JMAC_v3

Platinum Group Modification of TiO2 forEnvironmental Photocatalysis

Timothy Johnson

Prof. Matt Rosseinsky

University of Liverpool

JMAC: April 2015

Timothy Johnson University of Liverpool

Platinum Group Modification of TiO2 for Environmental Photocatalysis

Table of Contents

IntroductionApplications, TiO2, PGMs and CatalysisSynthetic Routes

Loading PGMs onto TiO2

Introduction to ImpregnationMicroscopy, Methyl Orange tests and XPSPt PrefabricatedConclusions

Flame SprayLiverpool SetupXRDMicroscopyXPSConclusions

Acknowledgments



Applications, TiO2, PGMs and Catalysis

The Waste Water Problem

The Vally of the Drums (1960s)

I 23-acre toxic waste site in the USI Organic compounds leached into local

environmentI Clean up is still ongoingI Massive cost to environment and local populations

Environmental cost of The Pill

I Ethinyl estradiol (EE2) is the main drug in thecontraceptive pill.

I EE2 leads to intersex conditions in fishI £30bn to clean Britain’s contaminated water

Is there a viable way to remove organic pollutantsfrom waste water sources?

1980. EPA photo.

2013, ABC News




Aims, Applications and Limitations

Can Photocatalysis solve some of these problems?

Applications of Photocatalysis

I Self Cleaning SurfacesI AntibacterialI Cancer treatmentsI Waste Water Remediation

I The oxidative breakdown of methyl orangeI Light stable model pollutantI Used in textile industry and is carcinogenic,

toxic and a mutagen [1]

N

N S

NO

O

O

Aim: to increase photocatalytic activity ofphotocatalysts by PGM modification

TiO2 as a photo-active support

[1] J. Kaur, S. Bansal, S. Singhal, Physica B: Cond. Matt., 416, 2013, 33-38.

Images: K. Hashimoto, H. Irie and A. Fujishima,Jpn J Appl Phys, 2005, 44, 8269-8285.




TiO2

TiO2 has 3 main polymorphs:

I Rutile, Anatase and BrookiteI Difference in TiO6 octahedra

distortion and edge/cornersharing

I Band structure differs betweenpolymorphs

Many commercial TiO2 productsare available including EvonikAeroxide R© P25:

I Anatase : Rutile (≈80:20)I Made via flame spray pyrolysisI Particles ≈20nm

Ma, X. et. al., Chemical reviews, 2014,114, 9987–10043.




Photocatalysis

TiO2 is a n-type semiconductor:

I Distinct Electronic StuctureI Conduction band and Valence band

separated by Bandgap (Eg)I Irradiation can lead to charge

separation (if hv > Eg)I Separated charges can be used to do

chemical work

I Eg is dependent on phase [1]I Between 3.0eV and 3.21eV

I TiO2, due to Eg energy, requires UVlight to separate charge

Image Adapted From P. K. J. Robertson et. al., Hdb Env.Chem., 2005, 2, 367-423. and A. Ajmal, et.al., RSC Adv,2014, 4,37003-37026.

[1] D. Reyes-Coronado et. al., Nanotechnology, 2008, 19,1-10

Is there a way to modify TiO2 so either visible light can be used orimprove efficiency with UV light?




PGMs and TiO2 Photocatalysis

Why use PGMs?I Valence 0

I PGM loaded onto surfaceI Act as electron sinks, increasing

charge species life times

I Valence 3+/4+I PGM doped into latticeI Alter Eg so lower energy photons can

be absorbedI Addition of impurity bands between

CB and VB which account for VLactivity

I Rh examples shown right

I Pt can also act as preferred O2

absorption sites[1]I Leads to increased production of H2O2

I Key driver in oxidation of organiccompounds

Calculation by M. S. Dyer from B. Kiss et. al., AngewandteChemie, 2014, 126, 52, 14708-14712.

Image adapted from: J. Kuncewicz et. al., Chem. Commun.2015, 51, 298-301.

[1] C.B. Musgrave, et. al., J. Phys. Chem. C., 2014, 116,10138-10149.



Synthetic Routes

Synthetic Routes

I Rh and Pt have beenadded to TiO2 tomodify itsphotocatalytic activity

I Both loading thesurface and doping intothe lattice has beenachieved for Rh

I Pt has been loaded onto the surface in avariety of ways

I Different methods =different advantagesand disadvantages

This talk will focus on two loadingmethods and doping via flame spray



Microscopy, Methyl Orange tests and XPS

Impregnation Method

The first and simplest method to add Rh to a support is theimpregnation method

I Addition of Rh(NO3)3.H2O solution to TiO2 followed bydrying and thermal pretreatment

I Deposition of nanoparticles on surface

I Capillary action is responsible to take upI Photocatalyst testing can be conducted on pretreated samples

I 0.1,0.5 and 1 wt% Rh samples producedI Pretreated under flowing air or N2\H2




XPS

I Samples reduced underH2 shows mix ofoxidation states

I Large proportion ofRh0

I Consistent with Rh0

nanoparticles

I Rh3+ also seenI RhOx oxide coating

on the surface of Rhnanoparticles

I This occursspontaneously atroom temperature[1]

318 316 314 312 310 308 306 304 302

5520

5980

6440

6900

7360 Rh3+

308.4eV

Rh 3d 5/2

Cou

nts

per s

econ

d

Binding Energy (eV)

Rh 3d 3/2

Impreg.RSTD = 1.775 Rh0

307.1eV

Oxidation State Atomic %

0 50.45

3+ 49.55

[1] A. Munoz, Surface and Interface Analysis, 1988, 12, 247-252.




