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DRIFTS analysis of ICCU-DRM using Ni/Ce/Ca@Zr dual functional materials

DRIFTs is a common and relatively cost-effective tool to obtain insights into reactions. Ni/Ca@Zr and NiCe/Ca@Zr were investigated for ICCU-DRM using DRIFTs (https://doi.org/10.1016/j.apcatb.2020.119734). A CO2-He-CH4 sequence was used. As shown below, during CO2 capture, peaks related to CO2 and CaCO3 are clearly observed. Monodentate carbonate (1370, 1480 and 1560 cm-1) is also present, in addition to polydentate carbonate on ZrO2 (1430 and 1450 cm-1). Biodentate carbonate on CeO2 is around 1290 cm-1. In addition, the formation of trace amounts of CO is also observed ascribed to the dissociation of CO2 on the reduced catalyst. This CO2 dissociation is improved with the addition of Ce, due to the abundance of oxygen vacancies.

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“In-situ DRIFTS recorded during the sequence of CO2-He-CH4 over the reduced Ni/Ca@Zr (A) and NiCe/Ca@Zr (B) at 550 °C and 1 bar. The materials (25 mg) were reduced ex-situ at 750 °C for 30 min and then in-situ at 500 °C for 1 h. The feed gases (24 N mL min−1) for CO2, He and CH4 steps are 25 % CO2/He, pure He and 25 % CH4/He, respectively. Abbreviation: (g): gas phase; (ad): adsorbed; MCa: Monodentate carbonate on CaO surface; PZr: Polydentate carbonate on ZrO2 surface; BCe: Bidentate carbonate on CeO2 surface; Ni(CO)x: complex of CO and Ni species.” (https://doi.org/10.1016/j.apcatb.2020.119734)

By introducing inert He, CO2 gas is removed. However, surface and bulk CaCO3 are still present, indicating that the thermal decomposition of carbonates is limited under the inert atmosphere at 550 °C. When CH4 is introduced, IR peaks related to gaseous CH4 appear. The intensity of CO peaks is clearly increased, while the peak intensity of carbonates decreases. Importantly, formate species are observed, demonstrating that the adsorbed H2 species, produced from CH4 decomposition, react with carbonate species. The decomposition of formate species produces CO. The authors suggested that CaCO3 decomposed into CO2, which reacted with CH4 to produce CO and H2. Due to the Le Chatelier’s principle, the consumed CO2 promotes the decomposition of CaCO3. This is supported by Gibbs free energy calculations, where CaCO3 decomposition is easier to happen under the CH4 atmosphere compared to He. The authors also suggested that the addition of Ce promoted the activation of CH4, thus resulting in enhanced CO production.

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“Thermodynamic analysis on (A) the CO2 capture step: equilibrium CO2 consumption (RCO2, = uptake) under the evolution of CaO/CaCO3 system as a function of temperature and CO2 feed concentration, gas:CaO = 12 (molar ratio). (B) the following DRM step: equilibrium CH4 conversion (XCH4) at 720 °C as a function of CH4 feed concentration with (solid lines) or without (dashed lines) consideration of carbon formation. Pressure = 1 bar. The red circle-cross symbols (⊗) show the experimental conditions in this study, i.e., 5 vol% CO2 feed concentration for capture step and 8 vol% CH4 feed concentration for DRM step. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).” https://doi.org/10.1016/j.apcatb.2020.119734)

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