Thermally activated iron containing layered double hydroxides as potential catalyst for N2 O abatement q

Chemical Engineering Journal

journalhomepage:www.elsevier.com/locate/cej

Thermally activated iron containing layered double hydroxides as potential catalyst for N q

2 O abatement

Tatjana J. Vulic c ⇑ b , Andreas F.K. Reitzmann , Károly Lázár

b University of Novi Sad, Faculty of Technology, bul. Cara Lazara 1, 21000 Novi Sad, Serbia Sued-Chemie AG, Research and Development Catalysis and Energy, Waldheimerstr. 13, 83052 Bruckmühl, Germany c Institute of Isotopes of the Hungarian Academy of Sciences, Department of Catalysis and Tracer Studies, P.O. Box 77, Budapest H-1525, Hungary

highlights

" Reduction behavior mainly influences catalytic activity in N 2 O reduction with NH 3 .

" Extended M(III) substitution weakens Mg–Al–Fe–O interactions. " Extended M(III) substitution improves catalytic behavior. " LDH matrix with M(III) near the limit for incorporation gives the best results.

article info

abstract

Article history: Layered double hydroxides (LDHs) and derived mixed oxides with different Mg/Al/Fe contents were Available online xxxx

investigated. Two super-saturation precipitation methods were used for the synthesis of LDHs with gen- eral formula [Mg

2 O where M(III) presents Al and/or Fe. The content of triva- Keywords:

M(III) x (OH) 2 ](CO 3 ) x/2

lent ions x = M(III)/[M(II) + M(III)], was varied between 0.15 < x < 0.7. Such a wide range of trivalent ions Hydrotalcite like materials

was chosen with the aim to induce the formation of different multiphase mixed oxides. Iron was intro- Mg–Al–Fe mixed oxides

duced as constituent metal in order to obtain redox properties. LDHs and their derived mixed oxides were Mössbauer spectroscopy

TPR characterized with respect to their crystalline structure (XRD), thermal stability (TG/DTA), textural (N 2 Nitrous oxide

adsorption), redox (H 2 TPR) and acid properties (NH 3 TPD) as well as the nature of the iron species (Mössbauer spectroscopy). Catalytic behavior was studied in two test reactions: N 2 O decomposition and reduction with NH 3 . It has been demonstrated that extended M(III) substitution influences the struc- ture and surface properties of Mg–Al–Fe LDHs and derived mixed oxides, weakens Mg–Al–Fe–O interac- tions and improves catalytic behavior correlated with the presence of Fe–O–Fe–O–Fe entities providing possibility for facilitated extraction of oxygen with simultaneous redox Fe 3+ M Fe 2+ conversion. The cat- alytic behavior is mainly determined by redox properties, nature of iron species in mixed oxides and by structural properties of initial LDHs. The best catalytic results were obtained when the amount of M(III) was near the limit for the incorporation into LDH matrix.

Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction control emission and H 2 production, anion exchangers, adsorbents, fillers (stabilizers for polymers) to medical-pharmaceutical appli- Layered double hydroxides (LDHs) and their derived mixed

cations by making use of their specific properties [1–6] . oxides reached growing attention in research and engineering

LDHs are anionic clay materials and also known as hydrotalcite- because of possibility to vary a large number of synthesis parame-

like materials have layered structure which consists of Brucite-like ters and in that way modify and tailor various properties. Their

layers with octahedrally centred M(II) ions. Partial isomorphous application ranges from catalysts and catalyst supports in organic

substitution of M(II) with M(III) ions creates positive charge which synthesis, (photo) degradation of organic wastes, greenhouse gas

is compensated with different anions present in the interlayer re- gion together with water [3,5,7] . The general formula of LDH is:

2 O, where M(II) is a divalent cat- Chemical Process Engineering, Fritz-Haber-Weg 2, D-76131 Karlsruhe, Germany.

M(II)

M(III) x (OH) 2 (A )

Work was carried out in Karlsruhe Institute of Technology (KIT), Institute of

x/n

is anion (usually carbonate) ⇑ Tel.: +381 21 485 3750; fax: +381 21 450 413.

ion, M(III) is a trivalent cation, A

and x = M(III)/[(M(II)+M(III)]. It has been reported that the value E-mail addresses: tvulic@uns.ac.rs (T.J. Vulic), andreas.reitzmann@sud-chemie.

of x between 0.2 and 0.4 is optimal for the formation of single com (A.F.K. Reitzmann), lazar@iki.kfki.hu (K. Lázár).

1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.06.152

2 T.J. Vulic et al. / Chemical Engineering Journal xxx (2012) xxx–xxx

LDH phase and by exceeding this range, hydroxides or other com- Mg50Al45Fe5 is the denotation for the sample synthesized by pounds may be formed [3] . The nature and amount of M(II) and

the LS method having following initial metal amounts: 50 mol% M(III) ions influences the redox properties of LDHs and derived

of magnesium, 45 mol% of aluminium and 5 mol% of iron. mixed oxides. After thermal treatment of LDHs, mixed oxides are formed with homogeneous interdispersion of constituting ele-

2.2. Characterization

ments, larger surface area, developed porous structure, less diffu- sion resistance than LDHs and abundant acid and basic sites,

The X-ray diffraction measurements were performed in a Sie- which makes them more interesting as catalysts or catalyst precur-

mens D500 X-ray diffractometer (Cu K a radiation, =0.154 nm, sors [3,8] .

