Preparation of Hollow Spherical Ni-CaO-Ca₂SiO₄ Materials and Their Performance in Enhanced Hydrogen Production from Pine Sawdust

Biomass sorption-enhanced steam reforming is a promising route for the efficient conversion of biomass into H₂-rich syngas by improving H₂ selectivity and yield. This study investigates the design and performance of CaO-based hybrid materials for the sorption-enhanced steam reforming (SESR) of pine sawdust for H₂ production. The hybrid materials were synthesized by incorporating different proportions of polymorphic Ca₂SiO₄ into CaO via a hydrothermal method followed by carbon-template removal. A homogeneous precursor solution containing Ni, Ca, and Si species was transferred into a 50 mL autoclave and subjected to hydrothermal treatment at 200 °C for 36 h. The obtained samples were then dried and calcined in air by heating to 800 °C at a rate of 5 °C·min⁻¹, followed by a holding period of 2 h. After carbon-template removal, hollow-shell sorbents were obtained. The CO₂ sorption capacity and cyclic stability of the undoped sorbents were evaluated, and 10 wt.% Ni was subsequently introduced into the optimized sample. The results indicate that Ca₂SiO₄ loading significantly affects the balance between CO₂ uptake and cyclic stability. Among the tested sorbents, the sample containing 10 wt.% Ca₂SiO₄ exhibited the best overall performance, achieving the highest cumulative CO₂ uptake over 10 cycles while maintaining relatively high CaO utilization. Structural characterization revealed that the stabilization effect of Ca₂SiO₄ arises both from the dilution of the active phase and from its role as a spatially distributed inert framework between CaO grains. The results from X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area analysis, and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM–EDS) consistently suggest that Ca₂SiO₄ was uniformly dispersed within the hollow shell. This distribution physically separated adjacent CaO particles, restricted grain growth during repeated carbonation and calcination, and helped preserve pore volume and accessible surface area. Consequently, this microstructural stabilization effectively suppressed sintering and delayed the loss of fast-reaction sites. Furthermore, Ni incorporation reduced the CaO crystallite size and improved the utilization of active CaO while preserving the hollow-shell morphology. Upon further doping with 10 wt.% Ni, both H₂ production and purity were significantly enhanced compared with those of the undoped sorbents. After 10 carbonation cycles, the Ni₁₀Ca₉Si₁-HS sorbent maintained a H₂ yield of 1.80 mmol/(g_bm·g_mat·min), representing only a 4.32% decrease from the initial value of 1.88 mmol/(g_bm·g_mat·min). Meanwhile, the H₂ purity decreased slightly from 71.50% to 67.63%, demonstrating excellent cyclic stability. This study demonstrates that morphological control and the use of polymorphic stabilizers are crucial for improving the cyclic stability of catalyst-sorbent hybrid materials for sustainable H₂ production from biomass, providing guidance for the structural design of highly efficient CaO-based hybrid materials.