Cycle Stability and Hydration Behavior of Magnesium Oxide and Its Dependence on the Precursor-Related Particle Morphology

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Article Cycle Stability and Hydration Behavior of Magnesium Oxide and Its Dependence on the Precursor-Related Particle Morphology

Georg Gravogl 1,2, Christian Knoll 2,3 , Jan M. Welch 4, Werner Artner 5, Norbert Freiberger 6, Roland Nilica 6, Elisabeth Eitenberger 7, Gernot Friedbacher 7, Michael Harasek 3 , Andreas Werner 8, Klaudia Hradil 5, Herwig Peterlik 9, Peter Weinberger 2 , Danny Müller 2,* and Ronald Miletich 1 1 Department of Mineralogy and Crystallography, University of Vienna, Althanstraße 14, 1090 Vienna, Austria; [email protected] (G.G.); [email protected] (R.M.) 2 Institute of Applied Synthetic Chemistry, TU Wien, Getreidemarkt 9, 1060 Vienna, Austria; [email protected] (C.K.); [email protected] (P.W.) 3 Institute of Chemical, Environmental & Biological Engineering, TU Wien, Getreidemarkt 9, 1060 Vienna, Austria; [email protected] 4 Atominstitut, TU Wien, Stadionallee 2, 1020 Vienna, Austria; [email protected] 5 X-ray Center, TU Wien, Getreidemarkt 9, 1060 Vienna, Austria; [email protected] (W.A.); [email protected] (K.H.) 6 RHI-AG, Magnesitstraße 2, 8700 Leoben, Austria; [email protected] (N.F.); [email protected] (R.N.) 7 Institute of Chemical Technologies and Analytics, TU Wien, Getreidemarkt 9, 1060 Vienna, Austria; [email protected] (E.E); [email protected] (G.F.) 8 Institute for Energy Systems and Thermodynamics, TU Wien, Getreidemarkt 9, 1060 Vienna, Austria; [email protected] 9 Faculty of Physics, University of Vienna, Boltzmanngasse 5, 1090 Vienna, Austria; [email protected] * Correspondence: [email protected]; Tel.: +43-1-5880-1163-740

Received: 31 August 2018; Accepted: 2 October 2018; Published: 7 October 2018

Abstract: Thermochemical energy storage is considered as an auspicious method for the recycling of medium-temperature waste heat. The reaction couple Mg(OH)2–MgO is intensely investigated for this purpose, suffering so far from limited cycle stability. To overcome this issue, Mg(OH)2, MgCO3, and MgC2O4·2H2O were compared as precursor materials for MgO production. Depending on the precursor, the particle morphology of the resulting MgO changes, resulting in different hydration behavior and cycle stability. Agglomeration of the material during cyclization was identiﬁed as main reason for the decreased reactivity. Immersion of the spent material in liquid H2O decomposes the agglomerates restoring the initial reactivity of the material, thus serving as a regeneration step.

Keywords: particle morphology; magnesium hydroxide; magnesium carbonate; magnesium oxalate; magnesium oxide; cycle stability; in-situ powder X-ray diffraction (PXRD); hydration reactivity; thermochemical energy storage; thermochemistry

1. Introduction Energy management is a major challenge for our society, requiring equal measures of political and scientiﬁc involvement [1]. Energy supply, sustainable, environmentally benign energy production, and efﬁcient utilization are key issues in managing global energy use [2]. Energy management may, in many cases, be better expressed as ‘heat management’, as heat is the most ubiquitous form of energy.

Nanomaterials 2018, 8, 795; doi:10.3390/nano8100795 www.mdpi.com/journal/nanomaterials Nanomaterials 2018, 8, x FOR PEER REVIEW 2 of 14

Nanomaterials 2018, 8, 795 2 of 14 management may, in many cases, be better expressed as ‘heat management’, as heat is the most ubiquitous form of energy. In nearly all types of electrical power plants, as well as in most industrial Inprocesses, nearly all heat types is used of electrical as the driving power plants,force and as welloperating as in mostmedium. industrial Within processes, this context, heat the is usedutilization as the drivingof waste force heat, and accounting operating for medium. two-thirds Within of this overall context, global the energy utilization production, of waste is heat, an extensively accounting forinvestigated two-thirds field of overall [3]. The global use energy of waste production, heat flows is includes an extensively several investigated aspects, one field of [ them3]. The being use oftemporal waste heatdecoupling flows includes of waste several heat availability aspects, one and of demand, them being as the temporal two are not decoupling necessarily of wastecorrelated. heat availabilityThe necessary and storage demand, may as be the realized two are using not materials necessarily for correlated.sensible, latent The, or necessary thermoch storageemical maystorage be realizedof energy using (heat) materials [4–9]. forAll sensible, three energy latent, or storage thermochemical concepts offer storage advantages of energy (heat) in specific [4–9]. Allareas three of energyapplication storage [6,9,10] concepts. offer advantages in specific areas of application [6,9,10]. Thermochemical energy storage (TCES) features long long-term-term storage, a wide range of compatible temperatures, applicability as a heat heat pump pump system system,, and and finally, finally, high energy energy storage storage dens densitiesities [10 [10––13]13].. Based on on these these aspects, aspects, medium medium-temperature-temperature waste waste heat heat (up (up to 450 to 450°C and◦C extensively and extensively available available from fromindustrial industrial processes) processes) is perfectly is perfectly suitable suitable for TCES for systems. TCES systems. An attractive An attractive TCES material TCES for material medium for- medium-temperaturetemperature applications applications is the system is the Mg(OH) system Mg(OH)2–MgO with2–MgO a storage with a storagetemperature temperature around around350 °C ◦ 350[14]. C[Both14 ]. Mg(OH) Both Mg(OH)2 and 2 MgOand MgO are industrial are industrial base base materials materials and and are are,, therefore therefore,, available available in in large quantities at low prices. Mg(OH)22––MgOMgO as aa TCESTCES materialmaterial is is well well known known for for this this purpose, purpose, with with many many aspects aspect relateds related to itsto applicationits application in energy in energy storage storage already already investigated investigated in literature. in literature. Kinetic investigations Kinetic investigations of dehydration of anddehydration rehydration and [ 15rehydration,16], mechanistic [15,16] aspects, mechanistic of the conversionaspects of the [17 ,conversion18], modification [17,18] of, modification the material byof additionsthe material of lithiumby additions salts [of16 lithium,19,20], bysalts coating [16,19,20] or use, by of coating composite or use material of composite [21,22], bymaterial dotation [21,22] [23],, andby dotation finally also[23], applicabilityand finally also in formapplicability of a chemical in form heat of a pumpchemical [24 ]heat were pump reported. [24] were Nonetheless, reported. twoNonetheless, key issues two preventing key issues industrial preventing application industrial remain unaddressed: application remain First, rehydration unaddressed: reactivity First, (completeness),rehydration reactivity and second, (completeness) the cycle stability., and second, Whereas the for the cycle limited stability. cyclability Whereas observed for the thus limited far, no satisfyingcyclability solution observed has thus been far found,, no satisfying the rehydration solution reactivity has been is addressedfound, the by rehydration the addition reactivity of lithium is saltsaddressed [16,19 b,20y the], which addition are of quite lithium expensive. salts [16,19,20] On a molecular, which are level, quite reactivity expensive. could On a also molecular be tuned level, by 2+ 2+ dotationreactivity of could Mg(OH) also2 bewith tuned Ca by-ions dotation [23]. of Mg(OH)2 with Ca -ions [23]. OnOn an an industrial industrial sca scalele MgO MgO is is produced produced via via calcination calcination of Mg(OH) of Mg(OH)2 or 2 MgCOor MgCO3 [25]3. Both[25]. Bothprecursors precursors are found are foundin natural in natural deposits, deposits, but whereas but whereas MgCO3 MgCOis an industrially3 is an industrially mined raw mined material, raw material,Mg(OH)2 is Mg(OH) produced2 is from produced serpentinite from serpentinite or processing or seawater processing [26] seawater. However, [26 aerobic]. However, calcination aerobic of calcinationany other Mg of anycompound other Mg may compound result in formation may result of in MgO formation by stepwise of MgO decomposition. by stepwise decomposition. In Scheme 1, Inthis Scheme is shown1, thisat the is example shown atof thethe mentioned example of industrial the mentioned precursor, industrial as well precursor,as for magnesium as well oxalate as for magnesiumdihydrate. oxalate dihydrate.

Scheme 1. ThermalThermal decomposition decomposition of of various various MgO MgO precursors: precursors: (1) (1) Mg(OH) Mg(OH)2, (22), (2) MgCO MgCO3, (3)3,

(3)MgC MgC2O42∙2HO42·O2H. 2O.

