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Journal of Materials Chemistry A View Article Online FEATURE ARTICLE Downloaded by Lanzhou University on 26 March 2013 Published on 18 December 2012 on http://pubs.rsc.org | doi:10.1039/C2TA00700B Cite this: J Mater Chem A, 2013, 1, 3540 View Journal | View Issue Syntheses and structures of lithium zirconates for high-temperature CO2 absorption Shutao Wang,a Changhua An*ab and Qin-Hui Zhang*b The sorption and further application of CO2 is a highlight in the field of environmental protection and sustainable development Lithium-containing zirconates (LixZryOz) are promising materials for high- Received 15th October 2012 Accepted 18th December 2012 temperature chemisorption of CO2 and have attracted tremendous interest This review presents discussion on the recent status of LixZryOz based CO2 sorbents at high temperature Special attention is focused on the solid state chemistry, synthetic strategies, high-temperature CO2 capture–regeneration DOI: 10.1039/c2ta00700b properties and possible sorption mechanisms for LixZryOz with different Li/Zr ratios, including Li2ZrO3, www.rsc.org/MaterialsA Li6Zr2O7, Li8ZrO6, and Li2ZrO3 doping with alkali metals Introduction Climate change and green house gas (GHG) emission regulation have recently attracted much attention, and were recognized as critical issues requiring action long before Studies have shown that increased GHG levels would lead to global warming Among the various affecting species, carbon dioxide (CO2) makes up a high proportion with respect to its amount in the atmosphere, contributing 60 percent of the a State Key Laboratory of Heavy Oil Processing, Key Laboratory of New Energy Physics & Materials Science in Universities of Shandong, College of Science, China University of Petroleum, Qingdao 266580, P R China E-mail: anchh@upc.edu.cn; Fax: +86-53286981787 b State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum, Qingdao 266580, P R China E-mail: qhzhang@upc.edu.cn; Fax: +86-532-86981855 Shutao Wang received her PhD in Chemistry from University of Science and Technology of China in 2006 with Prof Zude Zhang Then she joined the faculty at China University of Petroleum, where she is currently an associate professor From 2012, she held a postdoctoral position with Prof Zhang at the State Key Laboratory of Heavy Oil Processing, China University of Petroleum Her research involves functional inorganic nanocrystals, with emphasis on energy-related photocatalysts and hydrogenation nanocatalysts 3540 | J Mater Chem A, 2013, 1, 3540–3550 global warming effect,1 although methane and chlorouorocarbons have much higher warming potentials as per mass of gases According to the prediction of Intergovernmental Panel on Climate Change (IPCC), by year 2100, the atmosphere may contain up to 570 ppm of CO2, causing a rise of the mean temperature around 1.9 C and an increase in the mean sea level of 3.8 m.2 Accordingly, serious issues such as accompanied species extinction may occur In the IPCC report, approximately three quarters of the increase in atmospheric CO2 is caused by burning of the fossil fuels Fig shows the change of global-mean CO2 concentration between the years 1850 and 2100.3 It is clearly shown that the global climate change is mainly caused by the increase of CO2 concentration, due to the combustion of fossil fuels to sustain industry and maintain the rapid development rate of economy and technology The conditions will become more severe Changhua An received his PhD degree from University of Science and Technology of China in 2003 with Prof Yitai Qian, then he worked as postdoctoral research fellow at Seoul National University with Prof Taeghwan Hyeon, Korea from 2004 to 2005 He has been an associate professor at China University of Petroleum from 2005 He worked as a visiting scholar at University of Illinois at Urbana-Champaign, USA from 2009 to 2010 His research interests focus on the synthesis, characterization, modication, and application of nanomaterials used in the solar-chemical energy transformation This journal is ª The Royal Society of Chemistry 2013 View Article Online Downloaded by Lanzhou University on 26 March 2013 Published on 18 December 2012 on http://pubs.rsc.org | doi:10.1039/C2TA00700B Feature Article Fig Effect of climate/carbon-cycle feedback on CO2 increase and global warming (a) Global-mean CO2 concentration, and (b) global-mean and landmean temperature, versus year Three simulations are shown; the fully coupled simulation with interactive CO2 and dynamic vegetation (red lines), a standard GCM climate change simulation with prescribed (IS92a) CO2 concentration and fixed vegetation (dot-dashed lines), and the simulation which neglects direct CO2-induced climate change (blue lines) The slight warming in the latter is due to CO2-induced changes in stomatal conductance and vegetation distribution Reproduced from ref with permission Copyright ª 2000, Nature Publishing Group without control of CO2 emissions For both developed countries like USA and developing countries like China and India, strategies to reduce CO2 emissions are now Qin-Hui Zhang obtained his PhD degree from Tianjin University in 2002 From 2002 to 2004, he held a postdoctoral position at Tsinghua University, China From 2004 to 2011, he worked at the State Key Lab of Chemical Engineering, College of Chemical Engineering, East China University of Science and Technology Since 2010, he has been a professor of chemical engineering and chemistry In 2011, he joined the State Key Laboratory of Heavy Oil Processing, China University of Petroleum His research includes energy-related applications of nanomaterials, in particular carbon capture, photocatalysts, adsorption/extraction of lithium from brine or seawater This journal is ª The Royal Society of Chemistry 2013 Journal of Materials Chemistry A more important than ever Furthermore, the problems associated with CO2 emission may be a primary factor in restricting economic growth of many countries, e.g., China Therefore, developing reasonable strategies to reduce CO2 emission and enforcing sequestration are essential to mitigate global warming However, the low concentration