Particulate Formation, Evolution, and Fate - Hope Michelson Day 1 Part 2
Summary
TLDRThe script discusses various aspects of soot formation in flames, including size distributions, extinction spectra, and the absorption characteristics of soot at different flame heights. It delves into the maturity of soot particles, the significance of the dispersion exponent, and the presence of aliphatic and oxygenated species within them. The role of radicals, resonance-stabilized radicals, and their association with five-membered rings in soot particle inception is also explored. The talk highlights the importance of kinetics over thermodynamics in soot formation and the unexpected presence of certain radicals and compounds, such as furans, in flame-generated particles.
Takeaways
- 🔍 The script discusses the properties and behavior of soot particles, particularly their size distribution, absorption characteristics, and composition at different heights within a flame.
- 🧐 The absorption spectrum of soot changes as it matures, with stronger absorption at shorter wavelengths at the base of the flame and a broader, flatter spectrum as the soot matures higher up.
- 📊 The dispersion exponent, also known as the Angstrom exponent, is a crucial parameter that indicates the maturity of soot particles and is used to differentiate between flaming and smoldering combustion sources in atmospheric science.
- 🌐 The optical band gap and the absorption cross-section are related to the dispersion exponent and provide insights into the structure and maturity of soot particles.
- 📚 The script mentions that as soot matures, it exhibits an increase in long-range order and conjugation length, which are associated with the stacking and ordering of graphene-like sheets within the particles.
- 🔬 High-resolution TEM and AFM imaging, along with density functional calculations, reveal the presence of spherical, waxy particles and their structural evolution within a flame.
- 🌐 The composition of soot particles is explored through various experiments, including the use of scanning mobility particle sizer and aerosol mass spectrometry, which show a significant abundance of aliphatic groups and a carbon to hydrogen ratio around 1.4.
- 🔍 The presence of oxygenated species and aliphatic side chains in soot particles was unexpected and suggests a more complex chemical composition than previously thought.
- 🧪 An experiment by High Wong's group using IR spectroscopy on extracted soot revealed the presence of oxygen, challenging the assumption that soot should be primarily carbon and hydrogen.
- 🔬 The script also touches on the use of X-ray photoelectron spectroscopy to investigate the bonding within soot particles and the unexpected discovery of furan structures in flame-generated particles.
Q & A
What are the issues with measuring extinction in the flame itself?
-The script mentions that there are some technical issues with performing extinction measurements directly in the flame. While it does not specify the issues, common challenges can include flame instability, high temperatures, and the presence of other reactive species that can interfere with the measurements.
How does the absorption cross-section of soot change as the soot matures in a flame?
-As soot matures in a flame, the absorption cross-section becomes broader and flatter. This is indicated by the spectrum changing from stronger absorption at shorter wavelengths at the lower parts of the flame to a more even distribution as the soot particles grow and mature.
What is the significance of the dispersion exponent or Angstrom exponent in the study of soot?
-The dispersion exponent, also known as the Angstrom exponent, is a measure of how the absorption cross-section of a particle changes with wavelength. It is used to indicate the maturity of soot particles, with values close to one suggesting fully mature soot, often found in large flames. Higher values suggest less mature particles.
How does the optical band gap relate to the dispersion exponent?
-The optical band gap, derived from spectral measurements, increases as the dispersion exponent increases. This suggests that as soot particles mature and their absorption cross-section broadens, the range of wavelengths they absorb also increases.
What does the term 'Rayleigh particles' refer to in the context of soot?
-Rayleigh particles refer to particles that are much smaller than the wavelength of light they scatter. The script mentions that for Rayleigh particles with long wavelengths, the absorption cross-section is proportional to the particle diameter cubed over six times the wavelength of light.
What are the challenges in extracting and measuring soot particles from a flame?
-The script alludes to the difficulty of making measurements on soot particles extracted from a flame. Challenges include maintaining the integrity of the particles during extraction, avoiding contamination, and accurately measuring properties such as size, structure, and composition.
What compositional information can be derived from high-resolution AFM images of soot particles?
-High-resolution AFM images can reveal the structure of soot particles, including the presence of six-membered and five-membered rings, as well as bridges between aromatic structures. These images can provide insights into the chemical composition and maturity of the soot particles.
How does the carbon to hydrogen ratio in soot particles change as they mature?
-As soot particles mature, the carbon to hydrogen ratio increases. This is due to the formation of additional aromatic rings, which add carbon atoms without a proportional increase in hydrogen atoms, leading to a higher carbon to hydrogen ratio.
What is the significance of the presence of aliphatic side chains in soot particles?
-The presence of aliphatic side chains in soot particles is significant as it affects the carbon to hydrogen ratio and the overall composition of the particles. It also suggests that soot formation may involve different chemical pathways than previously thought, with aliphatic structures playing a more prominent role.
What role do resonance-stabilized radicals (RSRs) play in the formation of soot particles?
-Resonance-stabilized radicals, or persistent radicals, are believed to play a significant role in the formation of soot particles. They are more stable than non-resonance stabilized radicals and are associated with incipient particles, suggesting they may be involved in the initial stages of soot nucleation and growth.
Outlines
🔬 Soot Formation and Maturity Analysis
The paragraph discusses the study of soot formation in premixed flames using sethalene as fuel. It delves into the size distributions and extinction spectra of soot at different flame heights, noting the shift in absorption spectra as soot matures. The concept of the dispersion exponent, or angstrom exponent, is introduced as a measure of soot maturity, with values close to one indicating mature soot typical of flaming combustion. The paragraph also touches on the atmospheric implications of these findings, relating the dispersion exponent to the source of atmospheric particles, differentiating between smoldering and flaming combustion.
📊 Optical Band Gap and Soot Particle Structure
This section explores the relationship between the optical band gap and the dispersion exponent, highlighting how the band gap increases with the exponent. The optical band gap is used to infer the size of graphene sheets stacked on top of each other. The paragraph also discusses the increase in long-range order and conjugation length with soot maturity. It presents data from TEM and AFM imaging, showing the structure of incipient particles and how they appear spherical and waxy. The difficulty of extracting and measuring these particles from a flame is also acknowledged.
