TiO2 nanoparticlesfor CO2 photoreduction: a combined...

TiO2 nanoparticles for CO2 photoreduction: a

combined experimental and

periodic DFT study of the active sites

Lorenzo MinoC. Deiana, A. M. Ferrari, V. Maurino, G. Martra, G. Spoto

Department of Chemistry, University of Turin

Photocatalysis for energy

Lyon, 15‐17 October 2014

Background of the study and investigation techniques

Study of the TiO2 exposed surface sites by CO adsorption

CO2 interaction with dehydroxylatedand hydrated TiO2 anatase surfaces

Conclusions and perspectives

Background of the study and investigationtechniques

Section



CO2 interaction with dehydroxylated and hydrated TiO2 anatase surfaces


4Background of the study and investigation techniques

TiO2 for CO2 photoreductionA long story

surface study atan atomic/molecular level

Already in 1979 Honda and coworkers reported the photoreductionof CO2 to form formic acid, formaldehyde, methyl alcohol andmethane in aqueous suspension of TiO2.

Despite a considerable amount ofsubsequent studies, the conversionefficiency of CO2 into usefulhydrocarbons is still too low for large‐scale technological applications.


The key role of surface sitesShape dependent properties

M.V. Dozzi et al., Catalysts 2013, 3, 455


Nanoparticle morphologyElectron microscopy

TiO2 HT TiO2 T‐SP (Solaronix)TiO2 P25 (Evonik)

50 nm

study at an atomic level

Information on exposed surfaces

more complex morphology:how can we study the surface?

C. Deiana, et al., Phys.

Chem. Chem. Phys., 2013, 15, 307


Nanoparticle morphology and surface propertiesFTIR spectroscopy of adsorbed probe molecules

TiO2 surface

OTi


Nanoparticle morphology and surface propertiesFTIR spectroscopy of adsorbed probe molecules

TiO2 surface

OTi

C

O

OTi

TiO2 surfaceeffect of the interaction with surface sites on the ν of the internal mode


FTIR spectroscopy of adsorbed probe molecules Carbon monoxide

The higher is the blue‐shift, the greater is the Lewis

acidity of the metal cation

Stretching frequency of adsorbed CO is also influenced

by the lateral interactions occuring in the adlayers

Information on surfaceproperties and morphology


FTIR spectroscopy of adsorbed probe moleculesCryostat

G. Spoto et al., Prog. Surf. Sci., 2004, 76, 71

The use of a cryostat allowed us go below the liquid nitrogen

temperature obtaining spectra of considerably

improved quality

Study of the TiO2exposed surface sites by CO adsorption

Section





2220 2200 2180 2160 2140 2120

Abso

rban

ce 0.5 a.u.

Wavenumbers (cm-1)

12Study of the TiO2 exposed surface sites by CO adsorption

Anatase TiO2 nanocrystalsHRTEM and FTIR of CO adsorbed at 60 K

Ti

L. Mino, G. Spoto, S. Bordiga, A. Zecchina, J. Phys. Chem. C, 2012, 116, 17008

3800 3700 3600 3500

Wavenumbers (cm-1)

need of modeling for full assignment

Physisorbed CO

OH ∙∙ COAnatase extended surfaces Outgassedfor 2 h at 773 K

8 9 10 11 12 13 14 15 16 17 18 19 200.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

E's

(J/m

2 )

Slab thickness (Å)

(101)

(100)

(103)(110)

(001)

(112)

13

Periodic DFT calculationsAnatase surface energies

Anatase surfaces modeled with bidimensional slabs

characterized by two infinite dimensions (x, y) and a finite

thickness exploiting the CRYSTAL09 code and the PBE0

functional

(001)

(101)

L. Mino et al., J. Phys. Chem. C 2011, 115, 7694

6 Ti‐layers for CO adsorption


14

Periodic DFT calculationsCO adsorption on the main anatase surfaces

OTi


15

Periodic DFT calculationsMain computed parameters

OTi



2220 2200 2180 2160 2140 2120

Abso

rban

ce

0.5 a.u.

Wavenumbers (cm-1)

16Oxide surface properties and particle morphology

CO adsorbed at 60 K on nanoanataseComplete spectral assignment

L. Mino, G. Spoto, S. Bordiga, A. Zecchina, J. Phys. Chem. C, 2012, 116, 17008

Physisorbed CO

(112)

(101)

(001)

OH ∙∙ CO


Rutile TiO2Modeling of CO adsorption

(001) (100)

(110)

(011)



Rutile TiO2 model systemFESEM and FTIR of CO adsorbed at 60 K

OTi

BET surface area: 2 m2/g

2220 2200 2180 2160 2140 2120

0.3 a.u.