of CO2 available in the atmosphere and from the emission sources makes it difficult to full its further applications Selective CO2 absorption and sequestration could be considered for some scenarios as a viable choice to limit the emission of CO2 into the atmosphere Such a choice is favourable for all the parties as it has minimal interference with the prosperous fossil fuel economy, giving time for the burgeoning alternative energy sources to be optimized for easy implementation and accessibility For example, membrane separation processes requiring a lesser input of energy, only in the range of 0.04– 0.07 kWh per kg CO2, have been used in some systems, like the production of fuel gas from coal However, insufficient downstream purity and low removal rates of CO2 restrict further applications of this technique The capture and storage of CO2 from ue gas is an effective approach for the reduction of CO2 emitted to the atmosphere since the coal burning power plant is one of the largest sources of CO2 emission In light of the fact that the temperature of the ue gas between the turbine and the vent is usually in the range of 625–900 K, if CO2 is separated from ue gas at high temperature and further used as feedstock for the synthesis of fuels through optional techniques, i.e., hydrogenation or photoreduction with the assistance of photocatalysts, the efficiency and economics of the entire process of the power plant might be improved The availability of new materials with better separation performances to enhance the efficiency of CO2 removal and hence the efficiency of power generating systems is highly desirable In terms of the in situ removal processes like sorption, the high-temperature stability of the material is of immense importance as well Lithium based CO2 absorbents have attracted much attention in this eld Many good reviews are available for CO2 separation/absorption,4–9 among which Yamaguchi et al in 2009 have detailed the performance of lithium based ceramic materials, particularly on Li2ZrO3 and silicates,5 and related membranes for high temperature CO2 separation Strategies to enhance the performance of lithium compounds in terms of their stability and the amount and rate of CO2 absorption include improvement of the zirconates and investigation of new ceramic systems other than zirconates, which is out of the scope of the current review This review mainly focuses on the lithium-containing zirconates (LixZryOz) with different Li/Zr ratios for high-temperature chemisorption of CO2 Firstly, the solid chemistry behavior and syntheses of LixZryOz are briey discussed Then their characteristics of high-temperature CO2 absorption are summarized, which is varied with their preparation conditions At the same time, Li2ZrO3 doping with alkali metals is also discussed At the end of the review, future directions of the resourceful use of CO2 and a perspective on the eld are given J Mater Chem A, 2013, 1, 3540–3550 | 3541 View Article Online Journal of Materials Chemistry A High-temperature CO2 chemisorption on lithium-containing zirconates Downloaded by Lanzhou University on 26 March 2013 Published on 18 December 2012 on http://pubs.rsc.org | doi:10.1039/C2TA00700B 2.1 Structure and synthesis of LixZryOz Recently, LixZryOz with various stoichiometries have attracted interest as a class of novel in situ high-temperature CO2 sorbents These materials react with CO2 in the temperature range 723–823 K, resembling the actual fuel reforming process, with the advantages of high CO2 capture capacity, innite CO2/N2 or CO2/H2 selectivity, high stability, and ease of CO2 absorption/desorption reversibility Great effort has been made to gain insight into LixZryOz compounds with different Li/Zr molar ratios Investigations of their electronic structures, lattice dynamics, synthesis, and high-temperature CO2 capture properties have been widely performed And also various synthetic strategies for LixZryOz compounds have been developed, aiming to achieve more reactive phases with improved CO2 absorption kinetics The results have shown that the starting reagents, calcination temperature and time are all crucial to the nal stoichiometries and structures of the LixZryOz products In addition, the preparation history of the samples is also closely related to the reduced particle size, change of the composition, and incorporation of appropriate dopants, and hence the CO2 sorption performances of LixZryOz Generally, there are three kinds of lithium zirconates in the temperature range investigated, including Li2ZrO3, Li6Zr2O7, and Li8ZrO6, progresses on which will be summarized in the following text 2.1.1 Structure and synthesis of Li2ZrO3 There are two phases of Li2ZrO3, namely tetragonal t-Li2ZrO3 (JCPDS 20-0647, ˚ c ¼ 3.43 A) ˚ and monoclinic m-Li2ZrO3 (JCPDS 33a ¼ b ¼ 9.0 A, ˚ b ¼ 9.03 A, ˚ c ¼ 5.42 A) ˚ t-Li2ZrO3 is metastable 0843, a ¼ 5.43 A, and will undergo phase transformation at 1173 K to stable mLi2ZrO3 Accordingly, the characteristics associated with particle size, surface free energy, ionic conductivity, and the reactivity of the materials are also varied Moreover, Li2ZrO3 can be decomposed rapidly above 1773 K, though stable below 1473 K The Li2ZrO3 crystal is usually closely packed with all the cations octahedrally coordinated (Fig 2).10 Nakagawa and coworkers rst proposed Li2ZrO3 as a candidate for hightemperature CO2 absorption,11 which absorbs and desorbs CO2 cyclically with a theoretical uptake capacity of 0.28 g per g of Fig Snapshots of m-Li2ZrO3 and Na2ZrO3 structures The spheres represent, from the brightest to the darkest, the alkaline element (Li or Na), oxygen, and zirconium atoms, respectively Reproduced from ref 10 with permission Copyright ª 2006, American Chemical Society 3542 | J Mater Chem A, 2013, 1, 3540–3550 Feature Article acceptor Furthermore, t-Li2ZrO3 exhibits better performance as a CO2 absorbent than its monoclinic counterpart with higher stability, faster uptake rate, and higher absorption capacity Traditional synthesis of Li2ZrO3 via solid state reaction involves mechanical mixing of zirconium oxide (ZrO2) and lithium carbonate (Li2CO3) at high temperatures It is found that as Li2CO3 was