45 kV, 25 mA) in 2 range from 3° to 63°. Atomic absorption spec- The potential of these materials has also been studied in waste

troscopy, AAS using Hitachi Z-6100 instrument was used for the

elemental chemical analysis of constituent metals (Mg, Al and noble metal containing LHD derived mixed oxides have been re-

gas catalysis, namely in N 2 O abatement. Different transition and

Fe) in calcined samples. A semi-quantitative chemical analysis of ported to be active in N 2 O and NO x decomposition with different

calcined sample surface was performed by JEOL, JSM-460 LV combinations of divalent (Mg, Co, Ni, Mn, Cu, Ca, Zn) and trivalent

instrument equipped with energy dispersive spectroscopy, EDS, (Al, La, Rh) ions [9–14] . Zeolites, exchanged with transition metal,

Oxford Instruments INCA X-sight system operating at 25 kV.

TG/DTA thermal analysis of all synthesized samples was carried [15] . Iron exchanged zeolites exhibit also noticeable efficiencies in

especially with iron, showed high activity in decomposition of N 2 O

out in Baehr STA503 instrument from ambient temperature to

1000 °C with the heating rate of 5 °C min , in static air atmo- with various hydrocarbon reducing agents [16–18] . Therefore, in

the selective catalytic reduction of nitrous oxide with NH 3 and

sphere. The BET surface areas and pore radius distributions were our investigations iron was chosen as an active component to

determined by N 2

study its activity in N 2 O abetment reactions. Fe containing LDH de-

2010 instrument.

rived mixed oxides were reported to be catalytically active in var- Acidity of the calcined samples was determined by temperature ious reactions. Mg–Al–Fe and Mg–Fe mixed oxides were

programmed desorption (TPD) of ammonia in a Micromeritics successfully applied in Fischer–Tropsch synthesis [19] , in catalytic

AutoChem 2910 apparatus using 10 vol% NH 3 in He and ca. dehydrogenation of ethylbenzene [20] and in ethylbenzene dehy-

200 mg of calcined samples, flow rate of gas mixture of drogenation in presence of carbon dioxide [21,22] . In addition,

and heating rate of 10 °C min in temperature Mg–Al–Fe–LDHs themselves (without calcination to mixed oxides)

25 cm 3 min

range from 50° to 600 °C. The gases evolved during TPD measure- have been shown to be efficient catalyst for the reduction of 4-

ments were analyzed using Pfeiffer Vacuum QMS422 Mass Spec- nitrotoluene using phenylhydrazine or hydrazine hydrate as reduc-

trophotometer. Before TPD measurements the catalysts were ing agent [23] and in the reduction of aromatic nitro compound

preheated in a He gas flow (25 cm 3 min ) to 500 °C and kept at with hydrazine hydrate [24] .

that temperature for 1 h. Then the flow was switched to 10% vol For the synthesis of Mg–Al–Fe and Mg–Fe layered double

NH 3 in He (25 cm 3 min ) and the temperature was lowered to hydroxides two coprecipitation methods, low supersaturation, LS,

50 °C (20 °C min ). The catalysts were than purged from physi- and high supersaturation, HS, were chosen. The content of trivalent

sorbed ammonia at 50 °C in a He gas flow (25 cm 3 min ) for 1 h. ions was varied in a wide range between 0.15 < x < 0.7 with the

Mössbauer spectra were recorded with a KFKI spectrometer at intention to induce the formation of different LDHs and, after ther-

ambient temperature in constant acceleration mode. The values mal treatment, different multiphase mixed oxides. The objective

of isomer shift, IS, are given with respect to metallic a -iron. The

was to study the properties of different iron containing LDHs and estimated accuracy of positional parameters (IS, and quadrupole derived mixed oxides in correlation to their performance in two re-

splitting, QS) is c.a. ±0.03 mm s . Relative intensity, RI, in% was

calculated as relative contribution of the given component to the with ammonia.

dox processes: the decomposition of N 2 O, and the reduction of N 2 O

full area of the spectrum. Intensity per base line, I/BL, represents the total area of spectrum (sum of components) related to the base line. Mössbauer spectra were recorded for each sample in a se-

2. Materials and methods quence of three measurements: (i) as synthesized samples; (ii) samples calcined at 500 °C and (iii) after in situ treatment with

2.1. Synthesis

CO at 340 °C for 2 h.

