supporting information for · s7 xrd results 20 30 40 50 60 70 ca 500 oc - 6h lafeo 3 (no. 74-2203)...

S1

Supporting Information for

Design of nanocrystalline mixed oxides with improved oxygen

mobility: a simple non-aqueous route to nano-LaFeO3 and the

consequences on the catalytic oxidation performances

Wei Yang,1 Runduo Zhang,*

,1 Biaohua Chen,

1 Nicolas Bion,

2 Daniel Duprez,

2 Liwei

Hou,2,3

Hui Zhang3 and Sébastien Royer*

,2

1 State Key Laboratory of Chemical Resource Engineering, Beijing University of

Chemical Technology, Beijing, 100029, China 2 Universite de Poitiers, UMR 7285 CNRS, IC2MP, 4 Rue Michel Brunet, Poitiers,

86022 Poitiers Cedex, France 3 Department of Environmental Engineering, Hubei Biomass-Resource Chemistry and

Environmental Biotechnology Key Laboratory, Wuhan University, Wuhan 430079,

China

Corresponding Author:

Runduo Zhang; Email: [email protected]; Phone: +86(0)10-64412054; Fax:

+86(0)10-64419619

Sébastien Royer; Email: [email protected]; Phone: +33(0)549453479;

Fax: +33(0)549453499

TABLE OF CONTENT PAGE

Materials S2

Catalyst characterizations and activity tests S3

Reference material of BA-BaTiO3 S5

N2 physisorption S6

XRD results S7

XPS results S8

H2-TPR results S9-10

TPOIE S11

Activity tests in CO or CH4 oxidation S12-13

References S14

Electronic Supplementary Material (ESI) for Chemical CommunicationsThis journal is © The Royal Society of Chemistry 2013

mailto:[email protected]

mailto:[email protected]

S2

Materials

Citric acid route LaFeO3:

Stoichiometric amounts of La(NO3)3·6H2O and Fe(NO3)3·9H2O were dissolved in

the distilled water. Thereafter, an aqueous solution of citric acid (CA), containing a

number of mol of CA equal to the number of ionic equivalents of cations, was slowly

added to the precursor solution under gentle stirring. Solvent evaporation was

performed at 80 oC until the formation of a gel. Afterwards the gel was dried in an

oven at 110 oC during 24 h. Subsequently, the foam solid was crushed and calcined at

500-800 oC for 6 h (ramp rate = 5

oC min

-1).

[S1]

Benzyl alcohol route BaTiO3:

5 mmol of metallic Ba was first dissolved in a vial containing 60 mL of anhydrous

benzyl alcohol at 80 oC. Next, 1 mol equivalent of Ti isopropoxide was added

dropwise to the solution. The reaction mixture was stirred for another few minutes

and then transferred into the autoclave with a Teflon cup of 100 mL, followed by a

thermal treatment at 200 oC for 48 h in oven. Afterwards, the resulting milky

suspension was centrifuged, and the obtained precipitate was thoroughly washed with

ethanol and then dried in air at 80 °C overnight. The operations before the

solvothermal reaction were conducted in a glovebox, preventing the oxidation of Ba

by air.[S2]

Benzyl alcohol route LaFeO3:

5 mmol of La(NO3)3·6H2O was first dissolved in a vial with 60 mL of benzyl

alcohol at 80 oC. Subsequently, 1 mol equivalent of iron acetylacetonate Fe(C5H7O2)3

was added into the solution. The reaction mixture was stirred for a few minutes and

then transferred into the autoclave with a Teflon cup of 100 mL. The autoclave was

heated at 200 °C for 24 h. The resulting brown suspension was centrifuged, and the

precipitate was thoroughly washed with ethanol and thereafter dried in air at 80 °C

overnight. Due to an amorphous phase of the as-prepared solid (detected by XRD, not

shown), further calcination was necessary. Subsequently, the powder was calcined at

450 oC for 12 h or 500-800

oC for 6 h (ramp rate = 5

oC min

-1).

