Magnesium oxide (MgO) nanoparticles have emerged as promising adsorbents for post-combustion carbon dioxide (CO2) capture due to their high surface reactivity, moderate regeneration energy requirements, and potential for surface functionalization. Their ability to chemisorb CO2 through carbonate formation, combined with tunable synthesis methods and surface modifications, makes them a viable alternative to conventional amine-based solvents. This article explores the chemisorption mechanisms, synthesis routes, surface modification strategies, regeneration methods, and pilot-scale performance of MgO nanoparticles for CO2 adsorption.

The chemisorption of CO2 on MgO nanoparticles primarily occurs through the formation of magnesium carbonate (MgCO3) species. The process involves the reaction between CO2 and surface hydroxyl groups or oxygen vacancies on MgO. At temperatures between 50°C and 200°C, CO2 adsorbs onto basic sites of MgO, forming surface carbonates. The mechanism proceeds in two steps: physisorption of CO2 on MgO surfaces, followed by a chemical reaction leading to carbonate formation. The presence of hydroxyl groups enhances CO2 uptake by providing active sites for chemisorption. The theoretical CO2 adsorption capacity of pure MgO ranges between 0.5 to 1.0 mmol/g, depending on surface area and defect density. However, unmodified MgO suffers from slow kinetics and limited capacity due to surface passivation after initial carbonate layer formation.

Synthesis of MgO nanoparticles for CO2 adsorption typically involves thermal decomposition of magnesium precursors. The most common method is the calcination of magnesium hydroxide (Mg(OH)2) or magnesium carbonate (MgCO3) at temperatures between 400°C and 600°C. The calcination temperature directly influences the surface area and porosity of the resulting MgO nanoparticles. Lower calcination temperatures (400-450°C) produce nanoparticles with higher surface areas (100-200 m²/g) but lower crystallinity, while higher temperatures (500-600°C) yield more crystalline particles with reduced surface area (50-100 m²/g). Alternative synthesis methods include sol-gel processes using magnesium alkoxides, which allow better control over particle size and morphology. The sol-gel-derived MgO nanoparticles often exhibit higher surface areas and improved CO2 adsorption performance compared to conventionally calcined samples.

Surface modification of MgO nanoparticles significantly enhances their CO2 capture performance by introducing additional adsorption sites and improving surface basicity. Two primary modification strategies have been explored: amine functionalization and alkali salt promotion. Amine grafting involves treating MgO with organosilanes or alkanolamines, which provide additional CO2 binding sites through carbamate formation. The amine-modified MgO can achieve CO2 capacities up to 2.0 mmol/g at 75°C, with improved adsorption kinetics compared to unmodified MgO. Alkali salt promotion, particularly with sodium or potassium salts, creates molten carbonate layers on the MgO surface at elevated temperatures. These molten carbonates act as both CO2 carriers and reaction media, facilitating the diffusion of CO2 to the MgO surface. Potassium-promoted MgO has demonstrated CO2 capacities exceeding 3.0 mmol/g at 300°C, making it suitable for higher temperature applications.

Regeneration of spent MgO adsorbents is crucial for practical implementation. The most common regeneration method is thermal swing adsorption, where temperatures between 300°C and 500°C are applied to decompose the magnesium carbonate and release CO2. The energy requirement for regeneration typically ranges between 1.5 and 2.5 MJ/kg CO2, which is lower than conventional amine scrubbing systems. However, repeated thermal cycling can lead to particle sintering and gradual capacity loss. Alternative regeneration strategies include pressure swing adsorption and steam-assisted regeneration, which operate at lower temperatures but may require more complex system designs. The stability of MgO-based adsorbents varies with modification type, with alkali-promoted systems generally showing better cycling stability than amine-functionalized ones over multiple adsorption-regeneration cycles.

Pilot-scale studies have demonstrated the feasibility of MgO nanoparticles for post-combustion CO2 capture. Fixed-bed reactors packed with potassium-promoted MgO nanoparticles have achieved CO2 capture efficiencies above 80% from simulated flue gas streams (10-15% CO2) at temperatures between 200°C and 300°C. The adsorption kinetics in these systems follow a shrinking core model, where the reaction front moves inward as the carbonate layer forms. Challenges in scale-up include maintaining adsorbent performance over thousands of cycles, managing pressure drops across adsorption beds, and integrating the capture system with existing power plant infrastructure. Economic analyses suggest that MgO-based systems could reduce capture costs by 20-30% compared to amine scrubbing, primarily due to lower regeneration energy requirements and reduced solvent degradation issues.

The performance of MgO nanoparticles in real flue gas conditions depends on several factors. The presence of water vapor generally enhances CO2 adsorption by maintaining surface hydroxyl groups, while sulfur oxides (SOx) can compete with CO2 for adsorption sites and reduce capacity. Long-term exposure tests have shown that alkali-promoted MgO maintains over 70% of its initial capacity after 1000 hours of operation in simulated flue gas containing 100 ppm SO2. The particle size distribution and bed configuration also influence mass transfer limitations, with smaller particles (below 100 nm) providing faster kinetics but potentially causing higher pressure drops in packed beds.

Future developments in MgO-based CO2 adsorbents focus on improving cyclic stability and reducing production costs. Hybrid materials combining MgO with mesoporous silica or carbon matrices show promise for enhancing dispersion and preventing sintering during regeneration. Advanced synthesis techniques that precisely control defect density and surface chemistry could further optimize the CO2 adsorption performance. The integration of MgO adsorbents with emerging capture technologies, such as calcium looping or chemical looping combustion, may provide additional synergies for large-scale deployment.

In conclusion, MgO nanoparticles offer a technically viable pathway for post-combustion CO2 capture through chemisorption mechanisms that are fundamentally different from conventional liquid amine systems. While challenges remain in long-term stability and system integration, the combination of surface modification strategies and optimized regeneration methods positions MgO as a competitive adsorbent material for carbon capture applications. Continued research and pilot-scale validation will be essential to fully realize the potential of these nanomaterials in industrial CO2 capture systems.