Download Chemical Dynamics at Surfaces

9/15/2014
Dynamics at Surfaces

Goal of the field of dynamics at surfaces
– to interpret macroscopic surface chemistry with first principles
concepts at an atomic level.
Chemical Dynamics at Surfaces

Goals of this course:
– to describe the basic concepts needed to understand surface
chemistry
– as well as the modern experimental methods used to probe the
limits of this understanding.
Dalian Institute for Chemical Physics
16 September – 1 December 2014
© 2014 D.J. Auerbach – All rights reserved
Sept. 13, 2014
Course Description

20 1.5 hour lectures from 16 September to 20
November 2014.

Prerequisites: a knowledge of introductory kinetics,
quantum mechanics, thermodynamics and statistical
mechanics.

Text book – none.

The course is based on a survey of classic and current
scientific papers.

The course outline has the reading list. This will be
updated as we proceed through the semester.
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Course outline, lecture notes, and other material will be
posted
Sept. 13, 2014
Lecture 1 -- Chemical Dynamics at Surfaces -- Dalian 2014
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Format of Outline and Reading List
Outline
1. Early Work on reactions at surfaces (Lectures 1 and 2)
Key Papers: (1; 2)
a. Historical background – early work on surface chemistry
and the discovery of catalysis
b. Importance of Catalysis: Haber-Bosch process of ammonia
synthesis(3; 4)
c. Irving Langmuir and birth of modern surface chemistry (1;
2; 5-8)
Reading List
1. Langmuir I. 1906. The dissociation of water vapor and carbon
dioxide at high temperatures. J. Am. Chem. Soc. 28:1357-79
2. Langmuir I. 1932. Nobel Lecture - Surface Chemistry
3. Haber F, van Oordt G. 1905. On the formation of ammonia
from the elements. Zeitschrift Fur Anorganische Chemie
44:341-78
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Lecture Schedule
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Grades
Tuesday and Thursday, 9-10:30 AM
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Homework for lectures 1 and 2 assigned
– Lecture 3, Tuesday September 23
– Lecture 4, Thursday September 25
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Homework for lectures 1 and 2 due
Homework for lectures 3 and 4 assigned
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Date:
November 27
Homework for lectures 3 and 4 due
Homework for lectures 5, 6, 7, and 8 assigned
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Philosophy of the class

The lectures provide fence posts
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You have to fill in between through external reading and
study.

You will never fully master this (or any other) field. You
can only continue to broaden your knowledge. Learning
is a life-long process.

3 hour written exam
Lecture 5, Sunday September 28
Lecture 6, Tuesday September 30 (no homework)
Lecture 7, Thursday October 9
Lecture 8, Saturday October 11
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Sept. 13, 2014
Format:
Location: TBD
Except for national holiday, October 1-7
–
–
–
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Grades will be based on one final exam and homework
– 30% homework and 70% final exam
– Or 100% final exam whichever is greater
– Lecture 1, Tuesday September 16
– Lecture 2, Thursday September 18
Sept. 13, 2014

Early Work on reactions at surfaces (Lectures 1 and 2)
– Historical background – early work on surface chemistry and the
discovery of catalysis
– Importance of Catalysis: Haber-Bosch process of ammonia
synthesis
– Irving Langmuir and birth of modern surface chemistry

Framing the Problem: formulation of the modern
approach (Lectures 3 and 4)
– Born-Oppenheimer Approximation and potential energy surfaces
(PES)
– Standard Model of Chemical Reactivity
– Gas phase reaction dynamics – methods to probe the PES

– Key papers for each lecture
– Additional reading to broaden your knowledge
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Course Outline
I will strive to provide an excellent reading list and to
indicate which outside reading fits each lecture best.
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Energy requirements and energy disposal
Molecular beam scattering techniques
Comparison to ab initio theory
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Course Outline
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Course Outline
What is different about surface dynamics (Lectures 5-6)
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– It is not possible to ensure single collision conditions
– Complex target:
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– IR linewidths
– Energy pooling
– Real time measurements of vibrational damping
geometrical structure
electronic structure
complex elementary excitations: phonons, electron hole pairs
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– The need for Ultra High Vacuum (UHV) and surface analysis
– Reactions often involve a complex sequence of events
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Adsorbate Dynamics (Lectures 9-10)
Energy transfer processes involving electron hole pairs
(Lectures 11 and 12)
–
–
–
–
–
Energy transfer processes involving phonons in nonreactive systems (Lectures 7-8)
– Molecular Translation (T) coupling to phonons (ph), rotation (R),
vibration (V)
– Trapping and detailed balance
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Single quantum vibrational excitation
Multi quantum vibrational excitation
Vibrational Relaxation
Electronic friction and Surface Hoping Theory
Vibrationally promoted electron emission
Reactive Processes at Surfaces (Lecture 13-14)
– paradigm of reaction mechanisms from kinetics
– probing reaction mechanisms with dynamics experiments
– translational and vibrational activation of dissociative adsorption
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Course Outline
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Lecture 1 -- Chemical Dynamics at Surfaces -- Dalian 2014
Acknowledgement
Provisional Model of Surface Reactions Dynamics (15 –
18)
These lectures are based on a
course and possibly a book that
Alec Wodtke and I are developing.
– Comparison of experiment and ab initio theory
– Dissociative adsorption and associative desorption of Hydrogen
on Copper