According to the synthesis problems [(1) the distinct difficulties for the formation

of the Ba(Sr)TiO3 and LaFeO3 perovskites, due to the nature of the constituting

elements; (2) the diverse properties of the metallic precursors for each synthesis, e.g.

solubility, reactivity in benzyl alcohol], a series of different chemicals for La source

{La(NO3)3·6H2O, La[OCH(CH3)2]3, LaCl3·7H2O, metallic La} and Fe source

[Fe(NO3)3·9H2O, Fe(C5H7O2)3, Fe(C2H3O2)2, FeCl3·6H2O, metallic Fe] were used for

this solvothermal reaction, under different solvothermal reaction conditions (at 160,

180, 200 oC; for 12, 24, 36, 48 h). However, few of the attempts allowed obtaining the

perovskite structure, and a pure LaFeO3 was only achieved in the process mentioned

above.

All the chemicals used are ordered from Sigma-Aldrich.


S3

Catalyst characterizations and activity tests

X-ray diffraction (XRD) was carried out using X-Ray diffractometer equipped with

a CuKα radiation (λ = 0.15406 nm) (D8FOCUS, Bruker). Diffractograms were

collected in the 2θ range between 20 and 80o by step of 0.05

o (step time = 5 s). Phase

identification was made by comparison with JCPDS database while the crystallite

sizes were calculated using the Scherrer equation after Warren’s correction for the

instrumental broadening.

A Sorptomatic 1990 instrument (Thermo Electron) was used to measure the

specific surface area (SBET) of the samples at liquid N2 temperature (-196 oC), using

the Brunauer-Emmett-Teller (BET) method, with outgas pretreatment at 200 oC under

vacuum.

Transmission electron microscopy (TEM) investigations were performed on a

JEOL J-2100 instrument (operated at 200 kV with a LaB6 source and equipped with a

Gatan UltraScan camera).

A Supra 55 (Carl Zeiss) scanning electron microscope (SEM) was used to

characterize the surface morphology of those LaFeO3 (the accelerating voltage

applied was 20 kV). Cation homogeneity and composition of the crystallized particles

were evaluated by the coupled energy dispersive X-ray (EDX) spectroscopy.

X-ray photoelectron spectroscopy (XPS) was conducted on a Thermo-Fisher

ESCALAB 250 system with AlKα radiation under ultrahigh vacuum (UHV).

Prior to H2-TPR experiment, each material (90 mg) was pretreated at 600 oC under

20 % O2 in He in a flow rate of 20 mL min-1

for 30 min. After cooling down to 30 oC,

a He flow containing 5 vol.% H2 with a flow rate of 20 mL min-1

was stabilized, and

then the temperature of the reactor increased from 30 oC to 800

oC with a ramp of 5

oC min

-1. Evolution of the H2 concentration along with temperature was on-line

recorded by TCD, and a water trap was used before the TCD. The quantification of H2

consumed was performed after the calibration of TCD.

Temperature-programmed oxygen isotopic exchange (TPOIE):

Theory: oxygen isotopic exchange (OIE) technique was used to evaluate oxygen

mobility. Theory and data treatment are described in the references[S3-7]

and are

shortly summarized herein. The exchange reaction can be summarized as described in

Eq. S1: 18

O2 (g) + 2 16

O (s) → 2 18

O (s) + 16

O2 (g) (S1)

where (s) and (g) refer respectively to the solid and the gas phase. Nevertheless,

different mechanisms are observed depending on the oxides studied. For LaFeO3, the

simple heteroexchange mechanism (only one oxygen atom from the solid exchanged

at each step, as described in Eqs. S2 and S3) has been observed for this material in our

previous study.[S8]

18

O18

O (g) + 16

O (s) ↔ 18

O16

O (g) + 18

O (s) (S2) 18

O16

O (g) + 16

O (s) ↔ 16

O16

O (g) + 18

O (s) (S3)

Oxygen mobility for each LaFeO3 material was determined using TPOIE.


