mL/(g·h)) on MnO_X/Sep-H catalyst activity at 100 ppm of HCHO and RH of 50% (Figure 38b). results are as follows: 100% at 70 °C and 3000 mL/(g·h)) > 100% at ~80 °C and 6000 mL/(g·h)) > 100% at ~95 °C and 12,000 mL/(g·h)) > 100% at ~120 °C and 30,000 mL/(g·h)).

4.2.3. Initial HCHO Concentration

In real indoor air scenarios, the HCHO concentration often fluctuates dramatically within the range of several tens to several hundred parts per million (ppm), depending on the season, ventilation mode, and the intensity of pollution sources. This poses an extensive concentration window adaptability requirement for catalytic oxidation systems. Hua [96] studied the effect of different initial HCHO concentrations (50, 250, 500 ppm) on 1-MnCo₂O₄ catalyst activity. Figure 39a shows that the HCHO is completely converted at 30 °C,90 °C, and 100 °C for 50, 250, and 500 ppm, respectively. Analogously, Hua [97] studied the effect of different initial HCHO concentrations (50, 250, 500 ppm) on 1-CuMn₂O₄ catalyst activity (Figure 39b), the HCHO complete conversion at 30 °C, 100°C, and 100 °C for 50, 250, 500 ppm, respectively. In conclusion, the conversion rate of formaldehyde decreases as the initial concentration of formaldehyde increases.

4.2.4. Catalyst Dosage

The amount of catalyst not only directly affects the economic efficiency and operating cost of the reactor, but also has a significant impact on the catalytic activity. An appropriate amount of catalyst can ensure that reactants have sufficient contact with the active sites, thereby increasing the reaction rate and conversion rate; while an excessive or insufficient amount of catalyst may lead to the waste of active sites or incomplete conversion of reactants. Therefore, in-depth exploration of the influence of catalyst dosage on the catalytic oxidation performance of formaldehyde has important scientific significance and application value for the catalytic system. Hua [96] studied the effect of different catalyst dosages (20,40,60 mg) on 1-MnCo₂O₄ catalyst activity at room temperature, as shown in Figure 40a, the catalytic activity sequence is 100% at 60 mg, 28.8% at 40 mg, and 5.7% at 20 mg. Zhang [82] studied the effect of different catalyst dosage (0.05, 0.1,0.2 g) on Ag₅-LCO-I catalyst activity at room temperature. Figure 40c shows that the HCHO removal is ~20%, ~50%, ~60% for 0.05 g, 0.1 g, and 0.2 g, respectively. Hua [97] studied the effect of different catalyst dosages (30, 60, 120 mg) on 1-CuMn₂O₄ catalyst activity at room temperature. The catalytic activity sequence was 100% at 60 mg and 120 mg, ~20% at 30 mg (Figure 40b). Increasing the amount of catalyst can enhance the removal rate of formaldehyde. However, there is a saturation point beyond which the improvement in removal rate becomes limited.

In conclusion, an increase in temperature, moderate humidity (30–75%), low space velocity, low initial concentration, and an appropriate amount of catalyst are conducive to the catalytic oxidation of formaldehyde.

5. Reaction Mechanism

A clear and definite catalytic mechanism for the oxidation of formaldehyde can not only clarify the catalytic performance but also enable researchers to better understand the reaction process and provide guidance for designing efficient catalysts to eliminate formaldehyde. Currently, discussions on the mechanism of catalytic oxidation of formaldehyde have been relatively complete. There are mainly reaction mechanisms for the catalyst system of precious metals and metal oxides.

In the catalytic reaction mechanism, the Eley-Rideal (E-R) mechanism, the Langmuir-Hinshelwood (L-H) mechanism, and the Mars-Van Krevelen (MVK) mechanism respectively correspond to three classic surface reaction models. In the oxidation reaction of formaldehyde, if HCHO and O₂ share the same adsorption sites, the reaction mainly proceeds through the E-R mechanism; When the adsorption of HCHO and O₂ occurs at separate sites (such as hydroxyl groups and active metals), the reaction proceeds through the L-H mechanism. Additionally, when oxygen vacancies are easily formed on the catalyst surface (such as the oxygen storage-discharge cycle of CeO₂), the main reaction mechanism is the MVK mechanism [75,76,116,117].

