Hydrogen production by hydrothermal oxidation of FeO under acidic conditions

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Hydrogen production by hydrothermal oxidation of FeO under acidic conditions

Camille Crouzet, Fabrice Brunet, Nadir Recham, N. Findling, M Lanson, François Guyot, Jean-Henry Ferrasse, Bruno Goffé

To cite this version:

Camille Crouzet, Fabrice Brunet, Nadir Recham, N. Findling, M Lanson, et al.. Hydrogen production

by hydrothermal oxidation of FeO under acidic conditions . International Journal of Hydrogen Energy,

Elsevier, 2016, 42 (2), pp.795-806. �10.1016/j.ijhydene.2016.10.019�. �hal-01468080�

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H y d r o g e n p r o d u c t i o n b y h y d r o t h e r m a l o x i d a t i o n of FeO u n d e r acidic c o n d i t i o n s

C. Crouzet

, F. Brunet

, N. Recham

, N. Findling

, M. Lanson

, F. Guyot

, J.-H. Ferrasse

, B. Goffe

a Universite Grenoble Alpes, CNRS, ISTerre, Maison des Geosciences, F-38000 Grenoble, France

b Laboratoire de Reactivite et Chimie des Solides (LRCS), Universite de Picardie Jules Verne, CNRS UMR 7314, 80039 Amiens Cedex, France

c Institut de mineralogie, de physique des materiaux et de cosmochimie, Sorbonne Universite, Museum National d'Histoire Naturelle, UMR 7590, CNRS, UPMC, MNHN, IRD, F-75005 Paris, France

d Mecanique, Modelisation & Procedes Propres (M2P2), Aix-Marseille Universite, Centrale Marseille, CNRS, M2P2 UMR 7340, Europole de l'Arbois, 13545 Aix-en-Provence Cedex 4, France

e CEREGE, Aix-Marseille Universite, CNRS UMR 7330, Europole de l'Arbois, 13545 Aix-en-Provence, France

Keywords:

Hyd r o ge n p r o d u c t i o n FeO o x i d a t i o n Steel slags

H y d r o t h e r m a l c o n d i t i o n s Acet ic a c i d

I n t r o d u c t i o n

T h e p r o d u c t i o n of H2 by o x i d a t i o n of FeO, t a k e n h e r e a s m o d e l c o m p o u n d for s t e e l slags, h a s b e e n i n ve s t i g a t e d b o t h i n p u r e w a t e r a n d u n d e r acidi c a q u e o u s c on d i t i o n s i n t h e 373 e573 K t e m p e r a t u r e r a n ge . W h e r e a s a ft e r 65 h , H2 yield w a s negligible i n p u r e w a t e r a t 423 K, t h e r e a c t i o n 3 FeO(s) þ H2O(l) / Fe3O4 ( s) þ H2 ( a q ) r e a c h e d n e a r c o m p l e t i o n a t t h e s a m e t e m p e r a t u r e w i t h i n 10 h i n a s o l u t i on c on t a i n i n g 0.05 m ol / l a c e t i c acid. I n c r e a s i n g a c e t i c a c i d c o n c e n t r a t i o n by o n e o r d e r of m a g n i t u d e d i d n o t yield significant ly m o r e H2. At i d ent i c a l initial pH, a c e t i c a c i d w a s f o u n d t o b e m o r e effic ie nt t h a n oxalic a c i d a n d h y- d r oc h l or i c a c i d a t e n h a n c i n g H2 p r o d u c t i o n . Acidic c o n d i t i o n s i n c r e a s e d FeO d i s s o l u t i on ki n e t i c s a n d , c o n s e q u e n t l y , i m p r o v e d H2 yield. T h e specific efficiency of a c e t i c a c i d r e s i d e s i n its t h e r m a l stab ility a s well a s i n t h e p o t e n t i a l of l i g a n d - p r o m o t e d Fe(II) d i s s ol u t i on . We s h o w t h a t t h e positi ve k i n e t i c s effect of m i l d a c et i c a c i d s o l u t i o n s ove r H2 yield e v i d e n c e d o n FeO d o e s n o t ap p l y directly t o s t e e l slags w h i c h bu ffe r t h e pH t o h i gh va l u e s d u e t o t h e p r e s e n c e of large a m o u n t s of CaO.

el ec troly sis. H o wev er , w i t h t h e a i m of d ev e lop in g n e w s u s - t a i n a b l e h y d r o g e n p r o d u c t i o n m e t h o d s , a l t e r n a t i v e w a y s a r e A m o n g a l t e r n a t i v e en er g y s o u r c e s , d i h y d r o g e n (H2) h a s a n

i m p o r t a n t ro le t o play, espec ia l ly w i t h t h e d e v e l o p m e n t of fu el cell t e c h n o l o g i e s . N o wa d a y s , t w o m a i n p r o c e s s e s a r e u s e d t o p r o d u c e H2 w i t h t h e r e q u i r e d pu rity , s t e a m m e t h a n e r e f o r m i n g w i t h a n a d d i t i o n a l p u r i fi c a t i o n s t e p a n d w a t e r

a l s o b ei n g in v est i ga t ed . For e x a m p l e , e x t e n s i v e r e s e a r c h is b ei n g c a r r i e d o u t o n h y d r o g e n p r o d u c t i o n f r o m b i o m a s s a n d p r o c e s s e s b a s e d o n t h e u s e of r e n e w a b l e e n e r g y s o u r c e s for el ec tr oly sis o r t h e r m o c h e m i c a l cycle p a t h s [1e3]. T h e H2

p r o d u c t i o n m e t h o d p r e s e n t e d h e r e is i n s p i r e d f r o m a n

* Corresponding author. P r e s e nt ad d r e ss : ISTerre, Maison d e s Geosciences, BP 35, F-38041 Grenoble Cedex 9, France.

E-mail a d d r e s s : camille.crouzet@univ-grenoble-alpes.fr (C. Crouzet).

¨

¨

¨

€ abiogenic geo chemical pro ce ss wh i ch co rre spo n ds to t h e format i on of nat i ve dih ydrogen by i nt era cti on b e t we e n rock a n d h o t s e a w a t e r a t mi d-ocean ic ridges. U n d er t h e s e n a t u r a l h yd r o t h e r ma l conditions, H2 is a by-product of t h e h y d rat i on of olivine (Mg, Fe)2SiO4, t h e m a i n mi n e r a l co n st i tu e n t of t h e E a rt h's u p p e r - m a n t l e . F errou s iron co n ta in ed i n olivine is partl y inco rporated a s F e^3þ in ma g n e t i t e i n t h e co urse of t h e s e hy drat i on reactio ns (serp ent i ni za tio n ) wh i ch ta ke place in t h e oceanic cr u st [4e6]. Malvoisin e t al. (2013, [7]) s h o we d t h a t h igh-pu rity h ydr o g en ca n be p rod u ce d f rom st eel slag, a ma s si v e st eel i n du st ry by-product, u n d e r t h e p r e s s u r e a n d t e m p e r a t u r e conditions of serp e n t in i za ti o n , i.e., 350 C a n d 50 MPa, by oxidation of t h e iron (II) co n t a i n e d by wu s t i t e (Fe, Mg)O, i n st eel slags a n d by t h e s u b s e q u e n t red uct i on of w a t e r [7]. This m e t h o d of geo-in spired H2 p roduct ion from st eel slags, is also re mi n i sc e n t of t h e H2 p ro du ction m e t h o d u s e d a t t h e beginning of t h e 20th century. Hydrogen w a s p ro d u ce d from 823 to 1173 K according to t h e so-called s t e a m -i r o n process b a s ed o n t h e s a m e iron-oxidation principle, th ro u g h cycles of iron m e t a l oxidation i n t h e p r e s e n c e of s t e a m a n d iron oxide redu cti on by gasified coal [8e11]. More recently, several s t u d i e s h a v e focused on ch emi ca l looping combu s- tio n (CLC). CLC is a cyclic ro u t e w h e r e m e t a l oxide particles, s u c h a s iron oxides, a r e r e d u c e d du rin g t h e combu st ion in a fuel reacto r a n d t h e n re-oxidized in a se co nd ai r reactor. If t h i s m e t h o d w a s first devel ope d to ca p t u r e t h e CO2 p rod u ce d by t h e fuel combustion, th i s process ca n also be u s e d to prod uce H2 by p erf o rmi n g t h e oxidation s t e p i n t h e p r e s e n c e of s t e a m [12e15].