All so so far far performed performed investigations investigations on theon rehydrationthe rehydration of MgO of MgOfor thermochemical for thermochemical energy storage energy · purposesstorage purposes have largely have largely neglected neglected the origin the oforigin theMgO. of the AsMgO Mg(OH). As Mg(OH)2, MgCO2, MgCO3, MgC3,2 MgCO4 2H2O24O,∙2H and2O, MgOand MgO crystallize crystallize in crystallographically in crystallographically and stereochemically and stereochemically different different systems systems (Table 1(),Table and feature1), and notablyfeature notably different different particle particle morphologies, morphologies, MgO samples MgO samples originating originating from different from different precursors precursors can not necessarilycan not necessarily be expected be expected to have theto have same the properties same properties with respect with torespect rehydration to rehydration and cycle and stability. cycle Thisstability. assumption This assumption is supported is supported by previous by kinetic previous studies kinetic on studies the H2O-dissociation on the H2O-dissociation on MgO. Compared on MgO. toCompared the isotypic to the CaO isotypic [27], the CaO lower [27], hydration the lower reactivityhydration of reactivity MgO [28 of] is MgO mainly [28] caused is mainly by thecaused kinetic by barrierthe kinetic of the barrier water of dissociation the water dissociation on the surface on [the29]. surface The disfavored [29]. The Hdisfavored2O-dissociation H2O-dissocia as ﬁrst steption as in formationfirst step in of formation Mg(OH) 2ofoccurs Mg(OH) mainly2 occurs at surface mainly defects, at surface edges, defects, step edges, edges, step or corner edges, sites, or corner exhibiting sites,

Nanomaterials 2018, 8, 795 3 of 14 a lower dissociation energy barrier [30]. This suggests that by variation of the particle morphology and origin of the MgO, the rehydration behavior should be affected. While all precursors result in compositionally indistinguishable MgO sample stoichiometries, the particle size and morphology, crystallographic orientation, and thus the orientation of the reactive surfaces of the material are not necessarily the same. To verify this hypothesis, MgO obtained by calcination of Mg(OH)2, MgCO3, and MgC2O4·2H2O was investigated regarding hydration reactivity and cycle stability.

Table 1. Comparison of the crystallographic parameters of selected MgO precursors and MgO.

Mg(OH)2 [31] MgCO3 [32] MgC2O4·2H2O[33] MgO [34] Space group P3m1 (164) R3c (167) Fddd (70) Fm3m (225) a [Å] 3.1486(1) 4.637(1) 5.3940(11) 4.2113(5) b [Å] 3.1486(1 4.637(1) 12.691(3) 4.2113(5) c [Å] 4.7713(1) 15.023(3) 15.399(3) 4.2113(5) α [◦] 90 90 90 90 γ [◦] 120 120 90 90 V [Å3] 40.96 279.74 1054.14 74.69

2. Materials and Methods

2.1. Material

Mg(OH)2 powder (particle size ≤5 µm) and MgCO3 (particle size ≤200 µm) were supplied by RHI-AG (X-ray ﬂuorescence analysis (Bruker AXS GmbH, 76187 Karlsruhe, Germany)) of the materials revealed no signiﬁcant impurities). MgC2O4·2H2O (98.5% purity) was purchased from abcr (GmBH, 76187 Karlsruhe, Germany) and the particle fraction ≤200 µm was used as supplied. The materials were calcined in an electric furnace under air and a static atmosphere for 4 h at variable temperatures ◦ ◦ ◦ ◦ ◦ (Mg(OH)2: at 375 C; MgCO3: at 550 C, 600 C, 650 C; MgC2O4·2H2O: at 650 C). For subsequent rehydration, the in-situ calcined material from the (powder X-ray diffraction) P-XRD measurement was kept for 24 h in liquid water under ambient pressure-temperature conditions.

2.2. BET Surface The speciﬁc surface of the samples was determined by nitrogen sorption measurements, which were performed on an ASAP 2020 (Micromeritics) instrument. The samples (amounting between 100–200 mg) were degassed under vacuum at 80 ◦C overnight prior to measurement. The surface area was calculated according to Brunauer, Emmett, and Teller (BET, Micromeritics Instrument Corp., Norcross, GA, USA) and t-plot methods [35].

2.3. Powder X-ray Diffraction with In-Situ Hydration (P-XRD) Hydration of calcined samples was performed in an Anton Paar XRK 900 (Bruker AXS GmbH, 76187 Karlsruhe, Germany) sample chamber, connected to an evaporation coil kept at 300 ◦C (see −1 Figure S1a). Using an HPLC-pump, water was evaporated at rates from 1 g H2O min up to 3 g min−1 and the resulting steam was passed through the sample (1 mm thickness) with 0.2 L min−1 nitrogen as carrier gas. The sample is mounted on a hollow ceramic powder sample holder, allowing for complete perfusion of the sample with the water vapour (see Figure S1b). As the sample is completely penetrated by the X-rays, the obtained diffractograms represent an average across the total sample with respect to the quantitative phase proportions. The diffractograms were evaluated using the PANalytical program suite HighScorePlus v3.0d. A background correction and a Kα2 strip were performed. Phase assignment is based on the ICDD-PDF4+ database (International Diffraction Data-Powder Diffraction File), the exact phase composition, shown in the conversion plots, was obtained via Rietveld-reﬁnement [36] in the program suite HighScorePlus v3.0d. All quantiﬁcations based on P-XRD are accurate within of ±2%. The rehydration rates were calculated based on the phase Nanomaterials 2018, 8, 795 4 of 14

composition derived from the diffractograms, normalizing the percentages of Mg(OH)2 and MgO to a total of 100%.

2.4. Scanning Electron Microscopy (SEM) SEM (Thermo Fisher Scientiﬁc, 168 Third Avenue, Waltham, MA 02451, USA)) images were recorded on gold coated samples with a Quanta 200 SEM instrument from FEI under low-vacuum at a water vapour pressure of 80 Pa to prevent electrostatic charging.

2.5. Small-Angle X-ray Scattering (SAXS) The samples were prepared either as powder between two pieces of tape or in a sealed capillary. Patterns were recorded using a microsource with X-rays from a copper target (Incoatec High Brilliance, wavelength 0.1542 nm, CuKα), a point focus (Nanostar from Bruker AXS) and a 2D detector (VÅNTEC 2000). The X-ray patterns were radially averaged and background corrected to obtain scattering intensities in dependence on the scattering vector q = (4π/λ) sinθ, with 2θ being the scattering angle. The ﬁt function from Beaucage [37] to describe scattering intensities of complex systems with a broad size distribution consists of a power law and Guinier’s exponential form,

 √ d f 2 2 ! ( )3 −q Rg er f qRg/ 6 I(q) ∝ Gexp + B   (1) 3 q where G and B are the numerical prefactors, df is the fractal dimension, Rg is the radius of gyration and erf (x) is the error function. To describe the particle interference and thus the tendency of particles to agglomerate, additionally a structure factor from a hard sphere model was used [38,39],

  √ d f  2 2 ! ( )3 −q Rg er f qRg/ 6 I(q) ∝ G exp + B    S(q) (2)  3 q  with S(q) = 1/(1 + 24η Gint(2qRHS)/(2qRHS)) (3) and RHS being the hard sphere radius describing a typical distance of objects, η the hard sphere volume factor for characterizing the amount of agglomeration, and Gint a function derived in Kinning et al. [38].

3. Discussion and Results To combine the apparent particle morphology with the crystallographic features of the lattice as given in Table1, SEM-images of the original and calcined materials are compared in Figure1. The ﬁrst row corresponds to SEM-images of the various MgO precursors; in the second row the resulting MgO samples, obtained after thermal decomposition, are shown. Whereas Mg(OH)2 particles feature euhedral idiomorphic shapes with characteristic faces following hexagonal symmetry (Figure1a), both MgCO3 (Figure1b) and MgC 2O4·2H2O (Figure1c) reveal subidiomorphic irregular particle shapes occasionally showing typical rhombohedral (Figure1b) or foliated (Figure1c) cleavage faces, which in the case of MgC2O4·2H2O correspond to its layer structure. The particle morphology of the materials changes during calcination (Figure1, second row), leading to three differently textured MgO samples. Whereas for using Mg(OH)2 as the precursor material (Figure1a), calcination results in an apparently unchanged particle morphologies, the MgO crystallites obtained from both MgCO3 (Figure1b) and MgC 2O4·2H2O (Figure1c) precursors are characterized by a clear surface fragmentation, which can be attributed to larger degree of structural reconstruction on the release of volatile components. In contrast, the H2O release from Mg(OH)2 to Nanomaterials 2018, 8, x FOR PEER REVIEW 5 of 14

Nanomaterials 2018, 8, 795 5 of 14

MgO follows a simple change from hcp to ccp arrangement of the octahedral subunits and hence preservesNanomaterials the 2018 particles, 8, x FOR inPEER its REVIEW shape to a large extent. 5 of 14

Figure 1. SEM pictures of (a) Mg(OH)2; (b) MgCO3; (c) MgC2O4∙2H2O before calcination (first row) and after calcination (second row).