5.1. Reaction Mechanism on Noble Metal Catalysts

The mechanism of catalytic oxidation of formaldehyde involves multiple steps, including the adsorption, activation, oxidation reaction of formaldehyde molecules, and the desorption of the final products. A thorough understanding of these steps is crucial for the design and optimization of catalysts. Zeng et al. [118] proposed a mechanism for the catalytic oxidation of HCHO over the Pt/NiO-HNF catalyst, as shown in Figure 41. Firstly, HCHO is adsorbed onto the OH groups on the surface of the catalyst through hydrogen bonds, while the O₂ molecule is adsorbed onto the Pt nanoparticles and subsequently activated to generate reactive oxygen species. Subsequently, one of the oxygen species participates in the oxidation of HCHO, converting it into dioxymethylene (DOM). Another reactive oxygen species then interacts with one of the hydrogen atoms in DOM to form an OH group, further promoting the oxidation of DOM into formic acid salt substances. Finally, under the action of the active OH group, the formic acid salt substances undergo further oxidation and decomposition, ultimately generating CO₂ and H₂O, which are released from the catalyst surface and thereby restoring the active sites on the catalyst. Ye [119] proposed a mechanism for the catalytic oxidation of HCHO over the Pt-Au/TiO₂ dual noble metal catalyst. Firstly, gold nanoparticles can activate the surface oxygen species on titanium dioxide, while formaldehyde molecules are adsorbed onto the active oxygen species on the titanium dioxide surface and rapidly transformed into dissolved organic matter (DOM) species. Meanwhile, oxygen molecules are adsorbed onto Pt nanoparticles and are activated. Then, the activated oxygen can further oxidize the DOM into formate species. Subsequently, the formate species are completely oxidized to the adsorbed CO₂ and H₂O under the action of the active oxygen species generated by the Pt nanoparticles. Jiang [45] proposed a mechanism for the catalytic oxidation of HCHO over the Ag/CeO₂ catalyst. Interestingly, during the adsorption process, the adsorption occurs through the oxygen vacancy sites on CeO₂ rather than through hydrogen bonding; moreover, the main intermediates formed for different morphologies of cerium oxide are also different. For example, on the CeO₂-com surface, HCHO mainly forms DOM intermediates, while on the CeO₂-rod surface, due to the presence of oxygen vacancies, HCHO is more likely to form formate intermediates. This is quite novel and slightly different from previous studies. Analogously, Bu [50] proposed a mechanism for the catalytic oxidation of HCHO over the Au/CeO₂ catalyst. He mentioned that the strong interaction between Au and CeO₂ leads to the formation of Au³⁺ and more oxygen vacancies at the interface, which greatly enhances the activation of formaldehyde. In addition, the strong Au-CeO₂ interaction promotes the release of a large amount of active surface oxygen, enhancing the ability to form formate species.

In conclusion, first of all, we can observe that the adsorption methods of different materials may vary. For instance, they may be adsorbed through hydrogen bonds or at the oxygen vacancies on the surface. Secondly, different precious metals have different functions. For example, Au can enhance adsorption through interaction with the carrier and form more oxygen vacancies, promoting the release of a large amount of active surface oxygen, while Pt can activate oxygen molecules and provide more oxygen species to facilitate the oxidation reaction.

5.2. Reaction Mechanism on Transition/Rare Earth Metal Catalysts

After discussing the reaction mechanism of the catalytic oxidation of formaldehyde by precious metal catalysts, this chapter will systematically review the interactions between the surface-active sites of non-precious metal catalysts and the formaldehyde molecules, revealing the formation and transformation pathways of reaction intermediates, as well as the generation and desorption processes of the final products. These studies are helpful in understanding the microscopic mechanism of non-precious metal catalysts in the formaldehyde oxidation reaction.

Chang [108] proposed a mechanism for the catalytic oxidation of HCHO over the CeO₂ catalyst. It is worth noting, as shown in Figure 42, that they provided a detailed explanation of the three adsorption behaviors of the formaldehyde material surface rather than just one adsorption behavior. The intermediates generated by different adsorption behaviors are slightly different, which is similar to the catalytic oxidation mechanism of formaldehyde by the aforementioned noble metal catalysts. Firstly, HCHO adsorbs onto the surface of CeO₂ through hydrogen bonding with the OH on the surface, generating monodentate acids and hydroxide ions. Secondly, HCHO is adsorbed on the bridging sites on the surface of CeO₂ by the interaction with the C and O atoms in HCHO and the O and Ce atoms in CeO₂. This forms a bidentate acid. Thirdly, the oxygen atom in HCHO will insert into the oxygen vacancy, causing the C-H bond to break and forming the CHO group. Subsequently, the surface oxygen species reacts with CHO to form a monodentate acid. Zheng [120] proposed a mechanism for the catalytic oxidation of HCHO over the Mn/Ti catalyst. They summarized the entire process of formaldehyde catalytic oxidation into a 10-step cycle and pointed out that the MVK reaction pathway was carried out on the Mn/Ti surface. Lu [121] proposed a mechanism for the catalytic oxidation of HCHO over the CeO₂@MnO₂-5 catalyst. They proposed the reaction mechanisms under both water and non-water conditions. Different conditions led to different adsorption behaviors. In the presence of water, adsorption occurred through hydrogen bonding with the surface hydroxyl groups. In the absence of water, adsorption took place through surface-active oxygen species and Mn sites. The former oxidizes the entire process through the surface hydroxyl groups, while the latter oxidizes the entire process through the surface adsorption of oxygen. Fang [122] proposed a mechanism for the catalytic oxidation of HCHO over the MnO₂-Iso catalyst. They explained that oxygen vacancies activate oxygen molecules to generate reactive oxygen species. These reactive oxygen species react with water to form hydroxyl groups, and ultimately, under the action of hydroxyl groups, they are oxidized into carbon dioxide and water.

