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Review

Non-Noble Metal Catalysts for Efficient Formaldehyde Removal at Room Temperature

by
Yiqing Feng
and
Rui Wang
*
School of Environmental Science and Engineering, Shandong University, Qingdao 266237, China
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(8), 723; https://doi.org/10.3390/catal15080723
Submission received: 14 June 2025 / Revised: 26 July 2025 / Accepted: 28 July 2025 / Published: 30 July 2025
(This article belongs to the Special Issue Environmentally Friendly Catalysis for Green Future)
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Abstract

This review examines the research progress on non-noble-metal-based catalysts for formaldehyde (HCHO) oxidation at room temperature. It begins with an introduction to the hazards of HCHO as an indoor pollutant and the urgency of its removal, comparing several HCHO removal technologies and highlighting the advantages of room-temperature catalytic oxidation. It delves into the classification, preparation methods, and regulation strategies for non-precious metal catalysts, with a focus on manganese-based, cobalt-based, and other transition metal-based catalysts. The effects of catalyst preparation methods, morphological structure, and specific surface area on catalytic performance are discussed, and the catalytic oxidation mechanisms of HCHO, including the Eley–Rideal, Langmuir–Hinshelwood, and Mars–van Krevelen mechanisms, are analyzed. Finally, the challenges faced by non-precious metal catalysts are summarized, such as issues related to the powder form of catalysts in practical applications, lower catalytic activity at room temperature, and insufficient research in the presence of multiple VOC molecules. Suggestions for future research directions are also provided.

1. Introduction

Formaldehyde (HCHO) is becoming one of the most concerning indoor pollutants, originating from building materials, decoration materials, furniture, and consumables [1,2,3]. Based on formaldehyde-inhalation-induced squamous cell carcinoma (SCC) in rats and human cancers, the International Agency for Research on Cancer (IARC) classifies HCHO as Group 1 (human carcinogens), because the carcinogenic disability adjusted life (DALY) factor of HCHO is much higher than that of indoor pollutants such as ammonia, benzene, or toluene [4,5]. Therefore, removing indoor HCHO is an urgent issue at present in order to ensure public health security. Currently, several approaches exist for eliminating HCHO, including adsorption [6,7,8], photocatalytic oxidation [9,10], plasma techniques [11,12], and thermal or low-temperature catalytic oxidation [13,14,15]. Among the above technologies, adsorption can only temporarily fix HCHO on the surface of porous materials, which may cause secondary pollution during desorption. For photocatalytic oxidation, plasma techniques, and thermal catalytic oxidation, external devices such as light sources, plasma emission devices, and heat sources may be used in practical operations, which may increase processing costs and energy consumption. For low-temperature catalysis, especially at room temperature, it is increasingly favored as it can efficiently convert HCHO into CO2 and H2O without the need for external devices.
The existing HCHO oxidation catalysts at room temperature are divided into two categories: noble metal catalysts and non-noble metal catalysts. Noble metal catalysts typically contain Au, Ag, and platinum group elements (such as Pt, Rh, Pd, Ru, etc.) [16,17,18,19], while noble metal catalysts are generally replaced by non-noble metals such as Co and Mn [1,2,20,21], or materials such as graphene [22,23] and carbon nanotubes [24,25]. Although the use of noble metal catalysts can achieve good results in HCHO removal, the low content in the Earth and high price of noble metals determine that noble metal catalysts cannot be widely applied. Non-noble metal catalysts, with their wide availability and low cost, are showing their potential in replacing noble metal catalysts for eliminating HCHO. In this work, a review on room temperature catalysts for HCHO oxidation, non-noble metal catalyst classification, preparation methods, and regulation strategies, the mechanism of room temperature catalytic oxidation of HCHO, current research progress, challenges, and research directions are systematically discussed.

2. Catalytic Materials

Sekine et al. [26] first used synthesized MnO2 to efficiently degrade indoor HCHO. Subsequently, research on the use of transition metal oxides for HCHO oxidation has received widespread attention. Transition metal oxides have shown promising application prospects in the removal of volatile organic compounds due to their low cost and abundant reserves. So far, researchers have developed a large number of transition metal oxide-containing elements such as Mn, Co, and Cr. Among them, MnOx and Co3O4 can be doped with other metal elements such as Ce, Sn, Cu, and Zr. As research deepens, the types of elements that can remove HCHO will become increasingly diverse.