The particle morphology of the materials changes during calcination (Figure 1, second row), leading to three differently textured MgO samples. Whereas for using Mg(OH)2 as the precursor material (Figure 1a), calcination results in an apparently unchanged particle morphologies, the MgO crystallites obtained from both MgCO3 (Figure 1b) and MgC2O4∙2H2O (Figure 1c) precursors are characterized by a clear surface fragmentation, which can be attributed to larger degree of structural reconstruction on the release of volatile components. In contrast, the H2O release from Mg(OH)2 to Figure 1.1. SEMSEM picturespictures ofof ((aa)) Mg(OH) Mg(OH)22;(; (bb)) MgCOMgCO33;(; (cc)) MgCMgC22OO44∙2H·2H2OO before before calcination (first (first row) MgO follows a simple change from hcp to ccp arrangement of the octahedral subunits and hence and after calcination (second row).row). preserves the particles in its shape to a large extent. OnThe theparticle nanoscale, morphology small-anglesmall-angle of the X-rayX -raymaterials scattering changes (SAXS) during reveals calcination a transformation (Figure 1, of second the material row), fromleading a dense to three solid differently to aa highlyhighly textured porousporous MgO materialmaterial samples. onon calcinationcalcination Whereas (see(see for FigureusingFigure Mg(OH) S2).S2). TheThe2 nanostructureas the precursor of MgOmaterial was ( Figure modelled 1a) , by calcination a unifiedunified results Guinier/powerGuinier/power in an apparently law law [37] [37], ,unchanged resulting resulting in particle a radius morph of gyration ologies for , the the MgO size ofcrystallites the particlesparticles obtained andand anan from agglomeration agglomeration both MgCO with with3 (Figure a a structure structure 1b) factorand factor MgC from from2O a4∙2H harda hard2O sphere ( Figuresphere model, 1c)model precursors describing, describing theare agglomerationthecharacterized agglomerat byion of aparticles ofclear particles surface with with fragmentation a typical a typical distance distance, which 2R 2HS RcanHSand and be theattributed the packing packing to density densitylarger degree withwith aa of hard structural sphere  volumereconstruction ratioratio η [ on38[38,39] ,the39]. release. The The detailed detailed of volatile fit fit parameters parameters components. are are foundIn found contrast, in in the the the supporting supporting H2O release information information from Mg(OH) (Table (Table2 S1). to InS1).MgO general, In follows gene theral, a gyrationthe simple gyration change radius radius offrom MgOof MgOhcp particles to particles ccp arrangement calcined calcined from from of MgCO theMgCO octahedral3 and3 and MgC MgC subunits2O2O4·42H∙2H2 2andOO isis abouthence 6.6preserves and 5.1 the nm, particles respectively, in its shape inin comparisoncomparison to a large toto extent. aboutabout 22 nmnm ifif calcinedcalcined fromfrom Mg(OH)Mg(OH)22.. In contrast, the valuesOn of the η = =nanoscale, 0.18 and asmall fractal-angle dimensiondimension X-ray scattering ofof ddff = 2.8 (SAXS) indicate,indicate reveals, that MgOaMgO transformation fromfrom Mg(OH)Mg(OH) of 2the2 consists material of small,from a agglomerated dense solid to particles a highly with porous a wide material size distribution, on calcination whereas (see Figure MgO S from 2). T heother nanostructure precursors ofis builtMgO upwas of modelled larger, denserdenser by a unified nanoparticlesnanoparticles Guinier/power ((η closeclose to tolaw zero,zero, [37] dd,f f resulting= = 4).4). in a radius of gyration for the size of theIn particles order toto and allow an a agglomeration betterbetter comparability with a betweenstructure the factor differentdifferent from precursoraprecursor hard sphere materialsmaterials model investigatedinvestigated, describing 2 withinthe agglomerat this study,ion MgOMgOof particles obtained with by a typical calcination distance from 2 R Mg(OH)HS and the2 waswas packing used used densityas as reference reference with amaterial material hard sphere [28] [28].. Involume Scheme ratio 22,, schematic schematic [38,39]. representationTherepresentation detailed fit ofof parameters thethe calcinationcalcination are found andand rehydrationrehydration in the supporting conditionsconditions information appliedapplied for(forTable itsits preparationS1). In general, is shown. the gyration radius of MgO particles calcined from MgCO3 and MgC2O4∙2H2O is about 6.6 and 5.1 nm, respectively, in comparison to about 2 nm if calcined from Mg(OH)2. In contrast, the values of  =  and a fractal dimension of df = 2.8 indicate, that MgO from Mg(OH)2 consists of small, agglomerated particles with a wide size distribution, whereas MgO from other precursors is built up of larger, denser nanoparticles ( close to zero, df = 4). Scheme 2. Conditions for calcination and rehydration of MgO obtained from Mg(OH)2. In orderScheme to allow 2. Condit a betterions comparabilityfor calcination and between rehydration the d ofifferent MgO obtainedprecursor from materials Mg(OH) investigated2. within this study, MgO obtained by calcination from Mg(OH)2 was used as reference material [28]. To correlate the particle morphologies with rehydration reactivity and cycle stability, rehydration In SchemeTo correlate 2, schematic the particlerepresentation morpholog of theies calcination with rehydration and rehydration reactivity conditions and cycle applied stability for its, experiments using the different MgO samples were monitored by in situ powder X-ray diffraction rehydrationpreparation isexperiments shown. using the different MgO samples were monitored by in situ powder X-ray (P-XRD). This allows for a direct observation and quantification of the reaction progress. As in previous experiments, the rehydration reactivity of the MgO produced from Mg(OH)2 was found quite limited [28]. To eventually increase the reaction rate, an even larger excess of water vapour was introduced into the reaction chamber. Increasing the vapour flow from 1 g min−1 to 3 g min−1

Scheme 2. Conditions for calcination and rehydration of MgO obtained from Mg(OH)2.

To correlate the particle morphologies with rehydration reactivity and cycle stability, rehydration experiments using the different MgO samples were monitored by in situ powder X-ray

Nanomaterials 2018, 8, x FOR PEER REVIEW 6 of 14

diffractionNanomaterials 2018(P-XRD), 8, x FOR. This PEER allows REVIEW for a direct observation and quantification of the reaction6 progress.of 14 As in previous experiments, the rehydration reactivity of the MgO produced from Mg(OH)2 was diffraction (P-XRD). This allows for a direct observation and quantification of the reaction progress. found quite limited [28]. To eventually increase the reaction rate, an even larger excess of water As in previous experiments, the rehydration reactivity of the MgO produced from Mg(OH)2 was Nanomaterialsvapour was2018 introduced, 8, 795 into the reaction chamber. Increasing the vapour flow from 1 g min−1 6to of 3 14 g found quite limited [28]. To eventually increase the reaction rate, an even larger excess of water min−1 enhanced the rehydration conversion of MgO from 44% to 67% (Figure 2a). To assess the cycle vapour was introduced into the reaction chamber. Increasing the vapour flow from 1 g min−1 to 3 g stability for the increased vapour flow, five consecutive rehydration–calcination cycles were enhancedmin−1 enha thenced rehydration the rehydration conversion conversion of MgOof MgO from from 44% 44% to to 67%67% (Figure(Figure 22a).a). To To assess assess the the cycle cycle performed (Figure 2b). Similar to previous experiments with lower vapour flows the rehydration stabilitystability for for the the increased increased vapour vapour flow, flow, five consecutive five consecutive rehydration–calcination rehydration–calcination cycles were cycles performed were yield decreased over 5 cycles to a final Mg(OH)2 conversion of only 14%. Even after the first cycle the (Figureperformed2b). Similar(Figure to2b). previous Similar experiments to previous withexperiments lower vapour with lower flows vapour the rehydration flows the yieldrehydration decreased rehydratioyield decreasedn conversion over 5 cycles was to depleted a final Mg(OH) to 43%.2 conversion of only 14%. Even after the first cycle the over 5 cycles to a final Mg(OH)2 conversion of only 14%. Even after the first cycle the rehydration conversionrehydration was conversion depleted was to 43%.depleted to 43%.

Figure 2. (a) Rehydration of Mg(OH)2-originating MgO with various water vapour flow rates; (b) FigureFigure 2. 2.( a(a)) Rehydration Rehydration of of Mg(OH) Mg(OH)2-originating2-originating MgOMgO with with various various water water vapour vapour ﬂow flow rates; rates (b; )(b Cycle) Cycle stability of Mg(OH)2-originating MgO. stabilityCycle stability of Mg(OH) of Mg(OH)2-originating2-originating MgO. MgO.