S4

Experimental setup and analysis conditions: To prevent kinetic limitation due to

gas-phase diffusion, experiments were carried out in a recycling U-shaped

microreactor. The recirculation volume (V = 60 cm3; recirculation rate = 170 cm

3 s

-1)

was coupled to a quadrupolar mass spectrometer (Pfeiffer Vacuum). The gas sampling

was regulated by a thermo-valve adjusted to maintain a constant pressure of 1 × 10-6

mbar in the ionization chamber of the mass spectrometer. A constant mass of catalyst

(0.020 g) was introduced in the reactor between two quartz wool plugs. The sample

was heated up to its calcination temperature under O2 flow (ramp = 10 °C min-1

; total

flow rate = 20 mL min-1

) and cooled down to 200 oC. After temperature stabilization,

the sample was evacuated under dynamic vacuum for 30 min. Next, 52.0 ± 1.5 mbar

of pure 18

O2 was introduced into the recirculation volume and the sample was heated

up to 600 °C with a ramp of 2 °C min-1

. The evolutions of 18

O2 (P36), 16

O2 (P32), 16

O18

O (P34) partial pressures were recorded on a mass spectrometer during the

heating process. N2 (mass 28) was also recorded to detect any possible leak.

Activity tests

CO or CH4 oxidation reactions were performed in a fixed-bed type reactor, using a

reaction flow of 100 mL min-1

(GHSV of 60 000 h-1

), composed of 1% CO or CH4,

10% O2, and balanced with He, over 100 mg of each sample. Reactant and product

quantifications were conducted using a gas chromatograph from VARIAN (model

CP-3800) equipped with a TCD and a Porapak column for separation.


S5

Reference material of BA-BaTiO3

20 30 40 50 60 70

Inte

nsi

ty /

a.u

.

2 / o

BA - BaTiO3

Figure S1. XRD pattern for BA-BaTiO3.

Figure S2. Representative TEM picture for BA-BaTiO3.

As shown in Figure S1, pure perovskite XRD pattern is recorded, with the main

peak position at 31.5o. The mean crystal size was calculated to be 6.2 nm (applying

the Scherrer equation). These results are in agreement with those reported in

reference.[S2]

The TEM observation also evidenced the formation of BaTiO3

nanoparticles in quasi-spherical shape, with diameters ranging from 5-8 nm, which is

a little larger than the value reported in reference[S2]

(4-5 nm). The results above

confirm the practicability of the benzyl alcohol mediated procedure for the

crystallization of mixed-oxides, which is the premise to prepare LaFeO3 in this

procedure.


S6

N2-physisorption results

0

100

200

300

400

500

600

0 0.2 0.4 0.6 0.8 1

Vo

lum

e ad

sorb

ed /

cm

3g

-1,

ST

P

Relative pressure, P/P0 / -

CA 500 oC

BA 500 oC

Figure S3. N2-physisorption isotherms recorded over BA-500 and CA-500 materials.


S7

XRD results

20 30 40 50 60 70

CA 500 oC - 6h

LaFeO3 (no. 74-2203)

2 / o

Inte

nsity

/ a

.u.

BA 800 oC - 6h

BA 700 oC - 6h

BA 600 oC - 6h

BA 500 oC - 6h

BA 450 oC - 12h

Figure S4. XRD diffractogramms of LaFeO3 samples prepared through BA at different calcination temperatures, as

well as LaFeO3 obtained from CA at 500 oC. Reference X-ray lines belonging to LaFeO3 (JCPDS card no. 74-2203)

are given at the bottom of the figure.

