Particulate Formation, Evolution, and Fate- Hope Michelson Lecture Day 1 Part 3
Summary
TLDRThe script delves into the complex mechanisms of molecular weight growth and particle inception in flames, focusing on the Haka mechanism for modeling these processes. It introduces the concept of hydrogen abstraction and carbon addition, explaining how these reactions contribute to the formation of multi-ring hydrocarbons. The speaker also explores different inception mechanisms, contrasting thermodynamically driven nucleation with kinetically controlled covalent bond formation. The discussion includes recent theories and experimental evidence supporting the role of resonance-stabilized radicals (RSRs) in particle inception through radical chain reactions. The presentation aims to provide clarity on these intricate topics and stimulate further investigation into the underlying chemical processes.
Takeaways
- 🔬 The Haka mechanism, introduced by Michael Frenklach's group, is used to model molecular weight growth and surface growth, involving hydrogen abstraction and acetylene addition to hydrocarbon species.
- 🔍 The Gibbs free energy for the hydrogen abstraction carbon addition mechanism shows sequential growth from benzene to larger hydrocarbons like pyrene through the abstraction of hydrogens and addition of acetylene.
- 🔴 Molecular weight growth occurs at high temperatures in flames, leading to the simultaneous growth of various hydrocarbons, while inception is the process of gas-phase hydrocarbons clumping together to form particles.
- 🌡️ Inception is distinguished from molecular weight growth by the clumping of hydrocarbons into particles, which can be thermodynamically driven (like nucleation) or kinetically controlled (involving covalent bond formation).
- 🌟 The thermodynamically driven inception may involve large species, possibly larger than 11 aromatic rings, which could nucleate at flame temperatures, but their high carbon to hydrogen ratio and low concentration in flames make this less likely.
- 🚫 Kinetically controlled particle inception is favored at lower temperatures and involves reactions that form covalent bonds, leading to the growth of particles that cannot easily be vaporized.
- 🛑 The difference between dimerization and inception is highlighted, with dimerization being a step towards inception but not the complete process, which also involves continued growth and attraction of more species.
- 🧬 The potential role of resonance-stabilized radicals (RSRs) in particle inception is suggested, with the hypothesis that RSRs could lead to rapid polymerization and particle formation through radical chain reactions.
- 🧪 Pyrolysis experiments with ethylene and indene indicate that RSRs can seed particle formation at lower temperatures, supporting the hypothesis that RSRs are involved in the inception process.
- 📉 The mass spectrometry analysis of particles from flames and pyrolysis experiments show a distribution of species with varying carbon to hydrogen ratios, suggesting the presence of aliphatic character and potential branching in the particles.
Q & A
What is the Haka mechanism?
-The Haka mechanism, introduced by Michael Frenklach's group, is a multi-step process used to model molecular weight growth and surface growth in flames. It involves the abstraction of a hydrogen atom from carbon, followed by the addition of acetylene, leading to the formation of larger hydrocarbon species.
How does molecular weight growth differ from inception in the context of flames?
-Molecular weight growth refers to the process where hydrocarbons grow in size through chemical reactions at high temperatures in a flame. Inception, on the other hand, is the process where gas-phase hydrocarbons clump together to form particles.
What are the two main classes of inception mechanisms?
-The two main classes of inception mechanisms are thermodynamically driven mechanisms, such as nucleation and condensation, and kinetically controlled mechanisms, which involve reactions that lead to covalent bond formation.
What is the significance of the carbon to hydrogen (C/H) ratio in understanding particle formation in flames?
-The C/H ratio is significant as it helps determine the types of hydrocarbon species present in a flame and their potential to form particles. A high C/H ratio indicates larger, less volatile species that might be more prone to nucleation, while a lower ratio suggests smaller, more reactive species.
What is the role of dispersion forces in the thermodynamically driven inception mechanisms?
-Dispersion forces, such as van der Waals forces, play a role in the thermodynamically driven inception mechanisms by providing an attractive force between nonpolar molecules, which can lead to the clumping of gas-phase species into droplets or particles.
How does the kinetically controlled mechanism differ from the thermodynamically driven mechanism in terms of particle formation?
-In the kinetically controlled mechanism, particle formation is driven by reactions that lead to the covalent bonding of hydrocarbon species, creating a solid-like structure that cannot be easily vaporized. This is different from the thermodynamically driven mechanism, which relies on the physical clumping of species through dispersion forces.
What is the significance of the experiment where particles were extracted directly from a flame?
-The experiment where particles were extracted directly from a flame provides valuable insights into the types of hydrocarbon species present in the particles and their growth patterns. It helps researchers understand the processes of molecular weight growth and inception by analyzing the mass spectrum of the extracted particles.
What evidence suggests that resonance-stabilized radicals (RSRs) may play a role in particle inception in flames?
-Evidence from experiments, such as pyrolysis studies and mass spectrometry, show the presence of RSRs in the gas phase and their potential to react with closed-shell hydrocarbons like ethylene. This suggests that RSRs could be involved in radical chain reactions that lead to particle inception.
How do the concepts of molecular weight growth and inception relate to the formation of soot in flames?
-Molecular weight growth and inception are precursor steps to soot formation in flames. As hydrocarbons grow in size through molecular weight growth and clump together through inception, they eventually form larger aggregates that can lead to the development of soot particles.
What challenges do researchers face in modeling the inception and growth of particles in flames?
-Researchers face challenges such as the lack of kinetic data for the reactions involved in inception, the need to accurately represent the complex chemistry of hydrocarbon growth, and the difficulty in measuring the properties of incipient particles at the onset of formation.