Following t h e work by Malvoisin e t al. (2013, [7]) o n st e e l slags, w e i n ve st i g a t ed h e r e t h e role of acidic conditions o n h yd ro ge n yield a n d p ro d u cti o n kinetics f ro m p u r e FeO oxidation. Du e to t h e complex c h e m i st r y of slags, w e fo cus ed h e r e o n t h e beha vior of p u r e FeO i n o rd e r to u n - ra v e l t h e ch emi ca l p ro c es se s wh i c h l e a d to H2 producti on.

A f e w a dd i ti o na l e x p e r i m e n t s w e r e h o w e v e r p e r f o r m e d by a d d i n g CaO, t h e m a i n c o n s t i t u e n t of s t e e l slags, i n o rd er to a p p r o a c h FeO beha vior i n steel-slag-li ke compositions.

Initially p r e s e n t i n t h e f o rm of l i me i n f r e s h slags, CaO t r a n s f o r m s into Ca -hydro xi de a n d c a r b o n a t e by aging i n air. Therefo re, t h e CaO c o m p o n e n t w a s i n t r o d u c ed e i t h e r i n

p erf o rm ed a t a m b i e n t conditions, a n d little is k n o wn about Fe(II) a q u e o u s oxidation a t h i gh er t e m p e r a t u r e a n d p r e ssu r e .

M aterials a n d m e t h o d s

Starting materials

R ea gen t gra d e wu st it e, FeO (99.9%, ALDRICH^®), w a s c r u s h e d a n d sieved to a particle size of 50e100 mm wi t h a specific surfa ce a r e a of 0.70 m²/ g a s m e a s u r e d by N2-BET. Th e oxida- tion s t a t e of iron in t h e sta rtin g m a t e ri a l w a s q u a n t ifi ed by Mossbauer spectroscopy to be 91.6% F e^2þ, 5.6% F e^3þ a n d 2.8%

Fe⁰. Average iron oxidation s t a t e corresponds to p u r e Fe(II), co n si st ent wi t h t h e FeO r e a g e n t grade. Both ferric iron a n d m e t a l iron a r e r e s i d u es of indu st ri al FeO syn t h es i s wh i ch consists in a high t e m p e r a t u r e reaction b e t w e e n iron m e t a l a n d h e m a t i t e (Fe2O3).

Sealed gold capsules

A s e t of e x p e ri me n t s w a s p erf o rm ed in cold-seal vessels. Th e st a rt in g ma t e r i a l (80 mg) w a s load ed in a gold tu b e (4.0 m m o u t er d i a m e t e r a n d 3.6 m m i n n e r d i a met er) wi t h de-i oni zed w a t e r in a co n st a n t m a s s ratio of 1:1. Th e t wo e n d s of t h e t u b e w e r e we l d e d s h u t to form a cap sul e wh i ch w a s placed in t h e p r e s s u r e vessel, itself int rodu ced i n a horizontal furnace.

T e m p e r a t u r e s in t h e 373e473 K r a n g e w e r e investigated a t 30 MPa argon p r e s su r e. Since gold is ductile, t h e w a t e r p r e s - s u r e in both t h e autoclave a n d t h e i n n e r ca psu l e p r e s s u r e a r e t h e s a m e (see B ru n et a n d Chopin (1996, [21]) for e x p e ri m en t a l details); t h i s p r e s s u r e is kept co n st a n t all along t h e ex p eri - men t s . At t h e e n d of ex p er i men t , t h e p r e s s u r e vessel w a s q u e n c h e d u n d e r a c o mp r e sse d air s t r e a m . This t yp e of ex p eri m en t i s ea s y to s e t - u p a n d s e v e r a l sa mp l e s c a n b e r u n in a ra w. It is th eref o re con veni ent to investigate t h e effect of t e m p e r a t u r e or acid concentrat ion on H2 yield in a m i n i m u m a m o u n t of ti me. On t h e o t h er h a n d , c o mp a r ed to t h e sa mp l i n g autoclave, (1) only large st a rting ma t eria l : w a t e r ratio c a n be investigated (typically 1:1), (2) fluids ca n n o t be s a m p l e d in-situ a n d ca n only be an a l yz ed a t t h e e n d of t h e ru n . Th e gas pro-

t h e f o rm of Ca(OH)2 or CaCO3 in t h e s e add it i o n al d u c e d by t h e s a m p l e a n d enclosed in t h e gold ca psu l e is e x p e r i m en t s .

Th e role of pH on H2 production kinetics is s o m e h o w difficult to predict. H2 production from FeO interaction wi t h w a t e r is t h e resu l t of a t l ea st t wo st ep s , wu st i t e dissolution a n d ma gn e t it e precipitation. B e t w e e n t h e s e two st ep s , ferrous iron m u s t be partly oxidized into ferric iron. Potentially, e a c h of t h e s e t h r e e s t e p s (FeO dissolution, F e^2þ oxidation a n d Fe3O4

precipitation) ca n be r a t e limiting. In d eed , low pH h a s b een s h o w n to favor Fe solubilization [16] wh i ch m a y h a v e a posi- tive effect on H2 yield. Precipitation of iron oxides ( a n d hy- droxides) by oxidation of a q u e o u s ferrous sa lt s h a s b e en extensively s t u d i e d [17]. Pro perties of t h e solvent (ionic st ren gt h) a n d t h e n a t u r e of t h e ion p ai r h a v e a clear i n fl u en ce on t h e kinetics of a q u e o u s Fe(II) oxidation. It h a s also b e en s h o w n t h a t Fe(II) a q u e o u s oxidation kinetics by O2 is positively correlated to pH for 4 < pH < 8 [18]. F u rt h erm o re, surfa ce of h yd rou s oxides ca n catalyze t h e redox process (auto-oxida-

tion process, [19,20]). All t h e s e st udi es, however, w e r e

recovered following t h e m e t h o d described i n Malvoisin (2013, [7]) a n d injected wi t h a syringe t h ro u gh t h e s e p t u m of a gas ch ro ma t o g ra p h for analysis.

Sampling autoclave

An o t h er s e t of e x p e ri me n t s w a s carried out wi t h a sa mp l i n g autoclave in ord er to allow time-resol ved monitoring of t h e H2 production. A 500 m L autoclave m a d e of hastelloy™ (nickel b a sed alloy) is e q u i p p e d wi t h a s e t of h i g h -p re s su r e connec-tions for gas a n d solution sa mp l in g (Fig. 1). Th e autoclave is h e a t e d by t wo tight resistive collars ( u p p e r a n d lower). Both gas a n d a q u e o u s solutions a r e st i rred a t a s p e e d of 800 r p m . Th e e x p e r i m en t s w e r e carried out wi t h a solid: solution m a s s ratio of 1:200.

HP-HT ga s is s a m p l e d in a water-co oled co n d en s er prior to its injection in t h e gas ch r o m a t o gra p h (GC) for analysis.

Th e

a q u e o u s solution is s a m p l e d thro u gh a plunging capillary a n d

prior to carbon coating, t h e s a m p l e w a s ei t h e r m o u n t e d on a double-sided carbon t a p e or e m b e d d e d in epoxy a n d polished.

For TEM, a d rop of t h e p o wd er s a m p l e d i sp e r s ed in et h an o l w a s d ep o sit ed on a Lacey carbon-coated grid.

Derivation of total H2 production from the experimental data

Fig. 1 e 500 m L s t i r r ed aut ocl av e w i t h a q u e o u s a n d g a s s a m p l in g directly plugg ed t o t h e g a s c h r o m a t o g r a p h (GC).

filtered at high p r essu r e a n d high t em p er a t u r e by a titan iu m frit wi t h 0.2 mm pores. S a mp l ed solutions w e r e st o red in a fridge until th ey w e r e a n a l yz ed using ICP.