🌟 Composition and Structure of Soot Particles
The paragraph investigates the composition of soot particles, focusing on the carbon to hydrogen ratio and the presence of aliphatic side chains. It describes an experiment using Raman spectroscopy to analyze the material, revealing a structure similar to graphite with defects. The analysis includes the conjugation length, indicative of particle maturity, and the slope of the photoluminescence background, which provides a measure of the carbon to hydrogen ratio. The presence of aliphatic groups is also noted, suggesting a more complex composition than previously thought.
🔍 Carbon to Hydrogen Ratio and Soot Maturation
This section examines how the carbon to hydrogen ratio changes as soot particles mature. It discusses experiments conducted at different heights within a flame, showing an increase in the number of carbons relative to hydrogens as the particles mature. The paragraph explains that the formation of additional rings in the particle structure leads to a higher carbon hydrogen ratio, as hydrogens are replaced by carbons. The importance of considering aliphatic side chains in these measurements is emphasized, as they can significantly alter the ratio.
🔥 Pyrolysis Experiments and Hydrogen Elimination
The paragraph details pyrolysis experiments conducted with propane and propene, where particles were extracted from the flame and analyzed using aerosol mass spectrometry. The results showed a low carbon to hydrogen ratio even for large molecules, suggesting the presence of aliphatic side chains or alpha species. The paragraph also discusses the increase in the carbon to hydrogen ratio with temperature, indicating the elimination of hydrogen as particles mature in the flame.
🌿 Aliphatic Side Chains and Oxygenated Species in Soot
This section presents findings from experiments that revealed a significant presence of aliphatic side chains and oxygenated species in soot particles, contradicting previous assumptions. It discusses the use of IR spectroscopy to detect oxygen in soot and the observation of aliphatic to aromatic ratios that were higher than expected. The paragraph also mentions experiments that showed particle size distributions changing with flame height, indicating particle growth and maturation.
🌀 Unanticipated Presence of Oxygenated Species in Soot
The paragraph discusses the unexpected discovery of oxygenated species within soot particles, even in high-temperature environments where oxidation should lead to CO or CO2 emissions. It describes an experiment that identified mass peaks corresponding to oxygenated species, suggesting the presence of ether groups and furans, which are known pollutants. The use of x-ray photoelectron spectroscopy to confirm the presence of these species is also mentioned.
🔬 Mass Spectrometry Analysis of Soot Composition
This section delves into the use of mass spectrometry to analyze the composition of soot particles. It describes experiments where particles were collected from flames and vaporized for mass spectrometry analysis, revealing common peaks associated with pericondensed polycyclic aromatic hydrocarbons. The paragraph discusses the assumption that these peaks represent the most thermodynamically stable species, known as stabilimers, at specific carbon to hydrogen ratios.
🚫 Challenge to Stabilimer Assumption in Soot Formation
The paragraph presents evidence challenging the assumption that stabilimers are the primary species seen in mass spectra of soot. It describes experiments conducted at varying photon energies to ionize molecules, which revealed that the species present were not the expected stabilimers. The use of photoionization efficiency curves to identify isomers at specific masses is discussed, showing that the actual species present were a combination of different compounds, indicating that kinetics, not just thermodynamics, plays a significant role in soot formation.
🌱 Insights into Soot Inception and Radical Species
This section explores the role of radicals in soot inception, noting the presence of odd-numbered mass peaks in mass spectrometry data, which are indicative of radical species. It discusses the use of electron paramagnetic resonance (EPR) to detect radicals in incipient particles, and the association of these radicals with five-membered rings. The paragraph also highlights the concept of resonance-stabilized radicals (RSRs) and their potential involvement in combustion chemistry and soot formation.
📉 Molecular Weight Growth and Soot Particle Evolution
The final paragraph discusses the growth of molecular weight in soot particles, suggesting a process that involves radical species and five-membered rings. It acknowledges the contributions of audience members and hints at further exploration of the topic, possibly in subsequent discussions or presentations.
Mindmap
Keywords
💡Soot
💡Extinction Spectrum
💡Dispersion Exponent (Angstrom Exponent)
💡Rayleigh Particles
💡Maturity of Soot
💡Optical Band Gap
💡Conjugation Length
💡Aromatic Hydrocarbons
💡Resonance-Stabilized Radicals (RSR)
💡Photoionization Efficiency (PIE) Curves
Highlights
Analysis of soot size distributions at different flame heights and the challenges of measuring extinction in the flame.
The absorption cross-section and its relation to the dispersion exponent, which indicates soot maturity.
The significance of the Angstrom exponent in determining the source of atmospheric particles.
The relationship between optical band gap and dispersion exponent in understanding particle maturity.
High-resolution AFM and TEM imaging revealing the structure of incipient soot particles.
The composition of soot particles, including the presence of aliphatic side chains and their impact on carbon-to-hydrogen ratios.
Raman spectroscopy insights into the conjugation length and carbon-to-hydrogen ratio of soot particles.
The abundance of aliphatic groups in soot particles and their role in particle formation.
The unexpected presence of oxygenated species within soot particles as revealed by IR spectroscopy.
The role of kinetic factors in soot formation, challenging the assumption of thermodynamic control.
The identification of resonance-stabilized radicals in soot particles and their potential involvement in nucleation.
The use of X-ray photoelectron spectroscopy to confirm the presence of oxygenated species in soot.
The discovery of unexpected furan formation in ethylene flames and its implications for pollutant formation.
The application of aerosol mass spectrometry to study the composition of soot particles in flames.
The importance of considering aliphatic side chains in understanding the carbon-to-hydrogen ratios in soot particles.
The observation of radicals in soot particles using electron paramagnetic resonance spectroscopy.
The potential role of five-membered rings in the formation and stabilization of soot particles.