Normally, higher thermal treatment temperatures were used to ensure the complete

crystallization of the perovskite involving the citric acid procedure (600°C and

higher[S8,S9]

). However, such a high temperature treatment is accompanied with lower

SSA, larger D and consequently lower catalytic activities. It has been proved that

moderate thermal treatment (550 oC) can also bring pure LaFeO3 perovskite structure,

leading to more adequate physicochemical properties and consequently higher

catalytic activity.[S10]

Actually, the CA samples calcined at higher temperatures (600,

700 and 800 oC) were also analyzed by XRD, showing pure perovskite structures (not

shown). For BA samples, complete perovskite structure was attained at temperature as

low as 450 oC (Figure S4, BA 450

oC-12 h), while La or Fe simple oxides could not

disappear completely at this temperature though CA procedure (not shown). The

reflections intensities increased gradually along with calcination temperature,

implying the increase of crystal size. This observation fits well with the corresponding

evolutions of the values obtained from BET (SSA) and XRD (D), as presented in

Figure 1.


S8

XPS results

Table S1. The XPS results for CA and BA LaFeO3 calcined at different temperatures.

Samples Surface atom concentration from XPS / %

La Fe O

CA 500 oC 15.91 6.70 77.41

CA 600 oC 14.45 6.32 79.23

CA 700 oC 15.03 6.11 78.86

BA 450 oC 13.24 8.06 78.72

BA 500 oC 13.88 8.96 77.15

BA 600 oC 13.58 8.26 78.20

BA 700 oC 15.05 8.58 76.35

The XPS results for CA and BA samples obtained at each temperature exhibit an

obvious difference in surface atomic concentration of the active Fe cation, as shown

in Table S1. The highest surface Fe concentration among CA or BA samples was both

achieved at 500 oC, with the values of 6.70 and 8.96, respectively. Clearly, BA route

resulted in the surface enrichment by Fe atom, which is apparently favorable to the

catalytic properties. Roughly, the concentration of Fe decreased slightly along with

the calcination temperature increase among the samples prepared through the same

route, which may be attributed to the increase of D and the progressive decrease of

surface abundance in Fe-rich crystallographic planes.


S9

H2-TPR results

100 200 300 400 500 600 700 800

Bulk Fe3+

reduction

Surface/subsurface

Fe3+

reduction

Bulk Fe3+

reduction

CA 500 oC

TCD

Sig

nal

Temperature / oC

BA 500 oC

Surface/subsurface

Fe3+

reduction

Figure S5. H2-TPR profiles obtained for BA 500 oC and CA 500

oC LaFeO3.

Table S2. H2-TPR results for BA 500 oC and CA 500

oC LaFeO3.

LaFeO3

Theoretical H2

consumption[a]

/ mmol g-1

Experimental H2

consumption / mmol g-1

Reduction proportion

of Fe3+

→Fe2+[d]

/ %

Fe3+

→Fe2+

Fe2+

→Fe0 Total

[b] surface

[c] bulk

[c] total surface bulk

CA-500oC

2.06 4.12 0.789 0.322 0.467 38.3 15.6 22.7

BA-500oC 1.389 0.433 0.956 67.4 21.0 46.4

[a] calculated values of H2 to be consumed for the complete reduction of the different cations (reduction reactions

depicted in the Table); [b] total molar amount of H2 consumed per gram of materials, issued from H2-TPR experiment;

[c] the H2 consumption of surface/subsurface or bulk Fe3+

reduction, as presented in Figure S5; [d] reduction

proportion calculated by comparing each experimental H2 consumptions with Fe3+

→ Fe2+

theoretical H2 consumption.

In order to investigate the influence of the morphology on the sample properties,

H2-TPR experiment was used to evaluate their redox abilities. The H2 consumption

profiles are presented in Figure S5, while the calculated results are gathered in Table

S2. In the case of CA-500 oC LaFeO3, the Fe

3+ reduction started around 270

oC.

Subsequently, a broad peak centered at 415 oC was obtained, followed by a reduction

platform above 510 oC. Considering that the total experimental H2 consumption is

much lower than the theoretical H2 consumed for the total Fe3+→Fe

2+ reduction

(0.789 vs. 2.06 mmol g-1

, Table S2), the Fe3+

reduction below 800 oC can be safely

assigned to partial Fe3+

→ Fe2+

, and only 38.3 % of Fe3+

participated in this reduction

step. For the BA-500 oC sample, the reduction starting temperature shifted slightly to

250 oC, and two intense reduction peaks centered at 380

oC and 570

oC are observed.

