Catalytic degradation of formaldehyde in advanced composite materials: The role of modification treatments and heterogeneous interfaces

As a common volatile organic compound, formaldehyde (HCHO) is not only significantly toxic, but also comes from a wide range of sources. In addition to the traditional interior decoration material release, organic solid waste incineration and other channels, some emerging and complex multi-component composite solid waste, such as retired photovoltaic panels, waste printed circuit board and waste enameled wire, in the pyrolysis or incineration process, its contained organic components (such as ethylene–vinyl acetate copolymer, polyethylene terephthalate and brominated epoxy resin) may also become the source of HCHO generation [[1], [2], [3]]. Once this potential HCHO accumulates in the enclosed space, their threat to human health would rise dramatically, possibly in a geometric progression [4]. These potential health risks have spurred a substantial increase in research focused on the removal of HCHO. Among the available technologies, catalytic oxidation had emerged as one of the most promising methods for mitigating HCHO pollution, with its end products being almost harmless CO₂ and H₂O [5]. However, common catalysts were often limited in practical applications due to their own characteristics, such as the high cost of precious metals and the relatively low catalytic activity of transition metal oxides, which made the development of high-performance catalytic materials face major challenges.

To enhance the degradation performance of HCHO, numerous studies had focused on technological design, structural innovation, and modification of composite materials to develop multifunctional composites with stable structures and superior properties. For instance, various novel composite materials had been synthesized using ingenious preparation methods, including dip-coating (e.g., (ε-MnO₂/CeO₂)@CeO₂) [6], electrospinning (e.g., MnO₂@SiO₂-TiO₂) [7], in-situ deposition (e.g., MnO₂/UiO-66-NH₂) [8], step-by-step self-assembly (e.g., Pt/ZSM-5) [9], forming some interesting structures like core–shell structures (e.g., Au@Co₃O₄) [10], embedding structures and nanoflower types (e.g., CeO₂@LDHs) [11]. These structural innovations had increased the specific surface area of materials or mitigated the deactivation of catalytic components to some extent. To achieve a larger specific surface area and better dispersion of catalytic components, 3DOM structures (e.g., 3DOM Au/CeO₂-Co₃O₄) were developed as an alternative to traditional bulk structures [12]. In addition, a diverse array of surface vacancy defects, including oxygen vacancies (OVs) and manganese vacancies, were meticulously engineered to obtain a large number of unsaturated sites and increase the number of active sites in composite material [[13], [14], [15]]. Moreover, the electronic structure of catalytic component in composite material could also be modulated to facilitate charge separation and migration, thereby enhancing the degradation efficiency of HCHO. Among various defect formation factors, the inter-substitution of metal oxides caused by ion doping (e.g., K⁺, Mg²⁺, Ca²⁺, and Fe³⁺) was regarded as an effective strategy for altering the electronic structure of composite materials and generating defect sites [16]. Studies had demonstrated that different ion dopants, such as M−MnO_x (where M represented Ce or Co), could significantly influence the formation and distribution of surface vacancies, establishing a direct correlation between ion dopants and OVs [17]. The formation and distribution of OVs could be effectively controlled through the judicious selection of doping ions and precise regulation of their concentration to optimize the properties of materials. While these structural innovations and element doping methods had significantly enhanced the degradation performance of HCHO composites, some crystalline phases in composites would probably exhibit uncertain activity and low stability. To alleviate this problem, crystal facet engineering strategy was proposed to expose high surface energy and highly reactive crystal facets (e.g., {310} faces of α-MnO₂) [18]. By precisely controlling the crystal facets and material morphology, the surface properties of composite and the exposure of active sites could be effectively regulated, leading to substantial improvements in catalytic efficiency and selectivity.To further achieve low cost, high catalytic efficiency, and optimal metal utilization, single-atom catalyst (SAC) systems (e.g., Ag-HMO) had been developed to catalyze reactions at the atomic level, thereby maximizing the efficiency of metal atom utilization [19]. Despite the numerous advantages of SAC, challenges remained in its design and preparation, including precise control over the distribution and coordination structure of metal atoms, as well as improvements in stability and loading capacity. Additionally, the modification treatment of composite materials represented another new approach to improving the degradation performance of HCHO. Common modification techniques encompassed heat treatment, acid treatments (e.g., sulfuric acid, nitric acid, hydrochloric acid) [20,21], alkali treatments (e.g., NaOH, KOH, ammonia, tetrabutylammonium hydroxide) [22], alkaline metal ion and anion modifications, and functional group modifications (e.g., –OH, –COOH, –NH₂) [23].

Although the design, preparation, modification and other methods of new composite materials offered boundless opportunities for material structure innovation and performance improvement, it was found that these superior properties were also related to the heterogeneous interface, OVs (OVs) and other microscopic characteristics of composite materials in many cases, which made interface engineering a new focus of attention [[24], [25], [26]]. The influence of heterogeneous interface and OVs on catalysis in composites (e.g., Au/CeO₂-550 [27], Pt/ZSM-5 [28], MnO₂@PMIA-6 [29]) was a complex research field, and its effect on HCHO catalysis involved many aspects. Specifically, their influence on HCHO catalysis encompassed several dimensions, including the structural effects of heterogeneous interfaces, the synergistic interactions between support and catalyst, the combination of physical and chemical adsorption, and interfacial electronic phenomena [[27], [28], [29]]. In other words, the adsorption properties of heterogeneous interfaces had an important effect on gas aggregation, and the high reactivity of heterogeneous interfaces would significantly accelerate the catalytic degradation of HCHO on composite materials. By fine-tuning the heterogeneous interface, subtle modifications to the electronic structure and charge transport characteristics could be achieved, thereby enhancing the overall catalytic performance of composites. Compared with a mono-existent heterogeneous interface or OVs, the OVs at or near the heterogeneous interfaces of composite materials (e.g., ε-MnO₂/Mn₂V₂O₇, Pt/TiO_2-x, Pt/NaInO₂, 0.1Pt/TiO₂-NS) were considered to play a more critical role in the degradation of HCHO [[30], [31], [32]]. In short, the heterogeneous interface influenced the macroscopic properties of the material through transport and induction effects, while OVs enhanced HCHO degradation efficiency by promoting oxygen adsorption and activation, improving electron transport, and stabilizing the interface. Therefore, the influence of heterogeneous interface and oxygen vacancy should be considered in the design and optimization of composite materials to achieve efficient HCHO degradation performance.

Currently, there are hardly review articles related to heterogeneous interfaces and OVs in advanced composites to enhance HCHO degradation. Given the exponential growth of the promising subject field of composite design and its impact on air pollution control, it is timely and necessary to study the topic of efficient removal of HCHO by composite materials. In this work, we firstly introduce the structural design and modification methods of advanced composite materials. Subsequently, we conclude the influences of heterogeneous interfaces and OVs on the degradation of HCHO. This work aims to provide critical insights and references for the development of advanced composite materials optimized for effective HCHO removal through structural refinement, modification, and interface engineering.