Transcript Surface Structures of 4-Chlorocatechol Adsorbed on

Surface Structures of 4-Chlorocatechol
Adsorbed on Titanium Dioxide
Scott T. Martin, Janet M. Kesselman, David S. Park, Nathan S. Lewis, and
Michael R. Hoffman
An Oral Presentation for CE 468
February 8, 2000
Professor: J.F. Gaillard
By: Mike Marsolek
Overview of the Presentation
I - Motivation for the presentation
II - Goals of the Paper
III - Experimental Procedure
IV - Results and Applications
Motivation - Why are we interested?
•
•
Titanium Dioxide (TiO2) is used extensively in
photocatalysis
Adsorption of reactants onto the TiO2 surface is central to
understanding the mechanism as a whole
• 4-Chlorocatechol (CT) is an intermediate in the TiO2
catalyzed photooxidation of 4-chlorophenol
• I will be doing research on photobiocatalysis, which will
examine the effect of using photocatalysis as either a pre or
post treatment to bioremediation
What is Photocatalysis?
•
Photocatalysis is a technique used to degrade toxic species
into more environmentally friendly forms
• Absorption of light with energy equal to or greater then
the band gap energy results in elevation of an electron
from the “valence band” to the “conduction band”
• This elevation results in a positively charged hole in the
valence band
•
When these charge carriers occur at the surface there is
potential for oxidation/reduction reactions
Diagram of Generalized
Photocatalysis
Redox Reactions
Electron Mobilization and Hole Formation
B
B-
Conduction Band
hv
e-
++
Valence Band
A-
A
Characteristics of TiO2
• Ti forms HCP Structures
• For TiO2: pKa1 = 8.8,
pKa2 = 12.7
• TiO2 has a large band
gap energy, 3.3 eV, which
means it must be activated
by UV light
What Does Adsorption Have to
do With Photocatalysis?
• TiO2 can oxidize chlorinated hydrocarbons only if the
hydrocarbon is sorbed onto the surface
• Therefore, adsorption is of fundamental interest in the
study of photocatalysis
• A better understanding of adsorption can therefore lead
to better models and more successful applications
Why 4-Chlorocatechol?
• 4-Chlorocatechol is an
intermediate in the
oxidation of 4chlorophenol
OH
Cl
OH
4-Chlorocatechol
OH
Cl
4-chlorophenol
Goals of the Paper and Research
• “To investigate the surface structures formed between an
organic substrate (CT) and TiO2 in the context of
understanding how these specific surface interactions affect
photoreactivity.”
• To understand how pH and substrate concentration affects
adsorption
• To develop an adequate model using a generalized double
layer (Gouy-Chapman) approach
Experimental
• What types of experiments were
run, and why
• What tools were necessary to
perform these experiments
Materials
• Titanium Dioxide - Degussa brand, P25 mesh
• 4-Chlorocatechol - TCI America, recrystallized in heptane
• 1 and 10 mM KNO3
• 80 mM NaF
• 10 mM KCl
• 1 M HCl
Acid Base Titration
Procedure
• Into a sample beaker is placed a TiO2 dispersion (800 mL,
1.25 g/L), a pH electrode, a bubbler for Ar sparging, and a
tube for acid delivery
• Ionic strength is adjusted to 1 mM KNO3
• pH is adjusted to 10 with NaOH, and 0.1 N HNO3 is
introduced at 1 mL/min
• Once pH reaches 4, the ionic strength is increased, and the
titration is repeated
Acid Base Titration Purpose
• The titrations were carried out in order to determine the
moles of H+ adsorbed onto the P25 at a given pH
• This was done by calculating the difference between the
moles of H+ required to achieve a given pH in the slurry
solution vs. the moles of H+ required to reach the same pH
in a blank solution
Fluoride Titration
• Into a 1L Teflon beaker is added an 800 mL soln of 1.25
g/L P25 and 10 mM KNO3
• A pH electrode and fluoride electrode are inserted
• The pH is adjusted to 5.5 with HNO3
• 80 mM NaF is added at a rate of 233.4 M/h
• Fluoride ion adsorbed is calculated by subtracting the
solution concentration from the total amount of fluoride
added
• This provides a measure of the total capacity for adsorption
onto the TiO2
Batch Adsorption - Purpose
• To determine the amount of CT adsorbed onto TiO2 at a
given pH
• Multiple runs are done at varying pH so you can monitor
how the adsorption of CT is influenced by pH
• Can be used as a check for later measurements which will
also measure how CT adsorption varies as a function of pH
Batch Adsorption - Procedure
• Into a 250 mL three neck RB flask is added 100 mL of 1
g/L TiO2 and 10 mM KCl
• Into the necks are inserted a pH electrode, 10 mL burette,