Microscopy

I Large clusters of nanoparticles seenI Dispersion is poor, few isolated

nanoparticles on surface

I Isolated nanoparticles are very small(≈2nm)

I Hemisphere size ≈20nm by COmetal surface area analysis

I Large aggregates account for highervalue

I Exposed Rh surface good forphotocatalyisis

I Allows for O2 absorption

Does this translate into photocatalyticactivity?




Methyl Orange degradation testing

I Example UV spectra collected:I Shows MO absorbed onto surface during

stirring in the darkI Rate calculated from spectra collected

I Samples pretreated under reducinghydrogen show peak activity at 0.5 wt%

I Lower activity than unmodified support(rate = ≈0.0155min-1)

0.0 0.2 0.4 0.6 0.8 1.0 1.20.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

Rat

e (m

in-1)

Rh wt%

300 400 500 600 700

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Abs

.

Wavelength (nm)

Irradiation Time

Dark Absorption

MO abs.

0 20 40 60 80 100 120

0.0

0.1

0.2

0.3

0.4

0.5

0.6

-ln(C

/Co)

time (mins)




Effect of Pretreatment Temperature

I Samples treated at low temperatures havehigher rates than support

I Methyl orange tests show decreasingactivity as reducing thermal treatmentsincrease.

I Large drop in activity seen in recycledcatalyst

I Nitrate IR signal diminished after one runI Nitrate seems to be responsible for high

activity

1000 1500 2000

0.6

1.2

1.8

2.4

3.0

Wavenumber (cm-1)

abs

P25

150oC

150oC after 1 run

100 200 300 400 5000.006

0.008

0.010

0.012

0.014

0.016

0.018

150oC after one run rate

rate

(min

-1)

Temp (oC)

P25

1000 1500 2000

0.00

0.62

1.24

1.86

2.48

P25

150

200

250

300

Wavenumber | cm-1

Abs

Tem

p | o C



Pt Prefabricated

Prefabricated Pt

To improve control over PGM particle size wehave employed a method to produce and depositprefabricated nanoparticles onto TiO2

I Surfactant controlled methodI pH = 11 solution results in deposition onto

TiO2 surfaceI Surfactant removed with extensive cleaning

I Thermal treatment, Soxhlet and UVcleaning.

I Removes surfactant so catalytic testing canbe conducted.

Dodecenyl succinic anhydride



Pt Prefabricated

Pt Prefabricated Microscopy

I Individual nanoparticles seendeposited onto TiO2 surface

I ≈2nm in size and welldispersed

I No evidence of largeclusters observed

I This should increaseactive metal surface area

I Fourier transform showshexagonal pattern

I Indicative of Pt (111)face [1]

[1] H.B. Lyon, G.A. Somorjai,J. Chem. Phys., 1967, 46, 2539-2550



Pt Prefabricated

Pt Prefabricated Catalytic testing

I Samples pretreated inreducing N2/H2 show peakat 0.5wt% loading

I Overall activity is lower thanthat of bare support

I Nonlinear C/Co plot seen:I Initial rate is much higher

than P25I Deactivation occurs with

increasing PtOx [1]I Pt acts as preferential

O2 absorption site[2]

[1] A.P. Markusse, et. al., Cat. Lett., 1998, 55, 141-145.[2] C.B. Musgrave, et. al., J. Phys. Chem. C., 2014, 116,10138-10149.

0.00 0.25 0.50 0.75 1.000.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

rate

| m

in-1

wt% Pt

-20 0 20 40 60 80 100 120 1400.00

0.25

0.50

0.75

1.00

C/C

o

Time | mins

P25 0.5wt% Pt-TiO2



Conclusions

PGM loaded TiO2 Conclusions

Impregnation method of PGMs onto TiO2 does not improveactivity over support

I Rh0 and Rh 3+ detected

I Poor dispersion observed

I False positive seen when nitrates are present on surface

Higher initial rate seen when prefabricated Pt nanoparticles areloaded onto TiO2

I Better control over nanoparticle dispersion

I Deactivation of catalysts was observed



Flame Spray Objectives

Sample preparation via impregnation orhydrothermal synthesis can take days to weeks

I Can we speed up material screening by usingFlame Spray Pyrolysis?

I Can we use readily available cheaper watersoluble precursors?