ForFor MgO MgOMgO producedproduced produced by by calcination calcination calcination of of of Mg(OH) Mg(OH) Mg(OH)2 a2 2stronga a strongstrong correlation correlationcorrelation between betweenbetween reactivity, reactivity,reactivity, accessible accessible accessible surfacesurface area areaarea and andand calcinationcalcination calcination temperature temperature has has has been been been established. established established.[28] [28[28] Higher]. Higher Higher calcination calcination calcination temperatures temperatures temperatures promotepromote sintering sinteringsintering ofof of the the particles, particles, leading leading leading to to toa a decreaseda decreased decreased porosity, porosity, porosity, increased increased increased MgO MgO MgO crystal crystal crystal size size, , sizeand, and and decreaseddecreased rehydration rehydrationrehydration yield. yield. ToTo assess assessassess the thethe possibilitypossibility possibility of of a asimilar similar effect effect effect for for for MgO MgO MgO originating originating originating from from from MgCO MgCO MgCO3, initial3,3, initial initial studies studies studies of of of thethe correlation correlation betweenbetween between calcination calcination temperature, temperature, temperature, BET BET BET surface surface surface and and and rehydration rehydrationrehydration reactivity reactivityreactivity were werewere made. made.made. 3 ◦ ◦ ◦ ForFor this this purpose, purpose,purpose, samplessamples samples of of MgCO MgCO3 were3were were calcined calcined calcined at at 550at 550 550 °C, °C,C, 600 600 600 °C, °CC,and,and and 650 650 65°C0 for °CC for3,for 6, 3, 3,9, 6, 6,and 9,9, and 12and h. 12 12 h.h. The MgO formed from MgCO3 calcined for 6 h at 600 °C◦ had the highest surface area (Figure S3). TheThe MgO MgO formed formed from from MgCO MgCO33calcined calcined forfor 66 hh atat 600600 °CC hadhad thethe highesthighest surfacesurface areaarea (Figure(Figure S3).S3). Nevertheless,Nevertheless, attemptedattemptedattempted rehydration rehydrationrehydration of ofof all allall samples samples samples by by bywater water water vapo vapour vapour inur inthe in the theP-XRD P-XRD P-XRD failed, failed, failed, showing showing showing no Mg(OH)2 formation within 120 minutes. In Scheme 3, representation of the conditions applied for nono Mg(OH) Mg(OH)2 2formation formation within within 120 120 minutes. minutes. In In Scheme Scheme3 ,3 representation, representation of of the the conditions conditions applied applied for for calcination and rehydration of MgCO3-derived MgO is given. calcinationcalcination and and rehydration rehydration of of MgCO MgCO3-derived3-derived MgO MgO is is given. given.

Scheme 3. Conditions for calcination and rehydration of MgO obtained from MgCO3. Scheme 3. Conditions for calcination and rehydration of MgO obtained from MgCO . Scheme 3. Conditions for calcination and rehydration of MgO obtained from MgCO3 3. BasedBased onon thethe assumption, that that a a different different chemical chemical history history of ofthe the MgO MgO would would have have an impact an impact on on the reactivityBased on during the assumption, rehydration, that a varieda different rehydration chemical rate history would of havethe MgO been would expected. have Observing an impact on the reactivity during rehydration, a varied rehydration rate would have been expected. Observing no theno conversionreactivity during to Mg(OH) rehydration,2 under thea varied applied rehydration conditions rate was would, howeve haver, quitebeen expected. unexpected. Observing On conversion to Mg(OH)2 under the applied conditions was, however, quite unexpected. On prolonged noprolonged conversion exposure to Mg(OH) to water2 under vapour the over applied 24 h for conditions MgO CO 3 was-1 (see, howeve Schemer, 3)quite a very unexpected. sluggish On exposure to water vapour over 24 h for MgO CO3-1 (see Scheme3) a very sluggish formation of prolongedformation of exposure Mg(OH) 2 tobelow water 10 % vapour was observed over 24. To h forascertain MgO whetherCO3-1 (see rehydration Scheme of3) thisa very material sluggish Mg(OH)2 below 10% was observed. To ascertain whether rehydration of this material could be driven formationcould be driven of Mg(OH) by longer2 below exposure 10% wasto a observedvast excess. To of ascertainreactant, thewhether samples rehydration were stored of inthis liquid material by longer exposure to a vast excess of reactant, the samples were stored in liquid water. After 24 h could be driven by longer exposure to a vast excess of reactant, the samples were stored in liquid reaction time in water at room-temperature, according to P-XRD measurements the material had been completely transformed to Mg(OH)2 (CO3-3) To repeat the in situ rehydration study for this material, ◦ the calcination step was repeated at 375 C using the conditions developed for Mg(OH)2 [28]. After a new calcination step the BET surface of the various samples CO3-4 was found to be slightly higher NanomaterialsNanomaterials 20182018,, 88,, xx FORFOR PEERPEER REVIEWREVIEW 77 ofof 1414 water.water. AfterAfter 2424 hh reactionreaction timetime inin waterwater atat roomroom--temperature,temperature, accordingaccording toto PP--XRDXRD measurementsmeasurements thethe materialmaterial had had been been completely completely transformed transformed to to Mg(OH) Mg(OH)22 ((COCO33--33)) ToTo repeat repeat the the in in situsitu rehydration rehydration studyNanomaterialsstudy forfor thth2018isis material,material,, 8, 795 thethe calcinationcalcination stepstep waswas repeatedrepeated atat 375375 °C°C usingusing thethe conditionsconditions developeddeveloped7 of 14 forfor Mg(OH)Mg(OH)22 [28][28].. AAfterfter aa newnew calcinationcalcination stepstep thethe BETBET surfacesurface ofof thethe variousvarious samplessamples COCO33--44 waswas foundfound toto bebe slightlyslightly higherhigher thanthan forfor COCO33--11,, thethe MgOMgO originatinoriginatingg directlydirectly fromfrom MgCOMgCO33 ((FigureFigure SS44).). InIn contrastthancontrast for toCOto thethe3-1 first,first the attempt,attempt, MgO originating nownow forfor thosethose directly materialsmaterials from MgCO tthehe rehydrationrehydration3 (Figure S4). experimentsexperiments In contrast inin to the the P firstP--XRDXRD attempt, werewere repeatednowrepeated for those successfully successfully materials for for the all all rehydration materials materials experiments (for (for detailed detailed in therehydration rehydration P-XRD were rates rates repeated see see FigureFigure successfully SS55).). Ranked Ranked for all accordingmaterialsaccording (for to to their their detailed fin finalal rehydration conversion conversion rates to to Mg(OH) Mg(OH) see Figure22,, the the S5). most most Ranked reactive reactive according material material totheir within within final this this conversion series series was was to ◦ obtainedMg(OH)obtained2 by,by the calcinationcalcination most reactive ofof MgCOMgCO material33 atat within 600600 °C°C this forfor series 66 hh ((FigureFigure was obtained 3)3) andand subsequent bysubsequent calcination rehydrationrehydration of MgCO3 inatin 600liquidliquidC water.forwater. 6 h AA (Figure finalfinal 3conversionconversion) and subsequent toto 8484%% rehydration Mg(OH)Mg(OH)22 waswas in liquid not not onlyonly water. byby farfar A finalthethe highesthighest conversion yieldyield to forfor 84% thethe Mg(OH) MgCOMgCO332-- originatingwasoriginating not only series, series, by far butthe but highest also also notably notably yield for more more the MgCO than than for for3-originating MgO MgO originating originating series, but from from also notablyMg(OH) Mg(OH) more22 (67(67% than% final final for conversion).MgOconversion). originating from Mg(OH)2 (67% final conversion).

FigureFigure 3. 3. FinalFinal conversion conversion for for rehydration rehydration of of the the variousvarious MgCOMgCO33--originating-originatingoriginating MgO MgO samples samples COCO33---444 inin thethe P P-XRD.P--XRDXRD..

