Improvement of the kinetics of hydrogen release from ammonia borane confined in silica aerogel

1 1 2 3 4 5 Imp o emen o he kine ics o hyd ogen elease om ammonia 6 bo ane con ined in silica ae ogel 7 8 9 10 Mi iam Rueda, Luis Miguel Sanz-Mo al, José Juan Sego ia, Ángel Ma ín* 11 Depa men o Chemical Enginee ing and En i onmen al Technology - Uni e si y o 12 Valladolid 13 TERMOCAL Resea ch G oup – Uni e si y o Valladolid 14 15 16 c/ Doc o Me gelina s/n 47011 Valladolid (Spain) 17 Tel: +34 983423174, e-mail: [email p o ec ed] (Á. Ma ín) 18 19 20 2 Imp o emen o he kine ics o hyd ogen elease om ammonia 21 bo ane con ined in silica ae ogel 22 Mi iam Rueda1, Luis Miguel Sanz-Mo al1, José Juan Sego ia2, Ángel Ma ín1* 23 1Depa men o Chemical Enginee ing and En i onmen al Technology - Uni e si y o 24 Valladolid, c/ Doc o Me gelina s/n 47011 Valladolid (Spain) 25 2TERMOCAL Resea ch G oup – Uni e si y o Valladolid, c/ Paseo del Cauce 59 47011 26 Valladolid (Spain) 27 Tel: +34 983423174, e-mail: [email p o ec ed] (Á. Ma ín) 28 29 Abs ac 30 Ammonia bo ane is a p omising hyd ogen s o age ma e ial due o i s high g a ime ic 31 capaci y (19.6 % w ), bu i also p esen s limi a ions such as a slow hyd ogen elease wi h 32 a long induc ion ime, a di icul egene a ion, o he o ma ion o oams and gaseous by- 33 p oduc s du ing he molysis. P e ious s udies ha e shown ha by nanocon inemen o 34 ammonia bo ane wi hin a po ous suppo some o hese limi a ions can be o e come due 35 o he educ ion and s abiliza ion o ammonia bo ane pa icle size. Howe e , his e ec 36 was only obse ed wi h mode a e ammonia bo ane loadings, as wi h highe loadings he 37 po es o he suppo became obs uc ed. In his wo k, silica ae ogels p oduced by CO2 38 d ying, wi h po e olumes up o 2 cm3/g, ha e been used o con ine ammonia bo ane. 39 The in luence o he amoun o ammonia bo ane loaded on he ae ogel suppo on he 40 he mal and s uc u al p ope ies o he ma e ial has been analyzed. I has been ound ha 41 mo e han 60 w % o ammonia bo ane can be e ec i ely s o ed in he po es o he 42 ae ogel suppo . The esul ing ma e ial shows as e hyd ogen elease kine ics by 43 he molysis a 80ºC, due o a signi ican educ ion in he mea size o ammonia bo ane 44 3 a e con inemen and he pa icipa ion o SiOH and SiOSi g oups o silica ae ogel in he 45 decomposi ion mechanism. 46 47 Keywo ds: solid s a e hyd ogen s o age, ammonia bo ane, silica ae ogel, 48 nanocon ined, supe c i ical ca bon dioxide. 49 50 1. In oduc ion 51 As ossil uel ese es a e inc easingly limi ed and hei use cons i u e a cons an sou ce 52 o g eenhouse gases and o he en i onmen al p oblems, he de elopmen o al e na i e 53 ene gy sou ces is a ac ing a conside able a en ion [1, 2]. In con as wi h ossil uels, 54 which can be easily s o ed and used when needed, he p oduc ion o ene gy om mos 55 enewable sou ces is a iable and i canno be di ec ly con olled. A possible solu ion 56 o his limi a ion could be o use hyd ogen (H2) as an ene gy ec o , acco ding o he 57 app oach commonly known as ‘hyd ogen economy’ o ‘hyd ogen socie y’ [3]: 58 Hyd ogen can be p oduced om wa e (by elec olysis, he mal decomposi ion, 59 he mochemical p ocesses, pho olysis e c.) using enewable ene gy sou ces, and s o ed 60 un il needed. Fu he mo e, he e iciency o he combus ion o hyd ogen (by 61 combus ion in in e nal combus ion engines, ca aly ic combus ion o uel cells) is high, 62 and i is one o he mos en i onmen ally a o able uels, as i p oduces nea ly ze o 63 gaseous emissions. 64 A la ge scales, hyd ogen can be anspo ed h ough pipelines (gas H2) o anke s 65 (liquid H2), and i is a good ene gy ec o wi h an ene gy densi y o 33 kwh/kg, 66 con aining h ee imes mo e ene gy han any hyd oca bon uel on a weigh basis [4]. 67 Howe e , he s o age o hyd ogen in small mobile uni s such as ehicles o small 68 elec onic equipmen is mo e challenging. 69 4 These challenges can be quan i ied analyzing he a ge s se by he US Depa men o 70 Ene gy o 2020 o au omo i e hyd ogen sys ems: a hyd ogen s o age g a ime ic 71 capaci y o 5.5 w % and a olume ic capaci y o 0.040 kg/L, wi h a maximum cos o 72 333$/kg H2 s o ed [5]. Hyd ogen s o age by some o mos ob ious sys ems using 73 comp essed o condensed H2 canno ul ill hese equi emen s due o he physical 74 p ope ies o hyd ogen. In he o me case, high p essu es (700 ba o each a olume ic 75 concen a ion o 0.042 kg H2/L) o huge olumes a e needed, and in he la e , high 76 ene gy consump ion is una oidable in o de o main ain he equi ed c yogenic 77 empe a u es [6]. Fo hese easons, solid s a e hyd ogen s o age ma e ials ha e been 78 in ensi ely s udied, as hyd a es [7], me al-o ganic amewo ks [8] o me allic and 79 chemical hyd ides [9]. 