Recent Advances in ZrO₂-Based Catalysts for the Catalytic Oxidation of Formaldehyde

Abstract

Formaldehyde (HCHO) is a typical volatile organic compound (VOC) that poses significant risks to human health. Long-term exposure, even at low concentrations, has been associated with various malignant diseases, including nasopharyngeal, colon, and brain cancers. Common technologies for HCHO abatement include ventilation, adsorption, photocatalysis, and catalytic oxidation. Among these methods, catalytic oxidation is regarded as the most promising due to its high removal efficiency, low cost, minimal energy consumption, and no toxic by-products. In recent years, supported catalysts with excellent room-temperature activity and high dispersibility have attracted considerable attention. These catalysts can usually be divided into two categories: noble metal catalysts and non-noble metal catalysts. Zirconia (ZrO₂) has become an ideal support owing to its advantages of high specific surface area, abundant and tunable acid–base sites, and strong metal–support interaction (SMSI). Various modification strategies have been developed to improve the catalytic performance of ZrO₂-based systems, such as the construction of phase interfaces and the stabilization of single-atom species. This review summarizes the recent research progress of ZrO₂-based systems for the catalytic oxidation of formaldehyde. It provides a detailed discussion of the physicochemical properties of ZrO₂ supports and the reaction mechanisms involved, and highlights achievements in crystal phase regulation, elemental doping, metal–support interaction, and composite modification. Finally, future challenges and development directions for these catalysts are also outlined.

1. Introduction

Formaldehyde (HCHO), also known as methanal, is the simplest saturated aldehyde whose molecular structure is illustrated in Figure 1 [1]. The bond length of C=O is approximately 1.207 Å, while the bond length of C–H is around 1.102 Å. Formaldehyde usually exists as a colorless gas or liquid. It is difficult to detect at low concentrations, whereas at elevated concentrations it exhibits a strong, pungent, and irritating odor. Because the C=O bond in formaldehyde is strongly polarized and the α–C–H bond is readily activated, many heterogeneous catalytic oxidation pathways are initiated by the activation of HCHO with reactive oxygen species or hydroxyl radicals on the surface, typically accompanied by C–H bond cleavage or H–atom abstraction, followed by the continuous formation of surface species such as formates and carbonates, and ultimately complete conversion to CO₂ and H₂O [2]. It should be emphasized that formaldehyde is not intrinsically resistant to degradation in the atmosphere. It can be rapidly removed through photochemical processes and interactions with hydroxyl radicals, resulting in an atmospheric lifetime on the order of only a few hours. However, once continuous emission sources such as wood-based panels and related products are present, indoor concentrations over extended periods can remain detectable, or even exceed regulatory limits. Meanwhile, formaldehyde plays a significant role in atmospheric chemistry as a key oxidation intermediate and an important observational tracer for volatile organic compounds (VOCs). It is widely used to constrain emissions of biogenic VOC, such as isoprene, and to better understand associated photochemical processes [3].

In modern society, people spend approximately 80–90% of their time indoors, making indoor air quality a critical concern. As a key component of resin adhesives, formaldehyde is extensively used in the furniture and building-material industries, and its release can persist for up to 15 years. In response to its health risks, many countries and regions have implemented increasingly stringent regulations. For example, under the Toxic Substances Control Act (TSCA), the United States has established mandatory limits on formaldehyde emissions from composite wood products, which fully entered into force on 22 March 2019. These limits restrict emissions from hardwood plywood, medium-density fiberboard, and particleboard within 0.05 ppm, 0.11 ppm, and 0.09 ppm, respectively. Similarly, China implemented the revised Indoor Air Quality Standard on 1 February 2023, updating and refining concentration limits and analytical methods for pollutants, including formaldehyde. The European Union also strengthened its regulatory framework. On 17 July 2023, it published an amendment to the REACH regulation in the Official Journal that added restrictions on formaldehyde released from furniture, articles, and the interiors of road vehicles. Typical HCHO concentrations and the corresponding health risks are summarized in

Table 1. HCHO concentrations and associated hazards.

HCHO Concentration (mg·m−3)	Hazard
0.06–0.07	Detectable odor irritation
<1.2	Only slight irritation to the human body
>3.6	Markedly enhanced irritation to the human body
4.8–6.0	Thirty minutes of exposure may cause tearing, itchy eyes, and throat dryness; exposure to 5 mg·m−3 HCHO can immediately lower blood pressure
12–24	May cause dyspnea, coughing, and headache
≥60	May cause pneumonia, pulmonary edema, or even death

. Formaldehyde has been classified as a Group 1 carcinogen by the International Agency for Research on Cancer (IARC) [4,5]. Long-term exposure can damage the respiratory system and increase mortality risk [6], and has also been associated with malignant diseases such as leukemia, brain cancer, and nasopharyngeal carcinoma [7,8]. Indoor formaldehyde mainly originates from decoration and construction materials [9,10], especially engineered wood products such as plywood and blockboard. Additional sources include decorative materials, such as wall fabrics, wallpaper, synthetic-fiber carpets, foamed plastics, paints, and coatings, as well as cigarette smoke and the combustion of certain organic materials. Given its widespread sources and persistent emissions, formaldehyde pollution remains a serious environmental and public health issue. The growing global emphasis on regulatory control highlights the urgent need to mitigate exposure while advancing greener industrial practices. As a result, further research on efficient HCHO abatement technologies is of considerable importance.

At present, technologies for HCHO abatement can be broadly categorized into physical, chemical, and biological approaches [11]. These include phytoremediation [12], physical adsorption [13,14], plasma treatment [15], photocatalytic oxidation [16,17,18,19,20,21,22,23,24], and thermal catalytic oxidation [25,26,27,28,29,30,31,32,33,34,35], as summarized in

Table 2. Major HCHO abatement technologies and their characteristics.

Category	Technology	Mechanism	Advantages	Disadvantages	Ref.
Biological method	Phytoremediation	Plants absorb formaldehyde through respiration and then decompose, transform, or assimilate it through their own metabolic activities	Simple operation; environmentally friendly; can also improve indoor decoration	Limited uptake capacity; plants are prone to disease or death at high HCHO concentrations; regular maintenance is required, so it can only serve as an auxiliary measure	[12,36]
Physical method	Physical adsorption	Formaldehyde molecules are retained on the surface of porous adsorbents through van der Waals forces, thereby lowering indoor HCHO concentration	Materials are readily available; common adsorbents include porous materials such as activated carbon, molecular sieves, porous clay minerals, and activated alumina, and the cost is relatively low	Weak binding may lead to desorption; adsorption capacity is limited, and once saturated, the adsorbent loses effectiveness and may cause secondary HCHO release	[13,14,37,38,39,40,41,42]
Chemical method	Plasma treatment	1. High-energy electrons interact directly with formaldehyde molecules; 2. reactive groups generated by the plasma chemically react with formaldehyde to achieve degradation	Can be carried out at room temperature; HCHO purification efficiency can exceed 80%	High equipment cost; secondary pollution may be generated during the process	[15,43]
Chemical method	Photocatalytic oxidation	Semiconductor materials are photoexcited under irradiation and catalyze HCHO oxidation, thereby accelerating degradation	Environmentally friendly; relatively high catalytic efficiency	Mainly relies on ultraviolet light, so the utilization of natural light is low; by-products such as formic acid and methanol may be generated, causing secondary pollution	[16,17,18,19,20,21,22,23,24]
Chemical method	Thermal catalytic oxidation	The catalyst lowers the energy barrier for HCHO oxidation and promotes the complete oxidation of formaldehyde to H2O and CO2 under thermal conditions	Complete HCHO degradation can be achieved at room temperature, with no secondary pollution; high catalytic activity and fast reaction rate	High requirements on catalyst performance; catalytic materials with both low-temperature activity and long-term stability still need to be developed	[25,26,27,28,29,30,31,32,33,34,35]

. Photocatalytic oxidation usually depends on photogenerated electron–hole pairs together with radical species such as ·O₂⁻ and ·OH, and therefore operates under relatively mild conditions, although its gas-phase reaction rate and solar-energy utilization are often limited. Plasma treatment, by contrast, activates pollutants through energetic electrons, ions, ozone, and radical species, which gives rapid initial decomposition capability but may also increase the risk of by-product formation in the absence of appropriate catalytic synergy. Thermocatalytic oxidation mainly relies on surface reactive oxygen species, lattice oxygen, and hydroxyl groups, and is generally more advantageous in terms of low-temperature activity and long-term stability.