substituted by LiOH, both t-Li2ZrO3 and hexa-lithium zirconate (h-Li6Zr2O7) could be synthesized through calcination at 873 and 1073 K, respectively In this process, the initial Li/Zr molar ratios should be a little greater than the chemical stoichiometric amount,12,13 since lithium (Li) sublimes easily at high temperature Unfortunately, this method oen needs high energy input and it is difficult to control the sizes and phases of the nal products Therefore, great efforts have been made in the development of so-chemistry strategies, such as sol–gel or other liquid phase methods at low temperatures using water-soluble precursors to ensure improved mixing of reagents on the molecular/atomic level.14–21 For example, Nakagawa et al synthesized Li2ZrO3 using a sol–gel procedure and compared its properties with powders obtained by the powder-mixing route and a commercial-grade powder.14 They found that the CO2 absorption and membrane separation properties of Li2ZrO3 are closely connected to its particle size, crystal structure and agglomeration state A gelatin assisted biomimetic so solution method has been used to prepare nanoparticles of t-Li2ZrO3 containing monoclinic mLi6Zr2O7.19 A citrate sol–gel method (or modied Pechini method) via spray drying has been developed to prepare Li2ZrO3 nanocrystals starting from zirconium oxynitrate [ZrO(NO3)2$2H2O] and lithium nitrate (LiNO3) with improved CO2 capture efficiency,18 at a temperature (923 K) much lower than that required in solid state reactions In this process, heterogeneous spatial distribution of the Li and Zr elements is oen encountered, leading to incomplete reaction and thus reduced CO2 absorption capacity In addition, porous Li2ZrO3 has been prepared through aqueous reactions using an ultrasound assisted surfactanttemplate method from feedstocks of ZrO(NO3)2$3xH2O, lithium acetate, and cetyltrimethylammonium bromide (CTAB).22 The prepared Li2ZrO3 presented higher absorption rate, capacity, and cyclic stability than those obtained by the simple surfactant-template method (without sonication) and the conventional so-chemistry route The crystallite size and surface area of Li2ZrO3 could be controlled by the sonication time and the CTAB concentration Nanotubes of t-Li2ZrO3 with high aspectratio can also be prepared using a hydrothermal method in LiOH solution with anodic ZrO2 nanotubes as templates.23,24 In comparison with bulk and nanoparticles of Li2ZrO3, the nanotubes of Li2ZrO3 containing a small amount of ZrO2 exhibit enhanced CO2 capture properties It is found that the addition of LiOH can promote ZrO2 dissolution and result in the Li2ZrO3 formation on the surfaces of the nanotubes Namely, the alkalinity of the solution plays an important role in guiding the growth of the Li2ZrO3 nanotubes.24 2.1.2 Structure and synthesis of Li6Zr2O7 Similar to Li2ZrO3, two phases of Li6Zr2O7 exist, thermodynamically stable ˚ b ¼ 5.99 A, ˚ monoclinic m-Li6Zr2O7 (JCPDS 34-0312, a ¼ 10.45 A, ˚ ˚ c ¼ 10.20 A) and meta-stable triclinic tri-Li6Zr2O7 (a ¼ 6.0153 A, This journal is ª The Royal Society of Chemistry 2013 View Article Online Downloaded by Lanzhou University on 26 March 2013 Published on 18 December 2012 on http://pubs.rsc.org | doi:10.1039/C2TA00700B Feature Article ˚ c ¼ 5.3112 A) ˚ 25 Tri-Li6Zr2O7 transforms slowly into b ¼ 9.1941 A, m-Li6Zr2O7 and is rapidly sintered at 1223 K.13,26,27 Interestingly, this phase transformation is associated with a volumeincreasing process due to the larger cell parameters of the monoclinic phase The structures of both Li2ZrO3 and Li6Zr2O7 are of the NaCl type,26,28 although the distributions of cations are different in these two phases A more open layer structure than that of Li2ZrO3 has been suggested for Li6Zr2O7 (Fig 3),26 ) direction and each Zr where the Zr layer locates along the (111 atom is coordinated with six O atoms Consequently, both Li+ and O2À ions have higher mobility, rendering Li6Zr2O7 with good ionic conductivity Reactions between Zr(OH)4 and LiNO3 would produce phasepure m-Li6Zr2O7 under appropriate conditions However, acquiring m-Li6Zr2O7 with high purity through solid reactions of Li2CO3 and ZrO2 is not easy, since decomposition of Li2CO3 occurs under the reaction conditions,12,13 which would release CO2 and further react with Li6Zr2O7 to form Li2ZrO3 On the other hand, thermal analyses of Li6Zr2O7 showed continuous decomposition behaviour owing to the sublimation of lithium.29 Consequently, the poor thermal stability of Li6Zr2O7 makes it suffer from arduous regeneration.27 Theoretically, phase pure high-lithia zirconates of LixZryOz, i.e Li6Zr2O7 and Li8ZrO6, can be produced if the Li/Zr ratio is high enough However, few reports concern the structures and syntheses of these two compounds because of the rigorous conditions required, such as calcination under ultra-high vacuum, high temperature, and with prolonged heating time The substitution of the lithium source of Li2CO3 with LiOH$H2O could produce m-Li6Zr2O7, tri-Li6Zr2O7, and their mixtures with Li2ZrO3 under suitable conditions.13,29 Further replacement of ZrO2 with Zr(NO3)4$5H2O leads to the production of pure m-Li6Zr2O7 in a remarkably shortened recrystallization time from 96 h to 24 h.13 However, the zirconium sources of ZrOCl2$8H2O are accompanied with a greater lithium loss through LiCl volatilization It is noted that Li6Zr2O7 exhibits smaller surface area than t-Li2ZrO3 synthesized with similar parameters,27 since serious agglomeration happened for Li6Zr2O7 when calcined with higher temperature, longer time, and larger initial Li/Zr molar ratios 2.1.3 Structure and synthesis of Li8ZrO6 Few investigations have been performed on the detailed structure and CO2 Fig The crystal structures of Li6ZrO7 The biggest ball represents Zr, the smallest O c axis is vertical Reproduced from ref 26 with permission Copyright ª 2011, American Institute of Physics This journal is ª The Royal Society of Chemistry 2013 Journal of Materials Chemistry A absorption properties of Li8ZrO6 because of the difficulty in its preparation, conrmed both experimentally and theoretically through calculations of the standard Gibbs free energies of relevant reactions.30 