Unraveling the role of CaO in H2 production via sorption-enhanced steam gasification of biochar

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The integration of in-situ CO 2 capture utilizing CaO-based catalyst/sorbents in conventional biomass steam gasification (SG) processes for “green hydrogen” production can address the limitations of low hydrogen yield, elevated CO levels, and inherent high endothermicity. Nevertheless, the lack of a definitive molecular understanding of the synergistic interactions between the catalytic and sorption behavior hinders their development. Here, with thermodynamic analysis, online hydrogen tests, in-situ characterization and periodic density functional theory (DFT) calculations, we demonstrate that CaO-based sorbents transform the inherently endothermic SG process into a net exothermic one over a broad temperature window via thermodynamic coupling with the highly exothermic carbonation reaction, thereby markedly reducing the requirement for external heat input. The BC-Ca sample (bamboo char with CaO) achieves a nearly 80% average volumetric relative concentration of hydrogen at 700 °C, representing a 33.3% increase over pure char, with a 17% improvement in overall yield, thereby confirming the dual catalytic and sorbent roles of CaO. DFT calculations, supported by in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) further reveal that the dominant reaction pathway involves direct reaction of monatomic carbon (C*) simultaneously with two surface OH* groups (water dissociation on CaO (1 0 0) surface), forming CO*, COOH*, CO 2 *, and two H 2 *, with the formation of CO* (1.52 eV) identified as the rate-limiting step. Conversely, the indirect pathway, involving sequential reactions with two surface OH* one by one, exhibits higher activation barriers for both H 2 * evolution (1.76 and 1.41 eV), making it thermodynamically less favorable.