In t h e ca se of t h e e x p e r i me n t s p erf o rm ed in sea l ed gold cap- sules, t h e s a m p l e is q u e n c h e d before t h e cap su l e is pierced a n d t h e gas is s a m p l e d a n d analyzed. Owing to t h e limited solubility of H2 in w a t e r a t a m b i en t conditions, it ca n be considered t h a t all t h e H2 p ro du ced i n t h e e x p e r i m en t s is con centrated, a t a m b i en t conditions, in t h e gas p h a s e. Th e situation is totally different in t h e ca se of sa mp l i n g autoclave e x p e r i men t s w h e r e t h e g a s p h a s e i s s a m p l e d a t high T and P. A significant a m o u n t of H2 m a y be c o n c en t ra t ed in t h e solution.

Th e H2 distribution b e t w e e n gas a n d solution ca n be e s t i - m a t e d by using t h e Henry's l aw c o n st a n t a n d its t e m p e r a t u r e d ep e n d en c y a s calculated using t h e SUPCRT92 d a t a b a s e [22].

F u rt h erm o r e, mul ti pl e sa mp l ing of t h e ga s p h a s e l ea d s to a progressive extraction of H2 from t h e s yst em , wh i ch m u s t also be t a k en into account.

Finally, it is con venient to ex p ress t h e total H2 production a s H2 p rodu ced p e r kg of initial FeO according to reaction:

Analysis of the fluids

3FeO(s) þ H2O(l) / Fe3O4 ( s) þ H2 ( a q ) (1) Gas c o mp o n en t s (H2, CO2, N2, O2, CO, CH4) w e r e an al yzed wi t h

a Cla rus 500 gas ch r o ma t o g r a p h (Perkin Elmer^®) e q u i p p e d wi t h a polymer filled co l umn (Restek ShinCarbon^®) a n d a t h e r m a l conductivity detecto r (TCD). Th e t e m p e r a t u r e of t h e detector, t h e injection s y s t e m a n d t h e oven w e r e respectively s e t to 523, 373 a n d 353 K. Argon w a s u s e d a s gas carrier. Each gas s a m p l e w a s an a l yz ed a t l ea st t h r e e t i m e s consecutively.

Iron c o n t en t in t h e solution w a s d e t e r m i n e d right af t er samplin g, on 2 m L aliquots, by UV-spectroscopy a f t er complexation by o rt h op hen antr o l in e. This m e t h o d allowed a f a st quantification, compatible wi t h t h e sa mp l i n g frequency, wi t h a det ection limit of 0.1 p p m . K ept in t h e fridge, all solu- tions w e r e m e a s u r e d a t a n o t h er t i m e for total iron co n t en t by ICP-OES.

Solid characterization

T h e recovered solid p rod uct s w e r e first w a s h e d t h ro u gh r e p e a t e d w a t e r rinsing a n d t h e n c r u s h e d for X-ray p o wd e r diffraction (XRPD). XRPD p a t t e r n s w e r e collected wi t h a D8 diffractometer (Bruker, CuKa radiation) o p e ra t e d wi t h a 2q s t e p size of 0.026a n d a counting t i m e of 8 s.

Reaction progresses a r e t h eref o re cal culat ed as t h e a m o u n t of H2 p rod uced per kg of FeO in the starting material divided by 9.28 g of H2 p e r kg of FeO. However, in t h e ca se of e x p e ri me n t s p erf o rm ed a t low pH, p a r t of t h e iron int ro d uced in t h e s y s t e m is s e q u e s t e r e d a s a q u e o u s F e^2þ d u e to t h e reaction:

FeOðsÞ þ2H^þðaqÞ%Fe^2þðaqÞ þH2OðlÞ (2)

a n d will n ot be available for H2 production or ma g n et i t e.

Consequently, in order to co m p a re a t various pH valu es t h e e x t e n t of Reaction (1) deri ved from H2 c o n t en t in t h e gas p h a s e , a correction for F e^2þ h a s b e en applied.

Thermodynamic background

Stability of iron oxides in w a t e r h a s b e en calculated a t 423 K a n d 20 MPa, in t h e absence of a gas p h a s e ; using t h e SUPCRT92 t h e r m o d yn a mi c d a ta b a se. Two t yp es of reactions h a v e b e en considered, redox reactions a mo n g iron oxides, e.g., Reaction (1) a n d oxide e w a t e r equilibria, e.g., Reaction (2). Aq u eo u s F e^3þ w a s n egl ect ed since it is far less a b u n d a n t t h a n F e^2þ. Consequently, oxide e w a t e r equilibria involving iron oxides Pa rt of t h e recovered solid s a m p l e w a s k ept u n g r o u n d for containing ferric iron a r e also redox reactions, e.g., f u r t h e r electron microscopy characterization, Field Emission

Scanning Electron Microscopy (FE-SEM) a n d Tra n sm i ssi o n Electronic Microscopy (TEM). FE-SEM chara cteri zati on w a s p er f o rm ed wi t h a ZEISS Ultra 55 using both secondary a n d back-scattered electrons. TEM w a s p erf o r med on a Jeol FEG

2100F o p e r a t e d a t 200 kV. Both FE-SEM a n d TEM w e r e e q u i p - p ed wi t h a n Energy-Dispersive X-ray spectroscopy (EDS) d e - tector for chemical analysis. For FE-SEM characterization,

Fe3O4ðsÞ þH2ðaqÞ þ 6H^þðaqÞ%3Fe^2þðaqÞ þ4H2OðlÞ: Th e s y s t e m is d efi n e d by t h r e e activity variables, aH^þ , aH2;aq a n d aFe^2þ . All equilibria ca n be plott ed in a logaFe2þ=a_Hþ² log aH2a q d i a g ra m (Fig. 2). In t h i s d ia gra m, t h e H2 ( a q ) % H2(g) equilibrium boundary falls in a r a n g e of H2 ( a q ) activity w h e r e Fe3O4

is stable m e a n i n g t h a t t h e triple point FeO( s )eFe3O4 ( s )eFe^2þ( a q ) will n ev er be r e a c h e d w h e n reacting FeO wi t h w a t e r (see so-

lution reaction p a t h , Fig. 2). In o t h e r words, a t co n st a n t

Fig. 3 e C o m p a r i s o n of h y d r o g e n p r o d u ct i o n b e t w e e n t h r e e d ifferen t a cid s a ft er 11 d a y s of rea ct i on (423 K e 30 MPa).

Fig. 2 e Stability d i a g r a m of ir on o x i d e s in w a t e r a s a fu n ct ion of t h e activity of d issol v ed F e^2þ activity divided b y s q u a r e d H^þ activity a n d t h e activity of d issol v ed H2. T h e d o t t e d line c o r r e s p o n d s t o t h e s o l u t i on rea ct ion p a t h fol l owed b y a n “o x i d i z e d”s o l u t i o n t o w h i c h FeO is a d d e d a s calculat ed w i t h PHREEQC [32]. H e m a t it e (Fe2O3) s a t u r a t i o n will fi rst b e r e a ch ed . T h e n , a s s u m i n g t h a t h e m a t i t e n u cl e a ti on a n d g r o w t h kinetics a r e n o t limiting, FeO d i s s ol u t i on will b e a c c o m p a n i e d b y Fe2O3 p recip it at ion a n d H2 p rod u ct ion . T h e s o l u t i on will get e n r i c h e d in H2 ( a q ) a n d will evolve on the Fe2O3 s a t u r a t i o n l in e u p t o t h e t r i p l e p o i n t , Fe2O3 e Fe3O4 e a q u e o u s sp ecies . At t h i s p oin t , h e m a t i t e s h o u l d b e totally c o n v e r t ed in t o m a g n e t i t e b ef or e FeO d i s s ol u t i on a l l o ws t h e s ol u t i on t o get f u r t h e r e n r i c h e d in H2 ( a q ). M a g n et it e a n d H2 , a q a re prod u ced u n t il H2 ( g ) s a t u r a t i o n is achiev ed . At H2 s a t u r a t i o n , f u r t h e r Fe3O4 f o r m a t i o n is a cc omp a n ied b y H2 ( g ) p r o d u ct i on . I f P a n d T a r e k ep t c on s t a nt t h e n aH2;aq in t h e a q u e o u s s o l u t i on is fixed. N ote t h a t t h e triple p o i n t Fe OeF e3O4- a q u e o u s sp e c i e s is n e v e r r e a ch e d , m e a n i n g t h a t FeO s a t u r a t i o n will n o t b e achiev ed .

p r e s s u r e a n d t e m p e r a t u r e , FeO will n ev e r be stable wi t h w a t e r a n d sh o uld th eref o re rea ct until d i sa p p ea r a n ce.