Among these methods, thermal catalytic oxidation is a purely thermocatalytic process. The essence of thermal catalytic HCHO oxidation is to regulate the adsorption configuration and electronic-state distribution of reactants through active sites, thereby lowering the energy barriers for key bond-breaking and bond-forming steps and facilitating the complete conversion of reaction intermediates into CO₂ and H₂O [44,45,46,47]. Notably, this technology can achieve complete HCHO decomposition even at room temperature and offers several advantages, including high purification efficiency, controllable selectivity, no requirement for external energy input, and no secondary pollution [48,49,50]. It is therefore both efficient and economical and has become a research hotspot in recent years. In essence, the catalyst lowers the overall activation barrier and regulates the generation of reactive oxygen species and hydroxyl groups, thereby enhancing HCHO activation and accelerating the oxidation process [51]. Within such systems, catalyst selection directly determines process performance, and the central challenge lies in developing highly efficient catalysts that combine excellent low-temperature activity with long-term stability.

Current research on catalytic materials mainly focuses on three categories. The first comprises noble metal catalysts based on Pt [52,53,54,55], Au [56,57,58], Pd [59,60,61,62], and Ag [63,64,65,66,67,68]. The second category involves transition-metal catalysts based on Cu [69,70,71], Mn [72,73,74], and Co [71,75,76]. The third includes composite metal oxides, such as MnO_x [77,78,79] and Co₃O₄ [80,81,82]. The catalytic performance of both transition-metal-based catalysts and supported noble metal catalysts is strongly influenced by the properties of the oxide component. Although transition metals are abundant and cost-effective, their low-temperature activities are often inferior to those of noble metals, primarily due to their relatively weak ability to generate reactive surface oxygen [83]. In contrast, supported noble metal catalysts typically display higher activity and can achieve complete HCHO oxidation at lower temperatures. However, their practical application is constrained by the high cost and limited availability of noble metals. Therefore, it is imperative to develop catalysts that utilize only trace amounts of precious metals while maintaining high activity and stability. In this context, the role of support has evolved beyond that of a simple carrier for noble metal nanoparticles; it is now expected to actively facilitate oxygen activation and ensure favorable metal dispersion.

Common catalyst supports include Al₂O₃ [30,68,84,85], SiO₂ [30,55,57,62], TiO₂ [21,31,76,86,87], and ZrO₂ [83,88,89,90,91,92]. Among these, ZrO₂ exhibits a high specific surface area and a surface rich in hydroxyl groups as well as tunable acidic/basic sites, which are beneficial for HCHO adsorption and activation. In addition, ZrO₂ can establish strong metal–support interactions with active components, thereby enhancing catalytic efficiency in HCHO oxidation. It also possesses excellent mechanical properties and thermal resistance. Given the relatively high abundance of zirconium in the Earth’s crust, ZrO₂ is comparatively cost-effective, further increasing its attractiveness as a catalyst support.

To further clarify the performance and advantages of ZrO₂ supports in the formaldehyde oxidation system, we conducted a comparative analysis of ZrO₂ with other commonly used supports, including TiO₂, CeO_2, and Al₂O₃. According to the literature [83], under identical catalyst preparation and formaldehyde catalytic oxidation reaction conditions, the formaldehyde conversion activity follows the order: Pt/Al₂O₃ > Pt/ZrO₂ > Pt/CeO₂ > Pt/TiO₂. While this order is specific to the experimental system of the study, it clarifies the performance differences and underlying action mechanisms of different supports, and provides valuable reference for optimizing the performance of ZrO₂-based catalysts and selecting their application scenarios. This study confirms that, as a non-reducible oxide, the core advantage of ZrO₂ lies in its effective inhibition of the partial oxidation of Pt species. This addresses a key limitation of reducible supports such as CeO₂ and TiO₂, which readily induce Pt oxidation and result in a significant decline in room-temperature catalytic activity. Meanwhile, the type of metal precursor has a negligible effect on the catalytic performance of ZrO₂-supported catalysts, giving these catalysts enhanced robustness and stability in the preparation process compared with CeO₂ and TiO₂, whose catalytic properties are highly susceptible to precursor residues and inert by-products. In terms of comprehensive catalytic performance, ZrO₂-supported catalysts achieve a favorable balance between initial activity and cycling stability. Its room-temperature initial activity is second only to that of Al₂O₃ with a high specific surface area, and significantly higher than those of CeO₂ and TiO₂. During the oxidation-reduction cycle regeneration process, ZrO₂-based catalysts deactivate far less than Pt/Al₂O₃, which effectively compensates for the defect of Al₂O₃ that is vulnerable to irreversible deactivation caused by Pt particle sintering. In contrast, CeO₂ facilitates low-temperature oxidation of formaldehyde intermediates through lattice oxygen and achieves excellent cycling anti-deactivation performance via strong metal–support interaction. TiO₂ enables high dispersion of Pt nanoparticles and serves as a classic support for formaldehyde catalytic oxidation. Although the initial activity of CeO₂ and TiO₂ is inferior to that of ZrO₂, they exhibit unique advantages in long-term cycling application scenarios. Al₂O₃ achieves the highest initial activity via its ultrahigh specific surface area and abundant surface hydroxyl groups, making it preferable for short-term high-activity applications. Overall, the ZrO₂ support exhibits distinct advantages in its excellent ability to maintain the active metallic state of Pt, preparation stability, and well-balanced comprehensive catalytic performance. It is a promising support material in the room-temperature formaldehyde oxidation system, and provides broad optimization space for the structural design and performance regulation of high-efficiency formaldehyde catalytic oxidation catalysts.

In recent years, substantial progress has been achieved in ZrO₂-based catalysts through modifications such as crystal-phase regulation, elemental doping, and the incorporation of active components. Even so, dedicated reviews focusing specifically on ZrO₂ as a support for HCHO oxidation remain scarce, and a systematic understanding of its interactions with active phases, as well as the underlying catalytic mechanisms, is still lacking. This review, therefore, summarizes recent progress in ZrO₂-based catalysts for HCHO oxidation, to clarify structure–performance relationships and deepen mechanistic understanding to support practical applications. It begins with an overview of the basic structural characteristics of ZrO₂ supports, followed by a brief discussion of the catalytic mechanisms of HCHO oxidation. Subsequently, recent advances in ZrO₂-based catalysts are highlighted, with particular emphasis on crystal-phase regulation, elemental doping, metal–support interactions, and composite modification. It is expected that this review can provide useful insights for the rational design and synthesis of advanced zirconia-based catalytic materials.