Transcripts
so back to soot
um so here's the uh you you saw you saw
the like size distributions
um before for premix flame sethalene
um as a fuel
um at different heights of the flame the
right hand side is the extinction
Spectrum
um I think for this one they
I don't know if they extracted the soot
and then measured the extinction on this
group some of them extracted and some of
them are in the flame itself Extinction
in the flame there are some issues with
doing Extinction in the flame itself but
just you know this is let's say this is
a spectrum of this from the flame
uh
as you go up in the flame notice that
the Spectrum changes so low in the flame
you're going to have stronger absorption
at shorter wavelengths and then as you
go up in the flame
um your spectrum gets broader right as
the soot matures
so this is you know basically absorbance
or absorption cross-section
um and you know that notice that it gets
flatter that whole distribution gets
flatter
um this is
um leads to
what we call these numbers
um we in in the combustion Community we
call it the dispersion exponent and the
atmosphere Community they call it the
angstrom exponent it's the same thing
and it relates to this parameter that so
the absorption cross so this equation up
at the top is the absorption cross
section
um and it's related to some constants
times the the diameter of the particle
cubed
for this for Rayleigh particles you have
long pretty long wavelengths so Ray um
they under the particle cubed over six
times the wavelength of light to this
parameter
so they call it n in the figure and I
tend to make it that c
that's the dispersion exponent so you
can see it's wavelength to the N so that
is going to tell you something about how
broad this the absorption is
um so when soot is actually
pretty mature like when it's almost
fully mature it has an angstrom exponent
or a dispersion exponent close to one
it's almost always very close to one
um I've measured it less than one and
other people have measured it less than
one when you get it really like in the
most mature you in in a flame on the
very edge of a flame of a diffusion
flame you can see that it's less than
one but it's you can generally think of
in your head if you're if someone asks
you what this version exponent is from a
church just say one so um
as you get less mature that angstrom
exponent gets larger
and this can tell you something about
how mature the particles are so if
you're going to make a measurement you
can make a measurement in a flame if you
can measure that dispersion exponent you
have a a measure of how mature the soot
is so let's call it kind of a maturity
type parameter
and I think this is this is a good
parameter to keep in mind the
atmospheric scientists use this
parameter to also tell them about
whether or not they they mostly use it
to say whether or not if they're
measuring particles in the atmosphere
they've come from smoldering combustion
or flaming combustion so when they see
something as close to one they go okay
we have flaming combustion which means
that you actually have a big flame and
the particles are going out through the
flame front there they've gotten hot and
and they've matured
um when you have smoldering combustion
you actually don't really have
um a flame it's you can have flameless
combustion and it's it's the reactions
that are happening on the surface so
this you'll you'll see
um
um like peat bogs will often have
smoldering combustion
um and there are different conditions
like you know when you don't have the
hot fire going yet and you just have
oxidation go on the surface of the
biomass you get smoldering combustion
and you and and people measure you know
these angstrom exponents of three four
five under those conditions so um so for
us it's actually a good measure if we
want to measure inside a combustor and
we have a way of doing it
um then we can just make that
measurement and have some idea of what's
Happening inside the combustor okay
um
people off also often talk about this
Optical band Gap this is another
measurement they usually drive it from
these these Spectra and it tells them
basically what kind of absorption they
can get from a particular material so
um but you'll notice what I wanted to
point out is the optical band Gap also
gets larger as the dispersion exponent
gets larger
okay and and people back out from this
Optical band Gap they back out how big
the the you know the sheets of graphene
are they're stacked on top of each other
okay so increasing maturity increases
what we call the long range order
um and that
um and we call that also a conjugation
length so I'll use that term a little
bit later
um uh and it also increases the stacking
so we have stacking in the particle
okay
okay so when we have an incipient
particle
um again let's go back and look at some
of the data here are some more data for
tem like spherical particles this is
um this is high Wong's group
um they they appear uh spherical and
kind of like mushy
um they look like they spread like so
here it looks like it spreads a little
bit on the upper left-hand tem image and
there's the corresponding AFM image they
also did this experiment there's the AFM
image that makes the particle look like
it spreads out so the the bottom right
hand one is what I showed you uh earlier
um uh and and and so people just
basically see all of this
see very similar results
but it's really really really hard to
make these measurements
um and we'll talk a little bit about why
it's hard to make the measurements
um and
um what happens when you try to extract
something from a flame when you stick a
probe into the flame
oh that's for me
thank you thank you
you want to go do you want to have a cup
of coffee like did everyone yeah yeah
um is it okay if we take another short
break Ed so people can have some coffee
oh do we have another when is it coming
up
okay okay we'll wait okay okay okay okay
so okay
I'll try to I'll try to keep you awake
um so we have so now we have um the
particles of spherical waxy right um
they absorb in the UV kind of weekly at
longer wavelengths um they photo ionize
it whatever we don't know 6.3 EV about
um and they have disordered fine
structure
okay so
um what about their composition how do
we know anything about what's in them
right we know kind of like what's in the
gas phase and we know kind of like what
they end up looking like when we look at
high res tem what how what is actually
in those particles okay so we have this
scanning Mobility particle size again
for this experiment I'm going to show
you this is actually really beautiful
experiment
um and you see that we have our few
nanometer sized particles
um this is also Atomic Force microscopy
it's high resolution tonic Force
microscopy this is I I just love this
experiment they also did tem it's on the
bottom uh right along with those those
ones that are next to it that look looks
like they're plastic those are
calculations okay so there's a density
functional
calculations on to match the tem images
on the on the lower right but or no STM
sorry STM images on the right so but on
the left hand side and then the top four
squares on the right those are Atomic
Force microscopy so this is high res so
what they did is they they um extracted
from a flame
and then they so they had these things
on it they actually stuck a grid into a
flame and pulled it out
um they put that under vacuum and then
they put another grid under vacuum and
they and they cooled the second grid
down
um and heated the first grid up so they
vaporized some of the stuff that was on
the first grid onto the second grid and
that's which was this and the the reason
they did this they wanted a really clean
surface to do this so they did this
under high vacuum put it in front of
their AFM machine under high vacuum and
then did high resolution so they could
actually see atoms so this was a really
beautiful experiment so you can see when
they did that you can see the structure
of some of the species they saw from the
particles when they grabbed them in the
