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Wet Surface Chemistry from Infrared
Spectroscopy
Professor Jim McQuillan FRSNZ FNZIC
Department of Chemistry, University of Otago
Dunedin, New Zealand
 46 S
Preview
• Wet surface infrared spectroscopy
• Toxic mine waste water remediation
• Electron polaron behaviour in TiO2 photocatalysis
Modern Infrared Spectroscopy Deals Easily with
Wet Solid Interfaces
• Infrared (IR) spectroscopy sees molecules and their environments
through dipolar molecular vibrations eg. this water vibration has
frequency 1.13 x 1013 per sec or 3756 waves per cm (cm-1)
• Water absorbs IR strongly and this has prevented IR studies of
aqueous real world systems until the adoption of a new approach
• Internal reflection or attenuated total reflection (ATR-IR)
glass
cover
O- ring
solution
sample
ZnSe
IR beam
Total Internal Reflection at a ZnSe/H2O interface
45 ° Angle
of incidence
~ m
H 2O
ZnSe
Refractive indices
H2O
1.33
ZnSe
2.42
Critical angle 33°
IR
• evanescent wave samples ~ into less dense phase (sample)
• short pathlength creates no problem for aqueous systems
• thin films of solid particles immersed in water can now be studied
solution
ZnSe
infrared
Applications in Mine Waste Environmental Chemistry
• Gold (Au) is often found associated with minerals containing
sulfur (S), arsenic (As), and antimony (Sb).
• Arsenic and antimony are very toxic and occur underground
in reduced form eg. as arsenopyrite FeAsS and stibnite SbS3.
• When the gold ore containing these elements is brought to the
surface for processing the reduced elements are oxidised.
•
The final products of oxidation are the toxic water-soluble
arsenate AsO43- and antimonate Sb(OH)6- as well as
insoluble hydrous ferric oxide-hydroxide Fe(OH)3
AsO43- & Sb(OH)6- potential environmental disaster
Macraes Gold Mine in Otago, New Zealand
Oxidation
pond
• Gold extracted with cyanide. Waste water to oxidation pond
• Arsenate and antimonate adsorbed to hydrous ferric oxide
(adsorb means “stick to”)
Arsenate speciation varies with pH
pKa
H3AsO4
2.2
6.9
H2AsO4-
• Arsenate exists as H2AsO4- and
HAsO4 in paraneutral solution
11.5
HAsO42-
IR solution spectra
• Antisymmetric As-O stretch of
tetrahedral (Td) ion is strong IR
absorption
pH 3
• Some deprotonation of solution
arsenate with strong adsorption to
iron oxide
pH 7
Adsorption to hydrous ferric oxide (HFO)/ ferrihydrite
• Hydrous ferric oxide (Fe2O3.xH2O) generated from oxidation
of Fe(II) in mining extraction or precipitated from Fe(III)
chloride under neutral-alkaline conditions.
• Also known as ferrihydrite.
• HFO has a very large specific surface area (as
~ 500 -1000 m2 g-1)
• Very suitable substrate for sequestration of
toxic ligand species which coordinatively
adsorb to Fe(III) metal ions (oxide surface).
• Arsenate adsorption monodentate/bidentate
IR spectra of arsenate adsorbed on hydrous iron oxide
10-3 mol L-1 arsenate, pH 6.6
(a) Synthetic HFO
(b) &(c)Natural
mine site HFO
•
Arsenate peak ~830 cm-1
• Arsenate displaces adsorbed
carbonate ~1600-1400 cm-1
• Arsenate coordination to
Fe(III) probably bidentate
Carbonate displaced by arsenate
pH dependence of arsenate adsorption
• Follows typical
anion adsorption
pH variation
• Adsorbed arsenate
peak shifts a little
with pH –
probably due to
