Chemical Looping Hydrogen Generation over Ceria/Zirconia-Enhanced NiO–NiFe₂O₄ Oxygen Carrier

Abstract

The objective of this paper is to systematically investigate the influences of different metal modification on the performance of Ni-ferrite as oxygen carrier (OCs) in chemical looping hydrogen (CLH) production. Ni-ferrite OCs were modified with single-metal NiO, bimetallic NiO/ZrO2, and bimetallic NiO/CeO2. The sample was characterized by CO thermogravimetric analysis, X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy, X-ray photoelectron spectroscopy, and Brunauer–Emmett–Teller method. The results showed that NiO could promote the release of oxygen from Ni-ferrite, and the release of oxygen can reach about 99% at 800 °C. The addition of ZrO2 and CeO2 could further increase the oxygen release rate of the oxygen carrier, but the oxygen storage amount would decrease. However, the bimetallic modified OCs exhibited higher hydrogen yield, and the hydrogen yields of NiO/ZrO2- and NiO/CeO2-modified Ni-ferrite could reach 9800.0 and 8786.2 μmol g(OC) –1, respectively. Furthermore, the bimetallic modified oxygen carrier showed higher structural stability, in which the ZrO2 particles weaken the active particle contact by coating effect, while the CeO2 is mixed into the active particles to form steric hindrance, both of which can inhibit the sintering of the oxygen carrier to maintain structural stability. Finally, the oxygen carrier reaction mechanism is analyzed, and the results show that the oxygen atoms around Ni2+ first migrate through the existing vacancies. And the residual vacancies provided the space for the immigration of other nearby oxygen atoms, which enhanced migration velocity. Moreover, Fe3+ gradually transferred to the adjacent Ni2+ to generate Fe0.64Ni0.36 after its oxygen migration.

Developing Oxygen Carriers for Chemical Looping Biomass Processing: Challenges and Opportunities

“…[77] TPR of NiFe 2 O 4 in 5 vol% CO (N 2 balance) shows two reduction peaks around 800 °C, which are assigned to the reduction of NiFe 2 O 4 spinel and FeO, respectively. [71] In contrast, TPR conducted on the same material in 10 vol% CO (Ar balance) revealed several more DTG peaks, [91] suggesting the reduction pathway of: [92] Similarly, TPR of ilmenite in low and high CO partial pressures result in one and two DTG reduction peaks, respectively, further signifying the importance of using appropriate reducing gases for TPR. [54,73] Wei et al studied Fe-Al-Ni OCs using a combined TGA-XRD approach, which directly established the correlation between the DTG peaks during TPR and the specific phase transformations.…”

“…Typical reducing gases used by TGA include H 2 , CO, or methane, diluted in a balancing inert gas. [69][70][71] In specific cases, biochars have been used as the reducing agents in TGA. [72] As shown in Figure 5, OTCs measured by TGA vary from 1 wt% (e.g., in the case of iron oxide-based LD slag) to 12 wt% (e.g., in the case of 60 wt% NiO supported on MgAl 2 O 4 ).…”

“…[200] In general, coking is favored on oxygen carriers whose concentration of redox-active lattice oxygen is low. [200] For instance, carbon deposition is absence during the reduction of highly redox active NiFe 2 O 4 by CO, [71] whereas an early onset of carbon deposition was observed when methane was used as the fuel during the reduction of Ca 2 Fe 2 O 5 . [101] Carbon deposition also depends on the metal's ability to catalyze CC bond cleavage.…”

“…For example, the TPR of NiFe 2 O 4 in 5 vol% CO (N 2 balance) shows two reduction peaks around 800 °C, which are assigned to the reduction of NiFe 2 O 4 spinel and FeO, respectively. [ 71 ] In contrast, TPR conducted on the same material in 10 vol% CO (Ar balance) revealed several more DTG peaks, [ 91 ] suggesting the reduction pathway of: NiFe 2 O 4 → Ni−Fe 2 O 3 → Ni−Fe 3 O 4 → Ni‐FeO → Ni−Fe. [ 92 ] Similarly, TPR of ilmenite in low and high CO partial pressures result in one and two DTG reduction peaks, respectively, further signifying the importance of using appropriate reducing gases for TPR.…”

“…During coprecipitation, the pH of the metal precursors solution is raised by adding alkaline (e.g., NaOH, Na 2 CO 3 , and ammonia), resulting in the precipitation of insoluble oxides, hydroxides and, or carbonates of the metals of interest. [ 71 ] Aging of the precipitates may lead to the formation of composite metal salts with unique meso‐ and macro‐ pore structures, such as layered double hydroxides. [ 113 ] Hydrothermal and solvothermal syntheses represent energy‐intensive alternatives to co‐precipitation that offers access to unique meso‐ and macro‐porous structures, which can potentially benefit the heat and mass transfer during CLBP cycles.…”