flame
and this is really nice like you can
make out the six-membered Rings you can
make out five membered rings I'll show
you in a little bit I'll show you other
ones that were they saw like like you
know
um alphabetic side chains like chains of
carbon atoms coming off of them
they saw Bridges so there's M A the one
in the middle is a bridge between two
um
benzene-like structures right they saw
really interesting structures the bottom
one on the left is a bridge between two
larger PHS
um so this was actually fascinating this
is um kind of backs up some of the stuff
that people were doing right before they
did this they're like oh I mean it seems
like there should be five member to
Rings should be important you know
radicals should be important you know so
so this experiment was kind of like
helped crystallize some of that work
okay
um
yeah so I've added below each of those
species they're carbon to hydrogen ratio
okay so remember when people extract
extract out of the flame and they do the
elemental analysis on these particles
they see a carbon hydrogen ratio of like
1.3 to 2 somewhere in that range right
so they already knew kind of like from
that bulk measurement the carbon
hydrogen ratio so here are some of the
carbohydrate ratios of species that were
just you know observed imaged in that
experiment and they're kind of right in
that right range so it it kind of does
look like maybe they are part of the
particles right
okay
um
they did another experiment that
um this is all in the same paper they
did another experiment where they used
Raman spectroscopy to look at the
material so they could actually see
um the Raman spectrum of the material
cell okay Raman Spectra when you take a
Raman spectrum of graphite single
Crystal graphite what you see is the
peak on the right I don't know if you
can see where the one that says the
arrow has SP2 carbon that's called the G
Peak
so you can remember that is that's the
graphite Peak and when you see single
Crystal graphite it will be a sharp Peak
um
and and the one on the left is
associated with defects
so this is just tell this is basically
telling you we do not have
pure graph right but we have something
that's like like kind of like graphite
from from that from the RAM on Spectrum
you can actually get see that L A equals
that d i d so the intensity of the D
Peak which is the left-hand Peak over
the intensity of the G Peak the right
hand Peak
um
gives you la that's the conjugation
length that's how big these sheets are
in that
um that's a measure of how big the on
average the sheets are inside that
particle okay and that's close to 1.1
nanometers many many many experiments
have demonstrated that
for the not not incipient particles
actually for the particles that at least
partially matured is on the order of one
one nanometer or so
the bottom number so the MPL that's the
slope of the photoluminescence
background so you see where I've drawn a
little error that says slope
the slope of that luminescence
background divided by the intensity of
the G Peak gives you a measure of the
carbon to hydrogen ratio and that's 2.3
so that's also indicates if our
incipient particles are one point carbon
to hydrogen
1.3 to 1.02
um that kind of indicates that you have
kind of a maturing particle and then the
size the conjugation length together
indicates you have what we would call
partially mature or young particle okay
not fully mature like not even all that
mature but probably not completely
incipient so this is a particle that's
kind of like you know just starting to
like grow its little
pahs that are probably part of the
particle okay so this is kind of
evidence like what's happening to to
form these particles
okay
um
and
this is demonstrate some significant
abundance of alphatic groups so I'm
going to talk about this so remember
when we think about soot when we think
about soot we're thinking about pH is
coming together it's not necessarily pH
is coming together
okay
so here's another from the same group
just a different paper
um
so this
um I usually think of car I do carbon
over hydrogen because it's I find it
easier to remember numbers that are over
one
um instead of fractions
um
but a lot of people do H over c
um
there are some reasons to do use H over
c
um if you're doing a calculation but um
C over H is it's easier so on the right
hand side I put C over H for for you for
me actually for me too
um so you see that the number so they
took up their data and they they said
okay let's count the number of carbons
and then calculate our our ratio of H
over c c over H
and they did two different experiments
one low in the flame and one high in the
flame or you know two different heights
I'm not sure eight millimeters is low it
seems kind of high to me it seems like
you probably have some maturity there
but um but they did that and they they
said okay that's we see this curve and
so we get larger number of carbons We
have basically a higher carbon hydrogen
ratio
does that make sense to you
how would you think that that would be
sure
if you're thinking about these these um
molecules growing
how would you think that carbon would be
increasing relative to hydrogen
where who said that
elimination and how would you eliminate
uh-huh and what would be happening to
what the particle looks like
yes and no more number of rings right
what's your name
John excellent yes
um so yes you're growing more Rings
right so as you grow more if you think
about it I would probably oh yeah I
think I I did this out
okay here's naphthalene right so that
naphthalene is C10 h8 carbon hydrogen
ratio of 1.25
okay here's anthracine have had an extra
ring right
um c14h10
higher carbon hydrogen ratio right
as you go up as you add Rings you're
basically taking away hydrogens and
adding carbons with with not as many
hydrogens right the Rings are are you're
taking spots that had hydrogen on them
okay
um
okay does everyone see why those are see
like where the hydrogens are no yes no
the Hunter Insurance okay yeah the
hydrogens are implied so when you have
something that looks like this
remember each carbon has to have three
bonds right so this is there has to be a
hydrogen here because as one two this is
a double bond so this carbon has two
bonds to that carbon one bond to that
carbon and one bond to the hydrogen so
it has four bonds each carbon has to
have four bonds so these are so when you
see structures like that drawn out
um
those those are implied hydrogens as as
Mani sarathi said this morning I don't
bother to draw those hydrogens
um they're there okay so so those are so
each time you're adding a ring you're
taking away
some hydrogens but adding more carbons
with the with your hydrogens okay so
that's what's happening you're growing
that in your head you should be thinking
okay that makes sense as I grow these
these structures that's the way I do
this okay this is the way we normally
think about this stuff
so that all makes sense
but when they did this these
calculations
they didn't they actually said we're not
going to take into account the aliphatic
side chains
and remember there are a whole bunch of
them in the the last slide right there
are a lot of aliphatic side chains
so what's what's going to happen to that
that curve right is going to go up your
your carbon to hydrogen ratio will go
down your hydrogen carbon ratio will go
up so that curve actually goes up when
you take into account the aliphatic side
chains
okay so now we have to start we have to
like it doesn't make sense not to
consider the aliphatic side chains
because if you're going to compare to
the measurements where people just do
Elemental analysis where they just count
the number of hydrogens and and carbons
just by like you know reacting the
hydrogens away and making them water or
however they do the measurement carbon's
away and making CO2 measuring the CO2
um then you that's how that measurement
they don't care if it's an aliphatic
side chain or an aromatic ring right so
you have to if you're going to compare