Traditionally, Li8ZrO6 was prepared via solid state reactions between ZrO2 (or Li6Zr2O7) and Li2O (or Li2O2).13,25,30 Zou and Petric calculated the thermodynamic data and proposed the temperature range of 773–1373 K for the synthesis of pure Li8ZrO6 in air.30 They prepared Li8ZrO6 from ZrO2 and Li2O2 through complicated heating procedures Wyers and Cordfunke synthesized Li8ZrO6 through reactions between ZrO2 and Li2O in a vacuum.25 It is reported that rhombohedral r-Li8ZrO6 (JCPDS 26-0867, ˚ c ¼ 15.45 A) ˚ would decompose slowly into Li6Zr2O7 a ¼ 5.48 A, and Li2O upon prolonged heating above 1073 K.30 A mixture of Li6Zr2O7 and Li8ZrO6 was observed aer the calcination of Li2CO3 and ZrO2,13,29 and also of LiOH and Zr(NO3)4$5H2O The authors' group has synthesized r-Li8ZrO6 coexisting with a fraction of Li6Zr2O7 via coupling a liquid-phase coprecipitation and calcination at 1223 K for 72 h, using LiOH$H2O, NH3, and Zr(NO3)4$H2O as feedstocks.12 Furthermore, we synthesized pure Li8ZrO6 with a liquid mixture of the reagents followed by a simple three step calcination.31 The choice of LiNO3 as the lithium source with the advantages of safety, availability, and low melting point (873 K), is the key to successful production of phase-pure Li8ZrO6 The formation of homogeneous mixture of Li+ and Zr4+ is favoured by LiNO3 For Li2CO3, generation of CO2 from its decomposition interfere Li8ZrO6 preparation through a carbonate reaction just as the conditions of Li6Zr2O7 Unfortunately, the side effects in high-temperature processes also occur for the preparation of Li8ZrO6,12,13,30 i.e., lithium loss, serious particle aggregation, large particle size, low surface area, and thus poor CO2 absorption properties 2.2 High-temperature CO2 chemisorption on LixZryOz The high-temperature stability and CO2 capture–regeneration (absorption–desorption) kinetics of LixZryOz compounds with diverse stoichiometries have attracted much attention, particularly for Li2ZrO3, Li6Zr2O7, and Li8ZrO6 The CO2 absorption originates from the reaction between CO2 molecules and Li+ ions derived from LixZryOz Hence, the reactivity and absorption capacity of LixZryOz towards CO2 are greatly dependent on the diffusion and amount of Li+ ions Theoretically, Li6Zr2O7 and Li8ZrO6 might absorb more CO2 than Li2ZrO3 under similar conditions, since the Li/Zr ratio of Li6Zr2O7 and Li8ZrO6 is 1.5 and 4.0 times higher than that of Li2ZrO3, respectively This conclusion has been proved through simulation of CO2 absorption kinetics under various CO2 partial pressures following a double-exponential model as shown in eqn (1),22,27,32–34 where y is the weight percentage of CO2 absorbed, t is the absorption time, kCO2 is the rate constant for CO2 diffusion on the surfaces of the particles, kLi is the rate constant of Li+ diffusion from the core to the reaction interface, and A, B, and C are pre-exponential factors y ¼ AexpÀkCO2t + BexpÀkLit + C (1) J Mater Chem A, 2013, 1, 3540–3550 | 3543 View Article Online Downloaded by Lanzhou University on 26 March 2013 Published on 18 December 2012 on http://pubs.rsc.org | doi:10.1039/C2TA00700B Journal of Materials Chemistry A Feature Article It is desired that the CO2 absorbents at high temperature have high selectivity and absorption capacity, as well as good absorption–regeneration kinetics Theoretically, the absorption performance of the absorbents is determined mainly by their internal structures Furthermore, the sorbents should possess high stability during recycling That is, the absorbents should not only easily absorb CO2 in the rst half cycle, but also to release CO2 in the second half cycle Usually, the absorbents with accelerated rate of gas diffusion and CO2 chemisorption have the characteristics of decreased particle sizes and enlarged surface areas, facilitating the access of CO2 molecules into the internal layers of active sites On the other hand, the CO2 uptake capacity and kinetics of LixZryOz depend strongly on the temperature and CO2 partial pressure or concentration of the atmosphere, which could be judged by Le Chatelier's principle of equilibrium 2.2.1 High-temperature CO2 chemisorption on Li2ZrO3 The CO2 absorption–desorption process for Li2ZrO3 is ascribed to a reaction model whereby lithium oxide in the Li2ZrO3 structure reacts reversibly with CO2 As demonstrated by eqn (2), the stoichiometric capacity of CO2 absorption for Li2ZrO3 is up to 28.7 wt%, since mol Li2ZrO3 compensates mol CO2 Apart from the selective absorption capability irrespective of the gas species other than CO2, Li2ZrO3 has the advantages of hightemperature stability over other known CO2 absorbers, coupled with a low morphologic/volumetric change no more than 134%.11,14 Moreover, Ochoa-Fern´ andez et al demonstrated that t-Li2ZrO3 has superior CO2 capture–regeneration performance to that of m-Li2ZrO3,35 i.e., a faster uptake rate and a higher absorption capacity around 26 wt%, which is about 90% of its stoichiometric capacity (28.7 wt%) Li2ZrO3 (s) + CO2 (g) ! ZrO2 (s) + Li2CO3 (s) (2) Selective removal of CO2 from gas mixtures according to eqn (2) oen works at a specic temperature, because the absorption process takes place normally in the temperature range of 723– 923 K,5,11,16,34,36 meanwhile, the regeneration reaction initiated above 923 K Ida et al proposed a double shell model to describe the mechanism of the CO2 absorption/desorption process on both Li2ZrO3 and potassium (K) modied Li2ZrO3.4,34,36 The forward direction of eqn (2) would be accelerated through a carbonation mechanism,11–14,22,32,34,36–38 with the formation of a solid Li2CO3–ZrO2 layer surrounding an unreacted core of Li2ZrO3 (Fig 4) Then the formation and growth of the external shell limits the diffusion of gases and ions (Li+ and O2À), leading to a decreased rate of CO2 sorption Desorption of CO2 takes place accompanied by the regeneration of Li2ZrO3 through the reverse process of eqn (2),26,39,40 favoured by the generation of Li2O through Li2CO3 decomposition at the operating temperature Fig gives a general scheme for the possible external Li2CO3 shells with different compositions (phase pure Li2CO3, coexist with metal oxide or other lithium secondary phases of different