R e s u l t s

Sealed gold capsules

Acetic, oxalic a n d hydrochloric acid solutions w e r e p r e p a r e d a t con centration s of 0.05, 0.001 a n d 0.001 mol/L respectively, corresponding to a sta rtin g pH of 3. Th e s e solutions w e r e sea l ed t o get h er wi t h u n si ev e d FeO p o wd e r from t h e s a m e st a rt in g m a t e ri a l in gold cap sul es a n d rea ct ed for 10 d a ys a t 423 K a n d 30 MPa. After q u enc h in g, t h e ca p sul e containing acetic acid w a s significantly m o r e i n fl at ed t h a n t h e others . Th e a m o u n t of H2 recovered from acetic acid e x p e ri m en t s w a s about 10 t i m e s h i gh er t h a n in e x p e r i men t s wi t h hydrochloric a n d oxalic acids (Fig. 3).

Th re e e x p e r i m en t s w e r e p r e p a r e d wi t h t h e s a m e protocol but wi t h acetic acid concentrations of 0.005, 0.05 a n d 0.5 mol

p e r liter corresponding to sta rti n g pH of 3.5, 3.0, 2.5, respec- tively. After t h r e e d a y s a t 423 Ke30 MPa, only 0.074 g H2/kg FeO w a s p rodu ced for t h e 0.005 mol/L e x p e r i men t w h e r e a s 2.58 a n d 1.91 g H2/kg FeO w e r e recovered for initial acetic acid con centrations of 0.05 a n d 0.5 mol/L, respectively, i.e., about 25% of reaction. For t h e s e two e x p e ri m en t s , t h e final pH va lues a r e calculated to be 5.0, 4.7 a n d 4.5, respectively.

Three r u n t e mp er a t u r e s w e r e t e st ed a t 30 MPa, 373, 423 a n d 473 K wi t h capsules containing FeO a n d acetic acid a t a con- centration of 0.05 mol/L a n d for r u n durat i on s from 3 to 172 h (Table 1, Fig. 4). After 72 h, the amount of H2 produced at 373, 423 a n d 473 K resu lted, respectively, in 0.058, 2.58 a n d 5.34 g H2/kg FeO (Table 1). Data could not be successfully fitted to a first-order kinetic model, a sq u a re root function yielded better fits (Fig. 4) whi ch allowed to derive a n activation energy of 27 kJ/mol.

Sampling autoclave: effect of pH

Two e x p er i m en ts w e r e c onducted by introducing FeO i n distilled wa t e r a t 423 a n d 573 K in t h e samp li ng autoclave.

Hydrogen w a s p rodu ced a t both t e mp e ra t u re s following two different kinetic models. At 423 K, after a first period of hydrogen production in t h e first 10 h u p to a value of 0.14 g H2/kg FeO, H2

production cea sed (Fig. 5). Magnetite grains w er e identified t hrou gh XRPD. At 573 K, corrected H2 production i ncrea sed during t h e first 24 h a n d slowed d o wn progressively to rea ch 1.28 g H2/kg FeO after 144 h. M a x i mu m H2 production is equiv- alent to 23% of the reaction progress. H2 production at 573 K has b een fi tted to a p se ud o first-order kinetics (Fig. 5). In both ex- p eri ment s, iron (II) concentration w a s too low to be d et ect ed by UV-spectrophotometry. Calculations using PHREEQC predicted a concentration of a ro un d 10⁷ mol p er liter a t 573 K, i.e., far below t h e detection limit of UV-spectrophotometry.

FeO dissolution in a qu eo u s solution containing 0.05 mol/L acetic acid was monitored in a sampling autoclave by analyzing a qu eo u s F e^2þ a n d H2 in t h e gas p ha se . FeO dissolution occurred according to a t wo -st ep process (Fig. 6a). Fast dissolution w a s e n c ou nt er ed during t h e first 10 h until a m a x i mu m [Fe^2þ] con- centration of about 0.02 mol/L w a s reach ed . At th is stage, m or e t h a n 10%w of t h e initial s am pl e h a d dissolved. Within t h e n ex t following 10 h [Fe^2þ] decreased to reach a plateau at 0.01 mol/L. It

Table 1 e E x p e r i m e n t a l cond ition s , H2 yield (GC) a n d p r e s e n c e of iron o x i d e s (b e s id e s FeO a n d Fe3O4) a s d e t e c t e d b y XRPD.

S a mp l e n a m e Method Solution C on centrati on T e m p e r a t u r e Pr e ssu r e E n d t i m e Starting Normalized hydro gen Go ethite (G) (mol/L) (K) (MPa) (hours) solution pHa production a t t h e e n d or Lepidocrocite (L)

of e x p e r i m e n t^b FeO(OH) p r e s en c e^c (g H2/kg FeO e g/kg)

CT-H2-Ac^d C aps ule Acetic acid 0.05 423 30 240 3 2.62 (28%) e

CT-H2-Ox^d C aps ule Oxalic a cid 0.001 423 30 240 3 0.24 (3%) e

CT-H2-HCl^d C aps ule HCl 0.001 423 30 240 3 0.20 (2%) e

CAc-H2_01 C aps ule Acetic acid 0.005 423 30 72 3.5 0.074 (1%) e

CAc-H2_02 C aps ule Acetic acid 0.05 423 30 72 3 2.58 (28%) e

CAc-H2_03 C aps ule Acetic acid 0.5 423 30 72 2.5 1.91 (21%) G

CAc-H2_04 C aps ule Acetic acid 0.05 373 30 72 3 0.058 (<1%) G

CAc-H2_05 C aps ule Acetic acid 0.05 473 30 72 3 5.34 (58%) L

CAc-H2_06 C aps ule Acetic acid 0.05 423 30 24 3 1.34 (14%) G þ L

CAc-H2_07 C aps ule Acetic acid 0.05 423 30 3 3 0.068 (<1%) G

CAc-H2_08 C aps ule Acetic acid 0.05 423 30 168 3 4.08 (44%) e

CAc-H2_10 C aps ule Acetic acid 0.05 423 30 72 3 2.46 (27%) e

CAc-H2_12 C aps ule Acetic acid 0.05 423 30 8 3 0.30 (3%) G

CAc-H2_13 C aps ule Acetic acid 0.05 373 30 172 3 0.23 (2%) G

CAc-H2_14 C aps ule Acetic acid 0.05 473 30 24 3 3.74 (40%) L

CAc-H2_15 C aps ule Acetic acid 0.05 473 30 3 3 2.86 (31%) e

CAc-H2_16 C aps ule Acetic acid 0.05 473 30 48 3 3.26 (35%) e

CHCl-H2_11 C aps ule HCl 0.001 423 30 72 3 0.084 (<1%) e

CHCl-H2_17 C aps ule HCl 0.001 473 30 72 3 0.19 (2%) e

CAc-Ca(OH)2 C aps ule Acetic acid þ Ca(OH)2* 0.05 423 30 72 3 0.0005 (<1%) e

CAc-CaCO3 C aps ule Acetic a cid þ CaCO3* 0.05 423 30 72 3 0.05 (<1%) e APAc-H2-150 Sa mpl i ng Acetic acid 0.05 423 16 48 3 8.06^e (87%) e

autoclave

APW-H2-150 Sa mpl i ng W a te r e 423 15 64.5 6 0.26^e (3%) e

autoclave

APW-H2-300 Sa mpl i ng W a te r e 573 18 144 6 2.18^e (23%) e

autoclave

* Additiona l solid u n u s e d for s tarti ng pH calculations.

a Start ing pH ca lc ulated fro m acidic solution initial concentration.

b M as s of H2 p rod uc e d d e d u c e d by ga s chro ma to g rap hy divided by initial m a s s of solid rea ge nt, ca lculated rea ction p ro g res s es a r e p r e s e n t e d i n brackets.

c Observation or not of goethite a n d / o r lepidocrocite by XRPD o n retri e ve d solid s a mp l e s .

d E xpe r i me nts co nd ucted o n uns i e ve d s tarti ng mater ia ls .

e Corre cted hydroge n production.

