Chemical Activation and Controlled Carbonization

• Chemical activation and controlled carbonization are processes that convert organic precursors into engineered carbon materials with tunable porosity and surface properties.
• These techniques use activating agents and precise heat-treatment protocols to create hierarchical pore networks ideal for gas adsorption, catalysis, and energy storage.
• Optimized processing yields high BET surface areas, controlled pore sizes, and enhanced functional performances, supporting applications from CO₂ capture to supercapacitors.

Chemical activation and controlled carbonization are integrated processes employed to engineer the porosity, specific surface area, surface functionality, and bulk properties of carbonaceous materials derived from organic precursors. These processes underpin the rational design of microporous and mesoporous carbons, nitrogen-doped frameworks, activated carbons, carbon nanoparticles, and graphene-like structures for use in gas adsorption, catalysis, supercapacitor electrodes, and advanced functional materials.

1. Fundamental Principles and Process Overview

Chemical activation refers to the use of activating agents—typically alkali/alkaline earth hydroxides, acids, or salts—that selectively react with or etch the carbon matrix, generating a hierarchical network of micro- (<2 nm), meso- (2–10 nm), and macropores. Controlled carbonization is the thermal conversion of organic precursors to carbon under inert or reducing atmospheres, defining the carbon skeleton’s degree of graphitization, nitrogen retention, and aromaticity.

The prototypical workflow comprises precursor preparation (including functionalization and/or blending with activators), heat treatment (involving precisely defined ramp rates, dwell temperatures, and durations), and systematic post-treatment to remove residual inorganic species and optimize physicochemical properties. Synergistic integration of these steps enables precise tuning of pore architecture, surface chemistry, and, consequently, material performance in target applications (Gong et al., 2018, Mansurov et al., 2022, Sharifi et al., 24 Jul 2025).

2. Chemical Activation Strategies

2.1 Alkali Activation (KOH)

KOH activation—pioneered across a variety of precursors (e.g., rice husk, poly(ionic liquid), cellulose derivatives)—remains dominant due to its efficacy in micropore generation and creation of ultrahigh surface areas.

The canonical reaction scheme during high-temperature activation (700–900 °C) is:

$\begin{align*} 6\,\mathrm{KOH} + 2\,\mathrm{C} &\longrightarrow 2\,\mathrm{K} + 3\,\mathrm{H_2} + 2\,\mathrm{K_2CO_3} \tag{1} \ K_2CO_3 + 3\,\mathrm{C} &\longrightarrow K_2O + 2\,\mathrm{CO} \tag{2} \ K_2CO_3 &\longrightarrow K_2O + CO_2 \tag{3} \ 2\,\mathrm{K} + CO_2 &\longrightarrow K_2O + CO \tag{4} \ K_2O + C &\longrightarrow 2\,K + CO \tag{5} \end{align*}$

Metallic potassium intercalation and gaseous product evolution disrupt the carbon matrix, expand interlayer spacing, and develop micro- and mesoporosity. Alkali activation achieves BET surface areas up to 2 087 m²/g for rice husk-derived carbons (1:3 RH:KOH at 750 °C) and 1 742 m²/g for nitrogen-doped carbons from PIL precursors (KOH:PIL=4:1 at 900 °C) (Gong et al., 2018, Mansurov et al., 2022).

2.2 Acid and Salt Activation (H₃PO₄, ZnCl₂)

Activation via H₃PO₄ (impregnation ratio 1:1) favors dehydration and crosslinking, creating an all-carbon matrix with narrow microporosity and BET area up to 361 m²/g. ZnCl₂ (1:2) induces partial hydrolysis and aromatization, yielding materials that combine micro- and mesoporosity (surface area ≈ 732 m²/g) (Mansurov et al., 2022).

2.3 Nitrogen Source Functionalization

Hydrothermal carbonization of carboxymethyl cellulose in the presence of urea demonstrates that mild amination, Maillard-type chemistry, and dehydration of amide intermediates can install –NH₂, –NH–, and –C≡N sites, increasing the nitrogen content and promoting micropore formation (max N ~4.2 wt % in urea–assisted CNPs) (Sharifi et al., 24 Jul 2025).

3. Controlled Carbonization: Protocols and Mechanistic Considerations

The carbonization step dictates yield, structure, and chemical functionality. Standard heat-treatment protocols involve:

• Ramp Rate: 5–10 °C min⁻¹, enabling uniform heating and preserving framework integrity
• Target Temperatures: 700–900 °C for KOH, 700 °C for H₃PO₄/ZnCl₂, 900 °C for CNPs derived from CMC
• Dwell Time: 1–2 h, sufficient for complete decomposition/aromatization and activation
• Atmosphere: Inert (N₂, ~100 mL min⁻¹), to suppress combustion and control oxygen content
• Post-Activation: Sequential acid/base/water washing removes residual salts (K₂CO₃, K₂SiO₃) and silica (for RH), ensuring high purity and stable porosity (Gong et al., 2018, Mansurov et al., 2022, Sharifi et al., 24 Jul 2025)

At high temperature, cyano- or amide-containing groups trimerize or dehydrate to yield polytriazine and aromatic domains, respectively, preserving nitrogen in pyridinic, pyrrolic, or graphitic forms—a key feature for catalytic/basic site generation (Gong et al., 2018, Sharifi et al., 24 Jul 2025).

4. Structure, Properties, and Performance Metrics

4.1 Yield and Composition

Process yields depend on precursor, activation ratio, and temperature. For PIL-derived carbons, carbonization yield decreases from 67 wt % (no KOH) to 47 wt % (KOH:PIL=6:1). For rice husk, typical yields range 20–25 % for KOH activation and 10–15 % for graphene-like carbon at 900 °C (Gong et al., 2018, Mansurov et al., 2022).