80 Ammonia bo ane (AB) is conside ed as a p omising chemical hyd ide, due o i s high 81 hyd ogen g a ime ic capaci y (19.6%w H2) and olume ic capaci y (140 g/L), 82 mode a e decomposi ion empe a u e, non- oxici y [10] and s abili y a oom 83 empe a u e, e en in he ange 50-60ºC, which is impo an om he poin o iew o 84 sa e y and enginee ing implica ions [11]. The he mal decomposi ion o nea AB 85 eleases one mole o hyd ogen pe mole o AB in each o he ollowing eac ions (1), 86 (2) and (3) [12]. In he i s decomposi ion s ep, AB eleases H2 o ming a complex 87 polyme ic aminobo ane (PAB) below i s mel ing poin (114 ºC): 88 BH3NH3 → BH2NH2 + H2 T > 120ºC (1) 89 The ea e , PAB decomposes abo e 120 ºC, o ming polyme ic iminobo ane and small 90 ac ions o undesi able ola ile byp oduc s: 91 BH2NH2 → 1/3 (BHNH)3 + H2 T > 120ºC (2) 92 (BHNH) → BN + H2 T > 500ºC (3) 93 Ne e heless, he use o his compound as hyd ogen s o age ma e ial also aces some 94 5 impo an limi a ions. One o hem is he kine ic limi a ion due o he long induc ion 95 ime needed o dis up he dihyd ogen bonding and ini ia e he elease o hyd ogen. 96 Fu he mo e, AB is di icul o egene a e: a e he molysis, i is no possible o es o e 97 he ini ial AB by di ec hyd ogena ion, and complex chemical egene a ion ou es 98 comp ising se e al s eps a e needed. Addi ionally, i o ms oams du ing he molysis 99 ha also complica e he egene a ion due o he dis up ion o he physical s uc u e o 100 he ma e ial. Mo eo e , du ing he decomposi ion p ocess, he emission o some ola ile 101 byp oduc s as bo azine, dibo ane o ammonia can be eleased which could be poisonous 102 o downs eam p ocesses and, pa icula ly, hyd ogen uel cells. 103 Se e al s a egies ha e been es ed o o e come hese ba ie s, including he addi ion o 104 ca alys s as silicon (Si), nickel (Ni), u henium (Ru), palladium (Pd) o zinc (Zn) [13- 105 16], con inemen o ammonia bo ane in o po ous solid suppo s [17-21], dissolu ion o 106 AB in ionic liquids [22,23] o using polyme s composi es [24]. Rega ding con inemen 107 o AB, di e en suppo s ha e been es ed: silica sca olds as SBA-15 o MCM-48 [18], 108 me al o ganic amewo ks (MOF) [16, 25] o ca bon based ma e ials [17] among o he s. 109 The amoun o AB ha has been success ully loaded in he suppo depends on hei 110 s uc u al p ope ies (SBET and Vpo es), ge ing a maximum amoun o 50%w using silica 111 sca olds [17]. In all he cases ha ha e been epo ed, hyd ogen kine ic and 112 he modynamic p ope ies ha e been imp o ed espec o nea hyd ide due o he 113 educ ion in mean size. In ou p e ious wo k [26], mic opa icles o silica ae ogel we e 114 used as suppo , ge ing a maximum concen a ion o 5%w AB loaded in hyd ophobic 115 silica ae ogel. Liquid an isol en echnique was used o p ecipi a e he hyd ide p io 116 supe c i ical ca bon dioxide d ying, esul ing in enhanced hyd ogen elease kine ic 117 compa ed o pu e AB. 118 In his wo k, we epo he con inemen o AB using silica ae ogel as po ous hos . The 119 6 ae ogel has been p oduced by liquid o supe c i ical CO2 d ying, a echnique ha 120 enables o p oduce a silica ma e ial wi h a high po e olume, and a co espondingly 121 high po en ial capaci y o s o age o ammonia bo ane inside i s po es. Se e al samples 122 wi h di e en concen a ions o AB up o 60 w % ha e been p epa ed in o de o 123 analyze he in luence o he loading o AB on he he mal and s uc u al p ope ies o 124 he ma e ial. Scanning elec on mic oscopy, N2 adso p ion iso he ms, FT-IR 125 spec oscopy and X- ay di ac ion ha e been used in o de o cha ac e ize he inal 126 p oduc p io o he measu emen o hyd ogen elease kine ics by decomposi ion a 127 80ºC. 128 129 2. Expe imen al me hods 130 2.1 Ma e ials 131 Te ame hylo hosilica e (TMOS, 98.0% pu i y), ammonium hyd oxide (NH4OH, 28.0- 132 30.0% ammonia pu i y) and ammonia bo ane (AB, 97% pu i y) we e supplied by 133 Sigma-Ald ich. Figu e 1 shows a mic og aph o nea AB as ecei ed. As shown in his 134 Figu e, AB was cons i u ed by agglome a ed pa icles wi h sizes in he ange o 100 m 135 and wi h a po ous s uc u e. Me hanol (MeOH; 99.8% pu i y), n-hexane (95% pu i y) 136 and d y e ahyd o u an (THF; wi h maximum wa e o 0.0075w %) we e pu chased 137 om Pan eac. Ca bon dioxide (CO2, 99.95% pu i y) was supplied by Ca bu os 138 Me álicos S.A. 139 (FIGURE 1) 140 2.2 P epa a ion o AB loaded in silica ae ogel mic opa icles 141 As p esen ed in Figu e 2, he p ocedu e o he p epa a ion o AB-loaded silica ae ogel 142 mic opa icles consis s o h ee key s eps: p epa a ion o silica gel mic opa icles, 143 7 addi ion o ammonia bo ane by a we imp egna ion me hod, and d ying o he AB- 144 loaded gel pa icles wi h p essu ized ca bon dioxide in o de o p oduce he inal, d y 145 AB-loaded ae ogel mic opa icles. 146 (FIGURE 2) 147 In he i s s ep o his p ocedu e, hyd ophilic silica alcogel was p epa ed using he well- 148 known me hod o hyd olysis condensa ion sol-gel eac ion, using TMOS as p ecu so 149 and me hanol as sol en . While wi h his p ocedu e i is common o p epa e la ge gel 150 monoli hs, in his wo k he gel was syn he ized as mic opa icles, in o de o educe he 151 possible hea and mass ans e esis ances ha could be caused by la ge ae ogel 152 monoli hs. To do his, and acco ding o he p ocedu e desc ibed in a p e ious wo k [26], 153 he sol-gel eac ion media was dispe sed in hexane unde mechanical s i ing, in o de o 154 ob ain small d ople s o TMOS in me hanol dispe sed wi hin he hexane con inuous 155 phase. A e 10 minu es o mechanical s i ing o his mix u e wi h a wo bladed axial 156 s i e se a 600 pm, an aqueous solu ion o NH4OH was added as condensa ion 157 ca alys , which induced he gela ion o TMOS. The mola a io used was he ollowing: 158 1 mol TMOS: 4.4 mol MeOH: 3.3 mol H2O: 4.5 mol hexane: 0.08 mol NH4OH. As 159 me hanol o wa e p oduced du ing he condensa ion eac ion can induce he 160 decomposi ion o ammonia bo ane du ing he subsequen d ying p ocesses [26], a e 2 161 hou s o gela ion he alcogel mic opa icles we e e ie ed and imme sed in THF. Gel 162 pa icles we e hen kep du ing 7 days imme sed in THF in a closed essel, o le he gel 163 age and s eng hen i s s uc u e. Du ing his ageing pe iod, he THF sol en was 164 enewed a leas wice in o de o emo e he las aces o me hanol and wa e . 165 A e he ageing p ocess, mic opa icles o alcogel a e eady o we imp egna ion, 166 adding a solu ion o ammonia bo ane dissol ed in THF. This me hod has he ad an age 167 ha imp egna ion can be pe o med unde milde empe a u e condi ions compa ed o 168 8 mel in il a ion, and only one imp egna ion s ep is necessa y in con as o incipien 169 imp egna ion me hods [27]. Again, THF was used ins ead o me hanol as sol en in 170 o de o a oid me hanolysis and he e o e he decomposi ion p ocess o he hyd ide no 171 only du ing we imp egna ion (due o SiOH g oups) bu also du ing d ying p ocess [26]. 172 Di e en samples wi h di e en concen a ions o ammonia bo ane we e p epa ed, 173 adding di e en amoun s o hyd ide (0-0.4g AB dissol ed in 5mL o THF) o 2 g o 174 mic opa icles o alcogel (gel be o e d ying, he e o e wi h he po es illed wi h he 175 o ganic sol en ) in o de o s udy his in luence on he p ope ies o he inal solid 176 p oduc . Wi h his, concen a ions o AB in he inal p oduc anging om 10 o 60 w % 177 AB we e ob ained, whe e he concen a ion o AB is de ined as p esen ed in equa ion 178 (4): 179 (4) 180 The hi d and las s ep is he emo al o he o ganic sol en in o de o ob ain he inal, 181 d y AB/SiO2 pa icles. The d ying me hod employed is a key aspec ha de e mines he 182 ex u al p ope ies o he po ous suppo . I he sol en is emo ed by e apo a ion o 183 lyophiliza ion, he capilla y s esses associa ed o he o ma ion o apo -liquid 184 in e aces inside he po es o he suppo cause ac u es and a pa ial collapse o he 185 po e s uc u e o he ma e ial. In he case o SiO2 ma ixes, he ma e ials ob ained by 186 hese d ying me hods usually show po e olumes below 0.5 – 1.0 cm3/g. Some 187 examples a e he well-known SBA-15 o MCM-41 mesopo ous silica ma ixes. In 188 con as , i p essu ized o supe c i ical ca bon dioxide is used o ex ac he sol en , he 189 collapse o he po e s uc u e is minimized, because unde hese condi ions ca bon 190 dioxide can be comple ely miscible wi h he o ganic sol en , and he e o e he 191 ex ac ion p oceeds wi hou o ma ion o gas-liquid in e aces and wi hou capilla y 192 9 s esses. Due o his enhanced p ese a ion o he po e s uc u e, wi h his me hod i is 193 possible o