Comparing with CA material, BA route leads to a strongly increased total H2

consumption (1.389 vs. 0.789 mmol g-1

). However, metallic Fe was still not available,

due to the limited reducibility of Fe3+

itself (over BA-500°C, 67.4 % of Fe3+

is found

to be reduced into Fe2+

).[S10]

Furthermore, the two reduction peaks can be accordingly


S10

assigned to the surface/subsurface and bulk Fe3+

reductions, corresponding to 21.0 %

and 46.4 % of Fe3+

reduced, respectively. Such an assignment is also applied to the

former sample, with a demarcation temperature of 510 oC. Nevertheless, both of the

reductions decreased largely in contrast to BA LaFeO3. The redox capacity of LaFeO3

was obviously promoted through BA route, and more Fe3+

can be easily reduced over

BA-LaFeO3, no matter located in the surface/subsurface or in the bulk. Such an

improvement can be attributed to the excellent physicochemical properties – much

higher SSA, lower D as well as the suppression of agglomeration – in the final

BA-material. These led to a significant increment in the proportion of Fe3+

located in

surface/subsurface, while bulk Fe3+

reactivity also profited to the increased surface

site number and decreased crystal size.


S11

TPOIE

Oxygen mobility has been considered as a crucial factor for all oxygen involving

reactions, especially in oxidation reactions such as CO or CH4 combustion. Being a

conventional evaluation, O2-TPD has been proved to be not appropriate to measure

oxygen mobility or surface/bulk reactivity. Thus, oxygen isotopic exchange (OIE) has

been developed as a kinetic measure of the oxygen mobility on the mixed oxide

surface or/and in the bulk. This technique was applied to LaFe- and LaCo-based

perovskites in our previous studies, correlating successfully to their different CO and

CH4 oxidation activities, respectively.[S6,S10]

Obviously, the oxygen activation temperatures for both LaFeO3 were similar, at

around 260 oC, which is close to the reduction starting temperatures in H2-TPR

experiments (270 oC for CA, 250

oC for BA, Figure S5), illustrating that the redox

capacity of Fe3+

is very important to the oxygen activation. However, it is noted that

the redox ability was not the exclusive factor for oxygen mobility. Indeed, much

lower oxygen activation temperature was achieved in Cu2+

substituted LaFeO3

structure, but Pd2+

incorporation led to the worst oxygen activation temperature which

is completely different from the redox capacity order: Pd2+

> Cu2+

> Fe3+

.[S10]

Below

330 oC, the two evolutions of Ne were essentially overlapped, showing similar oxygen

mobility for the first 25% of oxygen atoms over these two LaFeO3, which could be

assigned to the surface/subsurface oxygen mobility. Subsequently, important

difference was observed, more oxygen atoms were available in the medium

temperature zone over BA sample.

It is also observed that in the second exchange step at higher temperature, the Re

values for CA sample were much higher than those for BA one, which was owing to

the insufficiency of the original (not already exchanged) bulk oxygen in BA material

(25% for BA vs. 59% for CA), rather than to its lower bulk oxygen exchange rate.

Besides, the total numbers of oxygen atoms exchanged at 600 oC were very close for

the both LaFeO3, implying that almost all the oxygen atoms participated in the

exchange reaction until this temperature, although scarce oxygen desorptions can be

detected in O2-TPD over LaFeO3 structure.[S10]

In conclusion, the excellent physicochemical properties of BA material obviously

led to a huge enhancement in oxygen mobility, more oxygen atoms (both from

surface/subsurface and bulk) are available at medium temperatures while higher

temperatures are needed over CA material. Therefore, the better oxidation activities

can be reasonably awaited over BA LaFeO3.