Outlines
🔬 Introduction to Mechanisms and Concepts in Flame Chemistry
The speaker begins by introducing the topic of different types of mechanisms and concepts used in the study of flame chemistry. They discuss the Haka mechanism, which is used to model molecular weight growth and surface growth, and is attributed to Michael Franklock's group. The mechanism involves hydrogen abstraction and carbon addition, particularly in the presence of acetylene, a common species in flames. The process is illustrated with a stepwise chemical reaction that results in the formation of multi-ring hydrocarbons. The concept of molecular weight growth is differentiated from inception, where hydrocarbons clump together to form particles. The speaker uses an analogy of people joining a group at tables to explain the clumping process in inception.
🌡️ Exploring Molecular Weight Growth and Inception Mechanisms
The speaker delves into the two classes of inception mechanisms: thermodynamically driven nucleation and kinetically controlled covalent bond formation. The former is likened to water vapor forming droplets during humid conditions, while the latter is compared to the phase transition in baking a cake. The speaker references Hai Wong's paper from 2011, which outlines different types of mechanisms, including covalent addition and thermodynamically driven nucleation. The challenges in achieving nucleation at flame temperatures are discussed, along with the potential factors that could influence the clumping of gas-phase species.
🔍 Analyzing Hydrocarbon Growth and Inception Theories
The speaker continues to explore the theories behind hydrocarbon growth and inception, discussing the evidence and challenges associated with each. They mention the difficulty in observing certain mechanisms and the need to understand the conditions that lead to particle formation. The speaker also discusses experiments that extracted particles from flames and analyzed them using mass spectrometry, revealing interesting patterns in the mass spectrum that suggest the presence of larger, layered structures on particles. The potential for covalently binding different species to form particles is also considered.
🧩 Investigating Kinetically Controlled Particle Formation
The speaker focuses on the kinetically controlled mechanism for particle formation, which they believe to be a likely candidate for the inception process. They discuss a review paper by Martin Edel and Marcus Crafts that examines various mechanisms published in the literature. The speaker argues against certain mechanisms due to their slow chemistry and high carbon-to-hydrogen ratios, and supports the kinetically controlled mechanism as a more plausible explanation for particle inception in flames.
🔬 The Role of Chemistry in Slow Inception Processes
The speaker investigates why certain chemical processes are too slow to contribute to inception. They discuss the sequential growth of molecular weight through mechanisms like the Haka mechanism, which involves repeated addition and abstraction of hydrogen and carbon. The speaker points out that reactions involving stable species with closed shells, such as soot precursors, are relatively slow, which may hinder the inception process at lower temperatures where such reactions are necessary.
🌟 The Importance of Dimerization and Inception in Particle Formation
The speaker clarifies the difference between dimerization and inception, emphasizing that dimerization alone does not equate to inception. They discuss various mechanisms that have been proposed in the literature for chemical covalent particle inception, including radical reactions that generate new species and potentially lead to particle formation. The speaker also addresses misconceptions in modeling papers that confuse dimerization with inception and the importance of distinguishing between the two processes.
🔎 Examining Radical Species and Their Role in Particle Inception
The speaker explores the role of radical species in particle inception, referencing gas phase measurements of radical species in flames. They discuss experiments that extracted particles from flames and used collision-induced dissociation to analyze them, revealing the presence of rsrs (resonance-stabilized radicals). The speaker proposes a hypothesis that these rsrs could be involved in a chain reaction that leads to rapid polymerization and particle formation, suggesting a potential mechanism for inception.
🧪 Pyrolysis Experiments and Particle Formation
The speaker presents results from pyrolysis experiments that provide insights into particle formation. They discuss the use of smps (Scanning Mobility Particle Sizer) to measure particle size distributions at various temperatures and the addition of indene, a direct precursor to rsrs, to ethylene flames. The experiments show particle formation at lower temperatures with the addition of indene, supporting the hypothesis that rsrs may play a role in particle inception. The speaker also discusses mass spectrometry analysis that reveals the presence of rsrs in the particles.
📊 Mass Spectrometry Analysis of Propane and Propene Pyrolysis
The speaker analyzes mass spectrometry data from pyrolysis experiments involving propane and propene. They note the presence of aliphatic character and extra hydrogens, suggesting the formation of branched structures. The data shows distributions of rsrs at particle onset temperatures, with a significant peak at mass 165. The speaker also discusses the collapse of mass spectra peaks to the lowest mass as temperature increases, indicating a shift towards more saturated species. The findings suggest that radical chain reactions may be driving inception, and the speaker invites further questions and discussion on the topic.
Mindmap
Keywords
💡Hydrogen Abstraction C2H2 Addition (HACA)
💡Molecular Weight Growth
💡Inception
💡Thermodynamically Driven Nucleation
💡Kinetically Controlled Reactions
💡Resonance-Stabilized Radicals (RSRs)
💡Pyrolysis
💡Aromatic Hydrocarbons
💡Carbon to Hydrogen Ratio
💡Surface Growth
Highlights
Introduction to the Haka mechanism for modeling molecular weight growth and surface growth in flames.
Explanation of the hydrogen abstraction carbon addition (HACA) mechanism involving acetylene.
Gibbs free energy illustration for the HACA mechanism showing the growth of multi-ring hydrocarbons.
Differentiation between molecular weight growth and inception in flames.
Discussion on thermodynamically driven nucleation and condensation type mechanisms.
Kinetically controlled reactions leading to covalent bond formation in particle inception.
Review of different inception mechanisms and their plausibility based on Hai Wong's paper.
Analysis of the carbon to hydrogen ratio in particle formation and its implications.
Investigation into the role of radicals and their potential impact on particle inception.
Experiments extracting particles from flames and analyzing their mass spectra.
The Turkerbest hypothesis for particle inception involving radical chain reactions.
Evidence from pyrolysis experiments supporting the Turkerbest hypothesis.
Mass spectrometry analysis revealing the presence of RSRs in particle formation.
Theoretical and experimental challenges in verifying the Turkerbest hypothesis.