lithium diffusion capacities) However, this model is applied only for the case where Li2CO3 is in a solid state.41 It has been reported that small particle sizes are preferred for a faster rate of CO2 uptake on Li2ZrO3 particles,42 while large 3544 | J Mater Chem A, 2013, 1, 3540–3550 Fig Schematic illustration of carbonation mechanism on (A and B) pure and (C and D) potassium modified Li2ZrO3 Reproduced from ref 36 with permission Copyright ª 2003, American Chemical Society particle sizes and serious aggregations restrict the migration of gases and ions The smaller particle size always means a thinner Li2CO3–ZrO2 shell/layer and a shorter diffusion distance, showing that the CO2 absorption is diffusion controlled.36,37 For example, porous Li2ZrO3 with signicantly reduced aggregation exhibited the maximum capacity of 22 wt% at 100% CO2 compared to 15.2 wt% for conventional aggregated samples.22 The temperature effect is observed with a faster absorption rate at a lower temperature for solid CO2 absorbents This effect is closely related to the size effect mentioned above,14,22 as high-temperature oen leads to aggregation and sintering of Fig Scheme of the lithium diffusion processes controlled by different possible external shell compositions (A) Lithium diffusion controlled exclusively by Li2CO3 in solid state; (B) lithium diffusion controlled by Li2CO3, but limited by the metal oxide presence; (C) lithium diffusion controlled by Li2CO3, which is reduce by the presence of other lithium secondary phase with a smaller lithium diffusion capacity; (D) lithium diffusion controlled by Li2CO3, which is enhanced, at a determined temperature, by the presence of the other lithium secondary phase with a larger lithium diffusion capacity Reproduced from ref 41 with permission ´miai Kiado ´ , Budapest, Hungary Copyright ª 2011, Akade This journal is ª The Royal Society of Chemistry 2013 View Article Online Downloaded by Lanzhou University on 26 March 2013 Published on 18 December 2012 on http://pubs.rsc.org | doi:10.1039/C2TA00700B Feature Article Journal of Materials Chemistry A adjacent particles Similarly, the synthetic temperature also affects the structure and the morphology of Li2ZrO3,43 and thus its CO2 absorption ability In addition, sublimation of lithium as Li2O occurs through decomposition of Li2CO3 when the temperature increased to 1173 K as eqn (3),39 resulting in an incomplete precursor reaction and loss of absorption capacity aer cyclic operations On the other hand, Li2O migrate to the particle surfaces and absorb CO2 efficiently through reverse reaction of eqn (3),39,40,44 since this process needs very low CO2 pressure and/or very high temperature For ZrO2, it might act as a dispersant and introduce more reactive boundaries, the presence of which in the lithium outer shell will decrease the rate of lithium diffusion to the surface which controls the rate of absorption.41 Therefore, the ratio of Li2O to ZrO2 considerably inuences not only the CO2 capture rate, but also the CO2 capture capacity of Li2ZrO3.21 Moreover, it is found that the absorption capacity of Li2ZrO3 prepared from tetragonal ZrO2 (tZrO2, metastable) (25 Æ 0.6 wt%) is higher than that produced from monoclinic ZrO2 (m-ZrO2, stable) (only Æ 0.6 wt%) under the same sorption conditions (at 773 K for h, atmosphere of 20% CO2 and 80% air).45 Li2CO3 (s) ! Li2O (s) + CO2 (g) (3) The main obstacle for practical application of Li2ZrO3 as a CO2 absorbent is the kinetic limitation.4 Many efforts have been made to improve its performance for CO2 capture/release Duan and coworkers studied the CO2 capture capabilities of various alkali metal zirconates by calculating the chemical potential change Dm (T, P) for the capture reactions under various CO2 pressures and temperatures.26,39,46 A kinetic equation for the sorption on Li2ZrO3 as a function of CO2 partial pressure and temperature has been acquired.47 As shown in Fig 6, the curve of Dm ¼ means the equilibrium between the absorption and desorption process at given temperatures and pressures Above the curve (Dm < 0), the forward reactions are favourable for both Li2ZrO3 and Li6Zr2O7 to absorb CO2 and form Li2CO3 Below the curve (Dm > 0), the reversed reaction to release CO2 is favourable, indicating decomposition of Li2CO3 to release CO2 and regeneration of the sorbents Consequently, Li2ZrO3 can absorb CO2 over a wide range of CO2 pressures (10À25 to 102 atm) below 700 K The reverse reaction to release CO2 happens by increasing the temperature over a CO2 pressure range of 10À25 to 10À1 atm.14 Practically, the CO2 absorption rate and capacity of Li2ZrO3 are also conrmed to be strongly dependent on the working temperature and CO2 concentrations.17,27 A higher percentage of CO2 in the atmosphere is benecial to the CO2 absorption by Li2ZrO3,43 while low CO2 partial pressure ( 0, which means the CO2 starts to release and the reaction goes backward to regenerate the sorbents Reproduced from ref 26 with permission Copyright ª 2011, American Institute of Physics is a large decay of the capacity of the absorbents compared to dry conditions, due to sintering of the particles, vaporization of alkali metals, and phase segregation 2.2.2 High-temperature CO2 chemisorption on Li6Zr2O7 Theoretically, the LixZryOz compounds with richer lithium content would exhibit a larger capacity for CO2 absorption than Li2ZrO3.12,29 In fact, both tri/m-Li6Zr2O7 show smaller CO2 absorption capacity compared with t-Li2ZrO3.12,26 Results from many researchers indicated that Li6Zr2O7 can be fully converted into ZrO2 and Li2CO3 only in the rst cycle of CO2 capture through eqn (4),12,26,29 with the maximum theoretical capacity of 39.28 wt% Aer CO2 desorption, Li2ZrO3 rather than Li6Zr2O7 would be regenerated as eqn (5) shows, when the temperature is not high enough to regenerate Li6Zr2O7 In the following cycles, Li2ZrO3 absorbs/desorbs CO2 following eqn (2) Consequently, the CO2 absorption capacity of Li6Zr2O7 through eqn (5) gains only 13 wt%.29 Although the capacity reduced gradually, multi-cycle tests demonstrate that mLi6Zr2O7 in low CO2 concentration stream (10% CO2 stream) exhibits fast CO2 uptake and release rates.27 There