2.1. Manganese-Based Catalysts

Mn-based catalysts have multiple valence states (+2, +3, +4, +6, +7, etc.), strong oxygen storage/release capacity, and excellent redox performance, and are considered among the most promising transition metal oxide catalysts [27]. Manganese compounds with different valence states have different physical and chemical properties and catalytic performance, and are therefore widely used in the removal of volatile organic compounds (VOCs) [28].
Since Sekine et al. [26] assembled activated carbon and MnOx into a purification material, researchers have been more concerned with MnO2 catalysts because their catalytic activity toward HCHO is stronger than that of other transition metal oxides. like Ag2O, TiO2, CeO2, CoO, CuO, and V2O5 [29]. In fact, MnO2 contains several structural forms, such as α, β, γ, δ, and ε. Among them, MnOx catalysts with [MnO6] octahedral structures have different physical and chemical properties, such as stronger adsorption desorption ability, more active sites, and larger surface area, which in turn determine the adsorption effect of the catalyst [3]. Zhang et al. [30] prepared α-, β-, γ-, and δ-MnO2 catalysts and compared their activity in catalyzing HCHO. They found that the δ-MnO2 catalyst had the best activity and could achieve almost complete HCHO conversion at lower temperatures. They also pointed out that it was the special two-dimensional layer tunnel structure, abundant active oxygen species, and lattice oxygen species on the δ-MnO2 surface that led to the highest activity of the catalyst. Han et al. [31] synthesized a δ-MnO2 catalyst for the catalytic oxidation of HCHO. Due to the highly defective structure of the catalyst, abundant hydroxyl groups, abundant lattice oxygen, and Mn3+ species, it has an almost 100% removal rate for HCHO. In addition to preparing the above structural forms, Wen et al. [32] prepared a MnOx catalyst (MnOx-PMMA-SSM) with 3D nanocomposite structure, which achieved a HCHO removal efficiency of up to 98.82% after 110 min at room temperature, and also had a long service life. The tunnel structure will also affect the catalytic effects of catalysts on HCHO to a certain extent. As shown in Figure 1, due to different tunnel structures, there are some differences in the treatment effects of catalysts on HCHO, which are reflected in three completely different curves [33]. Except for regulating the tunnel structure, Tao et al. [34] prepared a special interconnected network structure M-MnO2 for HCHO oxidation, consisting of δ-MnO2 nanosheets and c-MnOOH nanowires, using a one-step hydrothermal method. After 10 h of oxidation of 100 ppm HCHO, its catalytic activity reached 50%. The special network structure provides sufficient surface active oxygen species and surface-adsorbed water, exhibiting excellent catalytic activity and stability toward HCHO at room temperature. It can be seen that for a single form of manganese-based catalyst, δ-MnO2 has outstanding performance, with its unique structure and active oxygen species, and has become an excellent HCHO catalyst.
In addition to single MnO2-based catalysts, doping noble metals, rare earth elements, and transition or ordinary metal elements with MnO2 to form a solid solution can increase the surface active sites, specific surface area, and redox ability of the catalyst, thereby enhancing the treatment effect on HCHO. Among them, CeO2 is a good oxygen storage material. When CeO2 is doped with MnO2 in a certain proportion as a solid solution, this may change the oxidation state of Mn, thereby altering the physical and chemical properties of the catalyst itself. Zhang et al. [35] synthesized MnCeOx and analyzed the properties of the solid solution and the influence of water vapor on the catalytic effect of HCHO. As shown in Figure 2a, with the increase in processing time, MnCeOx has a significant improvement in the conversion rate of HCHO and ultimately stabilizes at around 80%. The results show that the catalytic performance of the MnCeOx catalyst is better than those of MnO2 and CeO2 alone, and even better than that of the mixture of MnO2 and CeO2. This is because the solid solution exhibits relatively abundant oxygen vacancies and the most surface lattice oxygen species, significantly enhancing the adsorption and redox performance. Additionally, from Figure 2b, we can observe that the catalyst can completely convert HCHO into CO2 under conditions of relative humidity above 50%, which may be suitable for indoor air purification. In addition to combining Mn and Ce elements, Duan et al. [36] added Ag to prepare an Ag/Mn/CeO2 catalyst for catalytic oxidation of low-concentration HCHO at room temperature. They showed that the introduction of Ag did not change the mesoporous structure of the Mn/CeO2 catalyst, but reduced the pore size and specific surface area; meanwhile, the introduction of Ag increased the lattice defects and oxygen vacancies generated by the interaction between Ag and Mn/CeO2. Therefore, the catalyst achieved a removal efficiency of 96.76% within 22 h, exhibiting high catalytic activity toward HCHO at room temperature.
Also, Xing et al. [37] synthesized a new type of K-Mn3O4@CeO2 catalyst. It achieved a conversion rate of 99.3% for 10 mL of 40 mg/L HCHO solution at 30 °C, while also exhibiting strong cycling stability. Based on DFT calculations, it was found that K provides more active oxygen species and richer oxygen vacancies on the catalyst surface, enhancing the fluidity of lattice oxygen and the room temperature reduction performance of oxygen species, resulting in lower oxygen vacancy formation and HCHO adsorption energy in aqueous solution. Liu et al. [38] prepared MnCeNiOx (MCN) and treated it with FeOx using the impregnation method to form an F-MCN catalyst. The results showed that under dry conditions, the F-MCN catalyst exhibited better HCHO conversion ability than the MCN and M2CN catalysts, due to the rich Mn3+ and Ce3+ increasing the oxygen vacancies, surface adsorption strength, and SBET, as well as strong interactions between Mn-Ce, Ce-Ni, and Fe-Mn mixed crystals. In addition, under wet conditions, the addition of water vapor enhances surface hydroxyl radicals, leading to complete oxidation of HCHO on the F-MCN catalyst at room temperature. Furthermore, the stability demonstrated by the 72 h continuous experiment shows the enormous potential for application of this new catalyst.
Mn can also be mixed with more components to synthesize more diverse HCHO catalysts. For instance, Jia et al. [39] developed a three-dimensional radiation nanostructure catalyst (PtSA-MnOOH/MnO2) self-assembled by Pt single-atom anchored MnOOH/MnO2 nanosheets using a two-step electrochemical method. Through the synergistic effect of active hydroxyl groups (OH*) and active oxygen species (O2*) and the decomposition of intermediates, a long-term continuous HCHO removal rate (about 98%) was achieved within 100 h in a 15 ppm HCHO atmosphere at 25 °C and an hourly rate of 30000 mL/gcat·h. Also, Zhu et al. [40] synthesized a series of CeO2-x-MnOx catalysts using Ce metal organic frameworks (MOFs) as precursors, and found that the gases released during the MOF pyrolysis process significantly affect the valence states of Ce and Mn, which is a key factor in catalytic activity. Furthermore, DFT results indicate that oxygen vacancies not only effectively suppress the charge loss of Mn and Ce atoms, but also significantly enhance the adsorption strength of CeO2-x-MnOx-2.5 for HCHO and O2, thereby achieving a catalytic conversion rate of 63% at room temperature. In addition, Wang et al. [41] synthesized a Mn/HZSM-5 catalyst using an HZSM-5 molecular sieve as a carrier. The activity test results showed that the catalyst with 0.4% Mn/HZSM-5 had an HCHO removal rate close to 100% under stable conditions of 30 °C and 20 h. The analysis results also showed that Mn4+ and hydroxyl groups can provide more active sites for HCHO oxidation, and the interaction between the two enables HCHO to have good activity during the reaction process, effectively activating HCHO and oxygen. In addition, Mn4+ increases the concentration of -OH on the catalyst surface, making it easier for the catalyst to adsorb and oxidize HCHO. Another interesting case is the one in which Wang et al. [42] used Al-Si-fiber-woven ceramic filter paper (CFP) as the substrate, Mn(NO3)2 as the oxidant, and glycine as the fuel, and in situ-coated Mn2O3 on it to prepare a Mn2O3/CFP catalyst. The Mn2O3/CFP catalyst was characterized by its more porous structure, higher oxidation-reduction ability, abundant surface active oxygen, and enhanced surface lattice oxygen mobility. The Mn2O3/CFP catalyst exhibits excellent HCHO oxidation performance with a high conversion rate (90%) of HCHO at room temperature. In addition, in the durability test conducted under room temperature dynamic flow mode, the Mn2O3/CFP catalyst maintained a high HCHO conversion rate of 66% within 11 days, demonstrating its potential in practical indoor air purification applications.
Generally speaking, manganese-based catalysts have shown great potential in the field of HCHO catalytic oxidation. Manganese’s variable valence state and rich redox performance provide a broad space for catalyst design. From a single form of MnO2 to doping modification and then to special preparation methods, by accurately adjusting the composition and structure of Mn-based catalysts, the catalytic oxidation efficiency of HCHO at room temperature can be effectively improved, providing an efficient solution for indoor air HCHO pollution control.