SEMSEM images images demonstrate, demonstrate, thatthat thethe particle particle morphology morphology ofof MgCO MgCO33 (((FigureFigureFigure 44a)4a)a) isisis retainedretainedretained afterafterafter calcination,calcination, although although the the formerly formerly distinct distinct edges edges and and surfaces surfaces are are now now covered covered by by smaller smaller scales scales ((Figure(FigureFigure4 b).4b).4b). During DuringDuring the the the hydration hydration hydration of the of of calcined the the calcined calcined material mater mater in liquidialial in in water liquid liquid the waterlarge water particles the the large large disintegrate particles particles disintegrateintodisintegrate smaller into intoplatelets smaller smaller (Figure platelets platelets4c), although ( (FigureFigure lacking4c)4c),, although although the characteristic lacking lacking thethe hexagonal characteristic characteristic morphology hexagonal hexagonal as morphologycharacteristicmorphology asas for characteristiccharacteristic euhedrally forfor grown euhedrallyeuhedrally Mg(OH) growngrown2 (see Mg(OHMg(OH Figure1))2).2 (see(see A subsequent FigureFigure 1).1). AA calcination subsequentsubsequent of calcinationcalcination material ofrehydratedof materialmaterial rehydratedrehydrated in liquid water inin liquidliquid retains waterwater the aforeretainsretains mentioned thethe aforeafore plateletmentionedmentioned morphology plateletplatelet morphologymorphology (Figure4d). ((FigureFigure 4d).4d).

Figure 4. SEM images of (a) MgCO3,(b) calcined MgCO3 (CO3-1), (c) calcined MgCO3, rehydrated for 24 h in liquid water (CO3-3), (d) material from image c after calcination (CO3-4). NanomaterialsNanomaterials 2018 2018, 8,, 8x, FORx FOR PEER PEER REVIEW REVIEW 8 8of of 14 14

Figure 4. SEM images of (a) MgCO3, (b) calcined MgCO3 (CO3-1), (c) calcined MgCO3, rehydrated for NanomaterialsFigure 4.2018 SEM, 8, 795images of (a) MgCO3, (b) calcined MgCO3 (CO3-1), (c) calcined MgCO3, rehydrated for8 of 14 24 h in liquid water (CO3-3), (d) material from image c after calcination (CO3-4). 24 h in liquid water (CO3-3), (d) material from image c after calcination (CO3-4).

TheTheThe changing changing changing particle particle particle shape shape shape observed observed observed in in in the the the SEM SEM SEM images images images is is is attributed attributed attributed to to to the the the volume volume volume work work work going going going along with the Mg(OH)2 formation. To enable this rehydration-related rearrangement of the material, alongalong with with the the Mg(OH) Mg(OH)22 formation.formation. To To enable enable this this rehydration rehydration-related-related rearrangement rearrangement of of the the material, material, waterwater in in its its its liquid liquid liquid form form form seems seems seems crucial crucial crucial as as as an an an agent agent agent triggering triggering triggering the the the rehydration rehydration rehydration process process. process. SAXS SAXS. SAXS intensities intensities intensities (Figure S6) show that on (repeated) rehydration of the MgCO3-derived MgO-samples the particle ((FigureFigure S S6)6) show that on (repeated) rehydration of thethe MgCOMgCO3--derivedderived MgO MgO-samples-samples the the particle morphologymorphologymorphology is is is widely widely widely unchanged. unchanged. unchanged. From From From the the the larger larger larger scattering scattering scattering intensity intensity intensity a a ahighly highly highly porous porous porous nanostructu nanostructure, nanostructure,re, retainedretainedretained during during rehydration,rehydration, rehydration, maymay may be be be extrapolated. extrapolated. extrapolated. At At theAt the the same same same time, time, time, a general a ageneral general decrease decrease decrease in particle in in particle particle size sizewassize was also was also observed, also observed observed being, being, being in good in in good agreementgood agreement agreement with withthe withSEM the the SEM imagesSEM images images (Figure (Figure (Figure4). 4). 4). The carbonate-derived MgO CO3-4 was also investigated in terms of cycle stability (Figure 5). TheThe carbonate carbonate-derived-derived MgO MgO COCO33-4-4 waswas also also investigated investigated in in terms terms of of cycle cycle s stabilitytability ( (FigureFigure 5).5). Similar to the material OH-1 originating from Mg(OH)2, also in the case of CO3-4 a decrease in SimilarSimilar to to the material OHOH-1-1 originating from Mg(OH)22,, also also in in the the case case of of CO3--44 aa decrease in rehydrationrehydrationrehydration reactivity reactivity reactivity was was was detected, detected, detected, although although although to to to a a alesser lesser lesser extent extent extent than than than observed observed observed for for for OHOH-1 OH-1-.1 Over. Over Over five ﬁve five cyclescyclescycles the the the rehydration rehydration rehydration conversion conversion drops drops t too t o57% 57% (84% (84% in in the the 1st, 1st, 75% 75% in in the the 2nd 2nd cycle). cycle).

Figure 5. Cycle stability of MgCO3-originating MgO CO3-4. Figure 5. Cycle stability of MgCO 3--originatingoriginating MgO MgO CO33--44..

As a third precursor for preparation of reactive MgO, MgC2O4∙2H2O was investigated (see AsAs a third precursor for preparation preparation of of reactive reactive MgO, MgO, MgCMgC22OO44∙2H·2H22OO was was investigated (see SchemeSchemeScheme 44)). 4). .

Scheme 4. Conditions for calcination and rehydration of MgO obtained from MgC O ·2H O. Scheme 4. Conditions for calcination and rehydration of MgO obtained from MgC2 2O4 4∙2H2 2O. Scheme 4. Conditions for calcination and rehydration of MgO obtained from MgC2O4∙2H2O.

Since MgC2O4·2H2O decomposes stepwise via MgCO3 (see Scheme1), only samples calcined Since MgC2O4∙2H2O decomposes stepwise via MgCO3 (see Scheme 1), only samples calcined in in theSince furnace MgC at2O 6004∙2H◦2OC decomposes for 6 h were stepwise investigated. via MgCO A comparison3 (see Scheme of the 1), SEMonly samples images incalcined Figure in6, thethe furnace furnace at at 600 600 °C °C for for 6 6h hwere were investigated. investigated. A A comparison comparison of of the the SEM SEM images images in in Figure Figure 6, 6, compares compares compares the morphology of the different samples: initial oxalate material C2O4-0 (Figure6a), the the morphology of the different samples: initial oxalate material C2O4-0 (Figure 6a), the calcined the morphology of the different samples: initial oxalate material C2O4-0 (Figure 6a), the calcined calcined material C2O4-1 (Figure6b), MgO after rehydration in liquid water C2O4-3 (Figure6c), and material C2O4-1 (Figure 6b), MgO after rehydration in liquid water C2O4-3 (Figure 6c), and a new material C2O4-1 (Figure 6b), MgO after rehydration in liquid water C2O4-3 (Figure 6c), and a new a new calcined material C2O4-4 (Figure6d). Similar to the MgCO 3-case, calcination of the initial calcined material C2O4-4 (Figure 6d). Similar to the MgCO3-case, calcination of the initial material calcinedmaterial material resulted C in2O partial4-4 (Figure fragmentation, 6d). Similar whereas to the MgCO subsequent3-case, treatmentcalcination with of the liquid initial water material and resulted in partial fragmentation, whereas subsequent treatment with liquid water and re-calcination resultedre-calcination in partial forced fragmentation, the material whereas to adopt subsequent a lamellar-structured treatment with particle liquid morphology. water and re-calcination In contrast forcedforced the the material material to to adopt adopt a a lamellar lamellar-structured-structured particle particle morphology morphology. . In In contrast contrast to to the the MgO MgO to the MgO originating from MgCO3, thinner platelets were formed, those structure is preserved originating from MgCO3, thinner platelets were formed, those structure is preserved after calcination. originatingafter calcination. from MgCO Moreover,3, thinner even platelets the rehydration were formed, of the those oxalate-based structure MgOis preserved did not after yield calcination. the typical Moreover, even the rehydration of the oxalate-based MgO did not yield the typical hexagonally Moreover,hexagonally even shaped the morphologies rehydration of of the euhedral oxalate brucite-based crystallites. MgO did not yield the typical hexagonally shapedshaped morphologies morphologies of of euhedral euhedral brucite brucite crystallites. crystallites.

Nanomaterials 2018, 8, 795 9 of 14 Nanomaterials 2018, 8, x FOR PEER REVIEW 9 of 14

Nanomaterials 2018, 8, x FOR PEER REVIEW 9 of 14

Figure 6. 6. SEM images ofof (a(a)) MgC MgC2O2O44·∙2H2H22OO,, ((bb)) calcined calcined MgC MgC22OO44·∙2H2H2OO( (C2O44--11)),, ( (cc)) calcined Figure 6. SEM images of (a) MgC2O4∙2H2O, (b) calcined MgC2O4∙2H2O (C2O4-1), (c) calcined MgC2OO44∙2H·2H22O,O, rehydrated rehydrated for for 24 24 h h in in liquid liquid water water ( C22OO44-3-3),), (d (d) )material material from from image image cc afterafter calcination calcination MgC2O4∙2H2O, rehydrated for 24 h in liquid water (C2O4-3), (d) material from image c after calcination (C2OO44--44)).. (C2O4-4).