Temperature programmed reduction (TPR) was conducted in a Two different coprecipitation methods, low supersaturation, LS,

Micromeritics AutoChem 2910 apparatus using ca. 200 mg of cal- and high supersaturation, HS, were chosen for the synthesis of

cined samples, flow rate of gas mixture (5% vol H 2 in N 2 ) of LDHs. In the HS method, the solution containing magnesium, alu-

and heating rate of 10 °C min in temperature range minium and/or iron nitrate salts was quickly added to the second

20 cm 3 min

from 25° to 1000 °C. Before the TPR measurements the samples

were preheated in a nitrogen gas flow (20 cm 3 min ) from ambi- Mg–Al–Fe containing solution was added drop wise at a constant

solution containing Na 2 CO 3 and NaOH. For the LS synthesis, the

ent temperature to 500 °C (30 °Cmin ) and then cooled to 50 °C. rate into 1 dm 3 of distilled water and the pH of the solution was maintained between 9.6 and 9.9 by the simultaneous addition of

2.3. Catalytic tests

the second solution containing Na 2 CO 3 and NaOH. In both cases,

the reaction solution was vigorously stirred, the samples were Catalytic properties were studied in a quartz fixed bed flow aged, washed and filtered, dried for 24 h, at 100 °C in air, and after-

reactor (ø = 8 mm, L = 19 cm). The experimental conditions were: wards calcined for 5 h, at 500 °C in air. A detailed explanation of

temperature from 300 to 500 °C, pressure 101 kPa, reactant con- synthesis methods and thermal activation is given elswere [25] .

centrations of N 2 O 1000 ppm (vol.) and of NH 3 1000 ppm (vol.), Two series of Mg–Al–Fe LDHs with different Mg/Al ratio, wide

:25 cm g ðcatÞ s range of trivalent ions between 0.15 < x < 0.7 and 5 mol% of iron

modified space velocity, GHSV mod from 2.17 to 6

(NTP).

was prepared. Besides that, Mg–Fe LDHs with higher iron content Before tests, all catalysts were first heated and held 2 h at (30 mol%) and without aluminum were also synthesized.

500 °C in a He stream. The measurements were performed at dif- Samples were denoted according to the synthesis method used

ferent reaction temperatures starting at 500 °C and lowering it (HS or LS) and the initial molar metal ratio. For example, LS-

stepwise by 25 °C. The temperature and the gas flows were

3 maintained constant until reaching steady state (waiting peri-

T.J. Vulic et al. / Chemical Engineering Journal xxx (2012) xxx–xxx

tions with corresponding mass losses typical for the LDHs, the first od > 1 h). The concentrations were measured at each reaction tem-

from the loss of physisorbed and interlayer water and the second perature after reaching steady state. The concentrations of gas

due to the loss of hydroxyl groups and interlayer anions. A third mixtures N 2 O, NH 3 and He, before and after reaction, were mea-

endothermic transition without significant mass loss is also ob- sured with nondispersive infrared spectroscope BINOS Leybold

served at temperatures higher that 700 °C indicating stoichiome- Heräus.

tric spinel phase and single magnesium-oxide phase formation [25] . Both Mg–Fe samples have lower temperatures of first two

3. Results and discussion endothermic transitional stages than corresponding Mg–Al–Fe samples, indicating lower thermal stability of Mg–Fe LDHs, and

3.1. Structural and elemental analysis also smaller mass losses, indicating smaller amount of physisorbed and interlayered water in Mg–Fe samples. Nevertheless, sample

It was reported in our previous work that all Mg–Al–Fe copre- HS-Mg70Fe30 has to some extent higher thermal stability than cipitation products have XRD patterns typical for LDH compounds,

sample LS-Mg70Fe30 since it has higher temperatures of both that HS samples have partially disordered structure, particularly in

transitional stages.

stacking of the layers and that extended M(III)ion substitution (x P 0.5) leads to the formation of additional aluminum hydroxide,

3.3. Textural analysis

Al(OH) 3 – Bayerite phase [25] . Also, after thermal treatment at 500 °C layered structure was destroyed and only the presence of

Nitrogen adsorption isotherms of all Mg–Al–Fe samples cal- Mg,M(III) mixed oxide phase was detected. The XRD patterns of

cined at 500 °C are presented in Fig. 2 . The isotherms of Mg–Fe the Mg–Fe as-synthesized samples are presented in Fig. 1 . Sharp

samples correspond to the isotherms of Mg–Al–Fe samples iso- and symmetric reflections from (0 0 3), (0 0 6), (1 1 0) and (1 1 3)

therms with same M(III) amount and the same synthesis method, planes were observed as well as broad, non-symmetric reflections

only the amount of nitrogen adsorbed is smaller in case of Mg– from (1 0 2), (1 0 5) and (1 0 8) planes. The lattice parameters were

Fe samples. In accordance to IUPAC classification [29] , all HS sam- calculated for a hexagonal unit cell on the basis of rhombohedral

ples have adsorption isotherms of the Type IV, with the hysteresis

loop and the plateau at high p/p o , typical for mesoporous oxide thickness of one layer constituted of one brucite-like sheet and