2. Properties of ZrO₂ Supports

ZrO₂ is an important transition-metal oxide that exists in three polymorphs. These crystal phases can be transformed through temperature regulation or doping [93], and each exhibits good thermal stability within a specific temperature range. In addition, the surface of ZrO₂ contains abundant hydroxyl groups and acidic/basic sites that can interact strongly with active components [94], thereby regulating the dispersion and electronic states of active species. Because ZrO₂ is relatively inexpensive [95] and also exhibits excellent thermal and chemical stability [96], it can maintain structural integrity under harsh reaction conditions, particularly at elevated temperatures. These characteristics make it an attractive and reliable support material for catalytic oxidation reactions.

Physicochemical Properties

As shown in Figure 2, the crystal structure of ZrO₂ is strongly temperature-dependent, with three distinct polymorphs appearing over different temperature ranges. Monoclinic ZrO₂ (m-ZrO₂) is stable below 1175 °C and contains four Zr atoms and eight O atoms per unit cell. Tetragonal ZrO₂ (t-ZrO₂), stable from 1175 °C to 2370 °C, exhibits a fluorite-like structure in which each Zr atom is coordinated to eight O atoms with unequal bond lengths. Cubic ZrO₂ (c-ZrO₂), stable from 2370 °C to 2706 °C, adopts an ideal fluorite structure, where each Zr atom is symmetrically coordinated by eight oxygen atoms [94,97]. Among these polymorphs, only the monoclinic phase is thermodynamically stable at room temperature [98,99]. The tetragonal phase is metastable under ambient conditions, yet it is valuable in catalysis because of its superior mechanical strength and favorable physicochemical properties. Although the tetragonal phase typically forms at high temperatures, its formation temperature can be significantly reduced by decreasing the crystallite size or introducing dopants into the lattice [89]. Doping not only facilitates phase transformation but also stabilizes high-temperature phases, enabling tetragonal or cubic ZrO₂ to persist at room temperature [92]. Common dopants include Ni, Mg, Ce, La, and Y. By modifying the lattice structure, these dopants can alter the phase-transition temperatures [100,101] and further enhance the thermal stability and catalytic performance of ZrO₂-based materials.

Due to its chemical stability, ZrO₂-based catalysts generally retain their structural and catalytic integrity after repeated use [102]. The surface of ZrO₂ contains both acidic and basic sites, which contribute to its excellent catalytic performance in HCHO oxidation. As illustrated in Figure 3, the acidic centers on ZrO₂ include both Lewis and Brönsted acid sites [94]. Lewis’s acid sites mainly arise from Zr⁴⁺ species generated when the ZrO₂ lattice is disrupted, and these sites are not bound to hydroxyl groups. Brönsted sites, on the other hand, originate from the Zr–OH bond formed with dissociated water that interacts with Zr⁴⁺, producing Zr–O–H structures. Basic sites are associated with unpaired O²⁻ species. Compared with the tetragonal and cubic phases, in which oxygen atoms are eightfold coordinated, the monoclinic phase contains a higher proportion of O²⁻ species. Specifically, among the seven oxygen ions in monoclinic ZrO₂, three possess unpaired electrons capable of accepting protons from acidic compounds, making the monoclinic phase more basic [103]. Functionally, the acidic sites on ZrO₂ can adsorb and activate the carbonyl oxygen of formaldehyde, whereas basic sites facilitate water activation to generate hydroxyl radicals. The synergistic interaction between these two types of sites promotes the oxidative decomposition of HCHO. Although acid–base balance has been well recognized to play a crucial role in HCHO activation and intermediate conversion over ZrO₂-based catalysts, a universally applicable quantitative criterion for optimal acid–base balance across various systems has not been established yet. Most existing studies provide qualitative or semi-quantitative evidence based on spectroscopic characterization, adsorption behavior analysis, or catalytic activity trends, rather than developing a transferable descriptor that can be directly applied to different ZrO₂ phases, defect structures, and supported metal configurations. Therefore, the quantitative optimal ratio of effective acid–base balance still awaits further systematic investigation and establishment.

ZrO₂ possesses a high specific surface area and rich surface hydroxyl groups (–OH), which endow the material with abundant active adsorption sites for HCHO. In addition, its basic surface facilitates the generation and stability of key intermediates during HCHO adsorption and activation. For example, Colussi et al. [83] studied the TPD experiments and found that bare ZrO₂ adsorbs HCHO differently from acidic supports such as Al₂O₃ and TiO₂. As shown in Figure 4, the signals that are characteristic of formaldehyde at m/z = 29 and 30 are almost negligible, whereas the characteristic of CO shows a strong signal at m/z = 28, which appears in the range of 230–300 °C. This finding indicates that ZrO₂, as a basic support, tends to form stable intermediates with HCHO rather than desorbing intact HCHO molecules. Watanabe et al. [104] studied the catalytic behavior of ZrO₂ in supercritical water at 673 K and 25–40 MPa. Using the CH₃OH/CO yield ratio as an indicator of the product distribution of HCHO conversion, they quantified the effective OH⁻ concentration on the ZrO₂ surface as 10⁻⁸ mol/L. This finding confirms that ZrO₂ behaves as a typical basic catalyst. The surface hydroxyls on ZrO₂ serve as basic sites, which facilitate the dissociation of adsorbed water and increase the concentration of OH⁻. Consequently, the Cannizzaro reaction (2HCHO + H₂O → CH₃OH + HCOOH) is promoted under these specific supercritical water and alkaline conditions described in the study, driving the conversion of HCHO in this experimental system. Critically, this pathway is mainly validated for basic ZrO₂ surfaces under specific supercritical water/alkaline environments and cannot be generalized as the dominant reaction route for all ZrO₂-based HCHO oxidation systems. This study further posited that, based on the electronegativities of metal ions, the strong basicity of ZrO₂ originates from the low electronegativity and large ionic radius of Zr⁴⁺. Compared with other oxides, the basicity order CeO₂ > ZrO₂ > MoO₃ > TiO₂ (rutile) > TiO₂ (anatase) demonstrates that ZrO₂ surpasses acidic supports such as TiO₂ in catalytic HCHO conversion. Overall, the catalytic performance of ZrO₂ is highly dependent on the balance between acidic and basic sites. Besides, the acid–base ratio on the catalyst will influence its performance in the formaldehyde catalytic reaction. Therefore, identifying an appropriate acid–base ratio is a crucial approach to enhancing the oxidation efficiency of HCHO.