to data you want to be able to take into
account alphabetic side chains
normally we think that they're not
present or important in so formation but
now let's start to think like maybe they
are present so here's if you add
analysis Paddock side chain so if you
added
um
what a kind of
so here's your now you take it you have
to take away that hydrogen right
you add this side chain now
um you have one two three four bonds to
that carbon this one has two hydrogens
if that's a single one and that's a
single bond that has to have two
hydrogens and this is going to have
three okay so you're going to have
a lot of hydrogens and that's going to
lower your carbon hydrogen ratio okay
does that make sense Okay so
um so here's an experiment my group did
um where we did pyrolysis of propene and
propine and
um
we did aerosol Mass Spec so we took we
extracted particles from the flame
and we vaporized them on a a Target
I'll talk a little bit more about this
tomorrow we vaporized them on a Target
and under vacuum and the particle the
molecules that came off we ionized those
and then we did Mass Effect okay so
um what we saw and this was uh
for um if if we'd pyrolyzed the two
separate experiments parallels propion
or propene
um and the different colors are
different temperatures so the lowest
temperature is
um
so I'll say squares and circles in case
you're colorblind the lower lines are
the lower lowest temperatures
and notice that how low the carbon
hydrogen ratio is even for the large
molecules
right
so now you must be thinking Hmm
that must be something like
alphabetic side chains right or Alpha
species like how are you going to get
that high that low of carbon hydrogen
ratio with that many carbons with these
pretty large hydrocarbon species they
have to have some some kind of
interesting character to them that we
don't normally think about okay so and
we see that for both propane propane and
propine
um so the um species that are all these
six-membered Rings
um lumped you know put together like say
if you know what pyrene is as four
aromatic rings together and a lot we
call this pericondensed hydrocarbons
aromatic hydrocarbons
okay
um
so uh on the right hand side we see that
like
um well so the top figure is the average
carbon to hydrogen ratio
um for as a function of temperature for
the the experiment so this is like now
we're increasing we're taking our fuel
we're increasing the temperature and
then counting all the carbon to hydrogen
ratios for all the molecules that we see
in in our mass spec
um and you see that the
um
carbon hydrogen ratio increases with
temperature
does that make sense
I see some nodding
why does that make sense you're naughty
you're getting rid of the hydrogen yeah
exactly
right so what's your name
Dan
Tanner Tanner so that's exactly right
you're you're getting rid of the
hydrogens as you go to higher
temperature this is what's going to
happen when you have remember remember
when you're causing the particles to
mature and you're so what's happening in
the flame is they're heating up right
they're having pretty long residence
time at higher temperatures you're
getting rid of those hydrogens when you
do that you that happens also in
pyrolysis okay so so that's and we see
that actually the most important effects
are for the larger species so if we take
only carbon number 17 and above so the
right hand side and just ignore the left
two points and just take the right hand
points those are the ones that are
changing the most as we heat up the
large species are they're reorganizing
and getting rid of hydrogens so you have
these probably have these aliphatic side
chains they're reacting
um as they as they form
um rings and stuff they're going to be
getting rid of the Hunter surgeons okay
however they have however that happens
that chemistry magic happens
okay
um
so yeah we have a significant abundance
of aliphatic side genes and our acetah H
ratio is about 1.4 for all those
experiments
um here's another experiment that was
kind of a shock when it was first done
by high Wong's group so
um this is where they took soot out of a
flame and did just IR spectroscopy on
the soot
and what they saw was like they saw
oxygen embedded in the particles it
looks like oxygen it was at least on the
surface
um and you know the uh the left-hand
Peaks where it says aliphatic CH that
was kind of a surprise like people were
like whoa why is there so much
alphabetic you know that can't be that
can't be right we just didn't expect it
right so this was kind of a shock of an
experiment
um
uh so here are the size distributions
um some of these size distributions
um indicate as you can see this is like
the bottom axis x-axis is is particle
size right the y-axis is the you know
number of particles in that size bin so
these are size distributions
um and you can see these are different
Flames like um they're actually all the
same equivalence ratio but different
flow rates in a pre-mixed flame and and
this is uh um the burner where you have
the stabilization plate that goes down
and acts as a probe as well
um so uh but just ignoring what the
different flow rates are
um you see that as you go the bottom one
HP means height of the height of the
plate so base height of a burner is 0.6
and you go up to one centimeter
as you go up your size distributions get
larger right so you're starting to get
more mature right your particles are
getting bigger they're growing
um and at one of those I don't know
which where that sampling was it didn't
say in the paper but I just wanted to
show you
um this group
um was seeing that
this is what they kind of like summed up
their experiments so
um and there is the data kind of noisy
but still it's interesting that they see
on the left hand side is the alphabetic
carbon to hydrogen to the aromatic
now we would normally think for as you
go up in the flame your alphabetic
should be like going to zero right
that's normally what we were thinking
all along but that's not what they were
seeing they're seeing actually uh like
of alphabetic to aromatic
um in this experiment of way more than
one like almost like like at the lowest
Heights you know three that's that's
really kind of amazing
um so this is this was kind of this is
just totally unexpected but now I'm
thinking maybe that's just like what you
know this is our new kind of route to
figure out what's happening okay we need
to be doing different types of
experiments
okay so particles have high aliphatic
content and oxygenated species in the
particles we didn't expect that one
either okay and then a lot of people
have seen the oxygenated species using
different uh um using IR spectroscopy
and particles
okay
um in fact here's an experiment we did
we're trying to understand so here's
here's an example of of us trying to put
together different techniques to
understand what's going on okay so we
saw okay in this that experiment they
saw oxygen right what the heck why is
there oxygen inside the particles right
this it just doesn't like it seems like
it shouldn't be there if you go to high
temperatures you you should have it
seems like a chef oxidation and you emit
Co or CO2 right
um okay so we did this experiment and we
didn't mean to do it was just we're
taking data trying to figure out well
isn't these sensibian particles and
um this is uh also a premix flame at
different heights
um uh actually the lowest one is at the
top sorry
it's opposite um
but uh
the red Peaks or Peaks that we
identified so they're Mass Peaks that we
identified with oxygenated species
um the blue Peaks are the ones that are
pure hydrocarbon no oxygen so we
actually saw a lot of Peaks that looked
like they had oxygen in them
and we could tell
um by the mass difference like we could
tell by the mass of the oxygen
it was slightly different from a carbon
so Oxygen 16 our carbon is 12. if you
have 12 plus four hydrogens it should be
the mass of an oxygen but we we had just
enough resolution to distinguish those
so we could tell those Peaks were
oxygenated
um
and we're trying to understand what was
