# Spherical MgSiO3–NH2 Adsorbents with Optimized Surface Chemistry for Humidity-Enhanced Direct Air CO2 Capture

## TL;DR

This study develops spherical MgSiO3–NH2 adsorbents that capture more CO2 in humid conditions, improving efficiency and stability for direct air capture.

## Contribution

The paper introduces an integrated design strategy combining particle morphology control and surface chemistry optimization for enhanced CO2 capture.

## Key findings

- Spherical MgSiO3 particles with 15 μm diameter reduce fines and hydrodynamic resistance.
- Adsorbents achieved 1.7–1.8 mmol/g CO2 capture at 50% humidity, a fourfold increase over dry conditions.
- Humidity shifts CO2 capture mechanism to bicarbonate formation, enabling stable regeneration at 100 °C.

## Abstract

Amine-functionalized solid adsorbents are widely recognized as promising candidates for direct air capture of CO2; however, their practical deployment remains constrained by humidity-dependent adsorption behavior and poor packed-bed operability arising from irregular particle morphology and fines generation. Rather than focusing solely on maximizing intrinsic adsorption capacity, this study addresses these process-level limitations through an integrated design strategy combining particle morphology control with surface chemistry optimization. Uniform spherical magnesium silicate particles with a mean diameter of approximately 15 μm were synthesized via a water-in-oil emulsion route to suppress fines formation and reduce hydrodynamic resistance. Controlled acid pretreatment was subsequently applied to adjust surface hydroxyl accessibility and enable efficient amine grafting without altering bulk composition. The optimized spherical magnesium silicate amine adsorbents exhibited pronounced humidity-enhanced carbon dioxide capture, achieving capacities of 1.7 to 1.8 millimoles/g at 50% relative humidity, representing an approximately fourfold increase compared with dry conditions. This enhancement is attributed to a humidity-induced mechanistic transition from carbamate formation under dry conditions to water-assisted bicarbonate formation under humid conditions. Complete regeneration was achieved at 100 °C, with stable adsorption desorption behavior maintained over ten consecutive cycles, demonstrating short-term reversibility. These findings highlight morphology controlled scalability. Future work should prioritize durability beyond 100 cycles, mechanical robustness, and techno-economic viability at scale.