2.2. Cobalt-Based Catalysts

Cobalt, as a common transition metal, has been explored by researchers for its potential application in low-temperature HCHO catalytic oxidation. Co3O4, as a common cobalt oxide, has various forms including tubular, sheet-like, strip, rod, spherical, and porous [43,44,45]. It is precisely its diverse forms and structures that promote different chemical reactions and catalytic abilities due to the surface exposure of active sites. Bai et al. [46] synthesized nano-Co3O4, 2D-Co3O4, and 3D-Co3O4 catalysts and compared their performance in oxidizing HCHO. From the TEM image in Figure 3, it can be seen that nano-Co3O4 is composed of single crystals without pores, while 2D-Co3O4 and 3D-Co3O4 have polycrystalline pore walls, and all catalysts exhibit ordered mesopores while retaining the template structure during synthesis. Therefore, the catalytic activity order is 3D-Co3O4 > 2D-Co3O4 > nano-Co3O4, because 2D-Co3O4 and 3D-Co3O4 retain the mesoporous characteristics and channel structure of the hard template, and have a large specific surface area. In particular, 3D-Co3O4 contains abundant surface active oxygen species and the surface contains more Co3+, which improves HCHO oxidation ability. Additionally Jiao et al. [47] synthesized Co3O4 porous nanofiber monolithic catalysts using heat-resistant polyacrylonitrile (PAN) as a template through electrospinning. At room temperature (10–12 ppm, 25 °C) and with a gas space velocity of 60000 mL·g−1 h−1, the catalytic activity for HCHO exceeded 99%. At the same time, the authors used catalysts prepared by direct calcination as controls to further demonstrate the catalytic oxidation ability and stability of porous nanofiber catalysts. In general, for Co3O4, its different morphologies and structures (such as nano, two-dimensional, three-dimensional, etc.) have a significant impact on the catalytic activity. Studies show that Co3O4 catalysts with porous structures and high specific surface areas have higher HCHO oxidation ability.
In addition to utilizing Co3O4, synthesizing cobalt single atom catalysts (SACs) is also an interesting strategy, as SACs can achieve 100% atomic utilization and possess unique electronic/geometric structures [48]. Du et al. [49] synthesized a single Co atom on ultra-thin carbon nanosheets (Co-SAs/NCS), which has a two-dimensional graphene-like morphology, high specific surface area, and enhanced nitrogen doping level, providing many active sites to firmly fix separated Co atoms. Figure 4 intuitively demonstrates the excellent ability of the Co-SAs/NCS catalyst in both HCHO conversion rate and catalytic activity, showing excellent application prospects. Mechanism studies also indicate that the planar structure of Co-N4 enhances the ability to activate molecular oxygen, resulting in a removal rate of 86% for 500 ppm HCHO at room temperature. Zhang et al. [50] prepared a novel Co-SAC that can completely convert 50 ppm of HCHO at room temperature, and the catalyst remains stable for at least 120 h. DFT calculations and isotope experiments reveal that the excellent formic acid intermediate decomposition ability of Co-SACs is a key factor in obtaining excellent HCHO oxidation performance, which determines the kinetic rate of HCHO catalytic oxidation reactions. In conclusion, cobalt SACs have attracted much attention due to their 100% atomic utilization and unique electronic/geometric structure. Their special planar structure can enhance the activation ability of molecular oxygen, showing high removal rate and good stability for HCHO at room temperature.
In summary, cobalt-based catalysts have great application potential in the field of low-temperature HCHO catalytic oxidation. The above studies show that the catalytic oxidation performance of cobalt-based catalysts for HCHO can be effectively improved by adjusting the morphology, structure, and composition of cobalt-based catalysts, which provides a variety of options for cobalt’s application in indoor air HCHO purification.