going on so we collaborated with Angela
violi's group to do some modeling so uh
they they
um use their kinetic Monte Carlo
um liquid Dynamics um simulations and
and ran some calculations and they said
okay actually
um what they saw was like oh if there's
oh in the flame
then you're gonna have you can actually
have oh attach to your molecule right
um so it generates
um an O like a COC so remember that's
called an ether group right
um so you have see that aliphatic side
chain see uh
that you have the
um the second one from the left you have
an ether group and then on a radical the
radical end and that goes in bonds and
makes a ring
um with the the rest of the molecule
right this is called a furion
furians are highly toxic they're one of
these pollutants people really worry
about
you find them anytime you have like
carbonaceous materials that you heat
like in coffee people find them in baby
food these are species that people worry
about health they're very worried about
furans in fact I think coffee actually
one of the flavors you like is a furan
um hopefully hopefully we're not
ingesting a lot of it but um but there's
something that no one expected to see in
a flame right from and this is ethylene
we're just burning ethylene like what
like you know and this is furians are
actually also proposed as you know
alternative fuels like biofuel derived
fuels so this is kind of fascinating
that we actually just generated with
ethylene we we totally didn't expect it
so we're like okay but we have to prove
it we can't just like rest on our
Laurels and say we just have a model
that shows us we have to prove that it's
um is probably furion so he did an
experiment we did we took these
particles and we did x-ray photoelectron
spectroscopy this helps us figure out
the binding we'll talk more about this
tomorrow this technique that helps you
figure out bonding between atoms and a
sample
um and we see lower in the sample
um if you so these are three different
heights in the burner and then
summarized on the right hand side
um
were the the Peaks uh so this peak
shifts so you can it's the car it's the
um oxygen binding energy so you have Co
um Co the ethylene the Aether COC and
the coh so low in the flame you have a
high coh peak right as you go up and you
have a little bit of The Ether Peak as
you go up in the flame that Co Peak goes
away and that The Ether Peak comes in
The Ether Peak is for the furan
so it's important I think to kind of
like when we think we know something to
try to confirm it
um and this is still an experiment we'd
like to confirm but we've we when we do
this we totally swamp the um the mass
spec with water
um because uh you know when you combust
stuff like you generate water and mass
specs don't like that very much and we
do these experiments at a synchrotron
and we've a couple times almost shut
down the sinkertron by like having like
all of a sudden like the pressure goes
up too high and then there's an
emergency and it's like it's really
embarrassing so um
yeah it's an experiment I'd like to redo
for different conditions and see if we
still see it
um but right now we actually uh dry the
um our samples to before we send them
into the mass spec
okay okay so um so let's talk more about
particle composition
um so here are a number of different
experiments
um where people have used Mass
spectrometry right so um and in this
particular type of mass spectrometry you
collect a sample
um and then you vaporize it so you can
see the individual molecules
um from that from the sample
um so our stuff is on like we have this
is a pre-mixed
um flame and a counterfeit flame with
two different fuels and it looks very
similar to the one in the middle
um which is uh also from how long's
group
um
you see all these a lot of the very same
Peaks
um and then on the right hand side an
older one from uh Dobbins at Al from
their group also showing
um a mass spec so the one on the right
hand side
and the one in the middle they actually
um vaporized and ionized with the laser
okay so they they took a sample they
stuck a plate into a flame got a sample
and then stuck it into an instrument
where they vaporized and ionized
um the sample
um and then got the Mass Spectrum of the
gas phase species and on our side we
actually have uh we generate a focused
beam of particles that hits a hot Target
and vaporized using thermally not with
the laser but we see the very similar
things and what we see are these Peaks
this is really common you see these even
numbered Peaks and you know they appear
to be associated with these
pericondensed
um hydrocarbons pericondense polycyclic
aromatic hydrocarbons
and
um
for decades we've all assumed that these
are what these are the stabilimers so
stabilomers are the most
thermodynamically stable species at a
particular carbon and hydrogen ratio so
so if the way you read this table is you
say at the top is a list of the number
of carbon atoms in the molecule and on
the left is the number of hydrogen atoms
in the molecule
and then the species
um are in are the ones that are
associated with those different
hydrocarbons different carbon to
hydrogen ratio okay
um
I've actually put along the top the
number of six-membered rings that are
associated with them and I've I've added
in red the carbon hydrogen ratio for
each of those species so as you get to
bigger and bigger stabilimers you get
higher and higher carbon to hydrogen
ratio okay
okay so
um
so this is the stabilimer grid and we've
all we all assumed that what we're
seeing in all those Mass Spectra were
the stabilimers like each one has a mass
Peak and we would expect to see that the
most thermodynamically stable species at
that Mass
so
um
so um one of the things we did was
um
we when we do our experiment at the
synchrotron
we can actually tune the photon energy
that we use to ionize the molecule okay
so we can you tune that and and you know
we can we start at you know an Ina
Photon energy where we don't see any
signal and as we go up in photon energy
at some point we're going to start
seeing signal and usually that's
associated with the species we're
actually looking at like so what we
normally the way we do this though is we
don't just sit on a peak and tune we um
take Mass Specter a whole bunch of
energies Photon energies so we basically
have all of the whole range of masses at
each energy and at some energies you
don't you see some Peaks but not all of
them and then as you go up you start to
they start to grow in and then
eventually if you get to too high energy
you start to fragment and then you start
to see smaller
um species okay so that's how we do the
experiment but we can use what we call
the photo ionization
um cross-section curves Spectra or
people call it photonization efficiency
we can use that to help identify isomers
um at a particular Mass so in a massive
room you just see a mass right and we
can assume that it's this or assume that
it's that because we expect it to be
there but it's nice to know exactly what
it is right so there here are the um
followed ionization efficiency or
photonization cross-section curves for
um
a number of different polycyclic
aromatocardial currents right so those
all have different masses but we went
and recorded those
um and then a lot of people have
recorded them for a whole bunch of
different species so this is a really
common technique in this um doing Mass
Spec at the synchrotron so here's an
example of how you might use them
um so we have flame sampled particles we
do take the Mass Spectrum we get our
Peaks and we get the pie curves for all
the different Peaks
um
see that we sample it here we have two
different heights in the flame
and
the two different heights in the flame
actually look very similar that
indicates that those isomers are
probably very similar in the two
different heights in the flame like
we're not seeing very very much
difference between the two different
heights where we extracted
but you see if we compare them in the
top
panel We compare them to the pie curve
for pyrene
this is Mass 202.