Abso

rban

ce

Wavenumbers (cm-1)

Reconstructed(110)

Rutile Wulffconstruction

(110)


OH ∙∙ CO


Mixture of different polymorphsEvonik (Degussa) TiO2 P25

Mixture of about 85% anatase and 15% rutile with a surface area of 60 m2/g prepared by flame hydrolysis of TiCl4.Its outstanding photocatalytic activity makes it a benchmark for testing new synthesized materials.

HRTEM particles in 10‐50 nm range

XRD anatase 26 nm – rutile 42 nm


TiO2 P25Ingredients for complete spectral assignment


TiO2 P25Complete spectral assignment

L. Mino, G. Spoto, S. Bordiga, A. Zecchina, J. Phys. Chem. C, 2012,

116, 17008

Informatio on face distribution,

surface regularity and crystallinity


Anatase TiO2 model systemFTIR of CO adsorbed at 100 K

C. Deiana, M. Minella, G. Tabacchi, V. Maurino, E. Fois, G. Martra, Phys. Chem. Chem. Phys., 2013, 15, 307‐315

CO2 interaction with dehydroxylated and hydrated TiO2anatase surfaces

Section





24

Dehydroxylated TiO2 anatase surfacesThe CO surface picture


Physisorbed CO

(101)

Low coordinated

Ti sites

OH ∙∙ CO

(001)

Nanoanataseoutgassed for 2h

at 823 K

25

Dehydroxylated TiO2 anatase surfacesFTIR of adsorbed CO2 (pressure up to 10 mbar)


L. Mino, G. Spoto, A.M. Ferrari, J. Phys. Chem. C, 2014, DOI: 10.1021/jp507443k in press

Nanoanataseoutgassed for 2h

at 823 K

26

Dehydroxylated TiO2 anatase surfacesPossible adsorption geometries on the (101) surface


ΔEform = ‐9.5 Kcal/mol ‐7.8 Kcal/mol

‐8.5 Kcal/mol2.7 Kcal/mol

27

Dehydroxylated TiO2 anatase surfacesPossible adsorption geometries on the (001) surface


ΔEform = ‐5.3 Kcal/mol ‐30.1 Kcal/mol

‐3.4 Kcal/mol ‐20.9 Kcal/mol

28

Dehydroxylated TiO2 anatase surfacesComputed vibrational frequencies



2400 23001700 1600 1500 1400 1300 1200

Experimental

Abso

rban

ce

Wavenumbers (cm-1)

0.5 a.u.

29

Dehydroxylated TiO2 anatase surfacesExperimental vs simulated FTIR spectra


101-L1

001-MC

2220 2200 2180 2160 2140 2120Wavenumbers (cm-1)

0.5 a.u.

Abso

rban

ce

30

Hydrated TiO2 anatase surfacesThe CO surface picture


Physisorbed CO

(101) halved intensity

Low coordinated Ti sites missing

OH ∙∙ COmore intense

(001) missing

Nanoanataseoutgassed for 2h at 823 K and then

partially rehydrated

31

Hydrated TiO2 anatase surfacesFTIR of adsorbed CO2 (pressure up to 10 mbar)



Nanoanataseoutgassed for 2h at 823 K and then

partially rehydrated

32

Hydrated TiO2 anatase surfacesPossible bicarbonates on the (001) surface


‐14.6 Kcal/mol

‐4.3 Kcal/mol

‐1.5 Kcal/mol

0.1 Kcal/mol 3.5 Kcal/mol 8.6 Kcal/mol

ΔEform = ‐10.3 Kcal/mol

33

Hydrated TiO2 anatase surfacesBicarbonates computed vibrational frequencies



34

Hydrated TiO2 anatase surfacesExperimental vs simulated FTIR spectra


101-L1

001-MB1 001-BB2 001-BB3


Section

‐ The use of adsorbed CO as probe moleculeallowed us to highlight the exposed surface sitesand to determine the average nanoparticlemorphology.

‐ Study of mixtures of different polymorphs ormixtures of different oxides.

- On the most stable anatase (101) surface CO2 isweakly adsorbed and retains its molecularproperties.

‐ On the dehydroxylated anatase (001) surface theformation of monodentate carbonates occurs.

‐ On the partially hydrated (001) surface theformation of bidentate bicarbonates is favored.

‐ Basis for future in situ study under UV irradiation.





Acknowledgments

A. Zecchina

S. Bordiga

C. Negri

Thank you for the kind attention

TiO2 nanoparticlesfor CO2 photoreduction: a combined...

Documents

Transcript of TiO2 nanoparticlesfor CO2 photoreduction: a combined...