each po e olumes in he ange 2 – 4 cm3/g [28]. Addi ionally, du ing his 194 d ying p ocess CO2 can ac as an isol en o solu es dissol ed in he o ganic sol en , as 195 i is comple ely miscible wi h he o ganic sol en , bu i canno dissol e high-molecula 196 weigh solu es dissol ed in he o ganic sol en . This p ecipi a ion me hod is commonly 197 e e ed in he li e a u e as “Gas An i Sol en ” (GAS) o “Supe c i ical An i Sol en ” 198 (SAS) p ecipi a ion [29,30]. In he case o his wo k, as ammonia bo ane is insoluble in 199 CO2 [26], when he THF-imme sed alcogels a e mixed wi h CO2, AB dissol ed in he 200 THF ha ills he po es o he alcogels quickly p ecipi a es acco ding o a GAS 201 p ecipi a ion mechanism, hus a o ing he o ma ion o small pa icles wi hin he po es 202 o he gels. 203 In a p e ious wo k, i was obse ed ha due he in e ac ion o AB wi h he SiO2 ma ix, 204 he empe a u e needed o ini ia e he decomposi ion o AB is d as ically dec eased [26]. 205 Thus, in o de o a oid he he molysis o AB, he d ying p ocess was ca ied ou a a 206 nea -ambien empe a u e o 25ºC and a a p essu e o 100 ba , hus employing 207 p essu ized, liquid CO2. As in he p e ious wo k [26], a ba ch d ying appa a us, 208 depic ed in Figu e 2, was used. Using his appa a us, he alcogels imme sed in THF 209 we e loaded in o he ex ac ion essel and he sys em was p essu ized wi h CO2 using an 210 ai -d i en pis on pump. The sys em was slowly p essu ized a a a e o 0.5 ba /min in 211 o de o a oid b eakages in he alcogel/ae ogel and mechanical s esses ha could 212 damage he s uc u al p ope ies o he inal p oduc . Once he desi ed p essu e and 213 empe a u e we e eached, he eci cula ion pump ha connec s he ex ac ion essel 214 wi h he CO2 ese oi was connec ed, hus bubbling CO2 h ough he THF-imme sed 215 alcogels and enhancing he ex ac ion o he sol en . As CO2 g adually becomes 216 sa u a ed wi h he sol en du ing he ex ac ion p ocess, CO2 in he sys em was enewed 217 16 he mapping o samples wi h highe concen a ion o AB, up o he maximum 359 concen a ion o 60 w % es ed in his wo k. 360 (FIGURE 5) 361 3.2.2 XRD pa e ns 362 Figu e 6 shows he XRD pa e n o nea ammonia bo ane, which ma ches well wi h 363 JCPDS e e ence 01-074-0894 sugges ing he ypical polyc ys alline s uc u e wi h 364 e agonal la ice symme y, in ag eemen wi h li e a u e in o ma ion abou he s uc u e 365 o AB a ambien empe a u e [42]. The c ys alli e size es ima ed using he Sche e ’s 366 equa ion o mula is 40 nm, and he dominan sha p peak o he pa e n is loca ed a 367 23.75º, co esponding o (110) planes. A e ec ys alliza ion by GAS p ocess, sligh 368 modi ica ions in he XRD pa e n can be obse ed a 2 = 17 and 30º. A simila 369 modi ica ion can be obse ed in silica-loaded AB samples, al hough wi h less de ined 370 peaks due o he s ong signal p oduced by he silica suppo . This modi ica ion in he 371 pa e n can sugges he o ma ion o diammonia e o dibo ane (DADB), an isome o 372 AB. This compound shows some di e ences ega ding he hyd ogen elease mechanism 373 compa ed o i s isome AB: he empe a u e o decomposi ion o DADB is abou 10ºC 374 lowe han ha o AB, and DADB unde goes solid-phase decomposi ion wi hou 375 mel ing o induc ion pe iod e en a mode a e empe a u e, while AB su e s om a long 376 induc ion pe iod p io o H2 elease [43]. The e o e, he o ma ion o his compound 377 may jus i y some o he obse ed he mal p ope ies, as i will be discussed in he 378 ollowing sec ions. 379 Ne e heless, in GAS ec ys allized samples, he dominan pa e n is equi alen o ha 380 o nea AB, also co esponding o e agonal c ys al s uc u e is obse ed, al hough 381 di ac ion peaks a e no so well de ined, and he es ima ed c ys alli e size inc eases o 382 17 75 – 150 nm. In he case o AB loaded in silica ae ogel, he peaks a e me ged, he e o e 383 sugges ing a educ ion o c ys allini y o an inc eased inhomogenei y in he p ope ies 384 o he c ys als due o he inco po a ion in he po es [44]. Addi ionally, as shown in 385 Figu e 5b i can be seen ha he XRD pa e n o amo phous silica, cha ac e ized by a 386 b oad peak a ound 24º, is o e laid o he dominan peaks ela ed o AB. The e o e 387 esul s show ha AB e ains i s c ys alline s uc u e a e nanocon ina ion wi hin he 388 po es o he ae ogel, wi h es ima ed c ys alli e sizes in he ange 100 – 200 nm, simila 389 o hose ob ained by ec ys alliza ion o AB by GAS p ocess. 