are also reports that the absorbent is composed of tri-Li6Zr2O7 instead of m-Li6Zr2O7 aer the rst cycle,27 whereas the desorption temperature is too low for the phase transformation of Li6Zr2O7 from triclinic to monoclinic For tri-Li6Zr2O7, most of the absorbed CO2 could be released, while no obvious desorption was observed for m-Li6Zr2O7, even at high temperature The regeneration of Li6Zr2O7 could be realized through eqn (6) between Li2O and Li2ZrO3.12,29 In other words, the capture behaviour of both Li6Zr2O7 and Li2ZrO3 is similar aer the rst cycle of the CO2 sorption/ desorption process The weak cycle stability and gradually reduced CO2 capacity of Li6Zr2O7 thus become the disadvantages to use Li6Zr2O7 over Li2ZrO3 as CO2 sorbents.12 These conclusions are in agreement with Pfeiffer's work that Li6Zr2O7 absorbed four times the amount of CO2 with faster sorption rate J Mater Chem A, 2013, 1, 3540–3550 | 3545 View Article Online Journal of Materials Chemistry A Feature Article than Li2ZrO3 in short times, but they became similar aer long reaction times.29 Li6Zr2O7 (s) + 3CO2 (g) ! 3Li2CO3 (s) + 2ZrO2 (s) (4) Li6Zr2O7 (s) + CO2 (g) ! Li2CO3 (s) + 2Li2ZrO3 (s) (5) Li2O (s) + 2Li2ZrO3 (s) ! Li6Zr2O7 (s) (6) Downloaded by Lanzhou University on 26 March 2013 Published on 18 December 2012 on http://pubs.rsc.org | doi:10.1039/C2TA00700B Li6Zr2O7 (s) + 2CO2 (g) ! 2Li2CO3 (s) + ZrO2 + Li2ZrO3 (s)(7) The possible pathway for CO2 absorption–desorption on tri/ m-Li6Zr2O7 at high temperature has been suggested as a series of reactions as shown by eqn (4)–(7),12,26,27,29 with the nal products of ZrO2, Li2ZrO3, and Li2CO3 There are two different sections during the multi-cycle processes on tri/m-Li6Zr2O7 at the operating temperatures, including the absorption process and desorption process with similar diffusion behaviours of CO2 and ions (Li+ and O2À) (Fig 7),27 corresponding to a carbonation and decarbonation mechanism similar to that of Li2ZrO3 (Fig 8).29 The absorption of CO2 on Li6Zr2O7 happens when lithium from Li6Zr2O7 structures reacts with CO2 to produce a Li2CO3 shell, rstly on the surfaces of the particles, as shown in eqn (4) Then Li2CO3 further decomposes into CO2 and Li2O through eqn (3) when the temperature is higher than 973 K, reacting continuously with ZrO2 to form an internal Li2ZrO3 core, and the other parts sublimate as Li2O (g) The desorption process to release CO2 happens with the regeneration of tri-Li6Zr2O7 through diffusion of lithium and CO2 in opposite ways Moreover, the activation energies for the diffusion of CO2 and Li+ in m-Li6Zr2O7 was estimated to be 22.684 and 56.084 kJ molÀ1 under 10% CO2 atmosphere,27 respectively These data prove that the diffusion of lithium is a dominating step in the process of whole CO2 absorption.41 Fig Schematic illustration of CO2 absorption (#1073 K) and desorption ($1123 K) on Li6Zr2O7 Section (A) is the first cycle processes including the absorption process from (a) to (c) in CO2 flow, and desorption process from (d) to (f) in N2 flow, for monoclinic phase Li6Zr2O7; section (B) is the following multicycle process for triclinic phase Li6Zr2O7 with the absorption process from (g) to (i) in CO2 flow and the regeneration process from (d) to (f) in N2 flow Reproduced from ref 27 with permission Copyright ª 2010, American Chemical Society 3546 | J Mater Chem A, 2013, 1, 3540–3550 Fig Schematic illustration of the carbonation and decarbonation mechanisms of Li6Zr2O7 at high temperatures Reproduced from ref 29 with permission Copyright ª 2005, American Chemical Society The authors' group also studied the effect of temperatures and CO2 partial pressures on CO2 absorption for m-Li6Zr2O7.27 The results indicated that about 86.7% of the capacity is preserved for m-Li6Zr2O7 at 1023 K as the CO2 partial pressure decreases from 1.0 to 0.1 bar Calculations of the chemical potentials versus CO2 pressures and temperatures for the CO2 capture reaction on Li6Zr2O7 are shown in Fig 6.26 It is proved that the optimal temperature for CO2 absorption on Li6Zr2O7 is around 823 K.29 Higher temperature would cause loss of lithium and reduced capacity of CO2 absorption during the multi-cycle processes.12,30 Kinetic studies ensure that there are no mass transfer limitations for CO2 absorption So control over the gas ow rate with a minimum value is essential The CO2 absorption rate for mLi6Zr2O7 at 1023 K increases obviously with the gas ow rate switched from 50 to 100 ml minÀ1.27 But upon further switching the ow rate up to 150 ml minÀ1, no more increase can be observed 2.2.3 High-temperature CO2 chemisorption on Li8ZrO6 Higher capacity of CO2 absorption is expected for rhombohedral r-Li8ZrO6 because of its higher lithium content than Li2ZrO3 and Li6Zr2O7 However, reports about the CO2 absorption properties of Li8ZrO6 are few because of the difficulty in obtaining pure Li8ZrO6 The CO2 absorption capacity of Li8ZrO6 could be well maintained within a wide range of CO2 partial pressures,31 which is very different from that of Li2ZrO3 The temperature effect on CO2 sorption is dependent on both kinetic and thermodynamic factors.34 Investigations of the temperature effect indicated that both Li6Zr2O7 and Li8ZrO6 showed slow CO2 uptake rates below 973 K.12,31 The limitation of ion (e.g Li+ and O2À) migration and CO2 diffusion at low temperatures are ascribed to the formation of the same solid carbonate shell and aggregation of the particles as described for Li6Zr2O7 Similar to Li6Zr2O7, as the operating temperature was enhanced above the melting point of Li2CO3 (983 K),31,36 the CO2 uptake rates would be dramatically increased for Li8ZrO6 with a capacity of 52 wt%, resulting from the facile diffusion of CO2 and ions Thermal stability tests demonstrated that Li8ZrO6 exhibited gradually reduced uptake capacity during the multicycle processes of CO2 absorption–desorption,12 due to the This journal is ª The Royal Society of Chemistry 2013 View Article Online Feature Article Journal of Materials Chemistry A Downloaded by Lanzhou University on 26 March 2013 Published on 18 December 2012 on http://pubs.rsc.org | doi:10.1039/C2TA00700B volatilization of Li2O at high temperature Fig illustrates the schematic processes of CO2 absorption on Li8ZrO6 at 1023 K and desorption at 1173 K.12,31 Aer the rst cycle of CO2 absorption, Li8ZrO6 decomposes as proposed in equations (8) and (9), producing more Li2CO3 than Li6Zr2O7 and Li2ZrO3 Then the CO2 absorption proceeds following the same pathway as that for Li6Zr2O7 and Li2ZrO3, with a theoretical CO2 absorption capacity of 54.4 wt% 2Li8ZrO6 + 5CO2 ! 