S12

Activity tests in CO or CH4 oxidation

150 180 210 240 270 300 330 3600

20

40

60

80

100

C

O c

on

vers

ion

/ %

Temperature / oC

CA 500 oC

BA 500 oC

(A)

250 300 350 400 450 500 550 6000

20

40

60

80

100

CH

4 c

on

vers

ion

/ %

Temperature / oC

CA 500 oC

BA 500 oC

(B)

Figure. S6 CO (A) or CH4 (B) conversion versus reaction temperature for CA or BA 500 oC LaFeO3.

The temperature dependence of CO or CH4 conversion obtained over the different

LaFeO3 are presented in Figure S6, and the corresponding Arrhenius plots (X < 30%)

are depicted in Figure 5, with values of pre-exponential factor and activation energy

being listed inside. It is observed that, in the low conversion region, BA sample

achieved much higher activities (Figure 5), with a little lower values of EA in

comparison with CA LaFeO3. Additionally, for each oxidation reaction, the

conversion over BA LaFeO3 catalysts always followed the same evolution as the one

over CA LaFeO3, but located at temperatures of 20-30 oC lower (Figure S6).

Apparently, similar EA and evolutions could be attributed to the same catalytic active

site (Fe3+

surface sites) over CA and BA samples, although with different

physicochemical properties. Indeed, we also obtained a similar situation for CH4

oxidation over a series of LaCoO3 prepared by different procedure,[S11]

while the

different CO conversion evolutions were observed after Cu2+

or Pd2+

substitution,

leading to a broader or narrower temperature window, respectively.[S10]


S13

0

20

40

60

80

100

150 180 210 240 270 300 330 360

CO

co

nv

ersi

on

/ %

Temperature / oC

BA 450

BA 500

Figure. S7 Evolution of the catalytic activity in CO oxidation for BA-derived LaFeO3 with the calcination temperature.

Results of CO oxidation obtained for BA-derived materials calcined at 450 °C and

500 °C are presented in Figure S7. The increase in calcination temperature is observed

to result in increase in catalytic activity, despite the observed decrease in surface area

(Figure 1). This evolution is suggesting, as previously observed,[S12]

the presence of

residual Fe2O3 external phase over BA-450°C even if not observed by XRD (Figure

S4). Then, a minimal temperature of 500 °C is proposed to ensure the complete

disappearance of external surface Fe2O3 phase.


S14

References

[S1] P. Courty, H. Ajot, C. Marcilly, B. Delmon, Powder Technol. 1973, 7, 21-38.

[S2] M. Niederberger, G. Garnweitner, N. Pinna, M. Antonietti, J. Am. Chem. Soc.

2004, 126, 9120-9126.

[S3] G. K. Boreskov, Adv. Catal. Related Subject 1964, 15, 285-339.

[S4] J. Novakova, Catal. Rev. 1971, 4, 77-113.

[S5] E. S. R. Winter, Adv. Catal. Related Subject 1958, 10, 196-241.

[S6] S. Royer, D. Duprez, S. Kaliaguine, J. Catal. 2005, 234, 364-375.

[S7] D. Martin, D. Duprez, J. Phys. Chem. 1996, 100, 9429-9438.

[S8] P. Ciambelli, S. Cimino, G. Lasorella, L. Lisi, S. De Rossi, M. Faticanti, G.

Minelli, Appl. Catal. B 2002, 37, 231-241.

[S9] G. Shabbir, A.H. Qureshi, K. Saeed, Mater. Lett. 2006, 60, 3706-3709.

[S10] W. Yang, R. Zhang, B. Chen, N. Bion, D. Duprez, S. Royer, J. Catal. 2012, 295,

45–58.

[S11] S. Royer, F. Bérubé, S. Kaliaguine, Appl. Catal. A 2005, 282, 273-284.

[S12] J. Faye, A. Baylet, M. Trentesaux, S. Royer, F. Dumeignil, D. Duprez, S.

Valange, J.-M. Tatibouët, Appl. Catal. B 2012, 126, 134-143.


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