3. Overview of ZrO₂-Based Catalysts

ZrO₂-based catalysts show excellent performance in the catalytic oxidation of HCHO. According to the properties of active components, their dispersion states, and the structural characteristics of supports, these catalysts can be broadly divided into four categories: noble metal-loaded systems, transition-metal-oxide-loaded systems, crystal-phase/phase-interface engineering systems, and single-atom catalyst (SAC) systems. Among them, noble metal-loaded catalysts exhibit the most prominent advantages in the low-temperature range, particularly Pt-based ones. Their catalytic performance is not solely determined by the intrinsic activity of noble metals, but also relies on the continuous supply of reactive oxygen species from surface defects and SMSI interfaces of ZrO₂, as well as the accelerated deep oxidation of intermediates. Transition-metal-oxide-loaded catalysts (such as MnO_x, Co₃O₄) outperform in cost and resource availability. These catalysts usually maintain oxidation capacity through reversible valence cycling, lattice oxygen migration, and replenishment. Given their activity is relatively limited at low temperatures, strategies such as doping, compositing, and defect engineering are commonly adopted to enhance the efficiency of oxygen migration and activation. Crystal and phase-interface engineering can modulate the surface coordination environment and charge distribution to optimize interfacial electron transfer and lower the initial C–H activation energy barrier. This, in turn, improves the low-temperature kinetics and inhibits the accumulation of inert intermediates such as some carbonates. Single-atom catalyst (SAC) systems hold the core advantages of maximum atomic utilization and well-defined active sites. However, their performance is highly dependent on the matching between continuous oxidation capacity and anchoring sites such as oxygen vacancies, defects, and a stable coordination environment.

Table 3. Overview of typical ZrO₂-based catalysts for HCHO oxidation.

Catalyst	Metal Loading	Preparation Method	Reaction Conditions	Removal Efficiency	T90	Stability Duration	Ref.
Pt/ZrO2	0.93 wt.%	Incipient-wetness impregnation of commercial ZrO2 using H2PtCl6·6H2O or (NH3)4Pt(NO3)2, followed by drying at 110 °C overnight and calcination at 500 °C for 3 h.	100 ppm HCHO, 20 vol.% O2; total flow rate = 300 mL/min	58% at R.T.	/	2 h catalytic test on reduced catalyst; lower conversion after oxidation–reduction regeneration	[83]
Pt–ZrO2	0.85 wt.%	Deposition of PVP-capped Pt colloid on m-ZrO2 prepared by a modified P123-assisted hydrothermal method.	100 ppm HCHO; 21 vol.% O2; GHSV = 60,000 mL·(g·h)−1; RH = 30%	79.3% at 20 °C	/	/	[105]
0.87Pt–VO–ZrO2	0.87 wt.%	Pt–ZrO2 precursor further calcined in static air at 200 °C for 2 h and 450 °C for 4 h to generate the oxygen-vacancy-associated Pt–VO–ZrO2 interface.	100 ppm HCHO; 21 vol.% O2; GHSV = 60,000 mL·(g·h)−1; RH = 30% and 75%	95% at 20 °C	≤20	720 min at 20 °C	[105]
Pt/ZrO2–M	0.84 wt.%	Pure m-ZrO2 support prepared hydrothermally from ZrOCl2·8H2O with NH4OH (140 °C, 24 h), then calcined at 500 °C for 4 h; Pt loaded by PtCl4 impregnation followed by NaBH4 reduction.	100 ppm HCHO; 21 vol.% O2; GHSV = 60,000 mL·(g·h)−1; RH = 30% and 75%	100% at 85 °C	≤85	/	[90]
Pt/ZrO2–K	0.88 wt.%	Mixed-phase ZrO2–K support prepared by precipitation of ZrOCl2·8H2O into 1.8 mol L−1 KOH solution at room temperature, then calcined at 500 °C for 4 h; Pt loaded by PtCl4 impregnation followed by NaBH4 reduction.	100 ppm HCHO; 21 vol.% O2; GHSV = 60,000 mL·(g·h)−1; RH = 30% and 75%	100% at 30 °C	≤30	60 h at 30 °C; 88% conversion after 60 h	[90]
Ir1–N–C/ZrO2	/	Carbonization of Ir/UiO–66–NH2 under flowing N2 at 700 °C for 3 h gives the black Ir1–N–C/ZrO2 intermediate.	100 ppm HCHO; 20 vol.% O2; WHSV = 600,000 mL·h−1·gcat−1; RH = 30%	85% at R.T.	/	/	[106]
Pt/ZrO2–GA–MOF-5	7 wt.%	Surface-casting synthesis of Pt/ZrO2 nanotube arrays using SBA-15-OH as template, boiling-water-bath assembly into graphene aerogel, followed by KH550/succinic-anhydride surface functionalization and stepwise self-assembly growth of MOF-5.	50 ppm HCHO; HCHO flow rate = 20 mL/min	90% at 70 °C	= 70	24 h continuous reaction at 100 °C; efficiency remained above 99%	[107]
N–C/ZrO2	/	/	100 ppm HCHO; 20 vol.% O2; WHSV = 600,000 mL·h−1·gcat−1; RH = 30%	80% at R.T.	/	/	[106]
Nano-ZrO2 phase junction	/	Co-precipitation of ZrOCl2·8H2O with NH3·H2O followed by calcination; TMZ-400 is the optimal tetragonal–monoclinic phase-junction sample.	50 and 100 ppm HCHO; 21 vol.% O2; WHSV = 600,000 mL·h−1·gcat−1; RH = 30%	92% at 40 °C	≤40	/	[108]

Note: “V_O” stands for oxygen vacancies. “RH” stands for relative humidity. “R.T.” stands for room temperature.

presents a summary of the research on the use of ZrO₂-based catalysts for the catalytic oxidation of formaldehyde.

3.1. Noble Metal-Loaded ZrO₂

In recent years, noble metals have attracted the interest of many researchers in HCHO catalysis due to their outstanding low-temperature activity and high selectivity [11]. Noble metals can act synergistically with acidic and basic sites, hydroxyl groups, and defect sites on the surface of ZrO₂, enabling precise regulation of the adsorption configuration of reactants as well as the generation efficiency of reactive oxygen species and hydroxyl groups. However, in ZrO₂-based catalytic systems, the dominant factor governing catalytic performance is not the intrinsic activity of the noble metal. Instead, it relies heavily on the continuous generation of reactive oxygen species available for the reaction via strong metal–support interactions (SMSIs) and on the efficient conversion of reaction intermediates into CO₂ and H₂O. The classification of SMSI mechanisms is displayed in Figure 5. It has been reported that the formation of specific metal–support interfaces facilitates SMSIs [109], and the corresponding interfacial sites are favorable for the activation of reactive oxygen species. Accordingly, constructing tailored SMSI interfaces is a critical strategy for improving the HCHO oxidation efficiency of noble metal-loaded ZrO₂ catalysts.

3.2. Transition-Metal-Oxide-Loaded ZrO₂

Although noble metal-loaded catalysts exhibit excellent catalytic activity, noble metals themselves suffer from high cost and limited natural reserves. In contrast, transition-metal oxides offer distinct advantages of low cost, more abundant raw material reserves, simple preparation procedures, and good stability, with particularly prominent superiority in fabrication cost and raw material availability. Moreover, under harsh reaction conditions such as high temperature, high pressure, and strongly corrosive reaction environments, the active noble metal phases are prone to sintering or poisoning, which directly leads to catalyst deactivation. For these reasons, the use of transition-metal oxides as active components is an important direction to reduce the dosage of noble metals and even realize noble metal-free catalytic systems. Common transition-metal oxides include MnO_x, Co₃O₄, and CuO. These oxides usually rely on reversible valence cycling and the migration and replenishment of lattice oxygen to maintain sustained oxidation capacity. When coupled with ZrO₂, the ZrO₂ support fulfills two core functions. On one hand, the support can inhibit aggregation and sintering of the active phase through metal–support interactions. On the other hand, the acidic/basic sites and hydroxyl groups on the ZrO₂ surface can modify the adsorption and desorption behaviors of HCHO and H₂O and thus influence the formation and removal rates of intermediates such as formates and carbonates.