390 (FIGURE 6) 391 3.3 The mal cha ac e iza ion o AB-loaded silica ae ogels 392 Figu e 7 shows he di e en ial scanning calo ime y (DSC) aces o nea and 393 ec ys allized AB compa ed o samples in which he hyd ide is loaded in silica ae ogel. 394 In he case o he cu e o nea AB, a sha p endo he mic peak is obse ed whose onse 395 empe a u e (Ton=108.5 ºC) and peak empe a u e (Tp=110.8ºC) is d ama ically educed 396 when AB is loaded in silica ae ogel (see able 2). This i s peak is associa ed o he 397 mel ing poin [14] o he dissocia ion o he in e molecula hyd ogen bonding [18]. The 398 educ ion o elimina ion in his i s peak sugges s ha he deg ee o hyd ogen bond in 399 he samples in which AB is embedded in silica ae ogel is dec eased, a o ing he 400 educ ion o he induc ion ime. A simila esul has been obse ed when AB was 401 embedded in o he silica suppo s [18]. Rega ding ec ys allized AB, DSC esul s also 402 show educ ion in he onse and peak endo he mic empe a u es, main aining he shape 403 o he cu e o nea AB. In his case, a ia ions in cha ac e is ic empe a u es o he 404 DSC aces can be associa ed o he educ ion in he mean pa icle size achie ed by 405 ec ys alliza ion o AB. As desc ibed by Va in e al [45], a educ ion o pa icle size 406 in o he subic ome ic o nanome ic scale is gene ally associa ed o a educ ion o he 407 18 onse and peak empe a u es o hyd ogen e olu ion he mal e en s, due o he 408 des abiliza ion o he ma e ial induced by he inc eased pa icle su ace. Addi ionally, 409 he educ ion o onse empe a u es and induc ion ime can be associa ed o he 410 o ma ion o DADB by ec ys alliza ion sugges ed by XRD assays. 411 Simila esul s ha e been ob ained in o he wo ks whe e AB is con ined in di e en 412 suppo s. In he case o SBA and MCM silica suppo s, educ ions in he onse and peak 413 empe a u es o 48ºC and 100ºC ha e been epo ed [18], bu as p e iously desc ibed 414 lowe empe a u es ha e been ob ained in his wo k. This ac can be due o he highe 415 olume o po es o ae ogel suppo ha a oids he agglome a ion o AB in meso- 416 channels. The e o e highe con ac be ween he pa icle and he su ace o he silica 417 ae ogel akes place, enhancing he in luence o silica su ace g oups on he 418 decomposi ion mechanism. In expe imen s wi h MOFs [42] o ca bon c yogels [17, 20] 419 as suppo s, simila modi ica ions in he he mal esponse o he ma e ial ha e been 420 epo ed. Howe e , S ini as e al. [16] obse ed a educ ion o 30ºC in he onse and 421 peak empe a u es o decomposi ion using MOFs, whe eas in ou case a displacemen o 422 almos 70ºC is ob ained. 423 Mo eo e , DSC esul s indica e a educ ion o he exo he mic en halpy associa ed wi h 424 hyd ogen elease as he p opo ion silica/AB is inc eased. The measu ed en halpy o 425 eac ion o H2 elease om nea AB is -24.9 kJ/mol AB, which is in good ag eemen 426 wi h esul s epo ed in li e a u e. Howe e , when he p opo ion silica/AB is inc eased, 427 he co esponding en halpy o eac ion changes o -10.9 kJ/mol AB o he sample wi h 428 60% o AB, and -5.5 kJ/mol AB o he sample wi h 30% o AB. A educ ion o he 429 en halpy o eac ion o H2 elease om AB was also obse ed by Gu owska e al. [40] 430 in hei s udies o inco po a ion o AB in mesopo ous SBA-15 silica sca old. Howe e , 431 hese au ho s epo a mo e d as ic a ia ion o he en halpy o -1.0 kJ/mol AB. These 432 19 au ho s indica e ha he eason o he educed exo he mici y is he supp ession o he 433 o ma ion o bo on compounds as byp oduc s o he PAB ha is he main 434 decomposi ion p oduc o AB acco ding o eac ion 1, which had he a o able 435 consequence o educing he p oduc ion o gaseous byp oduc s. 436 (TABLE 2) 437 (FIGURE 7) 438 This hypo hesis ag ees well wi h he esul s ob ained in his wo k by TGA assays. 439 Figu e 8 shows he esul s o TGA analysis pe o med on nea AB and AB-loaded silica 440 ae ogel. In he case o nea AB, wo impo an weigh loss s eps, which co espond o 441 he decomposi ion o he hyd ide, a e obse ed: he i s one ill 129ºC co esponds o a 442 weigh loss o 12.7 w % and he second one, which inishes a 213ºC, co esponds o a 443 weigh loss o 27.5 w %. In compa ison, and in ag eemen wi h he esul s o DSC 444 assays, TGA analysis shows ha he AB con ined in silica ae ogel ini ia es i s 445 decomposi ion a lowe empe a u es. I is also no iceable ha in his case, weigh loss 446 is no con ined o sha p s eps a de ined empe a u es, bu i p oceeds con inuously o e 447 he empe a