R–3 m symmetry. Basal spacing d 0 =d 003 was calculated as the

catalysts and supports. The samples HS-Mg85Al10Fe5, HS- one interlayer, cation–cation distance within the brucite-like layer

Mg70Al25Fe5 and HS-Mg70Fe30 have hysteresis loop Type H1 rep-

resentative of an adsorbent with a narrow distribution of relatively composition and lattice parameter of Mg–Fe–LDHs are given in Ta-

as a 0 110 and lattice parameter c 0 as c 0 003 . The phase

uniform cylindrical mesopores. The samples HS-Mg50Al45Fe5 and ble 1 and are in good agreement with results published by other

HS-Mg30Al65Fe5, have somewhat different shape of hysteresis authors [26–28] .

loop, belonging to Type H2 associated with a more complex pore Elemental chemical analysis of constituent metals of the Mg–

structure with narrow slit-shaped pores, probably due to the for- Al–Fe samples is also reported in our previous work [25] and the

mation of additional Bayerite phase in these samples. results of the Mg–Fe samples are given in Table 1 . The AAS analysis

All LS samples have adsorption isotherms of the Type II and hys- showed good agreement between initial amounts of Mg and Fe and

teresis loop H3 with no limiting adsorption at high relative pres- the bulk metal amount. Nevertheless, the surface-enhanced

sure and no well-defined mesopore structure, typical for micro information about metal composition obtained by the EDS analysis

and mesoporous materials with plate like aggregates and nonuni- revealed higher presence of iron on the surface of sample

form slit-shaped pores, such as clays. The sample LS-Mg30Al65Fe5 HS-Mg70Fe30 when compared with the sample LS-Mg70Fe30.

has similar but somewhat different shape of hysteresis loop, com- pared to other LS samples. It has the combination of H2 and H3 hysteresis loop which could be explained with the presence of

3.2. Thermal analysis the additional Bayerite phase similar as in case of multiphase HS samples.

A detailed thermal analysis of Mg–Al–Fe samples is already re- The most commonly used procedure for determination of ported [25] . The data from TG/DTA analysis of Mg–Fe samples in

mesopore size distribution is the BJH method, proposed by Barrett, comparison to the Mg–Al–Fe samples having the same M(III)ion

Joyner and Halenda, based on the notional emptying of the pores – amount is given in Table 2 . Samples have two endothermic transi-

desorption branch. The steep region of the desorption branch for the hysteresis loops Type H2 and H3 (obtained for the samples with extended M(III) substitution) is a feature depending on the nature of adsorptive rather then the distribution of pore size and also the reason not to take this curve region for the calculation of mesopore size distribution [29] . In these cases adsorption branch corresponds better to equilibrium and should be used for calcula- tions of pore size distribution [30] . The distribution based on the desorption data is indicative of the pore opening/mouth while the adsorption data provides information of the actual (interior) pore size. To enable the comparison of samples with different M(III) content and taking into account that the samples with ex- tended M(III) substitution have this type of hysteresis loop, the adsorption branch was chosen for the calculation of pore size dis- tribution using BJH method ( Figs. 3 and 4 ).

The BET surface areas of synthesized layer double hydroxides are between 70 and 100 m 2 g . The BET surface area values of cal- cined samples are given in Table 3 . The calcination results in in- crease of BET surface area caused by the formation of small

Fig. 1. XRD patterns of the Mg–Fe as-synthesized samples. venting holes (crater) at the crystal surface built during the ex-

4 T.J. Vulic et al. / Chemical Engineering Journal xxx (2012) xxx–xxx

Table 1 Precipitation products phase composition and lattice parameters, initial metal amounts and the metal amounts measured by AAS and EDS of Mg–Fe samples.

Sample XRD analysis

EDS amounts (mol%) Phase

Initial amounts (mol%)

AAS amounts (mol%)

Fe Mg Fe HS-Mg70Fe30

d 0 (nm)

a 0 (nm)

c 0 (nm)

Mg

Fe Mg

70 30 72.2 27.8 66.7 33.3 LS-Mg70Fe30

Table 2 Data from TG/DTA analysis: mass loss of the first m 1 , second m 2 and third transition stage m 3 , total mass loss m tot , temperatures of endothermic peaks corresponding to the first

T 1 , the second T 2 and to the third transition T 3 .

T 2 and T 2 0 (°C) T 3 (°C) HS-Mg70Fe30

T 1 (°C)

336, 379 >1000 HS-Mg70Al25Fe5

827 LS-Mg70Fe30

324, 373 >1000 LS-Mg70Al25Fe5

Fig. 2. Adsorption isotherms of Mg–Al–Fe samples after calcination.

