INTRODUCTION OF XPS MECHANISM OF XPS CHEMICAL SHIFT IN XPS STRENGTHS AND LIMITATIONS OF XPS INTRODUCTION OF ESR MECHANISM OF ESR PRESENTATION OF ESR SPECTRUM APPLICATION OF ESR ADVANTAGES AND DISADVANTAGES OF ESR

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SEMINAR ON :
XPS AND ESR
CATALYTIC REACTION
ENGINEERING
(HCE23)
DONE BY:
KISHAN KASUNDRA

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CONTENTS:-
INTRODUCTION OF XPS
MECHANISM OF XPS
CHEMICAL SHIFT IN XPS
STRENGTHS AND LIMITATIONS OF XPS
INTRODUCTION OF ESR
MECHANISM OF ESR
PRESENTATION OF ESR SPECTRUM
APPLICATION OF ESR
ADVANTAGES AND DISADVANTAGES OF ESR

3. XPS (X-RAY PHOTOELECTRON SPECTROSCOPY):
INTRODUCTION:-
X-ray photoelectron spectroscopy (XPS) is a surface-sensitive
technique that measures the elemental composition, empirical
formula, chemical state and electronic state of the elements that
exist within a material.
In 1887, Heinrich Rudolf Hertz discovered but could not explain the
photoelectric effect, which was later explained in 1905 by Albert
Einstein (Nobel Prize in Physics 1921). Two years after Einstein's
publication, in 1907.
A few years later in 1967, Siegbahn published a comprehensive study
of XPS, bringing instant recognition of the utility of XPS, which he
referred to as ESCA (Electron Spectroscopy for Chemical Analysis).
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4. MECHANISM:-
Photoelectron spectroscopy works by directing a beam of
monoenergeticphotons in the direction of a sample.
The photons have certain energy that when they hit electrons in the atoms
of the sample with the energy necessary the electrons from the atoms in
the sample are ejected from the atom.
The electrons ejected are analyzed in the XPS detector by measuring
electrons kinetic energy which provides the information to determine the
kind of elements present in the sample.
A typical XPS spectrum is a plot of the number of electrons detected
(sometimes per unit time) (Y-axis, ordinate) versus the binding energy of
the electrons detected (X-axis, abscissa).
Each element produces a characteristic set of XPS peaks at characteristic
binding energy values that directly identify each element that exists in or
on the surface of the material being analyzed.
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5. The number of detected electrons in each of the characteristic peaks is
directly related to the amount of element within the XPS sampling
volume.
To count the number of electrons during the acquisition of a spectrum
with a minimum of error, XPS detectors must be operated under ultra-
high vacuum(UHV) conditions because electron counting detectors in
XPS instruments are typically one meter away from the material
irradiated with X-rays. This long path length for detection requires such
low pressures.
Generally for X-Ray source Mg Kα (1253.6 eV) and Al Kα (1486.6eV) are
used.
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7. CHEMICAL SHIFT IN XPS:
It is change in binding energy of a core electron of an element due to a
change in the chemical bonding of that element.
Withdrawal of valence electron charge increase in Binding
(oxidation) Energy
Addition of valence electron charge decrease in Binding Energy
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8. STRENGTHS OF X-RAY PHOTOELECTRON
SPECTROSCOPY:-
XPS is routinely used to analyze inorganic compounds, metal alloys,
semiconductors, polymers, elements, catalysts, glasses, ceramics, paints,
papers, inks, woods, plant parts, make-up, teeth, bones, medical implants,
bio-materials, viscous oils, glues, ion-modified materials and many others.
It provides information about chemical bonding.
XPS detects all elements with an atomic number (Z) of 3 (lithium) and
above.
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9. LIMITATIONS FOR X-RAY PHOTOELECTRON
SPECTROSCOPY:-
It gives limited organic information (short-range bonding only)
Samples must be ultra high vacuum compatible.
Samples that decompose under X-ray irradiation cannot be studied.
It is very expensive technique.
It is slow process to analyze the sample (1/2 to 8 hours/sample).
Large area analysis is required.
It cannot easily detect hydrogen (Z = 1) or helium (Z = 2).
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10. ESR (ELECTRON SPIN RESONANCE):-
ESR was first observed in Kazan State University by Soviet physicist
Yevgeny Zavoisky in 1944 and was developed independently at the same
time by Brebis Bleaney at the University of Oxford.
Electron Spin Resonance (ESR), sometimes referred to as Electron
Paramagnetic Resonance (EPR), is a widely accepted spectroscopic
technique in various research fields.
It is a branch of absorption spectroscopy in which radiation having
frequency in microwave region is absorbed by paramagnetic substance.
This technique is used to study paramagnetic centers on various oxide
surfaces, which are frequently encountered in heterogeneous catalysis.