Observations of particle formation in propene and propine pyrolysis experiments.
The unique behavior of propargyl (propine) in particle formation despite higher barriers.
The presence of aliphatic character in the mass spectra of particles and its significance.
Final thoughts on the potential role of resonance-stabilized radicals in driving inception.
Transcripts
okay are we ready
okay so um
yeah so let's oh we've been talking
about different types of mechanisms or
you know what do we know what don't we
know
um let me just introduce
um some kind of like
um mechanisms and Concepts we've been
you know working with for a while and
um hakka the Haka mechanism you probably
heard about it it is the mechanism like
the main mechanism people have been
using to model
um molecular weight growth and to talk
about surface growth and all these and
it's a highly useful mechanism so um
this is a so this is a mechanism where
you like um it was introduced by Michael
franklock's group
um many years ago decades ago
um
and it basically is so is it the name of
it the acronym is hydrogen abstraction
the original paper was C2 H2 addition
which is acetylene Edition so you could
even call it hydrogen abstraction carbon
addition okay so
um it's a a multi-step but not that many
steps a couple step um mechanism where
you take a hydrogen off of a carbon
and then acetylene um as is a really
um uh is is one of the most
um concentrated species in a flame you
generate a lot of acetylene uh so
acetylene is usually floating around in
a flame so you once you abstract a
hydrogen from your carbon the acetylene
can attach to your species where you
abstracted the hydrogen where you have
that radical site
and add two carbons with the hydrogen
coming off so basically the chemical
reaction looks like something like your
whatever hydrocarbon species you have
plus a hydrogen you abstract it and you
end up with that radical
plus I said H2 right that radical the
acetylene and the Second Step plus an
acetylene attaches to that
um carbon and with the release of a an H
atom you end up
um in this case you generate a ring some
cases you just generate the the chain
and then a second acetylene comes in and
makes a six-membered ring so
um in this case you did the addition in
um one of these sites that's like a boat
okay so um so that's the basic uh Haka
mechanism
um this is uh a figure that shows kind
of like what you'd get for the Gibbs
free energy for this hydrogen
abstraction
um carbon Edition
mechanism where you'd actually showing
the growth from say a Benzene starting
from a Benzene and then sequentially
adding acetylene to the Benzene and
growing the species to Benzene and then
the two ring thing is anaphylene and
then you grow to
um
phenanthrene that three-ring structure
to a pyrene so just by sequentially
abstracting hydrogens and adding
acetylene you start to grow these these
multi-ring hydrocarbons okay so this is
molecular weight growth
um
this is not Inception right this is
molecular so this is like you you when
in a flame what you're you're if when
this is happening it's usually happening
at higher high temperature and you're
you're growing basically all of the
hydrocarbons
um at once right your your each of those
um say bent so you have a whole bunch of
benzene Rings each Benzene is growing
more and more rings so you have
basically distributed molecular weight
growth of the different gas phase
species in the flame so that's called
molecular weight growth
when you have Inception
now the difference between molecular
weight growth and Inception is
um
the Inception is when you take now all
those hydrocarbons and you Clump them
so you now are taking gas phase and
turning it into a particle
by like basically combining them all
together so it so you can kind of think
about it as
um say you're sitting out on the mall in
you know wherever you live Boston right
and you have a whole bunch of tables out
there and people are you know different
restaurants in the middle of summer it's
really fun okay let's say Paris and
you're and people are sitting out and
you know someone walks by and they join
your group then you're like so each each
little table is getting more and more
people just kind of like chatting and
having a nice time and then a street
performer comes out and everyone rushes
over to the street performer there's
your Inception okay so you're going from
growth of your individual little groups
too like like what is it that's causing
everyone to Clump to one spot okay
that's what you're trying to figure out
why is this happening so so we're trying
to figure out both how do you get
molecular weight growth and how you get
Inception
okay so so there are two classes of
inception mechanisms like you can think
of this as
um two kind of like
um ends and in between there are other
you know like can be a whole bunch of
other mechanisms but there too like
really distinct classes one is this um
thermodynamically driven like nucleation
condensation type of you know the same
way you'd get a droplet forming from
um when it rains when it's humid outside
and it starts to rain your water vapor
is coming up into droplets right that's
your thermodynamically driven type of
mechanism so what you're trying to
figure out is why are your gaseway
species clumping up into your droplets
and in the flame right
um what is causing them their bat not
they're kind of like water you you can
vaporize it back into a vapor right this
would be the same type of thing you're
just like physically binding their
electrostatic bonds
um you're not actually covalent
violently binding them with sharing
electrons between bonds right
um so we call that dispersed Bound by
dispersion forces
so dispersion forces like a van der
waals force between nonpolar
molecules okay so and then the other
class is um this kinetically controlled
type of reactions causing gum covalent
bond formation
um that in the end you have so you have
uh reactions that are making covalent
bonds so your particle at the end is now
covalently bound and you can't just
vaporize it it's more like the phase
transition that happens when you bake a
cake and you're going from a liquid to a
solid you're not going to go back to a
liquid right so this is like a very
different type of of mechanism so the
question is
what are we having kinetic or
thermodynamically driven
um
particle formation
so let's look at the different
um
uh types of mechanisms this is from Hai
Wong's paper is review paper this is a
great paper actually it's written in
2011 came out in 2011 from the
proceedings
um one of the review papers
um
here are three different types of
mechanisms
on the bottom an a is um uh you
basically have this covalently like
adding adding adding probably CO2 maybe
Haka type formation of these like curved
type of structure it's possibly going
into Bucky Balls or other fullering type
structures
and then in the second so in that one
we can pretty well say that it's not
happening it's too slow that mechanism
is too slow we don't see this type of
product normally and um the carbon
dehydrogen ratio for this type of one
particle formation is way too high so