Alec Wodtke presented the first
version of these lectures last fall at
the University of Gottingen and
prepared initial versions of many
of the slides that I will be using.
“The H+H2 reaction of surface chemistry”
– CH4 dissociation on Ni surfaces
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Importance of reagent vibration and surface motion
Reactions of radicals with surfaces (19-20)
Alec Wodtke
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Outline
 History
Lecture 1 - Introduction
of early work on reactions on surfaces
 The
Haber-Bosch process of ammonia synthesis
from the elements
 Langmuir
chemistry
and the birth of modern surface
– Langmuir’s first paper: surface reactions are fast
History of early work on reactions at
surfaces and the birth of modern
surface chemistry
– “Hydrogen clean up”: concept of molecular monolayer
– “Oxygen clean up”: the Langmuir-Hinschelwood
mechanism of surface reactions
© 2014 D.J. Auerbach – All rights reserved
Sept. 13, 2014
Davy – Explosion Proof Lamp for Miners
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Davy – Explosion Proof Lamp (1817)
Idea was to isolate flame from
explosive mixture by material
that would cool the flame
– Initially experiments with small
tubes
– Later, metal gauze surrounding
the flame
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Pt was special: Pt gauze
glowed when exposed to
gasses from coal mines
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References – Davy Safety Lamp
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Döbereiner’s Lighter – first catalysis
“Some New Experiments and Observations on the Combustion of
Gaseous Mixtures, with an Account of a Method of Preserving a
Continued Light in Mixtures of Inflammable Gases and Air without
Flame”
Humphry Davy, Philosophical Transactions of the
Royal Society of London, Vol. 107 (1817), pp. 77-85
– URL: http://www.jstor.org/stable/107574 .

“Some Researches on Flame”,
Humphry Davy, Abstracts of the Papers Printed in the Philosophical
Transactions of the Royal Society of London, Vol. 2 (1815 - 1830),
pp. 59-61
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Pt gauze glowed on exposure to
hydrogen and oxygen
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Temperature increased with
porosity of gauze
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Tremendous excitement - "cold
fusion" of 1823
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Generally regarded as the 1st
example of catalysis
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Used to make a lighter
– URL: http://www.jstor.org/stable/109871
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“On the Safety Lamp for Coal Miners with some Researches
on Flame”, Sir Humphry Davy, 1825
– Book available from Google Books
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Döbereiner’s Lighter (1823)
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Reactions
Zn + H2SO4 → Zn2+ + SO42- + H2
2 H2 + O2 --> H2O
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Modern view of mechanism
O2 + Pt → Pt-O2 → Pt-O (a)
H2 + Pt → Pt-H (a)
H2S04
Zn + H2SO4 → Zn2+ + SO42- + H2
Lecture 1 -- Chemical Dynamics at Surfaces -- Dalian 2014
Döbereiner’s Lighter
Pt Sponge
Zn
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1829, Berlin manufacturer offered
"… as a pleasant and useful
Christmas present a lighting
machine, outfitted with platinum,
elegant, clean, and sturdily
constructed, with Chinese and
other decoration, insensitive to
wetness and cold…."
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Early Work on Surface Reactions

Davy (1817)
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– explosion proof lamp for miners
– Pt gauze glowed when exposed
to gasses from coal mines
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Faraday (1834)
– reacting gasses held on Pt by
electrical forces