Both SEM and SAXS data show a comparablecomparable picturepicture asas observedobserved forfor MgCOMgCO33. Rehydration of Both SEM and SAXS data show a comparable picture as observed for MgCO3. Rehydration of MgO C2OO44--11 inin liquid liquid water water results results in in conservation conservation of of the the original original particle particle morphology morphology to to a a wide MgO C2O4-1 in liquid water results in conservation of the original particle morphology to a wide extent ( (FigureFigure S S7).7). The porous nanostructure formed formed is is preserved preserved during during rehydration. rehydration. Accordingly, Accordingly, extent (Figure S7). The porous nanostructure formed is preserved during rehydration. Accordingly, the increased SAXS intensities observed for th thee rehydrated material seem to arise from the formation the increased SAXS intensities observed for the rehydrated material seem to arise from the formation of smaller particles (SEM images, Figure6 6).). of smaller particles (SEM images, Figure 6). To determine determine the the reactivity reactivity of of the the MgC MgC22OO44∙2H·2H2OO-based-based MgO, MgO, both both the the material obtained by To determine the reactivity of the MgC2O4∙2H2O-based MgO, both the material obtained by direct calcination of of MgC MgC22O4∙2H·2H22OO( (CC22OO4-41-1) )and and that that resulting resulting from the rehydration–calcination rehydration–calcination direct calcination of MgC2O4∙2H2O (C2O4-1) and that resulting from the rehydration–calcination sequence ( C2OO44--44)) w wereere subject subject to to rehydration rehydration in in the the P P-XRD.-XRD. Unlike the case of MgCO3,, the the directly directly sequence (C2O4-4) were subject to rehydration in the P-XRD. Unlike the case of MgCO3, the directly calcined material C2OO4--11 waswas found found to to be be reactive reactive to to rehydration, rehydration, resulting resulting in in a final ﬁnal conversion of calcined material C22O4-1 was found to be reactive to rehydration, resulting in a final conversion of 68%68% ( (Figure Figure(Figure7 7a). a).7a).

FigureFigure 7.7. (a()a ) Direct Direct rehydration rehydration and and cycle cycle stability stability of MgC of MgC2O4-originating2O4-originating MgO C MgO2O4-1; C(2bO) 4-1; Figure 7. (a) Direct rehydration and cycle stability of MgC2O4-originating MgO C2O4-1; (b) Rehydration and cycle stability of MgC2O4-originating MgO C2O4-4. (b) Rehydration and cycle stability of MgC2O4-originating MgO C2O4-4. Rehydration and cycle stability of MgC2O4-originating MgO C2O4-4.

TheThe most most remarkableremarkable ﬁndingfinding is, that that several several batches batches of of CC2O2O4-44,-4 the, the sample sample previously previously rehydrated rehydrated 2 4 in liquidThe most H2O remarkable and subsequently finding calcined is, that ,several could be batches fully rehydratedof C O -4, tinhe thesample 1st cycle, previously but in rehydratedthe 2nd in liquid H2O and subsequently calcined, could be fully rehydrated in the 1st cycle, but in the 2nd incycle liquid the H rehydra2O andtion subsequently conversion calcineddecreased, couldto 64% be (Figure fully rehydrated7b). After five in cycles the 1st both cycle, samples but in gave the a 2nd cyclecomparable the rehydra finaltion conv conversionersion of slightly decreased less than to 64 50%% ( FigureMg(OH) 7b).2. After five cycles both samples gave a comparable final conversion of slightly less than 50% Mg(OH)2.

Nanomaterials 2018, 8, 795 10 of 14

cycleNanomaterials the rehydration 2018, 8, x FOR conversion PEER REVIEW decreased to 64% (Figure7b). After five cycles both samples10 of gave 14 a comparable final conversion of slightly less than 50% Mg(OH)2. AssessingAssessing thethe technologicaltechnological feasibility of of MgC MgC2O2O4∙2H4·2H2O2 Oas asprecursor precursor material, material, on onthe theone one hand hand thethe material material was was completelycompletely rehydrated to to Mg(OH) Mg(OH)2 2withinwithin the the first first cycle cycle after after an aninitial initial treatment treatment withwith liquid liquid water. water.On On thethe other hand, due due to to a a large large decrease decrease in in conversion conversion rate rate during during the thesuccessive successive cycles, a modest overall performance, and a relatively higher price compared to Mg(OH)2 and cycles, a modest overall performance, and a relatively higher price compared to Mg(OH)2 and MgCO3, MgCO3, MgC2O4∙2H2O is most likely not suitable as a competitive MgO precursor. Therefore, MgC2O4·2H2O is most likely not suitable as a competitive MgO precursor. Therefore, MgC2O4·2H2O wasMgC not2O subjected4∙2H2O was further not subjected studies. further studies. Based on the conversion-enhancing effect of rehydrating calcined material in liquid water, both Based on the conversion-enhancing effect of rehydrating calcined material in liquid water, both a a sample of spent MgO originating from Mg(OH)2 and one from MgCO3 after the 5th rehydration sample of spent MgO originating from Mg(OH) and one from MgCO after the 5th rehydration cycle cycle was calcined a further time and then rehydrated2 for 24 h in liquid3 water. At that point, P-XRD was calcined a further time and then rehydrated for 24 h in liquid water. At that point, P-XRD analysis analysis showed complete transformation to Mg(OH)2 for both samples. Both Mg(OH)2-samples were showed complete transformation to Mg(OH) for both samples. Both Mg(OH) -samples were now now subjected further five rehydration–calcination2 cycles in the P-XRD, followed2 by a further subjectedregeneration further in fiveliquid rehydration–calcination water and another five cycles rehydration in the P-XRD,–calcination followed cycles by in a further the P-XRD. regeneration The inconversion liquid water rates and for another 15 consecutive five rehydration–calcination cycles, including two cycles regeneration in the P-XRD. steps The after conversion five cycles rates are for 15shown consecutive in Figure cycles, 8. including two regeneration steps after five cycles are shown in Figure8. InIn the the case caseof of MgOMgO originatingoriginating from from M Mg(OH)g(OH)2,2 shown, shown in in Figure Figure 8a,8a, the the conversion conversion rate rate after after thethe firstfirst regeneration regeneration ( (CycleCycle 6) 6) was was slightly slightly enhanced enhanced compared compared to the to very the 1st very cycle. 1st This cycle. effect This was effect even was evenmore more pronounced pronounced after after the thesecond second regeneration regeneration (Cycle (Cycle 10), 10), revealing revealing an even an even further further increased increased reactivity.reactivity. Nevertheless, Nevertheless, the the depletion depletion evidenced evidenced in in the the second second cycle cycle was was retained retained even even after after the second the cyclessecond after cycles regeneration, after regeneration, as observed as observed for Cycles for Cycles 6 and 6 and 12. 12. These These results results could could be be reproducedreproduced on variouson various batches. batches.

FigureFigure 8. 8. SelectedSelected conversion conversion rates rates from from a series a series of 15 consecutive of 15 consecutive calcination calcination–hydration–hydration cycles, cycles,including including two regeneration two regeneration steps in liquid steps water in liquid after the water 5th andafter the the 10th 5th cycle. and (a the) Mg(OH) 10th cycle.2- originating MgO after regeneration; (b) MgCO3-originating MgO after regeneration. (a) Mg(OH)2-originating MgO after regeneration; (b) MgCO3-originating MgO after regeneration.

InIn the the case case of ofMgO MgO derived derived from from MgCO MgCO3, a different3, a different but even but even more more promising promising effect was effect observed: was Theobserved: spent material The spent could material be completely could be completely regenerated regenerated to reproduce to reproduce the reactivity the reactivity observed observed for the first cycle.for the Even first the cycle. second Even cycle the after second each cycle regeneration after each process regeneration (Cycle process 7 and 12) (C wasycle comparable 7 and 12) was to the “first”comparable second to cycle. the “first”Even in second this case cycle. the Even effect in was this reproducible case the effect on various was reproducible batches. on various batches.To better understand the physical processes during regeneration, SEM images of material during To better understand the physical processes during regeneration, SEM images of material during several stages of regeneration and cycling were compared. Despite differences in initial particle several stages of regeneration and cycling were compared. Despite differences in initial particle morphology (Figure9a,d), MgO originating from Mg(OH) 2 or MgCO3 shows similar evolution of morphology (Figure 9a,d), MgO originating from Mg(OH)2 or MgCO3 shows similar evolution of reactivity during repeated calcination–rehydration cycles. After five consecutive cycles, resulting in reactivity during repeated calcination–rehydration cycles. After five consecutive cycles, resulting in aged material of depleted rehydration reactivity (see Figures2 and5), the particle morphology of aged material of depleted rehydration reactivity (see Figures 2 and 5), the particle morphology of Mg(OH)2-derived MgO (Figure9b) seems nearly unaffected. In contrast, for material originating from Mg(OH)2-derived MgO (Figure 9b) seems nearly unaffected. In contrast, for material originating from MgCO , the larger spherical aggregates are retained (Figure9e). After regeneration for 24 h in liquid MgCO3 3, the larger spherical aggregates are retained (Figure 9e). After regeneration for 24 h in liquid water both materials reveal a lamellar, platelet morphology devoid of the characteristic hexagonal brucite particle shape (Figure 9c,f).