carbon hydrogen ratio would be way way
over two and that's not what we see okay
the second mechanism is now our new are
the one I said was thermodynamically
driven you basically condense ph's or
something
into particles
so that's or nucleate you know we we
might call it nucleating those into
particles
um
the and
um
theory has shown that in order to get
this type of nucleation at flame
temperatures where you see Inception
it's going to take pretty large species
usually larger than 11 aromatic rings to
form this so the question is can that be
happening in the flame do we have
species large enough or is there
something else that's causing those
species to be less volatile so that they
want to
nucleate so and that could be something
like you know it doesn't have oxygen
does oxygen want to cause these things
to be attracted to each other is it
aliphatic side chains is causing them to
be attractive are they radicals are the
radicals the ones that are want to be
attracted so there's a ton of work in
this area trying to figure out what's
going on there
so in order to have something that's
larger if we just have plain
pahs that are kind of nucleating so if
you go back to our stabilimer grid this
would be way over so I know remember I
put the number of six member rings at
the top in blue this would be way over
to the right hand side of our figure
here our chart and that has a really
high carbon to hydrogen ratio these are
large species and we actually don't even
see them in Flames normally right so so
this indicates this doesn't mean that it
doesn't happen but it indicates at least
under atmospheric pressure it's not
happening
um so
um the carbon hydrogen ratio is greater
than 2.4 as probably not the main
mechanism for shift formation for
Inception
okay
so
um
in addition as you go let's see
as you go up
um
The Binding so this is why people think
it's over that over larger larger than
11 Rings why the big you know part
particle masses are good for this type
of mechanism as you go up in number of
aromatic Rings The Binding energy
increases so you basically have more
attraction between these these molecules
so that's on the top curve plot on the
on the bottom plot
we see
um the uh
the
red curve is The Binding sublimation
temperature and that increases because
these things are attracted to each other
right this is thermodynamics because the
larger species are more attracted to
each other The Binding energy is higher
than the sublimation temperature is
going to be higher okay or the
vaporization temperature is going to be
higher
okay
um
so that increases with the number of
carbon atoms
um and but on the other hand the
concentration of these species for each
additional ring you add it decreases
exponentially
so these larger species are just not as
concentrated in the flame so even if you
could you somehow get one of these these
two things together they're not going to
find each other very well and they're
probably not going to be responsible at
least this mechan is not going to be
responsible fully responsible for
Inception
okay
so
um
is so here's a figure if you've been
studying so you've probably seen this
figure multiple times this is an
experiment where they sucked the
particles right out of the flame right
um and they did the mess back on the
particles themselves and they see this
interesting these interesting humps and
the Mass Spectrum of the particle
and it kind of indicates that you have
on the left hand side you have all these
like what we would typically see if
you're going to vaporize the particle
you see like all those Peaks
and then you see like almost like you're
adding layers onto the onto the particle
this doesn't mean that you're seeing all
those individual
um
um sheets in your Mass Spectrum you're
actually adding them onto the particles
this is seeing the entire particle
itself so this is a really interesting
experiment
um but I'm not sure if I've seen it
reproduced
um it would be really nice if someone
reproduced it and and delved into what
was going on
um it's a really interesting experiment
uh so this is the the same group and
what they saw is if they ionize so on
the left hand
um side a graph this is ionizing with um
248 nanometers and a Mass Spectrum from
the particle so the particle and the
ionize the 248 nanometers and then they
see actually what what you normally see
in a Mass Spectrum aerosol Mass Spectrum
or laser desorption ionization Mass
Spectrum where you collect the sample
and then you like vaporize it somehow
and then ionize that's very typical of
what you see but if you ionize it a
shorter wavelength where you might be
actually more able to ionize the
particle you actually see these larger
masses and I think that's because you're
actually starting to see the entire
you're seeing the entire particle almost
like grow between and it would be nice
if you could have these data at
different heights in the flame like this
would be a really cool experiment but it
I I don't they didn't seem to do that
measurement yeah
okay so
um
so let's go back to this guy
um so okay so this one isn't we don't
think is is happening
um and this one may be happening but we
need to figure out a way that it could
be happening but it you know our data
seem to indicate that it's not
um and this one
um at the top this is covalently binding
um different so this is the kinetic
mechanism the kinetically controlled
mechanism so that one
may be happening and
um and we think that it probably is at
least I do so this is I'm going to say
like this is this is Hope's Theory
um that um we we're gonna that it's it I
think it's a kinetically control room
okay so
um because I think the other ones seem
to have a lot of issues and we have a
lot of evidence that this is what's
happening so let me just go into a
little bit more detail on that
okay so here's another paper a review
paper that just came out last year by um
Martin edel
this um Marcus crafts group kind of
going into all the different mechanisms
that have been published in the
literature okay so
um so they're they went there are a
whole bunch of them so
um the top one we just talked about
that's the nucleation one it seems like
the Carbon High generation is too large
um and the density is too low for these
species to be large species to be around
unless there's some other way to make
them more less volatile
um and then the second one to the right
is the the kinetically controlled one
that um that I'm voting for okay so
um the ones over to the left
um those are just too slow like the
bottom left one is the one that was I
showed you from Hai Wong's paper
um the first one we talked about that
you know make the curve you know
probably through a Haka type mechanism
um we think that's just too slow
um and these other ones the carbon to
hydrogen ratio if you get species that
are like huge your carbon hydrogen ratio
is just going to be too large right we
just don't see that in a flame
um okay so uh
so let's talk about the now the ones
over to the uh lower right
so where it says c p a h that means
curved pah so the fpah up at the top
right that means flat pah
the so so the question is if you have a