¨

can be s ee n on Fig. 6a t h a t [Fe^2þ] of 0.02 a n d 0.01 mol/L a re t h e expected concentrations a t FeO a n d Fe3O4 satu ra ti on, respec- tively, in t h e conditions of t h e exp eri ment.

Evolution of H2 production (Fig. 6b) is ch a ra ct eri zed by a first st ep of fast production, again within t h e 10 first hours.

About 8.06 g of H2 w e r e p ro du ced p e r kg of s a m p l e du rin g t h i s st ep, equi val en t to a reaction pro gress of 87%. After t h i s first st ep, dihydrogen w a s no longer p rod uced in t h e gas p h a s e . A smal l d ecrea s e w a s e v en observed wh i ch is a tt ri but ed to H2

remo val by gas samplin g. In d eed , for e a ch gas sampling, e s t i m a t e d to 27 mL, t h e total p r e s s u r e is lo wered a n d p a r t of hydro gen is rem o v ed from t h e ex p e ri men t a l syst em .

Fig. 4 e H yd r og en p r o d u c t i o n f r o m FeOeH2O in gold c a p s u l e s y s t e m a t 373 (sq u a re ), 423 (circle) a n d 473 K ( d i a m o n d ) e 30 MPa.

Fig. 5 e C orr ect ed H2 p r o d u c t io n k in et ics in p u r e w a t e r a t 423 K ( s q u a r e ) a n d 573 K (circle); D a s h e d lin e c o r r e s p o n d s t o a p s e u d o fi r st - o rd e r kinetic fit t o t h e 573 K d a t a , log k is e s t i m a t e d t o ¡3 .2 6 .

Hydrogen partial p r e s su r e, i.e., its concentration, r e m a i n s co n st a n t e v en t h o u gh its mo la r a m o u n t is redu ced. H2 pro- duction d a t a p r e s e n t e d on Fig. 6b a r e corrected accordingly.

Characterization of the solid products

XR PD on reco vered solid samples indicates, beside wustite and ma gn et i t e, t h e production of goethite, FeO(OH) a n d , possibly, lepidocrocite (Table 1). Inspection of t h e r u n p rodu ct s wi t h FE- SEM in back-scattered electro n m o d e allowed to distinguish b e t w e e n FeO a n d ma gn et i t e. FeO of hi gh er avera ge atomic number is bri ght er than ma gneti te (F i g. 7a). R esidual iron metal particles were found to occur as bright spots within FeO grains.

FeO oxidation in acetic acid proceeded from FeO grain rim to core ei t h e r t h ro u gh progressive r e p l a c e m e n t of t h e FeO g rains without significant morphological change (Fig. 7a), this process will be called p seu d o mo rp h i c r e p l a c e m e n t in t h e following or by FeO dissolution (Fig. 7b) followed by t h e precipitation of coronae of m a gn et it e nan o pa rti cl es (Fig. 7c a n d d). Magnetit e precipitation w a s also observed wi t h t h e fo rmati on of nano- particle aggregates (Fig. 8a). Oxidation through dissolution and precipitation d o m i n a t e s in s a m p l e s p ro du ced in t h e sa mp l i n g autoclave. R esidual FeO recovered from cap su l e e x p e r i m en t s preferentially s h o ws p s eu d o mo rp h i c r e p l a c e m e n t pointing t h eref o re t o wa rd s a possible role of stirring on t h e oxidation reaction process. In p u r e wa t e r , oxidized p rod u ct s occurred a s a m u l t i t u d e of d e n d r i t e s growing pervasively wi t h i n t h e FeO grains (Fig. 7g a n d h). Compa ri son a t t h e s a m e magnification b e t w e e n res i du al FeO grains recovered from both acetic acid a n d distillated w a t e r e x p e r i men t s (APAc-H2-150 a n d APW-H2-

Fig. 6 e T im e- res ol v ed m o n it or i n g of of a q u e o u s Fe a n d H2 of t h e 423 Ke15 MPa e x p e r i m e n t w i t h a 0.05 mol / L acetic a cid s o l u t i on (a) a q u e o u s i r on (II þ III) c o n c en t ra t i on f r o m ICP d a t a (circle) a n d UV s p e c t r o p h o t o m e t r y (sq u a re ); (b) C orrect ed H2

p r o d u ct i o n a s m e a s u r e d b y GC.

Fig. 7 e FE-SEM b a c k - s c a t t e re d imaging of t h e e x p e r i m e n t a l p r od u ct s . a) a n d b) FeO o x i d a t i o n f e a t u r e s a t t h e g ra in sc al e in acetic acid in gold c a p s u l e (CAc-H2-2) a n d s a m p l i n g au t ocla v e (APAc-H2-150), respectivel y; c) a n d d) n a n o m a g n e t i t e covering r e s i d u a l FeO g ra in s (CAc-H2-14); e) a n d f) FeO o x i d a t i o n f e a t u r e s a t t h e s a m p l e sca l e in acetic acid (APAc-H2-150) a n d w a t e r (APW-H2-300), respectivel y; g) a n d h ) m ic r o t e xt u r a l d et a il s of t h e FeO (light grey) r e p l a c e m e n t b y Fe3O4 ( d a rk grey) i n w a t e r (APW-H2-300). E x p e r i m e n t a l c on d it i on s for e a c h s a m p l e a r e f o u n d in Table 1.

300 respectively, Table 1) are presented on Fig. 7e and f. Particle a t t h e interface b e t w e e n s a m p l e a n d gold-capsule wall. Lep- si zes close to initial (50e100 mm) a r e still p r e s e n t in both s a m - idocrocite w a s d et e ct ed using XRPD in s a m p l e s wi t h ples, however, w h e r e a s particles h a v e nicely k ept t h ei r s h a p e

in wa t e r, in acetic acid particles a r e split u p a n d n e w reactive surfa ce a r e a h a s b een creat ed.

Goethite, FeO(OH), w a s d et e ct e d by XRPD in about half of t h e e x p e r i m en t s p e r f o rm ed wi t h acetic acid (Table 1). Goethite g rains w e r e easily identified by FE-SEM d u e to th ei r lower avera ge atomic n u m b e r a n d t h ei r distinctive morphology ( n eedl es or very t h i n plates, Fig. 9). Goethite w a s ma i nl y f o und

comparatively hi gh er hydro gen yield.

In acetic acid ex p eri m en t s , na n opa rti cl es of ma g n e t i t e were o bserved in cluded in a carbon matrix (Fig. 8b), suggesting t h a t t h e carbon-rich co mp o u n d precipitated d u rin g or a ft er FeO oxidation. EDS d a t a a n d electron diffraction indicated t h a t t h i s solid is co mp o sed of a m o r p h o u s carbon. This lack of crystallinity explain s wh y t h i s solid w a s n o t identified by X- ray p o wd e r diffraction. To quantif y t h e a m o u n t of a m o r p h o u s

¨

Fig. 8 e TEM i m a g e s of s a m p l e APAc-H2-150. a) M a g n et it e n a n op a r t ic l e aggreg ates; b) in cl u s ion s of m a g n e t i t e n a n op a r t i c l e s in a n a m o r p h o u s c a r b on p h a s e .

Fig. 9 e FE-SEM i m a g e s of g o e t h it e (a) CAc-H2-14 in b a c k - s c a t t e r e d m o d e , (b) CAc-H2-12 i n s e c o n d a r y el e ct r on m o d e .

carbon, carbon e l e m e n t a r y analysis w a s conduct ed a n d r e t u r n e d a carbon m a s s proportion of 1e1.5%w. Solid carbon h a s b een d et e c t ed in both cap su l e a n d sa mp l in g autoclave r u n s (CAc-H2_03 a n d APAc-H2-150 sa mpl es) .