Nanomaterials 2018, 8, 795 11 of 14 water both materials reveal a lamellar, platelet morphology devoid of the characteristic hexagonal

Nanomaterialsbrucite particle 2018, shape8, x FOR (Figure PEER REVIEW9c,f). 11 of 14

Figure 9. SEMSEM images images of of various various intermediates intermediates during during calcination/rehydration/regeneration calcination/rehydration/regeneration for

Mg(OH)for Mg(OH)2- (left)2- and (left) MgCO and3- MgCOoriginating3-originating MgO (right) MgO; (a) (right);Mg(OH)2 (-aoriginating) Mg(OH) 2MgO-originating; (b) Mg(OH) MgO;2- (originatingb) Mg(OH) MgO2-originating after 5 rehydration MgO after–calcination 5 rehydration–calcination cycles; (c) material cycles; of image (c) material b after ofregeneration image b after for 24regeneration h in liquid forH2O 24; ( hd) in MgCO liquid3- Horiginating2O; (d) MgCO MgO3-originating; (e) MgCO3- MgO;originating (e) MgCO MgO3 -originatingafter five rehydration MgO after– calcinationﬁve rehydration–calcination cycles; (f) material cycles; of image (f) material e after regeneration of image e after for 24 regeneration h in liquid forH2O 24. h in liquid H2O.

To directlydirectly monitormonitor thethe rehydration/regenerationrehydration/regeneration process, process, both both a a sample of MgO originating from Mg(OH) Mg(OH)22 andand from from MgCO MgCO3 were3 were observed observed during during rehydration rehydration in liquid in liquid water water by in bysitu in SAXS situ (SAXSFigure (Figuress S8,S9). S8 Initial and S9 SAXS). Initial curves SAXS (black) curves and (black) final SAXS and ﬁnal curves SAXS (red) curves are (red)highlighted are highlighted to better visualizeto better visualizethe data and the clearlydata and show clearly the showchange the in change the inner in structure the inner of structure the particles. of the The particles. main Thedifference main differenceis the time required is the time for required structural for recovery, structural which recovery, is about which three is-fold about shorter three-fold for Mg(OH) shorter2- derivedfor Mg(OH) material2-derived compared material to that compared from MgCO to that3 (Figure from S8), MgCO most3 likely(Figure due S8), to the most considerably likely due larger to the particleconsiderably sizes largerfavored particle by the sizeslatter. favored by the latter. Within various repeated experiments the regeneration process described for spent M MgOgO was found to bebe reproducible—onreproducible—on one one hand hand for for the the material material of of the the same same origin, origin, on on the the other other for for materials materials of ofdifferent different origins. origins. We We suggest suggest that duringthat during the regeneration the regeneration process process due to due the comparablyto the comparably long reaction long reactiontime and time the vast and excess the vast of water, excess a complete of water, conversion a complete to Mg(OH) conversion2 as well to Mg(OH) as a regeneration2 as well of as the a particleregeneration morphology of the parti occurs.cle morphology Both effects complementoccurs. Both eacheffects other, complement restoring each theoriginal other, restoring reactivity the of originalthe material. reactivity of the material. The possibility of a regeneration of spent material is of utmost importance for assessing the economic feasibility feasibility of of a a TCES TCES material material and and energy energy storage storage process, process, as by as byprolonging prolonging the thelife- life-timetime the materialsthe materials investment investment costs costs are are minimized. minimized. Additionally, Additionally, by by implementation implementation of of a a continuous regeneration step into the process, regenerating after each discharging–charging cycle a defined amount of material, permanent high activity of the TCES material circumventing efficiency losses by ageing would be ensured.

Nanomaterials 2018, 8, 795 12 of 14 regeneration step into the process, regenerating after each discharging–charging cycle a deﬁned amount of material, permanent high activity of the TCES material circumventing efﬁciency losses by ageing would be ensured.

4. Conclusions

MgO obtained by calcination of Mg(OH)2, MgCO3, and MgC2O4·2H2O was compared regarding its rehydration reactivity and cycle stability to assess its applicability in thermochemical energy storage. The three different MgO-precursors led to three MgO samples featuring different particle morphologies with identical chemical compositions. Whereas Mg(OH)2 and MgC2O4·2H2O resulted in reactive MgO that could be rehydrated by water vapour to Mg(OH)2 directly following calcination, material originating from MgCO3 resulted in no conversion on contact with water vapour. Only after rehydration in liquid water and subsequent calcination of the thus formed Mg(OH)2, 84% of the resulting material could be rehydrated by water vapour. All materials investigated showed decreased rehydration reactivity during consecutive calcination–rehydration cycles, with MgCO3-derived MgO showing the smallest decline in reactivity. A regeneration step, consisting of rehydration of the spent material in liquid water over 24 h, restored the initial reactivity allowing for recycling of the material. In the case of Mg(OH)2 derived material, the initial reactivity could even be improved by repeated regeneration of the material in liquid water. The results reported herein conﬁrm, that the reactivity of MgO towards rehydration is strongly correlated to origin and physicochemical history of the material–an aspect so far neglected in the research on TCES materials. The correlation between chemical history and performance of storage materials may stimulate additional to coating, chemical dotation, etc., the consideration of a further, easily tunable parameter for the research on novel TCES systems.

Supplementary Materials: The following are available online at http://www.mdpi.com/2079-4991/8/10/795/s1, Figure S1: Rehydration setup, reaction chamber and sample holder used for the in situ studies. Figure S2: SAXS intensities of starting materials and materials after calcination. Figure S3: BET surfaces of the MgCO3-originating MgO samples. Figure S4: BET surfaces of the MgCO3-originating MgO samples after rehydration. Figure S5: Rehydration rates of MgCO3-originating MgO samples in the P-XRD. Figure S6: SAXS intensities of materials from MgCO3 precursor. Figure S7: SAXS intensities of materials from Mg2C2O4·2H2O precursor. Figure S8: In situ SAXS intensities during regeneration in liquid water for 24 h. Figure S9: Kinetics of conversion to hydroxide during regeneration in liquid water. Table S1: Fit data for calcined materials Author Contributions: Experimental investigation: G.G., C.K.; Proof-reading and language: J.M.W.; Evaluation of P-XRD data: W.A., K.H.; Provision of samples and scientific contribution: N.F., R.N.; SEM images and interpretation: E.E., G.F.; SAXS measurements and interpretation: H.P.; Project administration: A.W.; Conception of the study, writing, review, and editing: D.M.; Supervision and funding acquisition: M.H., P.W., R.M. Funding: This research was funded by the Austrian Research Promotion Agency (FFG Forschungsförderungsgesellschaft), project 845020, 841150 and project 848876. Acknowledgments: The X-ray center (XRC) of TU Wien is kindly acknowledged for the access to the powder X-ray diffractometer. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

References

1. Rahm, D. Sustainable Energy and the States, Essay on Politics Markets and Leadership, 1st ed.; McFarland: Jefferson, NC, USA, 2002. 2. IEA. Heating without Global Warming: Market Developments and Policy Considerations for Renewable Heat. Available online: https://www.google.com.hk/url?sa=t&rct=j&q=&esrc=s&source=web&cd= 1&cad=rja&uact=8&ved=2ahUKEwiWq76i5-ndAhULUd4KHYqlAxoQFjAAegQICRAC&url=https% 3A%2F%2Fwww.iea.org%2Fpublications%2Ffreepublications%2Fpublication%2FFeaturedInsight_ HeatingWithoutGlobalWarming_FINAL.pdf&usg=AOvVaw1PEQh4uZihy8UNUEKlI6EJ (accessed on 31 August 2018). Nanomaterials 2018, 8, 795 13 of 14

