ACTIVATED CARBON FIBERS WITH IRON-CONTAINING NANOPHASES

ABSTRACT

Activated carbon fibers are widely employed as supports for iron-containing nanophases due to their accessible porous structure and continuous fibrous morphology. This review presents a systematic analysis of approaches to the preparation of iron-modified activated carbon fibers with a focus on the role of the carbon fiber matrix in governing phase composition, dispersion, and stability of iron-containing nanophases. Particular attention is given to structure–property relationships that determine adsorption, catalytic, and magnetic behavior of these materials. The analysis shows that surface chemistry, pore accessibility, and matrix confinement effects inherent to activated carbon fibers enable efficient stabilization of nanoscale iron phases at moderate metal loadings while preserving the porous structure of the fibers. The presented overview highlights the importance of the fibrous carbon matrix as an active structure-directing component and provides a basis for the rational design of iron-containing carbon fiber materials for functional applications.

АННОТАЦИЯ

Активированные углеродные волокна (УВ) широко используются в качестве носителей железосодержащих нанофаз благодаря доступной пористой структуре и непрерывной волокнистой морфологии. В данном обзоре представлен систематический анализ подходов к получению железосодержащих активированных УВ с акцентом на роль углеродной волокнистой матрицы в формировании фазового состава, дисперсности и стабильности железосодержащих нанофаз. Особое внимание уделено взаимосвязям «структура–свойства», определяющим адсорбционные, каталитические и магнитные характеристики этих материалов. Показано, что поверхностная химия, доступность порового пространства и эффекты матричного ограничения, присущие активированным УВ, обеспечивают эффективную стабилизацию нанодисперсных железосодержащих фаз при умеренных содержаниях металла при сохранении пористой структуры. Представленный обзор подчёркивает значение волокнистой углеродной матрицы как активного структурообразующего компонента и создаёт основу для рационального проектирования железосодержащих углеволокнистых материалов функционального назначения.

Keywords: carbon fibers; iron-containing nanophases; adsorption; catalysis.

Ключевые слова: углеродные волокна; железосодержащие нанофазы; адсорбция; катализ.

INTRODUCTION

Activated carbon fibers (ACFs) are porous carbon materials with a continuous fibrous morphology and high surface accessibility. Compared to granular activated carbons, ACFs provide efficient mass transfer and structural integrity, which makes them suitable matrices for the formation and stabilization of functional nanophases.

Iron-containing nanophases are widely applied in adsorption, heterogeneous catalysis, and oxidation processes. When supported on activated carbon fibers, the formation and behavior of iron phases are strongly influenced by the structural features and surface chemistry of the fibrous carbon matrix. Despite extensive research on iron-modified carbon materials, the specific role of activated carbon fibers as matrices for iron-containing nanophases remains insufficiently systematized. In particular, relationships between fiber structure, iron phase composition, and functional properties are often discussed in an application-oriented manner. Recent reviews emphasize the growing importance of activated carbon fibers as functional supports; however, they primarily focus on performance aspects, while the structure-directing role of the fibrous carbon matrix in metal phase stabilization remains largely unaddressed [1].

Accordingly, this review summarizes methods for the preparation of iron-modified ACFs and analyzes their structural characteristics with a particular emphasis on structure–property relationships governed by the fibrous carbon matrix.

CARBON FIBER AND ACTIVATED CARBON FIBER SUBSTRATES

Unlike granular or powdered activated carbons, ACFs,  exhibit uniform fiber diameters and direct pore accessibility from the external surface, which provides reduced diffusion resistance and rapid mass transfer in adsorption and catalytic processes [2,3]. The porous structure of ACFs is predominantly microporous, although the pore size distribution strongly depends on the precursor and activation method. Physical activation typically yields narrow micropore distributions, whereas chemical activation results in broader porosity and higher specific surface areas, often exceeding several hundred square meters per gram [4,5]. Due to the open pore architecture, a larger fraction of the internal surface of ACFs is effectively utilized compared to granular carbons.

An important feature of ACFs is their mechanical integrity. Fiber-based materials can be processed into fabrics or monolithic structures without binders, preserving porosity and avoiding additional diffusion barriers [6]. These characteristics are particularly advantageous for applications under dynamic flow conditions and repeated
operation cycles.

Surface chemistry plays a key role in the interaction of activated carbon fibers with iron species. Activation introduces oxygen-containing functional groups associated with edge sites and structural defects, which act as anchoring sites for iron ions and promote uniform nucleation of iron-containing nanophases [7]. Thermal treatment modifies metal–support interactions: moderate temperatures preserve surface functionalities, whereas high-temperature treatment reduces surface oxygen content and may induce carbothermal reduction of iron oxides [8]. Due to short diffusion paths and favorable hydrodynamics, activated carbon fibers exhibit faster adsorption kinetics and more efficient utilization of the active surface than granular carbons [9]. In iron-modified systems, these features favor uniform dispersion of iron-containing nanophases and improved resistance to agglomeration, enabling high functional efficiency at relatively low metal loadings [10]. Overall, the structural and chemical properties of activated carbon fibers make them effective matrices for the formation and stabilization of iron-containing nanophases, providing the basis for the synthesis strategies and structure–property relationships discussed in the following sections [11].

