Surface Structures of 4-Chlorocatechol Adsorbed on Titaniu

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Surface Structures of 4-Chlorocatechol Adsorbed
on Titanium DioxideScott 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

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Overview of the Presentation

• I - Motivation for the presentation
• II - Goals of the Paper
• III - Experimental Procedure
• IV - Results and Applications

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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

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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

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Diagram of Generalized Photocatalysis
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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

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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

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Why 4-Chlorocatechol?

• 4-Chlorocatechol is an intermediate in the
  oxidation of 4-chlorophenol

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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

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Experimental

• What types of experiments were run, and why
• What tools were necessary to perform these
  experiments

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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

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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

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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

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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

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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

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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

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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

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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

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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 pHs
• 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

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IR Spectra of H2CT (a), HCT- (b), and CT2-
(c)

• Deprotonation shifts bands to lower energy
• Increased negative charge electrostatically
  destabilizes the molecule
• Disrupts resonance

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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

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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

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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 pHs - contrary to
  batch adsorption
• Predictive models based upon generalized double
  layer theory

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Mass Law Equations

• Ks not given are assumed to be 1
• Using mass law relationships, several models can
  be tried
• Relationships are constrained by adsorption
  isotherms

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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

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Governing Mass Law and Mole Balance Equations

• ?Ti2CT H2CT ? CTAds (i)
• H2CT ? CTAds (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

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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 charge-potential 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

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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

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Recognition

• Dr. J.F. Gaillard
• Stumm and Morgan
• Dr. Kimberly Gray