Fig. 3. Pore size distribution of Mg–Al–Fe samples after calcinations.

haust of water and CO 2 [31] . The increase in M(III) ion content in- ores shifts from 30 nm to 6 nm. The changes in the pore size distri- creases in general the BET surface area evidenced also in pore size

bution could be related to the initial ordering of the LDH structure distribution.

prior to calcination. The XRD analysis showed that with the With the increase of M(III) ion content, the bimodal size distri-

increasing amount of aluminum in the LDH samples, the intensity bution changes simultaneously ( Figs. 3 and 4 ), the fraction of small

of characteristic XRD reflections decreases and diffraction lines mesopores (2–3 nm) increases and the fraction of bigger mesop-

broaden, suggesting a decrease in the ordering of the LDH struc-

T.J. Vulic et al. / Chemical Engineering Journal xxx (2012) xxx–xxx

Fig. 4. Pore size distribution of Mg–Al–Fe and Mg–Fe samples after calcinations having x = 0.3.

Table 3 The initial M(III) molar ratio, x, BET surface area of calcined samples, S, the temperatures of the first, T 1-TPR and second TPR peak maxima, T 2-TPR , the temperature of the first TPD peak maxima T 1-TPD and area of the first TPD peak A 1-TPD.

Sample

A 1-TPD (a.u. g ) HS-Mg70Fe30

x, –

S (m 2 g )

T 1-TPR (°C)

T 2-TPR (°C)

T 1-TPD (°C)

130 HS-Mg85Al10-Fe5

122 HS-Mg70Al25Fe5

229 HS-Mg50Al45Fe5

243 HS-Mg30Al65Fe5

292 LS-Mg70Fe30

146 LS-Mg85Al10-Fe5

101 LS-Mg70Al25Fe5

187 LS-Mg50Al45Fe5

239 LS-Mg30Al65Fe5

ture. The formation of additional Bayerite phase causes a decrease in the amount of carbonate bounded in LDH interlayer and conse- quently a decrease in surface area of the calcined samples. Lower surface area of HS samples with extended M(III)ion substitution (x P 0.5) when compared to corresponding LS samples, which have no Bayerite phase of smaller amount of it, also supports this state- ment. The increase of surface area with increasing amount of alu- minum in the LDH samples eventually leads to increased amounts of alumina after calcination, which contributes to the higher sur- face area compared to the Mg/M(III) mixed oxides.

The pore size distribution of Mg–Fe samples is similar to the pore size distribution of the Mg–Al–Fe samples with the same amount of M(III) ions samples ( Fig. 4 ). The Mg–Fe samples have smaller fraction of small mesopores (ca. 2.5 nm) and consequently lower surface area, which could be explained with the smaller amount of physisorbed and interlayer water in these samples de- tected by TG analysis ( Table 2 ).

3.4. Acidic properties Fig. 5. TPD profiles of LS-Mg–Al–Fe samples. Ammonia TPD profile of all LS-Mg–Al–Fe samples are presented in Fig. 5 . All other samples have similar TPD profiles. Simulta- neously with TPD measurement the evolved gases were analyzed by mass spectrometry, MS. The MS analysis showed that the first

the first peak maxima and its area. The amount of iron in samples peak with temperatures between 120 and 140 °C corresponds to

decreases acidity compared to samples with aluminum. The in- the amount of ammonia desorbed and that the second one at tem-

crease of M(III) content increases also the acidity. All samples have peratures around 590 °C corresponds to the amount of water

acid sites of similar strength, although the strength of acid sites in desorbed.

LS-Mg–Al–Fe samples is slightly higher than in HS-Mg–Al–Fe sam- The conclusions about strength and the amount of acid sites,

ples. According to reported findings [32,33] LDHs have Lewis acid presented in Table 3 , were taken considering the temperature of

sites with medium–high acidic strength.

6 T.J. Vulic et al. / Chemical Engineering Journal xxx (2012) xxx–xxx

3.5. Mössbauer spectroscopy analysis The Mössbauer spectra of the HS and LS samples are shown in

the as synthesized, calcined and CO treated sequence in Figs. 6 and 7 , the data obtained from the corresponding evaluations are presented in Tables 4–6 .

The as synthesized samples exhibit asymmetric Fe 3+ doublet. This asymmetry in the doublet of the as synthesized materials has been reported for other LDHs as well. The asymmetry can be correlated either with relaxation effects emerging between distant and separated Fe 3+ ions at lower iron content [34] , or by distribu- tion of quadrupole splitting at higher iron contents, as suggested in [26,35] . The presence of asymmetric doublet is prevalent in the x = 0.3 samples, thus fitting with regard to relaxation phenomena was applied for them. For samples of larger extent of substitution (x = 0.5) existence and separation of two different iron environ- ments are assumed, in correspondence with results of previous studies where Fe 3+ components with a smaller and a larger qudru- pole splittings are distinguished [35–37] . These two Fe 3+ compo- nents are shown as Fe 3+ (1) and Fe 3+ (2) in the corresponding tables. An additional parameter, I/BL is also displayed. This param-

Fig. 7. Mössbauer spectra of samples LS-Mg70Fe30 (right), LS-Mg70Al25Fe5 eter is the relative intensity of the total spectrum related to the