2.3. Other Transition Metal-Based Catalysts

Besides Mn-based and Co-based catalysts, other transition metal-based catalysts are gradually receiving attention from researchers. The addition of other transition non-noble metals, such as Fe, Ni, Zn, etc., can not only replace the use of noble metals, but also enhance catalytic performance. Another reason is that they are easy to obtain and reduce costs. For example, Chen et al. [1] synthesized a series of layered double hydroxides (LDHs) containing multiple bimetallic species (MgAl, ZnAl, NiAl, NiFe, and NiTi), among which the NiTi-LDH catalyst exhibited a nearly 100% HCHO removal rate and excellent water resistance and chemical stability. The abundant hydroxyl groups in LDH directly bind with HCHO to generate CO2 and H2O, even in the absence of O2 and H2O. Additionally, the coexistence of O2 effectively reduces the reaction barrier for the dissociation of H2O molecules, facilitating the formation of hydroxyl groups and their subsequent backfilling on the catalyst surface. Yang et al. [51] synthesized P-CexZr1-xOy (PCZ) mesoporous nanoparticle catalysts using Pluronic P123 as a template. The templating caused by P123 and the effective doping of Zr atoms adjusted the oxygen vacancies in the catalyst, changed the reduction characteristics and chemical state of cerium, and formed a Ce3+/Ce4+ redox ring pair, successfully achieving a 100% conversion rate of HCHO at room temperature. Furthermore, it is also a good idea to synthesize catalysts using transition metals or non-metals as carriers. Wang et al. [52] developed a Pt-Ni bimetallic catalyst using γ-Al2O3 as a carrier, with a conversion rate of 97% at room temperature and a stability of 100 h. The presence of Pt sites stimulated more surface-adsorbed oxygen, improving the conversion rate of HCHO, and the doping of Ni helps to adsorb hydroxyl groups, thereby providing sustained activity. In addition, Zhou et al. [53] synthesized CuNi alloy carbon layer core–shell nanoparticles (CuNi@C) to achieve efficient conversion from formaldehyde to hydrogen. The H2 generation rate of CN(1:1) is 110.98 mmol·g−1. For H2, the values for Cu@C and Ni@C are 1.47 times and 4.93 times, respectively. On the one hand, this is due to the synergistic effects of Cu and Ni metals on the alloy resulting in very high catalytic performance, with Cu generating a large number of active substances for the decomposition of HCHO, and Ni having excellent H2 evolution performance. On the other hand, the carbon layer of CuNi@C protects Cu and Ni from excessive oxidation, greatly enhancing their stability.
The synthesis of HCHO catalysts using MOFs materials as carriers is also receiving more attention. For example, Duan et al. [54] added MOF UiO-66 to synthesize an Ag@Zr-TiO2-1000U6 catalyst during the process of doping TiO2 with Zr and achieved a degradation rate of 83.4% for HCHO. The doped Zr replaced Ti4+ in TiO2 in the form of Zr4+, resulting in a left shift of the TiO2 peak and a decrease in bandgap energy. The composite material of Zr-TiO2 and ZrO2 effectively increases the pore volume of the catalyst and provides more active sites. Furthermore, Zhang et al. [55] used defective MIL-88B (Fe) nanorods as Fe-containing MOFs catalysts to activate molecular oxygen and trigger effective catalytic oxidation of HCHO. Defective MIL-88B (Fe) nanorods grow along the [001] direction, exposing a large number of coordinated unsaturated iron sites (Fe-cuss), sufficient for the diffusion of HCHO and O2. The Lewis acid–base interaction between Fe-cuss and HCHO, as well as the Fenton-like catalysis of O2, enable the adsorbed HCHO to be oxidized to CO2, with a relatively high mineralization efficiency (over 80%). These studies show that reasonable selection and modification of other transition metal-based catalysts can further improve the catalytic oxidation efficiency of HCHO at room temperature, and provide more possibilities for HCHO pollution control.

3. Factors Affecting Catalytic Performance

The catalytic oxidation process of HCHO largely depends on the activity of the catalyst, which in turn is related to factors such as preparation methods, morphological structure, and specific surface area.

3.1. Preparation Methods

The physicochemical properties of the catalyst determine its ability to catalyze the oxidation of HCHO at room temperature, while the synthesis method and reaction conditions also affect the physicochemical properties of catalysts. At present, the synthesis methods for HCHO catalytic oxidants include the sol-gel method [56], the hydrothermal method [57], electrostatic spinning [58], the electrochemical method [39], the precipitation and coprecipitation method [59], template casting [60], etc. Different improvement methods have different effects on the valence state, morphology, and specific surface area of catalyst elements (as shown in Figure 5), thereby affecting the activity of catalysts.
For example, Liu et al. [62] used an impregnation/carbonization process to synthesize PdZn alloy and a Pd-catalyst-modified 3D layered porous carbon (HPC) network catalyst from energetic MOFs of MET-6 as raw material. The extremely fine PdZn and Pd nanoparticles were uniformly distributed on or derocated in the HPC network, creating strong interaction between metal nanoparticles and HPC. The prepared HPC-PdZn has excellent HCHO catalytic performance at room temperature, and its hydrogen production rate is four times that of the pure Pd catalyst. Wang et al. [63] used freeze-drying-pyrolysis technology to prepare alkali-modified hierarchical porous Na-CoOx/CN materials using SAP/ZIF-67 composite materials as templates, achieving engineering of the pore structure and surface properties. The presence of Na+ and CO32− plays a crucial role in the formation of the layered porous structure of the catalyst, the enhancement of the surface area, and the formation of active hydroxyl groups, enabling the complete conversion of 1 mg/m3 HCHO at room temperature at a high air velocity of 240000 mL/(gcat h), and outstanding catalytic stability at higher air velocities (480000 mL/(gcat h)). Furthermore, Sun et al. [64] prepared a series of MnOx/γ-Al2O3 catalysts by the impregnation method and treated them with acid, alkali, and H2O2 to study the effects of different treatment conditions on the catalytic activity. The results showed that the conversion rate of the MnAl-II catalyst to HCHO after the acid treatment was close to 100% (O3/HCHO=2.0), because after the acid treatment, the better metal dispersion, smaller particle size, and surface microporous structure increased the surface area of the MnAl-II, and the corresponding active sites promoted the deep oxidation of the HCHO. In addition, the high Mn3+ and Oad content of the catalyst is conducive to the formation of oxygen vacancies, accelerating the decomposition of O3 and the ozonation process of HCHO.
Except for different reaction conditions, Liu et al. [65] synthesized a series of ε-MnO2/Mn2V2O7 composite catalysts at different V/Mn ratios. Different V/Mn ratios have a significant impact on catalytic performance: when the V/Mn molar ratio is 0.03, the degradation rate of 10 mL of 10 mg/L HCHO by the catalyst is 37.0%; when the V/Mn molar ratio is 0.06, the degradation rate of HCHO by the catalyst increases to 72.0%, which is attributed to the addition of a certain amount of V increasing the specific surface area of the catalyst. The reaction temperature can also affect the effectiveness of HCHO catalysts. Wang et al. [66] synthesized a series of layered MnO catalysts at different calcination temperatures. When the calcination temperature was below 300 °C, the catalyst took on a spherical shape, and when the calcination temperature was increased to 500 °C, the morphology of the catalyst transformed into a flower-like structure composed of nanorod clusters. Although the change in shape increased the specific surface area of the catalyst, evaluation experiments showed that an increase in calcination temperature led to a decrease in the internal water content of the catalyst, resulting in a decrease in the conversion rate of formaldehyde from 83.7% (S-30) to 43.1% (S-300).
Therefore, the preparation method has a significant effect on the performance of non-noble metal catalysts. Common methods include the impregnation method, precipitation method, hydrothermal method, and so on. Each method optimizes the catalytic performance by affecting the physical and chemical properties of the catalysts. These methods can improve the catalytic oxidation efficiency of HCHO by accurately controlling the preparation conditions and adjusting the active sites and redox performance of the catalysts.