this is where this is what we always
assumed was piring and it's not piring
so
um it doesn't parting doesn't agree with
our peaks with our with our curves at
that mass okay
okay this this was kind of like
um
kind of
disturbing in some ways like we've
always assumed that we have a lot of
pyrene right and this is so if we
compare it with fluoranthine it does
it's not fluoranthe either that also has
a mass 202. if we use a linear
combination of pyrene fluoranthe we can
fit our curves so we think that it's
probably a combination of pyrene and
fluorinethine at Mass 202. so all this
time you know everyone's been focusing
on piring as kind of like the first
species like you if you if you look at
strip models often they're pyrene
dimerization is the initial step and
should Inception
but if two two isn't even pirating if
we're not having a lot of power in it so
what what the heck's going on right
um okay
so what we did is we then we were like
okay
that's pyrene but
um we actually see that that Peak that
mass is fluoranthine and pyrene and
sometimes we see that it's mostly
floranthine that it looks a lot like
Florentine okay Laura Anthony is not the
most stable but it's a fascinating
molecule and this can give us
information about what's going on
so this is the first step in realizing
aha it's not the most we're not all
driven by thermodynamics right
there's something else going on now we
have to start thinking
kinetics is is really important in soot
formation
okay and you're saying uh we should have
thought of that before but you know
it's nice to know like okay we have some
evidence that it's not thermodynamics
okay here's another one here's
anthracine we find that is actually
mostly phenanthrine
there's another one
we find
I don't know how to say this Ace
naphalene awesomeene is mostly one
ethanyl naphthalene
um
all these ones
are not what we expected I mean we have
and then chlorinine that's another one
people actually focus on a lot with
respect to nucleation type mechanism for
Inception it's not corny we don't know
what it is but it's definitely not
quarantine and then here's a fascinating
one I added this to the grid
this is a radical species
so this this species is at Mass 91.
it's c7h7 and
um when we went to measure we're like
um oh looks like vinyl cycle pentadional
like so a pentadino has five membered
rings plus like a two two carbons off
chain off of
um cycle cycle pentadiene
um but everyone people were like no no
you are totally wrong it is benzyl
benzyl is the most stable species it has
to be benzyl so we're like uh you know I
don't know it's you know our data are
kind of you know
uncertain yeah okay maybe it's benzyl
maybe it's benzyl so we ended up doing a
study and I'll talk a little bit more
about this in a bit
um it turns out it's not it part of it's
benzo probably but but we also have at
that mass we also see think we see
tropal and orthotyle and vinyl
psychopendidaneyl is almost always there
so
that tells you something about what's
happening
um in the kinetics if we're seeing all
these different species
there's something really interesting
going on it's not even even in the
radicals we're not seeing it sink to the
most thermodynamically stable
so we're actually skipping over a lot of
the most thermodynamically stable
species we're not we're not
falling into the well and staying there
we're doing something that's kind of
interesting chemically Okay so so here's
our next like what do we know about
inception is is sebia particles
they have aliphatic and oxygenated
species content
um
the
um
the masses that we see we see both in
pyrolysis and Flames with different
under different conditions with
different fuels we're seeing a lot of
the same remember I was saying how
exciting it is that you know we have the
same type of morphology and fine
structure at the end I mean now we're
seeing a lot of consistency so it's nice
to like try to track that down so we can
follow the trail okay the precursors are
not apparently not the most
thermodynamically stable
um and um they have carbon hydrogen
ratios probably close probably less than
two point
um closer to 1.3 to 2.0 which is what we
were saying earlier Okay so
so now let's talk about the radicals so
we had been doing this um uh okay when
we started doing these experiments
um in 2011 we were like okay we just
want to see these pahs that are that are
causing nucleation of particles to
nucleate
and we couldn't figure out what was
nucleating the flame because the species
we're seeing were too small to nucleate
so we did these experiments did them
under all these different conditions at
different flames and you know different
fuels and we kept seeing the same thing
and we kept seeing this you know our um
our Mass distribution I showed you
earlier you you saw this like this is
what we were seeing and almost every
single time the same Peaks like what
like we're like ah how are we going to
figure this out
um what are these things they're not
they're not the stabilimers what you
know what's going on so then we started
to look at where we these these um odd
Peaks odd Mass Peaks kept popping up and
were like
like those are just a nuisance like what
are they
um we see them very strongly in our Mass
Spec because
um I think what happens is we're not
really sensitive to the smaller masses
because
are in order to get masses into our Mass
Spec they have to go through our
sampling line into the vacuum chamber
through an aerodynamic lens system which
focuses the particles into a beam then
they hit the um the plate
um our heated plate and are vaporized
the smaller ones we don't see the
stickier but these apparently are sticky
enough that they stick to our particles
and make it all the way through they
don't just vaporize like the smaller
ones just vaporize before they get to
our Target to our detection region okay
okay so we see these um radical Peaks
um so these odd number Peaks are
radicals so they're missing an electron
um and uh and and we see them okay
so
um
we see them even when we do pyrolysis so
we see them in the flame we see them
during pyrolysis
um you don't see them so clearly when
you do gas phase look in the gas phase
but we see them very strongly when we do
the aerosol Mass Spec because we
basically are concentrating them so the
gas phase is like kind of swamped with
