Reversible CO2 absorption by the 6H perovskite Ba4Sb2O9

Matthew T. Dunstan,1 Wen Liu,2 Adriano F. Pavan,3 Justin A. Kimpton,4

Chris D. Ling,3 Stuart A. Scott,5 John S. Dennis,2 and Clare P. Grey1

1Department of Chemistry, University of Cambridge, Lensfield Road, CB2 1EW, UK2Department of Chemical Engineering and Biotechnology, University of Cambridge, Pembroke Street, CB2 3RA, UK

3School of Chemistry, The University of Sydney, Sydney NSW 2006, Australia 4Australian Synchrotron, 800 Blackburn Road, Clayton VIC 3168, Australia

5Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK Email: mtd33@cam.ac.uk

What is CO2 Chemical Looping?

Amines (MEA, DEA):

- Most developed technology

- React at low temperatures (~40◦C)

- Inefficient; consumes ~30% of power output

Perovskites

- Excellent thermal and mechanical stability

- Can be made from abundant elements

- Stable framework even after the formation of carbonate

- Doping of different elements gives a great deal of control over physical properties

- No studies to show whether they are able to be regenerated after

carbonation

Why Perovskites?

Rodriguez-Gomez, N., PhD Thesis,

Instituto Nacional del Carbón

(INCAR) (2010)

Allows for the separation of CO2 from flue gases for the purposes of CO2 sequestration.

Use a solid carrier to chemically absorb CO2:

Synthesis and Carbonation of Ba4Sb2O9

In-situ synchrotron XRD of carbonation and regeneration reactionsTGA cycling studies

Structural changes of Ba4Sb2O9 monitored by Scanning Electron Microscopy (SEM)

Acknowledgements

M.T.D thanks the Cambridge Commonwealth Trusts for the award of a PhD scholarship, and Trinity College, University of Cambridge, UK for the award of an External Research Studentship. We would also like to thank the Australian Synchrotron for the award of beamtime.

Conclusions

Ba4Sb2O9, a perovskite-type material, is able to absorb CO2 at ~600°C forming BaCO3 and BaSb2O6, and is able to be regenerated upon heating to 950°C.

S-XRD and TGA make it possible to monitor the carbonation and regeneration reactions in-situ, identifying the phases present at each stage and the relative rates and capacities of absorption.

Importantly, Ba4Sb2O9 is able to be cycled between 600-950°C to reversibly absorb CO2, and retains its capacity over 100 cycles.

It appears that the stability of Ba4Sb2O9 upon cycling comes in part from its ability to regenerate its original particle morphology, recreating a porous structure from a dense shell of BaCO3 in each cycle.

This study demonstrates the potential to employ a whole new class of inorganic oxide materials with stable and flexible chemical compositions and structures for applications in carbon capture and storage.

CarbonatorCalciner

650°C

Flue gases(FCO2)

O2 +Coal

Gases “without CO2”

CaCO3+ CaO

CO2 concentrated

900-930°C

CaCO3 (F0, fresh sorbent)

CaO (purge)

Existing power plant HinCaO

(FCaO)

CarbonatorCalciner

650°C

Flue gases(FCO2)

O2 +Coal

Gases “without CO2”

CaCO3+ CaO

CO2 concentrated

900-930°C

CaCO3 (F0, fresh sorbent)

CaO (purge)

Existing power plant HinCaO

(FCaO)

MxO + CO2→ MxCO3

MxCO3→ MxO + CO2

CaO and derivatives:

- Cheap and abundant material

- Excellent reaction kinetics

- Sintering over many cycles leads to a loss in capacity

Alkali Oxides (Li2ZrO3, Li4SiO4):

- Good stability at high temperatures (400◦C-700◦C)

- Retains capacity over many cycles

- Many different compounds allow tuning of absorption temperature

- Carbonation of pure compounds is slow

The carbonation and regeneration reactions were observed with in-situ synchrotron XRD carried out on the PD beamline at the Australian Synchrotron, performed under flowing CO2 using a capillary gas-flow cell.

We were able to show that we are able to regenerate crystalline Ba4Sb2O9 after heating to 950°C.

J.C. Abanades et al.., Energy & Fuels,, 17, 308 (2003)G. T. Rochelle, Science, 325, 1652 (2009)

Perovskite ABO3:12 coordinate A site 6 coordinate B site

Aims

- Observe carbonation reactions in-situ

- Determine if regeneration after carbonation is possible

- Monitor the structural and morphological changes upon cycling

373

473

573

673

773

873

973

1073

100%

102%

104%

106%

108%

110%

112%

114%

116%

0 100 200 300 400

Temperature (K

)

Perc

enta

ge w

eigh

t Cha

nge

Time (min)

Ba4Sb2O9

N2 purge 42 vol.% CO2 in N2

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

873 973 1073 1173

CO

2 par

tial p

ress

ure (

atm

)

Temperature (K) (a)

-8

-7

-6

-5

-4

-3

-2

-1

0

1

873 973 1073 1173

CO

2 par

tial p

ress

ure (

atm

)

Temperature (K) (b)

BaO − BaCO3

12.0 13.0

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BaSb 2O6

0 10 20 30 40 50 600.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Ba4Sb2O9BaCO3BaSb2O6