3.2. Catalyst Morphological Structure

The morphology and structure of catalysts are of great significance for improving their activity. By regulating the morphology and structure of catalysts, various factors such as pore size, specific surface area, and surface active sites can be changed. So far, various forms such as nanotubes, nanofibers, nanosheets, nanobelts, nanoflowers, and nanoscale prism structures have been developed. Wang et al. [67] prepared nanorod-like, wire-like, tubular α-MnO2, and flower-like spherical Mn2O3 using hydrothermal and CCl4 solution methods. The oxygen radical concentration and low-temperature reducibility were in the order of rod-like α-MnO2 > tubular α-MnO2 > flower-like Mn2O3 > filamentous α-MnO2, consistent with the catalytic performance of the samples. Meanwhile, surface active sites can affect the catalytic activity of catalysts. Li et al. [68] prepared MnOx/C composite catalysts with abundant oxygen vacancies. After adding fructose to the synthesized raw materials, the crystal structure of the catalyst changed and the oxygen vacancies increased, providing more adsorption sites.
Changing the catalyst structure or constructing crystal defects can also enhance the catalytic activities of catalysts. Liao et al. [69] prepared six prisms (SP), short six pyramids (SSP), and long six pyramids (LSP) ZnO with different defect contents using low-temperature liquid-phase method. The results showed that due to the presence of donor defects, LSP ZnO had a high degradation efficiency for HCHO after 10 h. At the same time, crystal morphology also had an impact on catalysis, and the differences in morphology and defect content jointly led to different catalytic performances for ZnO. In addition, Yu et al. [70] synthesized MnO2 nanostructured catalysts with cocoon-like, urchin-like, and nest-like morphologies using a simple method. For example, Figure 6 shows TEM images of different Pt/urchin-like MnO2 catalysts with varied Pt loadings. It can be observed that as the Pt content increases, the overall size of the catalyst increases (Figure 6c,f,i,l). This is due to the increase in precursor concentration (H2PtCl6·6H2O) during the loading process, which causes changes in the formation process. Compared with mesoporous cocoon-like and sea urchin-like MnO2, nest-like MnO2 has higher catalytic oxidation activity due to its higher dispersion of metal ions and specific surface area.
So, the morphological structure of HCHO catalysts has an important influence on their catalytic performance. A variety of catalysts have been developed, such as nanotubes, nanofibers, and so on. The pore size, specific surface area, and surface active sites can be changed by adjusting the structure. The common point of these studies is to enhance the adsorption capacity of HCHO and the number of active sites by reasonably designing the morphology and structure of the catalysts, so as to improve the catalytic oxidation performance.

3.3. Specific Surface Area

The specific surface area has a significant impact on catalysts’ ability to oxidize HCHO. A higher specific surface area can increase the probability of capturing HCHO and provide more active sites and oxygen vacancies, which to some extent determines the catalytic conversion rate of HCHO, so an important regulatory strategy is to develop HCHO catalysts with high specific surface area. Tian et al. [56] showed that compared with other catalysts synthesized in their experimental system, Mn-based catalysts with a birnesite (sodalite) structure and high specific surface area (154 m2/g) exhibited better catalytic activity. Similarly, the MnO2 catalyst synthesized by Sekine et al. [26] with a specific surface area of 163 m2/g exhibited higher HCHO oxidation activity.
Fang et al. [71] prepared porous manganese oxide nanowires (MnOx NWs) with large surface area and multiple manganese valence states using a simple electrospinning thermal calcination potassium permanganate solution post-treatment (C/S process) method. Subsequently, silver-oxide-coated silver nanowires were synthesized with the assistance of oxygen plasma (Ag@Ag2O NWs). The nanowire diameters and BET surface areas of untreated MnOx NWs, C-MnOx NWs, S-MnOx NWs, and C/S-MnOx NWs were measured under the experimental conditions. It was found that the specific surface areas of C-MnOx NWs and C/S-MnOx NWs are higher than those of other nanowires. Generally speaking, a higher specific surface area can provide more oxidation reaction sites and improve the activity of the catalyst. Experimental results also showed that the catalytic conversion rate of HCHO is positively correlated with the specific surface area of the catalyst. Wu et al. [72] prepared Co3O4 nano necklaces and studied the state of the mesoporous structure by regulating calcination conditions. It was found that when the temperature exceeded 500 °C, the mesoporous structure and high specific surface area of the catalyst were disrupted, resulting in a decrease in catalyst activity. Additionally, Tian et al. [73] synthesized different forms of MnOx of cryptomelane and tested the catalytic activity of a catalyst with a specific surface area of 206 m2/g. They found that the catalytic ability was higher than that of a nanorod cryptomelane catalyst with a specific surface area of 68 m2/g, indicating that an increase in specific surface area has a certain promoting effect on the improvement of catalytic ability.

4. Catalytic Oxidation Mechanism of HCHO

Understanding the catalytic oxidation mechanism of HCHO is of great significance for developing low-cost, high-stability, and high-activity catalysts. To understand the mechanism of HCHO catalytic oxidation, it is necessary to detect the distribution of reactive oxygen species on the reaction intermediates and catalyst surfaces. Generally, changes in the type and content of reactive oxygen species are detected by electron paramagnetic resonance (EPR) or electron spin resonance (ESR) [74]. Additionally, the formation of oxygen vacancies and oxygen mobility are determined by H2 temperature programmed reduction (H2-TPR), O2 temperature programmed desorption (O2-TPD), and X-ray photoelectron spectroscopy (XPS), in order to infer the generation of active substances [75]. In addition, in situ Fourier transform infrared spectroscopy (In-suit FTIR) or in situ diffuse reflectance infrared Fourier transform spectroscopy (In-suit DRIFTS) are also commonly used to analyze the intermediates produced by catalytic reactions [76,77]. According to these characterization analysis methods, the catalytic oxidation mechanism of HCHO can be divided into the Eley–Rideal mechanism (E-R), Langmuir–Hinshelwood mechanism (L-H), and Mars–Van Krevelen mechanism (MVK) [78,79,80].