3. IEA. Co-Generation and Renewables. Solutions for a Low-Carbon Energy Future. Available online: https://www.iea.org/publications/freepublications/publication/co-generation-and-renewables- solutions-for-a-low-carbon-energy-future.html (accessed on 31 August 2018). 4. Bauer, T.; Steinmann, W.-D.; Laing, D.; Tamme, R. Thermal energy storage materials and systems. Annu. Rev. Heat Transf. 2012, 15, 131–177. [CrossRef] 5. Hasnain, S.M. Review on sustainable thermal energy storage technologies, part i: Heat storage materials and techniques. Energy Convers. Manag. 1998, 39, 1127–1138. [CrossRef] 6. Xu, J.; Wang, R.Z.; Li, Y. A review of available technologies for seasonal thermal energy storage. Sol. Energy 2014, 103, 610–638. [CrossRef] 7. Zhang, H.; Baeyens, J.; Cáceres, G.; Degrève, J.; Lv, Y. Thermal energy storage: Recent developments and practical aspects. Prog. Energy Combust. Sci. 2016, 53, 1–40. [CrossRef] 8. Cabeza, L.F.; Castell, A.; Barreneche, C.; de Gracia, A.; Fernández, A.I. Materials used as PCM in thermal energy storage in buildings: A review. Renew. Sustain. Energy Rev. 2011, 15, 1675–1695. [CrossRef] 9. Dinker, A.; Agarwal, M.; Agarwal, G.D. Heat storage materials, geometry and applications: A review. J. Energy Inst. 2015, 90, 1–11. [CrossRef] 10. Abedin, A.H.; Rosen, M.R. A critical review of thermochemical energy storage systems. Open Renew. Energy J. 2011, 4, 42–46. [CrossRef] 11. Cot-Gores, J.; Castell, A.; Cabeza, L.F. Thermochemical energy storage and conversion: A-state-of-the-art review of the experimental research under practical conditions. Renew. Sustain. Energy Rev. 2012, 16, 5207–5224. [CrossRef] 12. Solé, A.; Fontanet, X.; Barreneche, C.; Martorell, I.; Fernández, A.I.; Cabeza, L.F. Parameters to take into account when developing a new thermochemical energy storage system. Energy Procedia 2012, 30, 380–387. [CrossRef] 13. Yan, T.; Wang, R.Z.; Li, T.X.; Wang, L.W.; Fred, I.T. A review of promising candidate reactions for chemical heat storage. Renew. Sustain. Energy Rev. 2015, 43, 13–31. [CrossRef] 14. Pan, Z.; Zhao, C.Y. Dehydration/hydration of mgo/h2o chemical thermal storage system. Energy 2015, 82, 611–618. [CrossRef] 15. Kuleci, H.; Schmidt, C.; Rybacki, E.; Petrishcheva, E.; Abart, R. Hydration of periclase at 350 ◦C to 620 ◦C and 200 mpa: Experimental calibration of reaction rate. Mineral. Petrol. 2015, 110, 1–10. [CrossRef] 16. Myagmarjav, O.; Ryu, J.; Kato, Y. Dehydration kinetic study of a chemical heat storage material with lithium bromide for a magnesium oxide/water chemical heat pump. Prog. Nucl. Energy 2015, 82, 153–158. [CrossRef] 17. Birchal, V.S.; Rocha, S.D.F.; Mansur, M.B.; Ciminelli, V.S.T. A simpliﬁed mechanistic analysis of the hydration of magnesia. Can. J. Chem. Eng. 2001, 79, 507–511. [CrossRef] 18. Zhou, S. Hydration Mechanisms of Magnesia-Based Refractory Bricks. Ph.D. Thesis, The University of Brithish Columbia, Vancouver, BC, Canada, December 2004. 19. Ishitobi, H.; Hirao, N.; Ryu, J.; Kato, Y. Evaluation of heat output densities of lithium chloride-modiﬁed magnesium hydroxide for thermochemical energy storage. Ind. Eng. Chem. Res. 2013, 52, 5321–5325. [CrossRef] 20. Myagmarjav, O.; Ryu, J.; Kato, Y. Lithium bromide-mediated reaction performance enhancement of a chemical heat-storage material for magnesium oxide/water chemical heat pumps. Appl. Therm. Eng. 2014, 63, 170–176. [CrossRef]

21. Shkatulov, A.; Ryu, J.; Kato, Y.; Aristov, Y. Composite material “Mg(OH)2/vermiculite”: A promising new candidate for storage of middle temperature heat. Energy 2012, 44, 1028–1034. [CrossRef] 22. Zamengo, M.; Ryu, J.; Kato, Y. Composite block of magnesium hydroxide – expanded graphite for chemical heat storage and heat pump. Appl. Therm. Eng. 2014, 69, 29–38. [CrossRef] 23. Müller, D.; Knoll, C.; Ruh, T.; Artner, W.; Welch, J.M.; Peterlik, H.; Eitenberger, E.; Friedbacher, G.; Harasek, M.; Blaha, P.; et al. Calcium doping facilitates water dissociation in magnesium oxide. Adv. Sustain. Syst. 2018, 2, 1700096. [CrossRef] 24. Kato, Y.; Sasaki, Y.; Yoshizawa, Y. Magnesium oxide/water chemical heat pump to enhance energy utilization of a cogeneration system. Energy 2005, 30, 2144–2155. [CrossRef] 25. Morozov, S.A. Synthesis of porous magnesium oxide by thermal decomposition of basic magnesium carbonate. Russ. J. Gen. Chem. 2003, 73, 37–42. [CrossRef] Nanomaterials 2018, 8, 795 14 of 14

26. Shand, M.A. The Chemistry and Technology of Magnesia; John Wiley & Sons Inc.: Hoboken, NJ, USA, 2006. Available online: https://www.wiley.com/en-us/The+Chemistry+and+Technology+of+Magnesia- p-9780471656036 (accessed on 31 August 2018).

27. Criado, Y.A.; Alonso, M.; Abanades, J.C. Kinetics of the CaO/Ca(OH)2 hydration/dehydration reaction for thermochemical energy storage applications. Ind. Eng. Chem. Res. 2014, 53, 12594–12601. [CrossRef] 28. Müller, D.; Knoll, C.; Artner, W.; Welch, J.M.; Freiberger, N.; Nilica, R.; Eitenberger, E.; Friedbacher, G.; Harasek, M.; Hradil, K.; et al. Influence of the particle morphology on cycle stability and hydration behavior of magnesium oxide. Appl. Energy 2017, submitted. 29. Hu, X.L.; Carrasco, J.; Klimeš, J.; Michaelides, A. Trends in water monomer adsorption and dissociation on flat insulating surfaces. Phys. Chem. Chem. Phys. 2011, 13, 12447–12453. [CrossRef][PubMed] 30. Stankic, S.; Bernardi, J.; Diwald, O.; Knözinger, E. Optical surface properties and morphology of Mgo and Cao nanocrystals. J. Phys. Chem. B 2006, 110, 13866–13871. [CrossRef][PubMed] 31. Kazimirov, V.Y.; Smirnov, M.B.; Bourgeois, L.; Guerlou-Demourgues, L.; Servant, L.; Balagurov, A.M.; Natkaniec, I.; Khasanova, N.R.; Antipov, E.V. Atomic structure and lattice dynamics of Ni and Mg hydroxides. Solid State Ion. 2010, 181, 1764–1770. [CrossRef] 32. Effenberger, H.; Mereiter, K.; Zemann, J. Crystal structure refinements of magnesite, calcite, rhodochrosite, siderite, smithonite, and dolomite, with discussion of some aspects of the stereochemistry of calcite type carbonates. Zeitschrift für Kristallographie–Cryst. Mater. 1981, 156.[CrossRef] 33. Chen, X.-A.; Song, F.-P.; Chang, X.-A.; Zang, H.-G.; Xiao, W.-Q. A new polymorph of magnesium oxalate dihydrate. Acta Crystallogr. Sect. E Struct. Rep. Online 2008, 64, m863. [CrossRef][PubMed] 34. Boiocchi, M.; Caucia, F.; Merli, M.; Prella, D.; Ungaretti, L. Crystal-chemical reasons for the immiscibility of periclase and wüstite under lithospheric p,t conditions. Eur. J. Mineral. 2001, 13, 871–881. [CrossRef] 35. Brunauer, S.; Emmett, P.H.; Teller, E. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 1938, 60, 309–319. [CrossRef] 36. Degen, T.; Sadki, M.; Bron, E.; König, U.; Nénert, G. The highscore suite. Powder Diffr. 2014, 29, S13–S18. [CrossRef] 37. Beaucage, G. Approximations leading to a unified exponential/power-law approach to small-angle scattering. J. Appl. Crystallogr. 1995, 28, 717–728. [CrossRef] 38. Kinning, D.J.; Thomas, E.L. Hard-sphere interactions between spherical domains in diblock copolymers. Macromolecules 1984, 17, 1712–1718. [CrossRef] 39. Pabisch, S.; Feichtenschlager, B.; Kickelbick, G.; Peterlik, H. Effect of interparticle interactions on size determination of zirconia and silica based systems—A comparison of SAXS, DLS, BET, XRD and TEM. Chem. Phys. Lett. 2012, 521, 91–97. [CrossRef][PubMed]

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