curved pah can you have a dipole moment
because your pah is curved
so how do you think you could get a
curved pah
any ideas
I know you know this
because you've looked at a soccer ball
before
a football
yes you have five membered rings if you
embed yes thank you what was your name
again Tanner
Dalton okay don't okay I am it's okay
I'm gonna remember Dalton yes
I should remember that that's a good
name
um okay so so you're gonna as as soon as
you embed a five-membered ring you can
get curvature right like you look at a
soccer ball it has five membered rings
embedded in all that those six-membered
Rings right okay
um so so here's the theory you have like
a dipole moment associated with this
curvature
um and here's a theoretical study
um that you know kind of assesses what
is The Binding energy for these
different
um molecules whether they're flat or
curved okay so they went through and let
me just point out a couple of them so
this one's corny and that's one's flat
okay and then
um this one is coranuline and that one
is curved
um they're not exactly the same size but
they're close
um and you notice that actually
curvature doesn't seem to have a big
effect even though even though they
calculate that the dipole moment for
karanuline should be comparable to water
um and higher than for chlorine so well
then you know it was a good idea but it
didn't actually work so it turns out
that um
that The Binding energy is is similar
for the two
um and uh and it just didn't help on top
of the fact that when we do the
experiments when we look at the mass for
coranuline and chlorinine we actually
don't see karanuline or chlorine I don't
know what they actually are but they're
not those species so they're not the
stabilimers yes
so why do you think that we do have
kinetics or don't have kinetics
kinetically
yes
yes
I yeah I think that it is kinetic I
think it's chemistry
right
why is the chemistry so oh here's why
the chemistry is slow is because
um uh for okay let's go back to the one
where I said chemistry is too slow for
that
yeah that's a good question
um this one at the bottom left like why
you mean why is that chemistry too slow
okay look at that particle that you're
actually forming on the right hand side
if you say okay we want to get to the
point where I have a particle
that chemistry that would cause that
formation is probably something like the
Haka mechanism
okay so now you're taking a molecule and
you're repeatedly so now instead of
doing like you're repeatedly adding
you're trying you're extracting a
hydrogen you're adding a carbon you're
attraction hydrogen adding a carbon
stretch you know so you're repeatedly
doing this
um so in order for that to so you have a
whole bunch of steps is causing that to
happen we call that sequential growth
molecular weight growth
um on top of the fact that a lot of the
species that are like have these like
closed um shells six-membered rings are
pretty stable so to get over a barrier
the barriers are relatively High to get
over the barrier the reactions are
relatively slow
where you have should Inception
temperatures
this will this like these are these um
the soot uh these Hawker type mechanisms
are faster at high temperatures
uh this is after where you have
Inception
it's lower yeah where you see Inception
it's lower temperatures
not not the high not like 2000 Kelvin or
1500 Kelvin yeah yeah yeah it's right
it's like in that high temperature
regime
yeah so it's it you need to be faster at
lower temperatures
an Inception yeah okay yeah good
question that's an excellent question
okay so so we think this is not
happening
um the density is too low for these to
begin with and
um the carbon hydrogen ratio is just too
high and and they the people who did
this you know Marcus's group
um concluded that like I'm not
judging their work they they actually
conclude made that conclusion
okay
um so
um and I want to okay here so
um
so uh okay so here's another set they
had in that same paper right
um the upper left hand corner with
polyline
um so you basically have
um a whole bunch of
um I guess settling like lumping
together type of mechanism that that we
don't see
um it's that's just not observed because
so we can rule that guy out
um
the bottom two left ones are a
kinetically controlled type mechanisms I
think those are possible
um the um
uh the upper the those three that on the
right those are not Inception mechanisms
those are dimerization mechanisms so yes
those you could have dimerization
mechanisms but that's not the same as
Inception Inception would be dimer plus
growth plus you know plus you know it's
it's um it's kind of like you're sitting
at your tables and and okay two two
groups actually
um notice each other and go hey look
who's over there and then they get
together that doesn't mean everyone else
is going to come around and get together
with them right so so you have to be
careful when someone says they see
dimerization that does not unless they
have a follow-on mechanism that says oh
we're going to keep growing we're going
to keep sucking in all the carbons
be careful because people sometimes
confuse dimerization with Inception
okay and you see this over and over and
over and over again in modeling papers
like we do Inception by pyrene
dimerization
so you just have to be very careful with
with like think be be look at it
carefully when you see that
okay so um and then the right hand is a
possible the right hand is basically the
kinetically controlled for if it were
smaller species okay so let's let's um
talk a little bit more about that
um okay we're back to here our two kind
of classes of mechanisms
so I think that here on the left hand
side the species are too small to
condense or nucleate or whatever you
however you want to say it
um at these Inception temperatures
actually Inception temperatures I think
this 1400 to 1700 Kelvin comes from
measurements and I think those are
really those are really hard
measurements
how do you know when the particles have
been when you get Inception you have to
be able to measure the incipient
particles and that's a really hard
measurement to make so I think actually
it's at low much lower temperatures
where you see particle Inception
um so that's another thing figure out
how to measure incipient particles
um uh and then um reactions between
stable precursors are too slow
um and remember dimerization is not the
same as Inception okay
so what are the what are the
possibilities now let's go back in the
literature and see what other people
what people have proposed for
um chemical
um covalent
um particle Inception okay here's a nice
mechanism where you start out with those
rsrs up at the upper left-hand Corners
cycle pentadieneal and
um
into Neil right those have radical so
it's radical radical reactions generate
another species and then that continues
to react and in in the process you end
up generating a whole bunch of different
radicals
um and this one this is this is
molecular weight growth but you can
imagine that maybe that mechanism could
keep going but in the end what you end
up with here is
um two closed shell very stable species
that are probably not all that reactive