u e ange s udied. In pa icula , a empe a u es abo e 200ºC, whe e as 448 p e iously discussed nea AB does no expe ience any addi ional weigh losses, a 449 con inuous weigh loss is s ill obse ed in he case o AB con ined in ae ogel. This 450 esul indica es ha he hi d s ep o he he mal decomposi ion mechanism, indica ed in 451 eac ion (3), also begins a lowe empe a u es in he case o con ined AB compa ed o 452 nea AB. Mo eo e , as shown in Table 2, in bo h cases he o al weigh loss pe uni 453 mass o AB in he sample is signi ican ly la ge han he maximum amoun o hyd ogen 454 ha can s o ed in he compound. Simila esul s ha e been epo ed in [18], sugges ing 455 ha when AB is hea ed o high empe a u es abo e 200ºC, o he gases apa om 456 hyd ogen a e p oduced, as bo azine, dibo ane, ammonia, e c. Howe e , analyzing he 457 20 esul s epo ed in Table 2, i is no iceable ha he o al amoun o ola ile compounds 458 p oduced by hea ing up o 300ºC is educed when AB is con ined in silica ae ogel. This 459 esul , oge he wi h he a ia ions in DSC assays o con ined AB p esen ed be o e, 460 sugges ha in e ac ions be ween AB and he silica suppo a e aking place ha 461 in luence he decomposi ion mechanism o AB. Such in e ac ions we e sugges ed o 462 happen be ween AB and hyd oxyl g oups om he silica su ace o he hos by Lai e al. 463 [18]. These g oups can in e ac wi h he BH3 g oup, loosening he co alen bond 464 be ween BH3 and NH3 g oups o AB, hus des abilizing and p omo ing he 465 decomposi ion o he compound. Fu he mo e, by his in e ac ion BH3 is kep bound o 466 he sca old educing he p oduc ion o bo azine and p ecluding he o ma ion o 467 poliiminobo ane 468 (FIGURE 8) 469 In igu e 9, FTIR o nea AB and AB loaded in silica ae ogel is shown be o e and a e 470 dehyd ogena ion a 80ºC. I is obse ed ha mos o he peaks a equencies ela ed o 471 N-H and B-H bonds a e b oadened, shi ed and dec eased o in ensi y which indica es 472 he dis up ion o he bonds due o he elease o hyd ogen [13] in bo h samples. The 473 same beha io is obse ed o all he concen a ions o AB loaded in silica ae ogel, 474 al hough i is mo e p onounced a highe concen a ions o AB. B-N band in he ange 475 700-900 cm-1, which is obse ed in all he samples, is weakened bu is s ill de ec ed 476 a e dehyd ogena ion; his ac cla i ies ha B-N is no dis up ed and ammonia 477 o ma ion is a oided du ing he decomposi ion [14]. On he o he hand, in he sample in 478 which AB is loaded, he bonds ela ed o silica a e p esen wi hou any change a e 479 he mal ea men due o i s s abili y a hese condi ions. 480 (FIGURE 9) 481 21 Rega ding he c ys allini y o he samples a e he mal dehyd ogena ion, igu e 10 482 shows XRD analyses o byp oduc a e iso he mal dehyd ogena ion a 80ºC. Acco ding 483 o ICDD 00-019-0418, 2θ= 20.1º, 23.6º and 41.1º a e assigned o amo phous PAB 484 (NH2BH2)5 [46]. In he case o AB/SiO2 samples, amo phous silica peak is p esen apa 485 om amo phous PAB byp oduc . 486 (FIGURE 10) 487 3.4 Kine ics o hyd ogen elease by he molysis a 80ºC 488 Figu e 11 shows he kine ics o hyd ogen elease by he molysis a 80ºC o nea AB 489 compa ed wi h AB con inemen in silica ae ogel. Resul s in his igu e a e no malized 490 epo ing he amoun o hyd ogen eleased by uni mass o AB in he sample. Due o he 491 design o he cell used o measu e kine ics, i was no possible o analyze samples o he 492 gas e ol ed du ing hemolysis. Howe e , i is assumed ha a his empe a u e, he gas 493 which is eleased is H2 [12, 19, 41, 47] and no o he ola ile gases a e p esen in he gas 494 s eam in nea AB no con ined in silica ae ogel. Rega ding he shape o he cu e in 495 nea AB, i ollows a sigmoidal kine ic, ypical o nuclea ion and g ow h pa hway, wi h 496 a long induc ion ime o mo e han 2 h, as epo ed in p e ious wo ks [48]. A e 497 con inemen in silica, he induc ion ime is educed signi ican ly. This means ha he 498 silica could ac as a ca alys (SiOH g oups) [40] c ea ing de ec si es in he suppo ha 499 ini ia e he decomposi ion a lowe empe a u e. On he o he hand, a lowe 500 concen a ion o AB in he solu ion (p io o he d ying), he hyd ide could p ecipi a e in 501 he po es o he silica wi h lowe mean size as was also sugges ed wi h XRD analyses 502 and co obo a ed in BET esul s. This sugges s ha smalle pa icles ha e mo e con ac 503 wi h he su ace o he silica and an easie way o o m di e en bonds be ween he wo 504 ee pai o elec ons o O in he Lewis base o Si-O-Si o Si-OH bond om he silica 505 22 wi h BH3 o NH3 om AB. As esul , he in e molecula hyd ogen bond is educed, 506 ollowing he mechanism p oposed by Lai e al. [18]. 