and needles for sampling and Ar sparging
• Experiments are run at a fixed pH
• CT is added as a 1 mL aliquot, allowed to come to
equilibrium, filtered, and analyzed with UV/Vis
spectrometry
FTIR -ATR Measurements
• Stands for “Fourier Transform Infrared - Atenuated Total
Reflectance” Spectroscopy
• Allows for spectra to be taken of adsorbed species at the
catalyst-solution interface to determine what species are
present, and in what form
• Can be run continuously so that the effects of changing the
pH can be analyzed in situ (and can be compared to batch
adsorption measurements)
• Uses ZnSe crystal coated with 50 L of 53 g/L TiO2
FTIR- ATR Set Up
• A Perkin-Elmer FTIR
Spectrophotometer was
used to collect the data
• ATR serves as a modern
salt plate for studies of
non-traditional
spectroscopy experiments
IR Spectra of Adsorbed CT and Change
in Adsorbed CT As A Function of pH
• IR spectra (A) indicate the same
ionic species at all pH’s
• Amount of adsorbed species
changes with pH
• Batch Adsorption also indicates that
the amount of CT adsorbed changes
with pH, again with max at pH = 8
• Changes are due to altering the
speciation of surface TiO2 and
H2CT, and changes in surface
charge with pH
IR Spectra of H2CT (a), HCT- (b),
CT2- (c)
• Deprotonation shifts bands to
lower energy
• Increased negative charge
electrostatically destabilizes the
molecule
• Disrupts resonance
and
IR Spectra of CT With Changes in [KCl],
and component spectra
• Changes in ionic strength
affect adsorption of CT
• Residuals shown for single
Langmuirian site
• Spectra of adsorbed CT
forms
• Indicate multiple forms may
exist near surface
Component Spectra Suggest Bidentate
Surface Structure of CT
• Similarities between component 1
and CT2- suggest a bidentate
formation at the surface
• Peaks between 1268 and 1484 lie
between those of singly and doubly
deprotonated spectra.
•Indicate a net ionic charge of -1.2
Adsorption of CT as a Function of Total
Solution Concentration
• Saturation effects before 50 M
• Adsorption is directly proportional
to solution concentration after 50
M
• As solution concentration is
increased, there is greater
adsorption at lower pH’s - contrary
to batch adsorption
• Predictive models based upon
generalized double layer theory
Mass Law Equations
• K’s not given are
assumed to be 1
• Using mass law
relationships, several
models can be tried
• Relationships are
constrained by
adsorption isotherms
Agreement Between Data and Proposed
Isotherms
• Goodness of fit is
indicated by Vy
• Fit determined by
agreement with data,
and a consistent
amount of sites as
found from proton
adsorption
Description
Variance
Vy
[=TiOH]a
(M)
[=TiOH]b
(M)
Log
Ka
(-)
-1.96
Log
Kb
(-)
34.1
-3.46
-.66
Log
Ka’
(M-1)
1 bidentate
mononuclear
2 bidentate
mononuclear
1 bidentate
mononuclear
1 nonspecific
binding
1 bidentate
binuclear
1 bidentate
binuclear
1 nonspecific
binding
2 bidentate
binuclear
1 bidentate
mononuclear
1 bidentate
binuclear
1.55
264
.54
751
1.57
259
1.25
179
4.38
.24
88.5
5.27
1.13
86.0
1.27
179
Log
Kb’
(M-1)
-1.93
108
Log
Kads’
(M-1)
-0.20
4.65
-5.17
Log
Kads
(M-1)
4.38
8.93
3.97
Governing Mass Law and Mole Balance
Equations
Ti2CT + H2CT  CT’Ads (i)
H2CT  CT’Ads (ii)
• (i) Indicates that bound CT increases the affinity for further
adsorption
• (ii) Indicates that a solution phase CT (H2CT) is adsorbed
without changing the concentration of Ti2CT
- Nonspecific Adsorption
Comments About Double Layer Model
• The generalized double layer approach worked adequately
at low ionic strengths
• It underestimated the surface charge at high ionic strengths
and with strongly negative surfaces
• The double layer model used here, Gouy Chapman theory,
is based upon a flat plane when calculating chargepotential relationships
• However, the TiO2 surface structure depends on the local
geometry and therefore is highly heterogeneous
• Thus, Gouy Chapman gives an averaged value of the
structure of TiO2
Conclusions
• TiO2 Adsorption is strongly influenced by pH, and its pH
history
• Most efficient processes are expected to occur near pH 5
• CT adsorbs as a binuclear surface complex monolayer at
concentrations less than or equal to 50  M
• Above a concentration of 50  M, CT adsorbs
nonspecifically as a multilayer complex
• Gouy Chapman double layer theory adequately predicts
adsorption at low ionic strengths, but fails as ionic strength
is increased due to the flat plane assumption
Recognition
• Dr. J.F. Gaillard
• Stumm and Morgan
• Dr. Kimberly Gray