right so now you've hit a dead end right
so now you have to figure out how to
excite those again they're not they're
not prone to be like they're not in the
configuration to generate an rsr so
these are hard to like activate again to
get them going
okay
um
so here's another mechanism again
um we're starting with naphthalene
pretty stable species you know
generating a radical out of it reacting
with a snaffling
um and then generating but now you can
see like maybe in this case you can
regenerate a radical
um it's not an an rsr but but maybe this
is a way to keep the reaction going if
you have you end up with a radical
product right okay so um so this is
another potential Inception mechanism
um here's a mechanism that was just
published recently this comes from Mani
sarathi's group this is published like
last week I think so it's not in your
notes because I added it because it
wasn't you know just came out
um so uh so this is pyrene dimerization
um so it's it's like and
um so on the upper left hand
um Cur uh is a is a mechanism for
physical powering dimerization what
people talk about all the time is is
pyrene nucleating which we know doesn't
happen like it's not nucleating in a
flame at these temperatures
um it's just not thermodynamically
stable the upper right hand one is a
mechanism that was proposed by
um Andrea Diana's group
um with a
propaneal so a radical Pro sorry radical
parine perineal
physically
dimerizing with a pyrene so there's an
attraction because one of them is a
radical right so that that's a little
bit that's hard to do because it's
pretty stable to pull off a hydrogen but
if you're a high enough temperature
maybe you have it happen enough times
and then
um Mani sarathi's group proposes that
you generate this you know attraction
and then you have enough time for them
to react so now you have a covalently
bound pyrene parine type of dimer
so can you think of any reason why this
is not
a really important mechanism as a
formation
just based on what I've told you
[Music]
yeah and and
um what's one reason why it wouldn't
um react a lot when
yeah and you actually have to have
pyrene rather than fluoranthine too
right like especially if you like we
might have some priorine but not a lot
of pyrene if you if a lot of us
floranthem
um but it may be that at some you know
some of them may like do that and you
end up with a dimer right so this isn't
something we should rule out the fact is
that we don't even see Mass 404 in our
Mass Spectra so we don't see this dimer
but you know it's possible that it can
happen but um but you're right I think
that the fact that you're not seeing not
seeing a lot of the pyrene you're not
you know it and it's also pretty stable
Coast shell very stable molecule
so but it's possible that it does happen
yeah
okay
um
and then oh yeah this is what they and
it turns out that when they do the
calculations it happens at pretty high
temperature if it happens it happens at
pretty high temperature
um so you're like but not super high
right 1100 Kelvin you could that's
probably not too far from the Inception
region right so you know it's possible
that it does happen but
okay
so rsrs so these are gas phase
measurements of radical species of of
species in a flame and you see there are
a couple of rsrs here in the gas phase
so you actually can see them in the gas
phase they're there they're there so the
question is how important are they in in
formation and should we start to think
about how to to incorporate them okay so
here's your fluorineal and you have
endoneal so fluorineal is as we see we
see this Mass huge like it's huge for us
165 we all we see it all the time and
it's huge and this is one of the main
Peaks that other people see so if you're
looking for a mechanism if you're trying
to think of something this is a good
mask to start with like what's happening
with this mask why do we always see it
and what's it doing
okay
um
and this is an experiment that was done
by Niels Hansen's group
um where they extracted from a flame
um I know I'm just throwing a lot of
data at you I so like absorb as much as
you can and start thinking about how to
put it all together and make a story
because that's where we are right now
um so this from Niels Hansen's group
where they extract it from a flame
and did
um this Mass Spec where they did
um Collision induced dissociation so
they they basically extracted from a
flame
um uh ionized and then slammed their
ions into the gas gas gas in a gas cell
and they could
um control how fast you know how much
kinetic energy they used to slam their
molecules into the gas phase
so here they did one to five EV
Collision energy so I was
um somewhere in the range of one to five
EV of energy of the ions that they
generated slamming into these um uh
argon or whatever gas the camera which
gas they used in this gas cell and you
can see a whole bunch of Peaks so this
is relatively low energy Collision
energy but they they saw a whole bunch
of Peaks some of those Peaks were rsrs
I've labeled them okay they're in the
dark the darker ones the blue ones
if they used higher Collision Energies
boom a lot of those other Peaks went
away and look what happened the rsrs
popped out right
what does that mean to you
have any ideas
I know slightly in the day it's been
many hours of people talking at you
but if you're if you're if you saw the
these this result like you slammed your
your molecules into these gas and you
saw like a lot of the piece went away
but these ones stuck
okay were you yawning or raising your
hand
okay so just think about it say say you
had like
um two benzenes stuck together
um and they were Mass what
141 like 156 right and then
um low energy they're 156 and then you
slam them harder into
um an argon and boom
right then you'd basically get fennel I
should have done an rsr okay an rsr like
psychopended psychopendidino right
um two C5 uh so that's what 65 Mass 65
two of them 130 140. okay so you see
Mass 40 in your Mass Spectrum you know
low energy you slam into it nothing
happens high energy boom and you break
apart and now you have cyclopedicineal
it kind of so the way they interpreted
the results is it indicated that what
they had in the particles they extracted
were bound
um species that looked like when they
broke apart it they could they could had
larger they had larger curves of
covalently bound things that they could
break apart into
um rsrs
um so they concluded that they had a lot
of like
um bound you know covalently linked
species of rsrs
or actually I think they might have said
other things but I interpret it as rsrs
so I think there are as ours um but they
um interpret as bridged and and branched
things okay that broke apart when the
energy was high enough
okay so there's more evidence that you
have rsrs that are bound together in
these particles okay
so what's the mechanism that would allow
you to do this so here's a proposal
and this is just a hypothesis at this
point this is a paper we published a few
years ago
um where we said okay we have we see all
these rsrs what we think might be
happening is I we have an rsr it reacts
with something and this is an example