change in
protonation
Adsorption isotherm for arsenate at pH of ....
Antimonate exists mainly as Sb(OH)6pKa = 2.7
HSb(OH)6
IR spectrum of
0.05 mol L-1
KSb(OH)6
aqueous
solution
Sb(OH)6-
OH stretch
OH bend
IR spectrum of antimonate adsorbed to hydrous ferric oxide
over first 30 min,10-4 mol L-1, pH 3
• Adsorbed species
clearly very similar to
antimonate in solution
• Antimonate OH stretch
mode must lie ~3100
cm-1 beneath water
• Sharp absorption loss
~3600 cm-1 may be
from loss of Fe surface
OH when antimonate
adsorbs
pH dependence of antimonate adsorbed to hydrous
ferric oxide
10-4 mol L-1, adsorption for 30 mins at each pH
• Spectrum of
adsorbed
antimonate 9001220 cm-1 quite
irregular and not
understood
• Irregularity may be
related to of innersphere complex
formation and
change of symmety
Adsorption at pH 3 then different desorption conditions
pH
3
6.5
10
Arsenate and antimonate adsorption
competition on hydrous ferric oxide
•
Adsorption of individual species can be measured
and studied but what happens with them competing?
• Strong IR absorptions of arsenate ~850-900 cm-1
and antimonate ~1100 cm-1 are separate
Adsorption competition for 10-3 mol L-1 mixture
Ads.
Sb(OH)6 AsO4
Adsorption/desorption kinetics
AsO4
Sb(OH)6
Analysis of the spectra and kinetics
• Arsenate IR absorption is about 5x stronger (oscillator) than
antimonate so more antimonate adsorbed than arsenate at pH 3
• Little antimonate adsorbed pH 7
• At pH 7, 25% arsenate desorbs but
76% remains adsorbed - why?
• Spectra show that the 25% desorbed
arsenate has a different spectrum from
arsenate remaining
• Outer-sphere arsenate desorbs while
inner-sphere arsenate stays adsorbed
What about 10-5 mol L-1 - more typical of mine waste?
Spectra similar to those for
10-3 mol L-1. Arsenate
adsorbs across pH range
Kinetics show very little arsenate
desorption – but antimonate poorly
adsorbed from neutral solution
References
1. Roddick-Lanzilotta, A. J., McQuillan, A. J., Craw, D.
Infrared spectroscopic characterisation of arsenate (V) ion
adsorption from mine waters, Macraes mine, New Zealand
Journal of Applied Geochemistry, 2002, 17, 445-454.
2. McComb, K., Craw, D., McQuillan, A. J. ATR-IR spectroscopic study
of antimonate adsorption to iron oxide
Langmuir, 2007, 23, 12125-12130.
3. Muller, T., Craw, D., McQuillan, A. J. Arsenate and antimonate
adsorption competition on hydrous ferric oxide monitored by infrared
spectroscopy, Langmuir in preparation
TiO2 Photocatalytic Processes
CB
O2 + e-  O2- et seq
Eg = 3.2 eV
electron traps
(~380 nm)
hole traps
VB
recombination
2H2O + 4h+  O2 + 4H+
Organics  CO2 + H2O
Leary & Westwood, Carbon 49, 741 (2011)
21
General features of TiO2 photocatalysis
• Typically observed with Degussa P25 TiO2 ~ 80% anatase, 20%
rutile. ~25 nm particles. Anatase more photoactive.
• Electron-hole recombination in bulk is dominant. Less so in small
particles.
• Cationic aromatic species such as methylene blue and crystal violet
widely used as hole scavengers to calibrate cataysts.
• Molecular details of surface and bulk processes still poorly
understood - electron and hole trapping, role of interfacial water,
surface groups /adsorbed species, particularly at aqueous surfaces.
What can in situ IR spectroscopy reveal?
Possible
electron and
hole surface
traps
H
H