(middle) and LS-Mg50Al45Fe5 (left) recorded: (a) as synthesized; (b) calcined at base line (spectral area in count numbers related to that of the base

500 °C and (c) treated with CO at 340 °C Decomposition of spectra containing Fe 2+ line), it carries information on the average value of the probability

components is also displayed in row (c) for illustration. of the Mössbauer effect and is strongly correlated to the bonding

strength of the iron species present in the sample [38] . After calcination at 500 °C, profound changes in structure are

Thus, e.g. spinel structure may already appear in the local environ- observed: iron ions became more strongly bonded, as the signifi-

ment of iron at the 500 °C treatment, however the transformation cant increase of the I/BL parameter attests. The ca. 50% increase

resulting in the formation of separate extended spinel phase pro- is significant and it clearly demonstrates that upon calcination

ceeds only at 800 °C as TG and XRD studies revealed. the open, layered structure transforms to a close, three dimen-

The third stage, the in situ treatment with CO at 340 °C, was sional matrix. LDHs with similar compositions may be transformed

intended originally to demonstrate the existence of Fe 3+ – to fluorite-type periclase or spinel oxides, with corresponding

O–Fe 3+ –O–Fe 3+ chains, with the assumption that reduction of iron Mössbauer parameters different from those of LDHs [20,34] . Our

with CO may proceed via extraction of oxygen from the chain with spectra were decomposed to three components Fe 3+ (3), Fe 3+ (4)

simultaneous reduction of two neighboring Fe 3+ to Fe 2+ . This and Fe 3+ (5). The first of them can probably be assigned to the com-

assumption seems to be proven in different extents. This kind of ponent of periclase structure [20] , the second and third can be

reduction is less expressed in the samples of low M(III) substitu- attributed to the different sites in the spinel matrix [20,24,26] . It

tion (x = 0.3), it reaches only 10% for the HS-Mg70–Al25–Fe5 and should be emphasized here that the method of Mössbauer spec-

23% for the LS-Mg70–Al25–Fe5 sample. The increase in aluminum troscopy provides local information from the very vicinity of iron.

amount increases also the extent of reduction, as the results from x = 0.5 samples measurements show. The Fe–O–Fe–O chains are probably originated from the presence of a minor ferrihydrite phase formed already at the synthesis of LDHs. This ferrihydrite may probably be incorporated into the bayerite phase detected by XRD. Its assignment by the Mössbauer method is less successful, since its parameters are similar to those of Fe 3+ (1). Further, the brownish color of the samples is a convincing indication for the presence of an amorphous ferric oxihydroxide. The same experi- ences were collected and the assumptions proven by low temper-

[24,36] . The effect of the CO treatment is not restricted only to the assumed ferrihy- drite-bayerite phase, the spinel oxide is also modified. For instance, the Fe 3+ (5) component fully disappears (is reduced to Fe 2+ ), and the quadrupole splittings of the Fe 3+ (3) and Fe 3+ (4) components are noticeably reduced.

3.6. Redox properties The TPR measurements of all iron containing materials have

two characteristic peaks at temperatures lower than 900 °C ( Fig. 8 , e.g. HS samples). For all of the samples, TPR signal did not reach the base line until 1000 °C and presumably the complete

Fig. 6. Mössbauer spectra of samples HS-Mg70Fe30 (right), HS-Mg70Al25Fe5 reduction of iron was not achieved. The hydrogen consumption

(middle) and HS-Mg50Al45Fe5 (left) recorded: (a) as synthesized; (b) calcined at was represented per mol iron to enable the comparison of samples 500 °C and (c) treated with CO at 340 °C. Decomposition of spectra containing Fe 2+

with different iron content. All HS and LS samples with the same components is also displayed in row (c) for illustration.

initial chemical composition have almost the same TPR profiles

7 Table 4

T.J. Vulic et al. / Chemical Engineering Journal xxx (2012) xxx–xxx

Mössbauer parameters and assignments of iron species recorded for samples HS-Mg70Al25Fe5 and LS-Mg70Al25Fe5 as synthesized; calcined at 500 °C and after treatment with CO at 340 °C.

Sample HS-Mg70Al25Fe5

LS-Mg70Al25Fe5

RI (%) I/BL (a.u.) LDH

Fe species IS (mm s )

QS (mm s )

RI (%)

I/BL (a.u.)

IS (mm s )

QS (mm s

Fe 3+ (sp) a 0.34 0.58 50 0.34 0.67 50

8.17 Calcined at 500 °C Fe 3+ 3+ (3)

13.86 Treated with CO at 340 °C

Fe 3+ 0.32 1.12 55 0.31 1.10 53 Fe 3+ Fe 2+

a Special fitting: the doublet is fitted assuming the same intensity but the same line width is not imposed (as otherwise is the common case at all the other fits).

Table 5 Mössbauer parameters and assignments of iron species recorded for samples HS-Mg50Al45Fe5 and LS-Mg50Al45Fe5 as synthesized; calcined at 500 °C and after treatment with CO at 340 °C.