METHODS FOR THE PREPARATION OF IRON-MODIFIED ACTIVATED CARBON FIBERS

Iron-containing nanophases are introduced onto activated carbon fibers using preparation routes that take advantage of their accessible porosity and chemically active surface [12]. The most widely used approach is wet impregnation with iron salt solutions, followed by controlled drying and thermal treatment. This method enables reproducible control of iron loading and dispersion and is commonly applied for catalytic and environmental applications [13].

The open pore architecture of activated carbon fibers facilitates penetration of iron precursors and promotes a more uniform distribution compared to granular carbon supports. At low precursor concentrations, finely dispersed iron-containing nanophases are formed, whereas higher loadings may lead to partial pore blocking and particle agglomeration [13].

In situ synthesis routes represent an alternative strategy in which iron oxides are generated directly on the fiber surface or within the pore system through controlled precipitation or growth processes. Such approaches strengthen the interaction between iron species and surface functional groups and improve the stability of the resulting nanophases, as shown for Fe₃O₄-containing carbon fiber composites [14,15]. Calcination in inert or oxidative atmospheres favors the formation of iron oxide phases, whereas reducing or carbothermal conditions can induce partial or complete reduction to metallic iron [16]. The fibrous carbon matrix restricts particle growth and influences local redox conditions, thereby affecting phase evolution [17].

Surface oxygen-containing functional groups act as anchoring sites for iron species during impregnation and in situ growth. These groups promote heterogeneous nucleation and suppress excessive sintering during subsequent thermal treatment, contributing to improved dispersion and stability of iron-containing nanophases [7]. By appropriate selection of synthesis route and thermal treatment parameters, activated carbon fibers can be converted into iron-modified materials with tailored phase composition, dispersion, and functionality. This versatility makes ACFs an effective platform for the controlled formation of iron-containing nanophases while preserving the structural advantages of the fibrous carbon matrix [12, 16].

PHASE COMPOSITION, STRUCTURAL CHARACTERISTICS, AND FUNCTIONAL BEHAVIOR OF IRON-CONTAINING NANOPHASES

The phase composition of iron-containing nanophases supported on activated carbon fibers is mainly determined by iron loading and thermal treatment conditions. Depending on atmosphere and temperature, iron species may be present as oxides or metallic phases. Confinement within the fibrous carbon matrix restricts particle growth and alters transformation pathways compared to bulk or granular systems, contributing to the stabilization of nanoscale iron phases [16,18].

Thermal treatment plays a decisive role in structural evolution. Heat treatment under inert or reducing conditions promotes partial or complete reduction of iron oxides, whereas oxidative environments stabilize oxide phases. Structural investigations show that the porous carbon fiber matrix suppresses sintering and limits crystallite growth, resulting in well-dispersed iron-containing nanophases with enhanced stability [19]. Iron loading strongly influences dispersion and textural properties. At low loadings, iron-containing phases are finely dispersed and largely preserve the original porosity of the fibers [20]. Increasing iron content leads to particle growth and partial pore blocking, which reduces accessible surface area and modifies pore size distribution [21]. Comparative studies demonstrate that activated carbon fibers provide more uniform dispersion of iron-containing phases than granular activated carbons due to higher pore accessibility and reduced diffusion limitations [22].

The structural features of iron-modified activated carbon fibers directly govern their functional behavior. Fine dispersion of iron-containing nanophases enhances adsorption performance toward organic pollutants [23], particularly when active sites remain accessible near the fiber surface [24]. In catalytic applications, iron-modified fibers exhibit activity in advanced oxidation processes, where the carbon matrix facilitates mass transfer while iron species participate in redox cycles [25,26].

Magnetic functionality represents an additional advantage of iron-modified activated carbon fibers. Stabilization of magnetite or metallic iron phases enables magnetic separation and recovery of spent materials without significant loss of structural integrity. Effective magnetic response can be achieved at moderate iron loadings when particle growth is controlled by matrix confinement effects [27,28]. Overall, the properties of iron-modified activated carbon fibers reflect a balance between phase composition, dispersion, and preservation of the fibrous carbon matrix. Controlled synthesis and thermal treatment allow tuning of structural and functional characteristics while maintaining the intrinsic advantages of activated carbon fibers [16, 26]. 

CONCLUSIONS

Activated carbon fibers represent an effective matrix for the formation and stabilization of iron-containing nanophases due to their accessible porous structure, surface functionality, and continuous fibrous morphology. The analysis demonstrates that the carbon fiber matrix plays an active structure-directing role by governing phase composition, dispersion, and stability of iron-containing species during synthesis and thermal treatment. Matrix confinement effects and surface chemical interactions suppress excessive particle growth and enable stabilization of nanoscale iron phases at moderate metal loadings while preserving pore accessibility. As a result, iron-modified activated carbon fibers exhibit enhanced adsorption, catalytic, and magnetic performance without significant loss of the intrinsic structural advantages of the fibrous carbon support. These features provide a rational basis for the design of iron-containing carbon fiber materials for functional and environmental applications [29].

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