just we did fennel but we did a whole
bunch of other like types of examples
and
um in the paper
um
but this uh um so in denial is an rsr
reacts with phenol
um it generates this adduct
um this adduct
um so remember how you stabilize why
aromatics are stable is because they
have they are able they have to have to
share their electron density with with
um they're with bonds with double bonds
the pi orbitals and double bonds right
so that stabilizes and make these things
really like Rock Solid you know it's
hard to break them apart okay so in the
middle here
um
in the at the attic between the um in
denial and the fennel
um is a bond that is not
um is what we call C CP3 so it's a
carbon with three atoms on it so it's
singly bonded to two carbons and two
hydrogens
if you pull off a hydrogen now you're
going to have a radical with an electron
that can pop into a the p orbital on
that carbon and then
interact with all the pi orbitals so
basically
popping off hydrogen on that c
um that tetrahedral or CP3 hybridized
carbon
will give you an rsr back
so what this does is a raction that
regenerates an rsr
and it looks like the Bears are low
enough that that hydrogen can just come
off on its own it doesn't even have to
be abstracted so the barriers act so the
hydrogens can go off on their own so now
you're generating hydrogens hydrogen
atoms which are radicals you're
generating a new rsr radical in this
whole reaction and so this is basically
a chain reaction so it's like a
polymerization so we're kind of like
rapidly polymerizing the carbon that's
um in the The Flame
and generating a particle and this is
how you could get a disordered structure
okay so this is just a hypothesis and
now we're trying to figure out if it's
right so now we're trying to work on
okay how do we verify it we have some
theoretical estimate you know I you know
what okay what we don't have so we have
some theory that suggests it should be
work we have some experiments that show
that we have all these rsrs in the OR
associated with the particles
um we don't have a Model A kinetic model
because in order to run a kinetic model
you have to have rate constants right we
don't have rate constants for any of
these reactions that could be happening
right so now we have to go figure out
how to generate these rate constants so
so we'll be talking a little bit more
about that but
um and we also need experiments like is
is this even okay we have
um we see the rsrs but is there a way we
can kind of really test this this
hypothesis
okay this height this hypothesis called
the turkerbest mechanism which is a
clustering of hydrocarbons by radical
chain reactions
okay so
um and what we think is we have these
um uh
resonance stabilize radicals they get
you have a chain reaction they quickly
generate an um an incipient particle and
then we also think that um we have rsrs
that are associated with the particle
itself and maybe that's how we're
getting surface growth
that is total total speculation total
hypothesis out there
um
so we we need to figure out all of this
um and and we're we're working on other
people a lot of other people are working
on this
um a lot of people are trying to prove
us wrong
um that's okay oh that's okay
um okay
so what's our evidence I don't want to
run over like this is uh we're we end at
5 30 I think okay so we're gonna
so if I if I get close just start waving
because I get too excited about this
stuff okay so let's do pyrolysis
experiment remember smps allows us to
get on particle size distributions right
okay so this is a figure that shows if
we take ethylene so we normally run
ethylene flames it's a really good
sitting fuel it's been studied a lot a
lot of people in our community study it
um so uh so we do paralysis of Ethylene
it's been done 100 times before no one's
really done the looking at um the mass
spec so where I'll show you some Mass
back in a minute
um but so here's the smps
at a number of temperatures so that's on
the axis that's going back over to the
right Mobility size so there's size
distributions as a function of
temperature so the temperature is color
coded okay so you see that there are no
particles we don't see particles in the
smps at temperatures below about 1100
okay so we don't we just don't see
particles for ethylene at those
temperatures but we do see them and they
grow in the size distribution like you
see that the particles are growing in
size as you go up in temperature okay
okay so
what we did is we added so we did this
experiment also with with ending we just
took Indian ending remember makes
indones so it's a what we call Direct
precursor to an rsr so notice that we
see
um particle formation at much lower
temperatures for in for ending
so we generate indino and it generates
it starts to generate particles right
okay so right there it's like okay
that's good we kind of predicted that
something like that would happen if we
did in Indy you know according to our
Theory we should have particles starting
at it more easily form particles with
ending
than with Ethylene okay
um so and so yeah just that's what you
just said and that's an RZR right okay
so now what does it look like with the
mass spec right why are we so now we can
go to the mass spec the aerosol Mass
Spec and say oh why why is this
happening can we can we show what's
happening okay so here's a mass spec on
the left for ethylene notice there are
no particle when there are no particles
we don't see signal okay when we have
particles we see signal okay so we don't
see particles at the lower temperatures
only at the higher temperatures ending
we see particles at all these
temperatures we see signal at all those
temperatures but we have isolated little
clumps of signals one right there and
one right there it's hard to see this on
this slide
um but but let me show you the mass
Spectra at these temperatures
okay
okay so ethylene on the left ending on
the right at 1073 Kelvin okay no signal
of ethylene we have signal for ending on
the right hand side
in danil is mass 115 okay and you see
the Indian opaque
okay
um
so let's see oh yeah oh wait I was going
to show you this
yeah
um if we go up to higher temperatures
now we see ethylene pyrolysis
um at 1173 we see a whole bunch of Peaks
because it's now starting to form
particles
um and ending actually internal ending
looks the same about the same
um okay
if we take a little tiny bit of ending
and put it into ethylene so now we seed
the ethylene with indine so we see that
in in ethylene with a little bit of rsr
we start to see particles at the lower
temperature
so
um so it's like oh well that's that's
kind of a good thing like compared to
like if we're trying to see that our
theories right like oh that means that
just a little bit of rsr is starting to
seed particle formation with ethylene
okay so we have this little bit of
mixture and what's interesting is
if you see it 1073 Kelvin for the ending
alone and for the ethylene plus indine
it's not just ending
thus reacting the Indian has to be
reacting with the Ethylene
with with Ethylene ethylene isn't
falling apart yet so in denil has to be
reacting with ethylene to generate