3.3. ZrO₂ Crystal-Phase/Phase-Interface Engineering Systems

It is widely recognized that ZrO₂ exists in three polymorphs. Among these, monoclinic ZrO₂ (m-ZrO₂), tetragonal ZrO₂ (t-ZrO₂), and the phase interface formed between them can alter the surface coordination environment and electronic structure. This, in turn, affects oxygen activation and the initial C–H activation step during the catalytic oxidation of HCHO. The core of phase-interface engineering lies in improving interfacial charge transfer efficiency and the generation rate of reactive oxygen species by modifying the structure and charge distribution, while also lowering the energy barriers of key reaction steps. It has been reported that the t-m phase interface can significantly enhance catalytic HCHO oxidation at room temperature. This enhancement is attributed to the optimized interfacial electron transfer characteristics and gas adsorption configurations, which facilitate C-H bond cleavage and subsequent oxidation at low temperature [108]. When crystal and phase-interface engineering are combined with noble metal loading, the resulting interfacial effect often becomes even more pronounced. Yang et al. prepared Pt/ZrO₂ catalysts on the mixed m/t phase of ZrO₂ supports, and reported that the activities of these catalysts were markedly higher than that of catalysts supported on pure m-ZrO₂. Besides, the catalysts achieved complete HCHO oxidation near ambient temperature [90]. This report indicated that mixed-phase interfaces favor the construction of more efficient interfacial active sites and improve the generation of reactive oxygen species, thereby enhancing low-temperature kinetics. These studies further confirm that the crystal phase ratio and interface structure of ZrO₂ itself can strongly influence the rate of low-temperature HCHO oxidation.

3.4. Single-Atom Catalyst (SAC) Systems

The defining feature of single-atom catalysts (SACs) is that isolated or individual noble metal atoms are usually anchored to a specific support. Owing to their unique coordination environments and electronic structures, SACs are expected to offer superior catalytic activity and selectivity [111]. Among the supports of SACs, transition-metal oxides such as ZrO₂, TiO₂, and CeO₂ can trigger strong metal–support interactions with single-atom noble metals [112], thus enhancing activity and long-term stability in oxidation reactions [113,114,115]. ZrO₂ simultaneously possesses excellent thermal stability, chemical resistance, redox surface properties, and low cost, making it a competent support for loading single atoms. Currently, Pt-loaded ZrO₂ catalysts have already been successfully prepared [116], demonstrating promising catalytic applications, including ammonia oxidation [117], photocatalysis [118,119], and hydrogenation reactions [120,121]. Peng et al. [88] proposed that ZrO₂ nanoparticles can regulate the electronic structure of Ir single atoms and thus enhance HCHO oxidation at room temperature. Their results suggest that ZrO₂ not only modifies the electronic state of the single-atom center but also promotes the activation of adsorbed oxygen, thereby providing a facile strategy to design low-loading single-atom catalysts with high catalytic activity toward HCHO oxidation. Overall, ZrO₂ can serve both as a defect-rich anchoring platform that stabilizes single-atom centers and inhibits migration/aggregation, and as an interactive support that triggers charge redistribution through SMSI, thus modulating the ability of single atoms to activate O₂ and H₂O and to generate reactive oxygen species. When the room-temperature reaction is limited by O₂ activation and the deep oxidation of intermediates, this strategy can provide a more sustainable solution to simultaneously balance activity, stability, and noble metal utilization.

3.5. Mechanisms of HCHO Oxidation over ZrO₂-Based Catalysts

In the catalytic reaction mechanisms, three classical surface reaction models are broadly recognized: the Eley-Rideal (E-R), Langmuir-Hinshelwood (L-H), and Mars-van Krevelen (MVK) mechanisms. In the oxidation reaction of HCHO, the reaction mainly proceeds through the E-R mechanism if HCHO and O₂ share the same adsorption site. When HCHO and O₂ are adsorbed on separate active sites, such as hydroxyl sites and active metal sites, the reactions proceed 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, HCHO oxidation is mainly described by the MVK mechanism [122,123,124,125].

According to Sun’s report [11], the mechanism of catalytic HCHO oxidation involves multiple steps, including the adsorption, activation, and oxidation reaction of formaldehyde molecules. Adsorption is the first step of the entire reaction process. As directly verified by in situ characterization experiments, in the presence of water, surface reactive oxygen species can generate hydroxyl groups, and HCHO molecules can be adsorbed by hydrogen bonds with these hydroxyl groups. This step is in line with the classic MVK, L-H, and E-R models. When strong metal–support interactions are present, HCHO may also adsorb on oxygen vacancies or the bridging sites of certain material surface atoms. These adsorption behaviors are also identified and confirmed by in situ spectroscopic observations. On ZrO₂-based catalysts, catalytic HCHO oxidation is initially driven by OH⁻ species on the basic ZrO₂ surface, which is revealed by in situ characterization of surface species under reaction conditions. Watanabe et al. [104] demonstrated that HCHO first undergoes a Cannizzaro reaction on ZrO₂ to form methanol and formic acid (2HCHO + H₂O → CH₃OH + HCOOH), which constitutes a key intermediate step in HCHO activation. It should be noted that the methanol and formic acid generated via the Cannizzaro reaction of formaldehyde on ZrO₂ surfaces are both transient intermediates, and no significant accumulation of these species occurs during the reaction process. This reaction pathway was derived from product distribution analysis and kinetic experiments. Formic acid then decomposes by decarboxylation to form CO₂ and H₂ (HCOOH → H₂ + CO₂). A small fraction can also dehydrate to CO and H₂O, and the resulting CO can be further oxidized to CO₂ through the water–gas shift reaction (H₂O + CO → H₂ + CO₂), ultimately realizing the complete conversion of formaldehyde to CO₂ and H₂O. In other words, an effective catalyst in this process must fulfill the following core functions: efficient HCHO adsorption, sufficient supply of reactive oxygen species, promotion of electron transfer, and acceleration of the decomposition of surface intermediates.

Peng et al. [105] further elucidated the oxygen-vacancy-mediated reaction pathway via in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) and density functional theory (DFT) calculations. As directly observed by in situ DRIFTS experiments, HCHO is first adsorbed near the oxygen vacancies on the ZrO₂ surface. The active oxygen species (O*) at the Pt–ZrO₂ interface attack the C–H bond of HCHO to directly form a formate intermediate rather than the DOM intermediate, thus avoiding the accumulation of inert species. As the DFT calculations show in Figure 6G, the activation barrier of O₂ dissociation on the Pt–V_O–ZrO₂ system is only 0.3 eV, which is far lower than the 0.9 eV calculated for the defect-free Pt–ZrO₂. Therefore, oxygen vacancies substantially reduce the reaction barrier, enabling the direct formation of formate without the extra energy barrier consumption caused by the generation of inert DOM intermediates. Meanwhile, both in situ observations and DFT results confirm that oxygen vacancies promote the dissociation of H₂O to form hydroxyl groups (–OH), which react rapidly with formate species to produce CO₂ and H₂O (HCOO⁻ + ·OH → CO₂ + H₂O). The consumed oxygen vacancies can be quickly replenished via the adsorption and dissociation of O₂, forming a cycle of oxygen vacancies, active oxygen, and hydroxyl groups. As further revealed by DFT charge density and electron transfer analysis (Figure 6C–F, Figure 7 and Figure 8), oxygen vacancies act as an electron transport channel to connect the ZrO₂ (110) and Pt (111) crystal planes. A total of 1.56 electrons are transferred from the Zr/Pt sites to the co-adsorption sites of HCHO and O₂, which is a quantitative result from DFT, not only activating O₂ to generate O* but also promoting the dissociation of H₂O into –OH, realizing the synergistic oxidation of O* and –OH.