4.1. Eley–Rideal Mechanism

Firstly, the oxidation process takes place between the oxygen that is adsorbed on the catalyst surface and the gaseous reactants, and during this process, chemisorption occurs on the catalyst surface. Then, the products that are formed when the reactive oxygen species react with the reactants are desorbed from the catalyst surface. The above is a typical E−R reaction process. In line with the E−R reaction model, HCHO is capable of dissociating in the presence of reactive oxygen species that are produced by molecular oxygen on the catalyst surface. Zhou et al. [81] studied the Eley–Rideal (ER) mechanism of HCHO oxidation on the Ti/Ti3C2O2 catalyst through DFT calculation. In the ER mechanism, O2 molecules are first adsorbed and activated into two O atoms, and then HCHO molecules approach the activated O atoms. HCHO spontaneously dissociates into CO molecules on the Ti/Ti3C2O2 surface, releasing 4.05 eV of energy, which helps overcome the energy barrier (1.04 eV) of subsequent reaction steps. Key steps include O-O bond cleavage, HCHO dissociation into CO and H atoms, H atoms combining with O atoms to form *OH groups, hydrogen transfer between *OH groups to form H2O, and CO combining with O atoms to form CO2. The ER mechanism has a low energy span of 1.04 eV, showing good catalytic cycle efficiency. The significant energy released in the initial step can promote subsequent reactions even without external heating. Liu et al. [80] used density functional theory (DFT) to study the mechanism of the oxidation of HCHO by three doped-carbon-based single-atom catalysts, Fe/DV-N4, Co/DV-N4, and Pd/DV-N4. They found that the oxidation mechanism of HCHO is basically consistent with the Eley–Rideal (E-R) mechanism, as shown in Figure 7.

4.2. Langmuir–Hinshelwood Mechanism

In the L−H catalytic mechanism, the reaction between adsorbed oxygen species and HCHO molecules on the surface serves as the rate-controlling step. In order to clarify the specific process of this mechanism, many researchers have put forward their own insights. Mang et al. [82] proposed five possible reaction pathways (including three L-H mechanisms and two E-R mechanisms), and using the transition state search method and other thermodynamic and kinetic analyses, they found that pathway 1 of the L-H mechanism (HCHO+O2 → HCOOOH → HCOH+OH → CO2+H2O) is the most likely reaction pathway to occur because it requires the lowest reaction energy barrier. During the entire catalytic process, IM-Co-2 to IM-Co-5 in Co-MnO2, IM-Fe-2 to IM-Fe-5 in Fe-MnO2, and IM-Ni-2 to IM-Ni-5 in Ni-MnO2 have the highest energy barriers, and pathway 1 is considered the rate controlling step. In addition, Ding et al. [83] studied the L-H reaction mechanism of Pt/TiO2 catalyst and found five basic steps: HCHO* → HCO*, HCO* → CO*, H2O desorption, CO* → CO2*, and CO2 desorption. The potential energy diagram is shown in Figure 8. In the first step (IM1 → TS1 → IM2), the H atom on HCHO transfers to form HOO species, which overcomes the 412.5 KJ/mol energy barrier and releases 20.05 KJ/mol of heat. Second, HCO reacts with HOO to generate H2O and activated O in step IM2 → TS2 → IM3, which requires overcoming an energy barrier of 146.09 kJ/mol and releasing a reaction heat of −217.71 kJ/mol. Then, H2O desorbs directly from the surface with a desorption energy of 1.72 kJ/mol (IM3 → IM4). In the following step, CO reacts with adjacent O atoms to generate CO2, releasing 57.29 kJ/mol of heat (IM4 → TS3 → IM5). Finally, CO2 desorption from the catalyst surface (IM5 → TS) takes place. This reaction process shows that the HCHO* → HCO* step has the highest activation energy, of 412.50 kJ/mol, indicating that the reaction between adsorbed HCHO and O2 is the rate-determining step in HCHO oxidation.

4.3. Mars–Van Krevelen Mechanism

Compared with the previous two reaction mechanisms, the MVK mechanism, also known as the two-step oxidation-reduction model, appears more frequently in the process of metal oxide catalysts catalyzing HCHO. This reaction consists of two steps: the catalyst surface is oxidized by gas-phase molecular oxygen to form surface-adsorbed oxygen, and the surface-adsorbed oxygen subsequently reduces pollutants. Sekine et al. [26] believed that HCHO first adsorbs and oxidizes on metal oxide catalysts, and then forms formic acid intermediates on the surface and decomposes, before, finally, the intermediates decompose into H2O and CO2, as shown in the following formulas:
HCHO (g) + O (a) → HCHOO (a)
HCHOO (a) → O (a) + HCHO (g)
HCOO (a) → H (a) + CO2 (g)
2H (a) + O (a) → H2O (g)
where (g) represents the gaseous state, (a) represents the adsorbed state.
Through the above formulas, it can be found that formic acid intermediate is produced in the catalytic oxidation process of HCHO, which can be seen in many articles. For instance, Qian et al. [84] found that oxygen first adsorbed on the catalyst surface and reacted with acid sites and oxygen vacancies to generate superoxide radicals, which then reacted with water molecules adsorbed on the surface to generate hydroxyl radicals. Under the action of ·OH, HCHO decomposed into formate, which is gradually deeply oxidized into CO2 and H2O. Zhao et al. [85] found that doping alkali metal ions significantly increased the amount of surface -OH in MnOx catalysts. The -OH groups can form hydrogen bonds with hydrogen atoms in HCHO and react with intermediate products such as formate, accelerating the decomposition of formate and promoting the generation of bridging formic acid intermediate products, thereby improving the catalytic oxidation activity of HCHO.
The properties of surface active substances have a significant impact on catalytic activity and, therefore, the forms of active substances have received high levels of attention in analyzing reaction mechanisms. Zhang et al. [30] showed that the catalytic activity of four MnO2 catalysts (i.e., α, β, γ, and δ-MnO2) is closely related to the amount of surface lattice oxygen (relative to surface-adsorbed oxygen) on each of them. Zhang’s team [86] synthesized a range of zirconia quadrilateral monoclinic (TMZ) catalysts and clarified the HCHO catalytic oxidation mechanism using DFT calculations and DRIFTS measurements. Initially, active *H atoms transfer from HCHO to O2, forming OOH groups. Then, lattice O atoms and OOH groups from TMZ dissociate into active *O and *H atoms, generating HCOOH. Finally, active O atoms from the OOH group facilitate HCOOH’s conversion to CO2 and H2O. This reaction follows a novel D-H and MVK pathway, with the rate-determining step being the transfer of the H atom of HCHO to the O-Zr bond’s lattice O atom in TMZ.
Wang et al. [87] analyzed the catalytic cycle of HCHO oxidation on black manganese ore, as shown in Figure 9. The water contained in birnite forms hydrogen bonds with HCHO, and the adsorbed HCHO and its hydrate (CH2(OH)2) are oxidized by surface reactive oxygen species (O2−, O−, or terminal OH groups), producing oxygen vacancies (V0). Then, the dioxygen molecules in the air are adsorbed on V0 and dissociated (O2 + V0 → O2−, O−), and finally, O2− reacts with H2O (O2− + H2O → 2OH) to supplement the surface OH. In this cyclic reaction, K+ can compensate for the charge imbalance caused by the presence of manganese vacancies, promote the formation of surface reactive oxygen species, and become active sites for formaldehyde oxidation.
In summary, three main catalytic oxidation mechanisms are discussed: Eley–Rideal (E-R), Langmuir–Hinshelwood (L-H), and Mars van–Krevelen (MVK). These mechanisms explain the oxidation process of HCHO on the surfaces of the catalysts from different perspectives. They jointly reveal the complex reaction path of HCHO catalytic oxidation, and provide theoretical guidance for the design of the catalysts. Through the in-depth understanding of these mechanisms, the composition and structure of the catalysts can be more targeted and optimized, and their performance and stability in practical applications can be improved.