Effect of calcium addition

In order to simulate the oxidation behavior of FeO in steel slags, CaO, wh i ch is a maj o r c o mp o n e n t of st eel slag, w a s a d d e d to wu st it e. Two mi x t u re s of FeO e Ca(OH)2 (portlandite), a n d FeO e CaCO3 (calcite), in a m a s s ratio of 1:1 w e r e r u n in gold sea l ed capsules with 0.05 mol/L acetic acid at 423 Ke30 MPa for 3 days.

A t h i rd cap su l e containing FeO a n d p u r e w a t e r w a s also r u n in parallel a s a reference. Corresponding H2 yields a r e p r e s e n t e d in Fig. 10. Th e addition of calcium ei t h er a s hydroxide or car- bonat e clearly inhibited FeO oxidation, hydro gen production w a s very low, i.e., about two orders of m a g n i t u d e lower t h a n i n t h e Ca-free s y s t e m u n d e r t h e s a m e conditions (Fig. 10).

At 423 K, H2 is only p ro du c ed wi t h i n t h e fi rs t 10 h to a n a m o u n t wh i c h cor re sp o nd s to 3% rea ct io n progress. Minor m a g n e t i t e w a s i d en ti fi ed by XRPD a m o n g re si d u a l FeO. After t h i s first H2 prod u ct ion st a ge a n d u nt i l t h e e n d of t h e r u n (65 h), n o m o r e H2 is p ro d u ce d. This t y p e of H2 pro d u cti o n behavior is i n t e rp r e t e d a s e i t h e r d u e to (1) t h e dissoluti on/

oxidation of s ma l l FeO part icles wi t h h i gh su rf a ce a r e a

D is c u s s io n

Effect of acidic conditions and temperature on H2 yield

In p u r e wa t e r , hydro gen production th ro u gh FeO a q u e o u s oxidation h a s b een mo ni t o red in a sa mp l in g autoclave a t t wo t e m p e r a t u r e s , 423 a n d 573 K (Fig. 5), a t a pH of ca. 6 (Table 1).

Fig. 10 e Effect of t h e a d d it i on of calcium t o FeO a s e i t h e r p o r t l a n d i t e o r calcite o n t h e H2 yield in a 0.05 mol / L acetic acid s ol u t i on a t 423 K e 30 MPa for 3 d a y s .

¨ m i x ed wi t h t h e 50e100 mm fraction, (2) t h e react ion of re -

s i du a l gra i n s of Fe m e t a l or (3) t h e p re fe re nt i al dissolution ( a n d oxidation) of h i gh -e n erg y si t e s a t t h e su rf a ce of FeO grains. Co nseq u ent ly, it will be co ns i d ered h e r e t h a t FeO g ra i n s i n t h e 50e100 mm si ze r a n g e do n o t re a c t wi t h w a t e r to p ro d u ce H2 i n t h e p r e s en c e of p u r e w a t e r a t 423 K a t t h e t i mes ca l e of a day.

At 573 K, wi t h i n t h e first 10 h, dihydrogen production ki- netics is four t i m e s hi gh er t h a n a t 423 K. By co n t ra st to t h e 423 K ex p eri men t , H2 is still p rodu ced after 10 h a t a r a t e wh i ch d e c r e a s es progressively wi t h t i m e (Fig. 5). After 144 h , a re- action pro gress of 23% is at t ai n ed . At t h e micro-scale, FeO oxidation into m a g n e t i t e is mainly localized in c h a n n e l s ho- mogeneously dist ri buted i n t h e bulk of t h e grain (Fig. 7h).

Magn etite s e e m s to nu cl ea t e o n st ru ct u ra l def ects or cracks.

Magn etite f ormati on mi g h t be rel a t ed to a n auto-oxidation process a s alread y e m p h a s i z e d for Fe(II) a q u e o u s oxidation a t t h e surfa ce of h yd ro u s iron oxides [19]. Wh at ever t h e exact

sa t u ra t i o n is calculated, a t 423 K a n d 30 MPa, to be 4.7 a n d 5.2, respectively. Either t h i s H2 production difference is d u e to a prot on p ro mo t ed Fe(II) dissolution in relation to t h e 0.5 pH u n i t difference b e t w e e n t h e t wo solutions or it is d u e to a ligand -promot ed Fe(II) dissolution by acetic acid. W h e r e a s oxalate-ligand h a s b een s h o we d to e n h a n c e t h e dissolution of Fe(III) c o mp o u n d s [26], a cet a t e h a s a p ro n o u n ced affinity for Fe(II) to form a complex su c h a s FeCH3COOH^þ wh i ch becomes t h e d o m i n a n t Fe(II) a q u e o u s form for pH > 5 (Fig. 11). Disso- lution of iron-bearing silicates in acetic acid solutions s e e m s to converge t o wa rd s a p rot on -p ro mo t ed r a t h e r t h a n a ligand- promot ed dissolution [27,28]. However, d u e t h e stability of a c et a t e e iron a q u e o u s complexes a t 423 K (Fig. 11) a n d above, ligand -promot ed Fe(II) dissolution in acetic acid solutions a t high p r e s s u r e a n d t e m p e r a t u r e ca n n o t be ru l ed out.

H2 production a n d Fe(II) dissolution in acetic acid a t 423 K h a v e both b een mo n it o red in t h e sa mp l i n g autoclave a t 150 MPa. Fe(II) con centrati on r e a c h ed a m a x i m u m af t er 10 h oxidation process, formati on of ma gn et i t e in t h e bulk of t h e wh i ch co rresponds to wu st i t e satu rat i on . Th en , Fe(II) FeO grains su ggests t h a t oxidation kinetics will n ot be directly

rel a t ed to t h e surface a r e a of t h e FeO sta rtin g mat eria l. In o t h er word s, t h e reducti on of t h e FeO grain size by grinding m i gh t n ot significantly e n h a n c e t h e reaction kinetics.

FeO oxidation w a s f o und to be strongly e n h a n c e d in t h e p r e s en c e of acetic acid (Fig. 6b). W h e r e a s w e s h o w e d t h a t FeO g rains i n t h e 50e100 mm size r a n g e do n ot rea ct wi t h p u r e w a t e r a t 423 K, t h e s a m e exp eri en ce p e rf o rm ed wi t h 0.05 mol/

L of acetic acid (starting pH of 3) r e a c h ed completion (or n e a r

d e c r e a s ed to r e a ch ma g n et it e sa t u ra t i o n in a t i m e interval of less t h a n 15 h (Fig. 6a). All t h e H2 is p rodu ced wi t h i n t h e s e first 10 h (Fig. 6b). XRPD anal ysis of p o s t - m o r t e m solid indicates full conversion of FeO into Fe3O4.

