Embedding Ni in Ni–Al Mixed-Metal Alkoxide for the Synthesis of Efficient Coking Resistant Ni–CaO-Based Catalyst-Sorbent Bifunctional Materials for Sorption-Enhanced Steam Reforming of Glycerol

Abstract

The sorption-enhanced steam reforming of glycerol (SESRG) is a sustainable approach to produce high-purity hydrogen in a single step by valorizing waste raw material. Fabricating Ni–CaO-based catalyst-sorbent bifunctional materials with highly dispersed Ni active sites is vital in suppressing coke formation during this process. While embedding Ni in the spinel structure of NiAl2O4 can provide a high distribution of Ni active sites, it can prevent an intimate contact between Ni, Al, and Ca species and adversely affect the efficiency of the SESRG process for high-purity hydrogen production. Due to the easiness to react in protic reagents and provide uniform distribution of metals, Ni–Al mixed-metal alkoxides can act as potential alternatives to NiAl2O4 to provide both the high distribution of Ni active sites and the close contact between Ni, Al, and Ca species. Herein, we developed two new Ni–CaO-based bifunctional materials, by adding the NiAl2O4 and Ni–Al mixed-metal alkoxide synthesized through a solvothermal approach during the treatment of the limestone-derived CaO sorbent with ethanol/water solution, to investigate the effects of using these kinds of Ni-containing structures on the development of coke-resistant efficient materials for SESRG. Testing the developed materials in 10 SESRG/regeneration cycles revealed that using Ni–Al mixed-metal alkoxide during the synthesis of Ca3Al2O6–CaO/NiO–CeO2 led to a higher hydrogen purity (96.30 vs 90.18%) and stability than using NiAl2O4 during the synthesis of NiAl2O4–CaO/NiAl2O4–CeO2. This indicates a more efficient coupling of reforming reaction and CO2 capture over Ca3Al2O6–CaO/NiO–CeO2 due to a far better distribution of active Ni and CaO sites. Almost no carbon deposits were detected on the materials surface after the reaction, attributed to the uniform distribution of small Ni particles achieved by using Ni–Al mixed-metal alkoxide and NiAl2O4 during the synthesis. These results confirm that using Ni–Al mixed-metal alkoxide in alternative to NiAl2O4 during the synthesis of Ni–CaO-based bifunctional materials can guarantee a more efficient coke-resistant material for the SESRG process by providing both the high distribution of small Ni particles and the intimate contact between active Ni and CaO sites.

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