Sample HS-Mg50Al45Fe5

LS-Mg50Al45Fe5

RI (%) I/BL (a.u.) LDH

Fe species IS (mm s )

QS (mm s )

RI (%)

I/BL (a.u.)

IS (mm s )

QS (mm s )

Fe 3+ 3+ (1) 0.27 0.68 46 0.34 0.61 58 Fe Fe 3+

8.23 Calcined at 500 °C Fe 3+ (3)

11.40 Treated with CO at 340 °C

Fe 3+ 3+ 0.36 1.19 21 0.35 1.22 30 Fe 2+

Table 6 Mössbauer parameters and assignments of iron species recorded for samples HS- Mg70Fe30 and LS- Mg70Fe30 as synthesized; calcined at 500 °C and after treatment with CO at 340 o C.

Sample HS-Mg70Fe30

LS-Mg70Fe30

RI (%) I/BL (a.u.) LDH

Fe species IS (mm s )

QS (mm s )

RI (%)

I/BL (a.u.)

IS (mm s )

QS (mm s )

Fe 3+ (sp.) a 0.34 0.54 50 0.35 0.56 50

7.39 Calcined at 500 °C

11.15 Treated with CO at 340 °C Fe 3+ 3+

a Special fitting: the doublet is fitted assuming the same intensity but the same line width is not imposed (as otherwise is the common case at all the other fits).

8 T.J. Vulic et al. / Chemical Engineering Journal xxx (2012) xxx–xxx

mation of solid solutions after calcination and in case of samples with additional Bayerite phase, evidenced by both Mössbauer and TPR analysis, extra LDH framework aluminum cations support the reduction of iron. An increase in temperature of the first TPR peak is followed by a decrease of the second reduction peak tem-

perature (corresponding to the reduction of Fe 2+ Fe 0 and the rest of Fe 3+ Fe 2+ ). This behavior favors the reduction of the first stage (Fe 3+ Fe 2+ ). Since the limit for the incorporation of M(III) ions into the LDH matrix is around x = 0.5, the samples with this M(III) ion content have the weakest interaction among different Mg– Al–Fe–O components and therefore the lowest temperature of the first TPR maxima, compared to other Mg–Al–Fe containing samples. On the contrary, the samples with x = 0.3 present the samples with single LDH phase, the most intensive XRD peaks, the strongest interaction among different Mg–Al–Fe–O compo- nents and in return the highest temperature of the first TPR maxima.

3.7. Catalytic tests The nitrous oxide molecule is quite stable at room temperature

and its simplest form of catalytic decomposition can be described as an adsorption of N 2 O at the active center followed by a decom- position giving N 2 (4) ). This surface oxygen desorbs in combination with another oxygen atom or in di- rect reaction with another N 2 O (Eqs. (5) and (6)) [15] :

Fig. 8. TPR profiles of HS samples with different Mg:Al:Fe content.

N 2 2 ð4Þ

ð5Þ with small differences in temperatures of both peak maxima listed

þO

in Table 3 . The Mg–Fe samples with 30% of iron have two separated peaks,

ð6Þ first sharp and symmetrical and second broad and nonsymmetri-

2O $O 2 þ2

The experimental results of catalytic N 2 O decomposition are cal. This type of reduction behavior was already reported for Mg–

presented in Fig. 9 . Low iron containing samples (5%), independent Fe–LDH derived mixed oxides with different iron amount between

on Mg/Al–ratio, have very low N 2 O conversion. The increase of iron

10 and 50% [19] and with 50% of iron [39] concluding that the pres- content (30%) improves conversion and confirms the crucial role of ence of Mg 2+ cations retards the reduction of iron and suggesting

iron in the catalytic act, proved also with a very small conversion in that the two reduction peaks in TPR profiles correspond to the

case of iron free sample (at 500 °C <0.04). This is in correspondence sequentional reduction of iron species from Fe 3+ to Fe 2+ in first

with reduction behavior, since the decomposition of N

2 O is based

Fe [15] . The influence of Mg/Al ratio on equations:

stage and from Fe 2+ to Fe 0 in second, presented with following

on the redox cycle Fe

N 2 O decomposition for the samples with 5% iron cannot be estab-

MgFe 2 III O 4 þH 2 ¼ Mg Fe x II OþH 2 O

lished because no significant difference in the range of measure- ment accuracy was observed. These samples all have very low

Mg Fe x II OþH 2 ¼ a 2 O

conversion probably because, according to TPR results, their reduc- tion cycle starts at temperatures above 450 °C. Better performance

On the contrary, the TPR profiles of the Mg–Al–Fe–LDH derived of LS-Mg70Fe30 sample compared to HS-Mg70Fe30 sample can be mixed oxides with 5% of iron have broad and overlapping peaks

explained with their reduction behavior. Both samples have the shifted to higher temperatures showing that the second reduction

same temperature of the peak maxima in the first reduction stage stage begins before the first stage is completed. This indicates stron-