Berzelius (1836)
J. B. A. Dumas
1800-1884
– reacting gasses held on Pt by a
"catalytic force ... not
independent of the affinities of
matter, but only a new
manifestation of the same".
– Origin of term catalysis
Döbereiner(1823)
– Pt gauze glowed on exposure
to hydrogen and oxygen
– Increased with porosity of
gauze
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Early Work on Hydrogen adsorption: I
Henry (1824)
– Davy lamp reactions
– 2 H2 + O2 --> H2O
– 2 CO + O2 --> CO2
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Dumas (1843)
– Quantitative study
of adsorption of
H2 on Cu
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Early work on Hydrogen Adsorption II
Ammonia synthesis from its
elements
Nobel Prize Chemistry, 1918
Fritz Haber
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One of the most important discoveries in
the field of chemistry in the last 100 years
N2+ 3H2→2NH3
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Fritz Haber
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Chemical Reactions are typically faster at elevated
temperatures
– Usual approach: Use a catalyst and heat it up
Fully 40% of the N atoms
found in all foodstuffs
world wide were converted
in a Haber reaction
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An active catalysts accelerates the reaction in both
directions
– That is it speeds up the path to equilibrium
Ammonia is the feedstock
for the fertilizer industry
1-2% of the worlds energy
supply is used to run this
reaction
Equilibrium in heterogeneous catalysis

– you don’t have to heat it up as much to get the desired rate
– selectively produces the desired product with high yield
– is resistant to poisoning by products and trace contaminants
The Nobel Prize in Chemistry
1918 was awarded to Fritz
Haber "for the synthesis of
ammonia from its elements".
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What makes a ‘good’ catalyst?
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Haber-Bosch: N2+ 3H2→2NH3
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Equilibrium measurements
ΔG/RT
Keq
½ N2+ 3/2 H2→NH3
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Haber-Bosch: N2+ 3H2→2NH3
Equilibrium measurements
Keq
Lecture 1 -- Chemical Dynamics at Surfaces -- Dalian 2014
ΔG/RT
½ N2+ 3/2 H2→NH3
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Haber-Bosch: N2+ 3H2→2NH3
Haber-Bosch: A closer look
Equilibrium measurements
Equilibrium measurements
Keq
ΔG/RT
N2+ 3 H2→2 NH3
Haber had a problem:
The equilibrium is unfavorable
Le Chatelier’s principle wants high
pressure and low temperature.
He needed high temperature to
get adequate rates. At high
temperature the equilibrium
favors reactants. Only at very
high pressure could progress be
made
Haber had a problem:
The equilibrium is unfavorable
He needed high temperature to get
reaction to speed up. At high temperature
the equilibrium favors reactants. Only at
very high pressure (and relatively low
temperature) could progress be made.
½ N2+ 3/2 H2→NH3
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Eventually Haber and Bosch got to this regime
Nobel Citation
 The Nobel Prize in
Chemistry 1931 was
awarded jointly to Carl
Bosch and Friedrich
Bergius "in recognition of
their contributions to the
invention and
development of
chemical high pressure
methods"
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High pressure chemistry was essential to
ammonia synthesis
An engineering feat of the time
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ΔH = -92.4 kJ/mole
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Carl Bosch
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How does the full process really work
1.
Gas Cleaning: methane from natural gas is cleaned, mainly
to remove sulfur impurities that would poison the catalysts
2.
Steam Reforming: the cleaned methane is reacted
with steam over a catalyst of nickel oxide.
3.
Secondary reforming: addition of air to convert the methane
that did not react during steam reforming.
Finally -- Ammonia Synthesis
Haber Process: synthesis of ammonia using a form
of magnetite, iron oxide, as the catalyst:
6.
CH4 + H2O → CO + 3 H2
N2 (g) + 3 H2 (g)
Conditions of Operation
2 CH4 + O2 → 2 CO + 4 H2 as well as
CH4 + 2 O2 → CO2 + 2 H2O
– 15–25 MPa (150–250 bar)
– 300 and 550 °C
Water gas shift reaction yields more hydrogen from CO and
steam.
4.
– four beds of catalyst, with cooling between each bed to maintain a
reasonable equilibrium constant. On each pass only about 15%
conversion occurs
CO + H2O → CO2 + H2
Methanator: Remove any residual CO by converting most of
the remaining CO into methane for recycling to avoid CO
poisoning of the catalyst
5.
2 NH3 (g) (∆H = -92.4 kJ·mol−1)
– Recycle unreacted gases so that eventually an overall conversion
of 98% can be achieved.
CO + 3 H2 → CH4 + H2O
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Engineering Side
Conditions: 150–250 bar and 300 to 550 °C
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Multiple passes over four beds of catalyst, with cooling
between each pass
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On each pass only about 15% conversion occurs, but
recycling any unreacted gases gives 98% conversion
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Impact of the Haber-Bosch Process
N2 (g) + 3 H2 (g) → 2 NH3 (g) (∆H = -92.4 kJ·mol−1)
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The Haber process now produces 100 million tons of
nitrogen fertilizer per year, mostly in the form of
anhydrous ammonia, ammonium nitrate, and urea.
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This fertilizer is responsible for sustaining one-third of the
Earth's population

3–5% of world natural gas production is consumed in the
Haber process (~1–2% of the world's annual energy
supply).