these larger Peaks as smaller masses so
this is a gas phase or the same system
as the one on the left and purple the
one on the right is the gas phase
um Mass Spectrum that goes along with it
and you see that they're there but
they're little
um mostly because they're swamped by
these other Peaks and we just I think
accumulate them so we condense we we
basically see them stronger than you
might think they should be there other
people have seen them too so this is a
whole list of people who had seen them
previously it's not just us it's not
just our technique it's basically every
time you collect the particles and do
mass spec on the composition
you see these Peaks and other people had
ignored them you know people have talked
about you know what they could be doing
but you know for the most part they kind
of were like focusing on on what are the
big masses that could be condensing or
nucleating in the flame
yeah so here are some other Mass Specter
examples people have seen them
um and here's a paper we wrote where we
actually focused on them
um and we're like okay there's a whole
series of them and if you notice they
start we think they start with um
propardial which is
um C3 so has a mass of 39 you add uh an
acetylene and we'll talk about the Haka
mechanism but you add an acetylene or
see some kind of C2 make 65 out of C2
makes 91 you add a C2 it makes 115 Etc
and you go out all the way up so we see
this whole like selection of these Peaks
okay
um and then again the benzyl this is
where we said the 91 Peak was funnel
cyclophantodynamic kind of came under
Fire
um okay and then other people have seen
the evidence for okay so we see these
radicals when we vaporize but do we know
that they're actually part of the
particle where are they coming from
other people have done experiments where
they have extracted particles and done
epr electron paramagnetic resonance or
electron paragomatic spin spectroscopy
um and seeing that you have very strong
and and this is a technique that's
really sensitive to radicals and they
see strong signature radicals for these
incipient particles but it's a particles
age that signal goes away so the mature
particles don't show this the strong
radical signal so it seems like these
radicals are associated with these
incipient particles
and then we go back to this experiment
right we saw earlier here the AFM
um
images and here once it picked out as
being radical species so even in the AFM
they start to see these radical species
they also see that they're associated
with five-membered rings
and what they see is also a lot of
bridged aromatic groups okay so this is
this just
um like okay so we're starting to
accumulate accumulate evidence right
okay
so um these turn out to be what we call
resonance stabilized radicals
or other people call them persistent
radicals and you'll see this in combust
the combustion chemistry literature
dating back you know eons
cyclopentadieneal is a really common
species that people think about the
kinetics for you know how is it involved
it's a commonly observed species or you
know it comes if people spend a lot of
time worrying about these types of
radicals and what their involvement in
in combustion is and combustion
chemistry okay so so we're not the first
ones to think about this but an rsr is
so in denial so Indian when it loses
hydrogen becomes in Danilo
is an rsr so the difference between
um an rsr and a regular a resonance
stabilized radical and a regular radical
is
it's
um
it's stabilized so it's not as reactive
as say an a non-resident stabilized
radical so it's it's not like oh
um so it's less reactive than oh
um and it's uh more stable than oh
um but it's less stable than pyrene than
a what we call closed shell which has
completely paired electrons so a radical
has an unpaired electron so what happens
in generating a radical so here you you
lose a hydrogen atom right off of this
in the nil so it ends up that having an
electron that used to be sharing you
know binding with hydrogen like in a
covalent bond so there are two electrons
are going between the carbon and the
hydrogen and that CH Bond on the side of
of ending you pull off that hydrogen now
it has one electron just sitting out in
space and that electron really wants to
have companies just sitting there alone
it really wants to be with other
electrons doesn't want to be unpaired so
what happens is so this is you remove
that hydrogen there and these are
molecular orbitals so that those green
and red blobs show you electron density
right so you pull off that hydrogen on
the on the ending
and um and then you have this electron
sitting there it that electron instead
of it used to be in we call a sigma bond
with the hydrogen in in the plane of the
molecule so now what it does is
surrounded by double bonds double bonds
also have they have Sigma character when
they're binding with the other carbons
they also have Pi the the second double
bond gives it Pi character so they have
electron density like this and the
electrons are sharing so when you have
Benzene the electrons are sharing in the
their their Pi bonds with their pot
they're sharing electron density in the
second bond which is what cause makes
them aromatic right makes them very
stable
so in the same way an rsr takes that
lone electron props it up into the pi
it's p orbital right and then it shares
its p orbital with all the other P
orbitals that are sharing with the
double bonds that's what stabilizes it
now it's like has delocalized electron
density now that says okay I'm happy I'm
happier than I was right but they're
still more reactive than a completely
closed shell molecule
okay so so it looks like now we have
five membered rings
are are common in these
um particles
um they have they seem to have a lot of
radicals
um
um and these radicals are connected
these resin stabilized radicals are
connected with these
um five-membered rings and in some way
okay and that makes sense because just
like ending losing that electron the
five-membered ring if it were just a
six-membered ring it probably already
would be in a double bond okay okay so
um so let's talk about molecular weight
growth so yeah
oh you are awesome thank you what's your
name
Dalton
thank you Dalton
I missed it again okay break break time
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