5Li2CO3 + Li6Zr2O7 (8) Li8ZrO6 + 3CO2 ! 3Li2CO3 + Li2ZrO3 (9) 2.2.4 High-temperature CO2 chemisorption on Li2ZrO3 doped with other alkali metals Recently, Li2ZrO3 sorbents doped with other alkali metals (Na and/or K) have attracted much interest, with a noticeably improved CO2 uptake rate and sorption capacity than the unmodied counterparts As far as we know, there are no reports about doped Li6Zr2O7 and Li8ZrO6 involved in CO2 chemisorption Na2ZrO3 and Na promoted absorbents are good candidates for increased CO2 capture Duan reported that the structure of Na2ZrO3 is isotypic to that of Li2TiO3,46 and both of them are different from that of K2ZrO3 The CO2 capture performance of Na2ZrO3 has been proved to be greater than that of Li2ZrO3,48–50 though they have a similar enthalpy of reaction.46 Furthermore, the introduction or doping of alkaline elements into Li2ZrO3 will change the melting points of the system apparently and produce a liquid eutectic mixed-salt molten shell (Fig and 10) on the outer surfaces of Li2ZrO3,14,16,21,36,51 hence signicantly improving the diffusion rate of CO2 on the samples The molten carbonate shell allows diffusion and sorption of CO2, which changes the viscoelastic properties of the sorbent and determines the effectiveness of the sorbent for CO2 uptake Nevertheless, particle coarsening leads to low capacities and poor stability of the absorbent Nakagawa et al pointed out that the amount of molten alkali carbonates may improve the CO2 diffusion rate in the carbonate shell and enhance the CO2 uptake rate compared with that happened in solid carbonate Fig Schematic illustration of CO2 absorption (at 1023 K) and desorption (at 1173 K) on Li8ZrO6 Steps (A)–(C) correspond to the first cycle of CO2 absorption–desorption, and steps (D)–(F) correspond to the second cycle of CO2 absorption–desorption Reproduced from ref 31 with permission Copyright ª 2011, American Chemical Society This journal is ª The Royal Society of Chemistry 2013 shell.14,52 The rheological properties of pure Li2ZrO3 and K-doped Li2ZrO3 under CO2 atmosphere have been studied to investigate the inuence of the molten carbonate in the sorbent mixture on the CO2 sorption.51,52 Pfeiffer et al found that the Li2ÀxNaxZrO3 solid solutions rstly chemisorbed CO2 through the formation of a carbonate shell on the surface of the particles at low temperatures (473–573 K).53 Then lithium and/or sodium atoms diffuse from the core to the surfaces of the particles through the external carbonate shell when the temperature reached 673 K or higher A compromise between the kinetic enhancement and thermal stability should be considered to develop efficient CO2 absorbents promoted with elements of Na and/or K Therefore, a number of binary and ternary eutectic salt-modied Li2ZrO3 sorbents through addition of NaF in combination with K2CO3 and Na2CO3 have been identied and evaluated as CO2 absorbents at temperatures between 723 and 973 K,10,14,45,52–54 with enhanced chemisorption capacity and diffusion kinetics Theoretically, Duan calculated the reaction heats and the relationships of Dm (T, P) versus temperatures and CO2 pressures of the M2ZrO3 (M ¼ K, Na, Li) system as shown in Fig 11 and eqn (10).46 Above the line of Dm < 0, M2ZrO3 (M ¼ K, Na, Li) tends to absorb CO2 to form M2CO3, while below the line of Dm > 0, M2CO3 is easy to decompose to release CO2 and regenerate M2ZrO3 again The molar ratios of Li to alkali metals (Na and/or K) are vital in determining the absorption/regeneration features of CO2 at high temperature M2ZrO3 (M ¼ K, Na, Li) + CO2 ! ZrO2 + M2CO3 (10) ˚ The atomic radii of lithium and sodium are 2.05 and 2.23 A, respectively The solubility limits of sodium into Li2ZrO3 and lithium into Na2ZrO3 are different,10 with smaller and lighter lithium atoms diffusing more easily into the Na2ZrO3 lattice than the sodium atoms into the Li2ZrO3 network (Fig 2) As a consequence, the maximum amount of sodium in Li2ÀxNaxZrO3 is 0.2, and the amount of lithium in the lattice of Na2ÀxLixZrO3 is 0.6 Li2ÀxNaxZrO3 (0 # x # 2) enclosed Na2ÀxLixZrO3 as in a cherry model has been prepared by a precipitation method.10 It was found that Li2(1Àx)Na2xZrO3 with x ¼ 0.02 possesses the best performance with a CO2 absorption capacity of 25 Æ 0.4 wt% in Fig 10 Phase diagram of the Li2CO3–K2CO3 binary systems Reproduced from ref 21 with permission Copyright ª 2008, American Chemical Society J Mater Chem A, 2013, 1, 3540–3550 | 3547 View Article Online Journal of Materials Chemistry A Feature Article coordinated with the LiO6 polyhedra distorted along the c-axis, other lithium atoms are lonely pentacoordinated without distortion to the LiO5 polyhedra Downloaded by Lanzhou University on 26 March 2013 Published on 18 December 2012 on http://pubs.rsc.org | doi:10.1039/C2TA00700B Fig 11 The contour plotting of calculated chemical potentials versus CO2 pressures and temperatures of the reactions of M2ZrO3 (M ¼ K, Na, Li) capturing CO2 Y-axis plotted in logarithm scale Only Dm ¼ curve (van't Hoff relation) is shown explicitly For each reaction, above its Dm ¼ curve, their Dm < 0, which means the alkali metal zirconates absorb CO2 and the reaction goes forward, whereas below the Dm ¼ curve, their Dm > 0, which means the CO2 starts to release and the reaction goes backward to regenerate the sorbents Reproduced from ref 46 with permission Copyright ª 2012, American Institute of Physics an atmosphere of CO2 (20 wt%) and air (80 wt%) at 773 K within h,55 losing only 0.9% of its CO2-absorption capacity aer 10 cycles of absorption–desorption Although it has a similar structure, the CO2 capture performance of K2ZrO3 is inferior to that of Na2ZrO3 and Li2ZrO3,46 because a high regeneration temperature is required However, resembling the cases of Na-doped Li2ZrO3, Xiong