6. Conclusions and Outlook

This review provides the latest research in the catalytic oxidation of HCHO over noble metal and Transition oxide catalysts, and the factors affecting the catalytic performance of the catalyst and mechanisms have been discussed. Future research on catalyst development should focus on exploring novel materials and synthesis methods to improve the catalytic performance. For example, designing catalysts with hierarchical structures, core-shell architectures, or nano-sized particles can enhance the surface area and active sites. Additionally, combining experimental studies with theoretical calculations can provide deeper insights into the reaction mechanisms and guide the rational design of catalysts.

For the mechanism study of the HCHO catalytic oxidation process, the first step of the entire catalytic oxidation process is adsorption. This step is in line with the three classic adsorption models (MVK, L-H, E-R). In this paper, we propose a summary that in the presence of water, surface active oxygen species can generate hydroxyl groups. The formaldehyde molecule is adsorbed by hydrogen bonds with surface hydroxyl groups. When there is strong metal interaction, it may adsorb on oxygen vacancies or the bridging points of certain material surface atoms. The entire mechanism process can be summarized as follows: Formaldehyde adsorption, formaldehyde reacts with reactive oxygen species to form DOM, which is then oxidized into formate species. Finally, it may directly generate carbon dioxide and water, or it may first generate carbon monoxide and then be oxidized to indirectly generate carbon dioxide.

In summary, the conclusion is as follows:

• Elevating the temperature is conducive to the catalytic oxidation of formaldehyde. This is not only beneficial for the single aspect of formaldehyde catalytic oxidation, but it can also be extended to the field of catalysis
• Regarding humidity, in the field of catalytic oxidation of formaldehyde, the optimal humidity range is between 30% and 75%. However, this conclusion cannot yet be drawn for other reactions
• Regarding the initial concentration of target pollutants, it is generally believed that the lower the initial concentration, the easier it is to remove. Space velocity is also a major factor affecting catalytic efficiency. When the space velocity is low, the contact time between reactants and catalysts is longer, which is conducive to a more thorough reaction
• The amount of catalyst used is generally appropriate at 100–200 mg, and an increase in dosage does not have a particularly significant impact on the catalytic oxidation efficiency of formaldehyde

Strategies for improving catalyst stability:

• Surface modification and functionalization, the introduction of specific functional groups or protective layers on the surface of catalysts, can improve their resistance to corrosion and poisoning.
• Optimization of carrier structure, by regulating the pore structure, surface defects, or morphology of the carrier, the interaction between the active component and the carrier can be enhanced, inhibiting its migration and agglomeration.
• Doping modification, introducing heteroatoms (such as transition metals or non-metallic elements) into the catalyst, can optimize the electronic structure of the active sites, inhibit sintering, and agglomeration.
• Core-shell structure design, constructing a structure with an active component as the core and an inert material as the shell (such as a carbon shell or an oxide shell) can effectively isolate the reactants from the active sites, preventing their loss or poisoning.

However, there are still some challenges in the field of catalytic oxidation of formaldehyde and catalytic oxidation in general. Therefore, we propose the following outlook for the future:

• Upgrade of catalyst system: noble metals moving towards single atomization: Au, Pt, Pd, etc., will be anchored on defective carriers such as TiO₂, CeO₂, MnO₂, etc., in the form of single atoms or sub-nanometer clusters, capable of completely oxidizing formaldehyde at room temperature or even below 0 °C.
• Defect engineering of transition metal oxides: Utilizing oxygen vacancies, lattice distortions, and interfacial heterojunctions (Co₃O₄-CeO₂, MnO_x-Fe₂O₃) to enhance the concentration of active oxygen species, achieving noble metal-free or low noble metal formulations.
• MOF/COF-derived porous catalysts: By pyrolyzing precursors such as ZIF-8 and MIL-100, N-doped carbon-coated metal nanoparticles are prepared, featuring both high specific surface area and hierarchical pores, addressing the issue of mass transfer limitations.
• Photo-thermal synergistic catalysis: Coupling visible light-responsive Ag/TiO_2-x with traditional thermal catalysis, utilizing surface plasmon localized heating to reduce the apparent activation energy, and achieving room temperature natural light-driven formaldehyde oxidation.