(Fe 3+ Fe 2+ ) but the temperature of the second reduction stage is for ger interaction among different Mg–Al–Fe–O components of mixed

the sample LS-Mg70Fe30 about 110 °C higher enabling better com- oxide. Similar reduction behavior was reported for Mg–Al–Fe–LDH

pletion of the first reduction stage responsible for the catalytic derived mixed oxides (with 50% of Mg and Fe content varied be-

reaction. The Mössbauer analysis confirmed it also showing that tween 12.5% and 50%) [39] explaining the first stage as the partial

after calcination sample LS-Mg70Fe30 has higher amount (40%) reduction of iron species from Fe

to Fe 2+ :

of the most easily reduced Fe 3+

3+ (5) component, than

MgFe III AlO 4 þH 2 ¼ Mg Fe II OþH 2 O þ MgðAlÞO

HS-Mg70Fe30 sample (29%).

The experimental results of the catalytic N 2 O reduction with and the second stage as the reduction from the rest of Fe species

NH 3 are presented in Fig. 10 . The other reaction path with reducing to Fe , proceeding according to Eq. (3) , and the reduction of Fe species to Fe 0

agent has overall higher activity, since the presence of reducing , proceeding according to Eq. (2) .

agent e.g. ammonia, boosts the removal of surface oxygen O⁄ The presence of aluminum intensifies interactions among dif-

[16] (Eq. (7) ), which is the reaction rate determining step in cata- ferent Mg–Al–Fe–O species, since it retards the reduction of iron.

lytic decomposition of N 2 O [15] .

This also agrees with the assumption derived from the thermal analysis that the presence of aluminum stabilizes layered structure

ð7Þ of LDHs. It has also been reported that the presence of aluminum

2NH 3 þ 3O $N 2 þ 3H 2 Oþ3

In this reaction path, the crucial role of iron is also confirmed, cations inside LDH framework retards the reduction of iron [39] ,

but the activity of samples with lower iron content is generally but the reduction of complex mixed oxides is affected by the for-

increased being dependent on the Mg/Al-ratio among the series.

T.J. Vulic et al. / Chemical Engineering Journal xxx (2012) xxx–xxx

Fig. 9. N 2 O decomposition at GHSV mod ¼2 :17 cm 3 NTP g cat

with all Mg–Al–Fe and Mg–Fe catalysts.

Fig. 10. N 2 O reduction with NH 3 at GHSV mod ¼2 :17 cm NTP 3 g cat

with all Mg–Al–Fe and Mg–Fe catalysts.

Differentiation of lower iron content samples could be made in cor-

4. Conclusions

respondence with reduction behavior supplemented with Mössbauer analysis. Other sample properties such as BET surface,

Mg–Al–Fe and Mg–Fe layered double hydroxides were success- acid properties and thermal behavior, do not influence the catalytic

fully synthesized by two different coprecipitation methods and in a behavior in both reactions significantly. The order of the activity

wide range of M(III) ions. The extended M(III) substitution influ- within the Mg–Al–Fe series follows the order obtained with TPR

ences the structure and surface properties of Mg–Al–Fe–LDHs analysis of the first reduction stage.

and their derived mixed oxides, weakens Mg-Al-Fe-O interactions Better performance between the samples with the same chem-

and improves catalytic behavior. Thermally activated iron- ical composition, synthesized with different methods, having the

containing LDHs exhibited catalytic activity in N 2 O decomposition same temperature of the first TRP maxima was already mentioned

and reduction with NH 3 . This feature is mainly influenced by

reduction behavior and can be correlated with the presence of tion reaction and explained with the strength of Mg–Fe–O interac-

for LS-Mg70Fe30 and HS-Mg70Fe30 samples in N 2 O decomposi-

Fe–O–Fe–O–Fe entities providing possibility for facilitated extrac- tions. This can also be applied within the Mg–Al–Fe–LDH derived

tion of oxygen with simultaneous redox Fe 3+ Fe 2+ conversion. mixed oxide series taking into account the strength of Mg–

The best results are obtained when the amount M(III) substitution Al–Fe–O interactions. In case of HS-Mg50Al45Fe5 and LS-

is near the limit for the incorporation into the LDH matrix (x = 0.5) Mg50Al45Fe5 samples having the same temperature of the first

and the methastabile mixed oxides are formed with the weakest TRP maxima, catalytically better performing sample, HS-

interactions between constituent metals and oxygen. The presence Mg50Al45Fe5, has higher temperature of the second reduction

of small amount of additional Bayerite phase, e.g. extra-LDH- stage and, as Mössbauer analysis showed, higher amount of Fe 3+

framework aluminum (and iron), in x = 0.5 samples, positively (5) component (29% compared to 23% in LS-Mg50Al45Fe5)

influences catalytic behavior, but higher concentrations of

responsible for the Fe 3+ Fe 2+

redox cycle.

extra-LDH-framework aluminum (x = 0.7) lead to the significant

10 T.J. Vulic et al. / Chemical Engineering Journal xxx (2012) xxx–xxx

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