507 In he case o nea AB, 2 hou s a e needed o s a eleasing H2, and mo e han 4 hou s 508 o ge hal o i s con en in H2 a his empe a u e (0.025gH2/gAB) whe eas only i akes 509 22 minu es in he case o 13%AB loaded in silica ae ogel. This ac shows he 510 imp o emen o silica ae ogel as suppo o his chemical hyd ide sys em. 511 Rega ding o ec ys allized sample, i can be obse ed ha he elease p o ile main ains 512 he sigmoidal shape cha ac e is ic o nea AB, wi h a slowe hyd ogen elease han 513 samples loaded in silica ae ogel du ing he i s 30 min o he molysis. Howe e , 514 compa ed wi h nea AB, wi h he pa icle size educ ion achie ed by GAS 515 ec ys alliza ion he induc ion ime is d as ically educed and he elease o hyd ogen is 516 accele a ed, o he poin ha a e one hou an equi alen amoun o hyd ogen is 517 eleased om GAS- ec ys allized AB as om AB loaded silica ae ogel wi h a 30 w % 518 o AB. 519 (FIGURE 11) 520 As a complemen o Figu e 11, Table 3 p esen s he o al hyd ogen elease pe uni mass 521 o solid p oduc (AB + silica ae ogel suppo ). Resul s in his able clea ly indica e he 522 weigh penal y caused by he use o silica ae ogel as po ous hos , as his ma e ial does 523 no con ibu e o he hyd ogen s o age capaci y, hus educing he o al g a ime ic 524 capaci y o he ma e ial [27]. Howe e , i can be seen ha his disad an age is 525 coun e balanced by a as e hyd ogen elease du ing he i s 1-2 h o he molysis. 526 (TABLE 3) 527 Mo eo e , he e is a signi ican isual change in he mo phology o nea AB in con as 528 o AB encapsula ed in silica ae ogel. Figu e 12 shows he di e en esul o AB a e 529 23 he mal decomposi ion a 80ºC. In he case o nea AB, oaming p ocess akes place a 530 he same ime ha H2 is eleased om he hyd ide. Thus, e en i he ma e ial was 531 mic onized be o e he molysis, his mo phology and i s associa ed ad an ages a e 532 comple ely los du ing he he molysis and he e o e in possible u u e hyd ogen cycles, 533 i he ma e ial is egene a ed [49]. Howe e , when AB is encapsula ed in silica ae ogel, 534 his p ocess is a oided ob aining pa icles wi h he same physical appea ance. To 535 con i m his obse a ion, Figu e 13 p esen s SEM/EDX mic og aphs o he 536 60%AB/SiO2 sample a e he molysis. As shown in his igu e, he o iginal 537 mo phology o he ma e ial is p ese ed a e he he molysis. Fu he mo e, as indica ed 538 by he esul s o ni ogen mapping, he decomposi ion p oduc s o AB emain 539 homogeneously dispe sed wi hin he SiO2 ae ogel ma ix. The e o e i can be concluded 540 ha by inco po a ion o AB inside he ae ogel he mo phological a ia ions o he 541 ma e ial due o oaming a e a oided. 542 (FIGURE 12) 543 (FIGURE 13) 544 4. Conclusions 545 Ammonia Bo ane has been ec ys allized and nanocon ined inside he po es o silica 546 ae ogel by a no el p ocess, based on a simul aneous ae ogel d ying and ammonia 547 bo ane gas an isol en p ecipi a ion using comp essed ca bon dioxide. Due o he 548 a o able ex u al p ope ies o he ae ogel ma e ials ob ained wi h his me hod, i has 549 been possible o load ae ogels wi h up o 60 w % o ammonia bo ane, wi hou blocking 550 o po es and wi h a homogeneous dispe sion o ammonia bo ane wi hin he ae ogel. By 551 analysis o he he molysis p ocess, i has been obse ed ha by nanocon inemen he 552 empe a u e equi ed o ini ia e he he molysis p ocess is educed and he elease 553 kine ics a e accele a ed as hey do no show induc ion ime. Fu he mo e, by 554 24 nanocon ina ion o AB, he mo phological p ope ies o he ma e ial a e s abilized and 555 oaming is elimina ed, which could be a o able p ope ies o a subsequen ma e ial 556 egene a ion p ocess. The easibili y o implemen a ion o he mul i-s ep chemical 557 p ocess equi ed o e-hyd ogena e AB when his compound is embedded in he silica 558 ma ix emains o be es ed. 559 560 Supplemen a y In o ma ion 561 Video 1: The molysis o nea ammonia bo ane a 80ºC. Time is accele a ed by a ac o 562 o 64. 563 Video 2: The molysis o 60%AB/SiO2 sample a 80ºC. Time is accele a ed by a ac o 564 o 64. 