Nitrogen retention is affected by activation severity, with surface N ranging from 12.1 at % (no KOH) to 4.5 at % (KOH:PIL=6:1) in PIL-derived carbons (Gong et al., 2018). In CMC-derived nanoparticles, urea increases bulk N (1.4 → 4.2 wt %) in the raw CNP and 3 wt % after 900 °C activation (Sharifi et al., 24 Jul 2025).

4.2 Porosity and Surface Area

A combination of BET and NLDFT/BJH methods is used:

Sample/Method	S_BET (m²/g)	V_tot (cm³/g)	Pore Type
NPC-4 (KOH:PIL=4, 900 °C)	1 742	1.415	micro+mesoporous
RHAC (RH:KOH=1:3, 750 °C)	2 087	1.22	micropore-rich
ACNPsu (CMC+urea, 900 °C)	351	0.28	microporous (3–4 nm pores)
ACNPs (CMC, 900 °C)	553	0.46	microporous
H₃PO₄–RH	361	0.30	narrow micropores
ZnCl₂–RH	732	0.58	micro+mesopores

Surface areas span <20 m²/g (non-activated) to >2 000 m²/g (optimized KOH activation), with pore structures ranging from type I isotherms (micropores) to mixed I/IV (hierarchical) (Gong et al., 2018, Mansurov et al., 2022, Sharifi et al., 24 Jul 2025).

4.3 Functional Performance

• CO₂ adsorption: Up to 6.2 mmol g⁻¹ at 273 K, 1 bar for PIL-KOH carbons (NPC-4), with selectivity CO₂/N₂ ~14–17, isosteric heat Q_st 15–32 kJ mol⁻¹, and high cycle stability (ca. 95 % retention after 10 cycles) (Gong et al., 2018).
• Catalysis: Metal-free oxidation of benzyl alcohol reaches 99.5 % conversion and 100 % selectivity with N-doped NPC-4 (Gong et al., 2018).
• Drug delivery: CMC-derived ACNPs/ACNPsu with tailored –NH₂, –C≡N surface groups and high specific area demonstrate favorable encapsulation of cationic drugs via electrostatic and π–π stacking interactions (Sharifi et al., 24 Jul 2025).

5. Pore Formation Mechanisms and Precursor Effects

5.1 Redox–Gasification in Alkali Activation

The alkali activation mechanism involves oxidative etching and gas evolution. For rice-husk, residual silica acts as a physical template, stabilizing the pore network during high-temperature exposure. Post-carbonization acid extraction of SiO₂ yields higher mesoporosity and superior mass transport (Mansurov et al., 2022). The presence of nitrogen heteroatoms (from PIL, urea) biases the distribution of electronic and chemical basic sites, advantageous for CO₂ capture and catalysis (Gong et al., 2018, Sharifi et al., 24 Jul 2025).

5.2 Hydrothermal Carbonization and Functionalization

In hydrothermal conditions, urea decomposes to NH₃/HNCO, which engages in amination and dehydration reactions with oxygenated carbohydrate fragments. Maillard-type crosslinking and nucleation at lower temperature reduce particle size and introduce nitrogen functionalities. Subsequent high-temperature activation consolidates aromatic clusters while preserving surface N species (Sharifi et al., 24 Jul 2025).

5.3 Effect of Activation Ratio and Temperature

KOH:precursor ratio modulates the micro/mesopore balance (optimal mesoporosity at KOH:PIL=4:1; excessive KOH leads to pore coalescence and diminished surface area). Carbonization at 900 °C drives partial graphitization and stabilizes triazine or aromatic domains (Gong et al., 2018).

6. Methodological Advances and Design Guidelines

Rational process optimization exploits the following controls:

• Precursor selection: Nitrogen-rich, functionalized polymers (PIL, CMC+urea) produce N-doped frameworks; biomass (rice husk) leverages in-situ templating by silica.
• Activation agent: KOH for maximized surface area and microporosity; acid/salt activation for targeted pore size distribution.
• Thermal regime: Balancing activation temperature and dwell time to optimize trade-offs between surface area, graphitic content, and heteroatom retention.
• Structure-property mapping: Systematic BET/NLDFT characterization aligned with performance criteria (adsorption capacity, catalytic selectivity, conductivity) (Gong et al., 2018, Mansurov et al., 2022, Sharifi et al., 24 Jul 2025).

Generalizable design rules extracted include leveraging Maillard or cyano chemistries for nitrogen installation, using in-situ templates (SiO₂), and harnessing precise mass ratios of activating agents to modulate pore hierarchy and total surface area.

7. Applications and Performance Optimization

Chemically activated, controllably carbonized carbons are foundational in:

• Gas capture and storage: Ultrapure, micro/mesoporous carbons for high-capacity CO₂ and H₂ uptake.
• Heterogeneous catalysis: Metal-free, N-doped carbons for aerobic oxidation and other redox transformations.
• Drug delivery/sorbents: Microporous nanoparticles with surface amine/nitrile functionality for encapsulation, release kinetics, and selective sorption.
• Energy storage: Electrodes for supercapacitors—mesopore-rich frameworks for rapid ion transport, high surface area for electric double-layer capacitance.
• Photonic and emerging applications: C–H activation by TMDCs enables photopatternable synthesis of luminescent carbon dots, demonstrating the frontier of light-driven carbonization and spatially programmable nanocarbon architectures (Li et al., 2022).

These materials’ performance is intimately linked to process variables (precursor, activation chemistry, carbonization protocol), with evidence-based metrics and structure–property frameworks guiding ongoing optimization across application domains (Gong et al., 2018, Mansurov et al., 2022, Sharifi et al., 24 Jul 2025, Li et al., 2022).