It ca n be concluded from t h e s e resu l t s t h a t (1) FeO h a s fully rea ct ed wi t h i n t h e first 10 h of ex p eri m ent s, (2) a s long a s FeO is p r e s ent , it controls t h e a q u e o u s Fe(II) content, (3) m o st , if n o t all, of both m a g n e t i t e a n d H2 is p ro du ced wi t h i n t h e first 10 h. C onsequ ently, FeO dissolution s t e p is fa st er t h a n Fe(II)

completion) wi t h i n 10 h. oxidation a n d / o r ma g n e t i t e precipitation step(s) wh i ch

For t e m p e r a t u r e s in t h e 293e303 K r a n g e in acidic solu- tions, dissolution rat es of simple oxide mi n e r a l s w e r e found to be proportional to ðaH^þ Þ^0:40:7 by Casey e t al. (1993, [23]) w h e r e ðaH^þ Þ d en o t e s H^þ activity. This m e a n s t h a t , a t a m b i en t con- ditions, a pH d ecr ea s e by 3 u n i t s b e t w e e n p u r e w a t e r a n d acetic acid e x p e ri me n t s is expect ed to r esu l t in a h i gh er dissolution r a t e by 1e2 o rd ers of ma g n i t ud e . Th e effect of t e m p e r a t u r e a n d p r e s s u r e on t h e d e p e n d en cy of FeO disso - lution rate with pH is not kno wn but a difference by 1e2 ord ers of m a g n i t u d e for a difference of 3 pH u n i t s is co n sist ent wi t h w h a t is observed h e r e a t 423 Ke15 MPa. Preliminary t e s t s in sea l ed gold cap su l es containing t h e s a m e u n si ev ed FeO p o wd e r along wi t h t h r e e different acidic solutions, acetic, oxalic a n d hydrochloric a t pH ¼ 3 gave co n t ra st ed H2 yields a t 423 K a n d 30 MPa (Fig. 3). Actually, th i s resu l t is n o t incon- sist ent wi t h a first order pH effect on FeO dissolution ( a n d oxidation). First, a q u e o u s oxalic acid is th ermal l y unstable. A reaction c o n st a n t of 10⁹ is calculated for oxalic acid decom- position into 2CO2(aq) a n d 1H2 (aq ) a t 423 K a n d 20 MPa. Crossey (1991, [24]) s h o we d t h a t a t 433 K, pH ¼3.6, about half t h e initial oxalate is d eco mp os ed a ft er 45 h . On t h e contrary, a q u e o u s acetic acid is stable a t high t e m p e r a t u r e wi t h a decompo- sition reaction co n st a n t of 10¹⁷ (C2H4O2( aq ) þ 2H2O(l) %

2CO2(aq) þ 4H2(aq)). In t h e a b senc e of a catalyst, Bell e t al. (1994,

[25]) s h o we d t h a t a t a t e m p e r a t u r e a s high a s 608 K, only 1% of t h e initial acetic acid solution (ca. 1 mol/L) d eco mp osed a f t er 10 days. It ca n t h eref o re be co nsidered acetic acid decompo- sition as negligible in our exper iment p erf o rmed at 423 K for 11 days. H2 production is h i gh er by 1.5 o rd er of m a g n i t u d e for acetic acid solution (initial pH ¼3) t h a n for HCl solution (initial

pH ¼ 3). Th e in -situ pH of t h e s e acidic solution a t m a g n et i t e

t h eref o re occur to be t h e rate-limiting step(s). This limitation is co nfi rmed by t h e formation of m a g n et i t e nanoparticl es. Th e quick dissolution s t e p p r e v e n t s ma g n e t i t e to grow leading to a q u e o u s iron oversaturati on a n d t h e forced precipitation of m a gn et i t e a s n an op a rti cl es (Fig. 8a). However, a t t h e resolu-tion of our sa mp l in g frequency, t h e ex p eri men t a l conditions a p p e a r to be close to optimal since all t h e s e reaction s t e p s s e e m to pro ceed a t similar r a t e s since FeO t ra n s f o rma t i o n into Fe3O4 is achieved w h e n a q u e o u s Fe(II) r e a c h e s FeO sa tu rati o n. If t h e kinetics of t h e s t e p s leading to t h e formati on of m a gn e t i t e would h a v e b e en lower t h e n H2 ( a n d ma gn et i t e)

Fig. 11 e Speciat ion of a q u e o u s i r on (II) a t 423 Ke15 MPa a s s u m i n g 0.01 mol / L of tot a l Fe ( n o m i n e r a l equilibria) in 0.05 mol / L of acetic acid.

¨ would h a v e still b een p ro du ced a ft er FeO sa t u ra t i o n h a d b een

achieved. E x p eri men t s p erf o rm ed in gold cap sul es wi t h acetic acid a t 0.005, 0.05 a n d 0.5 mol/L u n d e r identical condi- tions (423 K, 10 days, 30 MPa) co n fi rmed t h a t n e a r optimal conditions h a v e b e en r e a ch e d for a concentration of 0.05 mol/

L a t 423 K. As a m a t t e r of fact, w h e r e a s H2 yield i n crea s ed by about t wo o rd ers of m a g n i t u d e b e t w e e n 0.005 a n d 0.05 mol/L ex p eri m en t s , H2 yield b e t we e n 0.05 a n d 0.5 mol/L is n o t significantly different.

Iro n d i sso lu t i on flux h a s b e e n ca l c u l a t ed acco rding to m e t h o d d e t a i l ed by Jang e t al. (2009, [16]) f r o m iron (II) c o n c e n t r a t i o n d a t a (Fig. 6a). T h e slope, s, h a s b e e n e v a l u - a t e d f r o m t h e t wo fi r st di sso l u t i on p o i n t s , d r e p r e s e n t s t h e m a s s c o n c e n t r a t i o n of FeO a n d A t h e s u rf a c e a r e a m e a s u r e d by N2-BET.

F ¼ dA (3)

A log dissolution flux (mol/m²/s) of 3.3 h a s b een o btain ed for our ex p eri m en t a l conditions. At a m b i e n t conditions, for a n eq ui va len t pH of 4.7, a log flux (mol/m²/s) value of 8.0 is ex- pected. Such a difference m a y be r el a t ed to t h e t h ermal l y activated ch a ra ct er of t h e dissolution process. B a sed on t h e s e t wo dissolution fluxes a t t wo different t e m p e r a t u r e s , a n acti- vation en ergy of ca. 90 kJ/mol is calculated, wh i ch is in good agreement with values obtained for iron (III) oxides dissolution [29]. Moreover, iron solubility is largely i n c r ea s ed wi t h t h e addition of acetic acid at 423 K and 30 MPa from 2.2 10⁶ mol/L in w a t e r to 9.1 10³ mol/L in a 0.05 mol/L acetic acid solution.

In parallel to t h e effect of pH, w e co n fi rmed t h e resu l t of

i mp o rta ntl y (3), t h e lack of stirring. In d eed , stirring e n h a n c e s iron dissolution, identified a s limiting factor to dihydrogen production, t h ro u gh homo genizati on of t h e solution. Wha t - ev er t h e n a t u r e of t h e solvent, oxidation f e a t u r e s of iron oxide grains reco vered from gold ca p su l es also occurred to be different. In particular, oxidation is f ound to mainly pro ceed according to a progressive p seud o mo rp h i c r e p l a c e m e n t of t h e initial FeO gra ins (Fig. 7a). Stirring h a s also a mechan i cal effect wh i ch could account for t h e reaction t ex t u re differences observed b e t w e e n sti rred a n d static ex p e ri m en t s .

Goethite and solid carbon

Goethite, FeO(OH), w a s observed by XPRD in s o m e of t h e sa m- p l e s p rod uced in acetic acid solutions (Fig. 9). As a n Fe(III) hy- droxide, t h e precipitation of which, i nst e ad of magnetite, can potentially improve t h e H2 yield. However, goethite w a s only d et e ct ed in e xp er im en t s r u n in gold capsules, mainly a tt a ch ed to t h e cap su le walls. Its proportion w a s n ot q ua nt ifi ed but seemed l o w c ompa red to t ho se o f magn eti te a nd wu sti te. At t he e n d of ru n, gold cap su les we r e d ri ed a t 353 K wi th ou t was h i n g contrarily to t h e p owd er s a m pl es recovered from t h e sampl in g autoclave. As t h e rm od yn am ic s predicts t h a t ma gn et i t e sho uld be t h e only stable iron oxide/hydroxide p h a s e in t h e investi- gated h yd ro th er ma l conditions, goethite formation is likely to be t h e resu lt of evaporating Fe-rich acetic acid solution in air.