Rel

ativ

e Ph

ase

Frac

tion

Time (min)

Time0 min

54 min

30 min

6 min12 min18 min24 min

36 min42 min48 min

13.0 14.0 15.0

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> BaS

b 2O6

<102

> BaC

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Thermogravimetric studies of carbonation

1. air, 600°C, 12 h x 22. air, 1000°C, 12 h x 2

4 BaCO3 + Sb2O3

C.D.Ling et al., Acta Cryst. B, 63, 584 (2007)

M.T.Dunstan et al., Solid State Ionics, 235, 1 (2013)

Solid State synthesis route

600

650

700

750

800

850

900

950

1000

21.5

22

22.5

23

23.5

24

24.5

25

25.5

0 2000 4000 6000 8000 10000

Tem

pera

ture

(deg

rees

C)

Wei

ght (

mg)

Time (s)

Cycles 1,2 Cycles 9,10 Cycles 19,20 Cycles 49,50 Cycles 99,100 Temperature

We tested the cycling capacity of Ba4Sb2O9 over 100 cyc les of carbonation at 600°C and desorption at 950°C in 100% CO2. In later cycles it appeared that the material was more reactive towards absorption, indicating that changes in morphology or structure may activate the material for better absorption.

Importantly, we found that Ba4Sb2O9 retains its absorption capacity even after 100 cycles, making it suitable for chemical looping applications.

0%

20%

40%

60%

80%

100%

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 20 40 60 80 100

Precentage CO

2 uptake (mol C

O2 / m

ol Ba)

CO

2 upt

ake

(g C

O2/

g sor

bent

)

Cycle Number

Desirable Properties

- High absorption capacity at high temperatures (400-900◦C) to allow for heat recovery

- Fast reaction kinetics

- Retention of capacity upon many carbonation-regeneration cycles

Current Materials of Interest

K. Nakagawa et al., J. Electrochem. Soc., 145, 1344 (1998)M. Kato et al., J. Mater. Sci. Lett., 21, 485 (2002)

Pfeiffer, H. in Advances in CO2 Conversion and Utilization, ACS Symposium Series; American Chemical Society: Washington, DC, (2010).

Carbonation

(~600°C)

Ba4Sb2O9 + 3CO2 BaSb2O6 + 3BaCO3

Regeneration

(~900°C)

TGA Reaction ChamberDesign

Initial mass loss would indicate some prior carbonation under ambient conditions

Initial Carbonation Experiment

13.75% mass increase after carbonation indicates the carbonation products must have stoichiometry BaCO3 and ~BaSb1.89O5.72 (BaSb2O6)

M.T.Dunstan et al., submitted

Purge gas

Protective gas

Capillary

Balance cantilever

Reactive gas (N2/CO2)

Sample holderThermocouple

Crucible

Carbonation Equilibrium Curves

Solid line: calculated CaO-CaCO3-CO2 equilibrium

Square symbols: Ba4Sb2O9-BaCO3-CO2 equilibrium

Dashed line: BaO-BaCO3-CO2 equilibrium

Unlike the BaO-BaCO3 system, which requires unfeasibly high temperatures to regenerate, Ba4Sb2O9 is able to regenerate at much lower temperatures, mimicking the thermodynamic properties of CaO.

Ba4Sb2O9-BaCO3 equilibrium fit:

Carbonation

650°C

Regeneration

950°C

Upon carbonation, reflections indexed to BaCO3 and BaSb2O6 slowly appear

Some of the phases formed are amorphous or poorly crystalline, as seen in the increase in the background

Relative phase fractions extracted from Rietveld refinements show the reaction has not reached completion after 1 hour

Even after 1 hour, BaCO3 and BaSb2O6 remain , potentially due to poor gas mixing or insufficient time to reach equilibrium

(a) Freshly synthesised Ba4Sb2O9

(b) Fully carbonated Ba4Sb2O9 after 85 cycles

(c) Partially regenerated Ba4Sb2O9 after 85 cycles

(d) Fully regenerated Ba4Sb2O9 after 85 cycles

SEM-EDX elemental analysis: Partially regenerated Ba4Sb2O9

Spectra C O Sb Ba Phase

1 20.58 47.62 9.58 22.21 Ba4Sb2O9

2 34.25 43.96 0.58 21.21 BaCO3

3 34.29 47.24 - 18.47 BaCO3

4 17.93 45.00 10.83 26.24 Ba4Sb2O9

Loss of porosity on forming surface layer of BaCO3

Evidence of surface cracking associated with particle strain - possible route to further carbonation

Significant porosity recreated upon full regeneration - an explanation for the stable cycling capacity of Ba4Sb2O9

As-synthesised particle shows a large amount of porosity conducive to reaction with gaseous CO2

Upon regenerat ion , crystallites of Ba4Sb2O9 can be seen to nucleate on the BaCO3 surface

Rotameters Mass flowmeters

Needlevalves

Solenoidvalve controls

Reactive gas 3

Rotameters

Reactive gas 2

Reactive gas 1

Reactive gas

Purge gas

Protective gas

PCTo ventTGA

N2

Monday, 1 July 13