3.6. Practical Catalytic Performance of ZrO₂-Based Catalysts

To clarify the actual performance of ZrO₂-based catalysts in the catalytic oxidation of formaldehyde, this study further evaluated their catalytic performance under conditions of ultra-low formaldehyde concentrations and closer to realistic indoor environments. Most reported studies on ZrO₂-based catalysts are conducted under laboratory conditions, and specific tests under actual indoor conditions have not yet been carried out. Even so, several practically relevant trends can be identified from these laboratory experimental results. For instance, the Pt–V_O–ZrO₂ catalyst achieved over 95% HCHO removal and conversion at 20 °C [105]. Under continuous flow conditions, approximately 1 ppm of HCHO could be reduced to below 0.05 ppm, which demonstrates the potential of this interfacial system for purifying formaldehyde at ultra-low concentrations. Meanwhile, for Ce₀.₈Zr₀.₂O_y, the HCHO removal efficiency gradually increased as relative humidity rose to 50%, and remained above 90% for 300 min at room temperature [126]. This finding suggests that moderate humidity facilitates oxygen activation, hydroxyl generation, and the subsequent oxidation of intermediates. In recent research [105], a ZrO₂-coupled Ir single-atom catalyst exhibited excellent room-temperature activity, achieving over 95% HCHO conversion at 20 °C with only 0.25 wt% Ir loading, while maintaining high activity across a broad humidity range of 20% to 75%. These results confirm that ZrO₂-based catalysts hold good potential for indoor formaldehyde abatement under room-temperature, low-concentration, and variable-humidity conditions, though further testing under real indoor conditions is still required.

4. Structural Regulation and Modification Strategies for ZrO₂-Based Catalysts

4.1. Crystal-Phase Regulation

It is widely recognized that the catalytic performance is generally closely related to polymorphism. Crystal structure and the associated transformations can exert a pronounced influence on the physicochemical and catalytic properties of the material [127,128,129]. In the catalytic HCHO experiments performed in supercritical water at 673 K and 30 MPa, Watanabe et al. [104] found that a ZrO₂ catalyst initially composed of a mixed monoclinic-tetragonal (m-t) phase with a BET surface area of 68 m²/g transformed completely into pure monoclinic ZrO₂ after reaction, yet no structural collapse was observed for the post-reaction catalyst. Its effective surface OH⁻ concentration remained at 10⁻⁸ mol/L, and the CH₃OH/CO yield ratio from catalytic HCHO conversion stayed at 8.0, showing no significant change from the pre-reaction value. These observations confirm the outstanding hydrothermal stability of ZrO₂ and its tolerance toward phase transformation, which provides an important experimental basis for the high-temperature modification of ZrO₂ supports and their use in complex aqueous environments. Building on the understanding of crystal structure, several groups have sought to improve catalytic performance by altering the crystal phase or introducing mixed-phase structures. For example, Yang and co-workers [90] prepared a special m-t mixed-phase Pt/ZrO₂ catalyst and found that the Pt/ZrO₂–K sample with the greatest amount of interfacial structure could completely oxidize 100 ppm HCHO near room temperature, as shown in Figure 9(1). Moreover, this m-t mixed-phase structure suppressed the formation of inert carbonate species during HCHO oxidation. As shown in Figure 9(2), in situ DRIFTS detected only the characteristic peaks of formate species (1582 and 1379 cm⁻¹ for υ(COO⁻), 2986, 2862 and 2765 cm⁻¹ for υ(CH)) and dioxymethylene (DOM) species (1479, 1324, 1180 and 1117 cm⁻¹), with no characteristic peaks associated with carbonate species observed. In contrast, the pure monoclinic Pt/ZrO₂–M catalyst exhibited distinct carbonate characteristic peaks at 1667 cm⁻¹ (υ(C=O)) and 1178 cm⁻¹ (υₐₛ(COO)), confirming that the mixed-phase structure effectively inhibits inert intermediates and maintains an efficient catalytic cycle. The m-t interface can induce more defect structures and cationic Pt^δ+ species, which exhibit higher activity for HCHO oxidation than the metallic Pt⁰ species located near the pure monoclinic or pure tetragonal phases. These findings demonstrate that the crystal phase of ZrO₂ has a pronounced effect on catalytic behavior and that regulating the phase composition is an effective strategy to optimize surface properties and the distribution of active sites.

4.2. Support–Active-Component Interactions

As a support, ZrO₂ can modulate the dispersion, electronic state, and chemical properties of active components through strong metal–support interactions (SMSIs), thereby enhancing catalytic activity [130]. Oxygen vacancies serve as the key bridge for constructing high-efficiency SMSI interfaces. The ZrO₂ surface is rich in oxygen vacancies, and it is well established that the active centers of heterogeneous catalysts are closely correlated to surface defects [131]. An oxygen vacancy is an electron-rich center, with two excess electrons generated upon the formation of one vacancy. If H₂O or O₂ adsorbs near these vacancies, the corresponding electrons can be transferred to produce abundant reactive surface oxygen species, thereby improving adsorption and catalytic performance [132]. Similar conclusions have also been reached for other transition-metal-oxide supports. Tong et al. [133], based on the EPR and Raman spectroscopy characterizations, found that the sulfate-derived Pt/MnO₂–S catalyst, with its rich oxygen-vacancy population, achieved complete conversion of 200 ppm HCHO at 50 °C and performed much better than catalysts prepared from other precursors. The situ DRIFTS further revealed that oxygen vacancies promote HCHO adsorption and activation to form DOM intermediates while accelerating lattice oxygen migration and replenishment, thereby creating a stable cycle between oxygen vacancies and reactive oxygen species.

During catalytic oxidation, HCHO is activated through interactions with reactive oxygen species and gradually decomposes into intermediates such as DOM, formates, and carbonates before eventually being oxidized to CO₂ and H₂O. The reaction pathway of this process is strongly dependent on the oxygen content of the system, which is consistent with the “butterfly-shape dual-pathway mechanism” reported by Tong et al. [133] for Pt/MnO₂ catalysts. Under oxygen-rich conditions, surface reactive oxygen species dominate HCHO oxidation and enable rapid conversion, whereas in oxygen-deficient environments, lattice oxygen participates in the reaction, leading to the transient accumulation of formate intermediates, which are then gradually oxidized. DFT calculations further confirmed that oxygen vacancies can significantly reduce the adsorption energies of O₂ and HCHO (Eads(O₂) = −5.992 eV, Eads(HCHO) = −10.370 eV), thus promoting the activation of reactants. This rule is equally applicable to ZrO₂-based catalysts, for which surface oxygen vacancies can accelerate electron transfer, generate reactive oxygen species, and promote the rapid conversion of intermediates. Accordingly, enhancing the oxygen-vacancy concentration is an effective strategy to enhance the catalytic performance of ZrO₂-based catalysts.