5. Deactivation and Regeneration of HCHO Catalysts

The long-term use of HCHO catalysts will change their activity and selectivity, leading to deactivation. The catalyst presents a large cost in the operation of a catalytic device, and deactivation undoubtedly incurs many additional expenses [88]. Bartholomew summarized the deactivation process of a catalyst and divided it into the following six situations: pollution or coking, poisoning, gas–solid and/or solid–solid reaction, thermal degradation, crushing, and vapor compound formation [89]. Coking is due to the side reactions on the catalyst surface, and the carbon-containing by-products generated cover the catalysts’ surface or block the pores of the catalysts. Poisoning is mostly caused by the loss of active sites due to the chemical adsorption of catalyst impurities [90,91]. Poisons can block the active sites or change the catalyst activity. At the same time, because the adsorption coefficient of poisons is higher than that of reactants, it is difficult for reactants to approach the active sites [91].
Of course, the catalyst for the catalytic oxidation of HCHO may also be deactivated due to other uncommon factors. For example, when dealing with HCHO gas containing chlorine, the interaction between chlorine and catalyst will make chlorine deposit on the surface of the catalyst, resulting in the deactivation of the catalyst. Unfortunately, almost all catalysts are affected by chlorine deposition [92], such as cerium oxide catalysts [93], vanadium-based catalysts [94], transition metal oxides, perovskite [95], and mixed metal catalysts. The presence of water may also lead to catalyst deactivation. Such as the competition between HCHO and H2O for active sites caused by high gas humidity. Once H2O molecules form a stable hydroxyl structure with the active site, the catalytic effect of HCHO is affected. Temperature and space velocity may also lead to catalyst deactivation.
In order to ensure the activity of the catalyst and reduce unnecessary cost consumption, the regeneration of the catalyst is an effective method. The regeneration of the catalyst is determined by the reversibility of the deactivation process. For example, deactivation caused by coking can be reversed, while that caused by sintering cannot. Toxicants can be selectively removed by mechanical, oxidation, or chemical washing [96]. Therefore, there are many methods for catalyst regeneration [78], such as heat treatment, pressure swing, chemical regeneration, and plasma [97,98]. For example, oxygen plasma and ozone injection can effectively regenerate Au/TiO2 catalysts [99]. Considering the cost, although it is feasible to use air as the discharge gas for plasma regeneration, the presence of N2 will generate nitrogen oxides and cause new poisoning. In order to solve this problem, the cold plasma technology of moist air came into being [98]. In a word, modern regeneration technology is gradually replacing the conventional heat treatment or mechanical treatment techniques.

6. Conclusions and Outlook

The catalytic oxidation of HCHO at room temperature is one of the major topics in the field of environmental catalysis. This article discusses the application of non-noble metal catalysts in the catalytic oxidation of HCHO, and summarizes the influence of experimental conditions and catalytic machines on the catalytic oxidation ability of HCHO. Among the reported non-precious-metal catalysts, Mn has attracted widespread attention due to its unique properties, such as abundant valence states, strong oxidation adsorption capacity, high abundance, acid resistance, low toxicity, low cost, and high environmental compatibility. There are currently three possible mechanisms for the catalytic oxidation of formaldehyde, among which the MVK mechanism is the most frequently discovered and widely accepted theory among researchers. An important feature of this mechanism is the conversion of HCHO carbonyl (-C=O) to carboxyl (-COOH), and the intermediate products are known to be formate, DOM, CO, etc., after in situ DRIFTS analysis. At the same time, hydroxyl (-OH) groups are generated on the surface of the catalyst with the participation of water vapor, promoting the adsorption of HCHO by the catalyst and, thus, improving the catalytic oxidation activity. The surface structure and properties of the catalyst also affect its catalytic ability.
Many catalysts for HCHO catalytic oxidation are often prepared as powders. However, in practical applications, powder catalysts can encounter significant air resistance, which affects their catalytic efficiency. To mitigate this, they are typically pressed into pellets or flakes. This process not only complicates the preparation method but also leads to the agglomeration of active component particles and their attachment to the carrier surface, thereby reducing the effective surface area. Additionally, non-noble metal oxide catalysts often exhibit low catalytic activity for HCHO oxidation at room temperature. Moreover, there is a lack of studies on the catalytic activity, deactivation, and regeneration of catalysts for HCHO catalytic oxidation in the presence of multiple VOC molecules.
Although non-precious-metal catalysts have made progress in catalyzing HCHO at room temperature, there are still some challenges that can be studied in the following areas:
1. Improve materials and prepare catalysts with higher specific surface area. At present, a considerable portion of catalysts exist in powder form, and the agglomeration of powders can reduce catalytic efficiency. Using nanomaterials as carriers or preparing catalysts at the nanoscale is a potential development direction. At the same time, when preparing novel catalysts, it is important to actively consider their feasibility for practical applications. Some current catalysts suffer from problems such as high pressure drop, low mechanical strength, and poor physical or chemical stability. Although some catalysts contain non-noble metal components, they only partially replace noble metal elements, which still results in high costs. Overall, the preparation of catalysts with low pressure loss, good mechanical strength and stability, and low cost is of great significance.
2. Change the experimental conditions to gain a deeper understanding of the mechanism of HCHO catalytic oxidation. It is of great significance to design HCHO catalysts for room temperature oxidation, and to gain a deeper understanding of the catalytic steps through appropriate experimental design, in order to regulate the quantity and structural defect morphology of surface active substances in the future. In addition, the deactivation mechanism of catalysts can be further explored. Many factors, such as the nature of the catalyst, other components in the mixed gas, humidity, temperature, etc., will affect the activity and selectivity of the HCHO catalysts. Therefore, by combining these possible causes and conducting in-depth research using mixed gases, not only can the effectiveness of the catalyst be improved in a targeted manner, but the approach can also have positive significance for future large-scale applications.
3. Develop efficient materials to pave the way for future practical applications. At present, most of the reported catalysts are tested under laboratory conditions, but the concentration changes of HCHO in the actual environment and the interactions between HCHO and other atmospheric pollutants can complicate the catalytic process. Therefore, by combining theory with practice to develop catalysts that can be truly applied, contributions to human health can be made.