565 566 Acknowledgemen s 567 This esea ch has been inanced by he Spanish Minis y o Economy and 568 Compe i i eness h ough p ojec ENE2011-24547. Á. Ma ín hanks he Spanish 569 Minis y o Economy and Compe i i eness o a Ramón y Cajal esea ch ellowship. M. 570 Rueda hanks he Uni e si y o Valladolid o a FPI p edoc o al g an . L. M. Sanz 571 hanks he Spanish Minis y o Economy and Compe i i eness o a FPI p edoc o al 572 g an . 573 Re e ences 574 [1] M.Bala , In . J. Hyd ogen Ene g. 33 (2008) 4013-4029 575 [2] M.Hook, X.Tang, Ene g. Policy 52 (2013) 797-809 576 [3] S. A. She i , F. Ba bi , T. N. Vezi oglu, Elec ici y J. 18 (2005) 62-76 577 [4] J. G ae z, Chem. Soc. Re . 38 (2009) 73-82 578 25 [5] DOE: US Depa men o Ene gy. Web si e: h p://www.doe.go . 579 [6] L. O. Williams, D. E. Spond, Appl. Ene g. 6 (1980) 99-112. 580 [7] H. P. Veluswamy, R. Kuma , P. Linga, Appl. Ene g.122 (2014) 112-132. 581 [8] M. Paik Suh, H. J. Pa k, T. K. P asad, D. Lim, Chem. Re . 112 (2012) 782-835 582 [9] B.Sakin una, F.Lama i-Da k im, M.Hi sche , In . J. Hyd ogen Ene g. 32 (2007) 583 1121-1140 584 [10] B. Peng, J. Chen, Ene g. En i on. Sci. 1 (2008) 479-483 585 [11] S. D. Rassa , C. L. Aa dahl, T. Au ey, R. S. Smi h, Ene g. Fuel. 24 (2010) 2596- 586 2606 587 [12] G. Wol , J. Baumann, F. Bai alow, F. P. Ho mann, The mochim. Ac a 343 (2000) 588 19-25 589 [13] D. Kuma , H. A. Mangal edeka , S. K. Mahajan, Ma e . Renew. Sus ain. Ene gy 3 590 (2014) 23 591 [14] A. C. Gangal, P. Kale, R. Edla, J. Manna, P. Sha ma, In . J. Hyd ogen Ene g. 37 592 (2012) 6741-6748 593 [15] B. Roy, J. Manna, P. Sha ma, J. Alloy Compd. 645 (2015) S234-S238. 594 [16] G. S ini as, J. Fo d, W. Zhou, T. Yildi im, In . J. Hyd ogen Ene g. 37 (2012) 595 3633-3638 596 [17] A. Fea e , S. Sepeh i, P. Shambe ge , A. S owe, T. Au ey, G. Cao, J. Phys. Chem. 597 B 111, (2007) 7469-7472 598 [18] S-W Lai, H-L. Lin, T.L.Yu, L-P. Lee, B-J. Weng, In . J. Hyd ogen Ene g. 37 599 (2012) 14393-14404 600 [19] T.Au ey, A.Gu owska, L.Li, J.Linehan, M.Gu owski, P ep in s o Pape s- 601 Ame ican Chemical Socie y, Di ision o Fuel Chemis y 49 (2004) 150-151 602 [20] S.Sepeh i, B.B.Ga cia, G.Cao, J. Ma e . Chem. 18 (2008) 4034- 603 32 Table 2. - Tempe a u e da a and weigh losses o nea and ec ys allized AB s loaded 699 in silica ae ogel wi h di e en concen a ion ob ained om DSC and TGA analyses 700 espec i ely. 701 702 Sample Ton1 (ºC) Tp1 (ºC) Ton2 (ºC) Tp2 (ºC) To al w loss pe w AB Nea AB 108.5 110.8 113.9 114.1 40.2 Rec ys allized AB 70.5 77.9 102.4 113.4 - 60% AB/SiO2 37.6 62.3 84.8 108.9 33.0 30% AB/SiO2 39.5 68.7 83.1 99.0 - 33 Table 3. - Amoun o H2 eleased a di e en imes by iso he mal he molysis a 80ºC in 703 nea and ec ys allized s. AB loaded in silica ae ogel wi h di e en concen a ion 704 15min 30min 45min 1h 2h inal Sample mgH2/g o al mgH2/g o al mgH2/g o al mgH2/g o al mgH2/g o al mgH2/g o al nea AB 0 0 0 0 2 52 AB ec ys allized 1,66 7,45 20,19 30,45 43,98 52 13%AB/SiO2 2,26 4,28 5,28 5,80 6,38 6,76 30%AB/SiO2 1,69 4,43 7,30 9,07 12,31 14,56 60%AB/SiO2 2,48 9,42 17,79 20,65 27,27 32,24 705 34 Figu es 706 707 708 Figu e 1. - SEM mic og aph o nea ammonia bo ane as ecei ed 709 710 35 1. Syn hesis alcogel 3. L-CO2 d ying (100ba /25ºC) 2. Addi ion o AB solu ion PI TI co2 PI Liquid en Reci cula ion pump CO2 bu e Ex ac o CO2 pump Ho ai o en 711 Figu e 2. - S eps o p epa a ion o silica ae ogel mic opa icles loaded wi h ammonia 712 bo ane. 713 714 36 715 Figu e 3. FTIR spec a o a) silica ae ogel b) nea AB c) ec ys allized AB 716 d)30%AB/SiO2 e)60%AB/SiO2. Cu es a e e ically displaced o cla i y. 717 718 37 719 Figu e 4.-Ni ogen adso p ion-deso p ion iso he ms and BJH po e size dis ibu ion o 720 silica ae ogel and AB loaded in silica wi h di e en concen a ion ( ) adso p ion ( ) 721 deso p ion 722 723 38 724 Figu e 5.- SEM images o A) Rec ys allized AB a e ec ys alliza ion in THF using 725 liquid CO2 as d ying me hod B) 30%AB loaded in silica ae ogel and mapping o sample 726 B (blue is e e ed o silica and ed o Ni ogen) 727 728 39 729 Figu e 6. –a) XRD o nea ammonia bo ane, ec ys allized ammonia bo ane and AB 730 loaded in silica ae ogel wi h di e en concen a ions b) Ampli ica ion o XRD signal 731 showing he cha ac e is ic pa e n o silica ae ogel. Cu es a e e ically displaced o 732 cla i y 733 734 40 735 Figu e 7. - DSC cu es o AB and AB loaded in silica ae ogel wi h di e en 736 concen a ion. The cu es a e no malized acco ding o he weigh o AB, and e ically 737 displaced o cla i y. 738 739 41 740 Figu e 8. - TGA cu es o AB and AB loaded in silica ae ogel 741 742