Hydrogen production using electrolysis of
water powered by renewable energy is not yet costcompetitive with hydrogen from fossil fuels, such as
natural gas, and so has been responsible for only 4% of
current hydrogen production.
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Impact of the Haber-Bosch Process
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Deleterious environmental effects
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The cheap and efficient industrial production of NH3 also
played a crucial role in munitions manufacture, literally
lighting the fire that would become World War I.
The Birth of Modern Surface
Chemistry
– Without Haber-Bosch process Germanys production of
ammunition and fertilizer would have collapsed in 1914.
It happened in Göttingen
© 2014 D.J. Auerbach – All rights reserved
Sept. 13, 2014
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Irving Langmuir
1927 Solvay Conference
The Nobel Prize in Chemistry
1932 was awarded to Irving
Langmuir "for his discoveries
and investigations in surface
chemistry".

“Summit conference” of outstanding physicists and chemists
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Topic “Electrons and Photons”
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Einstein: “God does not play dice”; Bohr: “Einstein, stop telling God what to do!”
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19 of 27 participants became Nobel prize laureates.
Irving Langmuir was born in Brooklyn,
New York, on January 31, 1881, as
the third of four sons of Charles
Langmuir and Sadie, neé Comings.
His early education was obtained in
various schools and institutes in the
USA, and in Paris (1892-1895).
He graduated as a metallurgical
engineer from the School of Mines at
Columbia University in 1903.
Postgraduate work in Physical
Chemistry under Nernst in Göttingen
earned him the degrees of M.A. and
Ph.D. in 1906.
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Returning to America, Dr. Langmuir became
Instructor in Chemistry at Stevens
Institute of Technology, Hoboken, New
Jersey, where he taught until July 1909. He
then entered the Research Laboratory of
the General Electric Company at
Schenectady where he eventually became
Associate Director.
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Irving Langmuir: A pioneer of surface science

Langmuir left his mark in physical
chemistry including
Irving Langmuir’s first paper
– Langmuir Isotherm
– Langmuir-Hinshelwood mechanism
– Langmuir is the unit of exposure

Approach: look at historical
papers describing the
experiments Langmuir performed
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Lecture 1 -- Chemical Dynamics at Surfaces -- Dalian 2014
As a graduate student at the
Institute for Physical Chemistry in
Göttingen under the mentorship of
Walter Nernst
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What was Nernst doing in
Göttingen at that time?
He was studying Equilibrium
JACS 28 1357 (1906)
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Irving Langmuir’s first paper:

Langmuir’s Method
Introduction - describe the state of the art – two methods
1. Measure at temperature

Heat a sample to high
temperature to establish
chemical equilibrium
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2. Go to high temperature, establish equilibrium,
quench, measure
H2 + ½ O2
Lecture 1 -- Chemical Dynamics at Surfaces -- Dalian 2014
Rapidly quench and hope
kinetics are too slow to
keep up
H2O
H2 + ½ O2
(favors H2O at low T)
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Irving Langmuir’s First paper
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H2O
(favors H2 / O2 at High T)
…
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Equilibrium
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Irving Langmuir’s first paper
Stating the central problem
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Describe the new method to overcome the problem
…
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Nernst’s Suggestion
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Langmuir’s hot wire apparatus
Rapidly flowing gas. Before exposure to
the wire: gaseous water vapor

At ‘F’, water was
electrolyzed with platinum
electrodes to produce H2
and O2 vapor.
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The gas mixture flowed
over the hot platinum wire

Water was condensed in
Tube ‘T’
After exposure: equilibrium mixture of
H2, O2 and H2O at the wire temperature
If mixing is fast enough, eventually the
whole sample comes to the chemical
equilibrium at the temperature of the
wire.
In reality it was more convenient to run
the experiment backwards: H2 and O2
as educts.
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Chemical Analysis at the time
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Water could be condensed by an ice bath and separated
from the mixture.
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O2 could be condensed with liquid Nitrogen
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Results: astonishingly fast surface
chemistry
– Remaining vapor pressure due to H2
– In absence of LN total pressure of H2 and O2 could be measured.
– Ideal gas Law and Henry law analysis.
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What Langmuir must have been
thinking
End of Lecture 1
Wow!!!!
These surface
reactions are so
fast. That must
be something
nice for future
study in its own
right.
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Lecture 1 -- Chemical Dynamics at Surfaces -- Dalian 2014
History of early work on reactions at
surfaces and the birth of modern
surface chemistry
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