CO2 Chemisorption and Cyclability Analyses of Lithium Aluminate Polymorphs (α- and β-Li5AlO4)
Tatiana Á valos-Rendón,† Víctor H. Lara,‡ and Heriberto Pfeiffer*,†
Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Circuito exterior s/n, Ciudad Universitaria, Delegación Coyoacán, CP 04510, México D.F., Mexico ‡Departamento de Química, Universidad Autónoma Metropolitan-Iztapalapa, Av. Michoacán y la Purísima, Del. Iztapalapa, C.P. 09340, México D.F., Mexico ABSTRACT: The α- and β-Li5AlO4 polymorphs were synthesized using a solid-state reaction. The polymorphs were then characterized by X-ray diffraction (XRD), X-ray thermodiffraction (XRTD), and N2 adsorption. To determine the CO2 chemisorption capacity, thelithium aluminate polymorphs were analyzed thermogravimetrically in the presence of a CO2 flux. In addition, a cyclability study was performed on these ceramic materials. Although the results appear very similar for the two phases, α-Li5AlO4 exhibits a better CO2 chemisorption performance. The cyclic performance tests indicate that both materials exhibit a gradually reduced chemisorption capacityafter multicycle processes. However, even after many cycles, the chemisorption capacity is considerably high in comparison to other lithium ceramics tested as CO2 absorbents.
1. INTRODUCTION Carbon dioxide (CO2) is a major anthropogenic greenhouse gas, which causes global warming and climate change. The rapid increase in Earth’s population over the last few decades, combined with an improvement inthe quality of life, are directly related to the dramatic increase in the concentration of man-made CO2, as both are related to the production and consumption of energy primarily obtained from fossil fuels.1−3 To mitigate the impact of greenhouse gases, it is critical to trap CO2 from fossil fuel power plants. The first step in carbon sequestration is the CO2 capture from flue gas.3−6 To thisend, a variety of lithium ceramics have been tested as possible CO2capturing materials.7−48 Among these ceramics, lithium aluminates (LiAlO2 and βLi5AlO4) were recently tested as possible CO2-capturing materials.49 Although LiAlO2 was unable to chemisorb CO2, β-Li5AlO4 is able to capture CO2 by a mechanism similar to the one reported for other lithium ceramics. Additionally, Li5AlO4 appears to be oneof the best options as a CO2-capturing material because of its high theoretical CO2 chemisorption capacity due to its Li/Al molar ratio of 5 and the fact that aluminum is lighter than any other element tested, including zirconium, copper, and even silicon. The maximum CO2 chemisorption capacity of β-Li5AlO4 is 19.77 mmol/g, assuming that five lithium atoms react with CO2 to produce Li2CO3. Itshould also be noted that β-Li5AlO4 is able to chemisorb CO2 over a wide range of temperatures (200−700 °C).49 Therefore, because β-Li5 AlO4 has the best CO 2 chemisorption capacity per gram among various lithium ceramics, Li5AlO4 is an important case of study as a CO2 chemisorbent. Two Li5AlO4 polymorphs have been identified−the α- and βLi5AlO4−and detailed structural analyses have beenperformed.50,51 The low temperature phase (α-Li5AlO4) transforms into a high temperature phase (β-Li 5 AlO 4 ) at approximately 780 °C.52 These structures are ordered
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derivatives of the antifluorite (Li2O) structure with vacancies that occupy distinct lattice positions and can be formulated as Li5V2AlO4, where V represents a vacancy.53,54 The α-Li5AlO4 phase crystallizesin the orthorhombic space group Pbca with a = 9.087, b = 8.947, and c = 9.120 Å, with Z = 8. In contrast, the β-Li5AlO4 phase crystallizes in the orthorhombic space group Pmmn with a = 6.420, b = 6.302, and c = 4.620 Å, where Z = 2.50,51 In both polymorphs, the metallic atoms (Li and Al) and the vacancies occupy tetrahedral sites and their ordered arrangement causes an orthorhombic distortion...