particles because we see other Peaks
that are coming up in there that we
don't see under just ending alone so
it's not just ending reacting with
itself okay so so that's actually kind
of fascinating
um so we see also under for the ending
we see the 115 Peak
we see
two endonils making the 230 Peak we see
three endonos we actually see four into
Nils two I just didn't put it on here
um and then our Mass Spec you know
doesn't see anything else
um uh but you know we also see those
same Peaks actually at
um for the mixture
um but they're not actually all that big
right so it's the endonil is reacting
with the ethylene we think in that
mixture okay
cool so that kind of is really kind of
convincing that maybe something like
this is working
um okay
and and that the radicals are are
reacting with a closed shell species the
radicals are reacting with ethylene
which is closed shells not a radical
so that means it could be sucking up a
whole bunch of the hydrocarbons yeah
oh um it's like
um uh it's uh
you would ask something like so hard
um uh is
um
it's like a maybe five percent by
um a number of of ending in ethylene and
then it's diluted in in um argon yeah so
it's massively diluted it's only it's
actually not that much ethylene it's
massively diluted and argon yeah
okay
so that's that's kind of a fun
experiment so what happens I don't I'm
not going to run I'm not going to run
over so um if we have uh other rsers so
we can do this with propene and propine
propine is nice because it makes
a very important molecule in combustion
you've probably heard about perpardulin
Benzene and it's and and the the drama
associated with it so per partial plus
propardial is likely what what people
think causes Benzene formation in a
flame so perpartual is a nice ours it is
an rsr so we can take propane
generate per partial propane also
generates an rsr called allyl so they're
very similar
the barrier to generating
um propardial from propine is higher
than the barrier for generating aloe
propane okay so you might expect more
particles easier formation from propane
than propine
um okay so
um
sorry
so um here it is
even though like okay so propine has uh
lower
um particle formation temperature than
propene even though the barrier for Aloe
formation is lower for propane than for
propylene so there's something magical
about propagal propagal is is a happy
molecule
um it so so this is this is a an
interesting
um uh observation and we don't
understand it okay so this is another
one of those things that we're working
on
um and other people are working on so
it's kind of a fun fun result
um but we were we're gonna we're gonna
nail it we're gonna figure it out
um okay so just looking in more detail
at the mass Spectra
um this is at at three different
temperatures the lowest of the top
highest at the bottom and notice that on
on these Mass Spectra for each number of
carbons they're at the lowest
temperature there's a distribution of
Peaks
um so the
um Peak with the highest carbon to
hydrogen ratio would be on your left
because each of those Peaks on the right
is adding a hydrogen atom to the that
mass whatever the number of carbons so
it'd be like kind of like 18 carbons
with a distribution of numbers hydrogens
okay and then as you go up in
temperature
those collapse to the lowest
to the least number of carbons does that
make sense
you're supposed to go yeah yeah it does
yeah because remember as we're going up
in temperature like we saw this result
before right we saw this result like as
we go up 100 and and temperature we
eliminate those hydrogens and we come
collapse back down to like pericondensed
um pH probably something like that we
rearrange and get rid of hydrogens and
we start to be a happy pH okay
so that's for propane for propine
um
uh this is for propane see the same
things
um so uh this exact same thing you have
clusters of Peaks up at the top at the
lowest temperatures go up in temperature
and we collapse that back down to the
lowest Mass
the highest carbon to hydrogen at a
particular carbon okay
so that's all making sense that's all
good but the interesting thing is we
actually have a lot of aliphatic
character right what is causing it's
extra hydrogens how would you get that
aliphatic branching right all those
extra you know
um branches that add hydrogen onto your
your whatever your molecule okay
um so we think that these are are
examples of that of that happening
okay
um
and now we see for propane and propine
the same rsrs right this is a lower
lower
um masses we see the same rsrs
um and um so at the particle onset
temperatures
um we see the same types of
distributions of not exactly the same
rsrs we see in the flame like all the
Flames like many of the Flames but we
see a lot of them again 165 is huge so
if you figure that one out let me know
let me know what's going on there
um okay so let's see
um
okay I'll go for a couple minutes but
I'm not going to go over I'm not going
to go over okay so we saw this one
before
um
again this is the number of carbon
hydrogen carbons
um and the carbon to hydrogen ratio is
the lowest for the largest masses right
so you get that distribution for the
large masses right because you can have
that
um carbon you you have a lot more
probably a lot more branching at these
higher masses I mean I think that might
be what's going on
um and that dashed line is if you took
the stabilimers and you added up the
carbon to hydrogen ratio for the
stabilimers for each of the numbers of
carbons okay okay
um oh yeah that's what I just said that
the large
um larger species are more saturated
that have more hydrogen right saturated
means having more hydrogen
um okay uh the uh and it decreases with
temperature as we've said before right
we keep seeing this is an interesting
thing
um and that it's not driven by entirely
by thermodynamics right it's not that
most of thermodynamically stable species
by a long shot at these um at these
different masses okay so so there's
something that's going on that's keeping
us from sinking into those thermodynamic
Wells and staying there where we have
some kind of mechanism that's like
Dynamic it's exciting things are
reacting right so we're making particles
um okay so uh hey sorry if I'm yelling
at you
um okay so these residently stable
stabilized radicals may be driven
um the Inception may be driven by
radical chain reactions
okay so um I think I'll stop there
because it's a good stopping place for
today and then we'll pick up on how do
you model this like the next step is
like how would you put this into a model
what would be the first step because
right now no one's modeling it
um because we haven't actually gotten to
the point where we can
okay so are there any questions on any
of this
um
you're and if you just really want to
speak into this that microphone
okay so remember to meet each other
remember to say hi if you haven't if
there's someone in here you don't know
walk over and say hi my name is
um these are your colleagues for the
future
um these are people who are going to
help you and you can help them it's
gonna be fun
okay
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