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11. MECHANISM:-
In ESR the energy levels are produced by the interaction of magnetic
moment of an unpaired electron in a molecule ion with an applied
magnetic field.
The unpaired electrons are excited to a high energy state under the
magnetic field by the absorption of microwave radiations.
The excited electron changes its direction of spin and relaxes in to the
ground state by emitting its energy.
When a molecule or compound with an unpaired electron is placed in a
strong magnetic field, the spin of the unpaired electron can align in two
different ways creating two spin states, ms = ± ½.
The alignment can either be along the direction (parallel) to the magnetic
field which corresponds to the lower energy state ms = - ½ or opposite
(anti parallel) to the direction of the applied magnetic field ms = + ½ .
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13. The energy difference is given by:
∆ E = E+ - E- = hv = gµbB
Where, h = Plank’s constant = 6.626 x 10-34 J s-1
µb = Bohr’s magneton which is a factor for converting angular
momentum into magnetic moment. (9.274 x 10-24 J T-1)
g = the g-factor = 2.00232 for a free electron
B = strength of the magnetic field in Tesla
The value of µb is given as µb = eh/4πmc
Where, e = electric charge
m = mass of electron = 9.10938356 × 10-31 kg
c = velocity of light = 3 x 108 m/s
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15. The Klystron oscillator is set to produce microwaves.
After passing though the isolator, wave meter and attenuator the
microwaves are entered into the circulator on T.
Then it reaches the detector which acts as a rectifier, i.e. converting the
microwave power into the direct current.
If the magnetic field around the resonating cavity having the sample is
changed to the value required for the resonance, the recorder will show an
absorption peak.
If the magnetic field is swept slowly over a period of several minutes, the
recorder will show the derivative of the microwave absorption spectrum
against magnetic field as shown below:
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17. PRESENTATION OF ESR SPECTRUM:-
The ESR spectrum is obtained by plotting intensity against the strength of
a magnetic field.
The better way is to represent ESR spectrum as a derivative curve in which
the first derivative (slope) of the absorption curve is plotted against the
strength of the magnetic field.
The total area covered by either the absorption or derivative curve is
proportional to the number of unpaired electrons in the sample.
In order to find out the Number of electron in an unknown sample,
comparison is made with a standard sample having a known number of
unpaired electrons.
The most widely used standard is 1,1-diphenyl-2-picrylhydrazyl free
radical(DDPH).
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18. APPLICATION OF ELECTRON SPIN RESONANCE:-
ESR spectroscopy is used in various branches of science, such as biology,
chemistry and physics, for the detection and identification of free radicals
and paramagnetic centers.
It can be applied to a wide range of materials such as carbonates, sulfates,
phosphates, silica or other silicates.
EPR/ESR spectroscopy has been used to measure properties of crude oil, in
particular asphaltene and vanadium content.
In certain cases ESR provides the information about the shape of the
radicals.
In the biological field, the presence of free radicals can be find out in
healthy and diseased condition and functioning of most of the oxidative
enzymes can be confirmed.
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19. ADVANTAGES OF ESR:-
EPR is a very sensitive technique and is capable of providing useful data in
volumes as low as 300μL and concentrations as low as 1 μM.
Furthermore, EPR spectra can be readily taken in 15-20 minutes once the
equipment is prepared.
DISADVANTAGES OF ESR:-
Although EPR has high specificity - that specificity relies on unpaired
electrons which might not be relevant to every system being studied.
Most paramagnetic materials need temperatures as low as 20K for
detection which can be an expensive constraint.
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20. REFERENCES:-
https://en.wikipedia.org/wiki/Electron_paramagnetic_resonance
https://en.wikipedia.org/wiki/X-ray_photoelectron_spectroscopy
http://iiith.vlab.co.in/?sub=19&brch=206&sim=583&cnt=1
http://phys.ubbcluj.ro/~vchis/lab/esr1.pdf
http://nptel.ac.in/courses/115101003/downloads/module2/lecture23.pdf
http://faculty.georgetown.edu/popem/esr01.pdf
http://www.authorstream.com/Presentation/samjescy-1392513-electron-
spin-resonance-spectroscopy/
https://phys.libretexts.org/LibreTexts/University_of_California_Davis/U
CD%3A_Biophysics_200A/Electron_Paramagnetic_Resonance
http://www.ummto.dz/IMG/pdf/TEZKRATT_Said.pdf
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21. THANK YOU !!
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