I Spray onto substrates for analysis by:

I XRDI MicroscopyI Optical Measurements

I Promising materials can then be scaled upfor testing.

I Longer deposition times to collect moreproduct

T. Karhunen, A. Lahde, J. Leskinen, and R.Buchel, International Scholarly, 2011,2011, 1-6.



Liverpool Setup

Setup

I Bespoke set-up at Liverpool(Saflame)

I Aerosol pumped into mixingchamber, then into flame

I Si or quartz wafers used asdeposition medium

I Allows for direct anaylsisI XRD, SEM and DR

measurements

I Water soluble Ti precursors andRh(NO3)3.H2O used

Titanium(IV) bis(ammonium lactato)dihydroxide



XRD

Flame Spray: PXRD

A mixed phase TiO2 wasproduced.

Anatase and rutile phaseformed.

I Si acts as internalstandard

I Mixed phase TiO2

producedI ≈ 80:20, similar to

P25

I Sharp peaks suggests wedon’t have nano material

I Patterns collected afterjust 10 mins ofdeposition.

0.1 wt%

0.5 wt%

1 wt%A

R

Si

20 30 40 50 60

2th(o)

Cou

nts

(arb

. uni

ts)



XRD

Have we Doped?

To determine if we have dopedinto the TiO2 lattice we conducteda series of Pawley fittings.

I Increase in anatase a parameterI c remains constantI Error bars represent three

standard deviations (3σ)

Atom Size (A100)

Rh 3+ 0.665

Rh 4+ 0.600

Ti 4+ 0.605

0.0 0.5 1.0

3.783

3.784

3.785

a(Å)

Rh doping (wt%)

0.0 0.5 1.09.503

9.504

9.505

9.506

9.507

9.508

9.509

9.510

9.511

c(Å)

Rh doping (wt%)



Microscopy

1wt% Rh Microscopy: SEM

I Spherical TiO2 observedI Polydispersed particle size, ranging from several µm to tens of

nm.I Optimization to improve size dispersion.

I Collection plate distanceI Gas flow rateI Flame composition



Microscopy

1wt% Rh Microscopy: TEM

I Dark spots seen on the surface of some spheresI They are embedded in the surface



Microscopy

1wt% Rh Microscopy: EDX

I EDX Line scan shows dark spotsare Rh rich

I Exposed Rh mean size= 3.8nmI Rh signal detected over all sampleI Clustering may be due to high

loadingI Plateau seen in lattice parameters

2 3 4 5 6 7 8 9 10 11 12 130

5

10

15

20

25

30

35

40

num

ber o

f par

ticle

s

size (nm)

Mean = 3.8nm



XPS

XPS

I Fitting shows mix of Oxidation statesI No evidence of Rh0 unlike loaded

samplesI For effective photochemistry:

I Rh3+- Rh4+ can act as ”built-in redoxcouple” [1]

I Electron excited to CB from Rh3+

I Rh 4+ can oxidize compounds on itssurface

I Rh 4+ has also been reported as arecombination center [2]

I Combination of states shows promisefor further photocatalytic work

I Altering flame conditions and samplepretreatments will allow us to tune andoptimize oxidation states.

[1] J. Kuncewicz et. al., Chem. Commun. 2015, 51, 298-301.[2] S. Kawasaki, et. al. J. Phys. Chem. C, 2012, 116, 24445-24448.

318 316 314 312 310 308 306

3900

7800

11700

15600

19500 Rh3+

308.4eV

Rh 3d 5/2

Cou

nts

per s

econ

d

Binding Energy (eV)

Rh 3d 3/2

Flame SprayRSTD = 3.65

Rh4+

309.7eV

Oxidation State Atomic %4+ 87.973+ 12.03



Conclusions

Flame Spray: Conclusion

I Rh has been doped into TiO2

I TEM analysis of 1wt% Rh TiO2 also shows exposed Rh rich clustersembedded into TiO2

I XPS shows Rh oxidation states are of interest photocatalytically

We have demonstrated a rapid method to screen doped materials viaflame spray pyrolysis.

I Time to screen samples has been reduced from days to minutesI Screening can be done on lab-bench setupI Determine if samples are of interest before scaling up reactionsI Scaling up will allow for catalytic testingI Pretreatment and flame conditions will have to be optimised.

I Collection plate distanceI Gas flow rateI Flame composition



Acknowledgments

LiverpoolProf. Matt RosseinskyDr. John ClaridgeDr. Troy ManningDr. Noemie PerretDr. Alexandros KatsoulidisDr. Mike PitcherDr. Marco ZanellaDr. Matthew DyerMJR Group

JMTCDr. Stephen PoulstonDr. Sonia GarciaDr. Richard SmithDr. Tugce Eralp-ErdenDr. Martha Briceno De Gutierrez



Solid State RhxTi1-xO2 Optical Measurements

300 400 500 600 700

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.0 wt%

0.5 wt%

-lo

g(1/

R)

Wavelength nm.

0.1 wt%

0 wt%



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