O

Ti(III)
e-

O

Ti(IV)
H
h+

O
Ti(IV)
22
Broad IR absorption from UV irradiated P25 under vacuum
Szczepankiewicz et al (J. Phys.
Chem. B 2002) - diffuse reflectance
IR of P25. Band attributed to free
carrier absorption (a) or shallow
trap excitation(c)
Yamakata et al (J. Phys. Chem. B
2001) - time-resolved IR gave
similar broad absorption. Attributed
to shallow trap excitation.
Exposure to oxygen quenched the
broad absorption.
Both studies restricted to
wavenumber >1000 cm-1
23
Diamond/ZnSe accessory/flow cell for in situ ATR-IR
studies of TiO2 photocatalysis
370 nm
SEM image of TiO2 particle film
1 m
1 m
Inert gas saturated solutions
flowed ~ 2 mL min-1 or static
24
IR absorption from UV irradiation of wet ‘anoxic’ anatase TiO2
Anatase 400 nm particle film in N2 sparged dilute aqueous flowed HCl solution,
pH of 2.3, 80 s UV
• Cutoff suggested shallow electron trap absorption into continuum of CB
states with 875 cm-1 (0.1 eV) threshold + hole oxidation of water to O2
• Absorption loss ~1660 cm-1 related to surface water bending mode?
Change in surface H2O molar absorption coefficient with charge trapping?
Warren, McQuillan J Phys Chem B 2004, 108, 19373, Savory et al J Phys Chem C 2011, 115, 90225
pH variation of broad infrared absorption
Anatase
particles
(45 nm)
Flowed
solutions
Absorption more
pronounced for
larger particle
anatase
pH sensitivity
suggests bulk
electron trap
formation
accompanied by
charge-balancing
proton (H+)
intercalation
26
Adsorbed oxalate peaks
Constant UV, then (a)
adsorption of oxalate,
then (b) desorption
adsorption
Flowed 10-3 mol L-1 oxalic in
pH 2.3 HCl, N2 sparged, P25 TiO2 particles,
constant 375 nm, 1 mW cm-2
Oxalate hole trapping
makes possible long lived
electron trapping (broad IR)
desorption
kinetics
desorption
27
Outer-sphere adsorbed oxalate (1308 ) decays faster than inner-sphere (1410)
Formate hole trapping influence on broad IR band at pH 3
Diamond
cut-out
absorptions
23 nm
particles
Flowed
solutions
In absence of an
efficient hole trap
(formate),
holes must oxidise
water to O2 at the
surface, which in
turn capture
electrons
Savory & McQuillan J Phys Chem C 2013, 117, 23645
28
Schematic of possible electron trapping + H+ intercalation
Ti(III) represents shallow trapped electron but nature of protonstabilised trap state and proton location not yet established.
Can we detect with IR the hypothesised O-H bond of the
bridged oxygen?
29
Broad IR absorption is from an electron polaron?
• Electron traps generally considered to
lie ~ 0.2 - 1.6 eV below the CB
• Peak in broad absorption coincident
with anatase LO (Eu) phonon mode
Polaron - ions move in
the lattice in response to
the electron charge – the
phonon cloud
• Electron polarons involve electron-
phonon coupling with optical modes
• The broad IR absorption arises from
the trapped electron polaron absorbing
a photon to excite the LO phonon with
the excess energy being scattered into
the polaron
• Absorbance of polaron provides a
measure of trapped electron
concentration
Wikipedia
30
Calculated IR absorption spectra for electron polarons
The real part of the optical conductivity as a function of frequency for
different densities of an interacting large-polaron gas.
31
UV excitation/decay kinetics of polaronic absorption (PA)
Absorbance
at 875 cm-1
Anatase
45
nm
particles in static HCl
solutions pH of 3
with sequential UV
irradiation times of 5,
10, 15, 30, 60, 120,
300
and
600
seconds. Final trace
with the introduction
of 10-5 mol L-1
formate ion.
Near linear
initial decay
Hole capture by formate reduces rate of H2O to O2 photogeneration and
stabilises polaronic electrons in bulk (not defects)
32
PA excitation/decay kinetics in 0.1 mol L-1 formate solution
Anatase
45 nm
• Hole trapping from
high formate
concentration drives up
trapped electron
concentration
• Higher trapped
electron concentration
at lower pH from H+
intercalation influence
• Sustained perfectly
linear decay for higher
trapped electron
loadings
33
Mechanistic insights from kinetics for TiO2 photocatalysis
Generally recognised rate-determining reaction in photocatalysis
O2(ads) + e-(surf)  O2-
Rate = k [O2]ads[e-]surf
Constant rate for a zeroth order reaction – a reaction bottleneck!
Corresponds to both adsorbed oxygen and surface electron
concentrations being saturated – giving the maximum reaction rate
First time such
zeroth order
reactions in
photocatalysis
recognised
TiO2
H2O
e- (H+) O2
O2
e- (H+-) +
e (H ) O O2
2 O2
e+- (H+-) + O
e (H ) e (H ) O 2
2 O2
e- (H+)
O
2
e- (H+) e- (H+)
O2
O
+
2O
e (H )
2
e- (H+) e- (H+)
O2
e- (H+)
e- (H+) O2
Savory & McQuillan
J Phys Chem C
2014, 118, 13680-13692
34
PA spectra from 0.1 mol L-1 formate hole-trapping solution
45 nm
particles
Difference
spectra
from B
Peak
normalised
time
dependence
pH 6.7
Temporal
variation
of
different
peaks
Secondary absorption from ‘crowded’ trap states at high concentration?
36
The ATR-IR spectra of H2O/D2O/10-3 mol L-1 formate
mixture overlaid with the PA spectrum
37
Absorbance losses for each water absorption relative to
those from the spectrum of the liquid
Calculated from the spectral data shown in A.
Suggests a polaron absorption related refractive index effect
altering the sample penetration depth in the internal reflection
38