Peng et al. [105] prepared a monoclinic ZrO₂ support (m-ZrO₂) by a modified P123-assisted hydrothermal method, introduced abundant oxygen vacancies onto its surface through a two-step calcination procedure, followed by loading Pt nanoparticles to fabricate a Pt–V_O–ZrO₂ catalyst. On this basis, they proposed the concept of an oxygen-vacancy-regulated SMSI interface, in which V_O sites on the ZrO₂ surface capture and activate O₂ to form active oxygen atoms (O*) with an activation barrier of only 0.3 eV, far lower than that of a defect-free ZrO₂ system. Meanwhile, V_O acts as an electron-transport channel connecting the ZrO₂ (110) surface and Pt (111), which constructs a stable Pt–V_O–ZrO₂ interface and markedly strengthens charge transfer between the metal and the support. The preparation procedure and structural characterization are shown in Figure 10. The synergy between oxygen vacancies and SMSI enables high dispersion of noble metal nanoparticles, with an average particle size of 2–5 nm, and induces the partial conversion of Pt⁰ to oxidized species such as Pt²⁺ and Pt⁴⁺. These oxidized species, together with surface hydroxyls and reactive oxygen on ZrO₂, achieve efficient activation of HCHO and O₂. As a result, the 0.87 wt% Pt–V_O–ZrO₂ catalyst achieved a HCHO conversion of 95% for 100 ppm HCHO at 20 °C, with no obvious decay in activity after 720 min of continuous reaction. It can also reduce even 1 ppm HCHO to below 0.05 ppm, demonstrating outstanding room temperature activity and stability. Similarly, Peng et al. [88] prepared an Ir₁–N–C/ZrO₂ catalyst and found that ZrO₂ nanoparticles can tune the electronic properties of single Ir atoms through charge redistribution, which synergistically promotes the activation of adsorbed oxygen and lattice oxygen to generate reactive oxygen species. In addition, the Ir–C–Zr channel accelerates the decomposition of ·O₂⁻ into O*, and the resulting reactive oxygen species can rapidly oxidize HCHO into DOM, formate species, CO₂, and H₂O. The TPO experiments conducted by Colussi et al. [83] also showed that bare ZrO₂ generates a CO₂ characteristic peak in the range of 250–300 °C, whereas Pt/ZrO₂ shows a significantly lower CO₂ formation temperature with no HCHO signal detected. This further confirms that the synergistic interaction between Pt and ZrO₂ can effectively promote the decomposition and oxidation of adsorbed HCHO. In summary, strong interfacial interactions are crucial for anchoring metal nanoparticles, weakening Zr–O bonds, and enabling interfacial electron or oxygen atom transfer, all of which are core guidelines for the design of high-efficiency ZrO₂-based catalysts [134,135].

4.3. Elemental Doping

Elemental doping is an important strategy for regulating the electronic structure and surface properties of ZrO₂-based catalysts. The introduction of heteroatoms can alter the lattice structure of ZrO₂, the density of acidic/basic sites, and the oxygen-vacancy concentration, thereby enhancing catalytic performance. Metal dopants can be mainly divided into two categories: transition metal elements (Fe, Co, Ni, Cu, etc.) and rare earth elements (Ce, La, Nd, Y, etc.). Transition-metal dopants can introduce new active sites, simultaneously tailor the electronic structure of ZrO₂, and strengthen its interaction with HCHO molecules. Beyond single-element doping, compositing ZrO₂ with transition-metal oxides or rare earth oxides is also an effective modification strategy. Liu et al. [136] prepared an Fe₃O₄/ZrO₂ composite catalyst by low-temperature co-precipitation. The introduction of ZrO₂ increased the specific surface area of Fe₃O₄ from 92.96 to 128.96 m²/g, thereby effectively suppressing the loss of active sites caused by the magnetic aggregation of Fe₃O₄. Besides, the electronic interaction between Fe and Zr also promoted the generation of reactive oxygen species. As shown in Figure 11, Fe₃O₄/ZrO₂ exhibits a typical mesoporous structure with pores concentrated in the range of 0–15 nm and an average pore size of 7.509 nm. Its high specific surface area and rational pore structure effectively suppress the loss of active sites caused by the magnetic aggregation of Fe₃O₄. In this composite system, ZrO₂ acts as the support to provide a high specific surface area and stable structural framework, while the transition-metal oxide supplies active centers; the synergy between the two optimizes catalytic behavior. Similarly, this composite strategy can be extended to HCHO catalytic oxidation. Composites of ZrO₂ with transition-metal oxides such as MnO_x and Co₃O₄ are expected to improve catalytic activity by suppressing aggregation and promoting electron transfer, opening new avenues for the composite modification of ZrO₂-based catalysts. Wyrwalski et al. [92] confirmed that 5 mol% Y₂O₃ doping can fully stabilize ZrO₂ in the tetragonal phase, preventing phase transformation even at low temperature. Meanwhile, the ionic radius mismatch between Y³⁺ (0.090 nm) and Zr⁴⁺ (0.072 nm) induces lattice distortion and generates abundant anion vacancies. These oxygen vacancies not only enhance the oxygen-storage/release capacity of the support but also improve the adsorption and anchoring of active components, thereby providing abundant active sites for subsequent catalytic reactions. Shi et al. [137] prepared Ce_xZr_1−xO_y composite catalysts by a hydrothermal method using Pluronic F127 as the template, which achieved an HCHO conversion of 84.69% at room temperature after 6 h, and maintained more than 80% of its initial activity after 17 h. The core advantage of this system lies in the Ce–Zr synergy: the valence mismatch between Ce³⁺ and Zr⁴⁺ induces abundant oxygen vacancies, which activate O₂ to generate reactive adsorbed oxygen species that accelerate HCHO oxidation in synergy with hydroxyl groups. This strategy provides a useful quantitative reference for the precise structural regulation of ZrO₂-based catalysts.

4.4. Other Modification Strategies

Besides elemental doping, grain size is another key factor governing the stability of t-ZrO₂. Li et al. [138] confirmed by low-temperature XRD and HRTEM characterization that the martensitic transformation temperature (M_S) of t-ZrO₂ exhibits a linear correlation with the negative square root of its grain size (M_S = D·$d_{\mathrm{t}-\mathrm{Z}\mathrm{r}{\mathrm{O}}_2}^{-1/2}$ + E). When $d_{\mathrm{t}-\mathrm{Z}\mathrm{r}{\mathrm{O}}_2}$ is reduced to the critical size of 20 nm, t-ZrO₂ can remain completely stable in the temperature range of 203.15–293.15 K without transforming into the monoclinic phase. This finding provides a new strategy for crystal-phase regulation: by reducing the grain size to 20 nm or below, low-temperature stabilization of t-ZrO₂ can be achieved without elemental doping, thereby avoiding the influence of dopant-induced lattice distortion on catalytic sites. In addition, compositing ZrO₂ with mineral materials is a promising modification route. Divakar et al. [139] prepared an FAp@ZrO₂ composite catalyst through a facile preparation process. The combination of ZrO₂ and fluorapatite (FAp) not only preserves the Lewis acidic sites of ZrO₂ and the basic sites of FAp, but also constructs a synergistic acid–base bifunctional surface. Meanwhile, it also endows the catalyst with a favorable mesoporous structure, featuring a specific surface area of 47.2 m²/g and an average pore size of 14 nm, which enhances substrate adsorption and reaction efficiency. The system integrates a stable structural framework, basic sites, and acidic active centers to deliver improved catalytic performance. And this strategy could be extended to catalytic HCHO oxidation: combining ZrO₂ with mineral materials such as fluorapatite and zeolites is expected to strengthen the adsorption and activation of HCHO through acid–base-site synergy, thereby opening up a new direction for the composite modification of ZrO₂-based catalysts.