Nominally, t h e only source of carbon in our ex p eri m en t a l s y s t e m is acetic acid, t h ro u gh t h e reaction,

Malvoisin e t al. (2013, [7]) obtained on steel slags wh i ch is t h a t

h y d ro t h er ma l hydrogen production is a th ermal l y activated CH3COOH(aq) % 2C(s) þ 2H2O(l). (4) process. In wa t e r , e x p e ri me n t s p erf o rm ed in t h e sa mp l i n g

autoclave led to a m a x i m u m of p rod u ced hydro gen 9 t i m e s h i gh er a t 573 K t h a n a t 423 K (Fig. 5). T e m p e r a t u r e is expect ed to play a role o n both oxidation a n d dissolution ra t es, t wo

A possible ro ut e for t h e formation of solid carbon is t h e t h e r m a l decomposition of acetic acid

t h erma l ly activated processes. However, e ven by applying CH3COOH(aq) % 2CO2(g) þ 4H2(g) (5) t e m p e r a t u r e from 423 to 573 K in wa t e r, r a t e a n d m a x i m u m

h yd ro gen p rod u ct i on remain s fa r l o wer t h an o b served a t 423 K in acetic acid (Fig. 6b). In o t h e r word, i n order to a t t a i n sig- nificant kinetics i mp ro v em en t , t h e u s e of mil d acidic solvent is a far b ett er solution t h a n increasing t e m p e r a t u r e .

H2 yield difference between capsule a nd sampling autoclave H2 yield differences h a v e b e en e n c o u n t er e d b e t w e e n ca psu l e a n d sa mp l i n g autoclave ex p eri m en t s . U n d e r t h e s a m e condi- tions of t e m p e r a t u r e , p r e s s u r e a n d acetic acid concentration, total a m o u n t of dihydrogen pro d u ced a ft er 8 h is 27 t i m e s lower for e x p e r i m e n t s co n du cted in cap su l es (Fig. 4) t h a n for e x p e r i men t s in t h e sa mp l i n g autoclave (Fig. 6b). Th e overall s h a p e of t h e H2 production curves is also different wi t h a lower production r a t e a t t h e beginning of t h e r u n in t h e gold capsules. Th e reaction t ex t u res a r e also different a n d involve p seu d o mo rp h i c r e p l a c e m e n t of st a rt in g FeO grains. Th e dif- ferences in H2 yield b e t we e n t h e t wo ex p eri m en t a l m e t h o d s ca n be potentially account ed for by (1) a large difference in

t h ei r respective solid/solution m a s s ratio (1:200 in sa mp l in g autoclave a n d 1:1 in gold capsules), (2) t h e likely a b sen ce of a gas p h a s e in e x p e ri men t s p erf o rm ed in ca p sul es a n d , m o s t

followed by t h e reduct ion of CO2 into solid carbon.

Magne-tite surface h a s b een s h o w n to catalyze t h a t lat er s t e p u n d e r similar P-T conditions [30]. However, in co nt rast wi t h t h e re -su l t s of Milesi e t al. (2015, [30]), carbon h a s b e en only observed in ma gn et it e-ca rb o n agglomerates, a n d n o carbon coating on m a g n e t i t e grains h a s b e en observed in t h e s a mp l e . It sh ou ld be n o t e d t h a t t h e overall C forming Reaction (4) h a s no direct i mp ac t on t h e H2 budget. It ca n be calculated t h a t 4%m o l of acetic acid m u s t h a v e d eco mp o sed according to Reaction (4) in o rd er to account for t h e 1e2 wei gh t p e r c en t of carbon a n a l yz ed in s a m p l e (APAc-H2-150). This e s t i m a t e is signifi-cantly h i gh er t h a n t h e resu l t s by Bell e t al. (1994, [25]), alread y m e n t i o n e d , on t h e kinetics of acetic acid decomposition wi t h o u t catalyst. A possible catalytic effect of m a g n e t i t e n an opa rti cl es m a y be p u t forward in our ex p eri men t s .

From FeO to steel slag

Steel slags contain in a vera ge 46%w of CaO [31]. Initially p r e - s e n t in t h e fo rm of lime, CaO, in freshly p ro du ced slags, aging of t h e slag in air l ea d s to t h e formation of portlandite, Ca(OH)2

a n d calcium carbonate, CaCO3.

€

~

~

~

¨ Three experiments performed at 423 K and 30 MPa for 3 days

in gold capsules with pure FeO, FeOþCaCO3 and FeO þCa(OH)2

in a 0.05 mol/L acetic acid solution yielded co ntrast ed H2 pro- duction of 1.234, 0.025 a n d 0.0005 g H2/FeO kg, respectively (Fig. 10). The addition of a Ca-phase such as Ca(OH)2 or CaCO3, in t h e p re se nc e of acetic acid strongly modifies t h e speciation a n d pH of t h e a qu eo u s solution. In particular, w h e n portlandite is p re se nt , along wi t h Fe3O4, t h e pH of t h e solution is high (pH ¼ 9.1) d u e to t h e relatively high solubility of Ca(OH)2. In t h e case where calcite i s t he only Ca-bearing solid, pH is buffered to a value of 5.8, i.e., slightly above the measured final pH in the Ca- free Fe3O4 e acetic acid s ys t e m (pH ¼ 4.7). In addition to t h e increase of pH which will tend to slow do wn the FeO dissolution (see discussion above), t h e total a m o u n t of a qu eo u s Fe(II) in equilibrium with Fe3O4 is drastically lowered, from ca 10² mol/

L to 10⁶ mol/L in the presence of CaC O3 a nd d own t o 10^{1 1} mol/ L w h e n Ca(OH)2 is p re se nt .

C o n c lu s io n

For t h e sa k e of testin g factors t h a t could in crea se t h e kinetics of H2 production by h y d r o t h erm a l t r e a t m e n t of st eel slags according to t h e 3 FeO(s) þH2O( l) /F e3O4 ( s ) þH2 ( a q ) reaction[7], p u r e FeO w a s u s e d a s mod el co mp o u n d a n d t e s t e d in t h e 373e573 K r a n g e in t h e p r e s en c e of mi ld acidic solutions.

Th e effect on t h e kinetics of H2 production from FeO of acetic acid a t concent ration s far below t h a t of vinegar is remarkably strong. In 0.05 mol/L acetic acid a t 423 K, FeO oxidation r ea c h e s n e a r completion wi t h i n 10 h wh e r e a s , in p u r e wa t er, H2 yield is negligible, ev en a f t er 65 h. We identifi ed FeO dissolution a s t h e rate-limiting st ep; therefore, t h e ki- netics effect of acetic acid is primarily i n t e rp r e t ed a s rel a t ed to t h e d ep e n d en cy of FeO dissolution r a t e wi t h pH. A log disso- lution flux (mol/m²/s) a s hi gh a s 3.3 h a s b e en obtain ed a t 423 K wi t h a n acetic acid con c entrati on of 0.05 mol/L. At t h e microscale, fa st reaction kinetics is asso ciat ed wi t h a disso- lution/precipitation process wh i ch gives rise to a mu l t i t u d e of smal l m a g n e t i t e particles covering t h e f ast dissolving FeO grains. H yd rot h ermal H2 production from FeO oxidation is t h erma ll y activated; however, t h e kinetics e n h a n c e m e n t asso ciated wi t h t h e u s e of mi ld acidic solutions is f oun d to be e n o r m o u s in compari son to t h a t of t e m p e r a t u r e . Acetic acid being th erma l l y stable, it allows t h e combination of acidic conditions a t high t e m p e r a t u r e . In ord er to t a k e a d v a n t a g e of t h e positive effect of acetic acid on H2 yield, solutions a r e being explored to m a i n t a i n acidic conditions in t h e p r e s en c e of Ca-bearing p h a s e s wh i ch a r e ubiquitous in st eel slags. In particula r t h e p r e s e n c e Ca(OH)2 sh ou ld be avoided since it t e n d s to n eu t ra l i ze acetic acid a n d i mp o se basic conditions wh i ch h a v e a d ra m a t i c effect on H2 production.

A c k n o w l e d g e m e n t s

This work is su p p o rt ed by CNRS t h ro u gh t h e “Mission Inter- disciplinaire: Defi Transition Energetique: Ressources, Societe, E n v i r o n n em e n t e ENRS” pro g ram. This work is p a r t of t h e HyMag'In project f u n d e d by t h e SATT Link sium (Grenoble).

Labex OSUG@2020 is also t h a n k e d for financial supp o rt . Th e

a u t h o r s would like to t h a n k Moulay-Tahar Sougrati for Mossbauer spectroscopy. Valerie Magnin is t h a n k e d for t h e BET m e a s u r e m e n t a t ISTerre.

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