Particulate Formation, Evolution, and Fate - Hope Michelson Day 1 Part 2

Summary

The 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

00:00

🔬 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.

05:03

📊 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.

10:04

🌟 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.

15:05

🔍 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.

20:05

🔥 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.

25:07

🌿 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.

30:09

🌀 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.

35:10

🔬 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.

40:12

🚫 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.

45:13

🌱 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.

50:14

📉 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.

Keywords

💡Soot

Soot is a black carbonaceous material produced by the incomplete combustion of organic matter. In the video's context, soot formation and its properties are central to understanding combustion processes. The script discusses the size distribution, absorption characteristics, and maturity of soot particles, highlighting its importance in both combustion science and atmospheric science.

💡Extinction Spectrum

The extinction spectrum refers to the measurement of how much light is absorbed or scattered by a substance, in this case, soot particles at different heights of a flame. The script mentions that the spectrum changes as the soot matures, indicating broader absorption as the particles grow and become more developed within the flame.

💡Dispersion Exponent (Angstrom Exponent)

The dispersion exponent, also known as the Angstrom exponent in atmospheric science, is a measure of how the absorption of light by particles changes with wavelength. The script explains that a mature soot particle typically has an exponent close to one, indicating a broad and flat absorption spectrum, which is a key parameter in assessing the maturity of soot particles.

💡Rayleigh Particles

Rayleigh particles are small particles that scatter light in a way that is wavelength-dependent, with shorter wavelengths being scattered more efficiently than longer ones. The script uses the concept of Rayleigh scattering to explain the absorption characteristics of soot particles, particularly at the early stages of their formation in a flame.

💡Maturity of Soot

The maturity of soot refers to the degree of development or growth of soot particles during combustion. The script discusses how the absorption spectrum and the dispersion exponent can be used to assess the maturity of soot, with more mature soot particles having a flatter absorption spectrum and an Angstrom exponent closer to one.

💡Optical Band Gap

The optical band gap is a measure of the energy range between the highest valence band and the lowest conduction band of a material. In the script, it is mentioned that the optical band gap increases with the dispersion exponent, which is related to the size and stacking of graphene-like sheets in soot particles, indicating an increase in maturity.

💡Conjugation Length

Conjugation length refers to the extent of alternating single and double bonds in a molecule, which is associated with delocalized electrons and can affect optical and electronic properties. The script mentions that increasing maturity in soot particles increases the conjugation length, which is a measure of the size of the graphene-like sheets within the particles.

💡Aromatic Hydrocarbons

Aromatic hydrocarbons, specifically polycyclic aromatic hydrocarbons (PAHs), are organic compounds that contain multiple fused aromatic rings. The script discusses the presence of these compounds in soot particles, their role in the growth and structure of soot, and how their composition can be analyzed through techniques like mass spectrometry.

💡Resonance-Stabilized Radicals (RSR)

Resonance-stabilized radicals, or persistent radicals, are molecules with an unpaired electron that is stabilized through resonance structures. The script highlights the presence of RSRs in soot particles, suggesting they play a role in the formation and growth of these particles, and their association with five-membered rings.

💡Photoionization Efficiency (PIE) Curves

Photoionization efficiency curves, or PIE curves, are used in mass spectrometry to identify and differentiate molecular isomers based on their ionization efficiency at different photon energies. The script describes the use of PIE curves to confirm the identity of species observed in the mass spectra of soot particles, challenging previous assumptions about the most thermodynamically stable species.

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