Understanding the synergistic effect of Ni/Al2O3 on CO2 adsorption and sintering properties of CaO in sorption-enhanced steam reforming process

Abstract

Sorption-enhanced steam reforming with in-situ CO2 removal by high temperature solid sorbents is an environmentally friendly and sustainable approach for hydrogen production. Ca-Ni-Al bifunctional materials with both catalytic activity and sorption activity have received extensive attention recently. However, the synergistic effect of Ni/Al2O3 on the CO2 adsorption and sintering properties of CaO sorbents have not been studied, and the atomistic mechanisms behind them still remain unclear. Here, we investigated the synergistic effect of Ni/Al2O3 on the carbonation properties and sintering resistance of CaO sorbents by experiments combined with density functional theory (DFT) calculations. Results show that the phase transformation for reduction of NiO and NiAl2O4 to Ni promotes the reaction between CaO and Al2O3, which is beneficial to the formation of the inert framework Ca12Al14O33. After adding 5 % and 15 % Ni, the BET specific surface area decreases from 11.85 to 7.59 and 4.51 m2/g, respectively. The catalytic component Ni is dispersed on the CaO surface, which blocks the pore structure and eventually leads to a decrease in CO2 capture capacity. Under high temperature calcination, Ni aggravates the sintering of CaO, which is not conducive to cyclic adsorption. The Al2O3 support can effectively improve the pore structure and maintain cycle stability of CaO sorbents. DFT calculations suggest that the catalytic component Ni, the Al2O3 support and the Ni-Al2O3 bi-component have little effect on the CO2 adsorption activity. The Ni significantly weakens the adsorption energy of Ca4O4 cluster on CaO surface from −2.58 eV to −1.57 eV, resulting in the growth of CaO grains. The strong interaction between Ca4O4 cluster and Al2O3 surface with the adsorption energy of −7.34 eV makes Ca4O4 cluster migration difficult, thereby inhibiting the agglomeration and sintering of CaO particles. Meanwhile, the addition of Ni component does not significantly reduce the anti-sintering ability of Al2O3 support on CaO sorbents. For Ni-Ca-Al bifunctional materials, the strong interaction between Ni-Ca is not conducive to the carbonation performance of CaO sorbents and the strong interaction between Ni-Al should be appropriately avoided to weaken the efficacy of Al2O3 support, which was confirmed by experiments.