and Ida et al reported the synthesis of K-doped Li2ZrO3 with remarkably enhanced CO2 sorption kinetics compared to Li2ZrO3 alone.34,36,37,56 Solid solutions of Li2ÀxKxZrO3 (0 # x # 2) have been prepared by a coprecipitation method and tested as CO2 captors,54 with the solubility limit of potassium in Li2ZrO3 about x ¼ 0.2 Li2ÀxKxZrO3 has ve times the CO2 absorption rate of Li2ZrO3 alone in short times.54 On the other hand, the CO2 absorption rate for Li2ZrO3 synthesized from m-ZrO2 is evidently slower and impacted more obviously by K doping than that from t-ZrO2.45 Size effects also take place here, since the CO2 sorption rate on K-doped Li2ZrO3 increases with decreasing sizes of the CO2 sorbents.34 The K-doped Li2ZrO3 synthesized via a citrate route have been investigated at different temperatures and CO2 partial pressures,56 which possess better CO2 capture performance especially at low CO2 partial pressures However, kinetic analyses demonstrated that the CO2 absorption of Li2ÀxKxZrO3 was similar to that of Li2ZrO3 aer long times,46 due to the diffusion of lithium and potassium through the external carbonate shell of Li2CO3 and K2CO3 It was also found that increase of the amount of doping elements can enhance the CO2 uptake rate, associating with decreased CO2 absorption capacity of Li2ZrO3 For example, higher potassium concentrations lead to the formation of a new phase of Li2.27K1.19Zr2.16O6.05,54 or even different phases of ZrO2 In the structure of Li2.27K1.19Zr2.16O6.05, the lithium atoms are located on different positions, caused by the coulombic repulsion energies associated with the proximity of lithium and potassium atoms While some lithium atoms are hexa- 3548 | J Mater Chem A, 2013, 1, 3540–3550 Perspective and outlook Design and synthesis of novel CO2 absorbents represents a rapidly expanding research area with respect to environmental protection and resourceful application of CO2 This review briey summarized recent developments, with special attention focused on the synthesis and modication of lithium containing zirconates LixZryOz and their applications for CO2 sorption Currently, the main challenges of most of the absorbents, including LixZryOz, are their insufficient absorption capacity, kinetics, and stability The development of in situ high temperature precise measurement of weight variation of absorbents before and aer absorption of CO2 and the determination of internal correlation between the absorbent structures and their performance will provide a rational strategy to explore efficient absorbents LixZryOz reacts with CO2 and releases CO2 following reverse reactions Analyses conrm that high lithium containing LixZryOz does not regenerate easily aer repeated CO2 capture–regeneration cycles, since it suffers severely from textural degradation and gradually reduced uptake capacity because of the volatilization of Li2O Therefore, the long-term stability of the sorbents during the multi-cycle processes is a limiting issue affecting continuous CO2 separation and practical applications The CO2 capture performance of LixZryOz could be tuned via controlling the synthetic parameters or introduction of external elements with the dual effects of phase stabilization and increase in conductivity Meanwhile, partial substitution of Li by doping elements within the LixZryOz structures would form new structures to enhance the reactivity of Li+ with CO2, and consequently producing faster CO2 absorption kinetics with respect to non-doped LixZryOz More detailed studies on the synthesis of new phases as well as the determination of the doping effects on the structures and CO2 capture properties of these materials are necessary in the future Furthermore, possible sorption pathways and mechanisms would provide guidelines for further research in the search for more efficient high-temperature CO2 absorbents The effect of temperature and pressure on the sorption performances is complicated, depending on both thermodynamic and kinetic factors It is desired to reach an optimized balance between the absorption thermodynamics and regeneration kinetics Consequently, special attention should be paid to the structures of the active sites and the structural evolutions of these materials during the CO2 sorption and desorption, which are not yet clear and signicant for the process However, to date, most of the research has been performed at laboratory scale For realistic CO2 capture operations, there are a lot of complexities For example, both steam and other gases such as SO2 will be present in the ue gas.5,57 On the other hand, although kinetics would be enhanced, smaller particle size could always provoke important problems in industrial scale operations, such as pressure drop due to particle This journal is ª The Royal Society of Chemistry 2013 View Article Online Downloaded by Lanzhou University on 26 March 2013 Published on 18 December 2012 on http://pubs.rsc.org | doi:10.1039/C2TA00700B Feature Article agglomerations So, the complexity of real system is of signicance and in the next few years, scalable and controllable sample preparation should be paid more attentions Careful investigations should be made to explore the effect of real conditions with as many of the factors present as possible on CO2 capture efficiency in both laboratory and batch scale, such as the coexisting species of water vapor, etc where caution should be exercised to evaluate the sorbent performance before drawing conclusions In summary, the exciting applications of CO2 sorbent materials with high capacity and good recycling stabilities are strongly desired To make them available in the near future to address the pressing need for CO2 xation, particularly at 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A Frommell, J S Hoffman, R P Reasbeck and H W Pennline, Fuel Process Technol., 2005, 86, 1503–1521 53 H Pfeiffer, C V´ azquez, V H Lara and P Bosch, Chem Mater., 2007, 19, 922–926 54 M Y Veliz-Enriquez, G Gonzalez and H Pfeiffer, J Solid State Chem., 2007, 180, 2485–2492 Feature Article 3550 | J Mater Chem A, 2013, 1, 3540–3550 This journal is ª The Royal Society of Chemistry 2013
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