Author Contributions

R.W.: Conceptualization, methodology, validation, writing—review and editing, funding acquisition, supervision, project administration. Y.F.: investigation, writing—original draft preparation. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Natural Science Foundation of Shandong Province (ZR2018MEE048), China.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Tunnel effects of the 1 × 1, 2 × 2, and 3 × 3 tunnel manganese oxide catalysts (corresponding to right, middle, and left curves, respectively) on complete oxidation of HCHO. Reproduced with permission from [33].
Figure 1. Tunnel effects of the 1 × 1, 2 × 2, and 3 × 3 tunnel manganese oxide catalysts (corresponding to right, middle, and left curves, respectively) on complete oxidation of HCHO. Reproduced with permission from [33].
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Figure 2. The effect of relative humidity on the MnCeOx catalyst: (a) Effect of catalyst type on HCHO conversion; (b) CO2 yield of MnCeOx catalyst at different humidity. Reproduced with permission from [35].
Figure 2. The effect of relative humidity on the MnCeOx catalyst: (a) Effect of catalyst type on HCHO conversion; (b) CO2 yield of MnCeOx catalyst at different humidity. Reproduced with permission from [35].
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Figure 3. TEM images and HCHO conversions over 3D Co3O4 catalyst. Reproduced with permission from [46].
Figure 3. TEM images and HCHO conversions over 3D Co3O4 catalyst. Reproduced with permission from [46].
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Figure 4. (a) HCHO conversion and specific activity of the catalysts tested after 12 h of reaction; and (b) stability of the as-prepared catalysts. Reproduced with permission from [49].
Figure 4. (a) HCHO conversion and specific activity of the catalysts tested after 12 h of reaction; and (b) stability of the as-prepared catalysts. Reproduced with permission from [49].
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Figure 5. Schematic illustration of the growth of different manganese oxide nanostructures under varied conditions. Reproduced with permission from [61].
Figure 5. Schematic illustration of the growth of different manganese oxide nanostructures under varied conditions. Reproduced with permission from [61].
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Figure 6. TEM images and histograms of Pt nanoparticles in Pt/urchin-like MnO2 catalysts. Pt loading: (a–c) 0.5 wt%; (d–f) 1 wt%; (g–i) 2 wt%; (j–l) 3 wt%. Reproduced with permission from Elsevier from [70].
Figure 6. TEM images and histograms of Pt nanoparticles in Pt/urchin-like MnO2 catalysts. Pt loading: (a–c) 0.5 wt%; (d–f) 1 wt%; (g–i) 2 wt%; (j–l) 3 wt%. Reproduced with permission from Elsevier from [70].
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Figure 7. Optimized structures of intermediates, transition states, and final states of HCHO oxidation on Pd/DV-N4 surface and energy distribution of each reaction path. Reproduced with permission from [80].
Figure 7. Optimized structures of intermediates, transition states, and final states of HCHO oxidation on Pd/DV-N4 surface and energy distribution of each reaction path. Reproduced with permission from [80].
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Figure 8. The energy profile and optimized structures of HCHO oxidation through the Langmuir–Hinshelwood mechanism over the Pt/TiO2 (101) surface. Reproduced with permission from [83].
Figure 8. The energy profile and optimized structures of HCHO oxidation through the Langmuir–Hinshelwood mechanism over the Pt/TiO2 (101) surface. Reproduced with permission from [83].
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Figure 9. Reaction pathway of HCHO on layered manganese dioxide with K+-compensated manganese vacancies. Reproduced with permission from [87].
Figure 9. Reaction pathway of HCHO on layered manganese dioxide with K+-compensated manganese vacancies. Reproduced with permission from [87].
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Feng, Y.; Wang, R. Non-Noble Metal Catalysts for Efficient Formaldehyde Removal at Room Temperature. Catalysts 2025, 15, 723. https://doi.org/10.3390/catal15080723

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Feng Y, Wang R. Non-Noble Metal Catalysts for Efficient Formaldehyde Removal at Room Temperature. Catalysts. 2025; 15(8):723. https://doi.org/10.3390/catal15080723

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Feng, Yiqing, and Rui Wang. 2025. "Non-Noble Metal Catalysts for Efficient Formaldehyde Removal at Room Temperature" Catalysts 15, no. 8: 723. https://doi.org/10.3390/catal15080723

APA Style

Feng, Y., & Wang, R. (2025). Non-Noble Metal Catalysts for Efficient Formaldehyde Removal at Room Temperature. Catalysts, 15(8), 723. https://doi.org/10.3390/catal15080723

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