5. Conclusions and Outlook

This review systematically summarizes recent research on ZrO₂-based catalysts for catalytic HCHO oxidation, with a focus on the physicochemical properties of ZrO₂ supports, catalytic reaction mechanisms, as well as structural regulation and modification strategies. It also provides an in-depth discussion on the influence of laws of key factors, including crystal-phase regulation, elemental doping, strong metal–support interactions (SMSIs), and composite modification on catalytic performance. Benefiting from its excellent thermal stability, unique surface acid–base properties, and synergistic interactions with active components, ZrO₂ has become an ideal support for catalytic HCHO oxidation, and rational structural design enables highly efficient HCHO conversion at room temperature. The main structure–activity relationships of ZrO₂-based catalysts for HCHO oxidation are illustrated in Figure 12.

The core mechanism of catalytic HCHO oxidation follows the three classical adsorption–reaction models: MVK, L-H, and E-R. In the presence of water, reactive oxygen species on the catalyst surface can be converted into hydroxyl groups. HCHO molecules adsorb onto these surface hydroxyl groups through hydrogen bonding, and are then gradually oxidized to CO₂ and H₂O through key intermediates including DOM and formate species, or alternatively first converted to CO followed by deep oxidation via the water–gas shift reaction. A preliminary consensus has been reached regarding the influence of reaction conditions on catalytic performance: elevated temperature facilitates HCHO activation and intermediate conversion; an optimal relative humidity window of 30–75% can balance reactant adsorption and active site exposure; HCHO with a low initial concentration is more liable to be completely removed; a lower space velocity prolongs the contact time between reactants and the catalyst; the optimal dosage of the catalyst ranges from 100 mg to 200 mg, and excessive addition yields limited improvement in catalytic efficiency.

At present, it is difficult to identify a single catalyst family that can be universally recognized as optimal across all evaluation criteria for ZrO₂-based catalysts. Instead, the selection of a more suitable catalyst family is contingent on the specific evaluation metrics that are prioritized, especially when considering the balance among activity, noble metal loading, water resistance, and long-term stability. The Ir single-atom system anchored on ZrO₂ nanoparticles stands out as one of the most well-balanced options available to date when minimal noble metal loading is prioritized alongside high room temperature activity and broad humidity tolerance. This system achieves over 95% HCHO conversion at 20 °C with only 0.25 wt% Ir loading and maintains conversion rates above 80% even at 75% relative humidity (RH) [88], though its long-term stability under complex real-world operating conditions requires further investigation. For researchers prioritizing the classical Pt/ZrO₂ platform with comprehensive mechanistic evidence, the oxygen-vacancy-associated Pt–V_O–ZrO₂ system remains a highly reliable benchmark. It exhibits room temperature catalytic activity, and direct experimental evidence confirms V_O-assisted strong metal–support interaction (SMSI), the generation of reactive O* species, and rapid mineralization of reaction intermediates [105]. Nevertheless, its activity declines under high humidity conditions, indicating that moisture tolerance has not been fully optimized, and additional in-depth studies are needed to assess its long-term stability during continuous operation [105]. Ce–Zr mixed oxides represent a promising avenue for reducing catalyst costs while preserving room temperature activity. Through oxygen-vacancy engineering and the Ce³⁺/Ce⁴⁺ redox cycle, these oxides deliver high catalytic activity without relying on high noble metal loading [126,140]. However, their water resistance and long-term stability under harsh operating conditions still need to be enhanced to compete with noble metal-based catalyst systems. Overall, it remains challenging to identify a catalyst system that exhibits perfect performance across all aspects, including activity, precious metal loading, water resistance, and long-term stability. Each type of system has its own inherent advantages and drawbacks. Taking multiple factors into comprehensive consideration, the Ir single-atom system coupled with ZrO₂ nanoparticles may be closer to the level of comprehensive optimization compared with other catalyst systems. Nevertheless, the long-term stability of this system under complex practical operating conditions still needs to be improved, and there remains room for further optimization of its overall performance.

Key strategies to improve the stability of ZrO₂-based catalysts include the following aspects: optimizing the pore structure, surface defects, and morphology of the support; strengthening the interaction between active components and the support to suppress migration and aggregation of active species; and doping with transition metals or non-metal elements to regulate the electronic structure of active sites and improve sintering resistance. Nevertheless, ZrO₂-based catalysts still face challenges in practical applications, including the high usage of noble metals, insufficient efficiency for the removal of ultralow-concentration HCHO, and limited resistance to interference under complex environmental conditions. Future work may therefore focus on these directions.

Overall, although no single universally transferable descriptor has yet been established across all systems, a relatively consistent trend has emerged: a higher density of effective oxygen vacancies, more favorable interfacial charge redistribution, and more stable regulation of the active-metal oxidation state generally correspond to stronger O₂ activation, faster deep oxidation of intermediates, and better room temperature activity. In Pt–V_O–ZrO₂, for example, oxygen vacancies not only favor the formation of interfacial reactive oxygen species, but also alter the co-adsorption behavior of HCHO and O₂ and the subsequent reaction pathway. Similar roles of defect structure, electron transfer, and local coordination environment have also been implicated in Ce–Zr mixed oxides and ZrO₂-coupled single-atom systems, where they affect both the reaction rate and the humidity response.

5.1. Optimization of Catalytic Systems

Future catalyst design should focus on the anchoring of single atoms or subnanometer noble metal species (including Pt, Pd, and Ag) on defect-rich ZrO₂ or ZrO₂-based composite supports (such as ZrO₂–CeO₂ and ZrO₂–MnO₂). By exploiting strong metal–support interactions together with oxygen-vacancy effects, this design strategy is expected to achieve complete HCHO oxidation at room temperature or even below 0 °C while minimizing the loading of noble metals. Another important direction is the development of noble metal-free or low-noble metal formulations, which can be realized by constructing interfacial heterojunctions between ZrO₂ and transition-metal oxides such as Co₃O₄ and MnO_x and by utilizing defect engineering, including oxygen vacancies and lattice distortion, to increase the concentration of reactive oxygen species and improve electron-transfer efficiency.

5.2. Machine Learning-Assisted Design

Data-driven methods such as machine learning (ML) [141] can be introduced to construct dedicated databases for ZrO₂-based catalysts by integrating preparation parameters, including dopant ratio, template dosage, and calcination temperature, with structural descriptors such as grain size, oxygen-vacancy concentration, specific surface area, and acid–base-site density, together with reaction conditions and catalytic performance data. Algorithms such as extreme gradient boosting (XGBoost) can then be employed to construct performance-prediction models, while SHAP analysis can be applied to quantify the contributions of metal–support interactions, active-component electronegativity, specific surface area, and related factors. This data-driven framework enables rapid screening of the optimal structural combinations, and effectively avoids blind trial and error in conventional experiments. In the future, an integrated loop of experimental validation, data accumulation, model prediction, and process optimization could be established, together with a standardized catalytic-evaluation protocol that minimizes interference caused by differences in testing conditions, thus accelerating the industrial deployment of high-performance ZrO₂-based catalysts for HCHO oxidation.

In summary, ZrO₂-based catalysts have broad application prospects in the field of HCHO catalytic oxidation. Through the deep integration of materials innovation, process optimization, and data-driven research and development, these catalysts are expected to realize large-scale application in real indoor air purification scenarios and provide efficient, stable, and economical solutions for the control of formaldehyde pollution.